dhcp/dhcp4.cpp

#include <string.h>
#include <assert.h>
#include <stdio.h>

#include <arpa/inet.h>

#include "dhcp4.h"

namespace dhcp4 {

  int add_parameter_request_list(message * msg, int index)
  {
    uint8_t * opt = &msg->options[index];
    int length = 3;
    index += 2 + length;
    opt[0] = option::parameter_request_list;
    opt[1] = length;
    opt[2] = option::subnet_mask;
    opt[3] = option::router;
    opt[4] = option::domain_name_server;
    return index;
  }

  int add_uint32(message * msg, int index, uint8_t option, uint32_t value)
  {
    uint8_t * opt = &msg->options[index];
    int length = 4;
    index += 2 + length;
    opt[0] = option;
    opt[1] = length;
    memcpy(&opt[2], &value, 4);
    return index;
  }

  int make_message(void * buf,
                   uint32_t bootp_message_type,
                   uint32_t transaction_id,
                   uint8_t const * chaddr,
                   uint32_t dhcp_message_type,
                   message_parameters const * params)
  {
    memset(buf, 0, (sizeof (message)));
    message * msg = reinterpret_cast<message *>(buf);

    msg->op = bootp_message_type;
    msg->htype = htype::ethernet;
    msg->hlen = 6;
    msg->xid = htonl(transaction_id);

    msg->yiaddr = params->your_ip_address;
    msg->siaddr = params->next_server_ip_address;

    memcpy(msg->chaddr, chaddr, 6);

    int index = 0;

    uint8_t * cookie = &msg->options[index];
    index += 4;
    cookie[0] = 0x63;
    cookie[1] = 0x82;
    cookie[2] = 0x53;
    cookie[3] = 0x63;

    uint8_t * message_type = &msg->options[index];
    index += 2 + 1;
    message_type[0] = option::message_type;
    message_type[1] = 1; // length
    message_type[2] = dhcp_message_type;

    if (dhcp_message_type == message_type::dhcpdiscover ||
        dhcp_message_type == message_type::dhcprequest) {
      index = add_parameter_request_list(msg, index);
    }

    if (params->subnet_mask != 0) {
      index = add_uint32(msg, index, option::subnet_mask, params->subnet_mask);
    }

    if (params->server_identifier != 0) {
      index = add_uint32(msg, index, option::server_identifier, params->server_identifier);

      index = add_uint32(msg, index, option::ip_address_lease_time, htonl(86400));
    }

    if (params->router != 0) {
      index = add_uint32(msg, index, option::router, params->router);
    }

    if (params->domain_name_server != 0) {
      index = add_uint32(msg, index, option::domain_name_server, params->domain_name_server);
    }

    if (params->requested_ip_address != 0) {
      index = add_uint32(msg, index, option::requested_ip_address, params->requested_ip_address);
    }

    uint8_t * end = &msg->options[index];
    index += 1;
    end[0] = 255;

    return (sizeof (message)) + index;
  }

  char const * message_type_tostr(int message_type)
  {
    switch (message_type) {
    case message_type::dhcpdiscover: return "dhcpdiscover";
    case message_type::dhcpoffer: return "dhcpoffer";
    case message_type::dhcprequest: return "dhcprequest";
    case message_type::dhcpdecline: return "dhcpdecline";
    case message_type::dhcpack: return "dhcpack";
    case message_type::dhcpnak: return "dhcpnak";
    case message_type::dhcprelease: return "dhcprelease";
    case message_type::dhcpinform: return "dhcpinform";
    default:
      assert(false);
      return nullptr;
    }
  }

  void parse_options(message const * m, int options_length, options * o)
  {
    uint8_t const * options = m->options;

    int index = 4;
    while (index + 2 <= options_length) {
      int option = options[index + 0];
      if (option == option::end) {
        break;
      }
      int option_length = options[index + 1];

      switch (option) {
      case option::message_type:
        o->message_type = options[index + 2];
        break;
      case option::subnet_mask:
        o->subnet_mask = *reinterpret_cast<uint32_t const *>(&options[index + 2]);
        break;
      case option::router:
        o->router = *reinterpret_cast<uint32_t const *>(&options[index + 2]);
        break;
      case option::domain_name_server:
        o->domain_name_server = *reinterpret_cast<uint32_t const *>(&options[index + 2]);
        break;
      case option::requested_ip_address:
        o->requested_ip_address = *reinterpret_cast<uint32_t const *>(&options[index + 2]);
        break;
      default:
        break;
      }

      index += 2 + option_length;
    }

    assert(options[index] == option::end);
    for (index = index + 1; index < options_length; index++) {
      assert(options[index] == 0);
    }
  }
}