Add UpstreamHostOverride support for plugins and enhance reverse proxy handling

- Introduced UpstreamHostOverride constant in ctx package for plugin use.
- Implemented logic in reverse_proxy.go to allow plugins to override upstream host or URL.
- Added a new test file for URL redirect plugin to demonstrate UpstreamHostOverride functionality.

Author: sedkis

ctx/ctx.go

@@ -53,6 +53,8 @@ const (
 	CacheOptions
 	OASDefinition
 	SelfLooping
+	// Used by plugins to override the upstream host
+	UpstreamHostOverride
 )
 
 func ctxSetSession(r *http.Request, s *user.SessionState, scheduleUpdate bool, hashKey bool) {

gateway/reverse_proxy.go

@@ -293,8 +293,27 @@ func (gw *Gateway) TykNewSingleHostReverseProxy(target *url.URL, spec *APISpec,
 			}
 		}
 
+		// Simple upstream host override from plugins via context.
+		// Accepts either a host (e.g. "api.example.com") or full URL (e.g. "https://api.example.com").
+		if v := req.Context().Value(ctx.UpstreamHostOverride); v != nil {
+			if hostOrURL, ok := v.(string); ok && hostOrURL != "" {
+				if strings.Contains(hostOrURL, "://") {
+					if u, err := url.Parse(hostOrURL); err == nil {
+						if u.Scheme != "" {
+							req.URL.Scheme = u.Scheme
+						}
+						if u.Host != "" {
+							req.URL.Host = u.Host
+						}
+					}
+				} else {
+					req.URL.Host = hostOrURL
+				}
+			}
+		}
+
 		if !spec.Proxy.PreserveHostHeader {
-			req.Host = targetToUse.Host
+			req.Host = req.URL.Host
 		}
 
 		if targetQuery == "" || req.URL.RawQuery == "" {

test/goplugins/url_redirect_plugin.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/TykTechnologies/tyk/ctx"
+	"github.com/TykTechnologies/tyk/log"
+)
+
+var logger = log.Get()
+
+func HeaderBasedRedirect(rw http.ResponseWriter, r *http.Request) {
+	routeTo := r.Header.Get("X-Route-To")
+	if routeTo == "" {
+		return
+	}
+
+	logger.Info("[PLUGIN] X-Route-To header: ", routeTo)
+
+	// Use the UpstreamHostOverride context key
+	// This can be either a host or a full URL
+	newCtx := context.WithValue(r.Context(), ctx.UpstreamHostOverride, routeTo)
+	*r = *r.WithContext(newCtx)
+
+	logger.Info("[PLUGIN] Set UpstreamHostOverride to: ", routeTo)
+}
+
+func main() {}