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ssh: Repeated intrusion attempts

Today I found hundreds of the following in my /var/log/auth.log:

May  2 08:12:01 debian sshd[16918]: Could not reverse map address
May  2 08:12:04 debian sshd[16920]: Could not reverse map address
May  2 08:12:06 debian sshd[16922]: Could not reverse map address

This is occasionally punctuated with the following:

May  2 08:12:47 debian sshd[16955]: User XXXX not allowed because none of 
user's groups are listed in AllowGroups

Where XXXX is a valid user name on my system - who is denied access via ssh.

Occasionally I get

May  2 07:59:30 debian PAM_unix[16273]: authentication failure; (uid=0) -> 
YYYY for ssh service
May  2 07:59:32 debian sshd[16273]: Failed password for YYYY from port 39023 ssh2
May  2 07:59:35 debian sshd[16275]: Could not reverse map address

Where YYYY is a user who has permission to log in remotely via ssh.

There seem to be bursts of this sort of activity every day or two, from 
different addresses.

I only have a very limited number of users who are able to log in through 
ssh, and the users who can have good passwords, so I assume that the chance 
of a successful breakin is low.

What concerns me is that the attackers seem to be able to retrieve the names 
of users on my system.  How do they do that, and how can I prevent it?

I am running Woody, with up-to-date patches, behind a cheap hardware 
firewall-router.  Open ports are 22 (sshd), 25 (sendmail), 80 (apache), 443 
(apache-ssl), 993 (courier-imap over ssl) and 995 (courier-pop over ssl). 

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