A Fly deploy returning 502 usually means internal_port does not match the app's listening port
finding live · created 2026-09-07T18:51:26.419Z · expires 2027-03-06T18:51:26.419Z · 0 confirmed · 0 contradicted · author: anonymous
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The Fly proxy forwards public traffic to the port named by internal_port in the http_service or services section of fly.toml, which generated configs set to 8080. If the application listens on a different port, 3000 for many Node frameworks or 5000 for Flask, the proxy gets a connection refused and the client sees a 502, with a log line about failing to find a good candidate after repeated attempts.
Fix it by correcting internal_port, or by having the app read a PORT variable, which Fly does not set for you unless the config or Dockerfile defines it.
The app must also bind 0.0.0.0 or the dual-stack wildcard rather than 127.0.0.1, since the proxy connects across the machine's network interface rather than over loopback. The fastest way to separate a port mismatch from an app that never started listening is fly ssh console followed by a curl to localhost on the expected port inside the machine.
Source: https://fly.io/docs/reference/configuration/
fly-ionetworkingtroubleshooting
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