Annotation of src/usr.bin/ssh/ssh.1, Revision 1.64.2.8
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.59 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.64.2.3 jason 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.59 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.59 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.64.2.8! brad 37: .\" $OpenBSD: ssh.1,v 1.148 2002/02/18 17:55:20 markus Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.64.2.3 jason 43: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
46: .Op Fl l Ar login_name
1.64.2.5 miod 47: .Ar hostname | user@hostname
1.2 deraadt 48: .Op Ar command
49: .Pp
50: .Nm ssh
1.64.2.1 jason 51: .Op Fl afgknqstvxACNPTX1246
1.64.2.5 miod 52: .Op Fl b Ar bind_address
1.51 markus 53: .Op Fl c Ar cipher_spec
1.2 deraadt 54: .Op Fl e Ar escape_char
55: .Op Fl i Ar identity_file
56: .Op Fl l Ar login_name
1.64.2.1 jason 57: .Op Fl m Ar mac_spec
1.2 deraadt 58: .Op Fl o Ar option
59: .Op Fl p Ar port
1.64.2.5 miod 60: .Op Fl F Ar configfile
1.12 aaron 61: .Oo Fl L Xo
62: .Sm off
1.33 markus 63: .Ar port :
1.12 aaron 64: .Ar host :
65: .Ar hostport
66: .Sm on
67: .Xc
68: .Oc
69: .Oo Fl R Xo
70: .Sm off
1.33 markus 71: .Ar port :
1.12 aaron 72: .Ar host :
73: .Ar hostport
74: .Sm on
75: .Xc
76: .Oc
1.64.2.5 miod 77: .Op Fl D Ar port
78: .Ar hostname | user@hostname
1.2 deraadt 79: .Op Ar command
1.44 aaron 80: .Sh DESCRIPTION
1.2 deraadt 81: .Nm
1.64.2.3 jason 82: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 83: executing commands on a remote machine.
84: It is intended to replace
1.1 deraadt 85: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 86: two untrusted hosts over an insecure network.
87: X11 connections and
1.1 deraadt 88: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 89: .Pp
90: .Nm
1.44 aaron 91: connects and logs into the specified
1.2 deraadt 92: .Ar hostname .
1.1 deraadt 93: The user must prove
1.49 markus 94: his/her identity to the remote machine using one of several methods
95: depending on the protocol version used:
96: .Pp
97: .Ss SSH protocol version 1
1.2 deraadt 98: .Pp
1.1 deraadt 99: First, if the machine the user logs in from is listed in
1.2 deraadt 100: .Pa /etc/hosts.equiv
1.1 deraadt 101: or
1.2 deraadt 102: .Pa /etc/shosts.equiv
1.1 deraadt 103: on the remote machine, and the user names are
104: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 105: Second, if
1.2 deraadt 106: .Pa \&.rhosts
1.1 deraadt 107: or
1.2 deraadt 108: .Pa \&.shosts
1.1 deraadt 109: exists in the user's home directory on the
110: remote machine and contains a line containing the name of the client
111: machine and the name of the user on that machine, the user is
1.40 aaron 112: permitted to log in.
113: This form of authentication alone is normally not
1.1 deraadt 114: allowed by the server because it is not secure.
1.2 deraadt 115: .Pp
1.64.2.4 jason 116: The second authentication method is the
1.2 deraadt 117: .Pa rhosts
1.1 deraadt 118: or
1.2 deraadt 119: .Pa hosts.equiv
1.40 aaron 120: method combined with RSA-based host authentication.
121: It means that if the login would be permitted by
1.49 markus 122: .Pa $HOME/.rhosts ,
123: .Pa $HOME/.shosts ,
1.2 deraadt 124: .Pa /etc/hosts.equiv ,
1.1 deraadt 125: or
1.2 deraadt 126: .Pa /etc/shosts.equiv ,
1.11 deraadt 127: and if additionally the server can verify the client's
1.44 aaron 128: host key (see
1.2 deraadt 129: .Pa /etc/ssh_known_hosts
1.23 markus 130: and
131: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 132: in the
1.2 deraadt 133: .Sx FILES
1.40 aaron 134: section), only then login is permitted.
135: This authentication method closes security holes due to IP
136: spoofing, DNS spoofing and routing spoofing.
137: [Note to the administrator:
1.2 deraadt 138: .Pa /etc/hosts.equiv ,
1.49 markus 139: .Pa $HOME/.rhosts ,
1.1 deraadt 140: and the rlogin/rsh protocol in general, are inherently insecure and should be
141: disabled if security is desired.]
1.2 deraadt 142: .Pp
1.44 aaron 143: As a third authentication method,
1.2 deraadt 144: .Nm
1.1 deraadt 145: supports RSA based authentication.
146: The scheme is based on public-key cryptography: there are cryptosystems
147: where encryption and decryption are done using separate keys, and it
148: is not possible to derive the decryption key from the encryption key.
1.40 aaron 149: RSA is one such system.
1.44 aaron 150: The idea is that each user creates a public/private
1.40 aaron 151: key pair for authentication purposes.
152: The server knows the public key, and only the user knows the private key.
1.44 aaron 153: The file
1.2 deraadt 154: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 155: lists the public keys that are permitted for logging
1.40 aaron 156: in.
157: When the user logs in, the
1.2 deraadt 158: .Nm
1.1 deraadt 159: program tells the server which key pair it would like to use for
1.40 aaron 160: authentication.
161: The server checks if this key is permitted, and if
1.1 deraadt 162: so, sends the user (actually the
1.2 deraadt 163: .Nm
1.1 deraadt 164: program running on behalf of the user) a challenge, a random number,
1.40 aaron 165: encrypted by the user's public key.
166: The challenge can only be
167: decrypted using the proper private key.
168: The user's client then decrypts the
1.1 deraadt 169: challenge using the private key, proving that he/she knows the private
170: key but without disclosing it to the server.
1.2 deraadt 171: .Pp
172: .Nm
1.40 aaron 173: implements the RSA authentication protocol automatically.
174: The user creates his/her RSA key pair by running
1.2 deraadt 175: .Xr ssh-keygen 1 .
1.44 aaron 176: This stores the private key in
1.49 markus 177: .Pa $HOME/.ssh/identity
1.1 deraadt 178: and the public key in
1.49 markus 179: .Pa $HOME/.ssh/identity.pub
1.40 aaron 180: in the user's home directory.
181: The user should then copy the
1.2 deraadt 182: .Pa identity.pub
1.44 aaron 183: to
1.49 markus 184: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 185: in his/her home directory on the remote machine (the
1.2 deraadt 186: .Pa authorized_keys
1.44 aaron 187: file corresponds to the conventional
1.49 markus 188: .Pa $HOME/.rhosts
1.1 deraadt 189: file, and has one key
1.40 aaron 190: per line, though the lines can be very long).
191: After this, the user can log in without giving the password.
192: RSA authentication is much
1.1 deraadt 193: more secure than rhosts authentication.
1.2 deraadt 194: .Pp
1.1 deraadt 195: The most convenient way to use RSA authentication may be with an
1.40 aaron 196: authentication agent.
197: See
1.2 deraadt 198: .Xr ssh-agent 1
1.1 deraadt 199: for more information.
1.2 deraadt 200: .Pp
1.44 aaron 201: If other authentication methods fail,
1.2 deraadt 202: .Nm
1.40 aaron 203: prompts the user for a password.
204: The password is sent to the remote
1.1 deraadt 205: host for checking; however, since all communications are encrypted,
206: the password cannot be seen by someone listening on the network.
1.2 deraadt 207: .Pp
1.49 markus 208: .Ss SSH protocol version 2
209: .Pp
1.64.2.8! brad 210: When a user connects using protocol version 2
! 211: similar authentication methods are available.
1.64.2.4 jason 212: Using the default values for
213: .Cm PreferredAuthentications ,
1.64.2.5 miod 214: the client will try to authenticate first using the hostbased method;
215: if this method fails public key authentication is attempted,
216: and finally if this method fails keyboard-interactive and
217: password authentication are tried.
1.49 markus 218: .Pp
219: The public key method is similar to RSA authentication described
1.64.2.4 jason 220: in the previous section and allows the RSA or DSA algorithm to be used:
221: The client uses his private key,
1.49 markus 222: .Pa $HOME/.ssh/id_dsa
1.64.2.4 jason 223: or
224: .Pa $HOME/.ssh/id_rsa ,
1.49 markus 225: to sign the session identifier and sends the result to the server.
226: The server checks whether the matching public key is listed in
1.64.2.5 miod 227: .Pa $HOME/.ssh/authorized_keys
1.49 markus 228: and grants access if both the key is found and the signature is correct.
229: The session identifier is derived from a shared Diffie-Hellman value
230: and is only known to the client and the server.
231: .Pp
232: If public key authentication fails or is not available a password
233: can be sent encrypted to the remote host for proving the user's identity.
1.64.2.4 jason 234: .Pp
235: Additionally,
236: .Nm
237: supports hostbased or challenge response authentication.
1.49 markus 238: .Pp
239: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 240: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.64.2.3 jason 241: and integrity (hmac-md5, hmac-sha1).
1.49 markus 242: Note that protocol 1 lacks a strong mechanism for ensuring the
243: integrity of the connection.
244: .Pp
245: .Ss Login session and remote execution
246: .Pp
1.1 deraadt 247: When the user's identity has been accepted by the server, the server
248: either executes the given command, or logs into the machine and gives
1.40 aaron 249: the user a normal shell on the remote machine.
250: All communication with
1.1 deraadt 251: the remote command or shell will be automatically encrypted.
1.2 deraadt 252: .Pp
1.1 deraadt 253: If a pseudo-terminal has been allocated (normal login session), the
1.64.2.4 jason 254: user may use the escape characters noted below.
1.2 deraadt 255: .Pp
1.1 deraadt 256: If no pseudo tty has been allocated, the
257: session is transparent and can be used to reliably transfer binary
1.40 aaron 258: data.
259: On most systems, setting the escape character to
1.2 deraadt 260: .Dq none
261: will also make the session transparent even if a tty is used.
262: .Pp
1.64.2.1 jason 263: The session terminates when the command or shell on the remote
1.64.2.3 jason 264: machine exits and all X11 and TCP/IP connections have been closed.
1.1 deraadt 265: The exit status of the remote program is returned as the exit status
266: of
1.2 deraadt 267: .Nm ssh .
268: .Pp
1.64.2.4 jason 269: .Ss Escape Characters
270: .Pp
271: When a pseudo terminal has been requested, ssh supports a number of functions
1.64.2.5 miod 272: through the use of an escape character.
1.64.2.4 jason 273: .Pp
274: A single tilde character can be sent as
275: .Ic ~~
1.64.2.5 miod 276: or by following the tilde by a character other than those described below.
1.64.2.4 jason 277: The escape character must always follow a newline to be interpreted as
278: special.
279: The escape character can be changed in configuration files using the
280: .Cm EscapeChar
1.64.2.5 miod 281: configuration directive or on the command line by the
1.64.2.4 jason 282: .Fl e
283: option.
284: .Pp
285: The supported escapes (assuming the default
286: .Ql ~ )
287: are:
288: .Bl -tag -width Ds
289: .It Cm ~.
290: Disconnect
291: .It Cm ~^Z
292: Background ssh
293: .It Cm ~#
294: List forwarded connections
295: .It Cm ~&
296: Background ssh at logout when waiting for forwarded connection / X11 sessions
1.64.2.7 miod 297: to terminate
1.64.2.4 jason 298: .It Cm ~?
299: Display a list of escape characters
300: .It Cm ~R
301: Request rekeying of the connection (only useful for SSH protocol version 2
302: and if the peer supports it)
303: .El
304: .Pp
1.49 markus 305: .Ss X11 and TCP forwarding
306: .Pp
1.64.2.5 miod 307: If the
308: .Cm ForwardX11
309: variable is set to
310: .Dq yes
311: (or, see the description of the
312: .Fl X
313: and
314: .Fl x
315: options described later)
316: and the user is using X11 (the
1.2 deraadt 317: .Ev DISPLAY
1.1 deraadt 318: environment variable is set), the connection to the X11 display is
319: automatically forwarded to the remote side in such a way that any X11
320: programs started from the shell (or command) will go through the
321: encrypted channel, and the connection to the real X server will be made
1.40 aaron 322: from the local machine.
323: The user should not manually set
1.2 deraadt 324: .Ev DISPLAY .
1.1 deraadt 325: Forwarding of X11 connections can be
326: configured on the command line or in configuration files.
1.2 deraadt 327: .Pp
328: The
1.44 aaron 329: .Ev DISPLAY
1.2 deraadt 330: value set by
331: .Nm
1.1 deraadt 332: will point to the server machine, but with a display number greater
1.40 aaron 333: than zero.
334: This is normal, and happens because
1.2 deraadt 335: .Nm
336: creates a
337: .Dq proxy
338: X server on the server machine for forwarding the
1.1 deraadt 339: connections over the encrypted channel.
1.2 deraadt 340: .Pp
341: .Nm
1.1 deraadt 342: will also automatically set up Xauthority data on the server machine.
343: For this purpose, it will generate a random authorization cookie,
344: store it in Xauthority on the server, and verify that any forwarded
345: connections carry this cookie and replace it by the real cookie when
1.40 aaron 346: the connection is opened.
347: The real authentication cookie is never
1.1 deraadt 348: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 349: .Pp
1.1 deraadt 350: If the user is using an authentication agent, the connection to the agent
351: is automatically forwarded to the remote side unless disabled on
1.64.2.5 miod 352: the command line or in a configuration file.
1.2 deraadt 353: .Pp
1.1 deraadt 354: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.64.2.5 miod 355: be specified either on the command line or in a configuration file.
1.40 aaron 356: One possible application of TCP/IP forwarding is a secure connection to an
1.64.2.3 jason 357: electronic purse; another is going through firewalls.
1.2 deraadt 358: .Pp
1.49 markus 359: .Ss Server authentication
360: .Pp
1.2 deraadt 361: .Nm
1.49 markus 362: automatically maintains and checks a database containing
1.40 aaron 363: identifications for all hosts it has ever been used with.
1.64.2.5 miod 364: Host keys are stored in
1.49 markus 365: .Pa $HOME/.ssh/known_hosts
1.40 aaron 366: in the user's home directory.
1.64.2.5 miod 367: Additionally, the file
1.2 deraadt 368: .Pa /etc/ssh_known_hosts
1.64.2.5 miod 369: is automatically checked for known hosts.
1.40 aaron 370: Any new hosts are automatically added to the user's file.
371: If a host's identification
1.1 deraadt 372: ever changes,
1.2 deraadt 373: .Nm
1.1 deraadt 374: warns about this and disables password authentication to prevent a
1.40 aaron 375: trojan horse from getting the user's password.
376: Another purpose of
1.1 deraadt 377: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 378: otherwise be used to circumvent the encryption.
379: The
1.2 deraadt 380: .Cm StrictHostKeyChecking
1.1 deraadt 381: option (see below) can be used to prevent logins to machines whose
382: host key is not known or has changed.
1.64.2.1 jason 383: .Pp
384: The options are as follows:
1.2 deraadt 385: .Bl -tag -width Ds
1.4 dugsong 386: .It Fl a
1.42 aaron 387: Disables forwarding of the authentication agent connection.
1.54 markus 388: .It Fl A
389: Enables forwarding of the authentication agent connection.
390: This can also be specified on a per-host basis in a configuration file.
1.64.2.5 miod 391: .It Fl b Ar bind_address
392: Specify the interface to transmit from on machines with multiple
393: interfaces or aliased addresses.
394: .It Fl c Ar blowfish|3des|des
1.44 aaron 395: Selects the cipher to use for encrypting the session.
1.2 deraadt 396: .Ar 3des
1.40 aaron 397: is used by default.
1.44 aaron 398: It is believed to be secure.
1.5 deraadt 399: .Ar 3des
400: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
401: .Ar blowfish
402: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 403: .Ar 3des .
1.64.2.5 miod 404: .Ar des
405: is only supported in the
406: .Nm
407: client for interoperability with legacy protocol 1 implementations
408: that do not support the
409: .Ar 3des
410: cipher. Its use is strongly discouraged due to cryptographic
411: weaknesses.
1.64.2.1 jason 412: .It Fl c Ar cipher_spec
1.51 markus 413: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 414: be specified in order of preference.
1.64.2.1 jason 415: See
416: .Cm Ciphers
417: for more information.
1.2 deraadt 418: .It Fl e Ar ch|^ch|none
419: Sets the escape character for sessions with a pty (default:
420: .Ql ~ ) .
1.40 aaron 421: The escape character is only recognized at the beginning of a line.
422: The escape character followed by a dot
1.2 deraadt 423: .Pq Ql \&.
424: closes the connection, followed
1.1 deraadt 425: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 426: escape character once.
427: Setting the character to
1.2 deraadt 428: .Dq none
429: disables any escapes and makes the session fully transparent.
430: .It Fl f
431: Requests
432: .Nm
1.40 aaron 433: to go to background just before command execution.
434: This is useful if
1.2 deraadt 435: .Nm
436: is going to ask for passwords or passphrases, but the user
1.40 aaron 437: wants it in the background.
1.44 aaron 438: This implies
1.2 deraadt 439: .Fl n .
1.1 deraadt 440: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 441: something like
442: .Ic ssh -f host xterm .
1.34 markus 443: .It Fl g
444: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 445: .It Fl i Ar identity_file
1.64.2.8! brad 446: Selects a file from which the identity (private key) for
1.64.2.1 jason 447: RSA or DSA authentication is read.
1.64.2.8! brad 448: The default is
1.49 markus 449: .Pa $HOME/.ssh/identity
1.64.2.8! brad 450: for protocol version 1, and
! 451: .Pa $HOME/.ssh/id_rsa
! 452: and
! 453: .Pa $HOME/.ssh/id_dsa
! 454: for protocol version 2.
1.40 aaron 455: Identity files may also be specified on
456: a per-host basis in the configuration file.
457: It is possible to have multiple
1.2 deraadt 458: .Fl i
459: options (and multiple identities specified in
1.1 deraadt 460: configuration files).
1.64.2.5 miod 461: .It Fl I Ar smartcard_device
462: Specifies which smartcard device to use. The argument is
463: the device
464: .Nm
465: should use to communicate with a smartcard used for storing the user's
466: private RSA key.
1.2 deraadt 467: .It Fl k
1.42 aaron 468: Disables forwarding of Kerberos tickets and AFS tokens.
469: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 470: .It Fl l Ar login_name
1.40 aaron 471: Specifies the user to log in as on the remote machine.
472: This also may be specified on a per-host basis in the configuration file.
1.64.2.1 jason 473: .It Fl m Ar mac_spec
474: Additionally, for protocol version 2 a comma-separated list of MAC
475: (message authentication code) algorithms can
476: be specified in order of preference.
477: See the
478: .Cm MACs
479: keyword for more information.
1.2 deraadt 480: .It Fl n
481: Redirects stdin from
482: .Pa /dev/null
483: (actually, prevents reading from stdin).
1.1 deraadt 484: This must be used when
1.2 deraadt 485: .Nm
1.40 aaron 486: is run in the background.
487: A common trick is to use this to run X11 programs on a remote machine.
488: For example,
1.2 deraadt 489: .Ic ssh -n shadows.cs.hut.fi emacs &
490: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 491: connection will be automatically forwarded over an encrypted channel.
492: The
1.2 deraadt 493: .Nm
1.1 deraadt 494: program will be put in the background.
495: (This does not work if
1.2 deraadt 496: .Nm
497: needs to ask for a password or passphrase; see also the
498: .Fl f
499: option.)
1.53 markus 500: .It Fl N
501: Do not execute a remote command.
1.64.2.5 miod 502: This is useful for just forwarding ports
1.53 markus 503: (protocol version 2 only).
1.2 deraadt 504: .It Fl o Ar option
1.64.2.5 miod 505: Can be used to give options in the format used in the configuration file.
1.1 deraadt 506: This is useful for specifying options for which there is no separate
1.40 aaron 507: command-line flag.
1.2 deraadt 508: .It Fl p Ar port
1.40 aaron 509: Port to connect to on the remote host.
510: This can be specified on a
1.1 deraadt 511: per-host basis in the configuration file.
1.16 markus 512: .It Fl P
513: Use a non-privileged port for outgoing connections.
1.64.2.5 miod 514: This can be used if a firewall does
1.16 markus 515: not permit connections from privileged ports.
1.30 provos 516: Note that this option turns off
1.16 markus 517: .Cm RhostsAuthentication
518: and
1.64.2.1 jason 519: .Cm RhostsRSAAuthentication
520: for older servers.
1.2 deraadt 521: .It Fl q
1.40 aaron 522: Quiet mode.
523: Causes all warning and diagnostic messages to be suppressed.
1.64.2.1 jason 524: .It Fl s
1.64.2.5 miod 525: May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use
526: of SSH as a secure transport for other applications (eg. sftp). The
1.64.2.1 jason 527: subsystem is specified as the remote command.
1.2 deraadt 528: .It Fl t
1.40 aaron 529: Force pseudo-tty allocation.
1.43 brad 530: This can be used to execute arbitrary
1.40 aaron 531: screen-based programs on a remote machine, which can be very useful,
532: e.g., when implementing menu services.
1.64.2.1 jason 533: Multiple
534: .Fl t
535: options force tty allocation, even if
536: .Nm
537: has no local tty.
1.53 markus 538: .It Fl T
1.64.2.1 jason 539: Disable pseudo-tty allocation.
1.2 deraadt 540: .It Fl v
1.40 aaron 541: Verbose mode.
542: Causes
1.2 deraadt 543: .Nm
1.40 aaron 544: to print debugging messages about its progress.
545: This is helpful in
1.1 deraadt 546: debugging connection, authentication, and configuration problems.
1.64.2.1 jason 547: Multiple
548: .Fl v
549: options increases the verbosity.
1.61 aaron 550: Maximum is 3.
1.2 deraadt 551: .It Fl x
1.40 aaron 552: Disables X11 forwarding.
1.2 deraadt 553: .It Fl X
1.1 deraadt 554: Enables X11 forwarding.
1.54 markus 555: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 556: .It Fl C
1.1 deraadt 557: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 558: data for forwarded X11 and TCP/IP connections).
559: The compression algorithm is the same used by
1.34 markus 560: .Xr gzip 1 ,
561: and the
1.2 deraadt 562: .Dq level
563: can be controlled by the
564: .Cm CompressionLevel
1.40 aaron 565: option (see below).
566: Compression is desirable on modem lines and other
1.1 deraadt 567: slow connections, but will only slow down things on fast networks.
568: The default value can be set on a host-by-host basis in the
569: configuration files; see the
1.64.2.5 miod 570: .Cm Compression
1.1 deraadt 571: option below.
1.64.2.5 miod 572: .It Fl F Ar configfile
573: Specifies an alternative per-user configuration file.
574: If a configuration file is given on the command line,
575: the system-wide configuration file
576: .Pq Pa /etc/ssh_config
577: will be ignored.
578: The default for the per-user configuration file is
579: .Pa $HOME/.ssh/config .
1.2 deraadt 580: .It Fl L Ar port:host:hostport
1.1 deraadt 581: Specifies that the given port on the local (client) host is to be
1.40 aaron 582: forwarded to the given host and port on the remote side.
583: This works by allocating a socket to listen to
1.2 deraadt 584: .Ar port
1.1 deraadt 585: on the local side, and whenever a connection is made to this port, the
586: connection is forwarded over the secure channel, and a connection is
587: made to
1.32 markus 588: .Ar host
589: port
590: .Ar hostport
1.40 aaron 591: from the remote machine.
592: Port forwardings can also be specified in the configuration file.
593: Only root can forward privileged ports.
1.32 markus 594: IPv6 addresses can be specified with an alternative syntax:
595: .Ar port/host/hostport
1.2 deraadt 596: .It Fl R Ar port:host:hostport
1.1 deraadt 597: Specifies that the given port on the remote (server) host is to be
1.40 aaron 598: forwarded to the given host and port on the local side.
599: This works by allocating a socket to listen to
1.2 deraadt 600: .Ar port
1.1 deraadt 601: on the remote side, and whenever a connection is made to this port, the
602: connection is forwarded over the secure channel, and a connection is
603: made to
1.32 markus 604: .Ar host
605: port
606: .Ar hostport
1.40 aaron 607: from the local machine.
608: Port forwardings can also be specified in the configuration file.
609: Privileged ports can be forwarded only when
1.1 deraadt 610: logging in as root on the remote machine.
1.64.2.4 jason 611: IPv6 addresses can be specified with an alternative syntax:
612: .Ar port/host/hostport
1.64.2.5 miod 613: .It Fl D Ar port
614: Specifies a local
615: .Dq dynamic
616: application-level port forwarding.
617: This works by allocating a socket to listen to
618: .Ar port
619: on the local side, and whenever a connection is made to this port, the
620: connection is forwarded over the secure channel, and the application
621: protocol is then used to determine where to connect to from the
622: remote machine. Currently the SOCKS4 protocol is supported, and
623: .Nm
624: will act as a SOCKS4 server.
625: Only root can forward privileged ports.
626: Dynamic port forwardings can also be specified in the configuration file.
1.64.2.1 jason 627: .It Fl 1
628: Forces
629: .Nm
630: to try protocol version 1 only.
1.46 markus 631: .It Fl 2
632: Forces
633: .Nm
1.50 markus 634: to try protocol version 2 only.
1.32 markus 635: .It Fl 4
636: Forces
637: .Nm
638: to use IPv4 addresses only.
639: .It Fl 6
640: Forces
641: .Nm
642: to use IPv6 addresses only.
1.2 deraadt 643: .El
644: .Sh CONFIGURATION FILES
645: .Nm
1.64.2.5 miod 646: obtains configuration data from the following sources in
647: the following order:
1.1 deraadt 648: command line options, user's configuration file
1.2 deraadt 649: .Pq Pa $HOME/.ssh/config ,
650: and system-wide configuration file
651: .Pq Pa /etc/ssh_config .
652: For each parameter, the first obtained value
1.40 aaron 653: will be used.
654: The configuration files contain sections bracketed by
655: .Dq Host
656: specifications, and that section is only applied for hosts that
657: match one of the patterns given in the specification.
658: The matched host name is the one given on the command line.
1.2 deraadt 659: .Pp
1.1 deraadt 660: Since the first obtained value for each parameter is used, more
661: host-specific declarations should be given near the beginning of the
662: file, and general defaults at the end.
1.2 deraadt 663: .Pp
1.1 deraadt 664: The configuration file has the following format:
1.2 deraadt 665: .Pp
666: Empty lines and lines starting with
667: .Ql #
668: are comments.
669: .Pp
670: Otherwise a line is of the format
671: .Dq keyword arguments .
1.64.2.5 miod 672: Configuration options may be separated by whitespace or
673: optional whitespace and exactly one
674: .Ql = ;
675: the latter format is useful to avoid the need to quote whitespace
676: when specifying configuration options using the
677: .Nm ssh ,
678: .Nm scp
679: and
680: .Nm sftp
681: .Fl o
682: option.
683: .Pp
1.2 deraadt 684: The possible
1.64.2.5 miod 685: keywords and their meanings are as follows (note that
686: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 687: .Bl -tag -width Ds
688: .It Cm Host
1.1 deraadt 689: Restricts the following declarations (up to the next
1.2 deraadt 690: .Cm Host
1.1 deraadt 691: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 692: given after the keyword.
693: .Ql \&*
694: and
695: .Ql ?
696: can be used as wildcards in the
1.40 aaron 697: patterns.
698: A single
1.2 deraadt 699: .Ql \&*
700: as a pattern can be used to provide global
1.40 aaron 701: defaults for all hosts.
702: The host is the
1.2 deraadt 703: .Ar hostname
1.1 deraadt 704: argument given on the command line (i.e., the name is not converted to
705: a canonicalized host name before matching).
1.2 deraadt 706: .It Cm AFSTokenPassing
1.42 aaron 707: Specifies whether to pass AFS tokens to remote host.
708: The argument to this keyword must be
1.2 deraadt 709: .Dq yes
710: or
711: .Dq no .
1.64.2.4 jason 712: This option applies to protocol version 1 only.
1.2 deraadt 713: .It Cm BatchMode
714: If set to
715: .Dq yes ,
1.40 aaron 716: passphrase/password querying will be disabled.
1.64.2.5 miod 717: This option is useful in scripts and other batch jobs where no user
718: is present to supply the password.
1.40 aaron 719: The argument must be
1.2 deraadt 720: .Dq yes
721: or
722: .Dq no .
1.64.2.4 jason 723: The default is
724: .Dq no .
1.64.2.5 miod 725: .It Cm BindAddress
726: Specify the interface to transmit from on machines with multiple
727: interfaces or aliased addresses.
728: Note that this option does not work if
729: .Cm UsePrivilegedPort
730: is set to
731: .Dq yes .
1.34 markus 732: .It Cm CheckHostIP
733: If this flag is set to
734: .Dq yes ,
1.64.2.4 jason 735: ssh will additionally check the host IP address in the
1.34 markus 736: .Pa known_hosts
1.42 aaron 737: file.
738: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 739: If the option is set to
740: .Dq no ,
741: the check will not be executed.
1.64.2.4 jason 742: The default is
743: .Dq yes .
1.2 deraadt 744: .It Cm Cipher
1.62 markus 745: Specifies the cipher to use for encrypting the session
1.64 markus 746: in protocol version 1.
1.40 aaron 747: Currently,
1.64.2.5 miod 748: .Dq blowfish ,
749: .Dq 3des ,
1.1 deraadt 750: and
1.64.2.5 miod 751: .Dq des
1.40 aaron 752: are supported.
1.64.2.5 miod 753: .Ar des
754: is only supported in the
755: .Nm
756: client for interoperability with legacy protocol 1 implementations
757: that do not support the
758: .Ar 3des
759: cipher. Its use is strongly discouraged due to cryptographic
760: weaknesses.
1.40 aaron 761: The default is
1.2 deraadt 762: .Dq 3des .
1.45 markus 763: .It Cm Ciphers
764: Specifies the ciphers allowed for protocol version 2
765: in order of preference.
766: Multiple ciphers must be comma-separated.
767: The default is
1.64.2.1 jason 768: .Pp
769: .Bd -literal
1.64.2.3 jason 770: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.64.2.4 jason 771: aes192-cbc,aes256-cbc''
1.64.2.1 jason 772: .Ed
1.64.2.5 miod 773: .It Cm ClearAllForwardings
774: Specifies that all local, remote and dynamic port forwardings
775: specified in the configuration files or on the command line be
776: cleared. This option is primarily useful when used from the
777: .Nm
778: command line to clear port forwardings set in
779: configuration files, and is automatically set by
780: .Xr scp 1
781: and
782: .Xr sftp 1 .
783: The argument must be
784: .Dq yes
785: or
786: .Dq no .
787: The default is
788: .Dq no .
1.2 deraadt 789: .It Cm Compression
1.40 aaron 790: Specifies whether to use compression.
791: The argument must be
1.2 deraadt 792: .Dq yes
793: or
794: .Dq no .
1.64.2.4 jason 795: The default is
796: .Dq no .
1.2 deraadt 797: .It Cm CompressionLevel
1.64.2.4 jason 798: Specifies the compression level to use if compression is enabled.
1.40 aaron 799: The argument must be an integer from 1 (fast) to 9 (slow, best).
800: The default level is 6, which is good for most applications.
801: The meaning of the values is the same as in
1.34 markus 802: .Xr gzip 1 .
1.64.2.4 jason 803: Note that this option applies to protocol version 1 only.
1.2 deraadt 804: .It Cm ConnectionAttempts
1.1 deraadt 805: Specifies the number of tries (one per second) to make before falling
1.40 aaron 806: back to rsh or exiting.
807: The argument must be an integer.
808: This may be useful in scripts if the connection sometimes fails.
1.64.2.5 miod 809: The default is 1.
810: .It Cm DynamicForward
811: Specifies that a TCP/IP port on the local machine be forwarded
812: over the secure channel, and the application
813: protocol is then used to determine where to connect to from the
814: remote machine. The argument must be a port number.
815: Currently the SOCKS4 protocol is supported, and
816: .Nm
817: will act as a SOCKS4 server.
818: Multiple forwardings may be specified, and
819: additional forwardings can be given on the command line. Only
820: the superuser can forward privileged ports.
1.2 deraadt 821: .It Cm EscapeChar
822: Sets the escape character (default:
823: .Ql ~ ) .
824: The escape character can also
1.40 aaron 825: be set on the command line.
826: The argument should be a single character,
1.2 deraadt 827: .Ql ^
828: followed by a letter, or
829: .Dq none
830: to disable the escape
1.1 deraadt 831: character entirely (making the connection transparent for binary
832: data).
1.44 aaron 833: .It Cm FallBackToRsh
1.1 deraadt 834: Specifies that if connecting via
1.2 deraadt 835: .Nm
1.1 deraadt 836: fails due to a connection refused error (there is no
1.2 deraadt 837: .Xr sshd 8
1.44 aaron 838: listening on the remote host),
1.2 deraadt 839: .Xr rsh 1
1.1 deraadt 840: should automatically be used instead (after a suitable warning about
1.40 aaron 841: the session being unencrypted).
842: The argument must be
1.2 deraadt 843: .Dq yes
844: or
845: .Dq no .
1.64.2.4 jason 846: The default is
847: .Dq no .
1.2 deraadt 848: .It Cm ForwardAgent
1.1 deraadt 849: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 850: will be forwarded to the remote machine.
851: The argument must be
1.2 deraadt 852: .Dq yes
853: or
1.54 markus 854: .Dq no .
855: The default is
1.2 deraadt 856: .Dq no .
857: .It Cm ForwardX11
1.1 deraadt 858: Specifies whether X11 connections will be automatically redirected
1.44 aaron 859: over the secure channel and
1.2 deraadt 860: .Ev DISPLAY
1.40 aaron 861: set.
1.44 aaron 862: The argument must be
1.2 deraadt 863: .Dq yes
864: or
1.38 markus 865: .Dq no .
866: The default is
1.3 deraadt 867: .Dq no .
868: .It Cm GatewayPorts
869: Specifies whether remote hosts are allowed to connect to local
870: forwarded ports.
1.64.2.5 miod 871: By default,
872: .Nm
873: binds local port forwardings to the loopback addresss. This
874: prevents other remote hosts from connecting to forwarded ports.
875: .Cm GatewayPorts
876: can be used to specify that
877: .Nm
878: should bind local port forwardings to the wildcard address,
879: thus allowing remote hosts to connect to forwarded ports.
1.3 deraadt 880: The argument must be
881: .Dq yes
882: or
883: .Dq no .
884: The default is
1.2 deraadt 885: .Dq no .
886: .It Cm GlobalKnownHostsFile
1.64.2.5 miod 887: Specifies a file to use for the global
1.64.2.3 jason 888: host key database instead of
1.2 deraadt 889: .Pa /etc/ssh_known_hosts .
1.64.2.4 jason 890: .It Cm HostbasedAuthentication
891: Specifies whether to try rhosts based authentication with public key
892: authentication.
893: The argument must be
894: .Dq yes
895: or
896: .Dq no .
897: The default is
1.64.2.5 miod 898: .Dq no .
1.64.2.4 jason 899: This option applies to protocol version 2 only and
900: is similar to
901: .Cm RhostsRSAAuthentication .
902: .It Cm HostKeyAlgorithms
1.64.2.5 miod 903: Specifies the protocol version 2 host key algorithms
1.64.2.4 jason 904: that the client wants to use in order of preference.
905: The default for this option is:
1.64.2.8! brad 906: .Dq ssh-rsa,ssh-dss .
1.64.2.1 jason 907: .It Cm HostKeyAlias
908: Specifies an alias that should be used instead of the
909: real host name when looking up or saving the host key
1.64.2.4 jason 910: in the host key database files.
1.64.2.1 jason 911: This option is useful for tunneling ssh connections
1.64.2.5 miod 912: or for multiple servers running on a single host.
1.2 deraadt 913: .It Cm HostName
1.40 aaron 914: Specifies the real host name to log into.
915: This can be used to specify nicknames or abbreviations for hosts.
916: Default is the name given on the command line.
917: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 918: .Cm HostName
1.1 deraadt 919: specifications).
1.2 deraadt 920: .It Cm IdentityFile
1.64.2.8! brad 921: Specifies a file from which the user's RSA or DSA authentication identity
! 922: is read. The default is
1.49 markus 923: .Pa $HOME/.ssh/identity
1.64.2.8! brad 924: for protocol version 1, and
! 925: .Pa $HOME/.ssh/id_rsa
! 926: and
! 927: .Pa $HOME/.ssh/id_dsa
! 928: for protocol version 2.
1.1 deraadt 929: Additionally, any identities represented by the authentication agent
1.40 aaron 930: will be used for authentication.
931: The file name may use the tilde
932: syntax to refer to a user's home directory.
933: It is possible to have
1.1 deraadt 934: multiple identity files specified in configuration files; all these
935: identities will be tried in sequence.
1.2 deraadt 936: .It Cm KeepAlive
1.64.2.8! brad 937: Specifies whether the system should send TCP keepalive messages to the
1.40 aaron 938: other side.
939: If they are sent, death of the connection or crash of one
940: of the machines will be properly noticed.
941: However, this means that
1.1 deraadt 942: connections will die if the route is down temporarily, and some people
1.41 aaron 943: find it annoying.
1.2 deraadt 944: .Pp
945: The default is
946: .Dq yes
947: (to send keepalives), and the client will notice
1.40 aaron 948: if the network goes down or the remote host dies.
949: This is important in scripts, and many users want it too.
1.2 deraadt 950: .Pp
951: To disable keepalives, the value should be set to
1.64.2.8! brad 952: .Dq no .
1.2 deraadt 953: .It Cm KerberosAuthentication
1.42 aaron 954: Specifies whether Kerberos authentication will be used.
955: The argument to this keyword must be
1.4 dugsong 956: .Dq yes
957: or
958: .Dq no .
1.2 deraadt 959: .It Cm KerberosTgtPassing
1.42 aaron 960: Specifies whether a Kerberos TGT will be forwarded to the server.
961: This will only work if the Kerberos server is actually an AFS kaserver.
962: The argument to this keyword must be
1.4 dugsong 963: .Dq yes
964: or
965: .Dq no .
1.2 deraadt 966: .It Cm LocalForward
1.1 deraadt 967: Specifies that a TCP/IP port on the local machine be forwarded over
1.64.2.5 miod 968: the secure channel to the specified host and port from the remote machine.
1.40 aaron 969: The first argument must be a port number, and the second must be
1.64.2.5 miod 970: .Ar host:port .
971: IPv6 addresses can be specified with an alternative syntax:
972: .Ar host/port .
1.40 aaron 973: Multiple forwardings may be specified, and additional
974: forwardings can be given on the command line.
975: Only the superuser can forward privileged ports.
1.24 markus 976: .It Cm LogLevel
977: Gives the verbosity level that is used when logging messages from
978: .Nm ssh .
979: The possible values are:
1.64.2.8! brad 980: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
! 981: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
! 982: and DEBUG3 each specify higher levels of verbose output.
1.64.2.1 jason 983: .It Cm MACs
1.64.2.5 miod 984: Specifies the MAC (message authentication code) algorithms
1.64.2.1 jason 985: in order of preference.
986: The MAC algorithm is used in protocol version 2
987: for data integrity protection.
988: Multiple algorithms must be comma-separated.
989: The default is
1.64.2.5 miod 990: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.64.2.6 miod 991: .It Cm NoHostAuthenticationForLocalhost
992: This option can be used if the home directory is shared across machines.
993: In this case localhost will refer to a different machine on each of
994: the machines and the user will get many warnings about changed host keys.
995: However, this option disables host authentication for localhost.
996: The argument to this keyword must be
997: .Dq yes
998: or
999: .Dq no .
1000: The default is to check the host key for localhost.
1.14 dugsong 1001: .It Cm NumberOfPasswordPrompts
1.42 aaron 1002: Specifies the number of password prompts before giving up.
1003: The argument to this keyword must be an integer.
1004: Default is 3.
1.34 markus 1005: .It Cm PasswordAuthentication
1.40 aaron 1006: Specifies whether to use password authentication.
1007: The argument to this keyword must be
1.34 markus 1008: .Dq yes
1009: or
1010: .Dq no .
1.64.2.4 jason 1011: The default is
1012: .Dq yes .
1.2 deraadt 1013: .It Cm Port
1.40 aaron 1014: Specifies the port number to connect on the remote host.
1015: Default is 22.
1.64.2.3 jason 1016: .It Cm PreferredAuthentications
1.64.2.5 miod 1017: Specifies the order in which the client should try protocol 2
1018: authentication methods. This allows a client to prefer one method (e.g.
1.64.2.3 jason 1019: .Cm keyboard-interactive )
1020: over another method (e.g.
1021: .Cm password )
1022: The default for this option is:
1.64.2.8! brad 1023: .Dq hostbased,publickey,keyboard-interactive,password .
1.45 markus 1024: .It Cm Protocol
1025: Specifies the protocol versions
1026: .Nm
1027: should support in order of preference.
1028: The possible values are
1029: .Dq 1
1030: and
1031: .Dq 2 .
1032: Multiple versions must be comma-separated.
1033: The default is
1.64.2.4 jason 1034: .Dq 2,1 .
1.49 markus 1035: This means that
1036: .Nm
1.64.2.4 jason 1037: tries version 2 and falls back to version 1
1038: if version 2 is not available.
1.2 deraadt 1039: .It Cm ProxyCommand
1.40 aaron 1040: Specifies the command to use to connect to the server.
1041: The command
1042: string extends to the end of the line, and is executed with
1043: .Pa /bin/sh .
1044: In the command string,
1045: .Ql %h
1046: will be substituted by the host name to
1047: connect and
1048: .Ql %p
1049: by the port.
1050: The command can be basically anything,
1051: and should read from its standard input and write to its standard output.
1052: It should eventually connect an
1.2 deraadt 1053: .Xr sshd 8
1.1 deraadt 1054: server running on some machine, or execute
1.2 deraadt 1055: .Ic sshd -i
1.40 aaron 1056: somewhere.
1057: Host key management will be done using the
1.1 deraadt 1058: HostName of the host being connected (defaulting to the name typed by
1059: the user).
1.29 markus 1060: Note that
1061: .Cm CheckHostIP
1062: is not available for connects with a proxy command.
1.2 deraadt 1063: .Pp
1.64.2.4 jason 1064: .It Cm PubkeyAuthentication
1065: Specifies whether to try public key authentication.
1066: The argument to this keyword must be
1067: .Dq yes
1068: or
1069: .Dq no .
1070: The default is
1071: .Dq yes .
1072: This option applies to protocol version 2 only.
1.2 deraadt 1073: .It Cm RemoteForward
1.1 deraadt 1074: Specifies that a TCP/IP port on the remote machine be forwarded over
1.64.2.5 miod 1075: the secure channel to the specified host and port from the local machine.
1.40 aaron 1076: The first argument must be a port number, and the second must be
1.64.2.5 miod 1077: .Ar host:port .
1078: IPv6 addresses can be specified with an alternative syntax:
1079: .Ar host/port .
1.40 aaron 1080: Multiple forwardings may be specified, and additional
1081: forwardings can be given on the command line.
1082: Only the superuser can forward privileged ports.
1.2 deraadt 1083: .It Cm RhostsAuthentication
1.40 aaron 1084: Specifies whether to try rhosts based authentication.
1085: Note that this
1.1 deraadt 1086: declaration only affects the client side and has no effect whatsoever
1.40 aaron 1087: on security.
1088: Disabling rhosts authentication may reduce
1.1 deraadt 1089: authentication time on slow connections when rhosts authentication is
1.40 aaron 1090: not used.
1091: Most servers do not permit RhostsAuthentication because it
1.64.2.5 miod 1092: is not secure (see
1093: .Cm RhostsRSAAuthentication ) .
1.40 aaron 1094: The argument to this keyword must be
1.2 deraadt 1095: .Dq yes
1096: or
1097: .Dq no .
1.64.2.4 jason 1098: The default is
1099: .Dq yes .
1100: This option applies to protocol version 1 only.
1.2 deraadt 1101: .It Cm RhostsRSAAuthentication
1.1 deraadt 1102: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 1103: authentication.
1104: The argument must be
1.2 deraadt 1105: .Dq yes
1106: or
1107: .Dq no .
1.64.2.4 jason 1108: The default is
1109: .Dq yes .
1110: This option applies to protocol version 1 only.
1.2 deraadt 1111: .It Cm RSAAuthentication
1.40 aaron 1112: Specifies whether to try RSA authentication.
1113: The argument to this keyword must be
1.2 deraadt 1114: .Dq yes
1115: or
1116: .Dq no .
1.1 deraadt 1117: RSA authentication will only be
1118: attempted if the identity file exists, or an authentication agent is
1119: running.
1.64.2.4 jason 1120: The default is
1121: .Dq yes .
1.50 markus 1122: Note that this option applies to protocol version 1 only.
1.64.2.1 jason 1123: .It Cm ChallengeResponseAuthentication
1124: Specifies whether to use challenge response authentication.
1.40 aaron 1125: The argument to this keyword must be
1.27 markus 1126: .Dq yes
1127: or
1128: .Dq no .
1129: The default is
1.64.2.5 miod 1130: .Dq yes .
1131: .It Cm SmartcardDevice
1132: Specifies which smartcard device to use. The argument to this keyword is
1133: the device
1134: .Nm
1135: should use to communicate with a smartcard used for storing the user's
1136: private RSA key. By default, no device is specified and smartcard support
1137: is not activated.
1.2 deraadt 1138: .It Cm StrictHostKeyChecking
1139: If this flag is set to
1.44 aaron 1140: .Dq yes ,
1.2 deraadt 1141: .Nm
1.64.2.1 jason 1142: will never automatically add host keys to the
1.2 deraadt 1143: .Pa $HOME/.ssh/known_hosts
1.64.2.5 miod 1144: file, and refuses to connect to hosts whose host key has changed.
1145: This provides maximum protection against trojan horse attacks,
1146: however, can be annoying when the
1.2 deraadt 1147: .Pa /etc/ssh_known_hosts
1.64.2.5 miod 1148: file is poorly maintained, or connections to new hosts are
1149: frequently made.
1.64.2.1 jason 1150: This option forces the user to manually
1151: add all new hosts.
1152: If this flag is set to
1153: .Dq no ,
1154: .Nm
1155: will automatically add new host keys to the
1156: user known hosts files.
1157: If this flag is set to
1158: .Dq ask ,
1159: new host keys
1160: will be added to the user known host files only after the user
1161: has confirmed that is what they really want to do, and
1162: .Nm
1163: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1164: The host keys of
1.64.2.1 jason 1165: known hosts will be verified automatically in all cases.
1.40 aaron 1166: The argument must be
1.64.2.1 jason 1167: .Dq yes ,
1168: .Dq no
1.2 deraadt 1169: or
1.64.2.1 jason 1170: .Dq ask .
1171: The default is
1172: .Dq ask .
1.16 markus 1173: .It Cm UsePrivilegedPort
1174: Specifies whether to use a privileged port for outgoing connections.
1175: The argument must be
1176: .Dq yes
1177: or
1178: .Dq no .
1179: The default is
1.64.2.3 jason 1180: .Dq no .
1.64.2.5 miod 1181: Note that this option must be set to
1.64.2.4 jason 1182: .Dq yes
1.64.2.5 miod 1183: if
1.16 markus 1184: .Cm RhostsAuthentication
1185: and
1.64.2.1 jason 1186: .Cm RhostsRSAAuthentication
1.64.2.5 miod 1187: authentications are needed with older servers.
1.34 markus 1188: .It Cm User
1.40 aaron 1189: Specifies the user to log in as.
1.64.2.5 miod 1190: This can be useful when a different user name is used on different machines.
1.40 aaron 1191: This saves the trouble of
1.34 markus 1192: having to remember to give the user name on the command line.
1193: .It Cm UserKnownHostsFile
1.64.2.5 miod 1194: Specifies a file to use for the user
1.64.2.3 jason 1195: host key database instead of
1.34 markus 1196: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 1197: .It Cm UseRsh
1.40 aaron 1198: Specifies that rlogin/rsh should be used for this host.
1199: It is possible that the host does not at all support the
1.2 deraadt 1200: .Nm
1.40 aaron 1201: protocol.
1202: This causes
1.2 deraadt 1203: .Nm
1.40 aaron 1204: to immediately execute
1.2 deraadt 1205: .Xr rsh 1 .
1.1 deraadt 1206: All other options (except
1.2 deraadt 1207: .Cm HostName )
1.40 aaron 1208: are ignored if this has been specified.
1209: The argument must be
1.2 deraadt 1210: .Dq yes
1211: or
1212: .Dq no .
1.55 markus 1213: .It Cm XAuthLocation
1214: Specifies the location of the
1215: .Xr xauth 1
1216: program.
1217: The default is
1218: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1219: .El
1.2 deraadt 1220: .Sh ENVIRONMENT
1221: .Nm
1.1 deraadt 1222: will normally set the following environment variables:
1.2 deraadt 1223: .Bl -tag -width Ds
1224: .It Ev DISPLAY
1225: The
1226: .Ev DISPLAY
1.40 aaron 1227: variable indicates the location of the X11 server.
1.44 aaron 1228: It is automatically set by
1.2 deraadt 1229: .Nm
1230: to point to a value of the form
1231: .Dq hostname:n
1232: where hostname indicates
1.40 aaron 1233: the host where the shell runs, and n is an integer >= 1.
1234: .Nm
1235: uses this special value to forward X11 connections over the secure
1236: channel.
1.64.2.4 jason 1237: The user should normally not set
1238: .Ev DISPLAY
1239: explicitly, as that
1.1 deraadt 1240: will render the X11 connection insecure (and will require the user to
1241: manually copy any required authorization cookies).
1.2 deraadt 1242: .It Ev HOME
1.1 deraadt 1243: Set to the path of the user's home directory.
1.2 deraadt 1244: .It Ev LOGNAME
1245: Synonym for
1.12 aaron 1246: .Ev USER ;
1247: set for compatibility with systems that use this variable.
1.2 deraadt 1248: .It Ev MAIL
1.64.2.5 miod 1249: Set to the path of the user's mailbox.
1.40 aaron 1250: .It Ev PATH
1.2 deraadt 1251: Set to the default
1252: .Ev PATH ,
1253: as specified when compiling
1.12 aaron 1254: .Nm ssh .
1.64.2.5 miod 1255: .It Ev SSH_ASKPASS
1256: If
1257: .Nm
1258: needs a passphrase, it will read the passphrase from the current
1259: terminal if it was run from a terminal.
1260: If
1261: .Nm
1262: does not have a terminal associated with it but
1263: .Ev DISPLAY
1264: and
1265: .Ev SSH_ASKPASS
1266: are set, it will execute the program specified by
1267: .Ev SSH_ASKPASS
1268: and open an X11 window to read the passphrase.
1269: This is particularly useful when calling
1270: .Nm
1271: from a
1272: .Pa .Xsession
1273: or related script.
1274: (Note that on some machines it
1275: may be necessary to redirect the input from
1276: .Pa /dev/null
1277: to make this work.)
1.18 markus 1278: .It Ev SSH_AUTH_SOCK
1.64.2.5 miod 1279: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 1280: agent.
1.2 deraadt 1281: .It Ev SSH_CLIENT
1.40 aaron 1282: Identifies the client end of the connection.
1283: The variable contains
1.1 deraadt 1284: three space-separated values: client ip-address, client port number,
1285: and server port number.
1.64.2.1 jason 1286: .It Ev SSH_ORIGINAL_COMMAND
1287: The variable contains the original command line if a forced command
1288: is executed.
1289: It can be used to extract the original arguments.
1.2 deraadt 1290: .It Ev SSH_TTY
1.1 deraadt 1291: This is set to the name of the tty (path to the device) associated
1.40 aaron 1292: with the current shell or command.
1293: If the current session has no tty,
1.1 deraadt 1294: this variable is not set.
1.2 deraadt 1295: .It Ev TZ
1.1 deraadt 1296: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1297: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1298: on to new connections).
1.2 deraadt 1299: .It Ev USER
1.1 deraadt 1300: Set to the name of the user logging in.
1.2 deraadt 1301: .El
1302: .Pp
1.44 aaron 1303: Additionally,
1.2 deraadt 1304: .Nm
1.44 aaron 1305: reads
1306: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1307: and adds lines of the format
1308: .Dq VARNAME=value
1.12 aaron 1309: to the environment.
1.2 deraadt 1310: .Sh FILES
1.36 markus 1311: .Bl -tag -width Ds
1.64.2.5 miod 1312: .It Pa $HOME/.ssh/known_hosts
1313: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 1314: in
1.64.2.5 miod 1315: .Pa /etc/ssh_known_hosts .
1.2 deraadt 1316: See
1317: .Xr sshd 8 .
1.64.2.4 jason 1318: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1319: Contains the authentication identity of the user.
1320: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1321: These files
1322: contain sensitive data and should be readable by the user but not
1.15 markus 1323: accessible by others (read/write/execute).
1324: Note that
1325: .Nm
1.48 markus 1326: ignores a private key file if it is accessible by others.
1.15 markus 1327: It is possible to specify a passphrase when
1.1 deraadt 1328: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1329: sensitive part of this file using 3DES.
1.64.2.4 jason 1330: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1331: Contains the public key for authentication (public part of the
1.40 aaron 1332: identity file in human-readable form).
1.48 markus 1333: The contents of the
1334: .Pa $HOME/.ssh/identity.pub
1335: file should be added to
1.2 deraadt 1336: .Pa $HOME/.ssh/authorized_keys
1337: on all machines
1.64.2.5 miod 1338: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 1339: The contents of the
1340: .Pa $HOME/.ssh/id_dsa.pub
1.64.2.4 jason 1341: and
1342: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1343: file should be added to
1.64.2.5 miod 1344: .Pa $HOME/.ssh/authorized_keys
1.48 markus 1345: on all machines
1.64.2.5 miod 1346: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1347: These files are not
1.40 aaron 1348: sensitive and can (but need not) be readable by anyone.
1.48 markus 1349: These files are
1.64.2.1 jason 1350: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1351: the convenience of the user.
1.2 deraadt 1352: .It Pa $HOME/.ssh/config
1.40 aaron 1353: This is the per-user configuration file.
1354: The format of this file is described above.
1355: This file is used by the
1.2 deraadt 1356: .Nm
1.40 aaron 1357: client.
1358: This file does not usually contain any sensitive information,
1.1 deraadt 1359: but the recommended permissions are read/write for the user, and not
1360: accessible by others.
1.2 deraadt 1361: .It Pa $HOME/.ssh/authorized_keys
1.64.2.5 miod 1362: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 1363: The format of this file is described in the
1.2 deraadt 1364: .Xr sshd 8
1.40 aaron 1365: manual page.
1366: In the simplest form the format is the same as the .pub
1.64.2.5 miod 1367: identity files.
1.40 aaron 1368: This file is not highly sensitive, but the recommended
1.1 deraadt 1369: permissions are read/write for the user, and not accessible by others.
1.64.2.5 miod 1370: .It Pa /etc/ssh_known_hosts
1.40 aaron 1371: Systemwide list of known host keys.
1.64.2.5 miod 1372: This file should be prepared by the
1.1 deraadt 1373: system administrator to contain the public host keys of all machines in the
1.40 aaron 1374: organization.
1375: This file should be world-readable.
1376: This file contains
1.1 deraadt 1377: public keys, one per line, in the following format (fields separated
1.64.2.5 miod 1378: by spaces): system name, public key and optional comment field.
1.40 aaron 1379: When different names are used
1.1 deraadt 1380: for the same machine, all such names should be listed, separated by
1.40 aaron 1381: commas.
1382: The format is described on the
1.2 deraadt 1383: .Xr sshd 8
1.1 deraadt 1384: manual page.
1.2 deraadt 1385: .Pp
1.1 deraadt 1386: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1387: .Xr sshd 8
1.1 deraadt 1388: to verify the client host when logging in; other names are needed because
1.2 deraadt 1389: .Nm
1.1 deraadt 1390: does not convert the user-supplied name to a canonical name before
1391: checking the key, because someone with access to the name servers
1392: would then be able to fool host authentication.
1.2 deraadt 1393: .It Pa /etc/ssh_config
1.40 aaron 1394: Systemwide configuration file.
1395: This file provides defaults for those
1.1 deraadt 1396: values that are not specified in the user's configuration file, and
1.40 aaron 1397: for those users who do not have a configuration file.
1398: This file must be world-readable.
1.64.2.7 miod 1399: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1400: These three files contain the private parts of the host keys
1401: and are used for
1402: .Cm RhostsRSAAuthentication
1403: and
1404: .Cm HostbasedAuthentication .
1405: Since they are readable only by root
1406: .Nm
1407: must be setuid root if these authentication methods are desired.
1.2 deraadt 1408: .It Pa $HOME/.rhosts
1409: This file is used in
1410: .Pa \&.rhosts
1411: authentication to list the
1.40 aaron 1412: host/user pairs that are permitted to log in.
1413: (Note that this file is
1.1 deraadt 1414: also used by rlogin and rsh, which makes using this file insecure.)
1415: Each line of the file contains a host name (in the canonical form
1416: returned by name servers), and then a user name on that host,
1.40 aaron 1417: separated by a space.
1.64.2.3 jason 1418: On some machines this file may need to be
1.1 deraadt 1419: world-readable if the user's home directory is on a NFS partition,
1420: because
1.2 deraadt 1421: .Xr sshd 8
1.40 aaron 1422: reads it as root.
1423: Additionally, this file must be owned by the user,
1424: and must not have write permissions for anyone else.
1425: The recommended
1.1 deraadt 1426: permission for most machines is read/write for the user, and not
1427: accessible by others.
1.2 deraadt 1428: .Pp
1.1 deraadt 1429: Note that by default
1.2 deraadt 1430: .Xr sshd 8
1.1 deraadt 1431: will be installed so that it requires successful RSA host
1.40 aaron 1432: authentication before permitting \s+2.\s0rhosts authentication.
1.64.2.5 miod 1433: If the server machine does not have the client's host key in
1.2 deraadt 1434: .Pa /etc/ssh_known_hosts ,
1.64.2.5 miod 1435: it can be stored in
1.2 deraadt 1436: .Pa $HOME/.ssh/known_hosts .
1437: The easiest way to do this is to
1.1 deraadt 1438: connect back to the client from the server machine using ssh; this
1.48 markus 1439: will automatically add the host key to
1.2 deraadt 1440: .Pa $HOME/.ssh/known_hosts .
1441: .It Pa $HOME/.shosts
1442: This file is used exactly the same way as
1443: .Pa \&.rhosts .
1444: The purpose for
1.1 deraadt 1445: having this file is to be able to use rhosts authentication with
1.2 deraadt 1446: .Nm
1447: without permitting login with
1448: .Xr rlogin 1
1449: or
1450: .Xr rsh 1 .
1451: .It Pa /etc/hosts.equiv
1452: This file is used during
1.40 aaron 1453: .Pa \&.rhosts authentication.
1454: It contains
1.1 deraadt 1455: canonical hosts names, one per line (the full format is described on
1456: the
1.2 deraadt 1457: .Xr sshd 8
1.40 aaron 1458: manual page).
1459: If the client host is found in this file, login is
1.1 deraadt 1460: automatically permitted provided client and server user names are the
1.40 aaron 1461: same.
1462: Additionally, successful RSA host authentication is normally
1463: required.
1464: This file should only be writable by root.
1.2 deraadt 1465: .It Pa /etc/shosts.equiv
1.44 aaron 1466: This file is processed exactly as
1.2 deraadt 1467: .Pa /etc/hosts.equiv .
1.1 deraadt 1468: This file may be useful to permit logins using
1.2 deraadt 1469: .Nm
1.1 deraadt 1470: but not using rsh/rlogin.
1.2 deraadt 1471: .It Pa /etc/sshrc
1.1 deraadt 1472: Commands in this file are executed by
1.2 deraadt 1473: .Nm
1.1 deraadt 1474: when the user logs in just before the user's shell (or command) is started.
1475: See the
1.2 deraadt 1476: .Xr sshd 8
1.1 deraadt 1477: manual page for more information.
1.2 deraadt 1478: .It Pa $HOME/.ssh/rc
1.1 deraadt 1479: Commands in this file are executed by
1.2 deraadt 1480: .Nm
1.1 deraadt 1481: when the user logs in just before the user's shell (or command) is
1482: started.
1.44 aaron 1483: See the
1.2 deraadt 1484: .Xr sshd 8
1.1 deraadt 1485: manual page for more information.
1.31 markus 1486: .It Pa $HOME/.ssh/environment
1487: Contains additional definitions for environment variables, see section
1488: .Sx ENVIRONMENT
1489: above.
1.58 itojun 1490: .El
1.64.2.8! brad 1491: .Sh DIAGNOSTICS
! 1492: .Nm
! 1493: exits with the exit status of the remote command or with 255
! 1494: if an error occurred.
1.64.2.1 jason 1495: .Sh AUTHORS
1496: OpenSSH is a derivative of the original and free
1497: ssh 1.2.12 release by Tatu Ylonen.
1498: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1499: Theo de Raadt and Dug Song
1500: removed many bugs, re-added newer features and
1501: created OpenSSH.
1502: Markus Friedl contributed the support for SSH
1503: protocol versions 1.5 and 2.0.
1.2 deraadt 1504: .Sh SEE ALSO
1505: .Xr rlogin 1 ,
1506: .Xr rsh 1 ,
1507: .Xr scp 1 ,
1.64.2.1 jason 1508: .Xr sftp 1 ,
1.2 deraadt 1509: .Xr ssh-add 1 ,
1510: .Xr ssh-agent 1 ,
1511: .Xr ssh-keygen 1 ,
1512: .Xr telnet 1 ,
1.64.2.1 jason 1513: .Xr sshd 8
1.64.2.4 jason 1514: .Rs
1515: .%A T. Ylonen
1516: .%A T. Kivinen
1517: .%A M. Saarinen
1518: .%A T. Rinne
1519: .%A S. Lehtinen
1520: .%T "SSH Protocol Architecture"
1.64.2.5 miod 1521: .%N draft-ietf-secsh-architecture-09.txt
1522: .%D July 2001
1.64.2.4 jason 1523: .%O work in progress material
1524: .Re
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