Simple SMTP Banner Grabber in PERL

Here is a script which scans a subnet for SMTP servers running and returns the SMTP Banner. This is helpful in identifying possible vulnerable mail servers on a remote network.

#!/usr/bin/env perl
use strict;
use warnings;
use Net::SMTP;
use Net::IP;

my $CIDR    = '192.168.0.0/24';
my $domain  = 'my.mail.com';
my $timeout = 10;

my $ip = new Net::IP($CIDR);

print "[*] Searching $CIDR for SMTP Servers... (This may take a while)\n";

while (++$ip) {

    my $smtp = Net::SMTP->new(
        Host  => $ip->ip(), 
        Hello => $domain, 
        Timeout => $timeout
    );

    if (defined($smtp)) {    
        print "[+] ".$ip->ip()." @ ".$smtp->banner();
        $smtp->quit();
   }
}

If we run the script we get the following...

root@asus:~/public_html% perl post.pl
[*] Searching 192.168.0.0/24 for SMTP Servers... (This may take a while)
[+] 192.168.0.38 @ asus ESMTP Postfix (Ubuntu)
[+] 192.168.0.140 @ mail.acme.com Microsoft ESMTP MAIL Service ready at Sun, 11 Jun 2019 17:45:54 -0400
root@asus:~/public_html%

As you can see we identified two SMTP server on the network range 192.168.0.0.