Annotation of src/usr.bin/ssh/ssh.1, Revision 1.69
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: .\"
1.69 ! markus 37: .\" $OpenBSD: ssh.1,v 1.68 2000/11/12 19:50:38 markus 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
1.54 markus 51: .Op Fl afgknqtvxACNPTX246
1.51 markus 52: .Op Fl c Ar cipher_spec
1.2 deraadt 53: .Op Fl e Ar escape_char
54: .Op Fl i Ar identity_file
55: .Op Fl l Ar login_name
56: .Op Fl o Ar option
57: .Op Fl p Ar port
1.12 aaron 58: .Oo Fl L Xo
59: .Sm off
1.33 markus 60: .Ar port :
1.12 aaron 61: .Ar host :
62: .Ar hostport
63: .Sm on
64: .Xc
65: .Oc
66: .Oo Fl R Xo
67: .Sm off
1.33 markus 68: .Ar port :
1.12 aaron 69: .Ar host :
70: .Ar hostport
71: .Sm on
72: .Xc
73: .Oc
1.5 deraadt 74: .Op Ar hostname | user@hostname
1.2 deraadt 75: .Op Ar command
1.44 aaron 76: .Sh DESCRIPTION
1.2 deraadt 77: .Nm
1.5 deraadt 78: (Secure Shell) is a program for logging into a remote machine and for
1.40 aaron 79: executing commands on a remote machine.
80: It is intended to replace
1.1 deraadt 81: rlogin and rsh, and provide secure encrypted communications between
1.40 aaron 82: two untrusted hosts over an insecure network.
83: X11 connections and
1.1 deraadt 84: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 85: .Pp
86: .Nm
1.44 aaron 87: connects and logs into the specified
1.2 deraadt 88: .Ar hostname .
1.1 deraadt 89: The user must prove
1.49 markus 90: his/her identity to the remote machine using one of several methods
91: depending on the protocol version used:
92: .Pp
93: .Ss SSH protocol version 1
1.2 deraadt 94: .Pp
1.1 deraadt 95: First, if the machine the user logs in from is listed in
1.2 deraadt 96: .Pa /etc/hosts.equiv
1.1 deraadt 97: or
1.2 deraadt 98: .Pa /etc/shosts.equiv
1.1 deraadt 99: on the remote machine, and the user names are
100: the same on both sides, the user is immediately permitted to log in.
1.44 aaron 101: Second, if
1.2 deraadt 102: .Pa \&.rhosts
1.1 deraadt 103: or
1.2 deraadt 104: .Pa \&.shosts
1.1 deraadt 105: exists in the user's home directory on the
106: remote machine and contains a line containing the name of the client
107: machine and the name of the user on that machine, the user is
1.40 aaron 108: permitted to log in.
109: This form of authentication alone is normally not
1.1 deraadt 110: allowed by the server because it is not secure.
1.2 deraadt 111: .Pp
1.1 deraadt 112: The second (and primary) authentication method is the
1.2 deraadt 113: .Pa rhosts
1.1 deraadt 114: or
1.2 deraadt 115: .Pa hosts.equiv
1.40 aaron 116: method combined with RSA-based host authentication.
117: It means that if the login would be permitted by
1.49 markus 118: .Pa $HOME/.rhosts ,
119: .Pa $HOME/.shosts ,
1.2 deraadt 120: .Pa /etc/hosts.equiv ,
1.1 deraadt 121: or
1.2 deraadt 122: .Pa /etc/shosts.equiv ,
1.11 deraadt 123: and if additionally the server can verify the client's
1.44 aaron 124: host key (see
1.2 deraadt 125: .Pa /etc/ssh_known_hosts
1.23 markus 126: and
127: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 128: in the
1.2 deraadt 129: .Sx FILES
1.40 aaron 130: section), only then login is permitted.
131: This authentication method closes security holes due to IP
132: spoofing, DNS spoofing and routing spoofing.
133: [Note to the administrator:
1.2 deraadt 134: .Pa /etc/hosts.equiv ,
1.49 markus 135: .Pa $HOME/.rhosts ,
1.1 deraadt 136: and the rlogin/rsh protocol in general, are inherently insecure and should be
137: disabled if security is desired.]
1.2 deraadt 138: .Pp
1.44 aaron 139: As a third authentication method,
1.2 deraadt 140: .Nm
1.1 deraadt 141: supports RSA based authentication.
142: The scheme is based on public-key cryptography: there are cryptosystems
143: where encryption and decryption are done using separate keys, and it
144: is not possible to derive the decryption key from the encryption key.
1.40 aaron 145: RSA is one such system.
1.44 aaron 146: The idea is that each user creates a public/private
1.40 aaron 147: key pair for authentication purposes.
148: The server knows the public key, and only the user knows the private key.
1.44 aaron 149: The file
1.2 deraadt 150: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 151: lists the public keys that are permitted for logging
1.40 aaron 152: in.
153: When the user logs in, the
1.2 deraadt 154: .Nm
1.1 deraadt 155: program tells the server which key pair it would like to use for
1.40 aaron 156: authentication.
157: The server checks if this key is permitted, and if
1.1 deraadt 158: so, sends the user (actually the
1.2 deraadt 159: .Nm
1.1 deraadt 160: program running on behalf of the user) a challenge, a random number,
1.40 aaron 161: encrypted by the user's public key.
162: The challenge can only be
163: decrypted using the proper private key.
164: The user's client then decrypts the
1.1 deraadt 165: challenge using the private key, proving that he/she knows the private
166: key but without disclosing it to the server.
1.2 deraadt 167: .Pp
168: .Nm
1.40 aaron 169: implements the RSA authentication protocol automatically.
170: The user creates his/her RSA key pair by running
1.2 deraadt 171: .Xr ssh-keygen 1 .
1.44 aaron 172: This stores the private key in
1.49 markus 173: .Pa $HOME/.ssh/identity
1.1 deraadt 174: and the public key in
1.49 markus 175: .Pa $HOME/.ssh/identity.pub
1.40 aaron 176: in the user's home directory.
177: The user should then copy the
1.2 deraadt 178: .Pa identity.pub
1.44 aaron 179: to
1.49 markus 180: .Pa $HOME/.ssh/authorized_keys
1.44 aaron 181: in his/her home directory on the remote machine (the
1.2 deraadt 182: .Pa authorized_keys
1.44 aaron 183: file corresponds to the conventional
1.49 markus 184: .Pa $HOME/.rhosts
1.1 deraadt 185: file, and has one key
1.40 aaron 186: per line, though the lines can be very long).
187: After this, the user can log in without giving the password.
188: RSA authentication is much
1.1 deraadt 189: more secure than rhosts authentication.
1.2 deraadt 190: .Pp
1.1 deraadt 191: The most convenient way to use RSA authentication may be with an
1.40 aaron 192: authentication agent.
193: See
1.2 deraadt 194: .Xr ssh-agent 1
1.1 deraadt 195: for more information.
1.2 deraadt 196: .Pp
1.44 aaron 197: If other authentication methods fail,
1.2 deraadt 198: .Nm
1.40 aaron 199: prompts the user for a password.
200: The password is sent to the remote
1.1 deraadt 201: host for checking; however, since all communications are encrypted,
202: the password cannot be seen by someone listening on the network.
1.2 deraadt 203: .Pp
1.49 markus 204: .Ss SSH protocol version 2
205: .Pp
206: When a user connects using the protocol version 2
207: different authentication methods are available:
208: At first, the client attempts to authenticate using the public key method.
209: If this method fails password authentication is tried.
210: .Pp
211: The public key method is similar to RSA authentication described
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
1.1 deraadt 275: The session terminates when the command or shell in on the remote
276: machine exists and all X11 and TCP/IP connections have been closed.
277: The exit status of the remote program is returned as the exit status
278: of
1.2 deraadt 279: .Nm ssh .
280: .Pp
1.49 markus 281: .Ss X11 and TCP forwarding
282: .Pp
1.1 deraadt 283: If the user is using X11 (the
1.2 deraadt 284: .Ev DISPLAY
1.1 deraadt 285: environment variable is set), the connection to the X11 display is
286: automatically forwarded to the remote side in such a way that any X11
287: programs started from the shell (or command) will go through the
288: encrypted channel, and the connection to the real X server will be made
1.40 aaron 289: from the local machine.
290: The user should not manually set
1.2 deraadt 291: .Ev DISPLAY .
1.1 deraadt 292: Forwarding of X11 connections can be
293: configured on the command line or in configuration files.
1.2 deraadt 294: .Pp
295: The
1.44 aaron 296: .Ev DISPLAY
1.2 deraadt 297: value set by
298: .Nm
1.1 deraadt 299: will point to the server machine, but with a display number greater
1.40 aaron 300: than zero.
301: This is normal, and happens because
1.2 deraadt 302: .Nm
303: creates a
304: .Dq proxy
305: X server on the server machine for forwarding the
1.1 deraadt 306: connections over the encrypted channel.
1.2 deraadt 307: .Pp
308: .Nm
1.1 deraadt 309: will also automatically set up Xauthority data on the server machine.
310: For this purpose, it will generate a random authorization cookie,
311: store it in Xauthority on the server, and verify that any forwarded
312: connections carry this cookie and replace it by the real cookie when
1.40 aaron 313: the connection is opened.
314: The real authentication cookie is never
1.1 deraadt 315: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 316: .Pp
1.1 deraadt 317: If the user is using an authentication agent, the connection to the agent
318: is automatically forwarded to the remote side unless disabled on
319: command line or in a configuration file.
1.2 deraadt 320: .Pp
1.1 deraadt 321: Forwarding of arbitrary TCP/IP connections over the secure channel can
1.40 aaron 322: be specified either on command line or in a configuration file.
323: One possible application of TCP/IP forwarding is a secure connection to an
1.1 deraadt 324: electronic purse; another is going trough firewalls.
1.2 deraadt 325: .Pp
1.49 markus 326: .Ss Server authentication
327: .Pp
1.2 deraadt 328: .Nm
1.49 markus 329: automatically maintains and checks a database containing
1.40 aaron 330: identifications for all hosts it has ever been used with.
1.49 markus 331: RSA host keys are stored in
332: .Pa $HOME/.ssh/known_hosts
333: and
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.
449: This is usefull if you just want to forward ports
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
467: .Cm RhostsRSAAuthentication .
1.2 deraadt 468: .It Fl q
1.40 aaron 469: Quiet mode.
470: Causes all warning and diagnostic messages to be suppressed.
471: Only fatal errors are displayed.
1.2 deraadt 472: .It Fl t
1.40 aaron 473: Force pseudo-tty allocation.
1.43 brad 474: This can be used to execute arbitrary
1.40 aaron 475: screen-based programs on a remote machine, which can be very useful,
476: e.g., when implementing menu services.
1.53 markus 477: .It Fl T
1.69 ! markus 478: Disable pseudo-tty allocation.
1.2 deraadt 479: .It Fl v
1.40 aaron 480: Verbose mode.
481: Causes
1.2 deraadt 482: .Nm
1.40 aaron 483: to print debugging messages about its progress.
484: This is helpful in
1.1 deraadt 485: debugging connection, authentication, and configuration problems.
1.22 provos 486: The verbose mode is also used to display
487: .Xr skey 1
488: challenges, if the user entered "s/key" as password.
1.61 aaron 489: Multiple -v options increases the verbosity.
490: Maximum is 3.
1.2 deraadt 491: .It Fl x
1.40 aaron 492: Disables X11 forwarding.
1.2 deraadt 493: .It Fl X
1.1 deraadt 494: Enables X11 forwarding.
1.54 markus 495: This can also be specified on a per-host basis in a configuration file.
1.2 deraadt 496: .It Fl C
1.1 deraadt 497: Requests compression of all data (including stdin, stdout, stderr, and
1.40 aaron 498: data for forwarded X11 and TCP/IP connections).
499: The compression algorithm is the same used by
1.34 markus 500: .Xr gzip 1 ,
501: and the
1.2 deraadt 502: .Dq level
503: can be controlled by the
504: .Cm CompressionLevel
1.40 aaron 505: option (see below).
506: Compression is desirable on modem lines and other
1.1 deraadt 507: slow connections, but will only slow down things on fast networks.
508: The default value can be set on a host-by-host basis in the
509: configuration files; see the
1.2 deraadt 510: .Cm Compress
1.1 deraadt 511: option below.
1.2 deraadt 512: .It Fl L Ar port:host:hostport
1.1 deraadt 513: Specifies that the given port on the local (client) host is to be
1.40 aaron 514: forwarded to the given host and port on the remote side.
515: This works by allocating a socket to listen to
1.2 deraadt 516: .Ar port
1.1 deraadt 517: on the local side, and whenever a connection is made to this port, the
518: connection is forwarded over the secure channel, and a connection is
519: made to
1.32 markus 520: .Ar host
521: port
522: .Ar hostport
1.40 aaron 523: from the remote machine.
524: Port forwardings can also be specified in the configuration file.
525: Only root can forward privileged ports.
1.32 markus 526: IPv6 addresses can be specified with an alternative syntax:
527: .Ar port/host/hostport
1.2 deraadt 528: .It Fl R Ar port:host:hostport
1.1 deraadt 529: Specifies that the given port on the remote (server) host is to be
1.40 aaron 530: forwarded to the given host and port on the local side.
531: This works by allocating a socket to listen to
1.2 deraadt 532: .Ar port
1.1 deraadt 533: on the remote side, and whenever a connection is made to this port, the
534: connection is forwarded over the secure channel, and a connection is
535: made to
1.32 markus 536: .Ar host
537: port
538: .Ar hostport
1.40 aaron 539: from the local machine.
540: Port forwardings can also be specified in the configuration file.
541: Privileged ports can be forwarded only when
1.1 deraadt 542: logging in as root on the remote machine.
1.46 markus 543: .It Fl 2
544: Forces
545: .Nm
1.50 markus 546: to try protocol version 2 only.
1.32 markus 547: .It Fl 4
548: Forces
549: .Nm
550: to use IPv4 addresses only.
551: .It Fl 6
552: Forces
553: .Nm
554: to use IPv6 addresses only.
1.2 deraadt 555: .El
1.66 aaron 556: .Pp
557: If
558: .Nm
559: is not invoked with one of the standard program names
560: .Pf ( Dq ssh ,
561: .Dq slogin ,
562: .Dq rsh ,
563: .Dq rlogin ,
564: or
565: .Dq remsh ) ,
566: it uses this name as its
567: .Ar hostname
568: argument.
569: This is consistent with traditional
570: .Xr rsh 1
571: behavior.
1.2 deraadt 572: .Sh CONFIGURATION FILES
573: .Nm
1.1 deraadt 574: obtains configuration data from the following sources (in this order):
575: command line options, user's configuration file
1.2 deraadt 576: .Pq Pa $HOME/.ssh/config ,
577: and system-wide configuration file
578: .Pq Pa /etc/ssh_config .
579: For each parameter, the first obtained value
1.40 aaron 580: will be used.
581: The configuration files contain sections bracketed by
582: .Dq Host
583: specifications, and that section is only applied for hosts that
584: match one of the patterns given in the specification.
585: The matched host name is the one given on the command line.
1.2 deraadt 586: .Pp
1.1 deraadt 587: Since the first obtained value for each parameter is used, more
588: host-specific declarations should be given near the beginning of the
589: file, and general defaults at the end.
1.2 deraadt 590: .Pp
1.1 deraadt 591: The configuration file has the following format:
1.2 deraadt 592: .Pp
593: Empty lines and lines starting with
594: .Ql #
595: are comments.
596: .Pp
597: Otherwise a line is of the format
598: .Dq keyword arguments .
599: The possible
1.1 deraadt 600: keywords and their meanings are as follows (note that the
601: configuration files are case-sensitive):
1.2 deraadt 602: .Bl -tag -width Ds
603: .It Cm Host
1.1 deraadt 604: Restricts the following declarations (up to the next
1.2 deraadt 605: .Cm Host
1.1 deraadt 606: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 607: given after the keyword.
608: .Ql \&*
609: and
610: .Ql ?
611: can be used as wildcards in the
1.40 aaron 612: patterns.
613: A single
1.2 deraadt 614: .Ql \&*
615: as a pattern can be used to provide global
1.40 aaron 616: defaults for all hosts.
617: The host is the
1.2 deraadt 618: .Ar hostname
1.1 deraadt 619: argument given on the command line (i.e., the name is not converted to
620: a canonicalized host name before matching).
1.2 deraadt 621: .It Cm AFSTokenPassing
1.42 aaron 622: Specifies whether to pass AFS tokens to remote host.
623: The argument to this keyword must be
1.2 deraadt 624: .Dq yes
625: or
626: .Dq no .
627: .It Cm BatchMode
628: If set to
629: .Dq yes ,
1.40 aaron 630: passphrase/password querying will be disabled.
631: This option is useful in scripts and other batch jobs where you have no
632: user to supply the password.
633: The argument must be
1.2 deraadt 634: .Dq yes
635: or
636: .Dq no .
1.34 markus 637: .It Cm CheckHostIP
638: If this flag is set to
639: .Dq yes ,
640: ssh will additionally check the host ip address in the
641: .Pa known_hosts
1.42 aaron 642: file.
643: This allows ssh to detect if a host key changed due to DNS spoofing.
1.34 markus 644: If the option is set to
645: .Dq no ,
646: the check will not be executed.
1.2 deraadt 647: .It Cm Cipher
1.62 markus 648: Specifies the cipher to use for encrypting the session
1.64 markus 649: in protocol version 1.
1.40 aaron 650: Currently,
1.64 markus 651: .Dq blowfish
1.1 deraadt 652: and
1.10 provos 653: .Dq 3des
1.40 aaron 654: are supported.
655: The default is
1.2 deraadt 656: .Dq 3des .
1.45 markus 657: .It Cm Ciphers
658: Specifies the ciphers allowed for protocol version 2
659: in order of preference.
660: Multiple ciphers must be comma-separated.
661: The default is
1.62 markus 662: .Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour .
1.2 deraadt 663: .It Cm Compression
1.40 aaron 664: Specifies whether to use compression.
665: The argument must be
1.2 deraadt 666: .Dq yes
667: or
668: .Dq no .
669: .It Cm CompressionLevel
1.40 aaron 670: Specifies the compression level to use if compression is enable.
671: The argument must be an integer from 1 (fast) to 9 (slow, best).
672: The default level is 6, which is good for most applications.
673: The meaning of the values is the same as in
1.34 markus 674: .Xr gzip 1 .
1.2 deraadt 675: .It Cm ConnectionAttempts
1.1 deraadt 676: Specifies the number of tries (one per second) to make before falling
1.40 aaron 677: back to rsh or exiting.
678: The argument must be an integer.
679: This may be useful in scripts if the connection sometimes fails.
1.68 markus 680: .It Cm PubkeyAuthentication
681: Specifies whether to try public key authentication.
1.50 markus 682: The argument to this keyword must be
683: .Dq yes
684: or
685: .Dq no .
686: Note that this option applies to protocol version 2 only.
1.2 deraadt 687: .It Cm EscapeChar
688: Sets the escape character (default:
689: .Ql ~ ) .
690: The escape character can also
1.40 aaron 691: be set on the command line.
692: The argument should be a single character,
1.2 deraadt 693: .Ql ^
694: followed by a letter, or
695: .Dq none
696: to disable the escape
1.1 deraadt 697: character entirely (making the connection transparent for binary
698: data).
1.44 aaron 699: .It Cm FallBackToRsh
1.1 deraadt 700: Specifies that if connecting via
1.2 deraadt 701: .Nm
1.1 deraadt 702: fails due to a connection refused error (there is no
1.2 deraadt 703: .Xr sshd 8
1.44 aaron 704: listening on the remote host),
1.2 deraadt 705: .Xr rsh 1
1.1 deraadt 706: should automatically be used instead (after a suitable warning about
1.40 aaron 707: the session being unencrypted).
708: The argument must be
1.2 deraadt 709: .Dq yes
710: or
711: .Dq no .
712: .It Cm ForwardAgent
1.1 deraadt 713: Specifies whether the connection to the authentication agent (if any)
1.40 aaron 714: will be forwarded to the remote machine.
715: The argument must be
1.2 deraadt 716: .Dq yes
717: or
1.54 markus 718: .Dq no .
719: The default is
1.2 deraadt 720: .Dq no .
721: .It Cm ForwardX11
1.1 deraadt 722: Specifies whether X11 connections will be automatically redirected
1.44 aaron 723: over the secure channel and
1.2 deraadt 724: .Ev DISPLAY
1.40 aaron 725: set.
1.44 aaron 726: The argument must be
1.2 deraadt 727: .Dq yes
728: or
1.38 markus 729: .Dq no .
730: The default is
1.3 deraadt 731: .Dq no .
732: .It Cm GatewayPorts
733: Specifies whether remote hosts are allowed to connect to local
734: forwarded ports.
735: The argument must be
736: .Dq yes
737: or
738: .Dq no .
739: The default is
1.2 deraadt 740: .Dq no .
741: .It Cm GlobalKnownHostsFile
1.44 aaron 742: Specifies a file to use instead of
1.2 deraadt 743: .Pa /etc/ssh_known_hosts .
744: .It Cm HostName
1.40 aaron 745: Specifies the real host name to log into.
746: This can be used to specify nicknames or abbreviations for hosts.
747: Default is the name given on the command line.
748: Numeric IP addresses are also permitted (both on the command line and in
1.2 deraadt 749: .Cm HostName
1.1 deraadt 750: specifications).
1.2 deraadt 751: .It Cm IdentityFile
1.1 deraadt 752: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 753: is read (default
1.49 markus 754: .Pa $HOME/.ssh/identity
1.2 deraadt 755: in the user's home directory).
1.1 deraadt 756: Additionally, any identities represented by the authentication agent
1.40 aaron 757: will be used for authentication.
758: The file name may use the tilde
759: syntax to refer to a user's home directory.
760: It is possible to have
1.1 deraadt 761: multiple identity files specified in configuration files; all these
762: identities will be tried in sequence.
1.2 deraadt 763: .It Cm KeepAlive
1.1 deraadt 764: Specifies whether the system should send keepalive messages to the
1.40 aaron 765: other side.
766: If they are sent, death of the connection or crash of one
767: of the machines will be properly noticed.
768: However, this means that
1.1 deraadt 769: connections will die if the route is down temporarily, and some people
1.41 aaron 770: find it annoying.
1.2 deraadt 771: .Pp
772: The default is
773: .Dq yes
774: (to send keepalives), and the client will notice
1.40 aaron 775: if the network goes down or the remote host dies.
776: This is important in scripts, and many users want it too.
1.2 deraadt 777: .Pp
778: To disable keepalives, the value should be set to
779: .Dq no
780: in both the server and the client configuration files.
781: .It Cm KerberosAuthentication
1.42 aaron 782: Specifies whether Kerberos authentication will be used.
783: The argument to this keyword must be
1.4 dugsong 784: .Dq yes
785: or
786: .Dq no .
1.2 deraadt 787: .It Cm KerberosTgtPassing
1.42 aaron 788: Specifies whether a Kerberos TGT will be forwarded to the server.
789: This will only work if the Kerberos server is actually an AFS kaserver.
790: The argument to this keyword must be
1.4 dugsong 791: .Dq yes
792: or
793: .Dq no .
1.2 deraadt 794: .It Cm LocalForward
1.1 deraadt 795: Specifies that a TCP/IP port on the local machine be forwarded over
1.40 aaron 796: the secure channel to given host:port from the remote machine.
797: The first argument must be a port number, and the second must be
798: host:port.
799: Multiple forwardings may be specified, and additional
800: forwardings can be given on the command line.
801: Only the superuser can forward privileged ports.
1.24 markus 802: .It Cm LogLevel
803: Gives the verbosity level that is used when logging messages from
804: .Nm ssh .
805: The possible values are:
1.39 djm 806: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.24 markus 807: The default is INFO.
1.14 dugsong 808: .It Cm NumberOfPasswordPrompts
1.42 aaron 809: Specifies the number of password prompts before giving up.
810: The argument to this keyword must be an integer.
811: Default is 3.
1.34 markus 812: .It Cm PasswordAuthentication
1.40 aaron 813: Specifies whether to use password authentication.
814: The argument to this keyword must be
1.34 markus 815: .Dq yes
816: or
817: .Dq no .
1.50 markus 818: Note that this option applies to both protocol version 1 and 2.
1.2 deraadt 819: .It Cm Port
1.40 aaron 820: Specifies the port number to connect on the remote host.
821: Default is 22.
1.45 markus 822: .It Cm Protocol
823: Specifies the protocol versions
824: .Nm
825: should support in order of preference.
826: The possible values are
827: .Dq 1
828: and
829: .Dq 2 .
830: Multiple versions must be comma-separated.
831: The default is
1.49 markus 832: .Dq 1,2 .
833: This means that
834: .Nm
835: tries version 1 and falls back to version 2
1.52 hugh 836: if version 1 is not available.
1.2 deraadt 837: .It Cm ProxyCommand
1.40 aaron 838: Specifies the command to use to connect to the server.
839: The command
840: string extends to the end of the line, and is executed with
841: .Pa /bin/sh .
842: In the command string,
843: .Ql %h
844: will be substituted by the host name to
845: connect and
846: .Ql %p
847: by the port.
848: The command can be basically anything,
849: and should read from its standard input and write to its standard output.
850: It should eventually connect an
1.2 deraadt 851: .Xr sshd 8
1.1 deraadt 852: server running on some machine, or execute
1.2 deraadt 853: .Ic sshd -i
1.40 aaron 854: somewhere.
855: Host key management will be done using the
1.1 deraadt 856: HostName of the host being connected (defaulting to the name typed by
857: the user).
1.29 markus 858: Note that
859: .Cm CheckHostIP
860: is not available for connects with a proxy command.
1.2 deraadt 861: .Pp
862: .It Cm RemoteForward
1.1 deraadt 863: Specifies that a TCP/IP port on the remote machine be forwarded over
1.40 aaron 864: the secure channel to given host:port from the local machine.
865: The first argument must be a port number, and the second must be
866: host:port.
867: Multiple forwardings may be specified, and additional
868: forwardings can be given on the command line.
869: Only the superuser can forward privileged ports.
1.2 deraadt 870: .It Cm RhostsAuthentication
1.40 aaron 871: Specifies whether to try rhosts based authentication.
872: Note that this
1.1 deraadt 873: declaration only affects the client side and has no effect whatsoever
1.40 aaron 874: on security.
875: Disabling rhosts authentication may reduce
1.1 deraadt 876: authentication time on slow connections when rhosts authentication is
1.40 aaron 877: not used.
878: Most servers do not permit RhostsAuthentication because it
879: is not secure (see RhostsRSAAuthentication).
880: The argument to this keyword must be
1.2 deraadt 881: .Dq yes
882: or
883: .Dq no .
884: .It Cm RhostsRSAAuthentication
1.1 deraadt 885: Specifies whether to try rhosts based authentication with RSA host
1.40 aaron 886: authentication.
887: This is the primary authentication method for most sites.
888: The argument must be
1.2 deraadt 889: .Dq yes
890: or
891: .Dq no .
892: .It Cm RSAAuthentication
1.40 aaron 893: Specifies whether to try RSA authentication.
894: The argument to this keyword must be
1.2 deraadt 895: .Dq yes
896: or
897: .Dq no .
1.1 deraadt 898: RSA authentication will only be
899: attempted if the identity file exists, or an authentication agent is
900: running.
1.50 markus 901: Note that this option applies to protocol version 1 only.
1.27 markus 902: .It Cm SkeyAuthentication
903: Specifies whether to use
904: .Xr skey 1
1.40 aaron 905: authentication.
906: The argument to this keyword must be
1.27 markus 907: .Dq yes
908: or
909: .Dq no .
910: The default is
911: .Dq no .
1.2 deraadt 912: .It Cm StrictHostKeyChecking
913: If this flag is set to
1.44 aaron 914: .Dq yes ,
1.2 deraadt 915: .Nm
1.1 deraadt 916: ssh will never automatically add host keys to the
1.2 deraadt 917: .Pa $HOME/.ssh/known_hosts
1.48 markus 918: and
919: .Pa $HOME/.ssh/known_hosts2
920: files, and refuses to connect hosts whose host key has changed.
1.40 aaron 921: This provides maximum protection against trojan horse attacks.
922: However, it can be somewhat annoying if you don't have good
1.2 deraadt 923: .Pa /etc/ssh_known_hosts
1.48 markus 924: and
925: .Pa /etc/ssh_known_hosts2
1.1 deraadt 926: files installed and frequently
1.40 aaron 927: connect new hosts.
928: Basically this option forces the user to manually
929: add any new hosts.
930: Normally this option is disabled, and new hosts
931: will automatically be added to the known host files.
932: The host keys of
933: known hosts will be verified automatically in either case.
934: The argument must be
1.2 deraadt 935: .Dq yes
936: or
937: .Dq no .
1.16 markus 938: .It Cm UsePrivilegedPort
939: Specifies whether to use a privileged port for outgoing connections.
940: The argument must be
941: .Dq yes
942: or
943: .Dq no .
944: The default is
945: .Dq yes .
946: Note that setting this option to
947: .Dq no
1.30 provos 948: turns off
1.16 markus 949: .Cm RhostsAuthentication
950: and
951: .Cm RhostsRSAAuthentication .
1.34 markus 952: .It Cm User
1.40 aaron 953: Specifies the user to log in as.
954: This can be useful if you have a different user name on different machines.
955: This saves the trouble of
1.34 markus 956: having to remember to give the user name on the command line.
957: .It Cm UserKnownHostsFile
958: Specifies a file to use instead of
959: .Pa $HOME/.ssh/known_hosts .
1.2 deraadt 960: .It Cm UseRsh
1.40 aaron 961: Specifies that rlogin/rsh should be used for this host.
962: It is possible that the host does not at all support the
1.2 deraadt 963: .Nm
1.40 aaron 964: protocol.
965: This causes
1.2 deraadt 966: .Nm
1.40 aaron 967: to immediately execute
1.2 deraadt 968: .Xr rsh 1 .
1.1 deraadt 969: All other options (except
1.2 deraadt 970: .Cm HostName )
1.40 aaron 971: are ignored if this has been specified.
972: The argument must be
1.2 deraadt 973: .Dq yes
974: or
975: .Dq no .
1.55 markus 976: .It Cm XAuthLocation
977: Specifies the location of the
978: .Xr xauth 1
979: program.
980: The default is
981: .Pa /usr/X11R6/bin/xauth .
1.58 itojun 982: .El
1.2 deraadt 983: .Sh ENVIRONMENT
984: .Nm
1.1 deraadt 985: will normally set the following environment variables:
1.2 deraadt 986: .Bl -tag -width Ds
987: .It Ev DISPLAY
988: The
989: .Ev DISPLAY
1.40 aaron 990: variable indicates the location of the X11 server.
1.44 aaron 991: It is automatically set by
1.2 deraadt 992: .Nm
993: to point to a value of the form
994: .Dq hostname:n
995: where hostname indicates
1.40 aaron 996: the host where the shell runs, and n is an integer >= 1.
997: .Nm
998: uses this special value to forward X11 connections over the secure
999: channel.
1000: The user should normally not set DISPLAY explicitly, as that
1.1 deraadt 1001: will render the X11 connection insecure (and will require the user to
1002: manually copy any required authorization cookies).
1.2 deraadt 1003: .It Ev HOME
1.1 deraadt 1004: Set to the path of the user's home directory.
1.2 deraadt 1005: .It Ev LOGNAME
1006: Synonym for
1.12 aaron 1007: .Ev USER ;
1008: set for compatibility with systems that use this variable.
1.2 deraadt 1009: .It Ev MAIL
1.1 deraadt 1010: Set to point the user's mailbox.
1.40 aaron 1011: .It Ev PATH
1.2 deraadt 1012: Set to the default
1013: .Ev PATH ,
1014: as specified when compiling
1.12 aaron 1015: .Nm ssh .
1.18 markus 1016: .It Ev SSH_AUTH_SOCK
1.17 markus 1017: indicates the path of a unix-domain socket used to communicate with the
1018: agent.
1.2 deraadt 1019: .It Ev SSH_CLIENT
1.40 aaron 1020: Identifies the client end of the connection.
1021: The variable contains
1.1 deraadt 1022: three space-separated values: client ip-address, client port number,
1023: and server port number.
1.2 deraadt 1024: .It Ev SSH_TTY
1.1 deraadt 1025: This is set to the name of the tty (path to the device) associated
1.40 aaron 1026: with the current shell or command.
1027: If the current session has no tty,
1.1 deraadt 1028: this variable is not set.
1.2 deraadt 1029: .It Ev TZ
1.1 deraadt 1030: The timezone variable is set to indicate the present timezone if it
1.56 deraadt 1031: was set when the daemon was started (i.e., the daemon passes the value
1.1 deraadt 1032: on to new connections).
1.2 deraadt 1033: .It Ev USER
1.1 deraadt 1034: Set to the name of the user logging in.
1.2 deraadt 1035: .El
1036: .Pp
1.44 aaron 1037: Additionally,
1.2 deraadt 1038: .Nm
1.44 aaron 1039: reads
1040: .Pa $HOME/.ssh/environment ,
1.2 deraadt 1041: and adds lines of the format
1042: .Dq VARNAME=value
1.12 aaron 1043: to the environment.
1.2 deraadt 1044: .Sh FILES
1.36 markus 1045: .Bl -tag -width Ds
1.2 deraadt 1046: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 1047: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 1048: in
1049: .Pa /etc/ssh_known_hosts ) .
1050: See
1051: .Xr sshd 8 .
1.48 markus 1052: .It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
1053: Contains the RSA and the DSA authentication identity of the user.
1054: These files
1055: contain sensitive data and should be readable by the user but not
1.15 markus 1056: accessible by others (read/write/execute).
1057: Note that
1058: .Nm
1.48 markus 1059: ignores a private key file if it is accessible by others.
1.15 markus 1060: It is possible to specify a passphrase when
1.1 deraadt 1061: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 1062: sensitive part of this file using 3DES.
1.48 markus 1063: .It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
1.1 deraadt 1064: Contains the public key for authentication (public part of the
1.40 aaron 1065: identity file in human-readable form).
1.48 markus 1066: The contents of the
1067: .Pa $HOME/.ssh/identity.pub
1068: file should be added to
1.2 deraadt 1069: .Pa $HOME/.ssh/authorized_keys
1070: on all machines
1.40 aaron 1071: where you wish to log in using RSA authentication.
1.48 markus 1072: The contents of the
1073: .Pa $HOME/.ssh/id_dsa.pub
1074: file should be added to
1075: .Pa $HOME/.ssh/authorized_keys2
1076: on all machines
1077: where you wish to log in using DSA authentication.
1078: These files are not
1.40 aaron 1079: sensitive and can (but need not) be readable by anyone.
1.48 markus 1080: These files are
1081: never used automatically and are not necessary; they is only provided for
1.1 deraadt 1082: the convenience of the user.
1.2 deraadt 1083: .It Pa $HOME/.ssh/config
1.40 aaron 1084: This is the per-user configuration file.
1085: The format of this file is described above.
1086: This file is used by the
1.2 deraadt 1087: .Nm
1.40 aaron 1088: client.
1089: This file does not usually contain any sensitive information,
1.1 deraadt 1090: but the recommended permissions are read/write for the user, and not
1091: accessible by others.
1.2 deraadt 1092: .It Pa $HOME/.ssh/authorized_keys
1.40 aaron 1093: Lists the RSA keys that can be used for logging in as this user.
1094: The format of this file is described in the
1.2 deraadt 1095: .Xr sshd 8
1.40 aaron 1096: manual page.
1097: In the simplest form the format is the same as the .pub
1.1 deraadt 1098: identity files (that is, each line contains the number of bits in
1099: modulus, public exponent, modulus, and comment fields, separated by
1.40 aaron 1100: spaces).
1101: This file is not highly sensitive, but the recommended
1.1 deraadt 1102: permissions are read/write for the user, and not accessible by others.
1.48 markus 1103: .It Pa $HOME/.ssh/authorized_keys2
1.68 markus 1104: Lists the public keys (DSA/RSA) that can be used for logging in as this user.
1.48 markus 1105: This file is not highly sensitive, but the recommended
1106: permissions are read/write for the user, and not accessible by others.
1107: .It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
1.40 aaron 1108: Systemwide list of known host keys.
1.48 markus 1109: .Pa /etc/ssh_known_hosts
1110: contains RSA and
1111: .Pa /etc/ssh_known_hosts2
1.68 markus 1112: contains DSA or RSA keys for protocol version 2.
1.48 markus 1113: These files should be prepared by the
1.1 deraadt 1114: system administrator to contain the public host keys of all machines in the
1.40 aaron 1115: organization.
1116: This file should be world-readable.
1117: This file contains
1.1 deraadt 1118: public keys, one per line, in the following format (fields separated
1119: by spaces): system name, number of bits in modulus, public exponent,
1.40 aaron 1120: modulus, and optional comment field.
1121: When different names are used
1.1 deraadt 1122: for the same machine, all such names should be listed, separated by
1.40 aaron 1123: commas.
1124: The format is described on the
1.2 deraadt 1125: .Xr sshd 8
1.1 deraadt 1126: manual page.
1.2 deraadt 1127: .Pp
1.1 deraadt 1128: The canonical system name (as returned by name servers) is used by
1.2 deraadt 1129: .Xr sshd 8
1.1 deraadt 1130: to verify the client host when logging in; other names are needed because
1.2 deraadt 1131: .Nm
1.1 deraadt 1132: does not convert the user-supplied name to a canonical name before
1133: checking the key, because someone with access to the name servers
1134: would then be able to fool host authentication.
1.2 deraadt 1135: .It Pa /etc/ssh_config
1.40 aaron 1136: Systemwide configuration file.
1137: This file provides defaults for those
1.1 deraadt 1138: values that are not specified in the user's configuration file, and
1.40 aaron 1139: for those users who do not have a configuration file.
1140: This file must be world-readable.
1.2 deraadt 1141: .It Pa $HOME/.rhosts
1142: This file is used in
1143: .Pa \&.rhosts
1144: authentication to list the
1.40 aaron 1145: host/user pairs that are permitted to log in.
1146: (Note that this file is
1.1 deraadt 1147: also used by rlogin and rsh, which makes using this file insecure.)
1148: Each line of the file contains a host name (in the canonical form
1149: returned by name servers), and then a user name on that host,
1.40 aaron 1150: separated by a space.
1151: One some machines this file may need to be
1.1 deraadt 1152: world-readable if the user's home directory is on a NFS partition,
1153: because
1.2 deraadt 1154: .Xr sshd 8
1.40 aaron 1155: reads it as root.
1156: Additionally, this file must be owned by the user,
1157: and must not have write permissions for anyone else.
1158: The recommended
1.1 deraadt 1159: permission for most machines is read/write for the user, and not
1160: accessible by others.
1.2 deraadt 1161: .Pp
1.1 deraadt 1162: Note that by default
1.2 deraadt 1163: .Xr sshd 8
1.1 deraadt 1164: will be installed so that it requires successful RSA host
1.40 aaron 1165: authentication before permitting \s+2.\s0rhosts authentication.
1166: If your server machine does not have the client's host key in
1.2 deraadt 1167: .Pa /etc/ssh_known_hosts ,
1168: you can store it in
1169: .Pa $HOME/.ssh/known_hosts .
1170: The easiest way to do this is to
1.1 deraadt 1171: connect back to the client from the server machine using ssh; this
1.48 markus 1172: will automatically add the host key to
1.2 deraadt 1173: .Pa $HOME/.ssh/known_hosts .
1174: .It Pa $HOME/.shosts
1175: This file is used exactly the same way as
1176: .Pa \&.rhosts .
1177: The purpose for
1.1 deraadt 1178: having this file is to be able to use rhosts authentication with
1.2 deraadt 1179: .Nm
1180: without permitting login with
1181: .Xr rlogin 1
1182: or
1183: .Xr rsh 1 .
1184: .It Pa /etc/hosts.equiv
1185: This file is used during
1.40 aaron 1186: .Pa \&.rhosts authentication.
1187: It contains
1.1 deraadt 1188: canonical hosts names, one per line (the full format is described on
1189: the
1.2 deraadt 1190: .Xr sshd 8
1.40 aaron 1191: manual page).
1192: If the client host is found in this file, login is
1.1 deraadt 1193: automatically permitted provided client and server user names are the
1.40 aaron 1194: same.
1195: Additionally, successful RSA host authentication is normally
1196: required.
1197: This file should only be writable by root.
1.2 deraadt 1198: .It Pa /etc/shosts.equiv
1.44 aaron 1199: This file is processed exactly as
1.2 deraadt 1200: .Pa /etc/hosts.equiv .
1.1 deraadt 1201: This file may be useful to permit logins using
1.2 deraadt 1202: .Nm
1.1 deraadt 1203: but not using rsh/rlogin.
1.2 deraadt 1204: .It Pa /etc/sshrc
1.1 deraadt 1205: Commands in this file are executed by
1.2 deraadt 1206: .Nm
1.1 deraadt 1207: when the user logs in just before the user's shell (or command) is started.
1208: See the
1.2 deraadt 1209: .Xr sshd 8
1.1 deraadt 1210: manual page for more information.
1.2 deraadt 1211: .It Pa $HOME/.ssh/rc
1.1 deraadt 1212: Commands in this file are executed by
1.2 deraadt 1213: .Nm
1.1 deraadt 1214: when the user logs in just before the user's shell (or command) is
1215: started.
1.44 aaron 1216: See the
1.2 deraadt 1217: .Xr sshd 8
1.1 deraadt 1218: manual page for more information.
1.31 markus 1219: .It Pa $HOME/.ssh/environment
1220: Contains additional definitions for environment variables, see section
1221: .Sx ENVIRONMENT
1222: above.
1.5 deraadt 1223: .It Pa libcrypto.so.X.1
1224: A version of this library which includes support for the RSA algorithm
1225: is required for proper operation.
1.58 itojun 1226: .El
1.67 aaron 1227: .Sh AUTHORS
1.20 provos 1228: OpenSSH
1.37 deraadt 1229: is a derivative of the original (free) ssh 1.2.12 release by Tatu Ylonen,
1.40 aaron 1230: but with bugs removed and newer features re-added.
1231: Rapidly after the
1.37 deraadt 1232: 1.2.12 release, newer versions of the original ssh bore successively
1233: more restrictive licenses, and thus demand for a free version was born.
1.47 markus 1234: .Pp
1.37 deraadt 1235: This version of OpenSSH
1.20 provos 1236: .Bl -bullet
1237: .It
1.35 aaron 1238: has all components of a restrictive nature (i.e., patents, see
1.21 deraadt 1239: .Xr ssl 8 )
1240: directly removed from the source code; any licensed or patented components
1241: are chosen from
1242: external libraries.
1.20 provos 1243: .It
1.47 markus 1244: has been updated to support SSH protocol 1.5 and 2, making it compatible with
1245: all other SSH clients and servers.
1.20 provos 1246: .It
1.44 aaron 1247: contains added support for
1.20 provos 1248: .Xr kerberos 8
1249: authentication and ticket passing.
1250: .It
1.21 deraadt 1251: supports one-time password authentication with
1.20 provos 1252: .Xr skey 1 .
1253: .El
1.25 provos 1254: .Pp
1.26 aaron 1255: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1.25 provos 1256: Niels Provos, Theo de Raadt, and Dug Song.
1.47 markus 1257: .Pp
1258: The support for SSH protocol 2 was written by Markus Friedl.
1.2 deraadt 1259: .Sh SEE ALSO
1260: .Xr rlogin 1 ,
1261: .Xr rsh 1 ,
1262: .Xr scp 1 ,
1263: .Xr ssh-add 1 ,
1264: .Xr ssh-agent 1 ,
1265: .Xr ssh-keygen 1 ,
1266: .Xr telnet 1 ,
1.5 deraadt 1267: .Xr sshd 8 ,
1268: .Xr ssl 8
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