An ethernet bridge is actually needed by virtual machines to have access to the real network.

The following commands suppose that you can type these commands as root directly (not through ssh) on the host.

• create a bridge :
  

  brctl addbr br0

• add normal network interface to this bridge (let's suppose that it is eth0) :
  

  brctl addif br0 eth0

• Bring up the bridge
  

  ifup br0

• Verify that the bridge has an internet address (if it is not the case, may be you'll have to fix it by hands)
  

  ifconfig br0

• run marionnet-daemon as root
• run marionnet as normal user

If all goes well, you may be able to see that the bridge include 2 interfaces (eth0 and gwtapnnnn where nnnn is an integer) :

brctl show br0