Routing outbound IP packets on multihomed box

Christopher Cowart ccowart at
Fri Jun 15 22:01:43 UTC 2007


I have a server with two NICs:


The default gateway is The router for the 126 subnet is 

netstat -rn:
| Destination        Gateway            Flags    Refs      Use  Netif Expire
| default       UGS         0   102537    em0
|          UH          0      217    lo0
|  link#1             UC          0        0    em0
|     00:15:c7:b9:f4:80  UHLW        2        4    em0   1193
|     00:11:25:ab:42:70  UHLW        1      589    lo0
| 169.229.126/24     link#9             UC          0        0 vlan52
|      00:15:c7:b9:f4:80  UHLW        1       34 vlan52   1200
|      00:18:f8:09:d3:a5  UHLW        1        8    lo0

The IP address on em0 works exactly as one would expect. I have full IP
connectivity to it from other subnets. 

The problem is I can't get 2-way connectivity with the IP address on

Using my workstation on a third subnet (, I cannot
ping I leave the ping running and do some tcpdumps on 
the server.

$ sudo tcpdump -ni vlan526 host
| 14:14:37.002920 IP > ICMP echo 
| request, id 15733, seq 35, length 64
| 14:14:38.003037 IP > ICMP echo 
| request, id 15733, seq 36, length 64

Notice there are no echo replies. That's because they're being sent 

$ sudo tcpdump -ni em0 host
| 14:15:42.006997 IP > ICMP echo reply, 
| id 15733, seq 100, length 64
| 14:15:43.007118 IP > ICMP echo reply, 
| id 15733, seq 101, length 64

I repeated this last snoop with a -w and loaded it into ethereal. The
echo replies being sent out on em0 indeed have a source address of The router ( drops these packets on the
floor, because their source address isn't routable on that interface.

Because routing is based on destination, not source address, I'm not
sure how to get packets sourced from the 126 subnet to the router on the
126 subnet. I tried the following ipfw rule right after allow loopback
traffic (my second rule):

fwd ip from to not

Still no luck. Has anyone set up a multihomed box on *different* subnets
before without routing them through the FreeBSD box? Does anyone have
any pointers or things I should be looking at?


Chris Cowart
Lead Systems Administrator
Network Infrastructure, RSSP-IT
UC Berkeley
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