Dealing with SMTP invalid command attack

Posted by mark on Server Fault See other posts from Server Fault or by mark
Published on 2012-09-03T16:47:06Z Indexed on 2012/09/03 21:39 UTC
Read the original article Hit count: 191

Filed under:

linux

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security

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smtp

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sendmail

One of our semi-busy mail servers (sendmail) has had a lot of inbound connections over the past few days from hosts that are issuing garbage commands.

In the past two days:

incoming smtp connections with invalid commands from 39,000 unique IPs
the IPs come from various ranges all over the world, not just a few networks that I can block
the mail server serves users throughout north america, so I can't just block connections from unknown IPs
sample bad commands: http://pastebin.com/4QUsaTXT

I am not sure what someone is trying to accomplish with this attack, besides annoy me.

any ideas what this is about, or how to effectively deal with it?

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