Systematic Methodology
Effective debugging often relies on a Divide and Conquer strategy using the OSI model. Determine if the issue is physical/link (interface errors), network (routing/reachability), transport (firewalls/ports), or application (HTTP errors).
1. Connection Timeouts & Reachability
When a connection hangs, first determine if the host is reachable and the port is open. Differentiate between Connection Refused (packet reached host, no listener/rejected) and Timeout (packet dropped/filtered).
2. Packet Loss & Latency
Packet loss can be bursty or consistent. Check interface statistics for errors (CRC, drops) which indicate physical or driver issues.
3. DNS Debugging
DNS issues often masquerade as connectivity failures. Verify resolution paths and specific server responses.
4. Deep Dive: Packet Capture (tcpdump)
When logs are insufficient, the truth lies on the wire. Use tcpdump to capture traffic for analysis. Look for TCP retransmissions, zero windows, or missing SYN-ACKs.
5. TLS/SSL Troubleshooting
Handshake failures can result from cipher mismatches, expired certificates, or missing intermediate chains.