My home install of VirtualBox is older than my work one, and the instructions for port forwarding elsewhere in this blog don't work on it. This is how to port-forward Jenkins on an older VirtualBox.

• Shut down both the VM and VirtualBox.
• Open the XML file for the VM.
• Add these lines to the <ExtraData> section:

        <ExtraDataItem name="VBoxInternal/Devices/e1000/0/LUN#0/Config/jenkins/GuestPort" value="8080"/>
        <ExtraDataItem name="VBoxInternal/Devices/e1000/0/LUN#0/Config/jenkins/HostPort" value="9494"/>
        <ExtraDataItem name="VBoxInternal/Devices/e1000/0/LUN#0/Config/jenkins/Protocol" value="TCP"/>
  

• Restart VB and the VM. If you get this error:

  Configuration error: Failed to get the "MAC" value (VERR_CFGM_VALUE_NOT_FOUND).
  

  then you have the wrong value for the virtual network adapter (i.e. "e1000" above should be something else). Search the logs to find lines like this:

  00:00:01.104 [/Devices/e1000/0/Config/] (level 4)
  00:00:01.104   AdapterType    <integer> = 0x0000000000000000 (0)
  00:00:01.104   CableConnected <integer> = 0x0000000000000001 (1)
  00:00:01.104   LineSpeed      <integer> = 0x0000000000000000 (0)
  00:00:01.104   MAC            <bytes>   = "08 00 27 27 01 f8" (cb=6)
  

  which will tell you the correct name of the device.

Jenkins4 (my home VM) is now running builds... fingers crossed...