[CentOS] Dual personality Server (network pass through)

I have 3 nics in a NATed gateway file server. Two nics (eth1, eth2)
provide dhcp and nfs services to an internal subnet via a dedicated
switch. The other nic (eth0) connects to an external WAN switch to
provide net access to the systems in the subnet.

                               --- eth1
WAN switch eth0 --- |               SUBNET switch
                               --- eth2

Question:
Is it possible to disable the NATing, nfs, dhcp and just somehow
bridge the external WAN nic to the internal ones such that it's just a
pass through? Basically having the server behave like a switch?
Allowing the internal systems to join the network on the WAN. I know
how to disable NAT, nfs and dhcp but not how to configure the nics.

I know I could simply unplug eth1, eth2 from the server and plug them
into the WAN switch but my goal is to script this so I don't have to
physically plug and unplug network cables each time.

Thanks

-- 
Robert Arkiletian
Eric Hamber Secondary, Vancouver, Canada