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Nginx FastCGI proxying sends a request to a FastCGI application server such as PHP-FPM instead of serving the application code itself. The fastcgi_pass directive selects the backend listener, while fastcgi_param builds the parameter environment the application needs. A working configuration therefore has three essential parts: a location that matches the request, a fastcgi_pass endpoint that really exists, and a correct SCRIPT_FILENAME mapping to the script on the FastCGI server’s filesystem.
The configuration below covers PHP-style deployments, but the same routing model applies to other FastCGI applications. Socket names, ports, document roots and parameter requirements are deployment-specific; copy the structure, then substitute values exposed by your own FastCGI service.
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The ngx_http_fastcgi_module allows Nginx to pass HTTP requests to a FastCGI server. Nginx remains the client-facing HTTP server: it accepts the connection, chooses a location, constructs FastCGI parameters, sends the request to the configured backend and returns the backend response to the client.
FastCGI is an application-backend protocol, not an HTTP reverse-proxy exchange. That distinction is why a FastCGI location uses fastcgi_pass and fastcgi_param rather than the HTTP-oriented proxy_pass and proxy_set_header pattern.
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| Directive or pattern | What it selects or sends | Typical use |
|---|---|---|
proxy_pass |
An HTTP-oriented upstream URL | Proxying to an application that speaks HTTP |
fastcgi_pass |
A FastCGI hostname and port, IP and port, UNIX socket, or upstream group | Sending requests to PHP-FPM or another FastCGI listener |
proxy_set_header |
HTTP request headers | Preserving headers for an HTTP backend |
fastcgi_param |
FastCGI name/value parameters, including request metadata | Providing script paths, query strings and method information |
In a PHP request, Nginx does not execute the PHP file. It calculates the script path, passes that path and other request information to PHP-FPM, and PHP-FPM executes the script. The response then travels back through Nginx, which applies its buffering, timeout and error-handling settings before sending the result to the browser.
The minimum PHP-style configuration
A practical baseline is:
location ~ .php$ {
fastcgi_pass unix:/run/php/php-fpm.sock;
fastcgi_index index.php;
include fastcgi_params;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
}The socket path in this example is not universal. Replace it with the UNIX socket or TCP listener that PHP-FPM actually exposes. The fastcgi_pass directive accepts a hostname or IP plus port, a UNIX-domain socket, or the name of an upstream server group.
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- Nginx must be installed and able to load the FastCGI module.
- A FastCGI server must be running and listening on the endpoint you plan to use.
- The Nginx worker and the FastCGI service must have compatible access to the application files. A path that exists in one isolated environment but not the other still produces a script-resolution failure.
- Your server block must define the intended
root, because the baselineSCRIPT_FILENAMEexpression uses$document_root.
Step 1: identify the real listener
Inspect the FastCGI service configuration or process manager and record whether it listens on a UNIX socket or TCP address. Use that exact value after fastcgi_pass. A stale socket filename, wrong port, or mismatch between a container and the host is a connection problem, not a PHP script problem.
Step 2: create a matching location
The regular-expression location above matches URIs ending in .php. If your application has a different entry-point convention, adjust the location deliberately. Nginx’s location-selection rules determine which directives run, so a more specific or differently ordered location can prevent the FastCGI block from handling the request you expected.
Step 3: include parameters and set the script path
include fastcgi_params supplies the standard parameter definitions available in your installation. Set SCRIPT_FILENAME explicitly to the filesystem path the FastCGI server should execute. The expression $document_root$fastcgi_script_name combines the configured document root with the script name derived from the request URI.
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Step 4: validate and reload
- Check the configuration with
nginx -t. Correct syntax errors before reloading. - Reload Nginx using the service manager used by your operating system, then request a known PHP script.
- If the request fails, inspect the Nginx error log and the FastCGI service log together. The two logs distinguish an unreachable listener from an invalid script path or an application-level failure.
How Nginx maps a URI to a script
The most important mapping is SCRIPT_FILENAME. PHP uses this parameter to determine which file to execute. For a document root of /data/www and a request for /index.php, the expression $document_root$fastcgi_script_name produces /data/www/index.php. That complete path is what the FastCGI server receives.
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$fastcgi_script_name and directory indexes
$fastcgi_script_name is based on the request URI. When the URI ends in a slash and fastcgi_index is configured, Nginx appends the index filename. With fastcgi_index index.php, a request for a directory can resolve to an index.php script. The resulting value must still be valid below the configured document root.
PATH_INFO applications
Some applications use a script followed by additional path information, such as /index.php/articles. In that case, fastcgi_split_path_info accepts a two-capture regular expression. The first capture becomes $fastcgi_script_name; the second becomes $fastcgi_path_info. Pass the latter as PATH_INFO when the application expects it.
location ~ ^(.+.php)(/.+)$ {
fastcgi_split_path_info ^(.+.php)(/.+)$;
fastcgi_pass unix:/run/php/php-fpm.sock;
include fastcgi_params;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_param PATH_INFO $fastcgi_path_info;
}The regular expression must match the URL shape your application actually uses. If it captures too much or too little, Nginx can hand the backend a script path that does not exist.
Why a wrong path causes “Primary script unknown”
That message generally means the FastCGI service received a SCRIPT_FILENAME that it could not resolve. Compare the value Nginx computes with the path visible from the PHP-FPM process’s filesystem perspective. Common causes include a document root typo, a container path that differs from the host path, a missing index suffix, or a PATH_INFO expression that captured the URI incorrectly.
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FastCGI parameters: what the application receives
fastcgi_param creates the FastCGI parameter environment. HTTP request headers are represented as FastCGI parameters, and you can supply additional application-specific values. The module documentation identifies SCRIPT_FILENAME and QUERY_STRING as the minimum PHP-related settings.
| Parameter | Purpose | When it matters |
|---|---|---|
SCRIPT_FILENAME |
Absolute script path to execute | Every PHP-style script request |
QUERY_STRING |
Request parameters after the question mark | Requests that use query arguments |
REQUEST_METHOD |
HTTP method such as GET or POST | Especially important for form submissions and APIs |
CONTENT_TYPE |
Media type of the request body | POST and other requests carrying encoded data |
CONTENT_LENGTH |
Length of the request body | POST requests and other body-bearing methods |
PATH_INFO |
Path suffix separated from the script name | Applications using PATH_INFO routing |
For POST failures, verify that REQUEST_METHOD, CONTENT_TYPE and CONTENT_LENGTH are being passed in addition to the script and query-string parameters. A page that works for GET but fails for a form submission often has an incomplete parameter environment rather than a broken socket.
Choosing a backend endpoint
UNIX-domain socket
A UNIX socket is addressed with a path such as unix:/run/php/php-fpm.sock. It must exist at the path Nginx uses, and permissions must allow the Nginx worker to connect. Distribution packages and service pools commonly choose different filenames, so do not treat the sample path as a standard.
TCP listener
A TCP endpoint uses a host and port, for example 127.0.0.1:9000, when the FastCGI service listens there. Confirm the address from the runtime configuration. A listener bound only to another interface or network namespace is unreachable from Nginx even when the port number looks correct.
Upstream server group
For multiple FastCGI backends, define an upstream group and pass its name:
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upstream php_backends {
server 127.0.0.1:9000;
server 127.0.0.1:9001;
}
location ~ .php$ {
fastcgi_pass php_backends;
include fastcgi_params;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
}An upstream group distributes requests among the listed FastCGI servers. Adding servers does not by itself solve application-state problems. Review backend health, session affinity requirements and whether all servers share the same application files, uploads, caches and other state. If one member has a different document root or code version, requests can fail intermittently.
Buffering and timeout behavior
FastCGI settings have different scopes. Change the directive implicated by the observed symptom instead of copying arbitrary “performance” values.
| Directive | Current default | What the setting controls | When to change it |
|---|---|---|---|
fastcgi_buffering |
on |
Whether Nginx reads the FastCGI response into buffers before sending it onward | Disable only when the application genuinely requires streaming behavior |
fastcgi_buffer_size and fastcgi_buffers |
Values depend on the Nginx build and configuration; not stated as one universal size | Memory available for response buffering | Adjust when headers or response chunks exceed the configured buffers |
fastcgi_read_timeout |
60s |
Maximum interval between successive reads from the FastCGI server | Increase for applications that legitimately pause longer than 60 seconds between output chunks |
fastcgi_connect_timeout |
60s |
Time allowed to establish the backend connection | Investigate listener reachability first; the documented practical limit is usually no more than 75 seconds |
fastcgi_keep_conn |
off |
Whether Nginx keeps the FastCGI connection open after a request | Set to on only when the FastCGI server and deployment support persistent connections |
fastcgi_request_buffering |
on |
Whether Nginx buffers the request body before sending it upstream | Disable for deliberate request streaming; once body transmission starts, retrying another server is no longer possible |
Read timeout is not a total request timer
fastcgi_read_timeout 60s applies between successive reads, not to the complete response duration. An application can therefore run longer than 60 seconds if it keeps producing data within each interval. Conversely, a request that emits no data for longer than the interval can time out even if the application is still working. Set the value from the application’s actual output pattern and execution behavior.
When buffering should remain enabled
With buffering enabled, Nginx stores response data in memory controlled by fastcgi_buffer_size and fastcgi_buffers. If the response does not fit, Nginx can write part of it to a temporary file. This is the normal behavior for ordinary dynamic pages. Turn buffering off only for an application whose protocol or user experience depends on incremental streaming, and test the resulting memory and client behavior.
Error responses and hidden headers
fastcgi_intercept_errors is off by default. When enabled, FastCGI responses with status codes of 300 or higher can be processed by Nginx’s error_page mechanism. This is useful when the web server must present a consistent error page, but it changes where those responses are handled, so enable it intentionally.
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Nginx hides the Status and X-Accel-* response headers from clients by default. Use fastcgi_pass_header only when a specific hidden field must be exposed. Do not expose internal control headers simply because an application emits them.
A systematic 502 and script-error checklist
1. Separate connection errors from script errors
A 502 generated because Nginx cannot connect to the FastCGI listener has a different remedy from “Primary script unknown.” Check the Nginx error log wording and then inspect the FastCGI service’s listener configuration. Confirm the socket exists or that the host and port accept connections from Nginx.
2. Check socket ownership and permissions
For a UNIX socket, verify that the Nginx worker can open it. A correct filename with restrictive ownership still produces a connection failure. Fix the service’s socket permissions or group arrangement according to your operating-system policy rather than making the socket globally writable.
3. Verify the computed script path
Logically expand $document_root$fastcgi_script_name for the failing URI and check that the resulting file exists where the FastCGI process can see it. In containerized deployments, compare mount points inside the Nginx and FastCGI environments; identical host paths are not guaranteed to be identical container paths.
4. Confirm location selection
Ensure the request actually enters the intended FastCGI location. A competing exact, prefix or regular-expression location can send the request to static-file handling or a different upstream. Review location ordering and test with a URI that should unambiguously match.
5. Check POST parameters
If GET requests succeed but POST requests fail, verify REQUEST_METHOD, CONTENT_TYPE and CONTENT_LENGTH, along with QUERY_STRING and SCRIPT_FILENAME. Missing body metadata can make an otherwise correct script appear broken.
6. Diagnose slow responses by phase
- Failure while opening the connection points to
fastcgi_pass, listener availability or permissions. - Failure after connection but before the first response points to application execution or
fastcgi_read_timeout. - Failure after periods of silence points specifically to the interval-based read timeout.
- Large or streaming responses point to buffering choices and buffer sizes.
Adjust only the implicated setting, then retest with the same URI and workload. Raising every timeout can hide an unavailable backend without fixing it.
Deployment practices that prevent recurring failures
- Keep the Nginx document root and the FastCGI process’s view of the application tree consistent.
- Record the active socket or TCP endpoint as part of deployment configuration so upgrades do not leave a stale
fastcgi_passvalue. - Use a deliberately scoped PHP location instead of allowing arbitrary files to be interpreted as scripts.
- Apply configuration changes with a syntax test before reload, and retain the previous working configuration for rollback.
- When introducing an upstream group, verify that every backend has the same code, configuration and required shared state.
- Set timeouts from measured application behavior and expected client interactions, not from copied values whose scope is unclear.
FastCGI proxying is therefore a translation and routing layer: Nginx chooses the endpoint, translates request information into FastCGI parameters, maps the URI to an executable script and manages the response lifecycle. Correct endpoint selection and correct path construction solve most configuration failures; buffering, timeout and scaling directives then refine behavior for the application’s actual workload.
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