Subject: per route MTU settings

From: "[email-masked]" <[email-masked]>
Date: Fri, 11 Jan 2013 17:29:28 +0100 (CET)

Hello everybody.

I have done some tests with per route
mtu settings.

Here is the results on the 10.81.104.254
linux box running 3.6.9 on Slackware 14
32bit:

ip route add 10.81.105.109/32 via 10.81.104.1 mtu lock 1450
ip route flush cache

ping -M do 10.81.105.109 -c 5 -s 1450
PING 10.81.105.109 (10.81.105.109) 1450(1478) bytes of data.
>>From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450)
>>From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450)
>>From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450)
>>From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450)
>>From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 1450)

Here is the results on my linux box with
ip address 10.81.104.126 (the default
gateway is 10.81.104.254) running linux
3.7.0 on Slackware 14 64bit

ping -M do 10.81.105.109 -c 5 -s 560
PING 10.81.105.109 (10.81.105.109) 560(588) bytes of data.
>>From 10.81.104.254 icmp_seq=1 Frag needed and DF set (mtu = 576)
>>From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576)
>>From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576)
>>From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576)
>>From 10.81.104.126 icmp_seq=2 Frag needed and DF set (mtu = 576)

When packets are generated locally (on
the 10.81.104.254 box), linux icmp message
'need to frag' report the correct mtu of
next hop. Instead on the forwarded packets
(those that aren't originated on the 10.81.104.254
box), I always get the 576 value on the mtu
of nexthop.
Is this the expected behaviour?

Any response are welcome

TIA