1 Shocking Trick To Add VLANs To Your Busybox ifupdown Configuration Developers Don’t Want You To Know! Number One Will Disintegrate You!

After scouring the internet for answers on howto use Busybox’s ifupdown binary and configure /etc/network/interfaces to set a vlan for an interface I looked in the ifupdown.c code and there are no references to vlans at all!

Don’t panic! Just because there isn’t built in support for setting the vlans doesn’t mean you can’t use /etc/network/interfaces or have to resort to using some custom cockamamie script to set your vlans up! The answer is in front of your nose!

Lately I’ve been making extensive use of the ifupdown pre-up and post-up options for doing insane things like appending iptables rules, bringing up and down other interfaces, etc… The simple trick is to use pre-up|up to use vconfig to create your vlan. The configuration file’s default location is /etc/network/interfaces

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual

auto eth0.192
iface eth0.192 inet static
	address 192.168.0.77
	netmask 255.255.255.0
	gateway 192.168.0.1
	dns-nameservers 4.2.2.1 4.2.2.2
	pre-up ifconfig eth0 up
	pre-up vconfig add eth0 192

I found that I had to add a second pre-up command to configure eth0 up because it has no configuration. This could be a fundamental misunderstanding on my part however; the configuration works impressively well.

For the ifupdown scripts to work we will need to have a service file be executed on startup to run ifup -a. This will be up to individual project’s requirements however; assuming the project will be using a pure Busybox solution, Busybox provides its own init that will walk through /etc/init.d and run start on the scripts it finds there. Here is an example script that buildroot provided my rootfs skeleton.

/etc/init.d/S40Network
#!/bin/sh
#
# Start the network....
#

# Debian ifupdown needs the /run/network lock directory
mkdir -p /run/network

case "$1" in
  start)
	printf "Starting network: "
	/sbin/ifup -a
	[ $? = 0 ] && echo "OK" || echo "FAIL"
	;;
  stop)
	printf "Stopping network: "
	/sbin/ifdown -a
	[ $? = 0 ] && echo "OK" || echo "FAIL"
	;;
  restart|reload)
	"$0" stop
	"$0" start
	;;
  *)
	echo "Usage: $0 {start|stop|restart}"
	exit 1
esac

exit $?

QEMU VNC: Change Your Password And Daemonize Your Monitor

I had a lovely time figuring this out. For obvious reasons you don’t pass the vnc password to the qemu command as it can be seen with a ps. The only other approved

unimplemented option is to have a password file. At least with a password file you can secure it with file

system permissions. My box has no users so to me whatever works works. Anyway…

The problem: daemonize your qemu while still allowing vnc with security! To change the vnc password you have to issue a command ‘change vnc password XXXXX’ to the qemu monitor. The monitor can work with stdio but that defeats the point of daemonize…

My solution: set the monitor to use a tcp port bound to localhost. This way you can continue to control the qemu monitor from the tcp socket. Then use netcat’s nc command to issue it the change vnc password command. I ran in to yet another problem. The nc command didn’t automatically exit and my scripts are non interactive start up scripts.

qemu-kvm -vnc 192.168.0.1:0,password -monitor telnet:127.0.0.1:4444,server,nowait –daemonize
echo change vnc password XXXXX; echo | nc 127.0.0.1 4444

Solution to new problem: Tell nc to execute a command rather than use stdio. The command would simply echo command then echo a new line and exit. This also solved my problem of how to tell qemu to stop running without simply issuing it a kill command. My script took arg1 as the vnc password.

nc 127.0.0.1 4444 -e /change_vnc.sh XXXXX

The fact that I’m using busybox to do all of this may have complicated the nc command

so I don’t know if actual netcat

would have had that problem. Regardless it was very simple.