Annotation of src/usr.bin/ssh/ssh.1, Revision 1.326
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: .\"
1.326 ! dtucker 36: .\" $OpenBSD: ssh.1,v 1.325 2012/06/18 12:07:07 dtucker Exp $
! 37: .Dd $Mdocdate: June 18 2012 $
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
1.318 jmc 395: (request forwardings without command execution),
1.323 okan 396: .Dq cancel
397: (cancel forwardings),
1.218 jmc 398: .Dq exit
1.318 jmc 399: (request the master to exit), and
1.317 djm 400: .Dq stop
401: (request the master to stop accepting further multiplexing requests).
1.218 jmc 402: .It Fl o Ar option
403: Can be used to give options in the format used in the configuration file.
404: This is useful for specifying options for which there is no separate
405: command-line flag.
406: For full details of the options listed below, and their possible values, see
407: .Xr ssh_config 5 .
1.2 deraadt 408: .Pp
1.218 jmc 409: .Bl -tag -width Ds -offset indent -compact
410: .It AddressFamily
411: .It BatchMode
412: .It BindAddress
413: .It ChallengeResponseAuthentication
414: .It CheckHostIP
415: .It Cipher
416: .It Ciphers
417: .It ClearAllForwardings
418: .It Compression
419: .It CompressionLevel
420: .It ConnectionAttempts
421: .It ConnectTimeout
422: .It ControlMaster
423: .It ControlPath
1.321 djm 424: .It ControlPersist
1.218 jmc 425: .It DynamicForward
426: .It EscapeChar
1.263 markus 427: .It ExitOnForwardFailure
1.218 jmc 428: .It ForwardAgent
429: .It ForwardX11
1.321 djm 430: .It ForwardX11Timeout
1.218 jmc 431: .It ForwardX11Trusted
432: .It GatewayPorts
433: .It GlobalKnownHostsFile
434: .It GSSAPIAuthentication
435: .It GSSAPIDelegateCredentials
436: .It HashKnownHosts
437: .It Host
438: .It HostbasedAuthentication
439: .It HostKeyAlgorithms
440: .It HostKeyAlias
441: .It HostName
442: .It IdentityFile
443: .It IdentitiesOnly
1.316 jmc 444: .It IPQoS
1.321 djm 445: .It KbdInteractiveAuthentication
1.218 jmc 446: .It KbdInteractiveDevices
1.314 jmc 447: .It KexAlgorithms
1.218 jmc 448: .It LocalCommand
449: .It LocalForward
450: .It LogLevel
451: .It MACs
452: .It NoHostAuthenticationForLocalhost
453: .It NumberOfPasswordPrompts
454: .It PasswordAuthentication
455: .It PermitLocalCommand
1.291 markus 456: .It PKCS11Provider
1.218 jmc 457: .It Port
458: .It PreferredAuthentications
459: .It Protocol
460: .It ProxyCommand
461: .It PubkeyAuthentication
1.251 dtucker 462: .It RekeyLimit
1.218 jmc 463: .It RemoteForward
1.319 jmc 464: .It RequestTTY
1.218 jmc 465: .It RhostsRSAAuthentication
466: .It RSAAuthentication
467: .It SendEnv
468: .It ServerAliveInterval
469: .It ServerAliveCountMax
470: .It StrictHostKeyChecking
471: .It TCPKeepAlive
472: .It Tunnel
473: .It TunnelDevice
474: .It UsePrivilegedPort
475: .It User
476: .It UserKnownHostsFile
477: .It VerifyHostKeyDNS
1.276 jmc 478: .It VisualHostKey
1.218 jmc 479: .It XAuthLocation
480: .El
481: .It Fl p Ar port
482: Port to connect to on the remote host.
483: This can be specified on a
484: per-host basis in the configuration file.
485: .It Fl q
486: Quiet mode.
1.271 djm 487: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 488: .It Fl R Xo
489: .Sm off
490: .Oo Ar bind_address : Oc
491: .Ar port : host : hostport
492: .Sm on
493: .Xc
494: Specifies that the given port on the remote (server) host is to be
495: forwarded to the given host and port on the local side.
496: This works by allocating a socket to listen to
497: .Ar port
498: on the remote side, and whenever a connection is made to this port, the
499: connection is forwarded over the secure channel, and a connection is
500: made to
501: .Ar host
502: port
503: .Ar hostport
504: from the local machine.
1.2 deraadt 505: .Pp
1.218 jmc 506: Port forwardings can also be specified in the configuration file.
507: Privileged ports can be forwarded only when
508: logging in as root on the remote machine.
1.324 jmc 509: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 510: .Pp
1.218 jmc 511: By default, the listening socket on the server will be bound to the loopback
512: interface only.
1.280 tobias 513: This may be overridden by specifying a
1.218 jmc 514: .Ar bind_address .
515: An empty
516: .Ar bind_address ,
517: or the address
518: .Ql * ,
519: indicates that the remote socket should listen on all interfaces.
520: Specifying a remote
521: .Ar bind_address
522: will only succeed if the server's
523: .Cm GatewayPorts
524: option is enabled (see
525: .Xr sshd_config 5 ) .
1.281 djm 526: .Pp
527: If the
528: .Ar port
529: argument is
1.282 djm 530: .Ql 0 ,
1.281 djm 531: the listen port will be dynamically allocated on the server and reported
532: to the client at run time.
1.305 markus 533: When used together with
534: .Ic -O forward
535: the allocated port will be printed to the standard output.
1.218 jmc 536: .It Fl S Ar ctl_path
1.304 jmc 537: Specifies the location of a control socket for connection sharing,
1.303 djm 538: or the string
539: .Dq none
540: to disable connection sharing.
1.218 jmc 541: Refer to the description of
542: .Cm ControlPath
543: and
544: .Cm ControlMaster
545: in
546: .Xr ssh_config 5
547: for details.
548: .It Fl s
549: May be used to request invocation of a subsystem on the remote system.
550: Subsystems are a feature of the SSH2 protocol which facilitate the use
551: of SSH as a secure transport for other applications (eg.\&
552: .Xr sftp 1 ) .
553: The subsystem is specified as the remote command.
554: .It Fl T
555: Disable pseudo-tty allocation.
556: .It Fl t
557: Force pseudo-tty allocation.
558: This can be used to execute arbitrary
559: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 560: e.g. when implementing menu services.
1.218 jmc 561: Multiple
562: .Fl t
563: options force tty allocation, even if
1.194 jakob 564: .Nm
1.218 jmc 565: has no local tty.
566: .It Fl V
567: Display the version number and exit.
568: .It Fl v
569: Verbose mode.
570: Causes
1.176 jmc 571: .Nm
1.218 jmc 572: to print debugging messages about its progress.
573: This is helpful in
574: debugging connection, authentication, and configuration problems.
575: Multiple
576: .Fl v
577: options increase the verbosity.
578: The maximum is 3.
1.290 dtucker 579: .It Fl W Ar host : Ns Ar port
580: Requests that standard input and output on the client be forwarded to
581: .Ar host
582: on
583: .Ar port
584: over the secure channel.
585: Implies
586: .Fl N ,
587: .Fl T ,
588: .Cm ExitOnForwardFailure
589: and
1.326 ! dtucker 590: .Cm ClearAllForwardings .
! 591: Works with Protocol version 2 only.
1.261 stevesk 592: .It Fl w Xo
593: .Ar local_tun Ns Op : Ns Ar remote_tun
594: .Xc
595: Requests
596: tunnel
597: device forwarding with the specified
1.218 jmc 598: .Xr tun 4
1.261 stevesk 599: devices between the client
600: .Pq Ar local_tun
601: and the server
602: .Pq Ar remote_tun .
603: .Pp
1.228 jmc 604: The devices may be specified by numerical ID or the keyword
605: .Dq any ,
606: which uses the next available tunnel device.
1.261 stevesk 607: If
608: .Ar remote_tun
609: is not specified, it defaults to
610: .Dq any .
1.228 jmc 611: See also the
1.218 jmc 612: .Cm Tunnel
1.261 stevesk 613: and
614: .Cm TunnelDevice
615: directives in
1.218 jmc 616: .Xr ssh_config 5 .
1.261 stevesk 617: If the
618: .Cm Tunnel
619: directive is unset, it is set to the default tunnel mode, which is
620: .Dq point-to-point .
1.218 jmc 621: .It Fl X
622: Enables X11 forwarding.
1.54 markus 623: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 624: .Pp
1.218 jmc 625: X11 forwarding should be enabled with caution.
1.168 jmc 626: Users with the ability to bypass file permissions on the remote host
1.218 jmc 627: (for the user's X authorization database)
628: can access the local X11 display through the forwarded connection.
629: An attacker may then be able to perform activities such as keystroke monitoring.
630: .Pp
631: For this reason, X11 forwarding is subjected to X11 SECURITY extension
632: restrictions by default.
633: Please refer to the
634: .Nm
635: .Fl Y
636: option and the
637: .Cm ForwardX11Trusted
638: directive in
639: .Xr ssh_config 5
640: for more information.
641: .It Fl x
642: Disables X11 forwarding.
643: .It Fl Y
644: Enables trusted X11 forwarding.
645: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
646: controls.
1.278 djm 647: .It Fl y
648: Send log information using the
649: .Xr syslog 3
650: system module.
651: By default this information is sent to stderr.
1.218 jmc 652: .El
1.224 jmc 653: .Pp
654: .Nm
655: may additionally obtain configuration data from
656: a per-user configuration file and a system-wide configuration file.
657: The file format and configuration options are described in
658: .Xr ssh_config 5 .
1.222 jmc 659: .Sh AUTHENTICATION
1.249 jmc 660: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 661: The default is to use protocol 2 only,
662: though this can be changed via the
1.222 jmc 663: .Cm Protocol
664: option in
1.284 jmc 665: .Xr ssh_config 5
666: or the
1.222 jmc 667: .Fl 1
668: and
669: .Fl 2
670: options (see above).
671: Both protocols support similar authentication methods,
1.284 jmc 672: but protocol 2 is the default since
1.222 jmc 673: it provides additional mechanisms for confidentiality
674: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 675: and integrity (hmac-md5, hmac-sha1,
676: hmac-sha2-256, hmac-sha2-512,
677: umac-64, hmac-ripemd160).
1.222 jmc 678: Protocol 1 lacks a strong mechanism for ensuring the
679: integrity of the connection.
680: .Pp
681: The methods available for authentication are:
1.260 jmc 682: GSSAPI-based authentication,
1.222 jmc 683: host-based authentication,
684: public key authentication,
685: challenge-response authentication,
686: and password authentication.
687: Authentication methods are tried in the order specified above,
688: though protocol 2 has a configuration option to change the default order:
689: .Cm PreferredAuthentications .
690: .Pp
691: Host-based authentication works as follows:
1.218 jmc 692: If the machine the user logs in from is listed in
693: .Pa /etc/hosts.equiv
694: or
695: .Pa /etc/shosts.equiv
696: on the remote machine, and the user names are
697: the same on both sides, or if the files
698: .Pa ~/.rhosts
699: or
700: .Pa ~/.shosts
701: exist in the user's home directory on the
702: remote machine and contain a line containing the name of the client
703: machine and the name of the user on that machine, the user is
1.222 jmc 704: considered for login.
705: Additionally, the server
706: .Em must
707: be able to verify the client's
708: host key (see the description of
1.218 jmc 709: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 710: and
1.222 jmc 711: .Pa ~/.ssh/known_hosts ,
712: below)
713: for login to be permitted.
1.218 jmc 714: This authentication method closes security holes due to IP
1.222 jmc 715: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 716: [Note to the administrator:
717: .Pa /etc/hosts.equiv ,
718: .Pa ~/.rhosts ,
719: and the rlogin/rsh protocol in general, are inherently insecure and should be
720: disabled if security is desired.]
1.189 dtucker 721: .Pp
1.222 jmc 722: Public key authentication works as follows:
723: The scheme is based on public-key cryptography,
724: using cryptosystems
725: where encryption and decryption are done using separate keys,
726: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 727: The idea is that each user creates a public/private
728: key pair for authentication purposes.
729: The server knows the public key, and only the user knows the private key.
1.222 jmc 730: .Nm
731: implements public key authentication protocol automatically,
1.310 djm 732: using one of the DSA, ECDSA or RSA algorithms.
1.222 jmc 733: Protocol 1 is restricted to using only RSA keys,
1.310 djm 734: but protocol 2 may use any.
1.222 jmc 735: The
736: .Sx HISTORY
737: section of
738: .Xr ssl 8
1.311 jmc 739: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 740: .Pp
1.218 jmc 741: The file
742: .Pa ~/.ssh/authorized_keys
743: lists the public keys that are permitted for logging in.
744: When the user logs in, the
1.2 deraadt 745: .Nm
1.218 jmc 746: program tells the server which key pair it would like to use for
747: authentication.
1.222 jmc 748: The client proves that it has access to the private key
749: and the server checks that the corresponding public key
750: is authorized to accept the account.
1.218 jmc 751: .Pp
1.222 jmc 752: The user creates his/her key pair by running
1.218 jmc 753: .Xr ssh-keygen 1 .
754: This stores the private key in
1.207 djm 755: .Pa ~/.ssh/identity
1.222 jmc 756: (protocol 1),
757: .Pa ~/.ssh/id_dsa
758: (protocol 2 DSA),
1.310 djm 759: .Pa ~/.ssh/id_ecdsa
760: (protocol 2 ECDSA),
1.222 jmc 761: or
762: .Pa ~/.ssh/id_rsa
763: (protocol 2 RSA)
1.218 jmc 764: and stores the public key in
765: .Pa ~/.ssh/identity.pub
1.222 jmc 766: (protocol 1),
767: .Pa ~/.ssh/id_dsa.pub
768: (protocol 2 DSA),
1.310 djm 769: .Pa ~/.ssh/id_ecdsa.pub
770: (protocol 2 ECDSA),
1.222 jmc 771: or
772: .Pa ~/.ssh/id_rsa.pub
773: (protocol 2 RSA)
1.218 jmc 774: in the user's home directory.
1.222 jmc 775: The user should then copy the public key
1.218 jmc 776: to
777: .Pa ~/.ssh/authorized_keys
1.222 jmc 778: in his/her home directory on the remote machine.
779: The
1.218 jmc 780: .Pa authorized_keys
781: file corresponds to the conventional
782: .Pa ~/.rhosts
783: file, and has one key
1.222 jmc 784: per line, though the lines can be very long.
1.218 jmc 785: After this, the user can log in without giving the password.
786: .Pp
1.301 jmc 787: A variation on public key authentication
788: is available in the form of certificate authentication:
789: instead of a set of public/private keys,
790: signed certificates are used.
791: This has the advantage that a single trusted certification authority
792: can be used in place of many public/private keys.
793: See the
794: .Sx CERTIFICATES
795: section of
796: .Xr ssh-keygen 1
797: for more information.
798: .Pp
799: The most convenient way to use public key or certificate authentication
800: may be with an authentication agent.
1.218 jmc 801: See
802: .Xr ssh-agent 1
803: for more information.
804: .Pp
1.222 jmc 805: Challenge-response authentication works as follows:
806: The server sends an arbitrary
807: .Qq challenge
808: text, and prompts for a response.
809: Protocol 2 allows multiple challenges and responses;
810: protocol 1 is restricted to just one challenge/response.
811: Examples of challenge-response authentication include
812: BSD Authentication (see
813: .Xr login.conf 5 )
814: and PAM (some non-OpenBSD systems).
815: .Pp
816: Finally, if other authentication methods fail,
1.218 jmc 817: .Nm
818: prompts the user for a password.
819: The password is sent to the remote
820: host for checking; however, since all communications are encrypted,
821: the password cannot be seen by someone listening on the network.
1.232 jmc 822: .Pp
823: .Nm
824: automatically maintains and checks a database containing
825: identification for all hosts it has ever been used with.
826: Host keys are stored in
827: .Pa ~/.ssh/known_hosts
828: in the user's home directory.
829: Additionally, the file
830: .Pa /etc/ssh/ssh_known_hosts
831: is automatically checked for known hosts.
832: Any new hosts are automatically added to the user's file.
833: If a host's identification ever changes,
834: .Nm
835: warns about this and disables password authentication to prevent
836: server spoofing or man-in-the-middle attacks,
837: which could otherwise be used to circumvent the encryption.
838: The
839: .Cm StrictHostKeyChecking
840: option can be used to control logins to machines whose
841: host key is not known or has changed.
842: .Pp
1.218 jmc 843: When the user's identity has been accepted by the server, the server
844: either executes the given command, or logs into the machine and gives
845: the user a normal shell on the remote machine.
846: All communication with
847: the remote command or shell will be automatically encrypted.
848: .Pp
849: If a pseudo-terminal has been allocated (normal login session), the
850: user may use the escape characters noted below.
851: .Pp
852: If no pseudo-tty has been allocated,
853: the session is transparent and can be used to reliably transfer binary data.
854: On most systems, setting the escape character to
855: .Dq none
856: will also make the session transparent even if a tty is used.
857: .Pp
858: The session terminates when the command or shell on the remote
1.247 jmc 859: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 860: .Sh ESCAPE CHARACTERS
1.218 jmc 861: When a pseudo-terminal has been requested,
1.2 deraadt 862: .Nm
1.218 jmc 863: supports a number of functions through the use of an escape character.
864: .Pp
865: A single tilde character can be sent as
866: .Ic ~~
867: or by following the tilde by a character other than those described below.
868: The escape character must always follow a newline to be interpreted as
869: special.
870: The escape character can be changed in configuration files using the
871: .Cm EscapeChar
872: configuration directive or on the command line by the
873: .Fl e
874: option.
875: .Pp
876: The supported escapes (assuming the default
877: .Ql ~ )
878: are:
879: .Bl -tag -width Ds
880: .It Cm ~.
881: Disconnect.
882: .It Cm ~^Z
883: Background
1.234 jmc 884: .Nm .
1.218 jmc 885: .It Cm ~#
886: List forwarded connections.
887: .It Cm ~&
888: Background
1.2 deraadt 889: .Nm
1.218 jmc 890: at logout when waiting for forwarded connection / X11 sessions to terminate.
891: .It Cm ~?
892: Display a list of escape characters.
893: .It Cm ~B
894: Send a BREAK to the remote system
895: (only useful for SSH protocol version 2 and if the peer supports it).
896: .It Cm ~C
897: Open command line.
898: Currently this allows the addition of port forwardings using the
1.279 stevesk 899: .Fl L ,
900: .Fl R
1.218 jmc 901: and
1.279 stevesk 902: .Fl D
1.225 jmc 903: options (see above).
1.322 markus 904: It also allows the cancellation of existing port-forwardings
905: with
1.262 stevesk 906: .Sm off
1.322 markus 907: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 908: .Sm on
1.322 markus 909: for local,
910: .Sm off
911: .Fl KR Oo Ar bind_address : Oc Ar port
912: .Sm on
913: for remote and
914: .Sm off
915: .Fl KD Oo Ar bind_address : Oc Ar port
916: .Sm on
917: for dynamic port-forwardings.
1.218 jmc 918: .Ic !\& Ns Ar command
919: allows the user to execute a local command if the
920: .Ic PermitLocalCommand
921: option is enabled in
1.176 jmc 922: .Xr ssh_config 5 .
1.218 jmc 923: Basic help is available, using the
924: .Fl h
925: option.
926: .It Cm ~R
927: Request rekeying of the connection
928: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 929: .El
1.246 jmc 930: .Sh TCP FORWARDING
931: Forwarding of arbitrary TCP connections over the secure channel can
932: be specified either on the command line or in a configuration file.
933: One possible application of TCP forwarding is a secure connection to a
934: mail server; another is going through firewalls.
935: .Pp
936: In the example below, we look at encrypting communication between
937: an IRC client and server, even though the IRC server does not directly
938: support encrypted communications.
939: This works as follows:
940: the user connects to the remote host using
941: .Nm ,
942: specifying a port to be used to forward connections
943: to the remote server.
944: After that it is possible to start the service which is to be encrypted
945: on the client machine,
946: connecting to the same local port,
947: and
948: .Nm
949: will encrypt and forward the connection.
950: .Pp
951: The following example tunnels an IRC session from client machine
952: .Dq 127.0.0.1
953: (localhost)
954: to remote server
955: .Dq server.example.com :
956: .Bd -literal -offset 4n
957: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
958: $ irc -c '#users' -p 1234 pinky 127.0.0.1
959: .Ed
960: .Pp
961: This tunnels a connection to IRC server
962: .Dq server.example.com ,
963: joining channel
964: .Dq #users ,
965: nickname
966: .Dq pinky ,
967: using port 1234.
968: It doesn't matter which port is used,
969: as long as it's greater than 1023
970: (remember, only root can open sockets on privileged ports)
971: and doesn't conflict with any ports already in use.
972: The connection is forwarded to port 6667 on the remote server,
973: since that's the standard port for IRC services.
974: .Pp
975: The
976: .Fl f
977: option backgrounds
978: .Nm
979: and the remote command
980: .Dq sleep 10
981: is specified to allow an amount of time
982: (10 seconds, in the example)
983: to start the service which is to be tunnelled.
984: If no connections are made within the time specified,
985: .Nm
986: will exit.
987: .Sh X11 FORWARDING
1.218 jmc 988: If the
989: .Cm ForwardX11
990: variable is set to
991: .Dq yes
992: (or see the description of the
1.227 jmc 993: .Fl X ,
994: .Fl x ,
1.218 jmc 995: and
1.227 jmc 996: .Fl Y
1.226 jmc 997: options above)
1.218 jmc 998: and the user is using X11 (the
999: .Ev DISPLAY
1000: environment variable is set), the connection to the X11 display is
1001: automatically forwarded to the remote side in such a way that any X11
1002: programs started from the shell (or command) will go through the
1003: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1004: from the local machine.
1.218 jmc 1005: The user should not manually set
1006: .Ev DISPLAY .
1007: Forwarding of X11 connections can be
1008: configured on the command line or in configuration files.
1009: .Pp
1010: The
1011: .Ev DISPLAY
1012: value set by
1013: .Nm
1014: will point to the server machine, but with a display number greater than zero.
1015: This is normal, and happens because
1016: .Nm
1017: creates a
1018: .Dq proxy
1019: X server on the server machine for forwarding the
1020: connections over the encrypted channel.
1.200 djm 1021: .Pp
1.218 jmc 1022: .Nm
1023: will also automatically set up Xauthority data on the server machine.
1024: For this purpose, it will generate a random authorization cookie,
1025: store it in Xauthority on the server, and verify that any forwarded
1026: connections carry this cookie and replace it by the real cookie when
1027: the connection is opened.
1028: The real authentication cookie is never
1029: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1030: .Pp
1.218 jmc 1031: If the
1032: .Cm ForwardAgent
1033: variable is set to
1034: .Dq yes
1035: (or see the description of the
1036: .Fl A
1.191 djm 1037: and
1.218 jmc 1038: .Fl a
1.226 jmc 1039: options above) and
1.218 jmc 1040: the user is using an authentication agent, the connection to the agent
1041: is automatically forwarded to the remote side.
1.252 jmc 1042: .Sh VERIFYING HOST KEYS
1043: When connecting to a server for the first time,
1044: a fingerprint of the server's public key is presented to the user
1045: (unless the option
1046: .Cm StrictHostKeyChecking
1047: has been disabled).
1048: Fingerprints can be determined using
1049: .Xr ssh-keygen 1 :
1050: .Pp
1051: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1052: .Pp
1.274 grunk 1053: If the fingerprint is already known, it can be matched
1054: and the key can be accepted or rejected.
1055: Because of the difficulty of comparing host keys
1056: just by looking at hex strings,
1057: there is also support to compare host keys visually,
1058: using
1059: .Em random art .
1060: By setting the
1.275 grunk 1061: .Cm VisualHostKey
1.274 grunk 1062: option to
1.275 grunk 1063: .Dq yes ,
1.274 grunk 1064: a small ASCII graphic gets displayed on every login to a server, no matter
1065: if the session itself is interactive or not.
1066: By learning the pattern a known server produces, a user can easily
1067: find out that the host key has changed when a completely different pattern
1068: is displayed.
1069: Because these patterns are not unambiguous however, a pattern that looks
1070: similar to the pattern remembered only gives a good probability that the
1071: host key is the same, not guaranteed proof.
1072: .Pp
1073: To get a listing of the fingerprints along with their random art for
1074: all known hosts, the following command line can be used:
1075: .Pp
1076: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1077: .Pp
1.252 jmc 1078: If the fingerprint is unknown,
1079: an alternative method of verification is available:
1080: SSH fingerprints verified by DNS.
1081: An additional resource record (RR),
1082: SSHFP,
1083: is added to a zonefile
1084: and the connecting client is able to match the fingerprint
1085: with that of the key presented.
1086: .Pp
1087: In this example, we are connecting a client to a server,
1088: .Dq host.example.com .
1089: The SSHFP resource records should first be added to the zonefile for
1090: host.example.com:
1091: .Bd -literal -offset indent
1.259 jakob 1092: $ ssh-keygen -r host.example.com.
1.252 jmc 1093: .Ed
1094: .Pp
1095: The output lines will have to be added to the zonefile.
1096: To check that the zone is answering fingerprint queries:
1097: .Pp
1098: .Dl $ dig -t SSHFP host.example.com
1099: .Pp
1100: Finally the client connects:
1101: .Bd -literal -offset indent
1102: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1103: [...]
1104: Matching host key fingerprint found in DNS.
1105: Are you sure you want to continue connecting (yes/no)?
1106: .Ed
1107: .Pp
1108: See the
1109: .Cm VerifyHostKeyDNS
1110: option in
1111: .Xr ssh_config 5
1112: for more information.
1.250 jmc 1113: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1114: .Nm
1115: contains support for Virtual Private Network (VPN) tunnelling
1116: using the
1117: .Xr tun 4
1118: network pseudo-device,
1119: allowing two networks to be joined securely.
1120: The
1121: .Xr sshd_config 5
1122: configuration option
1123: .Cm PermitTunnel
1124: controls whether the server supports this,
1125: and at what level (layer 2 or 3 traffic).
1126: .Pp
1127: The following example would connect client network 10.0.50.0/24
1.265 otto 1128: with remote network 10.0.99.0/24 using a point-to-point connection
1129: from 10.1.1.1 to 10.1.1.2,
1130: provided that the SSH server running on the gateway to the remote network,
1131: at 192.168.1.15, allows it.
1132: .Pp
1133: On the client:
1.250 jmc 1134: .Bd -literal -offset indent
1135: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1136: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1137: # route add 10.0.99.0/24 10.1.1.2
1138: .Ed
1139: .Pp
1140: On the server:
1141: .Bd -literal -offset indent
1142: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1143: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1144: .Ed
1145: .Pp
1146: Client access may be more finely tuned via the
1147: .Pa /root/.ssh/authorized_keys
1148: file (see below) and the
1149: .Cm PermitRootLogin
1150: server option.
1.255 jmc 1151: The following entry would permit connections on
1.250 jmc 1152: .Xr tun 4
1.255 jmc 1153: device 1 from user
1.250 jmc 1154: .Dq jane
1.255 jmc 1155: and on tun device 2 from user
1.250 jmc 1156: .Dq john ,
1157: if
1158: .Cm PermitRootLogin
1159: is set to
1160: .Dq forced-commands-only :
1161: .Bd -literal -offset 2n
1162: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1163: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1164: .Ed
1165: .Pp
1.264 ray 1166: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1167: it may be more suited to temporary setups,
1168: such as for wireless VPNs.
1169: More permanent VPNs are better provided by tools such as
1170: .Xr ipsecctl 8
1171: and
1172: .Xr isakmpd 8 .
1.2 deraadt 1173: .Sh ENVIRONMENT
1174: .Nm
1.1 deraadt 1175: will normally set the following environment variables:
1.237 jmc 1176: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1177: .It Ev DISPLAY
1178: The
1179: .Ev DISPLAY
1.40 aaron 1180: variable indicates the location of the X11 server.
1.44 aaron 1181: It is automatically set by
1.2 deraadt 1182: .Nm
1183: to point to a value of the form
1.233 jmc 1184: .Dq hostname:n ,
1185: where
1186: .Dq hostname
1187: indicates the host where the shell runs, and
1188: .Sq n
1189: is an integer \*(Ge 1.
1.40 aaron 1190: .Nm
1191: uses this special value to forward X11 connections over the secure
1192: channel.
1.107 markus 1193: The user should normally not set
1194: .Ev DISPLAY
1195: explicitly, as that
1.1 deraadt 1196: will render the X11 connection insecure (and will require the user to
1197: manually copy any required authorization cookies).
1.2 deraadt 1198: .It Ev HOME
1.1 deraadt 1199: Set to the path of the user's home directory.
1.2 deraadt 1200: .It Ev LOGNAME
1201: Synonym for
1.12 aaron 1202: .Ev USER ;
1203: set for compatibility with systems that use this variable.
1.2 deraadt 1204: .It Ev MAIL
1.129 stevesk 1205: Set to the path of the user's mailbox.
1.40 aaron 1206: .It Ev PATH
1.2 deraadt 1207: Set to the default
1208: .Ev PATH ,
1209: as specified when compiling
1.234 jmc 1210: .Nm .
1.118 markus 1211: .It Ev SSH_ASKPASS
1212: If
1213: .Nm
1214: needs a passphrase, it will read the passphrase from the current
1215: terminal if it was run from a terminal.
1216: If
1217: .Nm
1218: does not have a terminal associated with it but
1219: .Ev DISPLAY
1220: and
1221: .Ev SSH_ASKPASS
1222: are set, it will execute the program specified by
1223: .Ev SSH_ASKPASS
1224: and open an X11 window to read the passphrase.
1225: This is particularly useful when calling
1226: .Nm
1227: from a
1.196 jmc 1228: .Pa .xsession
1.118 markus 1229: or related script.
1230: (Note that on some machines it
1231: may be necessary to redirect the input from
1232: .Pa /dev/null
1233: to make this work.)
1.18 markus 1234: .It Ev SSH_AUTH_SOCK
1.233 jmc 1235: Identifies the path of a
1236: .Ux Ns -domain
1237: socket used to communicate with the agent.
1.166 stevesk 1238: .It Ev SSH_CONNECTION
1239: Identifies the client and server ends of the connection.
1.40 aaron 1240: The variable contains
1.233 jmc 1241: four space-separated values: client IP address, client port number,
1242: server IP address, and server port number.
1.73 markus 1243: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1244: This variable contains the original command line if a forced command
1.73 markus 1245: is executed.
1246: It can be used to extract the original arguments.
1.2 deraadt 1247: .It Ev SSH_TTY
1.1 deraadt 1248: This is set to the name of the tty (path to the device) associated
1.40 aaron 1249: with the current shell or command.
1250: If the current session has no tty,
1.1 deraadt 1251: this variable is not set.
1.2 deraadt 1252: .It Ev TZ
1.214 jmc 1253: This variable is set to indicate the present time zone if it
1.257 jmc 1254: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1255: on to new connections).
1.2 deraadt 1256: .It Ev USER
1.1 deraadt 1257: Set to the name of the user logging in.
1.2 deraadt 1258: .El
1259: .Pp
1.44 aaron 1260: Additionally,
1.2 deraadt 1261: .Nm
1.44 aaron 1262: reads
1.207 djm 1263: .Pa ~/.ssh/environment ,
1.2 deraadt 1264: and adds lines of the format
1265: .Dq VARNAME=value
1.233 jmc 1266: to the environment if the file exists and users are allowed to
1.161 marc 1267: change their environment.
1.176 jmc 1268: For more information, see the
1.161 marc 1269: .Cm PermitUserEnvironment
1.162 stevesk 1270: option in
1.161 marc 1271: .Xr sshd_config 5 .
1.2 deraadt 1272: .Sh FILES
1.236 jmc 1273: .Bl -tag -width Ds -compact
1.309 jmc 1274: .It Pa ~/.rhosts
1.240 jmc 1275: This file is used for host-based authentication (see above).
1.92 markus 1276: On some machines this file may need to be
1.240 jmc 1277: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1278: because
1.2 deraadt 1279: .Xr sshd 8
1.40 aaron 1280: reads it as root.
1281: Additionally, this file must be owned by the user,
1282: and must not have write permissions for anyone else.
1283: The recommended
1.1 deraadt 1284: permission for most machines is read/write for the user, and not
1285: accessible by others.
1.2 deraadt 1286: .Pp
1.309 jmc 1287: .It Pa ~/.shosts
1.240 jmc 1288: This file is used in exactly the same way as
1289: .Pa .rhosts ,
1290: but allows host-based authentication without permitting login with
1291: rlogin/rsh.
1.272 mcbride 1292: .Pp
1.309 jmc 1293: .It Pa ~/.ssh/
1.272 mcbride 1294: This directory is the default location for all user-specific configuration
1295: and authentication information.
1296: There is no general requirement to keep the entire contents of this directory
1297: secret, but the recommended permissions are read/write/execute for the user,
1298: and not accessible by others.
1.236 jmc 1299: .Pp
1.309 jmc 1300: .It Pa ~/.ssh/authorized_keys
1.310 djm 1301: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1302: this user.
1.238 jmc 1303: The format of this file is described in the
1304: .Xr sshd 8
1305: manual page.
1306: This file is not highly sensitive, but the recommended
1307: permissions are read/write for the user, and not accessible by others.
1308: .Pp
1.309 jmc 1309: .It Pa ~/.ssh/config
1.238 jmc 1310: This is the per-user configuration file.
1311: The file format and configuration options are described in
1312: .Xr ssh_config 5 .
1313: Because of the potential for abuse, this file must have strict permissions:
1314: read/write for the user, and not accessible by others.
1315: .Pp
1.309 jmc 1316: .It Pa ~/.ssh/environment
1.239 jmc 1317: Contains additional definitions for environment variables; see
1318: .Sx ENVIRONMENT ,
1.238 jmc 1319: above.
1320: .Pp
1.309 jmc 1321: .It Pa ~/.ssh/identity
1322: .It Pa ~/.ssh/id_dsa
1.310 djm 1323: .It Pa ~/.ssh/id_ecdsa
1.309 jmc 1324: .It Pa ~/.ssh/id_rsa
1.238 jmc 1325: Contains the private key for authentication.
1326: These files
1327: contain sensitive data and should be readable by the user but not
1328: accessible by others (read/write/execute).
1329: .Nm
1330: will simply ignore a private key file if it is accessible by others.
1331: It is possible to specify a passphrase when
1332: generating the key which will be used to encrypt the
1333: sensitive part of this file using 3DES.
1334: .Pp
1.309 jmc 1335: .It Pa ~/.ssh/identity.pub
1336: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1337: .It Pa ~/.ssh/id_ecdsa.pub
1.309 jmc 1338: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1339: Contains the public key for authentication.
1340: These files are not
1341: sensitive and can (but need not) be readable by anyone.
1342: .Pp
1.309 jmc 1343: .It Pa ~/.ssh/known_hosts
1.244 jmc 1344: Contains a list of host keys for all hosts the user has logged into
1345: that are not already in the systemwide list of known host keys.
1.238 jmc 1346: See
1.244 jmc 1347: .Xr sshd 8
1348: for further details of the format of this file.
1.238 jmc 1349: .Pp
1.309 jmc 1350: .It Pa ~/.ssh/rc
1.238 jmc 1351: Commands in this file are executed by
1352: .Nm
1.245 jmc 1353: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1354: started.
1355: See the
1356: .Xr sshd 8
1357: manual page for more information.
1358: .Pp
1.309 jmc 1359: .It Pa /etc/hosts.equiv
1.240 jmc 1360: This file is for host-based authentication (see above).
1361: It should only be writable by root.
1.236 jmc 1362: .Pp
1.309 jmc 1363: .It Pa /etc/shosts.equiv
1.240 jmc 1364: This file is used in exactly the same way as
1365: .Pa hosts.equiv ,
1366: but allows host-based authentication without permitting login with
1367: rlogin/rsh.
1.236 jmc 1368: .Pp
1.238 jmc 1369: .It Pa /etc/ssh/ssh_config
1370: Systemwide configuration file.
1371: The file format and configuration options are described in
1372: .Xr ssh_config 5 .
1373: .Pp
1.309 jmc 1374: .It Pa /etc/ssh/ssh_host_key
1375: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1376: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309 jmc 1377: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1378: These files contain the private parts of the host keys
1.245 jmc 1379: and are used for host-based authentication.
1380: If protocol version 1 is used,
1.238 jmc 1381: .Nm
1382: must be setuid root, since the host key is readable only by root.
1383: For protocol version 2,
1384: .Nm
1385: uses
1386: .Xr ssh-keysign 8
1.245 jmc 1387: to access the host keys,
1388: eliminating the requirement that
1.238 jmc 1389: .Nm
1.245 jmc 1390: be setuid root when host-based authentication is used.
1.238 jmc 1391: By default
1.2 deraadt 1392: .Nm
1.238 jmc 1393: is not setuid root.
1394: .Pp
1.309 jmc 1395: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1396: Systemwide list of known host keys.
1397: This file should be prepared by the
1398: system administrator to contain the public host keys of all machines in the
1399: organization.
1.244 jmc 1400: It should be world-readable.
1401: See
1.238 jmc 1402: .Xr sshd 8
1.244 jmc 1403: for further details of the format of this file.
1.236 jmc 1404: .Pp
1.309 jmc 1405: .It Pa /etc/ssh/sshrc
1.1 deraadt 1406: Commands in this file are executed by
1.2 deraadt 1407: .Nm
1.245 jmc 1408: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1409: See the
1.2 deraadt 1410: .Xr sshd 8
1.1 deraadt 1411: manual page for more information.
1.58 itojun 1412: .El
1.312 jmc 1413: .Sh EXIT STATUS
1414: .Nm
1415: exits with the exit status of the remote command or with 255
1416: if an error occurred.
1.2 deraadt 1417: .Sh SEE ALSO
1418: .Xr scp 1 ,
1.83 djm 1419: .Xr sftp 1 ,
1.2 deraadt 1420: .Xr ssh-add 1 ,
1421: .Xr ssh-agent 1 ,
1422: .Xr ssh-keygen 1 ,
1.242 jmc 1423: .Xr ssh-keyscan 1 ,
1.250 jmc 1424: .Xr tun 4 ,
1.176 jmc 1425: .Xr hosts.equiv 5 ,
1.159 stevesk 1426: .Xr ssh_config 5 ,
1.160 naddy 1427: .Xr ssh-keysign 8 ,
1.87 itojun 1428: .Xr sshd 8
1.106 markus 1429: .Rs
1.256 jmc 1430: .%R RFC 4250
1431: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1432: .%D 2006
1433: .Re
1434: .Rs
1435: .%R RFC 4251
1436: .%T "The Secure Shell (SSH) Protocol Architecture"
1437: .%D 2006
1438: .Re
1439: .Rs
1440: .%R RFC 4252
1441: .%T "The Secure Shell (SSH) Authentication Protocol"
1442: .%D 2006
1443: .Re
1444: .Rs
1445: .%R RFC 4253
1446: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1447: .%D 2006
1448: .Re
1449: .Rs
1450: .%R RFC 4254
1451: .%T "The Secure Shell (SSH) Connection Protocol"
1452: .%D 2006
1453: .Re
1454: .Rs
1455: .%R RFC 4255
1456: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1457: .%D 2006
1458: .Re
1459: .Rs
1460: .%R RFC 4256
1461: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1462: .%D 2006
1463: .Re
1464: .Rs
1465: .%R RFC 4335
1466: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1467: .%D 2006
1468: .Re
1469: .Rs
1470: .%R RFC 4344
1471: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1472: .%D 2006
1473: .Re
1474: .Rs
1475: .%R RFC 4345
1476: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1477: .%D 2006
1478: .Re
1479: .Rs
1480: .%R RFC 4419
1481: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1482: .%D 2006
1483: .Re
1484: .Rs
1485: .%R RFC 4716
1486: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1487: .%D 2006
1.313 djm 1488: .Re
1489: .Rs
1490: .%R RFC 5656
1491: .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
1492: .%D 2009
1.274 grunk 1493: .Re
1494: .Rs
1495: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1496: .%A A. Perrig
1497: .%A D. Song
1498: .%D 1999
1499: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1500: .Re
1.173 jmc 1501: .Sh AUTHORS
1502: OpenSSH is a derivative of the original and free
1503: ssh 1.2.12 release by Tatu Ylonen.
1504: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1505: Theo de Raadt and Dug Song
1506: removed many bugs, re-added newer features and
1507: created OpenSSH.
1508: Markus Friedl contributed the support for SSH
1509: protocol versions 1.5 and 2.0.
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