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This document describes how to efficiently serve an arbitrary number of virtual hosts with Apache 1.3.
mod_rewrite
mod_rewrite
The techniques described here are of interest if your
httpd.conf contains many
<VirtualHost> sections that are substantially the
same, for example:
NameVirtualHost 111.22.33.44 <VirtualHost 111.22.33.44> ServerName www.customer-1.com DocumentRoot /www/hosts/www.customer-1.com/docs ScriptAlias /cgi-bin/ /www/hosts/www.customer-1.com/cgi-bin </VirtualHost> <VirtualHost 111.22.33.44> ServerName www.customer-2.com DocumentRoot /www/hosts/www.customer-2.com/docs ScriptAlias /cgi-bin/ /www/hosts/www.customer-2.com/cgi-bin </VirtualHost> # blah blah blah <VirtualHost 111.22.33.44> ServerName www.customer-N.com DocumentRoot /www/hosts/www.customer-N.com/docs ScriptAlias /cgi-bin/ /www/hosts/www.customer-N.com/cgi-bin </VirtualHost>
The basic idea is to replace all of the static
<VirtualHost> configuration with a mechanism that
works it out dynamically. This has a number of advantages:
The main disadvantage is that you cannot have a different log file for each virtual host; however if you have very many virtual hosts then doing this is dubious anyway because it eats file descriptors. It is better to log to a pipe or a fifo and arrange for the process at the other end to distribute the logs to the customers (it can also accumulate statistics, etc.).
A virtual host is defined by two pieces of information: its IP
address, and the contents of the Host: header in the HTTP
request. The dynamic mass virtual hosting technique is based on
automatically inserting this information into the pathname of the file
that is used to satisfy the request. This is done most easily using
mod_vhost_alias,
but if you are using a version of Apache up to 1.3.6 then you must use
mod_rewrite. Both
of these modules are disabled by default; you must enable one of them
when configuring and building Apache if you want to use this technique.
A couple of things need to be `faked' to make the dynamic virtual
host look like a normal one. The most important is the server name
which is used by Apache to generate self-referential URLs, etc. It
is configured with the ServerName directive, and it is
available to CGIs via the SERVER_NAME environment
variable. The actual value used at run time is controlled by the
UseCanonicalName
setting. With UseCanonicalName Off the server name
comes from the contents of the Host: header in the
request. With UseCanonicalName DNS it comes from a
reverse DNS lookup of the virtual host's IP address. The former
setting is used for name-based dynamic virtual hosting, and the latter
is used for IP-based hosting. If Apache cannot work out the server
name because there is no Host: header or the DNS lookup
fails then the value configured with ServerName is used
instead.
The other thing to `fake' is the document root (configured
with DocumentRoot and available to CGIs via the
DOCUMENT_ROOT environment variable). In a normal
configuration this setting is used by the core module when mapping
URIs to filenames, but when the server is configured to do dynamic
virtual hosting that job is taken over by another module (either
mod_vhost_alias or mod_rewrite) which has
a different way of doing the mapping. Neither of these modules is
responsible for setting the DOCUMENT_ROOT environment
variable so if any CGIs or SSI documents make use of it they will
get a misleading value.
This extract from httpd.conf implements the virtual
host arrangement outlined in the Motivation
section above, but in a generic fashion using
mod_vhost_alias.
# get the server name from the Host: header UseCanonicalName Off # this log format can be split per-virtual-host based on the first field LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon CustomLog logs/access_log vcommon # include the server name in the filenames used to satisfy requests VirtualDocumentRoot /www/hosts/%0/docs VirtualScriptAlias /www/hosts/%0/cgi-bin
This configuration can be changed into an IP-based virtual hosting
solution by just turning UseCanonicalName Off into
UseCanonicalName DNS. The server name that is inserted
into the filename is then derived from the IP address of the virtual
host.
This is an adjustment of the above system tailored for an ISP's
homepages server. Using a slightly more complicated configuration we
can select substrings of the server name to use in the filename so
that e.g. the documents for www.user.isp.com are found in
/home/user/. It uses a single cgi-bin
directory instead of one per virtual host.
# all the preliminary stuff is the same as above, then # include part of the server name in the filenames VirtualDocumentRoot /www/hosts/%2/docs # single cgi-bin directory ScriptAlias /cgi-bin/ /www/std-cgi/
There are examples of more complicated
VirtualDocumentRoot settings in
the
mod_vhost_alias documentation.
With more complicated setups you can use Apache's normal
<VirtualHost> directives to control the scope of
the various virtual hosting configurations. For example, you could
have one IP address for homepages customers and another for commercial
customers with the following setup. This can of course be combined
with conventional <VirtualHost> configuration
sections.
UseCanonicalName Off LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon <Directory /www/commercial> Options FollowSymLinks AllowOverride All </Directory> <Directory /www/homepages> Options FollowSymLinks AllowOverride None </Directory> <VirtualHost 111.22.33.44> ServerName www.commercial.isp.com CustomLog logs/access_log.commercial vcommon VirtualDocumentRoot /www/commercial/%0/docs VirtualScriptAlias /www/commercial/%0/cgi-bin </VirtualHost> <VirtualHost 111.22.33.45> ServerName www.homepages.isp.com CustomLog logs/access_log.homepages vcommon VirtualDocumentRoot /www/homepages/%0/docs ScriptAlias /cgi-bin/ /www/std-cgi/ </VirtualHost>
After the first example I noted that it is easy to turn it into an IP-based virtual hosting setup. Unfortunately that configuration is not very efficient because it requires a DNS lookup for every request. This can be avoided by laying out the filesystem according to the IP addresses themselves rather than the corresponding names and changing the logging similarly. Apache will then usually not need to work out the server name and so incur a DNS lookup.
# get the server name from the reverse DNS of the IP address UseCanonicalName DNS # include the IP address in the logs so they may be split LogFormat "%A %h %l %u %t \"%r\" %s %b" vcommon CustomLog logs/access_log vcommon # include the IP address in the filenames VirtualDocumentRootIP /www/hosts/%0/docs VirtualScriptAliasIP /www/hosts/%0/cgi-bin
The examples above rely on mod_vhost_alias which
appeared after version 1.3.6. If you are using a version of Apache
without mod_vhost_alias then you can implement this
technique with mod_rewrite as illustrated below, but
only for Host:-header-based virtual hosts.
In addition there are some things to beware of with logging. Apache
1.3.6 is the first version to include the %V log format
directive; in versions 1.3.0 - 1.3.3 the %v option did
what %V does; version 1.3.4 has no equivalent. In
all these versions of Apache the UseCanonicalName
directive can appear in .htaccess files which means that
customers can cause the wrong thing to be logged. Therefore the best
thing to do is use the %{Host}i directive which logs the
Host: header directly; note that this may include
:port on the end which is not the case for
%V.
mod_rewriteThis extract from httpd.conf does the same thing as
the first example. The first half is very
similar to the corresponding part above but with some changes for
backward compatibility and to make the mod_rewrite part
work properly; the second half configures mod_rewrite to
do the actual work.
There are a couple of especially tricky bits: By default,
mod_rewrite runs before the other URI translation modules
(mod_alias etc.) so if they are used then
mod_rewrite must be configured to accommodate them.
Also, mome magic must be performed to do a per-dynamic-virtual-host
equivalent of ScriptAlias.
# get the server name from the Host: header
UseCanonicalName Off
# splittable logs
LogFormat "%{Host}i %h %l %u %t \"%r\" %s %b" vcommon
CustomLog logs/access_log vcommon
<Directory /www/hosts>
# ExecCGI is needed here because we can't force
# CGI execution in the way that ScriptAlias does
Options FollowSymLinks ExecCGI
</Directory>
# now for the hard bit
RewriteEngine On
# a ServerName derived from a Host: header may be any case at all
RewriteMap lowercase int:tolower
## deal with normal documents first:
# allow Alias /icons/ to work - repeat for other aliases
RewriteCond %{REQUEST_URI} !^/icons/
# allow CGIs to work
RewriteCond %{REQUEST_URI} !^/cgi-bin/
# do the magic
RewriteRule ^/(.*)$ /www/hosts/${lowercase:%{SERVER_NAME}}/docs/$1
## and now deal with CGIs - we have to force a MIME type
RewriteCond %{REQUEST_URI} ^/cgi-bin/
RewriteRule ^/(.*)$ /www/hosts/${lowercase:%{SERVER_NAME}}/cgi-bin/$1 [T=application/x-httpd-cgi]
# that's it!
mod_rewriteThis does the same thing as the second example.
RewriteEngine on
RewriteMap lowercase int:tolower
# allow CGIs to work
RewriteCond %{REQUEST_URI} !^/cgi-bin/
# check the hostname is right so that the RewriteRule works
RewriteCond ${lowercase:%{SERVER_NAME}} ^www\.[a-z-]+\.isp\.com$
# concatenate the virtual host name onto the start of the URI
# the [C] means do the next rewrite on the result of this one
RewriteRule ^(.+) ${lowercase:%{SERVER_NAME}}$1 [C]
# now create the real file name
RewriteRule ^www\.([a-z-]+)\.isp\.com/(.*) /home/$1/$2
# define the global CGI directory
ScriptAlias /cgi-bin/ /www/std-cgi/
This arrangement uses more advanced mod_rewrite
features to get the translation from virtual host to document root
from a separate configuration file. This provides more flexibility but
requires more complicated configuration.
The vhost.map file contains something like this:
www.customer-1.com /www/customers/1 www.customer-2.com /www/customers/2 # ... www.customer-N.com /www/customers/N
The http.conf contains this:
RewriteEngine on
RewriteMap lowercase int:tolower
# define the map file
RewriteMap vhost txt:/www/conf/vhost.map
# deal with aliases as above
RewriteCond %{REQUEST_URI} !^/icons/
RewriteCond %{REQUEST_URI} !^/cgi-bin/
RewriteCond ${lowercase:%{SERVER_NAME}} ^(.+)$
# this does the file-based remap
RewriteCond ${vhost:%1} ^(/.*)$
RewriteRule ^/(.*)$ %1/docs/$1
RewriteCond %{REQUEST_URI} ^/cgi-bin/
RewriteCond ${lowercase:%{SERVER_NAME}} ^(.+)$
RewriteCond ${vhost:%1} ^(/.*)$
RewriteRule ^/(.*)$ %1/cgi-bin/$1
