[SOLVED] Qemu and host-only networking

Question

Hi,
I want to setup qemu with one Windows 7 guest. This guest should be able to connect to the hosts samba and postgres server, except external networks such the internet. I've read the archlinux qemu wiki page and it seems i need to use a bridge and a tap device to accomplish this. So far i've did as root:
brctl addbr br0
ip tuntap add dev tap0 mode tap user me
ip link set tap0 up
brctl addif br0 tap0
'brctl show' gives
bridge name bridge id STP enabled interfaces
br0 8000.16cad3ab6bda no tap0
When i start the vm as a user i get
qemu-system-x86_64 -enable-kvm -m 1024 -vga std -device e1000,netdev=tap0,mac=DE:AD:BE:EF:96:32 -netdev tap,id=tap0,script=no,downscript=no hdd1.qcow2
qemu-system-x86_64: -netdev tap,id=tap0: could not configure /dev/net/tun: Operation not permitted
qemu-system-x86_64: -netdev tap,id=tap0: Device 'tap' could not be initialized
Ok. Need to handle some permission issues. If i launch the above command as root, the VM (Windows 7) starts, but i don't have any connection to the host (Windows 7 complains about an unindentified network).
With the example scripts 'qemu-ifup', 'qemu-ifdown', the ip forwarding and the sudo configuration as described in the QEMU - ArchWiki - Creating bridge manually section, Windows 7 can indeed connect to the host, but also to the internet, what i simply don't want. The reason might sound silly, but everytime a Windows guest tries to connect to the internet, thus trying to obtain an ip from the dhcp server, the host completely looses its connection to the router. It seems my network hardware doesn't like this.
Anyway, i think i'm only missing one little thing. I would appreciate any help.
Last edited by ulixes (2014-06-09 20:51:52)

Answer 1

Thank you for this, I've tried lots of docs&tutorial, finnaly got this and success.
ulixes wrote:
Ok, found a solution. It's surprisingly simple. I've discarded the libvirt way, because the official archlinux virt-manager package is pretty unusable and configuring a working vlan for libvirt seems pretty complicated. I still couldn't get a working network device on the guest side.
For completeness, here is what i've done. I use a tap and a bridge device and dnsmasq. I did not activate ipv4 forwarding like mentioned in the wiki! The following steps need to be done as root.
First i've created the bridge, set an ip address and started it
brctl addbr br0
ip addr add 192.168.179.1/24 broadcast 192.168.179.255 dev br0
ip link set br0 up
The subnet should differ from any other device. In my case the only other device eth0 has the ip 192.168.178.xxx.
Next i've created the tap device for use by qemu respectively the guest
ip tuntap add dev tap0 mode tap
ip link set tap0 up promisc on
I don't know if 'promics' is really necessary, but there's no need to assign an ip to the tap device.
Now i've connected the tap device to the bridge
brctl addif br0 tap0
Until now this was approximately my first try. I've tried to use static ip on the guest side, but had no succeess. The important point was to run dnsmasq on the bridge
dnsmasq --interface=br0 --bind-interfaces --dhcp-range=192.168.179.10,192.168.179.254
In effect, i finally got working ip on the guest network devices (this may take some seconds). Pinging the host succeded and neither of my guests (SliTaz Linux, Windows 7) were able to connect to external networks, thus the internet.
I've started the guests with
qemu-system-x86_64 -enable-kvm -m 1024 -netdev tap,id=t0,ifname=tap0,script=no,downscript=no -device e1000,netdev=t0,id=nic0 <imagefile>
In my case, i had to omit the automated exection of scripts. This way i was also able to execute the above qemu command as an ordinary user.
Just to finalize this description for samba:
By adding 'br0' to 'interfaces' in smb.conf, guests were able to connect to the host samba service as usual.