How to Troubleshoot Linux Network Issues Quickly?

Quick Answer: Use ip -c -br addr show && ip route && ip neigh to instantly check interfaces, routing, and ARP tables. The modern ip command replaces outdated tools like ifconfig and netstat for comprehensive network diagnostics.

Essential Network Troubleshooting Commands

# Complete network status in one command
ip -c -br addr show && echo "--- ROUTES ---" && ip -c route show && echo "--- ARP ---" && ip -c neigh show

# Check interface status quickly
ip -c -br link show

# Show only active interfaces
ip link show up

# Display routing table
ip route show

# Check default gateway
ip route show default

# View ARP table
ip neigh show

# Test connectivity to specific IP
ping -c 4 8.8.8.8

# Check DNS resolution
nslookup google.com

# Show interface statistics
ip -s link show eth0

# Monitor network changes in real-time
ip monitor

Frequently Asked Questions

Q: What command replaces ifconfig in modern Linux? A: The ip command replaces ifconfig. Use ip addr show instead of ifconfig -a to display network interfaces and IP addresses with better formatting and features.

Q: How do I check if my network interface is up? A: Use ip link show up to see only active interfaces, or ip -br link show for a brief overview of all interfaces with their status (UP/DOWN).

Q: What’s the fastest way to check network connectivity? A: Use ping -c 4 8.8.8.8 to test internet connectivity, then ip route show default to verify your default gateway configuration.

Q: How do I view the routing table in Linux? A: Use ip route show to display the complete routing table, or ip route show default to see only the default gateway information.

Q: How can I see all network information at once? A: Use the one-liner: ip -c -br addr show && ip route && ip neigh to display interfaces, routing, and ARP information in a single command.

Essential Steps to Diagnose Network Issues

1. Check interface status: Use ip -br link show to verify network interfaces are UP and configured
2. Verify IP configuration: Use ip addr show to confirm IP addresses are assigned correctly
3. Test local connectivity: Use ping gateway_ip to verify connection to your router/gateway
4. Check routing table: Use ip route show to ensure proper routing configuration
5. Test external connectivity: Use ping 8.8.8.8 to verify internet connectivity
6. Verify DNS resolution: Use nslookup domain.com to check domain name resolution

Most Important Network Diagnostic Commands

What Are the Most Important ip Command Options?

Essential Interface Commands

# Brief interface overview
ip -br addr show

# Detailed interface information
ip addr show eth0

# Show only active interfaces
ip link show up

# Interface statistics and errors
ip -s link show eth0

Critical Routing Commands

# Complete routing table
ip route show

# Default gateway only
ip route show default

# Route to specific destination
ip route get 8.8.8.8

# Add temporary route
sudo ip route add 192.168.1.0/24 via 10.0.0.1

How Do You Check Network Connectivity Step-by-Step?

1. Verify Physical and Link Layer

# Check if interfaces exist and are UP
ip link show

# Check cable connection (for Ethernet)
ethtool eth0 | grep "Link detected"

# View interface statistics for errors
ip -s link show eth0

2. Verify Network Layer Configuration

# Check IP address assignment
ip addr show

# Verify subnet configuration
ip route show | grep "scope link"

# Check for IP conflicts
ping -c 1 $(ip route show default | awk '{print $3}')

3. Test Network Connectivity

# Test local gateway connectivity
ping -c 4 $(ip route show default | awk '{print $3}')

# Test external connectivity
ping -c 4 8.8.8.8

# Test DNS connectivity
ping -c 4 google.com

What Network Information Does ip Command Provide?

Interface Information

# Show all network interfaces
ip addr show

# Brief format for quick overview
ip -br addr show

# Color output for better readability
ip -c addr show

# Show specific interface details
ip addr show dev eth0

Routing Information

# Complete routing table
ip route show

# IPv6 routing table
ip -6 route show

# Routing cache information
ip route show cache

# Policy routing rules
ip rule show

Neighbor Discovery (ARP)

# Show ARP table entries
ip neigh show

# Show only reachable neighbors
ip neigh show nud reachable

# IPv6 neighbor discovery
ip -6 neigh show

# Neighbors on specific interface
ip neigh show dev eth0

What Are Advanced Network Troubleshooting Techniques?

Performance Analysis

# Monitor interface bandwidth
ip -s link show eth0 | grep bytes

# Check for packet drops
ip -s link show | grep -E "(RX|TX).*dropped"

# Real-time network monitoring
watch -n 1 'ip -s link show eth0'

# Check interface errors
ethtool -S eth0 | grep error

DNS Troubleshooting

# Test DNS resolution
nslookup google.com

# Use different DNS server
nslookup google.com 8.8.8.8

# Check DNS configuration
cat /etc/resolv.conf

# Test reverse DNS lookup
nslookup 8.8.8.8

Port and Service Diagnostics

# Show listening ports (replaces netstat)
ss -tuln

# Show specific port
ss -tuln | grep :80

# Show processes using ports
ss -tulnp

# Check if specific port is open
nc -zv google.com 80

When Should You Use Different Diagnostic Commands?

Basic Connectivity Issues

# No internet connection
ping -c 4 8.8.8.8                    # Test external connectivity
ip route show default                # Check default gateway
ip addr show                         # Verify IP configuration

# Can't reach local devices
ip neigh show                        # Check ARP table
ping -c 4 192.168.1.1               # Test gateway
ip route show | grep "scope link"    # Check local routes

Application Connectivity Problems

# Web server not responding
ss -tuln | grep :80                  # Check if service is listening
curl -I http://server.com            # Test HTTP connectivity
telnet server.com 80                 # Test raw TCP connection

# Database connection issues
ss -tuln | grep :3306                # Check MySQL port
ping -c 4 db-server.com              # Test basic connectivity
nslookup db-server.com               # Verify DNS resolution

Performance Investigation

# Slow network performance
ip -s link show                      # Check for packet drops/errors
ethtool eth0 | grep Speed            # Verify link speed
mtr google.com                       # Traceroute with statistics

# Intermittent connectivity
ping -i 0.2 gateway_ip               # Rapid ping test
watch -n 1 'ip neigh show'           # Monitor ARP changes

What Are All ip Command Options?

What Are Essential Network Troubleshooting Practices?

Systematic Approach

# Always start with the one-liner overview
ip -c -br addr show && echo "--- ROUTES ---" && ip route show && echo "--- ARP ---" && ip neigh show

# Document baseline configurations
ip addr show > network_baseline_$(date +%Y%m%d).txt
ip route show >> network_baseline_$(date +%Y%m%d).txt

# Test in layers (OSI model approach)
# 1. Physical: Check cables and interface status
# 2. Network: Test IP connectivity
# 3. Transport: Check ports and services
# 4. Application: Test specific applications

Performance Monitoring

# Create monitoring aliases
alias netstat='ss -tuln'
alias netmon='watch -n 2 "ip -s link show"'
alias routecheck='ip route show && ip neigh show'

# Regular network health checks
#!/bin/bash
echo "=== Network Health Check $(date) ==="
ip -br link show
ping -c 1 $(ip route show default | awk '{print $3}') >/dev/null && echo "Gateway: OK" || echo "Gateway: FAIL"
ping -c 1 8.8.8.8 >/dev/null && echo "Internet: OK" || echo "Internet: FAIL"

How Can You Create Useful Network Aliases?

Essential Network Aliases

# Add to ~/.bashrc
alias netinfo='ip -c -br addr show && echo "--- ROUTES ---" && ip -c route show'
alias netstat='ss -tuln'
alias ports='ss -tulnp'
alias gateway='ip route show default'
alias myip='curl -s ifconfig.me'

Advanced Network Functions

# Network troubleshooting function
netcheck() {
    echo "=== Interface Status ==="
    ip -br link show
    echo -e "\n=== IP Configuration ==="
    ip -br addr show
    echo -e "\n=== Default Gateway ==="
    ip route show default
    echo -e "\n=== DNS Test ==="
    nslookup google.com | head -5
}

# Quick connectivity test
conntest() {
    local target=${1:-8.8.8.8}
    echo "Testing connectivity to $target..."
    if ping -c 3 "$target" >/dev/null 2>&1; then
        echo "✓ Connectivity to $target: OK"
    else
        echo "✗ Connectivity to $target: FAILED"
    fi
}

What Commands Are Related to Network Troubleshooting?

• ss – Socket statistics (replaces netstat)
• ping – Test network connectivity
• traceroute – Trace network path to destination
• mtr – Continuous traceroute with statistics
• nslookup/dig – DNS lookup and troubleshooting
• nc (netcat) – Network connection testing
• ethtool – Ethernet interface configuration
• tcpdump – Network packet capture and analysis

Common Network Troubleshooting Problems and Solutions

“Network is unreachable” Errors

Problem: Cannot connect to any external networks

# Error: Network is unreachable

Diagnosis:

# Check interface status
ip link show

# Check IP configuration
ip addr show

# Check routing table
ip route show

# Check default gateway
ip route show default

Solutions:

# Bring interface up if down
sudo ip link set eth0 up

# Add default route if missing
sudo ip route add default via 192.168.1.1

# Restart networking service
sudo systemctl restart networking

# Check DHCP configuration
sudo dhclient eth0

DNS Resolution Failures

Problem: Can ping IP addresses but not domain names

Diagnosis:

# Test basic connectivity
ping -c 4 8.8.8.8

# Test DNS resolution
nslookup google.com

# Check DNS configuration
cat /etc/resolv.conf

# Test different DNS server
nslookup google.com 8.8.8.8

Solutions:

# Fix DNS configuration
echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf

# Restart DNS service
sudo systemctl restart systemd-resolved

# Flush DNS cache
sudo systemctl flush-dns

# Use public DNS temporarily
dig @8.8.8.8 google.com

Interface Not Coming Up

Problem: Network interface remains DOWN

Diagnosis:

# Check interface status
ip link show eth0

# Check for hardware issues
dmesg | grep eth0

# Check interface configuration
cat /etc/network/interfaces

# Check NetworkManager status
systemctl status NetworkManager

Solutions:

# Manually bring interface up
sudo ip link set eth0 up

# Restart NetworkManager
sudo systemctl restart NetworkManager

# Check cable connection
ethtool eth0 | grep "Link detected"

# Reset interface configuration
sudo ifdown eth0 && sudo ifup eth0

No Default Gateway

Problem: No route to external networks

Diagnosis:

# Check current routes
ip route show

# Check for default gateway
ip route show default

# Check gateway connectivity
ping -c 4 192.168.1.1  # Replace with your gateway

Solutions:

# Add default gateway manually
sudo ip route add default via 192.168.1.1

# Request DHCP configuration
sudo dhclient eth0

# Check DHCP lease
cat /var/lib/dhcp/dhclient.leases

# Restart networking
sudo systemctl restart networking

ARP Table Issues

Problem: Cannot reach devices on local network

Diagnosis:

# Check ARP table
ip neigh show

# Look for failed entries
ip neigh show | grep FAILED

# Check interface on same subnet
ip route show | grep "scope link"

Solutions:

# Clear ARP cache
sudo ip neigh flush all

# Add static ARP entry
sudo ip neigh add 192.168.1.100 lladdr 00:11:22:33:44:55 dev eth0

# Delete problematic entry
sudo ip neigh del 192.168.1.100 dev eth0

# Force ARP refresh
ping -c 1 192.168.1.100 && ip neigh show 192.168.1.100

Slow Network Performance

Problem: Network connections are slow or dropping packets

Diagnosis:

# Check interface statistics
ip -s link show eth0

# Look for errors and drops
ip -s link show | grep -E "(errors|dropped)"

# Check link speed
ethtool eth0 | grep Speed

# Monitor real-time statistics
watch -n 1 'ip -s link show eth0'

Solutions:

# Check for duplex mismatch
ethtool eth0 | grep -E "(Speed|Duplex)"

# Reset interface statistics
sudo ethtool -S eth0 --reset

# Check MTU size
ip link show eth0 | grep mtu

# Test with different MTU
sudo ip link set eth0 mtu 1500

Port Connection Issues

Problem: Cannot connect to specific services/ports

Diagnosis:

# Check if service is listening
ss -tuln | grep :80

# Test port connectivity
nc -zv server.com 80

# Check firewall rules
sudo iptables -L

# Test local vs remote connectivity
telnet localhost 80
telnet server.com 80

Solutions:

# Start service if not running
sudo systemctl start apache2

# Check service status
sudo systemctl status apache2

# Open firewall port
sudo ufw allow 80

# Test with curl
curl -I http://server.com

# Check SELinux if applicable
sudo setsebool -P httpd_can_network_connect 1

IPv6 Connectivity Issues

Problem: IPv4 works but IPv6 doesn’t

Diagnosis:

# Check IPv6 configuration
ip -6 addr show

# Test IPv6 connectivity
ping6 google.com

# Check IPv6 routing
ip -6 route show

# Check IPv6 neighbors
ip -6 neigh show

Solutions:

# Enable IPv6 if disabled
echo 0 | sudo tee /proc/sys/net/ipv6/conf/all/disable_ipv6

# Request IPv6 from DHCP
sudo dhclient -6 eth0

# Check router advertisements
sudo radvdump

# Add IPv6 route manually
sudo ip -6 route add default via fe80::1 dev eth0

Network Namespace Issues

Problem: Connectivity works in one namespace but not another

Diagnosis:

# List network namespaces
ip netns list

# Check interface in namespace
sudo ip netns exec namespace1 ip addr show

# Check routing in namespace
sudo ip netns exec namespace1 ip route show

Solutions:

# Move interface to namespace
sudo ip link set eth1 netns namespace1

# Add route in namespace
sudo ip netns exec namespace1 ip route add default via 192.168.1.1

# Test connectivity in namespace
sudo ip netns exec namespace1 ping 8.8.8.8

# Create veth pair for namespace communication
sudo ip link add veth0 type veth peer name veth1
sudo ip link set veth1 netns namespace1

Mastering modern Linux network troubleshooting with the ip command and systematic diagnostic approaches enables rapid problem resolution and efficient network management in professional environments.