Annotation of src/usr.bin/ssh/ssh.1, Revision 1.346
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: .\"
1.346 ! djm 36: .\" $OpenBSD: ssh.1,v 1.345 2014/04/19 18:42:19 tedu Exp $
! 37: .Dd $Mdocdate: April 19 2014 $
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
1.331 dtucker 50: .Op Fl E Ar log_file
1.2 deraadt 51: .Op Fl e Ar escape_char
1.176 jmc 52: .Op Fl F Ar configfile
1.292 jmc 53: .Op Fl I Ar pkcs11
1.2 deraadt 54: .Op Fl i Ar identity_file
1.306 jmc 55: .Op Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.176 jmc 56: .Op Fl l Ar login_name
57: .Op Fl m Ar mac_spec
1.198 djm 58: .Op Fl O Ar ctl_cmd
1.176 jmc 59: .Op Fl o Ar option
60: .Op Fl p Ar port
1.342 jmc 61: .Op Fl Q Cm cipher | cipher-auth | mac | kex | key
1.306 jmc 62: .Op Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
1.198 djm 63: .Op Fl S Ar ctl_path
1.290 dtucker 64: .Op Fl W Ar host : Ns Ar port
1.306 jmc 65: .Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
1.176 jmc 66: .Oo Ar user Ns @ Oc Ns Ar hostname
1.2 deraadt 67: .Op Ar command
1.306 jmc 68: .Ek
1.44 aaron 69: .Sh DESCRIPTION
1.2 deraadt 70: .Nm
1.96 deraadt 71: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 72: executing commands on a remote machine.
1.176 jmc 73: It is intended to replace rlogin and rsh,
74: and provide secure encrypted communications between
1.40 aaron 75: two untrusted hosts over an insecure network.
1.247 jmc 76: X11 connections and arbitrary TCP ports
1.176 jmc 77: can also be forwarded over the secure channel.
1.2 deraadt 78: .Pp
79: .Nm
1.44 aaron 80: connects and logs into the specified
1.176 jmc 81: .Ar hostname
82: (with optional
83: .Ar user
84: name).
1.1 deraadt 85: The user must prove
1.49 markus 86: his/her identity to the remote machine using one of several methods
1.221 jmc 87: depending on the protocol version used (see below).
1.49 markus 88: .Pp
1.176 jmc 89: If
90: .Ar command
91: is specified,
1.219 jmc 92: it is executed on the remote host instead of a login shell.
1.2 deraadt 93: .Pp
1.218 jmc 94: The options are as follows:
95: .Bl -tag -width Ds
96: .It Fl 1
97: Forces
1.2 deraadt 98: .Nm
1.218 jmc 99: to try protocol version 1 only.
100: .It Fl 2
101: Forces
1.2 deraadt 102: .Nm
1.218 jmc 103: to try protocol version 2 only.
104: .It Fl 4
105: Forces
1.2 deraadt 106: .Nm
1.218 jmc 107: to use IPv4 addresses only.
108: .It Fl 6
109: Forces
1.2 deraadt 110: .Nm
1.218 jmc 111: to use IPv6 addresses only.
112: .It Fl A
113: Enables forwarding of the authentication agent connection.
114: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 115: .Pp
1.218 jmc 116: Agent forwarding should be enabled with caution.
117: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 118: (for the agent's
1.286 sobrado 119: .Ux Ns -domain
120: socket) can access the local agent through the forwarded connection.
1.218 jmc 121: An attacker cannot obtain key material from the agent,
122: however they can perform operations on the keys that enable them to
123: authenticate using the identities loaded into the agent.
124: .It Fl a
125: Disables forwarding of the authentication agent connection.
126: .It Fl b Ar bind_address
127: Use
128: .Ar bind_address
129: on the local machine as the source address
130: of the connection.
131: Only useful on systems with more than one address.
132: .It Fl C
133: Requests compression of all data (including stdin, stdout, stderr, and
1.247 jmc 134: data for forwarded X11 and TCP connections).
1.218 jmc 135: The compression algorithm is the same used by
136: .Xr gzip 1 ,
137: and the
138: .Dq level
139: can be controlled by the
140: .Cm CompressionLevel
141: option for protocol version 1.
142: Compression is desirable on modem lines and other
143: slow connections, but will only slow down things on fast networks.
144: The default value can be set on a host-by-host basis in the
145: configuration files; see the
146: .Cm Compression
147: option.
148: .It Fl c Ar cipher_spec
149: Selects the cipher specification for encrypting the session.
1.2 deraadt 150: .Pp
1.218 jmc 151: Protocol version 1 allows specification of a single cipher.
152: The supported values are
153: .Dq 3des ,
1.220 jmc 154: .Dq blowfish ,
1.218 jmc 155: and
156: .Dq des .
1.230 jmc 157: For protocol version 2,
1.218 jmc 158: .Ar cipher_spec
159: is a comma-separated list of ciphers
160: listed in order of preference.
1.283 jmc 161: See the
162: .Cm Ciphers
1.307 dtucker 163: keyword in
164: .Xr ssh_config 5
165: for more information.
1.218 jmc 166: .It Fl D Xo
167: .Sm off
168: .Oo Ar bind_address : Oc
169: .Ar port
170: .Sm on
171: .Xc
172: Specifies a local
173: .Dq dynamic
174: application-level port forwarding.
175: This works by allocating a socket to listen to
176: .Ar port
177: on the local side, optionally bound to the specified
178: .Ar bind_address .
179: Whenever a connection is made to this port, the
180: connection is forwarded over the secure channel, and the application
181: protocol is then used to determine where to connect to from the
182: remote machine.
183: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 184: .Nm
1.218 jmc 185: will act as a SOCKS server.
186: Only root can forward privileged ports.
187: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 188: .Pp
1.308 djm 189: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 190: Only the superuser can forward privileged ports.
191: By default, the local port is bound in accordance with the
192: .Cm GatewayPorts
193: setting.
194: However, an explicit
195: .Ar bind_address
196: may be used to bind the connection to a specific address.
197: The
198: .Ar bind_address
199: of
200: .Dq localhost
201: indicates that the listening port be bound for local use only, while an
202: empty address or
203: .Sq *
204: indicates that the port should be available from all interfaces.
1.331 dtucker 205: .It Fl E Ar log_file
206: Append debug logs to
207: .Ar log_file
208: instead of standard error.
1.229 jmc 209: .It Fl e Ar escape_char
1.218 jmc 210: Sets the escape character for sessions with a pty (default:
211: .Ql ~ ) .
212: The escape character is only recognized at the beginning of a line.
213: The escape character followed by a dot
214: .Pq Ql \&.
215: closes the connection;
216: followed by control-Z suspends the connection;
217: and followed by itself sends the escape character once.
218: Setting the character to
1.2 deraadt 219: .Dq none
1.218 jmc 220: disables any escapes and makes the session fully transparent.
221: .It Fl F Ar configfile
222: Specifies an alternative per-user configuration file.
223: If a configuration file is given on the command line,
224: the system-wide configuration file
225: .Pq Pa /etc/ssh/ssh_config
226: will be ignored.
227: The default for the per-user configuration file is
228: .Pa ~/.ssh/config .
229: .It Fl f
230: Requests
231: .Nm
232: to go to background just before command execution.
233: This is useful if
1.176 jmc 234: .Nm
1.218 jmc 235: is going to ask for passwords or passphrases, but the user
236: wants it in the background.
237: This implies
238: .Fl n .
239: The recommended way to start X11 programs at a remote site is with
240: something like
241: .Ic ssh -f host xterm .
1.277 djm 242: .Pp
243: If the
244: .Cm ExitOnForwardFailure
245: configuration option is set to
246: .Dq yes ,
247: then a client started with
248: .Fl f
249: will wait for all remote port forwards to be successfully established
250: before placing itself in the background.
1.218 jmc 251: .It Fl g
252: Allows remote hosts to connect to local forwarded ports.
1.346 ! djm 253: If used on a multiplexed connection, then this option must be specified
! 254: on the master process.
1.291 markus 255: .It Fl I Ar pkcs11
1.294 jmc 256: Specify the PKCS#11 shared library
1.176 jmc 257: .Nm
1.293 markus 258: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 259: private RSA key.
260: .It Fl i Ar identity_file
261: Selects a file from which the identity (private key) for
1.310 djm 262: public key authentication is read.
1.218 jmc 263: The default is
264: .Pa ~/.ssh/identity
265: for protocol version 1, and
1.310 djm 266: .Pa ~/.ssh/id_dsa ,
1.343 naddy 267: .Pa ~/.ssh/id_ecdsa ,
268: .Pa ~/.ssh/id_ed25519
1.310 djm 269: and
1.218 jmc 270: .Pa ~/.ssh/id_rsa
271: for protocol version 2.
272: Identity files may also be specified on
273: a per-host basis in the configuration file.
274: It is possible to have multiple
275: .Fl i
276: options (and multiple identities specified in
277: configuration files).
1.302 djm 278: .Nm
279: will also try to load certificate information from the filename obtained
280: by appending
281: .Pa -cert.pub
282: to identity filenames.
1.269 djm 283: .It Fl K
284: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
285: credentials to the server.
1.218 jmc 286: .It Fl k
287: Disables forwarding (delegation) of GSSAPI credentials to the server.
288: .It Fl L Xo
289: .Sm off
290: .Oo Ar bind_address : Oc
291: .Ar port : host : hostport
292: .Sm on
293: .Xc
294: Specifies that the given port on the local (client) host is to be
295: forwarded to the given host and port on the remote side.
296: This works by allocating a socket to listen to
297: .Ar port
298: on the local side, optionally bound to the specified
299: .Ar bind_address .
300: Whenever a connection is made to this port, the
301: connection is forwarded over the secure channel, and a connection is
302: made to
303: .Ar host
304: port
305: .Ar hostport
306: from the remote machine.
307: Port forwardings can also be specified in the configuration file.
1.308 djm 308: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 309: Only the superuser can forward privileged ports.
310: By default, the local port is bound in accordance with the
311: .Cm GatewayPorts
312: setting.
313: However, an explicit
314: .Ar bind_address
315: may be used to bind the connection to a specific address.
1.2 deraadt 316: The
1.218 jmc 317: .Ar bind_address
318: of
319: .Dq localhost
320: indicates that the listening port be bound for local use only, while an
321: empty address or
322: .Sq *
323: indicates that the port should be available from all interfaces.
324: .It Fl l Ar login_name
325: Specifies the user to log in as on the remote machine.
326: This also may be specified on a per-host basis in the configuration file.
327: .It Fl M
328: Places the
329: .Nm
330: client into
331: .Dq master
332: mode for connection sharing.
1.231 stevesk 333: Multiple
334: .Fl M
335: options places
336: .Nm
337: into
338: .Dq master
339: mode with confirmation required before slave connections are accepted.
1.218 jmc 340: Refer to the description of
341: .Cm ControlMaster
342: in
343: .Xr ssh_config 5
344: for details.
345: .It Fl m Ar mac_spec
346: Additionally, for protocol version 2 a comma-separated list of MAC
347: (message authentication code) algorithms can
348: be specified in order of preference.
349: See the
350: .Cm MACs
351: keyword for more information.
352: .It Fl N
353: Do not execute a remote command.
354: This is useful for just forwarding ports
355: (protocol version 2 only).
356: .It Fl n
357: Redirects stdin from
358: .Pa /dev/null
359: (actually, prevents reading from stdin).
360: This must be used when
1.2 deraadt 361: .Nm
1.218 jmc 362: is run in the background.
363: A common trick is to use this to run X11 programs on a remote machine.
364: For example,
365: .Ic ssh -n shadows.cs.hut.fi emacs &
366: will start an emacs on shadows.cs.hut.fi, and the X11
367: connection will be automatically forwarded over an encrypted channel.
368: The
1.2 deraadt 369: .Nm
1.218 jmc 370: program will be put in the background.
371: (This does not work if
1.2 deraadt 372: .Nm
1.218 jmc 373: needs to ask for a password or passphrase; see also the
374: .Fl f
375: option.)
376: .It Fl O Ar ctl_cmd
377: Control an active connection multiplexing master process.
378: When the
379: .Fl O
380: option is specified, the
381: .Ar ctl_cmd
382: argument is interpreted and passed to the master process.
383: Valid commands are:
384: .Dq check
1.305 markus 385: (check that the master process is running),
386: .Dq forward
1.318 jmc 387: (request forwardings without command execution),
1.323 okan 388: .Dq cancel
389: (cancel forwardings),
1.218 jmc 390: .Dq exit
1.318 jmc 391: (request the master to exit), and
1.317 djm 392: .Dq stop
393: (request the master to stop accepting further multiplexing requests).
1.218 jmc 394: .It Fl o Ar option
395: Can be used to give options in the format used in the configuration file.
396: This is useful for specifying options for which there is no separate
397: command-line flag.
398: For full details of the options listed below, and their possible values, see
399: .Xr ssh_config 5 .
1.2 deraadt 400: .Pp
1.218 jmc 401: .Bl -tag -width Ds -offset indent -compact
402: .It AddressFamily
403: .It BatchMode
404: .It BindAddress
1.338 djm 405: .It CanonicalDomains
1.339 djm 406: .It CanonicalizeFallbackLocal
407: .It CanonicalizeHostname
408: .It CanonicalizeMaxDots
409: .It CanonicalizePermittedCNAMEs
1.218 jmc 410: .It ChallengeResponseAuthentication
411: .It CheckHostIP
412: .It Cipher
413: .It Ciphers
414: .It ClearAllForwardings
415: .It Compression
416: .It CompressionLevel
417: .It ConnectionAttempts
418: .It ConnectTimeout
419: .It ControlMaster
420: .It ControlPath
1.321 djm 421: .It ControlPersist
1.218 jmc 422: .It DynamicForward
423: .It EscapeChar
1.263 markus 424: .It ExitOnForwardFailure
1.218 jmc 425: .It ForwardAgent
426: .It ForwardX11
1.321 djm 427: .It ForwardX11Timeout
1.218 jmc 428: .It ForwardX11Trusted
429: .It GatewayPorts
430: .It GlobalKnownHostsFile
431: .It GSSAPIAuthentication
432: .It GSSAPIDelegateCredentials
433: .It HashKnownHosts
434: .It Host
435: .It HostbasedAuthentication
436: .It HostKeyAlgorithms
437: .It HostKeyAlias
438: .It HostName
439: .It IdentityFile
440: .It IdentitiesOnly
1.316 jmc 441: .It IPQoS
1.321 djm 442: .It KbdInteractiveAuthentication
1.218 jmc 443: .It KbdInteractiveDevices
1.314 jmc 444: .It KexAlgorithms
1.218 jmc 445: .It LocalCommand
446: .It LocalForward
447: .It LogLevel
448: .It MACs
1.337 jmc 449: .It Match
1.218 jmc 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
1.336 jmc 459: .It ProxyUseFdpass
1.218 jmc 460: .It PubkeyAuthentication
1.251 dtucker 461: .It RekeyLimit
1.218 jmc 462: .It RemoteForward
1.319 jmc 463: .It RequestTTY
1.218 jmc 464: .It RhostsRSAAuthentication
465: .It RSAAuthentication
466: .It SendEnv
467: .It ServerAliveInterval
468: .It ServerAliveCountMax
469: .It StrictHostKeyChecking
470: .It TCPKeepAlive
471: .It Tunnel
472: .It TunnelDevice
473: .It UsePrivilegedPort
474: .It User
475: .It UserKnownHostsFile
476: .It VerifyHostKeyDNS
1.276 jmc 477: .It VisualHostKey
1.218 jmc 478: .It XAuthLocation
479: .El
480: .It Fl p Ar port
481: Port to connect to on the remote host.
482: This can be specified on a
483: per-host basis in the configuration file.
1.342 jmc 484: .It Fl Q Cm cipher | cipher-auth | mac | kex | key
1.332 djm 485: Queries
486: .Nm
1.341 deraadt 487: for the algorithms supported for the specified version 2.
488: The available features are:
489: .Ar cipher
1.332 djm 490: (supported symmetric ciphers),
1.341 deraadt 491: .Ar cipher-auth
1.340 djm 492: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 493: .Ar mac
1.332 djm 494: (supported message integrity codes),
1.341 deraadt 495: .Ar kex
1.332 djm 496: (key exchange algorithms),
1.341 deraadt 497: .Ar key
1.332 djm 498: (key types).
1.218 jmc 499: .It Fl q
500: Quiet mode.
1.271 djm 501: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 502: .It Fl R Xo
503: .Sm off
504: .Oo Ar bind_address : Oc
505: .Ar port : host : hostport
506: .Sm on
507: .Xc
508: Specifies that the given port on the remote (server) host is to be
509: forwarded to the given host and port on the local side.
510: This works by allocating a socket to listen to
511: .Ar port
512: on the remote side, and whenever a connection is made to this port, the
513: connection is forwarded over the secure channel, and a connection is
514: made to
515: .Ar host
516: port
517: .Ar hostport
518: from the local machine.
1.2 deraadt 519: .Pp
1.218 jmc 520: Port forwardings can also be specified in the configuration file.
521: Privileged ports can be forwarded only when
522: logging in as root on the remote machine.
1.324 jmc 523: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 524: .Pp
1.218 jmc 525: By default, the listening socket on the server will be bound to the loopback
526: interface only.
1.280 tobias 527: This may be overridden by specifying a
1.218 jmc 528: .Ar bind_address .
529: An empty
530: .Ar bind_address ,
531: or the address
532: .Ql * ,
533: indicates that the remote socket should listen on all interfaces.
534: Specifying a remote
535: .Ar bind_address
536: will only succeed if the server's
537: .Cm GatewayPorts
538: option is enabled (see
539: .Xr sshd_config 5 ) .
1.281 djm 540: .Pp
541: If the
542: .Ar port
543: argument is
1.282 djm 544: .Ql 0 ,
1.281 djm 545: the listen port will be dynamically allocated on the server and reported
546: to the client at run time.
1.305 markus 547: When used together with
548: .Ic -O forward
549: the allocated port will be printed to the standard output.
1.218 jmc 550: .It Fl S Ar ctl_path
1.304 jmc 551: Specifies the location of a control socket for connection sharing,
1.303 djm 552: or the string
553: .Dq none
554: to disable connection sharing.
1.218 jmc 555: Refer to the description of
556: .Cm ControlPath
557: and
558: .Cm ControlMaster
559: in
560: .Xr ssh_config 5
561: for details.
562: .It Fl s
563: May be used to request invocation of a subsystem on the remote system.
564: Subsystems are a feature of the SSH2 protocol which facilitate the use
565: of SSH as a secure transport for other applications (eg.\&
566: .Xr sftp 1 ) .
567: The subsystem is specified as the remote command.
568: .It Fl T
569: Disable pseudo-tty allocation.
570: .It Fl t
571: Force pseudo-tty allocation.
572: This can be used to execute arbitrary
573: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 574: e.g. when implementing menu services.
1.218 jmc 575: Multiple
576: .Fl t
577: options force tty allocation, even if
1.194 jakob 578: .Nm
1.218 jmc 579: has no local tty.
580: .It Fl V
581: Display the version number and exit.
582: .It Fl v
583: Verbose mode.
584: Causes
1.176 jmc 585: .Nm
1.218 jmc 586: to print debugging messages about its progress.
587: This is helpful in
588: debugging connection, authentication, and configuration problems.
589: Multiple
590: .Fl v
591: options increase the verbosity.
592: The maximum is 3.
1.290 dtucker 593: .It Fl W Ar host : Ns Ar port
594: Requests that standard input and output on the client be forwarded to
595: .Ar host
596: on
597: .Ar port
598: over the secure channel.
599: Implies
600: .Fl N ,
601: .Fl T ,
602: .Cm ExitOnForwardFailure
603: and
1.326 dtucker 604: .Cm ClearAllForwardings .
605: Works with Protocol version 2 only.
1.261 stevesk 606: .It Fl w Xo
607: .Ar local_tun Ns Op : Ns Ar remote_tun
608: .Xc
609: Requests
610: tunnel
611: device forwarding with the specified
1.218 jmc 612: .Xr tun 4
1.261 stevesk 613: devices between the client
614: .Pq Ar local_tun
615: and the server
616: .Pq Ar remote_tun .
617: .Pp
1.228 jmc 618: The devices may be specified by numerical ID or the keyword
619: .Dq any ,
620: which uses the next available tunnel device.
1.261 stevesk 621: If
622: .Ar remote_tun
623: is not specified, it defaults to
624: .Dq any .
1.228 jmc 625: See also the
1.218 jmc 626: .Cm Tunnel
1.261 stevesk 627: and
628: .Cm TunnelDevice
629: directives in
1.218 jmc 630: .Xr ssh_config 5 .
1.261 stevesk 631: If the
632: .Cm Tunnel
633: directive is unset, it is set to the default tunnel mode, which is
634: .Dq point-to-point .
1.218 jmc 635: .It Fl X
636: Enables X11 forwarding.
1.54 markus 637: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 638: .Pp
1.218 jmc 639: X11 forwarding should be enabled with caution.
1.168 jmc 640: Users with the ability to bypass file permissions on the remote host
1.218 jmc 641: (for the user's X authorization database)
642: can access the local X11 display through the forwarded connection.
643: An attacker may then be able to perform activities such as keystroke monitoring.
644: .Pp
645: For this reason, X11 forwarding is subjected to X11 SECURITY extension
646: restrictions by default.
647: Please refer to the
648: .Nm
649: .Fl Y
650: option and the
651: .Cm ForwardX11Trusted
652: directive in
653: .Xr ssh_config 5
654: for more information.
655: .It Fl x
656: Disables X11 forwarding.
657: .It Fl Y
658: Enables trusted X11 forwarding.
659: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
660: controls.
1.278 djm 661: .It Fl y
662: Send log information using the
663: .Xr syslog 3
664: system module.
665: By default this information is sent to stderr.
1.218 jmc 666: .El
1.224 jmc 667: .Pp
668: .Nm
669: may additionally obtain configuration data from
670: a per-user configuration file and a system-wide configuration file.
671: The file format and configuration options are described in
672: .Xr ssh_config 5 .
1.222 jmc 673: .Sh AUTHENTICATION
1.249 jmc 674: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 675: The default is to use protocol 2 only,
676: though this can be changed via the
1.222 jmc 677: .Cm Protocol
678: option in
1.284 jmc 679: .Xr ssh_config 5
680: or the
1.222 jmc 681: .Fl 1
682: and
683: .Fl 2
684: options (see above).
685: Both protocols support similar authentication methods,
1.284 jmc 686: but protocol 2 is the default since
1.222 jmc 687: it provides additional mechanisms for confidentiality
688: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 689: and integrity (hmac-md5, hmac-sha1,
690: hmac-sha2-256, hmac-sha2-512,
1.330 markus 691: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 692: Protocol 1 lacks a strong mechanism for ensuring the
693: integrity of the connection.
694: .Pp
695: The methods available for authentication are:
1.260 jmc 696: GSSAPI-based authentication,
1.222 jmc 697: host-based authentication,
698: public key authentication,
699: challenge-response authentication,
700: and password authentication.
701: Authentication methods are tried in the order specified above,
702: though protocol 2 has a configuration option to change the default order:
703: .Cm PreferredAuthentications .
704: .Pp
705: Host-based authentication works as follows:
1.218 jmc 706: If the machine the user logs in from is listed in
707: .Pa /etc/hosts.equiv
708: or
709: .Pa /etc/shosts.equiv
710: on the remote machine, and the user names are
711: the same on both sides, or if the files
712: .Pa ~/.rhosts
713: or
714: .Pa ~/.shosts
715: exist in the user's home directory on the
716: remote machine and contain a line containing the name of the client
717: machine and the name of the user on that machine, the user is
1.222 jmc 718: considered for login.
719: Additionally, the server
720: .Em must
721: be able to verify the client's
722: host key (see the description of
1.218 jmc 723: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 724: and
1.222 jmc 725: .Pa ~/.ssh/known_hosts ,
726: below)
727: for login to be permitted.
1.218 jmc 728: This authentication method closes security holes due to IP
1.222 jmc 729: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 730: [Note to the administrator:
731: .Pa /etc/hosts.equiv ,
732: .Pa ~/.rhosts ,
733: and the rlogin/rsh protocol in general, are inherently insecure and should be
734: disabled if security is desired.]
1.189 dtucker 735: .Pp
1.222 jmc 736: Public key authentication works as follows:
737: The scheme is based on public-key cryptography,
738: using cryptosystems
739: where encryption and decryption are done using separate keys,
740: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 741: The idea is that each user creates a public/private
742: key pair for authentication purposes.
743: The server knows the public key, and only the user knows the private key.
1.222 jmc 744: .Nm
745: implements public key authentication protocol automatically,
1.343 naddy 746: using one of the DSA, ECDSA, ED25519 or RSA algorithms.
1.222 jmc 747: Protocol 1 is restricted to using only RSA keys,
1.310 djm 748: but protocol 2 may use any.
1.333 jmc 749: The HISTORY section of
1.222 jmc 750: .Xr ssl 8
1.311 jmc 751: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 752: .Pp
1.218 jmc 753: The file
754: .Pa ~/.ssh/authorized_keys
755: lists the public keys that are permitted for logging in.
756: When the user logs in, the
1.2 deraadt 757: .Nm
1.218 jmc 758: program tells the server which key pair it would like to use for
759: authentication.
1.222 jmc 760: The client proves that it has access to the private key
761: and the server checks that the corresponding public key
762: is authorized to accept the account.
1.218 jmc 763: .Pp
1.222 jmc 764: The user creates his/her key pair by running
1.218 jmc 765: .Xr ssh-keygen 1 .
766: This stores the private key in
1.207 djm 767: .Pa ~/.ssh/identity
1.222 jmc 768: (protocol 1),
769: .Pa ~/.ssh/id_dsa
770: (protocol 2 DSA),
1.310 djm 771: .Pa ~/.ssh/id_ecdsa
772: (protocol 2 ECDSA),
1.343 naddy 773: .Pa ~/.ssh/id_ed25519
774: (protocol 2 ED25519),
1.222 jmc 775: or
776: .Pa ~/.ssh/id_rsa
777: (protocol 2 RSA)
1.218 jmc 778: and stores the public key in
779: .Pa ~/.ssh/identity.pub
1.222 jmc 780: (protocol 1),
781: .Pa ~/.ssh/id_dsa.pub
782: (protocol 2 DSA),
1.310 djm 783: .Pa ~/.ssh/id_ecdsa.pub
784: (protocol 2 ECDSA),
1.343 naddy 785: .Pa ~/.ssh/id_ed25519.pub
786: (protocol 2 ED25519),
1.222 jmc 787: or
788: .Pa ~/.ssh/id_rsa.pub
789: (protocol 2 RSA)
1.218 jmc 790: in the user's home directory.
1.222 jmc 791: The user should then copy the public key
1.218 jmc 792: to
793: .Pa ~/.ssh/authorized_keys
1.222 jmc 794: in his/her home directory on the remote machine.
795: The
1.218 jmc 796: .Pa authorized_keys
797: file corresponds to the conventional
798: .Pa ~/.rhosts
799: file, and has one key
1.222 jmc 800: per line, though the lines can be very long.
1.218 jmc 801: After this, the user can log in without giving the password.
802: .Pp
1.301 jmc 803: A variation on public key authentication
804: is available in the form of certificate authentication:
805: instead of a set of public/private keys,
806: signed certificates are used.
807: This has the advantage that a single trusted certification authority
808: can be used in place of many public/private keys.
1.333 jmc 809: See the CERTIFICATES section of
1.301 jmc 810: .Xr ssh-keygen 1
811: for more information.
812: .Pp
813: The most convenient way to use public key or certificate authentication
814: may be with an authentication agent.
1.218 jmc 815: See
816: .Xr ssh-agent 1
817: for more information.
818: .Pp
1.222 jmc 819: Challenge-response authentication works as follows:
820: The server sends an arbitrary
821: .Qq challenge
822: text, and prompts for a response.
823: Protocol 2 allows multiple challenges and responses;
824: protocol 1 is restricted to just one challenge/response.
825: Examples of challenge-response authentication include
1.335 jmc 826: .Bx
827: Authentication (see
1.222 jmc 828: .Xr login.conf 5 )
1.335 jmc 829: and PAM (some
830: .Pf non- Ox
831: systems).
1.222 jmc 832: .Pp
833: Finally, if other authentication methods fail,
1.218 jmc 834: .Nm
835: prompts the user for a password.
836: The password is sent to the remote
837: host for checking; however, since all communications are encrypted,
838: the password cannot be seen by someone listening on the network.
1.232 jmc 839: .Pp
840: .Nm
841: automatically maintains and checks a database containing
842: identification for all hosts it has ever been used with.
843: Host keys are stored in
844: .Pa ~/.ssh/known_hosts
845: in the user's home directory.
846: Additionally, the file
847: .Pa /etc/ssh/ssh_known_hosts
848: is automatically checked for known hosts.
849: Any new hosts are automatically added to the user's file.
850: If a host's identification ever changes,
851: .Nm
852: warns about this and disables password authentication to prevent
853: server spoofing or man-in-the-middle attacks,
854: which could otherwise be used to circumvent the encryption.
855: The
856: .Cm StrictHostKeyChecking
857: option can be used to control logins to machines whose
858: host key is not known or has changed.
859: .Pp
1.218 jmc 860: When the user's identity has been accepted by the server, the server
861: either executes the given command, or logs into the machine and gives
862: the user a normal shell on the remote machine.
863: All communication with
864: the remote command or shell will be automatically encrypted.
865: .Pp
866: If a pseudo-terminal has been allocated (normal login session), the
867: user may use the escape characters noted below.
868: .Pp
869: If no pseudo-tty has been allocated,
870: the session is transparent and can be used to reliably transfer binary data.
871: On most systems, setting the escape character to
872: .Dq none
873: will also make the session transparent even if a tty is used.
874: .Pp
875: The session terminates when the command or shell on the remote
1.247 jmc 876: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 877: .Sh ESCAPE CHARACTERS
1.218 jmc 878: When a pseudo-terminal has been requested,
1.2 deraadt 879: .Nm
1.218 jmc 880: supports a number of functions through the use of an escape character.
881: .Pp
882: A single tilde character can be sent as
883: .Ic ~~
884: or by following the tilde by a character other than those described below.
885: The escape character must always follow a newline to be interpreted as
886: special.
887: The escape character can be changed in configuration files using the
888: .Cm EscapeChar
889: configuration directive or on the command line by the
890: .Fl e
891: option.
892: .Pp
893: The supported escapes (assuming the default
894: .Ql ~ )
895: are:
896: .Bl -tag -width Ds
897: .It Cm ~.
898: Disconnect.
899: .It Cm ~^Z
900: Background
1.234 jmc 901: .Nm .
1.218 jmc 902: .It Cm ~#
903: List forwarded connections.
904: .It Cm ~&
905: Background
1.2 deraadt 906: .Nm
1.218 jmc 907: at logout when waiting for forwarded connection / X11 sessions to terminate.
908: .It Cm ~?
909: Display a list of escape characters.
910: .It Cm ~B
911: Send a BREAK to the remote system
912: (only useful for SSH protocol version 2 and if the peer supports it).
913: .It Cm ~C
914: Open command line.
915: Currently this allows the addition of port forwardings using the
1.279 stevesk 916: .Fl L ,
917: .Fl R
1.218 jmc 918: and
1.279 stevesk 919: .Fl D
1.225 jmc 920: options (see above).
1.322 markus 921: It also allows the cancellation of existing port-forwardings
922: with
1.262 stevesk 923: .Sm off
1.322 markus 924: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 925: .Sm on
1.322 markus 926: for local,
927: .Sm off
928: .Fl KR Oo Ar bind_address : Oc Ar port
929: .Sm on
930: for remote and
931: .Sm off
932: .Fl KD Oo Ar bind_address : Oc Ar port
933: .Sm on
934: for dynamic port-forwardings.
1.218 jmc 935: .Ic !\& Ns Ar command
936: allows the user to execute a local command if the
937: .Ic PermitLocalCommand
938: option is enabled in
1.176 jmc 939: .Xr ssh_config 5 .
1.218 jmc 940: Basic help is available, using the
941: .Fl h
942: option.
943: .It Cm ~R
944: Request rekeying of the connection
945: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 946: .It Cm ~V
947: Decrease the verbosity
948: .Pq Ic LogLevel
949: when errors are being written to stderr.
950: .It Cm ~v
1.328 jmc 951: Increase the verbosity
1.327 dtucker 952: .Pq Ic LogLevel
953: when errors are being written to stderr.
1.176 jmc 954: .El
1.246 jmc 955: .Sh TCP FORWARDING
956: Forwarding of arbitrary TCP connections over the secure channel can
957: be specified either on the command line or in a configuration file.
958: One possible application of TCP forwarding is a secure connection to a
959: mail server; another is going through firewalls.
960: .Pp
961: In the example below, we look at encrypting communication between
962: an IRC client and server, even though the IRC server does not directly
963: support encrypted communications.
964: This works as follows:
965: the user connects to the remote host using
966: .Nm ,
967: specifying a port to be used to forward connections
968: to the remote server.
969: After that it is possible to start the service which is to be encrypted
970: on the client machine,
971: connecting to the same local port,
972: and
973: .Nm
974: will encrypt and forward the connection.
975: .Pp
976: The following example tunnels an IRC session from client machine
977: .Dq 127.0.0.1
978: (localhost)
979: to remote server
980: .Dq server.example.com :
981: .Bd -literal -offset 4n
982: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
983: $ irc -c '#users' -p 1234 pinky 127.0.0.1
984: .Ed
985: .Pp
986: This tunnels a connection to IRC server
987: .Dq server.example.com ,
988: joining channel
989: .Dq #users ,
990: nickname
991: .Dq pinky ,
992: using port 1234.
993: It doesn't matter which port is used,
994: as long as it's greater than 1023
995: (remember, only root can open sockets on privileged ports)
996: and doesn't conflict with any ports already in use.
997: The connection is forwarded to port 6667 on the remote server,
998: since that's the standard port for IRC services.
999: .Pp
1000: The
1001: .Fl f
1002: option backgrounds
1003: .Nm
1004: and the remote command
1005: .Dq sleep 10
1006: is specified to allow an amount of time
1007: (10 seconds, in the example)
1008: to start the service which is to be tunnelled.
1009: If no connections are made within the time specified,
1010: .Nm
1011: will exit.
1012: .Sh X11 FORWARDING
1.218 jmc 1013: If the
1014: .Cm ForwardX11
1015: variable is set to
1016: .Dq yes
1017: (or see the description of the
1.227 jmc 1018: .Fl X ,
1019: .Fl x ,
1.218 jmc 1020: and
1.227 jmc 1021: .Fl Y
1.226 jmc 1022: options above)
1.218 jmc 1023: and the user is using X11 (the
1024: .Ev DISPLAY
1025: environment variable is set), the connection to the X11 display is
1026: automatically forwarded to the remote side in such a way that any X11
1027: programs started from the shell (or command) will go through the
1028: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1029: from the local machine.
1.218 jmc 1030: The user should not manually set
1031: .Ev DISPLAY .
1032: Forwarding of X11 connections can be
1033: configured on the command line or in configuration files.
1034: .Pp
1035: The
1036: .Ev DISPLAY
1037: value set by
1038: .Nm
1039: will point to the server machine, but with a display number greater than zero.
1040: This is normal, and happens because
1041: .Nm
1042: creates a
1043: .Dq proxy
1044: X server on the server machine for forwarding the
1045: connections over the encrypted channel.
1.200 djm 1046: .Pp
1.218 jmc 1047: .Nm
1048: will also automatically set up Xauthority data on the server machine.
1049: For this purpose, it will generate a random authorization cookie,
1050: store it in Xauthority on the server, and verify that any forwarded
1051: connections carry this cookie and replace it by the real cookie when
1052: the connection is opened.
1053: The real authentication cookie is never
1054: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1055: .Pp
1.218 jmc 1056: If the
1057: .Cm ForwardAgent
1058: variable is set to
1059: .Dq yes
1060: (or see the description of the
1061: .Fl A
1.191 djm 1062: and
1.218 jmc 1063: .Fl a
1.226 jmc 1064: options above) and
1.218 jmc 1065: the user is using an authentication agent, the connection to the agent
1066: is automatically forwarded to the remote side.
1.252 jmc 1067: .Sh VERIFYING HOST KEYS
1068: When connecting to a server for the first time,
1069: a fingerprint of the server's public key is presented to the user
1070: (unless the option
1071: .Cm StrictHostKeyChecking
1072: has been disabled).
1073: Fingerprints can be determined using
1074: .Xr ssh-keygen 1 :
1075: .Pp
1076: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1077: .Pp
1.274 grunk 1078: If the fingerprint is already known, it can be matched
1079: and the key can be accepted or rejected.
1080: Because of the difficulty of comparing host keys
1081: just by looking at hex strings,
1082: there is also support to compare host keys visually,
1083: using
1084: .Em random art .
1085: By setting the
1.275 grunk 1086: .Cm VisualHostKey
1.274 grunk 1087: option to
1.275 grunk 1088: .Dq yes ,
1.274 grunk 1089: a small ASCII graphic gets displayed on every login to a server, no matter
1090: if the session itself is interactive or not.
1091: By learning the pattern a known server produces, a user can easily
1092: find out that the host key has changed when a completely different pattern
1093: is displayed.
1094: Because these patterns are not unambiguous however, a pattern that looks
1095: similar to the pattern remembered only gives a good probability that the
1096: host key is the same, not guaranteed proof.
1097: .Pp
1098: To get a listing of the fingerprints along with their random art for
1099: all known hosts, the following command line can be used:
1100: .Pp
1101: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1102: .Pp
1.252 jmc 1103: If the fingerprint is unknown,
1104: an alternative method of verification is available:
1105: SSH fingerprints verified by DNS.
1106: An additional resource record (RR),
1107: SSHFP,
1108: is added to a zonefile
1109: and the connecting client is able to match the fingerprint
1110: with that of the key presented.
1111: .Pp
1112: In this example, we are connecting a client to a server,
1113: .Dq host.example.com .
1114: The SSHFP resource records should first be added to the zonefile for
1115: host.example.com:
1116: .Bd -literal -offset indent
1.259 jakob 1117: $ ssh-keygen -r host.example.com.
1.252 jmc 1118: .Ed
1119: .Pp
1120: The output lines will have to be added to the zonefile.
1121: To check that the zone is answering fingerprint queries:
1122: .Pp
1123: .Dl $ dig -t SSHFP host.example.com
1124: .Pp
1125: Finally the client connects:
1126: .Bd -literal -offset indent
1127: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1128: [...]
1129: Matching host key fingerprint found in DNS.
1130: Are you sure you want to continue connecting (yes/no)?
1131: .Ed
1132: .Pp
1133: See the
1134: .Cm VerifyHostKeyDNS
1135: option in
1136: .Xr ssh_config 5
1137: for more information.
1.250 jmc 1138: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1139: .Nm
1140: contains support for Virtual Private Network (VPN) tunnelling
1141: using the
1142: .Xr tun 4
1143: network pseudo-device,
1144: allowing two networks to be joined securely.
1145: The
1146: .Xr sshd_config 5
1147: configuration option
1148: .Cm PermitTunnel
1149: controls whether the server supports this,
1150: and at what level (layer 2 or 3 traffic).
1151: .Pp
1152: The following example would connect client network 10.0.50.0/24
1.265 otto 1153: with remote network 10.0.99.0/24 using a point-to-point connection
1154: from 10.1.1.1 to 10.1.1.2,
1155: provided that the SSH server running on the gateway to the remote network,
1156: at 192.168.1.15, allows it.
1157: .Pp
1158: On the client:
1.250 jmc 1159: .Bd -literal -offset indent
1160: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1161: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1162: # route add 10.0.99.0/24 10.1.1.2
1163: .Ed
1164: .Pp
1165: On the server:
1166: .Bd -literal -offset indent
1167: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1168: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1169: .Ed
1170: .Pp
1171: Client access may be more finely tuned via the
1172: .Pa /root/.ssh/authorized_keys
1173: file (see below) and the
1174: .Cm PermitRootLogin
1175: server option.
1.255 jmc 1176: The following entry would permit connections on
1.250 jmc 1177: .Xr tun 4
1.255 jmc 1178: device 1 from user
1.250 jmc 1179: .Dq jane
1.255 jmc 1180: and on tun device 2 from user
1.250 jmc 1181: .Dq john ,
1182: if
1183: .Cm PermitRootLogin
1184: is set to
1185: .Dq forced-commands-only :
1186: .Bd -literal -offset 2n
1187: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1188: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1189: .Ed
1190: .Pp
1.264 ray 1191: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1192: it may be more suited to temporary setups,
1193: such as for wireless VPNs.
1194: More permanent VPNs are better provided by tools such as
1195: .Xr ipsecctl 8
1196: and
1197: .Xr isakmpd 8 .
1.2 deraadt 1198: .Sh ENVIRONMENT
1199: .Nm
1.1 deraadt 1200: will normally set the following environment variables:
1.237 jmc 1201: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1202: .It Ev DISPLAY
1203: The
1204: .Ev DISPLAY
1.40 aaron 1205: variable indicates the location of the X11 server.
1.44 aaron 1206: It is automatically set by
1.2 deraadt 1207: .Nm
1208: to point to a value of the form
1.233 jmc 1209: .Dq hostname:n ,
1210: where
1211: .Dq hostname
1212: indicates the host where the shell runs, and
1213: .Sq n
1214: is an integer \*(Ge 1.
1.40 aaron 1215: .Nm
1216: uses this special value to forward X11 connections over the secure
1217: channel.
1.107 markus 1218: The user should normally not set
1219: .Ev DISPLAY
1220: explicitly, as that
1.1 deraadt 1221: will render the X11 connection insecure (and will require the user to
1222: manually copy any required authorization cookies).
1.2 deraadt 1223: .It Ev HOME
1.1 deraadt 1224: Set to the path of the user's home directory.
1.2 deraadt 1225: .It Ev LOGNAME
1226: Synonym for
1.12 aaron 1227: .Ev USER ;
1228: set for compatibility with systems that use this variable.
1.2 deraadt 1229: .It Ev MAIL
1.129 stevesk 1230: Set to the path of the user's mailbox.
1.40 aaron 1231: .It Ev PATH
1.2 deraadt 1232: Set to the default
1233: .Ev PATH ,
1234: as specified when compiling
1.234 jmc 1235: .Nm .
1.118 markus 1236: .It Ev SSH_ASKPASS
1237: If
1238: .Nm
1239: needs a passphrase, it will read the passphrase from the current
1240: terminal if it was run from a terminal.
1241: If
1242: .Nm
1243: does not have a terminal associated with it but
1244: .Ev DISPLAY
1245: and
1246: .Ev SSH_ASKPASS
1247: are set, it will execute the program specified by
1248: .Ev SSH_ASKPASS
1249: and open an X11 window to read the passphrase.
1250: This is particularly useful when calling
1251: .Nm
1252: from a
1.196 jmc 1253: .Pa .xsession
1.118 markus 1254: or related script.
1255: (Note that on some machines it
1256: may be necessary to redirect the input from
1257: .Pa /dev/null
1258: to make this work.)
1.18 markus 1259: .It Ev SSH_AUTH_SOCK
1.233 jmc 1260: Identifies the path of a
1261: .Ux Ns -domain
1262: socket used to communicate with the agent.
1.166 stevesk 1263: .It Ev SSH_CONNECTION
1264: Identifies the client and server ends of the connection.
1.40 aaron 1265: The variable contains
1.233 jmc 1266: four space-separated values: client IP address, client port number,
1267: server IP address, and server port number.
1.73 markus 1268: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1269: This variable contains the original command line if a forced command
1.73 markus 1270: is executed.
1271: It can be used to extract the original arguments.
1.2 deraadt 1272: .It Ev SSH_TTY
1.1 deraadt 1273: This is set to the name of the tty (path to the device) associated
1.40 aaron 1274: with the current shell or command.
1275: If the current session has no tty,
1.1 deraadt 1276: this variable is not set.
1.2 deraadt 1277: .It Ev TZ
1.214 jmc 1278: This variable is set to indicate the present time zone if it
1.257 jmc 1279: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1280: on to new connections).
1.2 deraadt 1281: .It Ev USER
1.1 deraadt 1282: Set to the name of the user logging in.
1.2 deraadt 1283: .El
1284: .Pp
1.44 aaron 1285: Additionally,
1.2 deraadt 1286: .Nm
1.44 aaron 1287: reads
1.207 djm 1288: .Pa ~/.ssh/environment ,
1.2 deraadt 1289: and adds lines of the format
1290: .Dq VARNAME=value
1.233 jmc 1291: to the environment if the file exists and users are allowed to
1.161 marc 1292: change their environment.
1.176 jmc 1293: For more information, see the
1.161 marc 1294: .Cm PermitUserEnvironment
1.162 stevesk 1295: option in
1.161 marc 1296: .Xr sshd_config 5 .
1.2 deraadt 1297: .Sh FILES
1.236 jmc 1298: .Bl -tag -width Ds -compact
1.309 jmc 1299: .It Pa ~/.rhosts
1.240 jmc 1300: This file is used for host-based authentication (see above).
1.92 markus 1301: On some machines this file may need to be
1.240 jmc 1302: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1303: because
1.2 deraadt 1304: .Xr sshd 8
1.40 aaron 1305: reads it as root.
1306: Additionally, this file must be owned by the user,
1307: and must not have write permissions for anyone else.
1308: The recommended
1.1 deraadt 1309: permission for most machines is read/write for the user, and not
1310: accessible by others.
1.2 deraadt 1311: .Pp
1.309 jmc 1312: .It Pa ~/.shosts
1.240 jmc 1313: This file is used in exactly the same way as
1314: .Pa .rhosts ,
1315: but allows host-based authentication without permitting login with
1316: rlogin/rsh.
1.272 mcbride 1317: .Pp
1.309 jmc 1318: .It Pa ~/.ssh/
1.272 mcbride 1319: This directory is the default location for all user-specific configuration
1320: and authentication information.
1321: There is no general requirement to keep the entire contents of this directory
1322: secret, but the recommended permissions are read/write/execute for the user,
1323: and not accessible by others.
1.236 jmc 1324: .Pp
1.309 jmc 1325: .It Pa ~/.ssh/authorized_keys
1.343 naddy 1326: Lists the public keys (DSA, ECDSA, ED25519, RSA)
1327: that can be used for logging in as this user.
1.238 jmc 1328: The format of this file is described in the
1329: .Xr sshd 8
1330: manual page.
1331: This file is not highly sensitive, but the recommended
1332: permissions are read/write for the user, and not accessible by others.
1333: .Pp
1.309 jmc 1334: .It Pa ~/.ssh/config
1.238 jmc 1335: This is the per-user configuration file.
1336: The file format and configuration options are described in
1337: .Xr ssh_config 5 .
1338: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1339: read/write for the user, and not writable by others.
1.238 jmc 1340: .Pp
1.309 jmc 1341: .It Pa ~/.ssh/environment
1.239 jmc 1342: Contains additional definitions for environment variables; see
1343: .Sx ENVIRONMENT ,
1.238 jmc 1344: above.
1345: .Pp
1.309 jmc 1346: .It Pa ~/.ssh/identity
1347: .It Pa ~/.ssh/id_dsa
1.310 djm 1348: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1349: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1350: .It Pa ~/.ssh/id_rsa
1.238 jmc 1351: Contains the private key for authentication.
1352: These files
1353: contain sensitive data and should be readable by the user but not
1354: accessible by others (read/write/execute).
1355: .Nm
1356: will simply ignore a private key file if it is accessible by others.
1357: It is possible to specify a passphrase when
1358: generating the key which will be used to encrypt the
1359: sensitive part of this file using 3DES.
1360: .Pp
1.309 jmc 1361: .It Pa ~/.ssh/identity.pub
1362: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1363: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1364: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1365: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1366: Contains the public key for authentication.
1367: These files are not
1368: sensitive and can (but need not) be readable by anyone.
1369: .Pp
1.309 jmc 1370: .It Pa ~/.ssh/known_hosts
1.244 jmc 1371: Contains a list of host keys for all hosts the user has logged into
1372: that are not already in the systemwide list of known host keys.
1.238 jmc 1373: See
1.244 jmc 1374: .Xr sshd 8
1375: for further details of the format of this file.
1.238 jmc 1376: .Pp
1.309 jmc 1377: .It Pa ~/.ssh/rc
1.238 jmc 1378: Commands in this file are executed by
1379: .Nm
1.245 jmc 1380: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1381: started.
1382: See the
1383: .Xr sshd 8
1384: manual page for more information.
1385: .Pp
1.309 jmc 1386: .It Pa /etc/hosts.equiv
1.240 jmc 1387: This file is for host-based authentication (see above).
1388: It should only be writable by root.
1.236 jmc 1389: .Pp
1.309 jmc 1390: .It Pa /etc/shosts.equiv
1.240 jmc 1391: This file is used in exactly the same way as
1392: .Pa hosts.equiv ,
1393: but allows host-based authentication without permitting login with
1394: rlogin/rsh.
1.236 jmc 1395: .Pp
1.238 jmc 1396: .It Pa /etc/ssh/ssh_config
1397: Systemwide configuration file.
1398: The file format and configuration options are described in
1399: .Xr ssh_config 5 .
1400: .Pp
1.309 jmc 1401: .It Pa /etc/ssh/ssh_host_key
1402: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1403: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1404: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1405: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1406: These files contain the private parts of the host keys
1.245 jmc 1407: and are used for host-based authentication.
1408: If protocol version 1 is used,
1.238 jmc 1409: .Nm
1410: must be setuid root, since the host key is readable only by root.
1411: For protocol version 2,
1412: .Nm
1413: uses
1414: .Xr ssh-keysign 8
1.245 jmc 1415: to access the host keys,
1416: eliminating the requirement that
1.238 jmc 1417: .Nm
1.245 jmc 1418: be setuid root when host-based authentication is used.
1.238 jmc 1419: By default
1.2 deraadt 1420: .Nm
1.238 jmc 1421: is not setuid root.
1422: .Pp
1.309 jmc 1423: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1424: Systemwide list of known host keys.
1425: This file should be prepared by the
1426: system administrator to contain the public host keys of all machines in the
1427: organization.
1.244 jmc 1428: It should be world-readable.
1429: See
1.238 jmc 1430: .Xr sshd 8
1.244 jmc 1431: for further details of the format of this file.
1.236 jmc 1432: .Pp
1.309 jmc 1433: .It Pa /etc/ssh/sshrc
1.1 deraadt 1434: Commands in this file are executed by
1.2 deraadt 1435: .Nm
1.245 jmc 1436: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1437: See the
1.2 deraadt 1438: .Xr sshd 8
1.1 deraadt 1439: manual page for more information.
1.58 itojun 1440: .El
1.312 jmc 1441: .Sh EXIT STATUS
1442: .Nm
1443: exits with the exit status of the remote command or with 255
1444: if an error occurred.
1.2 deraadt 1445: .Sh SEE ALSO
1446: .Xr scp 1 ,
1.83 djm 1447: .Xr sftp 1 ,
1.2 deraadt 1448: .Xr ssh-add 1 ,
1449: .Xr ssh-agent 1 ,
1450: .Xr ssh-keygen 1 ,
1.242 jmc 1451: .Xr ssh-keyscan 1 ,
1.250 jmc 1452: .Xr tun 4 ,
1.159 stevesk 1453: .Xr ssh_config 5 ,
1.160 naddy 1454: .Xr ssh-keysign 8 ,
1.87 itojun 1455: .Xr sshd 8
1.329 jmc 1456: .Sh STANDARDS
1.106 markus 1457: .Rs
1.329 jmc 1458: .%A S. Lehtinen
1459: .%A C. Lonvick
1460: .%D January 2006
1.256 jmc 1461: .%R RFC 4250
1.329 jmc 1462: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1463: .Re
1.329 jmc 1464: .Pp
1.256 jmc 1465: .Rs
1.329 jmc 1466: .%A T. Ylonen
1467: .%A C. Lonvick
1468: .%D January 2006
1.256 jmc 1469: .%R RFC 4251
1.329 jmc 1470: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1471: .Re
1.329 jmc 1472: .Pp
1.256 jmc 1473: .Rs
1.329 jmc 1474: .%A T. Ylonen
1475: .%A C. Lonvick
1476: .%D January 2006
1.256 jmc 1477: .%R RFC 4252
1.329 jmc 1478: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1479: .Re
1.329 jmc 1480: .Pp
1.256 jmc 1481: .Rs
1.329 jmc 1482: .%A T. Ylonen
1483: .%A C. Lonvick
1484: .%D January 2006
1.256 jmc 1485: .%R RFC 4253
1.329 jmc 1486: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1487: .Re
1.329 jmc 1488: .Pp
1.256 jmc 1489: .Rs
1.329 jmc 1490: .%A T. Ylonen
1491: .%A C. Lonvick
1492: .%D January 2006
1.256 jmc 1493: .%R RFC 4254
1.329 jmc 1494: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1495: .Re
1.329 jmc 1496: .Pp
1.256 jmc 1497: .Rs
1.329 jmc 1498: .%A J. Schlyter
1499: .%A W. Griffin
1500: .%D January 2006
1.256 jmc 1501: .%R RFC 4255
1.329 jmc 1502: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1503: .Re
1.329 jmc 1504: .Pp
1.256 jmc 1505: .Rs
1.329 jmc 1506: .%A F. Cusack
1507: .%A M. Forssen
1508: .%D January 2006
1.256 jmc 1509: .%R RFC 4256
1.329 jmc 1510: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1511: .Re
1.329 jmc 1512: .Pp
1.256 jmc 1513: .Rs
1.329 jmc 1514: .%A J. Galbraith
1515: .%A P. Remaker
1516: .%D January 2006
1.256 jmc 1517: .%R RFC 4335
1.329 jmc 1518: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1519: .Re
1.329 jmc 1520: .Pp
1.256 jmc 1521: .Rs
1.329 jmc 1522: .%A M. Bellare
1523: .%A T. Kohno
1524: .%A C. Namprempre
1525: .%D January 2006
1.256 jmc 1526: .%R RFC 4344
1.329 jmc 1527: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1528: .Re
1.329 jmc 1529: .Pp
1.256 jmc 1530: .Rs
1.329 jmc 1531: .%A B. Harris
1532: .%D January 2006
1.256 jmc 1533: .%R RFC 4345
1.329 jmc 1534: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1535: .Re
1.329 jmc 1536: .Pp
1.258 djm 1537: .Rs
1.329 jmc 1538: .%A M. Friedl
1539: .%A N. Provos
1540: .%A W. Simpson
1541: .%D March 2006
1.258 djm 1542: .%R RFC 4419
1.329 jmc 1543: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1544: .Re
1.329 jmc 1545: .Pp
1.266 markus 1546: .Rs
1.329 jmc 1547: .%A J. Galbraith
1548: .%A R. Thayer
1549: .%D November 2006
1.266 markus 1550: .%R RFC 4716
1.329 jmc 1551: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1552: .Re
1.329 jmc 1553: .Pp
1.313 djm 1554: .Rs
1.329 jmc 1555: .%A D. Stebila
1556: .%A J. Green
1557: .%D December 2009
1.313 djm 1558: .%R RFC 5656
1.329 jmc 1559: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1560: .Re
1.329 jmc 1561: .Pp
1.274 grunk 1562: .Rs
1563: .%A A. Perrig
1564: .%A D. Song
1565: .%D 1999
1.329 jmc 1566: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1567: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1568: .Re
1.173 jmc 1569: .Sh AUTHORS
1570: OpenSSH is a derivative of the original and free
1571: ssh 1.2.12 release by Tatu Ylonen.
1572: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1573: Theo de Raadt and Dug Song
1574: removed many bugs, re-added newer features and
1575: created OpenSSH.
1576: Markus Friedl contributed the support for SSH
1577: protocol versions 1.5 and 2.0.