Recipe 17.20 Program: backsniff
This program logs attempts to connect to ports. It uses the Sys::Syslog module to log the connection attempt through a Unix socket, with logging level LOG_NOTICE and facility LOG_DAEMON. It uses getsockname to find out what port was connected to and getpeername to find out what machine made the connection. It uses getservbyport to convert the local port number (e.g., 7) into a service name (e.g, "echo").
It produces entries in the system log file like this:
May 25 15:50:22 coprolith sniffer: Connection from 126.96.36.199 to 188.8.131.52:echo
Install it in the inetd.conf file with a line like this:
echo stream tcp nowait nobody /usr/scripts/snfsqrd sniffer
The program is shown in Example 17-9.
#!/usr/bin/perl -w # backsniff - log attempts to connect to particular ports use strict; use Sys::Syslog qw(:DEFAULT setlogsock); use Socket; # identify my port and address my $sockname = getsockname(STDIN) or die "Couldn't identify myself: $!\n"; my ($port, $iaddr) = sockaddr_in($sockname); my $my_address = inet_ntoa($iaddr); # get a name for the service my $service = (getservbyport ($port, "tcp")) || $port; # now identify remote address $sockname = getpeername(STDIN) or die "Couldn't identify other end: $!\n"; ($port, $iaddr) = sockaddr_in($sockname); my $ex_address = inet_ntoa($iaddr); # and log the information setlogsock("unix"); openlog("sniffer", "ndelay", "daemon"); syslog("notice", "Connection from %s to %s:%s\n", $ex_address, $my_address, $service); closelog( );