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In FreeBSD, an ifconfig call seems to delete the default gateway route, even when given the same IP-address (e.g. when updating the netmask only). Is it a bug or a feature?
As fas as I remember Linux doesn't change the default gateway in this case.

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up vote 4 down vote accepted

In FreeBSD an ifconfig that removes direct connectivity to a subnet also zaps routes containing that subnet (IIRC updating netmasks is reduced to a remove-and-replace). This is a "feature" so as not to confuse your routing configuration by trying to talk to a host it can't reach anymore, though it causes its share of confusion when users encounter disappearing routes.

See also: http://forums.freebsd.org/showthread.php?t=12132

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Thanks. This is something to keep in mind when running ifconfig on a remote host. –  eugene y Jun 14 '10 at 18:58
Oh yes - I've been bitten enough times that I now set a safety reboot (shutdown -r +10) before doing any network-affecting stuff on a remote host. Great if you trust the machine to reboot correctly since if you break something it will come back with its boot-time config pretty quickly :) –  voretaq7 Jun 14 '10 at 21:29
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I believe this happens because the ifconfig command is actually deleting the old IP and creating the new one, not just changing it. "It's a feature!"

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I was looking for a way to keep the route to default gw in place, but since I saw this thread and tried on my own I guess there's no way. The only thing I had to do is set default gw manually right after setting the ip:

sudo ifconfig eth0 subnet
sudo route add default gw

so keep in mind if you're doing this remotely better run both commands with && or you'll get stuck out

sudo ifconfig eth0 subnet && sudo route add default gw
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