Annotation of src/usr.bin/ssh/ssh.1, Revision 1.343
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: .\"
1.343 ! naddy 36: .\" $OpenBSD: ssh.1,v 1.342 2013/11/26 12:14:54 jmc Exp $
! 37: .Dd $Mdocdate: November 26 2013 $
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 .
157: .Ar 3des
158: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
159: It is believed to be secure.
160: .Ar blowfish
161: is a fast block cipher; it appears very secure and is much faster than
162: .Ar 3des .
163: .Ar des
164: is only supported in the
1.2 deraadt 165: .Nm
1.218 jmc 166: client for interoperability with legacy protocol 1 implementations
167: that do not support the
168: .Ar 3des
169: cipher.
170: Its use is strongly discouraged due to cryptographic weaknesses.
171: The default is
172: .Dq 3des .
1.49 markus 173: .Pp
1.230 jmc 174: For protocol version 2,
1.218 jmc 175: .Ar cipher_spec
176: is a comma-separated list of ciphers
177: listed in order of preference.
1.283 jmc 178: See the
179: .Cm Ciphers
1.307 dtucker 180: keyword in
181: .Xr ssh_config 5
182: for more information.
1.218 jmc 183: .It Fl D Xo
184: .Sm off
185: .Oo Ar bind_address : Oc
186: .Ar port
187: .Sm on
188: .Xc
189: Specifies a local
190: .Dq dynamic
191: application-level port forwarding.
192: This works by allocating a socket to listen to
193: .Ar port
194: on the local side, optionally bound to the specified
195: .Ar bind_address .
196: Whenever a connection is made to this port, the
197: connection is forwarded over the secure channel, and the application
198: protocol is then used to determine where to connect to from the
199: remote machine.
200: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 201: .Nm
1.218 jmc 202: will act as a SOCKS server.
203: Only root can forward privileged ports.
204: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 205: .Pp
1.308 djm 206: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 207: Only the superuser can forward privileged ports.
208: By default, the local port is bound in accordance with the
209: .Cm GatewayPorts
210: setting.
211: However, an explicit
212: .Ar bind_address
213: may be used to bind the connection to a specific address.
214: The
215: .Ar bind_address
216: of
217: .Dq localhost
218: indicates that the listening port be bound for local use only, while an
219: empty address or
220: .Sq *
221: indicates that the port should be available from all interfaces.
1.331 dtucker 222: .It Fl E Ar log_file
223: Append debug logs to
224: .Ar log_file
225: instead of standard error.
1.229 jmc 226: .It Fl e Ar escape_char
1.218 jmc 227: Sets the escape character for sessions with a pty (default:
228: .Ql ~ ) .
229: The escape character is only recognized at the beginning of a line.
230: The escape character followed by a dot
231: .Pq Ql \&.
232: closes the connection;
233: followed by control-Z suspends the connection;
234: and followed by itself sends the escape character once.
235: Setting the character to
1.2 deraadt 236: .Dq none
1.218 jmc 237: disables any escapes and makes the session fully transparent.
238: .It Fl F Ar configfile
239: Specifies an alternative per-user configuration file.
240: If a configuration file is given on the command line,
241: the system-wide configuration file
242: .Pq Pa /etc/ssh/ssh_config
243: will be ignored.
244: The default for the per-user configuration file is
245: .Pa ~/.ssh/config .
246: .It Fl f
247: Requests
248: .Nm
249: to go to background just before command execution.
250: This is useful if
1.176 jmc 251: .Nm
1.218 jmc 252: is going to ask for passwords or passphrases, but the user
253: wants it in the background.
254: This implies
255: .Fl n .
256: The recommended way to start X11 programs at a remote site is with
257: something like
258: .Ic ssh -f host xterm .
1.277 djm 259: .Pp
260: If the
261: .Cm ExitOnForwardFailure
262: configuration option is set to
263: .Dq yes ,
264: then a client started with
265: .Fl f
266: will wait for all remote port forwards to be successfully established
267: before placing itself in the background.
1.218 jmc 268: .It Fl g
269: Allows remote hosts to connect to local forwarded ports.
1.291 markus 270: .It Fl I Ar pkcs11
1.294 jmc 271: Specify the PKCS#11 shared library
1.176 jmc 272: .Nm
1.293 markus 273: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 274: private RSA key.
275: .It Fl i Ar identity_file
276: Selects a file from which the identity (private key) for
1.310 djm 277: public key authentication is read.
1.218 jmc 278: The default is
279: .Pa ~/.ssh/identity
280: for protocol version 1, and
1.310 djm 281: .Pa ~/.ssh/id_dsa ,
1.343 ! naddy 282: .Pa ~/.ssh/id_ecdsa ,
! 283: .Pa ~/.ssh/id_ed25519
1.310 djm 284: and
1.218 jmc 285: .Pa ~/.ssh/id_rsa
286: for protocol version 2.
287: Identity files may also be specified on
288: a per-host basis in the configuration file.
289: It is possible to have multiple
290: .Fl i
291: options (and multiple identities specified in
292: configuration files).
1.302 djm 293: .Nm
294: will also try to load certificate information from the filename obtained
295: by appending
296: .Pa -cert.pub
297: to identity filenames.
1.269 djm 298: .It Fl K
299: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
300: credentials to the server.
1.218 jmc 301: .It Fl k
302: Disables forwarding (delegation) of GSSAPI credentials to the server.
303: .It Fl L Xo
304: .Sm off
305: .Oo Ar bind_address : Oc
306: .Ar port : host : hostport
307: .Sm on
308: .Xc
309: Specifies that the given port on the local (client) host is to be
310: forwarded to the given host and port on the remote side.
311: This works by allocating a socket to listen to
312: .Ar port
313: on the local side, optionally bound to the specified
314: .Ar bind_address .
315: Whenever a connection is made to this port, the
316: connection is forwarded over the secure channel, and a connection is
317: made to
318: .Ar host
319: port
320: .Ar hostport
321: from the remote machine.
322: Port forwardings can also be specified in the configuration file.
1.308 djm 323: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 324: Only the superuser can forward privileged ports.
325: By default, the local port is bound in accordance with the
326: .Cm GatewayPorts
327: setting.
328: However, an explicit
329: .Ar bind_address
330: may be used to bind the connection to a specific address.
1.2 deraadt 331: The
1.218 jmc 332: .Ar bind_address
333: of
334: .Dq localhost
335: indicates that the listening port be bound for local use only, while an
336: empty address or
337: .Sq *
338: indicates that the port should be available from all interfaces.
339: .It Fl l Ar login_name
340: Specifies the user to log in as on the remote machine.
341: This also may be specified on a per-host basis in the configuration file.
342: .It Fl M
343: Places the
344: .Nm
345: client into
346: .Dq master
347: mode for connection sharing.
1.231 stevesk 348: Multiple
349: .Fl M
350: options places
351: .Nm
352: into
353: .Dq master
354: mode with confirmation required before slave connections are accepted.
1.218 jmc 355: Refer to the description of
356: .Cm ControlMaster
357: in
358: .Xr ssh_config 5
359: for details.
360: .It Fl m Ar mac_spec
361: Additionally, for protocol version 2 a comma-separated list of MAC
362: (message authentication code) algorithms can
363: be specified in order of preference.
364: See the
365: .Cm MACs
366: keyword for more information.
367: .It Fl N
368: Do not execute a remote command.
369: This is useful for just forwarding ports
370: (protocol version 2 only).
371: .It Fl n
372: Redirects stdin from
373: .Pa /dev/null
374: (actually, prevents reading from stdin).
375: This must be used when
1.2 deraadt 376: .Nm
1.218 jmc 377: is run in the background.
378: A common trick is to use this to run X11 programs on a remote machine.
379: For example,
380: .Ic ssh -n shadows.cs.hut.fi emacs &
381: will start an emacs on shadows.cs.hut.fi, and the X11
382: connection will be automatically forwarded over an encrypted channel.
383: The
1.2 deraadt 384: .Nm
1.218 jmc 385: program will be put in the background.
386: (This does not work if
1.2 deraadt 387: .Nm
1.218 jmc 388: needs to ask for a password or passphrase; see also the
389: .Fl f
390: option.)
391: .It Fl O Ar ctl_cmd
392: Control an active connection multiplexing master process.
393: When the
394: .Fl O
395: option is specified, the
396: .Ar ctl_cmd
397: argument is interpreted and passed to the master process.
398: Valid commands are:
399: .Dq check
1.305 markus 400: (check that the master process is running),
401: .Dq forward
1.318 jmc 402: (request forwardings without command execution),
1.323 okan 403: .Dq cancel
404: (cancel forwardings),
1.218 jmc 405: .Dq exit
1.318 jmc 406: (request the master to exit), and
1.317 djm 407: .Dq stop
408: (request the master to stop accepting further multiplexing requests).
1.218 jmc 409: .It Fl o Ar option
410: Can be used to give options in the format used in the configuration file.
411: This is useful for specifying options for which there is no separate
412: command-line flag.
413: For full details of the options listed below, and their possible values, see
414: .Xr ssh_config 5 .
1.2 deraadt 415: .Pp
1.218 jmc 416: .Bl -tag -width Ds -offset indent -compact
417: .It AddressFamily
418: .It BatchMode
419: .It BindAddress
1.338 djm 420: .It CanonicalDomains
1.339 djm 421: .It CanonicalizeFallbackLocal
422: .It CanonicalizeHostname
423: .It CanonicalizeMaxDots
424: .It CanonicalizePermittedCNAMEs
1.218 jmc 425: .It ChallengeResponseAuthentication
426: .It CheckHostIP
427: .It Cipher
428: .It Ciphers
429: .It ClearAllForwardings
430: .It Compression
431: .It CompressionLevel
432: .It ConnectionAttempts
433: .It ConnectTimeout
434: .It ControlMaster
435: .It ControlPath
1.321 djm 436: .It ControlPersist
1.218 jmc 437: .It DynamicForward
438: .It EscapeChar
1.263 markus 439: .It ExitOnForwardFailure
1.218 jmc 440: .It ForwardAgent
441: .It ForwardX11
1.321 djm 442: .It ForwardX11Timeout
1.218 jmc 443: .It ForwardX11Trusted
444: .It GatewayPorts
445: .It GlobalKnownHostsFile
446: .It GSSAPIAuthentication
447: .It GSSAPIDelegateCredentials
448: .It HashKnownHosts
449: .It Host
450: .It HostbasedAuthentication
451: .It HostKeyAlgorithms
452: .It HostKeyAlias
453: .It HostName
454: .It IdentityFile
455: .It IdentitiesOnly
1.316 jmc 456: .It IPQoS
1.321 djm 457: .It KbdInteractiveAuthentication
1.218 jmc 458: .It KbdInteractiveDevices
1.314 jmc 459: .It KexAlgorithms
1.218 jmc 460: .It LocalCommand
461: .It LocalForward
462: .It LogLevel
463: .It MACs
1.337 jmc 464: .It Match
1.218 jmc 465: .It NoHostAuthenticationForLocalhost
466: .It NumberOfPasswordPrompts
467: .It PasswordAuthentication
468: .It PermitLocalCommand
1.291 markus 469: .It PKCS11Provider
1.218 jmc 470: .It Port
471: .It PreferredAuthentications
472: .It Protocol
473: .It ProxyCommand
1.336 jmc 474: .It ProxyUseFdpass
1.218 jmc 475: .It PubkeyAuthentication
1.251 dtucker 476: .It RekeyLimit
1.218 jmc 477: .It RemoteForward
1.319 jmc 478: .It RequestTTY
1.218 jmc 479: .It RhostsRSAAuthentication
480: .It RSAAuthentication
481: .It SendEnv
482: .It ServerAliveInterval
483: .It ServerAliveCountMax
484: .It StrictHostKeyChecking
485: .It TCPKeepAlive
486: .It Tunnel
487: .It TunnelDevice
488: .It UsePrivilegedPort
489: .It User
490: .It UserKnownHostsFile
491: .It VerifyHostKeyDNS
1.276 jmc 492: .It VisualHostKey
1.218 jmc 493: .It XAuthLocation
494: .El
495: .It Fl p Ar port
496: Port to connect to on the remote host.
497: This can be specified on a
498: per-host basis in the configuration file.
1.342 jmc 499: .It Fl Q Cm cipher | cipher-auth | mac | kex | key
1.332 djm 500: Queries
501: .Nm
1.341 deraadt 502: for the algorithms supported for the specified version 2.
503: The available features are:
504: .Ar cipher
1.332 djm 505: (supported symmetric ciphers),
1.341 deraadt 506: .Ar cipher-auth
1.340 djm 507: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 508: .Ar mac
1.332 djm 509: (supported message integrity codes),
1.341 deraadt 510: .Ar kex
1.332 djm 511: (key exchange algorithms),
1.341 deraadt 512: .Ar key
1.332 djm 513: (key types).
1.218 jmc 514: .It Fl q
515: Quiet mode.
1.271 djm 516: Causes most warning and diagnostic messages to be suppressed.
1.218 jmc 517: .It Fl R Xo
518: .Sm off
519: .Oo Ar bind_address : Oc
520: .Ar port : host : hostport
521: .Sm on
522: .Xc
523: Specifies that the given port on the remote (server) host is to be
524: forwarded to the given host and port on the local side.
525: This works by allocating a socket to listen to
526: .Ar port
527: on the remote side, and whenever a connection is made to this port, the
528: connection is forwarded over the secure channel, and a connection is
529: made to
530: .Ar host
531: port
532: .Ar hostport
533: from the local machine.
1.2 deraadt 534: .Pp
1.218 jmc 535: Port forwardings can also be specified in the configuration file.
536: Privileged ports can be forwarded only when
537: logging in as root on the remote machine.
1.324 jmc 538: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 539: .Pp
1.218 jmc 540: By default, the listening socket on the server will be bound to the loopback
541: interface only.
1.280 tobias 542: This may be overridden by specifying a
1.218 jmc 543: .Ar bind_address .
544: An empty
545: .Ar bind_address ,
546: or the address
547: .Ql * ,
548: indicates that the remote socket should listen on all interfaces.
549: Specifying a remote
550: .Ar bind_address
551: will only succeed if the server's
552: .Cm GatewayPorts
553: option is enabled (see
554: .Xr sshd_config 5 ) .
1.281 djm 555: .Pp
556: If the
557: .Ar port
558: argument is
1.282 djm 559: .Ql 0 ,
1.281 djm 560: the listen port will be dynamically allocated on the server and reported
561: to the client at run time.
1.305 markus 562: When used together with
563: .Ic -O forward
564: the allocated port will be printed to the standard output.
1.218 jmc 565: .It Fl S Ar ctl_path
1.304 jmc 566: Specifies the location of a control socket for connection sharing,
1.303 djm 567: or the string
568: .Dq none
569: to disable connection sharing.
1.218 jmc 570: Refer to the description of
571: .Cm ControlPath
572: and
573: .Cm ControlMaster
574: in
575: .Xr ssh_config 5
576: for details.
577: .It Fl s
578: May be used to request invocation of a subsystem on the remote system.
579: Subsystems are a feature of the SSH2 protocol which facilitate the use
580: of SSH as a secure transport for other applications (eg.\&
581: .Xr sftp 1 ) .
582: The subsystem is specified as the remote command.
583: .It Fl T
584: Disable pseudo-tty allocation.
585: .It Fl t
586: Force pseudo-tty allocation.
587: This can be used to execute arbitrary
588: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 589: e.g. when implementing menu services.
1.218 jmc 590: Multiple
591: .Fl t
592: options force tty allocation, even if
1.194 jakob 593: .Nm
1.218 jmc 594: has no local tty.
595: .It Fl V
596: Display the version number and exit.
597: .It Fl v
598: Verbose mode.
599: Causes
1.176 jmc 600: .Nm
1.218 jmc 601: to print debugging messages about its progress.
602: This is helpful in
603: debugging connection, authentication, and configuration problems.
604: Multiple
605: .Fl v
606: options increase the verbosity.
607: The maximum is 3.
1.290 dtucker 608: .It Fl W Ar host : Ns Ar port
609: Requests that standard input and output on the client be forwarded to
610: .Ar host
611: on
612: .Ar port
613: over the secure channel.
614: Implies
615: .Fl N ,
616: .Fl T ,
617: .Cm ExitOnForwardFailure
618: and
1.326 dtucker 619: .Cm ClearAllForwardings .
620: Works with Protocol version 2 only.
1.261 stevesk 621: .It Fl w Xo
622: .Ar local_tun Ns Op : Ns Ar remote_tun
623: .Xc
624: Requests
625: tunnel
626: device forwarding with the specified
1.218 jmc 627: .Xr tun 4
1.261 stevesk 628: devices between the client
629: .Pq Ar local_tun
630: and the server
631: .Pq Ar remote_tun .
632: .Pp
1.228 jmc 633: The devices may be specified by numerical ID or the keyword
634: .Dq any ,
635: which uses the next available tunnel device.
1.261 stevesk 636: If
637: .Ar remote_tun
638: is not specified, it defaults to
639: .Dq any .
1.228 jmc 640: See also the
1.218 jmc 641: .Cm Tunnel
1.261 stevesk 642: and
643: .Cm TunnelDevice
644: directives in
1.218 jmc 645: .Xr ssh_config 5 .
1.261 stevesk 646: If the
647: .Cm Tunnel
648: directive is unset, it is set to the default tunnel mode, which is
649: .Dq point-to-point .
1.218 jmc 650: .It Fl X
651: Enables X11 forwarding.
1.54 markus 652: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 653: .Pp
1.218 jmc 654: X11 forwarding should be enabled with caution.
1.168 jmc 655: Users with the ability to bypass file permissions on the remote host
1.218 jmc 656: (for the user's X authorization database)
657: can access the local X11 display through the forwarded connection.
658: An attacker may then be able to perform activities such as keystroke monitoring.
659: .Pp
660: For this reason, X11 forwarding is subjected to X11 SECURITY extension
661: restrictions by default.
662: Please refer to the
663: .Nm
664: .Fl Y
665: option and the
666: .Cm ForwardX11Trusted
667: directive in
668: .Xr ssh_config 5
669: for more information.
670: .It Fl x
671: Disables X11 forwarding.
672: .It Fl Y
673: Enables trusted X11 forwarding.
674: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
675: controls.
1.278 djm 676: .It Fl y
677: Send log information using the
678: .Xr syslog 3
679: system module.
680: By default this information is sent to stderr.
1.218 jmc 681: .El
1.224 jmc 682: .Pp
683: .Nm
684: may additionally obtain configuration data from
685: a per-user configuration file and a system-wide configuration file.
686: The file format and configuration options are described in
687: .Xr ssh_config 5 .
1.222 jmc 688: .Sh AUTHENTICATION
1.249 jmc 689: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 690: The default is to use protocol 2 only,
691: though this can be changed via the
1.222 jmc 692: .Cm Protocol
693: option in
1.284 jmc 694: .Xr ssh_config 5
695: or the
1.222 jmc 696: .Fl 1
697: and
698: .Fl 2
699: options (see above).
700: Both protocols support similar authentication methods,
1.284 jmc 701: but protocol 2 is the default since
1.222 jmc 702: it provides additional mechanisms for confidentiality
703: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 704: and integrity (hmac-md5, hmac-sha1,
705: hmac-sha2-256, hmac-sha2-512,
1.330 markus 706: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 707: Protocol 1 lacks a strong mechanism for ensuring the
708: integrity of the connection.
709: .Pp
710: The methods available for authentication are:
1.260 jmc 711: GSSAPI-based authentication,
1.222 jmc 712: host-based authentication,
713: public key authentication,
714: challenge-response authentication,
715: and password authentication.
716: Authentication methods are tried in the order specified above,
717: though protocol 2 has a configuration option to change the default order:
718: .Cm PreferredAuthentications .
719: .Pp
720: Host-based authentication works as follows:
1.218 jmc 721: If the machine the user logs in from is listed in
722: .Pa /etc/hosts.equiv
723: or
724: .Pa /etc/shosts.equiv
725: on the remote machine, and the user names are
726: the same on both sides, or if the files
727: .Pa ~/.rhosts
728: or
729: .Pa ~/.shosts
730: exist in the user's home directory on the
731: remote machine and contain a line containing the name of the client
732: machine and the name of the user on that machine, the user is
1.222 jmc 733: considered for login.
734: Additionally, the server
735: .Em must
736: be able to verify the client's
737: host key (see the description of
1.218 jmc 738: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 739: and
1.222 jmc 740: .Pa ~/.ssh/known_hosts ,
741: below)
742: for login to be permitted.
1.218 jmc 743: This authentication method closes security holes due to IP
1.222 jmc 744: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 745: [Note to the administrator:
746: .Pa /etc/hosts.equiv ,
747: .Pa ~/.rhosts ,
748: and the rlogin/rsh protocol in general, are inherently insecure and should be
749: disabled if security is desired.]
1.189 dtucker 750: .Pp
1.222 jmc 751: Public key authentication works as follows:
752: The scheme is based on public-key cryptography,
753: using cryptosystems
754: where encryption and decryption are done using separate keys,
755: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 756: The idea is that each user creates a public/private
757: key pair for authentication purposes.
758: The server knows the public key, and only the user knows the private key.
1.222 jmc 759: .Nm
760: implements public key authentication protocol automatically,
1.343 ! naddy 761: using one of the DSA, ECDSA, ED25519 or RSA algorithms.
1.222 jmc 762: Protocol 1 is restricted to using only RSA keys,
1.310 djm 763: but protocol 2 may use any.
1.333 jmc 764: The HISTORY section of
1.222 jmc 765: .Xr ssl 8
1.311 jmc 766: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 767: .Pp
1.218 jmc 768: The file
769: .Pa ~/.ssh/authorized_keys
770: lists the public keys that are permitted for logging in.
771: When the user logs in, the
1.2 deraadt 772: .Nm
1.218 jmc 773: program tells the server which key pair it would like to use for
774: authentication.
1.222 jmc 775: The client proves that it has access to the private key
776: and the server checks that the corresponding public key
777: is authorized to accept the account.
1.218 jmc 778: .Pp
1.222 jmc 779: The user creates his/her key pair by running
1.218 jmc 780: .Xr ssh-keygen 1 .
781: This stores the private key in
1.207 djm 782: .Pa ~/.ssh/identity
1.222 jmc 783: (protocol 1),
784: .Pa ~/.ssh/id_dsa
785: (protocol 2 DSA),
1.310 djm 786: .Pa ~/.ssh/id_ecdsa
787: (protocol 2 ECDSA),
1.343 ! naddy 788: .Pa ~/.ssh/id_ed25519
! 789: (protocol 2 ED25519),
1.222 jmc 790: or
791: .Pa ~/.ssh/id_rsa
792: (protocol 2 RSA)
1.218 jmc 793: and stores the public key in
794: .Pa ~/.ssh/identity.pub
1.222 jmc 795: (protocol 1),
796: .Pa ~/.ssh/id_dsa.pub
797: (protocol 2 DSA),
1.310 djm 798: .Pa ~/.ssh/id_ecdsa.pub
799: (protocol 2 ECDSA),
1.343 ! naddy 800: .Pa ~/.ssh/id_ed25519.pub
! 801: (protocol 2 ED25519),
1.222 jmc 802: or
803: .Pa ~/.ssh/id_rsa.pub
804: (protocol 2 RSA)
1.218 jmc 805: in the user's home directory.
1.222 jmc 806: The user should then copy the public key
1.218 jmc 807: to
808: .Pa ~/.ssh/authorized_keys
1.222 jmc 809: in his/her home directory on the remote machine.
810: The
1.218 jmc 811: .Pa authorized_keys
812: file corresponds to the conventional
813: .Pa ~/.rhosts
814: file, and has one key
1.222 jmc 815: per line, though the lines can be very long.
1.218 jmc 816: After this, the user can log in without giving the password.
817: .Pp
1.301 jmc 818: A variation on public key authentication
819: is available in the form of certificate authentication:
820: instead of a set of public/private keys,
821: signed certificates are used.
822: This has the advantage that a single trusted certification authority
823: can be used in place of many public/private keys.
1.333 jmc 824: See the CERTIFICATES section of
1.301 jmc 825: .Xr ssh-keygen 1
826: for more information.
827: .Pp
828: The most convenient way to use public key or certificate authentication
829: may be with an authentication agent.
1.218 jmc 830: See
831: .Xr ssh-agent 1
832: for more information.
833: .Pp
1.222 jmc 834: Challenge-response authentication works as follows:
835: The server sends an arbitrary
836: .Qq challenge
837: text, and prompts for a response.
838: Protocol 2 allows multiple challenges and responses;
839: protocol 1 is restricted to just one challenge/response.
840: Examples of challenge-response authentication include
1.335 jmc 841: .Bx
842: Authentication (see
1.222 jmc 843: .Xr login.conf 5 )
1.335 jmc 844: and PAM (some
845: .Pf non- Ox
846: systems).
1.222 jmc 847: .Pp
848: Finally, if other authentication methods fail,
1.218 jmc 849: .Nm
850: prompts the user for a password.
851: The password is sent to the remote
852: host for checking; however, since all communications are encrypted,
853: the password cannot be seen by someone listening on the network.
1.232 jmc 854: .Pp
855: .Nm
856: automatically maintains and checks a database containing
857: identification for all hosts it has ever been used with.
858: Host keys are stored in
859: .Pa ~/.ssh/known_hosts
860: in the user's home directory.
861: Additionally, the file
862: .Pa /etc/ssh/ssh_known_hosts
863: is automatically checked for known hosts.
864: Any new hosts are automatically added to the user's file.
865: If a host's identification ever changes,
866: .Nm
867: warns about this and disables password authentication to prevent
868: server spoofing or man-in-the-middle attacks,
869: which could otherwise be used to circumvent the encryption.
870: The
871: .Cm StrictHostKeyChecking
872: option can be used to control logins to machines whose
873: host key is not known or has changed.
874: .Pp
1.218 jmc 875: When the user's identity has been accepted by the server, the server
876: either executes the given command, or logs into the machine and gives
877: the user a normal shell on the remote machine.
878: All communication with
879: the remote command or shell will be automatically encrypted.
880: .Pp
881: If a pseudo-terminal has been allocated (normal login session), the
882: user may use the escape characters noted below.
883: .Pp
884: If no pseudo-tty has been allocated,
885: the session is transparent and can be used to reliably transfer binary data.
886: On most systems, setting the escape character to
887: .Dq none
888: will also make the session transparent even if a tty is used.
889: .Pp
890: The session terminates when the command or shell on the remote
1.247 jmc 891: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 892: .Sh ESCAPE CHARACTERS
1.218 jmc 893: When a pseudo-terminal has been requested,
1.2 deraadt 894: .Nm
1.218 jmc 895: supports a number of functions through the use of an escape character.
896: .Pp
897: A single tilde character can be sent as
898: .Ic ~~
899: or by following the tilde by a character other than those described below.
900: The escape character must always follow a newline to be interpreted as
901: special.
902: The escape character can be changed in configuration files using the
903: .Cm EscapeChar
904: configuration directive or on the command line by the
905: .Fl e
906: option.
907: .Pp
908: The supported escapes (assuming the default
909: .Ql ~ )
910: are:
911: .Bl -tag -width Ds
912: .It Cm ~.
913: Disconnect.
914: .It Cm ~^Z
915: Background
1.234 jmc 916: .Nm .
1.218 jmc 917: .It Cm ~#
918: List forwarded connections.
919: .It Cm ~&
920: Background
1.2 deraadt 921: .Nm
1.218 jmc 922: at logout when waiting for forwarded connection / X11 sessions to terminate.
923: .It Cm ~?
924: Display a list of escape characters.
925: .It Cm ~B
926: Send a BREAK to the remote system
927: (only useful for SSH protocol version 2 and if the peer supports it).
928: .It Cm ~C
929: Open command line.
930: Currently this allows the addition of port forwardings using the
1.279 stevesk 931: .Fl L ,
932: .Fl R
1.218 jmc 933: and
1.279 stevesk 934: .Fl D
1.225 jmc 935: options (see above).
1.322 markus 936: It also allows the cancellation of existing port-forwardings
937: with
1.262 stevesk 938: .Sm off
1.322 markus 939: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 940: .Sm on
1.322 markus 941: for local,
942: .Sm off
943: .Fl KR Oo Ar bind_address : Oc Ar port
944: .Sm on
945: for remote and
946: .Sm off
947: .Fl KD Oo Ar bind_address : Oc Ar port
948: .Sm on
949: for dynamic port-forwardings.
1.218 jmc 950: .Ic !\& Ns Ar command
951: allows the user to execute a local command if the
952: .Ic PermitLocalCommand
953: option is enabled in
1.176 jmc 954: .Xr ssh_config 5 .
1.218 jmc 955: Basic help is available, using the
956: .Fl h
957: option.
958: .It Cm ~R
959: Request rekeying of the connection
960: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 961: .It Cm ~V
962: Decrease the verbosity
963: .Pq Ic LogLevel
964: when errors are being written to stderr.
965: .It Cm ~v
1.328 jmc 966: Increase the verbosity
1.327 dtucker 967: .Pq Ic LogLevel
968: when errors are being written to stderr.
1.176 jmc 969: .El
1.246 jmc 970: .Sh TCP FORWARDING
971: Forwarding of arbitrary TCP connections over the secure channel can
972: be specified either on the command line or in a configuration file.
973: One possible application of TCP forwarding is a secure connection to a
974: mail server; another is going through firewalls.
975: .Pp
976: In the example below, we look at encrypting communication between
977: an IRC client and server, even though the IRC server does not directly
978: support encrypted communications.
979: This works as follows:
980: the user connects to the remote host using
981: .Nm ,
982: specifying a port to be used to forward connections
983: to the remote server.
984: After that it is possible to start the service which is to be encrypted
985: on the client machine,
986: connecting to the same local port,
987: and
988: .Nm
989: will encrypt and forward the connection.
990: .Pp
991: The following example tunnels an IRC session from client machine
992: .Dq 127.0.0.1
993: (localhost)
994: to remote server
995: .Dq server.example.com :
996: .Bd -literal -offset 4n
997: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
998: $ irc -c '#users' -p 1234 pinky 127.0.0.1
999: .Ed
1000: .Pp
1001: This tunnels a connection to IRC server
1002: .Dq server.example.com ,
1003: joining channel
1004: .Dq #users ,
1005: nickname
1006: .Dq pinky ,
1007: using port 1234.
1008: It doesn't matter which port is used,
1009: as long as it's greater than 1023
1010: (remember, only root can open sockets on privileged ports)
1011: and doesn't conflict with any ports already in use.
1012: The connection is forwarded to port 6667 on the remote server,
1013: since that's the standard port for IRC services.
1014: .Pp
1015: The
1016: .Fl f
1017: option backgrounds
1018: .Nm
1019: and the remote command
1020: .Dq sleep 10
1021: is specified to allow an amount of time
1022: (10 seconds, in the example)
1023: to start the service which is to be tunnelled.
1024: If no connections are made within the time specified,
1025: .Nm
1026: will exit.
1027: .Sh X11 FORWARDING
1.218 jmc 1028: If the
1029: .Cm ForwardX11
1030: variable is set to
1031: .Dq yes
1032: (or see the description of the
1.227 jmc 1033: .Fl X ,
1034: .Fl x ,
1.218 jmc 1035: and
1.227 jmc 1036: .Fl Y
1.226 jmc 1037: options above)
1.218 jmc 1038: and the user is using X11 (the
1039: .Ev DISPLAY
1040: environment variable is set), the connection to the X11 display is
1041: automatically forwarded to the remote side in such a way that any X11
1042: programs started from the shell (or command) will go through the
1043: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1044: from the local machine.
1.218 jmc 1045: The user should not manually set
1046: .Ev DISPLAY .
1047: Forwarding of X11 connections can be
1048: configured on the command line or in configuration files.
1049: .Pp
1050: The
1051: .Ev DISPLAY
1052: value set by
1053: .Nm
1054: will point to the server machine, but with a display number greater than zero.
1055: This is normal, and happens because
1056: .Nm
1057: creates a
1058: .Dq proxy
1059: X server on the server machine for forwarding the
1060: connections over the encrypted channel.
1.200 djm 1061: .Pp
1.218 jmc 1062: .Nm
1063: will also automatically set up Xauthority data on the server machine.
1064: For this purpose, it will generate a random authorization cookie,
1065: store it in Xauthority on the server, and verify that any forwarded
1066: connections carry this cookie and replace it by the real cookie when
1067: the connection is opened.
1068: The real authentication cookie is never
1069: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1070: .Pp
1.218 jmc 1071: If the
1072: .Cm ForwardAgent
1073: variable is set to
1074: .Dq yes
1075: (or see the description of the
1076: .Fl A
1.191 djm 1077: and
1.218 jmc 1078: .Fl a
1.226 jmc 1079: options above) and
1.218 jmc 1080: the user is using an authentication agent, the connection to the agent
1081: is automatically forwarded to the remote side.
1.252 jmc 1082: .Sh VERIFYING HOST KEYS
1083: When connecting to a server for the first time,
1084: a fingerprint of the server's public key is presented to the user
1085: (unless the option
1086: .Cm StrictHostKeyChecking
1087: has been disabled).
1088: Fingerprints can be determined using
1089: .Xr ssh-keygen 1 :
1090: .Pp
1091: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1092: .Pp
1.274 grunk 1093: If the fingerprint is already known, it can be matched
1094: and the key can be accepted or rejected.
1095: Because of the difficulty of comparing host keys
1096: just by looking at hex strings,
1097: there is also support to compare host keys visually,
1098: using
1099: .Em random art .
1100: By setting the
1.275 grunk 1101: .Cm VisualHostKey
1.274 grunk 1102: option to
1.275 grunk 1103: .Dq yes ,
1.274 grunk 1104: a small ASCII graphic gets displayed on every login to a server, no matter
1105: if the session itself is interactive or not.
1106: By learning the pattern a known server produces, a user can easily
1107: find out that the host key has changed when a completely different pattern
1108: is displayed.
1109: Because these patterns are not unambiguous however, a pattern that looks
1110: similar to the pattern remembered only gives a good probability that the
1111: host key is the same, not guaranteed proof.
1112: .Pp
1113: To get a listing of the fingerprints along with their random art for
1114: all known hosts, the following command line can be used:
1115: .Pp
1116: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1117: .Pp
1.252 jmc 1118: If the fingerprint is unknown,
1119: an alternative method of verification is available:
1120: SSH fingerprints verified by DNS.
1121: An additional resource record (RR),
1122: SSHFP,
1123: is added to a zonefile
1124: and the connecting client is able to match the fingerprint
1125: with that of the key presented.
1126: .Pp
1127: In this example, we are connecting a client to a server,
1128: .Dq host.example.com .
1129: The SSHFP resource records should first be added to the zonefile for
1130: host.example.com:
1131: .Bd -literal -offset indent
1.259 jakob 1132: $ ssh-keygen -r host.example.com.
1.252 jmc 1133: .Ed
1134: .Pp
1135: The output lines will have to be added to the zonefile.
1136: To check that the zone is answering fingerprint queries:
1137: .Pp
1138: .Dl $ dig -t SSHFP host.example.com
1139: .Pp
1140: Finally the client connects:
1141: .Bd -literal -offset indent
1142: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1143: [...]
1144: Matching host key fingerprint found in DNS.
1145: Are you sure you want to continue connecting (yes/no)?
1146: .Ed
1147: .Pp
1148: See the
1149: .Cm VerifyHostKeyDNS
1150: option in
1151: .Xr ssh_config 5
1152: for more information.
1.250 jmc 1153: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1154: .Nm
1155: contains support for Virtual Private Network (VPN) tunnelling
1156: using the
1157: .Xr tun 4
1158: network pseudo-device,
1159: allowing two networks to be joined securely.
1160: The
1161: .Xr sshd_config 5
1162: configuration option
1163: .Cm PermitTunnel
1164: controls whether the server supports this,
1165: and at what level (layer 2 or 3 traffic).
1166: .Pp
1167: The following example would connect client network 10.0.50.0/24
1.265 otto 1168: with remote network 10.0.99.0/24 using a point-to-point connection
1169: from 10.1.1.1 to 10.1.1.2,
1170: provided that the SSH server running on the gateway to the remote network,
1171: at 192.168.1.15, allows it.
1172: .Pp
1173: On the client:
1.250 jmc 1174: .Bd -literal -offset indent
1175: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1176: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1177: # route add 10.0.99.0/24 10.1.1.2
1178: .Ed
1179: .Pp
1180: On the server:
1181: .Bd -literal -offset indent
1182: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1183: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1184: .Ed
1185: .Pp
1186: Client access may be more finely tuned via the
1187: .Pa /root/.ssh/authorized_keys
1188: file (see below) and the
1189: .Cm PermitRootLogin
1190: server option.
1.255 jmc 1191: The following entry would permit connections on
1.250 jmc 1192: .Xr tun 4
1.255 jmc 1193: device 1 from user
1.250 jmc 1194: .Dq jane
1.255 jmc 1195: and on tun device 2 from user
1.250 jmc 1196: .Dq john ,
1197: if
1198: .Cm PermitRootLogin
1199: is set to
1200: .Dq forced-commands-only :
1201: .Bd -literal -offset 2n
1202: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1203: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1204: .Ed
1205: .Pp
1.264 ray 1206: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1207: it may be more suited to temporary setups,
1208: such as for wireless VPNs.
1209: More permanent VPNs are better provided by tools such as
1210: .Xr ipsecctl 8
1211: and
1212: .Xr isakmpd 8 .
1.2 deraadt 1213: .Sh ENVIRONMENT
1214: .Nm
1.1 deraadt 1215: will normally set the following environment variables:
1.237 jmc 1216: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1217: .It Ev DISPLAY
1218: The
1219: .Ev DISPLAY
1.40 aaron 1220: variable indicates the location of the X11 server.
1.44 aaron 1221: It is automatically set by
1.2 deraadt 1222: .Nm
1223: to point to a value of the form
1.233 jmc 1224: .Dq hostname:n ,
1225: where
1226: .Dq hostname
1227: indicates the host where the shell runs, and
1228: .Sq n
1229: is an integer \*(Ge 1.
1.40 aaron 1230: .Nm
1231: uses this special value to forward X11 connections over the secure
1232: channel.
1.107 markus 1233: The user should normally not set
1234: .Ev DISPLAY
1235: explicitly, as that
1.1 deraadt 1236: will render the X11 connection insecure (and will require the user to
1237: manually copy any required authorization cookies).
1.2 deraadt 1238: .It Ev HOME
1.1 deraadt 1239: Set to the path of the user's home directory.
1.2 deraadt 1240: .It Ev LOGNAME
1241: Synonym for
1.12 aaron 1242: .Ev USER ;
1243: set for compatibility with systems that use this variable.
1.2 deraadt 1244: .It Ev MAIL
1.129 stevesk 1245: Set to the path of the user's mailbox.
1.40 aaron 1246: .It Ev PATH
1.2 deraadt 1247: Set to the default
1248: .Ev PATH ,
1249: as specified when compiling
1.234 jmc 1250: .Nm .
1.118 markus 1251: .It Ev SSH_ASKPASS
1252: If
1253: .Nm
1254: needs a passphrase, it will read the passphrase from the current
1255: terminal if it was run from a terminal.
1256: If
1257: .Nm
1258: does not have a terminal associated with it but
1259: .Ev DISPLAY
1260: and
1261: .Ev SSH_ASKPASS
1262: are set, it will execute the program specified by
1263: .Ev SSH_ASKPASS
1264: and open an X11 window to read the passphrase.
1265: This is particularly useful when calling
1266: .Nm
1267: from a
1.196 jmc 1268: .Pa .xsession
1.118 markus 1269: or related script.
1270: (Note that on some machines it
1271: may be necessary to redirect the input from
1272: .Pa /dev/null
1273: to make this work.)
1.18 markus 1274: .It Ev SSH_AUTH_SOCK
1.233 jmc 1275: Identifies the path of a
1276: .Ux Ns -domain
1277: socket used to communicate with the agent.
1.166 stevesk 1278: .It Ev SSH_CONNECTION
1279: Identifies the client and server ends of the connection.
1.40 aaron 1280: The variable contains
1.233 jmc 1281: four space-separated values: client IP address, client port number,
1282: server IP address, and server port number.
1.73 markus 1283: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1284: This variable contains the original command line if a forced command
1.73 markus 1285: is executed.
1286: It can be used to extract the original arguments.
1.2 deraadt 1287: .It Ev SSH_TTY
1.1 deraadt 1288: This is set to the name of the tty (path to the device) associated
1.40 aaron 1289: with the current shell or command.
1290: If the current session has no tty,
1.1 deraadt 1291: this variable is not set.
1.2 deraadt 1292: .It Ev TZ
1.214 jmc 1293: This variable is set to indicate the present time zone if it
1.257 jmc 1294: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1295: on to new connections).
1.2 deraadt 1296: .It Ev USER
1.1 deraadt 1297: Set to the name of the user logging in.
1.2 deraadt 1298: .El
1299: .Pp
1.44 aaron 1300: Additionally,
1.2 deraadt 1301: .Nm
1.44 aaron 1302: reads
1.207 djm 1303: .Pa ~/.ssh/environment ,
1.2 deraadt 1304: and adds lines of the format
1305: .Dq VARNAME=value
1.233 jmc 1306: to the environment if the file exists and users are allowed to
1.161 marc 1307: change their environment.
1.176 jmc 1308: For more information, see the
1.161 marc 1309: .Cm PermitUserEnvironment
1.162 stevesk 1310: option in
1.161 marc 1311: .Xr sshd_config 5 .
1.2 deraadt 1312: .Sh FILES
1.236 jmc 1313: .Bl -tag -width Ds -compact
1.309 jmc 1314: .It Pa ~/.rhosts
1.240 jmc 1315: This file is used for host-based authentication (see above).
1.92 markus 1316: On some machines this file may need to be
1.240 jmc 1317: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1318: because
1.2 deraadt 1319: .Xr sshd 8
1.40 aaron 1320: reads it as root.
1321: Additionally, this file must be owned by the user,
1322: and must not have write permissions for anyone else.
1323: The recommended
1.1 deraadt 1324: permission for most machines is read/write for the user, and not
1325: accessible by others.
1.2 deraadt 1326: .Pp
1.309 jmc 1327: .It Pa ~/.shosts
1.240 jmc 1328: This file is used in exactly the same way as
1329: .Pa .rhosts ,
1330: but allows host-based authentication without permitting login with
1331: rlogin/rsh.
1.272 mcbride 1332: .Pp
1.309 jmc 1333: .It Pa ~/.ssh/
1.272 mcbride 1334: This directory is the default location for all user-specific configuration
1335: and authentication information.
1336: There is no general requirement to keep the entire contents of this directory
1337: secret, but the recommended permissions are read/write/execute for the user,
1338: and not accessible by others.
1.236 jmc 1339: .Pp
1.309 jmc 1340: .It Pa ~/.ssh/authorized_keys
1.343 ! naddy 1341: Lists the public keys (DSA, ECDSA, ED25519, RSA)
! 1342: that can be used for logging in as this user.
1.238 jmc 1343: The format of this file is described in the
1344: .Xr sshd 8
1345: manual page.
1346: This file is not highly sensitive, but the recommended
1347: permissions are read/write for the user, and not accessible by others.
1348: .Pp
1.309 jmc 1349: .It Pa ~/.ssh/config
1.238 jmc 1350: This is the per-user configuration file.
1351: The file format and configuration options are described in
1352: .Xr ssh_config 5 .
1353: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1354: read/write for the user, and not writable by others.
1.238 jmc 1355: .Pp
1.309 jmc 1356: .It Pa ~/.ssh/environment
1.239 jmc 1357: Contains additional definitions for environment variables; see
1358: .Sx ENVIRONMENT ,
1.238 jmc 1359: above.
1360: .Pp
1.309 jmc 1361: .It Pa ~/.ssh/identity
1362: .It Pa ~/.ssh/id_dsa
1.310 djm 1363: .It Pa ~/.ssh/id_ecdsa
1.343 ! naddy 1364: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1365: .It Pa ~/.ssh/id_rsa
1.238 jmc 1366: Contains the private key for authentication.
1367: These files
1368: contain sensitive data and should be readable by the user but not
1369: accessible by others (read/write/execute).
1370: .Nm
1371: will simply ignore a private key file if it is accessible by others.
1372: It is possible to specify a passphrase when
1373: generating the key which will be used to encrypt the
1374: sensitive part of this file using 3DES.
1375: .Pp
1.309 jmc 1376: .It Pa ~/.ssh/identity.pub
1377: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1378: .It Pa ~/.ssh/id_ecdsa.pub
1.343 ! naddy 1379: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1380: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1381: Contains the public key for authentication.
1382: These files are not
1383: sensitive and can (but need not) be readable by anyone.
1384: .Pp
1.309 jmc 1385: .It Pa ~/.ssh/known_hosts
1.244 jmc 1386: Contains a list of host keys for all hosts the user has logged into
1387: that are not already in the systemwide list of known host keys.
1.238 jmc 1388: See
1.244 jmc 1389: .Xr sshd 8
1390: for further details of the format of this file.
1.238 jmc 1391: .Pp
1.309 jmc 1392: .It Pa ~/.ssh/rc
1.238 jmc 1393: Commands in this file are executed by
1394: .Nm
1.245 jmc 1395: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1396: started.
1397: See the
1398: .Xr sshd 8
1399: manual page for more information.
1400: .Pp
1.309 jmc 1401: .It Pa /etc/hosts.equiv
1.240 jmc 1402: This file is for host-based authentication (see above).
1403: It should only be writable by root.
1.236 jmc 1404: .Pp
1.309 jmc 1405: .It Pa /etc/shosts.equiv
1.240 jmc 1406: This file is used in exactly the same way as
1407: .Pa hosts.equiv ,
1408: but allows host-based authentication without permitting login with
1409: rlogin/rsh.
1.236 jmc 1410: .Pp
1.238 jmc 1411: .It Pa /etc/ssh/ssh_config
1412: Systemwide configuration file.
1413: The file format and configuration options are described in
1414: .Xr ssh_config 5 .
1415: .Pp
1.309 jmc 1416: .It Pa /etc/ssh/ssh_host_key
1417: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1418: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 ! naddy 1419: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1420: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1421: These files contain the private parts of the host keys
1.245 jmc 1422: and are used for host-based authentication.
1423: If protocol version 1 is used,
1.238 jmc 1424: .Nm
1425: must be setuid root, since the host key is readable only by root.
1426: For protocol version 2,
1427: .Nm
1428: uses
1429: .Xr ssh-keysign 8
1.245 jmc 1430: to access the host keys,
1431: eliminating the requirement that
1.238 jmc 1432: .Nm
1.245 jmc 1433: be setuid root when host-based authentication is used.
1.238 jmc 1434: By default
1.2 deraadt 1435: .Nm
1.238 jmc 1436: is not setuid root.
1437: .Pp
1.309 jmc 1438: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1439: Systemwide list of known host keys.
1440: This file should be prepared by the
1441: system administrator to contain the public host keys of all machines in the
1442: organization.
1.244 jmc 1443: It should be world-readable.
1444: See
1.238 jmc 1445: .Xr sshd 8
1.244 jmc 1446: for further details of the format of this file.
1.236 jmc 1447: .Pp
1.309 jmc 1448: .It Pa /etc/ssh/sshrc
1.1 deraadt 1449: Commands in this file are executed by
1.2 deraadt 1450: .Nm
1.245 jmc 1451: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1452: See the
1.2 deraadt 1453: .Xr sshd 8
1.1 deraadt 1454: manual page for more information.
1.58 itojun 1455: .El
1.312 jmc 1456: .Sh EXIT STATUS
1457: .Nm
1458: exits with the exit status of the remote command or with 255
1459: if an error occurred.
1.2 deraadt 1460: .Sh SEE ALSO
1461: .Xr scp 1 ,
1.83 djm 1462: .Xr sftp 1 ,
1.2 deraadt 1463: .Xr ssh-add 1 ,
1464: .Xr ssh-agent 1 ,
1465: .Xr ssh-keygen 1 ,
1.242 jmc 1466: .Xr ssh-keyscan 1 ,
1.250 jmc 1467: .Xr tun 4 ,
1.176 jmc 1468: .Xr hosts.equiv 5 ,
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.
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