The Tool Desk
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What the 502 tells you
Nginx is acting as a reverse proxy. It accepts the browser or API connection, forwards the request to Gunicorn, waits for an upstream response, and sends that response back to the client. A 502 is generated when the handoff fails: the listener is absent, the address is wrong, the connection is refused or closed, the Nginx worker cannot access a Unix socket, or the upstream speaks an unexpected protocol.
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The status alone is not specific enough to choose a fix. A missing socket and an application import failure can both appear as 502 to the client. The Nginx error-log line is the first branch in the diagnosis, and Gunicorn and systemd logs provide the second half. Gunicorn’s FAQ notes that a proxy can report 502 while Gunicorn shows only worker-startup messages, so inspect both ends rather than assuming the application logged the fault.
Use this triage order
- Record the timestamp and URL that returned 502.
- Watch the Nginx error log while reproducing the request; copy the complete
connect(), timeout, or upstream-response message. - Check the Gunicorn service state and recent journal entries for import errors, bind failures, crashes, or workers that exit immediately.
- Compare the listener in Gunicorn’s
bindsetting with the address in Nginx’supstreamandproxy_pass. - Run a direct socket or TCP request as the account used by Nginx. If that direct request fails, repair Gunicorn, the listener, or access controls before editing application code.
- Validate Nginx syntax, reload only the changed service, and reproduce the same URL while tailing both logs.
Read the logs before changing configuration
Nginx error log
On many distributions the file is /var/log/nginx/error.log; a systemd journal may contain the same records. Follow it in one terminal:
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sudo tail -f /var/log/nginx/error.logIn another terminal, request the failing URL. Common messages and their implications are:
| Observed message | Likely fault | First check |
|---|---|---|
connect() failed (2: No such file or directory) while connecting to upstream |
The configured Unix-socket path does not exist, or it was removed during a restart. | Compare the path character-for-character and inspect the socket unit or Gunicorn startup sequence. |
connect() failed (13: Permission denied) while connecting to upstream |
The Nginx worker cannot traverse a parent directory or access the socket’s owner, group, or mode. | Check directory execute permissions and the socket’s ownership and mode as the Nginx user. |
connect() failed (111: Connection refused) while connecting to upstream |
No process is listening at the specified TCP address, or Gunicorn has stopped accepting connections. | Inspect Gunicorn status and verify the listening address with the service configuration. |
upstream timed out |
The listener was reached but did not produce a response within the configured timeout. | Check worker starvation, long requests, application dependencies, and whether the workload is streaming or long-polling. |
upstream prematurely closed connection |
Gunicorn or a worker closed the connection before Nginx received a complete response. | Inspect Gunicorn worker exits, application exceptions, memory pressure, and protocol settings. |
Keep the original line when troubleshooting. The upstream name, address, and error number often reveal a typo that is not visible in a browser’s generic 502 page.
Gunicorn and systemd logs
Replace gunicorn.service with the unit used by your deployment:
sudo systemctl status gunicorn.service
sudo journalctl -u gunicorn.service -n 100 --no-pager
sudo journalctl -u gunicorn.service -fLook for a Python import or settings error, an invalid working directory, a port already in use, a socket bind failure, permission errors, and workers that start and then die. A service can be marked active while every worker is failing, so read the recent journal rather than relying only on the green service-state line. If a separate socket unit is used, inspect it too:
sudo systemctl status gunicorn.socket
sudo journalctl -u gunicorn.socket -n 100 --no-pagerMake Nginx and Gunicorn name the same endpoint
There are two normal transports. A Unix socket is a filesystem object on the same host; TCP uses an IP address and port. Nginx must point to exactly the listener Gunicorn creates.
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| Transport | Gunicorn listener | Nginx form | What can break |
|---|---|---|---|
| Unix socket | A path such as /run/gunicorn.sock |
server unix:/run/gunicorn.sock fail_timeout=0; inside an upstream block, then proxy_pass http://app_server; |
Different paths, a socket deleted on restart, parent-directory permissions, wrong owner or mode, or a stale socket left by another process. |
| TCP | An address such as 127.0.0.1:8000 or 192.168.0.7:8000 |
server 127.0.0.1:8000; (or the actual address) and proxy_pass http://app_server; |
Wrong port, wrong interface, firewall policy, service bound only to loopback, or Nginx and Gunicorn running on different hosts. |
Gunicorn’s deployment example shows both a Unix-socket server declaration, server unix:/tmp/gunicorn.sock fail_timeout=0;, and a TCP declaration such as 192.168.0.7:8000. The important detail is not the sample path or address; it is that one value is used consistently by the process that listens and the proxy that connects.
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Inspect the effective settings
Find the actual service command, environment file, or configuration module used at boot. A manually launched Gunicorn process may be listening on a different port than the systemd service. Likewise, an Nginx include file can override the location you edited. Dump the loaded Nginx configuration and search for the upstream and location:
sudo nginx -T | lessThen compare the resulting proxy_pass target with Gunicorn’s --bind option or equivalent configuration. Do not fix a mismatch by adding a second listener until you know which unit owns the production endpoint.
Test the upstream directly as Nginx
Unix socket test
For a systemd-managed socket at /run/gunicorn.sock, run the request under the same account as the Nginx worker:
sudo -u www-data curl --unix-socket /run/gunicorn.sock httpIf your application expects a path, query string, or a particular Host header, append that request data after confirming the basic connection. A failure here separates proxy routing from application behavior: fix the listener, path, ownership, group, mode, or service lifecycle before changing Nginx headers.
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TCP test
For a local TCP listener, test the exact address and port:
sudo -u www-data curl -v http://127.0.0.1:8000/Use the real non-loopback address when Nginx connects to another host. A successful response from a shell account does not prove Nginx can connect; the Nginx-user test is what exposes access-control and network-policy differences.
Repair systemd socket activation
When systemd owns the Unix socket, the socket unit creates it and can activate the service when a connection arrives. A representative socket unit uses these documented settings:
[Unit]
Description=gunicorn socket
[Socket]
ListenStream=/run/gunicorn.sock
SocketUser=www-data
SocketGroup=www-data
SocketMode=0660
[Install]
WantedBy=sockets.targetEnable and start it with:
sudo systemctl enable --now gunicorn.socketEnsure the companion service requires the socket, starts after the intended prerequisites, uses the correct working directory, and executes the Gunicorn binary from the environment that contains your application and dependencies. The service and socket must agree on who creates the listener. If Gunicorn itself binds a path while systemd also owns that path, the second bind can fail or leave you diagnosing the wrong process.
After editing a unit, reload systemd’s unit definitions, restart the changed unit, and verify the resulting object:
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sudo systemctl daemon-reload
sudo systemctl restart gunicorn.socket
sudo systemctl restart gunicorn.service
ls -l /run/gunicorn.sockThe final listing should show the expected owner, group, and mode. If the socket disappears after a service restart, determine which unit removes and recreates it; Nginx cannot connect during that gap unless the lifecycle is coordinated.
Fix filesystem permissions and confinement
Unix-socket access is a two-part check. The Nginx worker needs execute permission on every parent directory in the path and read/write access to the socket itself. A mode of 0660 is useful only when Nginx belongs to the socket’s group.
- Check each directory in the path with
namei -l /run/gunicorn.sockand confirm that the Nginx account can traverse it. - Check the socket with
ls -l /run/gunicorn.sock; verify owner, group, and mode after every restart. - Do not assume a manually created socket survives reboot or service replacement. Let the service or socket unit recreate it with deterministic metadata.
- If mandatory access control or a service sandbox is enabled, inspect its denial logs and permitted paths. A Unix permission that looks correct does not override a confinement policy.
- For TCP, check host firewalls, security groups, bind interfaces, and routing when Nginx and Gunicorn are on different machines.
Changing the socket to a world-writable mode may hide the symptom while creating an avoidable security exposure. Prefer the intended service account and group.
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Use the required proxy headers and protocol
Once the connection works, pass the request context Gunicorn and the application expect. A typical Nginx location includes:
location / {
proxy_set_header Host $host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_pass http://app_server;
}The Host header preserves virtual-host routing. X-Forwarded-For carries the client chain, and X-Forwarded-Proto tells the application whether the original request was HTTP or HTTPS. Use the application’s documented trusted-proxy settings so it accepts forwarded headers only from your Nginx addresses. When Nginx and Gunicorn are on different hosts, restrict trusted forwarded-header sources to those proxy addresses; otherwise a client could supply values that the application treats as authoritative.
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Headers normally do not cause a low-level connect() failure, but incorrect protocol or forwarded-header handling can produce redirects, invalid absolute URLs, or application errors after the upstream connection has been fixed. Diagnose the transport first, then the request semantics.
Choose buffering and workers for the workload
Keep Nginx proxy buffering enabled for ordinary synchronous Gunicorn workers and normal request/response traffic. Buffering protects the upstream worker from slow clients and is the default pattern in the deployment guidance.
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Recognize the common failure patterns
| Symptom | Interpretation | Corrective action |
|---|---|---|
| The socket path in the error log is not present. | Nginx is routing to a path Gunicorn or systemd did not create. | Correct the path on one side, enable the socket unit if appropriate, and verify creation after restart. |
The socket exists, but the direct request as www-data returns permission denied. |
Directory traversal or socket metadata blocks the Nginx account. | Correct owner, group, mode, and parent-directory permissions; then retest without changing application code. |
| TCP curl returns connection refused. | Nothing is listening at that address and port, or Gunicorn is bound to another interface. | Inspect the service’s bind setting and listener state, then restart the service after correcting it. |
| Gunicorn repeatedly logs worker boot failures. | The process reaches startup but cannot import or initialize the application. | Fix the import, environment, settings, or dependency error shown in the journal. |
| Direct upstream curl succeeds but the proxied URL still returns 502. | The listener is healthy; Nginx routing, reload state, headers, confinement, or a different server block is likely wrong. | Inspect nginx -T, confirm the selected server block and upstream, test configuration, and reload Nginx. |
| Short requests work but streaming or long-polling requests time out. | Buffering, worker class, or timeout policy does not fit the response pattern. | Use asynchronous workers and the documented unbuffered proxy arrangement for that location. |
Validate the repair without masking it
- Run the distribution’s Nginx configuration test. On standard installations this is
sudo nginx -t; do not reload if it reports an error. - Reload Nginx after a valid configuration change:
sudo systemctl reload nginx. Reloading avoids unnecessarily dropping established connections. - Restart only the service whose configuration or executable changed. A blanket reboot can make a race disappear without correcting the lifecycle.
- Keep
sudo tail -f /var/log/nginx/error.logandsudo journalctl -u gunicorn.service -frunning while requesting the exact failing URL. - Confirm the direct socket or TCP curl succeeds, then confirm the same request through Nginx. The two successful tests demonstrate that the listener and the proxy route now agree.
If the error returns after deployment or reboot, compare the socket’s post-start owner, mode, and path with the values from a working run. A recurring 502 usually indicates a startup-order or socket-lifecycle defect rather than a transient browser problem.
Choose the deployment model deliberately
| Decision | Unix socket | TCP listener |
|---|---|---|
| Best fit | Nginx and Gunicorn on the same host. | Separate hosts, containers, or a network boundary. |
| Main dependency | Filesystem path, directory traversal, owner, group, and mode. | Address, port, interface binding, routing, and firewall policy. |
| Lifecycle risk | Socket can vanish or be recreated with different metadata during restarts. | Port can be free while the service is down or bound only to loopback. |
| Debug test | sudo -u www-data curl --unix-socket /run/gunicorn.sock http |
sudo -u www-data curl -v http://127.0.0.1:8000/ |
| Supervision choice | Who owns the listener | Operational consequence |
|---|---|---|
| Systemd socket activation | The socket unit creates the endpoint and activates the service. | Socket settings and service requirements must agree; enable the socket and verify recreation after restarts. |
| Long-running Gunicorn service | Gunicorn creates and keeps the listener while systemd supervises the process. | The service must start reliably, bind the expected address, and recreate a Unix socket with usable permissions if one is used. |
Do not mix these models casually. Decide whether systemd or Gunicorn owns creation of the endpoint, then configure Nginx for that single source of truth.
Quick Recap
Final recovery checklist
- The Nginx error line has been captured during a reproduced failure.
- Gunicorn’s service and worker logs show a stable process with no import or bind errors.
proxy_passresolves to the same Unix path or TCP address that Gunicorn actually listens on.- The direct upstream request succeeds when run as the Nginx worker account.
- Socket parent directories, owner, group, mode, and any confinement policy permit access.
- Systemd socket activation, if used, is enabled and the service requires the intended socket.
- Forwarded headers and trusted-proxy settings match the network layout.
- Buffering and worker class match ordinary responses, streaming, or long-polling requirements.
nginx -tpasses, Nginx has been reloaded, and the proxied request succeeds while both logs are being watched.
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