# Nmap Scripting Engine

## Introduction

The Nmap Scripting Engine (NSE) is one of Nmap's most powerful and flexible features. It allows users to write (and share) simple scripts to automate a wide variety of networking tasks. Those scripts are then executed in parallel with the speed and efficiency you expect from Nmap. Users can rely on the growing and diverse set of scripts distributed with Nmap, or write their own to meet custom needs.

We designed NSE to be versatile, with the following tasks in mind:

Network discovery

  This is Nmap's bread and butter. Examples include looking up whois data based on the target domain, querying ARIN, RIPE, or APNIC for the target IP to determine ownership, performing identd lookups on open ports, SNMP queries, and listing available NFS/SMB/RPC shares and services.

More sophisticated version detection

  The Nmap version detection system () is able to recognize thousands of different services through its probe and regular expression signature based matching system, but it cannot recognize everything. For example, identifying the Skype v2 service requires two independent probes, which version detection isn't flexible enough to handle. Nmap could also recognize more SNMP services if it tried a few hundred different community names by brute force. Neither of these tasks are well suited to traditional Nmap version detection, but both are easily accomplished with NSE. For these reasons, version detection now calls NSE by default to handle some tricky services. This is described in .

Vulnerability detection

  When a new vulnerability is discovered, you often want to scan your networks quickly to identify vulnerable systems before the bad guys do. While Nmap isn't a comprehensive vulnerability scanner, NSE is powerful enough to handle even demanding vulnerability checks. Many vulnerability detection scripts are already available and we plan to distribute more as they are written.

Backdoor detection

  Many attackers and some automated worms leave backdoors to enable later reentry. Some of these can be detected by Nmap's regular expression based version detection. For example, within hours of the MyDoom worm hitting the Internet, Jay Moran
  posted an Nmap version detection probe and signature so that others could quickly scan their networks for MyDoom infections. NSE is needed to reliably detect more complex worms and backdoors.

Vulnerability exploitation

  As a general scripting language, NSE can even be used to exploit vulnerabilities rather than just find them. The capability to add custom exploit scripts may be valuable for some people (particularly penetration testers),
  though we aren't planning to turn Nmap into an exploitation framework such as Metasploit.

These listed items were our initial goals, and we expect Nmap users to come up with even more inventive uses for NSE.

Scripts are written in the embedded Lua programming language.
The language itself is well documented in the books Programming in Lua, Second Edition and Lua 5.1 Reference Manual. Programming in Lua, Second Edition and Lua 5.1 Reference Manual.

The reference manual is also freely available online, as is the first edition of Programming in Lua. Given the availability of these excellent general Lua programming references, this document only covers aspects and extensions specific to Nmap's scripting engine.

NSE is activated with the `-sC` option (or `--script` if you wish to specify a custom set of scripts) and results are integrated into Nmap normal
and XML output.
Two types of scripts are supported: service and host scripts. Service scripts relate to a certain open port (service) on the target host, and any results they produce are included next to that port in the Nmap output port table. Host scripts, on the other hand, run no more than once against each target IP and produce results below the port table. shows a typical script scan. Service scripts producing output in this example are `ssh-hostkey`, which provides the system's RSA and DSA SSH keys, and `rpcinfo`, which queries portmapper to enumerate available services. The only host script producing output in this example is `smb-os-discovery`, which collects a variety of information from SMB servers.
Nmap discovered all of this information in a third of a second.

### Typical NSE output

    # nmap -sC -p22,111,139 -T4 localhost

    Starting Nmap ( http://nmap.org )
    Interesting ports on flog (127.0.0.1):
    PORT     STATE SERVICE
    22/tcp   open  ssh
    |  ssh-hostkey: 1024 b1:36:0d:3f:50:dc:13:96:b2:6e:34:39:0d:9b:1a:38 (DSA)
    |_ 2048 77:d0:20:1c:44:1f:87:a0:30:aa:85:cf:e8:ca:4c:11 (RSA)
    111/tcp  open  rpcbind
    |  rpcinfo:
    |  100000  2,3,4    111/udp  rpcbind
    |  100024  1      56454/udp  status
    |_ 100000  2,3,4    111/tcp  rpcbind
    139/tcp  open  netbios-ssn

    Host script results:
    |  smb-os-discovery: Unix
    |  LAN Manager: Samba 3.0.31-0.fc8
    |_ Name: WORKGROUP

    Nmap done: 1 IP address (1 host up) scanned in 0.33 seconds

## Usage and Examples

While NSE has a complex implementation for efficiency, it is strikingly easy to use. Simply specify `-sC`
to enable the most common scripts. Or specify the `--script`
option to choose your own scripts to execute by providing categories, script file names, or the name of directories full of scripts you wish to execute. You can customize some scripts by providing arguments to them via the `--script-args`
option. The two remaining options, `--script-trace`
and `--script-updatedb`,
are generally only used for script debugging and development. Script scanning is also included as part of the `-A` (aggressive scan) option.

### Script Categories

NSE scripts define a list of categories they belong to. Currently defined categories are `auth`, `default`, `discovery`, `external`, `intrusive`, `malware`, `safe`, `version`, and `vuln`. Category names are not case sensitive. The following list describes each category.

`auth`

  These scripts try to determine authentication credentials on the target system, often through a brute-force attack. Examples include `snmp-brute`, `http-auth`, and `ftp-anon`.

`default`

  These scripts are the default set and are run when using the `-sC` or `-A` options rather than listing scripts with `--script`. This category can also be specified explicitly like any other using `--script=default`. Many factors are considered in deciding whether a script should be run by default:

  Speed

    A default scan must finish quickly, which excludes brute force authentication crackers, web spiders, and any other scripts which can take minutes or hours to scan a single service.

  Usefulness

    Default scans need to produce valuable and actionable information. If even the script author has trouble explaining why an average networking or security professional would find the output valuable, the script should not run by default. The script may still be worth including in Nmap so that administrators can run for those occasions when they do need the extra information.

  Verbosity

    Nmap output is used for a wide variety of purposes and needs to be readable and concise. A script which frequently produces pages full of output should not be added to the `default` category. When there is no important information to report, NSE scripts (particularly default ones) should return nothing. Checking for an obscure vulnerability may be OK by default as long as it only produces output when that vulnerability discovered.

  Reliability

    Many scripts use heuristics and fuzzy signature matching to reach conclusions about the target host or service. Examples include `sniffer-detect` and `sql-injection`. If the script is often wrong, it doesn't belong in the `default` category where it may confuse or mislead casual users. Users who specify a script or category directly are generally more advanced and likely know how the script works or at least where to find its documentation.

  Intrusiveness

    Some scripts are very intrusive because they use significant resources on the remote system, are likely to crash the system or service, or are likely to be perceived as an attack by the remote administrators. The more intrusive a script is, the less suitable it is for the `default` category.

  Privacy

    Some scripts, particularly those in the `external` category described later, divulge information to third parties by their very nature. For example, the `whois` script must divulge the target IP address to regional whois registries. We have also considered (and decided against) adding scripts which check target SSH and SSL key fingerprints against Internet weak key databases. The more privacy-invasive a script is, the less suitable it is for `default` category inclusion.

  We don't have exact thresholds for each of these criteria, and many of them are subjective. All of these factors are considered together when making a decision whether to promote a script into the `default` category. A few default scripts are `identd-owners` (determines the username running remote services using identd), `http-auth` (obtains authentication scheme and realm of web sites requiring authentication), and `ftp-anon` (tests whether an FTP server allows anonymous access).

`discovery`

  These scripts try to actively discover more about the network by querying public registries, SNMP-enabled devices, directory services, and the like. Examples include `html-title` (obtains the title of the root path of web sites), `smb-enum-shares` (enumerates Windows shares), and `snmp-sysdescr` (extracts system details via SNMP).

`external`

  Scripts in this category may send data to a third-party database or other network resource. An example of this is `whois`, which makes a connection to whois
  servers to learn about the address of the target. There is always the possibility that operators of the third-party database will record anything you send to them, which in many cases will include your IP address and the address of the target. Most scripts involve traffic strictly between the scanning computer and the client; any that do not are placed in this category.

`intrusive`

  These are scripts that cannot be classified in the `safe` category because the risks are too high that they will crash the target system, use up significant resources on the target host (such as bandwidth or CPU time), or otherwise be perceived as malicious by the target's system administrators. Examples are `http-open-proxy` (which attempts to use the target server as an HTTP proxy) and `snmp-brute` (which tries to guess a device's SNMP community string by sending common values such as `public`, `private`, and `cisco`).

`malware`

  These scripts test whether the target platform is infected by malware or backdoors. Examples include `smtp-strangeport`, which watches for SMTP servers running on unusual port numbers, and `auth-spoof`, which detects identd spoofing daemons which provide a fake answer before even receiving a query. Both of these behaviors are commonly associated with malware infections.

`safe`

  Scripts which weren't designed to crash services, use large amounts of network bandwidth or other resources, or exploit security holes are categorized as `safe`. These are less likely to offend remote administrators, though (as with all other Nmap features) we cannot guarantee that they won't ever cause adverse reactions. Most of these perform general network discovery. Examples are `ssh-hostkey` (retrieves an SSH host key) and `html-title` (grabs the title from a web page).

`version`

  The scripts in this special category are an extension to the version detection feature and cannot be selected explicitly. They are selected to run only if version detection (`-sV`) was requested. Their output cannot be distinguished from version detection output and they do not produce service or host script results. Examples are `skypev2-version`, `pptp-version`, and `iax2-version`.

`vuln`

  These scripts check for specific known vulnerabilities and generally only report results if they are found. Examples include `realvnc-auth-bypass` and `xampp-default-auth`.

### Command-line Arguments

These are the five command line arguments specific to script-scanning:

`-sC`

  Performs a script scan using the default set of scripts. It is equivalent to `--script=default`. Some of the scripts in this `default` category are considered intrusive and should not be run against a target network without permission.

`--script script-categories|directory|filename|all`

  Runs a script scan (like `-sC`) using the comma-separated list of script categories, individual scripts, or directories containing scripts, rather than the default set. Nmap first tries to interpret the arguments as categories, then (if that fails) as files or directories. A script or directory of scripts may be specified as an absolute or relative path. Absolute paths are used as supplied. Relative paths are searched for in the following places until found:
  `--datadir/`; `$NMAPDIR/`;
  `~/.nmap/` (not searched on Windows);
  `NMAPDATADIR`/ or
  `./`. A `scripts/` subdirectory is also tried in each of these.

  If a directory is specified and found, Nmap loads all NSE scripts (any filenames ending with `.nse`) from that directory. Filenames without the `nse` extension are ignored. Nmap does not search recursively into subdirectories to find scripts. If individual file names are specified, the file extension does not have to be `nse`.

  Nmap scripts are stored in a `scripts` subdirectory of the Nmap data directory by default (see ). For efficiency, scripts are indexed in a database stored in `scripts/script.db`,
  which lists the category or categories in which each script belongs. Give the argument `all` to execute all scripts in the Nmap script database.

  Scripts are not run in a sandbox and thus could accidentally or maliciously damage your system or invade your privacy. Never run scripts from third parties unless you trust the authors or have carefully audited the scripts yourself.

`--script-args`

  provides arguments to the scripts. See for a detailed explanation.

`--script-trace`

  This option is similar to `--packet-trace`, but works at the application level rather than packet by packet. If this option is specified, all incoming and outgoing communication performed by scripts is printed. The displayed information includes the communication protocol, source and target addresses, and the transmitted data. If more than 5% of transmitted data is unprintable, hex dumps are given instead. Specifying `--packet-trace` enables script tracing too.

`--script-updatedb`

  This option updates the script database found in `scripts/script.db` which is used by Nmap to determine the available default scripts and categories. It is only necessary to update the database if you have added or removed NSE scripts from the default `scripts` directory or if you have changed the categories of any script. This option is used by itself without arguments: `nmap --script-updatedb`.

Some other Nmap options have effects on script scans. The most prominent of these is `-sV`.
A version scan automatically executes the scripts in the `version` category.
The scripts in this category are slightly different than other scripts because their output blends in with the version scan results and they do not produce any script scan output.

Another option which affects the scripting engine is `-A`.
The aggressive Nmap mode implies the `-sC` option.

### Arguments to Scripts

Arguments may be passed to NSE scripts using the `--script-args` option. The script arguments are generally name-value pairs. They are provided to scripts as a Lua table named `args` inside `nmap.registry`. The argument names are keys for the corresponding values. The values can be either strings or tables. Subtables can be used to pass arguments to scripts with finer granularity, such as passing different usernames for different scripts. Here is a typical Nmap invocation with script arguments:

`$ nmap -sC --script-args user=foo,pass=bar,whois={whodb=nofollow+ripe}`

The aforementioned command results in this Lua table:

    {user="foo",pass="bar",whois={whodb="nofollow+ripe"}}

You could therefore access the username (`foo`) inside your script with this statement:

`local username = nmap.registry.args.user`

Subtables used to override options for scripts are usually named after the script to ease retrieval.

### Usage Examples

A simple script scan using the default set of scripts:

`$ nmap -sC example.com`

Executing a specific script with tracing enabled:

`$ nmap --script=./showSSHVersion.nse --script-trace example.com`

Execute all scripts in the `mycustomscripts` directory as well as all default scripts in the `safe` category:

`$ nmap --script=mycustomscripts,safe example.com`

## Script Format

NSE scripts consist of two–five descriptive fields along with either a port or host rule defining when the script should be executed and an action block containing the actual script instructions. Values can be assigned to the descriptive fields just as you would assign any other Lua variables. Their names must be lowercase as shown in this section.

### `description` Field

The `description` field describes what a script is testing for and any important notes the user should be aware of. Depending on script complexity, the description may vary from a few sentences to a few paragraphs. The first paragraph should be a brief synopsis of the script function suitable for stand-alone presentation to the user. Further paragraphs may provide much more script detail.

### `categories` Field

The `categories` field defines one or more categories to which a script belongs (see ). The categories are case-insensitive and may be specified in any order. They are listed in an array-style Lua table as in this example:

    categories = {"default", "discovery", "safe"}

### `author` Field

The `author` field contains the script authors' names and contact information. If you are worried about spam, feel free to omit or obscure your email address, or give your home page URL instead. This optional field is not used by NSE, but gives script authors due credit or blame.

### `license` Field

Nmap is a community project and we welcome all sorts of code contributions, including NSE scripts. So if you write a valuable script, don't keep it to yourself! The optional `license` field helps ensure that we have legal permission to distribute all the scripts which come with Nmap. All of those scripts currently use the standard Nmap license (described in ). They include the following line:

    license = "Same as Nmap--See http://nmap.org/book/man-legal.html"

The Nmap license is similar to the GNU GPL. Script authors may use a BSD-style license (no advertising clause) instead if they prefer that.

### `runlevel` Field

This optional field determines script execution order. When this section is absent, the run level defaults to 1.0. Scripts with a given `runlevel` execute after any with a lower `runlevel` and before any scripts with a higher `runlevel` against a single target machine. The order of scripts with the same `runlevel` is undefined and they often run concurrently. One application of run levels is allowing scripts to depend on each other. If `script A` relies on some information gathered by `script B`, give `B` a lower run level than `A`. `Script B` can store information in the NSE registry for `A` to retrieve later. For information on the NSE registry, see .

### Port and Host Rules

Nmap uses the script rules to determine whether a script should be run against a target. A script contains either a port rule, which governs which ports of a target the scripts may run against, or a host rule, which specifies that the script should be run only once against a target IP and only if the given conditions are met. A rule is a Lua function that returns either `true` or `false`. The script action is only performed if its rule evaluates to `true`. Host rules accept a host table as their argument and may test, for example, the IP address or hostname of the target. A port rule accepts both host and port tables as arguments for any TCP or UDP port in the `open`
, `open|filtered`
, or `unfiltered`
port states. Port rules generally test factors such as the port number, port state, or listening service name in deciding whether to run against a port. Example rules are shown in .

### Action

The action is the heart of an NSE script. It contains all of the instructions to be executed when the script's port or host rule triggers. It is a Lua function which accepts the same arguments as the rule and can return either `nil` or a string. If a string is returned by a service script, the string and script's filename are printed in the Nmap port table output. A string returned by a host script is printed below the port table. No output is produced if the script returns `nil`. For an example of an NSE action refer to .

## Script Language

The core of the Nmap Scripting Engine is an embeddable Lua interpreter. Lua is a lightweight language designed for extensibility. It offers a powerful and well documented API for interfacing with other software such as Nmap.

The second part of the Nmap Scripting Engine is the NSE Library, which connects Lua and Nmap. This layer handles issues such as initialization of the Lua interpreter, scheduling of parallel script execution, script retrieval and more. It is also the heart of the NSE network I/O framework and the exception handling mechanism. It also includes utility libraries to make scripts more powerful and convenient. The utility library modules and extensions are described in .

### Lua Base Language

The Nmap scripting language is an embedded Lua interpreter which was extended with libraries for interfacing with Nmap. The Nmap API is in the Lua namespace `nmap`. This means that all calls to resources provided by Nmap have an `nmap` prefix.
`nmap.new_socket()`, for example, returns a new socket wrapper object. The Nmap library layer also takes care of initializing the Lua context, scheduling parallel scripts and collecting the output produced by completed scripts.

During the planning stages, we considered several programming languages as the base for Nmap scripting. Another option was to implement a completely new programming language. Our criteria were strict: NSE had to be easy to use, small in size, compatible with the Nmap license, scalable, fast and parallelizable. Several previous efforts (by other projects) to design their own security auditing language from scratch resulted in awkward solutions, so we decided early not to follow that route. First the Guile Scheme interpreter was considered, but the preference drifted towards the Elk interpreter due to its more favorable license. But parallelizing Elk scripts would have been difficult. In addition, we expect that most Nmap users prefer procedural programming over functional languages such as Scheme. Larger interpreters such as Perl, Python, and Ruby are well-known and loved, but are difficult to embed efficiently. In the end, Lua excelled in all of our criteria. It is small, distributed under the liberal MIT open source license, has coroutines for efficient parallel script execution, was designed with embeddability in mind, has excellent documentation, and is actively developed by a large and committed community. Lua is now even embedded in other popular open source security tools including the Wireshark sniffer and Snort IDS.

## NSE Scripts

This section lists (alphabetically) all NSE scripts packaged with Nmap at the time of this writing. It comes straight from the script source code thanks to the NSEDoc documentation system described in . Of course no paper documentation can stay current with software developed as actively as NSE is. For the most comprehensive and up-to-date documentation, see the online NSE Documentation Portal at .

&nse-scripts;

This section (a long list of NSE scripts with brief summaries) is only provided in the printed edition of this book because we already provide a better online interface to the information at the NSE Documentation Portal.

## NSE Libraries

In addition to the significant built-in capabilities of Lua, we have written or integrated many extension libraries which make script writing more powerful and convenient. These libraries (sometimes called modules) are compiled if necessary and installed along with Nmap. They have their own directory, `nselib`, which is installed in the configured Nmap data directory. Scripts need only `require` the default libraries in order to use them.

### List of All Libraries

This list is just an overview to give an idea of what libraries are available. Developers will want to consult the complete documentation at .

&nse-modules;

### Adding C Modules to Nselib

A few of the modules included in nselib are written in C or C++ rather than Lua. Two examples are `bit` and `pcre`. We recommend that modules be written in Lua if possible, but C and C++ may be more appropriate if performance is critical or (as with the `pcre` and `openssl` modules) you are linking to an existing C library. This section describes how to write your own compiled extensions to nselib.

The Lua C API is described at length in Programming in Lua, Second Edition, Programming in Lua, Second Edition, so this is a short summary. C modules consist of functions that follow the protocol of the lua_CFunction type. The functions are registered with Lua and assembled into a library by calling the `luaL_register`
function. A special initialization function provides the interface between the module and the rest of the NSE code. By convention the initialization function is named in the form `luaopen_module`.

The smallest compiled module that comes with NSE is `bit`,
and one of the most straightforward is `openssl`.
These modules serve as good examples for a beginning module writer. The source code for `bit` is found in `nse_bit.cc` and `nse_bit.h`, while the `openssl` source is in `nse_openssl.cc` and `nse_openssl.h`. Most of the other compiled modules follow this `nse_module name.cc` naming convention.

Reviewing the `openssl` module shows that one of the functions in `nse_openssl.cc` is `l_md5`, which calculates an MD5 digest. Its function prototype is:

    static int l_md5(lua_State *L);

The prototype shows that `l_md5` matches the lua_CFunction type. The function is static because it does not have to be visible to other compiled code. Only an address is required to register it with Lua. Later in the file, `l_md5` is entered into an array of type luaL_reg and associated with the name `md5`:

    static const struct luaL_reg openssllib[] = {
      { "md5", l_md5 },
      { NULL, NULL }
    };

This function will now be known as `md5` to NSE. Next the library is registered with a call to `luaL_register` inside the initialization function `luaopen_openssl`, as shown next. Some lines relating to the registration of OpenSSL BIGNUM types have been omitted:

    LUALIB_API int luaopen_openssl(lua_State *L) {
      luaL_register(L, OPENSSLLIBNAME, openssllib);
      return 1;
    }

The function `luaopen_openssl` is the only function in the file that is exposed in `nse_openssl.h`. `OPENSSLLIBNAME` is simply the string `"openssl"`.

After a compiled module is written, it must be added to NSE by including it in the list of standard libraries in `nse_init.cc`. Then the module's source file names must be added to `Makefile.in` in the appropriate places. For both these tasks you can simply follow the example of the other C modules. For the Windows build, the new source files must be added to the `mswin32/nmap.vcproj` project file using MS Visual Studio (see ).

## Nmap API

NSE scripts have access to several Nmap facilities for writing flexible and elegant scripts. The API provides target host details such as port states and version detection results. It also offers an interface to the Nsock
library for efficient network I/O.

### Information Passed to a Script

An effective Nmap scripting engine requires more than just a Lua interpreter. Users need easy access to the information Nmap has learned about the target hosts. This data is passed as arguments to the NSE script's `action` method.
The arguments, `host` and `port`, are Lua tables which contain information on the target against which the script is executed. If a script matched a hostrule, it gets only the `host` table, and if it matched a portrule it gets both `host` and `port`. The following list describes each variable in these two tables.

`host`

  This table is passed as a parameter to the rule and action functions. It contains information on the operating system run by the host (if the `-O` switch was supplied), the IP address and the host name of the scanned target.

`host.os`

  The `os` entry in the host table is an array of strings. The strings (as many as eight) are the names of the operating systems the target is possibly running. Strings are only entered in this array if the target machine is a perfect match for one or more OS database entries. If Nmap was run without the `-O` option, then `host.os` is `nil`.

`host.ip`

  Contains a string representation of the IP address of the target host. If the scan was run against a host name and the reverse DNS query returned more than one IP addresses then the same IP address is used as the one chosen for the scan.

`host.name`

  Contains the reverse DNS entry of the scanned target host represented as a string. If the host has no reverse DNS entry, the value of the field is an empty string.

`host.targetname`

  Contains the name of the host as specified on the command line. If the target given on the command line contains a netmask or is an IP address the value of the field is `nil`.

`host.directly_connected`

  A Boolean value indicating whether or not the target host is directly connected to (i.e. on the same network segment as) the host running Nmap.

`host.mac_addr`

  MAC address
  of the destination host (six-byte long binary string) or `nil`, if the host is not directly connected.

`host.mac_addr_src`

  Our own MAC address, which was used to connect to the host (either our network card's, or (with `--spoof-mac`)
  the spoofed address).

`host.interface`

  A string containing the interface name (dnet-style)
  through which packets to the host are sent.

`host.bin_ip`

  The target host's IPv4 address as a 32-bit binary value.

`host.bin_ip_src`

  Our host's (running Nmap) source IPv4 address as a 32-bit binary value.

`port`

  The port table is passed to an NSE service script (i.e. only those with a portrule rather than a hostrule) in the same fashion as the host table. It contains information about the port against which the script is running. While this table is not passed to host scripts, port states on the target can still be requested from Nmap using the `nmap.get_port_state()` call.

`port.number`

  Contains the port number of the target port.

`port.protocol`

  Defines the protocol of the target port. Valid values are `"tcp"` and `"udp"`.

`port.service`

  Contains a string representation of the service running on `port.number` as detected by the Nmap service detection. If the `port.version` field is `nil`, Nmap has guessed the service based on the port number. Otherwise version detection was able to determine the listening service and this field is equal to `port.version.name`.

`port.version`

  This entry is a table which contains information retrieved by the Nmap version scanning engine. Some of the values (such as service name, service type confidence, and the RPC-related values) may be retrieved by Nmap even if a version scan was not performed. Values which were not determined default to `nil`. The meaning of each value is given in the following table:

  #### `port.version` values

  Name | Description
  --- | ---
  `name` | Contains the service name Nmap decided on for the port.
  `name_confidence` | Evaluates how confident Nmap is about the accuracy of `name`, from 1 (least confident) to 10.
  `product`, `version`, `extrainfo`, `hostname`, `ostype`, `devicetype` | These five variables are the same as those described under versioninfo in .
  `service_tunnel` | Contains the string `"none"` or `"ssl"` based on whether or not Nmap used SSL tunneling to detect the service.
  `service_fp` | The service fingerprint, if any, is provided in this value. This is described in .
  `rpc_status` | Contains a string value of `good_prog` if we were able to determine the program number of an RPC service listening on the port, `unknown` if the port appears to be RPC but we couldn't determine the program number, `not_rpc` if the port doesn't appear be RPC, or `untested` if we haven't checked for RPC status.
  `rpc_program`, `rpc_lowver`, `rpc_highver` | The detected RPC program number and the range of version numbers supported by that program. These will be `nil` if `rpc_status` is anything other than `good_prog`.

`port.state`

  Contains information on the state of the port. Service scripts are only run against ports in the `open` or `open|filtered` states, so `port.state` generally contains one of those values. Other values might appear if the port table is a result of the `get_port_state` function. You can adjust the port state using the `nmap.set_port_state()` call. This is normally done when an `open|filtered` port is determined to be `open`.

### Network I/O API

To allow for efficient and parallelizable network I/O, NSE provides an interface to Nsock, the Nmap socket library. The smart callback mechanism Nsock uses is fully transparent to NSE scripts. The main benefit of NSE's sockets is that they never block on I/O operations, allowing many scripts to be run in parallel. The I/O parallelism is fully transparent to authors of NSE scripts. In NSE you can either program as if you were using a single non-blocking socket or you can program as if your connection is blocking. Even blocking I/O calls return once a specified timeout has been exceeded. Two flavors of Network I/O are supported: connect-style and raw packet.

#### Connect-style network I/O

This part of the network API should be suitable for most classical network uses: Users create a socket, connect it to a remote address, send and receive data and finally close the socket. Everything up to the Transport layer (which is either TCP, UDP or SSL) is handled by the library.

An NSE socket is created by calling `nmap.new_socket`, which returns a socket object. The socket object supports the usual `connect`, `send`, `receive`, and `close` methods. Additionally the functions `receive_bytes`, `receive_lines`, and `receive_buf` allow greater control over data reception. shows the use of connect-style network operations. The `try` function is used for error handling, as described in .

##### Connect-style I/O

    require("nmap")

    local socket = nmap.new_socket()
    socket:set_timeout(1000)
    try = nmap.new_try(function() socket:close() end)
    try(socket:connect(host.ip, port.number))
    try(socket:send("login"))
    response = try(socket:receive())
    socket:close()

#### Raw packet network I/O

For those cases where the connection-oriented approach is too high-level, NSE provides script developers with the option of raw packet network I/O.

Raw packet reception is handled through a Libpcap
wrapper inside the Nsock library.
The steps are to open a capture device, register listeners with the device, and then process packets as they are received.

The `pcap_open` method creates a handle for raw socket reads from an ordinary socket object. This method takes a callback function, which computes a packet hash from a packet (including its headers). This hash can return any binary string, which is later compared to the strings registered with the `pcap_register` function. The packet hash callback will normally extract some portion of the packet, such as its source address.

The pcap reader is instructed to listen for certain packets using the `pcap_register` function. The function takes a binary string which is compared against the hash value of every packet received. Those packets whose hashes match any registered strings will be returned by the `pcap_receive` method. Register the empty string to receive all packets.

A script receives all packets for which a listener has been registered by calling the `pcap_receive` method. The method blocks until a packet is received or a timeout occurs.

The more general the packet hash computing function is kept, the more scripts may receive the packet and proceed with their execution. To handle packet capture inside your script you first have to create a socket with `nmap.new_socket` and later close the socket with `socket_object:close`—just like with the connection-based network I/O.

Receiving raw packets is important, but sending them is a key feature as well. To accomplish this, NSE can access a wrapper around the `libdnet` library.
Raw packet writes do not use a standard socket object like reads do. Instead, call the function `nmap.new_dnet` to create a dnet object with ethernet sending methods. Then open an interface with the `ethernet_open` method. Raw ethernet frames can then be sent with `ethernet_send`. When you're done, close the ethernet handle with `ethernet_close`.

Sometimes the easiest ways to understand complex APIs is by example. The `sniffer-detect.nse` script included with Nmap uses raw packet capture and sending in an attempt to detect promiscuous-mode machines on the network (those running sniffers).

### Thread Mutexes

Each script execution thread (e.g. `ftp-anon` running against an FTP server on the target host) yields to other scripts whenever it makes a call on network objects (sending or receiving data). Some scripts require finer concurrency control over thread execution. An example is the `whois` script which queries whois
servers for each target IP address. Because many concurrent queries often result in getting one's IP banned for abuse, and because a single query may return additional information for targets other threads are running against, it is useful to have other threads pause while one thread performs a query.

To solve this problem, NSE includes a `mutex` function which provides a mutex (mutual exclusion object) usable by scripts. The mutex allows for only one thread to be working on an object. Competing threads waiting to work on this object are put in the waiting queue until they can get a "lock" on the mutex. A solution for the `whois` problem above is to have each thread block on a mutex using a common string, thus ensuring that only one thread is querying whois servers at once. That thread can store the results in the NSE registry before releasing unlocking the mutex. The next script in the waiting queue can then run. It will first check the registry and only query whois servers if the previous results were insufficient.

The first step is to create a mutex object using a statement such as:

`mutexfn = nmap.mutex(object)`

The `mutexfn` returned is a function which works as a mutex for the `object` passed in. This object can be any Lua data type except `nil`, `booleans`, and `numbers`. The returned function allows you to lock, try to lock, and release the mutex. Its first and only parameter must be one of the following:

`"lock"`

  Make a blocking lock on the mutex. If the mutex is busy (another thread has a lock on it), then the thread will yield and wait. The function returns with the mutex locked.

`"trylock"`

  Makes a non-blocking lock on the mutex. If the mutex is busy then it immediately returns with a return value of `false`. Otherwise the mutex locks the mutex and returns `true`.

`"done"`

  Releases the mutex and allows another thread to lock it. If the thread does not have a lock on the mutex, an error will be raised.

`"running"`

  Returns the thread locked on the mutex or `nil` if the mutex is not locked. This should only be used for debugging as it interferes with garbage collection of finished threads.

A simple example of using the API is provided in . For real-life examples, read the `asn-query.nse` and `whois.nse` scripts in the Nmap distribution.

#### Mutex manipulation

    local mutex = nmap.mutex("My Script's Unique ID");
    function action(host, port)
      mutex "lock";
      -- Do critical section work - only one thread at a time executes this.
      mutex "done";
      return script_output;
    end

### Exception Handling

NSE provides an exception handling mechanism which is not present in the base Lua language. It is tailored specifically for network I/O operations, and follows a functional programming paradigm rather than an object oriented one. The `nmap.new_try` API method is used to create an exception handler. This method returns a function which takes a variable number of arguments that are assumed to be the return values of another function. If an exception is detected in the return values (the first return value is false), then the script execution is aborted and no output is produced. Optionally, you can pass a function to `new_try` which will be called if an exception is caught. The function would generally perform any required cleanup operations.

shows cleanup exception handling at work. A new function named `catch` is defined to simply close the newly created socket in case of an error. It is then used to protect connection and communication attempts on that socket. If no catch function is specified, execution of the script aborts without further ado—open sockets will remain open until the next run of Lua's garbage collector. If the verbosity level is at least one or if the scan is performed in debugging mode a description of the uncaught error condition is printed on standard output. Note that it is currently not easily possible to group several statements in one try block.

#### Exception handling example

    local result, socket, try, catch

    result = ""
    socket = nmap.new_socket()
    catch = function()
    socket:close()
    end
    try = nmap.new_try(catch)

    try(socket:connect(host.ip, port.number))
    result = try(socket:receive_lines(1))
    try(socket:send(result))

Writing a function which is treated properly by the try/catch mechanism is straightforward. The function should return multiple values. The first value should be a Boolean which is `true` upon successful completion of the function and `false` otherwise. If the function completed successfully, the try construct consumes the indicator value and returns the remaining values. If the function failed then the second returned value must be a string describing the error condition. Note that if the value is not `nil` or `false` it is treated as `true` so you can return your value in the normal case and return `nil, error description` if an error occurs.

### The Registry

The registry is a Lua table (accessible as `nmap.registry`) with the special property that it is visible by all scripts and retains its state between script executions. The registry is transient—it is not stored between Nmap executions. Every script can read and write to the registry. Scripts commonly use it to save information for other instances of the same script. For example, the `whois` and `asn-query` scripts may query one IP address, but receive information which may apply to tens of thousands of IPs on that network. Saving the information in the registry may prevent other script threads from having to repeat the query.

The registry may also be used to hand information to completely different scripts. For example, the `snmp-brute` script saves a discovered community name in the registry where it may be used by other SNMP scripts. Scripts which leave information behind for a second script must have a lower `runlevel` than that second script, or there is no guarantee that they will run first.

Because every script can write to the registry table, it is important to avoid conflicts by choosing keys wisely (uniquely).

## Script Writing Tutorial

Suppose that you are convinced of the power of NSE. How do you go about writing your own script? Let's say that you want to extract information from an identification server
to determine the owner of the process listening on a TCP port. This is not really the purpose of identd (it is meant for querying the owner of outgoing connections, not listening daemons), but many identd servers allow it anyway. Nmap used to have this functionality (called ident scan), but it was removed while transitioning to a new scan engine architecture. The protocol identd uses is pretty simple, but still too complicated to handle with Nmap's version detection language. First, you connect to the identification server and send a query of the form `port-on-server, port-on-client` and terminated with a newline character. The server should then respond with a string containing the server port, client port, response type, and address information. The address information is omitted if there is an error. More details are available in RFC 1413, but this description is sufficient for our purposes. The protocol cannot be modeled in Nmap's version detection language for two reasons. The first is that you need to know both the local and the remote port of a connection. Version detection does not provide this data. The second, more severe obstacle, is that you need two open connections to the target—one to the identification server and one to the listening port you wish to query. Both obstacles are easily overcome with NSE.

The anatomy of a script is described in . In this section we will show how the described structure is utilized.

### The Head

The head of the script is essentially its meta information. This includes the fields: `description`, `categories`, `runlevel`, `author`, and `license` as well as initial NSEDoc information such as usage, args, and output tags (see ).

The description field should contain a paragraph or more describing what the script does. If anything about the script results might confuse or mislead users, and you can't eliminate the issue by improving the script or results text, it should be documented in the `description`. If there are multiple paragraphs, the first is used as a short summary where necessary. Make sure that first paragraph can serve as a stand alone abstract. This description is short because it is such a simple script:

    description = [[
    Attempts to find the owner of an open TCP port by querying an auth
    (identd - port 113) daemon which must also be open on the target system.
    ]]

Next comes NSEDoc information. This script is missing the common `@usage` and `@args` tags since it is so simple, but it does have an NSEDoc `@output` tag:

    ---
    --@output
    -- 21/tcp   open     ftp       ProFTPD 1.3.1
    -- |_ auth-owners: nobody
    -- 22/tcp   open     ssh       OpenSSH 4.3p2 Debian 9etch2 (protocol 2.0)
    -- |_ auth-owners: root
    -- 25/tcp   open     smtp      Postfix smtpd
    -- |_ auth-owners: postfix
    -- 80/tcp   open     http      Apache httpd 2.0.61 ((Unix) PHP/4.4.7 ...)
    -- |_ auth-owners: dhapache
    -- 113/tcp  open     auth?
    -- |_ auth-owners: nobody
    -- 587/tcp  open     submission Postfix smtpd
    -- |_ auth-owners: postfix
    -- 5666/tcp open     unknown
    -- |_ auth-owners: root

Next come the `author`, `license`, and `categories` tags. This script belongs to the `safe`
because we are not using the service for anything it was not intended for. Because this script is one that should run by default it is also in the `default`
category. Here are the variables in context:

    author = "Diman Todorov <diman.todorov@gmail.com>"

    license = "Same as Nmap--See http://nmap.org/book/man-legal.html"

    categories = {"default", "safe"}

### The Rule

The rule section is a Lua method which decides whether to skip or execute the script's action method against a particular service or host. This decision is usually based on the host and port information passed to the rule function. In the case of the identification script, it is slightly more complicated than that. To decide whether to run the identification script against a given port we need to know if there is an auth server running on the target machine. In other words, the script should be run only if the currently scanned TCP port is open and TCP port 113 is also open. For now we will rely on the fact that identification servers listen on TCP port 113. Unfortunately NSE only gives us information about the currently scanned port.

To find out if port 113 is open, we use the `nmap.get_port_state` function. If the auth port was not scanned, the `get_port_state` function returns `nil`. So we check that the table is not `nil`. We also check that both ports are in the `open` state. If this is the case, the action is executed, otherwise we skip the action.

    portrule = function(host, port)
            local auth_port = { number=113, protocol="tcp" }
            local identd = nmap.get_port_state(host, auth_port)

            if
                    identd ~= nil
                    and identd.state == "open"
                    and port.protocol == "tcp"
                    and port.state == "open"
            then
                    return true
            else
                    return false
            end
    end

### The Mechanism

At last we implement the actual functionality! The script first connects to the port on which we expect to find the identification server, then it will connect to the port we want information about. Doing so involves first creating two socket options by calling `nmap.new_socket`. Next we define an error-handling `catch` function which closes those sockets if failure is detected. At this point we can safely use object methods such as `open`, `close`, `send` and `receive` to operate on the network socket. In this case we call `connect` to make the connections. NSE's exception handling mechanism
is used to avoid excessive error-handling code. We simply wrap the networking calls in a `try` call which will in turn call our `catch` function if anything goes wrong.

If the two connections succeed, we construct a query string and parse the response. If we received a satisfactory response, we return the retrieved information.

    action = function(host, port)
            local owner = ""

            local client_ident = nmap.new_socket()
            local client_service = nmap.new_socket()

            local catch = function()
                    client_ident:close()
                    client_service:close()
            end

            local try = nmap.new_try(catch)

            try(client_ident:connect(host.ip, 113))
            try(client_service:connect(host.ip, port.number))

            local localip, localport, remoteip, remoteport =
                    try(client_service:get_info())

            local request = port.number .. ", " .. localport .. "\n"

            try(client_ident:send(request))

            owner = try(client_ident:receive_lines(1))

            if string.match(owner, "ERROR") then
                    owner = nil
            else
                    owner = string.match(owner, "USERID : .+ : (.+)\n", 1)
            end

            try(client_ident:close())
            try(client_service:close())

            return owner
    end

Note that because we know that the remote port is stored in `port.number`, we could have ignored the last two return values of `client_service:get_info()` like this:

    local localip, localport = try(client_service:get_info())

In this example we exit quietly if the service responds with an error. This is done by assigning `nil` to the `owner` variable which will be returned. NSE scripts generally only return messages when they succeed, so they don't flood the user with pointless alerts.

## Writing Script Documentation (NSEDoc)

Scripts are used by more than just their authors, so they require good documentation. NSE modules need documentation so developers can use them in their scripts. NSE's documentation system, described in this section, aims to meet both these needs. While reading this section, you may want to browse NSE's online documentation, which is generated using this system. It is at .

NSE uses a customized version of the LuaDoc
documentation system called NSEDoc. The documentation for scripts and modules is contained in their source code, as comments with a special form. is an NSEDoc comment taken from the `stdnse.print_debug()` function.

### An NSEDoc comment for a function

    --- Prints a formatted debug message if the current verbosity level is greater
    -- than or equal to a given level.
    --
    -- This is a convenience wrapper around
    -- <code>nmap.print_debug_unformatted()</code>. The first optional numeric
    -- argument, <code>verbosity</code>, is used as the verbosity level necessary
    -- to print the message (it defaults to 1 if omitted). All remaining arguments
    -- are processed with Lua's <code>string.format()</code> function.
    -- @param level Optional verbosity level.
    -- @param fmt Format string.
    -- @param ... Arguments to format.

Documentation comments start with three dashes: `---`. The body of the comment is the description of the following code. The first paragraph of the description should be a brief summary, with the following paragraphs providing more detail. Special tags starting with `@` mark off other parts of the documentation. In the above example you see `@param`, which is used to describe each parameter of a function. A complete list of the documentation tags is found in .

Text enclosed in the HTML-like `<code>` and `</code>` tags will be rendered in a monospace font. This should be used for variable and function names, as well as multi-line code examples. When a sequence of lines start with the characters `*` , they will be rendered as a bulleted list.

It is good practice to document every public function and table in a script or module. Additionally every script and module should have its own file-level documentation. A documentation comment at the beginning of a file (one that is not followed by a function or table definition) applies to the entire file. File-level documentation can and should be several paragraphs long, with all the high-level information useful to a developer using a module or a user running a script. shows documentation for the `comm` module (with a few paragraphs removed to save space).

### An NSEDoc comment for a module

    --- Common communication functions for network discovery tasks like
    -- banner grabbing and data exchange.
    --
    -- These functions may be passed a table of options, but it's not required. The
    -- keys for the options table are <code>"bytes"</code>, <code>"lines"</code>,
    -- <code>"proto"</code>, and <code>"timeout"</code>. <code>"bytes"</code> sets
    -- a minimum number of bytes to read. <code>"lines"</code> does the same for
    -- lines. <code>"proto"</code> sets the protocol to communicate with,
    -- defaulting to <code>"tcp"</code> if not provided. <code>"timeout"</code>
    -- sets the socket timeout (see the socket function <code>set_timeout()</code>
    -- for details).
    -- @author Kris Katterjohn 04/2008
    -- @copyright Same as Nmap--See http://nmap.org/book/man-legal.html

There are some special considerations for documenting scripts rather than functions and modules. In particular, scripts have special variables for some information which would otherwise belongs in @-tag comments (script variables are described in ). In particular, a script's description belongs in the `description` variable rather than in a documentation comment, and the information that would go in `@author` and `@copyright` belong in the variables `author` and `license` instead. NSEDoc knows about these variables and will use them in preference to fields in the comments. Scripts should also have an `@output` tag showing sample output, as well as `@args` and `@usage` where appropriate. shows proper form for script-level documentation, using a combination of documentation comments and NSE variables.

### An NSEDoc comment for a script

    description = [[
    Maps IP addresses to autonomous system (AS) numbers.

    The script works by sending DNS TXT queries to a DNS server which in
    turn queries a third-party service provided by Team Cymru
    (team-cymru.org) using an in-addr.arpa style zone set up especially for
    use by Nmap.
    ]]

    ---
    -- @usage
    -- nmap --script asn-query.nse [--script-args dns=<DNS server>] <target>
    -- @args dns The address of a recursive nameserver to use (optional).
    -- @output
    -- Host script results:
    -- |  AS Numbers:
    -- |  BGP: 64.13.128.0/21 | Country: US
    -- |    Origin AS: 10565 SVCOLO-AS - Silicon Valley Colocation, Inc.
    -- |      Peer AS: 3561 6461
    -- |  BGP: 64.13.128.0/18 | Country: US
    -- |    Origin AS: 10565 SVCOLO-AS - Silicon Valley Colocation, Inc.
    -- |_     Peer AS: 174 2914 6461

    author = "jah, Michael"
    license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
    categories = {"discovery", "external"}

Compiled NSE modules are also documented with NSEDoc, even though they have no Lua source code. Each compiled module has a file `modulename.luadoc`
that is kept in the `nselib` directory alongside the Lua modules. This file lists and documents the functions and tables in the compiled module as though they were written in Lua. Only the name of each function is required, not its definition (not even `end`). You must use the `@name` and `@class` tags when documenting a table to assist the documentation parser in identifying it. There are several examples of this method of documentation in the Nmap source distribution (including `nmap.luadoc`, `bit.luadoc`, and `pcre.luadoc`).

### NSE Documentation Tags

The following tags are understood by NSEDoc:

`@param`

  Describes a function parameter. The first word following `@param` is the name of the parameter being described. The tag should appear once for each parameter of a function.

`@see`

  Adds a cross-reference to another function or table.

`@return`

  Describes a return value of a function. `@return` may be used multiple times for multiple return values.

`@usage`

  Provides a usage example of a function or script. In the case of a function, the example is Lua code; for a script it is an Nmap command line. `@usage` may be given more than once.

`@name`

  Defines a name for the function or table being documented. This tag is normally not necessary because NSEDoc infers names through code analysis.

`@class`

  Defines the class of the object being documented: `function`, `table`, or `module`. Like `@name`, this is normally inferred automatically.

`@field`

  In the documentation of a table, `@field` describes the value of a named field.

`@args`

  Describes a script argument, as used with the `--script-args` option (see ). The first word after `@args` is the name of the argument, and everything following that is the description. This tag is special to script-level comments.

`@output`

  This tag, which is exclusive to script-level comments, shows sample output from a script.

`@author`

  This tag, which may be given multiple times, lists the authors of an NSE module. For scripts, use the `author` variable instead.

`@copyright`

  This tag describes the copyright status of a module. For scripts, use the `license` variable instead.

## Version Detection Using NSE

The version detection system built into Nmap was designed to efficiently recognize the vast majority of protocols with a simple probe and pattern matching syntax. Some protocols require more complex communication than version detection can handle. A generalized scripting language as provided by NSE is perfect for these tough cases.

NSE's `version`
category contains scripts that enhance standard version detection. Scripts in this category are run whenever you request version detection with `-sV`; you don't need to use `-sC` to run these. This cuts the other way too: if you use `-sC`, you won't get `version` scripts unless you also use `-sV`.

One protocol which we were unable to detect with normal version detection is Skype version 2. The protocol was likely designed to frustrate detection out of a fear that telecom-affiliated Internet service providers might consider Skype competition and interfere with the traffic. Yet we did find one way to detect it. If Skype receives an HTTP GET request, it pretends to be a web server and returns a 404 error. But for other requests, it sends back a chunk of random-looking data. Proper identification requires sending two probes and comparing the two responses—an ideal task for NSE. The simple NSE script which accomplishes this is shown in .

### A typical version detection script (Skype version 2 detection)

    description = [[
    Detects the Skype version 2 service.
    ]]
    author = "Brandon Enright <bmenrigh@ucsd.edu>"
    license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
    categories = {"version"}

    require "comm"

    portrule = function(host, port)
            return (port.number == 80 or port.number == 443 or
                    port.service == nil or port.service == "" or
                    port.service == "unknown")
                   and port.protocol == "tcp" and port.state == "open"
                   and port.service ~= "http" and port.service ~= "ssl/http"
    end

    action = function(host, port)
            local status, result = comm.exchange(host, port,
                    "GET / HTTP/1.0\r\n\r\n", {bytes=26, proto=port.protocol})
            if (not status) then
                    return
            end
            if (result ~= "HTTP/1.0 404 Not Found\r\n\r\n") then
                    return
            end
            -- So far so good, now see if we get random data for another request
            status, result = comm.exchange(host, port,
                    "random data\r\n\r\n", {bytes=15, proto=port.protocol})

            if (not status) then
                    return
            end
            if string.match(result, "[^%s!-~].*[^%s!-~].*[^%s!-~]") then
                    -- Detected
                    port.version.name = "skype2"
                    port.version.product = "Skype"
                    nmap.set_port_version(host, port, "hardmatched")
                    return
            end
            return
    end

If the script detects Skype, it augments its `port` table with now-known `name` and `product` fields. It then sends this new information to Nmap by calling `nmap.set_port_version`. Several other version fields are available to be set if they are known, but in this case we only have the name and product. For the full list of version fields, refer to the `nmap.set_port_version` documentation.

Notice that this script does nothing unless it detects the protocol. A script shouldn't produce output (other than debug output) just to say it didn't learn anything.

## Example Script: `finger.nse`

The finger script (`finger.nse`) is a perfect example of a short and simple NSE script.

First the information fields are assigned. A detailed description of what the script actually does goes in the `description` field.

    description = [[
    Attempts to get a list of usernames via the finger service.
    ]]
    author = "Eddie Bell <ejlbell@gmail.com>"
    license = "Same as Nmap--See http://nmap.org/book/man-legal.html"

The `categories` field is a table containing all the categories the script belongs to—These are used for script selection with the `--script` option:

    categories = {"default", "discovery"}

You can use the facilities provided by the nselib () with `require`. Here we want to use common communication functions and shorter port rules:

    require "comm"
    require "shortport"

We want to run the script against the finger service. So we test whether it is using the well-known finger port (`79/tcp`), or whether the service is named finger based on version detection results or in the port number's listing in `nmap-services`:

    portrule = shortport.port_or_service(79, "finger")

First, the script uses `nmap.new_try` to create an exception handler that will quit the script in case of an error. Next, it passes control to `comm.exchange`, which handles the network transaction. Here we have asked to wait in the communication exchange until we receive at least 100 lines, wait at least 5 seconds, or until the remote side closes the connection. Any errors are handled by the `try` exception handler. The script returns a string if the call to `comm.exchange()` was successful.

    action = function(host, port)
    	local try = nmap.new_try()

    	return try(comm.exchange(host, port, "\r\n",
            	{lines=100, proto=port.protocol, timeout=5000}))
    end

## Implementation Details

Now it is time to explore the NSE implementation details in depth. Understanding how NSE works is useful for designing efficient scripts and libraries. The canonical reference to the NSE implementation is the source code, but this section provides an overview of key details. It should be valuable to folks trying to understand and extend the NSE source code, as well as to script authors who want to better-understand how their scripts are executed.

### Initialization Phase

During its initialization stage, Nmap loads the Lua interpreter and its provided libraries. These libraries are fully documented in the Lua Reference Manual. Here is a summary of the libraries, listed alphabetically by their namespace name:

`debug`

  The debug library provides a low-level API to the Lua interpreter, allowing you to access functions along the execution stack, retrieve function closures and object metatables, and more.

`io`

  The Input/Output library offers functions such as reading from files or from the output from programs you execute.

`math`

  Numbers in Lua usually correspond to the `double` C type, so the math library provides access to rounding functions, trigonometric functions, random number generation, and more.

`os`

  The Operating System library provides system facilities such as filesystem operations (including file renaming or removal and temporary file creation) and system environment access.

`package`

  Among the functions provided by Lua's package-lib is `require`, which is used to load nselib modules.

`string`

  The string library provides functions for manipulating Lua strings, including printf-style string formatting, pattern matching using Lua-style patterns, substring extraction, and more.

`table`

  The table manipulation library is essential for operating on Lua's central data structure (tables).

In addition to loading the libraries provided by Lua, the `nmap` namespace functions are loaded. The search paths are the same directories that Nmap searches for its data files, except that the `nselib` directory is appended to each. At this stage any provided script arguments are stored inside the registry.

The next phase of NSE initialization is loading the selected scripts, based on the defaults or arguments provided to the `--script`
option. The `version`
category scripts are loaded as well if version detection was enabled. NSE first tries to interpret each `--script` argument as a category. This is done with a Lua C function in `nse_init.cc` named `entry` based on data from the `script.db` script categorization database.
If the category is found, those scripts are loaded. Otherwise Nmap tries to interpret `--script` arguments as files or directories. If no files or directories with a given name are found in Nmap's search path, an error is raised and the Script Engine aborts.

If a directory is specified, all of the `.nse` files inside it are loaded. Each loaded file is executed by Lua. If a portrule is present, it is saved in the porttests table with a portrule key and file closure value. Otherwise, if the script has a hostrule, it is saved in the hosttests table in the same manner.

### Matching Scripts with Targets

After initialization is finished, the `hostrules`
and `portrules`
are evaluated for each host in the current target group. The rules of every chosen script is tested against every host and (in the case of service scripts) each `open`
and `open|filtered`
port on the hosts. The combination can grow quite large, so portrules should be kept as simple as possible. Save any heavy computation for the script's `action`.

Next, a Lua thread is created for each of the matching script-target combinations. Each thread is stored with pertinent information such as the runlevel, target, target port (if applicable), host and port tables (passed to the `action`), and the script type (service or host script). The `mainloop` function then processes each runlevel grouping of threads in order.

### Script Execution

Nmap performs NSE script scanning in parallel
by taking advantage of Nmap's Nsock parallel I/O library and the Lua coroutines language feature. Coroutines offer collaborative multi-threading so that scripts can suspend themselves at defined points and allow other coroutines to execute. Network I/O, particularly waiting for responses from remote hosts, often involves long wait times, so this is when scripts yield to others. Key functions of the Nsock wrapper cause scripts to yield (pause). When Nsock finishes processing such a request, it makes a callback which causes the script to be pushed from the waiting queue back into the running queue so it can resume operations when its turn comes up again.

The `mainloop` function moves threads between the waiting and running queues as needed. A thread which yields is moved from the running queue into the waiting list. Running threads execute until they either yield, complete, or fail with an error. Threads are made ready to run (placed in the running queue) by calling `process_waiting2running`. This process of scheduling running threads and moving threads between queues continues until no threads exist in either queue.