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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsNGINX if is a conditional from the ngx_http_rewrite_module, not a general-purpose configuration language. Put a condition in parentheses, then place rewrite-module actions in braces. A method check, for example, can reject every POST request in a location:
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location / {
if ($request_method = POST) {
return 405;
}
}The condition can test a variable’s truthiness, compare strings, match a regular expression, or test whether a filesystem path exists. The important safety rule is to keep the block focused on response and rewrite actions. Do not put proxying or content-handler directives inside a location-level if; use map, try_files, explicit locations, or separate server blocks when those constructs describe the decision more accurately.
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What an NGINX if block actually does
The if directive is implemented by NGINX’s HTTP rewrite module. It is valid in both server and location contexts. NGINX evaluates the condition and, if it is true, processes the directives between the braces.
Its syntax is:
if (condition) {
rewrite-module-directive;
}The braces do not create a general programming-language branch in which any NGINX directive is safe. They create a rewrite-module configuration context. That distinction explains both the useful cases and the warnings surrounding if.
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A minimal conditional response
location / {
if ($request_method = POST) {
return 405;
}
# Normal content handling remains outside the if block.
}For a matching POST request, return 405 ends request processing with HTTP status 405. Requests using other methods continue through the rest of the location configuration.
Condition forms supported by if
Use one of the documented condition forms below. The operator determines whether NGINX performs a truthiness test, a string comparison, a regular-expression match, or a filesystem test.
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A variable by itself is true when it is neither an empty string nor the string 0. An empty value and 0 are false.
if ($maintenance_mode) {
return 503;
}This form is useful when another part of the configuration deliberately sets a flag variable. Because both an empty string and 0 evaluate as false, choose the variable’s values consistently rather than mixing arbitrary text, numbers, and empty values.
String comparison
Compare a variable with a literal value by using = or !=:
if ($request_method = POST) {
return 405;
}
if ($host != www.example.com) {
return 400;
}The first condition is true only when $request_method is exactly POST. The second is true for every host value other than the stated string. Comparisons are useful for a small, direct decision involving one value; they become harder to maintain when many accepted values or combinations are involved.
Regular-expression matching
Use ~ for a case-sensitive regular-expression match and ~* for a case-insensitive match. Prefix either operator with ! to negate it: !~ and !~*.
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if ($request_uri ~ ^/old/(.*)$) {
rewrite ^/old/(.*)$ /new/$1 permanent;
}
if ($host ~* ^api.) {
return 403;
}In the first example, a URI beginning with /old/ is rewritten to the corresponding /new/ path and sent as a permanent redirect. The expression captures the remainder of the path in $1. In the second example, the case-insensitive expression matches hosts whose value begins with api..
Negative forms let you reject or reroute values that do not match:
if ($request_uri !~ ^/public/) {
return 404;
}Keep regular expressions narrowly scoped. A pattern that is too broad can match more requests than intended, while a pattern that assumes a particular case can fail when the request value differs in capitalization.
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NGINX also supports tests for paths on the server:
-ftests for a regular file;!-fis its negative form.-dtests for a directory;!-dis its negative form.-etests whether the path exists;!-eis its negative form.-xtests for an executable path;!-xis its negative form.
if (-f $request_filename) {
return 200;
}These tests inspect the path represented by the expression. For ordinary static-file selection, however, try_files communicates the intent more clearly and avoids putting file selection logic in an if.
What belongs inside the braces
Keep the body to rewrite-module directives such as return, rewrite, set, and break. The safest uses make an immediate response or perform a deliberate URI rewrite.
Rejecting a method
location /api/ {
if ($request_method = POST) {
return 405;
}
proxy_pass http://backend;
}The conditional response is inside the block; the proxying directive remains in the location itself. This separation matters because content handling is not the purpose of the rewrite conditional.
Redirecting an old host
server {
listen 443 ssl;
server_name old.example.com;
if ($host = old.example.com) {
return 301 https://new.example.com$request_uri;
}
# Other server configuration follows outside the if.
}This works as a response-only condition, but a dedicated server block is the clearer choice for a host-wide redirect; that alternative is shown below.
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Rewriting a URI
location / {
if ($request_uri ~ ^/old/(.*)$) {
rewrite ^/old/(.*)$ /new/$1 permanent;
}
}A rewrite can use a redirect flag such as permanent, as in the example, or can route internally. When the purpose is to select another location, use rewrite ... last. When processing should continue in the current location, use break.
“The only 100% safe things which may be done inside if in a location context are: return …; rewrite … last;”
— NGINX Wiki, “If Is Evil”
The quotation is intentionally narrow: it describes the safest location-level actions, not a promise that every other rewrite-module directive is invalid. Treat it as a design boundary. Put response and routing decisions in the conditional, and leave proxying, static content, FastCGI, and other content handlers outside it.
Why location-level if is considered unsafe
A location-level if can create a separate configuration context while NGINX builds the request-processing configuration. Some non-rewrite directives may be accepted syntactically, yet their behavior can be surprising because the resulting context is not equivalent to writing those directives directly in the location.
The practical failure mode is usually a conditional that appears to work for one request but interferes with content handling, proxying, or another location on a different path. Avoid embedding a proxy or content handler in the block:
# Avoid this pattern
location / {
if ($request_method = POST) {
proxy_pass http://backend;
}
}Instead, let the if return or rewrite, and place the content handler in an explicit location or named fallback. If the decision is really about a derived value, a filesystem choice, or a host name, use the construct designed for that decision rather than extending if into a general branch.
How rewrite processing is evaluated
NGINX first runs rewrite-module directives at the server level. It then searches for a location and runs the location-level rewrite directives. If a rewrite changes the URI, NGINX can perform another location lookup and repeat the cycle.
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last versus break
lastdeliberately starts a new location lookup after the rewrite. Use it when the rewritten URI should be handled by another location.breakstops rewrite processing for the current cycle and continues in the current location. Use it when the current location should keep handling the request.
NGINX limits rewrite processing to 10 iterations. A chain that keeps changing the URI can therefore stop after the limit instead of reaching the intended handler. Design rewrites so that the new URI leaves the matching condition, or route once to a stable named or explicit location.
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Use map for derived values and multiple conditions
map creates a variable whose value depends on another variable. Define it at the http level, then use the resulting variable in a server or location. It supports string matches and regular-expression matches, and the mapped value is evaluated when the variable is used.
http {
map $request_method $method_allowed {
default 0;
GET 1;
HEAD 1;
}
server {
if ($method_allowed = 0) {
return 405;
}
}
}Here, the mapping expresses the method policy in one place. The conditional only checks the derived flag and returns the response. Add more methods or match patterns in the map rather than building a long sequence of nested conditions.
Use try_files for file and directory selection
For static content, try_files is the purpose-built alternative to filesystem tests in an if. It checks the specified files or directories in order. If it finds a match, NGINX uses that result; if none matches, it internally redirects to the configured fallback.
location / {
try_files $uri $uri/ @app;
}
location @app {
proxy_pass http://backend;
}The first location tests the requested URI as a file and then as a directory. If neither exists, processing goes to the named @app fallback, where the proxy directive is safely outside an if block.
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Use a separate server block for a host redirect
A host-wide redirect does not need a request-time condition in the destination server. Give the old host its own server block and return the redirect directly:
server {
listen 80;
server_name old.example.com;
return 301 https://new.example.com$request_uri;
}This makes host selection explicit and avoids evaluating a host condition for every request that reaches a broader server block. The rewrite-conversion guidance from NGINX describes an if-based host rewrite as wrong, cumbersome, and ineffective compared with a separate server.
Choosing between if, map, try_files, and server
| Construct | Decision input | Configuration context | Location lookup behavior | Safe primary role |
|---|---|---|---|---|
if |
Request variable, string, regular expression, or filesystem test | server or location |
A URI rewrite can trigger another lookup; processing can repeat up to 10 cycles | Return a response or perform a controlled rewrite |
map |
A derived value based on a variable, string match, or regular expression | http |
No lookup by itself; its variable is evaluated when used | Compute a flag or value for later routing or response logic |
try_files |
Whether candidate files or directories exist | location |
Uses the first match or internally redirects to the fallback | Select static content or a named application fallback |
Separate server block |
Host name | server |
Chooses the server configuration for the host | Return a host-wide redirect directly |
Use the smallest construct that matches the decision. A single method rejection is a good if. A reusable policy for several methods belongs in map. File selection belongs in try_files. A redirect for an entire old host belongs in its own server.
Putting the patterns together
The following configuration combines the constructs without putting a content handler inside a conditional:
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map $request_method $method_allowed {
default 0;
GET 1;
HEAD 1;
}
server {
listen 80;
server_name old.example.com;
return 301 https://new.example.com$request_uri;
}
server {
listen 80;
server_name example.com;
location / {
if ($method_allowed = 0) {
return 405;
}
try_files $uri $uri/ @app;
}
location @app {
proxy_pass http://backend;
}
}
}The old host is handled before it reaches the application server. The application server derives method policy with map, uses a response-only if, and delegates file selection and application handling to try_files and a named location.
Testing before deployment
Validate the configuration before reloading NGINX:
- Run
nginx -tand resolve every syntax or context error it reports. - Send representative requests for each branch: an allowed method, a rejected method, a matching URI, a nonmatching URI, an old host, and a new host.
- Check the status and destination for every redirect or rejection, and verify that requests falling through the condition still reach the intended location or named fallback.
- For rewrites, include a URI that could match again. Confirm that the rewritten URI becomes stable rather than starting an unintended chain.
Configuration validation proves that NGINX can parse the file; representative requests prove that the condition, rewrite, and fallback behavior match the intended request flow.
Troubleshooting common mistakes
The condition never matches
Check the operator and the exact value. A string comparison with = is not a regular-expression match, and a truthiness test treats both an empty string and 0 as false. For pattern matching, verify whether you need case-sensitive ~ or case-insensitive ~*, and whether the negative operator is the one you intended.
A rewrite reaches the wrong location
Inspect whether the rule uses last or break. last starts a new location lookup; break stays in the current location. Choose the flag according to where the rewritten request should be handled.
Requests loop or stop after several rewrites
Look for a rule whose replacement URI still satisfies its own condition or another rule in the chain. Rewrite processing is capped at 10 iterations, so make the destination unambiguously different from the source pattern or route once to a stable location.
Proxying behaves unexpectedly inside an if
Move proxy_pass and other content-handler directives outside the block. Keep the conditional to a response or rewrite, then use an explicit location or named fallback for proxying.
File checks are becoming complicated
Replace a series of existence tests with try_files. It expresses ordered file and directory checks and provides an explicit fallback when none exists.
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