qemu-ifup

#!/bin/bash
# qemu-ifup script for QEMU/KVM with NAT netowrk mode

# set your bridge name
BRIDGE=virbr0

# Network information
NETWORK=192.168.122.0
NETMASK=255.255.255.0
# GATEWAY for internal guests is the bridge in host
GATEWAY=192.168.122.1
DHCPRANGE=192.168.122.2,192.168.122.254

# Optionally parameters to enable PXE support
TFTPROOT=
BOOTP=

function check_bridge() 
{
	if brctl show | grep "^$BRIDGE" &> /dev/null; then
		return 1
	else
		return 0
	fi
}

function create_bridge() 
{
	    brctl addbr "$BRIDGE"
	    brctl stp "$BRIDGE" on
	    brctl setfd "$BRIDGE" 0
	    ifconfig "$BRIDGE" "$GATEWAY" netmask "$NETMASK" up
}

function enable_ip_forward() 
{
	echo 1 > /proc/sys/net/ipv4/ip_forward
}

function add_filter_rules() 
{
	iptables -t nat -A POSTROUTING -s "$NETWORK"/"$NETMASK" \
		! -d "$NETWORK"/"$NETMASK" -j MASQUERADE
}

function start_dnsmasq() 
{
	# don't run dnsmasq repeatedly
	ps -ef | grep "dnsmasq" | grep -v "grep" &> /dev/null
	if [ $? -eq 0 ]; then
		echo "Warning:dnsmasq is already running. No need to run it again."
		return 1
	fi

	dnsmasq \
		--strict-order \
		--except-interface=lo \
		--interface=$BRIDGE \
		--listen-address=$GATEWAY \
		--bind-interfaces \
		--dhcp-range=$DHCPRANGE \
		--conf-file="" \
		--pid-file=/var/run/qemu-dnsmasq-$BRIDGE.pid \
		--dhcp-leasefile=/var/run/qemu-dnsmasq-$BRIDGE.leases \
		--dhcp-no-override \
		${TFTPROOT:+"--enable-tftp"} \
		${TFTPROOT:+"--tftp-root=$TFTPROOT"} \
		${BOOTP:+"--dhcp-boot=$BOOTP"}
}

function setup_bridge_nat()
{
	check_bridge "$BRIDGE"
	if [ $? -eq 0 ]; then
		create_bridge
	fi
	enable_ip_forward
	add_filter_rules "$BRIDGE"
	start_dnsmasq "$BRIDGE"
}

# need to check $1 arg before setup
if [ -n "$1" ]; then
	setup_bridge_nat
	ifconfig "$1" 0.0.0.0 up
	brctl addif "$BRIDGE" "$1"
	exit 0
else
	echo "Error: no interface specified."
	exit 1
fi