ident-user-enum is a simple PERL script to query the ident service (113/TCP) in order to determine the owner of the process listening on each TCP port of a target system.

This can help to prioritise target service during a pentest (you might want to attack services running as root first).  Alternatively, the list of usernames gathered can be used for password guessing attacks on other network services.

Download

Install

The INSTALL file in the distribution tarball contains the instructions you need to get up and running:

$ cat INSTALL

1. Install the dependencies

Install PERL using your package manager.

Install the following PERL modules:
* IO::Socket::INET
* Net::Ident

These may be available through your package manager.  If not, then
install them from CPAN:
# perl -MCPAN -e shell
# install IO::Socket::INET
# install Net::Ident

2. "Install" ident-user-enum.pl

It's just a PERL script, so no installation is necessary, but probably
want to copy it into your path and set appropriate permissions:
# install -o root -m 0755 ident-user-enum.pl /usr/local/bin/

Running

$ ./ident-user-enum.pl 10.0.0.1 21 80 113 443
ident-user-enum v1.0 ( http://pentestmonkey.net/tools/ident-user-enum )

10.0.0.1:21    <unknown>
10.0.0.1:80    www-run
10.0.0.1:113   root
10.0.0.1:443  www-run

Note that some ident services lie, e.g. return "nobody" for all services.  This is good from a security perspective, but bad for you as a pentester. 

You should only get an "<unknown>" response if the target port is closed or firewalled.