How to Configure Multiple Network Interfaces on Ubuntu 20.04

Linux Serverspace servers are designed to make life easier for users. They have the function of configuring network interfaces in automatic mode when they are added in the control panel or during server creation. But in order to ensure the operation of policy-based routing and as a result of the server's availability on several network interfaces at once, we need to make changes to the Netplan settings. To prevent our settings from being overwritten during a reboot, we need to disable automatic network configuration. After you make sure that the required number of interfaces has been added to the server, create a file on it:

And add a parameter to it:

Now subsequent changes in Netplan will not be automatically overwritten on reboot.

Policy-Based routing powered by tables. Each table has its own routes and policies. Let's check the presence of the iproute2 package in the system. If not, install it.

Open the file with routing tables:

And add at the end the number of tables corresponding to the amount of interfaces you are configuring:

Record format: priority (number) space table name (number).
Now open the Netplan file with network interface settings. It is located along the path /etc/netplan/ and has the .yaml extension. In my case, the path is as follows:

Each interface has a settings block in this file. At the end of this block, we will add settings for policies and routing rules for the interface. In this file, it is important to observe the number of spaces before each parameter and value, and not to use tabs instead of spaces. Focus on indentation in existing entries and add by analogy.

For each network interface, add routes and routing-policy blocks. The final record for 1 interface will look like this:

For other interfaces, the entries will be the same, starting with the interface name (enp0s5), but others should not have the gateway4 parameter, since there should be one default gateway. Routes and routing-policy settings:

• to - destination address of the packet. For our purposes - any (0.0.0.0/0)
• via - gateway address for this interface
• table - name of the routing table. The same values must be specified in routes and routing-policy for one interface, but be different from interface to interface. We created these tables in the rt_tables file
• from - the source address of the package. For the purposes of this manual, this is the IP address of the interface
• priority - a required numeric value

After making the settings and saving the file, check the configuration for errors:

Output like this will indicate an error if present (in this case, a space is missing.):

If the config is compiled correctly, then the output will be absent. This means it's time to apply the changes:

After applying the changes, the network interfaces configured in this way will become available from the outside at their public IP addresses.