packet.c

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#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 "debug.h"
#include "dhcpd.h"
#include "options.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] == (unsigned char) 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;
}


u_int16_t checksum(void *addr, int count)
{
        /* Compute Internet Checksum for "count" bytes
         *         beginning at location "addr".
         */
        register int32_t sum = 0;
        u_int16_t *source = (u_int16_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 */
                u_int16_t tmp = 0;
                *(unsigned char *) (&tmp) = * (unsigned char *) source;
                sum += tmp;
        }
        /*  Fold 32-bit sum to 16 bits */
        while (sum >> 16)
                sum = (sum & 0xffff) + (sum >> 16);

        return ~sum;
}


/* Constuct a ip/udp header for a packet, and specify the source and dest hardware address */
int raw_packet(struct dhcpMessage *payload, u_int32_t source_ip, int source_port,
                   u_int32_t dest_ip, int dest_port, unsigned char *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: %s", strerror(errno));
                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: %s", strerror(errno));
                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: %s", strerror(errno));
        }
        close(fd);
        return result;
}


/* Let the kernel do all the work for packet generation */
int kernel_packet(struct dhcpMessage *payload, u_int32_t source_ip, int source_port,
                   u_int32_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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