sshd warning, "POSSIBLE BREAK-IN ATTEMPT!" for failed reverse DNS

Posted by rking on Server Fault See other posts from Server Fault or by rking
Published on 2012-10-10T16:03:47Z Indexed on 2012/10/14 3:39 UTC
Read the original article Hit count: 491

Filed under:

ssh

|

reverse-dns

Whenever I SSH somewhere I get something like this in the logs:

sshd[16734]: reverse mapping checking getaddrinfo for
    1.2.3.4.crummyisp.net [1.2.3.4] failed - POSSIBLE BREAK-IN ATTEMPT!

And it is right: if I do host 1.2.3.4 it returns 1.2.3.4.crummyisp.net, but if I do host 1.2.3.4.crummyisp.net it is not found.

I have two questions:

What security threat is there? How could anyone fake a one-way DNS in some threatening way?
Do I have any recourse for fixing this? I'll send my ISP a bug report, but who knows where that'll go.

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