xref: /freebsd/sys/netinet/tcp_syncache.c (revision 2710751bc309af25c6dea1171781678258e83840)
1 /*-
2  * Copyright (c) 2001 McAfee, Inc.
3  * Copyright (c) 2006 Andre Oppermann, Internet Business Solutions AG
4  * All rights reserved.
5  *
6  * This software was developed for the FreeBSD Project by Jonathan Lemon
7  * and McAfee Research, the Security Research Division of McAfee, Inc. under
8  * DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
9  * DARPA CHATS research program.
10  *
11  * Redistribution and use in source and binary forms, with or without
12  * modification, are permitted provided that the following conditions
13  * are met:
14  * 1. Redistributions of source code must retain the above copyright
15  *    notice, this list of conditions and the following disclaimer.
16  * 2. Redistributions in binary form must reproduce the above copyright
17  *    notice, this list of conditions and the following disclaimer in the
18  *    documentation and/or other materials provided with the distribution.
19  *
20  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
21  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
22  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
23  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
24  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
25  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
26  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
27  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
28  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
29  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
30  * SUCH DAMAGE.
31  */
32 
33 #include <sys/cdefs.h>
34 __FBSDID("$FreeBSD$");
35 
36 #include "opt_inet.h"
37 #include "opt_inet6.h"
38 #include "opt_ipsec.h"
39 #include "opt_pcbgroup.h"
40 
41 #include <sys/param.h>
42 #include <sys/systm.h>
43 #include <sys/kernel.h>
44 #include <sys/sysctl.h>
45 #include <sys/limits.h>
46 #include <sys/lock.h>
47 #include <sys/mutex.h>
48 #include <sys/malloc.h>
49 #include <sys/mbuf.h>
50 #include <sys/md5.h>
51 #include <sys/proc.h>		/* for proc0 declaration */
52 #include <sys/random.h>
53 #include <sys/socket.h>
54 #include <sys/socketvar.h>
55 #include <sys/syslog.h>
56 #include <sys/ucred.h>
57 
58 #include <vm/uma.h>
59 
60 #include <net/if.h>
61 #include <net/route.h>
62 #include <net/vnet.h>
63 
64 #include <netinet/in.h>
65 #include <netinet/in_systm.h>
66 #include <netinet/ip.h>
67 #include <netinet/in_var.h>
68 #include <netinet/in_pcb.h>
69 #include <netinet/ip_var.h>
70 #include <netinet/ip_options.h>
71 #ifdef INET6
72 #include <netinet/ip6.h>
73 #include <netinet/icmp6.h>
74 #include <netinet6/nd6.h>
75 #include <netinet6/ip6_var.h>
76 #include <netinet6/in6_pcb.h>
77 #endif
78 #include <netinet/tcp.h>
79 #include <netinet/tcp_fsm.h>
80 #include <netinet/tcp_seq.h>
81 #include <netinet/tcp_timer.h>
82 #include <netinet/tcp_var.h>
83 #include <netinet/tcp_syncache.h>
84 #ifdef INET6
85 #include <netinet6/tcp6_var.h>
86 #endif
87 #ifdef TCP_OFFLOAD
88 #include <netinet/toecore.h>
89 #endif
90 
91 #ifdef IPSEC
92 #include <netipsec/ipsec.h>
93 #ifdef INET6
94 #include <netipsec/ipsec6.h>
95 #endif
96 #include <netipsec/key.h>
97 #endif /*IPSEC*/
98 
99 #include <machine/in_cksum.h>
100 
101 #include <security/mac/mac_framework.h>
102 
103 static VNET_DEFINE(int, tcp_syncookies) = 1;
104 #define	V_tcp_syncookies		VNET(tcp_syncookies)
105 SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_RW,
106     &VNET_NAME(tcp_syncookies), 0,
107     "Use TCP SYN cookies if the syncache overflows");
108 
109 static VNET_DEFINE(int, tcp_syncookiesonly) = 0;
110 #define	V_tcp_syncookiesonly		VNET(tcp_syncookiesonly)
111 SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_RW,
112     &VNET_NAME(tcp_syncookiesonly), 0,
113     "Use only TCP SYN cookies");
114 
115 #ifdef TCP_OFFLOAD
116 #define ADDED_BY_TOE(sc) ((sc)->sc_tod != NULL)
117 #endif
118 
119 static void	 syncache_drop(struct syncache *, struct syncache_head *);
120 static void	 syncache_free(struct syncache *);
121 static void	 syncache_insert(struct syncache *, struct syncache_head *);
122 struct syncache *syncache_lookup(struct in_conninfo *, struct syncache_head **);
123 static int	 syncache_respond(struct syncache *);
124 static struct	 socket *syncache_socket(struct syncache *, struct socket *,
125 		    struct mbuf *m);
126 static int	 syncache_sysctl_count(SYSCTL_HANDLER_ARGS);
127 static void	 syncache_timeout(struct syncache *sc, struct syncache_head *sch,
128 		    int docallout);
129 static void	 syncache_timer(void *);
130 static void	 syncookie_generate(struct syncache_head *, struct syncache *,
131 		    u_int32_t *);
132 static struct syncache
133 		*syncookie_lookup(struct in_conninfo *, struct syncache_head *,
134 		    struct syncache *, struct tcpopt *, struct tcphdr *,
135 		    struct socket *);
136 
137 /*
138  * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
139  * 3 retransmits corresponds to a timeout of 3 * (1 + 2 + 4 + 8) == 45 seconds,
140  * the odds are that the user has given up attempting to connect by then.
141  */
142 #define SYNCACHE_MAXREXMTS		3
143 
144 /* Arbitrary values */
145 #define TCP_SYNCACHE_HASHSIZE		512
146 #define TCP_SYNCACHE_BUCKETLIMIT	30
147 
148 static VNET_DEFINE(struct tcp_syncache, tcp_syncache);
149 #define	V_tcp_syncache			VNET(tcp_syncache)
150 
151 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0,
152     "TCP SYN cache");
153 
154 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_RDTUN,
155     &VNET_NAME(tcp_syncache.bucket_limit), 0,
156     "Per-bucket hash limit for syncache");
157 
158 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_RDTUN,
159     &VNET_NAME(tcp_syncache.cache_limit), 0,
160     "Overall entry limit for syncache");
161 
162 SYSCTL_VNET_PROC(_net_inet_tcp_syncache, OID_AUTO, count, (CTLTYPE_UINT|CTLFLAG_RD),
163     NULL, 0, &syncache_sysctl_count, "IU",
164     "Current number of entries in syncache");
165 
166 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_RDTUN,
167     &VNET_NAME(tcp_syncache.hashsize), 0,
168     "Size of TCP syncache hashtable");
169 
170 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_RW,
171     &VNET_NAME(tcp_syncache.rexmt_limit), 0,
172     "Limit on SYN/ACK retransmissions");
173 
174 VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;
175 SYSCTL_VNET_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail,
176     CTLFLAG_RW, &VNET_NAME(tcp_sc_rst_sock_fail), 0,
177     "Send reset on socket allocation failure");
178 
179 static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");
180 
181 #define SYNCACHE_HASH(inc, mask)					\
182 	((V_tcp_syncache.hash_secret ^					\
183 	  (inc)->inc_faddr.s_addr ^					\
184 	  ((inc)->inc_faddr.s_addr >> 16) ^				\
185 	  (inc)->inc_fport ^ (inc)->inc_lport) & mask)
186 
187 #define SYNCACHE_HASH6(inc, mask)					\
188 	((V_tcp_syncache.hash_secret ^					\
189 	  (inc)->inc6_faddr.s6_addr32[0] ^				\
190 	  (inc)->inc6_faddr.s6_addr32[3] ^				\
191 	  (inc)->inc_fport ^ (inc)->inc_lport) & mask)
192 
193 #define ENDPTS_EQ(a, b) (						\
194 	(a)->ie_fport == (b)->ie_fport &&				\
195 	(a)->ie_lport == (b)->ie_lport &&				\
196 	(a)->ie_faddr.s_addr == (b)->ie_faddr.s_addr &&			\
197 	(a)->ie_laddr.s_addr == (b)->ie_laddr.s_addr			\
198 )
199 
200 #define ENDPTS6_EQ(a, b) (memcmp(a, b, sizeof(*a)) == 0)
201 
202 #define	SCH_LOCK(sch)		mtx_lock(&(sch)->sch_mtx)
203 #define	SCH_UNLOCK(sch)		mtx_unlock(&(sch)->sch_mtx)
204 #define	SCH_LOCK_ASSERT(sch)	mtx_assert(&(sch)->sch_mtx, MA_OWNED)
205 
206 /*
207  * Requires the syncache entry to be already removed from the bucket list.
208  */
209 static void
210 syncache_free(struct syncache *sc)
211 {
212 
213 	if (sc->sc_ipopts)
214 		(void) m_free(sc->sc_ipopts);
215 	if (sc->sc_cred)
216 		crfree(sc->sc_cred);
217 #ifdef MAC
218 	mac_syncache_destroy(&sc->sc_label);
219 #endif
220 
221 	uma_zfree(V_tcp_syncache.zone, sc);
222 }
223 
224 void
225 syncache_init(void)
226 {
227 	int i;
228 
229 	V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
230 	V_tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
231 	V_tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
232 	V_tcp_syncache.hash_secret = arc4random();
233 
234 	TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
235 	    &V_tcp_syncache.hashsize);
236 	TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
237 	    &V_tcp_syncache.bucket_limit);
238 	if (!powerof2(V_tcp_syncache.hashsize) ||
239 	    V_tcp_syncache.hashsize == 0) {
240 		printf("WARNING: syncache hash size is not a power of 2.\n");
241 		V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
242 	}
243 	V_tcp_syncache.hashmask = V_tcp_syncache.hashsize - 1;
244 
245 	/* Set limits. */
246 	V_tcp_syncache.cache_limit =
247 	    V_tcp_syncache.hashsize * V_tcp_syncache.bucket_limit;
248 	TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
249 	    &V_tcp_syncache.cache_limit);
250 
251 	/* Allocate the hash table. */
252 	V_tcp_syncache.hashbase = malloc(V_tcp_syncache.hashsize *
253 	    sizeof(struct syncache_head), M_SYNCACHE, M_WAITOK | M_ZERO);
254 
255 	/* Initialize the hash buckets. */
256 	for (i = 0; i < V_tcp_syncache.hashsize; i++) {
257 #ifdef VIMAGE
258 		V_tcp_syncache.hashbase[i].sch_vnet = curvnet;
259 #endif
260 		TAILQ_INIT(&V_tcp_syncache.hashbase[i].sch_bucket);
261 		mtx_init(&V_tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
262 			 NULL, MTX_DEF);
263 		callout_init_mtx(&V_tcp_syncache.hashbase[i].sch_timer,
264 			 &V_tcp_syncache.hashbase[i].sch_mtx, 0);
265 		V_tcp_syncache.hashbase[i].sch_length = 0;
266 	}
267 
268 	/* Create the syncache entry zone. */
269 	V_tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
270 	    NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
271 	V_tcp_syncache.cache_limit = uma_zone_set_max(V_tcp_syncache.zone,
272 	    V_tcp_syncache.cache_limit);
273 }
274 
275 #ifdef VIMAGE
276 void
277 syncache_destroy(void)
278 {
279 	struct syncache_head *sch;
280 	struct syncache *sc, *nsc;
281 	int i;
282 
283 	/* Cleanup hash buckets: stop timers, free entries, destroy locks. */
284 	for (i = 0; i < V_tcp_syncache.hashsize; i++) {
285 
286 		sch = &V_tcp_syncache.hashbase[i];
287 		callout_drain(&sch->sch_timer);
288 
289 		SCH_LOCK(sch);
290 		TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc)
291 			syncache_drop(sc, sch);
292 		SCH_UNLOCK(sch);
293 		KASSERT(TAILQ_EMPTY(&sch->sch_bucket),
294 		    ("%s: sch->sch_bucket not empty", __func__));
295 		KASSERT(sch->sch_length == 0, ("%s: sch->sch_length %d not 0",
296 		    __func__, sch->sch_length));
297 		mtx_destroy(&sch->sch_mtx);
298 	}
299 
300 	KASSERT(uma_zone_get_cur(V_tcp_syncache.zone) == 0,
301 	    ("%s: cache_count not 0", __func__));
302 
303 	/* Free the allocated global resources. */
304 	uma_zdestroy(V_tcp_syncache.zone);
305 	free(V_tcp_syncache.hashbase, M_SYNCACHE);
306 }
307 #endif
308 
309 static int
310 syncache_sysctl_count(SYSCTL_HANDLER_ARGS)
311 {
312 	int count;
313 
314 	count = uma_zone_get_cur(V_tcp_syncache.zone);
315 	return (sysctl_handle_int(oidp, &count, 0, req));
316 }
317 
318 /*
319  * Inserts a syncache entry into the specified bucket row.
320  * Locks and unlocks the syncache_head autonomously.
321  */
322 static void
323 syncache_insert(struct syncache *sc, struct syncache_head *sch)
324 {
325 	struct syncache *sc2;
326 
327 	SCH_LOCK(sch);
328 
329 	/*
330 	 * Make sure that we don't overflow the per-bucket limit.
331 	 * If the bucket is full, toss the oldest element.
332 	 */
333 	if (sch->sch_length >= V_tcp_syncache.bucket_limit) {
334 		KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
335 			("sch->sch_length incorrect"));
336 		sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
337 		syncache_drop(sc2, sch);
338 		TCPSTAT_INC(tcps_sc_bucketoverflow);
339 	}
340 
341 	/* Put it into the bucket. */
342 	TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
343 	sch->sch_length++;
344 
345 #ifdef TCP_OFFLOAD
346 	if (ADDED_BY_TOE(sc)) {
347 		struct toedev *tod = sc->sc_tod;
348 
349 		tod->tod_syncache_added(tod, sc->sc_todctx);
350 	}
351 #endif
352 
353 	/* Reinitialize the bucket row's timer. */
354 	if (sch->sch_length == 1)
355 		sch->sch_nextc = ticks + INT_MAX;
356 	syncache_timeout(sc, sch, 1);
357 
358 	SCH_UNLOCK(sch);
359 
360 	TCPSTAT_INC(tcps_sc_added);
361 }
362 
363 /*
364  * Remove and free entry from syncache bucket row.
365  * Expects locked syncache head.
366  */
367 static void
368 syncache_drop(struct syncache *sc, struct syncache_head *sch)
369 {
370 
371 	SCH_LOCK_ASSERT(sch);
372 
373 	TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
374 	sch->sch_length--;
375 
376 #ifdef TCP_OFFLOAD
377 	if (ADDED_BY_TOE(sc)) {
378 		struct toedev *tod = sc->sc_tod;
379 
380 		tod->tod_syncache_removed(tod, sc->sc_todctx);
381 	}
382 #endif
383 
384 	syncache_free(sc);
385 }
386 
387 /*
388  * Engage/reengage time on bucket row.
389  */
390 static void
391 syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
392 {
393 	sc->sc_rxttime = ticks +
394 		TCPTV_RTOBASE * (tcp_syn_backoff[sc->sc_rxmits]);
395 	sc->sc_rxmits++;
396 	if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {
397 		sch->sch_nextc = sc->sc_rxttime;
398 		if (docallout)
399 			callout_reset(&sch->sch_timer, sch->sch_nextc - ticks,
400 			    syncache_timer, (void *)sch);
401 	}
402 }
403 
404 /*
405  * Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
406  * If we have retransmitted an entry the maximum number of times, expire it.
407  * One separate timer for each bucket row.
408  */
409 static void
410 syncache_timer(void *xsch)
411 {
412 	struct syncache_head *sch = (struct syncache_head *)xsch;
413 	struct syncache *sc, *nsc;
414 	int tick = ticks;
415 	char *s;
416 
417 	CURVNET_SET(sch->sch_vnet);
418 
419 	/* NB: syncache_head has already been locked by the callout. */
420 	SCH_LOCK_ASSERT(sch);
421 
422 	/*
423 	 * In the following cycle we may remove some entries and/or
424 	 * advance some timeouts, so re-initialize the bucket timer.
425 	 */
426 	sch->sch_nextc = tick + INT_MAX;
427 
428 	TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
429 		/*
430 		 * We do not check if the listen socket still exists
431 		 * and accept the case where the listen socket may be
432 		 * gone by the time we resend the SYN/ACK.  We do
433 		 * not expect this to happens often. If it does,
434 		 * then the RST will be sent by the time the remote
435 		 * host does the SYN/ACK->ACK.
436 		 */
437 		if (TSTMP_GT(sc->sc_rxttime, tick)) {
438 			if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc))
439 				sch->sch_nextc = sc->sc_rxttime;
440 			continue;
441 		}
442 		if (sc->sc_rxmits > V_tcp_syncache.rexmt_limit) {
443 			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
444 				log(LOG_DEBUG, "%s; %s: Retransmits exhausted, "
445 				    "giving up and removing syncache entry\n",
446 				    s, __func__);
447 				free(s, M_TCPLOG);
448 			}
449 			syncache_drop(sc, sch);
450 			TCPSTAT_INC(tcps_sc_stale);
451 			continue;
452 		}
453 		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
454 			log(LOG_DEBUG, "%s; %s: Response timeout, "
455 			    "retransmitting (%u) SYN|ACK\n",
456 			    s, __func__, sc->sc_rxmits);
457 			free(s, M_TCPLOG);
458 		}
459 
460 		(void) syncache_respond(sc);
461 		TCPSTAT_INC(tcps_sc_retransmitted);
462 		syncache_timeout(sc, sch, 0);
463 	}
464 	if (!TAILQ_EMPTY(&(sch)->sch_bucket))
465 		callout_reset(&(sch)->sch_timer, (sch)->sch_nextc - tick,
466 			syncache_timer, (void *)(sch));
467 	CURVNET_RESTORE();
468 }
469 
470 /*
471  * Find an entry in the syncache.
472  * Returns always with locked syncache_head plus a matching entry or NULL.
473  */
474 struct syncache *
475 syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
476 {
477 	struct syncache *sc;
478 	struct syncache_head *sch;
479 
480 #ifdef INET6
481 	if (inc->inc_flags & INC_ISIPV6) {
482 		sch = &V_tcp_syncache.hashbase[
483 		    SYNCACHE_HASH6(inc, V_tcp_syncache.hashmask)];
484 		*schp = sch;
485 
486 		SCH_LOCK(sch);
487 
488 		/* Circle through bucket row to find matching entry. */
489 		TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
490 			if (ENDPTS6_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
491 				return (sc);
492 		}
493 	} else
494 #endif
495 	{
496 		sch = &V_tcp_syncache.hashbase[
497 		    SYNCACHE_HASH(inc, V_tcp_syncache.hashmask)];
498 		*schp = sch;
499 
500 		SCH_LOCK(sch);
501 
502 		/* Circle through bucket row to find matching entry. */
503 		TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
504 #ifdef INET6
505 			if (sc->sc_inc.inc_flags & INC_ISIPV6)
506 				continue;
507 #endif
508 			if (ENDPTS_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
509 				return (sc);
510 		}
511 	}
512 	SCH_LOCK_ASSERT(*schp);
513 	return (NULL);			/* always returns with locked sch */
514 }
515 
516 /*
517  * This function is called when we get a RST for a
518  * non-existent connection, so that we can see if the
519  * connection is in the syn cache.  If it is, zap it.
520  */
521 void
522 syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th)
523 {
524 	struct syncache *sc;
525 	struct syncache_head *sch;
526 	char *s = NULL;
527 
528 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
529 	SCH_LOCK_ASSERT(sch);
530 
531 	/*
532 	 * Any RST to our SYN|ACK must not carry ACK, SYN or FIN flags.
533 	 * See RFC 793 page 65, section SEGMENT ARRIVES.
534 	 */
535 	if (th->th_flags & (TH_ACK|TH_SYN|TH_FIN)) {
536 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
537 			log(LOG_DEBUG, "%s; %s: Spurious RST with ACK, SYN or "
538 			    "FIN flag set, segment ignored\n", s, __func__);
539 		TCPSTAT_INC(tcps_badrst);
540 		goto done;
541 	}
542 
543 	/*
544 	 * No corresponding connection was found in syncache.
545 	 * If syncookies are enabled and possibly exclusively
546 	 * used, or we are under memory pressure, a valid RST
547 	 * may not find a syncache entry.  In that case we're
548 	 * done and no SYN|ACK retransmissions will happen.
549 	 * Otherwise the RST was misdirected or spoofed.
550 	 */
551 	if (sc == NULL) {
552 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
553 			log(LOG_DEBUG, "%s; %s: Spurious RST without matching "
554 			    "syncache entry (possibly syncookie only), "
555 			    "segment ignored\n", s, __func__);
556 		TCPSTAT_INC(tcps_badrst);
557 		goto done;
558 	}
559 
560 	/*
561 	 * If the RST bit is set, check the sequence number to see
562 	 * if this is a valid reset segment.
563 	 * RFC 793 page 37:
564 	 *   In all states except SYN-SENT, all reset (RST) segments
565 	 *   are validated by checking their SEQ-fields.  A reset is
566 	 *   valid if its sequence number is in the window.
567 	 *
568 	 *   The sequence number in the reset segment is normally an
569 	 *   echo of our outgoing acknowlegement numbers, but some hosts
570 	 *   send a reset with the sequence number at the rightmost edge
571 	 *   of our receive window, and we have to handle this case.
572 	 */
573 	if (SEQ_GEQ(th->th_seq, sc->sc_irs) &&
574 	    SEQ_LEQ(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
575 		syncache_drop(sc, sch);
576 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
577 			log(LOG_DEBUG, "%s; %s: Our SYN|ACK was rejected, "
578 			    "connection attempt aborted by remote endpoint\n",
579 			    s, __func__);
580 		TCPSTAT_INC(tcps_sc_reset);
581 	} else {
582 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
583 			log(LOG_DEBUG, "%s; %s: RST with invalid SEQ %u != "
584 			    "IRS %u (+WND %u), segment ignored\n",
585 			    s, __func__, th->th_seq, sc->sc_irs, sc->sc_wnd);
586 		TCPSTAT_INC(tcps_badrst);
587 	}
588 
589 done:
590 	if (s != NULL)
591 		free(s, M_TCPLOG);
592 	SCH_UNLOCK(sch);
593 }
594 
595 void
596 syncache_badack(struct in_conninfo *inc)
597 {
598 	struct syncache *sc;
599 	struct syncache_head *sch;
600 
601 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
602 	SCH_LOCK_ASSERT(sch);
603 	if (sc != NULL) {
604 		syncache_drop(sc, sch);
605 		TCPSTAT_INC(tcps_sc_badack);
606 	}
607 	SCH_UNLOCK(sch);
608 }
609 
610 void
611 syncache_unreach(struct in_conninfo *inc, struct tcphdr *th)
612 {
613 	struct syncache *sc;
614 	struct syncache_head *sch;
615 
616 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
617 	SCH_LOCK_ASSERT(sch);
618 	if (sc == NULL)
619 		goto done;
620 
621 	/* If the sequence number != sc_iss, then it's a bogus ICMP msg */
622 	if (ntohl(th->th_seq) != sc->sc_iss)
623 		goto done;
624 
625 	/*
626 	 * If we've rertransmitted 3 times and this is our second error,
627 	 * we remove the entry.  Otherwise, we allow it to continue on.
628 	 * This prevents us from incorrectly nuking an entry during a
629 	 * spurious network outage.
630 	 *
631 	 * See tcp_notify().
632 	 */
633 	if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxmits < 3 + 1) {
634 		sc->sc_flags |= SCF_UNREACH;
635 		goto done;
636 	}
637 	syncache_drop(sc, sch);
638 	TCPSTAT_INC(tcps_sc_unreach);
639 done:
640 	SCH_UNLOCK(sch);
641 }
642 
643 /*
644  * Build a new TCP socket structure from a syncache entry.
645  */
646 static struct socket *
647 syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
648 {
649 	struct inpcb *inp = NULL;
650 	struct socket *so;
651 	struct tcpcb *tp;
652 	int error;
653 	char *s;
654 
655 	INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
656 
657 	/*
658 	 * Ok, create the full blown connection, and set things up
659 	 * as they would have been set up if we had created the
660 	 * connection when the SYN arrived.  If we can't create
661 	 * the connection, abort it.
662 	 */
663 	so = sonewconn(lso, SS_ISCONNECTED);
664 	if (so == NULL) {
665 		/*
666 		 * Drop the connection; we will either send a RST or
667 		 * have the peer retransmit its SYN again after its
668 		 * RTO and try again.
669 		 */
670 		TCPSTAT_INC(tcps_listendrop);
671 		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
672 			log(LOG_DEBUG, "%s; %s: Socket create failed "
673 			    "due to limits or memory shortage\n",
674 			    s, __func__);
675 			free(s, M_TCPLOG);
676 		}
677 		goto abort2;
678 	}
679 #ifdef MAC
680 	mac_socketpeer_set_from_mbuf(m, so);
681 #endif
682 
683 	inp = sotoinpcb(so);
684 	inp->inp_inc.inc_fibnum = so->so_fibnum;
685 	INP_WLOCK(inp);
686 	INP_HASH_WLOCK(&V_tcbinfo);
687 
688 	/* Insert new socket into PCB hash list. */
689 	inp->inp_inc.inc_flags = sc->sc_inc.inc_flags;
690 #ifdef INET6
691 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
692 		inp->in6p_laddr = sc->sc_inc.inc6_laddr;
693 	} else {
694 		inp->inp_vflag &= ~INP_IPV6;
695 		inp->inp_vflag |= INP_IPV4;
696 #endif
697 		inp->inp_laddr = sc->sc_inc.inc_laddr;
698 #ifdef INET6
699 	}
700 #endif
701 
702 	/*
703 	 * Install in the reservation hash table for now, but don't yet
704 	 * install a connection group since the full 4-tuple isn't yet
705 	 * configured.
706 	 */
707 	inp->inp_lport = sc->sc_inc.inc_lport;
708 	if ((error = in_pcbinshash_nopcbgroup(inp)) != 0) {
709 		/*
710 		 * Undo the assignments above if we failed to
711 		 * put the PCB on the hash lists.
712 		 */
713 #ifdef INET6
714 		if (sc->sc_inc.inc_flags & INC_ISIPV6)
715 			inp->in6p_laddr = in6addr_any;
716 		else
717 #endif
718 			inp->inp_laddr.s_addr = INADDR_ANY;
719 		inp->inp_lport = 0;
720 		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
721 			log(LOG_DEBUG, "%s; %s: in_pcbinshash failed "
722 			    "with error %i\n",
723 			    s, __func__, error);
724 			free(s, M_TCPLOG);
725 		}
726 		INP_HASH_WUNLOCK(&V_tcbinfo);
727 		goto abort;
728 	}
729 #ifdef IPSEC
730 	/* Copy old policy into new socket's. */
731 	if (ipsec_copy_policy(sotoinpcb(lso)->inp_sp, inp->inp_sp))
732 		printf("syncache_socket: could not copy policy\n");
733 #endif
734 #ifdef INET6
735 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
736 		struct inpcb *oinp = sotoinpcb(lso);
737 		struct in6_addr laddr6;
738 		struct sockaddr_in6 sin6;
739 		/*
740 		 * Inherit socket options from the listening socket.
741 		 * Note that in6p_inputopts are not (and should not be)
742 		 * copied, since it stores previously received options and is
743 		 * used to detect if each new option is different than the
744 		 * previous one and hence should be passed to a user.
745 		 * If we copied in6p_inputopts, a user would not be able to
746 		 * receive options just after calling the accept system call.
747 		 */
748 		inp->inp_flags |= oinp->inp_flags & INP_CONTROLOPTS;
749 		if (oinp->in6p_outputopts)
750 			inp->in6p_outputopts =
751 			    ip6_copypktopts(oinp->in6p_outputopts, M_NOWAIT);
752 
753 		sin6.sin6_family = AF_INET6;
754 		sin6.sin6_len = sizeof(sin6);
755 		sin6.sin6_addr = sc->sc_inc.inc6_faddr;
756 		sin6.sin6_port = sc->sc_inc.inc_fport;
757 		sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
758 		laddr6 = inp->in6p_laddr;
759 		if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
760 			inp->in6p_laddr = sc->sc_inc.inc6_laddr;
761 		if ((error = in6_pcbconnect_mbuf(inp, (struct sockaddr *)&sin6,
762 		    thread0.td_ucred, m)) != 0) {
763 			inp->in6p_laddr = laddr6;
764 			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
765 				log(LOG_DEBUG, "%s; %s: in6_pcbconnect failed "
766 				    "with error %i\n",
767 				    s, __func__, error);
768 				free(s, M_TCPLOG);
769 			}
770 			INP_HASH_WUNLOCK(&V_tcbinfo);
771 			goto abort;
772 		}
773 		/* Override flowlabel from in6_pcbconnect. */
774 		inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
775 		inp->inp_flow |= sc->sc_flowlabel;
776 	}
777 #endif /* INET6 */
778 #if defined(INET) && defined(INET6)
779 	else
780 #endif
781 #ifdef INET
782 	{
783 		struct in_addr laddr;
784 		struct sockaddr_in sin;
785 
786 		inp->inp_options = (m) ? ip_srcroute(m) : NULL;
787 
788 		if (inp->inp_options == NULL) {
789 			inp->inp_options = sc->sc_ipopts;
790 			sc->sc_ipopts = NULL;
791 		}
792 
793 		sin.sin_family = AF_INET;
794 		sin.sin_len = sizeof(sin);
795 		sin.sin_addr = sc->sc_inc.inc_faddr;
796 		sin.sin_port = sc->sc_inc.inc_fport;
797 		bzero((caddr_t)sin.sin_zero, sizeof(sin.sin_zero));
798 		laddr = inp->inp_laddr;
799 		if (inp->inp_laddr.s_addr == INADDR_ANY)
800 			inp->inp_laddr = sc->sc_inc.inc_laddr;
801 		if ((error = in_pcbconnect_mbuf(inp, (struct sockaddr *)&sin,
802 		    thread0.td_ucred, m)) != 0) {
803 			inp->inp_laddr = laddr;
804 			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
805 				log(LOG_DEBUG, "%s; %s: in_pcbconnect failed "
806 				    "with error %i\n",
807 				    s, __func__, error);
808 				free(s, M_TCPLOG);
809 			}
810 			INP_HASH_WUNLOCK(&V_tcbinfo);
811 			goto abort;
812 		}
813 	}
814 #endif /* INET */
815 	INP_HASH_WUNLOCK(&V_tcbinfo);
816 	tp = intotcpcb(inp);
817 	tp->t_state = TCPS_SYN_RECEIVED;
818 	tp->iss = sc->sc_iss;
819 	tp->irs = sc->sc_irs;
820 	tcp_rcvseqinit(tp);
821 	tcp_sendseqinit(tp);
822 	tp->snd_wl1 = sc->sc_irs;
823 	tp->snd_max = tp->iss + 1;
824 	tp->snd_nxt = tp->iss + 1;
825 	tp->rcv_up = sc->sc_irs + 1;
826 	tp->rcv_wnd = sc->sc_wnd;
827 	tp->rcv_adv += tp->rcv_wnd;
828 	tp->last_ack_sent = tp->rcv_nxt;
829 
830 	tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH|TF_NODELAY);
831 	if (sc->sc_flags & SCF_NOOPT)
832 		tp->t_flags |= TF_NOOPT;
833 	else {
834 		if (sc->sc_flags & SCF_WINSCALE) {
835 			tp->t_flags |= TF_REQ_SCALE|TF_RCVD_SCALE;
836 			tp->snd_scale = sc->sc_requested_s_scale;
837 			tp->request_r_scale = sc->sc_requested_r_scale;
838 		}
839 		if (sc->sc_flags & SCF_TIMESTAMP) {
840 			tp->t_flags |= TF_REQ_TSTMP|TF_RCVD_TSTMP;
841 			tp->ts_recent = sc->sc_tsreflect;
842 			tp->ts_recent_age = tcp_ts_getticks();
843 			tp->ts_offset = sc->sc_tsoff;
844 		}
845 #ifdef TCP_SIGNATURE
846 		if (sc->sc_flags & SCF_SIGNATURE)
847 			tp->t_flags |= TF_SIGNATURE;
848 #endif
849 		if (sc->sc_flags & SCF_SACK)
850 			tp->t_flags |= TF_SACK_PERMIT;
851 	}
852 
853 	if (sc->sc_flags & SCF_ECN)
854 		tp->t_flags |= TF_ECN_PERMIT;
855 
856 	/*
857 	 * Set up MSS and get cached values from tcp_hostcache.
858 	 * This might overwrite some of the defaults we just set.
859 	 */
860 	tcp_mss(tp, sc->sc_peer_mss);
861 
862 	/*
863 	 * If the SYN,ACK was retransmitted, indicate that CWND to be
864 	 * limited to one segment in cc_conn_init().
865 	 * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
866 	 */
867 	if (sc->sc_rxmits > 1)
868 		tp->snd_cwnd = 1;
869 
870 #ifdef TCP_OFFLOAD
871 	/*
872 	 * Allow a TOE driver to install its hooks.  Note that we hold the
873 	 * pcbinfo lock too and that prevents tcp_usr_accept from accepting a
874 	 * new connection before the TOE driver has done its thing.
875 	 */
876 	if (ADDED_BY_TOE(sc)) {
877 		struct toedev *tod = sc->sc_tod;
878 
879 		tod->tod_offload_socket(tod, sc->sc_todctx, so);
880 	}
881 #endif
882 	/*
883 	 * Copy and activate timers.
884 	 */
885 	tp->t_keepinit = sototcpcb(lso)->t_keepinit;
886 	tp->t_keepidle = sototcpcb(lso)->t_keepidle;
887 	tp->t_keepintvl = sototcpcb(lso)->t_keepintvl;
888 	tp->t_keepcnt = sototcpcb(lso)->t_keepcnt;
889 	tcp_timer_activate(tp, TT_KEEP, TP_KEEPINIT(tp));
890 
891 	INP_WUNLOCK(inp);
892 
893 	TCPSTAT_INC(tcps_accepts);
894 	return (so);
895 
896 abort:
897 	INP_WUNLOCK(inp);
898 abort2:
899 	if (so != NULL)
900 		soabort(so);
901 	return (NULL);
902 }
903 
904 /*
905  * This function gets called when we receive an ACK for a
906  * socket in the LISTEN state.  We look up the connection
907  * in the syncache, and if its there, we pull it out of
908  * the cache and turn it into a full-blown connection in
909  * the SYN-RECEIVED state.
910  */
911 int
912 syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
913     struct socket **lsop, struct mbuf *m)
914 {
915 	struct syncache *sc;
916 	struct syncache_head *sch;
917 	struct syncache scs;
918 	char *s;
919 
920 	/*
921 	 * Global TCP locks are held because we manipulate the PCB lists
922 	 * and create a new socket.
923 	 */
924 	INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
925 	KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
926 	    ("%s: can handle only ACK", __func__));
927 
928 	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
929 	SCH_LOCK_ASSERT(sch);
930 	if (sc == NULL) {
931 		/*
932 		 * There is no syncache entry, so see if this ACK is
933 		 * a returning syncookie.  To do this, first:
934 		 *  A. See if this socket has had a syncache entry dropped in
935 		 *     the past.  We don't want to accept a bogus syncookie
936 		 *     if we've never received a SYN.
937 		 *  B. check that the syncookie is valid.  If it is, then
938 		 *     cobble up a fake syncache entry, and return.
939 		 */
940 		if (!V_tcp_syncookies) {
941 			SCH_UNLOCK(sch);
942 			if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
943 				log(LOG_DEBUG, "%s; %s: Spurious ACK, "
944 				    "segment rejected (syncookies disabled)\n",
945 				    s, __func__);
946 			goto failed;
947 		}
948 		bzero(&scs, sizeof(scs));
949 		sc = syncookie_lookup(inc, sch, &scs, to, th, *lsop);
950 		SCH_UNLOCK(sch);
951 		if (sc == NULL) {
952 			if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
953 				log(LOG_DEBUG, "%s; %s: Segment failed "
954 				    "SYNCOOKIE authentication, segment rejected "
955 				    "(probably spoofed)\n", s, __func__);
956 			goto failed;
957 		}
958 	} else {
959 		/* Pull out the entry to unlock the bucket row. */
960 		TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
961 		sch->sch_length--;
962 #ifdef TCP_OFFLOAD
963 		if (ADDED_BY_TOE(sc)) {
964 			struct toedev *tod = sc->sc_tod;
965 
966 			tod->tod_syncache_removed(tod, sc->sc_todctx);
967 		}
968 #endif
969 		SCH_UNLOCK(sch);
970 	}
971 
972 	/*
973 	 * Segment validation:
974 	 * ACK must match our initial sequence number + 1 (the SYN|ACK).
975 	 */
976 	if (th->th_ack != sc->sc_iss + 1) {
977 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
978 			log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "
979 			    "rejected\n", s, __func__, th->th_ack, sc->sc_iss);
980 		goto failed;
981 	}
982 
983 	/*
984 	 * The SEQ must fall in the window starting at the received
985 	 * initial receive sequence number + 1 (the SYN).
986 	 */
987 	if (SEQ_LEQ(th->th_seq, sc->sc_irs) ||
988 	    SEQ_GT(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
989 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
990 			log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u, segment "
991 			    "rejected\n", s, __func__, th->th_seq, sc->sc_irs);
992 		goto failed;
993 	}
994 
995 	if (!(sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS)) {
996 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
997 			log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
998 			    "segment rejected\n", s, __func__);
999 		goto failed;
1000 	}
1001 	/*
1002 	 * If timestamps were negotiated the reflected timestamp
1003 	 * must be equal to what we actually sent in the SYN|ACK.
1004 	 */
1005 	if ((to->to_flags & TOF_TS) && to->to_tsecr != sc->sc_ts) {
1006 		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
1007 			log(LOG_DEBUG, "%s; %s: TSECR %u != TS %u, "
1008 			    "segment rejected\n",
1009 			    s, __func__, to->to_tsecr, sc->sc_ts);
1010 		goto failed;
1011 	}
1012 
1013 	*lsop = syncache_socket(sc, *lsop, m);
1014 
1015 	if (*lsop == NULL)
1016 		TCPSTAT_INC(tcps_sc_aborted);
1017 	else
1018 		TCPSTAT_INC(tcps_sc_completed);
1019 
1020 /* how do we find the inp for the new socket? */
1021 	if (sc != &scs)
1022 		syncache_free(sc);
1023 	return (1);
1024 failed:
1025 	if (sc != NULL && sc != &scs)
1026 		syncache_free(sc);
1027 	if (s != NULL)
1028 		free(s, M_TCPLOG);
1029 	*lsop = NULL;
1030 	return (0);
1031 }
1032 
1033 /*
1034  * Given a LISTEN socket and an inbound SYN request, add
1035  * this to the syn cache, and send back a segment:
1036  *	<SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
1037  * to the source.
1038  *
1039  * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
1040  * Doing so would require that we hold onto the data and deliver it
1041  * to the application.  However, if we are the target of a SYN-flood
1042  * DoS attack, an attacker could send data which would eventually
1043  * consume all available buffer space if it were ACKed.  By not ACKing
1044  * the data, we avoid this DoS scenario.
1045  */
1046 void
1047 syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
1048     struct inpcb *inp, struct socket **lsop, struct mbuf *m, void *tod,
1049     void *todctx)
1050 {
1051 	struct tcpcb *tp;
1052 	struct socket *so;
1053 	struct syncache *sc = NULL;
1054 	struct syncache_head *sch;
1055 	struct mbuf *ipopts = NULL;
1056 	u_int32_t flowtmp;
1057 	u_int ltflags;
1058 	int win, sb_hiwat, ip_ttl, ip_tos;
1059 	char *s;
1060 #ifdef INET6
1061 	int autoflowlabel = 0;
1062 #endif
1063 #ifdef MAC
1064 	struct label *maclabel;
1065 #endif
1066 	struct syncache scs;
1067 	struct ucred *cred;
1068 
1069 	INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
1070 	INP_WLOCK_ASSERT(inp);			/* listen socket */
1071 	KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN,
1072 	    ("%s: unexpected tcp flags", __func__));
1073 
1074 	/*
1075 	 * Combine all so/tp operations very early to drop the INP lock as
1076 	 * soon as possible.
1077 	 */
1078 	so = *lsop;
1079 	tp = sototcpcb(so);
1080 	cred = crhold(so->so_cred);
1081 
1082 #ifdef INET6
1083 	if ((inc->inc_flags & INC_ISIPV6) &&
1084 	    (inp->inp_flags & IN6P_AUTOFLOWLABEL))
1085 		autoflowlabel = 1;
1086 #endif
1087 	ip_ttl = inp->inp_ip_ttl;
1088 	ip_tos = inp->inp_ip_tos;
1089 	win = sbspace(&so->so_rcv);
1090 	sb_hiwat = so->so_rcv.sb_hiwat;
1091 	ltflags = (tp->t_flags & (TF_NOOPT | TF_SIGNATURE));
1092 
1093 	/* By the time we drop the lock these should no longer be used. */
1094 	so = NULL;
1095 	tp = NULL;
1096 
1097 #ifdef MAC
1098 	if (mac_syncache_init(&maclabel) != 0) {
1099 		INP_WUNLOCK(inp);
1100 		INP_INFO_WUNLOCK(&V_tcbinfo);
1101 		goto done;
1102 	} else
1103 		mac_syncache_create(maclabel, inp);
1104 #endif
1105 	INP_WUNLOCK(inp);
1106 	INP_INFO_WUNLOCK(&V_tcbinfo);
1107 
1108 	/*
1109 	 * Remember the IP options, if any.
1110 	 */
1111 #ifdef INET6
1112 	if (!(inc->inc_flags & INC_ISIPV6))
1113 #endif
1114 #ifdef INET
1115 		ipopts = (m) ? ip_srcroute(m) : NULL;
1116 #else
1117 		ipopts = NULL;
1118 #endif
1119 
1120 	/*
1121 	 * See if we already have an entry for this connection.
1122 	 * If we do, resend the SYN,ACK, and reset the retransmit timer.
1123 	 *
1124 	 * XXX: should the syncache be re-initialized with the contents
1125 	 * of the new SYN here (which may have different options?)
1126 	 *
1127 	 * XXX: We do not check the sequence number to see if this is a
1128 	 * real retransmit or a new connection attempt.  The question is
1129 	 * how to handle such a case; either ignore it as spoofed, or
1130 	 * drop the current entry and create a new one?
1131 	 */
1132 	sc = syncache_lookup(inc, &sch);	/* returns locked entry */
1133 	SCH_LOCK_ASSERT(sch);
1134 	if (sc != NULL) {
1135 		TCPSTAT_INC(tcps_sc_dupsyn);
1136 		if (ipopts) {
1137 			/*
1138 			 * If we were remembering a previous source route,
1139 			 * forget it and use the new one we've been given.
1140 			 */
1141 			if (sc->sc_ipopts)
1142 				(void) m_free(sc->sc_ipopts);
1143 			sc->sc_ipopts = ipopts;
1144 		}
1145 		/*
1146 		 * Update timestamp if present.
1147 		 */
1148 		if ((sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS))
1149 			sc->sc_tsreflect = to->to_tsval;
1150 		else
1151 			sc->sc_flags &= ~SCF_TIMESTAMP;
1152 #ifdef MAC
1153 		/*
1154 		 * Since we have already unconditionally allocated label
1155 		 * storage, free it up.  The syncache entry will already
1156 		 * have an initialized label we can use.
1157 		 */
1158 		mac_syncache_destroy(&maclabel);
1159 #endif
1160 		/* Retransmit SYN|ACK and reset retransmit count. */
1161 		if ((s = tcp_log_addrs(&sc->sc_inc, th, NULL, NULL))) {
1162 			log(LOG_DEBUG, "%s; %s: Received duplicate SYN, "
1163 			    "resetting timer and retransmitting SYN|ACK\n",
1164 			    s, __func__);
1165 			free(s, M_TCPLOG);
1166 		}
1167 		if (syncache_respond(sc) == 0) {
1168 			sc->sc_rxmits = 0;
1169 			syncache_timeout(sc, sch, 1);
1170 			TCPSTAT_INC(tcps_sndacks);
1171 			TCPSTAT_INC(tcps_sndtotal);
1172 		}
1173 		SCH_UNLOCK(sch);
1174 		goto done;
1175 	}
1176 
1177 	sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
1178 	if (sc == NULL) {
1179 		/*
1180 		 * The zone allocator couldn't provide more entries.
1181 		 * Treat this as if the cache was full; drop the oldest
1182 		 * entry and insert the new one.
1183 		 */
1184 		TCPSTAT_INC(tcps_sc_zonefail);
1185 		if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL)
1186 			syncache_drop(sc, sch);
1187 		sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
1188 		if (sc == NULL) {
1189 			if (V_tcp_syncookies) {
1190 				bzero(&scs, sizeof(scs));
1191 				sc = &scs;
1192 			} else {
1193 				SCH_UNLOCK(sch);
1194 				if (ipopts)
1195 					(void) m_free(ipopts);
1196 				goto done;
1197 			}
1198 		}
1199 	}
1200 
1201 	/*
1202 	 * Fill in the syncache values.
1203 	 */
1204 #ifdef MAC
1205 	sc->sc_label = maclabel;
1206 #endif
1207 	sc->sc_cred = cred;
1208 	cred = NULL;
1209 	sc->sc_ipopts = ipopts;
1210 	bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
1211 #ifdef INET6
1212 	if (!(inc->inc_flags & INC_ISIPV6))
1213 #endif
1214 	{
1215 		sc->sc_ip_tos = ip_tos;
1216 		sc->sc_ip_ttl = ip_ttl;
1217 	}
1218 #ifdef TCP_OFFLOAD
1219 	sc->sc_tod = tod;
1220 	sc->sc_todctx = todctx;
1221 #endif
1222 	sc->sc_irs = th->th_seq;
1223 	sc->sc_iss = arc4random();
1224 	sc->sc_flags = 0;
1225 	sc->sc_flowlabel = 0;
1226 
1227 	/*
1228 	 * Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
1229 	 * win was derived from socket earlier in the function.
1230 	 */
1231 	win = imax(win, 0);
1232 	win = imin(win, TCP_MAXWIN);
1233 	sc->sc_wnd = win;
1234 
1235 	if (V_tcp_do_rfc1323) {
1236 		/*
1237 		 * A timestamp received in a SYN makes
1238 		 * it ok to send timestamp requests and replies.
1239 		 */
1240 		if (to->to_flags & TOF_TS) {
1241 			sc->sc_tsreflect = to->to_tsval;
1242 			sc->sc_ts = tcp_ts_getticks();
1243 			sc->sc_flags |= SCF_TIMESTAMP;
1244 		}
1245 		if (to->to_flags & TOF_SCALE) {
1246 			int wscale = 0;
1247 
1248 			/*
1249 			 * Pick the smallest possible scaling factor that
1250 			 * will still allow us to scale up to sb_max, aka
1251 			 * kern.ipc.maxsockbuf.
1252 			 *
1253 			 * We do this because there are broken firewalls that
1254 			 * will corrupt the window scale option, leading to
1255 			 * the other endpoint believing that our advertised
1256 			 * window is unscaled.  At scale factors larger than
1257 			 * 5 the unscaled window will drop below 1500 bytes,
1258 			 * leading to serious problems when traversing these
1259 			 * broken firewalls.
1260 			 *
1261 			 * With the default maxsockbuf of 256K, a scale factor
1262 			 * of 3 will be chosen by this algorithm.  Those who
1263 			 * choose a larger maxsockbuf should watch out
1264 			 * for the compatiblity problems mentioned above.
1265 			 *
1266 			 * RFC1323: The Window field in a SYN (i.e., a <SYN>
1267 			 * or <SYN,ACK>) segment itself is never scaled.
1268 			 */
1269 			while (wscale < TCP_MAX_WINSHIFT &&
1270 			    (TCP_MAXWIN << wscale) < sb_max)
1271 				wscale++;
1272 			sc->sc_requested_r_scale = wscale;
1273 			sc->sc_requested_s_scale = to->to_wscale;
1274 			sc->sc_flags |= SCF_WINSCALE;
1275 		}
1276 	}
1277 #ifdef TCP_SIGNATURE
1278 	/*
1279 	 * If listening socket requested TCP digests, and received SYN
1280 	 * contains the option, flag this in the syncache so that
1281 	 * syncache_respond() will do the right thing with the SYN+ACK.
1282 	 * XXX: Currently we always record the option by default and will
1283 	 * attempt to use it in syncache_respond().
1284 	 */
1285 	if (to->to_flags & TOF_SIGNATURE || ltflags & TF_SIGNATURE)
1286 		sc->sc_flags |= SCF_SIGNATURE;
1287 #endif
1288 	if (to->to_flags & TOF_SACKPERM)
1289 		sc->sc_flags |= SCF_SACK;
1290 	if (to->to_flags & TOF_MSS)
1291 		sc->sc_peer_mss = to->to_mss;	/* peer mss may be zero */
1292 	if (ltflags & TF_NOOPT)
1293 		sc->sc_flags |= SCF_NOOPT;
1294 	if ((th->th_flags & (TH_ECE|TH_CWR)) && V_tcp_do_ecn)
1295 		sc->sc_flags |= SCF_ECN;
1296 
1297 	if (V_tcp_syncookies) {
1298 		syncookie_generate(sch, sc, &flowtmp);
1299 #ifdef INET6
1300 		if (autoflowlabel)
1301 			sc->sc_flowlabel = flowtmp;
1302 #endif
1303 	} else {
1304 #ifdef INET6
1305 		if (autoflowlabel)
1306 			sc->sc_flowlabel =
1307 			    (htonl(ip6_randomflowlabel()) & IPV6_FLOWLABEL_MASK);
1308 #endif
1309 	}
1310 	SCH_UNLOCK(sch);
1311 
1312 	/*
1313 	 * Do a standard 3-way handshake.
1314 	 */
1315 	if (syncache_respond(sc) == 0) {
1316 		if (V_tcp_syncookies && V_tcp_syncookiesonly && sc != &scs)
1317 			syncache_free(sc);
1318 		else if (sc != &scs)
1319 			syncache_insert(sc, sch);   /* locks and unlocks sch */
1320 		TCPSTAT_INC(tcps_sndacks);
1321 		TCPSTAT_INC(tcps_sndtotal);
1322 	} else {
1323 		if (sc != &scs)
1324 			syncache_free(sc);
1325 		TCPSTAT_INC(tcps_sc_dropped);
1326 	}
1327 
1328 done:
1329 	if (cred != NULL)
1330 		crfree(cred);
1331 #ifdef MAC
1332 	if (sc == &scs)
1333 		mac_syncache_destroy(&maclabel);
1334 #endif
1335 	if (m) {
1336 
1337 		*lsop = NULL;
1338 		m_freem(m);
1339 	}
1340 }
1341 
1342 static int
1343 syncache_respond(struct syncache *sc)
1344 {
1345 	struct ip *ip = NULL;
1346 	struct mbuf *m;
1347 	struct tcphdr *th = NULL;
1348 	int optlen, error = 0;	/* Make compiler happy */
1349 	u_int16_t hlen, tlen, mssopt;
1350 	struct tcpopt to;
1351 #ifdef INET6
1352 	struct ip6_hdr *ip6 = NULL;
1353 #endif
1354 
1355 	hlen =
1356 #ifdef INET6
1357 	       (sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
1358 #endif
1359 		sizeof(struct ip);
1360 	tlen = hlen + sizeof(struct tcphdr);
1361 
1362 	/* Determine MSS we advertize to other end of connection. */
1363 	mssopt = tcp_mssopt(&sc->sc_inc);
1364 	if (sc->sc_peer_mss)
1365 		mssopt = max( min(sc->sc_peer_mss, mssopt), V_tcp_minmss);
1366 
1367 	/* XXX: Assume that the entire packet will fit in a header mbuf. */
1368 	KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
1369 	    ("syncache: mbuf too small"));
1370 
1371 	/* Create the IP+TCP header from scratch. */
1372 	m = m_gethdr(M_NOWAIT, MT_DATA);
1373 	if (m == NULL)
1374 		return (ENOBUFS);
1375 #ifdef MAC
1376 	mac_syncache_create_mbuf(sc->sc_label, m);
1377 #endif
1378 	m->m_data += max_linkhdr;
1379 	m->m_len = tlen;
1380 	m->m_pkthdr.len = tlen;
1381 	m->m_pkthdr.rcvif = NULL;
1382 
1383 #ifdef INET6
1384 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
1385 		ip6 = mtod(m, struct ip6_hdr *);
1386 		ip6->ip6_vfc = IPV6_VERSION;
1387 		ip6->ip6_nxt = IPPROTO_TCP;
1388 		ip6->ip6_src = sc->sc_inc.inc6_laddr;
1389 		ip6->ip6_dst = sc->sc_inc.inc6_faddr;
1390 		ip6->ip6_plen = htons(tlen - hlen);
1391 		/* ip6_hlim is set after checksum */
1392 		ip6->ip6_flow &= ~IPV6_FLOWLABEL_MASK;
1393 		ip6->ip6_flow |= sc->sc_flowlabel;
1394 
1395 		th = (struct tcphdr *)(ip6 + 1);
1396 	}
1397 #endif
1398 #if defined(INET6) && defined(INET)
1399 	else
1400 #endif
1401 #ifdef INET
1402 	{
1403 		ip = mtod(m, struct ip *);
1404 		ip->ip_v = IPVERSION;
1405 		ip->ip_hl = sizeof(struct ip) >> 2;
1406 		ip->ip_len = htons(tlen);
1407 		ip->ip_id = 0;
1408 		ip->ip_off = 0;
1409 		ip->ip_sum = 0;
1410 		ip->ip_p = IPPROTO_TCP;
1411 		ip->ip_src = sc->sc_inc.inc_laddr;
1412 		ip->ip_dst = sc->sc_inc.inc_faddr;
1413 		ip->ip_ttl = sc->sc_ip_ttl;
1414 		ip->ip_tos = sc->sc_ip_tos;
1415 
1416 		/*
1417 		 * See if we should do MTU discovery.  Route lookups are
1418 		 * expensive, so we will only unset the DF bit if:
1419 		 *
1420 		 *	1) path_mtu_discovery is disabled
1421 		 *	2) the SCF_UNREACH flag has been set
1422 		 */
1423 		if (V_path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
1424 		       ip->ip_off |= htons(IP_DF);
1425 
1426 		th = (struct tcphdr *)(ip + 1);
1427 	}
1428 #endif /* INET */
1429 	th->th_sport = sc->sc_inc.inc_lport;
1430 	th->th_dport = sc->sc_inc.inc_fport;
1431 
1432 	th->th_seq = htonl(sc->sc_iss);
1433 	th->th_ack = htonl(sc->sc_irs + 1);
1434 	th->th_off = sizeof(struct tcphdr) >> 2;
1435 	th->th_x2 = 0;
1436 	th->th_flags = TH_SYN|TH_ACK;
1437 	th->th_win = htons(sc->sc_wnd);
1438 	th->th_urp = 0;
1439 
1440 	if (sc->sc_flags & SCF_ECN) {
1441 		th->th_flags |= TH_ECE;
1442 		TCPSTAT_INC(tcps_ecn_shs);
1443 	}
1444 
1445 	/* Tack on the TCP options. */
1446 	if ((sc->sc_flags & SCF_NOOPT) == 0) {
1447 		to.to_flags = 0;
1448 
1449 		to.to_mss = mssopt;
1450 		to.to_flags = TOF_MSS;
1451 		if (sc->sc_flags & SCF_WINSCALE) {
1452 			to.to_wscale = sc->sc_requested_r_scale;
1453 			to.to_flags |= TOF_SCALE;
1454 		}
1455 		if (sc->sc_flags & SCF_TIMESTAMP) {
1456 			/* Virgin timestamp or TCP cookie enhanced one. */
1457 			to.to_tsval = sc->sc_ts;
1458 			to.to_tsecr = sc->sc_tsreflect;
1459 			to.to_flags |= TOF_TS;
1460 		}
1461 		if (sc->sc_flags & SCF_SACK)
1462 			to.to_flags |= TOF_SACKPERM;
1463 #ifdef TCP_SIGNATURE
1464 		if (sc->sc_flags & SCF_SIGNATURE)
1465 			to.to_flags |= TOF_SIGNATURE;
1466 #endif
1467 		optlen = tcp_addoptions(&to, (u_char *)(th + 1));
1468 
1469 		/* Adjust headers by option size. */
1470 		th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
1471 		m->m_len += optlen;
1472 		m->m_pkthdr.len += optlen;
1473 
1474 #ifdef TCP_SIGNATURE
1475 		if (sc->sc_flags & SCF_SIGNATURE)
1476 			tcp_signature_compute(m, 0, 0, optlen,
1477 			    to.to_signature, IPSEC_DIR_OUTBOUND);
1478 #endif
1479 #ifdef INET6
1480 		if (sc->sc_inc.inc_flags & INC_ISIPV6)
1481 			ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
1482 		else
1483 #endif
1484 			ip->ip_len = htons(ntohs(ip->ip_len) + optlen);
1485 	} else
1486 		optlen = 0;
1487 
1488 	M_SETFIB(m, sc->sc_inc.inc_fibnum);
1489 	m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
1490 #ifdef INET6
1491 	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
1492 		m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
1493 		th->th_sum = in6_cksum_pseudo(ip6, tlen + optlen - hlen,
1494 		    IPPROTO_TCP, 0);
1495 		ip6->ip6_hlim = in6_selecthlim(NULL, NULL);
1496 #ifdef TCP_OFFLOAD
1497 		if (ADDED_BY_TOE(sc)) {
1498 			struct toedev *tod = sc->sc_tod;
1499 
1500 			error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
1501 
1502 			return (error);
1503 		}
1504 #endif
1505 		error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
1506 	}
1507 #endif
1508 #if defined(INET6) && defined(INET)
1509 	else
1510 #endif
1511 #ifdef INET
1512 	{
1513 		m->m_pkthdr.csum_flags = CSUM_TCP;
1514 		th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
1515 		    htons(tlen + optlen - hlen + IPPROTO_TCP));
1516 #ifdef TCP_OFFLOAD
1517 		if (ADDED_BY_TOE(sc)) {
1518 			struct toedev *tod = sc->sc_tod;
1519 
1520 			error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
1521 
1522 			return (error);
1523 		}
1524 #endif
1525 		error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
1526 	}
1527 #endif
1528 	return (error);
1529 }
1530 
1531 /*
1532  * The purpose of SYN cookies is to avoid keeping track of all SYN's we
1533  * receive and to be able to handle SYN floods from bogus source addresses
1534  * (where we will never receive any reply).  SYN floods try to exhaust all
1535  * our memory and available slots in the SYN cache table to cause a denial
1536  * of service to legitimate users of the local host.
1537  *
1538  * The idea of SYN cookies is to encode and include all necessary information
1539  * about the connection setup state within the SYN-ACK we send back and thus
1540  * to get along without keeping any local state until the ACK to the SYN-ACK
1541  * arrives (if ever).  Everything we need to know should be available from
1542  * the information we encoded in the SYN-ACK.
1543  *
1544  * More information about the theory behind SYN cookies and its first
1545  * discussion and specification can be found at:
1546  *  http://cr.yp.to/syncookies.html    (overview)
1547  *  http://cr.yp.to/syncookies/archive (gory details)
1548  *
1549  * This implementation extends the orginal idea and first implementation
1550  * of FreeBSD by using not only the initial sequence number field to store
1551  * information but also the timestamp field if present.  This way we can
1552  * keep track of the entire state we need to know to recreate the session in
1553  * its original form.  Almost all TCP speakers implement RFC1323 timestamps
1554  * these days.  For those that do not we still have to live with the known
1555  * shortcomings of the ISN only SYN cookies.
1556  *
1557  * Cookie layers:
1558  *
1559  * Initial sequence number we send:
1560  * 31|................................|0
1561  *    DDDDDDDDDDDDDDDDDDDDDDDDDMMMRRRP
1562  *    D = MD5 Digest (first dword)
1563  *    M = MSS index
1564  *    R = Rotation of secret
1565  *    P = Odd or Even secret
1566  *
1567  * The MD5 Digest is computed with over following parameters:
1568  *  a) randomly rotated secret
1569  *  b) struct in_conninfo containing the remote/local ip/port (IPv4&IPv6)
1570  *  c) the received initial sequence number from remote host
1571  *  d) the rotation offset and odd/even bit
1572  *
1573  * Timestamp we send:
1574  * 31|................................|0
1575  *    DDDDDDDDDDDDDDDDDDDDDDSSSSRRRRA5
1576  *    D = MD5 Digest (third dword) (only as filler)
1577  *    S = Requested send window scale
1578  *    R = Requested receive window scale
1579  *    A = SACK allowed
1580  *    5 = TCP-MD5 enabled (not implemented yet)
1581  *    XORed with MD5 Digest (forth dword)
1582  *
1583  * The timestamp isn't cryptographically secure and doesn't need to be.
1584  * The double use of the MD5 digest dwords ties it to a specific remote/
1585  * local host/port, remote initial sequence number and our local time
1586  * limited secret.  A received timestamp is reverted (XORed) and then
1587  * the contained MD5 dword is compared to the computed one to ensure the
1588  * timestamp belongs to the SYN-ACK we sent.  The other parameters may
1589  * have been tampered with but this isn't different from supplying bogus
1590  * values in the SYN in the first place.
1591  *
1592  * Some problems with SYN cookies remain however:
1593  * Consider the problem of a recreated (and retransmitted) cookie.  If the
1594  * original SYN was accepted, the connection is established.  The second
1595  * SYN is inflight, and if it arrives with an ISN that falls within the
1596  * receive window, the connection is killed.
1597  *
1598  * Notes:
1599  * A heuristic to determine when to accept syn cookies is not necessary.
1600  * An ACK flood would cause the syncookie verification to be attempted,
1601  * but a SYN flood causes syncookies to be generated.  Both are of equal
1602  * cost, so there's no point in trying to optimize the ACK flood case.
1603  * Also, if you don't process certain ACKs for some reason, then all someone
1604  * would have to do is launch a SYN and ACK flood at the same time, which
1605  * would stop cookie verification and defeat the entire purpose of syncookies.
1606  */
1607 static int tcp_sc_msstab[] = { 0, 256, 468, 536, 996, 1452, 1460, 8960 };
1608 
1609 static void
1610 syncookie_generate(struct syncache_head *sch, struct syncache *sc,
1611     u_int32_t *flowlabel)
1612 {
1613 	MD5_CTX ctx;
1614 	u_int32_t md5_buffer[MD5_DIGEST_LENGTH / sizeof(u_int32_t)];
1615 	u_int32_t data;
1616 	u_int32_t *secbits;
1617 	u_int off, pmss, mss;
1618 	int i;
1619 
1620 	SCH_LOCK_ASSERT(sch);
1621 
1622 	/* Which of the two secrets to use. */
1623 	secbits = sch->sch_oddeven ?
1624 			sch->sch_secbits_odd : sch->sch_secbits_even;
1625 
1626 	/* Reseed secret if too old. */
1627 	if (sch->sch_reseed < time_uptime) {
1628 		sch->sch_oddeven = sch->sch_oddeven ? 0 : 1;	/* toggle */
1629 		secbits = sch->sch_oddeven ?
1630 				sch->sch_secbits_odd : sch->sch_secbits_even;
1631 		for (i = 0; i < SYNCOOKIE_SECRET_SIZE; i++)
1632 			secbits[i] = arc4random();
1633 		sch->sch_reseed = time_uptime + SYNCOOKIE_LIFETIME;
1634 	}
1635 
1636 	/* Secret rotation offset. */
1637 	off = sc->sc_iss & 0x7;			/* iss was randomized before */
1638 
1639 	/* Maximum segment size calculation. */
1640 	pmss =
1641 	    max( min(sc->sc_peer_mss, tcp_mssopt(&sc->sc_inc)),	V_tcp_minmss);
1642 	for (mss = sizeof(tcp_sc_msstab) / sizeof(int) - 1; mss > 0; mss--)
1643 		if (tcp_sc_msstab[mss] <= pmss)
1644 			break;
1645 
1646 	/* Fold parameters and MD5 digest into the ISN we will send. */
1647 	data = sch->sch_oddeven;/* odd or even secret, 1 bit */
1648 	data |= off << 1;	/* secret offset, derived from iss, 3 bits */
1649 	data |= mss << 4;	/* mss, 3 bits */
1650 
1651 	MD5Init(&ctx);
1652 	MD5Update(&ctx, ((u_int8_t *)secbits) + off,
1653 	    SYNCOOKIE_SECRET_SIZE * sizeof(*secbits) - off);
1654 	MD5Update(&ctx, secbits, off);
1655 	MD5Update(&ctx, &sc->sc_inc, sizeof(sc->sc_inc));
1656 	MD5Update(&ctx, &sc->sc_irs, sizeof(sc->sc_irs));
1657 	MD5Update(&ctx, &data, sizeof(data));
1658 	MD5Final((u_int8_t *)&md5_buffer, &ctx);
1659 
1660 	data |= (md5_buffer[0] << 7);
1661 	sc->sc_iss = data;
1662 
1663 #ifdef INET6
1664 	*flowlabel = md5_buffer[1] & IPV6_FLOWLABEL_MASK;
1665 #endif
1666 
1667 	/* Additional parameters are stored in the timestamp if present. */
1668 	if (sc->sc_flags & SCF_TIMESTAMP) {
1669 		data =  ((sc->sc_flags & SCF_SIGNATURE) ? 1 : 0); /* TCP-MD5, 1 bit */
1670 		data |= ((sc->sc_flags & SCF_SACK) ? 1 : 0) << 1; /* SACK, 1 bit */
1671 		data |= sc->sc_requested_s_scale << 2;  /* SWIN scale, 4 bits */
1672 		data |= sc->sc_requested_r_scale << 6;  /* RWIN scale, 4 bits */
1673 		data |= md5_buffer[2] << 10;		/* more digest bits */
1674 		data ^= md5_buffer[3];
1675 		sc->sc_ts = data;
1676 		sc->sc_tsoff = data - tcp_ts_getticks();	/* after XOR */
1677 	}
1678 
1679 	TCPSTAT_INC(tcps_sc_sendcookie);
1680 }
1681 
1682 static struct syncache *
1683 syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch,
1684     struct syncache *sc, struct tcpopt *to, struct tcphdr *th,
1685     struct socket *so)
1686 {
1687 	MD5_CTX ctx;
1688 	u_int32_t md5_buffer[MD5_DIGEST_LENGTH / sizeof(u_int32_t)];
1689 	u_int32_t data = 0;
1690 	u_int32_t *secbits;
1691 	tcp_seq ack, seq;
1692 	int off, mss, wnd, flags;
1693 
1694 	SCH_LOCK_ASSERT(sch);
1695 
1696 	/*
1697 	 * Pull information out of SYN-ACK/ACK and
1698 	 * revert sequence number advances.
1699 	 */
1700 	ack = th->th_ack - 1;
1701 	seq = th->th_seq - 1;
1702 	off = (ack >> 1) & 0x7;
1703 	mss = (ack >> 4) & 0x7;
1704 	flags = ack & 0x7f;
1705 
1706 	/* Which of the two secrets to use. */
1707 	secbits = (flags & 0x1) ? sch->sch_secbits_odd : sch->sch_secbits_even;
1708 
1709 	/*
1710 	 * The secret wasn't updated for the lifetime of a syncookie,
1711 	 * so this SYN-ACK/ACK is either too old (replay) or totally bogus.
1712 	 */
1713 	if (sch->sch_reseed + SYNCOOKIE_LIFETIME < time_uptime) {
1714 		return (NULL);
1715 	}
1716 
1717 	/* Recompute the digest so we can compare it. */
1718 	MD5Init(&ctx);
1719 	MD5Update(&ctx, ((u_int8_t *)secbits) + off,
1720 	    SYNCOOKIE_SECRET_SIZE * sizeof(*secbits) - off);
1721 	MD5Update(&ctx, secbits, off);
1722 	MD5Update(&ctx, inc, sizeof(*inc));
1723 	MD5Update(&ctx, &seq, sizeof(seq));
1724 	MD5Update(&ctx, &flags, sizeof(flags));
1725 	MD5Final((u_int8_t *)&md5_buffer, &ctx);
1726 
1727 	/* Does the digest part of or ACK'ed ISS match? */
1728 	if ((ack & (~0x7f)) != (md5_buffer[0] << 7))
1729 		return (NULL);
1730 
1731 	/* Does the digest part of our reflected timestamp match? */
1732 	if (to->to_flags & TOF_TS) {
1733 		data = md5_buffer[3] ^ to->to_tsecr;
1734 		if ((data & (~0x3ff)) != (md5_buffer[2] << 10))
1735 			return (NULL);
1736 	}
1737 
1738 	/* Fill in the syncache values. */
1739 	bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
1740 	sc->sc_ipopts = NULL;
1741 
1742 	sc->sc_irs = seq;
1743 	sc->sc_iss = ack;
1744 
1745 #ifdef INET6
1746 	if (inc->inc_flags & INC_ISIPV6) {
1747 		if (sotoinpcb(so)->inp_flags & IN6P_AUTOFLOWLABEL)
1748 			sc->sc_flowlabel = md5_buffer[1] & IPV6_FLOWLABEL_MASK;
1749 	} else
1750 #endif
1751 	{
1752 		sc->sc_ip_ttl = sotoinpcb(so)->inp_ip_ttl;
1753 		sc->sc_ip_tos = sotoinpcb(so)->inp_ip_tos;
1754 	}
1755 
1756 	/* Additional parameters that were encoded in the timestamp. */
1757 	if (data) {
1758 		sc->sc_flags |= SCF_TIMESTAMP;
1759 		sc->sc_tsreflect = to->to_tsval;
1760 		sc->sc_ts = to->to_tsecr;
1761 		sc->sc_tsoff = to->to_tsecr - tcp_ts_getticks();
1762 		sc->sc_flags |= (data & 0x1) ? SCF_SIGNATURE : 0;
1763 		sc->sc_flags |= ((data >> 1) & 0x1) ? SCF_SACK : 0;
1764 		sc->sc_requested_s_scale = min((data >> 2) & 0xf,
1765 		    TCP_MAX_WINSHIFT);
1766 		sc->sc_requested_r_scale = min((data >> 6) & 0xf,
1767 		    TCP_MAX_WINSHIFT);
1768 		if (sc->sc_requested_s_scale || sc->sc_requested_r_scale)
1769 			sc->sc_flags |= SCF_WINSCALE;
1770 	} else
1771 		sc->sc_flags |= SCF_NOOPT;
1772 
1773 	wnd = sbspace(&so->so_rcv);
1774 	wnd = imax(wnd, 0);
1775 	wnd = imin(wnd, TCP_MAXWIN);
1776 	sc->sc_wnd = wnd;
1777 
1778 	sc->sc_rxmits = 0;
1779 	sc->sc_peer_mss = tcp_sc_msstab[mss];
1780 
1781 	TCPSTAT_INC(tcps_sc_recvcookie);
1782 	return (sc);
1783 }
1784 
1785 /*
1786  * Returns the current number of syncache entries.  This number
1787  * will probably change before you get around to calling
1788  * syncache_pcblist.
1789  */
1790 
1791 int
1792 syncache_pcbcount(void)
1793 {
1794 	struct syncache_head *sch;
1795 	int count, i;
1796 
1797 	for (count = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
1798 		/* No need to lock for a read. */
1799 		sch = &V_tcp_syncache.hashbase[i];
1800 		count += sch->sch_length;
1801 	}
1802 	return count;
1803 }
1804 
1805 /*
1806  * Exports the syncache entries to userland so that netstat can display
1807  * them alongside the other sockets.  This function is intended to be
1808  * called only from tcp_pcblist.
1809  *
1810  * Due to concurrency on an active system, the number of pcbs exported
1811  * may have no relation to max_pcbs.  max_pcbs merely indicates the
1812  * amount of space the caller allocated for this function to use.
1813  */
1814 int
1815 syncache_pcblist(struct sysctl_req *req, int max_pcbs, int *pcbs_exported)
1816 {
1817 	struct xtcpcb xt;
1818 	struct syncache *sc;
1819 	struct syncache_head *sch;
1820 	int count, error, i;
1821 
1822 	for (count = 0, error = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
1823 		sch = &V_tcp_syncache.hashbase[i];
1824 		SCH_LOCK(sch);
1825 		TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
1826 			if (count >= max_pcbs) {
1827 				SCH_UNLOCK(sch);
1828 				goto exit;
1829 			}
1830 			if (cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
1831 				continue;
1832 			bzero(&xt, sizeof(xt));
1833 			xt.xt_len = sizeof(xt);
1834 			if (sc->sc_inc.inc_flags & INC_ISIPV6)
1835 				xt.xt_inp.inp_vflag = INP_IPV6;
1836 			else
1837 				xt.xt_inp.inp_vflag = INP_IPV4;
1838 			bcopy(&sc->sc_inc, &xt.xt_inp.inp_inc, sizeof (struct in_conninfo));
1839 			xt.xt_tp.t_inpcb = &xt.xt_inp;
1840 			xt.xt_tp.t_state = TCPS_SYN_RECEIVED;
1841 			xt.xt_socket.xso_protocol = IPPROTO_TCP;
1842 			xt.xt_socket.xso_len = sizeof (struct xsocket);
1843 			xt.xt_socket.so_type = SOCK_STREAM;
1844 			xt.xt_socket.so_state = SS_ISCONNECTING;
1845 			error = SYSCTL_OUT(req, &xt, sizeof xt);
1846 			if (error) {
1847 				SCH_UNLOCK(sch);
1848 				goto exit;
1849 			}
1850 			count++;
1851 		}
1852 		SCH_UNLOCK(sch);
1853 	}
1854 exit:
1855 	*pcbs_exported = count;
1856 	return error;
1857 }
1858