[SQU] Manipulating Headers

Hi,
I am new to Squid and would like to know how to manipulate HTTP headers.
Do I get a call back function or some hook where I can place my code for modifying headers when a HTTP request is send to the origin server.
Also I would like to parse the response header for the request which I have modified. So how I do I get the context of the Request which I have modified.
I would be grateful for any help.

Thanks and Regards
Sitangshu

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