Vagrant on Ubuntu 17.04: no communication between host and guest (private network)

This problem took several hours to work out, so am posting the solution here. The problem was that, with an Ubuntu 17.04 host and Vagrant (1.9.2) and an Ubuntu 16.04 guest (ubuntu/xenial64) communication between the host and guest wasn't working when networking was setup with a private network like this:

It worked after the first vagrant up but after rebooting and/or running vagrant halt — then vagrant up — the guest and host could not communicate with each other over the private network.

Wouldn't work from the guest and:

Wouldn't work from the host.

Solution

Open a command prompt, enter:

In the output, look for the VirtualBox network, probably vboxnet0 (or something similar). If it looks something like this:

Where 'state' in the first line is 'DOWN' then the interface needs to be brought up; run this command (changing vboxnet0 to the name found by running ip addr if required):

Other stuff I tried (might be useful if the above didn't work):

1. In VirtualBox, going to File -> Preferences -> Network -> Host-only Networks -> vboxnet0 and clicking the screwdriver/spanner to edit the network, then click 'OK' and 'OK' on both open dialogues,
   this gets VirtualBox to recreate the network.
2. Running vagrant plugin install vagrant-vbguest at the command prompt on the host machine. This seems to ensure the VirtualBox Guest Additions are installed.
3. Updating guest additions to the latest version.
4. Re-installing the VirtualBox driver by running the command: sudo rcvboxdrv setup on the host.
5. Ensuring firewall is disabled using the commands sudo ufw disable and disabling iptables.