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These variables all correspond to the similarly named HTTP MIME-headers, C variables of the Apache HTTP Server or struct tm fields of the Unix system. Most are documented here or elsewhere in the Manual or in the CGI specification.

SERVER_NAME and SERVER_PORT depend on the values of UseCanonicalName and UseCanonicalPhysicalPort respectively.

Those that are special to mod_rewrite include those below.

If the TestString has the special value expr , the CondPattern will be treated as an ap_expr. HTTP headers referenced in the expression will be added to the Vary header if the novary flag is not given.

Other things you should be aware of:

The variables SCRIPT_FILENAME and REQUEST_FILENAME contain the same value — the value of the filename field of the internal request_rec structure of the Apache HTTP Server. The first name is the commonly known CGI variable name while the second is the appropriate counterpart of REQUEST_URI (which contains the value of the uri field of request_rec ).

If a substitution occurred and the rewriting continues, the value of both variables will be updated accordingly.

If used in per-server context (i.e., before the request is mapped to the filesystem) SCRIPT_FILENAME and REQUEST_FILENAME cannot contain the full local filesystem path since the path is unknown at this stage of processing. Both variables will initially contain the value of REQUEST_URI in that case. In order to obtain the full local filesystem path of the request in per-server context, use an URL-based look-ahead % to determine the final value of REQUEST_FILENAME.

If a HTTP header is used in a condition this header is added to the Vary header of the response in case the condition evaluates to true for the request. It is not added if the condition evaluates to false for the request. Adding the HTTP header to the Vary header of the response is needed for proper caching.

It has to be kept in mind that conditions follow a short circuit logic in the case of the ‘ ornext|OR ‘ flag so that certain conditions might not be evaluated at all.

For instance, to rewrite according to the REMOTE_USER variable from within the per-server context ( httpd.conf file) you must use % — this variable is set by the authorization phases, which come after the URL translation phase (during which mod_rewrite operates).

On the other hand, because mod_rewrite implements its per-directory context ( .htaccess file) via the Fixup phase of the API and because the authorization phases come before this phase, you just can use % in that context.

CondPattern is the condition pattern, a regular expression which is applied to the current instance of the TestString. TestString is first evaluated, before being matched against CondPattern.

CondPattern is usually a perl compatible regular expression, but there is additional syntax available to perform other useful tests against the Teststring:

1. You can prefix the pattern string with a ‘ ! ‘ character (exclamation mark) to negate the result of the condition, no matter what kind of CondPattern is used.
2. You can perform lexicographical string comparisons: <CondPattern Lexicographically precedes
   Treats the CondPattern as a plain string and compares it lexicographically to TestString. True if TestString lexicographically precedes CondPattern. >CondPattern Lexicographically follows
   Treats the CondPattern as a plain string and compares it lexicographically to TestString. True if TestString lexicographically follows CondPattern. =CondPattern Lexicographically equal
   Treats the CondPattern as a plain string and compares it lexicographically to TestString. True if TestString is lexicographically equal to CondPattern (the two strings are exactly equal, character for character). If CondPattern is «» (two quotation marks) this compares TestString to the empty string. <=CondPattern Lexicographically less than or equal to
   Treats the CondPattern as a plain string and compares it lexicographically to TestString. True if TestString lexicographically precedes CondPattern, or is equal to CondPattern (the two strings are equal, character for character). >=CondPattern Lexicographically greater than or equal to
   Treats the CondPattern as a plain string and compares it lexicographically to TestString. True if TestString lexicographically follows CondPattern, or is equal to CondPattern (the two strings are equal, character for character).

Is existing URL, via subrequest.
Checks whether or not TestString is a valid URL, accessible via all the server’s currently-configured access controls for that path. This uses an internal subrequest to do the check, so use it with care — it can impact your server’s performance!

This flag only returns information about things like access control, authentication, and authorization. This flag does not return information about the status code the configured handler (static file, CGI, proxy, etc.) would have returned.

If the TestString has the special value expr , the CondPattern will be treated as an ap_expr.

In the below example, -strmatch is used to compare the REFERER against the site hostname, to block unwanted hotlinking.

You can also set special flags for CondPattern by appending [ flags ] as the third argument to the RewriteCond directive, where flags is a comma-separated list of any of the following flags:

• ‘ nocase|NC ‘ (no case)
  This makes the test case-insensitive — differences between ‘A-Z’ and ‘a-z’ are ignored, both in the expanded TestString and the CondPattern. This flag is effective only for comparisons between TestString and CondPattern. It has no effect on filesystem and subrequest checks.
• ‘ ornext|OR ‘ (or next condition)
  Use this to combine rule conditions with a local OR instead of the implicit AND. Typical example: Without this flag you would have to write the condition/rule pair three times.
• ‘ novary|NV ‘ (no vary)
  If a HTTP header is used in the condition, this flag prevents this header from being added to the Vary header of the response.
  Using this flag might break proper caching of the response if the representation of this response varies on the value of this header. So this flag should be only used if the meaning of the Vary header is well understood.

Example:

To rewrite the Homepage of a site according to the « User-Agent: » header of the request, you can use the following:

Explanation: If you use a browser which identifies itself as a mobile browser (note that the example is incomplete, as there are many other mobile platforms), the mobile version of the homepage is served. Otherwise, the standard page is served.

By default, multiple RewriteCond s are evaluated in sequence with an implied logical AND. If a condition fails, in the absence of an OR flag, the entire ruleset is abandoned, and further conditions are not evaluated.

RewriteEngine Directive

The RewriteEngine directive enables or disables the runtime rewriting engine. If it is set to off this module does no runtime processing at all. It does not even update the SCRIPT_URx environment variables.

Use this directive to disable rules in a particular context, rather than commenting out all the RewriteRule directives.

Note that rewrite configurations are not inherited by virtual hosts. This means that you need to have a RewriteEngine on directive for each virtual host in which you wish to use rewrite rules.

RewriteMap directives of the type prg are not started during server initialization if they’re defined in a context that does not have RewriteEngine set to on

RewriteMap Directive

The RewriteMap directive defines a Rewriting Map which can be used inside rule substitution strings by the mapping-functions to insert/substitute fields through a key lookup. The source of this lookup can be of various types.

The MapName is the name of the map and will be used to specify a mapping-function for the substitution strings of a rewriting rule via one of the following constructs:

$< MapName : LookupKey >
$< MapName : LookupKey | DefaultValue >

When such a construct occurs, the map MapName is consulted and the key LookupKey is looked-up. If the key is found, the map-function construct is substituted by SubstValue. If the key is not found then it is substituted by DefaultValue or by the empty string if no DefaultValue was specified. Empty values behave as if the key was absent, therefore it is not possible to distinguish between empty-valued keys and absent keys.

For example, you might define a RewriteMap as:

You would then be able to use this map in a RewriteRule as follows:

The meaning of the MapTypeOptions argument depends on particular MapType. See the Using RewriteMap for more information.

The following combinations for MapType and MapSource can be used:

txt A plain text file containing space-separated key-value pairs, one per line. (Details . ) rnd Randomly selects an entry from a plain text file (Details . ) dbm Looks up an entry in a dbm file containing name, value pairs. Hash is constructed from a plain text file format using the httxt2dbm utility. (Details . ) int One of the four available internal functions provided by RewriteMap : toupper, tolower, escape or unescape. (Details . ) prg Calls an external program or script to process the rewriting. (Details . ) dbd or fastdbd A SQL SELECT statement to be performed to look up the rewrite target. (Details . )

Further details, and numerous examples, may be found in the RewriteMap HowTo

RewriteOptions Directive

The RewriteOptions directive sets some special options for the current per-server or per-directory configuration. The Option string can currently only be one of the following:

This forces the current configuration to inherit the configuration of the parent. In per-virtual-server context, this means that the maps, conditions and rules of the main server are inherited. In per-directory context this means that conditions and rules of the parent directory’s .htaccess configuration or <Directory> sections are inherited. The inherited rules are virtually copied to the section where this directive is being used. If used in combination with local rules, the inherited rules are copied behind the local rules. The position of this directive — below or above of local rules — has no influence on this behavior. If local rules forced the rewriting to stop, the inherited rules won’t be processed.

Like Inherit above, but the rules from the parent scope are applied before rules specified in the child scope.
Available in Apache HTTP Server 2.3.10 and later.

If this option is enabled, all child configurations will inherit the configuration of the current configuration. It is equivalent to specifying RewriteOptions Inherit in all child configurations. See the Inherit option for more details on how the parent-child relationships are handled.
Available in Apache HTTP Server 2.4.8 and later.

Like InheritDown above, but the rules from the current scope are applied before rules specified in any child’s scope.
Available in Apache HTTP Server 2.4.8 and later.

This option forces the current and child configurations to ignore all rules that would be inherited from a parent specifying InheritDown or InheritDownBefore .
Available in Apache HTTP Server 2.4.8 and later.

By default, mod_rewrite will ignore URLs that map to a directory on disk but lack a trailing slash, in the expectation that the mod_dir module will issue the client with a redirect to the canonical URL with a trailing slash.

When the DirectorySlash directive is set to off, the AllowNoSlash option can be enabled to ensure that rewrite rules are no longer ignored. This option makes it possible to apply rewrite rules within .htaccess files that match the directory without a trailing slash, if so desired.
Available in Apache HTTP Server 2.4.0 and later.

When RewriteRule is used in VirtualHost or server context with version 2.2.22 or later of httpd, mod_rewrite will only process the rewrite rules if the request URI is a URL-path. This avoids some security issues where particular rules could allow «surprising» pattern expansions (see CVE-2011-3368 and CVE-2011-4317). To lift the restriction on matching a URL-path, the AllowAnyURI option can be enabled, and mod_rewrite will apply the rule set to any request URI string, regardless of whether that string matches the URL-path grammar required by the HTTP specification.
Available in Apache HTTP Server 2.4.3 and later.

Security Warning

Enabling this option will make the server vulnerable to security issues if used with rewrite rules which are not carefully authored. It is strongly recommended that this option is not used. In particular, beware of input strings containing the ‘ @ ‘ character which could change the interpretation of the transformed URI, as per the above CVE names.

With this option, the value of RewriteBase is copied from where it’s explicitly defined into any sub-directory or sub-location that doesn’t define its own RewriteBase . This was the default behavior in 2.4.0 through 2.4.3, and the flag to restore it is available Apache HTTP Server 2.4.4 and later.

When a relative substitution is made in directory (htaccess) context and RewriteBase has not been set, this module uses some extended URL and filesystem context information to change the relative substitution back into a URL. Modules such as mod_userdir and mod_alias supply this extended context info. Available in 2.4.16 and later.

Prior to 2.4.26, if a substitution was an absolute URL that matched the current virtual host, the URL might first be reduced to a URL-path and then later reduced to a local path. Since the URL can be reduced to a local path, the path should be prefixed with the document root. This prevents a file such as /tmp/myfile from being accessed when a request is made to http://host/file/myfile with the following RewriteRule .

This option allows the old behavior to be used where the document root is not prefixed to a local path that was reduced from a URL. Available in 2.4.26 and later.

RewriteRule Directive

The RewriteRule directive is the real rewriting workhorse. The directive can occur more than once, with each instance defining a single rewrite rule. The order in which these rules are defined is important — this is the order in which they will be applied at run-time.

Pattern is a perl compatible regular expression. What this pattern is compared against varies depending on where the RewriteRule directive is defined.

What is matched?

In VirtualHost context, The Pattern will initially be matched against the part of the URL after the hostname and port, and before the query string (e.g. «/app1/index.html»). This is the (%-decoded) URL-path.

In per-directory context ( Directory and .htaccess), the Pattern is matched against only a partial path, for example a request of «/app1/index.html» may result in comparison against «app1/index.html» or «index.html» depending on where the RewriteRule is defined.

The directory path where the rule is defined is stripped from the currently mapped filesystem path before comparison (up to and including a trailing slash). The net result of this per-directory prefix stripping is that rules in this context only match against the portion of the currently mapped filesystem path «below» where the rule is defined.

Directives such as DocumentRoot and Alias , or even the result of previous RewriteRule substitutions, determine the currently mapped filesystem path.

If you wish to match against the hostname, port, or query string, use a RewriteCond with the % , % , or % variables respectively.

Per-directory Rewrites

• The rewrite engine may be used in .htaccess files and in <Directory> sections, with some additional complexity.
• To enable the rewrite engine in this context, you need to set » RewriteEngine On » and » Options FollowSymLinks » must be enabled. If your administrator has disabled override of FollowSymLinks for a user’s directory, then you cannot use the rewrite engine. This restriction is required for security reasons.
• See the RewriteBase directive for more information regarding what prefix will be added back to relative substitutions.
• If you wish to match against the full URL-path in a per-directory (htaccess) RewriteRule, use the % variable in a RewriteCond .
• The removed prefix always ends with a slash, meaning the matching occurs against a string which never has a leading slash. Therefore, a Pattern with ^/ never matches in per-directory context.
• Although rewrite rules are syntactically permitted in <Location> and <Files> sections (including their regular expression counterparts), this should never be necessary and is unsupported. A likely feature to break in these contexts is relative substitutions.
• The If blocks follow the rules of the directory context.
• By default, mod_rewrite overrides rules when merging sections belonging to the same context. The RewriteOptions directive can change this behavior, for example using the Inherit setting.
• The RewriteOptions also regulates the behavior of sections that are stated at the same nesting level of the configuration. In the following example, by default only the RewriteRules stated in the second If block are considered, since the first ones are overridden. Using RewriteOptions Inherit forces mod_rewrite to merge the two sections and consider both set of statements, rather than only the last one.

In mod_rewrite , the NOT character (‘ ! ‘) is also available as a possible pattern prefix. This enables you to negate a pattern; to say, for instance: «if the current URL does NOT match this pattern». This can be used for exceptional cases, where it is easier to match the negative pattern, or as a last default rule.

The Substitution of a rewrite rule is the string that replaces the original URL-path that was matched by Pattern. The Substitution may be a:

file-system path Designates the location on the file-system of the resource to be delivered to the client. Substitutions are only treated as a file-system path when the rule is configured in server (virtualhost) context and the first component of the path in the substitution exists in the file-system URL-path A DocumentRoot -relative path to the resource to be served. Note that mod_rewrite tries to guess whether you have specified a file-system path or a URL-path by checking to see if the first segment of the path exists at the root of the file-system. For example, if you specify a Substitution string of /www/file.html , then this will be treated as a URL-path unless a directory named www exists at the root or your file-system (or, in the case of using rewrites in a .htaccess file, relative to your document root), in which case it will be treated as a file-system path. If you wish other URL-mapping directives (such as Alias ) to be applied to the resulting URL-path, use the [PT] flag as described below. Absolute URL

If an absolute URL is specified, mod_rewrite checks to see whether the hostname matches the current host. If it does, the scheme and hostname are stripped out and the resulting path is treated as a URL-path. Otherwise, an external redirect is performed for the given URL. To force an external redirect back to the current host, see the [R] flag below.

Note that a redirect (implicit or not) using an absolute URI will include the requested query-string, to prevent this see the [QSD] flag below.

— (dash) A dash indicates that no substitution should be performed (the existing path is passed through untouched). This is used when a flag (see below) needs to be applied without changing the path.

In addition to plain text, the Substitution string can include

1. back-references ( $N ) to the RewriteRule pattern
2. back-references ( %N ) to the last matched RewriteCond pattern
3. server-variables as in rule condition test-strings ( % ) calls ( $ )

Back-references are identifiers of the form $ N (N=0..9), which will be replaced by the contents of the Nth group of the matched Pattern. The server-variables are the same as for the TestString of a RewriteCond directive. The mapping-functions come from the RewriteMap directive and are explained there. These three types of variables are expanded in the order above.

Rewrite rules are applied to the results of previous rewrite rules, in the order in which they are defined in the config file. The URL-path or file-system path (see «What is matched?», above) is completely replaced by the Substitution and the rewriting process continues until all rules have been applied, or it is explicitly terminated by an L flag, or other flag which implies immediate termination, such as END or F .

Modifying the Query String

By default, the query string is passed through unchanged. You can, however, create URLs in the substitution string containing a query string part. Simply use a question mark inside the substitution string to indicate that the following text should be re-injected into the query string. When you want to erase an existing query string, end the substitution string with just a question mark. To combine new and old query strings, use the [QSA] flag.

Additionally you can set special actions to be performed by appending [ flags ] as the third argument to the RewriteRule directive. Flags is a comma-separated list, surround by square brackets, of any of the flags in the following table. More details, and examples, for each flag, are available in the Rewrite Flags document.

Home directory expansion

When the substitution string begins with a string resembling «/

user» (via explicit text or backreferences), mod_rewrite performs home directory expansion independent of the presence or configuration of mod_userdir .

This expansion does not occur when the PT flag is used on the RewriteRule directive.

Here are all possible substitution combinations and their meanings:

Inside per-server configuration ( httpd.conf )
for request « GET /somepath/pathinfo »:

Inside per-directory configuration for /somepath
( /physical/path/to/somepath/.htaccess , with RewriteBase «/somepath» )
for request « GET /somepath/localpath/pathinfo »:

Apache Module mod_rewrite

This module uses a rule-based rewriting engine (based on a regular-expression parser) to rewrite requested URLs on the fly. It supports an unlimited number of rules and an unlimited number of attached rule conditions for each rule, to provide a really flexible and powerful URL manipulation mechanism. The URL manipulations can depend on various tests, of server variables, environment variables, HTTP headers, or time stamps. Even external database lookups in various formats can be used to achieve highly granular URL matching.

This module operates on the full URLs (including the path-info part) both in per-server context ( httpd.conf ) and per-directory context ( .htaccess ) and can generate query-string parts on result. The rewritten result can lead to internal sub-processing, external request redirection or even to an internal proxy throughput.

Further details, discussion, and examples, are provided in the detailed mod_rewrite documentation.

Directives

• RewriteBase
• RewriteCond
• RewriteEngine
• RewriteLock
• RewriteLog
• RewriteLogLevel
• RewriteMap
• RewriteOptions
• RewriteRule

Topics

• API Phases
• Ruleset Processing
• Regex Back-Reference Availability
• Quoting Special Characters
• Environment Variables
• Practical Solutions

API Phases

Apache processes a HTTP request in several phases. A hook for each of these phases is provided by the Apache API. mod_rewrite uses two of these hooks: the URL-to-filename translation hook (used after the HTTP request has been read, but before any authorization starts) and the Fixup hook (triggered after the authorization phases, and after the per-directory config files ( .htaccess ) have been read, but before the content handler is activated).

Once a request comes in, and Apache has determined the appropriate server (or virtual server), the rewrite engine starts the URL-to-filename translation, processing the mod_rewrite directives from the per-server configuration. A few steps later, when the final data directories are found, the per-directory configuration directives of mod_rewrite are triggered in the Fixup phase.

Ruleset Processing

When mod_rewrite is triggered during these two API phases, it reads the relevant rulesets from its configuration structure (which was either created on startup, for per-server context, or during the directory traversal for per-directory context). The URL rewriting engine is started with the appropriate ruleset (one or more rules together with their conditions), and its operation is exactly the same for both configuration contexts. Only the final result processing is different.

The order of rules in the ruleset is important because the rewrite engine processes them in a particular (not always obvious) order, as follows: The rewrite engine loops through the rulesets (each ruleset being made up of RewriteRule directives, with or without RewriteCond s), rule by rule. When a particular rule is matched, mod_rewrite also checks the corresponding conditions ( RewriteCond directives). For historical reasons the conditions are given first, making the control flow a little bit long-winded. See Figure 1 for more details.

The control flow of the rewrite engine through a rewrite ruleset

As above, first the URL is matched against the Pattern of a rule. If it does not match, mod_rewrite immediately stops processing that rule, and goes on to the next rule. If the Pattern matches, mod_rewrite checks for rule conditions. If none are present, the URL will be replaced with a new string, constructed from the Substitution string, and mod_rewrite goes on to the next rule.

If RewriteCond s exist, an inner loop is started, processing them in the order that they are listed. Conditions are not matched against the current URL directly. A TestString is constructed by expanding variables, back-references, map lookups, etc., against which the CondPattern is matched. If the pattern fails to match one of the conditions, the complete set of rule and associated conditions fails. If the pattern matches a given condition, then matching continues to the next condition, until no more conditions are available. If all conditions match, processing is continued with the substitution of the Substitution string for the URL.

Regex Back-Reference Availability

Using parentheses in Pattern or in one of the CondPatterns causes back-references to be internally created. These can later be referenced using the strings $N and %N (see below), for creating the Substitution and TestString strings. Figure 2 attempts to show how the back-references are transferred through the process for later expansion.

The back-reference flow through a rule.

Quoting Special Characters

As of Apache 1.3.20, special characters in TestString and Substitution strings can be escaped (that is, treated as normal characters without their usual special meaning) by prefixing them with a slash (‘\’) character. In other words, you can include an actual dollar-sign character in a Substitution string by using ‘ \$ ‘; this keeps mod_rewrite from trying to treat it as a backreference.

Environment Variables

This module keeps track of two additional (non-standard) CGI/SSI environment variables named SCRIPT_URL and SCRIPT_URI . These contain the logical Web-view to the current resource, while the standard CGI/SSI variables SCRIPT_NAME and SCRIPT_FILENAME contain the physical System-view.

Notice: These variables hold the URI/URL as they were initially requested, that is, before any rewriting. This is important to note because the rewriting process is primarily used to rewrite logical URLs to physical pathnames.

Example

Practical Solutions

For numerous examples of common, and not-so-common, uses for mod_rewrite, see the Rewrite Guide, and the Advanced Rewrite Guide documents.

RewriteBase Directive

The RewriteBase directive explicitly sets the base URL for per-directory rewrites. As you will see below, RewriteRule can be used in per-directory config files ( .htaccess ). In such a case, it will act locally, stripping the local directory prefix before processing, and applying rewrite rules only to the remainder. When processing is complete, the prefix is automatically added back to the path. The default setting is; RewriteBase physical-directory-path

When a substitution occurs for a new URL, this module has to re-inject the URL into the server processing. To be able to do this it needs to know what the corresponding URL-prefix or URL-base is. By default this prefix is the corresponding filepath itself. However, for most websites, URLs are NOT directly related to physical filename paths, so this assumption will often be wrong! Therefore, you can use the RewriteBase directive to specify the correct URL-prefix.

For example, assume the following per-directory config file:

In the above example, a request to /xyz/oldstuff.html gets correctly rewritten to the physical file /abc/def/newstuff.html .

For Apache Hackers

The following list gives detailed information about the internal processing steps:

This seems very complicated, but is in fact correct Apache internal processing. Because the per-directory rewriting comes late in the process, the rewritten request has to be re-injected into the Apache kernel. This is not the serious overhead it may seem to be — this re-injection is completely internal to the Apache server (and the same procedure is used by many other operations within Apache).

RewriteCond Directive

The RewriteCond directive defines a rule condition. One or more RewriteCond can precede a RewriteRule directive. The following rule is then only used if both the current state of the URI matches its pattern, and if these conditions are met.

TestString is a string which can contain the following expanded constructs in addition to plain text:

• RewriteRule backreferences: These are backreferences of the form $N (0 <= N <= 9), which provide access to the grouped parts (in parentheses) of the pattern, from the RewriteRule which is subject to the current set of RewriteCond conditions..
• RewriteCond backreferences: These are backreferences of the form %N (1 <= N <= 9), which provide access to the grouped parts (again, in parentheses) of the pattern, from the last matched RewriteCond in the current set of conditions.
• RewriteMap expansions: These are expansions of the form $ . See the documentation for RewriteMap for more details.
• Server-Variables: These are variables of the form %< NAME_OF_VARIABLE > where NAME_OF_VARIABLE can be a string taken from the following list:
  

  HTTP headers:	connection & request:
  HTTP_USER_AGENT HTTP_REFERER HTTP_COOKIE HTTP_FORWARDED HTTP_HOST HTTP_PROXY_CONNECTION HTTP_ACCEPT	REMOTE_ADDR REMOTE_HOST REMOTE_PORT REMOTE_USER REMOTE_IDENT REQUEST_METHOD SCRIPT_FILENAME PATH_INFO QUERY_STRING AUTH_TYPE
  server internals:	system stuff:	specials:
  DOCUMENT_ROOT SERVER_ADMIN SERVER_NAME SERVER_ADDR SERVER_PORT SERVER_PROTOCOL SERVER_SOFTWARE	TIME_YEAR TIME_MON TIME_DAY TIME_HOUR TIME_MIN TIME_SEC TIME_WDAY TIME	API_VERSION THE_REQUEST REQUEST_URI REQUEST_FILENAME IS_SUBREQ HTTPS