From mboxrd@z Thu Jan  1 00:00:00 1970
From: arnd@arndb.de
Subject: Re: Multiple TAP Interfaces, with multiple bridges
Date: Wed, 3 Feb 2010 18:16:01 +0100
Message-ID: <201002031816.02035.arnd@arndb.de>
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To: J L <lists@rrod.net>
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On Wednesday 03 February 2010 17:56:43 J L wrote:
> I am having an odd networking issue. It is one of those "it used to
> work, and now it doesn't" kind of things. I can't work out what I am
> doing differently.
> 
> I have a virtual machine, started with (among other things):
>   -net nic,macaddr=fa:9e:0b:53:d2:7d,model=rtl8139 -net
> tap,script=/images/1/ifup-eth0,downscript=/images/1/ifdown-eth0
>   -net nic,macaddr=fa:02:4e:86:ed:ce,model=e1000 -net
> tap,script=/images/1/ifup-eth1,downscript=/images/1/ifdown-eth1

This seems to be missing a vlan= option at least for the second pair:

> What I actually get:
>   * VM: eth1, dest MAC of Host's tap1/br0
>   * Host: tap1, dest MAC of Host's tap1/br0
>   * Host: br1, dest MAC of Host's tap1/br0
>   * Host should, but does not route from br0 to br1
>   * Host: tap0, dest MAC of ***Host's tap1/br0***
>   * Host: br0, dest MAC of ***Host's tap1/br0**
>   * Host: eth0, no packet
>   * Server: eth0, no packet
> 
> As you can see, the packet has egressed both tap interfaces! Is this
> expected behaviour? What can I do about this?

Qemu forwards this packet to everything inside of the same vlan, which
is 0 by default. Does it work with this?

   -net nic,vlan=1,macaddr=fa:9e:0b:53:d2:7d,model=rtl8139 -net tap,vlan=1,script=/images/1/ifup-eth0,downscript=/images/1/ifdown-eth0
   -net nic,vlan=2,macaddr=fa:02:4e:86:ed:ce,model=e1000 -net tap,vlan=2,script=/images/1/ifup-eth1,downscript=/images/1/ifdown-eth1

> If I remove tap0 from the bridge, I then get:
>   * VM: eth1, dest MAC of Host's tap1/br0
>   * Host: tap1, dest MAC of Host's tap1/br0
>   * Host: br1, dest MAC of Host's tap1/br0
>   * Host should, but does not, route from br0 to br1
>   * Host: tap0, no packet
>   * Host: br0, no packet
>   * Host: eth0, no packet
>   * Server: eth0, no packet
> 
> This is the other half of my problem: in this case, with effectively
> only one tap, the host is not routing between br1 and br0. The packet
> just gets silently dropped. Does anyone know what I am doing wrong?

Maybe /proc/sys/net/ipv4/ip_forward is disabled?

	Arnd