HAProxy set authorization header from cookie - haproxy

Question

I have a backend that I don't control that works using magic links. If I use the magic link, I don't have to log on. I want to share the magic link externally without exposing the actual key as the key changes from time to time and I was hoping to use haproxy as a reverse proxy. I don't want the link to be open to the entire internet and would like to use basic auth as well. The problem I am facing is that the backend overwrites the Authorization header (it's required for the magic link to work) and I get stuck in a loop and need to log on every time
My workaround:
On first request I request the basic auth (works)
Then I write it to a cookie (this part works)
On each subsequent request, if the cookie exists, I read the cookie and set the Authorization header to the cookie value (this part does not work)
Then I run http_auth which in my mind should now work since I have overwritten the header
But it does not work. Any suggestions?
userlist auth-list
user myuser insecure-password mypass
frontend myfrontend
bind *:80
mode http
acl is-path path -i -m beg /publiclink
acl has_cookie req.hdr(X-MyAuth) -m found
http-request set-header Authorization %[req.hdr(X-MyAuth)] if has_cookie
http-request auth unless { http_auth(auth-list) }
http-request set-var(txn.myhostheader) req.hdr(Authorization) if { http_auth(auth-list) !has_cookie }
default_backend node
backend node
mode http
server dcnode1 192.168.0.1:8000 check
http-response set-header set-cookie "X-MyAuth=%[var(txn.myhostheader)]; Path=/" if { var(txn.myhostheader) -m found }
http-request replace-path /publiclink1(.*) /magiclink\1
http-request set-header Authorization "Key magiclink"

Answer 1

My answer is related to this point :
On each subsequent request, if the cookie exists, I read the cookie and set the Authorization header to the cookie value (this part does not work)
In your backend, you set the Set-Cookie: X-MyAuth=... header :
backend node
http-response set-header set-cookie "X-MyAuth=%[var(txn.myhostheader)]; Path=/" if { var(txn.myhostheader) -m found }
So the next request contains a Cookie header like this one : Cookie: .* X-MyAuth=.*.
But in your frontend, you use the X-MyAuth header (which probably does not exist) :
frontend myfrontend
acl has_cookie req.hdr(X-MyAuth) -m found
http-request set-header Authorization %[req.hdr(X-MyAuth)] if has_cookie
You may use the X-MyAuth cookie value like this :
frontend myfrontend
acl has_cookie cook(X-MyAuth) -m found
http-request set-header Authorization %[cook(X-MyAuth)] if has_cookie