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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 limit the block to rewrite-module actions—most reliably return or rewrite ... last. For example:
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location / {
if ($request_method = POST) {
return 405;
}
if ($request_uri ~ ^/old/(.*)$) {
rewrite ^/old/(.*)$ /new/$1 permanent;
}
}The condition can test a variable, compare strings, match a regular expression, or check whether a file-system object exists. The decision you are making determines whether if is appropriate: use map for derived values, try_files for static-file selection, and a separate server block for host redirects.
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An Nginx if belongs to the rewrite module. It evaluates a condition while Nginx processes a request and then runs rewrite-module directives in the braces when that condition is true. It is valid in both server and location contexts.
This makes if useful for request-time gates and redirects, but it does not turn the configuration file into a shell script or a conventional programming language. Directives that select content, proxy a request, or create a full request handler should remain outside the block.
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Condition forms you can use
The syntax always places the expression in parentheses:
if (condition) {
rewrite-module-directive;
}Testing whether a variable is true
A variable condition is false when the resulting value is an empty string or the string 0. Any other value is treated as true.
if ($arg_preview) {
return 302 /preview$request_uri;
}Here, a missing preview argument produces an empty value and skips the block; an argument value of 0 also skips it.
Exact string comparison
Use = for equality and != for inequality. The right-hand side is a literal value in the condition.
if ($request_method = POST) {
return 405;
}
if ($host != www.example.com) {
return 301 https://www.example.com$request_uri;
}String comparisons are a good fit for a small number of explicit request values, such as one method or one host. If the list grows, derive a value with map instead of adding a long chain of conditionals.
Regular-expression matching
~ performs a case-sensitive regular-expression match and ~* performs a case-insensitive match. Prefixing either operator with ! negates the test: !~ is a case-sensitive non-match and !~* is a case-insensitive non-match.
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rewrite ^/old/(.*)$ /new/$1 permanent;
}
if ($http_user_agent ~* "bot") {
return 403;
}Keep the regular expression narrowly scoped. Captures from a rewrite expression are used by the rewrite directive shown in the same block; test representative paths, including paths with query strings and unexpected characters, before enabling a permanent redirect.
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Checking the file system
File tests evaluate a path and can check for a regular file, directory, any existing object, or an executable file:
-fis true when a regular file exists;!-fis true when it does not.-dis true when a directory exists;!-dis true when it does not.-eis true when any file-system object exists;!-eis true when none exists.-xis true when the path is executable;!-xis true when it is not.
For static-file selection, however, try_files expresses the intent more directly and avoids putting file handling inside an if.
What is safe to put inside the block?
return for an immediate response
A return ends processing with a status code, optionally with a redirect target or response text. This is the clearest use of if because the conditional action is a response rather than a content handler.
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location /api/legacy {
if ($request_method = POST) {
return 405;
}
return 410;
}The method check above rejects only POST; the unconditional 410 then handles requests that were not rejected by the condition.
rewrite ... last for a deliberate location change
Use rewrite when a request URI should change. The last flag stops the current rewrite phase and starts a new location lookup using the rewritten URI.
location / {
if ($request_uri ~ ^/old/(.*)$) {
rewrite ^/old/(.*)$ /new/$1 last;
}
}
location /new/ {
proxy_pass http://backend;
}The rewrite changes routing; it does not itself proxy the request. Keep the proxying directive in the destination location.
Other rewrite-module directives
set can assign a variable and break can stop the current rewrite phase while continuing in the current location. These directives are part of the rewrite module, but a conditional set or break still needs careful testing because it changes how later directives in that request are reached.
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Nginx-maintained guidance summarizes the location-level safety rule as: “The only 100% safe things which may be done inside if in a location context are: return …; rewrite … last;” Treat that as the default boundary. Do not place a content handler, proxy configuration, or other non-rewrite directive inside a location-level conditional merely because the parser accepts it.
Why Nginx if is considered unsafe
The warning is about configuration context, not about the condition operators themselves. A location-level if can create a separate configuration context. A directive that appears harmless may therefore inherit or combine settings differently from the surrounding location, producing behavior that is valid syntactically but not what you intended.
Typical risky patterns put request handlers inside the block:
location / {
if ($request_uri ~ ^/private/) {
proxy_pass http://private_backend;
}
}Instead, let the condition perform a redirect or rewrite to an explicit location and keep proxy_pass there:
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if ($request_uri ~ ^/private/) {
rewrite ^/private/(.*)$ /_private/$1 last;
}
}
location /_private/ {
proxy_pass http://private_backend;
}For more complicated routing, use a derived variable, named location, or separate server and location blocks. The goal is to make each location have one clear content handler rather than conditionally replacing its handler.
How rewrite processing is executed
Nginx runs rewrite-module directives at the server level first. It then searches for a matching location and runs the location-level rewrite directives. If a rewrite changes the URI with last, Nginx performs a new location lookup and repeats the relevant processing.
There is a limit of 10 rewrite cycles. A rule that repeatedly rewrites a URI back into another matching form can therefore stop with an internal redirect error instead of looping forever.
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Choosing last or break
- Use
lastwhen the rewritten URI should be matched against the location tree again. - Use
breakwhen processing should remain in the current location after the rewrite phase stops.
Do not use last simply because it is familiar: an intentional location change is its purpose. If no new location should be selected, break may be the appropriate behavior.
Safer constructs for common decisions
Choose the construct that matches the kind of decision you are making:
| Construct | Best decision type | Context | New location lookup | Typical action |
|---|---|---|---|---|
if |
One request-variable test, comparison, regular expression, or file test | server or location |
Only when a rewrite uses last |
Response or rewrite |
map |
Derived value from one or more request variables, including multiple matches | http |
No; it creates a variable | Set a value later consumed by routing or response logic |
try_files |
Whether a requested file or directory exists | server or location |
Internal redirect or configured fallback | Serve a file or select an application fallback |
Separate server block |
Canonical host or domain redirect | http |
No conditional lookup is needed | Redirect the old host to the canonical host |
Use map for derived values and multiple conditions
The map module creates a variable whose value depends on another variable. Declare it at http level, then consume the result where the request is handled. Values are evaluated 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 keeps the list of accepted methods in one map. The remaining if is only a simple response gate. You can also use a mapped value in other directives that accept variables, avoiding a chain of conditionals for every possible input.
Use try_files for file and directory selection
try_files checks the specified paths in order. When it finds a matching file or directory, Nginx uses that result; when none matches, it internally redirects to the final fallback.
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try_files $uri $uri/ @app;
}
location @app {
proxy_pass http://backend;
}The first candidate is the request path as a file, the second as a directory, and @app is the named fallback when neither exists. This is preferable to a file-existence if when the real requirement is “serve a static asset if present, otherwise send the request to the application.”
Use a separate server block for host redirects
A host redirect is a property of the virtual server, so express it in a server dedicated to the old host:
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server {
listen 80;
server_name old.example.com;
return 301 https://new.example.com$request_uri;
}This avoids matching every request in a general location just to discover that the host is wrong. The canonical server can then contain the normal locations and content handlers.
Practical conditional patterns
Rejecting a method
location /upload {
if ($request_method = POST) {
return 405;
}
}Use this when one method must receive an immediate status response. If several methods are allowed or denied, a map makes the policy easier to maintain.
Redirecting one host
server {
listen 80;
server_name old.example.com;
if ($host = old.example.com) {
return 301 https://new.example.com$request_uri;
}
}The conditional form works as a narrow redirect, but a separate server block is clearer and is the preferred design for a host-wide redirect.
Moving an old URI tree
location / {
if ($request_uri ~ ^/old/(.*)$) {
rewrite ^/old/(.*)$ /new/$1 permanent;
}
}permanent sends a permanent redirect to the client. If the destination must be handled internally by another location instead, use last rather than a client-visible redirect.
Testing before deployment
- Check the configuration syntax with
nginx -tusing the same configuration file and includes that the service will load. - Exercise the exact request method, host, path, query string, and representative headers that the condition tests.
- For redirects, inspect both the status code and the
Locationheader. Confirm that the destination does not match the original redirect rule. - For internal rewrites, verify which location ultimately serves the request and that the expected proxy or content directive is outside the
if. - Test a non-matching request as well as a matching one; a rule that appears correct in isolation may alter the fallback path.
- Reload only after the syntax check succeeds, then repeat the representative requests against the reloaded configuration.
Testing is especially important when a rewrite uses last, because each URI change can trigger another location lookup and contribute to the 10-cycle limit.
Troubleshooting conditional logic
The condition never matches
- Check whether the variable is empty or equals
0, which is false for a bare variable test. - Confirm exact spelling and case when using
=or~. - Use
~*only when case-insensitive matching is intentional. - Verify whether the variable contains the path, the full URI, or a host value before writing the expression.
The request reaches the wrong location
- Use
lastonly when a new location lookup is required. - Use
breakwhen processing should stay in the current location. - Inspect every rule that can rewrite the same URI; two matching rules can cause an unexpected chain.
Nginx reports an internal redirect cycle
Look for rules that rewrite a URI into another URI that still matches the original rule. Remove the overlap or make the destination pattern distinct. Nginx stops rewrite processing after 10 iterations, but the request still fails if the configuration keeps sending it back through the same cycle.
A proxy or handler behaves differently inside if
Move the handler into a dedicated location. Have the conditional block return a response or rewrite to that location instead of embedding proxy_pass or another content directive in the block.
Quick Recap
A decision checklist
- Is the requirement an immediate response or a URI rewrite? A narrowly scoped
ifmay be suitable. - Does the rule derive a value from several request cases? Define a
mapathttplevel. - Does it choose between an existing file and an application? Use
try_files. - Does it redirect an entire hostname? Create a separate
serverblock. - Will the block contain a proxy, content handler, or other non-rewrite directive? Redesign it with explicit locations.
- Could the rewrite match its own destination? Test for a loop before using
lastor a permanent redirect.
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