this is obsolete doc -- see http://doc.nethence.com/ instead

XEN -- NAT network configuration 

Dom0 

Create a custom wrapper, 

cd /etc/xen/scripts

vi nat

like, 

#!/bin/ksh

op=$1

[[ $op != start && $op != stop ]] && print "start|stop" && exit 1

br=/etc/xen/scripts/network-bridge

cmd="$br $op bridge=natbr netdev=dummy0 vifnum=1"

echo -n $cmd...

$cmd && echo done

unset op br

fix perms, 

chmod +x nat

cd ..

Enable forwarding, on RHEL or Slackware (create if doesn't exist), 

vi /etc/sysctl.conf

add, 

net.ipv4.ip_forward = 1

apply, 

sysctl -p

Create a dummy interface. On RHEL, 

cd /etc/sysconfig/network-scripts

vi ifcfg-dummy0

like, 

DEVICE=dummy0

BOOTPROTO=static

IPADDR=10.1.1.254

NETMASK=255.255.255.0

TYPE=Ethernet

ONBOOT=yes

Add those filter rules, 

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -P FORWARD ACCEPT

iptables -t nat -A POSTROUTING -s 10.1.1.0/24 -o eth0 -j MASQUERADE

iptables -A INPUT -i dummy0 -s 10.1.1.0/24 -j ACCEPT

iptables -A OUTPUT -o dummy0 -d 10.1.1.0/24 -j ACCEPT

and to redirect port 80 to 10.1.1.1, 

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 \

        -j DNAT --to-destination 10.1.1.1:80