Date: Wed, 31 Oct 2007 15:22:44 +0200 From: Nikos Vassiliadis <nvass@teledomenet.gr> To: freebsd-questions@freebsd.org Cc: Eric Boudrand <eric@boudrand.net> Subject: Re: ICMP and discard oversize frame Message-ID: <200710311522.45352.nvass@teledomenet.gr> In-Reply-To: <47285AF7.4060905@boudrand.net> References: <47285AF7.4060905@boudrand.net>
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On Wednesday 31 October 2007 12:37:43 Eric Boudrand wrote: > Hello, > > I am running a FreeBSD router with two ethernet cards. I have reduced > the MTU to 800 in order to generate ICMP packet "Fragmentation needed > but DF was set". I can find in log : > > rl1: discard oversize frame (ether type 800 flag 3 len 875 > max 814) > > But there is no ICMP sent. By default, is this feature disabled ? How > can I see these ICMP packets generated ? It's discarded before IP processing, the kernel knows nothing about the IPs involved in that packet(well, actually ethernet frame). MTU must be set equally across your link layer... It *will* send a fragmentation needed message if a received packet is to be forwarded to a link with smaller MTU, not big enough for the packet... That's not the same case with the one you've posted. So, long story short, FreeBSD sends fragment. needed messages. But, this is not the correct way to test such a situation. HTH Nikos
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