[FR]: I want to ban every host in log but I can't figure it out. about fail2ban HOT 3 CLOSED

Our issue template says clearly and explicitly what is needed for a filter request.
At least an excerpt with relevant log-lines is expected.
Thus closed, Please extend your request with needed info and I'll reopen it.

from fail2ban.

My apologies for not following the rules, at the time of writing my filter request I felt (incorrectly) that because every entry in the log file was to be banned; then the version, example log etc. was unnecessary. I will ensure I provide this information in the future.

from fail2ban.

Everything is important... Especially fail2ban version and log excerpt.
For instance if you don't have a timestamp in your monitored log, you'd need to set datepattern = {NONE} and it'd work only for fail2ban >= v.0.11.2 (or v.0.10.6), where it was initially implemented. Without that the filter would not find any message, even with correct failregex.

Here is an example for this log:

192.0.2.1 # evil
192.0.2.2 # also evil

[jail]
filter =
datepattern = {NONE}
failregex = ^<ADDR>

PoC:

$ fail2ban-regex '192.0.2.1 # evil' '^\s*<ADDR>'
... Lines: 1 lines, 0 ignored, 0 matched, 1 missed
$ fail2ban-regex -d '{NONE}' '192.0.2.1 # evil' '^\s*<ADDR>'
... Lines: 1 lines, 0 ignored, 1 matched, 0 missed

For log with timestamp the datepattern (or default date-templates) need to match it firstly and failregex must match a string without part cut out by datepattern. For example:

2024-03-20 10:00:00 192.0.2.1 # evil
2024-03-20 10:01:00 192.0.2.2 # also evil

[jail]
filter =
datepattern = %%Y-%%m-%%d\s{0,2}%%H:%%M:%%S
failregex = ^\s*<ADDR>

whereby the datepattern is optional here (just for illustration, as how-to), because this timestamp will be detected by fail2ban by default.
PoC:

$ fail2ban-regex -d '%Y-%m-%d\s{0,2}%H:%M:%S' '2024-03-20 10:00:00 192.0.2.1 # evil' '^\s*<ADDR>'
... Lines: 1 lines, 0 ignored, 1 matched, 0 missed
$ fail2ban-regex '2024-03-20 10:00:00 192.0.2.1 # evil' '^\s*<ADDR>'
... Lines: 1 lines, 0 ignored, 1 matched, 0 missed

By the way, <HOST> will also match the IP-address, but also a hostname (or anything that looks like a hostname), so if only IP-address expected, <ADDR> will be better choice.
If you rather want a subnet, see #2560

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