Running Alpine Linux As a QEMU networked Guest

To run an Alpine Linux iso from within qemu is trivial:

 qemu  -boot d -cdrom alpine*.iso

To get networking running correctly, you can use the tun/tap interface, which then becomes a real interface. The key is to define the virtual network interface on the correct virtual vlan, and the correct ifup script.

You need 2 net commands on the command line interface, one for the host:

  -net tap,vlan=[somenumber],ifname=[host if],script=[some script]

one for the guest

  -net nic,vlan=[samenumber]

So to have a single nic on the virtual box that is connected to tap0 on the physical host:

 qemu -net tap,vlan=0,ifname=tap0,script=./qemu-ifup -net nic,vlan0 \
    -boot d -cdrom alpine*.iso

To create a qemu guest with more than one nic, just repeat the -net commands

 qemu -net tap,vlan=0,ifname=tap0,script=./qemu-ifup -net nic,vlan0 \
      -net tap,vlan=0,ifname=tap1,script=./qemu-ifup -net nic,vlan0 \
      -net tap,vlan=0,ifname=tap2,script=./qemu-ifup -net nic,vlan0 \
      -boot d -cdrom alpine*.iso

Now your alpine guest will have 3 nics, mapped to tap0, tap1, and tap2 respectively.

What's actually happening is you are effectively creating a point-to-point tunnel, with the phys tap0 device being one endpoint, and the virtual box's eth0 being on the other point of the tunnel.

So you need to assign ip addresses to BOTH sides of the tunnel. The qemu-ifup script is what does that for the host. Here's an example:

 #!/bin/sh
 case $1 in
       tun0 | tap0 )
               sudo /sbin/ip addr add 192.168.1.100/24 dev $1
               sudo /sbin/ip link set $1 up
               ;;
       tap1 | tun1 )
               sudo /sbin/ip addr add 192.168.2.100/24 dev $1
               sudo /sbin/ip link set $1 up
               ;;
       tap2 | tun2 )
               sudo /sbin/ip addr add 192.168.3.100/24 dev $1
               sudo /sbin/ip link set $1 up
               ;;
       esac

In your alpinebox, create an interfaces file like this:

 iface eth0 inet static
       address 192.168.1.1
       netmask 255.255.255.0
       gateway 192.168.1.100

 iface eth1 inet static
       address 192.168.2.1
       netmask 255.255.255.0

 iface eth0 inet static
       address 192.168.3.1
       netmask 255.255.255.0

If on your host you now add a MASQUERADE rule for tap0 to your host's default nic, and you turn on ip_forward on your host, you can now do svn updates, surf, run tranmission, etc right from your qemu guest.