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A 502 Bad Gateway from nginx means the reverse proxy could not obtain a valid response from the Node.js upstream. In practice, the cause is usually a stopped Node process, a port or bind-address mismatch, an incorrect proxy_pass, a path rewrite that sends the wrong URI, or an upstream that exceeds its timeout.
Work from the inside out: read the nginx error log, prove that Node is listening where nginx expects, test that socket directly, correct the proxy block, change timeouts only when the log justifies it, and then validate and reload nginx. The procedure below covers same-host deployments and notes the version-dependent behavior that can affect keepalive connections.
What the 502 means in this setup
The request path is client → nginx → Node.js. Nginx accepts the client request, opens a connection to the address in proxy_pass, and waits for an HTTP response. A 502 is generated when that upstream exchange fails or the response is not valid HTTP. It is therefore a proxy-to-application problem, not proof that the browser or the public TLS certificate is broken.
Public HTTPS and the upstream connection are separate. Nginx may terminate TLS for visitors while speaking ordinary HTTP to a Node process on localhost. The upstream should use https:// only when the Node server itself is configured to serve TLS.
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Start with the nginx error log
The error log normally identifies the failure class faster than inspecting application code. On many Linux distributions the file is /var/log/nginx/error.log; installations can choose another path, so use the error_log directive in the active configuration if that file does not exist.
- Open a second terminal and watch the log:
sudo tail -f /var/log/nginx/error.log. - In a browser or with
curl, request the URL that returns 502. - Match the new log line to the request time, upstream address, and location block handling the request.
| Log wording or symptom | Likely meaning | First check |
|---|---|---|
connect() failed (111: Connection refused) or similar “connection refused” text |
Nothing is accepting connections at the configured address and port, or the port in nginx is wrong. | Check the Node service state and listening socket. |
upstream timed out |
Node did not produce the expected response before the proxy timeout elapsed. | Measure the endpoint directly, then investigate slow application work before raising a timeout. |
| 502 with an invalid or malformed upstream response message | Something answered, but it did not return a response nginx could parse as HTTP. | Call the upstream directly and inspect Node’s output and protocol settings. |
Exact wording varies by nginx version and distribution. Treat the examples as diagnostic patterns and confirm the precise message in your installed build. Do not change several unrelated settings before collecting the log line; doing so can hide the original failure.
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Check the process manager
Use the manager that starts your application. For a systemd service, an example is:
sudo systemctl status your-node-serviceFor a PM2-managed process, inspect its status with:
pm2 statusA failed, stopped, or repeatedly restarting process must be fixed before nginx can proxy to it. Read the service or process-manager log for startup errors such as a missing environment variable, an unavailable database, a failed migration, or an exception during module loading. Restarting nginx cannot bring a crashed Node process back.
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Confirm the listening port
Find the socket that is actually open. If the application is expected on port 3000:
ss -tlnp | grep 3000The output should show a listening TCP socket and, where permissions allow, the owning process. Compare that port with the number in nginx’s proxy_pass. A common deployment mistake is starting Node with an environment value such as PORT=8080 while leaving nginx pointed at 3000.
Check the bind address
Node must bind to an interface nginx can reach. On the same host, a process listening on 127.0.0.1:3000 is reachable through that loopback address. A process bound only to an unrelated interface is not. If nginx and Node are on different hosts, a loopback address on the Node host is not reachable from the nginx host; use the appropriate private address and firewall rules instead.
Call Node without nginx
A direct request separates application failures from proxy failures:
curl -i http://127.0.0.1:3000/Use the same path that failed publicly when possible. A normal HTTP status and headers from this command show that Node is answering on that socket; they do not yet prove that nginx is using the same address, port, or URI. A refusal, reset, or hang points to Node, its bind address, or the network path and should be resolved there first.
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Use a correct proxy_pass block
For a Node app on the same machine, this is a standard starting point:
location / {
proxy_pass http://127.0.0.1:3000;
proxy_http_version 1.1;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_read_timeout 60s;
}Replace the port with the one confirmed by ss. The http:// scheme is intentional when Node serves plain HTTP. Nginx can still provide HTTPS to clients; encryption on that public leg does not require HTTPS on the local upstream leg.
Check the address, scheme, and port together
- Address: it must resolve to an interface where Node is listening.
- Port: it must match the runtime configuration, not merely the port you intended to use.
- Scheme: use
http://for a plain-HTTP Node listener andhttps://only for a Node listener configured with TLS.
Sending HTTPS to a plain-HTTP Node socket can produce handshake or invalid-response errors that surface as a 502. Conversely, using HTTP for a TLS-only upstream will fail the connection. Test the chosen scheme directly with curl before changing nginx.
Preserve useful request headers
proxy_set_header Host $host; passes the requested host name to Node, which matters when the application selects a tenant or route from the host header. proxy_set_header X-Real-IP $remote_addr; supplies the client address visible to nginx. Keep these directives in the location that handles the failing request so another inherited or competing block does not override them.
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The presence of a URI portion after the upstream host and port changes how nginx constructs the request sent to Node. This is a frequent source of path errors after a proxy block is copied between applications.
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| Configuration | Request from client | Path sent upstream |
|---|---|---|
location /api/ { proxy_pass http://127.0.0.1:3000; } |
/api/users |
/api/users (the original request URI is retained) |
location /api/ { proxy_pass http://127.0.0.1:3000/; } |
/api/users |
/users (the matched /api/ prefix is replaced by /) |
location / { proxy_pass http://127.0.0.1:3000/app/; } |
/login |
The location prefix is replaced with /app/, producing /app/login. |
When proxy_pass has no URI part, nginx forwards the request URI as received. When it has a URI part, nginx replaces the portion that matched the location prefix. The trailing slash is therefore significant. Use the examples in the nginx proxy module documentation for complex rewrites rather than inferring behavior from the directive’s wording.
A path mismatch normally causes a Node 404 or application routing error rather than a 502, but it can make a healthy upstream appear broken. Test the exact rewritten path directly with curl and align the location and application router deliberately.
Raise proxy timeouts only when the log shows a timeout
If the error log says upstream timed out, first determine whether the endpoint is legitimately long-running or simply stuck. Compare the direct request to Node with the public request, inspect application and database timings, and check whether the process is CPU- or memory-constrained.
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The example block uses proxy_read_timeout 60s. Many nginx guides cite 60 seconds as the usual default, but the effective default is version- and configuration-dependent; verify it in the reference for the nginx build you run. If one report or route needs more time, scope the change to that route:
location /reports/ {
proxy_pass http://127.0.0.1:3000;
proxy_http_version 1.1;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_read_timeout 120s;
}Do not raise the timeout globally merely because one request is slow. A larger value keeps client connections open while a failing application consumes resources and can conceal a deadlock or overloaded dependency. Prefer asynchronous jobs, pagination, caching, or application-level performance fixes when the operation does not need to remain synchronous.
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Use upstream keepalive deliberately
Under load, repeatedly opening upstream connections can add latency and connection churn. The documented pattern for nginx-to-upstream keepalive uses HTTP/1.1 and removes the hop-by-hop Connection header:
location / {
proxy_pass http://127.0.0.1:3000;
proxy_http_version 1.1;
proxy_set_header Connection "";
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
}The empty Connection value prevents nginx from sending a Connection: close header to the upstream. Keepalive is a connection-management optimization, not a cure for a stopped process, wrong port, or malformed response. Add it when logs, connection counts, or load behavior indicate churn, and verify that the Node server and any intermediary handle persistent HTTP/1.1 connections correctly.
Account for nginx version differences
Nginx’s blog has announced a change in which upstream keepalive became the default, with HTTP/1.1 used by default for HTTP upstreams, starting in version 1.29.7. Confirm that behavior in the release notes and build documentation for your installation; distributions may ship an older or patched version. Before that change, explicitly setting proxy_http_version 1.1 and proxy_set_header Connection "" was the usual way to request this behavior.
Summaries of the upstream module reference have cited an idle keepalive limit of 32 connections per worker process. Treat that figure as version-dependent until verified against your installed reference, and do not size capacity from it without checking worker count, traffic, and upstream limits. Keepalive also interacts with upstream idle timeouts; a backend that closes idle sockets can create errors if connection reuse is misconfigured.
Validate the configuration and reload safely
- Test syntax and referenced files:
sudo nginx -t. - Do not reload if the test reports an error. Correct the reported file and line first.
- Apply a successful configuration with
sudo systemctl reload nginx. - Repeat the direct Node request and then test through the nginx hostname:
curl -i https://example.com/path(replace the hostname and path with your own). - Watch the error log during the test and confirm that the request reaches the intended upstream.
A reload applies configuration changes without a full nginx restart, allowing existing connections to drain under normal operation. Keep a copy of the last known-good server block so you can restore it quickly if the new location, rewrite, or timeout causes a regression.
Follow the symptom-specific recovery path
“Connection refused” after a deployment
- Check the Node service or PM2 process state.
- Read its startup log for crashes and missing configuration.
- Run
ss -tlnpand record the actual address and port. - Make
proxy_passmatch that socket, or fix the application’sPORTand bind settings. - Call the socket with
curl, then runnginx -tand reload.
“Upstream timed out” on one endpoint
- Call the endpoint directly on the Node port and measure how long it takes.
- Inspect application, database, and external-service timings.
- Fix the slow operation if it should be fast.
- If the duration is intentional, set a larger
proxy_read_timeoutonly on that location and document why. - Retest under representative concurrency rather than relying on a single request.
Node works locally but the public URL still returns 502
- Confirm that nginx is loading the server block you edited, not another file or virtual host.
- Check that the public location matches the request path and that a more specific location is not selecting a different upstream.
- Compare the address in the log with the address tested by
curl. - Check the upstream scheme for an HTTP-versus-HTTPS mismatch.
- Run
sudo nginx -tand reload after every accepted change.
The error began after adding a trailing slash
Inspect the resulting URI, not just the status code. A slash in proxy_pass can remove the location prefix before the request reaches Node. If the application expects /api/users but receives /users, either remove the URI portion or adjust the Node route intentionally. A direct curl request to both candidate paths makes the difference visible.
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Failures are intermittent under load
Check whether Node is restarting, exhausting file descriptors, or taking longer as concurrency rises. Correlate nginx error-log timestamps with process-manager events. If the failures coincide with a high rate of short-lived upstream connections, evaluate HTTP/1.1 keepalive and the empty Connection header, while also checking backend idle timeouts. Do not treat keepalive as a substitute for capacity planning or application health checks.
A repeatable production checklist
- Record the exact nginx error line for a failed request.
- Verify the Node process is running and not restarting.
- Verify the listening address and port with
ss -tlnp. - Request the same endpoint directly from the upstream socket.
- Match
proxy_passaddress, port, and scheme to that socket. - Check whether a URI portion and trailing slash rewrite the path your router expects.
- Change
proxy_read_timeoutonly for logged upstream timeouts. - Configure keepalive only when connection churn or load behavior warrants it, and account for nginx version defaults.
- Run
sudo nginx -tbefore every reload. - Reload with
sudo systemctl reload nginx, then test through nginx while watching the log.
The Bottom Line
Fix an nginx 502 by proving the upstream path one link at a time: identify the logged failure, make sure Node is listening on the configured socket, test it directly, correct proxy_pass and URI behavior, and only then tune timeouts or keepalive. Validate with nginx -t and reload rather than restarting blindly.
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