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Networking Troubleshooting Cheat Sheet

A handy cheat sheet covering essential networking troubleshooting commands, techniques, and concepts for network administrators and IT professionals.

Basic Network Troubleshooting Tools

Ping

Description:

Tests basic network connectivity by sending ICMP echo requests to a target host.

Command:

ping <destination>

Example:

ping google.com or ping 192.168.1.1

Troubleshooting Use:

Verify network connectivity, check for packet loss, and measure round-trip time.

Common Issues:

Destination unreachable, request timeout, high latency.

Solutions:

Check network configuration, verify DNS resolution, investigate network congestion or hardware issues.

Traceroute/Tracepath

Description:

Traces the route taken by packets to reach a destination, displaying each hop along the path.

Command:

traceroute <destination> (or tracepath <destination> on Linux)

Example:

traceroute google.com

Troubleshooting Use:

Identify network bottlenecks, locate points of failure, and map the network path.

Common Issues:

Hops timing out, unexpected routing paths, excessive latency at specific hops.

Solutions:

Investigate problematic hops, check firewall configurations, and review routing tables.

Nslookup/Dig

Description:

Queries DNS servers to obtain domain name or IP address information.

Command:

nslookup <hostname> or dig <hostname>

Example:

nslookup google.com or dig google.com

Troubleshooting Use:

Verify DNS resolution, check DNS records, and troubleshoot DNS-related issues.

Common Issues:

Incorrect DNS resolution, DNS server unreachable, incorrect DNS records.

Solutions:

Verify DNS server settings, check DNS records, and troubleshoot DNS server connectivity.

Advanced Network Analysis

Tcpdump/Wireshark

Description:

Packet capture and analysis tools used to inspect network traffic.

Command:

tcpdump -i <interface> <filter> or Wireshark GUI

Example:

tcpdump -i eth0 port 80

Troubleshooting Use:

Analyze network traffic, identify protocols, troubleshoot network performance issues, and detect security threats.

Common Issues:

Excessive traffic, unexpected protocols, suspicious activity, performance bottlenecks.

Solutions:

Filter traffic, analyze packet contents, and identify root causes of network issues.

Netstat/Ss

Description:

Displays network connections, routing tables, interface statistics, and masquerade connections.

Command:

netstat -an or ss -an

Example:

netstat -an | grep :80

Troubleshooting Use:

Identify listening ports, check connection states, and monitor network traffic.

Common Issues:

High number of connections, connections in CLOSE_WAIT state, unauthorized listening ports.

Solutions:

Investigate suspicious connections, identify resource-intensive processes, and secure listening ports.

Iperf/Nuttcp

Description:

Network bandwidth measurement tools used to test network throughput and performance.

Command:

iperf -s (server) and iperf -c <server_ip> (client)

Example:

iperf -c 192.168.1.100

Troubleshooting Use:

Measure network bandwidth, identify network bottlenecks, and evaluate network performance.

Common Issues:

Low bandwidth, high latency, packet loss.

Solutions:

Check network infrastructure, identify bandwidth-intensive applications, and optimize network configuration.

Common Network Issues and Solutions

IP Address Conflicts

Issue: Two or more devices are configured with the same IP address.

Symptoms: Intermittent connectivity issues, inability to access network resources.

Solutions:

  • Use DHCP to dynamically assign IP addresses.
  • Manually configure static IP addresses, ensuring each device has a unique address.
  • Use ping to identify the conflicting IP address.
  • Check ARP tables to determine the MAC address associated with the conflicting IP address.

DNS Resolution Problems

Issue: Inability to resolve domain names to IP addresses.

Symptoms: Cannot access websites by name, but can access them by IP address.

Solutions:

  • Verify DNS server settings.
  • Use nslookup or dig to query DNS servers.
  • Flush the DNS cache using ipconfig /flushdns (Windows) or sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder (macOS).
  • Check the host file for incorrect entries.

Gateway Issues

Issue: Devices are unable to communicate outside the local network.

Symptoms: Cannot access the internet, cannot ping external IP addresses.

Solutions:

  • Verify the default gateway setting.
  • Ensure the gateway device is reachable.
  • Check the gateway device’s configuration.
  • Traceroute to a known external IP address to identify the point of failure.

Wireless Network Troubleshooting

Signal Strength and Interference

Issue: Weak wireless signal or interference affecting network performance.

Symptoms: Slow connection speeds, intermittent disconnections, high latency.

Solutions:

  • Check the wireless signal strength using a Wi-Fi analyzer tool.
  • Identify sources of interference (e.g., microwave ovens, cordless phones).
  • Move closer to the wireless access point.
  • Change the wireless channel to avoid overlapping with other networks.

Authentication Problems

Issue: Inability to connect to the wireless network due to incorrect credentials or authentication failures.

Symptoms: Incorrect password error, authentication timeout.

Solutions:

  • Verify the wireless password.
  • Check the wireless security settings (e.g., WPA2, WPA3).
  • Ensure the wireless client is configured to use the correct authentication method.
  • Restart the wireless access point and client device.

DHCP Issues

Issue: Devices are unable to obtain an IP address from the DHCP server.

Symptoms: APIPA address (169.254.x.x), no internet access.

Solutions:

  • Verify the DHCP server is running and reachable.
  • Check the DHCP scope and lease time.
  • Release and renew the IP address on the client device.
  • Restart the DHCP server.
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