Annotation of src/usr.bin/ssh/ssh.1, Revision 1.424
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: .\"
1.424 ! djm 36: .\" $OpenBSD: ssh.1,v 1.423 2021/07/23 04:00:59 djm Exp $
! 37: .Dd $Mdocdate: July 23 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.
1.218 jmc 291: The default is
1.310 djm 292: .Pa ~/.ssh/id_dsa ,
1.343 naddy 293: .Pa ~/.ssh/id_ecdsa ,
1.404 naddy 294: .Pa ~/.ssh/id_ecdsa_sk ,
1.406 naddy 295: .Pa ~/.ssh/id_ed25519 ,
296: .Pa ~/.ssh/id_ed25519_sk
1.310 djm 297: and
1.381 naddy 298: .Pa ~/.ssh/id_rsa .
1.218 jmc 299: Identity files may also be specified on
300: a per-host basis in the configuration file.
301: It is possible to have multiple
302: .Fl i
303: options (and multiple identities specified in
304: configuration files).
1.364 jmc 305: If no certificates have been explicitly specified by the
1.363 djm 306: .Cm CertificateFile
307: directive,
1.302 djm 308: .Nm
309: will also try to load certificate information from the filename obtained
310: by appending
311: .Pa -cert.pub
312: to identity filenames.
1.360 millert 313: .Pp
1.386 millert 314: .It Fl J Ar destination
1.375 djm 315: Connect to the target host by first making a
316: .Nm
1.386 millert 317: connection to the jump host described by
318: .Ar destination
1.376 jmc 319: and then establishing a TCP forwarding to the ultimate destination from
1.375 djm 320: there.
321: Multiple jump hops may be specified separated by comma characters.
322: This is a shortcut to specify a
323: .Cm ProxyJump
324: configuration directive.
1.400 djm 325: Note that configuration directives supplied on the command-line generally
326: apply to the destination host and not any specified jump hosts.
327: Use
328: .Pa ~/.ssh/config
329: to specify configuration for jump hosts.
1.375 djm 330: .Pp
1.269 djm 331: .It Fl K
332: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
333: credentials to the server.
1.360 millert 334: .Pp
1.218 jmc 335: .It Fl k
336: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 337: .Pp
1.218 jmc 338: .It Fl L Xo
339: .Sm off
340: .Oo Ar bind_address : Oc
341: .Ar port : host : hostport
342: .Sm on
343: .Xc
1.360 millert 344: .It Fl L Xo
345: .Sm off
346: .Oo Ar bind_address : Oc
347: .Ar port : remote_socket
348: .Sm on
349: .Xc
350: .It Fl L Xo
351: .Sm off
352: .Ar local_socket : host : hostport
353: .Sm on
354: .Xc
355: .It Fl L Xo
356: .Sm off
357: .Ar local_socket : remote_socket
358: .Sm on
359: .Xc
360: Specifies that connections to the given TCP port or Unix socket on the local
361: (client) host are to be forwarded to the given host and port, or Unix socket,
362: on the remote side.
363: This works by allocating a socket to listen to either a TCP
1.218 jmc 364: .Ar port
365: on the local side, optionally bound to the specified
1.360 millert 366: .Ar bind_address ,
367: or to a Unix socket.
368: Whenever a connection is made to the local port or socket, the
1.218 jmc 369: connection is forwarded over the secure channel, and a connection is
1.360 millert 370: made to either
1.218 jmc 371: .Ar host
372: port
1.360 millert 373: .Ar hostport ,
374: or the Unix socket
375: .Ar remote_socket ,
1.218 jmc 376: from the remote machine.
1.360 millert 377: .Pp
1.218 jmc 378: Port forwardings can also be specified in the configuration file.
1.360 millert 379: Only the superuser can forward privileged ports.
1.308 djm 380: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 381: .Pp
1.218 jmc 382: By default, the local port is bound in accordance with the
383: .Cm GatewayPorts
384: setting.
385: However, an explicit
386: .Ar bind_address
387: may be used to bind the connection to a specific address.
1.2 deraadt 388: The
1.218 jmc 389: .Ar bind_address
390: of
391: .Dq localhost
392: indicates that the listening port be bound for local use only, while an
393: empty address or
394: .Sq *
395: indicates that the port should be available from all interfaces.
1.360 millert 396: .Pp
1.218 jmc 397: .It Fl l Ar login_name
398: Specifies the user to log in as on the remote machine.
399: This also may be specified on a per-host basis in the configuration file.
1.360 millert 400: .Pp
1.218 jmc 401: .It Fl M
402: Places the
403: .Nm
404: client into
405: .Dq master
406: mode for connection sharing.
1.231 stevesk 407: Multiple
408: .Fl M
409: options places
410: .Nm
411: into
412: .Dq master
1.397 djm 413: mode but with confirmation required using
414: .Xr ssh-askpass 1
415: before each operation that changes the multiplexing state
416: (e.g. opening a new session).
1.218 jmc 417: Refer to the description of
418: .Cm ControlMaster
419: in
420: .Xr ssh_config 5
421: for details.
1.360 millert 422: .Pp
1.218 jmc 423: .It Fl m Ar mac_spec
1.369 jmc 424: A comma-separated list of MAC (message authentication code) algorithms,
425: specified in order of preference.
1.218 jmc 426: See the
427: .Cm MACs
428: keyword for more information.
1.360 millert 429: .Pp
1.218 jmc 430: .It Fl N
431: Do not execute a remote command.
1.369 jmc 432: This is useful for just forwarding ports.
1.422 djm 433: Refer to the description of
434: .Cm SessionType
435: in
436: .Xr ssh_config 5
437: for details.
1.360 millert 438: .Pp
1.218 jmc 439: .It Fl n
440: Redirects stdin from
441: .Pa /dev/null
442: (actually, prevents reading from stdin).
443: This must be used when
1.2 deraadt 444: .Nm
1.218 jmc 445: is run in the background.
446: A common trick is to use this to run X11 programs on a remote machine.
447: For example,
448: .Ic ssh -n shadows.cs.hut.fi emacs &
449: will start an emacs on shadows.cs.hut.fi, and the X11
450: connection will be automatically forwarded over an encrypted channel.
451: The
1.2 deraadt 452: .Nm
1.218 jmc 453: program will be put in the background.
454: (This does not work if
1.2 deraadt 455: .Nm
1.218 jmc 456: needs to ask for a password or passphrase; see also the
457: .Fl f
458: option.)
1.423 djm 459: Refer to the description of
460: .Cm StdinNull
461: in
462: .Xr ssh_config 5
463: for details.
1.360 millert 464: .Pp
1.218 jmc 465: .It Fl O Ar ctl_cmd
466: Control an active connection multiplexing master process.
467: When the
468: .Fl O
469: option is specified, the
470: .Ar ctl_cmd
471: argument is interpreted and passed to the master process.
472: Valid commands are:
473: .Dq check
1.305 markus 474: (check that the master process is running),
475: .Dq forward
1.318 jmc 476: (request forwardings without command execution),
1.323 okan 477: .Dq cancel
478: (cancel forwardings),
1.218 jmc 479: .Dq exit
1.318 jmc 480: (request the master to exit), and
1.317 djm 481: .Dq stop
482: (request the master to stop accepting further multiplexing requests).
1.360 millert 483: .Pp
1.218 jmc 484: .It Fl o Ar option
485: Can be used to give options in the format used in the configuration file.
486: This is useful for specifying options for which there is no separate
487: command-line flag.
488: For full details of the options listed below, and their possible values, see
489: .Xr ssh_config 5 .
1.2 deraadt 490: .Pp
1.218 jmc 491: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 492: .It AddKeysToAgent
1.218 jmc 493: .It AddressFamily
494: .It BatchMode
495: .It BindAddress
1.338 djm 496: .It CanonicalDomains
1.339 djm 497: .It CanonicalizeFallbackLocal
498: .It CanonicalizeHostname
499: .It CanonicalizeMaxDots
500: .It CanonicalizePermittedCNAMEs
1.399 jmc 501: .It CASignatureAlgorithms
1.363 djm 502: .It CertificateFile
1.218 jmc 503: .It CheckHostIP
504: .It Ciphers
505: .It ClearAllForwardings
506: .It Compression
1.380 naddy 507: .It ConnectionAttempts
1.218 jmc 508: .It ConnectTimeout
509: .It ControlMaster
510: .It ControlPath
1.321 djm 511: .It ControlPersist
1.218 jmc 512: .It DynamicForward
513: .It EscapeChar
1.263 markus 514: .It ExitOnForwardFailure
1.353 jmc 515: .It FingerprintHash
1.424 ! djm 516: .It ForkAfterAuthentication
1.218 jmc 517: .It ForwardAgent
518: .It ForwardX11
1.321 djm 519: .It ForwardX11Timeout
1.218 jmc 520: .It ForwardX11Trusted
521: .It GatewayPorts
522: .It GlobalKnownHostsFile
523: .It GSSAPIAuthentication
524: .It GSSAPIDelegateCredentials
525: .It HashKnownHosts
526: .It Host
1.418 naddy 527: .It HostbasedAcceptedAlgorithms
1.218 jmc 528: .It HostbasedAuthentication
529: .It HostKeyAlgorithms
530: .It HostKeyAlias
1.403 jmc 531: .It Hostname
1.374 jmc 532: .It IdentitiesOnly
1.371 markus 533: .It IdentityAgent
1.218 jmc 534: .It IdentityFile
1.316 jmc 535: .It IPQoS
1.321 djm 536: .It KbdInteractiveAuthentication
1.218 jmc 537: .It KbdInteractiveDevices
1.314 jmc 538: .It KexAlgorithms
1.415 djm 539: .It KnownHostsCommand
1.218 jmc 540: .It LocalCommand
541: .It LocalForward
542: .It LogLevel
543: .It MACs
1.337 jmc 544: .It Match
1.218 jmc 545: .It NoHostAuthenticationForLocalhost
546: .It NumberOfPasswordPrompts
547: .It PasswordAuthentication
548: .It PermitLocalCommand
1.419 markus 549: .It PermitRemoteOpen
1.291 markus 550: .It PKCS11Provider
1.218 jmc 551: .It Port
552: .It PreferredAuthentications
553: .It ProxyCommand
1.375 djm 554: .It ProxyJump
1.336 jmc 555: .It ProxyUseFdpass
1.416 dtucker 556: .It PubkeyAcceptedAlgorithms
1.218 jmc 557: .It PubkeyAuthentication
1.251 dtucker 558: .It RekeyLimit
1.382 bluhm 559: .It RemoteCommand
1.218 jmc 560: .It RemoteForward
1.319 jmc 561: .It RequestTTY
1.218 jmc 562: .It SendEnv
563: .It ServerAliveInterval
564: .It ServerAliveCountMax
1.422 djm 565: .It SessionType
1.395 jmc 566: .It SetEnv
1.423 djm 567: .It StdinNull
1.347 jmc 568: .It StreamLocalBindMask
569: .It StreamLocalBindUnlink
1.218 jmc 570: .It StrictHostKeyChecking
571: .It TCPKeepAlive
572: .It Tunnel
573: .It TunnelDevice
1.354 djm 574: .It UpdateHostKeys
1.218 jmc 575: .It User
576: .It UserKnownHostsFile
577: .It VerifyHostKeyDNS
1.276 jmc 578: .It VisualHostKey
1.218 jmc 579: .It XAuthLocation
580: .El
1.360 millert 581: .Pp
1.218 jmc 582: .It Fl p Ar port
583: Port to connect to on the remote host.
584: This can be specified on a
585: per-host basis in the configuration file.
1.360 millert 586: .Pp
1.362 djm 587: .It Fl Q Ar query_option
1.332 djm 588: Queries
589: .Nm
1.341 deraadt 590: for the algorithms supported for the specified version 2.
591: The available features are:
592: .Ar cipher
1.332 djm 593: (supported symmetric ciphers),
1.341 deraadt 594: .Ar cipher-auth
1.340 djm 595: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 596: .Ar help
597: (supported query terms for use with the
598: .Fl Q
599: flag),
1.341 deraadt 600: .Ar mac
1.332 djm 601: (supported message integrity codes),
1.341 deraadt 602: .Ar kex
1.332 djm 603: (key exchange algorithms),
1.341 deraadt 604: .Ar key
1.362 djm 605: (key types),
606: .Ar key-cert
607: (certificate key types),
608: .Ar key-plain
1.398 djm 609: (non-certificate key types),
1.410 dtucker 610: .Ar key-sig
611: (all key types and signature algorithms),
1.356 djm 612: .Ar protocol-version
1.398 djm 613: (supported SSH protocol versions), and
614: .Ar sig
615: (supported signature algorithms).
1.410 dtucker 616: Alternatively, any keyword from
617: .Xr ssh_config 5
618: or
619: .Xr sshd_config 5
620: that takes an algorithm list may be used as an alias for the corresponding
621: query_option.
1.360 millert 622: .Pp
1.218 jmc 623: .It Fl q
624: Quiet mode.
1.271 djm 625: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 626: .Pp
1.218 jmc 627: .It Fl R Xo
628: .Sm off
629: .Oo Ar bind_address : Oc
630: .Ar port : host : hostport
631: .Sm on
632: .Xc
1.360 millert 633: .It Fl R Xo
634: .Sm off
635: .Oo Ar bind_address : Oc
636: .Ar port : local_socket
637: .Sm on
638: .Xc
639: .It Fl R Xo
640: .Sm off
641: .Ar remote_socket : host : hostport
642: .Sm on
643: .Xc
644: .It Fl R Xo
645: .Sm off
646: .Ar remote_socket : local_socket
647: .Sm on
648: .Xc
1.384 markus 649: .It Fl R Xo
650: .Sm off
651: .Oo Ar bind_address : Oc
652: .Ar port
653: .Sm on
654: .Xc
1.360 millert 655: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 656: (server) host are to be forwarded to the local side.
657: .Pp
1.360 millert 658: This works by allocating a socket to listen to either a TCP
1.218 jmc 659: .Ar port
1.360 millert 660: or to a Unix socket on the remote side.
661: Whenever a connection is made to this port or Unix socket, the
662: connection is forwarded over the secure channel, and a connection
1.384 markus 663: is made from the local machine to either an explicit destination specified by
1.218 jmc 664: .Ar host
665: port
1.360 millert 666: .Ar hostport ,
667: or
668: .Ar local_socket ,
1.384 markus 669: or, if no explicit destination was specified,
670: .Nm
671: will act as a SOCKS 4/5 proxy and forward connections to the destinations
672: requested by the remote SOCKS client.
1.2 deraadt 673: .Pp
1.218 jmc 674: Port forwardings can also be specified in the configuration file.
675: Privileged ports can be forwarded only when
676: logging in as root on the remote machine.
1.324 jmc 677: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 678: .Pp
1.360 millert 679: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 680: interface only.
1.280 tobias 681: This may be overridden by specifying a
1.218 jmc 682: .Ar bind_address .
683: An empty
684: .Ar bind_address ,
685: or the address
686: .Ql * ,
687: indicates that the remote socket should listen on all interfaces.
688: Specifying a remote
689: .Ar bind_address
690: will only succeed if the server's
691: .Cm GatewayPorts
692: option is enabled (see
693: .Xr sshd_config 5 ) .
1.281 djm 694: .Pp
695: If the
696: .Ar port
697: argument is
1.282 djm 698: .Ql 0 ,
1.281 djm 699: the listen port will be dynamically allocated on the server and reported
700: to the client at run time.
1.305 markus 701: When used together with
702: .Ic -O forward
703: the allocated port will be printed to the standard output.
1.360 millert 704: .Pp
1.218 jmc 705: .It Fl S Ar ctl_path
1.304 jmc 706: Specifies the location of a control socket for connection sharing,
1.303 djm 707: or the string
708: .Dq none
709: to disable connection sharing.
1.218 jmc 710: Refer to the description of
711: .Cm ControlPath
712: and
713: .Cm ControlMaster
714: in
715: .Xr ssh_config 5
716: for details.
1.360 millert 717: .Pp
1.218 jmc 718: .It Fl s
719: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 720: Subsystems facilitate the use of SSH
721: as a secure transport for other applications (e.g.\&
1.218 jmc 722: .Xr sftp 1 ) .
723: The subsystem is specified as the remote command.
1.422 djm 724: Refer to the description of
725: .Cm SessionType
726: in
727: .Xr ssh_config 5
728: for details.
1.360 millert 729: .Pp
1.218 jmc 730: .It Fl T
1.357 dtucker 731: Disable pseudo-terminal allocation.
1.360 millert 732: .Pp
1.218 jmc 733: .It Fl t
1.357 dtucker 734: Force pseudo-terminal allocation.
1.218 jmc 735: This can be used to execute arbitrary
736: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 737: e.g. when implementing menu services.
1.218 jmc 738: Multiple
739: .Fl t
740: options force tty allocation, even if
1.194 jakob 741: .Nm
1.218 jmc 742: has no local tty.
1.360 millert 743: .Pp
1.218 jmc 744: .It Fl V
745: Display the version number and exit.
1.360 millert 746: .Pp
1.218 jmc 747: .It Fl v
748: Verbose mode.
749: Causes
1.176 jmc 750: .Nm
1.218 jmc 751: to print debugging messages about its progress.
752: This is helpful in
753: debugging connection, authentication, and configuration problems.
754: Multiple
755: .Fl v
756: options increase the verbosity.
757: The maximum is 3.
1.360 millert 758: .Pp
1.290 dtucker 759: .It Fl W Ar host : Ns Ar port
760: Requests that standard input and output on the client be forwarded to
761: .Ar host
762: on
763: .Ar port
764: over the secure channel.
765: Implies
766: .Fl N ,
767: .Fl T ,
768: .Cm ExitOnForwardFailure
769: and
1.373 jmc 770: .Cm ClearAllForwardings ,
771: though these can be overridden in the configuration file or using
1.372 dtucker 772: .Fl o
773: command line options.
1.360 millert 774: .Pp
1.261 stevesk 775: .It Fl w Xo
776: .Ar local_tun Ns Op : Ns Ar remote_tun
777: .Xc
778: Requests
779: tunnel
780: device forwarding with the specified
1.218 jmc 781: .Xr tun 4
1.261 stevesk 782: devices between the client
783: .Pq Ar local_tun
784: and the server
785: .Pq Ar remote_tun .
786: .Pp
1.228 jmc 787: The devices may be specified by numerical ID or the keyword
788: .Dq any ,
789: which uses the next available tunnel device.
1.261 stevesk 790: If
791: .Ar remote_tun
792: is not specified, it defaults to
793: .Dq any .
1.228 jmc 794: See also the
1.218 jmc 795: .Cm Tunnel
1.261 stevesk 796: and
797: .Cm TunnelDevice
798: directives in
1.218 jmc 799: .Xr ssh_config 5 .
1.393 djm 800: .Pp
1.261 stevesk 801: If the
802: .Cm Tunnel
1.393 djm 803: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 804: .Dq point-to-point .
1.393 djm 805: If a different
806: .Cm Tunnel
807: forwarding mode it desired, then it should be specified before
808: .Fl w .
1.360 millert 809: .Pp
1.218 jmc 810: .It Fl X
811: Enables X11 forwarding.
1.54 markus 812: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 813: .Pp
1.218 jmc 814: X11 forwarding should be enabled with caution.
1.168 jmc 815: Users with the ability to bypass file permissions on the remote host
1.218 jmc 816: (for the user's X authorization database)
817: can access the local X11 display through the forwarded connection.
818: An attacker may then be able to perform activities such as keystroke monitoring.
819: .Pp
820: For this reason, X11 forwarding is subjected to X11 SECURITY extension
821: restrictions by default.
822: Please refer to the
823: .Nm
824: .Fl Y
825: option and the
826: .Cm ForwardX11Trusted
827: directive in
828: .Xr ssh_config 5
829: for more information.
1.360 millert 830: .Pp
1.218 jmc 831: .It Fl x
832: Disables X11 forwarding.
1.360 millert 833: .Pp
1.218 jmc 834: .It Fl Y
835: Enables trusted X11 forwarding.
836: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
837: controls.
1.360 millert 838: .Pp
1.278 djm 839: .It Fl y
840: Send log information using the
841: .Xr syslog 3
842: system module.
843: By default this information is sent to stderr.
1.218 jmc 844: .El
1.224 jmc 845: .Pp
846: .Nm
847: may additionally obtain configuration data from
848: a per-user configuration file and a system-wide configuration file.
849: The file format and configuration options are described in
850: .Xr ssh_config 5 .
1.222 jmc 851: .Sh AUTHENTICATION
1.377 djm 852: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 853: .Pp
854: The methods available for authentication are:
1.260 jmc 855: GSSAPI-based authentication,
1.222 jmc 856: host-based authentication,
857: public key authentication,
1.421 dtucker 858: keyboard-interactive authentication,
1.222 jmc 859: and password authentication.
860: Authentication methods are tried in the order specified above,
1.369 jmc 861: though
862: .Cm PreferredAuthentications
863: can be used to change the default order.
1.222 jmc 864: .Pp
865: Host-based authentication works as follows:
1.218 jmc 866: If the machine the user logs in from is listed in
867: .Pa /etc/hosts.equiv
868: or
869: .Pa /etc/shosts.equiv
1.412 djm 870: on the remote machine, the user is non-root and the user names are
1.218 jmc 871: the same on both sides, or if the files
872: .Pa ~/.rhosts
873: or
874: .Pa ~/.shosts
875: exist in the user's home directory on the
876: remote machine and contain a line containing the name of the client
877: machine and the name of the user on that machine, the user is
1.222 jmc 878: considered for login.
879: Additionally, the server
880: .Em must
881: be able to verify the client's
882: host key (see the description of
1.218 jmc 883: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 884: and
1.222 jmc 885: .Pa ~/.ssh/known_hosts ,
886: below)
887: for login to be permitted.
1.218 jmc 888: This authentication method closes security holes due to IP
1.222 jmc 889: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 890: [Note to the administrator:
891: .Pa /etc/hosts.equiv ,
892: .Pa ~/.rhosts ,
893: and the rlogin/rsh protocol in general, are inherently insecure and should be
894: disabled if security is desired.]
1.189 dtucker 895: .Pp
1.222 jmc 896: Public key authentication works as follows:
897: The scheme is based on public-key cryptography,
898: using cryptosystems
899: where encryption and decryption are done using separate keys,
900: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 901: The idea is that each user creates a public/private
902: key pair for authentication purposes.
903: The server knows the public key, and only the user knows the private key.
1.222 jmc 904: .Nm
905: implements public key authentication protocol automatically,
1.349 sobrado 906: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 907: The HISTORY section of
1.222 jmc 908: .Xr ssl 8
1.311 jmc 909: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 910: .Pp
1.218 jmc 911: The file
912: .Pa ~/.ssh/authorized_keys
913: lists the public keys that are permitted for logging in.
914: When the user logs in, the
1.2 deraadt 915: .Nm
1.218 jmc 916: program tells the server which key pair it would like to use for
917: authentication.
1.222 jmc 918: The client proves that it has access to the private key
919: and the server checks that the corresponding public key
920: is authorized to accept the account.
1.383 djm 921: .Pp
922: The server may inform the client of errors that prevented public key
923: authentication from succeeding after authentication completes using a
924: different method.
925: These may be viewed by increasing the
926: .Cm LogLevel
927: to
928: .Cm DEBUG
929: or higher (e.g. by using the
930: .Fl v
931: flag).
1.218 jmc 932: .Pp
1.420 dtucker 933: The user creates their key pair by running
1.218 jmc 934: .Xr ssh-keygen 1 .
935: This stores the private key in
1.222 jmc 936: .Pa ~/.ssh/id_dsa
1.369 jmc 937: (DSA),
1.310 djm 938: .Pa ~/.ssh/id_ecdsa
1.369 jmc 939: (ECDSA),
1.404 naddy 940: .Pa ~/.ssh/id_ecdsa_sk
1.409 naddy 941: (authenticator-hosted ECDSA),
1.343 naddy 942: .Pa ~/.ssh/id_ed25519
1.369 jmc 943: (Ed25519),
1.406 naddy 944: .Pa ~/.ssh/id_ed25519_sk
1.409 naddy 945: (authenticator-hosted Ed25519),
1.222 jmc 946: or
947: .Pa ~/.ssh/id_rsa
1.369 jmc 948: (RSA)
1.218 jmc 949: and stores the public key in
1.222 jmc 950: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 951: (DSA),
1.310 djm 952: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 953: (ECDSA),
1.404 naddy 954: .Pa ~/.ssh/id_ecdsa_sk.pub
1.409 naddy 955: (authenticator-hosted ECDSA),
1.343 naddy 956: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 957: (Ed25519),
1.406 naddy 958: .Pa ~/.ssh/id_ed25519_sk.pub
1.409 naddy 959: (authenticator-hosted Ed25519),
1.222 jmc 960: or
961: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 962: (RSA)
1.218 jmc 963: in the user's home directory.
1.222 jmc 964: The user should then copy the public key
1.218 jmc 965: to
966: .Pa ~/.ssh/authorized_keys
1.420 dtucker 967: in their home directory on the remote machine.
1.222 jmc 968: The
1.218 jmc 969: .Pa authorized_keys
970: file corresponds to the conventional
971: .Pa ~/.rhosts
972: file, and has one key
1.222 jmc 973: per line, though the lines can be very long.
1.218 jmc 974: After this, the user can log in without giving the password.
975: .Pp
1.301 jmc 976: A variation on public key authentication
977: is available in the form of certificate authentication:
978: instead of a set of public/private keys,
979: signed certificates are used.
980: This has the advantage that a single trusted certification authority
981: can be used in place of many public/private keys.
1.333 jmc 982: See the CERTIFICATES section of
1.301 jmc 983: .Xr ssh-keygen 1
984: for more information.
985: .Pp
986: The most convenient way to use public key or certificate authentication
987: may be with an authentication agent.
1.218 jmc 988: See
989: .Xr ssh-agent 1
1.366 jcs 990: and (optionally) the
991: .Cm AddKeysToAgent
992: directive in
993: .Xr ssh_config 5
1.218 jmc 994: for more information.
995: .Pp
1.421 dtucker 996: Keyboard-interactive authentication works as follows:
1.222 jmc 997: The server sends an arbitrary
998: .Qq challenge
1.421 dtucker 999: text and prompts for a response, possibly multiple times.
1000: Examples of keyboard-interactive authentication include
1.335 jmc 1001: .Bx
1002: Authentication (see
1.222 jmc 1003: .Xr login.conf 5 )
1.335 jmc 1004: and PAM (some
1005: .Pf non- Ox
1006: systems).
1.222 jmc 1007: .Pp
1008: Finally, if other authentication methods fail,
1.218 jmc 1009: .Nm
1010: prompts the user for a password.
1011: The password is sent to the remote
1012: host for checking; however, since all communications are encrypted,
1013: the password cannot be seen by someone listening on the network.
1.232 jmc 1014: .Pp
1015: .Nm
1016: automatically maintains and checks a database containing
1017: identification for all hosts it has ever been used with.
1018: Host keys are stored in
1019: .Pa ~/.ssh/known_hosts
1020: in the user's home directory.
1021: Additionally, the file
1022: .Pa /etc/ssh/ssh_known_hosts
1023: is automatically checked for known hosts.
1024: Any new hosts are automatically added to the user's file.
1025: If a host's identification ever changes,
1026: .Nm
1027: warns about this and disables password authentication to prevent
1028: server spoofing or man-in-the-middle attacks,
1029: which could otherwise be used to circumvent the encryption.
1030: The
1031: .Cm StrictHostKeyChecking
1032: option can be used to control logins to machines whose
1033: host key is not known or has changed.
1034: .Pp
1.218 jmc 1035: When the user's identity has been accepted by the server, the server
1.357 dtucker 1036: either executes the given command in a non-interactive session or,
1037: if no command has been specified, logs into the machine and gives
1038: the user a normal shell as an interactive session.
1.218 jmc 1039: All communication with
1040: the remote command or shell will be automatically encrypted.
1041: .Pp
1.357 dtucker 1042: If an interactive session is requested
1043: .Nm
1044: by default will only request a pseudo-terminal (pty) for interactive
1045: sessions when the client has one.
1046: The flags
1047: .Fl T
1048: and
1049: .Fl t
1050: can be used to override this behaviour.
1051: .Pp
1052: If a pseudo-terminal has been allocated the
1.218 jmc 1053: user may use the escape characters noted below.
1054: .Pp
1.357 dtucker 1055: If no pseudo-terminal has been allocated,
1.218 jmc 1056: the session is transparent and can be used to reliably transfer binary data.
1057: On most systems, setting the escape character to
1058: .Dq none
1059: will also make the session transparent even if a tty is used.
1060: .Pp
1061: The session terminates when the command or shell on the remote
1.247 jmc 1062: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1063: .Sh ESCAPE CHARACTERS
1.218 jmc 1064: When a pseudo-terminal has been requested,
1.2 deraadt 1065: .Nm
1.218 jmc 1066: supports a number of functions through the use of an escape character.
1067: .Pp
1068: A single tilde character can be sent as
1069: .Ic ~~
1070: or by following the tilde by a character other than those described below.
1071: The escape character must always follow a newline to be interpreted as
1072: special.
1073: The escape character can be changed in configuration files using the
1074: .Cm EscapeChar
1075: configuration directive or on the command line by the
1076: .Fl e
1077: option.
1078: .Pp
1079: The supported escapes (assuming the default
1080: .Ql ~ )
1081: are:
1082: .Bl -tag -width Ds
1083: .It Cm ~.
1084: Disconnect.
1085: .It Cm ~^Z
1086: Background
1.234 jmc 1087: .Nm .
1.218 jmc 1088: .It Cm ~#
1089: List forwarded connections.
1090: .It Cm ~&
1091: Background
1.2 deraadt 1092: .Nm
1.218 jmc 1093: at logout when waiting for forwarded connection / X11 sessions to terminate.
1094: .It Cm ~?
1095: Display a list of escape characters.
1096: .It Cm ~B
1097: Send a BREAK to the remote system
1.369 jmc 1098: (only useful if the peer supports it).
1.218 jmc 1099: .It Cm ~C
1100: Open command line.
1101: Currently this allows the addition of port forwardings using the
1.279 stevesk 1102: .Fl L ,
1103: .Fl R
1.218 jmc 1104: and
1.279 stevesk 1105: .Fl D
1.225 jmc 1106: options (see above).
1.322 markus 1107: It also allows the cancellation of existing port-forwardings
1108: with
1.262 stevesk 1109: .Sm off
1.322 markus 1110: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1111: .Sm on
1.322 markus 1112: for local,
1113: .Sm off
1114: .Fl KR Oo Ar bind_address : Oc Ar port
1115: .Sm on
1116: for remote and
1117: .Sm off
1118: .Fl KD Oo Ar bind_address : Oc Ar port
1119: .Sm on
1120: for dynamic port-forwardings.
1.218 jmc 1121: .Ic !\& Ns Ar command
1122: allows the user to execute a local command if the
1123: .Ic PermitLocalCommand
1124: option is enabled in
1.176 jmc 1125: .Xr ssh_config 5 .
1.218 jmc 1126: Basic help is available, using the
1127: .Fl h
1128: option.
1129: .It Cm ~R
1130: Request rekeying of the connection
1.369 jmc 1131: (only useful if the peer supports it).
1.327 dtucker 1132: .It Cm ~V
1133: Decrease the verbosity
1134: .Pq Ic LogLevel
1135: when errors are being written to stderr.
1136: .It Cm ~v
1.328 jmc 1137: Increase the verbosity
1.327 dtucker 1138: .Pq Ic LogLevel
1139: when errors are being written to stderr.
1.176 jmc 1140: .El
1.246 jmc 1141: .Sh TCP FORWARDING
1.402 jmc 1142: Forwarding of arbitrary TCP connections over a secure channel
1143: can be specified either on the command line or in a configuration file.
1.246 jmc 1144: One possible application of TCP forwarding is a secure connection to a
1145: mail server; another is going through firewalls.
1146: .Pp
1.402 jmc 1147: In the example below, we look at encrypting communication for an IRC client,
1148: even though the IRC server it connects to does not directly
1149: support encrypted communication.
1.246 jmc 1150: This works as follows:
1151: the user connects to the remote host using
1152: .Nm ,
1.402 jmc 1153: specifying the ports to be used to forward the connection.
1154: After that it is possible to start the program locally,
1.246 jmc 1155: and
1156: .Nm
1.402 jmc 1157: will encrypt and forward the connection to the remote server.
1.246 jmc 1158: .Pp
1.402 jmc 1159: The following example tunnels an IRC session from the client
1160: to an IRC server at
1.246 jmc 1161: .Dq server.example.com ,
1162: joining channel
1163: .Dq #users ,
1164: nickname
1165: .Dq pinky ,
1.402 jmc 1166: using the standard IRC port, 6667:
1167: .Bd -literal -offset 4n
1168: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1169: $ irc -c '#users' pinky IRC/127.0.0.1
1170: .Ed
1.246 jmc 1171: .Pp
1172: The
1173: .Fl f
1174: option backgrounds
1175: .Nm
1176: and the remote command
1177: .Dq sleep 10
1178: is specified to allow an amount of time
1179: (10 seconds, in the example)
1.402 jmc 1180: to start the program which is going to use the tunnel.
1.246 jmc 1181: If no connections are made within the time specified,
1182: .Nm
1183: will exit.
1184: .Sh X11 FORWARDING
1.218 jmc 1185: If the
1186: .Cm ForwardX11
1187: variable is set to
1188: .Dq yes
1189: (or see the description of the
1.227 jmc 1190: .Fl X ,
1191: .Fl x ,
1.218 jmc 1192: and
1.227 jmc 1193: .Fl Y
1.226 jmc 1194: options above)
1.218 jmc 1195: and the user is using X11 (the
1196: .Ev DISPLAY
1197: environment variable is set), the connection to the X11 display is
1198: automatically forwarded to the remote side in such a way that any X11
1199: programs started from the shell (or command) will go through the
1200: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1201: from the local machine.
1.218 jmc 1202: The user should not manually set
1203: .Ev DISPLAY .
1204: Forwarding of X11 connections can be
1205: configured on the command line or in configuration files.
1206: .Pp
1207: The
1208: .Ev DISPLAY
1209: value set by
1210: .Nm
1211: will point to the server machine, but with a display number greater than zero.
1212: This is normal, and happens because
1213: .Nm
1214: creates a
1215: .Dq proxy
1216: X server on the server machine for forwarding the
1217: connections over the encrypted channel.
1.200 djm 1218: .Pp
1.218 jmc 1219: .Nm
1220: will also automatically set up Xauthority data on the server machine.
1221: For this purpose, it will generate a random authorization cookie,
1222: store it in Xauthority on the server, and verify that any forwarded
1223: connections carry this cookie and replace it by the real cookie when
1224: the connection is opened.
1225: The real authentication cookie is never
1226: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1227: .Pp
1.218 jmc 1228: If the
1229: .Cm ForwardAgent
1230: variable is set to
1231: .Dq yes
1232: (or see the description of the
1233: .Fl A
1.191 djm 1234: and
1.218 jmc 1235: .Fl a
1.226 jmc 1236: options above) and
1.218 jmc 1237: the user is using an authentication agent, the connection to the agent
1238: is automatically forwarded to the remote side.
1.252 jmc 1239: .Sh VERIFYING HOST KEYS
1240: When connecting to a server for the first time,
1241: a fingerprint of the server's public key is presented to the user
1242: (unless the option
1243: .Cm StrictHostKeyChecking
1244: has been disabled).
1245: Fingerprints can be determined using
1246: .Xr ssh-keygen 1 :
1247: .Pp
1248: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1249: .Pp
1.274 grunk 1250: If the fingerprint is already known, it can be matched
1251: and the key can be accepted or rejected.
1.358 djm 1252: If only legacy (MD5) fingerprints for the server are available, the
1253: .Xr ssh-keygen 1
1254: .Fl E
1255: option may be used to downgrade the fingerprint algorithm to match.
1256: .Pp
1.274 grunk 1257: Because of the difficulty of comparing host keys
1.352 djm 1258: just by looking at fingerprint strings,
1.274 grunk 1259: there is also support to compare host keys visually,
1260: using
1261: .Em random art .
1262: By setting the
1.275 grunk 1263: .Cm VisualHostKey
1.274 grunk 1264: option to
1.275 grunk 1265: .Dq yes ,
1.274 grunk 1266: a small ASCII graphic gets displayed on every login to a server, no matter
1267: if the session itself is interactive or not.
1268: By learning the pattern a known server produces, a user can easily
1269: find out that the host key has changed when a completely different pattern
1270: is displayed.
1271: Because these patterns are not unambiguous however, a pattern that looks
1272: similar to the pattern remembered only gives a good probability that the
1273: host key is the same, not guaranteed proof.
1274: .Pp
1275: To get a listing of the fingerprints along with their random art for
1276: all known hosts, the following command line can be used:
1277: .Pp
1278: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1279: .Pp
1.252 jmc 1280: If the fingerprint is unknown,
1281: an alternative method of verification is available:
1282: SSH fingerprints verified by DNS.
1283: An additional resource record (RR),
1284: SSHFP,
1285: is added to a zonefile
1286: and the connecting client is able to match the fingerprint
1287: with that of the key presented.
1288: .Pp
1289: In this example, we are connecting a client to a server,
1290: .Dq host.example.com .
1291: The SSHFP resource records should first be added to the zonefile for
1292: host.example.com:
1293: .Bd -literal -offset indent
1.259 jakob 1294: $ ssh-keygen -r host.example.com.
1.252 jmc 1295: .Ed
1296: .Pp
1297: The output lines will have to be added to the zonefile.
1298: To check that the zone is answering fingerprint queries:
1299: .Pp
1300: .Dl $ dig -t SSHFP host.example.com
1301: .Pp
1302: Finally the client connects:
1303: .Bd -literal -offset indent
1304: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1305: [...]
1306: Matching host key fingerprint found in DNS.
1307: Are you sure you want to continue connecting (yes/no)?
1308: .Ed
1309: .Pp
1310: See the
1311: .Cm VerifyHostKeyDNS
1312: option in
1313: .Xr ssh_config 5
1314: for more information.
1.250 jmc 1315: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1316: .Nm
1317: contains support for Virtual Private Network (VPN) tunnelling
1318: using the
1319: .Xr tun 4
1320: network pseudo-device,
1321: allowing two networks to be joined securely.
1322: The
1323: .Xr sshd_config 5
1324: configuration option
1325: .Cm PermitTunnel
1326: controls whether the server supports this,
1327: and at what level (layer 2 or 3 traffic).
1328: .Pp
1329: The following example would connect client network 10.0.50.0/24
1.265 otto 1330: with remote network 10.0.99.0/24 using a point-to-point connection
1331: from 10.1.1.1 to 10.1.1.2,
1332: provided that the SSH server running on the gateway to the remote network,
1333: at 192.168.1.15, allows it.
1334: .Pp
1335: On the client:
1.250 jmc 1336: .Bd -literal -offset indent
1337: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1338: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1339: # route add 10.0.99.0/24 10.1.1.2
1340: .Ed
1341: .Pp
1342: On the server:
1343: .Bd -literal -offset indent
1344: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1345: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1346: .Ed
1347: .Pp
1348: Client access may be more finely tuned via the
1349: .Pa /root/.ssh/authorized_keys
1350: file (see below) and the
1351: .Cm PermitRootLogin
1352: server option.
1.255 jmc 1353: The following entry would permit connections on
1.250 jmc 1354: .Xr tun 4
1.255 jmc 1355: device 1 from user
1.250 jmc 1356: .Dq jane
1.255 jmc 1357: and on tun device 2 from user
1.250 jmc 1358: .Dq john ,
1359: if
1360: .Cm PermitRootLogin
1361: is set to
1362: .Dq forced-commands-only :
1363: .Bd -literal -offset 2n
1364: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1365: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1366: .Ed
1367: .Pp
1.264 ray 1368: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1369: it may be more suited to temporary setups,
1370: such as for wireless VPNs.
1371: More permanent VPNs are better provided by tools such as
1372: .Xr ipsecctl 8
1373: and
1374: .Xr isakmpd 8 .
1.2 deraadt 1375: .Sh ENVIRONMENT
1376: .Nm
1.1 deraadt 1377: will normally set the following environment variables:
1.237 jmc 1378: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1379: .It Ev DISPLAY
1380: The
1381: .Ev DISPLAY
1.40 aaron 1382: variable indicates the location of the X11 server.
1.44 aaron 1383: It is automatically set by
1.2 deraadt 1384: .Nm
1385: to point to a value of the form
1.233 jmc 1386: .Dq hostname:n ,
1387: where
1388: .Dq hostname
1389: indicates the host where the shell runs, and
1390: .Sq n
1391: is an integer \*(Ge 1.
1.40 aaron 1392: .Nm
1393: uses this special value to forward X11 connections over the secure
1394: channel.
1.107 markus 1395: The user should normally not set
1396: .Ev DISPLAY
1397: explicitly, as that
1.1 deraadt 1398: will render the X11 connection insecure (and will require the user to
1399: manually copy any required authorization cookies).
1.2 deraadt 1400: .It Ev HOME
1.1 deraadt 1401: Set to the path of the user's home directory.
1.2 deraadt 1402: .It Ev LOGNAME
1403: Synonym for
1.12 aaron 1404: .Ev USER ;
1405: set for compatibility with systems that use this variable.
1.2 deraadt 1406: .It Ev MAIL
1.129 stevesk 1407: Set to the path of the user's mailbox.
1.40 aaron 1408: .It Ev PATH
1.2 deraadt 1409: Set to the default
1410: .Ev PATH ,
1411: as specified when compiling
1.234 jmc 1412: .Nm .
1.118 markus 1413: .It Ev SSH_ASKPASS
1414: If
1415: .Nm
1416: needs a passphrase, it will read the passphrase from the current
1417: terminal if it was run from a terminal.
1418: If
1419: .Nm
1420: does not have a terminal associated with it but
1421: .Ev DISPLAY
1422: and
1423: .Ev SSH_ASKPASS
1424: are set, it will execute the program specified by
1425: .Ev SSH_ASKPASS
1426: and open an X11 window to read the passphrase.
1427: This is particularly useful when calling
1428: .Nm
1429: from a
1.196 jmc 1430: .Pa .xsession
1.118 markus 1431: or related script.
1432: (Note that on some machines it
1433: may be necessary to redirect the input from
1434: .Pa /dev/null
1435: to make this work.)
1.413 djm 1436: .It Ev SSH_ASKPASS_REQUIRE
1.414 jmc 1437: Allows further control over the use of an askpass program.
1.413 djm 1438: If this variable is set to
1439: .Dq never
1440: then
1441: .Nm
1442: will never attempt to use one.
1443: If it is set to
1444: .Dq prefer ,
1445: then
1446: .Nm
1447: will prefer to use the askpass program instead of the TTY when requesting
1448: passwords.
1449: Finally, if the variable is set to
1450: .Dq force ,
1451: then the askpass program will be used for all passphrase input regardless
1452: of whether
1453: .Ev DISPLAY
1454: is set.
1.18 markus 1455: .It Ev SSH_AUTH_SOCK
1.233 jmc 1456: Identifies the path of a
1457: .Ux Ns -domain
1458: socket used to communicate with the agent.
1.166 stevesk 1459: .It Ev SSH_CONNECTION
1460: Identifies the client and server ends of the connection.
1.40 aaron 1461: The variable contains
1.233 jmc 1462: four space-separated values: client IP address, client port number,
1463: server IP address, and server port number.
1.73 markus 1464: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1465: This variable contains the original command line if a forced command
1.73 markus 1466: is executed.
1467: It can be used to extract the original arguments.
1.2 deraadt 1468: .It Ev SSH_TTY
1.1 deraadt 1469: This is set to the name of the tty (path to the device) associated
1.40 aaron 1470: with the current shell or command.
1471: If the current session has no tty,
1.1 deraadt 1472: this variable is not set.
1.387 djm 1473: .It Ev SSH_TUNNEL
1474: Optionally set by
1475: .Xr sshd 8
1476: to contain the interface names assigned if tunnel forwarding was
1477: requested by the client.
1.385 djm 1478: .It Ev SSH_USER_AUTH
1479: Optionally set by
1480: .Xr sshd 8 ,
1481: this variable may contain a pathname to a file that lists the authentication
1482: methods successfully used when the session was established, including any
1483: public keys that were used.
1.2 deraadt 1484: .It Ev TZ
1.214 jmc 1485: This variable is set to indicate the present time zone if it
1.257 jmc 1486: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1487: on to new connections).
1.2 deraadt 1488: .It Ev USER
1.1 deraadt 1489: Set to the name of the user logging in.
1.2 deraadt 1490: .El
1491: .Pp
1.44 aaron 1492: Additionally,
1.2 deraadt 1493: .Nm
1.44 aaron 1494: reads
1.207 djm 1495: .Pa ~/.ssh/environment ,
1.2 deraadt 1496: and adds lines of the format
1497: .Dq VARNAME=value
1.233 jmc 1498: to the environment if the file exists and users are allowed to
1.161 marc 1499: change their environment.
1.176 jmc 1500: For more information, see the
1.161 marc 1501: .Cm PermitUserEnvironment
1.162 stevesk 1502: option in
1.161 marc 1503: .Xr sshd_config 5 .
1.2 deraadt 1504: .Sh FILES
1.236 jmc 1505: .Bl -tag -width Ds -compact
1.309 jmc 1506: .It Pa ~/.rhosts
1.240 jmc 1507: This file is used for host-based authentication (see above).
1.92 markus 1508: On some machines this file may need to be
1.240 jmc 1509: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1510: because
1.2 deraadt 1511: .Xr sshd 8
1.40 aaron 1512: reads it as root.
1513: Additionally, this file must be owned by the user,
1514: and must not have write permissions for anyone else.
1515: The recommended
1.1 deraadt 1516: permission for most machines is read/write for the user, and not
1517: accessible by others.
1.2 deraadt 1518: .Pp
1.309 jmc 1519: .It Pa ~/.shosts
1.240 jmc 1520: This file is used in exactly the same way as
1521: .Pa .rhosts ,
1522: but allows host-based authentication without permitting login with
1523: rlogin/rsh.
1.272 mcbride 1524: .Pp
1.309 jmc 1525: .It Pa ~/.ssh/
1.272 mcbride 1526: This directory is the default location for all user-specific configuration
1527: and authentication information.
1528: There is no general requirement to keep the entire contents of this directory
1529: secret, but the recommended permissions are read/write/execute for the user,
1530: and not accessible by others.
1.236 jmc 1531: .Pp
1.309 jmc 1532: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1533: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1534: that can be used for logging in as this user.
1.238 jmc 1535: The format of this file is described in the
1536: .Xr sshd 8
1537: manual page.
1538: This file is not highly sensitive, but the recommended
1539: permissions are read/write for the user, and not accessible by others.
1540: .Pp
1.309 jmc 1541: .It Pa ~/.ssh/config
1.238 jmc 1542: This is the per-user configuration file.
1543: The file format and configuration options are described in
1544: .Xr ssh_config 5 .
1545: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1546: read/write for the user, and not writable by others.
1.238 jmc 1547: .Pp
1.309 jmc 1548: .It Pa ~/.ssh/environment
1.239 jmc 1549: Contains additional definitions for environment variables; see
1550: .Sx ENVIRONMENT ,
1.238 jmc 1551: above.
1552: .Pp
1.309 jmc 1553: .It Pa ~/.ssh/id_dsa
1.310 djm 1554: .It Pa ~/.ssh/id_ecdsa
1.404 naddy 1555: .It Pa ~/.ssh/id_ecdsa_sk
1.343 naddy 1556: .It Pa ~/.ssh/id_ed25519
1.406 naddy 1557: .It Pa ~/.ssh/id_ed25519_sk
1.309 jmc 1558: .It Pa ~/.ssh/id_rsa
1.238 jmc 1559: Contains the private key for authentication.
1560: These files
1561: contain sensitive data and should be readable by the user but not
1562: accessible by others (read/write/execute).
1563: .Nm
1564: will simply ignore a private key file if it is accessible by others.
1565: It is possible to specify a passphrase when
1566: generating the key which will be used to encrypt the
1.389 djm 1567: sensitive part of this file using AES-128.
1.238 jmc 1568: .Pp
1.309 jmc 1569: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1570: .It Pa ~/.ssh/id_ecdsa.pub
1.404 naddy 1571: .It Pa ~/.ssh/id_ecdsa_sk.pub
1.343 naddy 1572: .It Pa ~/.ssh/id_ed25519.pub
1.406 naddy 1573: .It Pa ~/.ssh/id_ed25519_sk.pub
1.309 jmc 1574: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1575: Contains the public key for authentication.
1576: These files are not
1577: sensitive and can (but need not) be readable by anyone.
1578: .Pp
1.309 jmc 1579: .It Pa ~/.ssh/known_hosts
1.244 jmc 1580: Contains a list of host keys for all hosts the user has logged into
1581: that are not already in the systemwide list of known host keys.
1.238 jmc 1582: See
1.244 jmc 1583: .Xr sshd 8
1584: for further details of the format of this file.
1.238 jmc 1585: .Pp
1.309 jmc 1586: .It Pa ~/.ssh/rc
1.238 jmc 1587: Commands in this file are executed by
1588: .Nm
1.245 jmc 1589: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1590: started.
1591: See the
1592: .Xr sshd 8
1593: manual page for more information.
1594: .Pp
1.309 jmc 1595: .It Pa /etc/hosts.equiv
1.240 jmc 1596: This file is for host-based authentication (see above).
1597: It should only be writable by root.
1.236 jmc 1598: .Pp
1.309 jmc 1599: .It Pa /etc/shosts.equiv
1.240 jmc 1600: This file is used in exactly the same way as
1601: .Pa hosts.equiv ,
1602: but allows host-based authentication without permitting login with
1603: rlogin/rsh.
1.236 jmc 1604: .Pp
1.238 jmc 1605: .It Pa /etc/ssh/ssh_config
1606: Systemwide configuration file.
1607: The file format and configuration options are described in
1608: .Xr ssh_config 5 .
1609: .Pp
1.309 jmc 1610: .It Pa /etc/ssh/ssh_host_key
1611: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1612: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1613: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1614: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1615: These files contain the private parts of the host keys
1.245 jmc 1616: and are used for host-based authentication.
1.238 jmc 1617: .Pp
1.309 jmc 1618: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1619: Systemwide list of known host keys.
1620: This file should be prepared by the
1621: system administrator to contain the public host keys of all machines in the
1622: organization.
1.244 jmc 1623: It should be world-readable.
1624: See
1.238 jmc 1625: .Xr sshd 8
1.244 jmc 1626: for further details of the format of this file.
1.236 jmc 1627: .Pp
1.309 jmc 1628: .It Pa /etc/ssh/sshrc
1.1 deraadt 1629: Commands in this file are executed by
1.2 deraadt 1630: .Nm
1.245 jmc 1631: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1632: See the
1.2 deraadt 1633: .Xr sshd 8
1.1 deraadt 1634: manual page for more information.
1.58 itojun 1635: .El
1.312 jmc 1636: .Sh EXIT STATUS
1637: .Nm
1638: exits with the exit status of the remote command or with 255
1639: if an error occurred.
1.2 deraadt 1640: .Sh SEE ALSO
1641: .Xr scp 1 ,
1.83 djm 1642: .Xr sftp 1 ,
1.2 deraadt 1643: .Xr ssh-add 1 ,
1644: .Xr ssh-agent 1 ,
1645: .Xr ssh-keygen 1 ,
1.242 jmc 1646: .Xr ssh-keyscan 1 ,
1.250 jmc 1647: .Xr tun 4 ,
1.159 stevesk 1648: .Xr ssh_config 5 ,
1.160 naddy 1649: .Xr ssh-keysign 8 ,
1.87 itojun 1650: .Xr sshd 8
1.329 jmc 1651: .Sh STANDARDS
1.106 markus 1652: .Rs
1.329 jmc 1653: .%A S. Lehtinen
1654: .%A C. Lonvick
1655: .%D January 2006
1.256 jmc 1656: .%R RFC 4250
1.329 jmc 1657: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1658: .Re
1.329 jmc 1659: .Pp
1.256 jmc 1660: .Rs
1.329 jmc 1661: .%A T. Ylonen
1662: .%A C. Lonvick
1663: .%D January 2006
1.256 jmc 1664: .%R RFC 4251
1.329 jmc 1665: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1666: .Re
1.329 jmc 1667: .Pp
1.256 jmc 1668: .Rs
1.329 jmc 1669: .%A T. Ylonen
1670: .%A C. Lonvick
1671: .%D January 2006
1.256 jmc 1672: .%R RFC 4252
1.329 jmc 1673: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1674: .Re
1.329 jmc 1675: .Pp
1.256 jmc 1676: .Rs
1.329 jmc 1677: .%A T. Ylonen
1678: .%A C. Lonvick
1679: .%D January 2006
1.256 jmc 1680: .%R RFC 4253
1.329 jmc 1681: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1682: .Re
1.329 jmc 1683: .Pp
1.256 jmc 1684: .Rs
1.329 jmc 1685: .%A T. Ylonen
1686: .%A C. Lonvick
1687: .%D January 2006
1.256 jmc 1688: .%R RFC 4254
1.329 jmc 1689: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1690: .Re
1.329 jmc 1691: .Pp
1.256 jmc 1692: .Rs
1.329 jmc 1693: .%A J. Schlyter
1694: .%A W. Griffin
1695: .%D January 2006
1.256 jmc 1696: .%R RFC 4255
1.329 jmc 1697: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1698: .Re
1.329 jmc 1699: .Pp
1.256 jmc 1700: .Rs
1.329 jmc 1701: .%A F. Cusack
1702: .%A M. Forssen
1703: .%D January 2006
1.256 jmc 1704: .%R RFC 4256
1.329 jmc 1705: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1706: .Re
1.329 jmc 1707: .Pp
1.256 jmc 1708: .Rs
1.329 jmc 1709: .%A J. Galbraith
1710: .%A P. Remaker
1711: .%D January 2006
1.256 jmc 1712: .%R RFC 4335
1.329 jmc 1713: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1714: .Re
1.329 jmc 1715: .Pp
1.256 jmc 1716: .Rs
1.329 jmc 1717: .%A M. Bellare
1718: .%A T. Kohno
1719: .%A C. Namprempre
1720: .%D January 2006
1.256 jmc 1721: .%R RFC 4344
1.329 jmc 1722: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1723: .Re
1.329 jmc 1724: .Pp
1.256 jmc 1725: .Rs
1.329 jmc 1726: .%A B. Harris
1727: .%D January 2006
1.256 jmc 1728: .%R RFC 4345
1.329 jmc 1729: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1730: .Re
1.329 jmc 1731: .Pp
1.258 djm 1732: .Rs
1.329 jmc 1733: .%A M. Friedl
1734: .%A N. Provos
1735: .%A W. Simpson
1736: .%D March 2006
1.258 djm 1737: .%R RFC 4419
1.329 jmc 1738: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1739: .Re
1.329 jmc 1740: .Pp
1.266 markus 1741: .Rs
1.329 jmc 1742: .%A J. Galbraith
1743: .%A R. Thayer
1744: .%D November 2006
1.266 markus 1745: .%R RFC 4716
1.329 jmc 1746: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1747: .Re
1.329 jmc 1748: .Pp
1.313 djm 1749: .Rs
1.329 jmc 1750: .%A D. Stebila
1751: .%A J. Green
1752: .%D December 2009
1.313 djm 1753: .%R RFC 5656
1.329 jmc 1754: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1755: .Re
1.329 jmc 1756: .Pp
1.274 grunk 1757: .Rs
1758: .%A A. Perrig
1759: .%A D. Song
1760: .%D 1999
1.329 jmc 1761: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1762: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1763: .Re
1.173 jmc 1764: .Sh AUTHORS
1765: OpenSSH is a derivative of the original and free
1766: ssh 1.2.12 release by Tatu Ylonen.
1767: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1768: Theo de Raadt and Dug Song
1769: removed many bugs, re-added newer features and
1770: created OpenSSH.
1771: Markus Friedl contributed the support for SSH
1772: protocol versions 1.5 and 2.0.
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