To limit the fragmentation of packets and optimize your network, it can be necessary to find the best MTU size to set up on your interface. In order to find this best value, you can use a simple ping command.
We’re first trying with a MTU size of 1500 bytes:
$ ping -M do -s 1500 mysite.com PING mysite.com (1.2.3.4) 1500(1528) bytes of data. ping: local error: Message too long, mtu=1500 ping: local error: Message too long, mtu=1500 ping: local error: Message too long, mtu=1500
We can clearly see that with the overhead, it’s sending 1528 bytes, too long for the MTU size allowed on the network (1500). Message will be fragmented.
We’re changing the value to 1472 bytes:
$ ping -M do -s 1472 mysite.com PING mysite.com (1.2.3.4) 1472(1500) bytes of data. 1480 bytes from 1.2.3.4: icmp_seq=1 ttl=58 time=0.94 ms 1480 bytes from 1.2.3.4: icmp_seq=2 ttl=58 time=0.62 ms 1480 bytes from 1.2.3.4: icmp_seq=3 ttl=58 time=0.53 ms
Right now, we can see that packets are not fragmented anymore, exactly what we were expected!
On most of Linux distributions, MTU size can be set with this command:
ifconfig eth0 mtu 1472 up
You can check the results with the ip addr show command:
$ ip addr show
[...]
2: eth0: <no-carrier,broadcast,multicast,up> mtu 1492 qdisc fq_codel state DOWN group default qlen 1000
link/ether 12:34:56:78:90:12 brd ff:ff:ff:ff:ff:ff
[...]
</no-carrier,broadcast,multicast,up>