Annotation of src/usr.bin/ssh/ssh.1, Revision 1.63
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: .\"
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.
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.
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31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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1.1 deraadt 36: .\"
1.63 ! djm 37: .\" $OpenBSD$
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSH 1
40: .Os
41: .Sh NAME
42: .Nm ssh
1.20 provos 43: .Nd OpenSSH secure shell 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
1.54 markus 51: .Op Fl afgknqtvxACNPTX246
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
56: .Op Fl o Ar option
57: .Op Fl p Ar port
1.12 aaron 58: .Oo Fl L Xo
59: .Sm off
1.33 markus 60: .Ar port :
1.12 aaron 61: .Ar host :
62: .Ar hostport
63: .Sm on
64: .Xc
65: .Oc
66: .Oo Fl R Xo
67: .Sm off
1.33 markus 68: .Ar port :
1.12 aaron 69: .Ar host :
70: .Ar hostport
71: .Sm on
72: .Xc
73: .Oc
1.5 deraadt 74: .Op Ar hostname | user@hostname
1.2 deraadt 75: .Op Ar command
1.44 aaron 76: .Sh DESCRIPTION
1.2 deraadt 77: .Nm
1.5 deraadt 78: (Secure Shell) is a program for logging into a remote machine and for
1.40 aaron 79: executing commands on a remote machine.
80: It is intended to replace
1.1 deraadt 81: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 82: two untrusted hosts over an insecure network.
83: X11 connections and
1.1 deraadt 84: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 85: .Pp
86: .Nm
1.44 aaron 87: connects and logs into the specified
1.2 deraadt 88: .Ar hostname .
1.1 deraadt 89: The user must prove
1.49 markus 90: his/her identity to the remote machine using one of several methods
91: depending on the protocol version used:
92: .Pp
93: .Ss SSH protocol version 1
1.2 deraadt 94: .Pp
1.1 deraadt 95: First, if the machine the user logs in from is listed in
1.2 deraadt 96: .Pa /etc/hosts.equiv
1.1 deraadt 97: or
1.2 deraadt 98: .Pa /etc/shosts.equiv
1.1 deraadt 99: on the remote machine, and the user names are
100: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 101: Second, if
1.2 deraadt 102: .Pa \&.rhosts
1.1 deraadt 103: or
1.2 deraadt 104: .Pa \&.shosts
1.1 deraadt 105: exists in the user's home directory on the
106: remote machine and contains a line containing the name of the client
107: machine and the name of the user on that machine, the user is
1.40 aaron 108: permitted to log in.
109: This form of authentication alone is normally not
1.1 deraadt 110: allowed by the server because it is not secure.
1.2 deraadt 111: .Pp
1.1 deraadt 112: The second (and primary) authentication method is the
1.2 deraadt 113: .Pa rhosts
1.1 deraadt 114: or
1.2 deraadt 115: .Pa hosts.equiv
1.40 aaron 116: method combined with RSA-based host authentication.
117: It means that if the login would be permitted by
1.49 markus 118: .Pa $HOME/.rhosts ,
119: .Pa $HOME/.shosts ,
1.2 deraadt 120: .Pa /etc/hosts.equiv ,
1.1 deraadt 121: or
1.2 deraadt 122: .Pa /etc/shosts.equiv ,
1.11 deraadt 123: and if additionally the server can verify the client's
1.44 aaron 124: host key (see
1.2 deraadt 125: .Pa /etc/ssh_known_hosts
1.23 markus 126: and
127: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 128: in the
1.2 deraadt 129: .Sx FILES
1.40 aaron 130: section), only then login is permitted.
131: This authentication method closes security holes due to IP
132: spoofing, DNS spoofing and routing spoofing.
133: [Note to the administrator:
1.2 deraadt 134: .Pa /etc/hosts.equiv ,
1.49 markus 135: .Pa $HOME/.rhosts ,
1.1 deraadt 136: and the rlogin/rsh protocol in general, are inherently insecure and should be
137: disabled if security is desired.]
1.2 deraadt 138: .Pp
1.44 aaron 139: As a third authentication method,
1.2 deraadt 140: .Nm
1.1 deraadt 141: supports RSA based authentication.
142: The scheme is based on public-key cryptography: there are cryptosystems
143: where encryption and decryption are done using separate keys, and it
144: is not possible to derive the decryption key from the encryption key.
1.40 aaron 145: RSA is one such system.
1.44 aaron 146: The idea is that each user creates a public/private
1.40 aaron 147: key pair for authentication purposes.
148: The server knows the public key, and only the user knows the private key.
1.44 aaron 149: The file
1.2 deraadt 150: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 151: lists the public keys that are permitted for logging
1.40 aaron 152: in.
153: When the user logs in, the
1.2 deraadt 154: .Nm
1.1 deraadt 155: program tells the server which key pair it would like to use for
1.40 aaron 156: authentication.
157: The server checks if this key is permitted, and if
1.1 deraadt 158: so, sends the user (actually the
1.2 deraadt 159: .Nm
1.1 deraadt 160: program running on behalf of the user) a challenge, a random number,
1.40 aaron 161: encrypted by the user's public key.
162: The challenge can only be
163: decrypted using the proper private key.
164: The user's client then decrypts the
1.1 deraadt 165: challenge using the private key, proving that he/she knows the private
166: key but without disclosing it to the server.
1.2 deraadt 167: .Pp
168: .Nm
1.40 aaron 169: implements the RSA authentication protocol automatically.
170: The user creates his/her RSA key pair by running
1.2 deraadt 171: .Xr ssh-keygen 1 .
1.44 aaron 172: This stores the private key in
1.49 markus 173: .Pa $HOME/.ssh/identity
1.1 deraadt 174: and the public key in
1.49 markus 175: .Pa $HOME/.ssh/identity.pub
1.40 aaron 176: in the user's home directory.
177: The user should then copy the
1.2 deraadt 178: .Pa identity.pub
1.44 aaron 179: to
1.49 markus 180: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 181: in his/her home directory on the remote machine (the
1.2 deraadt 182: .Pa authorized_keys
1.44 aaron 183: file corresponds to the conventional
1.49 markus 184: .Pa $HOME/.rhosts
1.1 deraadt 185: file, and has one key
1.40 aaron 186: per line, though the lines can be very long).
187: After this, the user can log in without giving the password.
188: RSA authentication is much
1.1 deraadt 189: more secure than rhosts authentication.
1.2 deraadt 190: .Pp
1.1 deraadt 191: The most convenient way to use RSA authentication may be with an
1.40 aaron 192: authentication agent.
193: See
1.2 deraadt 194: .Xr ssh-agent 1
1.1 deraadt 195: for more information.
1.2 deraadt 196: .Pp
1.44 aaron 197: If other authentication methods fail,
1.2 deraadt 198: .Nm
1.40 aaron 199: prompts the user for a password.
200: The password is sent to the remote
1.1 deraadt 201: host for checking; however, since all communications are encrypted,
202: the password cannot be seen by someone listening on the network.
1.2 deraadt 203: .Pp
1.49 markus 204: .Ss SSH protocol version 2
205: .Pp
206: When a user connects using the protocol version 2
207: different authentication methods are available:
208: At first, the client attempts to authenticate using the public key method.
209: If this method fails password authentication is tried.
210: .Pp
211: The public key method is similar to RSA authentication described
212: in the previous section except that the DSA algorithm is used
213: instead of the patented RSA algorithm.
214: The client uses his private DSA key
215: .Pa $HOME/.ssh/id_dsa
216: to sign the session identifier and sends the result to the server.
217: The server checks whether the matching public key is listed in
218: .Pa $HOME/.ssh/authorized_keys2
219: and grants access if both the key is found and the signature is correct.
220: The session identifier is derived from a shared Diffie-Hellman value
221: and is only known to the client and the server.
222: .Pp
223: If public key authentication fails or is not available a password
224: can be sent encrypted to the remote host for proving the user's identity.
225: This protocol 2 implementation does not yet support Kerberos or
226: S/Key authentication.
227: .Pp
228: Protocol 2 provides additional mechanisms for confidentiality
1.51 markus 229: (the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
1.49 markus 230: and integrity (hmac-sha1, hmac-md5).
231: Note that protocol 1 lacks a strong mechanism for ensuring the
232: integrity of the connection.
233: .Pp
234: .Ss Login session and remote execution
235: .Pp
1.1 deraadt 236: When the user's identity has been accepted by the server, the server
237: either executes the given command, or logs into the machine and gives
1.40 aaron 238: the user a normal shell on the remote machine.
239: All communication with
1.1 deraadt 240: the remote command or shell will be automatically encrypted.
1.2 deraadt 241: .Pp
1.1 deraadt 242: If a pseudo-terminal has been allocated (normal login session), the
1.2 deraadt 243: user can disconnect with
244: .Ic ~. ,
245: and suspend
246: .Nm
247: with
248: .Ic ~^Z .
249: All forwarded connections can be listed with
1.44 aaron 250: .Ic ~#
1.2 deraadt 251: and if
1.1 deraadt 252: the session blocks waiting for forwarded X11 or TCP/IP
1.2 deraadt 253: connections to terminate, it can be backgrounded with
254: .Ic ~&
255: (this should not be used while the user shell is active, as it can cause the
1.40 aaron 256: shell to hang).
257: All available escapes can be listed with
1.2 deraadt 258: .Ic ~? .
259: .Pp
260: A single tilde character can be sent as
261: .Ic ~~
262: (or by following the tilde by a character other than those described above).
1.1 deraadt 263: The escape character must always follow a newline to be interpreted as
1.40 aaron 264: special.
265: The escape character can be changed in configuration files
266: or on the command line.
1.2 deraadt 267: .Pp
1.1 deraadt 268: If no pseudo tty has been allocated, the
269: session is transparent and can be used to reliably transfer binary
1.40 aaron 270: data.
271: On most systems, setting the escape character to
1.2 deraadt 272: .Dq none
273: will also make the session transparent even if a tty is used.
274: .Pp
1.1 deraadt 275: The session terminates when the command or shell in on the remote
276: machine exists and all X11 and TCP/IP connections have been closed.
277: The exit status of the remote program is returned as the exit status
278: of
1.2 deraadt 279: .Nm ssh .
280: .Pp
1.49 markus 281: .Ss X11 and TCP forwarding
282: .Pp
1.1 deraadt 283: If the user is using X11 (the
1.2 deraadt 284: .Ev DISPLAY
1.1 deraadt 285: environment variable is set), the connection to the X11 display is
286: automatically forwarded to the remote side in such a way that any X11
287: programs started from the shell (or command) will go through the
288: encrypted channel, and the connection to the real X server will be made
1.40 aaron 289: from the local machine.
290: The user should not manually set
1.2 deraadt 291: .Ev DISPLAY .
1.1 deraadt 292: Forwarding of X11 connections can be
293: configured on the command line or in configuration files.
1.2 deraadt 294: .Pp
295: The
1.44 aaron 296: .Ev DISPLAY
1.2 deraadt 297: value set by
298: .Nm
1.1 deraadt 299: will point to the server machine, but with a display number greater
1.40 aaron 300: than zero.
301: This is normal, and happens because
1.2 deraadt 302: .Nm
303: creates a
304: .Dq proxy
305: X server on the server machine for forwarding the
1.1 deraadt 306: connections over the encrypted channel.
1.2 deraadt 307: .Pp
308: .Nm
1.1 deraadt 309: will also automatically set up Xauthority data on the server machine.
310: For this purpose, it will generate a random authorization cookie,
311: store it in Xauthority on the server, and verify that any forwarded
312: connections carry this cookie and replace it by the real cookie when
1.40 aaron 313: the connection is opened.
314: The real authentication cookie is never
1.1 deraadt 315: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 316: .Pp
1.1 deraadt 317: If the user is using an authentication agent, the connection to the agent
318: is automatically forwarded to the remote side unless disabled on
319: command line or in a configuration file.
1.2 deraadt 320: .Pp
1.1 deraadt 321: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.40 aaron 322: be specified either on command line or in a configuration file.
323: One possible application of TCP/IP forwarding is a secure connection to an
1.1 deraadt 324: electronic purse; another is going trough firewalls.
1.2 deraadt 325: .Pp
1.49 markus 326: .Ss Server authentication
327: .Pp
1.2 deraadt 328: .Nm
1.49 markus 329: automatically maintains and checks a database containing
1.40 aaron 330: identifications for all hosts it has ever been used with.
1.49 markus 331: RSA host keys are stored in
332: .Pa $HOME/.ssh/known_hosts
333: and
334: DSA host keys are stored in
335: .Pa $HOME/.ssh/known_hosts2
1.40 aaron 336: in the user's home directory.
1.49 markus 337: Additionally, the files
1.2 deraadt 338: .Pa /etc/ssh_known_hosts
1.49 markus 339: and
340: .Pa /etc/ssh_known_hosts2
341: are automatically checked for known hosts.
1.40 aaron 342: Any new hosts are automatically added to the user's file.
343: If a host's identification
1.1 deraadt 344: ever changes,
1.2 deraadt 345: .Nm
1.1 deraadt 346: warns about this and disables password authentication to prevent a
1.40 aaron 347: trojan horse from getting the user's password.
348: Another purpose of
1.1 deraadt 349: this mechanism is to prevent man-in-the-middle attacks which could
1.40 aaron 350: otherwise be used to circumvent the encryption.
351: The
1.2 deraadt 352: .Cm StrictHostKeyChecking
1.1 deraadt 353: option (see below) can be used to prevent logins to machines whose
354: host key is not known or has changed.
1.2 deraadt 355: .Sh OPTIONS
356: .Bl -tag -width Ds
1.4 dugsong 357: .It Fl a
1.42 aaron 358: Disables forwarding of the authentication agent connection.
1.54 markus 359: .It Fl A
360: Enables forwarding of the authentication agent connection.
361: This can also be specified on a per-host basis in a configuration file.
1.6 deraadt 362: .It Fl c Ar blowfish|3des
1.44 aaron 363: Selects the cipher to use for encrypting the session.
1.2 deraadt 364: .Ar 3des
1.40 aaron 365: is used by default.
1.44 aaron 366: It is believed to be secure.
1.5 deraadt 367: .Ar 3des
368: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
369: It is presumably more secure than the
1.2 deraadt 370: .Ar des
1.51 markus 371: cipher which is no longer supported in
372: .Nm ssh .
1.5 deraadt 373: .Ar blowfish
374: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 375: .Ar 3des .
1.51 markus 376: .It Fl c Ar "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"
377: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 378: be specified in order of preference.
379: Protocol version 2 supports 3DES, Blowfish, and CAST128 in CBC mode
380: and Arcfour.
1.2 deraadt 381: .It Fl e Ar ch|^ch|none
382: Sets the escape character for sessions with a pty (default:
383: .Ql ~ ) .
1.40 aaron 384: The escape character is only recognized at the beginning of a line.
385: The escape character followed by a dot
1.2 deraadt 386: .Pq Ql \&.
387: closes the connection, followed
1.1 deraadt 388: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 389: escape character once.
390: Setting the character to
1.2 deraadt 391: .Dq none
392: disables any escapes and makes the session fully transparent.
393: .It Fl f
394: Requests
395: .Nm
1.40 aaron 396: to go to background just before command execution.
397: This is useful if
1.2 deraadt 398: .Nm
399: is going to ask for passwords or passphrases, but the user
1.40 aaron 400: wants it in the background.
1.44 aaron 401: This implies
1.2 deraadt 402: .Fl n .
1.1 deraadt 403: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 404: something like
405: .Ic ssh -f host xterm .
1.34 markus 406: .It Fl g
407: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 408: .It Fl i Ar identity_file
1.44 aaron 409: Selects the file from which the identity (private key) for
1.40 aaron 410: RSA authentication is read.
1.44 aaron 411: Default is
1.49 markus 412: .Pa $HOME/.ssh/identity
1.40 aaron 413: in the user's home directory.
414: Identity files may also be specified on
415: a per-host basis in the configuration file.
416: It is possible to have multiple
1.2 deraadt 417: .Fl i
418: options (and multiple identities specified in
1.1 deraadt 419: configuration files).
1.2 deraadt 420: .It Fl k
1.42 aaron 421: Disables forwarding of Kerberos tickets and AFS tokens.
422: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 423: .It Fl l Ar login_name
1.40 aaron 424: Specifies the user to log in as on the remote machine.
425: This also may be specified on a per-host basis in the configuration file.
1.2 deraadt 426: .It Fl n
427: Redirects stdin from
428: .Pa /dev/null
429: (actually, prevents reading from stdin).
1.1 deraadt 430: This must be used when
1.2 deraadt 431: .Nm
1.40 aaron 432: is run in the background.
433: A common trick is to use this to run X11 programs on a remote machine.
434: For example,
1.2 deraadt 435: .Ic ssh -n shadows.cs.hut.fi emacs &
436: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 437: connection will be automatically forwarded over an encrypted channel.
438: The
1.2 deraadt 439: .Nm
1.1 deraadt 440: program will be put in the background.
441: (This does not work if
1.2 deraadt 442: .Nm
443: needs to ask for a password or passphrase; see also the
444: .Fl f
445: option.)
1.53 markus 446: .It Fl N
447: Do not execute a remote command.
448: This is usefull if you just want to forward ports
449: (protocol version 2 only).
1.2 deraadt 450: .It Fl o Ar option
1.1 deraadt 451: Can be used to give options in the format used in the config file.
452: This is useful for specifying options for which there is no separate
1.40 aaron 453: command-line flag.
454: The option has the same format as a line in the configuration file.
1.2 deraadt 455: .It Fl p Ar port
1.40 aaron 456: Port to connect to on the remote host.
457: This can be specified on a
1.1 deraadt 458: per-host basis in the configuration file.
1.16 markus 459: .It Fl P
460: Use a non-privileged port for outgoing connections.
461: This can be used if your firewall does
462: not permit connections from privileged ports.
1.30 provos 463: Note that this option turns off
1.16 markus 464: .Cm RhostsAuthentication
465: and
466: .Cm RhostsRSAAuthentication .
1.2 deraadt 467: .It Fl q
1.40 aaron 468: Quiet mode.
469: Causes all warning and diagnostic messages to be suppressed.
470: Only fatal errors are displayed.
1.2 deraadt 471: .It Fl t
1.40 aaron 472: Force pseudo-tty allocation.
1.43 brad 473: This can be used to execute arbitrary
1.40 aaron 474: screen-based programs on a remote machine, which can be very useful,
475: e.g., when implementing menu services.
1.53 markus 476: .It Fl T
477: Disable pseudo-tty allocation (protocol version 2 only).
1.2 deraadt 478: .It Fl v
1.40 aaron 479: Verbose mode.
480: Causes
1.2 deraadt 481: .Nm
1.40 aaron 482: to print debugging messages about its progress.
483: This is helpful in
1.1 deraadt 484: debugging connection, authentication, and configuration problems.
1.22 provos 485: The verbose mode is also used to display
486: .Xr skey 1
487: challenges, if the user entered "s/key" as password.
1.61 aaron 488: Multiple -v options increases the verbosity.
489: Maximum is 3.
1.2 deraadt 490: .It Fl x
1.40 aaron 491: Disables X11 forwarding.
1.2 deraadt 492: .It Fl X
1.1 deraadt 493: Enables X11 forwarding.
1.54 markus 494: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 495: .It Fl C
1.1 deraadt 496: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 497: data for forwarded X11 and TCP/IP connections).
498: The compression algorithm is the same used by
1.34 markus 499: .Xr gzip 1 ,
500: and the
1.2 deraadt 501: .Dq level
502: can be controlled by the
503: .Cm CompressionLevel
1.40 aaron 504: option (see below).
505: Compression is desirable on modem lines and other
1.1 deraadt 506: slow connections, but will only slow down things on fast networks.
507: The default value can be set on a host-by-host basis in the
508: configuration files; see the
1.2 deraadt 509: .Cm Compress
1.1 deraadt 510: option below.
1.2 deraadt 511: .It Fl L Ar port:host:hostport
1.1 deraadt 512: Specifies that the given port on the local (client) host is to be
1.40 aaron 513: forwarded to the given host and port on the remote side.
514: This works by allocating a socket to listen to
1.2 deraadt 515: .Ar port
1.1 deraadt 516: on the local side, and whenever a connection is made to this port, the
517: connection is forwarded over the secure channel, and a connection is
518: made to
1.32 markus 519: .Ar host
520: port
521: .Ar hostport
1.40 aaron 522: from the remote machine.
523: Port forwardings can also be specified in the configuration file.
524: Only root can forward privileged ports.
1.32 markus 525: IPv6 addresses can be specified with an alternative syntax:
526: .Ar port/host/hostport
1.2 deraadt 527: .It Fl R Ar port:host:hostport
1.1 deraadt 528: Specifies that the given port on the remote (server) host is to be
1.40 aaron 529: forwarded to the given host and port on the local side.
530: This works by allocating a socket to listen to
1.2 deraadt 531: .Ar port
1.1 deraadt 532: on the remote side, and whenever a connection is made to this port, the
533: connection is forwarded over the secure channel, and a connection is
534: made to
1.32 markus 535: .Ar host
536: port
537: .Ar hostport
1.40 aaron 538: from the local machine.
539: Port forwardings can also be specified in the configuration file.
540: Privileged ports can be forwarded only when
1.1 deraadt 541: logging in as root on the remote machine.
1.46 markus 542: .It Fl 2
543: Forces
544: .Nm
1.50 markus 545: to try protocol version 2 only.
1.32 markus 546: .It Fl 4
547: Forces
548: .Nm
549: to use IPv4 addresses only.
550: .It Fl 6
551: Forces
552: .Nm
553: to use IPv6 addresses only.
1.2 deraadt 554: .El
555: .Sh CONFIGURATION FILES
556: .Nm
1.1 deraadt 557: obtains configuration data from the following sources (in this order):
558: command line options, user's configuration file
1.2 deraadt 559: .Pq Pa $HOME/.ssh/config ,
560: and system-wide configuration file
561: .Pq Pa /etc/ssh_config .
562: For each parameter, the first obtained value
1.40 aaron 563: will be used.
564: The configuration files contain sections bracketed by
565: .Dq Host
566: specifications, and that section is only applied for hosts that
567: match one of the patterns given in the specification.
568: The matched host name is the one given on the command line.
1.2 deraadt 569: .Pp
1.1 deraadt 570: Since the first obtained value for each parameter is used, more
571: host-specific declarations should be given near the beginning of the
572: file, and general defaults at the end.
1.2 deraadt 573: .Pp
1.1 deraadt 574: The configuration file has the following format:
1.2 deraadt 575: .Pp
576: Empty lines and lines starting with
577: .Ql #
578: are comments.
579: .Pp
580: Otherwise a line is of the format
581: .Dq keyword arguments .
582: The possible
1.1 deraadt 583: keywords and their meanings are as follows (note that the
584: configuration files are case-sensitive):
1.2 deraadt 585: .Bl -tag -width Ds
586: .It Cm Host
1.1 deraadt 587: Restricts the following declarations (up to the next
1.2 deraadt 588: .Cm Host
1.1 deraadt 589: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 590: given after the keyword.
591: .Ql \&*
592: and
593: .Ql ?
594: can be used as wildcards in the
1.40 aaron 595: patterns.
596: A single
1.2 deraadt 597: .Ql \&*
598: as a pattern can be used to provide global
1.40 aaron 599: defaults for all hosts.
600: The host is the
1.2 deraadt 601: .Ar hostname
1.1 deraadt 602: argument given on the command line (i.e., the name is not converted to
603: a canonicalized host name before matching).
1.2 deraadt 604: .It Cm AFSTokenPassing
1.42 aaron 605: Specifies whether to pass AFS tokens to remote host.
606: The argument to this keyword must be
1.2 deraadt 607: .Dq yes
608: or
609: .Dq no .
610: .It Cm BatchMode
611: If set to
612: .Dq yes ,
1.40 aaron 613: passphrase/password querying will be disabled.
614: This option is useful in scripts and other batch jobs where you have no
615: user to supply the password.
616: The argument must be
1.2 deraadt 617: .Dq yes
618: or
619: .Dq no .
1.34 markus 620: .It Cm CheckHostIP
621: If this flag is set to
622: .Dq yes ,
623: ssh will additionally check the host ip address in the
624: .Pa known_hosts
1.42 aaron 625: file.
626: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 627: If the option is set to
628: .Dq no ,
629: the check will not be executed.
1.2 deraadt 630: .It Cm Cipher
1.62 markus 631: Specifies the cipher to use for encrypting the session
632: in protocol version 1
1.40 aaron 633: Currently,
1.2 deraadt 634: .Dq blowfish ,
1.1 deraadt 635: and
1.10 provos 636: .Dq 3des
1.40 aaron 637: are supported.
638: The default is
1.2 deraadt 639: .Dq 3des .
1.45 markus 640: .It Cm Ciphers
641: Specifies the ciphers allowed for protocol version 2
642: in order of preference.
643: Multiple ciphers must be comma-separated.
644: The default is
1.62 markus 645: .Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour .
1.2 deraadt 646: .It Cm Compression
1.40 aaron 647: Specifies whether to use compression.
648: The argument must be
1.2 deraadt 649: .Dq yes
650: or
651: .Dq no .
652: .It Cm CompressionLevel
1.40 aaron 653: Specifies the compression level to use if compression is enable.
654: The argument must be an integer from 1 (fast) to 9 (slow, best).
655: The default level is 6, which is good for most applications.
656: The meaning of the values is the same as in
1.34 markus 657: .Xr gzip 1 .
1.2 deraadt 658: .It Cm ConnectionAttempts
1.1 deraadt 659: Specifies the number of tries (one per second) to make before falling
1.40 aaron 660: back to rsh or exiting.
661: The argument must be an integer.
662: This may be useful in scripts if the connection sometimes fails.
1.50 markus 663: .It Cm DSAAuthentication
664: Specifies whether to try DSA authentication.
665: The argument to this keyword must be
666: .Dq yes
667: or
668: .Dq no .
669: DSA authentication will only be
670: attempted if a DSA identity file exists.
671: Note that this option applies to protocol version 2 only.
1.2 deraadt 672: .It Cm EscapeChar
673: Sets the escape character (default:
674: .Ql ~ ) .
675: The escape character can also
1.40 aaron 676: be set on the command line.
677: The argument should be a single character,
1.2 deraadt 678: .Ql ^
679: followed by a letter, or
680: .Dq none
681: to disable the escape
1.1 deraadt 682: character entirely (making the connection transparent for binary
683: data).
1.44 aaron 684: .It Cm FallBackToRsh
1.1 deraadt 685: Specifies that if connecting via
1.2 deraadt 686: .Nm
1.1 deraadt 687: fails due to a connection refused error (there is no
1.2 deraadt 688: .Xr sshd 8
1.44 aaron 689: listening on the remote host),
1.2 deraadt 690: .Xr rsh 1
1.1 deraadt 691: should automatically be used instead (after a suitable warning about
1.40 aaron 692: the session being unencrypted).
693: The argument must be
1.2 deraadt 694: .Dq yes
695: or
696: .Dq no .
697: .It Cm ForwardAgent
1.1 deraadt 698: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 699: will be forwarded to the remote machine.
700: The argument must be
1.2 deraadt 701: .Dq yes
702: or
1.54 markus 703: .Dq no .
704: The default is
1.2 deraadt 705: .Dq no .
706: .It Cm ForwardX11
1.1 deraadt 707: Specifies whether X11 connections will be automatically redirected
1.44 aaron 708: over the secure channel and
1.2 deraadt 709: .Ev DISPLAY
1.40 aaron 710: set.
1.44 aaron 711: The argument must be
1.2 deraadt 712: .Dq yes
713: or
1.38 markus 714: .Dq no .
715: The default is
1.3 deraadt 716: .Dq no .
717: .It Cm GatewayPorts
718: Specifies whether remote hosts are allowed to connect to local
719: forwarded ports.
720: The argument must be
721: .Dq yes
722: or
723: .Dq no .
724: The default is
1.2 deraadt 725: .Dq no .
726: .It Cm GlobalKnownHostsFile
1.44 aaron 727: Specifies a file to use instead of
1.2 deraadt 728: .Pa /etc/ssh_known_hosts .
729: .It Cm HostName
1.40 aaron 730: Specifies the real host name to log into.
731: This can be used to specify nicknames or abbreviations for hosts.
732: Default is the name given on the command line.
733: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 734: .Cm HostName
1.1 deraadt 735: specifications).
1.2 deraadt 736: .It Cm IdentityFile
1.1 deraadt 737: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 738: is read (default
1.49 markus 739: .Pa $HOME/.ssh/identity
1.2 deraadt 740: in the user's home directory).
1.1 deraadt 741: Additionally, any identities represented by the authentication agent
1.40 aaron 742: will be used for authentication.
743: The file name may use the tilde
744: syntax to refer to a user's home directory.
745: It is possible to have
1.1 deraadt 746: multiple identity files specified in configuration files; all these
747: identities will be tried in sequence.
1.48 markus 748: .It Cm IdentityFile2
749: Specifies the file from which the user's DSA authentication identity
750: is read (default
1.49 markus 751: .Pa $HOME/.ssh/id_dsa
1.48 markus 752: in the user's home directory).
753: The file name may use the tilde
754: syntax to refer to a user's home directory.
755: It is possible to have
756: multiple identity files specified in configuration files; all these
757: identities will be tried in sequence.
1.2 deraadt 758: .It Cm KeepAlive
1.1 deraadt 759: Specifies whether the system should send keepalive messages to the
1.40 aaron 760: other side.
761: If they are sent, death of the connection or crash of one
762: of the machines will be properly noticed.
763: However, this means that
1.1 deraadt 764: connections will die if the route is down temporarily, and some people
1.41 aaron 765: find it annoying.
1.2 deraadt 766: .Pp
767: The default is
768: .Dq yes
769: (to send keepalives), and the client will notice
1.40 aaron 770: if the network goes down or the remote host dies.
771: This is important in scripts, and many users want it too.
1.2 deraadt 772: .Pp
773: To disable keepalives, the value should be set to
774: .Dq no
775: in both the server and the client configuration files.
776: .It Cm KerberosAuthentication
1.42 aaron 777: Specifies whether Kerberos authentication will be used.
778: The argument to this keyword must be
1.4 dugsong 779: .Dq yes
780: or
781: .Dq no .
1.2 deraadt 782: .It Cm KerberosTgtPassing
1.42 aaron 783: Specifies whether a Kerberos TGT will be forwarded to the server.
784: This will only work if the Kerberos server is actually an AFS kaserver.
785: The argument to this keyword must be
1.4 dugsong 786: .Dq yes
787: or
788: .Dq no .
1.2 deraadt 789: .It Cm LocalForward
1.1 deraadt 790: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 791: the secure channel to given host:port from the remote machine.
792: The first argument must be a port number, and the second must be
793: host:port.
794: Multiple forwardings may be specified, and additional
795: forwardings can be given on the command line.
796: Only the superuser can forward privileged ports.
1.24 markus 797: .It Cm LogLevel
798: Gives the verbosity level that is used when logging messages from
799: .Nm ssh .
800: The possible values are:
1.39 djm 801: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.24 markus 802: The default is INFO.
1.14 dugsong 803: .It Cm NumberOfPasswordPrompts
1.42 aaron 804: Specifies the number of password prompts before giving up.
805: The argument to this keyword must be an integer.
806: Default is 3.
1.34 markus 807: .It Cm PasswordAuthentication
1.40 aaron 808: Specifies whether to use password authentication.
809: The argument to this keyword must be
1.34 markus 810: .Dq yes
811: or
812: .Dq no .
1.50 markus 813: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 814: .It Cm Port
1.40 aaron 815: Specifies the port number to connect on the remote host.
816: Default is 22.
1.45 markus 817: .It Cm Protocol
818: Specifies the protocol versions
819: .Nm
820: should support in order of preference.
821: The possible values are
822: .Dq 1
823: and
824: .Dq 2 .
825: Multiple versions must be comma-separated.
826: The default is
1.49 markus 827: .Dq 1,2 .
828: This means that
829: .Nm
830: tries version 1 and falls back to version 2
1.52 hugh 831: if version 1 is not available.
1.2 deraadt 832: .It Cm ProxyCommand
1.40 aaron 833: Specifies the command to use to connect to the server.
834: The command
835: string extends to the end of the line, and is executed with
836: .Pa /bin/sh .
837: In the command string,
838: .Ql %h
839: will be substituted by the host name to
840: connect and
841: .Ql %p
842: by the port.
843: The command can be basically anything,
844: and should read from its standard input and write to its standard output.
845: It should eventually connect an
1.2 deraadt 846: .Xr sshd 8
1.1 deraadt 847: server running on some machine, or execute
1.2 deraadt 848: .Ic sshd -i
1.40 aaron 849: somewhere.
850: Host key management will be done using the
1.1 deraadt 851: HostName of the host being connected (defaulting to the name typed by
852: the user).
1.29 markus 853: Note that
854: .Cm CheckHostIP
855: is not available for connects with a proxy command.
1.2 deraadt 856: .Pp
857: .It Cm RemoteForward
1.1 deraadt 858: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 859: the secure channel to given host:port from the local machine.
860: The first argument must be a port number, and the second must be
861: host:port.
862: Multiple forwardings may be specified, and additional
863: forwardings can be given on the command line.
864: Only the superuser can forward privileged ports.
1.2 deraadt 865: .It Cm RhostsAuthentication
1.40 aaron 866: Specifies whether to try rhosts based authentication.
867: Note that this
1.1 deraadt 868: declaration only affects the client side and has no effect whatsoever
1.40 aaron 869: on security.
870: Disabling rhosts authentication may reduce
1.1 deraadt 871: authentication time on slow connections when rhosts authentication is
1.40 aaron 872: not used.
873: Most servers do not permit RhostsAuthentication because it
874: is not secure (see RhostsRSAAuthentication).
875: The argument to this keyword must be
1.2 deraadt 876: .Dq yes
877: or
878: .Dq no .
879: .It Cm RhostsRSAAuthentication
1.1 deraadt 880: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 881: authentication.
882: This is the primary authentication method for most sites.
883: The argument must be
1.2 deraadt 884: .Dq yes
885: or
886: .Dq no .
887: .It Cm RSAAuthentication
1.40 aaron 888: Specifies whether to try RSA authentication.
889: The argument to this keyword must be
1.2 deraadt 890: .Dq yes
891: or
892: .Dq no .
1.1 deraadt 893: RSA authentication will only be
894: attempted if the identity file exists, or an authentication agent is
895: running.
1.50 markus 896: Note that this option applies to protocol version 1 only.
1.27 markus 897: .It Cm SkeyAuthentication
898: Specifies whether to use
899: .Xr skey 1
1.40 aaron 900: authentication.
901: The argument to this keyword must be
1.27 markus 902: .Dq yes
903: or
904: .Dq no .
905: The default is
906: .Dq no .
1.2 deraadt 907: .It Cm StrictHostKeyChecking
908: If this flag is set to
1.44 aaron 909: .Dq yes ,
1.2 deraadt 910: .Nm
1.1 deraadt 911: ssh will never automatically add host keys to the
1.2 deraadt 912: .Pa $HOME/.ssh/known_hosts
1.48 markus 913: and
914: .Pa $HOME/.ssh/known_hosts2
915: files, and refuses to connect hosts whose host key has changed.
1.40 aaron 916: This provides maximum protection against trojan horse attacks.
917: However, it can be somewhat annoying if you don't have good
1.2 deraadt 918: .Pa /etc/ssh_known_hosts
1.48 markus 919: and
920: .Pa /etc/ssh_known_hosts2
1.1 deraadt 921: files installed and frequently
1.40 aaron 922: connect new hosts.
923: Basically this option forces the user to manually
924: add any new hosts.
925: Normally this option is disabled, and new hosts
926: will automatically be added to the known host files.
927: The host keys of
928: known hosts will be verified automatically in either case.
929: The argument must be
1.2 deraadt 930: .Dq yes
931: or
932: .Dq no .
1.16 markus 933: .It Cm UsePrivilegedPort
934: Specifies whether to use a privileged port for outgoing connections.
935: The argument must be
936: .Dq yes
937: or
938: .Dq no .
939: The default is
940: .Dq yes .
941: Note that setting this option to
942: .Dq no
1.30 provos 943: turns off
1.16 markus 944: .Cm RhostsAuthentication
945: and
946: .Cm RhostsRSAAuthentication .
1.34 markus 947: .It Cm User
1.40 aaron 948: Specifies the user to log in as.
949: This can be useful if you have a different user name on different machines.
950: This saves the trouble of
1.34 markus 951: having to remember to give the user name on the command line.
952: .It Cm UserKnownHostsFile
953: Specifies a file to use instead of
954: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 955: .It Cm UseRsh
1.40 aaron 956: Specifies that rlogin/rsh should be used for this host.
957: It is possible that the host does not at all support the
1.2 deraadt 958: .Nm
1.40 aaron 959: protocol.
960: This causes
1.2 deraadt 961: .Nm
1.40 aaron 962: to immediately execute
1.2 deraadt 963: .Xr rsh 1 .
1.1 deraadt 964: All other options (except
1.2 deraadt 965: .Cm HostName )
1.40 aaron 966: are ignored if this has been specified.
967: The argument must be
1.2 deraadt 968: .Dq yes
969: or
970: .Dq no .
1.55 markus 971: .It Cm XAuthLocation
972: Specifies the location of the
973: .Xr xauth 1
974: program.
975: The default is
976: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 977: .El
1.2 deraadt 978: .Sh ENVIRONMENT
979: .Nm
1.1 deraadt 980: will normally set the following environment variables:
1.2 deraadt 981: .Bl -tag -width Ds
982: .It Ev DISPLAY
983: The
984: .Ev DISPLAY
1.40 aaron 985: variable indicates the location of the X11 server.
1.44 aaron 986: It is automatically set by
1.2 deraadt 987: .Nm
988: to point to a value of the form
989: .Dq hostname:n
990: where hostname indicates
1.40 aaron 991: the host where the shell runs, and n is an integer >= 1.
992: .Nm
993: uses this special value to forward X11 connections over the secure
994: channel.
995: The user should normally not set DISPLAY explicitly, as that
1.1 deraadt 996: will render the X11 connection insecure (and will require the user to
997: manually copy any required authorization cookies).
1.2 deraadt 998: .It Ev HOME
1.1 deraadt 999: Set to the path of the user's home directory.
1.2 deraadt 1000: .It Ev LOGNAME
1001: Synonym for
1.12 aaron 1002: .Ev USER ;
1003: set for compatibility with systems that use this variable.
1.2 deraadt 1004: .It Ev MAIL
1.1 deraadt 1005: Set to point the user's mailbox.
1.40 aaron 1006: .It Ev PATH
1.2 deraadt 1007: Set to the default
1008: .Ev PATH ,
1009: as specified when compiling
1.12 aaron 1010: .Nm ssh .
1.18 markus 1011: .It Ev SSH_AUTH_SOCK
1.17 markus 1012: indicates the path of a unix-domain socket used to communicate with the
1013: agent.
1.2 deraadt 1014: .It Ev SSH_CLIENT
1.40 aaron 1015: Identifies the client end of the connection.
1016: The variable contains
1.1 deraadt 1017: three space-separated values: client ip-address, client port number,
1018: and server port number.
1.2 deraadt 1019: .It Ev SSH_TTY
1.1 deraadt 1020: This is set to the name of the tty (path to the device) associated
1.40 aaron 1021: with the current shell or command.
1022: If the current session has no tty,
1.1 deraadt 1023: this variable is not set.
1.2 deraadt 1024: .It Ev TZ
1.1 deraadt 1025: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1026: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1027: on to new connections).
1.2 deraadt 1028: .It Ev USER
1.1 deraadt 1029: Set to the name of the user logging in.
1.2 deraadt 1030: .El
1031: .Pp
1.44 aaron 1032: Additionally,
1.2 deraadt 1033: .Nm
1.44 aaron 1034: reads
1035: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1036: and adds lines of the format
1037: .Dq VARNAME=value
1.12 aaron 1038: to the environment.
1.2 deraadt 1039: .Sh FILES
1.36 markus 1040: .Bl -tag -width Ds
1.2 deraadt 1041: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1042: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1043: in
1044: .Pa /etc/ssh_known_hosts ) .
1045: See
1046: .Xr sshd 8 .
1.48 markus 1047: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1048: Contains the RSA and the DSA authentication identity of the user.
1049: These files
1050: contain sensitive data and should be readable by the user but not
1.15 markus 1051: accessible by others (read/write/execute).
1052: Note that
1053: .Nm
1.48 markus 1054: ignores a private key file if it is accessible by others.
1.15 markus 1055: It is possible to specify a passphrase when
1.1 deraadt 1056: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1057: sensitive part of this file using 3DES.
1.48 markus 1058: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1.1 deraadt 1059: Contains the public key for authentication (public part of the
1.40 aaron 1060: identity file in human-readable form).
1.48 markus 1061: The contents of the
1062: .Pa $HOME/.ssh/identity.pub
1063: file should be added to
1.2 deraadt 1064: .Pa $HOME/.ssh/authorized_keys
1065: on all machines
1.40 aaron 1066: where you wish to log in using RSA authentication.
1.48 markus 1067: The contents of the
1068: .Pa $HOME/.ssh/id_dsa.pub
1069: file should be added to
1070: .Pa $HOME/.ssh/authorized_keys2
1071: on all machines
1072: where you wish to log in using DSA authentication.
1073: These files are not
1.40 aaron 1074: sensitive and can (but need not) be readable by anyone.
1.48 markus 1075: These files are
1076: never used automatically and are not necessary; they is only provided for
1.1 deraadt 1077: the convenience of the user.
1.2 deraadt 1078: .It Pa $HOME/.ssh/config
1.40 aaron 1079: This is the per-user configuration file.
1080: The format of this file is described above.
1081: This file is used by the
1.2 deraadt 1082: .Nm
1.40 aaron 1083: client.
1084: This file does not usually contain any sensitive information,
1.1 deraadt 1085: but the recommended permissions are read/write for the user, and not
1086: accessible by others.
1.2 deraadt 1087: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1088: Lists the RSA keys that can be used for logging in as this user.
1089: The format of this file is described in the
1.2 deraadt 1090: .Xr sshd 8
1.40 aaron 1091: manual page.
1092: In the simplest form the format is the same as the .pub
1.1 deraadt 1093: identity files (that is, each line contains the number of bits in
1094: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1095: spaces).
1096: This file is not highly sensitive, but the recommended
1.1 deraadt 1097: permissions are read/write for the user, and not accessible by others.
1.48 markus 1098: .It Pa $HOME/.ssh/authorized_keys2
1099: Lists the DSA keys that can be used for logging in as this user.
1100: This file is not highly sensitive, but the recommended
1101: permissions are read/write for the user, and not accessible by others.
1102: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1103: Systemwide list of known host keys.
1.48 markus 1104: .Pa /etc/ssh_known_hosts
1105: contains RSA and
1106: .Pa /etc/ssh_known_hosts2
1107: contains DSA keys.
1108: These files should be prepared by the
1.1 deraadt 1109: system administrator to contain the public host keys of all machines in the
1.40 aaron 1110: organization.
1111: This file should be world-readable.
1112: This file contains
1.1 deraadt 1113: public keys, one per line, in the following format (fields separated
1114: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1115: modulus, and optional comment field.
1116: When different names are used
1.1 deraadt 1117: for the same machine, all such names should be listed, separated by
1.40 aaron 1118: commas.
1119: The format is described on the
1.2 deraadt 1120: .Xr sshd 8
1.1 deraadt 1121: manual page.
1.2 deraadt 1122: .Pp
1.1 deraadt 1123: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1124: .Xr sshd 8
1.1 deraadt 1125: to verify the client host when logging in; other names are needed because
1.2 deraadt 1126: .Nm
1.1 deraadt 1127: does not convert the user-supplied name to a canonical name before
1128: checking the key, because someone with access to the name servers
1129: would then be able to fool host authentication.
1.2 deraadt 1130: .It Pa /etc/ssh_config
1.40 aaron 1131: Systemwide configuration file.
1132: This file provides defaults for those
1.1 deraadt 1133: values that are not specified in the user's configuration file, and
1.40 aaron 1134: for those users who do not have a configuration file.
1135: This file must be world-readable.
1.2 deraadt 1136: .It Pa $HOME/.rhosts
1137: This file is used in
1138: .Pa \&.rhosts
1139: authentication to list the
1.40 aaron 1140: host/user pairs that are permitted to log in.
1141: (Note that this file is
1.1 deraadt 1142: also used by rlogin and rsh, which makes using this file insecure.)
1143: Each line of the file contains a host name (in the canonical form
1144: returned by name servers), and then a user name on that host,
1.40 aaron 1145: separated by a space.
1146: One some machines this file may need to be
1.1 deraadt 1147: world-readable if the user's home directory is on a NFS partition,
1148: because
1.2 deraadt 1149: .Xr sshd 8
1.40 aaron 1150: reads it as root.
1151: Additionally, this file must be owned by the user,
1152: and must not have write permissions for anyone else.
1153: The recommended
1.1 deraadt 1154: permission for most machines is read/write for the user, and not
1155: accessible by others.
1.2 deraadt 1156: .Pp
1.1 deraadt 1157: Note that by default
1.2 deraadt 1158: .Xr sshd 8
1.1 deraadt 1159: will be installed so that it requires successful RSA host
1.40 aaron 1160: authentication before permitting \s+2.\s0rhosts authentication.
1161: If your server machine does not have the client's host key in
1.2 deraadt 1162: .Pa /etc/ssh_known_hosts ,
1163: you can store it in
1164: .Pa $HOME/.ssh/known_hosts .
1165: The easiest way to do this is to
1.1 deraadt 1166: connect back to the client from the server machine using ssh; this
1.48 markus 1167: will automatically add the host key to
1.2 deraadt 1168: .Pa $HOME/.ssh/known_hosts .
1169: .It Pa $HOME/.shosts
1170: This file is used exactly the same way as
1171: .Pa \&.rhosts .
1172: The purpose for
1.1 deraadt 1173: having this file is to be able to use rhosts authentication with
1.2 deraadt 1174: .Nm
1175: without permitting login with
1176: .Xr rlogin 1
1177: or
1178: .Xr rsh 1 .
1179: .It Pa /etc/hosts.equiv
1180: This file is used during
1.40 aaron 1181: .Pa \&.rhosts authentication.
1182: It contains
1.1 deraadt 1183: canonical hosts names, one per line (the full format is described on
1184: the
1.2 deraadt 1185: .Xr sshd 8
1.40 aaron 1186: manual page).
1187: If the client host is found in this file, login is
1.1 deraadt 1188: automatically permitted provided client and server user names are the
1.40 aaron 1189: same.
1190: Additionally, successful RSA host authentication is normally
1191: required.
1192: This file should only be writable by root.
1.2 deraadt 1193: .It Pa /etc/shosts.equiv
1.44 aaron 1194: This file is processed exactly as
1.2 deraadt 1195: .Pa /etc/hosts.equiv .
1.1 deraadt 1196: This file may be useful to permit logins using
1.2 deraadt 1197: .Nm
1.1 deraadt 1198: but not using rsh/rlogin.
1.2 deraadt 1199: .It Pa /etc/sshrc
1.1 deraadt 1200: Commands in this file are executed by
1.2 deraadt 1201: .Nm
1.1 deraadt 1202: when the user logs in just before the user's shell (or command) is started.
1203: See the
1.2 deraadt 1204: .Xr sshd 8
1.1 deraadt 1205: manual page for more information.
1.2 deraadt 1206: .It Pa $HOME/.ssh/rc
1.1 deraadt 1207: Commands in this file are executed by
1.2 deraadt 1208: .Nm
1.1 deraadt 1209: when the user logs in just before the user's shell (or command) is
1210: started.
1.44 aaron 1211: See the
1.2 deraadt 1212: .Xr sshd 8
1.1 deraadt 1213: manual page for more information.
1.31 markus 1214: .It Pa $HOME/.ssh/environment
1215: Contains additional definitions for environment variables, see section
1216: .Sx ENVIRONMENT
1217: above.
1.5 deraadt 1218: .It Pa libcrypto.so.X.1
1219: A version of this library which includes support for the RSA algorithm
1220: is required for proper operation.
1.58 itojun 1221: .El
1.2 deraadt 1222: .Sh AUTHOR
1.20 provos 1223: OpenSSH
1.37 deraadt 1224: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.40 aaron 1225: but with bugs removed and newer features re-added.
1226: Rapidly after the
1.37 deraadt 1227: 1.2.12 release, newer versions of the original ssh bore successively
1228: more restrictive licenses, and thus demand for a free version was born.
1.47 markus 1229: .Pp
1.37 deraadt 1230: This version of OpenSSH
1.20 provos 1231: .Bl -bullet
1232: .It
1.35 aaron 1233: has all components of a restrictive nature (i.e., patents, see
1.21 deraadt 1234: .Xr ssl 8 )
1235: directly removed from the source code; any licensed or patented components
1236: are chosen from
1237: external libraries.
1.20 provos 1238: .It
1.47 markus 1239: has been updated to support SSH protocol 1.5 and 2, making it compatible with
1240: all other SSH clients and servers.
1.20 provos 1241: .It
1.44 aaron 1242: contains added support for
1.20 provos 1243: .Xr kerberos 8
1244: authentication and ticket passing.
1245: .It
1.21 deraadt 1246: supports one-time password authentication with
1.20 provos 1247: .Xr skey 1 .
1248: .El
1.25 provos 1249: .Pp
1.26 aaron 1250: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1.25 provos 1251: Niels Provos, Theo de Raadt, and Dug Song.
1.47 markus 1252: .Pp
1253: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1254: .Sh SEE ALSO
1255: .Xr rlogin 1 ,
1256: .Xr rsh 1 ,
1257: .Xr scp 1 ,
1258: .Xr ssh-add 1 ,
1259: .Xr ssh-agent 1 ,
1260: .Xr ssh-keygen 1 ,
1261: .Xr telnet 1 ,
1.5 deraadt 1262: .Xr sshd 8 ,
1263: .Xr ssl 8
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