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Command documentation sourced from the linux-command project This comprehensive command reference is part of the linux-command documentation project.

ifconfig - Network Interface Configuration

The ifconfig command is a system administration utility for Unix-like operating systems used to configure, control, and query TCP/IP network interface parameters. It allows administrators to configure network interfaces, assign IP addresses, enable/disable interfaces, and view network interface statistics. While newer systems increasingly use the ip command from the iproute2 package, ifconfig remains widely used and supported, especially in older systems and embedded devices.

Basic Syntax​

ifconfig [interface] [options] [address]
ifconfig [interface] [up|down]
ifconfig -a

Common Operations​

  • ifconfig - Display active network interfaces
  • ifconfig -a - Display all network interfaces (including inactive)
  • ifconfig interface up - Bring network interface up
  • ifconfig interface down - Take network interface down
  • ifconfig interface address - Configure IP address for interface
  • ifconfig interface netmask mask - Set subnet mask
  • ifconfig interface broadcast addr - Set broadcast address

Interface Configuration Options​

Address Configuration​

  • address - Set IP address for interface
  • netmask mask - Set subnet mask (e.g., 255.255.255.0)
  • broadcast addr - Set broadcast address
  • pointopoint addr - Set point-to-point address
  • dstaddr addr - Set destination address (for point-to-point)

Interface Control​

  • up - Activate the interface
  • down - Deactivate the interface
  • -arp - Disable ARP on this interface
  • arp - Enable ARP on this interface (default)
  • -promisc - Disable promiscuous mode
  • promisc - Enable promiscuous mode

MTU and Metric​

  • mtu n - Set Maximum Transmission Unit
  • metric n - Set interface metric

Hardware Address​

  • hw class address - Set hardware address (MAC)
    • ether - Ethernet address
    • ax25 - AX.25 address
    • arcnet - ARCnet address
    • netrom - NET/ROM address

IP Configuration​

  • add addr - Add IPv6 address
  • del addr - Delete IPv6 address
  • tunnel addr - Create tunnel interface

Usage Examples​

Basic Interface Operations​

Viewing Interface Information​

# Display active interfaces
ifconfig

# Display all interfaces (including inactive)
ifconfig -a

# Display specific interface
ifconfig eth0

# Display interface with IPv6
ifconfig -a inet6

# Show interface statistics continuously
watch ifconfig

Bringing Interfaces Up/Down​

# Bring interface up
ifconfig eth0 up

# Take interface down
ifconfig eth0 down

# Restart interface
ifconfig eth0 down && ifconfig eth0 up

# Check if interface is up
ifconfig eth0 | grep "UP"

IP Address Configuration​

Basic IP Configuration​

# Configure IP address with subnet mask
ifconfig eth0 192.168.1.100 netmask 255.255.255.0

# Configure IP address with broadcast
ifconfig eth0 192.168.1.100 broadcast 192.168.1.255

# Configure IP address with full network parameters
ifconfig eth0 192.168.1.100 netmask 255.255.255.0 broadcast 192.168.1.255 up

# Configure multiple IP addresses
ifconfig eth0:0 192.168.1.100
ifconfig eth0:1 192.168.1.101
ifconfig eth0:2 192.168.1.102

IPv6 Configuration​

# Add IPv6 address
ifconfig eth0 add 2001:db8::1/64

# Remove IPv6 address
ifconfig eth0 del 2001:db8::1/64

# Configure IPv6 address
ifconfig eth0 inet6 add 2001:db8::1/64

# Display IPv6 information
ifconfig -a inet6

Advanced Network Configuration​

MTU and Performance Tuning​

# Set MTU for jumbo frames
ifconfig eth0 mtu 9000

# Set standard MTU
ifconfig eth0 mtu 1500

# Set interface metric for routing
ifconfig eth0 metric 10

# Optimize for high-latency networks
ifconfig eth0 mtu 1400

Hardware Configuration​

# Change MAC address (requires root)
ifconfig eth0 hw ether 00:11:22:33:44:55

# Set interface to promiscuous mode
ifconfig eth0 promisc

# Disable promiscuous mode
ifconfig eth0 -promisc

# Enable/disable ARP
ifconfig eth0 arp
ifconfig eth0 -arp

Point-to-Point Configuration​

# Configure point-to-point interface
ifconfig ppp0 192.168.1.1 pointopoint 192.168.1.2

# Configure tunnel interface
ifconfig tun0 10.0.0.1 pointopoint 10.0.0.2

# Configure GRE tunnel
ifconfig gre0 tunnel 192.168.1.1 192.168.1.2

Practical Examples​

Network Interface Management​

System Network Setup​

# Configure primary network interface
ifconfig eth0 192.168.1.100 netmask 255.255.255.0 broadcast 192.168.1.255 up

# Configure loopback interface (usually set by system)
ifconfig lo 127.0.0.1 netmask 255.0.0.0 up

# Configure multiple interfaces
ifconfig eth0 192.168.1.100 netmask 255.255.255.0 up
ifconfig eth1 10.0.0.100 netmask 255.255.255.0 up

# Configure VLAN interface
ifconfig eth0.100 192.168.100.100 netmask 255.255.255.0 up

Virtual and Alias Interfaces​

# Create virtual interfaces for multiple IPs
ifconfig eth0:0 192.168.1.100 netmask 255.255.255.0
ifconfig eth0:1 192.168.1.101 netmask 255.255.255.0
ifconfig eth0:2 192.168.1.102 netmask 255.255.255.0

# Create bridge interface
ifconfig br0 192.168.1.10 netmask 255.255.255.0 up
brctl addbr br0
brctl addif br0 eth0
brctl addif br0 eth1

Network Diagnostics​

Interface Status Monitoring​

# Check interface status
ifconfig eth0 | grep "UP"

# Check interface statistics
ifconfig eth0 | grep "RX packets"
ifconfig eth0 | grep "TX packets"

# Monitor interface errors
ifconfig eth0 | grep "errors"
ifconfig eth0 | grep "dropped"

# Show interface statistics in human-readable format
ifconfig -s eth0

Network Connectivity Testing​

# Configure interface for testing
ifconfig eth0 192.168.1.100 netmask 255.255.255.0 up

# Test with loopback
ifconfig lo up
ping 127.0.0.1

# Test interface connectivity
ifconfig eth0 up && ping -c 4 192.168.1.1

# Test with different MTU sizes
ifconfig eth0 mtu 1400 && ping -c 4 -M do -s 1372 8.8.8.8

Server and System Administration​

Network Server Configuration​

# Configure web server interface
ifconfig eth0 192.168.1.100 netmask 255.255.255.0 up
ifconfig eth0:0 192.168.1.101 netmask 255.255.255.0  # Virtual IP

# Configure database server
ifconfig eth1 10.0.0.100 netmask 255.255.255.0 up
ifconfig eth1 mtu 9000  # Enable jumbo frames

# Configure backup network
ifconfig eth2 172.16.1.100 netmask 255.255.255.0 up

High Availability Setup​

# Configure floating IP
ifconfig eth0:0 192.168.1.200 netmask 255.255.255.0 up

# Configure heartbeat interface
ifconfig eth1 10.0.0.1 netmask 255.255.255.252 up

# Configure cluster communication
ifconfig eth0 192.168.1.10 netmask 255.255.255.0 up
ifconfig eth0:1 192.168.1.11 netmask 255.255.255.0 up

Advanced Usage​

Interface Bonding and Teaming​

Network Bonding Configuration​

# Configure bonding interface
ifconfig bond0 192.168.1.100 netmask 255.255.255.0 up
ifenslave bond0 eth0 eth1

# Check bonding status
cat /proc/net/bonding/bond0

# Configure active-backup mode
ifconfig bond0 up
echo +eth0 > /sys/class/net/bond0/bonding/slaves
echo +eth1 > /sys/class/net/bond0/bonding/slaves

Virtualization and Containers​

Bridge Configuration for VMs​

# Create bridge for virtual machines
ifconfig br0 192.168.1.10 netmask 255.255.255.0 up
brctl addbr br0
brctl addif br0 eth0
brctl stp br0 on

# Configure tap interface for VM
ifconfig tap0 up
brctl addif br0 tap0

# Create bridge for Docker
ifconfig docker0 172.17.42.1 netmask 255.255.0.0 up

Wireless Network Configuration​

Wireless Interface Setup​

# Configure wireless interface
ifconfig wlan0 up
iwconfig wlan0 essid "NetworkName"
iwconfig wlan0 key s:YourPassword

# Configure wireless with DHCP
ifconfig wlan0 up
dhclient wlan0

# Monitor wireless interface
ifconfig wlan0 promisc
iwconfig wlan0 mode monitor

Integration and Automation​

Shell Scripts​

Network Configuration Script​

#!/bin/bash
# Network interface configuration script

INTERFACE="eth0"
IP_ADDRESS="192.168.1.100"
NETMASK="255.255.255.0"
BROADCAST="192.168.1.255"

# Configure interface
ifconfig $INTERFACE $IP_ADDRESS netmask $NETMASK broadcast $BROADCAST up

# Verify configuration
if ifconfig $INTERFACE | grep -q "$IP_ADDRESS"; then
    echo "Interface $INTERFACE configured successfully"
    echo "IP Address: $IP_ADDRESS"
    echo "Netmask: $NETMASK"
    echo "Broadcast: $BROADCAST"
else
    echo "Failed to configure $INTERFACE"
    exit 1
fi

# Test connectivity
ping -c 4 $(echo $IP_ADDRESS | awk -F. '{print $1"."$2"."$3".1"}')

Network Monitoring Script​

#!/bin/bash
# Monitor network interface statistics

INTERFACE="eth0"
LOG_FILE="/var/log/network_monitor.log"

while true; do
    TIMESTAMP=$(date "+%Y-%m-%d %H:%M:%S")

    # Get interface statistics
    RX_PACKETS=$(ifconfig $INTERFACE | grep "RX packets" | awk '{print $2}' | cut -d: -f2)
    TX_PACKETS=$(ifconfig $INTERFACE | grep "TX packets" | awk '{print $2}' | cut -d: -f2)
    RX_BYTES=$(ifconfig $INTERFACE | grep "RX packets" | awk '{print $5}')
    TX_BYTES=$(ifconfig $INTERFACE | grep "TX packets" | awk '{print $5}')

    # Log statistics
    echo "$TIMESTAMP - RX: $RX_PACKETS packets ($RX_BYTES bytes), TX: $TX_PACKETS packets ($TX_BYTES bytes)" >> $LOG_FILE

    sleep 60
done

Network Backup and Recovery​

Configuration Backup Script​

#!/bin/bash
# Backup network interface configuration

BACKUP_DIR="/etc/network/backup"
DATE=$(date +%Y%m%d_%H%M%S)

# Create backup directory
mkdir -p $BACKUP_DIR

# Backup current configuration
ifconfig -a > $BACKUP_DIR/interfaces_$DATE.txt

# Backup routing table
route -n > $BACKUP_DIR/routes_$DATE.txt

# Backup ARP table
arp -n > $BACKUP_DIR/arp_$DATE.txt

echo "Network configuration backed up to $BACKUP_DIR"

Configuration Restore Script​

#!/bin/bash
# Restore network interface configuration

BACKUP_FILE=$1

if [ -z "$BACKUP_FILE" ]; then
    echo "Usage: $0 <backup_file>"
    exit 1
fi

if [ ! -f "$BACKUP_FILE" ]; then
    echo "Backup file not found: $BACKUP_FILE"
    exit 1
fi

# Parse and restore interfaces
while read -r line; do
    if [[ $line =~ ^[a-zA-Z0-9]+: ]]; then
        INTERFACE=$(echo $line | cut -d: -f1)
        if echo $line | grep -q "UP"; then
            # Restore interface configuration
            IP=$(echo $line | grep "inet " | awk '{print $2}')
            if [ -n "$IP" ]; then
                ifconfig $INTERFACE $IP up
            else
                ifconfig $INTERFACE up
            fi
        fi
    fi
done < "$BACKUP_FILE"

echo "Network configuration restored from $BACKUP_FILE"

Troubleshooting​

Common Issues​

Interface Not Found​

# List available interfaces
ifconfig -a

# Check if interface module is loaded
lsmod | grep driver_name

# Load network driver
modprobe driver_name

# Check device status
ethtool eth0
mii-tool eth0

Permission Denied​

# Check if running as root
whoami

# Use sudo
sudo ifconfig eth0 192.168.1.100

# Check capabilities
capsh --print

# Add net_admin capability (advanced)
sudo setcap cap_net_admin+ep /path/to/binary

Interface Won't Come Up​

# Check interface status
ifconfig eth0
ethtool eth0

# Check for link
mii-tool eth0
ethtool eth0 | grep "Link detected"

# Check driver status
dmesg | grep eth0
lspci | grep -i ethernet

Network Connectivity Issues​

# Check interface configuration
ifconfig eth0

# Check routing
route -n

# Check DNS
cat /etc/resolv.conf

# Test connectivity
ping -c 4 8.8.8.8
ping -c 4 google.com

# Check for conflicts
arp -n
ip neigh show

Performance Issues​

Slow Network Performance​

# Check interface statistics
ifconfig eth0 | grep -E "(RX|TX) packets"
ifconfig eth0 | grep -E "(RX|TX) bytes"

# Check for errors
ifconfig eth0 | grep -E "(errors|dropped|overruns)"

# Check duplex and speed
ethtool eth0

# Optimize MTU
ifconfig eth0 mtu 9000

# Check for packet loss
ping -c 100 -i 0.1 192.168.1.1
  • ip - Modern network interface configuration utility
  • route - Display and manipulate routing tables
  • netstat - Network statistics and connections
  • ss - Socket statistics utility
  • ethtool - Ethernet settings utility
  • mii-tool - Media Independent Interface utility
  • arp - Display and manipulate ARP cache
  • iwconfig - Wireless interface configuration
  • brctl - Ethernet bridge administration
  • dhclient - DHCP client

Best Practices​

  1. Use ip command for new systems - ifconfig is being deprecated
  2. Always specify netmask when configuring IP addresses
  3. Test connectivity after configuration changes
  4. Backup configuration before making changes
  5. Use interface aliases for multiple IP addresses on same interface
  6. Set appropriate MTU based on network requirements
  7. Monitor interface statistics for performance tuning
  8. Document network configuration for disaster recovery
  9. Use static addressing for servers and critical infrastructure
  10. Secure management interfaces with proper access controls

Performance Tips​

  1. Enable jumbo frames (MTU 9000) for high-performance networks
  2. Use appropriate duplex settings (auto-negotiation vs. fixed)
  3. Monitor interface statistics regularly for bottlenecks
  4. Optimize MTU size based on network topology
  5. Use bonding/teaming for redundancy and performance
  6. Configure appropriate buffer sizes for high-throughput applications
  7. Disable unnecessary features to reduce CPU overhead
  8. Use interrupt coalescing for better performance
  9. Optimize NIC offload features (TSO, LRO, GSO)
  10. Monitor queue lengths for network congestion

The ifconfig command remains a fundamental tool for network interface configuration and troubleshooting in Linux systems. While newer utilities like ip offer enhanced capabilities, ifconfig provides a straightforward and reliable method for managing network interfaces, making it essential knowledge for system administrators and network engineers.

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