Annotation of src/usr.bin/ssh/ssh.1, Revision 1.307
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: .\"
1.307 ! dtucker 37: .\" $OpenBSD: ssh.1,v 1.306 2010/07/14 17:06:58 jmc Exp $
! 38: .Dd $Mdocdate: July 14 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
1.306 jmc 46: .Bk -words
1.278 djm 47: .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
1.108 markus 48: .Op Fl b Ar bind_address
1.51 markus 49: .Op Fl c Ar cipher_spec
1.306 jmc 50: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
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
1.306 jmc 61: .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.198 djm 62: .Op Fl S Ar ctl_path
1.290 dtucker 63: .Op Fl W Ar host : Ns Ar port
1.306 jmc 64: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.176 jmc 65: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 66: .Op Ar command
1.306 jmc 67: .Ek
1.44 aaron 68: .Sh DESCRIPTION
1.2 deraadt 69: .Nm
1.96 deraadt 70: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 71: executing commands on a remote machine.
1.176 jmc 72: It is intended to replace rlogin and rsh,
73: and provide secure encrypted communications between
1.40 aaron 74: two untrusted hosts over an insecure network.
1.247 jmc 75: X11 connections and arbitrary TCP ports
1.176 jmc 76: can also be forwarded over the secure channel.
1.2 deraadt 77: .Pp
78: .Nm
1.44 aaron 79: connects and logs into the specified
1.176 jmc 80: .Ar hostname
81: (with optional
82: .Ar user
83: name).
1.1 deraadt 84: The user must prove
1.49 markus 85: his/her identity to the remote machine using one of several methods
1.221 jmc 86: depending on the protocol version used (see below).
1.49 markus 87: .Pp
1.176 jmc 88: If
89: .Ar command
90: is specified,
1.219 jmc 91: it is executed on the remote host instead of a login shell.
1.2 deraadt 92: .Pp
1.218 jmc 93: The options are as follows:
94: .Bl -tag -width Ds
95: .It Fl 1
96: Forces
1.2 deraadt 97: .Nm
1.218 jmc 98: to try protocol version 1 only.
99: .It Fl 2
100: Forces
1.2 deraadt 101: .Nm
1.218 jmc 102: to try protocol version 2 only.
103: .It Fl 4
104: Forces
1.2 deraadt 105: .Nm
1.218 jmc 106: to use IPv4 addresses only.
107: .It Fl 6
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv6 addresses only.
111: .It Fl A
112: Enables forwarding of the authentication agent connection.
113: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 114: .Pp
1.218 jmc 115: Agent forwarding should be enabled with caution.
116: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 117: (for the agent's
1.286 sobrado 118: .Ux Ns -domain
119: socket) can access the local agent through the forwarded connection.
1.218 jmc 120: An attacker cannot obtain key material from the agent,
121: however they can perform operations on the keys that enable them to
122: authenticate using the identities loaded into the agent.
123: .It Fl a
124: Disables forwarding of the authentication agent connection.
125: .It Fl b Ar bind_address
126: Use
127: .Ar bind_address
128: on the local machine as the source address
129: of the connection.
130: Only useful on systems with more than one address.
131: .It Fl C
132: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 133: data for forwarded X11 and TCP connections).
1.218 jmc 134: The compression algorithm is the same used by
135: .Xr gzip 1 ,
136: and the
137: .Dq level
138: can be controlled by the
139: .Cm CompressionLevel
140: option for protocol version 1.
141: Compression is desirable on modem lines and other
142: slow connections, but will only slow down things on fast networks.
143: The default value can be set on a host-by-host basis in the
144: configuration files; see the
145: .Cm Compression
146: option.
147: .It Fl c Ar cipher_spec
148: Selects the cipher specification for encrypting the session.
1.2 deraadt 149: .Pp
1.218 jmc 150: Protocol version 1 allows specification of a single cipher.
151: The supported values are
152: .Dq 3des ,
1.220 jmc 153: .Dq blowfish ,
1.218 jmc 154: and
155: .Dq des .
156: .Ar 3des
157: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
158: It is believed to be secure.
159: .Ar blowfish
160: is a fast block cipher; it appears very secure and is much faster than
161: .Ar 3des .
162: .Ar des
163: is only supported in the
1.2 deraadt 164: .Nm
1.218 jmc 165: client for interoperability with legacy protocol 1 implementations
166: that do not support the
167: .Ar 3des
168: cipher.
169: Its use is strongly discouraged due to cryptographic weaknesses.
170: The default is
171: .Dq 3des .
1.49 markus 172: .Pp
1.230 jmc 173: For protocol version 2,
1.218 jmc 174: .Ar cipher_spec
175: is a comma-separated list of ciphers
176: listed in order of preference.
1.283 jmc 177: See the
178: .Cm Ciphers
1.307 ! dtucker 179: keyword in
! 180: .Xr ssh_config 5
! 181: for more information.
1.218 jmc 182: .It Fl D Xo
183: .Sm off
184: .Oo Ar bind_address : Oc
185: .Ar port
186: .Sm on
187: .Xc
188: Specifies a local
189: .Dq dynamic
190: application-level port forwarding.
191: This works by allocating a socket to listen to
192: .Ar port
193: on the local side, optionally bound to the specified
194: .Ar bind_address .
195: Whenever a connection is made to this port, the
196: connection is forwarded over the secure channel, and the application
197: protocol is then used to determine where to connect to from the
198: remote machine.
199: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 200: .Nm
1.218 jmc 201: will act as a SOCKS server.
202: Only root can forward privileged ports.
203: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 204: .Pp
1.218 jmc 205: IPv6 addresses can be specified with an alternative syntax:
206: .Sm off
207: .Xo
208: .Op Ar bind_address No /
209: .Ar port
210: .Xc
211: .Sm on
212: or by enclosing the address in square brackets.
213: Only the superuser can forward privileged ports.
214: By default, the local port is bound in accordance with the
215: .Cm GatewayPorts
216: setting.
217: However, an explicit
218: .Ar bind_address
219: may be used to bind the connection to a specific address.
220: The
221: .Ar bind_address
222: of
223: .Dq localhost
224: indicates that the listening port be bound for local use only, while an
225: empty address or
226: .Sq *
227: indicates that the port should be available from all interfaces.
1.229 jmc 228: .It Fl e Ar escape_char
1.218 jmc 229: Sets the escape character for sessions with a pty (default:
230: .Ql ~ ) .
231: The escape character is only recognized at the beginning of a line.
232: The escape character followed by a dot
233: .Pq Ql \&.
234: closes the connection;
235: followed by control-Z suspends the connection;
236: and followed by itself sends the escape character once.
237: Setting the character to
1.2 deraadt 238: .Dq none
1.218 jmc 239: disables any escapes and makes the session fully transparent.
240: .It Fl F Ar configfile
241: Specifies an alternative per-user configuration file.
242: If a configuration file is given on the command line,
243: the system-wide configuration file
244: .Pq Pa /etc/ssh/ssh_config
245: will be ignored.
246: The default for the per-user configuration file is
247: .Pa ~/.ssh/config .
248: .It Fl f
249: Requests
250: .Nm
251: to go to background just before command execution.
252: This is useful if
1.176 jmc 253: .Nm
1.218 jmc 254: is going to ask for passwords or passphrases, but the user
255: wants it in the background.
256: This implies
257: .Fl n .
258: The recommended way to start X11 programs at a remote site is with
259: something like
260: .Ic ssh -f host xterm .
1.277 djm 261: .Pp
262: If the
263: .Cm ExitOnForwardFailure
264: configuration option is set to
265: .Dq yes ,
266: then a client started with
267: .Fl f
268: will wait for all remote port forwards to be successfully established
269: before placing itself in the background.
1.218 jmc 270: .It Fl g
271: Allows remote hosts to connect to local forwarded ports.
1.291 markus 272: .It Fl I Ar pkcs11
1.294 jmc 273: Specify the PKCS#11 shared library
1.176 jmc 274: .Nm
1.293 markus 275: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 276: private RSA key.
277: .It Fl i Ar identity_file
278: Selects a file from which the identity (private key) for
279: RSA or DSA authentication is read.
280: The default is
281: .Pa ~/.ssh/identity
282: for protocol version 1, and
283: .Pa ~/.ssh/id_rsa
1.149 jakob 284: and
1.218 jmc 285: .Pa ~/.ssh/id_dsa
286: for protocol version 2.
287: Identity files may also be specified on
288: a per-host basis in the configuration file.
289: It is possible to have multiple
290: .Fl i
291: options (and multiple identities specified in
292: configuration files).
1.302 djm 293: .Nm
294: will also try to load certificate information from the filename obtained
295: by appending
296: .Pa -cert.pub
297: to identity filenames.
1.269 djm 298: .It Fl K
299: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
300: credentials to the server.
1.218 jmc 301: .It Fl k
302: Disables forwarding (delegation) of GSSAPI credentials to the server.
303: .It Fl L Xo
304: .Sm off
305: .Oo Ar bind_address : Oc
306: .Ar port : host : hostport
307: .Sm on
308: .Xc
309: Specifies that the given port on the local (client) host is to be
310: forwarded to the given host and port on the remote side.
311: This works by allocating a socket to listen to
312: .Ar port
313: on the local side, optionally bound to the specified
314: .Ar bind_address .
315: Whenever a connection is made to this port, the
316: connection is forwarded over the secure channel, and a connection is
317: made to
318: .Ar host
319: port
320: .Ar hostport
321: from the remote machine.
322: Port forwardings can also be specified in the configuration file.
323: IPv6 addresses can be specified with an alternative syntax:
324: .Sm off
325: .Xo
326: .Op Ar bind_address No /
327: .Ar port No / Ar host No /
328: .Ar hostport
329: .Xc
330: .Sm on
331: or by enclosing the address in square brackets.
332: Only the superuser can forward privileged ports.
333: By default, the local port is bound in accordance with the
334: .Cm GatewayPorts
335: setting.
336: However, an explicit
337: .Ar bind_address
338: may be used to bind the connection to a specific address.
1.2 deraadt 339: The
1.218 jmc 340: .Ar bind_address
341: of
342: .Dq localhost
343: indicates that the listening port be bound for local use only, while an
344: empty address or
345: .Sq *
346: indicates that the port should be available from all interfaces.
347: .It Fl l Ar login_name
348: Specifies the user to log in as on the remote machine.
349: This also may be specified on a per-host basis in the configuration file.
350: .It Fl M
351: Places the
352: .Nm
353: client into
354: .Dq master
355: mode for connection sharing.
1.231 stevesk 356: Multiple
357: .Fl M
358: options places
359: .Nm
360: into
361: .Dq master
362: mode with confirmation required before slave connections are accepted.
1.218 jmc 363: Refer to the description of
364: .Cm ControlMaster
365: in
366: .Xr ssh_config 5
367: for details.
368: .It Fl m Ar mac_spec
369: Additionally, for protocol version 2 a comma-separated list of MAC
370: (message authentication code) algorithms can
371: be specified in order of preference.
372: See the
373: .Cm MACs
374: keyword for more information.
375: .It Fl N
376: Do not execute a remote command.
377: This is useful for just forwarding ports
378: (protocol version 2 only).
379: .It Fl n
380: Redirects stdin from
381: .Pa /dev/null
382: (actually, prevents reading from stdin).
383: This must be used when
1.2 deraadt 384: .Nm
1.218 jmc 385: is run in the background.
386: A common trick is to use this to run X11 programs on a remote machine.
387: For example,
388: .Ic ssh -n shadows.cs.hut.fi emacs &
389: will start an emacs on shadows.cs.hut.fi, and the X11
390: connection will be automatically forwarded over an encrypted channel.
391: The
1.2 deraadt 392: .Nm
1.218 jmc 393: program will be put in the background.
394: (This does not work if
1.2 deraadt 395: .Nm
1.218 jmc 396: needs to ask for a password or passphrase; see also the
397: .Fl f
398: option.)
399: .It Fl O Ar ctl_cmd
400: Control an active connection multiplexing master process.
401: When the
402: .Fl O
403: option is specified, the
404: .Ar ctl_cmd
405: argument is interpreted and passed to the master process.
406: Valid commands are:
407: .Dq check
1.305 markus 408: (check that the master process is running),
409: .Dq forward
410: (request forwardings without command execution) and
1.218 jmc 411: .Dq exit
412: (request the master to exit).
413: .It Fl o Ar option
414: Can be used to give options in the format used in the configuration file.
415: This is useful for specifying options for which there is no separate
416: command-line flag.
417: For full details of the options listed below, and their possible values, see
418: .Xr ssh_config 5 .
1.2 deraadt 419: .Pp
1.218 jmc 420: .Bl -tag -width Ds -offset indent -compact
421: .It AddressFamily
422: .It BatchMode
423: .It BindAddress
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
435: .It DynamicForward
436: .It EscapeChar
1.263 markus 437: .It ExitOnForwardFailure
1.218 jmc 438: .It ForwardAgent
439: .It ForwardX11
440: .It ForwardX11Trusted
441: .It GatewayPorts
442: .It GlobalKnownHostsFile
443: .It GSSAPIAuthentication
444: .It GSSAPIDelegateCredentials
445: .It HashKnownHosts
446: .It Host
447: .It HostbasedAuthentication
448: .It HostKeyAlgorithms
449: .It HostKeyAlias
450: .It HostName
451: .It IdentityFile
452: .It IdentitiesOnly
453: .It KbdInteractiveDevices
454: .It LocalCommand
455: .It LocalForward
456: .It LogLevel
457: .It MACs
458: .It NoHostAuthenticationForLocalhost
459: .It NumberOfPasswordPrompts
460: .It PasswordAuthentication
461: .It PermitLocalCommand
1.291 markus 462: .It PKCS11Provider
1.218 jmc 463: .It Port
464: .It PreferredAuthentications
465: .It Protocol
466: .It ProxyCommand
467: .It PubkeyAuthentication
1.251 dtucker 468: .It RekeyLimit
1.218 jmc 469: .It RemoteForward
470: .It RhostsRSAAuthentication
471: .It RSAAuthentication
472: .It SendEnv
473: .It ServerAliveInterval
474: .It ServerAliveCountMax
475: .It StrictHostKeyChecking
476: .It TCPKeepAlive
477: .It Tunnel
478: .It TunnelDevice
479: .It UsePrivilegedPort
480: .It User
481: .It UserKnownHostsFile
482: .It VerifyHostKeyDNS
1.276 jmc 483: .It VisualHostKey
1.218 jmc 484: .It XAuthLocation
485: .El
486: .It Fl p Ar port
487: Port to connect to on the remote host.
488: This can be specified on a
489: per-host basis in the configuration file.
490: .It Fl q
491: Quiet mode.
1.271 djm 492: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 493: .It Fl R Xo
494: .Sm off
495: .Oo Ar bind_address : Oc
496: .Ar port : host : hostport
497: .Sm on
498: .Xc
499: Specifies that the given port on the remote (server) host is to be
500: forwarded to the given host and port on the local side.
501: This works by allocating a socket to listen to
502: .Ar port
503: on the remote side, and whenever a connection is made to this port, the
504: connection is forwarded over the secure channel, and a connection is
505: made to
506: .Ar host
507: port
508: .Ar hostport
509: from the local machine.
1.2 deraadt 510: .Pp
1.218 jmc 511: Port forwardings can also be specified in the configuration file.
512: Privileged ports can be forwarded only when
513: logging in as root on the remote machine.
514: IPv6 addresses can be specified by enclosing the address in square braces or
515: using an alternative syntax:
516: .Sm off
517: .Xo
518: .Op Ar bind_address No /
519: .Ar host No / Ar port No /
520: .Ar hostport
521: .Xc .
522: .Sm on
1.194 jakob 523: .Pp
1.218 jmc 524: By default, the listening socket on the server will be bound to the loopback
525: interface only.
1.280 tobias 526: This may be overridden by specifying a
1.218 jmc 527: .Ar bind_address .
528: An empty
529: .Ar bind_address ,
530: or the address
531: .Ql * ,
532: indicates that the remote socket should listen on all interfaces.
533: Specifying a remote
534: .Ar bind_address
535: will only succeed if the server's
536: .Cm GatewayPorts
537: option is enabled (see
538: .Xr sshd_config 5 ) .
1.281 djm 539: .Pp
540: If the
541: .Ar port
542: argument is
1.282 djm 543: .Ql 0 ,
1.281 djm 544: the listen port will be dynamically allocated on the server and reported
545: to the client at run time.
1.305 markus 546: When used together with
547: .Ic -O forward
548: the allocated port will be printed to the standard output.
1.218 jmc 549: .It Fl S Ar ctl_path
1.304 jmc 550: Specifies the location of a control socket for connection sharing,
1.303 djm 551: or the string
552: .Dq none
553: to disable connection sharing.
1.218 jmc 554: Refer to the description of
555: .Cm ControlPath
556: and
557: .Cm ControlMaster
558: in
559: .Xr ssh_config 5
560: for details.
561: .It Fl s
562: May be used to request invocation of a subsystem on the remote system.
563: Subsystems are a feature of the SSH2 protocol which facilitate the use
564: of SSH as a secure transport for other applications (eg.\&
565: .Xr sftp 1 ) .
566: The subsystem is specified as the remote command.
567: .It Fl T
568: Disable pseudo-tty allocation.
569: .It Fl t
570: Force pseudo-tty allocation.
571: This can be used to execute arbitrary
572: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 573: e.g. when implementing menu services.
1.218 jmc 574: Multiple
575: .Fl t
576: options force tty allocation, even if
1.194 jakob 577: .Nm
1.218 jmc 578: has no local tty.
579: .It Fl V
580: Display the version number and exit.
581: .It Fl v
582: Verbose mode.
583: Causes
1.176 jmc 584: .Nm
1.218 jmc 585: to print debugging messages about its progress.
586: This is helpful in
587: debugging connection, authentication, and configuration problems.
588: Multiple
589: .Fl v
590: options increase the verbosity.
591: The maximum is 3.
1.290 dtucker 592: .It Fl W Ar host : Ns Ar port
593: Requests that standard input and output on the client be forwarded to
594: .Ar host
595: on
596: .Ar port
597: over the secure channel.
598: Implies
599: .Fl N ,
600: .Fl T ,
601: .Cm ExitOnForwardFailure
602: and
603: .Cm ClearAllForwardings
604: and works with Protocol version 2 only.
1.261 stevesk 605: .It Fl w Xo
606: .Ar local_tun Ns Op : Ns Ar remote_tun
607: .Xc
608: Requests
609: tunnel
610: device forwarding with the specified
1.218 jmc 611: .Xr tun 4
1.261 stevesk 612: devices between the client
613: .Pq Ar local_tun
614: and the server
615: .Pq Ar remote_tun .
616: .Pp
1.228 jmc 617: The devices may be specified by numerical ID or the keyword
618: .Dq any ,
619: which uses the next available tunnel device.
1.261 stevesk 620: If
621: .Ar remote_tun
622: is not specified, it defaults to
623: .Dq any .
1.228 jmc 624: See also the
1.218 jmc 625: .Cm Tunnel
1.261 stevesk 626: and
627: .Cm TunnelDevice
628: directives in
1.218 jmc 629: .Xr ssh_config 5 .
1.261 stevesk 630: If the
631: .Cm Tunnel
632: directive is unset, it is set to the default tunnel mode, which is
633: .Dq point-to-point .
1.218 jmc 634: .It Fl X
635: Enables X11 forwarding.
1.54 markus 636: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 637: .Pp
1.218 jmc 638: X11 forwarding should be enabled with caution.
1.168 jmc 639: Users with the ability to bypass file permissions on the remote host
1.218 jmc 640: (for the user's X authorization database)
641: can access the local X11 display through the forwarded connection.
642: An attacker may then be able to perform activities such as keystroke monitoring.
643: .Pp
644: For this reason, X11 forwarding is subjected to X11 SECURITY extension
645: restrictions by default.
646: Please refer to the
647: .Nm
648: .Fl Y
649: option and the
650: .Cm ForwardX11Trusted
651: directive in
652: .Xr ssh_config 5
653: for more information.
654: .It Fl x
655: Disables X11 forwarding.
656: .It Fl Y
657: Enables trusted X11 forwarding.
658: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
659: controls.
1.278 djm 660: .It Fl y
661: Send log information using the
662: .Xr syslog 3
663: system module.
664: By default this information is sent to stderr.
1.218 jmc 665: .El
1.224 jmc 666: .Pp
667: .Nm
668: may additionally obtain configuration data from
669: a per-user configuration file and a system-wide configuration file.
670: The file format and configuration options are described in
671: .Xr ssh_config 5 .
672: .Pp
673: .Nm
674: exits with the exit status of the remote command or with 255
675: if an error occurred.
1.222 jmc 676: .Sh AUTHENTICATION
1.249 jmc 677: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 678: The default is to use protocol 2 only,
679: though this can be changed via the
1.222 jmc 680: .Cm Protocol
681: option in
1.284 jmc 682: .Xr ssh_config 5
683: or the
1.222 jmc 684: .Fl 1
685: and
686: .Fl 2
687: options (see above).
688: Both protocols support similar authentication methods,
1.284 jmc 689: but protocol 2 is the default since
1.222 jmc 690: it provides additional mechanisms for confidentiality
691: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 692: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 693: Protocol 1 lacks a strong mechanism for ensuring the
694: integrity of the connection.
695: .Pp
696: The methods available for authentication are:
1.260 jmc 697: GSSAPI-based authentication,
1.222 jmc 698: host-based authentication,
699: public key authentication,
700: challenge-response authentication,
701: and password authentication.
702: Authentication methods are tried in the order specified above,
703: though protocol 2 has a configuration option to change the default order:
704: .Cm PreferredAuthentications .
705: .Pp
706: Host-based authentication works as follows:
1.218 jmc 707: If the machine the user logs in from is listed in
708: .Pa /etc/hosts.equiv
709: or
710: .Pa /etc/shosts.equiv
711: on the remote machine, and the user names are
712: the same on both sides, or if the files
713: .Pa ~/.rhosts
714: or
715: .Pa ~/.shosts
716: exist in the user's home directory on the
717: remote machine and contain a line containing the name of the client
718: machine and the name of the user on that machine, the user is
1.222 jmc 719: considered for login.
720: Additionally, the server
721: .Em must
722: be able to verify the client's
723: host key (see the description of
1.218 jmc 724: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 725: and
1.222 jmc 726: .Pa ~/.ssh/known_hosts ,
727: below)
728: for login to be permitted.
1.218 jmc 729: This authentication method closes security holes due to IP
1.222 jmc 730: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 731: [Note to the administrator:
732: .Pa /etc/hosts.equiv ,
733: .Pa ~/.rhosts ,
734: and the rlogin/rsh protocol in general, are inherently insecure and should be
735: disabled if security is desired.]
1.189 dtucker 736: .Pp
1.222 jmc 737: Public key authentication works as follows:
738: The scheme is based on public-key cryptography,
739: using cryptosystems
740: where encryption and decryption are done using separate keys,
741: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 742: The idea is that each user creates a public/private
743: key pair for authentication purposes.
744: The server knows the public key, and only the user knows the private key.
1.222 jmc 745: .Nm
746: implements public key authentication protocol automatically,
747: using either the RSA or DSA algorithms.
748: Protocol 1 is restricted to using only RSA keys,
749: but protocol 2 may use either.
750: The
751: .Sx HISTORY
752: section of
753: .Xr ssl 8
754: contains a brief discussion of the two algorithms.
1.210 djm 755: .Pp
1.218 jmc 756: The file
757: .Pa ~/.ssh/authorized_keys
758: lists the public keys that are permitted for logging in.
759: When the user logs in, the
1.2 deraadt 760: .Nm
1.218 jmc 761: program tells the server which key pair it would like to use for
762: authentication.
1.222 jmc 763: The client proves that it has access to the private key
764: and the server checks that the corresponding public key
765: is authorized to accept the account.
1.218 jmc 766: .Pp
1.222 jmc 767: The user creates his/her key pair by running
1.218 jmc 768: .Xr ssh-keygen 1 .
769: This stores the private key in
1.207 djm 770: .Pa ~/.ssh/identity
1.222 jmc 771: (protocol 1),
772: .Pa ~/.ssh/id_dsa
773: (protocol 2 DSA),
774: or
775: .Pa ~/.ssh/id_rsa
776: (protocol 2 RSA)
1.218 jmc 777: and stores the public key in
778: .Pa ~/.ssh/identity.pub
1.222 jmc 779: (protocol 1),
780: .Pa ~/.ssh/id_dsa.pub
781: (protocol 2 DSA),
782: or
783: .Pa ~/.ssh/id_rsa.pub
784: (protocol 2 RSA)
1.218 jmc 785: in the user's home directory.
1.222 jmc 786: The user should then copy the public key
1.218 jmc 787: to
788: .Pa ~/.ssh/authorized_keys
1.222 jmc 789: in his/her home directory on the remote machine.
790: The
1.218 jmc 791: .Pa authorized_keys
792: file corresponds to the conventional
793: .Pa ~/.rhosts
794: file, and has one key
1.222 jmc 795: per line, though the lines can be very long.
1.218 jmc 796: After this, the user can log in without giving the password.
797: .Pp
1.301 jmc 798: A variation on public key authentication
799: is available in the form of certificate authentication:
800: instead of a set of public/private keys,
801: signed certificates are used.
802: This has the advantage that a single trusted certification authority
803: can be used in place of many public/private keys.
804: See the
805: .Sx CERTIFICATES
806: section of
807: .Xr ssh-keygen 1
808: for more information.
809: .Pp
810: The most convenient way to use public key or certificate authentication
811: may be with an authentication agent.
1.218 jmc 812: See
813: .Xr ssh-agent 1
814: for more information.
815: .Pp
1.222 jmc 816: Challenge-response authentication works as follows:
817: The server sends an arbitrary
818: .Qq challenge
819: text, and prompts for a response.
820: Protocol 2 allows multiple challenges and responses;
821: protocol 1 is restricted to just one challenge/response.
822: Examples of challenge-response authentication include
823: BSD Authentication (see
824: .Xr login.conf 5 )
825: and PAM (some non-OpenBSD systems).
826: .Pp
827: Finally, if other authentication methods fail,
1.218 jmc 828: .Nm
829: prompts the user for a password.
830: The password is sent to the remote
831: host for checking; however, since all communications are encrypted,
832: the password cannot be seen by someone listening on the network.
1.232 jmc 833: .Pp
834: .Nm
835: automatically maintains and checks a database containing
836: identification for all hosts it has ever been used with.
837: Host keys are stored in
838: .Pa ~/.ssh/known_hosts
839: in the user's home directory.
840: Additionally, the file
841: .Pa /etc/ssh/ssh_known_hosts
842: is automatically checked for known hosts.
843: Any new hosts are automatically added to the user's file.
844: If a host's identification ever changes,
845: .Nm
846: warns about this and disables password authentication to prevent
847: server spoofing or man-in-the-middle attacks,
848: which could otherwise be used to circumvent the encryption.
849: The
850: .Cm StrictHostKeyChecking
851: option can be used to control logins to machines whose
852: host key is not known or has changed.
853: .Pp
1.218 jmc 854: When the user's identity has been accepted by the server, the server
855: either executes the given command, or logs into the machine and gives
856: the user a normal shell on the remote machine.
857: All communication with
858: the remote command or shell will be automatically encrypted.
859: .Pp
860: If a pseudo-terminal has been allocated (normal login session), the
861: user may use the escape characters noted below.
862: .Pp
863: If no pseudo-tty has been allocated,
864: the session is transparent and can be used to reliably transfer binary data.
865: On most systems, setting the escape character to
866: .Dq none
867: will also make the session transparent even if a tty is used.
868: .Pp
869: The session terminates when the command or shell on the remote
1.247 jmc 870: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 871: .Sh ESCAPE CHARACTERS
1.218 jmc 872: When a pseudo-terminal has been requested,
1.2 deraadt 873: .Nm
1.218 jmc 874: supports a number of functions through the use of an escape character.
875: .Pp
876: A single tilde character can be sent as
877: .Ic ~~
878: or by following the tilde by a character other than those described below.
879: The escape character must always follow a newline to be interpreted as
880: special.
881: The escape character can be changed in configuration files using the
882: .Cm EscapeChar
883: configuration directive or on the command line by the
884: .Fl e
885: option.
886: .Pp
887: The supported escapes (assuming the default
888: .Ql ~ )
889: are:
890: .Bl -tag -width Ds
891: .It Cm ~.
892: Disconnect.
893: .It Cm ~^Z
894: Background
1.234 jmc 895: .Nm .
1.218 jmc 896: .It Cm ~#
897: List forwarded connections.
898: .It Cm ~&
899: Background
1.2 deraadt 900: .Nm
1.218 jmc 901: at logout when waiting for forwarded connection / X11 sessions to terminate.
902: .It Cm ~?
903: Display a list of escape characters.
904: .It Cm ~B
905: Send a BREAK to the remote system
906: (only useful for SSH protocol version 2 and if the peer supports it).
907: .It Cm ~C
908: Open command line.
909: Currently this allows the addition of port forwardings using the
1.279 stevesk 910: .Fl L ,
911: .Fl R
1.218 jmc 912: and
1.279 stevesk 913: .Fl D
1.225 jmc 914: options (see above).
1.218 jmc 915: It also allows the cancellation of existing remote port-forwardings
916: using
1.262 stevesk 917: .Sm off
918: .Fl KR Oo Ar bind_address : Oc Ar port .
919: .Sm on
1.218 jmc 920: .Ic !\& Ns Ar command
921: allows the user to execute a local command if the
922: .Ic PermitLocalCommand
923: option is enabled in
1.176 jmc 924: .Xr ssh_config 5 .
1.218 jmc 925: Basic help is available, using the
926: .Fl h
927: option.
928: .It Cm ~R
929: Request rekeying of the connection
930: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 931: .El
1.246 jmc 932: .Sh TCP FORWARDING
933: Forwarding of arbitrary TCP connections over the secure channel can
934: be specified either on the command line or in a configuration file.
935: One possible application of TCP forwarding is a secure connection to a
936: mail server; another is going through firewalls.
937: .Pp
938: In the example below, we look at encrypting communication between
939: an IRC client and server, even though the IRC server does not directly
940: support encrypted communications.
941: This works as follows:
942: the user connects to the remote host using
943: .Nm ,
944: specifying a port to be used to forward connections
945: to the remote server.
946: After that it is possible to start the service which is to be encrypted
947: on the client machine,
948: connecting to the same local port,
949: and
950: .Nm
951: will encrypt and forward the connection.
952: .Pp
953: The following example tunnels an IRC session from client machine
954: .Dq 127.0.0.1
955: (localhost)
956: to remote server
957: .Dq server.example.com :
958: .Bd -literal -offset 4n
959: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
960: $ irc -c '#users' -p 1234 pinky 127.0.0.1
961: .Ed
962: .Pp
963: This tunnels a connection to IRC server
964: .Dq server.example.com ,
965: joining channel
966: .Dq #users ,
967: nickname
968: .Dq pinky ,
969: using port 1234.
970: It doesn't matter which port is used,
971: as long as it's greater than 1023
972: (remember, only root can open sockets on privileged ports)
973: and doesn't conflict with any ports already in use.
974: The connection is forwarded to port 6667 on the remote server,
975: since that's the standard port for IRC services.
976: .Pp
977: The
978: .Fl f
979: option backgrounds
980: .Nm
981: and the remote command
982: .Dq sleep 10
983: is specified to allow an amount of time
984: (10 seconds, in the example)
985: to start the service which is to be tunnelled.
986: If no connections are made within the time specified,
987: .Nm
988: will exit.
989: .Sh X11 FORWARDING
1.218 jmc 990: If the
991: .Cm ForwardX11
992: variable is set to
993: .Dq yes
994: (or see the description of the
1.227 jmc 995: .Fl X ,
996: .Fl x ,
1.218 jmc 997: and
1.227 jmc 998: .Fl Y
1.226 jmc 999: options above)
1.218 jmc 1000: and the user is using X11 (the
1001: .Ev DISPLAY
1002: environment variable is set), the connection to the X11 display is
1003: automatically forwarded to the remote side in such a way that any X11
1004: programs started from the shell (or command) will go through the
1005: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1006: from the local machine.
1.218 jmc 1007: The user should not manually set
1008: .Ev DISPLAY .
1009: Forwarding of X11 connections can be
1010: configured on the command line or in configuration files.
1011: .Pp
1012: The
1013: .Ev DISPLAY
1014: value set by
1015: .Nm
1016: will point to the server machine, but with a display number greater than zero.
1017: This is normal, and happens because
1018: .Nm
1019: creates a
1020: .Dq proxy
1021: X server on the server machine for forwarding the
1022: connections over the encrypted channel.
1.200 djm 1023: .Pp
1.218 jmc 1024: .Nm
1025: will also automatically set up Xauthority data on the server machine.
1026: For this purpose, it will generate a random authorization cookie,
1027: store it in Xauthority on the server, and verify that any forwarded
1028: connections carry this cookie and replace it by the real cookie when
1029: the connection is opened.
1030: The real authentication cookie is never
1031: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1032: .Pp
1.218 jmc 1033: If the
1034: .Cm ForwardAgent
1035: variable is set to
1036: .Dq yes
1037: (or see the description of the
1038: .Fl A
1.191 djm 1039: and
1.218 jmc 1040: .Fl a
1.226 jmc 1041: options above) and
1.218 jmc 1042: the user is using an authentication agent, the connection to the agent
1043: is automatically forwarded to the remote side.
1.252 jmc 1044: .Sh VERIFYING HOST KEYS
1045: When connecting to a server for the first time,
1046: a fingerprint of the server's public key is presented to the user
1047: (unless the option
1048: .Cm StrictHostKeyChecking
1049: has been disabled).
1050: Fingerprints can be determined using
1051: .Xr ssh-keygen 1 :
1052: .Pp
1053: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1054: .Pp
1.274 grunk 1055: If the fingerprint is already known, it can be matched
1056: and the key can be accepted or rejected.
1057: Because of the difficulty of comparing host keys
1058: just by looking at hex strings,
1059: there is also support to compare host keys visually,
1060: using
1061: .Em random art .
1062: By setting the
1.275 grunk 1063: .Cm VisualHostKey
1.274 grunk 1064: option to
1.275 grunk 1065: .Dq yes ,
1.274 grunk 1066: a small ASCII graphic gets displayed on every login to a server, no matter
1067: if the session itself is interactive or not.
1068: By learning the pattern a known server produces, a user can easily
1069: find out that the host key has changed when a completely different pattern
1070: is displayed.
1071: Because these patterns are not unambiguous however, a pattern that looks
1072: similar to the pattern remembered only gives a good probability that the
1073: host key is the same, not guaranteed proof.
1074: .Pp
1075: To get a listing of the fingerprints along with their random art for
1076: all known hosts, the following command line can be used:
1077: .Pp
1078: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1079: .Pp
1.252 jmc 1080: If the fingerprint is unknown,
1081: an alternative method of verification is available:
1082: SSH fingerprints verified by DNS.
1083: An additional resource record (RR),
1084: SSHFP,
1085: is added to a zonefile
1086: and the connecting client is able to match the fingerprint
1087: with that of the key presented.
1088: .Pp
1089: In this example, we are connecting a client to a server,
1090: .Dq host.example.com .
1091: The SSHFP resource records should first be added to the zonefile for
1092: host.example.com:
1093: .Bd -literal -offset indent
1.259 jakob 1094: $ ssh-keygen -r host.example.com.
1.252 jmc 1095: .Ed
1096: .Pp
1097: The output lines will have to be added to the zonefile.
1098: To check that the zone is answering fingerprint queries:
1099: .Pp
1100: .Dl $ dig -t SSHFP host.example.com
1101: .Pp
1102: Finally the client connects:
1103: .Bd -literal -offset indent
1104: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1105: [...]
1106: Matching host key fingerprint found in DNS.
1107: Are you sure you want to continue connecting (yes/no)?
1108: .Ed
1109: .Pp
1110: See the
1111: .Cm VerifyHostKeyDNS
1112: option in
1113: .Xr ssh_config 5
1114: for more information.
1.250 jmc 1115: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1116: .Nm
1117: contains support for Virtual Private Network (VPN) tunnelling
1118: using the
1119: .Xr tun 4
1120: network pseudo-device,
1121: allowing two networks to be joined securely.
1122: The
1123: .Xr sshd_config 5
1124: configuration option
1125: .Cm PermitTunnel
1126: controls whether the server supports this,
1127: and at what level (layer 2 or 3 traffic).
1128: .Pp
1129: The following example would connect client network 10.0.50.0/24
1.265 otto 1130: with remote network 10.0.99.0/24 using a point-to-point connection
1131: from 10.1.1.1 to 10.1.1.2,
1132: provided that the SSH server running on the gateway to the remote network,
1133: at 192.168.1.15, allows it.
1134: .Pp
1135: On the client:
1.250 jmc 1136: .Bd -literal -offset indent
1137: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1138: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1139: # route add 10.0.99.0/24 10.1.1.2
1140: .Ed
1141: .Pp
1142: On the server:
1143: .Bd -literal -offset indent
1144: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1145: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1146: .Ed
1147: .Pp
1148: Client access may be more finely tuned via the
1149: .Pa /root/.ssh/authorized_keys
1150: file (see below) and the
1151: .Cm PermitRootLogin
1152: server option.
1.255 jmc 1153: The following entry would permit connections on
1.250 jmc 1154: .Xr tun 4
1.255 jmc 1155: device 1 from user
1.250 jmc 1156: .Dq jane
1.255 jmc 1157: and on tun device 2 from user
1.250 jmc 1158: .Dq john ,
1159: if
1160: .Cm PermitRootLogin
1161: is set to
1162: .Dq forced-commands-only :
1163: .Bd -literal -offset 2n
1164: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1165: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1166: .Ed
1167: .Pp
1.264 ray 1168: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1169: it may be more suited to temporary setups,
1170: such as for wireless VPNs.
1171: More permanent VPNs are better provided by tools such as
1172: .Xr ipsecctl 8
1173: and
1174: .Xr isakmpd 8 .
1.2 deraadt 1175: .Sh ENVIRONMENT
1176: .Nm
1.1 deraadt 1177: will normally set the following environment variables:
1.237 jmc 1178: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1179: .It Ev DISPLAY
1180: The
1181: .Ev DISPLAY
1.40 aaron 1182: variable indicates the location of the X11 server.
1.44 aaron 1183: It is automatically set by
1.2 deraadt 1184: .Nm
1185: to point to a value of the form
1.233 jmc 1186: .Dq hostname:n ,
1187: where
1188: .Dq hostname
1189: indicates the host where the shell runs, and
1190: .Sq n
1191: is an integer \*(Ge 1.
1.40 aaron 1192: .Nm
1193: uses this special value to forward X11 connections over the secure
1194: channel.
1.107 markus 1195: The user should normally not set
1196: .Ev DISPLAY
1197: explicitly, as that
1.1 deraadt 1198: will render the X11 connection insecure (and will require the user to
1199: manually copy any required authorization cookies).
1.2 deraadt 1200: .It Ev HOME
1.1 deraadt 1201: Set to the path of the user's home directory.
1.2 deraadt 1202: .It Ev LOGNAME
1203: Synonym for
1.12 aaron 1204: .Ev USER ;
1205: set for compatibility with systems that use this variable.
1.2 deraadt 1206: .It Ev MAIL
1.129 stevesk 1207: Set to the path of the user's mailbox.
1.40 aaron 1208: .It Ev PATH
1.2 deraadt 1209: Set to the default
1210: .Ev PATH ,
1211: as specified when compiling
1.234 jmc 1212: .Nm .
1.118 markus 1213: .It Ev SSH_ASKPASS
1214: If
1215: .Nm
1216: needs a passphrase, it will read the passphrase from the current
1217: terminal if it was run from a terminal.
1218: If
1219: .Nm
1220: does not have a terminal associated with it but
1221: .Ev DISPLAY
1222: and
1223: .Ev SSH_ASKPASS
1224: are set, it will execute the program specified by
1225: .Ev SSH_ASKPASS
1226: and open an X11 window to read the passphrase.
1227: This is particularly useful when calling
1228: .Nm
1229: from a
1.196 jmc 1230: .Pa .xsession
1.118 markus 1231: or related script.
1232: (Note that on some machines it
1233: may be necessary to redirect the input from
1234: .Pa /dev/null
1235: to make this work.)
1.18 markus 1236: .It Ev SSH_AUTH_SOCK
1.233 jmc 1237: Identifies the path of a
1238: .Ux Ns -domain
1239: socket used to communicate with the agent.
1.166 stevesk 1240: .It Ev SSH_CONNECTION
1241: Identifies the client and server ends of the connection.
1.40 aaron 1242: The variable contains
1.233 jmc 1243: four space-separated values: client IP address, client port number,
1244: server IP address, and server port number.
1.73 markus 1245: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1246: This variable contains the original command line if a forced command
1.73 markus 1247: is executed.
1248: It can be used to extract the original arguments.
1.2 deraadt 1249: .It Ev SSH_TTY
1.1 deraadt 1250: This is set to the name of the tty (path to the device) associated
1.40 aaron 1251: with the current shell or command.
1252: If the current session has no tty,
1.1 deraadt 1253: this variable is not set.
1.2 deraadt 1254: .It Ev TZ
1.214 jmc 1255: This variable is set to indicate the present time zone if it
1.257 jmc 1256: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1257: on to new connections).
1.2 deraadt 1258: .It Ev USER
1.1 deraadt 1259: Set to the name of the user logging in.
1.2 deraadt 1260: .El
1261: .Pp
1.44 aaron 1262: Additionally,
1.2 deraadt 1263: .Nm
1.44 aaron 1264: reads
1.207 djm 1265: .Pa ~/.ssh/environment ,
1.2 deraadt 1266: and adds lines of the format
1267: .Dq VARNAME=value
1.233 jmc 1268: to the environment if the file exists and users are allowed to
1.161 marc 1269: change their environment.
1.176 jmc 1270: For more information, see the
1.161 marc 1271: .Cm PermitUserEnvironment
1.162 stevesk 1272: option in
1.161 marc 1273: .Xr sshd_config 5 .
1.2 deraadt 1274: .Sh FILES
1.236 jmc 1275: .Bl -tag -width Ds -compact
1276: .It ~/.rhosts
1.240 jmc 1277: This file is used for host-based authentication (see above).
1.92 markus 1278: On some machines this file may need to be
1.240 jmc 1279: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1280: because
1.2 deraadt 1281: .Xr sshd 8
1.40 aaron 1282: reads it as root.
1283: Additionally, this file must be owned by the user,
1284: and must not have write permissions for anyone else.
1285: The recommended
1.1 deraadt 1286: permission for most machines is read/write for the user, and not
1287: accessible by others.
1.2 deraadt 1288: .Pp
1.236 jmc 1289: .It ~/.shosts
1.240 jmc 1290: This file is used in exactly the same way as
1291: .Pa .rhosts ,
1292: but allows host-based authentication without permitting login with
1293: rlogin/rsh.
1.272 mcbride 1294: .Pp
1295: .It ~/.ssh/
1296: This directory is the default location for all user-specific configuration
1297: and authentication information.
1298: There is no general requirement to keep the entire contents of this directory
1299: secret, but the recommended permissions are read/write/execute for the user,
1300: and not accessible by others.
1.236 jmc 1301: .Pp
1.238 jmc 1302: .It ~/.ssh/authorized_keys
1303: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1304: The format of this file is described in the
1305: .Xr sshd 8
1306: manual page.
1307: This file is not highly sensitive, but the recommended
1308: permissions are read/write for the user, and not accessible by others.
1309: .Pp
1310: .It ~/.ssh/config
1311: This is the per-user configuration file.
1312: The file format and configuration options are described in
1313: .Xr ssh_config 5 .
1314: Because of the potential for abuse, this file must have strict permissions:
1315: read/write for the user, and not accessible by others.
1316: .Pp
1317: .It ~/.ssh/environment
1.239 jmc 1318: Contains additional definitions for environment variables; see
1319: .Sx ENVIRONMENT ,
1.238 jmc 1320: above.
1321: .Pp
1322: .It ~/.ssh/identity
1323: .It ~/.ssh/id_dsa
1324: .It ~/.ssh/id_rsa
1325: Contains the private key for authentication.
1326: These files
1327: contain sensitive data and should be readable by the user but not
1328: accessible by others (read/write/execute).
1329: .Nm
1330: will simply ignore a private key file if it is accessible by others.
1331: It is possible to specify a passphrase when
1332: generating the key which will be used to encrypt the
1333: sensitive part of this file using 3DES.
1334: .Pp
1335: .It ~/.ssh/identity.pub
1336: .It ~/.ssh/id_dsa.pub
1337: .It ~/.ssh/id_rsa.pub
1338: Contains the public key for authentication.
1339: These files are not
1340: sensitive and can (but need not) be readable by anyone.
1341: .Pp
1342: .It ~/.ssh/known_hosts
1.244 jmc 1343: Contains a list of host keys for all hosts the user has logged into
1344: that are not already in the systemwide list of known host keys.
1.238 jmc 1345: See
1.244 jmc 1346: .Xr sshd 8
1347: for further details of the format of this file.
1.238 jmc 1348: .Pp
1349: .It ~/.ssh/rc
1350: Commands in this file are executed by
1351: .Nm
1.245 jmc 1352: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1353: started.
1354: See the
1355: .Xr sshd 8
1356: manual page for more information.
1357: .Pp
1.236 jmc 1358: .It /etc/hosts.equiv
1.240 jmc 1359: This file is for host-based authentication (see above).
1360: It should only be writable by root.
1.236 jmc 1361: .Pp
1362: .It /etc/shosts.equiv
1.240 jmc 1363: This file is used in exactly the same way as
1364: .Pa hosts.equiv ,
1365: but allows host-based authentication without permitting login with
1366: rlogin/rsh.
1.236 jmc 1367: .Pp
1.238 jmc 1368: .It Pa /etc/ssh/ssh_config
1369: Systemwide configuration file.
1370: The file format and configuration options are described in
1371: .Xr ssh_config 5 .
1372: .Pp
1373: .It /etc/ssh/ssh_host_key
1374: .It /etc/ssh/ssh_host_dsa_key
1375: .It /etc/ssh/ssh_host_rsa_key
1376: These three files contain the private parts of the host keys
1.245 jmc 1377: and are used for host-based authentication.
1378: If protocol version 1 is used,
1.238 jmc 1379: .Nm
1380: must be setuid root, since the host key is readable only by root.
1381: For protocol version 2,
1382: .Nm
1383: uses
1384: .Xr ssh-keysign 8
1.245 jmc 1385: to access the host keys,
1386: eliminating the requirement that
1.238 jmc 1387: .Nm
1.245 jmc 1388: be setuid root when host-based authentication is used.
1.238 jmc 1389: By default
1.2 deraadt 1390: .Nm
1.238 jmc 1391: is not setuid root.
1392: .Pp
1393: .It /etc/ssh/ssh_known_hosts
1394: Systemwide list of known host keys.
1395: This file should be prepared by the
1396: system administrator to contain the public host keys of all machines in the
1397: organization.
1.244 jmc 1398: It should be world-readable.
1399: See
1.238 jmc 1400: .Xr sshd 8
1.244 jmc 1401: for further details of the format of this file.
1.236 jmc 1402: .Pp
1.238 jmc 1403: .It /etc/ssh/sshrc
1.1 deraadt 1404: Commands in this file are executed by
1.2 deraadt 1405: .Nm
1.245 jmc 1406: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1407: See the
1.2 deraadt 1408: .Xr sshd 8
1.1 deraadt 1409: manual page for more information.
1.58 itojun 1410: .El
1.2 deraadt 1411: .Sh SEE ALSO
1412: .Xr scp 1 ,
1.83 djm 1413: .Xr sftp 1 ,
1.2 deraadt 1414: .Xr ssh-add 1 ,
1415: .Xr ssh-agent 1 ,
1416: .Xr ssh-keygen 1 ,
1.242 jmc 1417: .Xr ssh-keyscan 1 ,
1.250 jmc 1418: .Xr tun 4 ,
1.176 jmc 1419: .Xr hosts.equiv 5 ,
1.159 stevesk 1420: .Xr ssh_config 5 ,
1.160 naddy 1421: .Xr ssh-keysign 8 ,
1.87 itojun 1422: .Xr sshd 8
1.106 markus 1423: .Rs
1.256 jmc 1424: .%R RFC 4250
1425: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1426: .%D 2006
1427: .Re
1428: .Rs
1429: .%R RFC 4251
1430: .%T "The Secure Shell (SSH) Protocol Architecture"
1431: .%D 2006
1432: .Re
1433: .Rs
1434: .%R RFC 4252
1435: .%T "The Secure Shell (SSH) Authentication Protocol"
1436: .%D 2006
1437: .Re
1438: .Rs
1439: .%R RFC 4253
1440: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1441: .%D 2006
1442: .Re
1443: .Rs
1444: .%R RFC 4254
1445: .%T "The Secure Shell (SSH) Connection Protocol"
1446: .%D 2006
1447: .Re
1448: .Rs
1449: .%R RFC 4255
1450: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1451: .%D 2006
1452: .Re
1453: .Rs
1454: .%R RFC 4256
1455: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1456: .%D 2006
1457: .Re
1458: .Rs
1459: .%R RFC 4335
1460: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1461: .%D 2006
1462: .Re
1463: .Rs
1464: .%R RFC 4344
1465: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1466: .%D 2006
1467: .Re
1468: .Rs
1469: .%R RFC 4345
1470: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1471: .%D 2006
1472: .Re
1473: .Rs
1474: .%R RFC 4419
1475: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1476: .%D 2006
1477: .Re
1478: .Rs
1479: .%R RFC 4716
1480: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1481: .%D 2006
1.274 grunk 1482: .Re
1483: .Rs
1484: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1485: .%A A. Perrig
1486: .%A D. Song
1487: .%D 1999
1488: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1489: .Re
1.173 jmc 1490: .Sh AUTHORS
1491: OpenSSH is a derivative of the original and free
1492: ssh 1.2.12 release by Tatu Ylonen.
1493: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1494: Theo de Raadt and Dug Song
1495: removed many bugs, re-added newer features and
1496: created OpenSSH.
1497: Markus Friedl contributed the support for SSH
1498: protocol versions 1.5 and 2.0.