Date: Fri, 13 Jul 2007 14:40:58 +0200 From: Sten Daniel Soersdal <netslists@gmail.com> To: Stephen.Clark@seclark.us Cc: freebsd-net@freebsd.org Subject: Re: 6.2 mtu now limits size of incomming packet Message-ID: <469772DA.1000700@gmail.com> In-Reply-To: <46967C5C.5040505@seclark.us> References: <46967C5C.5040505@seclark.us>
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Stephen Clark wrote: > Hello, > > Did something change in 6.2? If my mtu size on rl0 is 1280 it won't > accept a larger incomming packet. > > kernel: rl0: discard oversize frame (ether type 800 flags 3 len 1514 > max > 1294) That is what to be expected. Incoming interface must have mtu set to the same mtu as all other hosts on the same L2 network. If mtu is set to the same as all other hosts, then it is impossible to receive a frame that is too large (assuming everything works). > > I don't think it worked this way in the past. > > Won't this affect pmtud? Incoming interface must have its mtu set to large enough to receive the frame. Outgoing interface, on the other hand, can be lower. For pmtud to work you need to be able to receive packets on an interface with sufficiently set mtu, but the exitting interface can have a lower mtu configured. Thus the router can accept the incoming packet but may drop and notify on a frame that is too large to exit the outgoing interface (assuming DF is set). > > man page for ifconfig says mtu limits size of "transmission" not reception. > > "mtu n Set the maximum transmission unit of the interface to n, > default > is interface specific." Perhaps the man author considered reception to be implied? In any case, enforcing this on incoming packets is correct behavior. -- Sten Daniel Soersdal
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