I have file download site. What I look for is limiting bandwidth per IP (!). Limit should be set dynamically by HTTP header from backend.
My current implementation uses X-Accel-Limit-Rate (I can change that header, it's not hard-coded anywhere), but it does limit only current connection/request.
Is my idea doable in G-Wan?
Yes, this can be done.
Write a G-WAN handler to extract the X-Accel-Limit-Rate HTTP header. Then enforce this policy by using the throttle_reply() G-WAN API call documented here.
An example available called throttle.c might help you further.
The throttle_reply() G-WAN function lets you apply throttling on a global basis or per connection, so you will just apply the relevant throttling values for either IP addresses or authenticated users, depending on your needs.
throttle_reply() can change the download speed dynamically during the lifespan of each connection so you can slow-down old connections and create new ones with an adaptive download rate.
Of course, this can be enforced on a per client IP address (or cookie, or even ISP/Datacenter AS record) to deal with huge workloads.