One of the most well-kept secrets in technology is the extensibility of HTTP. It's one of the reasons it became the de facto application transport protocol and it was instrumental in getting SOAP off the ground before SOAP 1.2 and WS-I Basic Profile made the requirement for the SOAP Action header obsolete.

Web browsers aren't capable of adding custom HTTP headers on their own; that functionality comes from the use of client-side scripting languages such as JavaScript or VBScript. Other RIA (Rich Internet Applications) client platforms such as Adobe AIR and Flash are also capable of adding HTTP headers, though both have limitations on which (if any) custom headers you can use.

There are valid reasons for wanting to set a custom header. The most common use of custom HTTP headers is to preserve in some way the source IP address of the client for logging purposes in a load-balanced environment using the X-Forwarded-For custom header.  Custom HTTP headers can be set by the client or set by the server or intermediary (load-balancer, application delivery controller, cache) as well and often are to indicate that the content has passed through a proxy. A quick perusal of the web shows developers desiring to use custom HTTP headers for a variety of reasons including security, SSO (single sign on) functionality, and to transfer data between pages/applications.

Unfortunately, a class of vulnerabilities known as "HTTP header injection" often causes platform providers like Adobe to limit or completely remove the ability to manipulate HTTP headers on the client. And adding custom headers using JavaScript or VBScript may require modification of the application and relies on the user allowing scripts to run in the first place, the consistency of which can no longer be relied upon.

But what if you really need those custom headers to either address a problem or enable some functionality?

All is not lost; you can generally use an intelligent proxy-based load balancer (application delivery controller) to insert the headers for you.If the load balancer/application delivery controller has the ability to inspect requests and modify the requests and responses with a technology like iRules, you can easily add your custom headers at the intermediary without losing the functionality desired or needing to change the request method from GET to POST, as some have done to get around these limitations.

Using your load balancer/application delivery controller to insert, delete, or modify custom HTTP headers has other advantages as well:

1. You don't need to modify the client or the server-side application or script that served the client
2. The load balancer can add the required custom HTTP header(s) for all applications at one time in one place
3. Your application will still work even if the client disables scripting

Custom HTTP headers are often used for valid reasons when developing applications. The inability to manipulate them easily on the client can interfere with the development lifecycle and make it more difficult to address vulnerabilities and quirks with packaged applications and the platforms on which applications are often deployed. Taking advantage of more advanced features available in modern load balancers/application delivery controllers makes implementing such workarounds simple.