[Nottingham] NIC MACs aliases & IP aliases

Martin martin at ml1.co.uk
Wed May 13 19:45:06 UTC 2009

Graeme Fowler wrote:
> Go back to the "clean" system. Bring up the ethernet and two tun
> interfaces:
> tunctl -t tap0
> tunctl -t tap1
> ifconfig eth0 up
> ifconfig tap0 up
ifconfig tap1 up
> At this point you have the three interfaces running "bare" - they're up,
> but unconfigured. Now bring up the bridge:
> brctl addbr br0
> brctl addif br0 eth0
> brctl addif br0 tap0
> brctl addif br0 tap1
> ifconfig br0 up
> So the bridge itself is up, running and configured. Can you now reach
> your default gateway or other devices on the same network?

Yep, when going via br0.

> If you now "tcpdump -qnitap0" while traffic is hitting br0 from a
> separate machine on the same network, do you see that traffic being
> "sent out" of tap0?

OK... Pinging from the gateway, I can ping any of br0, tap0 or tap1 by 
IP address and the reply always comes back from the MAC address of the 
eth0. (br0 has the same MAC address as eth0.)

The tcpdump for tap0 shows repeatedly for example:

18:44:53.722965 arp who-has tell

> Once you've got the basic bridge up and running you can start tinkering
> with IP addresses!

OK, so outside inwards looks ok for pings, and outwards is fine via the 
br0. Still no beer for making the eth0 and br0 transparent to reveal the 
tap0 and tap1 MACs...

Errr, more arp tweaks?


Martin Lomas
martin at ml1.co.uk

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