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An In-Depth Discussion of Virtual Host Matching - Apache HTTP Server Version 2.4








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Apache HTTP Server Version 2.4



Apache > HTTP Server > Documentation > Version 2.4 > Virtual HostsAn In-Depth Discussion of Virtual Host Matching

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    This document attempts to explain
    exactly what Apache HTTP Server does when deciding what virtual host to
    serve a request from.

    Most users should read about 
    Name-based vs. IP-based Virtual Hosts to decide which type they
    want to use, then read more about name-based
    or IP-based virtualhosts, and then see
    some examples.

    If you want to understand all the details, then you can
    come back to this page.


 Configuration File
 Virtual Host Matching
 Tips
See alsoIP-based Virtual Host SupportName-based Virtual Hosts SupportVirtual Host examples for common setupsDynamically configured mass virtual hostingComments


Configuration File

    There is a main server which consists of all the
    definitions appearing outside of
    <VirtualHost> sections.

    There are virtual
    servers, called vhosts, which are defined by
    <VirtualHost>
    sections.

    Each VirtualHost directive includes one
    or more addresses and optional ports.

    Hostnames can be used in place of IP addresses in a virtual
    host definition, but they are resolved at startup and if any name
    resolutions fail, those virtual host definitions are ignored.
    This is, therefore, not recommended.

    The address can be specified as
    *, which will match a request if no
    other vhost has the explicit address on which the request was
    received. 

    The address appearing in the VirtualHost
    directive can have an optional port. If the port is unspecified,
    it is treated as a wildcard port, which can also be indicated
    explicitly using *.
    The wildcard port matches any port.

    (Port numbers specified in the VirtualHost directive do
    not influence what port numbers Apache will listen on, they only control
    which VirtualHost will be selected to handle a request.
    Use the Listen directive to
    control the addresses and ports on which the server listens.)
    

    Collectively the
    entire set of addresses (including multiple
    results from DNS lookups) are called the vhost's
    address set.

    Apache automatically discriminates on the
    basis of the HTTP Host header supplied by the client
    whenever the most specific match for an IP address and port combination
    is listed in multiple virtual hosts.

    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, the server
    attempts to deduce it from the server's IP address.

    The first name-based vhost in the configuration file for a
    given IP:port pair is significant because it is used for all
    requests received on that address and port for which no other
    vhost for that IP:port pair has a matching ServerName or
    ServerAlias.  It is also used for all SSL connections if the
    server does not support Server Name Indication.

    The complete list of names in the VirtualHost
    directive are treated just like a (non wildcard) ServerAlias 
    (but are not overridden by any ServerAlias statement).

    For every vhost various default values are set. In
    particular:

    
      If a vhost has no ServerAdmin,
      Timeout,
      KeepAliveTimeout,
      KeepAlive,
      MaxKeepAliveRequests,
      ReceiveBufferSize,
      or SendBufferSize
      directive then the respective value is inherited from the
      main server. (That is, inherited from whatever the final
      setting of that value is in the main server.)

      The "lookup defaults" that define the default directory
      permissions for a vhost are merged with those of the
      main server. This includes any per-directory configuration
      information for any module.

      The per-server configs for each module from the
      main server are merged into the vhost server.
    

    Essentially, the main server is treated as "defaults" or a
    "base" on which to build each vhost. But the positioning of
    these main server definitions in the config file is largely
    irrelevant -- the entire config of the main server has been
    parsed when this final merging occurs. So even if a main server
    definition appears after a vhost definition it might affect the
    vhost definition.

    If the main server has no ServerName at this
    point, then the hostname of the machine that httpd
    is running on is used instead. We will call the main server address
    set those IP addresses returned by a DNS lookup on the
    ServerName of the main server.

    For any undefined ServerName fields, a
    name-based vhost defaults to the address given first in the
    VirtualHost statement defining the vhost.

    Any vhost that includes the magic _default_
    wildcard is given the same ServerName as the
    main server.



Virtual Host Matching

    The server determines which vhost to use for a request as
    follows:

    IP address lookup

    When the connection is first received on some address and port,
    the server looks for all the VirtualHost definitions
    that have the same IP address and port.

    If there are no exact matches for the address and port, then
    wildcard (*) matches are considered.

    If no matches are found, the request is served by the
    main server.

    If there are VirtualHost definitions for
    the IP address, the next step is to decide if we have to
    deal with an IP-based or a name-based vhost.

    

    IP-based vhost

    If there is exactly one VirtualHost directive
    listing the IP address and port combination that was determined
    to be the best match, no further actions are performed and
    the request is served from the matching vhost.

    

    Name-based vhost

    If there are multiple VirtualHost directives listing
    the IP address and port combination that was determined to be the
    best match, the "list" in the remaining steps refers to the list of vhosts
    that matched, in the order they were in the configuration file.

    If the connection is using SSL, the server supports Server Name Indication, and
    the SSL client handshake includes the TLS extension with the
    requested hostname, then that hostname is used below just like the
    Host: header would be used on a non-SSL connection.
    Otherwise, the first name-based vhost whose address matched is
    used for SSL connections.  This is significant because the
    vhost determines which certificate the server will use for the
    connection.

    If the request contains a Host: header field, the
    list is searched for the first vhost with a matching
    ServerName or ServerAlias, and the
    request is served from that vhost. A Host: header
    field can contain a port number, but Apache always ignores it and
    matches against the real port to which the client sent the
    request.

    The first vhost in the config
    file with the specified IP address has the highest priority
    and catches any request to an unknown server name, or a request
    without a Host: header field (such as a HTTP/1.0
    request).

    

    Persistent connections

    The IP lookup described above is only done once for a
    particular TCP/IP session while the name lookup is done on
    every request during a KeepAlive/persistent
    connection. In other words, a client may request pages from
    different name-based vhosts during a single persistent
    connection.

    

    Absolute URI

    If the URI from the request is an absolute URI, and its
    hostname and port match the main server or one of the
    configured virtual hosts and match the address and
    port to which the client sent the request, then the
    scheme/hostname/port prefix is stripped off and the remaining
    relative URI is served by the corresponding main server or
    virtual host. If it does not match, then the URI remains
    untouched and the request is taken to be a proxy request.


Observations

    
      Name-based virtual hosting is a process applied after
      the server has selected the best matching IP-based virtual
      host.

      If you don't care what IP address the client has connected to, use a
      "*" as the address of every virtual host, and name-based virtual hosting
      is applied across all configured virtual hosts.

      ServerName and ServerAlias
      checks are never performed for an IP-based vhost.

      Only the ordering of
      name-based vhosts for a specific address set is significant.
      The one name-based vhosts that comes first in the
      configuration file has the highest priority for its
      corresponding address set.

      Any port in the Host: header field is never used during the
      matching process. Apache always uses the real port to which
      the client sent the request.

      If two vhosts have an address in common, those common addresses
      act as name-based virtual hosts implicitly.  This is new behavior as of
      2.3.11.

      The main server is only used to serve a request if the IP
      address and port number to which the client connected
      does not match any vhost (including a
      * vhost). In other words, the main server
      only catches a request for an unspecified address/port
      combination (unless there is a _default_ vhost
      which matches that port).

      You should never specify DNS names in
      VirtualHost directives because it will force
      your server to rely on DNS to boot. Furthermore it poses a
      security threat if you do not control the DNS for all the
      domains listed. There's more
      information available on this and the next two
      topics.

      ServerName should always be set for each
      vhost. Otherwise A DNS lookup is required for each
      vhost.
      
      



Tips

    In addition to the tips on the DNS Issues page, here are
    some further tips:

    
      Place all main server definitions before any
      VirtualHost definitions. (This is to aid the
      readability of the configuration -- the post-config merging
      process makes it non-obvious that definitions mixed in around
      virtual hosts might affect all virtual hosts.)
    



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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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