Oct 6, 2015

Overriding the Default Forwarded SSH Port in Vagrant

TLDR:

# Must specified `id: "ssh"` in order to override the default forwarded SSH port.
config.vm.network :forwarded_port, guest: 22, host: 2322, id: "ssh"

What’s the problem? If running multiple virtual machines managed by Vagrant, SSH port collision will happen:

Vagrant cannot forward the specified ports on this VM, since they
would collide with some other application that is already listening
on these ports. The forwarded port to 2222 is already in use
on the host machine.

To fix this, modify your current projects Vagrantfile to use another
port. Example, where '1234' would be replaced by a unique host port:

  config.vm.network :forwarded_port, guest: 22, host: 1234

Sometimes, Vagrant will attempt to auto-correct this for you. In this
case, Vagrant was unable to. This is usually because the guest machine
is in a state which doesn't allow modifying port forwarding.

The example is correct but incomplete. By default the SSH port (22) on the guest machine is forwarded to port 2222 on the host machine:

# Source: https://github.com/mitchellh/vagrant/blob/v1.7.2/plugins/kernel_v2/config/vm.rb
if !@__networks["forwarded_port-ssh"]
  network :forwarded_port,
    guest: 22,
    host: 2222,
    host_ip: "127.0.0.1",
    id: "ssh",
    auto_correct: true
end

Changing to another port in Vagrantfile does not solve the problem. For example, switching from 2222 to 2322:

# Create a forwarded port mapping which allows access to a specific port
# within the machine from a port on the host machine. In the example below,
# accessing "localhost:8080" will access port 80 on the guest machine.
config.vm.network "forwarded_port", guest: 22, host: 2322

In this situation, you are forwarding both ports 2222 and 2322 to the SSH port on the guest machine, as shown in VirtualBox port forwarding settings:

When bringing up the machine, there will be multiple host machine ports forwarding to the same SSH port.

$ vagrant up
==> default: Forwarding ports...
    default: 22 => 2322 (adapter 1)
    default: 22 => 2222 (adapter 1)

The SSH configuration shows the same outcome:

$ vagrant ssh-config
Host default
  HostName 127.0.0.1
  User chao
  Port 2222

Of course, you can do:

$ ssh -p 2322 chao@localhost

This works, but we like the simple vagrant ssh command, and we just want to retain a single port 2322.

The solution is appending id: "ssh", and it must be specified in order to override the default forwarded SSH port. See < https://github.com/mitchellh/vagrant/issues/3232#issuecomment-39354659>:

# Must specified `id: "ssh"` in order to override the default forwarded SSH port.
config.vm.network :forwarded_port, guest: 22, host: 2322, id: "ssh"

After appending id: "ssh", the SSH configuration has also been updated:

$ vagrant ssh-config
Host default
  HostName 127.0.0.1
  User chao
  Port 2322

Now, you are good to go:

$ vagrant ssh