C[omp]ute

From: andrew cooke <andrew@...>

Date: Sat, 13 Apr 2013 09:27:15 -0300

With my bridged modem I am now receiving connections directly from the
internet.  Which is great, but means that people are trying to hack me.  Since
the only port I have open is ssh that means people are trying to log-in all
day - the system log is full of wanrnings.

So I installed fail2ban.  Which is very neat.  It checks the log(s) and blocks
any IPs that appear there.  However, the configuration out-of-the-box is
useless.  Hence this post, which shows what I did to make it work.

First, I added the service:
  systemctl enable fail2ban.service

Then I added the file jail.local to /etc/fail2ban:

  [DEFAULT]

  # 12 hours
  bantime = 43200


  [ssh-iptables]

  enabled = true
  action   = hostsdeny
	     sendmail-whois[name=SSH, dest=andrew, sender=fail2ban]
  logpath  = /var/log/messages

Which does the following:
 - It bans people (IP addresses) for 12 hours
 - It enables the ssh-iptables "jail" (task, or whatever)
 - It calls the hostsdeny function to ban people (more on that below)
 - It sends me an email when someone is bannde
 - It checks the log file /var/log/messages

Note that all the things changed above are different from the default!  The
base install doesn't enable anything, uses iptables to block, only blocks for
10 minutes, doesn't check the correct logs for ssh, etc etc.

I also modified /etc/fail2ban/filter.d/sshd.conf to include more matcher
lines.  These are fairly self-explanatory:

 ^%(__prefix_line)sAddress <HOST> maps to \S*, but this does not map back to the address - POSSIBLE BREAK-IN ATTEMPT!$
 ^%(__prefix_line)sReceived disconnect from <HOST>: 11: Bye Bye \[preauth\]$

The first appears to have been remove from the default distribution because it
can also be triggered by bad DNS; the second may result in some addresses
being flagged twice for a single connection (default config requres 5 flags
for blocking).

Finally, the reason I used hostsdeny as the action is that the iptables action
doesn't work (from the fail2ban.log):

  2013-04-12 20:23:46,782 fail2ban.actions.action: ERROR  iptables -n -L INPUT | grep -q 'fail2ban-SSH[ \t]' returned 100
  2013-04-12 20:23:46,783 fail2ban.actions.action: ERROR  Invariant check failed. Trying to restore a sane environment
  2013-04-12 20:23:46,792 fail2ban.actions.action: ERROR  iptables -D INPUT -p tcp --dport ssh -j fail2ban-SSH
  iptables -F fail2ban-SSH
  iptables -X fail2ban-SSH returned 100

Using hostsdeny changes the /etc/hosts/deny file instead:

  # cat /etc/hosts.deny

  http-rman : ALL EXCEPT LOCAL

  ALL: 221.224.33.70
  ALL: 202.164.33.106
  ALL: 183.60.20.36

(the last three lines have been added).

Oh, and don't forget to start the service:
  systemctl start fail2ban.service

Andrew