[Nottingham] NIC MACs aliases & IP aliases

Martin martin at ml1.co.uk
Sat May 23 12:44:41 UTC 2009

Martin wrote:
> Chris Burton wrote:
> [---]
>> http://www.primianotucci.com/default.php?view=57
> Well... This one is proving to be a strange one indeed.
> ... And, it seems to work, sort of...
> Sending out data through one of the tap interfaces gives the MAC address 
> of that tap. Yea, it works!!!
> Pinging from externally the IP address of whichever tap also works, so 
> all looks good.
> BUT... "arp -e" still stubbornly lists the same one eth0/br0 MAC address 
> for all the host IPs.
> Also, there are "martian source" in the log for all the tap interfaces 
> except tap0.
> ifconfig shows that each tap has it's own unique MAC.
> Sooo... It /might/ be working although the arp table is still not 
> listing the expected MACs.
> Anyone with any ideas?

OK, so playing with:

# arp_accept
# arp_announce
# arp_filter
# arp_ignore
# forwarding
# proxy_arp

and I've managed to properly break it :-p

The various comments on:


suggests the default behaviour that I've seen for the defaults. (See the 
arp_announce and arp_ignore.)

So what is the magic combination of settings to get proxy_arp (and/or 
whatever else is needed) to work transparently across eth0 -- br0 -- 
tap0 so that the other taps become visible to a remote machine. Or is 
the MAC address of the eth0 actually the correct one to give for return 

To summarise:

eth0 -- br0 -- tap0 -- ( tap1 tap2 tap3 tap4 tap5 )

Outgoing packets via tap[1-5] show the MAC of the respective tap interface.

However, on a remote machine, the IP addresses of tap[1-5] have the MAC 
of eth0/br0 in that host's arp table.

Any ideas?

Or do I not need the remote host arp table to list unique MACs to still 
gain multiple DHCP addresses for the one host? Are the unique source 
MACs enough to work ok?

But then, there's still the question of how to get the eth0 -- br0 -- 
tap0 to appear transparent just like a true network switch...


Martin Lomas
martin at ml1.co.uk

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