Writing a filter for a regex that works in fail2ban-regex on the command line

Question

I have entries like these in apache2 error.log

[Thu Jan 12 09:18:51.078445 2023] [core:error] [pid 47992] [client 152.89.196.211:53158] AH10244: invalid URI path (/cgi-bin/.%2e/.%2e/.%2e/.%2e/bin/sh)
[Wed Jan 11 06:01:09.820582 2023] [core:error] [pid 30833] [client 185.225.74.55:39856] AH10244: invalid URI path (/cgi-bin/.%%%%32%%65/.%%%%32%%65/.%%%%32%%65/.%%%%32%%65/.%%%%32%%65/bin/sh)
[Wed Jan 11 17:16:49.643509 2023] [core:error] [pid 41882] [client 152.89.196.211:52746] AH10244: invalid URI path (/cgi-bin/.%2e/.%2e/.%2e/.%2e/bin/sh)

I got this to work on the command line:

fail2ban-regex test.log '.*\[client <HOST>:\d+\] AH10244.*$'

Every time I try to stick the regex into a .conf file like so:

[Definition]
 
failregex = .*\[client <HOST>:\d+\] AH10244.*$
 
ignoreregex =

fail2ban complains:

Running tests
=============

Use   failregex line : filter.conf
ERROR: No failure-id group in 'filter.conf'

I've looked in the man pages and online but I can't find an explanation of what this message is trying to say, or how to fix it.

The Questions

1. How do I wrap a .conf file around this regex?
2. What does that error mean?
3. Could I (how would I) use the pre-defined stuff in apache-common.conf to make this regex more robust?

Answer 1

This fixed it:

fail2ban-regex test.log ./filter.conf

I had my test files (test.log and filter.conf) in my home dir. When I ( from the home dir ) issued the command:

fail2ban-regex test.log filter.conf

I assumed that I was referencing ./test.log and ./filter.conf but I think that fail2ban was looking in the filter.d/ folder to try to find filter.conf.

I found that if filter.conf was in the /etc/fail2ban/filter.d/ folder, then fail2ban-regex test.log filter.conf succeeded.