Annotation of src/usr.bin/ssh/ssh.1, Revision 1.86
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: .\"
1.86 ! jakob 37: .\" $OpenBSD: ssh.1,v 1.85 2001/02/06 23:03:24 jakob 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
1.86 ! jakob 51: .Op Fl afgknqstvxACNPTX1246
1.51 markus 52: .Op Fl c Ar cipher_spec
1.2 deraadt 53: .Op Fl e Ar escape_char
54: .Op Fl i Ar identity_file
55: .Op Fl l Ar login_name
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.
1.85 jakob 552: .It Fl 1
553: Forces
554: .Nm
555: to try protocol version 1 only.
1.46 markus 556: .It Fl 2
557: Forces
558: .Nm
1.50 markus 559: to try protocol version 2 only.
1.32 markus 560: .It Fl 4
561: Forces
562: .Nm
563: to use IPv4 addresses only.
564: .It Fl 6
565: Forces
566: .Nm
567: to use IPv6 addresses only.
1.2 deraadt 568: .El
1.66 aaron 569: .Pp
570: If
571: .Nm
572: is not invoked with one of the standard program names
573: .Pf ( Dq ssh ,
574: .Dq slogin ,
575: .Dq rsh ,
576: .Dq rlogin ,
577: or
578: .Dq remsh ) ,
579: it uses this name as its
580: .Ar hostname
581: argument.
582: This is consistent with traditional
583: .Xr rsh 1
584: behavior.
1.2 deraadt 585: .Sh CONFIGURATION FILES
586: .Nm
1.1 deraadt 587: obtains configuration data from the following sources (in this order):
588: command line options, user's configuration file
1.2 deraadt 589: .Pq Pa $HOME/.ssh/config ,
590: and system-wide configuration file
591: .Pq Pa /etc/ssh_config .
592: For each parameter, the first obtained value
1.40 aaron 593: will be used.
594: The configuration files contain sections bracketed by
595: .Dq Host
596: specifications, and that section is only applied for hosts that
597: match one of the patterns given in the specification.
598: The matched host name is the one given on the command line.
1.2 deraadt 599: .Pp
1.1 deraadt 600: Since the first obtained value for each parameter is used, more
601: host-specific declarations should be given near the beginning of the
602: file, and general defaults at the end.
1.2 deraadt 603: .Pp
1.1 deraadt 604: The configuration file has the following format:
1.2 deraadt 605: .Pp
606: Empty lines and lines starting with
607: .Ql #
608: are comments.
609: .Pp
610: Otherwise a line is of the format
611: .Dq keyword arguments .
612: The possible
1.1 deraadt 613: keywords and their meanings are as follows (note that the
614: configuration files are case-sensitive):
1.2 deraadt 615: .Bl -tag -width Ds
616: .It Cm Host
1.1 deraadt 617: Restricts the following declarations (up to the next
1.2 deraadt 618: .Cm Host
1.1 deraadt 619: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 620: given after the keyword.
621: .Ql \&*
622: and
623: .Ql ?
624: can be used as wildcards in the
1.40 aaron 625: patterns.
626: A single
1.2 deraadt 627: .Ql \&*
628: as a pattern can be used to provide global
1.40 aaron 629: defaults for all hosts.
630: The host is the
1.2 deraadt 631: .Ar hostname
1.1 deraadt 632: argument given on the command line (i.e., the name is not converted to
633: a canonicalized host name before matching).
1.2 deraadt 634: .It Cm AFSTokenPassing
1.42 aaron 635: Specifies whether to pass AFS tokens to remote host.
636: The argument to this keyword must be
1.2 deraadt 637: .Dq yes
638: or
639: .Dq no .
640: .It Cm BatchMode
641: If set to
642: .Dq yes ,
1.40 aaron 643: passphrase/password querying will be disabled.
644: This option is useful in scripts and other batch jobs where you have no
645: user to supply the password.
646: The argument must be
1.2 deraadt 647: .Dq yes
648: or
649: .Dq no .
1.34 markus 650: .It Cm CheckHostIP
651: If this flag is set to
652: .Dq yes ,
653: ssh will additionally check the host ip address in the
654: .Pa known_hosts
1.42 aaron 655: file.
656: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 657: If the option is set to
658: .Dq no ,
659: the check will not be executed.
1.2 deraadt 660: .It Cm Cipher
1.62 markus 661: Specifies the cipher to use for encrypting the session
1.64 markus 662: in protocol version 1.
1.40 aaron 663: Currently,
1.64 markus 664: .Dq blowfish
1.1 deraadt 665: and
1.10 provos 666: .Dq 3des
1.40 aaron 667: are supported.
668: The default is
1.2 deraadt 669: .Dq 3des .
1.45 markus 670: .It Cm Ciphers
671: Specifies the ciphers allowed for protocol version 2
672: in order of preference.
673: Multiple ciphers must be comma-separated.
674: The default is
1.75 djm 675: .Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc .
1.2 deraadt 676: .It Cm Compression
1.40 aaron 677: Specifies whether to use compression.
678: The argument must be
1.2 deraadt 679: .Dq yes
680: or
681: .Dq no .
682: .It Cm CompressionLevel
1.40 aaron 683: Specifies the compression level to use if compression is enable.
684: The argument must be an integer from 1 (fast) to 9 (slow, best).
685: The default level is 6, which is good for most applications.
686: The meaning of the values is the same as in
1.34 markus 687: .Xr gzip 1 .
1.2 deraadt 688: .It Cm ConnectionAttempts
1.1 deraadt 689: Specifies the number of tries (one per second) to make before falling
1.40 aaron 690: back to rsh or exiting.
691: The argument must be an integer.
692: This may be useful in scripts if the connection sometimes fails.
1.68 markus 693: .It Cm PubkeyAuthentication
694: Specifies whether to try public key authentication.
1.50 markus 695: The argument to this keyword must be
696: .Dq yes
697: or
698: .Dq no .
699: Note that this option applies to protocol version 2 only.
1.2 deraadt 700: .It Cm EscapeChar
701: Sets the escape character (default:
702: .Ql ~ ) .
703: The escape character can also
1.40 aaron 704: be set on the command line.
705: The argument should be a single character,
1.2 deraadt 706: .Ql ^
707: followed by a letter, or
708: .Dq none
709: to disable the escape
1.1 deraadt 710: character entirely (making the connection transparent for binary
711: data).
1.44 aaron 712: .It Cm FallBackToRsh
1.1 deraadt 713: Specifies that if connecting via
1.2 deraadt 714: .Nm
1.1 deraadt 715: fails due to a connection refused error (there is no
1.2 deraadt 716: .Xr sshd 8
1.44 aaron 717: listening on the remote host),
1.2 deraadt 718: .Xr rsh 1
1.1 deraadt 719: should automatically be used instead (after a suitable warning about
1.40 aaron 720: the session being unencrypted).
721: The argument must be
1.2 deraadt 722: .Dq yes
723: or
724: .Dq no .
725: .It Cm ForwardAgent
1.1 deraadt 726: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 727: will be forwarded to the remote machine.
728: The argument must be
1.2 deraadt 729: .Dq yes
730: or
1.54 markus 731: .Dq no .
732: The default is
1.2 deraadt 733: .Dq no .
734: .It Cm ForwardX11
1.1 deraadt 735: Specifies whether X11 connections will be automatically redirected
1.44 aaron 736: over the secure channel and
1.2 deraadt 737: .Ev DISPLAY
1.40 aaron 738: set.
1.44 aaron 739: The argument must be
1.2 deraadt 740: .Dq yes
741: or
1.38 markus 742: .Dq no .
743: The default is
1.3 deraadt 744: .Dq no .
745: .It Cm GatewayPorts
746: Specifies whether remote hosts are allowed to connect to local
747: forwarded ports.
748: The argument must be
749: .Dq yes
750: or
751: .Dq no .
752: The default is
1.2 deraadt 753: .Dq no .
754: .It Cm GlobalKnownHostsFile
1.44 aaron 755: Specifies a file to use instead of
1.2 deraadt 756: .Pa /etc/ssh_known_hosts .
1.74 markus 757: .It Cm HostKeyAlias
758: Specifies an alias that should be used instead of the
759: real host name when looking up or saving the host key
1.82 stevesk 760: in the known_hosts files.
761: This option is useful for tunneling ssh connections
1.74 markus 762: or if you have multiple servers running on a single host.
1.2 deraadt 763: .It Cm HostName
1.40 aaron 764: Specifies the real host name to log into.
765: This can be used to specify nicknames or abbreviations for hosts.
766: Default is the name given on the command line.
767: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 768: .Cm HostName
1.1 deraadt 769: specifications).
1.2 deraadt 770: .It Cm IdentityFile
1.1 deraadt 771: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 772: is read (default
1.49 markus 773: .Pa $HOME/.ssh/identity
1.2 deraadt 774: in the user's home directory).
1.1 deraadt 775: Additionally, any identities represented by the authentication agent
1.40 aaron 776: will be used for authentication.
777: The file name may use the tilde
778: syntax to refer to a user's home directory.
779: It is possible to have
1.1 deraadt 780: multiple identity files specified in configuration files; all these
781: identities will be tried in sequence.
1.2 deraadt 782: .It Cm KeepAlive
1.1 deraadt 783: Specifies whether the system should send keepalive messages to the
1.40 aaron 784: other side.
785: If they are sent, death of the connection or crash of one
786: of the machines will be properly noticed.
787: However, this means that
1.1 deraadt 788: connections will die if the route is down temporarily, and some people
1.41 aaron 789: find it annoying.
1.2 deraadt 790: .Pp
791: The default is
792: .Dq yes
793: (to send keepalives), and the client will notice
1.40 aaron 794: if the network goes down or the remote host dies.
795: This is important in scripts, and many users want it too.
1.2 deraadt 796: .Pp
797: To disable keepalives, the value should be set to
798: .Dq no
799: in both the server and the client configuration files.
800: .It Cm KerberosAuthentication
1.42 aaron 801: Specifies whether Kerberos authentication will be used.
802: The argument to this keyword must be
1.4 dugsong 803: .Dq yes
804: or
805: .Dq no .
1.2 deraadt 806: .It Cm KerberosTgtPassing
1.42 aaron 807: Specifies whether a Kerberos TGT will be forwarded to the server.
808: This will only work if the Kerberos server is actually an AFS kaserver.
809: The argument to this keyword must be
1.4 dugsong 810: .Dq yes
811: or
812: .Dq no .
1.2 deraadt 813: .It Cm LocalForward
1.1 deraadt 814: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 815: the secure channel to given host:port from the remote machine.
816: The first argument must be a port number, and the second must be
817: host:port.
818: Multiple forwardings may be specified, and additional
819: forwardings can be given on the command line.
820: Only the superuser can forward privileged ports.
1.24 markus 821: .It Cm LogLevel
822: Gives the verbosity level that is used when logging messages from
823: .Nm ssh .
824: The possible values are:
1.77 markus 825: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
826: The default is INFO.
1.14 dugsong 827: .It Cm NumberOfPasswordPrompts
1.42 aaron 828: Specifies the number of password prompts before giving up.
829: The argument to this keyword must be an integer.
830: Default is 3.
1.34 markus 831: .It Cm PasswordAuthentication
1.40 aaron 832: Specifies whether to use password authentication.
833: The argument to this keyword must be
1.34 markus 834: .Dq yes
835: or
836: .Dq no .
1.50 markus 837: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 838: .It Cm Port
1.40 aaron 839: Specifies the port number to connect on the remote host.
840: Default is 22.
1.45 markus 841: .It Cm Protocol
842: Specifies the protocol versions
843: .Nm
844: should support in order of preference.
845: The possible values are
846: .Dq 1
847: and
848: .Dq 2 .
849: Multiple versions must be comma-separated.
850: The default is
1.49 markus 851: .Dq 1,2 .
852: This means that
853: .Nm
854: tries version 1 and falls back to version 2
1.52 hugh 855: if version 1 is not available.
1.2 deraadt 856: .It Cm ProxyCommand
1.40 aaron 857: Specifies the command to use to connect to the server.
858: The command
859: string extends to the end of the line, and is executed with
860: .Pa /bin/sh .
861: In the command string,
862: .Ql %h
863: will be substituted by the host name to
864: connect and
865: .Ql %p
866: by the port.
867: The command can be basically anything,
868: and should read from its standard input and write to its standard output.
869: It should eventually connect an
1.2 deraadt 870: .Xr sshd 8
1.1 deraadt 871: server running on some machine, or execute
1.2 deraadt 872: .Ic sshd -i
1.40 aaron 873: somewhere.
874: Host key management will be done using the
1.1 deraadt 875: HostName of the host being connected (defaulting to the name typed by
876: the user).
1.29 markus 877: Note that
878: .Cm CheckHostIP
879: is not available for connects with a proxy command.
1.2 deraadt 880: .Pp
881: .It Cm RemoteForward
1.1 deraadt 882: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 883: the secure channel to given host:port from the local machine.
884: The first argument must be a port number, and the second must be
885: host:port.
886: Multiple forwardings may be specified, and additional
887: forwardings can be given on the command line.
888: Only the superuser can forward privileged ports.
1.2 deraadt 889: .It Cm RhostsAuthentication
1.40 aaron 890: Specifies whether to try rhosts based authentication.
891: Note that this
1.1 deraadt 892: declaration only affects the client side and has no effect whatsoever
1.40 aaron 893: on security.
894: Disabling rhosts authentication may reduce
1.1 deraadt 895: authentication time on slow connections when rhosts authentication is
1.40 aaron 896: not used.
897: Most servers do not permit RhostsAuthentication because it
898: is not secure (see RhostsRSAAuthentication).
899: The argument to this keyword must be
1.2 deraadt 900: .Dq yes
901: or
902: .Dq no .
903: .It Cm RhostsRSAAuthentication
1.1 deraadt 904: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 905: authentication.
906: This is the primary authentication method for most sites.
907: The argument must be
1.2 deraadt 908: .Dq yes
909: or
910: .Dq no .
911: .It Cm RSAAuthentication
1.40 aaron 912: Specifies whether to try RSA authentication.
913: The argument to this keyword must be
1.2 deraadt 914: .Dq yes
915: or
916: .Dq no .
1.1 deraadt 917: RSA authentication will only be
918: attempted if the identity file exists, or an authentication agent is
919: running.
1.50 markus 920: Note that this option applies to protocol version 1 only.
1.81 markus 921: .It Cm ChallengeResponseAuthentication
922: Specifies whether to use challenge response authentication.
923: Currently there is only support for
1.27 markus 924: .Xr skey 1
1.40 aaron 925: authentication.
926: The argument to this keyword must be
1.27 markus 927: .Dq yes
928: or
929: .Dq no .
930: The default is
931: .Dq no .
1.2 deraadt 932: .It Cm StrictHostKeyChecking
933: If this flag is set to
1.44 aaron 934: .Dq yes ,
1.2 deraadt 935: .Nm
1.79 stevesk 936: will never automatically add host keys to the
1.2 deraadt 937: .Pa $HOME/.ssh/known_hosts
1.48 markus 938: and
939: .Pa $HOME/.ssh/known_hosts2
1.79 stevesk 940: files, and refuses to connect to hosts whose host key has changed.
1.40 aaron 941: This provides maximum protection against trojan horse attacks.
942: However, it can be somewhat annoying if you don't have good
1.2 deraadt 943: .Pa /etc/ssh_known_hosts
1.48 markus 944: and
945: .Pa /etc/ssh_known_hosts2
1.1 deraadt 946: files installed and frequently
1.79 stevesk 947: connect to new hosts.
948: This option forces the user to manually
949: add all new hosts.
950: If this flag is set to
951: .Dq no ,
952: .Nm
953: will automatically add new host keys to the
954: user known hosts files.
955: If this flag is set to
956: .Dq ask ,
957: new host keys
958: will be added to the user known host files only after the user
959: has confirmed that is what they really want to do, and
960: .Nm
961: will refuse to connect to hosts whose host key has changed.
1.40 aaron 962: The host keys of
1.79 stevesk 963: known hosts will be verified automatically in all cases.
1.40 aaron 964: The argument must be
1.79 stevesk 965: .Dq yes ,
966: .Dq no
1.2 deraadt 967: or
1.79 stevesk 968: .Dq ask .
969: The default is
970: .Dq ask .
1.16 markus 971: .It Cm UsePrivilegedPort
972: Specifies whether to use a privileged port for outgoing connections.
973: The argument must be
974: .Dq yes
975: or
976: .Dq no .
977: The default is
978: .Dq yes .
979: Note that setting this option to
980: .Dq no
1.30 provos 981: turns off
1.16 markus 982: .Cm RhostsAuthentication
983: and
1.72 markus 984: .Cm RhostsRSAAuthentication
985: for older servers.
1.34 markus 986: .It Cm User
1.40 aaron 987: Specifies the user to log in as.
988: This can be useful if you have a different user name on different machines.
989: This saves the trouble of
1.34 markus 990: having to remember to give the user name on the command line.
991: .It Cm UserKnownHostsFile
992: Specifies a file to use instead of
993: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 994: .It Cm UseRsh
1.40 aaron 995: Specifies that rlogin/rsh should be used for this host.
996: It is possible that the host does not at all support the
1.2 deraadt 997: .Nm
1.40 aaron 998: protocol.
999: This causes
1.2 deraadt 1000: .Nm
1.40 aaron 1001: to immediately execute
1.2 deraadt 1002: .Xr rsh 1 .
1.1 deraadt 1003: All other options (except
1.2 deraadt 1004: .Cm HostName )
1.40 aaron 1005: are ignored if this has been specified.
1006: The argument must be
1.2 deraadt 1007: .Dq yes
1008: or
1009: .Dq no .
1.55 markus 1010: .It Cm XAuthLocation
1011: Specifies the location of the
1012: .Xr xauth 1
1013: program.
1014: The default is
1015: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 1016: .El
1.2 deraadt 1017: .Sh ENVIRONMENT
1018: .Nm
1.1 deraadt 1019: will normally set the following environment variables:
1.2 deraadt 1020: .Bl -tag -width Ds
1021: .It Ev DISPLAY
1022: The
1023: .Ev DISPLAY
1.40 aaron 1024: variable indicates the location of the X11 server.
1.44 aaron 1025: It is automatically set by
1.2 deraadt 1026: .Nm
1027: to point to a value of the form
1028: .Dq hostname:n
1029: where hostname indicates
1.40 aaron 1030: the host where the shell runs, and n is an integer >= 1.
1031: .Nm
1032: uses this special value to forward X11 connections over the secure
1033: channel.
1034: The user should normally not set DISPLAY explicitly, as that
1.1 deraadt 1035: will render the X11 connection insecure (and will require the user to
1036: manually copy any required authorization cookies).
1.2 deraadt 1037: .It Ev HOME
1.1 deraadt 1038: Set to the path of the user's home directory.
1.2 deraadt 1039: .It Ev LOGNAME
1040: Synonym for
1.12 aaron 1041: .Ev USER ;
1042: set for compatibility with systems that use this variable.
1.2 deraadt 1043: .It Ev MAIL
1.1 deraadt 1044: Set to point the user's mailbox.
1.40 aaron 1045: .It Ev PATH
1.2 deraadt 1046: Set to the default
1047: .Ev PATH ,
1048: as specified when compiling
1.12 aaron 1049: .Nm ssh .
1.18 markus 1050: .It Ev SSH_AUTH_SOCK
1.17 markus 1051: indicates the path of a unix-domain socket used to communicate with the
1052: agent.
1.2 deraadt 1053: .It Ev SSH_CLIENT
1.40 aaron 1054: Identifies the client end of the connection.
1055: The variable contains
1.1 deraadt 1056: three space-separated values: client ip-address, client port number,
1057: and server port number.
1.73 markus 1058: .It Ev SSH_ORIGINAL_COMMAND
1059: The variable contains the original command line if a forced command
1060: is executed.
1061: It can be used to extract the original arguments.
1.2 deraadt 1062: .It Ev SSH_TTY
1.1 deraadt 1063: This is set to the name of the tty (path to the device) associated
1.40 aaron 1064: with the current shell or command.
1065: If the current session has no tty,
1.1 deraadt 1066: this variable is not set.
1.2 deraadt 1067: .It Ev TZ
1.1 deraadt 1068: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1069: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1070: on to new connections).
1.2 deraadt 1071: .It Ev USER
1.1 deraadt 1072: Set to the name of the user logging in.
1.2 deraadt 1073: .El
1074: .Pp
1.44 aaron 1075: Additionally,
1.2 deraadt 1076: .Nm
1.44 aaron 1077: reads
1078: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1079: and adds lines of the format
1080: .Dq VARNAME=value
1.12 aaron 1081: to the environment.
1.2 deraadt 1082: .Sh FILES
1.36 markus 1083: .Bl -tag -width Ds
1.2 deraadt 1084: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1085: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1086: in
1087: .Pa /etc/ssh_known_hosts ) .
1088: See
1089: .Xr sshd 8 .
1.48 markus 1090: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1091: Contains the RSA and the DSA authentication identity of the user.
1092: These files
1093: contain sensitive data and should be readable by the user but not
1.15 markus 1094: accessible by others (read/write/execute).
1095: Note that
1096: .Nm
1.48 markus 1097: ignores a private key file if it is accessible by others.
1.15 markus 1098: It is possible to specify a passphrase when
1.1 deraadt 1099: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1100: sensitive part of this file using 3DES.
1.48 markus 1101: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1.1 deraadt 1102: Contains the public key for authentication (public part of the
1.40 aaron 1103: identity file in human-readable form).
1.48 markus 1104: The contents of the
1105: .Pa $HOME/.ssh/identity.pub
1106: file should be added to
1.2 deraadt 1107: .Pa $HOME/.ssh/authorized_keys
1108: on all machines
1.40 aaron 1109: where you wish to log in using RSA authentication.
1.48 markus 1110: The contents of the
1111: .Pa $HOME/.ssh/id_dsa.pub
1112: file should be added to
1113: .Pa $HOME/.ssh/authorized_keys2
1114: on all machines
1115: where you wish to log in using DSA authentication.
1116: These files are not
1.40 aaron 1117: sensitive and can (but need not) be readable by anyone.
1.48 markus 1118: These files are
1.84 markus 1119: never used automatically and are not necessary; they are only provided for
1.1 deraadt 1120: the convenience of the user.
1.2 deraadt 1121: .It Pa $HOME/.ssh/config
1.40 aaron 1122: This is the per-user configuration file.
1123: The format of this file is described above.
1124: This file is used by the
1.2 deraadt 1125: .Nm
1.40 aaron 1126: client.
1127: This file does not usually contain any sensitive information,
1.1 deraadt 1128: but the recommended permissions are read/write for the user, and not
1129: accessible by others.
1.2 deraadt 1130: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1131: Lists the RSA keys that can be used for logging in as this user.
1132: The format of this file is described in the
1.2 deraadt 1133: .Xr sshd 8
1.40 aaron 1134: manual page.
1135: In the simplest form the format is the same as the .pub
1.1 deraadt 1136: identity files (that is, each line contains the number of bits in
1137: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1138: spaces).
1139: This file is not highly sensitive, but the recommended
1.1 deraadt 1140: permissions are read/write for the user, and not accessible by others.
1.48 markus 1141: .It Pa $HOME/.ssh/authorized_keys2
1.68 markus 1142: Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1.48 markus 1143: This file is not highly sensitive, but the recommended
1144: permissions are read/write for the user, and not accessible by others.
1145: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1146: Systemwide list of known host keys.
1.48 markus 1147: .Pa /etc/ssh_known_hosts
1148: contains RSA and
1149: .Pa /etc/ssh_known_hosts2
1.68 markus 1150: contains DSA or RSA keys for protocol version 2.
1.48 markus 1151: These files should be prepared by the
1.1 deraadt 1152: system administrator to contain the public host keys of all machines in the
1.40 aaron 1153: organization.
1154: This file should be world-readable.
1155: This file contains
1.1 deraadt 1156: public keys, one per line, in the following format (fields separated
1157: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1158: modulus, and optional comment field.
1159: When different names are used
1.1 deraadt 1160: for the same machine, all such names should be listed, separated by
1.40 aaron 1161: commas.
1162: The format is described on the
1.2 deraadt 1163: .Xr sshd 8
1.1 deraadt 1164: manual page.
1.2 deraadt 1165: .Pp
1.1 deraadt 1166: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1167: .Xr sshd 8
1.1 deraadt 1168: to verify the client host when logging in; other names are needed because
1.2 deraadt 1169: .Nm
1.1 deraadt 1170: does not convert the user-supplied name to a canonical name before
1171: checking the key, because someone with access to the name servers
1172: would then be able to fool host authentication.
1.2 deraadt 1173: .It Pa /etc/ssh_config
1.40 aaron 1174: Systemwide configuration file.
1175: This file provides defaults for those
1.1 deraadt 1176: values that are not specified in the user's configuration file, and
1.40 aaron 1177: for those users who do not have a configuration file.
1178: This file must be world-readable.
1.2 deraadt 1179: .It Pa $HOME/.rhosts
1180: This file is used in
1181: .Pa \&.rhosts
1182: authentication to list the
1.40 aaron 1183: host/user pairs that are permitted to log in.
1184: (Note that this file is
1.1 deraadt 1185: also used by rlogin and rsh, which makes using this file insecure.)
1186: Each line of the file contains a host name (in the canonical form
1187: returned by name servers), and then a user name on that host,
1.40 aaron 1188: separated by a space.
1189: One some machines this file may need to be
1.1 deraadt 1190: world-readable if the user's home directory is on a NFS partition,
1191: because
1.2 deraadt 1192: .Xr sshd 8
1.40 aaron 1193: reads it as root.
1194: Additionally, this file must be owned by the user,
1195: and must not have write permissions for anyone else.
1196: The recommended
1.1 deraadt 1197: permission for most machines is read/write for the user, and not
1198: accessible by others.
1.2 deraadt 1199: .Pp
1.1 deraadt 1200: Note that by default
1.2 deraadt 1201: .Xr sshd 8
1.1 deraadt 1202: will be installed so that it requires successful RSA host
1.40 aaron 1203: authentication before permitting \s+2.\s0rhosts authentication.
1204: If your server machine does not have the client's host key in
1.2 deraadt 1205: .Pa /etc/ssh_known_hosts ,
1206: you can store it in
1207: .Pa $HOME/.ssh/known_hosts .
1208: The easiest way to do this is to
1.1 deraadt 1209: connect back to the client from the server machine using ssh; this
1.48 markus 1210: will automatically add the host key to
1.2 deraadt 1211: .Pa $HOME/.ssh/known_hosts .
1212: .It Pa $HOME/.shosts
1213: This file is used exactly the same way as
1214: .Pa \&.rhosts .
1215: The purpose for
1.1 deraadt 1216: having this file is to be able to use rhosts authentication with
1.2 deraadt 1217: .Nm
1218: without permitting login with
1219: .Xr rlogin 1
1220: or
1221: .Xr rsh 1 .
1222: .It Pa /etc/hosts.equiv
1223: This file is used during
1.40 aaron 1224: .Pa \&.rhosts authentication.
1225: It contains
1.1 deraadt 1226: canonical hosts names, one per line (the full format is described on
1227: the
1.2 deraadt 1228: .Xr sshd 8
1.40 aaron 1229: manual page).
1230: If the client host is found in this file, login is
1.1 deraadt 1231: automatically permitted provided client and server user names are the
1.40 aaron 1232: same.
1233: Additionally, successful RSA host authentication is normally
1234: required.
1235: This file should only be writable by root.
1.2 deraadt 1236: .It Pa /etc/shosts.equiv
1.44 aaron 1237: This file is processed exactly as
1.2 deraadt 1238: .Pa /etc/hosts.equiv .
1.1 deraadt 1239: This file may be useful to permit logins using
1.2 deraadt 1240: .Nm
1.1 deraadt 1241: but not using rsh/rlogin.
1.2 deraadt 1242: .It Pa /etc/sshrc
1.1 deraadt 1243: Commands in this file are executed by
1.2 deraadt 1244: .Nm
1.1 deraadt 1245: when the user logs in just before the user's shell (or command) is started.
1246: See the
1.2 deraadt 1247: .Xr sshd 8
1.1 deraadt 1248: manual page for more information.
1.2 deraadt 1249: .It Pa $HOME/.ssh/rc
1.1 deraadt 1250: Commands in this file are executed by
1.2 deraadt 1251: .Nm
1.1 deraadt 1252: when the user logs in just before the user's shell (or command) is
1253: started.
1.44 aaron 1254: See the
1.2 deraadt 1255: .Xr sshd 8
1.1 deraadt 1256: manual page for more information.
1.31 markus 1257: .It Pa $HOME/.ssh/environment
1258: Contains additional definitions for environment variables, see section
1259: .Sx ENVIRONMENT
1260: above.
1.5 deraadt 1261: .It Pa libcrypto.so.X.1
1262: A version of this library which includes support for the RSA algorithm
1263: is required for proper operation.
1.58 itojun 1264: .El
1.67 aaron 1265: .Sh AUTHORS
1.78 markus 1266: OpenSSH is a derivative of the original and free
1267: ssh 1.2.12 release by Tatu Ylonen.
1268: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1269: Theo de Raadt and Dug Song
1270: removed many bugs, re-added newer features and
1271: created OpenSSH.
1272: Markus Friedl contributed the support for SSH
1273: protocol versions 1.5 and 2.0.
1.2 deraadt 1274: .Sh SEE ALSO
1275: .Xr rlogin 1 ,
1276: .Xr rsh 1 ,
1277: .Xr scp 1 ,
1.83 djm 1278: .Xr sftp 1 ,
1.2 deraadt 1279: .Xr ssh-add 1 ,
1280: .Xr ssh-agent 1 ,
1281: .Xr ssh-keygen 1 ,
1282: .Xr telnet 1 ,
1.5 deraadt 1283: .Xr sshd 8 ,
1284: .Xr ssl 8
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