Annotation of src/usr.bin/ssh/ssh.1, Revision 1.355
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: .\"
1.355 ! djm 36: .\" $OpenBSD: ssh.1,v 1.354 2015/01/26 12:16:36 djm Exp $
! 37: .Dd $Mdocdate: January 26 2015 $
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
1.351 jmc 46: .Op Fl 1246AaCfGgKkMNnqsTtVvXxYy
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.
1.348 millert 76: X11 connections, arbitrary TCP ports and
77: .Ux Ns -domain
78: sockets can also be forwarded over the secure channel.
1.2 deraadt 79: .Pp
80: .Nm
1.44 aaron 81: connects and logs into the specified
1.176 jmc 82: .Ar hostname
83: (with optional
84: .Ar user
85: name).
1.1 deraadt 86: The user must prove
1.49 markus 87: his/her identity to the remote machine using one of several methods
1.221 jmc 88: depending on the protocol version used (see below).
1.49 markus 89: .Pp
1.176 jmc 90: If
91: .Ar command
92: is specified,
1.219 jmc 93: it is executed on the remote host instead of a login shell.
1.2 deraadt 94: .Pp
1.218 jmc 95: The options are as follows:
96: .Bl -tag -width Ds
97: .It Fl 1
98: Forces
1.2 deraadt 99: .Nm
1.218 jmc 100: to try protocol version 1 only.
101: .It Fl 2
102: Forces
1.2 deraadt 103: .Nm
1.218 jmc 104: to try protocol version 2 only.
105: .It Fl 4
106: Forces
1.2 deraadt 107: .Nm
1.218 jmc 108: to use IPv4 addresses only.
109: .It Fl 6
110: Forces
1.2 deraadt 111: .Nm
1.218 jmc 112: to use IPv6 addresses only.
113: .It Fl A
114: Enables forwarding of the authentication agent connection.
115: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 116: .Pp
1.218 jmc 117: Agent forwarding should be enabled with caution.
118: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 119: (for the agent's
1.286 sobrado 120: .Ux Ns -domain
121: socket) can access the local agent through the forwarded connection.
1.218 jmc 122: An attacker cannot obtain key material from the agent,
123: however they can perform operations on the keys that enable them to
124: authenticate using the identities loaded into the agent.
125: .It Fl a
126: Disables forwarding of the authentication agent connection.
127: .It Fl b Ar bind_address
128: Use
129: .Ar bind_address
130: on the local machine as the source address
131: of the connection.
132: Only useful on systems with more than one address.
133: .It Fl C
134: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 135: data for forwarded X11, TCP and
136: .Ux Ns -domain
137: connections).
1.218 jmc 138: The compression algorithm is the same used by
139: .Xr gzip 1 ,
140: and the
141: .Dq level
142: can be controlled by the
143: .Cm CompressionLevel
144: option for protocol version 1.
145: Compression is desirable on modem lines and other
146: slow connections, but will only slow down things on fast networks.
147: The default value can be set on a host-by-host basis in the
148: configuration files; see the
149: .Cm Compression
150: option.
151: .It Fl c Ar cipher_spec
152: Selects the cipher specification for encrypting the session.
1.2 deraadt 153: .Pp
1.218 jmc 154: Protocol version 1 allows specification of a single cipher.
155: The supported values are
156: .Dq 3des ,
1.220 jmc 157: .Dq blowfish ,
1.218 jmc 158: and
159: .Dq des .
1.230 jmc 160: For protocol version 2,
1.218 jmc 161: .Ar cipher_spec
162: is a comma-separated list of ciphers
163: listed in order of preference.
1.283 jmc 164: See the
165: .Cm Ciphers
1.307 dtucker 166: keyword in
167: .Xr ssh_config 5
168: for more information.
1.218 jmc 169: .It Fl D Xo
170: .Sm off
171: .Oo Ar bind_address : Oc
172: .Ar port
173: .Sm on
174: .Xc
175: Specifies a local
176: .Dq dynamic
177: application-level port forwarding.
178: This works by allocating a socket to listen to
179: .Ar port
180: on the local side, optionally bound to the specified
181: .Ar bind_address .
182: Whenever a connection is made to this port, the
183: connection is forwarded over the secure channel, and the application
184: protocol is then used to determine where to connect to from the
185: remote machine.
186: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 187: .Nm
1.218 jmc 188: will act as a SOCKS server.
189: Only root can forward privileged ports.
190: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 191: .Pp
1.308 djm 192: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 193: Only the superuser can forward privileged ports.
194: By default, the local port is bound in accordance with the
195: .Cm GatewayPorts
196: setting.
197: However, an explicit
198: .Ar bind_address
199: may be used to bind the connection to a specific address.
200: The
201: .Ar bind_address
202: of
203: .Dq localhost
204: indicates that the listening port be bound for local use only, while an
205: empty address or
206: .Sq *
207: indicates that the port should be available from all interfaces.
1.331 dtucker 208: .It Fl E Ar log_file
209: Append debug logs to
210: .Ar log_file
211: instead of standard error.
1.229 jmc 212: .It Fl e Ar escape_char
1.218 jmc 213: Sets the escape character for sessions with a pty (default:
214: .Ql ~ ) .
215: The escape character is only recognized at the beginning of a line.
216: The escape character followed by a dot
217: .Pq Ql \&.
218: closes the connection;
219: followed by control-Z suspends the connection;
220: and followed by itself sends the escape character once.
221: Setting the character to
1.2 deraadt 222: .Dq none
1.218 jmc 223: disables any escapes and makes the session fully transparent.
224: .It Fl F Ar configfile
225: Specifies an alternative per-user configuration file.
226: If a configuration file is given on the command line,
227: the system-wide configuration file
228: .Pq Pa /etc/ssh/ssh_config
229: will be ignored.
230: The default for the per-user configuration file is
231: .Pa ~/.ssh/config .
232: .It Fl f
233: Requests
234: .Nm
235: to go to background just before command execution.
236: This is useful if
1.176 jmc 237: .Nm
1.218 jmc 238: is going to ask for passwords or passphrases, but the user
239: wants it in the background.
240: This implies
241: .Fl n .
242: The recommended way to start X11 programs at a remote site is with
243: something like
244: .Ic ssh -f host xterm .
1.277 djm 245: .Pp
246: If the
247: .Cm ExitOnForwardFailure
248: configuration option is set to
249: .Dq yes ,
250: then a client started with
251: .Fl f
252: will wait for all remote port forwards to be successfully established
253: before placing itself in the background.
1.350 djm 254: .It Fl G
255: Causes
256: .Nm
257: to print its configuration after evaluating
258: .Cm Host
259: and
260: .Cm Match
261: blocks and exit.
1.218 jmc 262: .It Fl g
263: Allows remote hosts to connect to local forwarded ports.
1.346 djm 264: If used on a multiplexed connection, then this option must be specified
265: on the master process.
1.291 markus 266: .It Fl I Ar pkcs11
1.294 jmc 267: Specify the PKCS#11 shared library
1.176 jmc 268: .Nm
1.293 markus 269: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 270: private RSA key.
271: .It Fl i Ar identity_file
272: Selects a file from which the identity (private key) for
1.310 djm 273: public key authentication is read.
1.218 jmc 274: The default is
275: .Pa ~/.ssh/identity
276: for protocol version 1, and
1.310 djm 277: .Pa ~/.ssh/id_dsa ,
1.343 naddy 278: .Pa ~/.ssh/id_ecdsa ,
279: .Pa ~/.ssh/id_ed25519
1.310 djm 280: and
1.218 jmc 281: .Pa ~/.ssh/id_rsa
282: for protocol version 2.
283: Identity files may also be specified on
284: a per-host basis in the configuration file.
285: It is possible to have multiple
286: .Fl i
287: options (and multiple identities specified in
288: configuration files).
1.302 djm 289: .Nm
290: will also try to load certificate information from the filename obtained
291: by appending
292: .Pa -cert.pub
293: to identity filenames.
1.269 djm 294: .It Fl K
295: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
296: credentials to the server.
1.218 jmc 297: .It Fl k
298: Disables forwarding (delegation) of GSSAPI credentials to the server.
299: .It Fl L Xo
300: .Sm off
301: .Oo Ar bind_address : Oc
302: .Ar port : host : hostport
303: .Sm on
304: .Xc
305: Specifies that the given port on the local (client) host is to be
306: forwarded to the given host and port on the remote side.
307: This works by allocating a socket to listen to
308: .Ar port
309: on the local side, optionally bound to the specified
310: .Ar bind_address .
311: Whenever a connection is made to this port, the
312: connection is forwarded over the secure channel, and a connection is
313: made to
314: .Ar host
315: port
316: .Ar hostport
317: from the remote machine.
318: Port forwardings can also be specified in the configuration file.
1.308 djm 319: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 320: Only the superuser can forward privileged ports.
321: By default, the local port is bound in accordance with the
322: .Cm GatewayPorts
323: setting.
324: However, an explicit
325: .Ar bind_address
326: may be used to bind the connection to a specific address.
1.2 deraadt 327: The
1.218 jmc 328: .Ar bind_address
329: of
330: .Dq localhost
331: indicates that the listening port be bound for local use only, while an
332: empty address or
333: .Sq *
334: indicates that the port should be available from all interfaces.
335: .It Fl l Ar login_name
336: Specifies the user to log in as on the remote machine.
337: This also may be specified on a per-host basis in the configuration file.
338: .It Fl M
339: Places the
340: .Nm
341: client into
342: .Dq master
343: mode for connection sharing.
1.231 stevesk 344: Multiple
345: .Fl M
346: options places
347: .Nm
348: into
349: .Dq master
350: mode with confirmation required before slave connections are accepted.
1.218 jmc 351: Refer to the description of
352: .Cm ControlMaster
353: in
354: .Xr ssh_config 5
355: for details.
356: .It Fl m Ar mac_spec
357: Additionally, for protocol version 2 a comma-separated list of MAC
358: (message authentication code) algorithms can
359: be specified in order of preference.
360: See the
361: .Cm MACs
362: keyword for more information.
363: .It Fl N
364: Do not execute a remote command.
365: This is useful for just forwarding ports
366: (protocol version 2 only).
367: .It Fl n
368: Redirects stdin from
369: .Pa /dev/null
370: (actually, prevents reading from stdin).
371: This must be used when
1.2 deraadt 372: .Nm
1.218 jmc 373: is run in the background.
374: A common trick is to use this to run X11 programs on a remote machine.
375: For example,
376: .Ic ssh -n shadows.cs.hut.fi emacs &
377: will start an emacs on shadows.cs.hut.fi, and the X11
378: connection will be automatically forwarded over an encrypted channel.
379: The
1.2 deraadt 380: .Nm
1.218 jmc 381: program will be put in the background.
382: (This does not work if
1.2 deraadt 383: .Nm
1.218 jmc 384: needs to ask for a password or passphrase; see also the
385: .Fl f
386: option.)
387: .It Fl O Ar ctl_cmd
388: Control an active connection multiplexing master process.
389: When the
390: .Fl O
391: option is specified, the
392: .Ar ctl_cmd
393: argument is interpreted and passed to the master process.
394: Valid commands are:
395: .Dq check
1.305 markus 396: (check that the master process is running),
397: .Dq forward
1.318 jmc 398: (request forwardings without command execution),
1.323 okan 399: .Dq cancel
400: (cancel forwardings),
1.218 jmc 401: .Dq exit
1.318 jmc 402: (request the master to exit), and
1.317 djm 403: .Dq stop
404: (request the master to stop accepting further multiplexing requests).
1.218 jmc 405: .It Fl o Ar option
406: Can be used to give options in the format used in the configuration file.
407: This is useful for specifying options for which there is no separate
408: command-line flag.
409: For full details of the options listed below, and their possible values, see
410: .Xr ssh_config 5 .
1.2 deraadt 411: .Pp
1.218 jmc 412: .Bl -tag -width Ds -offset indent -compact
413: .It AddressFamily
414: .It BatchMode
415: .It BindAddress
1.338 djm 416: .It CanonicalDomains
1.339 djm 417: .It CanonicalizeFallbackLocal
418: .It CanonicalizeHostname
419: .It CanonicalizeMaxDots
420: .It CanonicalizePermittedCNAMEs
1.218 jmc 421: .It ChallengeResponseAuthentication
422: .It CheckHostIP
423: .It Cipher
424: .It Ciphers
425: .It ClearAllForwardings
426: .It Compression
427: .It CompressionLevel
428: .It ConnectionAttempts
429: .It ConnectTimeout
430: .It ControlMaster
431: .It ControlPath
1.321 djm 432: .It ControlPersist
1.218 jmc 433: .It DynamicForward
434: .It EscapeChar
1.263 markus 435: .It ExitOnForwardFailure
1.353 jmc 436: .It FingerprintHash
1.218 jmc 437: .It ForwardAgent
438: .It ForwardX11
1.321 djm 439: .It ForwardX11Timeout
1.218 jmc 440: .It ForwardX11Trusted
441: .It GatewayPorts
442: .It GlobalKnownHostsFile
443: .It GSSAPIAuthentication
444: .It GSSAPIDelegateCredentials
445: .It HashKnownHosts
446: .It Host
447: .It HostbasedAuthentication
1.355 ! djm 448: .It HostbasedKeyTypes
1.218 jmc 449: .It HostKeyAlgorithms
450: .It HostKeyAlias
451: .It HostName
452: .It IdentityFile
453: .It IdentitiesOnly
1.316 jmc 454: .It IPQoS
1.321 djm 455: .It KbdInteractiveAuthentication
1.218 jmc 456: .It KbdInteractiveDevices
1.314 jmc 457: .It KexAlgorithms
1.218 jmc 458: .It LocalCommand
459: .It LocalForward
460: .It LogLevel
461: .It MACs
1.337 jmc 462: .It Match
1.218 jmc 463: .It NoHostAuthenticationForLocalhost
464: .It NumberOfPasswordPrompts
465: .It PasswordAuthentication
466: .It PermitLocalCommand
1.291 markus 467: .It PKCS11Provider
1.218 jmc 468: .It Port
469: .It PreferredAuthentications
470: .It Protocol
471: .It ProxyCommand
1.336 jmc 472: .It ProxyUseFdpass
1.218 jmc 473: .It PubkeyAuthentication
1.251 dtucker 474: .It RekeyLimit
1.218 jmc 475: .It RemoteForward
1.319 jmc 476: .It RequestTTY
1.218 jmc 477: .It RhostsRSAAuthentication
478: .It RSAAuthentication
479: .It SendEnv
480: .It ServerAliveInterval
481: .It ServerAliveCountMax
1.347 jmc 482: .It StreamLocalBindMask
483: .It StreamLocalBindUnlink
1.218 jmc 484: .It StrictHostKeyChecking
485: .It TCPKeepAlive
486: .It Tunnel
487: .It TunnelDevice
1.354 djm 488: .It UpdateHostKeys
1.218 jmc 489: .It UsePrivilegedPort
490: .It User
491: .It UserKnownHostsFile
492: .It VerifyHostKeyDNS
1.276 jmc 493: .It VisualHostKey
1.218 jmc 494: .It XAuthLocation
495: .El
496: .It Fl p Ar port
497: Port to connect to on the remote host.
498: This can be specified on a
499: per-host basis in the configuration file.
1.342 jmc 500: .It Fl Q Cm cipher | cipher-auth | mac | kex | key
1.332 djm 501: Queries
502: .Nm
1.341 deraadt 503: for the algorithms supported for the specified version 2.
504: The available features are:
505: .Ar cipher
1.332 djm 506: (supported symmetric ciphers),
1.341 deraadt 507: .Ar cipher-auth
1.340 djm 508: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 509: .Ar mac
1.332 djm 510: (supported message integrity codes),
1.341 deraadt 511: .Ar kex
1.332 djm 512: (key exchange algorithms),
1.341 deraadt 513: .Ar key
1.332 djm 514: (key types).
1.218 jmc 515: .It Fl q
516: Quiet mode.
1.271 djm 517: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 518: .It Fl R Xo
519: .Sm off
520: .Oo Ar bind_address : Oc
521: .Ar port : host : hostport
522: .Sm on
523: .Xc
524: Specifies that the given port on the remote (server) host is to be
525: forwarded to the given host and port on the local side.
526: This works by allocating a socket to listen to
527: .Ar port
528: on the remote side, and whenever a connection is made to this port, the
529: connection is forwarded over the secure channel, and a connection is
530: made to
531: .Ar host
532: port
533: .Ar hostport
534: from the local machine.
1.2 deraadt 535: .Pp
1.218 jmc 536: Port forwardings can also be specified in the configuration file.
537: Privileged ports can be forwarded only when
538: logging in as root on the remote machine.
1.324 jmc 539: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 540: .Pp
1.218 jmc 541: By default, the listening socket on the server will be bound to the loopback
542: interface only.
1.280 tobias 543: This may be overridden by specifying a
1.218 jmc 544: .Ar bind_address .
545: An empty
546: .Ar bind_address ,
547: or the address
548: .Ql * ,
549: indicates that the remote socket should listen on all interfaces.
550: Specifying a remote
551: .Ar bind_address
552: will only succeed if the server's
553: .Cm GatewayPorts
554: option is enabled (see
555: .Xr sshd_config 5 ) .
1.281 djm 556: .Pp
557: If the
558: .Ar port
559: argument is
1.282 djm 560: .Ql 0 ,
1.281 djm 561: the listen port will be dynamically allocated on the server and reported
562: to the client at run time.
1.305 markus 563: When used together with
564: .Ic -O forward
565: the allocated port will be printed to the standard output.
1.218 jmc 566: .It Fl S Ar ctl_path
1.304 jmc 567: Specifies the location of a control socket for connection sharing,
1.303 djm 568: or the string
569: .Dq none
570: to disable connection sharing.
1.218 jmc 571: Refer to the description of
572: .Cm ControlPath
573: and
574: .Cm ControlMaster
575: in
576: .Xr ssh_config 5
577: for details.
578: .It Fl s
579: May be used to request invocation of a subsystem on the remote system.
580: Subsystems are a feature of the SSH2 protocol which facilitate the use
581: of SSH as a secure transport for other applications (eg.\&
582: .Xr sftp 1 ) .
583: The subsystem is specified as the remote command.
584: .It Fl T
585: Disable pseudo-tty allocation.
586: .It Fl t
587: Force pseudo-tty allocation.
588: This can be used to execute arbitrary
589: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 590: e.g. when implementing menu services.
1.218 jmc 591: Multiple
592: .Fl t
593: options force tty allocation, even if
1.194 jakob 594: .Nm
1.218 jmc 595: has no local tty.
596: .It Fl V
597: Display the version number and exit.
598: .It Fl v
599: Verbose mode.
600: Causes
1.176 jmc 601: .Nm
1.218 jmc 602: to print debugging messages about its progress.
603: This is helpful in
604: debugging connection, authentication, and configuration problems.
605: Multiple
606: .Fl v
607: options increase the verbosity.
608: The maximum is 3.
1.290 dtucker 609: .It Fl W Ar host : Ns Ar port
610: Requests that standard input and output on the client be forwarded to
611: .Ar host
612: on
613: .Ar port
614: over the secure channel.
615: Implies
616: .Fl N ,
617: .Fl T ,
618: .Cm ExitOnForwardFailure
619: and
1.326 dtucker 620: .Cm ClearAllForwardings .
621: Works with Protocol version 2 only.
1.261 stevesk 622: .It Fl w Xo
623: .Ar local_tun Ns Op : Ns Ar remote_tun
624: .Xc
625: Requests
626: tunnel
627: device forwarding with the specified
1.218 jmc 628: .Xr tun 4
1.261 stevesk 629: devices between the client
630: .Pq Ar local_tun
631: and the server
632: .Pq Ar remote_tun .
633: .Pp
1.228 jmc 634: The devices may be specified by numerical ID or the keyword
635: .Dq any ,
636: which uses the next available tunnel device.
1.261 stevesk 637: If
638: .Ar remote_tun
639: is not specified, it defaults to
640: .Dq any .
1.228 jmc 641: See also the
1.218 jmc 642: .Cm Tunnel
1.261 stevesk 643: and
644: .Cm TunnelDevice
645: directives in
1.218 jmc 646: .Xr ssh_config 5 .
1.261 stevesk 647: If the
648: .Cm Tunnel
649: directive is unset, it is set to the default tunnel mode, which is
650: .Dq point-to-point .
1.218 jmc 651: .It Fl X
652: Enables X11 forwarding.
1.54 markus 653: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 654: .Pp
1.218 jmc 655: X11 forwarding should be enabled with caution.
1.168 jmc 656: Users with the ability to bypass file permissions on the remote host
1.218 jmc 657: (for the user's X authorization database)
658: can access the local X11 display through the forwarded connection.
659: An attacker may then be able to perform activities such as keystroke monitoring.
660: .Pp
661: For this reason, X11 forwarding is subjected to X11 SECURITY extension
662: restrictions by default.
663: Please refer to the
664: .Nm
665: .Fl Y
666: option and the
667: .Cm ForwardX11Trusted
668: directive in
669: .Xr ssh_config 5
670: for more information.
671: .It Fl x
672: Disables X11 forwarding.
673: .It Fl Y
674: Enables trusted X11 forwarding.
675: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
676: controls.
1.278 djm 677: .It Fl y
678: Send log information using the
679: .Xr syslog 3
680: system module.
681: By default this information is sent to stderr.
1.218 jmc 682: .El
1.224 jmc 683: .Pp
684: .Nm
685: may additionally obtain configuration data from
686: a per-user configuration file and a system-wide configuration file.
687: The file format and configuration options are described in
688: .Xr ssh_config 5 .
1.222 jmc 689: .Sh AUTHENTICATION
1.249 jmc 690: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 691: The default is to use protocol 2 only,
692: though this can be changed via the
1.222 jmc 693: .Cm Protocol
694: option in
1.284 jmc 695: .Xr ssh_config 5
696: or the
1.222 jmc 697: .Fl 1
698: and
699: .Fl 2
700: options (see above).
701: Both protocols support similar authentication methods,
1.284 jmc 702: but protocol 2 is the default since
1.222 jmc 703: it provides additional mechanisms for confidentiality
704: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 705: and integrity (hmac-md5, hmac-sha1,
706: hmac-sha2-256, hmac-sha2-512,
1.330 markus 707: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 708: Protocol 1 lacks a strong mechanism for ensuring the
709: integrity of the connection.
710: .Pp
711: The methods available for authentication are:
1.260 jmc 712: GSSAPI-based authentication,
1.222 jmc 713: host-based authentication,
714: public key authentication,
715: challenge-response authentication,
716: and password authentication.
717: Authentication methods are tried in the order specified above,
718: though protocol 2 has a configuration option to change the default order:
719: .Cm PreferredAuthentications .
720: .Pp
721: Host-based authentication works as follows:
1.218 jmc 722: If the machine the user logs in from is listed in
723: .Pa /etc/hosts.equiv
724: or
725: .Pa /etc/shosts.equiv
726: on the remote machine, and the user names are
727: the same on both sides, or if the files
728: .Pa ~/.rhosts
729: or
730: .Pa ~/.shosts
731: exist in the user's home directory on the
732: remote machine and contain a line containing the name of the client
733: machine and the name of the user on that machine, the user is
1.222 jmc 734: considered for login.
735: Additionally, the server
736: .Em must
737: be able to verify the client's
738: host key (see the description of
1.218 jmc 739: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 740: and
1.222 jmc 741: .Pa ~/.ssh/known_hosts ,
742: below)
743: for login to be permitted.
1.218 jmc 744: This authentication method closes security holes due to IP
1.222 jmc 745: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 746: [Note to the administrator:
747: .Pa /etc/hosts.equiv ,
748: .Pa ~/.rhosts ,
749: and the rlogin/rsh protocol in general, are inherently insecure and should be
750: disabled if security is desired.]
1.189 dtucker 751: .Pp
1.222 jmc 752: Public key authentication works as follows:
753: The scheme is based on public-key cryptography,
754: using cryptosystems
755: where encryption and decryption are done using separate keys,
756: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 757: The idea is that each user creates a public/private
758: key pair for authentication purposes.
759: The server knows the public key, and only the user knows the private key.
1.222 jmc 760: .Nm
761: implements public key authentication protocol automatically,
1.349 sobrado 762: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.222 jmc 763: Protocol 1 is restricted to using only RSA keys,
1.310 djm 764: but protocol 2 may use any.
1.333 jmc 765: The HISTORY section of
1.222 jmc 766: .Xr ssl 8
1.311 jmc 767: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 768: .Pp
1.218 jmc 769: The file
770: .Pa ~/.ssh/authorized_keys
771: lists the public keys that are permitted for logging in.
772: When the user logs in, the
1.2 deraadt 773: .Nm
1.218 jmc 774: program tells the server which key pair it would like to use for
775: authentication.
1.222 jmc 776: The client proves that it has access to the private key
777: and the server checks that the corresponding public key
778: is authorized to accept the account.
1.218 jmc 779: .Pp
1.222 jmc 780: The user creates his/her key pair by running
1.218 jmc 781: .Xr ssh-keygen 1 .
782: This stores the private key in
1.207 djm 783: .Pa ~/.ssh/identity
1.222 jmc 784: (protocol 1),
785: .Pa ~/.ssh/id_dsa
786: (protocol 2 DSA),
1.310 djm 787: .Pa ~/.ssh/id_ecdsa
788: (protocol 2 ECDSA),
1.343 naddy 789: .Pa ~/.ssh/id_ed25519
1.349 sobrado 790: (protocol 2 Ed25519),
1.222 jmc 791: or
792: .Pa ~/.ssh/id_rsa
793: (protocol 2 RSA)
1.218 jmc 794: and stores the public key in
795: .Pa ~/.ssh/identity.pub
1.222 jmc 796: (protocol 1),
797: .Pa ~/.ssh/id_dsa.pub
798: (protocol 2 DSA),
1.310 djm 799: .Pa ~/.ssh/id_ecdsa.pub
800: (protocol 2 ECDSA),
1.343 naddy 801: .Pa ~/.ssh/id_ed25519.pub
1.349 sobrado 802: (protocol 2 Ed25519),
1.222 jmc 803: or
804: .Pa ~/.ssh/id_rsa.pub
805: (protocol 2 RSA)
1.218 jmc 806: in the user's home directory.
1.222 jmc 807: The user should then copy the public key
1.218 jmc 808: to
809: .Pa ~/.ssh/authorized_keys
1.222 jmc 810: in his/her home directory on the remote machine.
811: The
1.218 jmc 812: .Pa authorized_keys
813: file corresponds to the conventional
814: .Pa ~/.rhosts
815: file, and has one key
1.222 jmc 816: per line, though the lines can be very long.
1.218 jmc 817: After this, the user can log in without giving the password.
818: .Pp
1.301 jmc 819: A variation on public key authentication
820: is available in the form of certificate authentication:
821: instead of a set of public/private keys,
822: signed certificates are used.
823: This has the advantage that a single trusted certification authority
824: can be used in place of many public/private keys.
1.333 jmc 825: See the CERTIFICATES section of
1.301 jmc 826: .Xr ssh-keygen 1
827: for more information.
828: .Pp
829: The most convenient way to use public key or certificate authentication
830: may be with an authentication agent.
1.218 jmc 831: See
832: .Xr ssh-agent 1
833: for more information.
834: .Pp
1.222 jmc 835: Challenge-response authentication works as follows:
836: The server sends an arbitrary
837: .Qq challenge
838: text, and prompts for a response.
839: Protocol 2 allows multiple challenges and responses;
840: protocol 1 is restricted to just one challenge/response.
841: Examples of challenge-response authentication include
1.335 jmc 842: .Bx
843: Authentication (see
1.222 jmc 844: .Xr login.conf 5 )
1.335 jmc 845: and PAM (some
846: .Pf non- Ox
847: systems).
1.222 jmc 848: .Pp
849: Finally, if other authentication methods fail,
1.218 jmc 850: .Nm
851: prompts the user for a password.
852: The password is sent to the remote
853: host for checking; however, since all communications are encrypted,
854: the password cannot be seen by someone listening on the network.
1.232 jmc 855: .Pp
856: .Nm
857: automatically maintains and checks a database containing
858: identification for all hosts it has ever been used with.
859: Host keys are stored in
860: .Pa ~/.ssh/known_hosts
861: in the user's home directory.
862: Additionally, the file
863: .Pa /etc/ssh/ssh_known_hosts
864: is automatically checked for known hosts.
865: Any new hosts are automatically added to the user's file.
866: If a host's identification ever changes,
867: .Nm
868: warns about this and disables password authentication to prevent
869: server spoofing or man-in-the-middle attacks,
870: which could otherwise be used to circumvent the encryption.
871: The
872: .Cm StrictHostKeyChecking
873: option can be used to control logins to machines whose
874: host key is not known or has changed.
875: .Pp
1.218 jmc 876: When the user's identity has been accepted by the server, the server
877: either executes the given command, or logs into the machine and gives
878: the user a normal shell on the remote machine.
879: All communication with
880: the remote command or shell will be automatically encrypted.
881: .Pp
882: If a pseudo-terminal has been allocated (normal login session), the
883: user may use the escape characters noted below.
884: .Pp
885: If no pseudo-tty has been allocated,
886: the session is transparent and can be used to reliably transfer binary data.
887: On most systems, setting the escape character to
888: .Dq none
889: will also make the session transparent even if a tty is used.
890: .Pp
891: The session terminates when the command or shell on the remote
1.247 jmc 892: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 893: .Sh ESCAPE CHARACTERS
1.218 jmc 894: When a pseudo-terminal has been requested,
1.2 deraadt 895: .Nm
1.218 jmc 896: supports a number of functions through the use of an escape character.
897: .Pp
898: A single tilde character can be sent as
899: .Ic ~~
900: or by following the tilde by a character other than those described below.
901: The escape character must always follow a newline to be interpreted as
902: special.
903: The escape character can be changed in configuration files using the
904: .Cm EscapeChar
905: configuration directive or on the command line by the
906: .Fl e
907: option.
908: .Pp
909: The supported escapes (assuming the default
910: .Ql ~ )
911: are:
912: .Bl -tag -width Ds
913: .It Cm ~.
914: Disconnect.
915: .It Cm ~^Z
916: Background
1.234 jmc 917: .Nm .
1.218 jmc 918: .It Cm ~#
919: List forwarded connections.
920: .It Cm ~&
921: Background
1.2 deraadt 922: .Nm
1.218 jmc 923: at logout when waiting for forwarded connection / X11 sessions to terminate.
924: .It Cm ~?
925: Display a list of escape characters.
926: .It Cm ~B
927: Send a BREAK to the remote system
928: (only useful for SSH protocol version 2 and if the peer supports it).
929: .It Cm ~C
930: Open command line.
931: Currently this allows the addition of port forwardings using the
1.279 stevesk 932: .Fl L ,
933: .Fl R
1.218 jmc 934: and
1.279 stevesk 935: .Fl D
1.225 jmc 936: options (see above).
1.322 markus 937: It also allows the cancellation of existing port-forwardings
938: with
1.262 stevesk 939: .Sm off
1.322 markus 940: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 941: .Sm on
1.322 markus 942: for local,
943: .Sm off
944: .Fl KR Oo Ar bind_address : Oc Ar port
945: .Sm on
946: for remote and
947: .Sm off
948: .Fl KD Oo Ar bind_address : Oc Ar port
949: .Sm on
950: for dynamic port-forwardings.
1.218 jmc 951: .Ic !\& Ns Ar command
952: allows the user to execute a local command if the
953: .Ic PermitLocalCommand
954: option is enabled in
1.176 jmc 955: .Xr ssh_config 5 .
1.218 jmc 956: Basic help is available, using the
957: .Fl h
958: option.
959: .It Cm ~R
960: Request rekeying of the connection
961: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 962: .It Cm ~V
963: Decrease the verbosity
964: .Pq Ic LogLevel
965: when errors are being written to stderr.
966: .It Cm ~v
1.328 jmc 967: Increase the verbosity
1.327 dtucker 968: .Pq Ic LogLevel
969: when errors are being written to stderr.
1.176 jmc 970: .El
1.246 jmc 971: .Sh TCP FORWARDING
972: Forwarding of arbitrary TCP connections over the secure channel can
973: be specified either on the command line or in a configuration file.
974: One possible application of TCP forwarding is a secure connection to a
975: mail server; another is going through firewalls.
976: .Pp
977: In the example below, we look at encrypting communication between
978: an IRC client and server, even though the IRC server does not directly
979: support encrypted communications.
980: This works as follows:
981: the user connects to the remote host using
982: .Nm ,
983: specifying a port to be used to forward connections
984: to the remote server.
985: After that it is possible to start the service which is to be encrypted
986: on the client machine,
987: connecting to the same local port,
988: and
989: .Nm
990: will encrypt and forward the connection.
991: .Pp
992: The following example tunnels an IRC session from client machine
993: .Dq 127.0.0.1
994: (localhost)
995: to remote server
996: .Dq server.example.com :
997: .Bd -literal -offset 4n
998: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
999: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1000: .Ed
1001: .Pp
1002: This tunnels a connection to IRC server
1003: .Dq server.example.com ,
1004: joining channel
1005: .Dq #users ,
1006: nickname
1007: .Dq pinky ,
1008: using port 1234.
1009: It doesn't matter which port is used,
1010: as long as it's greater than 1023
1011: (remember, only root can open sockets on privileged ports)
1012: and doesn't conflict with any ports already in use.
1013: The connection is forwarded to port 6667 on the remote server,
1014: since that's the standard port for IRC services.
1015: .Pp
1016: The
1017: .Fl f
1018: option backgrounds
1019: .Nm
1020: and the remote command
1021: .Dq sleep 10
1022: is specified to allow an amount of time
1023: (10 seconds, in the example)
1024: to start the service which is to be tunnelled.
1025: If no connections are made within the time specified,
1026: .Nm
1027: will exit.
1028: .Sh X11 FORWARDING
1.218 jmc 1029: If the
1030: .Cm ForwardX11
1031: variable is set to
1032: .Dq yes
1033: (or see the description of the
1.227 jmc 1034: .Fl X ,
1035: .Fl x ,
1.218 jmc 1036: and
1.227 jmc 1037: .Fl Y
1.226 jmc 1038: options above)
1.218 jmc 1039: and the user is using X11 (the
1040: .Ev DISPLAY
1041: environment variable is set), the connection to the X11 display is
1042: automatically forwarded to the remote side in such a way that any X11
1043: programs started from the shell (or command) will go through the
1044: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1045: from the local machine.
1.218 jmc 1046: The user should not manually set
1047: .Ev DISPLAY .
1048: Forwarding of X11 connections can be
1049: configured on the command line or in configuration files.
1050: .Pp
1051: The
1052: .Ev DISPLAY
1053: value set by
1054: .Nm
1055: will point to the server machine, but with a display number greater than zero.
1056: This is normal, and happens because
1057: .Nm
1058: creates a
1059: .Dq proxy
1060: X server on the server machine for forwarding the
1061: connections over the encrypted channel.
1.200 djm 1062: .Pp
1.218 jmc 1063: .Nm
1064: will also automatically set up Xauthority data on the server machine.
1065: For this purpose, it will generate a random authorization cookie,
1066: store it in Xauthority on the server, and verify that any forwarded
1067: connections carry this cookie and replace it by the real cookie when
1068: the connection is opened.
1069: The real authentication cookie is never
1070: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1071: .Pp
1.218 jmc 1072: If the
1073: .Cm ForwardAgent
1074: variable is set to
1075: .Dq yes
1076: (or see the description of the
1077: .Fl A
1.191 djm 1078: and
1.218 jmc 1079: .Fl a
1.226 jmc 1080: options above) and
1.218 jmc 1081: the user is using an authentication agent, the connection to the agent
1082: is automatically forwarded to the remote side.
1.252 jmc 1083: .Sh VERIFYING HOST KEYS
1084: When connecting to a server for the first time,
1085: a fingerprint of the server's public key is presented to the user
1086: (unless the option
1087: .Cm StrictHostKeyChecking
1088: has been disabled).
1089: Fingerprints can be determined using
1090: .Xr ssh-keygen 1 :
1091: .Pp
1092: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1093: .Pp
1.274 grunk 1094: If the fingerprint is already known, it can be matched
1095: and the key can be accepted or rejected.
1096: Because of the difficulty of comparing host keys
1.352 djm 1097: just by looking at fingerprint strings,
1.274 grunk 1098: there is also support to compare host keys visually,
1099: using
1100: .Em random art .
1101: By setting the
1.275 grunk 1102: .Cm VisualHostKey
1.274 grunk 1103: option to
1.275 grunk 1104: .Dq yes ,
1.274 grunk 1105: a small ASCII graphic gets displayed on every login to a server, no matter
1106: if the session itself is interactive or not.
1107: By learning the pattern a known server produces, a user can easily
1108: find out that the host key has changed when a completely different pattern
1109: is displayed.
1110: Because these patterns are not unambiguous however, a pattern that looks
1111: similar to the pattern remembered only gives a good probability that the
1112: host key is the same, not guaranteed proof.
1113: .Pp
1114: To get a listing of the fingerprints along with their random art for
1115: all known hosts, the following command line can be used:
1116: .Pp
1117: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1118: .Pp
1.252 jmc 1119: If the fingerprint is unknown,
1120: an alternative method of verification is available:
1121: SSH fingerprints verified by DNS.
1122: An additional resource record (RR),
1123: SSHFP,
1124: is added to a zonefile
1125: and the connecting client is able to match the fingerprint
1126: with that of the key presented.
1127: .Pp
1128: In this example, we are connecting a client to a server,
1129: .Dq host.example.com .
1130: The SSHFP resource records should first be added to the zonefile for
1131: host.example.com:
1132: .Bd -literal -offset indent
1.259 jakob 1133: $ ssh-keygen -r host.example.com.
1.252 jmc 1134: .Ed
1135: .Pp
1136: The output lines will have to be added to the zonefile.
1137: To check that the zone is answering fingerprint queries:
1138: .Pp
1139: .Dl $ dig -t SSHFP host.example.com
1140: .Pp
1141: Finally the client connects:
1142: .Bd -literal -offset indent
1143: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1144: [...]
1145: Matching host key fingerprint found in DNS.
1146: Are you sure you want to continue connecting (yes/no)?
1147: .Ed
1148: .Pp
1149: See the
1150: .Cm VerifyHostKeyDNS
1151: option in
1152: .Xr ssh_config 5
1153: for more information.
1.250 jmc 1154: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1155: .Nm
1156: contains support for Virtual Private Network (VPN) tunnelling
1157: using the
1158: .Xr tun 4
1159: network pseudo-device,
1160: allowing two networks to be joined securely.
1161: The
1162: .Xr sshd_config 5
1163: configuration option
1164: .Cm PermitTunnel
1165: controls whether the server supports this,
1166: and at what level (layer 2 or 3 traffic).
1167: .Pp
1168: The following example would connect client network 10.0.50.0/24
1.265 otto 1169: with remote network 10.0.99.0/24 using a point-to-point connection
1170: from 10.1.1.1 to 10.1.1.2,
1171: provided that the SSH server running on the gateway to the remote network,
1172: at 192.168.1.15, allows it.
1173: .Pp
1174: On the client:
1.250 jmc 1175: .Bd -literal -offset indent
1176: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1177: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1178: # route add 10.0.99.0/24 10.1.1.2
1179: .Ed
1180: .Pp
1181: On the server:
1182: .Bd -literal -offset indent
1183: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1184: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1185: .Ed
1186: .Pp
1187: Client access may be more finely tuned via the
1188: .Pa /root/.ssh/authorized_keys
1189: file (see below) and the
1190: .Cm PermitRootLogin
1191: server option.
1.255 jmc 1192: The following entry would permit connections on
1.250 jmc 1193: .Xr tun 4
1.255 jmc 1194: device 1 from user
1.250 jmc 1195: .Dq jane
1.255 jmc 1196: and on tun device 2 from user
1.250 jmc 1197: .Dq john ,
1198: if
1199: .Cm PermitRootLogin
1200: is set to
1201: .Dq forced-commands-only :
1202: .Bd -literal -offset 2n
1203: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1204: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1205: .Ed
1206: .Pp
1.264 ray 1207: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1208: it may be more suited to temporary setups,
1209: such as for wireless VPNs.
1210: More permanent VPNs are better provided by tools such as
1211: .Xr ipsecctl 8
1212: and
1213: .Xr isakmpd 8 .
1.2 deraadt 1214: .Sh ENVIRONMENT
1215: .Nm
1.1 deraadt 1216: will normally set the following environment variables:
1.237 jmc 1217: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1218: .It Ev DISPLAY
1219: The
1220: .Ev DISPLAY
1.40 aaron 1221: variable indicates the location of the X11 server.
1.44 aaron 1222: It is automatically set by
1.2 deraadt 1223: .Nm
1224: to point to a value of the form
1.233 jmc 1225: .Dq hostname:n ,
1226: where
1227: .Dq hostname
1228: indicates the host where the shell runs, and
1229: .Sq n
1230: is an integer \*(Ge 1.
1.40 aaron 1231: .Nm
1232: uses this special value to forward X11 connections over the secure
1233: channel.
1.107 markus 1234: The user should normally not set
1235: .Ev DISPLAY
1236: explicitly, as that
1.1 deraadt 1237: will render the X11 connection insecure (and will require the user to
1238: manually copy any required authorization cookies).
1.2 deraadt 1239: .It Ev HOME
1.1 deraadt 1240: Set to the path of the user's home directory.
1.2 deraadt 1241: .It Ev LOGNAME
1242: Synonym for
1.12 aaron 1243: .Ev USER ;
1244: set for compatibility with systems that use this variable.
1.2 deraadt 1245: .It Ev MAIL
1.129 stevesk 1246: Set to the path of the user's mailbox.
1.40 aaron 1247: .It Ev PATH
1.2 deraadt 1248: Set to the default
1249: .Ev PATH ,
1250: as specified when compiling
1.234 jmc 1251: .Nm .
1.118 markus 1252: .It Ev SSH_ASKPASS
1253: If
1254: .Nm
1255: needs a passphrase, it will read the passphrase from the current
1256: terminal if it was run from a terminal.
1257: If
1258: .Nm
1259: does not have a terminal associated with it but
1260: .Ev DISPLAY
1261: and
1262: .Ev SSH_ASKPASS
1263: are set, it will execute the program specified by
1264: .Ev SSH_ASKPASS
1265: and open an X11 window to read the passphrase.
1266: This is particularly useful when calling
1267: .Nm
1268: from a
1.196 jmc 1269: .Pa .xsession
1.118 markus 1270: or related script.
1271: (Note that on some machines it
1272: may be necessary to redirect the input from
1273: .Pa /dev/null
1274: to make this work.)
1.18 markus 1275: .It Ev SSH_AUTH_SOCK
1.233 jmc 1276: Identifies the path of a
1277: .Ux Ns -domain
1278: socket used to communicate with the agent.
1.166 stevesk 1279: .It Ev SSH_CONNECTION
1280: Identifies the client and server ends of the connection.
1.40 aaron 1281: The variable contains
1.233 jmc 1282: four space-separated values: client IP address, client port number,
1283: server IP address, and server port number.
1.73 markus 1284: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1285: This variable contains the original command line if a forced command
1.73 markus 1286: is executed.
1287: It can be used to extract the original arguments.
1.2 deraadt 1288: .It Ev SSH_TTY
1.1 deraadt 1289: This is set to the name of the tty (path to the device) associated
1.40 aaron 1290: with the current shell or command.
1291: If the current session has no tty,
1.1 deraadt 1292: this variable is not set.
1.2 deraadt 1293: .It Ev TZ
1.214 jmc 1294: This variable is set to indicate the present time zone if it
1.257 jmc 1295: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1296: on to new connections).
1.2 deraadt 1297: .It Ev USER
1.1 deraadt 1298: Set to the name of the user logging in.
1.2 deraadt 1299: .El
1300: .Pp
1.44 aaron 1301: Additionally,
1.2 deraadt 1302: .Nm
1.44 aaron 1303: reads
1.207 djm 1304: .Pa ~/.ssh/environment ,
1.2 deraadt 1305: and adds lines of the format
1306: .Dq VARNAME=value
1.233 jmc 1307: to the environment if the file exists and users are allowed to
1.161 marc 1308: change their environment.
1.176 jmc 1309: For more information, see the
1.161 marc 1310: .Cm PermitUserEnvironment
1.162 stevesk 1311: option in
1.161 marc 1312: .Xr sshd_config 5 .
1.2 deraadt 1313: .Sh FILES
1.236 jmc 1314: .Bl -tag -width Ds -compact
1.309 jmc 1315: .It Pa ~/.rhosts
1.240 jmc 1316: This file is used for host-based authentication (see above).
1.92 markus 1317: On some machines this file may need to be
1.240 jmc 1318: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1319: because
1.2 deraadt 1320: .Xr sshd 8
1.40 aaron 1321: reads it as root.
1322: Additionally, this file must be owned by the user,
1323: and must not have write permissions for anyone else.
1324: The recommended
1.1 deraadt 1325: permission for most machines is read/write for the user, and not
1326: accessible by others.
1.2 deraadt 1327: .Pp
1.309 jmc 1328: .It Pa ~/.shosts
1.240 jmc 1329: This file is used in exactly the same way as
1330: .Pa .rhosts ,
1331: but allows host-based authentication without permitting login with
1332: rlogin/rsh.
1.272 mcbride 1333: .Pp
1.309 jmc 1334: .It Pa ~/.ssh/
1.272 mcbride 1335: This directory is the default location for all user-specific configuration
1336: and authentication information.
1337: There is no general requirement to keep the entire contents of this directory
1338: secret, but the recommended permissions are read/write/execute for the user,
1339: and not accessible by others.
1.236 jmc 1340: .Pp
1.309 jmc 1341: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1342: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1343: that can be used for logging in as this user.
1.238 jmc 1344: The format of this file is described in the
1345: .Xr sshd 8
1346: manual page.
1347: This file is not highly sensitive, but the recommended
1348: permissions are read/write for the user, and not accessible by others.
1349: .Pp
1.309 jmc 1350: .It Pa ~/.ssh/config
1.238 jmc 1351: This is the per-user configuration file.
1352: The file format and configuration options are described in
1353: .Xr ssh_config 5 .
1354: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1355: read/write for the user, and not writable by others.
1.238 jmc 1356: .Pp
1.309 jmc 1357: .It Pa ~/.ssh/environment
1.239 jmc 1358: Contains additional definitions for environment variables; see
1359: .Sx ENVIRONMENT ,
1.238 jmc 1360: above.
1361: .Pp
1.309 jmc 1362: .It Pa ~/.ssh/identity
1363: .It Pa ~/.ssh/id_dsa
1.310 djm 1364: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1365: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1366: .It Pa ~/.ssh/id_rsa
1.238 jmc 1367: Contains the private key for authentication.
1368: These files
1369: contain sensitive data and should be readable by the user but not
1370: accessible by others (read/write/execute).
1371: .Nm
1372: will simply ignore a private key file if it is accessible by others.
1373: It is possible to specify a passphrase when
1374: generating the key which will be used to encrypt the
1375: sensitive part of this file using 3DES.
1376: .Pp
1.309 jmc 1377: .It Pa ~/.ssh/identity.pub
1378: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1379: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1380: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1381: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1382: Contains the public key for authentication.
1383: These files are not
1384: sensitive and can (but need not) be readable by anyone.
1385: .Pp
1.309 jmc 1386: .It Pa ~/.ssh/known_hosts
1.244 jmc 1387: Contains a list of host keys for all hosts the user has logged into
1388: that are not already in the systemwide list of known host keys.
1.238 jmc 1389: See
1.244 jmc 1390: .Xr sshd 8
1391: for further details of the format of this file.
1.238 jmc 1392: .Pp
1.309 jmc 1393: .It Pa ~/.ssh/rc
1.238 jmc 1394: Commands in this file are executed by
1395: .Nm
1.245 jmc 1396: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1397: started.
1398: See the
1399: .Xr sshd 8
1400: manual page for more information.
1401: .Pp
1.309 jmc 1402: .It Pa /etc/hosts.equiv
1.240 jmc 1403: This file is for host-based authentication (see above).
1404: It should only be writable by root.
1.236 jmc 1405: .Pp
1.309 jmc 1406: .It Pa /etc/shosts.equiv
1.240 jmc 1407: This file is used in exactly the same way as
1408: .Pa hosts.equiv ,
1409: but allows host-based authentication without permitting login with
1410: rlogin/rsh.
1.236 jmc 1411: .Pp
1.238 jmc 1412: .It Pa /etc/ssh/ssh_config
1413: Systemwide configuration file.
1414: The file format and configuration options are described in
1415: .Xr ssh_config 5 .
1416: .Pp
1.309 jmc 1417: .It Pa /etc/ssh/ssh_host_key
1418: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1419: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1420: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1421: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1422: These files contain the private parts of the host keys
1.245 jmc 1423: and are used for host-based authentication.
1424: If protocol version 1 is used,
1.238 jmc 1425: .Nm
1426: must be setuid root, since the host key is readable only by root.
1427: For protocol version 2,
1428: .Nm
1429: uses
1430: .Xr ssh-keysign 8
1.245 jmc 1431: to access the host keys,
1432: eliminating the requirement that
1.238 jmc 1433: .Nm
1.245 jmc 1434: be setuid root when host-based authentication is used.
1.238 jmc 1435: By default
1.2 deraadt 1436: .Nm
1.238 jmc 1437: is not setuid root.
1438: .Pp
1.309 jmc 1439: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1440: Systemwide list of known host keys.
1441: This file should be prepared by the
1442: system administrator to contain the public host keys of all machines in the
1443: organization.
1.244 jmc 1444: It should be world-readable.
1445: See
1.238 jmc 1446: .Xr sshd 8
1.244 jmc 1447: for further details of the format of this file.
1.236 jmc 1448: .Pp
1.309 jmc 1449: .It Pa /etc/ssh/sshrc
1.1 deraadt 1450: Commands in this file are executed by
1.2 deraadt 1451: .Nm
1.245 jmc 1452: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1453: See the
1.2 deraadt 1454: .Xr sshd 8
1.1 deraadt 1455: manual page for more information.
1.58 itojun 1456: .El
1.312 jmc 1457: .Sh EXIT STATUS
1458: .Nm
1459: exits with the exit status of the remote command or with 255
1460: if an error occurred.
1.2 deraadt 1461: .Sh SEE ALSO
1462: .Xr scp 1 ,
1.83 djm 1463: .Xr sftp 1 ,
1.2 deraadt 1464: .Xr ssh-add 1 ,
1465: .Xr ssh-agent 1 ,
1466: .Xr ssh-keygen 1 ,
1.242 jmc 1467: .Xr ssh-keyscan 1 ,
1.250 jmc 1468: .Xr tun 4 ,
1.159 stevesk 1469: .Xr ssh_config 5 ,
1.160 naddy 1470: .Xr ssh-keysign 8 ,
1.87 itojun 1471: .Xr sshd 8
1.329 jmc 1472: .Sh STANDARDS
1.106 markus 1473: .Rs
1.329 jmc 1474: .%A S. Lehtinen
1475: .%A C. Lonvick
1476: .%D January 2006
1.256 jmc 1477: .%R RFC 4250
1.329 jmc 1478: .%T The Secure Shell (SSH) Protocol Assigned Numbers
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 4251
1.329 jmc 1486: .%T The Secure Shell (SSH) Protocol Architecture
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 4252
1.329 jmc 1494: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1495: .Re
1.329 jmc 1496: .Pp
1.256 jmc 1497: .Rs
1.329 jmc 1498: .%A T. Ylonen
1499: .%A C. Lonvick
1500: .%D January 2006
1.256 jmc 1501: .%R RFC 4253
1.329 jmc 1502: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1503: .Re
1.329 jmc 1504: .Pp
1.256 jmc 1505: .Rs
1.329 jmc 1506: .%A T. Ylonen
1507: .%A C. Lonvick
1508: .%D January 2006
1.256 jmc 1509: .%R RFC 4254
1.329 jmc 1510: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1511: .Re
1.329 jmc 1512: .Pp
1.256 jmc 1513: .Rs
1.329 jmc 1514: .%A J. Schlyter
1515: .%A W. Griffin
1516: .%D January 2006
1.256 jmc 1517: .%R RFC 4255
1.329 jmc 1518: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1519: .Re
1.329 jmc 1520: .Pp
1.256 jmc 1521: .Rs
1.329 jmc 1522: .%A F. Cusack
1523: .%A M. Forssen
1524: .%D January 2006
1.256 jmc 1525: .%R RFC 4256
1.329 jmc 1526: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1527: .Re
1.329 jmc 1528: .Pp
1.256 jmc 1529: .Rs
1.329 jmc 1530: .%A J. Galbraith
1531: .%A P. Remaker
1532: .%D January 2006
1.256 jmc 1533: .%R RFC 4335
1.329 jmc 1534: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1535: .Re
1.329 jmc 1536: .Pp
1.256 jmc 1537: .Rs
1.329 jmc 1538: .%A M. Bellare
1539: .%A T. Kohno
1540: .%A C. Namprempre
1541: .%D January 2006
1.256 jmc 1542: .%R RFC 4344
1.329 jmc 1543: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1544: .Re
1.329 jmc 1545: .Pp
1.256 jmc 1546: .Rs
1.329 jmc 1547: .%A B. Harris
1548: .%D January 2006
1.256 jmc 1549: .%R RFC 4345
1.329 jmc 1550: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1551: .Re
1.329 jmc 1552: .Pp
1.258 djm 1553: .Rs
1.329 jmc 1554: .%A M. Friedl
1555: .%A N. Provos
1556: .%A W. Simpson
1557: .%D March 2006
1.258 djm 1558: .%R RFC 4419
1.329 jmc 1559: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1560: .Re
1.329 jmc 1561: .Pp
1.266 markus 1562: .Rs
1.329 jmc 1563: .%A J. Galbraith
1564: .%A R. Thayer
1565: .%D November 2006
1.266 markus 1566: .%R RFC 4716
1.329 jmc 1567: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1568: .Re
1.329 jmc 1569: .Pp
1.313 djm 1570: .Rs
1.329 jmc 1571: .%A D. Stebila
1572: .%A J. Green
1573: .%D December 2009
1.313 djm 1574: .%R RFC 5656
1.329 jmc 1575: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1576: .Re
1.329 jmc 1577: .Pp
1.274 grunk 1578: .Rs
1579: .%A A. Perrig
1580: .%A D. Song
1581: .%D 1999
1.329 jmc 1582: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1583: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1584: .Re
1.173 jmc 1585: .Sh AUTHORS
1586: OpenSSH is a derivative of the original and free
1587: ssh 1.2.12 release by Tatu Ylonen.
1588: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1589: Theo de Raadt and Dug Song
1590: removed many bugs, re-added newer features and
1591: created OpenSSH.
1592: Markus Friedl contributed the support for SSH
1593: protocol versions 1.5 and 2.0.