mod_rewrite - Apache HTTP Server

If all you want to do is a simple redirect, insert the following in the <head> tag of your html file:

The mod_rewrite module uses a rule-based rewriting engine (based on a regular-expression parser) to rewrite requested URLs on the fly. The URL manipulations can depend on various tests, for instance...

Directives

Topics

Description:	Provides a rule-based rewriting engine to rewrite requested URLs on the fly
Status:	Extension
Module Identifier:	rewrite_module
Source File:	mod_rewrite.c
Compatibility:	Available in Apache 1.3 and later

API Phases

Apache processes HTTP request in phases. A hook for each of these phases is provided by the Apache API. Mod_rewrite uses two of these hooks:

After a request comes in and Apache has determined the corresponding server (or virtual server) the rewriting engine starts processing of all mod_rewrite directives from the per-server configuration in the URL-to-filename phase. 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.

URL manipulations in per-server context are really fast and efficient. Per-directory rewrites are slow and inefficient

Ruleset Processing

Quoting Special Characters

Special characters in TestString and Substitution strings can be escaped by prefixing them with a backslash ('\').

Regex Back-Reference Availability

When you use parentheses in Pattern or in one of the CondPattern, back-references are internally created which can be used with the strings $N and %N. These are available for creating the strings Substitution and TestString.

Environment Variables

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

The standard CGI/SSI variables SCRIPT_NAME and SCRIPT_FILENAME contain the physical System-view.

Practical Solutions

We also have an URL Rewriting Guide available, which provides a collection of practical solutions for URL-based problems. There you can find real-life rulesets and additional information about mod_rewrite.

RewriteBase Directive

Description:	Sets the base URL for per-directory rewrites
Syntax:	RewriteBase URL-path
Default:	See usage for information.
Context:	directory, .htaccess
Override:	FileInfo
Status:	Extension
Module:	mod_rewrite

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). There it will act locally, i.e., the local directory prefix is stripped at this stage of processing and your rewriting rules act only on the remainder. At the end it 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. But at most websites URLs are NOT directly related to physical filename paths, so this assumption will usually be wrong! There you have to use the RewriteBase directive to specify the correct URL-prefix.

If your webserver's URLs are not directly related to physical file paths, you have to use RewriteBase in every .htaccess files where you want to use RewriteRule directives.

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

#
#  /abc/def/.htaccess -- per-dir config file for directory /abc/def
#  Remember: /abc/def is the physical path of /xyz, i.e., the server
#            has a 'Alias /xyz /abc/def' directive e.g.
#

RewriteEngine On

#  let the server know that we were reached via /xyz and not
#  via the physical path prefix /abc/def
RewriteBase   /xyz

#  now the rewriting rules
RewriteRule   ^oldstuff\.html$  newstuff.html

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:

Request:
  /xyz/oldstuff.html

Internal Processing:
  /xyz/oldstuff.html     -> /abc/def/oldstuff.html  (per-server Alias)
  /abc/def/oldstuff.html -> /abc/def/newstuff.html  (per-dir    RewriteRule)
  /abc/def/newstuff.html -> /xyz/newstuff.html      (per-dir    RewriteBase)
  /xyz/newstuff.html     -> /abc/def/newstuff.html  (per-server Alias)

Result:
  /abc/def/newstuff.html

This seems very complicated but is the correct Apache internal processing, because the per-directory rewriting comes too late in the process. So, when it occurs the (rewritten) request has to be re-injected into the Apache kernel! BUT: While this seems like a serious overhead, it really isn't, because this re-injection happens fully internally to the Apache server and the same procedure is used by many other operations inside Apache. So, you can be sure the design and implementation is correct.

RewriteCond Directive

Description:	Defines a condition under which rewriting will take place
Syntax:	RewriteCond TestString CondPattern
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Extension
Module:	mod_rewrite

Precede a RewriteRule directive with one or more RewriteCond directives. The following rewriting rule is only used if its pattern matches the current state of the URI and if these additional conditions apply too.

TestString is a string which can contains 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 (parenthesis!) of the pattern from the corresponding RewriteRule directive (the one following the current bunch of RewriteCond directives).
RewriteCond backreferences:
These are backreferences of the form
%N (1 <= N <= 9)

...which provide access to the grouped parts (parentheses!) of the pattern from the last matched RewriteCond directive in the current bunch of conditions.
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_USER_AGENT HTTP_REFERER HTTP_COOKIE HTTP_FORWARDED HTTP_HOST HTTP_PROXY_CONNECTION HTTP_ACCEPT	REMOTE_ADDR REMOTE_HOST 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

These variables all correspond to the similarly named HTTP MIME-headers, C variables of the Apache server or struct tm fields of the Unix system. Most are documented elsewhere in the Manual or in the CGI specification. Those that are special to mod_rewrite include:

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.
API_VERSION: This is the version of the Apache 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 in use (in the release version of Apache 1.3.14, for instance, it is 19990320:10), but is mainly of interest to module authors.
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.
REQUEST_URI: The resource requested in the HTTP request line. (In the example above, this would be "/index.html".)
REQUEST_FILENAME: The full local filesystem path to the file or script matching the request.

The variables SCRIPT_FILENAME and REQUEST_FILENAME contain the same value, i.e., the value of the filename field of the internal request_rec structure of the Apache server. The first name is just the commonly known CGI variable name while the second is the consistent counterpart to REQUEST_URI (which contains the value of the uri field of request_rec).

There is the special format: %{ENV:variable} where variable can be any environment variable. This is looked-up via internal Apache structures and (if not found there) via getenv() from the Apache server process.

There is the special format: %{HTTP:header} where header can be any HTTP MIME-header name. This is looked-up from the HTTP request. Example:

%{HTTP:Proxy-Connection}

...is the value of the HTTP header

``Proxy-Connection:''

There is the special format %{LA-U:variable} for look-aheads which perform an internal (URL-based) sub-request to determine the final value of variable. Use this when you want to use a variable for rewriting which is actually set later in an API phase and thus is not available at the current stage. For instance when you want to rewrite according to the REMOTE_USER variable from within the per-server context (httpd.conf file) you have to use %{LA-U:REMOTE_USER} because this variable is set by the authorization phases which come after the URL translation phase where 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} there.

There is the special format:

%{LA-F:variable}

...which performs 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, i.e., a regular expression which is applied to the current instance of the TestString, i.e., TestString is evaluated and then matched against CondPattern.

CondPattern is a perl compatible regular expression with some additions:

You can prefix the pattern string with a '!' character (exclamation mark) to specify a non-matching pattern.

There are some special variants of CondPatterns. Instead of real regular expression strings you can also use one of the following:

'<CondPattern' (is lexically lower)
Treats the CondPattern as a plain string and compares it lexically to TestString. True if TestString is lexically lower than CondPattern.
'>CondPattern' (is lexically greater)
Treats the CondPattern as a plain string and compares it lexically to TestString. True if TestString is lexically greater than CondPattern.
'=CondPattern' (is lexically equal)
Treats the CondPattern as a plain string and compares it lexically to TestString. True if TestString is lexically equal to CondPattern, i.e the two strings are exactly equal (character by character). If CondPattern is just "" (two quotation marks) this compares TestString to the empty string.
'-d' (is directory)
Treats the TestString as a pathname and tests if it exists and is a directory.
'-f' (is regular file)
Treats the TestString as a pathname and tests if it exists and is a regular file.
'-s' (is regular file with size)
Treats the TestString as a pathname and tests if it exists and is a regular file with size greater than zero.
'-l' (is symbolic link)
Treats the TestString as a pathname and tests if it exists and is a symbolic link.
'-F' (is existing file via subrequest)
Checks if TestString is a valid file and accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to determine the check, so use it with care because it decreases your servers performance!
'-U' (is existing URL via subrequest)
Checks if TestString is a valid URL and accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to determine the check, so use it with care because it decreases your server's performance!

All of these tests can also be prefixed by an exclamation mark ('!') to negate their meaning.

Additionally you can set special flags for CondPattern by appending [flags] as the third argument to the RewriteCond directive. Flags is a comma-separated list of the following flags:

'nocase|NC'
No case
Make the test case-insensitive. There is no difference between 'A-Z' and 'a-z' 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'
Next condition. Combine rule conditions with a local OR instead of the implicit AND. For 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 cond/rule three times.

To rewrite the Homepage of a site according to the ``User-Agent:'' header of the request...

RewriteCond %{HTTP_USER_AGENT} ^Mozilla.*
RewriteRule ^/$ /homepage.max.html [L]
RewriteCond %{HTTP_USER_AGENT} ^Lynx.*
RewriteRule ^/$ /homepage.min.html [L]
RewriteRule ^/$ /homepage.std.html [L]

If you use Netscape Navigator as your browser (which identifies itself as 'Mozilla'), then you get the max homepage, which includes Frames, etc. If you use the Lynx browser (which is Terminal-based), then you get the min homepage, which contains no images, no tables, etc. If you use any other browser you get the standard homepage.

RewriteEngine Directive

Description:	Enables or disables runtime rewriting engine
Syntax:	RewriteEngine on\|off
Default:	RewriteEngine off
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Extension
Module:	mod_rewrite

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 the module instead of commenting out all the RewriteRule directives!

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

RewriteLock Directive

Description:	Sets the name of the lock file used for RewriteMap synchronization
Syntax:	RewriteLock file-path
Context:	server config
Status:	Extension
Module:	mod_rewrite

This directive sets the filename for a synchronization lockfile which mod_rewrite needs to communicate with RewriteMap programs. Set this lockfile to a local path (not on a NFS-mounted device) when you want to use a rewriting map-program. It is not required for other types of rewriting maps.

RewriteLog Directive

Description:	Sets the name of the file used for logging rewrite engine processing
Syntax:	RewriteLog file-path
Context:	server config, virtual host
Status:	Extension
Module:	mod_rewrite

The RewriteLog directive sets the name of the file to which the server logs any rewriting actions it performs. If the name does not begin with a slash ('/') then it is assumed to be relative to the Server Root. The directive should occur only once per server config.

To disable the logging of rewriting actions it is not recommended to set Filename to /dev/null, because although the rewriting engine does not then output to a logfile it still creates the logfile output internally. This will slow down the server with no advantage to the administrator! To disable logging either remove or comment out the RewriteLog directive or use RewriteLogLevel 0!

Security

See the Apache Security Tips document for details on why your security could be compromised if the directory where logfiles are stored is writable by anyone other than the user that starts the server.

Example...

RewriteLog "/usr/local/var/apache/logs/rewrite.log"

RewriteLogLevel Directive

Description:	Sets the verbosity of the log file used by the rewrite engine
Syntax:	RewriteLogLevel Level
Default:	RewriteLogLevel 0
Context:	server config, virtual host
Status:	Extension
Module:	mod_rewrite

The RewriteLogLevel directive sets the verbosity level of the rewriting logfile. The default level 0 means no logging, while 9 or more means that practically all actions are logged.

To disable the logging of rewriting actions simply set Level to 0. This disables all rewrite action logs.

Using a high value for Level will slow down your Apache server dramatically! Use the rewriting logfile at a Level greater than 2 only for debugging!

Example

RewriteLogLevel 3

RewriteMap Directive

Description:	Defines a mapping function for key-lookup
Syntax:	RewriteMap MapName MapType:MapSource
Context:	server config, virtual host
Status:	Extension
Module:	mod_rewrite
Compatibility:	The choice of different dbm types is available in Apache 2.0.41 and later

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.

The following combinations for MapType and MapSource can be used:

Standard Plain Text
MapType: txt, MapSource: Unix filesystem path to valid regular file
This is the standard rewriting map feature where the MapSource is a plain ASCII file containing either blank lines, comment lines (starting with a '#' character) or pairs like the following - one per line.
MatchingKey SubstValue
Example...
```
##
##  map.txt -- rewriting map
##

Ralf.S.Engelschall    rse   # Bastard Operator From Hell
Mr.Joe.Average        joe   # Mr. Average
```
RewriteMap real-to-user txt:/path/to/file/map.txt
Randomized Plain Text
MapType: rnd, MapSource: Unix filesystem path to valid regular file
This is identical to the Standard Plain Text variant above but with a special post-processing feature: After looking up a value it is parsed according to contained ``|'' characters which have the meaning of ``or''. In other words they indicate a set of alternatives from which the actual returned value is chosen randomly. Although this sounds crazy and useless, it was actually designed for load balancing in a reverse proxy situation where the looked up values are server names. Example:
```
##
##  map.txt -- rewriting map
##

static   www1|www2|www3|www4
dynamic  www5|www6
```
RewriteMap servers rnd:/path/to/file/map.txt
Hash File
MapType: dbm[=type], MapSource: Unix filesystem path to valid regular file
Here the source is a binary format DBM file containing the same contents as a Plain Text format file, but in a special representation which is optimized for really fast lookups. The type can be sdbm, gdbm, ndbm, or db depending on compile-time settings. If the type is ommitted, the compile-time default will be chosen. You can create such a file with any DBM tool or with the following Perl script. Be sure to adjust it to create the appropriate type of DBM. The example creates an NDBM file.
```
#!/path/to/bin/perl
##
##  txt2dbm -- convert txt map to dbm format
##

use NDBM_File;
use Fcntl;

($txtmap, $dbmmap) = @ARGV;

open(TXT, "<$txtmap") or die "Couldn't open $txtmap!\n";
tie (%DB, 'NDBM_File', $dbmmap,O_RDWR|O_TRUNC|O_CREAT, 0644)
  or die "Couldn't create $dbmmap!\n";

while (<TXT>) {
  next if (/^\s*#/ or /^\s*$/);
  $DB{$1} = $2 if (/^\s*(\S+)\s+(\S+)/);
}

untie %DB;
close(TXT);
```
$ txt2dbm map.txt map.db
Internal Function
MapType: int, MapSource: Internal Apache function
Here the source is an internal Apache function. Currently you cannot create your own, but the following functions already exists:
- toupper:
  Converts the looked up key to all upper case.
- tolower:
  Converts the looked up key to all lower case.
- escape:
  Translates special characters in the looked up key to hex-encodings.
- unescape:
  Translates hex-encodings in the looked up key back to special characters.
External Rewriting Program
MapType: prg, MapSource: Unix filesystem path to valid regular file
Here the source is a program, not a map file. To create it you can use the language of your choice, but the result has to be a executable (i.e., either object-code or a script with the magic cookie trick '#!/path/to/interpreter' as the first line).
This program is started once at startup of the Apache servers and then communicates with the rewriting engine over its stdin and stdout file-handles. For each map-function lookup it will receive the key to lookup as a newline-terminated string on stdin. It then has to give back the looked-up value as a newline-terminated string on stdout or the four-character string ``NULL'' if it fails (i.e., there is no corresponding value for the given key). A trivial program which will implement a 1:1 map (i.e., key == value) could be:
```
#!/usr/bin/perl
$| = 1;
while (<STDIN>) {
    # ...put here any transformations or lookups...
    print $_;
}
```
But be very careful:
1. ``Keep it simple, stupid'' (KISS), because if this program hangs it will hang the Apache server when the rule occurs.
2. Avoid one common mistake: never do buffered I/O on stdout! This will cause a deadloop! Hence the ``$|=1'' in the above example...
3. Use the RewriteLock directive to define a lockfile mod_rewrite can use to synchronize the communication to the program. By default no such synchronization takes place.

The RewriteMap directive can occur more than once. For each mapping-function use one RewriteMap directive to declare its rewriting mapfile. While you cannot declare a map in per-directory context it is of course possible to use this map in per-directory context.

For plain text and DBM format files the looked-up keys are cached in-core until the mtime of the mapfile changes or the server does a restart. This way you can have map-functions in rules which are used for every request. This is no problem, because the external lookup only happens once!

RewriteOptions Directive

Description:	Sets some special options for the rewrite engine
Syntax:	RewriteOptions `Options`
Default:	RewriteOptions MaxRedirects=5
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Extension
Module:	mod_rewrite
Compatibility:	MaxRedirects is available in Apache 2.0.45 and later

The RewriteOptions directive sets some special options for the current per-server or per-directory configuration. The Option strings can 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 are inherited.
MaxRedirects=number: In order to prevent endless loops of internal redirects issued by per-directory RewriteRules, mod_rewrite aborts the request after reaching a maximum number of such redirects and responds with an 500 Internal Server Error. If you really need more internal redirects than 10 per request, you may increase the default to the desired value.

RewriteRule Directive

Description:	Defines rules for the rewriting engine
Syntax:	RewriteRule Pattern Substitution
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Extension
Module:	mod_rewrite
Compatibility:	The cookie-flag is available in Apache 2.0.40 and later.

RewriteRule can occur more than once.

Pattern is a perl compatible regular expression which gets applied to the current URL. Here ``current'' means the value of the URL when this rule gets applied. This may not be the originally requested URL, because any number of rules may already have matched and made alterations to it.

Some hints about the syntax of regular expressions:

. Any single character
[chars] Character class: One of chars
[^chars] Character class: None of chars
text1|text2 Alternative: text1 or text2
? 0 or 1 of the preceding text
* 0 or N of the preceding text (N > 0)
+ 1 or N of the preceding text (N > 1)
(text) Grouping of text (either to set the borders of an alternative or for making backreferences where the Nth group can be used on the RHS of a RewriteRule with $N)
^ Start of line anchor
$ End of line anchor
\char escape that particular char (for instance to specify the chars ".[]()" etc.)

The NOT character ('!') is a pattern prefix that gives you the ability to negate a pattern. For instance: ``if the current URL does NOT match this pattern''. This can be used to match the negative pattern, or as a last default rule. When using the NOT character to negate a pattern you cannot have grouped wildcard parts. If negated patterns are used, you cannot use $N in the substitution string.

Substitution of a rewriting rule is the string which is substituted for (or replaces) the original URL for which Pattern matched. Beside plain text you can use...

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

Back-references are $N (N=0..9) identifiers 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 of the above list.

As already mentioned above, all the rewriting rules are applied to the Substitution (in the order of definition in the config file). The URL is completely replaced by the Substitution and the rewriting process goes on until there are no more rules unless explicitly terminated by a L flag - see below.

The substitution string '-' means: NO substitution. It is useful to provide rewriting rules which only match some URLs but do no substitution, e.g., in conjunction with the C (chain) flag to be able to have more than one pattern to be applied before a substitution occurs.

You can create URLs in the substitution string containing a query string part. Just use a question mark inside the substitution string to indicate that the following stuff should be re-injected into the QUERY_STRING. When you want to erase an existing query string, end the substitution string with just the question mark.

When you prefix a substitution field with http://thishost[:thisport] then mod_rewrite automatically strips it out. This auto-reduction on implicit external redirect URLs is a useful and important feature when used in combination with a mapping-function which generates the hostname part. Have a look at the first example in the example section below to understand this.

An unconditional external redirect to your own server will not work with the prefix http://thishost because of this feature. To achieve such a self-redirect, you have to use the R-flag (see below).

You can set special flags for Substitution by appending...

[flags]

...as the third argument to the RewriteRule directive. Flags is a comma-separated list of the following flags:

'redirect|R [=code]' (force redirect)
Prefix Substitution with...
http://thishost[:thisport]/

...which makes the new URL a URI, to force a external redirection.
If no code is given a HTTP response of 302 (MOVED TEMPORARILY) is used. If you want to use other response codes in the range 300-400 just specify them as a number or use one of the following symbolic names: temp (default), permanent, seeother. Use it for rules which should canonicalize the URL and give it back to the client, e.g., translate ``/~'' into ``/u/'' or always append a slash to /u/user, etc.
This flag itself only prefixes the URL with...
http://thishost[:thisport]/

...rewriting continues. Usually you also want to stop and do the redirection immediately. To stop the rewriting you also have to provide the 'L' flag.
'forbidden|F' (force URL to be forbidden)
This forces the current URL to be forbidden, i.e., it immediately sends back a HTTP response of 403 (FORBIDDEN). Use this flag in conjunction with appropriate RewriteConds to conditionally block some URLs.
'gone|G' (force URL to be gone)
This forces the current URL to be gone, i.e., it immediately sends back a HTTP response of 410 (GONE). Use this flag to mark pages which no longer exist as gone.
'proxy|P' (force proxy)
This flag forces the substitution part to be internally forced as a proxy request and immediately (i.e., rewriting rule processing stops here) put through the proxy module. You have to make sure that the substitution string is a valid URI (e.g., typically starting with http://hostname) which can be handled by the Apache proxy module. If not you get an error from the proxy module. Use this flag to achieve a more powerful implementation of the ProxyPass directive, to map some remote stuff into the namespace of the local server.
Notice: To use this functionality make sure you have the proxy module compiled into your Apache server program. If you don't know please check whether mod_proxy.c is part of the ``httpd -l'' output. If yes, this functionality is available to mod_rewrite. If not, then you first have to rebuild the ``httpd'' program with mod_proxy enabled.
'last|L' (last rule)
Stop the rewriting process here and don't apply any more rewriting rules. This corresponds to the Perl last command or the break command from the C language. Use this flag to prevent the currently rewritten URL from being rewritten further by following rules. For example, use it to rewrite the root-path URL ('/') to a real one, e.g., '/e/www/'.
'next|N' (next round)
Re-run the rewriting process (starting again with the first rewriting rule). Here the URL to match is again not the original URL but the URL from the last rewriting rule. This corresponds to the Perl next command or the continue command from the C language. Use this flag to restart the rewriting process, i.e., to immediately go to the top of the loop.
But be careful not to create an infinite loop!
'chain|C' (chained with next rule)
This flag chains the current rule with the next rule (which itself can be chained with the following rule, etc.). This has the following effect: if a rule matches, then processing continues as usual, i.e., the flag has no effect. If the rule does not match, then all following chained rules are skipped. For instance, use it to remove the ``.www'' part inside a per-directory rule set when you let an external redirect happen (where the ``.www'' part should not to occur!).
'type|T=MIME-type' (force MIME type)
Force the MIME-type of the target file to be MIME-type. For instance, this can be used to simulate the mod_alias directive ScriptAlias which internally forces all files inside the mapped directory to have a MIME type of ``application/x-httpd-cgi''.
'nosubreq|NS' (used only if no internal sub-request)
This flag forces the rewriting engine to skip a rewriting rule if the current request is an internal sub-request. For instance, sub-requests occur internally in Apache when mod_include tries to find out information about possible directory default files (index.xxx). On sub-requests it is not always useful and even sometimes causes a failure to if the complete set of rules are applied. Use this flag to exclude some rules.

Use the following rule for your decision: whenever you prefix some URLs with CGI-scripts to force them to be processed by the CGI-script, the chance is high that you will run into problems (or even overhead) on sub-requests. In these cases, use this flag.
'nocase|NC' (no case)
This makes the Pattern case-insensitive, i.e., there is no difference between 'A-Z' and 'a-z' when Pattern is matched against the current URL.
'qsappend|QSA' (query string append)
This flag forces the rewriting engine to append a query string part in the substitution string to the existing one instead of replacing it. Use this when you want to add more data to the query string via a rewrite rule.
'noescape|NE' (no URI escaping of output)
This flag keeps mod_rewrite from applying the usual URI escaping rules to the result of a rewrite. Ordinarily, special characters (such as '%', '$', ';', and so on) will be escaped into their hexcode equivalents ('%25', '%24', and '%3B', respectively); this flag prevents this from being done. This allows percent symbols to appear in the output, as in
RewriteRule /foo/(.*) /bar?arg=P1\%3d$1 [R,NE] which would turn '/foo/zed' into a safe request for '/bar?arg=P1=zed'.
'passthrough|PT' (pass through to next handler)
This flag forces the rewriting engine to set the uri field of the internal request_rec structure to the value of the filename field. This flag is just a hack to be able to post-process the output of RewriteRule directives by Alias, ScriptAlias, Redirect, etc. directives from other URI-to-filename translators. A trivial example to show the semantics: If you want to rewrite /abc to /def via the rewriting engine of mod_rewrite and then /def to /ghi with mod_alias:
RewriteRule ^/abc(.*) /def$1 [PT]
Alias /def /ghi
If you omit the PT flag then mod_rewrite will do its job fine, i.e., it rewrites uri=/abc/... to filename=/def/... as a full API-compliant URI-to-filename translator should do. Then mod_alias comes and tries to do a URI-to-filename transition which will not work.
You have to use this flag if you want to intermix directives of different modules which contain URL-to-filename translators. The typical example is the use of mod_alias and mod_rewrite..
'skip|S=num' (skip next rule(s))
This flag forces the rewriting engine to skip the next num rules in sequence when the current rule matches. Use this to make pseudo if-then-else constructs: The last rule of the then-clause becomes skip=N where N is the number of rules in the else-clause. (This is not the same as the 'chain|C' flag!)
'env|E=VAR:VAL' (set environment variable)
This forces an environment variable named VAR to be set to the value VAL, where VAL can contain regexp backreferences $N and %N which will be expanded. You can use this flag more than once to set more than one variable. The variables can be later dereferenced in many situations, but usually from within XSSI (via ) or CGI (e.g. $ENV{'VAR'}). Additionally you can dereference it in a following RewriteCond pattern via %{ENV:VAR}. Use this to strip but remember information from URLs.
'cookie|CO=NAME:VAL:domain[:lifetime[:path]]' (set cookie)
This sets a cookie on the client's browser. The cookie's name is specified by NAME and the value is VAL. The domain field is the domain of the cookie, such as '.apache.org',the optional lifetime is the lifetime of the cookie in minutes, and the optional path is the path of the cookie

Pattern is applied to a complete URL in per-server configuration files. But in per-directory configuration files, the per-directory prefix (which always is the same for a specific directory!) is automatically removed for the pattern matching and automatically added after the substitution has been done. This feature is essential for many sorts of rewriting, because without this prefix stripping you have to match the parent directory which is not always possible.

There is one exception: If a substitution string starts with ``http://'' then the directory prefix will not be added and an external redirect or proxy throughput (if flag P is used!) is forced!

To enable the rewriting engine for per-directory configuration files you need to set ``RewriteEngine On'' in these files and ``Options FollowSymLinks'' must be enabled. If your administrator has disabled override of FollowSymLinks for a user's directory, then you cannot use the rewriting engine. This restriction is needed for security reasons.

Here are all possible substitution combinations and their meanings:

Inside per-server configuration (httpd.conf)

for request ``GET /somepath/pathinfo'':

Given Rule	Resulting Substitution
^/somepath(.*) otherpath$1	not supported, because invalid!
^/somepath(.*) otherpath$1 [R]	not supported, because invalid!
^/somepath(.*) otherpath$1 [P]	not supported, because invalid!
^/somepath(.*) /otherpath$1	/otherpath/pathinfo
^/somepath(.*) /otherpath$1 [R]	http://thishost/otherpath/pathinfo via external redirection
^/somepath(.*) /otherpath$1 [P]	not supported, because silly!
^/somepath(.*) http://thishost/otherpath$1	/otherpath/pathinfo
^/somepath(.*) http://thishost/otherpath$1 [R]	http://thishost/otherpath/pathinfo via external redirection
^/somepath(.*) http://thishost/otherpath$1 [P]	not supported, because silly!
^/somepath(.*) http://otherhost/otherpath$1	http://otherhost/otherpath/pathinfo via external redirection
^/somepath(.*) http://otherhost/otherpath$1 [R]	http://otherhost/otherpath/pathinfo via external redirection (the [R] flag is redundant)
^/somepath(.*) http://otherhost/otherpath$1 [P]	http://otherhost/otherpath/pathinfo via internal proxy

Inside per-directory configuration for /somepath

(i.e., file .htaccess in dir /physical/path/to/somepath containing RewriteBase /somepath)

for request ``GET /somepath/localpath/pathinfo'':

Given Rule	Resulting Substitution
^localpath(.*) otherpath$1	/somepath/otherpath/pathinfo
^localpath(.*) otherpath$1 [R]	http://thishost/somepath/otherpath/pathinfo via external redirection
^localpath(.*) otherpath$1 [P]	not supported, because silly!
^localpath(.*) /otherpath$1	/otherpath/pathinfo
^localpath(.*) /otherpath$1 [R]	http://thishost/otherpath/pathinfo via external redirection
^localpath(.*) /otherpath$1 [P]	not supported, because silly!
^localpath(.*) http://thishost/otherpath$1	/otherpath/pathinfo
^localpath(.*) http://thishost/otherpath$1 [R]	http://thishost/otherpath/pathinfo via external redirection
^localpath(.*) http://thishost/otherpath$1 [P]	not supported, because silly!
^localpath(.*) http://otherhost/otherpath$1	http://otherhost/otherpath/pathinfo via external redirection
^localpath(.*) http://otherhost/otherpath$1 [R]	http://otherhost/otherpath/pathinfo via external redirection (the [R] flag is redundant)
^localpath(.*) http://otherhost/otherpath$1 [P]	http://otherhost/otherpath/pathinfo via internal proxy

Example:

We want to rewrite URLs of the form

/Language/~Realname/.../File

...into...

/u/Username/.../File.Language

We take the rewrite mapfile from above and save it under /path/to/file/map.txt. Then we only have to add the following lines to the Apache server configuration file:

RewriteLog /path/to/file/rewrite.log
RewriteMap real-to-user txt:/path/to/file/map.txt
RewriteRule ^/([^/]+)/~([^/]+)/(.*)$ /u/${real-to-user:$2|nobody}/$3.$1

Apache Module mod_rewrite

Summary