[libpcap] / pcap-linux.c

/*
 *  pcap-linux.c: Packet capture interface to the Linux kernel
 *
 *  Copyright (c) 2000 Torsten Landschoff <torsten@debian.org>
 *                     Sebastian Krahmer  <krahmer@cs.uni-potsdam.de>
 *  
 *  License: BSD
 *  
 *  Redistribution and use in source and binary forms, with or without
 *  modification, are permitted provided that the following conditions
 *  are met:
 *  
 *  1. Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *  2. Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in
 *     the documentation and/or other materials provided with the
 *     distribution.
 *  3. The names of the authors may not be used to endorse or promote
 *     products derived from this software without specific prior
 *     written permission.
 *  
 *  THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
 *  IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
 *  WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */
#ifndef lint
static const char rcsid[] =
    "@(#) $Header: /tcpdump/master/libpcap/pcap-linux.c,v 1.36 2000-10-25 05:59:04 guy Exp $ (LBL)";
#endif

/*
 * Known bugs:
 *   - setting promiscuous on loopback gives every packet twice
 */


#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "pcap-int.h"

#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <netinet/in.h>
#include <linux/if_ether.h>
#include <netinet/if_ether.h>

#ifdef HAVE_NETPACKET_PACKET_H
#include <netpacket/packet.h>
#endif
#ifdef SO_ATTACH_FILTER
#include <linux/types.h>
#include <linux/filter.h>
#endif

#ifndef __GLIBC__
typedef int             socklen_t;
#endif

#ifndef MSG_TRUNC
#define MSG_TRUNC       0
#endif

#define MAX_LINKHEADER_SIZE     256

/* 
 * When capturing on all interfaces we use this as the buffer size. 
 * Should be bigger then all MTUs that occur in real life.
 * 64kB should be enough for now.
 */
#define BIGGER_THAN_ALL_MTUS    (64*1024)

/*
 * Prototypes for internal functions
 */
static int map_arphrd_to_dlt(int arptype );
static int live_open_old(pcap_t *, char *, int, int, char *);
static int live_open_new(pcap_t *, char *, int, int, char *);
static int pcap_read_packet(pcap_t *, pcap_handler, u_char *);

/*
 * Wrap some ioctl calls
 */
#ifdef HAVE_NETPACKET_PACKET_H
static int      iface_get_id(int fd, const char *device, char *ebuf);
#endif
static int      iface_get_mtu(int fd, const char *device, char *ebuf);
static int      iface_get_arptype(int fd, const char *device, char *ebuf);
#ifdef HAVE_NETPACKET_PACKET_H
static int      iface_bind(int fd, int ifindex, char *ebuf);
#endif
static int      iface_bind_old(int fd, const char *device, char *ebuf);

/*
 *  Get a handle for a live capture from the given device. You can 
 *  pass NULL as device to get all packages (without link level 
 *  information of course). If you pass 1 as promisc the interface
 *  will be set to promiscous mode (XXX: I think this usage should 
 *  be deprecated and functions be added to select that later allow
 *  modification of that values -- Torsten).
 *  
 *  See also pcap(3).
 */
pcap_t *
pcap_open_live(char *device, int snaplen, int promisc, int to_ms, char *ebuf)
{
        /* Allocate a handle for this session. */
        
        pcap_t  *handle = malloc(sizeof(*handle));
        if (handle == NULL) {
                snprintf(ebuf, PCAP_ERRBUF_SIZE, "malloc: %s",
                         pcap_strerror(errno));
                return NULL;
        }

        /* Initialize some components of the pcap structure. */

        memset(handle, 0, sizeof(*handle));
        handle->snapshot        = snaplen;
        handle->md.timeout      = to_ms;
        handle->md.promisc      = promisc;

        /*
         * NULL and "any" are special devices which give us the hint to 
         * monitor all devices.
         */
        if (!device || strcmp(device, "any") == 0) {
                device                  = NULL;
                handle->md.device       = strdup("any");
        } else
                handle->md.device       = strdup(device);

        if (handle->md.device == NULL) {
                snprintf(ebuf, PCAP_ERRBUF_SIZE, "strdup: %s",
                         pcap_strerror(errno) );
                free(handle);
                return NULL;
        }

        /* 
         * Current Linux kernels use the protocol family PF_PACKET to 
         * allow direct access to all packets on the network while 
         * older kernels had a special socket type SOCK_PACKET to 
         * implement this feature.
         * While this old implementation is kind of obsolete we need
         * to be compatible with older kernels for a while so we are 
         * trying both methods with the newer method preferred.
         */
        
        if (! (live_open_new(handle, device, promisc, to_ms, ebuf) ||
               live_open_old(handle, device, promisc, to_ms, ebuf)) )
        {
                /* 
                 * Both methods to open the packet socket failed. Tidy
                 * up and report our failure (ebuf is expected to be
                 * set by the functions above). 
                 */

                free(handle->md.device);
                free(handle);
                return NULL;
        }
        
        /* 
         * Okay, now we have a packet stream open. Maybe we need to handle 
         * a timeout? In that case we set the filehandle to nonblocking 
         * so pcap_read can try reading the fd and call select if no data
         * is available at first. 
         */

        if (to_ms > 0) {
                int     flags = fcntl(handle->fd, F_GETFL);
                if (flags != -1) {
                        flags |= O_NONBLOCK;
                        flags = fcntl(handle->fd, F_SETFL, flags);
                }
                if (flags == -1) {
                        snprintf(ebuf, PCAP_ERRBUF_SIZE, "fcntl: %s",
                                 pcap_strerror(errno));
                        pcap_close(handle);
                        return NULL;
                }
        }

        return handle;
}

/*
 *  Read at most max_packets from the capture stream and call the callback
 *  for each of them. Returns the number of packets handled or -1 if an
 *  error occured. 
 *  
 *  XXX: Can I rely on the Linux-specified behaviour of select (returning
 *  the time left in the timeval structure)? I really don't want to query
 *  the system time before each select call...
 *  
 *  pcap_read currently gets not only a packet from the kernel but also
 *  the sockaddr_ll returned as source of the packet. This way we can at
 *  some time extend tcpdump and libpcap to sniff on all devices at a time
 *  and find the right printing routine by using the information in the
 *  sockaddr_ll structure.
 */
int
pcap_read(pcap_t *handle, int max_packets, pcap_handler callback, u_char *user)
{
        int             status, packets;
        fd_set          read_fds;
        struct timeval  tv;

        /*
         * Fill in a timeval structure for select if we need to obeye a
         * timeout.
         */
        if (handle->md.timeout > 0) {
                tv.tv_usec      = (handle->md.timeout % 1000) * 1000;
                tv.tv_sec       = (handle->md.timeout / 1000);
        }
        
        /*
         * Read packets until the packet limit has been reached or 
         * an error occured while reading. Call the user function 
         * for each received packet.
         */
        for (packets = 0; max_packets == -1 || packets < max_packets;)
        {
                status = pcap_read_packet(handle, callback, user);

                if (status > 0) {
                        packets += status;
                        continue;
                } else if (status == -1)
                        return -1;
                        
                /* 
                 * If no packet is available we go to sleep. FIXME: This
                 * might be better implemented using poll(?)
                 */
                FD_ZERO(&read_fds);
                FD_SET(handle->fd, &read_fds);
                status = select(handle->fd + 1, 
                                &read_fds, NULL, NULL, &tv);

                if (status == -1) {
                        if (errno == EINTR)
                                return packets;
                        snprintf(handle->errbuf, sizeof(handle->errbuf),
                                 "select: %s", pcap_strerror(errno));
                        return -1;
                } 
                else if (status == 0 || 
                           (tv.tv_usec == 0 && tv.tv_sec == 0))
                        return packets;
        }

        return packets;
}
                
/*
 *  Read a packet from the socket calling the handler provided by 
 *  the user. Returns the number of packets received or -1 if an
 *  error occured.
 */
static int
pcap_read_packet(pcap_t *handle, pcap_handler callback, u_char *userdata)
{
        struct sockaddr         from;
        socklen_t               fromlen;
        int                     packet_len, caplen;
        struct pcap_pkthdr      pcap_header;

        /*
         * We don't currently use the from return value of recvfrom but
         * this will probably be implemented in the future.
         */
        
        /* Receive a single packet from the kernel */

        do {
                fromlen = sizeof(from);
                packet_len = recvfrom( 
                        handle->fd, handle->buffer + handle->offset, 
                        handle->snapshot, MSG_TRUNC, 
                        (struct sockaddr *) &from, &fromlen);
        } while (packet_len == -1 && errno == EINTR);

        /* Check if an error occured */

        if (packet_len == -1) {
                if (errno == EAGAIN)
                        return 0;       /* no packet there */
                else {
                        snprintf(handle->errbuf, sizeof(handle->errbuf),
                                 "recvfrom: %s", pcap_strerror(errno));
                        return -1;
                }
        }

        /*
         * XXX: According to the kernel source we should get the real 
         * packet len if calling recvfrom with MSG_TRUNC set. It does 
         * not seem to work here :(, but it is supported by this code
         * anyway. 
         * To be honest the code RELIES on that feature so this is really
         * broken with 2.2.x kernels.
         * I spend a day to figure out what's going on and I found out
         * that the following is happening: 
         *
         * The packet comes from a random interface and the packet_rcv 
         * hook is called with a clone of the packet. That code inserts
         * the packet into the receive queue of the packet socket.
         * If a filter is attached to that socket that filter is run
         * first - and there lies the problem. The default filter always
         * cuts the packet at the snaplen:
         *
         * # tcpdump -d
         * (000) ret      #68
         *
         * So the packet filter cuts down the packet. The recvfrom call 
         * says "hey, it's only 68 bytes, it fits into the buffer" with
         * the result that we don't get the real packet length. This 
         * is valid at least until kernel 2.2.17pre6. 
         *
         * tcpdump is currently fixed by changing the BPF code generator
         * to not truncate the received packet. 
         */
        
        caplen = packet_len;
        if (caplen > handle->snapshot)
                caplen = handle->snapshot;

        /* Run the packet filter if not using kernel filter */
        if (!handle->md.use_bpf && handle->fcode.bf_insns) {
                if (bpf_filter(handle->fcode.bf_insns, handle->buffer, 
                                packet_len, caplen) == 0)
                {
                        /* rejected by filter */
                        return 0;
                }
        }
        
        /* Fill in our own header data */
        
        if (ioctl(handle->fd, SIOCGSTAMP, &pcap_header.ts) == -1) {
                snprintf(handle->errbuf, sizeof(handle->errbuf),
                         "ioctl: %s", pcap_strerror(errno));
                return -1;
        }
        pcap_header.caplen      = caplen;
        pcap_header.len         = packet_len;
        
        /* Call the user supplied callback function */
        handle->md.stat.ps_recv++;
        callback(userdata, &pcap_header, handle->buffer + handle->offset);

        return 1;
}

/*
 *  Get the statistics for the given packet capture handle.
 *  FIXME: Currently does not report the number of dropped packets.
 */
int
pcap_stats(pcap_t *handle, struct pcap_stat *stats)
{
        *stats = handle->md.stat;
        return 0;
}

/*
 *  Attach the given BPF code to the packet capture device. 
 */
int
pcap_setfilter(pcap_t *handle, struct bpf_program *filter)
{
#ifdef SO_ATTACH_FILTER
        struct sock_fprog       fcode;
#endif

        if (!handle)
                return -1;
        if (!filter) {
                strncpy(handle->errbuf, "setfilter: No filter specified",
                        sizeof(handle->errbuf));
                return -1;
        }

        /* Free old filter code if existing */

        handle->fcode.bf_len    = 0;
        if (handle->fcode.bf_insns) {
                free(handle->fcode.bf_insns);
                handle->fcode.bf_insns = NULL;
        }


        /* Make our private copy of the filter */

        handle->fcode.bf_len   = filter->bf_len;
        handle->fcode.bf_insns = 
                malloc(filter->bf_len * sizeof(*filter->bf_insns));
        if (handle->fcode.bf_insns == NULL) {
                snprintf(handle->errbuf, sizeof(handle->errbuf),
                         "malloc: %s", pcap_strerror(errno));
                return -1;
        } 
        memcpy(handle->fcode.bf_insns, filter->bf_insns, 
               filter->bf_len * sizeof(*filter->bf_insns));

        /* 
         * Run user level packet filter by default. Will be overriden if 
         * installing a kernel filter succeeds. 
         */
        handle->md.use_bpf = 0;

        /*
         * If we're reading from a savefile, don't try to install
         * a kernel filter.
         */
        if (handle->sf.rfile != NULL)
                return 0;

        /* Install kernel level filter if possible */
        
#ifdef SO_ATTACH_FILTER
        /*
         * Oh joy, the Linux kernel uses struct sock_fprog instead of 
         * struct bpf_program and of course the length field is of 
         * different size. Pointed out by Sebastian
         */

        fcode.filter    = (struct sock_filter *) handle->fcode.bf_insns;
        fcode.len       = filter->bf_len;
#ifdef USHRT_MAX
        if (filter->bf_len > USHRT_MAX) {
                /*
                 * fcode.len is an unsigned short for current kernel. 
                 * I have yet to see BPF-Code with that much instructions
                 * but still it is possible. So for the sake of 
                 * correctness I added this check.
                 */
                fprintf(stderr, "Warning: Filter to complex for kernel\n");
        } 
        else
#endif
        if (setsockopt(handle->fd, SOL_SOCKET, SO_ATTACH_FILTER, 
                       &fcode, sizeof(fcode)) == 0)
        {
                /* Installation succeded - using kernel filter. */
                handle->md.use_bpf = 1;
        } 
        else
        {
                /* 
                 * Print a warning if kernel filter available but a problem
                 * occured using it. 
                 */
                if (errno != ENOPROTOOPT && errno != EOPNOTSUPP) {
                        fprintf(stderr, "Warning: Kernel filter failed: %s\n", 
                                pcap_strerror(errno));
                }
        }
#endif

        return 0;
}

/*
 *  Linux uses the ARP hardware type to identify the type of an 
 *  interface. pcap uses the DLT_xxx constants for this. This 
 *  function maps the ARPHRD_xxx constant to an appropriate
 *  DLT_xxx constant.
 *  
 *  Returns -1 if unable to map the type.
 */
static int map_arphrd_to_dlt(int arptype)
{
        switch (arptype) {
        case ARPHRD_ETHER:
        case ARPHRD_METRICOM:
        case ARPHRD_LOOPBACK:   return DLT_EN10MB;
        case ARPHRD_EETHER:     return DLT_EN3MB;
        case ARPHRD_AX25:       return DLT_AX25;
        case ARPHRD_PRONET:     return DLT_PRONET;
        case ARPHRD_CHAOS:      return DLT_CHAOS;
        case ARPHRD_IEEE802:    return DLT_IEEE802;
        case ARPHRD_ARCNET:     return DLT_ARCNET;
        case ARPHRD_FDDI:       return DLT_FDDI;

#ifndef ARPHRD_ATM  /* FIXME: How to #include this? */
#define ARPHRD_ATM 19
#endif
        case ARPHRD_ATM:        return DLT_ATM_CLIP;

        case ARPHRD_PPP:
        case ARPHRD_CSLIP:
        case ARPHRD_SLIP6:
        case ARPHRD_CSLIP6:
        case ARPHRD_SLIP:       return DLT_RAW;
        }

        return -1;
}

/* ===== Functions to interface to the newer kernels ================== */

/*
 *  Try to open a packet socket using the new kernel interface.
 *  Returns 0 on failure.
 *  FIXME: 0 uses to mean success (Sebastian)
 */
static int
live_open_new(pcap_t *handle, char *device, int promisc, 
              int to_ms, char *ebuf)
{
#ifdef HAVE_NETPACKET_PACKET_H
        int                     sock_fd = -1, device_id, mtu, arptype;
        struct packet_mreq      mr;

        /* One shot loop used for error handling - bail out with break */

        do {
                /*
                 * Open a socket with protocol family packet. If a device is
                 * given we try to open it in raw mode otherwise we use 
                 * the cooked interface. 
                 */
                sock_fd = device ? 
                        socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL))
                      : socket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_ALL));

                if (sock_fd == -1) {
                        snprintf(ebuf, PCAP_ERRBUF_SIZE, "socket: %s",
                                 pcap_strerror(errno) );
                        break;
                }

                /* It seems the kernel supports the new interface. */
                handle->md.sock_packet = 0;

                /*
                 * What kind of frames do we have to deal with? Fall back 
                 * to cooked mode if we have an unknown interface type. 
                 */

                if (device) {
                        arptype = iface_get_arptype(sock_fd, device, ebuf);
                        if (arptype == -1) 
                                break;
                        handle->linktype = map_arphrd_to_dlt(arptype);
                } else 
                        handle->linktype = DLT_RAW;
                        
                if (handle->linktype == -1) {
                        /* Unknown interface type - reopen in cooked mode */
                        
                        if (close(sock_fd) == -1) {
                                snprintf(ebuf, PCAP_ERRBUF_SIZE,
                                         "close: %s", pcap_strerror(errno));
                                break;
                        }
                        sock_fd = socket(PF_PACKET, SOCK_DGRAM, 
                                         htons(ETH_P_ALL));
                        if (sock_fd == -1) {
                                snprintf(ebuf, PCAP_ERRBUF_SIZE,
                                         "socket: %s", pcap_strerror(errno));
                                break;
                        }

                        fprintf(stderr, 
                                "Warning: Falling back to cooked socket\n");
                        handle->linktype = DLT_RAW;
                }


                if (device) {
                        device_id = iface_get_id(sock_fd, device, ebuf);
                        if (device_id == -1)
                                break;

                        if (iface_bind(sock_fd, device_id, ebuf) == -1)
                                break;
                }

                /* Select promiscuous mode on/off */

#ifdef SOL_PACKET
                /* 
                 * Hmm, how can we set promiscuous mode on all interfaces?
                 * I am not sure if that is possible at all.
                 */

                if (device) {
                        memset(&mr, 0, sizeof(mr));
                        mr.mr_ifindex = device_id;
                        mr.mr_type    = promisc ? 
                                PACKET_MR_PROMISC : PACKET_MR_ALLMULTI;
                        if (setsockopt(sock_fd, SOL_PACKET, 
                                PACKET_ADD_MEMBERSHIP, &mr, sizeof(mr)) == -1)
                        {
                                snprintf(ebuf, PCAP_ERRBUF_SIZE, 
                                        "setsockopt: %s", pcap_strerror(errno));
                                break;
                        }
                }
#endif
                
                /* Compute the buffersize */

                mtu     = iface_get_mtu(sock_fd, device, ebuf);
                if (mtu == -1)
                        break;
                handle->bufsize  = MAX_LINKHEADER_SIZE + mtu;
                
                /* Fill in the pcap structure */

                handle->fd       = sock_fd;
                handle->offset   = 0;

                handle->buffer   = malloc(handle->bufsize);
                if (!handle->buffer) {
                        snprintf(ebuf, PCAP_ERRBUF_SIZE,
                                 "malloc: %s", pcap_strerror(errno));
                        break;
                }

                return 1;

        } while(0);

        if (sock_fd != -1)
                close(sock_fd);
        return 0;
#else
        strncpy(ebuf, 
                "New packet capturing interface not supported by build " 
                "environment", PCAP_ERRBUF_SIZE);
        return 0;
#endif
}

#ifdef HAVE_NETPACKET_PACKET_H
/*
 *  Return the index of the given device name. Fill ebuf and return 
 *  -1 on failure.
 */
static int
iface_get_id(int fd, const char *device, char *ebuf)
{
        struct ifreq    ifr;

        memset(&ifr, 0, sizeof(ifr));
        strncpy(ifr.ifr_name, device, sizeof(ifr.ifr_name));

        if (ioctl(fd, SIOCGIFINDEX, &ifr) == -1) {
                snprintf(ebuf, PCAP_ERRBUF_SIZE,
                         "ioctl: %s", pcap_strerror(errno));
                return -1;
        }

        return ifr.ifr_ifindex;
}

/*
 *  Bind the socket associated with FD to the given device. 
 */
static int
iface_bind(int fd, int ifindex, char *ebuf)
{
        struct sockaddr_ll      sll;

        memset(&sll, 0, sizeof(sll));
        sll.sll_family          = AF_PACKET;
        sll.sll_ifindex         = ifindex;
        sll.sll_protocol        = htons(ETH_P_ALL);

        if (bind(fd, (struct sockaddr *) &sll, sizeof(sll)) == -1) {
                snprintf(ebuf, PCAP_ERRBUF_SIZE,
                         "bind: %s", pcap_strerror(errno));
                return -1;
        }

        return 0;
}

#endif


/* ===== Functions to interface to the older kernels ================== */

/*
 * With older kernels promiscuous mode is kind of interesting because we
 * have to reset the interface before exiting. The problem can't really
 * be solved without some daemon taking care of managing usage counts. 
 * We save the promiscuous state of the device when opening the capture
 * stream and arrange for it to be reset on process exit.
 *
 * XXX: This solution is still not correct even for this case. The 
 * devices stay in promiscuous mode until the process exits. I need to 
 * modify pcap_close to solve this.
 */

/* 
 * The device name and the interface flags to be restored at exit
 */
struct ifreq    restore_ifr;

static void     restore_interface( void )
{
        int     status = socket(PF_INET, SOCK_PACKET, 0);

        if (status != -1)
                status = ioctl(status, SIOCSIFFLAGS, &restore_ifr);

        if (status == -1) {
                fprintf(stderr, 
                "Can't restore interface flags. Please adjust manually. \n"
                "Hint: This can't happen with Linux >= 2.2.0.\n");
        }
}

/*
 *  Try to open a packet socket using the old kernel interface.
 *  Returns 0 on failure.
 *  FIXME: 0 uses to mean success (Sebastian)
 */
static int
live_open_old(pcap_t *handle, char *device, int promisc, 
              int to_ms, char *ebuf)
{
        int             sock_fd = -1, mtu, arptype;
        struct ifreq    ifr;

        do {
                /* Open the socket */
                
                sock_fd = socket(PF_INET, SOCK_PACKET, htons(ETH_P_ALL));
                if (sock_fd == -1) {
                        snprintf(ebuf, PCAP_ERRBUF_SIZE,
                                 "socket: %s", pcap_strerror(errno));
                        break;
                }

                /* It worked - we are using the old interface */
                handle->md.sock_packet = 1;

                /* Bind to the given device */

                if (!device) {
                        strncpy(ebuf, "pcap_open_live: No interface given",
                                PCAP_ERRBUF_SIZE);
                        break;
                }
                if (iface_bind_old(sock_fd, device, ebuf) == -1)
                        break;

                /* Go to promisc mode */
                if (promisc) {
                        memset(&ifr, 0, sizeof(ifr));
                        strncpy(ifr.ifr_name, device, sizeof(ifr.ifr_name));
                        if (ioctl(sock_fd, SIOCGIFFLAGS, &ifr) == -1) {
                                snprintf(ebuf, PCAP_ERRBUF_SIZE,
                                         "ioctl: %s", pcap_strerror(errno));
                                break;
                        }
                        if ((ifr.ifr_flags & IFF_PROMISC) == 0) {
                                restore_ifr    = ifr;
                                ifr.ifr_flags |= IFF_PROMISC;
                                if (ioctl(sock_fd, SIOCSIFFLAGS, &ifr) == -1) {
                                        snprintf(ebuf, PCAP_ERRBUF_SIZE,
                                                 "ioctl: %s",
                                                 pcap_strerror(errno));
                                        break;
                                }
                                if (atexit(restore_interface) == -1) {
                                        restore_interface();
                                        strncpy(ebuf, "atexit failed",
                                                PCAP_ERRBUF_SIZE);
                                        break;
                                }
                        }
                }

                
                /* Compute the buffersize */

                mtu     = iface_get_mtu(sock_fd, device, ebuf);
                if (mtu == -1)
                        break;
                handle->bufsize  = MAX_LINKHEADER_SIZE + mtu;
                if (handle->bufsize < handle->snapshot)
                        handle->bufsize = handle->snapshot;
                
                /* All done - fill in the pcap handle */

                arptype = iface_get_arptype(sock_fd, device, ebuf);
                if (arptype == -1)
                        break;

                handle->fd       = sock_fd;
                handle->offset   = 0;
                handle->linktype = map_arphrd_to_dlt(arptype);
                if (handle->linktype == -1) {
                        snprintf(ebuf, PCAP_ERRBUF_SIZE,
                                 "interface type of %s not supported", device);
                        break;
                }
                handle->buffer   = malloc(handle->bufsize);
                if (!handle->buffer) {
                        snprintf(ebuf, PCAP_ERRBUF_SIZE,
                                 "malloc: %s", pcap_strerror(errno));
                        break;
                }

                return 1;
                
        } while (0);
                
        if (sock_fd != -1)
                close(sock_fd);
        return 0;
}

/*
 *  Bind the socket associated with FD to the given device using the 
 *  interface of the old kernels.
 */
static int
iface_bind_old(int fd, const char *device, char *ebuf)
{
        struct sockaddr saddr;

        memset(&saddr, 0, sizeof(saddr));
        strncpy(saddr.sa_data, device, sizeof(saddr.sa_data));
        if (bind(fd, &saddr, sizeof(saddr)) == -1) {
                snprintf(ebuf, PCAP_ERRBUF_SIZE,
                         "bind: %s", pcap_strerror(errno));
                return -1;
        }

        return 0;
}


/* ===== System calls available on all supported kernels ============== */

/*
 *  Query the kernel for the MTU of the given interface. 
 */
static int
iface_get_mtu(int fd, const char *device, char *ebuf)
{
        struct ifreq    ifr;

        if (!device)
                return BIGGER_THAN_ALL_MTUS;

        memset(&ifr, 0, sizeof(ifr));
        strncpy(ifr.ifr_name, device, sizeof(ifr.ifr_name));

        if (ioctl(fd, SIOCGIFMTU, &ifr) == -1) {
                snprintf(ebuf, PCAP_ERRBUF_SIZE,
                         "ioctl: %s", pcap_strerror(errno));
                return -1;
        }

        return ifr.ifr_mtu;
}

/*
 *  Get the hardware type of the given interface as ARPHRD_xxx constant.
 */
static int
iface_get_arptype(int fd, const char *device, char *ebuf)
{
        struct ifreq    ifr;

        memset(&ifr, 0, sizeof(ifr));
        strncpy(ifr.ifr_name, device, sizeof(ifr.ifr_name));

        if (ioctl(fd, SIOCGIFHWADDR, &ifr) == -1) {
                snprintf(ebuf, PCAP_ERRBUF_SIZE,
                         "ioctl: %s", pcap_strerror(errno));
                return -1;
        }

        return ifr.ifr_hwaddr.sa_family;
}