Limitations of Virtual Subhosting

Virtual Subhosting is a great feature of our Virtual Private Server System. However, there are some limitations to this capability which you should understand. These limitation include the following:

1. Virtual Subhosting is made possible by the introduction of HTTP/1.1. In order to view subhosted domains you must have a browser which is HTTP/1.1 compliant. Generally speaking, Virtual Subhosting is supported by Netscape Navigator 2.0+ and MSIE 3.0+. Any other browser that is HTTP/1.1 compliant will be able to access a subhosted domain.

   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 Subhosting. However, considering that together Netscape and MSIE have 90-95% of the market share, this is generally not major a problem. Nonetheless, it is good to be familiar with this limitation.

2. A Virtual Private 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; the term hits refers to any request for a file (whether it's a .html, .gif, or any other type). 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) there will likely be a problem. This slowdown will affect all of your clients on the Virtual Private Server you are using to Subhost.

   When a slowdown occurs a wise administrator will properly manage his or her Virtual Private Server by reducing the number of Subhosts on the server by either upgrading one of the especially high traffic subhosted sites to its own Virtual Private Server or by moving some Subhosts to a less busy server. Either way, proper load balancing is a science that an administrator must understand in order to succeed with serious Virtual Subhosting.

3. A Virtual Private Server can only host a finite number of Virtual Subhosts due to performance reasons. Consider the following recommendations when deciding how many Subhosts to place on a single Virtual Private Server.

   Server A:	around 5 low volume subhosts
   Server B:	around 25 low volume subhosts
   Server C:	around 60 low volume subhosts

   We cannot guarantee the number of Virtual Subhosts you will be able to host since each site uses a different amount of resources. It may be that you can only host one other Virtual Subhost before resources are exhausted on a Virtual Private Server. It is up to you to monitor Virtual Subhosts and upgrade high load Virtual Subhosts to their own Virtual Private Servers.

4. Virtual Subhosting obviously uses the resources of a single Virtual Private 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 Private 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 Private Server can only support a single Digital Certificate. This can make the use of SSL difficult since all Subhosts must use the same Digital Certificate and only one domain name can be associated with a Digital Certificate.

6. A Virtual Subhost does not have telnet access to the Virtual Private Server.

7. There are some limitations to the e-mail capability of Subhosts, namely how the Virtual Private Servers interpret e-mail addresses. For instance, if you send an e-mail to john@abc.com and john@xyz.com, the server will view these as the same address. This is because SMTP programs (such as sendmail on the Virtual Private Server) do not differentiate between locally delivered domains. We have developed a way to get around this limitation by using a proprietary utility called virtmaps. See our document, Providing E-Mail for Virtual Subhosts for more information.

8. It is important to understand that giving cgi-bin access to your subhosted sites is a potential security risk. This is because the CGIs your users upload and execute have all of the rights and privileges of the CGIs you execute. Therefore, it is possible for a subhosted user, which has been granted CGI privileges, to read or remove any file in your directory hierarchy. Moreover, it is possible for a malicious Subhosted user to crack weak passwords and gain shell access to your Virtual Private Server. see our document, Virtual Subhosting Security Issues for more information.

Please note: the information on this page applies to ITS web hosting plans. It may or may not apply to other environments. If you are looking for a feature described here, or better support from your hosting provider, please consider hosting your site with ITS!

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