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Q&A
Q&AThe connection attempt left the observed point, but no response returned there.
It narrows the investigation but does not identify one root cause: route failure, filtering, destination outage, asymmetric routing, or capture at the wrong interface can all look similar. Capture at both ends, check socket listeners and network policy, and correlate with route and firewall telemetry.
Q&A
Q&AUDP has no connection handshake and applications may not answer an invalid or empty probe.
Unlike TCP, UDP does not establish a transport connection before sending a datagram. A missing response can mean filtering, packet loss, an invalid request, or an application that does not reply. Use a valid protocol-level health check and capture at sender and receiver when diagnosing the path.
Q&A
Q&ATCP establishes an ordered byte stream; UDP sends datagrams without delivery acknowledgement from the transport.
TCP connection establishment and retransmission make a port-connectivity test meaningful, but application message boundaries still need framing. UDP has no handshake, so silence may mean a dropped packet, invalid request, or a service that does not answer probes. Test using a valid protocol request and inspect both ends when possible.
unix
9 minSeparate resolver configuration, caching, search domains, authoritative records, and application-level DNS behavior.
unix
11 minDiagnose connection failures by separating DNS, routing, transport, listening sockets, and application protocol behavior.
http
5 minKeep list traversal stable, validate cached representations, and bound retry work when networks fail at ambiguous points.
distributed systems
9 minLearn why network retries create duplicate effects and how idempotency keys and an outbox close common reliability gaps.
Q&A
Q&AInspect local socket state and bind address before changing firewall policy.
On Linux, `ss -lntup` lists listening sockets; `netstat` may be available via net-tools on older systems. Confirm the process, address, and network namespace. A service bound only to 127.0.0.1 is not externally reachable; also check container port mapping and host policy.