[lug] Two NICs, one IP

[Scott Herod]

On Wed, 11 Aug 2004, Jeff Schroeder wrote:

> Hey all--
> 
> I manage a Linux server for a client, and he has asked me to set things 
> up so the server will be able to withstand the failure of a physical 
> cable on the LAN.  The server has two NICs (eth0 and eth1, in 
> Linuxspeak) and he wants both to have the same IP address.
> 
> I don't believe this is possible, since the box won't know where to 
> route outgoing packets (eth0? eth1? both?), and I've seen instances 
> where a network interface won't even come up if the system thinks 
> there's another machine with that IP on the network.
> 
> However, before I tell him it isn't possible, I wanted to confirm it 
> with the group here.  I don't have any hard-and-fast documentation to 
> support my claim, but it just doesn't seem right to me.  Can anyone 
> tell me for sure that it's not possible?  Or, if it is, how to do it 
> properly?
> 
> TIA,
> Jeff

The system that I am working on is set up this way.  (We have two
interfaces connecting two machines that use the linux-ha package to add
reliability.)  Basically it uses the brctl (bridge-utils RPM) command to
create a bridge and add physical interfaces to it.  In order to avoid
network loops you may have to turn on brctl's spanning tree protocol.  
The commands are roughly:

PMI_BRIDGE=pmi_if

INTERFACE_1=eth1
INTERFACE_2=eth2

ifconfig ${INTERFACE_1} 0.0.0.0
ifconfig ${INTERFACE_2} 0.0.0.0
# Create the bridge interface.
brctl addbr ${PMI_BRIDGE}
# Add interfaces to the bridge.
brctl addif ${PMI_BRIDGE} ${INTERFACE_1}
brctl addif ${PMI_BRIDGE} ${INTERFACE_2}

# Bring up the bridge.
ifconfig ${PMI_BRIDGE} ${HEARTBEAT_ADDR} netmask 255.255.255.0 broadcast 1.2.3.255 up

# Add route for the communication
/sbin/route add -net 1.2.3.0 netmask 255.255.255.0 dev ${PMI_BRIDGE}


Use "brctl delif" to remove interfaces.  The person setting up the bridge 
did have to modify the bridge module in kernel 2.4.20 to make the spanning 
tree work correctly.