Annotation of src/usr.bin/ssh/ssh.1, Revision 1.317
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: .\"
1.317 ! djm 36: .\" $OpenBSD: ssh.1,v 1.316 2010/11/18 15:01:00 jmc Exp $
! 37: .Dd $Mdocdate: November 18 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).
1.317 ! djm 398: .Dq stop
! 399: (request the master to stop accepting further multiplexing requests).
1.218 jmc 400: .It Fl o Ar option
401: Can be used to give options in the format used in the configuration file.
402: This is useful for specifying options for which there is no separate
403: command-line flag.
404: For full details of the options listed below, and their possible values, see
405: .Xr ssh_config 5 .
1.2 deraadt 406: .Pp
1.218 jmc 407: .Bl -tag -width Ds -offset indent -compact
408: .It AddressFamily
409: .It BatchMode
410: .It BindAddress
411: .It ChallengeResponseAuthentication
412: .It CheckHostIP
413: .It Cipher
414: .It Ciphers
415: .It ClearAllForwardings
416: .It Compression
417: .It CompressionLevel
418: .It ConnectionAttempts
419: .It ConnectTimeout
420: .It ControlMaster
421: .It ControlPath
422: .It DynamicForward
423: .It EscapeChar
1.263 markus 424: .It ExitOnForwardFailure
1.218 jmc 425: .It ForwardAgent
426: .It ForwardX11
427: .It ForwardX11Trusted
428: .It GatewayPorts
429: .It GlobalKnownHostsFile
430: .It GSSAPIAuthentication
431: .It GSSAPIDelegateCredentials
432: .It HashKnownHosts
433: .It Host
434: .It HostbasedAuthentication
435: .It HostKeyAlgorithms
436: .It HostKeyAlias
437: .It HostName
438: .It IdentityFile
439: .It IdentitiesOnly
1.316 jmc 440: .It IPQoS
1.218 jmc 441: .It KbdInteractiveDevices
1.314 jmc 442: .It KexAlgorithms
1.218 jmc 443: .It LocalCommand
444: .It LocalForward
445: .It LogLevel
446: .It MACs
447: .It NoHostAuthenticationForLocalhost
448: .It NumberOfPasswordPrompts
449: .It PasswordAuthentication
450: .It PermitLocalCommand
1.291 markus 451: .It PKCS11Provider
1.218 jmc 452: .It Port
453: .It PreferredAuthentications
454: .It Protocol
455: .It ProxyCommand
456: .It PubkeyAuthentication
1.251 dtucker 457: .It RekeyLimit
1.218 jmc 458: .It RemoteForward
459: .It RhostsRSAAuthentication
460: .It RSAAuthentication
461: .It SendEnv
462: .It ServerAliveInterval
463: .It ServerAliveCountMax
464: .It StrictHostKeyChecking
465: .It TCPKeepAlive
466: .It Tunnel
467: .It TunnelDevice
468: .It UsePrivilegedPort
469: .It User
470: .It UserKnownHostsFile
471: .It VerifyHostKeyDNS
1.276 jmc 472: .It VisualHostKey
1.218 jmc 473: .It XAuthLocation
474: .El
475: .It Fl p Ar port
476: Port to connect to on the remote host.
477: This can be specified on a
478: per-host basis in the configuration file.
479: .It Fl q
480: Quiet mode.
1.271 djm 481: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 482: .It Fl R Xo
483: .Sm off
484: .Oo Ar bind_address : Oc
485: .Ar port : host : hostport
486: .Sm on
487: .Xc
488: Specifies that the given port on the remote (server) host is to be
489: forwarded to the given host and port on the local side.
490: This works by allocating a socket to listen to
491: .Ar port
492: on the remote side, and whenever a connection is made to this port, the
493: connection is forwarded over the secure channel, and a connection is
494: made to
495: .Ar host
496: port
497: .Ar hostport
498: from the local machine.
1.2 deraadt 499: .Pp
1.218 jmc 500: Port forwardings can also be specified in the configuration file.
501: Privileged ports can be forwarded only when
502: logging in as root on the remote machine.
1.308 djm 503: IPv6 addresses can be specified by enclosing the address in square braces.
1.194 jakob 504: .Pp
1.218 jmc 505: By default, the listening socket on the server will be bound to the loopback
506: interface only.
1.280 tobias 507: This may be overridden by specifying a
1.218 jmc 508: .Ar bind_address .
509: An empty
510: .Ar bind_address ,
511: or the address
512: .Ql * ,
513: indicates that the remote socket should listen on all interfaces.
514: Specifying a remote
515: .Ar bind_address
516: will only succeed if the server's
517: .Cm GatewayPorts
518: option is enabled (see
519: .Xr sshd_config 5 ) .
1.281 djm 520: .Pp
521: If the
522: .Ar port
523: argument is
1.282 djm 524: .Ql 0 ,
1.281 djm 525: the listen port will be dynamically allocated on the server and reported
526: to the client at run time.
1.305 markus 527: When used together with
528: .Ic -O forward
529: the allocated port will be printed to the standard output.
1.218 jmc 530: .It Fl S Ar ctl_path
1.304 jmc 531: Specifies the location of a control socket for connection sharing,
1.303 djm 532: or the string
533: .Dq none
534: to disable connection sharing.
1.218 jmc 535: Refer to the description of
536: .Cm ControlPath
537: and
538: .Cm ControlMaster
539: in
540: .Xr ssh_config 5
541: for details.
542: .It Fl s
543: May be used to request invocation of a subsystem on the remote system.
544: Subsystems are a feature of the SSH2 protocol which facilitate the use
545: of SSH as a secure transport for other applications (eg.\&
546: .Xr sftp 1 ) .
547: The subsystem is specified as the remote command.
548: .It Fl T
549: Disable pseudo-tty allocation.
550: .It Fl t
551: Force pseudo-tty allocation.
552: This can be used to execute arbitrary
553: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 554: e.g. when implementing menu services.
1.218 jmc 555: Multiple
556: .Fl t
557: options force tty allocation, even if
1.194 jakob 558: .Nm
1.218 jmc 559: has no local tty.
560: .It Fl V
561: Display the version number and exit.
562: .It Fl v
563: Verbose mode.
564: Causes
1.176 jmc 565: .Nm
1.218 jmc 566: to print debugging messages about its progress.
567: This is helpful in
568: debugging connection, authentication, and configuration problems.
569: Multiple
570: .Fl v
571: options increase the verbosity.
572: The maximum is 3.
1.290 dtucker 573: .It Fl W Ar host : Ns Ar port
574: Requests that standard input and output on the client be forwarded to
575: .Ar host
576: on
577: .Ar port
578: over the secure channel.
579: Implies
580: .Fl N ,
581: .Fl T ,
582: .Cm ExitOnForwardFailure
583: and
584: .Cm ClearAllForwardings
585: and works with Protocol version 2 only.
1.261 stevesk 586: .It Fl w Xo
587: .Ar local_tun Ns Op : Ns Ar remote_tun
588: .Xc
589: Requests
590: tunnel
591: device forwarding with the specified
1.218 jmc 592: .Xr tun 4
1.261 stevesk 593: devices between the client
594: .Pq Ar local_tun
595: and the server
596: .Pq Ar remote_tun .
597: .Pp
1.228 jmc 598: The devices may be specified by numerical ID or the keyword
599: .Dq any ,
600: which uses the next available tunnel device.
1.261 stevesk 601: If
602: .Ar remote_tun
603: is not specified, it defaults to
604: .Dq any .
1.228 jmc 605: See also the
1.218 jmc 606: .Cm Tunnel
1.261 stevesk 607: and
608: .Cm TunnelDevice
609: directives in
1.218 jmc 610: .Xr ssh_config 5 .
1.261 stevesk 611: If the
612: .Cm Tunnel
613: directive is unset, it is set to the default tunnel mode, which is
614: .Dq point-to-point .
1.218 jmc 615: .It Fl X
616: Enables X11 forwarding.
1.54 markus 617: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 618: .Pp
1.218 jmc 619: X11 forwarding should be enabled with caution.
1.168 jmc 620: Users with the ability to bypass file permissions on the remote host
1.218 jmc 621: (for the user's X authorization database)
622: can access the local X11 display through the forwarded connection.
623: An attacker may then be able to perform activities such as keystroke monitoring.
624: .Pp
625: For this reason, X11 forwarding is subjected to X11 SECURITY extension
626: restrictions by default.
627: Please refer to the
628: .Nm
629: .Fl Y
630: option and the
631: .Cm ForwardX11Trusted
632: directive in
633: .Xr ssh_config 5
634: for more information.
635: .It Fl x
636: Disables X11 forwarding.
637: .It Fl Y
638: Enables trusted X11 forwarding.
639: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
640: controls.
1.278 djm 641: .It Fl y
642: Send log information using the
643: .Xr syslog 3
644: system module.
645: By default this information is sent to stderr.
1.218 jmc 646: .El
1.224 jmc 647: .Pp
648: .Nm
649: may additionally obtain configuration data from
650: a per-user configuration file and a system-wide configuration file.
651: The file format and configuration options are described in
652: .Xr ssh_config 5 .
1.222 jmc 653: .Sh AUTHENTICATION
1.249 jmc 654: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 655: The default is to use protocol 2 only,
656: though this can be changed via the
1.222 jmc 657: .Cm Protocol
658: option in
1.284 jmc 659: .Xr ssh_config 5
660: or the
1.222 jmc 661: .Fl 1
662: and
663: .Fl 2
664: options (see above).
665: Both protocols support similar authentication methods,
1.284 jmc 666: but protocol 2 is the default since
1.222 jmc 667: it provides additional mechanisms for confidentiality
668: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.268 pvalchev 669: and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
1.222 jmc 670: Protocol 1 lacks a strong mechanism for ensuring the
671: integrity of the connection.
672: .Pp
673: The methods available for authentication are:
1.260 jmc 674: GSSAPI-based authentication,
1.222 jmc 675: host-based authentication,
676: public key authentication,
677: challenge-response authentication,
678: and password authentication.
679: Authentication methods are tried in the order specified above,
680: though protocol 2 has a configuration option to change the default order:
681: .Cm PreferredAuthentications .
682: .Pp
683: Host-based authentication works as follows:
1.218 jmc 684: If the machine the user logs in from is listed in
685: .Pa /etc/hosts.equiv
686: or
687: .Pa /etc/shosts.equiv
688: on the remote machine, and the user names are
689: the same on both sides, or if the files
690: .Pa ~/.rhosts
691: or
692: .Pa ~/.shosts
693: exist in the user's home directory on the
694: remote machine and contain a line containing the name of the client
695: machine and the name of the user on that machine, the user is
1.222 jmc 696: considered for login.
697: Additionally, the server
698: .Em must
699: be able to verify the client's
700: host key (see the description of
1.218 jmc 701: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 702: and
1.222 jmc 703: .Pa ~/.ssh/known_hosts ,
704: below)
705: for login to be permitted.
1.218 jmc 706: This authentication method closes security holes due to IP
1.222 jmc 707: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 708: [Note to the administrator:
709: .Pa /etc/hosts.equiv ,
710: .Pa ~/.rhosts ,
711: and the rlogin/rsh protocol in general, are inherently insecure and should be
712: disabled if security is desired.]
1.189 dtucker 713: .Pp
1.222 jmc 714: Public key authentication works as follows:
715: The scheme is based on public-key cryptography,
716: using cryptosystems
717: where encryption and decryption are done using separate keys,
718: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 719: The idea is that each user creates a public/private
720: key pair for authentication purposes.
721: The server knows the public key, and only the user knows the private key.
1.222 jmc 722: .Nm
723: implements public key authentication protocol automatically,
1.310 djm 724: using one of the DSA, ECDSA or RSA algorithms.
1.222 jmc 725: Protocol 1 is restricted to using only RSA keys,
1.310 djm 726: but protocol 2 may use any.
1.222 jmc 727: The
728: .Sx HISTORY
729: section of
730: .Xr ssl 8
1.311 jmc 731: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 732: .Pp
1.218 jmc 733: The file
734: .Pa ~/.ssh/authorized_keys
735: lists the public keys that are permitted for logging in.
736: When the user logs in, the
1.2 deraadt 737: .Nm
1.218 jmc 738: program tells the server which key pair it would like to use for
739: authentication.
1.222 jmc 740: The client proves that it has access to the private key
741: and the server checks that the corresponding public key
742: is authorized to accept the account.
1.218 jmc 743: .Pp
1.222 jmc 744: The user creates his/her key pair by running
1.218 jmc 745: .Xr ssh-keygen 1 .
746: This stores the private key in
1.207 djm 747: .Pa ~/.ssh/identity
1.222 jmc 748: (protocol 1),
749: .Pa ~/.ssh/id_dsa
750: (protocol 2 DSA),
1.310 djm 751: .Pa ~/.ssh/id_ecdsa
752: (protocol 2 ECDSA),
1.222 jmc 753: or
754: .Pa ~/.ssh/id_rsa
755: (protocol 2 RSA)
1.218 jmc 756: and stores the public key in
757: .Pa ~/.ssh/identity.pub
1.222 jmc 758: (protocol 1),
759: .Pa ~/.ssh/id_dsa.pub
760: (protocol 2 DSA),
1.310 djm 761: .Pa ~/.ssh/id_ecdsa.pub
762: (protocol 2 ECDSA),
1.222 jmc 763: or
764: .Pa ~/.ssh/id_rsa.pub
765: (protocol 2 RSA)
1.218 jmc 766: in the user's home directory.
1.222 jmc 767: The user should then copy the public key
1.218 jmc 768: to
769: .Pa ~/.ssh/authorized_keys
1.222 jmc 770: in his/her home directory on the remote machine.
771: The
1.218 jmc 772: .Pa authorized_keys
773: file corresponds to the conventional
774: .Pa ~/.rhosts
775: file, and has one key
1.222 jmc 776: per line, though the lines can be very long.
1.218 jmc 777: After this, the user can log in without giving the password.
778: .Pp
1.301 jmc 779: A variation on public key authentication
780: is available in the form of certificate authentication:
781: instead of a set of public/private keys,
782: signed certificates are used.
783: This has the advantage that a single trusted certification authority
784: can be used in place of many public/private keys.
785: See the
786: .Sx CERTIFICATES
787: section of
788: .Xr ssh-keygen 1
789: for more information.
790: .Pp
791: The most convenient way to use public key or certificate authentication
792: may be with an authentication agent.
1.218 jmc 793: See
794: .Xr ssh-agent 1
795: for more information.
796: .Pp
1.222 jmc 797: Challenge-response authentication works as follows:
798: The server sends an arbitrary
799: .Qq challenge
800: text, and prompts for a response.
801: Protocol 2 allows multiple challenges and responses;
802: protocol 1 is restricted to just one challenge/response.
803: Examples of challenge-response authentication include
804: BSD Authentication (see
805: .Xr login.conf 5 )
806: and PAM (some non-OpenBSD systems).
807: .Pp
808: Finally, if other authentication methods fail,
1.218 jmc 809: .Nm
810: prompts the user for a password.
811: The password is sent to the remote
812: host for checking; however, since all communications are encrypted,
813: the password cannot be seen by someone listening on the network.
1.232 jmc 814: .Pp
815: .Nm
816: automatically maintains and checks a database containing
817: identification for all hosts it has ever been used with.
818: Host keys are stored in
819: .Pa ~/.ssh/known_hosts
820: in the user's home directory.
821: Additionally, the file
822: .Pa /etc/ssh/ssh_known_hosts
823: is automatically checked for known hosts.
824: Any new hosts are automatically added to the user's file.
825: If a host's identification ever changes,
826: .Nm
827: warns about this and disables password authentication to prevent
828: server spoofing or man-in-the-middle attacks,
829: which could otherwise be used to circumvent the encryption.
830: The
831: .Cm StrictHostKeyChecking
832: option can be used to control logins to machines whose
833: host key is not known or has changed.
834: .Pp
1.218 jmc 835: When the user's identity has been accepted by the server, the server
836: either executes the given command, or logs into the machine and gives
837: the user a normal shell on the remote machine.
838: All communication with
839: the remote command or shell will be automatically encrypted.
840: .Pp
841: If a pseudo-terminal has been allocated (normal login session), the
842: user may use the escape characters noted below.
843: .Pp
844: If no pseudo-tty has been allocated,
845: the session is transparent and can be used to reliably transfer binary data.
846: On most systems, setting the escape character to
847: .Dq none
848: will also make the session transparent even if a tty is used.
849: .Pp
850: The session terminates when the command or shell on the remote
1.247 jmc 851: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 852: .Sh ESCAPE CHARACTERS
1.218 jmc 853: When a pseudo-terminal has been requested,
1.2 deraadt 854: .Nm
1.218 jmc 855: supports a number of functions through the use of an escape character.
856: .Pp
857: A single tilde character can be sent as
858: .Ic ~~
859: or by following the tilde by a character other than those described below.
860: The escape character must always follow a newline to be interpreted as
861: special.
862: The escape character can be changed in configuration files using the
863: .Cm EscapeChar
864: configuration directive or on the command line by the
865: .Fl e
866: option.
867: .Pp
868: The supported escapes (assuming the default
869: .Ql ~ )
870: are:
871: .Bl -tag -width Ds
872: .It Cm ~.
873: Disconnect.
874: .It Cm ~^Z
875: Background
1.234 jmc 876: .Nm .
1.218 jmc 877: .It Cm ~#
878: List forwarded connections.
879: .It Cm ~&
880: Background
1.2 deraadt 881: .Nm
1.218 jmc 882: at logout when waiting for forwarded connection / X11 sessions to terminate.
883: .It Cm ~?
884: Display a list of escape characters.
885: .It Cm ~B
886: Send a BREAK to the remote system
887: (only useful for SSH protocol version 2 and if the peer supports it).
888: .It Cm ~C
889: Open command line.
890: Currently this allows the addition of port forwardings using the
1.279 stevesk 891: .Fl L ,
892: .Fl R
1.218 jmc 893: and
1.279 stevesk 894: .Fl D
1.225 jmc 895: options (see above).
1.218 jmc 896: It also allows the cancellation of existing remote port-forwardings
897: using
1.262 stevesk 898: .Sm off
899: .Fl KR Oo Ar bind_address : Oc Ar port .
900: .Sm on
1.218 jmc 901: .Ic !\& Ns Ar command
902: allows the user to execute a local command if the
903: .Ic PermitLocalCommand
904: option is enabled in
1.176 jmc 905: .Xr ssh_config 5 .
1.218 jmc 906: Basic help is available, using the
907: .Fl h
908: option.
909: .It Cm ~R
910: Request rekeying of the connection
911: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 912: .El
1.246 jmc 913: .Sh TCP FORWARDING
914: Forwarding of arbitrary TCP connections over the secure channel can
915: be specified either on the command line or in a configuration file.
916: One possible application of TCP forwarding is a secure connection to a
917: mail server; another is going through firewalls.
918: .Pp
919: In the example below, we look at encrypting communication between
920: an IRC client and server, even though the IRC server does not directly
921: support encrypted communications.
922: This works as follows:
923: the user connects to the remote host using
924: .Nm ,
925: specifying a port to be used to forward connections
926: to the remote server.
927: After that it is possible to start the service which is to be encrypted
928: on the client machine,
929: connecting to the same local port,
930: and
931: .Nm
932: will encrypt and forward the connection.
933: .Pp
934: The following example tunnels an IRC session from client machine
935: .Dq 127.0.0.1
936: (localhost)
937: to remote server
938: .Dq server.example.com :
939: .Bd -literal -offset 4n
940: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
941: $ irc -c '#users' -p 1234 pinky 127.0.0.1
942: .Ed
943: .Pp
944: This tunnels a connection to IRC server
945: .Dq server.example.com ,
946: joining channel
947: .Dq #users ,
948: nickname
949: .Dq pinky ,
950: using port 1234.
951: It doesn't matter which port is used,
952: as long as it's greater than 1023
953: (remember, only root can open sockets on privileged ports)
954: and doesn't conflict with any ports already in use.
955: The connection is forwarded to port 6667 on the remote server,
956: since that's the standard port for IRC services.
957: .Pp
958: The
959: .Fl f
960: option backgrounds
961: .Nm
962: and the remote command
963: .Dq sleep 10
964: is specified to allow an amount of time
965: (10 seconds, in the example)
966: to start the service which is to be tunnelled.
967: If no connections are made within the time specified,
968: .Nm
969: will exit.
970: .Sh X11 FORWARDING
1.218 jmc 971: If the
972: .Cm ForwardX11
973: variable is set to
974: .Dq yes
975: (or see the description of the
1.227 jmc 976: .Fl X ,
977: .Fl x ,
1.218 jmc 978: and
1.227 jmc 979: .Fl Y
1.226 jmc 980: options above)
1.218 jmc 981: and the user is using X11 (the
982: .Ev DISPLAY
983: environment variable is set), the connection to the X11 display is
984: automatically forwarded to the remote side in such a way that any X11
985: programs started from the shell (or command) will go through the
986: encrypted channel, and the connection to the real X server will be made
1.176 jmc 987: from the local machine.
1.218 jmc 988: The user should not manually set
989: .Ev DISPLAY .
990: Forwarding of X11 connections can be
991: configured on the command line or in configuration files.
992: .Pp
993: The
994: .Ev DISPLAY
995: value set by
996: .Nm
997: will point to the server machine, but with a display number greater than zero.
998: This is normal, and happens because
999: .Nm
1000: creates a
1001: .Dq proxy
1002: X server on the server machine for forwarding the
1003: connections over the encrypted channel.
1.200 djm 1004: .Pp
1.218 jmc 1005: .Nm
1006: will also automatically set up Xauthority data on the server machine.
1007: For this purpose, it will generate a random authorization cookie,
1008: store it in Xauthority on the server, and verify that any forwarded
1009: connections carry this cookie and replace it by the real cookie when
1010: the connection is opened.
1011: The real authentication cookie is never
1012: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1013: .Pp
1.218 jmc 1014: If the
1015: .Cm ForwardAgent
1016: variable is set to
1017: .Dq yes
1018: (or see the description of the
1019: .Fl A
1.191 djm 1020: and
1.218 jmc 1021: .Fl a
1.226 jmc 1022: options above) and
1.218 jmc 1023: the user is using an authentication agent, the connection to the agent
1024: is automatically forwarded to the remote side.
1.252 jmc 1025: .Sh VERIFYING HOST KEYS
1026: When connecting to a server for the first time,
1027: a fingerprint of the server's public key is presented to the user
1028: (unless the option
1029: .Cm StrictHostKeyChecking
1030: has been disabled).
1031: Fingerprints can be determined using
1032: .Xr ssh-keygen 1 :
1033: .Pp
1034: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1035: .Pp
1.274 grunk 1036: If the fingerprint is already known, it can be matched
1037: and the key can be accepted or rejected.
1038: Because of the difficulty of comparing host keys
1039: just by looking at hex strings,
1040: there is also support to compare host keys visually,
1041: using
1042: .Em random art .
1043: By setting the
1.275 grunk 1044: .Cm VisualHostKey
1.274 grunk 1045: option to
1.275 grunk 1046: .Dq yes ,
1.274 grunk 1047: a small ASCII graphic gets displayed on every login to a server, no matter
1048: if the session itself is interactive or not.
1049: By learning the pattern a known server produces, a user can easily
1050: find out that the host key has changed when a completely different pattern
1051: is displayed.
1052: Because these patterns are not unambiguous however, a pattern that looks
1053: similar to the pattern remembered only gives a good probability that the
1054: host key is the same, not guaranteed proof.
1055: .Pp
1056: To get a listing of the fingerprints along with their random art for
1057: all known hosts, the following command line can be used:
1058: .Pp
1059: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1060: .Pp
1.252 jmc 1061: If the fingerprint is unknown,
1062: an alternative method of verification is available:
1063: SSH fingerprints verified by DNS.
1064: An additional resource record (RR),
1065: SSHFP,
1066: is added to a zonefile
1067: and the connecting client is able to match the fingerprint
1068: with that of the key presented.
1069: .Pp
1070: In this example, we are connecting a client to a server,
1071: .Dq host.example.com .
1072: The SSHFP resource records should first be added to the zonefile for
1073: host.example.com:
1074: .Bd -literal -offset indent
1.259 jakob 1075: $ ssh-keygen -r host.example.com.
1.252 jmc 1076: .Ed
1077: .Pp
1078: The output lines will have to be added to the zonefile.
1079: To check that the zone is answering fingerprint queries:
1080: .Pp
1081: .Dl $ dig -t SSHFP host.example.com
1082: .Pp
1083: Finally the client connects:
1084: .Bd -literal -offset indent
1085: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1086: [...]
1087: Matching host key fingerprint found in DNS.
1088: Are you sure you want to continue connecting (yes/no)?
1089: .Ed
1090: .Pp
1091: See the
1092: .Cm VerifyHostKeyDNS
1093: option in
1094: .Xr ssh_config 5
1095: for more information.
1.250 jmc 1096: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1097: .Nm
1098: contains support for Virtual Private Network (VPN) tunnelling
1099: using the
1100: .Xr tun 4
1101: network pseudo-device,
1102: allowing two networks to be joined securely.
1103: The
1104: .Xr sshd_config 5
1105: configuration option
1106: .Cm PermitTunnel
1107: controls whether the server supports this,
1108: and at what level (layer 2 or 3 traffic).
1109: .Pp
1110: The following example would connect client network 10.0.50.0/24
1.265 otto 1111: with remote network 10.0.99.0/24 using a point-to-point connection
1112: from 10.1.1.1 to 10.1.1.2,
1113: provided that the SSH server running on the gateway to the remote network,
1114: at 192.168.1.15, allows it.
1115: .Pp
1116: On the client:
1.250 jmc 1117: .Bd -literal -offset indent
1118: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1119: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1120: # route add 10.0.99.0/24 10.1.1.2
1121: .Ed
1122: .Pp
1123: On the server:
1124: .Bd -literal -offset indent
1125: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1126: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1127: .Ed
1128: .Pp
1129: Client access may be more finely tuned via the
1130: .Pa /root/.ssh/authorized_keys
1131: file (see below) and the
1132: .Cm PermitRootLogin
1133: server option.
1.255 jmc 1134: The following entry would permit connections on
1.250 jmc 1135: .Xr tun 4
1.255 jmc 1136: device 1 from user
1.250 jmc 1137: .Dq jane
1.255 jmc 1138: and on tun device 2 from user
1.250 jmc 1139: .Dq john ,
1140: if
1141: .Cm PermitRootLogin
1142: is set to
1143: .Dq forced-commands-only :
1144: .Bd -literal -offset 2n
1145: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1146: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1147: .Ed
1148: .Pp
1.264 ray 1149: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1150: it may be more suited to temporary setups,
1151: such as for wireless VPNs.
1152: More permanent VPNs are better provided by tools such as
1153: .Xr ipsecctl 8
1154: and
1155: .Xr isakmpd 8 .
1.2 deraadt 1156: .Sh ENVIRONMENT
1157: .Nm
1.1 deraadt 1158: will normally set the following environment variables:
1.237 jmc 1159: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1160: .It Ev DISPLAY
1161: The
1162: .Ev DISPLAY
1.40 aaron 1163: variable indicates the location of the X11 server.
1.44 aaron 1164: It is automatically set by
1.2 deraadt 1165: .Nm
1166: to point to a value of the form
1.233 jmc 1167: .Dq hostname:n ,
1168: where
1169: .Dq hostname
1170: indicates the host where the shell runs, and
1171: .Sq n
1172: is an integer \*(Ge 1.
1.40 aaron 1173: .Nm
1174: uses this special value to forward X11 connections over the secure
1175: channel.
1.107 markus 1176: The user should normally not set
1177: .Ev DISPLAY
1178: explicitly, as that
1.1 deraadt 1179: will render the X11 connection insecure (and will require the user to
1180: manually copy any required authorization cookies).
1.2 deraadt 1181: .It Ev HOME
1.1 deraadt 1182: Set to the path of the user's home directory.
1.2 deraadt 1183: .It Ev LOGNAME
1184: Synonym for
1.12 aaron 1185: .Ev USER ;
1186: set for compatibility with systems that use this variable.
1.2 deraadt 1187: .It Ev MAIL
1.129 stevesk 1188: Set to the path of the user's mailbox.
1.40 aaron 1189: .It Ev PATH
1.2 deraadt 1190: Set to the default
1191: .Ev PATH ,
1192: as specified when compiling
1.234 jmc 1193: .Nm .
1.118 markus 1194: .It Ev SSH_ASKPASS
1195: If
1196: .Nm
1197: needs a passphrase, it will read the passphrase from the current
1198: terminal if it was run from a terminal.
1199: If
1200: .Nm
1201: does not have a terminal associated with it but
1202: .Ev DISPLAY
1203: and
1204: .Ev SSH_ASKPASS
1205: are set, it will execute the program specified by
1206: .Ev SSH_ASKPASS
1207: and open an X11 window to read the passphrase.
1208: This is particularly useful when calling
1209: .Nm
1210: from a
1.196 jmc 1211: .Pa .xsession
1.118 markus 1212: or related script.
1213: (Note that on some machines it
1214: may be necessary to redirect the input from
1215: .Pa /dev/null
1216: to make this work.)
1.18 markus 1217: .It Ev SSH_AUTH_SOCK
1.233 jmc 1218: Identifies the path of a
1219: .Ux Ns -domain
1220: socket used to communicate with the agent.
1.166 stevesk 1221: .It Ev SSH_CONNECTION
1222: Identifies the client and server ends of the connection.
1.40 aaron 1223: The variable contains
1.233 jmc 1224: four space-separated values: client IP address, client port number,
1225: server IP address, and server port number.
1.73 markus 1226: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1227: This variable contains the original command line if a forced command
1.73 markus 1228: is executed.
1229: It can be used to extract the original arguments.
1.2 deraadt 1230: .It Ev SSH_TTY
1.1 deraadt 1231: This is set to the name of the tty (path to the device) associated
1.40 aaron 1232: with the current shell or command.
1233: If the current session has no tty,
1.1 deraadt 1234: this variable is not set.
1.2 deraadt 1235: .It Ev TZ
1.214 jmc 1236: This variable is set to indicate the present time zone if it
1.257 jmc 1237: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1238: on to new connections).
1.2 deraadt 1239: .It Ev USER
1.1 deraadt 1240: Set to the name of the user logging in.
1.2 deraadt 1241: .El
1242: .Pp
1.44 aaron 1243: Additionally,
1.2 deraadt 1244: .Nm
1.44 aaron 1245: reads
1.207 djm 1246: .Pa ~/.ssh/environment ,
1.2 deraadt 1247: and adds lines of the format
1248: .Dq VARNAME=value
1.233 jmc 1249: to the environment if the file exists and users are allowed to
1.161 marc 1250: change their environment.
1.176 jmc 1251: For more information, see the
1.161 marc 1252: .Cm PermitUserEnvironment
1.162 stevesk 1253: option in
1.161 marc 1254: .Xr sshd_config 5 .
1.2 deraadt 1255: .Sh FILES
1.236 jmc 1256: .Bl -tag -width Ds -compact
1.309 jmc 1257: .It Pa ~/.rhosts
1.240 jmc 1258: This file is used for host-based authentication (see above).
1.92 markus 1259: On some machines this file may need to be
1.240 jmc 1260: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1261: because
1.2 deraadt 1262: .Xr sshd 8
1.40 aaron 1263: reads it as root.
1264: Additionally, this file must be owned by the user,
1265: and must not have write permissions for anyone else.
1266: The recommended
1.1 deraadt 1267: permission for most machines is read/write for the user, and not
1268: accessible by others.
1.2 deraadt 1269: .Pp
1.309 jmc 1270: .It Pa ~/.shosts
1.240 jmc 1271: This file is used in exactly the same way as
1272: .Pa .rhosts ,
1273: but allows host-based authentication without permitting login with
1274: rlogin/rsh.
1.272 mcbride 1275: .Pp
1.309 jmc 1276: .It Pa ~/.ssh/
1.272 mcbride 1277: This directory is the default location for all user-specific configuration
1278: and authentication information.
1279: There is no general requirement to keep the entire contents of this directory
1280: secret, but the recommended permissions are read/write/execute for the user,
1281: and not accessible by others.
1.236 jmc 1282: .Pp
1.309 jmc 1283: .It Pa ~/.ssh/authorized_keys
1.310 djm 1284: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1285: this user.
1.238 jmc 1286: The format of this file is described in the
1287: .Xr sshd 8
1288: manual page.
1289: This file is not highly sensitive, but the recommended
1290: permissions are read/write for the user, and not accessible by others.
1291: .Pp
1.309 jmc 1292: .It Pa ~/.ssh/config
1.238 jmc 1293: This is the per-user configuration file.
1294: The file format and configuration options are described in
1295: .Xr ssh_config 5 .
1296: Because of the potential for abuse, this file must have strict permissions:
1297: read/write for the user, and not accessible by others.
1298: .Pp
1.309 jmc 1299: .It Pa ~/.ssh/environment
1.239 jmc 1300: Contains additional definitions for environment variables; see
1301: .Sx ENVIRONMENT ,
1.238 jmc 1302: above.
1303: .Pp
1.309 jmc 1304: .It Pa ~/.ssh/identity
1305: .It Pa ~/.ssh/id_dsa
1.310 djm 1306: .It Pa ~/.ssh/id_ecdsa
1.309 jmc 1307: .It Pa ~/.ssh/id_rsa
1.238 jmc 1308: Contains the private key for authentication.
1309: These files
1310: contain sensitive data and should be readable by the user but not
1311: accessible by others (read/write/execute).
1312: .Nm
1313: will simply ignore a private key file if it is accessible by others.
1314: It is possible to specify a passphrase when
1315: generating the key which will be used to encrypt the
1316: sensitive part of this file using 3DES.
1317: .Pp
1.309 jmc 1318: .It Pa ~/.ssh/identity.pub
1319: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1320: .It Pa ~/.ssh/id_ecdsa.pub
1.309 jmc 1321: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1322: Contains the public key for authentication.
1323: These files are not
1324: sensitive and can (but need not) be readable by anyone.
1325: .Pp
1.309 jmc 1326: .It Pa ~/.ssh/known_hosts
1.244 jmc 1327: Contains a list of host keys for all hosts the user has logged into
1328: that are not already in the systemwide list of known host keys.
1.238 jmc 1329: See
1.244 jmc 1330: .Xr sshd 8
1331: for further details of the format of this file.
1.238 jmc 1332: .Pp
1.309 jmc 1333: .It Pa ~/.ssh/rc
1.238 jmc 1334: Commands in this file are executed by
1335: .Nm
1.245 jmc 1336: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1337: started.
1338: See the
1339: .Xr sshd 8
1340: manual page for more information.
1341: .Pp
1.309 jmc 1342: .It Pa /etc/hosts.equiv
1.240 jmc 1343: This file is for host-based authentication (see above).
1344: It should only be writable by root.
1.236 jmc 1345: .Pp
1.309 jmc 1346: .It Pa /etc/shosts.equiv
1.240 jmc 1347: This file is used in exactly the same way as
1348: .Pa hosts.equiv ,
1349: but allows host-based authentication without permitting login with
1350: rlogin/rsh.
1.236 jmc 1351: .Pp
1.238 jmc 1352: .It Pa /etc/ssh/ssh_config
1353: Systemwide configuration file.
1354: The file format and configuration options are described in
1355: .Xr ssh_config 5 .
1356: .Pp
1.309 jmc 1357: .It Pa /etc/ssh/ssh_host_key
1358: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1359: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309 jmc 1360: .It Pa /etc/ssh/ssh_host_rsa_key
1.238 jmc 1361: These three files contain the private parts of the host keys
1.245 jmc 1362: and are used for host-based authentication.
1363: If protocol version 1 is used,
1.238 jmc 1364: .Nm
1365: must be setuid root, since the host key is readable only by root.
1366: For protocol version 2,
1367: .Nm
1368: uses
1369: .Xr ssh-keysign 8
1.245 jmc 1370: to access the host keys,
1371: eliminating the requirement that
1.238 jmc 1372: .Nm
1.245 jmc 1373: be setuid root when host-based authentication is used.
1.238 jmc 1374: By default
1.2 deraadt 1375: .Nm
1.238 jmc 1376: is not setuid root.
1377: .Pp
1.309 jmc 1378: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1379: Systemwide list of known host keys.
1380: This file should be prepared by the
1381: system administrator to contain the public host keys of all machines in the
1382: organization.
1.244 jmc 1383: It should be world-readable.
1384: See
1.238 jmc 1385: .Xr sshd 8
1.244 jmc 1386: for further details of the format of this file.
1.236 jmc 1387: .Pp
1.309 jmc 1388: .It Pa /etc/ssh/sshrc
1.1 deraadt 1389: Commands in this file are executed by
1.2 deraadt 1390: .Nm
1.245 jmc 1391: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1392: See the
1.2 deraadt 1393: .Xr sshd 8
1.1 deraadt 1394: manual page for more information.
1.58 itojun 1395: .El
1.312 jmc 1396: .Sh EXIT STATUS
1397: .Nm
1398: exits with the exit status of the remote command or with 255
1399: if an error occurred.
1.2 deraadt 1400: .Sh SEE ALSO
1401: .Xr scp 1 ,
1.83 djm 1402: .Xr sftp 1 ,
1.2 deraadt 1403: .Xr ssh-add 1 ,
1404: .Xr ssh-agent 1 ,
1405: .Xr ssh-keygen 1 ,
1.242 jmc 1406: .Xr ssh-keyscan 1 ,
1.250 jmc 1407: .Xr tun 4 ,
1.176 jmc 1408: .Xr hosts.equiv 5 ,
1.159 stevesk 1409: .Xr ssh_config 5 ,
1.160 naddy 1410: .Xr ssh-keysign 8 ,
1.87 itojun 1411: .Xr sshd 8
1.106 markus 1412: .Rs
1.256 jmc 1413: .%R RFC 4250
1414: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1415: .%D 2006
1416: .Re
1417: .Rs
1418: .%R RFC 4251
1419: .%T "The Secure Shell (SSH) Protocol Architecture"
1420: .%D 2006
1421: .Re
1422: .Rs
1423: .%R RFC 4252
1424: .%T "The Secure Shell (SSH) Authentication Protocol"
1425: .%D 2006
1426: .Re
1427: .Rs
1428: .%R RFC 4253
1429: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1430: .%D 2006
1431: .Re
1432: .Rs
1433: .%R RFC 4254
1434: .%T "The Secure Shell (SSH) Connection Protocol"
1435: .%D 2006
1436: .Re
1437: .Rs
1438: .%R RFC 4255
1439: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1440: .%D 2006
1441: .Re
1442: .Rs
1443: .%R RFC 4256
1444: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1445: .%D 2006
1446: .Re
1447: .Rs
1448: .%R RFC 4335
1449: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1450: .%D 2006
1451: .Re
1452: .Rs
1453: .%R RFC 4344
1454: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1455: .%D 2006
1456: .Re
1457: .Rs
1458: .%R RFC 4345
1459: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1460: .%D 2006
1461: .Re
1462: .Rs
1463: .%R RFC 4419
1464: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1465: .%D 2006
1466: .Re
1467: .Rs
1468: .%R RFC 4716
1469: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1470: .%D 2006
1.313 djm 1471: .Re
1472: .Rs
1473: .%R RFC 5656
1474: .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
1475: .%D 2009
1.274 grunk 1476: .Re
1477: .Rs
1478: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1479: .%A A. Perrig
1480: .%A D. Song
1481: .%D 1999
1482: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1483: .Re
1.173 jmc 1484: .Sh AUTHORS
1485: OpenSSH is a derivative of the original and free
1486: ssh 1.2.12 release by Tatu Ylonen.
1487: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1488: Theo de Raadt and Dug Song
1489: removed many bugs, re-added newer features and
1490: created OpenSSH.
1491: Markus Friedl contributed the support for SSH
1492: protocol versions 1.5 and 2.0.