Apache Module mod_alias

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Description:	Provides for mapping different parts of the host filesystem in the document tree and for URL redirection
Status:	Base
Module Identifier:	alias_module
Source File:	mod_alias.c

Summary

The directives contained in this module allow for manipulation and control of URLs as requests arrive at the server. The Alias and ScriptAlias directives are used to map between URLs and filesystem paths. This allows for content which is not directly under the DocumentRoot served as part of the web document tree. The ScriptAlias directive has the additional effect of marking the target directory as containing only CGI scripts.

The Redirect directives are used to instruct clients to make a new request with a different URL. They are often used when a resource has moved to a new location.

When the Alias, ScriptAlias and Redirect directives are used within a <Location> or <LocationMatch> section, expression syntax can be used to manipulate the destination path or URL.

mod_alias is designed to handle simple URL manipulation tasks. For more complicated tasks such as manipulating the query string, use the tools provided by mod_rewrite.

Topics

Order of Processing

Bugfix checklist

Order of Processing

Aliases and Redirects occurring in different contexts are processed like other directives according to standard merging rules. But when multiple Aliases or Redirects occur in the same context (for example, in the same <VirtualHost> section) they are processed in a particular order.

First, all Redirects are processed before Aliases are processed, and therefore a request that matches a Redirect or RedirectMatch will never have Aliases applied. Second, the Aliases and Redirects are processed in the order they appear in the configuration files, with the first match taking precedence.

For this reason, when two or more of these directives apply to the same sub-path, you must list the most specific path first in order for all the directives to have an effect. For example, the following configuration will work as expected:

Alias "/foo/bar" "/baz"
Alias "/foo" "/gaq"

But if the above two directives were reversed in order, the /foo Alias would always match before the /foo/bar Alias, so the latter directive would be ignored.

When the Alias, ScriptAlias and Redirect directives are used within a <Location> or <LocationMatch> section, these directives will take precedence over any globally defined Alias, ScriptAlias and Redirect directives.

Alias Directive

Description:	Maps URLs to filesystem locations
Syntax:	`Alias [URL-path] file-path\|directory-path`
Context:	server config, virtual host, directory
Status:	Base
Module:	mod_alias

The Alias directive allows documents to be stored in the local filesystem other than under the DocumentRoot. URLs with a (%-decoded) path beginning with URL-path will be mapped to local files beginning with directory-path. The URL-path is case-sensitive, even on case-insensitive file systems.

Alias "/image" "/ftp/pub/image"

A request for https://example.com/image/foo.gif would cause the server to return the file /ftp/pub/image/foo.gif. Only complete path segments are matched, so the above alias would not match a request for https://example.com/imagefoo.gif. For more complex matching using regular expressions, see the AliasMatch directive.

Note that if you include a trailing / on the URL-path then the server will require a trailing / in order to expand the alias. That is, if you use

Alias "/icons/" "/usr/local/apache/icons/"

then the URL /icons will not be aliased, as it lacks that trailing /. Likewise, if you omit the slash on the URL-path then you must also omit it from the file-path.

Note that you may need to specify additional <Directory> sections which cover the destination of aliases. Aliasing occurs before <Directory> sections are checked, so only the destination of aliases are affected. (Note however <Location> sections are run through once before aliases are performed, so they will apply.)

In particular, if you are creating an Alias to a directory outside of your DocumentRoot, you may need to explicitly permit access to the target directory.

Alias "/image" "/ftp/pub/image"
<Directory "/ftp/pub/image">
    Require all granted
</Directory>

Any number slashes in the URL-path parameter matches any number of slashes in the requested URL-path.

If the Alias directive is used within a <Location> or <LocationMatch> section the URL-path is omitted, and the file-path is interpreted using expression syntax.
This syntax is available in Apache 2.4.19 and later.

<Location "/image">
    Alias "/ftp/pub/image"
</Location>
<LocationMatch "/error/(?<NUMBER>[0-9]+)">
    Alias "/usr/local/apache/errors/%{env:MATCH_NUMBER}.html"
</LocationMatch>

AliasMatch Directive

Description:	Maps URLs to filesystem locations using regular expressions
Syntax:	`AliasMatch regex file-path\|directory-path`
Context:	server config, virtual host
Status:	Base
Module:	mod_alias

This directive is equivalent to Alias, but makes use of regular expressions, instead of simple prefix matching. The supplied regular expression is matched against the URL-path, and if it matches, the server will substitute any parenthesized matches into the given string and use it as a filename. For example, to activate the /icons directory, one might use:

AliasMatch "^/icons(/|$)(.*)" "/usr/local/apache/icons$1$2"

The full range of regular expression power is available. For example, it is possible to construct an alias with case-insensitive matching of the URL-path:

AliasMatch "(?i)^/image(.*)" "/ftp/pub/image$1"

One subtle difference between Alias and AliasMatch is that Alias will automatically copy any additional part of the URI, past the part that matched, onto the end of the file path on the right side, while AliasMatch will not. This means that in almost all cases, you will want the regular expression to match the entire request URI from beginning to end, and to use substitution on the right side.

In other words, just changing Alias to AliasMatch will not have the same effect. At a minimum, you need to add ^ to the beginning of the regular expression and add (.*)$ to the end, and add $1 to the end of the replacement.

For example, suppose you want to replace this with AliasMatch:

Alias "/image/" "/ftp/pub/image/"

This is NOT equivalent - don't do this! This will send all requests that have /image/ anywhere in them to /ftp/pub/image/:

AliasMatch "/image/" "/ftp/pub/image/"

This is what you need to get the same effect:

AliasMatch "^/image/(.*)$" "/ftp/pub/image/$1"

Of course, there's no point in using AliasMatch where Alias would work. AliasMatch lets you do more complicated things. For example, you could serve different kinds of files from different directories:

AliasMatch "^/image/(.*)\.jpg$" "/files/jpg.images/$1.jpg"
AliasMatch "^/image/(.*)\.gif$" "/files/gif.images/$1.gif"

Multiple leading slashes in the requested URL are discarded by the server before directives from this module compares against the requested URL-path.

Redirect Directive

Description:	Sends an external redirect asking the client to fetch a different URL
Syntax:	`Redirect [status] [URL-path] URL`
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Base
Module:	mod_alias

The Redirect directive maps an old URL into a new one by asking the client to refetch the resource at the new location.

The old URL-path is a case-sensitive (%-decoded) path beginning with a slash. A relative path is not allowed.

The new URL may be either an absolute URL beginning with a scheme and hostname, or a URL-path beginning with a slash. In this latter case the scheme and hostname of the current server will be added.

Then any request beginning with URL-path will return a redirect request to the client at the location of the target URL. Additional path information beyond the matched URL-path will be appended to the target URL.

# Redirect to a URL on a different host
Redirect "/service" "https://foo2.example.com/service"

# Redirect to a URL on the same host
Redirect "/one" "/two"

If the client requests https://example.com/service/foo.txt, it will be told to access https://foo2.example.com/service/foo.txt instead. This includes requests with GET parameters, such as https://example.com/service/foo.pl?q=23&a=42, it will be redirected to https://foo2.example.com/service/foo.pl?q=23&a=42. Note that POSTs will be discarded.
Only complete path segments are matched, so the above example would not match a request for https://example.com/servicefoo.txt. For more complex matching using the expression syntax, omit the URL-path argument as described below. Alternatively, for matching using regular expressions, see the RedirectMatch directive.

Note

Redirect directives take precedence over Alias and ScriptAlias directives, irrespective of their ordering in the configuration file. Redirect directives inside a Location take precedence over Redirect and Alias directives with an URL-path.

If no status argument is given, the redirect will be "temporary" (HTTP status 302). This indicates to the client that the resource has moved temporarily. The status argument can be used to return other HTTP status codes:

permanent: Returns a permanent redirect status (301) indicating that the resource has moved permanently.
temp: Returns a temporary redirect status (302). This is the default.
seeother: Returns a "See Other" status (303) indicating that the resource has been replaced.
gone: Returns a "Gone" status (410) indicating that the resource has been permanently removed. When this status is used the URL argument should be omitted.

Other status codes can be returned by giving the numeric status code as the value of status. If the status is between 300 and 399, the URL argument must be present. If the status is not between 300 and 399, the URL argument must be omitted. The status must be a valid HTTP status code, known to the Apache HTTP Server (see the function send_error_response in http_protocol.c).

Redirect permanent "/one" "https://example.com/two"
Redirect 303 "/three" "https://example.com/other"

If the Redirect directive is used within a <Location> or <LocationMatch> section with the URL-path omitted, then the URL parameter will be interpreted using expression syntax.
This syntax is available in Apache 2.4.19 and later.

<Location "/one">
    Redirect permanent "https://example.com/two"
</Location>
<Location "/three">
    Redirect 303 "https://example.com/other"
</Location>
<LocationMatch "/error/(?<NUMBER>[0-9]+)">
    Redirect permanent "https://example.com/errors/%{env:MATCH_NUMBER}.html"
</LocationMatch>

RedirectMatch Directive

Description:	Sends an external redirect based on a regular expression match of the current URL
Syntax:	`RedirectMatch [status] regex URL`
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Base
Module:	mod_alias

This directive is equivalent to Redirect, but makes use of regular expressions, instead of simple prefix matching. The supplied regular expression is matched against the URL-path, and if it matches, the server will substitute any parenthesized matches into the given string and use it as a filename. For example, to redirect all GIF files to like-named JPEG files on another server, one might use:

RedirectMatch "(.*)\.gif$" "https://other.example.com$1.jpg"

The considerations related to the difference between Alias and AliasMatch also apply to the difference between Redirect and RedirectMatch. See AliasMatch for details.

RedirectPermanent Directive

Description:	Sends an external permanent redirect asking the client to fetch a different URL
Syntax:	`RedirectPermanent URL-path URL`
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Base
Module:	mod_alias

This directive makes the client know that the Redirect is permanent (status 301). Exactly equivalent to Redirect permanent.

RedirectTemp Directive

Description:	Sends an external temporary redirect asking the client to fetch a different URL
Syntax:	`RedirectTemp URL-path URL`
Context:	server config, virtual host, directory, .htaccess
Override:	FileInfo
Status:	Base
Module:	mod_alias

This directive makes the client know that the Redirect is only temporary (status 302). Exactly equivalent to Redirect temp.

ScriptAlias Directive

Description:	Maps a URL to a filesystem location and designates the target as a CGI script
Syntax:	`ScriptAlias [URL-path] file-path\|directory-path`
Context:	server config, virtual host, directory
Status:	Base
Module:	mod_alias

The ScriptAlias directive has the same behavior as the Alias directive, except that in addition it marks the target directory as containing CGI scripts that will be processed by mod_cgi's cgi-script handler. URLs with a case-sensitive (%-decoded) path beginning with URL-path will be mapped to scripts beginning with the second argument, which is a full pathname in the local filesystem.

ScriptAlias "/cgi-bin/" "/web/cgi-bin/"

A request for https://example.com/cgi-bin/foo would cause the server to run the script /web/cgi-bin/foo. This configuration is essentially equivalent to:

Alias "/cgi-bin/" "/web/cgi-bin/"
<Location "/cgi-bin">
    SetHandler cgi-script
    Options +ExecCGI
</Location>

ScriptAlias can also be used in conjunction with a script or handler you have. For example:

ScriptAlias "/cgi-bin/" "/web/cgi-handler.pl"

In this scenario all files requested in /cgi-bin/ will be handled by the file you have configured, this allows you to use your own custom handler. You may want to use this as a wrapper for CGI so that you can add content, or some other bespoke action.

It is safer to avoid placing CGI scripts under the DocumentRoot in order to avoid accidentally revealing their source code if the configuration is ever changed. The ScriptAlias makes this easy by mapping a URL and designating CGI scripts at the same time. If you do choose to place your CGI scripts in a directory already accessible from the web, do not use ScriptAlias. Instead, use <Directory>, SetHandler, and Options as in:

<Directory "/usr/local/apache2/htdocs/cgi-bin">
    SetHandler cgi-script
    Options ExecCGI
</Directory>

This is necessary since multiple URL-paths can map to the same filesystem location, potentially bypassing the ScriptAlias and revealing the source code of the CGI scripts if they are not restricted by a Directory section.

If the ScriptAlias directive is used within a <Location> or <LocationMatch> section with the URL-path omitted, then the URL parameter will be interpreted using expression syntax.
This syntax is available in Apache 2.4.19 and later.

<Location "/cgi-bin">
    ScriptAlias "/web/cgi-bin/"
</Location>
<LocationMatch "/cgi-bin/errors/(?<NUMBER>[0-9]+)">
    ScriptAlias "/web/cgi-bin/errors/%{env:MATCH_NUMBER}.cgi"
</LocationMatch>

ScriptAliasMatch Directive

Description:	Maps a URL to a filesystem location using a regular expression and designates the target as a CGI script
Syntax:	`ScriptAliasMatch regex file-path\|directory-path`
Context:	server config, virtual host
Status:	Base
Module:	mod_alias

This directive is equivalent to ScriptAlias, but makes use of regular expressions, instead of simple prefix matching. The supplied regular expression is matched against the URL-path, and if it matches, the server will substitute any parenthesized matches into the given string and use it as a filename. For example, to activate the standard /cgi-bin, one might use:

ScriptAliasMatch "^/cgi-bin(.*)" "/usr/local/apache/cgi-bin$1"

As for AliasMatch, the full range of regular expression power is available. For example, it is possible to construct an alias with case-insensitive matching of the URL-path:

ScriptAliasMatch "(?i)^/cgi-bin(.*)" "/usr/local/apache/cgi-bin$1"

The considerations related to the difference between Alias and AliasMatch also apply to the difference between ScriptAlias and ScriptAliasMatch. See AliasMatch for details.