The ngx_http_rewrite_module module allows to change URIs using regular expressions, return redirects, and conditionally select configurations.

The ngx_http_rewrite_module module directives are processed in the following order:

• the directives of this module specified on the server level are processed;
• a location for a request is searched;
• the directives of this module specified in the selected location are processed, and if they changed a URI, a new location is searched for the new URI. This cycle may be repeated up to 10 times after which the error 500 (Internal Server Error) is returned.

Stops processing the current set of ngx_http_rewrite_module directives.

Example:

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

The specified condition is evaluated. If true, the directives of this module specified inside the braces are executed, and a 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:

• a variable name; false if the value of a variable is an empty string or any string starting with “0”;
• comparing a variable with a string using the “=” and “!=” operators;
• matching a variable against a regular expression using the “~” (for case-sensitive matching) and “~*” (for case-insensitive matching) operators. Regular expressions can contain captures that are made available for later reuse in the $1..$9 variables. Negative operators “!~” and “!~*” are also available. If a regular expression includes the characters “}” or “;”, the whole expressions should be enclosed in single or double quotes.
• checking a file existence with the “-f” and “!-f” operators;
• checking a directory existence with the “-d” and “!-d” operators;
• checking a file, directory, or symbolic link existence with the “-e” and “!-e” operators;
• checking for an executable file with operators “-x” and “!-x” operators.

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 embedded variable $invalid_referer is set by the valid_referers directive.

Stops processing and returns the specified code to a client. The following codes can be returned: 204, 400, 402 — 406, 408, 410, 411, 413, 416, and 500 — 504. In addition, the non-standard code 444 closes a connection without sending a response header.

If the specified regular expression matches a URI, the URI is changed as specified in the replacement string. The rewrite directives are executed sequentially in order of their appearance in the configuration file. Flags make it possible to terminate further processing of directives. If a replacement string starts with “http://” or “https://”, the processing stops and a 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 followed by a search for a new location matching the changed URI;

break

stops processing the current set of ngx_http_rewrite_module directives;

redirect

returns a temporary redirect with the code 302; used if a replacement string does not start with “http://” or “https://”;

permanent

returns a permanent redirect with the code 301.

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, otherwise nginx will make 10 cycles and return the error 500:

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 characters “}” or “;”, the whole expressions should be enclosed in single or double quotes.

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

uninitialized_variable_warn on;

Controls whether warnings about uninitialized variables are logged.

The ngx_http_rewrite_module module directives are compiled during 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