Annotation of src/usr.bin/ssh/ssh.1, Revision 1.107.2.1
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: .\"
1.93 deraadt 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
33: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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: .\"
1.107.2.1! jason 37: .\" $OpenBSD: ssh.1,v 1.138 2001/09/19 19:24:19 stevesk Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.96 deraadt 43: .Nd OpenSSH SSH client (remote login program)
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm ssh
46: .Op Fl l Ar login_name
1.107.2.1! jason 47: .Ar hostname | user@hostname
1.2 deraadt 48: .Op Ar command
49: .Pp
50: .Nm ssh
1.86 jakob 51: .Op Fl afgknqstvxACNPTX1246
1.107.2.1! jason 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
1.91 markus 57: .Op Fl m Ar mac_spec
1.2 deraadt 58: .Op Fl o Ar option
59: .Op Fl p Ar port
1.107.2.1! jason 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
1.107.2.1! jason 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
1.96 deraadt 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
1.107 markus 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
210: When a user connects using the protocol version 2
1.107 markus 211: different authentication methods are available.
212: Using the default values for
213: .Cm PreferredAuthentications ,
1.107.2.1! jason 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
1.107 markus 220: in the previous section and allows the RSA or DSA algorithm to be used:
1.102 itojun 221: The client uses his private key,
1.49 markus 222: .Pa $HOME/.ssh/id_dsa
1.102 itojun 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
1.107.2.1! jason 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.
1.107 markus 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)
1.94 deraadt 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
1.104 djm 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
1.71 djm 263: The session terminates when the command or shell on the remote
1.92 markus 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 .
1.104 djm 268: .Pp
269: .Ss Escape Characters
270: .Pp
271: When a pseudo terminal has been requested, ssh supports a number of functions
1.107.2.1! jason 272: through the use of an escape character.
1.104 djm 273: .Pp
274: A single tilde character can be sent as
275: .Ic ~~
1.107.2.1! jason 276: or by following the tilde by a character other than those described below.
1.104 djm 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
1.107.2.1! jason 281: configuration directive or on the command line by the
1.104 djm 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
1.107 markus 297: to terminate (protocol version 1 only)
1.104 djm 298: .It Cm ~?
299: Display a list of escape characters
300: .It Cm ~R
1.105 djm 301: Request rekeying of the connection (only useful for SSH protocol version 2
302: and if the peer supports it)
1.104 djm 303: .El
1.2 deraadt 304: .Pp
1.49 markus 305: .Ss X11 and TCP forwarding
306: .Pp
1.107.2.1! jason 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
1.107.2.1! jason 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
1.107.2.1! jason 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
1.92 markus 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.
1.107.2.1! jason 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.
1.107.2.1! jason 367: Additionally, the file
1.2 deraadt 368: .Pa /etc/ssh_known_hosts
1.107.2.1! jason 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.
1.65 aaron 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.
1.107.2.1! jason 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 .
1.107.2.1! jason 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.
1.90 markus 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.
1.90 markus 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
1.44 aaron 446: Selects the file from which the identity (private key) for
1.68 markus 447: RSA or DSA authentication is read.
1.44 aaron 448: Default is
1.49 markus 449: .Pa $HOME/.ssh/identity
1.40 aaron 450: in the user's home directory.
451: Identity files may also be specified on
452: a per-host basis in the configuration file.
453: It is possible to have multiple
1.2 deraadt 454: .Fl i
455: options (and multiple identities specified in
1.1 deraadt 456: configuration files).
1.107.2.1! jason 457: .It Fl I Ar smartcard_device
! 458: Specifies which smartcard device to use. The argument is
! 459: the device
! 460: .Nm
! 461: should use to communicate with a smartcard used for storing the user's
! 462: private RSA key.
1.2 deraadt 463: .It Fl k
1.42 aaron 464: Disables forwarding of Kerberos tickets and AFS tokens.
465: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 466: .It Fl l Ar login_name
1.40 aaron 467: Specifies the user to log in as on the remote machine.
468: This also may be specified on a per-host basis in the configuration file.
1.91 markus 469: .It Fl m Ar mac_spec
470: Additionally, for protocol version 2 a comma-separated list of MAC
471: (message authentication code) algorithms can
472: be specified in order of preference.
473: See the
474: .Cm MACs
475: keyword for more information.
1.2 deraadt 476: .It Fl n
477: Redirects stdin from
478: .Pa /dev/null
479: (actually, prevents reading from stdin).
1.1 deraadt 480: This must be used when
1.2 deraadt 481: .Nm
1.40 aaron 482: is run in the background.
483: A common trick is to use this to run X11 programs on a remote machine.
484: For example,
1.2 deraadt 485: .Ic ssh -n shadows.cs.hut.fi emacs &
486: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 487: connection will be automatically forwarded over an encrypted channel.
488: The
1.2 deraadt 489: .Nm
1.1 deraadt 490: program will be put in the background.
491: (This does not work if
1.2 deraadt 492: .Nm
493: needs to ask for a password or passphrase; see also the
494: .Fl f
495: option.)
1.53 markus 496: .It Fl N
497: Do not execute a remote command.
1.107.2.1! jason 498: This is useful for just forwarding ports
1.53 markus 499: (protocol version 2 only).
1.2 deraadt 500: .It Fl o Ar option
1.107.2.1! jason 501: Can be used to give options in the format used in the configuration file.
1.1 deraadt 502: This is useful for specifying options for which there is no separate
1.40 aaron 503: command-line flag.
1.2 deraadt 504: .It Fl p Ar port
1.40 aaron 505: Port to connect to on the remote host.
506: This can be specified on a
1.1 deraadt 507: per-host basis in the configuration file.
1.16 markus 508: .It Fl P
509: Use a non-privileged port for outgoing connections.
1.107.2.1! jason 510: This can be used if a firewall does
1.16 markus 511: not permit connections from privileged ports.
1.30 provos 512: Note that this option turns off
1.16 markus 513: .Cm RhostsAuthentication
514: and
1.72 markus 515: .Cm RhostsRSAAuthentication
516: for older servers.
1.2 deraadt 517: .It Fl q
1.40 aaron 518: Quiet mode.
519: Causes all warning and diagnostic messages to be suppressed.
520: Only fatal errors are displayed.
1.80 djm 521: .It Fl s
1.107.2.1! jason 522: 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
! 523: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 524: subsystem is specified as the remote command.
1.2 deraadt 525: .It Fl t
1.40 aaron 526: Force pseudo-tty allocation.
1.43 brad 527: This can be used to execute arbitrary
1.40 aaron 528: screen-based programs on a remote machine, which can be very useful,
529: e.g., when implementing menu services.
1.73 markus 530: Multiple
531: .Fl t
532: options force tty allocation, even if
533: .Nm
534: has no local tty.
1.53 markus 535: .It Fl T
1.69 markus 536: Disable pseudo-tty allocation.
1.2 deraadt 537: .It Fl v
1.40 aaron 538: Verbose mode.
539: Causes
1.2 deraadt 540: .Nm
1.40 aaron 541: to print debugging messages about its progress.
542: This is helpful in
1.1 deraadt 543: debugging connection, authentication, and configuration problems.
1.73 markus 544: Multiple
545: .Fl v
546: options increases the verbosity.
1.61 aaron 547: Maximum is 3.
1.2 deraadt 548: .It Fl x
1.40 aaron 549: Disables X11 forwarding.
1.2 deraadt 550: .It Fl X
1.1 deraadt 551: Enables X11 forwarding.
1.54 markus 552: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 553: .It Fl C
1.1 deraadt 554: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 555: data for forwarded X11 and TCP/IP connections).
556: The compression algorithm is the same used by
1.34 markus 557: .Xr gzip 1 ,
558: and the
1.2 deraadt 559: .Dq level
560: can be controlled by the
561: .Cm CompressionLevel
1.40 aaron 562: option (see below).
563: Compression is desirable on modem lines and other
1.1 deraadt 564: slow connections, but will only slow down things on fast networks.
565: The default value can be set on a host-by-host basis in the
566: configuration files; see the
1.107.2.1! jason 567: .Cm Compression
1.1 deraadt 568: option below.
1.107.2.1! jason 569: .It Fl F Ar configfile
! 570: Specifies an alternative per-user configuration file.
! 571: If a configuration file is given on the command line,
! 572: the system-wide configuration file
! 573: .Pq Pa /etc/ssh_config
! 574: will be ignored.
! 575: The default for the per-user configuration file is
! 576: .Pa $HOME/.ssh/config .
1.2 deraadt 577: .It Fl L Ar port:host:hostport
1.1 deraadt 578: Specifies that the given port on the local (client) host is to be
1.40 aaron 579: forwarded to the given host and port on the remote side.
580: This works by allocating a socket to listen to
1.2 deraadt 581: .Ar port
1.1 deraadt 582: on the local side, and whenever a connection is made to this port, the
583: connection is forwarded over the secure channel, and a connection is
584: made to
1.32 markus 585: .Ar host
586: port
587: .Ar hostport
1.40 aaron 588: from the remote machine.
589: Port forwardings can also be specified in the configuration file.
590: Only root can forward privileged ports.
1.32 markus 591: IPv6 addresses can be specified with an alternative syntax:
592: .Ar port/host/hostport
1.2 deraadt 593: .It Fl R Ar port:host:hostport
1.1 deraadt 594: Specifies that the given port on the remote (server) host is to be
1.40 aaron 595: forwarded to the given host and port on the local side.
596: This works by allocating a socket to listen to
1.2 deraadt 597: .Ar port
1.1 deraadt 598: on the remote side, and whenever a connection is made to this port, the
599: connection is forwarded over the secure channel, and a connection is
600: made to
1.32 markus 601: .Ar host
602: port
603: .Ar hostport
1.40 aaron 604: from the local machine.
605: Port forwardings can also be specified in the configuration file.
606: Privileged ports can be forwarded only when
1.1 deraadt 607: logging in as root on the remote machine.
1.107 markus 608: IPv6 addresses can be specified with an alternative syntax:
609: .Ar port/host/hostport
1.107.2.1! jason 610: .It Fl D Ar port
! 611: Specifies a local
! 612: .Dq dynamic
! 613: application-level port forwarding.
! 614: This works by allocating a socket to listen to
! 615: .Ar port
! 616: on the local side, and whenever a connection is made to this port, the
! 617: connection is forwarded over the secure channel, and the application
! 618: protocol is then used to determine where to connect to from the
! 619: remote machine. Currently the SOCKS4 protocol is supported, and
! 620: .Nm
! 621: will act as a SOCKS4 server.
! 622: Only root can forward privileged ports.
! 623: Dynamic port forwardings can also be specified in the configuration file.
1.85 jakob 624: .It Fl 1
625: Forces
626: .Nm
627: to try protocol version 1 only.
1.46 markus 628: .It Fl 2
629: Forces
630: .Nm
1.50 markus 631: to try protocol version 2 only.
1.32 markus 632: .It Fl 4
633: Forces
634: .Nm
635: to use IPv4 addresses only.
636: .It Fl 6
637: Forces
638: .Nm
639: to use IPv6 addresses only.
1.2 deraadt 640: .El
641: .Sh CONFIGURATION FILES
642: .Nm
1.107.2.1! jason 643: obtains configuration data from the following sources in
! 644: the following order:
1.1 deraadt 645: command line options, user's configuration file
1.2 deraadt 646: .Pq Pa $HOME/.ssh/config ,
647: and system-wide configuration file
648: .Pq Pa /etc/ssh_config .
649: For each parameter, the first obtained value
1.40 aaron 650: will be used.
651: The configuration files contain sections bracketed by
652: .Dq Host
653: specifications, and that section is only applied for hosts that
654: match one of the patterns given in the specification.
655: The matched host name is the one given on the command line.
1.2 deraadt 656: .Pp
1.1 deraadt 657: Since the first obtained value for each parameter is used, more
658: host-specific declarations should be given near the beginning of the
659: file, and general defaults at the end.
1.2 deraadt 660: .Pp
1.1 deraadt 661: The configuration file has the following format:
1.2 deraadt 662: .Pp
663: Empty lines and lines starting with
664: .Ql #
665: are comments.
666: .Pp
667: Otherwise a line is of the format
668: .Dq keyword arguments .
1.107.2.1! jason 669: Configuration options may be separated by whitespace or
! 670: optional whitespace and exactly one
! 671: .Ql = ;
! 672: the latter format is useful to avoid the need to quote whitespace
! 673: when specifying configuration options using the
! 674: .Nm ssh ,
! 675: .Nm scp
! 676: and
! 677: .Nm sftp
! 678: .Fl o
! 679: option.
! 680: .Pp
1.2 deraadt 681: The possible
1.107.2.1! jason 682: keywords and their meanings are as follows (note that
! 683: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 684: .Bl -tag -width Ds
685: .It Cm Host
1.1 deraadt 686: Restricts the following declarations (up to the next
1.2 deraadt 687: .Cm Host
1.1 deraadt 688: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 689: given after the keyword.
690: .Ql \&*
691: and
692: .Ql ?
693: can be used as wildcards in the
1.40 aaron 694: patterns.
695: A single
1.2 deraadt 696: .Ql \&*
697: as a pattern can be used to provide global
1.40 aaron 698: defaults for all hosts.
699: The host is the
1.2 deraadt 700: .Ar hostname
1.1 deraadt 701: argument given on the command line (i.e., the name is not converted to
702: a canonicalized host name before matching).
1.2 deraadt 703: .It Cm AFSTokenPassing
1.42 aaron 704: Specifies whether to pass AFS tokens to remote host.
705: The argument to this keyword must be
1.2 deraadt 706: .Dq yes
707: or
708: .Dq no .
1.107 markus 709: This option applies to protocol version 1 only.
1.2 deraadt 710: .It Cm BatchMode
711: If set to
712: .Dq yes ,
1.40 aaron 713: passphrase/password querying will be disabled.
1.107.2.1! jason 714: This option is useful in scripts and other batch jobs where no user
! 715: is present to supply the password.
1.40 aaron 716: The argument must be
1.2 deraadt 717: .Dq yes
718: or
719: .Dq no .
1.100 stevesk 720: The default is
721: .Dq no .
1.107.2.1! jason 722: .It Cm BindAddress
! 723: Specify the interface to transmit from on machines with multiple
! 724: interfaces or aliased addresses.
! 725: Note that this option does not work if
! 726: .Cm UsePrivilegedPort
! 727: is set to
! 728: .Dq yes .
1.34 markus 729: .It Cm CheckHostIP
730: If this flag is set to
731: .Dq yes ,
1.100 stevesk 732: ssh will additionally check the host IP address in the
1.34 markus 733: .Pa known_hosts
1.42 aaron 734: file.
735: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 736: If the option is set to
737: .Dq no ,
738: the check will not be executed.
1.100 stevesk 739: The default is
740: .Dq yes .
1.2 deraadt 741: .It Cm Cipher
1.62 markus 742: Specifies the cipher to use for encrypting the session
1.64 markus 743: in protocol version 1.
1.40 aaron 744: Currently,
1.107.2.1! jason 745: .Dq blowfish ,
! 746: .Dq 3des ,
1.1 deraadt 747: and
1.107.2.1! jason 748: .Dq des
1.40 aaron 749: are supported.
1.107.2.1! jason 750: .Ar des
! 751: is only supported in the
! 752: .Nm
! 753: client for interoperability with legacy protocol 1 implementations
! 754: that do not support the
! 755: .Ar 3des
! 756: cipher. Its use is strongly discouraged due to cryptographic
! 757: weaknesses.
1.40 aaron 758: The default is
1.2 deraadt 759: .Dq 3des .
1.45 markus 760: .It Cm Ciphers
761: Specifies the ciphers allowed for protocol version 2
762: in order of preference.
763: Multiple ciphers must be comma-separated.
764: The default is
1.88 provos 765: .Pp
766: .Bd -literal
1.94 deraadt 767: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.107 markus 768: aes192-cbc,aes256-cbc''
1.88 provos 769: .Ed
1.107.2.1! jason 770: .It Cm ClearAllForwardings
! 771: Specifies that all local, remote and dynamic port forwardings
! 772: specified in the configuration files or on the command line be
! 773: cleared. This option is primarily useful when used from the
! 774: .Nm
! 775: command line to clear port forwardings set in
! 776: configuration files, and is automatically set by
! 777: .Xr scp 1
! 778: and
! 779: .Xr sftp 1 .
! 780: The argument must be
! 781: .Dq yes
! 782: or
! 783: .Dq no .
! 784: The default is
! 785: .Dq no .
1.2 deraadt 786: .It Cm Compression
1.40 aaron 787: Specifies whether to use compression.
788: The argument must be
1.2 deraadt 789: .Dq yes
790: or
791: .Dq no .
1.100 stevesk 792: The default is
793: .Dq no .
1.2 deraadt 794: .It Cm CompressionLevel
1.100 stevesk 795: Specifies the compression level to use if compression is enabled.
1.40 aaron 796: The argument must be an integer from 1 (fast) to 9 (slow, best).
797: The default level is 6, which is good for most applications.
798: The meaning of the values is the same as in
1.34 markus 799: .Xr gzip 1 .
1.107 markus 800: Note that this option applies to protocol version 1 only.
1.2 deraadt 801: .It Cm ConnectionAttempts
1.1 deraadt 802: Specifies the number of tries (one per second) to make before falling
1.40 aaron 803: back to rsh or exiting.
804: The argument must be an integer.
805: This may be useful in scripts if the connection sometimes fails.
1.107.2.1! jason 806: The default is 1.
! 807: .It Cm DynamicForward
! 808: Specifies that a TCP/IP port on the local machine be forwarded
! 809: over the secure channel, and the application
! 810: protocol is then used to determine where to connect to from the
! 811: remote machine. The argument must be a port number.
! 812: Currently the SOCKS4 protocol is supported, and
! 813: .Nm
! 814: will act as a SOCKS4 server.
! 815: Multiple forwardings may be specified, and
! 816: additional forwardings can be given on the command line. Only
! 817: the superuser can forward privileged ports.
1.2 deraadt 818: .It Cm EscapeChar
819: Sets the escape character (default:
820: .Ql ~ ) .
821: The escape character can also
1.40 aaron 822: be set on the command line.
823: The argument should be a single character,
1.2 deraadt 824: .Ql ^
825: followed by a letter, or
826: .Dq none
827: to disable the escape
1.1 deraadt 828: character entirely (making the connection transparent for binary
829: data).
1.44 aaron 830: .It Cm FallBackToRsh
1.1 deraadt 831: Specifies that if connecting via
1.2 deraadt 832: .Nm
1.1 deraadt 833: fails due to a connection refused error (there is no
1.2 deraadt 834: .Xr sshd 8
1.44 aaron 835: listening on the remote host),
1.2 deraadt 836: .Xr rsh 1
1.1 deraadt 837: should automatically be used instead (after a suitable warning about
1.40 aaron 838: the session being unencrypted).
839: The argument must be
1.2 deraadt 840: .Dq yes
841: or
842: .Dq no .
1.100 stevesk 843: The default is
844: .Dq no .
1.2 deraadt 845: .It Cm ForwardAgent
1.1 deraadt 846: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 847: will be forwarded to the remote machine.
848: The argument must be
1.2 deraadt 849: .Dq yes
850: or
1.54 markus 851: .Dq no .
852: The default is
1.2 deraadt 853: .Dq no .
854: .It Cm ForwardX11
1.1 deraadt 855: Specifies whether X11 connections will be automatically redirected
1.44 aaron 856: over the secure channel and
1.2 deraadt 857: .Ev DISPLAY
1.40 aaron 858: set.
1.44 aaron 859: The argument must be
1.2 deraadt 860: .Dq yes
861: or
1.38 markus 862: .Dq no .
863: The default is
1.3 deraadt 864: .Dq no .
865: .It Cm GatewayPorts
866: Specifies whether remote hosts are allowed to connect to local
867: forwarded ports.
1.107.2.1! jason 868: By default,
! 869: .Nm
! 870: binds local port forwardings to the loopback addresss. This
! 871: prevents other remote hosts from connecting to forwarded ports.
! 872: .Cm GatewayPorts
! 873: can be used to specify that
! 874: .Nm
! 875: should bind local port forwardings to the wildcard address,
! 876: thus allowing remote hosts to connect to forwarded ports.
1.3 deraadt 877: The argument must be
878: .Dq yes
879: or
880: .Dq no .
881: The default is
1.2 deraadt 882: .Dq no .
883: .It Cm GlobalKnownHostsFile
1.107.2.1! jason 884: Specifies a file to use for the global
1.95 stevesk 885: host key database instead of
1.2 deraadt 886: .Pa /etc/ssh_known_hosts .
1.107 markus 887: .It Cm HostbasedAuthentication
888: Specifies whether to try rhosts based authentication with public key
889: authentication.
890: The argument must be
891: .Dq yes
892: or
893: .Dq no .
894: The default is
1.107.2.1! jason 895: .Dq no .
1.107 markus 896: This option applies to protocol version 2 only and
897: is similar to
898: .Cm RhostsRSAAuthentication .
899: .It Cm HostKeyAlgorithms
1.107.2.1! jason 900: Specifies the protocol version 2 host key algorithms
1.107 markus 901: that the client wants to use in order of preference.
902: The default for this option is:
903: .Dq ssh-rsa,ssh-dss
1.74 markus 904: .It Cm HostKeyAlias
905: Specifies an alias that should be used instead of the
906: real host name when looking up or saving the host key
1.107 markus 907: in the host key database files.
1.82 stevesk 908: This option is useful for tunneling ssh connections
1.107.2.1! jason 909: or for multiple servers running on a single host.
1.2 deraadt 910: .It Cm HostName
1.40 aaron 911: Specifies the real host name to log into.
912: This can be used to specify nicknames or abbreviations for hosts.
913: Default is the name given on the command line.
914: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 915: .Cm HostName
1.1 deraadt 916: specifications).
1.2 deraadt 917: .It Cm IdentityFile
1.107 markus 918: Specifies the file from which the user's RSA or DSA authentication identity
1.2 deraadt 919: is read (default
1.49 markus 920: .Pa $HOME/.ssh/identity
1.2 deraadt 921: in the user's home directory).
1.1 deraadt 922: Additionally, any identities represented by the authentication agent
1.40 aaron 923: will be used for authentication.
924: The file name may use the tilde
925: syntax to refer to a user's home directory.
926: It is possible to have
1.1 deraadt 927: multiple identity files specified in configuration files; all these
928: identities will be tried in sequence.
1.2 deraadt 929: .It Cm KeepAlive
1.1 deraadt 930: Specifies whether the system should send keepalive messages to the
1.40 aaron 931: other side.
932: If they are sent, death of the connection or crash of one
933: of the machines will be properly noticed.
934: However, this means that
1.1 deraadt 935: connections will die if the route is down temporarily, and some people
1.41 aaron 936: find it annoying.
1.2 deraadt 937: .Pp
938: The default is
939: .Dq yes
940: (to send keepalives), and the client will notice
1.40 aaron 941: if the network goes down or the remote host dies.
942: This is important in scripts, and many users want it too.
1.2 deraadt 943: .Pp
944: To disable keepalives, the value should be set to
945: .Dq no
946: in both the server and the client configuration files.
947: .It Cm KerberosAuthentication
1.42 aaron 948: Specifies whether Kerberos authentication will be used.
949: The argument to this keyword must be
1.4 dugsong 950: .Dq yes
951: or
952: .Dq no .
1.2 deraadt 953: .It Cm KerberosTgtPassing
1.42 aaron 954: Specifies whether a Kerberos TGT will be forwarded to the server.
955: This will only work if the Kerberos server is actually an AFS kaserver.
956: The argument to this keyword must be
1.4 dugsong 957: .Dq yes
958: or
959: .Dq no .
1.2 deraadt 960: .It Cm LocalForward
1.1 deraadt 961: Specifies that a TCP/IP port on the local machine be forwarded over
1.107.2.1! jason 962: the secure channel to the specified host and port from the remote machine.
1.40 aaron 963: The first argument must be a port number, and the second must be
1.107.2.1! jason 964: .Ar host:port .
! 965: IPv6 addresses can be specified with an alternative syntax:
! 966: .Ar host/port .
1.40 aaron 967: Multiple forwardings may be specified, and additional
968: forwardings can be given on the command line.
969: Only the superuser can forward privileged ports.
1.24 markus 970: .It Cm LogLevel
971: Gives the verbosity level that is used when logging messages from
972: .Nm ssh .
973: The possible values are:
1.77 markus 974: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
975: The default is INFO.
1.91 markus 976: .It Cm MACs
1.107.2.1! jason 977: Specifies the MAC (message authentication code) algorithms
1.91 markus 978: in order of preference.
979: The MAC algorithm is used in protocol version 2
980: for data integrity protection.
981: Multiple algorithms must be comma-separated.
982: The default is
1.107.2.1! jason 983: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.14 dugsong 984: .It Cm NumberOfPasswordPrompts
1.42 aaron 985: Specifies the number of password prompts before giving up.
986: The argument to this keyword must be an integer.
987: Default is 3.
1.34 markus 988: .It Cm PasswordAuthentication
1.40 aaron 989: Specifies whether to use password authentication.
990: The argument to this keyword must be
1.34 markus 991: .Dq yes
992: or
993: .Dq no .
1.100 stevesk 994: The default is
995: .Dq yes .
1.2 deraadt 996: .It Cm Port
1.40 aaron 997: Specifies the port number to connect on the remote host.
998: Default is 22.
1.99 djm 999: .It Cm PreferredAuthentications
1.107.2.1! jason 1000: Specifies the order in which the client should try protocol 2
! 1001: authentication methods. This allows a client to prefer one method (e.g.
1.99 djm 1002: .Cm keyboard-interactive )
1003: over another method (e.g.
1004: .Cm password )
1005: The default for this option is:
1.107.2.1! jason 1006: .Dq hostbased,publickey,keyboard-interactive,password
1.45 markus 1007: .It Cm Protocol
1008: Specifies the protocol versions
1009: .Nm
1010: should support in order of preference.
1011: The possible values are
1012: .Dq 1
1013: and
1014: .Dq 2 .
1015: Multiple versions must be comma-separated.
1016: The default is
1.101 markus 1017: .Dq 2,1 .
1.49 markus 1018: This means that
1019: .Nm
1.101 markus 1020: tries version 2 and falls back to version 1
1021: if version 2 is not available.
1.2 deraadt 1022: .It Cm ProxyCommand
1.40 aaron 1023: Specifies the command to use to connect to the server.
1024: The command
1025: string extends to the end of the line, and is executed with
1026: .Pa /bin/sh .
1027: In the command string,
1028: .Ql %h
1029: will be substituted by the host name to
1030: connect and
1031: .Ql %p
1032: by the port.
1033: The command can be basically anything,
1034: and should read from its standard input and write to its standard output.
1035: It should eventually connect an
1.2 deraadt 1036: .Xr sshd 8
1.1 deraadt 1037: server running on some machine, or execute
1.2 deraadt 1038: .Ic sshd -i
1.40 aaron 1039: somewhere.
1040: Host key management will be done using the
1.1 deraadt 1041: HostName of the host being connected (defaulting to the name typed by
1042: the user).
1.29 markus 1043: Note that
1044: .Cm CheckHostIP
1045: is not available for connects with a proxy command.
1.2 deraadt 1046: .Pp
1.107 markus 1047: .It Cm PubkeyAuthentication
1048: Specifies whether to try public key authentication.
1049: The argument to this keyword must be
1050: .Dq yes
1051: or
1052: .Dq no .
1053: The default is
1054: .Dq yes .
1055: This option applies to protocol version 2 only.
1.2 deraadt 1056: .It Cm RemoteForward
1.1 deraadt 1057: Specifies that a TCP/IP port on the remote machine be forwarded over
1.107.2.1! jason 1058: the secure channel to the specified host and port from the local machine.
1.40 aaron 1059: The first argument must be a port number, and the second must be
1.107.2.1! jason 1060: .Ar host:port .
! 1061: IPv6 addresses can be specified with an alternative syntax:
! 1062: .Ar host/port .
1.40 aaron 1063: Multiple forwardings may be specified, and additional
1064: forwardings can be given on the command line.
1065: Only the superuser can forward privileged ports.
1.2 deraadt 1066: .It Cm RhostsAuthentication
1.40 aaron 1067: Specifies whether to try rhosts based authentication.
1068: Note that this
1.1 deraadt 1069: declaration only affects the client side and has no effect whatsoever
1.40 aaron 1070: on security.
1071: Disabling rhosts authentication may reduce
1.1 deraadt 1072: authentication time on slow connections when rhosts authentication is
1.40 aaron 1073: not used.
1074: Most servers do not permit RhostsAuthentication because it
1.107.2.1! jason 1075: is not secure (see
! 1076: .Cm RhostsRSAAuthentication ) .
1.40 aaron 1077: The argument to this keyword must be
1.2 deraadt 1078: .Dq yes
1079: or
1080: .Dq no .
1.100 stevesk 1081: The default is
1082: .Dq yes .
1.107 markus 1083: This option applies to protocol version 1 only.
1.2 deraadt 1084: .It Cm RhostsRSAAuthentication
1.1 deraadt 1085: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 1086: authentication.
1087: The argument must be
1.2 deraadt 1088: .Dq yes
1089: or
1090: .Dq no .
1.100 stevesk 1091: The default is
1092: .Dq yes .
1.107 markus 1093: This option applies to protocol version 1 only.
1.2 deraadt 1094: .It Cm RSAAuthentication
1.40 aaron 1095: Specifies whether to try RSA authentication.
1096: The argument to this keyword must be
1.2 deraadt 1097: .Dq yes
1098: or
1099: .Dq no .
1.1 deraadt 1100: RSA authentication will only be
1101: attempted if the identity file exists, or an authentication agent is
1102: running.
1.100 stevesk 1103: The default is
1104: .Dq yes .
1.50 markus 1105: Note that this option applies to protocol version 1 only.
1.81 markus 1106: .It Cm ChallengeResponseAuthentication
1107: Specifies whether to use challenge response authentication.
1.40 aaron 1108: The argument to this keyword must be
1.27 markus 1109: .Dq yes
1110: or
1111: .Dq no .
1112: The default is
1.107.2.1! jason 1113: .Dq yes .
! 1114: .It Cm SmartcardDevice
! 1115: Specifies which smartcard device to use. The argument to this keyword is
! 1116: the device
! 1117: .Nm
! 1118: should use to communicate with a smartcard used for storing the user's
! 1119: private RSA key. By default, no device is specified and smartcard support
! 1120: is not activated.
1.2 deraadt 1121: .It Cm StrictHostKeyChecking
1122: If this flag is set to
1.44 aaron 1123: .Dq yes ,
1.2 deraadt 1124: .Nm
1.79 stevesk 1125: will never automatically add host keys to the
1.2 deraadt 1126: .Pa $HOME/.ssh/known_hosts
1.107.2.1! jason 1127: file, and refuses to connect to hosts whose host key has changed.
! 1128: This provides maximum protection against trojan horse attacks,
! 1129: however, can be annoying when the
1.2 deraadt 1130: .Pa /etc/ssh_known_hosts
1.107.2.1! jason 1131: file is poorly maintained, or connections to new hosts are
! 1132: frequently made.
1.79 stevesk 1133: This option forces the user to manually
1134: add all new hosts.
1135: If this flag is set to
1136: .Dq no ,
1137: .Nm
1138: will automatically add new host keys to the
1139: user known hosts files.
1140: If this flag is set to
1141: .Dq ask ,
1142: new host keys
1143: will be added to the user known host files only after the user
1144: has confirmed that is what they really want to do, and
1145: .Nm
1146: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1147: The host keys of
1.79 stevesk 1148: known hosts will be verified automatically in all cases.
1.40 aaron 1149: The argument must be
1.79 stevesk 1150: .Dq yes ,
1151: .Dq no
1.2 deraadt 1152: or
1.79 stevesk 1153: .Dq ask .
1154: The default is
1155: .Dq ask .
1.16 markus 1156: .It Cm UsePrivilegedPort
1157: Specifies whether to use a privileged port for outgoing connections.
1158: The argument must be
1159: .Dq yes
1160: or
1161: .Dq no .
1162: The default is
1.98 markus 1163: .Dq no .
1.107.2.1! jason 1164: Note that this option must be set to
1.107 markus 1165: .Dq yes
1.107.2.1! jason 1166: if
1.16 markus 1167: .Cm RhostsAuthentication
1168: and
1.72 markus 1169: .Cm RhostsRSAAuthentication
1.107.2.1! jason 1170: authentications are needed with older servers.
1.34 markus 1171: .It Cm User
1.40 aaron 1172: Specifies the user to log in as.
1.107.2.1! jason 1173: This can be useful when a different user name is used on different machines.
1.40 aaron 1174: This saves the trouble of
1.34 markus 1175: having to remember to give the user name on the command line.
1176: .It Cm UserKnownHostsFile
1.107.2.1! jason 1177: Specifies a file to use for the user
1.95 stevesk 1178: host key database instead of
1.34 markus 1179: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 1180: .It Cm UseRsh
1.40 aaron 1181: Specifies that rlogin/rsh should be used for this host.
1182: It is possible that the host does not at all support the
1.2 deraadt 1183: .Nm
1.40 aaron 1184: protocol.
1185: This causes
1.2 deraadt 1186: .Nm
1.40 aaron 1187: to immediately execute
1.2 deraadt 1188: .Xr rsh 1 .
1.1 deraadt 1189: All other options (except
1.2 deraadt 1190: .Cm HostName )
1.40 aaron 1191: are ignored if this has been specified.
1192: The argument must be
1.2 deraadt 1193: .Dq yes
1194: or
1195: .Dq no .
1.55 markus 1196: .It Cm XAuthLocation
1197: Specifies the location of the
1198: .Xr xauth 1
1199: program.
1200: The default is
1201: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1202: .El
1.2 deraadt 1203: .Sh ENVIRONMENT
1204: .Nm
1.1 deraadt 1205: will normally set the following environment variables:
1.2 deraadt 1206: .Bl -tag -width Ds
1207: .It Ev DISPLAY
1208: The
1209: .Ev DISPLAY
1.40 aaron 1210: variable indicates the location of the X11 server.
1.44 aaron 1211: It is automatically set by
1.2 deraadt 1212: .Nm
1213: to point to a value of the form
1214: .Dq hostname:n
1215: where hostname indicates
1.40 aaron 1216: the host where the shell runs, and n is an integer >= 1.
1217: .Nm
1218: uses this special value to forward X11 connections over the secure
1219: channel.
1.107 markus 1220: The user should normally not set
1221: .Ev DISPLAY
1222: explicitly, as that
1.1 deraadt 1223: will render the X11 connection insecure (and will require the user to
1224: manually copy any required authorization cookies).
1.2 deraadt 1225: .It Ev HOME
1.1 deraadt 1226: Set to the path of the user's home directory.
1.2 deraadt 1227: .It Ev LOGNAME
1228: Synonym for
1.12 aaron 1229: .Ev USER ;
1230: set for compatibility with systems that use this variable.
1.2 deraadt 1231: .It Ev MAIL
1.107.2.1! jason 1232: Set to the path of the user's mailbox.
1.40 aaron 1233: .It Ev PATH
1.2 deraadt 1234: Set to the default
1235: .Ev PATH ,
1236: as specified when compiling
1.12 aaron 1237: .Nm ssh .
1.107.2.1! jason 1238: .It Ev SSH_ASKPASS
! 1239: If
! 1240: .Nm
! 1241: needs a passphrase, it will read the passphrase from the current
! 1242: terminal if it was run from a terminal.
! 1243: If
! 1244: .Nm
! 1245: does not have a terminal associated with it but
! 1246: .Ev DISPLAY
! 1247: and
! 1248: .Ev SSH_ASKPASS
! 1249: are set, it will execute the program specified by
! 1250: .Ev SSH_ASKPASS
! 1251: and open an X11 window to read the passphrase.
! 1252: This is particularly useful when calling
! 1253: .Nm
! 1254: from a
! 1255: .Pa .Xsession
! 1256: or related script.
! 1257: (Note that on some machines it
! 1258: may be necessary to redirect the input from
! 1259: .Pa /dev/null
! 1260: to make this work.)
1.18 markus 1261: .It Ev SSH_AUTH_SOCK
1.107.2.1! jason 1262: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 1263: agent.
1.2 deraadt 1264: .It Ev SSH_CLIENT
1.40 aaron 1265: Identifies the client end of the connection.
1266: The variable contains
1.1 deraadt 1267: three space-separated values: client ip-address, client port number,
1268: and server port number.
1.73 markus 1269: .It Ev SSH_ORIGINAL_COMMAND
1270: The variable contains the original command line if a forced command
1271: is executed.
1272: It can be used to extract the original arguments.
1.2 deraadt 1273: .It Ev SSH_TTY
1.1 deraadt 1274: This is set to the name of the tty (path to the device) associated
1.40 aaron 1275: with the current shell or command.
1276: If the current session has no tty,
1.1 deraadt 1277: this variable is not set.
1.2 deraadt 1278: .It Ev TZ
1.1 deraadt 1279: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1280: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1281: on to new connections).
1.2 deraadt 1282: .It Ev USER
1.1 deraadt 1283: Set to the name of the user logging in.
1.2 deraadt 1284: .El
1285: .Pp
1.44 aaron 1286: Additionally,
1.2 deraadt 1287: .Nm
1.44 aaron 1288: reads
1289: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1290: and adds lines of the format
1291: .Dq VARNAME=value
1.12 aaron 1292: to the environment.
1.2 deraadt 1293: .Sh FILES
1.36 markus 1294: .Bl -tag -width Ds
1.107.2.1! jason 1295: .It Pa $HOME/.ssh/known_hosts
! 1296: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 1297: in
1.107.2.1! jason 1298: .Pa /etc/ssh_known_hosts .
1.2 deraadt 1299: See
1300: .Xr sshd 8 .
1.102 itojun 1301: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1302: Contains the authentication identity of the user.
1303: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1304: These files
1305: contain sensitive data and should be readable by the user but not
1.15 markus 1306: accessible by others (read/write/execute).
1307: Note that
1308: .Nm
1.48 markus 1309: ignores a private key file if it is accessible by others.
1.15 markus 1310: It is possible to specify a passphrase when
1.1 deraadt 1311: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1312: sensitive part of this file using 3DES.
1.102 itojun 1313: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1314: Contains the public key for authentication (public part of the
1.40 aaron 1315: identity file in human-readable form).
1.48 markus 1316: The contents of the
1317: .Pa $HOME/.ssh/identity.pub
1318: file should be added to
1.2 deraadt 1319: .Pa $HOME/.ssh/authorized_keys
1320: on all machines
1.107.2.1! jason 1321: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 1322: The contents of the
1323: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1324: and
1325: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1326: file should be added to
1.107.2.1! jason 1327: .Pa $HOME/.ssh/authorized_keys
1.48 markus 1328: on all machines
1.107.2.1! jason 1329: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1330: These files are not
1.40 aaron 1331: sensitive and can (but need not) be readable by anyone.
1.48 markus 1332: These files are
1.84 markus 1333: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1334: the convenience of the user.
1.2 deraadt 1335: .It Pa $HOME/.ssh/config
1.40 aaron 1336: This is the per-user configuration file.
1337: The format of this file is described above.
1338: This file is used by the
1.2 deraadt 1339: .Nm
1.40 aaron 1340: client.
1341: This file does not usually contain any sensitive information,
1.1 deraadt 1342: but the recommended permissions are read/write for the user, and not
1343: accessible by others.
1.2 deraadt 1344: .It Pa $HOME/.ssh/authorized_keys
1.107.2.1! jason 1345: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 1346: The format of this file is described in the
1.2 deraadt 1347: .Xr sshd 8
1.40 aaron 1348: manual page.
1349: In the simplest form the format is the same as the .pub
1.107.2.1! jason 1350: identity files.
1.40 aaron 1351: This file is not highly sensitive, but the recommended
1.1 deraadt 1352: permissions are read/write for the user, and not accessible by others.
1.107.2.1! jason 1353: .It Pa /etc/ssh_known_hosts
1.40 aaron 1354: Systemwide list of known host keys.
1.107.2.1! jason 1355: This file should be prepared by the
1.1 deraadt 1356: system administrator to contain the public host keys of all machines in the
1.40 aaron 1357: organization.
1358: This file should be world-readable.
1359: This file contains
1.1 deraadt 1360: public keys, one per line, in the following format (fields separated
1.107.2.1! jason 1361: by spaces): system name, public key and optional comment field.
1.40 aaron 1362: When different names are used
1.1 deraadt 1363: for the same machine, all such names should be listed, separated by
1.40 aaron 1364: commas.
1365: The format is described on the
1.2 deraadt 1366: .Xr sshd 8
1.1 deraadt 1367: manual page.
1.2 deraadt 1368: .Pp
1.1 deraadt 1369: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1370: .Xr sshd 8
1.1 deraadt 1371: to verify the client host when logging in; other names are needed because
1.2 deraadt 1372: .Nm
1.1 deraadt 1373: does not convert the user-supplied name to a canonical name before
1374: checking the key, because someone with access to the name servers
1375: would then be able to fool host authentication.
1.2 deraadt 1376: .It Pa /etc/ssh_config
1.40 aaron 1377: Systemwide configuration file.
1378: This file provides defaults for those
1.1 deraadt 1379: values that are not specified in the user's configuration file, and
1.40 aaron 1380: for those users who do not have a configuration file.
1381: This file must be world-readable.
1.2 deraadt 1382: .It Pa $HOME/.rhosts
1383: This file is used in
1384: .Pa \&.rhosts
1385: authentication to list the
1.40 aaron 1386: host/user pairs that are permitted to log in.
1387: (Note that this file is
1.1 deraadt 1388: also used by rlogin and rsh, which makes using this file insecure.)
1389: Each line of the file contains a host name (in the canonical form
1390: returned by name servers), and then a user name on that host,
1.40 aaron 1391: separated by a space.
1.92 markus 1392: On some machines this file may need to be
1.1 deraadt 1393: world-readable if the user's home directory is on a NFS partition,
1394: because
1.2 deraadt 1395: .Xr sshd 8
1.40 aaron 1396: reads it as root.
1397: Additionally, this file must be owned by the user,
1398: and must not have write permissions for anyone else.
1399: The recommended
1.1 deraadt 1400: permission for most machines is read/write for the user, and not
1401: accessible by others.
1.2 deraadt 1402: .Pp
1.1 deraadt 1403: Note that by default
1.2 deraadt 1404: .Xr sshd 8
1.1 deraadt 1405: will be installed so that it requires successful RSA host
1.40 aaron 1406: authentication before permitting \s+2.\s0rhosts authentication.
1.107.2.1! jason 1407: If the server machine does not have the client's host key in
1.2 deraadt 1408: .Pa /etc/ssh_known_hosts ,
1.107.2.1! jason 1409: it can be stored in
1.2 deraadt 1410: .Pa $HOME/.ssh/known_hosts .
1411: The easiest way to do this is to
1.1 deraadt 1412: connect back to the client from the server machine using ssh; this
1.48 markus 1413: will automatically add the host key to
1.2 deraadt 1414: .Pa $HOME/.ssh/known_hosts .
1415: .It Pa $HOME/.shosts
1416: This file is used exactly the same way as
1417: .Pa \&.rhosts .
1418: The purpose for
1.1 deraadt 1419: having this file is to be able to use rhosts authentication with
1.2 deraadt 1420: .Nm
1421: without permitting login with
1422: .Xr rlogin 1
1423: or
1424: .Xr rsh 1 .
1425: .It Pa /etc/hosts.equiv
1426: This file is used during
1.40 aaron 1427: .Pa \&.rhosts authentication.
1428: It contains
1.1 deraadt 1429: canonical hosts names, one per line (the full format is described on
1430: the
1.2 deraadt 1431: .Xr sshd 8
1.40 aaron 1432: manual page).
1433: If the client host is found in this file, login is
1.1 deraadt 1434: automatically permitted provided client and server user names are the
1.40 aaron 1435: same.
1436: Additionally, successful RSA host authentication is normally
1437: required.
1438: This file should only be writable by root.
1.2 deraadt 1439: .It Pa /etc/shosts.equiv
1.44 aaron 1440: This file is processed exactly as
1.2 deraadt 1441: .Pa /etc/hosts.equiv .
1.1 deraadt 1442: This file may be useful to permit logins using
1.2 deraadt 1443: .Nm
1.1 deraadt 1444: but not using rsh/rlogin.
1.2 deraadt 1445: .It Pa /etc/sshrc
1.1 deraadt 1446: Commands in this file are executed by
1.2 deraadt 1447: .Nm
1.1 deraadt 1448: when the user logs in just before the user's shell (or command) is started.
1449: See the
1.2 deraadt 1450: .Xr sshd 8
1.1 deraadt 1451: manual page for more information.
1.2 deraadt 1452: .It Pa $HOME/.ssh/rc
1.1 deraadt 1453: Commands in this file are executed by
1.2 deraadt 1454: .Nm
1.1 deraadt 1455: when the user logs in just before the user's shell (or command) is
1456: started.
1.44 aaron 1457: See the
1.2 deraadt 1458: .Xr sshd 8
1.1 deraadt 1459: manual page for more information.
1.31 markus 1460: .It Pa $HOME/.ssh/environment
1461: Contains additional definitions for environment variables, see section
1462: .Sx ENVIRONMENT
1463: above.
1.58 itojun 1464: .El
1.67 aaron 1465: .Sh AUTHORS
1.78 markus 1466: OpenSSH is a derivative of the original and free
1467: ssh 1.2.12 release by Tatu Ylonen.
1468: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1469: Theo de Raadt and Dug Song
1470: removed many bugs, re-added newer features and
1471: created OpenSSH.
1472: Markus Friedl contributed the support for SSH
1473: protocol versions 1.5 and 2.0.
1.2 deraadt 1474: .Sh SEE ALSO
1475: .Xr rlogin 1 ,
1476: .Xr rsh 1 ,
1477: .Xr scp 1 ,
1.83 djm 1478: .Xr sftp 1 ,
1.2 deraadt 1479: .Xr ssh-add 1 ,
1480: .Xr ssh-agent 1 ,
1481: .Xr ssh-keygen 1 ,
1482: .Xr telnet 1 ,
1.87 itojun 1483: .Xr sshd 8
1.106 markus 1484: .Rs
1485: .%A T. Ylonen
1486: .%A T. Kivinen
1487: .%A M. Saarinen
1488: .%A T. Rinne
1489: .%A S. Lehtinen
1490: .%T "SSH Protocol Architecture"
1.107.2.1! jason 1491: .%N draft-ietf-secsh-architecture-09.txt
! 1492: .%D July 2001
1.106 markus 1493: .%O work in progress material
1494: .Re