Glossary - Apache HTTP Server Version 2.4

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Apache HTTP Server Version 2.4

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    This glossary defines some of the common terminology related to Apache in
      particular, and web serving in general. More information on each concept
      is provided in the links.


    Access Control
    The restriction of access to network realms. In an Apache context
      usually the restriction of access to certain URLs. See:  Authentication, Authorization, and Access

    An unambiguous formula or set of rules for solving a problem in a finite
      number of steps. Algorithms for encryption are usually called

        eXtension Tool (apxs)
    A perl script that aids in compiling module sources into Dynamic Shared Objects
      (DSOs) and helps install them in the
      Apache Web server.
      See: Manual Page: apxs

    Apache Portable Runtime (APR)
    A set of libraries providing many of the basic interfaces
      between the server and the operating system.  APR is developed
      parallel to the Apache HTTP Server as an independent project.
      See: Apache Portable Runtime

    The positive identification of a network entity such as a server, a
      client, or a user.
      See: Authentication, Authorization, and Access

    A data record used for authenticating network entities such
      as a server or a client. A certificate contains X.509 information pieces
      about its owner (called the subject) and the signing Certification Authority (called
      the issuer), plus the owner's public
      key and the
      signature made by the CA. Network entities verify these signatures
      using CA certificates.
      See: SSL/TLS Encryption

    Certificate Signing Request
    An unsigned certificate for
      submission to a Certification
      Authority, which signs it with the Private Key of their CA
      Certificate. Once the CSR is signed, it becomes a real
      See: SSL/TLS Encryption

    Certification Authority
    A trusted third party whose purpose is to sign certificates for network
      entities it has authenticated using secure means. Other network entities
      can check the signature to verify that a CA has authenticated the bearer
      of a certificate.
      See: SSL/TLS Encryption

    An algorithm or system for data encryption. Examples are DES, IDEA, RC4,
      See: SSL/TLS Encryption

    The result after Plaintext is
      passed through a Cipher. See: SSL/TLS Encryption

        Gateway Interface (CGI)
    A standard definition for an interface between a web server and an
      external program that allows the external program to service requests.
      There is an Informational
      RFC which covers the specifics.
      See: Dynamic Content with CGI

    Configuration Directive
    See: Directive

    A text file containing Directives
      that control the configuration of Apache.
      See: Configuration Files

    An HTTP method for proxying raw data
      channels over HTTP. It can be used to encapsulate other protocols, such as
      the SSL protocol.

    An area in the configuration
      files where certain types of directives are allowed.
      See: Terms Used to Describe
      Apache Directives

    An encrypted text block that validates a certificate or other file. A
      Certification Authority
      creates a signature by generating a hash of the Public Key
      embedded in a Certificate, then encrypting the hash with its own
      Private Key. Only the CA's public key can decrypt the signature,
      verifying that the CA has authenticated the network entity that owns the
      See: SSL/TLS Encryption

    A configuration command that controls one or more aspects of Apache's
      behavior.  Directives are placed in the Configuration File
    See: Directive Index

        Shared Object (DSO)
    Modules compiled separately from the
      Apache httpd binary that can be loaded on-demand.
      See: Dynamic Shared Object Support

        Variable (env-variable)
    Named variables managed by the operating system shell and used to store
      information and communicate between programs.  Apache also contains
      internal variables that are referred to as environment variables, but are
      stored in internal Apache structures, rather than in the shell
      See: Environment Variables in Apache

    Diminished in cryptographic strength (and security) in order to comply
      with the United States' Export Administration Regulations (EAR).
      Export-crippled cryptographic software is limited to a small key size,
      resulting in Ciphertext which usually can be decrypted by brute
      See: SSL/TLS Encryption

    A process that is applied to data that is sent or received by the
      server.  Input filters process data sent by the client to the server,
      while output filters process documents on the server before they are sent
      to the client.  For example, the INCLUDES output filter
      processes documents for Server Side
      See: Filters

    Fully-Qualified Domain-Name
    The unique name of a network entity, consisting of a hostname and a
      domain name that can resolve to an IP address. For example,
      www is a hostname, is a domain name,
      and is a fully-qualified domain name.

    An internal Apache representation of the action to be performed when a
      file is called. Generally, files have implicit handlers, based on the file
      type. Normally, all files are simply served by the server, but certain
      file types are "handled" separately.  For example, the
      cgi-script handler designates files to be processed as
      See: Apache's Handler Use

    A mathematical one-way, irreversible algorithm generating a string with
      fixed-length from another string of any length. Different input strings
      will usually produce different hashes (depending on the hash function).

    The part of the HTTP request and
      response that is sent before the actual content, and that contains
      meta-information describing the content.

    A configuration file that
      is placed inside the web tree and applies configuration directives to the directory where it is
      placed and all sub-directories.  Despite its name, this file can hold
      almost any type of directive, not just access-control directives.
      See: Configuration Files

    The main Apache configuration
      file.  The default location is
      /usr/local/apache2/conf/httpd.conf, but it may be moved using
      run-time or compile-time configuration.
      See: Configuration Files

    HyperText Transfer Protocol
    The standard transmission protocol used on the World Wide Web.  Apache
      implements version 1.1 of the protocol, referred to as HTTP/1.1 and
      defined by RFC 2616.

    The HyperText Transfer Protocol (Secure), the standard encrypted
      communication mechanism on the World Wide Web. This is actually just HTTP
      over SSL.
      See: SSL/TLS Encryption

    In the context of HTTP, an action to
      perform on a resource, specified on the request line by the client.  Some
      of the methods available in HTTP are GET, POST,
      and PUT.

    Message Digest
    A hash of a message, which can be used to verify that the contents of
      the message have not been altered in transit.
      See: SSL/TLS Encryption

    A way to describe the kind of document being transmitted.  Its name
      comes from that fact that its format is borrowed from the Multipurpose
      Internet Mail Extensions.  It consists of a major type and a minor type,
      separated by a slash.  Some examples are text/html,
      image/gif, and application/octet-stream.  In
      HTTP, the MIME-type is transmitted in the Content-Type
      See: mod_mime

    An independent part of a program.  Much of Apache's functionality is
      contained in modules that you can choose to include or exclude.  Modules
      that are compiled into the Apache httpd binary are
      called static modules, while modules that are stored
      separately and can be optionally loaded at run-time are called
      dynamic modules or DSOs.
      Modules that are included by default
      are called base modules. Many modules are available for Apache
      that are not distributed as part of the Apache HTTP Server tarball.  These are referred to as
      third-party modules.
      See: Module Index

    Module Magic
      Number (MMN)
    Module Magic Number is a constant defined in the Apache source code that
      is associated with binary compatibility of modules. It is changed when
      internal Apache structures, function calls and other significant parts of
      API change in such a way that binary compatibility cannot be guaranteed
      any more. On MMN change, all third party modules have to be at least
      recompiled, sometimes even slightly changed in order to work with the new
      version of Apache.

    The Open Source toolkit for SSL/TLS

    Pass Phrase
    The word or phrase that protects private key files. It prevents
      unauthorized users from encrypting them. Usually it's just the secret
      encryption/decryption key used for Ciphers.
      See: SSL/TLS Encryption

    The unencrypted text.

    Private Key
    The secret key in a Public Key
      Cryptography system, used to decrypt incoming messages and
      sign outgoing ones.
      See: SSL/TLS Encryption

    An intermediate server that sits between the client and the origin
        server.  It accepts requests from clients, transmits those requests
      on to the origin server, and then returns the response from the origin
      server to the client.  If several clients request the same content, the
      proxy can deliver that content from its cache, rather than requesting it
      from the origin server each time, thereby reducing response time.
      See: mod_proxy

    Public Key
    The publicly available key in a Public Key Cryptography system,
      used to encrypt messages bound for its owner and to decrypt signatures
      made by its owner.
      See: SSL/TLS Encryption

    Public Key Cryptography
    The study and application of asymmetric encryption systems, which use
      one key for encryption and another for decryption. A corresponding pair of
      such keys constitutes a key pair. Also called Asymmetric Cryptography.
      See: SSL/TLS Encryption

    Regular Expression
    A way of describing a pattern in text - for example, "all the words that
      begin with the letter A" or "every 10-digit phone number" or even "Every
      sentence with two commas in it, and no capital letter Q". Regular
      expressions are useful in Apache because they let you apply certain
      attributes against collections of files or resources in very flexible ways
      - for example, all .gif and .jpg files under any "images" directory could
      be written as "/images/.*(jpg|gif)$".  In places where
      regular expressions are used to replace strings, the special variables
      $1 ... $9 contain backreferences to the grouped parts (in parentheses) of
      the matched expression. The special variable $0 contains a backreference
      to the whole matched expression. To write a literal dollar sign in a
      replacement string, it can be escaped with a backslash. Historically, the
      variable & could be used as alias for $0 in some places. This is no
      longer possible since version 2.3.6.  Apache uses Perl Compatible Regular
      Expressions provided by the PCRE
      library.  You can find more documentation about PCRE's regular expression
      syntax at that site, or at

    Reverse Proxy
    A proxy server that appears to the client
      as if it is an origin server.  This is useful to hide the real
      origin server from the client for security reasons, or to load balance.

    Secure Sockets
        Layer (SSL)
    A protocol created by Netscape Communications Corporation for general
      communication authentication and encryption over TCP/IP networks.  The most
      popular usage is HTTPS, i.e. the HyperText Transfer Protocol (HTTP)
      over SSL.
      See: SSL/TLS Encryption

    Server Name
        Indication (SNI)
    An SSL function that allows passing the desired server
      hostname in the initial SSL handshake message, so that the web
      server can select the correct virtual host configuration to use
      in processing the SSL handshake.  It was added to SSL starting
      with the TLS extensions, RFC 3546.  
      See: the SSL FAQ
      and RFC 3546

    Server Side
        Includes (SSI)
    A technique for embedding processing directives inside HTML files.
      See: Introduction to Server Side Includes

    The context information of a communication in general.

    The original SSL/TLS implementation library developed by Eric A.

    Apache provides a subrequest API to modules that allows other
        filesystem or URL paths to be partially or fully evaluated by
        the server. Example consumers of this API are 
        mod_autoindex, and mod_include.

    The study and application of Ciphers that use a single secret key
      for both encryption and decryption operations.
      See: SSL/TLS Encryption

    A package of files gathered together using the tar utility.
      Apache distributions are stored in compressed tar archives or using

        Layer Security (TLS)
    The successor protocol to SSL, created by the Internet Engineering Task
      Force (IETF) for general communication authentication and encryption over
      TCP/IP networks. TLS version 1 is nearly identical with SSL version 3.
      See: SSL/TLS Encryption

        Resource Locator (URL)
    The name/address of a resource on the Internet.  This is the common
      informal term for what is formally called a Uniform Resource Identifier.
      URLs are usually made up of a scheme, like http or
      https, a hostname, and a path.  A URL for this page might

    Uniform Resource Identifier
    A compact string of characters for identifying an abstract or physical
      resource.  It is formally defined by RFC 2396.  URIs used on the
      world-wide web are commonly referred to as URLs.

    Virtual Hosting
    Serving multiple websites using a single instance of Apache.  IP
      virtual hosting differentiates between websites based on their IP
      address, while name-based virtual hosting uses only the name of the
      host and can therefore host many sites on the same IP address.
      See: Apache Virtual Host documentation

    An authentication certificate scheme recommended by the International
      Telecommunication Union (ITU-T) which is used for SSL/TLS authentication. See: SSL/TLS Encryption

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CommentsNotice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.

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