3 files changed, 55 insertions, 6 deletions

diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index 52f5af777..8ae83437e 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -83,6 +83,7 @@ type endpoint struct {
 	route          stack.Route `state:"manual"`
 	dstPort        uint16
 	v6only         bool
+	ttl            uint8
 	multicastTTL   uint8
 	multicastAddr  tcpip.Address
 	multicastNICID tcpip.NICID
@@ -374,12 +375,16 @@ func (e *endpoint) Write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, <-c
 		return 0, nil, tcpip.ErrMessageTooLong
 	}
 
-	ttl := route.DefaultTTL()
+	ttl := e.ttl
+	useDefaultTTL := ttl == 0
+
 	if header.IsV4MulticastAddress(route.RemoteAddress) || header.IsV6MulticastAddress(route.RemoteAddress) {
 		ttl = e.multicastTTL
+		// Multicast allows a 0 TTL.
+		useDefaultTTL = false
 	}
 
-	if err := sendUDP(route, buffer.View(v).ToVectorisedView(), e.id.LocalPort, dstPort, ttl); err != nil {
+	if err := sendUDP(route, buffer.View(v).ToVectorisedView(), e.id.LocalPort, dstPort, ttl, useDefaultTTL); err != nil {
 		return 0, nil, err
 	}
 	return int64(len(v)), nil, nil
@@ -414,6 +419,11 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 
 		e.v6only = v != 0
 
+	case tcpip.TTLOption:
+		e.mu.Lock()
+		e.ttl = uint8(v)
+		e.mu.Unlock()
+
 	case tcpip.MulticastTTLOption:
 		e.mu.Lock()
 		e.multicastTTL = uint8(v)
@@ -628,6 +638,12 @@ 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.MulticastTTLOption:
 		e.mu.Lock()
 		*o = tcpip.MulticastTTLOption(e.multicastTTL)
@@ -694,7 +710,7 @@ func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 
 // sendUDP sends a UDP segment via the provided network endpoint and under the
 // provided identity.
-func sendUDP(r *stack.Route, data buffer.VectorisedView, localPort, remotePort uint16, ttl uint8) *tcpip.Error {
+func sendUDP(r *stack.Route, data buffer.VectorisedView, localPort, remotePort uint16, ttl uint8, useDefaultTTL bool) *tcpip.Error {
 	// Allocate a buffer for the UDP header.
 	hdr := buffer.NewPrependable(header.UDPMinimumSize + int(r.MaxHeaderLength()))
 
@@ -720,7 +736,7 @@ func sendUDP(r *stack.Route, data buffer.VectorisedView, localPort, remotePort u
 	// Track count of packets sent.
 	r.Stats().UDP.PacketsSent.Increment()
 
-	return r.WritePacket(nil /* gso */, hdr, data, ProtocolNumber, ttl)
+	return r.WritePacket(nil /* gso */, hdr, data, ProtocolNumber, ttl, useDefaultTTL)
 }
 
 func (e *endpoint) checkV4Mapped(addr *tcpip.FullAddress, allowMismatch bool) (tcpip.NetworkProtocolNumber, *tcpip.Error) {
diff --git a/pkg/tcpip/transport/udp/protocol.go b/pkg/tcpip/transport/udp/protocol.go
index f5cc932dd..0b24bbc85 100644
--- a/pkg/tcpip/transport/udp/protocol.go
+++ b/pkg/tcpip/transport/udp/protocol.go
@@ -130,7 +130,7 @@ func (p *protocol) HandleUnknownDestinationPacket(r *stack.Route, id stack.Trans
 		pkt.SetType(header.ICMPv4DstUnreachable)
 		pkt.SetCode(header.ICMPv4PortUnreachable)
 		pkt.SetChecksum(header.ICMPv4Checksum(pkt, payload))
-		r.WritePacket(nil /* gso */, hdr, payload, header.ICMPv4ProtocolNumber, r.DefaultTTL())
+		r.WritePacket(nil /* gso */, hdr, payload, header.ICMPv4ProtocolNumber, 0, true /* useDefaultTTL */)
 
 	case header.IPv6AddressSize:
 		if !r.Stack().AllowICMPMessage() {
@@ -164,7 +164,7 @@ func (p *protocol) HandleUnknownDestinationPacket(r *stack.Route, id stack.Trans
 		pkt.SetType(header.ICMPv6DstUnreachable)
 		pkt.SetCode(header.ICMPv6PortUnreachable)
 		pkt.SetChecksum(header.ICMPv6Checksum(pkt, r.LocalAddress, r.RemoteAddress, payload))
-		r.WritePacket(nil /* gso */, hdr, payload, header.ICMPv6ProtocolNumber, r.DefaultTTL())
+		r.WritePacket(nil /* gso */, hdr, payload, header.ICMPv6ProtocolNumber, 0, true /* useDefaultTTL */)
 	}
 	return true
 }
diff --git a/pkg/tcpip/transport/udp/udp_test.go b/pkg/tcpip/transport/udp/udp_test.go
index d1ba8d2ce..faa728b68 100644
--- a/pkg/tcpip/transport/udp/udp_test.go
+++ b/pkg/tcpip/transport/udp/udp_test.go
@@ -1207,6 +1207,39 @@ func TestTTL(t *testing.T) {
 	}
 }
 
+func TestSetTTL(t *testing.T) {
+	for _, flow := range []testFlow{unicastV4, unicastV4in6, unicastV6, unicastV6Only, broadcast, broadcastIn6} {
+		t.Run(fmt.Sprintf("flow:%s", flow), func(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 := newDualTestContext(t, defaultMTU)
+					defer c.cleanup()
+
+					c.createEndpointForFlow(flow)
+
+					if err := c.ep.SetSockOpt(tcpip.TTLOption(wantTTL)); err != nil {
+						c.t.Fatalf("SetSockOpt failed: %v", err)
+					}
+
+					var p stack.NetworkProtocol
+					if flow.isV4() {
+						p = ipv4.NewProtocol()
+					} else {
+						p = ipv6.NewProtocol()
+					}
+					ep, err := p.NewEndpoint(0, tcpip.AddressWithPrefix{}, nil, nil, nil)
+					if err != nil {
+						t.Fatal(err)
+					}
+					ep.Close()
+
+					testWrite(c, flow, checker.TTL(wantTTL))
+				})
+			}
+		})
+	}
+}
+
 func TestMulticastInterfaceOption(t *testing.T) {
 	for _, flow := range []testFlow{multicastV4, multicastV4in6, multicastV6, multicastV6Only} {
 		t.Run(fmt.Sprintf("flow:%s", flow), func(t *testing.T) {