How to Set Host Header Dynamically After Backend Server Selection in HAProxy

Help!
1

Hello Team

I’m working on a setup where I need to dynamically set the Host header based on the backend server that HAProxy selects upon load balancing. My backend configuration looks like this:

backend be
    mode http
    option forwardfor
    balance leastconn
    option httpchk
    http-check send meth GET uri /health
    http-request set-header X-Forwarded-Port %[dst_port]
    http-request add-header X-Forwarded-Proto https if { ssl_fc }
    http-request add-header X-Forwarded-Host %[req.hdr(host)]

    http-request set-header Host [SERVER_NAME]

    server srv-instance1.com srv-instance1.com:443 check inter 10s fastinter 1s downinter 1m resolvers nimbus init-addr last,libc,none ssl verify none check-sni srv-instance1.com sni str(srv-instance1.com) alpn h2 check-alpn h2
    server srv-instance2.com srv-instance2.com:443 check inter 10s fastinter 1s downinter 1m resolvers nimbus init-addr last,libc,none ssl verify none check-sni srv-instance2.com sni str(srv-instance2.com) alpn h2 check-alpn h2

HAProxy version: 2.6.12
And I am using grpc http2 with tls

In this setup, I want the Host header to match the name of the server that HAProxy has chosen (e.g., srv-instance1.com or srv-instance2.com)

What I’ve Tried:
I attempted to use http-send-name-header to set the Host header, but it doesn’t seem to be working in my scenario.
Also variables like srv_name and srv_id also does not help.

Question:
How can I set the Host header dynamically based on the backend server that HAProxy selects?
Is there a way to achieve this ? Any guidance or suggestions would be greatly appreciated!

Thanks in advance!

2

Your scenario matches the use case of http-send-name-header. Can you elaborate what exactly happens when you use this feature, without any competing configurations like http-request set-header Host ?