ssh: Repeated intrusion attempts

From: Robert S (
Date: 05/02/05

  • Next message: Lars Roland: "Silence apt-get for real"
    Date: Mon, 2 May 2005 19:27:08 +1000

    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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