The term Virtual Host refers to the practice of running more than one web site (such as company1.example.com and company2.example.com) on a single machine. Virtual hosts can be "IP-based", meaning that you have a different IP address for every web site, or "name-based", meaning that you have multiple names running on each IP address. The fact that they are running on the same physical server is not apparent to the end user.
Apache was one of the first servers to support IP-based virtual hosts right out of the box. Versions 1.1 and later of Apache support both IP-based and name-based virtual hosts (vhosts). The latter variant of virtual hosts is sometimes also called host-based or non-IP virtual hosts.
Below is a list of documentation pages which explain all details of virtual host support in Apache HTTP Server:
Virtual Host Support
- Name-based Virtual Hosts (More than one web site per IP address)
- IP-based Virtual Hosts (An IP address for each web site)
- Virtual Host examples for common setups
- File Descriptor Limits (or, Too many log files)
- Dynamically Configured Mass Virtual Hosting
- In-Depth Discussion of Virtual Host Matching
Configuration directives
NameVirtualHost<VirtualHost>ServerNameServerAliasServerPath
If you are trying to debug your virtual host configuration, you may find the Apache -S command line switch useful. That is, type the following command:
/usr/local/apache2/bin/httpd -S
This command will dump out a description of how Apache parsed the configuration file. Careful examination of the IP addresses and server names may help uncover configuration mistakes. (See the docs for the httpd program for other command line options)
Apache IP-based Virtual Host Support
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 one httpd 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.
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 192.168.0.1:80>
ServerAdmin webmaster@smallco.example.com
DocumentRoot /groups/smallco/www
ServerName smallco.example.com
ErrorLog /groups/smallco/logs/error_log
TransferLog /groups/smallco/logs/access_log
</VirtualHost>
<VirtualHost 192.168.0.2:80>
ServerAdmin webmaster@baygroup.example.org
DocumentRoot /groups/baygroup/www
ServerName baygroup.example.com
ErrorLog /groups/baygroup/logs/error_log
TransferLog /groups/baygroup/logs/access_log
</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.
SuexecUserGroup may be used inside a VirtualHost directive if the suEXEC wrapper is used.
SECURITY: 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 security tips document for details.
Name-based Virtual Host Support
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, unless you are using a
mod_ssl version without SNI support (standard in Apache releases since 2.2.12).
Using Name-based Virtual Hosts
To use name-based virtual hosting, you must designate the IP address (and possibly port) on the server that will be accepting requests for the hosts. This is configured using the NameVirtualHost directive. In the normal case where any and all IP addresses on the server should be used, you can use * as the argument toNameVirtualHost. If you're planning to use multiple ports (e.g. running SSL) you should add a Port to the argument, such as *:80. Note that mentioning an IP address in a NameVirtualHost directive does not automatically make the server listen to that IP address. See Setting which addresses and ports Apache uses for more details. In addition, any IP address specified here must be associated with a network interface on the server.
The next step is to create a <VirtualHost> block for each different host that you would like to serve. The argument to the <VirtualHost> directive must match a defined NameVirtualHost directive. (In this usual case, this will be "*:80"). Inside each <VirtualHost> block, you will need at minimum a ServerName 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
If you are adding virtual hosts to an existing web server, you must also create a <VirtualHost> block for the existing host. The ServerName andDocumentRoot included in this virtual host should be the same as the global ServerName and DocumentRoot. List this virtual host first in the configuration file so that it will act as the default host.
For example, suppose that you are serving the domain www.domain.tld and you wish to add the virtual host www.otherdomain.tld, which points at the same IP address. Then you simply add the following to httpd.conf:
NameVirtualHost *:80
<VirtualHost *:80>
ServerName www.domain.tld
ServerAlias domain.tld *.domain.tld
DocumentRoot /www/domain</VirtualHost>
<VirtualHost *:80>
ServerName www.otherdomain.tld
DocumentRoot /www/otherdomain</VirtualHost>
You can alternatively specify an explicit IP address in place of the * in both the NameVirtualHost and <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 domain.tld *.domain.tld
then requests for all hosts in the domain.tld domain will be served by the www.domain.tld 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.
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.
Now when a request arrives, the server will first check if it is using an IP address that matches the NameVirtualHost. If it is, then it will look at each <VirtualHost>section with a matching IP address and try to find one where the ServerName or ServerAlias matches the requested hostname. If it finds one, then it uses the configuration for that server. If no matching virtual host is found, then the first listed virtual host that matches the IP address will be used.
As a consequence, the first listed virtual host is the default virtual host. The DocumentRoot from the main server will never be used when an IP address matches theNameVirtualHost directive. If you would like to have a special configuration for requests that do not match any particular virtual host, simply put that configuration in a<VirtualHost> container and list it first in the configuration file.
Compatibility with Older Browsers
As mentioned earlier, there are some clients who do not send the required data for the name-based virtual hosts to work properly. These clients will always be sent the pages from the first virtual host listed for that IP address (the primary name-based virtual host).
How much older?
Please note that when we say older, we really do mean older. You are very unlikely to encounter one of these browsers in use today. All current versions of any browser send the Host header as required for name-based virtual hosts.
There is a possible workaround with the ServerPath directive, albeit a slightly cumbersome one:
Example configuration:
NameVirtualHost 111.22.33.44
<VirtualHost 111.22.33.44>
ServerName www.domain.tld
ServerPath /domain
DocumentRoot /web/domain</VirtualHost>
What does this mean? It means that a request for any URI beginning with "/domain" will be served from the virtual host www.domain.tld. This means that the pages can be accessed as http://www.domain.tld/domain/ for all clients, although clients sending a Host: header can also access it as http://www.domain.tld/.
In order to make this work, put a link on your primary virtual host's page to http://www.domain.tld/domain/. Then, in the virtual host's pages, be sure to use either purely relative links (e.g., "file.html" or "../icons/image.gif") or links containing the prefacing /domain/ (e.g., "http://www.domain.tld/domain/misc/file.html" or "/domain/misc/file.html").
This requires a bit of discipline, but adherence to these guidelines will, for the most part, ensure that your pages will work with all browsers, new and old.
Parallel
Advance Technology Attachment (PATA) drives are also known as
Integrated Drive Electronics (IDE) developed by Western Digital under
the name Integrated Drive Electronics (IDE) 
Small
Computer System Interface or SCSI for short and can be pronounce as
"skuzzy" are high performance storage drives. SCSI drives are commonly
found in servers were real time performance or 24/7 operation is
required. SCSI drive are much faster than the standard drive Parallel
ATA (PATA) with 5400RPM - 7200RPM access time compared to 10000RPM -
15000RPM for SCSI. Revolution Per Minute "RPM" of hard drives is a
measurement of how many complete revolutions a computer's hard drive
makes in a single minute. Fast rotation speed of the spindle means high
data transfer , the higher the RPM, the faster the data will be accessed
Parallel
Advance Technology Attachment (PATA) drives are also known as
Integrated Drive Electronics (IDE) with 2.5 inch form factor. Due to
its smaller size it is commonly used in notebook PCs, external storage,
gaming consoles, and portable video players.
Serial
ATA was designed to replace the older parallel ATA (PATA). SATA offers
several advantages over the older IDE drive such as reduced cable size
reducing from 40 pins in IDE to 7 pins in SATA, native hot swapping and
has faster data transfer through higher signaling rates.
The
Serial Attached SCSI (SAS) is a new generation serial communication
protocol for devices designed to allow for much higher speed data
transfers and is compatible with SATA. The serial transmission of data
requires fewer connections and eliminates the SCSI bus and still uses
the very capable SCSI protocol. SAS uses a mechanically identical data
and power connector to standard 3.5-inch SATA1/SATA2 HDDs and many
server-oriented SAS RAID controllers are also capable of addressing SATA
hard drives. SATA drives can be attached to a SAS drive controller and
they will work perfectly because they are compatible but not SAS drive
connected to a SATA controller this configuration will not work.
