Annotation of src/usr.bin/ssh/ssh.1, Revision 1.109
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: .\"
1.109 ! stevesk 37: .\" $OpenBSD: ssh.1,v 1.108 2001/04/30 11:18:52 markus 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
1.5 deraadt 47: .Op 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
1.12 aaron 60: .Oo Fl L Xo
61: .Sm off
1.33 markus 62: .Ar port :
1.12 aaron 63: .Ar host :
64: .Ar hostport
65: .Sm on
66: .Xc
67: .Oc
68: .Oo Fl R Xo
69: .Sm off
1.33 markus 70: .Ar port :
1.12 aaron 71: .Ar host :
72: .Ar hostport
73: .Sm on
74: .Xc
75: .Oc
1.5 deraadt 76: .Op Ar hostname | user@hostname
1.2 deraadt 77: .Op Ar command
1.44 aaron 78: .Sh DESCRIPTION
1.2 deraadt 79: .Nm
1.96 deraadt 80: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 81: executing commands on a remote machine.
82: It is intended to replace
1.1 deraadt 83: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 84: two untrusted hosts over an insecure network.
85: X11 connections and
1.1 deraadt 86: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 87: .Pp
88: .Nm
1.44 aaron 89: connects and logs into the specified
1.2 deraadt 90: .Ar hostname .
1.1 deraadt 91: The user must prove
1.49 markus 92: his/her identity to the remote machine using one of several methods
93: depending on the protocol version used:
94: .Pp
95: .Ss SSH protocol version 1
1.2 deraadt 96: .Pp
1.1 deraadt 97: First, if the machine the user logs in from is listed in
1.2 deraadt 98: .Pa /etc/hosts.equiv
1.1 deraadt 99: or
1.2 deraadt 100: .Pa /etc/shosts.equiv
1.1 deraadt 101: on the remote machine, and the user names are
102: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 103: Second, if
1.2 deraadt 104: .Pa \&.rhosts
1.1 deraadt 105: or
1.2 deraadt 106: .Pa \&.shosts
1.1 deraadt 107: exists in the user's home directory on the
108: remote machine and contains a line containing the name of the client
109: machine and the name of the user on that machine, the user is
1.40 aaron 110: permitted to log in.
111: This form of authentication alone is normally not
1.1 deraadt 112: allowed by the server because it is not secure.
1.2 deraadt 113: .Pp
1.107 markus 114: The second authentication method is the
1.2 deraadt 115: .Pa rhosts
1.1 deraadt 116: or
1.2 deraadt 117: .Pa hosts.equiv
1.40 aaron 118: method combined with RSA-based host authentication.
119: It means that if the login would be permitted by
1.49 markus 120: .Pa $HOME/.rhosts ,
121: .Pa $HOME/.shosts ,
1.2 deraadt 122: .Pa /etc/hosts.equiv ,
1.1 deraadt 123: or
1.2 deraadt 124: .Pa /etc/shosts.equiv ,
1.11 deraadt 125: and if additionally the server can verify the client's
1.44 aaron 126: host key (see
1.2 deraadt 127: .Pa /etc/ssh_known_hosts
1.23 markus 128: and
129: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 130: in the
1.2 deraadt 131: .Sx FILES
1.40 aaron 132: section), only then login is permitted.
133: This authentication method closes security holes due to IP
134: spoofing, DNS spoofing and routing spoofing.
135: [Note to the administrator:
1.2 deraadt 136: .Pa /etc/hosts.equiv ,
1.49 markus 137: .Pa $HOME/.rhosts ,
1.1 deraadt 138: and the rlogin/rsh protocol in general, are inherently insecure and should be
139: disabled if security is desired.]
1.2 deraadt 140: .Pp
1.44 aaron 141: As a third authentication method,
1.2 deraadt 142: .Nm
1.1 deraadt 143: supports RSA based authentication.
144: The scheme is based on public-key cryptography: there are cryptosystems
145: where encryption and decryption are done using separate keys, and it
146: is not possible to derive the decryption key from the encryption key.
1.40 aaron 147: RSA is one such system.
1.44 aaron 148: The idea is that each user creates a public/private
1.40 aaron 149: key pair for authentication purposes.
150: The server knows the public key, and only the user knows the private key.
1.44 aaron 151: The file
1.2 deraadt 152: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 153: lists the public keys that are permitted for logging
1.40 aaron 154: in.
155: When the user logs in, the
1.2 deraadt 156: .Nm
1.1 deraadt 157: program tells the server which key pair it would like to use for
1.40 aaron 158: authentication.
159: The server checks if this key is permitted, and if
1.1 deraadt 160: so, sends the user (actually the
1.2 deraadt 161: .Nm
1.1 deraadt 162: program running on behalf of the user) a challenge, a random number,
1.40 aaron 163: encrypted by the user's public key.
164: The challenge can only be
165: decrypted using the proper private key.
166: The user's client then decrypts the
1.1 deraadt 167: challenge using the private key, proving that he/she knows the private
168: key but without disclosing it to the server.
1.2 deraadt 169: .Pp
170: .Nm
1.40 aaron 171: implements the RSA authentication protocol automatically.
172: The user creates his/her RSA key pair by running
1.2 deraadt 173: .Xr ssh-keygen 1 .
1.44 aaron 174: This stores the private key in
1.49 markus 175: .Pa $HOME/.ssh/identity
1.1 deraadt 176: and the public key in
1.49 markus 177: .Pa $HOME/.ssh/identity.pub
1.40 aaron 178: in the user's home directory.
179: The user should then copy the
1.2 deraadt 180: .Pa identity.pub
1.44 aaron 181: to
1.49 markus 182: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 183: in his/her home directory on the remote machine (the
1.2 deraadt 184: .Pa authorized_keys
1.44 aaron 185: file corresponds to the conventional
1.49 markus 186: .Pa $HOME/.rhosts
1.1 deraadt 187: file, and has one key
1.40 aaron 188: per line, though the lines can be very long).
189: After this, the user can log in without giving the password.
190: RSA authentication is much
1.1 deraadt 191: more secure than rhosts authentication.
1.2 deraadt 192: .Pp
1.1 deraadt 193: The most convenient way to use RSA authentication may be with an
1.40 aaron 194: authentication agent.
195: See
1.2 deraadt 196: .Xr ssh-agent 1
1.1 deraadt 197: for more information.
1.2 deraadt 198: .Pp
1.44 aaron 199: If other authentication methods fail,
1.2 deraadt 200: .Nm
1.40 aaron 201: prompts the user for a password.
202: The password is sent to the remote
1.1 deraadt 203: host for checking; however, since all communications are encrypted,
204: the password cannot be seen by someone listening on the network.
1.2 deraadt 205: .Pp
1.49 markus 206: .Ss SSH protocol version 2
207: .Pp
208: When a user connects using the protocol version 2
1.107 markus 209: different authentication methods are available.
210: Using the default values for
211: .Cm PreferredAuthentications ,
212: the client will try to authenticate first using the public key method;
213: if this method fails password authentication is attempted,
214: and finally if this method fails keyboard-interactive authentication
215: is attempted.
216: If this method fails password authentication is
217: 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
227: .Pa $HOME/.ssh/authorized_keys2
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
1.105 djm 272: through the use of an escape character.
1.104 djm 273: .Pp
274: A single tilde character can be sent as
275: .Ic ~~
276: (or by following the tilde by a character other than those described above).
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
281: configuration directive or on the command line by the
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
1.1 deraadt 307: If the user is using X11 (the
1.2 deraadt 308: .Ev DISPLAY
1.1 deraadt 309: environment variable is set), the connection to the X11 display is
310: automatically forwarded to the remote side in such a way that any X11
311: programs started from the shell (or command) will go through the
312: encrypted channel, and the connection to the real X server will be made
1.40 aaron 313: from the local machine.
314: The user should not manually set
1.2 deraadt 315: .Ev DISPLAY .
1.1 deraadt 316: Forwarding of X11 connections can be
317: configured on the command line or in configuration files.
1.2 deraadt 318: .Pp
319: The
1.44 aaron 320: .Ev DISPLAY
1.2 deraadt 321: value set by
322: .Nm
1.1 deraadt 323: will point to the server machine, but with a display number greater
1.40 aaron 324: than zero.
325: This is normal, and happens because
1.2 deraadt 326: .Nm
327: creates a
328: .Dq proxy
329: X server on the server machine for forwarding the
1.1 deraadt 330: connections over the encrypted channel.
1.2 deraadt 331: .Pp
332: .Nm
1.1 deraadt 333: will also automatically set up Xauthority data on the server machine.
334: For this purpose, it will generate a random authorization cookie,
335: store it in Xauthority on the server, and verify that any forwarded
336: connections carry this cookie and replace it by the real cookie when
1.40 aaron 337: the connection is opened.
338: The real authentication cookie is never
1.1 deraadt 339: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 340: .Pp
1.1 deraadt 341: If the user is using an authentication agent, the connection to the agent
342: is automatically forwarded to the remote side unless disabled on
343: command line or in a configuration file.
1.2 deraadt 344: .Pp
1.1 deraadt 345: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.40 aaron 346: be specified either on command line or in a configuration file.
347: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 348: electronic purse; another is going through firewalls.
1.2 deraadt 349: .Pp
1.49 markus 350: .Ss Server authentication
351: .Pp
1.2 deraadt 352: .Nm
1.49 markus 353: automatically maintains and checks a database containing
1.40 aaron 354: identifications for all hosts it has ever been used with.
1.49 markus 355: RSA host keys are stored in
356: .Pa $HOME/.ssh/known_hosts
357: and
1.68 markus 358: host keys used in the protocol version 2 are stored in
1.49 markus 359: .Pa $HOME/.ssh/known_hosts2
1.40 aaron 360: in the user's home directory.
1.49 markus 361: Additionally, the files
1.2 deraadt 362: .Pa /etc/ssh_known_hosts
1.49 markus 363: and
364: .Pa /etc/ssh_known_hosts2
365: are automatically checked for known hosts.
1.40 aaron 366: Any new hosts are automatically added to the user's file.
367: If a host's identification
1.1 deraadt 368: ever changes,
1.2 deraadt 369: .Nm
1.1 deraadt 370: warns about this and disables password authentication to prevent a
1.40 aaron 371: trojan horse from getting the user's password.
372: Another purpose of
1.1 deraadt 373: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 374: otherwise be used to circumvent the encryption.
375: The
1.2 deraadt 376: .Cm StrictHostKeyChecking
1.1 deraadt 377: option (see below) can be used to prevent logins to machines whose
378: host key is not known or has changed.
1.65 aaron 379: .Pp
380: The options are as follows:
1.2 deraadt 381: .Bl -tag -width Ds
1.4 dugsong 382: .It Fl a
1.42 aaron 383: Disables forwarding of the authentication agent connection.
1.54 markus 384: .It Fl A
385: Enables forwarding of the authentication agent connection.
386: This can also be specified on a per-host basis in a configuration file.
1.108 markus 387: .It Fl b Ar bind_address
388: Specify the interface to transmit from on machines with multiple
389: interfaces or aliased addresses.
1.6 deraadt 390: .It Fl c Ar blowfish|3des
1.44 aaron 391: Selects the cipher to use for encrypting the session.
1.2 deraadt 392: .Ar 3des
1.40 aaron 393: is used by default.
1.44 aaron 394: It is believed to be secure.
1.5 deraadt 395: .Ar 3des
396: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
397: It is presumably more secure than the
1.2 deraadt 398: .Ar des
1.64 markus 399: cipher which is no longer fully supported in
1.51 markus 400: .Nm ssh .
1.5 deraadt 401: .Ar blowfish
402: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 403: .Ar 3des .
1.90 markus 404: .It Fl c Ar cipher_spec
1.51 markus 405: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 406: be specified in order of preference.
1.90 markus 407: See
408: .Cm Ciphers
409: for more information.
1.2 deraadt 410: .It Fl e Ar ch|^ch|none
411: Sets the escape character for sessions with a pty (default:
412: .Ql ~ ) .
1.40 aaron 413: The escape character is only recognized at the beginning of a line.
414: The escape character followed by a dot
1.2 deraadt 415: .Pq Ql \&.
416: closes the connection, followed
1.1 deraadt 417: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 418: escape character once.
419: Setting the character to
1.2 deraadt 420: .Dq none
421: disables any escapes and makes the session fully transparent.
422: .It Fl f
423: Requests
424: .Nm
1.40 aaron 425: to go to background just before command execution.
426: This is useful if
1.2 deraadt 427: .Nm
428: is going to ask for passwords or passphrases, but the user
1.40 aaron 429: wants it in the background.
1.44 aaron 430: This implies
1.2 deraadt 431: .Fl n .
1.1 deraadt 432: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 433: something like
434: .Ic ssh -f host xterm .
1.34 markus 435: .It Fl g
436: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 437: .It Fl i Ar identity_file
1.44 aaron 438: Selects the file from which the identity (private key) for
1.68 markus 439: RSA or DSA authentication is read.
1.44 aaron 440: Default is
1.49 markus 441: .Pa $HOME/.ssh/identity
1.40 aaron 442: in the user's home directory.
443: Identity files may also be specified on
444: a per-host basis in the configuration file.
445: It is possible to have multiple
1.2 deraadt 446: .Fl i
447: options (and multiple identities specified in
1.1 deraadt 448: configuration files).
1.2 deraadt 449: .It Fl k
1.42 aaron 450: Disables forwarding of Kerberos tickets and AFS tokens.
451: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 452: .It Fl l Ar login_name
1.40 aaron 453: Specifies the user to log in as on the remote machine.
454: This also may be specified on a per-host basis in the configuration file.
1.91 markus 455: .It Fl m Ar mac_spec
456: Additionally, for protocol version 2 a comma-separated list of MAC
457: (message authentication code) algorithms can
458: be specified in order of preference.
459: See the
460: .Cm MACs
461: keyword for more information.
1.2 deraadt 462: .It Fl n
463: Redirects stdin from
464: .Pa /dev/null
465: (actually, prevents reading from stdin).
1.1 deraadt 466: This must be used when
1.2 deraadt 467: .Nm
1.40 aaron 468: is run in the background.
469: A common trick is to use this to run X11 programs on a remote machine.
470: For example,
1.2 deraadt 471: .Ic ssh -n shadows.cs.hut.fi emacs &
472: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 473: connection will be automatically forwarded over an encrypted channel.
474: The
1.2 deraadt 475: .Nm
1.1 deraadt 476: program will be put in the background.
477: (This does not work if
1.2 deraadt 478: .Nm
479: needs to ask for a password or passphrase; see also the
480: .Fl f
481: option.)
1.53 markus 482: .It Fl N
483: Do not execute a remote command.
1.70 markus 484: This is useful if you just want to forward ports
1.53 markus 485: (protocol version 2 only).
1.2 deraadt 486: .It Fl o Ar option
1.1 deraadt 487: Can be used to give options in the format used in the config file.
488: This is useful for specifying options for which there is no separate
1.40 aaron 489: command-line flag.
490: The option has the same format as a line in the configuration file.
1.2 deraadt 491: .It Fl p Ar port
1.40 aaron 492: Port to connect to on the remote host.
493: This can be specified on a
1.1 deraadt 494: per-host basis in the configuration file.
1.16 markus 495: .It Fl P
496: Use a non-privileged port for outgoing connections.
497: This can be used if your firewall does
498: not permit connections from privileged ports.
1.30 provos 499: Note that this option turns off
1.16 markus 500: .Cm RhostsAuthentication
501: and
1.72 markus 502: .Cm RhostsRSAAuthentication
503: for older servers.
1.2 deraadt 504: .It Fl q
1.40 aaron 505: Quiet mode.
506: Causes all warning and diagnostic messages to be suppressed.
507: Only fatal errors are displayed.
1.80 djm 508: .It Fl s
509: 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
1.109 ! stevesk 510: of SSH as a secure transport for other applications (eg. sftp). The
1.80 djm 511: subsystem is specified as the remote command.
1.2 deraadt 512: .It Fl t
1.40 aaron 513: Force pseudo-tty allocation.
1.43 brad 514: This can be used to execute arbitrary
1.40 aaron 515: screen-based programs on a remote machine, which can be very useful,
516: e.g., when implementing menu services.
1.73 markus 517: Multiple
518: .Fl t
519: options force tty allocation, even if
520: .Nm
521: has no local tty.
1.53 markus 522: .It Fl T
1.69 markus 523: Disable pseudo-tty allocation.
1.2 deraadt 524: .It Fl v
1.40 aaron 525: Verbose mode.
526: Causes
1.2 deraadt 527: .Nm
1.40 aaron 528: to print debugging messages about its progress.
529: This is helpful in
1.1 deraadt 530: debugging connection, authentication, and configuration problems.
1.73 markus 531: Multiple
532: .Fl v
533: options increases the verbosity.
1.61 aaron 534: Maximum is 3.
1.2 deraadt 535: .It Fl x
1.40 aaron 536: Disables X11 forwarding.
1.2 deraadt 537: .It Fl X
1.1 deraadt 538: Enables X11 forwarding.
1.54 markus 539: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 540: .It Fl C
1.1 deraadt 541: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 542: data for forwarded X11 and TCP/IP connections).
543: The compression algorithm is the same used by
1.34 markus 544: .Xr gzip 1 ,
545: and the
1.2 deraadt 546: .Dq level
547: can be controlled by the
548: .Cm CompressionLevel
1.40 aaron 549: option (see below).
550: Compression is desirable on modem lines and other
1.1 deraadt 551: slow connections, but will only slow down things on fast networks.
552: The default value can be set on a host-by-host basis in the
553: configuration files; see the
1.2 deraadt 554: .Cm Compress
1.1 deraadt 555: option below.
1.2 deraadt 556: .It Fl L Ar port:host:hostport
1.1 deraadt 557: Specifies that the given port on the local (client) host is to be
1.40 aaron 558: forwarded to the given host and port on the remote side.
559: This works by allocating a socket to listen to
1.2 deraadt 560: .Ar port
1.1 deraadt 561: on the local side, and whenever a connection is made to this port, the
562: connection is forwarded over the secure channel, and a connection is
563: made to
1.32 markus 564: .Ar host
565: port
566: .Ar hostport
1.40 aaron 567: from the remote machine.
568: Port forwardings can also be specified in the configuration file.
569: Only root can forward privileged ports.
1.32 markus 570: IPv6 addresses can be specified with an alternative syntax:
571: .Ar port/host/hostport
1.2 deraadt 572: .It Fl R Ar port:host:hostport
1.1 deraadt 573: Specifies that the given port on the remote (server) host is to be
1.40 aaron 574: forwarded to the given host and port on the local side.
575: This works by allocating a socket to listen to
1.2 deraadt 576: .Ar port
1.1 deraadt 577: on the remote side, and whenever a connection is made to this port, the
578: connection is forwarded over the secure channel, and a connection is
579: made to
1.32 markus 580: .Ar host
581: port
582: .Ar hostport
1.40 aaron 583: from the local machine.
584: Port forwardings can also be specified in the configuration file.
585: Privileged ports can be forwarded only when
1.1 deraadt 586: logging in as root on the remote machine.
1.107 markus 587: IPv6 addresses can be specified with an alternative syntax:
588: .Ar port/host/hostport
1.85 jakob 589: .It Fl 1
590: Forces
591: .Nm
592: to try protocol version 1 only.
1.46 markus 593: .It Fl 2
594: Forces
595: .Nm
1.50 markus 596: to try protocol version 2 only.
1.32 markus 597: .It Fl 4
598: Forces
599: .Nm
600: to use IPv4 addresses only.
601: .It Fl 6
602: Forces
603: .Nm
604: to use IPv6 addresses only.
1.2 deraadt 605: .El
606: .Sh CONFIGURATION FILES
607: .Nm
1.1 deraadt 608: obtains configuration data from the following sources (in this order):
609: command line options, user's configuration file
1.2 deraadt 610: .Pq Pa $HOME/.ssh/config ,
611: and system-wide configuration file
612: .Pq Pa /etc/ssh_config .
613: For each parameter, the first obtained value
1.40 aaron 614: will be used.
615: The configuration files contain sections bracketed by
616: .Dq Host
617: specifications, and that section is only applied for hosts that
618: match one of the patterns given in the specification.
619: The matched host name is the one given on the command line.
1.2 deraadt 620: .Pp
1.1 deraadt 621: Since the first obtained value for each parameter is used, more
622: host-specific declarations should be given near the beginning of the
623: file, and general defaults at the end.
1.2 deraadt 624: .Pp
1.1 deraadt 625: The configuration file has the following format:
1.2 deraadt 626: .Pp
627: Empty lines and lines starting with
628: .Ql #
629: are comments.
630: .Pp
631: Otherwise a line is of the format
632: .Dq keyword arguments .
633: The possible
1.1 deraadt 634: keywords and their meanings are as follows (note that the
635: configuration files are case-sensitive):
1.2 deraadt 636: .Bl -tag -width Ds
637: .It Cm Host
1.1 deraadt 638: Restricts the following declarations (up to the next
1.2 deraadt 639: .Cm Host
1.1 deraadt 640: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 641: given after the keyword.
642: .Ql \&*
643: and
644: .Ql ?
645: can be used as wildcards in the
1.40 aaron 646: patterns.
647: A single
1.2 deraadt 648: .Ql \&*
649: as a pattern can be used to provide global
1.40 aaron 650: defaults for all hosts.
651: The host is the
1.2 deraadt 652: .Ar hostname
1.1 deraadt 653: argument given on the command line (i.e., the name is not converted to
654: a canonicalized host name before matching).
1.2 deraadt 655: .It Cm AFSTokenPassing
1.42 aaron 656: Specifies whether to pass AFS tokens to remote host.
657: The argument to this keyword must be
1.2 deraadt 658: .Dq yes
659: or
660: .Dq no .
1.107 markus 661: This option applies to protocol version 1 only.
1.2 deraadt 662: .It Cm BatchMode
663: If set to
664: .Dq yes ,
1.40 aaron 665: passphrase/password querying will be disabled.
666: This option is useful in scripts and other batch jobs where you have no
667: user to supply the password.
668: The argument must be
1.2 deraadt 669: .Dq yes
670: or
671: .Dq no .
1.100 stevesk 672: The default is
673: .Dq no .
1.108 markus 674: .It Cm BindAddress
675: Specify the interface to transmit from on machines with multiple
676: interfaces or aliased addresses.
677: Note that this option does not work if
678: .Cm UsePrivilegedPort
679: is set to
680: .Dq yes .
1.34 markus 681: .It Cm CheckHostIP
682: If this flag is set to
683: .Dq yes ,
1.100 stevesk 684: ssh will additionally check the host IP address in the
1.34 markus 685: .Pa known_hosts
1.42 aaron 686: file.
687: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 688: If the option is set to
689: .Dq no ,
690: the check will not be executed.
1.100 stevesk 691: The default is
692: .Dq yes .
1.2 deraadt 693: .It Cm Cipher
1.62 markus 694: Specifies the cipher to use for encrypting the session
1.64 markus 695: in protocol version 1.
1.40 aaron 696: Currently,
1.64 markus 697: .Dq blowfish
1.1 deraadt 698: and
1.10 provos 699: .Dq 3des
1.40 aaron 700: are supported.
701: The default is
1.2 deraadt 702: .Dq 3des .
1.45 markus 703: .It Cm Ciphers
704: Specifies the ciphers allowed for protocol version 2
705: in order of preference.
706: Multiple ciphers must be comma-separated.
707: The default is
1.88 provos 708: .Pp
709: .Bd -literal
1.94 deraadt 710: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.107 markus 711: aes192-cbc,aes256-cbc''
1.88 provos 712: .Ed
1.2 deraadt 713: .It Cm Compression
1.40 aaron 714: Specifies whether to use compression.
715: The argument must be
1.2 deraadt 716: .Dq yes
717: or
718: .Dq no .
1.100 stevesk 719: The default is
720: .Dq no .
1.2 deraadt 721: .It Cm CompressionLevel
1.100 stevesk 722: Specifies the compression level to use if compression is enabled.
1.40 aaron 723: The argument must be an integer from 1 (fast) to 9 (slow, best).
724: The default level is 6, which is good for most applications.
725: The meaning of the values is the same as in
1.34 markus 726: .Xr gzip 1 .
1.107 markus 727: Note that this option applies to protocol version 1 only.
1.2 deraadt 728: .It Cm ConnectionAttempts
1.1 deraadt 729: Specifies the number of tries (one per second) to make before falling
1.40 aaron 730: back to rsh or exiting.
731: The argument must be an integer.
732: This may be useful in scripts if the connection sometimes fails.
1.100 stevesk 733: The default is 4.
1.2 deraadt 734: .It Cm EscapeChar
735: Sets the escape character (default:
736: .Ql ~ ) .
737: The escape character can also
1.40 aaron 738: be set on the command line.
739: The argument should be a single character,
1.2 deraadt 740: .Ql ^
741: followed by a letter, or
742: .Dq none
743: to disable the escape
1.1 deraadt 744: character entirely (making the connection transparent for binary
745: data).
1.44 aaron 746: .It Cm FallBackToRsh
1.1 deraadt 747: Specifies that if connecting via
1.2 deraadt 748: .Nm
1.1 deraadt 749: fails due to a connection refused error (there is no
1.2 deraadt 750: .Xr sshd 8
1.44 aaron 751: listening on the remote host),
1.2 deraadt 752: .Xr rsh 1
1.1 deraadt 753: should automatically be used instead (after a suitable warning about
1.40 aaron 754: the session being unencrypted).
755: The argument must be
1.2 deraadt 756: .Dq yes
757: or
758: .Dq no .
1.100 stevesk 759: The default is
760: .Dq no .
1.2 deraadt 761: .It Cm ForwardAgent
1.1 deraadt 762: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 763: will be forwarded to the remote machine.
764: The argument must be
1.2 deraadt 765: .Dq yes
766: or
1.54 markus 767: .Dq no .
768: The default is
1.2 deraadt 769: .Dq no .
770: .It Cm ForwardX11
1.1 deraadt 771: Specifies whether X11 connections will be automatically redirected
1.44 aaron 772: over the secure channel and
1.2 deraadt 773: .Ev DISPLAY
1.40 aaron 774: set.
1.44 aaron 775: The argument must be
1.2 deraadt 776: .Dq yes
777: or
1.38 markus 778: .Dq no .
779: The default is
1.3 deraadt 780: .Dq no .
781: .It Cm GatewayPorts
782: Specifies whether remote hosts are allowed to connect to local
783: forwarded ports.
784: The argument must be
785: .Dq yes
786: or
787: .Dq no .
788: The default is
1.2 deraadt 789: .Dq no .
790: .It Cm GlobalKnownHostsFile
1.95 stevesk 791: Specifies a file to use for the protocol version 1 global
792: host key database instead of
1.2 deraadt 793: .Pa /etc/ssh_known_hosts .
1.95 stevesk 794: .It Cm GlobalKnownHostsFile2
795: Specifies a file to use for the protocol version 2 global
796: host key database instead of
797: .Pa /etc/ssh_known_hosts2 .
1.107 markus 798: .It Cm HostbasedAuthentication
799: Specifies whether to try rhosts based authentication with public key
800: authentication.
801: The argument must be
802: .Dq yes
803: or
804: .Dq no .
805: The default is
806: .Dq yes .
807: This option applies to protocol version 2 only and
808: is similar to
809: .Cm RhostsRSAAuthentication .
810: .It Cm HostKeyAlgorithms
1.109 ! stevesk 811: Specifies the protocol version 2 host key algorithms
1.107 markus 812: that the client wants to use in order of preference.
813: The default for this option is:
814: .Dq ssh-rsa,ssh-dss
1.74 markus 815: .It Cm HostKeyAlias
816: Specifies an alias that should be used instead of the
817: real host name when looking up or saving the host key
1.107 markus 818: in the host key database files.
1.82 stevesk 819: This option is useful for tunneling ssh connections
1.74 markus 820: or if you have multiple servers running on a single host.
1.2 deraadt 821: .It Cm HostName
1.40 aaron 822: Specifies the real host name to log into.
823: This can be used to specify nicknames or abbreviations for hosts.
824: Default is the name given on the command line.
825: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 826: .Cm HostName
1.1 deraadt 827: specifications).
1.2 deraadt 828: .It Cm IdentityFile
1.107 markus 829: Specifies the file from which the user's RSA or DSA authentication identity
1.2 deraadt 830: is read (default
1.49 markus 831: .Pa $HOME/.ssh/identity
1.2 deraadt 832: in the user's home directory).
1.1 deraadt 833: Additionally, any identities represented by the authentication agent
1.40 aaron 834: will be used for authentication.
835: The file name may use the tilde
836: syntax to refer to a user's home directory.
837: It is possible to have
1.1 deraadt 838: multiple identity files specified in configuration files; all these
839: identities will be tried in sequence.
1.2 deraadt 840: .It Cm KeepAlive
1.1 deraadt 841: Specifies whether the system should send keepalive messages to the
1.40 aaron 842: other side.
843: If they are sent, death of the connection or crash of one
844: of the machines will be properly noticed.
845: However, this means that
1.1 deraadt 846: connections will die if the route is down temporarily, and some people
1.41 aaron 847: find it annoying.
1.2 deraadt 848: .Pp
849: The default is
850: .Dq yes
851: (to send keepalives), and the client will notice
1.40 aaron 852: if the network goes down or the remote host dies.
853: This is important in scripts, and many users want it too.
1.2 deraadt 854: .Pp
855: To disable keepalives, the value should be set to
856: .Dq no
857: in both the server and the client configuration files.
858: .It Cm KerberosAuthentication
1.42 aaron 859: Specifies whether Kerberos authentication will be used.
860: The argument to this keyword must be
1.4 dugsong 861: .Dq yes
862: or
863: .Dq no .
1.2 deraadt 864: .It Cm KerberosTgtPassing
1.42 aaron 865: Specifies whether a Kerberos TGT will be forwarded to the server.
866: This will only work if the Kerberos server is actually an AFS kaserver.
867: The argument to this keyword must be
1.4 dugsong 868: .Dq yes
869: or
870: .Dq no .
1.2 deraadt 871: .It Cm LocalForward
1.1 deraadt 872: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 873: the secure channel to given host:port from the remote machine.
874: The first argument must be a port number, and the second must be
875: host:port.
876: Multiple forwardings may be specified, and additional
877: forwardings can be given on the command line.
878: Only the superuser can forward privileged ports.
1.24 markus 879: .It Cm LogLevel
880: Gives the verbosity level that is used when logging messages from
881: .Nm ssh .
882: The possible values are:
1.77 markus 883: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
884: The default is INFO.
1.91 markus 885: .It Cm MACs
886: Specifies the MAC (message authentication code) algorithms
887: in order of preference.
888: The MAC algorithm is used in protocol version 2
889: for data integrity protection.
890: Multiple algorithms must be comma-separated.
891: The default is
892: .Pp
893: .Bd -literal
1.94 deraadt 894: ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
1.91 markus 895: hmac-sha1-96,hmac-md5-96''
896: .Ed
1.14 dugsong 897: .It Cm NumberOfPasswordPrompts
1.42 aaron 898: Specifies the number of password prompts before giving up.
899: The argument to this keyword must be an integer.
900: Default is 3.
1.34 markus 901: .It Cm PasswordAuthentication
1.40 aaron 902: Specifies whether to use password authentication.
903: The argument to this keyword must be
1.34 markus 904: .Dq yes
905: or
906: .Dq no .
1.100 stevesk 907: The default is
908: .Dq yes .
1.2 deraadt 909: .It Cm Port
1.40 aaron 910: Specifies the port number to connect on the remote host.
911: Default is 22.
1.99 djm 912: .It Cm PreferredAuthentications
913: Specifies the order in which the client should try protocol 2
914: authentication methods. This allows a client to prefer one method (e.g.
915: .Cm keyboard-interactive )
916: over another method (e.g.
917: .Cm password )
918: The default for this option is:
919: .Dq publickey, password, keyboard-interactive
1.45 markus 920: .It Cm Protocol
921: Specifies the protocol versions
922: .Nm
923: should support in order of preference.
924: The possible values are
925: .Dq 1
926: and
927: .Dq 2 .
928: Multiple versions must be comma-separated.
929: The default is
1.101 markus 930: .Dq 2,1 .
1.49 markus 931: This means that
932: .Nm
1.101 markus 933: tries version 2 and falls back to version 1
934: if version 2 is not available.
1.2 deraadt 935: .It Cm ProxyCommand
1.40 aaron 936: Specifies the command to use to connect to the server.
937: The command
938: string extends to the end of the line, and is executed with
939: .Pa /bin/sh .
940: In the command string,
941: .Ql %h
942: will be substituted by the host name to
943: connect and
944: .Ql %p
945: by the port.
946: The command can be basically anything,
947: and should read from its standard input and write to its standard output.
948: It should eventually connect an
1.2 deraadt 949: .Xr sshd 8
1.1 deraadt 950: server running on some machine, or execute
1.2 deraadt 951: .Ic sshd -i
1.40 aaron 952: somewhere.
953: Host key management will be done using the
1.1 deraadt 954: HostName of the host being connected (defaulting to the name typed by
955: the user).
1.29 markus 956: Note that
957: .Cm CheckHostIP
958: is not available for connects with a proxy command.
1.2 deraadt 959: .Pp
1.107 markus 960: .It Cm PubkeyAuthentication
961: Specifies whether to try public key authentication.
962: The argument to this keyword must be
963: .Dq yes
964: or
965: .Dq no .
966: The default is
967: .Dq yes .
968: This option applies to protocol version 2 only.
1.2 deraadt 969: .It Cm RemoteForward
1.1 deraadt 970: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 971: the secure channel to given host:port from the local machine.
972: The first argument must be a port number, and the second must be
973: host:port.
974: Multiple forwardings may be specified, and additional
975: forwardings can be given on the command line.
976: Only the superuser can forward privileged ports.
1.2 deraadt 977: .It Cm RhostsAuthentication
1.40 aaron 978: Specifies whether to try rhosts based authentication.
979: Note that this
1.1 deraadt 980: declaration only affects the client side and has no effect whatsoever
1.40 aaron 981: on security.
982: Disabling rhosts authentication may reduce
1.1 deraadt 983: authentication time on slow connections when rhosts authentication is
1.40 aaron 984: not used.
985: Most servers do not permit RhostsAuthentication because it
1.107 markus 986: is not secure (see
987: .Cm RhostsRSAAuthentication ).
1.40 aaron 988: The argument to this keyword must be
1.2 deraadt 989: .Dq yes
990: or
991: .Dq no .
1.100 stevesk 992: The default is
993: .Dq yes .
1.107 markus 994: This option applies to protocol version 1 only.
1.2 deraadt 995: .It Cm RhostsRSAAuthentication
1.1 deraadt 996: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 997: authentication.
998: The argument must be
1.2 deraadt 999: .Dq yes
1000: or
1001: .Dq no .
1.100 stevesk 1002: The default is
1003: .Dq yes .
1.107 markus 1004: This option applies to protocol version 1 only.
1.2 deraadt 1005: .It Cm RSAAuthentication
1.40 aaron 1006: Specifies whether to try RSA authentication.
1007: The argument to this keyword must be
1.2 deraadt 1008: .Dq yes
1009: or
1010: .Dq no .
1.1 deraadt 1011: RSA authentication will only be
1012: attempted if the identity file exists, or an authentication agent is
1013: running.
1.100 stevesk 1014: The default is
1015: .Dq yes .
1.50 markus 1016: Note that this option applies to protocol version 1 only.
1.81 markus 1017: .It Cm ChallengeResponseAuthentication
1018: Specifies whether to use challenge response authentication.
1019: Currently there is only support for
1.27 markus 1020: .Xr skey 1
1.40 aaron 1021: authentication.
1022: The argument to this keyword must be
1.27 markus 1023: .Dq yes
1024: or
1025: .Dq no .
1026: The default is
1027: .Dq no .
1.2 deraadt 1028: .It Cm StrictHostKeyChecking
1029: If this flag is set to
1.44 aaron 1030: .Dq yes ,
1.2 deraadt 1031: .Nm
1.79 stevesk 1032: will never automatically add host keys to the
1.2 deraadt 1033: .Pa $HOME/.ssh/known_hosts
1.48 markus 1034: and
1035: .Pa $HOME/.ssh/known_hosts2
1.79 stevesk 1036: files, and refuses to connect to hosts whose host key has changed.
1.40 aaron 1037: This provides maximum protection against trojan horse attacks.
1038: However, it can be somewhat annoying if you don't have good
1.2 deraadt 1039: .Pa /etc/ssh_known_hosts
1.48 markus 1040: and
1041: .Pa /etc/ssh_known_hosts2
1.1 deraadt 1042: files installed and frequently
1.79 stevesk 1043: connect to new hosts.
1044: This option forces the user to manually
1045: add all new hosts.
1046: If this flag is set to
1047: .Dq no ,
1048: .Nm
1049: will automatically add new host keys to the
1050: user known hosts files.
1051: If this flag is set to
1052: .Dq ask ,
1053: new host keys
1054: will be added to the user known host files only after the user
1055: has confirmed that is what they really want to do, and
1056: .Nm
1057: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1058: The host keys of
1.79 stevesk 1059: known hosts will be verified automatically in all cases.
1.40 aaron 1060: The argument must be
1.79 stevesk 1061: .Dq yes ,
1062: .Dq no
1.2 deraadt 1063: or
1.79 stevesk 1064: .Dq ask .
1065: The default is
1066: .Dq ask .
1.16 markus 1067: .It Cm UsePrivilegedPort
1068: Specifies whether to use a privileged port for outgoing connections.
1069: The argument must be
1070: .Dq yes
1071: or
1072: .Dq no .
1073: The default is
1.98 markus 1074: .Dq no .
1.107 markus 1075: Note that you need to set this option to
1076: .Dq yes
1077: if you want to use
1.16 markus 1078: .Cm RhostsAuthentication
1079: and
1.72 markus 1080: .Cm RhostsRSAAuthentication
1.107 markus 1081: with older servers.
1.34 markus 1082: .It Cm User
1.40 aaron 1083: Specifies the user to log in as.
1084: This can be useful if you have a different user name on different machines.
1085: This saves the trouble of
1.34 markus 1086: having to remember to give the user name on the command line.
1087: .It Cm UserKnownHostsFile
1.95 stevesk 1088: Specifies a file to use for the protocol version 1 user
1089: host key database instead of
1.34 markus 1090: .Pa $HOME/.ssh/known_hosts .
1.95 stevesk 1091: .It Cm UserKnownHostsFile2
1092: Specifies a file to use for the protocol version 2 user
1093: host key database instead of
1094: .Pa $HOME/.ssh/known_hosts2 .
1.2 deraadt 1095: .It Cm UseRsh
1.40 aaron 1096: Specifies that rlogin/rsh should be used for this host.
1097: It is possible that the host does not at all support the
1.2 deraadt 1098: .Nm
1.40 aaron 1099: protocol.
1100: This causes
1.2 deraadt 1101: .Nm
1.40 aaron 1102: to immediately execute
1.2 deraadt 1103: .Xr rsh 1 .
1.1 deraadt 1104: All other options (except
1.2 deraadt 1105: .Cm HostName )
1.40 aaron 1106: are ignored if this has been specified.
1107: The argument must be
1.2 deraadt 1108: .Dq yes
1109: or
1110: .Dq no .
1.55 markus 1111: .It Cm XAuthLocation
1112: Specifies the location of the
1113: .Xr xauth 1
1114: program.
1115: The default is
1116: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1117: .El
1.2 deraadt 1118: .Sh ENVIRONMENT
1119: .Nm
1.1 deraadt 1120: will normally set the following environment variables:
1.2 deraadt 1121: .Bl -tag -width Ds
1122: .It Ev DISPLAY
1123: The
1124: .Ev DISPLAY
1.40 aaron 1125: variable indicates the location of the X11 server.
1.44 aaron 1126: It is automatically set by
1.2 deraadt 1127: .Nm
1128: to point to a value of the form
1129: .Dq hostname:n
1130: where hostname indicates
1.40 aaron 1131: the host where the shell runs, and n is an integer >= 1.
1132: .Nm
1133: uses this special value to forward X11 connections over the secure
1134: channel.
1.107 markus 1135: The user should normally not set
1136: .Ev DISPLAY
1137: explicitly, as that
1.1 deraadt 1138: will render the X11 connection insecure (and will require the user to
1139: manually copy any required authorization cookies).
1.2 deraadt 1140: .It Ev HOME
1.1 deraadt 1141: Set to the path of the user's home directory.
1.2 deraadt 1142: .It Ev LOGNAME
1143: Synonym for
1.12 aaron 1144: .Ev USER ;
1145: set for compatibility with systems that use this variable.
1.2 deraadt 1146: .It Ev MAIL
1.1 deraadt 1147: Set to point the user's mailbox.
1.40 aaron 1148: .It Ev PATH
1.2 deraadt 1149: Set to the default
1150: .Ev PATH ,
1151: as specified when compiling
1.12 aaron 1152: .Nm ssh .
1.18 markus 1153: .It Ev SSH_AUTH_SOCK
1.17 markus 1154: indicates the path of a unix-domain socket used to communicate with the
1155: agent.
1.2 deraadt 1156: .It Ev SSH_CLIENT
1.40 aaron 1157: Identifies the client end of the connection.
1158: The variable contains
1.1 deraadt 1159: three space-separated values: client ip-address, client port number,
1160: and server port number.
1.73 markus 1161: .It Ev SSH_ORIGINAL_COMMAND
1162: The variable contains the original command line if a forced command
1163: is executed.
1164: It can be used to extract the original arguments.
1.2 deraadt 1165: .It Ev SSH_TTY
1.1 deraadt 1166: This is set to the name of the tty (path to the device) associated
1.40 aaron 1167: with the current shell or command.
1168: If the current session has no tty,
1.1 deraadt 1169: this variable is not set.
1.2 deraadt 1170: .It Ev TZ
1.1 deraadt 1171: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1172: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1173: on to new connections).
1.2 deraadt 1174: .It Ev USER
1.1 deraadt 1175: Set to the name of the user logging in.
1.2 deraadt 1176: .El
1177: .Pp
1.44 aaron 1178: Additionally,
1.2 deraadt 1179: .Nm
1.44 aaron 1180: reads
1181: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1182: and adds lines of the format
1183: .Dq VARNAME=value
1.12 aaron 1184: to the environment.
1.2 deraadt 1185: .Sh FILES
1.36 markus 1186: .Bl -tag -width Ds
1.95 stevesk 1187: .It Pa $HOME/.ssh/known_hosts, $HOME/.ssh/known_hosts2
1.1 deraadt 1188: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1189: in
1.95 stevesk 1190: .Pa /etc/ssh_known_hosts
1191: for protocol version 1 or
1192: .Pa /etc/ssh_known_hosts2
1193: for protocol version 2).
1.2 deraadt 1194: See
1195: .Xr sshd 8 .
1.102 itojun 1196: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1197: Contains the authentication identity of the user.
1198: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1199: These files
1200: contain sensitive data and should be readable by the user but not
1.15 markus 1201: accessible by others (read/write/execute).
1202: Note that
1203: .Nm
1.48 markus 1204: ignores a private key file if it is accessible by others.
1.15 markus 1205: It is possible to specify a passphrase when
1.1 deraadt 1206: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1207: sensitive part of this file using 3DES.
1.102 itojun 1208: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1209: Contains the public key for authentication (public part of the
1.40 aaron 1210: identity file in human-readable form).
1.48 markus 1211: The contents of the
1212: .Pa $HOME/.ssh/identity.pub
1213: file should be added to
1.2 deraadt 1214: .Pa $HOME/.ssh/authorized_keys
1215: on all machines
1.102 itojun 1216: where you wish to log in using protocol version 1 RSA authentication.
1.48 markus 1217: The contents of the
1218: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1219: and
1220: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1221: file should be added to
1222: .Pa $HOME/.ssh/authorized_keys2
1223: on all machines
1.102 itojun 1224: where you wish to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1225: These files are not
1.40 aaron 1226: sensitive and can (but need not) be readable by anyone.
1.48 markus 1227: These files are
1.84 markus 1228: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1229: the convenience of the user.
1.2 deraadt 1230: .It Pa $HOME/.ssh/config
1.40 aaron 1231: This is the per-user configuration file.
1232: The format of this file is described above.
1233: This file is used by the
1.2 deraadt 1234: .Nm
1.40 aaron 1235: client.
1236: This file does not usually contain any sensitive information,
1.1 deraadt 1237: but the recommended permissions are read/write for the user, and not
1238: accessible by others.
1.2 deraadt 1239: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1240: Lists the RSA keys that can be used for logging in as this user.
1241: The format of this file is described in the
1.2 deraadt 1242: .Xr sshd 8
1.40 aaron 1243: manual page.
1244: In the simplest form the format is the same as the .pub
1.1 deraadt 1245: identity files (that is, each line contains the number of bits in
1246: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1247: spaces).
1248: This file is not highly sensitive, but the recommended
1.1 deraadt 1249: permissions are read/write for the user, and not accessible by others.
1.48 markus 1250: .It Pa $HOME/.ssh/authorized_keys2
1.107 markus 1251: Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1.48 markus 1252: This file is not highly sensitive, but the recommended
1253: permissions are read/write for the user, and not accessible by others.
1254: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1255: Systemwide list of known host keys.
1.48 markus 1256: .Pa /etc/ssh_known_hosts
1257: contains RSA and
1258: .Pa /etc/ssh_known_hosts2
1.107 markus 1259: contains RSA or DSA keys for protocol version 2.
1.48 markus 1260: These files should be prepared by the
1.1 deraadt 1261: system administrator to contain the public host keys of all machines in the
1.40 aaron 1262: organization.
1263: This file should be world-readable.
1264: This file contains
1.1 deraadt 1265: public keys, one per line, in the following format (fields separated
1266: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1267: modulus, and optional comment field.
1268: When different names are used
1.1 deraadt 1269: for the same machine, all such names should be listed, separated by
1.40 aaron 1270: commas.
1271: The format is described on the
1.2 deraadt 1272: .Xr sshd 8
1.1 deraadt 1273: manual page.
1.2 deraadt 1274: .Pp
1.1 deraadt 1275: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1276: .Xr sshd 8
1.1 deraadt 1277: to verify the client host when logging in; other names are needed because
1.2 deraadt 1278: .Nm
1.1 deraadt 1279: does not convert the user-supplied name to a canonical name before
1280: checking the key, because someone with access to the name servers
1281: would then be able to fool host authentication.
1.2 deraadt 1282: .It Pa /etc/ssh_config
1.40 aaron 1283: Systemwide configuration file.
1284: This file provides defaults for those
1.1 deraadt 1285: values that are not specified in the user's configuration file, and
1.40 aaron 1286: for those users who do not have a configuration file.
1287: This file must be world-readable.
1.2 deraadt 1288: .It Pa $HOME/.rhosts
1289: This file is used in
1290: .Pa \&.rhosts
1291: authentication to list the
1.40 aaron 1292: host/user pairs that are permitted to log in.
1293: (Note that this file is
1.1 deraadt 1294: also used by rlogin and rsh, which makes using this file insecure.)
1295: Each line of the file contains a host name (in the canonical form
1296: returned by name servers), and then a user name on that host,
1.40 aaron 1297: separated by a space.
1.92 markus 1298: On some machines this file may need to be
1.1 deraadt 1299: world-readable if the user's home directory is on a NFS partition,
1300: because
1.2 deraadt 1301: .Xr sshd 8
1.40 aaron 1302: reads it as root.
1303: Additionally, this file must be owned by the user,
1304: and must not have write permissions for anyone else.
1305: The recommended
1.1 deraadt 1306: permission for most machines is read/write for the user, and not
1307: accessible by others.
1.2 deraadt 1308: .Pp
1.1 deraadt 1309: Note that by default
1.2 deraadt 1310: .Xr sshd 8
1.1 deraadt 1311: will be installed so that it requires successful RSA host
1.40 aaron 1312: authentication before permitting \s+2.\s0rhosts authentication.
1313: If your server machine does not have the client's host key in
1.2 deraadt 1314: .Pa /etc/ssh_known_hosts ,
1315: you can store it in
1316: .Pa $HOME/.ssh/known_hosts .
1317: The easiest way to do this is to
1.1 deraadt 1318: connect back to the client from the server machine using ssh; this
1.48 markus 1319: will automatically add the host key to
1.2 deraadt 1320: .Pa $HOME/.ssh/known_hosts .
1321: .It Pa $HOME/.shosts
1322: This file is used exactly the same way as
1323: .Pa \&.rhosts .
1324: The purpose for
1.1 deraadt 1325: having this file is to be able to use rhosts authentication with
1.2 deraadt 1326: .Nm
1327: without permitting login with
1328: .Xr rlogin 1
1329: or
1330: .Xr rsh 1 .
1331: .It Pa /etc/hosts.equiv
1332: This file is used during
1.40 aaron 1333: .Pa \&.rhosts authentication.
1334: It contains
1.1 deraadt 1335: canonical hosts names, one per line (the full format is described on
1336: the
1.2 deraadt 1337: .Xr sshd 8
1.40 aaron 1338: manual page).
1339: If the client host is found in this file, login is
1.1 deraadt 1340: automatically permitted provided client and server user names are the
1.40 aaron 1341: same.
1342: Additionally, successful RSA host authentication is normally
1343: required.
1344: This file should only be writable by root.
1.2 deraadt 1345: .It Pa /etc/shosts.equiv
1.44 aaron 1346: This file is processed exactly as
1.2 deraadt 1347: .Pa /etc/hosts.equiv .
1.1 deraadt 1348: This file may be useful to permit logins using
1.2 deraadt 1349: .Nm
1.1 deraadt 1350: but not using rsh/rlogin.
1.2 deraadt 1351: .It Pa /etc/sshrc
1.1 deraadt 1352: Commands in this file are executed by
1.2 deraadt 1353: .Nm
1.1 deraadt 1354: when the user logs in just before the user's shell (or command) is started.
1355: See the
1.2 deraadt 1356: .Xr sshd 8
1.1 deraadt 1357: manual page for more information.
1.2 deraadt 1358: .It Pa $HOME/.ssh/rc
1.1 deraadt 1359: Commands in this file are executed by
1.2 deraadt 1360: .Nm
1.1 deraadt 1361: when the user logs in just before the user's shell (or command) is
1362: started.
1.44 aaron 1363: See the
1.2 deraadt 1364: .Xr sshd 8
1.1 deraadt 1365: manual page for more information.
1.31 markus 1366: .It Pa $HOME/.ssh/environment
1367: Contains additional definitions for environment variables, see section
1368: .Sx ENVIRONMENT
1369: above.
1.58 itojun 1370: .El
1.67 aaron 1371: .Sh AUTHORS
1.78 markus 1372: OpenSSH is a derivative of the original and free
1373: ssh 1.2.12 release by Tatu Ylonen.
1374: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1375: Theo de Raadt and Dug Song
1376: removed many bugs, re-added newer features and
1377: created OpenSSH.
1378: Markus Friedl contributed the support for SSH
1379: protocol versions 1.5 and 2.0.
1.2 deraadt 1380: .Sh SEE ALSO
1381: .Xr rlogin 1 ,
1382: .Xr rsh 1 ,
1383: .Xr scp 1 ,
1.83 djm 1384: .Xr sftp 1 ,
1.2 deraadt 1385: .Xr ssh-add 1 ,
1386: .Xr ssh-agent 1 ,
1387: .Xr ssh-keygen 1 ,
1388: .Xr telnet 1 ,
1.87 itojun 1389: .Xr sshd 8
1.106 markus 1390: .Rs
1391: .%A T. Ylonen
1392: .%A T. Kivinen
1393: .%A M. Saarinen
1394: .%A T. Rinne
1395: .%A S. Lehtinen
1396: .%T "SSH Protocol Architecture"
1397: .%N draft-ietf-secsh-architecture-07.txt
1398: .%D January 2001
1399: .%O work in progress material
1400: .Re
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