Nginx 502 proxy versus origin split response

When this happens

Use this when a cdn, load balancer, reverse proxy, or waf sits in front of the service. Do not stop at the screen message; validate whether the edge/proxy or the origin server creates the failure first.

Symptom checklist

• 502 appears repeatedly in the Nginx UI or logs.
• server_name, location, proxy_pass, upstream logs differs between successful and failed requests.
• The issue appears only after separating Nginx layer from backend application.
• It often follows deploys, permission changes, configuration edits, or data refreshes.

Likely causes

• 502 specifically changes the investigation surface for Nginx: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• The proxy can connect to the edge but not to the intended backend target.
• Health checks use a different path, port, host header, or protocol than real traffic.
• FastCGI, app server, or container workers are down while the front server is alive.
• Timeouts are configured inconsistently across proxy, load balancer, and app layers.
• A deploy changed backend port or protocol without updating the routing layer.
• For the proxy versus origin split case, the first useful clue is whether the edge/proxy or the origin server creates the failure.

First 1-minute checks

1. Write down the first failure time, latest change, affected user, path, and object ID.
2. Compare server_name, location, proxy_pass, upstream logs for success and failure in the same window.
3. Test the hypothesis: upstream failure caused by unhealthy target, wrong backend port, proxy timeout, FastCGI handoff, or load balancer health check mismatch.
4. Classify this as proxy versus origin split: whether the edge/proxy or the origin server creates the failure.
5. Capture current values before changing configuration.

First evidence

Treat 502 as a proxy versus origin split case. First collect evidence for whether the edge/proxy or the origin server creates the failure.

Output examples

Normal output

Direct origin and routed proxy return equivalent status and headers.

Failing output

Origin succeeds but proxy fails, or proxy succeeds while direct origin exposes the real backend error.

Output-to-action branches

• A CDN, load balancer, reverse proxy, or WAF sits in front of the service.
  Test direct origin and routed traffic separately, then change only the layer that differs.
• The working and failing outputs differ.
  Act on the differing layer first: For 502, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Command output is normal but users still fail.
  Separate browser cache, cookies, permissions, and network location before declaring it fixed.

Do not do this

• Do not assume the CDN error code is the application root cause.
• Do not change multiple layers before identifying the failing layer.
• Do not delete production data, grant broad permissions, or disable security controls as a first response.

Evidence quality

Auto-generated operator draft: includes issue-specific causes, commands, output branches, and unsafe-action warnings. Official-source links and real incident validation are queued for enrichment.

Commands to run first

grep -R "502" ./logs

nginx -T | grep -n "server_name\|location\|proxy_pass"

tail -n 200 /var/log/nginx/error.log

curl -Iv http://127.0.0.1:3000

ss -lntp

curl -Iv http://127.0.0.1:3000/health

grep -R "upstream\|502\|503\|504\|proxy_fcgi" ./logs /var/log/nginx

systemctl status nginx php-fpm

curl -Iv https://example.com --resolve example.com:443:ORIGIN_IP

Fix order

1. Record the full 502 message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for upstream failure caused by unhealthy target, wrong backend port, proxy timeout, FastCGI handoff, or load balancer health check mismatch.
3. Compare the failing case with a successful case before editing settings.
4. If this is the proxy versus origin split branch, Test direct origin and routed traffic separately, then change only the layer that differs.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For 502, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Test the backend directly before changing the public proxy.
• Align health check path, host header, port, and expected status.
• Restart or scale the failing worker pool after checking logs.
• Tune proxy and app timeouts together.
• Record the working direct-backend command in the runbook.
• For the proxy versus origin split branch, Test direct origin and routed traffic separately, then change only the layer that differs.

Evidence links

• NGINX Admin Guide official
• NGINX reverse proxy guide official

Verification metadata

• operator-draft
• official-reference-linked
• 2026-07-23

Update queue

• Review cadence
  weekly-source-review
• Next enrichment
  Add one official-source check and one real output example for Nginx 502.

Environment-specific checks

• Shared hosting, proxies, VPNs, or CDN layers can change Nginx layer from backend application results.
• Do not trust only the Nginx UI; compare command output.
• Japanese hosting panels may show completion before DNS or SSL fully propagates.
• Test from both office and external networks.

Prevent it next time

• Store normal examples for server_name, location, proxy_pass, upstream logs.
• Add location priority, proxy headers, and upstream timeout to the release checklist.
• Keep recurring errors in the same note format.
• Split alerts by error rate, latency, certificates, disk, and permission changes.

Related notes

• Nginx 502 Bad Gateway upstream connection failure
• Nginx 301 loop fix note
• Nginx 413 fix note
• Nginx 301 loop fix note 2
• Nginx 413 fix note 2
• Nginx 301 loop fix note 3
• Nginx 413 fix note 3
• Nginx 301 loop fix note 4