Cannot connect to the public internet from a UBOS container

/docs/users/faq-howto-troubleshooting/error-container-cannot-connect/

A variety of things can be the root cause of that, and we don’t have a full list. Chances are that your host operating system is not correctly configured for networking containers. Here is a list of things to check:

  1. On your host, a new network interface is generated just for the UBOS container. Using:

    % ip addr
    

    check that such an interface appears when you create the container, and it has a suitable IP address such as 10.0.0.1. If not, check that you are running systemd-networkd on the host with a suitable configuration file.

  2. In your UBOS container, using:

    % ip addr
    

    make sure your container has a corresponding IP address such as 10.0.0.2 on an interface called host0@ifX for some value of X. If your interface host0@ifX does not have an IP address, try ip dev set host0 up. There seem to be some combinations of systemd versions between host and container in which the host0 interface in the container does not automatically come up.

  3. Test that you can ping the container from the host, and the host from the container with a command such as:

    % ping 10.0.0.1
    

    If you can’t and both host and container have correct IP addresses, make sure your host does not run a firewall that prevents the communication from happening.

  4. If the container can communicate with the host, and the host with the public internet, but the container cannot communicate with the public internet, chances are that some of the involved network interfaces aren’t forwarding packets. This is common because most Linux distros deactivate packet forwarding by default. The simplest way to globally switch on packet forwarding on the host is to execute, on the host:

    % sudo sysctl net.ipv4.ip_forward=1
    

    If this fixes the problem, you can make it permanent by telling systemd about it with a configuration file such as /etc/systemd/network/wired.network with content:

    [Match]
    Name=en*
    
    [Network]
    DHCP=ipv4
    IPForward=1
    

    You may have a file like that already, except for the IPForward=1 statement.

  5. In your container, try to connect to some well-known IP address, like:

    % ping 8.8.8.8
    

    If you can connect, check DNS, with something like:

    % dig ubos.net