[nycbug-talk] FreeBSD Dual homed

Brian A. Seklecki lavalamp at spiritual-machines.org
Thu Dec 20 16:32:24 EST 2007

Okay dual verizon adsl lines.  A common question.

I'm assuming that you're trying to serve certain services off-of 
150.210.240/24 and wondering why packets from 3rd party nework sources get 
there but not reply.

There can only be one default gateway.  The gateway selection decision is 
not based on any type of "state" table (by default), so:

1) A packet received on em0 with a foreign network source address and a 
destination address of 150.210.240.x/32 will be received, decapsulated, 
processed by the kernel -- the reply packet will be generated and 
transmitted out of em1 with a source address of 150.210.160.xxx/32 and a 
destination of [foreign].

Its just the nature of the TCP/IP stack and "standard" routing.

I've seen discussion on misc at openbsd.org on a pf(4) feature that somehow 
works around this.


> Internet:
> Destination        Gateway            Flags    Refs      Use  Netif Expire
> default      UGS         0      415    em1
> 150.210.160/24     link#2             UC          0        0    em1
> 150.210.240/24     link#1             UC          0        0    em0
>     00:0c:29:62:78:63  UHLW        1        6    em0    547

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