ICMP and discard oversize frame

Nikos Vassiliadis nvass at teledomenet.gr
Wed Oct 31 06:27:17 PDT 2007

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.



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