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Nmap Network Scanning

Grepable Output (-oG)

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. One grepable output aficionado is Lee MadHat Heath, who contributed to this section.

Example 13.14 shows a typical example of grepable output. Normally each host takes only one line, but I split this entry into seven lines to fit on the page. There are also three lines starting with a hash prompt (not counting the Nmap command line). Those are comments describing when Nmap started, the command line options used, and completion time and statistics. One of the comment lines enumerates the port numbers that were scanned. I shortened it to avoid wasting dozens of lines. That particular comment is only printed in verbose (-v) mode. Increasing the verbosity level beyond one -v will not further change the grepable output. The times and dates have been replaced with [time] to reduce line length.

Example 13.14. A typical example of grepable output

# nmap -T4 -A -v -oG - scanme.nmap.org
# Nmap 5.35DC18 scan initiated [time] as: nmap -T4 -A -v -oG - scanme.nmap.org
# Ports scanned: TCP(1000;1,3-4,6-7,...,,65389) UDP(0;) SCTP(0;) PROTOCOLS(0;)
Host: 64.13.134.52 (scanme.nmap.org)    Status: Up
Host: 64.13.134.52 (scanme.nmap.org)    Ports: 22/open/tcp//ssh//OpenSSH 4.3 (protocol 2.0)/, 25/closed/tcp//smtp///, 53/open/tcp//domain///, 70/closed/tcp//gopher///, 80/open/tcp//http//Apache httpd 2.2.3 ((CentOS))/, 113/closed/tcp//auth///, 31337/closed/tcp//Elite///  Ignored State: filtered (993)   OS: Linux 2.6.13 - 2.6.31       Seq Index: 204  IP ID Seq: All zeros
# Nmap done at [time] -- 1 IP address (1 host up) scanned in 21.90 seconds

The command-line here requested that grepable output be sent to standard output with the - argument to -oG. Aggressive timing (-T4) as well as OS and version detection (-A) were requested. The comment lines are self-explanatory, leaving the meat of grepable output in the Host line. Had I scanned more hosts, each of the available ones would have its own Host line.

Grepable Output Fields

The host line is split into fields, each of which consist of a field name followed by a colon and space, then the field content. The fields are separated by tab characters (ASCII number nine, '\t'). Example 13.14, “A typical example of grepable output” shows six fields: Host, Status, Ports, Ignored State, OS, Seq Index, and IP ID Seq. A Protocols section is included in IP protocol (-sO) scans. The exact fields given depend on Nmap options used. For example, OS detection triggers the OS, Seq Index, and IP ID Seq fields. Because they are tab-delimited, you might split up the fields with a Perl line such as:

@fields = split("\t", $host_line);

In the case of Example 13.14, “A typical example of grepable output”, the array @fields would contain six members. $fields[0] would contain Host: 64.13.134.52 (scanme.nmap.org), and $fields[1] would contain the long Ports field. Scripts that parse grepable output should ignore fields they don't recognize, as new fields may be added to support Nmap enhancements.

The eight possible fields are described in the following sections.

Host field

Example: Host: 64.13.134.52 (scanme.nmap.org)

The Host field always comes first and is included no matter what Nmap options are chosen. The contents are the IP address (an IPv6 address if -6 was specified), a space, and then the reverse DNS name in parentheses. If no reverse name is available, the parentheses will be empty.

Status field

Example: Status: Up

The Status field indicates the whether the target host is Up, Down, or Unknown. List scan (-sL) always categorizes targets as Unknown because it does not perform any tests. Ping scan lists a host as Up if it responds to at least one ping probe, and Down if no responses are received. It used to also report Smurf if ping probes sent to the target resulted in one or more responses from other hosts, but that is no longer done. Example 13.15 demonstrates a ping scan of five random hosts, while Example 13.16 demonstrates a list scan of five hosts.

Example 13.15. Ping scan grepable output

# nmap -sn -oG - -iR 100
# Nmap 5.35DC18 scan initiated [time] as: nmap -sn -oG - -iR 5
Host: 93.182.218.153 () Status: Up
Host: 154.223.142.85 () Status: Down
Host: 120.128.8.97 ()   Status: Down
Host: 47.159.134.149 () Status: Down
Host: 24.172.4.19 ()    Status: Down
# Nmap done at [time] -- 5 IP addresses (1 host up) scanned in 4.25 seconds

Example 13.16. List scan grepable output

# nmap -sL -oG - -iR 5
# Nmap 5.35DC18 scan initiated [time] as: ./nmap -sL -oG - -iR 5
Host: 91.244.202.129 () Status: Unknown
Host: 216.36.141.91 (cm216036141091.wcgwave.ca) Status: Unknown
Host: 17.130.29.192 ()  Status: Unknown
Host: 45.89.164.99 ()   Status: Unknown
Host: 215.22.1.81 ()    Status: Unknown
# Nmap done at [time] -- 5 IP addresses (0 hosts up) scanned in 13.00 seconds

Ports field

Example: Ports: 111/open/tcp//rpcbind (rpcbind V2)/(rpcbind:100000*2-2)/2 (rpc #100000)/, 113/closed/tcp//auth///

The Ports field is by far the most complex, as can be seen in Example 13.14, “A typical example of grepable output”. It includes entries for every interesting port (the ones which would be included in the port table in normal Nmap output). The port entries are separated with a comma and a space character. Each port entry consists of seven subfields, separated by a forward slash (/). The subfields are: port number, state, protocol, owner, service, SunRPC info, and version info. Some subfields may be empty, particularly for basic port scans without OS or version detection. The consecutive slashes in Example 13.14, “A typical example of grepable output” reveal empty subfields. In Perl, you might split them up as so:

($port, $state, $protocol, $owner, $service, $rpc_info, $version) = 
        split('/', $ports);

Alternatively, you could grab the information from the command line using commands such as these:

cut -d/ -f<fieldnumbers>
awk -F/ '{print $<fieldnumber>}'

Certain subfields can contain a slash in other output modes. For example, an SSL-enabled web server would show up as ssl/http and the version info might contain strings such as mod_ssl/2.8.12. Since a slash is the subfield delimiter, this would screw up parsing. To avoid this problem, slashes are changed into the pipe character (|) when they would appear anywhere in the Port field.

Parsers should be written to allow more than seven slash-delimited subfields and to simply ignore the extras because future Nmap enhancements may call for new ones. The following list describes each of the seven currently defined Port subfields.

Port number

This is simply the numeric TCP or UDP port number.

State

The same port state which would appear in the normal output port table is shown here.

Protocol

This is tcp, udp, or sctp.

Owner

This used to specify the username that the remote server is running under based on results from querying an identd (auth) server of the target host. The ident scan (-I) is no longer available with Nmap, so this field is always empty. Ident data can still be obtained using the auth-owners NSE script, though results are not placed in this field.

Service

The service name, as obtained from an nmap-services lookup, or (more reliably) through version detection (-sV) if it was requested and succeeded. With version detection enabled, compound entries such as ssl|http and entries with a trailing question mark may be seen. The meaning is the same as for normal output, as discussed in Chapter 7, Service and Application Version Detection.

SunRPC info

If version detection (-sV) was requested and the port was found to use the SunRPC protocol, the RPC program number and accepted version numbers are included here. A typical example is (rpcbind:100000*2-2). The data is always returned inside parentheses. It starts with the program name, then a colon and the program number, then an asterisk followed by the low and high supported version numbers separated by a hyphen. So in this example, rpcbind (program number 100000) is listening on the port for rpcbind version 2 requests.

Version info

If version detection is requested and succeeds, the results are provided here in the same format used in interactive output. For SunRPC ports, the RPC data is printed here too. The format for RPC results in this column is <low version number>-<high version number> (rpc #<rpc program number>). When only one version number is supported, it is printed by itself rather than as a range. A port which shows (rpcbind:100000*2-2) in the SunRPC info subfield would show 2 (rpc #100000) in the version info subfield.

Protocols field

Example: Protocols: 1/open/icmp/, 2/open|filtered/igmp/

The IP protocol scan (-sO) has a Protocols field rather than Ports. Its contents are quite similar to the Ports field, but it has only three subfields rather than seven. They are delimited with slashes, just as with the Ports field. Any slashes that would appear in a subfield are changed into pipes (|), also as done in the Ports field. The subfields are protocol number, state, and protocol name. These correspond to the three fields shown in interactive output for a protocol scan. An example of IP protocol scan grepable output is shown in Example 13.17. The Host line, which would normally be all one line, is here wrapped for readability.

Example 13.17. Grepable output for IP protocol scan

# nmap -v -sO -oG - localhost
# Nmap 5.35DC18 scan initiated [time] as: nmap -v -sO -oG - localhost
# Ports scanned: TCP(0;) UDP(0;) SCTP(0;) PROTOCOLS(256;0-255)
Host: 127.0.0.1 (localhost)     Status: Up
Host: 127.0.0.1 (localhost)     Protocols: 1/open/icmp/, 2/open/igmp/, 4/open|filtered/ip/, 6/open/tcp/, 17/open/udp/, 41/open|filtered/ipv6/, 47/open|filtered/gre/, 50/open|filtered/esp/, 51/open|filtered/ah/, 108/open|filtered/ipcomp/, 132/open/sctp/, 255/open|filtered//       Ignored State: closed (244)
# Nmap done at [time] -- 1 IP address (1 host up) scanned in 2.36 seconds

Ignored State field

Example: Ignored State: filtered (1658)

To save space, Nmap may omit ports in one non-open state from the list in the Ports field. Nmap does this in interactive output too. Regular Nmap users are familiar with the lines such as Not shown: 993 closed ports. For grepable mode, that state is given in the Ignored State field. Following the state name is a space, then in parentheses is the number of ports found in that state.

OS field

Example: OS: Linux 2.4.0 - 2.5.20

Any perfect OS matches are listed here. If there are multiple matches, they are separated by a pipe character as shown in Example 13.14, “A typical example of grepable output”. Only the free-text descriptions are provided. Grepable mode does not provide the vendor, OS family, and device type classification shown in other output modes.

Seq Index field

Example: Seq Index: 3004446

This number is an estimate of the difficulty of performing TCP initial sequence number prediction attacks against the remote host. These are also known as blind spoofing attacks, and they allow an attacker to forge a full TCP connection to a remote host as if it was coming from some other IP address. This can always help an attacker hide his or her tracks, and it can lead to privilege escalation against services such as rlogin that commonly grant extra privileges to trusted IP addresses. The Seq Index value is only available when OS detection (-O) is requested and succeeds in probing for this. It is reported in interactive output when verbosity (-v) is requested. More details on the computation and meaning of this value are provided in Chapter 8, Remote OS Detection.

IP ID Seq field

Example: IP ID Seq: All zeros

This simply describes the remote host's IP ID generation algorithm. It is only available when OS detection (-O) is requested and succeeds in probing for it. Interactive mode reports this as well, and it is discussed in Chapter 8, Remote OS Detection.

Parsing Grepable Output on the Command Line

Grepable output really shines when you want to gather information quickly without the overhead of writing a script to parse XML output. Example 13.18 shows a typical example of this. The goal is to find all hosts on a class C sized network with port 80 open. Nmap is told to scan just that port of each host (skipping the ping stage) and to output a grepable report to stdout. The results are piped to a trivial awk command which finds lines containing /open/ and outputs fields two and three for each matching line. Those fields are the IP address and hostname (or empty parentheses if the hostname is unavailable).

Example 13.18. Parsing grepable output on the command line

> nmap -p80 -Pn -oG - 10.1.1.0/24 | awk '/open/{print $2 " " $3}'
10.1.1.72 (userA.corp.foocompany.biz)
10.1.1.73 (userB.corp.foocompany.biz)
10.1.1.75 (userC.corp.foocompany.biz)
10.1.1.149 (admin.corp.foocompany.biz)
10.1.1.152 (printer.corp.foocompany.biz)
10.1.1.160 (10-1-1-160.foocompany.biz)
10.1.1.161 (10-1-1-161.foocompany.biz)
10.1.1.201 (10-1-1-201.foocompany.biz)
10.1.1.254 (10-1-1-254.foocompany.biz)

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