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.
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.
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!"
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 10.0.0.2 subnet 255.255.255.0 sudo route add default gw 10.0.0.138
so keep in mind if you're doing this remotely better run both commands with && or you'll get stuck out
sudo ifconfig eth0 10.0.0.2 subnet 255.255.255.0 && sudo route add default gw 10.0.0.138