Move UDP-related functions and variables into separate file, add/fix some comments

Author: vista-

dhcpv4/nclient4/conn_unix.go

@@ -21,8 +21,6 @@ var rawConnectionConfig = &raw.Config{
 
 // 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, vlans ...uint16) (net.PacketConn, error) {
 	ifc, err := net.InterfaceByName(iface)
 	if err != nil {
@@ -68,8 +66,10 @@ func NewBroadcastUDPConn(rawPacketConn net.PacketConn, boundAddr *net.UDPAddr, v
 
 // 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.
+// ReadFrom reads raw Ethernet packets, parses the VLAN stack (if configured)
+// and will try to match the IP+UDP destinations against upc.boundAddr.
+//
+// Any matching packets are returned via the given buffer.
 func (upc *BroadcastRawUDPConn) ReadFrom(b []byte) (int, net.Addr, error) {
 	ethHdrLen := ethHdrMinimum
 	if len(upc.VLANs) > 0 {

dhcpv4/nclient4/ethernet.go

@@ -19,11 +19,18 @@ const (
 	vlanTPID   uint16 = 0x8100
 )
 
+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})
+)
+
 // processVLANStack receives a buffer starting at the first TPID/EtherType field, and walks through
 // the VLAN stack until either an unexpected VLAN is found, or if an IPv4 EtherType is found.
-// The data from the provided buffer is consumed until the end of the Ethernet header
+// The data from the provided buffer is consumed until the end of the Ethernet header.
 //
-// processVLANStack returns true if the VLAN stack in the packet corresponds to the VLAN configuration, false otherwise
+// processVLANStack returns true if the VLAN stack in the packet corresponds to the VLAN configuration, false otherwise.
 func processVLANStack(buf *uio.Lexer, vlans []uint16) bool {
 	var currentVLAN uint16
 	var vlanStackIsCorrect bool = true

dhcpv4/nclient4/conn.go renamed to dhcpv4/nclient4/udp.go

@@ -1,3 +1,6 @@
+//go:build go1.12
+// +build go1.12
+
 package nclient4
 
 import (
@@ -7,13 +10,6 @@ import (
 	"github.com/u-root/uio/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")