CODEWITHSUNDEEP
c
someone
someone

Reputation: 101

how do I build a protocol header and body over a tcp socket in C

I'm writing a tacacs+ client,

I'm trying to write a simple packet to send to the server, I checked the RFC page and I saw the protocol needs to get this header in every packet:

    +----------------+----------------+----------------+----------------+
    |major  | minor  |                |                |                |
    |version| version|      type      |     seq_no     |   flags        |
    +----------------+----------------+----------------+----------------+
    |                                                                   |
    |                            session_id                             |
    +----------------+----------------+----------------+----------------+
    |                                                                   |
    |                              length                               |
    +----------------+----------------+----------------+----------------+

now I'm pretty new with this, and I'm not sure how do I build this header, is creating a buffer like this good enough?:

snprintf(buffer, sizeof(buffer), "%x%x%x%x%x",version,version,type,seq,flags,session_id,length);

and then take this buffer and attach it the body in the same way and send:

send(socket,buffer,sizeof(buffer),0);

or is there another way of doing this whole packet building?

Upvotes: 2

Views: 1149

Answers (1)

ikegami
ikegami

Reputation: 385789

What you have doesn't produce the right output at all!

The following is portable:

#include <stdint.h>
#include <stdlib.h>
#include <string.h>

uint8_t* pack_uint8(uint8_t* dest, uint8_t src) {
   *(dest++) = src;
   return dest;
}

uint8_t* pack_uint32be(uint8_t* dest, uint32_t src) {
   *(dest++) = src >> 24;
   *(dest++) = (src >> 16) & 0xFF;
   *(dest++) = (src >> 8) & 0xFF;
   *(dest++) = src & 0xFF;
   return dest;
}

uint8_t* build_packet(
   uint8_t maj_version,
   uint8_t min_version,
   uint8_t type,
   uint8_t seq,
   uint8_t flags,
   uint32_t session_id,
   uint32_t body_length,
   const uint8_t* body
) {
   uint8_t* packet = malloc(12 + body_length);
   uint8_t* p = packet;
   p = pack_uint8(p, ( maj_version << 4 ) | min_version);
   p = pack_uint8(p, type);
   p = pack_uint8(p, seq);
   p = pack_uint8(p, flags);
   p = pack_uint32be(p, session_id);
   p = pack_uint32be(p, body_length);
   memcpy(p, body, body_length);
   return packet;
}

Upvotes: 3

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