The HostHeader setting is one of the AppSettings
that is contained in the web.config file.
The HostHeader setting is used by installs that want to remove a portion of every path. This is primarily used when a site when a site wants to escalate the level it is at, whilst preserving the hierarchy internally e.g. perhaps a site is deployed into a virtual directory called "johndoe", this means the final url will look similar to "www.mydomain.com/johndoe/Default.aspx".
Whilst the correct fix is to move the contents to the root folder and add a Portal Alias
for www.mydomain.com, sometimes this is not possible e.g. perhaps another asp.net application is in the root folder, or perhaps the hosting provider does not support this. If this is the case then it sometimes this can be resolved by using this parameter.
Note: this parameter provides a very simple replacement of the set text on the URL - if the text is used elsewhere e.g. you have a page also called "johndoe" this will not work.
The HostHeader default looks similar to:
<add key="HostHeader" value="" />
Using the example of above we would set this to:
<add key="HostHeader" value="johndoe" />