Currently, bridged interfaces of VirtualBox guests won't adapt any link
changes of their bridged counterparts.  There exists a "cable connected"
checkbox in the guest settings but manually setting or unsetting it is
not very practical if you need to roam often and would like your VM to
pick up the change automatically.

linktool is a small set of simple utilities that can make that process
transparent.  It relies on scutil(8) and will therefore only run on OSX
but that could easily be adjusted in future.

In order to use linktool, copy linktool.sh somewhere (you may omit the
suffix), make it executable, add the following line to your rc.local

    # linktool, absolute paths only
    /path/to/linktool setup /path/to/VirtualBox.app

and reboot the machine.  All VMs that contain bridged interfaces should
now automagically adapt any link changes on their next start.

Note that setup will replace VirtualBoxVM (a hard link within the
VirtualBox app directory) with a two-line shell wrapper and since VB
startup scripts will recreate VirtualBoxVM on each reboot, setup needs
to run on each reboot too.  On the other hand, this makes uninstalling
linktool as easy as removing one line from rc.local.


Additional information:

linktool consists of several helpers that do a smaller part of the whole
job mentioned in the introduction:

    $ linktool
    usage:
    linktool watch host-if [host-if..]
    linktool map guest host-if:guest-if[:guest-if..] [host-if:guest-if[:guest-if..]..]
    linktool mapper guest
    linktool wrapper --startvm guest
    linktool seesaw master-host-if slave-host-if
    linktool setup vbox-dir vbox-cmd

    guest      : name or UUID of any VM
    host-if    : host interface (see ifconfig)
    guest-if   : guest interface (integer, 1-8)
    vbox-dir   : path to VirtualBox.app directory
    vbox-cmd   : name of VirtualBox command

linkwatch uses scutil(8) and watches host interfaces:

    $ linktool watch en0 en1
    en0:on		# initial output
    en1:off		# initial output
    en1:on		# link status of en1 changed
    (...)

linkmap maps host interfaces to guest interfaces:

    $ linktool map "My Virtual Machine" en0:1:2
    Mapping link status (on): localhost/en0 -> My Virtual Machine/nic1
    Mapping link status (on): localhost/en0 -> My Virtual Machine/nic2

linkmapper maps all bridged interfaces of a guest.  Its output looks
similar to the output above.

linkwrapper is a wrapper around linkmapper and tries to act like
VirtualBoxVM i.e. it understands the --startvm flag.

linksetup replaces VirtualBoxVM with a plain-text hook that starts
linkwrapper first followed by VirtualBoxVM-$(uname -m).  It needs to
run on every boot since VirtualBoxStartup.sh will otherwise persist
(and remove/relink VirtualBoxVM).

linkseesaw doesn't have much to do with linktool or VirtualBox.  It
reuses linkwatch, downs the slave interface if the master interface is
up and vice versa.


Issues:

    * VBoxHeadless is not supported (shouldn't be hard to add)

    * Switching link status doesn't work well with all virtual
      network devices.  e1000 is the most reliable choice,  while
      virtio-net mostly works although it sometimes gets stuck
      in a state that is only recoverable by unloading and loading
      the module (on Linux guests).  In that state, showvminfo
      reports link status A while ethtool claims link status !A.

    * Complicated bridging setups (two guest interfaces bridged with
      one host interface for example) can be unreliable too.  If you
      can't get any packets through but tcpdump happily reports arp
      traffic.. simplify your setup.


[Originally posted to [email protected] on January 20, 2014]