if_bridge with two subnets

[Jay L. T. Cornwall]

H.fazaeli wrote:

> It does work. However, if I understand your setup correctly, the freebsd 
> box
> has been setup to act as a bridge, not as a router (routing is enabled with
> sysctl net.inet.ip.forwarding=1). Bridging works when the forwarding is
> between the same subnets.
> 
> For freebsd box to route between subnets:
> - enable routing: sysctl net.inet.ip.forwarding=1
> - clients must use the freebsd box as gateway.
> - IP addresses must be removed from the bridge and assigned to
>  the member interfaces. (the bridge is no longer needed).
> 
> You may have bridging & routing on the same box at the same time but
> note that a single packet coming into the system either goes through
> bridging _or_ routing code, but not both. The former case happens
> if packet's destination MAC address is not that of box. The
> latter case happens when destination MAC address is that of receiving
> interface.

Thank you very much for your explanation. I had misunderstood precisely 
how routing and bridging are done in FreeBSD but it now makes sense.

> If you provide a network diagram along with your requirements,
> we can better discuss the matter.

I now have a working network configuration. For completeness I will 
explain how it's set up.

I have a small, publicly routable netblock to serve a larger LAN of 
machines. Thus some of the machines draw IPs from a non-routable private 
pool and are NAT'd to a one of the public IPs. It looks like this:

[Gateway] XX.XX.XXX.22
     |
[FreeBSD] XX.XX.XXX.20-21
     |
[L2 Switch]
     |
    PCs    XX.XX.XXX.17-19
           192.168.1.0/24

The gateway must have one of the public IPs to communicate with its 
upstream correctly. On the other side of the FreeBSD server the rest of 
those IPs are used. So a traditional gateway setup would not work here: 
it would imply that the FreeBSD server has two interfaces from two 
different subnets, rather than one split subnet. Perhaps a static route 
for the gateway would work, but it would be messy.

So I bridge the two interfaces to join the public subnet. Following your 
advice I have set the FreeBSD server to the network gateway - previously 
it was the .22 gateway - and now all of the LAN PCs can communicate 
without additional routing information. Inter-subnet packets will bounce 
off the FreeBSD server, rather than staying inside the L2 switch, but 
that's OK.

-- 
Jay L. T. Cornwall
http://www.jcornwall.me.uk/