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| <title>An In-Depth Discussion of Virtual Host Matching - Apache HTTP Server</title> |
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| <p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p> |
| <p class="apache">Apache HTTP Server Version 2.0</p> |
| <img alt="" src="../images/feather.gif" /></div> |
| <div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div> |
| <div id="path"> |
| <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.0</a> > <a href="./">Virtual Hosts</a></div><div id="page-content"><div class="retired"><h4>Please note</h4> |
| <p>This document refers to the <strong>2.0</strong> version of Apache httpd, which <strong>is no longer maintained</strong>. Upgrade, and refer to the current version of httpd instead, documented at:</p> |
| <ul><li><a href="http://httpd.apache.org/docs/current/">Current release version of Apache HTTP Server documentation</a></li></ul><p>You may follow <a href="http://httpd.apache.org/docs/current/vhosts/details.html">this link</a> to go to the current version of this document.</p></div><div id="preamble"><h1>An In-Depth Discussion of Virtual Host Matching</h1> |
| <div class="toplang"> |
| <p><span>Available Languages: </span><a href="../en/vhosts/details.html" title="English"> en </a> | |
| <a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | |
| <a href="../tr/vhosts/details.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> |
| </div> |
| |
| |
| <p>The virtual host code was completely rewritten in |
| <strong>Apache 1.3</strong>. This document attempts to explain |
| exactly what Apache does when deciding what virtual host to |
| serve a hit from. With the help of the new |
| <code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code> |
| directive virtual host configuration should be a lot easier and |
| safer than with versions prior to 1.3.</p> |
| |
| <p>If you just want to <cite>make it work</cite> without |
| understanding how, here are <a href="examples.html">some |
| examples</a>.</p> |
| |
| </div> |
| <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#configparsing">Config File Parsing</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#hostmatching">Virtual Host Matching</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#tips">Tips</a></li> |
| </ul></div> |
| <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="configparsing" id="configparsing">Config File Parsing</a></h2> |
| |
| <p>There is a <em>main_server</em> which consists of all the |
| definitions appearing outside of |
| <code><VirtualHost></code> sections. There are virtual |
| servers, called <em>vhosts</em>, which are defined by |
| <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code> |
| sections.</p> |
| |
| <p>The directives |
| <code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code>, |
| <code class="directive"><a href="../mod/core.html#servername">ServerName</a></code>, |
| <code class="directive"><a href="../mod/core.html#serverpath">ServerPath</a></code>, |
| and <code class="directive"><a href="../mod/core.html#serveralias">ServerAlias</a></code> |
| can appear anywhere within the definition of a server. However, |
| each appearance overrides the previous appearance (within that |
| server).</p> |
| |
| <p>The default value of the <code>Listen</code> field for |
| main_server is 80. The main_server has no default |
| <code>ServerPath</code>, or <code>ServerAlias</code>. The |
| default <code>ServerName</code> is deduced from the server's IP |
| address.</p> |
| |
| <p>The main_server Listen directive has two functions. One |
| function is to determine the default network port Apache will |
| bind to. The second function is to specify the port number |
| which is used in absolute URIs during redirects.</p> |
| |
| <p>Unlike the main_server, vhost ports <em>do not</em> affect |
| what ports Apache listens for connections on.</p> |
| |
| <p>Each address appearing in the <code>VirtualHost</code> |
| directive can have an optional port. If the port is unspecified |
| it defaults to the value of the main_server's most recent |
| <code>Listen</code> statement. The special port <code>*</code> |
| indicates a wildcard that matches any port. Collectively the |
| entire set of addresses (including multiple <code>A</code> |
| record results from DNS lookups) are called the vhost's |
| <em>address set</em>.</p> |
| |
| <p>Unless a <code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code> |
| directive is used for a specific IP address the first vhost |
| with that address is treated as an IP-based vhost. The IP |
| address can also be the wildcard <code>*</code>.</p> |
| |
| <p>If name-based vhosts should be used a |
| <code>NameVirtualHost</code> directive <em>must</em> appear |
| with the IP address set to be used for the name-based vhosts. |
| In other words, you must specify the IP address that holds the |
| hostname aliases (CNAMEs) for your name-based vhosts via a |
| <code>NameVirtualHost</code> directive in your configuration |
| file.</p> |
| |
| <p>Multiple <code>NameVirtualHost</code> directives can be used |
| each with a set of <code>VirtualHost</code> directives but only |
| one <code>NameVirtualHost</code> directive should be used for |
| each specific IP:port pair.</p> |
| |
| <p>The ordering of <code>NameVirtualHost</code> and |
| <code>VirtualHost</code> directives is not important which |
| makes the following two examples identical (only the order of |
| the <code>VirtualHost</code> directives for <em>one</em> |
| address set is important, see below):</p> |
| |
| <table><tr> |
| <td><div class="example"><p><code> |
| NameVirtualHost 111.22.33.44<br /> |
| <VirtualHost 111.22.33.44><br /> |
| # server A<br /> |
| ...<br /> |
| </VirtualHost><br /> |
| <VirtualHost 111.22.33.44><br /> |
| # server B<br /> |
| ...<br /> |
| </VirtualHost><br /> |
| <br /> |
| NameVirtualHost 111.22.33.55<br /> |
| <VirtualHost 111.22.33.55><br /> |
| # server C<br /> |
| ...<br /> |
| </VirtualHost><br /> |
| <VirtualHost 111.22.33.55><br /> |
| # server D<br /> |
| ...<br /> |
| </VirtualHost> |
| </code></p></div></td> |
| <td><div class="example"><p><code> |
| <VirtualHost 111.22.33.44><br /> |
| # server A<br /> |
| </VirtualHost><br /> |
| <VirtualHost 111.22.33.55><br /> |
| # server C<br /> |
| ...<br /> |
| </VirtualHost><br /> |
| <VirtualHost 111.22.33.44><br /> |
| # server B<br /> |
| ...<br /> |
| </VirtualHost><br /> |
| <VirtualHost 111.22.33.55><br /> |
| # server D<br /> |
| ...<br /> |
| </VirtualHost><br /> |
| <br /> |
| NameVirtualHost 111.22.33.44<br /> |
| NameVirtualHost 111.22.33.55<br /> |
| <br /> |
| </code></p></div></td> |
| </tr></table> |
| |
| |
| <p>(To aid the readability of your configuration you should |
| prefer the left variant.)</p> |
| |
| <p>After parsing the <code>VirtualHost</code> directive, the |
| vhost server is given a default <code>Listen</code> equal to the |
| port assigned to the first name in its <code>VirtualHost</code> |
| directive.</p> |
| |
| <p>The complete list of names in the <code>VirtualHost</code> |
| directive are treated just like a <code>ServerAlias</code> (but |
| are not overridden by any <code>ServerAlias</code> statement) |
| if all names resolve to the same address set. Note that |
| subsequent <code>Listen</code> statements for this vhost will not |
| affect the ports assigned in the address set.</p> |
| |
| <p>During initialization a list for each IP address is |
| generated and inserted into an hash table. If the IP address is |
| used in a <code>NameVirtualHost</code> directive the list |
| contains all name-based vhosts for the given IP address. If |
| there are no vhosts defined for that address the |
| <code>NameVirtualHost</code> directive is ignored and an error |
| is logged. For an IP-based vhost the list in the hash table is |
| empty.</p> |
| |
| <p>Due to a fast hashing function the overhead of hashing an IP |
| address during a request is minimal and almost not existent. |
| Additionally the table is optimized for IP addresses which vary |
| in the last octet.</p> |
| |
| <p>For every vhost various default values are set. In |
| particular:</p> |
| |
| <ol> |
| <li>If a vhost has no <code class="directive"><a href="../mod/core.html#serveradmin">ServerAdmin</a></code>, |
| <code class="directive"><a href="../mod/core.html#timeout">Timeout</a></code>, |
| <code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code>, |
| <code class="directive"><a href="../mod/core.html#keepalive">KeepAlive</a></code>, |
| <code class="directive"><a href="../mod/core.html#maxkeepaliverequests">MaxKeepAliveRequests</a></code>, |
| <code class="directive"><a href="../mod/core.html#receivebuffersize">ReceiveBufferSize</a></code>, |
| or <code class="directive"><a href="../mod/core.html#sendbuffersize">SendBufferSize</a></code> |
| 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.)</li> |
| |
| <li>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.</li> |
| |
| <li>The per-server configs for each module from the |
| main_server are merged into the vhost server.</li> |
| </ol> |
| |
| <p>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.</p> |
| |
| <p>If the main_server has no <code>ServerName</code> at this |
| point, then the hostname of the machine that <code class="program"><a href="../programs/httpd.html">httpd</a></code> |
| is running on is used instead. We will call the <em>main_server address |
| set</em> those IP addresses returned by a DNS lookup on the |
| <code>ServerName</code> of the main_server.</p> |
| |
| <p>For any undefined <code>ServerName</code> fields, a |
| name-based vhost defaults to the address given first in the |
| <code>VirtualHost</code> statement defining the vhost.</p> |
| |
| <p>Any vhost that includes the magic <code>_default_</code> |
| wildcard is given the same <code>ServerName</code> as the |
| main_server.</p> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="hostmatching" id="hostmatching">Virtual Host Matching</a></h2> |
| |
| <p>The server determines which vhost to use for a request as |
| follows:</p> |
| |
| <h3><a name="hashtable" id="hashtable">Hash table lookup</a></h3> |
| |
| <p>When the connection is first made by a client, the IP |
| address to which the client connected is looked up in the |
| internal IP hash table.</p> |
| |
| <p>If the lookup fails (the IP address wasn't found) the |
| request is served from the <code>_default_</code> vhost if |
| there is such a vhost for the port to which the client sent the |
| request. If there is no matching <code>_default_</code> vhost |
| the request is served from the main_server.</p> |
| |
| <p>If the IP address is not found in the hash table then the |
| match against the port number may also result in an entry |
| corresponding to a <code>NameVirtualHost *</code>, which is |
| subsequently handled like other name-based vhosts.</p> |
| |
| <p>If the lookup succeeded (a corresponding list for the IP |
| address was found) the next step is to decide if we have to |
| deal with an IP-based or a name-base vhost.</p> |
| |
| |
| |
| <h3><a name="ipbased" id="ipbased">IP-based vhost</a></h3> |
| |
| <p>If the entry we found has an empty name list then we have |
| found an IP-based vhost, no further actions are performed and |
| the request is served from that vhost.</p> |
| |
| |
| |
| <h3><a name="namebased" id="namebased">Name-based vhost</a></h3> |
| |
| <p>If the entry corresponds to a name-based vhost the name list |
| contains one or more vhost structures. This list contains the |
| vhosts in the same order as the <code>VirtualHost</code> |
| directives appear in the config file.</p> |
| |
| <p>The first vhost on this list (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 <code>Host:</code> header field.</p> |
| |
| <p>If the client provided a <code>Host:</code> header field the |
| list is searched for a matching vhost and the first hit on a |
| <code>ServerName</code> or <code>ServerAlias</code> is taken |
| and the request is served from that vhost. A <code>Host:</code> |
| header field can contain a port number, but Apache always |
| matches against the real port to which the client sent the |
| request.</p> |
| |
| <p>If the client submitted a HTTP/1.0 request without |
| <code>Host:</code> header field we don't know to what server |
| the client tried to connect and any existing |
| <code>ServerPath</code> is matched against the URI from the |
| request. The first matching path on the list is used and the |
| request is served from that vhost.</p> |
| |
| <p>If no matching vhost could be found the request is served |
| from the first vhost with a matching port number that is on the |
| list for the IP to which the client connected (as already |
| mentioned before).</p> |
| |
| |
| |
| <h3><a name="persistent" id="persistent">Persistent connections</a></h3> |
| |
| <p>The IP lookup described above is only done <em>once</em> for a |
| particular TCP/IP session while the name lookup is done on |
| <em>every</em> request during a KeepAlive/persistent |
| connection. In other words a client may request pages from |
| different name-based vhosts during a single persistent |
| connection.</p> |
| |
| |
| |
| <h3><a name="absoluteURI" id="absoluteURI">Absolute URI</a></h3> |
| |
| <p>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 <em>and</em> 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.</p> |
| |
| |
| <h3><a name="observations" id="observations">Observations</a></h3> |
| |
| <ul> |
| <li>A name-based vhost can never interfere with an IP-base |
| vhost and vice versa. IP-based vhosts can only be reached |
| through an IP address of its own address set and never |
| through any other address. The same applies to name-based |
| vhosts, they can only be reached through an IP address of the |
| corresponding address set which must be defined with a |
| <code>NameVirtualHost</code> directive.</li> |
| |
| <li><code>ServerAlias</code> and <code>ServerPath</code> |
| checks are never performed for an IP-based vhost.</li> |
| |
| <li>The order of name-/IP-based, the <code>_default_</code> |
| vhost and the <code>NameVirtualHost</code> directive within |
| the config file is not important. 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.</li> |
| |
| <li>For security reasons the port number given in a |
| <code>Host:</code> header field is never used during the |
| matching process. Apache always uses the real port to which |
| the client sent the request.</li> |
| |
| <li>If a <code>ServerPath</code> directive exists which is a |
| prefix of another <code>ServerPath</code> directive that |
| appears later in the configuration file, then the former will |
| always be matched and the latter will never be matched. (That |
| is assuming that no <code>Host:</code> header field was |
| available to disambiguate the two.)</li> |
| |
| <li>If two IP-based vhosts have an address in common, the |
| vhost appearing first in the config file is always matched. |
| Such a thing might happen inadvertently. The server will give |
| a warning in the error logfile when it detects this.</li> |
| |
| <li>A <code>_default_</code> vhost catches a request only if |
| there is no other vhost with a matching IP address |
| <em>and</em> a matching port number for the request. The |
| request is only caught if the port number to which the client |
| sent the request matches the port number of your |
| <code>_default_</code> vhost which is your standard |
| <code>Listen</code> by default. A wildcard port can be |
| specified (<em>i.e.</em>, <code>_default_:*</code>) to catch |
| requests to any available port. This also applies to |
| <code>NameVirtualHost *</code> vhosts.</li> |
| |
| <li>The main_server is only used to serve a request if the IP |
| address and port number to which the client connected is |
| unspecified and does not match any other vhost (including a |
| <code>_default_</code> vhost). In other words the main_server |
| only catches a request for an unspecified address/port |
| combination (unless there is a <code>_default_</code> vhost |
| which matches that port).</li> |
| |
| <li>A <code>_default_</code> vhost or the main_server is |
| <em>never</em> matched for a request with an unknown or |
| missing <code>Host:</code> header field if the client |
| connected to an address (and port) which is used for |
| name-based vhosts, <em>e.g.</em>, in a |
| <code>NameVirtualHost</code> directive.</li> |
| |
| <li>You should never specify DNS names in |
| <code>VirtualHost</code> 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 <a href="../dns-caveats.html">more |
| information</a> available on this and the next two |
| topics.</li> |
| |
| <li><code>ServerName</code> should always be set for each |
| vhost. Otherwise A DNS lookup is required for each |
| vhost.</li> |
| </ul> |
| |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="tips" id="tips">Tips</a></h2> |
| |
| <p>In addition to the tips on the <a href="../dns-caveats.html#tips">DNS Issues</a> page, here are |
| some further tips:</p> |
| |
| <ul> |
| <li>Place all main_server definitions before any |
| <code>VirtualHost</code> 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.)</li> |
| |
| <li>Group corresponding <code>NameVirtualHost</code> and |
| <code>VirtualHost</code> definitions in your configuration to |
| ensure better readability.</li> |
| |
| <li>Avoid <code>ServerPaths</code> which are prefixes of |
| other <code>ServerPaths</code>. If you cannot avoid this then |
| you have to ensure that the longer (more specific) prefix |
| vhost appears earlier in the configuration file than the |
| shorter (less specific) prefix (<em>i.e.</em>, "ServerPath |
| /abc" should appear after "ServerPath /abc/def").</li> |
| </ul> |
| |
| </div></div> |
| <div class="bottomlang"> |
| <p><span>Available Languages: </span><a href="../en/vhosts/details.html" title="English"> en </a> | |
| <a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | |
| <a href="../tr/vhosts/details.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> |
| </div><div id="footer"> |
| <p class="apache">Copyright 2013 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p> |
| <p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div> |
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