Configuring transparent bridge

[Karl]

I want to explore Zentyal's capabilities for existing networks where all the usual infrastructure is already in place (where Zentyal may not be used as a gateway or typical server).  According to the Zentyal docs:

The bridged mode consists of associating two physical network interfaces attached to your server that are connected to two different networks. For example, one card connected to the router and another card connected to the local network. By using this association you can redirect the network traffic transparently from one card to the other.

The main advantage here, is that client configurations do not need changing when the Zentyal server gateway is deployed. Traffic that passes through the server can be managed using content filtering or the intrusion detection system.

I understood that to mean Zentyal could operate as a layer 2 segment bridge, meaning with two NICs it could be connected inline between a gateway router and the rest of the network and transparently apply content filtering.  In that configuration, the same subnet would be used on both interfaces.

I'm trying to configure this in a virtual lab environment where I have eth0 bridged to my main network and eth1 on a virtual segment where another VM will use it for internet access.  I've set eth0 and eth1 in bridged mode, creating br1.  I've tried br1 with both DHCP and static addressing (it shouldn't really matter in this scenario).

What's happening is I have no connectivity through br1.  My VM on the private segment can ping its br1 IP (connected to eth1) but Zentyal cannot ping any host regardless of which physical interface it's on.  The only Zentyal modules enabled are network, users and groups, and firewall (tried initially without firewall but then I thought maybe iptables had to be engaged for it to work properly -- made no difference).  I also noticed after setting this up that eth1 and br1 share a MAC address.  I don't know if that's normal or not.

How exactly is this supposed to be configured?

[Karl]

If I reboot after configuring the interfaces in bridge mode, br1 gets an IP address, but... Zentyal stops listening on port 443 and the web interface is no longer accessible, and I'm still negative for connectivity.

[Karl]

I added eth2 as a host-only interface in virtualbox and got around the "vanishing web GUI" bug.  When it disappeared before, it couldn't even be accessed on localhost.  It was quite literally not listening on 443.  This time it stayed up and was accessible on both eth2 and lo.

When I first set up the bridge, br1 gets an IP via DHCP, but has no connectivity otherwise.  Can't even ping the gateway that assigned its IP.  After a reboot, br1 no longer gets an IP.

Also, my interfaces are fully functional.  If eth0 is set to DHCP instead of bridged, it has internet access.  If eth1 is set to static, it can communicate with the other VM on the virtual segment and also provide internet access to it.  And I did try setting eth0 to NAT instead of bridged in virtualbox, didn't make any difference.  The NIC works both ways on DHCP, but I can't get anything through either interface when bridged.

Any help would be greatly appreciated...