Revert "plugin/forward: Continue waiting after receiving malformed responses (#6014)" (#6270)

This reverts commit 604a902e2c.

plugin/pkg/proxy/connect.go

@@ -7,7 +7,6 @@ package proxy
 import (
 	"context"
 	"io"
-	"net"
 	"strconv"
 	"sync/atomic"
 	"time"
@@ -118,20 +117,11 @@ func (p *Proxy) Connect(ctx context.Context, state request.Request, opts Options
 	for {
 		ret, err = pc.c.ReadMsg()
 		if err != nil {
-			// For UDP, if the error is not a network error keep waiting for a valid response to prevent malformed
-			// spoofs from blocking the upstream response.
-			// In the case this is a legitimate malformed response from the upstream, this will result in a timeout.
-			if proto == "udp" {
-				if _, ok := err.(net.Error); !ok {
-					continue
-				}
-			}
-			pc.c.Close() // connection closed by peer, close the persistent connection
+			pc.c.Close() // not giving it back
 			if err == io.EOF && cached {
 				return nil, ErrCachedClosed
 			}
-
-			// recover the origin Id after upstream.
+			// recovery the origin Id after upstream.
 			if ret != nil {
 				ret.Id = originId
 			}