Using fail2ban to mitigate simple DOS attacks against apache (or why I am a terrible sysop)

   Sam Nicholls    No Comments yet    Devops, System Administration

If you aren’t interested in the dramatic reconstruction of the events that led me to try and configure a simple DOS jail with fail2ban, you can skip to the fail2ban apache DOS jail mitigation.
If you just want to know what happened without the flair, skip to the timeline.

Earlier this afternoon, my server was upset. At 15:57, a duo of IP addresses begun making rapid and repeated POST requests to an auxiliary component of WordPress, forcing apache to begin consuming significant amounts of system memory. Disappointingly this went undetected, and less than half an hour later, at 16:24, the system ran out of memory, invoked the OOM killer and terminated mysqld. Thus at 16:24, denial of service to all applications requiring access to a database was successful.

Although the server dutifully restarted mysqld less than a minute later, the attack continued. Access to apache was denied intermittently (by virtue of the number of requests) and the OOM killer terminated mysqld again at 16:35. The database server daemon was respawned once more, only to be killed just short of half an hour later at 17:03.

It wasn’t until 17:13 that I was notified of an issue, by means of a Linode anomaly notification, disk I/O had been unusually high for a two hour period. I was away from my terminal but used my phone to check my netdata instance. Indeed I could confirm a spike in disk activity but it appeared to have subsided. I had run some scripts and updates (which can occasionally trigger these notifications) in the previous two hours so assumed causation and dismissed the notification. Retrospectively, it would be a good idea to have some sort of check list to run through upon receipt of such a message, even if the cause seems obvious.

The attack continued for the next hour and a half, maintaining denial of the mysqld service (despite the respawner’s best effort), at 18:35 (two and a half hours after the attack began) I returned from the field to my terminal and decided to double check the origin of the high disk I/O. I loaded the netdata visualiser (apache seemed to be responsive) and load seemed a little higher than usual. Disk I/O was actually higher than usual, too. It would seem that I had become a victim of y-axis scaling; the spike I had dismissed as a one-off burst in activity earlier had masked the increase in average disk I/O. Something was happening.

I checked system memory, we were bursting at the seams. The apache process was battling to consume as much memory on the system as possible. mysqld appeared to be in a state of flux, so I tried to reach database backed applications; Phabricator, and my blog – both returned some form of upset “where is my database” response. I opened the syslog and searched for evidence that the out of memory killer had been swinging its hammer. At this point I realised this was a denial of service.

I located the source of the high disk I/O when I opened the apache access log. My terminal spewed information on POST requests to xmlrpc.php aimed at two WordPress sites hosted on my server. I immediately added iptables rules for both IP addresses, and two different IPs from the same block took over the attack. I checked the whois and discovered all the origin IPs were in the same assigned /24 block, so I updated iptables with a rule to drop traffic from the whole block. The requests stopped and I restarted the seemingly mangled mysqld process.

I suspect the attack was not aimed at us particularly, but rather the result of a scan for WordPress sites (I am leaning towards for the purpose of spamming). However I was disappointed in my opsec-fu, not only did I prevent this from happening, but I failed to stop it happening for over two hours. I was running OSSEC, but any useful notifications failed to arrive in time as I had configured messages to be sent to a non-primary address that GMail must poll from intermittently. A level 12 notification was sent 28 minutes after the attack started as soon as the OOM was invoked for the first time, but the message was not pulled to my inbox until after the attack had been stopped.

The level of traffic was certainly abnormal and I was also frustrated that I had not considered configuring fail2ban or iptables to try and catch these sort of extreme cases. Admittedly, I had dabbled in this previously, but struggled to strike a balance with iptables that did not accidentally ban false positives attempting to use a client’s web application. Wanting to combat this happening in future, I set about to implement some mitigations:

Mitigation Implementation

Configure a crude fail2ban jail for apache DOS defence

My first instinct was to prevent ridiculous numbers of requests to apache from the same IP being permitted in future. Naturally I wanted to tie this into fail2ban, the daemon I use to block access to ssh, the mail servers, WordPress administration, and such. I found a widely distributed jail configuration for this purpose online but it did not work; it didn’t find any hosts to block. The hint is in the following error from fail2ban.log when reloading the service:

The regular expression provided by the filter (failregex) didn’t have a ‘host’ group to collect the source IP with, so although fail2ban was capable of processing the apache access.log for lines containing GET requests, all the events were discarded. This is somewhat unfortunate considering the prevalence of the script (perhaps it was not intended for the combined_vhost formatted log, I don’t know). I cheated and added a CustomLog to my apache configuration to make parsing simple whilst also avoiding interference with the LogFormat of the prime access.log (whose format is probably expected to be the default by other tooling):

The LogFormat for the CustomLog above encapsulates the source IP in the same manner as the default apache error.log, with square brackets and the word “client”. I updated my http-get-dos.conf file to provide a host group to capture IPs as below (I’ve provided the relevant lines from jail.local for completeness):

I tested the configuration with fail2ban-regex to confirm that IP addresses were now successfully captured:

It works! However when I restarted fail2ban, I encountered an issue whereby clients were almost instantly banned when making only a handful of requests, which leads me to…

How to badly configure fail2ban

This took some time to track down, but I had the feeling that for some reason my jail.conf was not correctly overriding maxretry – the number of times an event can occur before the jail action is applied, which by default is 3. I confirmed this by checking the fail2ban.log when restarting the service:

Turns out, the version of the http-get-conf jail I had copied from the internet into my jail.conf was an invalid configuration. fail2ban relies on the Python ConfigParser which does not support use of the # character for an in-line comment. Thus lines such as the following are ignored (and the default is applied instead):

Removing the offending comments (or switching them to correctly-styled inline comments with ‘;’) fixed the situation immediately. I must admit this had me stumped and seems pretty counter-intuitive especially as fail2ban doesn’t offer a warning or such on startup either. But indeed, it appears in the documentation, so RTFM, kids.

Note that my jail.local above has a jail for http-post-dos, too. The http-post-dos.conf is exactly the same as the GET counterpart, just the word GET is replaced with POST (who’d’ve thought). I’ve kept them separate as it means I can apply different rules (maxretry and findtime) to GET and POST requests. Note too, that even if I had been using http-get-dos today, this wouldn’t have saved me from denial of service, as the requests were POSTs!

Relay access denied when sending OSSEC notifications

As mentioned, OSSEC was capable of sending notifications but they were not delivered until it was far too late. I altered the global ossec.conf to set the email_to field to something more suitable, but when I tested a notification, it was not received. When I checked the ossec.log, I found the following error:

I fiddled some more and in my confounding, located some Relay access denied errors from postfix in the mail.log. Various searches told me to update my postfix with a key that is not used for my version of postfix. This was not particularly helpful advice, but I figured from the ossec-maild error above that OSSEC must be going out to the internet and back to reach my SMTP server and external entities must be authorised correctly to send mail in this way. To fix this, I just updated the smtp_server value in the global OSSEC configuration to localhost:

Deny traffic to xmlrpc.php entirely

WordPress provides an auxiliary script, xmlrpc.php which allows external entities to contact your WordPress instance over the XML-RPC protocol. This is typically used for processing pingbacks (a feature of WordPress where one blog can notify another that one of its posts has been mentioned), via the XML-RPC pingback API, but the script also supports a WordPress API that can be used to create new posts and the like. For me, I don’t particularly care about pingback notifications and so can mitigate this attack in future entirely by denying access to the file in question in the apache VirtualHost in question:



  • 1557 (+0'00"): POSTs aimed at xmlrpc.php for two WordPress VirtualHost begin
  • 1624 (+0'27"): mysqld terminated by OOM killer
  • 1625 (+0'28"): OSSEC Level 12 Notification sent
  • 1625 (+0'28"): mysqld respawns but attack persists
  • 1635 (+0'38"): mysqld terminated by OOM killer
  • 1636 (+0'39"): mysqld respawns
  • 1700 (+1'03"): OSSEC Level 12 Notification sent
  • 1703 (+1'06"): mysqld terminated by OOM killer
  • 1713 (+1'16"): Disk IO 2-Hour anomaly notification sent from Linode
  • 1713 (+1'16"): Linode notification X-Received and acknowledged by out of office sysop
  • 1835 (+2'38"): Sysop login, netdata accessed
  • 1837 (+2'40"): mysqld terminated by OOM killer, error during respawn
  • 1839 (+2'42"): iptables updated to drop traffic from IPs, attack is halted briefly
  • 1840 (+2'43"): Attack continues from new IP, iptables updated to drop traffic from block
  • 1841 (+2'44"): Attack halted, load returns to normal, mysqld service restarted
  • 1842 (+2'45"): All OSSEC notifications X-Received after poll from server


  • POST requests originate from IPs in an assigned /24 block
  • whois record served by LACNIC (Latin America and Caribbean NIC)
  • Owner company appears to be an “Offshore VPS Provider”
  • Owner address and phone number based in Seychelles
  • Owner website served via CloudFlare
  • GeoIP database places attacker addresses in Chile (or Moscow)
  • traceroute shows the connection is located in Amsterdam (10ms away from – this is particularly amusing considering the whois owner is an “offshore VPS provider”, though it could easily be tunneled via Amsterdam

Suspected Purpose

  • Spam: Attacker potentially attempting to create false pingbacks (to link to their websites) or forge posts on the WordPress blogs in question
  • Scan hit-and-run: Scan yielded two xmlrpc.php endpoints that could be abused for automatic DOS


  • Intermittent apache stability for ~3 hours
  • Full service denial of mysql for ~2.25 hours
  • Intermittent disruption to email for ~2 hours


  • No monitoring or responsive control configured for high levels of requests to apache
  • OSSEC configured to deliver notifications to non-primary address causing messages that would have prompted action much sooner to not arrive within actionable timeframe
  • Failed to recognise (or consider) disk I/O anomaly message as a red herring for something more serious
  • Forgetting that the attack surface for WordPress is always bigger than you think


  • Recently installed netdata instance immediately helped narrow the cause down to apache based activity
  • Attack mitigated in less than five minutes once I actually got to my desk


  • OSSEC reconfigured to send notifications to an account that does not need to poll from POP3 intermittently
  • Added simple GET and POST jails to fail2ban configuration to try and mitigate such attacks automatically in future
  • Drop traffic to offending WordPress script to reduce attack surface
  • Develop a check list to be followed after receipt of an anomaly notification
  • Develop a healthy paranoia against people who are out to get you and be inside your computer (or make it fall over)
  • Moan about WordPress

Mitigation Tips and Gotcha’s

  • Set your OSSEC notification smtp_server to localhost to bypass relay access denied errors
  • Make use of fail2ban-regex <log> <filter> to test your jails
  • NEVER use # for inline comments in fail2ban configurations, the entire line is ignored
  • If you are protecting yourself from GET attacks, have you forgotten POST?