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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsStart with the service status, then read the complete systemd journal for the web server and its backend. Next, inspect nginx’s error log, verify that the upstream is listening, and use the HTTP status code to choose the least risky fix. This sequence separates a stopped service, a broken upstream connection, an intentional limit, and a slow application without guessing.
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Use a fixed triage order
A website failure on a Linux server is usually visible in one of three places: the systemd unit that launches a service, nginx’s request/error logs, or the application service behind nginx. Checking those layers in order prevents a common mistake: changing nginx settings when the backend is stopped, or restarting a healthy web server when the application is the component failing.
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- Check nginx and the backend unit. Run
sudo systemctl status nginx, followed by the status command for the backend, such assudo systemctl status php-fpm. Read theResultfield and the recent lines shown at the bottom. Those lines are only a short excerpt, so do not treat them as the complete diagnosis. - Read the full unit journal. Run
sudo journalctl -u nginx. Narrow the time range when the failure is recent:sudo journalctl -u nginx --since "5 minutes ago". After a reboot or restart, inspect the previous boot withsudo journalctl -u nginx -b -1. To show only error-priority entries from the current boot, usesudo journalctl -u nginx -b -p err. - Inspect nginx’s error log. The commonly used path is
/var/log/nginx/error.log; use the path configured by your server if it differs. Match the error text and timestamp to the request that failed. - Confirm the upstream is listening. Run
ss -tlnpand check that the expected backend address, port, or Unix socket is present. Then read the backend’s own recent entries, for examplesudo journalctl -u <app-service> --since "5 minutes ago". - Apply the smallest change indicated by the evidence. That may mean restarting a stopped backend, correcting access to its socket, changing an operating-system policy, or adjusting a timeout for a genuinely slow response. Re-test and immediately check the logs again.
Run commands with sudo only when needed. Record the original error and the time of each change; this makes it possible to undo a change that does not help.
What the main HTTP errors mean
The status code is a routing clue, not a complete diagnosis. The same code can have several causes, so always confirm it against the nginx and service logs.
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| Status | What nginx is telling the browser | First evidence to collect | Typical fault location |
|---|---|---|---|
| 502 Bad Gateway | Nginx could not connect to the upstream, received an invalid response, or the upstream closed the connection before completing it. | The nginx error-log line, then backend status and listener output from ss -tlnp. |
Backend process, socket/port, permissions, or an operating-system policy blocking the connection. |
| 503 Service Unavailable | Nginx is refusing the request because a configured rate or connection limit was reached; it is not automatically evidence that the backend crashed. | Error-log entries containing “limiting requests,” the client IP, and the affected limit zone. | Nginx limit configuration or traffic pattern; investigate the backend only if the logs point there. |
| 504 Gateway Timeout | Nginx connected to the upstream but did not receive the next part of the response before the configured read timeout expired. | The request time, backend logs, and the effective proxy_read_timeout in the matching nginx block. |
Slow or stuck application, overloaded backend, or a timeout that is shorter than the application’s legitimate response time. |
Diagnose a 502 Bad Gateway
Check whether the backend is running
A stopped PHP-FPM service is a common example. Check it directly with sudo systemctl status php-fpm. If the unit is failed or inactive, read its journal before restarting: sudo journalctl -u php-fpm --since "5 minutes ago". The log may show a crash, a configuration error, or memory exhaustion. Restarting can restore service, but it does not explain why the unit stopped; preserve the failure details before they roll out of the short status excerpt.
Use your actual unit name when the application is not PHP-FPM. A generic command is sudo systemctl status <backend>, followed by sudo journalctl -u <backend> --since "5 minutes ago". If the unit starts and then exits again, treat that as an application or resource problem rather than an nginx problem.
Verify the listener, port, or Unix socket
Run ss -tlnp and compare its output with the upstream address in the nginx configuration. A TCP upstream should have a listening process on the expected address and port. A FastCGI or other local backend may use a Unix socket instead. If nothing is listening, nginx cannot establish the upstream connection and will commonly log a connection-refused message such as connect() failed (111: Connection refused).
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When the backend listens on a socket, check that the nginx worker user can reach that socket. A user or group mismatch can produce a 502 even while the backend process is healthy. The exact ownership and mode must match the account running nginx and the account or group that owns the socket. Make the correction in the backend’s service or pool configuration so it survives a restart; changing a socket manually is usually temporary.
Separate malformed responses from connection failures
A 502 also occurs when the upstream answers with data nginx cannot accept, or closes the connection before sending a complete response. The nginx error-log wording distinguishes this from a refusal to connect. Use the timestamp to find the corresponding backend entry. If the backend log shows a crash or forced termination at the same moment, fix that event first. If nginx reports an invalid upstream response while the backend remains running, inspect the application’s output and its protocol configuration rather than raising a timeout.
Check SELinux on RHEL-family systems
On systems using SELinux, the policy can prevent nginx from making a network connection to an upstream even when the process and port look correct. Check the relevant boolean with getsebool httpd_can_network_connect. If the policy is denying the connection, follow your distribution’s approved SELinux procedure and record the policy change. Do not disable SELinux broadly as a first response; confirm the denial and allow only the required operation.
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Diagnose a 503 Service Unavailable
Look for an intentional nginx limit
Nginx can deliberately return 503 when a configured request-rate or connection limit rejects traffic. In that case, the error log records “limiting requests” together with the offending IP and the name of the limit zone. This is a useful distinction: restarting PHP-FPM or increasing an application timeout will not remove a limit enforced before the request reaches the backend.
Capture several log lines around the event. If the same client address repeatedly triggers the same zone, review the limit’s intended capacity and scope. A limit may be protecting the server from bursts, or it may be too restrictive for a legitimate workload. Change it only after confirming which zone is involved, then watch the error log while reproducing the request.
When a 503 does point beyond nginx
If the error log does not show a limiting message, inspect the unit status and backend journal using the triage sequence. A backend can return an application-level 503, and the application log will identify that condition. Keep the two cases separate: an nginx-generated 503 is a traffic-control decision, while an application-generated 503 is a response from the upstream service.
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Diagnose a 504 Gateway Timeout
Understand the 60-second default
Nginx’s default proxy_read_timeout is 60 seconds. The setting measures the maximum gap between successive reads from the upstream; it is not a cap on the total time from request start to response completion. A WebSocket connection is also closed by default when the proxied server sends no data within that interval.
Therefore, a request that takes 70 seconds but sends data every 20 seconds can remain connected, while a request that sends nothing for 61 seconds can time out even if its eventual computation would have finished. This distinction matters when deciding whether a timeout change is appropriate.
Determine whether the application is genuinely slow
Match the 504 timestamp with the backend journal: sudo journalctl -u <app-service> --since "5 minutes ago". Look for a long-running operation, a blocked worker, a crash, or memory exhaustion. Check whether other requests are completing normally. If the backend is overloaded or stuck, fix that condition instead of masking it with a larger nginx timeout.
Change the timeout only when the workload requires it
If the backend is healthy and the operation legitimately has a long silent interval, set an appropriate proxy_read_timeout in the nginx block that handles the request. Keep the value as small as the workload allows; a very large value can tie up connections while an upstream is unavailable. After editing, validate and reload nginx using your distribution’s normal configuration-check and reload procedure, then confirm the effective block is the one serving the request.
An nginx ticket, #1471, documented a report of a 504 after 45 seconds even though a timeout override appeared to exist. The report was closed as invalid, and the reported workaround was to place the setting in the location block that actually matched the request. If an override appears ignored, inspect inheritance and the selected location rather than assuming nginx discarded the directive.
Read logs without losing the useful context
Why systemctl status is not enough
systemctl status is a fast health check. Its bottom lines are a recent excerpt, not the complete unit record. A service may have failed during an earlier restart, recovered, and then failed again; only the journal shows that sequence. Use journalctl -u nginx for the full unit history, and apply --since, -b -1, or -p err to focus the search.
Correlate three timestamps
- The browser or monitoring timestamp for the failed request.
- The nginx error-log timestamp and status code.
- The backend or systemd journal timestamp.
If the nginx entry appears without a corresponding backend event, the request may have been rejected by nginx itself or blocked before reaching the application. If all three align, the backend log usually contains the most actionable cause.
Read the complete error message
Copy the entire nginx line, including the upstream address, error number, client address, and request path. “Connection refused,” “timed out,” and “invalid response” lead to different checks. Searching only for the three-digit status code discards the detail that identifies the failing boundary.
Choose the least risky corrective action
| Evidence | Action to try first | Why this is the safer choice |
|---|---|---|
| Backend unit is inactive or failed | Read its journal, correct the reported cause, then restart that backend. | It addresses the component that is unavailable instead of changing proxy behavior. |
Connection refused and no listener in ss -tlnp |
Start or repair the backend and verify its configured port/socket. | Nginx cannot connect to a service that is not listening. |
| Socket access denied | Correct the backend socket’s user/group or mode in its persistent configuration. | A manual permission change may disappear on the next restart. |
| SELinux denial on a RHEL-family host | Confirm the denial and adjust the specific boolean or policy rule required. | It preserves mandatory access control rather than disabling it globally. |
| 503 with “limiting requests” | Review the named rate or connection zone and its intended capacity. | The request is being rejected intentionally at nginx. |
| 504 with a healthy but slow upstream | Optimize the operation first; increase proxy_read_timeout only for the matching location when the delay is legitimate. |
A larger timeout does not cure a crashed or overloaded backend. |
A repeatable recovery runbook
- Save the symptom. Note the URL, status code, approximate time, and whether all pages or only one operation fail.
- Check both units. Run
sudo systemctl status nginxand the relevant backend status command. - Pull focused journals. Use
sudo journalctl -u nginx --since "5 minutes ago"and the equivalent backend command. If the incident followed a reboot, add-b -1to inspect the previous boot. - Inspect the nginx error log. Find the matching timestamp and copy the complete line.
- Map the evidence to 502, 503, or 504. Use the distinctions above; do not infer a backend crash from a 503 limit message.
- Verify connectivity. Run
ss -tlnp; check the configured port or socket and its access permissions. - Inspect the backend journal. Look for crashes, exits, malformed output, or resource exhaustion at the same time.
- Apply one targeted change. Restart a failed unit, repair socket access, adjust the relevant SELinux control, or change a location-level timeout only when justified.
- Re-test and re-read logs. A successful page load is not proof that the cause is fixed; confirm that new requests no longer produce the original error.
- Document the final state. Record the changed setting, the service restarted, and the evidence that the error stopped.
Common troubleshooting mistakes
- Restarting nginx first every time. This can erase the timing of a backend failure without correcting it.
- Using only the last lines from
systemctl status. The excerpt can omit the original failure. - Treating every 503 as an outage. Check for nginx’s “limiting requests” entry before touching the backend.
- Raising a timeout to hide a crash. A timeout helps only when the upstream is functioning but legitimately pauses between reads.
- Editing the wrong nginx context. A directive in an unused server or location block cannot affect the failing request.
- Changing socket permissions by hand. A service restart may recreate the socket with the old ownership; fix the persistent backend configuration.
- Disabling SELinux globally. Confirm the specific denial and make the narrow policy change required.
- Making several edits at once. Multiple simultaneous changes remove the evidence needed to identify the effective fix.
What a successful diagnosis looks like
You should be able to state the failing boundary in one sentence: for example, “nginx returned 502 because the PHP-FPM socket had no listener,” “nginx returned 503 because the configured request zone limited this client,” or “nginx returned 504 because the application left the upstream silent for longer than the 60-second read timeout.” That statement ties the browser symptom to a log line and a service-level observation. If you cannot make that connection, gather the full journal and error-log context before changing configuration.
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