2 * INET An implementation of the TCP/IP protocol suite for the LINUX
3 * operating system. INET is implemented using the BSD Socket
4 * interface as the means of communication with the user level.
6 * Implementation of the Transmission Control Protocol(TCP).
9 * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
10 * Mark Evans, <evansmp@uhura.aston.ac.uk>
11 * Corey Minyard <wf-rch!minyard@relay.EU.net>
12 * Florian La Roche, <flla@stud.uni-sb.de>
13 * Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
14 * Linus Torvalds, <torvalds@cs.helsinki.fi>
15 * Alan Cox, <gw4pts@gw4pts.ampr.org>
16 * Matthew Dillon, <dillon@apollo.west.oic.com>
17 * Arnt Gulbrandsen, <agulbra@nvg.unit.no>
18 * Jorge Cwik, <jorge@laser.satlink.net>
22 #include <linux/module.h>
23 #include <linux/slab.h>
24 #include <linux/sysctl.h>
25 #include <linux/workqueue.h>
27 #include <net/inet_common.h>
30 int sysctl_tcp_syncookies __read_mostly
= 1;
31 EXPORT_SYMBOL(sysctl_tcp_syncookies
);
33 int sysctl_tcp_abort_on_overflow __read_mostly
;
35 struct inet_timewait_death_row tcp_death_row
= {
36 .sysctl_max_tw_buckets
= NR_FILE
* 2,
37 .period
= TCP_TIMEWAIT_LEN
/ INET_TWDR_TWKILL_SLOTS
,
38 .death_lock
= __SPIN_LOCK_UNLOCKED(tcp_death_row
.death_lock
),
39 .hashinfo
= &tcp_hashinfo
,
40 .tw_timer
= TIMER_INITIALIZER(inet_twdr_hangman
, 0,
41 (unsigned long)&tcp_death_row
),
42 .twkill_work
= __WORK_INITIALIZER(tcp_death_row
.twkill_work
,
43 inet_twdr_twkill_work
),
44 /* Short-time timewait calendar */
47 .twcal_timer
= TIMER_INITIALIZER(inet_twdr_twcal_tick
, 0,
48 (unsigned long)&tcp_death_row
),
50 EXPORT_SYMBOL_GPL(tcp_death_row
);
52 static bool tcp_in_window(u32 seq
, u32 end_seq
, u32 s_win
, u32 e_win
)
56 if (after(end_seq
, s_win
) && before(seq
, e_win
))
58 return seq
== e_win
&& seq
== end_seq
;
62 * * Main purpose of TIME-WAIT state is to close connection gracefully,
63 * when one of ends sits in LAST-ACK or CLOSING retransmitting FIN
64 * (and, probably, tail of data) and one or more our ACKs are lost.
65 * * What is TIME-WAIT timeout? It is associated with maximal packet
66 * lifetime in the internet, which results in wrong conclusion, that
67 * it is set to catch "old duplicate segments" wandering out of their path.
68 * It is not quite correct. This timeout is calculated so that it exceeds
69 * maximal retransmission timeout enough to allow to lose one (or more)
70 * segments sent by peer and our ACKs. This time may be calculated from RTO.
71 * * When TIME-WAIT socket receives RST, it means that another end
72 * finally closed and we are allowed to kill TIME-WAIT too.
73 * * Second purpose of TIME-WAIT is catching old duplicate segments.
74 * Well, certainly it is pure paranoia, but if we load TIME-WAIT
75 * with this semantics, we MUST NOT kill TIME-WAIT state with RSTs.
76 * * If we invented some more clever way to catch duplicates
77 * (f.e. based on PAWS), we could truncate TIME-WAIT to several RTOs.
79 * The algorithm below is based on FORMAL INTERPRETATION of RFCs.
80 * When you compare it to RFCs, please, read section SEGMENT ARRIVES
81 * from the very beginning.
83 * NOTE. With recycling (and later with fin-wait-2) TW bucket
84 * is _not_ stateless. It means, that strictly speaking we must
85 * spinlock it. I do not want! Well, probability of misbehaviour
86 * is ridiculously low and, seems, we could use some mb() tricks
87 * to avoid misread sequence numbers, states etc. --ANK
89 * We don't need to initialize tmp_out.sack_ok as we don't use the results
92 tcp_timewait_state_process(struct inet_timewait_sock
*tw
, struct sk_buff
*skb
,
93 const struct tcphdr
*th
)
95 struct tcp_options_received tmp_opt
;
96 struct tcp_timewait_sock
*tcptw
= tcp_twsk((struct sock
*)tw
);
97 bool paws_reject
= false;
99 tmp_opt
.saw_tstamp
= 0;
100 if (th
->doff
> (sizeof(*th
) >> 2) && tcptw
->tw_ts_recent_stamp
) {
101 tcp_parse_options(skb
, &tmp_opt
, 0, NULL
);
103 if (tmp_opt
.saw_tstamp
) {
104 tmp_opt
.rcv_tsecr
-= tcptw
->tw_ts_offset
;
105 tmp_opt
.ts_recent
= tcptw
->tw_ts_recent
;
106 tmp_opt
.ts_recent_stamp
= tcptw
->tw_ts_recent_stamp
;
107 paws_reject
= tcp_paws_reject(&tmp_opt
, th
->rst
);
111 if (tw
->tw_substate
== TCP_FIN_WAIT2
) {
112 /* Just repeat all the checks of tcp_rcv_state_process() */
114 /* Out of window, send ACK */
116 !tcp_in_window(TCP_SKB_CB(skb
)->seq
, TCP_SKB_CB(skb
)->end_seq
,
118 tcptw
->tw_rcv_nxt
+ tcptw
->tw_rcv_wnd
))
124 if (th
->syn
&& !before(TCP_SKB_CB(skb
)->seq
, tcptw
->tw_rcv_nxt
))
129 !after(TCP_SKB_CB(skb
)->end_seq
, tcptw
->tw_rcv_nxt
) ||
130 TCP_SKB_CB(skb
)->end_seq
== TCP_SKB_CB(skb
)->seq
) {
132 return TCP_TW_SUCCESS
;
135 /* New data or FIN. If new data arrive after half-duplex close,
139 TCP_SKB_CB(skb
)->end_seq
!= tcptw
->tw_rcv_nxt
+ 1) {
141 inet_twsk_deschedule(tw
, &tcp_death_row
);
146 /* FIN arrived, enter true time-wait state. */
147 tw
->tw_substate
= TCP_TIME_WAIT
;
148 tcptw
->tw_rcv_nxt
= TCP_SKB_CB(skb
)->end_seq
;
149 if (tmp_opt
.saw_tstamp
) {
150 tcptw
->tw_ts_recent_stamp
= get_seconds();
151 tcptw
->tw_ts_recent
= tmp_opt
.rcv_tsval
;
154 if (tcp_death_row
.sysctl_tw_recycle
&&
155 tcptw
->tw_ts_recent_stamp
&&
156 tcp_tw_remember_stamp(tw
))
157 inet_twsk_schedule(tw
, &tcp_death_row
, tw
->tw_timeout
,
160 inet_twsk_schedule(tw
, &tcp_death_row
, TCP_TIMEWAIT_LEN
,
166 * Now real TIME-WAIT state.
169 * "When a connection is [...] on TIME-WAIT state [...]
170 * [a TCP] MAY accept a new SYN from the remote TCP to
171 * reopen the connection directly, if it:
173 * (1) assigns its initial sequence number for the new
174 * connection to be larger than the largest sequence
175 * number it used on the previous connection incarnation,
178 * (2) returns to TIME-WAIT state if the SYN turns out
179 * to be an old duplicate".
183 (TCP_SKB_CB(skb
)->seq
== tcptw
->tw_rcv_nxt
&&
184 (TCP_SKB_CB(skb
)->seq
== TCP_SKB_CB(skb
)->end_seq
|| th
->rst
))) {
185 /* In window segment, it may be only reset or bare ack. */
188 /* This is TIME_WAIT assassination, in two flavors.
189 * Oh well... nobody has a sufficient solution to this
192 if (sysctl_tcp_rfc1337
== 0) {
194 inet_twsk_deschedule(tw
, &tcp_death_row
);
196 return TCP_TW_SUCCESS
;
199 inet_twsk_schedule(tw
, &tcp_death_row
, TCP_TIMEWAIT_LEN
,
202 if (tmp_opt
.saw_tstamp
) {
203 tcptw
->tw_ts_recent
= tmp_opt
.rcv_tsval
;
204 tcptw
->tw_ts_recent_stamp
= get_seconds();
208 return TCP_TW_SUCCESS
;
211 /* Out of window segment.
213 All the segments are ACKed immediately.
215 The only exception is new SYN. We accept it, if it is
216 not old duplicate and we are not in danger to be killed
217 by delayed old duplicates. RFC check is that it has
218 newer sequence number works at rates <40Mbit/sec.
219 However, if paws works, it is reliable AND even more,
220 we even may relax silly seq space cutoff.
222 RED-PEN: we violate main RFC requirement, if this SYN will appear
223 old duplicate (i.e. we receive RST in reply to SYN-ACK),
224 we must return socket to time-wait state. It is not good,
228 if (th
->syn
&& !th
->rst
&& !th
->ack
&& !paws_reject
&&
229 (after(TCP_SKB_CB(skb
)->seq
, tcptw
->tw_rcv_nxt
) ||
230 (tmp_opt
.saw_tstamp
&&
231 (s32
)(tcptw
->tw_ts_recent
- tmp_opt
.rcv_tsval
) < 0))) {
232 u32 isn
= tcptw
->tw_snd_nxt
+ 65535 + 2;
235 TCP_SKB_CB(skb
)->tcp_tw_isn
= isn
;
240 NET_INC_STATS_BH(twsk_net(tw
), LINUX_MIB_PAWSESTABREJECTED
);
243 /* In this case we must reset the TIMEWAIT timer.
245 * If it is ACKless SYN it may be both old duplicate
246 * and new good SYN with random sequence number <rcv_nxt.
247 * Do not reschedule in the last case.
249 if (paws_reject
|| th
->ack
)
250 inet_twsk_schedule(tw
, &tcp_death_row
, TCP_TIMEWAIT_LEN
,
253 /* Send ACK. Note, we do not put the bucket,
254 * it will be released by caller.
259 return TCP_TW_SUCCESS
;
261 EXPORT_SYMBOL(tcp_timewait_state_process
);
264 * Move a socket to time-wait or dead fin-wait-2 state.
266 void tcp_time_wait(struct sock
*sk
, int state
, int timeo
)
268 struct inet_timewait_sock
*tw
= NULL
;
269 const struct inet_connection_sock
*icsk
= inet_csk(sk
);
270 const struct tcp_sock
*tp
= tcp_sk(sk
);
271 bool recycle_ok
= false;
273 if (tcp_death_row
.sysctl_tw_recycle
&& tp
->rx_opt
.ts_recent_stamp
)
274 recycle_ok
= tcp_remember_stamp(sk
);
276 if (tcp_death_row
.tw_count
< tcp_death_row
.sysctl_max_tw_buckets
)
277 tw
= inet_twsk_alloc(sk
, state
);
280 struct tcp_timewait_sock
*tcptw
= tcp_twsk((struct sock
*)tw
);
281 const int rto
= (icsk
->icsk_rto
<< 2) - (icsk
->icsk_rto
>> 1);
282 struct inet_sock
*inet
= inet_sk(sk
);
284 tw
->tw_transparent
= inet
->transparent
;
285 tw
->tw_rcv_wscale
= tp
->rx_opt
.rcv_wscale
;
286 tcptw
->tw_rcv_nxt
= tp
->rcv_nxt
;
287 tcptw
->tw_snd_nxt
= tp
->snd_nxt
;
288 tcptw
->tw_rcv_wnd
= tcp_receive_window(tp
);
289 tcptw
->tw_ts_recent
= tp
->rx_opt
.ts_recent
;
290 tcptw
->tw_ts_recent_stamp
= tp
->rx_opt
.ts_recent_stamp
;
291 tcptw
->tw_ts_offset
= tp
->tsoffset
;
293 #if IS_ENABLED(CONFIG_IPV6)
294 if (tw
->tw_family
== PF_INET6
) {
295 struct ipv6_pinfo
*np
= inet6_sk(sk
);
297 tw
->tw_v6_daddr
= sk
->sk_v6_daddr
;
298 tw
->tw_v6_rcv_saddr
= sk
->sk_v6_rcv_saddr
;
299 tw
->tw_tclass
= np
->tclass
;
300 tw
->tw_flowlabel
= np
->flow_label
>> 12;
301 tw
->tw_ipv6only
= sk
->sk_ipv6only
;
305 #ifdef CONFIG_TCP_MD5SIG
307 * The timewait bucket does not have the key DB from the
308 * sock structure. We just make a quick copy of the
309 * md5 key being used (if indeed we are using one)
310 * so the timewait ack generating code has the key.
313 struct tcp_md5sig_key
*key
;
314 tcptw
->tw_md5_key
= NULL
;
315 key
= tp
->af_specific
->md5_lookup(sk
, sk
);
317 tcptw
->tw_md5_key
= kmemdup(key
, sizeof(*key
), GFP_ATOMIC
);
318 if (tcptw
->tw_md5_key
&& !tcp_alloc_md5sig_pool())
324 /* Linkage updates. */
325 __inet_twsk_hashdance(tw
, sk
, &tcp_hashinfo
);
327 /* Get the TIME_WAIT timeout firing. */
332 tw
->tw_timeout
= rto
;
334 tw
->tw_timeout
= TCP_TIMEWAIT_LEN
;
335 if (state
== TCP_TIME_WAIT
)
336 timeo
= TCP_TIMEWAIT_LEN
;
339 inet_twsk_schedule(tw
, &tcp_death_row
, timeo
,
343 /* Sorry, if we're out of memory, just CLOSE this
344 * socket up. We've got bigger problems than
345 * non-graceful socket closings.
347 NET_INC_STATS_BH(sock_net(sk
), LINUX_MIB_TCPTIMEWAITOVERFLOW
);
350 tcp_update_metrics(sk
);
354 void tcp_twsk_destructor(struct sock
*sk
)
356 #ifdef CONFIG_TCP_MD5SIG
357 struct tcp_timewait_sock
*twsk
= tcp_twsk(sk
);
359 if (twsk
->tw_md5_key
)
360 kfree_rcu(twsk
->tw_md5_key
, rcu
);
363 EXPORT_SYMBOL_GPL(tcp_twsk_destructor
);
365 void tcp_openreq_init_rwin(struct request_sock
*req
,
366 struct sock
*sk
, struct dst_entry
*dst
)
368 struct inet_request_sock
*ireq
= inet_rsk(req
);
369 struct tcp_sock
*tp
= tcp_sk(sk
);
371 int mss
= dst_metric_advmss(dst
);
373 if (tp
->rx_opt
.user_mss
&& tp
->rx_opt
.user_mss
< mss
)
374 mss
= tp
->rx_opt
.user_mss
;
376 /* Set this up on the first call only */
377 req
->window_clamp
= tp
->window_clamp
? : dst_metric(dst
, RTAX_WINDOW
);
379 /* limit the window selection if the user enforce a smaller rx buffer */
380 if (sk
->sk_userlocks
& SOCK_RCVBUF_LOCK
&&
381 (req
->window_clamp
> tcp_full_space(sk
) || req
->window_clamp
== 0))
382 req
->window_clamp
= tcp_full_space(sk
);
384 /* tcp_full_space because it is guaranteed to be the first packet */
385 tcp_select_initial_window(tcp_full_space(sk
),
386 mss
- (ireq
->tstamp_ok
? TCPOLEN_TSTAMP_ALIGNED
: 0),
391 dst_metric(dst
, RTAX_INITRWND
));
392 ireq
->rcv_wscale
= rcv_wscale
;
394 EXPORT_SYMBOL(tcp_openreq_init_rwin
);
396 static void tcp_ecn_openreq_child(struct tcp_sock
*tp
,
397 const struct request_sock
*req
)
399 tp
->ecn_flags
= inet_rsk(req
)->ecn_ok
? TCP_ECN_OK
: 0;
402 void tcp_ca_openreq_child(struct sock
*sk
, const struct dst_entry
*dst
)
404 struct inet_connection_sock
*icsk
= inet_csk(sk
);
405 u32 ca_key
= dst_metric(dst
, RTAX_CC_ALGO
);
406 bool ca_got_dst
= false;
408 if (ca_key
!= TCP_CA_UNSPEC
) {
409 const struct tcp_congestion_ops
*ca
;
412 ca
= tcp_ca_find_key(ca_key
);
413 if (likely(ca
&& try_module_get(ca
->owner
))) {
414 icsk
->icsk_ca_dst_locked
= tcp_ca_dst_locked(dst
);
415 icsk
->icsk_ca_ops
= ca
;
421 if (!ca_got_dst
&& !try_module_get(icsk
->icsk_ca_ops
->owner
))
422 tcp_assign_congestion_control(sk
);
424 tcp_set_ca_state(sk
, TCP_CA_Open
);
426 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child
);
428 /* This is not only more efficient than what we used to do, it eliminates
429 * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
431 * Actually, we could lots of memory writes here. tp of listening
432 * socket contains all necessary default parameters.
434 struct sock
*tcp_create_openreq_child(struct sock
*sk
, struct request_sock
*req
, struct sk_buff
*skb
)
436 struct sock
*newsk
= inet_csk_clone_lock(sk
, req
, GFP_ATOMIC
);
439 const struct inet_request_sock
*ireq
= inet_rsk(req
);
440 struct tcp_request_sock
*treq
= tcp_rsk(req
);
441 struct inet_connection_sock
*newicsk
= inet_csk(newsk
);
442 struct tcp_sock
*newtp
= tcp_sk(newsk
);
444 /* Now setup tcp_sock */
445 newtp
->pred_flags
= 0;
447 newtp
->rcv_wup
= newtp
->copied_seq
=
448 newtp
->rcv_nxt
= treq
->rcv_isn
+ 1;
450 newtp
->snd_sml
= newtp
->snd_una
=
451 newtp
->snd_nxt
= newtp
->snd_up
= treq
->snt_isn
+ 1;
453 tcp_prequeue_init(newtp
);
454 INIT_LIST_HEAD(&newtp
->tsq_node
);
456 tcp_init_wl(newtp
, treq
->rcv_isn
);
459 newtp
->mdev_us
= jiffies_to_usecs(TCP_TIMEOUT_INIT
);
460 newicsk
->icsk_rto
= TCP_TIMEOUT_INIT
;
462 newtp
->packets_out
= 0;
463 newtp
->retrans_out
= 0;
464 newtp
->sacked_out
= 0;
465 newtp
->fackets_out
= 0;
466 newtp
->snd_ssthresh
= TCP_INFINITE_SSTHRESH
;
467 tcp_enable_early_retrans(newtp
);
468 newtp
->tlp_high_seq
= 0;
469 newtp
->lsndtime
= treq
->snt_synack
;
470 newtp
->total_retrans
= req
->num_retrans
;
472 /* So many TCP implementations out there (incorrectly) count the
473 * initial SYN frame in their delayed-ACK and congestion control
474 * algorithms that we must have the following bandaid to talk
475 * efficiently to them. -DaveM
477 newtp
->snd_cwnd
= TCP_INIT_CWND
;
478 newtp
->snd_cwnd_cnt
= 0;
480 tcp_init_xmit_timers(newsk
);
481 __skb_queue_head_init(&newtp
->out_of_order_queue
);
482 newtp
->write_seq
= newtp
->pushed_seq
= treq
->snt_isn
+ 1;
484 newtp
->rx_opt
.saw_tstamp
= 0;
486 newtp
->rx_opt
.dsack
= 0;
487 newtp
->rx_opt
.num_sacks
= 0;
491 if (sock_flag(newsk
, SOCK_KEEPOPEN
))
492 inet_csk_reset_keepalive_timer(newsk
,
493 keepalive_time_when(newtp
));
495 newtp
->rx_opt
.tstamp_ok
= ireq
->tstamp_ok
;
496 if ((newtp
->rx_opt
.sack_ok
= ireq
->sack_ok
) != 0) {
498 tcp_enable_fack(newtp
);
500 newtp
->window_clamp
= req
->window_clamp
;
501 newtp
->rcv_ssthresh
= req
->rcv_wnd
;
502 newtp
->rcv_wnd
= req
->rcv_wnd
;
503 newtp
->rx_opt
.wscale_ok
= ireq
->wscale_ok
;
504 if (newtp
->rx_opt
.wscale_ok
) {
505 newtp
->rx_opt
.snd_wscale
= ireq
->snd_wscale
;
506 newtp
->rx_opt
.rcv_wscale
= ireq
->rcv_wscale
;
508 newtp
->rx_opt
.snd_wscale
= newtp
->rx_opt
.rcv_wscale
= 0;
509 newtp
->window_clamp
= min(newtp
->window_clamp
, 65535U);
511 newtp
->snd_wnd
= (ntohs(tcp_hdr(skb
)->window
) <<
512 newtp
->rx_opt
.snd_wscale
);
513 newtp
->max_window
= newtp
->snd_wnd
;
515 if (newtp
->rx_opt
.tstamp_ok
) {
516 newtp
->rx_opt
.ts_recent
= req
->ts_recent
;
517 newtp
->rx_opt
.ts_recent_stamp
= get_seconds();
518 newtp
->tcp_header_len
= sizeof(struct tcphdr
) + TCPOLEN_TSTAMP_ALIGNED
;
520 newtp
->rx_opt
.ts_recent_stamp
= 0;
521 newtp
->tcp_header_len
= sizeof(struct tcphdr
);
524 #ifdef CONFIG_TCP_MD5SIG
525 newtp
->md5sig_info
= NULL
; /*XXX*/
526 if (newtp
->af_specific
->md5_lookup(sk
, newsk
))
527 newtp
->tcp_header_len
+= TCPOLEN_MD5SIG_ALIGNED
;
529 if (skb
->len
>= TCP_MSS_DEFAULT
+ newtp
->tcp_header_len
)
530 newicsk
->icsk_ack
.last_seg_size
= skb
->len
- newtp
->tcp_header_len
;
531 newtp
->rx_opt
.mss_clamp
= req
->mss
;
532 tcp_ecn_openreq_child(newtp
, req
);
533 newtp
->fastopen_rsk
= NULL
;
534 newtp
->syn_data_acked
= 0;
536 TCP_INC_STATS_BH(sock_net(sk
), TCP_MIB_PASSIVEOPENS
);
540 EXPORT_SYMBOL(tcp_create_openreq_child
);
543 * Process an incoming packet for SYN_RECV sockets represented as a
544 * request_sock. Normally sk is the listener socket but for TFO it
545 * points to the child socket.
547 * XXX (TFO) - The current impl contains a special check for ack
548 * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
550 * We don't need to initialize tmp_opt.sack_ok as we don't use the results
553 struct sock
*tcp_check_req(struct sock
*sk
, struct sk_buff
*skb
,
554 struct request_sock
*req
,
555 struct request_sock
**prev
,
558 struct tcp_options_received tmp_opt
;
560 const struct tcphdr
*th
= tcp_hdr(skb
);
561 __be32 flg
= tcp_flag_word(th
) & (TCP_FLAG_RST
|TCP_FLAG_SYN
|TCP_FLAG_ACK
);
562 bool paws_reject
= false;
564 BUG_ON(fastopen
== (sk
->sk_state
== TCP_LISTEN
));
566 tmp_opt
.saw_tstamp
= 0;
567 if (th
->doff
> (sizeof(struct tcphdr
)>>2)) {
568 tcp_parse_options(skb
, &tmp_opt
, 0, NULL
);
570 if (tmp_opt
.saw_tstamp
) {
571 tmp_opt
.ts_recent
= req
->ts_recent
;
572 /* We do not store true stamp, but it is not required,
573 * it can be estimated (approximately)
576 tmp_opt
.ts_recent_stamp
= get_seconds() - ((TCP_TIMEOUT_INIT
/HZ
)<<req
->num_timeout
);
577 paws_reject
= tcp_paws_reject(&tmp_opt
, th
->rst
);
581 /* Check for pure retransmitted SYN. */
582 if (TCP_SKB_CB(skb
)->seq
== tcp_rsk(req
)->rcv_isn
&&
583 flg
== TCP_FLAG_SYN
&&
586 * RFC793 draws (Incorrectly! It was fixed in RFC1122)
587 * this case on figure 6 and figure 8, but formal
588 * protocol description says NOTHING.
589 * To be more exact, it says that we should send ACK,
590 * because this segment (at least, if it has no data)
593 * CONCLUSION: RFC793 (even with RFC1122) DOES NOT
594 * describe SYN-RECV state. All the description
595 * is wrong, we cannot believe to it and should
596 * rely only on common sense and implementation
599 * Enforce "SYN-ACK" according to figure 8, figure 6
600 * of RFC793, fixed by RFC1122.
602 * Note that even if there is new data in the SYN packet
603 * they will be thrown away too.
605 * Reset timer after retransmitting SYNACK, similar to
606 * the idea of fast retransmit in recovery.
608 if (!inet_rtx_syn_ack(sk
, req
))
609 req
->expires
= min(TCP_TIMEOUT_INIT
<< req
->num_timeout
,
610 TCP_RTO_MAX
) + jiffies
;
614 /* Further reproduces section "SEGMENT ARRIVES"
615 for state SYN-RECEIVED of RFC793.
616 It is broken, however, it does not work only
617 when SYNs are crossed.
619 You would think that SYN crossing is impossible here, since
620 we should have a SYN_SENT socket (from connect()) on our end,
621 but this is not true if the crossed SYNs were sent to both
622 ends by a malicious third party. We must defend against this,
623 and to do that we first verify the ACK (as per RFC793, page
624 36) and reset if it is invalid. Is this a true full defense?
625 To convince ourselves, let us consider a way in which the ACK
626 test can still pass in this 'malicious crossed SYNs' case.
627 Malicious sender sends identical SYNs (and thus identical sequence
628 numbers) to both A and B:
633 By our good fortune, both A and B select the same initial
634 send sequence number of seven :-)
636 A: sends SYN|ACK, seq=7, ack_seq=8
637 B: sends SYN|ACK, seq=7, ack_seq=8
639 So we are now A eating this SYN|ACK, ACK test passes. So
640 does sequence test, SYN is truncated, and thus we consider
643 If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
644 bare ACK. Otherwise, we create an established connection. Both
645 ends (listening sockets) accept the new incoming connection and try
646 to talk to each other. 8-)
648 Note: This case is both harmless, and rare. Possibility is about the
649 same as us discovering intelligent life on another plant tomorrow.
651 But generally, we should (RFC lies!) to accept ACK
652 from SYNACK both here and in tcp_rcv_state_process().
653 tcp_rcv_state_process() does not, hence, we do not too.
655 Note that the case is absolutely generic:
656 we cannot optimize anything here without
657 violating protocol. All the checks must be made
658 before attempt to create socket.
661 /* RFC793 page 36: "If the connection is in any non-synchronized state ...
662 * and the incoming segment acknowledges something not yet
663 * sent (the segment carries an unacceptable ACK) ...
666 * Invalid ACK: reset will be sent by listening socket.
667 * Note that the ACK validity check for a Fast Open socket is done
668 * elsewhere and is checked directly against the child socket rather
669 * than req because user data may have been sent out.
671 if ((flg
& TCP_FLAG_ACK
) && !fastopen
&&
672 (TCP_SKB_CB(skb
)->ack_seq
!=
673 tcp_rsk(req
)->snt_isn
+ 1))
676 /* Also, it would be not so bad idea to check rcv_tsecr, which
677 * is essentially ACK extension and too early or too late values
678 * should cause reset in unsynchronized states.
681 /* RFC793: "first check sequence number". */
683 if (paws_reject
|| !tcp_in_window(TCP_SKB_CB(skb
)->seq
, TCP_SKB_CB(skb
)->end_seq
,
684 tcp_rsk(req
)->rcv_nxt
, tcp_rsk(req
)->rcv_nxt
+ req
->rcv_wnd
)) {
685 /* Out of window: send ACK and drop. */
686 if (!(flg
& TCP_FLAG_RST
))
687 req
->rsk_ops
->send_ack(sk
, skb
, req
);
689 NET_INC_STATS_BH(sock_net(sk
), LINUX_MIB_PAWSESTABREJECTED
);
693 /* In sequence, PAWS is OK. */
695 if (tmp_opt
.saw_tstamp
&& !after(TCP_SKB_CB(skb
)->seq
, tcp_rsk(req
)->rcv_nxt
))
696 req
->ts_recent
= tmp_opt
.rcv_tsval
;
698 if (TCP_SKB_CB(skb
)->seq
== tcp_rsk(req
)->rcv_isn
) {
699 /* Truncate SYN, it is out of window starting
700 at tcp_rsk(req)->rcv_isn + 1. */
701 flg
&= ~TCP_FLAG_SYN
;
704 /* RFC793: "second check the RST bit" and
705 * "fourth, check the SYN bit"
707 if (flg
& (TCP_FLAG_RST
|TCP_FLAG_SYN
)) {
708 TCP_INC_STATS_BH(sock_net(sk
), TCP_MIB_ATTEMPTFAILS
);
709 goto embryonic_reset
;
712 /* ACK sequence verified above, just make sure ACK is
713 * set. If ACK not set, just silently drop the packet.
715 * XXX (TFO) - if we ever allow "data after SYN", the
716 * following check needs to be removed.
718 if (!(flg
& TCP_FLAG_ACK
))
721 /* For Fast Open no more processing is needed (sk is the
727 /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
728 if (req
->num_timeout
< inet_csk(sk
)->icsk_accept_queue
.rskq_defer_accept
&&
729 TCP_SKB_CB(skb
)->end_seq
== tcp_rsk(req
)->rcv_isn
+ 1) {
730 inet_rsk(req
)->acked
= 1;
731 NET_INC_STATS_BH(sock_net(sk
), LINUX_MIB_TCPDEFERACCEPTDROP
);
735 /* OK, ACK is valid, create big socket and
736 * feed this segment to it. It will repeat all
737 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
738 * ESTABLISHED STATE. If it will be dropped after
739 * socket is created, wait for troubles.
741 child
= inet_csk(sk
)->icsk_af_ops
->syn_recv_sock(sk
, skb
, req
, NULL
);
743 goto listen_overflow
;
745 inet_csk_reqsk_queue_unlink(sk
, req
, prev
);
746 inet_csk_reqsk_queue_removed(sk
, req
);
748 inet_csk_reqsk_queue_add(sk
, req
, child
);
752 if (!sysctl_tcp_abort_on_overflow
) {
753 inet_rsk(req
)->acked
= 1;
758 if (!(flg
& TCP_FLAG_RST
)) {
759 /* Received a bad SYN pkt - for TFO We try not to reset
760 * the local connection unless it's really necessary to
761 * avoid becoming vulnerable to outside attack aiming at
762 * resetting legit local connections.
764 req
->rsk_ops
->send_reset(sk
, skb
);
765 } else if (fastopen
) { /* received a valid RST pkt */
766 reqsk_fastopen_remove(sk
, req
, true);
770 inet_csk_reqsk_queue_drop(sk
, req
, prev
);
771 NET_INC_STATS_BH(sock_net(sk
), LINUX_MIB_EMBRYONICRSTS
);
775 EXPORT_SYMBOL(tcp_check_req
);
778 * Queue segment on the new socket if the new socket is active,
779 * otherwise we just shortcircuit this and continue with
782 * For the vast majority of cases child->sk_state will be TCP_SYN_RECV
783 * when entering. But other states are possible due to a race condition
784 * where after __inet_lookup_established() fails but before the listener
785 * locked is obtained, other packets cause the same connection to
789 int tcp_child_process(struct sock
*parent
, struct sock
*child
,
793 int state
= child
->sk_state
;
795 if (!sock_owned_by_user(child
)) {
796 ret
= tcp_rcv_state_process(child
, skb
, tcp_hdr(skb
),
798 /* Wakeup parent, send SIGIO */
799 if (state
== TCP_SYN_RECV
&& child
->sk_state
!= state
)
800 parent
->sk_data_ready(parent
);
802 /* Alas, it is possible again, because we do lookup
803 * in main socket hash table and lock on listening
804 * socket does not protect us more.
806 __sk_add_backlog(child
, skb
);
809 bh_unlock_sock(child
);
813 EXPORT_SYMBOL(tcp_child_process
);