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17.17. Program: backsniff

This program logs attempts to connect to ports. It uses the Sys::Syslog module (it in turn wants the syslog.ph library, which may or may not come with your system) to log the connection attempt as 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 to 

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 17.7.

Example 17.7: backsniff

#!/usr/bin/perl -w
# backsniff - log attempts to connect to particular ports

use Sys::Syslog;
use Socket;

# identify my port and address
$sockname          = getsockname(STDIN)
                     or die "Couldn't identify myself: $!\n";
($port, $iaddr)    = sockaddr_in($sockname);
$my_address        = inet_ntoa($iaddr);

# get a name for the service
$service = (getservbyport ($port, "tcp"))[0] || $port;
# now identify remote address
$sockname          = getpeername(STDIN)
                         or die "Couldn't identify other end: $!\n";
($port, $iaddr)    = sockaddr_in($sockname);
$ex_address        = inet_ntoa($iaddr);

# and log the information
openlog("sniffer", "ndelay", "daemon");
syslog("notice", "Connection from %s to %s:%s\n", $ex_address,
        $my_address, $service); 

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