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 BACKOFF 1
41 #define RETRIES 16
42 
43 #ifndef __OpenBSD__
44 #define pledge(p, e) 0
45 #endif
46 
47 const char	*addr;		/* our addr */
48 const char	*ssh_tflag;
49 const char	*ssh_dest;
50 
51 char		 ssh_host[256];
52 char		 ssh_port[16];
53 
54 struct event	 sockev[MAXSOCK];
55 int		 socks[MAXSOCK];
56 int		 nsock;
57 
58 int		 debug;
59 int		 verbose;
60 
61 struct event	 sighupev;
62 struct event	 sigintev;
63 struct event	 sigtermev;
64 struct event	 sigchldev;
65 struct event	 siginfoev;
66 
67 struct timeval	 timeout = {600, 0}; /* 10 minutes */
68 struct event	 timeoutev;
69 
70 pid_t		 ssh_pid = -1;
71 
72 int		 conn;
73 
74 struct conn {
75 	int			 ntentative;
76 	struct timeval		 retry;
77 	struct event		 waitev;
78 	int			 source;
79 	struct bufferevent	*sourcebev;
80 	int			 to;
81 	struct bufferevent	*tobev;
82 };
83 
84 static void
85 sig_handler(int sig, short event, void *data)
86 {
87 	int status;
88 
89 	switch (sig) {
90 	case SIGHUP:
91 	case SIGINT:
92 	case SIGTERM:
93 		event_loopbreak();
94 		break;
95 	case SIGCHLD:
96 		if (waitpid(ssh_pid, &status, WNOHANG) == -1)
97 			fatal("waitpid");
98 		ssh_pid = -1;
99 		break;
100 #ifdef SIGINFO
101 	case SIGINFO:
102 #else
103 	case SIGUSR1:
104 #endif
105 		log_info("connections: %d", conn);
106 	}
107 }
108 
109 static void
110 spawn_ssh(void)
111 {
112 	log_debug("spawning ssh");
113 
114 	switch (ssh_pid = fork()) {
115 	case -1:
116 		fatal("fork");
117 	case 0:
118 		execl(SSH_PATH, "ssh", "-L", ssh_tflag, "-NTq", ssh_dest,
119 		    NULL);
120 		fatal("exec");
121 	default:
122 		return;
123 	}
124 }
125 
126 static void
127 killing_time(int fd, short event, void *data)
128 {
129 	if (ssh_pid == -1)
130 		return;
131 
132 	log_debug("timeout expired, killing ssh (%d)", ssh_pid);
133 	kill(ssh_pid, SIGTERM);
134 	ssh_pid = -1;
135 }
136 
137 static void
138 nopcb(struct bufferevent *bev, void *d)
139 {
140 	return;
141 }
142 
143 static void
144 sreadcb(struct bufferevent *bev, void *d)
145 {
146 	struct conn *c = d;
147 
148 	bufferevent_write_buffer(c->tobev, EVBUFFER_INPUT(bev));
149 }
150 
151 static void
152 treadcb(struct bufferevent *bev, void *d)
153 {
154 	struct conn *c = d;
155 
156 	bufferevent_write_buffer(c->sourcebev, EVBUFFER_INPUT(bev));
157 }
158 
159 static void
160 errcb(struct bufferevent *bev, short event, void *d)
161 {
162 	struct conn *c = d;
163 
164 	log_info("closing connection (event=%x)", event);
165 
166 	bufferevent_free(c->sourcebev);
167 	bufferevent_free(c->tobev);
168 
169 	close(c->source);
170 	close(c->to);
171 
172 	free(c);
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 	struct conn *c;
268 	int s;
269 
270 	log_debug("incoming connection");
271 
272 	if (evtimer_pending(&timeoutev, NULL))
273 		evtimer_del(&timeoutev);
274 
275 	if ((s = accept(fd, NULL, 0)) == -1)
276 		fatal("accept");
277 
278 	conn++;
279 
280 	if (ssh_pid == -1)
281 		spawn_ssh();
282 
283 	if ((c = calloc(1, sizeof(*c))) == NULL) {
284 		log_warn("calloc");
285 		close(s);
286 		return;
287 	}
288 
289 	c->source = s;
290 	c->retry.tv_sec = BACKOFF;
291 	evtimer_set(&c->waitev, try_to_connect, c);
292 	evtimer_add(&c->waitev, &c->retry);
293 }
294 
295 static const char *
296 copysec(const char *s, char *d, size_t len)
297 {
298 	const char *c;
299 
300 	if ((c = strchr(s, ':')) == NULL)
301 		return NULL;
302 	if ((size_t)(c - s) >= len-1)
303 		return NULL;
304 	memset(d, 0, len);
305 	memcpy(d, s, c - s);
306 	return c;
307 }
308 
309 static void
310 bind_socket(void)
311 {
312 	struct addrinfo hints, *res, *res0;
313 	int v, r, saved_errno;
314 	char host[64];
315 	const char *c, *h, *port, *cause;
316 
317 	if ((c = strchr(addr, ':')) == NULL) {
318 		h = NULL;
319 		port = addr;
320 	} else {
321 		if ((c = copysec(addr, host, sizeof(host))) == NULL)
322 			fatalx("name too long: %s", addr);
323 
324 		h = host;
325 		port = c+1;
326 	}
327 
328 	memset(&hints, 0, sizeof(hints));
329 	hints.ai_family = AF_UNSPEC;
330 	hints.ai_socktype = SOCK_STREAM;
331 	hints.ai_flags = AI_PASSIVE;
332 
333 	r = getaddrinfo(h, port, &hints, &res0);
334 	if (r != 0)
335 		fatalx("getaddrinfo(%s): %s", addr, gai_strerror(r));
336 
337 	for (res = res0; res && nsock < MAXSOCK; res = res->ai_next) {
338 		socks[nsock] = socket(res->ai_family, res->ai_socktype,
339 		    res->ai_protocol);
340 		if (socks[nsock] == -1) {
341 			cause = "socket";
342 			continue;
343 		}
344 
345 		if (bind(socks[nsock], res->ai_addr, res->ai_addrlen) == -1) {
346 			cause = "bind";
347 			saved_errno = errno;
348 			close(socks[nsock]);
349 			errno = saved_errno;
350 			continue;
351 		}
352 
353 		v = 1;
354 		if (setsockopt(socks[nsock], SOL_SOCKET, SO_REUSEADDR, &v,
355 		    sizeof(v)) == -1)
356 			fatal("setsockopt(SO_REUSEADDR)");
357 
358 		v = 1;
359 		if (setsockopt(socks[nsock], SOL_SOCKET, SO_REUSEPORT, &v,
360 		    sizeof(v)) == -1)
361 			fatal("setsockopt(SO_REUSEPORT)");
362 
363 		listen(socks[nsock], 5);
364 
365 		nsock++;
366 	}
367 	if (nsock == 0)
368 		fatal("%s", cause);
369 
370 	freeaddrinfo(res0);
371 }
372 
373 static void
374 parse_tflag(void)
375 {
376 	const char *c;
377 
378 	if (isdigit((unsigned char)*ssh_tflag)) {
379 		strlcpy(ssh_host, "localhost", sizeof(ssh_host));
380 		if (copysec(ssh_tflag, ssh_port, sizeof(ssh_port)) == NULL)
381 			goto err;
382 		return;
383 	}
384 
385 	if ((c = copysec(ssh_tflag, ssh_host, sizeof(ssh_host))) == NULL)
386 		goto err;
387 	if (copysec(c+1, ssh_port, sizeof(ssh_port)) == NULL)
388 		goto err;
389 	return;
390 
391 err:
392 	fatal("wrong value for -B");
393 }
394 
395 static void __dead
396 usage(void)
397 {
398 	fprintf(stderr, "usage: %s [-dv] -B sshaddr -b addr [-t timeout]"
399 	    " destination\n", getprogname());
400 	exit(1);
401 }
402 
403 int
404 main(int argc, char **argv)
405 {
406 	int ch, i;
407 	const char *errstr;
408 
409 	log_init(1, LOG_DAEMON);
410 	log_setverbose(1);
411 
412 	while ((ch = getopt(argc, argv, "B:b:dt:v")) != -1) {
413 		switch (ch) {
414 		case 'B':
415 			ssh_tflag = optarg;
416 			parse_tflag();
417 			break;
418 		case 'b':
419 			addr = optarg;
420 			break;
421 		case 'd':
422 			debug = 1;
423 			break;
424 		case 't':
425 			timeout.tv_sec = strtonum(optarg, 0, INT_MAX, &errstr);
426 			if (errstr != NULL)
427 				fatalx("timeout is %s: %s", errstr, optarg);
428 			break;
429 		case 'v':
430 			verbose = 1;
431 			break;
432 		default:
433 			usage();
434 		}
435 	}
436 	argc -= optind;
437 	argv += optind;
438 
439 	if (argc != 1 || addr == NULL || ssh_tflag == NULL)
440 		usage();
441 
442 	ssh_dest = argv[0];
443 
444 	log_init(debug, LOG_DAEMON);
445 	log_setverbose(verbose);
446 
447 	if (!debug)
448 		daemon(1, 0);
449 
450 	bind_socket();
451 
452 	signal(SIGPIPE, SIG_IGN);
453 
454 	event_init();
455 
456 	/* initialize the timer */
457 	evtimer_set(&timeoutev, killing_time, NULL);
458 
459 	signal_set(&sighupev, SIGHUP, sig_handler, NULL);
460 	signal_set(&sigintev, SIGINT, sig_handler, NULL);
461 	signal_set(&sigtermev, SIGTERM, sig_handler, NULL);
462 	signal_set(&sigchldev, SIGCHLD, sig_handler, NULL);
463 #ifdef SIGINFO
464 	signal_set(&siginfoev, SIGINFO, sig_handler, NULL);
465 #else
466 	signal_set(&siginfoev, SIGUSR1, sig_handler, NULL);
467 #endif
468 
469 	signal_add(&sighupev, NULL);
470 	signal_add(&sigintev, NULL);
471 	signal_add(&sigtermev, NULL);
472 	signal_add(&sigchldev, NULL);
473 	signal_add(&siginfoev, NULL);
474 
475 	for (i = 0; i < nsock; ++i) {
476 		event_set(&sockev[i], socks[i], EV_READ|EV_PERSIST,
477 		    do_accept, NULL);
478 		event_add(&sockev[i], NULL);
479 	}
480 
481 	/*
482 	 * dns, inet: bind the socket and connect to the childs.
483 	 * proc, exec: execute ssh on demand.
484 	 */
485 	if (pledge("stdio dns inet proc exec", NULL) == -1)
486 		fatal("pledge");
487 
488 	log_info("starting");
489 	event_dispatch();
490 
491 	if (ssh_pid != -1)
492 		kill(ssh_pid, SIGINT);
493 
494 	return 0;
495 }