ifconfig Command: Tutorial & Examples

Display and configure network interfaces on Linux servers

The ifconfig command is a classic network configuration tool used on Linux systems to display and modify the parameters of network interfaces. It allows administrators to view interface status, assign IP addresses, enable or disable interfaces, and configure various network settings. Although now deprecated in favor of the more powerful ip command, ifconfig remains widely available and useful for quick network diagnostics and legacy scripts on many systems.

What ifconfig Does

The ifconfig command interacts directly with the kernel's network stack to display or change the configuration of network interfaces. It can:

Show detailed information about all or specific network interfaces, including IP addresses, MAC addresses, netmask, broadcast address, MTU, and transmitted/received packet statistics.
Enable or disable network interfaces (bring them up or down).
Assign or change IP addresses and network masks.
Enable or disable promiscuous mode on interfaces, useful for network monitoring.
Adjust other parameters such as the Maximum Transmission Unit (MTU).

This makes ifconfig an essential tool for managing network connectivity on Linux servers and virtual machines, especially when working in environments without graphical interfaces.

How ifconfig Works

ifconfig uses system calls to query and manipulate the kernel's network interface configuration. It accesses the interface's parameters via the ioctl system calls, communicating with the kernel's networking subsystem to retrieve or update settings. The command reads the current state of interfaces from the kernel and writes changes back to configure the network stack accordingly.

Each network interface on the system corresponds to a physical or virtual device, such as an Ethernet card (eth0), loopback device (lo), or wireless adapter (wlan0). ifconfig allows you to inspect and modify these interfaces' properties at a low level.

Common Command Line Parameters

Here are some frequently used options with ifconfig:

interface_name: Specify which network interface to operate on (e.g., eth0, lo).
up: Activate the interface (bring it up).
down: Deactivate the interface (bring it down).
IP_address: Set the IP address of the interface.
netmask: Set the network mask for the interface.
broadcast: Set the broadcast address for the interface.
promisc: Enable promiscuous mode (receive all packets).
-promisc: Disable promiscuous mode.
mtu: Set the Maximum Transmission Unit size.
alias: Add a secondary IP address to the interface.

Basic Usage Examples

Display all configured network interfaces with their details:

ifconfig

Sample output:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.10  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 fe80::a00:27ff:fe4e:66a1  prefixlen 64  scopeid 0x20<link>
        ether 08:00:27:4e:66:a1  txqueuelen 1000  (Ethernet)
        RX packets 12345  bytes 1234567 (1.2 MB)
        TX packets 6789  bytes 987654 (987.6 KB)

View information for a specific interface, e.g., eth0:

ifconfig eth0

Assign an IP address and netmask to an interface:

ifconfig eth0 192.168.1.10 netmask 255.255.255.0

Bring the interface up (activate it):

ifconfig eth0 up

Bring the interface down (deactivate it):

ifconfig eth0 down

Enable promiscuous mode to capture all network traffic (useful for packet sniffing):

ifconfig eth0 promisc

Disable promiscuous mode:

ifconfig eth0 -promisc

Set the MTU size of an interface:

ifconfig eth0 mtu 1400

Add an alias IP address to an interface:

ifconfig eth0:0 192.168.1.11 netmask 255.255.255.0

Understanding ifconfig Output

The output of ifconfig can be divided into several parts:

Interface Name: e.g., eth0, lo.
Flags: Indicate interface status (e.g., UP, BROADCAST, RUNNING, MULTICAST).
MTU: Maximum Transmission Unit size in bytes.
inet: IPv4 address assigned to the interface.
netmask: Subnet mask for the IPv4 address.
broadcast: Broadcast address for the subnet.
inet6: IPv6 address and prefix length.
ether: MAC (hardware) address.
RX/TX packets and bytes: Counters for data received and transmitted.
txqueuelen: Length of the transmit queue.

Understanding these fields helps diagnose network issues and verify configurations.

Potential Problems and Troubleshooting

Permission Denied: Running ifconfig to change settings requires superuser privileges. Use sudo to run commands that modify interfaces.
Interface Not Found: Specifying a non-existent interface name will cause an error.
Changes Not Persistent: Changes made with ifconfig are not permanent and will be lost after reboot unless saved in network configuration files.
Deprecated Command Warning: Some modern Linux distributions may not include ifconfig by default or show warnings about its obsolescence.
Incorrect IP or Netmask: Setting wrong IP or netmask can cause connectivity issues.

For troubleshooting, always verify interface status with ifconfig interface_name and confirm network connectivity with tools like ping or ip.

Deprecation and Alternatives

The ifconfig command is deprecated and has been replaced by the more versatile and powerful ip command from the iproute2 package. The ip command offers more features and a unified syntax for managing network interfaces, routing, and tunnels.

Example equivalent commands:

ip addr show eth0
ip link set eth0 up
ip addr add 192.168.1.10/24 dev eth0

Despite deprecation, ifconfig remains widely used for simple tasks and legacy scripts.

Tips and Best Practices

Always use sudo or run as root when changing interface settings.
Use ifconfig primarily for quick checks or legacy compatibility.
Learn and transition to the ip command for advanced network management.
Document any manual changes to network configuration to avoid losing settings after reboot.
Combine ifconfig with other tools like ping, netstat, or ss for comprehensive network troubleshooting.