Need write permission to /dev/net/tun, not just read.

contrib/bochs/README

 
 5.  As root, set up a TAP virtual network device:
       /sbin/modprobe tun
-      chmod o+r /dev/net/tun
+      chmod o+rw /dev/net/tun
       ./tunctl -u <username> -t tap0
       /sbin/ifconfig tap0 up 10.254.254.2 netmask 255.255.255.0