Module ngx_http_rewrite_module

Directives
     break
     if
     return
     rewrite
     rewrite_log
     set
     uninitialized_variable_warn
Internal Implementation

The ngx_http_rewrite_module module is used to change request URI using PCRE regular expressions, return redirects, and conditionally select configurations.

The ngx_http_rewrite_module module directives are processed in the following order:

Directives

Syntax: break;
Default:
Context: server, location, if

Stops processing the current set of ngx_http_rewrite_module directives.

If a directive is specified inside the location, further processing of the request continues in this location.

Example:

if ($slow) {
    limit_rate 10k;
    break;
}

Syntax: if (condition) { ... }
Default:
Context: server, location

The specified condition is evaluated. If true, this module directives specified inside the braces are executed, and the request is assigned the configuration inside the if directive. Configurations inside the if directives are inherited from the previous configuration level.

A condition may be any of the following:

Examples:

if ($http_user_agent ~ MSIE) {
    rewrite ^(.*)$ /msie/$1 break;
}

if ($http_cookie ~* "id=([^;]+)(?:;|$)") {
    set $id $1;
}

if ($request_method = POST) {
    return 405;
}

if ($slow) {
    limit_rate 10k;
}

if ($invalid_referer) {
    return 403;
}

A value of the $invalid_referer embedded variable is set by the valid_referers directive.

Syntax: return code [text];
return code URL;
return URL;
Default:
Context: server, location, if

Stops processing and returns the specified code to a client. The non-standard code 444 closes a connection without sending a response header.

Starting from version 0.8.42, it is possible to specify either a redirect URL (for codes 301, 302, 303, and 307), or the response body text (for other codes). A response body text and redirect URL can contain variables. As a special case, a redirect URL can be specified as a URI local to this server, in which case the full redirect URL is formed according to the request scheme ($scheme) and the server_name_in_redirect and port_in_redirect directives.

In addition, a URL for temporary redirect with the code 302 can be specified as the sole parameter. Such a parameter should start with the “http://”, “https://”, or “$scheme” string. A URL can contain variables.

Only the following codes could be returned before version 0.7.51: 204, 400, 402 — 406, 408, 410, 411, 413, 416, and 500 — 504.

The code 307 was not treated as a redirect until versions 1.1.16 and 1.0.13.

See also the error_page directive.

Syntax: rewrite regex replacement [flag];
Default:
Context: server, location, if

If the specified regular expression matches a request URI, URI is changed as specified in the replacement string. The rewrite directives are executed sequentially in order of their appearance in the configuration file. It is possible to terminate further processing of the directives using flags. If a replacement string starts with “http://” or “https://”, the processing stops and the redirect is returned to a client.

An optional flag parameter can be one of:

last
stops processing the current set of ngx_http_rewrite_module directives and starts a search for a new location matching the changed URI;
break
stops processing the current set of ngx_http_rewrite_module directives as with the break directive;
redirect
returns a temporary redirect with the 302 code; used if a replacement string does not start with “http://” or “https://”;
permanent
returns a permanent redirect with the 301 code.

The full redirect URL is formed according to the request scheme ($scheme) and the server_name_in_redirect and port_in_redirect directives.

Example:

server {
    ...
    rewrite ^(/download/.*)/media/(.*)\..*$ $1/mp3/$2.mp3 last;
    rewrite ^(/download/.*)/audio/(.*)\..*$ $1/mp3/$2.ra  last;
    return  403;
    ...
}

But if these directives are put inside the “/download/” location, the last flag should be replaced by break, or otherwise nginx will make 10 cycles and return the 500 error:

location /download/ {
    rewrite ^(/download/.*)/media/(.*)\..*$ $1/mp3/$2.mp3 break;
    rewrite ^(/download/.*)/audio/(.*)\..*$ $1/mp3/$2.ra  break;
    return  403;
}

If a replacement string includes the new request arguments, the previous request arguments are appended after them. If this is undesired, putting a question mark at the end of a replacement string avoids having them appended, for example:

rewrite ^/users/(.*)$ /show?user=$1? last;

If a regular expression includes the “}” or “;” characters, the whole expressions should be enclosed in single or double quotes.

Syntax: rewrite_log on | off;
Default:
rewrite_log off;
Context: http, server, location, if

Enables or disables logging of ngx_http_rewrite_module module directives processing results into the error_log at the notice level.

Syntax: set $variable value;
Default:
Context: server, location, if

Sets a value for the specified variable. The value can contain text, variables, and their combination.

Syntax: uninitialized_variable_warn on | off;
Default:
uninitialized_variable_warn on;
Context: http, server, location, if

Controls whether warnings about uninitialized variables are logged.

Internal Implementation

The ngx_http_rewrite_module module directives are compiled at the configuration stage into internal instructions that are interpreted during request processing. An interpreter is a simple virtual stack machine.

For example, the directives

location /download/ {
    if ($forbidden) {
        return 403;
    }

    if ($slow) {
        limit_rate 10k;
    }

    rewrite ^/(download/.*)/media/(.*)\..*$ /$1/mp3/$2.mp3 break;
}

will be translated into these instructions:

variable $forbidden
check against zero
    return 403
    end of code
variable $slow
check against zero
match of regular expression
copy "/"
copy $1
copy "/mp3/"
copy $2
copy ".mp3"
end of regular expression
end of code

Note that there are no instructions for the limit_rate directive above as it is unrelated to the ngx_http_rewrite_module module. A separate configuration is created for the if block. If the condition holds true, a request is assigned this configuration where limit_rate equals to 10k.

The directive

rewrite ^/(download/.*)/media/(.*)\..*$ /$1/mp3/$2.mp3 break;

can be made smaller by one instruction if the first slash in the regular expression is put inside the parentheses:

rewrite ^(/download/.*)/media/(.*)\..*$ $1/mp3/$2.mp3 break;

The corresponding instructions will then look like this:

match of regular expression
copy $1
copy "/mp3/"
copy $2
copy ".mp3"
end of regular expression
end of code