Re: [Xen-users] Basic Network Bridge

[Eric Lindsey <eslindsey@xxxxxxxxx>]

This from dom0 (www):
15:39:30.506858 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP,
Request from 00:16:3e:5a:65:3c, length 300
15:39:30.506993 IP 10.1.20.1.67 > 10.1.20.110.68: BOOTP/DHCP, Reply, length 305

Haha I can't install tcpdump on domU because it has no Internet access.
Assigned static information on domU in /etc/network/interfaces, and
everything is golden.  DNS works, Internet works, network works!
So, now that I've used static information to download tcpdump on guest
domU, let's switch back to DHCP and see what happens...

It looks like domU is seeing the DHCP reply, but not acting on it.
Here's from domU (note this is a mix of output from ifup and tcpdump):
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 4
15:50:07.082944 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP,
Request from 00:16:3e:5a:65:3c, length 300
15:50:07.083135 IP 10.1.20.1.67 > 10.1.20.110.68: BOOTP/DHCP, Reply, length 305
15:50:08.354312 IP6 fe80::9006:8743:d996:7029.57926 > ff02::c.1900:
UDP, length 146
15:50:11.354221 IP6 fe80::9006:8743:d996:7029.57926 > ff02::c.1900:
UDP, length 146
No DHCPOFFERS received.
Unable to obtain a lease on first try.  Exiting.
Failed to bring up eth0.

Is my dom0 trying to send the DHCPOFFER directly to a specific IP
address (10.1.20.110 from the look of things) when the eth0 interface
on domU doesn't have an address yet (hence the DHCP request)?

On Fri, Jun 22, 2012 at 12:53 AM, Alexandre Kouznetsov <alk@xxxxxxxxxx> wrote:
> Hello.
>
> El 21/06/2012 09:21 p.m., Eric Lindsey escribió:
>
>> My LAN computers are now successfully getting DHCP from www (my server)
>> again and are reaching the Internet.
>
> I assume, "www" is your Dom0, so the DHCP server is functional, after all.
>
>
>> Now, when I boot the VM I can see once again in my dom0 logs, a
>> DHCPREQUEST on br0, and a DHCPOFFER also on br0. But the VM never sees it.
>>
>> How can I use tcpdump to try and track down this problem?
>
> On Dom0, run "tcpdump -ni br0" and watch for DHCP negotiation. You should
> see at least the request and the offer. Then, do the same on your DomU,
> using appropiate interface to sniff on. As you describe it, you probably
> will not see the DHCPOFFER.
>
> That would isolate the problem to the DomU's failure to recieve traffic.
> iptables INPUT rule prohibiting it, maybe?
>
> What about static IP configuration on DomU, does it works?
>
>
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