i used to add proxy arp's on the router when i had problems like 
this.  Dunno if it's the recommended fix, but it worked.

http://en.wikipedia.org/wiki/Proxy_arp

Carlos Narváez wrote:
quoted text
>  This is starting to frustrate me, because it should be much simpler
> than it seems to be, and I feel like I'm missing something small and
> obvious.
> 
> I have two private networks, we'll say 192.168.254.0/24 and
> 192.168.251.0/24. And I have a linux box in the middle with addresses
> 192.168.254.17 and 192.168.251.10:
> 
> 
> +---------------+ . +----------------+
> ¦ 192.168.251.1 +---+ 192.168.251.10 ¦ . +----------------+
> +---------------+ . ¦ 192.168.254.17 +---+ 192.168.254.16 ¦
> . . . . . . . . . . +----------------+ . +----------------+
> 
> 
> There is no NAT involved.. I just want the box in the middle to pass
> traffic between the two networks. Here is what I have done:
> 
> - IP Forwarding has been enabled on the router via "echo 1 >
> /proc/sys/net/ipv4/ip_forward"
> 
> - A route has been configured on 192.168.251.1 to point all traffic
> for 192.168.254.0/24 to 192.168.251.10.
> 
> - A route has been configured on 192.168.254.16 to point all traffic
> for 192.168.251.0/24 to 192.168.254.17.
> 
> - The command "iptables -I FORWARD -j ACCEPT" has been executed.
> 
> Now.. here's what happens. 192.168.251.10 can ping both interfaces on
> the router. 192.168.254.16 can also ping both interfaces on the
> router. However, 192.168.251.1 cannot ping 192.168.254.16, and
> likewise, 192.168.254.16 cannot ping 192.168.251.1.
> 
> What have I forgotten?
> 

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