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Nmap Network Scanning

Output

Any security tool 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.

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.

Nmap makes output available in five different formats. The default is called interactive output, and it is sent to standard output (stdout). There is also normal output, which is similar to interactive except that it displays less runtime information and warnings since it is expected to be analyzed after the scan completes rather than interactively.

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.

The two remaining output types are the simple grepable output which includes most information for a target host on a single line, and sCRiPt KiDDi3 0utPUt for users who consider themselves |<-r4d.

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 -oX myscan.xml -oN myscan.nmap. While this chapter uses the simple names like myscan.xml 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.

While these options save results to files, Nmap still prints interactive output to stdout as usual. For example, the command nmap -oX myscan.xml target prints XML to myscan.xml and fills standard output with the same interactive results it would have printed if -oX 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 nmap -oX - target will send only XML output to stdout. Serious errors may still be printed to the normal error stream, stderr.

Unlike some Nmap arguments, the space between the logfile option flag (such as -oX) and the filename or hyphen is mandatory. If you omit the flags and give arguments such as -oG- or -oXscan.xml, a backwards compatibility feature of Nmap will cause the creation of normal format output files named G- and Xscan.xml respectively.

All of these arguments support strftime-like conversions in the filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the same as in strftime. %T is the same as %H%M%S, %R is the same as %H%M, and %D is the same as %m%d%y. A % followed by any other character just yields that character (%% gives you a percent symbol). So -oX 'scan-%T-%D.xml' will use an XML file with a name in the form of scan-144840-121307.xml.

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.

Nmap Output Formats

-oN <filespec> (normal output)

Requests that normal output be directed to the given filename. As discussed above, this differs slightly from interactive output.

-oX <filespec> (XML output)

Requests that XML output 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 https://svn.nmap.org/nmap/docs/nmap.dtd.

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 Nmap::Scanner and Nmap::Parser in Perl CPAN. In almost all cases that a non-trivial application interfaces with Nmap, XML is the preferred format.

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 nmap.xsl filesystem path. Use the --webxml or --stylesheet options to create portable XML files that render as HTML on any web-connected machine.

-oS <filespec> (ScRipT KIdd|3 oUTpuT)

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 helping them.

-oG <filespec> (grepable output)

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.

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.

Grepable output consists of comments (lines starting with a pound (#)) and target lines. A target line includes a combination of six labeled fields, separated by tabs and followed with a colon. The fields are Host, Ports, Protocols, Ignored State, OS, Seq Index, IP ID, and Status.

The most important of these fields is generally Ports, 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: Port number, State, Protocol, Owner, Service, SunRPC info, and Version info.

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 in the section called “Grepable Output (-oG)”.

-oA <basename> (Output to all formats)

As a convenience, you may specify -oA <basename> to store scan results in normal, XML, and grepable formats at once. They are stored in <basename>.nmap, <basename>.xml, and <basename>.gnmap, respectively. As with most programs, you can prefix the filenames with a directory path, such as ~/nmaplogs/foocorp/ on Unix or c:\hacking\sco on Windows.

Verbosity and debugging options

-v (Increase verbosity level) , -v<level> (Set verbosity level)

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 or more for even greater verbosity: -vv, or give a verbosity level directly, for example -v3.

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.

-d (Increase debugging level) , -d<level> (Set debugging level)

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 (-v), debugging is enabled with a command-line flag (-d) and the debug level can be increased by specifying it multiple times, as in -dd, or by setting a level directly. For example, -d9 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.

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: Timeout vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987 rttvar: 14987 to: 100000. 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.

--reason (Host and port state reasons)

Shows the reason each port is set to a specific state and the reason each host is up or down. This option displays the type of the packet that determined a port or hosts state. For example, A RST packet from a closed port or an echo reply from an alive host. The information Nmap can provide is determined by the type of scan or ping. The SYN scan and SYN ping (-sS and -PS) are very detailed, but the TCP connect scan (-sT) is limited by the implementation of the connect system call. This feature is automatically enabled by the debug option (-d) and the results are stored in XML log files even if this option is not specified.

--stats-every <time> (Print periodic timing stats)

Periodically prints a timing status message after each interval of <time>. The time is a specification of the kind described in the section called “Timing and Performance”; so for example, use --stats-every 10s to get a status update every 10 seconds. Updates are printed to interactive output (the screen) and XML output.

--packet-trace (Trace packets and data sent and received)

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 -p20-30. If you only care about the goings on of the version detection subsystem, use --version-trace instead. If you only care about script tracing, specify --script-trace. With --packet-trace, you get all of the above.

--open (Show only open (or possibly open) ports)

Sometimes you only care about ports you can actually connect to (open ones), and don't want results cluttered with closed, filtered, and closed|filtered ports. Output customization is normally done after the scan using tools such as grep, awk, and Perl, but this feature was added due to overwhelming requests. Specify --open to only see hosts with at least one open, open|filtered, or unfiltered port, and only see ports in those states. These three states are treated just as they normally are, which means that open|filtered and unfiltered may be condensed into counts if there are an overwhelming number of them.

--iflist (List interfaces and routes)

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).

Miscellaneous output options

--append-output (Append to rather than clobber output files)

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

--resume <filename> (Resume aborted scan)

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 administrator running Nmap could cancel it for any other reason as well, by pressing ctrl-C. Restarting the whole scan from the beginning may be undesirable. Fortunately, if normal (-oN) or grepable (-oG) logs were kept, the user can ask Nmap to resume scanning with the target it was working on when execution ceased. Simply specify the --resume 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 nmap --resume <logfilename>. 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.

--stylesheet <path or URL> (Set XSL stylesheet to transform XML output)

Nmap ships with an XSL stylesheet named nmap.xsl for viewing or translating XML output to HTML. The XML output includes an xml-stylesheet directive which points to nmap.xml where it was initially installed by Nmap. Run the XML file through an XSLT processor such as xsltproc to produce an HTML file. Directly opening the XML file in a browser no longer works well because modern browsers limit the locations a stylesheet may be loaded from. If you wish to use a different stylesheet, specify it as the argument to --stylesheet. You must pass the full pathname or URL. One common invocation is --stylesheet http://nmap.org/svn/docs/nmap.xsl. This tells an XSLT processor to load the latest version of the stylesheet from Nmap.Org. The --webxml option does the same thing with less typing and memorization. Loading the XSL from Nmap.Org makes it easier to view results on a machine that doesn't have Nmap (and thus nmap.xsl) installed. So the URL is often more useful, but the local filesystem location of nmap.xsl is used by default for privacy reasons.

--webxml (Load stylesheet from Nmap.Org)

This is a convenience option, nothing more than an alias for --stylesheet http://nmap.org/svn/docs/nmap.xsl.

--no-stylesheet (Omit XSL stylesheet declaration from XML)

Specify this option to prevent Nmap from associating any XSL stylesheet with its XML output. The xml-stylesheet directive is omitted.

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