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