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package hotline
import (
"bytes"
"encoding/binary"
"errors"
"fmt"
"io"
)
// Hotline handshake process
//
// After establishing TCP connection, both client and server start the handshake process
// in order to confirm that each of them comply with requirements of the other.
// The information provided in this initial data exchange identifies protocols,
// and their versions, used in the communication. In the case where, after inspection,
// the capabilities of one of the subjects do not comply with the requirements of the other,
// the connection is dropped.
//
// The following information is sent to the server:
// Description Size Data Note
// Protocol Type 4 TRTP 0x54525450
// Sub-protocol Type 4 HOTL User defined
// VERSION 2 1 Currently 1
// Sub-version 2 2 User defined
//
// The server replies with the following:
// Description Size Data Note
// Protocol Type 4 TRTP
// Error code 4 Error code returned by the server (0 = no error)
type handshake struct {
Protocol [4]byte // Must be 0x54525450 TRTP
SubProtocol [4]byte // Must be 0x484F544C HOTL
Version [2]byte // Always 1 (?)
SubVersion [2]byte // Always 2 (?)
}
// Write implements the io.Writer interface for handshake.
func (h *handshake) Write(p []byte) (n int, err error) {
if len(p) != handshakeSize {
return 0, errors.New("invalid handshake size")
}
_ = binary.Read(bytes.NewBuffer(p), binary.BigEndian, h)
return len(p), nil
}
// Valid checks if the handshake contains valid protocol and sub-protocol IDs.
func (h *handshake) Valid() bool {
return h.Protocol == trtp && h.SubProtocol == hotl
}
var (
// trtp represents the Protocol Type "TRTP" in hex
trtp = [4]byte{0x54, 0x52, 0x54, 0x50}
// hotl represents the Sub-protocol Type "HOTL" in hex
hotl = [4]byte{0x48, 0x4F, 0x54, 0x4C}
// handshakeResponse represents the server's response after a successful handshake
// Response with "TRTP" and no error code
handshakeResponse = [8]byte{0x54, 0x52, 0x54, 0x50, 0x00, 0x00, 0x00, 0x00}
)
const handshakeSize = 12
// performHandshake performs the handshake process.
func performHandshake(rw io.ReadWriter) error {
var h handshake
// Copy exactly handshakeSize bytes from rw to handshake
if _, err := io.CopyN(&h, rw, handshakeSize); err != nil {
return fmt.Errorf("read handshake: %w", err)
}
if !h.Valid() {
return errors.New("invalid protocol or sub-protocol in handshake")
}
if _, err := rw.Write(handshakeResponse[:]); err != nil {
return fmt.Errorf("send handshake response: %w", err)
}
return nil
}
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