ndhc/ndhc/packet.c

#include <unistd.h>
#include <string.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <features.h>
#include <netpacket/packet.h>
#include <net/ethernet.h>
#include <errno.h>

#include "packet.h"
#include "log.h"
#include "io.h"
#include "dhcpd.h"
#include "options.h"

/* 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;

    memset(packet, 0, sizeof(struct dhcpMessage));
    bytes = safe_read(fd, (char *)packet, sizeof(struct dhcpMessage));
    if (bytes == -1) {
	log_line("Read on listen socket failed: %s", strerror(errno));
	return -1;
    }

    if (ntohl(packet->cookie) != DHCP_MAGIC) {
	log_error("Packet with bad magic number, ignoring.");
	return -2;
    }
    log_line("Received a packet");

    return bytes;
}

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

    while (count > 1)  {
	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;
}

/* returns -1 on error, >= 0 and equal to # chars written on success */
static int safe_sendto(int fd, const char *buf, int len, int flags,
		       const struct sockaddr *dest_addr, socklen_t addrlen)
{
    int r, s = 0;
    while (s < len) {
        r = sendto(fd, buf + s, len - s, flags, dest_addr, addrlen);
        if (r == -1) {
            if (errno == EINTR)
                continue;
            else
                return -1;
        }
        s += r;
    }
    return s;
}

/* Constuct 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,
	       unsigned char *dest_arp, int ifindex)
{
    struct sockaddr_ll dest;
    struct ip_udp_dhcp_packet packet;
    int fd, r = -1;
    unsigned int padding;

    if ((fd = socket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_IP))) < 0) {
	log_error("raw_packet: socket failed: %s", strerror(errno));
	goto out;
    }

    memset(&dest, 0, sizeof dest);
    memset(&packet, 0, offsetof(struct ip_udp_dhcp_packet, data));
    packet.data = *payload; /* struct copy */

    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) {
	log_error("raw_packet: bind failed: %s", strerror(errno));
	goto out_fd;
    }

    /* We were sending full-sized DHCP packets (zero padded),
     * but some badly configured servers were seen dropping them.
     * Apparently they drop all DHCP packets >576 *ethernet* octets big,
     * whereas they may only drop packets >576 *IP* octets big
     * (which for typical Ethernet II means 590 octets: 6+6+2 + 576).
     *
     * In order to work with those buggy servers,
     * we truncate packets after end option byte.
     */
    padding = DHCP_OPTIONS_BUFSIZE - 1 - end_option(packet.data.options);

    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);
    /* size, excluding IP header: */
    packet.udp.len = htons(UPD_DHCP_SIZE - padding);
    /* for UDP checksumming, ip.len is set to UDP packet len */
    packet.ip.tot_len = packet.udp.len;
    packet.udp.check = checksum(&packet, IP_UPD_DHCP_SIZE - padding);
    /* but for sending, it is set to IP packet len */
    packet.ip.tot_len = htons(IP_UPD_DHCP_SIZE - padding);
    packet.ip.ihl = sizeof packet.ip >> 2;
    packet.ip.version = IPVERSION;
    packet.ip.ttl = IPDEFTTL;
    packet.ip.check = checksum(&packet.ip, sizeof packet.ip);

    r = safe_sendto(fd, (const char *)&packet, IP_UPD_DHCP_SIZE - padding,
		    0, (struct sockaddr *)&dest, sizeof dest);
    if (r == -1)
	log_error("raw_packet: sendto failed: %s", strerror(errno));
  out_fd:
    close(fd);
  out:
    return r;
}

/* 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)
{
    struct sockaddr_in client;
    int opt = 1, fd, result = -1;
    unsigned int padding;

    if ((fd = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
	goto out;

    if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof opt) == -1)
	goto out_fd;

    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)
	goto out_fd;

    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)
	goto out_fd;

    padding = DHCP_OPTIONS_BUFSIZE - 1 - end_option(payload->options);
    result = safe_write(fd, (const char *)payload, DHCP_SIZE - padding);
    if (result == -1)
	log_error("kernel_packet: write failed: %s", strerror(errno));
  out_fd:
    close(fd);
  out:
    return result;
}