Limitations of Virtual Hosting
Virtual hosting or subhosting is a great feature of the Blue Reef Virtual
Servers System. There are some limitations, however, to this capability
which you should understand. These limitation include the following:
- Non HTTP/1.1 browsers
Virtual subhosting is made possible by the introduction of HTTP/1.1.
In order to view subhosts 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 subhosted servers.
If your clients are using an older browser which is not HTTP/1.1-compliant
then they will not be able to view their sites, nor other sites which
are using virtual subhosting. The likelihood of someone using an non
HTTP/1.1 browser is very rare, but nonetheless,you should be
familiar with this limitation.
- Server Hits
A Virtual Server is capable of handling 30,000 to 50,000 hits per day.
"Hits" are not "visitors", but rather requests for files.For
example, one web page with 20 graphics equals 21 hits, or 21 file requests.
If you have 5 subhosted domain names, each which is trying to accommodate
10,000 hits per day, server performance will be adversely affected.
This "slowdown" will affect all of your clients on the Virtual Server
you are using to subhost.
Proper load balancing is a science that a sysadmin must learn in
order to succeed with serious virtual subhosting. When a slowdown
occurs due to server overload, the system administrator will properly
"manage" his or her Virtual Server by reducing the number of subhosts
on the Virtual Server by:
- upgrading one of the especially high-traffic virtual hosted sites
to its own Virtual Server;
- moving some subhosts to a less busy Virtual Server; or
- upgrading the Server type to a more powerful Server.
For performance reasons, we have established recommendations with regard
to the number of virtual hosts you should place on a single Virtual
Server System. Keep in mind that the purpose of these recommendations
is to ensure continued performance of your server and the other sites
that you host.
Server A: around 5
Server B: around
25 low-volume subhosts
Server C: around
60 low-volume subhosts
We cannot guarantee the number of virtual hosts you will be able
to successfully host since each site uses a different amount of server
resources. It may be that you can only host one other virtual host
before your resources are exhausted on your Virtual Server. It is
up to you to monitor virtual hosts and upgrade high load virtual hosts
to their own Virtual Servers.
- Single IP Address
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, however, and have been for several
- Single Digital Certificate
A Virtual Server can only support a single Digital
Certificate. This can make the use of shared SSL difficult since
all subhosts must use the same Digital Certificate and only one domain
name can be associated with a Digital Certificate.
There is a feasible workaround, however. Thawte
allows wildcard digital certificates, which means you can provide a
secure connection for all your clients by offering them a canonical
domain. Thus, "clientA.you.com," "clientB.you.com," and "clientC.you.com"
may all share the same digital certificate. VeriSign,
another certifying authority, does not provide wildcard digital certificates.
- Single Telnet Account
A virtual subhost does not have Telnet
access to the Virtual Server. Only the Virtual Server Administrator
receives a telnet account. This provides a more secure environment for
your server as a whole.
- Microsoft FrontPage Extensions
Microsoft® FrontPage® 97 child webs will not work within a
virtual subhosting environment. Please note that Blue Reef only supports
FrontPage 2000 extensions; any other FrontPage product you install on
your Virtual Server is at your risk.
- Email Addresses
There are some limitations to the email capability of subhosts, specifically
how the Virtual Server interprets email addresses. Blue Reef provides,
however, a way to get around this limitation by using a proprietary
utility called virtmaps.
It is important to understand that giving cgi-bin access to your virtually
subhosted clients is a potential security risk. This is because the
CGI scripts your customers install and execute have all of the rights
and privileges of the CGI scripts you execute. Therefore, it is possible
for a virtually 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 Virtual Server. Please see our document,
Virtual Subhosting and Security Issues
for more information.