Annotation of src/usr.bin/ssh/ssh.1, Revision 1.368
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: .\"
1.368 ! jmc 36: .\" $OpenBSD: ssh.1,v 1.367 2016/02/16 05:11:04 djm Exp $
! 37: .Dd $Mdocdate: February 16 2016 $
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).
1.367 djm 798: Protocol 1 should not be used - it suffers from a number of cryptographic
799: weaknesses and is only offered to support legacy devices.
1.222 jmc 800: .Pp
801: The methods available for authentication are:
1.260 jmc 802: GSSAPI-based authentication,
1.222 jmc 803: host-based authentication,
804: public key authentication,
805: challenge-response authentication,
806: and password authentication.
807: Authentication methods are tried in the order specified above,
808: though protocol 2 has a configuration option to change the default order:
809: .Cm PreferredAuthentications .
810: .Pp
811: Host-based authentication works as follows:
1.218 jmc 812: If the machine the user logs in from is listed in
813: .Pa /etc/hosts.equiv
814: or
815: .Pa /etc/shosts.equiv
816: on the remote machine, and the user names are
817: the same on both sides, or if the files
818: .Pa ~/.rhosts
819: or
820: .Pa ~/.shosts
821: exist in the user's home directory on the
822: remote machine and contain a line containing the name of the client
823: machine and the name of the user on that machine, the user is
1.222 jmc 824: considered for login.
825: Additionally, the server
826: .Em must
827: be able to verify the client's
828: host key (see the description of
1.218 jmc 829: .Pa /etc/ssh/ssh_known_hosts
1.189 dtucker 830: and
1.222 jmc 831: .Pa ~/.ssh/known_hosts ,
832: below)
833: for login to be permitted.
1.218 jmc 834: This authentication method closes security holes due to IP
1.222 jmc 835: spoofing, DNS spoofing, and routing spoofing.
1.218 jmc 836: [Note to the administrator:
837: .Pa /etc/hosts.equiv ,
838: .Pa ~/.rhosts ,
839: and the rlogin/rsh protocol in general, are inherently insecure and should be
840: disabled if security is desired.]
1.189 dtucker 841: .Pp
1.222 jmc 842: Public key authentication works as follows:
843: The scheme is based on public-key cryptography,
844: using cryptosystems
845: where encryption and decryption are done using separate keys,
846: and it is unfeasible to derive the decryption key from the encryption key.
1.218 jmc 847: The idea is that each user creates a public/private
848: key pair for authentication purposes.
849: The server knows the public key, and only the user knows the private key.
1.222 jmc 850: .Nm
851: implements public key authentication protocol automatically,
1.349 sobrado 852: using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
1.222 jmc 853: Protocol 1 is restricted to using only RSA keys,
1.310 djm 854: but protocol 2 may use any.
1.333 jmc 855: The HISTORY section of
1.222 jmc 856: .Xr ssl 8
1.311 jmc 857: contains a brief discussion of the DSA and RSA algorithms.
1.210 djm 858: .Pp
1.218 jmc 859: The file
860: .Pa ~/.ssh/authorized_keys
861: lists the public keys that are permitted for logging in.
862: When the user logs in, the
1.2 deraadt 863: .Nm
1.218 jmc 864: program tells the server which key pair it would like to use for
865: authentication.
1.222 jmc 866: The client proves that it has access to the private key
867: and the server checks that the corresponding public key
868: is authorized to accept the account.
1.218 jmc 869: .Pp
1.222 jmc 870: The user creates his/her key pair by running
1.218 jmc 871: .Xr ssh-keygen 1 .
872: This stores the private key in
1.207 djm 873: .Pa ~/.ssh/identity
1.222 jmc 874: (protocol 1),
875: .Pa ~/.ssh/id_dsa
876: (protocol 2 DSA),
1.310 djm 877: .Pa ~/.ssh/id_ecdsa
878: (protocol 2 ECDSA),
1.343 naddy 879: .Pa ~/.ssh/id_ed25519
1.349 sobrado 880: (protocol 2 Ed25519),
1.222 jmc 881: or
882: .Pa ~/.ssh/id_rsa
883: (protocol 2 RSA)
1.218 jmc 884: and stores the public key in
885: .Pa ~/.ssh/identity.pub
1.222 jmc 886: (protocol 1),
887: .Pa ~/.ssh/id_dsa.pub
888: (protocol 2 DSA),
1.310 djm 889: .Pa ~/.ssh/id_ecdsa.pub
890: (protocol 2 ECDSA),
1.343 naddy 891: .Pa ~/.ssh/id_ed25519.pub
1.349 sobrado 892: (protocol 2 Ed25519),
1.222 jmc 893: or
894: .Pa ~/.ssh/id_rsa.pub
895: (protocol 2 RSA)
1.218 jmc 896: in the user's home directory.
1.222 jmc 897: The user should then copy the public key
1.218 jmc 898: to
899: .Pa ~/.ssh/authorized_keys
1.222 jmc 900: in his/her home directory on the remote machine.
901: The
1.218 jmc 902: .Pa authorized_keys
903: file corresponds to the conventional
904: .Pa ~/.rhosts
905: file, and has one key
1.222 jmc 906: per line, though the lines can be very long.
1.218 jmc 907: After this, the user can log in without giving the password.
908: .Pp
1.301 jmc 909: A variation on public key authentication
910: is available in the form of certificate authentication:
911: instead of a set of public/private keys,
912: signed certificates are used.
913: This has the advantage that a single trusted certification authority
914: can be used in place of many public/private keys.
1.333 jmc 915: See the CERTIFICATES section of
1.301 jmc 916: .Xr ssh-keygen 1
917: for more information.
918: .Pp
919: The most convenient way to use public key or certificate authentication
920: may be with an authentication agent.
1.218 jmc 921: See
922: .Xr ssh-agent 1
1.366 jcs 923: and (optionally) the
924: .Cm AddKeysToAgent
925: directive in
926: .Xr ssh_config 5
1.218 jmc 927: for more information.
928: .Pp
1.222 jmc 929: Challenge-response authentication works as follows:
930: The server sends an arbitrary
931: .Qq challenge
932: text, and prompts for a response.
933: Protocol 2 allows multiple challenges and responses;
934: protocol 1 is restricted to just one challenge/response.
935: Examples of challenge-response authentication include
1.335 jmc 936: .Bx
937: Authentication (see
1.222 jmc 938: .Xr login.conf 5 )
1.335 jmc 939: and PAM (some
940: .Pf non- Ox
941: systems).
1.222 jmc 942: .Pp
943: Finally, if other authentication methods fail,
1.218 jmc 944: .Nm
945: prompts the user for a password.
946: The password is sent to the remote
947: host for checking; however, since all communications are encrypted,
948: the password cannot be seen by someone listening on the network.
1.232 jmc 949: .Pp
950: .Nm
951: automatically maintains and checks a database containing
952: identification for all hosts it has ever been used with.
953: Host keys are stored in
954: .Pa ~/.ssh/known_hosts
955: in the user's home directory.
956: Additionally, the file
957: .Pa /etc/ssh/ssh_known_hosts
958: is automatically checked for known hosts.
959: Any new hosts are automatically added to the user's file.
960: If a host's identification ever changes,
961: .Nm
962: warns about this and disables password authentication to prevent
963: server spoofing or man-in-the-middle attacks,
964: which could otherwise be used to circumvent the encryption.
965: The
966: .Cm StrictHostKeyChecking
967: option can be used to control logins to machines whose
968: host key is not known or has changed.
969: .Pp
1.218 jmc 970: When the user's identity has been accepted by the server, the server
1.357 dtucker 971: either executes the given command in a non-interactive session or,
972: if no command has been specified, logs into the machine and gives
973: the user a normal shell as an interactive session.
1.218 jmc 974: All communication with
975: the remote command or shell will be automatically encrypted.
976: .Pp
1.357 dtucker 977: If an interactive session is requested
978: .Nm
979: by default will only request a pseudo-terminal (pty) for interactive
980: sessions when the client has one.
981: The flags
982: .Fl T
983: and
984: .Fl t
985: can be used to override this behaviour.
986: .Pp
987: If a pseudo-terminal has been allocated the
1.218 jmc 988: user may use the escape characters noted below.
989: .Pp
1.357 dtucker 990: If no pseudo-terminal has been allocated,
1.218 jmc 991: the session is transparent and can be used to reliably transfer binary data.
992: On most systems, setting the escape character to
993: .Dq none
994: will also make the session transparent even if a tty is used.
995: .Pp
996: The session terminates when the command or shell on the remote
1.247 jmc 997: machine exits and all X11 and TCP connections have been closed.
1.223 jmc 998: .Sh ESCAPE CHARACTERS
1.218 jmc 999: When a pseudo-terminal has been requested,
1.2 deraadt 1000: .Nm
1.218 jmc 1001: supports a number of functions through the use of an escape character.
1002: .Pp
1003: A single tilde character can be sent as
1004: .Ic ~~
1005: or by following the tilde by a character other than those described below.
1006: The escape character must always follow a newline to be interpreted as
1007: special.
1008: The escape character can be changed in configuration files using the
1009: .Cm EscapeChar
1010: configuration directive or on the command line by the
1011: .Fl e
1012: option.
1013: .Pp
1014: The supported escapes (assuming the default
1015: .Ql ~ )
1016: are:
1017: .Bl -tag -width Ds
1018: .It Cm ~.
1019: Disconnect.
1020: .It Cm ~^Z
1021: Background
1.234 jmc 1022: .Nm .
1.218 jmc 1023: .It Cm ~#
1024: List forwarded connections.
1025: .It Cm ~&
1026: Background
1.2 deraadt 1027: .Nm
1.218 jmc 1028: at logout when waiting for forwarded connection / X11 sessions to terminate.
1029: .It Cm ~?
1030: Display a list of escape characters.
1031: .It Cm ~B
1032: Send a BREAK to the remote system
1033: (only useful for SSH protocol version 2 and if the peer supports it).
1034: .It Cm ~C
1035: Open command line.
1036: Currently this allows the addition of port forwardings using the
1.279 stevesk 1037: .Fl L ,
1038: .Fl R
1.218 jmc 1039: and
1.279 stevesk 1040: .Fl D
1.225 jmc 1041: options (see above).
1.322 markus 1042: It also allows the cancellation of existing port-forwardings
1043: with
1.262 stevesk 1044: .Sm off
1.322 markus 1045: .Fl KL Oo Ar bind_address : Oc Ar port
1.262 stevesk 1046: .Sm on
1.322 markus 1047: for local,
1048: .Sm off
1049: .Fl KR Oo Ar bind_address : Oc Ar port
1050: .Sm on
1051: for remote and
1052: .Sm off
1053: .Fl KD Oo Ar bind_address : Oc Ar port
1054: .Sm on
1055: for dynamic port-forwardings.
1.218 jmc 1056: .Ic !\& Ns Ar command
1057: allows the user to execute a local command if the
1058: .Ic PermitLocalCommand
1059: option is enabled in
1.176 jmc 1060: .Xr ssh_config 5 .
1.218 jmc 1061: Basic help is available, using the
1062: .Fl h
1063: option.
1064: .It Cm ~R
1065: Request rekeying of the connection
1066: (only useful for SSH protocol version 2 and if the peer supports it).
1.327 dtucker 1067: .It Cm ~V
1068: Decrease the verbosity
1069: .Pq Ic LogLevel
1070: when errors are being written to stderr.
1071: .It Cm ~v
1.328 jmc 1072: Increase the verbosity
1.327 dtucker 1073: .Pq Ic LogLevel
1074: when errors are being written to stderr.
1.176 jmc 1075: .El
1.246 jmc 1076: .Sh TCP FORWARDING
1077: Forwarding of arbitrary TCP connections over the secure channel can
1078: be specified either on the command line or in a configuration file.
1079: One possible application of TCP forwarding is a secure connection to a
1080: mail server; another is going through firewalls.
1081: .Pp
1082: In the example below, we look at encrypting communication between
1083: an IRC client and server, even though the IRC server does not directly
1084: support encrypted communications.
1085: This works as follows:
1086: the user connects to the remote host using
1087: .Nm ,
1088: specifying a port to be used to forward connections
1089: to the remote server.
1090: After that it is possible to start the service which is to be encrypted
1091: on the client machine,
1092: connecting to the same local port,
1093: and
1094: .Nm
1095: will encrypt and forward the connection.
1096: .Pp
1097: The following example tunnels an IRC session from client machine
1098: .Dq 127.0.0.1
1099: (localhost)
1100: to remote server
1101: .Dq server.example.com :
1102: .Bd -literal -offset 4n
1103: $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
1104: $ irc -c '#users' -p 1234 pinky 127.0.0.1
1105: .Ed
1106: .Pp
1107: This tunnels a connection to IRC server
1108: .Dq server.example.com ,
1109: joining channel
1110: .Dq #users ,
1111: nickname
1112: .Dq pinky ,
1113: using port 1234.
1114: It doesn't matter which port is used,
1115: as long as it's greater than 1023
1116: (remember, only root can open sockets on privileged ports)
1117: and doesn't conflict with any ports already in use.
1118: The connection is forwarded to port 6667 on the remote server,
1119: since that's the standard port for IRC services.
1120: .Pp
1121: The
1122: .Fl f
1123: option backgrounds
1124: .Nm
1125: and the remote command
1126: .Dq sleep 10
1127: is specified to allow an amount of time
1128: (10 seconds, in the example)
1129: to start the service which is to be tunnelled.
1130: If no connections are made within the time specified,
1131: .Nm
1132: will exit.
1133: .Sh X11 FORWARDING
1.218 jmc 1134: If the
1135: .Cm ForwardX11
1136: variable is set to
1137: .Dq yes
1138: (or see the description of the
1.227 jmc 1139: .Fl X ,
1140: .Fl x ,
1.218 jmc 1141: and
1.227 jmc 1142: .Fl Y
1.226 jmc 1143: options above)
1.218 jmc 1144: and the user is using X11 (the
1145: .Ev DISPLAY
1146: environment variable is set), the connection to the X11 display is
1147: automatically forwarded to the remote side in such a way that any X11
1148: programs started from the shell (or command) will go through the
1149: encrypted channel, and the connection to the real X server will be made
1.176 jmc 1150: from the local machine.
1.218 jmc 1151: The user should not manually set
1152: .Ev DISPLAY .
1153: Forwarding of X11 connections can be
1154: configured on the command line or in configuration files.
1155: .Pp
1156: The
1157: .Ev DISPLAY
1158: value set by
1159: .Nm
1160: will point to the server machine, but with a display number greater than zero.
1161: This is normal, and happens because
1162: .Nm
1163: creates a
1164: .Dq proxy
1165: X server on the server machine for forwarding the
1166: connections over the encrypted channel.
1.200 djm 1167: .Pp
1.218 jmc 1168: .Nm
1169: will also automatically set up Xauthority data on the server machine.
1170: For this purpose, it will generate a random authorization cookie,
1171: store it in Xauthority on the server, and verify that any forwarded
1172: connections carry this cookie and replace it by the real cookie when
1173: the connection is opened.
1174: The real authentication cookie is never
1175: sent to the server machine (and no cookies are sent in the plain).
1.200 djm 1176: .Pp
1.218 jmc 1177: If the
1178: .Cm ForwardAgent
1179: variable is set to
1180: .Dq yes
1181: (or see the description of the
1182: .Fl A
1.191 djm 1183: and
1.218 jmc 1184: .Fl a
1.226 jmc 1185: options above) and
1.218 jmc 1186: the user is using an authentication agent, the connection to the agent
1187: is automatically forwarded to the remote side.
1.252 jmc 1188: .Sh VERIFYING HOST KEYS
1189: When connecting to a server for the first time,
1190: a fingerprint of the server's public key is presented to the user
1191: (unless the option
1192: .Cm StrictHostKeyChecking
1193: has been disabled).
1194: Fingerprints can be determined using
1195: .Xr ssh-keygen 1 :
1196: .Pp
1197: .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1198: .Pp
1.274 grunk 1199: If the fingerprint is already known, it can be matched
1200: and the key can be accepted or rejected.
1.358 djm 1201: If only legacy (MD5) fingerprints for the server are available, the
1202: .Xr ssh-keygen 1
1203: .Fl E
1204: option may be used to downgrade the fingerprint algorithm to match.
1205: .Pp
1.274 grunk 1206: Because of the difficulty of comparing host keys
1.352 djm 1207: just by looking at fingerprint strings,
1.274 grunk 1208: there is also support to compare host keys visually,
1209: using
1210: .Em random art .
1211: By setting the
1.275 grunk 1212: .Cm VisualHostKey
1.274 grunk 1213: option to
1.275 grunk 1214: .Dq yes ,
1.274 grunk 1215: a small ASCII graphic gets displayed on every login to a server, no matter
1216: if the session itself is interactive or not.
1217: By learning the pattern a known server produces, a user can easily
1218: find out that the host key has changed when a completely different pattern
1219: is displayed.
1220: Because these patterns are not unambiguous however, a pattern that looks
1221: similar to the pattern remembered only gives a good probability that the
1222: host key is the same, not guaranteed proof.
1223: .Pp
1224: To get a listing of the fingerprints along with their random art for
1225: all known hosts, the following command line can be used:
1226: .Pp
1227: .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1228: .Pp
1.252 jmc 1229: If the fingerprint is unknown,
1230: an alternative method of verification is available:
1231: SSH fingerprints verified by DNS.
1232: An additional resource record (RR),
1233: SSHFP,
1234: is added to a zonefile
1235: and the connecting client is able to match the fingerprint
1236: with that of the key presented.
1237: .Pp
1238: In this example, we are connecting a client to a server,
1239: .Dq host.example.com .
1240: The SSHFP resource records should first be added to the zonefile for
1241: host.example.com:
1242: .Bd -literal -offset indent
1.259 jakob 1243: $ ssh-keygen -r host.example.com.
1.252 jmc 1244: .Ed
1245: .Pp
1246: The output lines will have to be added to the zonefile.
1247: To check that the zone is answering fingerprint queries:
1248: .Pp
1249: .Dl $ dig -t SSHFP host.example.com
1250: .Pp
1251: Finally the client connects:
1252: .Bd -literal -offset indent
1253: $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1254: [...]
1255: Matching host key fingerprint found in DNS.
1256: Are you sure you want to continue connecting (yes/no)?
1257: .Ed
1258: .Pp
1259: See the
1260: .Cm VerifyHostKeyDNS
1261: option in
1262: .Xr ssh_config 5
1263: for more information.
1.250 jmc 1264: .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1265: .Nm
1266: contains support for Virtual Private Network (VPN) tunnelling
1267: using the
1268: .Xr tun 4
1269: network pseudo-device,
1270: allowing two networks to be joined securely.
1271: The
1272: .Xr sshd_config 5
1273: configuration option
1274: .Cm PermitTunnel
1275: controls whether the server supports this,
1276: and at what level (layer 2 or 3 traffic).
1277: .Pp
1278: The following example would connect client network 10.0.50.0/24
1.265 otto 1279: with remote network 10.0.99.0/24 using a point-to-point connection
1280: from 10.1.1.1 to 10.1.1.2,
1281: provided that the SSH server running on the gateway to the remote network,
1282: at 192.168.1.15, allows it.
1283: .Pp
1284: On the client:
1.250 jmc 1285: .Bd -literal -offset indent
1286: # ssh -f -w 0:1 192.168.1.15 true
1.265 otto 1287: # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1288: # route add 10.0.99.0/24 10.1.1.2
1289: .Ed
1290: .Pp
1291: On the server:
1292: .Bd -literal -offset indent
1293: # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1294: # route add 10.0.50.0/24 10.1.1.1
1.250 jmc 1295: .Ed
1296: .Pp
1297: Client access may be more finely tuned via the
1298: .Pa /root/.ssh/authorized_keys
1299: file (see below) and the
1300: .Cm PermitRootLogin
1301: server option.
1.255 jmc 1302: The following entry would permit connections on
1.250 jmc 1303: .Xr tun 4
1.255 jmc 1304: device 1 from user
1.250 jmc 1305: .Dq jane
1.255 jmc 1306: and on tun device 2 from user
1.250 jmc 1307: .Dq john ,
1308: if
1309: .Cm PermitRootLogin
1310: is set to
1311: .Dq forced-commands-only :
1312: .Bd -literal -offset 2n
1313: tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1.254 msf 1314: tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1.250 jmc 1315: .Ed
1316: .Pp
1.264 ray 1317: Since an SSH-based setup entails a fair amount of overhead,
1.250 jmc 1318: it may be more suited to temporary setups,
1319: such as for wireless VPNs.
1320: More permanent VPNs are better provided by tools such as
1321: .Xr ipsecctl 8
1322: and
1323: .Xr isakmpd 8 .
1.2 deraadt 1324: .Sh ENVIRONMENT
1325: .Nm
1.1 deraadt 1326: will normally set the following environment variables:
1.237 jmc 1327: .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1.2 deraadt 1328: .It Ev DISPLAY
1329: The
1330: .Ev DISPLAY
1.40 aaron 1331: variable indicates the location of the X11 server.
1.44 aaron 1332: It is automatically set by
1.2 deraadt 1333: .Nm
1334: to point to a value of the form
1.233 jmc 1335: .Dq hostname:n ,
1336: where
1337: .Dq hostname
1338: indicates the host where the shell runs, and
1339: .Sq n
1340: is an integer \*(Ge 1.
1.40 aaron 1341: .Nm
1342: uses this special value to forward X11 connections over the secure
1343: channel.
1.107 markus 1344: The user should normally not set
1345: .Ev DISPLAY
1346: explicitly, as that
1.1 deraadt 1347: will render the X11 connection insecure (and will require the user to
1348: manually copy any required authorization cookies).
1.2 deraadt 1349: .It Ev HOME
1.1 deraadt 1350: Set to the path of the user's home directory.
1.2 deraadt 1351: .It Ev LOGNAME
1352: Synonym for
1.12 aaron 1353: .Ev USER ;
1354: set for compatibility with systems that use this variable.
1.2 deraadt 1355: .It Ev MAIL
1.129 stevesk 1356: Set to the path of the user's mailbox.
1.40 aaron 1357: .It Ev PATH
1.2 deraadt 1358: Set to the default
1359: .Ev PATH ,
1360: as specified when compiling
1.234 jmc 1361: .Nm .
1.118 markus 1362: .It Ev SSH_ASKPASS
1363: If
1364: .Nm
1365: needs a passphrase, it will read the passphrase from the current
1366: terminal if it was run from a terminal.
1367: If
1368: .Nm
1369: does not have a terminal associated with it but
1370: .Ev DISPLAY
1371: and
1372: .Ev SSH_ASKPASS
1373: are set, it will execute the program specified by
1374: .Ev SSH_ASKPASS
1375: and open an X11 window to read the passphrase.
1376: This is particularly useful when calling
1377: .Nm
1378: from a
1.196 jmc 1379: .Pa .xsession
1.118 markus 1380: or related script.
1381: (Note that on some machines it
1382: may be necessary to redirect the input from
1383: .Pa /dev/null
1384: to make this work.)
1.18 markus 1385: .It Ev SSH_AUTH_SOCK
1.233 jmc 1386: Identifies the path of a
1387: .Ux Ns -domain
1388: socket used to communicate with the agent.
1.166 stevesk 1389: .It Ev SSH_CONNECTION
1390: Identifies the client and server ends of the connection.
1.40 aaron 1391: The variable contains
1.233 jmc 1392: four space-separated values: client IP address, client port number,
1393: server IP address, and server port number.
1.73 markus 1394: .It Ev SSH_ORIGINAL_COMMAND
1.233 jmc 1395: This variable contains the original command line if a forced command
1.73 markus 1396: is executed.
1397: It can be used to extract the original arguments.
1.2 deraadt 1398: .It Ev SSH_TTY
1.1 deraadt 1399: This is set to the name of the tty (path to the device) associated
1.40 aaron 1400: with the current shell or command.
1401: If the current session has no tty,
1.1 deraadt 1402: this variable is not set.
1.2 deraadt 1403: .It Ev TZ
1.214 jmc 1404: This variable is set to indicate the present time zone if it
1.257 jmc 1405: was set when the daemon was started (i.e. the daemon passes the value
1.1 deraadt 1406: on to new connections).
1.2 deraadt 1407: .It Ev USER
1.1 deraadt 1408: Set to the name of the user logging in.
1.2 deraadt 1409: .El
1410: .Pp
1.44 aaron 1411: Additionally,
1.2 deraadt 1412: .Nm
1.44 aaron 1413: reads
1.207 djm 1414: .Pa ~/.ssh/environment ,
1.2 deraadt 1415: and adds lines of the format
1416: .Dq VARNAME=value
1.233 jmc 1417: to the environment if the file exists and users are allowed to
1.161 marc 1418: change their environment.
1.176 jmc 1419: For more information, see the
1.161 marc 1420: .Cm PermitUserEnvironment
1.162 stevesk 1421: option in
1.161 marc 1422: .Xr sshd_config 5 .
1.2 deraadt 1423: .Sh FILES
1.236 jmc 1424: .Bl -tag -width Ds -compact
1.309 jmc 1425: .It Pa ~/.rhosts
1.240 jmc 1426: This file is used for host-based authentication (see above).
1.92 markus 1427: On some machines this file may need to be
1.240 jmc 1428: world-readable if the user's home directory is on an NFS partition,
1.1 deraadt 1429: because
1.2 deraadt 1430: .Xr sshd 8
1.40 aaron 1431: reads it as root.
1432: Additionally, this file must be owned by the user,
1433: and must not have write permissions for anyone else.
1434: The recommended
1.1 deraadt 1435: permission for most machines is read/write for the user, and not
1436: accessible by others.
1.2 deraadt 1437: .Pp
1.309 jmc 1438: .It Pa ~/.shosts
1.240 jmc 1439: This file is used in exactly the same way as
1440: .Pa .rhosts ,
1441: but allows host-based authentication without permitting login with
1442: rlogin/rsh.
1.272 mcbride 1443: .Pp
1.309 jmc 1444: .It Pa ~/.ssh/
1.272 mcbride 1445: This directory is the default location for all user-specific configuration
1446: and authentication information.
1447: There is no general requirement to keep the entire contents of this directory
1448: secret, but the recommended permissions are read/write/execute for the user,
1449: and not accessible by others.
1.236 jmc 1450: .Pp
1.309 jmc 1451: .It Pa ~/.ssh/authorized_keys
1.349 sobrado 1452: Lists the public keys (DSA, ECDSA, Ed25519, RSA)
1.343 naddy 1453: that can be used for logging in as this user.
1.238 jmc 1454: The format of this file is described in the
1455: .Xr sshd 8
1456: manual page.
1457: This file is not highly sensitive, but the recommended
1458: permissions are read/write for the user, and not accessible by others.
1459: .Pp
1.309 jmc 1460: .It Pa ~/.ssh/config
1.238 jmc 1461: This is the per-user configuration file.
1462: The file format and configuration options are described in
1463: .Xr ssh_config 5 .
1464: Because of the potential for abuse, this file must have strict permissions:
1.334 djm 1465: read/write for the user, and not writable by others.
1.238 jmc 1466: .Pp
1.309 jmc 1467: .It Pa ~/.ssh/environment
1.239 jmc 1468: Contains additional definitions for environment variables; see
1469: .Sx ENVIRONMENT ,
1.238 jmc 1470: above.
1471: .Pp
1.309 jmc 1472: .It Pa ~/.ssh/identity
1473: .It Pa ~/.ssh/id_dsa
1.310 djm 1474: .It Pa ~/.ssh/id_ecdsa
1.343 naddy 1475: .It Pa ~/.ssh/id_ed25519
1.309 jmc 1476: .It Pa ~/.ssh/id_rsa
1.238 jmc 1477: Contains the private key for authentication.
1478: These files
1479: contain sensitive data and should be readable by the user but not
1480: accessible by others (read/write/execute).
1481: .Nm
1482: will simply ignore a private key file if it is accessible by others.
1483: It is possible to specify a passphrase when
1484: generating the key which will be used to encrypt the
1485: sensitive part of this file using 3DES.
1486: .Pp
1.309 jmc 1487: .It Pa ~/.ssh/identity.pub
1488: .It Pa ~/.ssh/id_dsa.pub
1.310 djm 1489: .It Pa ~/.ssh/id_ecdsa.pub
1.343 naddy 1490: .It Pa ~/.ssh/id_ed25519.pub
1.309 jmc 1491: .It Pa ~/.ssh/id_rsa.pub
1.238 jmc 1492: Contains the public key for authentication.
1493: These files are not
1494: sensitive and can (but need not) be readable by anyone.
1495: .Pp
1.309 jmc 1496: .It Pa ~/.ssh/known_hosts
1.244 jmc 1497: Contains a list of host keys for all hosts the user has logged into
1498: that are not already in the systemwide list of known host keys.
1.238 jmc 1499: See
1.244 jmc 1500: .Xr sshd 8
1501: for further details of the format of this file.
1.238 jmc 1502: .Pp
1.309 jmc 1503: .It Pa ~/.ssh/rc
1.238 jmc 1504: Commands in this file are executed by
1505: .Nm
1.245 jmc 1506: when the user logs in, just before the user's shell (or command) is
1.238 jmc 1507: started.
1508: See the
1509: .Xr sshd 8
1510: manual page for more information.
1511: .Pp
1.309 jmc 1512: .It Pa /etc/hosts.equiv
1.240 jmc 1513: This file is for host-based authentication (see above).
1514: It should only be writable by root.
1.236 jmc 1515: .Pp
1.309 jmc 1516: .It Pa /etc/shosts.equiv
1.240 jmc 1517: This file is used in exactly the same way as
1518: .Pa hosts.equiv ,
1519: but allows host-based authentication without permitting login with
1520: rlogin/rsh.
1.236 jmc 1521: .Pp
1.238 jmc 1522: .It Pa /etc/ssh/ssh_config
1523: Systemwide configuration file.
1524: The file format and configuration options are described in
1525: .Xr ssh_config 5 .
1526: .Pp
1.309 jmc 1527: .It Pa /etc/ssh/ssh_host_key
1528: .It Pa /etc/ssh/ssh_host_dsa_key
1.310 djm 1529: .It Pa /etc/ssh/ssh_host_ecdsa_key
1.343 naddy 1530: .It Pa /etc/ssh/ssh_host_ed25519_key
1.309 jmc 1531: .It Pa /etc/ssh/ssh_host_rsa_key
1.325 dtucker 1532: These files contain the private parts of the host keys
1.245 jmc 1533: and are used for host-based authentication.
1534: If protocol version 1 is used,
1.238 jmc 1535: .Nm
1536: must be setuid root, since the host key is readable only by root.
1537: For protocol version 2,
1538: .Nm
1539: uses
1540: .Xr ssh-keysign 8
1.245 jmc 1541: to access the host keys,
1542: eliminating the requirement that
1.238 jmc 1543: .Nm
1.245 jmc 1544: be setuid root when host-based authentication is used.
1.238 jmc 1545: By default
1.2 deraadt 1546: .Nm
1.238 jmc 1547: is not setuid root.
1548: .Pp
1.309 jmc 1549: .It Pa /etc/ssh/ssh_known_hosts
1.238 jmc 1550: Systemwide list of known host keys.
1551: This file should be prepared by the
1552: system administrator to contain the public host keys of all machines in the
1553: organization.
1.244 jmc 1554: It should be world-readable.
1555: See
1.238 jmc 1556: .Xr sshd 8
1.244 jmc 1557: for further details of the format of this file.
1.236 jmc 1558: .Pp
1.309 jmc 1559: .It Pa /etc/ssh/sshrc
1.1 deraadt 1560: Commands in this file are executed by
1.2 deraadt 1561: .Nm
1.245 jmc 1562: when the user logs in, just before the user's shell (or command) is started.
1.44 aaron 1563: See the
1.2 deraadt 1564: .Xr sshd 8
1.1 deraadt 1565: manual page for more information.
1.58 itojun 1566: .El
1.312 jmc 1567: .Sh EXIT STATUS
1568: .Nm
1569: exits with the exit status of the remote command or with 255
1570: if an error occurred.
1.2 deraadt 1571: .Sh SEE ALSO
1572: .Xr scp 1 ,
1.83 djm 1573: .Xr sftp 1 ,
1.2 deraadt 1574: .Xr ssh-add 1 ,
1575: .Xr ssh-agent 1 ,
1576: .Xr ssh-keygen 1 ,
1.242 jmc 1577: .Xr ssh-keyscan 1 ,
1.250 jmc 1578: .Xr tun 4 ,
1.159 stevesk 1579: .Xr ssh_config 5 ,
1.160 naddy 1580: .Xr ssh-keysign 8 ,
1.87 itojun 1581: .Xr sshd 8
1.329 jmc 1582: .Sh STANDARDS
1.106 markus 1583: .Rs
1.329 jmc 1584: .%A S. Lehtinen
1585: .%A C. Lonvick
1586: .%D January 2006
1.256 jmc 1587: .%R RFC 4250
1.329 jmc 1588: .%T The Secure Shell (SSH) Protocol Assigned Numbers
1.256 jmc 1589: .Re
1.329 jmc 1590: .Pp
1.256 jmc 1591: .Rs
1.329 jmc 1592: .%A T. Ylonen
1593: .%A C. Lonvick
1594: .%D January 2006
1.256 jmc 1595: .%R RFC 4251
1.329 jmc 1596: .%T The Secure Shell (SSH) Protocol Architecture
1.256 jmc 1597: .Re
1.329 jmc 1598: .Pp
1.256 jmc 1599: .Rs
1.329 jmc 1600: .%A T. Ylonen
1601: .%A C. Lonvick
1602: .%D January 2006
1.256 jmc 1603: .%R RFC 4252
1.329 jmc 1604: .%T The Secure Shell (SSH) Authentication Protocol
1.256 jmc 1605: .Re
1.329 jmc 1606: .Pp
1.256 jmc 1607: .Rs
1.329 jmc 1608: .%A T. Ylonen
1609: .%A C. Lonvick
1610: .%D January 2006
1.256 jmc 1611: .%R RFC 4253
1.329 jmc 1612: .%T The Secure Shell (SSH) Transport Layer Protocol
1.256 jmc 1613: .Re
1.329 jmc 1614: .Pp
1.256 jmc 1615: .Rs
1.329 jmc 1616: .%A T. Ylonen
1617: .%A C. Lonvick
1618: .%D January 2006
1.256 jmc 1619: .%R RFC 4254
1.329 jmc 1620: .%T The Secure Shell (SSH) Connection Protocol
1.256 jmc 1621: .Re
1.329 jmc 1622: .Pp
1.256 jmc 1623: .Rs
1.329 jmc 1624: .%A J. Schlyter
1625: .%A W. Griffin
1626: .%D January 2006
1.256 jmc 1627: .%R RFC 4255
1.329 jmc 1628: .%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
1.256 jmc 1629: .Re
1.329 jmc 1630: .Pp
1.256 jmc 1631: .Rs
1.329 jmc 1632: .%A F. Cusack
1633: .%A M. Forssen
1634: .%D January 2006
1.256 jmc 1635: .%R RFC 4256
1.329 jmc 1636: .%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
1.256 jmc 1637: .Re
1.329 jmc 1638: .Pp
1.256 jmc 1639: .Rs
1.329 jmc 1640: .%A J. Galbraith
1641: .%A P. Remaker
1642: .%D January 2006
1.256 jmc 1643: .%R RFC 4335
1.329 jmc 1644: .%T The Secure Shell (SSH) Session Channel Break Extension
1.256 jmc 1645: .Re
1.329 jmc 1646: .Pp
1.256 jmc 1647: .Rs
1.329 jmc 1648: .%A M. Bellare
1649: .%A T. Kohno
1650: .%A C. Namprempre
1651: .%D January 2006
1.256 jmc 1652: .%R RFC 4344
1.329 jmc 1653: .%T The Secure Shell (SSH) Transport Layer Encryption Modes
1.256 jmc 1654: .Re
1.329 jmc 1655: .Pp
1.256 jmc 1656: .Rs
1.329 jmc 1657: .%A B. Harris
1658: .%D January 2006
1.256 jmc 1659: .%R RFC 4345
1.329 jmc 1660: .%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
1.258 djm 1661: .Re
1.329 jmc 1662: .Pp
1.258 djm 1663: .Rs
1.329 jmc 1664: .%A M. Friedl
1665: .%A N. Provos
1666: .%A W. Simpson
1667: .%D March 2006
1.258 djm 1668: .%R RFC 4419
1.329 jmc 1669: .%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
1.266 markus 1670: .Re
1.329 jmc 1671: .Pp
1.266 markus 1672: .Rs
1.329 jmc 1673: .%A J. Galbraith
1674: .%A R. Thayer
1675: .%D November 2006
1.266 markus 1676: .%R RFC 4716
1.329 jmc 1677: .%T The Secure Shell (SSH) Public Key File Format
1.313 djm 1678: .Re
1.329 jmc 1679: .Pp
1.313 djm 1680: .Rs
1.329 jmc 1681: .%A D. Stebila
1682: .%A J. Green
1683: .%D December 2009
1.313 djm 1684: .%R RFC 5656
1.329 jmc 1685: .%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
1.274 grunk 1686: .Re
1.329 jmc 1687: .Pp
1.274 grunk 1688: .Rs
1689: .%A A. Perrig
1690: .%A D. Song
1691: .%D 1999
1.329 jmc 1692: .%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
1693: .%T Hash Visualization: a New Technique to improve Real-World Security
1.106 markus 1694: .Re
1.173 jmc 1695: .Sh AUTHORS
1696: OpenSSH is a derivative of the original and free
1697: ssh 1.2.12 release by Tatu Ylonen.
1698: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1699: Theo de Raadt and Dug Song
1700: removed many bugs, re-added newer features and
1701: created OpenSSH.
1702: Markus Friedl contributed the support for SSH
1703: protocol versions 1.5 and 2.0.
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