How to Diagnose 502, 504 and Connection Reset Errors in Nginx‑Powered Services

This guide explains how to distinguish the root causes of 502 Bad Gateway, 504 Gateway Timeout, and Connection Reset errors in Nginx reverse‑proxy deployments and provides a step‑by‑step, four‑segment troubleshooting workflow with concrete log patterns, shell commands, and configuration tweaks.

502504Backend

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How to Diagnose 502, 504 and Connection Reset Errors in Nginx‑Powered Services

Liangxu Linux

Feb 22, 2023 · Fundamentals

Why Does a TCP Client Send RST When SYN‑ACK Ack Number Mismatches?

The article explains that during TCP's three‑way handshake, if a client receives a SYN‑ACK with an unexpected acknowledgment number, it must send an RST to abort the stale connection, because silently discarding the packet would cause long retransmission delays and prevent the new connection from being established.

Connection ResetRSTSYN-ACK

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Why Does a TCP Client Send RST When SYN‑ACK Ack Number Mismatches?

360 Quality & Efficiency

Apr 24, 2020 · Backend Development

Root Cause Analysis of Connection Reset by Peer in a Go Backend Service

This article details a production incident where a Go backend service returned "connection reset by peer" due to exhausted process file descriptors caused by a saturated database connection pool, and describes the step‑by‑step troubleshooting, socket internals, and the eventual fix.

Connection ResetDatabase Connection PoolGo

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Root Cause Analysis of Connection Reset by Peer in a Go Backend Service