Annotation of src/usr.bin/ssh/ssh.1, Revision 1.316
1.1 deraadt 1: .\"
2: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4: .\" All rights reserved
5: .\"
1.59 deraadt 6: .\" As far as I am concerned, the code I have written for this software
7: .\" can be used freely for any purpose. Any derived versions of this
8: .\" software must be clearly marked as such, and if the derived work is
9: .\" incompatible with the protocol description in the RFC file, it must be
10: .\" called by a name other than "ssh" or "Secure Shell".
11: .\"
1.93 deraadt 12: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.59 deraadt 15: .\"
16: .\" Redistribution and use in source and binary forms, with or without
17: .\" modification, are permitted provided that the following conditions
18: .\" are met:
19: .\" 1. Redistributions of source code must retain the above copyright
20: .\" notice, this list of conditions and the following disclaimer.
21: .\" 2. Redistributions in binary form must reproduce the above copyright
22: .\" notice, this list of conditions and the following disclaimer in the
23: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 24: .\"
1.59 deraadt 25: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 35: .\"
1.316 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.315 2010/10/28 18:33:28 jmc Exp $
! 37: .Dd $Mdocdate: October 28 2010 $
1.2 deraadt 38: .Dt SSH 1
39: .Os
40: .Sh NAME
41: .Nm ssh
1.96 deraadt 42: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 43: .Sh SYNOPSIS
44: .Nm ssh
1.306 jmc 45: .Bk -words
1.278 djm 46: .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
1.108 markus 47: .Op Fl b Ar bind_address
1.51 markus 48: .Op Fl c Ar cipher_spec
1.306 jmc 49: .Op Fl D Oo Ar bind_address : Oc Ns Ar port
1.2 deraadt 50: .Op Fl e Ar escape_char
1.176 jmc 51: .Op Fl F Ar configfile
1.292 jmc 52: .Op Fl I Ar pkcs11
1.2 deraadt 53: .Op Fl i Ar identity_file
1.306 jmc 54: .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.176 jmc 55: .Op Fl l Ar login_name
56: .Op Fl m Ar mac_spec
1.198 djm 57: .Op Fl O Ar ctl_cmd
1.176 jmc 58: .Op Fl o Ar option
59: .Op Fl p Ar port
1.306 jmc 60: .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.198 djm 61: .Op Fl S Ar ctl_path
1.290 dtucker 62: .Op Fl W Ar host : Ns Ar port
1.306 jmc 63: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.176 jmc 64: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 65: .Op Ar command
1.306 jmc 66: .Ek
1.44 aaron 67: .Sh DESCRIPTION
1.2 deraadt 68: .Nm
1.96 deraadt 69: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 70: executing commands on a remote machine.
1.176 jmc 71: It is intended to replace rlogin and rsh,
72: and provide secure encrypted communications between
1.40 aaron 73: two untrusted hosts over an insecure network.
1.247 jmc 74: X11 connections and arbitrary TCP ports
1.176 jmc 75: can also be forwarded over the secure channel.
1.2 deraadt 76: .Pp
77: .Nm
1.44 aaron 78: connects and logs into the specified
1.176 jmc 79: .Ar hostname
80: (with optional
81: .Ar user
82: name).
1.1 deraadt 83: The user must prove
1.49 markus 84: his/her identity to the remote machine using one of several methods
1.221 jmc 85: depending on the protocol version used (see below).
1.49 markus 86: .Pp
1.176 jmc 87: If
88: .Ar command
89: is specified,
1.219 jmc 90: it is executed on the remote host instead of a login shell.
1.2 deraadt 91: .Pp
1.218 jmc 92: The options are as follows:
93: .Bl -tag -width Ds
94: .It Fl 1
95: Forces
1.2 deraadt 96: .Nm
1.218 jmc 97: to try protocol version 1 only.
98: .It Fl 2
99: Forces
1.2 deraadt 100: .Nm
1.218 jmc 101: to try protocol version 2 only.
102: .It Fl 4
103: Forces
1.2 deraadt 104: .Nm
1.218 jmc 105: to use IPv4 addresses only.
106: .It Fl 6
107: Forces
1.2 deraadt 108: .Nm
1.218 jmc 109: to use IPv6 addresses only.
110: .It Fl A
111: Enables forwarding of the authentication agent connection.
112: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 113: .Pp
1.218 jmc 114: Agent forwarding should be enabled with caution.
115: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 116: (for the agent's
1.286 sobrado 117: .Ux Ns -domain
118: socket) can access the local agent through the forwarded connection.
1.218 jmc 119: An attacker cannot obtain key material from the agent,
120: however they can perform operations on the keys that enable them to
121: authenticate using the identities loaded into the agent.
122: .It Fl a
123: Disables forwarding of the authentication agent connection.
124: .It Fl b Ar bind_address
125: Use
126: .Ar bind_address
127: on the local machine as the source address
128: of the connection.
129: Only useful on systems with more than one address.
130: .It Fl C
131: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 132: data for forwarded X11 and TCP connections).
1.218 jmc 133: The compression algorithm is the same used by
134: .Xr gzip 1 ,
135: and the
136: .Dq level
137: can be controlled by the
138: .Cm CompressionLevel
139: option for protocol version 1.
140: Compression is desirable on modem lines and other
141: slow connections, but will only slow down things on fast networks.
142: The default value can be set on a host-by-host basis in the
143: configuration files; see the
144: .Cm Compression
145: option.
146: .It Fl c Ar cipher_spec
147: Selects the cipher specification for encrypting the session.
1.2 deraadt 148: .Pp
1.218 jmc 149: Protocol version 1 allows specification of a single cipher.
150: The supported values are
151: .Dq 3des ,
1.220 jmc 152: .Dq blowfish ,
1.218 jmc 153: and
154: .Dq des .
155: .Ar 3des
156: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
157: It is believed to be secure.
158: .Ar blowfish
159: is a fast block cipher; it appears very secure and is much faster than
160: .Ar 3des .
161: .Ar des
162: is only supported in the
1.2 deraadt 163: .Nm
1.218 jmc 164: client for interoperability with legacy protocol 1 implementations
165: that do not support the
166: .Ar 3des
167: cipher.
168: Its use is strongly discouraged due to cryptographic weaknesses.
169: The default is
170: .Dq 3des .
1.49 markus 171: .Pp
1.230 jmc 172: For protocol version 2,
1.218 jmc 173: .Ar cipher_spec
174: is a comma-separated list of ciphers
175: listed in order of preference.
1.283 jmc 176: See the
177: .Cm Ciphers
1.307 dtucker 178: keyword in
179: .Xr ssh_config 5
180: for more information.
1.218 jmc 181: .It Fl D Xo
182: .Sm off
183: .Oo Ar bind_address : Oc
184: .Ar port
185: .Sm on
186: .Xc
187: Specifies a local
188: .Dq dynamic
189: application-level port forwarding.
190: This works by allocating a socket to listen to
191: .Ar port
192: on the local side, optionally bound to the specified
193: .Ar bind_address .
194: Whenever a connection is made to this port, the
195: connection is forwarded over the secure channel, and the application
196: protocol is then used to determine where to connect to from the
197: remote machine.
198: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 199: .Nm
1.218 jmc 200: will act as a SOCKS server.
201: Only root can forward privileged ports.
202: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 203: .Pp
1.308 djm 204: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 205: Only the superuser can forward privileged ports.
206: By default, the local port is bound in accordance with the
207: .Cm GatewayPorts
208: setting.
209: However, an explicit
210: .Ar bind_address
211: may be used to bind the connection to a specific address.
212: The
213: .Ar bind_address
214: of
215: .Dq localhost
216: indicates that the listening port be bound for local use only, while an
217: empty address or
218: .Sq *
219: indicates that the port should be available from all interfaces.
1.229 jmc 220: .It Fl e Ar escape_char
1.218 jmc 221: Sets the escape character for sessions with a pty (default:
222: .Ql ~ ) .
223: The escape character is only recognized at the beginning of a line.
224: The escape character followed by a dot
225: .Pq Ql \&.
226: closes the connection;
227: followed by control-Z suspends the connection;
228: and followed by itself sends the escape character once.
229: Setting the character to
1.2 deraadt 230: .Dq none
1.218 jmc 231: disables any escapes and makes the session fully transparent.
232: .It Fl F Ar configfile
233: Specifies an alternative per-user configuration file.
234: If a configuration file is given on the command line,
235: the system-wide configuration file
236: .Pq Pa /etc/ssh/ssh_config
237: will be ignored.
238: The default for the per-user configuration file is
239: .Pa ~/.ssh/config .
240: .It Fl f
241: Requests
242: .Nm
243: to go to background just before command execution.
244: This is useful if
1.176 jmc 245: .Nm
1.218 jmc 246: is going to ask for passwords or passphrases, but the user
247: wants it in the background.
248: This implies
249: .Fl n .
250: The recommended way to start X11 programs at a remote site is with
251: something like
252: .Ic ssh -f host xterm .
1.277 djm 253: .Pp
254: If the
255: .Cm ExitOnForwardFailure
256: configuration option is set to
257: .Dq yes ,
258: then a client started with
259: .Fl f
260: will wait for all remote port forwards to be successfully established
261: before placing itself in the background.
1.218 jmc 262: .It Fl g
263: Allows remote hosts to connect to local forwarded ports.
1.291 markus 264: .It Fl I Ar pkcs11
1.294 jmc 265: Specify the PKCS#11 shared library
1.176 jmc 266: .Nm
1.293 markus 267: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 268: private RSA key.
269: .It Fl i Ar identity_file
270: Selects a file from which the identity (private key) for
1.310 djm 271: public key authentication is read.
1.218 jmc 272: The default is
273: .Pa ~/.ssh/identity
274: for protocol version 1, and
1.310 djm 275: .Pa ~/.ssh/id_dsa ,
276: .Pa ~/.ssh/id_ecdsa
277: and
1.218 jmc 278: .Pa ~/.ssh/id_rsa
279: for protocol version 2.
280: Identity files may also be specified on
281: a per-host basis in the configuration file.
282: It is possible to have multiple
283: .Fl i
284: options (and multiple identities specified in
285: configuration files).
1.302 djm 286: .Nm
287: will also try to load certificate information from the filename obtained
288: by appending
289: .Pa -cert.pub
290: to identity filenames.
1.269 djm 291: .It Fl K
292: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
293: credentials to the server.
1.218 jmc 294: .It Fl k
295: Disables forwarding (delegation) of GSSAPI credentials to the server.
296: .It Fl L Xo
297: .Sm off
298: .Oo Ar bind_address : Oc
299: .Ar port : host : hostport
300: .Sm on
301: .Xc
302: Specifies that the given port on the local (client) host is to be
303: forwarded to the given host and port on the remote side.
304: This works by allocating a socket to listen to
305: .Ar port
306: on the local side, optionally bound to the specified
307: .Ar bind_address .
308: Whenever a connection is made to this port, the
309: connection is forwarded over the secure channel, and a connection is
310: made to
311: .Ar host
312: port
313: .Ar hostport
314: from the remote machine.
315: Port forwardings can also be specified in the configuration file.
1.308 djm 316: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 317: Only the superuser can forward privileged ports.
318: By default, the local port is bound in accordance with the
319: .Cm GatewayPorts
320: setting.
321: However, an explicit
322: .Ar bind_address
323: may be used to bind the connection to a specific address.
1.2 deraadt 324: The
1.218 jmc 325: .Ar bind_address
326: of
327: .Dq localhost
328: indicates that the listening port be bound for local use only, while an
329: empty address or
330: .Sq *
331: indicates that the port should be available from all interfaces.
332: .It Fl l Ar login_name
333: Specifies the user to log in as on the remote machine.
334: This also may be specified on a per-host basis in the configuration file.
335: .It Fl M
336: Places the
337: .Nm
338: client into
339: .Dq master
340: mode for connection sharing.
1.231 stevesk 341: Multiple
342: .Fl M
343: options places
344: .Nm
345: into
346: .Dq master
347: mode with confirmation required before slave connections are accepted.
1.218 jmc 348: Refer to the description of
349: .Cm ControlMaster
350: in
351: .Xr ssh_config 5
352: for details.
353: .It Fl m Ar mac_spec
354: Additionally, for protocol version 2 a comma-separated list of MAC
355: (message authentication code) algorithms can
356: be specified in order of preference.
357: See the
358: .Cm MACs
359: keyword for more information.
360: .It Fl N
361: Do not execute a remote command.
362: This is useful for just forwarding ports
363: (protocol version 2 only).
364: .It Fl n
365: Redirects stdin from
366: .Pa /dev/null
367: (actually, prevents reading from stdin).
368: This must be used when
1.2 deraadt 369: .Nm
1.218 jmc 370: is run in the background.
371: A common trick is to use this to run X11 programs on a remote machine.
372: For example,
373: .Ic ssh -n shadows.cs.hut.fi emacs &
374: will start an emacs on shadows.cs.hut.fi, and the X11
375: connection will be automatically forwarded over an encrypted channel.
376: The
1.2 deraadt 377: .Nm
1.218 jmc 378: program will be put in the background.
379: (This does not work if
1.2 deraadt 380: .Nm
1.218 jmc 381: needs to ask for a password or passphrase; see also the
382: .Fl f
383: option.)
384: .It Fl O Ar ctl_cmd
385: Control an active connection multiplexing master process.
386: When the
387: .Fl O
388: option is specified, the
389: .Ar ctl_cmd
390: argument is interpreted and passed to the master process.
391: Valid commands are:
392: .Dq check
1.305 markus 393: (check that the master process is running),
394: .Dq forward
395: (request forwardings without command execution) and
1.218 jmc 396: .Dq exit
397: (request the master to exit).
398: .It Fl o Ar option
399: Can be used to give options in the format used in the configuration file.
400: This is useful for specifying options for which there is no separate
401: command-line flag.
402: For full details of the options listed below, and their possible values, see
403: .Xr ssh_config 5 .
1.2 deraadt 404: .Pp
1.218 jmc 405: .Bl -tag -width Ds -offset indent -compact
406: .It AddressFamily
407: .It BatchMode
408: .It BindAddress
409: .It ChallengeResponseAuthentication
410: .It CheckHostIP
411: .It Cipher
412: .It Ciphers
413: .It ClearAllForwardings
414: .It Compression
415: .It CompressionLevel
416: .It ConnectionAttempts
417: .It ConnectTimeout
418: .It ControlMaster
419: .It ControlPath
420: .It DynamicForward
421: .It EscapeChar
1.263 markus 422: .It ExitOnForwardFailure
1.218 jmc 423: .It ForwardAgent
424: .It ForwardX11
425: .It ForwardX11Trusted
426: .It GatewayPorts
427: .It GlobalKnownHostsFile
428: .It GSSAPIAuthentication
429: .It GSSAPIDelegateCredentials
430: .It HashKnownHosts
431: .It Host
432: .It HostbasedAuthentication
433: .It HostKeyAlgorithms
434: .It HostKeyAlias
435: .It HostName
436: .It IdentityFile
437: .It IdentitiesOnly
1.316 ! jmc 438: .It IPQoS
1.218 jmc 439: .It KbdInteractiveDevices
1.314 jmc 440: .It KexAlgorithms
1.218 jmc 441: .It LocalCommand
442: .It LocalForward
443: .It LogLevel
444: .It MACs
445: .It NoHostAuthenticationForLocalhost
446: .It NumberOfPasswordPrompts
447: .It PasswordAuthentication
448: .It PermitLocalCommand
1.291 markus 449: .It PKCS11Provider
1.218 jmc 450: .It Port
451: .It PreferredAuthentications
452: .It Protocol
453: .It ProxyCommand
454: .It PubkeyAuthentication
1.251 dtucker 455: .It RekeyLimit
1.218 jmc 456: .It RemoteForward
457: .It RhostsRSAAuthentication
458: .It RSAAuthentication
459: .It SendEnv
460: .It ServerAliveInterval
461: .It ServerAliveCountMax
462: .It StrictHostKeyChecking
463: .It TCPKeepAlive
464: .It Tunnel
465: .It TunnelDevice
466: .It UsePrivilegedPort
467: .It User
468: .It UserKnownHostsFile
469: .It VerifyHostKeyDNS
1.276 jmc 470: .It VisualHostKey
1.218 jmc 471: .It XAuthLocation
472: .El
473: .It Fl p Ar port
474: Port to connect to on the remote host.
475: This can be specified on a
476: per-host basis in the configuration file.
477: .It Fl q
478: Quiet mode.
1.271 djm 479: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 480: .It Fl R Xo
481: .Sm off
482: .Oo Ar bind_address : Oc
483: .Ar port : host : hostport
484: .Sm on
485: .Xc
486: Specifies that the given port on the remote (server) host is to be
487: forwarded to the given host and port on the local side.
488: This works by allocating a socket to listen to
489: .Ar port
490: on the remote side, and whenever a connection is made to this port, the
491: connection is forwarded over the secure channel, and a connection is
492: made to
493: .Ar host
494: port
495: .Ar hostport
496: from the local machine.
1.2 deraadt 497: .Pp
1.218 jmc 498: Port forwardings can also be specified in the configuration file.
499: Privileged ports can be forwarded only when
500: logging in as root on the remote machine.
1.308 djm 501: IPv6 addresses can be specified by enclosing the address in square braces.
1.194 jakob 502: .Pp
1.218 jmc 503: By default, the listening socket on the server will be bound to the loopback
504: interface only.
1.280 tobias 505: This may be overridden by specifying a
1.218 jmc 506: .Ar bind_address .
507: An empty
508: .Ar bind_address ,
509: or the address
510: .Ql * ,
511: indicates that the remote socket should listen on all interfaces.
512: Specifying a remote
513: .Ar bind_address
514: will only succeed if the server's
515: .Cm GatewayPorts
516: option is enabled (see
517: .Xr sshd_config 5 ) .
1.281 djm 518: .Pp
519: If the
520: .Ar port
521: argument is
1.282 djm 522: .Ql 0 ,
1.281 djm 523: the listen port will be dynamically allocated on the server and reported
524: to the client at run time.
1.305 markus 525: When used together with
526: .Ic -O forward
527: the allocated port will be printed to the standard output.
1.218 jmc 528: .It Fl S Ar ctl_path
1.304 jmc 529: Specifies the location of a control socket for connection sharing,
1.303 djm 530: or the string
531: .Dq none
532: to disable connection sharing.
1.218 jmc 533: Refer to the description of
534: .Cm ControlPath
535: and
536: .Cm ControlMaster
537: in
538: .Xr ssh_config 5
539: for details.
540: .It Fl s
541: May be used to request invocation of a subsystem on the remote system.
542: Subsystems are a feature of the SSH2 protocol which facilitate the use
543: of SSH as a secure transport for other applications (eg.\&
544: .Xr sftp 1 ) .
545: The subsystem is specified as the remote command.
546: .It Fl T
547: Disable pseudo-tty allocation.
548: .It Fl t
549: Force pseudo-tty allocation.
550: This can be used to execute arbitrary
551: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 552: e.g. when implementing menu services.
1.218 jmc 553: Multiple
554: .Fl t
555: options force tty allocation, even if
1.194 jakob 556: .Nm
1.218 jmc 557: has no local tty.
558: .It Fl V
559: Display the version number and exit.
560: .It Fl v
561: Verbose mode.
562: Causes
1.176 jmc 563: .Nm
1.218 jmc 564: to print debugging messages about its progress.
565: This is helpful in
566: debugging connection, authentication, and configuration problems.
567: Multiple
568: .Fl v
569: options increase the verbosity.
570: The maximum is 3.
1.290 dtucker 571: .It Fl W Ar host : Ns Ar port
572: Requests that standard input and output on the client be forwarded to
573: .Ar host
574: on
575: .Ar port
576: over the secure channel.
577: Implies
578: .Fl N ,
579: .Fl T ,
580: .Cm ExitOnForwardFailure
581: and
582: .Cm ClearAllForwardings
583: and works with Protocol version 2 only.
1.261 stevesk 584: .It Fl w Xo
585: .Ar local_tun Ns Op : Ns Ar remote_tun
586: .Xc
587: Requests
588: tunnel
589: device forwarding with the specified
1.218 jmc 590: .Xr tun 4
1.261 stevesk 591: devices between the client
592: .Pq Ar local_tun
593: and the server
594: .Pq Ar remote_tun .
595: .Pp
1.228 jmc 596: The devices may be specified by numerical ID or the keyword
597: .Dq any ,
598: which uses the next available tunnel device.
1.261 stevesk 599: If
600: .Ar remote_tun
601: is not specified, it defaults to
602: .Dq any .
1.228 jmc 603: See also the
1.218 jmc 604: .Cm Tunnel
1.261 stevesk 605: and
606: .Cm TunnelDevice
607: directives in
1.218 jmc 608: .Xr ssh_config 5 .
1.261 stevesk 609: If the
610: .Cm Tunnel
611: directive is unset, it is set to the default tunnel mode, which is
612: .Dq point-to-point .
1.218 jmc 613: .It Fl X
614: Enables X11 forwarding.
1.54 markus 615: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 616: .Pp
1.218 jmc 617: X11 forwarding should be enabled with caution.
1.168 jmc 618: Users with the ability to bypass file permissions on the remote host
1.218 jmc 619: (for the user's X authorization database)
620: can access the local X11 display through the forwarded connection.
621: An attacker may then be able to perform activities such as keystroke monitoring.
622: .Pp
623: For this reason, X11 forwarding is subjected to X11 SECURITY extension
624: restrictions by default.
625: Please refer to the
626: .Nm
627: .Fl Y
628: option and the
629: .Cm ForwardX11Trusted
630: directive in
631: .Xr ssh_config 5
632: for more information.
633: .It Fl x
634: Disables X11 forwarding.
635: .It Fl Y
636: Enables trusted X11 forwarding.
637: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
638: controls.
1.278 djm 639: .It Fl y
640: Send log information using the
641: .Xr syslog 3
642: system module.
643: By default this information is sent to stderr.
1.218 jmc 644: .El
1.224 jmc 645: .Pp
646: .Nm
647: may additionally obtain configuration data from
648: a per-user configuration file and a system-wide configuration file.
649: The file format and configuration options are described in
650: .Xr ssh_config 5 .
1.222 jmc 651: .Sh AUTHENTICATION
1.249 jmc 652: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 653: The default is to use protocol 2 only,
654: though this can be changed via the
1.222 jmc 655: .Cm Protocol
656: option in
1.284 jmc 657: .Xr ssh_config 5
658: or the
1.222 jmc 659: .Fl 1
660: and
661: .Fl 2
662: options (see above).
663: Both protocols support similar authentication methods,
1.284 jmc 664: but protocol 2 is the default since
1.222 jmc 665: it provides additional mechanisms for confidentiality
666: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 667: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 668: Protocol 1 lacks a strong mechanism for ensuring the
669: integrity of the connection.
670: .Pp
671: The methods available for authentication are:
1.260 jmc 672: GSSAPI-based authentication,
1.222 jmc 673: host-based authentication,
674: public key authentication,
675: challenge-response authentication,
676: and password authentication.
677: Authentication methods are tried in the order specified above,
678: though protocol 2 has a configuration option to change the default order:
679: .Cm PreferredAuthentications .
680: .Pp
681: Host-based authentication works as follows:
1.218 jmc 682: If the machine the user logs in from is listed in
683: .Pa /etc/hosts.equiv
684: or
685: .Pa /etc/shosts.equiv
686: on the remote machine, and the user names are
687: the same on both sides, or if the files
688: .Pa ~/.rhosts
689: or
690: .Pa ~/.shosts
691: exist in the user's home directory on the
692: remote machine and contain a line containing the name of the client
693: machine and the name of the user on that machine, the user is
1.222 jmc 694: considered for login.
695: Additionally, the server
696: .Em must
697: be able to verify the client's
698: host key (see the description of
1.218 jmc 699: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 700: and
1.222 jmc 701: .Pa ~/.ssh/known_hosts ,
702: below)
703: for login to be permitted.
1.218 jmc 704: This authentication method closes security holes due to IP
1.222 jmc 705: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 706: [Note to the administrator:
707: .Pa /etc/hosts.equiv ,
708: .Pa ~/.rhosts ,
709: and the rlogin/rsh protocol in general, are inherently insecure and should be
710: disabled if security is desired.]
1.189 dtucker 711: .Pp
1.222 jmc 712: Public key authentication works as follows:
713: The scheme is based on public-key cryptography,
714: using cryptosystems
715: where encryption and decryption are done using separate keys,
716: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 717: The idea is that each user creates a public/private
718: key pair for authentication purposes.
719: The server knows the public key, and only the user knows the private key.
1.222 jmc 720: .Nm
721: implements public key authentication protocol automatically,
1.310 djm 722: using one of the DSA, ECDSA or RSA algorithms.
1.222 jmc 723: Protocol 1 is restricted to using only RSA keys,
1.310 djm 724: but protocol 2 may use any.
1.222 jmc 725: The
726: .Sx HISTORY
727: section of
728: .Xr ssl 8
1.311 jmc 729: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 730: .Pp
1.218 jmc 731: The file
732: .Pa ~/.ssh/authorized_keys
733: lists the public keys that are permitted for logging in.
734: When the user logs in, the
1.2 deraadt 735: .Nm
1.218 jmc 736: program tells the server which key pair it would like to use for
737: authentication.
1.222 jmc 738: The client proves that it has access to the private key
739: and the server checks that the corresponding public key
740: is authorized to accept the account.
1.218 jmc 741: .Pp
1.222 jmc 742: The user creates his/her key pair by running
1.218 jmc 743: .Xr ssh-keygen 1 .
744: This stores the private key in
1.207 djm 745: .Pa ~/.ssh/identity
1.222 jmc 746: (protocol 1),
747: .Pa ~/.ssh/id_dsa
748: (protocol 2 DSA),
1.310 djm 749: .Pa ~/.ssh/id_ecdsa
750: (protocol 2 ECDSA),
1.222 jmc 751: or
752: .Pa ~/.ssh/id_rsa
753: (protocol 2 RSA)
1.218 jmc 754: and stores the public key in
755: .Pa ~/.ssh/identity.pub
1.222 jmc 756: (protocol 1),
757: .Pa ~/.ssh/id_dsa.pub
758: (protocol 2 DSA),
1.310 djm 759: .Pa ~/.ssh/id_ecdsa.pub
760: (protocol 2 ECDSA),
1.222 jmc 761: or
762: .Pa ~/.ssh/id_rsa.pub
763: (protocol 2 RSA)
1.218 jmc 764: in the user's home directory.
1.222 jmc 765: The user should then copy the public key
1.218 jmc 766: to
767: .Pa ~/.ssh/authorized_keys
1.222 jmc 768: in his/her home directory on the remote machine.
769: The
1.218 jmc 770: .Pa authorized_keys
771: file corresponds to the conventional
772: .Pa ~/.rhosts
773: file, and has one key
1.222 jmc 774: per line, though the lines can be very long.
1.218 jmc 775: After this, the user can log in without giving the password.
776: .Pp
1.301 jmc 777: A variation on public key authentication
778: is available in the form of certificate authentication:
779: instead of a set of public/private keys,
780: signed certificates are used.
781: This has the advantage that a single trusted certification authority
782: can be used in place of many public/private keys.
783: See the
784: .Sx CERTIFICATES
785: section of
786: .Xr ssh-keygen 1
787: for more information.
788: .Pp
789: The most convenient way to use public key or certificate authentication
790: may be with an authentication agent.
1.218 jmc 791: See
792: .Xr ssh-agent 1
793: for more information.
794: .Pp
1.222 jmc 795: Challenge-response authentication works as follows:
796: The server sends an arbitrary
797: .Qq challenge
798: text, and prompts for a response.
799: Protocol 2 allows multiple challenges and responses;
800: protocol 1 is restricted to just one challenge/response.
801: Examples of challenge-response authentication include
802: BSD Authentication (see
803: .Xr login.conf 5 )
804: and PAM (some non-OpenBSD systems).
805: .Pp
806: Finally, if other authentication methods fail,
1.218 jmc 807: .Nm
808: prompts the user for a password.
809: The password is sent to the remote
810: host for checking; however, since all communications are encrypted,
811: the password cannot be seen by someone listening on the network.
1.232 jmc 812: .Pp
813: .Nm
814: automatically maintains and checks a database containing
815: identification for all hosts it has ever been used with.
816: Host keys are stored in
817: .Pa ~/.ssh/known_hosts
818: in the user's home directory.
819: Additionally, the file
820: .Pa /etc/ssh/ssh_known_hosts
821: is automatically checked for known hosts.
822: Any new hosts are automatically added to the user's file.
823: If a host's identification ever changes,
824: .Nm
825: warns about this and disables password authentication to prevent
826: server spoofing or man-in-the-middle attacks,
827: which could otherwise be used to circumvent the encryption.
828: The
829: .Cm StrictHostKeyChecking
830: option can be used to control logins to machines whose
831: host key is not known or has changed.
832: .Pp
1.218 jmc 833: When the user's identity has been accepted by the server, the server
834: either executes the given command, or logs into the machine and gives
835: the user a normal shell on the remote machine.
836: All communication with
837: the remote command or shell will be automatically encrypted.
838: .Pp
839: If a pseudo-terminal has been allocated (normal login session), the
840: user may use the escape characters noted below.
841: .Pp
842: If no pseudo-tty has been allocated,
843: the session is transparent and can be used to reliably transfer binary data.
844: On most systems, setting the escape character to
845: .Dq none
846: will also make the session transparent even if a tty is used.
847: .Pp
848: The session terminates when the command or shell on the remote
1.247 jmc 849: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 850: .Sh ESCAPE CHARACTERS
1.218 jmc 851: When a pseudo-terminal has been requested,
1.2 deraadt 852: .Nm
1.218 jmc 853: supports a number of functions through the use of an escape character.
854: .Pp
855: A single tilde character can be sent as
856: .Ic ~~
857: or by following the tilde by a character other than those described below.
858: The escape character must always follow a newline to be interpreted as
859: special.
860: The escape character can be changed in configuration files using the
861: .Cm EscapeChar
862: configuration directive or on the command line by the
863: .Fl e
864: option.
865: .Pp
866: The supported escapes (assuming the default
867: .Ql ~ )
868: are:
869: .Bl -tag -width Ds
870: .It Cm ~.
871: Disconnect.
872: .It Cm ~^Z
873: Background
1.234 jmc 874: .Nm .
1.218 jmc 875: .It Cm ~#
876: List forwarded connections.
877: .It Cm ~&
878: Background
1.2 deraadt 879: .Nm
1.218 jmc 880: at logout when waiting for forwarded connection / X11 sessions to terminate.
881: .It Cm ~?
882: Display a list of escape characters.
883: .It Cm ~B
884: Send a BREAK to the remote system
885: (only useful for SSH protocol version 2 and if the peer supports it).
886: .It Cm ~C
887: Open command line.
888: Currently this allows the addition of port forwardings using the
1.279 stevesk 889: .Fl L ,
890: .Fl R
1.218 jmc 891: and
1.279 stevesk 892: .Fl D
1.225 jmc 893: options (see above).
1.218 jmc 894: It also allows the cancellation of existing remote port-forwardings
895: using
1.262 stevesk 896: .Sm off
897: .Fl KR Oo Ar bind_address : Oc Ar port .
898: .Sm on
1.218 jmc 899: .Ic !\& Ns Ar command
900: allows the user to execute a local command if the
901: .Ic PermitLocalCommand
902: option is enabled in
1.176 jmc 903: .Xr ssh_config 5 .
1.218 jmc 904: Basic help is available, using the
905: .Fl h
906: option.
907: .It Cm ~R
908: Request rekeying of the connection
909: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 910: .El
1.246 jmc 911: .Sh TCP FORWARDING
912: Forwarding of arbitrary TCP connections over the secure channel can
913: be specified either on the command line or in a configuration file.
914: One possible application of TCP forwarding is a secure connection to a
915: mail server; another is going through firewalls.
916: .Pp
917: In the example below, we look at encrypting communication between
918: an IRC client and server, even though the IRC server does not directly
919: support encrypted communications.
920: This works as follows:
921: the user connects to the remote host using
922: .Nm ,
923: specifying a port to be used to forward connections
924: to the remote server.
925: After that it is possible to start the service which is to be encrypted
926: on the client machine,
927: connecting to the same local port,
928: and
929: .Nm
930: will encrypt and forward the connection.
931: .Pp
932: The following example tunnels an IRC session from client machine
933: .Dq 127.0.0.1
934: (localhost)
935: to remote server
936: .Dq server.example.com :
937: .Bd -literal -offset 4n
938: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
939: $ irc -c '#users' -p 1234 pinky 127.0.0.1
940: .Ed
941: .Pp
942: This tunnels a connection to IRC server
943: .Dq server.example.com ,
944: joining channel
945: .Dq #users ,
946: nickname
947: .Dq pinky ,
948: using port 1234.
949: It doesn't matter which port is used,
950: as long as it's greater than 1023
951: (remember, only root can open sockets on privileged ports)
952: and doesn't conflict with any ports already in use.
953: The connection is forwarded to port 6667 on the remote server,
954: since that's the standard port for IRC services.
955: .Pp
956: The
957: .Fl f
958: option backgrounds
959: .Nm
960: and the remote command
961: .Dq sleep 10
962: is specified to allow an amount of time
963: (10 seconds, in the example)
964: to start the service which is to be tunnelled.
965: If no connections are made within the time specified,
966: .Nm
967: will exit.
968: .Sh X11 FORWARDING
1.218 jmc 969: If the
970: .Cm ForwardX11
971: variable is set to
972: .Dq yes
973: (or see the description of the
1.227 jmc 974: .Fl X ,
975: .Fl x ,
1.218 jmc 976: and
1.227 jmc 977: .Fl Y
1.226 jmc 978: options above)
1.218 jmc 979: and the user is using X11 (the
980: .Ev DISPLAY
981: environment variable is set), the connection to the X11 display is
982: automatically forwarded to the remote side in such a way that any X11
983: programs started from the shell (or command) will go through the
984: encrypted channel, and the connection to the real X server will be made
1.176 jmc 985: from the local machine.
1.218 jmc 986: The user should not manually set
987: .Ev DISPLAY .
988: Forwarding of X11 connections can be
989: configured on the command line or in configuration files.
990: .Pp
991: The
992: .Ev DISPLAY
993: value set by
994: .Nm
995: will point to the server machine, but with a display number greater than zero.
996: This is normal, and happens because
997: .Nm
998: creates a
999: .Dq proxy
1000: X server on the server machine for forwarding the
1001: connections over the encrypted channel.
1.200 djm 1002: .Pp
1.218 jmc 1003: .Nm
1004: will also automatically set up Xauthority data on the server machine.
1005: For this purpose, it will generate a random authorization cookie,
1006: store it in Xauthority on the server, and verify that any forwarded
1007: connections carry this cookie and replace it by the real cookie when
1008: the connection is opened.
1009: The real authentication cookie is never
1010: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1011: .Pp
1.218 jmc 1012: If the
1013: .Cm ForwardAgent
1014: variable is set to
1015: .Dq yes
1016: (or see the description of the
1017: .Fl A
1.191 djm 1018: and
1.218 jmc 1019: .Fl a
1.226 jmc 1020: options above) and
1.218 jmc 1021: the user is using an authentication agent, the connection to the agent
1022: is automatically forwarded to the remote side.
1.252 jmc 1023: .Sh VERIFYING HOST KEYS
1024: When connecting to a server for the first time,
1025: a fingerprint of the server's public key is presented to the user
1026: (unless the option
1027: .Cm StrictHostKeyChecking
1028: has been disabled).
1029: Fingerprints can be determined using
1030: .Xr ssh-keygen 1 :
1031: .Pp
1032: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1033: .Pp
1.274 grunk 1034: If the fingerprint is already known, it can be matched
1035: and the key can be accepted or rejected.
1036: Because of the difficulty of comparing host keys
1037: just by looking at hex strings,
1038: there is also support to compare host keys visually,
1039: using
1040: .Em random art .
1041: By setting the
1.275 grunk 1042: .Cm VisualHostKey
1.274 grunk 1043: option to
1.275 grunk 1044: .Dq yes ,
1.274 grunk 1045: a small ASCII graphic gets displayed on every login to a server, no matter
1046: if the session itself is interactive or not.
1047: By learning the pattern a known server produces, a user can easily
1048: find out that the host key has changed when a completely different pattern
1049: is displayed.
1050: Because these patterns are not unambiguous however, a pattern that looks
1051: similar to the pattern remembered only gives a good probability that the
1052: host key is the same, not guaranteed proof.
1053: .Pp
1054: To get a listing of the fingerprints along with their random art for
1055: all known hosts, the following command line can be used:
1056: .Pp
1057: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1058: .Pp
1.252 jmc 1059: If the fingerprint is unknown,
1060: an alternative method of verification is available:
1061: SSH fingerprints verified by DNS.
1062: An additional resource record (RR),
1063: SSHFP,
1064: is added to a zonefile
1065: and the connecting client is able to match the fingerprint
1066: with that of the key presented.
1067: .Pp
1068: In this example, we are connecting a client to a server,
1069: .Dq host.example.com .
1070: The SSHFP resource records should first be added to the zonefile for
1071: host.example.com:
1072: .Bd -literal -offset indent
1.259 jakob 1073: $ ssh-keygen -r host.example.com.
1.252 jmc 1074: .Ed
1075: .Pp
1076: The output lines will have to be added to the zonefile.
1077: To check that the zone is answering fingerprint queries:
1078: .Pp
1079: .Dl $ dig -t SSHFP host.example.com
1080: .Pp
1081: Finally the client connects:
1082: .Bd -literal -offset indent
1083: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1084: [...]
1085: Matching host key fingerprint found in DNS.
1086: Are you sure you want to continue connecting (yes/no)?
1087: .Ed
1088: .Pp
1089: See the
1090: .Cm VerifyHostKeyDNS
1091: option in
1092: .Xr ssh_config 5
1093: for more information.
1.250 jmc 1094: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1095: .Nm
1096: contains support for Virtual Private Network (VPN) tunnelling
1097: using the
1098: .Xr tun 4
1099: network pseudo-device,
1100: allowing two networks to be joined securely.
1101: The
1102: .Xr sshd_config 5
1103: configuration option
1104: .Cm PermitTunnel
1105: controls whether the server supports this,
1106: and at what level (layer 2 or 3 traffic).
1107: .Pp
1108: The following example would connect client network 10.0.50.0/24
1.265 otto 1109: with remote network 10.0.99.0/24 using a point-to-point connection
1110: from 10.1.1.1 to 10.1.1.2,
1111: provided that the SSH server running on the gateway to the remote network,
1112: at 192.168.1.15, allows it.
1113: .Pp
1114: On the client:
1.250 jmc 1115: .Bd -literal -offset indent
1116: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1117: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1118: # route add 10.0.99.0/24 10.1.1.2
1119: .Ed
1120: .Pp
1121: On the server:
1122: .Bd -literal -offset indent
1123: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1124: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1125: .Ed
1126: .Pp
1127: Client access may be more finely tuned via the
1128: .Pa /root/.ssh/authorized_keys
1129: file (see below) and the
1130: .Cm PermitRootLogin
1131: server option.
1.255 jmc 1132: The following entry would permit connections on
1.250 jmc 1133: .Xr tun 4
1.255 jmc 1134: device 1 from user
1.250 jmc 1135: .Dq jane
1.255 jmc 1136: and on tun device 2 from user
1.250 jmc 1137: .Dq john ,
1138: if
1139: .Cm PermitRootLogin
1140: is set to
1141: .Dq forced-commands-only :
1142: .Bd -literal -offset 2n
1143: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1144: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1145: .Ed
1146: .Pp
1.264 ray 1147: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1148: it may be more suited to temporary setups,
1149: such as for wireless VPNs.
1150: More permanent VPNs are better provided by tools such as
1151: .Xr ipsecctl 8
1152: and
1153: .Xr isakmpd 8 .
1.2 deraadt 1154: .Sh ENVIRONMENT
1155: .Nm
1.1 deraadt 1156: will normally set the following environment variables:
1.237 jmc 1157: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1158: .It Ev DISPLAY
1159: The
1160: .Ev DISPLAY
1.40 aaron 1161: variable indicates the location of the X11 server.
1.44 aaron 1162: It is automatically set by
1.2 deraadt 1163: .Nm
1164: to point to a value of the form
1.233 jmc 1165: .Dq hostname:n ,
1166: where
1167: .Dq hostname
1168: indicates the host where the shell runs, and
1169: .Sq n
1170: is an integer \*(Ge 1.
1.40 aaron 1171: .Nm
1172: uses this special value to forward X11 connections over the secure
1173: channel.
1.107 markus 1174: The user should normally not set
1175: .Ev DISPLAY
1176: explicitly, as that
1.1 deraadt 1177: will render the X11 connection insecure (and will require the user to
1178: manually copy any required authorization cookies).
1.2 deraadt 1179: .It Ev HOME
1.1 deraadt 1180: Set to the path of the user's home directory.
1.2 deraadt 1181: .It Ev LOGNAME
1182: Synonym for
1.12 aaron 1183: .Ev USER ;
1184: set for compatibility with systems that use this variable.
1.2 deraadt 1185: .It Ev MAIL
1.129 stevesk 1186: Set to the path of the user's mailbox.
1.40 aaron 1187: .It Ev PATH
1.2 deraadt 1188: Set to the default
1189: .Ev PATH ,
1190: as specified when compiling
1.234 jmc 1191: .Nm .
1.118 markus 1192: .It Ev SSH_ASKPASS
1193: If
1194: .Nm
1195: needs a passphrase, it will read the passphrase from the current
1196: terminal if it was run from a terminal.
1197: If
1198: .Nm
1199: does not have a terminal associated with it but
1200: .Ev DISPLAY
1201: and
1202: .Ev SSH_ASKPASS
1203: are set, it will execute the program specified by
1204: .Ev SSH_ASKPASS
1205: and open an X11 window to read the passphrase.
1206: This is particularly useful when calling
1207: .Nm
1208: from a
1.196 jmc 1209: .Pa .xsession
1.118 markus 1210: or related script.
1211: (Note that on some machines it
1212: may be necessary to redirect the input from
1213: .Pa /dev/null
1214: to make this work.)
1.18 markus 1215: .It Ev SSH_AUTH_SOCK
1.233 jmc 1216: Identifies the path of a
1217: .Ux Ns -domain
1218: socket used to communicate with the agent.
1.166 stevesk 1219: .It Ev SSH_CONNECTION
1220: Identifies the client and server ends of the connection.
1.40 aaron 1221: The variable contains
1.233 jmc 1222: four space-separated values: client IP address, client port number,
1223: server IP address, and server port number.
1.73 markus 1224: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1225: This variable contains the original command line if a forced command
1.73 markus 1226: is executed.
1227: It can be used to extract the original arguments.
1.2 deraadt 1228: .It Ev SSH_TTY
1.1 deraadt 1229: This is set to the name of the tty (path to the device) associated
1.40 aaron 1230: with the current shell or command.
1231: If the current session has no tty,
1.1 deraadt 1232: this variable is not set.
1.2 deraadt 1233: .It Ev TZ
1.214 jmc 1234: This variable is set to indicate the present time zone if it
1.257 jmc 1235: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1236: on to new connections).
1.2 deraadt 1237: .It Ev USER
1.1 deraadt 1238: Set to the name of the user logging in.
1.2 deraadt 1239: .El
1240: .Pp
1.44 aaron 1241: Additionally,
1.2 deraadt 1242: .Nm
1.44 aaron 1243: reads
1.207 djm 1244: .Pa ~/.ssh/environment ,
1.2 deraadt 1245: and adds lines of the format
1246: .Dq VARNAME=value
1.233 jmc 1247: to the environment if the file exists and users are allowed to
1.161 marc 1248: change their environment.
1.176 jmc 1249: For more information, see the
1.161 marc 1250: .Cm PermitUserEnvironment
1.162 stevesk 1251: option in
1.161 marc 1252: .Xr sshd_config 5 .
1.2 deraadt 1253: .Sh FILES
1.236 jmc 1254: .Bl -tag -width Ds -compact
1.309 jmc 1255: .It Pa ~/.rhosts
1.240 jmc 1256: This file is used for host-based authentication (see above).
1.92 markus 1257: On some machines this file may need to be
1.240 jmc 1258: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1259: because
1.2 deraadt 1260: .Xr sshd 8
1.40 aaron 1261: reads it as root.
1262: Additionally, this file must be owned by the user,
1263: and must not have write permissions for anyone else.
1264: The recommended
1.1 deraadt 1265: permission for most machines is read/write for the user, and not
1266: accessible by others.
1.2 deraadt 1267: .Pp
1.309 jmc 1268: .It Pa ~/.shosts
1.240 jmc 1269: This file is used in exactly the same way as
1270: .Pa .rhosts ,
1271: but allows host-based authentication without permitting login with
1272: rlogin/rsh.
1.272 mcbride 1273: .Pp
1.309 jmc 1274: .It Pa ~/.ssh/
1.272 mcbride 1275: This directory is the default location for all user-specific configuration
1276: and authentication information.
1277: There is no general requirement to keep the entire contents of this directory
1278: secret, but the recommended permissions are read/write/execute for the user,
1279: and not accessible by others.
1.236 jmc 1280: .Pp
1.309 jmc 1281: .It Pa ~/.ssh/authorized_keys
1.310 djm 1282: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1283: this user.
1.238 jmc 1284: The format of this file is described in the
1285: .Xr sshd 8
1286: manual page.
1287: This file is not highly sensitive, but the recommended
1288: permissions are read/write for the user, and not accessible by others.
1289: .Pp
1.309 jmc 1290: .It Pa ~/.ssh/config
1.238 jmc 1291: This is the per-user configuration file.
1292: The file format and configuration options are described in
1293: .Xr ssh_config 5 .
1294: Because of the potential for abuse, this file must have strict permissions:
1295: read/write for the user, and not accessible by others.
1296: .Pp
1.309 jmc 1297: .It Pa ~/.ssh/environment
1.239 jmc 1298: Contains additional definitions for environment variables; see
1299: .Sx ENVIRONMENT ,
1.238 jmc 1300: above.
1301: .Pp
1.309 jmc 1302: .It Pa ~/.ssh/identity
1303: .It Pa ~/.ssh/id_dsa
1.310 djm 1304: .It Pa ~/.ssh/id_ecdsa
1.309 jmc 1305: .It Pa ~/.ssh/id_rsa
1.238 jmc 1306: Contains the private key for authentication.
1307: These files
1308: contain sensitive data and should be readable by the user but not
1309: accessible by others (read/write/execute).
1310: .Nm
1311: will simply ignore a private key file if it is accessible by others.
1312: It is possible to specify a passphrase when
1313: generating the key which will be used to encrypt the
1314: sensitive part of this file using 3DES.
1315: .Pp
1.309 jmc 1316: .It Pa ~/.ssh/identity.pub
1317: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1318: .It Pa ~/.ssh/id_ecdsa.pub
1.309 jmc 1319: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1320: Contains the public key for authentication.
1321: These files are not
1322: sensitive and can (but need not) be readable by anyone.
1323: .Pp
1.309 jmc 1324: .It Pa ~/.ssh/known_hosts
1.244 jmc 1325: Contains a list of host keys for all hosts the user has logged into
1326: that are not already in the systemwide list of known host keys.
1.238 jmc 1327: See
1.244 jmc 1328: .Xr sshd 8
1329: for further details of the format of this file.
1.238 jmc 1330: .Pp
1.309 jmc 1331: .It Pa ~/.ssh/rc
1.238 jmc 1332: Commands in this file are executed by
1333: .Nm
1.245 jmc 1334: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1335: started.
1336: See the
1337: .Xr sshd 8
1338: manual page for more information.
1339: .Pp
1.309 jmc 1340: .It Pa /etc/hosts.equiv
1.240 jmc 1341: This file is for host-based authentication (see above).
1342: It should only be writable by root.
1.236 jmc 1343: .Pp
1.309 jmc 1344: .It Pa /etc/shosts.equiv
1.240 jmc 1345: This file is used in exactly the same way as
1346: .Pa hosts.equiv ,
1347: but allows host-based authentication without permitting login with
1348: rlogin/rsh.
1.236 jmc 1349: .Pp
1.238 jmc 1350: .It Pa /etc/ssh/ssh_config
1351: Systemwide configuration file.
1352: The file format and configuration options are described in
1353: .Xr ssh_config 5 .
1354: .Pp
1.309 jmc 1355: .It Pa /etc/ssh/ssh_host_key
1356: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1357: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309 jmc 1358: .It Pa /etc/ssh/ssh_host_rsa_key
1.238 jmc 1359: These three files contain the private parts of the host keys
1.245 jmc 1360: and are used for host-based authentication.
1361: If protocol version 1 is used,
1.238 jmc 1362: .Nm
1363: must be setuid root, since the host key is readable only by root.
1364: For protocol version 2,
1365: .Nm
1366: uses
1367: .Xr ssh-keysign 8
1.245 jmc 1368: to access the host keys,
1369: eliminating the requirement that
1.238 jmc 1370: .Nm
1.245 jmc 1371: be setuid root when host-based authentication is used.
1.238 jmc 1372: By default
1.2 deraadt 1373: .Nm
1.238 jmc 1374: is not setuid root.
1375: .Pp
1.309 jmc 1376: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1377: Systemwide list of known host keys.
1378: This file should be prepared by the
1379: system administrator to contain the public host keys of all machines in the
1380: organization.
1.244 jmc 1381: It should be world-readable.
1382: See
1.238 jmc 1383: .Xr sshd 8
1.244 jmc 1384: for further details of the format of this file.
1.236 jmc 1385: .Pp
1.309 jmc 1386: .It Pa /etc/ssh/sshrc
1.1 deraadt 1387: Commands in this file are executed by
1.2 deraadt 1388: .Nm
1.245 jmc 1389: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1390: See the
1.2 deraadt 1391: .Xr sshd 8
1.1 deraadt 1392: manual page for more information.
1.58 itojun 1393: .El
1.312 jmc 1394: .Sh EXIT STATUS
1395: .Nm
1396: exits with the exit status of the remote command or with 255
1397: if an error occurred.
1.2 deraadt 1398: .Sh SEE ALSO
1399: .Xr scp 1 ,
1.83 djm 1400: .Xr sftp 1 ,
1.2 deraadt 1401: .Xr ssh-add 1 ,
1402: .Xr ssh-agent 1 ,
1403: .Xr ssh-keygen 1 ,
1.242 jmc 1404: .Xr ssh-keyscan 1 ,
1.250 jmc 1405: .Xr tun 4 ,
1.176 jmc 1406: .Xr hosts.equiv 5 ,
1.159 stevesk 1407: .Xr ssh_config 5 ,
1.160 naddy 1408: .Xr ssh-keysign 8 ,
1.87 itojun 1409: .Xr sshd 8
1.106 markus 1410: .Rs
1.256 jmc 1411: .%R RFC 4250
1412: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1413: .%D 2006
1414: .Re
1415: .Rs
1416: .%R RFC 4251
1417: .%T "The Secure Shell (SSH) Protocol Architecture"
1418: .%D 2006
1419: .Re
1420: .Rs
1421: .%R RFC 4252
1422: .%T "The Secure Shell (SSH) Authentication Protocol"
1423: .%D 2006
1424: .Re
1425: .Rs
1426: .%R RFC 4253
1427: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1428: .%D 2006
1429: .Re
1430: .Rs
1431: .%R RFC 4254
1432: .%T "The Secure Shell (SSH) Connection Protocol"
1433: .%D 2006
1434: .Re
1435: .Rs
1436: .%R RFC 4255
1437: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1438: .%D 2006
1439: .Re
1440: .Rs
1441: .%R RFC 4256
1442: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1443: .%D 2006
1444: .Re
1445: .Rs
1446: .%R RFC 4335
1447: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1448: .%D 2006
1449: .Re
1450: .Rs
1451: .%R RFC 4344
1452: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1453: .%D 2006
1454: .Re
1455: .Rs
1456: .%R RFC 4345
1457: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1458: .%D 2006
1459: .Re
1460: .Rs
1461: .%R RFC 4419
1462: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1463: .%D 2006
1464: .Re
1465: .Rs
1466: .%R RFC 4716
1467: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1468: .%D 2006
1.313 djm 1469: .Re
1470: .Rs
1471: .%R RFC 5656
1472: .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
1473: .%D 2009
1.274 grunk 1474: .Re
1475: .Rs
1476: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1477: .%A A. Perrig
1478: .%A D. Song
1479: .%D 1999
1480: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1481: .Re
1.173 jmc 1482: .Sh AUTHORS
1483: OpenSSH is a derivative of the original and free
1484: ssh 1.2.12 release by Tatu Ylonen.
1485: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1486: Theo de Raadt and Dug Song
1487: removed many bugs, re-added newer features and
1488: created OpenSSH.
1489: Markus Friedl contributed the support for SSH
1490: protocol versions 1.5 and 2.0.
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