.. _configure-custom-mtu:

====================================
Set custom MTU for Docker containers
====================================

If you have a custom MTU set on the upstream network interface on your host machine, you also need to configure a custom
MTU on your Fleio network containers. Otherwise, networking will not work correctly.

You can test with the following commands if networking is working correctly from within Fleio ``utils`` container:

.. code-block:: bash

    fleio bash
    curl https://google.com

The curl command above fails when there is a Docker networking error. If network is working correctly, you should see
few lines of HTML code, otherwise you get an error.

The default networking MTU is 1500. If you're not sure if you have a lower MTU, run this command on the host where you
have Fleio installed:

.. code-block:: bash

    ip link show
    ...
    2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1400 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether 00:15:5d:e5:44:77 brd ff:ff:ff:ff:ff:ff
    ...

This will list network interfaces. Your upstream interface may be named ``eth0`` or ``ens3`` etc. The MTU value will be
displayed on its line, ``mtu 1400`` in the example above. Ignore Docker related interfaces, names that start with
``veth...`` or ``br-...``.

Once you know that you have a lower than standard 1500 MTU on host, you need to configure Docker containers with the
same value.

Edit file ``/home/fleio/compose/docker-compose.override.yml`` and add the following content

.. code-block:: yaml

    networks:
      backend:
        driver: bridge
        driver_opts:
          com.docker.network.driver.mtu: 1400

To apply changes, run:

.. code-block:: bash

    fleio recreate

The ``curl`` command should succeed now:

.. code-block:: bash

    fleio bash
    curl https://google.com