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packet.c

#include <unistd.h>
#include <string.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <features.h>
#if __GLIBC__ >=2 && __GLIBC_MINOR >= 1
#include <netpacket/packet.h>
#include <net/ethernet.h>
#else
#include <asm/types.h>
#include <linux/if_packet.h>
#include <linux/if_ether.h>
#endif
#include <errno.h>

#include "packet.h"
#include "dhcpd.h"
#include "options.h"
#include "common.h"


void init_header(struct dhcpMessage *packet, char type)
{
      memset(packet, 0, sizeof(struct dhcpMessage));
      switch (type) {
      case DHCPDISCOVER:
      case DHCPREQUEST:
      case DHCPRELEASE:
      case DHCPINFORM:
            packet->op = BOOTREQUEST;
            break;
      case DHCPOFFER:
      case DHCPACK:
      case DHCPNAK:
            packet->op = BOOTREPLY;
      }
      packet->htype = ETH_10MB;
      packet->hlen = ETH_10MB_LEN;
      packet->cookie = htonl(DHCP_MAGIC);
      packet->options[0] = DHCP_END;
      add_simple_option(packet->options, DHCP_MESSAGE_TYPE, type);
}


/* read a packet from socket fd, return -1 on read error, -2 on packet error */
int get_packet(struct dhcpMessage *packet, int fd)
{
      int bytes;
      int i;
      const char broken_vendors[][8] = {
            "MSFT 98",
            ""
      };
      char unsigned *vendor;

      memset(packet, 0, sizeof(struct dhcpMessage));
      bytes = read(fd, packet, sizeof(struct dhcpMessage));
      if (bytes < 0) {
            DEBUG(LOG_INFO, "couldn't read on listening socket, ignoring");
            return -1;
      }

      if (ntohl(packet->cookie) != DHCP_MAGIC) {
            LOG(LOG_ERR, "received bogus message, ignoring");
            return -2;
      }
      DEBUG(LOG_INFO, "Received a packet");

      if (packet->op == BOOTREQUEST && (vendor = get_option(packet, DHCP_VENDOR))) {
            for (i = 0; broken_vendors[i][0]; i++) {
                  if (vendor[OPT_LEN - 2] == (uint8_t) strlen(broken_vendors[i]) &&
                      !strncmp(vendor, broken_vendors[i], vendor[OPT_LEN - 2])) {
                        DEBUG(LOG_INFO, "broken client (%s), forcing broadcast",
                              broken_vendors[i]);
                        packet->flags |= htons(BROADCAST_FLAG);
                  }
            }
      }
                  

      return bytes;
}


uint16_t checksum(void *addr, int count)
{
      /* Compute Internet Checksum for "count" bytes
       *         beginning at location "addr".
       */
      register int32_t sum = 0;
      uint16_t *source = (uint16_t *) addr;

      while (count > 1)  {
            /*  This is the inner loop */
            sum += *source++;
            count -= 2;
      }

      /*  Add left-over byte, if any */
      if (count > 0) {
            /* Make sure that the left-over byte is added correctly both
             * with little and big endian hosts */
            uint16_t tmp = 0;
            *(uint8_t *) (&tmp) = * (uint8_t *) source;
            sum += tmp;
      }
      /*  Fold 32-bit sum to 16 bits */
      while (sum >> 16)
            sum = (sum & 0xffff) + (sum >> 16);

      return ~sum;
}


/* Construct a ip/udp header for a packet, and specify the source and dest hardware address */
int raw_packet(struct dhcpMessage *payload, uint32_t source_ip, int source_port,
               uint32_t dest_ip, int dest_port, uint8_t *dest_arp, int ifindex)
{
      int fd;
      int result;
      struct sockaddr_ll dest;
      struct udp_dhcp_packet packet;

      if ((fd = socket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_IP))) < 0) {
            DEBUG(LOG_ERR, "socket call failed: %m");
            return -1;
      }

      memset(&dest, 0, sizeof(dest));
      memset(&packet, 0, sizeof(packet));

      dest.sll_family = AF_PACKET;
      dest.sll_protocol = htons(ETH_P_IP);
      dest.sll_ifindex = ifindex;
      dest.sll_halen = 6;
      memcpy(dest.sll_addr, dest_arp, 6);
      if (bind(fd, (struct sockaddr *)&dest, sizeof(struct sockaddr_ll)) < 0) {
            DEBUG(LOG_ERR, "bind call failed: %m");
            close(fd);
            return -1;
      }

      packet.ip.protocol = IPPROTO_UDP;
      packet.ip.saddr = source_ip;
      packet.ip.daddr = dest_ip;
      packet.udp.source = htons(source_port);
      packet.udp.dest = htons(dest_port);
      packet.udp.len = htons(sizeof(packet.udp) + sizeof(struct dhcpMessage)); /* cheat on the psuedo-header */
      packet.ip.tot_len = packet.udp.len;
      memcpy(&(packet.data), payload, sizeof(struct dhcpMessage));
      packet.udp.check = checksum(&packet, sizeof(struct udp_dhcp_packet));

      packet.ip.tot_len = htons(sizeof(struct udp_dhcp_packet));
      packet.ip.ihl = sizeof(packet.ip) >> 2;
      packet.ip.version = IPVERSION;
      packet.ip.ttl = IPDEFTTL;
      packet.ip.check = checksum(&(packet.ip), sizeof(packet.ip));

      result = sendto(fd, &packet, sizeof(struct udp_dhcp_packet), 0, (struct sockaddr *) &dest, sizeof(dest));
      if (result <= 0) {
            DEBUG(LOG_ERR, "write on socket failed: %m");
      }
      close(fd);
      return result;
}


/* Let the kernel do all the work for packet generation */
int kernel_packet(struct dhcpMessage *payload, uint32_t source_ip, int source_port,
               uint32_t dest_ip, int dest_port)
{
      int n = 1;
      int fd, result;
      struct sockaddr_in client;

      if ((fd = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
            return -1;

      if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, (char *) &n, sizeof(n)) == -1)
            return -1;

      memset(&client, 0, sizeof(client));
      client.sin_family = AF_INET;
      client.sin_port = htons(source_port);
      client.sin_addr.s_addr = source_ip;

      if (bind(fd, (struct sockaddr *)&client, sizeof(struct sockaddr)) == -1)
            return -1;

      memset(&client, 0, sizeof(client));
      client.sin_family = AF_INET;
      client.sin_port = htons(dest_port);
      client.sin_addr.s_addr = dest_ip;

      if (connect(fd, (struct sockaddr *)&client, sizeof(struct sockaddr)) == -1)
            return -1;

      result = write(fd, payload, sizeof(struct dhcpMessage));
      close(fd);
      return result;
}

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