xref: /freebsd-9.3-release/contrib/libpcap/sf-pcap-ng.c

/*
 * Copyright (c) 1993, 1994, 1995, 1996, 1997
 *	The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that: (1) source code distributions
 * retain the above copyright notice and this paragraph in its entirety, (2)
 * distributions including binary code include the above copyright notice and
 * this paragraph in its entirety in the documentation or other materials
 * provided with the distribution, and (3) all advertising materials mentioning
 * features or use of this software display the following acknowledgement:
 * ``This product includes software developed by the University of California,
 * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
 * the University nor the names of its contributors may 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.
 *
 * sf-pcap-ng.c - pcap-ng-file-format-specific code from savefile.c
 */

#ifndef lint
static const char rcsid[] _U_ =
    "@(#) $Header$ (LBL)";
#endif

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

#ifdef WIN32
#include <pcap-stdinc.h>
#else /* WIN32 */
#if HAVE_INTTYPES_H
#include <inttypes.h>
#elif HAVE_STDINT_H
#include <stdint.h>
#endif
#ifdef HAVE_SYS_BITYPES_H
#include <sys/bitypes.h>
#endif
#include <sys/types.h>
#endif /* WIN32 */

#include <errno.h>
#include <memory.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "pcap-int.h"

#include "pcap-common.h"

#ifdef HAVE_OS_PROTO_H
#include "os-proto.h"
#endif

#include "sf-pcap-ng.h"

/*
 * Block types.
 */

/*
 * Common part at the beginning of all blocks.
 */
struct block_header {
	bpf_u_int32	block_type;
	bpf_u_int32	total_length;
};

/*
 * Common trailer at the end of all blocks.
 */
struct block_trailer {
	bpf_u_int32	total_length;
};

/*
 * Common options.
 */
#define OPT_ENDOFOPT	0	/* end of options */
#define OPT_COMMENT	1	/* comment string */

/*
 * Option header.
 */
struct option_header {
	u_short		option_code;
	u_short		option_length;
};

/*
 * Structures for the part of each block type following the common
 * part.
 */

/*
 * Section Header Block.
 */
#define BT_SHB			0x0A0D0D0A

struct section_header_block {
	bpf_u_int32	byte_order_magic;
	u_short		major_version;
	u_short		minor_version;
	u_int64_t	section_length;
	/* followed by options and trailer */
};

/*
 * Byte-order magic value.
 */
#define BYTE_ORDER_MAGIC	0x1A2B3C4D

/*
 * Current version number.  If major_version isn't PCAP_NG_VERSION_MAJOR,
 * that means that this code can't read the file.
 */
#define PCAP_NG_VERSION_MAJOR	1

/*
 * Interface Description Block.
 */
#define BT_IDB			0x00000001

struct interface_description_block {
	u_short		linktype;
	u_short		reserved;
	bpf_u_int32	snaplen;
	/* followed by options and trailer */
};

/*
 * Options in the IDB.
 */
#define IF_NAME		2	/* interface name string */
#define IF_DESCRIPTION	3	/* interface description string */
#define IF_IPV4ADDR	4	/* interface's IPv4 address and netmask */
#define IF_IPV6ADDR	5	/* interface's IPv6 address and prefix length */
#define IF_MACADDR	6	/* interface's MAC address */
#define IF_EUIADDR	7	/* interface's EUI address */
#define IF_SPEED	8	/* interface's speed, in bits/s */
#define IF_TSRESOL	9	/* interface's time stamp resolution */
#define IF_TZONE	10	/* interface's time zone */
#define IF_FILTER	11	/* filter used when capturing on interface */
#define IF_OS		12	/* string OS on which capture on this interface was done */
#define IF_FCSLEN	13	/* FCS length for this interface */
#define IF_TSOFFSET	14	/* time stamp offset for this interface */

/*
 * Enhanced Packet Block.
 */
#define BT_EPB			0x00000006

struct enhanced_packet_block {
	bpf_u_int32	interface_id;
	bpf_u_int32	timestamp_high;
	bpf_u_int32	timestamp_low;
	bpf_u_int32	caplen;
	bpf_u_int32	len;
	/* followed by packet data, options, and trailer */
};

/*
 * Simple Packet Block.
 */
#define BT_SPB			0x00000003

struct simple_packet_block {
	bpf_u_int32	len;
	/* followed by packet data and trailer */
};

/*
 * Packet Block.
 */
#define BT_PB			0x00000002

struct packet_block {
	u_short		interface_id;
	u_short		drops_count;
	bpf_u_int32	timestamp_high;
	bpf_u_int32	timestamp_low;
	bpf_u_int32	caplen;
	bpf_u_int32	len;
	/* followed by packet data, options, and trailer */
};

/*
 * Block cursor - used when processing the contents of a block.
 * Contains a pointer into the data being processed and a count
 * of bytes remaining in the block.
 */
struct block_cursor {
	u_char		*data;
	size_t		data_remaining;
	bpf_u_int32	block_type;
};

static int pcap_ng_next_packet(pcap_t *p, struct pcap_pkthdr *hdr,
    u_char **data);

static int
read_bytes(FILE *fp, void *buf, size_t bytes_to_read, int fail_on_eof,
    char *errbuf)
{
	size_t amt_read;

	amt_read = fread(buf, 1, bytes_to_read, fp);
	if (amt_read != bytes_to_read) {
		if (ferror(fp)) {
			snprintf(errbuf, PCAP_ERRBUF_SIZE,
			    "error reading dump file: %s",
			    pcap_strerror(errno));
		} else {
			if (amt_read == 0 && !fail_on_eof)
				return (0);	/* EOF */
			snprintf(errbuf, PCAP_ERRBUF_SIZE,
			    "truncated dump file; tried to read %lu bytes, only got %lu",
			    (unsigned long)bytes_to_read,
			    (unsigned long)amt_read);
		}
		return (-1);
	}
	return (1);
}

static int
read_block(FILE *fp, pcap_t *p, struct block_cursor *cursor, char *errbuf)
{
	int status;
	struct block_header bhdr;

	status = read_bytes(fp, &bhdr, sizeof(bhdr), 0, errbuf);
	if (status <= 0)
		return (status);	/* error or EOF */

	if (p->sf.swapped) {
		bhdr.block_type = SWAPLONG(bhdr.block_type);
		bhdr.total_length = SWAPLONG(bhdr.total_length);
	}

	/*
	 * Is this block "too big"?
	 *
	 * We choose 16MB as "too big", for now, so that we handle
	 * "reasonably" large buffers but don't chew up all the
	 * memory if we read a malformed file.
	 */
	if (bhdr.total_length > 16*1024*1024) {
		snprintf(errbuf, PCAP_ERRBUF_SIZE,
		    "pcap-ng block size %u > maximum %u",
		    bhdr.total_length, 16*1024*1024);
		    return (-1);
	}

	/*
	 * Is this block "too small" - i.e., is it shorter than a block
	 * header plus a block trailer?
	 */
	if (bhdr.total_length < sizeof(struct block_header) +
	    sizeof(struct block_trailer)) {
		snprintf(errbuf, PCAP_ERRBUF_SIZE,
		    "block in pcap-ng dump file has a length of %u < %lu",
		    bhdr.total_length,
		    (unsigned long)(sizeof(struct block_header) + sizeof(struct block_trailer)));
		return (-1);
	}

	/*
	 * Is the buffer big enough?
	 */
	if (p->bufsize < bhdr.total_length) {
		/*
		 * No - make it big enough.
		 */
		p->buffer = realloc(p->buffer, bhdr.total_length);
		if (p->buffer == NULL) {
			snprintf(errbuf, PCAP_ERRBUF_SIZE, "out of memory");
			return (-1);
		}
	}

	/*
	 * Copy the stuff we've read to the buffer, and read the rest
	 * of the block.
	 */
	memcpy(p->buffer, &bhdr, sizeof(bhdr));
	if (read_bytes(fp, p->buffer + sizeof(bhdr),
	    bhdr.total_length - sizeof(bhdr), 1, errbuf) == -1)
		return (-1);

	/*
	 * Initialize the cursor.
	 */
	cursor->data = p->buffer + sizeof(bhdr);
	cursor->data_remaining = bhdr.total_length - sizeof(bhdr) -
	    sizeof(struct block_trailer);
	cursor->block_type = bhdr.block_type;
	return (1);
}

static void *
get_from_block_data(struct block_cursor *cursor, size_t chunk_size,
    char *errbuf)
{
	void *data;

	/*
	 * Make sure we have the specified amount of data remaining in
	 * the block data.
	 */
	if (cursor->data_remaining < chunk_size) {
		snprintf(errbuf, PCAP_ERRBUF_SIZE,
		    "block of type %u in pcap-ng dump file is too short",
		    cursor->block_type);
		return (NULL);
	}

	/*
	 * Return the current pointer, and skip past the chunk.
	 */
	data = cursor->data;
	cursor->data += chunk_size;
	cursor->data_remaining -= chunk_size;
	return (data);
}

static struct option_header *
get_opthdr_from_block_data(pcap_t *p, struct block_cursor *cursor, char *errbuf)
{
	struct option_header *opthdr;

	opthdr = get_from_block_data(cursor, sizeof(*opthdr), errbuf);
	if (opthdr == NULL) {
		/*
		 * Option header is cut short.
		 */
		return (NULL);
	}

	/*
	 * Byte-swap it if necessary.
	 */
	if (p->sf.swapped) {
		opthdr->option_code = SWAPSHORT(opthdr->option_code);
		opthdr->option_length = SWAPSHORT(opthdr->option_length);
	}

	return (opthdr);
}

static void *
get_optvalue_from_block_data(struct block_cursor *cursor,
    struct option_header *opthdr, char *errbuf)
{
	size_t padded_option_len;
	void *optvalue;

	/* Pad option length to 4-byte boundary */
	padded_option_len = opthdr->option_length;
	padded_option_len = ((padded_option_len + 3)/4)*4;

	optvalue = get_from_block_data(cursor, padded_option_len, errbuf);
	if (optvalue == NULL) {
		/*
		 * Option value is cut short.
		 */
		return (NULL);
	}

	return (optvalue);
}

static int
process_idb_options(pcap_t *p, struct block_cursor *cursor, u_int *tsresol,
    u_int64_t *tsoffset, char *errbuf)
{
	struct option_header *opthdr;
	void *optvalue;
	int saw_tsresol, saw_tsoffset;
	u_char tsresol_opt;
	u_int i;

	saw_tsresol = 0;
	saw_tsoffset = 0;
	while (cursor->data_remaining != 0) {
		/*
		 * Get the option header.
		 */
		opthdr = get_opthdr_from_block_data(p, cursor, errbuf);
		if (opthdr == NULL) {
			/*
			 * Option header is cut short.
			 */
			return (-1);
		}

		/*
		 * Get option value.
		 */
		optvalue = get_optvalue_from_block_data(cursor, opthdr,
		    errbuf);
		if (optvalue == NULL) {
			/*
			 * Option value is cut short.
			 */
			return (-1);
		}

		switch (opthdr->option_code) {

		case OPT_ENDOFOPT:
			if (opthdr->option_length != 0) {
				snprintf(errbuf, PCAP_ERRBUF_SIZE,
				    "Interface Description Block has opt_endofopt option with length %u != 0",
				    opthdr->option_length);
				return (-1);
			}
			goto done;

		case IF_TSRESOL:
			if (opthdr->option_length != 1) {
				snprintf(errbuf, PCAP_ERRBUF_SIZE,
				    "Interface Description Block has if_tsresol option with length %u != 1",
				    opthdr->option_length);
				return (-1);
			}
			if (saw_tsresol) {
				snprintf(errbuf, PCAP_ERRBUF_SIZE,
				    "Interface Description Block has more than one if_tsresol option");
				return (-1);
			}
			saw_tsresol = 1;
			tsresol_opt = *(u_int *)optvalue;
			if (tsresol_opt & 0x80) {
				/*
				 * Resolution is negative power of 2.
				 */
				*tsresol = 1 << (tsresol_opt & 0x7F);
			} else {
				/*
				 * Resolution is negative power of 10.
				 */
				*tsresol = 1;
				for (i = 0; i < tsresol_opt; i++)
					*tsresol *= 10;
			}
			if (*tsresol == 0) {
				/*
				 * Resolution is too high.
				 */
				if (tsresol_opt & 0x80) {
					snprintf(errbuf, PCAP_ERRBUF_SIZE,
					    "Interface Description Block if_tsresol option resolution 2^-%u is too high",
					    tsresol_opt & 0x7F);
				} else {
					snprintf(errbuf, PCAP_ERRBUF_SIZE,
					    "Interface Description Block if_tsresol option resolution 10^-%u is too high",
					    tsresol_opt);
				}
				return (-1);
			}
			break;

		case IF_TSOFFSET:
			if (opthdr->option_length != 8) {
				snprintf(errbuf, PCAP_ERRBUF_SIZE,
				    "Interface Description Block has if_tsoffset option with length %u != 8",
				    opthdr->option_length);
				return (-1);
			}
			if (saw_tsoffset) {
				snprintf(errbuf, PCAP_ERRBUF_SIZE,
				    "Interface Description Block has more than one if_tsoffset option");
				return (-1);
			}
			saw_tsoffset = 1;
			memcpy(tsoffset, optvalue, sizeof(*tsoffset));
			if (p->sf.swapped)
				*tsoffset = SWAPLL(*tsoffset);
			break;

		default:
			break;
		}
	}

done:
	return (0);
}

/*
 * Check whether this is a pcap-ng savefile and, if it is, extract the
 * relevant information from the header.
 */
int
pcap_ng_check_header(pcap_t *p, bpf_u_int32 magic, FILE *fp, char *errbuf)
{
	size_t amt_read;
	bpf_u_int32 total_length;
	bpf_u_int32 byte_order_magic;
	struct block_header *bhdrp;
	struct section_header_block *shbp;
	int status;
	struct block_cursor cursor;
	struct interface_description_block *idbp;

	/*
	 * Check whether the first 4 bytes of the file are the block
	 * type for a pcap-ng savefile.
	 */
	if (magic != BT_SHB) {
		/*
		 * XXX - check whether this looks like what the block
		 * type would be after being munged by mapping between
		 * UN*X and DOS/Windows text file format and, if it
		 * does, look for the byte-order magic number in
		 * the appropriate place and, if we find it, report
		 * this as possibly being a pcap-ng file transferred
		 * between UN*X and Windows in text file format?
		 */
		return (0);	/* nope */
	}

	/*
	 * OK, they are.  However, that's just \n\r\r\n, so it could,
	 * conceivably, be an ordinary text file.
	 *
	 * It could not, however, conceivably be any other type of
	 * capture file, so we can read the rest of the putative
	 * Section Header Block; put the block type in the common
	 * header, read the rest of the common header and the
	 * fixed-length portion of the SHB, and look for the byte-order
	 * magic value.
	 */
	amt_read = fread(&total_length, 1, sizeof(total_length), fp);
	if (amt_read < sizeof(total_length)) {
		if (ferror(fp)) {
			snprintf(errbuf, PCAP_ERRBUF_SIZE,
			    "error reading dump file: %s",
			    pcap_strerror(errno));
			return (-1);	/* fail */
		}

		/*
		 * Possibly a weird short text file, so just say
		 * "not pcap-ng".
		 */
		return (0);
	}
	amt_read = fread(&byte_order_magic, 1, sizeof(byte_order_magic), fp);
	if (amt_read < sizeof(byte_order_magic)) {
		if (ferror(fp)) {
			snprintf(errbuf, PCAP_ERRBUF_SIZE,
			    "error reading dump file: %s",
			    pcap_strerror(errno));
			return (-1);	/* fail */
		}

		/*
		 * Possibly a weird short text file, so just say
		 * "not pcap-ng".
		 */
		return (0);
	}
	if (byte_order_magic != BYTE_ORDER_MAGIC) {
		byte_order_magic = SWAPLONG(byte_order_magic);
		if (byte_order_magic != BYTE_ORDER_MAGIC) {
			/*
			 * Not a pcap-ng file.
			 */
			return (0);
		}
		p->sf.swapped = 1;
		total_length = SWAPLONG(total_length);
	}

	/*
	 * Check the sanity of the total length.
	 */
	if (total_length < sizeof(*bhdrp) + sizeof(*shbp) + sizeof(struct block_trailer)) {
		snprintf(errbuf, PCAP_ERRBUF_SIZE,
		    "Section Header Block in pcap-ng dump file has a length of %u < %lu",
		    total_length,
		    (unsigned long)(sizeof(*bhdrp) + sizeof(*shbp) + sizeof(struct block_trailer)));
		return (-1);
	}

	/*
	 * Allocate a buffer into which to read blocks.  We default to
	 * the maximum of:
	 *
	 *	the total length of the SHB for which we read the header;
	 *
	 *	2K, which should be more than large enough for an Enhanced
	 *	Packet Block containing a full-size Ethernet frame, and
	 *	leaving room for some options.
	 *
	 * If we find a bigger block, we reallocate the buffer.
	 */
	p->bufsize = 2048;
	if (p->bufsize < total_length)
		p->bufsize = total_length;
	p->buffer = malloc(p->bufsize);
	if (p->buffer == NULL) {
		snprintf(errbuf, PCAP_ERRBUF_SIZE, "out of memory");
		return (-1);
	}

	/*
	 * Copy the stuff we've read to the buffer, and read the rest
	 * of the SHB.
	 */
	bhdrp = (struct block_header *)p->buffer;
	shbp = (struct section_header_block *)(p->buffer + sizeof(struct block_header));
	bhdrp->block_type = magic;
	bhdrp->total_length = total_length;
	shbp->byte_order_magic = byte_order_magic;
	if (read_bytes(fp,
	    p->buffer + (sizeof(magic) + sizeof(total_length) + sizeof(byte_order_magic)),
	    total_length - (sizeof(magic) + sizeof(total_length) + sizeof(byte_order_magic)),
	    1, errbuf) == -1)
		goto fail;

	if (p->sf.swapped) {
		/*
		 * Byte-swap the fields we've read.
		 */
		shbp->major_version = SWAPSHORT(shbp->major_version);
		shbp->minor_version = SWAPSHORT(shbp->minor_version);

		/*
		 * XXX - we don't care about the section length.
		 */
	}
	if (shbp->major_version != PCAP_NG_VERSION_MAJOR) {
		snprintf(errbuf, PCAP_ERRBUF_SIZE,
		    "unknown pcap-ng savefile major version number %u",
		    shbp->major_version);
		goto fail;
	}
	p->sf.version_major = shbp->major_version;
	p->sf.version_minor = shbp->minor_version;

	/*
	 * Set the default time stamp resolution and offset.
	 */
	p->sf.tsresol = 1000000;	/* microsecond resolution */
	p->sf.tsscale = 1;		/* multiply by 1 to scale to microseconds */
	p->sf.tsoffset = 0;		/* absolute timestamps */

	/*
	 * Now start looking for an Interface Description Block.
	 */
	for (;;) {
		/*
		 * Read the next block.
		 */
		status = read_block(fp, p, &cursor, errbuf);
		if (status == 0) {
			/* EOF - no IDB in this file */
			snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
			    "the capture file has no Interface Description Blocks");
			goto fail;
		}
		if (status == -1)
			goto fail;	/* error */
		switch (cursor.block_type) {

		case BT_IDB:
			/*
			 * Get a pointer to the fixed-length portion of the
			 * IDB.
			 */
			idbp = get_from_block_data(&cursor, sizeof(*idbp),
			    errbuf);
			if (idbp == NULL)
				goto fail;	/* error */

			/*
			 * Byte-swap it if necessary.
			 */
			if (p->sf.swapped) {
				idbp->linktype = SWAPSHORT(idbp->linktype);
				idbp->snaplen = SWAPLONG(idbp->snaplen);
			}

			/*
			 * Count this interface.
			 */
			p->sf.ifcount++;

			/*
			 * Now look for various time stamp options, so
			 * we know how to interpret the time stamps.
			 */
			if (process_idb_options(p, &cursor, &p->sf.tsresol,
			    &p->sf.tsoffset, errbuf) == -1)
				goto fail;

			/*
			 * Compute the scaling factor to convert the
			 * sub-second part of the time stamp to
			 * microseconds.
			 */
			if (p->sf.tsresol > 1000000) {
				/*
				 * Higher than microsecond resolution;
				 * scale down to microseconds.
				 */
				p->sf.tsscale = (p->sf.tsresol / 1000000);
			} else {
				/*
				 * Lower than microsecond resolution;
				 * scale up to microseconds.
				 */
				p->sf.tsscale = (1000000 / p->sf.tsresol);
			}
			goto done;

		case BT_EPB:
		case BT_SPB:
		case BT_PB:
			/*
			 * Saw a packet before we saw any IDBs.  That's
			 * not valid, as we don't know what link-layer
			 * encapsulation the packet has.
			 */
			snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
			    "the capture file has a packet block before any Interface Description Blocks");
			goto fail;

		default:
			/*
			 * Just ignore it.
			 */
			break;
		}
	}

done:
	p->tzoff = 0;	/* XXX - not used in pcap */
	p->snapshot = idbp->snaplen;
	p->linktype = linktype_to_dlt(idbp->linktype);
	p->linktype_ext = 0;

	p->sf.next_packet_op = pcap_ng_next_packet;

	return (1);

fail:
	free(p->buffer);
	return (-1);
}

/*
 * Read and return the next packet from the savefile.  Return the header
 * in hdr and a pointer to the contents in data.  Return 0 on success, 1
 * if there were no more packets, and -1 on an error.
 */
static int
pcap_ng_next_packet(pcap_t *p, struct pcap_pkthdr *hdr, u_char **data)
{
	struct block_cursor cursor;
	int status;
	struct enhanced_packet_block *epbp;
	struct simple_packet_block *spbp;
	struct packet_block *pbp;
	bpf_u_int32 interface_id = 0xFFFFFFFF;
	struct interface_description_block *idbp;
	struct section_header_block *shbp;
	FILE *fp = p->sf.rfile;
	u_int tsresol;
	u_int64_t tsoffset;
	u_int64_t t, sec, frac;

	/*
	 * Look for an Enhanced Packet Block, a Simple Packet Block,
	 * or a Packet Block.
	 */
	for (;;) {
		/*
		 * Read the block type and length; those are common
		 * to all blocks.
		 */
		status = read_block(fp, p, &cursor, p->errbuf);
		if (status == 0)
			return (1);	/* EOF */
		if (status == -1)
			return (-1);	/* error */
		switch (cursor.block_type) {

		case BT_EPB:
			/*
			 * Get a pointer to the fixed-length portion of the
			 * EPB.
			 */
			epbp = get_from_block_data(&cursor, sizeof(*epbp),
			    p->errbuf);
			if (epbp == NULL)
				return (-1);	/* error */

			/*
			 * Byte-swap it if necessary.
			 */
			if (p->sf.swapped) {
				/* these were written in opposite byte order */
				interface_id = SWAPLONG(epbp->interface_id);
				hdr->caplen = SWAPLONG(epbp->caplen);
				hdr->len = SWAPLONG(epbp->len);
				t = ((u_int64_t)SWAPLONG(epbp->timestamp_high)) << 32 |
				    SWAPLONG(epbp->timestamp_low);
			} else {
				interface_id = epbp->interface_id;
				hdr->caplen = epbp->caplen;
				hdr->len = epbp->len;
				t = ((u_int64_t)epbp->timestamp_high) << 32 |
				    epbp->timestamp_low;
			}
			goto found;

		case BT_SPB:
			/*
			 * Get a pointer to the fixed-length portion of the
			 * SPB.
			 */
			spbp = get_from_block_data(&cursor, sizeof(*spbp),
			    p->errbuf);
			if (spbp == NULL)
				return (-1);	/* error */

			/*
			 * SPB packets are assumed to have arrived on
			 * the first interface.
			 */
			interface_id = 0;

			/*
			 * Byte-swap it if necessary.
			 */
			if (p->sf.swapped) {
				/* these were written in opposite byte order */
				hdr->len = SWAPLONG(spbp->len);
			} else
				hdr->len = spbp->len;

			/*
			 * The SPB doesn't give the captured length;
			 * it's the minimum of the snapshot length
			 * and the packet length.
			 */
			hdr->caplen = hdr->len;
			if (hdr->caplen > p->snapshot)
				hdr->caplen = p->snapshot;
			t = 0;	/* no time stamps */
			goto found;

		case BT_PB:
			/*
			 * Get a pointer to the fixed-length portion of the
			 * PB.
			 */
			pbp = get_from_block_data(&cursor, sizeof(*pbp),
			    p->errbuf);
			if (pbp == NULL)
				return (-1);	/* error */

			/*
			 * Byte-swap it if necessary.
			 */
			if (p->sf.swapped) {
				/* these were written in opposite byte order */
				interface_id = SWAPSHORT(pbp->interface_id);
				hdr->caplen = SWAPLONG(pbp->caplen);
				hdr->len = SWAPLONG(pbp->len);
				t = ((u_int64_t)SWAPLONG(pbp->timestamp_high)) << 32 |
				    SWAPLONG(pbp->timestamp_low);
			} else {
				interface_id = pbp->interface_id;
				hdr->caplen = pbp->caplen;
				hdr->len = pbp->len;
				t = ((u_int64_t)pbp->timestamp_high) << 32 |
				    pbp->timestamp_low;
			}
			goto found;

		case BT_IDB:
			/*
			 * Interface Description Block.  Get a pointer
			 * to its fixed-length portion.
			 */
			idbp = get_from_block_data(&cursor, sizeof(*idbp),
			    p->errbuf);
			if (idbp == NULL)
				return (-1);	/* error */

			/*
			 * Byte-swap it if necessary.
			 */
			if (p->sf.swapped) {
				idbp->linktype = SWAPSHORT(idbp->linktype);
				idbp->snaplen = SWAPLONG(idbp->snaplen);
			}

			/*
			 * If the link-layer type or snapshot length
			 * differ from the ones for the first IDB we
			 * saw, quit.
			 *
			 * XXX - just discard packets from those
			 * interfaces?
			 */
			if (p->linktype != idbp->linktype) {
				snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
				    "an interface has a type %u different from the type of the first interface",
				    idbp->linktype);
				return (-1);
			}
			if (p->snapshot != idbp->snaplen) {
				snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
				    "an interface has a snapshot length %u different from the type of the first interface",
				    idbp->snaplen);
				return (-1);
			}

			/*
			 * Count this interface.
			 */
			p->sf.ifcount++;

			/*
			 * Set the default time stamp resolution and offset.
			 */
			tsresol = 1000000;	/* microsecond resolution */
			tsoffset = 0;		/* absolute timestamps */

			/*
			 * Now look for various time stamp options, to
			 * make sure they're the same.
			 *
			 * XXX - we could, in theory, handle multiple
			 * different resolutions and offsets, but we
			 * don't do so for now.
			 */
			if (process_idb_options(p, &cursor, &tsresol, &tsoffset,
			    p->errbuf) == -1)
				return (-1);
			if (tsresol != p->sf.tsresol) {
				snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
				    "an interface has a time stamp resolution different from the time stamp resolution of the first interface");
				return (-1);
			}
			if (tsoffset != p->sf.tsoffset) {
				snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
				    "an interface has a time stamp offset different from the time stamp offset of the first interface");
				return (-1);
			}
			break;

		case BT_SHB:
			/*
			 * Section Header Block.  Get a pointer
			 * to its fixed-length portion.
			 */
			shbp = get_from_block_data(&cursor, sizeof(*shbp),
			    p->errbuf);
			if (shbp == NULL)
				return (-1);	/* error */

			/*
			 * Assume the byte order of this section is
			 * the same as that of the previous section.
			 * We'll check for that later.
			 */
			if (p->sf.swapped) {
				shbp->byte_order_magic =
				    SWAPLONG(shbp->byte_order_magic);
				shbp->major_version =
				    SWAPSHORT(shbp->major_version);
			}

			/*
			 * Make sure the byte order doesn't change;
			 * pcap_is_swapped() shouldn't change its
			 * return value in the middle of reading a capture.
			 */
			switch (shbp->byte_order_magic) {

			case BYTE_ORDER_MAGIC:
				/*
				 * OK.
				 */
				break;

			case SWAPLONG(BYTE_ORDER_MAGIC):
				/*
				 * Byte order changes.
				 */
				snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
				    "the file has sections with different byte orders");
				return (-1);

			default:
				/*
				 * Not a valid SHB.
				 */
				snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
				    "the file has a section with a bad byte order magic field");
				return (-1);
			}

			/*
			 * Make sure the major version is the version
			 * we handle.
			 */
			if (shbp->major_version != PCAP_NG_VERSION_MAJOR) {
				snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
				    "unknown pcap-ng savefile major version number %u",
				    shbp->major_version);
				return (-1);
			}

			/*
			 * Reset the interface count; this section should
			 * have its own set of IDBs.  If any of them
			 * don't have the same interface type, snapshot
			 * length, or resolution as the first interface
			 * we saw, we'll fail.  (And if we don't see
			 * any IDBs, we'll fail when we see a packet
			 * block.)
			 */
			p->sf.ifcount = 0;
			break;

		default:
			/*
			 * Not a packet block, IDB, or SHB; ignore it.
			 */
			break;
		}
	}

found:
	/*
	 * Is the interface ID an interface we know?
	 */
	if (interface_id >= p->sf.ifcount) {
		/*
		 * Yes.  Fail.
		 */
		snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
		    "a packet arrived on interface %u, but there's no Interface Description Block for that interface",
		    interface_id);
		return (-1);
	}

	/*
	 * Convert the time stamp to a struct timeval.
	 */
	sec = t / p->sf.tsresol + p->sf.tsoffset;
	frac = t % p->sf.tsresol;
	if (p->sf.tsresol > 1000000) {
		/*
		 * Higher than microsecond resolution; scale down to
		 * microseconds.
		 */
		frac /= p->sf.tsscale;
	} else {
		/*
		 * Lower than microsecond resolution; scale up to
		 * microseconds.
		 */
		frac *= p->sf.tsscale;
	}
	hdr->ts.tv_sec = sec;
	hdr->ts.tv_usec = frac;

	/*
	 * Get a pointer to the packet data.
	 */
	*data = get_from_block_data(&cursor, hdr->caplen, p->errbuf);
	if (*data == NULL)
		return (-1);

	if (p->sf.swapped) {
		/*
		 * Convert pseudo-headers from the byte order of
		 * the host on which the file was saved to our
		 * byte order, as necessary.
		 */
		switch (p->linktype) {

		case DLT_USB_LINUX:
			swap_linux_usb_header(hdr, *data, 0);
			break;

		case DLT_USB_LINUX_MMAPPED:
			swap_linux_usb_header(hdr, *data, 1);
			break;
		}
	}

	return (0);
}