Annotation of src/usr.bin/ssh/ssh.1, Revision 1.419
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: .\"
1.419 ! markus 36: .\" $OpenBSD: ssh.1,v 1.418 2021/01/26 15:40:17 naddy Exp $
1.418 naddy 37: .Dd $Mdocdate: January 26 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
1.49 markus 91: his/her identity to the remote machine using one of several methods
1.365 mmcc 92: (see below).
1.49 markus 93: .Pp
1.386 millert 94: If a
1.176 jmc 95: .Ar command
96: is specified,
1.219 jmc 97: it is executed on the remote host instead of a login shell.
1.2 deraadt 98: .Pp
1.218 jmc 99: The options are as follows:
1.361 millert 100: .Pp
1.360 millert 101: .Bl -tag -width Ds -compact
1.218 jmc 102: .It Fl 4
103: Forces
1.2 deraadt 104: .Nm
1.218 jmc 105: to use IPv4 addresses only.
1.360 millert 106: .Pp
1.218 jmc 107: .It Fl 6
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv6 addresses only.
1.360 millert 111: .Pp
1.218 jmc 112: .It Fl A
1.407 jmc 113: Enables forwarding of connections from an authentication agent such as
114: .Xr ssh-agent 1 .
1.218 jmc 115: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 116: .Pp
1.218 jmc 117: Agent forwarding should be enabled with caution.
118: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 119: (for the agent's
1.286 sobrado 120: .Ux Ns -domain
121: socket) can access the local agent through the forwarded connection.
1.218 jmc 122: An attacker cannot obtain key material from the agent,
123: however they can perform operations on the keys that enable them to
124: authenticate using the identities loaded into the agent.
1.407 jmc 125: A safer alternative may be to use a jump host
126: (see
127: .Fl J ) .
1.360 millert 128: .Pp
1.218 jmc 129: .It Fl a
130: Disables forwarding of the authentication agent connection.
1.390 djm 131: .Pp
1.391 jmc 132: .It Fl B Ar bind_interface
1.390 djm 133: Bind to the address of
1.391 jmc 134: .Ar bind_interface
1.390 djm 135: before attempting to connect to the destination host.
136: This is only useful on systems with more than one address.
1.360 millert 137: .Pp
1.218 jmc 138: .It Fl b Ar bind_address
139: Use
140: .Ar bind_address
141: on the local machine as the source address
142: of the connection.
143: Only useful on systems with more than one address.
1.360 millert 144: .Pp
1.218 jmc 145: .It Fl C
146: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 147: data for forwarded X11, TCP and
148: .Ux Ns -domain
149: connections).
1.218 jmc 150: The compression algorithm is the same used by
1.377 djm 151: .Xr gzip 1 .
1.218 jmc 152: Compression is desirable on modem lines and other
153: slow connections, but will only slow down things on fast networks.
154: The default value can be set on a host-by-host basis in the
155: configuration files; see the
156: .Cm Compression
157: option.
1.360 millert 158: .Pp
1.218 jmc 159: .It Fl c Ar cipher_spec
160: Selects the cipher specification for encrypting the session.
161: .Ar cipher_spec
162: is a comma-separated list of ciphers
163: listed in order of preference.
1.283 jmc 164: See the
165: .Cm Ciphers
1.307 dtucker 166: keyword in
167: .Xr ssh_config 5
168: for more information.
1.360 millert 169: .Pp
1.218 jmc 170: .It Fl D Xo
171: .Sm off
172: .Oo Ar bind_address : Oc
173: .Ar port
174: .Sm on
175: .Xc
176: Specifies a local
177: .Dq dynamic
178: application-level port forwarding.
179: This works by allocating a socket to listen to
180: .Ar port
181: on the local side, optionally bound to the specified
182: .Ar bind_address .
183: Whenever a connection is made to this port, the
184: connection is forwarded over the secure channel, and the application
185: protocol is then used to determine where to connect to from the
186: remote machine.
187: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 188: .Nm
1.218 jmc 189: will act as a SOCKS server.
190: Only root can forward privileged ports.
191: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 192: .Pp
1.308 djm 193: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 194: Only the superuser can forward privileged ports.
195: By default, the local port is bound in accordance with the
196: .Cm GatewayPorts
197: setting.
198: However, an explicit
199: .Ar bind_address
200: may be used to bind the connection to a specific address.
201: The
202: .Ar bind_address
203: of
204: .Dq localhost
205: indicates that the listening port be bound for local use only, while an
206: empty address or
207: .Sq *
208: indicates that the port should be available from all interfaces.
1.360 millert 209: .Pp
1.331 dtucker 210: .It Fl E Ar log_file
211: Append debug logs to
212: .Ar log_file
213: instead of standard error.
1.360 millert 214: .Pp
1.229 jmc 215: .It Fl e Ar escape_char
1.218 jmc 216: Sets the escape character for sessions with a pty (default:
217: .Ql ~ ) .
218: The escape character is only recognized at the beginning of a line.
219: The escape character followed by a dot
220: .Pq Ql \&.
221: closes the connection;
222: followed by control-Z suspends the connection;
223: and followed by itself sends the escape character once.
224: Setting the character to
1.2 deraadt 225: .Dq none
1.218 jmc 226: disables any escapes and makes the session fully transparent.
1.360 millert 227: .Pp
1.218 jmc 228: .It Fl F Ar configfile
229: Specifies an alternative per-user configuration file.
230: If a configuration file is given on the command line,
231: the system-wide configuration file
232: .Pq Pa /etc/ssh/ssh_config
233: will be ignored.
234: The default for the per-user configuration file is
235: .Pa ~/.ssh/config .
1.411 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 ChallengeResponseAuthentication
489: .It CheckHostIP
490: .It Ciphers
491: .It ClearAllForwardings
492: .It Compression
1.380 naddy 493: .It ConnectionAttempts
1.218 jmc 494: .It ConnectTimeout
495: .It ControlMaster
496: .It ControlPath
1.321 djm 497: .It ControlPersist
1.218 jmc 498: .It DynamicForward
499: .It EscapeChar
1.263 markus 500: .It ExitOnForwardFailure
1.353 jmc 501: .It FingerprintHash
1.218 jmc 502: .It ForwardAgent
503: .It ForwardX11
1.321 djm 504: .It ForwardX11Timeout
1.218 jmc 505: .It ForwardX11Trusted
506: .It GatewayPorts
507: .It GlobalKnownHostsFile
508: .It GSSAPIAuthentication
509: .It GSSAPIDelegateCredentials
510: .It HashKnownHosts
511: .It Host
1.418 naddy 512: .It HostbasedAcceptedAlgorithms
1.218 jmc 513: .It HostbasedAuthentication
514: .It HostKeyAlgorithms
515: .It HostKeyAlias
1.403 jmc 516: .It Hostname
1.374 jmc 517: .It IdentitiesOnly
1.371 markus 518: .It IdentityAgent
1.218 jmc 519: .It IdentityFile
1.316 jmc 520: .It IPQoS
1.321 djm 521: .It KbdInteractiveAuthentication
1.218 jmc 522: .It KbdInteractiveDevices
1.314 jmc 523: .It KexAlgorithms
1.415 djm 524: .It KnownHostsCommand
1.218 jmc 525: .It LocalCommand
526: .It LocalForward
527: .It LogLevel
528: .It MACs
1.337 jmc 529: .It Match
1.218 jmc 530: .It NoHostAuthenticationForLocalhost
531: .It NumberOfPasswordPrompts
532: .It PasswordAuthentication
533: .It PermitLocalCommand
1.419 ! markus 534: .It PermitRemoteOpen
1.291 markus 535: .It PKCS11Provider
1.218 jmc 536: .It Port
537: .It PreferredAuthentications
538: .It ProxyCommand
1.375 djm 539: .It ProxyJump
1.336 jmc 540: .It ProxyUseFdpass
1.416 dtucker 541: .It PubkeyAcceptedAlgorithms
1.218 jmc 542: .It PubkeyAuthentication
1.251 dtucker 543: .It RekeyLimit
1.382 bluhm 544: .It RemoteCommand
1.218 jmc 545: .It RemoteForward
1.319 jmc 546: .It RequestTTY
1.218 jmc 547: .It SendEnv
548: .It ServerAliveInterval
549: .It ServerAliveCountMax
1.395 jmc 550: .It SetEnv
1.347 jmc 551: .It StreamLocalBindMask
552: .It StreamLocalBindUnlink
1.218 jmc 553: .It StrictHostKeyChecking
554: .It TCPKeepAlive
555: .It Tunnel
556: .It TunnelDevice
1.354 djm 557: .It UpdateHostKeys
1.218 jmc 558: .It User
559: .It UserKnownHostsFile
560: .It VerifyHostKeyDNS
1.276 jmc 561: .It VisualHostKey
1.218 jmc 562: .It XAuthLocation
563: .El
1.360 millert 564: .Pp
1.218 jmc 565: .It Fl p Ar port
566: Port to connect to on the remote host.
567: This can be specified on a
568: per-host basis in the configuration file.
1.360 millert 569: .Pp
1.362 djm 570: .It Fl Q Ar query_option
1.332 djm 571: Queries
572: .Nm
1.341 deraadt 573: for the algorithms supported for the specified version 2.
574: The available features are:
575: .Ar cipher
1.332 djm 576: (supported symmetric ciphers),
1.341 deraadt 577: .Ar cipher-auth
1.340 djm 578: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 579: .Ar help
580: (supported query terms for use with the
581: .Fl Q
582: flag),
1.341 deraadt 583: .Ar mac
1.332 djm 584: (supported message integrity codes),
1.341 deraadt 585: .Ar kex
1.332 djm 586: (key exchange algorithms),
1.341 deraadt 587: .Ar key
1.362 djm 588: (key types),
589: .Ar key-cert
590: (certificate key types),
591: .Ar key-plain
1.398 djm 592: (non-certificate key types),
1.410 dtucker 593: .Ar key-sig
594: (all key types and signature algorithms),
1.356 djm 595: .Ar protocol-version
1.398 djm 596: (supported SSH protocol versions), and
597: .Ar sig
598: (supported signature algorithms).
1.410 dtucker 599: Alternatively, any keyword from
600: .Xr ssh_config 5
601: or
602: .Xr sshd_config 5
603: that takes an algorithm list may be used as an alias for the corresponding
604: query_option.
1.360 millert 605: .Pp
1.218 jmc 606: .It Fl q
607: Quiet mode.
1.271 djm 608: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 609: .Pp
1.218 jmc 610: .It Fl R Xo
611: .Sm off
612: .Oo Ar bind_address : Oc
613: .Ar port : host : hostport
614: .Sm on
615: .Xc
1.360 millert 616: .It Fl R Xo
617: .Sm off
618: .Oo Ar bind_address : Oc
619: .Ar port : local_socket
620: .Sm on
621: .Xc
622: .It Fl R Xo
623: .Sm off
624: .Ar remote_socket : host : hostport
625: .Sm on
626: .Xc
627: .It Fl R Xo
628: .Sm off
629: .Ar remote_socket : local_socket
630: .Sm on
631: .Xc
1.384 markus 632: .It Fl R Xo
633: .Sm off
634: .Oo Ar bind_address : Oc
635: .Ar port
636: .Sm on
637: .Xc
1.360 millert 638: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 639: (server) host are to be forwarded to the local side.
640: .Pp
1.360 millert 641: This works by allocating a socket to listen to either a TCP
1.218 jmc 642: .Ar port
1.360 millert 643: or to a Unix socket on the remote side.
644: Whenever a connection is made to this port or Unix socket, the
645: connection is forwarded over the secure channel, and a connection
1.384 markus 646: is made from the local machine to either an explicit destination specified by
1.218 jmc 647: .Ar host
648: port
1.360 millert 649: .Ar hostport ,
650: or
651: .Ar local_socket ,
1.384 markus 652: or, if no explicit destination was specified,
653: .Nm
654: will act as a SOCKS 4/5 proxy and forward connections to the destinations
655: requested by the remote SOCKS client.
1.2 deraadt 656: .Pp
1.218 jmc 657: Port forwardings can also be specified in the configuration file.
658: Privileged ports can be forwarded only when
659: logging in as root on the remote machine.
1.324 jmc 660: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 661: .Pp
1.360 millert 662: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 663: interface only.
1.280 tobias 664: This may be overridden by specifying a
1.218 jmc 665: .Ar bind_address .
666: An empty
667: .Ar bind_address ,
668: or the address
669: .Ql * ,
670: indicates that the remote socket should listen on all interfaces.
671: Specifying a remote
672: .Ar bind_address
673: will only succeed if the server's
674: .Cm GatewayPorts
675: option is enabled (see
676: .Xr sshd_config 5 ) .
1.281 djm 677: .Pp
678: If the
679: .Ar port
680: argument is
1.282 djm 681: .Ql 0 ,
1.281 djm 682: the listen port will be dynamically allocated on the server and reported
683: to the client at run time.
1.305 markus 684: When used together with
685: .Ic -O forward
686: the allocated port will be printed to the standard output.
1.360 millert 687: .Pp
1.218 jmc 688: .It Fl S Ar ctl_path
1.304 jmc 689: Specifies the location of a control socket for connection sharing,
1.303 djm 690: or the string
691: .Dq none
692: to disable connection sharing.
1.218 jmc 693: Refer to the description of
694: .Cm ControlPath
695: and
696: .Cm ControlMaster
697: in
698: .Xr ssh_config 5
699: for details.
1.360 millert 700: .Pp
1.218 jmc 701: .It Fl s
702: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 703: Subsystems facilitate the use of SSH
704: as a secure transport for other applications (e.g.\&
1.218 jmc 705: .Xr sftp 1 ) .
706: The subsystem is specified as the remote command.
1.360 millert 707: .Pp
1.218 jmc 708: .It Fl T
1.357 dtucker 709: Disable pseudo-terminal allocation.
1.360 millert 710: .Pp
1.218 jmc 711: .It Fl t
1.357 dtucker 712: Force pseudo-terminal allocation.
1.218 jmc 713: This can be used to execute arbitrary
714: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 715: e.g. when implementing menu services.
1.218 jmc 716: Multiple
717: .Fl t
718: options force tty allocation, even if
1.194 jakob 719: .Nm
1.218 jmc 720: has no local tty.
1.360 millert 721: .Pp
1.218 jmc 722: .It Fl V
723: Display the version number and exit.
1.360 millert 724: .Pp
1.218 jmc 725: .It Fl v
726: Verbose mode.
727: Causes
1.176 jmc 728: .Nm
1.218 jmc 729: to print debugging messages about its progress.
730: This is helpful in
731: debugging connection, authentication, and configuration problems.
732: Multiple
733: .Fl v
734: options increase the verbosity.
735: The maximum is 3.
1.360 millert 736: .Pp
1.290 dtucker 737: .It Fl W Ar host : Ns Ar port
738: Requests that standard input and output on the client be forwarded to
739: .Ar host
740: on
741: .Ar port
742: over the secure channel.
743: Implies
744: .Fl N ,
745: .Fl T ,
746: .Cm ExitOnForwardFailure
747: and
1.373 jmc 748: .Cm ClearAllForwardings ,
749: though these can be overridden in the configuration file or using
1.372 dtucker 750: .Fl o
751: command line options.
1.360 millert 752: .Pp
1.261 stevesk 753: .It Fl w Xo
754: .Ar local_tun Ns Op : Ns Ar remote_tun
755: .Xc
756: Requests
757: tunnel
758: device forwarding with the specified
1.218 jmc 759: .Xr tun 4
1.261 stevesk 760: devices between the client
761: .Pq Ar local_tun
762: and the server
763: .Pq Ar remote_tun .
764: .Pp
1.228 jmc 765: The devices may be specified by numerical ID or the keyword
766: .Dq any ,
767: which uses the next available tunnel device.
1.261 stevesk 768: If
769: .Ar remote_tun
770: is not specified, it defaults to
771: .Dq any .
1.228 jmc 772: See also the
1.218 jmc 773: .Cm Tunnel
1.261 stevesk 774: and
775: .Cm TunnelDevice
776: directives in
1.218 jmc 777: .Xr ssh_config 5 .
1.393 djm 778: .Pp
1.261 stevesk 779: If the
780: .Cm Tunnel
1.393 djm 781: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 782: .Dq point-to-point .
1.393 djm 783: If a different
784: .Cm Tunnel
785: forwarding mode it desired, then it should be specified before
786: .Fl w .
1.360 millert 787: .Pp
1.218 jmc 788: .It Fl X
789: Enables X11 forwarding.
1.54 markus 790: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 791: .Pp
1.218 jmc 792: X11 forwarding should be enabled with caution.
1.168 jmc 793: Users with the ability to bypass file permissions on the remote host
1.218 jmc 794: (for the user's X authorization database)
795: can access the local X11 display through the forwarded connection.
796: An attacker may then be able to perform activities such as keystroke monitoring.
797: .Pp
798: For this reason, X11 forwarding is subjected to X11 SECURITY extension
799: restrictions by default.
800: Please refer to the
801: .Nm
802: .Fl Y
803: option and the
804: .Cm ForwardX11Trusted
805: directive in
806: .Xr ssh_config 5
807: for more information.
1.360 millert 808: .Pp
1.218 jmc 809: .It Fl x
810: Disables X11 forwarding.
1.360 millert 811: .Pp
1.218 jmc 812: .It Fl Y
813: Enables trusted X11 forwarding.
814: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
815: controls.
1.360 millert 816: .Pp
1.278 djm 817: .It Fl y
818: Send log information using the
819: .Xr syslog 3
820: system module.
821: By default this information is sent to stderr.
1.218 jmc 822: .El
1.224 jmc 823: .Pp
824: .Nm
825: may additionally obtain configuration data from
826: a per-user configuration file and a system-wide configuration file.
827: The file format and configuration options are described in
828: .Xr ssh_config 5 .
1.222 jmc 829: .Sh AUTHENTICATION
1.377 djm 830: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 831: .Pp
832: The methods available for authentication are:
1.260 jmc 833: GSSAPI-based authentication,
1.222 jmc 834: host-based authentication,
835: public key authentication,
836: challenge-response authentication,
837: and password authentication.
838: Authentication methods are tried in the order specified above,
1.369 jmc 839: though
840: .Cm PreferredAuthentications
841: can be used to change the default order.
1.222 jmc 842: .Pp
843: Host-based authentication works as follows:
1.218 jmc 844: If the machine the user logs in from is listed in
845: .Pa /etc/hosts.equiv
846: or
847: .Pa /etc/shosts.equiv
1.412 djm 848: on the remote machine, the user is non-root and the user names are
1.218 jmc 849: the same on both sides, or if the files
850: .Pa ~/.rhosts
851: or
852: .Pa ~/.shosts
853: exist in the user's home directory on the
854: remote machine and contain a line containing the name of the client
855: machine and the name of the user on that machine, the user is
1.222 jmc 856: considered for login.
857: Additionally, the server
858: .Em must
859: be able to verify the client's
860: host key (see the description of
1.218 jmc 861: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 862: and
1.222 jmc 863: .Pa ~/.ssh/known_hosts ,
864: below)
865: for login to be permitted.
1.218 jmc 866: This authentication method closes security holes due to IP
1.222 jmc 867: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 868: [Note to the administrator:
869: .Pa /etc/hosts.equiv ,
870: .Pa ~/.rhosts ,
871: and the rlogin/rsh protocol in general, are inherently insecure and should be
872: disabled if security is desired.]
1.189 dtucker 873: .Pp
1.222 jmc 874: Public key authentication works as follows:
875: The scheme is based on public-key cryptography,
876: using cryptosystems
877: where encryption and decryption are done using separate keys,
878: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 879: The idea is that each user creates a public/private
880: key pair for authentication purposes.
881: The server knows the public key, and only the user knows the private key.
1.222 jmc 882: .Nm
883: implements public key authentication protocol automatically,
1.349 sobrado 884: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 885: The HISTORY section of
1.222 jmc 886: .Xr ssl 8
1.311 jmc 887: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 888: .Pp
1.218 jmc 889: The file
890: .Pa ~/.ssh/authorized_keys
891: lists the public keys that are permitted for logging in.
892: When the user logs in, the
1.2 deraadt 893: .Nm
1.218 jmc 894: program tells the server which key pair it would like to use for
895: authentication.
1.222 jmc 896: The client proves that it has access to the private key
897: and the server checks that the corresponding public key
898: is authorized to accept the account.
1.383 djm 899: .Pp
900: The server may inform the client of errors that prevented public key
901: authentication from succeeding after authentication completes using a
902: different method.
903: These may be viewed by increasing the
904: .Cm LogLevel
905: to
906: .Cm DEBUG
907: or higher (e.g. by using the
908: .Fl v
909: flag).
1.218 jmc 910: .Pp
1.222 jmc 911: The user creates his/her key pair by running
1.218 jmc 912: .Xr ssh-keygen 1 .
913: This stores the private key in
1.222 jmc 914: .Pa ~/.ssh/id_dsa
1.369 jmc 915: (DSA),
1.310 djm 916: .Pa ~/.ssh/id_ecdsa
1.369 jmc 917: (ECDSA),
1.404 naddy 918: .Pa ~/.ssh/id_ecdsa_sk
1.409 naddy 919: (authenticator-hosted ECDSA),
1.343 naddy 920: .Pa ~/.ssh/id_ed25519
1.369 jmc 921: (Ed25519),
1.406 naddy 922: .Pa ~/.ssh/id_ed25519_sk
1.409 naddy 923: (authenticator-hosted Ed25519),
1.222 jmc 924: or
925: .Pa ~/.ssh/id_rsa
1.369 jmc 926: (RSA)
1.218 jmc 927: and stores the public key in
1.222 jmc 928: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 929: (DSA),
1.310 djm 930: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 931: (ECDSA),
1.404 naddy 932: .Pa ~/.ssh/id_ecdsa_sk.pub
1.409 naddy 933: (authenticator-hosted ECDSA),
1.343 naddy 934: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 935: (Ed25519),
1.406 naddy 936: .Pa ~/.ssh/id_ed25519_sk.pub
1.409 naddy 937: (authenticator-hosted Ed25519),
1.222 jmc 938: or
939: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 940: (RSA)
1.218 jmc 941: in the user's home directory.
1.222 jmc 942: The user should then copy the public key
1.218 jmc 943: to
944: .Pa ~/.ssh/authorized_keys
1.222 jmc 945: in his/her home directory on the remote machine.
946: The
1.218 jmc 947: .Pa authorized_keys
948: file corresponds to the conventional
949: .Pa ~/.rhosts
950: file, and has one key
1.222 jmc 951: per line, though the lines can be very long.
1.218 jmc 952: After this, the user can log in without giving the password.
953: .Pp
1.301 jmc 954: A variation on public key authentication
955: is available in the form of certificate authentication:
956: instead of a set of public/private keys,
957: signed certificates are used.
958: This has the advantage that a single trusted certification authority
959: can be used in place of many public/private keys.
1.333 jmc 960: See the CERTIFICATES section of
1.301 jmc 961: .Xr ssh-keygen 1
962: for more information.
963: .Pp
964: The most convenient way to use public key or certificate authentication
965: may be with an authentication agent.
1.218 jmc 966: See
967: .Xr ssh-agent 1
1.366 jcs 968: and (optionally) the
969: .Cm AddKeysToAgent
970: directive in
971: .Xr ssh_config 5
1.218 jmc 972: for more information.
973: .Pp
1.222 jmc 974: Challenge-response authentication works as follows:
975: The server sends an arbitrary
976: .Qq challenge
977: text, and prompts for a response.
978: Examples of challenge-response authentication include
1.335 jmc 979: .Bx
980: Authentication (see
1.222 jmc 981: .Xr login.conf 5 )
1.335 jmc 982: and PAM (some
983: .Pf non- Ox
984: systems).
1.222 jmc 985: .Pp
986: Finally, if other authentication methods fail,
1.218 jmc 987: .Nm
988: prompts the user for a password.
989: The password is sent to the remote
990: host for checking; however, since all communications are encrypted,
991: the password cannot be seen by someone listening on the network.
1.232 jmc 992: .Pp
993: .Nm
994: automatically maintains and checks a database containing
995: identification for all hosts it has ever been used with.
996: Host keys are stored in
997: .Pa ~/.ssh/known_hosts
998: in the user's home directory.
999: Additionally, the file
1000: .Pa /etc/ssh/ssh_known_hosts
1001: is automatically checked for known hosts.
1002: Any new hosts are automatically added to the user's file.
1003: If a host's identification ever changes,
1004: .Nm
1005: warns about this and disables password authentication to prevent
1006: server spoofing or man-in-the-middle attacks,
1007: which could otherwise be used to circumvent the encryption.
1008: The
1009: .Cm StrictHostKeyChecking
1010: option can be used to control logins to machines whose
1011: host key is not known or has changed.
1012: .Pp
1.218 jmc 1013: When the user's identity has been accepted by the server, the server
1.357 dtucker 1014: either executes the given command in a non-interactive session or,
1015: if no command has been specified, logs into the machine and gives
1016: the user a normal shell as an interactive session.
1.218 jmc 1017: All communication with
1018: the remote command or shell will be automatically encrypted.
1019: .Pp
1.357 dtucker 1020: If an interactive session is requested
1021: .Nm
1022: by default will only request a pseudo-terminal (pty) for interactive
1023: sessions when the client has one.
1024: The flags
1025: .Fl T
1026: and
1027: .Fl t
1028: can be used to override this behaviour.
1029: .Pp
1030: If a pseudo-terminal has been allocated the
1.218 jmc 1031: user may use the escape characters noted below.
1032: .Pp
1.357 dtucker 1033: If no pseudo-terminal has been allocated,
1.218 jmc 1034: the session is transparent and can be used to reliably transfer binary data.
1035: On most systems, setting the escape character to
1036: .Dq none
1037: will also make the session transparent even if a tty is used.
1038: .Pp
1039: The session terminates when the command or shell on the remote
1.247 jmc 1040: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1041: .Sh ESCAPE CHARACTERS
1.218 jmc 1042: When a pseudo-terminal has been requested,
1.2 deraadt 1043: .Nm
1.218 jmc 1044: supports a number of functions through the use of an escape character.
1045: .Pp
1046: A single tilde character can be sent as
1047: .Ic ~~
1048: or by following the tilde by a character other than those described below.
1049: The escape character must always follow a newline to be interpreted as
1050: special.
1051: The escape character can be changed in configuration files using the
1052: .Cm EscapeChar
1053: configuration directive or on the command line by the
1054: .Fl e
1055: option.
1056: .Pp
1057: The supported escapes (assuming the default
1058: .Ql ~ )
1059: are:
1060: .Bl -tag -width Ds
1061: .It Cm ~.
1062: Disconnect.
1063: .It Cm ~^Z
1064: Background
1.234 jmc 1065: .Nm .
1.218 jmc 1066: .It Cm ~#
1067: List forwarded connections.
1068: .It Cm ~&
1069: Background
1.2 deraadt 1070: .Nm
1.218 jmc 1071: at logout when waiting for forwarded connection / X11 sessions to terminate.
1072: .It Cm ~?
1073: Display a list of escape characters.
1074: .It Cm ~B
1075: Send a BREAK to the remote system
1.369 jmc 1076: (only useful if the peer supports it).
1.218 jmc 1077: .It Cm ~C
1078: Open command line.
1079: Currently this allows the addition of port forwardings using the
1.279 stevesk 1080: .Fl L ,
1081: .Fl R
1.218 jmc 1082: and
1.279 stevesk 1083: .Fl D
1.225 jmc 1084: options (see above).
1.322 markus 1085: It also allows the cancellation of existing port-forwardings
1086: with
1.262 stevesk 1087: .Sm off
1.322 markus 1088: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1089: .Sm on
1.322 markus 1090: for local,
1091: .Sm off
1092: .Fl KR Oo Ar bind_address : Oc Ar port
1093: .Sm on
1094: for remote and
1095: .Sm off
1096: .Fl KD Oo Ar bind_address : Oc Ar port
1097: .Sm on
1098: for dynamic port-forwardings.
1.218 jmc 1099: .Ic !\& Ns Ar command
1100: allows the user to execute a local command if the
1101: .Ic PermitLocalCommand
1102: option is enabled in
1.176 jmc 1103: .Xr ssh_config 5 .
1.218 jmc 1104: Basic help is available, using the
1105: .Fl h
1106: option.
1107: .It Cm ~R
1108: Request rekeying of the connection
1.369 jmc 1109: (only useful if the peer supports it).
1.327 dtucker 1110: .It Cm ~V
1111: Decrease the verbosity
1112: .Pq Ic LogLevel
1113: when errors are being written to stderr.
1114: .It Cm ~v
1.328 jmc 1115: Increase the verbosity
1.327 dtucker 1116: .Pq Ic LogLevel
1117: when errors are being written to stderr.
1.176 jmc 1118: .El
1.246 jmc 1119: .Sh TCP FORWARDING
1.402 jmc 1120: Forwarding of arbitrary TCP connections over a secure channel
1121: can be specified either on the command line or in a configuration file.
1.246 jmc 1122: One possible application of TCP forwarding is a secure connection to a
1123: mail server; another is going through firewalls.
1124: .Pp
1.402 jmc 1125: In the example below, we look at encrypting communication for an IRC client,
1126: even though the IRC server it connects to does not directly
1127: support encrypted communication.
1.246 jmc 1128: This works as follows:
1129: the user connects to the remote host using
1130: .Nm ,
1.402 jmc 1131: specifying the ports to be used to forward the connection.
1132: After that it is possible to start the program locally,
1.246 jmc 1133: and
1134: .Nm
1.402 jmc 1135: will encrypt and forward the connection to the remote server.
1.246 jmc 1136: .Pp
1.402 jmc 1137: The following example tunnels an IRC session from the client
1138: to an IRC server at
1.246 jmc 1139: .Dq server.example.com ,
1140: joining channel
1141: .Dq #users ,
1142: nickname
1143: .Dq pinky ,
1.402 jmc 1144: using the standard IRC port, 6667:
1145: .Bd -literal -offset 4n
1146: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1147: $ irc -c '#users' pinky IRC/127.0.0.1
1148: .Ed
1.246 jmc 1149: .Pp
1150: The
1151: .Fl f
1152: option backgrounds
1153: .Nm
1154: and the remote command
1155: .Dq sleep 10
1156: is specified to allow an amount of time
1157: (10 seconds, in the example)
1.402 jmc 1158: to start the program which is going to use the tunnel.
1.246 jmc 1159: If no connections are made within the time specified,
1160: .Nm
1161: will exit.
1162: .Sh X11 FORWARDING
1.218 jmc 1163: If the
1164: .Cm ForwardX11
1165: variable is set to
1166: .Dq yes
1167: (or see the description of the
1.227 jmc 1168: .Fl X ,
1169: .Fl x ,
1.218 jmc 1170: and
1.227 jmc 1171: .Fl Y
1.226 jmc 1172: options above)
1.218 jmc 1173: and the user is using X11 (the
1174: .Ev DISPLAY
1175: environment variable is set), the connection to the X11 display is
1176: automatically forwarded to the remote side in such a way that any X11
1177: programs started from the shell (or command) will go through the
1178: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1179: from the local machine.
1.218 jmc 1180: The user should not manually set
1181: .Ev DISPLAY .
1182: Forwarding of X11 connections can be
1183: configured on the command line or in configuration files.
1184: .Pp
1185: The
1186: .Ev DISPLAY
1187: value set by
1188: .Nm
1189: will point to the server machine, but with a display number greater than zero.
1190: This is normal, and happens because
1191: .Nm
1192: creates a
1193: .Dq proxy
1194: X server on the server machine for forwarding the
1195: connections over the encrypted channel.
1.200 djm 1196: .Pp
1.218 jmc 1197: .Nm
1198: will also automatically set up Xauthority data on the server machine.
1199: For this purpose, it will generate a random authorization cookie,
1200: store it in Xauthority on the server, and verify that any forwarded
1201: connections carry this cookie and replace it by the real cookie when
1202: the connection is opened.
1203: The real authentication cookie is never
1204: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1205: .Pp
1.218 jmc 1206: If the
1207: .Cm ForwardAgent
1208: variable is set to
1209: .Dq yes
1210: (or see the description of the
1211: .Fl A
1.191 djm 1212: and
1.218 jmc 1213: .Fl a
1.226 jmc 1214: options above) and
1.218 jmc 1215: the user is using an authentication agent, the connection to the agent
1216: is automatically forwarded to the remote side.
1.252 jmc 1217: .Sh VERIFYING HOST KEYS
1218: When connecting to a server for the first time,
1219: a fingerprint of the server's public key is presented to the user
1220: (unless the option
1221: .Cm StrictHostKeyChecking
1222: has been disabled).
1223: Fingerprints can be determined using
1224: .Xr ssh-keygen 1 :
1225: .Pp
1226: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1227: .Pp
1.274 grunk 1228: If the fingerprint is already known, it can be matched
1229: and the key can be accepted or rejected.
1.358 djm 1230: If only legacy (MD5) fingerprints for the server are available, the
1231: .Xr ssh-keygen 1
1232: .Fl E
1233: option may be used to downgrade the fingerprint algorithm to match.
1234: .Pp
1.274 grunk 1235: Because of the difficulty of comparing host keys
1.352 djm 1236: just by looking at fingerprint strings,
1.274 grunk 1237: there is also support to compare host keys visually,
1238: using
1239: .Em random art .
1240: By setting the
1.275 grunk 1241: .Cm VisualHostKey
1.274 grunk 1242: option to
1.275 grunk 1243: .Dq yes ,
1.274 grunk 1244: a small ASCII graphic gets displayed on every login to a server, no matter
1245: if the session itself is interactive or not.
1246: By learning the pattern a known server produces, a user can easily
1247: find out that the host key has changed when a completely different pattern
1248: is displayed.
1249: Because these patterns are not unambiguous however, a pattern that looks
1250: similar to the pattern remembered only gives a good probability that the
1251: host key is the same, not guaranteed proof.
1252: .Pp
1253: To get a listing of the fingerprints along with their random art for
1254: all known hosts, the following command line can be used:
1255: .Pp
1256: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1257: .Pp
1.252 jmc 1258: If the fingerprint is unknown,
1259: an alternative method of verification is available:
1260: SSH fingerprints verified by DNS.
1261: An additional resource record (RR),
1262: SSHFP,
1263: is added to a zonefile
1264: and the connecting client is able to match the fingerprint
1265: with that of the key presented.
1266: .Pp
1267: In this example, we are connecting a client to a server,
1268: .Dq host.example.com .
1269: The SSHFP resource records should first be added to the zonefile for
1270: host.example.com:
1271: .Bd -literal -offset indent
1.259 jakob 1272: $ ssh-keygen -r host.example.com.
1.252 jmc 1273: .Ed
1274: .Pp
1275: The output lines will have to be added to the zonefile.
1276: To check that the zone is answering fingerprint queries:
1277: .Pp
1278: .Dl $ dig -t SSHFP host.example.com
1279: .Pp
1280: Finally the client connects:
1281: .Bd -literal -offset indent
1282: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1283: [...]
1284: Matching host key fingerprint found in DNS.
1285: Are you sure you want to continue connecting (yes/no)?
1286: .Ed
1287: .Pp
1288: See the
1289: .Cm VerifyHostKeyDNS
1290: option in
1291: .Xr ssh_config 5
1292: for more information.
1.250 jmc 1293: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1294: .Nm
1295: contains support for Virtual Private Network (VPN) tunnelling
1296: using the
1297: .Xr tun 4
1298: network pseudo-device,
1299: allowing two networks to be joined securely.
1300: The
1301: .Xr sshd_config 5
1302: configuration option
1303: .Cm PermitTunnel
1304: controls whether the server supports this,
1305: and at what level (layer 2 or 3 traffic).
1306: .Pp
1307: The following example would connect client network 10.0.50.0/24
1.265 otto 1308: with remote network 10.0.99.0/24 using a point-to-point connection
1309: from 10.1.1.1 to 10.1.1.2,
1310: provided that the SSH server running on the gateway to the remote network,
1311: at 192.168.1.15, allows it.
1312: .Pp
1313: On the client:
1.250 jmc 1314: .Bd -literal -offset indent
1315: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1316: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1317: # route add 10.0.99.0/24 10.1.1.2
1318: .Ed
1319: .Pp
1320: On the server:
1321: .Bd -literal -offset indent
1322: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1323: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1324: .Ed
1325: .Pp
1326: Client access may be more finely tuned via the
1327: .Pa /root/.ssh/authorized_keys
1328: file (see below) and the
1329: .Cm PermitRootLogin
1330: server option.
1.255 jmc 1331: The following entry would permit connections on
1.250 jmc 1332: .Xr tun 4
1.255 jmc 1333: device 1 from user
1.250 jmc 1334: .Dq jane
1.255 jmc 1335: and on tun device 2 from user
1.250 jmc 1336: .Dq john ,
1337: if
1338: .Cm PermitRootLogin
1339: is set to
1340: .Dq forced-commands-only :
1341: .Bd -literal -offset 2n
1342: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1343: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1344: .Ed
1345: .Pp
1.264 ray 1346: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1347: it may be more suited to temporary setups,
1348: such as for wireless VPNs.
1349: More permanent VPNs are better provided by tools such as
1350: .Xr ipsecctl 8
1351: and
1352: .Xr isakmpd 8 .
1.2 deraadt 1353: .Sh ENVIRONMENT
1354: .Nm
1.1 deraadt 1355: will normally set the following environment variables:
1.237 jmc 1356: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1357: .It Ev DISPLAY
1358: The
1359: .Ev DISPLAY
1.40 aaron 1360: variable indicates the location of the X11 server.
1.44 aaron 1361: It is automatically set by
1.2 deraadt 1362: .Nm
1363: to point to a value of the form
1.233 jmc 1364: .Dq hostname:n ,
1365: where
1366: .Dq hostname
1367: indicates the host where the shell runs, and
1368: .Sq n
1369: is an integer \*(Ge 1.
1.40 aaron 1370: .Nm
1371: uses this special value to forward X11 connections over the secure
1372: channel.
1.107 markus 1373: The user should normally not set
1374: .Ev DISPLAY
1375: explicitly, as that
1.1 deraadt 1376: will render the X11 connection insecure (and will require the user to
1377: manually copy any required authorization cookies).
1.2 deraadt 1378: .It Ev HOME
1.1 deraadt 1379: Set to the path of the user's home directory.
1.2 deraadt 1380: .It Ev LOGNAME
1381: Synonym for
1.12 aaron 1382: .Ev USER ;
1383: set for compatibility with systems that use this variable.
1.2 deraadt 1384: .It Ev MAIL
1.129 stevesk 1385: Set to the path of the user's mailbox.
1.40 aaron 1386: .It Ev PATH
1.2 deraadt 1387: Set to the default
1388: .Ev PATH ,
1389: as specified when compiling
1.234 jmc 1390: .Nm .
1.118 markus 1391: .It Ev SSH_ASKPASS
1392: If
1393: .Nm
1394: needs a passphrase, it will read the passphrase from the current
1395: terminal if it was run from a terminal.
1396: If
1397: .Nm
1398: does not have a terminal associated with it but
1399: .Ev DISPLAY
1400: and
1401: .Ev SSH_ASKPASS
1402: are set, it will execute the program specified by
1403: .Ev SSH_ASKPASS
1404: and open an X11 window to read the passphrase.
1405: This is particularly useful when calling
1406: .Nm
1407: from a
1.196 jmc 1408: .Pa .xsession
1.118 markus 1409: or related script.
1410: (Note that on some machines it
1411: may be necessary to redirect the input from
1412: .Pa /dev/null
1413: to make this work.)
1.413 djm 1414: .It Ev SSH_ASKPASS_REQUIRE
1.414 jmc 1415: Allows further control over the use of an askpass program.
1.413 djm 1416: If this variable is set to
1417: .Dq never
1418: then
1419: .Nm
1420: will never attempt to use one.
1421: If it is set to
1422: .Dq prefer ,
1423: then
1424: .Nm
1425: will prefer to use the askpass program instead of the TTY when requesting
1426: passwords.
1427: Finally, if the variable is set to
1428: .Dq force ,
1429: then the askpass program will be used for all passphrase input regardless
1430: of whether
1431: .Ev DISPLAY
1432: is set.
1.18 markus 1433: .It Ev SSH_AUTH_SOCK
1.233 jmc 1434: Identifies the path of a
1435: .Ux Ns -domain
1436: socket used to communicate with the agent.
1.166 stevesk 1437: .It Ev SSH_CONNECTION
1438: Identifies the client and server ends of the connection.
1.40 aaron 1439: The variable contains
1.233 jmc 1440: four space-separated values: client IP address, client port number,
1441: server IP address, and server port number.
1.73 markus 1442: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1443: This variable contains the original command line if a forced command
1.73 markus 1444: is executed.
1445: It can be used to extract the original arguments.
1.2 deraadt 1446: .It Ev SSH_TTY
1.1 deraadt 1447: This is set to the name of the tty (path to the device) associated
1.40 aaron 1448: with the current shell or command.
1449: If the current session has no tty,
1.1 deraadt 1450: this variable is not set.
1.387 djm 1451: .It Ev SSH_TUNNEL
1452: Optionally set by
1453: .Xr sshd 8
1454: to contain the interface names assigned if tunnel forwarding was
1455: requested by the client.
1.385 djm 1456: .It Ev SSH_USER_AUTH
1457: Optionally set by
1458: .Xr sshd 8 ,
1459: this variable may contain a pathname to a file that lists the authentication
1460: methods successfully used when the session was established, including any
1461: public keys that were used.
1.2 deraadt 1462: .It Ev TZ
1.214 jmc 1463: This variable is set to indicate the present time zone if it
1.257 jmc 1464: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1465: on to new connections).
1.2 deraadt 1466: .It Ev USER
1.1 deraadt 1467: Set to the name of the user logging in.
1.2 deraadt 1468: .El
1469: .Pp
1.44 aaron 1470: Additionally,
1.2 deraadt 1471: .Nm
1.44 aaron 1472: reads
1.207 djm 1473: .Pa ~/.ssh/environment ,
1.2 deraadt 1474: and adds lines of the format
1475: .Dq VARNAME=value
1.233 jmc 1476: to the environment if the file exists and users are allowed to
1.161 marc 1477: change their environment.
1.176 jmc 1478: For more information, see the
1.161 marc 1479: .Cm PermitUserEnvironment
1.162 stevesk 1480: option in
1.161 marc 1481: .Xr sshd_config 5 .
1.2 deraadt 1482: .Sh FILES
1.236 jmc 1483: .Bl -tag -width Ds -compact
1.309 jmc 1484: .It Pa ~/.rhosts
1.240 jmc 1485: This file is used for host-based authentication (see above).
1.92 markus 1486: On some machines this file may need to be
1.240 jmc 1487: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1488: because
1.2 deraadt 1489: .Xr sshd 8
1.40 aaron 1490: reads it as root.
1491: Additionally, this file must be owned by the user,
1492: and must not have write permissions for anyone else.
1493: The recommended
1.1 deraadt 1494: permission for most machines is read/write for the user, and not
1495: accessible by others.
1.2 deraadt 1496: .Pp
1.309 jmc 1497: .It Pa ~/.shosts
1.240 jmc 1498: This file is used in exactly the same way as
1499: .Pa .rhosts ,
1500: but allows host-based authentication without permitting login with
1501: rlogin/rsh.
1.272 mcbride 1502: .Pp
1.309 jmc 1503: .It Pa ~/.ssh/
1.272 mcbride 1504: This directory is the default location for all user-specific configuration
1505: and authentication information.
1506: There is no general requirement to keep the entire contents of this directory
1507: secret, but the recommended permissions are read/write/execute for the user,
1508: and not accessible by others.
1.236 jmc 1509: .Pp
1.309 jmc 1510: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1511: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1512: that can be used for logging in as this user.
1.238 jmc 1513: The format of this file is described in the
1514: .Xr sshd 8
1515: manual page.
1516: This file is not highly sensitive, but the recommended
1517: permissions are read/write for the user, and not accessible by others.
1518: .Pp
1.309 jmc 1519: .It Pa ~/.ssh/config
1.238 jmc 1520: This is the per-user configuration file.
1521: The file format and configuration options are described in
1522: .Xr ssh_config 5 .
1523: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1524: read/write for the user, and not writable by others.
1.238 jmc 1525: .Pp
1.309 jmc 1526: .It Pa ~/.ssh/environment
1.239 jmc 1527: Contains additional definitions for environment variables; see
1528: .Sx ENVIRONMENT ,
1.238 jmc 1529: above.
1530: .Pp
1.309 jmc 1531: .It Pa ~/.ssh/id_dsa
1.310 djm 1532: .It Pa ~/.ssh/id_ecdsa
1.404 naddy 1533: .It Pa ~/.ssh/id_ecdsa_sk
1.343 naddy 1534: .It Pa ~/.ssh/id_ed25519
1.406 naddy 1535: .It Pa ~/.ssh/id_ed25519_sk
1.309 jmc 1536: .It Pa ~/.ssh/id_rsa
1.238 jmc 1537: Contains the private key for authentication.
1538: These files
1539: contain sensitive data and should be readable by the user but not
1540: accessible by others (read/write/execute).
1541: .Nm
1542: will simply ignore a private key file if it is accessible by others.
1543: It is possible to specify a passphrase when
1544: generating the key which will be used to encrypt the
1.389 djm 1545: sensitive part of this file using AES-128.
1.238 jmc 1546: .Pp
1.309 jmc 1547: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1548: .It Pa ~/.ssh/id_ecdsa.pub
1.404 naddy 1549: .It Pa ~/.ssh/id_ecdsa_sk.pub
1.343 naddy 1550: .It Pa ~/.ssh/id_ed25519.pub
1.406 naddy 1551: .It Pa ~/.ssh/id_ed25519_sk.pub
1.309 jmc 1552: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1553: Contains the public key for authentication.
1554: These files are not
1555: sensitive and can (but need not) be readable by anyone.
1556: .Pp
1.309 jmc 1557: .It Pa ~/.ssh/known_hosts
1.244 jmc 1558: Contains a list of host keys for all hosts the user has logged into
1559: that are not already in the systemwide list of known host keys.
1.238 jmc 1560: See
1.244 jmc 1561: .Xr sshd 8
1562: for further details of the format of this file.
1.238 jmc 1563: .Pp
1.309 jmc 1564: .It Pa ~/.ssh/rc
1.238 jmc 1565: Commands in this file are executed by
1566: .Nm
1.245 jmc 1567: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1568: started.
1569: See the
1570: .Xr sshd 8
1571: manual page for more information.
1572: .Pp
1.309 jmc 1573: .It Pa /etc/hosts.equiv
1.240 jmc 1574: This file is for host-based authentication (see above).
1575: It should only be writable by root.
1.236 jmc 1576: .Pp
1.309 jmc 1577: .It Pa /etc/shosts.equiv
1.240 jmc 1578: This file is used in exactly the same way as
1579: .Pa hosts.equiv ,
1580: but allows host-based authentication without permitting login with
1581: rlogin/rsh.
1.236 jmc 1582: .Pp
1.238 jmc 1583: .It Pa /etc/ssh/ssh_config
1584: Systemwide configuration file.
1585: The file format and configuration options are described in
1586: .Xr ssh_config 5 .
1587: .Pp
1.309 jmc 1588: .It Pa /etc/ssh/ssh_host_key
1589: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1590: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1591: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1592: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1593: These files contain the private parts of the host keys
1.245 jmc 1594: and are used for host-based authentication.
1.238 jmc 1595: .Pp
1.309 jmc 1596: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1597: Systemwide list of known host keys.
1598: This file should be prepared by the
1599: system administrator to contain the public host keys of all machines in the
1600: organization.
1.244 jmc 1601: It should be world-readable.
1602: See
1.238 jmc 1603: .Xr sshd 8
1.244 jmc 1604: for further details of the format of this file.
1.236 jmc 1605: .Pp
1.309 jmc 1606: .It Pa /etc/ssh/sshrc
1.1 deraadt 1607: Commands in this file are executed by
1.2 deraadt 1608: .Nm
1.245 jmc 1609: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1610: See the
1.2 deraadt 1611: .Xr sshd 8
1.1 deraadt 1612: manual page for more information.
1.58 itojun 1613: .El
1.312 jmc 1614: .Sh EXIT STATUS
1615: .Nm
1616: exits with the exit status of the remote command or with 255
1617: if an error occurred.
1.2 deraadt 1618: .Sh SEE ALSO
1619: .Xr scp 1 ,
1.83 djm 1620: .Xr sftp 1 ,
1.2 deraadt 1621: .Xr ssh-add 1 ,
1622: .Xr ssh-agent 1 ,
1623: .Xr ssh-keygen 1 ,
1.242 jmc 1624: .Xr ssh-keyscan 1 ,
1.250 jmc 1625: .Xr tun 4 ,
1.159 stevesk 1626: .Xr ssh_config 5 ,
1.160 naddy 1627: .Xr ssh-keysign 8 ,
1.87 itojun 1628: .Xr sshd 8
1.329 jmc 1629: .Sh STANDARDS
1.106 markus 1630: .Rs
1.329 jmc 1631: .%A S. Lehtinen
1632: .%A C. Lonvick
1633: .%D January 2006
1.256 jmc 1634: .%R RFC 4250
1.329 jmc 1635: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1636: .Re
1.329 jmc 1637: .Pp
1.256 jmc 1638: .Rs
1.329 jmc 1639: .%A T. Ylonen
1640: .%A C. Lonvick
1641: .%D January 2006
1.256 jmc 1642: .%R RFC 4251
1.329 jmc 1643: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1644: .Re
1.329 jmc 1645: .Pp
1.256 jmc 1646: .Rs
1.329 jmc 1647: .%A T. Ylonen
1648: .%A C. Lonvick
1649: .%D January 2006
1.256 jmc 1650: .%R RFC 4252
1.329 jmc 1651: .%T The Secure Shell (SSH) Authentication Protocol
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 4253
1.329 jmc 1659: .%T The Secure Shell (SSH) Transport Layer Protocol
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 4254
1.329 jmc 1667: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1668: .Re
1.329 jmc 1669: .Pp
1.256 jmc 1670: .Rs
1.329 jmc 1671: .%A J. Schlyter
1672: .%A W. Griffin
1673: .%D January 2006
1.256 jmc 1674: .%R RFC 4255
1.329 jmc 1675: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1676: .Re
1.329 jmc 1677: .Pp
1.256 jmc 1678: .Rs
1.329 jmc 1679: .%A F. Cusack
1680: .%A M. Forssen
1681: .%D January 2006
1.256 jmc 1682: .%R RFC 4256
1.329 jmc 1683: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1684: .Re
1.329 jmc 1685: .Pp
1.256 jmc 1686: .Rs
1.329 jmc 1687: .%A J. Galbraith
1688: .%A P. Remaker
1689: .%D January 2006
1.256 jmc 1690: .%R RFC 4335
1.329 jmc 1691: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1692: .Re
1.329 jmc 1693: .Pp
1.256 jmc 1694: .Rs
1.329 jmc 1695: .%A M. Bellare
1696: .%A T. Kohno
1697: .%A C. Namprempre
1698: .%D January 2006
1.256 jmc 1699: .%R RFC 4344
1.329 jmc 1700: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1701: .Re
1.329 jmc 1702: .Pp
1.256 jmc 1703: .Rs
1.329 jmc 1704: .%A B. Harris
1705: .%D January 2006
1.256 jmc 1706: .%R RFC 4345
1.329 jmc 1707: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1708: .Re
1.329 jmc 1709: .Pp
1.258 djm 1710: .Rs
1.329 jmc 1711: .%A M. Friedl
1712: .%A N. Provos
1713: .%A W. Simpson
1714: .%D March 2006
1.258 djm 1715: .%R RFC 4419
1.329 jmc 1716: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1717: .Re
1.329 jmc 1718: .Pp
1.266 markus 1719: .Rs
1.329 jmc 1720: .%A J. Galbraith
1721: .%A R. Thayer
1722: .%D November 2006
1.266 markus 1723: .%R RFC 4716
1.329 jmc 1724: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1725: .Re
1.329 jmc 1726: .Pp
1.313 djm 1727: .Rs
1.329 jmc 1728: .%A D. Stebila
1729: .%A J. Green
1730: .%D December 2009
1.313 djm 1731: .%R RFC 5656
1.329 jmc 1732: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1733: .Re
1.329 jmc 1734: .Pp
1.274 grunk 1735: .Rs
1736: .%A A. Perrig
1737: .%A D. Song
1738: .%D 1999
1.329 jmc 1739: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1740: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1741: .Re
1.173 jmc 1742: .Sh AUTHORS
1743: OpenSSH is a derivative of the original and free
1744: ssh 1.2.12 release by Tatu Ylonen.
1745: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1746: Theo de Raadt and Dug Song
1747: removed many bugs, re-added newer features and
1748: created OpenSSH.
1749: Markus Friedl contributed the support for SSH
1750: protocol versions 1.5 and 2.0.
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