Extended Brain Storage

# Maximum Transmission Unit Determination

Posted on November 17, 2014

A couple of notes to determine the maximum transmission unit (MTU) of a network link...

$ping -s 1472 server.domain.tld -M do  ### macOS $ ping -D -s 1472 server.domain.tld


### Windows

C:\> ping -f -l 1472 server.domain.tld


### How-to

Normally, the 1472 should work (for MTU=1500). If not, the results will instruct about the maximum transmission unit, such as:

ping: local error: Message too long, mtu=1480


In that case, it is necessary to lower the MTU value, for example, of a tunnel interface, in iptables, etc.

Note: The static ICMP header size of 8 bytes was changed by RFC1812. Therefore, the size of 1472B is just for the initial testing phase and may differ.

### References

The above details are solely based on a king help provided by the DSLReports FAQ.