Mounting NFS folders in vagrant fails due to a timeout

I recently hit the following mysterious error when using NFS shares in vagrant.

Mounting NFS shared folders...
The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

It wasn’t the first time I solved it by myself and colleagues. It has nothing to do with the nfs-package being installed in the guest or not. It has also nothing to do with /etc/exports. The cause is OSX’s firewall. Disable the “block all incoming connections” option in your firewall settings and NFS will work without any issues.