Annotation of src/usr.bin/ssh/ssh.1, Revision 1.422
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: .\"
1.422 ! djm 36: .\" $OpenBSD: ssh.1,v 1.421 2021/07/02 05:11:21 dtucker Exp $
! 37: .Dd $Mdocdate: July 2 2021 $
1.2 deraadt 38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.408 jmc 42: .Nd OpenSSH remote login client
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
1.377 djm 45: .Op Fl 46AaCfGgKkMNnqsTtVvXxYy
1.390 djm 46: .Op Fl B Ar bind_interface
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.306 jmc 49: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
1.331 dtucker 50: .Op Fl E Ar log_file
1.2 deraadt 51: .Op Fl e Ar escape_char
1.176 jmc 52: .Op Fl F Ar configfile
1.292 jmc 53: .Op Fl I Ar pkcs11
1.2 deraadt 54: .Op Fl i Ar identity_file
1.386 millert 55: .Op Fl J Ar destination
1.360 millert 56: .Op Fl L Ar address
1.176 jmc 57: .Op Fl l Ar login_name
58: .Op Fl m Ar mac_spec
1.198 djm 59: .Op Fl O Ar ctl_cmd
1.176 jmc 60: .Op Fl o Ar option
61: .Op Fl p Ar port
1.362 djm 62: .Op Fl Q Ar query_option
1.360 millert 63: .Op Fl R Ar address
1.198 djm 64: .Op Fl S Ar ctl_path
1.290 dtucker 65: .Op Fl W Ar host : Ns Ar port
1.306 jmc 66: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.386 millert 67: .Ar destination
1.2 deraadt 68: .Op Ar command
1.44 aaron 69: .Sh DESCRIPTION
1.2 deraadt 70: .Nm
1.96 deraadt 71: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 72: executing commands on a remote machine.
1.365 mmcc 73: It is intended to provide secure encrypted communications between
1.40 aaron 74: two untrusted hosts over an insecure network.
1.348 millert 75: X11 connections, arbitrary TCP ports and
76: .Ux Ns -domain
77: sockets can also be forwarded over the secure channel.
1.2 deraadt 78: .Pp
79: .Nm
1.44 aaron 80: connects and logs into the specified
1.388 jmc 81: .Ar destination ,
1.386 millert 82: which may be specified as either
1.388 jmc 83: .Sm off
84: .Oo user @ Oc hostname
85: .Sm on
86: or a URI of the form
87: .Sm off
88: .No ssh:// Oo user @ Oc hostname Op : port .
89: .Sm on
1.1 deraadt 90: The user must prove
1.420 dtucker 91: their identity to the remote machine using one of several methods
1.365 mmcc 92: (see below).
1.49 markus 93: .Pp
1.386 millert 94: If a
1.176 jmc 95: .Ar command
96: is specified,
1.219 jmc 97: it is executed on the remote host instead of a login shell.
1.2 deraadt 98: .Pp
1.218 jmc 99: The options are as follows:
1.361 millert 100: .Pp
1.360 millert 101: .Bl -tag -width Ds -compact
1.218 jmc 102: .It Fl 4
103: Forces
1.2 deraadt 104: .Nm
1.218 jmc 105: to use IPv4 addresses only.
1.360 millert 106: .Pp
1.218 jmc 107: .It Fl 6
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv6 addresses only.
1.360 millert 111: .Pp
1.218 jmc 112: .It Fl A
1.407 jmc 113: Enables forwarding of connections from an authentication agent such as
114: .Xr ssh-agent 1 .
1.218 jmc 115: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 116: .Pp
1.218 jmc 117: Agent forwarding should be enabled with caution.
118: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 119: (for the agent's
1.286 sobrado 120: .Ux Ns -domain
121: socket) can access the local agent through the forwarded connection.
1.218 jmc 122: An attacker cannot obtain key material from the agent,
123: however they can perform operations on the keys that enable them to
124: authenticate using the identities loaded into the agent.
1.407 jmc 125: A safer alternative may be to use a jump host
126: (see
127: .Fl J ) .
1.360 millert 128: .Pp
1.218 jmc 129: .It Fl a
130: Disables forwarding of the authentication agent connection.
1.390 djm 131: .Pp
1.391 jmc 132: .It Fl B Ar bind_interface
1.390 djm 133: Bind to the address of
1.391 jmc 134: .Ar bind_interface
1.390 djm 135: before attempting to connect to the destination host.
136: This is only useful on systems with more than one address.
1.360 millert 137: .Pp
1.218 jmc 138: .It Fl b Ar bind_address
139: Use
140: .Ar bind_address
141: on the local machine as the source address
142: of the connection.
143: Only useful on systems with more than one address.
1.360 millert 144: .Pp
1.218 jmc 145: .It Fl C
146: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 147: data for forwarded X11, TCP and
148: .Ux Ns -domain
149: connections).
1.218 jmc 150: The compression algorithm is the same used by
1.377 djm 151: .Xr gzip 1 .
1.218 jmc 152: Compression is desirable on modem lines and other
153: slow connections, but will only slow down things on fast networks.
154: The default value can be set on a host-by-host basis in the
155: configuration files; see the
156: .Cm Compression
157: option.
1.360 millert 158: .Pp
1.218 jmc 159: .It Fl c Ar cipher_spec
160: Selects the cipher specification for encrypting the session.
161: .Ar cipher_spec
162: is a comma-separated list of ciphers
163: listed in order of preference.
1.283 jmc 164: See the
165: .Cm Ciphers
1.307 dtucker 166: keyword in
167: .Xr ssh_config 5
168: for more information.
1.360 millert 169: .Pp
1.218 jmc 170: .It Fl D Xo
171: .Sm off
172: .Oo Ar bind_address : Oc
173: .Ar port
174: .Sm on
175: .Xc
176: Specifies a local
177: .Dq dynamic
178: application-level port forwarding.
179: This works by allocating a socket to listen to
180: .Ar port
181: on the local side, optionally bound to the specified
182: .Ar bind_address .
183: Whenever a connection is made to this port, the
184: connection is forwarded over the secure channel, and the application
185: protocol is then used to determine where to connect to from the
186: remote machine.
187: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 188: .Nm
1.218 jmc 189: will act as a SOCKS server.
190: Only root can forward privileged ports.
191: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 192: .Pp
1.308 djm 193: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 194: Only the superuser can forward privileged ports.
195: By default, the local port is bound in accordance with the
196: .Cm GatewayPorts
197: setting.
198: However, an explicit
199: .Ar bind_address
200: may be used to bind the connection to a specific address.
201: The
202: .Ar bind_address
203: of
204: .Dq localhost
205: indicates that the listening port be bound for local use only, while an
206: empty address or
207: .Sq *
208: indicates that the port should be available from all interfaces.
1.360 millert 209: .Pp
1.331 dtucker 210: .It Fl E Ar log_file
211: Append debug logs to
212: .Ar log_file
213: instead of standard error.
1.360 millert 214: .Pp
1.229 jmc 215: .It Fl e Ar escape_char
1.218 jmc 216: Sets the escape character for sessions with a pty (default:
217: .Ql ~ ) .
218: The escape character is only recognized at the beginning of a line.
219: The escape character followed by a dot
220: .Pq Ql \&.
221: closes the connection;
222: followed by control-Z suspends the connection;
223: and followed by itself sends the escape character once.
224: Setting the character to
1.2 deraadt 225: .Dq none
1.218 jmc 226: disables any escapes and makes the session fully transparent.
1.360 millert 227: .Pp
1.218 jmc 228: .It Fl F Ar configfile
229: Specifies an alternative per-user configuration file.
230: If a configuration file is given on the command line,
231: the system-wide configuration file
232: .Pq Pa /etc/ssh/ssh_config
233: will be ignored.
234: The default for the per-user configuration file is
235: .Pa ~/.ssh/config .
1.411 naddy 236: If set to
237: .Dq none ,
238: no configuration files will be read.
1.360 millert 239: .Pp
1.218 jmc 240: .It Fl f
241: Requests
242: .Nm
243: to go to background just before command execution.
244: This is useful if
1.176 jmc 245: .Nm
1.218 jmc 246: is going to ask for passwords or passphrases, but the user
247: wants it in the background.
248: This implies
249: .Fl n .
250: The recommended way to start X11 programs at a remote site is with
251: something like
252: .Ic ssh -f host xterm .
1.277 djm 253: .Pp
254: If the
255: .Cm ExitOnForwardFailure
256: configuration option is set to
257: .Dq yes ,
258: then a client started with
259: .Fl f
260: will wait for all remote port forwards to be successfully established
261: before placing itself in the background.
1.360 millert 262: .Pp
1.350 djm 263: .It Fl G
264: Causes
265: .Nm
266: to print its configuration after evaluating
267: .Cm Host
268: and
269: .Cm Match
270: blocks and exit.
1.360 millert 271: .Pp
1.218 jmc 272: .It Fl g
273: Allows remote hosts to connect to local forwarded ports.
1.346 djm 274: If used on a multiplexed connection, then this option must be specified
275: on the master process.
1.360 millert 276: .Pp
1.291 markus 277: .It Fl I Ar pkcs11
1.294 jmc 278: Specify the PKCS#11 shared library
1.176 jmc 279: .Nm
1.401 naddy 280: should use to communicate with a PKCS#11 token providing keys for user
281: authentication.
1.360 millert 282: .Pp
1.218 jmc 283: .It Fl i Ar identity_file
284: Selects a file from which the identity (private key) for
1.310 djm 285: public key authentication is read.
1.218 jmc 286: The default is
1.310 djm 287: .Pa ~/.ssh/id_dsa ,
1.343 naddy 288: .Pa ~/.ssh/id_ecdsa ,
1.404 naddy 289: .Pa ~/.ssh/id_ecdsa_sk ,
1.406 naddy 290: .Pa ~/.ssh/id_ed25519 ,
291: .Pa ~/.ssh/id_ed25519_sk
1.310 djm 292: and
1.381 naddy 293: .Pa ~/.ssh/id_rsa .
1.218 jmc 294: Identity files may also be specified on
295: a per-host basis in the configuration file.
296: It is possible to have multiple
297: .Fl i
298: options (and multiple identities specified in
299: configuration files).
1.364 jmc 300: If no certificates have been explicitly specified by the
1.363 djm 301: .Cm CertificateFile
302: directive,
1.302 djm 303: .Nm
304: will also try to load certificate information from the filename obtained
305: by appending
306: .Pa -cert.pub
307: to identity filenames.
1.360 millert 308: .Pp
1.386 millert 309: .It Fl J Ar destination
1.375 djm 310: Connect to the target host by first making a
311: .Nm
1.386 millert 312: connection to the jump host described by
313: .Ar destination
1.376 jmc 314: and then establishing a TCP forwarding to the ultimate destination from
1.375 djm 315: there.
316: Multiple jump hops may be specified separated by comma characters.
317: This is a shortcut to specify a
318: .Cm ProxyJump
319: configuration directive.
1.400 djm 320: Note that configuration directives supplied on the command-line generally
321: apply to the destination host and not any specified jump hosts.
322: Use
323: .Pa ~/.ssh/config
324: to specify configuration for jump hosts.
1.375 djm 325: .Pp
1.269 djm 326: .It Fl K
327: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
328: credentials to the server.
1.360 millert 329: .Pp
1.218 jmc 330: .It Fl k
331: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 332: .Pp
1.218 jmc 333: .It Fl L Xo
334: .Sm off
335: .Oo Ar bind_address : Oc
336: .Ar port : host : hostport
337: .Sm on
338: .Xc
1.360 millert 339: .It Fl L Xo
340: .Sm off
341: .Oo Ar bind_address : Oc
342: .Ar port : remote_socket
343: .Sm on
344: .Xc
345: .It Fl L Xo
346: .Sm off
347: .Ar local_socket : host : hostport
348: .Sm on
349: .Xc
350: .It Fl L Xo
351: .Sm off
352: .Ar local_socket : remote_socket
353: .Sm on
354: .Xc
355: Specifies that connections to the given TCP port or Unix socket on the local
356: (client) host are to be forwarded to the given host and port, or Unix socket,
357: on the remote side.
358: This works by allocating a socket to listen to either a TCP
1.218 jmc 359: .Ar port
360: on the local side, optionally bound to the specified
1.360 millert 361: .Ar bind_address ,
362: or to a Unix socket.
363: Whenever a connection is made to the local port or socket, the
1.218 jmc 364: connection is forwarded over the secure channel, and a connection is
1.360 millert 365: made to either
1.218 jmc 366: .Ar host
367: port
1.360 millert 368: .Ar hostport ,
369: or the Unix socket
370: .Ar remote_socket ,
1.218 jmc 371: from the remote machine.
1.360 millert 372: .Pp
1.218 jmc 373: Port forwardings can also be specified in the configuration file.
1.360 millert 374: Only the superuser can forward privileged ports.
1.308 djm 375: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 376: .Pp
1.218 jmc 377: By default, the local port is bound in accordance with the
378: .Cm GatewayPorts
379: setting.
380: However, an explicit
381: .Ar bind_address
382: may be used to bind the connection to a specific address.
1.2 deraadt 383: The
1.218 jmc 384: .Ar bind_address
385: of
386: .Dq localhost
387: indicates that the listening port be bound for local use only, while an
388: empty address or
389: .Sq *
390: indicates that the port should be available from all interfaces.
1.360 millert 391: .Pp
1.218 jmc 392: .It Fl l Ar login_name
393: Specifies the user to log in as on the remote machine.
394: This also may be specified on a per-host basis in the configuration file.
1.360 millert 395: .Pp
1.218 jmc 396: .It Fl M
397: Places the
398: .Nm
399: client into
400: .Dq master
401: mode for connection sharing.
1.231 stevesk 402: Multiple
403: .Fl M
404: options places
405: .Nm
406: into
407: .Dq master
1.397 djm 408: mode but with confirmation required using
409: .Xr ssh-askpass 1
410: before each operation that changes the multiplexing state
411: (e.g. opening a new session).
1.218 jmc 412: Refer to the description of
413: .Cm ControlMaster
414: in
415: .Xr ssh_config 5
416: for details.
1.360 millert 417: .Pp
1.218 jmc 418: .It Fl m Ar mac_spec
1.369 jmc 419: A comma-separated list of MAC (message authentication code) algorithms,
420: specified in order of preference.
1.218 jmc 421: See the
422: .Cm MACs
423: keyword for more information.
1.360 millert 424: .Pp
1.218 jmc 425: .It Fl N
426: Do not execute a remote command.
1.369 jmc 427: This is useful for just forwarding ports.
1.422 ! djm 428: Refer to the description of
! 429: .Cm SessionType
! 430: in
! 431: .Xr ssh_config 5
! 432: for details.
1.360 millert 433: .Pp
1.218 jmc 434: .It Fl n
435: Redirects stdin from
436: .Pa /dev/null
437: (actually, prevents reading from stdin).
438: This must be used when
1.2 deraadt 439: .Nm
1.218 jmc 440: is run in the background.
441: A common trick is to use this to run X11 programs on a remote machine.
442: For example,
443: .Ic ssh -n shadows.cs.hut.fi emacs &
444: will start an emacs on shadows.cs.hut.fi, and the X11
445: connection will be automatically forwarded over an encrypted channel.
446: The
1.2 deraadt 447: .Nm
1.218 jmc 448: program will be put in the background.
449: (This does not work if
1.2 deraadt 450: .Nm
1.218 jmc 451: needs to ask for a password or passphrase; see also the
452: .Fl f
453: option.)
1.360 millert 454: .Pp
1.218 jmc 455: .It Fl O Ar ctl_cmd
456: Control an active connection multiplexing master process.
457: When the
458: .Fl O
459: option is specified, the
460: .Ar ctl_cmd
461: argument is interpreted and passed to the master process.
462: Valid commands are:
463: .Dq check
1.305 markus 464: (check that the master process is running),
465: .Dq forward
1.318 jmc 466: (request forwardings without command execution),
1.323 okan 467: .Dq cancel
468: (cancel forwardings),
1.218 jmc 469: .Dq exit
1.318 jmc 470: (request the master to exit), and
1.317 djm 471: .Dq stop
472: (request the master to stop accepting further multiplexing requests).
1.360 millert 473: .Pp
1.218 jmc 474: .It Fl o Ar option
475: Can be used to give options in the format used in the configuration file.
476: This is useful for specifying options for which there is no separate
477: command-line flag.
478: For full details of the options listed below, and their possible values, see
479: .Xr ssh_config 5 .
1.2 deraadt 480: .Pp
1.218 jmc 481: .Bl -tag -width Ds -offset indent -compact
1.366 jcs 482: .It AddKeysToAgent
1.218 jmc 483: .It AddressFamily
484: .It BatchMode
485: .It BindAddress
1.338 djm 486: .It CanonicalDomains
1.339 djm 487: .It CanonicalizeFallbackLocal
488: .It CanonicalizeHostname
489: .It CanonicalizeMaxDots
490: .It CanonicalizePermittedCNAMEs
1.399 jmc 491: .It CASignatureAlgorithms
1.363 djm 492: .It CertificateFile
1.218 jmc 493: .It CheckHostIP
494: .It Ciphers
495: .It ClearAllForwardings
496: .It Compression
1.380 naddy 497: .It ConnectionAttempts
1.218 jmc 498: .It ConnectTimeout
499: .It ControlMaster
500: .It ControlPath
1.321 djm 501: .It ControlPersist
1.218 jmc 502: .It DynamicForward
503: .It EscapeChar
1.263 markus 504: .It ExitOnForwardFailure
1.353 jmc 505: .It FingerprintHash
1.218 jmc 506: .It ForwardAgent
507: .It ForwardX11
1.321 djm 508: .It ForwardX11Timeout
1.218 jmc 509: .It ForwardX11Trusted
510: .It GatewayPorts
511: .It GlobalKnownHostsFile
512: .It GSSAPIAuthentication
513: .It GSSAPIDelegateCredentials
514: .It HashKnownHosts
515: .It Host
1.418 naddy 516: .It HostbasedAcceptedAlgorithms
1.218 jmc 517: .It HostbasedAuthentication
518: .It HostKeyAlgorithms
519: .It HostKeyAlias
1.403 jmc 520: .It Hostname
1.374 jmc 521: .It IdentitiesOnly
1.371 markus 522: .It IdentityAgent
1.218 jmc 523: .It IdentityFile
1.316 jmc 524: .It IPQoS
1.321 djm 525: .It KbdInteractiveAuthentication
1.218 jmc 526: .It KbdInteractiveDevices
1.314 jmc 527: .It KexAlgorithms
1.415 djm 528: .It KnownHostsCommand
1.218 jmc 529: .It LocalCommand
530: .It LocalForward
531: .It LogLevel
532: .It MACs
1.337 jmc 533: .It Match
1.218 jmc 534: .It NoHostAuthenticationForLocalhost
535: .It NumberOfPasswordPrompts
536: .It PasswordAuthentication
537: .It PermitLocalCommand
1.419 markus 538: .It PermitRemoteOpen
1.291 markus 539: .It PKCS11Provider
1.218 jmc 540: .It Port
541: .It PreferredAuthentications
542: .It ProxyCommand
1.375 djm 543: .It ProxyJump
1.336 jmc 544: .It ProxyUseFdpass
1.416 dtucker 545: .It PubkeyAcceptedAlgorithms
1.218 jmc 546: .It PubkeyAuthentication
1.251 dtucker 547: .It RekeyLimit
1.382 bluhm 548: .It RemoteCommand
1.218 jmc 549: .It RemoteForward
1.319 jmc 550: .It RequestTTY
1.218 jmc 551: .It SendEnv
552: .It ServerAliveInterval
553: .It ServerAliveCountMax
1.422 ! djm 554: .It SessionType
1.395 jmc 555: .It SetEnv
1.347 jmc 556: .It StreamLocalBindMask
557: .It StreamLocalBindUnlink
1.218 jmc 558: .It StrictHostKeyChecking
559: .It TCPKeepAlive
560: .It Tunnel
561: .It TunnelDevice
1.354 djm 562: .It UpdateHostKeys
1.218 jmc 563: .It User
564: .It UserKnownHostsFile
565: .It VerifyHostKeyDNS
1.276 jmc 566: .It VisualHostKey
1.218 jmc 567: .It XAuthLocation
568: .El
1.360 millert 569: .Pp
1.218 jmc 570: .It Fl p Ar port
571: Port to connect to on the remote host.
572: This can be specified on a
573: per-host basis in the configuration file.
1.360 millert 574: .Pp
1.362 djm 575: .It Fl Q Ar query_option
1.332 djm 576: Queries
577: .Nm
1.341 deraadt 578: for the algorithms supported for the specified version 2.
579: The available features are:
580: .Ar cipher
1.332 djm 581: (supported symmetric ciphers),
1.341 deraadt 582: .Ar cipher-auth
1.340 djm 583: (supported symmetric ciphers that support authenticated encryption),
1.398 djm 584: .Ar help
585: (supported query terms for use with the
586: .Fl Q
587: flag),
1.341 deraadt 588: .Ar mac
1.332 djm 589: (supported message integrity codes),
1.341 deraadt 590: .Ar kex
1.332 djm 591: (key exchange algorithms),
1.341 deraadt 592: .Ar key
1.362 djm 593: (key types),
594: .Ar key-cert
595: (certificate key types),
596: .Ar key-plain
1.398 djm 597: (non-certificate key types),
1.410 dtucker 598: .Ar key-sig
599: (all key types and signature algorithms),
1.356 djm 600: .Ar protocol-version
1.398 djm 601: (supported SSH protocol versions), and
602: .Ar sig
603: (supported signature algorithms).
1.410 dtucker 604: Alternatively, any keyword from
605: .Xr ssh_config 5
606: or
607: .Xr sshd_config 5
608: that takes an algorithm list may be used as an alias for the corresponding
609: query_option.
1.360 millert 610: .Pp
1.218 jmc 611: .It Fl q
612: Quiet mode.
1.271 djm 613: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 614: .Pp
1.218 jmc 615: .It Fl R Xo
616: .Sm off
617: .Oo Ar bind_address : Oc
618: .Ar port : host : hostport
619: .Sm on
620: .Xc
1.360 millert 621: .It Fl R Xo
622: .Sm off
623: .Oo Ar bind_address : Oc
624: .Ar port : local_socket
625: .Sm on
626: .Xc
627: .It Fl R Xo
628: .Sm off
629: .Ar remote_socket : host : hostport
630: .Sm on
631: .Xc
632: .It Fl R Xo
633: .Sm off
634: .Ar remote_socket : local_socket
635: .Sm on
636: .Xc
1.384 markus 637: .It Fl R Xo
638: .Sm off
639: .Oo Ar bind_address : Oc
640: .Ar port
641: .Sm on
642: .Xc
1.360 millert 643: Specifies that connections to the given TCP port or Unix socket on the remote
1.384 markus 644: (server) host are to be forwarded to the local side.
645: .Pp
1.360 millert 646: This works by allocating a socket to listen to either a TCP
1.218 jmc 647: .Ar port
1.360 millert 648: or to a Unix socket on the remote side.
649: Whenever a connection is made to this port or Unix socket, the
650: connection is forwarded over the secure channel, and a connection
1.384 markus 651: is made from the local machine to either an explicit destination specified by
1.218 jmc 652: .Ar host
653: port
1.360 millert 654: .Ar hostport ,
655: or
656: .Ar local_socket ,
1.384 markus 657: or, if no explicit destination was specified,
658: .Nm
659: will act as a SOCKS 4/5 proxy and forward connections to the destinations
660: requested by the remote SOCKS client.
1.2 deraadt 661: .Pp
1.218 jmc 662: Port forwardings can also be specified in the configuration file.
663: Privileged ports can be forwarded only when
664: logging in as root on the remote machine.
1.324 jmc 665: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 666: .Pp
1.360 millert 667: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 668: interface only.
1.280 tobias 669: This may be overridden by specifying a
1.218 jmc 670: .Ar bind_address .
671: An empty
672: .Ar bind_address ,
673: or the address
674: .Ql * ,
675: indicates that the remote socket should listen on all interfaces.
676: Specifying a remote
677: .Ar bind_address
678: will only succeed if the server's
679: .Cm GatewayPorts
680: option is enabled (see
681: .Xr sshd_config 5 ) .
1.281 djm 682: .Pp
683: If the
684: .Ar port
685: argument is
1.282 djm 686: .Ql 0 ,
1.281 djm 687: the listen port will be dynamically allocated on the server and reported
688: to the client at run time.
1.305 markus 689: When used together with
690: .Ic -O forward
691: the allocated port will be printed to the standard output.
1.360 millert 692: .Pp
1.218 jmc 693: .It Fl S Ar ctl_path
1.304 jmc 694: Specifies the location of a control socket for connection sharing,
1.303 djm 695: or the string
696: .Dq none
697: to disable connection sharing.
1.218 jmc 698: Refer to the description of
699: .Cm ControlPath
700: and
701: .Cm ControlMaster
702: in
703: .Xr ssh_config 5
704: for details.
1.360 millert 705: .Pp
1.218 jmc 706: .It Fl s
707: May be used to request invocation of a subsystem on the remote system.
1.369 jmc 708: Subsystems facilitate the use of SSH
709: as a secure transport for other applications (e.g.\&
1.218 jmc 710: .Xr sftp 1 ) .
711: The subsystem is specified as the remote command.
1.422 ! djm 712: Refer to the description of
! 713: .Cm SessionType
! 714: in
! 715: .Xr ssh_config 5
! 716: for details.
1.360 millert 717: .Pp
1.218 jmc 718: .It Fl T
1.357 dtucker 719: Disable pseudo-terminal allocation.
1.360 millert 720: .Pp
1.218 jmc 721: .It Fl t
1.357 dtucker 722: Force pseudo-terminal allocation.
1.218 jmc 723: This can be used to execute arbitrary
724: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 725: e.g. when implementing menu services.
1.218 jmc 726: Multiple
727: .Fl t
728: options force tty allocation, even if
1.194 jakob 729: .Nm
1.218 jmc 730: has no local tty.
1.360 millert 731: .Pp
1.218 jmc 732: .It Fl V
733: Display the version number and exit.
1.360 millert 734: .Pp
1.218 jmc 735: .It Fl v
736: Verbose mode.
737: Causes
1.176 jmc 738: .Nm
1.218 jmc 739: to print debugging messages about its progress.
740: This is helpful in
741: debugging connection, authentication, and configuration problems.
742: Multiple
743: .Fl v
744: options increase the verbosity.
745: The maximum is 3.
1.360 millert 746: .Pp
1.290 dtucker 747: .It Fl W Ar host : Ns Ar port
748: Requests that standard input and output on the client be forwarded to
749: .Ar host
750: on
751: .Ar port
752: over the secure channel.
753: Implies
754: .Fl N ,
755: .Fl T ,
756: .Cm ExitOnForwardFailure
757: and
1.373 jmc 758: .Cm ClearAllForwardings ,
759: though these can be overridden in the configuration file or using
1.372 dtucker 760: .Fl o
761: command line options.
1.360 millert 762: .Pp
1.261 stevesk 763: .It Fl w Xo
764: .Ar local_tun Ns Op : Ns Ar remote_tun
765: .Xc
766: Requests
767: tunnel
768: device forwarding with the specified
1.218 jmc 769: .Xr tun 4
1.261 stevesk 770: devices between the client
771: .Pq Ar local_tun
772: and the server
773: .Pq Ar remote_tun .
774: .Pp
1.228 jmc 775: The devices may be specified by numerical ID or the keyword
776: .Dq any ,
777: which uses the next available tunnel device.
1.261 stevesk 778: If
779: .Ar remote_tun
780: is not specified, it defaults to
781: .Dq any .
1.228 jmc 782: See also the
1.218 jmc 783: .Cm Tunnel
1.261 stevesk 784: and
785: .Cm TunnelDevice
786: directives in
1.218 jmc 787: .Xr ssh_config 5 .
1.393 djm 788: .Pp
1.261 stevesk 789: If the
790: .Cm Tunnel
1.393 djm 791: directive is unset, it will be set to the default tunnel mode, which is
1.261 stevesk 792: .Dq point-to-point .
1.393 djm 793: If a different
794: .Cm Tunnel
795: forwarding mode it desired, then it should be specified before
796: .Fl w .
1.360 millert 797: .Pp
1.218 jmc 798: .It Fl X
799: Enables X11 forwarding.
1.54 markus 800: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 801: .Pp
1.218 jmc 802: X11 forwarding should be enabled with caution.
1.168 jmc 803: Users with the ability to bypass file permissions on the remote host
1.218 jmc 804: (for the user's X authorization database)
805: can access the local X11 display through the forwarded connection.
806: An attacker may then be able to perform activities such as keystroke monitoring.
807: .Pp
808: For this reason, X11 forwarding is subjected to X11 SECURITY extension
809: restrictions by default.
810: Please refer to the
811: .Nm
812: .Fl Y
813: option and the
814: .Cm ForwardX11Trusted
815: directive in
816: .Xr ssh_config 5
817: for more information.
1.360 millert 818: .Pp
1.218 jmc 819: .It Fl x
820: Disables X11 forwarding.
1.360 millert 821: .Pp
1.218 jmc 822: .It Fl Y
823: Enables trusted X11 forwarding.
824: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
825: controls.
1.360 millert 826: .Pp
1.278 djm 827: .It Fl y
828: Send log information using the
829: .Xr syslog 3
830: system module.
831: By default this information is sent to stderr.
1.218 jmc 832: .El
1.224 jmc 833: .Pp
834: .Nm
835: may additionally obtain configuration data from
836: a per-user configuration file and a system-wide configuration file.
837: The file format and configuration options are described in
838: .Xr ssh_config 5 .
1.222 jmc 839: .Sh AUTHENTICATION
1.377 djm 840: The OpenSSH SSH client supports SSH protocol 2.
1.222 jmc 841: .Pp
842: The methods available for authentication are:
1.260 jmc 843: GSSAPI-based authentication,
1.222 jmc 844: host-based authentication,
845: public key authentication,
1.421 dtucker 846: keyboard-interactive authentication,
1.222 jmc 847: and password authentication.
848: Authentication methods are tried in the order specified above,
1.369 jmc 849: though
850: .Cm PreferredAuthentications
851: can be used to change the default order.
1.222 jmc 852: .Pp
853: Host-based authentication works as follows:
1.218 jmc 854: If the machine the user logs in from is listed in
855: .Pa /etc/hosts.equiv
856: or
857: .Pa /etc/shosts.equiv
1.412 djm 858: on the remote machine, the user is non-root and the user names are
1.218 jmc 859: the same on both sides, or if the files
860: .Pa ~/.rhosts
861: or
862: .Pa ~/.shosts
863: exist in the user's home directory on the
864: remote machine and contain a line containing the name of the client
865: machine and the name of the user on that machine, the user is
1.222 jmc 866: considered for login.
867: Additionally, the server
868: .Em must
869: be able to verify the client's
870: host key (see the description of
1.218 jmc 871: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 872: and
1.222 jmc 873: .Pa ~/.ssh/known_hosts ,
874: below)
875: for login to be permitted.
1.218 jmc 876: This authentication method closes security holes due to IP
1.222 jmc 877: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 878: [Note to the administrator:
879: .Pa /etc/hosts.equiv ,
880: .Pa ~/.rhosts ,
881: and the rlogin/rsh protocol in general, are inherently insecure and should be
882: disabled if security is desired.]
1.189 dtucker 883: .Pp
1.222 jmc 884: Public key authentication works as follows:
885: The scheme is based on public-key cryptography,
886: using cryptosystems
887: where encryption and decryption are done using separate keys,
888: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 889: The idea is that each user creates a public/private
890: key pair for authentication purposes.
891: The server knows the public key, and only the user knows the private key.
1.222 jmc 892: .Nm
893: implements public key authentication protocol automatically,
1.349 sobrado 894: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.333 jmc 895: The HISTORY section of
1.222 jmc 896: .Xr ssl 8
1.311 jmc 897: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 898: .Pp
1.218 jmc 899: The file
900: .Pa ~/.ssh/authorized_keys
901: lists the public keys that are permitted for logging in.
902: When the user logs in, the
1.2 deraadt 903: .Nm
1.218 jmc 904: program tells the server which key pair it would like to use for
905: authentication.
1.222 jmc 906: The client proves that it has access to the private key
907: and the server checks that the corresponding public key
908: is authorized to accept the account.
1.383 djm 909: .Pp
910: The server may inform the client of errors that prevented public key
911: authentication from succeeding after authentication completes using a
912: different method.
913: These may be viewed by increasing the
914: .Cm LogLevel
915: to
916: .Cm DEBUG
917: or higher (e.g. by using the
918: .Fl v
919: flag).
1.218 jmc 920: .Pp
1.420 dtucker 921: The user creates their key pair by running
1.218 jmc 922: .Xr ssh-keygen 1 .
923: This stores the private key in
1.222 jmc 924: .Pa ~/.ssh/id_dsa
1.369 jmc 925: (DSA),
1.310 djm 926: .Pa ~/.ssh/id_ecdsa
1.369 jmc 927: (ECDSA),
1.404 naddy 928: .Pa ~/.ssh/id_ecdsa_sk
1.409 naddy 929: (authenticator-hosted ECDSA),
1.343 naddy 930: .Pa ~/.ssh/id_ed25519
1.369 jmc 931: (Ed25519),
1.406 naddy 932: .Pa ~/.ssh/id_ed25519_sk
1.409 naddy 933: (authenticator-hosted Ed25519),
1.222 jmc 934: or
935: .Pa ~/.ssh/id_rsa
1.369 jmc 936: (RSA)
1.218 jmc 937: and stores the public key in
1.222 jmc 938: .Pa ~/.ssh/id_dsa.pub
1.369 jmc 939: (DSA),
1.310 djm 940: .Pa ~/.ssh/id_ecdsa.pub
1.369 jmc 941: (ECDSA),
1.404 naddy 942: .Pa ~/.ssh/id_ecdsa_sk.pub
1.409 naddy 943: (authenticator-hosted ECDSA),
1.343 naddy 944: .Pa ~/.ssh/id_ed25519.pub
1.369 jmc 945: (Ed25519),
1.406 naddy 946: .Pa ~/.ssh/id_ed25519_sk.pub
1.409 naddy 947: (authenticator-hosted Ed25519),
1.222 jmc 948: or
949: .Pa ~/.ssh/id_rsa.pub
1.369 jmc 950: (RSA)
1.218 jmc 951: in the user's home directory.
1.222 jmc 952: The user should then copy the public key
1.218 jmc 953: to
954: .Pa ~/.ssh/authorized_keys
1.420 dtucker 955: in their home directory on the remote machine.
1.222 jmc 956: The
1.218 jmc 957: .Pa authorized_keys
958: file corresponds to the conventional
959: .Pa ~/.rhosts
960: file, and has one key
1.222 jmc 961: per line, though the lines can be very long.
1.218 jmc 962: After this, the user can log in without giving the password.
963: .Pp
1.301 jmc 964: A variation on public key authentication
965: is available in the form of certificate authentication:
966: instead of a set of public/private keys,
967: signed certificates are used.
968: This has the advantage that a single trusted certification authority
969: can be used in place of many public/private keys.
1.333 jmc 970: See the CERTIFICATES section of
1.301 jmc 971: .Xr ssh-keygen 1
972: for more information.
973: .Pp
974: The most convenient way to use public key or certificate authentication
975: may be with an authentication agent.
1.218 jmc 976: See
977: .Xr ssh-agent 1
1.366 jcs 978: and (optionally) the
979: .Cm AddKeysToAgent
980: directive in
981: .Xr ssh_config 5
1.218 jmc 982: for more information.
983: .Pp
1.421 dtucker 984: Keyboard-interactive authentication works as follows:
1.222 jmc 985: The server sends an arbitrary
986: .Qq challenge
1.421 dtucker 987: text and prompts for a response, possibly multiple times.
988: Examples of keyboard-interactive authentication include
1.335 jmc 989: .Bx
990: Authentication (see
1.222 jmc 991: .Xr login.conf 5 )
1.335 jmc 992: and PAM (some
993: .Pf non- Ox
994: systems).
1.222 jmc 995: .Pp
996: Finally, if other authentication methods fail,
1.218 jmc 997: .Nm
998: prompts the user for a password.
999: The password is sent to the remote
1000: host for checking; however, since all communications are encrypted,
1001: the password cannot be seen by someone listening on the network.
1.232 jmc 1002: .Pp
1003: .Nm
1004: automatically maintains and checks a database containing
1005: identification for all hosts it has ever been used with.
1006: Host keys are stored in
1007: .Pa ~/.ssh/known_hosts
1008: in the user's home directory.
1009: Additionally, the file
1010: .Pa /etc/ssh/ssh_known_hosts
1011: is automatically checked for known hosts.
1012: Any new hosts are automatically added to the user's file.
1013: If a host's identification ever changes,
1014: .Nm
1015: warns about this and disables password authentication to prevent
1016: server spoofing or man-in-the-middle attacks,
1017: which could otherwise be used to circumvent the encryption.
1018: The
1019: .Cm StrictHostKeyChecking
1020: option can be used to control logins to machines whose
1021: host key is not known or has changed.
1022: .Pp
1.218 jmc 1023: When the user's identity has been accepted by the server, the server
1.357 dtucker 1024: either executes the given command in a non-interactive session or,
1025: if no command has been specified, logs into the machine and gives
1026: the user a normal shell as an interactive session.
1.218 jmc 1027: All communication with
1028: the remote command or shell will be automatically encrypted.
1029: .Pp
1.357 dtucker 1030: If an interactive session is requested
1031: .Nm
1032: by default will only request a pseudo-terminal (pty) for interactive
1033: sessions when the client has one.
1034: The flags
1035: .Fl T
1036: and
1037: .Fl t
1038: can be used to override this behaviour.
1039: .Pp
1040: If a pseudo-terminal has been allocated the
1.218 jmc 1041: user may use the escape characters noted below.
1042: .Pp
1.357 dtucker 1043: If no pseudo-terminal has been allocated,
1.218 jmc 1044: the session is transparent and can be used to reliably transfer binary data.
1045: On most systems, setting the escape character to
1046: .Dq none
1047: will also make the session transparent even if a tty is used.
1048: .Pp
1049: The session terminates when the command or shell on the remote
1.247 jmc 1050: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1051: .Sh ESCAPE CHARACTERS
1.218 jmc 1052: When a pseudo-terminal has been requested,
1.2 deraadt 1053: .Nm
1.218 jmc 1054: supports a number of functions through the use of an escape character.
1055: .Pp
1056: A single tilde character can be sent as
1057: .Ic ~~
1058: or by following the tilde by a character other than those described below.
1059: The escape character must always follow a newline to be interpreted as
1060: special.
1061: The escape character can be changed in configuration files using the
1062: .Cm EscapeChar
1063: configuration directive or on the command line by the
1064: .Fl e
1065: option.
1066: .Pp
1067: The supported escapes (assuming the default
1068: .Ql ~ )
1069: are:
1070: .Bl -tag -width Ds
1071: .It Cm ~.
1072: Disconnect.
1073: .It Cm ~^Z
1074: Background
1.234 jmc 1075: .Nm .
1.218 jmc 1076: .It Cm ~#
1077: List forwarded connections.
1078: .It Cm ~&
1079: Background
1.2 deraadt 1080: .Nm
1.218 jmc 1081: at logout when waiting for forwarded connection / X11 sessions to terminate.
1082: .It Cm ~?
1083: Display a list of escape characters.
1084: .It Cm ~B
1085: Send a BREAK to the remote system
1.369 jmc 1086: (only useful if the peer supports it).
1.218 jmc 1087: .It Cm ~C
1088: Open command line.
1089: Currently this allows the addition of port forwardings using the
1.279 stevesk 1090: .Fl L ,
1091: .Fl R
1.218 jmc 1092: and
1.279 stevesk 1093: .Fl D
1.225 jmc 1094: options (see above).
1.322 markus 1095: It also allows the cancellation of existing port-forwardings
1096: with
1.262 stevesk 1097: .Sm off
1.322 markus 1098: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1099: .Sm on
1.322 markus 1100: for local,
1101: .Sm off
1102: .Fl KR Oo Ar bind_address : Oc Ar port
1103: .Sm on
1104: for remote and
1105: .Sm off
1106: .Fl KD Oo Ar bind_address : Oc Ar port
1107: .Sm on
1108: for dynamic port-forwardings.
1.218 jmc 1109: .Ic !\& Ns Ar command
1110: allows the user to execute a local command if the
1111: .Ic PermitLocalCommand
1112: option is enabled in
1.176 jmc 1113: .Xr ssh_config 5 .
1.218 jmc 1114: Basic help is available, using the
1115: .Fl h
1116: option.
1117: .It Cm ~R
1118: Request rekeying of the connection
1.369 jmc 1119: (only useful if the peer supports it).
1.327 dtucker 1120: .It Cm ~V
1121: Decrease the verbosity
1122: .Pq Ic LogLevel
1123: when errors are being written to stderr.
1124: .It Cm ~v
1.328 jmc 1125: Increase the verbosity
1.327 dtucker 1126: .Pq Ic LogLevel
1127: when errors are being written to stderr.
1.176 jmc 1128: .El
1.246 jmc 1129: .Sh TCP FORWARDING
1.402 jmc 1130: Forwarding of arbitrary TCP connections over a secure channel
1131: can be specified either on the command line or in a configuration file.
1.246 jmc 1132: One possible application of TCP forwarding is a secure connection to a
1133: mail server; another is going through firewalls.
1134: .Pp
1.402 jmc 1135: In the example below, we look at encrypting communication for an IRC client,
1136: even though the IRC server it connects to does not directly
1137: support encrypted communication.
1.246 jmc 1138: This works as follows:
1139: the user connects to the remote host using
1140: .Nm ,
1.402 jmc 1141: specifying the ports to be used to forward the connection.
1142: After that it is possible to start the program locally,
1.246 jmc 1143: and
1144: .Nm
1.402 jmc 1145: will encrypt and forward the connection to the remote server.
1.246 jmc 1146: .Pp
1.402 jmc 1147: The following example tunnels an IRC session from the client
1148: to an IRC server at
1.246 jmc 1149: .Dq server.example.com ,
1150: joining channel
1151: .Dq #users ,
1152: nickname
1153: .Dq pinky ,
1.402 jmc 1154: using the standard IRC port, 6667:
1155: .Bd -literal -offset 4n
1156: $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
1157: $ irc -c '#users' pinky IRC/127.0.0.1
1158: .Ed
1.246 jmc 1159: .Pp
1160: The
1161: .Fl f
1162: option backgrounds
1163: .Nm
1164: and the remote command
1165: .Dq sleep 10
1166: is specified to allow an amount of time
1167: (10 seconds, in the example)
1.402 jmc 1168: to start the program which is going to use the tunnel.
1.246 jmc 1169: If no connections are made within the time specified,
1170: .Nm
1171: will exit.
1172: .Sh X11 FORWARDING
1.218 jmc 1173: If the
1174: .Cm ForwardX11
1175: variable is set to
1176: .Dq yes
1177: (or see the description of the
1.227 jmc 1178: .Fl X ,
1179: .Fl x ,
1.218 jmc 1180: and
1.227 jmc 1181: .Fl Y
1.226 jmc 1182: options above)
1.218 jmc 1183: and the user is using X11 (the
1184: .Ev DISPLAY
1185: environment variable is set), the connection to the X11 display is
1186: automatically forwarded to the remote side in such a way that any X11
1187: programs started from the shell (or command) will go through the
1188: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1189: from the local machine.
1.218 jmc 1190: The user should not manually set
1191: .Ev DISPLAY .
1192: Forwarding of X11 connections can be
1193: configured on the command line or in configuration files.
1194: .Pp
1195: The
1196: .Ev DISPLAY
1197: value set by
1198: .Nm
1199: will point to the server machine, but with a display number greater than zero.
1200: This is normal, and happens because
1201: .Nm
1202: creates a
1203: .Dq proxy
1204: X server on the server machine for forwarding the
1205: connections over the encrypted channel.
1.200 djm 1206: .Pp
1.218 jmc 1207: .Nm
1208: will also automatically set up Xauthority data on the server machine.
1209: For this purpose, it will generate a random authorization cookie,
1210: store it in Xauthority on the server, and verify that any forwarded
1211: connections carry this cookie and replace it by the real cookie when
1212: the connection is opened.
1213: The real authentication cookie is never
1214: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1215: .Pp
1.218 jmc 1216: If the
1217: .Cm ForwardAgent
1218: variable is set to
1219: .Dq yes
1220: (or see the description of the
1221: .Fl A
1.191 djm 1222: and
1.218 jmc 1223: .Fl a
1.226 jmc 1224: options above) and
1.218 jmc 1225: the user is using an authentication agent, the connection to the agent
1226: is automatically forwarded to the remote side.
1.252 jmc 1227: .Sh VERIFYING HOST KEYS
1228: When connecting to a server for the first time,
1229: a fingerprint of the server's public key is presented to the user
1230: (unless the option
1231: .Cm StrictHostKeyChecking
1232: has been disabled).
1233: Fingerprints can be determined using
1234: .Xr ssh-keygen 1 :
1235: .Pp
1236: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1237: .Pp
1.274 grunk 1238: If the fingerprint is already known, it can be matched
1239: and the key can be accepted or rejected.
1.358 djm 1240: If only legacy (MD5) fingerprints for the server are available, the
1241: .Xr ssh-keygen 1
1242: .Fl E
1243: option may be used to downgrade the fingerprint algorithm to match.
1244: .Pp
1.274 grunk 1245: Because of the difficulty of comparing host keys
1.352 djm 1246: just by looking at fingerprint strings,
1.274 grunk 1247: there is also support to compare host keys visually,
1248: using
1249: .Em random art .
1250: By setting the
1.275 grunk 1251: .Cm VisualHostKey
1.274 grunk 1252: option to
1.275 grunk 1253: .Dq yes ,
1.274 grunk 1254: a small ASCII graphic gets displayed on every login to a server, no matter
1255: if the session itself is interactive or not.
1256: By learning the pattern a known server produces, a user can easily
1257: find out that the host key has changed when a completely different pattern
1258: is displayed.
1259: Because these patterns are not unambiguous however, a pattern that looks
1260: similar to the pattern remembered only gives a good probability that the
1261: host key is the same, not guaranteed proof.
1262: .Pp
1263: To get a listing of the fingerprints along with their random art for
1264: all known hosts, the following command line can be used:
1265: .Pp
1266: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1267: .Pp
1.252 jmc 1268: If the fingerprint is unknown,
1269: an alternative method of verification is available:
1270: SSH fingerprints verified by DNS.
1271: An additional resource record (RR),
1272: SSHFP,
1273: is added to a zonefile
1274: and the connecting client is able to match the fingerprint
1275: with that of the key presented.
1276: .Pp
1277: In this example, we are connecting a client to a server,
1278: .Dq host.example.com .
1279: The SSHFP resource records should first be added to the zonefile for
1280: host.example.com:
1281: .Bd -literal -offset indent
1.259 jakob 1282: $ ssh-keygen -r host.example.com.
1.252 jmc 1283: .Ed
1284: .Pp
1285: The output lines will have to be added to the zonefile.
1286: To check that the zone is answering fingerprint queries:
1287: .Pp
1288: .Dl $ dig -t SSHFP host.example.com
1289: .Pp
1290: Finally the client connects:
1291: .Bd -literal -offset indent
1292: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1293: [...]
1294: Matching host key fingerprint found in DNS.
1295: Are you sure you want to continue connecting (yes/no)?
1296: .Ed
1297: .Pp
1298: See the
1299: .Cm VerifyHostKeyDNS
1300: option in
1301: .Xr ssh_config 5
1302: for more information.
1.250 jmc 1303: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1304: .Nm
1305: contains support for Virtual Private Network (VPN) tunnelling
1306: using the
1307: .Xr tun 4
1308: network pseudo-device,
1309: allowing two networks to be joined securely.
1310: The
1311: .Xr sshd_config 5
1312: configuration option
1313: .Cm PermitTunnel
1314: controls whether the server supports this,
1315: and at what level (layer 2 or 3 traffic).
1316: .Pp
1317: The following example would connect client network 10.0.50.0/24
1.265 otto 1318: with remote network 10.0.99.0/24 using a point-to-point connection
1319: from 10.1.1.1 to 10.1.1.2,
1320: provided that the SSH server running on the gateway to the remote network,
1321: at 192.168.1.15, allows it.
1322: .Pp
1323: On the client:
1.250 jmc 1324: .Bd -literal -offset indent
1325: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1326: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1327: # route add 10.0.99.0/24 10.1.1.2
1328: .Ed
1329: .Pp
1330: On the server:
1331: .Bd -literal -offset indent
1332: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1333: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1334: .Ed
1335: .Pp
1336: Client access may be more finely tuned via the
1337: .Pa /root/.ssh/authorized_keys
1338: file (see below) and the
1339: .Cm PermitRootLogin
1340: server option.
1.255 jmc 1341: The following entry would permit connections on
1.250 jmc 1342: .Xr tun 4
1.255 jmc 1343: device 1 from user
1.250 jmc 1344: .Dq jane
1.255 jmc 1345: and on tun device 2 from user
1.250 jmc 1346: .Dq john ,
1347: if
1348: .Cm PermitRootLogin
1349: is set to
1350: .Dq forced-commands-only :
1351: .Bd -literal -offset 2n
1352: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1353: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1354: .Ed
1355: .Pp
1.264 ray 1356: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1357: it may be more suited to temporary setups,
1358: such as for wireless VPNs.
1359: More permanent VPNs are better provided by tools such as
1360: .Xr ipsecctl 8
1361: and
1362: .Xr isakmpd 8 .
1.2 deraadt 1363: .Sh ENVIRONMENT
1364: .Nm
1.1 deraadt 1365: will normally set the following environment variables:
1.237 jmc 1366: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1367: .It Ev DISPLAY
1368: The
1369: .Ev DISPLAY
1.40 aaron 1370: variable indicates the location of the X11 server.
1.44 aaron 1371: It is automatically set by
1.2 deraadt 1372: .Nm
1373: to point to a value of the form
1.233 jmc 1374: .Dq hostname:n ,
1375: where
1376: .Dq hostname
1377: indicates the host where the shell runs, and
1378: .Sq n
1379: is an integer \*(Ge 1.
1.40 aaron 1380: .Nm
1381: uses this special value to forward X11 connections over the secure
1382: channel.
1.107 markus 1383: The user should normally not set
1384: .Ev DISPLAY
1385: explicitly, as that
1.1 deraadt 1386: will render the X11 connection insecure (and will require the user to
1387: manually copy any required authorization cookies).
1.2 deraadt 1388: .It Ev HOME
1.1 deraadt 1389: Set to the path of the user's home directory.
1.2 deraadt 1390: .It Ev LOGNAME
1391: Synonym for
1.12 aaron 1392: .Ev USER ;
1393: set for compatibility with systems that use this variable.
1.2 deraadt 1394: .It Ev MAIL
1.129 stevesk 1395: Set to the path of the user's mailbox.
1.40 aaron 1396: .It Ev PATH
1.2 deraadt 1397: Set to the default
1398: .Ev PATH ,
1399: as specified when compiling
1.234 jmc 1400: .Nm .
1.118 markus 1401: .It Ev SSH_ASKPASS
1402: If
1403: .Nm
1404: needs a passphrase, it will read the passphrase from the current
1405: terminal if it was run from a terminal.
1406: If
1407: .Nm
1408: does not have a terminal associated with it but
1409: .Ev DISPLAY
1410: and
1411: .Ev SSH_ASKPASS
1412: are set, it will execute the program specified by
1413: .Ev SSH_ASKPASS
1414: and open an X11 window to read the passphrase.
1415: This is particularly useful when calling
1416: .Nm
1417: from a
1.196 jmc 1418: .Pa .xsession
1.118 markus 1419: or related script.
1420: (Note that on some machines it
1421: may be necessary to redirect the input from
1422: .Pa /dev/null
1423: to make this work.)
1.413 djm 1424: .It Ev SSH_ASKPASS_REQUIRE
1.414 jmc 1425: Allows further control over the use of an askpass program.
1.413 djm 1426: If this variable is set to
1427: .Dq never
1428: then
1429: .Nm
1430: will never attempt to use one.
1431: If it is set to
1432: .Dq prefer ,
1433: then
1434: .Nm
1435: will prefer to use the askpass program instead of the TTY when requesting
1436: passwords.
1437: Finally, if the variable is set to
1438: .Dq force ,
1439: then the askpass program will be used for all passphrase input regardless
1440: of whether
1441: .Ev DISPLAY
1442: is set.
1.18 markus 1443: .It Ev SSH_AUTH_SOCK
1.233 jmc 1444: Identifies the path of a
1445: .Ux Ns -domain
1446: socket used to communicate with the agent.
1.166 stevesk 1447: .It Ev SSH_CONNECTION
1448: Identifies the client and server ends of the connection.
1.40 aaron 1449: The variable contains
1.233 jmc 1450: four space-separated values: client IP address, client port number,
1451: server IP address, and server port number.
1.73 markus 1452: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1453: This variable contains the original command line if a forced command
1.73 markus 1454: is executed.
1455: It can be used to extract the original arguments.
1.2 deraadt 1456: .It Ev SSH_TTY
1.1 deraadt 1457: This is set to the name of the tty (path to the device) associated
1.40 aaron 1458: with the current shell or command.
1459: If the current session has no tty,
1.1 deraadt 1460: this variable is not set.
1.387 djm 1461: .It Ev SSH_TUNNEL
1462: Optionally set by
1463: .Xr sshd 8
1464: to contain the interface names assigned if tunnel forwarding was
1465: requested by the client.
1.385 djm 1466: .It Ev SSH_USER_AUTH
1467: Optionally set by
1468: .Xr sshd 8 ,
1469: this variable may contain a pathname to a file that lists the authentication
1470: methods successfully used when the session was established, including any
1471: public keys that were used.
1.2 deraadt 1472: .It Ev TZ
1.214 jmc 1473: This variable is set to indicate the present time zone if it
1.257 jmc 1474: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1475: on to new connections).
1.2 deraadt 1476: .It Ev USER
1.1 deraadt 1477: Set to the name of the user logging in.
1.2 deraadt 1478: .El
1479: .Pp
1.44 aaron 1480: Additionally,
1.2 deraadt 1481: .Nm
1.44 aaron 1482: reads
1.207 djm 1483: .Pa ~/.ssh/environment ,
1.2 deraadt 1484: and adds lines of the format
1485: .Dq VARNAME=value
1.233 jmc 1486: to the environment if the file exists and users are allowed to
1.161 marc 1487: change their environment.
1.176 jmc 1488: For more information, see the
1.161 marc 1489: .Cm PermitUserEnvironment
1.162 stevesk 1490: option in
1.161 marc 1491: .Xr sshd_config 5 .
1.2 deraadt 1492: .Sh FILES
1.236 jmc 1493: .Bl -tag -width Ds -compact
1.309 jmc 1494: .It Pa ~/.rhosts
1.240 jmc 1495: This file is used for host-based authentication (see above).
1.92 markus 1496: On some machines this file may need to be
1.240 jmc 1497: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1498: because
1.2 deraadt 1499: .Xr sshd 8
1.40 aaron 1500: reads it as root.
1501: Additionally, this file must be owned by the user,
1502: and must not have write permissions for anyone else.
1503: The recommended
1.1 deraadt 1504: permission for most machines is read/write for the user, and not
1505: accessible by others.
1.2 deraadt 1506: .Pp
1.309 jmc 1507: .It Pa ~/.shosts
1.240 jmc 1508: This file is used in exactly the same way as
1509: .Pa .rhosts ,
1510: but allows host-based authentication without permitting login with
1511: rlogin/rsh.
1.272 mcbride 1512: .Pp
1.309 jmc 1513: .It Pa ~/.ssh/
1.272 mcbride 1514: This directory is the default location for all user-specific configuration
1515: and authentication information.
1516: There is no general requirement to keep the entire contents of this directory
1517: secret, but the recommended permissions are read/write/execute for the user,
1518: and not accessible by others.
1.236 jmc 1519: .Pp
1.309 jmc 1520: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1521: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1522: that can be used for logging in as this user.
1.238 jmc 1523: The format of this file is described in the
1524: .Xr sshd 8
1525: manual page.
1526: This file is not highly sensitive, but the recommended
1527: permissions are read/write for the user, and not accessible by others.
1528: .Pp
1.309 jmc 1529: .It Pa ~/.ssh/config
1.238 jmc 1530: This is the per-user configuration file.
1531: The file format and configuration options are described in
1532: .Xr ssh_config 5 .
1533: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1534: read/write for the user, and not writable by others.
1.238 jmc 1535: .Pp
1.309 jmc 1536: .It Pa ~/.ssh/environment
1.239 jmc 1537: Contains additional definitions for environment variables; see
1538: .Sx ENVIRONMENT ,
1.238 jmc 1539: above.
1540: .Pp
1.309 jmc 1541: .It Pa ~/.ssh/id_dsa
1.310 djm 1542: .It Pa ~/.ssh/id_ecdsa
1.404 naddy 1543: .It Pa ~/.ssh/id_ecdsa_sk
1.343 naddy 1544: .It Pa ~/.ssh/id_ed25519
1.406 naddy 1545: .It Pa ~/.ssh/id_ed25519_sk
1.309 jmc 1546: .It Pa ~/.ssh/id_rsa
1.238 jmc 1547: Contains the private key for authentication.
1548: These files
1549: contain sensitive data and should be readable by the user but not
1550: accessible by others (read/write/execute).
1551: .Nm
1552: will simply ignore a private key file if it is accessible by others.
1553: It is possible to specify a passphrase when
1554: generating the key which will be used to encrypt the
1.389 djm 1555: sensitive part of this file using AES-128.
1.238 jmc 1556: .Pp
1.309 jmc 1557: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1558: .It Pa ~/.ssh/id_ecdsa.pub
1.404 naddy 1559: .It Pa ~/.ssh/id_ecdsa_sk.pub
1.343 naddy 1560: .It Pa ~/.ssh/id_ed25519.pub
1.406 naddy 1561: .It Pa ~/.ssh/id_ed25519_sk.pub
1.309 jmc 1562: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1563: Contains the public key for authentication.
1564: These files are not
1565: sensitive and can (but need not) be readable by anyone.
1566: .Pp
1.309 jmc 1567: .It Pa ~/.ssh/known_hosts
1.244 jmc 1568: Contains a list of host keys for all hosts the user has logged into
1569: that are not already in the systemwide list of known host keys.
1.238 jmc 1570: See
1.244 jmc 1571: .Xr sshd 8
1572: for further details of the format of this file.
1.238 jmc 1573: .Pp
1.309 jmc 1574: .It Pa ~/.ssh/rc
1.238 jmc 1575: Commands in this file are executed by
1576: .Nm
1.245 jmc 1577: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1578: started.
1579: See the
1580: .Xr sshd 8
1581: manual page for more information.
1582: .Pp
1.309 jmc 1583: .It Pa /etc/hosts.equiv
1.240 jmc 1584: This file is for host-based authentication (see above).
1585: It should only be writable by root.
1.236 jmc 1586: .Pp
1.309 jmc 1587: .It Pa /etc/shosts.equiv
1.240 jmc 1588: This file is used in exactly the same way as
1589: .Pa hosts.equiv ,
1590: but allows host-based authentication without permitting login with
1591: rlogin/rsh.
1.236 jmc 1592: .Pp
1.238 jmc 1593: .It Pa /etc/ssh/ssh_config
1594: Systemwide configuration file.
1595: The file format and configuration options are described in
1596: .Xr ssh_config 5 .
1597: .Pp
1.309 jmc 1598: .It Pa /etc/ssh/ssh_host_key
1599: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1600: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1601: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1602: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1603: These files contain the private parts of the host keys
1.245 jmc 1604: and are used for host-based authentication.
1.238 jmc 1605: .Pp
1.309 jmc 1606: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1607: Systemwide list of known host keys.
1608: This file should be prepared by the
1609: system administrator to contain the public host keys of all machines in the
1610: organization.
1.244 jmc 1611: It should be world-readable.
1612: See
1.238 jmc 1613: .Xr sshd 8
1.244 jmc 1614: for further details of the format of this file.
1.236 jmc 1615: .Pp
1.309 jmc 1616: .It Pa /etc/ssh/sshrc
1.1 deraadt 1617: Commands in this file are executed by
1.2 deraadt 1618: .Nm
1.245 jmc 1619: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1620: See the
1.2 deraadt 1621: .Xr sshd 8
1.1 deraadt 1622: manual page for more information.
1.58 itojun 1623: .El
1.312 jmc 1624: .Sh EXIT STATUS
1625: .Nm
1626: exits with the exit status of the remote command or with 255
1627: if an error occurred.
1.2 deraadt 1628: .Sh SEE ALSO
1629: .Xr scp 1 ,
1.83 djm 1630: .Xr sftp 1 ,
1.2 deraadt 1631: .Xr ssh-add 1 ,
1632: .Xr ssh-agent 1 ,
1633: .Xr ssh-keygen 1 ,
1.242 jmc 1634: .Xr ssh-keyscan 1 ,
1.250 jmc 1635: .Xr tun 4 ,
1.159 stevesk 1636: .Xr ssh_config 5 ,
1.160 naddy 1637: .Xr ssh-keysign 8 ,
1.87 itojun 1638: .Xr sshd 8
1.329 jmc 1639: .Sh STANDARDS
1.106 markus 1640: .Rs
1.329 jmc 1641: .%A S. Lehtinen
1642: .%A C. Lonvick
1643: .%D January 2006
1.256 jmc 1644: .%R RFC 4250
1.329 jmc 1645: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1646: .Re
1.329 jmc 1647: .Pp
1.256 jmc 1648: .Rs
1.329 jmc 1649: .%A T. Ylonen
1650: .%A C. Lonvick
1651: .%D January 2006
1.256 jmc 1652: .%R RFC 4251
1.329 jmc 1653: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1654: .Re
1.329 jmc 1655: .Pp
1.256 jmc 1656: .Rs
1.329 jmc 1657: .%A T. Ylonen
1658: .%A C. Lonvick
1659: .%D January 2006
1.256 jmc 1660: .%R RFC 4252
1.329 jmc 1661: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1662: .Re
1.329 jmc 1663: .Pp
1.256 jmc 1664: .Rs
1.329 jmc 1665: .%A T. Ylonen
1666: .%A C. Lonvick
1667: .%D January 2006
1.256 jmc 1668: .%R RFC 4253
1.329 jmc 1669: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1670: .Re
1.329 jmc 1671: .Pp
1.256 jmc 1672: .Rs
1.329 jmc 1673: .%A T. Ylonen
1674: .%A C. Lonvick
1675: .%D January 2006
1.256 jmc 1676: .%R RFC 4254
1.329 jmc 1677: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1678: .Re
1.329 jmc 1679: .Pp
1.256 jmc 1680: .Rs
1.329 jmc 1681: .%A J. Schlyter
1682: .%A W. Griffin
1683: .%D January 2006
1.256 jmc 1684: .%R RFC 4255
1.329 jmc 1685: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1686: .Re
1.329 jmc 1687: .Pp
1.256 jmc 1688: .Rs
1.329 jmc 1689: .%A F. Cusack
1690: .%A M. Forssen
1691: .%D January 2006
1.256 jmc 1692: .%R RFC 4256
1.329 jmc 1693: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1694: .Re
1.329 jmc 1695: .Pp
1.256 jmc 1696: .Rs
1.329 jmc 1697: .%A J. Galbraith
1698: .%A P. Remaker
1699: .%D January 2006
1.256 jmc 1700: .%R RFC 4335
1.329 jmc 1701: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1702: .Re
1.329 jmc 1703: .Pp
1.256 jmc 1704: .Rs
1.329 jmc 1705: .%A M. Bellare
1706: .%A T. Kohno
1707: .%A C. Namprempre
1708: .%D January 2006
1.256 jmc 1709: .%R RFC 4344
1.329 jmc 1710: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1711: .Re
1.329 jmc 1712: .Pp
1.256 jmc 1713: .Rs
1.329 jmc 1714: .%A B. Harris
1715: .%D January 2006
1.256 jmc 1716: .%R RFC 4345
1.329 jmc 1717: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1718: .Re
1.329 jmc 1719: .Pp
1.258 djm 1720: .Rs
1.329 jmc 1721: .%A M. Friedl
1722: .%A N. Provos
1723: .%A W. Simpson
1724: .%D March 2006
1.258 djm 1725: .%R RFC 4419
1.329 jmc 1726: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1727: .Re
1.329 jmc 1728: .Pp
1.266 markus 1729: .Rs
1.329 jmc 1730: .%A J. Galbraith
1731: .%A R. Thayer
1732: .%D November 2006
1.266 markus 1733: .%R RFC 4716
1.329 jmc 1734: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1735: .Re
1.329 jmc 1736: .Pp
1.313 djm 1737: .Rs
1.329 jmc 1738: .%A D. Stebila
1739: .%A J. Green
1740: .%D December 2009
1.313 djm 1741: .%R RFC 5656
1.329 jmc 1742: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1743: .Re
1.329 jmc 1744: .Pp
1.274 grunk 1745: .Rs
1746: .%A A. Perrig
1747: .%A D. Song
1748: .%D 1999
1.329 jmc 1749: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1750: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1751: .Re
1.173 jmc 1752: .Sh AUTHORS
1753: OpenSSH is a derivative of the original and free
1754: ssh 1.2.12 release by Tatu Ylonen.
1755: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1756: Theo de Raadt and Dug Song
1757: removed many bugs, re-added newer features and
1758: created OpenSSH.
1759: Markus Friedl contributed the support for SSH
1760: protocol versions 1.5 and 2.0.