dhcpv4: fixes (#19)

* dhcpv4: fix several issues

* condition flipped in Encapsulate
* check for correct bounds in ForEachOption
* getMessageType didn't check for opt

* dhcpv4: preserve backing buffer in reset

dhcpv4/client.go

@@ -159,7 +159,7 @@ func (c *Client) Encapsulate(carrierData []byte, offsetToIP, offsetToFrame int)
 		}
 		n, _ = EncodeOption16(opts[numOpts:], OptMaximumMessageSize, uint16(maxlen))
 		numOpts += n
-		if !c.reqIP.valid {
+		if c.reqIP.valid {
 			n, _ = EncodeOption(opts[numOpts:], OptRequestedIPaddress, c.reqIP.addr[:]...)
 			numOpts += n
 		}
@@ -253,7 +253,7 @@ func (c *Client) getMessageType(frm Frame) MessageType {
 	c.auxbuf[0] = 255
 	ptrMsgType := &c.auxbuf[0]
 	frm.ForEachOption(func(_ int, opt OptNum, data []byte) error {
-		if len(data) == 1 {
+		if opt == OptMessageType && len(data) == 1 {
 			*ptrMsgType = data[0]
 			return io.EOF
 		}
@@ -340,6 +340,9 @@ func (c *Client) reset(xid uint32) {
 		reqIP:       c.reqIP,
 		clientMAC:   c.clientMAC,
 		clientID:    c.clientID,
+		dns:         c.dns[:0],
+		ntps:        c.ntps[:0],
+		hostname:    c.hostname[:0],
 	}
 }