cable modem + switch dhcp problem
 

Hi,

I'm trying to assign the public ip to a virtual machines guest os
by using a bridge and dhcp in linux. When connected I can
see broadacst traffic on the network, but when running the dhcp
client from the guest os no reply is seen. The dhcp discover
requests can be seen on the network.

I've also tried connecting the cable modem and a laptop
using a hardware switch, but I get the same problem.
This time running dhcp client using the laptops physical interface.

If I remove the switch and plug the modem directly into the
laptap, dhcp works fine.

Is there any reason why the modem can't be bridged ?

Thanks
Duncan

Re: cable modem + switch dhcp problem
 

:welcome: to the forum. Are you running a DHCP server on the host machine? If you are just doing a bridge from the VM to the cable modem then that may be the problem.

Re: cable modem + switch dhcp problem
 

VM residential cable modems allocate one WAN/Public IP, they also "bond" themselves to one MAC address. Which in your case in the MAC address of the physical host machine. The only way to force a re-association to a new MAC address it to power cycle the modem during kit swaps.

Connecting your cable modem to your PC via a switch will break the MAC association, as the cable modem will see the MAC address of the switch port that's connected to your cable modem. No point doing this anyway as the cable modem will only talk to one piece of equipment anyway.

Assuming the client and guest OS's NIC's are properly bridged, I suspect your cable modem will need a reboot so that it can associate itself with the MAC of the bridge.

One extra thought: I'm not 100% sure what MAC addy your CM will try and associate to, the MAC of the bridge or the MAC of the physical interface on the host machine, can't remember if one replaces the other.

Re: cable modem + switch dhcp problem
 

Ok, this might make sense now. It looks like the modem will associate
with the physical interface of the machine. If it will only accept frames
with this mac address then what I'm trying to achieve will not work
as it will see the mac of the guest os. So basically, you can't bridge the modem ?
Btw, the modem was power cycled each time it was plugged into a different
device.

Thanks
Duncan

Re: cable modem + switch dhcp problem
 

No

I suppose you could change the MAC of the bridge to be the same as the physical interface, but i'm not even sure this would even work bearing in mind the implication for ARP requests, etc.

Re: cable modem + switch dhcp problem
 

Ok, there is a hack to get past this.
I let the modem associate with the machines physical nic, then take
the interface down and assign a different mac address to it. Then
start the guest os up with the mac address just removed from the physical
interface. This seems to work without any problems. The guest os now
acquires the public ip though dhcp.

Thanks
Duncan