restructure package, rename module/repo

dhcpv4/frame.go

@@ -0,0 +1,140 @@
+package dhcpv4
+
+import (
+	"encoding/binary"
+	"errors"
+)
+
+const (
+	maxHostSize  = 16  // max size for hostname.
+	sizeSName    = 64  // Server name, part of BOOTP too.
+	sizeBootFile = 128 // Boot file name, Legacy.
+	sizeHeader   = 44
+	// Magic Cookie offset measured from the start of the UDP payload.
+	magicCookieOffset = sizeHeader + sizeSName + sizeBootFile
+	// Expected Magic Cookie value.
+	MagicCookie uint32 = 0x63825363
+	// DHCP Options offset measured from the start of the UDP payload.
+	optionsOffset = magicCookieOffset + 4
+
+	DefaultClientPort = 68
+	DefaultServerPort = 67
+)
+
+// NewFrame returns a new DHCPv4 Frame with data set to buf.
+// An error is returned if the buffer size is smaller than 240.
+func NewFrame(buf []byte) (Frame, error) {
+	if len(buf) < optionsOffset {
+		return Frame{}, errors.New("DHCPv4 short frame")
+	}
+	return Frame{buf: buf}, nil
+}
+
+// Frame encapsulates the raw data of a DHCP packet
+// and provides methods for manipulating, validating and
+// retrieving fields and payload data. See [RFC2131].
+//
+// [RFC2131]: https://tools.ietf.org/html/rfc2131
+type Frame struct {
+	buf []byte
+}
+
+// OptionsPayload returns the options portion of the DHCP frame. May be zero lengthed.
+func (frm Frame) OptionsPayload() []byte {
+	return frm.buf[:optionsOffset]
+}
+
+func (frm Frame) Op() Op      { return Op(frm.buf[0]) }
+func (frm Frame) SetOp(op Op) { frm.buf[0] = byte(op) }
+
+func (frm Frame) Hardware() (Type, Len, Ops uint8) {
+	return frm.buf[1], frm.buf[2], frm.buf[3]
+}
+
+func (frm Frame) SetHardware(Type, Len, Ops uint8) {
+	frm.buf[1], frm.buf[2], frm.buf[3] = Type, Len, Ops
+}
+
+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) }
+
+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) }
+
+func (frm Frame) Flags() Flags         { return Flags(binary.BigEndian.Uint16(frm.buf[10:12])) }
+func (frm Frame) SetFlags(flags Flags) { binary.BigEndian.PutUint16(frm.buf[10:12], uint16(flags)) }
+
+// CIAddr is the client IP address. If the client has not obtained an IP
+// address yet, this field is set to 0.
+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.
+func (frm Frame) YIAddr() *[4]byte {
+	return (*[4]byte)(frm.buf[16:20])
+}
+
+// SIAddr is the IP address of the next server to use in bootstrap. This
+// field is used in DHCPOFFER and DHCPACK messages.
+func (frm Frame) SIAddr() *[4]byte {
+	return (*[4]byte)(frm.buf[20:24])
+}
+
+// GIAddr is the gateway IP address.
+func (frm Frame) GIAddr() *[4]byte {
+	return (*[4]byte)(frm.buf[24:28])
+}
+
+// CHAddrAs6 returns [Frame.CHAddr] but limited to first 6 bytes.
+func (frm Frame) CHAddrAs6() *[6]byte {
+	return (*[6]byte)(frm.buf[28 : 28+6])
+}
+
+// CHAddr is the client hardware address. Can be up to 16 bytes in length but
+// is usually 6 bytes for Ethernet.
+func (frm Frame) CHAddr() *[16]byte {
+	return (*[16]byte)(frm.buf[28:44])
+}
+
+func (frm Frame) MagicCookie() uint32 { return binary.BigEndian.Uint32(frm.buf[magicCookieOffset:]) }
+func (frm Frame) SetMagicCookie(cookie uint32) {
+	binary.BigEndian.PutUint32(frm.buf[magicCookieOffset:], cookie)
+}
+
+// ClearHeader zeros out the header contents.
+func (frm Frame) ClearHeader() {
+	for i := range frm.buf[:optionsOffset] {
+		frm.buf[i] = 0
+	}
+}
+
+func (frm Frame) ForEachOption(fn func(op OptNum, data []byte) error) error {
+	if fn == nil {
+		return errors.New("nil function to parse DHCP")
+	}
+	// Parse DHCP options.
+	ptr := optionsOffset
+	if ptr >= len(frm.buf) {
+		return errors.New("short payload to parse DHCP options")
+	}
+	for ptr+1 < len(frm.buf) {
+		if int(frm.buf[ptr+1]) >= len(frm.buf) {
+			return errors.New("DHCP option length exceeds payload")
+		}
+		optnum := OptNum(frm.buf[ptr])
+		if optnum == 0xff {
+			break
+		} else if optnum == OptWordAligned {
+			ptr++
+			continue
+		}
+		optlen := frm.buf[ptr+1]
+		optionData := frm.buf[ptr+2 : ptr+2+int(optlen)]
+		if err := fn(optnum, optionData); err != nil {
+			return err
+		}
+		ptr += int(optlen) + 2
+	}
+	return nil
+}