When I ping a remote site with the DF bit set and a packet size that is too big for my router the first ICMP "fragmentation required" message is sent from the router. After that the message comes from my localhost.

Netstat -rC (on Linux) allows me to view the routing table cache, but

1) Seems to show MTUs under a column called MSS (which I would expect to be the lower TCP MSS of the link)

2) Always shows the value as 1500

My localhost must be caching the PMTU somewhere so it can generate the fragmentation required message. But how do I see that?

Here is an example on my machine (-n on netstat inhibits reverse DNS lookups):

[root@vbcentos ~]# ping -c 4 -M do -s 1431
PING ( 1431(1459) bytes of data.
From icmp_seq=1 Frag needed and DF set (mtu = 1458)
From icmp_seq=2 Frag needed and DF set (mtu = 1458)
From icmp_seq=2 Frag needed and DF set (mtu = 1458)
From icmp_seq=2 Frag needed and DF set (mtu = 1458)

--- ping statistics ---
1 packets transmitted, 0 received, +4 errors, 100% packet loss, time 1002ms

[root@vbcentos ~]# netstat -rCn
Kernel IP routing cache
Source          Destination     Gateway         Flags   MSS Window  irtt Iface   il        0 0          0 lo          1500 0          0 eth0   l     16436 0          0 lo ibl       0 0          0 lo          1500 0          0 eth0   il        0 0          0 lo   l         0 0          0 lo
[root@vbcentos ~]#

EDIT: As per suggestion:

ip route get to

gives via dev eth1  src
    cache  mtu 1500 advmss 1460 hoplimit 64

Which also seems wrong as the MSS is just 40 less than the mtu, which is the interface mtu rather than the PMTU

  • 1
    On Fedora 22, netstat -rCn returns nothing, but watch ip route get to $HOST shows what's up, including the cache TTL. ip route show cached shows probably also output something but does not. – David Tonhofer Feb 13 '16 at 20:53


ip route get to
  • Edited question to add this. – Neik Aug 28 '13 at 9:17

Under Windows, use the netsh command to view the "destination cache" which holds this information. For example (assuming IPv4):

netsh interface ipv4 show destinationcache

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