Developer Error Lookup
Find and Fix HTTP, Cloudflare, Nginx, and Browser Errors Fast
ErrorLookup is a practical troubleshooting reference for production incidents. Use it to decode failures, isolate likely root causes, and move from symptom to verified fix with less guesswork.
How ErrorLookup Helps During Triage
During active incidents, teams need short feedback loops: identify what failed, verify where it failed, and apply the least risky fix first. Each guide is structured to support that flow under pressure.
- Clear meaning of the error in protocol and runtime context.
- Likely causes across application, upstream, gateway, edge, and client layers.
- First checks that quickly confirm or rule out common failure paths.
- Deeper checks for logs, timeouts, header handling, and configuration drift.
- Actionable fixes that map to real deployment and infrastructure workflows.
- Related-error links to compare adjacent symptoms before changing production.
Start with all status codes when you know the code, or use comparison pages when two errors look similar in dashboards and alerts.
Common Error Families
Most outages are not single-layer failures. These groups help you map symptoms to the right layer quickly and avoid chasing the wrong fix.
HTTP client errors (4xx)
Client errors usually indicate request validity, authentication, authorization, routing, or rate-limit issues rather than origin crashes. Focus on request shape, auth state, route ownership, and policy gates. Start with 4xx client errors, then drill into 401, 403, 404, and 429.
HTTP server errors (5xx)
Server errors point to failures in application code, upstream dependencies, or reverse-proxy handoff. Separate malformed upstream responses from intentional overload and timeout behavior before tuning infrastructure. Use 5xx server errors and reference 500, 502, 503, and 504.
Cloudflare edge errors
Cloudflare 52x errors often mean edge-to-origin problems, not browser-to-edge problems. Validate origin reachability, TLS compatibility, firewall rules, and origin response timing in the same window as Cloudflare event logs. See 520, 522, and 524.
Nginx and reverse proxy errors
Proxy-layer errors expose handoff details between clients, load balancers, and upstream services. Inspect timeout values, upstream keepalive settings, header limits, and request cancellation behavior to explain intermittent failures. Review Nginx 499, Nginx 444, and related 502 gateway failures.
Browser, network, and SSL/TLS errors
When the browser reports a network error, HTTP may never have started. Prioritize DNS resolution, socket reachability, certificate identity, and TLS handshake state before debugging application handlers. Start with ERR_CONNECTION_REFUSED, ERR_NAME_NOT_RESOLVED, ERR_CERT_COMMON_NAME_INVALID, and SSL handshake failures.
What to Check First
Use this checklist to reduce mean time to isolate during live incidents.
- Confirm DNS answers and TTL behavior match the expected environment and failover state.
- Verify origin health endpoints from the same network path used by your edge or load balancer.
- Check reverse proxy and ingress config diffs for recent deployment drift.
- Validate certificate validity, SAN coverage, chain order, and protocol/cipher compatibility.
- Correlate access and error logs across CDN, proxy, and origin using timestamps and request IDs.
- Inspect upstream timeout settings at every hop; mismatched budgets create false 502/504 patterns.
- Audit authentication middleware, token expiration, and permission mapping for sudden 401/403 spikes.
- Review cache behavior and stale-content rules to separate cache misses from origin failures.
- Check WAF/CDN rules, bot controls, and geo policies for unintended blocks or challenge loops.
Popular Troubleshooting Paths
Follow these clusters when incident symptoms overlap.
502 / 503 / 504 gateway cluster
502 usually indicates an invalid upstream response, 503 often reflects overload or maintenance, and 504 indicates upstream wait exhaustion. Use 502 vs 503, 503 vs 504, and 502 vs 504 to select the right remediation path.
401 / 403 / 404 access and routing cluster
These errors can look identical to users while requiring different owners to fix them. Use 401 vs 403, 403 vs 404, and 401 vs 404 to distinguish authentication, authorization, and route resolution failures.
Cloudflare 520 / 522 / 524 edge cluster
Start from the edge code, then validate origin connectivity, TLS handshake behavior, and response timing in sequence. Compare 520, 522, and 524 when dashboards show mixed 52x bursts.
Browser network and SSL handshake cluster
ERR_CONNECTION_REFUSED implies socket-level refusal, ERR_NAME_NOT_RESOLVED points to DNS failure, and SSL handshake failed signals TLS negotiation breakdown. Use certificate mismatch guides to validate endpoint identity before rotating certs.
Why Errors That Look Similar Are Different
Superficially similar errors often happen at different layers and require different responders. Treating them as interchangeable increases incident duration and rollback risk.
A 502 can originate in gateway-upstream translation while a 504 is dominated by timeout budget exhaustion. A 401 is usually identity/authentication state, while 403 is policy/authorization state. Comparison guides like 500 vs 502, 500 vs 504, and 404 vs 410 make these distinctions explicit so teams can route incidents to the correct owner quickly.
Use Cases
Debugging production incidents
On-call engineers use ErrorLookup to translate alert noise into a concrete fault hypothesis, then validate it with layer-specific checks.
Investigating deployment failures
After releases, teams correlate new 4xx/5xx patterns with config and code changes to isolate regressions faster.
Validating reverse proxy behavior
Platform teams verify gateway and ingress behavior under timeout pressure, upstream restarts, and connection churn.
Understanding browser-reported connection failures
Support and SRE teams map browser error strings to DNS, TCP, and TLS checks to avoid app-layer misdiagnosis.
Top Developer Error Guides
High-frequency production failures with concrete remediation steps.
Need help fast?
Have questions? Visit the FAQ for direct answers and troubleshooting guidance.
Categories
Navigate by response class when auditing services, APIs, and reverse proxies.