Vagrant: Port Forwarding – Explained

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Vagrant port forwarding

Vagrant forwarded ports allow you to access a port on your host machine and have all data forwarded to a port on the guest machine, over either TCP or UDP.

For example: If the guest machine is running a web server listening on port 80, you can make a forwarded port mapping to port 9090 (or anything) on your host machine. You can then open your browser to localhost:9090 and browse the website, while all actual network data is being sent to the guest.

Defining a Forwarded Port

The forwarded port configuration expects two parameters, the port on the guest and the port on the host. Example:

Vagrant.configure("2") do |config|
  config.vm.network "forwarded_port", guest: 80, host: 8080
end

This will allow accessing port 80 on the guest via port 9090 on the host.

sample port forwarding

For most providers, forwarded ports by default bind to all interfaces. This means that other devices on your network can access the forwarded ports. If you want to restrict access, see the guest_ip and host_ip settings below.

Options

Here will see what are the options available for forward a port. For this only the guest and host options are required. Below the section, will see some examples to understand about this options.

  • auto_correct (boolean) – If true, the host port will be changed automatically in case it collides with a port already in use. By default, this is false.
  • guest (int) – The port on the guest that you want to be exposed on the host. This can be any port.
  • guest_ip (string) – The guest IP to bind the forwarded port to. If this is not set, the port will go to every IP interface. By default, this is empty.
  • host (int) – The port on the host that you want to use to access the port on the guest. This must be greater than port 1024 unless Vagrant is running as root (which is not recommended).
  • host_ip (string) – The IP on the host you want to bind the forwarded port to. If not specified, it will be bound to every IP. By default, this is empty.
  • protocol (string) – Either “udp” or “tcp”. This specifies the protocol that will be allowed through the forwarded port. By default, this is “tcp”.
  • id (string) – Name of the rule (can be visible in VirtualBox). By default this is “protocol””guest” (exemple : “tcp123”).

Port Protocols Forwarding

By default, any defined port will only forward the TCP protocol. As an optional third parameter, you may specify protocol: ‘udp’ in order to pass UDP traffic. If a given port needs to be able to listen to the same port on both protocols, you must define the port twice with each protocol specified, like so:

Vagrant.configure("2") do |config|
  config.vm.network "forwarded_port", guest: 22, host: 10222, protocol: "tcp"
  config.vm.network "forwarded_port", guest: 9004, host: 36987, protocol: "udp"
end

Port Collisions and Correction

It is common when running multiple Vagrant machines to unknowingly create forwarded port definitions that collide with each other (two separate Vagrant projects forwarded to port 9090, for example). Vagrant includes built-in mechanism to detect this and correct it, automatically.

Read More: What is Vagrant and How to provision a Ubuntu using it

Port collision detection is always done. Vagrant will not allow you to define a forwarded port where the port on the host appears to be accepting traffic or connections.

Port collision auto-correction must be manually enabled for each forwarded port, since it is often surprising when it occurs and can lead the Vagrant user to think that the port was not properly forwarded. Enabling auto correct is easy:

Vagrant.configure("2") do |config|
  config.vm.network "forwarded_port", guest: 80, host: 9090,
    auto_correct: true
end

The final :auto_correct parameter set to true tells Vagrant to auto correct any collisions. During a vagrant up or vagrant reload, Vagrant will output information about any collisions detections and auto corrections made, so you can take notice and act accordingly.

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