[PVE-User] NAT issues

Flavio Stanchina flavio.stanchina at ies.it
Fri Oct 19 14:39:15 CEST 2012

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.

Flavio Stanchina
Informatica e Servizi
Trento - Italy

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