pmtud + ipnat RELENG_6_2 appears to be broken

[Chuck Swiger]

On Jul 12, 2007, at 12:34 PM, Stephen Clark wrote:
> Did something change in 6.2?  If my mtu size on rl0 is 1280 it won't
> accept a larger incoming packet.

Nothing changed; that is the expected behavior.
(Modulo support for 4-byte VLAN tags.)

> kernel: rl0: discard oversize frame (ether type 800 flags 3 len  
> 1514 > max
> 1294)
>
> I don't think it worked this way in the past.

Well, it did.  :-)

> Won't this affect pmtud?

Nope.

> 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."

The MTU is actually defined in reference to a network segment such as  
an "ethernet collision domain", and applies to all machines sending  
traffic to that segment.  If the MTU is really 1280, nobody else  
should be sending larger packets, and the drivers will drop any  
larger packets they receive and generate the appropriate ICMP error....

-- 
-Chuck