Pull request: * all: move internal Go packages to internal/

Merge in DNS/adguard-home from 2234-move-to-internal to master

Squashed commit of the following:

commit d26a288cabeac86f9483fab307677b1027c78524
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 12:44:18 2020 +0300

    * all: move internal Go packages to internal/

    Closes #2234.

internal/dhcpd/nclient4/conn_unix.go

@@ -0,0 +1,144 @@
+// Copyright 2018 the u-root Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+// +build go1.12
+
+package nclient4
+
+import (
+	"errors"
+	"io"
+	"net"
+
+	"github.com/mdlayher/ethernet"
+	"github.com/mdlayher/raw"
+	"github.com/u-root/u-root/pkg/uio"
+)
+
+var (
+	// BroadcastMac is the broadcast MAC address.
+	//
+	// Any UDP packet sent to this address is broadcast on the subnet.
+	BroadcastMac = net.HardwareAddr([]byte{255, 255, 255, 255, 255, 255})
+)
+
+var (
+	// ErrUDPAddrIsRequired is an error used when a passed argument is not of type "*net.UDPAddr".
+	ErrUDPAddrIsRequired = errors.New("must supply UDPAddr")
+)
+
+// NewRawUDPConn returns a UDP connection bound to the interface and port
+// given based on a raw packet socket. All packets are broadcasted.
+//
+// The interface can be completely unconfigured.
+func NewRawUDPConn(iface string, port int) (net.PacketConn, error) {
+	ifc, err := net.InterfaceByName(iface)
+	if err != nil {
+		return nil, err
+	}
+	rawConn, err := raw.ListenPacket(ifc, uint16(ethernet.EtherTypeIPv4), &raw.Config{LinuxSockDGRAM: true})
+	if err != nil {
+		return nil, err
+	}
+	return NewBroadcastUDPConn(rawConn, &net.UDPAddr{Port: port}), nil
+}
+
+// BroadcastRawUDPConn uses a raw socket to send UDP packets to the broadcast
+// MAC address.
+type BroadcastRawUDPConn struct {
+	// PacketConn is a raw DGRAM socket.
+	net.PacketConn
+
+	// boundAddr is the address this RawUDPConn is "bound" to.
+	//
+	// Calls to ReadFrom will only return packets destined to this address.
+	boundAddr *net.UDPAddr
+}
+
+// NewBroadcastUDPConn returns a PacketConn that marshals and unmarshals UDP
+// packets, sending them to the broadcast MAC at on rawPacketConn.
+//
+// Calls to ReadFrom will only return packets destined to boundAddr.
+func NewBroadcastUDPConn(rawPacketConn net.PacketConn, boundAddr *net.UDPAddr) net.PacketConn {
+	return &BroadcastRawUDPConn{
+		PacketConn: rawPacketConn,
+		boundAddr:  boundAddr,
+	}
+}
+
+func udpMatch(addr *net.UDPAddr, bound *net.UDPAddr) bool {
+	if bound == nil {
+		return true
+	}
+	if bound.IP != nil && !bound.IP.Equal(addr.IP) {
+		return false
+	}
+	return bound.Port == addr.Port
+}
+
+// ReadFrom implements net.PacketConn.ReadFrom.
+//
+// ReadFrom reads raw IP packets and will try to match them against
+// upc.boundAddr. Any matching packets are returned via the given buffer.
+func (upc *BroadcastRawUDPConn) ReadFrom(b []byte) (int, net.Addr, error) {
+	ipHdrMaxLen := IPv4MaximumHeaderSize
+	udpHdrLen := UDPMinimumSize
+
+	for {
+		pkt := make([]byte, ipHdrMaxLen+udpHdrLen+len(b))
+		n, _, err := upc.PacketConn.ReadFrom(pkt)
+		if err != nil {
+			return 0, nil, err
+		}
+		if n == 0 {
+			return 0, nil, io.EOF
+		}
+		pkt = pkt[:n]
+		buf := uio.NewBigEndianBuffer(pkt)
+
+		// To read the header length, access data directly.
+		ipHdr := IPv4(buf.Data())
+		ipHdr = IPv4(buf.Consume(int(ipHdr.HeaderLength())))
+
+		if ipHdr.TransportProtocol() != UDPProtocolNumber {
+			continue
+		}
+		udpHdr := UDP(buf.Consume(udpHdrLen))
+
+		addr := &net.UDPAddr{
+			IP:   ipHdr.DestinationAddress(),
+			Port: int(udpHdr.DestinationPort()),
+		}
+		if !udpMatch(addr, upc.boundAddr) {
+			continue
+		}
+		srcAddr := &net.UDPAddr{
+			IP:   ipHdr.SourceAddress(),
+			Port: int(udpHdr.SourcePort()),
+		}
+		// Extra padding after end of IP packet should be ignored,
+		// if not dhcp option parsing will fail.
+		dhcpLen := int(ipHdr.PayloadLength()) - udpHdrLen
+		return copy(b, buf.Consume(dhcpLen)), srcAddr, nil
+	}
+}
+
+// WriteTo implements net.PacketConn.WriteTo and broadcasts all packets at the
+// raw socket level.
+//
+// WriteTo wraps the given packet in the appropriate UDP and IP header before
+// sending it on the packet conn.
+func (upc *BroadcastRawUDPConn) WriteTo(b []byte, addr net.Addr) (int, error) {
+	udpAddr, ok := addr.(*net.UDPAddr)
+	if !ok {
+		return 0, ErrUDPAddrIsRequired
+	}
+
+	// Using the boundAddr is not quite right here, but it works.
+	packet := udp4pkt(b, udpAddr, upc.boundAddr)
+
+	// Broadcasting is not always right, but hell, what the ARP do I know.
+	return upc.PacketConn.WriteTo(packet, &raw.Addr{HardwareAddr: BroadcastMac})
+}