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Network Troubleshooting Tools

Updated May 25, 2020 ·

Overview​

Troubleshooting application connectivity is easier when you move layer by layer. Start with local network state, confirm routing, test name resolution, and then test the application port.

Troubleshooting Flow​

Use this order when the problem is unclear:

  1. Check the local interface and IP address.
  2. Check the default gateway.
  3. Check routing to the destination.
  4. Check DNS resolution.
  5. Check the application port.
  6. Check firewalls, load balancers, proxies, and application health.

If a web service returns an HTTP 500 response, the network path is likely working and the next step is application or server troubleshooting.

Interface Checks​

Use ifconfig or ip address on Linux to inspect interface state, IP addresses, MAC addresses, and traffic counters.

ifconfig
ip address

Common checks:

  • Confirm the interface is up.
  • Confirm the IP address and subnet are correct.
  • Confirm the expected MAC address is present.
  • Review transmit and receive counters.
  • Check the MTU when fragmentation or large packet issues are suspected.

Note: Some Linux distributions have deprecated ifconfig. Prefer ip address on modern Linux systems.

Ping​

ping tests basic IP reachability with ICMP echo requests and replies.

ping -c 5 www.cisco.com

Ping can show:

  • Whether replies are received.
  • Round-trip latency.
  • Packet loss.
  • TTL values.
  • IPv4 or IPv6 reachability.

Note: A failed ping does not always mean the host is down. Firewalls often block ICMP while allowing application ports such as TCP 443.

Traceroute​

traceroute and Windows tracert show the path packets take toward a destination.

traceroute www.cisco.com
tracert www.cisco.com

Traceroute helps identify:

  • Routing loops.
  • Unexpected paths.
  • The hop where traffic stops.
  • Latency between hops.

Linux traceroute commonly uses UDP probes by default, while some platforms use ICMP. Firewalls can affect the output.

DNS Checks​

Use nslookup or dig to confirm that a name resolves to the expected address.

nslookup www.cisco.com
dig www.cisco.com

DNS checks help identify:

  • Missing records.
  • Unexpected IP addresses.
  • Incorrect DNS servers.
  • Cached stale records.

For more information, please see Analyzing DNS page.

Application Port Checks​

After IP reachability works, test the actual application port.

curl -I https://example.com
telnet example.com 443

If the port is blocked, check:

  • Host firewalls.
  • Network firewalls.
  • Security groups or network ACLs.
  • Load balancer listeners and health checks.
  • Proxy and reverse proxy rules.

Performance Checks​

Some problems only appear under load. Tools such as iperf can generate traffic to test throughput, delay, and packet loss.

When performance is the issue, also check:

  • Bandwidth saturation.
  • QoS policy.
  • Packet drops.
  • Load balancer health.
  • Backend database or application latency.