Annotation of src/usr.bin/ssh/ssh.1, Revision 1.347
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: .\"
1.347 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.346 2014/07/03 05:38:17 djm Exp $
! 37: .Dd $Mdocdate: July 3 2014 $
1.2 deraadt 38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.96 deraadt 42: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
1.306 jmc 45: .Bk -words
1.278 djm 46: .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
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
1.306 jmc 55: .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.176 jmc 56: .Op Fl l Ar login_name
57: .Op Fl m Ar mac_spec
1.198 djm 58: .Op Fl O Ar ctl_cmd
1.176 jmc 59: .Op Fl o Ar option
60: .Op Fl p Ar port
1.342 jmc 61: .Op Fl Q Cm cipher | cipher-auth | mac | kex | key
1.306 jmc 62: .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.198 djm 63: .Op Fl S Ar ctl_path
1.290 dtucker 64: .Op Fl W Ar host : Ns Ar port
1.306 jmc 65: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.176 jmc 66: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 67: .Op Ar command
1.306 jmc 68: .Ek
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.
1.176 jmc 73: It is intended to replace rlogin and rsh,
74: and provide secure encrypted communications between
1.40 aaron 75: two untrusted hosts over an insecure network.
1.247 jmc 76: X11 connections and arbitrary TCP ports
1.176 jmc 77: 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
1.176 jmc 81: .Ar hostname
82: (with optional
83: .Ar user
84: name).
1.1 deraadt 85: The user must prove
1.49 markus 86: his/her identity to the remote machine using one of several methods
1.221 jmc 87: depending on the protocol version used (see below).
1.49 markus 88: .Pp
1.176 jmc 89: If
90: .Ar command
91: is specified,
1.219 jmc 92: it is executed on the remote host instead of a login shell.
1.2 deraadt 93: .Pp
1.218 jmc 94: The options are as follows:
95: .Bl -tag -width Ds
96: .It Fl 1
97: Forces
1.2 deraadt 98: .Nm
1.218 jmc 99: to try protocol version 1 only.
100: .It Fl 2
101: Forces
1.2 deraadt 102: .Nm
1.218 jmc 103: to try protocol version 2 only.
104: .It Fl 4
105: Forces
1.2 deraadt 106: .Nm
1.218 jmc 107: to use IPv4 addresses only.
108: .It Fl 6
109: Forces
1.2 deraadt 110: .Nm
1.218 jmc 111: to use IPv6 addresses only.
112: .It Fl A
113: Enables forwarding of the authentication agent connection.
114: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 115: .Pp
1.218 jmc 116: Agent forwarding should be enabled with caution.
117: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 118: (for the agent's
1.286 sobrado 119: .Ux Ns -domain
120: socket) can access the local agent through the forwarded connection.
1.218 jmc 121: An attacker cannot obtain key material from the agent,
122: however they can perform operations on the keys that enable them to
123: authenticate using the identities loaded into the agent.
124: .It Fl a
125: Disables forwarding of the authentication agent connection.
126: .It Fl b Ar bind_address
127: Use
128: .Ar bind_address
129: on the local machine as the source address
130: of the connection.
131: Only useful on systems with more than one address.
132: .It Fl C
133: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 134: data for forwarded X11 and TCP connections).
1.218 jmc 135: The compression algorithm is the same used by
136: .Xr gzip 1 ,
137: and the
138: .Dq level
139: can be controlled by the
140: .Cm CompressionLevel
141: option for protocol version 1.
142: Compression is desirable on modem lines and other
143: slow connections, but will only slow down things on fast networks.
144: The default value can be set on a host-by-host basis in the
145: configuration files; see the
146: .Cm Compression
147: option.
148: .It Fl c Ar cipher_spec
149: Selects the cipher specification for encrypting the session.
1.2 deraadt 150: .Pp
1.218 jmc 151: Protocol version 1 allows specification of a single cipher.
152: The supported values are
153: .Dq 3des ,
1.220 jmc 154: .Dq blowfish ,
1.218 jmc 155: and
156: .Dq des .
1.230 jmc 157: For protocol version 2,
1.218 jmc 158: .Ar cipher_spec
159: is a comma-separated list of ciphers
160: listed in order of preference.
1.283 jmc 161: See the
162: .Cm Ciphers
1.307 dtucker 163: keyword in
164: .Xr ssh_config 5
165: for more information.
1.218 jmc 166: .It Fl D Xo
167: .Sm off
168: .Oo Ar bind_address : Oc
169: .Ar port
170: .Sm on
171: .Xc
172: Specifies a local
173: .Dq dynamic
174: application-level port forwarding.
175: This works by allocating a socket to listen to
176: .Ar port
177: on the local side, optionally bound to the specified
178: .Ar bind_address .
179: Whenever a connection is made to this port, the
180: connection is forwarded over the secure channel, and the application
181: protocol is then used to determine where to connect to from the
182: remote machine.
183: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 184: .Nm
1.218 jmc 185: will act as a SOCKS server.
186: Only root can forward privileged ports.
187: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 188: .Pp
1.308 djm 189: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 190: Only the superuser can forward privileged ports.
191: By default, the local port is bound in accordance with the
192: .Cm GatewayPorts
193: setting.
194: However, an explicit
195: .Ar bind_address
196: may be used to bind the connection to a specific address.
197: The
198: .Ar bind_address
199: of
200: .Dq localhost
201: indicates that the listening port be bound for local use only, while an
202: empty address or
203: .Sq *
204: indicates that the port should be available from all interfaces.
1.331 dtucker 205: .It Fl E Ar log_file
206: Append debug logs to
207: .Ar log_file
208: instead of standard error.
1.229 jmc 209: .It Fl e Ar escape_char
1.218 jmc 210: Sets the escape character for sessions with a pty (default:
211: .Ql ~ ) .
212: The escape character is only recognized at the beginning of a line.
213: The escape character followed by a dot
214: .Pq Ql \&.
215: closes the connection;
216: followed by control-Z suspends the connection;
217: and followed by itself sends the escape character once.
218: Setting the character to
1.2 deraadt 219: .Dq none
1.218 jmc 220: disables any escapes and makes the session fully transparent.
221: .It Fl F Ar configfile
222: Specifies an alternative per-user configuration file.
223: If a configuration file is given on the command line,
224: the system-wide configuration file
225: .Pq Pa /etc/ssh/ssh_config
226: will be ignored.
227: The default for the per-user configuration file is
228: .Pa ~/.ssh/config .
229: .It Fl f
230: Requests
231: .Nm
232: to go to background just before command execution.
233: This is useful if
1.176 jmc 234: .Nm
1.218 jmc 235: is going to ask for passwords or passphrases, but the user
236: wants it in the background.
237: This implies
238: .Fl n .
239: The recommended way to start X11 programs at a remote site is with
240: something like
241: .Ic ssh -f host xterm .
1.277 djm 242: .Pp
243: If the
244: .Cm ExitOnForwardFailure
245: configuration option is set to
246: .Dq yes ,
247: then a client started with
248: .Fl f
249: will wait for all remote port forwards to be successfully established
250: before placing itself in the background.
1.218 jmc 251: .It Fl g
252: Allows remote hosts to connect to local forwarded ports.
1.346 djm 253: If used on a multiplexed connection, then this option must be specified
254: on the master process.
1.291 markus 255: .It Fl I Ar pkcs11
1.294 jmc 256: Specify the PKCS#11 shared library
1.176 jmc 257: .Nm
1.293 markus 258: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 259: private RSA key.
260: .It Fl i Ar identity_file
261: Selects a file from which the identity (private key) for
1.310 djm 262: public key authentication is read.
1.218 jmc 263: The default is
264: .Pa ~/.ssh/identity
265: for protocol version 1, and
1.310 djm 266: .Pa ~/.ssh/id_dsa ,
1.343 naddy 267: .Pa ~/.ssh/id_ecdsa ,
268: .Pa ~/.ssh/id_ed25519
1.310 djm 269: and
1.218 jmc 270: .Pa ~/.ssh/id_rsa
271: for protocol version 2.
272: Identity files may also be specified on
273: a per-host basis in the configuration file.
274: It is possible to have multiple
275: .Fl i
276: options (and multiple identities specified in
277: configuration files).
1.302 djm 278: .Nm
279: will also try to load certificate information from the filename obtained
280: by appending
281: .Pa -cert.pub
282: to identity filenames.
1.269 djm 283: .It Fl K
284: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
285: credentials to the server.
1.218 jmc 286: .It Fl k
287: Disables forwarding (delegation) of GSSAPI credentials to the server.
288: .It Fl L Xo
289: .Sm off
290: .Oo Ar bind_address : Oc
291: .Ar port : host : hostport
292: .Sm on
293: .Xc
294: Specifies that the given port on the local (client) host is to be
295: forwarded to the given host and port on the remote side.
296: This works by allocating a socket to listen to
297: .Ar port
298: on the local side, optionally bound to the specified
299: .Ar bind_address .
300: Whenever a connection is made to this port, the
301: connection is forwarded over the secure channel, and a connection is
302: made to
303: .Ar host
304: port
305: .Ar hostport
306: from the remote machine.
307: Port forwardings can also be specified in the configuration file.
1.308 djm 308: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 309: Only the superuser can forward privileged ports.
310: By default, the local port is bound in accordance with the
311: .Cm GatewayPorts
312: setting.
313: However, an explicit
314: .Ar bind_address
315: may be used to bind the connection to a specific address.
1.2 deraadt 316: The
1.218 jmc 317: .Ar bind_address
318: of
319: .Dq localhost
320: indicates that the listening port be bound for local use only, while an
321: empty address or
322: .Sq *
323: indicates that the port should be available from all interfaces.
324: .It Fl l Ar login_name
325: Specifies the user to log in as on the remote machine.
326: This also may be specified on a per-host basis in the configuration file.
327: .It Fl M
328: Places the
329: .Nm
330: client into
331: .Dq master
332: mode for connection sharing.
1.231 stevesk 333: Multiple
334: .Fl M
335: options places
336: .Nm
337: into
338: .Dq master
339: mode with confirmation required before slave connections are accepted.
1.218 jmc 340: Refer to the description of
341: .Cm ControlMaster
342: in
343: .Xr ssh_config 5
344: for details.
345: .It Fl m Ar mac_spec
346: Additionally, for protocol version 2 a comma-separated list of MAC
347: (message authentication code) algorithms can
348: be specified in order of preference.
349: See the
350: .Cm MACs
351: keyword for more information.
352: .It Fl N
353: Do not execute a remote command.
354: This is useful for just forwarding ports
355: (protocol version 2 only).
356: .It Fl n
357: Redirects stdin from
358: .Pa /dev/null
359: (actually, prevents reading from stdin).
360: This must be used when
1.2 deraadt 361: .Nm
1.218 jmc 362: is run in the background.
363: A common trick is to use this to run X11 programs on a remote machine.
364: For example,
365: .Ic ssh -n shadows.cs.hut.fi emacs &
366: will start an emacs on shadows.cs.hut.fi, and the X11
367: connection will be automatically forwarded over an encrypted channel.
368: The
1.2 deraadt 369: .Nm
1.218 jmc 370: program will be put in the background.
371: (This does not work if
1.2 deraadt 372: .Nm
1.218 jmc 373: needs to ask for a password or passphrase; see also the
374: .Fl f
375: option.)
376: .It Fl O Ar ctl_cmd
377: Control an active connection multiplexing master process.
378: When the
379: .Fl O
380: option is specified, the
381: .Ar ctl_cmd
382: argument is interpreted and passed to the master process.
383: Valid commands are:
384: .Dq check
1.305 markus 385: (check that the master process is running),
386: .Dq forward
1.318 jmc 387: (request forwardings without command execution),
1.323 okan 388: .Dq cancel
389: (cancel forwardings),
1.218 jmc 390: .Dq exit
1.318 jmc 391: (request the master to exit), and
1.317 djm 392: .Dq stop
393: (request the master to stop accepting further multiplexing requests).
1.218 jmc 394: .It Fl o Ar option
395: Can be used to give options in the format used in the configuration file.
396: This is useful for specifying options for which there is no separate
397: command-line flag.
398: For full details of the options listed below, and their possible values, see
399: .Xr ssh_config 5 .
1.2 deraadt 400: .Pp
1.218 jmc 401: .Bl -tag -width Ds -offset indent -compact
402: .It AddressFamily
403: .It BatchMode
404: .It BindAddress
1.338 djm 405: .It CanonicalDomains
1.339 djm 406: .It CanonicalizeFallbackLocal
407: .It CanonicalizeHostname
408: .It CanonicalizeMaxDots
409: .It CanonicalizePermittedCNAMEs
1.218 jmc 410: .It ChallengeResponseAuthentication
411: .It CheckHostIP
412: .It Cipher
413: .It Ciphers
414: .It ClearAllForwardings
415: .It Compression
416: .It CompressionLevel
417: .It ConnectionAttempts
418: .It ConnectTimeout
419: .It ControlMaster
420: .It ControlPath
1.321 djm 421: .It ControlPersist
1.218 jmc 422: .It DynamicForward
423: .It EscapeChar
1.263 markus 424: .It ExitOnForwardFailure
1.218 jmc 425: .It ForwardAgent
426: .It ForwardX11
1.321 djm 427: .It ForwardX11Timeout
1.218 jmc 428: .It ForwardX11Trusted
429: .It GatewayPorts
430: .It GlobalKnownHostsFile
431: .It GSSAPIAuthentication
432: .It GSSAPIDelegateCredentials
433: .It HashKnownHosts
434: .It Host
435: .It HostbasedAuthentication
436: .It HostKeyAlgorithms
437: .It HostKeyAlias
438: .It HostName
439: .It IdentityFile
440: .It IdentitiesOnly
1.316 jmc 441: .It IPQoS
1.321 djm 442: .It KbdInteractiveAuthentication
1.218 jmc 443: .It KbdInteractiveDevices
1.314 jmc 444: .It KexAlgorithms
1.218 jmc 445: .It LocalCommand
446: .It LocalForward
447: .It LogLevel
448: .It MACs
1.337 jmc 449: .It Match
1.218 jmc 450: .It NoHostAuthenticationForLocalhost
451: .It NumberOfPasswordPrompts
452: .It PasswordAuthentication
453: .It PermitLocalCommand
1.291 markus 454: .It PKCS11Provider
1.218 jmc 455: .It Port
456: .It PreferredAuthentications
457: .It Protocol
458: .It ProxyCommand
1.336 jmc 459: .It ProxyUseFdpass
1.218 jmc 460: .It PubkeyAuthentication
1.251 dtucker 461: .It RekeyLimit
1.218 jmc 462: .It RemoteForward
1.319 jmc 463: .It RequestTTY
1.218 jmc 464: .It RhostsRSAAuthentication
465: .It RSAAuthentication
466: .It SendEnv
467: .It ServerAliveInterval
468: .It ServerAliveCountMax
1.347 ! jmc 469: .It StreamLocalBindMask
! 470: .It StreamLocalBindUnlink
1.218 jmc 471: .It StrictHostKeyChecking
472: .It TCPKeepAlive
473: .It Tunnel
474: .It TunnelDevice
475: .It UsePrivilegedPort
476: .It User
477: .It UserKnownHostsFile
478: .It VerifyHostKeyDNS
1.276 jmc 479: .It VisualHostKey
1.218 jmc 480: .It XAuthLocation
481: .El
482: .It Fl p Ar port
483: Port to connect to on the remote host.
484: This can be specified on a
485: per-host basis in the configuration file.
1.342 jmc 486: .It Fl Q Cm cipher | cipher-auth | mac | kex | key
1.332 djm 487: Queries
488: .Nm
1.341 deraadt 489: for the algorithms supported for the specified version 2.
490: The available features are:
491: .Ar cipher
1.332 djm 492: (supported symmetric ciphers),
1.341 deraadt 493: .Ar cipher-auth
1.340 djm 494: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 495: .Ar mac
1.332 djm 496: (supported message integrity codes),
1.341 deraadt 497: .Ar kex
1.332 djm 498: (key exchange algorithms),
1.341 deraadt 499: .Ar key
1.332 djm 500: (key types).
1.218 jmc 501: .It Fl q
502: Quiet mode.
1.271 djm 503: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 504: .It Fl R Xo
505: .Sm off
506: .Oo Ar bind_address : Oc
507: .Ar port : host : hostport
508: .Sm on
509: .Xc
510: Specifies that the given port on the remote (server) host is to be
511: forwarded to the given host and port on the local side.
512: This works by allocating a socket to listen to
513: .Ar port
514: on the remote side, and whenever a connection is made to this port, the
515: connection is forwarded over the secure channel, and a connection is
516: made to
517: .Ar host
518: port
519: .Ar hostport
520: from the local machine.
1.2 deraadt 521: .Pp
1.218 jmc 522: Port forwardings can also be specified in the configuration file.
523: Privileged ports can be forwarded only when
524: logging in as root on the remote machine.
1.324 jmc 525: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 526: .Pp
1.218 jmc 527: By default, the listening socket on the server will be bound to the loopback
528: interface only.
1.280 tobias 529: This may be overridden by specifying a
1.218 jmc 530: .Ar bind_address .
531: An empty
532: .Ar bind_address ,
533: or the address
534: .Ql * ,
535: indicates that the remote socket should listen on all interfaces.
536: Specifying a remote
537: .Ar bind_address
538: will only succeed if the server's
539: .Cm GatewayPorts
540: option is enabled (see
541: .Xr sshd_config 5 ) .
1.281 djm 542: .Pp
543: If the
544: .Ar port
545: argument is
1.282 djm 546: .Ql 0 ,
1.281 djm 547: the listen port will be dynamically allocated on the server and reported
548: to the client at run time.
1.305 markus 549: When used together with
550: .Ic -O forward
551: the allocated port will be printed to the standard output.
1.218 jmc 552: .It Fl S Ar ctl_path
1.304 jmc 553: Specifies the location of a control socket for connection sharing,
1.303 djm 554: or the string
555: .Dq none
556: to disable connection sharing.
1.218 jmc 557: Refer to the description of
558: .Cm ControlPath
559: and
560: .Cm ControlMaster
561: in
562: .Xr ssh_config 5
563: for details.
564: .It Fl s
565: May be used to request invocation of a subsystem on the remote system.
566: Subsystems are a feature of the SSH2 protocol which facilitate the use
567: of SSH as a secure transport for other applications (eg.\&
568: .Xr sftp 1 ) .
569: The subsystem is specified as the remote command.
570: .It Fl T
571: Disable pseudo-tty allocation.
572: .It Fl t
573: Force pseudo-tty allocation.
574: This can be used to execute arbitrary
575: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 576: e.g. when implementing menu services.
1.218 jmc 577: Multiple
578: .Fl t
579: options force tty allocation, even if
1.194 jakob 580: .Nm
1.218 jmc 581: has no local tty.
582: .It Fl V
583: Display the version number and exit.
584: .It Fl v
585: Verbose mode.
586: Causes
1.176 jmc 587: .Nm
1.218 jmc 588: to print debugging messages about its progress.
589: This is helpful in
590: debugging connection, authentication, and configuration problems.
591: Multiple
592: .Fl v
593: options increase the verbosity.
594: The maximum is 3.
1.290 dtucker 595: .It Fl W Ar host : Ns Ar port
596: Requests that standard input and output on the client be forwarded to
597: .Ar host
598: on
599: .Ar port
600: over the secure channel.
601: Implies
602: .Fl N ,
603: .Fl T ,
604: .Cm ExitOnForwardFailure
605: and
1.326 dtucker 606: .Cm ClearAllForwardings .
607: Works with Protocol version 2 only.
1.261 stevesk 608: .It Fl w Xo
609: .Ar local_tun Ns Op : Ns Ar remote_tun
610: .Xc
611: Requests
612: tunnel
613: device forwarding with the specified
1.218 jmc 614: .Xr tun 4
1.261 stevesk 615: devices between the client
616: .Pq Ar local_tun
617: and the server
618: .Pq Ar remote_tun .
619: .Pp
1.228 jmc 620: The devices may be specified by numerical ID or the keyword
621: .Dq any ,
622: which uses the next available tunnel device.
1.261 stevesk 623: If
624: .Ar remote_tun
625: is not specified, it defaults to
626: .Dq any .
1.228 jmc 627: See also the
1.218 jmc 628: .Cm Tunnel
1.261 stevesk 629: and
630: .Cm TunnelDevice
631: directives in
1.218 jmc 632: .Xr ssh_config 5 .
1.261 stevesk 633: If the
634: .Cm Tunnel
635: directive is unset, it is set to the default tunnel mode, which is
636: .Dq point-to-point .
1.218 jmc 637: .It Fl X
638: Enables X11 forwarding.
1.54 markus 639: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 640: .Pp
1.218 jmc 641: X11 forwarding should be enabled with caution.
1.168 jmc 642: Users with the ability to bypass file permissions on the remote host
1.218 jmc 643: (for the user's X authorization database)
644: can access the local X11 display through the forwarded connection.
645: An attacker may then be able to perform activities such as keystroke monitoring.
646: .Pp
647: For this reason, X11 forwarding is subjected to X11 SECURITY extension
648: restrictions by default.
649: Please refer to the
650: .Nm
651: .Fl Y
652: option and the
653: .Cm ForwardX11Trusted
654: directive in
655: .Xr ssh_config 5
656: for more information.
657: .It Fl x
658: Disables X11 forwarding.
659: .It Fl Y
660: Enables trusted X11 forwarding.
661: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
662: controls.
1.278 djm 663: .It Fl y
664: Send log information using the
665: .Xr syslog 3
666: system module.
667: By default this information is sent to stderr.
1.218 jmc 668: .El
1.224 jmc 669: .Pp
670: .Nm
671: may additionally obtain configuration data from
672: a per-user configuration file and a system-wide configuration file.
673: The file format and configuration options are described in
674: .Xr ssh_config 5 .
1.222 jmc 675: .Sh AUTHENTICATION
1.249 jmc 676: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 677: The default is to use protocol 2 only,
678: though this can be changed via the
1.222 jmc 679: .Cm Protocol
680: option in
1.284 jmc 681: .Xr ssh_config 5
682: or the
1.222 jmc 683: .Fl 1
684: and
685: .Fl 2
686: options (see above).
687: Both protocols support similar authentication methods,
1.284 jmc 688: but protocol 2 is the default since
1.222 jmc 689: it provides additional mechanisms for confidentiality
690: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 691: and integrity (hmac-md5, hmac-sha1,
692: hmac-sha2-256, hmac-sha2-512,
1.330 markus 693: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 694: Protocol 1 lacks a strong mechanism for ensuring the
695: integrity of the connection.
696: .Pp
697: The methods available for authentication are:
1.260 jmc 698: GSSAPI-based authentication,
1.222 jmc 699: host-based authentication,
700: public key authentication,
701: challenge-response authentication,
702: and password authentication.
703: Authentication methods are tried in the order specified above,
704: though protocol 2 has a configuration option to change the default order:
705: .Cm PreferredAuthentications .
706: .Pp
707: Host-based authentication works as follows:
1.218 jmc 708: If the machine the user logs in from is listed in
709: .Pa /etc/hosts.equiv
710: or
711: .Pa /etc/shosts.equiv
712: on the remote machine, and the user names are
713: the same on both sides, or if the files
714: .Pa ~/.rhosts
715: or
716: .Pa ~/.shosts
717: exist in the user's home directory on the
718: remote machine and contain a line containing the name of the client
719: machine and the name of the user on that machine, the user is
1.222 jmc 720: considered for login.
721: Additionally, the server
722: .Em must
723: be able to verify the client's
724: host key (see the description of
1.218 jmc 725: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 726: and
1.222 jmc 727: .Pa ~/.ssh/known_hosts ,
728: below)
729: for login to be permitted.
1.218 jmc 730: This authentication method closes security holes due to IP
1.222 jmc 731: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 732: [Note to the administrator:
733: .Pa /etc/hosts.equiv ,
734: .Pa ~/.rhosts ,
735: and the rlogin/rsh protocol in general, are inherently insecure and should be
736: disabled if security is desired.]
1.189 dtucker 737: .Pp
1.222 jmc 738: Public key authentication works as follows:
739: The scheme is based on public-key cryptography,
740: using cryptosystems
741: where encryption and decryption are done using separate keys,
742: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 743: The idea is that each user creates a public/private
744: key pair for authentication purposes.
745: The server knows the public key, and only the user knows the private key.
1.222 jmc 746: .Nm
747: implements public key authentication protocol automatically,
1.343 naddy 748: using one of the DSA, ECDSA, ED25519 or RSA algorithms.
1.222 jmc 749: Protocol 1 is restricted to using only RSA keys,
1.310 djm 750: but protocol 2 may use any.
1.333 jmc 751: The HISTORY section of
1.222 jmc 752: .Xr ssl 8
1.311 jmc 753: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 754: .Pp
1.218 jmc 755: The file
756: .Pa ~/.ssh/authorized_keys
757: lists the public keys that are permitted for logging in.
758: When the user logs in, the
1.2 deraadt 759: .Nm
1.218 jmc 760: program tells the server which key pair it would like to use for
761: authentication.
1.222 jmc 762: The client proves that it has access to the private key
763: and the server checks that the corresponding public key
764: is authorized to accept the account.
1.218 jmc 765: .Pp
1.222 jmc 766: The user creates his/her key pair by running
1.218 jmc 767: .Xr ssh-keygen 1 .
768: This stores the private key in
1.207 djm 769: .Pa ~/.ssh/identity
1.222 jmc 770: (protocol 1),
771: .Pa ~/.ssh/id_dsa
772: (protocol 2 DSA),
1.310 djm 773: .Pa ~/.ssh/id_ecdsa
774: (protocol 2 ECDSA),
1.343 naddy 775: .Pa ~/.ssh/id_ed25519
776: (protocol 2 ED25519),
1.222 jmc 777: or
778: .Pa ~/.ssh/id_rsa
779: (protocol 2 RSA)
1.218 jmc 780: and stores the public key in
781: .Pa ~/.ssh/identity.pub
1.222 jmc 782: (protocol 1),
783: .Pa ~/.ssh/id_dsa.pub
784: (protocol 2 DSA),
1.310 djm 785: .Pa ~/.ssh/id_ecdsa.pub
786: (protocol 2 ECDSA),
1.343 naddy 787: .Pa ~/.ssh/id_ed25519.pub
788: (protocol 2 ED25519),
1.222 jmc 789: or
790: .Pa ~/.ssh/id_rsa.pub
791: (protocol 2 RSA)
1.218 jmc 792: in the user's home directory.
1.222 jmc 793: The user should then copy the public key
1.218 jmc 794: to
795: .Pa ~/.ssh/authorized_keys
1.222 jmc 796: in his/her home directory on the remote machine.
797: The
1.218 jmc 798: .Pa authorized_keys
799: file corresponds to the conventional
800: .Pa ~/.rhosts
801: file, and has one key
1.222 jmc 802: per line, though the lines can be very long.
1.218 jmc 803: After this, the user can log in without giving the password.
804: .Pp
1.301 jmc 805: A variation on public key authentication
806: is available in the form of certificate authentication:
807: instead of a set of public/private keys,
808: signed certificates are used.
809: This has the advantage that a single trusted certification authority
810: can be used in place of many public/private keys.
1.333 jmc 811: See the CERTIFICATES section of
1.301 jmc 812: .Xr ssh-keygen 1
813: for more information.
814: .Pp
815: The most convenient way to use public key or certificate authentication
816: may be with an authentication agent.
1.218 jmc 817: See
818: .Xr ssh-agent 1
819: for more information.
820: .Pp
1.222 jmc 821: Challenge-response authentication works as follows:
822: The server sends an arbitrary
823: .Qq challenge
824: text, and prompts for a response.
825: Protocol 2 allows multiple challenges and responses;
826: protocol 1 is restricted to just one challenge/response.
827: Examples of challenge-response authentication include
1.335 jmc 828: .Bx
829: Authentication (see
1.222 jmc 830: .Xr login.conf 5 )
1.335 jmc 831: and PAM (some
832: .Pf non- Ox
833: systems).
1.222 jmc 834: .Pp
835: Finally, if other authentication methods fail,
1.218 jmc 836: .Nm
837: prompts the user for a password.
838: The password is sent to the remote
839: host for checking; however, since all communications are encrypted,
840: the password cannot be seen by someone listening on the network.
1.232 jmc 841: .Pp
842: .Nm
843: automatically maintains and checks a database containing
844: identification for all hosts it has ever been used with.
845: Host keys are stored in
846: .Pa ~/.ssh/known_hosts
847: in the user's home directory.
848: Additionally, the file
849: .Pa /etc/ssh/ssh_known_hosts
850: is automatically checked for known hosts.
851: Any new hosts are automatically added to the user's file.
852: If a host's identification ever changes,
853: .Nm
854: warns about this and disables password authentication to prevent
855: server spoofing or man-in-the-middle attacks,
856: which could otherwise be used to circumvent the encryption.
857: The
858: .Cm StrictHostKeyChecking
859: option can be used to control logins to machines whose
860: host key is not known or has changed.
861: .Pp
1.218 jmc 862: When the user's identity has been accepted by the server, the server
863: either executes the given command, or logs into the machine and gives
864: the user a normal shell on the remote machine.
865: All communication with
866: the remote command or shell will be automatically encrypted.
867: .Pp
868: If a pseudo-terminal has been allocated (normal login session), the
869: user may use the escape characters noted below.
870: .Pp
871: If no pseudo-tty has been allocated,
872: the session is transparent and can be used to reliably transfer binary data.
873: On most systems, setting the escape character to
874: .Dq none
875: will also make the session transparent even if a tty is used.
876: .Pp
877: The session terminates when the command or shell on the remote
1.247 jmc 878: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 879: .Sh ESCAPE CHARACTERS
1.218 jmc 880: When a pseudo-terminal has been requested,
1.2 deraadt 881: .Nm
1.218 jmc 882: supports a number of functions through the use of an escape character.
883: .Pp
884: A single tilde character can be sent as
885: .Ic ~~
886: or by following the tilde by a character other than those described below.
887: The escape character must always follow a newline to be interpreted as
888: special.
889: The escape character can be changed in configuration files using the
890: .Cm EscapeChar
891: configuration directive or on the command line by the
892: .Fl e
893: option.
894: .Pp
895: The supported escapes (assuming the default
896: .Ql ~ )
897: are:
898: .Bl -tag -width Ds
899: .It Cm ~.
900: Disconnect.
901: .It Cm ~^Z
902: Background
1.234 jmc 903: .Nm .
1.218 jmc 904: .It Cm ~#
905: List forwarded connections.
906: .It Cm ~&
907: Background
1.2 deraadt 908: .Nm
1.218 jmc 909: at logout when waiting for forwarded connection / X11 sessions to terminate.
910: .It Cm ~?
911: Display a list of escape characters.
912: .It Cm ~B
913: Send a BREAK to the remote system
914: (only useful for SSH protocol version 2 and if the peer supports it).
915: .It Cm ~C
916: Open command line.
917: Currently this allows the addition of port forwardings using the
1.279 stevesk 918: .Fl L ,
919: .Fl R
1.218 jmc 920: and
1.279 stevesk 921: .Fl D
1.225 jmc 922: options (see above).
1.322 markus 923: It also allows the cancellation of existing port-forwardings
924: with
1.262 stevesk 925: .Sm off
1.322 markus 926: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 927: .Sm on
1.322 markus 928: for local,
929: .Sm off
930: .Fl KR Oo Ar bind_address : Oc Ar port
931: .Sm on
932: for remote and
933: .Sm off
934: .Fl KD Oo Ar bind_address : Oc Ar port
935: .Sm on
936: for dynamic port-forwardings.
1.218 jmc 937: .Ic !\& Ns Ar command
938: allows the user to execute a local command if the
939: .Ic PermitLocalCommand
940: option is enabled in
1.176 jmc 941: .Xr ssh_config 5 .
1.218 jmc 942: Basic help is available, using the
943: .Fl h
944: option.
945: .It Cm ~R
946: Request rekeying of the connection
947: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 948: .It Cm ~V
949: Decrease the verbosity
950: .Pq Ic LogLevel
951: when errors are being written to stderr.
952: .It Cm ~v
1.328 jmc 953: Increase the verbosity
1.327 dtucker 954: .Pq Ic LogLevel
955: when errors are being written to stderr.
1.176 jmc 956: .El
1.246 jmc 957: .Sh TCP FORWARDING
958: Forwarding of arbitrary TCP connections over the secure channel can
959: be specified either on the command line or in a configuration file.
960: One possible application of TCP forwarding is a secure connection to a
961: mail server; another is going through firewalls.
962: .Pp
963: In the example below, we look at encrypting communication between
964: an IRC client and server, even though the IRC server does not directly
965: support encrypted communications.
966: This works as follows:
967: the user connects to the remote host using
968: .Nm ,
969: specifying a port to be used to forward connections
970: to the remote server.
971: After that it is possible to start the service which is to be encrypted
972: on the client machine,
973: connecting to the same local port,
974: and
975: .Nm
976: will encrypt and forward the connection.
977: .Pp
978: The following example tunnels an IRC session from client machine
979: .Dq 127.0.0.1
980: (localhost)
981: to remote server
982: .Dq server.example.com :
983: .Bd -literal -offset 4n
984: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
985: $ irc -c '#users' -p 1234 pinky 127.0.0.1
986: .Ed
987: .Pp
988: This tunnels a connection to IRC server
989: .Dq server.example.com ,
990: joining channel
991: .Dq #users ,
992: nickname
993: .Dq pinky ,
994: using port 1234.
995: It doesn't matter which port is used,
996: as long as it's greater than 1023
997: (remember, only root can open sockets on privileged ports)
998: and doesn't conflict with any ports already in use.
999: The connection is forwarded to port 6667 on the remote server,
1000: since that's the standard port for IRC services.
1001: .Pp
1002: The
1003: .Fl f
1004: option backgrounds
1005: .Nm
1006: and the remote command
1007: .Dq sleep 10
1008: is specified to allow an amount of time
1009: (10 seconds, in the example)
1010: to start the service which is to be tunnelled.
1011: If no connections are made within the time specified,
1012: .Nm
1013: will exit.
1014: .Sh X11 FORWARDING
1.218 jmc 1015: If the
1016: .Cm ForwardX11
1017: variable is set to
1018: .Dq yes
1019: (or see the description of the
1.227 jmc 1020: .Fl X ,
1021: .Fl x ,
1.218 jmc 1022: and
1.227 jmc 1023: .Fl Y
1.226 jmc 1024: options above)
1.218 jmc 1025: and the user is using X11 (the
1026: .Ev DISPLAY
1027: environment variable is set), the connection to the X11 display is
1028: automatically forwarded to the remote side in such a way that any X11
1029: programs started from the shell (or command) will go through the
1030: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1031: from the local machine.
1.218 jmc 1032: The user should not manually set
1033: .Ev DISPLAY .
1034: Forwarding of X11 connections can be
1035: configured on the command line or in configuration files.
1036: .Pp
1037: The
1038: .Ev DISPLAY
1039: value set by
1040: .Nm
1041: will point to the server machine, but with a display number greater than zero.
1042: This is normal, and happens because
1043: .Nm
1044: creates a
1045: .Dq proxy
1046: X server on the server machine for forwarding the
1047: connections over the encrypted channel.
1.200 djm 1048: .Pp
1.218 jmc 1049: .Nm
1050: will also automatically set up Xauthority data on the server machine.
1051: For this purpose, it will generate a random authorization cookie,
1052: store it in Xauthority on the server, and verify that any forwarded
1053: connections carry this cookie and replace it by the real cookie when
1054: the connection is opened.
1055: The real authentication cookie is never
1056: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1057: .Pp
1.218 jmc 1058: If the
1059: .Cm ForwardAgent
1060: variable is set to
1061: .Dq yes
1062: (or see the description of the
1063: .Fl A
1.191 djm 1064: and
1.218 jmc 1065: .Fl a
1.226 jmc 1066: options above) and
1.218 jmc 1067: the user is using an authentication agent, the connection to the agent
1068: is automatically forwarded to the remote side.
1.252 jmc 1069: .Sh VERIFYING HOST KEYS
1070: When connecting to a server for the first time,
1071: a fingerprint of the server's public key is presented to the user
1072: (unless the option
1073: .Cm StrictHostKeyChecking
1074: has been disabled).
1075: Fingerprints can be determined using
1076: .Xr ssh-keygen 1 :
1077: .Pp
1078: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1079: .Pp
1.274 grunk 1080: If the fingerprint is already known, it can be matched
1081: and the key can be accepted or rejected.
1082: Because of the difficulty of comparing host keys
1083: just by looking at hex strings,
1084: there is also support to compare host keys visually,
1085: using
1086: .Em random art .
1087: By setting the
1.275 grunk 1088: .Cm VisualHostKey
1.274 grunk 1089: option to
1.275 grunk 1090: .Dq yes ,
1.274 grunk 1091: a small ASCII graphic gets displayed on every login to a server, no matter
1092: if the session itself is interactive or not.
1093: By learning the pattern a known server produces, a user can easily
1094: find out that the host key has changed when a completely different pattern
1095: is displayed.
1096: Because these patterns are not unambiguous however, a pattern that looks
1097: similar to the pattern remembered only gives a good probability that the
1098: host key is the same, not guaranteed proof.
1099: .Pp
1100: To get a listing of the fingerprints along with their random art for
1101: all known hosts, the following command line can be used:
1102: .Pp
1103: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1104: .Pp
1.252 jmc 1105: If the fingerprint is unknown,
1106: an alternative method of verification is available:
1107: SSH fingerprints verified by DNS.
1108: An additional resource record (RR),
1109: SSHFP,
1110: is added to a zonefile
1111: and the connecting client is able to match the fingerprint
1112: with that of the key presented.
1113: .Pp
1114: In this example, we are connecting a client to a server,
1115: .Dq host.example.com .
1116: The SSHFP resource records should first be added to the zonefile for
1117: host.example.com:
1118: .Bd -literal -offset indent
1.259 jakob 1119: $ ssh-keygen -r host.example.com.
1.252 jmc 1120: .Ed
1121: .Pp
1122: The output lines will have to be added to the zonefile.
1123: To check that the zone is answering fingerprint queries:
1124: .Pp
1125: .Dl $ dig -t SSHFP host.example.com
1126: .Pp
1127: Finally the client connects:
1128: .Bd -literal -offset indent
1129: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1130: [...]
1131: Matching host key fingerprint found in DNS.
1132: Are you sure you want to continue connecting (yes/no)?
1133: .Ed
1134: .Pp
1135: See the
1136: .Cm VerifyHostKeyDNS
1137: option in
1138: .Xr ssh_config 5
1139: for more information.
1.250 jmc 1140: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1141: .Nm
1142: contains support for Virtual Private Network (VPN) tunnelling
1143: using the
1144: .Xr tun 4
1145: network pseudo-device,
1146: allowing two networks to be joined securely.
1147: The
1148: .Xr sshd_config 5
1149: configuration option
1150: .Cm PermitTunnel
1151: controls whether the server supports this,
1152: and at what level (layer 2 or 3 traffic).
1153: .Pp
1154: The following example would connect client network 10.0.50.0/24
1.265 otto 1155: with remote network 10.0.99.0/24 using a point-to-point connection
1156: from 10.1.1.1 to 10.1.1.2,
1157: provided that the SSH server running on the gateway to the remote network,
1158: at 192.168.1.15, allows it.
1159: .Pp
1160: On the client:
1.250 jmc 1161: .Bd -literal -offset indent
1162: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1163: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1164: # route add 10.0.99.0/24 10.1.1.2
1165: .Ed
1166: .Pp
1167: On the server:
1168: .Bd -literal -offset indent
1169: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1170: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1171: .Ed
1172: .Pp
1173: Client access may be more finely tuned via the
1174: .Pa /root/.ssh/authorized_keys
1175: file (see below) and the
1176: .Cm PermitRootLogin
1177: server option.
1.255 jmc 1178: The following entry would permit connections on
1.250 jmc 1179: .Xr tun 4
1.255 jmc 1180: device 1 from user
1.250 jmc 1181: .Dq jane
1.255 jmc 1182: and on tun device 2 from user
1.250 jmc 1183: .Dq john ,
1184: if
1185: .Cm PermitRootLogin
1186: is set to
1187: .Dq forced-commands-only :
1188: .Bd -literal -offset 2n
1189: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1190: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1191: .Ed
1192: .Pp
1.264 ray 1193: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1194: it may be more suited to temporary setups,
1195: such as for wireless VPNs.
1196: More permanent VPNs are better provided by tools such as
1197: .Xr ipsecctl 8
1198: and
1199: .Xr isakmpd 8 .
1.2 deraadt 1200: .Sh ENVIRONMENT
1201: .Nm
1.1 deraadt 1202: will normally set the following environment variables:
1.237 jmc 1203: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1204: .It Ev DISPLAY
1205: The
1206: .Ev DISPLAY
1.40 aaron 1207: variable indicates the location of the X11 server.
1.44 aaron 1208: It is automatically set by
1.2 deraadt 1209: .Nm
1210: to point to a value of the form
1.233 jmc 1211: .Dq hostname:n ,
1212: where
1213: .Dq hostname
1214: indicates the host where the shell runs, and
1215: .Sq n
1216: is an integer \*(Ge 1.
1.40 aaron 1217: .Nm
1218: uses this special value to forward X11 connections over the secure
1219: channel.
1.107 markus 1220: The user should normally not set
1221: .Ev DISPLAY
1222: explicitly, as that
1.1 deraadt 1223: will render the X11 connection insecure (and will require the user to
1224: manually copy any required authorization cookies).
1.2 deraadt 1225: .It Ev HOME
1.1 deraadt 1226: Set to the path of the user's home directory.
1.2 deraadt 1227: .It Ev LOGNAME
1228: Synonym for
1.12 aaron 1229: .Ev USER ;
1230: set for compatibility with systems that use this variable.
1.2 deraadt 1231: .It Ev MAIL
1.129 stevesk 1232: Set to the path of the user's mailbox.
1.40 aaron 1233: .It Ev PATH
1.2 deraadt 1234: Set to the default
1235: .Ev PATH ,
1236: as specified when compiling
1.234 jmc 1237: .Nm .
1.118 markus 1238: .It Ev SSH_ASKPASS
1239: If
1240: .Nm
1241: needs a passphrase, it will read the passphrase from the current
1242: terminal if it was run from a terminal.
1243: If
1244: .Nm
1245: does not have a terminal associated with it but
1246: .Ev DISPLAY
1247: and
1248: .Ev SSH_ASKPASS
1249: are set, it will execute the program specified by
1250: .Ev SSH_ASKPASS
1251: and open an X11 window to read the passphrase.
1252: This is particularly useful when calling
1253: .Nm
1254: from a
1.196 jmc 1255: .Pa .xsession
1.118 markus 1256: or related script.
1257: (Note that on some machines it
1258: may be necessary to redirect the input from
1259: .Pa /dev/null
1260: to make this work.)
1.18 markus 1261: .It Ev SSH_AUTH_SOCK
1.233 jmc 1262: Identifies the path of a
1263: .Ux Ns -domain
1264: socket used to communicate with the agent.
1.166 stevesk 1265: .It Ev SSH_CONNECTION
1266: Identifies the client and server ends of the connection.
1.40 aaron 1267: The variable contains
1.233 jmc 1268: four space-separated values: client IP address, client port number,
1269: server IP address, and server port number.
1.73 markus 1270: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1271: This variable contains the original command line if a forced command
1.73 markus 1272: is executed.
1273: It can be used to extract the original arguments.
1.2 deraadt 1274: .It Ev SSH_TTY
1.1 deraadt 1275: This is set to the name of the tty (path to the device) associated
1.40 aaron 1276: with the current shell or command.
1277: If the current session has no tty,
1.1 deraadt 1278: this variable is not set.
1.2 deraadt 1279: .It Ev TZ
1.214 jmc 1280: This variable is set to indicate the present time zone if it
1.257 jmc 1281: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1282: on to new connections).
1.2 deraadt 1283: .It Ev USER
1.1 deraadt 1284: Set to the name of the user logging in.
1.2 deraadt 1285: .El
1286: .Pp
1.44 aaron 1287: Additionally,
1.2 deraadt 1288: .Nm
1.44 aaron 1289: reads
1.207 djm 1290: .Pa ~/.ssh/environment ,
1.2 deraadt 1291: and adds lines of the format
1292: .Dq VARNAME=value
1.233 jmc 1293: to the environment if the file exists and users are allowed to
1.161 marc 1294: change their environment.
1.176 jmc 1295: For more information, see the
1.161 marc 1296: .Cm PermitUserEnvironment
1.162 stevesk 1297: option in
1.161 marc 1298: .Xr sshd_config 5 .
1.2 deraadt 1299: .Sh FILES
1.236 jmc 1300: .Bl -tag -width Ds -compact
1.309 jmc 1301: .It Pa ~/.rhosts
1.240 jmc 1302: This file is used for host-based authentication (see above).
1.92 markus 1303: On some machines this file may need to be
1.240 jmc 1304: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1305: because
1.2 deraadt 1306: .Xr sshd 8
1.40 aaron 1307: reads it as root.
1308: Additionally, this file must be owned by the user,
1309: and must not have write permissions for anyone else.
1310: The recommended
1.1 deraadt 1311: permission for most machines is read/write for the user, and not
1312: accessible by others.
1.2 deraadt 1313: .Pp
1.309 jmc 1314: .It Pa ~/.shosts
1.240 jmc 1315: This file is used in exactly the same way as
1316: .Pa .rhosts ,
1317: but allows host-based authentication without permitting login with
1318: rlogin/rsh.
1.272 mcbride 1319: .Pp
1.309 jmc 1320: .It Pa ~/.ssh/
1.272 mcbride 1321: This directory is the default location for all user-specific configuration
1322: and authentication information.
1323: There is no general requirement to keep the entire contents of this directory
1324: secret, but the recommended permissions are read/write/execute for the user,
1325: and not accessible by others.
1.236 jmc 1326: .Pp
1.309 jmc 1327: .It Pa ~/.ssh/authorized_keys
1.343 naddy 1328: Lists the public keys (DSA, ECDSA, ED25519, RSA)
1329: that can be used for logging in as this user.
1.238 jmc 1330: The format of this file is described in the
1331: .Xr sshd 8
1332: manual page.
1333: This file is not highly sensitive, but the recommended
1334: permissions are read/write for the user, and not accessible by others.
1335: .Pp
1.309 jmc 1336: .It Pa ~/.ssh/config
1.238 jmc 1337: This is the per-user configuration file.
1338: The file format and configuration options are described in
1339: .Xr ssh_config 5 .
1340: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1341: read/write for the user, and not writable by others.
1.238 jmc 1342: .Pp
1.309 jmc 1343: .It Pa ~/.ssh/environment
1.239 jmc 1344: Contains additional definitions for environment variables; see
1345: .Sx ENVIRONMENT ,
1.238 jmc 1346: above.
1347: .Pp
1.309 jmc 1348: .It Pa ~/.ssh/identity
1349: .It Pa ~/.ssh/id_dsa
1.310 djm 1350: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1351: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1352: .It Pa ~/.ssh/id_rsa
1.238 jmc 1353: Contains the private key for authentication.
1354: These files
1355: contain sensitive data and should be readable by the user but not
1356: accessible by others (read/write/execute).
1357: .Nm
1358: will simply ignore a private key file if it is accessible by others.
1359: It is possible to specify a passphrase when
1360: generating the key which will be used to encrypt the
1361: sensitive part of this file using 3DES.
1362: .Pp
1.309 jmc 1363: .It Pa ~/.ssh/identity.pub
1364: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1365: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1366: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1367: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1368: Contains the public key for authentication.
1369: These files are not
1370: sensitive and can (but need not) be readable by anyone.
1371: .Pp
1.309 jmc 1372: .It Pa ~/.ssh/known_hosts
1.244 jmc 1373: Contains a list of host keys for all hosts the user has logged into
1374: that are not already in the systemwide list of known host keys.
1.238 jmc 1375: See
1.244 jmc 1376: .Xr sshd 8
1377: for further details of the format of this file.
1.238 jmc 1378: .Pp
1.309 jmc 1379: .It Pa ~/.ssh/rc
1.238 jmc 1380: Commands in this file are executed by
1381: .Nm
1.245 jmc 1382: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1383: started.
1384: See the
1385: .Xr sshd 8
1386: manual page for more information.
1387: .Pp
1.309 jmc 1388: .It Pa /etc/hosts.equiv
1.240 jmc 1389: This file is for host-based authentication (see above).
1390: It should only be writable by root.
1.236 jmc 1391: .Pp
1.309 jmc 1392: .It Pa /etc/shosts.equiv
1.240 jmc 1393: This file is used in exactly the same way as
1394: .Pa hosts.equiv ,
1395: but allows host-based authentication without permitting login with
1396: rlogin/rsh.
1.236 jmc 1397: .Pp
1.238 jmc 1398: .It Pa /etc/ssh/ssh_config
1399: Systemwide configuration file.
1400: The file format and configuration options are described in
1401: .Xr ssh_config 5 .
1402: .Pp
1.309 jmc 1403: .It Pa /etc/ssh/ssh_host_key
1404: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1405: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1406: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1407: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1408: These files contain the private parts of the host keys
1.245 jmc 1409: and are used for host-based authentication.
1410: If protocol version 1 is used,
1.238 jmc 1411: .Nm
1412: must be setuid root, since the host key is readable only by root.
1413: For protocol version 2,
1414: .Nm
1415: uses
1416: .Xr ssh-keysign 8
1.245 jmc 1417: to access the host keys,
1418: eliminating the requirement that
1.238 jmc 1419: .Nm
1.245 jmc 1420: be setuid root when host-based authentication is used.
1.238 jmc 1421: By default
1.2 deraadt 1422: .Nm
1.238 jmc 1423: is not setuid root.
1424: .Pp
1.309 jmc 1425: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1426: Systemwide list of known host keys.
1427: This file should be prepared by the
1428: system administrator to contain the public host keys of all machines in the
1429: organization.
1.244 jmc 1430: It should be world-readable.
1431: See
1.238 jmc 1432: .Xr sshd 8
1.244 jmc 1433: for further details of the format of this file.
1.236 jmc 1434: .Pp
1.309 jmc 1435: .It Pa /etc/ssh/sshrc
1.1 deraadt 1436: Commands in this file are executed by
1.2 deraadt 1437: .Nm
1.245 jmc 1438: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1439: See the
1.2 deraadt 1440: .Xr sshd 8
1.1 deraadt 1441: manual page for more information.
1.58 itojun 1442: .El
1.312 jmc 1443: .Sh EXIT STATUS
1444: .Nm
1445: exits with the exit status of the remote command or with 255
1446: if an error occurred.
1.2 deraadt 1447: .Sh SEE ALSO
1448: .Xr scp 1 ,
1.83 djm 1449: .Xr sftp 1 ,
1.2 deraadt 1450: .Xr ssh-add 1 ,
1451: .Xr ssh-agent 1 ,
1452: .Xr ssh-keygen 1 ,
1.242 jmc 1453: .Xr ssh-keyscan 1 ,
1.250 jmc 1454: .Xr tun 4 ,
1.159 stevesk 1455: .Xr ssh_config 5 ,
1.160 naddy 1456: .Xr ssh-keysign 8 ,
1.87 itojun 1457: .Xr sshd 8
1.329 jmc 1458: .Sh STANDARDS
1.106 markus 1459: .Rs
1.329 jmc 1460: .%A S. Lehtinen
1461: .%A C. Lonvick
1462: .%D January 2006
1.256 jmc 1463: .%R RFC 4250
1.329 jmc 1464: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1465: .Re
1.329 jmc 1466: .Pp
1.256 jmc 1467: .Rs
1.329 jmc 1468: .%A T. Ylonen
1469: .%A C. Lonvick
1470: .%D January 2006
1.256 jmc 1471: .%R RFC 4251
1.329 jmc 1472: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1473: .Re
1.329 jmc 1474: .Pp
1.256 jmc 1475: .Rs
1.329 jmc 1476: .%A T. Ylonen
1477: .%A C. Lonvick
1478: .%D January 2006
1.256 jmc 1479: .%R RFC 4252
1.329 jmc 1480: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1481: .Re
1.329 jmc 1482: .Pp
1.256 jmc 1483: .Rs
1.329 jmc 1484: .%A T. Ylonen
1485: .%A C. Lonvick
1486: .%D January 2006
1.256 jmc 1487: .%R RFC 4253
1.329 jmc 1488: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1489: .Re
1.329 jmc 1490: .Pp
1.256 jmc 1491: .Rs
1.329 jmc 1492: .%A T. Ylonen
1493: .%A C. Lonvick
1494: .%D January 2006
1.256 jmc 1495: .%R RFC 4254
1.329 jmc 1496: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1497: .Re
1.329 jmc 1498: .Pp
1.256 jmc 1499: .Rs
1.329 jmc 1500: .%A J. Schlyter
1501: .%A W. Griffin
1502: .%D January 2006
1.256 jmc 1503: .%R RFC 4255
1.329 jmc 1504: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1505: .Re
1.329 jmc 1506: .Pp
1.256 jmc 1507: .Rs
1.329 jmc 1508: .%A F. Cusack
1509: .%A M. Forssen
1510: .%D January 2006
1.256 jmc 1511: .%R RFC 4256
1.329 jmc 1512: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1513: .Re
1.329 jmc 1514: .Pp
1.256 jmc 1515: .Rs
1.329 jmc 1516: .%A J. Galbraith
1517: .%A P. Remaker
1518: .%D January 2006
1.256 jmc 1519: .%R RFC 4335
1.329 jmc 1520: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1521: .Re
1.329 jmc 1522: .Pp
1.256 jmc 1523: .Rs
1.329 jmc 1524: .%A M. Bellare
1525: .%A T. Kohno
1526: .%A C. Namprempre
1527: .%D January 2006
1.256 jmc 1528: .%R RFC 4344
1.329 jmc 1529: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1530: .Re
1.329 jmc 1531: .Pp
1.256 jmc 1532: .Rs
1.329 jmc 1533: .%A B. Harris
1534: .%D January 2006
1.256 jmc 1535: .%R RFC 4345
1.329 jmc 1536: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1537: .Re
1.329 jmc 1538: .Pp
1.258 djm 1539: .Rs
1.329 jmc 1540: .%A M. Friedl
1541: .%A N. Provos
1542: .%A W. Simpson
1543: .%D March 2006
1.258 djm 1544: .%R RFC 4419
1.329 jmc 1545: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1546: .Re
1.329 jmc 1547: .Pp
1.266 markus 1548: .Rs
1.329 jmc 1549: .%A J. Galbraith
1550: .%A R. Thayer
1551: .%D November 2006
1.266 markus 1552: .%R RFC 4716
1.329 jmc 1553: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1554: .Re
1.329 jmc 1555: .Pp
1.313 djm 1556: .Rs
1.329 jmc 1557: .%A D. Stebila
1558: .%A J. Green
1559: .%D December 2009
1.313 djm 1560: .%R RFC 5656
1.329 jmc 1561: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1562: .Re
1.329 jmc 1563: .Pp
1.274 grunk 1564: .Rs
1565: .%A A. Perrig
1566: .%A D. Song
1567: .%D 1999
1.329 jmc 1568: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1569: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1570: .Re
1.173 jmc 1571: .Sh AUTHORS
1572: OpenSSH is a derivative of the original and free
1573: ssh 1.2.12 release by Tatu Ylonen.
1574: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1575: Theo de Raadt and Dug Song
1576: removed many bugs, re-added newer features and
1577: created OpenSSH.
1578: Markus Friedl contributed the support for SSH
1579: protocol versions 1.5 and 2.0.