To run apache/memcached in guest, and to run ab/memslap in host, we need to run the following commands

/sbin/ifconfig <args>
/usr/sbin/brctl <args>
/usr/sbin/tunctl <args>

Dushyant, etc. has configured sudo to run above in root. So, please try
sudo /sbin/ifconfig <args> instead of /sbin/ifconfig <args>, etc.

Steps at high level:
1. Use tunctl to create a new tap device owned by your userid. It'll return "tunN" device.
		1. sudo tunctl -u $USER # returns tunN
2. Give the tapN to qemu along with -net tun,ifname=tunN,script=$upscript,downscript=$downscript
		1. qemu calls $upscript tunN to configure tunN. This script is expected to configure tunN using sudo /sbin/ifconfig, and add the tunN to bridge "br0" using sudo /usr/sbin/brctl.
		2. After emulation,qemu calls $downscript tunN, you should give up the ip address etc.
3. Destroy the new tap device
		1. sudo tunctl -d tunN


What happens if you didn't give ifname=tunN to qemu?
Qemu will try to open /dev/net/tun directly to create tunN. And even though you are having read/write permission, it'll still fail - you need CAP_XXX capability or root permission to do that.

Note: I got stuck at sudo /sbin/ifconfig and while fixing entire server went down, and I had to get a physical access for dhcp38 and do a reboot. If you're able to get it working let us know :-)

--
Regards,
Deepak Ravi

To enable net in kvm, please use:
sudo <qemu or qemu-system-x86_64 or kvm> -net tap -net nic,model=<e1000 or pcnet etc.> ...


There was some issue with /etc/qemu-ifup original version and modified version.
And the original qemu version is computing the bridge as eth0 instead of br0. For time being we've hardcoded br0 in the scripts(both up&down).
We've moved the existing/modified version of /etc/qemu-ifup to /etc/qemu-ifup.ipsec temporarily.

--
Regards,
Deepak Ravi

PS: We could override script=<>,downscript=<> in qemu/kvm too.
Patch:
--- qemu-ifup.orig    2011-08-14 08:18:14.701284260 +0530
+++ qemu-ifup    2011-08-14 08:18:14.701284260 +0530
@@ -1,5 +1,6 @@
#!/bin/sh

switch=$(/sbin/ip route list | awk '/^default / { print $5 }')
+switch=br0
/sbin/ifconfig $1 0.0.0.0 up
/usr/sbin/brctl addif ${switch} $1