ifconfig (interface configuration) is a network management tool. It is used to configure and view the status of the network interfaces in Linux operating systems. With ifconfig, you can assign IP addresses, enable or disable interfaces, manage ARP cache, routes, and more.

In this article, we’ll explore how to use the ifconfig command.

How to Install ifconfig#

The ifconfig command is deprecated and replaced with ip and may not be included in the newer Linux distributions.

If you get an error message saying “ifconfig: command not found”, it means that the package that contains the command is not installed on your system.

Install ifconfig on Ubuntu/Debian#

On Ubuntu and Debian-based based Linux distributions, run the following command to install ifconfig:

sudo apt install net-tools -y

Install ifconfig on Centos#

To install ifconfig on CentOS and other RHEL based Linux distros, type:

sudo dnf install net-tools -y

How to Use the ifconfig Command#

The basic syntax of the ifconfig command is shown below:

ifconfig [-a] [-v] [-s] <interface> [[<AF>] <address>]

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Where:

• interface – is the name of the network interface.
• address – is the IP address that you want to assign.

The configurations set with the ifconfig command are not persistent. After a system restart, all changes are lost. To make the changes permanent, you need to edit the distro-specific configuration files or add the commands to a startup script.

Only root or users with sudo privileges can configure network interfaces.

Display Information of Network Interfaces#

When invoked without any options, ifconfig displays the configuration information of all network interfaces and associated ip address:

ifconfig -a

The output includes information about all active and inactive network interfaces:

docker0   Link encap:Ethernet  HWaddr 56:84:7a:fe:97:99  
          inet addr:172.17.42.1  Bcast:0.0.0.0  Mask:255.255.0.0
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:4198 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4198 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1 
          RX bytes:498729 (498.7 KB)  TX bytes:498729 (498.7 KB)

eth0      Link encap:Ethernet  HWaddr 4c:bb:58:9c:f5:55  
          inet addr:172.20.10.3  Bcast:172.20.10.15  Mask:255.255.255.240
          inet6 addr: 2401:4900:1d65:40a1:4ebb:58ff:fe9c:f555/64 Scope:Global
          inet6 addr: 2401:4900:1d65:40a1:f1c9:6a90:2d99:924e/64 Scope:Global
          inet6 addr: fe80::4ebb:58ff:fe9c:f555/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:84110 errors:0 dropped:0 overruns:0 frame:0
          TX packets:59727 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:70667629 (70.6 MB)  TX bytes:20886290 (20.8 MB)

To display the configuration information of any specific network interface, write the interface name after the command:

ifconfig eth0

The output will look something like this:

eth0      Link encap:Ethernet  HWaddr 4c:bb:58:9c:f5:55  
          inet addr:172.20.10.3  Bcast:172.20.10.15  Mask:255.255.255.240
          inet6 addr: 2401:4900:1d65:40a1:4ebb:58ff:fe9c:f555/64 Scope:Global
          inet6 addr: 2401:4900:1d65:40a1:f1c9:6a90:2d99:924e/64 Scope:Global
          inet6 addr: fe80::4ebb:58ff:fe9c:f555/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:84110 errors:0 dropped:0 overruns:0 frame:0
          TX packets:59727 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:70667629 (70.6 MB)  TX bytes:20886290 (20.8 MB)

Assign an IP address and Netmask to a Network Interface#

With the ifconfig command, you can assign an IP address and netmask to a network interface.

Use the following syntax to assign the IP address and netmask:

ifconfig [interface-name] [ip-address] netmask [subnet-mask]

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For example, to assign the IP address 192.168.0.101 and netmask 255.255.0.0 to the interface eth0, you would run:

ifconfig eth0 192.168.0.101 netmask 255.255.0.0

You can also assign a secondary IP address to a network interface using the interface aliasing:

ifconfig eth0:0 192.168.0.102 netmask 255.255.0.0

Enable and Disable a Network Interface#

Sometimes, you may need to reset the network interface. In this case, the ifconfig command can be used to enable or disable a network interface.

To disable an active network interface , enter the device name followed by the down flag:

ifconfig eth0 down

To enable an inactive network interface, use the up flag:

ifconfig eth0 up

Enable and Disable Promiscuous Mode#

Promiscuous allows a network interface to access and view all packets in a network. You can use the ifconfig command to enable and disable the promiscuous on a specific network device.

To enable the promiscuous mode on a network interface, enter the promisc flag after the device name:

ifconfig eth0 promisc

To disable the promiscuous mode, use the -promisc flag

ifconfig eth0 -promisc

Change MTU of a Network Interface#

The MTU “Maximum Transmission Unit” allows you to limit the size of packets that are transmitted on an interface.

You can change the MTU value using the syntax:

ifconfig [interface-name] mtu [mtu-value]

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For example, set the MTU value of a network interface eth0 to 500, run the following command:

ifconfig eth0 mtu 500

Change the MAC address of a Network Interface#

The MAC “Media Access Control” is the physical address that uniquely identifies the devices on a network.

To change the MAC address of a network interface, use the hw ether flag to set the new MAC address:

ifconfig eth0 hw ether 00:00:2d:3a:2a:28

Conclusion#