aboutsummaryrefslogtreecommitdiffstats
path: root/pcap-dag.c
blob: c918b3e10df0108333367b67a6113d567346cdbf (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
/*
 * pcap-dag.c: Packet capture interface for Endace DAG card.
 *
 * The functionality of this code attempts to mimic that of pcap-linux as much
 * as possible.  This code is compiled in several different ways depending on
 * whether DAG_ONLY and HAVE_DAG_API are defined.  If HAVE_DAG_API is not
 * defined it should not get compiled in, otherwise if DAG_ONLY is defined then
 * the 'dag_' function calls are renamed to 'pcap_' equivalents.  If DAG_ONLY
 * is not defined then nothing is altered - the dag_ functions will be
 * called as required from their pcap-linux/bpf equivalents.
 *
 * Authors: Richard Littin, Sean Irvine ({richard,sean}@reeltwo.com)
 * Modifications: Jesper Peterson, Koryn Grant <support@endace.com>
 */

#ifndef lint
static const char rcsid[] _U_ =
	"@(#) $Header: /tcpdump/master/libpcap/pcap-dag.c,v 1.18 2004-03-23 19:18:04 guy Exp $ (LBL)";
#endif

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

#include <sys/param.h>			/* optionally get BSD define */

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

#include "pcap-int.h"

#include <ctype.h>
#include <netinet/in.h>
#include <sys/mman.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

struct mbuf;		/* Squelch compiler warnings on some platforms for */
struct rtentry;		/* declarations in <net/if.h> */
#include <net/if.h>

#include <dagnew.h>
#include <dagapi.h>

#define MIN_DAG_SNAPLEN		12
#define MAX_DAG_SNAPLEN		2040
#define ATM_CELL_SIZE		52
#define ATM_HDR_SIZE		4

/* SunATM pseudo header */
struct sunatm_hdr {
	unsigned char	flags;		/* destination and traffic type */
	unsigned char	vpi;		/* VPI */
	unsigned short	vci;		/* VCI */
};

typedef struct pcap_dag_node {
	struct pcap_dag_node *next;
	pcap_t *p;
	pid_t pid;
} pcap_dag_node_t;

static pcap_dag_node_t *pcap_dags = NULL;
static int atexit_handler_installed = 0;
static const unsigned short endian_test_word = 0x0100;

#define IS_BIGENDIAN() (*((unsigned char *)&endian_test_word))

/*
 * Swap byte ordering of unsigned long long timestamp on a big endian
 * machine.
 */
#define SWAP_TS(ull)  ((ull & 0xff00000000000000LL) >> 56) | \
                      ((ull & 0x00ff000000000000LL) >> 40) | \
                      ((ull & 0x0000ff0000000000LL) >> 24) | \
                      ((ull & 0x000000ff00000000LL) >> 8)  | \
                      ((ull & 0x00000000ff000000LL) << 8)  | \
                      ((ull & 0x0000000000ff0000LL) << 24) | \
                      ((ull & 0x000000000000ff00LL) << 40) | \
                      ((ull & 0x00000000000000ffLL) << 56)


#ifdef DAG_ONLY
/* This code is required when compiling for a DAG device only. */
#include "pcap-dag.h"

/* Replace dag function names with pcap equivalent. */
#define dag_open_live pcap_open_live
#define dag_platform_finddevs pcap_platform_finddevs
#endif /* DAG_ONLY */

static int dag_setfilter(pcap_t *p, struct bpf_program *fp);
static int dag_stats(pcap_t *p, struct pcap_stat *ps);
static int dag_set_datalink(pcap_t *p, int dlt);
static int dag_get_datalink(pcap_t *p);
static int dag_setnonblock(pcap_t *p, int nonblock, char *errbuf);

static void
delete_pcap_dag(pcap_t *p)
{
	pcap_dag_node_t *curr = NULL, *prev = NULL;

	for (prev = NULL, curr = pcap_dags; curr != NULL && curr->p != p; prev = curr, curr = curr->next) {
		/* empty */
	}

	if (curr != NULL && curr->p == p) {
		if (prev != NULL) {
			prev->next = curr->next;
		} else {
			pcap_dags = curr->next;
		}
	}
}

/*
 * Performs a graceful shutdown of the DAG card, frees dynamic memory held
 * in the pcap_t structure, and closes the file descriptor for the DAG card.
 */

static void
dag_platform_close(pcap_t *p)
{

#ifdef linux
	if (p != NULL && p->md.device != NULL) {
		if(dag_stop(p->fd) < 0)
			fprintf(stderr,"dag_stop %s: %s\n", p->md.device, strerror(errno));
		if(dag_close(p->fd) < 0)
			fprintf(stderr,"dag_close %s: %s\n", p->md.device, strerror(errno));
		
		free(p->md.device);
	}
#else
	if (p != NULL) {
		if(dag_stop(p->fd) < 0)
			fprintf(stderr,"dag_stop: %s\n", strerror(errno));
		if(dag_close(p->fd) < 0)
			fprintf(stderr,"dag_close: %s\n", strerror(errno));
	}
#endif
	delete_pcap_dag(p);
	/* Note: don't need to call close(p->fd) here as dag_close(p->fd) does this. */
}

static void
atexit_handler(void)
{
	while (pcap_dags != NULL) {
		if (pcap_dags->pid == getpid()) {
			dag_platform_close(pcap_dags->p);
		} else {
			delete_pcap_dag(pcap_dags->p);
		}
	}
}

static int
new_pcap_dag(pcap_t *p)
{
	pcap_dag_node_t *node = NULL;

	if ((node = malloc(sizeof(pcap_dag_node_t))) == NULL) {
		return -1;
	}

	if (!atexit_handler_installed) {
		atexit(atexit_handler);
		atexit_handler_installed = 1;
	}

	node->next = pcap_dags;
	node->p = p;
	node->pid = getpid();

	pcap_dags = node;

	return 0;
}

/*
 *  Read at most max_packets from the capture stream and call the callback
 *  for each of them. Returns the number of packets handled, -1 if an
 *  error occured, or -2 if we were told to break out of the loop.
 */
static int
dag_read(pcap_t *p, int cnt, pcap_handler callback, u_char *user)
{
	unsigned int processed = 0;
	int flags = p->md.dag_offset_flags;
	unsigned int nonblocking = flags & DAGF_NONBLOCK;

	for (;;)
	{
		/* Get the next bufferful of packets (if necessary). */
		while (p->md.dag_mem_top - p->md.dag_mem_bottom < dag_record_size) {
 
			/*
			 * Has "pcap_breakloop()" been called?
			 */
			if (p->break_loop) {
				/*
				 * Yes - clear the flag that indicates that
				 * it has, and return -2 to indicate that
				 * we were told to break out of the loop.
				 */
				p->break_loop = 0;
				return -2;
			}

			p->md.dag_mem_top = dag_offset(p->fd, &(p->md.dag_mem_bottom), flags);
			if (nonblocking && (p->md.dag_mem_top - p->md.dag_mem_bottom < dag_record_size))
			{
				/* Pcap is configured to process only available packets, and there aren't any. */
				return 0;
			}
		}
	
		/* Process the packets. */
		while (p->md.dag_mem_top - p->md.dag_mem_bottom >= dag_record_size) {

			unsigned short packet_len = 0;
			int caplen = 0;
			struct pcap_pkthdr	pcap_header;

			dag_record_t *header = (dag_record_t *)(p->md.dag_mem_base + p->md.dag_mem_bottom);
			u_char *dp = ((u_char *)header) + dag_record_size;
			unsigned short rlen;
 
			/*
			 * Has "pcap_breakloop()" been called?
			 */
			if (p->break_loop) {
				/*
				 * Yes - clear the flag that indicates that
				 * it has, and return -2 to indicate that
				 * we were told to break out of the loop.
				 */
				p->break_loop = 0;
				return -2;
			}
 
			if (IS_BIGENDIAN()) {
				rlen = header->rlen;
			} else {
				rlen = ntohs(header->rlen);
			}
			p->md.dag_mem_bottom += rlen;

			switch(header->type) {
			case TYPE_AAL5:
			case TYPE_ATM:
				if (header->type == TYPE_AAL5) {
					if (IS_BIGENDIAN()) {
						packet_len = header->wlen;
					} else {
						packet_len = ntohs(header->wlen);
					}
					caplen = rlen - dag_record_size;
				} else {
					caplen = packet_len = ATM_CELL_SIZE;
				}
				if (p->linktype == DLT_SUNATM) {
					struct sunatm_hdr *sunatm = (struct sunatm_hdr *)dp;
					unsigned long rawatm;
					if (IS_BIGENDIAN()) {
						rawatm = *((unsigned long *)dp);
						sunatm->vci = (rawatm >>  4) & 0xffff;
					} else {
						rawatm = ntohl(*((unsigned long *)dp));
						sunatm->vci = htons((rawatm >>  4) & 0xffff);
					}
					sunatm->vpi = (rawatm >> 20) & 0x00ff;
					sunatm->flags = ((header->flags.iface & 1) ? 0x80 : 0x00) | 
						((sunatm->vpi == 0 && sunatm->vci == htons(5)) ? 6 :
						((sunatm->vpi == 0 && sunatm->vci == htons(16)) ? 5 : 
						((dp[ATM_HDR_SIZE] == 0xaa &&
							dp[ATM_HDR_SIZE+1] == 0xaa &&
							dp[ATM_HDR_SIZE+2] == 0x03) ? 2 : 1)));

				} else {
					packet_len -= ATM_HDR_SIZE;
					caplen -= ATM_HDR_SIZE;
					dp += ATM_HDR_SIZE;
				}
				break;

			case TYPE_ETH:
				if (IS_BIGENDIAN()) {
					packet_len = header->wlen;
				} else {
					packet_len = ntohs(header->wlen);
				}
				packet_len -= (p->md.dag_fcs_bits >> 3);
				caplen = rlen - dag_record_size - 2;
				if (caplen > packet_len) {
					caplen = packet_len;
				}
				dp += 2;
				break;

			case TYPE_HDLC_POS:
				if (IS_BIGENDIAN()) {
					packet_len = header->wlen;
				} else {
					packet_len = ntohs(header->wlen);
				}
				packet_len -= (p->md.dag_fcs_bits >> 3);
				caplen = rlen - dag_record_size;
				if (caplen > packet_len) {
					caplen = packet_len;
				}
				break;
			}
 
			if (caplen > p->snapshot)
				caplen = p->snapshot;

			/* Count lost packets. */
			if (header->lctr) {
				if (p->md.stat.ps_drop > (UINT_MAX - ntohs(header->lctr))) {
					p->md.stat.ps_drop = UINT_MAX;
				} else {
					p->md.stat.ps_drop += ntohs(header->lctr);
				}
			}

			/* Run the packet filter if there is one. */
			if ((p->fcode.bf_insns == NULL) || bpf_filter(p->fcode.bf_insns, dp, packet_len, caplen)) {

				/* convert between timestamp formats */
				register unsigned long long ts;
				
				if (IS_BIGENDIAN()) {
					ts = SWAP_TS(header->ts);
				} else {
					ts = header->ts;
				}

				pcap_header.ts.tv_sec = ts >> 32;
				ts = (ts & 0xffffffffULL) * 1000000;
				ts += 0x80000000; /* rounding */
				pcap_header.ts.tv_usec = ts >> 32;		
				if (pcap_header.ts.tv_usec >= 1000000) {
					pcap_header.ts.tv_usec -= 1000000;
					pcap_header.ts.tv_sec++;
				}

				/* Fill in our own header data */
				pcap_header.caplen = caplen;
				pcap_header.len = packet_len;
	
				/* Count the packet. */
				p->md.stat.ps_recv++;
	
				/* Call the user supplied callback function */
				callback(user, &pcap_header, dp);
	
				/* Only count packets that pass the filter, for consistency with standard Linux behaviour. */
				processed++;
				if (processed == cnt)
				{
					/* Reached the user-specified limit. */
					return cnt;
				}
			}
		}

		if (nonblocking || processed)
		{
			return processed;
		}
	}
	
	return processed;
}

static int
dag_inject(pcap_t *p, const void *buf _U_, size_t size _U_)
{
	strlcpy(p->errbuf, "Sending packets isn't supported on DAG cards",
	    PCAP_ERRBUF_SIZE);
	return (-1);
}

/*
 *  Get a handle for a live capture from the given DAG device.  Passing a NULL
 *  device will result in a failure.  The promisc flag is ignored because DAG
 *  cards are always promiscuous.  The to_ms parameter is also ignored as it is
 *  not supported in hardware.
 *  
 *  See also pcap(3).
 */
pcap_t *
dag_open_live(const char *device, int snaplen, int promisc, int to_ms, char *ebuf)
{
	char conf[30]; /* dag configure string */
	pcap_t *handle;
	char *s;
	int n;
	
	if (device == NULL) {
		snprintf(ebuf, PCAP_ERRBUF_SIZE, "device is NULL: %s", pcap_strerror(errno));
		return NULL;
	}
	/* Allocate a handle for this session. */

	handle = malloc(sizeof(*handle));
	if (handle == NULL) {
		snprintf(ebuf, PCAP_ERRBUF_SIZE, "malloc %s: %s", device, pcap_strerror(errno));
		return NULL;
	}
	
	/* Initialize some components of the pcap structure. */
	
	memset(handle, 0, sizeof(*handle));

	if (strstr(device, "/dev") == NULL) {
		char * newDev = (char *)malloc(strlen(device) + 6);
		newDev[0] = '\0';
		strcat(newDev, "/dev/");
		strcat(newDev,device);
		device = newDev;
	} else {
		device = strdup(device);
	}

	if (device == NULL) {
		snprintf(ebuf, PCAP_ERRBUF_SIZE, "str_dup: %s\n", pcap_strerror(errno));
		goto fail;
	}

	/* setup device parameters */
	if((handle->fd = dag_open((char *)device)) < 0) {
		snprintf(ebuf, PCAP_ERRBUF_SIZE, "dag_open %s: %s", device, pcap_strerror(errno));
		goto fail;
	}

	/* set the card snap length to the specified snaplen parameter */
	if (snaplen == 0 || snaplen > MAX_DAG_SNAPLEN) {
		snaplen = MAX_DAG_SNAPLEN;
	} else if (snaplen < MIN_DAG_SNAPLEN) {
		snaplen = MIN_DAG_SNAPLEN;
	}
	/* snap len has to be a multiple of 4 */
	snprintf(conf, 30, "varlen slen=%d", (snaplen + 3) & ~3); 

	if(dag_configure(handle->fd, conf) < 0) {
		snprintf(ebuf, PCAP_ERRBUF_SIZE,"dag_configure %s: %s\n", device, pcap_strerror(errno));
		goto fail;
	}
	
	if((handle->md.dag_mem_base = dag_mmap(handle->fd)) == MAP_FAILED) {
		snprintf(ebuf, PCAP_ERRBUF_SIZE,"dag_mmap %s: %s\n", device, pcap_strerror(errno));
		goto fail;
	}
	
	if(dag_start(handle->fd) < 0) {
		snprintf(ebuf, PCAP_ERRBUF_SIZE, "dag_start %s: %s\n", device, pcap_strerror(errno));
		goto fail;
	}

	/*
	 * Important! You have to ensure bottom is properly
	 * initialized to zero on startup, it won't give you
	 * a compiler warning if you make this mistake!
	 */
	handle->md.dag_mem_bottom = 0;
	handle->md.dag_mem_top = 0;

	/* TODO: query the card */
	handle->md.dag_fcs_bits = 32;
	if ((s = getenv("ERF_FCS_BITS")) != NULL) {
		if ((n = atoi(s)) == 0 || n == 16|| n == 32) {
			handle->md.dag_fcs_bits = n;
		} else {
			snprintf(ebuf, PCAP_ERRBUF_SIZE,
				"pcap_open_live %s: bad ERF_FCS_BITS value (%d) in environment\n", device, n);
			goto fail;
		}
	}

	handle->snapshot	= snaplen;
	/*handle->md.timeout	= to_ms; */

	handle->linktype = -1;
	if (dag_get_datalink(handle) < 0) {
		strcpy(ebuf, handle->errbuf);
		goto fail;
	}
	
	handle->bufsize = 0;

	if (new_pcap_dag(handle) < 0) {
		snprintf(ebuf, PCAP_ERRBUF_SIZE, "new_pcap_dag %s: %s\n", device, pcap_strerror(errno));
		goto fail;
	}

	/*
	 * "select()" and "poll()" don't (yet) work on DAG device descriptors.
	 */
	handle->selectable_fd = -1;

#ifdef linux
	handle->md.device = (char *)device;
#else
	free((char *)device);
	device = NULL;
#endif

	handle->read_op = dag_read;
	handle->inject_op = dag_inject;
	handle->setfilter_op = dag_setfilter;
	handle->set_datalink_op = dag_set_datalink;
	handle->getnonblock_op = pcap_getnonblock_fd;
	handle->setnonblock_op = dag_setnonblock;
	handle->stats_op = dag_stats;
	handle->close_op = dag_platform_close;

	return handle;

fail:
	if (device != NULL) {
		free((char *)device);
	}
	if (handle != NULL) {
		/*
		 * Get rid of any link-layer type list we allocated.
		 */
		if (handle->dlt_list != NULL) {
			free(handle->dlt_list);
		}
		free(handle);
	}

	return NULL;
}

static int
dag_stats(pcap_t *p, struct pcap_stat *ps) {
	/* This needs to be filled out correctly.  Hopefully a dagapi call will
		 provide all necessary information.
	*/
	/*p->md.stat.ps_recv = 0;*/
	/*p->md.stat.ps_drop = 0;*/
	
	*ps = p->md.stat;
 
	return 0;
}

/*
 * Get from "/proc/dag" all interfaces listed there; if they're
 * already in the list of interfaces we have, that won't add another
 * instance, but if they're not, that'll add them.
 *
 * We don't bother getting any addresses for them.
 *
 * We also don't fail if we couldn't open "/proc/dag"; we just leave
 * the list of interfaces as is.
 */
int
dag_platform_finddevs(pcap_if_t **devlistp, char *errbuf)
{
	FILE *proc_dag_f;
	char linebuf[512];
	int linenum;
	unsigned char *p;
	char name[512];	/* XXX - pick a size */
	char *q;
	int ret = 0;

	/* Quick exit if /proc/dag not readable */
	proc_dag_f = fopen("/proc/dag", "r");
	if (proc_dag_f == NULL)
	{
		int i;
		char dev[16] = "dagx";

		for (i = '0'; ret == 0 && i <= '9'; i++) {
			dev[3] = i;
			if (pcap_add_if(devlistp, dev, 0, NULL, errbuf) == -1) {
				/*
				 * Failure.
				 */
				ret = -1;
			}
		}

		return (ret);
	}

	for (linenum = 1; fgets(linebuf, sizeof linebuf, proc_dag_f) != NULL; linenum++) {
		
		/*
		 * Skip the first two lines - they're headers.
		 */
		if (linenum <= 2)
			continue;

		p = &linebuf[0];

		if (*p == '\0' || *p == '\n' || *p != 'D')
			continue;  /* not a Dag line */

		/*
		 * Get the interface name.
		 */
		q = &name[0];
		while (*p != '\0' && *p != ':') {
			if (*p != ' ')
				*q++ = tolower(*p++);
			else
				p++;
		}
		*q = '\0';

		/*
		 * Add an entry for this interface, with no addresses.
		 */
		p[strlen(p) - 1] = '\0'; /* get rid of \n */
		if (pcap_add_if(devlistp, name, 0, strdup(p + 2), errbuf) == -1) {
			/*
			 * Failure.
			 */
			ret = -1;
			break;
		}
	}
	if (ret != -1) {
		/*
		 * Well, we didn't fail for any other reason; did we
		 * fail due to an error reading the file?
		 */
		if (ferror(proc_dag_f)) {
			(void)snprintf(errbuf, PCAP_ERRBUF_SIZE,
				  "Error reading /proc/dag: %s",
				  pcap_strerror(errno));
			ret = -1;
		}
	}

	(void)fclose(proc_dag_f);
	return (ret);
}

/*
 * Installs the given bpf filter program in the given pcap structure.  There is
 * no attempt to store the filter in kernel memory as that is not supported
 * with DAG cards.
 */
static int
dag_setfilter(pcap_t *p, struct bpf_program *fp)
{
	if (!p)
		return -1;
	if (!fp) {
		strncpy(p->errbuf, "setfilter: No filter specified",
			sizeof(p->errbuf));
		return -1;
	}

	/* Make our private copy of the filter */

	if (install_bpf_program(p, fp) < 0) {
		snprintf(p->errbuf, sizeof(p->errbuf),
			 "malloc: %s", pcap_strerror(errno));
		return -1;
	}

	p->md.use_bpf = 0;

	return (0);
}

static int
dag_set_datalink(pcap_t *p, int dlt)
{
	p->linktype = dlt;

	return (0);
}

static int
dag_setnonblock(pcap_t *p, int nonblock, char *errbuf)
{
	/*
	 * Set non-blocking mode on the FD.
	 * XXX - is that necessary?  If not, don't bother calling it,
	 * and have a "dag_getnonblock()" function that looks at
	 * "p->md.dag_offset_flags".
	 */
	if (pcap_setnonblock_fd(p, nonblock, errbuf) < 0)
		return (-1);

	if (nonblock) {
		p->md.dag_offset_flags |= DAGF_NONBLOCK;
	} else {
		p->md.dag_offset_flags &= ~DAGF_NONBLOCK;
	}
	return (0);
}
		
static int
dag_get_datalink(pcap_t *p)
{
	int daglinktype;

	if (p->dlt_list == NULL && (p->dlt_list = malloc(2*sizeof(*(p->dlt_list)))) == NULL) {
		(void)snprintf(p->errbuf, sizeof(p->errbuf), "malloc: %s", pcap_strerror(errno));
		return (-1);
	}

	/* Check the type through a dagapi call. */
	daglinktype = dag_linktype(p->fd);

	switch(daglinktype) {

	case TYPE_HDLC_POS:
		if (p->dlt_list != NULL) {
			p->dlt_count = 2;
			p->dlt_list[0] = DLT_CHDLC;
			p->dlt_list[1] = DLT_PPP_SERIAL;
		}
		p->linktype = DLT_CHDLC;
		break;

	case TYPE_ETH:
		/*
		 * This is (presumably) a real Ethernet capture; give it a
		 * link-layer-type list with DLT_EN10MB and DLT_DOCSIS, so
		 * that an application can let you choose it, in case you're
		 * capturing DOCSIS traffic that a Cisco Cable Modem
		 * Termination System is putting out onto an Ethernet (it
		 * doesn't put an Ethernet header onto the wire, it puts raw
		 * DOCSIS frames out on the wire inside the low-level
		 * Ethernet framing).
		 */
		if (p->dlt_list != NULL) {
			p->dlt_count = 2;
			p->dlt_list[0] = DLT_EN10MB;
			p->dlt_list[1] = DLT_DOCSIS;
		}
		p->linktype = DLT_EN10MB;
		break;

	case TYPE_AAL5:
	case TYPE_ATM: 
		if (p->dlt_list != NULL) {
			p->dlt_count = 2;
			p->dlt_list[0] = DLT_ATM_RFC1483;
			p->dlt_list[1] = DLT_SUNATM;
		}
		p->linktype = DLT_ATM_RFC1483;
		break;


	case TYPE_LEGACY:
		p->linktype = DLT_NULL;
		break;

	default:
		snprintf(p->errbuf, sizeof(p->errbuf), "unknown DAG linktype %d\n", daglinktype);
		return (-1);

	}

	return p->linktype;
}