Mod rewrite

From Bauman National Library
Revision as of 13:58, 24 December 2016 by denis georgiadis (Talk | contribs) (Created page with "<big>'''mod_rewrite'''</big> is Apache module uses a rule-based rewriting engine, based on a PCRE regular-expression parser, to rewrite requested URLs on the fly. By default,...")

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

mod_rewrite is Apache module uses a rule-based rewriting engine, based on a PCRE regular-expression parser, to rewrite requested URLs on the fly. By default, mod_rewrite maps a URL to a filesystem path. However, it can also be used to redirect one URL to another URL, or to invoke an internal proxy fetch.

mod_rewrite provides a flexible and powerful way to manipulate URLs using an unlimited number of rules. Each rule can have an unlimited number of attached rule conditions, to allow you to rewrite URL based on server variables, environment variables, HTTP headers, or time stamps.

mod_rewrite operates on the full URL path, including the path-info section. A rewrite rule can be invoked in httpd.conf or in .htaccess. The path generated by a rewrite rule can include a query string, or can lead to internal sub-processing, external request redirection, or internal proxy throughput.


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

Using a high trace log level for mod_rewrite will slow down your Apache HTTP Server dramatically! Use a log level higher than trace2 only for debugging!


LogLevel alert rewrite:trace3


Those familiar with earlier versions of mod_rewrite will no doubt be looking for the RewriteLog and RewriteLogLevel directives. This functionality has been completely replaced by the new per-module logging configuration mentioned above. To get just the mod_rewrite-specific log messages, pipe the log file through grep: tail -f error_log|fgrep '[rewrite:'

RewriteBase Directive

Description: Sets the base URL for per-directory rewrites
Syntax: RewriteBase URL-path
Default: None
Context: directory, .htaccess
Override: FileInfo
Status: Extention
Module: mod_rewrite

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

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

The original request, and the substitution, are underneath the DocumentRoot (as opposed to reachable by other means, such as Alias). The filesystem path to the directory containing the RewriteRule, suffixed by the relative substitution is also valid as a URL path on the server (this is rare). In Apache HTTP Server 2.4.16 and later, this directive may be omitted when the request is mapped via Alias or mod_userdir. In the example below, RewriteBase is necessary to avoid rewriting to since the resource was not relative to the document root. This misconfiguration would normally cause the server to look for an "opt" directory under the document root.

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

RewriteCond Directive

Description: Defines a condition under which rewriting will take place
Syntax: RewriteCond TestString CondPattern [flags]
Context: server config, virtual host, directory, .htaccess
Override: FileInfo
Status: Extention
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). $1 to $9 provide access to the grouped parts (in parentheses) of the pattern, from the RewriteRule which is subject to the current set of RewriteCond conditions. $0 provides access to the whole string matched by that pattern.
  • RewriteCond backreferences: These are backreferences of the form %N (0 <= N <= 9). %1 to %9 provide access to the grouped parts (again, in parentheses) of the pattern, from the last matched RewriteCond in the current set of conditions. %0 provides access to the whole string matched by that pattern.
  • RewriteMap expansions: These are expansions of the form ${mapname:key|default}. See the documentation for RewriteMap for more details.
  • Server-Variables: These are variables of the form %{ NAME_OF_VARIABLE }.


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

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

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

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

RewriteEngine Directive

Description: Enables or disables runtime rewriting engine
Syntax: off
Default: RewriteEngine off
Context: server config, virtual host, directory, .htaccess
Override: FileInfo
Status: Extention
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 rules in a particular context, rather than commenting out all the RewriteRule directives.

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

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

RewriteMap Directive

Description: Defines a mapping function for key-lookup
Syntax: RewriteMap MapName MapType:MapSource
Context: server config, virtual host
Status: Extention
Module: mod_rewrite

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

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

${ MapName : LookupKey } ${ MapName : LookupKey | DefaultValue }

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

For example, you might define a RewriteMap as:

RewriteMap examplemap "txt:/path/to/file/map.txt" You would then be able to use this map in a RewriteRule as follows:

RewriteRule "^/ex/(.*)" "${examplemap:$1}" The following combinations for MapType and MapSource can be used:

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

RewriteOptions Directive

Description: Sets some special options for the rewrite engine
Syntax: RewriteOptions Options
Context: server config, virtual host, directory, .htaccess
Override: FileInfo
Status: Extention
Module: mod_rewrite

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

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

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

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

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

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

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

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

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

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

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

RewriteRule Directive

Description: Defines rules for the rewriting engine
Syntax: RewriteRule Pattern Substitution [flags]
Context: server config, virtual host, directory, .htaccess
Override: FileInfo
Status: Extention
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. What this pattern is compared against varies depending on where the RewriteRule directive is defined.

What is matched?

  • In VirtualHost context, The Pattern will initially be matched against the part of the URL after the hostname and port, and before the query string (e.g. "/app1/index.html"). This is the (%-decoded) URL-path.
  • In per-directory context (Directory and .htaccess), the Pattern is matched against only a partial path, for example a request of "/app1/index.html" may result in comparison against "app1/index.html" or "index.html" depending on where the RewriteRule is defined.
  • The directory path where the rule is defined is stripped from the currently mapped filesystem path before comparison (up to and including a trailing slash). The net result of this per-directory prefix stripping is that rules in this context only match against the portion of the currently mapped filesystem path "below" where the rule is defined.
  • Directives such as DocumentRoot and Alias, or even the result of previous RewriteRule substitutions, determine the currently mapped filesystem path.
  • If you wish to match against the hostname, port, or query string, use a RewriteCond with the %{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.
  • See the RewriteBase directive for more information regarding what prefix will be added back to relative substitutions.
  • If you wish to match against the full URL-path in a per-directory (htaccess) RewriteRule, use the %{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 (including their regular expression counterparts), this should never be necessary and is unsupported. A likely feature to break in these contexts is relative substitutions.

External Reference