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<h3>Apache HTTP Server Version 1.3</h3>
<p><small><em>Is this the version you want? For more recent
versions, check our <a href="/docs/">documentation
index</a>.</em></small></p>
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<h1 align="CENTER">Virtual Host Support</h1>
<strong>See Also:</strong> <a href="host.html">Non-IP based
virtual hosts</a>
<h2>What are virtual hosts?</h2>
This is the ability of a single machine to be a web server for
multiple domains. For example, an Internet service provider
might have a machine called <code>www.serve.com</code> which
provides Web space for several organizations including, say,
<em>smallco</em> and <em>baygroup</em>. Ordinarily, these
groups would be given parts of the Web tree on www.serve.com.
So smallco's home page would have the URL
<blockquote>
http://www.serve.com/smallco/
</blockquote>
and baygroup's home page would have the URL
<blockquote>
http://www.serve.com/baygroup/
</blockquote>
<p>For esthetic reasons, however, both organizations would
rather their home pages appeared under their own names rather
than that of the service provider's; but they do not want to
set up their own Internet links and servers.</p>
<p>Virtual hosts are the solution to this problem. smallco and
baygroup would have their own Internet name registrations,
<code>www.smallco.com</code> and <code>www.baygroup.org</code>
respectively. These hostnames would both correspond to the
service provider's machine (www.serve.com). Thus smallco's home
page would now have the URL</p>
<blockquote>
http://www.smallco.com/
</blockquote>
and baygroup's home page would have the URL
<blockquote>
http://www.baygroup.org/
</blockquote>
<h2>System requirements</h2>
Due to limitations in the HTTP/1.0 protocol, the web server
<strong>must have a different IP address for each virtual
host</strong>. This can be achieved by the machine having
several physical network connections, or by use of a <a
href="../misc/vif-info.html">virtual interface</a> on some
operating systems.
<h2>How to set up Apache</h2>
There are two ways of configuring apache to support multiple
hosts. Either by running a separate httpd daemon for each
hostname, or by running a single daemon which supports all the
virtual hosts.
<p>Use multiple daemons when:</p>
<ul>
<li>The different virtual hosts need very different httpd
configurations, such as different values for: <a
href="../mod/core.html#servertype">ServerType</a>, <a
href="../mod/core.html#user">User</a>, <a
href="../mod/core.html#group">Group</a>, <a
href="../mod/mod_mime.html#typesconfig">TypesConfig</a> or <a
href="../mod/core.html#serverroot">ServerRoot</a>.</li>
<li>The machine does not process a very high request
rate.</li>
</ul>
Use a single daemon when:
<ul>
<li>Sharing of the httpd configuration between virtual hosts
is acceptable.</li>
<li>The machine services a large number of requests, and so
the performance loss in running separate daemons may be
significant.</li>
</ul>
<h2>Setting up multiple daemons</h2>
Create a separate httpd installation for each virtual host. For
each installation, use the <a
href="../mod/core.html#bindaddress">BindAddress</a> directive
in the configuration file to select which IP address (or
virtual host) that daemon services. <em>E.g.</em>,
<blockquote>
<code>BindAddress www.smallco.com</code>
</blockquote>
This hostname can also be given as an IP address.
<h2>Setting up a single daemon</h2>
For this case, a single httpd will service requests for all the
virtual hosts. The <a
href="../mod/core.html#virtualhost">VirtualHost</a> directive
in the configuration file is used to set the values of <a
href="../mod/core.html#serveradmin">ServerAdmin</a>, <a
href="../mod/core.html#servername">ServerName</a>, <a
href="../mod/core.html#documentroot">DocumentRoot</a>, <a
href="../mod/core.html#errorlog">ErrorLog</a> and <a
href="../mod/mod_log_config.html#transferlog">TransferLog</a>
configuration directives to different values for each virtual
host. <em>E.g.</em>,
<blockquote>
<code><VirtualHost www.smallco.com><br />
ServerAdmin webmaster@mail.smallco.com<br />
DocumentRoot /groups/smallco/www<br />
ServerName www.smallco.com<br />
ErrorLog /groups/smallco/logs/error_log<br />
TransferLog /groups/smallco/logs/access_log<br />
</VirtualHost><br />
<br />
<VirtualHost www.baygroup.org><br />
ServerAdmin webmaster@mail.baygroup.org<br />
DocumentRoot /groups/baygroup/www<br />
ServerName www.baygroup.org<br />
ErrorLog /groups/baygroup/logs/error_log<br />
TransferLog /groups/baygroup/logs/access_log<br />
</VirtualHost><br />
</code>
</blockquote>
This VirtualHost hostnames can also be given as IP addresses.
<p>Almost <strong>ANY</strong> configuration directive can be
put in the VirtualHost directive, with the exception of <a
href="../mod/core.html#servertype">ServerType</a>, <a
href="../mod/core.html#user">User</a>, <a
href="../mod/core.html#group">Group</a>, <a
href="../mod/core.html#startservers">StartServers</a>, <a
href="../mod/core.html#maxspareservers">MaxSpareServers</a>, <a
href="../mod/core.html#minspareservers">MinSpareServers</a>, <a
href="../mod/core.html#maxrequestsperchild">MaxRequestsPerChild</a>,
<a href="../mod/core.html#bindaddress">BindAddress</a>, <a
href="../mod/core.html#pidfile">PidFile</a>, <a
href="../mod/mod_mime.html#typesconfig">TypesConfig</a>, and <a
href="../mod/core.html#serverroot">ServerRoot</a>.</p>
<p><em>SECURITY:</em> When specifying where to write log files,
be aware of some security risks which are present if anyone
other than the user that starts Apache has write access to the
directory where they are written. See the <a
href="../misc/security_tips.html">security tips</a> document
for details.</p>
<h2>File Handle/Resource Limits:</h2>
When using a large number of Virtual Hosts, Apache may run out
of available file descriptors if each Virtual Host specifies
different log files. The total number of file descriptors used
by Apache is one for each distinct error log file, one for
every other log file directive, plus 10-20 for internal use.
Unix operating systems limit the number of file descriptors
that may be used by a process; the limit is typically 64, and
may usually be increased up to a large hard-limit.
<p>Although Apache attempts to increase the limit as required,
this may not work if:</p>
<ol>
<li>Your system does not provide the setrlimit() system
call.</li>
<li>The setrlimit(RLIMIT_NOFILE) call does not function on
your system (such as Solaris 2.3)</li>
<li>The number of file descriptors required exceeds the hard
limit.</li>
<li>Your system imposes other limits on file descriptors,
such as a limit on stdio streams only using file descriptors
below 256. (Solaris 2)</li>
</ol>
In the event of problems you can:
<ul>
<li>Reduce the number of log files; don't specify log files
in the VirtualHost sections, but only log to the main log
files.</li>
<li>
If you system falls into 1 or 2 (above), then increase the
file descriptor limit before starting Apache, using a
script like
<blockquote>
<code>#!/bin/sh<br />
ulimit -S -n 100<br />
exec httpd</code>
</blockquote>
</li>
</ul>
The have been reports that Apache may start running out of
resources allocated for the root process. This will exhibit
itself as errors in the error log like "unable to fork". There
are two ways you can bump this up:
<ol>
<li>Have a <code>csh</code> script wrapper around httpd which
sets the "rlimit" to some large number, like 512.</li>
<li>
Edit http_main.c to add calls to setrlimit() from main(),
along the lines of
<pre>
struct rlimit rlp;
rlp.rlim_cur = rlp.rlim_max = 512;
if (setrlimit(RLIMIT_NPROC, &rlp)) {
fprintf(stderr, "setrlimit(RLIMIT_NPROC) failed.\n");
exit(1);
}
</pre>
(thanks to "Aaron Gifford <agifford@InfoWest.COM>"
for the patch)
</li>
</ol>
The latter will probably manifest itself in a later version of
Apache. <hr />
<h3 align="CENTER">Apache HTTP Server Version 1.3</h3>
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