net/packet: add checksum update tests

Updates tailscale/corp#8020

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/packet/packet.go

@@ -556,14 +556,18 @@ func updateV4PacketChecksums(p *Parsed, old, new netip.Addr) {
 	// TODO(maisem): more protocols (sctp, gre, dccp)
 }
 
-// updateV4Checksum calculates and updates the checksum in the packet buffer
-// for a change between old and new. The checksum is updated in place.
+// updateV4Checksum calculates and updates the checksum in the packet buffer for
+// a change between old and new. The oldSum must point to the 16-bit checksum
+// field in the packet buffer that holds the old checksum value, it will be
+// updated in place.
+//
+// The old and new must be the same length, and must be an even number of bytes.
 func updateV4Checksum(oldSum, old, new []byte) {
 	if len(old) != len(new) {
 		panic("old and new must be the same length")
 	}
 	if len(old)%2 != 0 {
-		panic("old and new must be even length")
+		panic("old and new must be of even length")
 	}
 	/*
 		RFC 1624

net/packet/packet_test.go

@@ -5,6 +5,7 @@ package packet
 
 import (
 	"bytes"
+	"encoding/binary"
 	"encoding/hex"
 	"net/netip"
 	"reflect"
@@ -29,6 +30,72 @@ const (
 	Fragment = ipproto.Fragment
 )
 
+func fullHeaderChecksumV4(b []byte) uint16 {
+	s := uint32(0)
+	for i := 0; i < len(b); i += 2 {
+		if i == 10 {
+			// Skip checksum field.
+			continue
+		}
+		s += uint32(binary.BigEndian.Uint16(b[i : i+2]))
+	}
+	for s>>16 > 0 {
+		s = s&0xFFFF + s>>16
+	}
+	return ^uint16(s)
+}
+
+func TestHeaderChecksums(t *testing.T) {
+	// This is not a good enough test, because it doesn't
+	// check the various packet types or the many edge cases
+	// of the checksum algorithm. But it's a start.
+
+	tests := []struct {
+		name   string
+		packet []byte
+	}{
+		{
+			name: "ICMPv4",
+			packet: []byte{
+				0x45, 0x00, 0x00, 0x54, 0xb7, 0x96, 0x40, 0x00, 0x40, 0x01, 0x7a, 0x06, 0x64, 0x7f, 0x3f, 0x4c, 0x64, 0x40, 0x01, 0x01, 0x08, 0x00, 0x47, 0x1a, 0x00, 0x11, 0x01, 0xac, 0xcc, 0xf5, 0x95, 0x63, 0x00, 0x00, 0x00, 0x00, 0x8d, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
+			},
+		},
+		{
+			name: "TLS",
+			packet: []byte{
+				0x45, 0x00, 0x00, 0x3c, 0x54, 0x29, 0x40, 0x00, 0x40, 0x06, 0xb1, 0xac, 0x64, 0x42, 0xd4, 0x33, 0x64, 0x61, 0x98, 0x0f, 0xb1, 0x94, 0x01, 0xbb, 0x0a, 0x51, 0xce, 0x7c, 0x00, 0x00, 0x00, 0x00, 0xa0, 0x02, 0xfb, 0xe0, 0x38, 0xf6, 0x00, 0x00, 0x02, 0x04, 0x04, 0xd8, 0x04, 0x02, 0x08, 0x0a, 0x86, 0x2b, 0xcc, 0xd5, 0x00, 0x00, 0x00, 0x00, 0x01, 0x03, 0x03, 0x07,
+			},
+		},
+		{
+			name: "DNS",
+			packet: []byte{
+				0x45, 0x00, 0x00, 0x74, 0xe2, 0x85, 0x00, 0x00, 0x40, 0x11, 0x96, 0xb5, 0x64, 0x64, 0x64, 0x64, 0x64, 0x42, 0xd4, 0x33, 0x00, 0x35, 0xec, 0x55, 0x00, 0x60, 0xd9, 0x19, 0xed, 0xfd, 0x81, 0x80, 0x00, 0x01, 0x00, 0x02, 0x00, 0x00, 0x00, 0x01, 0x08, 0x63, 0x6c, 0x69, 0x65, 0x6e, 0x74, 0x73, 0x34, 0x06, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x03, 0x63, 0x6f, 0x6d, 0x00, 0x00, 0x01, 0x00, 0x01, 0xc0, 0x0c, 0x00, 0x05, 0x00, 0x01, 0x00, 0x00, 0x01, 0x1e, 0x00, 0x0c, 0x07, 0x63, 0x6c, 0x69, 0x65, 0x6e, 0x74, 0x73, 0x01, 0x6c, 0xc0, 0x15, 0xc0, 0x31, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x01, 0x1e, 0x00, 0x04, 0x8e, 0xfa, 0xbd, 0xce, 0x00, 0x00, 0x29, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			},
+		},
+	}
+	var p Parsed
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			p.Decode(tt.packet)
+			t.Log(p.String())
+			p.UpdateSrcAddr(netip.MustParseAddr("100.64.0.1"))
+
+			got := binary.BigEndian.Uint16(tt.packet[10:12])
+			want := fullHeaderChecksumV4(tt.packet[:20])
+			if got != want {
+				t.Fatalf("got %x want %x", got, want)
+			}
+
+			p.UpdateDstAddr(netip.MustParseAddr("100.64.0.2"))
+			got = binary.BigEndian.Uint16(tt.packet[10:12])
+			want = fullHeaderChecksumV4(tt.packet[:20])
+			if got != want {
+				t.Fatalf("got %x want %x", got, want)
+			}
+		})
+	}
+}
+
 func mustIPPort(s string) netip.AddrPort {
 	ipp, err := netip.ParseAddrPort(s)
 	if err != nil {