HTTP 502 Bad Gateway

A proxy in front of the application could not get a valid response from it.

What it usually means

A proxy in front of the application could not get a valid response from it. Usually the app process is down or restarting behind nginx, Cloudflare or a load balancer.

What to check first

The application process behind nginx, Caddy, Cloudflare or the load balancer is running.
A restart or deploy in progress; 502s that last seconds are a deploy without a health check.
The upstream address and port the proxy points at.

How ErrorDetector reports it

The target answered, so this is reported as a failed check, not downtime: the alert names the reason, the uptime percentage is untouched, and a recovery notice follows when the check passes again.

Caught by: httpkeywordapi monitors. Read about the incident management feature.

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Other causes

DNS · DNS resolution failed (NXDOMAIN, no answer) REF · Connection refused TLS · TLS handshake failed (certificate error) RST · Connection reset by peer NET · Host unreachable (no route) T/O · Connection timed out RDR · Redirect loop (too many redirects) KEY · Keyword not found on the page SLOW · Response slower than the limit ASSERT · API response failed its body assertion BLK · Blocked by a firewall or bot filter 401 · HTTP 401 Unauthorized 403 · HTTP 403 Forbidden 404 · HTTP 404 Not Found 429 · HTTP 429 Too Many Requests 500 · HTTP 500 Internal Server Error 503 · HTTP 503 Service Unavailable 504 · HTTP 504 Gateway Timeout