sshd tries reverse DNS lookups with UseDNS no
Nowadays the most frequent culprit is GSSAPI:
/etc/ssh/sshd_config:
GSSAPIAuthentication no
The other three culprits for Linux platform have been mentioned in other answer:
- add to
sshda command line option-u0 - set
UseDNS no - don't use
from=hostnameinsideauthorized_keysfiles
Use DNS = no does not prevent sshd from performing DNS lookups, it prevents it from rejecting clients when PTR records don't match.
-u0 prevents sshd from logging DNS names in the utmp struct.
lookups might still happen depending one what a user has in their authorized_keys.
See this for a decent explanation:
http://lists.freebsd.org/pipermail/freebsd-stable/2006-November/030886.html