Annotation of src/usr.bin/ssh/ssh.1, Revision 1.342
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: .\"
1.342 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.341 2013/11/25 18:04:21 deraadt Exp $
! 37: .Dd $Mdocdate: November 25 2013 $
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 .
157: .Ar 3des
158: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
159: It is believed to be secure.
160: .Ar blowfish
161: is a fast block cipher; it appears very secure and is much faster than
162: .Ar 3des .
163: .Ar des
164: is only supported in the
1.2 deraadt 165: .Nm
1.218 jmc 166: client for interoperability with legacy protocol 1 implementations
167: that do not support the
168: .Ar 3des
169: cipher.
170: Its use is strongly discouraged due to cryptographic weaknesses.
171: The default is
172: .Dq 3des .
1.49 markus 173: .Pp
1.230 jmc 174: For protocol version 2,
1.218 jmc 175: .Ar cipher_spec
176: is a comma-separated list of ciphers
177: listed in order of preference.
1.283 jmc 178: See the
179: .Cm Ciphers
1.307 dtucker 180: keyword in
181: .Xr ssh_config 5
182: for more information.
1.218 jmc 183: .It Fl D Xo
184: .Sm off
185: .Oo Ar bind_address : Oc
186: .Ar port
187: .Sm on
188: .Xc
189: Specifies a local
190: .Dq dynamic
191: application-level port forwarding.
192: This works by allocating a socket to listen to
193: .Ar port
194: on the local side, optionally bound to the specified
195: .Ar bind_address .
196: Whenever a connection is made to this port, the
197: connection is forwarded over the secure channel, and the application
198: protocol is then used to determine where to connect to from the
199: remote machine.
200: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 201: .Nm
1.218 jmc 202: will act as a SOCKS server.
203: Only root can forward privileged ports.
204: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 205: .Pp
1.308 djm 206: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 207: Only the superuser can forward privileged ports.
208: By default, the local port is bound in accordance with the
209: .Cm GatewayPorts
210: setting.
211: However, an explicit
212: .Ar bind_address
213: may be used to bind the connection to a specific address.
214: The
215: .Ar bind_address
216: of
217: .Dq localhost
218: indicates that the listening port be bound for local use only, while an
219: empty address or
220: .Sq *
221: indicates that the port should be available from all interfaces.
1.331 dtucker 222: .It Fl E Ar log_file
223: Append debug logs to
224: .Ar log_file
225: instead of standard error.
1.229 jmc 226: .It Fl e Ar escape_char
1.218 jmc 227: Sets the escape character for sessions with a pty (default:
228: .Ql ~ ) .
229: The escape character is only recognized at the beginning of a line.
230: The escape character followed by a dot
231: .Pq Ql \&.
232: closes the connection;
233: followed by control-Z suspends the connection;
234: and followed by itself sends the escape character once.
235: Setting the character to
1.2 deraadt 236: .Dq none
1.218 jmc 237: disables any escapes and makes the session fully transparent.
238: .It Fl F Ar configfile
239: Specifies an alternative per-user configuration file.
240: If a configuration file is given on the command line,
241: the system-wide configuration file
242: .Pq Pa /etc/ssh/ssh_config
243: will be ignored.
244: The default for the per-user configuration file is
245: .Pa ~/.ssh/config .
246: .It Fl f
247: Requests
248: .Nm
249: to go to background just before command execution.
250: This is useful if
1.176 jmc 251: .Nm
1.218 jmc 252: is going to ask for passwords or passphrases, but the user
253: wants it in the background.
254: This implies
255: .Fl n .
256: The recommended way to start X11 programs at a remote site is with
257: something like
258: .Ic ssh -f host xterm .
1.277 djm 259: .Pp
260: If the
261: .Cm ExitOnForwardFailure
262: configuration option is set to
263: .Dq yes ,
264: then a client started with
265: .Fl f
266: will wait for all remote port forwards to be successfully established
267: before placing itself in the background.
1.218 jmc 268: .It Fl g
269: Allows remote hosts to connect to local forwarded ports.
1.291 markus 270: .It Fl I Ar pkcs11
1.294 jmc 271: Specify the PKCS#11 shared library
1.176 jmc 272: .Nm
1.293 markus 273: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 274: private RSA key.
275: .It Fl i Ar identity_file
276: Selects a file from which the identity (private key) for
1.310 djm 277: public key authentication is read.
1.218 jmc 278: The default is
279: .Pa ~/.ssh/identity
280: for protocol version 1, and
1.310 djm 281: .Pa ~/.ssh/id_dsa ,
282: .Pa ~/.ssh/id_ecdsa
283: and
1.218 jmc 284: .Pa ~/.ssh/id_rsa
285: for protocol version 2.
286: Identity files may also be specified on
287: a per-host basis in the configuration file.
288: It is possible to have multiple
289: .Fl i
290: options (and multiple identities specified in
291: configuration files).
1.302 djm 292: .Nm
293: will also try to load certificate information from the filename obtained
294: by appending
295: .Pa -cert.pub
296: to identity filenames.
1.269 djm 297: .It Fl K
298: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
299: credentials to the server.
1.218 jmc 300: .It Fl k
301: Disables forwarding (delegation) of GSSAPI credentials to the server.
302: .It Fl L Xo
303: .Sm off
304: .Oo Ar bind_address : Oc
305: .Ar port : host : hostport
306: .Sm on
307: .Xc
308: Specifies that the given port on the local (client) host is to be
309: forwarded to the given host and port on the remote side.
310: This works by allocating a socket to listen to
311: .Ar port
312: on the local side, optionally bound to the specified
313: .Ar bind_address .
314: Whenever a connection is made to this port, the
315: connection is forwarded over the secure channel, and a connection is
316: made to
317: .Ar host
318: port
319: .Ar hostport
320: from the remote machine.
321: Port forwardings can also be specified in the configuration file.
1.308 djm 322: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 323: Only the superuser can forward privileged ports.
324: By default, the local port is bound in accordance with the
325: .Cm GatewayPorts
326: setting.
327: However, an explicit
328: .Ar bind_address
329: may be used to bind the connection to a specific address.
1.2 deraadt 330: The
1.218 jmc 331: .Ar bind_address
332: of
333: .Dq localhost
334: indicates that the listening port be bound for local use only, while an
335: empty address or
336: .Sq *
337: indicates that the port should be available from all interfaces.
338: .It Fl l Ar login_name
339: Specifies the user to log in as on the remote machine.
340: This also may be specified on a per-host basis in the configuration file.
341: .It Fl M
342: Places the
343: .Nm
344: client into
345: .Dq master
346: mode for connection sharing.
1.231 stevesk 347: Multiple
348: .Fl M
349: options places
350: .Nm
351: into
352: .Dq master
353: mode with confirmation required before slave connections are accepted.
1.218 jmc 354: Refer to the description of
355: .Cm ControlMaster
356: in
357: .Xr ssh_config 5
358: for details.
359: .It Fl m Ar mac_spec
360: Additionally, for protocol version 2 a comma-separated list of MAC
361: (message authentication code) algorithms can
362: be specified in order of preference.
363: See the
364: .Cm MACs
365: keyword for more information.
366: .It Fl N
367: Do not execute a remote command.
368: This is useful for just forwarding ports
369: (protocol version 2 only).
370: .It Fl n
371: Redirects stdin from
372: .Pa /dev/null
373: (actually, prevents reading from stdin).
374: This must be used when
1.2 deraadt 375: .Nm
1.218 jmc 376: is run in the background.
377: A common trick is to use this to run X11 programs on a remote machine.
378: For example,
379: .Ic ssh -n shadows.cs.hut.fi emacs &
380: will start an emacs on shadows.cs.hut.fi, and the X11
381: connection will be automatically forwarded over an encrypted channel.
382: The
1.2 deraadt 383: .Nm
1.218 jmc 384: program will be put in the background.
385: (This does not work if
1.2 deraadt 386: .Nm
1.218 jmc 387: needs to ask for a password or passphrase; see also the
388: .Fl f
389: option.)
390: .It Fl O Ar ctl_cmd
391: Control an active connection multiplexing master process.
392: When the
393: .Fl O
394: option is specified, the
395: .Ar ctl_cmd
396: argument is interpreted and passed to the master process.
397: Valid commands are:
398: .Dq check
1.305 markus 399: (check that the master process is running),
400: .Dq forward
1.318 jmc 401: (request forwardings without command execution),
1.323 okan 402: .Dq cancel
403: (cancel forwardings),
1.218 jmc 404: .Dq exit
1.318 jmc 405: (request the master to exit), and
1.317 djm 406: .Dq stop
407: (request the master to stop accepting further multiplexing requests).
1.218 jmc 408: .It Fl o Ar option
409: Can be used to give options in the format used in the configuration file.
410: This is useful for specifying options for which there is no separate
411: command-line flag.
412: For full details of the options listed below, and their possible values, see
413: .Xr ssh_config 5 .
1.2 deraadt 414: .Pp
1.218 jmc 415: .Bl -tag -width Ds -offset indent -compact
416: .It AddressFamily
417: .It BatchMode
418: .It BindAddress
1.338 djm 419: .It CanonicalDomains
1.339 djm 420: .It CanonicalizeFallbackLocal
421: .It CanonicalizeHostname
422: .It CanonicalizeMaxDots
423: .It CanonicalizePermittedCNAMEs
1.218 jmc 424: .It ChallengeResponseAuthentication
425: .It CheckHostIP
426: .It Cipher
427: .It Ciphers
428: .It ClearAllForwardings
429: .It Compression
430: .It CompressionLevel
431: .It ConnectionAttempts
432: .It ConnectTimeout
433: .It ControlMaster
434: .It ControlPath
1.321 djm 435: .It ControlPersist
1.218 jmc 436: .It DynamicForward
437: .It EscapeChar
1.263 markus 438: .It ExitOnForwardFailure
1.218 jmc 439: .It ForwardAgent
440: .It ForwardX11
1.321 djm 441: .It ForwardX11Timeout
1.218 jmc 442: .It ForwardX11Trusted
443: .It GatewayPorts
444: .It GlobalKnownHostsFile
445: .It GSSAPIAuthentication
446: .It GSSAPIDelegateCredentials
447: .It HashKnownHosts
448: .It Host
449: .It HostbasedAuthentication
450: .It HostKeyAlgorithms
451: .It HostKeyAlias
452: .It HostName
453: .It IdentityFile
454: .It IdentitiesOnly
1.316 jmc 455: .It IPQoS
1.321 djm 456: .It KbdInteractiveAuthentication
1.218 jmc 457: .It KbdInteractiveDevices
1.314 jmc 458: .It KexAlgorithms
1.218 jmc 459: .It LocalCommand
460: .It LocalForward
461: .It LogLevel
462: .It MACs
1.337 jmc 463: .It Match
1.218 jmc 464: .It NoHostAuthenticationForLocalhost
465: .It NumberOfPasswordPrompts
466: .It PasswordAuthentication
467: .It PermitLocalCommand
1.291 markus 468: .It PKCS11Provider
1.218 jmc 469: .It Port
470: .It PreferredAuthentications
471: .It Protocol
472: .It ProxyCommand
1.336 jmc 473: .It ProxyUseFdpass
1.218 jmc 474: .It PubkeyAuthentication
1.251 dtucker 475: .It RekeyLimit
1.218 jmc 476: .It RemoteForward
1.319 jmc 477: .It RequestTTY
1.218 jmc 478: .It RhostsRSAAuthentication
479: .It RSAAuthentication
480: .It SendEnv
481: .It ServerAliveInterval
482: .It ServerAliveCountMax
483: .It StrictHostKeyChecking
484: .It TCPKeepAlive
485: .It Tunnel
486: .It TunnelDevice
487: .It UsePrivilegedPort
488: .It User
489: .It UserKnownHostsFile
490: .It VerifyHostKeyDNS
1.276 jmc 491: .It VisualHostKey
1.218 jmc 492: .It XAuthLocation
493: .El
494: .It Fl p Ar port
495: Port to connect to on the remote host.
496: This can be specified on a
497: per-host basis in the configuration file.
1.342 ! jmc 498: .It Fl Q Cm cipher | cipher-auth | mac | kex | key
1.332 djm 499: Queries
500: .Nm
1.341 deraadt 501: for the algorithms supported for the specified version 2.
502: The available features are:
503: .Ar cipher
1.332 djm 504: (supported symmetric ciphers),
1.341 deraadt 505: .Ar cipher-auth
1.340 djm 506: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 507: .Ar mac
1.332 djm 508: (supported message integrity codes),
1.341 deraadt 509: .Ar kex
1.332 djm 510: (key exchange algorithms),
1.341 deraadt 511: .Ar key
1.332 djm 512: (key types).
1.218 jmc 513: .It Fl q
514: Quiet mode.
1.271 djm 515: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 516: .It Fl R Xo
517: .Sm off
518: .Oo Ar bind_address : Oc
519: .Ar port : host : hostport
520: .Sm on
521: .Xc
522: Specifies that the given port on the remote (server) host is to be
523: forwarded to the given host and port on the local side.
524: This works by allocating a socket to listen to
525: .Ar port
526: on the remote side, and whenever a connection is made to this port, the
527: connection is forwarded over the secure channel, and a connection is
528: made to
529: .Ar host
530: port
531: .Ar hostport
532: from the local machine.
1.2 deraadt 533: .Pp
1.218 jmc 534: Port forwardings can also be specified in the configuration file.
535: Privileged ports can be forwarded only when
536: logging in as root on the remote machine.
1.324 jmc 537: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 538: .Pp
1.218 jmc 539: By default, the listening socket on the server will be bound to the loopback
540: interface only.
1.280 tobias 541: This may be overridden by specifying a
1.218 jmc 542: .Ar bind_address .
543: An empty
544: .Ar bind_address ,
545: or the address
546: .Ql * ,
547: indicates that the remote socket should listen on all interfaces.
548: Specifying a remote
549: .Ar bind_address
550: will only succeed if the server's
551: .Cm GatewayPorts
552: option is enabled (see
553: .Xr sshd_config 5 ) .
1.281 djm 554: .Pp
555: If the
556: .Ar port
557: argument is
1.282 djm 558: .Ql 0 ,
1.281 djm 559: the listen port will be dynamically allocated on the server and reported
560: to the client at run time.
1.305 markus 561: When used together with
562: .Ic -O forward
563: the allocated port will be printed to the standard output.
1.218 jmc 564: .It Fl S Ar ctl_path
1.304 jmc 565: Specifies the location of a control socket for connection sharing,
1.303 djm 566: or the string
567: .Dq none
568: to disable connection sharing.
1.218 jmc 569: Refer to the description of
570: .Cm ControlPath
571: and
572: .Cm ControlMaster
573: in
574: .Xr ssh_config 5
575: for details.
576: .It Fl s
577: May be used to request invocation of a subsystem on the remote system.
578: Subsystems are a feature of the SSH2 protocol which facilitate the use
579: of SSH as a secure transport for other applications (eg.\&
580: .Xr sftp 1 ) .
581: The subsystem is specified as the remote command.
582: .It Fl T
583: Disable pseudo-tty allocation.
584: .It Fl t
585: Force pseudo-tty allocation.
586: This can be used to execute arbitrary
587: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 588: e.g. when implementing menu services.
1.218 jmc 589: Multiple
590: .Fl t
591: options force tty allocation, even if
1.194 jakob 592: .Nm
1.218 jmc 593: has no local tty.
594: .It Fl V
595: Display the version number and exit.
596: .It Fl v
597: Verbose mode.
598: Causes
1.176 jmc 599: .Nm
1.218 jmc 600: to print debugging messages about its progress.
601: This is helpful in
602: debugging connection, authentication, and configuration problems.
603: Multiple
604: .Fl v
605: options increase the verbosity.
606: The maximum is 3.
1.290 dtucker 607: .It Fl W Ar host : Ns Ar port
608: Requests that standard input and output on the client be forwarded to
609: .Ar host
610: on
611: .Ar port
612: over the secure channel.
613: Implies
614: .Fl N ,
615: .Fl T ,
616: .Cm ExitOnForwardFailure
617: and
1.326 dtucker 618: .Cm ClearAllForwardings .
619: Works with Protocol version 2 only.
1.261 stevesk 620: .It Fl w Xo
621: .Ar local_tun Ns Op : Ns Ar remote_tun
622: .Xc
623: Requests
624: tunnel
625: device forwarding with the specified
1.218 jmc 626: .Xr tun 4
1.261 stevesk 627: devices between the client
628: .Pq Ar local_tun
629: and the server
630: .Pq Ar remote_tun .
631: .Pp
1.228 jmc 632: The devices may be specified by numerical ID or the keyword
633: .Dq any ,
634: which uses the next available tunnel device.
1.261 stevesk 635: If
636: .Ar remote_tun
637: is not specified, it defaults to
638: .Dq any .
1.228 jmc 639: See also the
1.218 jmc 640: .Cm Tunnel
1.261 stevesk 641: and
642: .Cm TunnelDevice
643: directives in
1.218 jmc 644: .Xr ssh_config 5 .
1.261 stevesk 645: If the
646: .Cm Tunnel
647: directive is unset, it is set to the default tunnel mode, which is
648: .Dq point-to-point .
1.218 jmc 649: .It Fl X
650: Enables X11 forwarding.
1.54 markus 651: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 652: .Pp
1.218 jmc 653: X11 forwarding should be enabled with caution.
1.168 jmc 654: Users with the ability to bypass file permissions on the remote host
1.218 jmc 655: (for the user's X authorization database)
656: can access the local X11 display through the forwarded connection.
657: An attacker may then be able to perform activities such as keystroke monitoring.
658: .Pp
659: For this reason, X11 forwarding is subjected to X11 SECURITY extension
660: restrictions by default.
661: Please refer to the
662: .Nm
663: .Fl Y
664: option and the
665: .Cm ForwardX11Trusted
666: directive in
667: .Xr ssh_config 5
668: for more information.
669: .It Fl x
670: Disables X11 forwarding.
671: .It Fl Y
672: Enables trusted X11 forwarding.
673: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
674: controls.
1.278 djm 675: .It Fl y
676: Send log information using the
677: .Xr syslog 3
678: system module.
679: By default this information is sent to stderr.
1.218 jmc 680: .El
1.224 jmc 681: .Pp
682: .Nm
683: may additionally obtain configuration data from
684: a per-user configuration file and a system-wide configuration file.
685: The file format and configuration options are described in
686: .Xr ssh_config 5 .
1.222 jmc 687: .Sh AUTHENTICATION
1.249 jmc 688: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 689: The default is to use protocol 2 only,
690: though this can be changed via the
1.222 jmc 691: .Cm Protocol
692: option in
1.284 jmc 693: .Xr ssh_config 5
694: or the
1.222 jmc 695: .Fl 1
696: and
697: .Fl 2
698: options (see above).
699: Both protocols support similar authentication methods,
1.284 jmc 700: but protocol 2 is the default since
1.222 jmc 701: it provides additional mechanisms for confidentiality
702: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 703: and integrity (hmac-md5, hmac-sha1,
704: hmac-sha2-256, hmac-sha2-512,
1.330 markus 705: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 706: Protocol 1 lacks a strong mechanism for ensuring the
707: integrity of the connection.
708: .Pp
709: The methods available for authentication are:
1.260 jmc 710: GSSAPI-based authentication,
1.222 jmc 711: host-based authentication,
712: public key authentication,
713: challenge-response authentication,
714: and password authentication.
715: Authentication methods are tried in the order specified above,
716: though protocol 2 has a configuration option to change the default order:
717: .Cm PreferredAuthentications .
718: .Pp
719: Host-based authentication works as follows:
1.218 jmc 720: If the machine the user logs in from is listed in
721: .Pa /etc/hosts.equiv
722: or
723: .Pa /etc/shosts.equiv
724: on the remote machine, and the user names are
725: the same on both sides, or if the files
726: .Pa ~/.rhosts
727: or
728: .Pa ~/.shosts
729: exist in the user's home directory on the
730: remote machine and contain a line containing the name of the client
731: machine and the name of the user on that machine, the user is
1.222 jmc 732: considered for login.
733: Additionally, the server
734: .Em must
735: be able to verify the client's
736: host key (see the description of
1.218 jmc 737: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 738: and
1.222 jmc 739: .Pa ~/.ssh/known_hosts ,
740: below)
741: for login to be permitted.
1.218 jmc 742: This authentication method closes security holes due to IP
1.222 jmc 743: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 744: [Note to the administrator:
745: .Pa /etc/hosts.equiv ,
746: .Pa ~/.rhosts ,
747: and the rlogin/rsh protocol in general, are inherently insecure and should be
748: disabled if security is desired.]
1.189 dtucker 749: .Pp
1.222 jmc 750: Public key authentication works as follows:
751: The scheme is based on public-key cryptography,
752: using cryptosystems
753: where encryption and decryption are done using separate keys,
754: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 755: The idea is that each user creates a public/private
756: key pair for authentication purposes.
757: The server knows the public key, and only the user knows the private key.
1.222 jmc 758: .Nm
759: implements public key authentication protocol automatically,
1.310 djm 760: using one of the DSA, ECDSA or RSA algorithms.
1.222 jmc 761: Protocol 1 is restricted to using only RSA keys,
1.310 djm 762: but protocol 2 may use any.
1.333 jmc 763: The HISTORY section of
1.222 jmc 764: .Xr ssl 8
1.311 jmc 765: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 766: .Pp
1.218 jmc 767: The file
768: .Pa ~/.ssh/authorized_keys
769: lists the public keys that are permitted for logging in.
770: When the user logs in, the
1.2 deraadt 771: .Nm
1.218 jmc 772: program tells the server which key pair it would like to use for
773: authentication.
1.222 jmc 774: The client proves that it has access to the private key
775: and the server checks that the corresponding public key
776: is authorized to accept the account.
1.218 jmc 777: .Pp
1.222 jmc 778: The user creates his/her key pair by running
1.218 jmc 779: .Xr ssh-keygen 1 .
780: This stores the private key in
1.207 djm 781: .Pa ~/.ssh/identity
1.222 jmc 782: (protocol 1),
783: .Pa ~/.ssh/id_dsa
784: (protocol 2 DSA),
1.310 djm 785: .Pa ~/.ssh/id_ecdsa
786: (protocol 2 ECDSA),
1.222 jmc 787: or
788: .Pa ~/.ssh/id_rsa
789: (protocol 2 RSA)
1.218 jmc 790: and stores the public key in
791: .Pa ~/.ssh/identity.pub
1.222 jmc 792: (protocol 1),
793: .Pa ~/.ssh/id_dsa.pub
794: (protocol 2 DSA),
1.310 djm 795: .Pa ~/.ssh/id_ecdsa.pub
796: (protocol 2 ECDSA),
1.222 jmc 797: or
798: .Pa ~/.ssh/id_rsa.pub
799: (protocol 2 RSA)
1.218 jmc 800: in the user's home directory.
1.222 jmc 801: The user should then copy the public key
1.218 jmc 802: to
803: .Pa ~/.ssh/authorized_keys
1.222 jmc 804: in his/her home directory on the remote machine.
805: The
1.218 jmc 806: .Pa authorized_keys
807: file corresponds to the conventional
808: .Pa ~/.rhosts
809: file, and has one key
1.222 jmc 810: per line, though the lines can be very long.
1.218 jmc 811: After this, the user can log in without giving the password.
812: .Pp
1.301 jmc 813: A variation on public key authentication
814: is available in the form of certificate authentication:
815: instead of a set of public/private keys,
816: signed certificates are used.
817: This has the advantage that a single trusted certification authority
818: can be used in place of many public/private keys.
1.333 jmc 819: See the CERTIFICATES section of
1.301 jmc 820: .Xr ssh-keygen 1
821: for more information.
822: .Pp
823: The most convenient way to use public key or certificate authentication
824: may be with an authentication agent.
1.218 jmc 825: See
826: .Xr ssh-agent 1
827: for more information.
828: .Pp
1.222 jmc 829: Challenge-response authentication works as follows:
830: The server sends an arbitrary
831: .Qq challenge
832: text, and prompts for a response.
833: Protocol 2 allows multiple challenges and responses;
834: protocol 1 is restricted to just one challenge/response.
835: Examples of challenge-response authentication include
1.335 jmc 836: .Bx
837: Authentication (see
1.222 jmc 838: .Xr login.conf 5 )
1.335 jmc 839: and PAM (some
840: .Pf non- Ox
841: systems).
1.222 jmc 842: .Pp
843: Finally, if other authentication methods fail,
1.218 jmc 844: .Nm
845: prompts the user for a password.
846: The password is sent to the remote
847: host for checking; however, since all communications are encrypted,
848: the password cannot be seen by someone listening on the network.
1.232 jmc 849: .Pp
850: .Nm
851: automatically maintains and checks a database containing
852: identification for all hosts it has ever been used with.
853: Host keys are stored in
854: .Pa ~/.ssh/known_hosts
855: in the user's home directory.
856: Additionally, the file
857: .Pa /etc/ssh/ssh_known_hosts
858: is automatically checked for known hosts.
859: Any new hosts are automatically added to the user's file.
860: If a host's identification ever changes,
861: .Nm
862: warns about this and disables password authentication to prevent
863: server spoofing or man-in-the-middle attacks,
864: which could otherwise be used to circumvent the encryption.
865: The
866: .Cm StrictHostKeyChecking
867: option can be used to control logins to machines whose
868: host key is not known or has changed.
869: .Pp
1.218 jmc 870: When the user's identity has been accepted by the server, the server
871: either executes the given command, or logs into the machine and gives
872: the user a normal shell on the remote machine.
873: All communication with
874: the remote command or shell will be automatically encrypted.
875: .Pp
876: If a pseudo-terminal has been allocated (normal login session), the
877: user may use the escape characters noted below.
878: .Pp
879: If no pseudo-tty has been allocated,
880: the session is transparent and can be used to reliably transfer binary data.
881: On most systems, setting the escape character to
882: .Dq none
883: will also make the session transparent even if a tty is used.
884: .Pp
885: The session terminates when the command or shell on the remote
1.247 jmc 886: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 887: .Sh ESCAPE CHARACTERS
1.218 jmc 888: When a pseudo-terminal has been requested,
1.2 deraadt 889: .Nm
1.218 jmc 890: supports a number of functions through the use of an escape character.
891: .Pp
892: A single tilde character can be sent as
893: .Ic ~~
894: or by following the tilde by a character other than those described below.
895: The escape character must always follow a newline to be interpreted as
896: special.
897: The escape character can be changed in configuration files using the
898: .Cm EscapeChar
899: configuration directive or on the command line by the
900: .Fl e
901: option.
902: .Pp
903: The supported escapes (assuming the default
904: .Ql ~ )
905: are:
906: .Bl -tag -width Ds
907: .It Cm ~.
908: Disconnect.
909: .It Cm ~^Z
910: Background
1.234 jmc 911: .Nm .
1.218 jmc 912: .It Cm ~#
913: List forwarded connections.
914: .It Cm ~&
915: Background
1.2 deraadt 916: .Nm
1.218 jmc 917: at logout when waiting for forwarded connection / X11 sessions to terminate.
918: .It Cm ~?
919: Display a list of escape characters.
920: .It Cm ~B
921: Send a BREAK to the remote system
922: (only useful for SSH protocol version 2 and if the peer supports it).
923: .It Cm ~C
924: Open command line.
925: Currently this allows the addition of port forwardings using the
1.279 stevesk 926: .Fl L ,
927: .Fl R
1.218 jmc 928: and
1.279 stevesk 929: .Fl D
1.225 jmc 930: options (see above).
1.322 markus 931: It also allows the cancellation of existing port-forwardings
932: with
1.262 stevesk 933: .Sm off
1.322 markus 934: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 935: .Sm on
1.322 markus 936: for local,
937: .Sm off
938: .Fl KR Oo Ar bind_address : Oc Ar port
939: .Sm on
940: for remote and
941: .Sm off
942: .Fl KD Oo Ar bind_address : Oc Ar port
943: .Sm on
944: for dynamic port-forwardings.
1.218 jmc 945: .Ic !\& Ns Ar command
946: allows the user to execute a local command if the
947: .Ic PermitLocalCommand
948: option is enabled in
1.176 jmc 949: .Xr ssh_config 5 .
1.218 jmc 950: Basic help is available, using the
951: .Fl h
952: option.
953: .It Cm ~R
954: Request rekeying of the connection
955: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 956: .It Cm ~V
957: Decrease the verbosity
958: .Pq Ic LogLevel
959: when errors are being written to stderr.
960: .It Cm ~v
1.328 jmc 961: Increase the verbosity
1.327 dtucker 962: .Pq Ic LogLevel
963: when errors are being written to stderr.
1.176 jmc 964: .El
1.246 jmc 965: .Sh TCP FORWARDING
966: Forwarding of arbitrary TCP connections over the secure channel can
967: be specified either on the command line or in a configuration file.
968: One possible application of TCP forwarding is a secure connection to a
969: mail server; another is going through firewalls.
970: .Pp
971: In the example below, we look at encrypting communication between
972: an IRC client and server, even though the IRC server does not directly
973: support encrypted communications.
974: This works as follows:
975: the user connects to the remote host using
976: .Nm ,
977: specifying a port to be used to forward connections
978: to the remote server.
979: After that it is possible to start the service which is to be encrypted
980: on the client machine,
981: connecting to the same local port,
982: and
983: .Nm
984: will encrypt and forward the connection.
985: .Pp
986: The following example tunnels an IRC session from client machine
987: .Dq 127.0.0.1
988: (localhost)
989: to remote server
990: .Dq server.example.com :
991: .Bd -literal -offset 4n
992: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
993: $ irc -c '#users' -p 1234 pinky 127.0.0.1
994: .Ed
995: .Pp
996: This tunnels a connection to IRC server
997: .Dq server.example.com ,
998: joining channel
999: .Dq #users ,
1000: nickname
1001: .Dq pinky ,
1002: using port 1234.
1003: It doesn't matter which port is used,
1004: as long as it's greater than 1023
1005: (remember, only root can open sockets on privileged ports)
1006: and doesn't conflict with any ports already in use.
1007: The connection is forwarded to port 6667 on the remote server,
1008: since that's the standard port for IRC services.
1009: .Pp
1010: The
1011: .Fl f
1012: option backgrounds
1013: .Nm
1014: and the remote command
1015: .Dq sleep 10
1016: is specified to allow an amount of time
1017: (10 seconds, in the example)
1018: to start the service which is to be tunnelled.
1019: If no connections are made within the time specified,
1020: .Nm
1021: will exit.
1022: .Sh X11 FORWARDING
1.218 jmc 1023: If the
1024: .Cm ForwardX11
1025: variable is set to
1026: .Dq yes
1027: (or see the description of the
1.227 jmc 1028: .Fl X ,
1029: .Fl x ,
1.218 jmc 1030: and
1.227 jmc 1031: .Fl Y
1.226 jmc 1032: options above)
1.218 jmc 1033: and the user is using X11 (the
1034: .Ev DISPLAY
1035: environment variable is set), the connection to the X11 display is
1036: automatically forwarded to the remote side in such a way that any X11
1037: programs started from the shell (or command) will go through the
1038: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1039: from the local machine.
1.218 jmc 1040: The user should not manually set
1041: .Ev DISPLAY .
1042: Forwarding of X11 connections can be
1043: configured on the command line or in configuration files.
1044: .Pp
1045: The
1046: .Ev DISPLAY
1047: value set by
1048: .Nm
1049: will point to the server machine, but with a display number greater than zero.
1050: This is normal, and happens because
1051: .Nm
1052: creates a
1053: .Dq proxy
1054: X server on the server machine for forwarding the
1055: connections over the encrypted channel.
1.200 djm 1056: .Pp
1.218 jmc 1057: .Nm
1058: will also automatically set up Xauthority data on the server machine.
1059: For this purpose, it will generate a random authorization cookie,
1060: store it in Xauthority on the server, and verify that any forwarded
1061: connections carry this cookie and replace it by the real cookie when
1062: the connection is opened.
1063: The real authentication cookie is never
1064: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1065: .Pp
1.218 jmc 1066: If the
1067: .Cm ForwardAgent
1068: variable is set to
1069: .Dq yes
1070: (or see the description of the
1071: .Fl A
1.191 djm 1072: and
1.218 jmc 1073: .Fl a
1.226 jmc 1074: options above) and
1.218 jmc 1075: the user is using an authentication agent, the connection to the agent
1076: is automatically forwarded to the remote side.
1.252 jmc 1077: .Sh VERIFYING HOST KEYS
1078: When connecting to a server for the first time,
1079: a fingerprint of the server's public key is presented to the user
1080: (unless the option
1081: .Cm StrictHostKeyChecking
1082: has been disabled).
1083: Fingerprints can be determined using
1084: .Xr ssh-keygen 1 :
1085: .Pp
1086: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1087: .Pp
1.274 grunk 1088: If the fingerprint is already known, it can be matched
1089: and the key can be accepted or rejected.
1090: Because of the difficulty of comparing host keys
1091: just by looking at hex strings,
1092: there is also support to compare host keys visually,
1093: using
1094: .Em random art .
1095: By setting the
1.275 grunk 1096: .Cm VisualHostKey
1.274 grunk 1097: option to
1.275 grunk 1098: .Dq yes ,
1.274 grunk 1099: a small ASCII graphic gets displayed on every login to a server, no matter
1100: if the session itself is interactive or not.
1101: By learning the pattern a known server produces, a user can easily
1102: find out that the host key has changed when a completely different pattern
1103: is displayed.
1104: Because these patterns are not unambiguous however, a pattern that looks
1105: similar to the pattern remembered only gives a good probability that the
1106: host key is the same, not guaranteed proof.
1107: .Pp
1108: To get a listing of the fingerprints along with their random art for
1109: all known hosts, the following command line can be used:
1110: .Pp
1111: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1112: .Pp
1.252 jmc 1113: If the fingerprint is unknown,
1114: an alternative method of verification is available:
1115: SSH fingerprints verified by DNS.
1116: An additional resource record (RR),
1117: SSHFP,
1118: is added to a zonefile
1119: and the connecting client is able to match the fingerprint
1120: with that of the key presented.
1121: .Pp
1122: In this example, we are connecting a client to a server,
1123: .Dq host.example.com .
1124: The SSHFP resource records should first be added to the zonefile for
1125: host.example.com:
1126: .Bd -literal -offset indent
1.259 jakob 1127: $ ssh-keygen -r host.example.com.
1.252 jmc 1128: .Ed
1129: .Pp
1130: The output lines will have to be added to the zonefile.
1131: To check that the zone is answering fingerprint queries:
1132: .Pp
1133: .Dl $ dig -t SSHFP host.example.com
1134: .Pp
1135: Finally the client connects:
1136: .Bd -literal -offset indent
1137: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1138: [...]
1139: Matching host key fingerprint found in DNS.
1140: Are you sure you want to continue connecting (yes/no)?
1141: .Ed
1142: .Pp
1143: See the
1144: .Cm VerifyHostKeyDNS
1145: option in
1146: .Xr ssh_config 5
1147: for more information.
1.250 jmc 1148: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1149: .Nm
1150: contains support for Virtual Private Network (VPN) tunnelling
1151: using the
1152: .Xr tun 4
1153: network pseudo-device,
1154: allowing two networks to be joined securely.
1155: The
1156: .Xr sshd_config 5
1157: configuration option
1158: .Cm PermitTunnel
1159: controls whether the server supports this,
1160: and at what level (layer 2 or 3 traffic).
1161: .Pp
1162: The following example would connect client network 10.0.50.0/24
1.265 otto 1163: with remote network 10.0.99.0/24 using a point-to-point connection
1164: from 10.1.1.1 to 10.1.1.2,
1165: provided that the SSH server running on the gateway to the remote network,
1166: at 192.168.1.15, allows it.
1167: .Pp
1168: On the client:
1.250 jmc 1169: .Bd -literal -offset indent
1170: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1171: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1172: # route add 10.0.99.0/24 10.1.1.2
1173: .Ed
1174: .Pp
1175: On the server:
1176: .Bd -literal -offset indent
1177: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1178: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1179: .Ed
1180: .Pp
1181: Client access may be more finely tuned via the
1182: .Pa /root/.ssh/authorized_keys
1183: file (see below) and the
1184: .Cm PermitRootLogin
1185: server option.
1.255 jmc 1186: The following entry would permit connections on
1.250 jmc 1187: .Xr tun 4
1.255 jmc 1188: device 1 from user
1.250 jmc 1189: .Dq jane
1.255 jmc 1190: and on tun device 2 from user
1.250 jmc 1191: .Dq john ,
1192: if
1193: .Cm PermitRootLogin
1194: is set to
1195: .Dq forced-commands-only :
1196: .Bd -literal -offset 2n
1197: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1198: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1199: .Ed
1200: .Pp
1.264 ray 1201: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1202: it may be more suited to temporary setups,
1203: such as for wireless VPNs.
1204: More permanent VPNs are better provided by tools such as
1205: .Xr ipsecctl 8
1206: and
1207: .Xr isakmpd 8 .
1.2 deraadt 1208: .Sh ENVIRONMENT
1209: .Nm
1.1 deraadt 1210: will normally set the following environment variables:
1.237 jmc 1211: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1212: .It Ev DISPLAY
1213: The
1214: .Ev DISPLAY
1.40 aaron 1215: variable indicates the location of the X11 server.
1.44 aaron 1216: It is automatically set by
1.2 deraadt 1217: .Nm
1218: to point to a value of the form
1.233 jmc 1219: .Dq hostname:n ,
1220: where
1221: .Dq hostname
1222: indicates the host where the shell runs, and
1223: .Sq n
1224: is an integer \*(Ge 1.
1.40 aaron 1225: .Nm
1226: uses this special value to forward X11 connections over the secure
1227: channel.
1.107 markus 1228: The user should normally not set
1229: .Ev DISPLAY
1230: explicitly, as that
1.1 deraadt 1231: will render the X11 connection insecure (and will require the user to
1232: manually copy any required authorization cookies).
1.2 deraadt 1233: .It Ev HOME
1.1 deraadt 1234: Set to the path of the user's home directory.
1.2 deraadt 1235: .It Ev LOGNAME
1236: Synonym for
1.12 aaron 1237: .Ev USER ;
1238: set for compatibility with systems that use this variable.
1.2 deraadt 1239: .It Ev MAIL
1.129 stevesk 1240: Set to the path of the user's mailbox.
1.40 aaron 1241: .It Ev PATH
1.2 deraadt 1242: Set to the default
1243: .Ev PATH ,
1244: as specified when compiling
1.234 jmc 1245: .Nm .
1.118 markus 1246: .It Ev SSH_ASKPASS
1247: If
1248: .Nm
1249: needs a passphrase, it will read the passphrase from the current
1250: terminal if it was run from a terminal.
1251: If
1252: .Nm
1253: does not have a terminal associated with it but
1254: .Ev DISPLAY
1255: and
1256: .Ev SSH_ASKPASS
1257: are set, it will execute the program specified by
1258: .Ev SSH_ASKPASS
1259: and open an X11 window to read the passphrase.
1260: This is particularly useful when calling
1261: .Nm
1262: from a
1.196 jmc 1263: .Pa .xsession
1.118 markus 1264: or related script.
1265: (Note that on some machines it
1266: may be necessary to redirect the input from
1267: .Pa /dev/null
1268: to make this work.)
1.18 markus 1269: .It Ev SSH_AUTH_SOCK
1.233 jmc 1270: Identifies the path of a
1271: .Ux Ns -domain
1272: socket used to communicate with the agent.
1.166 stevesk 1273: .It Ev SSH_CONNECTION
1274: Identifies the client and server ends of the connection.
1.40 aaron 1275: The variable contains
1.233 jmc 1276: four space-separated values: client IP address, client port number,
1277: server IP address, and server port number.
1.73 markus 1278: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1279: This variable contains the original command line if a forced command
1.73 markus 1280: is executed.
1281: It can be used to extract the original arguments.
1.2 deraadt 1282: .It Ev SSH_TTY
1.1 deraadt 1283: This is set to the name of the tty (path to the device) associated
1.40 aaron 1284: with the current shell or command.
1285: If the current session has no tty,
1.1 deraadt 1286: this variable is not set.
1.2 deraadt 1287: .It Ev TZ
1.214 jmc 1288: This variable is set to indicate the present time zone if it
1.257 jmc 1289: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1290: on to new connections).
1.2 deraadt 1291: .It Ev USER
1.1 deraadt 1292: Set to the name of the user logging in.
1.2 deraadt 1293: .El
1294: .Pp
1.44 aaron 1295: Additionally,
1.2 deraadt 1296: .Nm
1.44 aaron 1297: reads
1.207 djm 1298: .Pa ~/.ssh/environment ,
1.2 deraadt 1299: and adds lines of the format
1300: .Dq VARNAME=value
1.233 jmc 1301: to the environment if the file exists and users are allowed to
1.161 marc 1302: change their environment.
1.176 jmc 1303: For more information, see the
1.161 marc 1304: .Cm PermitUserEnvironment
1.162 stevesk 1305: option in
1.161 marc 1306: .Xr sshd_config 5 .
1.2 deraadt 1307: .Sh FILES
1.236 jmc 1308: .Bl -tag -width Ds -compact
1.309 jmc 1309: .It Pa ~/.rhosts
1.240 jmc 1310: This file is used for host-based authentication (see above).
1.92 markus 1311: On some machines this file may need to be
1.240 jmc 1312: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1313: because
1.2 deraadt 1314: .Xr sshd 8
1.40 aaron 1315: reads it as root.
1316: Additionally, this file must be owned by the user,
1317: and must not have write permissions for anyone else.
1318: The recommended
1.1 deraadt 1319: permission for most machines is read/write for the user, and not
1320: accessible by others.
1.2 deraadt 1321: .Pp
1.309 jmc 1322: .It Pa ~/.shosts
1.240 jmc 1323: This file is used in exactly the same way as
1324: .Pa .rhosts ,
1325: but allows host-based authentication without permitting login with
1326: rlogin/rsh.
1.272 mcbride 1327: .Pp
1.309 jmc 1328: .It Pa ~/.ssh/
1.272 mcbride 1329: This directory is the default location for all user-specific configuration
1330: and authentication information.
1331: There is no general requirement to keep the entire contents of this directory
1332: secret, but the recommended permissions are read/write/execute for the user,
1333: and not accessible by others.
1.236 jmc 1334: .Pp
1.309 jmc 1335: .It Pa ~/.ssh/authorized_keys
1.310 djm 1336: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1337: this user.
1.238 jmc 1338: The format of this file is described in the
1339: .Xr sshd 8
1340: manual page.
1341: This file is not highly sensitive, but the recommended
1342: permissions are read/write for the user, and not accessible by others.
1343: .Pp
1.309 jmc 1344: .It Pa ~/.ssh/config
1.238 jmc 1345: This is the per-user configuration file.
1346: The file format and configuration options are described in
1347: .Xr ssh_config 5 .
1348: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1349: read/write for the user, and not writable by others.
1.238 jmc 1350: .Pp
1.309 jmc 1351: .It Pa ~/.ssh/environment
1.239 jmc 1352: Contains additional definitions for environment variables; see
1353: .Sx ENVIRONMENT ,
1.238 jmc 1354: above.
1355: .Pp
1.309 jmc 1356: .It Pa ~/.ssh/identity
1357: .It Pa ~/.ssh/id_dsa
1.310 djm 1358: .It Pa ~/.ssh/id_ecdsa
1.309 jmc 1359: .It Pa ~/.ssh/id_rsa
1.238 jmc 1360: Contains the private key for authentication.
1361: These files
1362: contain sensitive data and should be readable by the user but not
1363: accessible by others (read/write/execute).
1364: .Nm
1365: will simply ignore a private key file if it is accessible by others.
1366: It is possible to specify a passphrase when
1367: generating the key which will be used to encrypt the
1368: sensitive part of this file using 3DES.
1369: .Pp
1.309 jmc 1370: .It Pa ~/.ssh/identity.pub
1371: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1372: .It Pa ~/.ssh/id_ecdsa.pub
1.309 jmc 1373: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1374: Contains the public key for authentication.
1375: These files are not
1376: sensitive and can (but need not) be readable by anyone.
1377: .Pp
1.309 jmc 1378: .It Pa ~/.ssh/known_hosts
1.244 jmc 1379: Contains a list of host keys for all hosts the user has logged into
1380: that are not already in the systemwide list of known host keys.
1.238 jmc 1381: See
1.244 jmc 1382: .Xr sshd 8
1383: for further details of the format of this file.
1.238 jmc 1384: .Pp
1.309 jmc 1385: .It Pa ~/.ssh/rc
1.238 jmc 1386: Commands in this file are executed by
1387: .Nm
1.245 jmc 1388: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1389: started.
1390: See the
1391: .Xr sshd 8
1392: manual page for more information.
1393: .Pp
1.309 jmc 1394: .It Pa /etc/hosts.equiv
1.240 jmc 1395: This file is for host-based authentication (see above).
1396: It should only be writable by root.
1.236 jmc 1397: .Pp
1.309 jmc 1398: .It Pa /etc/shosts.equiv
1.240 jmc 1399: This file is used in exactly the same way as
1400: .Pa hosts.equiv ,
1401: but allows host-based authentication without permitting login with
1402: rlogin/rsh.
1.236 jmc 1403: .Pp
1.238 jmc 1404: .It Pa /etc/ssh/ssh_config
1405: Systemwide configuration file.
1406: The file format and configuration options are described in
1407: .Xr ssh_config 5 .
1408: .Pp
1.309 jmc 1409: .It Pa /etc/ssh/ssh_host_key
1410: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1411: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309 jmc 1412: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1413: These files contain the private parts of the host keys
1.245 jmc 1414: and are used for host-based authentication.
1415: If protocol version 1 is used,
1.238 jmc 1416: .Nm
1417: must be setuid root, since the host key is readable only by root.
1418: For protocol version 2,
1419: .Nm
1420: uses
1421: .Xr ssh-keysign 8
1.245 jmc 1422: to access the host keys,
1423: eliminating the requirement that
1.238 jmc 1424: .Nm
1.245 jmc 1425: be setuid root when host-based authentication is used.
1.238 jmc 1426: By default
1.2 deraadt 1427: .Nm
1.238 jmc 1428: is not setuid root.
1429: .Pp
1.309 jmc 1430: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1431: Systemwide list of known host keys.
1432: This file should be prepared by the
1433: system administrator to contain the public host keys of all machines in the
1434: organization.
1.244 jmc 1435: It should be world-readable.
1436: See
1.238 jmc 1437: .Xr sshd 8
1.244 jmc 1438: for further details of the format of this file.
1.236 jmc 1439: .Pp
1.309 jmc 1440: .It Pa /etc/ssh/sshrc
1.1 deraadt 1441: Commands in this file are executed by
1.2 deraadt 1442: .Nm
1.245 jmc 1443: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1444: See the
1.2 deraadt 1445: .Xr sshd 8
1.1 deraadt 1446: manual page for more information.
1.58 itojun 1447: .El
1.312 jmc 1448: .Sh EXIT STATUS
1449: .Nm
1450: exits with the exit status of the remote command or with 255
1451: if an error occurred.
1.2 deraadt 1452: .Sh SEE ALSO
1453: .Xr scp 1 ,
1.83 djm 1454: .Xr sftp 1 ,
1.2 deraadt 1455: .Xr ssh-add 1 ,
1456: .Xr ssh-agent 1 ,
1457: .Xr ssh-keygen 1 ,
1.242 jmc 1458: .Xr ssh-keyscan 1 ,
1.250 jmc 1459: .Xr tun 4 ,
1.176 jmc 1460: .Xr hosts.equiv 5 ,
1.159 stevesk 1461: .Xr ssh_config 5 ,
1.160 naddy 1462: .Xr ssh-keysign 8 ,
1.87 itojun 1463: .Xr sshd 8
1.329 jmc 1464: .Sh STANDARDS
1.106 markus 1465: .Rs
1.329 jmc 1466: .%A S. Lehtinen
1467: .%A C. Lonvick
1468: .%D January 2006
1.256 jmc 1469: .%R RFC 4250
1.329 jmc 1470: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1471: .Re
1.329 jmc 1472: .Pp
1.256 jmc 1473: .Rs
1.329 jmc 1474: .%A T. Ylonen
1475: .%A C. Lonvick
1476: .%D January 2006
1.256 jmc 1477: .%R RFC 4251
1.329 jmc 1478: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1479: .Re
1.329 jmc 1480: .Pp
1.256 jmc 1481: .Rs
1.329 jmc 1482: .%A T. Ylonen
1483: .%A C. Lonvick
1484: .%D January 2006
1.256 jmc 1485: .%R RFC 4252
1.329 jmc 1486: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1487: .Re
1.329 jmc 1488: .Pp
1.256 jmc 1489: .Rs
1.329 jmc 1490: .%A T. Ylonen
1491: .%A C. Lonvick
1492: .%D January 2006
1.256 jmc 1493: .%R RFC 4253
1.329 jmc 1494: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1495: .Re
1.329 jmc 1496: .Pp
1.256 jmc 1497: .Rs
1.329 jmc 1498: .%A T. Ylonen
1499: .%A C. Lonvick
1500: .%D January 2006
1.256 jmc 1501: .%R RFC 4254
1.329 jmc 1502: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1503: .Re
1.329 jmc 1504: .Pp
1.256 jmc 1505: .Rs
1.329 jmc 1506: .%A J. Schlyter
1507: .%A W. Griffin
1508: .%D January 2006
1.256 jmc 1509: .%R RFC 4255
1.329 jmc 1510: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1511: .Re
1.329 jmc 1512: .Pp
1.256 jmc 1513: .Rs
1.329 jmc 1514: .%A F. Cusack
1515: .%A M. Forssen
1516: .%D January 2006
1.256 jmc 1517: .%R RFC 4256
1.329 jmc 1518: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1519: .Re
1.329 jmc 1520: .Pp
1.256 jmc 1521: .Rs
1.329 jmc 1522: .%A J. Galbraith
1523: .%A P. Remaker
1524: .%D January 2006
1.256 jmc 1525: .%R RFC 4335
1.329 jmc 1526: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1527: .Re
1.329 jmc 1528: .Pp
1.256 jmc 1529: .Rs
1.329 jmc 1530: .%A M. Bellare
1531: .%A T. Kohno
1532: .%A C. Namprempre
1533: .%D January 2006
1.256 jmc 1534: .%R RFC 4344
1.329 jmc 1535: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1536: .Re
1.329 jmc 1537: .Pp
1.256 jmc 1538: .Rs
1.329 jmc 1539: .%A B. Harris
1540: .%D January 2006
1.256 jmc 1541: .%R RFC 4345
1.329 jmc 1542: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1543: .Re
1.329 jmc 1544: .Pp
1.258 djm 1545: .Rs
1.329 jmc 1546: .%A M. Friedl
1547: .%A N. Provos
1548: .%A W. Simpson
1549: .%D March 2006
1.258 djm 1550: .%R RFC 4419
1.329 jmc 1551: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1552: .Re
1.329 jmc 1553: .Pp
1.266 markus 1554: .Rs
1.329 jmc 1555: .%A J. Galbraith
1556: .%A R. Thayer
1557: .%D November 2006
1.266 markus 1558: .%R RFC 4716
1.329 jmc 1559: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1560: .Re
1.329 jmc 1561: .Pp
1.313 djm 1562: .Rs
1.329 jmc 1563: .%A D. Stebila
1564: .%A J. Green
1565: .%D December 2009
1.313 djm 1566: .%R RFC 5656
1.329 jmc 1567: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1568: .Re
1.329 jmc 1569: .Pp
1.274 grunk 1570: .Rs
1571: .%A A. Perrig
1572: .%A D. Song
1573: .%D 1999
1.329 jmc 1574: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1575: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1576: .Re
1.173 jmc 1577: .Sh AUTHORS
1578: OpenSSH is a derivative of the original and free
1579: ssh 1.2.12 release by Tatu Ylonen.
1580: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1581: Theo de Raadt and Dug Song
1582: removed many bugs, re-added newer features and
1583: created OpenSSH.
1584: Markus Friedl contributed the support for SSH
1585: protocol versions 1.5 and 2.0.
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