Annotation of src/usr.bin/ssh/ssh.1, Revision 1.410
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: .\"
1.410 ! dtucker 36: .\" $OpenBSD: ssh.1,v 1.409 2019/12/21 20:22:34 naddy Exp $
! 37: .Dd $Mdocdate: December 21 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),
1.410 ! dtucker 588: .Ar key-sig
! 589: (all key types and signature algorithms),
1.356 djm 590: .Ar protocol-version
1.398 djm 591: (supported SSH protocol versions), and
592: .Ar sig
593: (supported signature algorithms).
1.410 ! dtucker 594: Alternatively, any keyword from
! 595: .Xr ssh_config 5
! 596: or
! 597: .Xr sshd_config 5
! 598: that takes an algorithm list may be used as an alias for the corresponding
! 599: query_option.
1.360 millert 600: .Pp
1.218 jmc 601: .It Fl q
602: Quiet mode.
1.271 djm 603: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 604: .Pp
1.218 jmc 605: .It Fl R Xo
606: .Sm off
607: .Oo Ar bind_address : Oc
608: .Ar port : host : hostport
609: .Sm on
610: .Xc
1.360 millert 611: .It Fl R Xo
612: .Sm off
613: .Oo Ar bind_address : Oc
614: .Ar port : local_socket
615: .Sm on
616: .Xc
617: .It Fl R Xo
618: .Sm off
619: .Ar remote_socket : host : hostport
620: .Sm on
621: .Xc
622: .It Fl R Xo
623: .Sm off
624: .Ar remote_socket : local_socket
625: .Sm on
626: .Xc
1.384 markus 627: .It Fl R Xo
628: .Sm off
629: .Oo Ar bind_address : Oc
630: .Ar port
631: .Sm on
632: .Xc
1.360 millert 633: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 634: (server) host are to be forwarded to the local side.
635: .Pp
1.360 millert 636: This works by allocating a socket to listen to either a TCP
1.218 jmc 637: .Ar port
1.360 millert 638: or to a Unix socket on the remote side.
639: Whenever a connection is made to this port or Unix socket, the
640: connection is forwarded over the secure channel, and a connection
1.384 markus 641: is made from the local machine to either an explicit destination specified by
1.218 jmc 642: .Ar host
643: port
1.360 millert 644: .Ar hostport ,
645: or
646: .Ar local_socket ,
1.384 markus 647: or, if no explicit destination was specified,
648: .Nm
649: will act as a SOCKS 4/5 proxy and forward connections to the destinations
650: requested by the remote SOCKS client.
1.2 deraadt 651: .Pp
1.218 jmc 652: Port forwardings can also be specified in the configuration file.
653: Privileged ports can be forwarded only when
654: logging in as root on the remote machine.
1.324 jmc 655: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 656: .Pp
1.360 millert 657: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 658: interface only.
1.280 tobias 659: This may be overridden by specifying a
1.218 jmc 660: .Ar bind_address .
661: An empty
662: .Ar bind_address ,
663: or the address
664: .Ql * ,
665: indicates that the remote socket should listen on all interfaces.
666: Specifying a remote
667: .Ar bind_address
668: will only succeed if the server's
669: .Cm GatewayPorts
670: option is enabled (see
671: .Xr sshd_config 5 ) .
1.281 djm 672: .Pp
673: If the
674: .Ar port
675: argument is
1.282 djm 676: .Ql 0 ,
1.281 djm 677: the listen port will be dynamically allocated on the server and reported
678: to the client at run time.
1.305 markus 679: When used together with
680: .Ic -O forward
681: the allocated port will be printed to the standard output.
1.360 millert 682: .Pp
1.218 jmc 683: .It Fl S Ar ctl_path
1.304 jmc 684: Specifies the location of a control socket for connection sharing,
1.303 djm 685: or the string
686: .Dq none
687: to disable connection sharing.
1.218 jmc 688: Refer to the description of
689: .Cm ControlPath
690: and
691: .Cm ControlMaster
692: in
693: .Xr ssh_config 5
694: for details.
1.360 millert 695: .Pp
1.218 jmc 696: .It Fl s
697: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 698: Subsystems facilitate the use of SSH
699: as a secure transport for other applications (e.g.\&
1.218 jmc 700: .Xr sftp 1 ) .
701: The subsystem is specified as the remote command.
1.360 millert 702: .Pp
1.218 jmc 703: .It Fl T
1.357 dtucker 704: Disable pseudo-terminal allocation.
1.360 millert 705: .Pp
1.218 jmc 706: .It Fl t
1.357 dtucker 707: Force pseudo-terminal allocation.
1.218 jmc 708: This can be used to execute arbitrary
709: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 710: e.g. when implementing menu services.
1.218 jmc 711: Multiple
712: .Fl t
713: options force tty allocation, even if
1.194 jakob 714: .Nm
1.218 jmc 715: has no local tty.
1.360 millert 716: .Pp
1.218 jmc 717: .It Fl V
718: Display the version number and exit.
1.360 millert 719: .Pp
1.218 jmc 720: .It Fl v
721: Verbose mode.
722: Causes
1.176 jmc 723: .Nm
1.218 jmc 724: to print debugging messages about its progress.
725: This is helpful in
726: debugging connection, authentication, and configuration problems.
727: Multiple
728: .Fl v
729: options increase the verbosity.
730: The maximum is 3.
1.360 millert 731: .Pp
1.290 dtucker 732: .It Fl W Ar host : Ns Ar port
733: Requests that standard input and output on the client be forwarded to
734: .Ar host
735: on
736: .Ar port
737: over the secure channel.
738: Implies
739: .Fl N ,
740: .Fl T ,
741: .Cm ExitOnForwardFailure
742: and
1.373 jmc 743: .Cm ClearAllForwardings ,
744: though these can be overridden in the configuration file or using
1.372 dtucker 745: .Fl o
746: command line options.
1.360 millert 747: .Pp
1.261 stevesk 748: .It Fl w Xo
749: .Ar local_tun Ns Op : Ns Ar remote_tun
750: .Xc
751: Requests
752: tunnel
753: device forwarding with the specified
1.218 jmc 754: .Xr tun 4
1.261 stevesk 755: devices between the client
756: .Pq Ar local_tun
757: and the server
758: .Pq Ar remote_tun .
759: .Pp
1.228 jmc 760: The devices may be specified by numerical ID or the keyword
761: .Dq any ,
762: which uses the next available tunnel device.
1.261 stevesk 763: If
764: .Ar remote_tun
765: is not specified, it defaults to
766: .Dq any .
1.228 jmc 767: See also the
1.218 jmc 768: .Cm Tunnel
1.261 stevesk 769: and
770: .Cm TunnelDevice
771: directives in
1.218 jmc 772: .Xr ssh_config 5 .
1.393 djm 773: .Pp
1.261 stevesk 774: If the
775: .Cm Tunnel
1.393 djm 776: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 777: .Dq point-to-point .
1.393 djm 778: If a different
779: .Cm Tunnel
780: forwarding mode it desired, then it should be specified before
781: .Fl w .
1.360 millert 782: .Pp
1.218 jmc 783: .It Fl X
784: Enables X11 forwarding.
1.54 markus 785: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 786: .Pp
1.218 jmc 787: X11 forwarding should be enabled with caution.
1.168 jmc 788: Users with the ability to bypass file permissions on the remote host
1.218 jmc 789: (for the user's X authorization database)
790: can access the local X11 display through the forwarded connection.
791: An attacker may then be able to perform activities such as keystroke monitoring.
792: .Pp
793: For this reason, X11 forwarding is subjected to X11 SECURITY extension
794: restrictions by default.
795: Please refer to the
796: .Nm
797: .Fl Y
798: option and the
799: .Cm ForwardX11Trusted
800: directive in
801: .Xr ssh_config 5
802: for more information.
1.360 millert 803: .Pp
1.218 jmc 804: .It Fl x
805: Disables X11 forwarding.
1.360 millert 806: .Pp
1.218 jmc 807: .It Fl Y
808: Enables trusted X11 forwarding.
809: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
810: controls.
1.360 millert 811: .Pp
1.278 djm 812: .It Fl y
813: Send log information using the
814: .Xr syslog 3
815: system module.
816: By default this information is sent to stderr.
1.218 jmc 817: .El
1.224 jmc 818: .Pp
819: .Nm
820: may additionally obtain configuration data from
821: a per-user configuration file and a system-wide configuration file.
822: The file format and configuration options are described in
823: .Xr ssh_config 5 .
1.222 jmc 824: .Sh AUTHENTICATION
1.377 djm 825: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 826: .Pp
827: The methods available for authentication are:
1.260 jmc 828: GSSAPI-based authentication,
1.222 jmc 829: host-based authentication,
830: public key authentication,
831: challenge-response authentication,
832: and password authentication.
833: Authentication methods are tried in the order specified above,
1.369 jmc 834: though
835: .Cm PreferredAuthentications
836: can be used to change the default order.
1.222 jmc 837: .Pp
838: Host-based authentication works as follows:
1.218 jmc 839: If the machine the user logs in from is listed in
840: .Pa /etc/hosts.equiv
841: or
842: .Pa /etc/shosts.equiv
843: on the remote machine, and the user names are
844: the same on both sides, or if the files
845: .Pa ~/.rhosts
846: or
847: .Pa ~/.shosts
848: exist in the user's home directory on the
849: remote machine and contain a line containing the name of the client
850: machine and the name of the user on that machine, the user is
1.222 jmc 851: considered for login.
852: Additionally, the server
853: .Em must
854: be able to verify the client's
855: host key (see the description of
1.218 jmc 856: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 857: and
1.222 jmc 858: .Pa ~/.ssh/known_hosts ,
859: below)
860: for login to be permitted.
1.218 jmc 861: This authentication method closes security holes due to IP
1.222 jmc 862: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 863: [Note to the administrator:
864: .Pa /etc/hosts.equiv ,
865: .Pa ~/.rhosts ,
866: and the rlogin/rsh protocol in general, are inherently insecure and should be
867: disabled if security is desired.]
1.189 dtucker 868: .Pp
1.222 jmc 869: Public key authentication works as follows:
870: The scheme is based on public-key cryptography,
871: using cryptosystems
872: where encryption and decryption are done using separate keys,
873: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 874: The idea is that each user creates a public/private
875: key pair for authentication purposes.
876: The server knows the public key, and only the user knows the private key.
1.222 jmc 877: .Nm
878: implements public key authentication protocol automatically,
1.349 sobrado 879: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 880: The HISTORY section of
1.222 jmc 881: .Xr ssl 8
1.311 jmc 882: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 883: .Pp
1.218 jmc 884: The file
885: .Pa ~/.ssh/authorized_keys
886: lists the public keys that are permitted for logging in.
887: When the user logs in, the
1.2 deraadt 888: .Nm
1.218 jmc 889: program tells the server which key pair it would like to use for
890: authentication.
1.222 jmc 891: The client proves that it has access to the private key
892: and the server checks that the corresponding public key
893: is authorized to accept the account.
1.383 djm 894: .Pp
895: The server may inform the client of errors that prevented public key
896: authentication from succeeding after authentication completes using a
897: different method.
898: These may be viewed by increasing the
899: .Cm LogLevel
900: to
901: .Cm DEBUG
902: or higher (e.g. by using the
903: .Fl v
904: flag).
1.218 jmc 905: .Pp
1.222 jmc 906: The user creates his/her key pair by running
1.218 jmc 907: .Xr ssh-keygen 1 .
908: This stores the private key in
1.222 jmc 909: .Pa ~/.ssh/id_dsa
1.369 jmc 910: (DSA),
1.310 djm 911: .Pa ~/.ssh/id_ecdsa
1.369 jmc 912: (ECDSA),
1.404 naddy 913: .Pa ~/.ssh/id_ecdsa_sk
1.409 naddy 914: (authenticator-hosted ECDSA),
1.343 naddy 915: .Pa ~/.ssh/id_ed25519
1.369 jmc 916: (Ed25519),
1.406 naddy 917: .Pa ~/.ssh/id_ed25519_sk
1.409 naddy 918: (authenticator-hosted Ed25519),
1.222 jmc 919: or
920: .Pa ~/.ssh/id_rsa
1.369 jmc 921: (RSA)
1.218 jmc 922: and stores the public key in
1.222 jmc 923: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 924: (DSA),
1.310 djm 925: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 926: (ECDSA),
1.404 naddy 927: .Pa ~/.ssh/id_ecdsa_sk.pub
1.409 naddy 928: (authenticator-hosted ECDSA),
1.343 naddy 929: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 930: (Ed25519),
1.406 naddy 931: .Pa ~/.ssh/id_ed25519_sk.pub
1.409 naddy 932: (authenticator-hosted Ed25519),
1.222 jmc 933: or
934: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 935: (RSA)
1.218 jmc 936: in the user's home directory.
1.222 jmc 937: The user should then copy the public key
1.218 jmc 938: to
939: .Pa ~/.ssh/authorized_keys
1.222 jmc 940: in his/her home directory on the remote machine.
941: The
1.218 jmc 942: .Pa authorized_keys
943: file corresponds to the conventional
944: .Pa ~/.rhosts
945: file, and has one key
1.222 jmc 946: per line, though the lines can be very long.
1.218 jmc 947: After this, the user can log in without giving the password.
948: .Pp
1.301 jmc 949: A variation on public key authentication
950: is available in the form of certificate authentication:
951: instead of a set of public/private keys,
952: signed certificates are used.
953: This has the advantage that a single trusted certification authority
954: can be used in place of many public/private keys.
1.333 jmc 955: See the CERTIFICATES section of
1.301 jmc 956: .Xr ssh-keygen 1
957: for more information.
958: .Pp
959: The most convenient way to use public key or certificate authentication
960: may be with an authentication agent.
1.218 jmc 961: See
962: .Xr ssh-agent 1
1.366 jcs 963: and (optionally) the
964: .Cm AddKeysToAgent
965: directive in
966: .Xr ssh_config 5
1.218 jmc 967: for more information.
968: .Pp
1.222 jmc 969: Challenge-response authentication works as follows:
970: The server sends an arbitrary
971: .Qq challenge
972: text, and prompts for a response.
973: Examples of challenge-response authentication include
1.335 jmc 974: .Bx
975: Authentication (see
1.222 jmc 976: .Xr login.conf 5 )
1.335 jmc 977: and PAM (some
978: .Pf non- Ox
979: systems).
1.222 jmc 980: .Pp
981: Finally, if other authentication methods fail,
1.218 jmc 982: .Nm
983: prompts the user for a password.
984: The password is sent to the remote
985: host for checking; however, since all communications are encrypted,
986: the password cannot be seen by someone listening on the network.
1.232 jmc 987: .Pp
988: .Nm
989: automatically maintains and checks a database containing
990: identification for all hosts it has ever been used with.
991: Host keys are stored in
992: .Pa ~/.ssh/known_hosts
993: in the user's home directory.
994: Additionally, the file
995: .Pa /etc/ssh/ssh_known_hosts
996: is automatically checked for known hosts.
997: Any new hosts are automatically added to the user's file.
998: If a host's identification ever changes,
999: .Nm
1000: warns about this and disables password authentication to prevent
1001: server spoofing or man-in-the-middle attacks,
1002: which could otherwise be used to circumvent the encryption.
1003: The
1004: .Cm StrictHostKeyChecking
1005: option can be used to control logins to machines whose
1006: host key is not known or has changed.
1007: .Pp
1.218 jmc 1008: When the user's identity has been accepted by the server, the server
1.357 dtucker 1009: either executes the given command in a non-interactive session or,
1010: if no command has been specified, logs into the machine and gives
1011: the user a normal shell as an interactive session.
1.218 jmc 1012: All communication with
1013: the remote command or shell will be automatically encrypted.
1014: .Pp
1.357 dtucker 1015: If an interactive session is requested
1016: .Nm
1017: by default will only request a pseudo-terminal (pty) for interactive
1018: sessions when the client has one.
1019: The flags
1020: .Fl T
1021: and
1022: .Fl t
1023: can be used to override this behaviour.
1024: .Pp
1025: If a pseudo-terminal has been allocated the
1.218 jmc 1026: user may use the escape characters noted below.
1027: .Pp
1.357 dtucker 1028: If no pseudo-terminal has been allocated,
1.218 jmc 1029: the session is transparent and can be used to reliably transfer binary data.
1030: On most systems, setting the escape character to
1031: .Dq none
1032: will also make the session transparent even if a tty is used.
1033: .Pp
1034: The session terminates when the command or shell on the remote
1.247 jmc 1035: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1036: .Sh ESCAPE CHARACTERS
1.218 jmc 1037: When a pseudo-terminal has been requested,
1.2 deraadt 1038: .Nm
1.218 jmc 1039: supports a number of functions through the use of an escape character.
1040: .Pp
1041: A single tilde character can be sent as
1042: .Ic ~~
1043: or by following the tilde by a character other than those described below.
1044: The escape character must always follow a newline to be interpreted as
1045: special.
1046: The escape character can be changed in configuration files using the
1047: .Cm EscapeChar
1048: configuration directive or on the command line by the
1049: .Fl e
1050: option.
1051: .Pp
1052: The supported escapes (assuming the default
1053: .Ql ~ )
1054: are:
1055: .Bl -tag -width Ds
1056: .It Cm ~.
1057: Disconnect.
1058: .It Cm ~^Z
1059: Background
1.234 jmc 1060: .Nm .
1.218 jmc 1061: .It Cm ~#
1062: List forwarded connections.
1063: .It Cm ~&
1064: Background
1.2 deraadt 1065: .Nm
1.218 jmc 1066: at logout when waiting for forwarded connection / X11 sessions to terminate.
1067: .It Cm ~?
1068: Display a list of escape characters.
1069: .It Cm ~B
1070: Send a BREAK to the remote system
1.369 jmc 1071: (only useful if the peer supports it).
1.218 jmc 1072: .It Cm ~C
1073: Open command line.
1074: Currently this allows the addition of port forwardings using the
1.279 stevesk 1075: .Fl L ,
1076: .Fl R
1.218 jmc 1077: and
1.279 stevesk 1078: .Fl D
1.225 jmc 1079: options (see above).
1.322 markus 1080: It also allows the cancellation of existing port-forwardings
1081: with
1.262 stevesk 1082: .Sm off
1.322 markus 1083: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1084: .Sm on
1.322 markus 1085: for local,
1086: .Sm off
1087: .Fl KR Oo Ar bind_address : Oc Ar port
1088: .Sm on
1089: for remote and
1090: .Sm off
1091: .Fl KD Oo Ar bind_address : Oc Ar port
1092: .Sm on
1093: for dynamic port-forwardings.
1.218 jmc 1094: .Ic !\& Ns Ar command
1095: allows the user to execute a local command if the
1096: .Ic PermitLocalCommand
1097: option is enabled in
1.176 jmc 1098: .Xr ssh_config 5 .
1.218 jmc 1099: Basic help is available, using the
1100: .Fl h
1101: option.
1102: .It Cm ~R
1103: Request rekeying of the connection
1.369 jmc 1104: (only useful if the peer supports it).
1.327 dtucker 1105: .It Cm ~V
1106: Decrease the verbosity
1107: .Pq Ic LogLevel
1108: when errors are being written to stderr.
1109: .It Cm ~v
1.328 jmc 1110: Increase the verbosity
1.327 dtucker 1111: .Pq Ic LogLevel
1112: when errors are being written to stderr.
1.176 jmc 1113: .El
1.246 jmc 1114: .Sh TCP FORWARDING
1.402 jmc 1115: Forwarding of arbitrary TCP connections over a secure channel
1116: can be specified either on the command line or in a configuration file.
1.246 jmc 1117: One possible application of TCP forwarding is a secure connection to a
1118: mail server; another is going through firewalls.
1119: .Pp
1.402 jmc 1120: In the example below, we look at encrypting communication for an IRC client,
1121: even though the IRC server it connects to does not directly
1122: support encrypted communication.
1.246 jmc 1123: This works as follows:
1124: the user connects to the remote host using
1125: .Nm ,
1.402 jmc 1126: specifying the ports to be used to forward the connection.
1127: After that it is possible to start the program locally,
1.246 jmc 1128: and
1129: .Nm
1.402 jmc 1130: will encrypt and forward the connection to the remote server.
1.246 jmc 1131: .Pp
1.402 jmc 1132: The following example tunnels an IRC session from the client
1133: to an IRC server at
1.246 jmc 1134: .Dq server.example.com ,
1135: joining channel
1136: .Dq #users ,
1137: nickname
1138: .Dq pinky ,
1.402 jmc 1139: using the standard IRC port, 6667:
1140: .Bd -literal -offset 4n
1141: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1142: $ irc -c '#users' pinky IRC/127.0.0.1
1143: .Ed
1.246 jmc 1144: .Pp
1145: The
1146: .Fl f
1147: option backgrounds
1148: .Nm
1149: and the remote command
1150: .Dq sleep 10
1151: is specified to allow an amount of time
1152: (10 seconds, in the example)
1.402 jmc 1153: to start the program which is going to use the tunnel.
1.246 jmc 1154: If no connections are made within the time specified,
1155: .Nm
1156: will exit.
1157: .Sh X11 FORWARDING
1.218 jmc 1158: If the
1159: .Cm ForwardX11
1160: variable is set to
1161: .Dq yes
1162: (or see the description of the
1.227 jmc 1163: .Fl X ,
1164: .Fl x ,
1.218 jmc 1165: and
1.227 jmc 1166: .Fl Y
1.226 jmc 1167: options above)
1.218 jmc 1168: and the user is using X11 (the
1169: .Ev DISPLAY
1170: environment variable is set), the connection to the X11 display is
1171: automatically forwarded to the remote side in such a way that any X11
1172: programs started from the shell (or command) will go through the
1173: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1174: from the local machine.
1.218 jmc 1175: The user should not manually set
1176: .Ev DISPLAY .
1177: Forwarding of X11 connections can be
1178: configured on the command line or in configuration files.
1179: .Pp
1180: The
1181: .Ev DISPLAY
1182: value set by
1183: .Nm
1184: will point to the server machine, but with a display number greater than zero.
1185: This is normal, and happens because
1186: .Nm
1187: creates a
1188: .Dq proxy
1189: X server on the server machine for forwarding the
1190: connections over the encrypted channel.
1.200 djm 1191: .Pp
1.218 jmc 1192: .Nm
1193: will also automatically set up Xauthority data on the server machine.
1194: For this purpose, it will generate a random authorization cookie,
1195: store it in Xauthority on the server, and verify that any forwarded
1196: connections carry this cookie and replace it by the real cookie when
1197: the connection is opened.
1198: The real authentication cookie is never
1199: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1200: .Pp
1.218 jmc 1201: If the
1202: .Cm ForwardAgent
1203: variable is set to
1204: .Dq yes
1205: (or see the description of the
1206: .Fl A
1.191 djm 1207: and
1.218 jmc 1208: .Fl a
1.226 jmc 1209: options above) and
1.218 jmc 1210: the user is using an authentication agent, the connection to the agent
1211: is automatically forwarded to the remote side.
1.252 jmc 1212: .Sh VERIFYING HOST KEYS
1213: When connecting to a server for the first time,
1214: a fingerprint of the server's public key is presented to the user
1215: (unless the option
1216: .Cm StrictHostKeyChecking
1217: has been disabled).
1218: Fingerprints can be determined using
1219: .Xr ssh-keygen 1 :
1220: .Pp
1221: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1222: .Pp
1.274 grunk 1223: If the fingerprint is already known, it can be matched
1224: and the key can be accepted or rejected.
1.358 djm 1225: If only legacy (MD5) fingerprints for the server are available, the
1226: .Xr ssh-keygen 1
1227: .Fl E
1228: option may be used to downgrade the fingerprint algorithm to match.
1229: .Pp
1.274 grunk 1230: Because of the difficulty of comparing host keys
1.352 djm 1231: just by looking at fingerprint strings,
1.274 grunk 1232: there is also support to compare host keys visually,
1233: using
1234: .Em random art .
1235: By setting the
1.275 grunk 1236: .Cm VisualHostKey
1.274 grunk 1237: option to
1.275 grunk 1238: .Dq yes ,
1.274 grunk 1239: a small ASCII graphic gets displayed on every login to a server, no matter
1240: if the session itself is interactive or not.
1241: By learning the pattern a known server produces, a user can easily
1242: find out that the host key has changed when a completely different pattern
1243: is displayed.
1244: Because these patterns are not unambiguous however, a pattern that looks
1245: similar to the pattern remembered only gives a good probability that the
1246: host key is the same, not guaranteed proof.
1247: .Pp
1248: To get a listing of the fingerprints along with their random art for
1249: all known hosts, the following command line can be used:
1250: .Pp
1251: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1252: .Pp
1.252 jmc 1253: If the fingerprint is unknown,
1254: an alternative method of verification is available:
1255: SSH fingerprints verified by DNS.
1256: An additional resource record (RR),
1257: SSHFP,
1258: is added to a zonefile
1259: and the connecting client is able to match the fingerprint
1260: with that of the key presented.
1261: .Pp
1262: In this example, we are connecting a client to a server,
1263: .Dq host.example.com .
1264: The SSHFP resource records should first be added to the zonefile for
1265: host.example.com:
1266: .Bd -literal -offset indent
1.259 jakob 1267: $ ssh-keygen -r host.example.com.
1.252 jmc 1268: .Ed
1269: .Pp
1270: The output lines will have to be added to the zonefile.
1271: To check that the zone is answering fingerprint queries:
1272: .Pp
1273: .Dl $ dig -t SSHFP host.example.com
1274: .Pp
1275: Finally the client connects:
1276: .Bd -literal -offset indent
1277: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1278: [...]
1279: Matching host key fingerprint found in DNS.
1280: Are you sure you want to continue connecting (yes/no)?
1281: .Ed
1282: .Pp
1283: See the
1284: .Cm VerifyHostKeyDNS
1285: option in
1286: .Xr ssh_config 5
1287: for more information.
1.250 jmc 1288: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1289: .Nm
1290: contains support for Virtual Private Network (VPN) tunnelling
1291: using the
1292: .Xr tun 4
1293: network pseudo-device,
1294: allowing two networks to be joined securely.
1295: The
1296: .Xr sshd_config 5
1297: configuration option
1298: .Cm PermitTunnel
1299: controls whether the server supports this,
1300: and at what level (layer 2 or 3 traffic).
1301: .Pp
1302: The following example would connect client network 10.0.50.0/24
1.265 otto 1303: with remote network 10.0.99.0/24 using a point-to-point connection
1304: from 10.1.1.1 to 10.1.1.2,
1305: provided that the SSH server running on the gateway to the remote network,
1306: at 192.168.1.15, allows it.
1307: .Pp
1308: On the client:
1.250 jmc 1309: .Bd -literal -offset indent
1310: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1311: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1312: # route add 10.0.99.0/24 10.1.1.2
1313: .Ed
1314: .Pp
1315: On the server:
1316: .Bd -literal -offset indent
1317: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1318: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1319: .Ed
1320: .Pp
1321: Client access may be more finely tuned via the
1322: .Pa /root/.ssh/authorized_keys
1323: file (see below) and the
1324: .Cm PermitRootLogin
1325: server option.
1.255 jmc 1326: The following entry would permit connections on
1.250 jmc 1327: .Xr tun 4
1.255 jmc 1328: device 1 from user
1.250 jmc 1329: .Dq jane
1.255 jmc 1330: and on tun device 2 from user
1.250 jmc 1331: .Dq john ,
1332: if
1333: .Cm PermitRootLogin
1334: is set to
1335: .Dq forced-commands-only :
1336: .Bd -literal -offset 2n
1337: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1338: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1339: .Ed
1340: .Pp
1.264 ray 1341: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1342: it may be more suited to temporary setups,
1343: such as for wireless VPNs.
1344: More permanent VPNs are better provided by tools such as
1345: .Xr ipsecctl 8
1346: and
1347: .Xr isakmpd 8 .
1.2 deraadt 1348: .Sh ENVIRONMENT
1349: .Nm
1.1 deraadt 1350: will normally set the following environment variables:
1.237 jmc 1351: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1352: .It Ev DISPLAY
1353: The
1354: .Ev DISPLAY
1.40 aaron 1355: variable indicates the location of the X11 server.
1.44 aaron 1356: It is automatically set by
1.2 deraadt 1357: .Nm
1358: to point to a value of the form
1.233 jmc 1359: .Dq hostname:n ,
1360: where
1361: .Dq hostname
1362: indicates the host where the shell runs, and
1363: .Sq n
1364: is an integer \*(Ge 1.
1.40 aaron 1365: .Nm
1366: uses this special value to forward X11 connections over the secure
1367: channel.
1.107 markus 1368: The user should normally not set
1369: .Ev DISPLAY
1370: explicitly, as that
1.1 deraadt 1371: will render the X11 connection insecure (and will require the user to
1372: manually copy any required authorization cookies).
1.2 deraadt 1373: .It Ev HOME
1.1 deraadt 1374: Set to the path of the user's home directory.
1.2 deraadt 1375: .It Ev LOGNAME
1376: Synonym for
1.12 aaron 1377: .Ev USER ;
1378: set for compatibility with systems that use this variable.
1.2 deraadt 1379: .It Ev MAIL
1.129 stevesk 1380: Set to the path of the user's mailbox.
1.40 aaron 1381: .It Ev PATH
1.2 deraadt 1382: Set to the default
1383: .Ev PATH ,
1384: as specified when compiling
1.234 jmc 1385: .Nm .
1.118 markus 1386: .It Ev SSH_ASKPASS
1387: If
1388: .Nm
1389: needs a passphrase, it will read the passphrase from the current
1390: terminal if it was run from a terminal.
1391: If
1392: .Nm
1393: does not have a terminal associated with it but
1394: .Ev DISPLAY
1395: and
1396: .Ev SSH_ASKPASS
1397: are set, it will execute the program specified by
1398: .Ev SSH_ASKPASS
1399: and open an X11 window to read the passphrase.
1400: This is particularly useful when calling
1401: .Nm
1402: from a
1.196 jmc 1403: .Pa .xsession
1.118 markus 1404: or related script.
1405: (Note that on some machines it
1406: may be necessary to redirect the input from
1407: .Pa /dev/null
1408: to make this work.)
1.18 markus 1409: .It Ev SSH_AUTH_SOCK
1.233 jmc 1410: Identifies the path of a
1411: .Ux Ns -domain
1412: socket used to communicate with the agent.
1.166 stevesk 1413: .It Ev SSH_CONNECTION
1414: Identifies the client and server ends of the connection.
1.40 aaron 1415: The variable contains
1.233 jmc 1416: four space-separated values: client IP address, client port number,
1417: server IP address, and server port number.
1.73 markus 1418: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1419: This variable contains the original command line if a forced command
1.73 markus 1420: is executed.
1421: It can be used to extract the original arguments.
1.2 deraadt 1422: .It Ev SSH_TTY
1.1 deraadt 1423: This is set to the name of the tty (path to the device) associated
1.40 aaron 1424: with the current shell or command.
1425: If the current session has no tty,
1.1 deraadt 1426: this variable is not set.
1.387 djm 1427: .It Ev SSH_TUNNEL
1428: Optionally set by
1429: .Xr sshd 8
1430: to contain the interface names assigned if tunnel forwarding was
1431: requested by the client.
1.385 djm 1432: .It Ev SSH_USER_AUTH
1433: Optionally set by
1434: .Xr sshd 8 ,
1435: this variable may contain a pathname to a file that lists the authentication
1436: methods successfully used when the session was established, including any
1437: public keys that were used.
1.2 deraadt 1438: .It Ev TZ
1.214 jmc 1439: This variable is set to indicate the present time zone if it
1.257 jmc 1440: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1441: on to new connections).
1.2 deraadt 1442: .It Ev USER
1.1 deraadt 1443: Set to the name of the user logging in.
1.2 deraadt 1444: .El
1445: .Pp
1.44 aaron 1446: Additionally,
1.2 deraadt 1447: .Nm
1.44 aaron 1448: reads
1.207 djm 1449: .Pa ~/.ssh/environment ,
1.2 deraadt 1450: and adds lines of the format
1451: .Dq VARNAME=value
1.233 jmc 1452: to the environment if the file exists and users are allowed to
1.161 marc 1453: change their environment.
1.176 jmc 1454: For more information, see the
1.161 marc 1455: .Cm PermitUserEnvironment
1.162 stevesk 1456: option in
1.161 marc 1457: .Xr sshd_config 5 .
1.2 deraadt 1458: .Sh FILES
1.236 jmc 1459: .Bl -tag -width Ds -compact
1.309 jmc 1460: .It Pa ~/.rhosts
1.240 jmc 1461: This file is used for host-based authentication (see above).
1.92 markus 1462: On some machines this file may need to be
1.240 jmc 1463: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1464: because
1.2 deraadt 1465: .Xr sshd 8
1.40 aaron 1466: reads it as root.
1467: Additionally, this file must be owned by the user,
1468: and must not have write permissions for anyone else.
1469: The recommended
1.1 deraadt 1470: permission for most machines is read/write for the user, and not
1471: accessible by others.
1.2 deraadt 1472: .Pp
1.309 jmc 1473: .It Pa ~/.shosts
1.240 jmc 1474: This file is used in exactly the same way as
1475: .Pa .rhosts ,
1476: but allows host-based authentication without permitting login with
1477: rlogin/rsh.
1.272 mcbride 1478: .Pp
1.309 jmc 1479: .It Pa ~/.ssh/
1.272 mcbride 1480: This directory is the default location for all user-specific configuration
1481: and authentication information.
1482: There is no general requirement to keep the entire contents of this directory
1483: secret, but the recommended permissions are read/write/execute for the user,
1484: and not accessible by others.
1.236 jmc 1485: .Pp
1.309 jmc 1486: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1487: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1488: that can be used for logging in as this user.
1.238 jmc 1489: The format of this file is described in the
1490: .Xr sshd 8
1491: manual page.
1492: This file is not highly sensitive, but the recommended
1493: permissions are read/write for the user, and not accessible by others.
1494: .Pp
1.309 jmc 1495: .It Pa ~/.ssh/config
1.238 jmc 1496: This is the per-user configuration file.
1497: The file format and configuration options are described in
1498: .Xr ssh_config 5 .
1499: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1500: read/write for the user, and not writable by others.
1.238 jmc 1501: .Pp
1.309 jmc 1502: .It Pa ~/.ssh/environment
1.239 jmc 1503: Contains additional definitions for environment variables; see
1504: .Sx ENVIRONMENT ,
1.238 jmc 1505: above.
1506: .Pp
1.309 jmc 1507: .It Pa ~/.ssh/id_dsa
1.310 djm 1508: .It Pa ~/.ssh/id_ecdsa
1.404 naddy 1509: .It Pa ~/.ssh/id_ecdsa_sk
1.343 naddy 1510: .It Pa ~/.ssh/id_ed25519
1.406 naddy 1511: .It Pa ~/.ssh/id_ed25519_sk
1.309 jmc 1512: .It Pa ~/.ssh/id_rsa
1.238 jmc 1513: Contains the private key for authentication.
1514: These files
1515: contain sensitive data and should be readable by the user but not
1516: accessible by others (read/write/execute).
1517: .Nm
1518: will simply ignore a private key file if it is accessible by others.
1519: It is possible to specify a passphrase when
1520: generating the key which will be used to encrypt the
1.389 djm 1521: sensitive part of this file using AES-128.
1.238 jmc 1522: .Pp
1.309 jmc 1523: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1524: .It Pa ~/.ssh/id_ecdsa.pub
1.404 naddy 1525: .It Pa ~/.ssh/id_ecdsa_sk.pub
1.343 naddy 1526: .It Pa ~/.ssh/id_ed25519.pub
1.406 naddy 1527: .It Pa ~/.ssh/id_ed25519_sk.pub
1.309 jmc 1528: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1529: Contains the public key for authentication.
1530: These files are not
1531: sensitive and can (but need not) be readable by anyone.
1532: .Pp
1.309 jmc 1533: .It Pa ~/.ssh/known_hosts
1.244 jmc 1534: Contains a list of host keys for all hosts the user has logged into
1535: that are not already in the systemwide list of known host keys.
1.238 jmc 1536: See
1.244 jmc 1537: .Xr sshd 8
1538: for further details of the format of this file.
1.238 jmc 1539: .Pp
1.309 jmc 1540: .It Pa ~/.ssh/rc
1.238 jmc 1541: Commands in this file are executed by
1542: .Nm
1.245 jmc 1543: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1544: started.
1545: See the
1546: .Xr sshd 8
1547: manual page for more information.
1548: .Pp
1.309 jmc 1549: .It Pa /etc/hosts.equiv
1.240 jmc 1550: This file is for host-based authentication (see above).
1551: It should only be writable by root.
1.236 jmc 1552: .Pp
1.309 jmc 1553: .It Pa /etc/shosts.equiv
1.240 jmc 1554: This file is used in exactly the same way as
1555: .Pa hosts.equiv ,
1556: but allows host-based authentication without permitting login with
1557: rlogin/rsh.
1.236 jmc 1558: .Pp
1.238 jmc 1559: .It Pa /etc/ssh/ssh_config
1560: Systemwide configuration file.
1561: The file format and configuration options are described in
1562: .Xr ssh_config 5 .
1563: .Pp
1.309 jmc 1564: .It Pa /etc/ssh/ssh_host_key
1565: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1566: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1567: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1568: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1569: These files contain the private parts of the host keys
1.245 jmc 1570: and are used for host-based authentication.
1.238 jmc 1571: .Pp
1.309 jmc 1572: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1573: Systemwide list of known host keys.
1574: This file should be prepared by the
1575: system administrator to contain the public host keys of all machines in the
1576: organization.
1.244 jmc 1577: It should be world-readable.
1578: See
1.238 jmc 1579: .Xr sshd 8
1.244 jmc 1580: for further details of the format of this file.
1.236 jmc 1581: .Pp
1.309 jmc 1582: .It Pa /etc/ssh/sshrc
1.1 deraadt 1583: Commands in this file are executed by
1.2 deraadt 1584: .Nm
1.245 jmc 1585: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1586: See the
1.2 deraadt 1587: .Xr sshd 8
1.1 deraadt 1588: manual page for more information.
1.58 itojun 1589: .El
1.312 jmc 1590: .Sh EXIT STATUS
1591: .Nm
1592: exits with the exit status of the remote command or with 255
1593: if an error occurred.
1.2 deraadt 1594: .Sh SEE ALSO
1595: .Xr scp 1 ,
1.83 djm 1596: .Xr sftp 1 ,
1.2 deraadt 1597: .Xr ssh-add 1 ,
1598: .Xr ssh-agent 1 ,
1599: .Xr ssh-keygen 1 ,
1.242 jmc 1600: .Xr ssh-keyscan 1 ,
1.250 jmc 1601: .Xr tun 4 ,
1.159 stevesk 1602: .Xr ssh_config 5 ,
1.160 naddy 1603: .Xr ssh-keysign 8 ,
1.87 itojun 1604: .Xr sshd 8
1.329 jmc 1605: .Sh STANDARDS
1.106 markus 1606: .Rs
1.329 jmc 1607: .%A S. Lehtinen
1608: .%A C. Lonvick
1609: .%D January 2006
1.256 jmc 1610: .%R RFC 4250
1.329 jmc 1611: .%T The Secure Shell (SSH) Protocol Assigned Numbers
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 4251
1.329 jmc 1619: .%T The Secure Shell (SSH) Protocol Architecture
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 4252
1.329 jmc 1627: .%T The Secure Shell (SSH) Authentication 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 4253
1.329 jmc 1635: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1636: .Re
1.329 jmc 1637: .Pp
1.256 jmc 1638: .Rs
1.329 jmc 1639: .%A T. Ylonen
1640: .%A C. Lonvick
1641: .%D January 2006
1.256 jmc 1642: .%R RFC 4254
1.329 jmc 1643: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1644: .Re
1.329 jmc 1645: .Pp
1.256 jmc 1646: .Rs
1.329 jmc 1647: .%A J. Schlyter
1648: .%A W. Griffin
1649: .%D January 2006
1.256 jmc 1650: .%R RFC 4255
1.329 jmc 1651: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1652: .Re
1.329 jmc 1653: .Pp
1.256 jmc 1654: .Rs
1.329 jmc 1655: .%A F. Cusack
1656: .%A M. Forssen
1657: .%D January 2006
1.256 jmc 1658: .%R RFC 4256
1.329 jmc 1659: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1660: .Re
1.329 jmc 1661: .Pp
1.256 jmc 1662: .Rs
1.329 jmc 1663: .%A J. Galbraith
1664: .%A P. Remaker
1665: .%D January 2006
1.256 jmc 1666: .%R RFC 4335
1.329 jmc 1667: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1668: .Re
1.329 jmc 1669: .Pp
1.256 jmc 1670: .Rs
1.329 jmc 1671: .%A M. Bellare
1672: .%A T. Kohno
1673: .%A C. Namprempre
1674: .%D January 2006
1.256 jmc 1675: .%R RFC 4344
1.329 jmc 1676: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1677: .Re
1.329 jmc 1678: .Pp
1.256 jmc 1679: .Rs
1.329 jmc 1680: .%A B. Harris
1681: .%D January 2006
1.256 jmc 1682: .%R RFC 4345
1.329 jmc 1683: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1684: .Re
1.329 jmc 1685: .Pp
1.258 djm 1686: .Rs
1.329 jmc 1687: .%A M. Friedl
1688: .%A N. Provos
1689: .%A W. Simpson
1690: .%D March 2006
1.258 djm 1691: .%R RFC 4419
1.329 jmc 1692: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1693: .Re
1.329 jmc 1694: .Pp
1.266 markus 1695: .Rs
1.329 jmc 1696: .%A J. Galbraith
1697: .%A R. Thayer
1698: .%D November 2006
1.266 markus 1699: .%R RFC 4716
1.329 jmc 1700: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1701: .Re
1.329 jmc 1702: .Pp
1.313 djm 1703: .Rs
1.329 jmc 1704: .%A D. Stebila
1705: .%A J. Green
1706: .%D December 2009
1.313 djm 1707: .%R RFC 5656
1.329 jmc 1708: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1709: .Re
1.329 jmc 1710: .Pp
1.274 grunk 1711: .Rs
1712: .%A A. Perrig
1713: .%A D. Song
1714: .%D 1999
1.329 jmc 1715: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1716: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1717: .Re
1.173 jmc 1718: .Sh AUTHORS
1719: OpenSSH is a derivative of the original and free
1720: ssh 1.2.12 release by Tatu Ylonen.
1721: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1722: Theo de Raadt and Dug Song
1723: removed many bugs, re-added newer features and
1724: created OpenSSH.
1725: Markus Friedl contributed the support for SSH
1726: protocol versions 1.5 and 2.0.
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