Understanding and Using the 'ifmetric' Command (with examples)

The ifmetric command is a powerful tool for manipulating IPv4 route metrics associated with network interfaces. It allows users to set or reset the priority of network interfaces by adjusting their metrics. Lower metrics suggest higher priority for outbound traffic. Applying the ifmetric command can therefore enable more efficient routing or aid in troubleshooting network issues on systems running Linux operating systems.

Use case 1: Set the priority of the specified network interface

Code:

sudo ifmetric eth0 10

Motivation:

In a situation where multiple network interfaces are configured on a device, there might be a need to prioritize one interface over another based on certain criteria such as bandwidth capability, reliability, or cost of connectivity. For instance, you might have both an Ethernet (cable) connection and a Wi-Fi (wireless) connection available. To ensure that the Ethernet connection is preferred due to its typically more stable performance and potential for higher bandwidth, you can set the routing metric for the Ethernet interface (eth0) to a low value compared to the Wi-Fi interface (wlan0). This configuration ensures that network traffic defaults to the wired connection when both are available.

Explanation:

• sudo: This command is required to gain superuser privileges necessary for modifying network configurations. Network settings are sensitive and require administrative permissions for any changes.
• ifmetric: This is the command being used, which stands for interface metric and is used to manipulate network route metrics.
• eth0: This indicates the specific network interface whose metric we want to set. In most Linux systems, eth0 commonly refers to the first Ethernet interface.
• 10: This is the metric value for the interface eth0. A lower metric value signifies a higher priority, so by setting this metric to 10, we are assigning a higher preference to this interface for network traffic routes, compared to interfaces with higher metric values.

Example Output:

After executing this command, there’s usually no direct text output. However, the priority of eth0 is effectively set to 10, and you can verify this by using additional tools like ip route or route -n to inspect the current routing table, showing that eth0 is now prioritized over others with higher metric values.

Use case 2: Reset the priority of the specified network interface

Code:

sudo ifmetric eth0 0

Motivation:

There might arise a situation where you wish to reset the priority of a network interface to its default value, or assign it the highest possible priority. This could be due to changes in network configuration, such as upgrading to a network service with better performance or troubleshooting issues where network flows are not behaving as expected. By resetting the metric of eth0 to zero, you effectively make it the most preferred interface, allowing for automatic route adjustment without any need for manual intervention, thus aligning network traffic flow with newly established preferences.

Explanation:

• sudo: Just like in the previous example, superuser permissions are necessary here since network configuration adjustments require administrative privileges.
• ifmetric: The ifmetric command is used to change the routing metric for the specified interface.
• eth0: This clarifies the specific network interface for which the metric is being reset. eth0 remains a typical designation for the primary Ethernet interface.
• 0: Assigning a metric value of 0 gives eth0 the highest possible route priority. Lower metrics have higher precedence, making this interface the default choice for routing outbound traffic when available.

Example Output:

Similar to setting the metric, there is generally no visible output once the command is run. However, utilizing tools like ip route can confirm that the eth0 interface has been assigned a metric of 0, making it the highest priority interface in the routing table by default.

Conclusion:

The ifmetric command offers a straightforward yet effective method to control the routing preferences among multiple network interfaces on Linux systems. By adjusting the metric values, users can dictate which interface should be prioritized for network traffic, thereby optimizing network performance or accommodating specific system requirements. Understanding how to assign and reset these metrics allows for flexibility and control over how systems handle their outbound network connections.