portable way to detect MTU?

Maybe I'm blind, but libnet doesn't seem to have a way to determine the MTU of the selected network card so you won't find out that the packet is too big until you actually send the packet.

Anyone know of a *portable* (Linux, BSD, Solaris, etc) way of getting the MTU?

-- 
Aaron Turner   
http://synfin.net/aturner
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