[libvirt-users] a VM does not get dhcp

[Ken D'Ambrosio]

Okay.  I may be missing something; from what you explicitly write about 
subnets, it sounds like you only have one, but if you don't have two 
subnets, why do you need a DHCP relay?  SO.  Assuming you have two 
subnets:

* It sounds as if your DHCP request is generating an ACK.
* I'm 99% sure your ACK has to traverse the DHCP helper and wend its way 
back to the originating host.

So I'd focus on the DHCP relay, to be sure that the ACK with lease info 
is both arriving at it, and then being unicast to the MAC.  tcpdump on 
respective interfaces should do the trick (something like "tcpdump -i 
<dhcp-server-facing interface> port 67" or somesuch to see if it 
arrives, then swap it around to the other interface to see if it 
leaves).

Good luck!

-Ken

P.S.  I don't claim to be an expert, but I like to think I have a decent 
handle on DHCP.  If someone sees a flaw in my thinking, please do pipe 
up -- I admit that this virtual stuff can be counterintuitive at times.


On 2015-11-10 12:11, lejeczek wrote:
> hi everybody
> 
> I'm looking at my setup and got stuck
> 
> I have a box with two NICs on the same subnet, and another box
> similar, also two NICs on one subnet, all four NICs are on the same
> subnet.
> 
> Now, that second box has also a virtual NIC (libvirtd's bridge route
> mode) and VMs guests are using it, traffic to that virtual net is
> routed via 1st real NIC. This second box dhcrelays to the first
> box(dhcpd).
> 
> I see box-dhcrelay forwards to box-dhcpd, I see box-dhcpd receives and
> offers a lease but that VM guest does not get it.
> 
> I have policy routing manually set in place so both boxes can ping
> each other all NICs. (including virtual NIC on the box-dhcrelay)
> Moreover, that VM guest can ping both boxes' all NICs when its IP
> address is set to manual.
> 
> It's RHEL7 and I'm only trying IPv4.
> I'm hoping some can rule out (or suggest what might be broken in)
> libvirt/policy based routing.
> I see those offers box-dhcpd makes are exactly for the subnet of
> box-dhcrelay's virtual NIC/subnet.
> 
> It's a pickle. An expert's thought would be great to hear.
> 
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