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6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd<!DOCTYPE manualpage SYSTEM "/style/manualpage.dtd">

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd<?xml-stylesheet type="text/xsl" href="/style/manual.en.xsl"?>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd

f3ec420152ca921e4c1ce77782f51b53f659018dnd<manualpage metafile="details.xml.meta">

f3ec420152ca921e4c1ce77782f51b53f659018dnd<parentdocument href="./">Virtual Hosts</parentdocument>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <title>An In-Depth Discussion of Virtual Host Matching</title>

e4ca72aa494fed7b6948012734b9c9c098fbba07nd

e4ca72aa494fed7b6948012734b9c9c098fbba07nd<summary>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>This document attempts to explain

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd exactly what Apache HTTP Server does when deciding what virtual host to

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd serve a request from.</p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>Most users should read about <a href="name-based.html#namevip">

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd Name-based vs. IP-based Virtual Hosts</a> to decide which type they

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd want to use, then read more about <a href="name-based.html">name-based</a>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd or <a href="ip-based.html">IP-based</a> virtualhosts, and then see

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <a href="examples.html">some examples</a>.</p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>If you want to understand all the details, then you can

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd come back to this page.</p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd</summary>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd<seealso><a href="ip-based.html">IP-based Virtual Host Support</a></seealso>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd<seealso><a href="name-based.html">Name-based Virtual Hosts Support</a></seealso>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd<seealso><a href="examples.html">Virtual Host examples for common setups</a></seealso>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd<seealso><a href="mass.html">Dynamically configured mass virtual hosting</a></seealso>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd<section id="configparsing"><title>Configuration File</title>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>There is a <em>main server</em> which consists of all the

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd definitions appearing outside of

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <code>&lt;VirtualHost&gt;</code> sections.</p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>There are virtual

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd servers, called <em>vhosts</em>, which are defined by

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <directive type="section" module="core">VirtualHost</directive>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd sections.</p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>Each <code>VirtualHost</code> directive includes one

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd or more addresses and optional ports.</p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>Hostnames can be used in place of IP addresses in a virtual

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd host definition, but they are resolved at startup and if any name

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd resolutions fail, those virtual host definitions are ignored.

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd This is, therefore, not recommended.</p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>The address can be specified as

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <code>*</code>, which will match a request if no

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd other vhost has the explicit address on which the request was

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd received. </p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>The address appearing in the <code>VirtualHost</code>

71fccc298df6a1540d408151a26aa22beed55d0bnd directive can have an optional port. If the port is unspecified,

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd it is treated as a wildcard port, which can also be indicated

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd explicitly using <code>*</code>.

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd The wildcard port matches any port.</p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>(Port numbers specified in the <code>VirtualHost</code> directive do

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd not influence what port numbers Apache will listen on, they only control

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd which <code>VirtualHost</code> will be selected to handle a request.

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd Use the <directive module="mpm_common">Listen</directive> directive to

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd control the addresses and ports on which the server listens.)

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd </p>

71fccc298df6a1540d408151a26aa22beed55d0bnd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>Collectively the

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd entire set of addresses (including multiple

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd results from DNS lookups) are called the vhost's

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd <em>address set</em>.</p>

ad74a0524a06bfe11b7de9e3b4ce7233ab3bd3f7nd

f3ec420152ca921e4c1ce77782f51b53f659018dnd <p>Apache automatically discriminates on the

f3ec420152ca921e4c1ce77782f51b53f659018dnd basis of the HTTP <code>Host</code> header supplied by the client

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd whenever the most specific match for an IP address and port combination

6eed902e5b4d3e016e220bfbf8769a87c4cb242enoodl is listed in multiple virtual hosts.</p>

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd

6116c12fdd3ed06d388fe6572e50a22e9320dfa5nd <p>The

<directive module="core">ServerName</directive> directive

may appear anywhere within the definition of a server. However,

each appearance overrides the previous appearance (within that

server). If no <code>ServerName</code> is specified, the server

attempts to deduce it from the server's IP address.</p>

<p>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 <glossary

ref="servernameindication">Server Name Indication</glossary>.</p>

<p>The complete list of names in the <code>VirtualHost</code>

directive are treated just like a (non wildcard) <code>ServerAlias</code>

(but are not overridden by any <code>ServerAlias</code> statement).</p>

<p>For every vhost various default values are set. In

particular:</p>

<ol>

<li>If a vhost has no <directive module="core">ServerAdmin</directive>,

<directive module="core">Timeout</directive>,

<directive module="core">KeepAliveTimeout</directive>,

<directive module="core">KeepAlive</directive>,

<directive module="core">MaxKeepAliveRequests</directive>,

<directive module="mpm_common">ReceiveBufferSize</directive>,

or <directive module="mpm_common">SendBufferSize</directive>

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 <program>httpd</program>

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>

</section>

<section id="hostmatching"><title>Virtual Host Matching</title>

<p>The server determines which vhost to use for a request as

follows:</p>

<section id="hashtable"><title>IP address lookup</title>

<p>When the connection is first received on some address and port,

the server looks for all the <code>VirtualHost</code> definitions

that have the same IP address and port.</p>

<p>If there are no exact matches for the address and port, then

wildcard (<code>*</code>) matches are considered.</p>

<p>If no matches are found, the request is served by the

main server.</p>

<p>If there are <code>VirtualHost</code> 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.</p>

</section>

<section id="ipbased"><title>IP-based vhost</title>

<p>If there is exactly one <code>VirtualHost</code> 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.</p>

</section>

<section id="namebased"><title>Name-based vhost</title>

<p>If there are multiple <code>VirtualHost</code> 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.</p>

<p>If the connection is using SSL, the server supports <glossary

ref="servernameindication">Server Name Indication</glossary>, and

the SSL client handshake includes the TLS extension with the

requested hostname, then that hostname is used below just like the

<code>Host:</code> 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.</p>

<p>If the request contains a <code>Host:</code> header field, the

list is searched for the first vhost with a matching

<code>ServerName</code> or <code>ServerAlias</code>, and the

request is served from that vhost. A <code>Host:</code> 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.</p>

<p>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 (such as a HTTP/1.0

request).</p>

</section>

<section id="persistent"><title>Persistent connections</title>

<p>The <em>IP lookup</em> described above is only done <em>once</em> for a

particular TCP/IP session while the <em>name lookup</em> 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>

</section>

<section id="absoluteURI"><title>Absolute URI</title>

<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>

</section>

<section id="observations"><title>Observations</title>

<ul>

<li>Name-based virtual hosting is a process applied after

the server has selected the best matching IP-based virtual

host.</li>

<li>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.</li>

<li><code>ServerName</code> and <code>ServerAlias</code>

checks are never performed for an IP-based vhost.</li>

<li>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>Any port in the <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 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.</li>

<li>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

<code>*</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>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>

</section>

</section>

<section id="tips"><title>Tips</title>

<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>

</ul>

</section>

</manualpage>