virtual network with qemu

marco.borsatino at marco.borsatino at
Thu May 14 11:39:19 UTC 2009

First, thank you.
You are right, qemu has an internal DHCP server, which should be perfect for me, becuse I would like to emulate a network without any contact with external (real) world. The problem is that my virtual PC can't ping the gateway. For my idea (this is just a way to study a project for a network without a real network) the communication is intended only among virtual PCs. But If I can't contact the (virtual) gateway will it be possible to contact another virtual PC on a different subnet? Or even on the same subnet? In my idea I would like to create a little but complex net with one master controller, a slave controller, a little number of client belonging to different subnets. Maybe with or without a DHCP server.

---------- Original Header -----------

>From      : "Nikos Vassiliadis" nvass9573 at
To          : "marco.borsatino at" marco.borsatino at
Cc          : freebsd-questions at
Date      : Thu, 14 May 2009 11:28:59 +0300
Subject : Re: virtual network with qemu

> marco.borsatino at wrote:
> > When the installation program asked for information about network configuration, as a 
> > first step, I chose DHCP configuration and, as usualy, the network has been set
> > like this:
> > IP
> If I recall correctly qemu has a built-in DHCP server.
> That's the one that served you, not a "real" DHCP server
> running on your LAN, that is, you are not in any way connected
> to the "real" network.
> > #qemu -L /usr/local/share/qemu -localtime -net nic,macaddr=00:15:f2:44:2d:01 -net socket,mcast= -hda pc01.img -cdrom /dev/acd0 &
> > but the network in the guest system does not work.
> It makes sense, that the multicast option will work
> between virtual hosts only. That is, it uses multicast
> to provide a virtual broadcast domain, which appears to
> the host operating system as a ethernet device.
> > ifconfig in the guest system tells:
> > #ifconfig -a
> > ed0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
> >   ether 00:15:f2:44:2d:01
> >   media: Ethernet 10baseT/UTP <full-duplex>
> > plip0: ...
> > lo0: ...
> > If I try:
> > #ping (the gateway)
> > all packets are lost. For this reason, I've tryed a static IP configuration like this:
> > IP
> > gateway
> > nameserver
> > but the gateway does not respond. So it is useless to try with a second guest system.
> No, infact it's the exact opposite. This type of device
> will work *only* if you add another virtual system.
> To get connected to the "real" network, you must use tap
> devices. Correction:
> Browsing the qemu's wiki I found out that there is a newer
> and simpler approach that I am not familiar with:
> So, if you do want internet access, just remove all network
> associated options and it will work automagically. If you just
> want to connect guest systems together use multicast or socket
> or ...
> HTH, Nikos

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