Annotation of src/usr.bin/ssh/ssh.1, Revision 1.426
1.1 deraadt 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
1.59 deraadt 6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
1.93 deraadt 12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.59 deraadt 15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 24: .\"
1.59 deraadt 25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 35: .\"
1.426 ! dtucker 36: .\" $OpenBSD: ssh.1,v 1.425 2021/07/28 05:57:42 jmc Exp $
! 37: .Dd $Mdocdate: July 28 2021 $
1.2 deraadt 38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.408 jmc 42: .Nd OpenSSH remote login client
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
1.377 djm 45: .Op Fl 46AaCfGgKkMNnqsTtVvXxYy
1.390 djm 46: .Op Fl B Ar bind_interface
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.306 jmc 49: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
1.331 dtucker 50: .Op Fl E Ar log_file
1.2 deraadt 51: .Op Fl e Ar escape_char
1.176 jmc 52: .Op Fl F Ar configfile
1.292 jmc 53: .Op Fl I Ar pkcs11
1.2 deraadt 54: .Op Fl i Ar identity_file
1.386 millert 55: .Op Fl J Ar destination
1.360 millert 56: .Op Fl L Ar address
1.176 jmc 57: .Op Fl l Ar login_name
58: .Op Fl m Ar mac_spec
1.198 djm 59: .Op Fl O Ar ctl_cmd
1.176 jmc 60: .Op Fl o Ar option
61: .Op Fl p Ar port
1.362 djm 62: .Op Fl Q Ar query_option
1.360 millert 63: .Op Fl R Ar address
1.198 djm 64: .Op Fl S Ar ctl_path
1.290 dtucker 65: .Op Fl W Ar host : Ns Ar port
1.306 jmc 66: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.386 millert 67: .Ar destination
1.2 deraadt 68: .Op Ar command
1.44 aaron 69: .Sh DESCRIPTION
1.2 deraadt 70: .Nm
1.96 deraadt 71: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 72: executing commands on a remote machine.
1.365 mmcc 73: It is intended to provide secure encrypted communications between
1.40 aaron 74: two untrusted hosts over an insecure network.
1.348 millert 75: X11 connections, arbitrary TCP ports and
76: .Ux Ns -domain
77: sockets can also be forwarded over the secure channel.
1.2 deraadt 78: .Pp
79: .Nm
1.44 aaron 80: connects and logs into the specified
1.388 jmc 81: .Ar destination ,
1.386 millert 82: which may be specified as either
1.388 jmc 83: .Sm off
84: .Oo user @ Oc hostname
85: .Sm on
86: or a URI of the form
87: .Sm off
88: .No ssh:// Oo user @ Oc hostname Op : port .
89: .Sm on
1.1 deraadt 90: The user must prove
1.420 dtucker 91: their identity to the remote machine using one of several methods
1.365 mmcc 92: (see below).
1.49 markus 93: .Pp
1.386 millert 94: If a
1.176 jmc 95: .Ar command
96: is specified,
1.219 jmc 97: it is executed on the remote host instead of a login shell.
1.2 deraadt 98: .Pp
1.218 jmc 99: The options are as follows:
1.361 millert 100: .Pp
1.360 millert 101: .Bl -tag -width Ds -compact
1.218 jmc 102: .It Fl 4
103: Forces
1.2 deraadt 104: .Nm
1.218 jmc 105: to use IPv4 addresses only.
1.360 millert 106: .Pp
1.218 jmc 107: .It Fl 6
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv6 addresses only.
1.360 millert 111: .Pp
1.218 jmc 112: .It Fl A
1.407 jmc 113: Enables forwarding of connections from an authentication agent such as
114: .Xr ssh-agent 1 .
1.218 jmc 115: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 116: .Pp
1.218 jmc 117: Agent forwarding should be enabled with caution.
118: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 119: (for the agent's
1.286 sobrado 120: .Ux Ns -domain
121: socket) can access the local agent through the forwarded connection.
1.218 jmc 122: An attacker cannot obtain key material from the agent,
123: however they can perform operations on the keys that enable them to
124: authenticate using the identities loaded into the agent.
1.407 jmc 125: A safer alternative may be to use a jump host
126: (see
127: .Fl J ) .
1.360 millert 128: .Pp
1.218 jmc 129: .It Fl a
130: Disables forwarding of the authentication agent connection.
1.390 djm 131: .Pp
1.391 jmc 132: .It Fl B Ar bind_interface
1.390 djm 133: Bind to the address of
1.391 jmc 134: .Ar bind_interface
1.390 djm 135: before attempting to connect to the destination host.
136: This is only useful on systems with more than one address.
1.360 millert 137: .Pp
1.218 jmc 138: .It Fl b Ar bind_address
139: Use
140: .Ar bind_address
141: on the local machine as the source address
142: of the connection.
143: Only useful on systems with more than one address.
1.360 millert 144: .Pp
1.218 jmc 145: .It Fl C
146: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 147: data for forwarded X11, TCP and
148: .Ux Ns -domain
149: connections).
1.218 jmc 150: The compression algorithm is the same used by
1.377 djm 151: .Xr gzip 1 .
1.218 jmc 152: Compression is desirable on modem lines and other
153: slow connections, but will only slow down things on fast networks.
154: The default value can be set on a host-by-host basis in the
155: configuration files; see the
156: .Cm Compression
157: option.
1.360 millert 158: .Pp
1.218 jmc 159: .It Fl c Ar cipher_spec
160: Selects the cipher specification for encrypting the session.
161: .Ar cipher_spec
162: is a comma-separated list of ciphers
163: listed in order of preference.
1.283 jmc 164: See the
165: .Cm Ciphers
1.307 dtucker 166: keyword in
167: .Xr ssh_config 5
168: for more information.
1.360 millert 169: .Pp
1.218 jmc 170: .It Fl D Xo
171: .Sm off
172: .Oo Ar bind_address : Oc
173: .Ar port
174: .Sm on
175: .Xc
176: Specifies a local
177: .Dq dynamic
178: application-level port forwarding.
179: This works by allocating a socket to listen to
180: .Ar port
181: on the local side, optionally bound to the specified
182: .Ar bind_address .
183: Whenever a connection is made to this port, the
184: connection is forwarded over the secure channel, and the application
185: protocol is then used to determine where to connect to from the
186: remote machine.
187: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 188: .Nm
1.218 jmc 189: will act as a SOCKS server.
190: Only root can forward privileged ports.
191: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 192: .Pp
1.308 djm 193: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 194: Only the superuser can forward privileged ports.
195: By default, the local port is bound in accordance with the
196: .Cm GatewayPorts
197: setting.
198: However, an explicit
199: .Ar bind_address
200: may be used to bind the connection to a specific address.
201: The
202: .Ar bind_address
203: of
204: .Dq localhost
205: indicates that the listening port be bound for local use only, while an
206: empty address or
207: .Sq *
208: indicates that the port should be available from all interfaces.
1.360 millert 209: .Pp
1.331 dtucker 210: .It Fl E Ar log_file
211: Append debug logs to
212: .Ar log_file
213: instead of standard error.
1.360 millert 214: .Pp
1.229 jmc 215: .It Fl e Ar escape_char
1.218 jmc 216: Sets the escape character for sessions with a pty (default:
217: .Ql ~ ) .
218: The escape character is only recognized at the beginning of a line.
219: The escape character followed by a dot
220: .Pq Ql \&.
221: closes the connection;
222: followed by control-Z suspends the connection;
223: and followed by itself sends the escape character once.
224: Setting the character to
1.2 deraadt 225: .Dq none
1.218 jmc 226: disables any escapes and makes the session fully transparent.
1.360 millert 227: .Pp
1.218 jmc 228: .It Fl F Ar configfile
229: Specifies an alternative per-user configuration file.
230: If a configuration file is given on the command line,
231: the system-wide configuration file
232: .Pq Pa /etc/ssh/ssh_config
233: will be ignored.
234: The default for the per-user configuration file is
235: .Pa ~/.ssh/config .
1.411 naddy 236: If set to
237: .Dq none ,
238: no configuration files will be read.
1.360 millert 239: .Pp
1.218 jmc 240: .It Fl f
241: Requests
242: .Nm
243: to go to background just before command execution.
244: This is useful if
1.176 jmc 245: .Nm
1.218 jmc 246: is going to ask for passwords or passphrases, but the user
247: wants it in the background.
248: This implies
249: .Fl n .
250: The recommended way to start X11 programs at a remote site is with
251: something like
252: .Ic ssh -f host xterm .
1.277 djm 253: .Pp
254: If the
255: .Cm ExitOnForwardFailure
256: configuration option is set to
257: .Dq yes ,
258: then a client started with
259: .Fl f
260: will wait for all remote port forwards to be successfully established
261: before placing itself in the background.
1.424 djm 262: Refer to the description of
263: .Cm ForkAfterAuthentication
264: in
265: .Xr ssh_config 5
266: for details.
1.360 millert 267: .Pp
1.350 djm 268: .It Fl G
269: Causes
270: .Nm
271: to print its configuration after evaluating
272: .Cm Host
273: and
274: .Cm Match
275: blocks and exit.
1.360 millert 276: .Pp
1.218 jmc 277: .It Fl g
278: Allows remote hosts to connect to local forwarded ports.
1.346 djm 279: If used on a multiplexed connection, then this option must be specified
280: on the master process.
1.360 millert 281: .Pp
1.291 markus 282: .It Fl I Ar pkcs11
1.294 jmc 283: Specify the PKCS#11 shared library
1.176 jmc 284: .Nm
1.401 naddy 285: should use to communicate with a PKCS#11 token providing keys for user
286: authentication.
1.360 millert 287: .Pp
1.218 jmc 288: .It Fl i Ar identity_file
289: Selects a file from which the identity (private key) for
1.310 djm 290: public key authentication is read.
1.426 ! dtucker 291: You can also specify a public key file to use the corresponding
! 292: private key that is loaded in
! 293: .Xr ssh-agent 1
! 294: when the private key file is not present locally.
1.218 jmc 295: The default is
1.310 djm 296: .Pa ~/.ssh/id_dsa ,
1.343 naddy 297: .Pa ~/.ssh/id_ecdsa ,
1.404 naddy 298: .Pa ~/.ssh/id_ecdsa_sk ,
1.406 naddy 299: .Pa ~/.ssh/id_ed25519 ,
300: .Pa ~/.ssh/id_ed25519_sk
1.310 djm 301: and
1.381 naddy 302: .Pa ~/.ssh/id_rsa .
1.218 jmc 303: Identity files may also be specified on
304: a per-host basis in the configuration file.
305: It is possible to have multiple
306: .Fl i
307: options (and multiple identities specified in
308: configuration files).
1.364 jmc 309: If no certificates have been explicitly specified by the
1.363 djm 310: .Cm CertificateFile
311: directive,
1.302 djm 312: .Nm
313: will also try to load certificate information from the filename obtained
314: by appending
315: .Pa -cert.pub
316: to identity filenames.
1.360 millert 317: .Pp
1.386 millert 318: .It Fl J Ar destination
1.375 djm 319: Connect to the target host by first making a
320: .Nm
1.386 millert 321: connection to the jump host described by
322: .Ar destination
1.376 jmc 323: and then establishing a TCP forwarding to the ultimate destination from
1.375 djm 324: there.
325: Multiple jump hops may be specified separated by comma characters.
326: This is a shortcut to specify a
327: .Cm ProxyJump
328: configuration directive.
1.400 djm 329: Note that configuration directives supplied on the command-line generally
330: apply to the destination host and not any specified jump hosts.
331: Use
332: .Pa ~/.ssh/config
333: to specify configuration for jump hosts.
1.375 djm 334: .Pp
1.269 djm 335: .It Fl K
336: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
337: credentials to the server.
1.360 millert 338: .Pp
1.218 jmc 339: .It Fl k
340: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 341: .Pp
1.218 jmc 342: .It Fl L Xo
343: .Sm off
344: .Oo Ar bind_address : Oc
345: .Ar port : host : hostport
346: .Sm on
347: .Xc
1.360 millert 348: .It Fl L Xo
349: .Sm off
350: .Oo Ar bind_address : Oc
351: .Ar port : remote_socket
352: .Sm on
353: .Xc
354: .It Fl L Xo
355: .Sm off
356: .Ar local_socket : host : hostport
357: .Sm on
358: .Xc
359: .It Fl L Xo
360: .Sm off
361: .Ar local_socket : remote_socket
362: .Sm on
363: .Xc
364: Specifies that connections to the given TCP port or Unix socket on the local
365: (client) host are to be forwarded to the given host and port, or Unix socket,
366: on the remote side.
367: This works by allocating a socket to listen to either a TCP
1.218 jmc 368: .Ar port
369: on the local side, optionally bound to the specified
1.360 millert 370: .Ar bind_address ,
371: or to a Unix socket.
372: Whenever a connection is made to the local port or socket, the
1.218 jmc 373: connection is forwarded over the secure channel, and a connection is
1.360 millert 374: made to either
1.218 jmc 375: .Ar host
376: port
1.360 millert 377: .Ar hostport ,
378: or the Unix socket
379: .Ar remote_socket ,
1.218 jmc 380: from the remote machine.
1.360 millert 381: .Pp
1.218 jmc 382: Port forwardings can also be specified in the configuration file.
1.360 millert 383: Only the superuser can forward privileged ports.
1.308 djm 384: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 385: .Pp
1.218 jmc 386: By default, the local port is bound in accordance with the
387: .Cm GatewayPorts
388: setting.
389: However, an explicit
390: .Ar bind_address
391: may be used to bind the connection to a specific address.
1.2 deraadt 392: The
1.218 jmc 393: .Ar bind_address
394: of
395: .Dq localhost
396: indicates that the listening port be bound for local use only, while an
397: empty address or
398: .Sq *
399: indicates that the port should be available from all interfaces.
1.360 millert 400: .Pp
1.218 jmc 401: .It Fl l Ar login_name
402: Specifies the user to log in as on the remote machine.
403: This also may be specified on a per-host basis in the configuration file.
1.360 millert 404: .Pp
1.218 jmc 405: .It Fl M
406: Places the
407: .Nm
408: client into
409: .Dq master
410: mode for connection sharing.
1.231 stevesk 411: Multiple
412: .Fl M
413: options places
414: .Nm
415: into
416: .Dq master
1.397 djm 417: mode but with confirmation required using
418: .Xr ssh-askpass 1
419: before each operation that changes the multiplexing state
420: (e.g. opening a new session).
1.218 jmc 421: Refer to the description of
422: .Cm ControlMaster
423: in
424: .Xr ssh_config 5
425: for details.
1.360 millert 426: .Pp
1.218 jmc 427: .It Fl m Ar mac_spec
1.369 jmc 428: A comma-separated list of MAC (message authentication code) algorithms,
429: specified in order of preference.
1.218 jmc 430: See the
431: .Cm MACs
432: keyword for more information.
1.360 millert 433: .Pp
1.218 jmc 434: .It Fl N
435: Do not execute a remote command.
1.369 jmc 436: This is useful for just forwarding ports.
1.422 djm 437: Refer to the description of
438: .Cm SessionType
439: in
440: .Xr ssh_config 5
441: for details.
1.360 millert 442: .Pp
1.218 jmc 443: .It Fl n
444: Redirects stdin from
445: .Pa /dev/null
446: (actually, prevents reading from stdin).
447: This must be used when
1.2 deraadt 448: .Nm
1.218 jmc 449: is run in the background.
450: A common trick is to use this to run X11 programs on a remote machine.
451: For example,
452: .Ic ssh -n shadows.cs.hut.fi emacs &
453: will start an emacs on shadows.cs.hut.fi, and the X11
454: connection will be automatically forwarded over an encrypted channel.
455: The
1.2 deraadt 456: .Nm
1.218 jmc 457: program will be put in the background.
458: (This does not work if
1.2 deraadt 459: .Nm
1.218 jmc 460: needs to ask for a password or passphrase; see also the
461: .Fl f
462: option.)
1.423 djm 463: Refer to the description of
464: .Cm StdinNull
465: in
466: .Xr ssh_config 5
467: for details.
1.360 millert 468: .Pp
1.218 jmc 469: .It Fl O Ar ctl_cmd
470: Control an active connection multiplexing master process.
471: When the
472: .Fl O
473: option is specified, the
474: .Ar ctl_cmd
475: argument is interpreted and passed to the master process.
476: Valid commands are:
477: .Dq check
1.305 markus 478: (check that the master process is running),
479: .Dq forward
1.318 jmc 480: (request forwardings without command execution),
1.323 okan 481: .Dq cancel
482: (cancel forwardings),
1.218 jmc 483: .Dq exit
1.318 jmc 484: (request the master to exit), and
1.317 djm 485: .Dq stop
486: (request the master to stop accepting further multiplexing requests).
1.360 millert 487: .Pp
1.218 jmc 488: .It Fl o Ar option
489: Can be used to give options in the format used in the configuration file.
490: This is useful for specifying options for which there is no separate
491: command-line flag.
492: For full details of the options listed below, and their possible values, see
493: .Xr ssh_config 5 .
1.2 deraadt 494: .Pp
1.218 jmc 495: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 496: .It AddKeysToAgent
1.218 jmc 497: .It AddressFamily
498: .It BatchMode
499: .It BindAddress
1.338 djm 500: .It CanonicalDomains
1.339 djm 501: .It CanonicalizeFallbackLocal
502: .It CanonicalizeHostname
503: .It CanonicalizeMaxDots
504: .It CanonicalizePermittedCNAMEs
1.399 jmc 505: .It CASignatureAlgorithms
1.363 djm 506: .It CertificateFile
1.218 jmc 507: .It CheckHostIP
508: .It Ciphers
509: .It ClearAllForwardings
510: .It Compression
1.380 naddy 511: .It ConnectionAttempts
1.218 jmc 512: .It ConnectTimeout
513: .It ControlMaster
514: .It ControlPath
1.321 djm 515: .It ControlPersist
1.218 jmc 516: .It DynamicForward
517: .It EscapeChar
1.263 markus 518: .It ExitOnForwardFailure
1.353 jmc 519: .It FingerprintHash
1.424 djm 520: .It ForkAfterAuthentication
1.218 jmc 521: .It ForwardAgent
522: .It ForwardX11
1.321 djm 523: .It ForwardX11Timeout
1.218 jmc 524: .It ForwardX11Trusted
525: .It GatewayPorts
526: .It GlobalKnownHostsFile
527: .It GSSAPIAuthentication
528: .It GSSAPIDelegateCredentials
529: .It HashKnownHosts
530: .It Host
1.418 naddy 531: .It HostbasedAcceptedAlgorithms
1.218 jmc 532: .It HostbasedAuthentication
533: .It HostKeyAlgorithms
534: .It HostKeyAlias
1.403 jmc 535: .It Hostname
1.374 jmc 536: .It IdentitiesOnly
1.371 markus 537: .It IdentityAgent
1.218 jmc 538: .It IdentityFile
1.316 jmc 539: .It IPQoS
1.321 djm 540: .It KbdInteractiveAuthentication
1.218 jmc 541: .It KbdInteractiveDevices
1.314 jmc 542: .It KexAlgorithms
1.415 djm 543: .It KnownHostsCommand
1.218 jmc 544: .It LocalCommand
545: .It LocalForward
546: .It LogLevel
547: .It MACs
1.337 jmc 548: .It Match
1.218 jmc 549: .It NoHostAuthenticationForLocalhost
550: .It NumberOfPasswordPrompts
551: .It PasswordAuthentication
552: .It PermitLocalCommand
1.419 markus 553: .It PermitRemoteOpen
1.291 markus 554: .It PKCS11Provider
1.218 jmc 555: .It Port
556: .It PreferredAuthentications
557: .It ProxyCommand
1.375 djm 558: .It ProxyJump
1.336 jmc 559: .It ProxyUseFdpass
1.416 dtucker 560: .It PubkeyAcceptedAlgorithms
1.218 jmc 561: .It PubkeyAuthentication
1.251 dtucker 562: .It RekeyLimit
1.382 bluhm 563: .It RemoteCommand
1.218 jmc 564: .It RemoteForward
1.319 jmc 565: .It RequestTTY
1.218 jmc 566: .It SendEnv
567: .It ServerAliveInterval
568: .It ServerAliveCountMax
1.422 djm 569: .It SessionType
1.395 jmc 570: .It SetEnv
1.423 djm 571: .It StdinNull
1.347 jmc 572: .It StreamLocalBindMask
573: .It StreamLocalBindUnlink
1.218 jmc 574: .It StrictHostKeyChecking
575: .It TCPKeepAlive
576: .It Tunnel
577: .It TunnelDevice
1.354 djm 578: .It UpdateHostKeys
1.218 jmc 579: .It User
580: .It UserKnownHostsFile
581: .It VerifyHostKeyDNS
1.276 jmc 582: .It VisualHostKey
1.218 jmc 583: .It XAuthLocation
584: .El
1.360 millert 585: .Pp
1.218 jmc 586: .It Fl p Ar port
587: Port to connect to on the remote host.
588: This can be specified on a
589: per-host basis in the configuration file.
1.360 millert 590: .Pp
1.362 djm 591: .It Fl Q Ar query_option
1.425 jmc 592: Queries for the algorithms supported by one of the following features:
1.341 deraadt 593: .Ar cipher
1.332 djm 594: (supported symmetric ciphers),
1.341 deraadt 595: .Ar cipher-auth
1.340 djm 596: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 597: .Ar help
598: (supported query terms for use with the
599: .Fl Q
600: flag),
1.341 deraadt 601: .Ar mac
1.332 djm 602: (supported message integrity codes),
1.341 deraadt 603: .Ar kex
1.332 djm 604: (key exchange algorithms),
1.341 deraadt 605: .Ar key
1.362 djm 606: (key types),
607: .Ar key-cert
608: (certificate key types),
609: .Ar key-plain
1.398 djm 610: (non-certificate key types),
1.410 dtucker 611: .Ar key-sig
612: (all key types and signature algorithms),
1.356 djm 613: .Ar protocol-version
1.398 djm 614: (supported SSH protocol versions), and
615: .Ar sig
616: (supported signature algorithms).
1.410 dtucker 617: Alternatively, any keyword from
618: .Xr ssh_config 5
619: or
620: .Xr sshd_config 5
621: that takes an algorithm list may be used as an alias for the corresponding
622: query_option.
1.360 millert 623: .Pp
1.218 jmc 624: .It Fl q
625: Quiet mode.
1.271 djm 626: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 627: .Pp
1.218 jmc 628: .It Fl R Xo
629: .Sm off
630: .Oo Ar bind_address : Oc
631: .Ar port : host : hostport
632: .Sm on
633: .Xc
1.360 millert 634: .It Fl R Xo
635: .Sm off
636: .Oo Ar bind_address : Oc
637: .Ar port : local_socket
638: .Sm on
639: .Xc
640: .It Fl R Xo
641: .Sm off
642: .Ar remote_socket : host : hostport
643: .Sm on
644: .Xc
645: .It Fl R Xo
646: .Sm off
647: .Ar remote_socket : local_socket
648: .Sm on
649: .Xc
1.384 markus 650: .It Fl R Xo
651: .Sm off
652: .Oo Ar bind_address : Oc
653: .Ar port
654: .Sm on
655: .Xc
1.360 millert 656: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 657: (server) host are to be forwarded to the local side.
658: .Pp
1.360 millert 659: This works by allocating a socket to listen to either a TCP
1.218 jmc 660: .Ar port
1.360 millert 661: or to a Unix socket on the remote side.
662: Whenever a connection is made to this port or Unix socket, the
663: connection is forwarded over the secure channel, and a connection
1.384 markus 664: is made from the local machine to either an explicit destination specified by
1.218 jmc 665: .Ar host
666: port
1.360 millert 667: .Ar hostport ,
668: or
669: .Ar local_socket ,
1.384 markus 670: or, if no explicit destination was specified,
671: .Nm
672: will act as a SOCKS 4/5 proxy and forward connections to the destinations
673: requested by the remote SOCKS client.
1.2 deraadt 674: .Pp
1.218 jmc 675: Port forwardings can also be specified in the configuration file.
676: Privileged ports can be forwarded only when
677: logging in as root on the remote machine.
1.324 jmc 678: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 679: .Pp
1.360 millert 680: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 681: interface only.
1.280 tobias 682: This may be overridden by specifying a
1.218 jmc 683: .Ar bind_address .
684: An empty
685: .Ar bind_address ,
686: or the address
687: .Ql * ,
688: indicates that the remote socket should listen on all interfaces.
689: Specifying a remote
690: .Ar bind_address
691: will only succeed if the server's
692: .Cm GatewayPorts
693: option is enabled (see
694: .Xr sshd_config 5 ) .
1.281 djm 695: .Pp
696: If the
697: .Ar port
698: argument is
1.282 djm 699: .Ql 0 ,
1.281 djm 700: the listen port will be dynamically allocated on the server and reported
701: to the client at run time.
1.305 markus 702: When used together with
703: .Ic -O forward
704: the allocated port will be printed to the standard output.
1.360 millert 705: .Pp
1.218 jmc 706: .It Fl S Ar ctl_path
1.304 jmc 707: Specifies the location of a control socket for connection sharing,
1.303 djm 708: or the string
709: .Dq none
710: to disable connection sharing.
1.218 jmc 711: Refer to the description of
712: .Cm ControlPath
713: and
714: .Cm ControlMaster
715: in
716: .Xr ssh_config 5
717: for details.
1.360 millert 718: .Pp
1.218 jmc 719: .It Fl s
720: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 721: Subsystems facilitate the use of SSH
722: as a secure transport for other applications (e.g.\&
1.218 jmc 723: .Xr sftp 1 ) .
724: The subsystem is specified as the remote command.
1.422 djm 725: Refer to the description of
726: .Cm SessionType
727: in
728: .Xr ssh_config 5
729: for details.
1.360 millert 730: .Pp
1.218 jmc 731: .It Fl T
1.357 dtucker 732: Disable pseudo-terminal allocation.
1.360 millert 733: .Pp
1.218 jmc 734: .It Fl t
1.357 dtucker 735: Force pseudo-terminal allocation.
1.218 jmc 736: This can be used to execute arbitrary
737: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 738: e.g. when implementing menu services.
1.218 jmc 739: Multiple
740: .Fl t
741: options force tty allocation, even if
1.194 jakob 742: .Nm
1.218 jmc 743: has no local tty.
1.360 millert 744: .Pp
1.218 jmc 745: .It Fl V
746: Display the version number and exit.
1.360 millert 747: .Pp
1.218 jmc 748: .It Fl v
749: Verbose mode.
750: Causes
1.176 jmc 751: .Nm
1.218 jmc 752: to print debugging messages about its progress.
753: This is helpful in
754: debugging connection, authentication, and configuration problems.
755: Multiple
756: .Fl v
757: options increase the verbosity.
758: The maximum is 3.
1.360 millert 759: .Pp
1.290 dtucker 760: .It Fl W Ar host : Ns Ar port
761: Requests that standard input and output on the client be forwarded to
762: .Ar host
763: on
764: .Ar port
765: over the secure channel.
766: Implies
767: .Fl N ,
768: .Fl T ,
769: .Cm ExitOnForwardFailure
770: and
1.373 jmc 771: .Cm ClearAllForwardings ,
772: though these can be overridden in the configuration file or using
1.372 dtucker 773: .Fl o
774: command line options.
1.360 millert 775: .Pp
1.261 stevesk 776: .It Fl w Xo
777: .Ar local_tun Ns Op : Ns Ar remote_tun
778: .Xc
779: Requests
780: tunnel
781: device forwarding with the specified
1.218 jmc 782: .Xr tun 4
1.261 stevesk 783: devices between the client
784: .Pq Ar local_tun
785: and the server
786: .Pq Ar remote_tun .
787: .Pp
1.228 jmc 788: The devices may be specified by numerical ID or the keyword
789: .Dq any ,
790: which uses the next available tunnel device.
1.261 stevesk 791: If
792: .Ar remote_tun
793: is not specified, it defaults to
794: .Dq any .
1.228 jmc 795: See also the
1.218 jmc 796: .Cm Tunnel
1.261 stevesk 797: and
798: .Cm TunnelDevice
799: directives in
1.218 jmc 800: .Xr ssh_config 5 .
1.393 djm 801: .Pp
1.261 stevesk 802: If the
803: .Cm Tunnel
1.393 djm 804: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 805: .Dq point-to-point .
1.393 djm 806: If a different
807: .Cm Tunnel
808: forwarding mode it desired, then it should be specified before
809: .Fl w .
1.360 millert 810: .Pp
1.218 jmc 811: .It Fl X
812: Enables X11 forwarding.
1.54 markus 813: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 814: .Pp
1.218 jmc 815: X11 forwarding should be enabled with caution.
1.168 jmc 816: Users with the ability to bypass file permissions on the remote host
1.218 jmc 817: (for the user's X authorization database)
818: can access the local X11 display through the forwarded connection.
819: An attacker may then be able to perform activities such as keystroke monitoring.
820: .Pp
821: For this reason, X11 forwarding is subjected to X11 SECURITY extension
822: restrictions by default.
823: Please refer to the
824: .Nm
825: .Fl Y
826: option and the
827: .Cm ForwardX11Trusted
828: directive in
829: .Xr ssh_config 5
830: for more information.
1.360 millert 831: .Pp
1.218 jmc 832: .It Fl x
833: Disables X11 forwarding.
1.360 millert 834: .Pp
1.218 jmc 835: .It Fl Y
836: Enables trusted X11 forwarding.
837: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
838: controls.
1.360 millert 839: .Pp
1.278 djm 840: .It Fl y
841: Send log information using the
842: .Xr syslog 3
843: system module.
844: By default this information is sent to stderr.
1.218 jmc 845: .El
1.224 jmc 846: .Pp
847: .Nm
848: may additionally obtain configuration data from
849: a per-user configuration file and a system-wide configuration file.
850: The file format and configuration options are described in
851: .Xr ssh_config 5 .
1.222 jmc 852: .Sh AUTHENTICATION
1.377 djm 853: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 854: .Pp
855: The methods available for authentication are:
1.260 jmc 856: GSSAPI-based authentication,
1.222 jmc 857: host-based authentication,
858: public key authentication,
1.421 dtucker 859: keyboard-interactive authentication,
1.222 jmc 860: and password authentication.
861: Authentication methods are tried in the order specified above,
1.369 jmc 862: though
863: .Cm PreferredAuthentications
864: can be used to change the default order.
1.222 jmc 865: .Pp
866: Host-based authentication works as follows:
1.218 jmc 867: If the machine the user logs in from is listed in
868: .Pa /etc/hosts.equiv
869: or
870: .Pa /etc/shosts.equiv
1.412 djm 871: on the remote machine, the user is non-root and the user names are
1.218 jmc 872: the same on both sides, or if the files
873: .Pa ~/.rhosts
874: or
875: .Pa ~/.shosts
876: exist in the user's home directory on the
877: remote machine and contain a line containing the name of the client
878: machine and the name of the user on that machine, the user is
1.222 jmc 879: considered for login.
880: Additionally, the server
881: .Em must
882: be able to verify the client's
883: host key (see the description of
1.218 jmc 884: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 885: and
1.222 jmc 886: .Pa ~/.ssh/known_hosts ,
887: below)
888: for login to be permitted.
1.218 jmc 889: This authentication method closes security holes due to IP
1.222 jmc 890: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 891: [Note to the administrator:
892: .Pa /etc/hosts.equiv ,
893: .Pa ~/.rhosts ,
894: and the rlogin/rsh protocol in general, are inherently insecure and should be
895: disabled if security is desired.]
1.189 dtucker 896: .Pp
1.222 jmc 897: Public key authentication works as follows:
898: The scheme is based on public-key cryptography,
899: using cryptosystems
900: where encryption and decryption are done using separate keys,
901: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 902: The idea is that each user creates a public/private
903: key pair for authentication purposes.
904: The server knows the public key, and only the user knows the private key.
1.222 jmc 905: .Nm
906: implements public key authentication protocol automatically,
1.349 sobrado 907: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 908: The HISTORY section of
1.222 jmc 909: .Xr ssl 8
1.311 jmc 910: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 911: .Pp
1.218 jmc 912: The file
913: .Pa ~/.ssh/authorized_keys
914: lists the public keys that are permitted for logging in.
915: When the user logs in, the
1.2 deraadt 916: .Nm
1.218 jmc 917: program tells the server which key pair it would like to use for
918: authentication.
1.222 jmc 919: The client proves that it has access to the private key
920: and the server checks that the corresponding public key
921: is authorized to accept the account.
1.383 djm 922: .Pp
923: The server may inform the client of errors that prevented public key
924: authentication from succeeding after authentication completes using a
925: different method.
926: These may be viewed by increasing the
927: .Cm LogLevel
928: to
929: .Cm DEBUG
930: or higher (e.g. by using the
931: .Fl v
932: flag).
1.218 jmc 933: .Pp
1.420 dtucker 934: The user creates their key pair by running
1.218 jmc 935: .Xr ssh-keygen 1 .
936: This stores the private key in
1.222 jmc 937: .Pa ~/.ssh/id_dsa
1.369 jmc 938: (DSA),
1.310 djm 939: .Pa ~/.ssh/id_ecdsa
1.369 jmc 940: (ECDSA),
1.404 naddy 941: .Pa ~/.ssh/id_ecdsa_sk
1.409 naddy 942: (authenticator-hosted ECDSA),
1.343 naddy 943: .Pa ~/.ssh/id_ed25519
1.369 jmc 944: (Ed25519),
1.406 naddy 945: .Pa ~/.ssh/id_ed25519_sk
1.409 naddy 946: (authenticator-hosted Ed25519),
1.222 jmc 947: or
948: .Pa ~/.ssh/id_rsa
1.369 jmc 949: (RSA)
1.218 jmc 950: and stores the public key in
1.222 jmc 951: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 952: (DSA),
1.310 djm 953: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 954: (ECDSA),
1.404 naddy 955: .Pa ~/.ssh/id_ecdsa_sk.pub
1.409 naddy 956: (authenticator-hosted ECDSA),
1.343 naddy 957: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 958: (Ed25519),
1.406 naddy 959: .Pa ~/.ssh/id_ed25519_sk.pub
1.409 naddy 960: (authenticator-hosted Ed25519),
1.222 jmc 961: or
962: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 963: (RSA)
1.218 jmc 964: in the user's home directory.
1.222 jmc 965: The user should then copy the public key
1.218 jmc 966: to
967: .Pa ~/.ssh/authorized_keys
1.420 dtucker 968: in their home directory on the remote machine.
1.222 jmc 969: The
1.218 jmc 970: .Pa authorized_keys
971: file corresponds to the conventional
972: .Pa ~/.rhosts
973: file, and has one key
1.222 jmc 974: per line, though the lines can be very long.
1.218 jmc 975: After this, the user can log in without giving the password.
976: .Pp
1.301 jmc 977: A variation on public key authentication
978: is available in the form of certificate authentication:
979: instead of a set of public/private keys,
980: signed certificates are used.
981: This has the advantage that a single trusted certification authority
982: can be used in place of many public/private keys.
1.333 jmc 983: See the CERTIFICATES section of
1.301 jmc 984: .Xr ssh-keygen 1
985: for more information.
986: .Pp
987: The most convenient way to use public key or certificate authentication
988: may be with an authentication agent.
1.218 jmc 989: See
990: .Xr ssh-agent 1
1.366 jcs 991: and (optionally) the
992: .Cm AddKeysToAgent
993: directive in
994: .Xr ssh_config 5
1.218 jmc 995: for more information.
996: .Pp
1.421 dtucker 997: Keyboard-interactive authentication works as follows:
1.222 jmc 998: The server sends an arbitrary
999: .Qq challenge
1.421 dtucker 1000: text and prompts for a response, possibly multiple times.
1001: Examples of keyboard-interactive authentication include
1.335 jmc 1002: .Bx
1003: Authentication (see
1.222 jmc 1004: .Xr login.conf 5 )
1.335 jmc 1005: and PAM (some
1006: .Pf non- Ox
1007: systems).
1.222 jmc 1008: .Pp
1009: Finally, if other authentication methods fail,
1.218 jmc 1010: .Nm
1011: prompts the user for a password.
1012: The password is sent to the remote
1013: host for checking; however, since all communications are encrypted,
1014: the password cannot be seen by someone listening on the network.
1.232 jmc 1015: .Pp
1016: .Nm
1017: automatically maintains and checks a database containing
1018: identification for all hosts it has ever been used with.
1019: Host keys are stored in
1020: .Pa ~/.ssh/known_hosts
1021: in the user's home directory.
1022: Additionally, the file
1023: .Pa /etc/ssh/ssh_known_hosts
1024: is automatically checked for known hosts.
1025: Any new hosts are automatically added to the user's file.
1026: If a host's identification ever changes,
1027: .Nm
1028: warns about this and disables password authentication to prevent
1029: server spoofing or man-in-the-middle attacks,
1030: which could otherwise be used to circumvent the encryption.
1031: The
1032: .Cm StrictHostKeyChecking
1033: option can be used to control logins to machines whose
1034: host key is not known or has changed.
1035: .Pp
1.218 jmc 1036: When the user's identity has been accepted by the server, the server
1.357 dtucker 1037: either executes the given command in a non-interactive session or,
1038: if no command has been specified, logs into the machine and gives
1039: the user a normal shell as an interactive session.
1.218 jmc 1040: All communication with
1041: the remote command or shell will be automatically encrypted.
1042: .Pp
1.357 dtucker 1043: If an interactive session is requested
1044: .Nm
1045: by default will only request a pseudo-terminal (pty) for interactive
1046: sessions when the client has one.
1047: The flags
1048: .Fl T
1049: and
1050: .Fl t
1051: can be used to override this behaviour.
1052: .Pp
1053: If a pseudo-terminal has been allocated the
1.218 jmc 1054: user may use the escape characters noted below.
1055: .Pp
1.357 dtucker 1056: If no pseudo-terminal has been allocated,
1.218 jmc 1057: the session is transparent and can be used to reliably transfer binary data.
1058: On most systems, setting the escape character to
1059: .Dq none
1060: will also make the session transparent even if a tty is used.
1061: .Pp
1062: The session terminates when the command or shell on the remote
1.247 jmc 1063: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1064: .Sh ESCAPE CHARACTERS
1.218 jmc 1065: When a pseudo-terminal has been requested,
1.2 deraadt 1066: .Nm
1.218 jmc 1067: supports a number of functions through the use of an escape character.
1068: .Pp
1069: A single tilde character can be sent as
1070: .Ic ~~
1071: or by following the tilde by a character other than those described below.
1072: The escape character must always follow a newline to be interpreted as
1073: special.
1074: The escape character can be changed in configuration files using the
1075: .Cm EscapeChar
1076: configuration directive or on the command line by the
1077: .Fl e
1078: option.
1079: .Pp
1080: The supported escapes (assuming the default
1081: .Ql ~ )
1082: are:
1083: .Bl -tag -width Ds
1084: .It Cm ~.
1085: Disconnect.
1086: .It Cm ~^Z
1087: Background
1.234 jmc 1088: .Nm .
1.218 jmc 1089: .It Cm ~#
1090: List forwarded connections.
1091: .It Cm ~&
1092: Background
1.2 deraadt 1093: .Nm
1.218 jmc 1094: at logout when waiting for forwarded connection / X11 sessions to terminate.
1095: .It Cm ~?
1096: Display a list of escape characters.
1097: .It Cm ~B
1098: Send a BREAK to the remote system
1.369 jmc 1099: (only useful if the peer supports it).
1.218 jmc 1100: .It Cm ~C
1101: Open command line.
1102: Currently this allows the addition of port forwardings using the
1.279 stevesk 1103: .Fl L ,
1104: .Fl R
1.218 jmc 1105: and
1.279 stevesk 1106: .Fl D
1.225 jmc 1107: options (see above).
1.322 markus 1108: It also allows the cancellation of existing port-forwardings
1109: with
1.262 stevesk 1110: .Sm off
1.322 markus 1111: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1112: .Sm on
1.322 markus 1113: for local,
1114: .Sm off
1115: .Fl KR Oo Ar bind_address : Oc Ar port
1116: .Sm on
1117: for remote and
1118: .Sm off
1119: .Fl KD Oo Ar bind_address : Oc Ar port
1120: .Sm on
1121: for dynamic port-forwardings.
1.218 jmc 1122: .Ic !\& Ns Ar command
1123: allows the user to execute a local command if the
1124: .Ic PermitLocalCommand
1125: option is enabled in
1.176 jmc 1126: .Xr ssh_config 5 .
1.218 jmc 1127: Basic help is available, using the
1128: .Fl h
1129: option.
1130: .It Cm ~R
1131: Request rekeying of the connection
1.369 jmc 1132: (only useful if the peer supports it).
1.327 dtucker 1133: .It Cm ~V
1134: Decrease the verbosity
1135: .Pq Ic LogLevel
1136: when errors are being written to stderr.
1137: .It Cm ~v
1.328 jmc 1138: Increase the verbosity
1.327 dtucker 1139: .Pq Ic LogLevel
1140: when errors are being written to stderr.
1.176 jmc 1141: .El
1.246 jmc 1142: .Sh TCP FORWARDING
1.402 jmc 1143: Forwarding of arbitrary TCP connections over a secure channel
1144: can be specified either on the command line or in a configuration file.
1.246 jmc 1145: One possible application of TCP forwarding is a secure connection to a
1146: mail server; another is going through firewalls.
1147: .Pp
1.402 jmc 1148: In the example below, we look at encrypting communication for an IRC client,
1149: even though the IRC server it connects to does not directly
1150: support encrypted communication.
1.246 jmc 1151: This works as follows:
1152: the user connects to the remote host using
1153: .Nm ,
1.402 jmc 1154: specifying the ports to be used to forward the connection.
1155: After that it is possible to start the program locally,
1.246 jmc 1156: and
1157: .Nm
1.402 jmc 1158: will encrypt and forward the connection to the remote server.
1.246 jmc 1159: .Pp
1.402 jmc 1160: The following example tunnels an IRC session from the client
1161: to an IRC server at
1.246 jmc 1162: .Dq server.example.com ,
1163: joining channel
1164: .Dq #users ,
1165: nickname
1166: .Dq pinky ,
1.402 jmc 1167: using the standard IRC port, 6667:
1168: .Bd -literal -offset 4n
1169: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1170: $ irc -c '#users' pinky IRC/127.0.0.1
1171: .Ed
1.246 jmc 1172: .Pp
1173: The
1174: .Fl f
1175: option backgrounds
1176: .Nm
1177: and the remote command
1178: .Dq sleep 10
1179: is specified to allow an amount of time
1180: (10 seconds, in the example)
1.402 jmc 1181: to start the program which is going to use the tunnel.
1.246 jmc 1182: If no connections are made within the time specified,
1183: .Nm
1184: will exit.
1185: .Sh X11 FORWARDING
1.218 jmc 1186: If the
1187: .Cm ForwardX11
1188: variable is set to
1189: .Dq yes
1190: (or see the description of the
1.227 jmc 1191: .Fl X ,
1192: .Fl x ,
1.218 jmc 1193: and
1.227 jmc 1194: .Fl Y
1.226 jmc 1195: options above)
1.218 jmc 1196: and the user is using X11 (the
1197: .Ev DISPLAY
1198: environment variable is set), the connection to the X11 display is
1199: automatically forwarded to the remote side in such a way that any X11
1200: programs started from the shell (or command) will go through the
1201: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1202: from the local machine.
1.218 jmc 1203: The user should not manually set
1204: .Ev DISPLAY .
1205: Forwarding of X11 connections can be
1206: configured on the command line or in configuration files.
1207: .Pp
1208: The
1209: .Ev DISPLAY
1210: value set by
1211: .Nm
1212: will point to the server machine, but with a display number greater than zero.
1213: This is normal, and happens because
1214: .Nm
1215: creates a
1216: .Dq proxy
1217: X server on the server machine for forwarding the
1218: connections over the encrypted channel.
1.200 djm 1219: .Pp
1.218 jmc 1220: .Nm
1221: will also automatically set up Xauthority data on the server machine.
1222: For this purpose, it will generate a random authorization cookie,
1223: store it in Xauthority on the server, and verify that any forwarded
1224: connections carry this cookie and replace it by the real cookie when
1225: the connection is opened.
1226: The real authentication cookie is never
1227: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1228: .Pp
1.218 jmc 1229: If the
1230: .Cm ForwardAgent
1231: variable is set to
1232: .Dq yes
1233: (or see the description of the
1234: .Fl A
1.191 djm 1235: and
1.218 jmc 1236: .Fl a
1.226 jmc 1237: options above) and
1.218 jmc 1238: the user is using an authentication agent, the connection to the agent
1239: is automatically forwarded to the remote side.
1.252 jmc 1240: .Sh VERIFYING HOST KEYS
1241: When connecting to a server for the first time,
1242: a fingerprint of the server's public key is presented to the user
1243: (unless the option
1244: .Cm StrictHostKeyChecking
1245: has been disabled).
1246: Fingerprints can be determined using
1247: .Xr ssh-keygen 1 :
1248: .Pp
1249: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1250: .Pp
1.274 grunk 1251: If the fingerprint is already known, it can be matched
1252: and the key can be accepted or rejected.
1.358 djm 1253: If only legacy (MD5) fingerprints for the server are available, the
1254: .Xr ssh-keygen 1
1255: .Fl E
1256: option may be used to downgrade the fingerprint algorithm to match.
1257: .Pp
1.274 grunk 1258: Because of the difficulty of comparing host keys
1.352 djm 1259: just by looking at fingerprint strings,
1.274 grunk 1260: there is also support to compare host keys visually,
1261: using
1262: .Em random art .
1263: By setting the
1.275 grunk 1264: .Cm VisualHostKey
1.274 grunk 1265: option to
1.275 grunk 1266: .Dq yes ,
1.274 grunk 1267: a small ASCII graphic gets displayed on every login to a server, no matter
1268: if the session itself is interactive or not.
1269: By learning the pattern a known server produces, a user can easily
1270: find out that the host key has changed when a completely different pattern
1271: is displayed.
1272: Because these patterns are not unambiguous however, a pattern that looks
1273: similar to the pattern remembered only gives a good probability that the
1274: host key is the same, not guaranteed proof.
1275: .Pp
1276: To get a listing of the fingerprints along with their random art for
1277: all known hosts, the following command line can be used:
1278: .Pp
1279: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1280: .Pp
1.252 jmc 1281: If the fingerprint is unknown,
1282: an alternative method of verification is available:
1283: SSH fingerprints verified by DNS.
1284: An additional resource record (RR),
1285: SSHFP,
1286: is added to a zonefile
1287: and the connecting client is able to match the fingerprint
1288: with that of the key presented.
1289: .Pp
1290: In this example, we are connecting a client to a server,
1291: .Dq host.example.com .
1292: The SSHFP resource records should first be added to the zonefile for
1293: host.example.com:
1294: .Bd -literal -offset indent
1.259 jakob 1295: $ ssh-keygen -r host.example.com.
1.252 jmc 1296: .Ed
1297: .Pp
1298: The output lines will have to be added to the zonefile.
1299: To check that the zone is answering fingerprint queries:
1300: .Pp
1301: .Dl $ dig -t SSHFP host.example.com
1302: .Pp
1303: Finally the client connects:
1304: .Bd -literal -offset indent
1305: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1306: [...]
1307: Matching host key fingerprint found in DNS.
1308: Are you sure you want to continue connecting (yes/no)?
1309: .Ed
1310: .Pp
1311: See the
1312: .Cm VerifyHostKeyDNS
1313: option in
1314: .Xr ssh_config 5
1315: for more information.
1.250 jmc 1316: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1317: .Nm
1318: contains support for Virtual Private Network (VPN) tunnelling
1319: using the
1320: .Xr tun 4
1321: network pseudo-device,
1322: allowing two networks to be joined securely.
1323: The
1324: .Xr sshd_config 5
1325: configuration option
1326: .Cm PermitTunnel
1327: controls whether the server supports this,
1328: and at what level (layer 2 or 3 traffic).
1329: .Pp
1330: The following example would connect client network 10.0.50.0/24
1.265 otto 1331: with remote network 10.0.99.0/24 using a point-to-point connection
1332: from 10.1.1.1 to 10.1.1.2,
1333: provided that the SSH server running on the gateway to the remote network,
1334: at 192.168.1.15, allows it.
1335: .Pp
1336: On the client:
1.250 jmc 1337: .Bd -literal -offset indent
1338: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1339: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1340: # route add 10.0.99.0/24 10.1.1.2
1341: .Ed
1342: .Pp
1343: On the server:
1344: .Bd -literal -offset indent
1345: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1346: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1347: .Ed
1348: .Pp
1349: Client access may be more finely tuned via the
1350: .Pa /root/.ssh/authorized_keys
1351: file (see below) and the
1352: .Cm PermitRootLogin
1353: server option.
1.255 jmc 1354: The following entry would permit connections on
1.250 jmc 1355: .Xr tun 4
1.255 jmc 1356: device 1 from user
1.250 jmc 1357: .Dq jane
1.255 jmc 1358: and on tun device 2 from user
1.250 jmc 1359: .Dq john ,
1360: if
1361: .Cm PermitRootLogin
1362: is set to
1363: .Dq forced-commands-only :
1364: .Bd -literal -offset 2n
1365: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1366: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1367: .Ed
1368: .Pp
1.264 ray 1369: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1370: it may be more suited to temporary setups,
1371: such as for wireless VPNs.
1372: More permanent VPNs are better provided by tools such as
1373: .Xr ipsecctl 8
1374: and
1375: .Xr isakmpd 8 .
1.2 deraadt 1376: .Sh ENVIRONMENT
1377: .Nm
1.1 deraadt 1378: will normally set the following environment variables:
1.237 jmc 1379: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1380: .It Ev DISPLAY
1381: The
1382: .Ev DISPLAY
1.40 aaron 1383: variable indicates the location of the X11 server.
1.44 aaron 1384: It is automatically set by
1.2 deraadt 1385: .Nm
1386: to point to a value of the form
1.233 jmc 1387: .Dq hostname:n ,
1388: where
1389: .Dq hostname
1390: indicates the host where the shell runs, and
1391: .Sq n
1392: is an integer \*(Ge 1.
1.40 aaron 1393: .Nm
1394: uses this special value to forward X11 connections over the secure
1395: channel.
1.107 markus 1396: The user should normally not set
1397: .Ev DISPLAY
1398: explicitly, as that
1.1 deraadt 1399: will render the X11 connection insecure (and will require the user to
1400: manually copy any required authorization cookies).
1.2 deraadt 1401: .It Ev HOME
1.1 deraadt 1402: Set to the path of the user's home directory.
1.2 deraadt 1403: .It Ev LOGNAME
1404: Synonym for
1.12 aaron 1405: .Ev USER ;
1406: set for compatibility with systems that use this variable.
1.2 deraadt 1407: .It Ev MAIL
1.129 stevesk 1408: Set to the path of the user's mailbox.
1.40 aaron 1409: .It Ev PATH
1.2 deraadt 1410: Set to the default
1411: .Ev PATH ,
1412: as specified when compiling
1.234 jmc 1413: .Nm .
1.118 markus 1414: .It Ev SSH_ASKPASS
1415: If
1416: .Nm
1417: needs a passphrase, it will read the passphrase from the current
1418: terminal if it was run from a terminal.
1419: If
1420: .Nm
1421: does not have a terminal associated with it but
1422: .Ev DISPLAY
1423: and
1424: .Ev SSH_ASKPASS
1425: are set, it will execute the program specified by
1426: .Ev SSH_ASKPASS
1427: and open an X11 window to read the passphrase.
1428: This is particularly useful when calling
1429: .Nm
1430: from a
1.196 jmc 1431: .Pa .xsession
1.118 markus 1432: or related script.
1433: (Note that on some machines it
1434: may be necessary to redirect the input from
1435: .Pa /dev/null
1436: to make this work.)
1.413 djm 1437: .It Ev SSH_ASKPASS_REQUIRE
1.414 jmc 1438: Allows further control over the use of an askpass program.
1.413 djm 1439: If this variable is set to
1440: .Dq never
1441: then
1442: .Nm
1443: will never attempt to use one.
1444: If it is set to
1445: .Dq prefer ,
1446: then
1447: .Nm
1448: will prefer to use the askpass program instead of the TTY when requesting
1449: passwords.
1450: Finally, if the variable is set to
1451: .Dq force ,
1452: then the askpass program will be used for all passphrase input regardless
1453: of whether
1454: .Ev DISPLAY
1455: is set.
1.18 markus 1456: .It Ev SSH_AUTH_SOCK
1.233 jmc 1457: Identifies the path of a
1458: .Ux Ns -domain
1459: socket used to communicate with the agent.
1.166 stevesk 1460: .It Ev SSH_CONNECTION
1461: Identifies the client and server ends of the connection.
1.40 aaron 1462: The variable contains
1.233 jmc 1463: four space-separated values: client IP address, client port number,
1464: server IP address, and server port number.
1.73 markus 1465: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1466: This variable contains the original command line if a forced command
1.73 markus 1467: is executed.
1468: It can be used to extract the original arguments.
1.2 deraadt 1469: .It Ev SSH_TTY
1.1 deraadt 1470: This is set to the name of the tty (path to the device) associated
1.40 aaron 1471: with the current shell or command.
1472: If the current session has no tty,
1.1 deraadt 1473: this variable is not set.
1.387 djm 1474: .It Ev SSH_TUNNEL
1475: Optionally set by
1476: .Xr sshd 8
1477: to contain the interface names assigned if tunnel forwarding was
1478: requested by the client.
1.385 djm 1479: .It Ev SSH_USER_AUTH
1480: Optionally set by
1481: .Xr sshd 8 ,
1482: this variable may contain a pathname to a file that lists the authentication
1483: methods successfully used when the session was established, including any
1484: public keys that were used.
1.2 deraadt 1485: .It Ev TZ
1.214 jmc 1486: This variable is set to indicate the present time zone if it
1.257 jmc 1487: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1488: on to new connections).
1.2 deraadt 1489: .It Ev USER
1.1 deraadt 1490: Set to the name of the user logging in.
1.2 deraadt 1491: .El
1492: .Pp
1.44 aaron 1493: Additionally,
1.2 deraadt 1494: .Nm
1.44 aaron 1495: reads
1.207 djm 1496: .Pa ~/.ssh/environment ,
1.2 deraadt 1497: and adds lines of the format
1498: .Dq VARNAME=value
1.233 jmc 1499: to the environment if the file exists and users are allowed to
1.161 marc 1500: change their environment.
1.176 jmc 1501: For more information, see the
1.161 marc 1502: .Cm PermitUserEnvironment
1.162 stevesk 1503: option in
1.161 marc 1504: .Xr sshd_config 5 .
1.2 deraadt 1505: .Sh FILES
1.236 jmc 1506: .Bl -tag -width Ds -compact
1.309 jmc 1507: .It Pa ~/.rhosts
1.240 jmc 1508: This file is used for host-based authentication (see above).
1.92 markus 1509: On some machines this file may need to be
1.240 jmc 1510: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1511: because
1.2 deraadt 1512: .Xr sshd 8
1.40 aaron 1513: reads it as root.
1514: Additionally, this file must be owned by the user,
1515: and must not have write permissions for anyone else.
1516: The recommended
1.1 deraadt 1517: permission for most machines is read/write for the user, and not
1518: accessible by others.
1.2 deraadt 1519: .Pp
1.309 jmc 1520: .It Pa ~/.shosts
1.240 jmc 1521: This file is used in exactly the same way as
1522: .Pa .rhosts ,
1523: but allows host-based authentication without permitting login with
1524: rlogin/rsh.
1.272 mcbride 1525: .Pp
1.309 jmc 1526: .It Pa ~/.ssh/
1.272 mcbride 1527: This directory is the default location for all user-specific configuration
1528: and authentication information.
1529: There is no general requirement to keep the entire contents of this directory
1530: secret, but the recommended permissions are read/write/execute for the user,
1531: and not accessible by others.
1.236 jmc 1532: .Pp
1.309 jmc 1533: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1534: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1535: that can be used for logging in as this user.
1.238 jmc 1536: The format of this file is described in the
1537: .Xr sshd 8
1538: manual page.
1539: This file is not highly sensitive, but the recommended
1540: permissions are read/write for the user, and not accessible by others.
1541: .Pp
1.309 jmc 1542: .It Pa ~/.ssh/config
1.238 jmc 1543: This is the per-user configuration file.
1544: The file format and configuration options are described in
1545: .Xr ssh_config 5 .
1546: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1547: read/write for the user, and not writable by others.
1.238 jmc 1548: .Pp
1.309 jmc 1549: .It Pa ~/.ssh/environment
1.239 jmc 1550: Contains additional definitions for environment variables; see
1551: .Sx ENVIRONMENT ,
1.238 jmc 1552: above.
1553: .Pp
1.309 jmc 1554: .It Pa ~/.ssh/id_dsa
1.310 djm 1555: .It Pa ~/.ssh/id_ecdsa
1.404 naddy 1556: .It Pa ~/.ssh/id_ecdsa_sk
1.343 naddy 1557: .It Pa ~/.ssh/id_ed25519
1.406 naddy 1558: .It Pa ~/.ssh/id_ed25519_sk
1.309 jmc 1559: .It Pa ~/.ssh/id_rsa
1.238 jmc 1560: Contains the private key for authentication.
1561: These files
1562: contain sensitive data and should be readable by the user but not
1563: accessible by others (read/write/execute).
1564: .Nm
1565: will simply ignore a private key file if it is accessible by others.
1566: It is possible to specify a passphrase when
1567: generating the key which will be used to encrypt the
1.389 djm 1568: sensitive part of this file using AES-128.
1.238 jmc 1569: .Pp
1.309 jmc 1570: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1571: .It Pa ~/.ssh/id_ecdsa.pub
1.404 naddy 1572: .It Pa ~/.ssh/id_ecdsa_sk.pub
1.343 naddy 1573: .It Pa ~/.ssh/id_ed25519.pub
1.406 naddy 1574: .It Pa ~/.ssh/id_ed25519_sk.pub
1.309 jmc 1575: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1576: Contains the public key for authentication.
1577: These files are not
1578: sensitive and can (but need not) be readable by anyone.
1579: .Pp
1.309 jmc 1580: .It Pa ~/.ssh/known_hosts
1.244 jmc 1581: Contains a list of host keys for all hosts the user has logged into
1582: that are not already in the systemwide list of known host keys.
1.238 jmc 1583: See
1.244 jmc 1584: .Xr sshd 8
1585: for further details of the format of this file.
1.238 jmc 1586: .Pp
1.309 jmc 1587: .It Pa ~/.ssh/rc
1.238 jmc 1588: Commands in this file are executed by
1589: .Nm
1.245 jmc 1590: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1591: started.
1592: See the
1593: .Xr sshd 8
1594: manual page for more information.
1595: .Pp
1.309 jmc 1596: .It Pa /etc/hosts.equiv
1.240 jmc 1597: This file is for host-based authentication (see above).
1598: It should only be writable by root.
1.236 jmc 1599: .Pp
1.309 jmc 1600: .It Pa /etc/shosts.equiv
1.240 jmc 1601: This file is used in exactly the same way as
1602: .Pa hosts.equiv ,
1603: but allows host-based authentication without permitting login with
1604: rlogin/rsh.
1.236 jmc 1605: .Pp
1.238 jmc 1606: .It Pa /etc/ssh/ssh_config
1607: Systemwide configuration file.
1608: The file format and configuration options are described in
1609: .Xr ssh_config 5 .
1610: .Pp
1.309 jmc 1611: .It Pa /etc/ssh/ssh_host_key
1612: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1613: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1614: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1615: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1616: These files contain the private parts of the host keys
1.245 jmc 1617: and are used for host-based authentication.
1.238 jmc 1618: .Pp
1.309 jmc 1619: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1620: Systemwide list of known host keys.
1621: This file should be prepared by the
1622: system administrator to contain the public host keys of all machines in the
1623: organization.
1.244 jmc 1624: It should be world-readable.
1625: See
1.238 jmc 1626: .Xr sshd 8
1.244 jmc 1627: for further details of the format of this file.
1.236 jmc 1628: .Pp
1.309 jmc 1629: .It Pa /etc/ssh/sshrc
1.1 deraadt 1630: Commands in this file are executed by
1.2 deraadt 1631: .Nm
1.245 jmc 1632: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1633: See the
1.2 deraadt 1634: .Xr sshd 8
1.1 deraadt 1635: manual page for more information.
1.58 itojun 1636: .El
1.312 jmc 1637: .Sh EXIT STATUS
1638: .Nm
1639: exits with the exit status of the remote command or with 255
1640: if an error occurred.
1.2 deraadt 1641: .Sh SEE ALSO
1642: .Xr scp 1 ,
1.83 djm 1643: .Xr sftp 1 ,
1.2 deraadt 1644: .Xr ssh-add 1 ,
1645: .Xr ssh-agent 1 ,
1646: .Xr ssh-keygen 1 ,
1.242 jmc 1647: .Xr ssh-keyscan 1 ,
1.250 jmc 1648: .Xr tun 4 ,
1.159 stevesk 1649: .Xr ssh_config 5 ,
1.160 naddy 1650: .Xr ssh-keysign 8 ,
1.87 itojun 1651: .Xr sshd 8
1.329 jmc 1652: .Sh STANDARDS
1.106 markus 1653: .Rs
1.329 jmc 1654: .%A S. Lehtinen
1655: .%A C. Lonvick
1656: .%D January 2006
1.256 jmc 1657: .%R RFC 4250
1.329 jmc 1658: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1659: .Re
1.329 jmc 1660: .Pp
1.256 jmc 1661: .Rs
1.329 jmc 1662: .%A T. Ylonen
1663: .%A C. Lonvick
1664: .%D January 2006
1.256 jmc 1665: .%R RFC 4251
1.329 jmc 1666: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1667: .Re
1.329 jmc 1668: .Pp
1.256 jmc 1669: .Rs
1.329 jmc 1670: .%A T. Ylonen
1671: .%A C. Lonvick
1672: .%D January 2006
1.256 jmc 1673: .%R RFC 4252
1.329 jmc 1674: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1675: .Re
1.329 jmc 1676: .Pp
1.256 jmc 1677: .Rs
1.329 jmc 1678: .%A T. Ylonen
1679: .%A C. Lonvick
1680: .%D January 2006
1.256 jmc 1681: .%R RFC 4253
1.329 jmc 1682: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1683: .Re
1.329 jmc 1684: .Pp
1.256 jmc 1685: .Rs
1.329 jmc 1686: .%A T. Ylonen
1687: .%A C. Lonvick
1688: .%D January 2006
1.256 jmc 1689: .%R RFC 4254
1.329 jmc 1690: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1691: .Re
1.329 jmc 1692: .Pp
1.256 jmc 1693: .Rs
1.329 jmc 1694: .%A J. Schlyter
1695: .%A W. Griffin
1696: .%D January 2006
1.256 jmc 1697: .%R RFC 4255
1.329 jmc 1698: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1699: .Re
1.329 jmc 1700: .Pp
1.256 jmc 1701: .Rs
1.329 jmc 1702: .%A F. Cusack
1703: .%A M. Forssen
1704: .%D January 2006
1.256 jmc 1705: .%R RFC 4256
1.329 jmc 1706: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1707: .Re
1.329 jmc 1708: .Pp
1.256 jmc 1709: .Rs
1.329 jmc 1710: .%A J. Galbraith
1711: .%A P. Remaker
1712: .%D January 2006
1.256 jmc 1713: .%R RFC 4335
1.329 jmc 1714: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1715: .Re
1.329 jmc 1716: .Pp
1.256 jmc 1717: .Rs
1.329 jmc 1718: .%A M. Bellare
1719: .%A T. Kohno
1720: .%A C. Namprempre
1721: .%D January 2006
1.256 jmc 1722: .%R RFC 4344
1.329 jmc 1723: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1724: .Re
1.329 jmc 1725: .Pp
1.256 jmc 1726: .Rs
1.329 jmc 1727: .%A B. Harris
1728: .%D January 2006
1.256 jmc 1729: .%R RFC 4345
1.329 jmc 1730: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1731: .Re
1.329 jmc 1732: .Pp
1.258 djm 1733: .Rs
1.329 jmc 1734: .%A M. Friedl
1735: .%A N. Provos
1736: .%A W. Simpson
1737: .%D March 2006
1.258 djm 1738: .%R RFC 4419
1.329 jmc 1739: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1740: .Re
1.329 jmc 1741: .Pp
1.266 markus 1742: .Rs
1.329 jmc 1743: .%A J. Galbraith
1744: .%A R. Thayer
1745: .%D November 2006
1.266 markus 1746: .%R RFC 4716
1.329 jmc 1747: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1748: .Re
1.329 jmc 1749: .Pp
1.313 djm 1750: .Rs
1.329 jmc 1751: .%A D. Stebila
1752: .%A J. Green
1753: .%D December 2009
1.313 djm 1754: .%R RFC 5656
1.329 jmc 1755: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1756: .Re
1.329 jmc 1757: .Pp
1.274 grunk 1758: .Rs
1759: .%A A. Perrig
1760: .%A D. Song
1761: .%D 1999
1.329 jmc 1762: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1763: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1764: .Re
1.173 jmc 1765: .Sh AUTHORS
1766: OpenSSH is a derivative of the original and free
1767: ssh 1.2.12 release by Tatu Ylonen.
1768: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1769: Theo de Raadt and Dug Song
1770: removed many bugs, re-added newer features and
1771: created OpenSSH.
1772: Markus Friedl contributed the support for SSH
1773: protocol versions 1.5 and 2.0.
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