Logging

mod_rewrite offers detailed logging of its actions at the trace1 to trace8 log levels. The log level can be set specifically for mod_rewrite using the LogLevel directive: Up to level debug, no actions are logged, while trace8 means that practically all actions are logged.

LogLevel alert rewrite:trace3

RewriteBase Directive

The RewriteBase directive specifies the URL prefix to be used for per-directory (htaccess) RewriteRule directives that substitute a relative path.

This directive is required when you use a relative path in a substitution in per-directory (htaccess) context unless any of the following conditions are true:

• The original request, and the substitution, are underneath the DocumentRoot (as opposed to reachable by other means, such as Alias).
• The filesystem path to the directory containing the RewriteRule, suffixed by the relative substitution is also valid as a URL path on the server (this is rare).
• In Apache HTTP Server 2.4.16 and later, this directive may be omitted when the request is mapped via Alias or mod_userdir.

In the example below, RewriteBase is necessary to avoid rewriting to http://example.com/opt/myapp-1.2.3/welcome.html since the resource was not relative to the document root. This misconfiguration would normally cause the server to look for an "opt" directory under the document root.

DocumentRoot "/var/www/example.com"
AliasMatch "^/myapp" "/opt/myapp-1.2.3"
<Directory "/opt/myapp-1.2.3">
    RewriteEngine On
    RewriteBase "/myapp/"
    RewriteRule "^index\.html$"  "welcome.html"
</Directory>

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). $1 to $9 provide access to the grouped parts (in parentheses) of the pattern, from the RewriteRule which is subject to the current set of RewriteCond conditions. $0 provides access to the whole string matched by that pattern.
• RewriteCond backreferences: These are backreferences of the form %N (0 <= N <= 9). %1 to %9 provide access to the grouped parts (again, in parentheses) of the pattern, from the last matched RewriteCond in the current set of conditions. %0 provides access to the whole string matched by that pattern.
• RewriteMap expansions: These are expansions of the form ${mapname:key|default}. 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_ACCEPT HTTP_COOKIE HTTP_FORWARDED HTTP_HOST HTTP_PROXY_CONNECTION HTTP_REFERER HTTP_USER_AGENT	AUTH_TYPE CONN_REMOTE_ADDR CONTEXT_PREFIX CONTEXT_DOCUMENT_ROOT IPV6 PATH_INFO QUERY_STRING REMOTE_ADDR REMOTE_HOST REMOTE_IDENT REMOTE_PORT REMOTE_USER REQUEST_METHOD SCRIPT_FILENAME
  server internals:	date and time:	specials:
  DOCUMENT_ROOT SCRIPT_GROUP SCRIPT_USER SERVER_ADDR SERVER_ADMIN SERVER_NAME SERVER_PORT SERVER_PROTOCOL SERVER_SOFTWARE	TIME_YEAR TIME_MON TIME_DAY TIME_HOUR TIME_MIN TIME_SEC TIME_WDAY TIME	API_VERSION CONN_REMOTE_ADDR HTTPS IS_SUBREQ REMOTE_ADDR REQUEST_FILENAME REQUEST_SCHEME REQUEST_URI THE_REQUEST

  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.

  API_VERSION

  This is the version of the Apache httpd module API (the internal interface between server and module) in the current httpd build, as defined in include/ap_mmn.h. The module API version corresponds to the version of Apache httpd in use (in the release version of Apache httpd 1.3.14, for instance, it is 19990320:10), but is mainly of interest to module authors.

  CONN_REMOTE_ADDR

  Since 2.4.8: The peer IP address of the connection (see the mod_remoteip module).

  HTTPS

  Will contain the text "on" if the connection is using SSL/TLS, or "off" otherwise. (This variable can be safely used regardless of whether or not mod_ssl is loaded).

  IS_SUBREQ

  Will contain the text "true" if the request currently being processed is a sub-request, "false" otherwise. Sub-requests may be generated by modules that need to resolve additional files or URIs in order to complete their tasks.

  REMOTE_ADDR

  The IP address of the remote host (see the mod_remoteip module).

  REQUEST_FILENAME

  The full local filesystem path to the file or script matching the request, if this has already been determined by the server at the time REQUEST_FILENAME is referenced. Otherwise, such as when used in virtual host context, the same value as REQUEST_URI. Depending on the value of AcceptPathInfo, the server may have only used some leading components of the REQUEST_URI to map the request to a file.

  REQUEST_SCHEME

  Will contain the scheme of the request (usually "http" or "https"). This value can be influenced with ServerName.

  REQUEST_URI

  The path component of the requested URI, such as "/index.html". This notably excludes the query string which is available as its own variable named QUERY_STRING.

  THE_REQUEST

  The full HTTP request line sent by the browser to the server (e.g., "GET /index.html HTTP/1.1"). This does not include any additional headers sent by the browser. This value has not been unescaped (decoded), unlike most other variables 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:

1. 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 %{LA-U:REQUEST_FILENAME} to determine the final value of REQUEST_FILENAME.

2. %{ENV:variable}, where variable can be any environment variable, is also available. This is looked-up via internal Apache httpd structures and (if not found there) via getenv() from the Apache httpd server process.
3. %{SSL:variable}, where variable is the name of an SSL environment variable, can be used whether or not mod_ssl is loaded, but will always expand to the empty string if it is not. Example: %{SSL:SSL_CIPHER_USEKEYSIZE} may expand to 128. These variables are available even without setting the StdEnvVars option of the SSLOptions directive.
4. %{HTTP:header}, where header can be any HTTP MIME-header name, can always be used to obtain the value of a header sent in the HTTP request. Example: %{HTTP:Proxy-Connection} is the value of the HTTP header ``Proxy-Connection:''.

   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.

5. %{LA-U:variable} can be used for look-aheads which perform an internal (URL-based) sub-request to determine the final value of variable. This can be used to access variable for rewriting which is not available at the current stage, but will be set in a later phase.

   For instance, to rewrite according to the REMOTE_USER variable from within the per-server context (httpd.conf file) you must use %{LA-U:REMOTE_USER} - 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 %{REMOTE_USER} in that context.

6. %{LA-F:variable} can be used to perform an internal (filename-based) sub-request, to determine the final value of variable. Most of the time, this is the same as LA-U above.

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).

3. You can perform integer comparisons:

   -eq

   Is numerically equal to
   The TestString is treated as an integer, and is numerically compared to the CondPattern. True if the two are numerically equal.

   -ge

   Is numerically greater than or equal to
   The TestString is treated as an integer, and is numerically compared to the CondPattern. True if the TestString is numerically greater than or equal to the CondPattern.

   -gt

   Is numerically greater than
   The TestString is treated as an integer, and is numerically compared to the CondPattern. True if the TestString is numerically greater than the CondPattern.

   -le

   Is numerically less than or equal to
   The TestString is treated as an integer, and is numerically compared to the CondPattern. True if the TestString is numerically less than or equal to the CondPattern. Avoid confusion with the -l by using the -L or -h variant.

   -lt

   Is numerically less than
   The TestString is treated as an integer, and is numerically compared to the CondPattern. True if the TestString is numerically less than the CondPattern. Avoid confusion with the -l by using the -L or -h variant.

   -ne

   Is numerically not equal to
   The TestString is treated as an integer, and is numerically compared to the CondPattern. True if the two are numerically different. This is equivalent to !-eq.

4. You can perform various file attribute tests:

   -d

   Is directory.
   Treats the TestString as a pathname and tests whether or not it exists, and is a directory.

   -f

   Is regular file.
   Treats the TestString as a pathname and tests whether or not it exists, and is a regular file.

   -F

   Is existing file, via subrequest.
   Checks whether or not TestString is a valid file, 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!

   -h

   Is symbolic link, bash convention.
   See -l.

   -l

   Is symbolic link.
   Treats the TestString as a pathname and tests whether or not it exists, and is a symbolic link. May also use the bash convention of -L or -h if there's a possibility of confusion such as when using the -lt or -le tests.

   -L

   Is symbolic link, bash convention.
   See -l.

   -s

   Is regular file, with size.
   Treats the TestString as a pathname and tests whether or not it exists, and is a regular file with size greater than zero.

   -U

   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.

   -x

   Has executable permissions.
   Treats the TestString as a pathname and tests whether or not it exists, and has executable permissions. These permissions are determined according to the underlying OS.

   For example:

   RewriteCond /var/www/%{REQUEST_URI} !-f
   RewriteRule ^(.+) /other/archive/$1 [R]

5. 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.

   RewriteCond expr "! %{HTTP_REFERER} -strmatch '*://%{HTTP_HOST}/*'"
   RewriteRule "^/images" "-" [F]

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:

  RewriteCond "%{REMOTE_HOST}"  "^host1"  [OR]
  RewriteCond "%{REMOTE_HOST}"  "^host2"  [OR]
  RewriteCond "%{REMOTE_HOST}"  "^host3"
  RewriteRule ...some special stuff for any of these hosts...

  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:

RewriteCond  "%{HTTP_USER_AGENT}"  "(iPhone|Blackberry|Android)"
RewriteRule  "^/$"                 "/homepage.mobile.html"  [L]

RewriteRule  "^/$"                 "/homepage.std.html"     [L]

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.

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:

RewriteMap examplemap "txt:/path/to/file/map.txt"

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

RewriteRule "^/ex/(.*)" "${examplemap:$1}"

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:

Inherit

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.

Rules inherited from the parent scope are applied after rules specified in the child scope.

InheritBefore

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.

InheritDown

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.

InheritDownBefore

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.

IgnoreInherit

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.

AllowNoSlash

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.

AllowAnyURI

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.

MergeBase

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.

IgnoreContextInfo

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.

LegacyPrefixDocRoot

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.

RewriteRule /file/(.*) http://localhost/tmp/$1

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.

For some hints on regular expressions, see the mod_rewrite Introduction.

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.

- (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 ($N) to the RewriteRule pattern
2. back-references (%N) to the last matched RewriteCond pattern
3. server-variables as in rule condition test-strings (%{VARNAME})
4. mapping-function calls (${mapname:key|default})

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.

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.

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'':