:title: Port redirection
:description: usage about port redirection
:keywords: Usage, basic port, docker, documentation, examples


.. _port_redirection:

Port redirection
================

Interacting with a service is commonly done through a connection to a
port. When this service runs inside a container, one can connect to
the port after finding the IP address of the container as follows:

.. code-block:: bash

    # Find IP address of container with ID <container_id>
    docker inspect <container_id> | grep IPAddress | cut -d '"' -f 4

However, this IP address is local to the host system and the container
port is not reachable by the outside world. Furthermore, even if the
port is used locally, e.g. by another container, this method is
tedious as the IP address of the container changes every time it
starts.

Docker addresses these two problems and give a simple and robust way
to access services running inside containers.

To allow non-local clients to reach the service running inside the
container, Docker provide ways to bind the container port to an
interface of the host system. To simplify communication between
containers, Docker provides the linking mechanism.

Binding a port to an host interface
-----------------------------------

To bind a port of the container to a specific interface of the host
system, use the ``-p`` parameter of the ``docker run`` command:

.. code-block:: bash

    # General syntax
    docker run -p [([<host_interface>:[host_port]])|(<host_port>):]<container_port>[/udp] <image> <cmd>

When no host interface is provided, the port is bound to all available
interfaces of the host machine (aka INADDR_ANY, or 0.0.0.0).When no host port is
provided, one is dynamically allocated. The possible combinations of options for
TCP port are the following:

.. code-block:: bash

    # Bind TCP port 8080 of the container to TCP port 80 on 127.0.0.1 of the host machine.
    docker run -p 127.0.0.1:80:8080 <image> <cmd>

    # Bind TCP port 8080 of the container to a dynamically allocated TCP port on 127.0.0.1 of the host machine.
    docker run -p 127.0.0.1::8080 <image> <cmd>

    # Bind TCP port 8080 of the container to TCP port 80 on all available interfaces of the host machine.
    docker run -p 80:8080 <image> <cmd>

    # Bind TCP port 8080 of the container to a dynamically allocated TCP port on all available interfaces of the host machine.
    docker run -p 8080 <image> <cmd>

UDP ports can also be bound by adding a trailing ``/udp``. All the
combinations described for TCP work. Here is only one example:

.. code-block:: bash

    # Bind UDP port 5353 of the container to UDP port 53 on 127.0.0.1 of the host machine.
    docker run -p 127.0.0.1:53:5353/udp <image> <cmd>

The command ``docker port`` lists the interface and port on the host
machine bound to a given container port. It is useful when using
dynamically allocated ports:

.. code-block:: bash

   # Bind to a dynamically allocated port
   docker run -p 127.0.0.1::8080 -name dyn-bound <image> <cmd>

   # Lookup the actual port
   docker port dyn-bound 8080
   127.0.0.1:49160


Linking a container
-------------------

Communication between two containers can also be established in a
docker-specific way called linking.

To briefly present the concept of linking, let us consider two
containers: ``server``, containing the service, and ``client``,
accessing the service.  Once ``server`` is running, ``client`` is
started and links to server. Linking sets environment variables in
``client`` giving it some information about ``server``. In this sense,
linking is a method of service discovery.

Let us now get back to our topic of interest; communication between
the two containers. We mentioned that the tricky part about this
communication was that the IP address of ``server`` was not
fixed. Therefore, some of the environment variables are going to be
used to inform ``client`` about this IP address. This process called
exposure, is possible because ``client`` is started after ``server``
has been started.

Here is a full example. On ``server``, the port of interest is
exposed. The exposure is done either through the ``-expose`` parameter
to the ``docker run`` command, or the ``EXPOSE`` build command in a
Dockerfile:

.. code-block:: bash

    # Expose port 80
    docker run -expose 80 -name server <image> <cmd>

The ``client`` then links to the ``server``:

.. code-block:: bash

    # Link
    docker run -name client -link server:linked-server <image> <cmd>

``client`` locally refers to ``server`` as ``linked-server``. The
following environment variables, among others, are available on
``client``:

.. code-block:: bash

    # The default protocol, ip, and port of the service running in the container
    LINKED-SERVER_PORT=tcp://172.17.0.8:80

    # A specific protocol, ip, and port of various services
    LINKED-SERVER_PORT_80_TCP=tcp://172.17.0.8:80
    LINKED-SERVER_PORT_80_TCP_PROTO=tcp
    LINKED-SERVER_PORT_80_TCP_ADDR=172.17.0.8
    LINKED-SERVER_PORT_80_TCP_PORT=80

This tells ``client`` that a service is running on port 80 of
``server`` and that ``server`` is accessible at the IP address
172.17.0.8

Note: Using the ``-p`` parameter also exposes the port..
