Re: [Qemu-discuss] Networking and vlan

[Mike Lovell]

On 04/17/2012 08:35 AM, Valerio Pachera wrote:
> Hi all, I'm trying to figure out how to manage networking in qemu.
>
> I already use qemu-kvm 0.12.5 on debian squeeze.
> I have a bridge on eth0 that "connects" the taps of the guests.
> -net nic,macaddr=xx.xx.xx.xx.xx.xx -net tap
> Now I need something to isolate the guests net from each other.
> So I tried to run a guest with
> -net nic,macaddr=xx.xx.xx.xx.xx.xx,vlan=2 -net tap,vlan=2
> But I can still ping the other guests (and the host).
>
> I know the sintax I used is obsolete. I read this:
> http://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDYQFjAB&url=http%3A%2F%2Fwiki.qemu.org%2FDocumentation%2FNetworking&ei=eHSNT9H2Au_44QTZ7ZG1Dw&usg=AFQjCNEDsUHr6iu7osfwPKn_YRFpq2MoMw
>
> May you explain me by simple words how vlan work and what are they usefull for?

questions related to this are very common. there is a clue in the page 
you linked about this under "Creating a network backend." it says "It is 
not an 802.1q VLAN."

vlans in the context of the qemu refers to a qemu internal construct to 
allow one or more network backends to be connected to one or more guest 
network interfaces. it also behaves like an ethernet hub in that all 
packets emitted from one device (network backend or guest interface) on 
the 'vlan' will be delivered to all the others.

these are useful for specifying multiple networks that a guest may be 
using. for example, say a guest needs to connect to two tap devices on 
the host, tap0 and tap1. these two tap devices connect to different 
networks. if you were to just specify

'-net tap,ifname=tap0 -net nic,macaddr=xx.xx.xx.xx.xx.xx -net 
tap,ifname=tap1 -net nic,macaddr=yy.yy.yy.yy.yy.yy'

the result would be that both of the network backends and the guest 
interfaces would all be connected together in 'vlan' 0. to isolate the 
two networks, you would use

'-net tap,ifname=tap0,vlan=0 -net nic,macaddr=xx.xx.xx.xx.xx.xx,vlan=0 
-net tap,ifname=tap1,vlan=1 -net nic,macaddr=yy.yy.yy.yy.yy.yy,vlan=1'

again. these are not 802.1q vlans. and they do not extend outside of the 
individual qemu processes. running a qemu guest with using vlan 1 would 
not mean that the vlan 1 on another qemu guest using vlan 1 in its options.

i hope that gives some clarification.

mike