Annotation of src/usr.bin/ssh/ssh.1, Revision 1.322
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: .\"
1.322 ! markus 36: .\" $OpenBSD: ssh.1,v 1.321 2011/08/26 01:45:15 djm Exp $
! 37: .Dd $Mdocdate: August 26 2011 $
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),
1.218 jmc 396: .Dq exit
1.318 jmc 397: (request the master to exit), and
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
1.321 djm 422: .It ControlPersist
1.218 jmc 423: .It DynamicForward
424: .It EscapeChar
1.263 markus 425: .It ExitOnForwardFailure
1.218 jmc 426: .It ForwardAgent
427: .It ForwardX11
1.321 djm 428: .It ForwardX11Timeout
1.218 jmc 429: .It ForwardX11Trusted
430: .It GatewayPorts
431: .It GlobalKnownHostsFile
432: .It GSSAPIAuthentication
433: .It GSSAPIDelegateCredentials
434: .It HashKnownHosts
435: .It Host
436: .It HostbasedAuthentication
437: .It HostKeyAlgorithms
438: .It HostKeyAlias
439: .It HostName
440: .It IdentityFile
441: .It IdentitiesOnly
1.316 jmc 442: .It IPQoS
1.321 djm 443: .It KbdInteractiveAuthentication
1.218 jmc 444: .It KbdInteractiveDevices
1.314 jmc 445: .It KexAlgorithms
1.218 jmc 446: .It LocalCommand
447: .It LocalForward
448: .It LogLevel
449: .It MACs
450: .It NoHostAuthenticationForLocalhost
451: .It NumberOfPasswordPrompts
452: .It PasswordAuthentication
453: .It PermitLocalCommand
1.291 markus 454: .It PKCS11Provider
1.218 jmc 455: .It Port
456: .It PreferredAuthentications
457: .It Protocol
458: .It ProxyCommand
459: .It PubkeyAuthentication
1.251 dtucker 460: .It RekeyLimit
1.218 jmc 461: .It RemoteForward
1.319 jmc 462: .It RequestTTY
1.218 jmc 463: .It RhostsRSAAuthentication
464: .It RSAAuthentication
465: .It SendEnv
466: .It ServerAliveInterval
467: .It ServerAliveCountMax
468: .It StrictHostKeyChecking
469: .It TCPKeepAlive
470: .It Tunnel
471: .It TunnelDevice
472: .It UsePrivilegedPort
473: .It User
474: .It UserKnownHostsFile
475: .It VerifyHostKeyDNS
1.276 jmc 476: .It VisualHostKey
1.218 jmc 477: .It XAuthLocation
478: .El
479: .It Fl p Ar port
480: Port to connect to on the remote host.
481: This can be specified on a
482: per-host basis in the configuration file.
483: .It Fl q
484: Quiet mode.
1.271 djm 485: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 486: .It Fl R Xo
487: .Sm off
488: .Oo Ar bind_address : Oc
489: .Ar port : host : hostport
490: .Sm on
491: .Xc
492: Specifies that the given port on the remote (server) host is to be
493: forwarded to the given host and port on the local side.
494: This works by allocating a socket to listen to
495: .Ar port
496: on the remote side, and whenever a connection is made to this port, the
497: connection is forwarded over the secure channel, and a connection is
498: made to
499: .Ar host
500: port
501: .Ar hostport
502: from the local machine.
1.2 deraadt 503: .Pp
1.218 jmc 504: Port forwardings can also be specified in the configuration file.
505: Privileged ports can be forwarded only when
506: logging in as root on the remote machine.
1.308 djm 507: IPv6 addresses can be specified by enclosing the address in square braces.
1.194 jakob 508: .Pp
1.218 jmc 509: By default, the listening socket on the server will be bound to the loopback
510: interface only.
1.280 tobias 511: This may be overridden by specifying a
1.218 jmc 512: .Ar bind_address .
513: An empty
514: .Ar bind_address ,
515: or the address
516: .Ql * ,
517: indicates that the remote socket should listen on all interfaces.
518: Specifying a remote
519: .Ar bind_address
520: will only succeed if the server's
521: .Cm GatewayPorts
522: option is enabled (see
523: .Xr sshd_config 5 ) .
1.281 djm 524: .Pp
525: If the
526: .Ar port
527: argument is
1.282 djm 528: .Ql 0 ,
1.281 djm 529: the listen port will be dynamically allocated on the server and reported
530: to the client at run time.
1.305 markus 531: When used together with
532: .Ic -O forward
533: the allocated port will be printed to the standard output.
1.218 jmc 534: .It Fl S Ar ctl_path
1.304 jmc 535: Specifies the location of a control socket for connection sharing,
1.303 djm 536: or the string
537: .Dq none
538: to disable connection sharing.
1.218 jmc 539: Refer to the description of
540: .Cm ControlPath
541: and
542: .Cm ControlMaster
543: in
544: .Xr ssh_config 5
545: for details.
546: .It Fl s
547: May be used to request invocation of a subsystem on the remote system.
548: Subsystems are a feature of the SSH2 protocol which facilitate the use
549: of SSH as a secure transport for other applications (eg.\&
550: .Xr sftp 1 ) .
551: The subsystem is specified as the remote command.
552: .It Fl T
553: Disable pseudo-tty allocation.
554: .It Fl t
555: Force pseudo-tty allocation.
556: This can be used to execute arbitrary
557: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 558: e.g. when implementing menu services.
1.218 jmc 559: Multiple
560: .Fl t
561: options force tty allocation, even if
1.194 jakob 562: .Nm
1.218 jmc 563: has no local tty.
564: .It Fl V
565: Display the version number and exit.
566: .It Fl v
567: Verbose mode.
568: Causes
1.176 jmc 569: .Nm
1.218 jmc 570: to print debugging messages about its progress.
571: This is helpful in
572: debugging connection, authentication, and configuration problems.
573: Multiple
574: .Fl v
575: options increase the verbosity.
576: The maximum is 3.
1.290 dtucker 577: .It Fl W Ar host : Ns Ar port
578: Requests that standard input and output on the client be forwarded to
579: .Ar host
580: on
581: .Ar port
582: over the secure channel.
583: Implies
584: .Fl N ,
585: .Fl T ,
586: .Cm ExitOnForwardFailure
587: and
588: .Cm ClearAllForwardings
589: and works with Protocol version 2 only.
1.261 stevesk 590: .It Fl w Xo
591: .Ar local_tun Ns Op : Ns Ar remote_tun
592: .Xc
593: Requests
594: tunnel
595: device forwarding with the specified
1.218 jmc 596: .Xr tun 4
1.261 stevesk 597: devices between the client
598: .Pq Ar local_tun
599: and the server
600: .Pq Ar remote_tun .
601: .Pp
1.228 jmc 602: The devices may be specified by numerical ID or the keyword
603: .Dq any ,
604: which uses the next available tunnel device.
1.261 stevesk 605: If
606: .Ar remote_tun
607: is not specified, it defaults to
608: .Dq any .
1.228 jmc 609: See also the
1.218 jmc 610: .Cm Tunnel
1.261 stevesk 611: and
612: .Cm TunnelDevice
613: directives in
1.218 jmc 614: .Xr ssh_config 5 .
1.261 stevesk 615: If the
616: .Cm Tunnel
617: directive is unset, it is set to the default tunnel mode, which is
618: .Dq point-to-point .
1.218 jmc 619: .It Fl X
620: Enables X11 forwarding.
1.54 markus 621: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 622: .Pp
1.218 jmc 623: X11 forwarding should be enabled with caution.
1.168 jmc 624: Users with the ability to bypass file permissions on the remote host
1.218 jmc 625: (for the user's X authorization database)
626: can access the local X11 display through the forwarded connection.
627: An attacker may then be able to perform activities such as keystroke monitoring.
628: .Pp
629: For this reason, X11 forwarding is subjected to X11 SECURITY extension
630: restrictions by default.
631: Please refer to the
632: .Nm
633: .Fl Y
634: option and the
635: .Cm ForwardX11Trusted
636: directive in
637: .Xr ssh_config 5
638: for more information.
639: .It Fl x
640: Disables X11 forwarding.
641: .It Fl Y
642: Enables trusted X11 forwarding.
643: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
644: controls.
1.278 djm 645: .It Fl y
646: Send log information using the
647: .Xr syslog 3
648: system module.
649: By default this information is sent to stderr.
1.218 jmc 650: .El
1.224 jmc 651: .Pp
652: .Nm
653: may additionally obtain configuration data from
654: a per-user configuration file and a system-wide configuration file.
655: The file format and configuration options are described in
656: .Xr ssh_config 5 .
1.222 jmc 657: .Sh AUTHENTICATION
1.249 jmc 658: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 659: The default is to use protocol 2 only,
660: though this can be changed via the
1.222 jmc 661: .Cm Protocol
662: option in
1.284 jmc 663: .Xr ssh_config 5
664: or the
1.222 jmc 665: .Fl 1
666: and
667: .Fl 2
668: options (see above).
669: Both protocols support similar authentication methods,
1.284 jmc 670: but protocol 2 is the default since
1.222 jmc 671: it provides additional mechanisms for confidentiality
672: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 673: and integrity (hmac-md5, hmac-sha1,
674: hmac-sha2-256, hmac-sha2-512,
675: umac-64, hmac-ripemd160).
1.222 jmc 676: Protocol 1 lacks a strong mechanism for ensuring the
677: integrity of the connection.
678: .Pp
679: The methods available for authentication are:
1.260 jmc 680: GSSAPI-based authentication,
1.222 jmc 681: host-based authentication,
682: public key authentication,
683: challenge-response authentication,
684: and password authentication.
685: Authentication methods are tried in the order specified above,
686: though protocol 2 has a configuration option to change the default order:
687: .Cm PreferredAuthentications .
688: .Pp
689: Host-based authentication works as follows:
1.218 jmc 690: If the machine the user logs in from is listed in
691: .Pa /etc/hosts.equiv
692: or
693: .Pa /etc/shosts.equiv
694: on the remote machine, and the user names are
695: the same on both sides, or if the files
696: .Pa ~/.rhosts
697: or
698: .Pa ~/.shosts
699: exist in the user's home directory on the
700: remote machine and contain a line containing the name of the client
701: machine and the name of the user on that machine, the user is
1.222 jmc 702: considered for login.
703: Additionally, the server
704: .Em must
705: be able to verify the client's
706: host key (see the description of
1.218 jmc 707: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 708: and
1.222 jmc 709: .Pa ~/.ssh/known_hosts ,
710: below)
711: for login to be permitted.
1.218 jmc 712: This authentication method closes security holes due to IP
1.222 jmc 713: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 714: [Note to the administrator:
715: .Pa /etc/hosts.equiv ,
716: .Pa ~/.rhosts ,
717: and the rlogin/rsh protocol in general, are inherently insecure and should be
718: disabled if security is desired.]
1.189 dtucker 719: .Pp
1.222 jmc 720: Public key authentication works as follows:
721: The scheme is based on public-key cryptography,
722: using cryptosystems
723: where encryption and decryption are done using separate keys,
724: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 725: The idea is that each user creates a public/private
726: key pair for authentication purposes.
727: The server knows the public key, and only the user knows the private key.
1.222 jmc 728: .Nm
729: implements public key authentication protocol automatically,
1.310 djm 730: using one of the DSA, ECDSA or RSA algorithms.
1.222 jmc 731: Protocol 1 is restricted to using only RSA keys,
1.310 djm 732: but protocol 2 may use any.
1.222 jmc 733: The
734: .Sx HISTORY
735: section of
736: .Xr ssl 8
1.311 jmc 737: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 738: .Pp
1.218 jmc 739: The file
740: .Pa ~/.ssh/authorized_keys
741: lists the public keys that are permitted for logging in.
742: When the user logs in, the
1.2 deraadt 743: .Nm
1.218 jmc 744: program tells the server which key pair it would like to use for
745: authentication.
1.222 jmc 746: The client proves that it has access to the private key
747: and the server checks that the corresponding public key
748: is authorized to accept the account.
1.218 jmc 749: .Pp
1.222 jmc 750: The user creates his/her key pair by running
1.218 jmc 751: .Xr ssh-keygen 1 .
752: This stores the private key in
1.207 djm 753: .Pa ~/.ssh/identity
1.222 jmc 754: (protocol 1),
755: .Pa ~/.ssh/id_dsa
756: (protocol 2 DSA),
1.310 djm 757: .Pa ~/.ssh/id_ecdsa
758: (protocol 2 ECDSA),
1.222 jmc 759: or
760: .Pa ~/.ssh/id_rsa
761: (protocol 2 RSA)
1.218 jmc 762: and stores the public key in
763: .Pa ~/.ssh/identity.pub
1.222 jmc 764: (protocol 1),
765: .Pa ~/.ssh/id_dsa.pub
766: (protocol 2 DSA),
1.310 djm 767: .Pa ~/.ssh/id_ecdsa.pub
768: (protocol 2 ECDSA),
1.222 jmc 769: or
770: .Pa ~/.ssh/id_rsa.pub
771: (protocol 2 RSA)
1.218 jmc 772: in the user's home directory.
1.222 jmc 773: The user should then copy the public key
1.218 jmc 774: to
775: .Pa ~/.ssh/authorized_keys
1.222 jmc 776: in his/her home directory on the remote machine.
777: The
1.218 jmc 778: .Pa authorized_keys
779: file corresponds to the conventional
780: .Pa ~/.rhosts
781: file, and has one key
1.222 jmc 782: per line, though the lines can be very long.
1.218 jmc 783: After this, the user can log in without giving the password.
784: .Pp
1.301 jmc 785: A variation on public key authentication
786: is available in the form of certificate authentication:
787: instead of a set of public/private keys,
788: signed certificates are used.
789: This has the advantage that a single trusted certification authority
790: can be used in place of many public/private keys.
791: See the
792: .Sx CERTIFICATES
793: section of
794: .Xr ssh-keygen 1
795: for more information.
796: .Pp
797: The most convenient way to use public key or certificate authentication
798: may be with an authentication agent.
1.218 jmc 799: See
800: .Xr ssh-agent 1
801: for more information.
802: .Pp
1.222 jmc 803: Challenge-response authentication works as follows:
804: The server sends an arbitrary
805: .Qq challenge
806: text, and prompts for a response.
807: Protocol 2 allows multiple challenges and responses;
808: protocol 1 is restricted to just one challenge/response.
809: Examples of challenge-response authentication include
810: BSD Authentication (see
811: .Xr login.conf 5 )
812: and PAM (some non-OpenBSD systems).
813: .Pp
814: Finally, if other authentication methods fail,
1.218 jmc 815: .Nm
816: prompts the user for a password.
817: The password is sent to the remote
818: host for checking; however, since all communications are encrypted,
819: the password cannot be seen by someone listening on the network.
1.232 jmc 820: .Pp
821: .Nm
822: automatically maintains and checks a database containing
823: identification for all hosts it has ever been used with.
824: Host keys are stored in
825: .Pa ~/.ssh/known_hosts
826: in the user's home directory.
827: Additionally, the file
828: .Pa /etc/ssh/ssh_known_hosts
829: is automatically checked for known hosts.
830: Any new hosts are automatically added to the user's file.
831: If a host's identification ever changes,
832: .Nm
833: warns about this and disables password authentication to prevent
834: server spoofing or man-in-the-middle attacks,
835: which could otherwise be used to circumvent the encryption.
836: The
837: .Cm StrictHostKeyChecking
838: option can be used to control logins to machines whose
839: host key is not known or has changed.
840: .Pp
1.218 jmc 841: When the user's identity has been accepted by the server, the server
842: either executes the given command, or logs into the machine and gives
843: the user a normal shell on the remote machine.
844: All communication with
845: the remote command or shell will be automatically encrypted.
846: .Pp
847: If a pseudo-terminal has been allocated (normal login session), the
848: user may use the escape characters noted below.
849: .Pp
850: If no pseudo-tty has been allocated,
851: the session is transparent and can be used to reliably transfer binary data.
852: On most systems, setting the escape character to
853: .Dq none
854: will also make the session transparent even if a tty is used.
855: .Pp
856: The session terminates when the command or shell on the remote
1.247 jmc 857: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 858: .Sh ESCAPE CHARACTERS
1.218 jmc 859: When a pseudo-terminal has been requested,
1.2 deraadt 860: .Nm
1.218 jmc 861: supports a number of functions through the use of an escape character.
862: .Pp
863: A single tilde character can be sent as
864: .Ic ~~
865: or by following the tilde by a character other than those described below.
866: The escape character must always follow a newline to be interpreted as
867: special.
868: The escape character can be changed in configuration files using the
869: .Cm EscapeChar
870: configuration directive or on the command line by the
871: .Fl e
872: option.
873: .Pp
874: The supported escapes (assuming the default
875: .Ql ~ )
876: are:
877: .Bl -tag -width Ds
878: .It Cm ~.
879: Disconnect.
880: .It Cm ~^Z
881: Background
1.234 jmc 882: .Nm .
1.218 jmc 883: .It Cm ~#
884: List forwarded connections.
885: .It Cm ~&
886: Background
1.2 deraadt 887: .Nm
1.218 jmc 888: at logout when waiting for forwarded connection / X11 sessions to terminate.
889: .It Cm ~?
890: Display a list of escape characters.
891: .It Cm ~B
892: Send a BREAK to the remote system
893: (only useful for SSH protocol version 2 and if the peer supports it).
894: .It Cm ~C
895: Open command line.
896: Currently this allows the addition of port forwardings using the
1.279 stevesk 897: .Fl L ,
898: .Fl R
1.218 jmc 899: and
1.279 stevesk 900: .Fl D
1.225 jmc 901: options (see above).
1.322 ! markus 902: It also allows the cancellation of existing port-forwardings
! 903: with
1.262 stevesk 904: .Sm off
1.322 ! markus 905: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 906: .Sm on
1.322 ! markus 907: for local,
! 908: .Sm off
! 909: .Fl KR Oo Ar bind_address : Oc Ar port
! 910: .Sm on
! 911: for remote and
! 912: .Sm off
! 913: .Fl KD Oo Ar bind_address : Oc Ar port
! 914: .Sm on
! 915: for dynamic port-forwardings.
1.218 jmc 916: .Ic !\& Ns Ar command
917: allows the user to execute a local command if the
918: .Ic PermitLocalCommand
919: option is enabled in
1.176 jmc 920: .Xr ssh_config 5 .
1.218 jmc 921: Basic help is available, using the
922: .Fl h
923: option.
924: .It Cm ~R
925: Request rekeying of the connection
926: (only useful for SSH protocol version 2 and if the peer supports it).
1.176 jmc 927: .El
1.246 jmc 928: .Sh TCP FORWARDING
929: Forwarding of arbitrary TCP connections over the secure channel can
930: be specified either on the command line or in a configuration file.
931: One possible application of TCP forwarding is a secure connection to a
932: mail server; another is going through firewalls.
933: .Pp
934: In the example below, we look at encrypting communication between
935: an IRC client and server, even though the IRC server does not directly
936: support encrypted communications.
937: This works as follows:
938: the user connects to the remote host using
939: .Nm ,
940: specifying a port to be used to forward connections
941: to the remote server.
942: After that it is possible to start the service which is to be encrypted
943: on the client machine,
944: connecting to the same local port,
945: and
946: .Nm
947: will encrypt and forward the connection.
948: .Pp
949: The following example tunnels an IRC session from client machine
950: .Dq 127.0.0.1
951: (localhost)
952: to remote server
953: .Dq server.example.com :
954: .Bd -literal -offset 4n
955: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
956: $ irc -c '#users' -p 1234 pinky 127.0.0.1
957: .Ed
958: .Pp
959: This tunnels a connection to IRC server
960: .Dq server.example.com ,
961: joining channel
962: .Dq #users ,
963: nickname
964: .Dq pinky ,
965: using port 1234.
966: It doesn't matter which port is used,
967: as long as it's greater than 1023
968: (remember, only root can open sockets on privileged ports)
969: and doesn't conflict with any ports already in use.
970: The connection is forwarded to port 6667 on the remote server,
971: since that's the standard port for IRC services.
972: .Pp
973: The
974: .Fl f
975: option backgrounds
976: .Nm
977: and the remote command
978: .Dq sleep 10
979: is specified to allow an amount of time
980: (10 seconds, in the example)
981: to start the service which is to be tunnelled.
982: If no connections are made within the time specified,
983: .Nm
984: will exit.
985: .Sh X11 FORWARDING
1.218 jmc 986: If the
987: .Cm ForwardX11
988: variable is set to
989: .Dq yes
990: (or see the description of the
1.227 jmc 991: .Fl X ,
992: .Fl x ,
1.218 jmc 993: and
1.227 jmc 994: .Fl Y
1.226 jmc 995: options above)
1.218 jmc 996: and the user is using X11 (the
997: .Ev DISPLAY
998: environment variable is set), the connection to the X11 display is
999: automatically forwarded to the remote side in such a way that any X11
1000: programs started from the shell (or command) will go through the
1001: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1002: from the local machine.
1.218 jmc 1003: The user should not manually set
1004: .Ev DISPLAY .
1005: Forwarding of X11 connections can be
1006: configured on the command line or in configuration files.
1007: .Pp
1008: The
1009: .Ev DISPLAY
1010: value set by
1011: .Nm
1012: will point to the server machine, but with a display number greater than zero.
1013: This is normal, and happens because
1014: .Nm
1015: creates a
1016: .Dq proxy
1017: X server on the server machine for forwarding the
1018: connections over the encrypted channel.
1.200 djm 1019: .Pp
1.218 jmc 1020: .Nm
1021: will also automatically set up Xauthority data on the server machine.
1022: For this purpose, it will generate a random authorization cookie,
1023: store it in Xauthority on the server, and verify that any forwarded
1024: connections carry this cookie and replace it by the real cookie when
1025: the connection is opened.
1026: The real authentication cookie is never
1027: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1028: .Pp
1.218 jmc 1029: If the
1030: .Cm ForwardAgent
1031: variable is set to
1032: .Dq yes
1033: (or see the description of the
1034: .Fl A
1.191 djm 1035: and
1.218 jmc 1036: .Fl a
1.226 jmc 1037: options above) and
1.218 jmc 1038: the user is using an authentication agent, the connection to the agent
1039: is automatically forwarded to the remote side.
1.252 jmc 1040: .Sh VERIFYING HOST KEYS
1041: When connecting to a server for the first time,
1042: a fingerprint of the server's public key is presented to the user
1043: (unless the option
1044: .Cm StrictHostKeyChecking
1045: has been disabled).
1046: Fingerprints can be determined using
1047: .Xr ssh-keygen 1 :
1048: .Pp
1049: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1050: .Pp
1.274 grunk 1051: If the fingerprint is already known, it can be matched
1052: and the key can be accepted or rejected.
1053: Because of the difficulty of comparing host keys
1054: just by looking at hex strings,
1055: there is also support to compare host keys visually,
1056: using
1057: .Em random art .
1058: By setting the
1.275 grunk 1059: .Cm VisualHostKey
1.274 grunk 1060: option to
1.275 grunk 1061: .Dq yes ,
1.274 grunk 1062: a small ASCII graphic gets displayed on every login to a server, no matter
1063: if the session itself is interactive or not.
1064: By learning the pattern a known server produces, a user can easily
1065: find out that the host key has changed when a completely different pattern
1066: is displayed.
1067: Because these patterns are not unambiguous however, a pattern that looks
1068: similar to the pattern remembered only gives a good probability that the
1069: host key is the same, not guaranteed proof.
1070: .Pp
1071: To get a listing of the fingerprints along with their random art for
1072: all known hosts, the following command line can be used:
1073: .Pp
1074: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1075: .Pp
1.252 jmc 1076: If the fingerprint is unknown,
1077: an alternative method of verification is available:
1078: SSH fingerprints verified by DNS.
1079: An additional resource record (RR),
1080: SSHFP,
1081: is added to a zonefile
1082: and the connecting client is able to match the fingerprint
1083: with that of the key presented.
1084: .Pp
1085: In this example, we are connecting a client to a server,
1086: .Dq host.example.com .
1087: The SSHFP resource records should first be added to the zonefile for
1088: host.example.com:
1089: .Bd -literal -offset indent
1.259 jakob 1090: $ ssh-keygen -r host.example.com.
1.252 jmc 1091: .Ed
1092: .Pp
1093: The output lines will have to be added to the zonefile.
1094: To check that the zone is answering fingerprint queries:
1095: .Pp
1096: .Dl $ dig -t SSHFP host.example.com
1097: .Pp
1098: Finally the client connects:
1099: .Bd -literal -offset indent
1100: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1101: [...]
1102: Matching host key fingerprint found in DNS.
1103: Are you sure you want to continue connecting (yes/no)?
1104: .Ed
1105: .Pp
1106: See the
1107: .Cm VerifyHostKeyDNS
1108: option in
1109: .Xr ssh_config 5
1110: for more information.
1.250 jmc 1111: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1112: .Nm
1113: contains support for Virtual Private Network (VPN) tunnelling
1114: using the
1115: .Xr tun 4
1116: network pseudo-device,
1117: allowing two networks to be joined securely.
1118: The
1119: .Xr sshd_config 5
1120: configuration option
1121: .Cm PermitTunnel
1122: controls whether the server supports this,
1123: and at what level (layer 2 or 3 traffic).
1124: .Pp
1125: The following example would connect client network 10.0.50.0/24
1.265 otto 1126: with remote network 10.0.99.0/24 using a point-to-point connection
1127: from 10.1.1.1 to 10.1.1.2,
1128: provided that the SSH server running on the gateway to the remote network,
1129: at 192.168.1.15, allows it.
1130: .Pp
1131: On the client:
1.250 jmc 1132: .Bd -literal -offset indent
1133: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1134: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1135: # route add 10.0.99.0/24 10.1.1.2
1136: .Ed
1137: .Pp
1138: On the server:
1139: .Bd -literal -offset indent
1140: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1141: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1142: .Ed
1143: .Pp
1144: Client access may be more finely tuned via the
1145: .Pa /root/.ssh/authorized_keys
1146: file (see below) and the
1147: .Cm PermitRootLogin
1148: server option.
1.255 jmc 1149: The following entry would permit connections on
1.250 jmc 1150: .Xr tun 4
1.255 jmc 1151: device 1 from user
1.250 jmc 1152: .Dq jane
1.255 jmc 1153: and on tun device 2 from user
1.250 jmc 1154: .Dq john ,
1155: if
1156: .Cm PermitRootLogin
1157: is set to
1158: .Dq forced-commands-only :
1159: .Bd -literal -offset 2n
1160: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1161: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1162: .Ed
1163: .Pp
1.264 ray 1164: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1165: it may be more suited to temporary setups,
1166: such as for wireless VPNs.
1167: More permanent VPNs are better provided by tools such as
1168: .Xr ipsecctl 8
1169: and
1170: .Xr isakmpd 8 .
1.2 deraadt 1171: .Sh ENVIRONMENT
1172: .Nm
1.1 deraadt 1173: will normally set the following environment variables:
1.237 jmc 1174: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1175: .It Ev DISPLAY
1176: The
1177: .Ev DISPLAY
1.40 aaron 1178: variable indicates the location of the X11 server.
1.44 aaron 1179: It is automatically set by
1.2 deraadt 1180: .Nm
1181: to point to a value of the form
1.233 jmc 1182: .Dq hostname:n ,
1183: where
1184: .Dq hostname
1185: indicates the host where the shell runs, and
1186: .Sq n
1187: is an integer \*(Ge 1.
1.40 aaron 1188: .Nm
1189: uses this special value to forward X11 connections over the secure
1190: channel.
1.107 markus 1191: The user should normally not set
1192: .Ev DISPLAY
1193: explicitly, as that
1.1 deraadt 1194: will render the X11 connection insecure (and will require the user to
1195: manually copy any required authorization cookies).
1.2 deraadt 1196: .It Ev HOME
1.1 deraadt 1197: Set to the path of the user's home directory.
1.2 deraadt 1198: .It Ev LOGNAME
1199: Synonym for
1.12 aaron 1200: .Ev USER ;
1201: set for compatibility with systems that use this variable.
1.2 deraadt 1202: .It Ev MAIL
1.129 stevesk 1203: Set to the path of the user's mailbox.
1.40 aaron 1204: .It Ev PATH
1.2 deraadt 1205: Set to the default
1206: .Ev PATH ,
1207: as specified when compiling
1.234 jmc 1208: .Nm .
1.118 markus 1209: .It Ev SSH_ASKPASS
1210: If
1211: .Nm
1212: needs a passphrase, it will read the passphrase from the current
1213: terminal if it was run from a terminal.
1214: If
1215: .Nm
1216: does not have a terminal associated with it but
1217: .Ev DISPLAY
1218: and
1219: .Ev SSH_ASKPASS
1220: are set, it will execute the program specified by
1221: .Ev SSH_ASKPASS
1222: and open an X11 window to read the passphrase.
1223: This is particularly useful when calling
1224: .Nm
1225: from a
1.196 jmc 1226: .Pa .xsession
1.118 markus 1227: or related script.
1228: (Note that on some machines it
1229: may be necessary to redirect the input from
1230: .Pa /dev/null
1231: to make this work.)
1.18 markus 1232: .It Ev SSH_AUTH_SOCK
1.233 jmc 1233: Identifies the path of a
1234: .Ux Ns -domain
1235: socket used to communicate with the agent.
1.166 stevesk 1236: .It Ev SSH_CONNECTION
1237: Identifies the client and server ends of the connection.
1.40 aaron 1238: The variable contains
1.233 jmc 1239: four space-separated values: client IP address, client port number,
1240: server IP address, and server port number.
1.73 markus 1241: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1242: This variable contains the original command line if a forced command
1.73 markus 1243: is executed.
1244: It can be used to extract the original arguments.
1.2 deraadt 1245: .It Ev SSH_TTY
1.1 deraadt 1246: This is set to the name of the tty (path to the device) associated
1.40 aaron 1247: with the current shell or command.
1248: If the current session has no tty,
1.1 deraadt 1249: this variable is not set.
1.2 deraadt 1250: .It Ev TZ
1.214 jmc 1251: This variable is set to indicate the present time zone if it
1.257 jmc 1252: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1253: on to new connections).
1.2 deraadt 1254: .It Ev USER
1.1 deraadt 1255: Set to the name of the user logging in.
1.2 deraadt 1256: .El
1257: .Pp
1.44 aaron 1258: Additionally,
1.2 deraadt 1259: .Nm
1.44 aaron 1260: reads
1.207 djm 1261: .Pa ~/.ssh/environment ,
1.2 deraadt 1262: and adds lines of the format
1263: .Dq VARNAME=value
1.233 jmc 1264: to the environment if the file exists and users are allowed to
1.161 marc 1265: change their environment.
1.176 jmc 1266: For more information, see the
1.161 marc 1267: .Cm PermitUserEnvironment
1.162 stevesk 1268: option in
1.161 marc 1269: .Xr sshd_config 5 .
1.2 deraadt 1270: .Sh FILES
1.236 jmc 1271: .Bl -tag -width Ds -compact
1.309 jmc 1272: .It Pa ~/.rhosts
1.240 jmc 1273: This file is used for host-based authentication (see above).
1.92 markus 1274: On some machines this file may need to be
1.240 jmc 1275: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1276: because
1.2 deraadt 1277: .Xr sshd 8
1.40 aaron 1278: reads it as root.
1279: Additionally, this file must be owned by the user,
1280: and must not have write permissions for anyone else.
1281: The recommended
1.1 deraadt 1282: permission for most machines is read/write for the user, and not
1283: accessible by others.
1.2 deraadt 1284: .Pp
1.309 jmc 1285: .It Pa ~/.shosts
1.240 jmc 1286: This file is used in exactly the same way as
1287: .Pa .rhosts ,
1288: but allows host-based authentication without permitting login with
1289: rlogin/rsh.
1.272 mcbride 1290: .Pp
1.309 jmc 1291: .It Pa ~/.ssh/
1.272 mcbride 1292: This directory is the default location for all user-specific configuration
1293: and authentication information.
1294: There is no general requirement to keep the entire contents of this directory
1295: secret, but the recommended permissions are read/write/execute for the user,
1296: and not accessible by others.
1.236 jmc 1297: .Pp
1.309 jmc 1298: .It Pa ~/.ssh/authorized_keys
1.310 djm 1299: Lists the public keys (DSA/ECDSA/RSA) that can be used for logging in as
1300: this user.
1.238 jmc 1301: The format of this file is described in the
1302: .Xr sshd 8
1303: manual page.
1304: This file is not highly sensitive, but the recommended
1305: permissions are read/write for the user, and not accessible by others.
1306: .Pp
1.309 jmc 1307: .It Pa ~/.ssh/config
1.238 jmc 1308: This is the per-user configuration file.
1309: The file format and configuration options are described in
1310: .Xr ssh_config 5 .
1311: Because of the potential for abuse, this file must have strict permissions:
1312: read/write for the user, and not accessible by others.
1313: .Pp
1.309 jmc 1314: .It Pa ~/.ssh/environment
1.239 jmc 1315: Contains additional definitions for environment variables; see
1316: .Sx ENVIRONMENT ,
1.238 jmc 1317: above.
1318: .Pp
1.309 jmc 1319: .It Pa ~/.ssh/identity
1320: .It Pa ~/.ssh/id_dsa
1.310 djm 1321: .It Pa ~/.ssh/id_ecdsa
1.309 jmc 1322: .It Pa ~/.ssh/id_rsa
1.238 jmc 1323: Contains the private key for authentication.
1324: These files
1325: contain sensitive data and should be readable by the user but not
1326: accessible by others (read/write/execute).
1327: .Nm
1328: will simply ignore a private key file if it is accessible by others.
1329: It is possible to specify a passphrase when
1330: generating the key which will be used to encrypt the
1331: sensitive part of this file using 3DES.
1332: .Pp
1.309 jmc 1333: .It Pa ~/.ssh/identity.pub
1334: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1335: .It Pa ~/.ssh/id_ecdsa.pub
1.309 jmc 1336: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1337: Contains the public key for authentication.
1338: These files are not
1339: sensitive and can (but need not) be readable by anyone.
1340: .Pp
1.309 jmc 1341: .It Pa ~/.ssh/known_hosts
1.244 jmc 1342: Contains a list of host keys for all hosts the user has logged into
1343: that are not already in the systemwide list of known host keys.
1.238 jmc 1344: See
1.244 jmc 1345: .Xr sshd 8
1346: for further details of the format of this file.
1.238 jmc 1347: .Pp
1.309 jmc 1348: .It Pa ~/.ssh/rc
1.238 jmc 1349: Commands in this file are executed by
1350: .Nm
1.245 jmc 1351: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1352: started.
1353: See the
1354: .Xr sshd 8
1355: manual page for more information.
1356: .Pp
1.309 jmc 1357: .It Pa /etc/hosts.equiv
1.240 jmc 1358: This file is for host-based authentication (see above).
1359: It should only be writable by root.
1.236 jmc 1360: .Pp
1.309 jmc 1361: .It Pa /etc/shosts.equiv
1.240 jmc 1362: This file is used in exactly the same way as
1363: .Pa hosts.equiv ,
1364: but allows host-based authentication without permitting login with
1365: rlogin/rsh.
1.236 jmc 1366: .Pp
1.238 jmc 1367: .It Pa /etc/ssh/ssh_config
1368: Systemwide configuration file.
1369: The file format and configuration options are described in
1370: .Xr ssh_config 5 .
1371: .Pp
1.309 jmc 1372: .It Pa /etc/ssh/ssh_host_key
1373: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1374: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.309 jmc 1375: .It Pa /etc/ssh/ssh_host_rsa_key
1.238 jmc 1376: These three files contain the private parts of the host keys
1.245 jmc 1377: and are used for host-based authentication.
1378: If protocol version 1 is used,
1.238 jmc 1379: .Nm
1380: must be setuid root, since the host key is readable only by root.
1381: For protocol version 2,
1382: .Nm
1383: uses
1384: .Xr ssh-keysign 8
1.245 jmc 1385: to access the host keys,
1386: eliminating the requirement that
1.238 jmc 1387: .Nm
1.245 jmc 1388: be setuid root when host-based authentication is used.
1.238 jmc 1389: By default
1.2 deraadt 1390: .Nm
1.238 jmc 1391: is not setuid root.
1392: .Pp
1.309 jmc 1393: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1394: Systemwide list of known host keys.
1395: This file should be prepared by the
1396: system administrator to contain the public host keys of all machines in the
1397: organization.
1.244 jmc 1398: It should be world-readable.
1399: See
1.238 jmc 1400: .Xr sshd 8
1.244 jmc 1401: for further details of the format of this file.
1.236 jmc 1402: .Pp
1.309 jmc 1403: .It Pa /etc/ssh/sshrc
1.1 deraadt 1404: Commands in this file are executed by
1.2 deraadt 1405: .Nm
1.245 jmc 1406: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1407: See the
1.2 deraadt 1408: .Xr sshd 8
1.1 deraadt 1409: manual page for more information.
1.58 itojun 1410: .El
1.312 jmc 1411: .Sh EXIT STATUS
1412: .Nm
1413: exits with the exit status of the remote command or with 255
1414: if an error occurred.
1.2 deraadt 1415: .Sh SEE ALSO
1416: .Xr scp 1 ,
1.83 djm 1417: .Xr sftp 1 ,
1.2 deraadt 1418: .Xr ssh-add 1 ,
1419: .Xr ssh-agent 1 ,
1420: .Xr ssh-keygen 1 ,
1.242 jmc 1421: .Xr ssh-keyscan 1 ,
1.250 jmc 1422: .Xr tun 4 ,
1.176 jmc 1423: .Xr hosts.equiv 5 ,
1.159 stevesk 1424: .Xr ssh_config 5 ,
1.160 naddy 1425: .Xr ssh-keysign 8 ,
1.87 itojun 1426: .Xr sshd 8
1.106 markus 1427: .Rs
1.256 jmc 1428: .%R RFC 4250
1429: .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1430: .%D 2006
1431: .Re
1432: .Rs
1433: .%R RFC 4251
1434: .%T "The Secure Shell (SSH) Protocol Architecture"
1435: .%D 2006
1436: .Re
1437: .Rs
1438: .%R RFC 4252
1439: .%T "The Secure Shell (SSH) Authentication Protocol"
1440: .%D 2006
1441: .Re
1442: .Rs
1443: .%R RFC 4253
1444: .%T "The Secure Shell (SSH) Transport Layer Protocol"
1445: .%D 2006
1446: .Re
1447: .Rs
1448: .%R RFC 4254
1449: .%T "The Secure Shell (SSH) Connection Protocol"
1450: .%D 2006
1451: .Re
1452: .Rs
1453: .%R RFC 4255
1454: .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1455: .%D 2006
1456: .Re
1457: .Rs
1458: .%R RFC 4256
1459: .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1460: .%D 2006
1461: .Re
1462: .Rs
1463: .%R RFC 4335
1464: .%T "The Secure Shell (SSH) Session Channel Break Extension"
1465: .%D 2006
1466: .Re
1467: .Rs
1468: .%R RFC 4344
1469: .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1470: .%D 2006
1471: .Re
1472: .Rs
1473: .%R RFC 4345
1474: .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1.258 djm 1475: .%D 2006
1476: .Re
1477: .Rs
1478: .%R RFC 4419
1479: .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1.266 markus 1480: .%D 2006
1481: .Re
1482: .Rs
1483: .%R RFC 4716
1484: .%T "The Secure Shell (SSH) Public Key File Format"
1.256 jmc 1485: .%D 2006
1.313 djm 1486: .Re
1487: .Rs
1488: .%R RFC 5656
1489: .%T "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer"
1490: .%D 2009
1.274 grunk 1491: .Re
1492: .Rs
1493: .%T "Hash Visualization: a New Technique to improve Real-World Security"
1494: .%A A. Perrig
1495: .%A D. Song
1496: .%D 1999
1497: .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1.106 markus 1498: .Re
1.173 jmc 1499: .Sh AUTHORS
1500: OpenSSH is a derivative of the original and free
1501: ssh 1.2.12 release by Tatu Ylonen.
1502: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1503: Theo de Raadt and Dug Song
1504: removed many bugs, re-added newer features and
1505: created OpenSSH.
1506: Markus Friedl contributed the support for SSH
1507: protocol versions 1.5 and 2.0.