Function udp4raw

wgengine/netstack/netstack_test.go:1649–1681

udp4raw constructs a valid raw IPv4+UDP packet with proper checksums.

(t testing.TB, src, dst netip.Addr, sport, dport uint16, payload []byte)

// udp4raw constructs a valid raw IPv4+UDP packet with proper checksums.
func udp4raw(t testing.TB, src, dst netip.Addr, sport, dport uint16, payload []byte) []byte {
	t.Helper()
	totalLen := header.IPv4MinimumSize + header.UDPMinimumSize + len(payload)
	buf := make([]byte, totalLen)

	ip := header.IPv4(buf)
	ip.Encode(&header.IPv4Fields{
		TotalLength: uint16(totalLen),
		Protocol:    uint8(header.UDPProtocolNumber),
		TTL:         64,
		SrcAddr:     tcpip.AddrFrom4Slice(src.AsSlice()),
		DstAddr:     tcpip.AddrFrom4Slice(dst.AsSlice()),
	})
	ip.SetChecksum(^ip.CalculateChecksum())

	// Build UDP header + payload.
	u := header.UDP(buf[header.IPv4MinimumSize:])
	u.Encode(&header.UDPFields{
		SrcPort: sport,
		DstPort: dport,
		Length:  uint16(header.UDPMinimumSize + len(payload)),
	})
	copy(buf[header.IPv4MinimumSize+header.UDPMinimumSize:], payload)

	xsum := header.PseudoHeaderChecksum(
		header.UDPProtocolNumber,
		tcpip.AddrFrom4Slice(src.AsSlice()),
		tcpip.AddrFrom4Slice(dst.AsSlice()),
		uint16(header.UDPMinimumSize+len(payload)),
	)
	u.SetChecksum(^header.UDP(buf[header.IPv4MinimumSize:]).CalculateChecksum(xsum))
	return buf
}

// TestInjectLoopback verifies that the inject goroutine delivers self-addressed
// packets back into gVisor (via DeliverLoopback) instead of sending them to