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sll.c

/* David Leonard, 2003. Public domain. */
/* $Id: sll.c,v 1.1 2003/08/17 11:38:13 d Exp $ */

#include <stdio.h>
#include <pcap.h>
#include <sys/types.h>
#include <sys/socket.h>

#include "tag.h"
#include "flow.h"

#if defined(DLT_LINUX_SLL)

/*
 * libpcap for Linux provides synthetic framing for AF_PACKET sockets
 * (cf. SOCK_RAW) by prefixing data with a struct sll_header structure
 * derived from the AF_PACKET 'from' address structure.
 *
 * From libpcap/tcpdump's (private) "sll.h", we are told the 
 *
 *    "fake header includes:
 *     a 2-byte 'packet type' which is one of:
 *          LINUX_SLL_HOST          [0] packet was sent to us
 *          LINUX_SLL_BROADCAST     [1] packet was broadcast
 *          LINUX_SLL_MULTICAST     [2] packet was multicast
 *          LINUX_SLL_OTHERHOST     [3] packet was sent to somebody else
 *          LINUX_SLL_OUTGOING      [4] packet was sent *by* us;
 *     a 2-byte Ethernet protocol field;
 *     a 2-byte link-layer type;
 *     a 2-byte link-layer address length;
 *     an 8-byte source link-layer address, whose actual length is
 *          specified by the previous value.
 *     All fields except for the link-layer address are in network
 *     byte order."
 *
 * This is misleading: the 'Ethernet protocol field' is actually last.
 * The 'Ethernet protocol field' also takes the same values as Linux's 
 * ETH_P_ values from <linux/if_ether.h>. Luckilly, these seem to be
 * the same as the standard ethernet protocol identifiers. The code below
 * may break if libpcap/linux-bpf.c or linux/if_ether.h change.
 */

#define SLL_ADDRLEN     8
struct sll_header {
      u_int16_t   sll_pkttype;
      u_int16_t   sll_hatype;
      u_int16_t   sll_halen;
      u_int8_t    sll_addr[SLL_ADDRLEN];
      u_int16_t   sll_protocol;
};

/* Return the tag for a Linux cooked socket packet */

const char *
sll_tag(p, end)
        const char *p;
      const char *end;
{
      struct sll_header *sll = (struct sll_header *)p;
      const char *tag;
      static char buf[TAGLEN];
      u_int16_t type;

      p += sizeof *sll;
      if (p > end)
            return "short sll";
      type = ntohs(sll->sll_protocol);

      tag = ether_tagx(type, p, end);
      if (tag)
            return tag;

      snprintf(buf, sizeof buf, "sll type 0x%04x hatype %04x",
            type, ntohs(sll->sll_hatype));
      return buf;
}

#endif /* defined(DLT_LINUX_SLL) */

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