Limitations of Virtual Hosting

Virtual hosting is a great feature of our Virtual Servers. However, there are some limitations to this capability which you should understand. These limitations include the following:

1. Virtual hosting is a feature of the Virtual Servers which is made possible by the introduction of HTTP/1.1. In order to view virtual hosts you must have a browser which is HTTP/1.1 compliant. Generally speaking, subhosts are supported by Netscape Navigator 2.0+ and MSIE 3.0+. Any other browser that is HTTP/1.1 compliant will be able to access virtual hosted servers.

   If your clients are using an older browser which is not HTTP/1.1 compliant they will not be able to view their sites, nor other sites which are using Virtual hosting. However, considering that together Netscape and MSIE have 90-95% of the market share (which includes AOL and CompuServe users), this is generally not major a problem. Nevertheless, it is good to be familiar with this limitation.

2. A Virtual Server is capable of handling 30,000 to 50,000 hits (assuming hits generally request about 5 kb of data) per day. That is not "visitors", rather hits or requests for files. For instance, if you have 5 subhosted domain names, each which is trying to accommodate 10,000 hits per day (which really isn't that much if you have a graphically intensive page; one request for a .gif or .jpeg equals one hit!) there will likely be a problem. This "slowdown" will affect all of your clients on the Virtual Server you are using to virtual host.

   When a slowdown occurs a wise system administrator will properly "manage" his or her Virtual Server by reducing the number of virtual hosts on the Virtual Server by either upgrading one of the especially high traffic virtual hosted sites to its own Virtual Server or by moving some virtual hosts to a less busy Virtual Server. Either way, proper load balancing is a science that a system administrator must have a feel for in order to succeed with serious virtual hosting.

3. A Virtual Server can only host a finite number of Virtual Hosts due to performance reasons. We provide the following guidelines as to what we have found as the average number of virtual hosts each virtual server type can support prior to noticable response degradation. However this number can vary depending on the type of sites hosted.

   Server Type	<Virtual Hosts>
   Basic	5
   Advanced	25
   Pro	60

4. Virtual hosting obviously uses the resources of a single Virtual Server to accommodate the needs of multiple web sites. Among the resources that are shared is the single IP address that is associated with the Virtual Server. Search engine spiders which are not HTTP/1.1 compliant will not be able to index the sites. Most major spiders and search engines are now HTTP/1.1 compliant.

5. A Virtual Server can only support a single Digital Certificate. This can make the use of SSL difficult since all virtual hosts must use the same Digital Certificate and only one domain name can be associated with a Digital Certificate. There are workarounds to this problem by using wilcard certificates. Please refer to our SSL extensions section for more information.

6. A virtual host does not have telnet access to the Virtual Server.

7. There are some limitations to the e-mail capability of virtual hosts, namely how the Virtual Server interprets e-mail addresses. For instance, if you send an e-mail to "john@abc.com" and "john@xyz.com" the Virtual Server will view these as the same address. This is because to the Virtual Server, "john@abc.com" and "john@xyz.com" both resolve to "john@192.168.1.1" because both domain names resolve to the same IP address. There is a workaround to this limitation by using virtual e-mail mapping using the virtmaps utility.

8. It is important to understand that giving cgi-bin access to your virtually hosted clients is a potential security risk. This is because the CGIs your customers upload and execute have all of the rights and privileges of the CGIs you execute. Therefore, it is possible for a virtually hosted client, which has been granted CGI privileges, to read or remove any file on your virtual server. Moreover, it is possible for a malicious virtual hosted client to crack weak passwords and gain shell access to your Virtual Server. Please see Virtual hosting and Security Issues for more information.