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<article id="man">
<artheader>
  <title>Nmap Reference Guide (Man Page)</title>
</artheader>
<refentry id="man-nmap1">
  <refmeta>
    <refentrytitle>nmap</refentrytitle>
    <manvolnum>1</manvolnum>
  </refmeta>
  <refnamediv id="man-name">
    <refname>nmap</refname>
    <refpurpose>Network exploration tool and security / port scanner</refpurpose>
  </refnamediv>
  <!-- body begins here -->
  <refsynopsisdiv id="man-synopsis">
    <cmdsynopsis sepchar=" ">
      <command moreinfo="none">nmap</command>
      <arg choice="opt" rep="repeat">
        <replaceable>Scan Type</replaceable>
      </arg>
      <arg choice="opt" rep="norepeat">
        <replaceable>Options</replaceable>
      </arg>
      <arg choice="req" rep="norepeat">
        <replaceable>target specification</replaceable>
      </arg>
    </cmdsynopsis>
  </refsynopsisdiv>
  <refsect1 id="man-description">
    <title>Description</title>

    <para>Nmap (<quote>Network Mapper</quote>) is an open source tool for network
    exploration and security auditing.  It was designed to rapidly
    scan large networks, although it works fine against single
    hosts. Nmap uses raw IP packets in novel ways to determine what
    hosts are available on the network, what services (application
    name and version) those hosts are offering, what operating systems
    (and OS versions) they are running, what type of packet
    filters/firewalls are in use, and dozens of other
    characteristics.  While Nmap is commonly used for security audits,
    many systems and network administrators find it useful for routine
    tasks such as network inventory, managing service upgrade
    schedules, and monitoring host or service uptime.</para>

    <para>The output from Nmap is a list of scanned targets, with
    supplemental information on each depending on the options
    used. Key among that information is the <quote>interesting ports
    table</quote>.  That table lists the port number and protocol,
    service name, and state.  The state is either
    <literal moreinfo="none">open</literal>, <literal moreinfo="none">filtered</literal>,
    <literal moreinfo="none">closed</literal>, or <literal moreinfo="none">unfiltered</literal>. Open
    means that an application on the target machine is listening for
    connections/packets on that port. <literal moreinfo="none">Filtered</literal> means that a firewall,
    filter, or other network obstacle is blocking the port so that
    Nmap cannot tell whether it is <literal moreinfo="none">open</literal> or <literal moreinfo="none">closed</literal>.  <literal moreinfo="none">Closed</literal> ports have
    no application listening on them, though they could open up at any
    time.  Ports are classified as <literal moreinfo="none">unfiltered</literal> when they are responsive
    to Nmap's probes, but Nmap cannot determine whether they are open
    or closed.  Nmap reports the state combinations
    <literal moreinfo="none">open|filtered</literal> and
    <literal moreinfo="none">closed|filtered</literal> when it cannot determine which
    of the two states describe a port.  The port table may also
    include software version details when version detection has been
    requested.  When an IP protocol scan is requested
    (<option>-sO</option>), Nmap provides information on supported IP
    protocols rather than listening ports.</para>

    <para>In addition to the interesting ports table, Nmap can provide
    further information on targets, including reverse DNS names,
    operating system guesses, device types, and MAC addresses.</para>

    <para>A typical Nmap scan is shown in <xref linkend="man-ex-repscan"/>.  The only Nmap arguments used in
    this example are <option>-A</option>, to enable OS and version
    detection, <option>-T4</option> for faster execution, and then the
    two target hostnames.</para>

<example id="man-ex-repscan"><title>A representative Nmap scan</title>
<screen format="linespecific">
# nmap -A -T4 scanme.nmap.org playground

Starting nmap ( http://www.insecure.org/nmap/ )
Interesting ports on scanme.nmap.org (205.217.153.62):
(The 1663 ports scanned but not shown below are in state: filtered)
PORT    STATE  SERVICE VERSION
22/tcp  open   ssh     OpenSSH 3.9p1 (protocol 1.99)
53/tcp  open   domain
70/tcp  closed gopher
80/tcp  open   http    Apache httpd 2.0.52 ((Fedora))
113/tcp closed auth
Device type: general purpose
Running: Linux 2.4.X|2.5.X|2.6.X
OS details: Linux 2.4.7 - 2.6.11, Linux 2.6.0 - 2.6.11
Uptime 33.908 days (since Thu Jul 21 03:38:03 2005)

Interesting ports on playground.nmap.org (192.168.0.40):
(The 1659 ports scanned but not shown below are in state: closed)
PORT     STATE SERVICE       VERSION
135/tcp  open  msrpc         Microsoft Windows RPC
139/tcp  open  netbios-ssn
389/tcp  open  ldap?
445/tcp  open  microsoft-ds  Microsoft Windows XP microsoft-ds
1002/tcp open  windows-icfw?
1025/tcp open  msrpc         Microsoft Windows RPC
1720/tcp open  H.323/Q.931   CompTek AquaGateKeeper
5800/tcp open  vnc-http      RealVNC 4.0 (Resolution 400x250; VNC TCP port: 5900)
5900/tcp open  vnc           VNC (protocol 3.8)
MAC Address: 00:A0:CC:63:85:4B (Lite-on Communications)
Device type: general purpose
Running: Microsoft Windows NT/2K/XP
OS details: Microsoft Windows XP Pro RC1+ through final release
Service Info: OSs: Windows, Windows XP

Nmap finished: 2 IP addresses (2 hosts up) scanned in 88.392 seconds
</screen>
</example>

<para>The newest version of Nmap can be obtained from <ulink url="http://www.insecure.org/nmap/"/>.  The newest version of the man
page is available from <ulink url="http://www.insecure.org/nmap/man/"/>.</para>

  </refsect1>

  <refsect1 id="man-briefoptions">
    <title>Options Summary</title>

<para>This options summary is printed when Nmap is run
with no arguments, and the latest version is always available at
<ulink url="http://www.insecure.org/nmap/data/nmap.usage.txt"/>.
It helps people remember the most common options, but is no
substitute for the in-depth documentation in the rest of this
manual.  Some obscure options aren't even included here.</para>

<para>
<literallayout format="linespecific" class="normal">Usage: nmap [Scan Type(s)] [Options] {target specification}
TARGET SPECIFICATION:
  Can pass hostnames, IP addresses, networks, etc.
  Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254
  -iL &lt;inputfilename&gt;: Input from list of hosts/networks
  -iR &lt;num hosts&gt;: Choose random targets
  --exclude &lt;host1[,host2][,host3],...&gt;: Exclude hosts/networks
  --excludefile &lt;exclude_file&gt;: Exclude list from file
HOST DISCOVERY:
  -sL: List Scan - simply list targets to scan
  -sP: Ping Scan - go no further than determining if host is online
  -P0: Treat all hosts as online -- skip host discovery
  -PS/PA/PU [portlist]: TCP SYN/ACK or UDP discovery to given ports
  -PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
  -n/-R: Never do DNS resolution/Always resolve [default: sometimes]
  --dns-servers &lt;serv1[,serv2],...&gt;: Specify custom DNS servers
  --system-dns: Use OS's DNS resolver
SCAN TECHNIQUES:
  -sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
  -sN/sF/sX: TCP Null, FIN, and Xmas scans
  --scanflags &lt;flags&gt;: Customize TCP scan flags
  -sI &lt;zombie host[:probeport]&gt;: Idlescan
  -sO: IP protocol scan
  -b &lt;ftp relay host&gt;: FTP bounce scan
PORT SPECIFICATION AND SCAN ORDER:
  -p &lt;port ranges&gt;: Only scan specified ports
    Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080
  -F: Fast - Scan only the ports listed in the nmap-services file)
  -r: Scan ports consecutively - don't randomize
SERVICE/VERSION DETECTION:
  -sV: Probe open ports to determine service/version info
  --version-intensity &lt;level&gt;: Set from 0 (light) to 9 (try all probes)
  --version-light: Limit to most likely probes (intensity 2)
  --version-all: Try every single probe (intensity 9)
  --version-trace: Show detailed version scan activity (for debugging)
OS DETECTION:
  -O: Enable OS detection
  --osscan-limit: Limit OS detection to promising targets
  --osscan-guess: Guess OS more aggressively
TIMING AND PERFORMANCE:
  -T[0-5]: Set timing template (higher is faster)
  --min-hostgroup/max-hostgroup &lt;size&gt;: Parallel host scan group sizes
  --min-parallelism/max-parallelism &lt;msec&gt;: Probe parallelization
  --min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout &lt;msec&gt;: Specifies
      probe round trip time.
  --max-retries &lt;tries&gt;: Caps number of port scan probe retransmissions.
  --host-timeout &lt;msec&gt;: Give up on target after this long
  --scan-delay/--max-scan-delay &lt;msec&gt;: Adjust delay between probes
FIREWALL/IDS EVASION AND SPOOFING:
  -f; --mtu &lt;val&gt;: fragment packets (optionally w/given MTU)
  -D &lt;decoy1,decoy2[,ME],...&gt;: Cloak a scan with decoys
  -S &lt;IP_Address&gt;: Spoof source address
  -e &lt;iface&gt;: Use specified interface
  -g/--source-port &lt;portnum&gt;: Use given port number
  --data-length &lt;num&gt;: Append random data to sent packets
  --ttl &lt;val&gt;: Set IP time-to-live field
  --spoof-mac &lt;mac address/prefix/vendor name&gt;: Spoof your MAC address
  --badsum: Send packets with a bogus TCP/UDP checksum
OUTPUT:
  -oN/-oX/-oS/-oG &lt;file&gt;: Output scan in normal, XML, s|&lt;rIpt kIddi3,
     and Grepable format, respectively, to the given filename.
  -oA &lt;basename&gt;: Output in the three major formats at once
  -v: Increase verbosity level (use twice for more effect)
  -d[level]: Set or increase debugging level (Up to 9 is meaningful)
  --packet-trace: Show all packets sent and received
  --iflist: Print host interfaces and routes (for debugging)
  --append-output: Append to rather than clobber specified output files
  --resume &lt;filename&gt;: Resume an aborted scan
  --stylesheet &lt;path/URL&gt;: XSL stylesheet to transform XML output to HTML
  --webxml: Reference stylesheet from Insecure.Org for more portable XML
  --no-stylesheet: Prevent associating of XSL stylesheet w/XML output
MISC:
  -6: Enable IPv6 scanning
  -A: Enables OS detection and Version detection
  --datadir &lt;dirname&gt;: Specify custom Nmap data file location
  --send-eth/--send-ip: Send using raw ethernet frames or IP packets
  --privileged: Assume that the user is fully privileged
  -V: Print version number
  -h: Print this help summary page.
EXAMPLES:
  nmap -v -A scanme.nmap.org
  nmap -v -sP 192.168.0.0/16 10.0.0.0/8
  nmap -v -iR 10000 -P0 -p 80
</literallayout>

</para>

  </refsect1>

  <refsect1 id="man-target-specification">
    <title>Target Specification</title>

<para>Everything on the Nmap command-line that isn't an option (or
option argument) is treated as a target host specification.  The
simplest case is to specify a target IP address or hostname for scanning.</para>

<para>Sometimes you wish to scan a whole network of adjacent hosts.
For this, Nmap supports CIDR-style addressing.  You can append
/<replaceable>numbits</replaceable> to an IP address or hostname and
Nmap will scan every IP address for which the first
<replaceable>numbits</replaceable> are the same as for the reference
IP or hostname given.  For example, 192.168.10.0/24 would scan the 256
hosts between 192.168.10.0 (binary: <literal moreinfo="none">11000000 10101000
00001010 00000000</literal>) and 192.168.10.255 (binary: <literal moreinfo="none">11000000 10101000
00001010 11111111</literal>), inclusive.
192.168.10.40/24 would do exactly the same thing.  Given that the host
scanme.nmap.org is at the IP address 205.217.153.62, the specification
scanme.nmap.org/16 would scan the 65,536 IP addresses between
205.217.0.0 and 205.217.255.255.  The smallest allowed value is /1,
which scans half the Internet.  The largest value is 32, which scans
just the named host or IP address because all address bits are fixed.</para>

<para>CIDR notation is short but not always flexible enough.  For example, you
might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or
.255 because they are commonly broadcast addresses.  Nmap supports
this through octet range addressing.  Rather than specify a normal IP
address, you can specify a comma separated list of numbers or ranges
for each octet.  For example, 192.168.0-255.1-254 will skip all
addresses in the range that end in .0 and or .255.  Ranges need not be
limited to the final octects: the specifier
0-255.0-255.13.37 will perform an Internet-wide scan for all IP
addresses ending in 13.37.  This sort of broad sampling can be useful
for Internet surveys and research.</para>

<para>IPv6 addresses can only be specified by their fully qualified IPv6
address or hostname.  CIDR and octet ranges aren't supported for
IPv6 because they are rarely useful.</para>

<para>Nmap accepts multiple host specifications on the command line,
and they don't need to be the same type.  The command <command moreinfo="none">nmap
scanme.nmap.org 192.168.0.0/8 10.0.0,1,3-7.0-255</command> does what
you would expect.</para>

<para>While targets are usually specified on the command lines, the following options are also available to control target selection:</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-iL &lt;inputfilename&gt;</option> (Input from list)
        </term>
        <listitem>
          <para>Reads target specifications from
          <replaceable>inputfilename</replaceable>.  Passing a huge
          list of hosts is often awkward on the command line, yet it
          is a common desire.  For example, your DHCP server might
          export a list of 10,000 current leases that you wish to
          scan.  Or maybe you want to scan all IP addresses
          <emphasis>except</emphasis> for those to locate hosts using
          unauthorized static IP addresses.  Simply generate the list
          of hosts to scan and pass that filename to Nmap as an
          argument to the <option>-iL</option> option.  Entries can be
          in any of the formats accepted by Nmap on the command line
          (IP address, hostname, CIDR, IPv6, or octet ranges).  Each
          entry must be separated by one or more spaces, tabs, or
          newlines.  You can specify a hyphen (<literal moreinfo="none">-</literal>)
          as the filename if you want Nmap to read hosts from standard
          input rather than an actual file.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-iR &lt;num hosts&gt;</option> (Choose random targets)
        </term>
        <listitem>
          <para>For Internet-wide surveys
          and other research, you may want to choose targets at
          random.  The <replaceable>num hosts</replaceable> argument
          tells Nmap how many IPs to generate.  Undesirable IPs such
          as those in certain private, multicast, or unallocated
          address ranges are automatically skipped.  The argument <literal moreinfo="none">0</literal>
          can be specified for a never-ending scan.  Keep in mind that
          some network administrators bristle at unauthorized scans of
          their networks and may complain.  Use this option at your
          own risk!  If you find yourself really bored one rainy
          afternoon, try the command <command moreinfo="none">nmap -sS -PS80 -iR 0 -p
          80</command> to locate random web servers for
          browsing.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--exclude
          &lt;host1[,host2][,host3],...&gt;</option> (Exclude hosts/networks)
        </term>
        <listitem>
          <para>Specifies a comma-separated list of targets to be
          excluded from the scan even if they are part of the overall
          network range you specify.  The list you pass in uses normal
          Nmap syntax, so it can include hostnames, CIDR netblocks,
          octet ranges, etc.  This can be useful when the network you
          wish to scan includes untouchable mission-critical servers,
          systems that are known to react adversely to port scans,
          or subnetworks administered by other people.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--excludefile &lt;exclude_file&gt;</option> (Exclude list from file)
        </term>
        <listitem>
          <para>This offers the same functionality as the <option>--exclude</option>
          option, except that the excluded targets are provided in a
          newline, space, or tab delimited
          <replaceable>exclude_file</replaceable> rather than on the
          command line.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id="man-host-discovery">
    <title>Host Discovery</title>
 
   <para>One of the very first steps in any network reconnaissance
    mission is to reduce a (sometimes huge) set of IP ranges into a
    list of active or interesting hosts. Scanning every port of
    every single IP address is slow and usually unnecessary. Of
    course what makes a host interesting depends greatly on the
    scan purposes. Network administrators may only be interested in
    hosts running a certain service, while security auditors may
    care about every single device with an IP address. An
    administrator may be comfortable using just an ICMP ping to
    locate hosts on his internal network, while an external
    penetration tester may use a diverse set of dozens of probes in
    an attempt to evade firewall restrictions.</para>

    <para>Because host discovery needs are so diverse, Nmap offers a
    wide variety of options for customizing the techniques used. Host
    discovery is sometimes called ping scan, but it goes well beyond
    the simple ICMP echo request packets associated with the
    ubiquitous <application moreinfo="none">ping</application> tool. Users can skip
    the ping step entirely with a list scan (<option>-sL</option>) or
    by disabling ping (<option>-P0</option>), or engage the network
    with arbitrary combinations of multi-port TCP SYN/ACK, UDP, and
    ICMP probes. The goal of these probes is to solicit responses
    which demonstrate that an IP address is actually active (is being
    used by a host or network device). On many networks, only a small
    percentage of IP addresses are active at any given time. This is
    particularly common with RFC1918-blessed private address space
    such as 10.0.0.0/8. That network has 16 million IPs, but I have
    seen it used by companies with less than a thousand machines. Host
    discovery can find those machines in a sparsely allocated sea of
    IP addresses.</para>

    <para>If no host discovery options are given, Nmap
          sends a TCP ACK
          packet destined for port 80 and an ICMP Echo Request query
          to each target machine.  An exception to this is that an ARP scan is
          used for any targets which are on a local ethernet network.
          For unprivileged UNIX shell users, a SYN packet is sent
          instead of the ack using the <function moreinfo="none">connect()</function>
          system call.  These defaults are equivalent to the
          <option>-PA -PE</option> options.  This host discovery is
          often sufficient when scanning local networks, but a more
          comprehensive set of discovery probes is recommended for
          security auditing.</para>
    
    <para>The <option>-P*</option> options (which select
    ping types) can be combined.  You can increase your odds of
    penetrating strict firewalls by sending many probe types using
    different TCP ports/flags and ICMP codes.  Also note that ARP
    discovery (<option>-PR</option>) is done by default against
    targets on a local ethernet network even if you specify other
    <option>-P*</option> options, because it is almost always faster
    and more effective.</para>

    <para>The following options control host discovery.</para>
    <variablelist>

      <varlistentry>
        <term>
        <option>-sL</option> (List Scan)</term>
        <listitem>
          <para>The list scan is a degenerate form of host discovery
          that simply lists each host of the network(s) specified,
          without sending any packets to the target hosts.  By
          default, Nmap still does reverse-DNS resolution on the hosts
          to learn their names.  It is often surprising how much
          useful information simple hostnames give out.  For example,
          <literal moreinfo="none">fw.chi.playboy.com</literal> is the firewall for the Chicago office of
          Playboy Enterprises.  Nmap also reports the total number of
          IP addresses at the end.  The list scan is a good sanity
          check to ensure that you have proper IP addresses for your
          targets.  If the hosts sport domain names you do not
          recognize, it is worth investigating further to prevent
          scanning the wrong company's network.</para>

          <para>Since the idea is to simply print a list of target
          hosts, options for higher level functionality such as port
          scanning, OS detection, or ping scanning cannot be combined
          with this.  If you wish to disable ping scanning while still
          performing such higher level functionality, read up on the
          <option>-P0</option> option.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sP</option> (Ping Scan)</term>
        <listitem>

           <para>This option tells Nmap to <emphasis>only</emphasis>
           perform a ping scan (host discovery), then print out the available hosts
           that responded to the scan.  No further testing (such as
           port scanning or OS detection) is performed.  This is one
           step more intrusive than the list scan, and can often be
           used for the same purposes.  It allows light reconnaissance
           of a target network without attracting much attention.
           Knowing how many hosts are up is more valuable to attackers
           than the list provided by list scan of every single IP and host name.</para>

           <para>Systems administrators often find this option
           valuable as well.  It can easily be used to count available
           machines on a network or monitor server availability.  This
           is often called a ping sweep, and is more reliable than
           pinging the broadcast address because many hosts do not
           reply to broadcast queries.</para>

           <para>The <option>-sP</option> option sends an ICMP echo
           request and a TCP packet to port 80 by default.  When
           executed by an unprivileged user, a SYN packet is sent
           (using a <function moreinfo="none">connect()</function> call) to port 80 on
           the target.  When a privileged user tries to scan targets
           on a local ethernet network, ARP requests
           (<option>-PR</option>) are used unless
           <option>--send-ip</option> was specified.
           The <option>-sP</option> option can be combined with any of the
           discovery probe types (the <option>-P*</option> options,
           excluding <option>-P0</option>) for greater flexibility.
           If any of those probe type and port number options are
           used, the default probes (ACK and echo request) are
           overridden.  When strict firewalls are in place between the
           source host running Nmap and the target network, using
           those advanced techniques is recommended.  Otherwise hosts
           could be missed when the firewall drops probes or their
           responses.</para>

        </listitem>
      </varlistentry>


      <varlistentry>
        <term>
          <option>-P0</option> (No ping)
        </term>
        <listitem>
          <para>This option skips the Nmap discovery stage altogether.
          Normally, Nmap uses this stage to determine active machines
          for heavier scanning.  By default, Nmap only performs heavy
          probing such as port scans, version detection, or OS
          detection against hosts that are found to be up.  Disabling
          host discovery with <option>-P0</option> causes Nmap to
          attempt the requested scanning functions against
          <emphasis>every</emphasis> target IP address specified.  So
          if a class B sized target address space (/16) is specified
          on the command line, all 65,536 IP addresses are scanned.
          That second option character in <option>-P0</option> is a
          zero and not the letter O.  Proper host discovery is skipped
          as with the list scan, but instead of stopping and printing
          the target list, Nmap continues to perform requested
          functions as if each target IP is active.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PS [portlist]</option> (TCP SYN Ping)</term>
        <listitem>

          <para>This option sends an empty TCP packet with the SYN
          flag set.  The default destination port is 80 (configurable
          at compile time by changing DEFAULT_TCP_PROBE_PORT in
          <filename moreinfo="none">nmap.h</filename>), but an alternate port can be
          specified as a parameter.  A comma separated list of ports
          can even be specified
          (e.g. <option>-PS22,23,25,80,113,1050,35000</option>), in
          which case probes will be attempted against each port in
          parallel.</para>

          <para>The SYN flag suggests to the remote system that you
          are attempting to establish a connection.  Normally the
          destination port will be closed, and a RST (reset) packet
          sent back.  If the port happens to be open, the target will
          take the second step of a TCP 3-way-handshake by responding
          with a SYN/ACK TCP packet. The machine running Nmap then
          tears down the nascent connection by responding with a RST
          rather than sending an ACK packet which would complete the
          3-way-handshake and establish a full
          connection.  The RST packet is sent by the
          kernel of the machine running Nmap in response to the
          unexpected SYN/ACK, not by Nmap itself.</para>

          <para>Nmap does not care whether the port is open or closed.
          Either the RST or SYN/ACK response discussed previously tell
          Nmap that the host is available and responsive.</para>

          <para>On UNIX boxes, only the privileged user
          <literal moreinfo="none">root</literal> is generally able to send and
          receive raw TCP packets.  For unprivileged users, a
          workaround is automatically employed whereby the connect()
          system call is initiated against each target port.  This has
          the effect of sending a SYN packet to the target host, in an
          attempt to establish a connection.  If connect() returns
          with a quick success or an ECONNREFUSED failure, the
          underlying TCP stack must have received a SYN/ACK or RST and
          the host is marked available.  If the connection attempt
          is left hanging until a timeout is reached, the host is
          marked as down.  This workaround is also used for IPv6
          connections, as raw IPv6 packet building support is not yet
          available in Nmap.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PA [portlist]</option> (TCP ACK Ping)</term>
        <listitem>
          <para>The TCP ACK ping is quite similar to the
          just-discussed SYN ping.  The difference, as you could
          likely guess, is that the TCP ACK flag is set instead of the
          SYN flag.  Such an ACK packet purports to be acknowledging
          data over an established TCP connection, but no such
          connection exists.  So remote hosts should always respond
          with a RST packet, disclosing their existence in the
          process.</para>

          <para>The <option>-PA</option> option uses the same default
          port as the SYN probe (80) and can also take a list of
          destination ports in the same format.  If an unprivileged
          user tries this, or an IPv6 target is specified, the
          connect() workaround discussed previously is used.  This
          workaround is imperfect because connect() is actually
          sending a SYN packet rather than an ACK.</para>

          <para>The reason for offering both SYN and ACK ping probes
          is to maximize the chances of bypassing firewalls.  Many
          administrators configure routers and other simple firewalls
          to block incoming SYN packets except for those destined for
          public services like the company web site or mail server.
          This prevents other incoming connections to the
          organization, while allowing users to make unobstructed
          outgoing connections to the Internet.  This non-stateful
          approach takes up few resources on the firewall/router and
          is widely supported by hardware and software filters.  The
          Linux Netfilter/iptables firewall software offers the
          <option>--syn</option> convenience option to implement this
          stateless approach.  When stateless firewall rules such as
          this are in place, SYN ping probes (<option>-PS</option>)
          are likely to be blocked when sent to closed target ports.
          In such cases, the ACK probe shines as it cuts right through
          these rules.</para>

          <para>Another common type of firewall uses stateful rules
          that drop unexpected packets.  This feature was initially
          found mostly on high-end firewalls, though it has become
          much more common over the years.  The Linux
          Netfilter/iptables system supports this through the
          <option>--state</option> option, which categorizes packets
          based on connection state.  A SYN probe is more likely to
          work against such a system, as unexpected ACK packets are
          generally recognized as bogus and dropped.  A solution to this quandary is
          to send both SYN and ACK probes by specifying
          <option>-PS</option> and <option>-PA</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PU [portlist]</option> (UDP Ping)</term>
        <listitem>

          <para>Another host discovery option is the UDP ping, which
          sends an empty (unless <option>--data-length</option> is
          specified) UDP packet to the given ports.  The portlist
          takes the same format as with the previously discussed
          <option>-PS</option> and <option>-PA</option> options.  If
          no ports are specified, the default is 31338.  This default
          can be configured at compile-time by changing
          DEFAULT_UDP_PROBE_PORT in <filename moreinfo="none">nmap.h</filename>.  A
          highly uncommon port is used by default because sending to
          open ports is often undesirable for this particular scan
          type.</para>

          <para>Upon hitting a closed port on the target machine, the
          UDP probe should elicit an ICMP port unreachable packet in
          return.  This signifies to Nmap that the machine is up and
          available.  Many other types of ICMP errors, such as
          host/network unreachables or TTL exceeded are indicative of
          a down or unreachable host.  A lack of response is also
          interpreted this way.  If an open port is reached, most
          services simply ignore the empty packet and fail to return
          any response.  This is why the default probe port is 31338,
          which is highly unlikely to be in use.  A few services, such
          as chargen, will respond to an empty UDP packet, and thus
          disclose to Nmap that the machine is available.</para>

          <para>The primary advantage of this scan type is that it
          bypasses firewalls and filters that only screen TCP.  For
          example, I once owned a Linksys BEFW11S4 wireless broadband
          router.  The external interface of this device filtered all
          TCP ports by default, but UDP probes would still elicit port
          unreachable messages and thus give away the device.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PE</option>;
        <option>-PP</option>;
        <option>-PM</option> (ICMP Ping Types)</term>
        <listitem>

          <para>In addition to the unusual TCP and UDP host discovery
          types discussed previously, Nmap can send the standard
          packets sent by the ubiquitous
          <application moreinfo="none">ping</application> program.  Nmap sends an ICMP
          type 8 (echo request) packet to the target IP addresses,
          expecting a type 0 (Echo Reply) in return from available
          hosts.  Unfortunately for network explorers, many hosts and
          firewalls now block these packets, rather than responding as
          required by <ulink url="http://www.rfc-editor.org/rfc/rfc1122.txt">RFC
          1122</ulink>.  For this reason, ICMP-only scans are rarely
          reliable enough against unknown targets over the Internet.
          But for system administrators monitoring an internal
          network, they can be a practical and efficient approach.
          Use the <option>-PE</option> option to enable this echo
          request behavior.</para>

          <para>While echo request is the standard ICMP ping query,
          Nmap does not stop there.  The ICMP standard (<ulink url="http://www.rfc-editor.org/rfc/rfc792.txt">RFC
          792</ulink>) also specifies timestamp request, information
          request, and address mask request packets as codes 13, 15,
          and 17, respectively.  While the ostensible purpose for
          these queries is to learn information such as address masks
          and current times, they can easily be used for host
          discovery.  A system that replies is up and available.  Nmap
          does not currently implement information request packets, as
          they are not widely supported.  RFC 1122 insists that
          <quote>a host SHOULD NOT implement these messages</quote>.
          Timestamp and address mask queries can be sent with the
          <option>-PP</option> and <option>-PM</option> options,
          respectively.  A timestamp reply (ICMP code 14) or address
          mask reply (code 18) discloses that the host is available.
          These two queries can be valuable when admins specifically
          block echo request packets while forgetting that other ICMP
          queries can be used for the same purpose.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-PR</option> (ARP Ping)</term>
        <listitem>

          <para>One of the most common Nmap usage scenarios is to scan
          an ethernet LAN. On most LANs, especially those using
          RFC1918-blessed private address ranges, the vast majority of
          IP addresses are unused at any given time.  When Nmap tries
          to send a raw IP packet such as an ICMP echo request, the
          operating system must determine the destination hardware
          (ARP) address corresponding to the target IP so that it can
          properly address the ethernet frame.  This is often slow and
          problematic, since operating systems weren't written with
          the expectation that they would need to do millions of ARP
          requests against unavailable hosts in a short time
          period.</para>

          <para>ARP scan puts Nmap and its optimized algorithms in
          charge of ARP requests.  And if it gets a response back,
          Nmap doesn't even need to worry about the IP-based ping
          packets since it already knows the host is up.  This makes
          ARP scan much faster and more reliable than IP-based scans.
          So it is done by default when scanning ethernet hosts that Nmap
          detects are on a local ethernet network.  Even if different
          ping types (such as <option>-PE</option> or
          <option>-PS</option>) are specified, Nmap uses ARP instead
          for any of the targets which are on the same LAN.  If you
          absolutely don't want to do an ARP scan, specify
          <option>--send-ip</option>.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-n</option> (No DNS resolution)
        </term>
        <listitem>
          <para>Tells Nmap to 
          <emphasis>never</emphasis> do reverse DNS
          resolution on the active IP addresses it finds. Since DNS
          is often slow, this speeds things up.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-R</option> (DNS resolution for all targets)
        </term>
        <listitem>
          <para>Tells Nmap to 
          <emphasis>always</emphasis> do reverse DNS
          resolution on the target IP addresses. Normally this is
          only performed when a machine is found to be alive.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--system_dns</option> (Use system DNS resolver)
        </term>
        <listitem>
          <para>By default, Nmap resolves IP addresses by sending
          queries directly to the name servers configured on your host
          and then listening for responses.  Many requests (often
          dozens) are performed in parallel for performance.  Specify
          this option if you wish to use your system resolver instead
          (one IP at a time via the getnameinfo() call).  This is
          slower and rarely useful unless there is a bug in the Nmap
          DNS code -- please contact us if that is the case.  The
          system resolver is always used for IPv6 scans.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--dns_servers &lt;server1[,server2],...&gt;
          </option> (Servers to use for reverse DNS queries)
        </term>
        <listitem>
          <para>Normally Nmap will try to determine the DNS servers
          from your resolv.conf file (UNIX) or the registry (Win32).
          Alternatively, you may use this option to specify your own
          servers.  This option is not honored if you are using
          <option>--system_dns</option> or an IPv6 scan. Using
          multiple DNS servers is often faster than querying
          just one.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id="man-port-scanning-basics">
    <title>Port Scanning Basics</title>

          <para>While Nmap has grown in functionality over the years,
          it began as an efficient port scanner, and that remains its
          core function.  The simple command <command moreinfo="none">nmap
          <replaceable>target</replaceable></command> scans more than
          1660 TCP ports on the host
          <replaceable>target</replaceable>.  While many port scanners
          have traditionally lumped all ports into the open or closed
          states, Nmap is much more granular.  It divides ports into
          six states: <literal moreinfo="none">open</literal>,
          <literal moreinfo="none">closed</literal>, <literal moreinfo="none">filtered</literal>,
          <literal moreinfo="none">unfiltered</literal>,
          <literal moreinfo="none">open|filtered</literal>, or
          <literal moreinfo="none">closed|filtered</literal>.</para>

<para>These states are not intrinsic
properties of the port itself, but describe how Nmap sees them.  For
example, an Nmap scan from the same network as the target may show
port 135/tcp as open, while a scan at the same time with the same
options from across the Internet might show that port as <literal moreinfo="none">filtered</literal>.</para>

<variablelist><title>The six port states recognized by Nmap</title>

  <varlistentry><term>open</term>
  <listitem><para>An application is actively accepting TCP
  connections or UDP packets on this port.  Finding these is often the
  primary goal of port scanning.  Security-minded people know that
  each open port is an avenue for attack.  Attackers and pen-testers
  want to exploit the open ports, while administrators try to close or
  protect them with firewalls without thwarting legitimate users.
  Open ports are also interesting for non-security scans because they show
  services available for use on the network.
  </para></listitem></varlistentry>

  <varlistentry><term>closed</term>

  <listitem><para>A closed port is accessible (it receives and
  responds to Nmap probe packets), but there is no application
  listening on it.  They can be helpful in showing that a host is up
  on an IP address (host discovery, or ping scanning), and as part
  of OS detection.  Because closed ports are reachable, it may be
  worth scanning later in case some open up.  Administrators may want
  to consider blocking such ports with a firewall.  Then they would
  appear in the filtered state, discussed next.
  </para></listitem></varlistentry>

  <varlistentry><term>filtered</term>

  <listitem><para>Nmap cannot determine whether the port is open
  because packet filtering prevents its probes from reaching the port.
  The filtering could be from a dedicated firewall device, router
  rules, or host-based firewall software.  These ports frustrate
  attackers because they provide so little information.  Sometimes
  they respond with ICMP error messages such as type 3 code 13
  (destination unreachable: communication administratively
  prohibited), but filters that simply drop probes without responding
  are far more common.  This forces Nmap to retry several times just
  in case the probe was dropped due to network congestion rather than
  filtering.  This slows down the scan dramatically.</para></listitem></varlistentry>

  <varlistentry><term>unfiltered</term>
  <listitem><para>The unfiltered state means that a port is accessible,
  but Nmap is unable to determine whether it is open or closed.  Only
  the ACK scan, which is used to map firewall rulesets, classifies
  ports into this state.  Scanning unfiltered ports with other scan
  types such as Window scan, SYN scan, or FIN scan, may help resolve
  whether the port is open.
  </para></listitem></varlistentry>

  <varlistentry><term>open|filtered</term>
  <listitem><para>Nmap places ports in this state when it is unable to
  determine whether a port is open or filtered.  This occurs for scan
  types in which open ports give no response.  The lack of
  response could also mean that a packet filter dropped the probe or
  any response it elicited.  So Nmap does not know for sure whether
  the port is open or being filtered.  The UDP, IP Protocol,
  FIN, Null, and Xmas scans classify ports this
  way.</para></listitem></varlistentry>

  <varlistentry><term>closed|filtered</term>
  <listitem><para>This state is used when Nmap is unable to determine
  whether a port is closed or filtered.  It is only used for the IPID
  Idle scan.</para></listitem></varlistentry>
  </variablelist>
</refsect1>

<refsect1 id="man-port-scanning-techniques">
  <title>Port Scanning Techniques</title>

<para>As a novice performing automotive repair, I can struggle
for hours trying to fit my rudimentary tools (hammer, duct tape,
wrench, etc.)  to the task at hand.  When I fail miserably and tow my
jalopy to a real mechanic, he invariably fishes around in a huge tool chest until
pulling out the perfect gizmo which makes the job seem effortless.  The
art of port scanning is similar.  Experts understand the dozens of
scan techniques and choose the appropriate one (or combination) for a
given task.  Inexperienced users and script kiddies, on the other
hand, try to solve every problem with the default SYN scan.  Since Nmap is
free, the only barrier to port scanning mastery is knowledge.  That
certainly beats the automotive world, where it may take great skill to
determine that you need a strut spring compressor, then you still
have to pay thousands of dollars for it.</para>

<para>Most of the scan types are only available to privileged users.
This is because they send and receive raw packets, which requires root
access on UNIX systems.  Using an administrator account on Windows is
recommended, though Nmap sometimes works for unprivileged users on that
platform when WinPcap has already been loaded into the OS.  Requiring
root privileges was a serious limitation when Nmap was released in
1997, as many users only had access to shared shell accounts.  Now,
the world is different.  Computers are cheaper, far more people have
always-on direct Internet access, and desktop UNIX systems (including
Linux and MAC OS X) are prevalent.  A Windows version of Nmap is now
available, allowing it to run on even more desktops.  For all these
reasons, users have less need to run Nmap from limited shared shell accounts.
This is fortunate, as the privileged options make Nmap far more
powerful and flexible.</para>

<para>While Nmap attempts to produce accurate results, keep in mind
that all of its insights are based on packets returned by the target
machines (or firewalls in front of them).  Such hosts may be
untrustworthy and send responses intended to confuse or mislead Nmap.
Much more common are non-RFC-compliant hosts that do not respond as
they should to Nmap probes.  FIN, Null, and Xmas scans are
particularly susceptible to this problem.  Such issues are specific to
certain scan types and so are
discussed in the individual scan type entries.</para>

<para>This section documents the dozen or so port scan
techniques supported by Nmap.  Only one method may be used at a time,
except that UDP scan (<option>-sU</option>) may be combined with any
one of the TCP scan types.  As a memory aid, port scan type options
are of the form <option>-s<replaceable>C</replaceable></option>, where
<replaceable>C</replaceable> is a prominent character in the scan
name, usually the first.  The one exception to this is the deprecated
FTP bounce scan (<option>-b</option>).  By default, Nmap performs a
SYN Scan, though it substitutes a Connect() scan if the user does not
have proper privileges to send raw packets (requires root access on
UNIX) or if IPv6 targets were specified.  Of the scans listed in this
section, unprivileged users can only execute connect() and ftp bounce
scans.</para>

    <variablelist>
      <varlistentry>
        <term>
        <option>-sS</option> (TCP SYN scan)</term>
        <listitem>

<para>SYN scan is the default and most popular scan option for good
reasons.  It can be performed quickly, scanning thousands of ports per
second on a fast network not hampered by intrusive firewalls. SYN scan
is relatively unobtrusive and stealthy, since it never completes TCP
connections.  It also works against any compliant TCP stack rather
than depending on idiosyncrasies of specific platforms as Nmap's
Fin/Null/Xmas, Maimon and Idle scans do.  It also allows clear,
reliable differentiation between the <literal moreinfo="none">open</literal>,
<literal moreinfo="none">closed</literal>, and <literal moreinfo="none">filtered</literal>
states.</para>

<para>This technique is often referred to as half-open scanning,
because you don't open a full TCP connection. You send a SYN packet,
as if you are going to open a real connection and then wait for a
response. A SYN/ACK indicates the port is listening (open), while a
RST (reset) is indicative of a non-listener. If no response is
received after several retransmissions, the port is marked as
filtered.  The port is also marked filtered if an ICMP unreachable
error (type 3, code 1,2, 3, 9, 10, or 13) is received.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sT</option> (TCP connect() scan)</term>
        <listitem>

<para>TCP Connect() scan is the default TCP scan type when SYN scan is
not an option.  This is the case when a user does not have raw packet
privileges or is scanning IPv6 networks.  Instead of writing raw
packets as most other scan types do, Nmap asks the underlying
operating system to establish a connection with the target machine and
port by issuing the <literal moreinfo="none">connect()</literal> system call.  This is
the same high-level system call that web browsers, P2P clients, and
most other network-enabled applications use to establish a connection.
It is part of a programming interface known as the Berkeley Sockets
API.  Rather than read raw packet responses off the wire, Nmap uses
this API to obtain status information on each connection attempt.
</para>

<para>When SYN scan is available, it is usually a better choice.  Nmap
has less control over the high level <literal moreinfo="none">connect()</literal> call
than with raw packets, making it less efficient.  The system call
completes connections to open target ports rather than performing the
half-open reset that SYN scan does.  Not only does this take longer
and require more packets to obtain the same information, but target
machines are more likely to log the connection.  A decent IDS will
catch either, but most machines have no such alarm system.  Many
services on your average UNIX system will add a note to syslog, and
sometimes a cryptic error message, when Nmap connects and then closes
the connection without sending data.  Truly pathetic services crash
when this happens, though that is uncommon.  An administrator who sees
a bunch of connection attempts in her logs from a single system should
know that she has been connect scanned.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sU</option> (UDP scans)</term>
        <listitem>

<para>While most popular services on the Internet run over the TCP
protocol, <ulink url="http://www.rfc-editor.org/rfc/rfc768.txt">UDP</ulink> services
are widely deployed.  DNS, SNMP, and DHCP
(registered ports 53, 161/162, and 67/68) are three of the most
common.  Because UDP scanning is generally slower and more difficult
than TCP, some security auditors ignore these ports.  This is a mistake, as
exploitable UDP services are quite common and attackers certainly
don't ignore the whole protocol.  Fortunately, Nmap can help inventory
UDP ports.</para>

<para>UDP scan is activated with the <option>-sU</option> option.  It
can be combined with a TCP scan type such as SYN scan
(<option>-sS</option>) to check both protocols during the same
run.</para>

<para>UDP scan works by sending an empty (no data) UDP header to every
targeted port.  If an ICMP port unreachable error (type 3, code 3) is
returned, the port is <literal moreinfo="none">closed</literal>.  Other ICMP unreachable errors (type 3,
codes 1, 2, 9, 10, or 13) mark the port as <literal moreinfo="none">filtered</literal>.  Occasionally, a
service will respond with a UDP packet, proving that it is <literal moreinfo="none">open</literal>.  If
no response is received after retransmissions, the port is classified
as <literal moreinfo="none">open|filtered</literal>.  This means that the port could be open, or perhaps
packet filters are blocking the communication. Versions scan
(<option>-sV</option>) can be used to help differentiate the truly
open ports from the filtered ones.</para>

<para>A big challenge with UDP scanning is doing it quickly.
Open and filtered ports rarely send any response, leaving Nmap to time
out and then conduct retransmissions just in case the probe or
response were lost.  Closed ports are often an even bigger problem.
They usually send back an ICMP port unreachable error.  But unlike the
RST packets sent by closed TCP ports in response to a SYN or Connect
scan, many hosts rate limit ICMP port unreachable messages by default.
Linux and Solaris are particularly strict about this.  For example, the
Linux 2.4.20 kernel limits destination unreachable messages to one per
second (in <filename moreinfo="none">net/ipv4/icmp.c</filename>).</para>

<para>Nmap detects rate limiting and slows down accordingly to avoid
flooding the network with useless packets that the target machine will
drop.  Unfortunately, a Linux-style limit of one packet per second
makes a 65,536-port scan take more than 18 hours.  Ideas for speeding
your UDP scans up include scanning more hosts in parallel, doing a
quick scan of just the popular ports first, scanning from behind the
firewall, and using <option>--host-timeout</option> to skip slow
hosts.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sN</option>; <option>-sF</option>; <option>-sX</option> (TCP Null, FIN, and Xmas scans)</term>
        <listitem>

<para>These three scan types (even more are possible with the
<option>--scanflags</option> option described in the next section)
exploit a subtle loophole in the <ulink url="http://www.rfc-editor.org/rfc/rfc793.txt">TCP RFC</ulink> to
differentiate between <literal moreinfo="none">open</literal> and
<literal moreinfo="none">closed</literal> ports.  Page 65 says that <quote>if the
[destination] port state is CLOSED .... an incoming segment not
containing a RST causes a RST to be sent in response.</quote>  Then the next
page discusses packets sent to open ports without the SYN, RST, or ACK
bits set, stating that: <quote>you are unlikely to get here, but if you do, drop the
segment, and return.</quote></para>

<para>When scanning systems compliant with this RFC text, any packet
not containing SYN, RST, or ACK bits will result in a returned RST if
the port is closed and no response at all if the port is open.  As
long as none of those three bits are included, any combination of the
other three (FIN, PSH, and URG) are OK.  Nmap exploits this with three
scan types:</para>

<variablelist>
  <varlistentry><term>Null scan (<option>-sN</option>)</term>
  <listitem><para>Does not set any bits (tcp flag header is 0)</para></listitem></varlistentry> 

  <varlistentry><term>FIN scan (<option>-sF</option>)</term>
  <listitem><para>Sets just the TCP FIN bit.</para></listitem></varlistentry> 

  <varlistentry><term>Xmas scan (<option>-sX</option>)</term>
  <listitem><para>Sets the FIN, PSH, and URG flags, lighting the
  packet up like a Christmas tree.</para></listitem></varlistentry>
</variablelist>

<para>These three scan types are exactly the same in behavior except
for the TCP flags set in probe packets.  If a RST packet is received,
the port is considered <literal moreinfo="none">closed</literal>, while no response
means it is <literal moreinfo="none">open|filtered</literal>.  The port is marked
<literal moreinfo="none">filtered</literal> if an ICMP unreachable error (type 3, code
1, 2, 3, 9, 10, or 13) is received.</para>

<para>The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a little more
stealthy than even a SYN scan.  Don't count on this though -- most
modern IDS products can be configured to detect them.  The big
downside is that not all systems follow RFC 793 to the letter.  A
number of systems send RST responses to the probes regardless of
whether the port is open or not.  This causes all of the ports to be
labeled <literal moreinfo="none">closed</literal>.  Major operating systems that do
this are Microsoft Windows, many Cisco devices, BSDI, and IBM OS/400.
This scan does work against most UNIX-based systems though. Another
downside of these scans is that they can't distinguish <literal moreinfo="none">open</literal> ports from
certain <literal moreinfo="none">filtered</literal> ones, leaving you with the response
<literal moreinfo="none">open|filtered</literal>.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sA</option> (TCP ACK scan)</term>
        <listitem>

<para>This scan is different than the others discussed so far in that
it never determines <literal moreinfo="none">open</literal> (or even
<literal moreinfo="none">open|filtered</literal>) ports.  It is used to map out
firewall rulesets, determining whether they are stateful or not and
which ports are filtered.</para>

<para>The ACK scan probe packet has only the ACK flag set (unless you
use <option>--scanflags</option>).  When scanning unfiltered systems,
<literal moreinfo="none">open</literal> and <literal moreinfo="none">closed</literal> ports will both
return a RST packet.  Nmap then labels them as
<literal moreinfo="none">unfiltered</literal>, meaning that they are reachable by the
ACK packet, but whether they are <literal moreinfo="none">open</literal> or
<literal moreinfo="none">closed</literal> is undetermined.  Ports that don't respond,
or send certain ICMP error messages back (type 3, code 1, 2, 3, 9, 10,
or 13), are labeled <literal moreinfo="none">filtered</literal>.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sW</option> (TCP Window scan)</term>
        <listitem>

<para>Window scan is exactly the same as ACK scan except that it
exploits an implementation detail of certain systems to differentiate
open ports from closed ones, rather than always printing
<literal moreinfo="none">unfiltered</literal> when a RST is returned.  It does this by
examining the TCP Window field of the RST packets returned.  On some
systems, open ports use a positive window size (even for RST packets)
while closed ones have a zero window.  So instead of always listing a
port as <literal moreinfo="none">unfiltered</literal> when it receives a RST back,
Window scan lists the port as <literal moreinfo="none">open</literal> or
<literal moreinfo="none">closed</literal> if the TCP Window value in that reset is
positive or zero, respectively.</para>

<para>This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can't always trust it.  Systems
that don't support it will usually return all ports
<literal moreinfo="none">closed</literal>.  Of course, it is possible that the machine
really has no open ports.  If most scanned ports are
<literal moreinfo="none">closed</literal> but a few common port numbers (such as 22,
25, 53) are <literal moreinfo="none">filtered</literal>, the system is most likely
susceptible.  Occasionally, systems will even show the exact opposite
behavior.  If your scan shows 1000 open ports and 3 closed or filtered
ports, then those three may very well be the truly open ones.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sM</option> (TCP Maimon scan)</term>
        <listitem>

<para>The Maimon scan is named after its discoverer, Uriel Maimon.  He
described the technique in Phrack Magazine issue #49 (November 1996).
Nmap, which included this technique, was released two issues later.
This technique is exactly the same as Null, FIN, and Xmas scans, except
that the probe is FIN/ACK.  According to RFC 793 (TCP), a RST packet
should be generated in response to such a probe whether the port is
open or closed.  However, Uriel noticed that many BSD-derived systems
simply drop the packet if the port is open.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--scanflags</option> (Custom TCP scan)</term>
        <listitem>

<para>Truly advanced Nmap users need not limit themselves to the
canned scan types offered.  The <option>--scanflags</option> option allows
you to design your own scan by specifying arbitrary TCP flags.  Let
your creative juices flow, while evading intrusion detection systems whose vendors simply paged through the Nmap man page adding specific rules!</para>

<para>The <option>--scanflags</option> argument can be a numerical
flag value such as 9 (PSH and FIN), but using symbolic names is
easier.  Just mash together any combination of  <literal moreinfo="none">URG</literal>,
<literal moreinfo="none">ACK</literal>, <literal moreinfo="none">PSH</literal>,
<literal moreinfo="none">RST</literal>, <literal moreinfo="none">SYN</literal>, and
<literal moreinfo="none">FIN</literal>.  For example, <option>--scanflags
URGACKPSHRSTSYNFIN</option> sets everything, though it's not very
useful for scanning.  The order these are specified in is
irrelevant.</para>

<para>In addition to specifying the desired flags, you can specify a
TCP scan type (such as <option>-sA</option> or <option>-sF</option>).
That base type tells Nmap how to interpret responses.  For
example, a SYN scan considers no-response to indicate a
<literal moreinfo="none">filtered</literal> port, while a FIN scan treats the same as
<literal moreinfo="none">open|filtered</literal>.  Nmap will behave the same way it
does for the base scan type, except that it will use the TCP flags you
specify instead.  If you don't specify a base type, SYN scan is
used.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sI &lt;zombie
        host[:probeport]&gt;</option> (Idlescan)</term>

        <listitem>

          <para>This advanced scan method allows for a truly blind TCP
          port scan of the target (meaning no packets are sent to the
          target from your real IP address). Instead, a unique
          side-channel attack exploits predictable IP fragmentation ID
          sequence generation on the zombie host to glean information
          about the open ports on the target.  IDS systems will
          display the scan as coming from the zombie machine you
          specify (which must be up and meet certain criteria). This
          fascinating scan type is too complex to fully describe in this reference
          guide, so I wrote and posted an informal paper with full details at
          <ulink url="http://www.insecure.org/nmap/idlescan.html"/>.</para>

          <para>Besides being extraordinarily stealthy (due to its
          blind nature), this scan type permits mapping out
          IP-based trust relationships between machines. The port
          listing shows open ports 
          <emphasis>from the perspective of the zombie
          host.</emphasis> So you can try scanning a target using
          various zombies that you think might be trusted (via
          router/packet filter rules).</para>

          <para>You can add a colon followed by a port number to the
          zombie host if you wish to probe a particular port on the
          zombie for IPID changes. Otherwise Nmap will use the port it
          uses by default for tcp pings (80).</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sO</option> (IP protocol scan)</term>
        <listitem>
          
<para>IP Protocol scan allows you to determine which IP protocols
(TCP, ICMP, IGMP, etc.) are supported by target machines.  This isn't
technically a port scan, since it cycles through IP protocol numbers
rather than TCP or UDP port numbers. Yet it still uses the
<option>-p</option> option to select scanned protocol numbers, reports
its results within the normal port table format, and even uses the same
underlying scan engine as the true port scanning methods. So it is
close enough to a port scan that it belongs here.</para>

<para>Besides being useful in its own right, protocol scan
demonstrates the power of open source software.  While the fundamental
idea is pretty simple, I had not thought to add it nor received any
requests for such functionality.  Then in the summer of 2000, Gerhard
Rieger conceived the idea, wrote an excellent patch implementing it,
and sent it to the nmap-hackers mailing list.  I incorporated that
patch into the Nmap tree and released a new version the next day.  Few
pieces of commercial software have users enthusiastic enough to design
and contribute their own improvements!</para>

<para>Protocol scan works in a similar fashion to UDP scan.  Instead
of iterating through the port number field of a UDP packet, it sends
IP packet headers and iterates through the 8-bit IP protocol field.
The headers are usually empty, containing no data and not even the
proper header for the claimed protocol.  The three exceptions are TCP,
UDP, and ICMP.  A proper protocol header for those is included since
some systems won't send them otherwise and because Nmap already has
functions to create them.  Instead of watching for ICMP port
unreachable messages, protocol scan is on the lookout for ICMP
<emphasis>protocol</emphasis> unreachable messages.  If Nmap receives
any response in any protocol from the target host, Nmap marks that
protocol as <literal moreinfo="none">open</literal>.  An ICMP protocol unreachable
error (type 3, code 2) causes the protocol to be marked as
<literal moreinfo="none">closed</literal> Other ICMP unreachable errors (type 3, code
1, 3, 9, 10, or 13) cause the protocol to be marked
<literal moreinfo="none">filtered</literal> (though they prove that ICMP is
<literal moreinfo="none">open</literal> at the same time).  If no response is received
after retransmissions, the protocol is marked
<literal moreinfo="none">open|filtered</literal></para>

        </listitem>
      </varlistentry>



      <varlistentry>
        <term>
        <option>-b &lt;ftp relay host&gt;</option> (FTP bounce scan)</term>
        <listitem>

<para>An interesting feature of the FTP protocol (<ulink url="http://www.rfc-editor.org/rfc/rfc959.txt">RFC 959</ulink>) is
support for so-called proxy ftp connections.  This allows a user to
connect to one FTP server, then ask that files be sent to a
third-party server.  Such a feature is ripe for abuse on many levels,
so most servers have ceased supporting it.  One of the abuses this
feature allows is causing the FTP server to port scan other hosts.
Simply ask the FTP server to send a file to each interesting port of a
target host in turn.  The error message will describe whether the port
is open or not.  This is a good way to bypass firewalls because
organizational FTP servers are often placed where they have
more access to other internal hosts than any old Internet host would.  Nmap supports ftp
bounce scan with the <option>-b</option> option.  It takes an argument
of the form
<replaceable>username</replaceable>:<replaceable>password</replaceable>@<replaceable>server</replaceable>:<replaceable>port</replaceable>.
<replaceable>Server</replaceable> is the name or IP address of a
vulnerable FTP server.  As with a normal URL, you may omit
<replaceable>username</replaceable>:<replaceable>password</replaceable>,
in which case anonymous login credentials (user:
<literal moreinfo="none">anonymous</literal> password:<literal moreinfo="none">-wwwuser@</literal>)
are used.  The port number (and preceding colon) may be omitted as
well, in which case the default FTP port (21) on
<replaceable>server</replaceable> is used.</para>

<para>This vulnerability was widespread in 1997 when Nmap was
released, but has largely been fixed.  Vulnerable servers are still
around, so it is worth trying when all else fails.  If bypassing a
firewall is your goal, scan the target network for open port 21 (or
even for any ftp services if you scan all ports with version
detection), then try a bounce scan using each.  Nmap will tell you
whether the host is vulnerable or not.  If you are just trying to
cover your tracks, you don't need to (and, in fact, shouldn't) limit
yourself to hosts on the target network.  Before you go scanning
random Internet addresses for vulnerable FTP servers, consider that
sysadmins may not appreciate you abusing their servers in this
way.</para>

        </listitem>
      </varlistentry>
    </variablelist>
   </refsect1>

   <refsect1 id="man-port-specification">
    <title>Port Specification and Scan Order</title>

    <para>In addition to all of the scan methods discussed previously,
    Nmap offers options for specifying which ports are scanned and
    whether the scan order is randomized or sequential.  By default, Nmap scans all ports up to and including 1024 as well as higher numbered ports listed in the <filename moreinfo="none">nmap-services</filename> file for the protocol(s) being scanned.</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-p &lt;port ranges&gt;</option> (Only scan specified ports)
        </term>
        <listitem>

          <para>This option specifies which ports you want to scan and
          overrides the default.  Individual port numbers are OK, as
          are ranges separated by a hyphen (e.g. 1-1023).  The
          beginning and/or end values of a range may be omitted,
          causing Nmap to use 1 and 65535, respectively.  So you can
          specify <option>-p-</option> to scan ports from 1 through
          65535.  Scanning port zero is allowed if you specify it
          explicitly. For IP protocol scanning (<option>-sO</option>), this option
          specifies the protocol numbers you wish to scan for
          (0-255).</para>

          <para>When scanning both TCP and UDP ports, you can specify
          a particular protocol by preceding the port numbers by <literal moreinfo="none">T:</literal>
          or <literal moreinfo="none">U:</literal>. The qualifier lasts until you specify another
          qualifier. For example, the argument <option>-p
          U:53,111,137,T:21-25,80,139,8080</option> would scan UDP
          ports 53,111,and 137, as well as the listed TCP ports. Note
          that to scan both UDP &amp; TCP, you have to specify
          <option>-sU</option> and at least one TCP scan type (such as
          <option>-sS</option>, <option>-sF</option>, or
          <option>-sT</option>). If no protocol qualifier is given,
          the port numbers are added to all protocol lists.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-F</option> (Fast (limited port) scan)
        </term>
        <listitem>

          <para>Specifies that you only wish to scan
          for ports listed in the <filename moreinfo="none">nmap-services</filename>
          file which comes with nmap (or the protocols file for
          <option>-sO</option>). This is much faster than scanning all 65535 ports on a
          host.  Because this list contains so many TCP ports (more
          than 1200), the speed difference from a default TCP scan
          (about 1650 ports) isn't dramatic.  The difference can be
          enormous if you specify your own tiny
          <filename moreinfo="none">nmap-services</filename> file using the
          <option>--datadir</option> option.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-r</option> (Don't randomize ports)
        </term>
        <listitem>
          <para>By default, Nmap randomizes the scanned port order
          (except that certain commonly accessible ports are moved
          near the beginning for efficiency reasons).  This
          randomization is normally desirable, but you can specify
          <option>-r</option> for sequential port scanning
          instead.</para>
        </listitem>
      </varlistentry>
    </variablelist>
   </refsect1>

  <refsect1 id="man-version-detection">
    <title>Service and Version Detection</title>

    <para>Point Nmap at a remote machine and it might tell you
    that ports 25/tcp, 80/tcp, and 53/udp are open. Using its
    <filename moreinfo="none">nmap-services</filename> database of about 2,200 well-known services,
    Nmap would report that those ports probably correspond to a
    mail server (SMTP), web server (HTTP), and name server (DNS)
    respectively. This lookup is usually accurate -- the vast
    majority of daemons listening on TCP port 25 are, in fact, mail
    servers. However, you should not bet your security on this!
    People can and do run services on strange ports.</para>

    <para>Even if Nmap is right, and the hypothetical server above is
    running SMTP, HTTP, and DNS servers, that is not a lot of
    information. When doing vulnerability assessments (or even simple
    network inventories) of your companies or clients, you really want
    to know which mail and DNS servers and versions are
    running. Having an accurate version number helps dramatically in
    determining which exploits a server is vulnerable to.  Version
    detection helps you obtain this information.
</para>

    <para>After TCP and/or UDP ports are discovered using one of the
    other scan methods, version detection interrogates those ports to
    determine more about what is actually running. The
    <filename moreinfo="none">nmap-service-probes</filename> database contains probes
    for querying various services and match expressions to recognize
    and parse responses. Nmap tries to determine the service protocol
    (e.g. ftp, ssh, telnet, http), the application name (e.g. ISC
    Bind, Apache httpd, Solaris telnetd), the version number,
    hostname, device type (e.g. printer, router), the OS family
    (e.g. Windows, Linux) and sometimes miscellaneous details like
    whether an X server is open to connections, the SSH protocol
    version, or the KaZaA user name).  Of course, most services don't
    provide all of this information.  If Nmap was compiled with
    OpenSSL support, it will connect to SSL servers to deduce the
    service listening behind that encryption layer. When RPC services are
    discovered, the Nmap RPC grinder (<option>-sR</option>) is
    automatically used to determine the RPC program and version
    numbers. Some UDP ports are left in the
    <literal moreinfo="none">open|filtered</literal> state after a UDP port scan is
    unable to determine whether the port is open or filtered. Version
    detection will try to elicit a response from these ports (just as
    it does with open ports), and change the state to open if it
    succeeds. <literal moreinfo="none">open|filtered</literal> TCP ports are treaded
    the same way.  Note that the Nmap <option>-A</option> option
    enables version detection among other things.  A paper documenting
    the workings, usage, and customization of version detection is
    available at <ulink url="http://www.insecure.org/nmap/vscan/"/>.</para>

    <para>When Nmap receives responses from a service but cannot match
    them to its database, it prints out a special fingerprint and
    a URL for you to submit if to if you know for sure what is running
    on the port.  Please take a couple minutes to make the submission
    so that your find can benefit everyone.  Thanks to these
    submissions, Nmap has about 3,000 pattern matches for more than
    350 protocols such as smtp, ftp, http, etc.</para>

    <para>Version detection is enabled and controlled with the
    following options:</para>


    <variablelist>

      <varlistentry>
        <term>
        <option>-sV</option> (Version detection)</term>
        <listitem>

          <para>Enables version detection, as discussed above.
          Alternatively, you can use <option>-A</option> to enable
          both OS detection and version detection.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--allports</option> (Don't exclude any ports from
          version detection)
        </term>
        <listitem>
          <para>By default, Nmap version detection skips TCP port 9100
          because some printers simply print anything sent to that
          port, leading to dozens of pages of HTTP get requests, binary
          SSL session requests, etc.  This behavior can be changed by
          modifying or removing the <literal moreinfo="none">Exclude</literal>
          directive in <filename moreinfo="none">nmap-service-probes</filename>, or
          you can specify <option>--allports</option> to scan all
          ports regardless of any <literal moreinfo="none">Exclude</literal>
          directive.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--version-intensity &lt;intensity&gt;</option> (Set
          version scan intensity)
        </term>
        <listitem>

          <para>When performing a version scan (<option>-sV</option>), nmap sends a
          series of probes, each of which is assigned a rarity value
          between 1 and 9. The lower-numbered probes are effective
          against a wide variety of common services, while the higher
          numbered ones are rarely useful. The intensity level
          specifies which probes should be applied. The higher the
          number, the more likely it is the service will be correctly
          identified. However, high intensity scans take longer.  The
          intensity must be between 0 and 9. The default is 7.  When a
          probe is registered to the target port via the
          <filename moreinfo="none">nmap-service-probes</filename> <literal moreinfo="none">ports</literal> directive, that probe is tried
          regardless of intensity level.  This ensures that the DNS
          probes will always be attempted against any open port 53,
          the SSL probe will be done against 443, etc.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--version-light</option> (Enablie light mode)
        </term>
        <listitem>
          <para>This is a convenience alias for
          <option>--version-intensity 2</option>.  This light mode
          makes version scanning much faster, but it is slightly less
          likely to identify services.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--version-all</option> (Try every single probe)
        </term>
        <listitem>
          <para>An alias for <option>--version-intensity 9</option>,
          ensuring that every single probe is attempted against each
          port.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--version-trace</option> (Trace version scan activity)
        </term>
        <listitem>
          <para>This causes Nmap to print out extensive debugging info
          about what version scanning is doing.  It is a subset of
          what you get with <option>--packet-trace</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-sR</option> (RPC scan)</term>
        <listitem>
          <para>This method works in conjunction with the various port
          scan methods of Nmap. It takes all the TCP/UDP ports found
          open and floods them with SunRPC program NULL commands in an
          attempt to determine whether they are RPC ports, and if so,
          what program and version number they serve up. Thus you can
          effectively obtain the same info as <command moreinfo="none">rpcinfo -p</command> even if the
          target's portmapper is behind a firewall (or protected by
          TCP wrappers). Decoys do not currently work with RPC scan.
          This is automatically enabled as part of version scan
          (<option>-sV</option>) if you request that.  As version
          detection includes this and is much more comprehensive,
          <option>-sR</option> is rarely needed.</para>
        </listitem>
      </varlistentry>
  
    </variablelist>
  </refsect1>

  <refsect1 id="man-os-detection">

    <title>OS Detection</title>

    <para>One of Nmap's best-known features is remote OS detection
    using TCP/IP stack fingerprinting.  Nmap sends a series of TCP and
    UDP packets to the remote host and examines practically every bit
    in the responses.  After performing dozens of tests such as TCP
    ISN sampling, TCP options support and ordering, IPID sampling, and
    the initial window size check, Nmap compares the results to its
    <filename moreinfo="none">nmap-os-fingerprints</filename> database of more than 1500 known
    OS fingerprints and prints out the OS details if there is a match.
    Each fingerprint includes a freeform textual description of the
    OS, and a classification which provides the vendor name
    (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g. 10),
    and device type (general purpose, router, switch, game console,
    etc).</para>

    <para>If Nmap is unable to guess the OS of a machine, and
    conditions are good (e.g. at least one open port and one closed
    port were found), Nmap will
    provide a URL you can use to submit the fingerprint if you know
    (for sure) the OS running on the machine.  By doing this you
    contribute to the pool of operating systems known to Nmap and thus
    it will be more accurate for everyone.</para>

    <para>OS detection enables several other tests which make use
    of information that is gathered during the process anyway.  One of these
    is uptime measurement, which uses the TCP timestamp option (RFC
    1323) to guess when a machine was last rebooted.  This is only
    reported for machines which provide this information.  Another is
    TCP Sequence Predictability Classification.  This measures
    approximately how hard it is to establish a forged
    TCP connection against the remote host.  It is useful for
    exploiting source-IP based trust relationships (rlogin, firewall
    filters, etc) or for hiding the source of an attack.  This sort of
    spoofing is rarely performed any more, but many machines are still
    vulnerable to it.  The actual
    difficulty number is based on statistical sampling and may
    fluctuate.  It is generally better to use the English
    classification such as <quote>worthy challenge</quote> or <quote>trivial joke</quote>.  This
    is only reported in normal output in verbose (<option>-v</option>)
    mode.  When verbose mode is enabled along with <option>-O</option>, IPID Sequence
    Generation is also reported.  Most machines are in the
    <quote>incremental</quote> class, which means that they increment the ID
    field in the IP header for each packet they send.  This makes them
    vulnerable to several advanced information gathering and
    spoofing attacks.</para>

    <para>A paper documenting the workings, usage, and customization
    of version detection is available in more than a dozen languages at <ulink url="http://www.insecure.org/nmap/nmap-fingerprinting-article.html"/>.</para>

    <para>OS detection is enabled and controlled with the following options:</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-O</option> (Enable OS detection)
        </term>
        <listitem>
          <para>Enables OS detection, as discussed above.
          Alternatively, you can use <option>-A</option> to enable
          both OS detection and version detection.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--osscan-limit</option> (Limit OS detection to
          promising targets)
        </term>
        <listitem>
          <para>OS detection is far more effective if at least one
          open and one closed TCP port are found. Set this option
          and Nmap will not even try OS detection against hosts
          that do not meet this criteria. This can save substantial
          time, particularly on <option>-P0</option> scans against many hosts. It
          only matters when OS detection is requested with <option>-O</option> or <option>-A</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--osscan-guess</option>; <option>--fuzzy</option> (Guess OS detection results)
        </term>
        <listitem>
          <para>When Nmap is unable to detect a perfect OS match, it
          sometimes offers up near-matches as possibilities.  The
          match has to be very close for Nmap to do this by default.  Either
          of these (equivalent) options make Nmap guess more
          aggressively.</para>
        </listitem>
      </varlistentry>


    </variablelist>
  </refsect1>

   <refsect1 id="man-performance">
    <title>Timing and Performance</title>
    <para>One of my highest Nmap development priorities has always been
performance.  A default scan (<command moreinfo="none">nmap
<replaceable>hostname</replaceable></command>) of a host on my local
network takes a fifth of a second.  That is barely enough time to
blink, but adds up when you are scanning tens or hundreds of thousands
of hosts.  Moreover, certain scan options such as UDP scanning and
version detection can increase scan times substantially.  So can
certain firewall configurations, particularly response rate limiting.
While Nmap utilizes parallelism and many advanced algorithms to
accelerate these scans, the user has ultimate control over how Nmap
runs.  Expert users carefully craft Nmap commands to obtain only the
information they care about while meeting their time
constraints.</para>

    <para>Techniques for improving scan times include omitting
    non-critical tests, and upgrading to the latest version of Nmap
    (performance enhancements are made frequently).  Optimizing timing
    parameters can also make a substantial difference.  Those options
    are listed below.</para>

    <variablelist>

      <varlistentry>
        <term>
        <option>--min-hostgroup &lt;numhosts&gt;</option>; 
        <option>--max-hostgroup
        &lt;numhosts&gt;</option> (Adjust parallel scan group sizes)</term>
        <listitem>
<para>Nmap has the ability to port scan or version scan multiple hosts
in parallel.  Nmap does this by dividing the target IP space into
groups and then scanning one group at a time.  In general, larger
groups are more efficient.  The downside is that host results can't be
provided until the whole group is finished.  So if Nmap started out
with a group size of 50, the user would not receive any reports
(except for the updates offered in verbose mode) until the first 50
hosts are completed.</para>

<para>By default, Nmap takes a compromise approach to this conflict.
It starts out with a group size as low as five so the first results
come quickly and then increases the groupsize to as high as 1024.  The
exact default numbers depend on the options given.  For efficiency
reasons, Nmap uses larger group sizes for UDP or few-port TCP
scans.</para>

<para>When a maximum group size is specified with
<option>--max-hostgroup</option>, Nmap will never exceed that size.
Specify a minimum size with <option>--min-hostgroup</option> and Nmap
will try to keep group sizes above that level.  Nmap may have to use
smaller groups than you specify if there are not enough target hosts
left on a given interface to fulfill the specified minimum.  Both may
be set to keep the group size within a specific range, though this is
rarely desired.</para>

<para>The primary use of these options is to specify a large minimum
group size so that the full scan runs more quickly.  A common choice
is 256 to scan a network in Class C sized chunks.  For a scan with
many ports, exceeding that number is unlikely to help much. For scans
of just a few port numbers, host group sizes of 2048 or more may be
helpful.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--min-parallelism &lt;numprobes&gt;</option>;
        <option>--max-parallelism
        &lt;numprobes&gt;</option> (Adjust probe parallelization)</term>
        <listitem>

<para>These options control the total number of probes that may
be outstanding for a host group.  They are used for port scanning and
host discovery.  By default, Nmap calculates an ever-changing ideal
parallelism based on network performance.  If packets are being dropped,
Nmap slows down and allows fewer outstanding probes.  The ideal probe
number slowly rises as the network proves itself worthy.  These
options place minimum or maximum bounds on that variable.  By default,
the ideal parallelism can drop to 1 if the network proves unreliable
and rise to several hundred in perfect conditions.</para>

<para>The most common usage is to set
<option>--min-parallelism</option> to a number higher than one to
speed up scans of poorly performing hosts or networks.  This is a
risky option to play with, as setting it too high may affect accuracy.
Setting this also reduces Nmap's ability to control parallelism
dynamically based on network conditions.  A value of ten might be
reasonable, though I only adjust this value as a last resort.</para>

<para>The <option>--max-parallelism</option> option is sometimes set to one
to prevent Nmap from sending more than one probe at a time to hosts.
This can be useful in combination with <option>--scan-delay</option>
(discussed later), although the latter usually serves the purpose well
enough by itself.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--min_rtt_timeout &lt;time&gt;</option>, 
        <option>--max-rtt-timeout &lt;time&gt;</option>, 
        <option>--initial-rtt-timeout
        &lt;time&gt;</option> (Adjust probe timeouts)</term>
        <listitem>

<para>Nmap maintains a
running timeout value for determining how long it will wait for a
probe response before giving up or retransmitting the probe.  This is
calculated based on the response times of previous probes.  If
the network latency shows itself to be significant and variable, this
timeout can grow to several seconds.  It also starts at a conservative
(high) level and may stay that way for a while when Nmap scans
unresponsive hosts.</para>

<para>These options take a value in milliseconds, or you can append
<literal moreinfo="none">s</literal>, <literal moreinfo="none">m</literal>, or <literal moreinfo="none">h</literal> to
the argument to specify a time in seconds, minutes, or hours.
Specifying a lower <option>--max-rtt-timeout</option> and
<option>--initial-rtt-timeout</option> than the defaults can cut scan
times significantly.  This is particularly true for pingless
(<option>-P0</option>) scans, and those against heavily filtered
networks.  Don't get too aggressive though.  The scan can end up
taking longer if you specify such a low value that many probes are
timing out and retransmitting while the response is in transit.</para>

<para>If all the hosts are on a local network, 100 milliseconds is a
reasonable aggressive <option>--max-rtt-timeout</option> value.  If
routing is involved, ping a host on the network first with the ICMP
ping utility, or with a custom packet crafter such as hping2 that is
more likely to get through a firewall.  Look at the maximum round trip
time out of ten packets or so.  You might want to double that for the
<option>--initial-rtt-timeout</option> and triple or quadruple it for
the <option>--max-rtt-timeout</option>.  I generally do not set the
maximum rtt below 100ms, no matter what the ping times are.  Nor do I
exceed 1000ms.</para>

<para><option>--min_rtt_timeout</option> is a rarely used option that
could be useful when a network is so unreliable that even Nmap's
default is too aggressive.  Since Nmap only reduces the timeout down to
the minimum when the network seems to be reliable, this need is
unusual and should be reported as a bug to the nmap-dev mailing
list.</para>

        </listitem>
      </varlistentry>


      <varlistentry>
        <term>
          <option>--max-retries &lt;numtries&gt;</option> (Specify the
          maximum number of port scan probe retransmissions)
        </term>
        <listitem>

<para>When Nmap receives no response to a port scan probe, it could
mean the port is filtered.  Or maybe the probe or response was simply
lost on the network.  It is also possible that the target host has
rate limiting enabled that temporarily blocked the response.  So Nmap
tries again by retransmitting the initial probe.  If Nmap detects poor
network reliability, it may try many more times before giving up on a
port.  While this benefits accuracy, it also lengthen scan times.
When performance is critical, scans may be sped up by limiting the
number of retransmissions allowed.  You can even specify
<option>--max-retries 0</option> to prevent any retransmissions,
though that is rarely recommended.
</para>

<para>The default (with no <option>-T</option> template) is to allow
ten retransmissions.  If a network seems reliable and the target hosts
aren't rate limiting, Nmap usually only does one retransmission.  So
most target scans aren't even affected by dropping
<option>--max-retries</option> to a low value such as three.  Such
values can substantially speed scans of slow (rate limited) hosts.
You usually lose some information when Nmap gives up on ports early,
though that may be preferable to letting the
<option>--host-timeout</option> expire and losing all information
about the target.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--host-timeout &lt;time&gt;</option> (Give
          up on slow target hosts)
        </term>
        <listitem>

<para>Some hosts simply take a <emphasis>long</emphasis> time to scan.
This may be due to poorly performing or unreliable networking hardware
or software, packet rate limiting, or a restrictive firewall.  The
slowest few percent of the scanned hosts can eat up a majority of the
scan time.  Sometimes it is best to cut your losses and skip those
hosts initially.  This can be done by specifying
<option>--host-timeout</option> with the number of milliseconds you
are willing to wait.  Alternatively, you can append
<literal moreinfo="none">s</literal>, <literal moreinfo="none">m</literal>, or <literal moreinfo="none">h</literal> to
the argument to specify a timeout in seconds, minutes, or hours.  I
often specify <literal moreinfo="none">30m</literal> to ensure that Nmap doesn't waste
more than half an hour on a single host.  Note that Nmap may be
scanning other hosts at the same time during that half an hour as
well, so it isn't a complete loss.  A host that times out is skipped.
No port table, OS detection, or version detection results are printed
for that host.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>--scan-delay &lt;time&gt;</option>;
        <option>--max_scan-delay
        &lt;time&gt;</option> (Adjust delay between probes)</term>
        <listitem>

<para>This option causes Nmap to wait at least the given number of
milliseconds between each probe it sends to a given host.  As with
many other timing options, you can append <literal moreinfo="none">s</literal>,
<literal moreinfo="none">m</literal>, or <literal moreinfo="none">h</literal> to the argument to
specify a delay in seconds, minutes, or hours instead.  This is
particularly useful in the case of rate limiting.  Solaris machines
(among many others) will usually respond to UDP scan probe packets
with only one ICMP message per second.  Any more than that sent by
Nmap will be wasteful.  A <option>--scan-delay</option> of
<literal moreinfo="none">1s</literal> will keep Nmap at that slow rate.  Nmap tries to
detect rate limiting and adjust the scan delay accordingly, but it
doesn't hurt to specify it explicitly if you already know what rate
works best.</para>

<para>When Nmap adjusts the scan delay upward to cope with rate
limiting, the scan slows down dramatically.  The
<option>--max_scan-delay</option> option specifies the largest delay
that Nmap will allow.  Setting this value too low can lead to wasteful
packet retransmissions and possible missed ports when the target
implements strict rate limiting.</para>

<para>Another use of <option>--scan-delay</option> is to evade
threshold based intrusion detection and prevention systems (IDS/IPS).</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-T
          &lt;Paranoid|Sneaky|Polite|Normal|Aggressive|Insane&gt;</option>
          (Set a timing template)
        </term>
        <listitem>

<para>While the fine grained timing controls discussed in the previous
section are powerful and effective, some people find them confusing.
Moreover, choosing the appropriate values can sometimes take more time
than the scan you are trying to optimize.  So Nmap offers a simpler
approach, with six timing templates.  You can specify them with the
<option>-T</option> option and their number (0 - 5) or their name.
The template names are paranoid (0), sneaky (1), polite (2), normal
(3), aggressive (4), and insane (5).  The first two are for IDS
evasion.  Polite mode slows down the scan to use less bandwidth and
target machine resources.  Normal mode is the default and so
<option>-T3</option> does nothing. Aggressive mode speeds scans up by
making the assumption that you are on a reasonably fast and reliable
network.  Finally Insane mode assumes that you are on an
extraordinarily fast network or are willing to sacrifice some accuracy
for speed.</para>

<para>These templates allow the user to specify how aggressive they
wish to be, while leaving Nmap to pick the exact timing values.  The
templates also make some minor speed adjustments for which fine
grained control options do not currently exist.  For example,
<option>-T4</option> prohibits the dynamic scan delay from exceeding
10ms for TCP ports and <option>-T5</option> caps that value at 5
milliseconds.  Templates can be used in combination with fine grained
controls, as long as the template is specified first.  Otherwise the
standard values for the template may override the values you specify.
I recommend using <option>-T4</option> when scanning reasonably modern
and reliable networks.  Keep that option (at the beginning of the
command line) even when you add fine grained controls so that you
benefit from those extra minor optimizations that it enables.</para>

<para>If you are on a decent broadband or ethernet connection, I would
recommend always using <option>-T4</option>.  Some people love
<option>-T5</option> though it is too aggressive for my taste.  People
sometimes specify <option>-T2</option> because they think it is less
likely to crash hosts or because they consider themselves to be polite
in general.  They often don't realize just how slow <option>-T
Polite</option> really is.  Their scan may take ten times longer than a
default scan.
Machine crashes and bandwidth problems are rare with the
default timing options (<option>-T3</option>) and so I normally
recommend that for cautious scanners.  Omitting version detection is
far more effective than playing with timing values at reducing these
problems.</para>

<para>While <option>-T0</option> and <option>-T1</option> may be
useful for avoiding IDS alerts, they will take an extraordinarily long
time to scan thousands of machines or ports.  For such a long scan,
you may prefer to set the exact timing values you need rather than
rely on the canned <option>-T0</option> and <option>-T1</option>
values.</para>

<para>The main effects of <option>T0</option> are serializing the scan
so only one port is scanned at a time, and waiting five minutes
between sending each probe.  <option>T1</option> and
<option>T2</option> are similar but they only wait 15 seconds and 0.4
seconds, respectively, between probes.  <option>T3</option> is Nmap's
default behavior, which includes parallelization.  <option>T4</option>
does the equivalent of <option>--max-rtt-timeout 1250
--initial-rtt-timeout 500 --max-retries 6</option> and sets the maximum TCP scan delay
to 10 milliseconds.  <option>T5</option> does the equivalent of
<option>--max-rtt-timeout 300 --min_rtt_timeout 50
--initial-rtt-timeout 250 --max-retries 2 --host-timeout 900000</option> as well as
setting the maximum TCP scan delay to 5ms.</para>

        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id="man-bypass-firewalls-ids">
    <title>Firewall/IDS Evasion and Spoofing</title>

<para>Many Internet pioneers envisioned a global open network with a
universal IP address space allowing virtual connections between any
two nodes.  This allows hosts to act as true peers, serving and
retrieving information from each other.  People could access all of
their home systems from work, changing the climate control settings or
unlocking the doors for early guests.  This vision of universal
connectivity has been stifled by address space shortages and security
concerns.  In the early 1990s, organizations began deploying
firewalls for the express purpose of reducing connectivity.  Huge
networks were cordoned off from the unfiltered Internet by application
proxies, network address translation, and packet filters.  The
unrestricted flow of information gave way to tight regulation of
approved communication channels and the content that passes over
them.</para>

<para>Network obstructions such as firewalls can make mapping a
network exceedingly difficult.  It will not get any easier, as
stifling casual reconnaissance is often a key goal of implementing the
devices.   Nevertheless, Nmap offers many features to help understand these
complex networks, and to verify that filters are working as intended.
It even supports mechanisms for bypassing poorly implemented
defenses.  One of the best methods of understanding your
network security posture is to try to defeat it.  Place yourself in
the mindset of an attacker, and deploy techniques from this section
against your networks.  Launch an FTP bounce scan, Idle scan,
fragmentation attack, or try to tunnel through one of your own
proxies.</para>

<para>In addition to restricting network activity, companies are
increasingly monitoring traffic with intrusion detection systems
(IDS).  All of the major IDSs ship with rules designed to detect Nmap
scans because scans are sometimes a precursor to attacks.  Many of
these products have recently morphed into intrusion
<emphasis>prevention</emphasis> systems (IPS) that actively block
traffic deemed malicious.  Unfortunately for network administrators
and IDS vendors, reliably detecting bad intentions by analyzing packet
data is a tough problem.  Attackers with patience, skill, and the help
of certain Nmap options can usually pass by IDSs undetected.
Meanwhile, administrators must cope with large numbers of false
positive results where innocent activity is misdiagnosed and alerted
on or blocked.</para>

<para>Occasionally people suggest that Nmap should not offer features
for evading firewall rules or sneaking past IDSs.  They argue
that these features are just as likely to be misused by attackers as
used by administrators to enhance security.  The problem with this
logic is that these methods would still be used by attackers, who
would just find other tools or patch the functionality into Nmap.
Meanwhile, administrators would find it that much harder to do their
jobs.  Deploying only modern, patched FTP servers is a far more
powerful defense than trying to prevent the distribution of tools
implementing the FTP bounce attack.
</para>

<para>There is no magic bullet (or Nmap option) for detecting and
subverting firewalls and IDS systems.  It takes skill and experience.
A tutorial is beyond the scope of this reference guide, which only
lists the relevant options and describes what they do.</para>

<variablelist>
      <varlistentry>
        <term>
          <option>-f</option> (fragment packets);
          <option>--mtu</option> (using the specified MTU)
        </term>

        <listitem>
          <para>The <option>-f</option> option causes the requested scan (including
          ping scans) to use tiny fragmented IP packets. The idea
          is to split up the TCP header over several packets to
          make it harder for packet filters, intrusion detection
          systems, and other annoyances to detect what you are
          doing. Be careful with this! Some programs have trouble
          handling these tiny packets. The old-school sniffer named
          Sniffit segmentation faulted immediately upon receiving
          the first fragment. Specify this option once, and Nmap
          splits the packets into 8 bytes or less after the IP
          header. So a 20-byte TCP header would be split into 3
          packets. Two with eight bytes of the TCP header, and one
          with the final four. Of course each fragment also has an
          IP header. Specify <option>-f</option> again to use 16 bytes per fragment
          (reducing the number of fragments). Or you can specify
          your own offset size with the <option>--mtu</option> option. Don't also
          specify <option>-f</option> if you use <option>--mtu</option>. The offset must be a
          multiple of 8. While fragmented packets won't get by
          packet filters and firewalls that queue all IP fragments,
          such as the CONFIG_IP_ALWAYS_DEFRAG option in the Linux
          kernel, some networks can't afford the performance hit
          this causes and thus leave it disabled. Others can't enable
          this because fragments may take different routes into their
          networks.  Some source
          systems defragment outgoing packets in the kernel. Linux
          with the iptables connection tracking module is one such
          example. Do a scan while a sniffer such as Ethereal
          is running to ensure that sent packets are
          fragmented.  If your host OS is causing problems, try the <option>--send-eth</option> option to bypass the IP layer and send raw ethernet frames.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-D &lt;decoy1 [,decoy2][,ME],...&gt;</option>
          (Cloak a scan with decoys)
        </term>
        <listitem>
          <para>Causes a decoy scan to be performed, which makes it
          appear to the remote host that the host(s) you specify as
          decoys are scanning the target network too. Thus their
          IDS might report 5-10 port scans from unique IP
          addresses, but they won't know which IP was scanning them
          and which were innocent decoys. While this can be
          defeated through router path tracing, response-dropping,
          and other active mechanisms, it is generally an
          effective technique for hiding your IP
          address.</para>

          <para>Separate each decoy host with commas, and you can
          optionally use <literal moreinfo="none">ME</literal> as one of the decoys to represent the
          position for your real IP address. If you put
          <literal moreinfo="none">ME</literal> in the 6th position or later, some common port scan
          detectors (such as Solar Designer's excellent scanlogd)
          are unlikely to show your IP address at all. If you don't
          use <literal moreinfo="none">ME</literal>, nmap will put you in a random position.</para>
          <para>Note that the hosts you use as decoys should be up
          or you might accidentally SYN flood your targets. Also it
          will be pretty easy to determine which host is scanning
          if only one is actually up on the network. You might want
          to use IP addresses instead of names (so the decoy
          networks don't see you in their nameserver logs).</para>

          <para>Decoys are used both in the initial ping scan (using
          ICMP, SYN, ACK, or whatever) and during the actual port
          scanning phase. Decoys are also used during remote OS
          detection (<option>-O</option>).  Decoys do not work with version detection
          or TCP connect() scan.</para>

          <para>It is worth noting that using too many decoys may
          slow your scan and potentially even make it less
          accurate. Also, some ISPs will filter out your spoofed
          packets, but many do not restrict
          spoofed IP packets at all.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-S &lt;IP_Address&gt;</option> (Spoof source address)
        </term>
        <listitem>
          <para>In some circumstances, 
          Nmap may not be able to determine your
          source address ( 
          Nmap will tell you if this is the
          case). In this situation, use <option>-S</option> with the IP address of
          the interface you wish to send packets through.</para>

          <para>Another possible use of this flag is to spoof the scan
          to make the targets think that <emphasis>someone
          else</emphasis> is scanning them. Imagine a company being
          repeatedly port scanned by a competitor!  The
          <option>-e</option> option would generally be required for
          this sort of usage, and <option>-P0</option> would normally
          be advisable as well.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-e &lt;interface&gt;</option> (Use specified interface)
        </term>
        <listitem>
          <para>Tells Nmap what interface to send and receive
          packets on. Nmap should be able to detect this
          automatically, but it
          will tell you if it cannot.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--source-port &lt;portnumber&gt;;</option>
          <option>-g &lt;portnumber&gt;</option> (Spoof source port number)
        </term>
        <listitem>

<para>One surprisingly common misconfiguration is to trust traffic
based only on the source port number.  It is easy to understand how
this comes about.  An administrator will set up a shiny new firewall,
only to be flooded with complains from ungrateful users whose
applications stopped working.  In particular, DNS may be broken
because the UDP DNS replies from external servers can no longer enter
the network.  FTP is another common example.  In active FTP transfers,
the remote server tries to establish a connection back to the client
to transfer the requested file.</para>

<para>Secure solutions to these problems exist, often in the form of
application-level proxies or protocol-parsing firewall modules.
Unfortunately there are also easier, insecure solutions.  Noting that
DNS replies come from port 53 and active ftp from port 20, many admins
have fallen into the trap of simply allowing incoming traffic from
those ports.  They often assume that no attacker would notice and
exploit such firewall holes.  In other cases, admins consider this a
short-term stop-gap measure until they can implement a more secure
solution.  Then they forget the security upgrade.
</para>

<para>Overworked network administrators are not the only ones to fall
into this trap.  Numerous products have shipped with these insecure
rules.  Even Microsoft has been guilty.  The IPsec filters that
shipped with Windows 2000 and Windows XP contain an implicit rule that
allows all TCP or UDP traffic from port 88 (Kerberos).  In another well-known
case, versions of the Zone Alarm personal firewall up to 2.1.25
allowed any incoming UDP packets with the source port 53 (DNS) or 67
(DHCP).</para>

<para>Nmap offers the <option>-g</option> and
<option>--source-port</option> options (they are equivalent) to exploit these
weaknesses.  Simply provide a port number and Nmap will send packets
from that port where possible.  Nmap must use different port numbers
for certain OS detection tests to work properly, and DNS requests
ignore the <option>--source-port</option> flag because Nmap relies on system
libraries to handle those.  Most TCP scans, including SYN scan,
support the option completely, as does UDP scan.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--data-length &lt;number&gt;</option> (Append random
          data to sent packets)
        </term>
        <listitem>
          <para>Normally Nmap sends minimalist packets containing only
          a header. So its TCP packets are generally 40
          bytes and ICMP echo requests are just 28. This option
          tells Nmap to append the given number of random bytes to
          most of the packets it sends. OS detection (<option>-O</option>) packets
          are not affected, but most pinging and portscan packets
          are. This slows things down, but can make a scan slightly less
          conspicuous.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--ttl &lt;value&gt;</option> (Set IP time-to-live field)
        </term>
        <listitem>
          <para>Sets the IPv4 time-to-live field in sent packets to
          the given value.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--randomize-hosts</option> (Randomize target host order)
        </term>
        <listitem>
          <para>Tells Nmap to shuffle each group of up to 8096 hosts
          before it scans them. This can make the scans less obvious
          to various network monitoring systems, especially when you
          combine it with slow timing options.  If you
          want to randomize over larger group sizes, increase
          PING_GROUP_SZ in <filename moreinfo="none">nmap.h</filename> and recompile.
          An alternative solution is to generate the target IP list
          with a list scan (<option>-sL -n -oN
          <replaceable>filename</replaceable></option>), randomize it
          with a Perl script, then provide the whole list to Nmap with
          <option>-iL</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--spoof-mac &lt;mac address, prefix, or vendor
          name&gt;</option> (Spoof MAC address)
        </term>
        <listitem>

          <para>Asks Nmap to use the given MAC address for all of the
  raw ethernet frames it sends.  This option implies
  <option>--send-eth</option> to ensure that Nmap actually sends
  ethernet-level packets.  The MAC given can take several formats.  If
  it is simply the string <quote>0</quote>, Nmap chooses a completely random MAC
  for the session.  If the given string is an even number of hex
  digits (with the pairs optionally separated by a colon), Nmap will
  use those as the MAC.  If less than 12 hex digits are provided, Nmap
  fills in the remainder of the 6 bytes with random values.  If the
  argument isn't a 0 or hex string, Nmap looks through
  <filename moreinfo="none">nmap-mac-prefixes</filename> to find a vendor name containing the given string
  (it is case insensitive).  If a match is found, Nmap uses the
  vendor's OUI (3-byte prefix) and fills out the remaining 3 bytes
  randomly.  Valid <option>--spoof-mac</option> argument examples are <literal moreinfo="none">Apple</literal>, <literal moreinfo="none">0</literal>,
  <literal moreinfo="none">01:02:03:04:05:06</literal>, <literal moreinfo="none">deadbeefcafe</literal>, <literal moreinfo="none">0020F2</literal>, and <literal moreinfo="none">Cisco</literal>.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--badsum</option> (Send packets with bogus TCP/UDP checksums)
        </term>
        <listitem>

          <para>Asks Nmap to use an invalid TCP or UDP checksum for
  packets sent to target hosts.  Since virtually all
  host IP stacks properly drop these packets, any responses received
  are likely coming from a firewall or IDS that didn't bother to
  verify the checksum.  For more details on this technique, see <ulink url="http://www.phrack.org/phrack/60/p60-0x0c.txt"/></para>
        </listitem>
      </varlistentry>

    </variablelist>

  </refsect1>

  <refsect1 id="man-output">
    <title>Output</title>

    <para>Any security tools is only as useful as the output it
    generates.  Complex tests and algorithms are of little value if
    they aren't presented in an organized and comprehensible fashion.
    Given the number of ways Nmap is used by people and other
    software, no single format can please everyone.  So Nmap offers
    several formats, including the interactive mode for humans to read
    directly and XML for easy parsing by software.</para>

<para>In addition to offering different output formats, Nmap provides
options for controlling the verbosity of output as well as debugging
messages.  Output types may be sent to standard output or to named
files, which Nmap can append to or clobber.  Output files may also be
used to resume aborted scans.</para>

<para>Nmap makes output available in five different formats.
The default is called <literal moreinfo="none">interactive output</literal>, and it is sent to standard
output (stdout).  There is also <literal moreinfo="none">normal output</literal>,
which is similar to <literal moreinfo="none">interactive</literal> except that it
displays less runtime information and warnings since it is expected to
be analyzed after the scan completes rather than interactively.</para>

<para>XML output is one of the most important output types, as it can
be converted to HTML, easily parsed by programs such as Nmap graphical
user interfaces, or imported into databases.</para>

<para>The two remaining output types are the simple <literal moreinfo="none">grepable
output</literal> which includes most information for a target host on
a single line, and <literal moreinfo="none">sCRiPt KiDDi3 0utPUt</literal> for users
who consider themselves |&lt;-r4d.</para>

<para>While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax.  They take one argument, which is the filename that results
should be stored in.  Multiple formats may be specified, but each
format may only be specified once.  For example, you may wish to save
normal output for your own review while saving XML of the same scan
for programmatic analysis.  You might do this with the options
<option>-oX myscan.xml -oN myscan.nmap</option>.  While this chapter
uses the simple names like <literal moreinfo="none">myscan.xml</literal> for brevity,
more descriptive names are generally recommended.  The names chosen
are a matter of personal preference, though I use long ones that
incorporate the scan date and a word or two describing the scan, placed
in a directory named after the company I'm scanning.</para>

<para>While these options save results to files, Nmap still prints
interactive output to stdout as usual.  For example, the command
<command moreinfo="none">nmap -oX myscan.xml target</command> prints XML to
<filename moreinfo="none">myscan.xml</filename> and fills standard output with the same interactive results it would have printed if <option>-oX</option>
wasn't specified at all.  You can change this by passing a hyphen
character as the argument to one of the format types.  This causes
Nmap to deactivate interactive output, and instead print
results in the format you specified to the standard output stream.  So the
command <command moreinfo="none">nmap -oX - target</command> will send only XML output to
stdout.  Serious errors may still be printed to the normal error
stream, stderr.</para>

<para>Unlike some Nmap arguments, the space between the logfile option
flag (such as <option>-oX</option>) and the filename or hyphen is
mandatory.  If you omit the flags and give arguments such as
<option>-oG-</option> or <option>-oXscan.xml</option>, a backwards
compatibility feature of Nmap will cause the creation of
<emphasis>normal format</emphasis> output files named
<filename moreinfo="none">G-</filename> and <filename moreinfo="none">Xscan.xml</filename>
respectively.</para>

<para>Nmap also offers options to control scan verbosity and to append
to output files rather than clobbering them.  All of these options are
described below.</para>

<variablelist><title>Nmap Output Formats</title>
      <varlistentry>
        <term>
        <option>-oN &lt;filespec&gt;</option> (Normal output)</term>
        <listitem>
          <para>Requests that <literal moreinfo="none">normal output</literal> be
          directed to the given filename.  As discussed above, this
          differs slightly from <literal moreinfo="none">interactive output</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-oX &lt;filespec&gt;</option> (XML output)</term>
        <listitem>

          <para>Requests that <literal moreinfo="none">XML output</literal> be
          directed to the given filename.  Nmap includes a document
          type definition (DTD) which allows XML parsers to validate
          Nmap XML output.  While it is primarily intended for
          programmatic use, it can also help humans interpret Nmap XML
          output.  The DTD defines the legal elements of the format,
          and often enumerates the attributes and values they can take
          on.  The latest version is always available from <ulink url="http://www.insecure.org/nmap/data/nmap.dtd"/>.</para>

          <para>XML offers a stable format that is easily parsed by
          software. Free XML parsers are available for all major
          computer languages, including C/C++, Perl, Python, and
          Java. People have even written bindings for most of these
          languages to handle Nmap output and execution specifically.
          Examples are <ulink url="http://sourceforge.net/projects/nmap-scanner/">Nmap::Scanner</ulink>
          and <ulink url="http://www.nmapparser.com">Nmap::Parser</ulink> in Perl
          CPAN.  In almost all cases that a non-trivial application
          interfaces with Nmap, XML is the preferred format.</para>

          <para>The XML output references an XSL stylesheet which can
          be used to format the results as HTML.  The easiest way to
          use this is simply to load the XML output in a web browser
          such as Firefox or IE.  By default, this will only work on
          the machine you ran Nmap on (or a similarly configured one)
          due to the hard-coded <filename moreinfo="none">nmap.xsl</filename>
          filesystem path.  Use the <option>--webxml</option> or
          <option>--stylesheet</option> options to create portable XML
          files that render as HTML on any web-connected
          machine.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-oS &lt;filespec&gt;</option> (ScRipT KIdd|3 oUTpuT)</term>
        <listitem>
   <para>Script kiddie output is like interactive output, except that
   it is post-processed to better suit the l33t HaXXorZ who
   previously looked down on Nmap due to its consistent capitalization
   and spelling.  Humor impaired people should note that this option
   is making fun of the script kiddies before flaming me for
   supposedly <quote>helping them</quote>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <option>-oG &lt;filespec&gt;</option> (Grepable output)</term>
        <listitem>

<para>This output format is covered last because it is deprecated.
The XML output format is far more powerful, and is nearly as
convenient for experienced users.  XML is a standard for which dozens
of excellent parsers are available, while grepable output is my own
simple hack.  XML is extensible to support new Nmap features as they
are released, while I often must omit those features from grepable
output for lack of a place to put them.</para>

<para>Nevertheless, grepable output is still quite popular.  It is a
simple format that lists each host on one line and can be trivially
searched and parsed with standard UNIX tools such as grep, awk, cut,
sed, diff, and Perl.  Even I usually use it for one-off tests done at the
command line.  Finding all the hosts with the ssh port open or that
are running Solaris takes only a simple grep to identify the hosts,
piped to an awk or cut command to print the desired fields.</para>

<para>Grepable output consists of comments (lines starting with a
pound (#)) and target lines.  A target line includes a combination
of 6 labeled fields, separated by tabs and followed with a colon.
The fields are <literal moreinfo="none">Host</literal>, <literal moreinfo="none">Ports</literal>,
<literal moreinfo="none">Protocols</literal>, <literal moreinfo="none">Ignored State</literal>,
<literal moreinfo="none">OS</literal>, <literal moreinfo="none">Seq Index</literal>,
<literal moreinfo="none">IPID</literal>, and <literal moreinfo="none">Status</literal>.</para>

<para>The most important of these fields is generally
<literal moreinfo="none">Ports</literal>, which gives details on each interesting
port.  It is a comma separated list of port entries.  Each port entry
represents one interesting port, and takes the form of seven slash
(/) separated subfields.  Those subfields are: <literal moreinfo="none">Port
number</literal>, <literal moreinfo="none">State</literal>, <literal moreinfo="none">Protocol</literal>,
<literal moreinfo="none">Owner</literal>, <literal moreinfo="none">Service</literal>, <literal moreinfo="none">SunRPC
info</literal>, and <literal moreinfo="none">Version info</literal>.</para>

<para>As with XML output, this man page does not allow for documenting
the entire format.  A more detailed look at the Nmap grepable output
format is available from <ulink url="http://www.unspecific.com/nmap-oG-output"/>.</para>
        </listitem>
      </varlistentry>

     <varlistentry>
        <term>
        <option>-oA &lt;basename&gt;</option> (Output to all formats)</term>
       <listitem><para>
           As a convenience, you may specify <option>-oA
           <replaceable>basename</replaceable></option> to store scan
           results in normal, XML, and grepable formats at once.  They
           are stored in <replaceable>basename</replaceable>.nmap,
           <replaceable>basename</replaceable>.xml, and
           <replaceable>basename</replaceable>.gnmap, respectively.
           As with most programs, you can prefix the filenames with a
           directory path, such as
           <filename moreinfo="none">~/nmaplogs/foocorp/</filename> on UNIX or
           <filename moreinfo="none">c:\hacking\sco</filename> on Windows.</para>
        </listitem>
      </varlistentry>

   </variablelist>

    <variablelist><title>Verbosity and debugging options</title>
      <varlistentry>
        <term>
          <option>-v</option> (Increase verbosity level)
        </term>
        <listitem>
          <para>Increases the verbosity level, causing Nmap to
          print more information about the scan in progress. Open
          ports are shown as they are found and completion time
          estimates are provided when Nmap thinks a scan will take
          more than a few minutes. Use it twice for even greater
          verbosity. Using it more than twice has no effect.</para>

          <para>Most changes only affect interactive output, and some
          also affect normal and script kiddie output.  The other
          output types are meant to be processed by machines, so Nmap
          can give substantial detail by default in those formats
          without fatiguing a human user.  However, there are a few
          changes in other modes where output size can be reduced
          substantially by omitting some detail.  For example, a
          comment line in the grepable output that provides a list of
          all ports scanned is only printed in verbose mode because it
          can be quite long.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-d [level]</option> (Increase or set debugging level)
        </term>
        <listitem>

<para>When even verbose mode doesn't provide sufficient data for you,
debugging is available to flood you with much more!  As with the
verbosity option (<option>-v</option>), debugging is enabled with a
command-line flag (<option>-d</option>) and the debug level can be
increased by specifying it multiple times.  Alternatively, you can set
a debug level by giving an argument to <option>-d</option>.  For
example, <option>-d9</option> sets level nine.  That is the highest
effective level and will produce thousands of lines unless you run a
very simple scan with very few ports and targets.</para>

<para>Debugging output is useful when a bug is suspected in Nmap,
or if you are simply confused as to what Nmap is doing and why.  As this
feature is mostly intended for developers, debug lines aren't always
self-explanatory.  You may get something like: <computeroutput moreinfo="none">Timeout
vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==&gt; srtt: 14987
rttvar: 14987 to: 100000</computeroutput>.  If you don't understand a line, your only recourses
are to ignore it, look it up in the source code, or request help from
the development list (nmap-dev).  Some lines are self explanatory, but
the messages become more obscure as the debug level is
increased.</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--packet-trace</option> (Trace packets and data sent and received)
        </term>
        <listitem>
          <para>Causes Nmap to print a summary of every packet sent
          or received. This is often used for debugging, but is
          also a valuable way for new users to understand exactly
          what Nmap is doing under the covers. To avoid printing
          thousands of lines, you may want to specify a limited
          number of ports to scan, such as <option>-p20-30</option>.  If you only care
          about the goings on of the version detection subsystem, use
          <option>--version-trace</option> instead.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--iflist</option> (List interfaces and routes)
        </term>
        <listitem>
          <para>Prints the interface list and system routes as
          detected by Nmap.  This is useful for debugging routing
          problems or device mischaracterization (such as
          Nmap treating a PPP connection as Ethernet).</para>
        </listitem>
      </varlistentry>

   </variablelist>

   <variablelist><title>Miscellaneous output options</title>

      <varlistentry>
        <term>
          <option>--append-output</option> (Append to rather than clobber output files)
        </term>
        <listitem>
           <para>When you specify a filename to an output format flag
           such as <option>-oX</option> or <option>-oN</option>, that
           file is overwritten by default.  If you prefer to keep the
           existing content of the file and append the new results,
           specify the <option>--append-output</option> option.  All
           output filenames specified in that Nmap execution will then
           be appended to rather than clobbered.  This doesn't work
           well for XML (<option>-oX</option>) scan data as the
           resultant file generally won't parse properly until you fix
           it up by hand.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--resume &lt;filename&gt;</option> (Resume aborted scan)
        </term>
        <listitem>

          <para>Some extensive Nmap runs take a very long time -- on
          the order of days.  Such scans don't always run to
          completion.  Restrictions may prevent Nmap from being run
          during working hours, the network could go down, the machine
          Nmap is running on might suffer a planned or unplanned
          reboot, or Nmap itself could crash.  The admin running Nmap
          could cancel it for any other reason as well, by pressing
          <keycap moreinfo="none">ctrl-C</keycap>.  Restarting the whole scan from the
          beginning may be undesirable.  Fortunately, if normal
          (<option>-oN</option>) or grepable (<option>-oG</option>)
          logs were kept, the user can ask Nmap to resume scanning
          with the target it was working on when execution ceased.
          Simply specify the <option>--resume</option> option and pass
          the normal/grepable output file as its argument.  No other
          arguments are permitted, as Nmap parses the output file to
          use the same ones specified previously.  Simply call Nmap as
          <command moreinfo="none">nmap --resume
          <replaceable>logfilename</replaceable></command>.  Nmap will
          append new results to the data files specified in the
          previous execution.  Resumption does not support the XML
          output format because combining the two runs into one valid
          XML file would be difficult.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--stylesheet &lt;path or URL&gt;</option> (Set XSL stylesheet to transform XML output)
        </term>
        <listitem>
          <para>Nmap ships with an XSL stylesheet named
          <filename moreinfo="none">nmap.xsl</filename> for viewing or translating XML
          output to HTML.  The XML output includes an <literal moreinfo="none">xml-stylesheet</literal>
          directive which points to <filename moreinfo="none">nmap.xml</filename>
          where it was initially installed by Nmap (or in the current
          working directory on Windows).  Simply load Nmap's XML
          output in a modern web browser and it should retrieve
          <filename moreinfo="none">nmap.xsl</filename> from the filesystem and use it
          to render results.  If you wish to use a different
          stylesheet, specify it as the argument to
          <option>--stylesheet</option>.  You must pass the full
          pathname or URL.  One common invocation is
          <option>--stylesheet
          http://www.insecure.org/nmap/data/nmap.xsl</option>.  This
          tells a browser to load the latest version of the stylesheet
          from Insecure.Org.  The <option>--webxml</option> option
          does the same thing with less typing and memorization.
          Loading the XSL from Insecure.Org makes it easier to view results on
          a machine that doesn't have Nmap (and thus
          <filename moreinfo="none">nmap.xsl</filename>) installed.  So the URL is
          often more useful, but the local filesystem location of
          nmap.xsl is used by default for privacy reasons.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--webxml</option> (Load stylesheet from Insecure.Org)
        </term>
        <listitem>
          <para>This convenience option is simply an alias for
          <option>--stylesheet http://www.insecure.org/nmap/data/nmap.xsl</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--no_stylesheet</option> (Omit XSL stylesheet declaration from XML)
        </term>
        <listitem>
          <para>Specify  this  option  to prevent Nmap from associating any XSL
              stylesheet with its XML output.  The  <literal moreinfo="none">xml-stylesheet</literal>  directive
              is omitted.</para>
        </listitem>
      </varlistentry>



    </variablelist>
  </refsect1>

  <refsect1 id="man-misc-options">
    <title>Miscellaneous Options</title>
    <para>This section describes some important (and not-so-important)
    options that don't really fit anywhere else.</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-6</option> (Enable IPv6 scanning)
        </term>
        <listitem>

          <para>Since 2002, Nmap has offered IPv6 support for its most
          popular features.  In particular, ping scanning (TCP-only),
          connect() scanning, and version detection all support IPv6.
          The command syntax is the same as usual except that you also
          add the <option>-6</option> option.  Of course, you must use
          IPv6 syntax if you specify an address rather than a
          hostname.  An address might look like
          <literal moreinfo="none">3ffe:7501:4819:2000:210:f3ff:fe03:14d0</literal>,
          so hostnames are recommended.  The output looks the same as
          usual, with the IPv6 address on the <quote>interesting
          ports</quote> line being the only IPv6 give away.</para>

          <para>While IPv6 hasn't exactly taken the world by storm, it
          gets significant use in some (usually Asian) countries and
          most modern operating systems support it.  To use Nmap
          with IPv6, both the source and target of your scan must be
          configured for IPv6.  If your ISP (like most of them) does
          not allocate IPv6 addresses to you, free tunnel brokers are
          widely available and work fine with Nmap.  One of the better
          ones is run by BT Exact at <ulink url="https://tb.ipv6.btexact.com/"/>.  I have also used one
          that Hurricane Electric provides at <ulink url="http://ipv6tb.he.net/"/>.  6to4 tunnels are another
          popular, free approach.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-A</option> (Aggressive scan options)
        </term>
        <listitem>
          <para>This option enables additional advanced and
          aggressive options. I haven't decided exactly which it
          stands for yet. Presently this enables OS Detection
          (<option>-O</option>) and version scanning (<option>-sV</option>). More features may be
          added in the future. The point is to enable a
          comprehensive set of scan options without people having
          to remember a large set of flags. This option only
          enables features, and not timing options (such as <option>-T4</option>) or
          verbosity options (<option>-v</option>) that you might want as
          well.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--datadir &lt;directoryname&gt;</option> (Specify custom Nmap data file location)
        </term>
        <listitem>
          <para>Nmap obtains some special data at runtime in files
          named <filename moreinfo="none">nmap-service-probes</filename>,
          <filename moreinfo="none">nmap-services</filename>,
          <filename moreinfo="none">nmap-protocols</filename>,
          <filename moreinfo="none">nmap-rpc</filename>,
          <filename moreinfo="none">nmap-mac-prefixes</filename>, and
          <filename moreinfo="none">nmap-os-fingerprints</filename>.  Nmap first
          searches these files in the directory specified with the
          <option>--datadir</option> option (if any). Any files not
          found there, are searched for in the directory specified by
          the NMAPDIR environmental variable. Next comes <filename moreinfo="none">~/.nmap</filename> for
          real and effective UIDs (POSIX systems only) or location of
          the Nmap executable (Win32 only), and then a compiled-in
          location such as <filename moreinfo="none">/usr/local/share/nmap</filename> or <filename moreinfo="none">/usr/share/nmap</filename>
          . As a last resort, Nmap will look in the current
          directory.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--send-eth</option> (Use raw ethernet sending)
        </term>
        <listitem>
          <para>Asks Nmap to send packets at the raw ethernet (data
          link) layer rather than the higher IP (network) layer. By
          default, Nmap chooses the one which is generally best for
          the platform it is running on. Raw sockets (IP layer) are
          generally most efficient for UNIX machines, while ethernet
          frames are required for Windows operation since Microsoft
          disabled raw socket support. Nmap still uses raw IP packets
          on UNIX despite this option when there is no other choice
          (such as non-ethernet connections).</para>

        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--send-ip</option> (Send at raw IP level)
        </term>
        <listitem>
          <para>Asks Nmap to send packets via raw IP sockets rather
          than sending lower level ethernet frames. It is the
          complement to the <option>--send-eth</option> option discussed
          previously.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--privileged</option> (Assume that the user is fully privileged)
        </term>
        <listitem>
          <para>Tells Nmap to simply assume that it is privileged
          enough to perform raw socket sends, packet sniffing, and
          similar operations that usually require root privileges on
          UNIX systems. By default Nmap quits if such operations are
          requested but geteuid() is not
          zero. <option>--privileged</option> is useful with Linux
          kernel capabilities and similar systems that may be
          configured to allow unprivileged users to perform raw-packet
          scans. Be sure to provide this option flag before any flags
          for options that require privileges (SYN scan, OS detection,
          etc.). The NMAP_PRIVILEGED variable may be set as an
          equivalent alternative to
          <option>--privileged</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--interactive</option> (Start in interactive mode)
        </term>
        <listitem>
          <para>Starts Nmap in interactive mode, which offers an
          interactive Nmap prompt allowing easy launching of
          multiple scans (either synchronously or in the
          background). This is useful for people who scan from
          multi-user systems as they often want to test their
          security without letting everyone else on the system know
          exactly which systems they are scanning. Use
          <option>--interactive</option> to activate this mode and then type <keycap moreinfo="none">h</keycap> for
          help. This option is rarely used because proper shells
          are usually more familiar and feature-complete.  This option
          includes a bang (!) operator for executing shell commands,
          which is one of many reasons not to install Nmap setuid root.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-V</option>; <option>--version</option> (Print version number)
        </term>
        <listitem>
          <para>Prints the Nmap version number and exits.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-h</option>; <option>--help</option> (Print help summary page)
        </term>
        <listitem>
          <para>Prints a short help screen with the most common
          command flags.  Running Nmap without any arguments does the
          same thing.</para>
        </listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1 id="man-runtime-interaction">
    <title>Runtime Interaction</title>
 
    <para>During the execution of nmap, all key presses are
    captured. This allows you to interact with the program
    without aborting and restarting it. Certain special
    keys will change options, while any other keys will print out a
    status message telling you about the scan. The convention is
    that 
    <emphasis>lowercase letters increase</emphasis> the
    amount of printing, and 
    <emphasis>uppercase letters decrease</emphasis> the
    printing.  You may also press &#x2018;<emphasis>?</emphasis>&#x2019; for </para>
    <variablelist>
      <varlistentry>
        <term>
          <option>v</option> / <option>V</option>
        </term>
        <listitem>
          <para>Increase / Decrease the Verbosity</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>d</option> / <option>D</option>
        </term>
        <listitem>
          <para>Increase / Decrease the Debugging Level</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>p</option> / <option>P</option>
        </term>
        <listitem>
          <para>Turn on / off Packet Tracing</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          <option>?</option>
        </term>
        <listitem>
          <para>Print a runtime interaction help screen</para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
          Anything else
        </term>
        <listitem>
          <para>Print out a status message like this:</para>
          <para>Stats: 0:00:08 elapsed; 111 hosts completed (5 up),
          5 undergoing Service Scan</para>
          <para>Service scan Timing: About 28.00% done; ETC: 16:18
          (0:00:15 remaining)</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id="man-examples">
    <title>Examples</title>
    <para>Here are some Nmap usage examples, from the simple and
    routine to a little more complex and esoteric. Some actual
    IP addresses and domain names are used to make things
    more concrete. In their place you should substitute
    addresses/names from 
    <emphasis>your own network.</emphasis>.  While I don't think
    port scanning other networks is or should be illegal, some network
    administrators don't appreciate unsolicited scanning of their networks and may
    complain.  Getting permission first is the best approach.</para>

    <para>For testing purposes, you have permission to scan the host
    <literal moreinfo="none">scanme.nmap.org</literal>.  This permission only includes
    scanning via Nmap and not testing exploits or denial of service
    attacks.  To conserve bandwidth, please do not initiate more than
    a dozen scans against that host per day.  If this free scanning
    target service is abused, it will be taken down and Nmap will
    report <computeroutput moreinfo="none">Failed to resolve given hostname/IP:
    scanme.nmap.org</computeroutput>.  These permissions also apply to
    the hosts <literal moreinfo="none">scanme2.nmap.org</literal>,
    <literal moreinfo="none">scanme3.nmap.org</literal>, and so on, though those hosts
    do not currently exist.</para>

    <para>
      <userinput moreinfo="none">nmap -v scanme.nmap.org</userinput>
    </para>
    <para>This option scans all reserved TCP ports on the machine
    <literal moreinfo="none">scanme.nmap.org</literal> . The <option>-v</option>
    option enables verbose mode.</para>
    <para>
      <userinput moreinfo="none">nmap -sS -O scanme.nmap.org/24</userinput>
    </para>
    <para>Launches a stealth SYN scan against each machine that is
    up out of the 255 machines on <quote>class C</quote> network where
    Scanme resides. It also tries to determine what
    operating system is running on each host that is up and
    running. This requires root privileges because of the SYN scan
    and OS detection.</para>
    <para>
      <userinput moreinfo="none">nmap -sV -p 22,53,110,143,4564
      198.116.0-255.1-127</userinput>
    </para>

    <para>Launches host enumeration and a TCP scan at the first half
    of each of the 255 possible 8 bit subnets in the 198.116 class B
    address space. This tests whether the systems run sshd, DNS,
    pop3d, imapd, or port 4564. For any of these ports found open,
    version detection is used to determine what application is
    running.</para>

    <para>
    <userinput moreinfo="none">nmap -v -iR 100000 -P0 -p 80</userinput>
    </para>

    <para>Asks Nmap to choose 100,000 hosts at random and scan them
    for web servers (port 80).  Host enumeration is disabled with
    <option>-P0</option> since first sending a couple probes to
    determine whether a host is up is wasteful when you are only
    probing one port on each target host anyway.</para>

    <para>
      <userinput moreinfo="none">nmap -P0 -p80 -oX logs/pb-port80scan.xml -oG
      logs/pb-port80scan.gnmap 216.163.128.20/20</userinput>
    </para>
    <para>This scans 4096 IPs for any webservers (without pinging
    them) and saves the output in grepable and XML formats.</para>

  </refsect1>

  <refsect1 id="man-bugs">
    <title>Bugs</title>

    <para>Like its author, Nmap isn't perfect.  But you can help make
    it better by sending bug reports or even writing patches.  If Nmap
    doesn't behave the way you expect, first upgrade to the latest
    version available from <ulink url="http://www.insecure.org/nmap/"/>.  If the problem persists,
    do some research to determine whether it has already been
    discovered and addressed.  Try Googling the error message or
    browsing the Nmap-dev archives at <ulink url="http://seclists.org/"/>.  Read this full munaual page as
    well.  If nothing comes of this, mail a bug report to
    <email>nmap-dev@insecure.org</email>.  Please include everything
    you have learned about the problem, as well as what version of
    Nmap you are running and what operating system version it is
    running on.  Problem reports and Nmap usage questions sent to
    nmap-dev@insecure.org are far more likely to be answered than
    those sent to Fyodor directly.</para>

    <para>Code patches to fix bugs are even better than bug reports.
    Basic instructions for creating patch files with your changes are
    available at <ulink url="http://www.insecure.org/nmap/data/HACKING"/>.  Patches may
    be sent to nmap-dev (recommended) or to Fyodor directly.</para>
  </refsect1>

  <refsect1 id="man-author">
    <title>Author</title>
    <para>Fyodor 
    <email>fyodor@insecure.org</email>
    (<ulink url="http://www.insecure.org"/>)
    </para>

    <para>Hundreds of people have made valuable contributions to Nmap
    over the years.  These are detailed in the
    <filename moreinfo="none">CHANGELOG</filename> file which is distributed with Nmap
    and also available from <ulink url="http://www.insecure.org/nmap/changelog.html"/>.</para>

  </refsect1>

  <refsect1 id="man-legal">
    <title>Legal Notices</title>

<refsect2 id="nmap-copyright">
  <title>Nmap Copyright and Licensing</title>

<para>The Nmap Security
Scanner is (C) 1996-2005 Insecure.Com LLC. Nmap is also a registered
trademark of Insecure.Com LLC.  This program is free software; you may
redistribute and/or modify it under the terms of the GNU General
Public License as published by the Free Software Foundation; Version
2. This guarantees your right to use, modify, and redistribute this
software under certain conditions. If you wish to embed Nmap
technology into proprietary software, we may be willing to sell
alternative licenses (contact <email>sales@insecure.com</email>). Many
security scanner vendors already license Nmap technology such as host
discovery, port scanning, OS detection, and service/version
detection.</para>

<para>Note that the GPL places important restrictions on
<quote>derived works</quote>, yet it does not provide a detailed
definition of that term. To avoid misunderstandings, we consider an
application to constitute a <quote>derivative work</quote> for the
purpose of this license if it does any of the following:</para>

<itemizedlist>
<listitem><para>Integrates source code from Nmap</para></listitem>
<listitem><para>Reads or includes Nmap copyrighted data files, such as
    <filename moreinfo="none">nmap-os-fingerprints</filename> or
    <filename moreinfo="none">nmap-service-probes</filename>.</para></listitem>
<listitem><para>Executes Nmap and parses the results (as opposed to
    typical shell or execution-menu apps, which simply display raw
    Nmap output and so are not derivative works.)</para></listitem>
<listitem><para>Integrates/includes/aggregates Nmap into a proprietary
    executable installer, such as those produced by
    InstallShield.</para></listitem>
<listitem><para>Links to a library or executes a program that does any
    of the above.</para></listitem>
</itemizedlist>

<para>The term <quote>Nmap</quote> should be taken to also include any
portions or derived works of Nmap. This list is not exclusive, but is
just meant to clarify our interpretation of derived works with some
common examples. These restrictions only apply when you actually
redistribute Nmap. For example, nothing stops you from writing and
selling a proprietary front-end to Nmap.  Just distribute it by
itself, and point people to <ulink url="http://www.insecure.org/nmap/"/> to download Nmap.</para>

<para>We don't consider these to be added restrictions on top of the
GPL, but just a clarification of how we interpret <quote>derived
works</quote> as it applies to our GPL-licensed Nmap product.  This is
similar to the way Linus Torvalds has announced his interpretation of
how <quote>derived works</quote> applies to Linux kernel modules. Our
interpretation refers only to Nmap - we don't speak for any other GPL
products.</para>

<para>If you have any questions about the GPL licensing restrictions
on using Nmap in non-GPL works, we would be happy to help. As
mentioned above, we also offer alternative license to integrate Nmap
into proprietary applications and appliances.  These contracts have
been sold to many security vendors, and generally include a perpetual
license as well as providing for priority support and updates as well
as helping to fund the continued development of Nmap
technology. Please email <email>sales@insecure.com</email> for further
information.</para>

<para>As a special exception to the GPL terms, Insecure.Com LLC grants
permission to link the code of this program with any version of the
OpenSSL library which is distributed under a license identical to that
listed in the included Copying.OpenSSL file, and distribute linked
combinations including the two. You must obey the GNU GPL in all
respects for all of the code used other than OpenSSL. If you modify
this file, you may extend this exception to your version of the file,
but you are not obligated to do so.</para>

<para>If you received these files with a written license agreement or
contract stating terms other than the terms above, then that
alternative license agreement takes precedence over these
comments.</para>
</refsect2>

<refsect2 id="man-copyright">
  <title>Creative Commons license for this Nmap guide</title>
  <para>This Nmap Reference Guide is (C) 2005 Insecure.Com LLC. It is
   hereby placed under version 2.5 of the <ulink url="http://creativecommons.org/licenses/by/2.5/">Creative Commons
   Attribution License</ulink>.  This allows you redistribute and modify
   the work as you desire, as long as you credit the original source.
   Alternatively, you may choose to treat this document as falling under
   the same license as Nmap itself (discussed previously).</para>
</refsect2>

<refsect2 id="source-contrib">
  <title>Source code availability and community contributions</title>

<para>Source is provided to this software because we believe users
have a right to know exactly what a program is going to do before they
run it. This also allows you to audit the software for security holes
(none have been found so far).</para>

<para>Source code also allows you to port Nmap to new platforms, fix
bugs, and add new features. You are highly encouraged to send your
changes to <email>fyodor@insecure.org</email> for possible
incorporation into the main distribution. By sending these changes to
Fyodor or one of the Insecure.Org development mailing lists, it is
assumed that you are offering Fyodor and Insecure.Com LLC the
unlimited, non-exclusive right to reuse, modify, and relicense the
code. Nmap will always be available Open Source, but this is important
because the inability to relicense code has caused devastating
problems for other Free Software projects (such as KDE and NASM). We
also occasionally relicense the code to third parties as discussed
above. If you wish to specify special license conditions of your
contributions, just say so when you send them.</para>
</refsect2>

<refsect2 id="no-warranty"><title>No Warranty</title>

<para>This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details at <ulink url="http://www.gnu.org/copyleft/gpl.html"/>, or in the COPYING file
included with Nmap.</para>

<para>It should also be noted that Nmap has occasionally been known to crash
poorly written applications, TCP/IP stacks, and even operating
systems.  While this is extremely rare, it is important to keep in
mind.  <emphasis>Nmap should never be run against mission
critical systems</emphasis> unless you are prepared to suffer
downtime. We acknowledge here that Nmap may crash your systems or
networks and we disclaim all liability for any damage or problems Nmap
could cause.</para>
</refsect2>

<refsect2 id="inappropriate-usage"><title>Inappropriate Usage</title>
<para>Because of the slight risk of crashes and because a few black
hats like to use Nmap for reconnaissance prior to attacking systems,
there are administrators who become upset and may complain when their
system is scanned. Thus, it is often advisable to request permission
before doing even a light scan of a network.</para>

<para>Nmap should never be installed with special privileges
(e.g. suid root) for security reasons.</para>
</refsect2>

<refsect2 id="third-party-soft"><title>Third-Party Software</title>
<para>This product includes software developed by the <ulink url="http://www.apache.org">Apache Software Foundation</ulink>.  A
modified version of the <ulink url="http://www.tcpdump.org">Libpcap
portable packet capture library</ulink> is distributed along with
nmap.  The Windows version of Nmap utilized the libpcap-derived <ulink url="http://www.winpcap.org">WinPcap library</ulink> instead.  Regular
expression support is provided by the <ulink url="http://www.pcre.org">PCRE library</ulink>, which is open source
software, written by Philip Hazel.  Certain raw networking functions
use the <ulink url="http://libdnet.sourceforge.net">Libdnet</ulink>
networking library, which was written by Dug Song.  A modified version
is distributed with Nmap.  Nmap can optionally link with the <ulink url="http://www.openssl.org">OpenSSL cryptography toolkit</ulink> for
SSL version detection support.  All of the third-party software
described in this paragraph is freely redistributable under BSD-style
software licenses.</para>
</refsect2>

<refsect2 id="us-export"><title>US Export Control Classification</title>
<para>US Export Control: Insecure.Com LLC believes that Nmap falls
under US ECCN (export control classification number) 5D992. This
category is called <quote>Information Security software not controlled
by 5D002</quote>. The only restriction of this classification is AT
(anti-terrorism), which applies to almost all goods and denies export
to a handful of rogue nations such as Iran and North Korea. Thus
exporting Nmap does not require any special license, permit, or other
governmental authorization.</para>
</refsect2>

  </refsect1>
</refentry>

</article>
