Re: xen networking

[Dima Veselov <kab00m%lich.phys.spbu.ru@localhost>]

On Fri, Jun 05, 2020 at 02:42:55PM +0100, Iain Hibbert wrote:

> I have a xen dom0 with external connectivity and wish to set up NAT to
> allow the domU network access but having a little difficulty with network
> setup. I have tried several variations and always hit a wall eventually.
> This is where I'm currently at:

If I understand right - you have one dom0 with several domU's and one
domU acting as a router/NAT for other domUs.

Real network - wm0 (dom0) br0 - xennet0 (domU) xennet1 - br1 (dom0) br1 - domU xennet0

> domU-router has dnsmasq set up to provide IP addresses onto bridge1 and
> this works fine, I can ping back and forth using hostnames. I also have
> dom0 ask for an IP on this network (might NAT that to a separate network
> instead, later)
>
> So currently I am stuck. I want to have domU-router get the IP address to
> the external interface with dhcpcd. Then bridge0 will do its job and
> domU-router will be the front end, right? (if so then I set up NAT)

Something stay undisclosed here. You say dnsmasq is working good but then it
sounds domU-router can't get xennet0 IP. Then what means it works good?

> Unfortunately, I think, if I get domU-router to issue a DHCP request, what
> happens is that it goes out onto bridge0 with the domU-router MAC address
> as source. 

Do you want domU-router to get an IP with mac-address of dom0 as it sounds?
This will never happen, bridge is like a switch - every server have its own
unique mac. 

There should be something about your network configuration. I suppose you
make this setup complexed because you have some limitations in real network
connected to wm0, but we need to know what they are.

As it sounds to me - DHCP server which you try to use accept only dom0's wm0
mac address. If it is so - you can either use dom0 as a router/NAT instead
or swap mac addresses between dom0 and domU-router.

> I can see it with tcpdump on wm0 but I don't know if it
> actually goes out on the wire, and nothing ever comes back.

Once you see it on dom0 wm0 you can be sure its on the wire. tcpdump capture
output packets after all processing. There are some problematic cases when
its not true but I would check it only as a last shot.

--
Sincerely yours,
Dima Veselov
Physics R&D Establishment of Saint-Petersburg University