[libvirt-users] a VM does not get dhcp

lejeczek peljasz at yahoo.co.uk
Thu Nov 12 10:30:52 UTC 2015

On 10/11/15 19:58, Ken D'Ambrosio wrote:
> 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:
yes, that - virtual NIC (libvirtd's bridge route mode) - I 
realize might have not been explicit, there is the second 
I' try to sniff that traffic.
> * 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.
>> _______________________________________________
>> libvirt-users mailing list
>> libvirt-users at redhat.com
>> https://www.redhat.com/mailman/listinfo/libvirt-users

More information about the libvirt-users mailing list