Understanding and Configuring Linux Network Interfaces

Last updated: March 18, 2024

Reviewed by: Saajan Nagendra

1. Overview

There are multiple ways to configure the network in modern Linux systems. The most popular are using Network Manager and Systemd. However, sometimes we need to do it manually using the /etc/network/interfaces configuration file.

In this tutorial, we’ll see how to configure the network using the configuration file. Firstly, we’ll see what a network interface is. Then we’ll learn what the /etc/network/interfaces file is. Finally, we’ll see a complete example for configuring the network using /etc/network/interfaces.

2. What Is a Network Interface?

Simply, a network interface is the point of connection between a computer and a network. In other words, how the Linux system links up the software side of networking to the hardware side.

2.1. Network Interface Types

The Linux system distinguishes two types of network interfaces – the physical network interface and the virtual network interface.

A physical network interface represents a network hardware device such as NIC (Network Interface Card), WNIC (Wireless Network Interface Card), or a modem.

A virtual network interface does not represent a hardware device but is linked to a network device. It can be associated with a physical or virtual interface.

2.2. Network Interface Name

Linux systems use two different styles of naming the network interfaces. The first style is the old-style name, such as eth0, eth1, and wlan0. The new ones are based on hardware locations like enp3s0 and wlp2s0.

So, we can use the ls command and the sys file system to quickly list the available network interfaces. Each entry in the /sys/class/net directory represents a physical or virtual network interface:

$ ls /sys/class/net
eth0 lo wlan0

To get more details about the network interfaces. we can use the ip link command:

$ ip link 
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 54:ee:74:c1:19:92 brd ff:ff:ff:ff:ff:ff
3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000
    link/ether 52:54:00:f0:27:9a brd ff:ff:ff:ff:ff:ff permaddr 94:e9:79:fd:51:5d

Here, we can see three network interfaces, their type, and their state. Alternatively, we can also use the ifconfig command.

In addition, to get the IP address and other related information, we use  the ip addr command:

$ ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 54:ee:74:c1:19:92 brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.122/24 brd 192.168.0.255 scope global dynamic noprefixroute enp3s0
       valid_lft 80953sec preferred_lft 80953sec
    inet6 fe80::bb4c:8096:6:3695/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever
3: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 52:54:00:f0:27:9a brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.13/24 brd 192.168.1.255 scope global dynamic noprefixroute wlp2s0
       valid_lft 42974sec preferred_lft 42974sec
    inet6 fe80::f73c:2d98:746f:9582/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

3. What Is the /etc/network/interfaces File?

To clarify,  /etc/network/interfaces file is a way to configure network interfaces. It’s mostly used by Linux Debian-like distributions.

The majority of the network setup can be done via this file. We can give an IP address to a network interface either statically or dynamically. Further, we can set up routing information, a DNS server, etc.

Moreover, when we use the network interface management commands, they bring up and down the interfaces and configure them based on the /etc/network/interfaces file: the ifup command brings the network interface up, while the ifdown command takes it down.

4. How to Configure Network Interface Using /etc/network/interfaces

Before going into the configuration details, let’s get familiar with the syntax of the /etc/network/interfaces file. We’ll understand some keywords so that we can configure the network correctly.

4.1. The /etc/network/interfaces File Syntax

To enable a network interface at boot time automatically, we use the following syntax:

auto <interface>

Here, <interface> is the network interface name, like, eth0.

We declare a network interface with the keyword iface:

iface lo inet loopback

The lo stands for loopback, a virtual network interface for local usage, and inet means internet protocol family (IPv4). In short, we assign a special IP address (127.0.0.1).

Here’s the format for the declaration of an interface:

iface <interface> <address_family> <method>

For example, we could declare an interface eth0 and get an IP address dynamically using DHCP:

iface eth0 inet dhcp

To configure an interface statically, we follow these steps. First, we define the interface eth1 as a static:

iface eth1 inet static

After that, we set the IP address, network mask, and gateway:

address 192.168.1.100 
netmask 255.255.255.0 
gateway 192.168.1.1

To configure the DNS servers, we add:

dns-nameservers 8.8.8.8 8.8.4.4

Here’s a way to define an interface manually without giving it an IP address:

iface <interface> inet manual

To be able to execute a command or a script after enabling the interface, we use:

post-up <command>

We can also run a command or a script before enabling the network interface:

pre-up <command>

In the same vein, we can use pre-down and post-down options. We run the command before taking the interface down:

pre-down <command>

Here’s how we can execute a command after taking the interface down:

post-down <command>

 4.2. Dynamic Configuration

Let’s configure a network interface dynamically by adding these two lines to our /etc/network/interfaces file:

auto eth0
iface eth0 inet dhcp

4.3. Static Configuration

To set up a static IP address to the eth0 interface, let’s first replace the line iface eth0 inet dhcp from earlier:

iface eth0 inet static

Then, we give our interface an IP address:

address 192.168.1.100
netmask 255.255.255.0
gateway 192.168.1.1

We can add more options, as we’ll see later.

4.4. Enabling and Disabling Network Interfaces

To manually activate or deactivate a network interface, we use ifup/ifdown commands. It’s necessary to have super-user privileges to execute these commands.

In order to enable the interface eth0, for example, we do:

$ sudo ifup eth0

In the same way, we can disable the network interface eth0:

$ sudo ifdown eth0

4.5. Full Example Configuration

Now, we will see a complete example of the static configuration step by step.

For this purpose, we open the /etc/network/interfaces file with a text editor like vi :

$ sudo vi /etc/network/interfaces

First, we enable the network interface eth0 at boot time.

Secondly, we set a static IP address, network, and gateway.

Next, we add two DNS servers.

In addition, we put a script to launch a firewall before starting the interface.

Lastly, we add a script to execute a backup after disabling the interface:

auto eth0
iface eth0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1
dns-nameservers 8.8.8.8 8.8.4.4
pre-up /usr/local/sbin/start-iptables.sh
post-up /usr/local/sbin/backup-log.sh

Finally, we disable and enable the network interface for changes to take effect:

sudo ifdown eth0; sudo ifup eth0

To configure a secondary interface, we just have to repeat the same steps we saw previously.

5. Why /etc/network/interface File Is Empty

As we saw earlier, there multiple ways to configure the network interfaces on the Linux system. So, if we find the config file empty, this probably means that our network is managed by another tool.

In many distributions and often in desktop installations, Linux systems use the network manager to manage the network. Therefore, the config file can be empty.

In this case, we’ve to disable the network manager to avoid interfering with the /etc/network/interfaces file :

$ sudo systemctl stop NetworkManager.service
$ sudo systemctl disable NetworkManager.service

6. Conclusion

In this article, we’ve covered what a network interface is and what its different types are.

In addition, we’ve seen how to set up networks in the Linux system using the /etc/network/interfaces file. Further, we’ve seen its syntax and how to configure it statically and dynamically. Finally, we looked at a complete example. While there are multiple ways to configure a network, we’ve seen just one of them here.

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