21.5. Configuration Directives in httpd.conf

21.5. Configuration Directives in httpd.conf

The Apache HTTP Server configuration file is /etc/httpd/conf/httpd.conf. The httpd.conf file is well-commented and mostly self-explanatory. The default configuration works for most situations; however, it is a good idea to become familiar some of the more important configuration options.

Warning

With the release of Apache HTTP Server 2.2, many configuration options have changed. If migrating from version 1.3 to 2.2, please firstly read Section 21.2.2, “Migrating Apache HTTP Server 1.3 Configuration Files to 2.0”.

21.5.1. General Configuration Tips

If configuring the Apache HTTP Server, edit /etc/httpd/conf/httpd.conf and then either reload, restart, or stop and start the httpd process as outlined in Section 21.3, “Starting and Stopping httpd”.

Before editing httpd.conf, make a copy the original file. Creating a backup makes it easier to recover from mistakes made while editing the configuration file.

If a mistake is made and the Web server does not work correctly, first review recently edited passages in httpd.conf to verify there are no typos.

Next look in the Web server's error log, /var/log/httpd/error_log. The error log may not be easy to interpret, depending on your level of expertise. However, the last entries in the error log should provide useful information.

The following subsections contain a list of short descriptions for many of the directives included in httpd.conf. These descriptions are not exhaustive. For more information, refer to the Apache documentation online at http://httpd.apache.org/docs/2.2/.

For more information about mod_ssl directives, refer to the documentation online at http://httpd.apache.org/docs/2.2/mod/mod_ssl.html.

AccessFileName

AccessFileName names the file which the server should use for access control information in each directory. The default is .htaccess.

Immediately after the AccessFileName directive, a set of Files tags apply access control to any file beginning with a .ht. These directives deny Web access to any .htaccess files (or other files which begin with .ht) for security reasons.

Action

Action specifies a MIME content type and CGI script pair, so that when a file of that media type is requested, a particular CGI script is executed.

AddDescription

When using FancyIndexing as an IndexOptions parameter, the AddDescription directive can be used to display user-specified descriptions for certain files or file types in a server generated directory listing. The AddDescription directive supports listing specific files, wildcard expressions, or file extensions.

AddEncoding

AddEncoding names file name extensions which should specify a particular encoding type. AddEncoding can also be used to instruct some browsers to uncompress certain files as they are downloaded.

AddHandler

AddHandler maps file extensions to specific handlers. For example, the cgi-script handler can be matched with the extension .cgi to automatically treat a file ending with .cgi as a CGI script. The following is a sample AddHandler directive for the .cgi extension.

AddHandler cgi-script .cgi

This directive enables CGIs outside of the cgi-bin to function in any directory on the server which has the ExecCGI option within the directories container. Refer to Directory for more information about setting the ExecCGI option for a directory.

In addition to CGI scripts, the AddHandler directive is used to process server-parsed HTML and image-map files.

AddIcon

AddIcon specifies which icon to show in server generated directory listings for files with certain extensions. For example, the Web server is set to show the icon binary.gif for files with .bin or .exe extensions.

AddIconByEncoding

This directive names icons which are displayed by files with MIME encoding in server generated directory listings. For example, by default, the Web server shows the compressed.gif icon next to MIME encoded x-compress and x-gzip files in server generated directory listings.

AddIconByType

This directive names icons which are displayed next to files with MIME types in server generated directory listings. For example, the server shows the icon text.gif next to files with a mime-type of text, in server generated directory listings.

AddLanguage

AddLanguage associates file name extensions with specific languages. This directive is useful for Apache HTTP Servers which serve content in multiple languages based on the client Web browser's language settings.

AddType

Use the AddType directive to define or override a default MIME type and file extension pairs. The following example directive tells the Apache HTTP Server to recognize the .tgz file extension:

AddType application/x-tar .tgz

Alias

The Alias setting allows directories outside the DocumentRoot directory to be accessible. Any URL ending in the alias automatically resolves to the alias' path. By default, one alias for an icons/ directory is already set up. An icons/ directory can be accessed by the Web server, but the directory is not in the DocumentRoot.

Allow

Allow specifies which client can access a given directory. The client can be all, a domain name, an IP address, a partial IP address, a network/netmask pair, and so on. The DocumentRoot directory is configured to Allow requests from all, meaning everyone has access.

AllowOverride

The AllowOverride directive sets whether any Options can be overridden by the declarations in an .htaccess file. By default, both the root directory and the DocumentRoot are set to allow no .htaccess overrides.

BrowserMatch

The BrowserMatch directive allows the server to define environment variables and take appropriate actions based on the User-Agent HTTP header field — which identifies the client's Web browser type. By default, the Web server uses BrowserMatch to deny connections to specific browsers with known problems and also to disable keepalives and HTTP header flushes for browsers that are known to have problems with those actions.

Cache Directives

A number of commented cache directives are supplied by the default Apache HTTP Server configuration file. In most cases, uncommenting these lines by removing the hash mark (#) from the beginning of the line is sufficient. The following, however, is a list of some of the more important cache-related directives.

  • CacheEnable — Specifies whether the cache is a disk, memory, or file descriptor cache. By default CacheEnable configures a disk cache for URLs at or below /.

  • CacheRoot — Specifies the name of the directory containing cached files. The default CacheRoot is the /var/httpd/proxy/ directory.

  • CacheSize — Specifies how much space the cache can use in kilobytes. The default CacheSize is 5 KB.

The following is a list of some of the other common cache-related directives.

  • CacheMaxExpire — Specifies how long HTML documents are retained (without a reload from the originating Web server) in the cache. The default is 24 hours (86400 seconds).

  • CacheLastModifiedFactor — Specifies the creation of an expiry (expiration) date for a document which did not come from its originating server with its own expiry set. The default CacheLastModifiedFactor is set to 0.1, meaning that the expiry date for such documents equals one-tenth of the amount of time since the document was last modified.

  • CacheDefaultExpire — Specifies the expiry time in hours for a document that was received using a protocol that does not support expiry times. The default is set to 1 hour (3600 seconds).

  • NoProxy — Specifies a space-separated list of subnets, IP addresses, domains, or hosts whose content is not cached. This setting is most useful for Intranet sites.

CacheNegotiatedDocs

By default, the Web server asks proxy servers not to cache any documents which were negotiated on the basis of content (that is, they may change over time or because of the input from the requester). If CacheNegotiatedDocs is set to on, this function is disabled and proxy servers are allowed to cache such documents.

CustomLog

CustomLog identifies the log file and the log file format. By default, the access log is recorded to the /var/log/httpd/access_log file while errors are recorded in the /var/log/httpd/error_log file.

The default CustomLog format is the combined log file format, as illustrated here:

remotehost rfc931 user date "request" status bytes referrer user-agent

DefaultIcon

DefaultIcon specifies the icon displayed in server generated directory listings for files which have no other icon specified. The unknown.gif image file is the default.

DefaultType

DefaultType sets a default content type for the Web server to use for documents whose MIME types cannot be determined. The default is text/plain.

Deny

Deny works similar to Allow, except it specifies who is denied access. The DocumentRoot is not configured to Deny requests from anyone by default.

Directory

<Directory /path/to/directory> and </Directory> tags create a container used to enclose a group of configuration directives which apply only to a specific directory and its subdirectories. Any directive which is applicable to a directory may be used within Directory tags.

By default, very restrictive parameters are applied to the root directory (/), using the Options (refer to Options) and AllowOverride (refer to AllowOverride) directives. Under this configuration, any directory on the system which needs more permissive settings has to be explicitly given those settings.

In the default configuration, another Directory container is configured for the DocumentRoot which assigns less rigid parameters to the directory tree so that the Apache HTTP Server can access the files residing there.

The Directory container can be also be used to configure additional cgi-bin directories for server-side applications outside of the directory specified in the ScriptAlias directive (refer to ScriptAlias for more information).

To accomplish this, the Directory container must set the ExecCGI option for that directory.

For example, if CGI scripts are located in /home/my_cgi_directory, add the following Directory container to the httpd.conf file:

<Directory /home/my_cgi_directory>     Options +ExecCGI </Directory>

Next, the AddHandler directive must be uncommented to identify files with the .cgi extension as CGI scripts. Refer to AddHandler for instructions on setting AddHandler.

For this to work, permissions for CGI scripts, and the entire path to the scripts, must be set to 0755.

DirectoryIndex

The DirectoryIndex is the default page served by the server when a user requests an index of a directory by specifying a forward slash (/) at the end of the directory name.

When a user requests the page http://example/this_directory/, they get either the DirectoryIndex page, if it exists, or a server-generated directory list. The default for DirectoryIndex is index.html and the index.html.var type map. The server tries to find either of these files and returns the first one it finds. If it does not find one of these files and Options Indexes is set for that directory, the server generates and returns a listing, in HTML format, of the subdirectories and files within the directory, unless the directory listing feature is turned off.

DocumentRoot

DocumentRoot is the directory which contains most of the HTML files which are served in response to requests. The default DocumentRoot, for both the non-secure and secure Web servers, is the /var/www/html directory. For example, the server might receive a request for the following document:

http://example.com/foo.html

The server looks for the following file in the default directory:

/var/www/html/foo.html

To change the DocumentRoot so that it is not shared by the secure and the non-secure Web servers, refer to Section 21.7, “Virtual Hosts”.

ErrorDocument

The ErrorDocument directive associates an HTTP response code with a message or a URL to be sent back to the client. By default, the Web server outputs a simple and usually cryptic error message when an error occurs. The ErrorDocument directive forces the Web server to instead output a customized message or page.

Important

To be valid, the message must be enclosed in a pair of double quotes ".

ErrorLog

ErrorLog specifies the file where server errors are logged. By default, this directive is set to /var/log/httpd/error_log.

ExtendedStatus

The ExtendedStatus directive controls whether Apache generates basic (off) or detailed server status information (on), when the server-status handler is called. The server-status handler is called using Location tags. More information on calling server-status is included in Location.

Group

Specifies the group name of the Apache HTTP Server processes.

This directive has been deprecated for the configuration of virtual hosts.

By default, Group is set to apache.

HeaderName

HeaderName names the file which, if it exists in the directory, is prepended to the start of server generated directory listings. Like ReadmeName, the server tries to include it as an HTML document if possible or in plain text if not.

HostnameLookups

HostnameLookups can be set to on, off, or double. If HostnameLookups is set to on, the server automatically resolves the IP address for each connection. Resolving the IP address means that the server makes one or more connections to a DNS server, adding processing overhead. If HostnameLookups is set to double, the server performs a double-reverse DNS look up adding even more processing overhead.

To conserve resources on the server, HostnameLookups is set to off by default.

If hostnames are required in server log files, consider running one of the many log analyzer tools that perform the DNS lookups more efficiently and in bulk when rotating the Web server log files.

IfDefine

The IfDefine tags surround configuration directives that are applied if the "test" stated in the IfDefine tag is true. The directives are ignored if the test is false.

The test in the IfDefine tags is a parameter name (for example, HAVE_PERL). If the parameter is defined, meaning that it is provided as an argument to the server's start-up command, then the test is true. In this case, when the Web server is started, the test is true and the directives contained in the IfDefine tags are applied.

IfModule

<IfModule> and </IfModule> tags create a conditional container which are only activated if the specified module is loaded. Directives within the IfModule container are processed under one of two conditions. The directives are processed if the module contained within the starting <IfModule> tag is loaded. Or, if an exclamation point ! appears before the module name, the directives are processed only if the module specified in the <IfModule> tag is not loaded.

For more information about Apache HTTP Server modules, refer to Section 21.6, “Adding Modules”.

Include

Include allows other configuration files to be included at runtime.

The path to these configuration files can be absolute or relative to the ServerRoot.

Important

For the server to use individually packaged modules, such as mod_ssl, mod_perl, and php, the following directive must be included in Section 1: Global Environment of httpd.conf:

Include conf.d/*.conf

IndexIgnore

IndexIgnore lists file extensions, partial file names, wildcard expressions, or full file names. The Web server does not include any files which match any of those parameters in server generated directory listings.

IndexOptions

IndexOptions controls the appearance of server generated directing listings, by adding icons, file descriptions, and so on. If Options Indexes is set (refer to Options), the Web server generates a directory listing when the Web server receives an HTTP request for a directory without an index.

First, the Web server looks in the requested directory for a file matching the names listed in the DirectoryIndex directive (usually, index.html). If an index.html file is not found, Apache HTTP Server creates an HTML directory listing of the requested directory. The appearance of this directory listing is controlled, in part, by the IndexOptions directive.

The default configuration turns on FancyIndexing. This means that a user can re-sort a directory listing by clicking on column headers. Another click on the same header switches from ascending to descending order. FancyIndexing also shows different icons for different files, based upon file extensions.

The AddDescription option, when used in conjunction with FancyIndexing, presents a short description for the file in server generated directory listings.

IndexOptions has a number of other parameters which can be set to control the appearance of server generated directories. The IconHeight and IconWidth parameters require the server to include HTML HEIGHT and WIDTH tags for the icons in server generated webpages. The IconsAreLinks parameter combines the graphical icon with the HTML link anchor, which contains the URL link target.

KeepAlive

KeepAlive sets whether the server allows more than one request per connection and can be used to prevent any one client from consuming too much of the server's resources.

By default Keepalive is set to off. If Keepalive is set to on and the server becomes very busy, the server can quickly spawn the maximum number of child processes. In this situation, the server slows down significantly. If Keepalive is enabled, it is a good idea to set the the KeepAliveTimeout low (refer to KeepAliveTimeout for more information about the KeepAliveTimeout directive) and monitor the /var/log/httpd/error_log log file on the server. This log reports when the server is running out of child processes.

KeepAliveTimeout

KeepAliveTimeout sets the number of seconds the server waits after a request has been served before it closes the connection. Once the server receives a request, the Timeout directive applies instead. The KeepAliveTimeout directive is set to 15 seconds by default.

LanguagePriority

LanguagePriority sets precedence for different languages in case the client Web browser has no language preference set.

Listen

The Listen command identifies the ports on which the Web server accepts incoming requests. By default, the Apache HTTP Server is set to listen to port 80 for non-secure Web communications and (in the /etc/httpd/conf.d/ssl.conf file which defines any secure servers) to port 443 for secure Web communications.

If the Apache HTTP Server is configured to listen to a port under 1024, only the root user can start it. For port 1024 and above, httpd can be started as a regular user.

The Listen directive can also be used to specify particular IP addresses over which the server accepts connections.

LoadModule

LoadModule is used to load Dynamic Shared Object (DSO) modules. More information on the Apache HTTP Server's DSO support, including instructions for using the LoadModule directive, can be found in Section 21.6, “Adding Modules”. Note, the load order of the modules is no longer important with Apache HTTP Server 2.0. Refer to Section 21.2.2.1.3, “Dynamic Shared Object (DSO) Support” for more information about Apache HTTP Server 2.0 DSO support.

Location

The <Location> and </Location> tags create a container in which access control based on URL can be specified.

For instance, to allow people connecting from within the server's domain to see status reports, use the following directives:

<Location /server-status>     SetHandler server-status      Order deny,allow     Deny from all      Allow from <.example.com> </Location>

Replace <.example.com> with the second-level domain name for the Web server.

To provide server configuration reports (including installed modules and configuration directives) to requests from inside the domain, use the following directives:

<Location /server-info>     SetHandler server-info     Order deny,allow     Deny from all     Allow from <.example.com> </Location>

Again, replace <.example.com> with the second-level domain name for the Web server.

LogFormat

The LogFormat directive configures the format of the various Web server log files. The actual LogFormat used depends on the settings given in the CustomLog directive (refer to CustomLog).

The following are the format options if the CustomLog directive is set to combined:

%h (remote host's IP address or hostname)

Lists the remote IP address of the requesting client. If HostnameLookups is set to on, the client hostname is recorded unless it is not available from DNS.

%l (rfc931)

Not used. A hyphen - appears in the log file for this field.

%u (authenticated user)

Lists the username of the user recorded if authentication was required. Usually, this is not used, so a hyphen - appears in the log file for this field.

%t (date)

Lists the date and time of the request.

%r (request string)

Lists the request string exactly as it came from the browser or client.

%s (status)

Lists the HTTP status code which was returned to the client host.

%b (bytes)

Lists the size of the document.

%\"%{Referer}i\" (referrer)

Lists the URL of the webpage which referred the client host to Web server.

%\"%{User-Agent}i\" (user-agent)

Lists the type of Web browser making the request.

LogLevel

LogLevel sets how verbose the error messages in the error logs are. LogLevel can be set (from least verbose to most verbose) to emerg, alert, crit, error, warn, notice, info, or debug. The default LogLevel is warn.

MaxKeepAliveRequests

This directive sets the maximum number of requests allowed per persistent connection. The Apache Project recommends a high setting, which improves the server's performance. MaxKeepAliveRequests is set to 100 by default, which should be appropriate for most situations.

NameVirtualHost

The NameVirtualHost directive associates an IP address and port number, if necessary, for any name-based virtual hosts. Name-based virtual hosting allows one Apache HTTP Server to serve different domains without using multiple IP addresses.

Note

Name-based virtual hosts only work with non-secure HTTP connections. If using virtual hosts with a secure server, use IP address-based virtual hosts instead.

To enable name-based virtual hosting, uncomment the NameVirtualHost configuration directive and add the correct IP address. Then add additional VirtualHost containers for each virtual host as is necessary for your configuration.

Options

The Options directive controls which server features are available in a particular directory. For example, under the restrictive parameters specified for the root directory, Options is only set to the FollowSymLinks directive. No features are enabled, except that the server is allowed to follow symbolic links in the root directory.

By default, in the DocumentRoot directory, Options is set to include Indexes and FollowSymLinks. Indexes permits the server to generate a directory listing for a directory if no DirectoryIndex (for example, index.html) is specified. FollowSymLinks allows the server to follow symbolic links in that directory.

Note

Options statements from the main server configuration section need to be replicated to each VirtualHost container individually. Refer to VirtualHost for more information.

Order

The Order directive controls the order in which allow and deny directives are evaluated. The server is configured to evaluate the Allow directives before the Deny directives for the DocumentRoot directory.

PidFile

PidFile names the file where the server records its process ID (PID). By default the PID is listed in /var/run/httpd.pid.

Proxy

<Proxy *> and </Proxy> tags create a container which encloses a group of configuration directives meant to apply only to the proxy server. Many directives which are allowed within a <Directory> container may also be used within <Proxy> container.

ProxyRequests

To configure the Apache HTTP Server to function as a proxy server, remove the hash mark (#) from the beginning of the <IfModule mod_proxy.c> line, the ProxyRequests, and each line in the <Proxy> stanza. Set the ProxyRequests directive to On, and set which domains are allowed access to the server in the Allow from directive of the <Proxy> stanza.

ReadmeName

ReadmeName names the file which, if it exists in the directory, is appended to the end of server generated directory listings. The Web server first tries to include the file as an HTML document and then tries to include it as plain text. By default, ReadmeName is set to README.html.

Redirect

When a webpage is moved, Redirect can be used to map the file location to a new URL. The format is as follows:

Redirect /<old-path>/<file-name> http://<current-domain>/<current-path>/<file-name>

In this example, replace <old-path> with the old path information for <file-name> and <current-domain> and <current-path> with the current domain and path information for <file-name>.

In this example, any requests for <file-name> at the old location is automatically redirected to the new location.

For more advanced redirection techniques, use the mod_rewrite module included with the Apache HTTP Server. For more information about configuring the mod_rewrite module, refer to the Apache Software Foundation documentation online at http://httpd.apache.org/docs/2.2/mod/mod_rewrite.html.

ScriptAlias

The ScriptAlias directive defines where CGI scripts are located. Generally, it is not good practice to leave CGI scripts within the DocumentRoot, where they can potentially be viewed as text documents. For this reason, a special directory outside of the DocumentRoot directory containing server-side executables and scripts is designated by the ScriptAlias directive. This directory is known as a cgi-bin and is set to /var/www/cgi-bin/ by default.

It is possible to establish directories for storing executables outside of the cgi-bin/ directory. For instructions on doing so, refer to AddHandler and Directory.

ServerAdmin

Sets the ServerAdmin directive to the email address of the Web server administrator. This email address shows up in error messages on server-generated Web pages, so users can report a problem by sending email to the server administrator.

By default, ServerAdmin is set to root@localhost.

A common way to set up ServerAdmin is to set it to webmaster@example.com. Once set, alias webmaster to the person responsible for the Web server in /etc/aliases and run /usr/bin/newaliases.

ServerName

ServerName specifies a hostname and port number (matching the Listen directive) for the server. The ServerName does not need to match the machine's actual hostname. For example, the Web server may be www.example.com, but the server's hostname is actually foo.example.com. The value specified in ServerName must be a valid Domain Name Service (DNS) name that can be resolved by the system — do not make something up.

The following is a sample ServerName directive:

ServerName www.example.com:80

When specifying a ServerName, be sure the IP address and server name pair are included in the /etc/hosts file.

ServerRoot

The ServerRoot directive specifies the top-level directory containing website content. By default, ServerRoot is set to "/etc/httpd" for both secure and non-secure servers.

ServerSignature

The ServerSignature directive adds a line containing the Apache HTTP Server server version and the ServerName to any server-generated documents, such as error messages sent back to clients. ServerSignature is set to on by default.

ServerSignature can be set to EMail which adds a mailto:ServerAdmin HTML tag to the signature line of auto-generated responses. ServerSignature can also be set to Off to stop Apache from sending out its version number and module information. Please also check the ServerTokens settings.

ServerTokens

The ServerTokens directive determines if the Server response header field sent back to clients should include details of the Operating System type and information about compiled-in modules. By default, ServerTokens is set to Full which sends information about the Operating System type and compiled-in modules. Setting the ServerTokens to Prod sends the product name only and is recommended as many hackers check information in the Server header when scanning for vulnerabilities. You can also set the ServerTokens to Min (minimal) or to OS (operating system).

SuexecUserGroup

The SuexecUserGroup directive, which originates from the mod_suexec module, allows the specification of user and group execution privileges for CGI programs. Non-CGI requests are still processed with the user and group specified in the User and Group directives.

Note

From version 2.0, the SuexecUserGroup directive replaced the Apache HTTP Server 1.3 configuration of using the User and Group directives inside the configuration of VirtualHosts sections.

Timeout

Timeout defines, in seconds, the amount of time that the server waits for receipts and transmissions during communications. Timeout is set to 300 seconds by default, which is appropriate for most situations.

TypesConfig

TypesConfig names the file which sets the default list of MIME type mappings (file name extensions to content types). The default TypesConfig file is /etc/mime.types. Instead of editing /etc/mime.types, the recommended way to add MIME type mappings is to use the AddType directive.

For more information about AddType, refer to AddType.

UseCanonicalName

When set to on, this directive configures the Apache HTTP Server to reference itself using the value specified in the ServerName and Port directives. When UseCanonicalName is set to off, the server instead uses the value used by the requesting client when referring to itself.

UseCanonicalName is set to off by default.

User

The User directive sets the username of the server process and determines what files the server is allowed to access. Any files inaccessible to this user are also inaccessible to clients connecting to the Apache HTTP Server.

By default User is set to apache.

This directive has been deprecated for the configuration of virtual hosts.

Note

For security reasons, the Apache HTTP Server does not run as the root user.

UserDir

UserDir is the subdirectory within each user's home directory where they should place personal HTML files which are served by the Web server. This directive is set to disable by default.

The name for the subdirectory is set to public_html in the default configuration. For example, the server might receive the following request:

http://example.com/~username/foo.html

The server would look for the file:

/home/username/public_html/foo.html

In the above example, /home/username/ is the user's home directory (note that the default path to users' home directories may vary).

Make sure that the permissions on the users' home directories are set correctly. Users' home directories must be set to 0711. The read (r) and execute (x) bits must be set on the users' public_html directories (0755 also works). Files that are served in a users' public_html directories must be set to at least 0644.

VirtualHost

<VirtualHost> and </VirtualHost> tags create a container outlining the characteristics of a virtual host. The VirtualHost container accepts most configuration directives.

A commented VirtualHost container is provided in httpd.conf, which illustrates the minimum set of configuration directives necessary for each virtual host. Refer to Section 21.7, “Virtual Hosts” for more information about virtual hosts.

Note

The default SSL virtual host container now resides in the file /etc/httpd/conf.d/ssl.conf.

21.5.2. Configuration Directives for SSL

The directives in /etc/httpd/conf.d/ssl.conf file can be configured to enable secure Web communications using SSL and TLS.

SetEnvIf

SetEnvIf sets environment variables based on the headers of incoming connections. It is not solely an SSL directive, though it is present in the supplied /etc/httpd/conf.d/ssl.conf file. It's purpose in this context is to disable HTTP keepalive and to allow SSL to close the connection without a closing notification from the client browser. This setting is necessary for certain browsers that do not reliably shut down the SSL connection.

For more information on other directives within the SSL configuration file, refer to the following URLs:

Note

In most cases, SSL directives are configured appropriately during the installation of Red Hat Enterprise Linux. Be careful when altering Apache HTTP Secure Server directives, misconfiguration can lead to security vulnerabilities.

21.5.3. MPM Specific Server-Pool Directives

As explained in Section 21.2.2.1.2, “Server-Pool Size Regulation”, the responsibility for managing characteristics of the server-pool falls to a module group called MPMs under Apache HTTP Server 2.0. The characteristics of the server-pool differ depending upon which MPM is used. For this reason, an IfModule container is necessary to define the server-pool for the MPM in use.

By default, Apache HTTP Server 2.0 defines the server-pool for both the prefork and worker MPMs.

The following section list directives found within the MPM-specific server-pool containers.

MaxClients

MaxClients sets a limit on the total number of server processes, or simultaneously connected clients, that can run at one time. The main purpose of this directive is to keep a runaway Apache HTTP Server from crashing the operating system. For busy servers this value should be set to a high value. The server's default is set to 150 regardless of the MPM in use. However, it is not recommended that the value for MaxClients exceeds 256 when using the prefork MPM.

MaxRequestsPerChild

MaxRequestsPerChild sets the total number of requests each child server process serves before the child dies. The main reason for setting MaxRequestsPerChild is to avoid long-lived process induced memory leaks. The default MaxRequestsPerChild for the prefork MPM is 4000 and for the worker MPM is 0.

MinSpareServers and MaxSpareServers

These values are only used with the prefork MPM. They adjust how the Apache HTTP Server dynamically adapts to the perceived load by maintaining an appropriate number of spare server processes based on the number of incoming requests. The server checks the number of servers waiting for a request and kills some if there are more than MaxSpareServers or creates some if the number of servers is less than MinSpareServers.

The default MinSpareServers value is 5; the default MaxSpareServers value is 20. These default settings should be appropriate for most situations. Be careful not to increase the MinSpareServers to a large number as doing so creates a heavy processing load on the server even when traffic is light.

MinSpareThreads and MaxSpareThreads

These values are only used with the worker MPM. They adjust how the Apache HTTP Server dynamically adapts to the perceived load by maintaining an appropriate number of spare server threads based on the number of incoming requests. The server checks the number of server threads waiting for a request and kills some if there are more than MaxSpareThreads or creates some if the number of servers is less than MinSpareThreads.

The default MinSpareThreads value is 25; the default MaxSpareThreads value is 75. These default settings should be appropriate for most situations. The value for MaxSpareThreads must be greater than or equal to the sum of MinSpareThreads and ThreadsPerChild, else the Apache HTTP Server automatically corrects it.

StartServers

The StartServers directive sets how many server processes are created upon startup. Since the Web server dynamically kills and creates server processes based on traffic load, it is not necessary to change this parameter. The Web server is set to start 8 server processes at startup for the prefork MPM and 2 for the worker MPM.

ThreadsPerChild

This value is only used with the worker MPM. It sets the number of threads within each child process. The default value for this directive is 25.


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