reverseproxy: Replace health header placeholders (#5861)

modules/caddyhttp/reverseproxy/caddyfile.go

@@ -372,7 +372,7 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 				if len(values) == 0 {
 					values = append(values, "")
 				}
-				healthHeaders[key] = values
+				healthHeaders[key] = append(healthHeaders[key], values...)
 			}
 			if h.HealthChecks == nil {
 				h.HealthChecks = new(HealthChecks)

modules/caddyhttp/reverseproxy/healthchecks.go

@@ -358,11 +358,17 @@ func (h *Handler) doActiveHealthCheck(dialInfo DialInfo, hostAddr string, upstre
 	}
 	ctx = context.WithValue(ctx, caddyhttp.OriginalRequestCtxKey, *req)
 	req = req.WithContext(ctx)
-	for key, hdrs := range h.HealthChecks.Active.Headers {
+
+	// set headers, using a replacer with only globals (env vars, system info, etc.)
+	repl := caddy.NewReplacer()
+	for key, vals := range h.HealthChecks.Active.Headers {
+		key = repl.ReplaceAll(key, "")
 		if key == "Host" {
-			req.Host = h.HealthChecks.Active.Headers.Get(key)
-		} else {
-			req.Header[key] = hdrs
+			req.Host = repl.ReplaceAll(h.HealthChecks.Active.Headers.Get(key), "")
+			continue
+		}
+		for _, val := range vals {
+			req.Header.Add(key, repl.ReplaceKnown(val, ""))
 		}
 	}