Annotation of src/usr.bin/ssh/ssh.1, Revision 1.406
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: .\"
1.406 ! naddy 36: .\" $OpenBSD: ssh.1,v 1.405 2019/11/14 21:27:30 djm Exp $
! 37: .Dd $Mdocdate: November 14 2019 $
1.2 deraadt 38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.96 deraadt 42: .Nd OpenSSH SSH client (remote login program)
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
113: Enables forwarding of the authentication agent connection.
114: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 115: .Pp
1.218 jmc 116: Agent forwarding should be enabled with caution.
117: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 118: (for the agent's
1.286 sobrado 119: .Ux Ns -domain
120: socket) can access the local agent through the forwarded connection.
1.218 jmc 121: An attacker cannot obtain key material from the agent,
122: however they can perform operations on the keys that enable them to
123: authenticate using the identities loaded into the agent.
1.360 millert 124: .Pp
1.218 jmc 125: .It Fl a
126: Disables forwarding of the authentication agent connection.
1.390 djm 127: .Pp
1.391 jmc 128: .It Fl B Ar bind_interface
1.390 djm 129: Bind to the address of
1.391 jmc 130: .Ar bind_interface
1.390 djm 131: before attempting to connect to the destination host.
132: This is only useful on systems with more than one address.
1.360 millert 133: .Pp
1.218 jmc 134: .It Fl b Ar bind_address
135: Use
136: .Ar bind_address
137: on the local machine as the source address
138: of the connection.
139: Only useful on systems with more than one address.
1.360 millert 140: .Pp
1.218 jmc 141: .It Fl C
142: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 143: data for forwarded X11, TCP and
144: .Ux Ns -domain
145: connections).
1.218 jmc 146: The compression algorithm is the same used by
1.377 djm 147: .Xr gzip 1 .
1.218 jmc 148: Compression is desirable on modem lines and other
149: slow connections, but will only slow down things on fast networks.
150: The default value can be set on a host-by-host basis in the
151: configuration files; see the
152: .Cm Compression
153: option.
1.360 millert 154: .Pp
1.218 jmc 155: .It Fl c Ar cipher_spec
156: Selects the cipher specification for encrypting the session.
157: .Ar cipher_spec
158: is a comma-separated list of ciphers
159: listed in order of preference.
1.283 jmc 160: See the
161: .Cm Ciphers
1.307 dtucker 162: keyword in
163: .Xr ssh_config 5
164: for more information.
1.360 millert 165: .Pp
1.218 jmc 166: .It Fl D Xo
167: .Sm off
168: .Oo Ar bind_address : Oc
169: .Ar port
170: .Sm on
171: .Xc
172: Specifies a local
173: .Dq dynamic
174: application-level port forwarding.
175: This works by allocating a socket to listen to
176: .Ar port
177: on the local side, optionally bound to the specified
178: .Ar bind_address .
179: Whenever a connection is made to this port, the
180: connection is forwarded over the secure channel, and the application
181: protocol is then used to determine where to connect to from the
182: remote machine.
183: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 184: .Nm
1.218 jmc 185: will act as a SOCKS server.
186: Only root can forward privileged ports.
187: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 188: .Pp
1.308 djm 189: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 190: Only the superuser can forward privileged ports.
191: By default, the local port is bound in accordance with the
192: .Cm GatewayPorts
193: setting.
194: However, an explicit
195: .Ar bind_address
196: may be used to bind the connection to a specific address.
197: The
198: .Ar bind_address
199: of
200: .Dq localhost
201: indicates that the listening port be bound for local use only, while an
202: empty address or
203: .Sq *
204: indicates that the port should be available from all interfaces.
1.360 millert 205: .Pp
1.331 dtucker 206: .It Fl E Ar log_file
207: Append debug logs to
208: .Ar log_file
209: instead of standard error.
1.360 millert 210: .Pp
1.229 jmc 211: .It Fl e Ar escape_char
1.218 jmc 212: Sets the escape character for sessions with a pty (default:
213: .Ql ~ ) .
214: The escape character is only recognized at the beginning of a line.
215: The escape character followed by a dot
216: .Pq Ql \&.
217: closes the connection;
218: followed by control-Z suspends the connection;
219: and followed by itself sends the escape character once.
220: Setting the character to
1.2 deraadt 221: .Dq none
1.218 jmc 222: disables any escapes and makes the session fully transparent.
1.360 millert 223: .Pp
1.218 jmc 224: .It Fl F Ar configfile
225: Specifies an alternative per-user configuration file.
226: If a configuration file is given on the command line,
227: the system-wide configuration file
228: .Pq Pa /etc/ssh/ssh_config
229: will be ignored.
230: The default for the per-user configuration file is
231: .Pa ~/.ssh/config .
1.360 millert 232: .Pp
1.218 jmc 233: .It Fl f
234: Requests
235: .Nm
236: to go to background just before command execution.
237: This is useful if
1.176 jmc 238: .Nm
1.218 jmc 239: is going to ask for passwords or passphrases, but the user
240: wants it in the background.
241: This implies
242: .Fl n .
243: The recommended way to start X11 programs at a remote site is with
244: something like
245: .Ic ssh -f host xterm .
1.277 djm 246: .Pp
247: If the
248: .Cm ExitOnForwardFailure
249: configuration option is set to
250: .Dq yes ,
251: then a client started with
252: .Fl f
253: will wait for all remote port forwards to be successfully established
254: before placing itself in the background.
1.360 millert 255: .Pp
1.350 djm 256: .It Fl G
257: Causes
258: .Nm
259: to print its configuration after evaluating
260: .Cm Host
261: and
262: .Cm Match
263: blocks and exit.
1.360 millert 264: .Pp
1.218 jmc 265: .It Fl g
266: Allows remote hosts to connect to local forwarded ports.
1.346 djm 267: If used on a multiplexed connection, then this option must be specified
268: on the master process.
1.360 millert 269: .Pp
1.291 markus 270: .It Fl I Ar pkcs11
1.294 jmc 271: Specify the PKCS#11 shared library
1.176 jmc 272: .Nm
1.401 naddy 273: should use to communicate with a PKCS#11 token providing keys for user
274: authentication.
1.360 millert 275: .Pp
1.218 jmc 276: .It Fl i Ar identity_file
277: Selects a file from which the identity (private key) for
1.310 djm 278: public key authentication is read.
1.218 jmc 279: The default is
1.310 djm 280: .Pa ~/.ssh/id_dsa ,
1.343 naddy 281: .Pa ~/.ssh/id_ecdsa ,
1.404 naddy 282: .Pa ~/.ssh/id_ecdsa_sk ,
1.406 ! naddy 283: .Pa ~/.ssh/id_ed25519 ,
! 284: .Pa ~/.ssh/id_ed25519_sk
1.310 djm 285: and
1.381 naddy 286: .Pa ~/.ssh/id_rsa .
1.218 jmc 287: Identity files may also be specified on
288: a per-host basis in the configuration file.
289: It is possible to have multiple
290: .Fl i
291: options (and multiple identities specified in
292: configuration files).
1.364 jmc 293: If no certificates have been explicitly specified by the
1.363 djm 294: .Cm CertificateFile
295: directive,
1.302 djm 296: .Nm
297: will also try to load certificate information from the filename obtained
298: by appending
299: .Pa -cert.pub
300: to identity filenames.
1.360 millert 301: .Pp
1.386 millert 302: .It Fl J Ar destination
1.375 djm 303: Connect to the target host by first making a
304: .Nm
1.386 millert 305: connection to the jump host described by
306: .Ar destination
1.376 jmc 307: and then establishing a TCP forwarding to the ultimate destination from
1.375 djm 308: there.
309: Multiple jump hops may be specified separated by comma characters.
310: This is a shortcut to specify a
311: .Cm ProxyJump
312: configuration directive.
1.400 djm 313: Note that configuration directives supplied on the command-line generally
314: apply to the destination host and not any specified jump hosts.
315: Use
316: .Pa ~/.ssh/config
317: to specify configuration for jump hosts.
1.375 djm 318: .Pp
1.269 djm 319: .It Fl K
320: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
321: credentials to the server.
1.360 millert 322: .Pp
1.218 jmc 323: .It Fl k
324: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 325: .Pp
1.218 jmc 326: .It Fl L Xo
327: .Sm off
328: .Oo Ar bind_address : Oc
329: .Ar port : host : hostport
330: .Sm on
331: .Xc
1.360 millert 332: .It Fl L Xo
333: .Sm off
334: .Oo Ar bind_address : Oc
335: .Ar port : remote_socket
336: .Sm on
337: .Xc
338: .It Fl L Xo
339: .Sm off
340: .Ar local_socket : host : hostport
341: .Sm on
342: .Xc
343: .It Fl L Xo
344: .Sm off
345: .Ar local_socket : remote_socket
346: .Sm on
347: .Xc
348: Specifies that connections to the given TCP port or Unix socket on the local
349: (client) host are to be forwarded to the given host and port, or Unix socket,
350: on the remote side.
351: This works by allocating a socket to listen to either a TCP
1.218 jmc 352: .Ar port
353: on the local side, optionally bound to the specified
1.360 millert 354: .Ar bind_address ,
355: or to a Unix socket.
356: Whenever a connection is made to the local port or socket, the
1.218 jmc 357: connection is forwarded over the secure channel, and a connection is
1.360 millert 358: made to either
1.218 jmc 359: .Ar host
360: port
1.360 millert 361: .Ar hostport ,
362: or the Unix socket
363: .Ar remote_socket ,
1.218 jmc 364: from the remote machine.
1.360 millert 365: .Pp
1.218 jmc 366: Port forwardings can also be specified in the configuration file.
1.360 millert 367: Only the superuser can forward privileged ports.
1.308 djm 368: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 369: .Pp
1.218 jmc 370: By default, the local port is bound in accordance with the
371: .Cm GatewayPorts
372: setting.
373: However, an explicit
374: .Ar bind_address
375: may be used to bind the connection to a specific address.
1.2 deraadt 376: The
1.218 jmc 377: .Ar bind_address
378: of
379: .Dq localhost
380: indicates that the listening port be bound for local use only, while an
381: empty address or
382: .Sq *
383: indicates that the port should be available from all interfaces.
1.360 millert 384: .Pp
1.218 jmc 385: .It Fl l Ar login_name
386: Specifies the user to log in as on the remote machine.
387: This also may be specified on a per-host basis in the configuration file.
1.360 millert 388: .Pp
1.218 jmc 389: .It Fl M
390: Places the
391: .Nm
392: client into
393: .Dq master
394: mode for connection sharing.
1.231 stevesk 395: Multiple
396: .Fl M
397: options places
398: .Nm
399: into
400: .Dq master
1.397 djm 401: mode but with confirmation required using
402: .Xr ssh-askpass 1
403: before each operation that changes the multiplexing state
404: (e.g. opening a new session).
1.218 jmc 405: Refer to the description of
406: .Cm ControlMaster
407: in
408: .Xr ssh_config 5
409: for details.
1.360 millert 410: .Pp
1.218 jmc 411: .It Fl m Ar mac_spec
1.369 jmc 412: A comma-separated list of MAC (message authentication code) algorithms,
413: specified in order of preference.
1.218 jmc 414: See the
415: .Cm MACs
416: keyword for more information.
1.360 millert 417: .Pp
1.218 jmc 418: .It Fl N
419: Do not execute a remote command.
1.369 jmc 420: This is useful for just forwarding ports.
1.360 millert 421: .Pp
1.218 jmc 422: .It Fl n
423: Redirects stdin from
424: .Pa /dev/null
425: (actually, prevents reading from stdin).
426: This must be used when
1.2 deraadt 427: .Nm
1.218 jmc 428: is run in the background.
429: A common trick is to use this to run X11 programs on a remote machine.
430: For example,
431: .Ic ssh -n shadows.cs.hut.fi emacs &
432: will start an emacs on shadows.cs.hut.fi, and the X11
433: connection will be automatically forwarded over an encrypted channel.
434: The
1.2 deraadt 435: .Nm
1.218 jmc 436: program will be put in the background.
437: (This does not work if
1.2 deraadt 438: .Nm
1.218 jmc 439: needs to ask for a password or passphrase; see also the
440: .Fl f
441: option.)
1.360 millert 442: .Pp
1.218 jmc 443: .It Fl O Ar ctl_cmd
444: Control an active connection multiplexing master process.
445: When the
446: .Fl O
447: option is specified, the
448: .Ar ctl_cmd
449: argument is interpreted and passed to the master process.
450: Valid commands are:
451: .Dq check
1.305 markus 452: (check that the master process is running),
453: .Dq forward
1.318 jmc 454: (request forwardings without command execution),
1.323 okan 455: .Dq cancel
456: (cancel forwardings),
1.218 jmc 457: .Dq exit
1.318 jmc 458: (request the master to exit), and
1.317 djm 459: .Dq stop
460: (request the master to stop accepting further multiplexing requests).
1.360 millert 461: .Pp
1.218 jmc 462: .It Fl o Ar option
463: Can be used to give options in the format used in the configuration file.
464: This is useful for specifying options for which there is no separate
465: command-line flag.
466: For full details of the options listed below, and their possible values, see
467: .Xr ssh_config 5 .
1.2 deraadt 468: .Pp
1.218 jmc 469: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 470: .It AddKeysToAgent
1.218 jmc 471: .It AddressFamily
472: .It BatchMode
473: .It BindAddress
1.338 djm 474: .It CanonicalDomains
1.339 djm 475: .It CanonicalizeFallbackLocal
476: .It CanonicalizeHostname
477: .It CanonicalizeMaxDots
478: .It CanonicalizePermittedCNAMEs
1.399 jmc 479: .It CASignatureAlgorithms
1.363 djm 480: .It CertificateFile
1.218 jmc 481: .It ChallengeResponseAuthentication
482: .It CheckHostIP
483: .It Ciphers
484: .It ClearAllForwardings
485: .It Compression
1.380 naddy 486: .It ConnectionAttempts
1.218 jmc 487: .It ConnectTimeout
488: .It ControlMaster
489: .It ControlPath
1.321 djm 490: .It ControlPersist
1.218 jmc 491: .It DynamicForward
492: .It EscapeChar
1.263 markus 493: .It ExitOnForwardFailure
1.353 jmc 494: .It FingerprintHash
1.218 jmc 495: .It ForwardAgent
496: .It ForwardX11
1.321 djm 497: .It ForwardX11Timeout
1.218 jmc 498: .It ForwardX11Trusted
499: .It GatewayPorts
500: .It GlobalKnownHostsFile
501: .It GSSAPIAuthentication
502: .It GSSAPIDelegateCredentials
503: .It HashKnownHosts
504: .It Host
505: .It HostbasedAuthentication
1.355 djm 506: .It HostbasedKeyTypes
1.218 jmc 507: .It HostKeyAlgorithms
508: .It HostKeyAlias
1.403 jmc 509: .It Hostname
1.374 jmc 510: .It IdentitiesOnly
1.371 markus 511: .It IdentityAgent
1.218 jmc 512: .It IdentityFile
1.316 jmc 513: .It IPQoS
1.321 djm 514: .It KbdInteractiveAuthentication
1.218 jmc 515: .It KbdInteractiveDevices
1.314 jmc 516: .It KexAlgorithms
1.218 jmc 517: .It LocalCommand
518: .It LocalForward
519: .It LogLevel
520: .It MACs
1.337 jmc 521: .It Match
1.218 jmc 522: .It NoHostAuthenticationForLocalhost
523: .It NumberOfPasswordPrompts
524: .It PasswordAuthentication
525: .It PermitLocalCommand
1.291 markus 526: .It PKCS11Provider
1.218 jmc 527: .It Port
528: .It PreferredAuthentications
529: .It ProxyCommand
1.375 djm 530: .It ProxyJump
1.336 jmc 531: .It ProxyUseFdpass
1.359 markus 532: .It PubkeyAcceptedKeyTypes
1.218 jmc 533: .It PubkeyAuthentication
1.251 dtucker 534: .It RekeyLimit
1.382 bluhm 535: .It RemoteCommand
1.218 jmc 536: .It RemoteForward
1.319 jmc 537: .It RequestTTY
1.218 jmc 538: .It SendEnv
539: .It ServerAliveInterval
540: .It ServerAliveCountMax
1.395 jmc 541: .It SetEnv
1.347 jmc 542: .It StreamLocalBindMask
543: .It StreamLocalBindUnlink
1.218 jmc 544: .It StrictHostKeyChecking
545: .It TCPKeepAlive
546: .It Tunnel
547: .It TunnelDevice
1.354 djm 548: .It UpdateHostKeys
1.218 jmc 549: .It User
550: .It UserKnownHostsFile
551: .It VerifyHostKeyDNS
1.276 jmc 552: .It VisualHostKey
1.218 jmc 553: .It XAuthLocation
554: .El
1.360 millert 555: .Pp
1.218 jmc 556: .It Fl p Ar port
557: Port to connect to on the remote host.
558: This can be specified on a
559: per-host basis in the configuration file.
1.360 millert 560: .Pp
1.362 djm 561: .It Fl Q Ar query_option
1.332 djm 562: Queries
563: .Nm
1.341 deraadt 564: for the algorithms supported for the specified version 2.
565: The available features are:
566: .Ar cipher
1.332 djm 567: (supported symmetric ciphers),
1.341 deraadt 568: .Ar cipher-auth
1.340 djm 569: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 570: .Ar help
571: (supported query terms for use with the
572: .Fl Q
573: flag),
1.341 deraadt 574: .Ar mac
1.332 djm 575: (supported message integrity codes),
1.341 deraadt 576: .Ar kex
1.332 djm 577: (key exchange algorithms),
1.341 deraadt 578: .Ar key
1.362 djm 579: (key types),
580: .Ar key-cert
581: (certificate key types),
582: .Ar key-plain
1.398 djm 583: (non-certificate key types),
1.356 djm 584: .Ar protocol-version
1.398 djm 585: (supported SSH protocol versions), and
586: .Ar sig
587: (supported signature algorithms).
1.360 millert 588: .Pp
1.218 jmc 589: .It Fl q
590: Quiet mode.
1.271 djm 591: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 592: .Pp
1.218 jmc 593: .It Fl R Xo
594: .Sm off
595: .Oo Ar bind_address : Oc
596: .Ar port : host : hostport
597: .Sm on
598: .Xc
1.360 millert 599: .It Fl R Xo
600: .Sm off
601: .Oo Ar bind_address : Oc
602: .Ar port : local_socket
603: .Sm on
604: .Xc
605: .It Fl R Xo
606: .Sm off
607: .Ar remote_socket : host : hostport
608: .Sm on
609: .Xc
610: .It Fl R Xo
611: .Sm off
612: .Ar remote_socket : local_socket
613: .Sm on
614: .Xc
1.384 markus 615: .It Fl R Xo
616: .Sm off
617: .Oo Ar bind_address : Oc
618: .Ar port
619: .Sm on
620: .Xc
1.360 millert 621: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 622: (server) host are to be forwarded to the local side.
623: .Pp
1.360 millert 624: This works by allocating a socket to listen to either a TCP
1.218 jmc 625: .Ar port
1.360 millert 626: or to a Unix socket on the remote side.
627: Whenever a connection is made to this port or Unix socket, the
628: connection is forwarded over the secure channel, and a connection
1.384 markus 629: is made from the local machine to either an explicit destination specified by
1.218 jmc 630: .Ar host
631: port
1.360 millert 632: .Ar hostport ,
633: or
634: .Ar local_socket ,
1.384 markus 635: or, if no explicit destination was specified,
636: .Nm
637: will act as a SOCKS 4/5 proxy and forward connections to the destinations
638: requested by the remote SOCKS client.
1.2 deraadt 639: .Pp
1.218 jmc 640: Port forwardings can also be specified in the configuration file.
641: Privileged ports can be forwarded only when
642: logging in as root on the remote machine.
1.324 jmc 643: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 644: .Pp
1.360 millert 645: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 646: interface only.
1.280 tobias 647: This may be overridden by specifying a
1.218 jmc 648: .Ar bind_address .
649: An empty
650: .Ar bind_address ,
651: or the address
652: .Ql * ,
653: indicates that the remote socket should listen on all interfaces.
654: Specifying a remote
655: .Ar bind_address
656: will only succeed if the server's
657: .Cm GatewayPorts
658: option is enabled (see
659: .Xr sshd_config 5 ) .
1.281 djm 660: .Pp
661: If the
662: .Ar port
663: argument is
1.282 djm 664: .Ql 0 ,
1.281 djm 665: the listen port will be dynamically allocated on the server and reported
666: to the client at run time.
1.305 markus 667: When used together with
668: .Ic -O forward
669: the allocated port will be printed to the standard output.
1.360 millert 670: .Pp
1.218 jmc 671: .It Fl S Ar ctl_path
1.304 jmc 672: Specifies the location of a control socket for connection sharing,
1.303 djm 673: or the string
674: .Dq none
675: to disable connection sharing.
1.218 jmc 676: Refer to the description of
677: .Cm ControlPath
678: and
679: .Cm ControlMaster
680: in
681: .Xr ssh_config 5
682: for details.
1.360 millert 683: .Pp
1.218 jmc 684: .It Fl s
685: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 686: Subsystems facilitate the use of SSH
687: as a secure transport for other applications (e.g.\&
1.218 jmc 688: .Xr sftp 1 ) .
689: The subsystem is specified as the remote command.
1.360 millert 690: .Pp
1.218 jmc 691: .It Fl T
1.357 dtucker 692: Disable pseudo-terminal allocation.
1.360 millert 693: .Pp
1.218 jmc 694: .It Fl t
1.357 dtucker 695: Force pseudo-terminal allocation.
1.218 jmc 696: This can be used to execute arbitrary
697: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 698: e.g. when implementing menu services.
1.218 jmc 699: Multiple
700: .Fl t
701: options force tty allocation, even if
1.194 jakob 702: .Nm
1.218 jmc 703: has no local tty.
1.360 millert 704: .Pp
1.218 jmc 705: .It Fl V
706: Display the version number and exit.
1.360 millert 707: .Pp
1.218 jmc 708: .It Fl v
709: Verbose mode.
710: Causes
1.176 jmc 711: .Nm
1.218 jmc 712: to print debugging messages about its progress.
713: This is helpful in
714: debugging connection, authentication, and configuration problems.
715: Multiple
716: .Fl v
717: options increase the verbosity.
718: The maximum is 3.
1.360 millert 719: .Pp
1.290 dtucker 720: .It Fl W Ar host : Ns Ar port
721: Requests that standard input and output on the client be forwarded to
722: .Ar host
723: on
724: .Ar port
725: over the secure channel.
726: Implies
727: .Fl N ,
728: .Fl T ,
729: .Cm ExitOnForwardFailure
730: and
1.373 jmc 731: .Cm ClearAllForwardings ,
732: though these can be overridden in the configuration file or using
1.372 dtucker 733: .Fl o
734: command line options.
1.360 millert 735: .Pp
1.261 stevesk 736: .It Fl w Xo
737: .Ar local_tun Ns Op : Ns Ar remote_tun
738: .Xc
739: Requests
740: tunnel
741: device forwarding with the specified
1.218 jmc 742: .Xr tun 4
1.261 stevesk 743: devices between the client
744: .Pq Ar local_tun
745: and the server
746: .Pq Ar remote_tun .
747: .Pp
1.228 jmc 748: The devices may be specified by numerical ID or the keyword
749: .Dq any ,
750: which uses the next available tunnel device.
1.261 stevesk 751: If
752: .Ar remote_tun
753: is not specified, it defaults to
754: .Dq any .
1.228 jmc 755: See also the
1.218 jmc 756: .Cm Tunnel
1.261 stevesk 757: and
758: .Cm TunnelDevice
759: directives in
1.218 jmc 760: .Xr ssh_config 5 .
1.393 djm 761: .Pp
1.261 stevesk 762: If the
763: .Cm Tunnel
1.393 djm 764: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 765: .Dq point-to-point .
1.393 djm 766: If a different
767: .Cm Tunnel
768: forwarding mode it desired, then it should be specified before
769: .Fl w .
1.360 millert 770: .Pp
1.218 jmc 771: .It Fl X
772: Enables X11 forwarding.
1.54 markus 773: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 774: .Pp
1.218 jmc 775: X11 forwarding should be enabled with caution.
1.168 jmc 776: Users with the ability to bypass file permissions on the remote host
1.218 jmc 777: (for the user's X authorization database)
778: can access the local X11 display through the forwarded connection.
779: An attacker may then be able to perform activities such as keystroke monitoring.
780: .Pp
781: For this reason, X11 forwarding is subjected to X11 SECURITY extension
782: restrictions by default.
783: Please refer to the
784: .Nm
785: .Fl Y
786: option and the
787: .Cm ForwardX11Trusted
788: directive in
789: .Xr ssh_config 5
790: for more information.
1.360 millert 791: .Pp
1.218 jmc 792: .It Fl x
793: Disables X11 forwarding.
1.360 millert 794: .Pp
1.218 jmc 795: .It Fl Y
796: Enables trusted X11 forwarding.
797: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
798: controls.
1.360 millert 799: .Pp
1.278 djm 800: .It Fl y
801: Send log information using the
802: .Xr syslog 3
803: system module.
804: By default this information is sent to stderr.
1.218 jmc 805: .El
1.224 jmc 806: .Pp
807: .Nm
808: may additionally obtain configuration data from
809: a per-user configuration file and a system-wide configuration file.
810: The file format and configuration options are described in
811: .Xr ssh_config 5 .
1.222 jmc 812: .Sh AUTHENTICATION
1.377 djm 813: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 814: .Pp
815: The methods available for authentication are:
1.260 jmc 816: GSSAPI-based authentication,
1.222 jmc 817: host-based authentication,
818: public key authentication,
819: challenge-response authentication,
820: and password authentication.
821: Authentication methods are tried in the order specified above,
1.369 jmc 822: though
823: .Cm PreferredAuthentications
824: can be used to change the default order.
1.222 jmc 825: .Pp
826: Host-based authentication works as follows:
1.218 jmc 827: If the machine the user logs in from is listed in
828: .Pa /etc/hosts.equiv
829: or
830: .Pa /etc/shosts.equiv
831: on the remote machine, and the user names are
832: the same on both sides, or if the files
833: .Pa ~/.rhosts
834: or
835: .Pa ~/.shosts
836: exist in the user's home directory on the
837: remote machine and contain a line containing the name of the client
838: machine and the name of the user on that machine, the user is
1.222 jmc 839: considered for login.
840: Additionally, the server
841: .Em must
842: be able to verify the client's
843: host key (see the description of
1.218 jmc 844: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 845: and
1.222 jmc 846: .Pa ~/.ssh/known_hosts ,
847: below)
848: for login to be permitted.
1.218 jmc 849: This authentication method closes security holes due to IP
1.222 jmc 850: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 851: [Note to the administrator:
852: .Pa /etc/hosts.equiv ,
853: .Pa ~/.rhosts ,
854: and the rlogin/rsh protocol in general, are inherently insecure and should be
855: disabled if security is desired.]
1.189 dtucker 856: .Pp
1.222 jmc 857: Public key authentication works as follows:
858: The scheme is based on public-key cryptography,
859: using cryptosystems
860: where encryption and decryption are done using separate keys,
861: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 862: The idea is that each user creates a public/private
863: key pair for authentication purposes.
864: The server knows the public key, and only the user knows the private key.
1.222 jmc 865: .Nm
866: implements public key authentication protocol automatically,
1.349 sobrado 867: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 868: The HISTORY section of
1.222 jmc 869: .Xr ssl 8
1.311 jmc 870: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 871: .Pp
1.218 jmc 872: The file
873: .Pa ~/.ssh/authorized_keys
874: lists the public keys that are permitted for logging in.
875: When the user logs in, the
1.2 deraadt 876: .Nm
1.218 jmc 877: program tells the server which key pair it would like to use for
878: authentication.
1.222 jmc 879: The client proves that it has access to the private key
880: and the server checks that the corresponding public key
881: is authorized to accept the account.
1.383 djm 882: .Pp
883: The server may inform the client of errors that prevented public key
884: authentication from succeeding after authentication completes using a
885: different method.
886: These may be viewed by increasing the
887: .Cm LogLevel
888: to
889: .Cm DEBUG
890: or higher (e.g. by using the
891: .Fl v
892: flag).
1.218 jmc 893: .Pp
1.222 jmc 894: The user creates his/her key pair by running
1.218 jmc 895: .Xr ssh-keygen 1 .
896: This stores the private key in
1.222 jmc 897: .Pa ~/.ssh/id_dsa
1.369 jmc 898: (DSA),
1.310 djm 899: .Pa ~/.ssh/id_ecdsa
1.369 jmc 900: (ECDSA),
1.404 naddy 901: .Pa ~/.ssh/id_ecdsa_sk
902: (security key-hosted ECDSA),
1.343 naddy 903: .Pa ~/.ssh/id_ed25519
1.369 jmc 904: (Ed25519),
1.406 ! naddy 905: .Pa ~/.ssh/id_ed25519_sk
! 906: (security key-hosted Ed25519),
1.222 jmc 907: or
908: .Pa ~/.ssh/id_rsa
1.369 jmc 909: (RSA)
1.218 jmc 910: and stores the public key in
1.222 jmc 911: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 912: (DSA),
1.310 djm 913: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 914: (ECDSA),
1.404 naddy 915: .Pa ~/.ssh/id_ecdsa_sk.pub
916: (security key-hosted ECDSA),
1.343 naddy 917: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 918: (Ed25519),
1.406 ! naddy 919: .Pa ~/.ssh/id_ed25519_sk.pub
! 920: (security key-hosted Ed25519),
1.222 jmc 921: or
922: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 923: (RSA)
1.218 jmc 924: in the user's home directory.
1.222 jmc 925: The user should then copy the public key
1.218 jmc 926: to
927: .Pa ~/.ssh/authorized_keys
1.222 jmc 928: in his/her home directory on the remote machine.
929: The
1.218 jmc 930: .Pa authorized_keys
931: file corresponds to the conventional
932: .Pa ~/.rhosts
933: file, and has one key
1.222 jmc 934: per line, though the lines can be very long.
1.218 jmc 935: After this, the user can log in without giving the password.
936: .Pp
1.301 jmc 937: A variation on public key authentication
938: is available in the form of certificate authentication:
939: instead of a set of public/private keys,
940: signed certificates are used.
941: This has the advantage that a single trusted certification authority
942: can be used in place of many public/private keys.
1.333 jmc 943: See the CERTIFICATES section of
1.301 jmc 944: .Xr ssh-keygen 1
945: for more information.
946: .Pp
947: The most convenient way to use public key or certificate authentication
948: may be with an authentication agent.
1.218 jmc 949: See
950: .Xr ssh-agent 1
1.366 jcs 951: and (optionally) the
952: .Cm AddKeysToAgent
953: directive in
954: .Xr ssh_config 5
1.218 jmc 955: for more information.
956: .Pp
1.222 jmc 957: Challenge-response authentication works as follows:
958: The server sends an arbitrary
959: .Qq challenge
960: text, and prompts for a response.
961: Examples of challenge-response authentication include
1.335 jmc 962: .Bx
963: Authentication (see
1.222 jmc 964: .Xr login.conf 5 )
1.335 jmc 965: and PAM (some
966: .Pf non- Ox
967: systems).
1.222 jmc 968: .Pp
969: Finally, if other authentication methods fail,
1.218 jmc 970: .Nm
971: prompts the user for a password.
972: The password is sent to the remote
973: host for checking; however, since all communications are encrypted,
974: the password cannot be seen by someone listening on the network.
1.232 jmc 975: .Pp
976: .Nm
977: automatically maintains and checks a database containing
978: identification for all hosts it has ever been used with.
979: Host keys are stored in
980: .Pa ~/.ssh/known_hosts
981: in the user's home directory.
982: Additionally, the file
983: .Pa /etc/ssh/ssh_known_hosts
984: is automatically checked for known hosts.
985: Any new hosts are automatically added to the user's file.
986: If a host's identification ever changes,
987: .Nm
988: warns about this and disables password authentication to prevent
989: server spoofing or man-in-the-middle attacks,
990: which could otherwise be used to circumvent the encryption.
991: The
992: .Cm StrictHostKeyChecking
993: option can be used to control logins to machines whose
994: host key is not known or has changed.
995: .Pp
1.218 jmc 996: When the user's identity has been accepted by the server, the server
1.357 dtucker 997: either executes the given command in a non-interactive session or,
998: if no command has been specified, logs into the machine and gives
999: the user a normal shell as an interactive session.
1.218 jmc 1000: All communication with
1001: the remote command or shell will be automatically encrypted.
1002: .Pp
1.357 dtucker 1003: If an interactive session is requested
1004: .Nm
1005: by default will only request a pseudo-terminal (pty) for interactive
1006: sessions when the client has one.
1007: The flags
1008: .Fl T
1009: and
1010: .Fl t
1011: can be used to override this behaviour.
1012: .Pp
1013: If a pseudo-terminal has been allocated the
1.218 jmc 1014: user may use the escape characters noted below.
1015: .Pp
1.357 dtucker 1016: If no pseudo-terminal has been allocated,
1.218 jmc 1017: the session is transparent and can be used to reliably transfer binary data.
1018: On most systems, setting the escape character to
1019: .Dq none
1020: will also make the session transparent even if a tty is used.
1021: .Pp
1022: The session terminates when the command or shell on the remote
1.247 jmc 1023: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1024: .Sh ESCAPE CHARACTERS
1.218 jmc 1025: When a pseudo-terminal has been requested,
1.2 deraadt 1026: .Nm
1.218 jmc 1027: supports a number of functions through the use of an escape character.
1028: .Pp
1029: A single tilde character can be sent as
1030: .Ic ~~
1031: or by following the tilde by a character other than those described below.
1032: The escape character must always follow a newline to be interpreted as
1033: special.
1034: The escape character can be changed in configuration files using the
1035: .Cm EscapeChar
1036: configuration directive or on the command line by the
1037: .Fl e
1038: option.
1039: .Pp
1040: The supported escapes (assuming the default
1041: .Ql ~ )
1042: are:
1043: .Bl -tag -width Ds
1044: .It Cm ~.
1045: Disconnect.
1046: .It Cm ~^Z
1047: Background
1.234 jmc 1048: .Nm .
1.218 jmc 1049: .It Cm ~#
1050: List forwarded connections.
1051: .It Cm ~&
1052: Background
1.2 deraadt 1053: .Nm
1.218 jmc 1054: at logout when waiting for forwarded connection / X11 sessions to terminate.
1055: .It Cm ~?
1056: Display a list of escape characters.
1057: .It Cm ~B
1058: Send a BREAK to the remote system
1.369 jmc 1059: (only useful if the peer supports it).
1.218 jmc 1060: .It Cm ~C
1061: Open command line.
1062: Currently this allows the addition of port forwardings using the
1.279 stevesk 1063: .Fl L ,
1064: .Fl R
1.218 jmc 1065: and
1.279 stevesk 1066: .Fl D
1.225 jmc 1067: options (see above).
1.322 markus 1068: It also allows the cancellation of existing port-forwardings
1069: with
1.262 stevesk 1070: .Sm off
1.322 markus 1071: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1072: .Sm on
1.322 markus 1073: for local,
1074: .Sm off
1075: .Fl KR Oo Ar bind_address : Oc Ar port
1076: .Sm on
1077: for remote and
1078: .Sm off
1079: .Fl KD Oo Ar bind_address : Oc Ar port
1080: .Sm on
1081: for dynamic port-forwardings.
1.218 jmc 1082: .Ic !\& Ns Ar command
1083: allows the user to execute a local command if the
1084: .Ic PermitLocalCommand
1085: option is enabled in
1.176 jmc 1086: .Xr ssh_config 5 .
1.218 jmc 1087: Basic help is available, using the
1088: .Fl h
1089: option.
1090: .It Cm ~R
1091: Request rekeying of the connection
1.369 jmc 1092: (only useful if the peer supports it).
1.327 dtucker 1093: .It Cm ~V
1094: Decrease the verbosity
1095: .Pq Ic LogLevel
1096: when errors are being written to stderr.
1097: .It Cm ~v
1.328 jmc 1098: Increase the verbosity
1.327 dtucker 1099: .Pq Ic LogLevel
1100: when errors are being written to stderr.
1.176 jmc 1101: .El
1.246 jmc 1102: .Sh TCP FORWARDING
1.402 jmc 1103: Forwarding of arbitrary TCP connections over a secure channel
1104: can be specified either on the command line or in a configuration file.
1.246 jmc 1105: One possible application of TCP forwarding is a secure connection to a
1106: mail server; another is going through firewalls.
1107: .Pp
1.402 jmc 1108: In the example below, we look at encrypting communication for an IRC client,
1109: even though the IRC server it connects to does not directly
1110: support encrypted communication.
1.246 jmc 1111: This works as follows:
1112: the user connects to the remote host using
1113: .Nm ,
1.402 jmc 1114: specifying the ports to be used to forward the connection.
1115: After that it is possible to start the program locally,
1.246 jmc 1116: and
1117: .Nm
1.402 jmc 1118: will encrypt and forward the connection to the remote server.
1.246 jmc 1119: .Pp
1.402 jmc 1120: The following example tunnels an IRC session from the client
1121: to an IRC server at
1.246 jmc 1122: .Dq server.example.com ,
1123: joining channel
1124: .Dq #users ,
1125: nickname
1126: .Dq pinky ,
1.402 jmc 1127: using the standard IRC port, 6667:
1128: .Bd -literal -offset 4n
1129: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1130: $ irc -c '#users' pinky IRC/127.0.0.1
1131: .Ed
1.246 jmc 1132: .Pp
1133: The
1134: .Fl f
1135: option backgrounds
1136: .Nm
1137: and the remote command
1138: .Dq sleep 10
1139: is specified to allow an amount of time
1140: (10 seconds, in the example)
1.402 jmc 1141: to start the program which is going to use the tunnel.
1.246 jmc 1142: If no connections are made within the time specified,
1143: .Nm
1144: will exit.
1145: .Sh X11 FORWARDING
1.218 jmc 1146: If the
1147: .Cm ForwardX11
1148: variable is set to
1149: .Dq yes
1150: (or see the description of the
1.227 jmc 1151: .Fl X ,
1152: .Fl x ,
1.218 jmc 1153: and
1.227 jmc 1154: .Fl Y
1.226 jmc 1155: options above)
1.218 jmc 1156: and the user is using X11 (the
1157: .Ev DISPLAY
1158: environment variable is set), the connection to the X11 display is
1159: automatically forwarded to the remote side in such a way that any X11
1160: programs started from the shell (or command) will go through the
1161: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1162: from the local machine.
1.218 jmc 1163: The user should not manually set
1164: .Ev DISPLAY .
1165: Forwarding of X11 connections can be
1166: configured on the command line or in configuration files.
1167: .Pp
1168: The
1169: .Ev DISPLAY
1170: value set by
1171: .Nm
1172: will point to the server machine, but with a display number greater than zero.
1173: This is normal, and happens because
1174: .Nm
1175: creates a
1176: .Dq proxy
1177: X server on the server machine for forwarding the
1178: connections over the encrypted channel.
1.200 djm 1179: .Pp
1.218 jmc 1180: .Nm
1181: will also automatically set up Xauthority data on the server machine.
1182: For this purpose, it will generate a random authorization cookie,
1183: store it in Xauthority on the server, and verify that any forwarded
1184: connections carry this cookie and replace it by the real cookie when
1185: the connection is opened.
1186: The real authentication cookie is never
1187: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1188: .Pp
1.218 jmc 1189: If the
1190: .Cm ForwardAgent
1191: variable is set to
1192: .Dq yes
1193: (or see the description of the
1194: .Fl A
1.191 djm 1195: and
1.218 jmc 1196: .Fl a
1.226 jmc 1197: options above) and
1.218 jmc 1198: the user is using an authentication agent, the connection to the agent
1199: is automatically forwarded to the remote side.
1.252 jmc 1200: .Sh VERIFYING HOST KEYS
1201: When connecting to a server for the first time,
1202: a fingerprint of the server's public key is presented to the user
1203: (unless the option
1204: .Cm StrictHostKeyChecking
1205: has been disabled).
1206: Fingerprints can be determined using
1207: .Xr ssh-keygen 1 :
1208: .Pp
1209: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1210: .Pp
1.274 grunk 1211: If the fingerprint is already known, it can be matched
1212: and the key can be accepted or rejected.
1.358 djm 1213: If only legacy (MD5) fingerprints for the server are available, the
1214: .Xr ssh-keygen 1
1215: .Fl E
1216: option may be used to downgrade the fingerprint algorithm to match.
1217: .Pp
1.274 grunk 1218: Because of the difficulty of comparing host keys
1.352 djm 1219: just by looking at fingerprint strings,
1.274 grunk 1220: there is also support to compare host keys visually,
1221: using
1222: .Em random art .
1223: By setting the
1.275 grunk 1224: .Cm VisualHostKey
1.274 grunk 1225: option to
1.275 grunk 1226: .Dq yes ,
1.274 grunk 1227: a small ASCII graphic gets displayed on every login to a server, no matter
1228: if the session itself is interactive or not.
1229: By learning the pattern a known server produces, a user can easily
1230: find out that the host key has changed when a completely different pattern
1231: is displayed.
1232: Because these patterns are not unambiguous however, a pattern that looks
1233: similar to the pattern remembered only gives a good probability that the
1234: host key is the same, not guaranteed proof.
1235: .Pp
1236: To get a listing of the fingerprints along with their random art for
1237: all known hosts, the following command line can be used:
1238: .Pp
1239: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1240: .Pp
1.252 jmc 1241: If the fingerprint is unknown,
1242: an alternative method of verification is available:
1243: SSH fingerprints verified by DNS.
1244: An additional resource record (RR),
1245: SSHFP,
1246: is added to a zonefile
1247: and the connecting client is able to match the fingerprint
1248: with that of the key presented.
1249: .Pp
1250: In this example, we are connecting a client to a server,
1251: .Dq host.example.com .
1252: The SSHFP resource records should first be added to the zonefile for
1253: host.example.com:
1254: .Bd -literal -offset indent
1.259 jakob 1255: $ ssh-keygen -r host.example.com.
1.252 jmc 1256: .Ed
1257: .Pp
1258: The output lines will have to be added to the zonefile.
1259: To check that the zone is answering fingerprint queries:
1260: .Pp
1261: .Dl $ dig -t SSHFP host.example.com
1262: .Pp
1263: Finally the client connects:
1264: .Bd -literal -offset indent
1265: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1266: [...]
1267: Matching host key fingerprint found in DNS.
1268: Are you sure you want to continue connecting (yes/no)?
1269: .Ed
1270: .Pp
1271: See the
1272: .Cm VerifyHostKeyDNS
1273: option in
1274: .Xr ssh_config 5
1275: for more information.
1.250 jmc 1276: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1277: .Nm
1278: contains support for Virtual Private Network (VPN) tunnelling
1279: using the
1280: .Xr tun 4
1281: network pseudo-device,
1282: allowing two networks to be joined securely.
1283: The
1284: .Xr sshd_config 5
1285: configuration option
1286: .Cm PermitTunnel
1287: controls whether the server supports this,
1288: and at what level (layer 2 or 3 traffic).
1289: .Pp
1290: The following example would connect client network 10.0.50.0/24
1.265 otto 1291: with remote network 10.0.99.0/24 using a point-to-point connection
1292: from 10.1.1.1 to 10.1.1.2,
1293: provided that the SSH server running on the gateway to the remote network,
1294: at 192.168.1.15, allows it.
1295: .Pp
1296: On the client:
1.250 jmc 1297: .Bd -literal -offset indent
1298: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1299: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1300: # route add 10.0.99.0/24 10.1.1.2
1301: .Ed
1302: .Pp
1303: On the server:
1304: .Bd -literal -offset indent
1305: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1306: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1307: .Ed
1308: .Pp
1309: Client access may be more finely tuned via the
1310: .Pa /root/.ssh/authorized_keys
1311: file (see below) and the
1312: .Cm PermitRootLogin
1313: server option.
1.255 jmc 1314: The following entry would permit connections on
1.250 jmc 1315: .Xr tun 4
1.255 jmc 1316: device 1 from user
1.250 jmc 1317: .Dq jane
1.255 jmc 1318: and on tun device 2 from user
1.250 jmc 1319: .Dq john ,
1320: if
1321: .Cm PermitRootLogin
1322: is set to
1323: .Dq forced-commands-only :
1324: .Bd -literal -offset 2n
1325: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1326: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1327: .Ed
1328: .Pp
1.264 ray 1329: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1330: it may be more suited to temporary setups,
1331: such as for wireless VPNs.
1332: More permanent VPNs are better provided by tools such as
1333: .Xr ipsecctl 8
1334: and
1335: .Xr isakmpd 8 .
1.2 deraadt 1336: .Sh ENVIRONMENT
1337: .Nm
1.1 deraadt 1338: will normally set the following environment variables:
1.237 jmc 1339: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1340: .It Ev DISPLAY
1341: The
1342: .Ev DISPLAY
1.40 aaron 1343: variable indicates the location of the X11 server.
1.44 aaron 1344: It is automatically set by
1.2 deraadt 1345: .Nm
1346: to point to a value of the form
1.233 jmc 1347: .Dq hostname:n ,
1348: where
1349: .Dq hostname
1350: indicates the host where the shell runs, and
1351: .Sq n
1352: is an integer \*(Ge 1.
1.40 aaron 1353: .Nm
1354: uses this special value to forward X11 connections over the secure
1355: channel.
1.107 markus 1356: The user should normally not set
1357: .Ev DISPLAY
1358: explicitly, as that
1.1 deraadt 1359: will render the X11 connection insecure (and will require the user to
1360: manually copy any required authorization cookies).
1.2 deraadt 1361: .It Ev HOME
1.1 deraadt 1362: Set to the path of the user's home directory.
1.2 deraadt 1363: .It Ev LOGNAME
1364: Synonym for
1.12 aaron 1365: .Ev USER ;
1366: set for compatibility with systems that use this variable.
1.2 deraadt 1367: .It Ev MAIL
1.129 stevesk 1368: Set to the path of the user's mailbox.
1.40 aaron 1369: .It Ev PATH
1.2 deraadt 1370: Set to the default
1371: .Ev PATH ,
1372: as specified when compiling
1.234 jmc 1373: .Nm .
1.118 markus 1374: .It Ev SSH_ASKPASS
1375: If
1376: .Nm
1377: needs a passphrase, it will read the passphrase from the current
1378: terminal if it was run from a terminal.
1379: If
1380: .Nm
1381: does not have a terminal associated with it but
1382: .Ev DISPLAY
1383: and
1384: .Ev SSH_ASKPASS
1385: are set, it will execute the program specified by
1386: .Ev SSH_ASKPASS
1387: and open an X11 window to read the passphrase.
1388: This is particularly useful when calling
1389: .Nm
1390: from a
1.196 jmc 1391: .Pa .xsession
1.118 markus 1392: or related script.
1393: (Note that on some machines it
1394: may be necessary to redirect the input from
1395: .Pa /dev/null
1396: to make this work.)
1.18 markus 1397: .It Ev SSH_AUTH_SOCK
1.233 jmc 1398: Identifies the path of a
1399: .Ux Ns -domain
1400: socket used to communicate with the agent.
1.166 stevesk 1401: .It Ev SSH_CONNECTION
1402: Identifies the client and server ends of the connection.
1.40 aaron 1403: The variable contains
1.233 jmc 1404: four space-separated values: client IP address, client port number,
1405: server IP address, and server port number.
1.73 markus 1406: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1407: This variable contains the original command line if a forced command
1.73 markus 1408: is executed.
1409: It can be used to extract the original arguments.
1.2 deraadt 1410: .It Ev SSH_TTY
1.1 deraadt 1411: This is set to the name of the tty (path to the device) associated
1.40 aaron 1412: with the current shell or command.
1413: If the current session has no tty,
1.1 deraadt 1414: this variable is not set.
1.387 djm 1415: .It Ev SSH_TUNNEL
1416: Optionally set by
1417: .Xr sshd 8
1418: to contain the interface names assigned if tunnel forwarding was
1419: requested by the client.
1.385 djm 1420: .It Ev SSH_USER_AUTH
1421: Optionally set by
1422: .Xr sshd 8 ,
1423: this variable may contain a pathname to a file that lists the authentication
1424: methods successfully used when the session was established, including any
1425: public keys that were used.
1.2 deraadt 1426: .It Ev TZ
1.214 jmc 1427: This variable is set to indicate the present time zone if it
1.257 jmc 1428: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1429: on to new connections).
1.2 deraadt 1430: .It Ev USER
1.1 deraadt 1431: Set to the name of the user logging in.
1.2 deraadt 1432: .El
1433: .Pp
1.44 aaron 1434: Additionally,
1.2 deraadt 1435: .Nm
1.44 aaron 1436: reads
1.207 djm 1437: .Pa ~/.ssh/environment ,
1.2 deraadt 1438: and adds lines of the format
1439: .Dq VARNAME=value
1.233 jmc 1440: to the environment if the file exists and users are allowed to
1.161 marc 1441: change their environment.
1.176 jmc 1442: For more information, see the
1.161 marc 1443: .Cm PermitUserEnvironment
1.162 stevesk 1444: option in
1.161 marc 1445: .Xr sshd_config 5 .
1.2 deraadt 1446: .Sh FILES
1.236 jmc 1447: .Bl -tag -width Ds -compact
1.309 jmc 1448: .It Pa ~/.rhosts
1.240 jmc 1449: This file is used for host-based authentication (see above).
1.92 markus 1450: On some machines this file may need to be
1.240 jmc 1451: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1452: because
1.2 deraadt 1453: .Xr sshd 8
1.40 aaron 1454: reads it as root.
1455: Additionally, this file must be owned by the user,
1456: and must not have write permissions for anyone else.
1457: The recommended
1.1 deraadt 1458: permission for most machines is read/write for the user, and not
1459: accessible by others.
1.2 deraadt 1460: .Pp
1.309 jmc 1461: .It Pa ~/.shosts
1.240 jmc 1462: This file is used in exactly the same way as
1463: .Pa .rhosts ,
1464: but allows host-based authentication without permitting login with
1465: rlogin/rsh.
1.272 mcbride 1466: .Pp
1.309 jmc 1467: .It Pa ~/.ssh/
1.272 mcbride 1468: This directory is the default location for all user-specific configuration
1469: and authentication information.
1470: There is no general requirement to keep the entire contents of this directory
1471: secret, but the recommended permissions are read/write/execute for the user,
1472: and not accessible by others.
1.236 jmc 1473: .Pp
1.309 jmc 1474: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1475: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1476: that can be used for logging in as this user.
1.238 jmc 1477: The format of this file is described in the
1478: .Xr sshd 8
1479: manual page.
1480: This file is not highly sensitive, but the recommended
1481: permissions are read/write for the user, and not accessible by others.
1482: .Pp
1.309 jmc 1483: .It Pa ~/.ssh/config
1.238 jmc 1484: This is the per-user configuration file.
1485: The file format and configuration options are described in
1486: .Xr ssh_config 5 .
1487: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1488: read/write for the user, and not writable by others.
1.238 jmc 1489: .Pp
1.309 jmc 1490: .It Pa ~/.ssh/environment
1.239 jmc 1491: Contains additional definitions for environment variables; see
1492: .Sx ENVIRONMENT ,
1.238 jmc 1493: above.
1494: .Pp
1.309 jmc 1495: .It Pa ~/.ssh/id_dsa
1.310 djm 1496: .It Pa ~/.ssh/id_ecdsa
1.404 naddy 1497: .It Pa ~/.ssh/id_ecdsa_sk
1.343 naddy 1498: .It Pa ~/.ssh/id_ed25519
1.406 ! naddy 1499: .It Pa ~/.ssh/id_ed25519_sk
1.309 jmc 1500: .It Pa ~/.ssh/id_rsa
1.238 jmc 1501: Contains the private key for authentication.
1502: These files
1503: contain sensitive data and should be readable by the user but not
1504: accessible by others (read/write/execute).
1505: .Nm
1506: will simply ignore a private key file if it is accessible by others.
1507: It is possible to specify a passphrase when
1508: generating the key which will be used to encrypt the
1.389 djm 1509: sensitive part of this file using AES-128.
1.238 jmc 1510: .Pp
1.309 jmc 1511: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1512: .It Pa ~/.ssh/id_ecdsa.pub
1.404 naddy 1513: .It Pa ~/.ssh/id_ecdsa_sk.pub
1.343 naddy 1514: .It Pa ~/.ssh/id_ed25519.pub
1.406 ! naddy 1515: .It Pa ~/.ssh/id_ed25519_sk.pub
1.309 jmc 1516: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1517: Contains the public key for authentication.
1518: These files are not
1519: sensitive and can (but need not) be readable by anyone.
1520: .Pp
1.309 jmc 1521: .It Pa ~/.ssh/known_hosts
1.244 jmc 1522: Contains a list of host keys for all hosts the user has logged into
1523: that are not already in the systemwide list of known host keys.
1.238 jmc 1524: See
1.244 jmc 1525: .Xr sshd 8
1526: for further details of the format of this file.
1.238 jmc 1527: .Pp
1.309 jmc 1528: .It Pa ~/.ssh/rc
1.238 jmc 1529: Commands in this file are executed by
1530: .Nm
1.245 jmc 1531: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1532: started.
1533: See the
1534: .Xr sshd 8
1535: manual page for more information.
1536: .Pp
1.309 jmc 1537: .It Pa /etc/hosts.equiv
1.240 jmc 1538: This file is for host-based authentication (see above).
1539: It should only be writable by root.
1.236 jmc 1540: .Pp
1.309 jmc 1541: .It Pa /etc/shosts.equiv
1.240 jmc 1542: This file is used in exactly the same way as
1543: .Pa hosts.equiv ,
1544: but allows host-based authentication without permitting login with
1545: rlogin/rsh.
1.236 jmc 1546: .Pp
1.238 jmc 1547: .It Pa /etc/ssh/ssh_config
1548: Systemwide configuration file.
1549: The file format and configuration options are described in
1550: .Xr ssh_config 5 .
1551: .Pp
1.309 jmc 1552: .It Pa /etc/ssh/ssh_host_key
1553: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1554: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1555: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1556: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1557: These files contain the private parts of the host keys
1.245 jmc 1558: and are used for host-based authentication.
1.238 jmc 1559: .Pp
1.309 jmc 1560: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1561: Systemwide list of known host keys.
1562: This file should be prepared by the
1563: system administrator to contain the public host keys of all machines in the
1564: organization.
1.244 jmc 1565: It should be world-readable.
1566: See
1.238 jmc 1567: .Xr sshd 8
1.244 jmc 1568: for further details of the format of this file.
1.236 jmc 1569: .Pp
1.309 jmc 1570: .It Pa /etc/ssh/sshrc
1.1 deraadt 1571: Commands in this file are executed by
1.2 deraadt 1572: .Nm
1.245 jmc 1573: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1574: See the
1.2 deraadt 1575: .Xr sshd 8
1.1 deraadt 1576: manual page for more information.
1.58 itojun 1577: .El
1.312 jmc 1578: .Sh EXIT STATUS
1579: .Nm
1580: exits with the exit status of the remote command or with 255
1581: if an error occurred.
1.2 deraadt 1582: .Sh SEE ALSO
1583: .Xr scp 1 ,
1.83 djm 1584: .Xr sftp 1 ,
1.2 deraadt 1585: .Xr ssh-add 1 ,
1586: .Xr ssh-agent 1 ,
1587: .Xr ssh-keygen 1 ,
1.242 jmc 1588: .Xr ssh-keyscan 1 ,
1.250 jmc 1589: .Xr tun 4 ,
1.159 stevesk 1590: .Xr ssh_config 5 ,
1.160 naddy 1591: .Xr ssh-keysign 8 ,
1.87 itojun 1592: .Xr sshd 8
1.329 jmc 1593: .Sh STANDARDS
1.106 markus 1594: .Rs
1.329 jmc 1595: .%A S. Lehtinen
1596: .%A C. Lonvick
1597: .%D January 2006
1.256 jmc 1598: .%R RFC 4250
1.329 jmc 1599: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1600: .Re
1.329 jmc 1601: .Pp
1.256 jmc 1602: .Rs
1.329 jmc 1603: .%A T. Ylonen
1604: .%A C. Lonvick
1605: .%D January 2006
1.256 jmc 1606: .%R RFC 4251
1.329 jmc 1607: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1608: .Re
1.329 jmc 1609: .Pp
1.256 jmc 1610: .Rs
1.329 jmc 1611: .%A T. Ylonen
1612: .%A C. Lonvick
1613: .%D January 2006
1.256 jmc 1614: .%R RFC 4252
1.329 jmc 1615: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1616: .Re
1.329 jmc 1617: .Pp
1.256 jmc 1618: .Rs
1.329 jmc 1619: .%A T. Ylonen
1620: .%A C. Lonvick
1621: .%D January 2006
1.256 jmc 1622: .%R RFC 4253
1.329 jmc 1623: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1624: .Re
1.329 jmc 1625: .Pp
1.256 jmc 1626: .Rs
1.329 jmc 1627: .%A T. Ylonen
1628: .%A C. Lonvick
1629: .%D January 2006
1.256 jmc 1630: .%R RFC 4254
1.329 jmc 1631: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1632: .Re
1.329 jmc 1633: .Pp
1.256 jmc 1634: .Rs
1.329 jmc 1635: .%A J. Schlyter
1636: .%A W. Griffin
1637: .%D January 2006
1.256 jmc 1638: .%R RFC 4255
1.329 jmc 1639: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1640: .Re
1.329 jmc 1641: .Pp
1.256 jmc 1642: .Rs
1.329 jmc 1643: .%A F. Cusack
1644: .%A M. Forssen
1645: .%D January 2006
1.256 jmc 1646: .%R RFC 4256
1.329 jmc 1647: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1648: .Re
1.329 jmc 1649: .Pp
1.256 jmc 1650: .Rs
1.329 jmc 1651: .%A J. Galbraith
1652: .%A P. Remaker
1653: .%D January 2006
1.256 jmc 1654: .%R RFC 4335
1.329 jmc 1655: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1656: .Re
1.329 jmc 1657: .Pp
1.256 jmc 1658: .Rs
1.329 jmc 1659: .%A M. Bellare
1660: .%A T. Kohno
1661: .%A C. Namprempre
1662: .%D January 2006
1.256 jmc 1663: .%R RFC 4344
1.329 jmc 1664: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1665: .Re
1.329 jmc 1666: .Pp
1.256 jmc 1667: .Rs
1.329 jmc 1668: .%A B. Harris
1669: .%D January 2006
1.256 jmc 1670: .%R RFC 4345
1.329 jmc 1671: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1672: .Re
1.329 jmc 1673: .Pp
1.258 djm 1674: .Rs
1.329 jmc 1675: .%A M. Friedl
1676: .%A N. Provos
1677: .%A W. Simpson
1678: .%D March 2006
1.258 djm 1679: .%R RFC 4419
1.329 jmc 1680: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1681: .Re
1.329 jmc 1682: .Pp
1.266 markus 1683: .Rs
1.329 jmc 1684: .%A J. Galbraith
1685: .%A R. Thayer
1686: .%D November 2006
1.266 markus 1687: .%R RFC 4716
1.329 jmc 1688: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1689: .Re
1.329 jmc 1690: .Pp
1.313 djm 1691: .Rs
1.329 jmc 1692: .%A D. Stebila
1693: .%A J. Green
1694: .%D December 2009
1.313 djm 1695: .%R RFC 5656
1.329 jmc 1696: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1697: .Re
1.329 jmc 1698: .Pp
1.274 grunk 1699: .Rs
1700: .%A A. Perrig
1701: .%A D. Song
1702: .%D 1999
1.329 jmc 1703: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1704: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1705: .Re
1.173 jmc 1706: .Sh AUTHORS
1707: OpenSSH is a derivative of the original and free
1708: ssh 1.2.12 release by Tatu Ylonen.
1709: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1710: Theo de Raadt and Dug Song
1711: removed many bugs, re-added newer features and
1712: created OpenSSH.
1713: Markus Friedl contributed the support for SSH
1714: protocol versions 1.5 and 2.0.