How to judge stun protocol

STUN Full name ：（Session Traversal Utilities for NAT）NAT Session traversal application , Allow in NAT（ Or multiple NAT） After the client to find out their own public network address , Find out what kind of NAT After that and NAT Bound to a local port Internet End port . This information is used to be in NAT Create... Between hosts after router UDP signal communication . Agreement by the RFC 5389 Definition .

1、stun Magic of message cookie

//fixed value 0x2112A442
const uint8_t stun_packet::magic_cookie[] = { 0x21, 0x12, 0xA4, 0x42 };

Four bytes , and RTP The agreement is similar to , But more , There are four , Namely 0x21,0x12, 0xA4, 0x42
according to RFC 5389, The protocol header is shown below ：

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0 0|     STUN Message Type     |         Message Length        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         Magic Cookie                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  |                     Transaction ID (96 bits)                  |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2、 How to determine

bool is_stun_pack(const uint8_t* data, size_t len) {
    
    if ((len >= STUN_HEADER_SIZE) && (data[0] < 3) &&
        (data[4] == 0x21) && (data[5] == 0x12) &&
        (data[6] == 0xA4) && (data[7] == 0x42)) {
    
        return true;
    } 
    return false;
}