htaccess y mod rewrite

Nuevo articulo para ver un poco como va el htaccess y una de las opciones mas utilizadas del htaccess, mod-rewrite.
Tras el salto unos tips muy utiles.

http://httpd.apache.org/docs/current/mod/mod_rewrite.html
http://httpd.apache.org/docs/current/rewrite/flags.html

Apache Module mod_rewrite

Available Languages:  en  |  fr 

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

Summary

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

• Quoting Special Characters
• Environment Variables
• Rewriting in Virtual Hosts
• Practical Solutions

See also

• Rewrite Flags

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 backslash ('\') 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

SCRIPT_NAME=/sw/lib/w3s/tree/global/u/rse/.www/index.html
SCRIPT_FILENAME=/u/rse/.www/index.html
SCRIPT_URL=/u/rse/
SCRIPT_URI=http://en1.engelschall.com/u/rse/

Rewriting in Virtual Hosts

By default, mod_rewrite configuration settings from the main server context are not inherited by virtual hosts. To make the main server settings apply to virtual hosts, you must place the following directives in each <VirtualHost> section:

RewriteEngine On
RewriteOptions Inherit

Practical Solutions

For numerous examples of common, and not-so-common, uses for mod_rewrite, see the extended rewrite documentation.

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). 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.

If your webserver's URLs are not directly related to physical file paths, you will need to use RewriteBase in every .htaccess file 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 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, as if it were a new request. (See mod_rewrite technical details.) 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

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

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 ${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_PORT REMOTE_USER REMOTE_IDENT REQUEST_METHOD SCRIPT_FILENAME PATH_INFO QUERY_STRING AUTH_TYPE
  server internals:	date and time:	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

  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.
  SERVER_NAME and SERVER_PORT depend on the values of UseCanonicalName and UseCanonicalPhysicalPort respectively.
  Those that are special to mod_rewrite include those below.
  
  
  

  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. This value has not been unescaped (decoded), unlike most other variables below.

  REQUEST_URI

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

  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.

  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).

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 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 structures and (if not found there) via getenv() from the Apache 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.
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 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.
Remember: CondPattern is a perl compatible regular expression with some additions:

1. You can prefix the pattern string with a '!' character (exclamation mark) to specify a non-matching pattern.
2. There are some special variants of CondPatterns. Instead of real regular expression strings you can also use one of the following:
   • '<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.
   • '-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.
   • '-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.
   • '-l' (is symbolic link)
     Treats the TestString as a pathname and tests whether or not it exists, and is a symbolic link.
   • '-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.
   • '-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!
   • '-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!

   Note:

   All of these tests can also be prefixed by an exclamation mark ('!') to negate their meaning.
3. 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}  ^Mozilla
RewriteRule  ^/$                 /homepage.max.html  [L]

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

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

Explanation: If you use a browser which identifies itself as 'Mozilla' (including Netscape Navigator, Mozilla etc), then you get the max homepage (which could include frames, or other special features). If you use the Lynx browser (which is terminal-based), then you get the min homepage (which could be a version designed for easy, text-only browsing). If neither of these conditions apply (you use any other browser, or your browser identifies itself as something non-standard), you get the std (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 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

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!

The RewriteLog log file format is as follows:

Description	Example
Remote host IP address	192.168.200.166
Remote login name	Will usually be "-"
HTTP user auth name	Username, or "-" if no auth
Date and time of request	[28/Aug/2009:13:09:09 --0400]
Virtualhost and virtualhost ID	[www.example.com/sid#84a650]
Request ID, and whether it's a subrequest	[rid#9f0e58/subreq]
Log entry severity level	(2)
Text error message	forcing proxy-throughput with http://127.0.0.1:8080/index.html

Security

See the Apache Security Tips document for details on how 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. 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:

• 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. For example, you might use the following map file and directives to provide a random load balancing between several back-end server, via a reverse-proxy. Images are sent to one of the servers in the 'static' pool, while everything else is sent to one of the 'dynamic' pool.
  Example:
  

  Rewrite map file

  ##
  ##  map.txt -- rewriting map
  ##
  
  static   www1|www2|www3|www4
  dynamic  www5|www6
  

  Configuration directives

  RewriteMap servers rnd:/path/to/file/map.txt

RewriteRule ^/(.*\.(png|gif|jpg)) http://${servers:static}/$1 [NC,P,L]
RewriteRule ^/(.*) http://${servers:dynamic}/$1 [P,L]
• 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 omitted, the compile-time default will be chosen.
  To create a dbm file from a source text file, use the httxt2dbm utility.
  
  $ httxt2dbm -i mapfile.txt -o mapfile.map
• Internal Function
  MapType: int, MapSource: Internal Apache function Here, the source is an internal Apache function. Module authors can provide additional internal functions by registering them with the ap_register_rewrite_mapfunc API. The functions that are provided by default are:
  
  • toupper:
    Converts the key to all upper case.
  • tolower:
    Converts the key to all lower case.
  • escape:
    Translates special characters in the key to hex-encodings.
  • unescape:
    Translates hex-encodings in the 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 a language of your choice, but the result has to be an executable program (either object-code or a script with the magic cookie trick '#!/path/to/interpreter' as the first line).
  This program is started once, when the Apache server is started, and then communicates with the rewriting engine via 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:
  External rewriting programs are not started if they're defined in a context that does not have RewriteEngine set to on
  .

  #!/usr/bin/perl
  $| = 1;
  while (<STDIN>) {
      # ...put here any transformations or lookups...
      print $_;
  }
  

  But be very careful:
  
  1. ``Keep it simple, stupid'' (KISS). If this program hangs, it will cause Apache to hang when trying to use the relevant rewrite rule.
  2. A common mistake is to use buffered I/O on stdout. Avoid this, as it will cause a deadloop! ``$|=1'' is used above, to prevent this.
  3. The RewriteLock directive can be used to define a lockfile which mod_rewrite can use to synchronize communication with the mapping 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.

Note

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
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Extension
Module:	mod_rewrite
Compatibility:	MaxRedirects is no longer available in version 2.1 and later

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 are inherited.

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

RewriteRule Directive

Description:	Defines rules for the rewriting engine
Syntax:	RewriteRule Pattern Substitution [flags]
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Extension
Module:	mod_rewrite

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. On the first RewriteRule it is applied to the (%-decoded) URL-path of the request; subsequent patterns are applied to the output of the last matched RewriteRule.

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").
In Directory and htaccess context, the Pattern will initially be matched against the filesystem path, after removing the prefix that lead the server to the current RewriteRule (e.g. "app1/index.html" or "index.html" depending on where the directives are defined).
If you wish to match against the hostname, port, or query string, use a RewriteCond with the %{HTTP_HOST}, %{SERVER_PORT}, or %{QUERY_STRING} 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.
• When using the rewrite engine in .htaccess files the per-directory prefix (which always is the same for a specific directory) is automatically removed for the RewriteRule pattern matching and automatically added after any relative (not starting with a slash or protocol name) substitution encounters the end of a rule set. See the RewriteBase directive for more information regarding what prefix will be added back to relative substitions.
• If you wish to match against the full URL-path in a per-directory (htaccess) RewriteRule, use the %{REQUEST_URI} 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, this should never be necessary and is unsupported.

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.

Note

When using the NOT character to negate a pattern, you cannot include grouped wildcard parts in that pattern. This is because, when the pattern does NOT match (ie, the negation matches), there are no contents for the groups. Thus, if negated patterns are used, you cannot use $N in the substitution string!

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.

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, 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 Substition 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 a applied to the results of previous rewrite rules, in the order in which they are defined in the config file. The URL is completely replaced by the Substitution and the rewriting process continues until all rules have been applied, or it is explicitly terminated by a 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.

Flag and syntax	Function
B	Escape non-alphanumeric characters before applying the transformation. details ...
chain|C	Rule is chained to the following rule. If the rule fails, the rule(s) chained to it will be skipped. details ...
cookie|CO=NAME:VAL	Sets a cookie in the client browser. Full syntax is: CO=NAME:VAL:domain[:lifetime[:path[:secure[:httponly]]]] details ...
discardpath|DPI	Causes the PATH_INFO portion of the rewritten URI to be discarded. details ...
env|E=[!]VAR[:VAL]	Causes an environment variable VAR to be set (to the value VAL if provided). The form !VAR causes the environment variable VAR to be unset.details ...
forbidden|F	Returns a 403 FORBIDDEN response to the client browser. details ...
gone|G	Returns a 410 GONE response to the client browser. details ...
Handler|H=Content-handler	Causes the resulting URI to be sent to the specified Content-handler for processing. details ...
last|L	Stop the rewriting process immediately and don't apply any more rules. Especially note caveats for per-directory and .htaccess context (see also the END flag). details ...
next|N	Re-run the rewriting process, starting again with the first rule, using the result of the ruleset so far as a starting point. details ...
nocase|NC	Makes the pattern pattern comparison case-insensitive. details ...
noescape|NE	Prevent mod_rewrite from applying hexcode escaping of special characters in the result of the rewrite. details ...
nosubreq|NS	Causes a rule to be skipped if the current request is an internal sub-request. details ...
proxy|P	Force the substitution URL to be internally sent as a proxy request. details ...
passthrough|PT	Forces the resulting URI to be passed back to the URL mapping engine for processing of other URI-to-filename translators, such as Alias or Redirect. details ...
qsappend|QSA	Appends any query string created in the rewrite target to any query string that was in the original request URL. details ...
redirect|R[=code]	Forces an external redirect, optionally with the specified HTTP status code. details ...
skip|S=num	Tells the rewriting engine to skip the next num rules if the current rule matches. details ...
type|T=MIME-type	Force the MIME-type of the target file to be the specified type. details ...

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'':

Given Rule                                      Resulting Substitution
----------------------------------------------  ----------------------------------
^/somepath(.*) otherpath$1                      invalid, not supported

^/somepath(.*) otherpath$1  [R]                 invalid, not supported

^/somepath(.*) otherpath$1  [P]                 invalid, not supported
----------------------------------------------  ----------------------------------
^/somepath(.*) /otherpath$1                     /otherpath/pathinfo

^/somepath(.*) /otherpath$1 [R]                 http://thishost/otherpath/pathinfo
                                                via external redirection

^/somepath(.*) /otherpath$1 [P]                 doesn't make sense, not supported
----------------------------------------------  ----------------------------------
^/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]  doesn't make sense, not supported
----------------------------------------------  ----------------------------------
^/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
(/physical/path/to/somepath/.htacccess, with 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]                 doesn't make sense, not supported
----------------------------------------------  ----------------------------------
^localpath(.*) /otherpath$1                     /otherpath/pathinfo

^localpath(.*) /otherpath$1 [R]                 http://thishost/otherpath/pathinfo
                                                via external redirection

^localpath(.*) /otherpath$1 [P]                 doesn't make sense, not supported
----------------------------------------------  ----------------------------------
^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]  doesn't make sense, not supported
----------------------------------------------  ----------------------------------
^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

RewriteRule Flags
This document discusses the flags which are available to the
RewriteRule directive,
providing detailed explanations and examples.
Introduction
A RewriteRule can have
its behavior modified by one or more flags. Flags are included in
square brackets at the end of the rule, and multiple flags are separated
by commas.


RewriteRule pattern target [Flag1,Flag2,Flag3]
The flags all have a short form, such as CO, as well as
a longer form, such as cookie. Some flags take one or more
arguments. Flags are not case sensitive.
Each flag (with a few exceptions) 
has a long and short form. While it is most common to use
the short form, it is recommended that you familiarize yourself with the
long form, so that you remember what each flag is supposed to do.
Flags that alter metadata associated with the request (T=, H=, E=)
have no affect in per-directory and htaccess context, when a substitution
(other than '-') is performed during the same round of rewrite processing.

Presented here are each of the available flags, along with an example
of how you might use them.
B (escape backreferences)
The [B] flag instructs RewriteRule to escape non-alphanumeric
characters before applying the transformation.

mod_rewrite has to unescape URLs before mapping them,
so backreferences will be unescaped at the time they are applied.
Using the B flag, non-alphanumeric characters in backreferences
will be escaped. For example, consider the rule:


RewriteRule ^(/.*)$ /index.php?show=$1
This will map /C++ to
/index.php?show=/C++. But it will also map
/C%2b%2b to /index.php?show=/C++, because
the %2b has been unescaped.  With the B flag, it will
instead map to /index.php?show=/C%2b%2b.
This escaping is particularly necessary in a proxy situation,
when the backend may break if presented with an unescaped URL.
C|chain
The [C] or [chain] flag indicates that the RewriteRule is chained to the next
rule. That is, if the rule matches, then it is processed as usual and
control moves on to the next rule. However, if it does not match, then
the next rule, and any other rules that are chained together, will be
skipped.
CO|cookie
The [CO], or [cookie] flag, allows you to set a cookie when a
particular RewriteRule
matches. The argument consists of three required fields and four optional
fields.
The full syntax for the flag, including all attributes, is as
follows:


[CO=NAME:VALUE:DOMAIN:lifetime:path:secure:httponly]
You must declare a name, a value, and a domain for the cookie to be set.

Domain
The domain for which you want the cookie to be valid. This may be a hostname, such as www.example.com, or it may be a domain, such as .example.com. It must be at least two parts separated by a dot. That is, it may not be merely .com or .net. Cookies of that kind are forbidden by the cookie security model.
 
You may optionally also set the following values:

Lifetime
The time for which the cookie will persist, in minutes.
 
A value of 0 indicates that the cookie will persist only for the current browser session. This is the default value if none is specified.
Path
The path, on the current website, for which the cookie is valid, such as /customers/ or /files/download/.
 
By default, this is set to / - that is, the entire website.
Secure
If set to secure, true, or 1, the cookie will only be permitted to be translated via secure (https) connections.
httponly
If set to HttpOnly, true, or 1, the cookie will have the HttpOnly flag set, which means that the cookie will be inaccessible to JavaScript code on browsers that support this feature.
 
Several examples are offered here:

 RewriteEngine On  RewriteRule ^/index\.html - [CO=frontdoor:yes:.example.com:1440:/] 
In the example give, the rule doesn't rewrite the request. The "-" rewrite target tells mod_rewrite to pass the request through unchanged. Instead, it sets a cookie called 'frontdoor' to a value of 'yes'. The cookie is valid for any host in the .example.com domain. It will be set to expire in 1440 minutes (24 hours) and will be returned for all URIs.
DPI|discardpathinfo
The DPI flag causes the PATH_INFO portion of the rewritten URI to be discarded.

This flag is available in version 2.2.12 and later.

In per-directory context, the URI each RewriteRule compares against is the concatenation of the current values of the URI and PATH_INFO.

The current URI can be the initial URI as requested by the client, the result of a previous round of mod_rewrite processing, or the result of a prior rule in the current round of mod_rewrite processing.

In contrast, the PATH_INFO that is appended to the URI before each rule reflects only the value of PATH_INFO before this round of mod_rewrite processing. As a consequence, if large portions of the URI are matched and copied into a substitution in multiple RewriteRule directives, without regard for which parts of the URI came from the current PATH_INFO, the final URI may have multiple copies of PATH_INFO appended to it.

Use this flag on any substitution where the PATH_INFO that resulted from the previous mapping of this request to the filesystem is not of interest.  This flag permanently forgets the PATH_INFO established before this round of mod_rewrite processing began. PATH_INFO will not be recalculated until the current round of mod_rewrite processing completes.  Subsequent rules during this round of processing will see only the direct result of substitutions, without any PATH_INFO appended.
E|env
With the [E], or [env] flag, you can set the value of an environment variable. Note that some environment variables may be set after the rule is run, thus unsetting what you have set. See the Environment Variables document for more details on how Environment variables work.

The full syntax for this flag is:

 [E=VAR:VAL] [E=!VAR] 
VAL may contain backreferences ($N or %N) which will be expanded.

Using the short form

 [E=VAR] 
you can set the environment variable named VAR to an empty value.

The form

 [E=!VAR] 
allows to unset a previously set environment variable named VAR.

Environment variables can then be used in a variety of contexts, including CGI programs, other RewriteRule directives, or CustomLog directives.

The following example sets an environment variable called 'image' to a value of '1' if the requested URI is an image file. Then, that environment variable is used to exclude those requests from the access log.

 RewriteRule \.(png|gif|jpg) - [E=image:1]  CustomLog logs/access_log combined env=!image 
Note that this same effect can be obtained using SetEnvIf. This technique is offered as an example, not as a recommendation.
F|forbidden
Using the [F] flag causes the server to return a 403 Forbidden status code to the client. While the same behavior can be accomplished using the Deny directive, this  allows more flexibility in assigning a Forbidden status.

The following rule will forbid .exe files from being downloaded from your server.

 RewriteRule \.exe - [F] 
This example uses the "-" syntax for the rewrite target, which means that the requested URI is not modified. There's no reason to rewrite to another URI, if you're going to forbid the request.

When using [F], an [L] is implied - that is, the response is returned immediately, and no further rules are evaluated.
G|gone
The [G] flag forces the server to return a 410 Gone status with the response. This indicates that a resource used to be available, but is no longer available.

As with the [F] flag, you will typically use the "-" syntax for the rewrite target when using the [G] flag:

 RewriteRule oldproduct - [G,NC] 
When using [F], an [L] is implied - that is, the response is returned immediately, and no further rules are evaluated.
H|handler
Forces the resulting request to be handled with the specified handler. For example, one might use this to force all files without a file extension to be parsed by the php handler:

 RewriteRule !\. - [H=application/x-httpd-php] 
The regular expression above - !\. - will match any request that does not contain the literal . character. 

This can be also used to force the handler based on some conditions. For example, the following snippet used in per-server context allows .php files to be displayed by mod_php if they are requested with the .phps extension:

 RewriteRule ^(/source/.+\.php)s$ $1 [H=application/x-httpd-php-source] 
The regular expression above - ^(/source/.+\.php)s$ - will match any request that starts with /source/ followed by 1 or n characters followed by .phps literally. The backreference $1 referrers to the captured match within parenthesis of the regular expression.
L|last
The [L] flag causes mod_rewrite to stop processing the rule set. In most contexts, this means that if the rule matches, no further rules will be processed. This corresponds to the last command in Perl, or the break command in C. Use this flag to indicate that the current rule should be applied immediately without considering further rules.

If you are using RewriteRule in either .htaccess files or in  <Directory> sections, it is important to have some understanding of how the rules are processed.  The simplified form of this is that once the rules have been processed, the rewritten request is handed back to the URL parsing engine to do what it may with it. It is possible that as the rewritten request is handled, the .htaccess file or  <Directory> section may be encountered again, and thus the ruleset may be run again from the start. Most commonly this will happen if one of the rules causes a redirect - either internal or external - causing the request process to start over.

It is therefore important, if you are using RewriteRule directives in one of these contexts, that you take explicit steps to avoid rules looping, and not count solely on the [L] flag to terminate execution of a series of rules, as shown below.

The example given here will rewrite any request to index.php, giving the original request as a query string argument to index.php, however, the RewriteCond ensures that if the request  is already for index.php, the RewriteRule will be skipped.

 RewriteBase /  RewriteCond %{REQUEST_URI} !=/index.php  RewriteRule ^(.*) /index.php?req=$1 [L,PT] 
N|next
The [N] flag causes the ruleset to start over again from the top, using the result of the ruleset so far as a starting point. Use with extreme caution, as it may result in loop. 

The [Next] flag could be used, for example, if you wished to replace a certain string or letter repeatedly in a request. The example shown here will replace A with B everywhere in a request, and will continue doing so until there are no more As to be replaced. 

 RewriteRule (.*)A(.*) $1B$2 [N] 
You can think of this as a while loop: While this pattern still matches (i.e., while the URI still contains an A), perform this substitution (i.e., replace the A with a B).
NC|nocase
Use of the [NC] flag causes the RewriteRule to be matched in a case-insensitive manner. That is, it doesn't care whether letters appear as upper-case or lower-case in the matched URI.

In the example below, any request for an image file will be proxied to your dedicated image server. The match is case-insensitive, so that .jpg and .JPG files are both acceptable, for example.

 RewriteRule (.*\.(jpg|gif|png))$ http://images.example.com$1 [P,NC] 
NE|noescape
By default, special characters, such as & and ?, for example, will be converted to their hexcode equivalent. Using the [NE] flag prevents that from happening. 

 RewriteRule ^/anchor/(.+) /bigpage.html#$1 [NE,R] 
The above example will redirect /anchor/xyz to /bigpage.html#xyz. Omitting the [NE] will result in the # being converted to its hexcode equivalent, %23, which will then result in a 404 Not Found error condition. 
NS|nosubreq
Use of the [NS] flag prevents the rule from being used on subrequests. For example, a page which is included using an SSI (Server Side Include) is a subrequest, and you may want to avoid rewrites happening on those subrequests. Also, when mod_dir tries to find out information about possible directory default files (such as index.html files), this is an internal subrequest, and you often want to avoid rewrites on such subrequests. On subrequests, it is not always useful, and can even cause errors, if the complete set of rules are applied. Use this flag to exclude problematic rules.

To decide whether or not to use this rule: if you prefix URLs with  CGI-scripts, to force them to be processed by the CGI-script, it's likely that you will run into problems (or significant overhead) on sub-requests. In these cases, use this flag.

Images, javascript files, or css files, loaded as part of an HTML page, are not subrequests - the browser requests them as separate HTTP requests. 
P|proxy
Use of the [P] flag causes the request to be handled by mod_proxy, and handled via a proxy request. For example, if you wanted all image requests to be handled by a back-end image server, you might do something like the following:

 RewriteRule /(.*)\.(jpg|gif|png) http://images.example.com/$1.$2 [P] 
Use of the [P] flag implies [L] - that is, the request is immediately pushed through the proxy, and any following rules will not be considered.

You must make sure that the substitution string is a valid URI (typically starting with http://hostname) which can be handled by the mod_proxy. If not, you will get an error from the proxy module. Use this flag to achieve a more powerful implementation of the ProxyPass directive, to map remote content into the namespace of the local server.

Security Warning
Take care when constructing the target URL of the rule, considering the security impact from allowing the client influence over the set of URLs to which your server will act as a proxy.  Ensure that the scheme and hostname part of the URL is either fixed, or does not allow the client undue influence.
Note: mod_proxy must be enabled in order to use this flag.
PT|passthrough
The target (or substitution string) in a RewriteRule is assumed to be a file path, by default. The use of the [PT] flag causes it to be treated as a URI instead. That is to say, the use of the [PT] flag causes the result of the RewriteRule to be passed back through URL mapping, so that location-based mappings, such as Alias, Redirect, or ScriptAlias, for example, might have a  chance to take effect. 

If, for example, you have an  Alias for /icons, and have a RewriteRule pointing there, you should use the [PT] flag to ensure that the  Alias is evaluated. 

 Alias /icons /usr/local/apache/icons  RewriteRule /pics/(.+)\.jpg /icons/$1.gif [PT] 
Omission of the [PT] flag in this case will cause the Alias to be ignored, resulting in a 'File not found' error being returned. 

The PT flag implies the L flag: rewriting will be stopped in order to pass the request to the next phase of processing.

Note that the PT flag is implied in per-directory contexts such as <Directory> sections or in .htaccess files. The only way to circumvent that is to rewrite to -.
QSA|qsappend
When the replacement URI contains a query string, the default behavior of RewriteRule is to discard the existing query string, and replace it with the newly generated one. Using the [QSA] flag causes the query strings to be combined. 

Consider the following rule:

 RewriteRule /pages/(.+) /page.php?page=$1 [QSA] 
With the [QSA] flag, a request for /pages/123?one=two will be mapped to /page.php?page=123&one=two. Without the [QSA] flag, that same request will be mapped to /page.php?page=123 - that is, the existing query string will be discarded. 
R|redirect
Use of the [R] flag causes a HTTP redirect to be issued to the browser. If a fully-qualified URL is specified (that is, including http://servername/) then a redirect will be issued to that location. Otherwise, the current protocol, servername, and port number will be used to generate the URL sent with the redirect. 

Any valid HTTP response  status code may be specified,  using the syntax [R=305], with a 302 status code being used by  default if none is specified. The status code specified need not necessarily be a redirect (3xx) status code. 

If a status code is outside the redirect range (300-399) then the substitution string is dropped entirely, and rewriting is stopped as if the L were used.

In addition to response status codes, you may also specify redirect status using their symbolic names: temp (default),  permanent, or seeother.

You will almost always want to use [R] in conjunction with [L] (that is, use [R,L]) because on its own, the [R] flag prepends http://thishost[:thisport] to the URI, but then passes this on to the next rule in the ruleset, which can often result in 'Invalid URI in request' warnings. 
S|skip
The [S] flag is used to skip rules that you don't want to run. This can be thought of as a goto statement in your rewrite ruleset. In the following example, we only want to run the RewriteRule if the requested URI doesn't correspond with an actual file.

 # Is the request for a non-existent file?  RewriteCond %{REQUEST_FILENAME} !-f  RewriteCond %{REQUEST_FILENAME} !-d  # If so, skip these two RewriteRules  RewriteRule .? - [S=2]    RewriteRule (.*\.gif) images.php?$1  RewriteRule (.*\.html) docs.php?$1 
This technique is useful because a RewriteCond only applies to the RewriteRule immediately following it. Thus, if you want to make a RewriteCond apply to several RewriteRules, one possible technique is to negate those conditions and use a [Skip] flag. So, you can  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.
T|type
Sets the MIME type with which the resulting response will be sent. This has the same effect as the AddType directive.

For example, you might use the following technique to serve Perl source code as plain text, if requested in a particular way:

 # Serve .pl files as plain text  RewriteRule \.pl$ - [T=text/plain] 
Or, perhaps, if you have a camera that produces jpeg images without file extensions, you could force those images to be served with the correct MIME type by virtue of their file names:

 # Files with 'IMG' in the name are jpg images.  RewriteRule IMG - [T=image/jpg] 
Please note that this is a trivial example, and could be better done using <FilesMatch> instead. Always consider the alternate solutions to a problem before resorting to rewrite, which will invariably be a less efficient solution than the alternatives.

If used in per-directory context, use only - (dash) as the substitution for the entire round of mod_rewrite processing,  otherwise the MIME-type set with this flag is lost due to an internal  re-processing (including subsequent rounds of mod_rewrite processing). The L flag can be useful in this context to end the  current round of mod_rewrite processing.