Version $Id: fail2ban.html,v 1.2 2014/01/07 15:16:33 madhatta Exp madhatta $
I'm suffering a lot from people trying to guess valid username/password duples for authenticating to my SMTP server. This is bad for several reasons: if they were to succeed, they could relay vast quantities of rubbish email off my server, taking advantage of my IP addresses' reasonably good reputation in the sender stakes. Some (though not all) valid SMTP login duples are also valid ssh login duples, and that would lead to an even worse compromise of the server. But even unsuccessful attempts take up resources on the server (mostly, memory) leading to a shortfall, and possible activation of the OOM-killer and consequent service disruption to legitimate users.
At the moment I have my NAGIOS server watching the logs for evidence of failures, and memory shortfalls, and SMSing me when they're found. This leads to notifications such as:
Service: smtp/imap authentication failures Host: teaparty Address: 126.96.36.199 State: CRITICAL Date/Time: Tue Jan 7 11:30:23 GMT 2014 Additional Info: CRITICAL: Found 51 lines (limit=4/10): Jan 7 11:30:11 lory saslauthd: pam_unix(smtp:auth): authentication failure: logname= uid=0 euid=0 tty= ruser= rhost=and
Service: memory Host: teaparty Address: 188.8.131.52 State: CRITICAL Date/Time: Tue Jan 7 11:31:12 GMT 2014 Additional Info: CRITICALs: committed is 7301853184.00 (outside range [:3000000000]).
But in addition to costing me money (SMS sending isn't free), when I get one of these I have to drop everything and sort the problem out. It would be much better if there were some daemon that ran on teaparty, watched the logs for evidence of brute-forcing, and ban that offending IP address from connecting to the service for a period.
Well, there is, and it's called
On teaparty, the RPM was available from the
RPMForge repository, which I have configured in to yum, so
installed failban with a simple
yum install fail2ban
I'm pleased to report that it ships with no "jails" (which appears to be fail2ban-speak for "log files that I'm watching for certain regexps with a view to taking certain actions") configured, so initially, it won't throw anyone out of anything. I know this because
[madhatta@lory fail2ban]$ sudo fail2ban-client status ERROR Unable to contact server. Is it running? [madhatta@lory fail2ban]$ sudo service fail2ban start Starting fail2ban: [ OK ] [madhatta@lory fail2ban]$ sudo fail2ban-client status Status |- Number of jail: 0 `- Jail list:
I have established that the signature I'm looking for in my logs is this
Jan 7 11:25:45 lory sendmail: s07BPgwK001558: AUTH failure (LOGIN): authentication failure (-13) SASL(-13): authentication failure: checkpass failed, relay=[184.108.40.206]in /var/log/facility/mail (my syslogging is a custom setup; you'll need to adapt this to wherever your sendmail logs its complaints).
[smtp-auth] enabled = true filter = smtp-auth action = iptables[name=SMTP, port=smtp, protocol=tcp] logpath = /var/log/facility/mail maxretry = 3and put the following in /etc/fail2ban/filter.d/smtp-auth.conf:
[Definition] failregex = sendmail.*authentication failure: checkpass failed, relay=\[<HOST>\]$ ignoreregex =I tested the filter file against the logfile with
sudo fail2ban-regex /var/log/facility/mail /etc/fail2ban/filter.d/smtp-auth.conf |less
and received much output like
Running tests ============= Use regex file : /etc/fail2ban/filter.d/smtp-auth.conf Use log file : /var/log/facility/mail Results ======= Failregex: 1249 total |- #) [# of hits] regular expression | 1)  sendmail.*authentication failure: checkpass failed, relay=\[and moreover saw the following rule in iptables:
\]$ `- Ignoreregex: 0 total Summary ======= Addresses found:  220.127.116.11 (Tue Jan 07 11:25:41 2014) 18.104.22.168 (Tue Jan 07 11:25:42 2014) [...] Date template hits: 52380 hit(s): MONTH Day Hour:Minute:Second Success, the total number of match is 1249
[madhatta@lory fail2ban]$ sudo iptables -L -n -v|head -3 Chain INPUT (policy DROP 8 packets, 392 bytes) pkts bytes target prot opt in out source destination 259 116K fail2ban-SMTP tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:25plus the client reports a single jail configured:
[madhatta@lory fail2ban]$ sudo fail2ban-client status Status |- Number of jail: 1 `- Jail list: smtp-auth
smtp-server=www.teaparty.net/tls/user=madhattaBut when I tried repeatedly to send an email with the wrong password, nothing happened. /var/log/facility/mail showed this:
Jan 7 14:53:46 lory sendmail: s07EriLI028424: AUTH failure (PLAIN): authentication failure (-13) SASL(-13): authentication failure: Password verification failed, relay=odessa.webform.co.uk [22.214.171.124]which is subtly different from the log entry above. I modified the regexp in /etc/fail2ban/filter.d/smtp-auth.conf to read
failregex = sendmail.*authentication failure \(-13\) SASL\(-13\): authentication failure:.*relay=.*\[<HOST>\]and confirmed it worked with fail2ban-regex. After kicking fail2ban, the following was logged in /var/log/fail2ban.log:
2014-01-07 14:59:04,912 fail2ban.actions: WARNING [smtp-auth] Ban 126.96.36.199and the following iptables rule appeared:
[madhatta@lory fail2ban]$ sudo iptables -L fail2ban-SMTP -n -v Chain fail2ban-SMTP (1 references) pkts bytes target prot opt in out source destination 5 300 DROP all -- * * 188.8.131.52 0.0.0.0/0 113 32955 RETURN all -- * * 0.0.0.0/0 0.0.0.0/0The alpine client was unable to connect to port 25 on the mail server. A few minutes later, the following log entry appeared in fail2ban.log
2014-01-07 15:09:05,643 fail2ban.actions: WARNING [smtp-auth] Unban 184.108.40.206and the iptables rule went away.
This HOWTO was most helpful when it came to understanding some of the basic concepts, but the project documentation was best of all for giving me a good grounding in the fail2ban way.
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