If proxy protocol is enabled, keepalive was disabled because there was no known way to differentiate requests to the same upstream from different clients.

But caddy actually updates the url in the requests to point to the upstream, this url is then used to dial to the upstream unless if a proxy is used for this request, in which case the proxy url is used for the dial.

We can update the url to add unique identifiers to create a mapping between the client address and the upstream address so that connections can be kept alive and requests from different clients to the same upstream won't interfere with each other.

Using the following caddyfile:

{
 debug
 skip_install_trust
 local_certs
 servers :9080 {
 listener_wrappers {
 proxy_protocol {
 allow 192.168.100.103/24
 }
 }
 }
}
localhost {
 reverse_proxy 192.168.100.103:9080 {
 header_up Host {upstream_hostport}
 transport http {
 proxy_protocol v2
 }
 }
}
http://192.168.100.103:9080 {
 respond "{http.request.remote.host}" 200
}

And curl command will output the same response;

$ curl -k https://localhost --interface 127.0.0.1
127.0.0.1
$ curl -k https://localhost --interface ::1
::1

Assistance Disclosure

No AI was used.