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c4f4t0r
Asked: 2013-08-22 00:42:44 +0800 CST

Fedora Linux doesn't show route cache entries

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i'm playing with ip command and today i saw my fedora 16 doesn't show the routing cache information after i did a connection to a host, on centos 5 and on suse 10,11 i have no problem if i use the command ip route show cache or route -Cen, my kernel version is 3.6.11-4.fc16.i686

I would like to know if it's something related with kernel version or if it's a fedora problem command utilities.

linux

2 Answers

  1. The IPv4 route cache was removed from kernel 3.6. You should be able to find any routing exceptions in the FIB trie (see /proc/net/fib_trie).

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  2. You can get detailed information on the path MTU for a host as follows. Note that the cache information must be populated first. This testing was performed with a 3.13 Ubuntu kernel. I first validate the current cache for a host, I haven't communicated with it and have no information:

    johnf@mtutest:~$ ip ro get 192.168.3.48
192.168.3.48 dev eth0  src 192.168.1.22
    cache

    I then try to ping it with a packet larger than the MTU (but not so large that the packet must be fragmented by the OS). You may miss the first few pings when you test, you should see the Frag needed message.

    johnf@mtutest:~$ ping -s 1460 192.168.3.48 -c 10
PING 192.168.3.48 (192.168.3.48) 1460(1488) bytes of data.
From 192.168.2.0 icmp_seq=1 Frag needed and DF set (mtu = 1220)
1468 bytes from 192.168.2.0: icmp_seq=2 ttl=252 time=1973 ms
[...]
--- 192.168.3.48 ping statistics ---
10 packets transmitted, 9 received, +1 errors, 10% packet loss, time 9016ms
rtt min/avg/max/mdev = 95.681/516.815/1973.697/568.969 ms, pipe 2

    After you receive the ICMP MTU Exceeded message the kernel should adjust your route cache to reflect the path limitations:

    johnf@mtutest:~$ ip ro get 192.168.3.48
192.168.3.48 dev eth0  src 192.168.1.22
    cache  expires 588sec mtu 1220
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