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MTU = maximum IP datagram that can be sent over an interface, including IP headers. This must be less than or equal to the maximum size data field of the lower layer (e.g., 1500 bytes for Ethernet).

One way to determine what MTU size to use is via the ping command. The general procedure is to send a ping with the “Don’t Fragment” bit set (-D option) to a host on the network using different size data fields (-s option).

ping -D -s n

where n is the ICMP data field size you want to use. The total IP packet size transmitted is “n + 28″ bytes.

Start with a value of n=1472 (equivalent to MTU = 1500) and lower the values for n until you get a valid response to your ping. The MTU for that interface will then be “n + 28″.

Substitute a real host for “testhost” in the examples below.

In the following examples, I am connected to a Corporate network via VPN (using SOHO router) over en0.

For example, the following setting returns a “Message too long” error indicating that it is larger than the MTU supported on that interface (1500 + 28 = 1528):

$ ping -D -s 1500 testhost PING testhost (x.x.x.x): 1500 data bytes ping: sendto: Message too long ping: sendto: Message too long ping: sendto: Message too long ^C --- testhost ping statistics --- 3 packets transmitted, 0 packets received, 100% packet loss

A Cisco 871 router is nice enough to return an ICMP “unreachable” message with a clue to the right MTU (1400). I just subtracted 28 from that value for my second try and verified that 1400 was the right MTU size.

Not all routers or networks will return this error. Sometimes the ping might just time out.

Also, be sure to ping a host that supports pinging and doesn’t have a firewall in front of it that blocks ICMP packets.

This approach does not apply to OS X versions 10.4 and up. Apple got rid of the /System/Library/StartupItems/Network directory in 10.4.

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