Re: ICMP_UNREACH_NEEDFRAG returns iface MTU instead of route?

[Darren Reed <darrenr%fastmail.net@localhost>]

On 20/12/2013 10:34 AM, Dave Huang wrote:

> It seems that the ICMP fragmentation needed packet contains the
> interface MTU rather than the route MTU if the route MTU is lower
> than the iface's: see sys/netinet/ip_input.c, ip_forward(), case
> EMSGSIZE--around line 1337: destmtu = rt->rt_ifp->if_mtu;
>
> Is that what it should be doing? It seems wrong and isn't what I
> expected... if I artificially lower the MTU of a route, e.g.,
> route add www.netbsd.org $my_gateway_ip -mtu 1200
>
> Then ping -Ds 1300 www.netbsd.org from another machine that routes
> through the above router, I get:
> PING www.netbsd.org (149.20.53.86): 1300 data bytes
> 36 bytes from foxy.azeotrope.org (10.1.1.67): frag needed and DF set.  Next 
> MTU=1500 for icmp_seq=0
>
> Shouldn't Next MTU=1200, rather than 1500?
Yes.

Otherwise PMTU Discovery does not work.

We can argue about RFC semantics forever and a day but if the

behaviour stays the same then PMTU discovery involving NetBSD

will remain broken in situations like this.

Darren