1 /*
2  * Copyright (c) 2021 Omar Polo <op@omarpolo.com>
3  *
4  * Permission to use, copy, modify, and distribute this software for any
5  * purpose with or without fee is hereby granted, provided that the above
6  * copyright notice and this permission notice appear in all copies.
7  *
8  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15  */
16 
17 #include <sys/types.h>
18 #include <sys/socket.h>
19 #include <sys/wait.h>
20 
21 #include <ctype.h>
22 #include <errno.h>
23 #include <event.h>
24 #include <limits.h>
25 #include <netdb.h>
26 #include <signal.h>
27 #include <stdio.h>
28 #include <stdlib.h>
29 #include <string.h>
30 #include <syslog.h>
31 #include <unistd.h>
32 
33 #include "log.h"
34 
35 #ifndef SSH_PATH
36 #define SSH_PATH "/usr/bin/ssh"
37 #endif
38 
39 #define MAXSOCK 32
40 #define MAXCONN 16
41 #define BACKOFF 1
42 #define RETRIES 8
43 
44 #ifndef __OpenBSD__
45 #define pledge(p, e) 0
46 #endif
47 
48 const char	*addr;		/* our addr */
49 const char	*ssh_tflag;
50 const char	*ssh_dest;
51 
52 char		 ssh_host[256];
53 char		 ssh_port[16];
54 
55 struct event	 sockev[MAXSOCK];
56 int		 socks[MAXSOCK];
57 int		 nsock;
58 
59 int		 debug;
60 int		 verbose;
61 
62 struct event	 sighupev;
63 struct event	 sigintev;
64 struct event	 sigtermev;
65 struct event	 sigchldev;
66 struct event	 siginfoev;
67 
68 struct timeval	 timeout = {600, 0}; /* 10 minutes */
69 struct event	 timeoutev;
70 
71 pid_t		 ssh_pid = -1;
72 
73 int		 conn;
74 
75 struct conn {
76 	int			 ntentative;
77 	struct timeval		 retry;
78 	struct event		 waitev;
79 	int			 source;
80 	struct bufferevent	*sourcebev;
81 	int			 to;
82 	struct bufferevent	*tobev;
83 } conns[MAXCONN];
84 
85 static void
86 terminate(int fd, short event, void *data)
87 {
88 	event_loopbreak();
89 }
90 
91 static void
92 chld(int fd, short event, void *data)
93 {
94 	int status;
95 
96 	if (waitpid(ssh_pid, &status, WNOHANG) == -1)
97 		fatal("waitpid");
98 
99 	ssh_pid = -1;
100 }
101 
102 static void
103 info(int fd, short event, void *data)
104 {
105 	log_info("connections: %d", conn);
106 }
107 
108 static void
109 spawn_ssh(void)
110 {
111 	log_debug("spawning ssh");
112 
113 	switch (ssh_pid = fork()) {
114 	case -1:
115 		fatal("fork");
116 	case 0:
117 		execl(SSH_PATH, "ssh", "-L", ssh_tflag, "-NTq", ssh_dest,
118 		    NULL);
119 		fatal("exec");
120 	default:
121 		return;
122 	}
123 }
124 
125 static void
126 killing_time(int fd, short event, void *data)
127 {
128 	if (ssh_pid == -1)
129 		return;
130 
131 	log_debug("timeout expired, killing ssh (%d)", ssh_pid);
132 	kill(ssh_pid, SIGTERM);
133 	ssh_pid = -1;
134 }
135 
136 static void
137 nopcb(struct bufferevent *bev, void *d)
138 {
139 	return;
140 }
141 
142 static void
143 sreadcb(struct bufferevent *bev, void *d)
144 {
145 	struct conn *c = d;
146 
147 	bufferevent_write_buffer(c->tobev, EVBUFFER_INPUT(bev));
148 }
149 
150 static void
151 treadcb(struct bufferevent *bev, void *d)
152 {
153 	struct conn *c = d;
154 
155 	bufferevent_write_buffer(c->sourcebev, EVBUFFER_INPUT(bev));
156 }
157 
158 static void
159 errcb(struct bufferevent *bev, short event, void *d)
160 {
161 	struct conn *c = d;
162 
163 	log_info("closing connection (event=%x)", event);
164 
165 	bufferevent_free(c->sourcebev);
166 	bufferevent_free(c->tobev);
167 
168 	close(c->source);
169 	close(c->to);
170 
171 	c->source = -1;
172 	c->to = -1;
173 
174 	if (--conn == 0) {
175 		log_debug("scheduling ssh termination (%llds)",
176 		    (long long)timeout.tv_sec);
177 		if (timeout.tv_sec != 0) {
178 			evtimer_set(&timeoutev, killing_time, NULL);
179 			evtimer_add(&timeoutev, &timeout);
180 		}
181 	}
182 }
183 
184 static int
185 connect_to_ssh(void)
186 {
187 	struct addrinfo hints, *res, *res0;
188 	int r, saved_errno, sock;
189 	const char *cause;
190 
191 	memset(&hints, 0, sizeof(hints));
192 	hints.ai_family = AF_UNSPEC;
193 	hints.ai_socktype = SOCK_STREAM;
194 
195 	r = getaddrinfo(ssh_host, ssh_port, &hints, &res0);
196 	if (r != 0)
197 		fatal("getaddrinfo(\"%s\", \"%s\"): %s",
198 		    ssh_host, ssh_port, gai_strerror(r));
199 
200 	for (res = res0; res; res = res->ai_next) {
201 		sock = socket(res->ai_family, res->ai_socktype,
202 		    res->ai_protocol);
203 		if (sock == -1) {
204 			cause = "socket";
205 			continue;
206 		}
207 
208 		if (connect(sock, res->ai_addr, res->ai_addrlen) == -1) {
209 			cause = "connect";
210 			saved_errno = errno;
211 			close(sock);
212 			errno = saved_errno;
213 			sock = -1;
214 			continue;
215 		}
216 
217 		break;
218 	}
219 
220 	if (sock == -1)
221 		log_warn("%s", cause);
222 
223 	freeaddrinfo(res0);
224 	return sock;
225 }
226 
227 static void
228 try_to_connect(int fd, short event, void *d)
229 {
230 	struct conn *c = d;
231 
232 	/* ssh may die in the meantime */
233 	if (ssh_pid == -1) {
234 		close(c->source);
235 		c->source = -1;
236 		return;
237 	}
238 
239 	c->ntentative++;
240 	log_info("trying to connect to %s:%s (%d/%d)", ssh_host, ssh_port,
241 	    c->ntentative, RETRIES);
242 
243 	if ((c->to = connect_to_ssh()) == -1) {
244 		if (c->ntentative == RETRIES) {
245 			log_warnx("giving up connecting");
246 			close(c->source);
247 			c->source = -1;
248 			return;
249 		}
250 
251 		evtimer_set(&c->waitev, try_to_connect, c);
252 		evtimer_add(&c->waitev, &c->retry);
253 		return;
254 	}
255 
256 	c->sourcebev = bufferevent_new(c->source, sreadcb, nopcb, errcb, c);
257 	c->tobev = bufferevent_new(c->to, treadcb, nopcb, errcb, c);
258 	if (c->sourcebev == NULL || c->tobev == NULL)
259 		fatal("bufferevent_new");
260 	bufferevent_enable(c->sourcebev, EV_READ|EV_WRITE);
261 	bufferevent_enable(c->tobev, EV_READ|EV_WRITE);
262 }
263 
264 static void
265 do_accept(int fd, short event, void *data)
266 {
267 	int s, i;
268 
269 	log_debug("incoming connection");
270 
271 	if (evtimer_pending(&timeoutev, NULL))
272 		evtimer_del(&timeoutev);
273 
274 	if ((s = accept(fd, NULL, 0)) == -1)
275 		fatal("accept");
276 
277 	if (conn == MAXCONN) {
278 		log_warnx("dropping the connection, too many already");
279 		close(s);
280 		return;
281 	}
282 
283 	conn++;
284 
285 	if (ssh_pid == -1)
286 		spawn_ssh();
287 
288 	for (i = 0; i < MAXCONN; ++i) {
289 		if (conns[i].source != -1)
290 			continue;
291 
292 		conns[i].source = s;
293 		conns[i].ntentative = 0;
294 		conns[i].retry.tv_sec = BACKOFF;
295 		conns[i].retry.tv_usec = 0;
296 		evtimer_set(&conns[i].waitev, try_to_connect, &conns[i]);
297 		evtimer_add(&conns[i].waitev, &conns[i].retry);
298 		break;
299 	}
300 }
301 
302 static const char *
303 copysec(const char *s, char *d, size_t len)
304 {
305 	const char *c;
306 
307 	if ((c = strchr(s, ':')) == NULL)
308 		return NULL;
309 	if ((size_t)(c - s) >= len-1)
310 		return NULL;
311 	memset(d, 0, len);
312 	memcpy(d, s, c - s);
313 	return c;
314 }
315 
316 static void
317 bind_socket(void)
318 {
319 	struct addrinfo hints, *res, *res0;
320 	int v, r, saved_errno;
321 	char host[64];
322 	const char *c, *h, *port, *cause;
323 
324 	if ((c = strchr(addr, ':')) == NULL) {
325 		h = NULL;
326 		port = addr;
327 	} else {
328 		if ((c = copysec(addr, host, sizeof(host))) == NULL)
329 			fatalx("name too long: %s", addr);
330 
331 		h = host;
332 		port = c+1;
333 	}
334 
335 	memset(&hints, 0, sizeof(hints));
336 	hints.ai_family = AF_UNSPEC;
337 	hints.ai_socktype = SOCK_STREAM;
338 	hints.ai_flags = AI_PASSIVE;
339 
340 	r = getaddrinfo(h, port, &hints, &res0);
341 	if (r != 0)
342 		fatalx("getaddrinfo(%s): %s", addr, gai_strerror(r));
343 
344 	for (res = res0; res && nsock < MAXSOCK; res = res->ai_next) {
345 		socks[nsock] = socket(res->ai_family, res->ai_socktype,
346 		    res->ai_protocol);
347 		if (socks[nsock] == -1) {
348 			cause = "socket";
349 			continue;
350 		}
351 
352 		if (bind(socks[nsock], res->ai_addr, res->ai_addrlen) == -1) {
353 			cause = "bind";
354 			saved_errno = errno;
355 			close(socks[nsock]);
356 			errno = saved_errno;
357 			continue;
358 		}
359 
360 		v = 1;
361 		if (setsockopt(socks[nsock], SOL_SOCKET, SO_REUSEADDR, &v,
362 		    sizeof(v)) == -1)
363 			fatal("setsockopt(SO_REUSEADDR)");
364 
365 		v = 1;
366 		if (setsockopt(socks[nsock], SOL_SOCKET, SO_REUSEPORT, &v,
367 		    sizeof(v)) == -1)
368 			fatal("setsockopt(SO_REUSEPORT)");
369 
370 		listen(socks[nsock], 5);
371 
372 		nsock++;
373 	}
374 	if (nsock == 0)
375 		fatal("%s", cause);
376 
377 	freeaddrinfo(res0);
378 }
379 
380 static void
381 parse_tflag(void)
382 {
383 	const char *c;
384 
385 	if (isdigit(*ssh_tflag)) {
386 		strlcpy(ssh_host, "localhost", sizeof(ssh_host));
387 		if (copysec(ssh_tflag, ssh_port, sizeof(ssh_port)) == NULL)
388 			goto err;
389 		return;
390 	}
391 
392 	if ((c = copysec(ssh_tflag, ssh_host, sizeof(ssh_host))) == NULL)
393 		goto err;
394 	if (copysec(c+1, ssh_port, sizeof(ssh_port)) == NULL)
395 		goto err;
396 	return;
397 
398 err:
399 	fatal("wrong value for -B");
400 }
401 
402 static void __dead
403 usage(void)
404 {
405 	fprintf(stderr, "usage: %s [-dv] -B sshaddr -b addr [-t timeout]"
406 	    " destination\n", getprogname());
407 	exit(1);
408 }
409 
410 int
411 main(int argc, char **argv)
412 {
413 	int ch, i;
414 	const char *errstr;
415 
416 	log_init(1, LOG_DAEMON);
417 	log_setverbose(1);
418 
419 	while ((ch = getopt(argc, argv, "B:b:dt:v")) != -1) {
420 		switch (ch) {
421 		case 'B':
422 			ssh_tflag = optarg;
423 			parse_tflag();
424 			break;
425 		case 'b':
426 			addr = optarg;
427 			break;
428 		case 'd':
429 			debug = 1;
430 			break;
431 		case 't':
432 			timeout.tv_sec = strtonum(optarg, 0, INT_MAX, &errstr);
433 			if (errstr != NULL)
434 				fatalx("timeout is %s: %s", errstr, optarg);
435 			break;
436 		case 'v':
437 			verbose = 1;
438 			break;
439 		default:
440 			usage();
441 		}
442 	}
443 	argc -= optind;
444 	argv += optind;
445 
446 	if (argc != 1 || addr == NULL || ssh_tflag == NULL)
447 		usage();
448 
449 	ssh_dest = argv[0];
450 
451 	for (i = 0; i < MAXCONN; ++i) {
452 		conns[i].source = -1;
453 		conns[i].to = -1;
454 	}
455 
456 	log_init(debug, LOG_DAEMON);
457 	log_setverbose(verbose);
458 
459 	if (!debug)
460 		daemon(1, 0);
461 
462 	bind_socket();
463 
464 	signal(SIGPIPE, SIG_IGN);
465 
466 	event_init();
467 
468 	/* initialize the timer */
469 	evtimer_set(&timeoutev, killing_time, NULL);
470 
471 	signal_set(&sighupev, SIGHUP, terminate, NULL);
472 	signal_set(&sigintev, SIGINT, terminate, NULL);
473 	signal_set(&sigtermev, SIGTERM, terminate, NULL);
474 	signal_set(&sigchldev, SIGCHLD, chld, NULL);
475 #ifdef SIGINFO
476 	signal_set(&siginfoev, SIGINFO, info, NULL);
477 #else
478 	signal_set(&siginfoev, SIGUSR1, info, NULL);
479 #endif
480 
481 	signal_add(&sighupev, NULL);
482 	signal_add(&sigintev, NULL);
483 	signal_add(&sigtermev, NULL);
484 	signal_add(&sigchldev, NULL);
485 	signal_add(&siginfoev, NULL);
486 
487 	for (i = 0; i < nsock; ++i) {
488 		event_set(&sockev[i], socks[i], EV_READ|EV_PERSIST,
489 		    do_accept, NULL);
490 		event_add(&sockev[i], NULL);
491 	}
492 
493 	/*
494 	 * dns, inet: bind the socket and connect to the childs.
495 	 * proc, exec: execute ssh on demand.
496 	 */
497 	if (pledge("stdio dns inet proc exec", NULL) == -1)
498 		fatal("pledge");
499 
500 	log_info("starting");
501 	event_dispatch();
502 
503 	if (ssh_pid != -1)
504 		kill(ssh_pid, SIGINT);
505 
506 	return 0;
507 }