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### Configuration from the outside

Changing the configuration of a virtual machine between boots can be inconvenient because of the need to change config files in the root filesystem that boots the machine. For example, if you want to change the filesystems that are mounted, you need to change the kernel command line to pass in the new filesystems, but you also need to change /etc/fstab to mount them.

You can mount loopback-mount the root filesystem and go in and edit the relevant files, but this is more manual than necessary, and it's impossible without root privileges on the host.

The solution that I'm using is to:

> - Generate the config files and whatever else needs to be changed or added to the filesystem that will boot the machine
> - Lay them out in a skeletal filesystem structure reflecting where they will go in the virtual machine
> - Tar up that directory
> - Add that tar file as one of the block devices on the kernel command line
> - Also add `

>                         CONFIG_DEV=device-name

>   ` to the command line

The first thing the machine does after it mounts the root filesystem read-write is check the CONFIG_DEV environment variable. If it has a value, then it tars the new files from the device after saving the originals. This is early enough to ensure that almost any new configuration options will take effect.

To enable this, you need to put this script in /etc/rc.d/rc.configfiles ``

    #!/bin/sh

    for ARG in $@
    do
        case $ARG in
        start)
            if [ "$CONFIG_DEV" != "" ]; then
                restore=""
                remove=""
                for file in `tar tf $CONFIG_DEV`; do
                    if [ -e $file ]
                    then restore="$restore $file"
                    else remove="$remove $file"
                    fi
                done
                [ "$restore" != "" ] && tar cf /tmp/restore.tar $restore
                echo $remove > /tmp/remove
                tar xf $CONFIG_DEV
            fi
        ;;
        stop)
            if [ "$CONFIG_DEV" != "" ]; then
                [ -e /tmp/restore.tar ] && tar xf /tmp/restore.tar
                [ -f /tmp/remove ] && rm -rf `cat /tmp/remove`
                rm -rf /tmp/restore.tar /tmp/remove
            fi
        ;;
        *)
            echo "usage: $0 (start|stop)"
            exit 1
            ;;
        esac
    done

    exit 0

and add a call to /etc/rc.S: `

                  /etc/rc.d/rc.configfiles start

` and a similar call to /etc/rc.0: `

                  /etc/rc.d/rc.configfiles stop

` The addition to /etc/rc.S needs to be immediately after the read-write remount of /: `

                  mount -o remount /

` and the /etc/rc.0 needs to be just before the read-only remount of /: `

                  mount -n -o remount,ro /

`

With this in place, it is possible to arbitrarily reconfigure a virtual machine without booting it up or loopback mounting its root filesystem beforehand.

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