[PVE-User] NAT issues

Samir Faci (Dev) dev at esamir.com
Fri Oct 19 18:09:00 CEST 2012

I didn't realize that QEmu segregated each NAT-ted VM into it's own
network.  That makes more sense, and it does explain
why multiple machines with different mac address would get the same IP

Though I was surprised at the fact that creating multiple nic on the
same VM still assigned the same IP address to both

As far as debugging... if you can tell me a bit more about what you'd
like to see I'd be happy to pass that info along.

I'm going to try the suggested more Manual mode of Natting and see if
it provides the behavior I was looking for.

Thank you for all the help.


On Fri, Oct 19, 2012 at 8:47 AM, Alexandre Kouznetsov <alk at ondore.com> wrote:
> Hello, Flavio.
> Yes, I saw your mail, but was surprised how QEMU networking was involved.
> Proxmox Wiki does not stays NAT _has_ to be manual, but since it mentions
> NAT only in the context of manual configuration... New for me, thank you.
> Sorry for that confusion.
> In Samir's case, it would be great to be able to see some logging or debug
> output of QEMU's DHCP subsystem. Anyway, we'll have to wait for his
> feedback.
> Greetings.
> El 19/10/12 07:39, Flavio Stanchina escribió:
>> Alexandre Kouznetsov wrote:
>>> El 18/10/12 17:01, Samir Faci (Dev) escribió:
>>>> Built in Proxmox support.  I didn't setup a DHCP server.
>>> I did not knew there is a built-in DHCP server in Proxmox NAT networking
>>> model. Did you got this from any documentation?
>> The NAT here is one of QEMU's networking modes and is selectable as an
>> alternative to bridging in PVE's network configuration for a VM. It does
>> contain a minimalist DHCP service for the VM and it *does* work out of
>> the box if networking is set up properly on the host: the NAT is, AFAIK,
>> internal to QEMU's code and doesn't depend on iptables. See mi previous
>> mail in this thread for links to QEMU documentation.
>>>> If I was using a DHCP server then I would be using Bridged mode.  That
>>>> seems to work fine.  I have an issue with the NAT support.
>>>> my understand was that proxmox would allocate IP and handle the
>>>> NATing.
>>> Ok.
>>> http://pve.proxmox.com/wiki/Network_Model#Masquerading_.28NAT.29
>>> describes that in a quite clear way. For NAT mode, you still use the
>>> good old bridge configuration, except that the bridge will not contain
>>> any physical network interface. [...]
>> This describes the setup of "standard" Linux NAT where the PVE host is
>> acting as a NATing gateway for the VMs via iptables, just like any other
>> physical Linux gateway NATing physical machines on an internal network.
>> The only difference is that one of the two interfaces is not a physical
>> Ethernet port, but a bridge that can be effectively described as a
>> virtual switch between the host and the VMs.
>> The two modes do the same thing in practice, the main difference being
>> that using a bridge and doing explicit NAT allows the VMs to see each
>> other as if they were physical hosts connected to the same switch, while
>> QEMU's internal NAT effectively segregates each VM in its own network.
> --
> Alexandre Kouznetsov
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Samir Faci
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