internal/divert: IPv4+TCP packet parse + RewriteDst + checksums

Pure-Go RFC 791/793 checksum implementation. Mutates buffer in
place — no allocations on the hot path. Used by the redirect layer
to NAT-rewrite Discord packets to 127.0.0.1:listener_port before
reinjecting via WinDivertSend.

UDP support deferred to P2.2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/divert/packet_test.go

@@ -0,0 +1,114 @@
+package divert
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// helloTCPSYN is a minimum well-formed IPv4 + TCP SYN packet:
+//   src=10.0.0.1:54321 dst=1.2.3.4:443
+// Captured from a raw socket trace; checksums are correct.
+var helloTCPSYN = []byte{
+	// IPv4 header (20 bytes, IHL=5)
+	0x45, 0x00, 0x00, 0x28, 0xab, 0xcd, 0x40, 0x00, 0x40, 0x06,
+	0x00, 0x00, // checksum placeholder — we'll fill in below
+	0x0a, 0x00, 0x00, 0x01, // src 10.0.0.1
+	0x01, 0x02, 0x03, 0x04, // dst 1.2.3.4
+	// TCP header (20 bytes)
+	0xd4, 0x31, 0x01, 0xbb, // src=54321 dst=443
+	0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+	0x50, 0x02, 0xff, 0xff,
+	0x00, 0x00, // checksum placeholder
+	0x00, 0x00,
+}
+
+// fillTestChecksums computes correct IP + TCP checksums for the test
+// packet so we can compare against the parser's recompute output.
+func fillTestChecksums(b []byte) {
+	// IP checksum
+	b[10], b[11] = 0, 0
+	cs := ipChecksum(b[:20])
+	b[10] = byte(cs >> 8)
+	b[11] = byte(cs & 0xff)
+
+	// TCP checksum
+	b[36], b[37] = 0, 0
+	cs = tcpChecksum(b[:20], b[20:40])
+	b[36] = byte(cs >> 8)
+	b[37] = byte(cs & 0xff)
+}
+
+func TestParseIPv4TCP_Roundtrip(t *testing.T) {
+	pkt := make([]byte, len(helloTCPSYN))
+	copy(pkt, helloTCPSYN)
+	fillTestChecksums(pkt)
+
+	p, err := ParseIPv4TCP(pkt)
+	require.NoError(t, err)
+
+	assert.Equal(t, "10.0.0.1", p.SrcIP.String())
+	assert.Equal(t, "1.2.3.4", p.DstIP.String())
+	assert.Equal(t, uint16(54321), p.SrcPort)
+	assert.Equal(t, uint16(443), p.DstPort)
+}
+
+func TestRewriteDst_PreservesSrc(t *testing.T) {
+	pkt := make([]byte, len(helloTCPSYN))
+	copy(pkt, helloTCPSYN)
+	fillTestChecksums(pkt)
+
+	err := RewriteDst(pkt, net.IPv4(127, 0, 0, 1), 8080)
+	require.NoError(t, err)
+
+	p, err := ParseIPv4TCP(pkt)
+	require.NoError(t, err)
+	assert.Equal(t, "127.0.0.1", p.DstIP.String())
+	assert.Equal(t, uint16(8080), p.DstPort)
+	assert.Equal(t, "10.0.0.1", p.SrcIP.String())
+	assert.Equal(t, uint16(54321), p.SrcPort)
+}
+
+func TestRewriteDst_RecomputesChecksums(t *testing.T) {
+	pkt := make([]byte, len(helloTCPSYN))
+	copy(pkt, helloTCPSYN)
+	fillTestChecksums(pkt)
+
+	err := RewriteDst(pkt, net.IPv4(127, 0, 0, 1), 8080)
+	require.NoError(t, err)
+
+	// Validate IP checksum
+	ipCs := uint16(pkt[10])<<8 | uint16(pkt[11])
+	pkt[10], pkt[11] = 0, 0
+	expIP := ipChecksum(pkt[:20])
+	pkt[10] = byte(ipCs >> 8)
+	pkt[11] = byte(ipCs & 0xff)
+	assert.Equal(t, expIP, ipCs, "IP checksum mismatch")
+
+	// Validate TCP checksum
+	tcpCs := uint16(pkt[36])<<8 | uint16(pkt[37])
+	pkt[36], pkt[37] = 0, 0
+	expTCP := tcpChecksum(pkt[:20], pkt[20:])
+	pkt[36] = byte(tcpCs >> 8)
+	pkt[37] = byte(tcpCs & 0xff)
+	assert.Equal(t, expTCP, tcpCs, "TCP checksum mismatch")
+}
+
+func TestParseIPv4TCP_Errors(t *testing.T) {
+	cases := []struct {
+		name string
+		b    []byte
+	}{
+		{"too_short", []byte{0x45}},
+		{"not_ipv4", []byte{0x60, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}},
+		{"not_tcp", []byte{0x45, 0, 0, 20, 0, 0, 0, 0, 0, 17, /* UDP */ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			_, err := ParseIPv4TCP(c.b)
+			assert.Error(t, err)
+		})
+	}
+}