Manually setup KVM public bridge

# My initial status
% ifconfig 
eth0      Link encap:Ethernet  HWaddr 00:1f:16:a4:d0:8c  
          inet addr:192.168.0.108  Bcast:192.168.0.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:16888 errors:0 dropped:0 overruns:0 frame:0
          TX packets:12404 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:24617836 (23.4 MiB)  TX bytes:921398 (899.8 KiB)
          Interrupt:16 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:2 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:100 (100.0 B)  TX bytes:100 (100.0 B)

# Disable any dhcp clients if any!

# Setup tun tap0
% tunctl -t tap0 -g kvm
Set 'tap0' persistent and owned by gid 103

# Create the bridge interface.
% brctl addbr br0

# Add interfaces to the bridge.
% brctl addif br0 eth0
% brctl addif br0 tap0

# Zero IP the interfaces.
% ip addr flush eth0
% ifconfig eth0 0.0.0.0
% ifconfig tap0 0.0.0.0

# Put up the bridge.
% ifconfig br0 192.168.0.108 netmask 255.255.255.0 up
% route add default gw 192.168.0.1 metric 1

# This gives you a fixed IP, just replace the last 2 lines with your normal DHCP command if you want a dynamic IP.

# Status scheck
% ifconfig
br0       Link encap:Ethernet  HWaddr 00:1f:16:a4:d0:8c  
          inet addr:192.168.0.108  Bcast:192.168.0.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

eth0      Link encap:Ethernet  HWaddr 00:1f:16:a4:d0:8c  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:66 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:12636 (12.3 KiB)  TX bytes:0 (0.0 B)
          Interrupt:16 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:4 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:200 (200.0 B)  TX bytes:200 (200.0 B)

tap0      Link encap:Ethernet  HWaddr 8a:0d:1d:f2:79:a9  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:500 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.0.0     *               255.255.255.0   U     0      0        0 br0
default         192.168.0.1     0.0.0.0         UG    1      0        0 br0

$ ping 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.
64 bytes from 192.168.0.1: icmp_seq=1 ttl=127 time=0.368 ms
64 bytes from 192.168.0.1: icmp_seq=2 ttl=127 time=0.356 ms
64 bytes from 192.168.0.1: icmp_seq=3 ttl=127 time=0.328 ms
^C

# To start kvm:
kvm -m 1024 -smp 2 -net nic,macaddr=52:54:00:39:81:49 -net tap,ifname=tap0,script=no,downscript=no . . .