dialers/desync: retry recvmsg once after a 5ms delay

intra/dialers/split_and_desync.go

@@ -1,10 +1,5 @@
 package dialers
 
-/*
-Combine direct_split with TCB (Transmission Control Block) Desynchronization Attack
-Inspired by byedpi
-*/
-
 import (
 	"io"
 	"math/rand"
@@ -37,12 +32,14 @@ const (
 // TODO: invalidate cache on network changes.
 var ttlcache = core.NewDefaultSieve[netip.Addr, int]()
 
+// Combines direct split with TCB Desynchronization Attack
+// Inspired by byedpi: github.com/hufrea/byedpi/blob/82e5229df00/desync.c#L69-L123
 type overwriteSplitter struct {
-	conn    *net.TCPConn
-	used    atomic.Bool
-	ttl     int
-	ip6     bool
-	payload []byte // must be smaller than 1st written packet
+	conn    *net.TCPConn // underlying connection
+	used    atomic.Bool  // ensures desync only runs once
+	ttl     int          // desync TTL
+	ip6     bool         // IPv6
+	payload []byte       // must be smaller than 1st written packet
 	// note: Normal ClientHello generated by browsers is 517 bytes. If kyber is enabled, the ClientHello can be larger.
 }
 
@@ -214,22 +211,29 @@ func desyncWithTraceroute(d *protect.RDial, ipp netip.AddrPort) (*overwriteSplit
 		core.Recycle(bptr)
 	}()
 
-	split1 := &overwriteSplitter{
+	oc := &overwriteSplitter{
 		conn:    tcpConn,
 		ttl:     DESYNC_NOOP_TTL,
 		payload: []byte(Http1_1String),
 		ip6:     isIPv6,
 	}
 
-	// skip desync if no measurement is done
-	defer split1.used.Store(!measureTTL)
-
+	processed := false
 	// after TCP handshake, check received ICMP messages, if measureTTL is true.
-	for i := 0; i < DESYNC_MAX_TTL-1 && measureTTL; i += DESYNC_DELTA_TTL {
+	for i := 0; i < desync_max_ttl-1 && measureTTL; i += desync_delta_ttl {
 		_, cmsgN, _, from, err := unix.Recvmsg(udpFD, msgBuf[:], cmsgBuf[:], unix.MSG_ERRQUEUE)
 		if err != nil {
-			log.V("split-desync: recvmsg %v failed: %v", ipp, err)
-			break // udpConn must be nonblocking
+			log.V("split-desync: recvmsg %v, processed? %t, err: %v", ipp, processed, err)
+			if processed { // recvmsg processed at least once
+				break // uc must be nonblocking
+			} else {
+				processed = true
+				i -= desync_delta_ttl
+				time.Sleep(5 * time.Millisecond)
+				continue
+			}
+		} else {
+			processed = true
 		}
 
 		cmsgs, err := unix.ParseSocketControlMessage(cmsgBuf[:cmsgN])
@@ -242,26 +246,29 @@ func desyncWithTraceroute(d *protect.RDial, ipp netip.AddrPort) (*overwriteSplit
 			if exceedsHopLimit(cmsgs) {
 				fromPort := from.(*unix.SockaddrInet6).Port
 				ttl := fromPort - basePort
-				if ttl > DESYNC_MAX_TTL {
+				if ttl > desync_max_ttl {
 					break
 				}
-				split1.ttl = max(split1.ttl, ttl)
+				oc.ttl = max(oc.ttl, ttl)
 			}
 		} else {
 			if exceedsTTL(cmsgs) {
 				fromPort := from.(*unix.SockaddrInet4).Port
 				ttl := fromPort - basePort
-				if ttl > DESYNC_MAX_TTL {
+				if ttl > desync_max_ttl {
 					break
 				}
-				split1.ttl = max(split1.ttl, ttl)
+				oc.ttl = max(oc.ttl, ttl)
 			}
 		}
 	}
 
-	log.D("split-desync: done: %v, ok? %t, ttl: %d", ipp, measureTTL, split1.ttl)
+	// skip desync if no measurement is done
+	oc.used.Store(!processed)
+
+	log.D("split-desync: done: %v, ok? %t, ttl: %d", ipp, processed, oc.ttl)
 
-	return split1, nil
+	return oc, nil
 }
 
 func desyncWithFixedTtl(d *protect.RDial, ipp netip.AddrPort, initialTTL int) (*overwriteSplitter, error) {
@@ -444,5 +451,7 @@ func (s *overwriteSplitter) ReadFrom(reader io.Reader) (bytes int64, err error)
 
 	b, err := s.conn.ReadFrom(reader)
 	bytes += b
+	log.V("split-desync: readfrom: done; sz: %d; err: %v", bytes, err)
+
 	return
 }