Annotation of src/usr.bin/ssh/ssh.1, Revision 1.80
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.
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: .\"
1.80 ! djm 37: .\" $OpenBSD: ssh.1,v 1.79 2001/01/28 20:36:16 stevesk Exp $
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
1.80 ! djm 51: .Op Fl afgknqstvxACNPTX246
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
1.68 markus 212: in the previous section except that the DSA or RSA algorithm is used
213: instead.
214: The client uses his private key
1.49 markus 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
1.71 djm 275: The session terminates when the command or shell on the remote
1.1 deraadt 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
1.68 markus 334: host keys used in the protocol version 2 are stored in
1.49 markus 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.
1.65 aaron 355: .Pp
356: The options are as follows:
1.2 deraadt 357: .Bl -tag -width Ds
1.4 dugsong 358: .It Fl a
1.42 aaron 359: Disables forwarding of the authentication agent connection.
1.54 markus 360: .It Fl A
361: Enables forwarding of the authentication agent connection.
362: This can also be specified on a per-host basis in a configuration file.
1.6 deraadt 363: .It Fl c Ar blowfish|3des
1.44 aaron 364: Selects the cipher to use for encrypting the session.
1.2 deraadt 365: .Ar 3des
1.40 aaron 366: is used by default.
1.44 aaron 367: It is believed to be secure.
1.5 deraadt 368: .Ar 3des
369: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
370: It is presumably more secure than the
1.2 deraadt 371: .Ar des
1.64 markus 372: cipher which is no longer fully supported in
1.51 markus 373: .Nm ssh .
1.5 deraadt 374: .Ar blowfish
375: is a fast block cipher, it appears very secure and is much faster than
1.40 aaron 376: .Ar 3des .
1.51 markus 377: .It Fl c Ar "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"
378: Additionally, for protocol version 2 a comma-separated list of ciphers can
1.61 aaron 379: be specified in order of preference.
380: Protocol version 2 supports 3DES, Blowfish, and CAST128 in CBC mode
381: and Arcfour.
1.2 deraadt 382: .It Fl e Ar ch|^ch|none
383: Sets the escape character for sessions with a pty (default:
384: .Ql ~ ) .
1.40 aaron 385: The escape character is only recognized at the beginning of a line.
386: The escape character followed by a dot
1.2 deraadt 387: .Pq Ql \&.
388: closes the connection, followed
1.1 deraadt 389: by control-Z suspends the connection, and followed by itself sends the
1.40 aaron 390: escape character once.
391: Setting the character to
1.2 deraadt 392: .Dq none
393: disables any escapes and makes the session fully transparent.
394: .It Fl f
395: Requests
396: .Nm
1.40 aaron 397: to go to background just before command execution.
398: This is useful if
1.2 deraadt 399: .Nm
400: is going to ask for passwords or passphrases, but the user
1.40 aaron 401: wants it in the background.
1.44 aaron 402: This implies
1.2 deraadt 403: .Fl n .
1.1 deraadt 404: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 405: something like
406: .Ic ssh -f host xterm .
1.34 markus 407: .It Fl g
408: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 409: .It Fl i Ar identity_file
1.44 aaron 410: Selects the file from which the identity (private key) for
1.68 markus 411: RSA or DSA authentication is read.
1.44 aaron 412: Default is
1.49 markus 413: .Pa $HOME/.ssh/identity
1.40 aaron 414: in the user's home directory.
415: Identity files may also be specified on
416: a per-host basis in the configuration file.
417: It is possible to have multiple
1.2 deraadt 418: .Fl i
419: options (and multiple identities specified in
1.1 deraadt 420: configuration files).
1.2 deraadt 421: .It Fl k
1.42 aaron 422: Disables forwarding of Kerberos tickets and AFS tokens.
423: This may also be specified on a per-host basis in the configuration file.
1.2 deraadt 424: .It Fl l Ar login_name
1.40 aaron 425: Specifies the user to log in as on the remote machine.
426: This also may be specified on a per-host basis in the configuration file.
1.2 deraadt 427: .It Fl n
428: Redirects stdin from
429: .Pa /dev/null
430: (actually, prevents reading from stdin).
1.1 deraadt 431: This must be used when
1.2 deraadt 432: .Nm
1.40 aaron 433: is run in the background.
434: A common trick is to use this to run X11 programs on a remote machine.
435: For example,
1.2 deraadt 436: .Ic ssh -n shadows.cs.hut.fi emacs &
437: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 438: connection will be automatically forwarded over an encrypted channel.
439: The
1.2 deraadt 440: .Nm
1.1 deraadt 441: program will be put in the background.
442: (This does not work if
1.2 deraadt 443: .Nm
444: needs to ask for a password or passphrase; see also the
445: .Fl f
446: option.)
1.53 markus 447: .It Fl N
448: Do not execute a remote command.
1.70 markus 449: This is useful if you just want to forward ports
1.53 markus 450: (protocol version 2 only).
1.2 deraadt 451: .It Fl o Ar option
1.1 deraadt 452: Can be used to give options in the format used in the config file.
453: This is useful for specifying options for which there is no separate
1.40 aaron 454: command-line flag.
455: The option has the same format as a line in the configuration file.
1.2 deraadt 456: .It Fl p Ar port
1.40 aaron 457: Port to connect to on the remote host.
458: This can be specified on a
1.1 deraadt 459: per-host basis in the configuration file.
1.16 markus 460: .It Fl P
461: Use a non-privileged port for outgoing connections.
462: This can be used if your firewall does
463: not permit connections from privileged ports.
1.30 provos 464: Note that this option turns off
1.16 markus 465: .Cm RhostsAuthentication
466: and
1.72 markus 467: .Cm RhostsRSAAuthentication
468: for older servers.
1.2 deraadt 469: .It Fl q
1.40 aaron 470: Quiet mode.
471: Causes all warning and diagnostic messages to be suppressed.
472: Only fatal errors are displayed.
1.80 ! djm 473: .It Fl s
! 474: 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
! 475: of SSH as a secure transport for other application (eg. sftp). The
! 476: subsystem is specified as the remote command.
1.2 deraadt 477: .It Fl t
1.40 aaron 478: Force pseudo-tty allocation.
1.43 brad 479: This can be used to execute arbitrary
1.40 aaron 480: screen-based programs on a remote machine, which can be very useful,
481: e.g., when implementing menu services.
1.73 markus 482: Multiple
483: .Fl t
484: options force tty allocation, even if
485: .Nm
486: has no local tty.
1.53 markus 487: .It Fl T
1.69 markus 488: Disable pseudo-tty allocation.
1.2 deraadt 489: .It Fl v
1.40 aaron 490: Verbose mode.
491: Causes
1.2 deraadt 492: .Nm
1.40 aaron 493: to print debugging messages about its progress.
494: This is helpful in
1.1 deraadt 495: debugging connection, authentication, and configuration problems.
1.73 markus 496: Multiple
497: .Fl v
498: options increases the verbosity.
1.61 aaron 499: Maximum is 3.
1.2 deraadt 500: .It Fl x
1.40 aaron 501: Disables X11 forwarding.
1.2 deraadt 502: .It Fl X
1.1 deraadt 503: Enables X11 forwarding.
1.54 markus 504: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 505: .It Fl C
1.1 deraadt 506: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 507: data for forwarded X11 and TCP/IP connections).
508: The compression algorithm is the same used by
1.34 markus 509: .Xr gzip 1 ,
510: and the
1.2 deraadt 511: .Dq level
512: can be controlled by the
513: .Cm CompressionLevel
1.40 aaron 514: option (see below).
515: Compression is desirable on modem lines and other
1.1 deraadt 516: slow connections, but will only slow down things on fast networks.
517: The default value can be set on a host-by-host basis in the
518: configuration files; see the
1.2 deraadt 519: .Cm Compress
1.1 deraadt 520: option below.
1.2 deraadt 521: .It Fl L Ar port:host:hostport
1.1 deraadt 522: Specifies that the given port on the local (client) host is to be
1.40 aaron 523: forwarded to the given host and port on the remote side.
524: This works by allocating a socket to listen to
1.2 deraadt 525: .Ar port
1.1 deraadt 526: on the local side, and whenever a connection is made to this port, the
527: connection is forwarded over the secure channel, and a connection is
528: made to
1.32 markus 529: .Ar host
530: port
531: .Ar hostport
1.40 aaron 532: from the remote machine.
533: Port forwardings can also be specified in the configuration file.
534: Only root can forward privileged ports.
1.32 markus 535: IPv6 addresses can be specified with an alternative syntax:
536: .Ar port/host/hostport
1.2 deraadt 537: .It Fl R Ar port:host:hostport
1.1 deraadt 538: Specifies that the given port on the remote (server) host is to be
1.40 aaron 539: forwarded to the given host and port on the local side.
540: This works by allocating a socket to listen to
1.2 deraadt 541: .Ar port
1.1 deraadt 542: on the remote side, and whenever a connection is made to this port, the
543: connection is forwarded over the secure channel, and a connection is
544: made to
1.32 markus 545: .Ar host
546: port
547: .Ar hostport
1.40 aaron 548: from the local machine.
549: Port forwardings can also be specified in the configuration file.
550: Privileged ports can be forwarded only when
1.1 deraadt 551: logging in as root on the remote machine.
1.46 markus 552: .It Fl 2
553: Forces
554: .Nm
1.50 markus 555: to try protocol version 2 only.
1.32 markus 556: .It Fl 4
557: Forces
558: .Nm
559: to use IPv4 addresses only.
560: .It Fl 6
561: Forces
562: .Nm
563: to use IPv6 addresses only.
1.2 deraadt 564: .El
1.66 aaron 565: .Pp
566: If
567: .Nm
568: is not invoked with one of the standard program names
569: .Pf ( Dq ssh ,
570: .Dq slogin ,
571: .Dq rsh ,
572: .Dq rlogin ,
573: or
574: .Dq remsh ) ,
575: it uses this name as its
576: .Ar hostname
577: argument.
578: This is consistent with traditional
579: .Xr rsh 1
580: behavior.
1.2 deraadt 581: .Sh CONFIGURATION FILES
582: .Nm
1.1 deraadt 583: obtains configuration data from the following sources (in this order):
584: command line options, user's configuration file
1.2 deraadt 585: .Pq Pa $HOME/.ssh/config ,
586: and system-wide configuration file
587: .Pq Pa /etc/ssh_config .
588: For each parameter, the first obtained value
1.40 aaron 589: will be used.
590: The configuration files contain sections bracketed by
591: .Dq Host
592: specifications, and that section is only applied for hosts that
593: match one of the patterns given in the specification.
594: The matched host name is the one given on the command line.
1.2 deraadt 595: .Pp
1.1 deraadt 596: Since the first obtained value for each parameter is used, more
597: host-specific declarations should be given near the beginning of the
598: file, and general defaults at the end.
1.2 deraadt 599: .Pp
1.1 deraadt 600: The configuration file has the following format:
1.2 deraadt 601: .Pp
602: Empty lines and lines starting with
603: .Ql #
604: are comments.
605: .Pp
606: Otherwise a line is of the format
607: .Dq keyword arguments .
608: The possible
1.1 deraadt 609: keywords and their meanings are as follows (note that the
610: configuration files are case-sensitive):
1.2 deraadt 611: .Bl -tag -width Ds
612: .It Cm Host
1.1 deraadt 613: Restricts the following declarations (up to the next
1.2 deraadt 614: .Cm Host
1.1 deraadt 615: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 616: given after the keyword.
617: .Ql \&*
618: and
619: .Ql ?
620: can be used as wildcards in the
1.40 aaron 621: patterns.
622: A single
1.2 deraadt 623: .Ql \&*
624: as a pattern can be used to provide global
1.40 aaron 625: defaults for all hosts.
626: The host is the
1.2 deraadt 627: .Ar hostname
1.1 deraadt 628: argument given on the command line (i.e., the name is not converted to
629: a canonicalized host name before matching).
1.2 deraadt 630: .It Cm AFSTokenPassing
1.42 aaron 631: Specifies whether to pass AFS tokens to remote host.
632: The argument to this keyword must be
1.2 deraadt 633: .Dq yes
634: or
635: .Dq no .
636: .It Cm BatchMode
637: If set to
638: .Dq yes ,
1.40 aaron 639: passphrase/password querying will be disabled.
640: This option is useful in scripts and other batch jobs where you have no
641: user to supply the password.
642: The argument must be
1.2 deraadt 643: .Dq yes
644: or
645: .Dq no .
1.34 markus 646: .It Cm CheckHostIP
647: If this flag is set to
648: .Dq yes ,
649: ssh will additionally check the host ip address in the
650: .Pa known_hosts
1.42 aaron 651: file.
652: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 653: If the option is set to
654: .Dq no ,
655: the check will not be executed.
1.2 deraadt 656: .It Cm Cipher
1.62 markus 657: Specifies the cipher to use for encrypting the session
1.64 markus 658: in protocol version 1.
1.40 aaron 659: Currently,
1.64 markus 660: .Dq blowfish
1.1 deraadt 661: and
1.10 provos 662: .Dq 3des
1.40 aaron 663: are supported.
664: The default is
1.2 deraadt 665: .Dq 3des .
1.45 markus 666: .It Cm Ciphers
667: Specifies the ciphers allowed for protocol version 2
668: in order of preference.
669: Multiple ciphers must be comma-separated.
670: The default is
1.75 djm 671: .Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc .
1.2 deraadt 672: .It Cm Compression
1.40 aaron 673: Specifies whether to use compression.
674: The argument must be
1.2 deraadt 675: .Dq yes
676: or
677: .Dq no .
678: .It Cm CompressionLevel
1.40 aaron 679: Specifies the compression level to use if compression is enable.
680: The argument must be an integer from 1 (fast) to 9 (slow, best).
681: The default level is 6, which is good for most applications.
682: The meaning of the values is the same as in
1.34 markus 683: .Xr gzip 1 .
1.2 deraadt 684: .It Cm ConnectionAttempts
1.1 deraadt 685: Specifies the number of tries (one per second) to make before falling
1.40 aaron 686: back to rsh or exiting.
687: The argument must be an integer.
688: This may be useful in scripts if the connection sometimes fails.
1.68 markus 689: .It Cm PubkeyAuthentication
690: Specifies whether to try public key authentication.
1.50 markus 691: The argument to this keyword must be
692: .Dq yes
693: or
694: .Dq no .
695: Note that this option applies to protocol version 2 only.
1.2 deraadt 696: .It Cm EscapeChar
697: Sets the escape character (default:
698: .Ql ~ ) .
699: The escape character can also
1.40 aaron 700: be set on the command line.
701: The argument should be a single character,
1.2 deraadt 702: .Ql ^
703: followed by a letter, or
704: .Dq none
705: to disable the escape
1.1 deraadt 706: character entirely (making the connection transparent for binary
707: data).
1.44 aaron 708: .It Cm FallBackToRsh
1.1 deraadt 709: Specifies that if connecting via
1.2 deraadt 710: .Nm
1.1 deraadt 711: fails due to a connection refused error (there is no
1.2 deraadt 712: .Xr sshd 8
1.44 aaron 713: listening on the remote host),
1.2 deraadt 714: .Xr rsh 1
1.1 deraadt 715: should automatically be used instead (after a suitable warning about
1.40 aaron 716: the session being unencrypted).
717: The argument must be
1.2 deraadt 718: .Dq yes
719: or
720: .Dq no .
721: .It Cm ForwardAgent
1.1 deraadt 722: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 723: will be forwarded to the remote machine.
724: The argument must be
1.2 deraadt 725: .Dq yes
726: or
1.54 markus 727: .Dq no .
728: The default is
1.2 deraadt 729: .Dq no .
730: .It Cm ForwardX11
1.1 deraadt 731: Specifies whether X11 connections will be automatically redirected
1.44 aaron 732: over the secure channel and
1.2 deraadt 733: .Ev DISPLAY
1.40 aaron 734: set.
1.44 aaron 735: The argument must be
1.2 deraadt 736: .Dq yes
737: or
1.38 markus 738: .Dq no .
739: The default is
1.3 deraadt 740: .Dq no .
741: .It Cm GatewayPorts
742: Specifies whether remote hosts are allowed to connect to local
743: forwarded ports.
744: The argument must be
745: .Dq yes
746: or
747: .Dq no .
748: The default is
1.2 deraadt 749: .Dq no .
750: .It Cm GlobalKnownHostsFile
1.44 aaron 751: Specifies a file to use instead of
1.2 deraadt 752: .Pa /etc/ssh_known_hosts .
1.74 markus 753: .It Cm HostKeyAlias
754: Specifies an alias that should be used instead of the
755: real host name when looking up or saving the host key
756: the kown_hosts files.
757: This option is useful for tunneling ssh connection
758: or if you have multiple servers running on a single host.
1.2 deraadt 759: .It Cm HostName
1.40 aaron 760: Specifies the real host name to log into.
761: This can be used to specify nicknames or abbreviations for hosts.
762: Default is the name given on the command line.
763: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 764: .Cm HostName
1.1 deraadt 765: specifications).
1.2 deraadt 766: .It Cm IdentityFile
1.1 deraadt 767: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 768: is read (default
1.49 markus 769: .Pa $HOME/.ssh/identity
1.2 deraadt 770: in the user's home directory).
1.1 deraadt 771: Additionally, any identities represented by the authentication agent
1.40 aaron 772: will be used for authentication.
773: The file name may use the tilde
774: syntax to refer to a user's home directory.
775: It is possible to have
1.1 deraadt 776: multiple identity files specified in configuration files; all these
777: identities will be tried in sequence.
1.2 deraadt 778: .It Cm KeepAlive
1.1 deraadt 779: Specifies whether the system should send keepalive messages to the
1.40 aaron 780: other side.
781: If they are sent, death of the connection or crash of one
782: of the machines will be properly noticed.
783: However, this means that
1.1 deraadt 784: connections will die if the route is down temporarily, and some people
1.41 aaron 785: find it annoying.
1.2 deraadt 786: .Pp
787: The default is
788: .Dq yes
789: (to send keepalives), and the client will notice
1.40 aaron 790: if the network goes down or the remote host dies.
791: This is important in scripts, and many users want it too.
1.2 deraadt 792: .Pp
793: To disable keepalives, the value should be set to
794: .Dq no
795: in both the server and the client configuration files.
796: .It Cm KerberosAuthentication
1.42 aaron 797: Specifies whether Kerberos authentication will be used.
798: The argument to this keyword must be
1.4 dugsong 799: .Dq yes
800: or
801: .Dq no .
1.2 deraadt 802: .It Cm KerberosTgtPassing
1.42 aaron 803: Specifies whether a Kerberos TGT will be forwarded to the server.
804: This will only work if the Kerberos server is actually an AFS kaserver.
805: The argument to this keyword must be
1.4 dugsong 806: .Dq yes
807: or
808: .Dq no .
1.2 deraadt 809: .It Cm LocalForward
1.1 deraadt 810: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 811: the secure channel to given host:port from the remote machine.
812: The first argument must be a port number, and the second must be
813: host:port.
814: Multiple forwardings may be specified, and additional
815: forwardings can be given on the command line.
816: Only the superuser can forward privileged ports.
1.24 markus 817: .It Cm LogLevel
818: Gives the verbosity level that is used when logging messages from
819: .Nm ssh .
820: The possible values are:
1.77 markus 821: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
822: The default is INFO.
1.14 dugsong 823: .It Cm NumberOfPasswordPrompts
1.42 aaron 824: Specifies the number of password prompts before giving up.
825: The argument to this keyword must be an integer.
826: Default is 3.
1.34 markus 827: .It Cm PasswordAuthentication
1.40 aaron 828: Specifies whether to use password authentication.
829: The argument to this keyword must be
1.34 markus 830: .Dq yes
831: or
832: .Dq no .
1.50 markus 833: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 834: .It Cm Port
1.40 aaron 835: Specifies the port number to connect on the remote host.
836: Default is 22.
1.45 markus 837: .It Cm Protocol
838: Specifies the protocol versions
839: .Nm
840: should support in order of preference.
841: The possible values are
842: .Dq 1
843: and
844: .Dq 2 .
845: Multiple versions must be comma-separated.
846: The default is
1.49 markus 847: .Dq 1,2 .
848: This means that
849: .Nm
850: tries version 1 and falls back to version 2
1.52 hugh 851: if version 1 is not available.
1.2 deraadt 852: .It Cm ProxyCommand
1.40 aaron 853: Specifies the command to use to connect to the server.
854: The command
855: string extends to the end of the line, and is executed with
856: .Pa /bin/sh .
857: In the command string,
858: .Ql %h
859: will be substituted by the host name to
860: connect and
861: .Ql %p
862: by the port.
863: The command can be basically anything,
864: and should read from its standard input and write to its standard output.
865: It should eventually connect an
1.2 deraadt 866: .Xr sshd 8
1.1 deraadt 867: server running on some machine, or execute
1.2 deraadt 868: .Ic sshd -i
1.40 aaron 869: somewhere.
870: Host key management will be done using the
1.1 deraadt 871: HostName of the host being connected (defaulting to the name typed by
872: the user).
1.29 markus 873: Note that
874: .Cm CheckHostIP
875: is not available for connects with a proxy command.
1.2 deraadt 876: .Pp
877: .It Cm RemoteForward
1.1 deraadt 878: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 879: the secure channel to given host:port from the local machine.
880: The first argument must be a port number, and the second must be
881: host:port.
882: Multiple forwardings may be specified, and additional
883: forwardings can be given on the command line.
884: Only the superuser can forward privileged ports.
1.2 deraadt 885: .It Cm RhostsAuthentication
1.40 aaron 886: Specifies whether to try rhosts based authentication.
887: Note that this
1.1 deraadt 888: declaration only affects the client side and has no effect whatsoever
1.40 aaron 889: on security.
890: Disabling rhosts authentication may reduce
1.1 deraadt 891: authentication time on slow connections when rhosts authentication is
1.40 aaron 892: not used.
893: Most servers do not permit RhostsAuthentication because it
894: is not secure (see RhostsRSAAuthentication).
895: The argument to this keyword must be
1.2 deraadt 896: .Dq yes
897: or
898: .Dq no .
899: .It Cm RhostsRSAAuthentication
1.1 deraadt 900: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 901: authentication.
902: This is the primary authentication method for most sites.
903: The argument must be
1.2 deraadt 904: .Dq yes
905: or
906: .Dq no .
907: .It Cm RSAAuthentication
1.40 aaron 908: Specifies whether to try RSA authentication.
909: The argument to this keyword must be
1.2 deraadt 910: .Dq yes
911: or
912: .Dq no .
1.1 deraadt 913: RSA authentication will only be
914: attempted if the identity file exists, or an authentication agent is
915: running.
1.50 markus 916: Note that this option applies to protocol version 1 only.
1.27 markus 917: .It Cm SkeyAuthentication
918: Specifies whether to use
919: .Xr skey 1
1.40 aaron 920: authentication.
921: The argument to this keyword must be
1.27 markus 922: .Dq yes
923: or
924: .Dq no .
925: The default is
926: .Dq no .
1.2 deraadt 927: .It Cm StrictHostKeyChecking
928: If this flag is set to
1.44 aaron 929: .Dq yes ,
1.2 deraadt 930: .Nm
1.79 stevesk 931: will never automatically add host keys to the
1.2 deraadt 932: .Pa $HOME/.ssh/known_hosts
1.48 markus 933: and
934: .Pa $HOME/.ssh/known_hosts2
1.79 stevesk 935: files, and refuses to connect to hosts whose host key has changed.
1.40 aaron 936: This provides maximum protection against trojan horse attacks.
937: However, it can be somewhat annoying if you don't have good
1.2 deraadt 938: .Pa /etc/ssh_known_hosts
1.48 markus 939: and
940: .Pa /etc/ssh_known_hosts2
1.1 deraadt 941: files installed and frequently
1.79 stevesk 942: connect to new hosts.
943: This option forces the user to manually
944: add all new hosts.
945: If this flag is set to
946: .Dq no ,
947: .Nm
948: will automatically add new host keys to the
949: user known hosts files.
950: If this flag is set to
951: .Dq ask ,
952: new host keys
953: will be added to the user known host files only after the user
954: has confirmed that is what they really want to do, and
955: .Nm
956: will refuse to connect to hosts whose host key has changed.
1.40 aaron 957: The host keys of
1.79 stevesk 958: known hosts will be verified automatically in all cases.
1.40 aaron 959: The argument must be
1.79 stevesk 960: .Dq yes ,
961: .Dq no
1.2 deraadt 962: or
1.79 stevesk 963: .Dq ask .
964: The default is
965: .Dq ask .
1.16 markus 966: .It Cm UsePrivilegedPort
967: Specifies whether to use a privileged port for outgoing connections.
968: The argument must be
969: .Dq yes
970: or
971: .Dq no .
972: The default is
973: .Dq yes .
974: Note that setting this option to
975: .Dq no
1.30 provos 976: turns off
1.16 markus 977: .Cm RhostsAuthentication
978: and
1.72 markus 979: .Cm RhostsRSAAuthentication
980: for older servers.
1.34 markus 981: .It Cm User
1.40 aaron 982: Specifies the user to log in as.
983: This can be useful if you have a different user name on different machines.
984: This saves the trouble of
1.34 markus 985: having to remember to give the user name on the command line.
986: .It Cm UserKnownHostsFile
987: Specifies a file to use instead of
988: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 989: .It Cm UseRsh
1.40 aaron 990: Specifies that rlogin/rsh should be used for this host.
991: It is possible that the host does not at all support the
1.2 deraadt 992: .Nm
1.40 aaron 993: protocol.
994: This causes
1.2 deraadt 995: .Nm
1.40 aaron 996: to immediately execute
1.2 deraadt 997: .Xr rsh 1 .
1.1 deraadt 998: All other options (except
1.2 deraadt 999: .Cm HostName )
1.40 aaron 1000: are ignored if this has been specified.
1001: The argument must be
1.2 deraadt 1002: .Dq yes
1003: or
1004: .Dq no .
1.55 markus 1005: .It Cm XAuthLocation
1006: Specifies the location of the
1007: .Xr xauth 1
1008: program.
1009: The default is
1010: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1011: .El
1.2 deraadt 1012: .Sh ENVIRONMENT
1013: .Nm
1.1 deraadt 1014: will normally set the following environment variables:
1.2 deraadt 1015: .Bl -tag -width Ds
1016: .It Ev DISPLAY
1017: The
1018: .Ev DISPLAY
1.40 aaron 1019: variable indicates the location of the X11 server.
1.44 aaron 1020: It is automatically set by
1.2 deraadt 1021: .Nm
1022: to point to a value of the form
1023: .Dq hostname:n
1024: where hostname indicates
1.40 aaron 1025: the host where the shell runs, and n is an integer >= 1.
1026: .Nm
1027: uses this special value to forward X11 connections over the secure
1028: channel.
1029: The user should normally not set DISPLAY explicitly, as that
1.1 deraadt 1030: will render the X11 connection insecure (and will require the user to
1031: manually copy any required authorization cookies).
1.2 deraadt 1032: .It Ev HOME
1.1 deraadt 1033: Set to the path of the user's home directory.
1.2 deraadt 1034: .It Ev LOGNAME
1035: Synonym for
1.12 aaron 1036: .Ev USER ;
1037: set for compatibility with systems that use this variable.
1.2 deraadt 1038: .It Ev MAIL
1.1 deraadt 1039: Set to point the user's mailbox.
1.40 aaron 1040: .It Ev PATH
1.2 deraadt 1041: Set to the default
1042: .Ev PATH ,
1043: as specified when compiling
1.12 aaron 1044: .Nm ssh .
1.18 markus 1045: .It Ev SSH_AUTH_SOCK
1.17 markus 1046: indicates the path of a unix-domain socket used to communicate with the
1047: agent.
1.2 deraadt 1048: .It Ev SSH_CLIENT
1.40 aaron 1049: Identifies the client end of the connection.
1050: The variable contains
1.1 deraadt 1051: three space-separated values: client ip-address, client port number,
1052: and server port number.
1.73 markus 1053: .It Ev SSH_ORIGINAL_COMMAND
1054: The variable contains the original command line if a forced command
1055: is executed.
1056: It can be used to extract the original arguments.
1.2 deraadt 1057: .It Ev SSH_TTY
1.1 deraadt 1058: This is set to the name of the tty (path to the device) associated
1.40 aaron 1059: with the current shell or command.
1060: If the current session has no tty,
1.1 deraadt 1061: this variable is not set.
1.2 deraadt 1062: .It Ev TZ
1.1 deraadt 1063: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1064: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1065: on to new connections).
1.2 deraadt 1066: .It Ev USER
1.1 deraadt 1067: Set to the name of the user logging in.
1.2 deraadt 1068: .El
1069: .Pp
1.44 aaron 1070: Additionally,
1.2 deraadt 1071: .Nm
1.44 aaron 1072: reads
1073: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1074: and adds lines of the format
1075: .Dq VARNAME=value
1.12 aaron 1076: to the environment.
1.2 deraadt 1077: .Sh FILES
1.36 markus 1078: .Bl -tag -width Ds
1.2 deraadt 1079: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1080: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1081: in
1082: .Pa /etc/ssh_known_hosts ) .
1083: See
1084: .Xr sshd 8 .
1.48 markus 1085: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1086: Contains the RSA and the DSA authentication identity of the user.
1087: These files
1088: contain sensitive data and should be readable by the user but not
1.15 markus 1089: accessible by others (read/write/execute).
1090: Note that
1091: .Nm
1.48 markus 1092: ignores a private key file if it is accessible by others.
1.15 markus 1093: It is possible to specify a passphrase when
1.1 deraadt 1094: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1095: sensitive part of this file using 3DES.
1.48 markus 1096: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1.1 deraadt 1097: Contains the public key for authentication (public part of the
1.40 aaron 1098: identity file in human-readable form).
1.48 markus 1099: The contents of the
1100: .Pa $HOME/.ssh/identity.pub
1101: file should be added to
1.2 deraadt 1102: .Pa $HOME/.ssh/authorized_keys
1103: on all machines
1.40 aaron 1104: where you wish to log in using RSA authentication.
1.48 markus 1105: The contents of the
1106: .Pa $HOME/.ssh/id_dsa.pub
1107: file should be added to
1108: .Pa $HOME/.ssh/authorized_keys2
1109: on all machines
1110: where you wish to log in using DSA authentication.
1111: These files are not
1.40 aaron 1112: sensitive and can (but need not) be readable by anyone.
1.48 markus 1113: These files are
1114: never used automatically and are not necessary; they is only provided for
1.1 deraadt 1115: the convenience of the user.
1.2 deraadt 1116: .It Pa $HOME/.ssh/config
1.40 aaron 1117: This is the per-user configuration file.
1118: The format of this file is described above.
1119: This file is used by the
1.2 deraadt 1120: .Nm
1.40 aaron 1121: client.
1122: This file does not usually contain any sensitive information,
1.1 deraadt 1123: but the recommended permissions are read/write for the user, and not
1124: accessible by others.
1.2 deraadt 1125: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1126: Lists the RSA keys that can be used for logging in as this user.
1127: The format of this file is described in the
1.2 deraadt 1128: .Xr sshd 8
1.40 aaron 1129: manual page.
1130: In the simplest form the format is the same as the .pub
1.1 deraadt 1131: identity files (that is, each line contains the number of bits in
1132: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1133: spaces).
1134: This file is not highly sensitive, but the recommended
1.1 deraadt 1135: permissions are read/write for the user, and not accessible by others.
1.48 markus 1136: .It Pa $HOME/.ssh/authorized_keys2
1.68 markus 1137: Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1.48 markus 1138: This file is not highly sensitive, but the recommended
1139: permissions are read/write for the user, and not accessible by others.
1140: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1141: Systemwide list of known host keys.
1.48 markus 1142: .Pa /etc/ssh_known_hosts
1143: contains RSA and
1144: .Pa /etc/ssh_known_hosts2
1.68 markus 1145: contains DSA or RSA keys for protocol version 2.
1.48 markus 1146: These files should be prepared by the
1.1 deraadt 1147: system administrator to contain the public host keys of all machines in the
1.40 aaron 1148: organization.
1149: This file should be world-readable.
1150: This file contains
1.1 deraadt 1151: public keys, one per line, in the following format (fields separated
1152: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1153: modulus, and optional comment field.
1154: When different names are used
1.1 deraadt 1155: for the same machine, all such names should be listed, separated by
1.40 aaron 1156: commas.
1157: The format is described on the
1.2 deraadt 1158: .Xr sshd 8
1.1 deraadt 1159: manual page.
1.2 deraadt 1160: .Pp
1.1 deraadt 1161: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1162: .Xr sshd 8
1.1 deraadt 1163: to verify the client host when logging in; other names are needed because
1.2 deraadt 1164: .Nm
1.1 deraadt 1165: does not convert the user-supplied name to a canonical name before
1166: checking the key, because someone with access to the name servers
1167: would then be able to fool host authentication.
1.2 deraadt 1168: .It Pa /etc/ssh_config
1.40 aaron 1169: Systemwide configuration file.
1170: This file provides defaults for those
1.1 deraadt 1171: values that are not specified in the user's configuration file, and
1.40 aaron 1172: for those users who do not have a configuration file.
1173: This file must be world-readable.
1.2 deraadt 1174: .It Pa $HOME/.rhosts
1175: This file is used in
1176: .Pa \&.rhosts
1177: authentication to list the
1.40 aaron 1178: host/user pairs that are permitted to log in.
1179: (Note that this file is
1.1 deraadt 1180: also used by rlogin and rsh, which makes using this file insecure.)
1181: Each line of the file contains a host name (in the canonical form
1182: returned by name servers), and then a user name on that host,
1.40 aaron 1183: separated by a space.
1184: One some machines this file may need to be
1.1 deraadt 1185: world-readable if the user's home directory is on a NFS partition,
1186: because
1.2 deraadt 1187: .Xr sshd 8
1.40 aaron 1188: reads it as root.
1189: Additionally, this file must be owned by the user,
1190: and must not have write permissions for anyone else.
1191: The recommended
1.1 deraadt 1192: permission for most machines is read/write for the user, and not
1193: accessible by others.
1.2 deraadt 1194: .Pp
1.1 deraadt 1195: Note that by default
1.2 deraadt 1196: .Xr sshd 8
1.1 deraadt 1197: will be installed so that it requires successful RSA host
1.40 aaron 1198: authentication before permitting \s+2.\s0rhosts authentication.
1199: If your server machine does not have the client's host key in
1.2 deraadt 1200: .Pa /etc/ssh_known_hosts ,
1201: you can store it in
1202: .Pa $HOME/.ssh/known_hosts .
1203: The easiest way to do this is to
1.1 deraadt 1204: connect back to the client from the server machine using ssh; this
1.48 markus 1205: will automatically add the host key to
1.2 deraadt 1206: .Pa $HOME/.ssh/known_hosts .
1207: .It Pa $HOME/.shosts
1208: This file is used exactly the same way as
1209: .Pa \&.rhosts .
1210: The purpose for
1.1 deraadt 1211: having this file is to be able to use rhosts authentication with
1.2 deraadt 1212: .Nm
1213: without permitting login with
1214: .Xr rlogin 1
1215: or
1216: .Xr rsh 1 .
1217: .It Pa /etc/hosts.equiv
1218: This file is used during
1.40 aaron 1219: .Pa \&.rhosts authentication.
1220: It contains
1.1 deraadt 1221: canonical hosts names, one per line (the full format is described on
1222: the
1.2 deraadt 1223: .Xr sshd 8
1.40 aaron 1224: manual page).
1225: If the client host is found in this file, login is
1.1 deraadt 1226: automatically permitted provided client and server user names are the
1.40 aaron 1227: same.
1228: Additionally, successful RSA host authentication is normally
1229: required.
1230: This file should only be writable by root.
1.2 deraadt 1231: .It Pa /etc/shosts.equiv
1.44 aaron 1232: This file is processed exactly as
1.2 deraadt 1233: .Pa /etc/hosts.equiv .
1.1 deraadt 1234: This file may be useful to permit logins using
1.2 deraadt 1235: .Nm
1.1 deraadt 1236: but not using rsh/rlogin.
1.2 deraadt 1237: .It Pa /etc/sshrc
1.1 deraadt 1238: Commands in this file are executed by
1.2 deraadt 1239: .Nm
1.1 deraadt 1240: when the user logs in just before the user's shell (or command) is started.
1241: See the
1.2 deraadt 1242: .Xr sshd 8
1.1 deraadt 1243: manual page for more information.
1.2 deraadt 1244: .It Pa $HOME/.ssh/rc
1.1 deraadt 1245: Commands in this file are executed by
1.2 deraadt 1246: .Nm
1.1 deraadt 1247: when the user logs in just before the user's shell (or command) is
1248: started.
1.44 aaron 1249: See the
1.2 deraadt 1250: .Xr sshd 8
1.1 deraadt 1251: manual page for more information.
1.31 markus 1252: .It Pa $HOME/.ssh/environment
1253: Contains additional definitions for environment variables, see section
1254: .Sx ENVIRONMENT
1255: above.
1.5 deraadt 1256: .It Pa libcrypto.so.X.1
1257: A version of this library which includes support for the RSA algorithm
1258: is required for proper operation.
1.58 itojun 1259: .El
1.67 aaron 1260: .Sh AUTHORS
1.78 markus 1261: OpenSSH is a derivative of the original and free
1262: ssh 1.2.12 release by Tatu Ylonen.
1263: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1264: Theo de Raadt and Dug Song
1265: removed many bugs, re-added newer features and
1266: created OpenSSH.
1267: Markus Friedl contributed the support for SSH
1268: protocol versions 1.5 and 2.0.
1.2 deraadt 1269: .Sh SEE ALSO
1270: .Xr rlogin 1 ,
1271: .Xr rsh 1 ,
1272: .Xr scp 1 ,
1273: .Xr ssh-add 1 ,
1274: .Xr ssh-agent 1 ,
1275: .Xr ssh-keygen 1 ,
1276: .Xr telnet 1 ,
1.5 deraadt 1277: .Xr sshd 8 ,
1278: .Xr ssl 8
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