Annotation of src/usr.bin/ssh/ssh.1, Revision 1.306
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: .\"
1.306 ! jmc 37: .\" $OpenBSD: ssh.1,v 1.305 2010/05/16 12:55:51 markus Exp $
! 38: .Dd $Mdocdate: May 16 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
179: keyword for more information.
1.218 jmc 180: .It Fl D Xo
181: .Sm off
182: .Oo Ar bind_address : Oc
183: .Ar port
184: .Sm on
185: .Xc
186: Specifies a local
187: .Dq dynamic
188: application-level port forwarding.
189: This works by allocating a socket to listen to
190: .Ar port
191: on the local side, optionally bound to the specified
192: .Ar bind_address .
193: Whenever a connection is made to this port, the
194: connection is forwarded over the secure channel, and the application
195: protocol is then used to determine where to connect to from the
196: remote machine.
197: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 198: .Nm
1.218 jmc 199: will act as a SOCKS server.
200: Only root can forward privileged ports.
201: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 202: .Pp
1.218 jmc 203: IPv6 addresses can be specified with an alternative syntax:
204: .Sm off
205: .Xo
206: .Op Ar bind_address No /
207: .Ar port
208: .Xc
209: .Sm on
210: or by enclosing the address in square brackets.
211: Only the superuser can forward privileged ports.
212: By default, the local port is bound in accordance with the
213: .Cm GatewayPorts
214: setting.
215: However, an explicit
216: .Ar bind_address
217: may be used to bind the connection to a specific address.
218: The
219: .Ar bind_address
220: of
221: .Dq localhost
222: indicates that the listening port be bound for local use only, while an
223: empty address or
224: .Sq *
225: indicates that the port should be available from all interfaces.
1.229 jmc 226: .It Fl e Ar escape_char
1.218 jmc 227: Sets the escape character for sessions with a pty (default:
228: .Ql ~ ) .
229: The escape character is only recognized at the beginning of a line.
230: The escape character followed by a dot
231: .Pq Ql \&.
232: closes the connection;
233: followed by control-Z suspends the connection;
234: and followed by itself sends the escape character once.
235: Setting the character to
1.2 deraadt 236: .Dq none
1.218 jmc 237: disables any escapes and makes the session fully transparent.
238: .It Fl F Ar configfile
239: Specifies an alternative per-user configuration file.
240: If a configuration file is given on the command line,
241: the system-wide configuration file
242: .Pq Pa /etc/ssh/ssh_config
243: will be ignored.
244: The default for the per-user configuration file is
245: .Pa ~/.ssh/config .
246: .It Fl f
247: Requests
248: .Nm
249: to go to background just before command execution.
250: This is useful if
1.176 jmc 251: .Nm
1.218 jmc 252: is going to ask for passwords or passphrases, but the user
253: wants it in the background.
254: This implies
255: .Fl n .
256: The recommended way to start X11 programs at a remote site is with
257: something like
258: .Ic ssh -f host xterm .
1.277 djm 259: .Pp
260: If the
261: .Cm ExitOnForwardFailure
262: configuration option is set to
263: .Dq yes ,
264: then a client started with
265: .Fl f
266: will wait for all remote port forwards to be successfully established
267: before placing itself in the background.
1.218 jmc 268: .It Fl g
269: Allows remote hosts to connect to local forwarded ports.
1.291 markus 270: .It Fl I Ar pkcs11
1.294 jmc 271: Specify the PKCS#11 shared library
1.176 jmc 272: .Nm
1.293 markus 273: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 274: private RSA key.
275: .It Fl i Ar identity_file
276: Selects a file from which the identity (private key) for
277: RSA or DSA authentication is read.
278: The default is
279: .Pa ~/.ssh/identity
280: for protocol version 1, and
281: .Pa ~/.ssh/id_rsa
1.149 jakob 282: and
1.218 jmc 283: .Pa ~/.ssh/id_dsa
284: for protocol version 2.
285: Identity files may also be specified on
286: a per-host basis in the configuration file.
287: It is possible to have multiple
288: .Fl i
289: options (and multiple identities specified in
290: configuration files).
1.302 djm 291: .Nm
292: will also try to load certificate information from the filename obtained
293: by appending
294: .Pa -cert.pub
295: to identity filenames.
1.269 djm 296: .It Fl K
297: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
298: credentials to the server.
1.218 jmc 299: .It Fl k
300: Disables forwarding (delegation) of GSSAPI credentials to the server.
301: .It Fl L Xo
302: .Sm off
303: .Oo Ar bind_address : Oc
304: .Ar port : host : hostport
305: .Sm on
306: .Xc
307: Specifies that the given port on the local (client) host is to be
308: forwarded to the given host and port on the remote side.
309: This works by allocating a socket to listen to
310: .Ar port
311: on the local side, optionally bound to the specified
312: .Ar bind_address .
313: Whenever a connection is made to this port, the
314: connection is forwarded over the secure channel, and a connection is
315: made to
316: .Ar host
317: port
318: .Ar hostport
319: from the remote machine.
320: Port forwardings can also be specified in the configuration file.
321: IPv6 addresses can be specified with an alternative syntax:
322: .Sm off
323: .Xo
324: .Op Ar bind_address No /
325: .Ar port No / Ar host No /
326: .Ar hostport
327: .Xc
328: .Sm on
329: or by enclosing the address in square brackets.
330: Only the superuser can forward privileged ports.
331: By default, the local port is bound in accordance with the
332: .Cm GatewayPorts
333: setting.
334: However, an explicit
335: .Ar bind_address
336: may be used to bind the connection to a specific address.
1.2 deraadt 337: The
1.218 jmc 338: .Ar bind_address
339: of
340: .Dq localhost
341: indicates that the listening port be bound for local use only, while an
342: empty address or
343: .Sq *
344: indicates that the port should be available from all interfaces.
345: .It Fl l Ar login_name
346: Specifies the user to log in as on the remote machine.
347: This also may be specified on a per-host basis in the configuration file.
348: .It Fl M
349: Places the
350: .Nm
351: client into
352: .Dq master
353: mode for connection sharing.
1.231 stevesk 354: Multiple
355: .Fl M
356: options places
357: .Nm
358: into
359: .Dq master
360: mode with confirmation required before slave connections are accepted.
1.218 jmc 361: Refer to the description of
362: .Cm ControlMaster
363: in
364: .Xr ssh_config 5
365: for details.
366: .It Fl m Ar mac_spec
367: Additionally, for protocol version 2 a comma-separated list of MAC
368: (message authentication code) algorithms can
369: be specified in order of preference.
370: See the
371: .Cm MACs
372: keyword for more information.
373: .It Fl N
374: Do not execute a remote command.
375: This is useful for just forwarding ports
376: (protocol version 2 only).
377: .It Fl n
378: Redirects stdin from
379: .Pa /dev/null
380: (actually, prevents reading from stdin).
381: This must be used when
1.2 deraadt 382: .Nm
1.218 jmc 383: is run in the background.
384: A common trick is to use this to run X11 programs on a remote machine.
385: For example,
386: .Ic ssh -n shadows.cs.hut.fi emacs &
387: will start an emacs on shadows.cs.hut.fi, and the X11
388: connection will be automatically forwarded over an encrypted channel.
389: The
1.2 deraadt 390: .Nm
1.218 jmc 391: program will be put in the background.
392: (This does not work if
1.2 deraadt 393: .Nm
1.218 jmc 394: needs to ask for a password or passphrase; see also the
395: .Fl f
396: option.)
397: .It Fl O Ar ctl_cmd
398: Control an active connection multiplexing master process.
399: When the
400: .Fl O
401: option is specified, the
402: .Ar ctl_cmd
403: argument is interpreted and passed to the master process.
404: Valid commands are:
405: .Dq check
1.305 markus 406: (check that the master process is running),
407: .Dq forward
408: (request forwardings without command execution) and
1.218 jmc 409: .Dq exit
410: (request the master to exit).
411: .It Fl o Ar option
412: Can be used to give options in the format used in the configuration file.
413: This is useful for specifying options for which there is no separate
414: command-line flag.
415: For full details of the options listed below, and their possible values, see
416: .Xr ssh_config 5 .
1.2 deraadt 417: .Pp
1.218 jmc 418: .Bl -tag -width Ds -offset indent -compact
419: .It AddressFamily
420: .It BatchMode
421: .It BindAddress
422: .It ChallengeResponseAuthentication
423: .It CheckHostIP
424: .It Cipher
425: .It Ciphers
426: .It ClearAllForwardings
427: .It Compression
428: .It CompressionLevel
429: .It ConnectionAttempts
430: .It ConnectTimeout
431: .It ControlMaster
432: .It ControlPath
433: .It DynamicForward
434: .It EscapeChar
1.263 markus 435: .It ExitOnForwardFailure
1.218 jmc 436: .It ForwardAgent
437: .It ForwardX11
438: .It ForwardX11Trusted
439: .It GatewayPorts
440: .It GlobalKnownHostsFile
441: .It GSSAPIAuthentication
442: .It GSSAPIDelegateCredentials
443: .It HashKnownHosts
444: .It Host
445: .It HostbasedAuthentication
446: .It HostKeyAlgorithms
447: .It HostKeyAlias
448: .It HostName
449: .It IdentityFile
450: .It IdentitiesOnly
451: .It KbdInteractiveDevices
452: .It LocalCommand
453: .It LocalForward
454: .It LogLevel
455: .It MACs
456: .It NoHostAuthenticationForLocalhost
457: .It NumberOfPasswordPrompts
458: .It PasswordAuthentication
459: .It PermitLocalCommand
1.291 markus 460: .It PKCS11Provider
1.218 jmc 461: .It Port
462: .It PreferredAuthentications
463: .It Protocol
464: .It ProxyCommand
465: .It PubkeyAuthentication
1.251 dtucker 466: .It RekeyLimit
1.218 jmc 467: .It RemoteForward
468: .It RhostsRSAAuthentication
469: .It RSAAuthentication
470: .It SendEnv
471: .It ServerAliveInterval
472: .It ServerAliveCountMax
473: .It StrictHostKeyChecking
474: .It TCPKeepAlive
475: .It Tunnel
476: .It TunnelDevice
477: .It UsePrivilegedPort
478: .It User
479: .It UserKnownHostsFile
480: .It VerifyHostKeyDNS
1.276 jmc 481: .It VisualHostKey
1.218 jmc 482: .It XAuthLocation
483: .El
484: .It Fl p Ar port
485: Port to connect to on the remote host.
486: This can be specified on a
487: per-host basis in the configuration file.
488: .It Fl q
489: Quiet mode.
1.271 djm 490: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 491: .It Fl R Xo
492: .Sm off
493: .Oo Ar bind_address : Oc
494: .Ar port : host : hostport
495: .Sm on
496: .Xc
497: Specifies that the given port on the remote (server) host is to be
498: forwarded to the given host and port on the local side.
499: This works by allocating a socket to listen to
500: .Ar port
501: on the remote side, and whenever a connection is made to this port, the
502: connection is forwarded over the secure channel, and a connection is
503: made to
504: .Ar host
505: port
506: .Ar hostport
507: from the local machine.
1.2 deraadt 508: .Pp
1.218 jmc 509: Port forwardings can also be specified in the configuration file.
510: Privileged ports can be forwarded only when
511: logging in as root on the remote machine.
512: IPv6 addresses can be specified by enclosing the address in square braces or
513: using an alternative syntax:
514: .Sm off
515: .Xo
516: .Op Ar bind_address No /
517: .Ar host No / Ar port No /
518: .Ar hostport
519: .Xc .
520: .Sm on
1.194 jakob 521: .Pp
1.218 jmc 522: By default, the listening socket on the server will be bound to the loopback
523: interface only.
1.280 tobias 524: This may be overridden by specifying a
1.218 jmc 525: .Ar bind_address .
526: An empty
527: .Ar bind_address ,
528: or the address
529: .Ql * ,
530: indicates that the remote socket should listen on all interfaces.
531: Specifying a remote
532: .Ar bind_address
533: will only succeed if the server's
534: .Cm GatewayPorts
535: option is enabled (see
536: .Xr sshd_config 5 ) .
1.281 djm 537: .Pp
538: If the
539: .Ar port
540: argument is
1.282 djm 541: .Ql 0 ,
1.281 djm 542: the listen port will be dynamically allocated on the server and reported
543: to the client at run time.
1.305 markus 544: When used together with
545: .Ic -O forward
546: the allocated port will be printed to the standard output.
1.218 jmc 547: .It Fl S Ar ctl_path
1.304 jmc 548: Specifies the location of a control socket for connection sharing,
1.303 djm 549: or the string
550: .Dq none
551: to disable connection sharing.
1.218 jmc 552: Refer to the description of
553: .Cm ControlPath
554: and
555: .Cm ControlMaster
556: in
557: .Xr ssh_config 5
558: for details.
559: .It Fl s
560: May be used to request invocation of a subsystem on the remote system.
561: Subsystems are a feature of the SSH2 protocol which facilitate the use
562: of SSH as a secure transport for other applications (eg.\&
563: .Xr sftp 1 ) .
564: The subsystem is specified as the remote command.
565: .It Fl T
566: Disable pseudo-tty allocation.
567: .It Fl t
568: Force pseudo-tty allocation.
569: This can be used to execute arbitrary
570: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 571: e.g. when implementing menu services.
1.218 jmc 572: Multiple
573: .Fl t
574: options force tty allocation, even if
1.194 jakob 575: .Nm
1.218 jmc 576: has no local tty.
577: .It Fl V
578: Display the version number and exit.
579: .It Fl v
580: Verbose mode.
581: Causes
1.176 jmc 582: .Nm
1.218 jmc 583: to print debugging messages about its progress.
584: This is helpful in
585: debugging connection, authentication, and configuration problems.
586: Multiple
587: .Fl v
588: options increase the verbosity.
589: The maximum is 3.
1.290 dtucker 590: .It Fl W Ar host : Ns Ar port
591: Requests that standard input and output on the client be forwarded to
592: .Ar host
593: on
594: .Ar port
595: over the secure channel.
596: Implies
597: .Fl N ,
598: .Fl T ,
599: .Cm ExitOnForwardFailure
600: and
601: .Cm ClearAllForwardings
602: and works with Protocol version 2 only.
1.261 stevesk 603: .It Fl w Xo
604: .Ar local_tun Ns Op : Ns Ar remote_tun
605: .Xc
606: Requests
607: tunnel
608: device forwarding with the specified
1.218 jmc 609: .Xr tun 4
1.261 stevesk 610: devices between the client
611: .Pq Ar local_tun
612: and the server
613: .Pq Ar remote_tun .
614: .Pp
1.228 jmc 615: The devices may be specified by numerical ID or the keyword
616: .Dq any ,
617: which uses the next available tunnel device.
1.261 stevesk 618: If
619: .Ar remote_tun
620: is not specified, it defaults to
621: .Dq any .
1.228 jmc 622: See also the
1.218 jmc 623: .Cm Tunnel
1.261 stevesk 624: and
625: .Cm TunnelDevice
626: directives in
1.218 jmc 627: .Xr ssh_config 5 .
1.261 stevesk 628: If the
629: .Cm Tunnel
630: directive is unset, it is set to the default tunnel mode, which is
631: .Dq point-to-point .
1.218 jmc 632: .It Fl X
633: Enables X11 forwarding.
1.54 markus 634: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 635: .Pp
1.218 jmc 636: X11 forwarding should be enabled with caution.
1.168 jmc 637: Users with the ability to bypass file permissions on the remote host
1.218 jmc 638: (for the user's X authorization database)
639: can access the local X11 display through the forwarded connection.
640: An attacker may then be able to perform activities such as keystroke monitoring.
641: .Pp
642: For this reason, X11 forwarding is subjected to X11 SECURITY extension
643: restrictions by default.
644: Please refer to the
645: .Nm
646: .Fl Y
647: option and the
648: .Cm ForwardX11Trusted
649: directive in
650: .Xr ssh_config 5
651: for more information.
652: .It Fl x
653: Disables X11 forwarding.
654: .It Fl Y
655: Enables trusted X11 forwarding.
656: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
657: controls.
1.278 djm 658: .It Fl y
659: Send log information using the
660: .Xr syslog 3
661: system module.
662: By default this information is sent to stderr.
1.218 jmc 663: .El
1.224 jmc 664: .Pp
665: .Nm
666: may additionally obtain configuration data from
667: a per-user configuration file and a system-wide configuration file.
668: The file format and configuration options are described in
669: .Xr ssh_config 5 .
670: .Pp
671: .Nm
672: exits with the exit status of the remote command or with 255
673: if an error occurred.
1.222 jmc 674: .Sh AUTHENTICATION
1.249 jmc 675: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 676: The default is to use protocol 2 only,
677: though this can be changed via the
1.222 jmc 678: .Cm Protocol
679: option in
1.284 jmc 680: .Xr ssh_config 5
681: or the
1.222 jmc 682: .Fl 1
683: and
684: .Fl 2
685: options (see above).
686: Both protocols support similar authentication methods,
1.284 jmc 687: but protocol 2 is the default since
1.222 jmc 688: it provides additional mechanisms for confidentiality
689: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 690: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 691: Protocol 1 lacks a strong mechanism for ensuring the
692: integrity of the connection.
693: .Pp
694: The methods available for authentication are:
1.260 jmc 695: GSSAPI-based authentication,
1.222 jmc 696: host-based authentication,
697: public key authentication,
698: challenge-response authentication,
699: and password authentication.
700: Authentication methods are tried in the order specified above,
701: though protocol 2 has a configuration option to change the default order:
702: .Cm PreferredAuthentications .
703: .Pp
704: Host-based authentication works as follows:
1.218 jmc 705: If the machine the user logs in from is listed in
706: .Pa /etc/hosts.equiv
707: or
708: .Pa /etc/shosts.equiv
709: on the remote machine, and the user names are
710: the same on both sides, or if the files
711: .Pa ~/.rhosts
712: or
713: .Pa ~/.shosts
714: exist in the user's home directory on the
715: remote machine and contain a line containing the name of the client
716: machine and the name of the user on that machine, the user is
1.222 jmc 717: considered for login.
718: Additionally, the server
719: .Em must
720: be able to verify the client's
721: host key (see the description of
1.218 jmc 722: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 723: and
1.222 jmc 724: .Pa ~/.ssh/known_hosts ,
725: below)
726: for login to be permitted.
1.218 jmc 727: This authentication method closes security holes due to IP
1.222 jmc 728: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 729: [Note to the administrator:
730: .Pa /etc/hosts.equiv ,
731: .Pa ~/.rhosts ,
732: and the rlogin/rsh protocol in general, are inherently insecure and should be
733: disabled if security is desired.]
1.189 dtucker 734: .Pp
1.222 jmc 735: Public key authentication works as follows:
736: The scheme is based on public-key cryptography,
737: using cryptosystems
738: where encryption and decryption are done using separate keys,
739: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 740: The idea is that each user creates a public/private
741: key pair for authentication purposes.
742: The server knows the public key, and only the user knows the private key.
1.222 jmc 743: .Nm
744: implements public key authentication protocol automatically,
745: using either the RSA or DSA algorithms.
746: Protocol 1 is restricted to using only RSA keys,
747: but protocol 2 may use either.
748: The
749: .Sx HISTORY
750: section of
751: .Xr ssl 8
752: contains a brief discussion of the two algorithms.
1.210 djm 753: .Pp
1.218 jmc 754: The file
755: .Pa ~/.ssh/authorized_keys
756: lists the public keys that are permitted for logging in.
757: When the user logs in, the
1.2 deraadt 758: .Nm
1.218 jmc 759: program tells the server which key pair it would like to use for
760: authentication.
1.222 jmc 761: The client proves that it has access to the private key
762: and the server checks that the corresponding public key
763: is authorized to accept the account.
1.218 jmc 764: .Pp
1.222 jmc 765: The user creates his/her key pair by running
1.218 jmc 766: .Xr ssh-keygen 1 .
767: This stores the private key in
1.207 djm 768: .Pa ~/.ssh/identity
1.222 jmc 769: (protocol 1),
770: .Pa ~/.ssh/id_dsa
771: (protocol 2 DSA),
772: or
773: .Pa ~/.ssh/id_rsa
774: (protocol 2 RSA)
1.218 jmc 775: and stores the public key in
776: .Pa ~/.ssh/identity.pub
1.222 jmc 777: (protocol 1),
778: .Pa ~/.ssh/id_dsa.pub
779: (protocol 2 DSA),
780: or
781: .Pa ~/.ssh/id_rsa.pub
782: (protocol 2 RSA)
1.218 jmc 783: in the user's home directory.
1.222 jmc 784: The user should then copy the public key
1.218 jmc 785: to
786: .Pa ~/.ssh/authorized_keys
1.222 jmc 787: in his/her home directory on the remote machine.
788: The
1.218 jmc 789: .Pa authorized_keys
790: file corresponds to the conventional
791: .Pa ~/.rhosts
792: file, and has one key
1.222 jmc 793: per line, though the lines can be very long.
1.218 jmc 794: After this, the user can log in without giving the password.
795: .Pp
1.301 jmc 796: A variation on public key authentication
797: is available in the form of certificate authentication:
798: instead of a set of public/private keys,
799: signed certificates are used.
800: This has the advantage that a single trusted certification authority
801: can be used in place of many public/private keys.
802: See the
803: .Sx CERTIFICATES
804: section of
805: .Xr ssh-keygen 1
806: for more information.
807: .Pp
808: The most convenient way to use public key or certificate authentication
809: may be with an authentication agent.
1.218 jmc 810: See
811: .Xr ssh-agent 1
812: for more information.
813: .Pp
1.222 jmc 814: Challenge-response authentication works as follows:
815: The server sends an arbitrary
816: .Qq challenge
817: text, and prompts for a response.
818: Protocol 2 allows multiple challenges and responses;
819: protocol 1 is restricted to just one challenge/response.
820: Examples of challenge-response authentication include
821: BSD Authentication (see
822: .Xr login.conf 5 )
823: and PAM (some non-OpenBSD systems).
824: .Pp
825: Finally, if other authentication methods fail,
1.218 jmc 826: .Nm
827: prompts the user for a password.
828: The password is sent to the remote
829: host for checking; however, since all communications are encrypted,
830: the password cannot be seen by someone listening on the network.
1.232 jmc 831: .Pp
832: .Nm
833: automatically maintains and checks a database containing
834: identification for all hosts it has ever been used with.
835: Host keys are stored in
836: .Pa ~/.ssh/known_hosts
837: in the user's home directory.
838: Additionally, the file
839: .Pa /etc/ssh/ssh_known_hosts
840: is automatically checked for known hosts.
841: Any new hosts are automatically added to the user's file.
842: If a host's identification ever changes,
843: .Nm
844: warns about this and disables password authentication to prevent
845: server spoofing or man-in-the-middle attacks,
846: which could otherwise be used to circumvent the encryption.
847: The
848: .Cm StrictHostKeyChecking
849: option can be used to control logins to machines whose
850: host key is not known or has changed.
851: .Pp
1.218 jmc 852: When the user's identity has been accepted by the server, the server
853: either executes the given command, or logs into the machine and gives
854: the user a normal shell on the remote machine.
855: All communication with
856: the remote command or shell will be automatically encrypted.
857: .Pp
858: If a pseudo-terminal has been allocated (normal login session), the
859: user may use the escape characters noted below.
860: .Pp
861: If no pseudo-tty has been allocated,
862: the session is transparent and can be used to reliably transfer binary data.
863: On most systems, setting the escape character to
864: .Dq none
865: will also make the session transparent even if a tty is used.
866: .Pp
867: The session terminates when the command or shell on the remote
1.247 jmc 868: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 869: .Sh ESCAPE CHARACTERS
1.218 jmc 870: When a pseudo-terminal has been requested,
1.2 deraadt 871: .Nm
1.218 jmc 872: supports a number of functions through the use of an escape character.
873: .Pp
874: A single tilde character can be sent as
875: .Ic ~~
876: or by following the tilde by a character other than those described below.
877: The escape character must always follow a newline to be interpreted as
878: special.
879: The escape character can be changed in configuration files using the
880: .Cm EscapeChar
881: configuration directive or on the command line by the
882: .Fl e
883: option.
884: .Pp
885: The supported escapes (assuming the default
886: .Ql ~ )
887: are:
888: .Bl -tag -width Ds
889: .It Cm ~.
890: Disconnect.
891: .It Cm ~^Z
892: Background
1.234 jmc 893: .Nm .
1.218 jmc 894: .It Cm ~#
895: List forwarded connections.
896: .It Cm ~&
897: Background
1.2 deraadt 898: .Nm
1.218 jmc 899: at logout when waiting for forwarded connection / X11 sessions to terminate.
900: .It Cm ~?
901: Display a list of escape characters.
902: .It Cm ~B
903: Send a BREAK to the remote system
904: (only useful for SSH protocol version 2 and if the peer supports it).
905: .It Cm ~C
906: Open command line.
907: Currently this allows the addition of port forwardings using the
1.279 stevesk 908: .Fl L ,
909: .Fl R
1.218 jmc 910: and
1.279 stevesk 911: .Fl D
1.225 jmc 912: options (see above).
1.218 jmc 913: It also allows the cancellation of existing remote port-forwardings
914: using
1.262 stevesk 915: .Sm off
916: .Fl KR Oo Ar bind_address : Oc Ar port .
917: .Sm on
1.218 jmc 918: .Ic !\& Ns Ar command
919: allows the user to execute a local command if the
920: .Ic PermitLocalCommand
921: option is enabled in
1.176 jmc 922: .Xr ssh_config 5 .
1.218 jmc 923: Basic help is available, using the
924: .Fl h
925: option.
926: .It Cm ~R
927: Request rekeying of the connection
928: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 929: .El
1.246 jmc 930: .Sh TCP FORWARDING
931: Forwarding of arbitrary TCP connections over the secure channel can
932: be specified either on the command line or in a configuration file.
933: One possible application of TCP forwarding is a secure connection to a
934: mail server; another is going through firewalls.
935: .Pp
936: In the example below, we look at encrypting communication between
937: an IRC client and server, even though the IRC server does not directly
938: support encrypted communications.
939: This works as follows:
940: the user connects to the remote host using
941: .Nm ,
942: specifying a port to be used to forward connections
943: to the remote server.
944: After that it is possible to start the service which is to be encrypted
945: on the client machine,
946: connecting to the same local port,
947: and
948: .Nm
949: will encrypt and forward the connection.
950: .Pp
951: The following example tunnels an IRC session from client machine
952: .Dq 127.0.0.1
953: (localhost)
954: to remote server
955: .Dq server.example.com :
956: .Bd -literal -offset 4n
957: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
958: $ irc -c '#users' -p 1234 pinky 127.0.0.1
959: .Ed
960: .Pp
961: This tunnels a connection to IRC server
962: .Dq server.example.com ,
963: joining channel
964: .Dq #users ,
965: nickname
966: .Dq pinky ,
967: using port 1234.
968: It doesn't matter which port is used,
969: as long as it's greater than 1023
970: (remember, only root can open sockets on privileged ports)
971: and doesn't conflict with any ports already in use.
972: The connection is forwarded to port 6667 on the remote server,
973: since that's the standard port for IRC services.
974: .Pp
975: The
976: .Fl f
977: option backgrounds
978: .Nm
979: and the remote command
980: .Dq sleep 10
981: is specified to allow an amount of time
982: (10 seconds, in the example)
983: to start the service which is to be tunnelled.
984: If no connections are made within the time specified,
985: .Nm
986: will exit.
987: .Sh X11 FORWARDING
1.218 jmc 988: If the
989: .Cm ForwardX11
990: variable is set to
991: .Dq yes
992: (or see the description of the
1.227 jmc 993: .Fl X ,
994: .Fl x ,
1.218 jmc 995: and
1.227 jmc 996: .Fl Y
1.226 jmc 997: options above)
1.218 jmc 998: and the user is using X11 (the
999: .Ev DISPLAY
1000: environment variable is set), the connection to the X11 display is
1001: automatically forwarded to the remote side in such a way that any X11
1002: programs started from the shell (or command) will go through the
1003: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1004: from the local machine.
1.218 jmc 1005: The user should not manually set
1006: .Ev DISPLAY .
1007: Forwarding of X11 connections can be
1008: configured on the command line or in configuration files.
1009: .Pp
1010: The
1011: .Ev DISPLAY
1012: value set by
1013: .Nm
1014: will point to the server machine, but with a display number greater than zero.
1015: This is normal, and happens because
1016: .Nm
1017: creates a
1018: .Dq proxy
1019: X server on the server machine for forwarding the
1020: connections over the encrypted channel.
1.200 djm 1021: .Pp
1.218 jmc 1022: .Nm
1023: will also automatically set up Xauthority data on the server machine.
1024: For this purpose, it will generate a random authorization cookie,
1025: store it in Xauthority on the server, and verify that any forwarded
1026: connections carry this cookie and replace it by the real cookie when
1027: the connection is opened.
1028: The real authentication cookie is never
1029: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1030: .Pp
1.218 jmc 1031: If the
1032: .Cm ForwardAgent
1033: variable is set to
1034: .Dq yes
1035: (or see the description of the
1036: .Fl A
1.191 djm 1037: and
1.218 jmc 1038: .Fl a
1.226 jmc 1039: options above) and
1.218 jmc 1040: the user is using an authentication agent, the connection to the agent
1041: is automatically forwarded to the remote side.
1.252 jmc 1042: .Sh VERIFYING HOST KEYS
1043: When connecting to a server for the first time,
1044: a fingerprint of the server's public key is presented to the user
1045: (unless the option
1046: .Cm StrictHostKeyChecking
1047: has been disabled).
1048: Fingerprints can be determined using
1049: .Xr ssh-keygen 1 :
1050: .Pp
1051: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1052: .Pp
1.274 grunk 1053: If the fingerprint is already known, it can be matched
1054: and the key can be accepted or rejected.
1055: Because of the difficulty of comparing host keys
1056: just by looking at hex strings,
1057: there is also support to compare host keys visually,
1058: using
1059: .Em random art .
1060: By setting the
1.275 grunk 1061: .Cm VisualHostKey
1.274 grunk 1062: option to
1.275 grunk 1063: .Dq yes ,
1.274 grunk 1064: a small ASCII graphic gets displayed on every login to a server, no matter
1065: if the session itself is interactive or not.
1066: By learning the pattern a known server produces, a user can easily
1067: find out that the host key has changed when a completely different pattern
1068: is displayed.
1069: Because these patterns are not unambiguous however, a pattern that looks
1070: similar to the pattern remembered only gives a good probability that the
1071: host key is the same, not guaranteed proof.
1072: .Pp
1073: To get a listing of the fingerprints along with their random art for
1074: all known hosts, the following command line can be used:
1075: .Pp
1076: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1077: .Pp
1.252 jmc 1078: If the fingerprint is unknown,
1079: an alternative method of verification is available:
1080: SSH fingerprints verified by DNS.
1081: An additional resource record (RR),
1082: SSHFP,
1083: is added to a zonefile
1084: and the connecting client is able to match the fingerprint
1085: with that of the key presented.
1086: .Pp
1087: In this example, we are connecting a client to a server,
1088: .Dq host.example.com .
1089: The SSHFP resource records should first be added to the zonefile for
1090: host.example.com:
1091: .Bd -literal -offset indent
1.259 jakob 1092: $ ssh-keygen -r host.example.com.
1.252 jmc 1093: .Ed
1094: .Pp
1095: The output lines will have to be added to the zonefile.
1096: To check that the zone is answering fingerprint queries:
1097: .Pp
1098: .Dl $ dig -t SSHFP host.example.com
1099: .Pp
1100: Finally the client connects:
1101: .Bd -literal -offset indent
1102: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1103: [...]
1104: Matching host key fingerprint found in DNS.
1105: Are you sure you want to continue connecting (yes/no)?
1106: .Ed
1107: .Pp
1108: See the
1109: .Cm VerifyHostKeyDNS
1110: option in
1111: .Xr ssh_config 5
1112: for more information.
1.250 jmc 1113: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1114: .Nm
1115: contains support for Virtual Private Network (VPN) tunnelling
1116: using the
1117: .Xr tun 4
1118: network pseudo-device,
1119: allowing two networks to be joined securely.
1120: The
1121: .Xr sshd_config 5
1122: configuration option
1123: .Cm PermitTunnel
1124: controls whether the server supports this,
1125: and at what level (layer 2 or 3 traffic).
1126: .Pp
1127: The following example would connect client network 10.0.50.0/24
1.265 otto 1128: with remote network 10.0.99.0/24 using a point-to-point connection
1129: from 10.1.1.1 to 10.1.1.2,
1130: provided that the SSH server running on the gateway to the remote network,
1131: at 192.168.1.15, allows it.
1132: .Pp
1133: On the client:
1.250 jmc 1134: .Bd -literal -offset indent
1135: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1136: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1137: # route add 10.0.99.0/24 10.1.1.2
1138: .Ed
1139: .Pp
1140: On the server:
1141: .Bd -literal -offset indent
1142: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1143: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1144: .Ed
1145: .Pp
1146: Client access may be more finely tuned via the
1147: .Pa /root/.ssh/authorized_keys
1148: file (see below) and the
1149: .Cm PermitRootLogin
1150: server option.
1.255 jmc 1151: The following entry would permit connections on
1.250 jmc 1152: .Xr tun 4
1.255 jmc 1153: device 1 from user
1.250 jmc 1154: .Dq jane
1.255 jmc 1155: and on tun device 2 from user
1.250 jmc 1156: .Dq john ,
1157: if
1158: .Cm PermitRootLogin
1159: is set to
1160: .Dq forced-commands-only :
1161: .Bd -literal -offset 2n
1162: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1163: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1164: .Ed
1165: .Pp
1.264 ray 1166: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1167: it may be more suited to temporary setups,
1168: such as for wireless VPNs.
1169: More permanent VPNs are better provided by tools such as
1170: .Xr ipsecctl 8
1171: and
1172: .Xr isakmpd 8 .
1.2 deraadt 1173: .Sh ENVIRONMENT
1174: .Nm
1.1 deraadt 1175: will normally set the following environment variables:
1.237 jmc 1176: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1177: .It Ev DISPLAY
1178: The
1179: .Ev DISPLAY
1.40 aaron 1180: variable indicates the location of the X11 server.
1.44 aaron 1181: It is automatically set by
1.2 deraadt 1182: .Nm
1183: to point to a value of the form
1.233 jmc 1184: .Dq hostname:n ,
1185: where
1186: .Dq hostname
1187: indicates the host where the shell runs, and
1188: .Sq n
1189: is an integer \*(Ge 1.
1.40 aaron 1190: .Nm
1191: uses this special value to forward X11 connections over the secure
1192: channel.
1.107 markus 1193: The user should normally not set
1194: .Ev DISPLAY
1195: explicitly, as that
1.1 deraadt 1196: will render the X11 connection insecure (and will require the user to
1197: manually copy any required authorization cookies).
1.2 deraadt 1198: .It Ev HOME
1.1 deraadt 1199: Set to the path of the user's home directory.
1.2 deraadt 1200: .It Ev LOGNAME
1201: Synonym for
1.12 aaron 1202: .Ev USER ;
1203: set for compatibility with systems that use this variable.
1.2 deraadt 1204: .It Ev MAIL
1.129 stevesk 1205: Set to the path of the user's mailbox.
1.40 aaron 1206: .It Ev PATH
1.2 deraadt 1207: Set to the default
1208: .Ev PATH ,
1209: as specified when compiling
1.234 jmc 1210: .Nm .
1.118 markus 1211: .It Ev SSH_ASKPASS
1212: If
1213: .Nm
1214: needs a passphrase, it will read the passphrase from the current
1215: terminal if it was run from a terminal.
1216: If
1217: .Nm
1218: does not have a terminal associated with it but
1219: .Ev DISPLAY
1220: and
1221: .Ev SSH_ASKPASS
1222: are set, it will execute the program specified by
1223: .Ev SSH_ASKPASS
1224: and open an X11 window to read the passphrase.
1225: This is particularly useful when calling
1226: .Nm
1227: from a
1.196 jmc 1228: .Pa .xsession
1.118 markus 1229: or related script.
1230: (Note that on some machines it
1231: may be necessary to redirect the input from
1232: .Pa /dev/null
1233: to make this work.)
1.18 markus 1234: .It Ev SSH_AUTH_SOCK
1.233 jmc 1235: Identifies the path of a
1236: .Ux Ns -domain
1237: socket used to communicate with the agent.
1.166 stevesk 1238: .It Ev SSH_CONNECTION
1239: Identifies the client and server ends of the connection.
1.40 aaron 1240: The variable contains
1.233 jmc 1241: four space-separated values: client IP address, client port number,
1242: server IP address, and server port number.
1.73 markus 1243: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1244: This variable contains the original command line if a forced command
1.73 markus 1245: is executed.
1246: It can be used to extract the original arguments.
1.2 deraadt 1247: .It Ev SSH_TTY
1.1 deraadt 1248: This is set to the name of the tty (path to the device) associated
1.40 aaron 1249: with the current shell or command.
1250: If the current session has no tty,
1.1 deraadt 1251: this variable is not set.
1.2 deraadt 1252: .It Ev TZ
1.214 jmc 1253: This variable is set to indicate the present time zone if it
1.257 jmc 1254: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1255: on to new connections).
1.2 deraadt 1256: .It Ev USER
1.1 deraadt 1257: Set to the name of the user logging in.
1.2 deraadt 1258: .El
1259: .Pp
1.44 aaron 1260: Additionally,
1.2 deraadt 1261: .Nm
1.44 aaron 1262: reads
1.207 djm 1263: .Pa ~/.ssh/environment ,
1.2 deraadt 1264: and adds lines of the format
1265: .Dq VARNAME=value
1.233 jmc 1266: to the environment if the file exists and users are allowed to
1.161 marc 1267: change their environment.
1.176 jmc 1268: For more information, see the
1.161 marc 1269: .Cm PermitUserEnvironment
1.162 stevesk 1270: option in
1.161 marc 1271: .Xr sshd_config 5 .
1.2 deraadt 1272: .Sh FILES
1.236 jmc 1273: .Bl -tag -width Ds -compact
1274: .It ~/.rhosts
1.240 jmc 1275: This file is used for host-based authentication (see above).
1.92 markus 1276: On some machines this file may need to be
1.240 jmc 1277: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1278: because
1.2 deraadt 1279: .Xr sshd 8
1.40 aaron 1280: reads it as root.
1281: Additionally, this file must be owned by the user,
1282: and must not have write permissions for anyone else.
1283: The recommended
1.1 deraadt 1284: permission for most machines is read/write for the user, and not
1285: accessible by others.
1.2 deraadt 1286: .Pp
1.236 jmc 1287: .It ~/.shosts
1.240 jmc 1288: This file is used in exactly the same way as
1289: .Pa .rhosts ,
1290: but allows host-based authentication without permitting login with
1291: rlogin/rsh.
1.272 mcbride 1292: .Pp
1293: .It ~/.ssh/
1294: This directory is the default location for all user-specific configuration
1295: and authentication information.
1296: There is no general requirement to keep the entire contents of this directory
1297: secret, but the recommended permissions are read/write/execute for the user,
1298: and not accessible by others.
1.236 jmc 1299: .Pp
1.238 jmc 1300: .It ~/.ssh/authorized_keys
1301: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1302: The format of this file is described in the
1303: .Xr sshd 8
1304: manual page.
1305: This file is not highly sensitive, but the recommended
1306: permissions are read/write for the user, and not accessible by others.
1307: .Pp
1308: .It ~/.ssh/config
1309: This is the per-user configuration file.
1310: The file format and configuration options are described in
1311: .Xr ssh_config 5 .
1312: Because of the potential for abuse, this file must have strict permissions:
1313: read/write for the user, and not accessible by others.
1314: .Pp
1315: .It ~/.ssh/environment
1.239 jmc 1316: Contains additional definitions for environment variables; see
1317: .Sx ENVIRONMENT ,
1.238 jmc 1318: above.
1319: .Pp
1320: .It ~/.ssh/identity
1321: .It ~/.ssh/id_dsa
1322: .It ~/.ssh/id_rsa
1323: Contains the private key for authentication.
1324: These files
1325: contain sensitive data and should be readable by the user but not
1326: accessible by others (read/write/execute).
1327: .Nm
1328: will simply ignore a private key file if it is accessible by others.
1329: It is possible to specify a passphrase when
1330: generating the key which will be used to encrypt the
1331: sensitive part of this file using 3DES.
1332: .Pp
1333: .It ~/.ssh/identity.pub
1334: .It ~/.ssh/id_dsa.pub
1335: .It ~/.ssh/id_rsa.pub
1336: Contains the public key for authentication.
1337: These files are not
1338: sensitive and can (but need not) be readable by anyone.
1339: .Pp
1340: .It ~/.ssh/known_hosts
1.244 jmc 1341: Contains a list of host keys for all hosts the user has logged into
1342: that are not already in the systemwide list of known host keys.
1.238 jmc 1343: See
1.244 jmc 1344: .Xr sshd 8
1345: for further details of the format of this file.
1.238 jmc 1346: .Pp
1347: .It ~/.ssh/rc
1348: Commands in this file are executed by
1349: .Nm
1.245 jmc 1350: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1351: started.
1352: See the
1353: .Xr sshd 8
1354: manual page for more information.
1355: .Pp
1.236 jmc 1356: .It /etc/hosts.equiv
1.240 jmc 1357: This file is for host-based authentication (see above).
1358: It should only be writable by root.
1.236 jmc 1359: .Pp
1360: .It /etc/shosts.equiv
1.240 jmc 1361: This file is used in exactly the same way as
1362: .Pa hosts.equiv ,
1363: but allows host-based authentication without permitting login with
1364: rlogin/rsh.
1.236 jmc 1365: .Pp
1.238 jmc 1366: .It Pa /etc/ssh/ssh_config
1367: Systemwide configuration file.
1368: The file format and configuration options are described in
1369: .Xr ssh_config 5 .
1370: .Pp
1371: .It /etc/ssh/ssh_host_key
1372: .It /etc/ssh/ssh_host_dsa_key
1373: .It /etc/ssh/ssh_host_rsa_key
1374: These three files contain the private parts of the host keys
1.245 jmc 1375: and are used for host-based authentication.
1376: If protocol version 1 is used,
1.238 jmc 1377: .Nm
1378: must be setuid root, since the host key is readable only by root.
1379: For protocol version 2,
1380: .Nm
1381: uses
1382: .Xr ssh-keysign 8
1.245 jmc 1383: to access the host keys,
1384: eliminating the requirement that
1.238 jmc 1385: .Nm
1.245 jmc 1386: be setuid root when host-based authentication is used.
1.238 jmc 1387: By default
1.2 deraadt 1388: .Nm
1.238 jmc 1389: is not setuid root.
1390: .Pp
1391: .It /etc/ssh/ssh_known_hosts
1392: Systemwide list of known host keys.
1393: This file should be prepared by the
1394: system administrator to contain the public host keys of all machines in the
1395: organization.
1.244 jmc 1396: It should be world-readable.
1397: See
1.238 jmc 1398: .Xr sshd 8
1.244 jmc 1399: for further details of the format of this file.
1.236 jmc 1400: .Pp
1.238 jmc 1401: .It /etc/ssh/sshrc
1.1 deraadt 1402: Commands in this file are executed by
1.2 deraadt 1403: .Nm
1.245 jmc 1404: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1405: See the
1.2 deraadt 1406: .Xr sshd 8
1.1 deraadt 1407: manual page for more information.
1.58 itojun 1408: .El
1.2 deraadt 1409: .Sh SEE ALSO
1410: .Xr scp 1 ,
1.83 djm 1411: .Xr sftp 1 ,
1.2 deraadt 1412: .Xr ssh-add 1 ,
1413: .Xr ssh-agent 1 ,
1414: .Xr ssh-keygen 1 ,
1.242 jmc 1415: .Xr ssh-keyscan 1 ,
1.250 jmc 1416: .Xr tun 4 ,
1.176 jmc 1417: .Xr hosts.equiv 5 ,
1.159 stevesk 1418: .Xr ssh_config 5 ,
1.160 naddy 1419: .Xr ssh-keysign 8 ,
1.87 itojun 1420: .Xr sshd 8
1.106 markus 1421: .Rs
1.256 jmc 1422: .%R RFC 4250
1423: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1424: .%D 2006
1425: .Re
1426: .Rs
1427: .%R RFC 4251
1428: .%T "The Secure Shell (SSH) Protocol Architecture"
1429: .%D 2006
1430: .Re
1431: .Rs
1432: .%R RFC 4252
1433: .%T "The Secure Shell (SSH) Authentication Protocol"
1434: .%D 2006
1435: .Re
1436: .Rs
1437: .%R RFC 4253
1438: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1439: .%D 2006
1440: .Re
1441: .Rs
1442: .%R RFC 4254
1443: .%T "The Secure Shell (SSH) Connection Protocol"
1444: .%D 2006
1445: .Re
1446: .Rs
1447: .%R RFC 4255
1448: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1449: .%D 2006
1450: .Re
1451: .Rs
1452: .%R RFC 4256
1453: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1454: .%D 2006
1455: .Re
1456: .Rs
1457: .%R RFC 4335
1458: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1459: .%D 2006
1460: .Re
1461: .Rs
1462: .%R RFC 4344
1463: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1464: .%D 2006
1465: .Re
1466: .Rs
1467: .%R RFC 4345
1468: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1469: .%D 2006
1470: .Re
1471: .Rs
1472: .%R RFC 4419
1473: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1474: .%D 2006
1475: .Re
1476: .Rs
1477: .%R RFC 4716
1478: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1479: .%D 2006
1.274 grunk 1480: .Re
1481: .Rs
1482: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1483: .%A A. Perrig
1484: .%A D. Song
1485: .%D 1999
1486: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1487: .Re
1.173 jmc 1488: .Sh AUTHORS
1489: OpenSSH is a derivative of the original and free
1490: ssh 1.2.12 release by Tatu Ylonen.
1491: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1492: Theo de Raadt and Dug Song
1493: removed many bugs, re-added newer features and
1494: created OpenSSH.
1495: Markus Friedl contributed the support for SSH
1496: protocol versions 1.5 and 2.0.