Virtual hosts
A virtual host is a configuration entity that enables a single host machine to resemble multiple host machines. It maintains a list of Multipurpose Internet Mail Extensions (MIME) types that it processes. You can associate a virtual host to one or more Web modules, but you can associate each web module with one and only one virtual host. Resources associated with one virtual host cannot share data with resources associated with another virtual host, even if the virtual hosts share the same physical machine.
Each virtual host has a logical name and a list of one or more
DNS aliases by which it is known. A DNS alias is the TCP/IP hostname
and port number that is used to request the servlet, for example yourHostName:80
.
When no port number is specified, 80 is assumed.
The virtual host configuration uses wildcard entries with the ports for its virtual host entries.
- The default alias is *:80, using an external port that is not secure.
- Aliases of the form *:9080 use the internal port that is not secure.
- Aliases of the form *:9443 use the secure internal port.
- Aliases of the form *:443 use the secure external port.
A client request for a servlet, JavaServer Pages file, or related resource contains a DNS alias and a Uniform Resource Indicator (URI) that is unique to that resource. When a client request for a servlet, JavaServer Pages file, or related resource is received, the DNS alias is compared to the list of all known virtual host groups to locate the correct virtual host, and the URI is compared to the list of all known URI groups to locate the correct URI group. If the virtual host group and URI group are found, the request is sent to the corresponding server group for processing and a response is returned to browser. If a matching virtual host group or URI group is not found, an error is returned to the browser.
A virtual host is not associated with a particular node (machine). It is a configuration, rather than a live object, which is why you can create it, but cannot start or stop it. A default virtual host, named default_host, is automatically configured the first time you start an application server. Unless you specifically want to isolate resources from one another on the same node (physical machine), you probably do not need any virtual hosts in addition to the default host.
The DNS aliases for the default virtual host are configured as *:80
and *:9080
,
where port 80 is the HTTP server port and port 9080 is the port for
the default server's HTTP transport. The default virtual host includes
common aliases, such as the machine's IP address, short host name,
and fully qualified host name. One of these aliases comprises the
first part of the path for accessing a resource such as a servlet.
For example, the alias localhost:80
is used in the
request http://localhost:80/myServlet.
Adding a localhost to the virtual hosts adds the host name and IP address of the localhost machine to the alias table. This allows a remote user to access the administrative console.
You can use the administrative console to add or change DNS aliases if you want to use ports other than the default ports. If you do make a change to a DNS alias, you must regenerate the web server plug-in configuration. You can use the administrative console to initiate the plug-in regeneration.
- The HTTP server instance is running on a port other than 80. Add
the correct port number to each of the aliases. For example, change
yourhost
toyourhost:8000
. - You want to make HTTPS requests, which use Secure Sockets Layer (SSL). To make HTTPS requests you must add port 443 to each of the aliases. Port 443 is the default port for SSL requests.
- Your web server instance is listening for SSL requests on a port other than 443. In this situation, you must add that port number to each of the aliases.
- You want to use a port other then default port (9080) for the application server.
- You want to use other aliases that are not listed.
When you request a resource, the product tries to map the request
to an alias of a defined virtual host. The http://host:port/
portion
of the virtual host is not case sensitive, but the URL that follows
is case sensitive. The match for the URL must be alphanumerically
exact. Different port numbers are treated as different aliases.
For example, the request http://www.myhost.com/myservlet maps
successfully to http://WWW.MYHOST.COM/myservlet but
not to http://WWW.MYHOST.COM/MYSERVLET or Www.Myhost.Com/Myservlet
.
In the latter two cases, these mappings fail because of case sensitivity.
The request http://www.myhost.com/myservlet does
not map successfully to http://myhost/myservlet or
to http://myhost:9876/myservlet. These mappings
fail because they are not alphanumerically correct.
You can use wildcard entries for aliases by port and specify that all valid host name and address combinations on a particular port map to a particular virtual host.
If you request a resource using an alias that cannot be mapped to an alias of a defined virtual host, you receive a 404 error in the browser that you used to issue the request. A message states that the virtual host could not be found.
- The web client asks for the snoop servlet: at web address
http://www.some_host.some_company.com:9080/snoop
- The some_host machine has the 9080 port assigned to the stand-alone application server, server1.
- server1 looks at the virtual host assignments to determine the
virtual host that is assigned to the alias
some_host.some_company.com:9080
. - The application server finds that no explicit alias for that DNS string exists. However, a wild card assignment for host name * at port 9080 does exist. This is a match. The virtual host that defines the match is default_host.
- The application server looks at the applications deployed on the default_host and finds the snoop servlet.
- The application server serves the application to the web client and the requester is able to use the snoop servlet.
Virtual host | Alias | Port |
---|---|---|
default_host | * | 9080 |
localhost | 9080 | |
my_machine | 9080 | |
my_machine.my_company.com | 9080 | |
localhost | 80 |
The Application Server looks for a match using the explicit address specified on the web client address. However, it might resolve the match to any other alias that matches the pattern before matching the explicit address. Simply defining an alias first in the list of aliases does not guarantee the search order whenever the product is looking for a matching alias.
A problem can occur if you use the same alias for two different virtual hosts. For example, assume that you installed the default application and the snoop servlet on the default_host. You also have another virtual host called the admin_host. However, you have not installed the default application or the snoop servlet on the admin_host.
Virtual host | Alias | Port |
---|---|---|
default_host | * | 9080 |
localhost | 9080 | |
admin_host | * | 9060 |
my_machine.com | 9080 |
Assume that a web client request comes in for http://my_machine.com:9080/snoop
.
If the application server matches the request against *:9080
,
the application is served from the default_host. If the application
server matches the request to my.machine.com:9080
,
the application cannot be found. A 404 error occurs in the browser
that issues the request. A message states that the virtual host could
not be found.
This problem is the result of not finding the requested application in the first virtual host that has a matching alias. The correct way to code aliases is for the alias name on an incoming request to match only one virtual host in all of your virtual host definitions. If the URL can match more than one virtual host, you can see the problem just described.