Esta directiva hace posible mejoras específicas a nivel de sistema operativo y a través del tipo de Protocolo para un socket que escucha. La premisa básica es que el kernel no envíe un socket al servidor hasta que o bien los datos se hayan recibido o bien se haya almacenado en el buffer una Respuesta HTTP completa. Actualmente sólo están soportados Accept Filters sobre FreeBSD, TCP_DEFER_ACCEPT sobre Linux, y AcceptEx() sobre Windows.

El uso de none para un argumento desactiva cualquier filtro aceptado para ese protocolo. Esto es útil para protocolos que requieren que un servidor envíe datos primeros, tales como ftp: o nntp:

Los nombres de protocolo por defecto son https para el puerto 443 y http para todos los demás puertos. Para especificar que se está utilizando otro protocolo con un puerto escuchando, añade el argumento protocol a la directiva Listen.

Sobre FreeBDS los valores por defecto:

El filtro httpready almacena en el buffer peticiones HTTP completas a nivel de kernel. Una vez que la petición es recibida, el kernel la envía al servidor. Consulta la página man de accf_http(9) para más detalles. Puesto que las peticiones HTTPS están encriptadas, sólo se utiliza el filtro accf_data(9).

Sobre Linux los valores por defecto son:

En Linux, TCP_DEFER_ACCEPT no soporta el buffering en peticiones http. Cualquier valor además de none habilitará TCP_DEFER_ACCEPT en ese socket. Para más detalles ver la página man de Linux tcp(7).

Sobre Windows los valores por defecto son:

Sobre Windows mpm_winnt interpreta el argumento AcceptFilter para conmutar la API AcceptEx(), y no soporta el buffering sobre el protocolo http. Hay dos valores que utilizan la API Windows AcceptEx() y que recuperan sockets de red entre conexciones. data espera hasta que los datos han sido transmitidos como se comentaba anteriormente, y el buffer inicial de datos y las direcciones de red son recuperadas a partir de una única llamada AcceptEx(). connect utiliza la API AcceptEx() API, y recupera también las direcciones de red, pero a diferencia de none la opción connect no espera a la transmisión inicial de los datos.

Sobre Windows, none prefiere accept() antes que AcceptEx() y no recuperará sockets entre las conexiones. Lo que es útil para los adaptadores de red con un soporte precario de drivers, así como para algunos proveedores de red tales como drivers vpn, o filtros de spam, de virus o de spyware.

Esta directiva controla si las peticiones que contienen información sobre la ruta que sigue un fichero que existe (o un fichero que no existe pero en un directorio que sí existe) serán aceptadas o denegadas. La información de ruta puede estar disponible para los scripts en la variable de entorno PATH_INFO.

Por ejemplo, asumamos que la ubicación /test/ apunta a un directorio que contiene únicamente el fichero here.html. Entonces, las peticiones tanto para /test/here.html/more como para /test/nothere.html/more recogen /more como PATH_INFO.

Los tres posibles argumentos para la directiva AcceptPathInfo son los siguientes:

Off

Una petición sólo será aceptada si se corresponde con una ruta literal que existe. Por lo tanto, una petición con una información de ruta después del nombre de fichero tal como /test/here.html/more en el ejemplo anterior devolverá un error 404 NOT FOUND.

On

Una petición será aceptada si una ruta principal de acceso se corresponde con un fichero que existe. El ejemplo anterior /test/here.html/more será aceptado si /test/here.html corresponde a un fichero válido.

Default

La gestión de las peticiones con información de ruta está determinada por el controlador responsable de la petición. El controlador principal para para ficheros normales rechaza por defecto peticiones PATH_INFO. Los controladores que sirven scripts, tales como cgi-script e isapi-handler, normalmente aceptan PATH_INFO por defecto.

El objetivo principal de la directiva AcceptPathInfo es permitirte sobreescribir la opción del controlador de aceptar or rechazar PATH_INFO. Este tipo de sobreescritura se necesita, por ejemplo, cuando utilizas un filtro, tal como INCLUDES, para generar contenido basado en PATH_INFO. El controlador principal normalmente rechazaría la petición, de modo que puedes utilizar la siguiente configuración para habilitarla como script:

Mientras que procesa una petición el servidor busca el primer fichero de configuración existente dentro de un listado de nombres en cada directorio de la ruta del documento, si los ficheros distribuidos de configuración están habilitados para ese directorio. Por ejemplo:

antes de servir el documento /usr/local/web/index.html, el servidor leerá /.acl, /usr/.acl, /usr/local/.acl and /usr/local/web/.acl para las directivas, salvo que estén deshabilitadas with

This directive specifies a default value for the media type charset parameter (the name of a character encoding) to be added to a response if and only if the response's content-type is either text/plain or text/html. This should override any charset specified in the body of the response via a META element, though the exact behavior is often dependent on the user's client configuration. A setting of AddDefaultCharset Off disables this functionality. AddDefaultCharset On enables a default charset of iso-8859-1. Any other value is assumed to be the charset to be used, which should be one of the IANA registered charset values for use in Internet media types (MIME types). For example:

AddDefaultCharset should only be used when all of the text resources to which it applies are known to be in that character encoding and it is too inconvenient to label their charset individually. One such example is to add the charset parameter to resources containing generated content, such as legacy CGI scripts, that might be vulnerable to cross-site scripting attacks due to user-provided data being included in the output. Note, however, that a better solution is to just fix (or delete) those scripts, since setting a default charset does not protect users that have enabled the "auto-detect character encoding" feature on their browser.

The AllowEncodedSlashes directive allows URLs which contain encoded path separators (%2F for / and additionally %5C for \ on according systems) to be used. Normally such URLs are refused with a 404 (Not found) error.

Turning AllowEncodedSlashes On is mostly useful when used in conjunction with PATH_INFO.

When the server finds an .htaccess file (as specified by AccessFileName) it needs to know which directives declared in that file can override earlier configuration directives.

When this directive is set to None, then .htaccess files are completely ignored. In this case, the server will not even attempt to read .htaccess files in the filesystem.

When this directive is set to All, then any directive which has the .htaccess Context is allowed in .htaccess files.

The directive-type can be one of the following groupings of directives.

AuthConfig

Allow use of the authorization directives (AuthDBMGroupFile, AuthDBMUserFile, AuthGroupFile, AuthName, AuthType, AuthUserFile, Require, etc.).

FileInfo

Allow use of the directives controlling document types (ErrorDocument, ForceType, LanguagePriority, SetHandler, SetInputFilter, SetOutputFilter, and mod_mime Add* and Remove* directives), document meta data (Header, RequestHeader, SetEnvIf, SetEnvIfNoCase, BrowserMatch, CookieExpires, CookieDomain, CookieStyle, CookieTracking, CookieName), mod_rewrite directives RewriteEngine, RewriteOptions, RewriteBase, RewriteCond, RewriteRule) and Action from mod_actions.

Indexes

Allow use of the directives controlling directory indexing (AddDescription, AddIcon, AddIconByEncoding, AddIconByType, DefaultIcon, DirectoryIndex, FancyIndexing, HeaderName, IndexIgnore, IndexOptions, ReadmeName, etc.).

Limit

Allow use of the directives controlling host access (Allow, Deny and Order).

Options[=Option,...]

Allow use of the directives controlling specific directory features (Options and XBitHack). An equal sign may be given followed by a comma (but no spaces) separated lists of options that may be set using the Options command.

Example:

In the example above all directives that are neither in the group AuthConfig nor Indexes cause an internal server error.

This directive is used to control how Apache httpd finds the interpreter used to run CGI scripts. For example, setting CGIMapExtension sys:\foo.nlm .foo will cause all CGI script files with a .foo extension to be passed to the FOO interpreter.

This directive enables the generation of Content-MD5 headers as defined in RFC1864 respectively RFC2616.

MD5 is an algorithm for computing a "message digest" (sometimes called "fingerprint") of arbitrary-length data, with a high degree of confidence that any alterations in the data will be reflected in alterations in the message digest.

The Content-MD5 header provides an end-to-end message integrity check (MIC) of the entity-body. A proxy or client may check this header for detecting accidental modification of the entity-body in transit. Example header:

Note that this can cause performance problems on your server since the message digest is computed on every request (the values are not cached).

Content-MD5 is only sent for documents served by the core, and not by any module. For example, SSI documents, output from CGI scripts, and byte range responses do not have this header.

This directive has been disabled. For backwards compatibility of configuration files, it may be specified with the value none, meaning no default media type. For example:

DefaultType None is only available in httpd-2.2.7 and later.

Use the mime.types configuration file and the AddType to configure media type assignments via file extensions, or the ForceType directive to configure the media type for specific resources. Otherwise, the server will send the response without a Content-Type header field and the recipient may attempt to guess the media type.

Equivalent to passing the -D argument to httpd.

This directive can be used to toggle the use of IfDefine sections without needing to alter -D arguments in any startup scripts.

Directory and </Directory> are used to enclose a group of directives that will apply only to the named directory, sub-directories of that directory, and the files within the respective directories. Any directive that is allowed in a directory context may be used. Directory-path is either the full path to a directory, or a wild-card string using Unix shell-style matching. In a wild-card string, ? matches any single character, and * matches any sequences of characters. You may also use [] character ranges. None of the wildcards match a `/' character, so <Directory /*/public_html> will not match /home/user/public_html, but <Directory /home/*/public_html> will match. Example:

Regular expressions can also be used, with the addition of the ~ character. For example:

would match directories in /www/ that consisted of three numbers.

If multiple (non-regular expression) Directory sections match the directory (or one of its parents) containing a document, then the directives are applied in the order of shortest match first, interspersed with the directives from the .htaccess files. For example, with

for access to the document /home/web/dir/doc.html the steps are:

• Apply directive AllowOverride None (disabling .htaccess files).
• Apply directive AllowOverride FileInfo (for directory /home).
• Apply any FileInfo directives in /home/.htaccess, /home/web/.htaccess and /home/web/dir/.htaccess in that order.

Regular expressions are not considered until after all of the normal sections have been applied. Then all of the regular expressions are tested in the order they appeared in the configuration file. For example, with

the regular expression section won't be considered until after all normal Directorys and .htaccess files have been applied. Then the regular expression will match on /home/abc/public_html/abc and the corresponding Directory will be applied.

Note that the default access for <Directory /> is Allow from All. This means that Apache httpd will serve any file mapped from an URL. It is recommended that you change this with a block such as

and then override this for directories you want accessible. See the Security Tips page for more details.

The directory sections occur in the httpd.conf file. Directory directives cannot nest, and cannot appear in a Limit or LimitExcept section.

DirectoryMatch and </DirectoryMatch> are used to enclose a group of directives which will apply only to the named directory (and the files within), the same as Directory. However, it takes as an argument a regular expression. For example:

would match directories in /www/ that consisted of three numbers.

This directive sets the directory from which httpd will serve files. Unless matched by a directive like Alias, the server appends the path from the requested URL to the document root to make the path to the document. Example:

then an access to http://www.my.host.com/index.html refers to /usr/web/index.html. If the directory-path is not absolute then it is assumed to be relative to the ServerRoot.

The DocumentRoot should be specified without a trailing slash.

This directive controls whether the httpd may use memory-mapping if it needs to read the contents of a file during delivery. By default, when the handling of a request requires access to the data within a file -- for example, when delivering a server-parsed file using mod_include -- Apache httpd memory-maps the file if the OS supports it.

This memory-mapping sometimes yields a performance improvement. But in some environments, it is better to disable the memory-mapping to prevent operational problems:

• On some multiprocessor systems, memory-mapping can reduce the performance of the httpd.
• Deleting or truncating a file while httpd has it memory-mapped can cause httpd to crash with a segmentation fault.

For server configurations that are vulnerable to these problems, you should disable memory-mapping of delivered files by specifying:

For NFS mounted files, this feature may be disabled explicitly for the offending files by specifying:

This directive controls whether httpd may use the sendfile support from the kernel to transmit file contents to the client. By default, when the handling of a request requires no access to the data within a file -- for example, when delivering a static file -- Apache httpd uses sendfile to deliver the file contents without ever reading the file if the OS supports it.

This sendfile mechanism avoids separate read and send operations, and buffer allocations. But on some platforms or within some filesystems, it is better to disable this feature to avoid operational problems:

• Some platforms may have broken sendfile support that the build system did not detect, especially if the binaries were built on another box and moved to such a machine with broken sendfile support.
• On Linux the use of sendfile triggers TCP-checksum offloading bugs on certain networking cards when using IPv6.
• On Linux on Itanium, sendfile may be unable to handle files over 2GB in size.
• With a network-mounted DocumentRoot (e.g., NFS, SMB, CIFS, FUSE), the kernel may be unable to serve the network file through its own cache.

For server configurations that are not vulnerable to these problems, you may enable this feature by specifying:

For network mounted files, this feature may be disabled explicitly for the offending files by specifying:

Please note that the per-directory and .htaccess configuration of EnableSendfile is not supported by mod_cache_disk. Only global definition of EnableSendfile is taken into account by the module.

If an error can be detected within the configuration, this directive can be used to generate a custom error message, and halt configuration parsing. The typical use is for reporting required modules which are missing from the configuration.

In the event of a problem or error, Apache httpd can be configured to do one of four things,

1. output a simple hardcoded error message
2. output a customized message
3. redirect to a local URL-path to handle the problem/error
4. redirect to an external URL to handle the problem/error

The first option is the default, while options 2-4 are configured using the ErrorDocument directive, which is followed by the HTTP response code and a URL or a message. Apache httpd will sometimes offer additional information regarding the problem/error.

URLs can begin with a slash (/) for local web-paths (relative to the DocumentRoot), or be a full URL which the client can resolve. Alternatively, a message can be provided to be displayed by the browser. Examples:

Additionally, the special value default can be used to specify Apache httpd's simple hardcoded message. While not required under normal circumstances, default will restore Apache httpd's simple hardcoded message for configurations that would otherwise inherit an existing ErrorDocument.

Note that when you specify an ErrorDocument that points to a remote URL (ie. anything with a method such as http in front of it), Apache HTTP Server will send a redirect to the client to tell it where to find the document, even if the document ends up being on the same server. This has several implications, the most important being that the client will not receive the original error status code, but instead will receive a redirect status code. This in turn can confuse web robots and other clients which try to determine if a URL is valid using the status code. In addition, if you use a remote URL in an ErrorDocument 401, the client will not know to prompt the user for a password since it will not receive the 401 status code. Therefore, if you use an ErrorDocument 401 directive then it must refer to a local document.

Microsoft Internet Explorer (MSIE) will by default ignore server-generated error messages when they are "too small" and substitute its own "friendly" error messages. The size threshold varies depending on the type of error, but in general, if you make your error document greater than 512 bytes, then MSIE will show the server-generated error rather than masking it. More information is available in Microsoft Knowledge Base article Q294807.

Although most error messages can be overriden, there are certain circumstances where the internal messages are used regardless of the setting of ErrorDocument. In particular, if a malformed request is detected, normal request processing will be immediately halted and the internal error message returned. This is necessary to guard against security problems caused by bad requests.

If you are using mod_proxy, you may wish to enable ProxyErrorOverride so that you can provide custom error messages on behalf of your Origin servers. If you don't enable ProxyErrorOverride, Apache httpd will not generate custom error documents for proxied content.

The ErrorLog directive sets the name of the file to which the server will log any errors it encounters. If the file-path is not absolute then it is assumed to be relative to the ServerRoot.

If the file-path begins with a pipe character "|" then it is assumed to be a command to spawn to handle the error log.

See the notes on piped logs for more information.

Using syslog instead of a filename enables logging via syslogd(8) if the system supports it. The default is to use syslog facility local7, but you can override this by using the syslog:facility syntax where facility can be one of the names usually documented in syslog(1). The facility is effectively global, and if it is changed in individual virtual hosts, the final facility specified affects the entire server.

SECURITY: See the security tips document for details on why your security could be compromised if the directory where log files are stored is writable by anyone other than the user that starts the server.

ErrorLogFormat allows to specify what supplementary information is logged in the error log in addition to the actual log message.

Specifying connection or request as first paramter allows to specify additional formats, causing additional information to be logged when the first message is logged for a specific connection or request, respectivly. This additional information is only logged once per connection/request. If a connection or request is processed without causing any log message, the additional information is not logged either.

It can happen that some format string items do not produce output. For example, the Referer header is only present if the log message is associated to a request and the log message happens at a time when the Referer header has already been read from the client. If no output is produced, the default behaviour is to delete everything from the preceeding space character to the next space character. This means the log line is implicitly divided into fields on non-whitespace to whitespace transitions. If a format string item does not produce output, the whole field is ommitted. For example, if the remote address %a in the log format [%t] [%l] [%a] %M  is not available, the surrounding brackets are not logged either. Space characters can be escaped with a backslash to prevent them from delimiting a field. The combination '% ' (percent space) is a zero-witdh field delimiter that does not produce any output.

The above behaviour can be changed by adding modifiers to the format string item. A - (minus) modifier causes a minus to be logged if the respective item does not produce any output. In once-per-connection/request formats, it is also possible to use the + (plus) modifier. If an item with the plus modifier does not produce any output, the whole line is ommitted.

A number as modifier can be used to assign a log severity level to a format item. The item will only be logged if the severity of the log message is not higher than the specified log severity level. The number can range from 1 (alert) over 4 (warn) and 7 (debug) to 15 (trace8).

Some format string items accept additional parameters in braces.

The log ID format %L produces a unique id for a connection or request. This can be used to correlate which log lines belong to the same connection or request, which request happens on which connection. A %L format string is also available in mod_log_config, to allow to correlate access log entries with error log lines. If mod_unique_id is loaded, its unique id will be used as log ID for requests.

This option tracks additional data per worker about the currently executing request, and a utilization summary; you can see these variables during runtime by configuring mod_status. Note that other modules may rely on this scoreboard.

This setting applies to the entire server, and cannot be enabled or disabled on a virtualhost-by-virtualhost basis. The collection of extended status information can slow down the server. Also note that this setting cannot be changed during a graceful restart.

The FileETag directive configures the file attributes that are used to create the ETag (entity tag) response header field when the document is based on a static file. (The ETag value is used in cache management to save network bandwidth.) The FileETag directive allows you to choose which of these -- if any -- should be used. The recognized keywords are:

INode

The file's i-node number will be included in the calculation

MTime

The date and time the file was last modified will be included

Size

The number of bytes in the file will be included

All

All available fields will be used. This is equivalent to: FileETag INode MTime Size

None

If a document is file-based, no ETag field will be included in the response

The INode, MTime, and Size keywords may be prefixed with either + or -, which allow changes to be made to the default setting inherited from a broader scope. Any keyword appearing without such a prefix immediately and completely cancels the inherited setting.

If a directory's configuration includes FileETag INode MTime Size, and a subdirectory's includes FileETag -INode, the setting for that subdirectory (which will be inherited by any sub-subdirectories that don't override it) will be equivalent to FileETag MTime Size.

The Files directive limits the scope of the enclosed directives by filename. It is comparable to the Directory and Location directives. It should be matched with a </Files> directive. The directives given within this section will be applied to any object with a basename (last component of filename) matching the specified filename. Files sections are processed in the order they appear in the configuration file, after the Directory sections and .htaccess files are read, but before Location sections. Note that Files can be nested inside Directory sections to restrict the portion of the filesystem they apply to.

The filename argument should include a filename, or a wild-card string, where ? matches any single character, and * matches any sequences of characters. Regular expressions can also be used, with the addition of the ~ character. For example:

would match most common Internet graphics formats. FilesMatch is preferred, however.

Note that unlike Directory and Location sections, Files sections can be used inside .htaccess files. This allows users to control access to their own files, at a file-by-file level.

The FilesMatch directive limits the scope of the enclosed directives by filename, just as the Files directive does. However, it accepts a regular expression. For example:

would match most common Internet graphics formats.

When placed into an .htaccess file or a Directory, or Location or Files section, this directive forces all matching files to be served with the content type identification given by media-type. For example, if you had a directory full of GIF files, but did not want to label them all with .gif, you might want to use:

Note that this directive overrides other indirect media type associations defined in mime.types or via the AddType.

You can also override more general ForceType settings by using the value of None:

This directive primarily overrides the content types generated for static files served out of the filesystem. For resources other than static files, where the generator of the response typically specifies a Content-Type, this directive has no effect.

When the server has been compiled with gprof profiling support, GprofDir causes gmon.out files to be written to the specified directory when the process exits. If the argument ends with a percent symbol ('%'), subdirectories are created for each process id.

This directive currently only works with the prefork MPM.

This directive enables DNS lookups so that host names can be logged (and passed to CGIs/SSIs in REMOTE_HOST). The value Double refers to doing double-reverse DNS lookup. That is, after a reverse lookup is performed, a forward lookup is then performed on that result. At least one of the IP addresses in the forward lookup must match the original address. (In "tcpwrappers" terminology this is called PARANOID.)

Regardless of the setting, when mod_authz_host is used for controlling access by hostname, a double reverse lookup will be performed. This is necessary for security. Note that the result of this double-reverse isn't generally available unless you set HostnameLookups Double. For example, if only HostnameLookups On and a request is made to an object that is protected by hostname restrictions, regardless of whether the double-reverse fails or not, CGIs will still be passed the single-reverse result in REMOTE_HOST.

The default is Off in order to save the network traffic for those sites that don't truly need the reverse lookups done. It is also better for the end users because they don't have to suffer the extra latency that a lookup entails. Heavily loaded sites should leave this directive Off, since DNS lookups can take considerable amounts of time. The utility logresolve, compiled by default to the bin subdirectory of your installation directory, can be used to look up host names from logged IP addresses offline.

The If directive evaluates an expression at runtime, and applies the enclosed directives if and only if the expression evaluates to true. For example:

would match HTTP/1.0 requests without a Host: header.

You may compare the value of any variable in the request headers ($req), response headers ($resp) or environment ($env) in your expression.

Apart from =, If can use the IN operator to compare if the expression is in a given range:

The <IfDefine test>...</IfDefine> section is used to mark directives that are conditional. The directives within an IfDefine section are only processed if the test is true. If test is false, everything between the start and end markers is ignored.

The test in the IfDefine section directive can be one of two forms:

• parameter-name
• !parameter-name

In the former case, the directives between the start and end markers are only processed if the parameter named parameter-name is defined. The second format reverses the test, and only processes the directives if parameter-name is not defined.

The parameter-name argument is a define as given on the httpd command line via -Dparameter at the time the server was started or by the Define directive.

IfDefine sections are nest-able, which can be used to implement simple multiple-parameter tests. Example:

The <IfModule test>...</IfModule> section is used to mark directives that are conditional on the presence of a specific module. The directives within an IfModule section are only processed if the test is true. If test is false, everything between the start and end markers is ignored.

The test in the IfModule section directive can be one of two forms:

• module
• !module

In the former case, the directives between the start and end markers are only processed if the module named module is included in Apache httpd -- either compiled in or dynamically loaded using LoadModule. The second format reverses the test, and only processes the directives if module is not included.

The module argument can be either the module identifier or the file name of the module, at the time it was compiled. For example, rewrite_module is the identifier and mod_rewrite.c is the file name. If a module consists of several source files, use the name of the file containing the string STANDARD20_MODULE_STUFF.

IfModule sections are nest-able, which can be used to implement simple multiple-module tests.

This directive allows inclusion of other configuration files from within the server configuration files.

Shell-style (fnmatch()) wildcard characters can be used in the filename or directory parts of the path to include several files at once, in alphabetical order. In addition, if Include points to a directory, rather than a file, Apache httpd will read all files in that directory and any subdirectory. However, including entire directories is not recommended, because it is easy to accidentally leave temporary files in a directory that can cause httpd to fail. Instead, we encourage you to use the wildcard syntax shown below, to include files that match a particular pattern, such as *.conf, for example.

When a wildcard is specified for a file component of the path, and no file matches the wildcard, the Include directive will be silently ignored. When a wildcard is specified for a directory component of the path, and no directory matches the wildcard, the Include directive will fail with an error saying the directory cannot be found.

For further control over the behaviour of the server when no files or directories match, prefix the path with the modifiers optional or strict. If optional is specified, any wildcard file or directory that does not match will be silently ignored. If strict is specified, any wildcard file or directory that does not match at least one file will cause server startup to fail.

When a directory or file component of the path is specified exactly, and that directory or file does not exist, Include directive will fail with an error saying the file or directory cannot be found.

The file path specified may be an absolute path, or may be relative to the ServerRoot directory.

Examples:

Or, providing paths relative to your ServerRoot directory:

Wildcards may be included in the directory or file portion of the path. In the following example, the server will fail to load if no directories match conf/vhosts/*, but will load successfully if no files match *.conf.

In this example, the server will fail to load if either conf/vhosts/* matches no directories, or if *.conf matches no files:

In this example, the server load successfully if either conf/vhosts/* matches no directories, or if *.conf matches no files:

The Keep-Alive extension to HTTP/1.0 and the persistent connection feature of HTTP/1.1 provide long-lived HTTP sessions which allow multiple requests to be sent over the same TCP connection. In some cases this has been shown to result in an almost 50% speedup in latency times for HTML documents with many images. To enable Keep-Alive connections, set KeepAlive On.

For HTTP/1.0 clients, Keep-Alive connections will only be used if they are specifically requested by a client. In addition, a Keep-Alive connection with an HTTP/1.0 client can only be used when the length of the content is known in advance. This implies that dynamic content such as CGI output, SSI pages, and server-generated directory listings will generally not use Keep-Alive connections to HTTP/1.0 clients. For HTTP/1.1 clients, persistent connections are the default unless otherwise specified. If the client requests it, chunked encoding will be used in order to send content of unknown length over persistent connections.

When a client uses a Keep-Alive connection it will be counted as a single "request" for the MaxConnectionsPerChild directive, regardless of how many requests are sent using the connection.

The number of seconds Apache httpd will wait for a subsequent request before closing the connection. By adding a postfix of ms the timeout can be also set in milliseconds. Once a request has been received, the timeout value specified by the Timeout directive applies.

Setting KeepAliveTimeout to a high value may cause performance problems in heavily loaded servers. The higher the timeout, the more server processes will be kept occupied waiting on connections with idle clients.

In a name-based virtual host context, the value of the first defined virtual host (the default host) in a set of NameVirtualHost will be used. The other values will be ignored.

Access controls are normally effective for all access methods, and this is the usual desired behavior. In the general case, access control directives should not be placed within a Limit section.

The purpose of the Limit directive is to restrict the effect of the access controls to the nominated HTTP methods. For all other methods, the access restrictions that are enclosed in the Limit bracket will have no effect. The following example applies the access control only to the methods POST, PUT, and DELETE, leaving all other methods unprotected:

The method names listed can be one or more of: GET, POST, PUT, DELETE, CONNECT, OPTIONS, PATCH, PROPFIND, PROPPATCH, MKCOL, COPY, MOVE, LOCK, and UNLOCK. The method name is case-sensitive. If GET is used it will also restrict HEAD requests. The TRACE method cannot be limited (see TraceEnable).

The Limit and LimitExcept directives may be nested. In this case, each successive level of Limit or LimitExcept directives must further restrict the set of methods to which access controls apply.

For example, given the following configuration, all users will be authorized for POST requests, and the Require group editors directive will be ignored in all cases:

LimitExcept and </LimitExcept> are used to enclose a group of access control directives which will then apply to any HTTP access method not listed in the arguments; i.e., it is the opposite of a Limit section and can be used to control both standard and nonstandard/unrecognized methods. See the documentation for Limit for more details.

For example:

An internal redirect happens, for example, when using the Action directive, which internally redirects the original request to a CGI script. A subrequest is Apache httpd's mechanism to find out what would happen for some URI if it were requested. For example, mod_dir uses subrequests to look for the files listed in the DirectoryIndex directive.

LimitInternalRecursion prevents the server from crashing when entering an infinite loop of internal redirects or subrequests. Such loops are usually caused by misconfigurations.

The directive stores two different limits, which are evaluated on per-request basis. The first number is the maximum number of internal redirects, that may follow each other. The second number determines, how deep subrequests may be nested. If you specify only one number, it will be assigned to both limits.

This directive specifies the number of bytes from 0 (meaning unlimited) to 2147483647 (2GB) that are allowed in a request body. See the note below for the limited applicability to proxy requests.

The LimitRequestBody directive allows the user to set a limit on the allowed size of an HTTP request message body within the context in which the directive is given (server, per-directory, per-file or per-location). If the client request exceeds that limit, the server will return an error response instead of servicing the request. The size of a normal request message body will vary greatly depending on the nature of the resource and the methods allowed on that resource. CGI scripts typically use the message body for retrieving form information. Implementations of the PUT method will require a value at least as large as any representation that the server wishes to accept for that resource.

This directive gives the server administrator greater control over abnormal client request behavior, which may be useful for avoiding some forms of denial-of-service attacks.

If, for example, you are permitting file upload to a particular location, and wish to limit the size of the uploaded file to 100K, you might use the following directive:

Number is an integer from 0 (meaning unlimited) to 32767. The default value is defined by the compile-time constant DEFAULT_LIMIT_REQUEST_FIELDS (100 as distributed).

The LimitRequestFields directive allows the server administrator to modify the limit on the number of request header fields allowed in an HTTP request. A server needs this value to be larger than the number of fields that a normal client request might include. The number of request header fields used by a client rarely exceeds 20, but this may vary among different client implementations, often depending upon the extent to which a user has configured their browser to support detailed content negotiation. Optional HTTP extensions are often expressed using request header fields.

This directive gives the server administrator greater control over abnormal client request behavior, which may be useful for avoiding some forms of denial-of-service attacks. The value should be increased if normal clients see an error response from the server that indicates too many fields were sent in the request.

For example:

This directive specifies the number of bytes that will be allowed in an HTTP request header.

The LimitRequestFieldSize directive allows the server administrator to reduce or increase the limit on the allowed size of an HTTP request header field. A server needs this value to be large enough to hold any one header field from a normal client request. The size of a normal request header field will vary greatly among different client implementations, often depending upon the extent to which a user has configured their browser to support detailed content negotiation. SPNEGO authentication headers can be up to 12392 bytes.

This directive gives the server administrator greater control over abnormal client request behavior, which may be useful for avoiding some forms of denial-of-service attacks.

For example:

This directive sets the number of bytes that will be allowed on the HTTP request-line.

The LimitRequestLine directive allows the server administrator to reduce or increase the limit on the allowed size of a client's HTTP request-line. Since the request-line consists of the HTTP method, URI, and protocol version, the LimitRequestLine directive places a restriction on the length of a request-URI allowed for a request on the server. A server needs this value to be large enough to hold any of its resource names, including any information that might be passed in the query part of a GET request.

This directive gives the server administrator greater control over abnormal client request behavior, which may be useful for avoiding some forms of denial-of-service attacks.

For example:

Limit (in bytes) on maximum size of an XML-based request body. A value of 0 will disable any checking.

Example:

The Location directive limits the scope of the enclosed directives by URL. It is similar to the Directory directive, and starts a subsection which is terminated with a </Location> directive. Location sections are processed in the order they appear in the configuration file, after the Directory sections and .htaccess files are read, and after the Files sections.

Location sections operate completely outside the filesystem. This has several consequences. Most importantly, Location directives should not be used to control access to filesystem locations. Since several different URLs may map to the same filesystem location, such access controls may by circumvented.

The enclosed directives will be applied to the request if the path component of the URL meets any of the following criteria:

• The specified location matches exactly the path component of the URL.
• The specified location, which ends in a forward slash, is a prefix of the path component of the URL (treated as a context root).
• The specified location, with the addition of a trailing slash, is a prefix of the path component of the URL (also treated as a context root).

In the example below, where no trailing slash is used, requests to /private1, /private1/ and /private1/file.txt will have the enclosed directives applied, but /private1other would not.

In the example below, where a trailing slash is used, requests to /private2/ and /private2/file.txt will have the enclosed directives applied, but /private2 and /private2other would not.

For all origin (non-proxy) requests, the URL to be matched is a URL-path of the form /path/. No scheme, hostname, port, or query string may be included. For proxy requests, the URL to be matched is of the form scheme://servername/path, and you must include the prefix.

The URL may use wildcards. In a wild-card string, ? matches any single character, and * matches any sequences of characters. Neither wildcard character matches a / in the URL-path.

Regular expressions can also be used, with the addition of the ~ character. For example:

would match URLs that contained the substring /extra/data or /special/data. The directive LocationMatch behaves identical to the regex version of Location, and is preferred, for the simple reason that ~ is hard to distinguish from - in many fonts.

The Location functionality is especially useful when combined with the SetHandler directive. For example, to enable status requests, but allow them only from browsers at example.com, you might use:

The LocationMatch directive limits the scope of the enclosed directives by URL, in an identical manner to Location. However, it takes a regular expression as an argument instead of a simple string. For example:

would match URLs that contained the substring /extra/data or /special/data.

LogLevel adjusts the verbosity of the messages recorded in the error logs (see ErrorLog directive). The following levels are available, in order of decreasing significance:

When a particular level is specified, messages from all other levels of higher significance will be reported as well. E.g., when LogLevel info is specified, then messages with log levels of notice and warn will also be posted.

Using a level of at least crit is recommended.

For example:

Specifying a level without a module name will reset the level for all modules to that level. Specifying a level with a module name will set the level for that module only. It is possible to use the module source file name, the module identifier, or the module identifier with the trailing _module omitted as module specification. This means the following three specifications are equivalent:

It is also possible to change the level per directory:

The MaxKeepAliveRequests directive limits the number of requests allowed per connection when KeepAlive is on. If it is set to 0, unlimited requests will be allowed. We recommend that this setting be kept to a high value for maximum server performance.

For example:

The Mutex directive sets the mechanism, and optionally the lock file location, that httpd and modules use to serialize access to resources. Specify default as the first argument to change the settings for all mutexes; specify a mutex name (see table below) as the first argument to override defaults only for that mutex.

The Mutex directive is typically used in the following exceptional situations:

• change the mutex mechanism when the default mechanism selected by APR has a functional or performance problem
• change the directory used by file-based mutexes when the default directory does not support locking

The following mutex mechanisms are available:

• default | yes

  This selects the default locking implementation, as determined by APR. The default locking implementation can be displayed by running httpd with the -V option.

• none | no

  This effectively disables the mutex, and is only allowed for a mutex if the module indicates that it is a valid choice. Consult the module documentation for more information.

• posixsem

  This is a mutex variant based on a Posix semaphore.

  The semaphore ownership is not recovered if a thread in the process holding the mutex segfaults, resulting in a hang of the web server.

• sysvsem

  This is a mutex variant based on a SystemV IPC semaphore.

  It is possible to "leak" SysV semaphores if processes crash before the semaphore is removed.

  The semaphore API allows for a denial of service attack by any CGIs running under the same uid as the webserver (i.e., all CGIs, unless you use something like suexec or cgiwrapper).

• sem

  This selects the "best" available semaphore implementation, choosing between Posix and SystemV IPC semaphores, in that order.

• pthread

  This is a mutex variant based on cross-process Posix thread mutexes.

  On most systems, if a child process terminates abnormally while holding a mutex that uses this implementation, the server will deadlock and stop responding to requests. When this occurs, the server will require a manual restart to recover.

  Solaris is a notable exception as it provides a mechanism which usually allows the mutex to be recovered after a child process terminates abnormally while holding a mutex.

  If your system implements the pthread_mutexattr_setrobust_np() function, you may be able to use the pthread option safely.

• fcntl:/path/to/mutex

  This is a mutex variant where a physical (lock-)file and the fcntl() function are used as the mutex.

  When multiple mutexes based on this mechanism are used within multi-threaded, multi-process environments, deadlock errors (EDEADLK) can be reported for valid mutex operations if fcntl() is not thread-aware, such as on Solaris.

• flock:/path/to/mutex

  This is similar to the fcntl:/path/to/mutex method with the exception that the flock() function is used to provide file locking.

• file:/path/to/mutex

  This selects the "best" available file locking implementation, choosing between fcntl and flock, in that order.

Most mechanisms are only available on selected platforms, where the underlying platform and APR support it. Mechanisms which aren't available on all platforms are posixsem, sysvsem, sem, pthread, fcntl, flock, and file.

With the file-based mechanisms fcntl and flock, the path, if provided, is a directory where the lock file will be created. The default directory is httpd's run-time file directory relative to ServerRoot. Always use a local disk filesystem for /path/to/mutex and never a directory residing on a NFS- or AFS-filesystem. The basename of the file will be the mutex type, an optional instance string provided by the module, and unless the OmitPID keyword is specified, the process id of the httpd parent process will be appended to to make the file name unique, avoiding conflicts when multiple httpd instances share a lock file directory. For example, if the mutex name is mpm-accept and the lock file directory is /var/httpd/locks, the lock file name for the httpd instance with parent process id 12345 would be /var/httpd/locks/mpm-accept.12345.

The following table documents the names of mutexes used by httpd and bundled modules.

The OmitPID keyword suppresses the addition of the httpd parent process id from the lock file name.

In the following example, the mutex mechanism for the MPM accept mutex will be changed from the compiled-in default to fcntl, with the associated lock file created in directory /var/httpd/locks. The mutex mechanism for all other mutexes will be changed from the compiled-in default to sysvsem.

A single NameVirtualHost directive identifies a set of identical virtual hosts on which the server will further select from on the basis of the hostname requested by the client. The NameVirtualHost directive is a required directive if you want to configure name-based virtual hosts.

This directive, and the corresponding VirtualHost, must be qualified with a port number if the server supports both HTTP and HTTPS connections.

Although addr can be a hostname, it is recommended that you always use an IP address or a wildcard. A wildcard NameVirtualHost matches only virtualhosts that also have a literal wildcard as their argument.

In cases where a firewall or other proxy receives the requests and forwards them on a different IP address to the server, you must specify the IP address of the physical interface on the machine which will be servicing the requests.

In the example below, requests received on interface 192.0.2.1 and port 80 will only select among the first two virtual hosts. Requests received on port 80 on any other interface will only select among the third and fourth virtual hosts. In the common case where the interface isn't important to the mapping, only the "*:80" NameVirtualHost and VirtualHost directives are necessary.

If no matching virtual host is found, then the first listed virtual host that matches the IP address and port will be used.

IPv6 addresses must be enclosed in square brackets, as shown in the following example:

The Options directive controls which server features are available in a particular directory.

option can be set to None, in which case none of the extra features are enabled, or one or more of the following:

All

All options except for MultiViews. This is the default setting.

ExecCGI

Execution of CGI scripts using mod_cgi is permitted.

FollowSymLinks

The server will follow symbolic links in this directory.

Even though the server follows the symlink it does not change the pathname used to match against Directory sections.

Note also, that this option gets ignored if set inside a Location section.

Omitting this option should not be considered a security restriction, since symlink testing is subject to race conditions that make it circumventable.

Includes

Server-side includes provided by mod_include are permitted.

IncludesNOEXEC

Server-side includes are permitted, but the #exec cmd and #exec cgi are disabled. It is still possible to #include virtual CGI scripts from ScriptAliased directories.

Indexes

If a URL which maps to a directory is requested, and there is no DirectoryIndex (e.g., index.html) in that directory, then mod_autoindex will return a formatted listing of the directory.

MultiViews

Content negotiated "MultiViews" are allowed using mod_negotiation.

This option gets ignored if set anywhere other than Directory, as mod_negotiation needs real resources to compare against and evaluate from.

SymLinksIfOwnerMatch

The server will only follow symbolic links for which the target file or directory is owned by the same user id as the link.

This option gets ignored if set inside a Location section.

This option should not be considered a security restriction, since symlink testing is subject to race conditions that make it circumventable.

Normally, if multiple Options could apply to a directory, then the most specific one is used and others are ignored; the options are not merged. (See how sections are merged.) However if all the options on the Options directive are preceded by a + or - symbol, the options are merged. Any options preceded by a + are added to the options currently in force, and any options preceded by a - are removed from the options currently in force.

For example, without any + and - symbols:

then only Includes will be set for the /web/docs/spec directory. However if the second Options directive uses the + and - symbols:

then the options FollowSymLinks and Includes are set for the /web/docs/spec directory.

The default in the absence of any other settings is All.

This directive specifies the protocol used for a specific listening socket. The protocol is used to determine which module should handle a request, and to apply protocol specific optimizations with the AcceptFilter directive.

You only need to set the protocol if you are running on non-standard ports, otherwise http is assumed for port 80 and https for port 443.

For example, if you are running https on a non-standard port, specify the protocol explicitly:

You can also specify the protocol using the Listen directive.

Takes 1 or 2 parameters. The first parameter sets the soft resource limit for all processes and the second parameter sets the maximum resource limit. Either parameter can be a number, or max to indicate to the server that the limit should be set to the maximum allowed by the operating system configuration. Raising the maximum resource limit requires that the server is running as root, or in the initial startup phase.

This applies to processes forked off from Apache httpd children servicing requests, not the Apache httpd children themselves. This includes CGI scripts and SSI exec commands, but not any processes forked off from the Apache httpd parent such as piped logs.

CPU resource limits are expressed in seconds per process.

Takes 1 or 2 parameters. The first parameter sets the soft resource limit for all processes and the second parameter sets the maximum resource limit. Either parameter can be a number, or max to indicate to the server that the limit should be set to the maximum allowed by the operating system configuration. Raising the maximum resource limit requires that the server is running as root, or in the initial startup phase.

This applies to processes forked off from Apache httpd children servicing requests, not the Apache httpd children themselves. This includes CGI scripts and SSI exec commands, but not any processes forked off from the Apache httpd parent such as piped logs.

Memory resource limits are expressed in bytes per process.

Takes 1 or 2 parameters. The first parameter sets the soft resource limit for all processes and the second parameter sets the maximum resource limit. Either parameter can be a number, or max to indicate to the server that the limit should be set to the maximum allowed by the operating system configuration. Raising the maximum resource limit requires that the server is running as root, or in the initial startup phase.

This applies to processes forked off from Apache httpd children servicing requests, not the Apache httpd children themselves. This includes CGI scripts and SSI exec commands, but not any processes forked off from the Apache httpd parent such as piped logs.

Process limits control the number of processes per user.

This directive is used to control how Apache httpd finds the interpreter used to run CGI scripts. The default setting is Script. This causes Apache httpd to use the interpreter pointed to by the shebang line (first line, starting with #!) in the script. On Win32 systems this line usually looks like:

or, if perl is in the PATH, simply:

Setting ScriptInterpreterSource Registry will cause the Windows Registry tree HKEY_CLASSES_ROOT to be searched using the script file extension (e.g., .pl) as a search key. The command defined by the registry subkey Shell\ExecCGI\Command or, if it does not exist, by the subkey Shell\Open\Command is used to open the script file. If the registry keys cannot be found, Apache httpd falls back to the behavior of the Script option.

The option Registry-Strict which is new in Apache HTTP Server 2.0 does the same thing as Registry but uses only the subkey Shell\ExecCGI\Command. The ExecCGI key is not a common one. It must be configured manually in the windows registry and hence prevents accidental program calls on your system.

mod_status with ExtendedStatus On displays the actual request being handled. For historical purposes, only 63 characters of the request are actually stored for display purposes. This directive controls whether the 1st 63 characters are stored (the previous behavior and the default) or if the last 63 characters are. This is only applicable, of course, if the length of the request is 64 characters or greater.

If Apache httpd is handling GET /disk1/storage/apache/htdocs/images/imagestore1/food/apples.jpg HTTP/1.1 mod_status displays as follows:

The ServerAdmin sets the contact address that the server includes in any error messages it returns to the client. If the httpd doesn't recognize the supplied argument as an URL, it assumes, that it's an email-address and prepends it with mailto: in hyperlink targets. However, it's recommended to actually use an email address, since there are a lot of CGI scripts that make that assumption. If you want to use an URL, it should point to another server under your control. Otherwise users may not be able to contact you in case of errors.

It may be worth setting up a dedicated address for this, e.g.

as users do not always mention that they are talking about the server!

The ServerAlias directive sets the alternate names for a host, for use with name-based virtual hosts. The ServerAlias may include wildcards, if appropriate.

The ServerName directive sets the request scheme, hostname and port that the server uses to identify itself. This is used when creating redirection URLs.

Additionally, ServerName is used (possibly in conjunction with ServerAlias) to uniquely identify a virtual host, when using name-based virtual hosts.

For example, if the name of the machine hosting the web server is simple.example.com, but the machine also has the DNS alias www.example.com and you wish the web server to be so identified, the following directive should be used:

The ServerName directive may appear anywhere within the definition of a server. However, each appearance overrides the previous appearance (within that server).

If no ServerName is specified, then the server attempts to deduce the hostname by performing a reverse lookup on the IP address. If no port is specified in the ServerName, then the server will use the port from the incoming request. For optimal reliability and predictability, you should specify an explicit hostname and port using the ServerName directive.

If you are using name-based virtual hosts, the ServerName inside a VirtualHost section specifies what hostname must appear in the request's Host: header to match this virtual host.

Sometimes, the server runs behind a device that processes SSL, such as a reverse proxy, load balancer or SSL offload appliance. When this is the case, specify the https:// scheme and the port number to which the clients connect in the ServerName directive to make sure that the server generates the correct self-referential URLs.

See the description of the UseCanonicalName and UseCanonicalPhysicalPort directives for settings which determine whether self-referential URLs (e.g., by the mod_dir module) will refer to the specified port, or to the port number given in the client's request.

The ServerPath directive sets the legacy URL pathname for a host, for use with name-based virtual hosts.

The ServerRoot directive sets the directory in which the server lives. Typically it will contain the subdirectories conf/ and logs/. Relative paths in other configuration directives (such as Include or LoadModule, for example) are taken as relative to this directory.

The ServerSignature directive allows the configuration of a trailing footer line under server-generated documents (error messages, mod_proxy ftp directory listings, mod_info output, ...). The reason why you would want to enable such a footer line is that in a chain of proxies, the user often has no possibility to tell which of the chained servers actually produced a returned error message.

The Off setting, which is the default, suppresses the footer line (and is therefore compatible with the behavior of Apache-1.2 and below). The On setting simply adds a line with the server version number and ServerName of the serving virtual host, and the EMail setting additionally creates a "mailto:" reference to the ServerAdmin of the referenced document.

After version 2.0.44, the details of the server version number presented are controlled by the ServerTokens directive.

This directive controls whether Server response header field which is sent back to clients includes a description of the generic OS-type of the server as well as information about compiled-in modules.

ServerTokens Full (or not specified)

Server sends (e.g.): Server: Apache/2.4.1 (Unix) PHP/4.2.2 MyMod/1.2

ServerTokens Prod[uctOnly]

Server sends (e.g.): Server: Apache

ServerTokens Major

Server sends (e.g.): Server: Apache/2

ServerTokens Minor

Server sends (e.g.): Server: Apache/2.4

ServerTokens Min[imal]

Server sends (e.g.): Server: Apache/2.4.1

ServerTokens OS

Server sends (e.g.): Server: Apache/2.4.1 (Unix)

This setting applies to the entire server, and cannot be enabled or disabled on a virtualhost-by-virtualhost basis.

After version 2.0.44, this directive also controls the information presented by the ServerSignature directive.

When placed into an .htaccess file or a Directory or Location section, this directive forces all matching files to be parsed through the handler given by handler-name. For example, if you had a directory you wanted to be parsed entirely as imagemap rule files, regardless of extension, you might put the following into an .htaccess file in that directory:

Another example: if you wanted to have the server display a status report whenever a URL of http://servername/status was called, you might put the following into httpd.conf:

You can override an earlier defined SetHandler directive by using the value None.

Note: because SetHandler overrides default handlers, normal behaviour such as handling of URLs ending in a slash (/) as directories or index files is suppressed.

The SetInputFilter directive sets the filter or filters which will process client requests and POST input when they are received by the server. This is in addition to any filters defined elsewhere, including the AddInputFilter directive.

If more than one filter is specified, they must be separated by semicolons in the order in which they should process the content.

The SetOutputFilter directive sets the filters which will process responses from the server before they are sent to the client. This is in addition to any filters defined elsewhere, including the AddOutputFilter directive.

For example, the following configuration will process all files in the /www/data/ directory for server-side includes.

If more than one filter is specified, they must be separated by semicolons in the order in which they should process the content.

The TimeOut directive defines the length of time Apache httpd will wait for I/O in various circumstances:

1. When reading data from the client, the length of time to wait for a TCP packet to arrive if the read buffer is empty.
2. When writing data to the client, the length of time to wait for an acknowledgement of a packet if the send buffer is full.
3. In mod_cgi, the length of time to wait for output from a CGI script.
4. In mod_ext_filter, the length of time to wait for output from a filtering process.
5. In mod_proxy, the default timeout value if ProxyTimeout is not configured.

This directive overrides the behavior of TRACE for both the core server and mod_proxy. The default TraceEnable on permits TRACE requests per RFC 2616, which disallows any request body to accompany the request. TraceEnable off causes the core server and mod_proxy to return a 405 (Method not allowed) error to the client.

Finally, for testing and diagnostic purposes only, request bodies may be allowed using the non-compliant TraceEnable extended directive. The core (as an origin server) will restrict the request body to 64k (plus 8k for chunk headers if Transfer-Encoding: chunked is used). The core will reflect the full headers and all chunk headers with the response body. As a proxy server, the request body is not restricted to 64k.

Undoes the effect of a Define or of passing a -D argument to httpd.

This directive can be used to toggle the use of IfDefine sections without needing to alter -D arguments in any startup scripts.

In many situations Apache httpd must construct a self-referential URL -- that is, a URL that refers back to the same server. With UseCanonicalName On Apache httpd will use the hostname and port specified in the ServerName directive to construct the canonical name for the server. This name is used in all self-referential URLs, and for the values of SERVER_NAME and SERVER_PORT in CGIs.

With UseCanonicalName Off Apache httpd will form self-referential URLs using the hostname and port supplied by the client if any are supplied (otherwise it will use the canonical name, as defined above). These values are the same that are used to implement name-based virtual hosts, and are available with the same clients. The CGI variables SERVER_NAME and SERVER_PORT will be constructed from the client supplied values as well.

An example where this may be useful is on an intranet server where you have users connecting to the machine using short names such as www. You'll notice that if the users type a shortname, and a URL which is a directory, such as http://www/splat, without the trailing slash then Apache httpd will redirect them to http://www.domain.com/splat/. If you have authentication enabled, this will cause the user to have to authenticate twice (once for www and once again for www.domain.com -- see the FAQ on this subject for more information). But if UseCanonicalName is set Off, then Apache httpd will redirect to http://www/splat/.

There is a third option, UseCanonicalName DNS, which is intended for use with mass IP-based virtual hosting to support ancient clients that do not provide a Host: header. With this option Apache httpd does a reverse DNS lookup on the server IP address that the client connected to in order to work out self-referential URLs.

In many situations Apache httpd must construct a self-referential URL -- that is, a URL that refers back to the same server. With UseCanonicalPhysicalPort On Apache httpd will, when constructing the canonical port for the server to honor the UseCanonicalName directive, provide the actual physical port number being used by this request as a potential port. With UseCanonicalPhysicalPort Off Apache httpd will not ever use the actual physical port number, instead relying on all configured information to construct a valid port number.

VirtualHost and </VirtualHost> are used to enclose a group of directives that will apply only to a particular virtual host. Any directive that is allowed in a virtual host context may be used. When the server receives a request for a document on a particular virtual host, it uses the configuration directives enclosed in the VirtualHost section. Addr can be:

• The IP address of the virtual host;
• A fully qualified domain name for the IP address of the virtual host (not recommended);
• The character *, which is used only in combination with NameVirtualHost * to match all IP addresses; or
• The string _default_, which is used only with IP virtual hosting to catch unmatched IP addresses.

IPv6 addresses must be specified in square brackets because the optional port number could not be determined otherwise. An IPv6 example is shown below:

Each Virtual Host must correspond to a different IP address, different port number or a different host name for the server, in the former case the server machine must be configured to accept IP packets for multiple addresses. (If the machine does not have multiple network interfaces, then this can be accomplished with the ifconfig alias command -- if your OS supports it).

When using IP-based virtual hosting, the special name _default_ can be specified in which case this virtual host will match any IP address that is not explicitly listed in another virtual host. In the absence of any _default_ virtual host the "main" server config, consisting of all those definitions outside any VirtualHost section, is used when no IP-match occurs.

You can specify a :port to change the port that is matched. If unspecified then it defaults to the same port as the most recent Listen statement of the main server. You may also specify :* to match all ports on that address. (This is recommended when used with _default_.)

A ServerName should be specified inside each VirtualHost block. If it is absent, the ServerName from the "main" server configuration will be inherited.

If no matching virtual host is found, then the first listed virtual host that matches the IP address will be used. As a consequence, the first listed virtual host is the default virtual host.