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A 502 Bad Gateway from Nginx means Nginx accepted the browser request but could not get a usable response from the Gunicorn endpoint configured in proxy_pass. The fault is usually a stopped Gunicorn service, a mismatched Unix socket or TCP port, socket permissions, a failed worker, or an incorrect proxy protocol. Find the exact upstream error first, then test the same endpoint as Nginx before changing application code.
What a 502 means in an Nginx–Gunicorn deployment
Nginx is the reverse proxy and Gunicorn is the upstream application server. A request normally travels through this chain:
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- Nginx selects a
locationblock and follows itsproxy_passtarget. - Nginx connects to Gunicorn over a Unix socket or TCP address.
- Gunicorn assigns the request to a worker and returns an HTTP response.
- Nginx sends that response to the client.
A 502 indicates that the handoff in steps 3 or 4 failed. It does not, by itself, prove that your application route is broken. Gunicorn can be running while listening on a different socket, port, or address, and it can also be starting workers that immediately exit. The Gunicorn deployment documentation states: “We strongly recommend running Gunicorn behind a proxy server.” That arrangement works only when both processes agree on the same endpoint and protocol.
Capture the exact failure before changing configuration
Start with the Nginx error log. Its message normally identifies whether the problem is connection refusal, a missing socket, permissions, a timeout, or an invalid upstream response.
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sudo tail -f /var/log/nginx/error.logIn another terminal, reproduce the failing URL. Typical messages include:
connect() failed (111: Connection refused) while connecting to upstream: nothing is listening at the configured TCP address, or Gunicorn is listening elsewhere.connect() to unix:/path/gunicorn.sock failed (2: No such file or directory): the socket was not created, was removed during a restart, or the path in Nginx is wrong.connect() to unix:/path/gunicorn.sock failed (13: Permission denied): the Nginx worker cannot traverse the directory or access the socket.- A timeout while connecting or reading: Gunicorn may be overloaded, blocked, bound to an unreachable address, or unable to complete the request.
- An invalid or prematurely closed response: a worker may have crashed, or Nginx and the upstream are using incompatible protocol settings.
Then inspect the Gunicorn service and its recent logs. Replace gunicorn with the actual systemd unit name if your deployment uses a different one.
sudo systemctl status gunicorn
sudo journalctl -u gunicorn -n 100 --no-pagerLook for import errors, a failure to bind, a missing working directory, an invalid executable path, configuration errors, and workers that start and immediately exit. Gunicorn’s FAQ notes that a reverse proxy can report 502 while Gunicorn shows only worker-startup messages, so inspect both logs rather than relying on the application log alone.
Make Nginx and Gunicorn use the same endpoint
Compare the address in Nginx with Gunicorn’s --bind setting or its systemd ExecStart command. The endpoint must match in path, address, port, and transport.
Unix-socket configuration
A Unix socket is a filesystem object, commonly placed under /run or /tmp. Gunicorn can bind to a path such as /tmp/gunicorn.sock, while Nginx names that socket in an upstream block:
upstream app_server {
server unix:/tmp/gunicorn.sock fail_timeout=0;
}
server {
listen 80;
server_name example.test;
location / {
proxy_set_header Host $http_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 corresponding Gunicorn bind might be --bind unix:/tmp/gunicorn.sock. A one-character difference in the path creates a different socket and produces a 502 even when Gunicorn is healthy.
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TCP configuration
For TCP, Gunicorn listens on an IP address and port, for example 192.168.0.7:8000. Nginx can define that listener in the upstream group and proxy to the group name:
upstream app_server {
server 192.168.0.7:8000;
}
location / {
proxy_pass http://app_server;
}Gunicorn must bind to the same address and port. Binding only to 127.0.0.1:8000 will not work if Nginx is on another host, and binding to a private interface will not work if Nginx resolves a different address.
| Deployment choice | What must match | Typical failure | Operational trade-off |
|---|---|---|---|
| Unix socket | Exact filesystem path, directory traversal, owner, group, and mode | Missing socket or permission denied | No network port is exposed, but the socket lifecycle and filesystem permissions must be maintained |
| TCP listener | IP address, port, routing, and firewall policy | Connection refused or unreachable upstream | Simpler path handling and suitable for separate hosts, but address and network access must be controlled |
Test the upstream as the Nginx user
A test made as root can hide the very permission problem that causes the 502. If the deployment uses the usual www-data Nginx account and a socket at /run/gunicorn.sock, run:
sudo -u www-data curl --unix-socket /run/gunicorn.sock httpUse the path requested by your application when necessary. A valid HTTP response, even a framework-level 404, proves that the account can open the socket and that Gunicorn is answering. A connection error means the fault is below Nginx routing: fix Gunicorn, the socket path, ownership, group membership, or mode first.
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curl -v http://192.168.0.7:8000/If this direct request fails, changing proxy_set_header or application routes will not solve the 502. If it succeeds but the proxied request fails, focus on the Nginx upstream definition, the Nginx worker’s identity, and the request protocol.
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Repair systemd socket activation
With socket activation, systemd creates and owns the listening socket, then starts Gunicorn when the socket receives traffic. A representative socket unit is:
[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.socketCheck the resulting object and unit state:
sudo systemctl status gunicorn.socket
ls -l /run/gunicorn.sockThe service unit should require the socket and start Gunicorn with the intended working directory and executable. Its unit section commonly includes:
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Requires=gunicorn.socket
After=network.targetVerify that the service’s WorkingDirectory, virtual-environment path, application module, and worker command are correct. An import error or an executable path from a deleted virtual environment can make the service fail even though the socket unit is active.
Socket activation also explains the common symptom “the socket exists, but Nginx still gets 502.” The file can exist while no healthy Gunicorn process is accepting requests, or it can have an owner and mode that prevent Nginx from opening it. Inspect both gunicorn.socket and gunicorn.service; restarting only the service may not recreate a damaged socket. If the path is recreated during a restart, confirm that systemd applies the expected owner, group, and mode every time.
Fix Unix-socket permissions and confinement
Nginx needs two kinds of access:
- Execute (traverse) permission on every parent directory in the socket path.
- Read/write access to the socket itself according to its owner, group, and mode.
Inspect each directory and the socket:
namei -l /run/gunicorn.sock
ls -l /run/gunicorn.sockConfirm the Nginx worker account. It is set by the user directive in the main Nginx configuration and may differ from www-data. The socket’s SocketUser, SocketGroup, and SocketMode must allow that account to connect. A mode of 0660 works only when Nginx belongs to the socket’s group (or is the owner).
Do not solve the problem by making the socket or its parent directories world-writable. Correct the systemd ownership and group, use a directory intended for runtime sockets, and ensure the service recreates the object consistently. If mandatory access controls or another confinement system is enabled, inspect its denial logs as well as normal Unix permissions; a path that looks accessible with ls can still be blocked by policy.
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Set the proxy headers and protocol deliberately
Gunicorn and the application need to know the original host, client chain, and scheme when Nginx terminates the public connection. A typical location includes:
proxy_set_header Host $http_host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;Use an appropriate Host value for your application. Forwarding the original host supports virtual-host routing and URL generation; forwarding the scheme prevents an HTTPS request from being treated as plain HTTP. The forwarded-for setting preserves the existing client chain while adding Nginx’s address.
When Nginx and Gunicorn run on different hosts, do not trust forwarded headers from every possible client. Restrict Gunicorn’s trusted forwarded-header sources to the proxy addresses. Otherwise an external client could supply a forged scheme or client address. Header mistakes more often cause redirects, incorrect absolute URLs, or security issues than a direct 502, but correcting them is part of a sound proxy handoff.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Use buffering that matches the worker model
For ordinary request/response traffic handled by synchronous workers, keep Nginx proxy buffering enabled. It lets Nginx absorb the upstream response and protects the client connection from tying up a Gunicorn worker unnecessarily.
Streaming responses, server-sent events, and long-polling need different behavior. For those patterns, the deployment guidance calls for an asynchronous Gunicorn worker class and disabled proxy buffering:
location /stream/ {
proxy_buffering off;
proxy_pass http://app_server;
}| Traffic pattern | Gunicorn workers | Nginx buffering | Reason |
|---|---|---|---|
| Normal short requests | Synchronous workers | Keep enabled | Default request handling and response buffering |
| Streaming or long-polling | Asynchronous worker class | Turn off for the streaming location | Allows incremental data to reach the client instead of waiting for a complete buffered response |
Disabling buffering is not a general cure for a connection-refused 502. First establish that Nginx can connect and Gunicorn can answer; then tune buffering for the endpoint’s traffic pattern.
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Validate configuration and retest in a controlled order
After making one related change, validate Nginx before reloading it. The exact binary path varies by distribution, but the standard test is:
sudo nginx -tOnly after a successful test should you reload Nginx:
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sudo systemctl reload nginxRestart only the changed Gunicorn service or socket unit, rather than restarting the entire server:
sudo systemctl restart gunicorn
sudo systemctl restart gunicorn.socketUse the command appropriate to the component you changed; restarting both is useful when repairing socket activation, but unnecessary restarts can erase useful diagnostic state. Tail the Nginx and Gunicorn logs while requesting the failing URL. Confirm in this order:
- Gunicorn starts without import, bind, or worker-exit errors.
- The expected Unix socket or TCP listener exists.
- The Nginx account can make a direct request to that endpoint.
nginx -tpasses and Nginx reloads cleanly.- The public URL returns the application’s response while both logs remain free of upstream errors.
A successful direct socket or TCP curl followed by a successful proxied request demonstrates that the original fault was in Nginx routing, permissions, or service wiring rather than the application endpoint itself.
Quick Recap
Choose the right fix from the symptom
| Observed symptom | Most likely cause | Next check |
|---|---|---|
| Connection refused to an IP and port | Gunicorn is stopped, bound to another port, or listening only on another interface | Compare --bind with proxy_pass; inspect service status and listening sockets |
| No such file for a Unix socket | Wrong path or socket not recreated after restart | Compare the systemd ListenStream and Nginx upstream path; inspect socket-unit state |
| Permission denied | Nginx cannot traverse a parent directory or open the socket | Run the Unix-socket curl as the Nginx account; inspect namei -l and socket ownership |
| Socket exists but requests fail | Gunicorn service is unhealthy, workers exit, or the socket belongs to a stale listener | Read the Gunicorn journal and test the socket directly |
| Direct upstream curl works, public URL fails | Nginx upstream block, location matching, headers, or reload state is wrong | Run nginx -t, verify the selected location, reload, and watch the error log |
| Streaming endpoint stalls or closes unexpectedly | Buffered proxying or synchronous workers are unsuitable for the traffic pattern | Use an asynchronous worker class and disable buffering only for the streaming location |
A compact recovery checklist
- Record the complete Nginx upstream error while reproducing the 502.
- Check the Gunicorn systemd status and recent journal entries for startup and worker failures.
- Write down the exact Gunicorn bind address or socket path.
- Compare it character-for-character with Nginx’s upstream and
proxy_pass. - Test the endpoint as the Nginx worker account, not only as root.
- For systemd sockets, verify
ListenStream, ownership, group, mode, and the service’sRequiresrelationship. - Check directory traversal, mandatory confinement, and socket recreation after restarts.
- Set the host and forwarded headers deliberately, and limit trusted proxy sources when hosts are separate.
- Keep buffering for synchronous ordinary requests; disable it only for streaming patterns with asynchronous workers.
- Run
nginx -t, reload Nginx, restart the changed unit, and retest while tailing both logs.
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