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FastCGI errors are failures in the conversation between a web server and an application process such as PHP-FPM. A 502 Bad Gateway, “connect() failed while connecting to upstream,” a missing socket, or a timeout each points to a different stage of that conversation. Capture the exact log line first, then verify that PHP-FPM is running, make the configured endpoint match the listener, correct socket permissions, and investigate application capacity before raising a timeout.
What a FastCGI error means
FastCGI is the interface that lets Nginx or Apache pass a request to a long-running application service. For PHP sites, that service is commonly PHP-FPM. The web server accepts the browser request, selects a script, sends request metadata and (when needed) a request body to PHP-FPM, waits for headers and content, and returns the result to the browser.
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An error can occur at any of those stages:
- Connection stage: the server cannot open a TCP connection or Unix-domain socket.
- Send stage: the connection exists, but the request cannot be delivered.
- Read stage: the backend does not return data within the configured interval.
- Response stage: the backend closes the connection or sends invalid FastCGI headers.
- Application stage: PHP starts but fails, hangs, exhausts workers, or encounters a database or code error.
The same browser symptom can result from different stages. Treat the complete web-server log entry—not only “502 Bad Gateway”—as the diagnosis starting point.
First response: capture and classify the exact error
Reproduce the failing URL once while watching the relevant error log. Preserve the upstream address, socket path, port, errno text, and wording such as “while connecting,” “while sending,” or “while reading response header.” Those details determine which check comes next.
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Nginx logs
sudo tail -f /var/log/nginx/error.logOn systems that use systemd or a custom log path, use the location configured in the Nginx error_log directive. A message such as connect() to unix:/run/php/php-fpm.sock failed (2: No such file or directory) identifies an endpoint or service-path problem. Connection refused usually means that nothing is accepting connections at the configured address, or that the listener has reached a condition where it refuses new work. A message that names while reading response header from upstream points to backend processing, capacity, or a read limit rather than a missing listener.
Apache logs
sudo tail -f /var/log/apache2/error.logDistribution paths vary; use the Apache virtual-host configuration if the error log is elsewhere. Record the PHP-FPM host, port, or socket shown in the proxy error, and note whether Apache reports a connection failure, a timeout, or an invalid response.
Use the classification to choose the next check
| Log evidence | Most likely area | First check |
|---|---|---|
| Connection refused | PHP-FPM stopped, wrong address or port, listener saturation, or firewall policy | Confirm the service and its listening endpoint |
| No such file or directory for a Unix socket | Socket path differs from the pool configuration, or the service has not created it | Inspect the actual socket and PHP-FPM pool file |
| Permission denied opening a socket | Web-server worker cannot access the socket or one of its parent directories | Check owner, group, mode, and directory traversal permissions |
| Timeout while connecting | Unreachable endpoint, overloaded listener, or network path problem | Test the address and inspect backend capacity |
| Timeout while reading | Slow PHP code, database or external call, blocked worker, or too-short read interval | Inspect PHP-FPM and application timing before changing limits |
| Invalid or incomplete upstream headers | PHP-FPM crashed, was killed, or emitted an invalid response | Read PHP-FPM and application logs at the same timestamp |
Verify that PHP-FPM is alive and listening
Do not change browser settings for a refused connection or missing socket. Check the backend process first.
Check service state
sudo systemctl status php-fpm
sudo systemctl status php8.2-fpmThe service name depends on the operating system and installed PHP version. Use the name shown by your package manager if neither command exists. If the service is stopped, inspect its journal before starting it:
sudo journalctl -u php-fpm --since "15 minutes ago"
sudo journalctl -u php8.2-fpm --since "15 minutes ago"Look for configuration parse errors, missing directories, exhausted resources, or a pool that failed to start. Correct the underlying error, then restart or start the service and reproduce the request.
Inspect TCP listeners
sudo ss -ltnp | grep ':9000'Replace 9000 with the port configured in the PHP-FPM pool. A healthy listener must be on an address reachable from the web-server process. A pool bound only to a different loopback address, a private interface, or IPv6 can be unreachable when the web server uses another address family.
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Inspect Unix-domain sockets
sudo find /run /var/run -type s -name '*php*' -ls 2>/dev/nullCompare the discovered path with the pool’s listen setting and the web server’s upstream directive. A socket is created only after its pool starts successfully; an absent file can therefore indicate a stopped pool or a path mismatch rather than a filesystem deletion.
Test the endpoint from the server
For TCP, verify that the port is reachable from the machine running Nginx or Apache:
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nc -vz 127.0.0.1 9000This confirms that a TCP listener accepts a connection; it does not by itself prove that PHP returns a valid response. For a Unix socket, inspect the file and permissions instead of trying to use an HTTP client against it.
Fix Nginx FastCGI configuration
Nginx’s FastCGI integration is provided by ngx_http_fastcgi_module. The two settings that most often get out of sync are fastcgi_pass, which identifies the backend endpoint, and fastcgi_param SCRIPT_FILENAME, which identifies the script PHP should execute.
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Use a TCP address when the pool listens on TCP:
location ~ .php$ {
include fastcgi_params;
fastcgi_pass 127.0.0.1:9000;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
}Use the exact Unix socket path when the pool listens on a socket:
location ~ .php$ {
include fastcgi_params;
fastcgi_pass unix:/run/php/php-fpm.sock;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
}The example paths are not universal. PHP package versions and pool names commonly create different filenames, so copy the path from the active pool configuration rather than assuming that a file with a similar name is correct. Do not use a TCP directive for a socket or a socket directive for a TCP listener.
Resolve the script identity
SCRIPT_FILENAME must resolve to the real filesystem path of the requested PHP file. If the document root is /srv/www/site/public, a request for /index.php should produce /srv/www/site/public/index.php. A wrong root can produce “primary script unknown,” a 404-like response, or an application failure even though the FastCGI connection itself works.
sudo nginx -T | lessReview the complete, expanded configuration rather than only the file you last edited. Another server block, an included snippet, or a more specific location can override the directive you are inspecting.
Pass the request parameters PHP needs
For ordinary PHP requests, include the distribution’s FastCGI parameter file or define the required parameters explicitly. POST requests in particular need the request method, content type, and content length passed through:
fastcgi_param REQUEST_METHOD $request_method;
fastcgi_param CONTENT_TYPE $content_type;
fastcgi_param CONTENT_LENGTH $content_length;Use the parameter names and values expected by your Nginx version and PHP-FPM setup. Omitting them can make a form submission behave differently from a simple GET while leaving the socket connection apparently healthy.
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Validate before reloading
sudo nginx -t
sudo systemctl reload nginxIf the test fails, Nginx keeps its previous configuration; fix the reported file and line before reloading. A reload applies configuration without unnecessarily terminating existing workers. Restart only when a service-level change requires it.
Fix Apache FastCGI proxying
Apache commonly reaches PHP-FPM through mod_proxy together with mod_proxy_fcgi. Some installations use mod_fcgid instead; do not mix directives from those modules without checking which integration is enabled.
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sudo apachectl -M | grep -E 'proxy_module|proxy_fcgi_module|fcgid_module'On systems that provide module-management commands, enable the modules there and reload Apache. A proxy rule cannot work when the module that implements its scheme is not loaded.
Point the proxy at the real PHP-FPM endpoint
A TCP backend rule must name the same host and port as the PHP-FPM pool. A common pattern is:
ProxyPassMatch "^/(.*.php)$" "fcgi://127.0.0.1:9000/var/www/site/public/"Adjust the URL and filesystem prefix to your virtual host. The prefix determines how Apache maps the requested script; a mismatch can connect successfully yet execute the wrong path. For a Unix socket, use the socket form documented for the installed Apache mod_proxy_fcgi version and ensure the path is the one created by PHP-FPM. Socket syntax and support details vary by version, so do not copy a TCP URL and merely replace its port.
Understand connection reuse
Apache’s FastCGI proxy can reuse backend connections in configurations that opt in to reuse. Reuse is appropriate only when the FastCGI application can safely handle concurrent reused connections. If PHP-FPM or another backend is not configured for that behavior, disable reuse according to the module’s directives rather than treating every reuse-related error as a timeout problem.
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Check and reload Apache
sudo apachectl configtest
sudo systemctl reload apache2Use httpd instead of apache2 where that is the service name. Resolve syntax errors before reloading. Then watch the Apache error log while making one fresh request.
Correct socket ownership and permissions
A socket can exist and still be unusable. The worker account running Nginx or Apache needs permission to open the socket and traverse every parent directory.
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Inspect the complete path
namei -l /run/php/php-fpm.sock
ls -l /run/php/php-fpm.socknamei reveals permissions on each directory component. Check the account under which the web server runs in its service configuration, commonly shown by the Nginx user directive or Apache’s service settings.
Align the PHP-FPM pool
PHP-FPM pool settings normally provide socket owner, group, and mode controls. Set the group to one that includes the web-server account, or use an appropriate owner and mode policy for your host. Change only the relevant pool and avoid making the socket world-writable. After editing the pool, restart PHP-FPM so it recreates the socket with the new metadata, then reload the web server if its endpoint changed.
sudo systemctl restart php-fpm
sudo systemctl reload nginxPackage upgrades, pool renames, and runtime-directory cleanup can recreate a socket with a different path or mode. Record the setting in configuration management and verify it after upgrades rather than applying a temporary chmod that will disappear at the next restart.
Diagnose timeouts by phase
Nginx has separate controls for connecting to the backend, sending the request, and reading the response:
| Directive | What it limits | What an expiry suggests |
|---|---|---|
fastcgi_connect_timeout |
Time allowed to establish the FastCGI connection | Wrong route, unavailable listener, network issue, or saturated backend |
fastcgi_send_timeout |
Time between writes while sending request data | Backend or network is not accepting data, or a large upload is blocked |
fastcgi_read_timeout |
Time between reads while waiting for the backend response | Slow code, a blocked database or external call, exhausted workers, or an interval that is too short |
These are inactivity intervals, not a promise that the entire request may run for exactly that many seconds. Raising a limit can hide a saturated pool and tie up more web-server workers. First establish that the operation is expected to be long-running, identify its slow dependency, and confirm that PHP-FPM has enough workers and memory.
Check PHP-FPM capacity
Review the pool’s process-manager settings, active and idle workers, slow-request log, and recent restarts. If all workers are busy, requests queue until the web server’s read interval expires. Increasing the pool limit without checking available memory can replace timeouts with swapping or out-of-memory kills. Compare worker counts with the host’s memory budget and the application’s observed peak usage.
Check the application path
Correlate the web-server timestamp with PHP-FPM, PHP, and application logs. A slow SQL query, an unavailable remote API, a lock, or a fatal error can prevent headers from being returned. Fix that operation or add an application-level limit before changing Nginx or Apache timeouts. Test a static file and a minimal PHP script separately to distinguish a server-wide problem from one route.
Apache connection and process controls
Apache’s FastCGI modules expose their own connection and process-lifetime settings. Identify which module is active and change the corresponding directives; an Nginx timeout setting has no effect on an Apache request. Apply a larger value only after measuring the expected operation and confirming backend health.
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Understand retries and incomplete responses
Nginx’s fastcgi_next_upstream can try another eligible upstream after connection errors, timeouts, denied connections, or invalid headers. It cannot repair a response after Nginx has already sent part of that response to the client. A retry is therefore a limited availability mechanism, not a cure for a broken PHP-FPM pool or a failing application.
Retries can also repeat a request. Be cautious with non-idempotent operations such as payments, account changes, or form submissions; confirm that the application and upstream policy make repetition safe. When a request has already produced response bytes, correct the backend or application rather than adding more retry conditions.
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“502 Bad Gateway” immediately after deployment
- Read the Nginx or Apache error log at the failure time.
- Compare the deployed configuration’s endpoint with the active PHP-FPM pool.
- Run the configuration test and verify that the new pool actually started.
- Check whether a release changed the document root, causing an incorrect
SCRIPT_FILENAMEor Apache path prefix.
“connect() failed while connecting to upstream”
Confirm that the listener exists at the exact address and port or socket path. A refused TCP connection calls for service and listener checks; a missing Unix socket calls for pool status and path comparison. Do not increase fastcgi_read_timeout; the request has not reached the read stage.
“No such file or directory” for a PHP-FPM socket
Find the socket PHP-FPM created, compare it with fastcgi_pass or Apache’s backend URL, and inspect the pool’s listen setting. Start or restart the correct pool if no socket exists. If the socket is created under a runtime directory that is cleaned at boot, ensure the service creates that directory before starting.
“Permission denied” for a socket
Inspect every parent directory and the socket’s owner, group, and mode. Make the web-server account a member of the intended group or adjust pool ownership, then restart PHP-FPM so the socket is recreated. Avoid broad permissions that expose the application to unrelated local users.
“upstream timed out while reading response header”
Check PHP-FPM worker utilization, slow-request records, database latency, and application logs. If the operation is valid but predictably slow, raise the read or process limit modestly and monitor worker occupancy. If the operation is unexpectedly slow, fix it instead of extending the limit indefinitely.
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Verify that the web server reaches the same pool you inspected. Multiple PHP versions can create multiple sockets or ports. Check the active virtual host, included configuration, container or chroot boundaries, and the account used by the web-server workers. Then test a minimal script and the failing application route independently.
Nginx and Apache: what differs
| Area | Nginx | Apache |
|---|---|---|
| FastCGI integration | ngx_http_fastcgi_module |
mod_proxy with mod_proxy_fcgi, or mod_fcgid |
| Backend address | fastcgi_pass to a TCP address or Unix socket |
FastCGI backend URL for the PHP-FPM host, port, or supported socket form |
| Script identity | fastcgi_param SCRIPT_FILENAME plus request parameters |
Proxy mapping and PHP-FPM environment parameters |
| Timeout controls | fastcgi_connect_timeout, fastcgi_send_timeout, and fastcgi_read_timeout |
Module-specific connection and process-lifetime settings |
| Retry behavior | fastcgi_next_upstream for eligible failures before response data is sent |
Determined by the proxy and backend configuration |
The diagnostic order is the same for both servers: capture the exact error, verify the backend, align the endpoint, fix permissions, validate script mapping, and investigate capacity before changing time limits.
After the fix: verify and prevent recurrence
- Run the web-server configuration test and reload the affected service.
- Make one request to a minimal PHP endpoint, then the previously failing URL.
- Watch the error log and PHP-FPM log during both requests.
- Confirm that the response status, headers, and body are complete rather than merely free of a connection error.
- Record the active socket or port, pool name, owner and group, document root, and timeout rationale.
- Add monitoring for PHP-FPM restarts, worker saturation, socket disappearance, and recurring upstream timeouts.
Keep Nginx or Apache configuration, PHP-FPM pool files, and runtime assumptions together in deployment management. When a PHP version, package, virtual host, or container changes, verify the endpoint and socket permissions as part of the rollout instead of waiting for a 502 report.
A compact decision checklist
- Exact message captured? Include stage, errno, endpoint, and timestamp.
- Backend running? The correct PHP-FPM pool is active and its logs show no startup failure.
- Listener aligned? TCP host and port or Unix socket path match exactly.
- Socket usable? The web-server account can traverse directories and open the socket.
- Script path correct? Nginx’s
SCRIPT_FILENAMEor Apache’s mapping points to the intended file. - Request parameters present? Method, content type, and content length are passed for requests that need them.
- Timeout explained? Capacity and application behavior were checked before increasing a limit.
- Retry appropriate? No policy relies on retries to repair a response that has already begun or to repeat unsafe writes.
Following this sequence turns a generic FastCGI 502 into a specific service, endpoint, permission, configuration, or application problem—the level at which it can be fixed permanently.
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