We have the following setup:
Redhat kernel 2.4.16
This is a very old, ~7 years, cobalt RaQ server and is not easily upgradeable. Currently, we are being hit by multiple IP addresses from Latin America, Russia, Pakistan and others that open SMTP connections to our mail server and attempt to send emails from malformed addresses. Always missing the top level domain and similar in format to:
Our sendmail responds with:
sendmail: n8JABr406973: ruleset=check_mail, arg1=<evangelinaii2@mysterious>, relay=wtl.worldcall.net.pk [220.127.116.11] (may be forged), reject=553 5.1.8 <evangelinaii2@mysterious>... Domain of sender address evangelinaii2@mysterious does not exist
At this point, the attacker sends a RSET command and trys another similar but different email address. We are getting ~2 attempts every second from each IP address. The reason I think this may be a DOS attack is that as soon as I kill a connection, within seconds a new IP address starts up another attack from a completely new geography. For example, I kill a connection from Chile and another connection from Columbia starts 1 second later.
Obviously, this is causing unnecessary load on our mail server and I would like to stop these connections at the gateway if possible. Does anyone have ideas on how to fix this? I realize our mail server is badly in need of replacement, and that is in the works, but not complete yet. What I am looking for is a band-aid on this issue.