Annotation of src/usr.bin/ssh/ssh.1, Revision 1.366
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: .\"
1.366 ! jcs 36: .\" $OpenBSD: ssh.1,v 1.365 2015/11/06 00:31:41 mmcc Exp $
! 37: .Dd $Mdocdate: November 6 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
1.360 millert 55: .Op Fl L Ar address
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
1.362 djm 61: .Op Fl Q Ar query_option
1.360 millert 62: .Op Fl R Ar address
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.
1.365 mmcc 73: It is intended to provide secure encrypted communications between
1.40 aaron 74: two untrusted hosts over an insecure network.
1.348 millert 75: X11 connections, arbitrary TCP ports and
76: .Ux Ns -domain
77: sockets 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
1.365 mmcc 87: (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:
1.361 millert 95: .Pp
1.360 millert 96: .Bl -tag -width Ds -compact
1.218 jmc 97: .It Fl 1
98: Forces
1.2 deraadt 99: .Nm
1.218 jmc 100: to try protocol version 1 only.
1.360 millert 101: .Pp
1.218 jmc 102: .It Fl 2
103: Forces
1.2 deraadt 104: .Nm
1.218 jmc 105: to try protocol version 2 only.
1.360 millert 106: .Pp
1.218 jmc 107: .It Fl 4
108: Forces
1.2 deraadt 109: .Nm
1.218 jmc 110: to use IPv4 addresses only.
1.360 millert 111: .Pp
1.218 jmc 112: .It Fl 6
113: Forces
1.2 deraadt 114: .Nm
1.218 jmc 115: to use IPv6 addresses only.
1.360 millert 116: .Pp
1.218 jmc 117: .It Fl A
118: Enables forwarding of the authentication agent connection.
119: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 120: .Pp
1.218 jmc 121: Agent forwarding should be enabled with caution.
122: Users with the ability to bypass file permissions on the remote host
1.285 sobrado 123: (for the agent's
1.286 sobrado 124: .Ux Ns -domain
125: socket) can access the local agent through the forwarded connection.
1.218 jmc 126: An attacker cannot obtain key material from the agent,
127: however they can perform operations on the keys that enable them to
128: authenticate using the identities loaded into the agent.
1.360 millert 129: .Pp
1.218 jmc 130: .It Fl a
131: Disables forwarding of the authentication agent connection.
1.360 millert 132: .Pp
1.218 jmc 133: .It Fl b Ar bind_address
134: Use
135: .Ar bind_address
136: on the local machine as the source address
137: of the connection.
138: Only useful on systems with more than one address.
1.360 millert 139: .Pp
1.218 jmc 140: .It Fl C
141: Requests compression of all data (including stdin, stdout, stderr, and
1.348 millert 142: data for forwarded X11, TCP and
143: .Ux Ns -domain
144: connections).
1.218 jmc 145: The compression algorithm is the same used by
146: .Xr gzip 1 ,
147: and the
148: .Dq level
149: can be controlled by the
150: .Cm CompressionLevel
151: option for protocol version 1.
152: Compression is desirable on modem lines and other
153: slow connections, but will only slow down things on fast networks.
154: The default value can be set on a host-by-host basis in the
155: configuration files; see the
156: .Cm Compression
157: option.
1.360 millert 158: .Pp
1.218 jmc 159: .It Fl c Ar cipher_spec
160: Selects the cipher specification for encrypting the session.
1.2 deraadt 161: .Pp
1.218 jmc 162: Protocol version 1 allows specification of a single cipher.
163: The supported values are
164: .Dq 3des ,
1.220 jmc 165: .Dq blowfish ,
1.218 jmc 166: and
167: .Dq des .
1.230 jmc 168: For protocol version 2,
1.218 jmc 169: .Ar cipher_spec
170: is a comma-separated list of ciphers
171: listed in order of preference.
1.283 jmc 172: See the
173: .Cm Ciphers
1.307 dtucker 174: keyword in
175: .Xr ssh_config 5
176: for more information.
1.360 millert 177: .Pp
1.218 jmc 178: .It Fl D Xo
179: .Sm off
180: .Oo Ar bind_address : Oc
181: .Ar port
182: .Sm on
183: .Xc
184: Specifies a local
185: .Dq dynamic
186: application-level port forwarding.
187: This works by allocating a socket to listen to
188: .Ar port
189: on the local side, optionally bound to the specified
190: .Ar bind_address .
191: Whenever a connection is made to this port, the
192: connection is forwarded over the secure channel, and the application
193: protocol is then used to determine where to connect to from the
194: remote machine.
195: Currently the SOCKS4 and SOCKS5 protocols are supported, and
1.107 markus 196: .Nm
1.218 jmc 197: will act as a SOCKS server.
198: Only root can forward privileged ports.
199: Dynamic port forwardings can also be specified in the configuration file.
1.49 markus 200: .Pp
1.308 djm 201: IPv6 addresses can be specified by enclosing the address in square brackets.
1.218 jmc 202: Only the superuser can forward privileged ports.
203: By default, the local port is bound in accordance with the
204: .Cm GatewayPorts
205: setting.
206: However, an explicit
207: .Ar bind_address
208: may be used to bind the connection to a specific address.
209: The
210: .Ar bind_address
211: of
212: .Dq localhost
213: indicates that the listening port be bound for local use only, while an
214: empty address or
215: .Sq *
216: indicates that the port should be available from all interfaces.
1.360 millert 217: .Pp
1.331 dtucker 218: .It Fl E Ar log_file
219: Append debug logs to
220: .Ar log_file
221: instead of standard error.
1.360 millert 222: .Pp
1.229 jmc 223: .It Fl e Ar escape_char
1.218 jmc 224: Sets the escape character for sessions with a pty (default:
225: .Ql ~ ) .
226: The escape character is only recognized at the beginning of a line.
227: The escape character followed by a dot
228: .Pq Ql \&.
229: closes the connection;
230: followed by control-Z suspends the connection;
231: and followed by itself sends the escape character once.
232: Setting the character to
1.2 deraadt 233: .Dq none
1.218 jmc 234: disables any escapes and makes the session fully transparent.
1.360 millert 235: .Pp
1.218 jmc 236: .It Fl F Ar configfile
237: Specifies an alternative per-user configuration file.
238: If a configuration file is given on the command line,
239: the system-wide configuration file
240: .Pq Pa /etc/ssh/ssh_config
241: will be ignored.
242: The default for the per-user configuration file is
243: .Pa ~/.ssh/config .
1.360 millert 244: .Pp
1.218 jmc 245: .It Fl f
246: Requests
247: .Nm
248: to go to background just before command execution.
249: This is useful if
1.176 jmc 250: .Nm
1.218 jmc 251: is going to ask for passwords or passphrases, but the user
252: wants it in the background.
253: This implies
254: .Fl n .
255: The recommended way to start X11 programs at a remote site is with
256: something like
257: .Ic ssh -f host xterm .
1.277 djm 258: .Pp
259: If the
260: .Cm ExitOnForwardFailure
261: configuration option is set to
262: .Dq yes ,
263: then a client started with
264: .Fl f
265: will wait for all remote port forwards to be successfully established
266: before placing itself in the background.
1.360 millert 267: .Pp
1.350 djm 268: .It Fl G
269: Causes
270: .Nm
271: to print its configuration after evaluating
272: .Cm Host
273: and
274: .Cm Match
275: blocks and exit.
1.360 millert 276: .Pp
1.218 jmc 277: .It Fl g
278: Allows remote hosts to connect to local forwarded ports.
1.346 djm 279: If used on a multiplexed connection, then this option must be specified
280: on the master process.
1.360 millert 281: .Pp
1.291 markus 282: .It Fl I Ar pkcs11
1.294 jmc 283: Specify the PKCS#11 shared library
1.176 jmc 284: .Nm
1.293 markus 285: should use to communicate with a PKCS#11 token providing the user's
1.218 jmc 286: private RSA key.
1.360 millert 287: .Pp
1.218 jmc 288: .It Fl i Ar identity_file
289: Selects a file from which the identity (private key) for
1.310 djm 290: public key authentication is read.
1.218 jmc 291: The default is
292: .Pa ~/.ssh/identity
293: for protocol version 1, and
1.310 djm 294: .Pa ~/.ssh/id_dsa ,
1.343 naddy 295: .Pa ~/.ssh/id_ecdsa ,
296: .Pa ~/.ssh/id_ed25519
1.310 djm 297: and
1.218 jmc 298: .Pa ~/.ssh/id_rsa
299: for protocol version 2.
300: Identity files may also be specified on
301: a per-host basis in the configuration file.
302: It is possible to have multiple
303: .Fl i
304: options (and multiple identities specified in
305: configuration files).
1.364 jmc 306: If no certificates have been explicitly specified by the
1.363 djm 307: .Cm CertificateFile
308: directive,
1.302 djm 309: .Nm
310: will also try to load certificate information from the filename obtained
311: by appending
312: .Pa -cert.pub
313: to identity filenames.
1.360 millert 314: .Pp
1.269 djm 315: .It Fl K
316: Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
317: credentials to the server.
1.360 millert 318: .Pp
1.218 jmc 319: .It Fl k
320: Disables forwarding (delegation) of GSSAPI credentials to the server.
1.360 millert 321: .Pp
1.218 jmc 322: .It Fl L Xo
323: .Sm off
324: .Oo Ar bind_address : Oc
325: .Ar port : host : hostport
326: .Sm on
327: .Xc
1.360 millert 328: .It Fl L Xo
329: .Sm off
330: .Oo Ar bind_address : Oc
331: .Ar port : remote_socket
332: .Sm on
333: .Xc
334: .It Fl L Xo
335: .Sm off
336: .Ar local_socket : host : hostport
337: .Sm on
338: .Xc
339: .It Fl L Xo
340: .Sm off
341: .Ar local_socket : remote_socket
342: .Sm on
343: .Xc
344: Specifies that connections to the given TCP port or Unix socket on the local
345: (client) host are to be forwarded to the given host and port, or Unix socket,
346: on the remote side.
347: This works by allocating a socket to listen to either a TCP
1.218 jmc 348: .Ar port
349: on the local side, optionally bound to the specified
1.360 millert 350: .Ar bind_address ,
351: or to a Unix socket.
352: Whenever a connection is made to the local port or socket, the
1.218 jmc 353: connection is forwarded over the secure channel, and a connection is
1.360 millert 354: made to either
1.218 jmc 355: .Ar host
356: port
1.360 millert 357: .Ar hostport ,
358: or the Unix socket
359: .Ar remote_socket ,
1.218 jmc 360: from the remote machine.
1.360 millert 361: .Pp
1.218 jmc 362: Port forwardings can also be specified in the configuration file.
1.360 millert 363: Only the superuser can forward privileged ports.
1.308 djm 364: IPv6 addresses can be specified by enclosing the address in square brackets.
1.360 millert 365: .Pp
1.218 jmc 366: By default, the local port is bound in accordance with the
367: .Cm GatewayPorts
368: setting.
369: However, an explicit
370: .Ar bind_address
371: may be used to bind the connection to a specific address.
1.2 deraadt 372: The
1.218 jmc 373: .Ar bind_address
374: of
375: .Dq localhost
376: indicates that the listening port be bound for local use only, while an
377: empty address or
378: .Sq *
379: indicates that the port should be available from all interfaces.
1.360 millert 380: .Pp
1.218 jmc 381: .It Fl l Ar login_name
382: Specifies the user to log in as on the remote machine.
383: This also may be specified on a per-host basis in the configuration file.
1.360 millert 384: .Pp
1.218 jmc 385: .It Fl M
386: Places the
387: .Nm
388: client into
389: .Dq master
390: mode for connection sharing.
1.231 stevesk 391: Multiple
392: .Fl M
393: options places
394: .Nm
395: into
396: .Dq master
397: mode with confirmation required before slave connections are accepted.
1.218 jmc 398: Refer to the description of
399: .Cm ControlMaster
400: in
401: .Xr ssh_config 5
402: for details.
1.360 millert 403: .Pp
1.218 jmc 404: .It Fl m Ar mac_spec
405: Additionally, for protocol version 2 a comma-separated list of MAC
406: (message authentication code) algorithms can
407: be specified in order of preference.
408: See the
409: .Cm MACs
410: keyword for more information.
1.360 millert 411: .Pp
1.218 jmc 412: .It Fl N
413: Do not execute a remote command.
414: This is useful for just forwarding ports
415: (protocol version 2 only).
1.360 millert 416: .Pp
1.218 jmc 417: .It Fl n
418: Redirects stdin from
419: .Pa /dev/null
420: (actually, prevents reading from stdin).
421: This must be used when
1.2 deraadt 422: .Nm
1.218 jmc 423: is run in the background.
424: A common trick is to use this to run X11 programs on a remote machine.
425: For example,
426: .Ic ssh -n shadows.cs.hut.fi emacs &
427: will start an emacs on shadows.cs.hut.fi, and the X11
428: connection will be automatically forwarded over an encrypted channel.
429: The
1.2 deraadt 430: .Nm
1.218 jmc 431: program will be put in the background.
432: (This does not work if
1.2 deraadt 433: .Nm
1.218 jmc 434: needs to ask for a password or passphrase; see also the
435: .Fl f
436: option.)
1.360 millert 437: .Pp
1.218 jmc 438: .It Fl O Ar ctl_cmd
439: Control an active connection multiplexing master process.
440: When the
441: .Fl O
442: option is specified, the
443: .Ar ctl_cmd
444: argument is interpreted and passed to the master process.
445: Valid commands are:
446: .Dq check
1.305 markus 447: (check that the master process is running),
448: .Dq forward
1.318 jmc 449: (request forwardings without command execution),
1.323 okan 450: .Dq cancel
451: (cancel forwardings),
1.218 jmc 452: .Dq exit
1.318 jmc 453: (request the master to exit), and
1.317 djm 454: .Dq stop
455: (request the master to stop accepting further multiplexing requests).
1.360 millert 456: .Pp
1.218 jmc 457: .It Fl o Ar option
458: Can be used to give options in the format used in the configuration file.
459: This is useful for specifying options for which there is no separate
460: command-line flag.
461: For full details of the options listed below, and their possible values, see
462: .Xr ssh_config 5 .
1.2 deraadt 463: .Pp
1.218 jmc 464: .Bl -tag -width Ds -offset indent -compact
1.366 ! jcs 465: .It AddKeysToAgent
1.218 jmc 466: .It AddressFamily
467: .It BatchMode
468: .It BindAddress
1.338 djm 469: .It CanonicalDomains
1.339 djm 470: .It CanonicalizeFallbackLocal
471: .It CanonicalizeHostname
472: .It CanonicalizeMaxDots
473: .It CanonicalizePermittedCNAMEs
1.363 djm 474: .It CertificateFile
1.218 jmc 475: .It ChallengeResponseAuthentication
476: .It CheckHostIP
477: .It Cipher
478: .It Ciphers
479: .It ClearAllForwardings
480: .It Compression
481: .It CompressionLevel
482: .It ConnectionAttempts
483: .It ConnectTimeout
484: .It ControlMaster
485: .It ControlPath
1.321 djm 486: .It ControlPersist
1.218 jmc 487: .It DynamicForward
488: .It EscapeChar
1.263 markus 489: .It ExitOnForwardFailure
1.353 jmc 490: .It FingerprintHash
1.218 jmc 491: .It ForwardAgent
492: .It ForwardX11
1.321 djm 493: .It ForwardX11Timeout
1.218 jmc 494: .It ForwardX11Trusted
495: .It GatewayPorts
496: .It GlobalKnownHostsFile
497: .It GSSAPIAuthentication
498: .It GSSAPIDelegateCredentials
499: .It HashKnownHosts
500: .It Host
501: .It HostbasedAuthentication
1.355 djm 502: .It HostbasedKeyTypes
1.218 jmc 503: .It HostKeyAlgorithms
504: .It HostKeyAlias
505: .It HostName
506: .It IdentityFile
507: .It IdentitiesOnly
1.316 jmc 508: .It IPQoS
1.321 djm 509: .It KbdInteractiveAuthentication
1.218 jmc 510: .It KbdInteractiveDevices
1.314 jmc 511: .It KexAlgorithms
1.218 jmc 512: .It LocalCommand
513: .It LocalForward
514: .It LogLevel
515: .It MACs
1.337 jmc 516: .It Match
1.218 jmc 517: .It NoHostAuthenticationForLocalhost
518: .It NumberOfPasswordPrompts
519: .It PasswordAuthentication
520: .It PermitLocalCommand
1.291 markus 521: .It PKCS11Provider
1.218 jmc 522: .It Port
523: .It PreferredAuthentications
524: .It Protocol
525: .It ProxyCommand
1.336 jmc 526: .It ProxyUseFdpass
1.359 markus 527: .It PubkeyAcceptedKeyTypes
1.218 jmc 528: .It PubkeyAuthentication
1.251 dtucker 529: .It RekeyLimit
1.218 jmc 530: .It RemoteForward
1.319 jmc 531: .It RequestTTY
1.218 jmc 532: .It RhostsRSAAuthentication
533: .It RSAAuthentication
534: .It SendEnv
535: .It ServerAliveInterval
536: .It ServerAliveCountMax
1.347 jmc 537: .It StreamLocalBindMask
538: .It StreamLocalBindUnlink
1.218 jmc 539: .It StrictHostKeyChecking
540: .It TCPKeepAlive
541: .It Tunnel
542: .It TunnelDevice
1.354 djm 543: .It UpdateHostKeys
1.218 jmc 544: .It UsePrivilegedPort
545: .It User
546: .It UserKnownHostsFile
547: .It VerifyHostKeyDNS
1.276 jmc 548: .It VisualHostKey
1.218 jmc 549: .It XAuthLocation
550: .El
1.360 millert 551: .Pp
1.218 jmc 552: .It Fl p Ar port
553: Port to connect to on the remote host.
554: This can be specified on a
555: per-host basis in the configuration file.
1.360 millert 556: .Pp
1.362 djm 557: .It Fl Q Ar query_option
1.332 djm 558: Queries
559: .Nm
1.341 deraadt 560: for the algorithms supported for the specified version 2.
561: The available features are:
562: .Ar cipher
1.332 djm 563: (supported symmetric ciphers),
1.341 deraadt 564: .Ar cipher-auth
1.340 djm 565: (supported symmetric ciphers that support authenticated encryption),
1.341 deraadt 566: .Ar mac
1.332 djm 567: (supported message integrity codes),
1.341 deraadt 568: .Ar kex
1.332 djm 569: (key exchange algorithms),
1.341 deraadt 570: .Ar key
1.362 djm 571: (key types),
572: .Ar key-cert
573: (certificate key types),
574: .Ar key-plain
575: (non-certificate key types), and
1.356 djm 576: .Ar protocol-version
577: (supported SSH protocol versions).
1.360 millert 578: .Pp
1.218 jmc 579: .It Fl q
580: Quiet mode.
1.271 djm 581: Causes most warning and diagnostic messages to be suppressed.
1.360 millert 582: .Pp
1.218 jmc 583: .It Fl R Xo
584: .Sm off
585: .Oo Ar bind_address : Oc
586: .Ar port : host : hostport
587: .Sm on
588: .Xc
1.360 millert 589: .It Fl R Xo
590: .Sm off
591: .Oo Ar bind_address : Oc
592: .Ar port : local_socket
593: .Sm on
594: .Xc
595: .It Fl R Xo
596: .Sm off
597: .Ar remote_socket : host : hostport
598: .Sm on
599: .Xc
600: .It Fl R Xo
601: .Sm off
602: .Ar remote_socket : local_socket
603: .Sm on
604: .Xc
605: Specifies that connections to the given TCP port or Unix socket on the remote
606: (server) host are to be forwarded to the given host and port, or Unix socket,
607: on the local side.
608: This works by allocating a socket to listen to either a TCP
1.218 jmc 609: .Ar port
1.360 millert 610: or to a Unix socket on the remote side.
611: Whenever a connection is made to this port or Unix socket, the
612: connection is forwarded over the secure channel, and a connection
613: is made to either
1.218 jmc 614: .Ar host
615: port
1.360 millert 616: .Ar hostport ,
617: or
618: .Ar local_socket ,
1.218 jmc 619: from the local machine.
1.2 deraadt 620: .Pp
1.218 jmc 621: Port forwardings can also be specified in the configuration file.
622: Privileged ports can be forwarded only when
623: logging in as root on the remote machine.
1.324 jmc 624: IPv6 addresses can be specified by enclosing the address in square brackets.
1.194 jakob 625: .Pp
1.360 millert 626: By default, TCP listening sockets on the server will be bound to the loopback
1.218 jmc 627: interface only.
1.280 tobias 628: This may be overridden by specifying a
1.218 jmc 629: .Ar bind_address .
630: An empty
631: .Ar bind_address ,
632: or the address
633: .Ql * ,
634: indicates that the remote socket should listen on all interfaces.
635: Specifying a remote
636: .Ar bind_address
637: will only succeed if the server's
638: .Cm GatewayPorts
639: option is enabled (see
640: .Xr sshd_config 5 ) .
1.281 djm 641: .Pp
642: If the
643: .Ar port
644: argument is
1.282 djm 645: .Ql 0 ,
1.281 djm 646: the listen port will be dynamically allocated on the server and reported
647: to the client at run time.
1.305 markus 648: When used together with
649: .Ic -O forward
650: the allocated port will be printed to the standard output.
1.360 millert 651: .Pp
1.218 jmc 652: .It Fl S Ar ctl_path
1.304 jmc 653: Specifies the location of a control socket for connection sharing,
1.303 djm 654: or the string
655: .Dq none
656: to disable connection sharing.
1.218 jmc 657: Refer to the description of
658: .Cm ControlPath
659: and
660: .Cm ControlMaster
661: in
662: .Xr ssh_config 5
663: for details.
1.360 millert 664: .Pp
1.218 jmc 665: .It Fl s
666: May be used to request invocation of a subsystem on the remote system.
667: Subsystems are a feature of the SSH2 protocol which facilitate the use
668: of SSH as a secure transport for other applications (eg.\&
669: .Xr sftp 1 ) .
670: The subsystem is specified as the remote command.
1.360 millert 671: .Pp
1.218 jmc 672: .It Fl T
1.357 dtucker 673: Disable pseudo-terminal allocation.
1.360 millert 674: .Pp
1.218 jmc 675: .It Fl t
1.357 dtucker 676: Force pseudo-terminal allocation.
1.218 jmc 677: This can be used to execute arbitrary
678: screen-based programs on a remote machine, which can be very useful,
1.257 jmc 679: e.g. when implementing menu services.
1.218 jmc 680: Multiple
681: .Fl t
682: options force tty allocation, even if
1.194 jakob 683: .Nm
1.218 jmc 684: has no local tty.
1.360 millert 685: .Pp
1.218 jmc 686: .It Fl V
687: Display the version number and exit.
1.360 millert 688: .Pp
1.218 jmc 689: .It Fl v
690: Verbose mode.
691: Causes
1.176 jmc 692: .Nm
1.218 jmc 693: to print debugging messages about its progress.
694: This is helpful in
695: debugging connection, authentication, and configuration problems.
696: Multiple
697: .Fl v
698: options increase the verbosity.
699: The maximum is 3.
1.360 millert 700: .Pp
1.290 dtucker 701: .It Fl W Ar host : Ns Ar port
702: Requests that standard input and output on the client be forwarded to
703: .Ar host
704: on
705: .Ar port
706: over the secure channel.
707: Implies
708: .Fl N ,
709: .Fl T ,
710: .Cm ExitOnForwardFailure
711: and
1.326 dtucker 712: .Cm ClearAllForwardings .
713: Works with Protocol version 2 only.
1.360 millert 714: .Pp
1.261 stevesk 715: .It Fl w Xo
716: .Ar local_tun Ns Op : Ns Ar remote_tun
717: .Xc
718: Requests
719: tunnel
720: device forwarding with the specified
1.218 jmc 721: .Xr tun 4
1.261 stevesk 722: devices between the client
723: .Pq Ar local_tun
724: and the server
725: .Pq Ar remote_tun .
726: .Pp
1.228 jmc 727: The devices may be specified by numerical ID or the keyword
728: .Dq any ,
729: which uses the next available tunnel device.
1.261 stevesk 730: If
731: .Ar remote_tun
732: is not specified, it defaults to
733: .Dq any .
1.228 jmc 734: See also the
1.218 jmc 735: .Cm Tunnel
1.261 stevesk 736: and
737: .Cm TunnelDevice
738: directives in
1.218 jmc 739: .Xr ssh_config 5 .
1.261 stevesk 740: If the
741: .Cm Tunnel
742: directive is unset, it is set to the default tunnel mode, which is
743: .Dq point-to-point .
1.360 millert 744: .Pp
1.218 jmc 745: .It Fl X
746: Enables X11 forwarding.
1.54 markus 747: This can also be specified on a per-host basis in a configuration file.
1.165 stevesk 748: .Pp
1.218 jmc 749: X11 forwarding should be enabled with caution.
1.168 jmc 750: Users with the ability to bypass file permissions on the remote host
1.218 jmc 751: (for the user's X authorization database)
752: can access the local X11 display through the forwarded connection.
753: An attacker may then be able to perform activities such as keystroke monitoring.
754: .Pp
755: For this reason, X11 forwarding is subjected to X11 SECURITY extension
756: restrictions by default.
757: Please refer to the
758: .Nm
759: .Fl Y
760: option and the
761: .Cm ForwardX11Trusted
762: directive in
763: .Xr ssh_config 5
764: for more information.
1.360 millert 765: .Pp
1.218 jmc 766: .It Fl x
767: Disables X11 forwarding.
1.360 millert 768: .Pp
1.218 jmc 769: .It Fl Y
770: Enables trusted X11 forwarding.
771: Trusted X11 forwardings are not subjected to the X11 SECURITY extension
772: controls.
1.360 millert 773: .Pp
1.278 djm 774: .It Fl y
775: Send log information using the
776: .Xr syslog 3
777: system module.
778: By default this information is sent to stderr.
1.218 jmc 779: .El
1.224 jmc 780: .Pp
781: .Nm
782: may additionally obtain configuration data from
783: a per-user configuration file and a system-wide configuration file.
784: The file format and configuration options are described in
785: .Xr ssh_config 5 .
1.222 jmc 786: .Sh AUTHENTICATION
1.249 jmc 787: The OpenSSH SSH client supports SSH protocols 1 and 2.
1.284 jmc 788: The default is to use protocol 2 only,
789: though this can be changed via the
1.222 jmc 790: .Cm Protocol
791: option in
1.284 jmc 792: .Xr ssh_config 5
793: or the
1.222 jmc 794: .Fl 1
795: and
796: .Fl 2
797: options (see above).
798: Both protocols support similar authentication methods,
1.284 jmc 799: but protocol 2 is the default since
1.222 jmc 800: it provides additional mechanisms for confidentiality
801: (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
1.320 djm 802: and integrity (hmac-md5, hmac-sha1,
803: hmac-sha2-256, hmac-sha2-512,
1.330 markus 804: umac-64, umac-128, hmac-ripemd160).
1.222 jmc 805: Protocol 1 lacks a strong mechanism for ensuring the
806: integrity of the connection.
807: .Pp
808: The methods available for authentication are:
1.260 jmc 809: GSSAPI-based authentication,
1.222 jmc 810: host-based authentication,
811: public key authentication,
812: challenge-response authentication,
813: and password authentication.
814: Authentication methods are tried in the order specified above,
815: though protocol 2 has a configuration option to change the default order:
816: .Cm PreferredAuthentications .
817: .Pp
818: Host-based authentication works as follows:
1.218 jmc 819: If the machine the user logs in from is listed in
820: .Pa /etc/hosts.equiv
821: or
822: .Pa /etc/shosts.equiv
823: on the remote machine, and the user names are
824: the same on both sides, or if the files
825: .Pa ~/.rhosts
826: or
827: .Pa ~/.shosts
828: exist in the user's home directory on the
829: remote machine and contain a line containing the name of the client
830: machine and the name of the user on that machine, the user is
1.222 jmc 831: considered for login.
832: Additionally, the server
833: .Em must
834: be able to verify the client's
835: host key (see the description of
1.218 jmc 836: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 837: and
1.222 jmc 838: .Pa ~/.ssh/known_hosts ,
839: below)
840: for login to be permitted.
1.218 jmc 841: This authentication method closes security holes due to IP
1.222 jmc 842: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 843: [Note to the administrator:
844: .Pa /etc/hosts.equiv ,
845: .Pa ~/.rhosts ,
846: and the rlogin/rsh protocol in general, are inherently insecure and should be
847: disabled if security is desired.]
1.189 dtucker 848: .Pp
1.222 jmc 849: Public key authentication works as follows:
850: The scheme is based on public-key cryptography,
851: using cryptosystems
852: where encryption and decryption are done using separate keys,
853: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 854: The idea is that each user creates a public/private
855: key pair for authentication purposes.
856: The server knows the public key, and only the user knows the private key.
1.222 jmc 857: .Nm
858: implements public key authentication protocol automatically,
1.349 sobrado 859: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.222 jmc 860: Protocol 1 is restricted to using only RSA keys,
1.310 djm 861: but protocol 2 may use any.
1.333 jmc 862: The HISTORY section of
1.222 jmc 863: .Xr ssl 8
1.311 jmc 864: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 865: .Pp
1.218 jmc 866: The file
867: .Pa ~/.ssh/authorized_keys
868: lists the public keys that are permitted for logging in.
869: When the user logs in, the
1.2 deraadt 870: .Nm
1.218 jmc 871: program tells the server which key pair it would like to use for
872: authentication.
1.222 jmc 873: The client proves that it has access to the private key
874: and the server checks that the corresponding public key
875: is authorized to accept the account.
1.218 jmc 876: .Pp
1.222 jmc 877: The user creates his/her key pair by running
1.218 jmc 878: .Xr ssh-keygen 1 .
879: This stores the private key in
1.207 djm 880: .Pa ~/.ssh/identity
1.222 jmc 881: (protocol 1),
882: .Pa ~/.ssh/id_dsa
883: (protocol 2 DSA),
1.310 djm 884: .Pa ~/.ssh/id_ecdsa
885: (protocol 2 ECDSA),
1.343 naddy 886: .Pa ~/.ssh/id_ed25519
1.349 sobrado 887: (protocol 2 Ed25519),
1.222 jmc 888: or
889: .Pa ~/.ssh/id_rsa
890: (protocol 2 RSA)
1.218 jmc 891: and stores the public key in
892: .Pa ~/.ssh/identity.pub
1.222 jmc 893: (protocol 1),
894: .Pa ~/.ssh/id_dsa.pub
895: (protocol 2 DSA),
1.310 djm 896: .Pa ~/.ssh/id_ecdsa.pub
897: (protocol 2 ECDSA),
1.343 naddy 898: .Pa ~/.ssh/id_ed25519.pub
1.349 sobrado 899: (protocol 2 Ed25519),
1.222 jmc 900: or
901: .Pa ~/.ssh/id_rsa.pub
902: (protocol 2 RSA)
1.218 jmc 903: in the user's home directory.
1.222 jmc 904: The user should then copy the public key
1.218 jmc 905: to
906: .Pa ~/.ssh/authorized_keys
1.222 jmc 907: in his/her home directory on the remote machine.
908: The
1.218 jmc 909: .Pa authorized_keys
910: file corresponds to the conventional
911: .Pa ~/.rhosts
912: file, and has one key
1.222 jmc 913: per line, though the lines can be very long.
1.218 jmc 914: After this, the user can log in without giving the password.
915: .Pp
1.301 jmc 916: A variation on public key authentication
917: is available in the form of certificate authentication:
918: instead of a set of public/private keys,
919: signed certificates are used.
920: This has the advantage that a single trusted certification authority
921: can be used in place of many public/private keys.
1.333 jmc 922: See the CERTIFICATES section of
1.301 jmc 923: .Xr ssh-keygen 1
924: for more information.
925: .Pp
926: The most convenient way to use public key or certificate authentication
927: may be with an authentication agent.
1.218 jmc 928: See
929: .Xr ssh-agent 1
1.366 ! jcs 930: and (optionally) the
! 931: .Cm AddKeysToAgent
! 932: directive in
! 933: .Xr ssh_config 5
1.218 jmc 934: for more information.
935: .Pp
1.222 jmc 936: Challenge-response authentication works as follows:
937: The server sends an arbitrary
938: .Qq challenge
939: text, and prompts for a response.
940: Protocol 2 allows multiple challenges and responses;
941: protocol 1 is restricted to just one challenge/response.
942: Examples of challenge-response authentication include
1.335 jmc 943: .Bx
944: Authentication (see
1.222 jmc 945: .Xr login.conf 5 )
1.335 jmc 946: and PAM (some
947: .Pf non- Ox
948: systems).
1.222 jmc 949: .Pp
950: Finally, if other authentication methods fail,
1.218 jmc 951: .Nm
952: prompts the user for a password.
953: The password is sent to the remote
954: host for checking; however, since all communications are encrypted,
955: the password cannot be seen by someone listening on the network.
1.232 jmc 956: .Pp
957: .Nm
958: automatically maintains and checks a database containing
959: identification for all hosts it has ever been used with.
960: Host keys are stored in
961: .Pa ~/.ssh/known_hosts
962: in the user's home directory.
963: Additionally, the file
964: .Pa /etc/ssh/ssh_known_hosts
965: is automatically checked for known hosts.
966: Any new hosts are automatically added to the user's file.
967: If a host's identification ever changes,
968: .Nm
969: warns about this and disables password authentication to prevent
970: server spoofing or man-in-the-middle attacks,
971: which could otherwise be used to circumvent the encryption.
972: The
973: .Cm StrictHostKeyChecking
974: option can be used to control logins to machines whose
975: host key is not known or has changed.
976: .Pp
1.218 jmc 977: When the user's identity has been accepted by the server, the server
1.357 dtucker 978: either executes the given command in a non-interactive session or,
979: if no command has been specified, logs into the machine and gives
980: the user a normal shell as an interactive session.
1.218 jmc 981: All communication with
982: the remote command or shell will be automatically encrypted.
983: .Pp
1.357 dtucker 984: If an interactive session is requested
985: .Nm
986: by default will only request a pseudo-terminal (pty) for interactive
987: sessions when the client has one.
988: The flags
989: .Fl T
990: and
991: .Fl t
992: can be used to override this behaviour.
993: .Pp
994: If a pseudo-terminal has been allocated the
1.218 jmc 995: user may use the escape characters noted below.
996: .Pp
1.357 dtucker 997: If no pseudo-terminal has been allocated,
1.218 jmc 998: the session is transparent and can be used to reliably transfer binary data.
999: On most systems, setting the escape character to
1000: .Dq none
1001: will also make the session transparent even if a tty is used.
1002: .Pp
1003: The session terminates when the command or shell on the remote
1.247 jmc 1004: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 1005: .Sh ESCAPE CHARACTERS
1.218 jmc 1006: When a pseudo-terminal has been requested,
1.2 deraadt 1007: .Nm
1.218 jmc 1008: supports a number of functions through the use of an escape character.
1009: .Pp
1010: A single tilde character can be sent as
1011: .Ic ~~
1012: or by following the tilde by a character other than those described below.
1013: The escape character must always follow a newline to be interpreted as
1014: special.
1015: The escape character can be changed in configuration files using the
1016: .Cm EscapeChar
1017: configuration directive or on the command line by the
1018: .Fl e
1019: option.
1020: .Pp
1021: The supported escapes (assuming the default
1022: .Ql ~ )
1023: are:
1024: .Bl -tag -width Ds
1025: .It Cm ~.
1026: Disconnect.
1027: .It Cm ~^Z
1028: Background
1.234 jmc 1029: .Nm .
1.218 jmc 1030: .It Cm ~#
1031: List forwarded connections.
1032: .It Cm ~&
1033: Background
1.2 deraadt 1034: .Nm
1.218 jmc 1035: at logout when waiting for forwarded connection / X11 sessions to terminate.
1036: .It Cm ~?
1037: Display a list of escape characters.
1038: .It Cm ~B
1039: Send a BREAK to the remote system
1040: (only useful for SSH protocol version 2 and if the peer supports it).
1041: .It Cm ~C
1042: Open command line.
1043: Currently this allows the addition of port forwardings using the
1.279 stevesk 1044: .Fl L ,
1045: .Fl R
1.218 jmc 1046: and
1.279 stevesk 1047: .Fl D
1.225 jmc 1048: options (see above).
1.322 markus 1049: It also allows the cancellation of existing port-forwardings
1050: with
1.262 stevesk 1051: .Sm off
1.322 markus 1052: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1053: .Sm on
1.322 markus 1054: for local,
1055: .Sm off
1056: .Fl KR Oo Ar bind_address : Oc Ar port
1057: .Sm on
1058: for remote and
1059: .Sm off
1060: .Fl KD Oo Ar bind_address : Oc Ar port
1061: .Sm on
1062: for dynamic port-forwardings.
1.218 jmc 1063: .Ic !\& Ns Ar command
1064: allows the user to execute a local command if the
1065: .Ic PermitLocalCommand
1066: option is enabled in
1.176 jmc 1067: .Xr ssh_config 5 .
1.218 jmc 1068: Basic help is available, using the
1069: .Fl h
1070: option.
1071: .It Cm ~R
1072: Request rekeying of the connection
1073: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 1074: .It Cm ~V
1075: Decrease the verbosity
1076: .Pq Ic LogLevel
1077: when errors are being written to stderr.
1078: .It Cm ~v
1.328 jmc 1079: Increase the verbosity
1.327 dtucker 1080: .Pq Ic LogLevel
1081: when errors are being written to stderr.
1.176 jmc 1082: .El
1.246 jmc 1083: .Sh TCP FORWARDING
1084: Forwarding of arbitrary TCP connections over the secure channel can
1085: be specified either on the command line or in a configuration file.
1086: One possible application of TCP forwarding is a secure connection to a
1087: mail server; another is going through firewalls.
1088: .Pp
1089: In the example below, we look at encrypting communication between
1090: an IRC client and server, even though the IRC server does not directly
1091: support encrypted communications.
1092: This works as follows:
1093: the user connects to the remote host using
1094: .Nm ,
1095: specifying a port to be used to forward connections
1096: to the remote server.
1097: After that it is possible to start the service which is to be encrypted
1098: on the client machine,
1099: connecting to the same local port,
1100: and
1101: .Nm
1102: will encrypt and forward the connection.
1103: .Pp
1104: The following example tunnels an IRC session from client machine
1105: .Dq 127.0.0.1
1106: (localhost)
1107: to remote server
1108: .Dq server.example.com :
1109: .Bd -literal -offset 4n
1110: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1111: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1112: .Ed
1113: .Pp
1114: This tunnels a connection to IRC server
1115: .Dq server.example.com ,
1116: joining channel
1117: .Dq #users ,
1118: nickname
1119: .Dq pinky ,
1120: using port 1234.
1121: It doesn't matter which port is used,
1122: as long as it's greater than 1023
1123: (remember, only root can open sockets on privileged ports)
1124: and doesn't conflict with any ports already in use.
1125: The connection is forwarded to port 6667 on the remote server,
1126: since that's the standard port for IRC services.
1127: .Pp
1128: The
1129: .Fl f
1130: option backgrounds
1131: .Nm
1132: and the remote command
1133: .Dq sleep 10
1134: is specified to allow an amount of time
1135: (10 seconds, in the example)
1136: to start the service which is to be tunnelled.
1137: If no connections are made within the time specified,
1138: .Nm
1139: will exit.
1140: .Sh X11 FORWARDING
1.218 jmc 1141: If the
1142: .Cm ForwardX11
1143: variable is set to
1144: .Dq yes
1145: (or see the description of the
1.227 jmc 1146: .Fl X ,
1147: .Fl x ,
1.218 jmc 1148: and
1.227 jmc 1149: .Fl Y
1.226 jmc 1150: options above)
1.218 jmc 1151: and the user is using X11 (the
1152: .Ev DISPLAY
1153: environment variable is set), the connection to the X11 display is
1154: automatically forwarded to the remote side in such a way that any X11
1155: programs started from the shell (or command) will go through the
1156: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1157: from the local machine.
1.218 jmc 1158: The user should not manually set
1159: .Ev DISPLAY .
1160: Forwarding of X11 connections can be
1161: configured on the command line or in configuration files.
1162: .Pp
1163: The
1164: .Ev DISPLAY
1165: value set by
1166: .Nm
1167: will point to the server machine, but with a display number greater than zero.
1168: This is normal, and happens because
1169: .Nm
1170: creates a
1171: .Dq proxy
1172: X server on the server machine for forwarding the
1173: connections over the encrypted channel.
1.200 djm 1174: .Pp
1.218 jmc 1175: .Nm
1176: will also automatically set up Xauthority data on the server machine.
1177: For this purpose, it will generate a random authorization cookie,
1178: store it in Xauthority on the server, and verify that any forwarded
1179: connections carry this cookie and replace it by the real cookie when
1180: the connection is opened.
1181: The real authentication cookie is never
1182: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1183: .Pp
1.218 jmc 1184: If the
1185: .Cm ForwardAgent
1186: variable is set to
1187: .Dq yes
1188: (or see the description of the
1189: .Fl A
1.191 djm 1190: and
1.218 jmc 1191: .Fl a
1.226 jmc 1192: options above) and
1.218 jmc 1193: the user is using an authentication agent, the connection to the agent
1194: is automatically forwarded to the remote side.
1.252 jmc 1195: .Sh VERIFYING HOST KEYS
1196: When connecting to a server for the first time,
1197: a fingerprint of the server's public key is presented to the user
1198: (unless the option
1199: .Cm StrictHostKeyChecking
1200: has been disabled).
1201: Fingerprints can be determined using
1202: .Xr ssh-keygen 1 :
1203: .Pp
1204: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1205: .Pp
1.274 grunk 1206: If the fingerprint is already known, it can be matched
1207: and the key can be accepted or rejected.
1.358 djm 1208: If only legacy (MD5) fingerprints for the server are available, the
1209: .Xr ssh-keygen 1
1210: .Fl E
1211: option may be used to downgrade the fingerprint algorithm to match.
1212: .Pp
1.274 grunk 1213: Because of the difficulty of comparing host keys
1.352 djm 1214: just by looking at fingerprint strings,
1.274 grunk 1215: there is also support to compare host keys visually,
1216: using
1217: .Em random art .
1218: By setting the
1.275 grunk 1219: .Cm VisualHostKey
1.274 grunk 1220: option to
1.275 grunk 1221: .Dq yes ,
1.274 grunk 1222: a small ASCII graphic gets displayed on every login to a server, no matter
1223: if the session itself is interactive or not.
1224: By learning the pattern a known server produces, a user can easily
1225: find out that the host key has changed when a completely different pattern
1226: is displayed.
1227: Because these patterns are not unambiguous however, a pattern that looks
1228: similar to the pattern remembered only gives a good probability that the
1229: host key is the same, not guaranteed proof.
1230: .Pp
1231: To get a listing of the fingerprints along with their random art for
1232: all known hosts, the following command line can be used:
1233: .Pp
1234: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1235: .Pp
1.252 jmc 1236: If the fingerprint is unknown,
1237: an alternative method of verification is available:
1238: SSH fingerprints verified by DNS.
1239: An additional resource record (RR),
1240: SSHFP,
1241: is added to a zonefile
1242: and the connecting client is able to match the fingerprint
1243: with that of the key presented.
1244: .Pp
1245: In this example, we are connecting a client to a server,
1246: .Dq host.example.com .
1247: The SSHFP resource records should first be added to the zonefile for
1248: host.example.com:
1249: .Bd -literal -offset indent
1.259 jakob 1250: $ ssh-keygen -r host.example.com.
1.252 jmc 1251: .Ed
1252: .Pp
1253: The output lines will have to be added to the zonefile.
1254: To check that the zone is answering fingerprint queries:
1255: .Pp
1256: .Dl $ dig -t SSHFP host.example.com
1257: .Pp
1258: Finally the client connects:
1259: .Bd -literal -offset indent
1260: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1261: [...]
1262: Matching host key fingerprint found in DNS.
1263: Are you sure you want to continue connecting (yes/no)?
1264: .Ed
1265: .Pp
1266: See the
1267: .Cm VerifyHostKeyDNS
1268: option in
1269: .Xr ssh_config 5
1270: for more information.
1.250 jmc 1271: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1272: .Nm
1273: contains support for Virtual Private Network (VPN) tunnelling
1274: using the
1275: .Xr tun 4
1276: network pseudo-device,
1277: allowing two networks to be joined securely.
1278: The
1279: .Xr sshd_config 5
1280: configuration option
1281: .Cm PermitTunnel
1282: controls whether the server supports this,
1283: and at what level (layer 2 or 3 traffic).
1284: .Pp
1285: The following example would connect client network 10.0.50.0/24
1.265 otto 1286: with remote network 10.0.99.0/24 using a point-to-point connection
1287: from 10.1.1.1 to 10.1.1.2,
1288: provided that the SSH server running on the gateway to the remote network,
1289: at 192.168.1.15, allows it.
1290: .Pp
1291: On the client:
1.250 jmc 1292: .Bd -literal -offset indent
1293: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1294: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1295: # route add 10.0.99.0/24 10.1.1.2
1296: .Ed
1297: .Pp
1298: On the server:
1299: .Bd -literal -offset indent
1300: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1301: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1302: .Ed
1303: .Pp
1304: Client access may be more finely tuned via the
1305: .Pa /root/.ssh/authorized_keys
1306: file (see below) and the
1307: .Cm PermitRootLogin
1308: server option.
1.255 jmc 1309: The following entry would permit connections on
1.250 jmc 1310: .Xr tun 4
1.255 jmc 1311: device 1 from user
1.250 jmc 1312: .Dq jane
1.255 jmc 1313: and on tun device 2 from user
1.250 jmc 1314: .Dq john ,
1315: if
1316: .Cm PermitRootLogin
1317: is set to
1318: .Dq forced-commands-only :
1319: .Bd -literal -offset 2n
1320: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1321: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1322: .Ed
1323: .Pp
1.264 ray 1324: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1325: it may be more suited to temporary setups,
1326: such as for wireless VPNs.
1327: More permanent VPNs are better provided by tools such as
1328: .Xr ipsecctl 8
1329: and
1330: .Xr isakmpd 8 .
1.2 deraadt 1331: .Sh ENVIRONMENT
1332: .Nm
1.1 deraadt 1333: will normally set the following environment variables:
1.237 jmc 1334: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1335: .It Ev DISPLAY
1336: The
1337: .Ev DISPLAY
1.40 aaron 1338: variable indicates the location of the X11 server.
1.44 aaron 1339: It is automatically set by
1.2 deraadt 1340: .Nm
1341: to point to a value of the form
1.233 jmc 1342: .Dq hostname:n ,
1343: where
1344: .Dq hostname
1345: indicates the host where the shell runs, and
1346: .Sq n
1347: is an integer \*(Ge 1.
1.40 aaron 1348: .Nm
1349: uses this special value to forward X11 connections over the secure
1350: channel.
1.107 markus 1351: The user should normally not set
1352: .Ev DISPLAY
1353: explicitly, as that
1.1 deraadt 1354: will render the X11 connection insecure (and will require the user to
1355: manually copy any required authorization cookies).
1.2 deraadt 1356: .It Ev HOME
1.1 deraadt 1357: Set to the path of the user's home directory.
1.2 deraadt 1358: .It Ev LOGNAME
1359: Synonym for
1.12 aaron 1360: .Ev USER ;
1361: set for compatibility with systems that use this variable.
1.2 deraadt 1362: .It Ev MAIL
1.129 stevesk 1363: Set to the path of the user's mailbox.
1.40 aaron 1364: .It Ev PATH
1.2 deraadt 1365: Set to the default
1366: .Ev PATH ,
1367: as specified when compiling
1.234 jmc 1368: .Nm .
1.118 markus 1369: .It Ev SSH_ASKPASS
1370: If
1371: .Nm
1372: needs a passphrase, it will read the passphrase from the current
1373: terminal if it was run from a terminal.
1374: If
1375: .Nm
1376: does not have a terminal associated with it but
1377: .Ev DISPLAY
1378: and
1379: .Ev SSH_ASKPASS
1380: are set, it will execute the program specified by
1381: .Ev SSH_ASKPASS
1382: and open an X11 window to read the passphrase.
1383: This is particularly useful when calling
1384: .Nm
1385: from a
1.196 jmc 1386: .Pa .xsession
1.118 markus 1387: or related script.
1388: (Note that on some machines it
1389: may be necessary to redirect the input from
1390: .Pa /dev/null
1391: to make this work.)
1.18 markus 1392: .It Ev SSH_AUTH_SOCK
1.233 jmc 1393: Identifies the path of a
1394: .Ux Ns -domain
1395: socket used to communicate with the agent.
1.166 stevesk 1396: .It Ev SSH_CONNECTION
1397: Identifies the client and server ends of the connection.
1.40 aaron 1398: The variable contains
1.233 jmc 1399: four space-separated values: client IP address, client port number,
1400: server IP address, and server port number.
1.73 markus 1401: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1402: This variable contains the original command line if a forced command
1.73 markus 1403: is executed.
1404: It can be used to extract the original arguments.
1.2 deraadt 1405: .It Ev SSH_TTY
1.1 deraadt 1406: This is set to the name of the tty (path to the device) associated
1.40 aaron 1407: with the current shell or command.
1408: If the current session has no tty,
1.1 deraadt 1409: this variable is not set.
1.2 deraadt 1410: .It Ev TZ
1.214 jmc 1411: This variable is set to indicate the present time zone if it
1.257 jmc 1412: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1413: on to new connections).
1.2 deraadt 1414: .It Ev USER
1.1 deraadt 1415: Set to the name of the user logging in.
1.2 deraadt 1416: .El
1417: .Pp
1.44 aaron 1418: Additionally,
1.2 deraadt 1419: .Nm
1.44 aaron 1420: reads
1.207 djm 1421: .Pa ~/.ssh/environment ,
1.2 deraadt 1422: and adds lines of the format
1423: .Dq VARNAME=value
1.233 jmc 1424: to the environment if the file exists and users are allowed to
1.161 marc 1425: change their environment.
1.176 jmc 1426: For more information, see the
1.161 marc 1427: .Cm PermitUserEnvironment
1.162 stevesk 1428: option in
1.161 marc 1429: .Xr sshd_config 5 .
1.2 deraadt 1430: .Sh FILES
1.236 jmc 1431: .Bl -tag -width Ds -compact
1.309 jmc 1432: .It Pa ~/.rhosts
1.240 jmc 1433: This file is used for host-based authentication (see above).
1.92 markus 1434: On some machines this file may need to be
1.240 jmc 1435: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1436: because
1.2 deraadt 1437: .Xr sshd 8
1.40 aaron 1438: reads it as root.
1439: Additionally, this file must be owned by the user,
1440: and must not have write permissions for anyone else.
1441: The recommended
1.1 deraadt 1442: permission for most machines is read/write for the user, and not
1443: accessible by others.
1.2 deraadt 1444: .Pp
1.309 jmc 1445: .It Pa ~/.shosts
1.240 jmc 1446: This file is used in exactly the same way as
1447: .Pa .rhosts ,
1448: but allows host-based authentication without permitting login with
1449: rlogin/rsh.
1.272 mcbride 1450: .Pp
1.309 jmc 1451: .It Pa ~/.ssh/
1.272 mcbride 1452: This directory is the default location for all user-specific configuration
1453: and authentication information.
1454: There is no general requirement to keep the entire contents of this directory
1455: secret, but the recommended permissions are read/write/execute for the user,
1456: and not accessible by others.
1.236 jmc 1457: .Pp
1.309 jmc 1458: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1459: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1460: that can be used for logging in as this user.
1.238 jmc 1461: The format of this file is described in the
1462: .Xr sshd 8
1463: manual page.
1464: This file is not highly sensitive, but the recommended
1465: permissions are read/write for the user, and not accessible by others.
1466: .Pp
1.309 jmc 1467: .It Pa ~/.ssh/config
1.238 jmc 1468: This is the per-user configuration file.
1469: The file format and configuration options are described in
1470: .Xr ssh_config 5 .
1471: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1472: read/write for the user, and not writable by others.
1.238 jmc 1473: .Pp
1.309 jmc 1474: .It Pa ~/.ssh/environment
1.239 jmc 1475: Contains additional definitions for environment variables; see
1476: .Sx ENVIRONMENT ,
1.238 jmc 1477: above.
1478: .Pp
1.309 jmc 1479: .It Pa ~/.ssh/identity
1480: .It Pa ~/.ssh/id_dsa
1.310 djm 1481: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1482: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1483: .It Pa ~/.ssh/id_rsa
1.238 jmc 1484: Contains the private key for authentication.
1485: These files
1486: contain sensitive data and should be readable by the user but not
1487: accessible by others (read/write/execute).
1488: .Nm
1489: will simply ignore a private key file if it is accessible by others.
1490: It is possible to specify a passphrase when
1491: generating the key which will be used to encrypt the
1492: sensitive part of this file using 3DES.
1493: .Pp
1.309 jmc 1494: .It Pa ~/.ssh/identity.pub
1495: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1496: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1497: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1498: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1499: Contains the public key for authentication.
1500: These files are not
1501: sensitive and can (but need not) be readable by anyone.
1502: .Pp
1.309 jmc 1503: .It Pa ~/.ssh/known_hosts
1.244 jmc 1504: Contains a list of host keys for all hosts the user has logged into
1505: that are not already in the systemwide list of known host keys.
1.238 jmc 1506: See
1.244 jmc 1507: .Xr sshd 8
1508: for further details of the format of this file.
1.238 jmc 1509: .Pp
1.309 jmc 1510: .It Pa ~/.ssh/rc
1.238 jmc 1511: Commands in this file are executed by
1512: .Nm
1.245 jmc 1513: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1514: started.
1515: See the
1516: .Xr sshd 8
1517: manual page for more information.
1518: .Pp
1.309 jmc 1519: .It Pa /etc/hosts.equiv
1.240 jmc 1520: This file is for host-based authentication (see above).
1521: It should only be writable by root.
1.236 jmc 1522: .Pp
1.309 jmc 1523: .It Pa /etc/shosts.equiv
1.240 jmc 1524: This file is used in exactly the same way as
1525: .Pa hosts.equiv ,
1526: but allows host-based authentication without permitting login with
1527: rlogin/rsh.
1.236 jmc 1528: .Pp
1.238 jmc 1529: .It Pa /etc/ssh/ssh_config
1530: Systemwide configuration file.
1531: The file format and configuration options are described in
1532: .Xr ssh_config 5 .
1533: .Pp
1.309 jmc 1534: .It Pa /etc/ssh/ssh_host_key
1535: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1536: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1537: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1538: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1539: These files contain the private parts of the host keys
1.245 jmc 1540: and are used for host-based authentication.
1541: If protocol version 1 is used,
1.238 jmc 1542: .Nm
1543: must be setuid root, since the host key is readable only by root.
1544: For protocol version 2,
1545: .Nm
1546: uses
1547: .Xr ssh-keysign 8
1.245 jmc 1548: to access the host keys,
1549: eliminating the requirement that
1.238 jmc 1550: .Nm
1.245 jmc 1551: be setuid root when host-based authentication is used.
1.238 jmc 1552: By default
1.2 deraadt 1553: .Nm
1.238 jmc 1554: is not setuid root.
1555: .Pp
1.309 jmc 1556: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1557: Systemwide list of known host keys.
1558: This file should be prepared by the
1559: system administrator to contain the public host keys of all machines in the
1560: organization.
1.244 jmc 1561: It should be world-readable.
1562: See
1.238 jmc 1563: .Xr sshd 8
1.244 jmc 1564: for further details of the format of this file.
1.236 jmc 1565: .Pp
1.309 jmc 1566: .It Pa /etc/ssh/sshrc
1.1 deraadt 1567: Commands in this file are executed by
1.2 deraadt 1568: .Nm
1.245 jmc 1569: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1570: See the
1.2 deraadt 1571: .Xr sshd 8
1.1 deraadt 1572: manual page for more information.
1.58 itojun 1573: .El
1.312 jmc 1574: .Sh EXIT STATUS
1575: .Nm
1576: exits with the exit status of the remote command or with 255
1577: if an error occurred.
1.2 deraadt 1578: .Sh SEE ALSO
1579: .Xr scp 1 ,
1.83 djm 1580: .Xr sftp 1 ,
1.2 deraadt 1581: .Xr ssh-add 1 ,
1582: .Xr ssh-agent 1 ,
1583: .Xr ssh-keygen 1 ,
1.242 jmc 1584: .Xr ssh-keyscan 1 ,
1.250 jmc 1585: .Xr tun 4 ,
1.159 stevesk 1586: .Xr ssh_config 5 ,
1.160 naddy 1587: .Xr ssh-keysign 8 ,
1.87 itojun 1588: .Xr sshd 8
1.329 jmc 1589: .Sh STANDARDS
1.106 markus 1590: .Rs
1.329 jmc 1591: .%A S. Lehtinen
1592: .%A C. Lonvick
1593: .%D January 2006
1.256 jmc 1594: .%R RFC 4250
1.329 jmc 1595: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1596: .Re
1.329 jmc 1597: .Pp
1.256 jmc 1598: .Rs
1.329 jmc 1599: .%A T. Ylonen
1600: .%A C. Lonvick
1601: .%D January 2006
1.256 jmc 1602: .%R RFC 4251
1.329 jmc 1603: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1604: .Re
1.329 jmc 1605: .Pp
1.256 jmc 1606: .Rs
1.329 jmc 1607: .%A T. Ylonen
1608: .%A C. Lonvick
1609: .%D January 2006
1.256 jmc 1610: .%R RFC 4252
1.329 jmc 1611: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1612: .Re
1.329 jmc 1613: .Pp
1.256 jmc 1614: .Rs
1.329 jmc 1615: .%A T. Ylonen
1616: .%A C. Lonvick
1617: .%D January 2006
1.256 jmc 1618: .%R RFC 4253
1.329 jmc 1619: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1620: .Re
1.329 jmc 1621: .Pp
1.256 jmc 1622: .Rs
1.329 jmc 1623: .%A T. Ylonen
1624: .%A C. Lonvick
1625: .%D January 2006
1.256 jmc 1626: .%R RFC 4254
1.329 jmc 1627: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1628: .Re
1.329 jmc 1629: .Pp
1.256 jmc 1630: .Rs
1.329 jmc 1631: .%A J. Schlyter
1632: .%A W. Griffin
1633: .%D January 2006
1.256 jmc 1634: .%R RFC 4255
1.329 jmc 1635: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1636: .Re
1.329 jmc 1637: .Pp
1.256 jmc 1638: .Rs
1.329 jmc 1639: .%A F. Cusack
1640: .%A M. Forssen
1641: .%D January 2006
1.256 jmc 1642: .%R RFC 4256
1.329 jmc 1643: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1644: .Re
1.329 jmc 1645: .Pp
1.256 jmc 1646: .Rs
1.329 jmc 1647: .%A J. Galbraith
1648: .%A P. Remaker
1649: .%D January 2006
1.256 jmc 1650: .%R RFC 4335
1.329 jmc 1651: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1652: .Re
1.329 jmc 1653: .Pp
1.256 jmc 1654: .Rs
1.329 jmc 1655: .%A M. Bellare
1656: .%A T. Kohno
1657: .%A C. Namprempre
1658: .%D January 2006
1.256 jmc 1659: .%R RFC 4344
1.329 jmc 1660: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1661: .Re
1.329 jmc 1662: .Pp
1.256 jmc 1663: .Rs
1.329 jmc 1664: .%A B. Harris
1665: .%D January 2006
1.256 jmc 1666: .%R RFC 4345
1.329 jmc 1667: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1668: .Re
1.329 jmc 1669: .Pp
1.258 djm 1670: .Rs
1.329 jmc 1671: .%A M. Friedl
1672: .%A N. Provos
1673: .%A W. Simpson
1674: .%D March 2006
1.258 djm 1675: .%R RFC 4419
1.329 jmc 1676: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1677: .Re
1.329 jmc 1678: .Pp
1.266 markus 1679: .Rs
1.329 jmc 1680: .%A J. Galbraith
1681: .%A R. Thayer
1682: .%D November 2006
1.266 markus 1683: .%R RFC 4716
1.329 jmc 1684: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1685: .Re
1.329 jmc 1686: .Pp
1.313 djm 1687: .Rs
1.329 jmc 1688: .%A D. Stebila
1689: .%A J. Green
1690: .%D December 2009
1.313 djm 1691: .%R RFC 5656
1.329 jmc 1692: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1693: .Re
1.329 jmc 1694: .Pp
1.274 grunk 1695: .Rs
1696: .%A A. Perrig
1697: .%A D. Song
1698: .%D 1999
1.329 jmc 1699: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1700: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1701: .Re
1.173 jmc 1702: .Sh AUTHORS
1703: OpenSSH is a derivative of the original and free
1704: ssh 1.2.12 release by Tatu Ylonen.
1705: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1706: Theo de Raadt and Dug Song
1707: removed many bugs, re-added newer features and
1708: created OpenSSH.
1709: Markus Friedl contributed the support for SSH
1710: protocol versions 1.5 and 2.0.
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