XEN with routed DomUs

With a reference to Martin Würtele:

Configure /etc/xen/xend-config.sxp for routing:

(network-script network-route)
(vif-script     vif-route)

and enable arp proxy with sysctl -w net.ipv4.conf.eth0.proxy_arp=1 for runtime and by adding

net.ipv4.conf.eth0.proxy_arp=1

to /etc/sysctl.conf permanently.