** Description changed: Hello, Looks like setting per route mtu (as described in [1]) is broken in linux kernel 3.2 (default kernel from ubuntu 12.04). I made script to illustrate this: - root@germany7:~# cat route-mtu + root@germany7:~# cat route-mtu #!/bin/bash uname -a ip tuntap add dev test mode tun ifconfig test 10.1.1.1/16 mtu 1500 echo ============================================================= echo No per route mtu entry ping -c1 -Mdo -s2000 10.1.1.2 echo ============================================================= echo Per route MTU is set to 800 ip route add 10.1.1.2 dev test proto static src 10.1.1.1 mtu lock 800 ip route flush cache ping -c1 -Mdo -s2000 10.1.1.2 ip route del 10.1.1.2 dev test proto static src 10.1.1.1 mtu lock 800 echo ============================================================= echo Per route MTU is set to 600 ip route add 10.1.1.2 dev test proto static src 10.1.1.1 mtu lock 600 ip route flush cache ping -c1 -Mdo -s2000 10.1.1.2 ip route del 10.1.1.2 dev test proto static src 10.1.1.1 mtu lock 600 echo ============================================================= echo No per route mtu entry ip route flush cache ping -c1 -Mdo -s2000 10.1.1.2 ip tuntap del dev test mode tun - With kernel 3.2 I get following output: Linux germany7 3.2.0-41-generic-pae #66-Ubuntu SMP Thu Apr 25 03:50:20 UTC 2013 i686 i686 i386 GNU/Linux ============================================================= No per route mtu entry PING 10.1.1.2 (10.1.1.2) 2000(2028) bytes of data. From 10.1.1.1 icmp_seq=1 Frag needed and DF set (mtu = 1500) --- 10.1.1.2 ping statistics --- 0 packets transmitted, 0 received, +1 errors ============================================================= Per route MTU is set to 800 PING 10.1.1.2 (10.1.1.2) 2000(2028) bytes of data. From 10.1.1.1 icmp_seq=1 Frag needed and DF set (mtu = 800) --- 10.1.1.2 ping statistics --- 0 packets transmitted, 0 received, +1 errors ============================================================= Per route MTU is set to 600 PING 10.1.1.2 (10.1.1.2) 2000(2028) bytes of data. From 10.1.1.1 icmp_seq=1 Frag needed and DF set (mtu = 800) --- 10.1.1.2 ping statistics --- 0 packets transmitted, 0 received, +1 errors ============================================================= No per route mtu entry PING 10.1.1.2 (10.1.1.2) 2000(2028) bytes of data. From 10.1.1.1 icmp_seq=1 Frag needed and DF set (mtu = 800) --- 10.1.1.2 ping statistics --- 0 packets transmitted, 0 received, +1 errors - After once set to 800 MTU is always 800, even if you add different route - and clean routing cache. + After once set to 800, MTU is always 800, even if you add different + route and clean routing cache. - - It looks like that problem is fixed in ubuntu kernel 3.5. With it everything works like expected: - root@germany7:~# ./route-mtu + It looks like that problem is fixed in ubuntu kernel 3.5. With it, everything works like expected: + root@germany7:~# ./route-mtu Linux germany7 3.5.0-28-generic #48~precise1-Ubuntu SMP Wed Apr 24 21:43:05 UTC 2013 i686 i686 i386 GNU/Linux ============================================================= No per route mtu entry PING 10.1.1.2 (10.1.1.2) 2000(2028) bytes of data. From 10.1.1.1 icmp_seq=1 Frag needed and DF set (mtu = 1500) --- 10.1.1.2 ping statistics --- 0 packets transmitted, 0 received, +1 errors ============================================================= Per route MTU is set to 800 PING 10.1.1.2 (10.1.1.2) 2000(2028) bytes of data. From 10.1.1.1 icmp_seq=1 Frag needed and DF set (mtu = 800) --- 10.1.1.2 ping statistics --- 0 packets transmitted, 0 received, +1 errors ============================================================= Per route MTU is set to 600 PING 10.1.1.2 (10.1.1.2) 2000(2028) bytes of data. From 10.1.1.1 icmp_seq=1 Frag needed and DF set (mtu = 600) --- 10.1.1.2 ping statistics --- 0 packets transmitted, 0 received, +1 errors ============================================================= No per route mtu entry PING 10.1.1.2 (10.1.1.2) 2000(2028) bytes of data. From 10.1.1.1 icmp_seq=1 Frag needed and DF set (mtu = 1500) --- 10.1.1.2 ping statistics --- 0 packets transmitted, 0 received, +1 errors - - I also tested with kernels 2.6.35 (in lucid) and 3.8 (in precise), results are same to 3.5. + I also tested with kernels 2.6.35 (in lucid) and 3.8 (in precise), + results are same to 3.5. Is there any chance to get fixes from 3.5/3.8 backported to the default precise kernel 3.2? - 1. http://lartc.org/howto/lartc.cookbook.mtu-discovery.html
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