Annotation of src/usr.bin/ssh/ssh.1, Revision 1.295
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.59 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.93 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.59 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.59 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.295 ! djm 37: .\" $OpenBSD: ssh.1,v 1.294 2010/02/11 13:23:29 jmc Exp $
! 38: .Dd $Mdocdate: February 11 2010 $
1.2 deraadt 39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.96 deraadt 43: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
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
1.210 djm 49: .Oo Fl D\ \&
50: .Sm off
51: .Oo Ar bind_address : Oc
52: .Ar port
53: .Sm on
54: .Oc
1.2 deraadt 55: .Op Fl e Ar escape_char
1.176 jmc 56: .Op Fl F Ar configfile
1.292 jmc 57: .Op Fl I Ar pkcs11
1.211 jmc 58: .Bk -words
1.2 deraadt 59: .Op Fl i Ar identity_file
1.211 jmc 60: .Ek
1.202 jmc 61: .Oo Fl L\ \&
1.12 aaron 62: .Sm off
1.200 djm 63: .Oo Ar bind_address : Oc
1.202 jmc 64: .Ar port : host : hostport
1.12 aaron 65: .Sm on
66: .Oc
1.211 jmc 67: .Bk -words
1.176 jmc 68: .Op Fl l Ar login_name
1.211 jmc 69: .Ek
1.176 jmc 70: .Op Fl m Ar mac_spec
1.198 djm 71: .Op Fl O Ar ctl_cmd
1.176 jmc 72: .Op Fl o Ar option
73: .Op Fl p Ar port
1.202 jmc 74: .Oo Fl R\ \&
1.12 aaron 75: .Sm off
1.200 djm 76: .Oo Ar bind_address : Oc
1.202 jmc 77: .Ar port : host : hostport
1.12 aaron 78: .Sm on
79: .Oc
1.198 djm 80: .Op Fl S Ar ctl_path
1.290 dtucker 81: .Op Fl W Ar host : Ns Ar port
1.261 stevesk 82: .Oo Fl w Ar local_tun Ns
83: .Op : Ns Ar remote_tun Oc
1.176 jmc 84: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 85: .Op Ar command
1.44 aaron 86: .Sh DESCRIPTION
1.2 deraadt 87: .Nm
1.96 deraadt 88: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 89: executing commands on a remote machine.
1.176 jmc 90: It is intended to replace rlogin and rsh,
91: and provide secure encrypted communications between
1.40 aaron 92: two untrusted hosts over an insecure network.
1.247 jmc 93: X11 connections and arbitrary TCP ports
1.176 jmc 94: can also be forwarded over the secure channel.
1.2 deraadt 95: .Pp
96: .Nm
1.44 aaron 97: connects and logs into the specified
1.176 jmc 98: .Ar hostname
99: (with optional
100: .Ar user
101: name).
1.1 deraadt 102: The user must prove
1.49 markus 103: his/her identity to the remote machine using one of several methods
1.221 jmc 104: depending on the protocol version used (see below).
1.49 markus 105: .Pp
1.176 jmc 106: If
107: .Ar command
108: is specified,
1.219 jmc 109: it is executed on the remote host instead of a login shell.
1.2 deraadt 110: .Pp
1.218 jmc 111: The options are as follows:
112: .Bl -tag -width Ds
113: .It Fl 1
114: Forces
1.2 deraadt 115: .Nm
1.218 jmc 116: to try protocol version 1 only.
117: .It Fl 2
118: Forces
1.2 deraadt 119: .Nm
1.218 jmc 120: to try protocol version 2 only.
121: .It Fl 4
122: Forces
1.2 deraadt 123: .Nm
1.218 jmc 124: to use IPv4 addresses only.
125: .It Fl 6
126: Forces
1.2 deraadt 127: .Nm
1.218 jmc 128: to use IPv6 addresses only.
129: .It Fl A
130: Enables forwarding of the authentication agent connection.
131: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 132: .Pp
1.218 jmc 133: Agent forwarding should be enabled with caution.
134: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 135: (for the agent's
1.286 sobrado 136: .Ux Ns -domain
137: socket) can access the local agent through the forwarded connection.
1.218 jmc 138: An attacker cannot obtain key material from the agent,
139: however they can perform operations on the keys that enable them to
140: authenticate using the identities loaded into the agent.
141: .It Fl a
142: Disables forwarding of the authentication agent connection.
143: .It Fl b Ar bind_address
144: Use
145: .Ar bind_address
146: on the local machine as the source address
147: of the connection.
148: Only useful on systems with more than one address.
149: .It Fl C
150: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 151: data for forwarded X11 and TCP connections).
1.218 jmc 152: The compression algorithm is the same used by
153: .Xr gzip 1 ,
154: and the
155: .Dq level
156: can be controlled by the
157: .Cm CompressionLevel
158: option for protocol version 1.
159: Compression is desirable on modem lines and other
160: slow connections, but will only slow down things on fast networks.
161: The default value can be set on a host-by-host basis in the
162: configuration files; see the
163: .Cm Compression
164: option.
165: .It Fl c Ar cipher_spec
166: Selects the cipher specification for encrypting the session.
1.2 deraadt 167: .Pp
1.218 jmc 168: Protocol version 1 allows specification of a single cipher.
169: The supported values are
170: .Dq 3des ,
1.220 jmc 171: .Dq blowfish ,
1.218 jmc 172: and
173: .Dq des .
174: .Ar 3des
175: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
176: It is believed to be secure.
177: .Ar blowfish
178: is a fast block cipher; it appears very secure and is much faster than
179: .Ar 3des .
180: .Ar des
181: is only supported in the
1.2 deraadt 182: .Nm
1.218 jmc 183: client for interoperability with legacy protocol 1 implementations
184: that do not support the
185: .Ar 3des
186: cipher.
187: Its use is strongly discouraged due to cryptographic weaknesses.
188: The default is
189: .Dq 3des .
1.49 markus 190: .Pp
1.230 jmc 191: For protocol version 2,
1.218 jmc 192: .Ar cipher_spec
193: is a comma-separated list of ciphers
194: listed in order of preference.
1.283 jmc 195: See the
196: .Cm Ciphers
197: keyword for more information.
1.218 jmc 198: .It Fl D Xo
199: .Sm off
200: .Oo Ar bind_address : Oc
201: .Ar port
202: .Sm on
203: .Xc
204: Specifies a local
205: .Dq dynamic
206: application-level port forwarding.
207: This works by allocating a socket to listen to
208: .Ar port
209: on the local side, optionally bound to the specified
210: .Ar bind_address .
211: Whenever a connection is made to this port, the
212: connection is forwarded over the secure channel, and the application
213: protocol is then used to determine where to connect to from the
214: remote machine.
215: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 216: .Nm
1.218 jmc 217: will act as a SOCKS server.
218: Only root can forward privileged ports.
219: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 220: .Pp
1.218 jmc 221: IPv6 addresses can be specified with an alternative syntax:
222: .Sm off
223: .Xo
224: .Op Ar bind_address No /
225: .Ar port
226: .Xc
227: .Sm on
228: or by enclosing the address in square brackets.
229: Only the superuser can forward privileged ports.
230: By default, the local port is bound in accordance with the
231: .Cm GatewayPorts
232: setting.
233: However, an explicit
234: .Ar bind_address
235: may be used to bind the connection to a specific address.
236: The
237: .Ar bind_address
238: of
239: .Dq localhost
240: indicates that the listening port be bound for local use only, while an
241: empty address or
242: .Sq *
243: indicates that the port should be available from all interfaces.
1.229 jmc 244: .It Fl e Ar escape_char
1.218 jmc 245: Sets the escape character for sessions with a pty (default:
246: .Ql ~ ) .
247: The escape character is only recognized at the beginning of a line.
248: The escape character followed by a dot
249: .Pq Ql \&.
250: closes the connection;
251: followed by control-Z suspends the connection;
252: and followed by itself sends the escape character once.
253: Setting the character to
1.2 deraadt 254: .Dq none
1.218 jmc 255: disables any escapes and makes the session fully transparent.
256: .It Fl F Ar configfile
257: Specifies an alternative per-user configuration file.
258: If a configuration file is given on the command line,
259: the system-wide configuration file
260: .Pq Pa /etc/ssh/ssh_config
261: will be ignored.
262: The default for the per-user configuration file is
263: .Pa ~/.ssh/config .
264: .It Fl f
265: Requests
266: .Nm
267: to go to background just before command execution.
268: This is useful if
1.176 jmc 269: .Nm
1.218 jmc 270: is going to ask for passwords or passphrases, but the user
271: wants it in the background.
272: This implies
273: .Fl n .
274: The recommended way to start X11 programs at a remote site is with
275: something like
276: .Ic ssh -f host xterm .
1.277 djm 277: .Pp
278: If the
279: .Cm ExitOnForwardFailure
280: configuration option is set to
281: .Dq yes ,
282: then a client started with
283: .Fl f
284: will wait for all remote port forwards to be successfully established
285: before placing itself in the background.
1.218 jmc 286: .It Fl g
287: Allows remote hosts to connect to local forwarded ports.
1.291 markus 288: .It Fl I Ar pkcs11
1.294 jmc 289: Specify the PKCS#11 shared library
1.176 jmc 290: .Nm
1.293 markus 291: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 292: private RSA key.
293: .It Fl i Ar identity_file
294: Selects a file from which the identity (private key) for
295: RSA or DSA authentication is read.
296: The default is
297: .Pa ~/.ssh/identity
298: for protocol version 1, and
299: .Pa ~/.ssh/id_rsa
1.149 jakob 300: and
1.218 jmc 301: .Pa ~/.ssh/id_dsa
302: for protocol version 2.
303: Identity files may also be specified on
304: a per-host basis in the configuration file.
305: It is possible to have multiple
306: .Fl i
307: options (and multiple identities specified in
308: configuration files).
1.269 djm 309: .It Fl K
310: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
311: credentials to the server.
1.218 jmc 312: .It Fl k
313: Disables forwarding (delegation) of GSSAPI credentials to the server.
314: .It Fl L Xo
315: .Sm off
316: .Oo Ar bind_address : Oc
317: .Ar port : host : hostport
318: .Sm on
319: .Xc
320: Specifies that the given port on the local (client) host is to be
321: forwarded to the given host and port on the remote side.
322: This works by allocating a socket to listen to
323: .Ar port
324: on the local side, optionally bound to the specified
325: .Ar bind_address .
326: Whenever a connection is made to this port, the
327: connection is forwarded over the secure channel, and a connection is
328: made to
329: .Ar host
330: port
331: .Ar hostport
332: from the remote machine.
333: Port forwardings can also be specified in the configuration file.
334: IPv6 addresses can be specified with an alternative syntax:
335: .Sm off
336: .Xo
337: .Op Ar bind_address No /
338: .Ar port No / Ar host No /
339: .Ar hostport
340: .Xc
341: .Sm on
342: or by enclosing the address in square brackets.
343: Only the superuser can forward privileged ports.
344: By default, the local port is bound in accordance with the
345: .Cm GatewayPorts
346: setting.
347: However, an explicit
348: .Ar bind_address
349: may be used to bind the connection to a specific address.
1.2 deraadt 350: The
1.218 jmc 351: .Ar bind_address
352: of
353: .Dq localhost
354: indicates that the listening port be bound for local use only, while an
355: empty address or
356: .Sq *
357: indicates that the port should be available from all interfaces.
358: .It Fl l Ar login_name
359: Specifies the user to log in as on the remote machine.
360: This also may be specified on a per-host basis in the configuration file.
361: .It Fl M
362: Places the
363: .Nm
364: client into
365: .Dq master
366: mode for connection sharing.
1.231 stevesk 367: Multiple
368: .Fl M
369: options places
370: .Nm
371: into
372: .Dq master
373: mode with confirmation required before slave connections are accepted.
1.218 jmc 374: Refer to the description of
375: .Cm ControlMaster
376: in
377: .Xr ssh_config 5
378: for details.
379: .It Fl m Ar mac_spec
380: Additionally, for protocol version 2 a comma-separated list of MAC
381: (message authentication code) algorithms can
382: be specified in order of preference.
383: See the
384: .Cm MACs
385: keyword for more information.
386: .It Fl N
387: Do not execute a remote command.
388: This is useful for just forwarding ports
389: (protocol version 2 only).
390: .It Fl n
391: Redirects stdin from
392: .Pa /dev/null
393: (actually, prevents reading from stdin).
394: This must be used when
1.2 deraadt 395: .Nm
1.218 jmc 396: is run in the background.
397: A common trick is to use this to run X11 programs on a remote machine.
398: For example,
399: .Ic ssh -n shadows.cs.hut.fi emacs &
400: will start an emacs on shadows.cs.hut.fi, and the X11
401: connection will be automatically forwarded over an encrypted channel.
402: The
1.2 deraadt 403: .Nm
1.218 jmc 404: program will be put in the background.
405: (This does not work if
1.2 deraadt 406: .Nm
1.218 jmc 407: needs to ask for a password or passphrase; see also the
408: .Fl f
409: option.)
410: .It Fl O Ar ctl_cmd
411: Control an active connection multiplexing master process.
412: When the
413: .Fl O
414: option is specified, the
415: .Ar ctl_cmd
416: argument is interpreted and passed to the master process.
417: Valid commands are:
418: .Dq check
419: (check that the master process is running) and
420: .Dq exit
421: (request the master to exit).
422: .It Fl o Ar option
423: Can be used to give options in the format used in the configuration file.
424: This is useful for specifying options for which there is no separate
425: command-line flag.
426: For full details of the options listed below, and their possible values, see
427: .Xr ssh_config 5 .
1.2 deraadt 428: .Pp
1.218 jmc 429: .Bl -tag -width Ds -offset indent -compact
430: .It AddressFamily
431: .It BatchMode
432: .It BindAddress
433: .It ChallengeResponseAuthentication
434: .It CheckHostIP
435: .It Cipher
436: .It Ciphers
437: .It ClearAllForwardings
438: .It Compression
439: .It CompressionLevel
440: .It ConnectionAttempts
441: .It ConnectTimeout
442: .It ControlMaster
443: .It ControlPath
444: .It DynamicForward
445: .It EscapeChar
1.263 markus 446: .It ExitOnForwardFailure
1.218 jmc 447: .It ForwardAgent
448: .It ForwardX11
449: .It ForwardX11Trusted
450: .It GatewayPorts
451: .It GlobalKnownHostsFile
452: .It GSSAPIAuthentication
453: .It GSSAPIDelegateCredentials
454: .It HashKnownHosts
455: .It Host
456: .It HostbasedAuthentication
457: .It HostKeyAlgorithms
458: .It HostKeyAlias
459: .It HostName
460: .It IdentityFile
461: .It IdentitiesOnly
462: .It KbdInteractiveDevices
463: .It LocalCommand
464: .It LocalForward
465: .It LogLevel
466: .It MACs
467: .It NoHostAuthenticationForLocalhost
468: .It NumberOfPasswordPrompts
469: .It PasswordAuthentication
470: .It PermitLocalCommand
1.291 markus 471: .It PKCS11Provider
1.218 jmc 472: .It Port
473: .It PreferredAuthentications
474: .It Protocol
475: .It ProxyCommand
476: .It PubkeyAuthentication
1.251 dtucker 477: .It RekeyLimit
1.218 jmc 478: .It RemoteForward
479: .It RhostsRSAAuthentication
480: .It RSAAuthentication
481: .It SendEnv
482: .It ServerAliveInterval
483: .It ServerAliveCountMax
484: .It StrictHostKeyChecking
485: .It TCPKeepAlive
486: .It Tunnel
487: .It TunnelDevice
488: .It UsePrivilegedPort
489: .It User
490: .It UserKnownHostsFile
491: .It VerifyHostKeyDNS
1.276 jmc 492: .It VisualHostKey
1.218 jmc 493: .It XAuthLocation
494: .El
495: .It Fl p Ar port
496: Port to connect to on the remote host.
497: This can be specified on a
498: per-host basis in the configuration file.
499: .It Fl q
500: Quiet mode.
1.271 djm 501: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 502: .It Fl R Xo
503: .Sm off
504: .Oo Ar bind_address : Oc
505: .Ar port : host : hostport
506: .Sm on
507: .Xc
508: Specifies that the given port on the remote (server) host is to be
509: forwarded to the given host and port on the local side.
510: This works by allocating a socket to listen to
511: .Ar port
512: on the remote side, and whenever a connection is made to this port, the
513: connection is forwarded over the secure channel, and a connection is
514: made to
515: .Ar host
516: port
517: .Ar hostport
518: from the local machine.
1.2 deraadt 519: .Pp
1.218 jmc 520: Port forwardings can also be specified in the configuration file.
521: Privileged ports can be forwarded only when
522: logging in as root on the remote machine.
523: IPv6 addresses can be specified by enclosing the address in square braces or
524: using an alternative syntax:
525: .Sm off
526: .Xo
527: .Op Ar bind_address No /
528: .Ar host No / Ar port No /
529: .Ar hostport
530: .Xc .
531: .Sm on
1.194 jakob 532: .Pp
1.218 jmc 533: By default, the listening socket on the server will be bound to the loopback
534: interface only.
1.280 tobias 535: This may be overridden by specifying a
1.218 jmc 536: .Ar bind_address .
537: An empty
538: .Ar bind_address ,
539: or the address
540: .Ql * ,
541: indicates that the remote socket should listen on all interfaces.
542: Specifying a remote
543: .Ar bind_address
544: will only succeed if the server's
545: .Cm GatewayPorts
546: option is enabled (see
547: .Xr sshd_config 5 ) .
1.281 djm 548: .Pp
549: If the
550: .Ar port
551: argument is
1.282 djm 552: .Ql 0 ,
1.281 djm 553: the listen port will be dynamically allocated on the server and reported
554: to the client at run time.
1.218 jmc 555: .It Fl S Ar ctl_path
556: Specifies the location of a control socket for connection sharing.
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
606: .Cm ClearAllForwardings
607: and 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 .
675: .Pp
676: .Nm
677: exits with the exit status of the remote command or with 255
678: if an error occurred.
1.222 jmc 679: .Sh AUTHENTICATION
1.249 jmc 680: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 681: The default is to use protocol 2 only,
682: though this can be changed via the
1.222 jmc 683: .Cm Protocol
684: option in
1.284 jmc 685: .Xr ssh_config 5
686: or the
1.222 jmc 687: .Fl 1
688: and
689: .Fl 2
690: options (see above).
691: Both protocols support similar authentication methods,
1.284 jmc 692: but protocol 2 is the default since
1.222 jmc 693: it provides additional mechanisms for confidentiality
694: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 695: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 696: Protocol 1 lacks a strong mechanism for ensuring the
697: integrity of the connection.
698: .Pp
699: The methods available for authentication are:
1.260 jmc 700: GSSAPI-based authentication,
1.222 jmc 701: host-based authentication,
702: public key authentication,
703: challenge-response authentication,
704: and password authentication.
705: Authentication methods are tried in the order specified above,
706: though protocol 2 has a configuration option to change the default order:
707: .Cm PreferredAuthentications .
708: .Pp
709: Host-based authentication works as follows:
1.218 jmc 710: If the machine the user logs in from is listed in
711: .Pa /etc/hosts.equiv
712: or
713: .Pa /etc/shosts.equiv
714: on the remote machine, and the user names are
715: the same on both sides, or if the files
716: .Pa ~/.rhosts
717: or
718: .Pa ~/.shosts
719: exist in the user's home directory on the
720: remote machine and contain a line containing the name of the client
721: machine and the name of the user on that machine, the user is
1.222 jmc 722: considered for login.
723: Additionally, the server
724: .Em must
725: be able to verify the client's
726: host key (see the description of
1.218 jmc 727: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 728: and
1.222 jmc 729: .Pa ~/.ssh/known_hosts ,
730: below)
731: for login to be permitted.
1.218 jmc 732: This authentication method closes security holes due to IP
1.222 jmc 733: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 734: [Note to the administrator:
735: .Pa /etc/hosts.equiv ,
736: .Pa ~/.rhosts ,
737: and the rlogin/rsh protocol in general, are inherently insecure and should be
738: disabled if security is desired.]
1.189 dtucker 739: .Pp
1.222 jmc 740: Public key authentication works as follows:
741: The scheme is based on public-key cryptography,
742: using cryptosystems
743: where encryption and decryption are done using separate keys,
744: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 745: The idea is that each user creates a public/private
746: key pair for authentication purposes.
747: The server knows the public key, and only the user knows the private key.
1.222 jmc 748: .Nm
749: implements public key authentication protocol automatically,
750: using either the RSA or DSA algorithms.
751: Protocol 1 is restricted to using only RSA keys,
752: but protocol 2 may use either.
753: The
754: .Sx HISTORY
755: section of
756: .Xr ssl 8
757: contains a brief discussion of the two algorithms.
1.210 djm 758: .Pp
1.218 jmc 759: The file
760: .Pa ~/.ssh/authorized_keys
761: lists the public keys that are permitted for logging in.
762: When the user logs in, the
1.2 deraadt 763: .Nm
1.218 jmc 764: program tells the server which key pair it would like to use for
765: authentication.
1.222 jmc 766: The client proves that it has access to the private key
767: and the server checks that the corresponding public key
768: is authorized to accept the account.
1.218 jmc 769: .Pp
1.222 jmc 770: The user creates his/her key pair by running
1.218 jmc 771: .Xr ssh-keygen 1 .
772: This stores the private key in
1.207 djm 773: .Pa ~/.ssh/identity
1.222 jmc 774: (protocol 1),
775: .Pa ~/.ssh/id_dsa
776: (protocol 2 DSA),
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),
785: or
786: .Pa ~/.ssh/id_rsa.pub
787: (protocol 2 RSA)
1.218 jmc 788: in the user's home directory.
1.222 jmc 789: The user should then copy the public key
1.218 jmc 790: to
791: .Pa ~/.ssh/authorized_keys
1.222 jmc 792: in his/her home directory on the remote machine.
793: The
1.218 jmc 794: .Pa authorized_keys
795: file corresponds to the conventional
796: .Pa ~/.rhosts
797: file, and has one key
1.222 jmc 798: per line, though the lines can be very long.
1.218 jmc 799: After this, the user can log in without giving the password.
800: .Pp
1.222 jmc 801: The most convenient way to use public key authentication may be with an
1.218 jmc 802: authentication agent.
803: See
804: .Xr ssh-agent 1
805: for more information.
806: .Pp
1.222 jmc 807: Challenge-response authentication works as follows:
808: The server sends an arbitrary
809: .Qq challenge
810: text, and prompts for a response.
811: Protocol 2 allows multiple challenges and responses;
812: protocol 1 is restricted to just one challenge/response.
813: Examples of challenge-response authentication include
814: BSD Authentication (see
815: .Xr login.conf 5 )
816: and PAM (some non-OpenBSD systems).
817: .Pp
818: Finally, if other authentication methods fail,
1.218 jmc 819: .Nm
820: prompts the user for a password.
821: The password is sent to the remote
822: host for checking; however, since all communications are encrypted,
823: the password cannot be seen by someone listening on the network.
1.232 jmc 824: .Pp
825: .Nm
826: automatically maintains and checks a database containing
827: identification for all hosts it has ever been used with.
828: Host keys are stored in
829: .Pa ~/.ssh/known_hosts
830: in the user's home directory.
831: Additionally, the file
832: .Pa /etc/ssh/ssh_known_hosts
833: is automatically checked for known hosts.
834: Any new hosts are automatically added to the user's file.
835: If a host's identification ever changes,
836: .Nm
837: warns about this and disables password authentication to prevent
838: server spoofing or man-in-the-middle attacks,
839: which could otherwise be used to circumvent the encryption.
840: The
841: .Cm StrictHostKeyChecking
842: option can be used to control logins to machines whose
843: host key is not known or has changed.
844: .Pp
1.218 jmc 845: When the user's identity has been accepted by the server, the server
846: either executes the given command, or logs into the machine and gives
847: the user a normal shell on the remote machine.
848: All communication with
849: the remote command or shell will be automatically encrypted.
850: .Pp
851: If a pseudo-terminal has been allocated (normal login session), the
852: user may use the escape characters noted below.
853: .Pp
854: If no pseudo-tty has been allocated,
855: the session is transparent and can be used to reliably transfer binary data.
856: On most systems, setting the escape character to
857: .Dq none
858: will also make the session transparent even if a tty is used.
859: .Pp
860: The session terminates when the command or shell on the remote
1.247 jmc 861: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 862: .Sh ESCAPE CHARACTERS
1.218 jmc 863: When a pseudo-terminal has been requested,
1.2 deraadt 864: .Nm
1.218 jmc 865: supports a number of functions through the use of an escape character.
866: .Pp
867: A single tilde character can be sent as
868: .Ic ~~
869: or by following the tilde by a character other than those described below.
870: The escape character must always follow a newline to be interpreted as
871: special.
872: The escape character can be changed in configuration files using the
873: .Cm EscapeChar
874: configuration directive or on the command line by the
875: .Fl e
876: option.
877: .Pp
878: The supported escapes (assuming the default
879: .Ql ~ )
880: are:
881: .Bl -tag -width Ds
882: .It Cm ~.
883: Disconnect.
884: .It Cm ~^Z
885: Background
1.234 jmc 886: .Nm .
1.218 jmc 887: .It Cm ~#
888: List forwarded connections.
889: .It Cm ~&
890: Background
1.2 deraadt 891: .Nm
1.218 jmc 892: at logout when waiting for forwarded connection / X11 sessions to terminate.
893: .It Cm ~?
894: Display a list of escape characters.
895: .It Cm ~B
896: Send a BREAK to the remote system
897: (only useful for SSH protocol version 2 and if the peer supports it).
898: .It Cm ~C
899: Open command line.
900: Currently this allows the addition of port forwardings using the
1.279 stevesk 901: .Fl L ,
902: .Fl R
1.218 jmc 903: and
1.279 stevesk 904: .Fl D
1.225 jmc 905: options (see above).
1.218 jmc 906: It also allows the cancellation of existing remote port-forwardings
907: using
1.262 stevesk 908: .Sm off
909: .Fl KR Oo Ar bind_address : Oc Ar port .
910: .Sm on
1.218 jmc 911: .Ic !\& Ns Ar command
912: allows the user to execute a local command if the
913: .Ic PermitLocalCommand
914: option is enabled in
1.176 jmc 915: .Xr ssh_config 5 .
1.218 jmc 916: Basic help is available, using the
917: .Fl h
918: option.
919: .It Cm ~R
920: Request rekeying of the connection
921: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 922: .El
1.246 jmc 923: .Sh TCP FORWARDING
924: Forwarding of arbitrary TCP connections over the secure channel can
925: be specified either on the command line or in a configuration file.
926: One possible application of TCP forwarding is a secure connection to a
927: mail server; another is going through firewalls.
928: .Pp
929: In the example below, we look at encrypting communication between
930: an IRC client and server, even though the IRC server does not directly
931: support encrypted communications.
932: This works as follows:
933: the user connects to the remote host using
934: .Nm ,
935: specifying a port to be used to forward connections
936: to the remote server.
937: After that it is possible to start the service which is to be encrypted
938: on the client machine,
939: connecting to the same local port,
940: and
941: .Nm
942: will encrypt and forward the connection.
943: .Pp
944: The following example tunnels an IRC session from client machine
945: .Dq 127.0.0.1
946: (localhost)
947: to remote server
948: .Dq server.example.com :
949: .Bd -literal -offset 4n
950: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
951: $ irc -c '#users' -p 1234 pinky 127.0.0.1
952: .Ed
953: .Pp
954: This tunnels a connection to IRC server
955: .Dq server.example.com ,
956: joining channel
957: .Dq #users ,
958: nickname
959: .Dq pinky ,
960: using port 1234.
961: It doesn't matter which port is used,
962: as long as it's greater than 1023
963: (remember, only root can open sockets on privileged ports)
964: and doesn't conflict with any ports already in use.
965: The connection is forwarded to port 6667 on the remote server,
966: since that's the standard port for IRC services.
967: .Pp
968: The
969: .Fl f
970: option backgrounds
971: .Nm
972: and the remote command
973: .Dq sleep 10
974: is specified to allow an amount of time
975: (10 seconds, in the example)
976: to start the service which is to be tunnelled.
977: If no connections are made within the time specified,
978: .Nm
979: will exit.
980: .Sh X11 FORWARDING
1.218 jmc 981: If the
982: .Cm ForwardX11
983: variable is set to
984: .Dq yes
985: (or see the description of the
1.227 jmc 986: .Fl X ,
987: .Fl x ,
1.218 jmc 988: and
1.227 jmc 989: .Fl Y
1.226 jmc 990: options above)
1.218 jmc 991: and the user is using X11 (the
992: .Ev DISPLAY
993: environment variable is set), the connection to the X11 display is
994: automatically forwarded to the remote side in such a way that any X11
995: programs started from the shell (or command) will go through the
996: encrypted channel, and the connection to the real X server will be made
1.176 jmc 997: from the local machine.
1.218 jmc 998: The user should not manually set
999: .Ev DISPLAY .
1000: Forwarding of X11 connections can be
1001: configured on the command line or in configuration files.
1002: .Pp
1003: The
1004: .Ev DISPLAY
1005: value set by
1006: .Nm
1007: will point to the server machine, but with a display number greater than zero.
1008: This is normal, and happens because
1009: .Nm
1010: creates a
1011: .Dq proxy
1012: X server on the server machine for forwarding the
1013: connections over the encrypted channel.
1.200 djm 1014: .Pp
1.218 jmc 1015: .Nm
1016: will also automatically set up Xauthority data on the server machine.
1017: For this purpose, it will generate a random authorization cookie,
1018: store it in Xauthority on the server, and verify that any forwarded
1019: connections carry this cookie and replace it by the real cookie when
1020: the connection is opened.
1021: The real authentication cookie is never
1022: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1023: .Pp
1.218 jmc 1024: If the
1025: .Cm ForwardAgent
1026: variable is set to
1027: .Dq yes
1028: (or see the description of the
1029: .Fl A
1.191 djm 1030: and
1.218 jmc 1031: .Fl a
1.226 jmc 1032: options above) and
1.218 jmc 1033: the user is using an authentication agent, the connection to the agent
1034: is automatically forwarded to the remote side.
1.252 jmc 1035: .Sh VERIFYING HOST KEYS
1036: When connecting to a server for the first time,
1037: a fingerprint of the server's public key is presented to the user
1038: (unless the option
1039: .Cm StrictHostKeyChecking
1040: has been disabled).
1041: Fingerprints can be determined using
1042: .Xr ssh-keygen 1 :
1043: .Pp
1044: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1045: .Pp
1.274 grunk 1046: If the fingerprint is already known, it can be matched
1047: and the key can be accepted or rejected.
1048: Because of the difficulty of comparing host keys
1049: just by looking at hex strings,
1050: there is also support to compare host keys visually,
1051: using
1052: .Em random art .
1053: By setting the
1.275 grunk 1054: .Cm VisualHostKey
1.274 grunk 1055: option to
1.275 grunk 1056: .Dq yes ,
1.274 grunk 1057: a small ASCII graphic gets displayed on every login to a server, no matter
1058: if the session itself is interactive or not.
1059: By learning the pattern a known server produces, a user can easily
1060: find out that the host key has changed when a completely different pattern
1061: is displayed.
1062: Because these patterns are not unambiguous however, a pattern that looks
1063: similar to the pattern remembered only gives a good probability that the
1064: host key is the same, not guaranteed proof.
1065: .Pp
1066: To get a listing of the fingerprints along with their random art for
1067: all known hosts, the following command line can be used:
1068: .Pp
1069: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1070: .Pp
1.252 jmc 1071: If the fingerprint is unknown,
1072: an alternative method of verification is available:
1073: SSH fingerprints verified by DNS.
1074: An additional resource record (RR),
1075: SSHFP,
1076: is added to a zonefile
1077: and the connecting client is able to match the fingerprint
1078: with that of the key presented.
1079: .Pp
1080: In this example, we are connecting a client to a server,
1081: .Dq host.example.com .
1082: The SSHFP resource records should first be added to the zonefile for
1083: host.example.com:
1084: .Bd -literal -offset indent
1.259 jakob 1085: $ ssh-keygen -r host.example.com.
1.252 jmc 1086: .Ed
1087: .Pp
1088: The output lines will have to be added to the zonefile.
1089: To check that the zone is answering fingerprint queries:
1090: .Pp
1091: .Dl $ dig -t SSHFP host.example.com
1092: .Pp
1093: Finally the client connects:
1094: .Bd -literal -offset indent
1095: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1096: [...]
1097: Matching host key fingerprint found in DNS.
1098: Are you sure you want to continue connecting (yes/no)?
1099: .Ed
1100: .Pp
1101: See the
1102: .Cm VerifyHostKeyDNS
1103: option in
1104: .Xr ssh_config 5
1105: for more information.
1.295 ! djm 1106: .Pp
! 1107: Host keys may also be presented as certificates signed by a trusted
! 1108: certification authority (CA).
! 1109: In this case, trust of the CA key alone is sufficient for the host key
! 1110: to be accepted.
! 1111: To specify a public key as a trusted CA key in a known hosts file,
! 1112: it should be added after a
! 1113: .Dq @cert-authority
! 1114: tag and a set of one or more domain-name wildcards separated by commas.
! 1115: For example:
! 1116: .Pp
! 1117: .Dl @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
! 1118: .Pp
! 1119: See the
! 1120: .Sx CERTIFICATES
! 1121: section of
! 1122: .Xr ssh-keygen 1
! 1123: for more details.
! 1124: .Pp
1.250 jmc 1125: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1126: .Nm
1127: contains support for Virtual Private Network (VPN) tunnelling
1128: using the
1129: .Xr tun 4
1130: network pseudo-device,
1131: allowing two networks to be joined securely.
1132: The
1133: .Xr sshd_config 5
1134: configuration option
1135: .Cm PermitTunnel
1136: controls whether the server supports this,
1137: and at what level (layer 2 or 3 traffic).
1138: .Pp
1139: The following example would connect client network 10.0.50.0/24
1.265 otto 1140: with remote network 10.0.99.0/24 using a point-to-point connection
1141: from 10.1.1.1 to 10.1.1.2,
1142: provided that the SSH server running on the gateway to the remote network,
1143: at 192.168.1.15, allows it.
1144: .Pp
1145: On the client:
1.250 jmc 1146: .Bd -literal -offset indent
1147: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1148: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1149: # route add 10.0.99.0/24 10.1.1.2
1150: .Ed
1151: .Pp
1152: On the server:
1153: .Bd -literal -offset indent
1154: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1155: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1156: .Ed
1157: .Pp
1158: Client access may be more finely tuned via the
1159: .Pa /root/.ssh/authorized_keys
1160: file (see below) and the
1161: .Cm PermitRootLogin
1162: server option.
1.255 jmc 1163: The following entry would permit connections on
1.250 jmc 1164: .Xr tun 4
1.255 jmc 1165: device 1 from user
1.250 jmc 1166: .Dq jane
1.255 jmc 1167: and on tun device 2 from user
1.250 jmc 1168: .Dq john ,
1169: if
1170: .Cm PermitRootLogin
1171: is set to
1172: .Dq forced-commands-only :
1173: .Bd -literal -offset 2n
1174: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1175: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1176: .Ed
1177: .Pp
1.264 ray 1178: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1179: it may be more suited to temporary setups,
1180: such as for wireless VPNs.
1181: More permanent VPNs are better provided by tools such as
1182: .Xr ipsecctl 8
1183: and
1184: .Xr isakmpd 8 .
1.2 deraadt 1185: .Sh ENVIRONMENT
1186: .Nm
1.1 deraadt 1187: will normally set the following environment variables:
1.237 jmc 1188: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1189: .It Ev DISPLAY
1190: The
1191: .Ev DISPLAY
1.40 aaron 1192: variable indicates the location of the X11 server.
1.44 aaron 1193: It is automatically set by
1.2 deraadt 1194: .Nm
1195: to point to a value of the form
1.233 jmc 1196: .Dq hostname:n ,
1197: where
1198: .Dq hostname
1199: indicates the host where the shell runs, and
1200: .Sq n
1201: is an integer \*(Ge 1.
1.40 aaron 1202: .Nm
1203: uses this special value to forward X11 connections over the secure
1204: channel.
1.107 markus 1205: The user should normally not set
1206: .Ev DISPLAY
1207: explicitly, as that
1.1 deraadt 1208: will render the X11 connection insecure (and will require the user to
1209: manually copy any required authorization cookies).
1.2 deraadt 1210: .It Ev HOME
1.1 deraadt 1211: Set to the path of the user's home directory.
1.2 deraadt 1212: .It Ev LOGNAME
1213: Synonym for
1.12 aaron 1214: .Ev USER ;
1215: set for compatibility with systems that use this variable.
1.2 deraadt 1216: .It Ev MAIL
1.129 stevesk 1217: Set to the path of the user's mailbox.
1.40 aaron 1218: .It Ev PATH
1.2 deraadt 1219: Set to the default
1220: .Ev PATH ,
1221: as specified when compiling
1.234 jmc 1222: .Nm .
1.118 markus 1223: .It Ev SSH_ASKPASS
1224: If
1225: .Nm
1226: needs a passphrase, it will read the passphrase from the current
1227: terminal if it was run from a terminal.
1228: If
1229: .Nm
1230: does not have a terminal associated with it but
1231: .Ev DISPLAY
1232: and
1233: .Ev SSH_ASKPASS
1234: are set, it will execute the program specified by
1235: .Ev SSH_ASKPASS
1236: and open an X11 window to read the passphrase.
1237: This is particularly useful when calling
1238: .Nm
1239: from a
1.196 jmc 1240: .Pa .xsession
1.118 markus 1241: or related script.
1242: (Note that on some machines it
1243: may be necessary to redirect the input from
1244: .Pa /dev/null
1245: to make this work.)
1.18 markus 1246: .It Ev SSH_AUTH_SOCK
1.233 jmc 1247: Identifies the path of a
1248: .Ux Ns -domain
1249: socket used to communicate with the agent.
1.166 stevesk 1250: .It Ev SSH_CONNECTION
1251: Identifies the client and server ends of the connection.
1.40 aaron 1252: The variable contains
1.233 jmc 1253: four space-separated values: client IP address, client port number,
1254: server IP address, and server port number.
1.73 markus 1255: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1256: This variable contains the original command line if a forced command
1.73 markus 1257: is executed.
1258: It can be used to extract the original arguments.
1.2 deraadt 1259: .It Ev SSH_TTY
1.1 deraadt 1260: This is set to the name of the tty (path to the device) associated
1.40 aaron 1261: with the current shell or command.
1262: If the current session has no tty,
1.1 deraadt 1263: this variable is not set.
1.2 deraadt 1264: .It Ev TZ
1.214 jmc 1265: This variable is set to indicate the present time zone if it
1.257 jmc 1266: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1267: on to new connections).
1.2 deraadt 1268: .It Ev USER
1.1 deraadt 1269: Set to the name of the user logging in.
1.2 deraadt 1270: .El
1271: .Pp
1.44 aaron 1272: Additionally,
1.2 deraadt 1273: .Nm
1.44 aaron 1274: reads
1.207 djm 1275: .Pa ~/.ssh/environment ,
1.2 deraadt 1276: and adds lines of the format
1277: .Dq VARNAME=value
1.233 jmc 1278: to the environment if the file exists and users are allowed to
1.161 marc 1279: change their environment.
1.176 jmc 1280: For more information, see the
1.161 marc 1281: .Cm PermitUserEnvironment
1.162 stevesk 1282: option in
1.161 marc 1283: .Xr sshd_config 5 .
1.2 deraadt 1284: .Sh FILES
1.236 jmc 1285: .Bl -tag -width Ds -compact
1286: .It ~/.rhosts
1.240 jmc 1287: This file is used for host-based authentication (see above).
1.92 markus 1288: On some machines this file may need to be
1.240 jmc 1289: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1290: because
1.2 deraadt 1291: .Xr sshd 8
1.40 aaron 1292: reads it as root.
1293: Additionally, this file must be owned by the user,
1294: and must not have write permissions for anyone else.
1295: The recommended
1.1 deraadt 1296: permission for most machines is read/write for the user, and not
1297: accessible by others.
1.2 deraadt 1298: .Pp
1.236 jmc 1299: .It ~/.shosts
1.240 jmc 1300: This file is used in exactly the same way as
1301: .Pa .rhosts ,
1302: but allows host-based authentication without permitting login with
1303: rlogin/rsh.
1.272 mcbride 1304: .Pp
1305: .It ~/.ssh/
1306: This directory is the default location for all user-specific configuration
1307: and authentication information.
1308: There is no general requirement to keep the entire contents of this directory
1309: secret, but the recommended permissions are read/write/execute for the user,
1310: and not accessible by others.
1.236 jmc 1311: .Pp
1.238 jmc 1312: .It ~/.ssh/authorized_keys
1313: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1314: The format of this file is described in the
1315: .Xr sshd 8
1316: manual page.
1317: This file is not highly sensitive, but the recommended
1318: permissions are read/write for the user, and not accessible by others.
1319: .Pp
1320: .It ~/.ssh/config
1321: This is the per-user configuration file.
1322: The file format and configuration options are described in
1323: .Xr ssh_config 5 .
1324: Because of the potential for abuse, this file must have strict permissions:
1325: read/write for the user, and not accessible by others.
1326: .Pp
1327: .It ~/.ssh/environment
1.239 jmc 1328: Contains additional definitions for environment variables; see
1329: .Sx ENVIRONMENT ,
1.238 jmc 1330: above.
1331: .Pp
1332: .It ~/.ssh/identity
1333: .It ~/.ssh/id_dsa
1334: .It ~/.ssh/id_rsa
1335: Contains the private key for authentication.
1336: These files
1337: contain sensitive data and should be readable by the user but not
1338: accessible by others (read/write/execute).
1339: .Nm
1340: will simply ignore a private key file if it is accessible by others.
1341: It is possible to specify a passphrase when
1342: generating the key which will be used to encrypt the
1343: sensitive part of this file using 3DES.
1344: .Pp
1345: .It ~/.ssh/identity.pub
1346: .It ~/.ssh/id_dsa.pub
1347: .It ~/.ssh/id_rsa.pub
1348: Contains the public key for authentication.
1349: These files are not
1350: sensitive and can (but need not) be readable by anyone.
1351: .Pp
1352: .It ~/.ssh/known_hosts
1.244 jmc 1353: Contains a list of host keys for all hosts the user has logged into
1354: that are not already in the systemwide list of known host keys.
1.238 jmc 1355: See
1.244 jmc 1356: .Xr sshd 8
1357: for further details of the format of this file.
1.238 jmc 1358: .Pp
1359: .It ~/.ssh/rc
1360: Commands in this file are executed by
1361: .Nm
1.245 jmc 1362: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1363: started.
1364: See the
1365: .Xr sshd 8
1366: manual page for more information.
1367: .Pp
1.236 jmc 1368: .It /etc/hosts.equiv
1.240 jmc 1369: This file is for host-based authentication (see above).
1370: It should only be writable by root.
1.236 jmc 1371: .Pp
1372: .It /etc/shosts.equiv
1.240 jmc 1373: This file is used in exactly the same way as
1374: .Pa hosts.equiv ,
1375: but allows host-based authentication without permitting login with
1376: rlogin/rsh.
1.236 jmc 1377: .Pp
1.238 jmc 1378: .It Pa /etc/ssh/ssh_config
1379: Systemwide configuration file.
1380: The file format and configuration options are described in
1381: .Xr ssh_config 5 .
1382: .Pp
1383: .It /etc/ssh/ssh_host_key
1384: .It /etc/ssh/ssh_host_dsa_key
1385: .It /etc/ssh/ssh_host_rsa_key
1386: These three files contain the private parts of the host keys
1.245 jmc 1387: and are used for host-based authentication.
1388: If protocol version 1 is used,
1.238 jmc 1389: .Nm
1390: must be setuid root, since the host key is readable only by root.
1391: For protocol version 2,
1392: .Nm
1393: uses
1394: .Xr ssh-keysign 8
1.245 jmc 1395: to access the host keys,
1396: eliminating the requirement that
1.238 jmc 1397: .Nm
1.245 jmc 1398: be setuid root when host-based authentication is used.
1.238 jmc 1399: By default
1.2 deraadt 1400: .Nm
1.238 jmc 1401: is not setuid root.
1402: .Pp
1403: .It /etc/ssh/ssh_known_hosts
1404: Systemwide list of known host keys.
1405: This file should be prepared by the
1406: system administrator to contain the public host keys of all machines in the
1407: organization.
1.244 jmc 1408: It should be world-readable.
1409: See
1.238 jmc 1410: .Xr sshd 8
1.244 jmc 1411: for further details of the format of this file.
1.236 jmc 1412: .Pp
1.238 jmc 1413: .It /etc/ssh/sshrc
1.1 deraadt 1414: Commands in this file are executed by
1.2 deraadt 1415: .Nm
1.245 jmc 1416: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1417: See the
1.2 deraadt 1418: .Xr sshd 8
1.1 deraadt 1419: manual page for more information.
1.58 itojun 1420: .El
1.2 deraadt 1421: .Sh SEE ALSO
1422: .Xr scp 1 ,
1.83 djm 1423: .Xr sftp 1 ,
1.2 deraadt 1424: .Xr ssh-add 1 ,
1425: .Xr ssh-agent 1 ,
1426: .Xr ssh-keygen 1 ,
1.242 jmc 1427: .Xr ssh-keyscan 1 ,
1.250 jmc 1428: .Xr tun 4 ,
1.176 jmc 1429: .Xr hosts.equiv 5 ,
1.159 stevesk 1430: .Xr ssh_config 5 ,
1.160 naddy 1431: .Xr ssh-keysign 8 ,
1.87 itojun 1432: .Xr sshd 8
1.106 markus 1433: .Rs
1.256 jmc 1434: .%R RFC 4250
1435: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1436: .%D 2006
1437: .Re
1438: .Rs
1439: .%R RFC 4251
1440: .%T "The Secure Shell (SSH) Protocol Architecture"
1441: .%D 2006
1442: .Re
1443: .Rs
1444: .%R RFC 4252
1445: .%T "The Secure Shell (SSH) Authentication Protocol"
1446: .%D 2006
1447: .Re
1448: .Rs
1449: .%R RFC 4253
1450: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1451: .%D 2006
1452: .Re
1453: .Rs
1454: .%R RFC 4254
1455: .%T "The Secure Shell (SSH) Connection Protocol"
1456: .%D 2006
1457: .Re
1458: .Rs
1459: .%R RFC 4255
1460: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1461: .%D 2006
1462: .Re
1463: .Rs
1464: .%R RFC 4256
1465: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1466: .%D 2006
1467: .Re
1468: .Rs
1469: .%R RFC 4335
1470: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1471: .%D 2006
1472: .Re
1473: .Rs
1474: .%R RFC 4344
1475: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1476: .%D 2006
1477: .Re
1478: .Rs
1479: .%R RFC 4345
1480: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1481: .%D 2006
1482: .Re
1483: .Rs
1484: .%R RFC 4419
1485: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1486: .%D 2006
1487: .Re
1488: .Rs
1489: .%R RFC 4716
1490: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1491: .%D 2006
1.274 grunk 1492: .Re
1493: .Rs
1494: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1495: .%A A. Perrig
1496: .%A D. Song
1497: .%D 1999
1498: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1499: .Re
1.173 jmc 1500: .Sh AUTHORS
1501: OpenSSH is a derivative of the original and free
1502: ssh 1.2.12 release by Tatu Ylonen.
1503: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1504: Theo de Raadt and Dug Song
1505: removed many bugs, re-added newer features and
1506: created OpenSSH.
1507: Markus Friedl contributed the support for SSH
1508: protocol versions 1.5 and 2.0.
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