Network Troubleshooting
A browser certificate warning may come from expiry, name mismatch, an incomplete chain or managed inspection. Inspect the presented chain using the correct hostname indication.
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อ่านบทความ : Inspect an HTTPS certificate chain with OpenSSL
Network Troubleshooting
A website that returns a deliberate HTTP error is different from one that never accepts a connection. Collect the response without assuming the code alone identifies the root cause.
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อ่านบทความ : Use HTTP status codes to route a support incident
Network Troubleshooting
A slow page can spend time resolving a name, establishing transport, negotiating TLS or waiting for the server. Timing a small request helps choose the next investigation.
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อ่านบทความ : Measure DNS, TCP and TLS stages with curl
Network Troubleshooting
A Linux workstation may use per-link DNS servers supplied by a VPN or NetworkManager. Reading only resolv.conf can miss the active resolver routing.
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อ่านบทความ : Inspect Linux resolver configuration with resolvectl
Network Troubleshooting
A local-link connectivity problem can stem from address resolution rather than DNS. Inspect the neighbor table for the destination or its gateway.
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อ่านบทความ : Inspect Linux ARP and NDP state with ip neigh
Network Troubleshooting
A resolver problem is easier to explain when the query type, answer and response status are visible together. Use a targeted lookup instead of changing system DNS settings.
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อ่านบทความ : Query a DNS record explicitly with dig
Network Troubleshooting
Two applications can report cannot connect while failing at different stages. A bounded connection attempt provides a clearer error for the network or service owner.
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อ่านบทความ : Distinguish a refused TCP connection from a timeout
Network Troubleshooting
Clock skew can disrupt Kerberos and certificate validation even when network transport works. Inspect the configured time source before changing credentials.
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อ่านบทความ : Check Windows time when authentication fails