minor dhcp changes to better match seqs exchange

dhcpv4/client.go

@@ -25,7 +25,8 @@ type Client struct {
 	currentXID uint32
 	state      ClientState
 	offer      [4]byte
-	svip       [4]byte
+	svip       [4]byte // OptServerIdentification.
+	siip       [4]byte // SIAddr.
 	reqIP      [4]byte
 	router     [4]byte
 	subnet     [4]byte
@@ -80,6 +81,18 @@ func (c *Client) setIP(b []byte, frameOffset int) {
 		const ecnmask = 0b1100_0000
 		ifrm.SetToS(ecnmask)
 	}
+	src := ifrm.SourceAddr()
+	for i := range src {
+		src[i] = 0
+	}
+	dst := ifrm.DestinationAddr()[:]
+	if c.svip == ([4]byte{}) {
+		for i := range dst {
+			dst[i] = 255
+		}
+	} else {
+		copy(dst, c.svip[:])
+	}
 }
 
 func (c *Client) Encapsulate(carrierFrame []byte, frameOffset int) (int, error) {
@@ -177,7 +190,7 @@ func (c *Client) Demux(carrierData []byte, frameOffset int) error {
 			// Lock in on this offer.
 			c.gateway = *frm.GIAddr()
 			c.offer = *frm.YIAddr()
-			c.svip = *frm.SIAddr()
+			c.siip = *frm.SIAddr()
 		}
 
 	case StateRequesting:
@@ -246,10 +259,16 @@ func (c *Client) setHeader(frm Frame) {
 	frm.SetXID(c.currentXID)
 	frm.SetHardware(1, 6, 0)
 	frm.SetSecs(1)
-	if c.state == StateRequesting || c.state == StateSelecting || c.state == StateBound || c.state == StateRenewing {
+	if c.state.HasIP() {
 		copy(frm.CIAddr()[:], c.offer[:])
 	}
-	copy(frm.SIAddr()[:], c.svip[:])
+	if c.state == StateInit {
+		siaddr := frm.SIAddr()[:]
+		for i := range siaddr {
+			siaddr[i] = 255
+		}
+	}
+
 	copy(frm.YIAddr()[:], c.offer[:])
 	copy(frm.CHAddrAs6()[:], c.clientMAC[:])
 	frm.SetMagicCookie(MagicCookie)

dhcpv4/definitions.go

@@ -30,6 +30,11 @@ const (
 	StateRebooting  // rebooting
 )
 
+// HasIP returns true if the state indicates the Client has an IP address assigned by server.
+func (state ClientState) HasIP() bool {
+	return state == StateBound || state == StateRenewing || state == StateRebinding
+}
+
 func AppendOption(dst []byte, opt OptNum, data ...byte) []byte {
 	if len(data) > 255 {
 		panic("option data too long")

dhcpv4/frame.go

@@ -57,9 +57,11 @@ func (frm Frame) SetHardware(Type, Len, Ops uint8) {
 	frm.buf[1], frm.buf[2], frm.buf[3] = Type, Len, Ops
 }
 
+// XID is the transaction ID. Is unique and constant for a DHCP request/response exchange of packets.
 func (frm Frame) XID() uint32       { return binary.BigEndian.Uint32(frm.buf[4:8]) }
 func (frm Frame) SetXID(xid uint32) { binary.BigEndian.PutUint32(frm.buf[4:8], xid) }
 
+// Secs is seconds elapsed.
 func (frm Frame) Secs() uint16        { return binary.BigEndian.Uint16(frm.buf[8:10]) }
 func (frm Frame) SetSecs(secs uint16) { binary.BigEndian.PutUint16(frm.buf[8:10], secs) }
 
@@ -72,7 +74,7 @@ func (frm Frame) CIAddr() *[4]byte {
 	return (*[4]byte)(frm.buf[12:16])
 }
 
-// YIAddr is the IP address offered by the server to the client.
+// YIAddr is the IP address offered by the server to the client. Your (client) IP Address.
 func (frm Frame) YIAddr() *[4]byte {
 	return (*[4]byte)(frm.buf[16:20])
 }
@@ -83,7 +85,7 @@ func (frm Frame) SIAddr() *[4]byte {
 	return (*[4]byte)(frm.buf[20:24])
 }
 
-// GIAddr is the gateway IP address.
+// GIAddr is the gateway IP address. Is also known as the Relay Agent IP Address.
 func (frm Frame) GIAddr() *[4]byte {
 	return (*[4]byte)(frm.buf[24:28])
 }