apache knowlege

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• What is IP-based virtual hosting
• System requirements
• How to set up Apache
• Setting up multiple daemons
• Setting up a single daemon with virtual hosts

See also

• Name-based Virtual Hosts Support

• Comments

What is IP-based virtual hosting

IP-based virtual hosting is a method to apply different directives based on the IP address and port a request is received on. Most commonly, this is used to serve different websites on different ports or interfaces.

In many cases, name-based virtual hosts are more convenient, because they allow many virtual hosts to share a single address/port. See Name-based vs. IP-based Virtual Hosts to help you decide.

System requirements

As the term IP-based indicates, the server must have a different IP address/port combination for each IP-based virtual host. This can be achieved by the machine having several physical network connections, or by use of virtual interfaces which are supported by most modern operating systems (see system documentation for details, these are frequently called "ip aliases", and the "ifconfig" command is most commonly used to set them up), and/or using multiple port numbers.

In the terminology of Apache HTTP Server, using a single IP address but multiple TCP ports, is also IP-based virtual hosting.

How to set up Apache

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.

Use multiple daemons when:

• There are security partitioning issues, such as company1 does not want anyone at company2 to be able to read their data except via the web. In this case you would need two daemons, each running with different User, Group, Listen, and ServerRoot settings.
• You can afford the memory and file descriptor requirements of listening to every IP alias on the machine. It's only possible to Listen to the "wildcard" address, or to specific addresses. So if you have a need to listen to a specific address for whatever reason, then you will need to listen to all specific addresses. (Although onehttpd could listen to N-1 of the addresses, and another could listen to the remaining address.)

Use a single daemon when:

• Sharing of the httpd configuration between virtual hosts is acceptable.
• The machine services a large number of requests, and so the performance loss in running separate daemons may be significant.

Setting up multiple daemons

Create a separate httpd installation for each virtual host. For each installation, use the Listen directive in the configuration file to select which IP address (or virtual host) that daemon services. e.g.

Listen 192.0.2.100:80

It is recommended that you use an IP address instead of a hostname (see DNS caveats).

Setting up a single daemon with virtual hosts

For this case, a single httpd will service requests for the main server and all the virtual hosts. The VirtualHost directive in the configuration file is used to set the values of ServerAdmin, ServerName, DocumentRoot, ErrorLog and TransferLog or CustomLog configuration directives to different values for each virtual host. e.g.

<VirtualHost 172.20.30.40:80>    ServerAdmin [e-mail]    DocumentRoot /www/vhosts/www1    ServerName www1.example.com    ErrorLog /www/logs/www1/error_log    CustomLog /www/logs/www1/access_log combined</VirtualHost><VirtualHost 172.20.30.50:80>    ServerAdmin [e-mail]    DocumentRoot /www/vhosts/www2    ServerName www2.example.org    ErrorLog /www/logs/www2/error_log    CustomLog /www/logs/www2/access_log combined</VirtualHost>

It is recommended that you use an IP address instead of a hostname in the <VirtualHost> directive (see DNS caveats).

Specific IP addresses or ports have precedence over their wildcard equivalents, and any virtual host that matches has precedence over the servers base configuration.

Almost any configuration directive can be put in the VirtualHost directive, with the exception of directives that control process creation and a few other directives. To find out if a directive can be used in the VirtualHost directive, check the Context using the directive index.

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Name-based vs. IP-based Virtual Hosts

IP-based virtual hosts 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.

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.

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.

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.

How the server selects the proper name-based virtual host

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.

When a request arrives, the server will find the best (most specific) matching <VirtualHost> 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 ServerName and ServerAlias directives to the server name present in the request.

The default name-based vhost for an IP and port combination

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 the first listed virtual host that matches that will be used.

Using Name-based Virtual Hosts

Related ModulesRelated Directives

• core

• DocumentRoot
• ServerAlias
• ServerName
• <VirtualHost>

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

Main host goes away

Any request that doesn't match an existing <VirtualHost> is handled by the global server configuration, regardless of the hostname or ServerName.

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 default virtual host with a ServerName 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.

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

<VirtualHost *:80>    # This first-listed virtual host is also the default for *:80    ServerName www.example.com    ServerAlias example.com     DocumentRoot /www/domain</VirtualHost><VirtualHost *:80>    ServerName other.example.com    DocumentRoot /www/otherdomain</VirtualHost>

You can alternatively specify an explicit IP address in place of the * in <VirtualHost> 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.

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

ServerAlias example.com *.example.com

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

Name-based virtual hosts for the best-matching set of <virtualhost>s are processed in the order they appear in the configuration. The first matching ServerName orServerAlias is used, with no different precedence for wildcards (nor for ServerName vs. ServerAlias).

The complete list of names in the VirtualHost directive are treated just like a (non wildcard) ServerAlias.

Finally, you can fine-tune the configuration of the virtual hosts by placing other directives inside the <VirtualHost> 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 Context of the directive. Configuration directives set in the main server context (outside any <VirtualHost> container) will be used only if they are not overridden by the virtual host settings.