My first custom Fail2Ban filter

written by Robin Schubert on 2022-10-01 | Tags:

On my servers that are meant to be world-accessible, the first things I set up are the firewall and Fail2Ban, a service that updates my firewall rules automatically to reject requests from IP addresses that have failed repeatedly before. The ban duration and number of failed attempts that trigger a ban can easily be customized; that way, bots- and hacker attacks that try to break into my system via brute force and trial and error can be blocked or at least delayed very effectively.

Luckily, many pre-defined and modules and filters already exist that I can use to secure my offered services. To set up a jail for sshd for instance and do some minor configurations, I only need a few lines in my /etc/fail2ban/jail.local file:

[DEFAULT]
bantime  = 4w
findtime = 1h
maxretry = 2
ignoreip  = 127.0.0.1/8 192.168.0.1/24


[sshd]
enabled   = true
maxretry  = 1
findtime  = 1d

Just be aware that you should not change /etc/fail2ban/jail.conf, as this will be overwritten by fail2ban. If a jail.local is not already present, create one.

As you can see, I set some default options about how long IPs should be banned and after how many failed tries. I also exclude local IP ranges from bans, so I'll not lock myself out every time I test a new service or setting. However, for sshd I even tighten the rules a bit, since I only use public key authentication where I don't expect a single failure from a client that is allowed to connect. All the others can happily be sent to jail.

It's always a joy but also kind of terrifying to check the jail for the currently banned IPs; the internet is not what I would call a safe place.


sudo fail2ban-client status sshd
Status for the jail:
|- Filter
|  |- Currently failed: 0
|  |- Total failed:     211
|  `- File list:        /var/log/auth.log
`- Actions
   |- Currently banned: 2016
   |- Total banned:     2202
   `- Banned IP list: ...

My own filter

Do identify IP addresses that should be banned, Fail2Ban scans the appropriate log files for failed attempts with a regular expression, as the sshd module does with my /var/log/auth.log.

Like mentioned above, there are already quite some pre-defined modules. For my nginx reverse proxy the modules nginx-botsearch, nginx-http-auth and nginx-limit-req are available; the log files they scan by default is /var/log/nginx/error.log.

However, having a look in my /var/log/nginx/access.log I regularly find lots of failed attempts that are probing my infrastructure. They look like this:

118.195.252.158 - - [01/Oct/2022:02:08:59 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
118.195.252.158 - - [01/Oct/2022:02:08:59 +0200] "GET http://xxx.xxx.xxx.xxx:80/mysql/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
118.195.252.158 - - [01/Oct/2022:02:08:59 +0200] "GET http://xxx.xxx.xxx.xxx:80/pma/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
118.195.252.158 - - [01/Oct/2022:02:08:59 +0200] "GET http://xxx.xxx.xxx.xxx:80/db/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
...
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.11.7/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.11.4/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.10.3/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin2/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/db/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.11.3/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/mysqladmin/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/myadmin/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.11.1.2/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.11.9.2/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/pma/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.10.2/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.10.0.2/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.8.0.2/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/phpMyAdmin-2.11.0/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
42.236.120.51 - - [01/Oct/2022:02:26:57 +0200] "GET http://xxx.xxx.xxx.xxx:80/mysql/scripts/setup.php HTTP/1.0" 404 162 "-" "-"
...
185.183.122.143 - - [30/Sep/2022:01:19:48 +0200] "GET /wp-login.php HTTP/1.1" 404 134 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:96.0) Gecko/20100101 Firefox/96"
198.98.59.132 - - [30/Sep/2022:01:51:59 +0200] "POST /boaform/admin/formLogin HTTP/1.1" 404 134 "http://xxx.xxx.xxx.xxx:80/admin/login.asp" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:71.0) Gecko/20100101 Firefox/71.0"
20.168.74.192 - - [30/Sep/2022:01:54:29 +0200] "GET /.env HTTP/1.1" 404 134 "-" "Mozilla/5.0 (Linux; U; Android 4.4.2; en-US; HM NOTE 1W Build/KOT49H) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 UCBrowser/11.0.5.850 U3/0.8.0 Mobile Safari/534.30"
20.168.74.192 - - [30/Sep/2022:01:54:29 +0200] "GET /_profiler/phpinfo HTTP/1.1" 404 134 "-" "Mozilla/5.0 (Linux; U; Android 4.4.2; en-US; HM NOTE 1W Build/KOT49H) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 UCBrowser/11.0.5.850 U3/0.8.0 Mobile Safari/534.30"
20.168.74.192 - - [30/Sep/2022:01:54:30 +0200] "GET /config.json HTTP/1.1" 404 134 "-" "Mozilla/5.0 (Linux; U; Android 4.4.2; en-US; HM NOTE 1W Build/KOT49H) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 UCBrowser/11.0.5.850 U3/0.8.0 Mobile Safari/534.30"
20.168.74.192 - - [30/Sep/2022:01:54:30 +0200] "GET /.git/config HTTP/1.1" 404 134 "-" "Mozilla/5.0 (Linux; U; Android 4.4.2; en-US; HM NOTE 1W Build/KOT49H) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 UCBrowser/11.0.5.850 U3/0.8.0 Mobile Safari/534.30"

I don't use PhpMyAdmin and I don't host a wordpress site (requests to wp-login and wp-admin are pretty common) and I would prefer to ban IPs that scan my infrastructure for these services. So I wrote a new filter to scan my nginx/access.log file for requests of that kind.

In /etc/fail2ban/filters.d/nginx-access.conf I added the following definition:

[Definition]

_daemon = nginx-access
failregex = (?i)^<HOST> .*(wp-login|xmlrpc|wp-admin|wp-content|phpmyadmin|mysql).* (404|403)

(?i) makes the whole regular expression case insensitive, so it will capture phpmyadmin and PhpMyAdmin equally.
^<HOST> will look from the start of each line to the first space for the IP address. <HOST> is a defined capture group from Fail2Ban, that must be present in failregexes to let Fail2Ban know who to ban.
.* matches any character, and an arbitrary number of them
(wp-login|wp-admin...) these are the request snippets to look for; in parentheses and separated with the pipe operator, it will look for matches of either of the given strings.
(404|403) are http responses for "file/page not found" and "forbidden". So if these pages are not available or not meant to be accessed, this rule will be triggered.

In my jail.local I add the following section to use the new filter:

[nginx-access]
enabled   = true
port      = http,https
filter    = nginx-access
logpath   = /var/log/nginx/access.log

Restart the fail2ban service (e.g. systemctl restart fail2ban) to enable the new rule.

I started with only a few keywords to filter, but the used regular expression can easily be expanded by further terms.