Implement IP_TTL.

Also change the default TTL to 64 to match Linux.

PiperOrigin-RevId: 273430341

4 files changed, 54 insertions, 9 deletions

diff --git a/pkg/tcpip/transport/tcp/accept.go b/pkg/tcpip/transport/tcp/accept.go
index 3ae4a5426..b8b4bcee8 100644
--- a/pkg/tcpip/transport/tcp/accept.go
+++ b/pkg/tcpip/transport/tcp/accept.go
@@ -439,7 +439,7 @@ func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
 				TSEcr: opts.TSVal,
 				MSS:   uint16(mss),
 			}
-			sendSynTCP(&s.route, s.id, header.TCPFlagSyn|header.TCPFlagAck, cookie, s.sequenceNumber+1, ctx.rcvWnd, synOpts)
+			sendSynTCP(&s.route, s.id, e.ttl, header.TCPFlagSyn|header.TCPFlagAck, cookie, s.sequenceNumber+1, ctx.rcvWnd, synOpts)
 			e.stack.Stats().TCP.ListenOverflowSynCookieSent.Increment()
 		}
 
diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index 21038a65a..1d6e7f5f3 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -238,6 +238,7 @@ func (h *handshake) synSentState(s *segment) *tcpip.Error {
 	h.state = handshakeSynRcvd
 	h.ep.mu.Lock()
 	h.ep.state = StateSynRecv
+	ttl := h.ep.ttl
 	h.ep.mu.Unlock()
 	synOpts := header.TCPSynOptions{
 		WS:    int(h.effectiveRcvWndScale()),
@@ -251,7 +252,10 @@ func (h *handshake) synSentState(s *segment) *tcpip.Error {
 		SACKPermitted: rcvSynOpts.SACKPermitted,
 		MSS:           h.ep.amss,
 	}
-	sendSynTCP(&s.route, h.ep.id, h.flags, h.iss, h.ackNum, h.rcvWnd, synOpts)
+	if ttl == 0 {
+		ttl = s.route.DefaultTTL()
+	}
+	sendSynTCP(&s.route, h.ep.id, ttl, h.flags, h.iss, h.ackNum, h.rcvWnd, synOpts)
 
 	return nil
 }
@@ -296,7 +300,7 @@ func (h *handshake) synRcvdState(s *segment) *tcpip.Error {
 			SACKPermitted: h.ep.sackPermitted,
 			MSS:           h.ep.amss,
 		}
-		sendSynTCP(&s.route, h.ep.id, h.flags, h.iss, h.ackNum, h.rcvWnd, synOpts)
+		sendSynTCP(&s.route, h.ep.id, h.ep.ttl, h.flags, h.iss, h.ackNum, h.rcvWnd, synOpts)
 		return nil
 	}
 
@@ -460,7 +464,7 @@ func (h *handshake) execute() *tcpip.Error {
 			synOpts.WS = -1
 		}
 	}
-	sendSynTCP(&h.ep.route, h.ep.id, h.flags, h.iss, h.ackNum, h.rcvWnd, synOpts)
+	sendSynTCP(&h.ep.route, h.ep.id, h.ep.ttl, h.flags, h.iss, h.ackNum, h.rcvWnd, synOpts)
 	for h.state != handshakeCompleted {
 		switch index, _ := s.Fetch(true); index {
 		case wakerForResend:
@@ -469,7 +473,7 @@ func (h *handshake) execute() *tcpip.Error {
 				return tcpip.ErrTimeout
 			}
 			rt.Reset(timeOut)
-			sendSynTCP(&h.ep.route, h.ep.id, h.flags, h.iss, h.ackNum, h.rcvWnd, synOpts)
+			sendSynTCP(&h.ep.route, h.ep.id, h.ep.ttl, h.flags, h.iss, h.ackNum, h.rcvWnd, synOpts)
 
 		case wakerForNotification:
 			n := h.ep.fetchNotifications()
@@ -579,9 +583,9 @@ func makeSynOptions(opts header.TCPSynOptions) []byte {
 	return options[:offset]
 }
 
-func sendSynTCP(r *stack.Route, id stack.TransportEndpointID, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size, opts header.TCPSynOptions) *tcpip.Error {
+func sendSynTCP(r *stack.Route, id stack.TransportEndpointID, ttl uint8, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size, opts header.TCPSynOptions) *tcpip.Error {
 	options := makeSynOptions(opts)
-	err := sendTCP(r, id, buffer.VectorisedView{}, r.DefaultTTL(), flags, seq, ack, rcvWnd, options, nil)
+	err := sendTCP(r, id, buffer.VectorisedView{}, ttl, flags, seq, ack, rcvWnd, options, nil)
 	putOptions(options)
 	return err
 }
@@ -629,7 +633,7 @@ func sendTCP(r *stack.Route, id stack.TransportEndpointID, data buffer.Vectorise
 		r.Stats().TCP.ResetsSent.Increment()
 	}
 
-	return r.WritePacket(gso, hdr, data, ProtocolNumber, ttl)
+	return r.WritePacket(gso, hdr, data, ProtocolNumber, ttl, ttl == 0 /* useDefaultTTL */)
 }
 
 // makeOptions makes an options slice.
@@ -678,7 +682,7 @@ func (e *endpoint) sendRaw(data buffer.VectorisedView, flags byte, seq, ack seqn
 		sackBlocks = e.sack.Blocks[:e.sack.NumBlocks]
 	}
 	options := e.makeOptions(sackBlocks)
-	err := sendTCP(&e.route, e.id, data, e.route.DefaultTTL(), flags, seq, ack, rcvWnd, options, e.gso)
+	err := sendTCP(&e.route, e.id, data, e.ttl, flags, seq, ack, rcvWnd, options, e.gso)
 	putOptions(options)
 	return err
 }
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index f9d5e0085..83d92b3e1 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -228,6 +228,7 @@ type endpoint struct {
 	isRegistered      bool
 	boundNICID        tcpip.NICID `state:"manual"`
 	route             stack.Route `state:"manual"`
+	ttl               uint8
 	v6only            bool
 	isConnectNotified bool
 	// TCP should never broadcast but Linux nevertheless supports enabling/
@@ -1116,6 +1117,12 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 		e.v6only = v != 0
 		return nil
 
+	case tcpip.TTLOption:
+		e.mu.Lock()
+		e.ttl = uint8(v)
+		e.mu.Unlock()
+		return nil
+
 	case tcpip.KeepaliveEnabledOption:
 		e.keepalive.Lock()
 		e.keepalive.enabled = v != 0
@@ -1313,6 +1320,12 @@ func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 		}
 		return nil
 
+	case *tcpip.TTLOption:
+		e.mu.Lock()
+		*o = tcpip.TTLOption(e.ttl)
+		e.mu.Unlock()
+		return nil
+
 	case *tcpip.TCPInfoOption:
 		*o = tcpip.TCPInfoOption{}
 		e.mu.RLock()
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 089826a88..a86123829 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -1752,6 +1752,34 @@ func TestSendGreaterThanMTU(t *testing.T) {
 	testBrokenUpWrite(t, c, maxPayload)
 }
 
+func TestSetTTL(t *testing.T) {
+	for _, wantTTL := range []uint8{1, 2, 50, 64, 128, 254, 255} {
+		t.Run(fmt.Sprintf("TTL:%d", wantTTL), func(t *testing.T) {
+			c := context.New(t, 65535)
+			defer c.Cleanup()
+
+			var err *tcpip.Error
+			c.EP, err = c.Stack().NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, &waiter.Queue{})
+			if err != nil {
+				t.Fatalf("NewEndpoint failed: %v", err)
+			}
+
+			if err := c.EP.SetSockOpt(tcpip.TTLOption(wantTTL)); err != nil {
+				t.Fatalf("SetSockOpt failed: %v", err)
+			}
+
+			if err := c.EP.Connect(tcpip.FullAddress{Addr: context.TestAddr, Port: context.TestPort}); err != tcpip.ErrConnectStarted {
+				t.Fatalf("Unexpected return value from Connect: %v", err)
+			}
+
+			// Receive SYN packet.
+			b := c.GetPacket()
+
+			checker.IPv4(t, b, checker.TTL(wantTTL))
+		})
+	}
+}
+
 func TestActiveSendMSSLessThanMTU(t *testing.T) {
 	const maxPayload = 100
 	c := context.New(t, 65535)