reverseproxy: Restore request's original host and header (fix #3509)

We already restore them within the retry loop, but after successful
proxy we didn't reset them, so as handlers bubble back up, they would
see the values used for proxying.

Thanks to @ziddey for identifying the cause.

1 files changed, 10 insertions, 3 deletions

diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
index 7971348..bb1453a 100644
--- a/modules/caddyhttp/reverseproxy/reverseproxy.go
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -329,10 +329,17 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyht
 			fmt.Errorf("preparing request for upstream round-trip: %v", err))
 	}
 
-	// we will need the original headers and Host
-	// value if header operations are configured
-	reqHeader := r.Header
+	// we will need the original headers and Host value if
+	// header operations are configured; and we should
+	// restore them after we're done if they are changed
+	// (for example, changing the outbound Host header
+	// should not permanently change r.Host; issue #3509)
 	reqHost := r.Host
+	reqHeader := r.Header
+	defer func() {
+		r.Host = reqHost
+		r.Header = reqHeader
+	}()
 
 	start := time.Now()