Automagic bridged networking with QEMU (tap)

[martinko]

Hallo list,

I spent a good portion of yesterday trying to find out how to change 
from using user mode to bridged networking with QEMU on FreeBSD 7.2.
As I found a few how-to's of differing quality and I couldn't manage to 
make it work for some time, I decided to share the final results of my 
journey with you as I believe it may save someone a few hours or hair.

The proposed solution does not need any manual steps and is fully 
transparent to users (like user mode networking is).  It does not even 
permanently bridge your real NIC, only when necessary, which is good 
because of some performance penalties when bridging.  And you can run as 
many guests as you want and it's all set up automagically.  :-)

OK, enough words, here's what to do:

/boot/loader.conf[.local]:
         if_bridge_load="YES"
         if_tap_load="YES"

/etc/sysctl.conf:
         net.link.tap.up_on_open=1
         net.link.tap.user_open=1

/etc/devfs.rules:
         [localrules=10]
         add path 'tap*' mode 0660

/etc/rc.conf[.local]:
         devfs_system_ruleset="localrules"
         kqemu_enable="YES"

/usr/local/etc/qemu-ifup        -- custom script
/usr/local/etc/qemu-ifdown      -- custom script

--- /usr/local/etc/qemu-ifup

#!/bin/sh
#
# /usr/local/etc/qemu-ifup : martinko [20-aug-2009]
#

IFNAME=re0

for BRIDGE in $(ifconfig -a | grep '^bridge' | cut -d: -f1)
do
         if [ -n "$(ifconfig "$BRIDGE" | grep -w "member: $IFNAME")" ]
         then
                 echo "${0##*/}: Adding $1 as a member of $BRIDGE"
                 sudo /sbin/ifconfig "$BRIDGE" addm "$1" up
                 exit
         fi
done

BRIDGE="$(sudo /sbin/ifconfig bridge create)"
sudo /sbin/ifconfig "$BRIDGE" addm "$IFNAME" addm "$1" up
echo "${0##*/}: Created $BRIDGE and added $1 as a member"

--- /usr/local/etc/qemu-ifdown

#!/bin/sh
#
# /usr/local/etc/qemu-ifdown : martinko [20-aug-2009]
#

for BRIDGE in $(ifconfig -a | grep '^bridge' | cut -d: -f1)
do
         if [ -n "$(ifconfig "$BRIDGE" | grep -w "member: $1")" ]
         then
                 if [ "$(ifconfig "$BRIDGE" | grep -c -w "member:")" -le 2 ]
                 then
                         echo "${0##*/}: Destroying $BRIDGE"
                         sudo /sbin/ifconfig "$BRIDGE" destroy
                 fi
                 echo "${0##*/}: Destroying $1"
                 sudo /sbin/ifconfig "$1" destroy
         fi
done

---

And that's all, folks! :)

Enjoy and if you find a better solution please let me/us know.

Cheers,

Martin

PS1: Of course change IFNAME in qemu-ifup according to your setup.
PS2: You man want/need to change the group in devfs.rules as well.