xref: /freebsd-11-stable/sys/netinet/tcp_syncache.c

/*-
 * Copyright (c) 2001 McAfee, Inc.
 * Copyright (c) 2006,2013 Andre Oppermann, Internet Business Solutions AG
 * All rights reserved.
 *
 * This software was developed for the FreeBSD Project by Jonathan Lemon
 * and McAfee Research, the Security Research Division of McAfee, Inc. under
 * DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
 * DARPA CHATS research program. [2001 McAfee, Inc.]
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD: stable/11/sys/netinet/tcp_syncache.c 332238 2018-04-07 20:47:25Z tuexen $");

#include "opt_inet.h"
#include "opt_inet6.h"
#include "opt_ipsec.h"
#include "opt_pcbgroup.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/hash.h>
#include <sys/refcount.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/limits.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/proc.h>		/* for proc0 declaration */
#include <sys/random.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/syslog.h>
#include <sys/ucred.h>

#include <sys/md5.h>
#include <crypto/siphash/siphash.h>

#include <vm/uma.h>

#include <net/if.h>
#include <net/if_var.h>
#include <net/route.h>
#include <net/vnet.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_var.h>
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#include <netinet/ip_options.h>
#ifdef INET6
#include <netinet/ip6.h>
#include <netinet/icmp6.h>
#include <netinet6/nd6.h>
#include <netinet6/ip6_var.h>
#include <netinet6/in6_pcb.h>
#endif
#include <netinet/tcp.h>
#ifdef TCP_RFC7413
#include <netinet/tcp_fastopen.h>
#endif
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/tcp_syncache.h>
#ifdef INET6
#include <netinet6/tcp6_var.h>
#endif
#ifdef TCP_OFFLOAD
#include <netinet/toecore.h>
#endif

#include <netipsec/ipsec_support.h>

#include <machine/in_cksum.h>

#include <security/mac/mac_framework.h>

static VNET_DEFINE(int, tcp_syncookies) = 1;
#define	V_tcp_syncookies		VNET(tcp_syncookies)
SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(tcp_syncookies), 0,
    "Use TCP SYN cookies if the syncache overflows");

static VNET_DEFINE(int, tcp_syncookiesonly) = 0;
#define	V_tcp_syncookiesonly		VNET(tcp_syncookiesonly)
SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(tcp_syncookiesonly), 0,
    "Use only TCP SYN cookies");

#ifdef TCP_OFFLOAD
#define ADDED_BY_TOE(sc) ((sc)->sc_tod != NULL)
#endif

static void	 syncache_drop(struct syncache *, struct syncache_head *);
static void	 syncache_free(struct syncache *);
static void	 syncache_insert(struct syncache *, struct syncache_head *);
static int	 syncache_respond(struct syncache *, struct syncache_head *, int,
		    const struct mbuf *);
static struct	 socket *syncache_socket(struct syncache *, struct socket *,
		    struct mbuf *m);
static void	 syncache_timeout(struct syncache *sc, struct syncache_head *sch,
		    int docallout);
static void	 syncache_timer(void *);

static uint32_t	 syncookie_mac(struct in_conninfo *, tcp_seq, uint8_t,
		    uint8_t *, uintptr_t);
static tcp_seq	 syncookie_generate(struct syncache_head *, struct syncache *);
static struct syncache
		*syncookie_lookup(struct in_conninfo *, struct syncache_head *,
		    struct syncache *, struct tcphdr *, struct tcpopt *,
		    struct socket *);
static void	 syncookie_reseed(void *);
#ifdef INVARIANTS
static int	 syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
		    struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
		    struct socket *lso);
#endif

/*
 * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
 * 3 retransmits corresponds to a timeout of 3 * (1 + 2 + 4 + 8) == 45 seconds,
 * the odds are that the user has given up attempting to connect by then.
 */
#define SYNCACHE_MAXREXMTS		3

/* Arbitrary values */
#define TCP_SYNCACHE_HASHSIZE		512
#define TCP_SYNCACHE_BUCKETLIMIT	30

static VNET_DEFINE(struct tcp_syncache, tcp_syncache);
#define	V_tcp_syncache			VNET(tcp_syncache)

static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0,
    "TCP SYN cache");

SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
    &VNET_NAME(tcp_syncache.bucket_limit), 0,
    "Per-bucket hash limit for syncache");

SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
    &VNET_NAME(tcp_syncache.cache_limit), 0,
    "Overall entry limit for syncache");

SYSCTL_UMA_CUR(_net_inet_tcp_syncache, OID_AUTO, count, CTLFLAG_VNET,
    &VNET_NAME(tcp_syncache.zone), "Current number of entries in syncache");

SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_VNET | CTLFLAG_RDTUN,
    &VNET_NAME(tcp_syncache.hashsize), 0,
    "Size of TCP syncache hashtable");

SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(tcp_syncache.rexmt_limit), 0,
    "Limit on SYN/ACK retransmissions");

VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;
SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail,
    CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_sc_rst_sock_fail), 0,
    "Send reset on socket allocation failure");

static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");

#define	SCH_LOCK(sch)		mtx_lock(&(sch)->sch_mtx)
#define	SCH_UNLOCK(sch)		mtx_unlock(&(sch)->sch_mtx)
#define	SCH_LOCK_ASSERT(sch)	mtx_assert(&(sch)->sch_mtx, MA_OWNED)

/*
 * Requires the syncache entry to be already removed from the bucket list.
 */
static void
syncache_free(struct syncache *sc)
{

	if (sc->sc_ipopts)
		(void) m_free(sc->sc_ipopts);
	if (sc->sc_cred)
		crfree(sc->sc_cred);
#ifdef MAC
	mac_syncache_destroy(&sc->sc_label);
#endif

	uma_zfree(V_tcp_syncache.zone, sc);
}

void
syncache_init(void)
{
	int i;

	V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
	V_tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
	V_tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
	V_tcp_syncache.hash_secret = arc4random();

	TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
	    &V_tcp_syncache.hashsize);
	TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
	    &V_tcp_syncache.bucket_limit);
	if (!powerof2(V_tcp_syncache.hashsize) ||
	    V_tcp_syncache.hashsize == 0) {
		printf("WARNING: syncache hash size is not a power of 2.\n");
		V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
	}
	V_tcp_syncache.hashmask = V_tcp_syncache.hashsize - 1;

	/* Set limits. */
	V_tcp_syncache.cache_limit =
	    V_tcp_syncache.hashsize * V_tcp_syncache.bucket_limit;
	TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
	    &V_tcp_syncache.cache_limit);

	/* Allocate the hash table. */
	V_tcp_syncache.hashbase = malloc(V_tcp_syncache.hashsize *
	    sizeof(struct syncache_head), M_SYNCACHE, M_WAITOK | M_ZERO);

#ifdef VIMAGE
	V_tcp_syncache.vnet = curvnet;
#endif

	/* Initialize the hash buckets. */
	for (i = 0; i < V_tcp_syncache.hashsize; i++) {
		TAILQ_INIT(&V_tcp_syncache.hashbase[i].sch_bucket);
		mtx_init(&V_tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
			 NULL, MTX_DEF);
		callout_init_mtx(&V_tcp_syncache.hashbase[i].sch_timer,
			 &V_tcp_syncache.hashbase[i].sch_mtx, 0);
		V_tcp_syncache.hashbase[i].sch_length = 0;
		V_tcp_syncache.hashbase[i].sch_sc = &V_tcp_syncache;
		V_tcp_syncache.hashbase[i].sch_last_overflow =
		    -(SYNCOOKIE_LIFETIME + 1);
	}

	/* Create the syncache entry zone. */
	V_tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
	    NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
	V_tcp_syncache.cache_limit = uma_zone_set_max(V_tcp_syncache.zone,
	    V_tcp_syncache.cache_limit);

	/* Start the SYN cookie reseeder callout. */
	callout_init(&V_tcp_syncache.secret.reseed, 1);
	arc4rand(V_tcp_syncache.secret.key[0], SYNCOOKIE_SECRET_SIZE, 0);
	arc4rand(V_tcp_syncache.secret.key[1], SYNCOOKIE_SECRET_SIZE, 0);
	callout_reset(&V_tcp_syncache.secret.reseed, SYNCOOKIE_LIFETIME * hz,
	    syncookie_reseed, &V_tcp_syncache);
}

#ifdef VIMAGE
void
syncache_destroy(void)
{
	struct syncache_head *sch;
	struct syncache *sc, *nsc;
	int i;

	/*
	 * Stop the re-seed timer before freeing resources.  No need to
	 * possibly schedule it another time.
	 */
	callout_drain(&V_tcp_syncache.secret.reseed);

	/* Cleanup hash buckets: stop timers, free entries, destroy locks. */
	for (i = 0; i < V_tcp_syncache.hashsize; i++) {

		sch = &V_tcp_syncache.hashbase[i];
		callout_drain(&sch->sch_timer);

		SCH_LOCK(sch);
		TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc)
			syncache_drop(sc, sch);
		SCH_UNLOCK(sch);
		KASSERT(TAILQ_EMPTY(&sch->sch_bucket),
		    ("%s: sch->sch_bucket not empty", __func__));
		KASSERT(sch->sch_length == 0, ("%s: sch->sch_length %d not 0",
		    __func__, sch->sch_length));
		mtx_destroy(&sch->sch_mtx);
	}

	KASSERT(uma_zone_get_cur(V_tcp_syncache.zone) == 0,
	    ("%s: cache_count not 0", __func__));

	/* Free the allocated global resources. */
	uma_zdestroy(V_tcp_syncache.zone);
	free(V_tcp_syncache.hashbase, M_SYNCACHE);
}
#endif

/*
 * Inserts a syncache entry into the specified bucket row.
 * Locks and unlocks the syncache_head autonomously.
 */
static void
syncache_insert(struct syncache *sc, struct syncache_head *sch)
{
	struct syncache *sc2;

	SCH_LOCK(sch);

	/*
	 * Make sure that we don't overflow the per-bucket limit.
	 * If the bucket is full, toss the oldest element.
	 */
	if (sch->sch_length >= V_tcp_syncache.bucket_limit) {
		KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
			("sch->sch_length incorrect"));
		sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
		sch->sch_last_overflow = time_uptime;
		syncache_drop(sc2, sch);
		TCPSTAT_INC(tcps_sc_bucketoverflow);
	}

	/* Put it into the bucket. */
	TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
	sch->sch_length++;

#ifdef TCP_OFFLOAD
	if (ADDED_BY_TOE(sc)) {
		struct toedev *tod = sc->sc_tod;

		tod->tod_syncache_added(tod, sc->sc_todctx);
	}
#endif

	/* Reinitialize the bucket row's timer. */
	if (sch->sch_length == 1)
		sch->sch_nextc = ticks + INT_MAX;
	syncache_timeout(sc, sch, 1);

	SCH_UNLOCK(sch);

	TCPSTATES_INC(TCPS_SYN_RECEIVED);
	TCPSTAT_INC(tcps_sc_added);
}

/*
 * Remove and free entry from syncache bucket row.
 * Expects locked syncache head.
 */
static void
syncache_drop(struct syncache *sc, struct syncache_head *sch)
{

	SCH_LOCK_ASSERT(sch);

	TCPSTATES_DEC(TCPS_SYN_RECEIVED);
	TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
	sch->sch_length--;

#ifdef TCP_OFFLOAD
	if (ADDED_BY_TOE(sc)) {
		struct toedev *tod = sc->sc_tod;

		tod->tod_syncache_removed(tod, sc->sc_todctx);
	}
#endif

	syncache_free(sc);
}

/*
 * Engage/reengage time on bucket row.
 */
static void
syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
{
	sc->sc_rxttime = ticks +
		TCPTV_RTOBASE * (tcp_syn_backoff[sc->sc_rxmits]);
	sc->sc_rxmits++;
	if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {
		sch->sch_nextc = sc->sc_rxttime;
		if (docallout)
			callout_reset(&sch->sch_timer, sch->sch_nextc - ticks,
			    syncache_timer, (void *)sch);
	}
}

/*
 * Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
 * If we have retransmitted an entry the maximum number of times, expire it.
 * One separate timer for each bucket row.
 */
static void
syncache_timer(void *xsch)
{
	struct syncache_head *sch = (struct syncache_head *)xsch;
	struct syncache *sc, *nsc;
	int tick = ticks;
	char *s;

	CURVNET_SET(sch->sch_sc->vnet);

	/* NB: syncache_head has already been locked by the callout. */
	SCH_LOCK_ASSERT(sch);

	/*
	 * In the following cycle we may remove some entries and/or
	 * advance some timeouts, so re-initialize the bucket timer.
	 */
	sch->sch_nextc = tick + INT_MAX;

	TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
		/*
		 * We do not check if the listen socket still exists
		 * and accept the case where the listen socket may be
		 * gone by the time we resend the SYN/ACK.  We do
		 * not expect this to happens often. If it does,
		 * then the RST will be sent by the time the remote
		 * host does the SYN/ACK->ACK.
		 */
		if (TSTMP_GT(sc->sc_rxttime, tick)) {
			if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc))
				sch->sch_nextc = sc->sc_rxttime;
			continue;
		}
		if (sc->sc_rxmits > V_tcp_syncache.rexmt_limit) {
			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
				log(LOG_DEBUG, "%s; %s: Retransmits exhausted, "
				    "giving up and removing syncache entry\n",
				    s, __func__);
				free(s, M_TCPLOG);
			}
			syncache_drop(sc, sch);
			TCPSTAT_INC(tcps_sc_stale);
			continue;
		}
		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
			log(LOG_DEBUG, "%s; %s: Response timeout, "
			    "retransmitting (%u) SYN|ACK\n",
			    s, __func__, sc->sc_rxmits);
			free(s, M_TCPLOG);
		}

		syncache_respond(sc, sch, 1, NULL);
		TCPSTAT_INC(tcps_sc_retransmitted);
		syncache_timeout(sc, sch, 0);
	}
	if (!TAILQ_EMPTY(&(sch)->sch_bucket))
		callout_reset(&(sch)->sch_timer, (sch)->sch_nextc - tick,
			syncache_timer, (void *)(sch));
	CURVNET_RESTORE();
}

/*
 * Find an entry in the syncache.
 * Returns always with locked syncache_head plus a matching entry or NULL.
 */
static struct syncache *
syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
{
	struct syncache *sc;
	struct syncache_head *sch;
	uint32_t hash;

	/*
	 * The hash is built on foreign port + local port + foreign address.
	 * We rely on the fact that struct in_conninfo starts with 16 bits
	 * of foreign port, then 16 bits of local port then followed by 128
	 * bits of foreign address.  In case of IPv4 address, the first 3
	 * 32-bit words of the address always are zeroes.
	 */
	hash = jenkins_hash32((uint32_t *)&inc->inc_ie, 5,
	    V_tcp_syncache.hash_secret) & V_tcp_syncache.hashmask;

	sch = &V_tcp_syncache.hashbase[hash];
	*schp = sch;
	SCH_LOCK(sch);

	/* Circle through bucket row to find matching entry. */
	TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash)
		if (bcmp(&inc->inc_ie, &sc->sc_inc.inc_ie,
		    sizeof(struct in_endpoints)) == 0)
			break;

	return (sc);	/* Always returns with locked sch. */
}

/*
 * This function is called when we get a RST for a
 * non-existent connection, so that we can see if the
 * connection is in the syn cache.  If it is, zap it.
 */
void
syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th)
{
	struct syncache *sc;
	struct syncache_head *sch;
	char *s = NULL;

	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
	SCH_LOCK_ASSERT(sch);

	/*
	 * Any RST to our SYN|ACK must not carry ACK, SYN or FIN flags.
	 * See RFC 793 page 65, section SEGMENT ARRIVES.
	 */
	if (th->th_flags & (TH_ACK|TH_SYN|TH_FIN)) {
		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
			log(LOG_DEBUG, "%s; %s: Spurious RST with ACK, SYN or "
			    "FIN flag set, segment ignored\n", s, __func__);
		TCPSTAT_INC(tcps_badrst);
		goto done;
	}

	/*
	 * No corresponding connection was found in syncache.
	 * If syncookies are enabled and possibly exclusively
	 * used, or we are under memory pressure, a valid RST
	 * may not find a syncache entry.  In that case we're
	 * done and no SYN|ACK retransmissions will happen.
	 * Otherwise the RST was misdirected or spoofed.
	 */
	if (sc == NULL) {
		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
			log(LOG_DEBUG, "%s; %s: Spurious RST without matching "
			    "syncache entry (possibly syncookie only), "
			    "segment ignored\n", s, __func__);
		TCPSTAT_INC(tcps_badrst);
		goto done;
	}

	/*
	 * If the RST bit is set, check the sequence number to see
	 * if this is a valid reset segment.
	 * RFC 793 page 37:
	 *   In all states except SYN-SENT, all reset (RST) segments
	 *   are validated by checking their SEQ-fields.  A reset is
	 *   valid if its sequence number is in the window.
	 *
	 *   The sequence number in the reset segment is normally an
	 *   echo of our outgoing acknowlegement numbers, but some hosts
	 *   send a reset with the sequence number at the rightmost edge
	 *   of our receive window, and we have to handle this case.
	 */
	if (SEQ_GEQ(th->th_seq, sc->sc_irs) &&
	    SEQ_LEQ(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
		syncache_drop(sc, sch);
		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
			log(LOG_DEBUG, "%s; %s: Our SYN|ACK was rejected, "
			    "connection attempt aborted by remote endpoint\n",
			    s, __func__);
		TCPSTAT_INC(tcps_sc_reset);
	} else {
		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
			log(LOG_DEBUG, "%s; %s: RST with invalid SEQ %u != "
			    "IRS %u (+WND %u), segment ignored\n",
			    s, __func__, th->th_seq, sc->sc_irs, sc->sc_wnd);
		TCPSTAT_INC(tcps_badrst);
	}

done:
	if (s != NULL)
		free(s, M_TCPLOG);
	SCH_UNLOCK(sch);
}

void
syncache_badack(struct in_conninfo *inc)
{
	struct syncache *sc;
	struct syncache_head *sch;

	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
	SCH_LOCK_ASSERT(sch);
	if (sc != NULL) {
		syncache_drop(sc, sch);
		TCPSTAT_INC(tcps_sc_badack);
	}
	SCH_UNLOCK(sch);
}

void
syncache_unreach(struct in_conninfo *inc, tcp_seq th_seq)
{
	struct syncache *sc;
	struct syncache_head *sch;

	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
	SCH_LOCK_ASSERT(sch);
	if (sc == NULL)
		goto done;

	/* If the sequence number != sc_iss, then it's a bogus ICMP msg */
	if (ntohl(th_seq) != sc->sc_iss)
		goto done;

	/*
	 * If we've rertransmitted 3 times and this is our second error,
	 * we remove the entry.  Otherwise, we allow it to continue on.
	 * This prevents us from incorrectly nuking an entry during a
	 * spurious network outage.
	 *
	 * See tcp_notify().
	 */
	if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxmits < 3 + 1) {
		sc->sc_flags |= SCF_UNREACH;
		goto done;
	}
	syncache_drop(sc, sch);
	TCPSTAT_INC(tcps_sc_unreach);
done:
	SCH_UNLOCK(sch);
}

/*
 * Build a new TCP socket structure from a syncache entry.
 *
 * On success return the newly created socket with its underlying inp locked.
 */
static struct socket *
syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
{
	struct tcp_function_block *blk;
	struct inpcb *inp = NULL;
	struct socket *so;
	struct tcpcb *tp;
	int error;
	char *s;

	INP_INFO_RLOCK_ASSERT(&V_tcbinfo);

	/*
	 * Ok, create the full blown connection, and set things up
	 * as they would have been set up if we had created the
	 * connection when the SYN arrived.  If we can't create
	 * the connection, abort it.
	 */
	so = sonewconn(lso, 0);
	if (so == NULL) {
		/*
		 * Drop the connection; we will either send a RST or
		 * have the peer retransmit its SYN again after its
		 * RTO and try again.
		 */
		TCPSTAT_INC(tcps_listendrop);
		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
			log(LOG_DEBUG, "%s; %s: Socket create failed "
			    "due to limits or memory shortage\n",
			    s, __func__);
			free(s, M_TCPLOG);
		}
		goto abort2;
	}
#ifdef MAC
	mac_socketpeer_set_from_mbuf(m, so);
#endif

	inp = sotoinpcb(so);
	inp->inp_inc.inc_fibnum = so->so_fibnum;
	INP_WLOCK(inp);
	/*
	 * Exclusive pcbinfo lock is not required in syncache socket case even
	 * if two inpcb locks can be acquired simultaneously:
	 *  - the inpcb in LISTEN state,
	 *  - the newly created inp.
	 *
	 * In this case, an inp cannot be at same time in LISTEN state and
	 * just created by an accept() call.
	 */
	INP_HASH_WLOCK(&V_tcbinfo);

	/* Insert new socket into PCB hash list. */
	inp->inp_inc.inc_flags = sc->sc_inc.inc_flags;
#ifdef INET6
	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
		inp->inp_vflag &= ~INP_IPV4;
		inp->inp_vflag |= INP_IPV6;
		inp->in6p_laddr = sc->sc_inc.inc6_laddr;
	} else {
		inp->inp_vflag &= ~INP_IPV6;
		inp->inp_vflag |= INP_IPV4;
#endif
		inp->inp_laddr = sc->sc_inc.inc_laddr;
#ifdef INET6
	}
#endif

	/*
	 * If there's an mbuf and it has a flowid, then let's initialise the
	 * inp with that particular flowid.
	 */
	if (m != NULL && M_HASHTYPE_GET(m) != M_HASHTYPE_NONE) {
		inp->inp_flowid = m->m_pkthdr.flowid;
		inp->inp_flowtype = M_HASHTYPE_GET(m);
	}

	/*
	 * Install in the reservation hash table for now, but don't yet
	 * install a connection group since the full 4-tuple isn't yet
	 * configured.
	 */
	inp->inp_lport = sc->sc_inc.inc_lport;
	if ((error = in_pcbinshash_nopcbgroup(inp)) != 0) {
		/*
		 * Undo the assignments above if we failed to
		 * put the PCB on the hash lists.
		 */
#ifdef INET6
		if (sc->sc_inc.inc_flags & INC_ISIPV6)
			inp->in6p_laddr = in6addr_any;
		else
#endif
			inp->inp_laddr.s_addr = INADDR_ANY;
		inp->inp_lport = 0;
		if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
			log(LOG_DEBUG, "%s; %s: in_pcbinshash failed "
			    "with error %i\n",
			    s, __func__, error);
			free(s, M_TCPLOG);
		}
		INP_HASH_WUNLOCK(&V_tcbinfo);
		goto abort;
	}
#ifdef INET6
	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
		struct inpcb *oinp = sotoinpcb(lso);
		struct in6_addr laddr6;
		struct sockaddr_in6 sin6;
		/*
		 * Inherit socket options from the listening socket.
		 * Note that in6p_inputopts are not (and should not be)
		 * copied, since it stores previously received options and is
		 * used to detect if each new option is different than the
		 * previous one and hence should be passed to a user.
		 * If we copied in6p_inputopts, a user would not be able to
		 * receive options just after calling the accept system call.
		 */
		inp->inp_flags |= oinp->inp_flags & INP_CONTROLOPTS;
		if (oinp->in6p_outputopts)
			inp->in6p_outputopts =
			    ip6_copypktopts(oinp->in6p_outputopts, M_NOWAIT);

		sin6.sin6_family = AF_INET6;
		sin6.sin6_len = sizeof(sin6);
		sin6.sin6_addr = sc->sc_inc.inc6_faddr;
		sin6.sin6_port = sc->sc_inc.inc_fport;
		sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
		laddr6 = inp->in6p_laddr;
		if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
			inp->in6p_laddr = sc->sc_inc.inc6_laddr;
		if ((error = in6_pcbconnect_mbuf(inp, (struct sockaddr *)&sin6,
		    thread0.td_ucred, m)) != 0) {
			inp->in6p_laddr = laddr6;
			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
				log(LOG_DEBUG, "%s; %s: in6_pcbconnect failed "
				    "with error %i\n",
				    s, __func__, error);
				free(s, M_TCPLOG);
			}
			INP_HASH_WUNLOCK(&V_tcbinfo);
			goto abort;
		}
		/* Override flowlabel from in6_pcbconnect. */
		inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
		inp->inp_flow |= sc->sc_flowlabel;
	}
#endif /* INET6 */
#if defined(INET) && defined(INET6)
	else
#endif
#ifdef INET
	{
		struct in_addr laddr;
		struct sockaddr_in sin;

		inp->inp_options = (m) ? ip_srcroute(m) : NULL;

		if (inp->inp_options == NULL) {
			inp->inp_options = sc->sc_ipopts;
			sc->sc_ipopts = NULL;
		}

		sin.sin_family = AF_INET;
		sin.sin_len = sizeof(sin);
		sin.sin_addr = sc->sc_inc.inc_faddr;
		sin.sin_port = sc->sc_inc.inc_fport;
		bzero((caddr_t)sin.sin_zero, sizeof(sin.sin_zero));
		laddr = inp->inp_laddr;
		if (inp->inp_laddr.s_addr == INADDR_ANY)
			inp->inp_laddr = sc->sc_inc.inc_laddr;
		if ((error = in_pcbconnect_mbuf(inp, (struct sockaddr *)&sin,
		    thread0.td_ucred, m)) != 0) {
			inp->inp_laddr = laddr;
			if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
				log(LOG_DEBUG, "%s; %s: in_pcbconnect failed "
				    "with error %i\n",
				    s, __func__, error);
				free(s, M_TCPLOG);
			}
			INP_HASH_WUNLOCK(&V_tcbinfo);
			goto abort;
		}
	}
#endif /* INET */
#if defined(IPSEC) || defined(IPSEC_SUPPORT)
	/* Copy old policy into new socket's. */
	if (ipsec_copy_pcbpolicy(sotoinpcb(lso), inp) != 0)
		printf("syncache_socket: could not copy policy\n");
#endif
	INP_HASH_WUNLOCK(&V_tcbinfo);
	tp = intotcpcb(inp);
	tcp_state_change(tp, TCPS_SYN_RECEIVED);
	tp->iss = sc->sc_iss;
	tp->irs = sc->sc_irs;
	tcp_rcvseqinit(tp);
	tcp_sendseqinit(tp);
	blk = sototcpcb(lso)->t_fb;
	if (blk != tp->t_fb) {
		/*
		 * Our parents t_fb was not the default,
		 * we need to release our ref on tp->t_fb and
		 * pickup one on the new entry.
		 */
		struct tcp_function_block *rblk;

		rblk = find_and_ref_tcp_fb(blk);
		KASSERT(rblk != NULL,
		    ("cannot find blk %p out of syncache?", blk));
		if (tp->t_fb->tfb_tcp_fb_fini)
			(*tp->t_fb->tfb_tcp_fb_fini)(tp);
		refcount_release(&tp->t_fb->tfb_refcnt);
		tp->t_fb = rblk;
		if (tp->t_fb->tfb_tcp_fb_init) {
			(*tp->t_fb->tfb_tcp_fb_init)(tp);
		}
	}
	tp->snd_wl1 = sc->sc_irs;
	tp->snd_max = tp->iss + 1;
	tp->snd_nxt = tp->iss + 1;
	tp->rcv_up = sc->sc_irs + 1;
	tp->rcv_wnd = sc->sc_wnd;
	tp->rcv_adv += tp->rcv_wnd;
	tp->last_ack_sent = tp->rcv_nxt;

	tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH|TF_NODELAY);
	if (sc->sc_flags & SCF_NOOPT)
		tp->t_flags |= TF_NOOPT;
	else {
		if (sc->sc_flags & SCF_WINSCALE) {
			tp->t_flags |= TF_REQ_SCALE|TF_RCVD_SCALE;
			tp->snd_scale = sc->sc_requested_s_scale;
			tp->request_r_scale = sc->sc_requested_r_scale;
		}
		if (sc->sc_flags & SCF_TIMESTAMP) {
			tp->t_flags |= TF_REQ_TSTMP|TF_RCVD_TSTMP;
			tp->ts_recent = sc->sc_tsreflect;
			tp->ts_recent_age = tcp_ts_getticks();
			tp->ts_offset = sc->sc_tsoff;
		}
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
		if (sc->sc_flags & SCF_SIGNATURE)
			tp->t_flags |= TF_SIGNATURE;
#endif
		if (sc->sc_flags & SCF_SACK)
			tp->t_flags |= TF_SACK_PERMIT;
	}

	if (sc->sc_flags & SCF_ECN)
		tp->t_flags |= TF_ECN_PERMIT;

	/*
	 * Set up MSS and get cached values from tcp_hostcache.
	 * This might overwrite some of the defaults we just set.
	 */
	tcp_mss(tp, sc->sc_peer_mss);

	/*
	 * If the SYN,ACK was retransmitted, indicate that CWND to be
	 * limited to one segment in cc_conn_init().
	 * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
	 */
	if (sc->sc_rxmits > 1)
		tp->snd_cwnd = 1;

#ifdef TCP_OFFLOAD
	/*
	 * Allow a TOE driver to install its hooks.  Note that we hold the
	 * pcbinfo lock too and that prevents tcp_usr_accept from accepting a
	 * new connection before the TOE driver has done its thing.
	 */
	if (ADDED_BY_TOE(sc)) {
		struct toedev *tod = sc->sc_tod;

		tod->tod_offload_socket(tod, sc->sc_todctx, so);
	}
#endif
	/*
	 * Copy and activate timers.
	 */
	tp->t_keepinit = sototcpcb(lso)->t_keepinit;
	tp->t_keepidle = sototcpcb(lso)->t_keepidle;
	tp->t_keepintvl = sototcpcb(lso)->t_keepintvl;
	tp->t_keepcnt = sototcpcb(lso)->t_keepcnt;
	tcp_timer_activate(tp, TT_KEEP, TP_KEEPINIT(tp));

	TCPSTAT_INC(tcps_accepts);
	return (so);

abort:
	INP_WUNLOCK(inp);
abort2:
	if (so != NULL)
		soabort(so);
	return (NULL);
}

/*
 * This function gets called when we receive an ACK for a
 * socket in the LISTEN state.  We look up the connection
 * in the syncache, and if its there, we pull it out of
 * the cache and turn it into a full-blown connection in
 * the SYN-RECEIVED state.
 *
 * On syncache_socket() success the newly created socket
 * has its underlying inp locked.
 */
int
syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
    struct socket **lsop, struct mbuf *m)
{
	struct syncache *sc;
	struct syncache_head *sch;
	struct syncache scs;
	char *s;

	/*
	 * Global TCP locks are held because we manipulate the PCB lists
	 * and create a new socket.
	 */
	INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
	KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
	    ("%s: can handle only ACK", __func__));

	sc = syncache_lookup(inc, &sch);	/* returns locked sch */
	SCH_LOCK_ASSERT(sch);

#ifdef INVARIANTS
	/*
	 * Test code for syncookies comparing the syncache stored
	 * values with the reconstructed values from the cookie.
	 */
	if (sc != NULL)
		syncookie_cmp(inc, sch, sc, th, to, *lsop);
#endif

	if (sc == NULL) {
		/*
		 * There is no syncache entry, so see if this ACK is
		 * a returning syncookie.  To do this, first:
		 *  A. Check if syncookies are used in case of syncache
		 *     overflows
		 *  B. See if this socket has had a syncache entry dropped in
		 *     the recent past. We don't want to accept a bogus
		 *     syncookie if we've never received a SYN or accept it
		 *     twice.
		 *  C. check that the syncookie is valid.  If it is, then
		 *     cobble up a fake syncache entry, and return.
		 */
		if (!V_tcp_syncookies) {
			SCH_UNLOCK(sch);
			if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
				log(LOG_DEBUG, "%s; %s: Spurious ACK, "
				    "segment rejected (syncookies disabled)\n",
				    s, __func__);
			goto failed;
		}
		if (!V_tcp_syncookiesonly &&
		    sch->sch_last_overflow < time_uptime - SYNCOOKIE_LIFETIME) {
			SCH_UNLOCK(sch);
			if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
				log(LOG_DEBUG, "%s; %s: Spurious ACK, "
				    "segment rejected (no syncache entry)\n",
				    s, __func__);
			goto failed;
		}
		bzero(&scs, sizeof(scs));
		sc = syncookie_lookup(inc, sch, &scs, th, to, *lsop);
		SCH_UNLOCK(sch);
		if (sc == NULL) {
			if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
				log(LOG_DEBUG, "%s; %s: Segment failed "
				    "SYNCOOKIE authentication, segment rejected "
				    "(probably spoofed)\n", s, __func__);
			goto failed;
		}
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
		/* If received ACK has MD5 signature, check it. */
		if ((to->to_flags & TOF_SIGNATURE) != 0 &&
		    (!TCPMD5_ENABLED() ||
		    TCPMD5_INPUT(m, th, to->to_signature) != 0)) {
			/* Drop the ACK. */
			if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
				log(LOG_DEBUG, "%s; %s: Segment rejected, "
				    "MD5 signature doesn't match.\n",
				    s, __func__);
				free(s, M_TCPLOG);
			}
			TCPSTAT_INC(tcps_sig_err_sigopt);
			return (-1); /* Do not send RST */
		}
#endif /* TCP_SIGNATURE */
	} else {
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
		/*
		 * If listening socket requested TCP digests, check that
		 * received ACK has signature and it is correct.
		 * If not, drop the ACK and leave sc entry in th cache,
		 * because SYN was received with correct signature.
		 */
		if (sc->sc_flags & SCF_SIGNATURE) {
			if ((to->to_flags & TOF_SIGNATURE) == 0) {
				/* No signature */
				TCPSTAT_INC(tcps_sig_err_nosigopt);
				SCH_UNLOCK(sch);
				if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
					log(LOG_DEBUG, "%s; %s: Segment "
					    "rejected, MD5 signature wasn't "
					    "provided.\n", s, __func__);
					free(s, M_TCPLOG);
				}
				return (-1); /* Do not send RST */
			}
			if (!TCPMD5_ENABLED() ||
			    TCPMD5_INPUT(m, th, to->to_signature) != 0) {
				/* Doesn't match or no SA */
				SCH_UNLOCK(sch);
				if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
					log(LOG_DEBUG, "%s; %s: Segment "
					    "rejected, MD5 signature doesn't "
					    "match.\n", s, __func__);
					free(s, M_TCPLOG);
				}
				return (-1); /* Do not send RST */
			}
		}
#endif /* TCP_SIGNATURE */
		/*
		 * Pull out the entry to unlock the bucket row.
		 *
		 * NOTE: We must decrease TCPS_SYN_RECEIVED count here, not
		 * tcp_state_change().  The tcpcb is not existent at this
		 * moment.  A new one will be allocated via syncache_socket->
		 * sonewconn->tcp_usr_attach in TCPS_CLOSED state, then
		 * syncache_socket() will change it to TCPS_SYN_RECEIVED.
		 */
		TCPSTATES_DEC(TCPS_SYN_RECEIVED);
		TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
		sch->sch_length--;
#ifdef TCP_OFFLOAD
		if (ADDED_BY_TOE(sc)) {
			struct toedev *tod = sc->sc_tod;

			tod->tod_syncache_removed(tod, sc->sc_todctx);
		}
#endif
		SCH_UNLOCK(sch);
	}

	/*
	 * Segment validation:
	 * ACK must match our initial sequence number + 1 (the SYN|ACK).
	 */
	if (th->th_ack != sc->sc_iss + 1) {
		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
			log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "
			    "rejected\n", s, __func__, th->th_ack, sc->sc_iss);
		goto failed;
	}

	/*
	 * The SEQ must fall in the window starting at the received
	 * initial receive sequence number + 1 (the SYN).
	 */
	if (SEQ_LEQ(th->th_seq, sc->sc_irs) ||
	    SEQ_GT(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
			log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u, segment "
			    "rejected\n", s, __func__, th->th_seq, sc->sc_irs);
		goto failed;
	}

	/*
	 * If timestamps were not negotiated during SYN/ACK they
	 * must not appear on any segment during this session.
	 */
	if (!(sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS)) {
		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
			log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
			    "segment rejected\n", s, __func__);
		goto failed;
	}

	/*
	 * If timestamps were negotiated during SYN/ACK they should
	 * appear on every segment during this session.
	 * XXXAO: This is only informal as there have been unverified
	 * reports of non-compliants stacks.
	 */
	if ((sc->sc_flags & SCF_TIMESTAMP) && !(to->to_flags & TOF_TS)) {
		if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
			log(LOG_DEBUG, "%s; %s: Timestamp missing, "
			    "no action\n", s, __func__);
			free(s, M_TCPLOG);
			s = NULL;
		}
	}

	/*
	 * If timestamps were negotiated, the reflected timestamp
	 * must be equal to what we actually sent in the SYN|ACK
	 * except in the case of 0. Some boxes are known for sending
	 * broken timestamp replies during the 3whs (and potentially
	 * during the connection also).
	 *
	 * Accept the final ACK of 3whs with reflected timestamp of 0
	 * instead of sending a RST and deleting the syncache entry.
	 */
	if ((to->to_flags & TOF_TS) && to->to_tsecr &&
	    to->to_tsecr != sc->sc_ts) {
		if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
			log(LOG_DEBUG, "%s; %s: TSECR %u != TS %u, "
			    "segment rejected\n",
			    s, __func__, to->to_tsecr, sc->sc_ts);
		goto failed;
	}

	*lsop = syncache_socket(sc, *lsop, m);

	if (*lsop == NULL)
		TCPSTAT_INC(tcps_sc_aborted);
	else
		TCPSTAT_INC(tcps_sc_completed);

/* how do we find the inp for the new socket? */
	if (sc != &scs)
		syncache_free(sc);
	return (1);
failed:
	if (sc != NULL && sc != &scs)
		syncache_free(sc);
	if (s != NULL)
		free(s, M_TCPLOG);
	*lsop = NULL;
	return (0);
}

#ifdef TCP_RFC7413
static void
syncache_tfo_expand(struct syncache *sc, struct socket **lsop, struct mbuf *m,
    uint64_t response_cookie)
{
	struct inpcb *inp;
	struct tcpcb *tp;
	unsigned int *pending_counter;

	/*
	 * Global TCP locks are held because we manipulate the PCB lists
	 * and create a new socket.
	 */
	INP_INFO_RLOCK_ASSERT(&V_tcbinfo);

	pending_counter = intotcpcb(sotoinpcb(*lsop))->t_tfo_pending;
	*lsop = syncache_socket(sc, *lsop, m);
	if (*lsop == NULL) {
		TCPSTAT_INC(tcps_sc_aborted);
		atomic_subtract_int(pending_counter, 1);
	} else {
		inp = sotoinpcb(*lsop);
		tp = intotcpcb(inp);
		tp->t_flags |= TF_FASTOPEN;
		tp->t_tfo_cookie = response_cookie;
		tp->snd_max = tp->iss;
		tp->snd_nxt = tp->iss;
		tp->t_tfo_pending = pending_counter;
		TCPSTAT_INC(tcps_sc_completed);
	}
}
#endif /* TCP_RFC7413 */

/*
 * Given a LISTEN socket and an inbound SYN request, add
 * this to the syn cache, and send back a segment:
 *	<SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
 * to the source.
 *
 * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
 * Doing so would require that we hold onto the data and deliver it
 * to the application.  However, if we are the target of a SYN-flood
 * DoS attack, an attacker could send data which would eventually
 * consume all available buffer space if it were ACKed.  By not ACKing
 * the data, we avoid this DoS scenario.
 *
 * The exception to the above is when a SYN with a valid TCP Fast Open (TFO)
 * cookie is processed, V_tcp_fastopen_enabled set to true, and the
 * TCP_FASTOPEN socket option is set.  In this case, a new socket is created
 * and returned via lsop, the mbuf is not freed so that tcp_input() can
 * queue its data to the socket, and 1 is returned to indicate the
 * TFO-socket-creation path was taken.
 */
int
syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
    struct inpcb *inp, struct socket **lsop, struct mbuf *m, void *tod,
    void *todctx)
{
	struct tcpcb *tp;
	struct socket *so;
	struct syncache *sc = NULL;
	struct syncache_head *sch;
	struct mbuf *ipopts = NULL;
	u_int ltflags;
	int win, sb_hiwat, ip_ttl, ip_tos;
	char *s;
	int rv = 0;
#ifdef INET6
	int autoflowlabel = 0;
#endif
#ifdef MAC
	struct label *maclabel;
#endif
	struct syncache scs;
	struct ucred *cred;
#ifdef TCP_RFC7413
	uint64_t tfo_response_cookie;
	int tfo_cookie_valid = 0;
	int tfo_response_cookie_valid = 0;
#endif

	INP_WLOCK_ASSERT(inp);			/* listen socket */
	KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN,
	    ("%s: unexpected tcp flags", __func__));

	/*
	 * Combine all so/tp operations very early to drop the INP lock as
	 * soon as possible.
	 */
	so = *lsop;
	tp = sototcpcb(so);
	cred = crhold(so->so_cred);

#ifdef INET6
	if ((inc->inc_flags & INC_ISIPV6) &&
	    (inp->inp_flags & IN6P_AUTOFLOWLABEL))
		autoflowlabel = 1;
#endif
	ip_ttl = inp->inp_ip_ttl;
	ip_tos = inp->inp_ip_tos;
	win = sbspace(&so->so_rcv);
	sb_hiwat = so->so_rcv.sb_hiwat;
	ltflags = (tp->t_flags & (TF_NOOPT | TF_SIGNATURE));

#ifdef TCP_RFC7413
	if (V_tcp_fastopen_enabled && (tp->t_flags & TF_FASTOPEN) &&
	    (tp->t_tfo_pending != NULL) && (to->to_flags & TOF_FASTOPEN)) {
		/*
		 * Limit the number of pending TFO connections to
		 * approximately half of the queue limit.  This prevents TFO
		 * SYN floods from starving the service by filling the
		 * listen queue with bogus TFO connections.
		 */
		if (atomic_fetchadd_int(tp->t_tfo_pending, 1) <=
		    (so->so_qlimit / 2)) {
			int result;

			result = tcp_fastopen_check_cookie(inc,
			    to->to_tfo_cookie, to->to_tfo_len,
			    &tfo_response_cookie);
			tfo_cookie_valid = (result > 0);
			tfo_response_cookie_valid = (result >= 0);
		} else
			atomic_subtract_int(tp->t_tfo_pending, 1);
	}
#endif

	/* By the time we drop the lock these should no longer be used. */
	so = NULL;
	tp = NULL;

#ifdef MAC
	if (mac_syncache_init(&maclabel) != 0) {
		INP_WUNLOCK(inp);
		goto done;
	} else
		mac_syncache_create(maclabel, inp);
#endif
#ifdef TCP_RFC7413
	if (!tfo_cookie_valid)
#endif
		INP_WUNLOCK(inp);

	/*
	 * Remember the IP options, if any.
	 */
#ifdef INET6
	if (!(inc->inc_flags & INC_ISIPV6))
#endif
#ifdef INET
		ipopts = (m) ? ip_srcroute(m) : NULL;
#else
		ipopts = NULL;
#endif

#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
	/*
	 * If listening socket requested TCP digests, check that received
	 * SYN has signature and it is correct. If signature doesn't match
	 * or TCP_SIGNATURE support isn't enabled, drop the packet.
	 */
	if (ltflags & TF_SIGNATURE) {
		if ((to->to_flags & TOF_SIGNATURE) == 0) {
			TCPSTAT_INC(tcps_sig_err_nosigopt);
			goto done;
		}
		if (!TCPMD5_ENABLED() ||
		    TCPMD5_INPUT(m, th, to->to_signature) != 0)
			goto done;
	}
#endif	/* TCP_SIGNATURE */
	/*
	 * See if we already have an entry for this connection.
	 * If we do, resend the SYN,ACK, and reset the retransmit timer.
	 *
	 * XXX: should the syncache be re-initialized with the contents
	 * of the new SYN here (which may have different options?)
	 *
	 * XXX: We do not check the sequence number to see if this is a
	 * real retransmit or a new connection attempt.  The question is
	 * how to handle such a case; either ignore it as spoofed, or
	 * drop the current entry and create a new one?
	 */
	sc = syncache_lookup(inc, &sch);	/* returns locked entry */
	SCH_LOCK_ASSERT(sch);
	if (sc != NULL) {
#ifdef TCP_RFC7413
		if (tfo_cookie_valid)
			INP_WUNLOCK(inp);
#endif
		TCPSTAT_INC(tcps_sc_dupsyn);
		if (ipopts) {
			/*
			 * If we were remembering a previous source route,
			 * forget it and use the new one we've been given.
			 */
			if (sc->sc_ipopts)
				(void) m_free(sc->sc_ipopts);
			sc->sc_ipopts = ipopts;
		}
		/*
		 * Update timestamp if present.
		 */
		if ((sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS))
			sc->sc_tsreflect = to->to_tsval;
		else
			sc->sc_flags &= ~SCF_TIMESTAMP;
#ifdef MAC
		/*
		 * Since we have already unconditionally allocated label
		 * storage, free it up.  The syncache entry will already
		 * have an initialized label we can use.
		 */
		mac_syncache_destroy(&maclabel);
#endif
		/* Retransmit SYN|ACK and reset retransmit count. */
		if ((s = tcp_log_addrs(&sc->sc_inc, th, NULL, NULL))) {
			log(LOG_DEBUG, "%s; %s: Received duplicate SYN, "
			    "resetting timer and retransmitting SYN|ACK\n",
			    s, __func__);
			free(s, M_TCPLOG);
		}
		if (syncache_respond(sc, sch, 1, m) == 0) {
			sc->sc_rxmits = 0;
			syncache_timeout(sc, sch, 1);
			TCPSTAT_INC(tcps_sndacks);
			TCPSTAT_INC(tcps_sndtotal);
		}
		SCH_UNLOCK(sch);
		goto done;
	}

#ifdef TCP_RFC7413
	if (tfo_cookie_valid) {
		bzero(&scs, sizeof(scs));
		sc = &scs;
		goto skip_alloc;
	}
#endif

	sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
	if (sc == NULL) {
		/*
		 * The zone allocator couldn't provide more entries.
		 * Treat this as if the cache was full; drop the oldest
		 * entry and insert the new one.
		 */
		TCPSTAT_INC(tcps_sc_zonefail);
		if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL) {
			sch->sch_last_overflow = time_uptime;
			syncache_drop(sc, sch);
		}
		sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
		if (sc == NULL) {
			if (V_tcp_syncookies) {
				bzero(&scs, sizeof(scs));
				sc = &scs;
			} else {
				SCH_UNLOCK(sch);
				if (ipopts)
					(void) m_free(ipopts);
				goto done;
			}
		}
	}

#ifdef TCP_RFC7413
skip_alloc:
	if (!tfo_cookie_valid && tfo_response_cookie_valid)
		sc->sc_tfo_cookie = &tfo_response_cookie;
#endif

	/*
	 * Fill in the syncache values.
	 */
#ifdef MAC
	sc->sc_label = maclabel;
#endif
	sc->sc_cred = cred;
	cred = NULL;
	sc->sc_ipopts = ipopts;
	bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
#ifdef INET6
	if (!(inc->inc_flags & INC_ISIPV6))
#endif
	{
		sc->sc_ip_tos = ip_tos;
		sc->sc_ip_ttl = ip_ttl;
	}
#ifdef TCP_OFFLOAD
	sc->sc_tod = tod;
	sc->sc_todctx = todctx;
#endif
	sc->sc_irs = th->th_seq;
	sc->sc_iss = arc4random();
	sc->sc_flags = 0;
	sc->sc_flowlabel = 0;

	/*
	 * Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
	 * win was derived from socket earlier in the function.
	 */
	win = imax(win, 0);
	win = imin(win, TCP_MAXWIN);
	sc->sc_wnd = win;

	if (V_tcp_do_rfc1323) {
		/*
		 * A timestamp received in a SYN makes
		 * it ok to send timestamp requests and replies.
		 */
		if (to->to_flags & TOF_TS) {
			sc->sc_tsreflect = to->to_tsval;
			sc->sc_ts = tcp_ts_getticks();
			sc->sc_flags |= SCF_TIMESTAMP;
		}
		if (to->to_flags & TOF_SCALE) {
			int wscale = 0;

			/*
			 * Pick the smallest possible scaling factor that
			 * will still allow us to scale up to sb_max, aka
			 * kern.ipc.maxsockbuf.
			 *
			 * We do this because there are broken firewalls that
			 * will corrupt the window scale option, leading to
			 * the other endpoint believing that our advertised
			 * window is unscaled.  At scale factors larger than
			 * 5 the unscaled window will drop below 1500 bytes,
			 * leading to serious problems when traversing these
			 * broken firewalls.
			 *
			 * With the default maxsockbuf of 256K, a scale factor
			 * of 3 will be chosen by this algorithm.  Those who
			 * choose a larger maxsockbuf should watch out
			 * for the compatibility problems mentioned above.
			 *
			 * RFC1323: The Window field in a SYN (i.e., a <SYN>
			 * or <SYN,ACK>) segment itself is never scaled.
			 */
			while (wscale < TCP_MAX_WINSHIFT &&
			    (TCP_MAXWIN << wscale) < sb_max)
				wscale++;
			sc->sc_requested_r_scale = wscale;
			sc->sc_requested_s_scale = to->to_wscale;
			sc->sc_flags |= SCF_WINSCALE;
		}
	}
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
	/*
	 * If listening socket requested TCP digests, flag this in the
	 * syncache so that syncache_respond() will do the right thing
	 * with the SYN+ACK.
	 */
	if (ltflags & TF_SIGNATURE)
		sc->sc_flags |= SCF_SIGNATURE;
#endif	/* TCP_SIGNATURE */
	if (to->to_flags & TOF_SACKPERM)
		sc->sc_flags |= SCF_SACK;
	if (to->to_flags & TOF_MSS)
		sc->sc_peer_mss = to->to_mss;	/* peer mss may be zero */
	if (ltflags & TF_NOOPT)
		sc->sc_flags |= SCF_NOOPT;
	if ((th->th_flags & (TH_ECE|TH_CWR)) && V_tcp_do_ecn)
		sc->sc_flags |= SCF_ECN;

	if (V_tcp_syncookies)
		sc->sc_iss = syncookie_generate(sch, sc);
#ifdef INET6
	if (autoflowlabel) {
		if (V_tcp_syncookies)
			sc->sc_flowlabel = sc->sc_iss;
		else
			sc->sc_flowlabel = ip6_randomflowlabel();
		sc->sc_flowlabel = htonl(sc->sc_flowlabel) & IPV6_FLOWLABEL_MASK;
	}
#endif
	SCH_UNLOCK(sch);

#ifdef TCP_RFC7413
	if (tfo_cookie_valid) {
		syncache_tfo_expand(sc, lsop, m, tfo_response_cookie);
		/* INP_WUNLOCK(inp) will be performed by the called */
		rv = 1;
		goto tfo_done;
	}
#endif

	/*
	 * Do a standard 3-way handshake.
	 */
	if (syncache_respond(sc, sch, 0, m) == 0) {
		if (V_tcp_syncookies && V_tcp_syncookiesonly && sc != &scs)
			syncache_free(sc);
		else if (sc != &scs)
			syncache_insert(sc, sch);   /* locks and unlocks sch */
		TCPSTAT_INC(tcps_sndacks);
		TCPSTAT_INC(tcps_sndtotal);
	} else {
		if (sc != &scs)
			syncache_free(sc);
		TCPSTAT_INC(tcps_sc_dropped);
	}

done:
	if (m) {
		*lsop = NULL;
		m_freem(m);
	}
#ifdef TCP_RFC7413
tfo_done:
#endif
	if (cred != NULL)
		crfree(cred);
#ifdef MAC
	if (sc == &scs)
		mac_syncache_destroy(&maclabel);
#endif
	return (rv);
}

/*
 * Send SYN|ACK to the peer.  Either in response to the peer's SYN,
 * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
 */
static int
syncache_respond(struct syncache *sc, struct syncache_head *sch, int locked,
    const struct mbuf *m0)
{
	struct ip *ip = NULL;
	struct mbuf *m;
	struct tcphdr *th = NULL;
	int optlen, error = 0;	/* Make compiler happy */
	u_int16_t hlen, tlen, mssopt;
	struct tcpopt to;
#ifdef INET6
	struct ip6_hdr *ip6 = NULL;
#endif
	hlen =
#ifdef INET6
	       (sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
#endif
		sizeof(struct ip);
	tlen = hlen + sizeof(struct tcphdr);

	/* Determine MSS we advertize to other end of connection. */
	mssopt = max(tcp_mssopt(&sc->sc_inc), V_tcp_minmss);

	/* XXX: Assume that the entire packet will fit in a header mbuf. */
	KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
	    ("syncache: mbuf too small"));

	/* Create the IP+TCP header from scratch. */
	m = m_gethdr(M_NOWAIT, MT_DATA);
	if (m == NULL)
		return (ENOBUFS);
#ifdef MAC
	mac_syncache_create_mbuf(sc->sc_label, m);
#endif
	m->m_data += max_linkhdr;
	m->m_len = tlen;
	m->m_pkthdr.len = tlen;
	m->m_pkthdr.rcvif = NULL;

#ifdef INET6
	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
		ip6 = mtod(m, struct ip6_hdr *);
		ip6->ip6_vfc = IPV6_VERSION;
		ip6->ip6_nxt = IPPROTO_TCP;
		ip6->ip6_src = sc->sc_inc.inc6_laddr;
		ip6->ip6_dst = sc->sc_inc.inc6_faddr;
		ip6->ip6_plen = htons(tlen - hlen);
		/* ip6_hlim is set after checksum */
		ip6->ip6_flow &= ~IPV6_FLOWLABEL_MASK;
		ip6->ip6_flow |= sc->sc_flowlabel;

		th = (struct tcphdr *)(ip6 + 1);
	}
#endif
#if defined(INET6) && defined(INET)
	else
#endif
#ifdef INET
	{
		ip = mtod(m, struct ip *);
		ip->ip_v = IPVERSION;
		ip->ip_hl = sizeof(struct ip) >> 2;
		ip->ip_len = htons(tlen);
		ip->ip_id = 0;
		ip->ip_off = 0;
		ip->ip_sum = 0;
		ip->ip_p = IPPROTO_TCP;
		ip->ip_src = sc->sc_inc.inc_laddr;
		ip->ip_dst = sc->sc_inc.inc_faddr;
		ip->ip_ttl = sc->sc_ip_ttl;
		ip->ip_tos = sc->sc_ip_tos;

		/*
		 * See if we should do MTU discovery.  Route lookups are
		 * expensive, so we will only unset the DF bit if:
		 *
		 *	1) path_mtu_discovery is disabled
		 *	2) the SCF_UNREACH flag has been set
		 */
		if (V_path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
		       ip->ip_off |= htons(IP_DF);

		th = (struct tcphdr *)(ip + 1);
	}
#endif /* INET */
	th->th_sport = sc->sc_inc.inc_lport;
	th->th_dport = sc->sc_inc.inc_fport;

	th->th_seq = htonl(sc->sc_iss);
	th->th_ack = htonl(sc->sc_irs + 1);
	th->th_off = sizeof(struct tcphdr) >> 2;
	th->th_x2 = 0;
	th->th_flags = TH_SYN|TH_ACK;
	th->th_win = htons(sc->sc_wnd);
	th->th_urp = 0;

	if (sc->sc_flags & SCF_ECN) {
		th->th_flags |= TH_ECE;
		TCPSTAT_INC(tcps_ecn_shs);
	}

	/* Tack on the TCP options. */
	if ((sc->sc_flags & SCF_NOOPT) == 0) {
		to.to_flags = 0;

		to.to_mss = mssopt;
		to.to_flags = TOF_MSS;
		if (sc->sc_flags & SCF_WINSCALE) {
			to.to_wscale = sc->sc_requested_r_scale;
			to.to_flags |= TOF_SCALE;
		}
		if (sc->sc_flags & SCF_TIMESTAMP) {
			/* Virgin timestamp or TCP cookie enhanced one. */
			to.to_tsval = sc->sc_ts;
			to.to_tsecr = sc->sc_tsreflect;
			to.to_flags |= TOF_TS;
		}
		if (sc->sc_flags & SCF_SACK)
			to.to_flags |= TOF_SACKPERM;
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
		if (sc->sc_flags & SCF_SIGNATURE)
			to.to_flags |= TOF_SIGNATURE;
#endif
#ifdef TCP_RFC7413
		if (sc->sc_tfo_cookie) {
			to.to_flags |= TOF_FASTOPEN;
			to.to_tfo_len = TCP_FASTOPEN_COOKIE_LEN;
			to.to_tfo_cookie = sc->sc_tfo_cookie;
			/* don't send cookie again when retransmitting response */
			sc->sc_tfo_cookie = NULL;
		}
#endif
		optlen = tcp_addoptions(&to, (u_char *)(th + 1));

		/* Adjust headers by option size. */
		th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
		m->m_len += optlen;
		m->m_pkthdr.len += optlen;
#ifdef INET6
		if (sc->sc_inc.inc_flags & INC_ISIPV6)
			ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
		else
#endif
			ip->ip_len = htons(ntohs(ip->ip_len) + optlen);
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
		if (sc->sc_flags & SCF_SIGNATURE) {
			KASSERT(to.to_flags & TOF_SIGNATURE,
			    ("tcp_addoptions() didn't set tcp_signature"));

			/* NOTE: to.to_signature is inside of mbuf */
			if (!TCPMD5_ENABLED() ||
			    TCPMD5_OUTPUT(m, th, to.to_signature) != 0) {
				m_freem(m);
				return (EACCES);
			}
		}
#endif
	} else
		optlen = 0;

	M_SETFIB(m, sc->sc_inc.inc_fibnum);
	m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
	/*
	 * If we have peer's SYN and it has a flowid, then let's assign it to
	 * our SYN|ACK.  ip6_output() and ip_output() will not assign flowid
	 * to SYN|ACK due to lack of inp here.
	 */
	if (m0 != NULL && M_HASHTYPE_GET(m0) != M_HASHTYPE_NONE) {
		m->m_pkthdr.flowid = m0->m_pkthdr.flowid;
		M_HASHTYPE_SET(m, M_HASHTYPE_GET(m0));
	}
#ifdef INET6
	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
		m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
		th->th_sum = in6_cksum_pseudo(ip6, tlen + optlen - hlen,
		    IPPROTO_TCP, 0);
		ip6->ip6_hlim = in6_selecthlim(NULL, NULL);
#ifdef TCP_OFFLOAD
		if (ADDED_BY_TOE(sc)) {
			struct toedev *tod = sc->sc_tod;

			error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);

			return (error);
		}
#endif
		error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
	}
#endif
#if defined(INET6) && defined(INET)
	else
#endif
#ifdef INET
	{
		m->m_pkthdr.csum_flags = CSUM_TCP;
		th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
		    htons(tlen + optlen - hlen + IPPROTO_TCP));
#ifdef TCP_OFFLOAD
		if (ADDED_BY_TOE(sc)) {
			struct toedev *tod = sc->sc_tod;

			error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);

			return (error);
		}
#endif
		error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
	}
#endif
	return (error);
}

/*
 * The purpose of syncookies is to handle spoofed SYN flooding DoS attacks
 * that exceed the capacity of the syncache by avoiding the storage of any
 * of the SYNs we receive.  Syncookies defend against blind SYN flooding
 * attacks where the attacker does not have access to our responses.
 *
 * Syncookies encode and include all necessary information about the
 * connection setup within the SYN|ACK that we send back.  That way we
 * can avoid keeping any local state until the ACK to our SYN|ACK returns
 * (if ever).  Normally the syncache and syncookies are running in parallel
 * with the latter taking over when the former is exhausted.  When matching
 * syncache entry is found the syncookie is ignored.
 *
 * The only reliable information persisting the 3WHS is our initial sequence
 * number ISS of 32 bits.  Syncookies embed a cryptographically sufficient
 * strong hash (MAC) value and a few bits of TCP SYN options in the ISS
 * of our SYN|ACK.  The MAC can be recomputed when the ACK to our SYN|ACK
 * returns and signifies a legitimate connection if it matches the ACK.
 *
 * The available space of 32 bits to store the hash and to encode the SYN
 * option information is very tight and we should have at least 24 bits for
 * the MAC to keep the number of guesses by blind spoofing reasonably high.
 *
 * SYN option information we have to encode to fully restore a connection:
 * MSS: is imporant to chose an optimal segment size to avoid IP level
 *   fragmentation along the path.  The common MSS values can be encoded
 *   in a 3-bit table.  Uncommon values are captured by the next lower value
 *   in the table leading to a slight increase in packetization overhead.
 * WSCALE: is necessary to allow large windows to be used for high delay-
 *   bandwidth product links.  Not scaling the window when it was initially
 *   negotiated is bad for performance as lack of scaling further decreases
 *   the apparent available send window.  We only need to encode the WSCALE
 *   we received from the remote end.  Our end can be recalculated at any
 *   time.  The common WSCALE values can be encoded in a 3-bit table.
 *   Uncommon values are captured by the next lower value in the table
 *   making us under-estimate the available window size halving our
 *   theoretically possible maximum throughput for that connection.
 * SACK: Greatly assists in packet loss recovery and requires 1 bit.
 * TIMESTAMP and SIGNATURE is not encoded because they are permanent options
 *   that are included in all segments on a connection.  We enable them when
 *   the ACK has them.
 *
 * Security of syncookies and attack vectors:
 *
 * The MAC is computed over (faddr||laddr||fport||lport||irs||flags||secmod)
 * together with the gloabl secret to make it unique per connection attempt.
 * Thus any change of any of those parameters results in a different MAC output
 * in an unpredictable way unless a collision is encountered.  24 bits of the
 * MAC are embedded into the ISS.
 *
 * To prevent replay attacks two rotating global secrets are updated with a
 * new random value every 15 seconds.  The life-time of a syncookie is thus
 * 15-30 seconds.
 *
 * Vector 1: Attacking the secret.  This requires finding a weakness in the
 * MAC itself or the way it is used here.  The attacker can do a chosen plain
 * text attack by varying and testing the all parameters under his control.
 * The strength depends on the size and randomness of the secret, and the
 * cryptographic security of the MAC function.  Due to the constant updating
 * of the secret the attacker has at most 29.999 seconds to find the secret
 * and launch spoofed connections.  After that he has to start all over again.
 *
 * Vector 2: Collision attack on the MAC of a single ACK.  With a 24 bit MAC
 * size an average of 4,823 attempts are required for a 50% chance of success
 * to spoof a single syncookie (birthday collision paradox).  However the
 * attacker is blind and doesn't know if one of his attempts succeeded unless
 * he has a side channel to interfere success from.  A single connection setup
 * success average of 90% requires 8,790 packets, 99.99% requires 17,578 packets.
 * This many attempts are required for each one blind spoofed connection.  For
 * every additional spoofed connection he has to launch another N attempts.
 * Thus for a sustained rate 100 spoofed connections per second approximately
 * 1,800,000 packets per second would have to be sent.
 *
 * NB: The MAC function should be fast so that it doesn't become a CPU
 * exhaustion attack vector itself.
 *
 * References:
 *  RFC4987 TCP SYN Flooding Attacks and Common Mitigations
 *  SYN cookies were first proposed by cryptographer Dan J. Bernstein in 1996
 *   http://cr.yp.to/syncookies.html    (overview)
 *   http://cr.yp.to/syncookies/archive (details)
 *
 *
 * Schematic construction of a syncookie enabled Initial Sequence Number:
 *  0        1         2         3
 *  12345678901234567890123456789012
 * |xxxxxxxxxxxxxxxxxxxxxxxxWWWMMMSP|
 *
 *  x 24 MAC (truncated)
 *  W  3 Send Window Scale index
 *  M  3 MSS index
 *  S  1 SACK permitted
 *  P  1 Odd/even secret
 */

/*
 * Distribution and probability of certain MSS values.  Those in between are
 * rounded down to the next lower one.
 * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
 *                            .2%  .3%   5%    7%    7%    20%   15%   45%
 */
static int tcp_sc_msstab[] = { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };

/*
 * Distribution and probability of certain WSCALE values.  We have to map the
 * (send) window scale (shift) option with a range of 0-14 from 4 bits into 3
 * bits based on prevalence of certain values.  Where we don't have an exact
 * match for are rounded down to the next lower one letting us under-estimate
 * the true available window.  At the moment this would happen only for the
 * very uncommon values 3, 5 and those above 8 (more than 16MB socket buffer
 * and window size).  The absence of the WSCALE option (no scaling in either
 * direction) is encoded with index zero.
 * [WSCALE values histograms, Allman, 2012]
 *                            X 10 10 35  5  6 14 10%   by host
 *                            X 11  4  5  5 18 49  3%   by connections
 */
static int tcp_sc_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };

/*
 * Compute the MAC for the SYN cookie.  SIPHASH-2-4 is chosen for its speed
 * and good cryptographic properties.
 */
static uint32_t
syncookie_mac(struct in_conninfo *inc, tcp_seq irs, uint8_t flags,
    uint8_t *secbits, uintptr_t secmod)
{
	SIPHASH_CTX ctx;
	uint32_t siphash[2];

	SipHash24_Init(&ctx);
	SipHash_SetKey(&ctx, secbits);
	switch (inc->inc_flags & INC_ISIPV6) {
#ifdef INET
	case 0:
		SipHash_Update(&ctx, &inc->inc_faddr, sizeof(inc->inc_faddr));
		SipHash_Update(&ctx, &inc->inc_laddr, sizeof(inc->inc_laddr));
		break;
#endif
#ifdef INET6
	case INC_ISIPV6:
		SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
		SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
		break;
#endif
	}
	SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
	SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
	SipHash_Update(&ctx, &irs, sizeof(irs));
	SipHash_Update(&ctx, &flags, sizeof(flags));
	SipHash_Update(&ctx, &secmod, sizeof(secmod));
	SipHash_Final((u_int8_t *)&siphash, &ctx);

	return (siphash[0] ^ siphash[1]);
}

static tcp_seq
syncookie_generate(struct syncache_head *sch, struct syncache *sc)
{
	u_int i, secbit, wscale;
	uint32_t iss, hash;
	uint8_t *secbits;
	union syncookie cookie;

	SCH_LOCK_ASSERT(sch);

	cookie.cookie = 0;

	/* Map our computed MSS into the 3-bit index. */
	for (i = nitems(tcp_sc_msstab) - 1;
	     tcp_sc_msstab[i] > sc->sc_peer_mss && i > 0;
	     i--)
		;
	cookie.flags.mss_idx = i;

	/*
	 * Map the send window scale into the 3-bit index but only if
	 * the wscale option was received.
	 */
	if (sc->sc_flags & SCF_WINSCALE) {
		wscale = sc->sc_requested_s_scale;
		for (i = nitems(tcp_sc_wstab) - 1;
		    tcp_sc_wstab[i] > wscale && i > 0;
		     i--)
			;
		cookie.flags.wscale_idx = i;
	}

	/* Can we do SACK? */
	if (sc->sc_flags & SCF_SACK)
		cookie.flags.sack_ok = 1;

	/* Which of the two secrets to use. */
	secbit = sch->sch_sc->secret.oddeven & 0x1;
	cookie.flags.odd_even = secbit;

	secbits = sch->sch_sc->secret.key[secbit];
	hash = syncookie_mac(&sc->sc_inc, sc->sc_irs, cookie.cookie, secbits,
	    (uintptr_t)sch);

	/*
	 * Put the flags into the hash and XOR them to get better ISS number
	 * variance.  This doesn't enhance the cryptographic strength and is
	 * done to prevent the 8 cookie bits from showing up directly on the
	 * wire.
	 */
	iss = hash & ~0xff;
	iss |= cookie.cookie ^ (hash >> 24);

	/* Randomize the timestamp. */
	if (sc->sc_flags & SCF_TIMESTAMP) {
		sc->sc_ts = arc4random();
		sc->sc_tsoff = sc->sc_ts - tcp_ts_getticks();
	}

	TCPSTAT_INC(tcps_sc_sendcookie);
	return (iss);
}

static struct syncache *
syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch,
    struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
    struct socket *lso)
{
	uint32_t hash;
	uint8_t *secbits;
	tcp_seq ack, seq;
	int wnd, wscale = 0;
	union syncookie cookie;

	SCH_LOCK_ASSERT(sch);

	/*
	 * Pull information out of SYN-ACK/ACK and revert sequence number
	 * advances.
	 */
	ack = th->th_ack - 1;
	seq = th->th_seq - 1;

	/*
	 * Unpack the flags containing enough information to restore the
	 * connection.
	 */
	cookie.cookie = (ack & 0xff) ^ (ack >> 24);

	/* Which of the two secrets to use. */
	secbits = sch->sch_sc->secret.key[cookie.flags.odd_even];

	hash = syncookie_mac(inc, seq, cookie.cookie, secbits, (uintptr_t)sch);

	/* The recomputed hash matches the ACK if this was a genuine cookie. */
	if ((ack & ~0xff) != (hash & ~0xff))
		return (NULL);

	/* Fill in the syncache values. */
	sc->sc_flags = 0;
	bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
	sc->sc_ipopts = NULL;

	sc->sc_irs = seq;
	sc->sc_iss = ack;

	switch (inc->inc_flags & INC_ISIPV6) {
#ifdef INET
	case 0:
		sc->sc_ip_ttl = sotoinpcb(lso)->inp_ip_ttl;
		sc->sc_ip_tos = sotoinpcb(lso)->inp_ip_tos;
		break;
#endif
#ifdef INET6
	case INC_ISIPV6:
		if (sotoinpcb(lso)->inp_flags & IN6P_AUTOFLOWLABEL)
			sc->sc_flowlabel = sc->sc_iss & IPV6_FLOWLABEL_MASK;
		break;
#endif
	}

	sc->sc_peer_mss = tcp_sc_msstab[cookie.flags.mss_idx];

	/* We can simply recompute receive window scale we sent earlier. */
	while (wscale < TCP_MAX_WINSHIFT && (TCP_MAXWIN << wscale) < sb_max)
		wscale++;

	/* Only use wscale if it was enabled in the orignal SYN. */
	if (cookie.flags.wscale_idx > 0) {
		sc->sc_requested_r_scale = wscale;
		sc->sc_requested_s_scale = tcp_sc_wstab[cookie.flags.wscale_idx];
		sc->sc_flags |= SCF_WINSCALE;
	}

	wnd = sbspace(&lso->so_rcv);
	wnd = imax(wnd, 0);
	wnd = imin(wnd, TCP_MAXWIN);
	sc->sc_wnd = wnd;

	if (cookie.flags.sack_ok)
		sc->sc_flags |= SCF_SACK;

	if (to->to_flags & TOF_TS) {
		sc->sc_flags |= SCF_TIMESTAMP;
		sc->sc_tsreflect = to->to_tsval;
		sc->sc_ts = to->to_tsecr;
		sc->sc_tsoff = to->to_tsecr - tcp_ts_getticks();
	}

	if (to->to_flags & TOF_SIGNATURE)
		sc->sc_flags |= SCF_SIGNATURE;

	sc->sc_rxmits = 0;

	TCPSTAT_INC(tcps_sc_recvcookie);
	return (sc);
}

#ifdef INVARIANTS
static int
syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
    struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
    struct socket *lso)
{
	struct syncache scs, *scx;
	char *s;

	bzero(&scs, sizeof(scs));
	scx = syncookie_lookup(inc, sch, &scs, th, to, lso);

	if ((s = tcp_log_addrs(inc, th, NULL, NULL)) == NULL)
		return (0);

	if (scx != NULL) {
		if (sc->sc_peer_mss != scx->sc_peer_mss)
			log(LOG_DEBUG, "%s; %s: mss different %i vs %i\n",
			    s, __func__, sc->sc_peer_mss, scx->sc_peer_mss);

		if (sc->sc_requested_r_scale != scx->sc_requested_r_scale)
			log(LOG_DEBUG, "%s; %s: rwscale different %i vs %i\n",
			    s, __func__, sc->sc_requested_r_scale,
			    scx->sc_requested_r_scale);

		if (sc->sc_requested_s_scale != scx->sc_requested_s_scale)
			log(LOG_DEBUG, "%s; %s: swscale different %i vs %i\n",
			    s, __func__, sc->sc_requested_s_scale,
			    scx->sc_requested_s_scale);

		if ((sc->sc_flags & SCF_SACK) != (scx->sc_flags & SCF_SACK))
			log(LOG_DEBUG, "%s; %s: SACK different\n", s, __func__);
	}

	if (s != NULL)
		free(s, M_TCPLOG);
	return (0);
}
#endif /* INVARIANTS */

static void
syncookie_reseed(void *arg)
{
	struct tcp_syncache *sc = arg;
	uint8_t *secbits;
	int secbit;

	/*
	 * Reseeding the secret doesn't have to be protected by a lock.
	 * It only must be ensured that the new random values are visible
	 * to all CPUs in a SMP environment.  The atomic with release
	 * semantics ensures that.
	 */
	secbit = (sc->secret.oddeven & 0x1) ? 0 : 1;
	secbits = sc->secret.key[secbit];
	arc4rand(secbits, SYNCOOKIE_SECRET_SIZE, 0);
	atomic_add_rel_int(&sc->secret.oddeven, 1);

	/* Reschedule ourself. */
	callout_schedule(&sc->secret.reseed, SYNCOOKIE_LIFETIME * hz);
}

/*
 * Exports the syncache entries to userland so that netstat can display
 * them alongside the other sockets.  This function is intended to be
 * called only from tcp_pcblist.
 *
 * Due to concurrency on an active system, the number of pcbs exported
 * may have no relation to max_pcbs.  max_pcbs merely indicates the
 * amount of space the caller allocated for this function to use.
 */
int
syncache_pcblist(struct sysctl_req *req, int max_pcbs, int *pcbs_exported)
{
	struct xtcpcb xt;
	struct syncache *sc;
	struct syncache_head *sch;
	int count, error, i;

	for (count = 0, error = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
		sch = &V_tcp_syncache.hashbase[i];
		SCH_LOCK(sch);
		TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
			if (count >= max_pcbs) {
				SCH_UNLOCK(sch);
				goto exit;
			}
			if (cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
				continue;
			bzero(&xt, sizeof(xt));
			xt.xt_len = sizeof(xt);
			if (sc->sc_inc.inc_flags & INC_ISIPV6)
				xt.xt_inp.inp_vflag = INP_IPV6;
			else
				xt.xt_inp.inp_vflag = INP_IPV4;
			bcopy(&sc->sc_inc, &xt.xt_inp.inp_inc, sizeof (struct in_conninfo));
			xt.xt_tp.t_inpcb = &xt.xt_inp;
			xt.xt_tp.t_state = TCPS_SYN_RECEIVED;
			xt.xt_socket.xso_protocol = IPPROTO_TCP;
			xt.xt_socket.xso_len = sizeof (struct xsocket);
			xt.xt_socket.so_type = SOCK_STREAM;
			xt.xt_socket.so_state = SS_ISCONNECTING;
			error = SYSCTL_OUT(req, &xt, sizeof xt);
			if (error) {
				SCH_UNLOCK(sch);
				goto exit;
			}
			count++;
		}
		SCH_UNLOCK(sch);
	}
exit:
	*pcbs_exported = count;
	return error;
}