How to get maximum supported MTU size for interface?

Question

I'm trying to find a way to check if nic supporting a specific MTU size or not without trying to set it first (actually, I want to find the theoretical maximum MTU on all interfaces on all my servers).

I've inspected all features of ethtool, looked in /sys/class/net, etc, but all I can find is the current MTU value.

Are there a way to see how high MTU can be on interface without trying it?

Answer 1

you can send a specific mtu size with ping

ping -M do -s <mtu-size> <ip-address>

<ip-address> being the local ip of the interface you wish to check.

Note there is an additional 28 bytes as a header when using this method.

Just keep increasing the mtu size (in the ping command) until you get a Message too long error or similar.

Current MTU setting and IP:

[root@centos7 ~]# ip l  | grep ens37 | awk '{print $4,$5}'
mtu 1500
[root@dev-worker1 ~]# ip addr show ens37 | grep "inet " | awk '{print $2}'
10.10.10.10/24

sending a packet larger than the current MTU setting, but is still accepted:

[root@centos7 ~]#  ping -M do -s 8972 10.10.10.10
PING 10.10.10.10 (10.10.10.10) 8972(9000) bytes of data.
8980 bytes from 10.10.10.10: icmp_seq=1 ttl=64 time=0.103 ms
8980 bytes from 10.10.10.10: icmp_seq=2 ttl=64 time=0.067 ms

Sending one too large. Some distros may actually tell you the maximum via this method. E.g Centos7:

[root@centos7 ~]#  ping -M do -s 118972 10.10.10.10
Error: packet size 118972 is too large. Maximum is 65507

Once done, you can set it to the maximum if that's what you desire using ip link

ip link set <interface name> mtu <mtu value>

edit:

• Clarified I'm referring to pinging a local IP and provided example.
• I do not know for sure that some distros will output the actual limit, as my testing environment interfaces have max capabilities of 65535 bytes.