Annotation of src/usr.bin/ssh/ssh.1, Revision 1.105
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: .\"
1.105 ! djm 37: .\" $OpenBSD: ssh.1,v 1.104 2001/04/20 07:17:51 djm 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
1.51 markus 52: .Op Fl c Ar cipher_spec
1.2 deraadt 53: .Op Fl e Ar escape_char
54: .Op Fl i Ar identity_file
55: .Op Fl l Ar login_name
1.91 markus 56: .Op Fl m Ar mac_spec
1.2 deraadt 57: .Op Fl o Ar option
58: .Op Fl p Ar port
1.12 aaron 59: .Oo Fl L Xo
60: .Sm off
1.33 markus 61: .Ar port :
1.12 aaron 62: .Ar host :
63: .Ar hostport
64: .Sm on
65: .Xc
66: .Oc
67: .Oo Fl R Xo
68: .Sm off
1.33 markus 69: .Ar port :
1.12 aaron 70: .Ar host :
71: .Ar hostport
72: .Sm on
73: .Xc
74: .Oc
1.5 deraadt 75: .Op Ar hostname | user@hostname
1.2 deraadt 76: .Op Ar command
1.44 aaron 77: .Sh DESCRIPTION
1.2 deraadt 78: .Nm
1.96 deraadt 79: (SSH client) is a program for logging into a remote machine and for
1.40 aaron 80: executing commands on a remote machine.
81: It is intended to replace
1.1 deraadt 82: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 83: two untrusted hosts over an insecure network.
84: X11 connections and
1.1 deraadt 85: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 86: .Pp
87: .Nm
1.44 aaron 88: connects and logs into the specified
1.2 deraadt 89: .Ar hostname .
1.1 deraadt 90: The user must prove
1.49 markus 91: his/her identity to the remote machine using one of several methods
92: depending on the protocol version used:
93: .Pp
94: .Ss SSH protocol version 1
1.2 deraadt 95: .Pp
1.1 deraadt 96: First, if the machine the user logs in from is listed in
1.2 deraadt 97: .Pa /etc/hosts.equiv
1.1 deraadt 98: or
1.2 deraadt 99: .Pa /etc/shosts.equiv
1.1 deraadt 100: on the remote machine, and the user names are
101: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 102: Second, if
1.2 deraadt 103: .Pa \&.rhosts
1.1 deraadt 104: or
1.2 deraadt 105: .Pa \&.shosts
1.1 deraadt 106: exists in the user's home directory on the
107: remote machine and contains a line containing the name of the client
108: machine and the name of the user on that machine, the user is
1.40 aaron 109: permitted to log in.
110: This form of authentication alone is normally not
1.1 deraadt 111: allowed by the server because it is not secure.
1.2 deraadt 112: .Pp
1.1 deraadt 113: The second (and primary) authentication method is the
1.2 deraadt 114: .Pa rhosts
1.1 deraadt 115: or
1.2 deraadt 116: .Pa hosts.equiv
1.40 aaron 117: method combined with RSA-based host authentication.
118: It means that if the login would be permitted by
1.49 markus 119: .Pa $HOME/.rhosts ,
120: .Pa $HOME/.shosts ,
1.2 deraadt 121: .Pa /etc/hosts.equiv ,
1.1 deraadt 122: or
1.2 deraadt 123: .Pa /etc/shosts.equiv ,
1.11 deraadt 124: and if additionally the server can verify the client's
1.44 aaron 125: host key (see
1.2 deraadt 126: .Pa /etc/ssh_known_hosts
1.23 markus 127: and
128: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 129: in the
1.2 deraadt 130: .Sx FILES
1.40 aaron 131: section), only then login is permitted.
132: This authentication method closes security holes due to IP
133: spoofing, DNS spoofing and routing spoofing.
134: [Note to the administrator:
1.2 deraadt 135: .Pa /etc/hosts.equiv ,
1.49 markus 136: .Pa $HOME/.rhosts ,
1.1 deraadt 137: and the rlogin/rsh protocol in general, are inherently insecure and should be
138: disabled if security is desired.]
1.2 deraadt 139: .Pp
1.44 aaron 140: As a third authentication method,
1.2 deraadt 141: .Nm
1.1 deraadt 142: supports RSA based authentication.
143: The scheme is based on public-key cryptography: there are cryptosystems
144: where encryption and decryption are done using separate keys, and it
145: is not possible to derive the decryption key from the encryption key.
1.40 aaron 146: RSA is one such system.
1.44 aaron 147: The idea is that each user creates a public/private
1.40 aaron 148: key pair for authentication purposes.
149: The server knows the public key, and only the user knows the private key.
1.44 aaron 150: The file
1.2 deraadt 151: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 152: lists the public keys that are permitted for logging
1.40 aaron 153: in.
154: When the user logs in, the
1.2 deraadt 155: .Nm
1.1 deraadt 156: program tells the server which key pair it would like to use for
1.40 aaron 157: authentication.
158: The server checks if this key is permitted, and if
1.1 deraadt 159: so, sends the user (actually the
1.2 deraadt 160: .Nm
1.1 deraadt 161: program running on behalf of the user) a challenge, a random number,
1.40 aaron 162: encrypted by the user's public key.
163: The challenge can only be
164: decrypted using the proper private key.
165: The user's client then decrypts the
1.1 deraadt 166: challenge using the private key, proving that he/she knows the private
167: key but without disclosing it to the server.
1.2 deraadt 168: .Pp
169: .Nm
1.40 aaron 170: implements the RSA authentication protocol automatically.
171: The user creates his/her RSA key pair by running
1.2 deraadt 172: .Xr ssh-keygen 1 .
1.44 aaron 173: This stores the private key in
1.49 markus 174: .Pa $HOME/.ssh/identity
1.1 deraadt 175: and the public key in
1.49 markus 176: .Pa $HOME/.ssh/identity.pub
1.40 aaron 177: in the user's home directory.
178: The user should then copy the
1.2 deraadt 179: .Pa identity.pub
1.44 aaron 180: to
1.49 markus 181: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 182: in his/her home directory on the remote machine (the
1.2 deraadt 183: .Pa authorized_keys
1.44 aaron 184: file corresponds to the conventional
1.49 markus 185: .Pa $HOME/.rhosts
1.1 deraadt 186: file, and has one key
1.40 aaron 187: per line, though the lines can be very long).
188: After this, the user can log in without giving the password.
189: RSA authentication is much
1.1 deraadt 190: more secure than rhosts authentication.
1.2 deraadt 191: .Pp
1.1 deraadt 192: The most convenient way to use RSA authentication may be with an
1.40 aaron 193: authentication agent.
194: See
1.2 deraadt 195: .Xr ssh-agent 1
1.1 deraadt 196: for more information.
1.2 deraadt 197: .Pp
1.44 aaron 198: If other authentication methods fail,
1.2 deraadt 199: .Nm
1.40 aaron 200: prompts the user for a password.
201: The password is sent to the remote
1.1 deraadt 202: host for checking; however, since all communications are encrypted,
203: the password cannot be seen by someone listening on the network.
1.2 deraadt 204: .Pp
1.49 markus 205: .Ss SSH protocol version 2
206: .Pp
207: When a user connects using the protocol version 2
208: different authentication methods are available:
209: At first, the client attempts to authenticate using the public key method.
210: If this method fails password authentication is tried.
211: .Pp
212: The public key method is similar to RSA authentication described
1.68 markus 213: in the previous section except that the DSA or RSA algorithm is used
214: instead.
1.102 itojun 215: The client uses his private key,
1.49 markus 216: .Pa $HOME/.ssh/id_dsa
1.102 itojun 217: or
218: .Pa $HOME/.ssh/id_rsa ,
1.49 markus 219: to sign the session identifier and sends the result to the server.
220: The server checks whether the matching public key is listed in
221: .Pa $HOME/.ssh/authorized_keys2
222: and grants access if both the key is found and the signature is correct.
223: The session identifier is derived from a shared Diffie-Hellman value
224: and is only known to the client and the server.
225: .Pp
226: If public key authentication fails or is not available a password
227: can be sent encrypted to the remote host for proving the user's identity.
228: This protocol 2 implementation does not yet support Kerberos or
229: S/Key authentication.
230: .Pp
231: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 232: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.94 deraadt 233: and integrity (hmac-md5, hmac-sha1).
1.49 markus 234: Note that protocol 1 lacks a strong mechanism for ensuring the
235: integrity of the connection.
236: .Pp
237: .Ss Login session and remote execution
238: .Pp
1.1 deraadt 239: When the user's identity has been accepted by the server, the server
240: either executes the given command, or logs into the machine and gives
1.40 aaron 241: the user a normal shell on the remote machine.
242: All communication with
1.1 deraadt 243: the remote command or shell will be automatically encrypted.
1.2 deraadt 244: .Pp
1.1 deraadt 245: If a pseudo-terminal has been allocated (normal login session), the
1.104 djm 246: user may use the escape characters noted below.
1.2 deraadt 247: .Pp
1.1 deraadt 248: If no pseudo tty has been allocated, the
249: session is transparent and can be used to reliably transfer binary
1.40 aaron 250: data.
251: On most systems, setting the escape character to
1.2 deraadt 252: .Dq none
253: will also make the session transparent even if a tty is used.
254: .Pp
1.71 djm 255: The session terminates when the command or shell on the remote
1.92 markus 256: machine exits and all X11 and TCP/IP connections have been closed.
1.1 deraadt 257: The exit status of the remote program is returned as the exit status
258: of
1.2 deraadt 259: .Nm ssh .
1.104 djm 260: .Pp
261: .Ss Escape Characters
262: .Pp
263: When a pseudo terminal has been requested, ssh supports a number of functions
1.105 ! djm 264: through the use of an escape character.
1.104 djm 265: .Pp
266: A single tilde character can be sent as
267: .Ic ~~
268: (or by following the tilde by a character other than those described above).
269: The escape character must always follow a newline to be interpreted as
270: special.
271: The escape character can be changed in configuration files using the
272: .Cm EscapeChar
273: configuration directive or on the command line by the
274: .Fl e
275: option.
276: .Pp
277: The supported escapes (assuming the default
278: .Ql ~ )
279: are:
280: .Bl -tag -width Ds
281: .It Cm ~.
282: Disconnect
283: .It Cm ~^Z
284: Background ssh
285: .It Cm ~#
286: List forwarded connections
287: .It Cm ~&
288: Background ssh at logout when waiting for forwarded connection / X11 sessions
289: to terminate (this does not currently work for SSH protocol version 2)
290: .It Cm ~?
291: Display a list of escape characters
292: .It Cm ~R
1.105 ! djm 293: Request rekeying of the connection (only useful for SSH protocol version 2
! 294: and if the peer supports it)
1.104 djm 295: .El
1.2 deraadt 296: .Pp
1.49 markus 297: .Ss X11 and TCP forwarding
298: .Pp
1.1 deraadt 299: If the user is using X11 (the
1.2 deraadt 300: .Ev DISPLAY
1.1 deraadt 301: environment variable is set), the connection to the X11 display is
302: automatically forwarded to the remote side in such a way that any X11
303: programs started from the shell (or command) will go through the
304: encrypted channel, and the connection to the real X server will be made
1.40 aaron 305: from the local machine.
306: The user should not manually set
1.2 deraadt 307: .Ev DISPLAY .
1.1 deraadt 308: Forwarding of X11 connections can be
309: configured on the command line or in configuration files.
1.2 deraadt 310: .Pp
311: The
1.44 aaron 312: .Ev DISPLAY
1.2 deraadt 313: value set by
314: .Nm
1.1 deraadt 315: will point to the server machine, but with a display number greater
1.40 aaron 316: than zero.
317: This is normal, and happens because
1.2 deraadt 318: .Nm
319: creates a
320: .Dq proxy
321: X server on the server machine for forwarding the
1.1 deraadt 322: connections over the encrypted channel.
1.2 deraadt 323: .Pp
324: .Nm
1.1 deraadt 325: will also automatically set up Xauthority data on the server machine.
326: For this purpose, it will generate a random authorization cookie,
327: store it in Xauthority on the server, and verify that any forwarded
328: connections carry this cookie and replace it by the real cookie when
1.40 aaron 329: the connection is opened.
330: The real authentication cookie is never
1.1 deraadt 331: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 332: .Pp
1.1 deraadt 333: If the user is using an authentication agent, the connection to the agent
334: is automatically forwarded to the remote side unless disabled on
335: command line or in a configuration file.
1.2 deraadt 336: .Pp
1.1 deraadt 337: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.40 aaron 338: be specified either on command line or in a configuration file.
339: One possible application of TCP/IP forwarding is a secure connection to an
1.92 markus 340: electronic purse; another is going through firewalls.
1.2 deraadt 341: .Pp
1.49 markus 342: .Ss Server authentication
343: .Pp
1.2 deraadt 344: .Nm
1.49 markus 345: automatically maintains and checks a database containing
1.40 aaron 346: identifications for all hosts it has ever been used with.
1.49 markus 347: RSA host keys are stored in
348: .Pa $HOME/.ssh/known_hosts
349: and
1.68 markus 350: host keys used in the protocol version 2 are stored in
1.49 markus 351: .Pa $HOME/.ssh/known_hosts2
1.40 aaron 352: in the user's home directory.
1.49 markus 353: Additionally, the files
1.2 deraadt 354: .Pa /etc/ssh_known_hosts
1.49 markus 355: and
356: .Pa /etc/ssh_known_hosts2
357: are automatically checked for known hosts.
1.40 aaron 358: Any new hosts are automatically added to the user's file.
359: If a host's identification
1.1 deraadt 360: ever changes,
1.2 deraadt 361: .Nm
1.1 deraadt 362: warns about this and disables password authentication to prevent a
1.40 aaron 363: trojan horse from getting the user's password.
364: Another purpose of
1.1 deraadt 365: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 366: otherwise be used to circumvent the encryption.
367: The
1.2 deraadt 368: .Cm StrictHostKeyChecking
1.1 deraadt 369: option (see below) can be used to prevent logins to machines whose
370: host key is not known or has changed.
1.65 aaron 371: .Pp
372: The options are as follows:
1.2 deraadt 373: .Bl -tag -width Ds
1.4 dugsong 374: .It Fl a
1.42 aaron 375: Disables forwarding of the authentication agent connection.
1.54 markus 376: .It Fl A
377: Enables forwarding of the authentication agent connection.
378: This can also be specified on a per-host basis in a configuration file.
1.6 deraadt 379: .It Fl c Ar blowfish|3des
1.44 aaron 380: Selects the cipher to use for encrypting the session.
1.2 deraadt 381: .Ar 3des
1.40 aaron 382: is used by default.
1.44 aaron 383: It is believed to be secure.
1.5 deraadt 384: .Ar 3des
385: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
386: It is presumably more secure than the
1.2 deraadt 387: .Ar des
1.64 markus 388: cipher which is no longer fully supported in
1.51 markus 389: .Nm ssh .
1.5 deraadt 390: .Ar blowfish
391: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 392: .Ar 3des .
1.90 markus 393: .It Fl c Ar cipher_spec
1.51 markus 394: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 395: be specified in order of preference.
1.90 markus 396: See
397: .Cm Ciphers
398: for more information.
1.2 deraadt 399: .It Fl e Ar ch|^ch|none
400: Sets the escape character for sessions with a pty (default:
401: .Ql ~ ) .
1.40 aaron 402: The escape character is only recognized at the beginning of a line.
403: The escape character followed by a dot
1.2 deraadt 404: .Pq Ql \&.
405: closes the connection, followed
1.1 deraadt 406: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 407: escape character once.
408: Setting the character to
1.2 deraadt 409: .Dq none
410: disables any escapes and makes the session fully transparent.
411: .It Fl f
412: Requests
413: .Nm
1.40 aaron 414: to go to background just before command execution.
415: This is useful if
1.2 deraadt 416: .Nm
417: is going to ask for passwords or passphrases, but the user
1.40 aaron 418: wants it in the background.
1.44 aaron 419: This implies
1.2 deraadt 420: .Fl n .
1.1 deraadt 421: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 422: something like
423: .Ic ssh -f host xterm .
1.34 markus 424: .It Fl g
425: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 426: .It Fl i Ar identity_file
1.44 aaron 427: Selects the file from which the identity (private key) for
1.68 markus 428: RSA or DSA authentication is read.
1.44 aaron 429: Default is
1.49 markus 430: .Pa $HOME/.ssh/identity
1.40 aaron 431: in the user's home directory.
432: Identity files may also be specified on
433: a per-host basis in the configuration file.
434: It is possible to have multiple
1.2 deraadt 435: .Fl i
436: options (and multiple identities specified in
1.1 deraadt 437: configuration files).
1.2 deraadt 438: .It Fl k
1.42 aaron 439: Disables forwarding of Kerberos tickets and AFS tokens.
440: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 441: .It Fl l Ar login_name
1.40 aaron 442: Specifies the user to log in as on the remote machine.
443: This also may be specified on a per-host basis in the configuration file.
1.91 markus 444: .It Fl m Ar mac_spec
445: Additionally, for protocol version 2 a comma-separated list of MAC
446: (message authentication code) algorithms can
447: be specified in order of preference.
448: See the
449: .Cm MACs
450: keyword for more information.
1.2 deraadt 451: .It Fl n
452: Redirects stdin from
453: .Pa /dev/null
454: (actually, prevents reading from stdin).
1.1 deraadt 455: This must be used when
1.2 deraadt 456: .Nm
1.40 aaron 457: is run in the background.
458: A common trick is to use this to run X11 programs on a remote machine.
459: For example,
1.2 deraadt 460: .Ic ssh -n shadows.cs.hut.fi emacs &
461: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 462: connection will be automatically forwarded over an encrypted channel.
463: The
1.2 deraadt 464: .Nm
1.1 deraadt 465: program will be put in the background.
466: (This does not work if
1.2 deraadt 467: .Nm
468: needs to ask for a password or passphrase; see also the
469: .Fl f
470: option.)
1.53 markus 471: .It Fl N
472: Do not execute a remote command.
1.70 markus 473: This is useful if you just want to forward ports
1.53 markus 474: (protocol version 2 only).
1.2 deraadt 475: .It Fl o Ar option
1.1 deraadt 476: Can be used to give options in the format used in the config file.
477: This is useful for specifying options for which there is no separate
1.40 aaron 478: command-line flag.
479: The option has the same format as a line in the configuration file.
1.2 deraadt 480: .It Fl p Ar port
1.40 aaron 481: Port to connect to on the remote host.
482: This can be specified on a
1.1 deraadt 483: per-host basis in the configuration file.
1.16 markus 484: .It Fl P
485: Use a non-privileged port for outgoing connections.
486: This can be used if your firewall does
487: not permit connections from privileged ports.
1.30 provos 488: Note that this option turns off
1.16 markus 489: .Cm RhostsAuthentication
490: and
1.72 markus 491: .Cm RhostsRSAAuthentication
492: for older servers.
1.2 deraadt 493: .It Fl q
1.40 aaron 494: Quiet mode.
495: Causes all warning and diagnostic messages to be suppressed.
496: Only fatal errors are displayed.
1.80 djm 497: .It Fl s
498: 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
499: of SSH as a secure transport for other application (eg. sftp). The
500: subsystem is specified as the remote command.
1.2 deraadt 501: .It Fl t
1.40 aaron 502: Force pseudo-tty allocation.
1.43 brad 503: This can be used to execute arbitrary
1.40 aaron 504: screen-based programs on a remote machine, which can be very useful,
505: e.g., when implementing menu services.
1.73 markus 506: Multiple
507: .Fl t
508: options force tty allocation, even if
509: .Nm
510: has no local tty.
1.53 markus 511: .It Fl T
1.69 markus 512: Disable pseudo-tty allocation.
1.2 deraadt 513: .It Fl v
1.40 aaron 514: Verbose mode.
515: Causes
1.2 deraadt 516: .Nm
1.40 aaron 517: to print debugging messages about its progress.
518: This is helpful in
1.1 deraadt 519: debugging connection, authentication, and configuration problems.
1.73 markus 520: Multiple
521: .Fl v
522: options increases the verbosity.
1.61 aaron 523: Maximum is 3.
1.2 deraadt 524: .It Fl x
1.40 aaron 525: Disables X11 forwarding.
1.2 deraadt 526: .It Fl X
1.1 deraadt 527: Enables X11 forwarding.
1.54 markus 528: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 529: .It Fl C
1.1 deraadt 530: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 531: data for forwarded X11 and TCP/IP connections).
532: The compression algorithm is the same used by
1.34 markus 533: .Xr gzip 1 ,
534: and the
1.2 deraadt 535: .Dq level
536: can be controlled by the
537: .Cm CompressionLevel
1.40 aaron 538: option (see below).
539: Compression is desirable on modem lines and other
1.1 deraadt 540: slow connections, but will only slow down things on fast networks.
541: The default value can be set on a host-by-host basis in the
542: configuration files; see the
1.2 deraadt 543: .Cm Compress
1.1 deraadt 544: option below.
1.2 deraadt 545: .It Fl L Ar port:host:hostport
1.1 deraadt 546: Specifies that the given port on the local (client) host is to be
1.40 aaron 547: forwarded to the given host and port on the remote side.
548: This works by allocating a socket to listen to
1.2 deraadt 549: .Ar port
1.1 deraadt 550: on the local side, and whenever a connection is made to this port, the
551: connection is forwarded over the secure channel, and a connection is
552: made to
1.32 markus 553: .Ar host
554: port
555: .Ar hostport
1.40 aaron 556: from the remote machine.
557: Port forwardings can also be specified in the configuration file.
558: Only root can forward privileged ports.
1.32 markus 559: IPv6 addresses can be specified with an alternative syntax:
560: .Ar port/host/hostport
1.2 deraadt 561: .It Fl R Ar port:host:hostport
1.1 deraadt 562: Specifies that the given port on the remote (server) host is to be
1.40 aaron 563: forwarded to the given host and port on the local side.
564: This works by allocating a socket to listen to
1.2 deraadt 565: .Ar port
1.1 deraadt 566: on the remote side, and whenever a connection is made to this port, the
567: connection is forwarded over the secure channel, and a connection is
568: made to
1.32 markus 569: .Ar host
570: port
571: .Ar hostport
1.40 aaron 572: from the local machine.
573: Port forwardings can also be specified in the configuration file.
574: Privileged ports can be forwarded only when
1.1 deraadt 575: logging in as root on the remote machine.
1.85 jakob 576: .It Fl 1
577: Forces
578: .Nm
579: to try protocol version 1 only.
1.46 markus 580: .It Fl 2
581: Forces
582: .Nm
1.50 markus 583: to try protocol version 2 only.
1.32 markus 584: .It Fl 4
585: Forces
586: .Nm
587: to use IPv4 addresses only.
588: .It Fl 6
589: Forces
590: .Nm
591: to use IPv6 addresses only.
1.2 deraadt 592: .El
593: .Sh CONFIGURATION FILES
594: .Nm
1.1 deraadt 595: obtains configuration data from the following sources (in this order):
596: command line options, user's configuration file
1.2 deraadt 597: .Pq Pa $HOME/.ssh/config ,
598: and system-wide configuration file
599: .Pq Pa /etc/ssh_config .
600: For each parameter, the first obtained value
1.40 aaron 601: will be used.
602: The configuration files contain sections bracketed by
603: .Dq Host
604: specifications, and that section is only applied for hosts that
605: match one of the patterns given in the specification.
606: The matched host name is the one given on the command line.
1.2 deraadt 607: .Pp
1.1 deraadt 608: Since the first obtained value for each parameter is used, more
609: host-specific declarations should be given near the beginning of the
610: file, and general defaults at the end.
1.2 deraadt 611: .Pp
1.1 deraadt 612: The configuration file has the following format:
1.2 deraadt 613: .Pp
614: Empty lines and lines starting with
615: .Ql #
616: are comments.
617: .Pp
618: Otherwise a line is of the format
619: .Dq keyword arguments .
620: The possible
1.1 deraadt 621: keywords and their meanings are as follows (note that the
622: configuration files are case-sensitive):
1.2 deraadt 623: .Bl -tag -width Ds
624: .It Cm Host
1.1 deraadt 625: Restricts the following declarations (up to the next
1.2 deraadt 626: .Cm Host
1.1 deraadt 627: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 628: given after the keyword.
629: .Ql \&*
630: and
631: .Ql ?
632: can be used as wildcards in the
1.40 aaron 633: patterns.
634: A single
1.2 deraadt 635: .Ql \&*
636: as a pattern can be used to provide global
1.40 aaron 637: defaults for all hosts.
638: The host is the
1.2 deraadt 639: .Ar hostname
1.1 deraadt 640: argument given on the command line (i.e., the name is not converted to
641: a canonicalized host name before matching).
1.2 deraadt 642: .It Cm AFSTokenPassing
1.42 aaron 643: Specifies whether to pass AFS tokens to remote host.
644: The argument to this keyword must be
1.2 deraadt 645: .Dq yes
646: or
647: .Dq no .
648: .It Cm BatchMode
649: If set to
650: .Dq yes ,
1.40 aaron 651: passphrase/password querying will be disabled.
652: This option is useful in scripts and other batch jobs where you have no
653: user to supply the password.
654: The argument must be
1.2 deraadt 655: .Dq yes
656: or
657: .Dq no .
1.100 stevesk 658: The default is
659: .Dq no .
1.34 markus 660: .It Cm CheckHostIP
661: If this flag is set to
662: .Dq yes ,
1.100 stevesk 663: ssh will additionally check the host IP address in the
1.34 markus 664: .Pa known_hosts
1.42 aaron 665: file.
666: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 667: If the option is set to
668: .Dq no ,
669: the check will not be executed.
1.100 stevesk 670: The default is
671: .Dq yes .
1.2 deraadt 672: .It Cm Cipher
1.62 markus 673: Specifies the cipher to use for encrypting the session
1.64 markus 674: in protocol version 1.
1.40 aaron 675: Currently,
1.64 markus 676: .Dq blowfish
1.1 deraadt 677: and
1.10 provos 678: .Dq 3des
1.40 aaron 679: are supported.
680: The default is
1.2 deraadt 681: .Dq 3des .
1.45 markus 682: .It Cm Ciphers
683: Specifies the ciphers allowed for protocol version 2
684: in order of preference.
685: Multiple ciphers must be comma-separated.
686: The default is
1.88 provos 687: .Pp
688: .Bd -literal
1.94 deraadt 689: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
1.88 provos 690: aes192-cbc,aes256-cbc,rijndael128-cbc,rijndael192-cbc,
691: rijndael256-cbc,rijndael-cbc@lysator.liu.se''
692: .Ed
1.2 deraadt 693: .It Cm Compression
1.40 aaron 694: Specifies whether to use compression.
695: The argument must be
1.2 deraadt 696: .Dq yes
697: or
698: .Dq no .
1.100 stevesk 699: The default is
700: .Dq no .
1.2 deraadt 701: .It Cm CompressionLevel
1.100 stevesk 702: Specifies the compression level to use if compression is enabled.
1.40 aaron 703: The argument must be an integer from 1 (fast) to 9 (slow, best).
704: The default level is 6, which is good for most applications.
705: The meaning of the values is the same as in
1.34 markus 706: .Xr gzip 1 .
1.2 deraadt 707: .It Cm ConnectionAttempts
1.1 deraadt 708: Specifies the number of tries (one per second) to make before falling
1.40 aaron 709: back to rsh or exiting.
710: The argument must be an integer.
711: This may be useful in scripts if the connection sometimes fails.
1.100 stevesk 712: The default is 4.
1.68 markus 713: .It Cm PubkeyAuthentication
714: Specifies whether to try public key authentication.
1.50 markus 715: The argument to this keyword must be
716: .Dq yes
717: or
718: .Dq no .
1.100 stevesk 719: The default is
720: .Dq yes .
1.50 markus 721: Note that this option applies to protocol version 2 only.
1.2 deraadt 722: .It Cm EscapeChar
723: Sets the escape character (default:
724: .Ql ~ ) .
725: The escape character can also
1.40 aaron 726: be set on the command line.
727: The argument should be a single character,
1.2 deraadt 728: .Ql ^
729: followed by a letter, or
730: .Dq none
731: to disable the escape
1.1 deraadt 732: character entirely (making the connection transparent for binary
733: data).
1.44 aaron 734: .It Cm FallBackToRsh
1.1 deraadt 735: Specifies that if connecting via
1.2 deraadt 736: .Nm
1.1 deraadt 737: fails due to a connection refused error (there is no
1.2 deraadt 738: .Xr sshd 8
1.44 aaron 739: listening on the remote host),
1.2 deraadt 740: .Xr rsh 1
1.1 deraadt 741: should automatically be used instead (after a suitable warning about
1.40 aaron 742: the session being unencrypted).
743: The argument must be
1.2 deraadt 744: .Dq yes
745: or
746: .Dq no .
1.100 stevesk 747: The default is
748: .Dq no .
1.2 deraadt 749: .It Cm ForwardAgent
1.1 deraadt 750: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 751: will be forwarded to the remote machine.
752: The argument must be
1.2 deraadt 753: .Dq yes
754: or
1.54 markus 755: .Dq no .
756: The default is
1.2 deraadt 757: .Dq no .
758: .It Cm ForwardX11
1.1 deraadt 759: Specifies whether X11 connections will be automatically redirected
1.44 aaron 760: over the secure channel and
1.2 deraadt 761: .Ev DISPLAY
1.40 aaron 762: set.
1.44 aaron 763: The argument must be
1.2 deraadt 764: .Dq yes
765: or
1.38 markus 766: .Dq no .
767: The default is
1.3 deraadt 768: .Dq no .
769: .It Cm GatewayPorts
770: Specifies whether remote hosts are allowed to connect to local
771: forwarded ports.
772: The argument must be
773: .Dq yes
774: or
775: .Dq no .
776: The default is
1.2 deraadt 777: .Dq no .
778: .It Cm GlobalKnownHostsFile
1.95 stevesk 779: Specifies a file to use for the protocol version 1 global
780: host key database instead of
1.2 deraadt 781: .Pa /etc/ssh_known_hosts .
1.95 stevesk 782: .It Cm GlobalKnownHostsFile2
783: Specifies a file to use for the protocol version 2 global
784: host key database instead of
785: .Pa /etc/ssh_known_hosts2 .
1.74 markus 786: .It Cm HostKeyAlias
787: Specifies an alias that should be used instead of the
788: real host name when looking up or saving the host key
1.82 stevesk 789: in the known_hosts files.
790: This option is useful for tunneling ssh connections
1.74 markus 791: or if you have multiple servers running on a single host.
1.103 markus 792: .It Cm HostKeyAlgorithms
793: Specfies the protocol version 2 host key algorithms
794: that the client wants to use in order of preference.
795: The default for this option is:
796: .Dq ssh-rsa,ssh-dss
1.2 deraadt 797: .It Cm HostName
1.40 aaron 798: Specifies the real host name to log into.
799: This can be used to specify nicknames or abbreviations for hosts.
800: Default is the name given on the command line.
801: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 802: .Cm HostName
1.1 deraadt 803: specifications).
1.2 deraadt 804: .It Cm IdentityFile
1.1 deraadt 805: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 806: is read (default
1.49 markus 807: .Pa $HOME/.ssh/identity
1.2 deraadt 808: in the user's home directory).
1.1 deraadt 809: Additionally, any identities represented by the authentication agent
1.40 aaron 810: will be used for authentication.
811: The file name may use the tilde
812: syntax to refer to a user's home directory.
813: It is possible to have
1.1 deraadt 814: multiple identity files specified in configuration files; all these
815: identities will be tried in sequence.
1.2 deraadt 816: .It Cm KeepAlive
1.1 deraadt 817: Specifies whether the system should send keepalive messages to the
1.40 aaron 818: other side.
819: If they are sent, death of the connection or crash of one
820: of the machines will be properly noticed.
821: However, this means that
1.1 deraadt 822: connections will die if the route is down temporarily, and some people
1.41 aaron 823: find it annoying.
1.2 deraadt 824: .Pp
825: The default is
826: .Dq yes
827: (to send keepalives), and the client will notice
1.40 aaron 828: if the network goes down or the remote host dies.
829: This is important in scripts, and many users want it too.
1.2 deraadt 830: .Pp
831: To disable keepalives, the value should be set to
832: .Dq no
833: in both the server and the client configuration files.
834: .It Cm KerberosAuthentication
1.42 aaron 835: Specifies whether Kerberos authentication will be used.
836: The argument to this keyword must be
1.4 dugsong 837: .Dq yes
838: or
839: .Dq no .
1.2 deraadt 840: .It Cm KerberosTgtPassing
1.42 aaron 841: Specifies whether a Kerberos TGT will be forwarded to the server.
842: This will only work if the Kerberos server is actually an AFS kaserver.
843: The argument to this keyword must be
1.4 dugsong 844: .Dq yes
845: or
846: .Dq no .
1.2 deraadt 847: .It Cm LocalForward
1.1 deraadt 848: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 849: the secure channel to given host:port from the remote machine.
850: The first argument must be a port number, and the second must be
851: host:port.
852: Multiple forwardings may be specified, and additional
853: forwardings can be given on the command line.
854: Only the superuser can forward privileged ports.
1.24 markus 855: .It Cm LogLevel
856: Gives the verbosity level that is used when logging messages from
857: .Nm ssh .
858: The possible values are:
1.77 markus 859: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
860: The default is INFO.
1.91 markus 861: .It Cm MACs
862: Specifies the MAC (message authentication code) algorithms
863: in order of preference.
864: The MAC algorithm is used in protocol version 2
865: for data integrity protection.
866: Multiple algorithms must be comma-separated.
867: The default is
868: .Pp
869: .Bd -literal
1.94 deraadt 870: ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
1.91 markus 871: hmac-sha1-96,hmac-md5-96''
872: .Ed
1.14 dugsong 873: .It Cm NumberOfPasswordPrompts
1.42 aaron 874: Specifies the number of password prompts before giving up.
875: The argument to this keyword must be an integer.
876: Default is 3.
1.34 markus 877: .It Cm PasswordAuthentication
1.40 aaron 878: Specifies whether to use password authentication.
879: The argument to this keyword must be
1.34 markus 880: .Dq yes
881: or
882: .Dq no .
1.100 stevesk 883: The default is
884: .Dq yes .
1.50 markus 885: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 886: .It Cm Port
1.40 aaron 887: Specifies the port number to connect on the remote host.
888: Default is 22.
1.99 djm 889: .It Cm PreferredAuthentications
890: Specifies the order in which the client should try protocol 2
891: authentication methods. This allows a client to prefer one method (e.g.
892: .Cm keyboard-interactive )
893: over another method (e.g.
894: .Cm password )
895: The default for this option is:
896: .Dq publickey, password, keyboard-interactive
1.45 markus 897: .It Cm Protocol
898: Specifies the protocol versions
899: .Nm
900: should support in order of preference.
901: The possible values are
902: .Dq 1
903: and
904: .Dq 2 .
905: Multiple versions must be comma-separated.
906: The default is
1.101 markus 907: .Dq 2,1 .
1.49 markus 908: This means that
909: .Nm
1.101 markus 910: tries version 2 and falls back to version 1
911: if version 2 is not available.
1.2 deraadt 912: .It Cm ProxyCommand
1.40 aaron 913: Specifies the command to use to connect to the server.
914: The command
915: string extends to the end of the line, and is executed with
916: .Pa /bin/sh .
917: In the command string,
918: .Ql %h
919: will be substituted by the host name to
920: connect and
921: .Ql %p
922: by the port.
923: The command can be basically anything,
924: and should read from its standard input and write to its standard output.
925: It should eventually connect an
1.2 deraadt 926: .Xr sshd 8
1.1 deraadt 927: server running on some machine, or execute
1.2 deraadt 928: .Ic sshd -i
1.40 aaron 929: somewhere.
930: Host key management will be done using the
1.1 deraadt 931: HostName of the host being connected (defaulting to the name typed by
932: the user).
1.29 markus 933: Note that
934: .Cm CheckHostIP
935: is not available for connects with a proxy command.
1.2 deraadt 936: .Pp
937: .It Cm RemoteForward
1.1 deraadt 938: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 939: the secure channel to given host:port from the local machine.
940: The first argument must be a port number, and the second must be
941: host:port.
942: Multiple forwardings may be specified, and additional
943: forwardings can be given on the command line.
944: Only the superuser can forward privileged ports.
1.2 deraadt 945: .It Cm RhostsAuthentication
1.40 aaron 946: Specifies whether to try rhosts based authentication.
947: Note that this
1.1 deraadt 948: declaration only affects the client side and has no effect whatsoever
1.40 aaron 949: on security.
950: Disabling rhosts authentication may reduce
1.1 deraadt 951: authentication time on slow connections when rhosts authentication is
1.40 aaron 952: not used.
953: Most servers do not permit RhostsAuthentication because it
954: is not secure (see RhostsRSAAuthentication).
955: The argument to this keyword must be
1.2 deraadt 956: .Dq yes
957: or
958: .Dq no .
1.100 stevesk 959: The default is
960: .Dq yes .
1.2 deraadt 961: .It Cm RhostsRSAAuthentication
1.1 deraadt 962: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 963: authentication.
964: The argument must be
1.2 deraadt 965: .Dq yes
966: or
967: .Dq no .
1.100 stevesk 968: The default is
969: .Dq yes .
1.2 deraadt 970: .It Cm RSAAuthentication
1.40 aaron 971: Specifies whether to try RSA authentication.
972: The argument to this keyword must be
1.2 deraadt 973: .Dq yes
974: or
975: .Dq no .
1.1 deraadt 976: RSA authentication will only be
977: attempted if the identity file exists, or an authentication agent is
978: running.
1.100 stevesk 979: The default is
980: .Dq yes .
1.50 markus 981: Note that this option applies to protocol version 1 only.
1.81 markus 982: .It Cm ChallengeResponseAuthentication
983: Specifies whether to use challenge response authentication.
984: Currently there is only support for
1.27 markus 985: .Xr skey 1
1.40 aaron 986: authentication.
987: The argument to this keyword must be
1.27 markus 988: .Dq yes
989: or
990: .Dq no .
991: The default is
992: .Dq no .
1.2 deraadt 993: .It Cm StrictHostKeyChecking
994: If this flag is set to
1.44 aaron 995: .Dq yes ,
1.2 deraadt 996: .Nm
1.79 stevesk 997: will never automatically add host keys to the
1.2 deraadt 998: .Pa $HOME/.ssh/known_hosts
1.48 markus 999: and
1000: .Pa $HOME/.ssh/known_hosts2
1.79 stevesk 1001: files, and refuses to connect to hosts whose host key has changed.
1.40 aaron 1002: This provides maximum protection against trojan horse attacks.
1003: However, it can be somewhat annoying if you don't have good
1.2 deraadt 1004: .Pa /etc/ssh_known_hosts
1.48 markus 1005: and
1006: .Pa /etc/ssh_known_hosts2
1.1 deraadt 1007: files installed and frequently
1.79 stevesk 1008: connect to new hosts.
1009: This option forces the user to manually
1010: add all new hosts.
1011: If this flag is set to
1012: .Dq no ,
1013: .Nm
1014: will automatically add new host keys to the
1015: user known hosts files.
1016: If this flag is set to
1017: .Dq ask ,
1018: new host keys
1019: will be added to the user known host files only after the user
1020: has confirmed that is what they really want to do, and
1021: .Nm
1022: will refuse to connect to hosts whose host key has changed.
1.40 aaron 1023: The host keys of
1.79 stevesk 1024: known hosts will be verified automatically in all cases.
1.40 aaron 1025: The argument must be
1.79 stevesk 1026: .Dq yes ,
1027: .Dq no
1.2 deraadt 1028: or
1.79 stevesk 1029: .Dq ask .
1030: The default is
1031: .Dq ask .
1.16 markus 1032: .It Cm UsePrivilegedPort
1033: Specifies whether to use a privileged port for outgoing connections.
1034: The argument must be
1035: .Dq yes
1036: or
1037: .Dq no .
1038: The default is
1.98 markus 1039: .Dq no .
1.16 markus 1040: Note that setting this option to
1041: .Dq no
1.30 provos 1042: turns off
1.16 markus 1043: .Cm RhostsAuthentication
1044: and
1.72 markus 1045: .Cm RhostsRSAAuthentication
1046: for older servers.
1.34 markus 1047: .It Cm User
1.40 aaron 1048: Specifies the user to log in as.
1049: This can be useful if you have a different user name on different machines.
1050: This saves the trouble of
1.34 markus 1051: having to remember to give the user name on the command line.
1052: .It Cm UserKnownHostsFile
1.95 stevesk 1053: Specifies a file to use for the protocol version 1 user
1054: host key database instead of
1.34 markus 1055: .Pa $HOME/.ssh/known_hosts .
1.95 stevesk 1056: .It Cm UserKnownHostsFile2
1057: Specifies a file to use for the protocol version 2 user
1058: host key database instead of
1059: .Pa $HOME/.ssh/known_hosts2 .
1.2 deraadt 1060: .It Cm UseRsh
1.40 aaron 1061: Specifies that rlogin/rsh should be used for this host.
1062: It is possible that the host does not at all support the
1.2 deraadt 1063: .Nm
1.40 aaron 1064: protocol.
1065: This causes
1.2 deraadt 1066: .Nm
1.40 aaron 1067: to immediately execute
1.2 deraadt 1068: .Xr rsh 1 .
1.1 deraadt 1069: All other options (except
1.2 deraadt 1070: .Cm HostName )
1.40 aaron 1071: are ignored if this has been specified.
1072: The argument must be
1.2 deraadt 1073: .Dq yes
1074: or
1075: .Dq no .
1.55 markus 1076: .It Cm XAuthLocation
1077: Specifies the location of the
1078: .Xr xauth 1
1079: program.
1080: The default is
1081: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1082: .El
1.2 deraadt 1083: .Sh ENVIRONMENT
1084: .Nm
1.1 deraadt 1085: will normally set the following environment variables:
1.2 deraadt 1086: .Bl -tag -width Ds
1087: .It Ev DISPLAY
1088: The
1089: .Ev DISPLAY
1.40 aaron 1090: variable indicates the location of the X11 server.
1.44 aaron 1091: It is automatically set by
1.2 deraadt 1092: .Nm
1093: to point to a value of the form
1094: .Dq hostname:n
1095: where hostname indicates
1.40 aaron 1096: the host where the shell runs, and n is an integer >= 1.
1097: .Nm
1098: uses this special value to forward X11 connections over the secure
1099: channel.
1100: The user should normally not set DISPLAY explicitly, as that
1.1 deraadt 1101: will render the X11 connection insecure (and will require the user to
1102: manually copy any required authorization cookies).
1.2 deraadt 1103: .It Ev HOME
1.1 deraadt 1104: Set to the path of the user's home directory.
1.2 deraadt 1105: .It Ev LOGNAME
1106: Synonym for
1.12 aaron 1107: .Ev USER ;
1108: set for compatibility with systems that use this variable.
1.2 deraadt 1109: .It Ev MAIL
1.1 deraadt 1110: Set to point the user's mailbox.
1.40 aaron 1111: .It Ev PATH
1.2 deraadt 1112: Set to the default
1113: .Ev PATH ,
1114: as specified when compiling
1.12 aaron 1115: .Nm ssh .
1.18 markus 1116: .It Ev SSH_AUTH_SOCK
1.17 markus 1117: indicates the path of a unix-domain socket used to communicate with the
1118: agent.
1.2 deraadt 1119: .It Ev SSH_CLIENT
1.40 aaron 1120: Identifies the client end of the connection.
1121: The variable contains
1.1 deraadt 1122: three space-separated values: client ip-address, client port number,
1123: and server port number.
1.73 markus 1124: .It Ev SSH_ORIGINAL_COMMAND
1125: The variable contains the original command line if a forced command
1126: is executed.
1127: It can be used to extract the original arguments.
1.2 deraadt 1128: .It Ev SSH_TTY
1.1 deraadt 1129: This is set to the name of the tty (path to the device) associated
1.40 aaron 1130: with the current shell or command.
1131: If the current session has no tty,
1.1 deraadt 1132: this variable is not set.
1.2 deraadt 1133: .It Ev TZ
1.1 deraadt 1134: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1135: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1136: on to new connections).
1.2 deraadt 1137: .It Ev USER
1.1 deraadt 1138: Set to the name of the user logging in.
1.2 deraadt 1139: .El
1140: .Pp
1.44 aaron 1141: Additionally,
1.2 deraadt 1142: .Nm
1.44 aaron 1143: reads
1144: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1145: and adds lines of the format
1146: .Dq VARNAME=value
1.12 aaron 1147: to the environment.
1.2 deraadt 1148: .Sh FILES
1.36 markus 1149: .Bl -tag -width Ds
1.95 stevesk 1150: .It Pa $HOME/.ssh/known_hosts, $HOME/.ssh/known_hosts2
1.1 deraadt 1151: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1152: in
1.95 stevesk 1153: .Pa /etc/ssh_known_hosts
1154: for protocol version 1 or
1155: .Pa /etc/ssh_known_hosts2
1156: for protocol version 2).
1.2 deraadt 1157: See
1158: .Xr sshd 8 .
1.102 itojun 1159: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
1160: Contains the authentication identity of the user.
1161: They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
1.48 markus 1162: These files
1163: contain sensitive data and should be readable by the user but not
1.15 markus 1164: accessible by others (read/write/execute).
1165: Note that
1166: .Nm
1.48 markus 1167: ignores a private key file if it is accessible by others.
1.15 markus 1168: It is possible to specify a passphrase when
1.1 deraadt 1169: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1170: sensitive part of this file using 3DES.
1.102 itojun 1171: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
1.1 deraadt 1172: Contains the public key for authentication (public part of the
1.40 aaron 1173: identity file in human-readable form).
1.48 markus 1174: The contents of the
1175: .Pa $HOME/.ssh/identity.pub
1176: file should be added to
1.2 deraadt 1177: .Pa $HOME/.ssh/authorized_keys
1178: on all machines
1.102 itojun 1179: where you wish to log in using protocol version 1 RSA authentication.
1.48 markus 1180: The contents of the
1181: .Pa $HOME/.ssh/id_dsa.pub
1.102 itojun 1182: and
1183: .Pa $HOME/.ssh/id_rsa.pub
1.48 markus 1184: file should be added to
1185: .Pa $HOME/.ssh/authorized_keys2
1186: on all machines
1.102 itojun 1187: where you wish to log in using protocol version 2 DSA/RSA authentication.
1.48 markus 1188: These files are not
1.40 aaron 1189: sensitive and can (but need not) be readable by anyone.
1.48 markus 1190: These files are
1.84 markus 1191: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1192: the convenience of the user.
1.2 deraadt 1193: .It Pa $HOME/.ssh/config
1.40 aaron 1194: This is the per-user configuration file.
1195: The format of this file is described above.
1196: This file is used by the
1.2 deraadt 1197: .Nm
1.40 aaron 1198: client.
1199: This file does not usually contain any sensitive information,
1.1 deraadt 1200: but the recommended permissions are read/write for the user, and not
1201: accessible by others.
1.2 deraadt 1202: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1203: Lists the RSA keys that can be used for logging in as this user.
1204: The format of this file is described in the
1.2 deraadt 1205: .Xr sshd 8
1.40 aaron 1206: manual page.
1207: In the simplest form the format is the same as the .pub
1.1 deraadt 1208: identity files (that is, each line contains the number of bits in
1209: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1210: spaces).
1211: This file is not highly sensitive, but the recommended
1.1 deraadt 1212: permissions are read/write for the user, and not accessible by others.
1.48 markus 1213: .It Pa $HOME/.ssh/authorized_keys2
1.68 markus 1214: Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1.48 markus 1215: This file is not highly sensitive, but the recommended
1216: permissions are read/write for the user, and not accessible by others.
1217: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1218: Systemwide list of known host keys.
1.48 markus 1219: .Pa /etc/ssh_known_hosts
1220: contains RSA and
1221: .Pa /etc/ssh_known_hosts2
1.68 markus 1222: contains DSA or RSA keys for protocol version 2.
1.48 markus 1223: These files should be prepared by the
1.1 deraadt 1224: system administrator to contain the public host keys of all machines in the
1.40 aaron 1225: organization.
1226: This file should be world-readable.
1227: This file contains
1.1 deraadt 1228: public keys, one per line, in the following format (fields separated
1229: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1230: modulus, and optional comment field.
1231: When different names are used
1.1 deraadt 1232: for the same machine, all such names should be listed, separated by
1.40 aaron 1233: commas.
1234: The format is described on the
1.2 deraadt 1235: .Xr sshd 8
1.1 deraadt 1236: manual page.
1.2 deraadt 1237: .Pp
1.1 deraadt 1238: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1239: .Xr sshd 8
1.1 deraadt 1240: to verify the client host when logging in; other names are needed because
1.2 deraadt 1241: .Nm
1.1 deraadt 1242: does not convert the user-supplied name to a canonical name before
1243: checking the key, because someone with access to the name servers
1244: would then be able to fool host authentication.
1.2 deraadt 1245: .It Pa /etc/ssh_config
1.40 aaron 1246: Systemwide configuration file.
1247: This file provides defaults for those
1.1 deraadt 1248: values that are not specified in the user's configuration file, and
1.40 aaron 1249: for those users who do not have a configuration file.
1250: This file must be world-readable.
1.2 deraadt 1251: .It Pa $HOME/.rhosts
1252: This file is used in
1253: .Pa \&.rhosts
1254: authentication to list the
1.40 aaron 1255: host/user pairs that are permitted to log in.
1256: (Note that this file is
1.1 deraadt 1257: also used by rlogin and rsh, which makes using this file insecure.)
1258: Each line of the file contains a host name (in the canonical form
1259: returned by name servers), and then a user name on that host,
1.40 aaron 1260: separated by a space.
1.92 markus 1261: On some machines this file may need to be
1.1 deraadt 1262: world-readable if the user's home directory is on a NFS partition,
1263: because
1.2 deraadt 1264: .Xr sshd 8
1.40 aaron 1265: reads it as root.
1266: Additionally, this file must be owned by the user,
1267: and must not have write permissions for anyone else.
1268: The recommended
1.1 deraadt 1269: permission for most machines is read/write for the user, and not
1270: accessible by others.
1.2 deraadt 1271: .Pp
1.1 deraadt 1272: Note that by default
1.2 deraadt 1273: .Xr sshd 8
1.1 deraadt 1274: will be installed so that it requires successful RSA host
1.40 aaron 1275: authentication before permitting \s+2.\s0rhosts authentication.
1276: If your server machine does not have the client's host key in
1.2 deraadt 1277: .Pa /etc/ssh_known_hosts ,
1278: you can store it in
1279: .Pa $HOME/.ssh/known_hosts .
1280: The easiest way to do this is to
1.1 deraadt 1281: connect back to the client from the server machine using ssh; this
1.48 markus 1282: will automatically add the host key to
1.2 deraadt 1283: .Pa $HOME/.ssh/known_hosts .
1284: .It Pa $HOME/.shosts
1285: This file is used exactly the same way as
1286: .Pa \&.rhosts .
1287: The purpose for
1.1 deraadt 1288: having this file is to be able to use rhosts authentication with
1.2 deraadt 1289: .Nm
1290: without permitting login with
1291: .Xr rlogin 1
1292: or
1293: .Xr rsh 1 .
1294: .It Pa /etc/hosts.equiv
1295: This file is used during
1.40 aaron 1296: .Pa \&.rhosts authentication.
1297: It contains
1.1 deraadt 1298: canonical hosts names, one per line (the full format is described on
1299: the
1.2 deraadt 1300: .Xr sshd 8
1.40 aaron 1301: manual page).
1302: If the client host is found in this file, login is
1.1 deraadt 1303: automatically permitted provided client and server user names are the
1.40 aaron 1304: same.
1305: Additionally, successful RSA host authentication is normally
1306: required.
1307: This file should only be writable by root.
1.2 deraadt 1308: .It Pa /etc/shosts.equiv
1.44 aaron 1309: This file is processed exactly as
1.2 deraadt 1310: .Pa /etc/hosts.equiv .
1.1 deraadt 1311: This file may be useful to permit logins using
1.2 deraadt 1312: .Nm
1.1 deraadt 1313: but not using rsh/rlogin.
1.2 deraadt 1314: .It Pa /etc/sshrc
1.1 deraadt 1315: Commands in this file are executed by
1.2 deraadt 1316: .Nm
1.1 deraadt 1317: when the user logs in just before the user's shell (or command) is started.
1318: See the
1.2 deraadt 1319: .Xr sshd 8
1.1 deraadt 1320: manual page for more information.
1.2 deraadt 1321: .It Pa $HOME/.ssh/rc
1.1 deraadt 1322: Commands in this file are executed by
1.2 deraadt 1323: .Nm
1.1 deraadt 1324: when the user logs in just before the user's shell (or command) is
1325: started.
1.44 aaron 1326: See the
1.2 deraadt 1327: .Xr sshd 8
1.1 deraadt 1328: manual page for more information.
1.31 markus 1329: .It Pa $HOME/.ssh/environment
1330: Contains additional definitions for environment variables, see section
1331: .Sx ENVIRONMENT
1332: above.
1.58 itojun 1333: .El
1.67 aaron 1334: .Sh AUTHORS
1.78 markus 1335: OpenSSH is a derivative of the original and free
1336: ssh 1.2.12 release by Tatu Ylonen.
1337: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1338: Theo de Raadt and Dug Song
1339: removed many bugs, re-added newer features and
1340: created OpenSSH.
1341: Markus Friedl contributed the support for SSH
1342: protocol versions 1.5 and 2.0.
1.2 deraadt 1343: .Sh SEE ALSO
1344: .Xr rlogin 1 ,
1345: .Xr rsh 1 ,
1346: .Xr scp 1 ,
1.83 djm 1347: .Xr sftp 1 ,
1.2 deraadt 1348: .Xr ssh-add 1 ,
1349: .Xr ssh-agent 1 ,
1350: .Xr ssh-keygen 1 ,
1351: .Xr telnet 1 ,
1.87 itojun 1352: .Xr sshd 8
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