OK here it is! Thanks to everyone who might help with this issue!

PS: I am aware of the FAQ page you mentioned, but sadly this doesn't
impact my problem.

Manuel Kasper a écrit :
> On Jul 23, 2008, at 10:57 AM, Yannick Bréhon wrote:
> 
>> I have fiddled some more with my m0n0wall, unsuccessfully. Would anybody
>> know why the m0n0, when there are "pass-all" rules (ie. any proto, any
>> source/dest, any port allowed) on all interfaces, might still reject
>> some traffic? Maybe this is a known bug??
> 
> Please post your (anonymized) config.xml, otherwise this will lead
> nowhere. Also, be aware of this:
> <http://doc.m0n0.ch/handbook/faq-legit-traffic-dropped.html>.
> 
> - Manuel
>