ifconfig Command Linux: Complete Guide to Network Interface Configuration (Deprecated)

The ifconfig (interface configuration) command has been a cornerstone of Linux network administration for decades. While officially deprecated in favor of the ip command, understanding ifconfig remains crucial for system administrators working with legacy systems and scripts.

What is ifconfig Command?

The ifconfig command is a system administration utility used to configure, control, and query TCP/IP network interface parameters from the command line. It allows administrators to assign IP addresses, enable or disable interfaces, and view network interface statistics.

ifconfig Command Syntax

The basic syntax of the ifconfig command is:

ifconfig [interface] [options] [address]

Parameters:

• interface: Network interface name (eth0, wlan0, lo, etc.)
• options: Configuration options like up, down, netmask, etc.
• address: IP address to assign

Installing ifconfig

On modern Linux distributions, ifconfig might not be installed by default. Here’s how to install it:

Ubuntu/Debian:

sudo apt update
sudo apt install net-tools

CentOS/RHEL/Fedora:

# For CentOS/RHEL
sudo yum install net-tools

# For Fedora
sudo dnf install net-tools

Basic ifconfig Commands with Examples

1. Display All Network Interfaces

To view all network interfaces and their configurations:

ifconfig

Sample Output:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.100  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 1052  bytes 153350 (149.7 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 805  bytes 67532 (65.9 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 68  bytes 5287 (5.1 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 68  bytes 5287 (5.1 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

2. Display Specific Interface

To view configuration of a specific interface:

ifconfig eth0

Sample Output:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.100  netmask 255.255.255.0  broadcast 192.168.1.255
        ether 08:00:27:4e:66:a1  txqueuelen 1000  (Ethernet)
        RX packets 1052  bytes 153350 (149.7 KiB)
        TX packets 805  bytes 67532 (65.9 KiB)

3. Display Only Active Interfaces

To show only interfaces that are currently up:

ifconfig -s

Sample Output:

Iface   MTU    RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0   1500     1052      0      0 0           805      0      0      0 BMRU
lo    65536       68      0      0 0            68      0      0      0 LRU

Network Interface Configuration

1. Assign IP Address

To assign an IP address to an interface:

sudo ifconfig eth0 192.168.1.150

2. Assign IP Address with Netmask

sudo ifconfig eth0 192.168.1.150 netmask 255.255.255.0

3. Assign IP Address with Broadcast

sudo ifconfig eth0 192.168.1.150 netmask 255.255.255.0 broadcast 192.168.1.255

4. Enable Network Interface

To bring an interface up (activate it):

sudo ifconfig eth0 up

5. Disable Network Interface

To bring an interface down (deactivate it):

sudo ifconfig eth0 down

Advanced ifconfig Operations

1. Change MAC Address

To change the hardware address (MAC) of an interface:

sudo ifconfig eth0 down
sudo ifconfig eth0 hw ether 00:11:22:33:44:55
sudo ifconfig eth0 up

2. Set MTU Size

To change the Maximum Transmission Unit:

sudo ifconfig eth0 mtu 1200

3. Add Alias Interface

To create a virtual interface alias:

sudo ifconfig eth0:0 192.168.1.200 netmask 255.255.255.0

This creates a virtual interface eth0:0 with a different IP address.

4. Remove Alias Interface

To remove a virtual interface:

sudo ifconfig eth0:0 down

Understanding ifconfig Output

Let’s break down the key components of ifconfig output:

Common Interface Flags

• UP: Interface is active
• BROADCAST: Interface supports broadcast
• RUNNING: Interface is operational
• MULTICAST: Interface supports multicast
• LOOPBACK: Interface is a loopback interface
• POINTOPOINT: Interface is a point-to-point link

Practical Examples and Use Cases

Example 1: Quick Network Diagnostics

Check if network interfaces are properly configured:

# Check all interfaces
ifconfig

# Check specific interface
ifconfig eth0

# Quick status overview
ifconfig -s

Example 2: Temporary IP Configuration

Temporarily assign a new IP address for testing:

# Backup current configuration
ifconfig eth0 > /tmp/eth0_backup.txt

# Assign temporary IP
sudo ifconfig eth0 10.0.0.50 netmask 255.255.255.0

# Test connectivity
ping -c 4 10.0.0.1

# Restore original configuration if needed
sudo ifconfig eth0 192.168.1.100 netmask 255.255.255.0

Example 3: Setting up Virtual Interface

Create multiple IP addresses on single interface:

# Create virtual interface
sudo ifconfig eth0:1 192.168.1.200 netmask 255.255.255.0 up

# Verify creation
ifconfig eth0:1

# Test both IPs work
ping -c 2 192.168.1.100  # Original IP
ping -c 2 192.168.1.200  # Virtual IP

Troubleshooting Common Issues

Interface Not Found

If you get “Device not found” error:

# List available interfaces
ip link show
# or
ls /sys/class/net/

Permission Denied

Most ifconfig operations require root privileges:

sudo ifconfig eth0 192.168.1.100

Interface Won’t Come Up

Check interface status and bring it up manually:

# Check current status
ifconfig eth0

# Bring interface down and up
sudo ifconfig eth0 down
sudo ifconfig eth0 up

Modern Alternatives to ifconfig

Since ifconfig is deprecated, here are modern alternatives:

Using ip command:

Security Considerations

• Root Privileges: Most ifconfig operations require sudo/root access
• Temporary Changes: Changes made with ifconfig are not persistent across reboots
• MAC Address Changes: Changing MAC addresses might trigger security alerts
• Network Isolation: Incorrect configuration can lead to network connectivity issues

Best Practices

1. Backup Configuration: Always backup current settings before making changes
2. Use Persistent Configuration: For permanent changes, modify network configuration files
3. Test Changes: Verify connectivity after making network changes
4. Document Changes: Keep records of network configuration modifications
5. Migrate to Modern Tools: Consider using ip command for new deployments

Conclusion

While the ifconfig command is deprecated, it remains an important tool for understanding Linux networking fundamentals. Its straightforward syntax and comprehensive output make it valuable for troubleshooting and learning network configuration concepts. However, system administrators should transition to modern alternatives like the ip command for production environments.

Understanding both ifconfig and its modern replacements ensures you can work effectively across different Linux environments, from legacy systems that still rely on net-tools to modern distributions that have fully embraced iproute2.