On Thu, Feb 22, 2007 at 09:15:27PM -0700, Jim wrote:
Last night I came home from work and sat down at the computer. I
noticed the lights on the DSL router were blinking very rapidly. I have
an ftp server running on my linux box (Slackware 10.2). So I thought
someone might have been uploading something.
Ftpwho showed no users logged in. I checked the incoming directory and
saw nothing there.
Tcpdump showed me that they were sending something using ssh.
I used find to look for anything they might have been uploading, but
found nothing.
/var/log/syslog contained the following over and over for about 4 hours
before I got home
Feb 22 20:43:56 ladmo smbd[6375]: [2007/02/22 20:43:56, 0]
printing/print_cups.c:cups_cache_reload(85)
Feb 22 20:43:56 ladmo smbd[6375]: Unable to connect to CUPS server
localhost - Connection refused
Then I found in /var/log/syslog this over and over
Feb 21 22:11:14 ladmo sshd[26255]: error: Could not get shadow
information for NOUSER
I stopped sshd and edited /etc/sshd_config by adding the following:
AllowUsers root jim
AllowGroups root
To test the change, I tried to log into the server via ssh and using
another account. It wouldn't let me log in using that other account via
ssh.
I also tried
find / -mmin 1200 -size +100k
and without the size option, but found nothing from the time this was
going on.
After all this I tried to send an email, but sendmail wasn't working. I
backed up my sendmail config files, uninstalled sendmail, reinstalled it
and restored the config files. Sendmail worked after that.
Is there anything else I should do?
Look for root kits. Reinstall?
Stop all services that you don't actively use. For the remainder,
consider restricting them to your local network (CUPS, etc).
If you have a home network, consider plugging your DSL modem directly
into one PC and using that as a firewall machine. Yes, you can also use
it as a desktop if you need.
Is there a compelling reason you need password authentication for ssh?
It's very easy to generate public keys and use those. You can even keep
one on a thumb drive to use if you have to. Then turn OFF password
authentication (PasswordAuthentication no) in your sshd_config.
I'm guessing they got in through some kind of guest account you
have setup (but maybe didn't know about) or another common account name
with a weak password. I constantly watch my system logs and several
times a week I'll get a ton of attempts to try to brute force passwords
to various accounts through SSH.