Annotation of src/usr.bin/ssh/ssh.1, Revision 1.421
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: .\"
1.421 ! dtucker 36: .\" $OpenBSD: ssh.1,v 1.420 2021/06/25 06:20:39 dtucker Exp $
! 37: .Dd $Mdocdate: June 25 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.
1.360 millert 428: .Pp
1.218 jmc 429: .It Fl n
430: Redirects stdin from
431: .Pa /dev/null
432: (actually, prevents reading from stdin).
433: This must be used when
1.2 deraadt 434: .Nm
1.218 jmc 435: is run in the background.
436: A common trick is to use this to run X11 programs on a remote machine.
437: For example,
438: .Ic ssh -n shadows.cs.hut.fi emacs &
439: will start an emacs on shadows.cs.hut.fi, and the X11
440: connection will be automatically forwarded over an encrypted channel.
441: The
1.2 deraadt 442: .Nm
1.218 jmc 443: program will be put in the background.
444: (This does not work if
1.2 deraadt 445: .Nm
1.218 jmc 446: needs to ask for a password or passphrase; see also the
447: .Fl f
448: option.)
1.360 millert 449: .Pp
1.218 jmc 450: .It Fl O Ar ctl_cmd
451: Control an active connection multiplexing master process.
452: When the
453: .Fl O
454: option is specified, the
455: .Ar ctl_cmd
456: argument is interpreted and passed to the master process.
457: Valid commands are:
458: .Dq check
1.305 markus 459: (check that the master process is running),
460: .Dq forward
1.318 jmc 461: (request forwardings without command execution),
1.323 okan 462: .Dq cancel
463: (cancel forwardings),
1.218 jmc 464: .Dq exit
1.318 jmc 465: (request the master to exit), and
1.317 djm 466: .Dq stop
467: (request the master to stop accepting further multiplexing requests).
1.360 millert 468: .Pp
1.218 jmc 469: .It Fl o Ar option
470: Can be used to give options in the format used in the configuration file.
471: This is useful for specifying options for which there is no separate
472: command-line flag.
473: For full details of the options listed below, and their possible values, see
474: .Xr ssh_config 5 .
1.2 deraadt 475: .Pp
1.218 jmc 476: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 477: .It AddKeysToAgent
1.218 jmc 478: .It AddressFamily
479: .It BatchMode
480: .It BindAddress
1.338 djm 481: .It CanonicalDomains
1.339 djm 482: .It CanonicalizeFallbackLocal
483: .It CanonicalizeHostname
484: .It CanonicalizeMaxDots
485: .It CanonicalizePermittedCNAMEs
1.399 jmc 486: .It CASignatureAlgorithms
1.363 djm 487: .It CertificateFile
1.218 jmc 488: .It CheckHostIP
489: .It Ciphers
490: .It ClearAllForwardings
491: .It Compression
1.380 naddy 492: .It ConnectionAttempts
1.218 jmc 493: .It ConnectTimeout
494: .It ControlMaster
495: .It ControlPath
1.321 djm 496: .It ControlPersist
1.218 jmc 497: .It DynamicForward
498: .It EscapeChar
1.263 markus 499: .It ExitOnForwardFailure
1.353 jmc 500: .It FingerprintHash
1.218 jmc 501: .It ForwardAgent
502: .It ForwardX11
1.321 djm 503: .It ForwardX11Timeout
1.218 jmc 504: .It ForwardX11Trusted
505: .It GatewayPorts
506: .It GlobalKnownHostsFile
507: .It GSSAPIAuthentication
508: .It GSSAPIDelegateCredentials
509: .It HashKnownHosts
510: .It Host
1.418 naddy 511: .It HostbasedAcceptedAlgorithms
1.218 jmc 512: .It HostbasedAuthentication
513: .It HostKeyAlgorithms
514: .It HostKeyAlias
1.403 jmc 515: .It Hostname
1.374 jmc 516: .It IdentitiesOnly
1.371 markus 517: .It IdentityAgent
1.218 jmc 518: .It IdentityFile
1.316 jmc 519: .It IPQoS
1.321 djm 520: .It KbdInteractiveAuthentication
1.218 jmc 521: .It KbdInteractiveDevices
1.314 jmc 522: .It KexAlgorithms
1.415 djm 523: .It KnownHostsCommand
1.218 jmc 524: .It LocalCommand
525: .It LocalForward
526: .It LogLevel
527: .It MACs
1.337 jmc 528: .It Match
1.218 jmc 529: .It NoHostAuthenticationForLocalhost
530: .It NumberOfPasswordPrompts
531: .It PasswordAuthentication
532: .It PermitLocalCommand
1.419 markus 533: .It PermitRemoteOpen
1.291 markus 534: .It PKCS11Provider
1.218 jmc 535: .It Port
536: .It PreferredAuthentications
537: .It ProxyCommand
1.375 djm 538: .It ProxyJump
1.336 jmc 539: .It ProxyUseFdpass
1.416 dtucker 540: .It PubkeyAcceptedAlgorithms
1.218 jmc 541: .It PubkeyAuthentication
1.251 dtucker 542: .It RekeyLimit
1.382 bluhm 543: .It RemoteCommand
1.218 jmc 544: .It RemoteForward
1.319 jmc 545: .It RequestTTY
1.218 jmc 546: .It SendEnv
547: .It ServerAliveInterval
548: .It ServerAliveCountMax
1.395 jmc 549: .It SetEnv
1.347 jmc 550: .It StreamLocalBindMask
551: .It StreamLocalBindUnlink
1.218 jmc 552: .It StrictHostKeyChecking
553: .It TCPKeepAlive
554: .It Tunnel
555: .It TunnelDevice
1.354 djm 556: .It UpdateHostKeys
1.218 jmc 557: .It User
558: .It UserKnownHostsFile
559: .It VerifyHostKeyDNS
1.276 jmc 560: .It VisualHostKey
1.218 jmc 561: .It XAuthLocation
562: .El
1.360 millert 563: .Pp
1.218 jmc 564: .It Fl p Ar port
565: Port to connect to on the remote host.
566: This can be specified on a
567: per-host basis in the configuration file.
1.360 millert 568: .Pp
1.362 djm 569: .It Fl Q Ar query_option
1.332 djm 570: Queries
571: .Nm
1.341 deraadt 572: for the algorithms supported for the specified version 2.
573: The available features are:
574: .Ar cipher
1.332 djm 575: (supported symmetric ciphers),
1.341 deraadt 576: .Ar cipher-auth
1.340 djm 577: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 578: .Ar help
579: (supported query terms for use with the
580: .Fl Q
581: flag),
1.341 deraadt 582: .Ar mac
1.332 djm 583: (supported message integrity codes),
1.341 deraadt 584: .Ar kex
1.332 djm 585: (key exchange algorithms),
1.341 deraadt 586: .Ar key
1.362 djm 587: (key types),
588: .Ar key-cert
589: (certificate key types),
590: .Ar key-plain
1.398 djm 591: (non-certificate key types),
1.410 dtucker 592: .Ar key-sig
593: (all key types and signature algorithms),
1.356 djm 594: .Ar protocol-version
1.398 djm 595: (supported SSH protocol versions), and
596: .Ar sig
597: (supported signature algorithms).
1.410 dtucker 598: Alternatively, any keyword from
599: .Xr ssh_config 5
600: or
601: .Xr sshd_config 5
602: that takes an algorithm list may be used as an alias for the corresponding
603: query_option.
1.360 millert 604: .Pp
1.218 jmc 605: .It Fl q
606: Quiet mode.
1.271 djm 607: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 608: .Pp
1.218 jmc 609: .It Fl R Xo
610: .Sm off
611: .Oo Ar bind_address : Oc
612: .Ar port : host : hostport
613: .Sm on
614: .Xc
1.360 millert 615: .It Fl R Xo
616: .Sm off
617: .Oo Ar bind_address : Oc
618: .Ar port : local_socket
619: .Sm on
620: .Xc
621: .It Fl R Xo
622: .Sm off
623: .Ar remote_socket : host : hostport
624: .Sm on
625: .Xc
626: .It Fl R Xo
627: .Sm off
628: .Ar remote_socket : local_socket
629: .Sm on
630: .Xc
1.384 markus 631: .It Fl R Xo
632: .Sm off
633: .Oo Ar bind_address : Oc
634: .Ar port
635: .Sm on
636: .Xc
1.360 millert 637: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 638: (server) host are to be forwarded to the local side.
639: .Pp
1.360 millert 640: This works by allocating a socket to listen to either a TCP
1.218 jmc 641: .Ar port
1.360 millert 642: or to a Unix socket on the remote side.
643: Whenever a connection is made to this port or Unix socket, the
644: connection is forwarded over the secure channel, and a connection
1.384 markus 645: is made from the local machine to either an explicit destination specified by
1.218 jmc 646: .Ar host
647: port
1.360 millert 648: .Ar hostport ,
649: or
650: .Ar local_socket ,
1.384 markus 651: or, if no explicit destination was specified,
652: .Nm
653: will act as a SOCKS 4/5 proxy and forward connections to the destinations
654: requested by the remote SOCKS client.
1.2 deraadt 655: .Pp
1.218 jmc 656: Port forwardings can also be specified in the configuration file.
657: Privileged ports can be forwarded only when
658: logging in as root on the remote machine.
1.324 jmc 659: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 660: .Pp
1.360 millert 661: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 662: interface only.
1.280 tobias 663: This may be overridden by specifying a
1.218 jmc 664: .Ar bind_address .
665: An empty
666: .Ar bind_address ,
667: or the address
668: .Ql * ,
669: indicates that the remote socket should listen on all interfaces.
670: Specifying a remote
671: .Ar bind_address
672: will only succeed if the server's
673: .Cm GatewayPorts
674: option is enabled (see
675: .Xr sshd_config 5 ) .
1.281 djm 676: .Pp
677: If the
678: .Ar port
679: argument is
1.282 djm 680: .Ql 0 ,
1.281 djm 681: the listen port will be dynamically allocated on the server and reported
682: to the client at run time.
1.305 markus 683: When used together with
684: .Ic -O forward
685: the allocated port will be printed to the standard output.
1.360 millert 686: .Pp
1.218 jmc 687: .It Fl S Ar ctl_path
1.304 jmc 688: Specifies the location of a control socket for connection sharing,
1.303 djm 689: or the string
690: .Dq none
691: to disable connection sharing.
1.218 jmc 692: Refer to the description of
693: .Cm ControlPath
694: and
695: .Cm ControlMaster
696: in
697: .Xr ssh_config 5
698: for details.
1.360 millert 699: .Pp
1.218 jmc 700: .It Fl s
701: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 702: Subsystems facilitate the use of SSH
703: as a secure transport for other applications (e.g.\&
1.218 jmc 704: .Xr sftp 1 ) .
705: The subsystem is specified as the remote command.
1.360 millert 706: .Pp
1.218 jmc 707: .It Fl T
1.357 dtucker 708: Disable pseudo-terminal allocation.
1.360 millert 709: .Pp
1.218 jmc 710: .It Fl t
1.357 dtucker 711: Force pseudo-terminal allocation.
1.218 jmc 712: This can be used to execute arbitrary
713: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 714: e.g. when implementing menu services.
1.218 jmc 715: Multiple
716: .Fl t
717: options force tty allocation, even if
1.194 jakob 718: .Nm
1.218 jmc 719: has no local tty.
1.360 millert 720: .Pp
1.218 jmc 721: .It Fl V
722: Display the version number and exit.
1.360 millert 723: .Pp
1.218 jmc 724: .It Fl v
725: Verbose mode.
726: Causes
1.176 jmc 727: .Nm
1.218 jmc 728: to print debugging messages about its progress.
729: This is helpful in
730: debugging connection, authentication, and configuration problems.
731: Multiple
732: .Fl v
733: options increase the verbosity.
734: The maximum is 3.
1.360 millert 735: .Pp
1.290 dtucker 736: .It Fl W Ar host : Ns Ar port
737: Requests that standard input and output on the client be forwarded to
738: .Ar host
739: on
740: .Ar port
741: over the secure channel.
742: Implies
743: .Fl N ,
744: .Fl T ,
745: .Cm ExitOnForwardFailure
746: and
1.373 jmc 747: .Cm ClearAllForwardings ,
748: though these can be overridden in the configuration file or using
1.372 dtucker 749: .Fl o
750: command line options.
1.360 millert 751: .Pp
1.261 stevesk 752: .It Fl w Xo
753: .Ar local_tun Ns Op : Ns Ar remote_tun
754: .Xc
755: Requests
756: tunnel
757: device forwarding with the specified
1.218 jmc 758: .Xr tun 4
1.261 stevesk 759: devices between the client
760: .Pq Ar local_tun
761: and the server
762: .Pq Ar remote_tun .
763: .Pp
1.228 jmc 764: The devices may be specified by numerical ID or the keyword
765: .Dq any ,
766: which uses the next available tunnel device.
1.261 stevesk 767: If
768: .Ar remote_tun
769: is not specified, it defaults to
770: .Dq any .
1.228 jmc 771: See also the
1.218 jmc 772: .Cm Tunnel
1.261 stevesk 773: and
774: .Cm TunnelDevice
775: directives in
1.218 jmc 776: .Xr ssh_config 5 .
1.393 djm 777: .Pp
1.261 stevesk 778: If the
779: .Cm Tunnel
1.393 djm 780: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 781: .Dq point-to-point .
1.393 djm 782: If a different
783: .Cm Tunnel
784: forwarding mode it desired, then it should be specified before
785: .Fl w .
1.360 millert 786: .Pp
1.218 jmc 787: .It Fl X
788: Enables X11 forwarding.
1.54 markus 789: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 790: .Pp
1.218 jmc 791: X11 forwarding should be enabled with caution.
1.168 jmc 792: Users with the ability to bypass file permissions on the remote host
1.218 jmc 793: (for the user's X authorization database)
794: can access the local X11 display through the forwarded connection.
795: An attacker may then be able to perform activities such as keystroke monitoring.
796: .Pp
797: For this reason, X11 forwarding is subjected to X11 SECURITY extension
798: restrictions by default.
799: Please refer to the
800: .Nm
801: .Fl Y
802: option and the
803: .Cm ForwardX11Trusted
804: directive in
805: .Xr ssh_config 5
806: for more information.
1.360 millert 807: .Pp
1.218 jmc 808: .It Fl x
809: Disables X11 forwarding.
1.360 millert 810: .Pp
1.218 jmc 811: .It Fl Y
812: Enables trusted X11 forwarding.
813: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
814: controls.
1.360 millert 815: .Pp
1.278 djm 816: .It Fl y
817: Send log information using the
818: .Xr syslog 3
819: system module.
820: By default this information is sent to stderr.
1.218 jmc 821: .El
1.224 jmc 822: .Pp
823: .Nm
824: may additionally obtain configuration data from
825: a per-user configuration file and a system-wide configuration file.
826: The file format and configuration options are described in
827: .Xr ssh_config 5 .
1.222 jmc 828: .Sh AUTHENTICATION
1.377 djm 829: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 830: .Pp
831: The methods available for authentication are:
1.260 jmc 832: GSSAPI-based authentication,
1.222 jmc 833: host-based authentication,
834: public key authentication,
1.421 ! dtucker 835: keyboard-interactive authentication,
1.222 jmc 836: and password authentication.
837: Authentication methods are tried in the order specified above,
1.369 jmc 838: though
839: .Cm PreferredAuthentications
840: can be used to change the default order.
1.222 jmc 841: .Pp
842: Host-based authentication works as follows:
1.218 jmc 843: If the machine the user logs in from is listed in
844: .Pa /etc/hosts.equiv
845: or
846: .Pa /etc/shosts.equiv
1.412 djm 847: on the remote machine, the user is non-root and the user names are
1.218 jmc 848: the same on both sides, or if the files
849: .Pa ~/.rhosts
850: or
851: .Pa ~/.shosts
852: exist in the user's home directory on the
853: remote machine and contain a line containing the name of the client
854: machine and the name of the user on that machine, the user is
1.222 jmc 855: considered for login.
856: Additionally, the server
857: .Em must
858: be able to verify the client's
859: host key (see the description of
1.218 jmc 860: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 861: and
1.222 jmc 862: .Pa ~/.ssh/known_hosts ,
863: below)
864: for login to be permitted.
1.218 jmc 865: This authentication method closes security holes due to IP
1.222 jmc 866: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 867: [Note to the administrator:
868: .Pa /etc/hosts.equiv ,
869: .Pa ~/.rhosts ,
870: and the rlogin/rsh protocol in general, are inherently insecure and should be
871: disabled if security is desired.]
1.189 dtucker 872: .Pp
1.222 jmc 873: Public key authentication works as follows:
874: The scheme is based on public-key cryptography,
875: using cryptosystems
876: where encryption and decryption are done using separate keys,
877: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 878: The idea is that each user creates a public/private
879: key pair for authentication purposes.
880: The server knows the public key, and only the user knows the private key.
1.222 jmc 881: .Nm
882: implements public key authentication protocol automatically,
1.349 sobrado 883: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 884: The HISTORY section of
1.222 jmc 885: .Xr ssl 8
1.311 jmc 886: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 887: .Pp
1.218 jmc 888: The file
889: .Pa ~/.ssh/authorized_keys
890: lists the public keys that are permitted for logging in.
891: When the user logs in, the
1.2 deraadt 892: .Nm
1.218 jmc 893: program tells the server which key pair it would like to use for
894: authentication.
1.222 jmc 895: The client proves that it has access to the private key
896: and the server checks that the corresponding public key
897: is authorized to accept the account.
1.383 djm 898: .Pp
899: The server may inform the client of errors that prevented public key
900: authentication from succeeding after authentication completes using a
901: different method.
902: These may be viewed by increasing the
903: .Cm LogLevel
904: to
905: .Cm DEBUG
906: or higher (e.g. by using the
907: .Fl v
908: flag).
1.218 jmc 909: .Pp
1.420 dtucker 910: The user creates their key pair by running
1.218 jmc 911: .Xr ssh-keygen 1 .
912: This stores the private key in
1.222 jmc 913: .Pa ~/.ssh/id_dsa
1.369 jmc 914: (DSA),
1.310 djm 915: .Pa ~/.ssh/id_ecdsa
1.369 jmc 916: (ECDSA),
1.404 naddy 917: .Pa ~/.ssh/id_ecdsa_sk
1.409 naddy 918: (authenticator-hosted ECDSA),
1.343 naddy 919: .Pa ~/.ssh/id_ed25519
1.369 jmc 920: (Ed25519),
1.406 naddy 921: .Pa ~/.ssh/id_ed25519_sk
1.409 naddy 922: (authenticator-hosted Ed25519),
1.222 jmc 923: or
924: .Pa ~/.ssh/id_rsa
1.369 jmc 925: (RSA)
1.218 jmc 926: and stores the public key in
1.222 jmc 927: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 928: (DSA),
1.310 djm 929: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 930: (ECDSA),
1.404 naddy 931: .Pa ~/.ssh/id_ecdsa_sk.pub
1.409 naddy 932: (authenticator-hosted ECDSA),
1.343 naddy 933: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 934: (Ed25519),
1.406 naddy 935: .Pa ~/.ssh/id_ed25519_sk.pub
1.409 naddy 936: (authenticator-hosted Ed25519),
1.222 jmc 937: or
938: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 939: (RSA)
1.218 jmc 940: in the user's home directory.
1.222 jmc 941: The user should then copy the public key
1.218 jmc 942: to
943: .Pa ~/.ssh/authorized_keys
1.420 dtucker 944: in their home directory on the remote machine.
1.222 jmc 945: The
1.218 jmc 946: .Pa authorized_keys
947: file corresponds to the conventional
948: .Pa ~/.rhosts
949: file, and has one key
1.222 jmc 950: per line, though the lines can be very long.
1.218 jmc 951: After this, the user can log in without giving the password.
952: .Pp
1.301 jmc 953: A variation on public key authentication
954: is available in the form of certificate authentication:
955: instead of a set of public/private keys,
956: signed certificates are used.
957: This has the advantage that a single trusted certification authority
958: can be used in place of many public/private keys.
1.333 jmc 959: See the CERTIFICATES section of
1.301 jmc 960: .Xr ssh-keygen 1
961: for more information.
962: .Pp
963: The most convenient way to use public key or certificate authentication
964: may be with an authentication agent.
1.218 jmc 965: See
966: .Xr ssh-agent 1
1.366 jcs 967: and (optionally) the
968: .Cm AddKeysToAgent
969: directive in
970: .Xr ssh_config 5
1.218 jmc 971: for more information.
972: .Pp
1.421 ! dtucker 973: Keyboard-interactive authentication works as follows:
1.222 jmc 974: The server sends an arbitrary
975: .Qq challenge
1.421 ! dtucker 976: text and prompts for a response, possibly multiple times.
! 977: Examples of keyboard-interactive authentication include
1.335 jmc 978: .Bx
979: Authentication (see
1.222 jmc 980: .Xr login.conf 5 )
1.335 jmc 981: and PAM (some
982: .Pf non- Ox
983: systems).
1.222 jmc 984: .Pp
985: Finally, if other authentication methods fail,
1.218 jmc 986: .Nm
987: prompts the user for a password.
988: The password is sent to the remote
989: host for checking; however, since all communications are encrypted,
990: the password cannot be seen by someone listening on the network.
1.232 jmc 991: .Pp
992: .Nm
993: automatically maintains and checks a database containing
994: identification for all hosts it has ever been used with.
995: Host keys are stored in
996: .Pa ~/.ssh/known_hosts
997: in the user's home directory.
998: Additionally, the file
999: .Pa /etc/ssh/ssh_known_hosts
1000: is automatically checked for known hosts.
1001: Any new hosts are automatically added to the user's file.
1002: If a host's identification ever changes,
1003: .Nm
1004: warns about this and disables password authentication to prevent
1005: server spoofing or man-in-the-middle attacks,
1006: which could otherwise be used to circumvent the encryption.
1007: The
1008: .Cm StrictHostKeyChecking
1009: option can be used to control logins to machines whose
1010: host key is not known or has changed.
1011: .Pp
1.218 jmc 1012: When the user's identity has been accepted by the server, the server
1.357 dtucker 1013: either executes the given command in a non-interactive session or,
1014: if no command has been specified, logs into the machine and gives
1015: the user a normal shell as an interactive session.
1.218 jmc 1016: All communication with
1017: the remote command or shell will be automatically encrypted.
1018: .Pp
1.357 dtucker 1019: If an interactive session is requested
1020: .Nm
1021: by default will only request a pseudo-terminal (pty) for interactive
1022: sessions when the client has one.
1023: The flags
1024: .Fl T
1025: and
1026: .Fl t
1027: can be used to override this behaviour.
1028: .Pp
1029: If a pseudo-terminal has been allocated the
1.218 jmc 1030: user may use the escape characters noted below.
1031: .Pp
1.357 dtucker 1032: If no pseudo-terminal has been allocated,
1.218 jmc 1033: the session is transparent and can be used to reliably transfer binary data.
1034: On most systems, setting the escape character to
1035: .Dq none
1036: will also make the session transparent even if a tty is used.
1037: .Pp
1038: The session terminates when the command or shell on the remote
1.247 jmc 1039: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1040: .Sh ESCAPE CHARACTERS
1.218 jmc 1041: When a pseudo-terminal has been requested,
1.2 deraadt 1042: .Nm
1.218 jmc 1043: supports a number of functions through the use of an escape character.
1044: .Pp
1045: A single tilde character can be sent as
1046: .Ic ~~
1047: or by following the tilde by a character other than those described below.
1048: The escape character must always follow a newline to be interpreted as
1049: special.
1050: The escape character can be changed in configuration files using the
1051: .Cm EscapeChar
1052: configuration directive or on the command line by the
1053: .Fl e
1054: option.
1055: .Pp
1056: The supported escapes (assuming the default
1057: .Ql ~ )
1058: are:
1059: .Bl -tag -width Ds
1060: .It Cm ~.
1061: Disconnect.
1062: .It Cm ~^Z
1063: Background
1.234 jmc 1064: .Nm .
1.218 jmc 1065: .It Cm ~#
1066: List forwarded connections.
1067: .It Cm ~&
1068: Background
1.2 deraadt 1069: .Nm
1.218 jmc 1070: at logout when waiting for forwarded connection / X11 sessions to terminate.
1071: .It Cm ~?
1072: Display a list of escape characters.
1073: .It Cm ~B
1074: Send a BREAK to the remote system
1.369 jmc 1075: (only useful if the peer supports it).
1.218 jmc 1076: .It Cm ~C
1077: Open command line.
1078: Currently this allows the addition of port forwardings using the
1.279 stevesk 1079: .Fl L ,
1080: .Fl R
1.218 jmc 1081: and
1.279 stevesk 1082: .Fl D
1.225 jmc 1083: options (see above).
1.322 markus 1084: It also allows the cancellation of existing port-forwardings
1085: with
1.262 stevesk 1086: .Sm off
1.322 markus 1087: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1088: .Sm on
1.322 markus 1089: for local,
1090: .Sm off
1091: .Fl KR Oo Ar bind_address : Oc Ar port
1092: .Sm on
1093: for remote and
1094: .Sm off
1095: .Fl KD Oo Ar bind_address : Oc Ar port
1096: .Sm on
1097: for dynamic port-forwardings.
1.218 jmc 1098: .Ic !\& Ns Ar command
1099: allows the user to execute a local command if the
1100: .Ic PermitLocalCommand
1101: option is enabled in
1.176 jmc 1102: .Xr ssh_config 5 .
1.218 jmc 1103: Basic help is available, using the
1104: .Fl h
1105: option.
1106: .It Cm ~R
1107: Request rekeying of the connection
1.369 jmc 1108: (only useful if the peer supports it).
1.327 dtucker 1109: .It Cm ~V
1110: Decrease the verbosity
1111: .Pq Ic LogLevel
1112: when errors are being written to stderr.
1113: .It Cm ~v
1.328 jmc 1114: Increase the verbosity
1.327 dtucker 1115: .Pq Ic LogLevel
1116: when errors are being written to stderr.
1.176 jmc 1117: .El
1.246 jmc 1118: .Sh TCP FORWARDING
1.402 jmc 1119: Forwarding of arbitrary TCP connections over a secure channel
1120: can be specified either on the command line or in a configuration file.
1.246 jmc 1121: One possible application of TCP forwarding is a secure connection to a
1122: mail server; another is going through firewalls.
1123: .Pp
1.402 jmc 1124: In the example below, we look at encrypting communication for an IRC client,
1125: even though the IRC server it connects to does not directly
1126: support encrypted communication.
1.246 jmc 1127: This works as follows:
1128: the user connects to the remote host using
1129: .Nm ,
1.402 jmc 1130: specifying the ports to be used to forward the connection.
1131: After that it is possible to start the program locally,
1.246 jmc 1132: and
1133: .Nm
1.402 jmc 1134: will encrypt and forward the connection to the remote server.
1.246 jmc 1135: .Pp
1.402 jmc 1136: The following example tunnels an IRC session from the client
1137: to an IRC server at
1.246 jmc 1138: .Dq server.example.com ,
1139: joining channel
1140: .Dq #users ,
1141: nickname
1142: .Dq pinky ,
1.402 jmc 1143: using the standard IRC port, 6667:
1144: .Bd -literal -offset 4n
1145: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1146: $ irc -c '#users' pinky IRC/127.0.0.1
1147: .Ed
1.246 jmc 1148: .Pp
1149: The
1150: .Fl f
1151: option backgrounds
1152: .Nm
1153: and the remote command
1154: .Dq sleep 10
1155: is specified to allow an amount of time
1156: (10 seconds, in the example)
1.402 jmc 1157: to start the program which is going to use the tunnel.
1.246 jmc 1158: If no connections are made within the time specified,
1159: .Nm
1160: will exit.
1161: .Sh X11 FORWARDING
1.218 jmc 1162: If the
1163: .Cm ForwardX11
1164: variable is set to
1165: .Dq yes
1166: (or see the description of the
1.227 jmc 1167: .Fl X ,
1168: .Fl x ,
1.218 jmc 1169: and
1.227 jmc 1170: .Fl Y
1.226 jmc 1171: options above)
1.218 jmc 1172: and the user is using X11 (the
1173: .Ev DISPLAY
1174: environment variable is set), the connection to the X11 display is
1175: automatically forwarded to the remote side in such a way that any X11
1176: programs started from the shell (or command) will go through the
1177: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1178: from the local machine.
1.218 jmc 1179: The user should not manually set
1180: .Ev DISPLAY .
1181: Forwarding of X11 connections can be
1182: configured on the command line or in configuration files.
1183: .Pp
1184: The
1185: .Ev DISPLAY
1186: value set by
1187: .Nm
1188: will point to the server machine, but with a display number greater than zero.
1189: This is normal, and happens because
1190: .Nm
1191: creates a
1192: .Dq proxy
1193: X server on the server machine for forwarding the
1194: connections over the encrypted channel.
1.200 djm 1195: .Pp
1.218 jmc 1196: .Nm
1197: will also automatically set up Xauthority data on the server machine.
1198: For this purpose, it will generate a random authorization cookie,
1199: store it in Xauthority on the server, and verify that any forwarded
1200: connections carry this cookie and replace it by the real cookie when
1201: the connection is opened.
1202: The real authentication cookie is never
1203: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1204: .Pp
1.218 jmc 1205: If the
1206: .Cm ForwardAgent
1207: variable is set to
1208: .Dq yes
1209: (or see the description of the
1210: .Fl A
1.191 djm 1211: and
1.218 jmc 1212: .Fl a
1.226 jmc 1213: options above) and
1.218 jmc 1214: the user is using an authentication agent, the connection to the agent
1215: is automatically forwarded to the remote side.
1.252 jmc 1216: .Sh VERIFYING HOST KEYS
1217: When connecting to a server for the first time,
1218: a fingerprint of the server's public key is presented to the user
1219: (unless the option
1220: .Cm StrictHostKeyChecking
1221: has been disabled).
1222: Fingerprints can be determined using
1223: .Xr ssh-keygen 1 :
1224: .Pp
1225: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1226: .Pp
1.274 grunk 1227: If the fingerprint is already known, it can be matched
1228: and the key can be accepted or rejected.
1.358 djm 1229: If only legacy (MD5) fingerprints for the server are available, the
1230: .Xr ssh-keygen 1
1231: .Fl E
1232: option may be used to downgrade the fingerprint algorithm to match.
1233: .Pp
1.274 grunk 1234: Because of the difficulty of comparing host keys
1.352 djm 1235: just by looking at fingerprint strings,
1.274 grunk 1236: there is also support to compare host keys visually,
1237: using
1238: .Em random art .
1239: By setting the
1.275 grunk 1240: .Cm VisualHostKey
1.274 grunk 1241: option to
1.275 grunk 1242: .Dq yes ,
1.274 grunk 1243: a small ASCII graphic gets displayed on every login to a server, no matter
1244: if the session itself is interactive or not.
1245: By learning the pattern a known server produces, a user can easily
1246: find out that the host key has changed when a completely different pattern
1247: is displayed.
1248: Because these patterns are not unambiguous however, a pattern that looks
1249: similar to the pattern remembered only gives a good probability that the
1250: host key is the same, not guaranteed proof.
1251: .Pp
1252: To get a listing of the fingerprints along with their random art for
1253: all known hosts, the following command line can be used:
1254: .Pp
1255: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1256: .Pp
1.252 jmc 1257: If the fingerprint is unknown,
1258: an alternative method of verification is available:
1259: SSH fingerprints verified by DNS.
1260: An additional resource record (RR),
1261: SSHFP,
1262: is added to a zonefile
1263: and the connecting client is able to match the fingerprint
1264: with that of the key presented.
1265: .Pp
1266: In this example, we are connecting a client to a server,
1267: .Dq host.example.com .
1268: The SSHFP resource records should first be added to the zonefile for
1269: host.example.com:
1270: .Bd -literal -offset indent
1.259 jakob 1271: $ ssh-keygen -r host.example.com.
1.252 jmc 1272: .Ed
1273: .Pp
1274: The output lines will have to be added to the zonefile.
1275: To check that the zone is answering fingerprint queries:
1276: .Pp
1277: .Dl $ dig -t SSHFP host.example.com
1278: .Pp
1279: Finally the client connects:
1280: .Bd -literal -offset indent
1281: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1282: [...]
1283: Matching host key fingerprint found in DNS.
1284: Are you sure you want to continue connecting (yes/no)?
1285: .Ed
1286: .Pp
1287: See the
1288: .Cm VerifyHostKeyDNS
1289: option in
1290: .Xr ssh_config 5
1291: for more information.
1.250 jmc 1292: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1293: .Nm
1294: contains support for Virtual Private Network (VPN) tunnelling
1295: using the
1296: .Xr tun 4
1297: network pseudo-device,
1298: allowing two networks to be joined securely.
1299: The
1300: .Xr sshd_config 5
1301: configuration option
1302: .Cm PermitTunnel
1303: controls whether the server supports this,
1304: and at what level (layer 2 or 3 traffic).
1305: .Pp
1306: The following example would connect client network 10.0.50.0/24
1.265 otto 1307: with remote network 10.0.99.0/24 using a point-to-point connection
1308: from 10.1.1.1 to 10.1.1.2,
1309: provided that the SSH server running on the gateway to the remote network,
1310: at 192.168.1.15, allows it.
1311: .Pp
1312: On the client:
1.250 jmc 1313: .Bd -literal -offset indent
1314: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1315: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1316: # route add 10.0.99.0/24 10.1.1.2
1317: .Ed
1318: .Pp
1319: On the server:
1320: .Bd -literal -offset indent
1321: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1322: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1323: .Ed
1324: .Pp
1325: Client access may be more finely tuned via the
1326: .Pa /root/.ssh/authorized_keys
1327: file (see below) and the
1328: .Cm PermitRootLogin
1329: server option.
1.255 jmc 1330: The following entry would permit connections on
1.250 jmc 1331: .Xr tun 4
1.255 jmc 1332: device 1 from user
1.250 jmc 1333: .Dq jane
1.255 jmc 1334: and on tun device 2 from user
1.250 jmc 1335: .Dq john ,
1336: if
1337: .Cm PermitRootLogin
1338: is set to
1339: .Dq forced-commands-only :
1340: .Bd -literal -offset 2n
1341: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1342: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1343: .Ed
1344: .Pp
1.264 ray 1345: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1346: it may be more suited to temporary setups,
1347: such as for wireless VPNs.
1348: More permanent VPNs are better provided by tools such as
1349: .Xr ipsecctl 8
1350: and
1351: .Xr isakmpd 8 .
1.2 deraadt 1352: .Sh ENVIRONMENT
1353: .Nm
1.1 deraadt 1354: will normally set the following environment variables:
1.237 jmc 1355: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1356: .It Ev DISPLAY
1357: The
1358: .Ev DISPLAY
1.40 aaron 1359: variable indicates the location of the X11 server.
1.44 aaron 1360: It is automatically set by
1.2 deraadt 1361: .Nm
1362: to point to a value of the form
1.233 jmc 1363: .Dq hostname:n ,
1364: where
1365: .Dq hostname
1366: indicates the host where the shell runs, and
1367: .Sq n
1368: is an integer \*(Ge 1.
1.40 aaron 1369: .Nm
1370: uses this special value to forward X11 connections over the secure
1371: channel.
1.107 markus 1372: The user should normally not set
1373: .Ev DISPLAY
1374: explicitly, as that
1.1 deraadt 1375: will render the X11 connection insecure (and will require the user to
1376: manually copy any required authorization cookies).
1.2 deraadt 1377: .It Ev HOME
1.1 deraadt 1378: Set to the path of the user's home directory.
1.2 deraadt 1379: .It Ev LOGNAME
1380: Synonym for
1.12 aaron 1381: .Ev USER ;
1382: set for compatibility with systems that use this variable.
1.2 deraadt 1383: .It Ev MAIL
1.129 stevesk 1384: Set to the path of the user's mailbox.
1.40 aaron 1385: .It Ev PATH
1.2 deraadt 1386: Set to the default
1387: .Ev PATH ,
1388: as specified when compiling
1.234 jmc 1389: .Nm .
1.118 markus 1390: .It Ev SSH_ASKPASS
1391: If
1392: .Nm
1393: needs a passphrase, it will read the passphrase from the current
1394: terminal if it was run from a terminal.
1395: If
1396: .Nm
1397: does not have a terminal associated with it but
1398: .Ev DISPLAY
1399: and
1400: .Ev SSH_ASKPASS
1401: are set, it will execute the program specified by
1402: .Ev SSH_ASKPASS
1403: and open an X11 window to read the passphrase.
1404: This is particularly useful when calling
1405: .Nm
1406: from a
1.196 jmc 1407: .Pa .xsession
1.118 markus 1408: or related script.
1409: (Note that on some machines it
1410: may be necessary to redirect the input from
1411: .Pa /dev/null
1412: to make this work.)
1.413 djm 1413: .It Ev SSH_ASKPASS_REQUIRE
1.414 jmc 1414: Allows further control over the use of an askpass program.
1.413 djm 1415: If this variable is set to
1416: .Dq never
1417: then
1418: .Nm
1419: will never attempt to use one.
1420: If it is set to
1421: .Dq prefer ,
1422: then
1423: .Nm
1424: will prefer to use the askpass program instead of the TTY when requesting
1425: passwords.
1426: Finally, if the variable is set to
1427: .Dq force ,
1428: then the askpass program will be used for all passphrase input regardless
1429: of whether
1430: .Ev DISPLAY
1431: is set.
1.18 markus 1432: .It Ev SSH_AUTH_SOCK
1.233 jmc 1433: Identifies the path of a
1434: .Ux Ns -domain
1435: socket used to communicate with the agent.
1.166 stevesk 1436: .It Ev SSH_CONNECTION
1437: Identifies the client and server ends of the connection.
1.40 aaron 1438: The variable contains
1.233 jmc 1439: four space-separated values: client IP address, client port number,
1440: server IP address, and server port number.
1.73 markus 1441: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1442: This variable contains the original command line if a forced command
1.73 markus 1443: is executed.
1444: It can be used to extract the original arguments.
1.2 deraadt 1445: .It Ev SSH_TTY
1.1 deraadt 1446: This is set to the name of the tty (path to the device) associated
1.40 aaron 1447: with the current shell or command.
1448: If the current session has no tty,
1.1 deraadt 1449: this variable is not set.
1.387 djm 1450: .It Ev SSH_TUNNEL
1451: Optionally set by
1452: .Xr sshd 8
1453: to contain the interface names assigned if tunnel forwarding was
1454: requested by the client.
1.385 djm 1455: .It Ev SSH_USER_AUTH
1456: Optionally set by
1457: .Xr sshd 8 ,
1458: this variable may contain a pathname to a file that lists the authentication
1459: methods successfully used when the session was established, including any
1460: public keys that were used.
1.2 deraadt 1461: .It Ev TZ
1.214 jmc 1462: This variable is set to indicate the present time zone if it
1.257 jmc 1463: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1464: on to new connections).
1.2 deraadt 1465: .It Ev USER
1.1 deraadt 1466: Set to the name of the user logging in.
1.2 deraadt 1467: .El
1468: .Pp
1.44 aaron 1469: Additionally,
1.2 deraadt 1470: .Nm
1.44 aaron 1471: reads
1.207 djm 1472: .Pa ~/.ssh/environment ,
1.2 deraadt 1473: and adds lines of the format
1474: .Dq VARNAME=value
1.233 jmc 1475: to the environment if the file exists and users are allowed to
1.161 marc 1476: change their environment.
1.176 jmc 1477: For more information, see the
1.161 marc 1478: .Cm PermitUserEnvironment
1.162 stevesk 1479: option in
1.161 marc 1480: .Xr sshd_config 5 .
1.2 deraadt 1481: .Sh FILES
1.236 jmc 1482: .Bl -tag -width Ds -compact
1.309 jmc 1483: .It Pa ~/.rhosts
1.240 jmc 1484: This file is used for host-based authentication (see above).
1.92 markus 1485: On some machines this file may need to be
1.240 jmc 1486: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1487: because
1.2 deraadt 1488: .Xr sshd 8
1.40 aaron 1489: reads it as root.
1490: Additionally, this file must be owned by the user,
1491: and must not have write permissions for anyone else.
1492: The recommended
1.1 deraadt 1493: permission for most machines is read/write for the user, and not
1494: accessible by others.
1.2 deraadt 1495: .Pp
1.309 jmc 1496: .It Pa ~/.shosts
1.240 jmc 1497: This file is used in exactly the same way as
1498: .Pa .rhosts ,
1499: but allows host-based authentication without permitting login with
1500: rlogin/rsh.
1.272 mcbride 1501: .Pp
1.309 jmc 1502: .It Pa ~/.ssh/
1.272 mcbride 1503: This directory is the default location for all user-specific configuration
1504: and authentication information.
1505: There is no general requirement to keep the entire contents of this directory
1506: secret, but the recommended permissions are read/write/execute for the user,
1507: and not accessible by others.
1.236 jmc 1508: .Pp
1.309 jmc 1509: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1510: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1511: that can be used for logging in as this user.
1.238 jmc 1512: The format of this file is described in the
1513: .Xr sshd 8
1514: manual page.
1515: This file is not highly sensitive, but the recommended
1516: permissions are read/write for the user, and not accessible by others.
1517: .Pp
1.309 jmc 1518: .It Pa ~/.ssh/config
1.238 jmc 1519: This is the per-user configuration file.
1520: The file format and configuration options are described in
1521: .Xr ssh_config 5 .
1522: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1523: read/write for the user, and not writable by others.
1.238 jmc 1524: .Pp
1.309 jmc 1525: .It Pa ~/.ssh/environment
1.239 jmc 1526: Contains additional definitions for environment variables; see
1527: .Sx ENVIRONMENT ,
1.238 jmc 1528: above.
1529: .Pp
1.309 jmc 1530: .It Pa ~/.ssh/id_dsa
1.310 djm 1531: .It Pa ~/.ssh/id_ecdsa
1.404 naddy 1532: .It Pa ~/.ssh/id_ecdsa_sk
1.343 naddy 1533: .It Pa ~/.ssh/id_ed25519
1.406 naddy 1534: .It Pa ~/.ssh/id_ed25519_sk
1.309 jmc 1535: .It Pa ~/.ssh/id_rsa
1.238 jmc 1536: Contains the private key for authentication.
1537: These files
1538: contain sensitive data and should be readable by the user but not
1539: accessible by others (read/write/execute).
1540: .Nm
1541: will simply ignore a private key file if it is accessible by others.
1542: It is possible to specify a passphrase when
1543: generating the key which will be used to encrypt the
1.389 djm 1544: sensitive part of this file using AES-128.
1.238 jmc 1545: .Pp
1.309 jmc 1546: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1547: .It Pa ~/.ssh/id_ecdsa.pub
1.404 naddy 1548: .It Pa ~/.ssh/id_ecdsa_sk.pub
1.343 naddy 1549: .It Pa ~/.ssh/id_ed25519.pub
1.406 naddy 1550: .It Pa ~/.ssh/id_ed25519_sk.pub
1.309 jmc 1551: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1552: Contains the public key for authentication.
1553: These files are not
1554: sensitive and can (but need not) be readable by anyone.
1555: .Pp
1.309 jmc 1556: .It Pa ~/.ssh/known_hosts
1.244 jmc 1557: Contains a list of host keys for all hosts the user has logged into
1558: that are not already in the systemwide list of known host keys.
1.238 jmc 1559: See
1.244 jmc 1560: .Xr sshd 8
1561: for further details of the format of this file.
1.238 jmc 1562: .Pp
1.309 jmc 1563: .It Pa ~/.ssh/rc
1.238 jmc 1564: Commands in this file are executed by
1565: .Nm
1.245 jmc 1566: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1567: started.
1568: See the
1569: .Xr sshd 8
1570: manual page for more information.
1571: .Pp
1.309 jmc 1572: .It Pa /etc/hosts.equiv
1.240 jmc 1573: This file is for host-based authentication (see above).
1574: It should only be writable by root.
1.236 jmc 1575: .Pp
1.309 jmc 1576: .It Pa /etc/shosts.equiv
1.240 jmc 1577: This file is used in exactly the same way as
1578: .Pa hosts.equiv ,
1579: but allows host-based authentication without permitting login with
1580: rlogin/rsh.
1.236 jmc 1581: .Pp
1.238 jmc 1582: .It Pa /etc/ssh/ssh_config
1583: Systemwide configuration file.
1584: The file format and configuration options are described in
1585: .Xr ssh_config 5 .
1586: .Pp
1.309 jmc 1587: .It Pa /etc/ssh/ssh_host_key
1588: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1589: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1590: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1591: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1592: These files contain the private parts of the host keys
1.245 jmc 1593: and are used for host-based authentication.
1.238 jmc 1594: .Pp
1.309 jmc 1595: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1596: Systemwide list of known host keys.
1597: This file should be prepared by the
1598: system administrator to contain the public host keys of all machines in the
1599: organization.
1.244 jmc 1600: It should be world-readable.
1601: See
1.238 jmc 1602: .Xr sshd 8
1.244 jmc 1603: for further details of the format of this file.
1.236 jmc 1604: .Pp
1.309 jmc 1605: .It Pa /etc/ssh/sshrc
1.1 deraadt 1606: Commands in this file are executed by
1.2 deraadt 1607: .Nm
1.245 jmc 1608: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1609: See the
1.2 deraadt 1610: .Xr sshd 8
1.1 deraadt 1611: manual page for more information.
1.58 itojun 1612: .El
1.312 jmc 1613: .Sh EXIT STATUS
1614: .Nm
1615: exits with the exit status of the remote command or with 255
1616: if an error occurred.
1.2 deraadt 1617: .Sh SEE ALSO
1618: .Xr scp 1 ,
1.83 djm 1619: .Xr sftp 1 ,
1.2 deraadt 1620: .Xr ssh-add 1 ,
1621: .Xr ssh-agent 1 ,
1622: .Xr ssh-keygen 1 ,
1.242 jmc 1623: .Xr ssh-keyscan 1 ,
1.250 jmc 1624: .Xr tun 4 ,
1.159 stevesk 1625: .Xr ssh_config 5 ,
1.160 naddy 1626: .Xr ssh-keysign 8 ,
1.87 itojun 1627: .Xr sshd 8
1.329 jmc 1628: .Sh STANDARDS
1.106 markus 1629: .Rs
1.329 jmc 1630: .%A S. Lehtinen
1631: .%A C. Lonvick
1632: .%D January 2006
1.256 jmc 1633: .%R RFC 4250
1.329 jmc 1634: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1635: .Re
1.329 jmc 1636: .Pp
1.256 jmc 1637: .Rs
1.329 jmc 1638: .%A T. Ylonen
1639: .%A C. Lonvick
1640: .%D January 2006
1.256 jmc 1641: .%R RFC 4251
1.329 jmc 1642: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1643: .Re
1.329 jmc 1644: .Pp
1.256 jmc 1645: .Rs
1.329 jmc 1646: .%A T. Ylonen
1647: .%A C. Lonvick
1648: .%D January 2006
1.256 jmc 1649: .%R RFC 4252
1.329 jmc 1650: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1651: .Re
1.329 jmc 1652: .Pp
1.256 jmc 1653: .Rs
1.329 jmc 1654: .%A T. Ylonen
1655: .%A C. Lonvick
1656: .%D January 2006
1.256 jmc 1657: .%R RFC 4253
1.329 jmc 1658: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1659: .Re
1.329 jmc 1660: .Pp
1.256 jmc 1661: .Rs
1.329 jmc 1662: .%A T. Ylonen
1663: .%A C. Lonvick
1664: .%D January 2006
1.256 jmc 1665: .%R RFC 4254
1.329 jmc 1666: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1667: .Re
1.329 jmc 1668: .Pp
1.256 jmc 1669: .Rs
1.329 jmc 1670: .%A J. Schlyter
1671: .%A W. Griffin
1672: .%D January 2006
1.256 jmc 1673: .%R RFC 4255
1.329 jmc 1674: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1675: .Re
1.329 jmc 1676: .Pp
1.256 jmc 1677: .Rs
1.329 jmc 1678: .%A F. Cusack
1679: .%A M. Forssen
1680: .%D January 2006
1.256 jmc 1681: .%R RFC 4256
1.329 jmc 1682: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1683: .Re
1.329 jmc 1684: .Pp
1.256 jmc 1685: .Rs
1.329 jmc 1686: .%A J. Galbraith
1687: .%A P. Remaker
1688: .%D January 2006
1.256 jmc 1689: .%R RFC 4335
1.329 jmc 1690: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1691: .Re
1.329 jmc 1692: .Pp
1.256 jmc 1693: .Rs
1.329 jmc 1694: .%A M. Bellare
1695: .%A T. Kohno
1696: .%A C. Namprempre
1697: .%D January 2006
1.256 jmc 1698: .%R RFC 4344
1.329 jmc 1699: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1700: .Re
1.329 jmc 1701: .Pp
1.256 jmc 1702: .Rs
1.329 jmc 1703: .%A B. Harris
1704: .%D January 2006
1.256 jmc 1705: .%R RFC 4345
1.329 jmc 1706: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1707: .Re
1.329 jmc 1708: .Pp
1.258 djm 1709: .Rs
1.329 jmc 1710: .%A M. Friedl
1711: .%A N. Provos
1712: .%A W. Simpson
1713: .%D March 2006
1.258 djm 1714: .%R RFC 4419
1.329 jmc 1715: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1716: .Re
1.329 jmc 1717: .Pp
1.266 markus 1718: .Rs
1.329 jmc 1719: .%A J. Galbraith
1720: .%A R. Thayer
1721: .%D November 2006
1.266 markus 1722: .%R RFC 4716
1.329 jmc 1723: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1724: .Re
1.329 jmc 1725: .Pp
1.313 djm 1726: .Rs
1.329 jmc 1727: .%A D. Stebila
1728: .%A J. Green
1729: .%D December 2009
1.313 djm 1730: .%R RFC 5656
1.329 jmc 1731: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1732: .Re
1.329 jmc 1733: .Pp
1.274 grunk 1734: .Rs
1735: .%A A. Perrig
1736: .%A D. Song
1737: .%D 1999
1.329 jmc 1738: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1739: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1740: .Re
1.173 jmc 1741: .Sh AUTHORS
1742: OpenSSH is a derivative of the original and free
1743: ssh 1.2.12 release by Tatu Ylonen.
1744: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1745: Theo de Raadt and Dug Song
1746: removed many bugs, re-added newer features and
1747: created OpenSSH.
1748: Markus Friedl contributed the support for SSH
1749: protocol versions 1.5 and 2.0.
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