Annotation of src/usr.bin/ssh/ssh.1, Revision 1.27
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" ssh.1.in
4: .\"
5: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
6: .\"
7: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
8: .\" All rights reserved
9: .\"
10: .\" Created: Sat Apr 22 21:55:14 1995 ylo
11: .\"
1.27 ! markus 12: .\" $Id: ssh.1,v 1.26 1999/11/17 21:08:07 aaron Exp $
1.1 deraadt 13: .\"
1.2 deraadt 14: .Dd September 25, 1999
15: .Dt SSH 1
16: .Os
17: .Sh NAME
18: .Nm ssh
1.20 provos 19: .Nd OpenSSH secure shell client (remote login program)
1.2 deraadt 20: .Sh SYNOPSIS
21: .Nm ssh
22: .Op Fl l Ar login_name
1.5 deraadt 23: .Op Ar hostname | user@hostname
1.2 deraadt 24: .Op Ar command
25: .Pp
26: .Nm ssh
1.19 markus 27: .Op Fl afgknqtvxCPX
1.12 aaron 28: .Op Fl c Ar blowfish | 3des
1.2 deraadt 29: .Op Fl e Ar escape_char
30: .Op Fl i Ar identity_file
31: .Op Fl l Ar login_name
32: .Op Fl o Ar option
33: .Op Fl p Ar port
1.12 aaron 34: .Oo Fl L Xo
35: .Sm off
36: .Ar host :
37: .Ar port :
38: .Ar hostport
39: .Sm on
40: .Xc
41: .Oc
42: .Oo Fl R Xo
43: .Sm off
44: .Ar host :
45: .Ar port :
46: .Ar hostport
47: .Sm on
48: .Xc
49: .Oc
1.5 deraadt 50: .Op Ar hostname | user@hostname
1.2 deraadt 51: .Op Ar command
52: .Sh DESCRIPTION
53: .Nm
1.5 deraadt 54: (Secure Shell) is a program for logging into a remote machine and for
55: executing commands on a remote machine. It is intended to replace
1.1 deraadt 56: rlogin and rsh, and provide secure encrypted communications between
57: two untrusted hosts over an insecure network. X11 connections and
58: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 59: .Pp
60: .Nm
1.1 deraadt 61: connects and logs into the specified
1.2 deraadt 62: .Ar hostname .
1.1 deraadt 63: The user must prove
64: his/her identity to the remote machine using one of several methods.
1.2 deraadt 65: .Pp
1.1 deraadt 66: First, if the machine the user logs in from is listed in
1.2 deraadt 67: .Pa /etc/hosts.equiv
1.1 deraadt 68: or
1.2 deraadt 69: .Pa /etc/shosts.equiv
1.1 deraadt 70: on the remote machine, and the user names are
71: the same on both sides, the user is immediately permitted to log in.
72: Second, if
1.2 deraadt 73: .Pa \&.rhosts
1.1 deraadt 74: or
1.2 deraadt 75: .Pa \&.shosts
1.1 deraadt 76: exists in the user's home directory on the
77: remote machine and contains a line containing the name of the client
78: machine and the name of the user on that machine, the user is
79: permitted to log in. This form of authentication alone is normally not
80: allowed by the server because it is not secure.
1.2 deraadt 81: .Pp
1.1 deraadt 82: The second (and primary) authentication method is the
1.2 deraadt 83: .Pa rhosts
1.1 deraadt 84: or
1.2 deraadt 85: .Pa hosts.equiv
1.1 deraadt 86: method combined with RSA-based host authentication. It
87: means that if the login would be permitted by
1.2 deraadt 88: .Pa \&.rhosts ,
89: .Pa \&.shosts ,
90: .Pa /etc/hosts.equiv ,
1.1 deraadt 91: or
1.2 deraadt 92: .Pa /etc/shosts.equiv ,
1.11 deraadt 93: and if additionally the server can verify the client's
1.1 deraadt 94: host key (see
1.2 deraadt 95: .Pa /etc/ssh_known_hosts
1.23 markus 96: and
97: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 98: in the
1.2 deraadt 99: .Sx FILES
1.1 deraadt 100: section), only then login is
101: permitted. This authentication method closes security holes due to IP
102: spoofing, DNS spoofing and routing spoofing. [Note to the
103: administrator:
1.2 deraadt 104: .Pa /etc/hosts.equiv ,
105: .Pa \&.rhosts ,
1.1 deraadt 106: and the rlogin/rsh protocol in general, are inherently insecure and should be
107: disabled if security is desired.]
1.2 deraadt 108: .Pp
1.1 deraadt 109: As a third authentication method,
1.2 deraadt 110: .Nm
1.1 deraadt 111: supports RSA based authentication.
112: The scheme is based on public-key cryptography: there are cryptosystems
113: where encryption and decryption are done using separate keys, and it
114: is not possible to derive the decryption key from the encryption key.
115: RSA is one such system. The idea is that each user creates a public/private
116: key pair for authentication purposes. The
117: server knows the public key, and only the user knows the private key.
118: The file
1.2 deraadt 119: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 120: lists the public keys that are permitted for logging
121: in. When the user logs in, the
1.2 deraadt 122: .Nm
1.1 deraadt 123: program tells the server which key pair it would like to use for
124: authentication. The server checks if this key is permitted, and if
125: so, sends the user (actually the
1.2 deraadt 126: .Nm
1.1 deraadt 127: program running on behalf of the user) a challenge, a random number,
128: encrypted by the user's public key. The challenge can only be
129: decrypted using the proper private key. The user's client then decrypts the
130: challenge using the private key, proving that he/she knows the private
131: key but without disclosing it to the server.
1.2 deraadt 132: .Pp
133: .Nm
1.1 deraadt 134: implements the RSA authentication protocol automatically. The user
135: creates his/her RSA key pair by running
1.2 deraadt 136: .Xr ssh-keygen 1 .
1.1 deraadt 137: This stores the private key in
1.2 deraadt 138: .Pa \&.ssh/identity
1.1 deraadt 139: and the public key in
1.2 deraadt 140: .Pa \&.ssh/identity.pub
1.1 deraadt 141: in the user's home directory. The user should then
142: copy the
1.2 deraadt 143: .Pa identity.pub
1.1 deraadt 144: to
1.2 deraadt 145: .Pa \&.ssh/authorized_keys
1.1 deraadt 146: in his/her home directory on the remote machine (the
1.2 deraadt 147: .Pa authorized_keys
1.1 deraadt 148: file corresponds to the conventional
1.2 deraadt 149: .Pa \&.rhosts
1.1 deraadt 150: file, and has one key
151: per line, though the lines can be very long). After this, the user
152: can log in without giving the password. RSA authentication is much
153: more secure than rhosts authentication.
1.2 deraadt 154: .Pp
1.1 deraadt 155: The most convenient way to use RSA authentication may be with an
156: authentication agent. See
1.2 deraadt 157: .Xr ssh-agent 1
1.1 deraadt 158: for more information.
1.2 deraadt 159: .Pp
1.1 deraadt 160: If other authentication methods fail,
1.2 deraadt 161: .Nm
1.1 deraadt 162: prompts the user for a password. The password is sent to the remote
163: host for checking; however, since all communications are encrypted,
164: the password cannot be seen by someone listening on the network.
1.2 deraadt 165: .Pp
1.1 deraadt 166: When the user's identity has been accepted by the server, the server
167: either executes the given command, or logs into the machine and gives
168: the user a normal shell on the remote machine. All communication with
169: the remote command or shell will be automatically encrypted.
1.2 deraadt 170: .Pp
1.1 deraadt 171: If a pseudo-terminal has been allocated (normal login session), the
1.2 deraadt 172: user can disconnect with
173: .Ic ~. ,
174: and suspend
175: .Nm
176: with
177: .Ic ~^Z .
178: All forwarded connections can be listed with
179: .Ic ~#
180: and if
1.1 deraadt 181: the session blocks waiting for forwarded X11 or TCP/IP
1.2 deraadt 182: connections to terminate, it can be backgrounded with
183: .Ic ~&
184: (this should not be used while the user shell is active, as it can cause the
185: shell to hang). All available escapes can be listed with
186: .Ic ~? .
187: .Pp
188: A single tilde character can be sent as
189: .Ic ~~
190: (or by following the tilde by a character other than those described above).
1.1 deraadt 191: The escape character must always follow a newline to be interpreted as
192: special. The escape character can be changed in configuration files
193: or on the command line.
1.2 deraadt 194: .Pp
1.1 deraadt 195: If no pseudo tty has been allocated, the
196: session is transparent and can be used to reliably transfer binary
1.2 deraadt 197: data. On most systems, setting the escape character to
198: .Dq none
199: will also make the session transparent even if a tty is used.
200: .Pp
1.1 deraadt 201: The session terminates when the command or shell in on the remote
202: machine exists and all X11 and TCP/IP connections have been closed.
203: The exit status of the remote program is returned as the exit status
204: of
1.2 deraadt 205: .Nm ssh .
206: .Pp
1.1 deraadt 207: If the user is using X11 (the
1.2 deraadt 208: .Ev DISPLAY
1.1 deraadt 209: environment variable is set), the connection to the X11 display is
210: automatically forwarded to the remote side in such a way that any X11
211: programs started from the shell (or command) will go through the
212: encrypted channel, and the connection to the real X server will be made
213: from the local machine. The user should not manually set
1.2 deraadt 214: .Ev DISPLAY .
1.1 deraadt 215: Forwarding of X11 connections can be
216: configured on the command line or in configuration files.
1.2 deraadt 217: .Pp
218: The
219: .Ev DISPLAY
220: value set by
221: .Nm
1.1 deraadt 222: will point to the server machine, but with a display number greater
223: than zero. This is normal, and happens because
1.2 deraadt 224: .Nm
225: creates a
226: .Dq proxy
227: X server on the server machine for forwarding the
1.1 deraadt 228: connections over the encrypted channel.
1.2 deraadt 229: .Pp
230: .Nm
1.1 deraadt 231: will also automatically set up Xauthority data on the server machine.
232: For this purpose, it will generate a random authorization cookie,
233: store it in Xauthority on the server, and verify that any forwarded
234: connections carry this cookie and replace it by the real cookie when
235: the connection is opened. The real authentication cookie is never
236: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 237: .Pp
1.1 deraadt 238: If the user is using an authentication agent, the connection to the agent
239: is automatically forwarded to the remote side unless disabled on
240: command line or in a configuration file.
1.2 deraadt 241: .Pp
1.1 deraadt 242: Forwarding of arbitrary TCP/IP connections over the secure channel can
243: be specified either on command line or in a configuration file. One
244: possible application of TCP/IP forwarding is a secure connection to an
245: electronic purse; another is going trough firewalls.
1.2 deraadt 246: .Pp
247: .Nm
1.1 deraadt 248: automatically maintains and checks a database containing RSA-based
249: identifications for all hosts it has ever been used with. The
250: database is stored in
1.2 deraadt 251: .Pa \&.ssh/known_hosts
1.1 deraadt 252: in the user's home directory. Additionally, the file
1.2 deraadt 253: .Pa /etc/ssh_known_hosts
1.1 deraadt 254: is automatically checked for known hosts. Any new hosts are
255: automatically added to the user's file. If a host's identification
256: ever changes,
1.2 deraadt 257: .Nm
1.1 deraadt 258: warns about this and disables password authentication to prevent a
259: trojan horse from getting the user's password. Another purpose of
260: this mechanism is to prevent man-in-the-middle attacks which could
261: otherwise be used to circumvent the encryption. The
1.2 deraadt 262: .Cm StrictHostKeyChecking
1.1 deraadt 263: option (see below) can be used to prevent logins to machines whose
264: host key is not known or has changed.
1.2 deraadt 265: .Sh OPTIONS
266: .Bl -tag -width Ds
1.4 dugsong 267: .It Fl a
268: Disables forwarding of the authentication agent connection. This may
269: also be specified on a per-host basis in the configuration file.
1.6 deraadt 270: .It Fl c Ar blowfish|3des
1.1 deraadt 271: Selects the cipher to use for encrypting the session.
1.2 deraadt 272: .Ar 3des
1.1 deraadt 273: is used by default. It is believed to be secure.
1.5 deraadt 274: .Ar 3des
275: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
276: It is presumably more secure than the
1.2 deraadt 277: .Ar des
1.5 deraadt 278: cipher which is no longer supported in ssh.
279: .Ar blowfish
280: is a fast block cipher, it appears very secure and is much faster than
281: .Ar 3des .
1.2 deraadt 282: .It Fl e Ar ch|^ch|none
283: Sets the escape character for sessions with a pty (default:
284: .Ql ~ ) .
285: The escape character is only recognized at the beginning of a line. The
286: escape character followed by a dot
287: .Pq Ql \&.
288: closes the connection, followed
1.1 deraadt 289: by control-Z suspends the connection, and followed by itself sends the
1.2 deraadt 290: escape character once. Setting the character to
291: .Dq none
292: disables any escapes and makes the session fully transparent.
293: .It Fl f
294: Requests
295: .Nm
296: to go to background after authentication. This is useful
297: if
298: .Nm
299: is going to ask for passwords or passphrases, but the user
1.1 deraadt 300: wants it in the background. This implies
1.2 deraadt 301: .Fl n .
1.1 deraadt 302: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 303: something like
304: .Ic ssh -f host xterm .
305: .It Fl i Ar identity_file
1.1 deraadt 306: Selects the file from which the identity (private key) for
1.2 deraadt 307: RSA authentication is read. Default is
308: .Pa \&.ssh/identity
1.1 deraadt 309: in the user's home directory. Identity files may also be specified on
310: a per-host basis in the configuration file. It is possible to have
1.2 deraadt 311: multiple
312: .Fl i
313: options (and multiple identities specified in
1.1 deraadt 314: configuration files).
1.3 deraadt 315: .It Fl g
316: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 317: .It Fl k
1.4 dugsong 318: Disables forwarding of Kerberos tickets and AFS tokens. This may
1.1 deraadt 319: also be specified on a per-host basis in the configuration file.
1.2 deraadt 320: .It Fl l Ar login_name
1.1 deraadt 321: Specifies the user to log in as on the remote machine. This may also
322: be specified on a per-host basis in the configuration file.
1.2 deraadt 323: .It Fl n
324: Redirects stdin from
325: .Pa /dev/null
326: (actually, prevents reading from stdin).
1.1 deraadt 327: This must be used when
1.2 deraadt 328: .Nm
1.1 deraadt 329: is run in the background. A common trick is to use this to run X11
1.2 deraadt 330: programs in a remote machine. For example,
331: .Ic ssh -n shadows.cs.hut.fi emacs &
332: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 333: connection will be automatically forwarded over an encrypted channel.
334: The
1.2 deraadt 335: .Nm
1.1 deraadt 336: program will be put in the background.
337: (This does not work if
1.2 deraadt 338: .Nm
339: needs to ask for a password or passphrase; see also the
340: .Fl f
341: option.)
342: .It Fl o Ar option
1.1 deraadt 343: Can be used to give options in the format used in the config file.
344: This is useful for specifying options for which there is no separate
345: command-line flag. The option has the same format as a line in the
346: configuration file.
1.2 deraadt 347: .It Fl p Ar port
1.1 deraadt 348: Port to connect to on the remote host. This can be specified on a
349: per-host basis in the configuration file.
1.16 markus 350: .It Fl P
351: Use a non-privileged port for outgoing connections.
352: This can be used if your firewall does
353: not permit connections from privileged ports.
354: Note that this option turns of
355: .Cm RhostsAuthentication
356: and
357: .Cm RhostsRSAAuthentication .
1.2 deraadt 358: .It Fl q
1.1 deraadt 359: Quiet mode. Causes all warning and diagnostic messages to be
360: suppressed. Only fatal errors are displayed.
1.2 deraadt 361: .It Fl t
1.1 deraadt 362: Force pseudo-tty allocation. This can be used to execute arbitary
363: screen-based programs on a remote machine, which can be very useful
364: e.g. when implementing menu services.
1.2 deraadt 365: .It Fl v
1.1 deraadt 366: Verbose mode. Causes
1.2 deraadt 367: .Nm
1.1 deraadt 368: to print debugging messages about its progress. This is helpful in
369: debugging connection, authentication, and configuration problems.
1.22 provos 370: The verbose mode is also used to display
371: .Xr skey 1
372: challenges, if the user entered "s/key" as password.
1.2 deraadt 373: .It Fl x
1.1 deraadt 374: Disables X11 forwarding. This can also be specified on a per-host
375: basis in a configuration file.
1.2 deraadt 376: .It Fl X
1.1 deraadt 377: Enables X11 forwarding.
1.2 deraadt 378: .It Fl C
1.1 deraadt 379: Requests compression of all data (including stdin, stdout, stderr, and
380: data for forwarded X11 and TCP/IP connections). The compression
1.2 deraadt 381: algorithm is the same used by gzip, and the
382: .Dq level
383: can be controlled by the
384: .Cm CompressionLevel
1.1 deraadt 385: option (see below). Compression is desirable on modem lines and other
386: slow connections, but will only slow down things on fast networks.
387: The default value can be set on a host-by-host basis in the
388: configuration files; see the
1.2 deraadt 389: .Cm Compress
1.1 deraadt 390: option below.
1.2 deraadt 391: .It Fl L Ar port:host:hostport
1.1 deraadt 392: Specifies that the given port on the local (client) host is to be
393: forwarded to the given host and port on the remote side. This works
394: by allocating a socket to listen to
1.2 deraadt 395: .Ar port
1.1 deraadt 396: on the local side, and whenever a connection is made to this port, the
397: connection is forwarded over the secure channel, and a connection is
398: made to
1.2 deraadt 399: .Ar host:hostport
1.1 deraadt 400: from the remote machine. Port forwardings can also be specified in the
401: configuration file. Only root can forward privileged ports.
1.2 deraadt 402: .It Fl R Ar port:host:hostport
1.1 deraadt 403: Specifies that the given port on the remote (server) host is to be
404: forwarded to the given host and port on the local side. This works
405: by allocating a socket to listen to
1.2 deraadt 406: .Ar port
1.1 deraadt 407: on the remote side, and whenever a connection is made to this port, the
408: connection is forwarded over the secure channel, and a connection is
409: made to
1.2 deraadt 410: .Ar host:hostport
1.1 deraadt 411: from the local machine. Port forwardings can also be specified in the
412: configuration file. Privileged ports can be forwarded only when
413: logging in as root on the remote machine.
1.2 deraadt 414: .El
415: .Sh CONFIGURATION FILES
416: .Nm
1.1 deraadt 417: obtains configuration data from the following sources (in this order):
418: command line options, user's configuration file
1.2 deraadt 419: .Pq Pa $HOME/.ssh/config ,
420: and system-wide configuration file
421: .Pq Pa /etc/ssh_config .
422: For each parameter, the first obtained value
1.1 deraadt 423: will be used. The configuration files contain sections bracketed by
424: "Host" specifications, and that section is only applied for hosts that
425: match one of the patterns given in the specification. The matched
426: host name is the one given on the command line.
1.2 deraadt 427: .Pp
1.1 deraadt 428: Since the first obtained value for each parameter is used, more
429: host-specific declarations should be given near the beginning of the
430: file, and general defaults at the end.
1.2 deraadt 431: .Pp
1.1 deraadt 432: The configuration file has the following format:
1.2 deraadt 433: .Pp
434: Empty lines and lines starting with
435: .Ql #
436: are comments.
437: .Pp
438: Otherwise a line is of the format
439: .Dq keyword arguments .
440: The possible
1.1 deraadt 441: keywords and their meanings are as follows (note that the
442: configuration files are case-sensitive):
1.2 deraadt 443: .Bl -tag -width Ds
444: .It Cm Host
1.1 deraadt 445: Restricts the following declarations (up to the next
1.2 deraadt 446: .Cm Host
1.1 deraadt 447: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 448: given after the keyword.
449: .Ql \&*
450: and
451: .Ql ?
452: can be used as wildcards in the
453: patterns. A single
454: .Ql \&*
455: as a pattern can be used to provide global
1.1 deraadt 456: defaults for all hosts. The host is the
1.2 deraadt 457: .Ar hostname
1.1 deraadt 458: argument given on the command line (i.e., the name is not converted to
459: a canonicalized host name before matching).
1.2 deraadt 460: .It Cm AFSTokenPassing
1.1 deraadt 461: Specifies whether to pass AFS tokens to remote host. The argument to
462: this keyword must be
1.2 deraadt 463: .Dq yes
464: or
465: .Dq no .
466: .It Cm BatchMode
467: If set to
468: .Dq yes ,
469: passphrase/password querying will be disabled. This
1.1 deraadt 470: option is useful in scripts and other batch jobs where you have no
471: user to supply the password. The argument must be
1.2 deraadt 472: .Dq yes
473: or
474: .Dq no .
475: .It Cm Cipher
1.1 deraadt 476: Specifies the cipher to use for encrypting the session. Currently,
1.2 deraadt 477: .Dq blowfish ,
1.1 deraadt 478: and
1.10 provos 479: .Dq 3des
1.2 deraadt 480: are supported. The default is
481: .Dq 3des .
482: .It Cm Compression
1.1 deraadt 483: Specifies whether to use compression. The argument must be
1.2 deraadt 484: .Dq yes
485: or
486: .Dq no .
487: .It Cm CompressionLevel
1.1 deraadt 488: Specifies the compression level to use if compression is enable. The
489: argument must be an integer from 1 (fast) to 9 (slow, best). The
490: default level is 6, which is good for most applications. The meaning
491: of the values is the same as in GNU GZIP.
1.2 deraadt 492: .It Cm ConnectionAttempts
1.1 deraadt 493: Specifies the number of tries (one per second) to make before falling
494: back to rsh or exiting. The argument must be an integer. This may be
495: useful in scripts if the connection sometimes fails.
1.2 deraadt 496: .It Cm EscapeChar
497: Sets the escape character (default:
498: .Ql ~ ) .
499: The escape character can also
1.1 deraadt 500: be set on the command line. The argument should be a single
1.2 deraadt 501: character,
502: .Ql ^
503: followed by a letter, or
504: .Dq none
505: to disable the escape
1.1 deraadt 506: character entirely (making the connection transparent for binary
507: data).
1.2 deraadt 508: .It Cm FallBackToRsh
1.1 deraadt 509: Specifies that if connecting via
1.2 deraadt 510: .Nm
1.1 deraadt 511: fails due to a connection refused error (there is no
1.2 deraadt 512: .Xr sshd 8
1.1 deraadt 513: listening on the remote host),
1.2 deraadt 514: .Xr rsh 1
1.1 deraadt 515: should automatically be used instead (after a suitable warning about
516: the session being unencrypted). The argument must be
1.2 deraadt 517: .Dq yes
518: or
519: .Dq no .
520: .It Cm ForwardAgent
1.1 deraadt 521: Specifies whether the connection to the authentication agent (if any)
522: will be forwarded to the remote machine. The argument must be
1.2 deraadt 523: .Dq yes
524: or
525: .Dq no .
526: .It Cm ForwardX11
1.1 deraadt 527: Specifies whether X11 connections will be automatically redirected
528: over the secure channel and
1.2 deraadt 529: .Ev DISPLAY
1.1 deraadt 530: set. The argument must be
1.2 deraadt 531: .Dq yes
532: or
1.3 deraadt 533: .Dq no .
534: .It Cm GatewayPorts
535: Specifies whether remote hosts are allowed to connect to local
536: forwarded ports.
537: The argument must be
538: .Dq yes
539: or
540: .Dq no .
541: The default is
1.2 deraadt 542: .Dq no .
543: .It Cm GlobalKnownHostsFile
1.1 deraadt 544: Specifies a file to use instead of
1.2 deraadt 545: .Pa /etc/ssh_known_hosts .
546: .It Cm HostName
1.1 deraadt 547: Specifies the real host name to log into. This can be used to specify
548: nicnames or abbreviations for hosts. Default is the name given on the
549: command line. Numeric IP addresses are also permitted (both on the
550: command line and in
1.2 deraadt 551: .Cm HostName
1.1 deraadt 552: specifications).
1.2 deraadt 553: .It Cm IdentityFile
1.1 deraadt 554: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 555: is read (default
556: .Pa .ssh/identity
557: in the user's home directory).
1.1 deraadt 558: Additionally, any identities represented by the authentication agent
559: will be used for authentication. The file name may use the tilde
560: syntax to refer to a user's home directory. It is possible to have
561: multiple identity files specified in configuration files; all these
562: identities will be tried in sequence.
1.2 deraadt 563: .It Cm KeepAlive
1.1 deraadt 564: Specifies whether the system should send keepalive messages to the
565: other side. If they are sent, death of the connection or crash of one
566: of the machines will be properly noticed. However, this means that
567: connections will die if the route is down temporarily, and some people
568: find it annoying.
1.2 deraadt 569: .Pp
570: The default is
571: .Dq yes
572: (to send keepalives), and the client will notice
1.1 deraadt 573: if the network goes down or the remote host dies. This is important
574: in scripts, and many users want it too.
1.2 deraadt 575: .Pp
576: To disable keepalives, the value should be set to
577: .Dq no
578: in both the server and the client configuration files.
579: .It Cm KerberosAuthentication
1.4 dugsong 580: Specifies whether Kerberos authentication will be used. The argument to
581: this keyword must be
582: .Dq yes
583: or
584: .Dq no .
1.2 deraadt 585: .It Cm KerberosTgtPassing
1.4 dugsong 586: Specifies whether a Kerberos TGT will be forwarded to the server. This
587: will only work if the Kerberos server is actually an AFS kaserver. The
588: argument to this keyword must be
589: .Dq yes
590: or
591: .Dq no .
1.2 deraadt 592: .It Cm LocalForward
1.1 deraadt 593: Specifies that a TCP/IP port on the local machine be forwarded over
594: the secure channel to given host:port from the remote machine. The
595: first argument must be a port number, and the second must be
596: host:port. Multiple forwardings may be specified, and additional
597: forwardings can be given on the command line. Only the root can
598: forward privileged ports.
1.2 deraadt 599: .It Cm PasswordAuthentication
1.1 deraadt 600: Specifies whether to use password authentication. The argument to
601: this keyword must be
1.2 deraadt 602: .Dq yes
603: or
604: .Dq no .
1.24 markus 605: .It Cm LogLevel
606: Gives the verbosity level that is used when logging messages from
607: .Nm ssh .
608: The possible values are:
609: QUIET, FATAL, ERROR, INFO, CHAT and DEBUG.
610: The default is INFO.
1.14 dugsong 611: .It Cm NumberOfPasswordPrompts
612: Specifies the number of password prompts before giving up. The
613: argument to this keyword must be an integer. Default is 3.
1.2 deraadt 614: .It Cm Port
1.1 deraadt 615: Specifies the port number to connect on the remote host. Default is
616: 22.
1.2 deraadt 617: .It Cm ProxyCommand
1.1 deraadt 618: Specifies the command to use to connect to the server. The command
619: string extends to the end of the line, and is executed with /bin/sh.
620: In the command string, %h will be substituted by the host name to
621: connect and %p by the port. The command can be basically anything,
622: and should read from its stdin and write to its stdout. It should
623: eventually connect an
1.2 deraadt 624: .Xr sshd 8
1.1 deraadt 625: server running on some machine, or execute
1.2 deraadt 626: .Ic sshd -i
627: somewhere. Host key management will be done using the
1.1 deraadt 628: HostName of the host being connected (defaulting to the name typed by
629: the user).
1.2 deraadt 630: .Pp
631: .It Cm RemoteForward
1.1 deraadt 632: Specifies that a TCP/IP port on the remote machine be forwarded over
633: the secure channel to given host:port from the local machine. The
634: first argument must be a port number, and the second must be
635: host:port. Multiple forwardings may be specified, and additional
636: forwardings can be given on the command line. Only the root can
637: forward privileged ports.
1.2 deraadt 638: .It Cm RhostsAuthentication
1.1 deraadt 639: Specifies whether to try rhosts based authentication. Note that this
640: declaration only affects the client side and has no effect whatsoever
641: on security. Disabling rhosts authentication may reduce
642: authentication time on slow connections when rhosts authentication is
643: not used. Most servers do not permit RhostsAuthentication because it
644: is not secure (see RhostsRSAAuthentication). The argument to this
645: keyword must be
1.2 deraadt 646: .Dq yes
647: or
648: .Dq no .
649: .It Cm RhostsRSAAuthentication
1.1 deraadt 650: Specifies whether to try rhosts based authentication with RSA host
651: authentication. This is the primary authentication method for most
652: sites. The argument must be
1.2 deraadt 653: .Dq yes
654: or
655: .Dq no .
656: .It Cm RSAAuthentication
1.1 deraadt 657: Specifies whether to try RSA authentication. The argument to this
658: keyword must be
1.2 deraadt 659: .Dq yes
660: or
661: .Dq no .
1.1 deraadt 662: RSA authentication will only be
663: attempted if the identity file exists, or an authentication agent is
664: running.
1.27 ! markus 665: .It Cm SkeyAuthentication
! 666: Specifies whether to use
! 667: .Xr skey 1
! 668: authentication. The argument to
! 669: this keyword must be
! 670: .Dq yes
! 671: or
! 672: .Dq no .
! 673: The default is
! 674: .Dq no .
1.13 provos 675: .It Cm CheckHostIP
676: If this flag is set to
677: .Dq yes ,
678: ssh will additionally check the host ip address in the
679: .Pa known_hosts
680: file. This allows ssh to detect if a host key changed due to DNS spoofing.
681: If the option is set to
682: .Dq no ,
683: the check will not be executed.
1.2 deraadt 684: .It Cm StrictHostKeyChecking
685: If this flag is set to
686: .Dq yes ,
687: .Nm
1.1 deraadt 688: ssh will never automatically add host keys to the
1.2 deraadt 689: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 690: file, and refuses to connect hosts whose host key has changed. This
691: provides maximum protection against trojan horse attacks. However, it
692: can be somewhat annoying if you don't have good
1.2 deraadt 693: .Pa /etc/ssh_known_hosts
1.1 deraadt 694: files installed and frequently
695: connect new hosts. Basically this option forces the user to manually
696: add any new hosts. Normally this option is disabled, and new hosts
697: will automatically be added to the known host files. The host keys of
698: known hosts will be verified automatically in either case. The
699: argument must be
1.2 deraadt 700: .Dq yes
701: or
702: .Dq no .
703: .It Cm User
1.1 deraadt 704: Specifies the user to log in as. This can be useful if you have a
705: different user name in different machines. This saves the trouble of
706: having to remember to give the user name on the command line.
1.2 deraadt 707: .It Cm UserKnownHostsFile
708: Specifies a file to use instead of
709: .Pa $HOME/.ssh/known_hosts .
1.16 markus 710: .It Cm UsePrivilegedPort
711: Specifies whether to use a privileged port for outgoing connections.
712: The argument must be
713: .Dq yes
714: or
715: .Dq no .
716: The default is
717: .Dq yes .
718: Note that setting this option to
719: .Dq no
720: turns of
721: .Cm RhostsAuthentication
722: and
723: .Cm RhostsRSAAuthentication .
1.2 deraadt 724: .It Cm UseRsh
1.1 deraadt 725: Specifies that rlogin/rsh should be used for this host. It is
726: possible that the host does not at all support the
1.2 deraadt 727: .Nm
1.1 deraadt 728: protocol. This causes
1.2 deraadt 729: .Nm
1.1 deraadt 730: to immediately exec
1.2 deraadt 731: .Xr rsh 1 .
1.1 deraadt 732: All other options (except
1.2 deraadt 733: .Cm HostName )
1.1 deraadt 734: are ignored if this has been specified. The argument must be
1.2 deraadt 735: .Dq yes
736: or
737: .Dq no .
738: .Sh ENVIRONMENT
739: .Nm
1.1 deraadt 740: will normally set the following environment variables:
1.2 deraadt 741: .Bl -tag -width Ds
742: .It Ev DISPLAY
743: The
744: .Ev DISPLAY
745: variable indicates the location of the X11 server. It is
1.1 deraadt 746: automatically set by
1.2 deraadt 747: .Nm
748: to point to a value of the form
749: .Dq hostname:n
750: where hostname indicates
1.1 deraadt 751: the host where the shell runs, and n is an integer >= 1. Ssh uses
752: this special value to forward X11 connections over the secure
753: channel. The user should normally not set DISPLAY explicitly, as that
754: will render the X11 connection insecure (and will require the user to
755: manually copy any required authorization cookies).
1.2 deraadt 756: .It Ev HOME
1.1 deraadt 757: Set to the path of the user's home directory.
1.2 deraadt 758: .It Ev LOGNAME
759: Synonym for
1.12 aaron 760: .Ev USER ;
761: set for compatibility with systems that use this variable.
1.2 deraadt 762: .It Ev MAIL
1.1 deraadt 763: Set to point the user's mailbox.
1.2 deraadt 764: .It Ev PATH
765: Set to the default
766: .Ev PATH ,
767: as specified when compiling
1.12 aaron 768: .Nm ssh .
1.18 markus 769: .It Ev SSH_AUTH_SOCK
1.17 markus 770: indicates the path of a unix-domain socket used to communicate with the
771: agent.
1.2 deraadt 772: .It Ev SSH_CLIENT
1.1 deraadt 773: Identifies the client end of the connection. The variable contains
774: three space-separated values: client ip-address, client port number,
775: and server port number.
1.2 deraadt 776: .It Ev SSH_TTY
1.1 deraadt 777: This is set to the name of the tty (path to the device) associated
778: with the current shell or command. If the current session has no tty,
779: this variable is not set.
1.2 deraadt 780: .It Ev TZ
1.1 deraadt 781: The timezone variable is set to indicate the present timezone if it
782: was set when the daemon was started (e.i., the daemon passes the value
783: on to new connections).
1.2 deraadt 784: .It Ev USER
1.1 deraadt 785: Set to the name of the user logging in.
1.2 deraadt 786: .El
787: .Pp
1.1 deraadt 788: Additionally,
1.2 deraadt 789: .Nm
1.1 deraadt 790: reads
1.2 deraadt 791: .Pa $HOME/.ssh/environment ,
792: and adds lines of the format
793: .Dq VARNAME=value
1.12 aaron 794: to the environment.
1.2 deraadt 795: .Sh FILES
796: .Bl -tag -width $HOME/.ssh/known_hosts
797: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 798: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 799: in
800: .Pa /etc/ssh_known_hosts ) .
801: See
802: .Xr sshd 8 .
803: .It Pa $HOME/.ssh/random_seed
1.1 deraadt 804: Used for seeding the random number generator. This file contains
805: sensitive data and should read/write for the user and not accessible
806: for others. This file is created the first time the program is run
807: and updated automatically. The user should never need to read or
808: modify this file.
1.2 deraadt 809: .It Pa $HOME/.ssh/identity
1.1 deraadt 810: Contains the RSA authentication identity of the user. This file
811: contains sensitive data and should be readable by the user but not
1.15 markus 812: accessible by others (read/write/execute).
813: Note that
814: .Nm
815: ignores this file if it is accessible by others.
816: It is possible to specify a passphrase when
1.1 deraadt 817: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 818: sensitive part of this file using 3DES.
1.2 deraadt 819: .It Pa $HOME/.ssh/identity.pub
1.1 deraadt 820: Contains the public key for authentication (public part of the
821: identity file in human-readable form). The contents of this file
1.2 deraadt 822: should be added to
823: .Pa $HOME/.ssh/authorized_keys
824: on all machines
1.1 deraadt 825: where you wish to log in using RSA authentication. This file is not
826: sensitive and can (but need not) be readable by anyone. This file is
827: never used automatically and is not necessary; it is only provided for
828: the convenience of the user.
1.2 deraadt 829: .It Pa $HOME/.ssh/config
1.1 deraadt 830: This is the per-user configuration file. The format of this file is
831: described above. This file is used by the
1.2 deraadt 832: .Nm
1.1 deraadt 833: client. This file does not usually contain any sensitive information,
834: but the recommended permissions are read/write for the user, and not
835: accessible by others.
1.2 deraadt 836: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 837: Lists the RSA keys that can be used for logging in as this user. The
838: format of this file is described in the
1.2 deraadt 839: .Xr sshd 8
1.1 deraadt 840: manual page. In the simplest form the format is the same as the .pub
841: identity files (that is, each line contains the number of bits in
842: modulus, public exponent, modulus, and comment fields, separated by
843: spaces). This file is not highly sensitive, but the recommended
844: permissions are read/write for the user, and not accessible by others.
1.2 deraadt 845: .It Pa /etc/ssh_known_hosts
1.1 deraadt 846: Systemwide list of known host keys. This file should be prepared by the
847: system administrator to contain the public host keys of all machines in the
848: organization. This file should be world-readable. This file contains
849: public keys, one per line, in the following format (fields separated
850: by spaces): system name, number of bits in modulus, public exponent,
851: modulus, and optional comment field. When different names are used
852: for the same machine, all such names should be listed, separated by
853: commas. The format is described on the
1.2 deraadt 854: .Xr sshd 8
1.1 deraadt 855: manual page.
1.2 deraadt 856: .Pp
1.1 deraadt 857: The canonical system name (as returned by name servers) is used by
1.2 deraadt 858: .Xr sshd 8
1.1 deraadt 859: to verify the client host when logging in; other names are needed because
1.2 deraadt 860: .Nm
1.1 deraadt 861: does not convert the user-supplied name to a canonical name before
862: checking the key, because someone with access to the name servers
863: would then be able to fool host authentication.
1.2 deraadt 864: .It Pa /etc/ssh_config
1.1 deraadt 865: Systemwide configuration file. This file provides defaults for those
866: values that are not specified in the user's configuration file, and
867: for those users who do not have a configuration file. This file must
868: be world-readable.
1.2 deraadt 869: .It Pa $HOME/.rhosts
870: This file is used in
871: .Pa \&.rhosts
872: authentication to list the
1.1 deraadt 873: host/user pairs that are permitted to log in. (Note that this file is
874: also used by rlogin and rsh, which makes using this file insecure.)
875: Each line of the file contains a host name (in the canonical form
876: returned by name servers), and then a user name on that host,
877: separated by a space. One some machines this file may need to be
878: world-readable if the user's home directory is on a NFS partition,
879: because
1.2 deraadt 880: .Xr sshd 8
1.1 deraadt 881: reads it as root. Additionally, this file must be owned by the user,
882: and must not have write permissions for anyone else. The recommended
883: permission for most machines is read/write for the user, and not
884: accessible by others.
1.2 deraadt 885: .Pp
1.1 deraadt 886: Note that by default
1.2 deraadt 887: .Xr sshd 8
1.1 deraadt 888: will be installed so that it requires successful RSA host
889: authentication before permitting \s+2.\s0rhosts authentication. If your
890: server machine does not have the client's host key in
1.2 deraadt 891: .Pa /etc/ssh_known_hosts ,
892: you can store it in
893: .Pa $HOME/.ssh/known_hosts .
894: The easiest way to do this is to
1.1 deraadt 895: connect back to the client from the server machine using ssh; this
1.2 deraadt 896: will automatically add the host key inxi
897: .Pa $HOME/.ssh/known_hosts .
898: .It Pa $HOME/.shosts
899: This file is used exactly the same way as
900: .Pa \&.rhosts .
901: The purpose for
1.1 deraadt 902: having this file is to be able to use rhosts authentication with
1.2 deraadt 903: .Nm
904: without permitting login with
905: .Xr rlogin 1
906: or
907: .Xr rsh 1 .
908: .It Pa /etc/hosts.equiv
909: This file is used during
910: .Pa \&.rhosts authentication. It contains
1.1 deraadt 911: canonical hosts names, one per line (the full format is described on
912: the
1.2 deraadt 913: .Xr sshd 8
1.1 deraadt 914: manual page). If the client host is found in this file, login is
915: automatically permitted provided client and server user names are the
916: same. Additionally, successful RSA host authentication is normally
917: required. This file should only be writable by root.
1.2 deraadt 918: .It Pa /etc/shosts.equiv
1.1 deraadt 919: This file is processed exactly as
1.2 deraadt 920: .Pa /etc/hosts.equiv .
1.1 deraadt 921: This file may be useful to permit logins using
1.2 deraadt 922: .Nm
1.1 deraadt 923: but not using rsh/rlogin.
1.2 deraadt 924: .It Pa /etc/sshrc
1.1 deraadt 925: Commands in this file are executed by
1.2 deraadt 926: .Nm
1.1 deraadt 927: when the user logs in just before the user's shell (or command) is started.
928: See the
1.2 deraadt 929: .Xr sshd 8
1.1 deraadt 930: manual page for more information.
1.2 deraadt 931: .It Pa $HOME/.ssh/rc
1.1 deraadt 932: Commands in this file are executed by
1.2 deraadt 933: .Nm
1.1 deraadt 934: when the user logs in just before the user's shell (or command) is
935: started.
936: See the
1.2 deraadt 937: .Xr sshd 8
1.1 deraadt 938: manual page for more information.
1.5 deraadt 939: .It Pa libcrypto.so.X.1
940: A version of this library which includes support for the RSA algorithm
941: is required for proper operation.
1.2 deraadt 942: .Sh AUTHOR
1.1 deraadt 943: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 944: .Pp
945: Issues can be found from the SSH WWW home page:
946: .Pp
947: .Dl http://www.cs.hut.fi/ssh
1.5 deraadt 948: .Pp
1.20 provos 949: OpenSSH
1.21 deraadt 950: is a derivative of the original (free) ssh 1.2.12 release, but with bugs
951: removed and newer features re-added. Rapidly after the 1.2.12 release,
952: newer versions bore successively more restrictive licenses. This version
953: of OpenSSH
1.20 provos 954: .Bl -bullet
955: .It
1.21 deraadt 956: has all components of a restrictive nature (ie. patents, see
957: .Xr ssl 8 )
958: directly removed from the source code; any licensed or patented components
959: are chosen from
960: external libraries.
1.20 provos 961: .It
1.21 deraadt 962: has been updated to support ssh protocol 1.5.
1.20 provos 963: .It
964: contains added support for
965: .Xr kerberos 8
966: authentication and ticket passing.
967: .It
1.21 deraadt 968: supports one-time password authentication with
1.20 provos 969: .Xr skey 1 .
970: .El
971: .Pp
972: The libraries described in
1.5 deraadt 973: .Xr ssl 8
974: are required for proper operation.
1.25 provos 975: .Pp
1.26 aaron 976: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1.25 provos 977: Niels Provos, Theo de Raadt, and Dug Song.
1.2 deraadt 978: .Sh SEE ALSO
979: .Xr rlogin 1 ,
980: .Xr rsh 1 ,
981: .Xr scp 1 ,
982: .Xr ssh-add 1 ,
983: .Xr ssh-agent 1 ,
984: .Xr ssh-keygen 1 ,
985: .Xr telnet 1 ,
1.5 deraadt 986: .Xr sshd 8 ,
987: .Xr ssl 8
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