Re: 6.2 mtu now limits size of incomming packet

Sten Daniel Soersdal wrote:

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.



But shouldn't an icmp be generated back to the system sending the packet that is
being dropped? This is not happening. So the connection stalls.

client mtu 1500 <-> |rl0 mtu 1500 FreeBSD Router rl1 mtu 1280| <-> some host on internet
client sends syn saying i can do mss=1460
host sends syn saying i can do mss=1460
host tries to send packet of 1460 it get silently dropped. connection stalls.

--

"They that give up essential liberty to obtain temporary safety, deserve neither liberty nor safety." (Ben Franklin)

"The course of history shows that as a government grows, liberty decreases." (Thomas Jefferson)



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