diff options
Diffstat (limited to 'sys/netinet/tcp_syncache.c')
| -rw-r--r-- | sys/netinet/tcp_syncache.c | 1553 |
1 files changed, 1553 insertions, 0 deletions
diff --git a/sys/netinet/tcp_syncache.c b/sys/netinet/tcp_syncache.c new file mode 100644 index 000000000000..9e03f32db9e8 --- /dev/null +++ b/sys/netinet/tcp_syncache.c @@ -0,0 +1,1553 @@ +/*- + * Copyright (c) 2001 McAfee, Inc. + * Copyright (c) 2006 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. + * + * 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. + * + * $FreeBSD$ + */ + +#include "opt_inet.h" +#include "opt_inet6.h" +#include "opt_ipsec.h" +#include "opt_mac.h" + +#include <sys/param.h> +#include <sys/systm.h> +#include <sys/kernel.h> +#include <sys/sysctl.h> +#include <sys/lock.h> +#include <sys/mutex.h> +#include <sys/malloc.h> +#include <sys/mbuf.h> +#include <sys/md5.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 <vm/uma.h> + +#include <net/if.h> +#include <net/route.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> +#include <netinet/tcp_fsm.h> +#include <netinet/tcp_seq.h> +#include <netinet/tcp_timer.h> +#include <netinet/tcp_var.h> +#ifdef INET6 +#include <netinet6/tcp6_var.h> +#endif + +#ifdef FAST_IPSEC +#include <netipsec/ipsec.h> +#ifdef INET6 +#include <netipsec/ipsec6.h> +#endif +#include <netipsec/key.h> +#endif /*FAST_IPSEC*/ + +#include <machine/in_cksum.h> + +#include <security/mac/mac_framework.h> + +static int tcp_syncookies = 1; +SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_RW, + &tcp_syncookies, 0, + "Use TCP SYN cookies if the syncache overflows"); + +static int tcp_syncookiesonly = 0; +SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_RW, + &tcp_syncookiesonly, 0, + "Use only TCP SYN cookies"); + +#define SYNCOOKIE_SECRET_SIZE 8 /* dwords */ +#define SYNCOOKIE_LIFETIME 16 /* seconds */ + +struct syncache { + TAILQ_ENTRY(syncache) sc_hash; + struct in_conninfo sc_inc; /* addresses */ + u_long sc_rxttime; /* retransmit time */ + u_int16_t sc_rxmits; /* retransmit counter */ + + u_int32_t sc_tsreflect; /* timestamp to reflect */ + u_int32_t sc_ts; /* our timestamp to send */ + u_int32_t sc_tsoff; /* ts offset w/ syncookies */ + u_int32_t sc_flowlabel; /* IPv6 flowlabel */ + tcp_seq sc_irs; /* seq from peer */ + tcp_seq sc_iss; /* our ISS */ + struct mbuf *sc_ipopts; /* source route */ + + u_int16_t sc_peer_mss; /* peer's MSS */ + u_int16_t sc_wnd; /* advertised window */ + u_int8_t sc_ip_ttl; /* IPv4 TTL */ + u_int8_t sc_ip_tos; /* IPv4 TOS */ + u_int8_t sc_requested_s_scale:4, + sc_requested_r_scale:4; + u_int8_t sc_flags; +#define SCF_NOOPT 0x01 /* no TCP options */ +#define SCF_WINSCALE 0x02 /* negotiated window scaling */ +#define SCF_TIMESTAMP 0x04 /* negotiated timestamps */ + /* MSS is implicit */ +#define SCF_UNREACH 0x10 /* icmp unreachable received */ +#define SCF_SIGNATURE 0x20 /* send MD5 digests */ +#define SCF_SACK 0x80 /* send SACK option */ +#ifdef MAC + struct label *sc_label; /* MAC label reference */ +#endif +}; + +struct syncache_head { + struct mtx sch_mtx; + TAILQ_HEAD(sch_head, syncache) sch_bucket; + struct callout sch_timer; + int sch_nextc; + u_int sch_length; + u_int sch_oddeven; + u_int32_t sch_secbits_odd[SYNCOOKIE_SECRET_SIZE]; + u_int32_t sch_secbits_even[SYNCOOKIE_SECRET_SIZE]; + u_int sch_reseed; /* time_uptime, seconds */ +}; + +static void syncache_drop(struct syncache *, struct syncache_head *); +static void syncache_free(struct syncache *); +static void syncache_insert(struct syncache *, struct syncache_head *); +struct syncache *syncache_lookup(struct in_conninfo *, struct syncache_head **); +static int syncache_respond(struct syncache *); +static struct socket *syncache_socket(struct syncache *, struct socket *, + struct mbuf *m); +static void syncache_timer(void *); +static void syncookie_generate(struct syncache_head *, struct syncache *, + u_int32_t *); +static struct syncache + *syncookie_lookup(struct in_conninfo *, struct syncache_head *, + struct syncache *, struct tcpopt *, struct tcphdr *, + struct socket *); + +/* + * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies. + * 3 retransmits corresponds to a timeout of (1 + 2 + 4 + 8 == 15) 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 + +struct tcp_syncache { + struct syncache_head *hashbase; + uma_zone_t zone; + u_int hashsize; + u_int hashmask; + u_int bucket_limit; + u_int cache_count; /* XXX: unprotected */ + u_int cache_limit; + u_int rexmt_limit; + u_int hash_secret; +}; +static struct tcp_syncache tcp_syncache; + +SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0, "TCP SYN cache"); + +SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_RDTUN, + &tcp_syncache.bucket_limit, 0, "Per-bucket hash limit for syncache"); + +SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_RDTUN, + &tcp_syncache.cache_limit, 0, "Overall entry limit for syncache"); + +SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, count, CTLFLAG_RD, + &tcp_syncache.cache_count, 0, "Current number of entries in syncache"); + +SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_RDTUN, + &tcp_syncache.hashsize, 0, "Size of TCP syncache hashtable"); + +SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_RW, + &tcp_syncache.rexmt_limit, 0, "Limit on SYN/ACK retransmissions"); + +int tcp_sc_rst_sock_fail = 1; +SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail, CTLFLAG_RW, + &tcp_sc_rst_sock_fail, 0, "Send reset on socket allocation failure"); + +static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache"); + +#define SYNCACHE_HASH(inc, mask) \ + ((tcp_syncache.hash_secret ^ \ + (inc)->inc_faddr.s_addr ^ \ + ((inc)->inc_faddr.s_addr >> 16) ^ \ + (inc)->inc_fport ^ (inc)->inc_lport) & mask) + +#define SYNCACHE_HASH6(inc, mask) \ + ((tcp_syncache.hash_secret ^ \ + (inc)->inc6_faddr.s6_addr32[0] ^ \ + (inc)->inc6_faddr.s6_addr32[3] ^ \ + (inc)->inc_fport ^ (inc)->inc_lport) & mask) + +#define ENDPTS_EQ(a, b) ( \ + (a)->ie_fport == (b)->ie_fport && \ + (a)->ie_lport == (b)->ie_lport && \ + (a)->ie_faddr.s_addr == (b)->ie_faddr.s_addr && \ + (a)->ie_laddr.s_addr == (b)->ie_laddr.s_addr \ +) + +#define ENDPTS6_EQ(a, b) (memcmp(a, b, sizeof(*a)) == 0) + +#define SYNCACHE_TIMEOUT(sc, sch, co) do { \ + (sc)->sc_rxmits++; \ + (sc)->sc_rxttime = ticks + \ + TCPTV_RTOBASE * tcp_backoff[(sc)->sc_rxmits - 1]; \ + if ((sch)->sch_nextc > (sc)->sc_rxttime) \ + (sch)->sch_nextc = (sc)->sc_rxttime; \ + if (!TAILQ_EMPTY(&(sch)->sch_bucket) && !(co)) \ + callout_reset(&(sch)->sch_timer, \ + (sch)->sch_nextc - ticks, \ + syncache_timer, (void *)(sch)); \ +} while (0) + +#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); +#ifdef MAC + mac_destroy_syncache(&sc->sc_label); +#endif + + uma_zfree(tcp_syncache.zone, sc); +} + +void +syncache_init(void) +{ + int i; + + tcp_syncache.cache_count = 0; + tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE; + tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT; + tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS; + tcp_syncache.hash_secret = arc4random(); + + TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize", + &tcp_syncache.hashsize); + TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit", + &tcp_syncache.bucket_limit); + if (!powerof2(tcp_syncache.hashsize) || tcp_syncache.hashsize == 0) { + printf("WARNING: syncache hash size is not a power of 2.\n"); + tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE; + } + tcp_syncache.hashmask = tcp_syncache.hashsize - 1; + + /* Set limits. */ + tcp_syncache.cache_limit = + tcp_syncache.hashsize * tcp_syncache.bucket_limit; + TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit", + &tcp_syncache.cache_limit); + + /* Allocate the hash table. */ + MALLOC(tcp_syncache.hashbase, struct syncache_head *, + tcp_syncache.hashsize * sizeof(struct syncache_head), + M_SYNCACHE, M_WAITOK | M_ZERO); + + /* Initialize the hash buckets. */ + for (i = 0; i < tcp_syncache.hashsize; i++) { + TAILQ_INIT(&tcp_syncache.hashbase[i].sch_bucket); + mtx_init(&tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head", + NULL, MTX_DEF); + callout_init_mtx(&tcp_syncache.hashbase[i].sch_timer, + &tcp_syncache.hashbase[i].sch_mtx, 0); + tcp_syncache.hashbase[i].sch_length = 0; + } + + /* Create the syncache entry zone. */ + tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache), + NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0); + uma_zone_set_max(tcp_syncache.zone, tcp_syncache.cache_limit); +} + +/* + * 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 >= tcp_syncache.bucket_limit) { + KASSERT(!TAILQ_EMPTY(&sch->sch_bucket), + ("sch->sch_length incorrect")); + sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head); + syncache_drop(sc2, sch); + tcpstat.tcps_sc_bucketoverflow++; + } + + /* Put it into the bucket. */ + TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash); + sch->sch_length++; + + /* Reinitialize the bucket row's timer. */ + SYNCACHE_TIMEOUT(sc, sch, 1); + + SCH_UNLOCK(sch); + + tcp_syncache.cache_count++; + tcpstat.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); + + TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash); + sch->sch_length--; + + syncache_free(sc); + tcp_syncache.cache_count--; +} + +/* + * 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; + + /* NB: syncache_head has already been locked by the callout. */ + SCH_LOCK_ASSERT(sch); + + 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 (sc->sc_rxttime >= tick) { + if (sc->sc_rxttime < sch->sch_nextc) + sch->sch_nextc = sc->sc_rxttime; + continue; + } + + if (sc->sc_rxmits > tcp_syncache.rexmt_limit) { + if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) { + log(LOG_DEBUG, "%s; %s: Response timeout\n", + s, __func__); + free(s, M_TCPLOG); + } + syncache_drop(sc, sch); + tcpstat.tcps_sc_stale++; + continue; + } + + (void) syncache_respond(sc); + tcpstat.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)); +} + +/* + * Find an entry in the syncache. + * Returns always with locked syncache_head plus a matching entry or NULL. + */ +struct syncache * +syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp) +{ + struct syncache *sc; + struct syncache_head *sch; + +#ifdef INET6 + if (inc->inc_isipv6) { + sch = &tcp_syncache.hashbase[ + SYNCACHE_HASH6(inc, tcp_syncache.hashmask)]; + *schp = sch; + + SCH_LOCK(sch); + + /* Circle through bucket row to find matching entry. */ + TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) { + if (ENDPTS6_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie)) + return (sc); + } + } else +#endif + { + sch = &tcp_syncache.hashbase[ + SYNCACHE_HASH(inc, tcp_syncache.hashmask)]; + *schp = sch; + + SCH_LOCK(sch); + + /* Circle through bucket row to find matching entry. */ + TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) { +#ifdef INET6 + if (sc->sc_inc.inc_isipv6) + continue; +#endif + if (ENDPTS_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie)) + return (sc); + } + } + SCH_LOCK_ASSERT(*schp); + return (NULL); /* 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; + + sc = syncache_lookup(inc, &sch); /* returns locked sch */ + SCH_LOCK_ASSERT(sch); + if (sc == NULL) + 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); + tcpstat.tcps_sc_reset++; + } +done: + 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.tcps_sc_badack++; + } + SCH_UNLOCK(sch); +} + +void +syncache_unreach(struct in_conninfo *inc, struct tcphdr *th) +{ + 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->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.tcps_sc_unreach++; +done: + SCH_UNLOCK(sch); +} + +/* + * Build a new TCP socket structure from a syncache entry. + */ +static struct socket * +syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m) +{ + struct inpcb *inp = NULL; + struct socket *so; + struct tcpcb *tp; + char *s; + + NET_ASSERT_GIANT(); + INP_INFO_WLOCK_ASSERT(&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, SS_ISCONNECTED); + 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.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 + SOCK_LOCK(so); + mac_set_socket_peer_from_mbuf(m, so); + SOCK_UNLOCK(so); +#endif + + inp = sotoinpcb(so); + INP_LOCK(inp); + + /* Insert new socket into PCB hash list. */ + inp->inp_inc.inc_isipv6 = sc->sc_inc.inc_isipv6; +#ifdef INET6 + if (sc->sc_inc.inc_isipv6) { + 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 + inp->inp_lport = sc->sc_inc.inc_lport; + if (in_pcbinshash(inp) != 0) { + /* + * Undo the assignments above if we failed to + * put the PCB on the hash lists. + */ +#ifdef INET6 + if (sc->sc_inc.inc_isipv6) + inp->in6p_laddr = in6addr_any; + else +#endif + inp->inp_laddr.s_addr = INADDR_ANY; + inp->inp_lport = 0; + goto abort; + } +#ifdef FAST_IPSEC + /* Copy old policy into new socket's. */ + if (ipsec_copy_policy(sotoinpcb(lso)->inp_sp, inp->inp_sp)) + printf("syncache_socket: could not copy policy\n"); +#endif +#ifdef INET6 + if (sc->sc_inc.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 (in6_pcbconnect(inp, (struct sockaddr *)&sin6, + thread0.td_ucred)) { + inp->in6p_laddr = laddr6; + goto abort; + } + /* Override flowlabel from in6_pcbconnect. */ + inp->in6p_flowinfo &= ~IPV6_FLOWLABEL_MASK; + inp->in6p_flowinfo |= sc->sc_flowlabel; + } else +#endif + { + struct in_addr laddr; + struct sockaddr_in sin; + + inp->inp_options = ip_srcroute(m); + 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 (in_pcbconnect(inp, (struct sockaddr *)&sin, + thread0.td_ucred)) { + inp->inp_laddr = laddr; + goto abort; + } + } + tp = intotcpcb(inp); + tp->t_state = TCPS_SYN_RECEIVED; + tp->iss = sc->sc_iss; + tp->irs = sc->sc_irs; + tcp_rcvseqinit(tp); + tcp_sendseqinit(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 = ticks; + tp->ts_offset = sc->sc_tsoff; + } +#ifdef 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; + } + + /* + * 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, reset cwnd to 1 segment. + */ + if (sc->sc_rxmits > 1) + tp->snd_cwnd = tp->t_maxseg; + tcp_timer_activate(tp, TT_KEEP, tcp_keepinit); + + INP_UNLOCK(inp); + + tcpstat.tcps_accepts++; + return (so); + +abort: + INP_UNLOCK(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. + */ +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_WLOCK_ASSERT(&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); + if (sc == NULL) { + /* + * There is no syncache entry, so see if this ACK is + * a returning syncookie. To do this, first: + * A. See if this socket has had a syncache entry dropped in + * the past. We don't want to accept a bogus syncookie + * if we've never received a SYN. + * B. check that the syncookie is valid. If it is, then + * cobble up a fake syncache entry, and return. + */ + if (!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; + } + bzero(&scs, sizeof(scs)); + sc = syncookie_lookup(inc, sch, &scs, to, th, *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; + } + tcpstat.tcps_sc_recvcookie++; + } else { + /* Pull out the entry to unlock the bucket row. */ + TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash); + sch->sch_length--; + tcp_syncache.cache_count--; + 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 match the received initial receive sequence + * number + 1 (the SYN) because we didn't ACK any data that + * may have come with the SYN. + */ + if (th->th_seq != sc->sc_irs + 1) { + 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 present in the SYN and we accepted + * them in our SYN|ACK we require them to be present from + * now on. And vice versa. + */ + 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, " + "segment rejected\n", s, __func__); + goto failed; + } + 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 the reflected timestamp + * must be equal to what we actually sent in the SYN|ACK. + */ + if ((to->to_flags & TOF_TS) && 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.tcps_sc_aborted++; + else + tcpstat.tcps_sc_completed++; + + 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); +} + +/* + * 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. + */ +void +syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th, + struct inpcb *inp, struct socket **lsop, struct mbuf *m) +{ + struct tcpcb *tp; + struct socket *so; + struct syncache *sc = NULL; + struct syncache_head *sch; + struct mbuf *ipopts = NULL; + u_int32_t flowtmp; + int win, sb_hiwat, ip_ttl, ip_tos, noopt; +#ifdef INET6 + int autoflowlabel = 0; +#endif +#ifdef MAC + struct label *maclabel; +#endif + struct syncache scs; + + INP_INFO_WLOCK_ASSERT(&tcbinfo); + INP_LOCK_ASSERT(inp); /* listen socket */ + + /* + * Combine all so/tp operations very early to drop the INP lock as + * soon as possible. + */ + so = *lsop; + tp = sototcpcb(so); + +#ifdef INET6 + if (inc->inc_isipv6 && + (inp->in6p_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; + noopt = (tp->t_flags & TF_NOOPT); + + so = NULL; + tp = NULL; + +#ifdef MAC + if (mac_init_syncache(&maclabel) != 0) { + INP_UNLOCK(inp); + INP_INFO_WUNLOCK(&tcbinfo); + goto done; + } else + mac_init_syncache_from_inpcb(maclabel, inp); +#endif + INP_UNLOCK(inp); + INP_INFO_WUNLOCK(&tcbinfo); + + /* + * Remember the IP options, if any. + */ +#ifdef INET6 + if (!inc->inc_isipv6) +#endif + ipopts = ip_srcroute(m); + + /* + * 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?) + */ + sc = syncache_lookup(inc, &sch); /* returns locked entry */ + SCH_LOCK_ASSERT(sch); + if (sc != NULL) { + tcpstat.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_destroy_syncache(&maclabel); + KASSERT(sc->sc_label != NULL, + ("%s: label not initialized", __func__)); +#endif + if (syncache_respond(sc) == 0) { + SYNCACHE_TIMEOUT(sc, sch, 1); + tcpstat.tcps_sndacks++; + tcpstat.tcps_sndtotal++; + } + SCH_UNLOCK(sch); + goto done; + } + + sc = uma_zalloc(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.tcps_sc_zonefail++; + if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL) + syncache_drop(sc, sch); + sc = uma_zalloc(tcp_syncache.zone, M_NOWAIT | M_ZERO); + if (sc == NULL) { + if (tcp_syncookies) { + bzero(&scs, sizeof(scs)); + sc = &scs; + } else { + SCH_UNLOCK(sch); + if (ipopts) + (void) m_free(ipopts); + goto done; + } + } + } + + /* + * Fill in the syncache values. + */ +#ifdef MAC + sc->sc_label = maclabel; +#endif + sc->sc_ipopts = ipopts; + bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo)); +#ifdef INET6 + if (!inc->inc_isipv6) +#endif + { + sc->sc_ip_tos = ip_tos; + sc->sc_ip_ttl = ip_ttl; + } + + 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 (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 = ticks; + sc->sc_flags |= SCF_TIMESTAMP; + } + if (to->to_flags & TOF_SCALE) { + int wscale = 0; + + /* + * Compute proper scaling value from buffer space. + * Leave enough room for the socket buffer to grow + * with auto sizing. This allows us to scale the + * receive buffer over a wide range while not losing + * any efficiency or fine granularity. + * + * RFC1323: The Window field in a SYN (i.e., a <SYN> + * or <SYN,ACK>) segment itself is never scaled. + */ + while (wscale < TCP_MAX_WINSHIFT && + (0x1 << wscale) < tcp_minmss) + wscale++; + sc->sc_requested_r_scale = wscale; + sc->sc_requested_s_scale = to->to_wscale; + sc->sc_flags |= SCF_WINSCALE; + } + } +#ifdef TCP_SIGNATURE + /* + * If listening socket requested TCP digests, and received SYN + * contains the option, flag this in the syncache so that + * syncache_respond() will do the right thing with the SYN+ACK. + * XXX: Currently we always record the option by default and will + * attempt to use it in syncache_respond(). + */ + if (to->to_flags & TOF_SIGNATURE) + sc->sc_flags |= SCF_SIGNATURE; +#endif + if (to->to_flags & TOF_SACK) + sc->sc_flags |= SCF_SACK; + if (to->to_flags & TOF_MSS) + sc->sc_peer_mss = to->to_mss; /* peer mss may be zero */ + if (noopt) + sc->sc_flags |= SCF_NOOPT; + + if (tcp_syncookies) { + syncookie_generate(sch, sc, &flowtmp); +#ifdef INET6 + if (autoflowlabel) + sc->sc_flowlabel = flowtmp; +#endif + } else { +#ifdef INET6 + if (autoflowlabel) + sc->sc_flowlabel = + (htonl(ip6_randomflowlabel()) & IPV6_FLOWLABEL_MASK); +#endif + } + SCH_UNLOCK(sch); + + /* + * Do a standard 3-way handshake. + */ + if (syncache_respond(sc) == 0) { + if (tcp_syncookies && tcp_syncookiesonly && sc != &scs) + syncache_free(sc); + else if (sc != &scs) + syncache_insert(sc, sch); /* locks and unlocks sch */ + tcpstat.tcps_sndacks++; + tcpstat.tcps_sndtotal++; + } else { + if (sc != &scs) + syncache_free(sc); + tcpstat.tcps_sc_dropped++; + } + +done: +#ifdef MAC + if (sc == &scs) + mac_destroy_syncache(&maclabel); +#endif + *lsop = NULL; + m_freem(m); + return; +} + +static int +syncache_respond(struct syncache *sc) +{ + struct ip *ip = NULL; + struct mbuf *m; + struct tcphdr *th; + int optlen, error; + u_int16_t hlen, tlen, mssopt; + struct tcpopt to; +#ifdef INET6 + struct ip6_hdr *ip6 = NULL; +#endif + + hlen = +#ifdef INET6 + (sc->sc_inc.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 = tcp_mssopt(&sc->sc_inc); + if (sc->sc_peer_mss) + mssopt = max( min(sc->sc_peer_mss, mssopt), 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_DONTWAIT, MT_DATA); + if (m == NULL) + return (ENOBUFS); +#ifdef MAC + mac_create_mbuf_from_syncache(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_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); + } else +#endif + { + ip = mtod(m, struct ip *); + ip->ip_v = IPVERSION; + ip->ip_hl = sizeof(struct ip) >> 2; + ip->ip_len = 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 (path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0)) + ip->ip_off |= IP_DF; + + th = (struct tcphdr *)(ip + 1); + } + 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; + + /* 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; +#ifdef TCP_SIGNATURE + if (sc->sc_flags & SCF_SIGNATURE) + to.to_flags |= TOF_SIGNATURE; +#endif + optlen = tcp_addoptions(&to, (u_char *)(th + 1)); + +#ifdef TCP_SIGNATURE + tcp_signature_compute(m, sizeof(struct ip), 0, optlen, + to.to_signature, IPSEC_DIR_OUTBOUND); +#endif + + /* 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_isipv6) + ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen); + else +#endif + ip->ip_len += optlen; + } else + optlen = 0; + +#ifdef INET6 + if (sc->sc_inc.inc_isipv6) { + th->th_sum = 0; + th->th_sum = in6_cksum(m, IPPROTO_TCP, hlen, + tlen + optlen - hlen); + ip6->ip6_hlim = in6_selecthlim(NULL, NULL); + error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL); + } else +#endif + { + th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr, + htons(tlen + optlen - hlen + IPPROTO_TCP)); + m->m_pkthdr.csum_flags = CSUM_TCP; + m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum); + error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL); + } + return (error); +} + +/* + * The purpose of SYN cookies is to avoid keeping track of all SYN's we + * receive and to be able to handle SYN floods from bogus source addresses + * (where we will never receive any reply). SYN floods try to exhaust all + * our memory and available slots in the SYN cache table to cause a denial + * of service to legitimate users of the local host. + * + * The idea of SYN cookies is to encode and include all necessary information + * about the connection setup state within the SYN-ACK we send back and thus + * to get along without keeping any local state until the ACK to the SYN-ACK + * arrives (if ever). Everything we need to know should be available from + * the information we encoded in the SYN-ACK. + * + * More information about the theory behind SYN cookies and its first + * discussion and specification can be found at: + * http://cr.yp.to/syncookies.html (overview) + * http://cr.yp.to/syncookies/archive (gory details) + * + * This implementation extends the orginal idea and first implementation + * of FreeBSD by using not only the initial sequence number field to store + * information but also the timestamp field if present. This way we can + * keep track of the entire state we need to know to recreate the session in + * its original form. Almost all TCP speakers implement RFC1323 timestamps + * these days. For those that do not we still have to live with the known + * shortcomings of the ISN only SYN cookies. + * + * Cookie layers: + * + * Initial sequence number we send: + * 31|................................|0 + * DDDDDDDDDDDDDDDDDDDDDDDDDMMMRRRP + * D = MD5 Digest (first dword) + * M = MSS index + * R = Rotation of secret + * P = Odd or Even secret + * + * The MD5 Digest is computed with over following parameters: + * a) randomly rotated secret + * b) struct in_conninfo containing the remote/local ip/port (IPv4&IPv6) + * c) the received initial sequence number from remote host + * d) the rotation offset and odd/even bit + * + * Timestamp we send: + * 31|................................|0 + * DDDDDDDDDDDDDDDDDDDDDDSSSSRRRRA5 + * D = MD5 Digest (third dword) (only as filler) + * S = Requested send window scale + * R = Requested receive window scale + * A = SACK allowed + * 5 = TCP-MD5 enabled (not implemented yet) + * XORed with MD5 Digest (forth dword) + * + * The timestamp isn't cryptographically secure and doesn't need to be. + * The double use of the MD5 digest dwords ties it to a specific remote/ + * local host/port, remote initial sequence number and our local time + * limited secret. A received timestamp is reverted (XORed) and then + * the contained MD5 dword is compared to the computed one to ensure the + * timestamp belongs to the SYN-ACK we sent. The other parameters may + * have been tampered with but this isn't different from supplying bogus + * values in the SYN in the first place. + * + * Some problems with SYN cookies remain however: + * Consider the problem of a recreated (and retransmitted) cookie. If the + * original SYN was accepted, the connection is established. The second + * SYN is inflight, and if it arrives with an ISN that falls within the + * receive window, the connection is killed. + * + * Notes: + * A heuristic to determine when to accept syn cookies is not necessary. + * An ACK flood would cause the syncookie verification to be attempted, + * but a SYN flood causes syncookies to be generated. Both are of equal + * cost, so there's no point in trying to optimize the ACK flood case. + * Also, if you don't process certain ACKs for some reason, then all someone + * would have to do is launch a SYN and ACK flood at the same time, which + * would stop cookie verification and defeat the entire purpose of syncookies. + */ +static int tcp_sc_msstab[] = { 0, 256, 468, 536, 996, 1452, 1460, 8960 }; + +static void +syncookie_generate(struct syncache_head *sch, struct syncache *sc, + u_int32_t *flowlabel) +{ + MD5_CTX ctx; + u_int32_t md5_buffer[MD5_DIGEST_LENGTH / sizeof(u_int32_t)]; + u_int32_t data; + u_int32_t *secbits; + u_int off, pmss, mss; + int i; + + SCH_LOCK_ASSERT(sch); + + /* Which of the two secrets to use. */ + secbits = sch->sch_oddeven ? + sch->sch_secbits_odd : sch->sch_secbits_even; + + /* Reseed secret if too old. */ + if (sch->sch_reseed < time_uptime) { + sch->sch_oddeven = sch->sch_oddeven ? 0 : 1; /* toggle */ + secbits = sch->sch_oddeven ? + sch->sch_secbits_odd : sch->sch_secbits_even; + for (i = 0; i < SYNCOOKIE_SECRET_SIZE; i++) + secbits[i] = arc4random(); + sch->sch_reseed = time_uptime + SYNCOOKIE_LIFETIME; + } + + /* Secret rotation offset. */ + off = sc->sc_iss & 0x7; /* iss was randomized before */ + + /* Maximum segment size calculation. */ + pmss = max( min(sc->sc_peer_mss, tcp_mssopt(&sc->sc_inc)), tcp_minmss); + for (mss = sizeof(tcp_sc_msstab) / sizeof(int) - 1; mss > 0; mss--) + if (tcp_sc_msstab[mss] <= pmss) + break; + + /* Fold parameters and MD5 digest into the ISN we will send. */ + data = sch->sch_oddeven;/* odd or even secret, 1 bit */ + data |= off << 1; /* secret offset, derived from iss, 3 bits */ + data |= mss << 4; /* mss, 3 bits */ + + MD5Init(&ctx); + MD5Update(&ctx, ((u_int8_t *)secbits) + off, + SYNCOOKIE_SECRET_SIZE * sizeof(*secbits) - off); + MD5Update(&ctx, secbits, off); + MD5Update(&ctx, &sc->sc_inc, sizeof(sc->sc_inc)); + MD5Update(&ctx, &sc->sc_irs, sizeof(sc->sc_irs)); + MD5Update(&ctx, &data, sizeof(data)); + MD5Final((u_int8_t *)&md5_buffer, &ctx); + + data |= (md5_buffer[0] << 7); + sc->sc_iss = data; + +#ifdef INET6 + *flowlabel = md5_buffer[1] & IPV6_FLOWLABEL_MASK; +#endif + + /* Additional parameters are stored in the timestamp if present. */ + if (sc->sc_flags & SCF_TIMESTAMP) { + data = ((sc->sc_flags & SCF_SIGNATURE) ? 1 : 0); /* TCP-MD5, 1 bit */ + data |= ((sc->sc_flags & SCF_SACK) ? 1 : 0) << 1; /* SACK, 1 bit */ + data |= sc->sc_requested_s_scale << 2; /* SWIN scale, 4 bits */ + data |= sc->sc_requested_r_scale << 6; /* RWIN scale, 4 bits */ + data |= md5_buffer[2] << 10; /* more digest bits */ + data ^= md5_buffer[3]; + sc->sc_ts = data; + sc->sc_tsoff = data - ticks; /* after XOR */ + } + + return; +} + +static struct syncache * +syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch, + struct syncache *sc, struct tcpopt *to, struct tcphdr *th, + struct socket *so) +{ + MD5_CTX ctx; + u_int32_t md5_buffer[MD5_DIGEST_LENGTH / sizeof(u_int32_t)]; + u_int32_t data = 0; + u_int32_t *secbits; + tcp_seq ack, seq; + int off, mss, wnd, flags; + + 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; + off = (ack >> 1) & 0x7; + mss = (ack >> 4) & 0x7; + flags = ack & 0x7f; + + /* Which of the two secrets to use. */ + secbits = (flags & 0x1) ? sch->sch_secbits_odd : sch->sch_secbits_even; + + /* + * The secret wasn't updated for the lifetime of a syncookie, + * so this SYN-ACK/ACK is either too old (replay) or totally bogus. + */ + if (sch->sch_reseed < time_uptime) { + return (NULL); + } + + /* Recompute the digest so we can compare it. */ + MD5Init(&ctx); + MD5Update(&ctx, ((u_int8_t *)secbits) + off, + SYNCOOKIE_SECRET_SIZE * sizeof(*secbits) - off); + MD5Update(&ctx, secbits, off); + MD5Update(&ctx, inc, sizeof(*inc)); + MD5Update(&ctx, &seq, sizeof(seq)); + MD5Update(&ctx, &flags, sizeof(flags)); + MD5Final((u_int8_t *)&md5_buffer, &ctx); + + /* Does the digest part of or ACK'ed ISS match? */ + if ((ack & (~0x7f)) != (md5_buffer[0] << 7)) + return (NULL); + + /* Does the digest part of our reflected timestamp match? */ + if (to->to_flags & TOF_TS) { + data = md5_buffer[3] ^ to->to_tsecr; + if ((data & (~0x3ff)) != (md5_buffer[2] << 10)) + return (NULL); + } + + /* Fill in the syncache values. */ + bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo)); + sc->sc_ipopts = NULL; + + sc->sc_irs = seq; + sc->sc_iss = ack; + +#ifdef INET6 + if (inc->inc_isipv6) { + if (sotoinpcb(so)->in6p_flags & IN6P_AUTOFLOWLABEL) + sc->sc_flowlabel = md5_buffer[1] & IPV6_FLOWLABEL_MASK; + } else +#endif + { + sc->sc_ip_ttl = sotoinpcb(so)->inp_ip_ttl; + sc->sc_ip_tos = sotoinpcb(so)->inp_ip_tos; + } + + /* Additional parameters that were encoded in the timestamp. */ + if (data) { + sc->sc_flags |= SCF_TIMESTAMP; + sc->sc_tsreflect = to->to_tsval; + sc->sc_ts = to->to_tsecr; + sc->sc_tsoff = to->to_tsecr - ticks; + sc->sc_flags |= (data & 0x1) ? SCF_SIGNATURE : 0; + sc->sc_flags |= ((data >> 1) & 0x1) ? SCF_SACK : 0; + sc->sc_requested_s_scale = min((data >> 2) & 0xf, + TCP_MAX_WINSHIFT); + sc->sc_requested_r_scale = min((data >> 6) & 0xf, + TCP_MAX_WINSHIFT); + if (sc->sc_requested_s_scale || sc->sc_requested_r_scale) + sc->sc_flags |= SCF_WINSCALE; + } else + sc->sc_flags |= SCF_NOOPT; + + wnd = sbspace(&so->so_rcv); + wnd = imax(wnd, 0); + wnd = imin(wnd, TCP_MAXWIN); + sc->sc_wnd = wnd; + + sc->sc_rxmits = 0; + sc->sc_peer_mss = tcp_sc_msstab[mss]; + + return (sc); +} |