Send a TCP RST in response to a TCP SYN-ACK on a listening endpoint

This change better follows what is outlined in RFC 793 section 3.4 figure 12
where a listening socket should not accept a SYN-ACK segment in response to a
(potentially) old SYN segment.

Tests: Test that checks the TCP RST segment sent in response to a TCP SYN-ACK
segment received on a listening TCP endpoint.
PiperOrigin-RevId: 278893114

1 files changed, 56 insertions, 0 deletions

diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 126f26ed3..beaa40210 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -598,6 +598,62 @@ func TestUserSuppliedMSSOnConnectV6(t *testing.T) {
 	}
 }
 
+func TestSendRstOnListenerRxSynAckV4(t *testing.T) {
+	c := context.New(t, defaultMTU)
+	defer c.Cleanup()
+
+	c.Create(-1)
+
+	if err := c.EP.Bind(tcpip.FullAddress{Port: context.StackPort}); err != nil {
+		t.Fatal("Bind failed:", err)
+	}
+
+	if err := c.EP.Listen(10); err != nil {
+		t.Fatal("Listen failed:", err)
+	}
+
+	c.SendPacket(nil, &context.Headers{
+		SrcPort: context.TestPort,
+		DstPort: context.StackPort,
+		Flags:   header.TCPFlagSyn | header.TCPFlagAck,
+		SeqNum:  100,
+		AckNum:  200,
+	})
+
+	checker.IPv4(t, c.GetPacket(), checker.TCP(
+		checker.DstPort(context.TestPort),
+		checker.TCPFlags(header.TCPFlagRst),
+		checker.SeqNum(200)))
+}
+
+func TestSendRstOnListenerRxSynAckV6(t *testing.T) {
+	c := context.New(t, defaultMTU)
+	defer c.Cleanup()
+
+	c.CreateV6Endpoint(true)
+
+	if err := c.EP.Bind(tcpip.FullAddress{Port: context.StackPort}); err != nil {
+		t.Fatal("Bind failed:", err)
+	}
+
+	if err := c.EP.Listen(10); err != nil {
+		t.Fatal("Listen failed:", err)
+	}
+
+	c.SendV6Packet(nil, &context.Headers{
+		SrcPort: context.TestPort,
+		DstPort: context.StackPort,
+		Flags:   header.TCPFlagSyn | header.TCPFlagAck,
+		SeqNum:  100,
+		AckNum:  200,
+	})
+
+	checker.IPv6(t, c.GetV6Packet(), checker.TCP(
+		checker.DstPort(context.TestPort),
+		checker.TCPFlags(header.TCPFlagRst),
+		checker.SeqNum(200)))
+}
+
 func TestTOSV4(t *testing.T) {
 	c := context.New(t, defaultMTU)
 	defer c.Cleanup()