Limitations of Virtual Subhosting

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 smalltextator 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 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 VPS you are using to Subhost.
   
   When a slowdown occurs a wise Virtual Server Administrator will properly "manage" his or her Virtual Server by reducing the number of Subhosts on the VPS by either upgrading one of the especially high traffic Virtual hosted sites to its own Virtual Server or by moving some Subhosts to a less busy Virtual Server. Either way, proper load balancing is a science that a Virtual Server Administrator must have a feel for in order to succeed with serious Virtual Subhosting.
3. A Virtual 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 Server.

   Server A:   around  5 low volume subhosts

   NOTE: The FreeBSD Standard Virtual Servers configuration is not suitable for multiple domain hosting.

   
   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 your Virtual Server. It is up to you to monitor Virtual Subhosts and upgrade high load Virtual Subhosts to their own Virtual Servers.
4. Virtual Subhosting 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 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 Server.
7. There are some limitations to the e-mail capability of Subhosts, 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 VPS 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@192.41.5.2" because both domain names resolve to the same IP address. However, we have developed a way to get around this limitation by using a proprietary utility titled "virtmaps". See our document, Configuring e-mail accounts for Virtual Subhosts for more information.
8. It is important to understand that giving cgi-bin access to your subhosted 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 subhosted client, which has been granted CGI privileges, to read or remove any file in your directory hierarchy. Moreover, it is possible for a malicious Subhosted client to crack weak passwords and gain shell access to your VPS. see our document, Virtual Subhosting security issues for more information.