2.4.x/docs/manual/vhosts/name-based.xml

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<manualpage metafile="name-based.xml.meta">
<parentdocument href="./">Virtual Hosts</parentdocument>
<title>Name-based Virtual Host Support</title>

<summary>
    <p>This document describes when and how to use name-based virtual hosts.</p>
</summary>

<seealso><a href="ip-based.html">IP-based Virtual Host Support</a></seealso>
<seealso><a href="details.html">An In-Depth Discussion of Virtual Host Matching</a></seealso>
<seealso><a href="mass.html">Dynamically configured mass virtual hosting</a></seealso>
<seealso><a href="examples.html">Virtual Host examples for common setups</a></seealso>

<section id="namevip"><title>Name-based vs. IP-based Virtual Hosts</title>

    <p><a href="ip-based.html">IP-based virtual hosts</a> use the IP address of the connection to
    determine the correct virtual host to serve.  Therefore you need to
    have a separate IP address for each host.</p>

    <p>With name-based virtual hosting, the server relies on the client to
    report the hostname as part of the HTTP headers.  Using this technique,
    many different hosts can share the same IP address.</p>

    <p>Name-based virtual hosting is usually simpler, since you need
    only configure your DNS server to map each hostname to the correct
    IP address and then configure the Apache HTTP Server to recognize
    the different hostnames. Name-based virtual hosting also eases
    the demand for scarce IP addresses. Therefore you should use
    name-based virtual hosting unless you are using equipment
    that explicitly demands IP-based hosting.  Historical reasons for
    IP-based virtual hosting based on client support are no longer
    applicable to a general-purpose web server.</p>

    <p> Name-based virtual hosting builds off of the IP-based virtual host
    selection algorithm, meaning that searches for the proper server name
    occur only between virtual hosts that have the best IP-based address.</p>

</section>

<section id="alg"><title>How the server selects the proper name-based virtual host</title>

    <p>It is important to recognize that the first step in name-based virtual
    host resolution is IP-based resolution.  Name-based virtual host
    resolution only chooses the most appropriate name-based virtual host
    after narrowing down the candidates to the best IP-based match.  Using a wildcard (*)
    for the IP address in all of the VirtualHost directives makes this
    IP-based mapping irrelevant.</p>

    <p>When a request arrives, the server will find the best (most specific) matching
    <directive type="section" module="core">VirtualHost</directive> argument based on
    the IP address and port used by the request.  If there is more than one virtual host
    containing this best-match address and port combination, Apache will further
    compare the <directive module="core" >ServerName</directive> and <directive
    module="core">ServerAlias</directive> directives to the server name
    present in the request.</p>

    <p>If you omit the <directive module="core">ServerName</directive> 
    directive from any name-based virtual host, the server will default
    to a fully qualified domain name (FQDN) derived from the system hostname.
    This implicitly set server name can lead to counter-intuitive virtual host
    matching and is discouraged.</p>
 
    <section id="defaultvhost"><title>The default name-based vhost for an IP and port combination </title>
    <p> If no matching ServerName or ServerAlias is found in the set of
    virtual hosts containing the most specific matching IP address and port
    combination, then <strong>the first listed virtual host</strong> that
    matches that will be used.</p></section>
</section>

<section id="using"><title>Using Name-based Virtual Hosts</title>

<related>
    <modulelist>
    <module>core</module>
    </modulelist>

    <directivelist>
        <directive module="core">DocumentRoot</directive>
        <directive module="core">ServerAlias</directive>
        <directive module="core">ServerName</directive>
        <directive module="core" type="section">VirtualHost</directive>
    </directivelist>
</related>

    <p>The first step is to create a <directive type="section"
    module="core">VirtualHost</directive> block for
    each different host that you would like to serve.  Inside each <directive type="section"
    module="core">VirtualHost</directive> block, you will need at minimum a
    <directive module="core">ServerName</directive> directive to designate
    which host is served and a <directive module="core">DocumentRoot</directive>
    directive to show where in the filesystem the content for that host
    lives.</p>

    <note><title>Main host goes away</title>
        <p> Any request that doesn't match an existing <directive type="section"
        module="core">VirtualHost</directive> is handled by the global
        server configuration, regardless of the hostname or ServerName.</p>

        <p> When you add a name-based virtual host to an existing server, and
        the virtual host arguments match preexisting IP and port combinations,
        requests will now be handled by an explicit virtual host.  In this case,
        it's usually wise to create a <a href="#defaultvhost">default virtual host</a>
        with a <directive module="core">ServerName</directive> matching that of
        the base server.  New domains on the same interface and port, but
        requiring separate configurations,  can then be added as subsequent (non-default)
        virtual hosts.</p>
    </note>

    <note><title>ServerName inheritance</title>
       <p> It is best to always explicitly list a  <directive module="core"
       >ServerName</directive> in every name-based virtual host.</p>
       <p>If a <directive module="core">VirtualHost</directive> doesn't specify 
       a <directive module="core">ServerName</directive>, a server name will be 
       inherited from the base server configuration.  If no server name was 
       specified globally, one is detected at startup through reverse DNS resolution
       of the first listening address.  In either case, this inherited server name
       will influence name-based virtual host resolution, so it is best to always
       explicitly list a  <directive module="core">ServerName</directive> in every
       name-based virtual host.</p>
    </note>

    <p>For example, suppose that you are serving the domain
    <code>www.example.com</code> and you wish to add the virtual host
    <code>other.example.com</code>, which points at the same IP address.
    Then you simply add the following to <code>httpd.conf</code>:</p>

    <highlight language="config">
&lt;VirtualHost *:80&gt;
    # This first-listed virtual host is also the default for *:80
    ServerName www.example.com
    ServerAlias example.com 
    DocumentRoot "/www/domain"
&lt;/VirtualHost&gt;

&lt;VirtualHost *:80&gt;
    ServerName other.example.com
    DocumentRoot "/www/otherdomain"
&lt;/VirtualHost&gt;
    </highlight>

    <p>You can alternatively specify an explicit IP address in place of the
    <code>*</code> in <directive type="section" module="core"
    >VirtualHost</directive> directives. For example, you might want to do this
    in order to run some name-based virtual hosts on one IP address, and either
    IP-based, or another set of name-based virtual hosts on another address.</p>

    <p>Many servers want to be accessible by more than one name. This is
    possible with the <directive module="core">ServerAlias</directive>
    directive, placed inside the <directive type="section" module="core"
    >VirtualHost</directive> section. For example in the first <directive
    type="section" module="core">VirtualHost</directive> block above, the
    <directive module="core">ServerAlias</directive> directive indicates that
    the listed names are other names which people can use to see that same
    web site:</p>

    <highlight language="config">
        ServerAlias example.com *.example.com
    </highlight>

    <p>then requests for all hosts in the <code>example.com</code> domain will
    be served by the <code>www.example.com</code> virtual host. The wildcard
    characters <code>*</code> and <code>?</code> can be used to match names.
    Of course, you can't just make up names and place them in <directive
    module="core">ServerName</directive> or <code>ServerAlias</code>. You must
    first have your DNS server properly configured to map those names to an IP
    address associated with your server.</p>

    <p>Name-based virtual hosts for the best-matching set of  <directive
    type="section" module="core">virtualhost</directive>s are processed 
    in the order they appear in the configuration.  The first matching <directive
    module="core">ServerName</directive> or <directive module="core"
    >ServerAlias</directive> is used, with no different precedence for wildcards
    (nor for ServerName vs. ServerAlias).  </p>

    <p>The complete list of names in the <directive module="core"
    >VirtualHost</directive>
    directive are treated just like a (non wildcard) 
    <directive module="core">ServerAlias</directive>.</p>

    <p>Finally, you can fine-tune the configuration of the virtual hosts
    by placing other directives inside the <directive type="section"
    module="core">VirtualHost</directive> containers. Most directives can be
    placed in these containers and will then change the configuration only of
    the relevant virtual host. To find out if a particular directive is allowed,
    check the <a href="../mod/directive-dict.html#Context">Context</a> of the
    directive. Configuration directives set in the <em>main server context</em>
    (outside any <directive type="section" module="core">VirtualHost</directive>
    container) will be used only if they are not overridden by the virtual host
    settings.</p>

</section>

</manualpage>