aboutsummaryrefslogtreecommitdiff
path: root/ssh-agent-filter.C
blob: 74b15ab0f1c1225862d73417d997c23cca0c20ac (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
/*
 * ssh-agent-filter.C -- filtering proxy for ssh-agent meant to be forwarded to untrusted servers
 *
 * Copyright (C) 2013-2018 Timo Weingärtner <timo@tiwe.de>
 *
 * This file is part of ssh-agent-filter.
 *
 * ssh-agent-filter is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * ssh-agent-filter is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with ssh-agent-filter.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <boost/program_options.hpp>
namespace po = boost::program_options;

#include <boost/filesystem.hpp>
namespace fs = boost::filesystem;

#include <boost/iostreams/stream.hpp>
#include <boost/iostreams/device/array.hpp>
#include <boost/iostreams/device/back_inserter.hpp>
#include <boost/iostreams/device/file_descriptor.hpp>
namespace io = boost::iostreams;

#include <string>
using std::string;

#include <vector>
using std::vector;

#include <set>
#include <map>
#include <iostream>
using std::cout;
using std::clog;
using std::endl;
using std::flush;

#include <stdexcept>
using std::runtime_error;
using std::length_error;
using std::invalid_argument;

#include <system_error>
using std::system_error;
using std::system_category;

#include <utility>
using std::pair;

#include <algorithm>

#include <thread>
#include <mutex>
using std::mutex;
using std::lock_guard;

#include <chrono>

#include <cerrno>
#include <csignal>
#include <cstdlib>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <sys/un.h>
#include <sys/wait.h>
#include <sysexits.h>
#include <nettle/md5.h>
#include <nettle/base64.h>
#include <nettle/base16.h>

#include "rfc4251.H"
#include "ssh-agent.h"
#include "version.h"

#ifndef SOCK_CLOEXEC
#define SOCK_CLOEXEC 0
#endif

vector<string> allowed_b64;
vector<string> allowed_md5;
vector<string> allowed_comment;
vector<string> confirmed_b64;
vector<string> confirmed_md5;
vector<string> confirmed_comment;
std::set<rfc4251::string> allowed_pubkeys;
std::map<rfc4251::string, string> confirmed_pubkeys;
bool debug{false};
bool all_confirmed{false};
string saf_name;
fs::path path;
mutex fd_fork_mutex;


string md5_hex (string const & s) {
	struct md5_ctx ctx;
	md5_init(&ctx);
	md5_update(&ctx, s.size(), reinterpret_cast<uint8_t const *>(s.data()));
	uint8_t bin[MD5_DIGEST_SIZE];
	md5_digest(&ctx, MD5_DIGEST_SIZE, bin);
	char hex[BASE16_ENCODE_LENGTH(MD5_DIGEST_SIZE)];
	base16_encode_update(hex, MD5_DIGEST_SIZE, bin);
	return {hex, sizeof(hex)};
}

string base64_encode (string const & s) {
	struct base64_encode_ctx ctx;
	base64_encode_init(&ctx);
	char b64[BASE64_ENCODE_LENGTH(s.size())];
	auto len = base64_encode_update(&ctx, b64, s.size(), reinterpret_cast<uint8_t const *>(s.data()));
	len += base64_encode_final(&ctx, b64 + len);
	return {b64, len};
}

void cloexec (int fd) {
	if (fcntl(fd, F_SETFD, fcntl(fd, F_GETFD) | FD_CLOEXEC))
		throw system_error(errno, system_category(), "fcntl");
}

void arm(std::ios & stream) {
	stream.exceptions(stream.badbit | stream.failbit);
}

int make_upstream_agent_conn () {
	char const * path;
	int sock;
	struct sockaddr_un addr;

	if (!(path = getenv("SSH_AUTH_SOCK")))
		throw invalid_argument("no $SSH_AUTH_SOCK");

	{
		lock_guard<mutex> lock{fd_fork_mutex};
		if ((sock = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0)) == -1)
			throw system_error(errno, system_category(), "socket");
		cloexec(sock);
	}
	
	addr.sun_family = AF_UNIX;
	
	if (strlen(path) >= sizeof(addr.sun_path))
		throw length_error("$SSH_AUTH_SOCK too long");

	strcpy(addr.sun_path, path);

	if (connect(sock, reinterpret_cast<struct sockaddr const *>(&addr), sizeof(addr)))
		throw system_error(errno, system_category(), "connect");

	return sock;
}

int make_listen_sock () {
	int sock;
	struct sockaddr_un addr;
	
	{
		lock_guard<mutex> lock{fd_fork_mutex};
		if ((sock = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0)) == -1)
			throw system_error(errno, system_category(), "socket");
		cloexec(sock);
	}

	if (fcntl(sock, F_SETFL, fcntl(sock, F_GETFL) | O_NONBLOCK))
		throw system_error(errno, system_category(), "fcntl");

	addr.sun_family = AF_UNIX;
	
	if (path.native().length() >= sizeof(addr.sun_path))
		throw length_error("path for listen socket too long");

	strcpy(addr.sun_path, path.c_str());

	if (bind(sock, reinterpret_cast<struct sockaddr const *>(&addr), sizeof(addr)))
		throw system_error(errno, system_category(), "bind");
	
	if (listen(sock, 0))
		throw system_error(errno, system_category(), "listen");

	return sock;
}

void parse_cmdline (int const argc, char const * const * const argv) {
	po::options_description opts{"Options"};
	opts.add_options()
		("all-confirmed,A",		po::bool_switch(&all_confirmed),"allow all other keys with confirmation")
		("comment,c",			po::value(&allowed_comment),	"key specified by comment")
		("comment-confirmed,C",		po::value(&confirmed_comment),	"key specified by comment, with confirmation")
		("debug,d",			po::bool_switch(&debug),	"show some debug info, don't fork")
		("fingerprint,fp,f",		po::value(&allowed_md5),	"key specified by pubkey's hex-encoded md5 fingerprint")
		("fingerprint-confirmed,F",	po::value(&confirmed_md5),	"key specified by pubkey's hex-encoded md5 fingerprint, with confirmation")
		("help,h",			"print this help message")
		("key,k",			po::value(&allowed_b64),	"key specified by base64-encoded pubkey")
		("key-confirmed,K",		po::value(&confirmed_b64),	"key specified by base64-encoded pubkey, with confirmation")
		("name,n",			po::value(&saf_name),		"name for this instance of ssh-agent-filter, for confirmation puposes")
		("version,V",			"print version information")
		;
	po::variables_map config;
	store(parse_command_line(argc, argv, opts), config);
	notify(config);
	
	if (config.count("help")) {
		cout << "Usage: ssh-agent-filter [ OPTIONS ]\n";
		cout << opts;
		cout << "Environment:\n";
		cout << "  SSH_AUTH_SOCK  socket of upstream ssh-agent\n";
		cout << "  SSH_ASKPASS    command to run for confirmation questions\n";
		exit(EX_OK);
	}
	
	if (config.count("version")) {
		cout << SSH_AGENT_FILTER_VERSION "\n";
		cout << "Written by Timo Weingärtner.\n";
		cout << "Report bugs to the Debian BTS at https://bugs.debian.org/\n";
		cout << "or by mail to timo@tiwe.de.\n";
		exit(EX_OK);
	}

	// canonicalize hashes
	for (auto & s : allowed_md5)
		for (auto it = s.begin(); it != s.end(); )
			if (isxdigit(*it)) {
				*it = tolower(*it);
				++it;
			} else
				it = s.erase(it);
}

void setup_filters () {
	io::stream<io::file_descriptor> agent{make_upstream_agent_conn(), io::close_handle};
	arm(agent);
	
	agent << rfc4251::string{string{SSH2_AGENTC_REQUEST_IDENTITIES}};
	rfc4251::string answer{agent};
	io::stream<io::array_source> answer_iss{answer.data(), answer.size()};
	arm(answer_iss);
	rfc4251::byte resp_code{answer_iss};
	if (resp_code != SSH2_AGENT_IDENTITIES_ANSWER)
		throw runtime_error{"unexpected answer from ssh-agent"};
	rfc4251::uint32 keycount{answer_iss};
	for (uint32_t i = keycount; i; --i) {
		rfc4251::string key{answer_iss};
		rfc4251::string comment{answer_iss};
		
		auto b64 = base64_encode(key);
		if (debug) clog << b64 << endl;
		
		auto md5 = md5_hex(key);
		if (debug) clog << md5 << endl;
		
		string comm(comment);
		if (debug) clog << comm << endl;
		
		bool allow{false};

		if (std::count(allowed_b64.begin(), allowed_b64.end(), b64)) {
			allow = true;
			if (debug) clog << "key allowed by equal base64 representation" << endl;
		}
		if (std::count(allowed_md5.begin(), allowed_md5.end(), md5)) {
			allow = true;
			if (debug) clog << "key allowed by matching md5 fingerprint" << endl;
		}
		if (std::count(allowed_comment.begin(), allowed_comment.end(), comm)) {
			allow = true;
			if (debug) clog << "key allowed by matching comment" << endl;
		}
		
		if (allow) allowed_pubkeys.emplace(std::move(key));
		else {
			bool confirm{false};
			
			if (std::count(confirmed_b64.begin(), confirmed_b64.end(), b64)) {
				confirm = true;
				if (debug) clog << "key allowed with confirmation by equal base64 representation" << endl;
			}
			if (std::count(confirmed_md5.begin(), confirmed_md5.end(), md5)) {
				confirm = true;
				if (debug) clog << "key allowed with confirmation by matching md5 fingerprint" << endl;
			}
			if (std::count(confirmed_comment.begin(), confirmed_comment.end(), comm)) {
				confirm = true;
				if (debug) clog << "key allowed with confirmation by matching comment" << endl;
			}
			if (all_confirmed) {
				confirm = true;
				if (debug) clog << "key allowed with confirmation by catch-all (-A)" << endl;
			}
			
			if (confirm) confirmed_pubkeys.emplace(std::move(key), std::move(comm));
		}

		if (debug) clog << endl;
	}
}

bool confirm (string const & question) {
	char const * sap;
	if (!(sap = getenv("SSH_ASKPASS")))
		sap = "ssh-askpass";
	pid_t pid;
	{
		lock_guard<mutex> lock{fd_fork_mutex};
		pid = fork();
	}
	if (pid < 0)
		throw runtime_error("fork()");
	if (pid == 0) {
		// child
		char const * args[3] = { sap, question.c_str(), nullptr };
		// see execvp(3p) for cast rationale
		execvp(sap, const_cast<char * const *>(args));
		throw system_error(errno, system_category(), "exec");
	} else {
		// parent
		int status;
		return waitpid(pid, &status, 0) > 0 && WIFEXITED(status) && WEXITSTATUS(status) == 0;
	}
}

bool dissect_auth_data_ssh_cert (rfc4251::string const & data, string & request_description) try {
	io::stream<io::array_source> datastream{data.data(), data.size()};
	arm(datastream);

	// Format specified in https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?annotate=1.13
	rfc4251::string		keytype{datastream};
	std::string		keytype_str{keytype};
	{
		// check for and remove suffix to get the base keytype
		std::string const	suffix{"-cert-v01@openssh.com"};
		if (keytype_str.length() <= suffix.length())
			return false;
		auto suffix_start = keytype_str.end() - suffix.length();
		if (!std::equal(suffix.begin(), suffix.end(), suffix_start))
			return false;
		keytype_str.erase(suffix_start, keytype_str.end());
	}
	rfc4251::string		nonce{datastream};
	std::ostringstream	key_to_be_signed{};
	if (keytype_str == "ssh-rsa") {
		rfc4251::string		e{datastream};
		rfc4251::string		n{datastream};
		key_to_be_signed << rfc4251::string{keytype_str} << e << n;
	} else if (keytype_str == "ssh-dss") {
		rfc4251::string		p{datastream};
		rfc4251::string		q{datastream};
		rfc4251::string		g{datastream};
		rfc4251::string		y{datastream};
		key_to_be_signed << rfc4251::string{keytype_str} << p << q << g << y;
	} else if (keytype_str == "ecdsa-sha2-nistp256"
			|| keytype_str == "ecdsa-sha2-nistp384"
			|| keytype_str == "ecdsa-sha2-nistp521") {
		rfc4251::string		curve{datastream};
		rfc4251::string		public_key{datastream};
		key_to_be_signed << rfc4251::string{keytype_str} << curve << public_key;
	} else if (keytype_str == "ssh-ed25519") {
		rfc4251::string		pk{datastream};
		key_to_be_signed << rfc4251::string{keytype_str} << pk;
	} else {
		return false;
	}
	rfc4251::uint64		serial{datastream};
	rfc4251::uint32		type{datastream};
	rfc4251::string		key_id{datastream};
	rfc4251::string		valid_principals{datastream};
	rfc4251::uint64		valid_after{datastream};
	rfc4251::uint64		valid_before{datastream};
	rfc4251::string		critical_options{datastream};
	rfc4251::string		extensions{datastream};
	rfc4251::string		reserved{datastream};
	rfc4251::string		signature_key{datastream};

	request_description = "The request is for a certificate signature on key " + base64_encode(key_to_be_signed.str()) + ".";

	return true;
} catch (...) {
	return false;
}

bool dissect_auth_data_ssh (rfc4251::string const & data, string & request_description) try {
	io::stream<io::array_source> datastream{data.data(), data.size()};
	arm(datastream);

	// Format specified in RFC 4252 Section 7
	rfc4251::string		session_identifier{datastream};
	rfc4251::byte		requesttype{datastream};
	rfc4251::string		username{datastream};
	rfc4251::string		servicename{datastream};
	rfc4251::string		publickeystring{datastream};
	rfc4251::boolean	shouldbetrue{datastream};
	rfc4251::string		publickeyalgorithm{datastream};
	rfc4251::string		publickey{datastream};

	request_description = "The request is for an ssh connection as user '" + string{username} + "' with service name '" + string{servicename} + "'.";

	if (string{servicename} == "pam_ssh_agent_auth") try {
		clog << base64_encode(session_identifier) << endl;
		io::stream<io::array_source> idstream{session_identifier.data(), session_identifier.size()};
		arm(idstream);

		rfc4251::uint32	type{idstream};
		if (type == 101) {
			// PAM_SSH_AGENT_AUTH_REQUESTv1
			rfc4251::string	cookie{idstream};
			rfc4251::string	user{idstream};
			rfc4251::string	ruser{idstream};
			rfc4251::string	pam_service{idstream};
			rfc4251::string	pwd{idstream};
			rfc4251::string	action{idstream};
			rfc4251::string	hostname{idstream};
			rfc4251::uint64	timestamp{idstream};

			string singleuser{user};
			if (user != ruser)
				singleuser += " (" + string{ruser} + ")";

			string additional;
			additional += "User '" + singleuser + "' wants to use '" + string{pam_service};
			additional += "' in '" + string{pwd};
			
			io::stream<io::array_source> actionstream{action.data(), action.size()};
			arm(actionstream);
			
			rfc4251::uint32	argc{actionstream};
			
			if (argc) {
				additional += " to run";
				for (uint32_t i = argc; i; --i) {
					rfc4251::string	argv{actionstream};
					additional += ' ' + string{argv};
				}
			}
			
			additional += " on " + string{hostname} + ".\n";
			
			auto now = std::chrono::system_clock::now();
			auto req_time = std::chrono::system_clock::from_time_t(static_cast<uint64_t>(timestamp));
			auto timediff = std::chrono::duration_cast<std::chrono::seconds>(now - req_time).count();
			
			additional += "The request was generated " + std::to_string(timediff) + " seconds ago.\n";
			request_description = move(additional);
		}
	} catch (...) {}
	
	return true;
} catch (...) {
	return false;
}

rfc4251::string handle_request (rfc4251::string const & r) {
	io::stream<io::array_source> request{r.data(), r.size()};
	rfc4251::string ret;
	io::stream<io::back_insert_device<vector<char>>> answer{ret.value};
	arm(request);
	arm(answer);
	rfc4251::byte request_code{request};
	switch (request_code) {
		case SSH2_AGENTC_REQUEST_IDENTITIES:
			{
				io::stream<io::file_descriptor> agent{make_upstream_agent_conn(), io::close_handle};
				arm(agent);
				agent << rfc4251::string{string{SSH2_AGENTC_REQUEST_IDENTITIES}};
				// temp to test key filtering when signing
				//return rfc4251::string{agent};
				rfc4251::string agent_answer{agent};
				io::stream<io::array_source> agent_answer_iss{agent_answer.data(), agent_answer.size()};
				arm(agent_answer_iss);
				rfc4251::byte answer_code{agent_answer_iss};
				rfc4251::uint32 keycount{agent_answer_iss};
				if (answer_code != SSH2_AGENT_IDENTITIES_ANSWER)
					throw runtime_error{"unexpected answer from ssh-agent"};
				vector<pair<rfc4251::string, rfc4251::string>> keys;
				for (uint32_t i = keycount; i; --i) {
					rfc4251::string key{agent_answer_iss};
					rfc4251::string comment{agent_answer_iss};
					if (allowed_pubkeys.count(key) or confirmed_pubkeys.count(key))
						keys.emplace_back(std::move(key), std::move(comment));
				}
				answer << answer_code << rfc4251::uint32{static_cast<uint32_t>(keys.size())};
				for (auto const & k : keys)
					answer << k.first << k.second;
			}
			break;
		case SSH2_AGENTC_SIGN_REQUEST:
			{
				rfc4251::string key{request};
				rfc4251::string data{request};
				rfc4251::uint32 flags{request};
				bool allow{false};
				
				if (allowed_pubkeys.count(key))
					allow = true;
				else {
					auto it = confirmed_pubkeys.find(key);
					if (it != confirmed_pubkeys.end()) {
						string request_description;
						bool dissect_ok{false};
						if (!dissect_ok)
							dissect_ok = dissect_auth_data_ssh_cert(data, request_description);
						if (!dissect_ok)
							dissect_ok = dissect_auth_data_ssh(data, request_description);
						if (!dissect_ok)
							request_description = "The request format is unknown.";
						
						string question = "Something behind the ssh-agent-filter";
						if (saf_name.length())
							question += " named '" + saf_name + "'";
						question += " requested use of the key named '" + it->second + "'.\n";
						question += request_description;
						allow = confirm(question);
					}
				}
				
				if (allow) {
					io::stream<io::file_descriptor> agent{make_upstream_agent_conn(), io::close_handle};
					arm(agent);
					rfc4251::string agent_answer;
					
					agent << r;
					return rfc4251::string{agent};
				} else
					answer << rfc4251::byte{SSH_AGENT_FAILURE};
			}
			break;
		case SSH_AGENTC_REQUEST_RSA_IDENTITIES:
			answer << rfc4251::byte{SSH_AGENT_RSA_IDENTITIES_ANSWER};
			// we got no SSHv1 keys
			answer << rfc4251::uint32{0};
			break;
		case SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES:
			answer << rfc4251::byte{SSH_AGENT_SUCCESS};
			break;
		case SSH_AGENTC_RSA_CHALLENGE:
		case SSH_AGENTC_ADD_RSA_IDENTITY:
		case SSH_AGENTC_REMOVE_RSA_IDENTITY:
		case SSH_AGENTC_ADD_RSA_ID_CONSTRAINED:
		case SSH2_AGENTC_ADD_IDENTITY:
		case SSH2_AGENTC_REMOVE_IDENTITY:
		case SSH2_AGENTC_REMOVE_ALL_IDENTITIES:
		case SSH2_AGENTC_ADD_ID_CONSTRAINED:
		case SSH_AGENTC_ADD_SMARTCARD_KEY:
		case SSH_AGENTC_REMOVE_SMARTCARD_KEY:
		case SSH_AGENTC_LOCK:
		case SSH_AGENTC_UNLOCK:
		case SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED:
		default:
			answer << rfc4251::byte{SSH_AGENT_FAILURE};
			break;
	}

	answer << flush;
	return ret;
}

void handle_client (int const sock) try {
	if (fcntl(sock, F_SETFL, fcntl(sock, F_GETFL) & ~O_NONBLOCK))
		throw system_error(errno, system_category(), "fcntl");

	io::stream<io::file_descriptor> client{sock, io::close_handle};
	arm(client);
	
	for (;;)
		client << handle_request(rfc4251::string{client}) << flush;
} catch (...) {
}

void sighandler (int sig) {
	switch (sig) {
		case SIGPIPE:
			break;
		default:
			remove(path);
			std::exit(0);
	}
}

int main (int const argc, char const * const * const argv) {
	parse_cmdline(argc, argv);
	
	setup_filters();

	path = fs::current_path() / ("agent." + std::to_string(getpid()));
	int listen_sock = make_listen_sock();

	if (not debug) {
		pid_t pid = fork();
		if (pid == -1)
			throw system_error(errno, system_category(), "fork");
		if (pid > 0) {
			cout << "SSH_AUTH_SOCK='" << path.native() << "'; export SSH_AUTH_SOCK;" << endl;
			cout << "SSH_AGENT_PID='" << pid << "'; export SSH_AGENT_PID;" << endl;
			cout << "echo 'Agent pid " << pid << "';" << endl;
			exit(EX_OK);
		}
	
		// the following stuff is optional, so we don't do error checking
		setsid();
		static_cast<void>(chdir("/"));
		int devnull = open("/dev/null", O_RDWR);
		dup2(devnull, 0);
		dup2(devnull, 1);
		dup2(devnull, 2);
		close(devnull);
	} else {
		cout << "copy this to another terminal:" << endl;
		cout << "SSH_AUTH_SOCK='" << path.native() << "'; export SSH_AUTH_SOCK;" << endl;
	}
	
	signal(SIGINT, sighandler);
	signal(SIGPIPE, sighandler);
	signal(SIGHUP, sighandler);
	signal(SIGTERM, sighandler);

	for (;;) {
		fd_set fds;
		FD_ZERO(&fds);
		FD_SET(listen_sock, &fds);
		select(listen_sock + 1, &fds, nullptr, nullptr, nullptr);
		int client_sock;
		{
			lock_guard<mutex> lock{fd_fork_mutex};
			if ((client_sock = accept(listen_sock, nullptr, nullptr)) == -1) {
				if (errno == EAGAIN || errno == EWOULDBLOCK)
					continue;
				else
					break;
			}
			cloexec(client_sock);
		}
		std::thread t{handle_client, client_sock};
		t.detach();
	}
}