Annotation of src/usr.bin/ssh/ssh.1, Revision 1.153
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: .\"
1.153 ! stevesk 37: .\" $OpenBSD: ssh.1,v 1.152 2002/05/26 20:35:10 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
1.130 stevesk 47: .Ar hostname | user@hostname
1.2 deraadt 48: .Op Ar command
49: .Pp
50: .Nm ssh
1.86 jakob 51: .Op Fl afgknqstvxACNPTX1246
1.108 markus 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
1.132 markus 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
1.135 naddy 77: .Op Fl D Ar port
1.130 stevesk 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
1.147 deraadt 129: .Pa /etc/ssh/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
1.145 markus 210: When a user connects using protocol version 2
211: similar authentication methods are available.
1.107 markus 212: Using the default values for
213: .Cm PreferredAuthentications ,
1.123 markus 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
1.115 markus 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
1.117 itojun 272: through the use of an escape character.
1.104 djm 273: .Pp
274: A single tilde character can be sent as
275: .Ic ~~
1.119 stevesk 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
1.117 itojun 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
1.140 markus 297: to terminate
1.104 djm 298: .It Cm ~?
299: Display a list of escape characters
1.149 jakob 300: .It Cm ~C
301: Open command line (only useful for adding port forwardings using the
302: .Fl L
303: and
304: .Fl R
305: options)
1.104 djm 306: .It Cm ~R
1.105 djm 307: Request rekeying of the connection (only useful for SSH protocol version 2
308: and if the peer supports it)
1.104 djm 309: .El
1.2 deraadt 310: .Pp
1.49 markus 311: .Ss X11 and TCP forwarding
312: .Pp
1.110 deraadt 313: If the
314: .Cm ForwardX11
315: variable is set to
316: .Dq yes
317: (or, see the description of the
318: .Fl X
319: and
320: .Fl x
321: options described later)
322: and the user is using X11 (the
1.2 deraadt 323: .Ev DISPLAY
1.1 deraadt 324: environment variable is set), the connection to the X11 display is
325: automatically forwarded to the remote side in such a way that any X11
326: programs started from the shell (or command) will go through the
327: encrypted channel, and the connection to the real X server will be made
1.40 aaron 328: from the local machine.
329: The user should not manually set
1.2 deraadt 330: .Ev DISPLAY .
1.1 deraadt 331: Forwarding of X11 connections can be
332: configured on the command line or in configuration files.
1.2 deraadt 333: .Pp
334: The
1.44 aaron 335: .Ev DISPLAY
1.2 deraadt 336: value set by
337: .Nm
1.1 deraadt 338: will point to the server machine, but with a display number greater
1.40 aaron 339: than zero.
340: This is normal, and happens because
1.2 deraadt 341: .Nm
342: creates a
343: .Dq proxy
344: X server on the server machine for forwarding the
1.1 deraadt 345: connections over the encrypted channel.
1.2 deraadt 346: .Pp
347: .Nm
1.1 deraadt 348: will also automatically set up Xauthority data on the server machine.
349: For this purpose, it will generate a random authorization cookie,
350: store it in Xauthority on the server, and verify that any forwarded
351: connections carry this cookie and replace it by the real cookie when
1.40 aaron 352: the connection is opened.
353: The real authentication cookie is never
1.1 deraadt 354: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 355: .Pp
1.1 deraadt 356: If the user is using an authentication agent, the connection to the agent
357: is automatically forwarded to the remote side unless disabled on
1.120 stevesk 358: the command line or in a configuration file.
1.2 deraadt 359: .Pp
1.1 deraadt 360: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.120 stevesk 361: be specified either on the command line or in a configuration file.
1.40 aaron 362: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 363: electronic purse; another is going through firewalls.
1.2 deraadt 364: .Pp
1.49 markus 365: .Ss Server authentication
366: .Pp
1.2 deraadt 367: .Nm
1.49 markus 368: automatically maintains and checks a database containing
1.40 aaron 369: identifications for all hosts it has ever been used with.
1.116 markus 370: Host keys are stored in
1.49 markus 371: .Pa $HOME/.ssh/known_hosts
1.40 aaron 372: in the user's home directory.
1.116 markus 373: Additionally, the file
1.147 deraadt 374: .Pa /etc/ssh/ssh_known_hosts
1.116 markus 375: is automatically checked for known hosts.
1.40 aaron 376: Any new hosts are automatically added to the user's file.
377: If a host's identification
1.1 deraadt 378: ever changes,
1.2 deraadt 379: .Nm
1.1 deraadt 380: warns about this and disables password authentication to prevent a
1.40 aaron 381: trojan horse from getting the user's password.
382: Another purpose of
1.1 deraadt 383: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 384: otherwise be used to circumvent the encryption.
385: The
1.2 deraadt 386: .Cm StrictHostKeyChecking
1.1 deraadt 387: option (see below) can be used to prevent logins to machines whose
388: host key is not known or has changed.
1.65 aaron 389: .Pp
390: The options are as follows:
1.2 deraadt 391: .Bl -tag -width Ds
1.4 dugsong 392: .It Fl a
1.42 aaron 393: Disables forwarding of the authentication agent connection.
1.54 markus 394: .It Fl A
395: Enables forwarding of the authentication agent connection.
396: This can also be specified on a per-host basis in a configuration file.
1.108 markus 397: .It Fl b Ar bind_address
398: Specify the interface to transmit from on machines with multiple
399: interfaces or aliased addresses.
1.131 stevesk 400: .It Fl c Ar blowfish|3des|des
1.44 aaron 401: Selects the cipher to use for encrypting the session.
1.2 deraadt 402: .Ar 3des
1.40 aaron 403: is used by default.
1.44 aaron 404: It is believed to be secure.
1.5 deraadt 405: .Ar 3des
406: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
407: .Ar blowfish
408: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 409: .Ar 3des .
1.131 stevesk 410: .Ar des
411: is only supported in the
412: .Nm
413: client for interoperability with legacy protocol 1 implementations
414: that do not support the
415: .Ar 3des
416: cipher. Its use is strongly discouraged due to cryptographic
417: weaknesses.
1.90 markus 418: .It Fl c Ar cipher_spec
1.51 markus 419: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 420: be specified in order of preference.
1.90 markus 421: See
422: .Cm Ciphers
423: for more information.
1.2 deraadt 424: .It Fl e Ar ch|^ch|none
425: Sets the escape character for sessions with a pty (default:
426: .Ql ~ ) .
1.40 aaron 427: The escape character is only recognized at the beginning of a line.
428: The escape character followed by a dot
1.2 deraadt 429: .Pq Ql \&.
430: closes the connection, followed
1.1 deraadt 431: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 432: escape character once.
433: Setting the character to
1.2 deraadt 434: .Dq none
435: disables any escapes and makes the session fully transparent.
436: .It Fl f
437: Requests
438: .Nm
1.40 aaron 439: to go to background just before command execution.
440: This is useful if
1.2 deraadt 441: .Nm
442: is going to ask for passwords or passphrases, but the user
1.40 aaron 443: wants it in the background.
1.44 aaron 444: This implies
1.2 deraadt 445: .Fl n .
1.1 deraadt 446: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 447: something like
448: .Ic ssh -f host xterm .
1.34 markus 449: .It Fl g
450: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 451: .It Fl i Ar identity_file
1.144 stevesk 452: Selects a file from which the identity (private key) for
1.68 markus 453: RSA or DSA authentication is read.
1.144 stevesk 454: The default is
1.49 markus 455: .Pa $HOME/.ssh/identity
1.144 stevesk 456: for protocol version 1, and
457: .Pa $HOME/.ssh/id_rsa
458: and
459: .Pa $HOME/.ssh/id_dsa
460: for protocol version 2.
1.40 aaron 461: Identity files may also be specified on
462: a per-host basis in the configuration file.
463: It is possible to have multiple
1.2 deraadt 464: .Fl i
465: options (and multiple identities specified in
1.1 deraadt 466: configuration files).
1.125 jakob 467: .It Fl I Ar smartcard_device
468: Specifies which smartcard device to use. The argument is
469: the device
470: .Nm
471: should use to communicate with a smartcard used for storing the user's
472: private RSA key.
1.2 deraadt 473: .It Fl k
1.42 aaron 474: Disables forwarding of Kerberos tickets and AFS tokens.
475: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 476: .It Fl l Ar login_name
1.40 aaron 477: Specifies the user to log in as on the remote machine.
478: This also may be specified on a per-host basis in the configuration file.
1.91 markus 479: .It Fl m Ar mac_spec
480: Additionally, for protocol version 2 a comma-separated list of MAC
481: (message authentication code) algorithms can
482: be specified in order of preference.
483: See the
484: .Cm MACs
485: keyword for more information.
1.2 deraadt 486: .It Fl n
487: Redirects stdin from
488: .Pa /dev/null
489: (actually, prevents reading from stdin).
1.1 deraadt 490: This must be used when
1.2 deraadt 491: .Nm
1.40 aaron 492: is run in the background.
493: A common trick is to use this to run X11 programs on a remote machine.
494: For example,
1.2 deraadt 495: .Ic ssh -n shadows.cs.hut.fi emacs &
496: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 497: connection will be automatically forwarded over an encrypted channel.
498: The
1.2 deraadt 499: .Nm
1.1 deraadt 500: program will be put in the background.
501: (This does not work if
1.2 deraadt 502: .Nm
503: needs to ask for a password or passphrase; see also the
504: .Fl f
505: option.)
1.53 markus 506: .It Fl N
507: Do not execute a remote command.
1.137 deraadt 508: This is useful for just forwarding ports
1.53 markus 509: (protocol version 2 only).
1.2 deraadt 510: .It Fl o Ar option
1.127 stevesk 511: Can be used to give options in the format used in the configuration file.
1.1 deraadt 512: This is useful for specifying options for which there is no separate
1.40 aaron 513: command-line flag.
1.2 deraadt 514: .It Fl p Ar port
1.40 aaron 515: Port to connect to on the remote host.
516: This can be specified on a
1.1 deraadt 517: per-host basis in the configuration file.
1.16 markus 518: .It Fl P
519: Use a non-privileged port for outgoing connections.
1.137 deraadt 520: This can be used if a firewall does
1.16 markus 521: not permit connections from privileged ports.
1.30 provos 522: Note that this option turns off
1.16 markus 523: .Cm RhostsAuthentication
524: and
1.72 markus 525: .Cm RhostsRSAAuthentication
526: for older servers.
1.2 deraadt 527: .It Fl q
1.40 aaron 528: Quiet mode.
529: Causes all warning and diagnostic messages to be suppressed.
1.80 djm 530: .It Fl s
1.117 itojun 531: 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
532: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 533: subsystem is specified as the remote command.
1.2 deraadt 534: .It Fl t
1.40 aaron 535: Force pseudo-tty allocation.
1.43 brad 536: This can be used to execute arbitrary
1.40 aaron 537: screen-based programs on a remote machine, which can be very useful,
538: e.g., when implementing menu services.
1.73 markus 539: Multiple
540: .Fl t
541: options force tty allocation, even if
542: .Nm
543: has no local tty.
1.53 markus 544: .It Fl T
1.69 markus 545: Disable pseudo-tty allocation.
1.2 deraadt 546: .It Fl v
1.40 aaron 547: Verbose mode.
548: Causes
1.2 deraadt 549: .Nm
1.40 aaron 550: to print debugging messages about its progress.
551: This is helpful in
1.1 deraadt 552: debugging connection, authentication, and configuration problems.
1.73 markus 553: Multiple
554: .Fl v
555: options increases the verbosity.
1.61 aaron 556: Maximum is 3.
1.2 deraadt 557: .It Fl x
1.40 aaron 558: Disables X11 forwarding.
1.2 deraadt 559: .It Fl X
1.1 deraadt 560: Enables X11 forwarding.
1.54 markus 561: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 562: .It Fl C
1.1 deraadt 563: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 564: data for forwarded X11 and TCP/IP connections).
565: The compression algorithm is the same used by
1.34 markus 566: .Xr gzip 1 ,
567: and the
1.2 deraadt 568: .Dq level
569: can be controlled by the
570: .Cm CompressionLevel
1.40 aaron 571: option (see below).
572: Compression is desirable on modem lines and other
1.1 deraadt 573: slow connections, but will only slow down things on fast networks.
574: The default value can be set on a host-by-host basis in the
575: configuration files; see the
1.121 pvalchev 576: .Cm Compression
1.1 deraadt 577: option below.
1.132 markus 578: .It Fl F Ar configfile
579: Specifies an alternative per-user configuration file.
580: If a configuration file is given on the command line,
581: the system-wide configuration file
1.147 deraadt 582: .Pq Pa /etc/ssh/ssh_config
1.132 markus 583: will be ignored.
584: The default for the per-user configuration file is
585: .Pa $HOME/.ssh/config .
1.2 deraadt 586: .It Fl L Ar port:host:hostport
1.1 deraadt 587: Specifies that the given port on the local (client) host is to be
1.40 aaron 588: forwarded to the given host and port on the remote side.
589: This works by allocating a socket to listen to
1.2 deraadt 590: .Ar port
1.1 deraadt 591: on the local side, and whenever a connection is made to this port, the
592: connection is forwarded over the secure channel, and a connection is
593: made to
1.32 markus 594: .Ar host
595: port
596: .Ar hostport
1.40 aaron 597: from the remote machine.
598: Port forwardings can also be specified in the configuration file.
599: Only root can forward privileged ports.
1.32 markus 600: IPv6 addresses can be specified with an alternative syntax:
601: .Ar port/host/hostport
1.2 deraadt 602: .It Fl R Ar port:host:hostport
1.1 deraadt 603: Specifies that the given port on the remote (server) host is to be
1.40 aaron 604: forwarded to the given host and port on the local side.
605: This works by allocating a socket to listen to
1.2 deraadt 606: .Ar port
1.1 deraadt 607: on the remote side, and whenever a connection is made to this port, the
608: connection is forwarded over the secure channel, and a connection is
609: made to
1.32 markus 610: .Ar host
611: port
612: .Ar hostport
1.40 aaron 613: from the local machine.
614: Port forwardings can also be specified in the configuration file.
615: Privileged ports can be forwarded only when
1.1 deraadt 616: logging in as root on the remote machine.
1.107 markus 617: IPv6 addresses can be specified with an alternative syntax:
618: .Ar port/host/hostport
1.133 stevesk 619: .It Fl D Ar port
620: Specifies a local
621: .Dq dynamic
622: application-level port forwarding.
623: This works by allocating a socket to listen to
624: .Ar port
625: on the local side, and whenever a connection is made to this port, the
626: connection is forwarded over the secure channel, and the application
627: protocol is then used to determine where to connect to from the
628: remote machine. Currently the SOCKS4 protocol is supported, and
629: .Nm
630: will act as a SOCKS4 server.
631: Only root can forward privileged ports.
632: Dynamic port forwardings can also be specified in the configuration file.
1.85 jakob 633: .It Fl 1
634: Forces
635: .Nm
636: to try protocol version 1 only.
1.46 markus 637: .It Fl 2
638: Forces
639: .Nm
1.50 markus 640: to try protocol version 2 only.
1.32 markus 641: .It Fl 4
642: Forces
643: .Nm
644: to use IPv4 addresses only.
645: .It Fl 6
646: Forces
647: .Nm
648: to use IPv6 addresses only.
1.2 deraadt 649: .El
650: .Sh CONFIGURATION FILES
651: .Nm
1.127 stevesk 652: obtains configuration data from the following sources in
653: the following order:
1.1 deraadt 654: command line options, user's configuration file
1.2 deraadt 655: .Pq Pa $HOME/.ssh/config ,
656: and system-wide configuration file
1.147 deraadt 657: .Pq Pa /etc/ssh/ssh_config .
1.2 deraadt 658: For each parameter, the first obtained value
1.40 aaron 659: will be used.
660: The configuration files contain sections bracketed by
661: .Dq Host
662: specifications, and that section is only applied for hosts that
663: match one of the patterns given in the specification.
664: The matched host name is the one given on the command line.
1.2 deraadt 665: .Pp
1.1 deraadt 666: Since the first obtained value for each parameter is used, more
667: host-specific declarations should be given near the beginning of the
668: file, and general defaults at the end.
1.2 deraadt 669: .Pp
1.1 deraadt 670: The configuration file has the following format:
1.2 deraadt 671: .Pp
672: Empty lines and lines starting with
673: .Ql #
674: are comments.
675: .Pp
676: Otherwise a line is of the format
677: .Dq keyword arguments .
1.127 stevesk 678: Configuration options may be separated by whitespace or
679: optional whitespace and exactly one
680: .Ql = ;
681: the latter format is useful to avoid the need to quote whitespace
682: when specifying configuration options using the
683: .Nm ssh ,
684: .Nm scp
685: and
686: .Nm sftp
687: .Fl o
688: option.
689: .Pp
1.2 deraadt 690: The possible
1.128 stevesk 691: keywords and their meanings are as follows (note that
692: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 693: .Bl -tag -width Ds
694: .It Cm Host
1.1 deraadt 695: Restricts the following declarations (up to the next
1.2 deraadt 696: .Cm Host
1.1 deraadt 697: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 698: given after the keyword.
699: .Ql \&*
700: and
701: .Ql ?
702: can be used as wildcards in the
1.40 aaron 703: patterns.
704: A single
1.2 deraadt 705: .Ql \&*
706: as a pattern can be used to provide global
1.40 aaron 707: defaults for all hosts.
708: The host is the
1.2 deraadt 709: .Ar hostname
1.1 deraadt 710: argument given on the command line (i.e., the name is not converted to
711: a canonicalized host name before matching).
1.2 deraadt 712: .It Cm AFSTokenPassing
1.42 aaron 713: Specifies whether to pass AFS tokens to remote host.
714: The argument to this keyword must be
1.2 deraadt 715: .Dq yes
716: or
717: .Dq no .
1.107 markus 718: This option applies to protocol version 1 only.
1.2 deraadt 719: .It Cm BatchMode
720: If set to
721: .Dq yes ,
1.40 aaron 722: passphrase/password querying will be disabled.
1.137 deraadt 723: This option is useful in scripts and other batch jobs where no user
724: is present to supply the password.
1.40 aaron 725: The argument must be
1.2 deraadt 726: .Dq yes
727: or
728: .Dq no .
1.100 stevesk 729: The default is
730: .Dq no .
1.108 markus 731: .It Cm BindAddress
732: Specify the interface to transmit from on machines with multiple
733: interfaces or aliased addresses.
734: Note that this option does not work if
735: .Cm UsePrivilegedPort
736: is set to
737: .Dq yes .
1.152 stevesk 738: .It Cm ChallengeResponseAuthentication
739: Specifies whether to use challenge response authentication.
740: The argument to this keyword must be
741: .Dq yes
742: or
743: .Dq no .
744: The default is
745: .Dq yes .
1.34 markus 746: .It Cm CheckHostIP
747: If this flag is set to
748: .Dq yes ,
1.100 stevesk 749: ssh will additionally check the host IP address in the
1.34 markus 750: .Pa known_hosts
1.42 aaron 751: file.
752: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 753: If the option is set to
754: .Dq no ,
755: the check will not be executed.
1.100 stevesk 756: The default is
757: .Dq yes .
1.2 deraadt 758: .It Cm Cipher
1.62 markus 759: Specifies the cipher to use for encrypting the session
1.64 markus 760: in protocol version 1.
1.40 aaron 761: Currently,
1.131 stevesk 762: .Dq blowfish ,
763: .Dq 3des ,
1.1 deraadt 764: and
1.131 stevesk 765: .Dq des
1.40 aaron 766: are supported.
1.131 stevesk 767: .Ar des
768: is only supported in the
769: .Nm
770: client for interoperability with legacy protocol 1 implementations
771: that do not support the
772: .Ar 3des
773: cipher. Its use is strongly discouraged due to cryptographic
774: weaknesses.
1.40 aaron 775: The default is
1.2 deraadt 776: .Dq 3des .
1.45 markus 777: .It Cm Ciphers
778: Specifies the ciphers allowed for protocol version 2
779: in order of preference.
780: Multiple ciphers must be comma-separated.
781: The default is
1.88 provos 782: .Pp
783: .Bd -literal
1.94 deraadt 784: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.107 markus 785: aes192-cbc,aes256-cbc''
1.88 provos 786: .Ed
1.138 stevesk 787: .It Cm ClearAllForwardings
788: Specifies that all local, remote and dynamic port forwardings
789: specified in the configuration files or on the command line be
790: cleared. This option is primarily useful when used from the
791: .Nm
792: command line to clear port forwardings set in
793: configuration files, and is automatically set by
794: .Xr scp 1
795: and
796: .Xr sftp 1 .
797: The argument must be
798: .Dq yes
799: or
800: .Dq no .
801: The default is
802: .Dq no .
1.2 deraadt 803: .It Cm Compression
1.40 aaron 804: Specifies whether to use compression.
805: The argument must be
1.2 deraadt 806: .Dq yes
807: or
808: .Dq no .
1.100 stevesk 809: The default is
810: .Dq no .
1.2 deraadt 811: .It Cm CompressionLevel
1.100 stevesk 812: Specifies the compression level to use if compression is enabled.
1.40 aaron 813: The argument must be an integer from 1 (fast) to 9 (slow, best).
814: The default level is 6, which is good for most applications.
815: The meaning of the values is the same as in
1.34 markus 816: .Xr gzip 1 .
1.107 markus 817: Note that this option applies to protocol version 1 only.
1.2 deraadt 818: .It Cm ConnectionAttempts
1.1 deraadt 819: Specifies the number of tries (one per second) to make before falling
1.40 aaron 820: back to rsh or exiting.
821: The argument must be an integer.
822: This may be useful in scripts if the connection sometimes fails.
1.124 markus 823: The default is 1.
1.133 stevesk 824: .It Cm DynamicForward
825: Specifies that a TCP/IP port on the local machine be forwarded
826: over the secure channel, and the application
827: protocol is then used to determine where to connect to from the
828: remote machine. The argument must be a port number.
829: Currently the SOCKS4 protocol is supported, and
830: .Nm
831: will act as a SOCKS4 server.
832: Multiple forwardings may be specified, and
833: additional forwardings can be given on the command line. Only
834: the superuser can forward privileged ports.
1.2 deraadt 835: .It Cm EscapeChar
836: Sets the escape character (default:
837: .Ql ~ ) .
838: The escape character can also
1.40 aaron 839: be set on the command line.
840: The argument should be a single character,
1.2 deraadt 841: .Ql ^
842: followed by a letter, or
843: .Dq none
844: to disable the escape
1.1 deraadt 845: character entirely (making the connection transparent for binary
846: data).
1.44 aaron 847: .It Cm FallBackToRsh
1.1 deraadt 848: Specifies that if connecting via
1.2 deraadt 849: .Nm
1.1 deraadt 850: fails due to a connection refused error (there is no
1.2 deraadt 851: .Xr sshd 8
1.44 aaron 852: listening on the remote host),
1.2 deraadt 853: .Xr rsh 1
1.1 deraadt 854: should automatically be used instead (after a suitable warning about
1.40 aaron 855: the session being unencrypted).
856: The argument must be
1.2 deraadt 857: .Dq yes
858: or
859: .Dq no .
1.100 stevesk 860: The default is
861: .Dq no .
1.2 deraadt 862: .It Cm ForwardAgent
1.1 deraadt 863: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 864: will be forwarded to the remote machine.
865: The argument must be
1.2 deraadt 866: .Dq yes
867: or
1.54 markus 868: .Dq no .
869: The default is
1.2 deraadt 870: .Dq no .
871: .It Cm ForwardX11
1.1 deraadt 872: Specifies whether X11 connections will be automatically redirected
1.44 aaron 873: over the secure channel and
1.2 deraadt 874: .Ev DISPLAY
1.40 aaron 875: set.
1.44 aaron 876: The argument must be
1.2 deraadt 877: .Dq yes
878: or
1.38 markus 879: .Dq no .
880: The default is
1.3 deraadt 881: .Dq no .
882: .It Cm GatewayPorts
883: Specifies whether remote hosts are allowed to connect to local
884: forwarded ports.
1.134 stevesk 885: By default,
886: .Nm
1.153 ! stevesk 887: binds local port forwardings to the loopback address. This
1.134 stevesk 888: prevents other remote hosts from connecting to forwarded ports.
889: .Cm GatewayPorts
890: can be used to specify that
891: .Nm
892: should bind local port forwardings to the wildcard address,
893: thus allowing remote hosts to connect to forwarded ports.
1.3 deraadt 894: The argument must be
895: .Dq yes
896: or
897: .Dq no .
898: The default is
1.2 deraadt 899: .Dq no .
900: .It Cm GlobalKnownHostsFile
1.116 markus 901: Specifies a file to use for the global
1.95 stevesk 902: host key database instead of
1.147 deraadt 903: .Pa /etc/ssh/ssh_known_hosts .
1.107 markus 904: .It Cm HostbasedAuthentication
905: Specifies whether to try rhosts based authentication with public key
906: authentication.
907: The argument must be
908: .Dq yes
909: or
910: .Dq no .
911: The default is
1.112 markus 912: .Dq no .
1.107 markus 913: This option applies to protocol version 2 only and
914: is similar to
915: .Cm RhostsRSAAuthentication .
916: .It Cm HostKeyAlgorithms
1.109 stevesk 917: Specifies the protocol version 2 host key algorithms
1.107 markus 918: that the client wants to use in order of preference.
919: The default for this option is:
1.143 stevesk 920: .Dq ssh-rsa,ssh-dss .
1.74 markus 921: .It Cm HostKeyAlias
922: Specifies an alias that should be used instead of the
923: real host name when looking up or saving the host key
1.107 markus 924: in the host key database files.
1.82 stevesk 925: This option is useful for tunneling ssh connections
1.137 deraadt 926: or for multiple servers running on a single host.
1.2 deraadt 927: .It Cm HostName
1.40 aaron 928: Specifies the real host name to log into.
929: This can be used to specify nicknames or abbreviations for hosts.
930: Default is the name given on the command line.
931: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 932: .Cm HostName
1.1 deraadt 933: specifications).
1.2 deraadt 934: .It Cm IdentityFile
1.144 stevesk 935: Specifies a file from which the user's RSA or DSA authentication identity
936: is read. The default is
1.49 markus 937: .Pa $HOME/.ssh/identity
1.144 stevesk 938: for protocol version 1, and
939: .Pa $HOME/.ssh/id_rsa
940: and
941: .Pa $HOME/.ssh/id_dsa
942: for protocol version 2.
1.1 deraadt 943: Additionally, any identities represented by the authentication agent
1.40 aaron 944: will be used for authentication.
945: The file name may use the tilde
946: syntax to refer to a user's home directory.
947: It is possible to have
1.1 deraadt 948: multiple identity files specified in configuration files; all these
949: identities will be tried in sequence.
1.2 deraadt 950: .It Cm KeepAlive
1.146 stevesk 951: Specifies whether the system should send TCP keepalive messages to the
1.40 aaron 952: other side.
953: If they are sent, death of the connection or crash of one
954: of the machines will be properly noticed.
955: However, this means that
1.1 deraadt 956: connections will die if the route is down temporarily, and some people
1.41 aaron 957: find it annoying.
1.2 deraadt 958: .Pp
959: The default is
960: .Dq yes
961: (to send keepalives), and the client will notice
1.40 aaron 962: if the network goes down or the remote host dies.
963: This is important in scripts, and many users want it too.
1.2 deraadt 964: .Pp
965: To disable keepalives, the value should be set to
1.146 stevesk 966: .Dq no .
1.2 deraadt 967: .It Cm KerberosAuthentication
1.42 aaron 968: Specifies whether Kerberos authentication will be used.
969: The argument to this keyword must be
1.4 dugsong 970: .Dq yes
971: or
972: .Dq no .
1.2 deraadt 973: .It Cm KerberosTgtPassing
1.42 aaron 974: Specifies whether a Kerberos TGT will be forwarded to the server.
975: This will only work if the Kerberos server is actually an AFS kaserver.
976: The argument to this keyword must be
1.4 dugsong 977: .Dq yes
978: or
979: .Dq no .
1.2 deraadt 980: .It Cm LocalForward
1.1 deraadt 981: Specifies that a TCP/IP port on the local machine be forwarded over
1.136 stevesk 982: the secure channel to the specified host and port from the remote machine.
1.40 aaron 983: The first argument must be a port number, and the second must be
1.136 stevesk 984: .Ar host:port .
985: IPv6 addresses can be specified with an alternative syntax:
986: .Ar host/port .
1.40 aaron 987: Multiple forwardings may be specified, and additional
988: forwardings can be given on the command line.
989: Only the superuser can forward privileged ports.
1.24 markus 990: .It Cm LogLevel
991: Gives the verbosity level that is used when logging messages from
992: .Nm ssh .
993: The possible values are:
1.142 stevesk 994: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
995: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
996: and DEBUG3 each specify higher levels of verbose output.
1.91 markus 997: .It Cm MACs
1.117 itojun 998: Specifies the MAC (message authentication code) algorithms
1.91 markus 999: in order of preference.
1000: The MAC algorithm is used in protocol version 2
1001: for data integrity protection.
1002: Multiple algorithms must be comma-separated.
1003: The default is
1.113 markus 1004: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.139 markus 1005: .It Cm NoHostAuthenticationForLocalhost
1006: This option can be used if the home directory is shared across machines.
1007: In this case localhost will refer to a different machine on each of
1008: the machines and the user will get many warnings about changed host keys.
1009: However, this option disables host authentication for localhost.
1010: The argument to this keyword must be
1011: .Dq yes
1012: or
1013: .Dq no .
1014: The default is to check the host key for localhost.
1.14 dugsong 1015: .It Cm NumberOfPasswordPrompts
1.42 aaron 1016: Specifies the number of password prompts before giving up.
1017: The argument to this keyword must be an integer.
1018: Default is 3.
1.34 markus 1019: .It Cm PasswordAuthentication
1.40 aaron 1020: Specifies whether to use password authentication.
1021: The argument to this keyword must be
1.34 markus 1022: .Dq yes
1023: or
1024: .Dq no .
1.100 stevesk 1025: The default is
1026: .Dq yes .
1.2 deraadt 1027: .It Cm Port
1.40 aaron 1028: Specifies the port number to connect on the remote host.
1029: Default is 22.
1.99 djm 1030: .It Cm PreferredAuthentications
1.117 itojun 1031: Specifies the order in which the client should try protocol 2
1032: authentication methods. This allows a client to prefer one method (e.g.
1.99 djm 1033: .Cm keyboard-interactive )
1034: over another method (e.g.
1035: .Cm password )
1036: The default for this option is:
1.143 stevesk 1037: .Dq hostbased,publickey,keyboard-interactive,password .
1.45 markus 1038: .It Cm Protocol
1039: Specifies the protocol versions
1040: .Nm
1041: should support in order of preference.
1042: The possible values are
1043: .Dq 1
1044: and
1045: .Dq 2 .
1046: Multiple versions must be comma-separated.
1047: The default is
1.101 markus 1048: .Dq 2,1 .
1.49 markus 1049: This means that
1050: .Nm
1.101 markus 1051: tries version 2 and falls back to version 1
1052: if version 2 is not available.
1.2 deraadt 1053: .It Cm ProxyCommand
1.40 aaron 1054: Specifies the command to use to connect to the server.
1055: The command
1056: string extends to the end of the line, and is executed with
1057: .Pa /bin/sh .
1058: In the command string,
1059: .Ql %h
1060: will be substituted by the host name to
1061: connect and
1062: .Ql %p
1063: by the port.
1064: The command can be basically anything,
1065: and should read from its standard input and write to its standard output.
1066: It should eventually connect an
1.2 deraadt 1067: .Xr sshd 8
1.1 deraadt 1068: server running on some machine, or execute
1.2 deraadt 1069: .Ic sshd -i
1.40 aaron 1070: somewhere.
1071: Host key management will be done using the
1.1 deraadt 1072: HostName of the host being connected (defaulting to the name typed by
1073: the user).
1.29 markus 1074: Note that
1075: .Cm CheckHostIP
1076: is not available for connects with a proxy command.
1.2 deraadt 1077: .Pp
1.107 markus 1078: .It Cm PubkeyAuthentication
1079: Specifies whether to try public key authentication.
1080: The argument to this keyword must be
1081: .Dq yes
1082: or
1083: .Dq no .
1084: The default is
1085: .Dq yes .
1086: This option applies to protocol version 2 only.
1.2 deraadt 1087: .It Cm RemoteForward
1.1 deraadt 1088: Specifies that a TCP/IP port on the remote machine be forwarded over
1.136 stevesk 1089: the secure channel to the specified host and port from the local machine.
1.40 aaron 1090: The first argument must be a port number, and the second must be
1.136 stevesk 1091: .Ar host:port .
1092: IPv6 addresses can be specified with an alternative syntax:
1093: .Ar host/port .
1.40 aaron 1094: Multiple forwardings may be specified, and additional
1095: forwardings can be given on the command line.
1096: Only the superuser can forward privileged ports.
1.2 deraadt 1097: .It Cm RhostsAuthentication
1.40 aaron 1098: Specifies whether to try rhosts based authentication.
1099: Note that this
1.1 deraadt 1100: declaration only affects the client side and has no effect whatsoever
1.40 aaron 1101: on security.
1102: Disabling rhosts authentication may reduce
1.1 deraadt 1103: authentication time on slow connections when rhosts authentication is
1.40 aaron 1104: not used.
1105: Most servers do not permit RhostsAuthentication because it
1.117 itojun 1106: is not secure (see
1.114 mpech 1107: .Cm RhostsRSAAuthentication ) .
1.40 aaron 1108: The argument to this keyword must be
1.2 deraadt 1109: .Dq yes
1110: or
1111: .Dq no .
1.100 stevesk 1112: The default is
1113: .Dq yes .
1.107 markus 1114: This option applies to protocol version 1 only.
1.2 deraadt 1115: .It Cm RhostsRSAAuthentication
1.1 deraadt 1116: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 1117: authentication.
1118: The argument must be
1.2 deraadt 1119: .Dq yes
1120: or
1121: .Dq no .
1.100 stevesk 1122: The default is
1123: .Dq yes .
1.107 markus 1124: This option applies to protocol version 1 only.
1.2 deraadt 1125: .It Cm RSAAuthentication
1.40 aaron 1126: Specifies whether to try RSA authentication.
1127: The argument to this keyword must be
1.2 deraadt 1128: .Dq yes
1129: or
1130: .Dq no .
1.1 deraadt 1131: RSA authentication will only be
1132: attempted if the identity file exists, or an authentication agent is
1133: running.
1.100 stevesk 1134: The default is
1135: .Dq yes .
1.50 markus 1136: Note that this option applies to protocol version 1 only.
1.125 jakob 1137: .It Cm SmartcardDevice
1138: Specifies which smartcard device to use. The argument to this keyword is
1139: the device
1140: .Nm
1141: should use to communicate with a smartcard used for storing the user's
1142: private RSA key. By default, no device is specified and smartcard support
1143: is not activated.
1.2 deraadt 1144: .It Cm StrictHostKeyChecking
1145: If this flag is set to
1.44 aaron 1146: .Dq yes ,
1.2 deraadt 1147: .Nm
1.79 stevesk 1148: will never automatically add host keys to the
1.2 deraadt 1149: .Pa $HOME/.ssh/known_hosts
1.116 markus 1150: file, and refuses to connect to hosts whose host key has changed.
1.137 deraadt 1151: This provides maximum protection against trojan horse attacks,
1152: however, can be annoying when the
1.147 deraadt 1153: .Pa /etc/ssh/ssh_known_hosts
1.137 deraadt 1154: file is poorly maintained, or connections to new hosts are
1155: frequently made.
1.79 stevesk 1156: This option forces the user to manually
1157: add all new hosts.
1158: If this flag is set to
1159: .Dq no ,
1160: .Nm
1161: will automatically add new host keys to the
1162: user known hosts files.
1163: If this flag is set to
1164: .Dq ask ,
1165: new host keys
1166: will be added to the user known host files only after the user
1167: has confirmed that is what they really want to do, and
1168: .Nm
1169: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1170: The host keys of
1.79 stevesk 1171: known hosts will be verified automatically in all cases.
1.40 aaron 1172: The argument must be
1.79 stevesk 1173: .Dq yes ,
1174: .Dq no
1.2 deraadt 1175: or
1.79 stevesk 1176: .Dq ask .
1177: The default is
1178: .Dq ask .
1.16 markus 1179: .It Cm UsePrivilegedPort
1180: Specifies whether to use a privileged port for outgoing connections.
1181: The argument must be
1182: .Dq yes
1183: or
1184: .Dq no .
1185: The default is
1.98 markus 1186: .Dq no .
1.137 deraadt 1187: Note that this option must be set to
1.107 markus 1188: .Dq yes
1.137 deraadt 1189: if
1.16 markus 1190: .Cm RhostsAuthentication
1191: and
1.72 markus 1192: .Cm RhostsRSAAuthentication
1.137 deraadt 1193: authentications are needed with older servers.
1.34 markus 1194: .It Cm User
1.40 aaron 1195: Specifies the user to log in as.
1.137 deraadt 1196: This can be useful when a different user name is used on different machines.
1.40 aaron 1197: This saves the trouble of
1.34 markus 1198: having to remember to give the user name on the command line.
1199: .It Cm UserKnownHostsFile
1.116 markus 1200: Specifies a file to use for the user
1.95 stevesk 1201: host key database instead of
1.34 markus 1202: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 1203: .It Cm UseRsh
1.40 aaron 1204: Specifies that rlogin/rsh should be used for this host.
1205: It is possible that the host does not at all support the
1.2 deraadt 1206: .Nm
1.40 aaron 1207: protocol.
1208: This causes
1.2 deraadt 1209: .Nm
1.40 aaron 1210: to immediately execute
1.2 deraadt 1211: .Xr rsh 1 .
1.1 deraadt 1212: All other options (except
1.2 deraadt 1213: .Cm HostName )
1.40 aaron 1214: are ignored if this has been specified.
1215: The argument must be
1.2 deraadt 1216: .Dq yes
1217: or
1218: .Dq no .
1.55 markus 1219: .It Cm XAuthLocation
1220: Specifies the location of the
1221: .Xr xauth 1
1222: program.
1223: The default is
1224: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1225: .El
1.2 deraadt 1226: .Sh ENVIRONMENT
1227: .Nm
1.1 deraadt 1228: will normally set the following environment variables:
1.2 deraadt 1229: .Bl -tag -width Ds
1230: .It Ev DISPLAY
1231: The
1232: .Ev DISPLAY
1.40 aaron 1233: variable indicates the location of the X11 server.
1.44 aaron 1234: It is automatically set by
1.2 deraadt 1235: .Nm
1236: to point to a value of the form
1237: .Dq hostname:n
1238: where hostname indicates
1.40 aaron 1239: the host where the shell runs, and n is an integer >= 1.
1240: .Nm
1241: uses this special value to forward X11 connections over the secure
1242: channel.
1.107 markus 1243: The user should normally not set
1244: .Ev DISPLAY
1245: explicitly, as that
1.1 deraadt 1246: will render the X11 connection insecure (and will require the user to
1247: manually copy any required authorization cookies).
1.2 deraadt 1248: .It Ev HOME
1.1 deraadt 1249: Set to the path of the user's home directory.
1.2 deraadt 1250: .It Ev LOGNAME
1251: Synonym for
1.12 aaron 1252: .Ev USER ;
1253: set for compatibility with systems that use this variable.
1.2 deraadt 1254: .It Ev MAIL
1.129 stevesk 1255: Set to the path of the user's mailbox.
1.40 aaron 1256: .It Ev PATH
1.2 deraadt 1257: Set to the default
1258: .Ev PATH ,
1259: as specified when compiling
1.12 aaron 1260: .Nm ssh .
1.118 markus 1261: .It Ev SSH_ASKPASS
1262: If
1263: .Nm
1264: needs a passphrase, it will read the passphrase from the current
1265: terminal if it was run from a terminal.
1266: If
1267: .Nm
1268: does not have a terminal associated with it but
1269: .Ev DISPLAY
1270: and
1271: .Ev SSH_ASKPASS
1272: are set, it will execute the program specified by
1273: .Ev SSH_ASKPASS
1274: and open an X11 window to read the passphrase.
1275: This is particularly useful when calling
1276: .Nm
1277: from a
1278: .Pa .Xsession
1279: or related script.
1280: (Note that on some machines it
1281: may be necessary to redirect the input from
1282: .Pa /dev/null
1283: to make this work.)
1.18 markus 1284: .It Ev SSH_AUTH_SOCK
1.129 stevesk 1285: Identifies the path of a unix-domain socket used to communicate with the
1.17 markus 1286: agent.
1.2 deraadt 1287: .It Ev SSH_CLIENT
1.40 aaron 1288: Identifies the client end of the connection.
1289: The variable contains
1.1 deraadt 1290: three space-separated values: client ip-address, client port number,
1291: and server port number.
1.73 markus 1292: .It Ev SSH_ORIGINAL_COMMAND
1293: The variable contains the original command line if a forced command
1294: is executed.
1295: It can be used to extract the original arguments.
1.2 deraadt 1296: .It Ev SSH_TTY
1.1 deraadt 1297: This is set to the name of the tty (path to the device) associated
1.40 aaron 1298: with the current shell or command.
1299: If the current session has no tty,
1.1 deraadt 1300: this variable is not set.
1.2 deraadt 1301: .It Ev TZ
1.1 deraadt 1302: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1303: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1304: on to new connections).
1.2 deraadt 1305: .It Ev USER
1.1 deraadt 1306: Set to the name of the user logging in.
1.2 deraadt 1307: .El
1308: .Pp
1.44 aaron 1309: Additionally,
1.2 deraadt 1310: .Nm
1.44 aaron 1311: reads
1312: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1313: and adds lines of the format
1314: .Dq VARNAME=value
1.12 aaron 1315: to the environment.
1.2 deraadt 1316: .Sh FILES
1.36 markus 1317: .Bl -tag -width Ds
1.116 markus 1318: .It Pa $HOME/.ssh/known_hosts
1.129 stevesk 1319: Records host keys for all hosts the user has logged into that are not
1.2 deraadt 1320: in
1.147 deraadt 1321: .Pa /etc/ssh/ssh_known_hosts .
1.2 deraadt 1322: See
1323: .Xr sshd 8 .
1.102 itojun 1324: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1325: Contains the authentication identity of the user.
1326: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1327: These files
1328: contain sensitive data and should be readable by the user but not
1.15 markus 1329: accessible by others (read/write/execute).
1330: Note that
1331: .Nm
1.48 markus 1332: ignores a private key file if it is accessible by others.
1.15 markus 1333: It is possible to specify a passphrase when
1.1 deraadt 1334: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1335: sensitive part of this file using 3DES.
1.102 itojun 1336: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1337: Contains the public key for authentication (public part of the
1.40 aaron 1338: identity file in human-readable form).
1.48 markus 1339: The contents of the
1340: .Pa $HOME/.ssh/identity.pub
1341: file should be added to
1.2 deraadt 1342: .Pa $HOME/.ssh/authorized_keys
1343: on all machines
1.137 deraadt 1344: where the user wishes to log in using protocol version 1 RSA authentication.
1.48 markus 1345: The contents of the
1346: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1347: and
1348: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1349: file should be added to
1.115 markus 1350: .Pa $HOME/.ssh/authorized_keys
1.48 markus 1351: on all machines
1.137 deraadt 1352: where the user wishes to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1353: These files are not
1.40 aaron 1354: sensitive and can (but need not) be readable by anyone.
1.48 markus 1355: These files are
1.84 markus 1356: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1357: the convenience of the user.
1.2 deraadt 1358: .It Pa $HOME/.ssh/config
1.40 aaron 1359: This is the per-user configuration file.
1360: The format of this file is described above.
1361: This file is used by the
1.2 deraadt 1362: .Nm
1.40 aaron 1363: client.
1364: This file does not usually contain any sensitive information,
1.1 deraadt 1365: but the recommended permissions are read/write for the user, and not
1366: accessible by others.
1.2 deraadt 1367: .It Pa $HOME/.ssh/authorized_keys
1.115 markus 1368: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.40 aaron 1369: The format of this file is described in the
1.2 deraadt 1370: .Xr sshd 8
1.40 aaron 1371: manual page.
1.116 markus 1372: In the simplest form the format is the same as the .pub
1373: identity files.
1.48 markus 1374: This file is not highly sensitive, but the recommended
1375: permissions are read/write for the user, and not accessible by others.
1.147 deraadt 1376: .It Pa /etc/ssh/ssh_known_hosts
1.40 aaron 1377: Systemwide list of known host keys.
1.116 markus 1378: This file should be prepared by the
1.1 deraadt 1379: system administrator to contain the public host keys of all machines in the
1.40 aaron 1380: organization.
1381: This file should be world-readable.
1382: This file contains
1.1 deraadt 1383: public keys, one per line, in the following format (fields separated
1.116 markus 1384: by spaces): system name, public key and optional comment field.
1.40 aaron 1385: When different names are used
1.1 deraadt 1386: for the same machine, all such names should be listed, separated by
1.40 aaron 1387: commas.
1388: The format is described on the
1.2 deraadt 1389: .Xr sshd 8
1.1 deraadt 1390: manual page.
1.2 deraadt 1391: .Pp
1.1 deraadt 1392: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1393: .Xr sshd 8
1.1 deraadt 1394: to verify the client host when logging in; other names are needed because
1.2 deraadt 1395: .Nm
1.1 deraadt 1396: does not convert the user-supplied name to a canonical name before
1397: checking the key, because someone with access to the name servers
1398: would then be able to fool host authentication.
1.147 deraadt 1399: .It Pa /etc/ssh/ssh_config
1.40 aaron 1400: Systemwide configuration file.
1401: This file provides defaults for those
1.1 deraadt 1402: values that are not specified in the user's configuration file, and
1.40 aaron 1403: for those users who do not have a configuration file.
1404: This file must be world-readable.
1.147 deraadt 1405: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.141 markus 1406: These three files contain the private parts of the host keys
1407: and are used for
1408: .Cm RhostsRSAAuthentication
1409: and
1410: .Cm HostbasedAuthentication .
1411: Since they are readable only by root
1412: .Nm
1413: must be setuid root if these authentication methods are desired.
1.2 deraadt 1414: .It Pa $HOME/.rhosts
1415: This file is used in
1416: .Pa \&.rhosts
1417: authentication to list the
1.40 aaron 1418: host/user pairs that are permitted to log in.
1419: (Note that this file is
1.1 deraadt 1420: also used by rlogin and rsh, which makes using this file insecure.)
1421: Each line of the file contains a host name (in the canonical form
1422: returned by name servers), and then a user name on that host,
1.40 aaron 1423: separated by a space.
1.92 markus 1424: On some machines this file may need to be
1.1 deraadt 1425: world-readable if the user's home directory is on a NFS partition,
1426: because
1.2 deraadt 1427: .Xr sshd 8
1.40 aaron 1428: reads it as root.
1429: Additionally, this file must be owned by the user,
1430: and must not have write permissions for anyone else.
1431: The recommended
1.1 deraadt 1432: permission for most machines is read/write for the user, and not
1433: accessible by others.
1.2 deraadt 1434: .Pp
1.1 deraadt 1435: Note that by default
1.2 deraadt 1436: .Xr sshd 8
1.1 deraadt 1437: will be installed so that it requires successful RSA host
1.40 aaron 1438: authentication before permitting \s+2.\s0rhosts authentication.
1.137 deraadt 1439: If the server machine does not have the client's host key in
1.147 deraadt 1440: .Pa /etc/ssh/ssh_known_hosts ,
1.137 deraadt 1441: it can be stored in
1.2 deraadt 1442: .Pa $HOME/.ssh/known_hosts .
1443: The easiest way to do this is to
1.1 deraadt 1444: connect back to the client from the server machine using ssh; this
1.48 markus 1445: will automatically add the host key to
1.2 deraadt 1446: .Pa $HOME/.ssh/known_hosts .
1447: .It Pa $HOME/.shosts
1448: This file is used exactly the same way as
1449: .Pa \&.rhosts .
1450: The purpose for
1.1 deraadt 1451: having this file is to be able to use rhosts authentication with
1.2 deraadt 1452: .Nm
1453: without permitting login with
1.151 millert 1454: .Nm rlogin
1.2 deraadt 1455: or
1456: .Xr rsh 1 .
1457: .It Pa /etc/hosts.equiv
1458: This file is used during
1.40 aaron 1459: .Pa \&.rhosts authentication.
1460: It contains
1.1 deraadt 1461: canonical hosts names, one per line (the full format is described on
1462: the
1.2 deraadt 1463: .Xr sshd 8
1.40 aaron 1464: manual page).
1465: If the client host is found in this file, login is
1.1 deraadt 1466: automatically permitted provided client and server user names are the
1.40 aaron 1467: same.
1468: Additionally, successful RSA host authentication is normally
1469: required.
1470: This file should only be writable by root.
1.2 deraadt 1471: .It Pa /etc/shosts.equiv
1.44 aaron 1472: This file is processed exactly as
1.2 deraadt 1473: .Pa /etc/hosts.equiv .
1.1 deraadt 1474: This file may be useful to permit logins using
1.2 deraadt 1475: .Nm
1.1 deraadt 1476: but not using rsh/rlogin.
1.147 deraadt 1477: .It Pa /etc/ssh/sshrc
1.1 deraadt 1478: Commands in this file are executed by
1.2 deraadt 1479: .Nm
1.1 deraadt 1480: when the user logs in just before the user's shell (or command) is started.
1481: See the
1.2 deraadt 1482: .Xr sshd 8
1.1 deraadt 1483: manual page for more information.
1.2 deraadt 1484: .It Pa $HOME/.ssh/rc
1.1 deraadt 1485: Commands in this file are executed by
1.2 deraadt 1486: .Nm
1.1 deraadt 1487: when the user logs in just before the user's shell (or command) is
1488: started.
1.44 aaron 1489: See the
1.2 deraadt 1490: .Xr sshd 8
1.1 deraadt 1491: manual page for more information.
1.31 markus 1492: .It Pa $HOME/.ssh/environment
1493: Contains additional definitions for environment variables, see section
1494: .Sx ENVIRONMENT
1495: above.
1.58 itojun 1496: .El
1.145 markus 1497: .Sh DIAGNOSTICS
1498: .Nm
1499: exits with the exit status of the remote command or with 255
1500: if an error occurred.
1.67 aaron 1501: .Sh AUTHORS
1.78 markus 1502: OpenSSH is a derivative of the original and free
1503: ssh 1.2.12 release by Tatu Ylonen.
1504: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1505: Theo de Raadt and Dug Song
1506: removed many bugs, re-added newer features and
1507: created OpenSSH.
1508: Markus Friedl contributed the support for SSH
1509: protocol versions 1.5 and 2.0.
1.2 deraadt 1510: .Sh SEE ALSO
1511: .Xr rsh 1 ,
1512: .Xr scp 1 ,
1.83 djm 1513: .Xr sftp 1 ,
1.2 deraadt 1514: .Xr ssh-add 1 ,
1515: .Xr ssh-agent 1 ,
1516: .Xr ssh-keygen 1 ,
1517: .Xr telnet 1 ,
1.87 itojun 1518: .Xr sshd 8
1.106 markus 1519: .Rs
1520: .%A T. Ylonen
1521: .%A T. Kivinen
1522: .%A M. Saarinen
1523: .%A T. Rinne
1524: .%A S. Lehtinen
1525: .%T "SSH Protocol Architecture"
1.150 markus 1526: .%N draft-ietf-secsh-architecture-12.txt
1527: .%D January 2002
1.106 markus 1528: .%O work in progress material
1529: .Re
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