Default proxy classes provide an adequate level of security. You typically derive your own classes if you want to change the values of certain attributes, like manually setting the contents (like browser type, operating system) of the request headers leaving the HTTP proxy. Complex proxy setups – such as virus filtering of HTTP, SMTP, traffic or proxy stacking – also require derived classes.

This process is somewhat complex involving many steps; therefore will be demonstrated using an example of changing the User-Agent HTTP request header output by a custom HTTP proxy component.

The customized proxy class you are defining is based on an already defined proxy class. There are quite a lot of predefined proxy classes that are available by default. For some protocols (for example HTTP and FTP) there are more than one to choose from, each with a specific intended purpose. FtpProxyRO, for instance, is for read-only FTP access, while FtpProxyRW is for read/write FTP access.