Annotation of src/usr.bin/ssh/ssh.1, Revision 1.409
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: .\"
1.409 ! naddy 36: .\" $OpenBSD: ssh.1,v 1.408 2019/11/30 07:07:59 jmc Exp $
! 37: .Dd $Mdocdate: November 30 2019 $
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
1.49 markus 91: his/her 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 .
1.360 millert 236: .Pp
1.218 jmc 237: .It Fl f
238: Requests
239: .Nm
240: to go to background just before command execution.
241: This is useful if
1.176 jmc 242: .Nm
1.218 jmc 243: is going to ask for passwords or passphrases, but the user
244: wants it in the background.
245: This implies
246: .Fl n .
247: The recommended way to start X11 programs at a remote site is with
248: something like
249: .Ic ssh -f host xterm .
1.277 djm 250: .Pp
251: If the
252: .Cm ExitOnForwardFailure
253: configuration option is set to
254: .Dq yes ,
255: then a client started with
256: .Fl f
257: will wait for all remote port forwards to be successfully established
258: before placing itself in the background.
1.360 millert 259: .Pp
1.350 djm 260: .It Fl G
261: Causes
262: .Nm
263: to print its configuration after evaluating
264: .Cm Host
265: and
266: .Cm Match
267: blocks and exit.
1.360 millert 268: .Pp
1.218 jmc 269: .It Fl g
270: Allows remote hosts to connect to local forwarded ports.
1.346 djm 271: If used on a multiplexed connection, then this option must be specified
272: on the master process.
1.360 millert 273: .Pp
1.291 markus 274: .It Fl I Ar pkcs11
1.294 jmc 275: Specify the PKCS#11 shared library
1.176 jmc 276: .Nm
1.401 naddy 277: should use to communicate with a PKCS#11 token providing keys for user
278: authentication.
1.360 millert 279: .Pp
1.218 jmc 280: .It Fl i Ar identity_file
281: Selects a file from which the identity (private key) for
1.310 djm 282: public key authentication is read.
1.218 jmc 283: The default is
1.310 djm 284: .Pa ~/.ssh/id_dsa ,
1.343 naddy 285: .Pa ~/.ssh/id_ecdsa ,
1.404 naddy 286: .Pa ~/.ssh/id_ecdsa_sk ,
1.406 naddy 287: .Pa ~/.ssh/id_ed25519 ,
288: .Pa ~/.ssh/id_ed25519_sk
1.310 djm 289: and
1.381 naddy 290: .Pa ~/.ssh/id_rsa .
1.218 jmc 291: Identity files may also be specified on
292: a per-host basis in the configuration file.
293: It is possible to have multiple
294: .Fl i
295: options (and multiple identities specified in
296: configuration files).
1.364 jmc 297: If no certificates have been explicitly specified by the
1.363 djm 298: .Cm CertificateFile
299: directive,
1.302 djm 300: .Nm
301: will also try to load certificate information from the filename obtained
302: by appending
303: .Pa -cert.pub
304: to identity filenames.
1.360 millert 305: .Pp
1.386 millert 306: .It Fl J Ar destination
1.375 djm 307: Connect to the target host by first making a
308: .Nm
1.386 millert 309: connection to the jump host described by
310: .Ar destination
1.376 jmc 311: and then establishing a TCP forwarding to the ultimate destination from
1.375 djm 312: there.
313: Multiple jump hops may be specified separated by comma characters.
314: This is a shortcut to specify a
315: .Cm ProxyJump
316: configuration directive.
1.400 djm 317: Note that configuration directives supplied on the command-line generally
318: apply to the destination host and not any specified jump hosts.
319: Use
320: .Pa ~/.ssh/config
321: to specify configuration for jump hosts.
1.375 djm 322: .Pp
1.269 djm 323: .It Fl K
324: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
325: credentials to the server.
1.360 millert 326: .Pp
1.218 jmc 327: .It Fl k
328: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 329: .Pp
1.218 jmc 330: .It Fl L Xo
331: .Sm off
332: .Oo Ar bind_address : Oc
333: .Ar port : host : hostport
334: .Sm on
335: .Xc
1.360 millert 336: .It Fl L Xo
337: .Sm off
338: .Oo Ar bind_address : Oc
339: .Ar port : remote_socket
340: .Sm on
341: .Xc
342: .It Fl L Xo
343: .Sm off
344: .Ar local_socket : host : hostport
345: .Sm on
346: .Xc
347: .It Fl L Xo
348: .Sm off
349: .Ar local_socket : remote_socket
350: .Sm on
351: .Xc
352: Specifies that connections to the given TCP port or Unix socket on the local
353: (client) host are to be forwarded to the given host and port, or Unix socket,
354: on the remote side.
355: This works by allocating a socket to listen to either a TCP
1.218 jmc 356: .Ar port
357: on the local side, optionally bound to the specified
1.360 millert 358: .Ar bind_address ,
359: or to a Unix socket.
360: Whenever a connection is made to the local port or socket, the
1.218 jmc 361: connection is forwarded over the secure channel, and a connection is
1.360 millert 362: made to either
1.218 jmc 363: .Ar host
364: port
1.360 millert 365: .Ar hostport ,
366: or the Unix socket
367: .Ar remote_socket ,
1.218 jmc 368: from the remote machine.
1.360 millert 369: .Pp
1.218 jmc 370: Port forwardings can also be specified in the configuration file.
1.360 millert 371: Only the superuser can forward privileged ports.
1.308 djm 372: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 373: .Pp
1.218 jmc 374: By default, the local port is bound in accordance with the
375: .Cm GatewayPorts
376: setting.
377: However, an explicit
378: .Ar bind_address
379: may be used to bind the connection to a specific address.
1.2 deraadt 380: The
1.218 jmc 381: .Ar bind_address
382: of
383: .Dq localhost
384: indicates that the listening port be bound for local use only, while an
385: empty address or
386: .Sq *
387: indicates that the port should be available from all interfaces.
1.360 millert 388: .Pp
1.218 jmc 389: .It Fl l Ar login_name
390: Specifies the user to log in as on the remote machine.
391: This also may be specified on a per-host basis in the configuration file.
1.360 millert 392: .Pp
1.218 jmc 393: .It Fl M
394: Places the
395: .Nm
396: client into
397: .Dq master
398: mode for connection sharing.
1.231 stevesk 399: Multiple
400: .Fl M
401: options places
402: .Nm
403: into
404: .Dq master
1.397 djm 405: mode but with confirmation required using
406: .Xr ssh-askpass 1
407: before each operation that changes the multiplexing state
408: (e.g. opening a new session).
1.218 jmc 409: Refer to the description of
410: .Cm ControlMaster
411: in
412: .Xr ssh_config 5
413: for details.
1.360 millert 414: .Pp
1.218 jmc 415: .It Fl m Ar mac_spec
1.369 jmc 416: A comma-separated list of MAC (message authentication code) algorithms,
417: specified in order of preference.
1.218 jmc 418: See the
419: .Cm MACs
420: keyword for more information.
1.360 millert 421: .Pp
1.218 jmc 422: .It Fl N
423: Do not execute a remote command.
1.369 jmc 424: This is useful for just forwarding ports.
1.360 millert 425: .Pp
1.218 jmc 426: .It Fl n
427: Redirects stdin from
428: .Pa /dev/null
429: (actually, prevents reading from stdin).
430: This must be used when
1.2 deraadt 431: .Nm
1.218 jmc 432: is run in the background.
433: A common trick is to use this to run X11 programs on a remote machine.
434: For example,
435: .Ic ssh -n shadows.cs.hut.fi emacs &
436: will start an emacs on shadows.cs.hut.fi, and the X11
437: connection will be automatically forwarded over an encrypted channel.
438: The
1.2 deraadt 439: .Nm
1.218 jmc 440: program will be put in the background.
441: (This does not work if
1.2 deraadt 442: .Nm
1.218 jmc 443: needs to ask for a password or passphrase; see also the
444: .Fl f
445: option.)
1.360 millert 446: .Pp
1.218 jmc 447: .It Fl O Ar ctl_cmd
448: Control an active connection multiplexing master process.
449: When the
450: .Fl O
451: option is specified, the
452: .Ar ctl_cmd
453: argument is interpreted and passed to the master process.
454: Valid commands are:
455: .Dq check
1.305 markus 456: (check that the master process is running),
457: .Dq forward
1.318 jmc 458: (request forwardings without command execution),
1.323 okan 459: .Dq cancel
460: (cancel forwardings),
1.218 jmc 461: .Dq exit
1.318 jmc 462: (request the master to exit), and
1.317 djm 463: .Dq stop
464: (request the master to stop accepting further multiplexing requests).
1.360 millert 465: .Pp
1.218 jmc 466: .It Fl o Ar option
467: Can be used to give options in the format used in the configuration file.
468: This is useful for specifying options for which there is no separate
469: command-line flag.
470: For full details of the options listed below, and their possible values, see
471: .Xr ssh_config 5 .
1.2 deraadt 472: .Pp
1.218 jmc 473: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 474: .It AddKeysToAgent
1.218 jmc 475: .It AddressFamily
476: .It BatchMode
477: .It BindAddress
1.338 djm 478: .It CanonicalDomains
1.339 djm 479: .It CanonicalizeFallbackLocal
480: .It CanonicalizeHostname
481: .It CanonicalizeMaxDots
482: .It CanonicalizePermittedCNAMEs
1.399 jmc 483: .It CASignatureAlgorithms
1.363 djm 484: .It CertificateFile
1.218 jmc 485: .It ChallengeResponseAuthentication
486: .It CheckHostIP
487: .It Ciphers
488: .It ClearAllForwardings
489: .It Compression
1.380 naddy 490: .It ConnectionAttempts
1.218 jmc 491: .It ConnectTimeout
492: .It ControlMaster
493: .It ControlPath
1.321 djm 494: .It ControlPersist
1.218 jmc 495: .It DynamicForward
496: .It EscapeChar
1.263 markus 497: .It ExitOnForwardFailure
1.353 jmc 498: .It FingerprintHash
1.218 jmc 499: .It ForwardAgent
500: .It ForwardX11
1.321 djm 501: .It ForwardX11Timeout
1.218 jmc 502: .It ForwardX11Trusted
503: .It GatewayPorts
504: .It GlobalKnownHostsFile
505: .It GSSAPIAuthentication
506: .It GSSAPIDelegateCredentials
507: .It HashKnownHosts
508: .It Host
509: .It HostbasedAuthentication
1.355 djm 510: .It HostbasedKeyTypes
1.218 jmc 511: .It HostKeyAlgorithms
512: .It HostKeyAlias
1.403 jmc 513: .It Hostname
1.374 jmc 514: .It IdentitiesOnly
1.371 markus 515: .It IdentityAgent
1.218 jmc 516: .It IdentityFile
1.316 jmc 517: .It IPQoS
1.321 djm 518: .It KbdInteractiveAuthentication
1.218 jmc 519: .It KbdInteractiveDevices
1.314 jmc 520: .It KexAlgorithms
1.218 jmc 521: .It LocalCommand
522: .It LocalForward
523: .It LogLevel
524: .It MACs
1.337 jmc 525: .It Match
1.218 jmc 526: .It NoHostAuthenticationForLocalhost
527: .It NumberOfPasswordPrompts
528: .It PasswordAuthentication
529: .It PermitLocalCommand
1.291 markus 530: .It PKCS11Provider
1.218 jmc 531: .It Port
532: .It PreferredAuthentications
533: .It ProxyCommand
1.375 djm 534: .It ProxyJump
1.336 jmc 535: .It ProxyUseFdpass
1.359 markus 536: .It PubkeyAcceptedKeyTypes
1.218 jmc 537: .It PubkeyAuthentication
1.251 dtucker 538: .It RekeyLimit
1.382 bluhm 539: .It RemoteCommand
1.218 jmc 540: .It RemoteForward
1.319 jmc 541: .It RequestTTY
1.218 jmc 542: .It SendEnv
543: .It ServerAliveInterval
544: .It ServerAliveCountMax
1.395 jmc 545: .It SetEnv
1.347 jmc 546: .It StreamLocalBindMask
547: .It StreamLocalBindUnlink
1.218 jmc 548: .It StrictHostKeyChecking
549: .It TCPKeepAlive
550: .It Tunnel
551: .It TunnelDevice
1.354 djm 552: .It UpdateHostKeys
1.218 jmc 553: .It User
554: .It UserKnownHostsFile
555: .It VerifyHostKeyDNS
1.276 jmc 556: .It VisualHostKey
1.218 jmc 557: .It XAuthLocation
558: .El
1.360 millert 559: .Pp
1.218 jmc 560: .It Fl p Ar port
561: Port to connect to on the remote host.
562: This can be specified on a
563: per-host basis in the configuration file.
1.360 millert 564: .Pp
1.362 djm 565: .It Fl Q Ar query_option
1.332 djm 566: Queries
567: .Nm
1.341 deraadt 568: for the algorithms supported for the specified version 2.
569: The available features are:
570: .Ar cipher
1.332 djm 571: (supported symmetric ciphers),
1.341 deraadt 572: .Ar cipher-auth
1.340 djm 573: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 574: .Ar help
575: (supported query terms for use with the
576: .Fl Q
577: flag),
1.341 deraadt 578: .Ar mac
1.332 djm 579: (supported message integrity codes),
1.341 deraadt 580: .Ar kex
1.332 djm 581: (key exchange algorithms),
1.341 deraadt 582: .Ar key
1.362 djm 583: (key types),
584: .Ar key-cert
585: (certificate key types),
586: .Ar key-plain
1.398 djm 587: (non-certificate key types),
1.356 djm 588: .Ar protocol-version
1.398 djm 589: (supported SSH protocol versions), and
590: .Ar sig
591: (supported signature algorithms).
1.360 millert 592: .Pp
1.218 jmc 593: .It Fl q
594: Quiet mode.
1.271 djm 595: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 596: .Pp
1.218 jmc 597: .It Fl R Xo
598: .Sm off
599: .Oo Ar bind_address : Oc
600: .Ar port : host : hostport
601: .Sm on
602: .Xc
1.360 millert 603: .It Fl R Xo
604: .Sm off
605: .Oo Ar bind_address : Oc
606: .Ar port : local_socket
607: .Sm on
608: .Xc
609: .It Fl R Xo
610: .Sm off
611: .Ar remote_socket : host : hostport
612: .Sm on
613: .Xc
614: .It Fl R Xo
615: .Sm off
616: .Ar remote_socket : local_socket
617: .Sm on
618: .Xc
1.384 markus 619: .It Fl R Xo
620: .Sm off
621: .Oo Ar bind_address : Oc
622: .Ar port
623: .Sm on
624: .Xc
1.360 millert 625: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 626: (server) host are to be forwarded to the local side.
627: .Pp
1.360 millert 628: This works by allocating a socket to listen to either a TCP
1.218 jmc 629: .Ar port
1.360 millert 630: or to a Unix socket on the remote side.
631: Whenever a connection is made to this port or Unix socket, the
632: connection is forwarded over the secure channel, and a connection
1.384 markus 633: is made from the local machine to either an explicit destination specified by
1.218 jmc 634: .Ar host
635: port
1.360 millert 636: .Ar hostport ,
637: or
638: .Ar local_socket ,
1.384 markus 639: or, if no explicit destination was specified,
640: .Nm
641: will act as a SOCKS 4/5 proxy and forward connections to the destinations
642: requested by the remote SOCKS client.
1.2 deraadt 643: .Pp
1.218 jmc 644: Port forwardings can also be specified in the configuration file.
645: Privileged ports can be forwarded only when
646: logging in as root on the remote machine.
1.324 jmc 647: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 648: .Pp
1.360 millert 649: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 650: interface only.
1.280 tobias 651: This may be overridden by specifying a
1.218 jmc 652: .Ar bind_address .
653: An empty
654: .Ar bind_address ,
655: or the address
656: .Ql * ,
657: indicates that the remote socket should listen on all interfaces.
658: Specifying a remote
659: .Ar bind_address
660: will only succeed if the server's
661: .Cm GatewayPorts
662: option is enabled (see
663: .Xr sshd_config 5 ) .
1.281 djm 664: .Pp
665: If the
666: .Ar port
667: argument is
1.282 djm 668: .Ql 0 ,
1.281 djm 669: the listen port will be dynamically allocated on the server and reported
670: to the client at run time.
1.305 markus 671: When used together with
672: .Ic -O forward
673: the allocated port will be printed to the standard output.
1.360 millert 674: .Pp
1.218 jmc 675: .It Fl S Ar ctl_path
1.304 jmc 676: Specifies the location of a control socket for connection sharing,
1.303 djm 677: or the string
678: .Dq none
679: to disable connection sharing.
1.218 jmc 680: Refer to the description of
681: .Cm ControlPath
682: and
683: .Cm ControlMaster
684: in
685: .Xr ssh_config 5
686: for details.
1.360 millert 687: .Pp
1.218 jmc 688: .It Fl s
689: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 690: Subsystems facilitate the use of SSH
691: as a secure transport for other applications (e.g.\&
1.218 jmc 692: .Xr sftp 1 ) .
693: The subsystem is specified as the remote command.
1.360 millert 694: .Pp
1.218 jmc 695: .It Fl T
1.357 dtucker 696: Disable pseudo-terminal allocation.
1.360 millert 697: .Pp
1.218 jmc 698: .It Fl t
1.357 dtucker 699: Force pseudo-terminal allocation.
1.218 jmc 700: This can be used to execute arbitrary
701: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 702: e.g. when implementing menu services.
1.218 jmc 703: Multiple
704: .Fl t
705: options force tty allocation, even if
1.194 jakob 706: .Nm
1.218 jmc 707: has no local tty.
1.360 millert 708: .Pp
1.218 jmc 709: .It Fl V
710: Display the version number and exit.
1.360 millert 711: .Pp
1.218 jmc 712: .It Fl v
713: Verbose mode.
714: Causes
1.176 jmc 715: .Nm
1.218 jmc 716: to print debugging messages about its progress.
717: This is helpful in
718: debugging connection, authentication, and configuration problems.
719: Multiple
720: .Fl v
721: options increase the verbosity.
722: The maximum is 3.
1.360 millert 723: .Pp
1.290 dtucker 724: .It Fl W Ar host : Ns Ar port
725: Requests that standard input and output on the client be forwarded to
726: .Ar host
727: on
728: .Ar port
729: over the secure channel.
730: Implies
731: .Fl N ,
732: .Fl T ,
733: .Cm ExitOnForwardFailure
734: and
1.373 jmc 735: .Cm ClearAllForwardings ,
736: though these can be overridden in the configuration file or using
1.372 dtucker 737: .Fl o
738: command line options.
1.360 millert 739: .Pp
1.261 stevesk 740: .It Fl w Xo
741: .Ar local_tun Ns Op : Ns Ar remote_tun
742: .Xc
743: Requests
744: tunnel
745: device forwarding with the specified
1.218 jmc 746: .Xr tun 4
1.261 stevesk 747: devices between the client
748: .Pq Ar local_tun
749: and the server
750: .Pq Ar remote_tun .
751: .Pp
1.228 jmc 752: The devices may be specified by numerical ID or the keyword
753: .Dq any ,
754: which uses the next available tunnel device.
1.261 stevesk 755: If
756: .Ar remote_tun
757: is not specified, it defaults to
758: .Dq any .
1.228 jmc 759: See also the
1.218 jmc 760: .Cm Tunnel
1.261 stevesk 761: and
762: .Cm TunnelDevice
763: directives in
1.218 jmc 764: .Xr ssh_config 5 .
1.393 djm 765: .Pp
1.261 stevesk 766: If the
767: .Cm Tunnel
1.393 djm 768: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 769: .Dq point-to-point .
1.393 djm 770: If a different
771: .Cm Tunnel
772: forwarding mode it desired, then it should be specified before
773: .Fl w .
1.360 millert 774: .Pp
1.218 jmc 775: .It Fl X
776: Enables X11 forwarding.
1.54 markus 777: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 778: .Pp
1.218 jmc 779: X11 forwarding should be enabled with caution.
1.168 jmc 780: Users with the ability to bypass file permissions on the remote host
1.218 jmc 781: (for the user's X authorization database)
782: can access the local X11 display through the forwarded connection.
783: An attacker may then be able to perform activities such as keystroke monitoring.
784: .Pp
785: For this reason, X11 forwarding is subjected to X11 SECURITY extension
786: restrictions by default.
787: Please refer to the
788: .Nm
789: .Fl Y
790: option and the
791: .Cm ForwardX11Trusted
792: directive in
793: .Xr ssh_config 5
794: for more information.
1.360 millert 795: .Pp
1.218 jmc 796: .It Fl x
797: Disables X11 forwarding.
1.360 millert 798: .Pp
1.218 jmc 799: .It Fl Y
800: Enables trusted X11 forwarding.
801: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
802: controls.
1.360 millert 803: .Pp
1.278 djm 804: .It Fl y
805: Send log information using the
806: .Xr syslog 3
807: system module.
808: By default this information is sent to stderr.
1.218 jmc 809: .El
1.224 jmc 810: .Pp
811: .Nm
812: may additionally obtain configuration data from
813: a per-user configuration file and a system-wide configuration file.
814: The file format and configuration options are described in
815: .Xr ssh_config 5 .
1.222 jmc 816: .Sh AUTHENTICATION
1.377 djm 817: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 818: .Pp
819: The methods available for authentication are:
1.260 jmc 820: GSSAPI-based authentication,
1.222 jmc 821: host-based authentication,
822: public key authentication,
823: challenge-response authentication,
824: and password authentication.
825: Authentication methods are tried in the order specified above,
1.369 jmc 826: though
827: .Cm PreferredAuthentications
828: can be used to change the default order.
1.222 jmc 829: .Pp
830: Host-based authentication works as follows:
1.218 jmc 831: If the machine the user logs in from is listed in
832: .Pa /etc/hosts.equiv
833: or
834: .Pa /etc/shosts.equiv
835: on the remote machine, and the user names are
836: the same on both sides, or if the files
837: .Pa ~/.rhosts
838: or
839: .Pa ~/.shosts
840: exist in the user's home directory on the
841: remote machine and contain a line containing the name of the client
842: machine and the name of the user on that machine, the user is
1.222 jmc 843: considered for login.
844: Additionally, the server
845: .Em must
846: be able to verify the client's
847: host key (see the description of
1.218 jmc 848: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 849: and
1.222 jmc 850: .Pa ~/.ssh/known_hosts ,
851: below)
852: for login to be permitted.
1.218 jmc 853: This authentication method closes security holes due to IP
1.222 jmc 854: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 855: [Note to the administrator:
856: .Pa /etc/hosts.equiv ,
857: .Pa ~/.rhosts ,
858: and the rlogin/rsh protocol in general, are inherently insecure and should be
859: disabled if security is desired.]
1.189 dtucker 860: .Pp
1.222 jmc 861: Public key authentication works as follows:
862: The scheme is based on public-key cryptography,
863: using cryptosystems
864: where encryption and decryption are done using separate keys,
865: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 866: The idea is that each user creates a public/private
867: key pair for authentication purposes.
868: The server knows the public key, and only the user knows the private key.
1.222 jmc 869: .Nm
870: implements public key authentication protocol automatically,
1.349 sobrado 871: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 872: The HISTORY section of
1.222 jmc 873: .Xr ssl 8
1.311 jmc 874: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 875: .Pp
1.218 jmc 876: The file
877: .Pa ~/.ssh/authorized_keys
878: lists the public keys that are permitted for logging in.
879: When the user logs in, the
1.2 deraadt 880: .Nm
1.218 jmc 881: program tells the server which key pair it would like to use for
882: authentication.
1.222 jmc 883: The client proves that it has access to the private key
884: and the server checks that the corresponding public key
885: is authorized to accept the account.
1.383 djm 886: .Pp
887: The server may inform the client of errors that prevented public key
888: authentication from succeeding after authentication completes using a
889: different method.
890: These may be viewed by increasing the
891: .Cm LogLevel
892: to
893: .Cm DEBUG
894: or higher (e.g. by using the
895: .Fl v
896: flag).
1.218 jmc 897: .Pp
1.222 jmc 898: The user creates his/her key pair by running
1.218 jmc 899: .Xr ssh-keygen 1 .
900: This stores the private key in
1.222 jmc 901: .Pa ~/.ssh/id_dsa
1.369 jmc 902: (DSA),
1.310 djm 903: .Pa ~/.ssh/id_ecdsa
1.369 jmc 904: (ECDSA),
1.404 naddy 905: .Pa ~/.ssh/id_ecdsa_sk
1.409 ! naddy 906: (authenticator-hosted ECDSA),
1.343 naddy 907: .Pa ~/.ssh/id_ed25519
1.369 jmc 908: (Ed25519),
1.406 naddy 909: .Pa ~/.ssh/id_ed25519_sk
1.409 ! naddy 910: (authenticator-hosted Ed25519),
1.222 jmc 911: or
912: .Pa ~/.ssh/id_rsa
1.369 jmc 913: (RSA)
1.218 jmc 914: and stores the public key in
1.222 jmc 915: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 916: (DSA),
1.310 djm 917: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 918: (ECDSA),
1.404 naddy 919: .Pa ~/.ssh/id_ecdsa_sk.pub
1.409 ! naddy 920: (authenticator-hosted ECDSA),
1.343 naddy 921: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 922: (Ed25519),
1.406 naddy 923: .Pa ~/.ssh/id_ed25519_sk.pub
1.409 ! naddy 924: (authenticator-hosted Ed25519),
1.222 jmc 925: or
926: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 927: (RSA)
1.218 jmc 928: in the user's home directory.
1.222 jmc 929: The user should then copy the public key
1.218 jmc 930: to
931: .Pa ~/.ssh/authorized_keys
1.222 jmc 932: in his/her home directory on the remote machine.
933: The
1.218 jmc 934: .Pa authorized_keys
935: file corresponds to the conventional
936: .Pa ~/.rhosts
937: file, and has one key
1.222 jmc 938: per line, though the lines can be very long.
1.218 jmc 939: After this, the user can log in without giving the password.
940: .Pp
1.301 jmc 941: A variation on public key authentication
942: is available in the form of certificate authentication:
943: instead of a set of public/private keys,
944: signed certificates are used.
945: This has the advantage that a single trusted certification authority
946: can be used in place of many public/private keys.
1.333 jmc 947: See the CERTIFICATES section of
1.301 jmc 948: .Xr ssh-keygen 1
949: for more information.
950: .Pp
951: The most convenient way to use public key or certificate authentication
952: may be with an authentication agent.
1.218 jmc 953: See
954: .Xr ssh-agent 1
1.366 jcs 955: and (optionally) the
956: .Cm AddKeysToAgent
957: directive in
958: .Xr ssh_config 5
1.218 jmc 959: for more information.
960: .Pp
1.222 jmc 961: Challenge-response authentication works as follows:
962: The server sends an arbitrary
963: .Qq challenge
964: text, and prompts for a response.
965: Examples of challenge-response authentication include
1.335 jmc 966: .Bx
967: Authentication (see
1.222 jmc 968: .Xr login.conf 5 )
1.335 jmc 969: and PAM (some
970: .Pf non- Ox
971: systems).
1.222 jmc 972: .Pp
973: Finally, if other authentication methods fail,
1.218 jmc 974: .Nm
975: prompts the user for a password.
976: The password is sent to the remote
977: host for checking; however, since all communications are encrypted,
978: the password cannot be seen by someone listening on the network.
1.232 jmc 979: .Pp
980: .Nm
981: automatically maintains and checks a database containing
982: identification for all hosts it has ever been used with.
983: Host keys are stored in
984: .Pa ~/.ssh/known_hosts
985: in the user's home directory.
986: Additionally, the file
987: .Pa /etc/ssh/ssh_known_hosts
988: is automatically checked for known hosts.
989: Any new hosts are automatically added to the user's file.
990: If a host's identification ever changes,
991: .Nm
992: warns about this and disables password authentication to prevent
993: server spoofing or man-in-the-middle attacks,
994: which could otherwise be used to circumvent the encryption.
995: The
996: .Cm StrictHostKeyChecking
997: option can be used to control logins to machines whose
998: host key is not known or has changed.
999: .Pp
1.218 jmc 1000: When the user's identity has been accepted by the server, the server
1.357 dtucker 1001: either executes the given command in a non-interactive session or,
1002: if no command has been specified, logs into the machine and gives
1003: the user a normal shell as an interactive session.
1.218 jmc 1004: All communication with
1005: the remote command or shell will be automatically encrypted.
1006: .Pp
1.357 dtucker 1007: If an interactive session is requested
1008: .Nm
1009: by default will only request a pseudo-terminal (pty) for interactive
1010: sessions when the client has one.
1011: The flags
1012: .Fl T
1013: and
1014: .Fl t
1015: can be used to override this behaviour.
1016: .Pp
1017: If a pseudo-terminal has been allocated the
1.218 jmc 1018: user may use the escape characters noted below.
1019: .Pp
1.357 dtucker 1020: If no pseudo-terminal has been allocated,
1.218 jmc 1021: the session is transparent and can be used to reliably transfer binary data.
1022: On most systems, setting the escape character to
1023: .Dq none
1024: will also make the session transparent even if a tty is used.
1025: .Pp
1026: The session terminates when the command or shell on the remote
1.247 jmc 1027: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1028: .Sh ESCAPE CHARACTERS
1.218 jmc 1029: When a pseudo-terminal has been requested,
1.2 deraadt 1030: .Nm
1.218 jmc 1031: supports a number of functions through the use of an escape character.
1032: .Pp
1033: A single tilde character can be sent as
1034: .Ic ~~
1035: or by following the tilde by a character other than those described below.
1036: The escape character must always follow a newline to be interpreted as
1037: special.
1038: The escape character can be changed in configuration files using the
1039: .Cm EscapeChar
1040: configuration directive or on the command line by the
1041: .Fl e
1042: option.
1043: .Pp
1044: The supported escapes (assuming the default
1045: .Ql ~ )
1046: are:
1047: .Bl -tag -width Ds
1048: .It Cm ~.
1049: Disconnect.
1050: .It Cm ~^Z
1051: Background
1.234 jmc 1052: .Nm .
1.218 jmc 1053: .It Cm ~#
1054: List forwarded connections.
1055: .It Cm ~&
1056: Background
1.2 deraadt 1057: .Nm
1.218 jmc 1058: at logout when waiting for forwarded connection / X11 sessions to terminate.
1059: .It Cm ~?
1060: Display a list of escape characters.
1061: .It Cm ~B
1062: Send a BREAK to the remote system
1.369 jmc 1063: (only useful if the peer supports it).
1.218 jmc 1064: .It Cm ~C
1065: Open command line.
1066: Currently this allows the addition of port forwardings using the
1.279 stevesk 1067: .Fl L ,
1068: .Fl R
1.218 jmc 1069: and
1.279 stevesk 1070: .Fl D
1.225 jmc 1071: options (see above).
1.322 markus 1072: It also allows the cancellation of existing port-forwardings
1073: with
1.262 stevesk 1074: .Sm off
1.322 markus 1075: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1076: .Sm on
1.322 markus 1077: for local,
1078: .Sm off
1079: .Fl KR Oo Ar bind_address : Oc Ar port
1080: .Sm on
1081: for remote and
1082: .Sm off
1083: .Fl KD Oo Ar bind_address : Oc Ar port
1084: .Sm on
1085: for dynamic port-forwardings.
1.218 jmc 1086: .Ic !\& Ns Ar command
1087: allows the user to execute a local command if the
1088: .Ic PermitLocalCommand
1089: option is enabled in
1.176 jmc 1090: .Xr ssh_config 5 .
1.218 jmc 1091: Basic help is available, using the
1092: .Fl h
1093: option.
1094: .It Cm ~R
1095: Request rekeying of the connection
1.369 jmc 1096: (only useful if the peer supports it).
1.327 dtucker 1097: .It Cm ~V
1098: Decrease the verbosity
1099: .Pq Ic LogLevel
1100: when errors are being written to stderr.
1101: .It Cm ~v
1.328 jmc 1102: Increase the verbosity
1.327 dtucker 1103: .Pq Ic LogLevel
1104: when errors are being written to stderr.
1.176 jmc 1105: .El
1.246 jmc 1106: .Sh TCP FORWARDING
1.402 jmc 1107: Forwarding of arbitrary TCP connections over a secure channel
1108: can be specified either on the command line or in a configuration file.
1.246 jmc 1109: One possible application of TCP forwarding is a secure connection to a
1110: mail server; another is going through firewalls.
1111: .Pp
1.402 jmc 1112: In the example below, we look at encrypting communication for an IRC client,
1113: even though the IRC server it connects to does not directly
1114: support encrypted communication.
1.246 jmc 1115: This works as follows:
1116: the user connects to the remote host using
1117: .Nm ,
1.402 jmc 1118: specifying the ports to be used to forward the connection.
1119: After that it is possible to start the program locally,
1.246 jmc 1120: and
1121: .Nm
1.402 jmc 1122: will encrypt and forward the connection to the remote server.
1.246 jmc 1123: .Pp
1.402 jmc 1124: The following example tunnels an IRC session from the client
1125: to an IRC server at
1.246 jmc 1126: .Dq server.example.com ,
1127: joining channel
1128: .Dq #users ,
1129: nickname
1130: .Dq pinky ,
1.402 jmc 1131: using the standard IRC port, 6667:
1132: .Bd -literal -offset 4n
1133: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1134: $ irc -c '#users' pinky IRC/127.0.0.1
1135: .Ed
1.246 jmc 1136: .Pp
1137: The
1138: .Fl f
1139: option backgrounds
1140: .Nm
1141: and the remote command
1142: .Dq sleep 10
1143: is specified to allow an amount of time
1144: (10 seconds, in the example)
1.402 jmc 1145: to start the program which is going to use the tunnel.
1.246 jmc 1146: If no connections are made within the time specified,
1147: .Nm
1148: will exit.
1149: .Sh X11 FORWARDING
1.218 jmc 1150: If the
1151: .Cm ForwardX11
1152: variable is set to
1153: .Dq yes
1154: (or see the description of the
1.227 jmc 1155: .Fl X ,
1156: .Fl x ,
1.218 jmc 1157: and
1.227 jmc 1158: .Fl Y
1.226 jmc 1159: options above)
1.218 jmc 1160: and the user is using X11 (the
1161: .Ev DISPLAY
1162: environment variable is set), the connection to the X11 display is
1163: automatically forwarded to the remote side in such a way that any X11
1164: programs started from the shell (or command) will go through the
1165: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1166: from the local machine.
1.218 jmc 1167: The user should not manually set
1168: .Ev DISPLAY .
1169: Forwarding of X11 connections can be
1170: configured on the command line or in configuration files.
1171: .Pp
1172: The
1173: .Ev DISPLAY
1174: value set by
1175: .Nm
1176: will point to the server machine, but with a display number greater than zero.
1177: This is normal, and happens because
1178: .Nm
1179: creates a
1180: .Dq proxy
1181: X server on the server machine for forwarding the
1182: connections over the encrypted channel.
1.200 djm 1183: .Pp
1.218 jmc 1184: .Nm
1185: will also automatically set up Xauthority data on the server machine.
1186: For this purpose, it will generate a random authorization cookie,
1187: store it in Xauthority on the server, and verify that any forwarded
1188: connections carry this cookie and replace it by the real cookie when
1189: the connection is opened.
1190: The real authentication cookie is never
1191: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1192: .Pp
1.218 jmc 1193: If the
1194: .Cm ForwardAgent
1195: variable is set to
1196: .Dq yes
1197: (or see the description of the
1198: .Fl A
1.191 djm 1199: and
1.218 jmc 1200: .Fl a
1.226 jmc 1201: options above) and
1.218 jmc 1202: the user is using an authentication agent, the connection to the agent
1203: is automatically forwarded to the remote side.
1.252 jmc 1204: .Sh VERIFYING HOST KEYS
1205: When connecting to a server for the first time,
1206: a fingerprint of the server's public key is presented to the user
1207: (unless the option
1208: .Cm StrictHostKeyChecking
1209: has been disabled).
1210: Fingerprints can be determined using
1211: .Xr ssh-keygen 1 :
1212: .Pp
1213: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1214: .Pp
1.274 grunk 1215: If the fingerprint is already known, it can be matched
1216: and the key can be accepted or rejected.
1.358 djm 1217: If only legacy (MD5) fingerprints for the server are available, the
1218: .Xr ssh-keygen 1
1219: .Fl E
1220: option may be used to downgrade the fingerprint algorithm to match.
1221: .Pp
1.274 grunk 1222: Because of the difficulty of comparing host keys
1.352 djm 1223: just by looking at fingerprint strings,
1.274 grunk 1224: there is also support to compare host keys visually,
1225: using
1226: .Em random art .
1227: By setting the
1.275 grunk 1228: .Cm VisualHostKey
1.274 grunk 1229: option to
1.275 grunk 1230: .Dq yes ,
1.274 grunk 1231: a small ASCII graphic gets displayed on every login to a server, no matter
1232: if the session itself is interactive or not.
1233: By learning the pattern a known server produces, a user can easily
1234: find out that the host key has changed when a completely different pattern
1235: is displayed.
1236: Because these patterns are not unambiguous however, a pattern that looks
1237: similar to the pattern remembered only gives a good probability that the
1238: host key is the same, not guaranteed proof.
1239: .Pp
1240: To get a listing of the fingerprints along with their random art for
1241: all known hosts, the following command line can be used:
1242: .Pp
1243: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1244: .Pp
1.252 jmc 1245: If the fingerprint is unknown,
1246: an alternative method of verification is available:
1247: SSH fingerprints verified by DNS.
1248: An additional resource record (RR),
1249: SSHFP,
1250: is added to a zonefile
1251: and the connecting client is able to match the fingerprint
1252: with that of the key presented.
1253: .Pp
1254: In this example, we are connecting a client to a server,
1255: .Dq host.example.com .
1256: The SSHFP resource records should first be added to the zonefile for
1257: host.example.com:
1258: .Bd -literal -offset indent
1.259 jakob 1259: $ ssh-keygen -r host.example.com.
1.252 jmc 1260: .Ed
1261: .Pp
1262: The output lines will have to be added to the zonefile.
1263: To check that the zone is answering fingerprint queries:
1264: .Pp
1265: .Dl $ dig -t SSHFP host.example.com
1266: .Pp
1267: Finally the client connects:
1268: .Bd -literal -offset indent
1269: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1270: [...]
1271: Matching host key fingerprint found in DNS.
1272: Are you sure you want to continue connecting (yes/no)?
1273: .Ed
1274: .Pp
1275: See the
1276: .Cm VerifyHostKeyDNS
1277: option in
1278: .Xr ssh_config 5
1279: for more information.
1.250 jmc 1280: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1281: .Nm
1282: contains support for Virtual Private Network (VPN) tunnelling
1283: using the
1284: .Xr tun 4
1285: network pseudo-device,
1286: allowing two networks to be joined securely.
1287: The
1288: .Xr sshd_config 5
1289: configuration option
1290: .Cm PermitTunnel
1291: controls whether the server supports this,
1292: and at what level (layer 2 or 3 traffic).
1293: .Pp
1294: The following example would connect client network 10.0.50.0/24
1.265 otto 1295: with remote network 10.0.99.0/24 using a point-to-point connection
1296: from 10.1.1.1 to 10.1.1.2,
1297: provided that the SSH server running on the gateway to the remote network,
1298: at 192.168.1.15, allows it.
1299: .Pp
1300: On the client:
1.250 jmc 1301: .Bd -literal -offset indent
1302: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1303: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1304: # route add 10.0.99.0/24 10.1.1.2
1305: .Ed
1306: .Pp
1307: On the server:
1308: .Bd -literal -offset indent
1309: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1310: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1311: .Ed
1312: .Pp
1313: Client access may be more finely tuned via the
1314: .Pa /root/.ssh/authorized_keys
1315: file (see below) and the
1316: .Cm PermitRootLogin
1317: server option.
1.255 jmc 1318: The following entry would permit connections on
1.250 jmc 1319: .Xr tun 4
1.255 jmc 1320: device 1 from user
1.250 jmc 1321: .Dq jane
1.255 jmc 1322: and on tun device 2 from user
1.250 jmc 1323: .Dq john ,
1324: if
1325: .Cm PermitRootLogin
1326: is set to
1327: .Dq forced-commands-only :
1328: .Bd -literal -offset 2n
1329: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1330: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1331: .Ed
1332: .Pp
1.264 ray 1333: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1334: it may be more suited to temporary setups,
1335: such as for wireless VPNs.
1336: More permanent VPNs are better provided by tools such as
1337: .Xr ipsecctl 8
1338: and
1339: .Xr isakmpd 8 .
1.2 deraadt 1340: .Sh ENVIRONMENT
1341: .Nm
1.1 deraadt 1342: will normally set the following environment variables:
1.237 jmc 1343: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1344: .It Ev DISPLAY
1345: The
1346: .Ev DISPLAY
1.40 aaron 1347: variable indicates the location of the X11 server.
1.44 aaron 1348: It is automatically set by
1.2 deraadt 1349: .Nm
1350: to point to a value of the form
1.233 jmc 1351: .Dq hostname:n ,
1352: where
1353: .Dq hostname
1354: indicates the host where the shell runs, and
1355: .Sq n
1356: is an integer \*(Ge 1.
1.40 aaron 1357: .Nm
1358: uses this special value to forward X11 connections over the secure
1359: channel.
1.107 markus 1360: The user should normally not set
1361: .Ev DISPLAY
1362: explicitly, as that
1.1 deraadt 1363: will render the X11 connection insecure (and will require the user to
1364: manually copy any required authorization cookies).
1.2 deraadt 1365: .It Ev HOME
1.1 deraadt 1366: Set to the path of the user's home directory.
1.2 deraadt 1367: .It Ev LOGNAME
1368: Synonym for
1.12 aaron 1369: .Ev USER ;
1370: set for compatibility with systems that use this variable.
1.2 deraadt 1371: .It Ev MAIL
1.129 stevesk 1372: Set to the path of the user's mailbox.
1.40 aaron 1373: .It Ev PATH
1.2 deraadt 1374: Set to the default
1375: .Ev PATH ,
1376: as specified when compiling
1.234 jmc 1377: .Nm .
1.118 markus 1378: .It Ev SSH_ASKPASS
1379: If
1380: .Nm
1381: needs a passphrase, it will read the passphrase from the current
1382: terminal if it was run from a terminal.
1383: If
1384: .Nm
1385: does not have a terminal associated with it but
1386: .Ev DISPLAY
1387: and
1388: .Ev SSH_ASKPASS
1389: are set, it will execute the program specified by
1390: .Ev SSH_ASKPASS
1391: and open an X11 window to read the passphrase.
1392: This is particularly useful when calling
1393: .Nm
1394: from a
1.196 jmc 1395: .Pa .xsession
1.118 markus 1396: or related script.
1397: (Note that on some machines it
1398: may be necessary to redirect the input from
1399: .Pa /dev/null
1400: to make this work.)
1.18 markus 1401: .It Ev SSH_AUTH_SOCK
1.233 jmc 1402: Identifies the path of a
1403: .Ux Ns -domain
1404: socket used to communicate with the agent.
1.166 stevesk 1405: .It Ev SSH_CONNECTION
1406: Identifies the client and server ends of the connection.
1.40 aaron 1407: The variable contains
1.233 jmc 1408: four space-separated values: client IP address, client port number,
1409: server IP address, and server port number.
1.73 markus 1410: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1411: This variable contains the original command line if a forced command
1.73 markus 1412: is executed.
1413: It can be used to extract the original arguments.
1.2 deraadt 1414: .It Ev SSH_TTY
1.1 deraadt 1415: This is set to the name of the tty (path to the device) associated
1.40 aaron 1416: with the current shell or command.
1417: If the current session has no tty,
1.1 deraadt 1418: this variable is not set.
1.387 djm 1419: .It Ev SSH_TUNNEL
1420: Optionally set by
1421: .Xr sshd 8
1422: to contain the interface names assigned if tunnel forwarding was
1423: requested by the client.
1.385 djm 1424: .It Ev SSH_USER_AUTH
1425: Optionally set by
1426: .Xr sshd 8 ,
1427: this variable may contain a pathname to a file that lists the authentication
1428: methods successfully used when the session was established, including any
1429: public keys that were used.
1.2 deraadt 1430: .It Ev TZ
1.214 jmc 1431: This variable is set to indicate the present time zone if it
1.257 jmc 1432: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1433: on to new connections).
1.2 deraadt 1434: .It Ev USER
1.1 deraadt 1435: Set to the name of the user logging in.
1.2 deraadt 1436: .El
1437: .Pp
1.44 aaron 1438: Additionally,
1.2 deraadt 1439: .Nm
1.44 aaron 1440: reads
1.207 djm 1441: .Pa ~/.ssh/environment ,
1.2 deraadt 1442: and adds lines of the format
1443: .Dq VARNAME=value
1.233 jmc 1444: to the environment if the file exists and users are allowed to
1.161 marc 1445: change their environment.
1.176 jmc 1446: For more information, see the
1.161 marc 1447: .Cm PermitUserEnvironment
1.162 stevesk 1448: option in
1.161 marc 1449: .Xr sshd_config 5 .
1.2 deraadt 1450: .Sh FILES
1.236 jmc 1451: .Bl -tag -width Ds -compact
1.309 jmc 1452: .It Pa ~/.rhosts
1.240 jmc 1453: This file is used for host-based authentication (see above).
1.92 markus 1454: On some machines this file may need to be
1.240 jmc 1455: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1456: because
1.2 deraadt 1457: .Xr sshd 8
1.40 aaron 1458: reads it as root.
1459: Additionally, this file must be owned by the user,
1460: and must not have write permissions for anyone else.
1461: The recommended
1.1 deraadt 1462: permission for most machines is read/write for the user, and not
1463: accessible by others.
1.2 deraadt 1464: .Pp
1.309 jmc 1465: .It Pa ~/.shosts
1.240 jmc 1466: This file is used in exactly the same way as
1467: .Pa .rhosts ,
1468: but allows host-based authentication without permitting login with
1469: rlogin/rsh.
1.272 mcbride 1470: .Pp
1.309 jmc 1471: .It Pa ~/.ssh/
1.272 mcbride 1472: This directory is the default location for all user-specific configuration
1473: and authentication information.
1474: There is no general requirement to keep the entire contents of this directory
1475: secret, but the recommended permissions are read/write/execute for the user,
1476: and not accessible by others.
1.236 jmc 1477: .Pp
1.309 jmc 1478: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1479: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1480: that can be used for logging in as this user.
1.238 jmc 1481: The format of this file is described in the
1482: .Xr sshd 8
1483: manual page.
1484: This file is not highly sensitive, but the recommended
1485: permissions are read/write for the user, and not accessible by others.
1486: .Pp
1.309 jmc 1487: .It Pa ~/.ssh/config
1.238 jmc 1488: This is the per-user configuration file.
1489: The file format and configuration options are described in
1490: .Xr ssh_config 5 .
1491: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1492: read/write for the user, and not writable by others.
1.238 jmc 1493: .Pp
1.309 jmc 1494: .It Pa ~/.ssh/environment
1.239 jmc 1495: Contains additional definitions for environment variables; see
1496: .Sx ENVIRONMENT ,
1.238 jmc 1497: above.
1498: .Pp
1.309 jmc 1499: .It Pa ~/.ssh/id_dsa
1.310 djm 1500: .It Pa ~/.ssh/id_ecdsa
1.404 naddy 1501: .It Pa ~/.ssh/id_ecdsa_sk
1.343 naddy 1502: .It Pa ~/.ssh/id_ed25519
1.406 naddy 1503: .It Pa ~/.ssh/id_ed25519_sk
1.309 jmc 1504: .It Pa ~/.ssh/id_rsa
1.238 jmc 1505: Contains the private key for authentication.
1506: These files
1507: contain sensitive data and should be readable by the user but not
1508: accessible by others (read/write/execute).
1509: .Nm
1510: will simply ignore a private key file if it is accessible by others.
1511: It is possible to specify a passphrase when
1512: generating the key which will be used to encrypt the
1.389 djm 1513: sensitive part of this file using AES-128.
1.238 jmc 1514: .Pp
1.309 jmc 1515: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1516: .It Pa ~/.ssh/id_ecdsa.pub
1.404 naddy 1517: .It Pa ~/.ssh/id_ecdsa_sk.pub
1.343 naddy 1518: .It Pa ~/.ssh/id_ed25519.pub
1.406 naddy 1519: .It Pa ~/.ssh/id_ed25519_sk.pub
1.309 jmc 1520: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1521: Contains the public key for authentication.
1522: These files are not
1523: sensitive and can (but need not) be readable by anyone.
1524: .Pp
1.309 jmc 1525: .It Pa ~/.ssh/known_hosts
1.244 jmc 1526: Contains a list of host keys for all hosts the user has logged into
1527: that are not already in the systemwide list of known host keys.
1.238 jmc 1528: See
1.244 jmc 1529: .Xr sshd 8
1530: for further details of the format of this file.
1.238 jmc 1531: .Pp
1.309 jmc 1532: .It Pa ~/.ssh/rc
1.238 jmc 1533: Commands in this file are executed by
1534: .Nm
1.245 jmc 1535: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1536: started.
1537: See the
1538: .Xr sshd 8
1539: manual page for more information.
1540: .Pp
1.309 jmc 1541: .It Pa /etc/hosts.equiv
1.240 jmc 1542: This file is for host-based authentication (see above).
1543: It should only be writable by root.
1.236 jmc 1544: .Pp
1.309 jmc 1545: .It Pa /etc/shosts.equiv
1.240 jmc 1546: This file is used in exactly the same way as
1547: .Pa hosts.equiv ,
1548: but allows host-based authentication without permitting login with
1549: rlogin/rsh.
1.236 jmc 1550: .Pp
1.238 jmc 1551: .It Pa /etc/ssh/ssh_config
1552: Systemwide configuration file.
1553: The file format and configuration options are described in
1554: .Xr ssh_config 5 .
1555: .Pp
1.309 jmc 1556: .It Pa /etc/ssh/ssh_host_key
1557: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1558: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1559: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1560: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1561: These files contain the private parts of the host keys
1.245 jmc 1562: and are used for host-based authentication.
1.238 jmc 1563: .Pp
1.309 jmc 1564: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1565: Systemwide list of known host keys.
1566: This file should be prepared by the
1567: system administrator to contain the public host keys of all machines in the
1568: organization.
1.244 jmc 1569: It should be world-readable.
1570: See
1.238 jmc 1571: .Xr sshd 8
1.244 jmc 1572: for further details of the format of this file.
1.236 jmc 1573: .Pp
1.309 jmc 1574: .It Pa /etc/ssh/sshrc
1.1 deraadt 1575: Commands in this file are executed by
1.2 deraadt 1576: .Nm
1.245 jmc 1577: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1578: See the
1.2 deraadt 1579: .Xr sshd 8
1.1 deraadt 1580: manual page for more information.
1.58 itojun 1581: .El
1.312 jmc 1582: .Sh EXIT STATUS
1583: .Nm
1584: exits with the exit status of the remote command or with 255
1585: if an error occurred.
1.2 deraadt 1586: .Sh SEE ALSO
1587: .Xr scp 1 ,
1.83 djm 1588: .Xr sftp 1 ,
1.2 deraadt 1589: .Xr ssh-add 1 ,
1590: .Xr ssh-agent 1 ,
1591: .Xr ssh-keygen 1 ,
1.242 jmc 1592: .Xr ssh-keyscan 1 ,
1.250 jmc 1593: .Xr tun 4 ,
1.159 stevesk 1594: .Xr ssh_config 5 ,
1.160 naddy 1595: .Xr ssh-keysign 8 ,
1.87 itojun 1596: .Xr sshd 8
1.329 jmc 1597: .Sh STANDARDS
1.106 markus 1598: .Rs
1.329 jmc 1599: .%A S. Lehtinen
1600: .%A C. Lonvick
1601: .%D January 2006
1.256 jmc 1602: .%R RFC 4250
1.329 jmc 1603: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1604: .Re
1.329 jmc 1605: .Pp
1.256 jmc 1606: .Rs
1.329 jmc 1607: .%A T. Ylonen
1608: .%A C. Lonvick
1609: .%D January 2006
1.256 jmc 1610: .%R RFC 4251
1.329 jmc 1611: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1612: .Re
1.329 jmc 1613: .Pp
1.256 jmc 1614: .Rs
1.329 jmc 1615: .%A T. Ylonen
1616: .%A C. Lonvick
1617: .%D January 2006
1.256 jmc 1618: .%R RFC 4252
1.329 jmc 1619: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1620: .Re
1.329 jmc 1621: .Pp
1.256 jmc 1622: .Rs
1.329 jmc 1623: .%A T. Ylonen
1624: .%A C. Lonvick
1625: .%D January 2006
1.256 jmc 1626: .%R RFC 4253
1.329 jmc 1627: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1628: .Re
1.329 jmc 1629: .Pp
1.256 jmc 1630: .Rs
1.329 jmc 1631: .%A T. Ylonen
1632: .%A C. Lonvick
1633: .%D January 2006
1.256 jmc 1634: .%R RFC 4254
1.329 jmc 1635: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1636: .Re
1.329 jmc 1637: .Pp
1.256 jmc 1638: .Rs
1.329 jmc 1639: .%A J. Schlyter
1640: .%A W. Griffin
1641: .%D January 2006
1.256 jmc 1642: .%R RFC 4255
1.329 jmc 1643: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1644: .Re
1.329 jmc 1645: .Pp
1.256 jmc 1646: .Rs
1.329 jmc 1647: .%A F. Cusack
1648: .%A M. Forssen
1649: .%D January 2006
1.256 jmc 1650: .%R RFC 4256
1.329 jmc 1651: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1652: .Re
1.329 jmc 1653: .Pp
1.256 jmc 1654: .Rs
1.329 jmc 1655: .%A J. Galbraith
1656: .%A P. Remaker
1657: .%D January 2006
1.256 jmc 1658: .%R RFC 4335
1.329 jmc 1659: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1660: .Re
1.329 jmc 1661: .Pp
1.256 jmc 1662: .Rs
1.329 jmc 1663: .%A M. Bellare
1664: .%A T. Kohno
1665: .%A C. Namprempre
1666: .%D January 2006
1.256 jmc 1667: .%R RFC 4344
1.329 jmc 1668: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1669: .Re
1.329 jmc 1670: .Pp
1.256 jmc 1671: .Rs
1.329 jmc 1672: .%A B. Harris
1673: .%D January 2006
1.256 jmc 1674: .%R RFC 4345
1.329 jmc 1675: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1676: .Re
1.329 jmc 1677: .Pp
1.258 djm 1678: .Rs
1.329 jmc 1679: .%A M. Friedl
1680: .%A N. Provos
1681: .%A W. Simpson
1682: .%D March 2006
1.258 djm 1683: .%R RFC 4419
1.329 jmc 1684: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1685: .Re
1.329 jmc 1686: .Pp
1.266 markus 1687: .Rs
1.329 jmc 1688: .%A J. Galbraith
1689: .%A R. Thayer
1690: .%D November 2006
1.266 markus 1691: .%R RFC 4716
1.329 jmc 1692: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1693: .Re
1.329 jmc 1694: .Pp
1.313 djm 1695: .Rs
1.329 jmc 1696: .%A D. Stebila
1697: .%A J. Green
1698: .%D December 2009
1.313 djm 1699: .%R RFC 5656
1.329 jmc 1700: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1701: .Re
1.329 jmc 1702: .Pp
1.274 grunk 1703: .Rs
1704: .%A A. Perrig
1705: .%A D. Song
1706: .%D 1999
1.329 jmc 1707: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1708: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1709: .Re
1.173 jmc 1710: .Sh AUTHORS
1711: OpenSSH is a derivative of the original and free
1712: ssh 1.2.12 release by Tatu Ylonen.
1713: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1714: Theo de Raadt and Dug Song
1715: removed many bugs, re-added newer features and
1716: created OpenSSH.
1717: Markus Friedl contributed the support for SSH
1718: protocol versions 1.5 and 2.0.
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