> I want to do source address based routing for some particular IPs.

I have a pseudo-interface driver that does exactly this:

[Truly-Delicious - root] 75> netstat -rn -f inet | egrep srt0
default            10.0.0.1           UGS         3   468844   1500  srt0
10.0.0.1           216.46.0.70        UH          1        0   1500  srt0

My default route points out an srt interface.  The interface is
configured as

[Truly-Delicious - root] 76> ifconfig srt0
srt0: flags=11<UP,POINTOPOINT> mtu 1500
	inet 216.46.0.70 --> 10.0.0.1 netmask 0xffffffff

[Truly-Delicious - root] 77> srtconfig srt0
0: 216.46.5.1 /32 rl0 216.46.5.9
1: 216.46.0.70 /32 ppp0 216.46.1.10

The srtconfig output indicates that packets whose source address is
216.46.5.1 are sent out rl0 with next-hop address 216.46.5.9; those
whose source address is 216.46.0.70 go out ppp0 with next-hop address
216.46.1.10.  (This is actually outdated; the next-hop address for ppp0
should be different.  The only reason this works is that ppp0 ignores
the next-hop address.)  Packets with other source addresses are
silently dropped; that machine should not be sending packets with any
other source addresses down its default route.

> But, how do I do this with an ethernet?

Install sys/dev/pseudo/if_srt* from my patch tree? :)

This assumes it's IPv4.  I haven't yet taught it about IPv6.

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