Linux KVM (virt) with DHCP and bridging

Serge van Ginderachter serge at
Wed Apr 21 06:48:22 UTC 2010

On 21 April 2010 03:27, <mailers at> wrote:

> auto br0
> iface br0 inet dhcp
>        pre-up /usr/sbin/ethtool -s eth0 speed 100 duplex full autoneg off
>        bridge_ports eth0
>        bridge_fd 9
>        bridge_hello 2
>        bridge_maxage 12
>        bridge_stp off
> Which all seems fine.. the link state is all fine there are no errors on
> the switch. But we still can't get an IP address despite it working
> perfectly in Windows 7 or default Linux. There are no iptables rules or
> anything like that which would possibly cause problems. If I run
> sudo tcpdump -i br0 -vvv -s0 ether host 54:52:00:22:d7:8f
> I can see that the DHCP requests are going out without any problems but I
> generally get an IP Address assigned then followed by a NAK. I've disabled
> the DHCP server and installed a variety of other dhcp servers to identify
> where the problem is and I get the same problems no matter what the
> configuration is.. Has anyone seen similar issues or knows where I can start
> to look further to diagnose the problems?

Check that your interface is in forwarding state with command:
brctl showstp br0

If it isn't you might have forgotten to make sure you also have this in the
interfaces file:

auto eth0
iface eth0 inet manual


    Met vriendelijke groet,

    Serge van Ginderachter
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