On Startup: WARNING 'allowipv6' not defined in 'Definition'. Using default one: 'auto'

[jhgarrison]

This is a newly installed Debian 12.4, and even with an unmodified fail2ban install I get this warning message on startup every time. This seems to be related to the allowipv6 option in fail2ban.conf being commented out. Curiously, the commented section containing allowipv6 is in the [Default] section, not [Definition].

I tried adding allowipv6=yes to jail.local in the [Default] section (my system is dual-stack 4/6), but it does not remove the warning. I then tried adding it in a [Definition] section in jail.local, also with no effect.

Finally, un-commenting the line in fail2ban.conf does prevent the warning, but I'd rather not modify fail2ban.conf unless absolutely necessary.

Is there a way to set the value in jail.conf and prevent the warning?

[sebres]

This is not an error, just warning...
(However I'd see to disable such warnings, targeting default values, in new version).

Finally, un-commenting the line in fail2ban.conf does prevent the warning, but I'd rather not modify fail2ban.conf unless absolutely necessary.

Just add it to local configuration fail2ban.local (emphasis on .local not .conf):

[DEFAULT]
allowipv6 = auto

(don't copy *.conf to *.local, create empty one and set only parameters you need to overwrite).

Is there a way to set the value in jail.conf ...

No. This parameter works globally and needed directly by start of fail2ban-server (before any jail gets started), so it belongs to fail2ban-config not to jail-config.
But normally one should anyway avoid to change stock *.conf (no matter jail.conf or fail2ban.conf) - there are local configs for that reason.
See https://github.com/fail2ban/fail2ban/wiki/Proper-fail2ban-configuration

[sebres]

As for the warning suppression, I forgotten that I already "fixed" it in 432e7e1 (gets out with the next release)