opnsense-src/share/examples/find_interface/README
Franco Fichtner 402e7dde73 src: initial commit based on FreeBSD-10.0
Taken from:	https://github.com/freebsd/freebsd.git
Commit id:	d44ce30d3054a38723f89a161c5e003e64d1aaae
2014-11-09 09:30:14 +01:00

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Text

This is a simple program which demonstrates how to query the kernel
routing mechanism using only a UDP socket. Pass it a hostname on
the command line (sorry, it doesn't parse dotted decimal) and it will
print out an IP address which names the interface over which UDP
packets intended for that destination would be sent.
A more sophisticated program might use the list obtained from SIOCGIFCONF
to match the address with an interface name, but applications programmers
much more often need to know the address of the interface rather than
the name.