Copying request headers to response header (1 reply)

Hello,

I'm using the unique-id-header, and i'm having some difficulty getting it
to work how i want it to.

First of all, we have a variety of backends, so i'd like if what i'm trying
to do could be handled entirely by haproxy.

We're trying to implement a header for storing a unique request id (and if
possible, making it nested), so it's possible to track the entire chain of
requests going through our proxies.

Basically that means if a request already contains a "X-ReqId" header, it
should just leave it be, and if there is none, it should tag on it's own.

Now i can it to set the header just fine, (not sure about the don't set one
if already there part), but i can't seem to figure out how to get this
shown in the response (without going outside haproxy).

I'm not entirely familar with the haproxy processing pipeline, and what
samples are available where.

I tried something like this in the frontend (and tried backends as well),
to no avail.

http-response set-header X-ReqId %[req.fhdr(x-reqid)]

Any pointers on how i can achieve my goal?