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NGINX if is a conditional from the ngx_http_rewrite_module, not a general-purpose configuration language. Put the condition in parentheses and use rewrite-module actions inside the braces. For example, this rejects POST requests while leaving normal content handling outside the conditional:
location / {
if ($request_method = POST) {
return 405;
}
}Use if for a small, immediate response or rewrite. For derived values, file selection, or host-wide routing, map, try_files, and separate server blocks are safer and easier to reason about.
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NGINX evaluates rewrite-module directives in a request-processing phase. An if condition is written in parentheses, followed by a block containing directives that the rewrite module supports:
if (condition) {
rewrite-or-response-directive;
}The directive is valid in both server and location contexts. At the server level, rewrite directives run before NGINX selects a location. After a location is selected, location-level rewrite directives run there. If a rewrite changes the URI and requests another location lookup, this process can repeat, but NGINX limits rewrite cycles to 10 iterations.
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That execution model explains both the usefulness and the danger of if: it is excellent for a narrowly scoped rewrite decision, but it is not an all-purpose programming construct for configuring proxying, content handlers, or an entire request pipeline.
Condition syntax you can use
NGINX supports four practical categories of tests inside an if. The condition must be enclosed in parentheses.
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Test whether a variable is true
If you put a variable by itself in the condition, NGINX treats an empty string or the value 0 as false. Any other value is true.
if ($some_flag) {
return 204;
}This form is useful when another directive has already produced a flag variable. Make sure the variable can actually be empty or zero when you want the condition to fail; a nonempty string such as false is still truthy.
Compare a variable with a string
Use = for equality and != for inequality:
if ($request_method = POST) {
return 405;
}
if ($host != example.com) {
return 444;
}These comparisons are string comparisons. The value on the right is written directly in the configuration, while the value on the left is normally an NGINX variable.
Match a regular expression
Use ~ for a case-sensitive regular-expression match and ~* for a case-insensitive match. Prefix either operator with ! to negate it: !~ and !~*.
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if ($request_uri ~ ^/old/(.*)$) {
rewrite ^/old/(.*)$ /new/$1 permanent;
}
if ($http_user_agent ~* mobile) {
return 302 /mobile$request_uri;
}Keep the expression focused on the routing decision. A broad expression can match more requests than intended, and a rewrite that repeatedly produces another matching URI can consume the available rewrite cycles.
Check whether a file-system path exists
File tests use these operators:
-fand!-ftest for a regular file or its absence.-dand!-dtest for a directory or its absence.-eand!-etest for any existing path or its absence.-xand!-xtest whether a path is executable or is not executable.
For example, a file-existence test can be written against a path variable:
if (-f $request_filename) {
return 200;
}For ordinary static-file dispatch, however, try_files expresses this requirement more directly and avoids putting file selection logic in an if block.
What belongs inside the braces
Keep the body to rewrite-module actions whose purpose is to produce a response, change the URI, assign a rewrite variable, or control rewrite processing. Common examples are:
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchreturnto send a status code, redirect, or short response immediately.rewriteto transform a URI or redirect it.setto assign a variable used by later rewrite logic.breakto stop rewrite processing in the current location when that behavior is intentional.
The NGINX-maintained “If Is Evil” guidance states: “The only 100% safe things which may be done inside if in a location context are: return …; rewrite … last;” Treat that as the conservative rule for location-level configuration. In particular, do not put proxying, content handlers, or other non-rewrite directives inside a location-level if merely because NGINX accepts the configuration.
A location-level if can create a separate configuration context. A directive that appears to inherit or combine with the surrounding location may therefore behave differently than expected. Keep the actual content handler outside the conditional and route requests to an explicit location or named fallback instead.
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Useful if patterns
Reject a method before content handling
This is the simplest safe pattern: test the request method and return a status without invoking the location’s content handler.
location /api/ {
if ($request_method = POST) {
return 405;
}
proxy_pass http://backend;
}The conditional only decides the response. The proxy directive remains in the location, outside the block. If the endpoint should advertise allowed methods, add the appropriate response headers in the surrounding configuration rather than trying to turn the if into a complete request policy engine.
Redirect a legacy host
An immediate return can redirect a request while preserving its path and query string:
if ($host = old.example.com) {
return 301 https://new.example.com$request_uri;
}This works as a narrow response rule, but a separate server block is the clearer design for a canonical-host redirect because the host decision belongs to virtual-server routing rather than to a location’s rewrite logic.
Redirect an old URI shape
Use a regular expression when the old path contains a variable suffix that must be carried into the new path:
location / {
if ($request_uri ~ ^/old/(.*)$) {
rewrite ^/old/(.*)$ /new/$1 permanent;
}
}The permanent flag sends a permanent redirect. If the intent is to continue processing internally under a different location, use last instead and understand that NGINX will perform another location lookup. Use break only when processing should remain in the current location.
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Why complex location-level if logic is risky
There are three recurring failure modes:
- Context surprises: a location-level conditional can create a separate configuration context, so non-rewrite directives may no longer combine with the surrounding location as you expect.
- Unexpected content handling: putting proxy, FastCGI, static, or other content directives inside a conditional can select an incomplete or unintended request handler.
- Rewrite loops: a rewrite can trigger another location search and run rewrite directives again. NGINX stops after 10 rewrite iterations, but the resulting request can still fail or return an unexpected URI.
Use an if when the decision is local and the action is a direct rewrite-module response. When the decision controls content selection or combines several independent inputs, move the decision into a construct designed for that job.
Choose the right construct instead
| Construct | Decision type | Configuration context | New location lookup | Typical action | Directive-safety guidance |
|---|---|---|---|---|---|
if |
Request variable, comparison, regular expression, or file test | server or location |
Only when its rewrite changes the URI and uses a flow that requests another lookup | Immediate response or rewrite | Keep location-level bodies to rewrite-module actions, especially return and rewrite ... last |
map |
Derived value from one or more variable matches | http |
No; it creates a variable evaluated when used | Set a flag or value consumed later by a location, upstream, or response rule | Safer for multiple conditions because decision-making is separated from request handling |
try_files |
Whether candidate files or directories exist | server or location |
Yes, when it internally redirects to a matching path or fallback | Serve a file, directory, or named fallback | Use for static-file selection instead of a file-test if |
Separate server block |
Host or virtual-server routing | http configuration containing servers |
NGINX selects the appropriate server for the request | Canonical-host redirect or host-specific handling | Preferred for old-host redirects; keeps host routing out of location rewrites |
Use map for derived values and multiple conditions
map creates a variable whose value depends on another variable’s value. It is declared at the http level, supports string and regular-expression matches, and evaluates the result when the mapped variable is used.
http {
map $request_method $method_allowed {
default 0;
GET 1;
HEAD 1;
}
server {
if ($method_allowed = 0) {
return 405;
}
}
}This example leaves one small if for the final response but moves the method matrix into a declarative map. The same pattern scales when the output must be reused by several locations or when matching involves more than one regular expression. The map itself does not invoke a content handler or perform a location lookup; it simply supplies a value to later configuration.
Use try_files for static-file selection
When the question is “does this request map to a file or directory, and what should happen if it does not?”, use try_files:
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location / {
try_files $uri $uri/ @app;
}
location @app {
proxy_pass http://backend;
}NGINX checks the listed paths in order. If it finds a file or directory, processing continues with that match. If none exists, NGINX uses the configured fallback, here the named location @app. This keeps static serving and application proxying as explicit locations rather than embedding a content decision in an if.
Use separate server blocks for host redirects
For an old hostname that should permanently redirect to a canonical hostname, define a server dedicated to the old host:
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server {
listen 80;
server_name old.example.com;
return 301 https://new.example.com$request_uri;
}The host is selected before location processing, so this arrangement avoids an if-based host rewrite and makes the redirect’s scope obvious. It also prevents the old host from accidentally reaching the normal content handler if the conditional is changed later.
Understand last, break, and rewrite cycles
last: rewrite and search again
Use last when the rewritten URI should be processed by a new location. NGINX performs another location lookup, then runs the rewrite phase for that new request cycle.
break: stay in the current location
Use break when rewrite processing should stop but the request should remain in the current location. This is different from last; choosing the wrong flag can leave the request in a location whose content handler was not intended for the rewritten URI.
The ten-iteration limit
Because a URI rewrite can start another cycle, NGINX limits this processing to 10 iterations. A configuration that reaches the limit usually has a rewrite that keeps matching its own output, two locations that rewrite back and forth, or a regular expression that is broader than intended. Make each rewrite’s input and output visibly different, and test a request that exercises the boundary.
Validate and test before deployment
- Save the configuration and run
nginx -t. Fix syntax or context errors before reloading. - Reload NGINX only after the test succeeds.
- Send representative requests for every branch: matching and nonmatching methods, hosts, URI patterns, file paths, and regex case variants.
- For redirects, inspect the status code and
Locationheader, and verify that the path and query string are preserved when expected. - For internal rewrites, confirm which location ultimately handles the request and that the rewrite does not repeat.
- Test a missing file, an existing directory, and an application fallback when using
try_files. - Check logs for unexpected status codes, repeated redirects, or a request that reaches the wrong upstream.
nginx -t checks configuration validity; it does not prove that the request-routing behavior matches your intention. Behavioral testing with representative requests is still required.
Troubleshoot common mistakes
The condition never matches
- Check whether the variable is empty, zero, or carrying a different value than expected.
- For string comparisons, verify exact spelling and case.
- For regular expressions, first test the simplest matching path and then add complexity.
- Remember that
~is case-sensitive while~*is case-insensitive.
The request reaches the wrong handler
Move proxying or other content directives out of the if. Put them in an explicit location, and use return, rewrite ... last, map, or try_files to route the request there.
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Compare the redirect target with the condition’s input. If the target still satisfies the same condition, the next request can redirect again. A host redirect is usually better implemented as a separate server block, and a path rewrite should have a pattern that excludes its final form.
A rewrite appears to do nothing
Check whether you used break when you needed a new location lookup. If the request must be handled by another location, use last and verify that the new URI actually matches that location.
Static files fall through unexpectedly
Replace a file-existence if with try_files and an explicit fallback. Confirm the order of candidate paths and that the fallback location has its own content handler.
Quick Recap
A practical decision checklist
- Is the rule a single request test with an immediate status or redirect? Use a small
ifwithreturn. - Does a URI need to be transformed and processed by another location? Use
rewrite ... last. - Does the rule combine many matches or produce a reusable flag? Define a
mapathttplevel. - Does the rule choose between files, directories, and an application fallback? Use
try_files. - Does the rule distinguish hostnames? Use separate
serverblocks. - Would the block need proxying, FastCGI, static serving, or another content directive? Do not put that content handler inside the
if. - Could the rewrite match its own output? Test for a loop and remember the 10-cycle limit.
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