Add documentation for network bridge

doc/NETWORK_BRIDGE.md

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+# Network Bridge
+
+## Introduction
+
+On a workstation or server it is easy to share your existing network interface
+with your VM's by setting up a network bridge, and attaching the external interface
+to it.
+
+VM's can subsequently be configured to connect to that bridge, and thus share the
+network interface. If you wish to set things up this way, please refer to documentation
+of your distro to see how to setup a network bridge on your ethernet device.
+
+Unfortunately that does not work with most wireless network cards, and so for a
+laptop you would have to do it a bit differently.
+
+## Setup a NAT bridge that forwards your VM's traffic
+
+(this is similar to how `virsh net-start default` works with the default libvirtd bridge)
+Configuration is slightly different now, typically it involves the following steps:
+
+1) Enable routing
+2) Create the bridge
+3) Setup iptables with NAT rules
+4) Start a DHCP server
+
+Note that to do this, the script below relies on executables form the `net-tools`,
+`bridge-utils` and `isc-dhcp-server` packages.
+
+The start_vmbr0.sh script:
+
+```
+#!/bin/env bash
+
+IP=192.168.191.1
+NET=192.168.191.0/24
+MASK=224.0.0.0/24
+PORTS=1024-65535
+
+# Allow routing
+echo 1 > /proc/sys/net/ipv4/ip_forward
+
+# Setup the bridge
+brctl addbr vmbr0
+ifconfig vmbr0 ${IP} up
+
+# Setup NAT
+iptables -t nat -N EZKVM_PRT
+iptables -t nat -A POSTROUTING -j EZKVM_PRT
+iptables -t nat -A EZKVM_PRT -s ${NET} -d ${MASK} -j RETURN
+iptables -t nat -A EZKVM_PRT -s ${NET} -d 255.255.255.255/32 -j RETURN
+iptables -t nat -A EZKVM_PRT -s ${NET} ! -d ${NET} -p tcp -j MASQUERADE --to-ports ${PORTS}
+iptables -t nat -A EZKVM_PRT -s ${NET} ! -d ${NET} -p udp -j MASQUERADE --to-ports ${PORTS}
+iptables -t nat -A EZKVM_PRT -s ${NET} ! -d ${NET} -j MASQUERADE
+
+touch /var/ezkvm/dhcpd.vmbr0.lease
+dhcpd -cf /etc/ezkvm/dhcpd.vmbr0.conf -pf /var/ezkvm/dhcpd.vmbr0.pid -lf /var/ezkvm/dhcpd.vmbr0.lease
+```
+
+And the /etc/ezkvm/dhcpd.vmbr0.conf contains:
+
+```
+option domain-name "ezkvm.net";
+option domain-name-servers 8.8.8.8;
+
+default-lease-time 600;
+max-lease-time 7200;
+
+ddns-update-style none;
+
+subnet 192.168.191.0 netmask 255.255.255.0 {
+  range 192.168.191.70 192.168.191.99;
+  option routers 192.168.191.1;
+}
+```