Annotation of src/usr.bin/ssh/ssh.1, Revision 1.411
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: .\"
1.411 ! naddy 36: .\" $OpenBSD: ssh.1,v 1.410 2020/02/07 03:54:44 dtucker Exp $
! 37: .Dd $Mdocdate: February 7 2020 $
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
512: .It HostbasedAuthentication
1.355 djm 513: .It HostbasedKeyTypes
1.218 jmc 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
1.218 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
1.291 markus 533: .It PKCS11Provider
1.218 jmc 534: .It Port
535: .It PreferredAuthentications
536: .It ProxyCommand
1.375 djm 537: .It ProxyJump
1.336 jmc 538: .It ProxyUseFdpass
1.359 markus 539: .It PubkeyAcceptedKeyTypes
1.218 jmc 540: .It PubkeyAuthentication
1.251 dtucker 541: .It RekeyLimit
1.382 bluhm 542: .It RemoteCommand
1.218 jmc 543: .It RemoteForward
1.319 jmc 544: .It RequestTTY
1.218 jmc 545: .It SendEnv
546: .It ServerAliveInterval
547: .It ServerAliveCountMax
1.395 jmc 548: .It SetEnv
1.347 jmc 549: .It StreamLocalBindMask
550: .It StreamLocalBindUnlink
1.218 jmc 551: .It StrictHostKeyChecking
552: .It TCPKeepAlive
553: .It Tunnel
554: .It TunnelDevice
1.354 djm 555: .It UpdateHostKeys
1.218 jmc 556: .It User
557: .It UserKnownHostsFile
558: .It VerifyHostKeyDNS
1.276 jmc 559: .It VisualHostKey
1.218 jmc 560: .It XAuthLocation
561: .El
1.360 millert 562: .Pp
1.218 jmc 563: .It Fl p Ar port
564: Port to connect to on the remote host.
565: This can be specified on a
566: per-host basis in the configuration file.
1.360 millert 567: .Pp
1.362 djm 568: .It Fl Q Ar query_option
1.332 djm 569: Queries
570: .Nm
1.341 deraadt 571: for the algorithms supported for the specified version 2.
572: The available features are:
573: .Ar cipher
1.332 djm 574: (supported symmetric ciphers),
1.341 deraadt 575: .Ar cipher-auth
1.340 djm 576: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 577: .Ar help
578: (supported query terms for use with the
579: .Fl Q
580: flag),
1.341 deraadt 581: .Ar mac
1.332 djm 582: (supported message integrity codes),
1.341 deraadt 583: .Ar kex
1.332 djm 584: (key exchange algorithms),
1.341 deraadt 585: .Ar key
1.362 djm 586: (key types),
587: .Ar key-cert
588: (certificate key types),
589: .Ar key-plain
1.398 djm 590: (non-certificate key types),
1.410 dtucker 591: .Ar key-sig
592: (all key types and signature algorithms),
1.356 djm 593: .Ar protocol-version
1.398 djm 594: (supported SSH protocol versions), and
595: .Ar sig
596: (supported signature algorithms).
1.410 dtucker 597: Alternatively, any keyword from
598: .Xr ssh_config 5
599: or
600: .Xr sshd_config 5
601: that takes an algorithm list may be used as an alias for the corresponding
602: query_option.
1.360 millert 603: .Pp
1.218 jmc 604: .It Fl q
605: Quiet mode.
1.271 djm 606: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 607: .Pp
1.218 jmc 608: .It Fl R Xo
609: .Sm off
610: .Oo Ar bind_address : Oc
611: .Ar port : host : hostport
612: .Sm on
613: .Xc
1.360 millert 614: .It Fl R Xo
615: .Sm off
616: .Oo Ar bind_address : Oc
617: .Ar port : local_socket
618: .Sm on
619: .Xc
620: .It Fl R Xo
621: .Sm off
622: .Ar remote_socket : host : hostport
623: .Sm on
624: .Xc
625: .It Fl R Xo
626: .Sm off
627: .Ar remote_socket : local_socket
628: .Sm on
629: .Xc
1.384 markus 630: .It Fl R Xo
631: .Sm off
632: .Oo Ar bind_address : Oc
633: .Ar port
634: .Sm on
635: .Xc
1.360 millert 636: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 637: (server) host are to be forwarded to the local side.
638: .Pp
1.360 millert 639: This works by allocating a socket to listen to either a TCP
1.218 jmc 640: .Ar port
1.360 millert 641: or to a Unix socket on the remote side.
642: Whenever a connection is made to this port or Unix socket, the
643: connection is forwarded over the secure channel, and a connection
1.384 markus 644: is made from the local machine to either an explicit destination specified by
1.218 jmc 645: .Ar host
646: port
1.360 millert 647: .Ar hostport ,
648: or
649: .Ar local_socket ,
1.384 markus 650: or, if no explicit destination was specified,
651: .Nm
652: will act as a SOCKS 4/5 proxy and forward connections to the destinations
653: requested by the remote SOCKS client.
1.2 deraadt 654: .Pp
1.218 jmc 655: Port forwardings can also be specified in the configuration file.
656: Privileged ports can be forwarded only when
657: logging in as root on the remote machine.
1.324 jmc 658: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 659: .Pp
1.360 millert 660: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 661: interface only.
1.280 tobias 662: This may be overridden by specifying a
1.218 jmc 663: .Ar bind_address .
664: An empty
665: .Ar bind_address ,
666: or the address
667: .Ql * ,
668: indicates that the remote socket should listen on all interfaces.
669: Specifying a remote
670: .Ar bind_address
671: will only succeed if the server's
672: .Cm GatewayPorts
673: option is enabled (see
674: .Xr sshd_config 5 ) .
1.281 djm 675: .Pp
676: If the
677: .Ar port
678: argument is
1.282 djm 679: .Ql 0 ,
1.281 djm 680: the listen port will be dynamically allocated on the server and reported
681: to the client at run time.
1.305 markus 682: When used together with
683: .Ic -O forward
684: the allocated port will be printed to the standard output.
1.360 millert 685: .Pp
1.218 jmc 686: .It Fl S Ar ctl_path
1.304 jmc 687: Specifies the location of a control socket for connection sharing,
1.303 djm 688: or the string
689: .Dq none
690: to disable connection sharing.
1.218 jmc 691: Refer to the description of
692: .Cm ControlPath
693: and
694: .Cm ControlMaster
695: in
696: .Xr ssh_config 5
697: for details.
1.360 millert 698: .Pp
1.218 jmc 699: .It Fl s
700: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 701: Subsystems facilitate the use of SSH
702: as a secure transport for other applications (e.g.\&
1.218 jmc 703: .Xr sftp 1 ) .
704: The subsystem is specified as the remote command.
1.360 millert 705: .Pp
1.218 jmc 706: .It Fl T
1.357 dtucker 707: Disable pseudo-terminal allocation.
1.360 millert 708: .Pp
1.218 jmc 709: .It Fl t
1.357 dtucker 710: Force pseudo-terminal allocation.
1.218 jmc 711: This can be used to execute arbitrary
712: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 713: e.g. when implementing menu services.
1.218 jmc 714: Multiple
715: .Fl t
716: options force tty allocation, even if
1.194 jakob 717: .Nm
1.218 jmc 718: has no local tty.
1.360 millert 719: .Pp
1.218 jmc 720: .It Fl V
721: Display the version number and exit.
1.360 millert 722: .Pp
1.218 jmc 723: .It Fl v
724: Verbose mode.
725: Causes
1.176 jmc 726: .Nm
1.218 jmc 727: to print debugging messages about its progress.
728: This is helpful in
729: debugging connection, authentication, and configuration problems.
730: Multiple
731: .Fl v
732: options increase the verbosity.
733: The maximum is 3.
1.360 millert 734: .Pp
1.290 dtucker 735: .It Fl W Ar host : Ns Ar port
736: Requests that standard input and output on the client be forwarded to
737: .Ar host
738: on
739: .Ar port
740: over the secure channel.
741: Implies
742: .Fl N ,
743: .Fl T ,
744: .Cm ExitOnForwardFailure
745: and
1.373 jmc 746: .Cm ClearAllForwardings ,
747: though these can be overridden in the configuration file or using
1.372 dtucker 748: .Fl o
749: command line options.
1.360 millert 750: .Pp
1.261 stevesk 751: .It Fl w Xo
752: .Ar local_tun Ns Op : Ns Ar remote_tun
753: .Xc
754: Requests
755: tunnel
756: device forwarding with the specified
1.218 jmc 757: .Xr tun 4
1.261 stevesk 758: devices between the client
759: .Pq Ar local_tun
760: and the server
761: .Pq Ar remote_tun .
762: .Pp
1.228 jmc 763: The devices may be specified by numerical ID or the keyword
764: .Dq any ,
765: which uses the next available tunnel device.
1.261 stevesk 766: If
767: .Ar remote_tun
768: is not specified, it defaults to
769: .Dq any .
1.228 jmc 770: See also the
1.218 jmc 771: .Cm Tunnel
1.261 stevesk 772: and
773: .Cm TunnelDevice
774: directives in
1.218 jmc 775: .Xr ssh_config 5 .
1.393 djm 776: .Pp
1.261 stevesk 777: If the
778: .Cm Tunnel
1.393 djm 779: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 780: .Dq point-to-point .
1.393 djm 781: If a different
782: .Cm Tunnel
783: forwarding mode it desired, then it should be specified before
784: .Fl w .
1.360 millert 785: .Pp
1.218 jmc 786: .It Fl X
787: Enables X11 forwarding.
1.54 markus 788: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 789: .Pp
1.218 jmc 790: X11 forwarding should be enabled with caution.
1.168 jmc 791: Users with the ability to bypass file permissions on the remote host
1.218 jmc 792: (for the user's X authorization database)
793: can access the local X11 display through the forwarded connection.
794: An attacker may then be able to perform activities such as keystroke monitoring.
795: .Pp
796: For this reason, X11 forwarding is subjected to X11 SECURITY extension
797: restrictions by default.
798: Please refer to the
799: .Nm
800: .Fl Y
801: option and the
802: .Cm ForwardX11Trusted
803: directive in
804: .Xr ssh_config 5
805: for more information.
1.360 millert 806: .Pp
1.218 jmc 807: .It Fl x
808: Disables X11 forwarding.
1.360 millert 809: .Pp
1.218 jmc 810: .It Fl Y
811: Enables trusted X11 forwarding.
812: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
813: controls.
1.360 millert 814: .Pp
1.278 djm 815: .It Fl y
816: Send log information using the
817: .Xr syslog 3
818: system module.
819: By default this information is sent to stderr.
1.218 jmc 820: .El
1.224 jmc 821: .Pp
822: .Nm
823: may additionally obtain configuration data from
824: a per-user configuration file and a system-wide configuration file.
825: The file format and configuration options are described in
826: .Xr ssh_config 5 .
1.222 jmc 827: .Sh AUTHENTICATION
1.377 djm 828: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 829: .Pp
830: The methods available for authentication are:
1.260 jmc 831: GSSAPI-based authentication,
1.222 jmc 832: host-based authentication,
833: public key authentication,
834: challenge-response authentication,
835: and password authentication.
836: Authentication methods are tried in the order specified above,
1.369 jmc 837: though
838: .Cm PreferredAuthentications
839: can be used to change the default order.
1.222 jmc 840: .Pp
841: Host-based authentication works as follows:
1.218 jmc 842: If the machine the user logs in from is listed in
843: .Pa /etc/hosts.equiv
844: or
845: .Pa /etc/shosts.equiv
846: on the remote machine, and the user names are
847: the same on both sides, or if the files
848: .Pa ~/.rhosts
849: or
850: .Pa ~/.shosts
851: exist in the user's home directory on the
852: remote machine and contain a line containing the name of the client
853: machine and the name of the user on that machine, the user is
1.222 jmc 854: considered for login.
855: Additionally, the server
856: .Em must
857: be able to verify the client's
858: host key (see the description of
1.218 jmc 859: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 860: and
1.222 jmc 861: .Pa ~/.ssh/known_hosts ,
862: below)
863: for login to be permitted.
1.218 jmc 864: This authentication method closes security holes due to IP
1.222 jmc 865: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 866: [Note to the administrator:
867: .Pa /etc/hosts.equiv ,
868: .Pa ~/.rhosts ,
869: and the rlogin/rsh protocol in general, are inherently insecure and should be
870: disabled if security is desired.]
1.189 dtucker 871: .Pp
1.222 jmc 872: Public key authentication works as follows:
873: The scheme is based on public-key cryptography,
874: using cryptosystems
875: where encryption and decryption are done using separate keys,
876: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 877: The idea is that each user creates a public/private
878: key pair for authentication purposes.
879: The server knows the public key, and only the user knows the private key.
1.222 jmc 880: .Nm
881: implements public key authentication protocol automatically,
1.349 sobrado 882: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 883: The HISTORY section of
1.222 jmc 884: .Xr ssl 8
1.311 jmc 885: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 886: .Pp
1.218 jmc 887: The file
888: .Pa ~/.ssh/authorized_keys
889: lists the public keys that are permitted for logging in.
890: When the user logs in, the
1.2 deraadt 891: .Nm
1.218 jmc 892: program tells the server which key pair it would like to use for
893: authentication.
1.222 jmc 894: The client proves that it has access to the private key
895: and the server checks that the corresponding public key
896: is authorized to accept the account.
1.383 djm 897: .Pp
898: The server may inform the client of errors that prevented public key
899: authentication from succeeding after authentication completes using a
900: different method.
901: These may be viewed by increasing the
902: .Cm LogLevel
903: to
904: .Cm DEBUG
905: or higher (e.g. by using the
906: .Fl v
907: flag).
1.218 jmc 908: .Pp
1.222 jmc 909: The user creates his/her key pair by running
1.218 jmc 910: .Xr ssh-keygen 1 .
911: This stores the private key in
1.222 jmc 912: .Pa ~/.ssh/id_dsa
1.369 jmc 913: (DSA),
1.310 djm 914: .Pa ~/.ssh/id_ecdsa
1.369 jmc 915: (ECDSA),
1.404 naddy 916: .Pa ~/.ssh/id_ecdsa_sk
1.409 naddy 917: (authenticator-hosted ECDSA),
1.343 naddy 918: .Pa ~/.ssh/id_ed25519
1.369 jmc 919: (Ed25519),
1.406 naddy 920: .Pa ~/.ssh/id_ed25519_sk
1.409 naddy 921: (authenticator-hosted Ed25519),
1.222 jmc 922: or
923: .Pa ~/.ssh/id_rsa
1.369 jmc 924: (RSA)
1.218 jmc 925: and stores the public key in
1.222 jmc 926: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 927: (DSA),
1.310 djm 928: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 929: (ECDSA),
1.404 naddy 930: .Pa ~/.ssh/id_ecdsa_sk.pub
1.409 naddy 931: (authenticator-hosted ECDSA),
1.343 naddy 932: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 933: (Ed25519),
1.406 naddy 934: .Pa ~/.ssh/id_ed25519_sk.pub
1.409 naddy 935: (authenticator-hosted Ed25519),
1.222 jmc 936: or
937: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 938: (RSA)
1.218 jmc 939: in the user's home directory.
1.222 jmc 940: The user should then copy the public key
1.218 jmc 941: to
942: .Pa ~/.ssh/authorized_keys
1.222 jmc 943: in his/her home directory on the remote machine.
944: The
1.218 jmc 945: .Pa authorized_keys
946: file corresponds to the conventional
947: .Pa ~/.rhosts
948: file, and has one key
1.222 jmc 949: per line, though the lines can be very long.
1.218 jmc 950: After this, the user can log in without giving the password.
951: .Pp
1.301 jmc 952: A variation on public key authentication
953: is available in the form of certificate authentication:
954: instead of a set of public/private keys,
955: signed certificates are used.
956: This has the advantage that a single trusted certification authority
957: can be used in place of many public/private keys.
1.333 jmc 958: See the CERTIFICATES section of
1.301 jmc 959: .Xr ssh-keygen 1
960: for more information.
961: .Pp
962: The most convenient way to use public key or certificate authentication
963: may be with an authentication agent.
1.218 jmc 964: See
965: .Xr ssh-agent 1
1.366 jcs 966: and (optionally) the
967: .Cm AddKeysToAgent
968: directive in
969: .Xr ssh_config 5
1.218 jmc 970: for more information.
971: .Pp
1.222 jmc 972: Challenge-response authentication works as follows:
973: The server sends an arbitrary
974: .Qq challenge
975: text, and prompts for a response.
976: Examples of challenge-response authentication include
1.335 jmc 977: .Bx
978: Authentication (see
1.222 jmc 979: .Xr login.conf 5 )
1.335 jmc 980: and PAM (some
981: .Pf non- Ox
982: systems).
1.222 jmc 983: .Pp
984: Finally, if other authentication methods fail,
1.218 jmc 985: .Nm
986: prompts the user for a password.
987: The password is sent to the remote
988: host for checking; however, since all communications are encrypted,
989: the password cannot be seen by someone listening on the network.
1.232 jmc 990: .Pp
991: .Nm
992: automatically maintains and checks a database containing
993: identification for all hosts it has ever been used with.
994: Host keys are stored in
995: .Pa ~/.ssh/known_hosts
996: in the user's home directory.
997: Additionally, the file
998: .Pa /etc/ssh/ssh_known_hosts
999: is automatically checked for known hosts.
1000: Any new hosts are automatically added to the user's file.
1001: If a host's identification ever changes,
1002: .Nm
1003: warns about this and disables password authentication to prevent
1004: server spoofing or man-in-the-middle attacks,
1005: which could otherwise be used to circumvent the encryption.
1006: The
1007: .Cm StrictHostKeyChecking
1008: option can be used to control logins to machines whose
1009: host key is not known or has changed.
1010: .Pp
1.218 jmc 1011: When the user's identity has been accepted by the server, the server
1.357 dtucker 1012: either executes the given command in a non-interactive session or,
1013: if no command has been specified, logs into the machine and gives
1014: the user a normal shell as an interactive session.
1.218 jmc 1015: All communication with
1016: the remote command or shell will be automatically encrypted.
1017: .Pp
1.357 dtucker 1018: If an interactive session is requested
1019: .Nm
1020: by default will only request a pseudo-terminal (pty) for interactive
1021: sessions when the client has one.
1022: The flags
1023: .Fl T
1024: and
1025: .Fl t
1026: can be used to override this behaviour.
1027: .Pp
1028: If a pseudo-terminal has been allocated the
1.218 jmc 1029: user may use the escape characters noted below.
1030: .Pp
1.357 dtucker 1031: If no pseudo-terminal has been allocated,
1.218 jmc 1032: the session is transparent and can be used to reliably transfer binary data.
1033: On most systems, setting the escape character to
1034: .Dq none
1035: will also make the session transparent even if a tty is used.
1036: .Pp
1037: The session terminates when the command or shell on the remote
1.247 jmc 1038: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1039: .Sh ESCAPE CHARACTERS
1.218 jmc 1040: When a pseudo-terminal has been requested,
1.2 deraadt 1041: .Nm
1.218 jmc 1042: supports a number of functions through the use of an escape character.
1043: .Pp
1044: A single tilde character can be sent as
1045: .Ic ~~
1046: or by following the tilde by a character other than those described below.
1047: The escape character must always follow a newline to be interpreted as
1048: special.
1049: The escape character can be changed in configuration files using the
1050: .Cm EscapeChar
1051: configuration directive or on the command line by the
1052: .Fl e
1053: option.
1054: .Pp
1055: The supported escapes (assuming the default
1056: .Ql ~ )
1057: are:
1058: .Bl -tag -width Ds
1059: .It Cm ~.
1060: Disconnect.
1061: .It Cm ~^Z
1062: Background
1.234 jmc 1063: .Nm .
1.218 jmc 1064: .It Cm ~#
1065: List forwarded connections.
1066: .It Cm ~&
1067: Background
1.2 deraadt 1068: .Nm
1.218 jmc 1069: at logout when waiting for forwarded connection / X11 sessions to terminate.
1070: .It Cm ~?
1071: Display a list of escape characters.
1072: .It Cm ~B
1073: Send a BREAK to the remote system
1.369 jmc 1074: (only useful if the peer supports it).
1.218 jmc 1075: .It Cm ~C
1076: Open command line.
1077: Currently this allows the addition of port forwardings using the
1.279 stevesk 1078: .Fl L ,
1079: .Fl R
1.218 jmc 1080: and
1.279 stevesk 1081: .Fl D
1.225 jmc 1082: options (see above).
1.322 markus 1083: It also allows the cancellation of existing port-forwardings
1084: with
1.262 stevesk 1085: .Sm off
1.322 markus 1086: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1087: .Sm on
1.322 markus 1088: for local,
1089: .Sm off
1090: .Fl KR Oo Ar bind_address : Oc Ar port
1091: .Sm on
1092: for remote and
1093: .Sm off
1094: .Fl KD Oo Ar bind_address : Oc Ar port
1095: .Sm on
1096: for dynamic port-forwardings.
1.218 jmc 1097: .Ic !\& Ns Ar command
1098: allows the user to execute a local command if the
1099: .Ic PermitLocalCommand
1100: option is enabled in
1.176 jmc 1101: .Xr ssh_config 5 .
1.218 jmc 1102: Basic help is available, using the
1103: .Fl h
1104: option.
1105: .It Cm ~R
1106: Request rekeying of the connection
1.369 jmc 1107: (only useful if the peer supports it).
1.327 dtucker 1108: .It Cm ~V
1109: Decrease the verbosity
1110: .Pq Ic LogLevel
1111: when errors are being written to stderr.
1112: .It Cm ~v
1.328 jmc 1113: Increase the verbosity
1.327 dtucker 1114: .Pq Ic LogLevel
1115: when errors are being written to stderr.
1.176 jmc 1116: .El
1.246 jmc 1117: .Sh TCP FORWARDING
1.402 jmc 1118: Forwarding of arbitrary TCP connections over a secure channel
1119: can be specified either on the command line or in a configuration file.
1.246 jmc 1120: One possible application of TCP forwarding is a secure connection to a
1121: mail server; another is going through firewalls.
1122: .Pp
1.402 jmc 1123: In the example below, we look at encrypting communication for an IRC client,
1124: even though the IRC server it connects to does not directly
1125: support encrypted communication.
1.246 jmc 1126: This works as follows:
1127: the user connects to the remote host using
1128: .Nm ,
1.402 jmc 1129: specifying the ports to be used to forward the connection.
1130: After that it is possible to start the program locally,
1.246 jmc 1131: and
1132: .Nm
1.402 jmc 1133: will encrypt and forward the connection to the remote server.
1.246 jmc 1134: .Pp
1.402 jmc 1135: The following example tunnels an IRC session from the client
1136: to an IRC server at
1.246 jmc 1137: .Dq server.example.com ,
1138: joining channel
1139: .Dq #users ,
1140: nickname
1141: .Dq pinky ,
1.402 jmc 1142: using the standard IRC port, 6667:
1143: .Bd -literal -offset 4n
1144: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1145: $ irc -c '#users' pinky IRC/127.0.0.1
1146: .Ed
1.246 jmc 1147: .Pp
1148: The
1149: .Fl f
1150: option backgrounds
1151: .Nm
1152: and the remote command
1153: .Dq sleep 10
1154: is specified to allow an amount of time
1155: (10 seconds, in the example)
1.402 jmc 1156: to start the program which is going to use the tunnel.
1.246 jmc 1157: If no connections are made within the time specified,
1158: .Nm
1159: will exit.
1160: .Sh X11 FORWARDING
1.218 jmc 1161: If the
1162: .Cm ForwardX11
1163: variable is set to
1164: .Dq yes
1165: (or see the description of the
1.227 jmc 1166: .Fl X ,
1167: .Fl x ,
1.218 jmc 1168: and
1.227 jmc 1169: .Fl Y
1.226 jmc 1170: options above)
1.218 jmc 1171: and the user is using X11 (the
1172: .Ev DISPLAY
1173: environment variable is set), the connection to the X11 display is
1174: automatically forwarded to the remote side in such a way that any X11
1175: programs started from the shell (or command) will go through the
1176: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1177: from the local machine.
1.218 jmc 1178: The user should not manually set
1179: .Ev DISPLAY .
1180: Forwarding of X11 connections can be
1181: configured on the command line or in configuration files.
1182: .Pp
1183: The
1184: .Ev DISPLAY
1185: value set by
1186: .Nm
1187: will point to the server machine, but with a display number greater than zero.
1188: This is normal, and happens because
1189: .Nm
1190: creates a
1191: .Dq proxy
1192: X server on the server machine for forwarding the
1193: connections over the encrypted channel.
1.200 djm 1194: .Pp
1.218 jmc 1195: .Nm
1196: will also automatically set up Xauthority data on the server machine.
1197: For this purpose, it will generate a random authorization cookie,
1198: store it in Xauthority on the server, and verify that any forwarded
1199: connections carry this cookie and replace it by the real cookie when
1200: the connection is opened.
1201: The real authentication cookie is never
1202: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1203: .Pp
1.218 jmc 1204: If the
1205: .Cm ForwardAgent
1206: variable is set to
1207: .Dq yes
1208: (or see the description of the
1209: .Fl A
1.191 djm 1210: and
1.218 jmc 1211: .Fl a
1.226 jmc 1212: options above) and
1.218 jmc 1213: the user is using an authentication agent, the connection to the agent
1214: is automatically forwarded to the remote side.
1.252 jmc 1215: .Sh VERIFYING HOST KEYS
1216: When connecting to a server for the first time,
1217: a fingerprint of the server's public key is presented to the user
1218: (unless the option
1219: .Cm StrictHostKeyChecking
1220: has been disabled).
1221: Fingerprints can be determined using
1222: .Xr ssh-keygen 1 :
1223: .Pp
1224: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1225: .Pp
1.274 grunk 1226: If the fingerprint is already known, it can be matched
1227: and the key can be accepted or rejected.
1.358 djm 1228: If only legacy (MD5) fingerprints for the server are available, the
1229: .Xr ssh-keygen 1
1230: .Fl E
1231: option may be used to downgrade the fingerprint algorithm to match.
1232: .Pp
1.274 grunk 1233: Because of the difficulty of comparing host keys
1.352 djm 1234: just by looking at fingerprint strings,
1.274 grunk 1235: there is also support to compare host keys visually,
1236: using
1237: .Em random art .
1238: By setting the
1.275 grunk 1239: .Cm VisualHostKey
1.274 grunk 1240: option to
1.275 grunk 1241: .Dq yes ,
1.274 grunk 1242: a small ASCII graphic gets displayed on every login to a server, no matter
1243: if the session itself is interactive or not.
1244: By learning the pattern a known server produces, a user can easily
1245: find out that the host key has changed when a completely different pattern
1246: is displayed.
1247: Because these patterns are not unambiguous however, a pattern that looks
1248: similar to the pattern remembered only gives a good probability that the
1249: host key is the same, not guaranteed proof.
1250: .Pp
1251: To get a listing of the fingerprints along with their random art for
1252: all known hosts, the following command line can be used:
1253: .Pp
1254: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1255: .Pp
1.252 jmc 1256: If the fingerprint is unknown,
1257: an alternative method of verification is available:
1258: SSH fingerprints verified by DNS.
1259: An additional resource record (RR),
1260: SSHFP,
1261: is added to a zonefile
1262: and the connecting client is able to match the fingerprint
1263: with that of the key presented.
1264: .Pp
1265: In this example, we are connecting a client to a server,
1266: .Dq host.example.com .
1267: The SSHFP resource records should first be added to the zonefile for
1268: host.example.com:
1269: .Bd -literal -offset indent
1.259 jakob 1270: $ ssh-keygen -r host.example.com.
1.252 jmc 1271: .Ed
1272: .Pp
1273: The output lines will have to be added to the zonefile.
1274: To check that the zone is answering fingerprint queries:
1275: .Pp
1276: .Dl $ dig -t SSHFP host.example.com
1277: .Pp
1278: Finally the client connects:
1279: .Bd -literal -offset indent
1280: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1281: [...]
1282: Matching host key fingerprint found in DNS.
1283: Are you sure you want to continue connecting (yes/no)?
1284: .Ed
1285: .Pp
1286: See the
1287: .Cm VerifyHostKeyDNS
1288: option in
1289: .Xr ssh_config 5
1290: for more information.
1.250 jmc 1291: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1292: .Nm
1293: contains support for Virtual Private Network (VPN) tunnelling
1294: using the
1295: .Xr tun 4
1296: network pseudo-device,
1297: allowing two networks to be joined securely.
1298: The
1299: .Xr sshd_config 5
1300: configuration option
1301: .Cm PermitTunnel
1302: controls whether the server supports this,
1303: and at what level (layer 2 or 3 traffic).
1304: .Pp
1305: The following example would connect client network 10.0.50.0/24
1.265 otto 1306: with remote network 10.0.99.0/24 using a point-to-point connection
1307: from 10.1.1.1 to 10.1.1.2,
1308: provided that the SSH server running on the gateway to the remote network,
1309: at 192.168.1.15, allows it.
1310: .Pp
1311: On the client:
1.250 jmc 1312: .Bd -literal -offset indent
1313: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1314: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1315: # route add 10.0.99.0/24 10.1.1.2
1316: .Ed
1317: .Pp
1318: On the server:
1319: .Bd -literal -offset indent
1320: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1321: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1322: .Ed
1323: .Pp
1324: Client access may be more finely tuned via the
1325: .Pa /root/.ssh/authorized_keys
1326: file (see below) and the
1327: .Cm PermitRootLogin
1328: server option.
1.255 jmc 1329: The following entry would permit connections on
1.250 jmc 1330: .Xr tun 4
1.255 jmc 1331: device 1 from user
1.250 jmc 1332: .Dq jane
1.255 jmc 1333: and on tun device 2 from user
1.250 jmc 1334: .Dq john ,
1335: if
1336: .Cm PermitRootLogin
1337: is set to
1338: .Dq forced-commands-only :
1339: .Bd -literal -offset 2n
1340: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1341: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1342: .Ed
1343: .Pp
1.264 ray 1344: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1345: it may be more suited to temporary setups,
1346: such as for wireless VPNs.
1347: More permanent VPNs are better provided by tools such as
1348: .Xr ipsecctl 8
1349: and
1350: .Xr isakmpd 8 .
1.2 deraadt 1351: .Sh ENVIRONMENT
1352: .Nm
1.1 deraadt 1353: will normally set the following environment variables:
1.237 jmc 1354: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1355: .It Ev DISPLAY
1356: The
1357: .Ev DISPLAY
1.40 aaron 1358: variable indicates the location of the X11 server.
1.44 aaron 1359: It is automatically set by
1.2 deraadt 1360: .Nm
1361: to point to a value of the form
1.233 jmc 1362: .Dq hostname:n ,
1363: where
1364: .Dq hostname
1365: indicates the host where the shell runs, and
1366: .Sq n
1367: is an integer \*(Ge 1.
1.40 aaron 1368: .Nm
1369: uses this special value to forward X11 connections over the secure
1370: channel.
1.107 markus 1371: The user should normally not set
1372: .Ev DISPLAY
1373: explicitly, as that
1.1 deraadt 1374: will render the X11 connection insecure (and will require the user to
1375: manually copy any required authorization cookies).
1.2 deraadt 1376: .It Ev HOME
1.1 deraadt 1377: Set to the path of the user's home directory.
1.2 deraadt 1378: .It Ev LOGNAME
1379: Synonym for
1.12 aaron 1380: .Ev USER ;
1381: set for compatibility with systems that use this variable.
1.2 deraadt 1382: .It Ev MAIL
1.129 stevesk 1383: Set to the path of the user's mailbox.
1.40 aaron 1384: .It Ev PATH
1.2 deraadt 1385: Set to the default
1386: .Ev PATH ,
1387: as specified when compiling
1.234 jmc 1388: .Nm .
1.118 markus 1389: .It Ev SSH_ASKPASS
1390: If
1391: .Nm
1392: needs a passphrase, it will read the passphrase from the current
1393: terminal if it was run from a terminal.
1394: If
1395: .Nm
1396: does not have a terminal associated with it but
1397: .Ev DISPLAY
1398: and
1399: .Ev SSH_ASKPASS
1400: are set, it will execute the program specified by
1401: .Ev SSH_ASKPASS
1402: and open an X11 window to read the passphrase.
1403: This is particularly useful when calling
1404: .Nm
1405: from a
1.196 jmc 1406: .Pa .xsession
1.118 markus 1407: or related script.
1408: (Note that on some machines it
1409: may be necessary to redirect the input from
1410: .Pa /dev/null
1411: to make this work.)
1.18 markus 1412: .It Ev SSH_AUTH_SOCK
1.233 jmc 1413: Identifies the path of a
1414: .Ux Ns -domain
1415: socket used to communicate with the agent.
1.166 stevesk 1416: .It Ev SSH_CONNECTION
1417: Identifies the client and server ends of the connection.
1.40 aaron 1418: The variable contains
1.233 jmc 1419: four space-separated values: client IP address, client port number,
1420: server IP address, and server port number.
1.73 markus 1421: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1422: This variable contains the original command line if a forced command
1.73 markus 1423: is executed.
1424: It can be used to extract the original arguments.
1.2 deraadt 1425: .It Ev SSH_TTY
1.1 deraadt 1426: This is set to the name of the tty (path to the device) associated
1.40 aaron 1427: with the current shell or command.
1428: If the current session has no tty,
1.1 deraadt 1429: this variable is not set.
1.387 djm 1430: .It Ev SSH_TUNNEL
1431: Optionally set by
1432: .Xr sshd 8
1433: to contain the interface names assigned if tunnel forwarding was
1434: requested by the client.
1.385 djm 1435: .It Ev SSH_USER_AUTH
1436: Optionally set by
1437: .Xr sshd 8 ,
1438: this variable may contain a pathname to a file that lists the authentication
1439: methods successfully used when the session was established, including any
1440: public keys that were used.
1.2 deraadt 1441: .It Ev TZ
1.214 jmc 1442: This variable is set to indicate the present time zone if it
1.257 jmc 1443: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1444: on to new connections).
1.2 deraadt 1445: .It Ev USER
1.1 deraadt 1446: Set to the name of the user logging in.
1.2 deraadt 1447: .El
1448: .Pp
1.44 aaron 1449: Additionally,
1.2 deraadt 1450: .Nm
1.44 aaron 1451: reads
1.207 djm 1452: .Pa ~/.ssh/environment ,
1.2 deraadt 1453: and adds lines of the format
1454: .Dq VARNAME=value
1.233 jmc 1455: to the environment if the file exists and users are allowed to
1.161 marc 1456: change their environment.
1.176 jmc 1457: For more information, see the
1.161 marc 1458: .Cm PermitUserEnvironment
1.162 stevesk 1459: option in
1.161 marc 1460: .Xr sshd_config 5 .
1.2 deraadt 1461: .Sh FILES
1.236 jmc 1462: .Bl -tag -width Ds -compact
1.309 jmc 1463: .It Pa ~/.rhosts
1.240 jmc 1464: This file is used for host-based authentication (see above).
1.92 markus 1465: On some machines this file may need to be
1.240 jmc 1466: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1467: because
1.2 deraadt 1468: .Xr sshd 8
1.40 aaron 1469: reads it as root.
1470: Additionally, this file must be owned by the user,
1471: and must not have write permissions for anyone else.
1472: The recommended
1.1 deraadt 1473: permission for most machines is read/write for the user, and not
1474: accessible by others.
1.2 deraadt 1475: .Pp
1.309 jmc 1476: .It Pa ~/.shosts
1.240 jmc 1477: This file is used in exactly the same way as
1478: .Pa .rhosts ,
1479: but allows host-based authentication without permitting login with
1480: rlogin/rsh.
1.272 mcbride 1481: .Pp
1.309 jmc 1482: .It Pa ~/.ssh/
1.272 mcbride 1483: This directory is the default location for all user-specific configuration
1484: and authentication information.
1485: There is no general requirement to keep the entire contents of this directory
1486: secret, but the recommended permissions are read/write/execute for the user,
1487: and not accessible by others.
1.236 jmc 1488: .Pp
1.309 jmc 1489: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1490: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1491: that can be used for logging in as this user.
1.238 jmc 1492: The format of this file is described in the
1493: .Xr sshd 8
1494: manual page.
1495: This file is not highly sensitive, but the recommended
1496: permissions are read/write for the user, and not accessible by others.
1497: .Pp
1.309 jmc 1498: .It Pa ~/.ssh/config
1.238 jmc 1499: This is the per-user configuration file.
1500: The file format and configuration options are described in
1501: .Xr ssh_config 5 .
1502: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1503: read/write for the user, and not writable by others.
1.238 jmc 1504: .Pp
1.309 jmc 1505: .It Pa ~/.ssh/environment
1.239 jmc 1506: Contains additional definitions for environment variables; see
1507: .Sx ENVIRONMENT ,
1.238 jmc 1508: above.
1509: .Pp
1.309 jmc 1510: .It Pa ~/.ssh/id_dsa
1.310 djm 1511: .It Pa ~/.ssh/id_ecdsa
1.404 naddy 1512: .It Pa ~/.ssh/id_ecdsa_sk
1.343 naddy 1513: .It Pa ~/.ssh/id_ed25519
1.406 naddy 1514: .It Pa ~/.ssh/id_ed25519_sk
1.309 jmc 1515: .It Pa ~/.ssh/id_rsa
1.238 jmc 1516: Contains the private key for authentication.
1517: These files
1518: contain sensitive data and should be readable by the user but not
1519: accessible by others (read/write/execute).
1520: .Nm
1521: will simply ignore a private key file if it is accessible by others.
1522: It is possible to specify a passphrase when
1523: generating the key which will be used to encrypt the
1.389 djm 1524: sensitive part of this file using AES-128.
1.238 jmc 1525: .Pp
1.309 jmc 1526: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1527: .It Pa ~/.ssh/id_ecdsa.pub
1.404 naddy 1528: .It Pa ~/.ssh/id_ecdsa_sk.pub
1.343 naddy 1529: .It Pa ~/.ssh/id_ed25519.pub
1.406 naddy 1530: .It Pa ~/.ssh/id_ed25519_sk.pub
1.309 jmc 1531: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1532: Contains the public key for authentication.
1533: These files are not
1534: sensitive and can (but need not) be readable by anyone.
1535: .Pp
1.309 jmc 1536: .It Pa ~/.ssh/known_hosts
1.244 jmc 1537: Contains a list of host keys for all hosts the user has logged into
1538: that are not already in the systemwide list of known host keys.
1.238 jmc 1539: See
1.244 jmc 1540: .Xr sshd 8
1541: for further details of the format of this file.
1.238 jmc 1542: .Pp
1.309 jmc 1543: .It Pa ~/.ssh/rc
1.238 jmc 1544: Commands in this file are executed by
1545: .Nm
1.245 jmc 1546: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1547: started.
1548: See the
1549: .Xr sshd 8
1550: manual page for more information.
1551: .Pp
1.309 jmc 1552: .It Pa /etc/hosts.equiv
1.240 jmc 1553: This file is for host-based authentication (see above).
1554: It should only be writable by root.
1.236 jmc 1555: .Pp
1.309 jmc 1556: .It Pa /etc/shosts.equiv
1.240 jmc 1557: This file is used in exactly the same way as
1558: .Pa hosts.equiv ,
1559: but allows host-based authentication without permitting login with
1560: rlogin/rsh.
1.236 jmc 1561: .Pp
1.238 jmc 1562: .It Pa /etc/ssh/ssh_config
1563: Systemwide configuration file.
1564: The file format and configuration options are described in
1565: .Xr ssh_config 5 .
1566: .Pp
1.309 jmc 1567: .It Pa /etc/ssh/ssh_host_key
1568: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1569: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1570: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1571: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1572: These files contain the private parts of the host keys
1.245 jmc 1573: and are used for host-based authentication.
1.238 jmc 1574: .Pp
1.309 jmc 1575: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1576: Systemwide list of known host keys.
1577: This file should be prepared by the
1578: system administrator to contain the public host keys of all machines in the
1579: organization.
1.244 jmc 1580: It should be world-readable.
1581: See
1.238 jmc 1582: .Xr sshd 8
1.244 jmc 1583: for further details of the format of this file.
1.236 jmc 1584: .Pp
1.309 jmc 1585: .It Pa /etc/ssh/sshrc
1.1 deraadt 1586: Commands in this file are executed by
1.2 deraadt 1587: .Nm
1.245 jmc 1588: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1589: See the
1.2 deraadt 1590: .Xr sshd 8
1.1 deraadt 1591: manual page for more information.
1.58 itojun 1592: .El
1.312 jmc 1593: .Sh EXIT STATUS
1594: .Nm
1595: exits with the exit status of the remote command or with 255
1596: if an error occurred.
1.2 deraadt 1597: .Sh SEE ALSO
1598: .Xr scp 1 ,
1.83 djm 1599: .Xr sftp 1 ,
1.2 deraadt 1600: .Xr ssh-add 1 ,
1601: .Xr ssh-agent 1 ,
1602: .Xr ssh-keygen 1 ,
1.242 jmc 1603: .Xr ssh-keyscan 1 ,
1.250 jmc 1604: .Xr tun 4 ,
1.159 stevesk 1605: .Xr ssh_config 5 ,
1.160 naddy 1606: .Xr ssh-keysign 8 ,
1.87 itojun 1607: .Xr sshd 8
1.329 jmc 1608: .Sh STANDARDS
1.106 markus 1609: .Rs
1.329 jmc 1610: .%A S. Lehtinen
1611: .%A C. Lonvick
1612: .%D January 2006
1.256 jmc 1613: .%R RFC 4250
1.329 jmc 1614: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1615: .Re
1.329 jmc 1616: .Pp
1.256 jmc 1617: .Rs
1.329 jmc 1618: .%A T. Ylonen
1619: .%A C. Lonvick
1620: .%D January 2006
1.256 jmc 1621: .%R RFC 4251
1.329 jmc 1622: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1623: .Re
1.329 jmc 1624: .Pp
1.256 jmc 1625: .Rs
1.329 jmc 1626: .%A T. Ylonen
1627: .%A C. Lonvick
1628: .%D January 2006
1.256 jmc 1629: .%R RFC 4252
1.329 jmc 1630: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1631: .Re
1.329 jmc 1632: .Pp
1.256 jmc 1633: .Rs
1.329 jmc 1634: .%A T. Ylonen
1635: .%A C. Lonvick
1636: .%D January 2006
1.256 jmc 1637: .%R RFC 4253
1.329 jmc 1638: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1639: .Re
1.329 jmc 1640: .Pp
1.256 jmc 1641: .Rs
1.329 jmc 1642: .%A T. Ylonen
1643: .%A C. Lonvick
1644: .%D January 2006
1.256 jmc 1645: .%R RFC 4254
1.329 jmc 1646: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1647: .Re
1.329 jmc 1648: .Pp
1.256 jmc 1649: .Rs
1.329 jmc 1650: .%A J. Schlyter
1651: .%A W. Griffin
1652: .%D January 2006
1.256 jmc 1653: .%R RFC 4255
1.329 jmc 1654: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1655: .Re
1.329 jmc 1656: .Pp
1.256 jmc 1657: .Rs
1.329 jmc 1658: .%A F. Cusack
1659: .%A M. Forssen
1660: .%D January 2006
1.256 jmc 1661: .%R RFC 4256
1.329 jmc 1662: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1663: .Re
1.329 jmc 1664: .Pp
1.256 jmc 1665: .Rs
1.329 jmc 1666: .%A J. Galbraith
1667: .%A P. Remaker
1668: .%D January 2006
1.256 jmc 1669: .%R RFC 4335
1.329 jmc 1670: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1671: .Re
1.329 jmc 1672: .Pp
1.256 jmc 1673: .Rs
1.329 jmc 1674: .%A M. Bellare
1675: .%A T. Kohno
1676: .%A C. Namprempre
1677: .%D January 2006
1.256 jmc 1678: .%R RFC 4344
1.329 jmc 1679: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1680: .Re
1.329 jmc 1681: .Pp
1.256 jmc 1682: .Rs
1.329 jmc 1683: .%A B. Harris
1684: .%D January 2006
1.256 jmc 1685: .%R RFC 4345
1.329 jmc 1686: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1687: .Re
1.329 jmc 1688: .Pp
1.258 djm 1689: .Rs
1.329 jmc 1690: .%A M. Friedl
1691: .%A N. Provos
1692: .%A W. Simpson
1693: .%D March 2006
1.258 djm 1694: .%R RFC 4419
1.329 jmc 1695: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1696: .Re
1.329 jmc 1697: .Pp
1.266 markus 1698: .Rs
1.329 jmc 1699: .%A J. Galbraith
1700: .%A R. Thayer
1701: .%D November 2006
1.266 markus 1702: .%R RFC 4716
1.329 jmc 1703: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1704: .Re
1.329 jmc 1705: .Pp
1.313 djm 1706: .Rs
1.329 jmc 1707: .%A D. Stebila
1708: .%A J. Green
1709: .%D December 2009
1.313 djm 1710: .%R RFC 5656
1.329 jmc 1711: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1712: .Re
1.329 jmc 1713: .Pp
1.274 grunk 1714: .Rs
1715: .%A A. Perrig
1716: .%A D. Song
1717: .%D 1999
1.329 jmc 1718: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1719: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1720: .Re
1.173 jmc 1721: .Sh AUTHORS
1722: OpenSSH is a derivative of the original and free
1723: ssh 1.2.12 release by Tatu Ylonen.
1724: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1725: Theo de Raadt and Dug Song
1726: removed many bugs, re-added newer features and
1727: created OpenSSH.
1728: Markus Friedl contributed the support for SSH
1729: protocol versions 1.5 and 2.0.
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