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Struggling with unwanted default routing in your embedded Linux setup? Learn how to configure your network interfaces and disable automatic routes effectively!
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This video is based on the question https://stackoverflow.com/q/63936058/ asked by the user 'Ziegfried' ( https://stackoverflow.com/u/3584297/ ) and on the answer https://stackoverflow.com/a/63937602/ provided by the user 'Ziegfried' ( https://stackoverflow.com/u/3584297/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Control Default Routing in Linux: A Step-by-Step Guide

If you've ever tried to configure your network interfaces in Linux, only to find that unwanted default routes are being added during boot, you're not alone. Many users face issues when setting up network configurations, especially involving multiple interfaces. In this guide, we will dive into a specific problem and guide you through the solution to ensure your desired default routing works correctly.

The Problem

You have two network interfaces defined:

eth0: a wired connection

wwan0: a wireless connection

You want to set wwan0 as the default route interface, but after a system boot or when establishing an SSH session, the system automatically adds a default route through eth0, which you don't want. Here's what your main configuration files look like:

Configuration Files

20-wire.network

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25-wireless.network

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When executing the ip route command, you notice something troubling:

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The Challenge

Your goal is to ensure that wwan0 is always used as the default gateway. This issue may be caused by several factors within the networking setup of Linux, particularly the involvement of additional network management tools such as NetworkManager or ConnMan.

The Solution

To resolve this default routing issue, you need to disable any automatic connection setups that might interfere with your configurations. Here's a step-by-step approach to fix this problem.

Step 1: Identify the Network Manager

In this case, you identified that ConnMan is set up to manage your network connections, which is likely causing the unwanted routes to be created.

Step 2: Modify ConnMan Settings

You need to change the settings in ConnMan to ensure it does not automatically connect and override your intended routes. Follow these steps:

Access the ConnMan settings file located at:

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Find the line that reads:

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Change this line to:

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This modification disables the automatic connection feature for the Ethernet interface, allowing you to maintain your desired routing without interference.

Step 3: Restart the Networking Service

After making the changes, restart the ConnMan service or reboot your Linux system to apply the new configurations. You can typically restart ConnMan using:

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Step 4: Verify Your Settings

After restarting, check to ensure that your intended routing is now in effect:

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You should no longer see any default routes pointing to eth0 unless you intentionally configure it that way.

Conclusion

By disabling the auto-connect feature in your ConnMan settings, you can effectively manage your default routing in Linux and ensure that your system always uses wwan0 as the default gateway. Systemd network configurations may be complex, but with the right adjustments, you can enjoy a stable network setup tailored to your needs.

Now you’re all set to manage your network interfaces like a pro!