Annotation of src/usr.bin/ssh/ssh.1, Revision 1.32
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: .\"
1.32 ! markus 12: .\" $Id: ssh.1,v 1.31 1999/12/25 13:35:55 markus 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
1.32 ! markus 27: .Op Fl afgknqtvxCPX46
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
1.28 markus 296: to go to background just before command execution. This is useful
1.2 deraadt 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.
1.30 provos 354: Note that this option turns off
1.16 markus 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
1.32 ! markus 399: .Ar host
! 400: port
! 401: .Ar hostport
1.1 deraadt 402: from the remote machine. Port forwardings can also be specified in the
403: configuration file. Only root can forward privileged ports.
1.32 ! markus 404: IPv6 addresses can be specified with an alternative syntax:
! 405: .Ar port/host/hostport
1.2 deraadt 406: .It Fl R Ar port:host:hostport
1.1 deraadt 407: Specifies that the given port on the remote (server) host is to be
408: forwarded to the given host and port on the local side. This works
409: by allocating a socket to listen to
1.2 deraadt 410: .Ar port
1.1 deraadt 411: on the remote side, and whenever a connection is made to this port, the
412: connection is forwarded over the secure channel, and a connection is
413: made to
1.32 ! markus 414: .Ar host
! 415: port
! 416: .Ar hostport
1.1 deraadt 417: from the local machine. Port forwardings can also be specified in the
418: configuration file. Privileged ports can be forwarded only when
419: logging in as root on the remote machine.
1.32 ! markus 420: .It Fl 4
! 421: Forces
! 422: .Nm
! 423: to use IPv4 addresses only.
! 424: .It Fl 6
! 425: Forces
! 426: .Nm
! 427: to use IPv6 addresses only.
1.2 deraadt 428: .El
429: .Sh CONFIGURATION FILES
430: .Nm
1.1 deraadt 431: obtains configuration data from the following sources (in this order):
432: command line options, user's configuration file
1.2 deraadt 433: .Pq Pa $HOME/.ssh/config ,
434: and system-wide configuration file
435: .Pq Pa /etc/ssh_config .
436: For each parameter, the first obtained value
1.1 deraadt 437: will be used. The configuration files contain sections bracketed by
438: "Host" specifications, and that section is only applied for hosts that
439: match one of the patterns given in the specification. The matched
440: host name is the one given on the command line.
1.2 deraadt 441: .Pp
1.1 deraadt 442: Since the first obtained value for each parameter is used, more
443: host-specific declarations should be given near the beginning of the
444: file, and general defaults at the end.
1.2 deraadt 445: .Pp
1.1 deraadt 446: The configuration file has the following format:
1.2 deraadt 447: .Pp
448: Empty lines and lines starting with
449: .Ql #
450: are comments.
451: .Pp
452: Otherwise a line is of the format
453: .Dq keyword arguments .
454: The possible
1.1 deraadt 455: keywords and their meanings are as follows (note that the
456: configuration files are case-sensitive):
1.2 deraadt 457: .Bl -tag -width Ds
458: .It Cm Host
1.1 deraadt 459: Restricts the following declarations (up to the next
1.2 deraadt 460: .Cm Host
1.1 deraadt 461: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 462: given after the keyword.
463: .Ql \&*
464: and
465: .Ql ?
466: can be used as wildcards in the
467: patterns. A single
468: .Ql \&*
469: as a pattern can be used to provide global
1.1 deraadt 470: defaults for all hosts. The host is the
1.2 deraadt 471: .Ar hostname
1.1 deraadt 472: argument given on the command line (i.e., the name is not converted to
473: a canonicalized host name before matching).
1.2 deraadt 474: .It Cm AFSTokenPassing
1.1 deraadt 475: Specifies whether to pass AFS tokens to remote host. The argument to
476: this keyword must be
1.2 deraadt 477: .Dq yes
478: or
479: .Dq no .
480: .It Cm BatchMode
481: If set to
482: .Dq yes ,
483: passphrase/password querying will be disabled. This
1.1 deraadt 484: option is useful in scripts and other batch jobs where you have no
485: user to supply the password. The argument must be
1.2 deraadt 486: .Dq yes
487: or
488: .Dq no .
489: .It Cm Cipher
1.1 deraadt 490: Specifies the cipher to use for encrypting the session. Currently,
1.2 deraadt 491: .Dq blowfish ,
1.1 deraadt 492: and
1.10 provos 493: .Dq 3des
1.2 deraadt 494: are supported. The default is
495: .Dq 3des .
496: .It Cm Compression
1.1 deraadt 497: Specifies whether to use compression. The argument must be
1.2 deraadt 498: .Dq yes
499: or
500: .Dq no .
501: .It Cm CompressionLevel
1.1 deraadt 502: Specifies the compression level to use if compression is enable. The
503: argument must be an integer from 1 (fast) to 9 (slow, best). The
504: default level is 6, which is good for most applications. The meaning
505: of the values is the same as in GNU GZIP.
1.2 deraadt 506: .It Cm ConnectionAttempts
1.1 deraadt 507: Specifies the number of tries (one per second) to make before falling
508: back to rsh or exiting. The argument must be an integer. This may be
509: useful in scripts if the connection sometimes fails.
1.2 deraadt 510: .It Cm EscapeChar
511: Sets the escape character (default:
512: .Ql ~ ) .
513: The escape character can also
1.1 deraadt 514: be set on the command line. The argument should be a single
1.2 deraadt 515: character,
516: .Ql ^
517: followed by a letter, or
518: .Dq none
519: to disable the escape
1.1 deraadt 520: character entirely (making the connection transparent for binary
521: data).
1.2 deraadt 522: .It Cm FallBackToRsh
1.1 deraadt 523: Specifies that if connecting via
1.2 deraadt 524: .Nm
1.1 deraadt 525: fails due to a connection refused error (there is no
1.2 deraadt 526: .Xr sshd 8
1.1 deraadt 527: listening on the remote host),
1.2 deraadt 528: .Xr rsh 1
1.1 deraadt 529: should automatically be used instead (after a suitable warning about
530: the session being unencrypted). The argument must be
1.2 deraadt 531: .Dq yes
532: or
533: .Dq no .
534: .It Cm ForwardAgent
1.1 deraadt 535: Specifies whether the connection to the authentication agent (if any)
536: will be forwarded to the remote machine. The argument must be
1.2 deraadt 537: .Dq yes
538: or
539: .Dq no .
540: .It Cm ForwardX11
1.1 deraadt 541: Specifies whether X11 connections will be automatically redirected
542: over the secure channel and
1.2 deraadt 543: .Ev DISPLAY
1.1 deraadt 544: set. The argument must be
1.2 deraadt 545: .Dq yes
546: or
1.3 deraadt 547: .Dq no .
548: .It Cm GatewayPorts
549: Specifies whether remote hosts are allowed to connect to local
550: forwarded ports.
551: The argument must be
552: .Dq yes
553: or
554: .Dq no .
555: The default is
1.2 deraadt 556: .Dq no .
557: .It Cm GlobalKnownHostsFile
1.1 deraadt 558: Specifies a file to use instead of
1.2 deraadt 559: .Pa /etc/ssh_known_hosts .
560: .It Cm HostName
1.1 deraadt 561: Specifies the real host name to log into. This can be used to specify
562: nicnames or abbreviations for hosts. Default is the name given on the
563: command line. Numeric IP addresses are also permitted (both on the
564: command line and in
1.2 deraadt 565: .Cm HostName
1.1 deraadt 566: specifications).
1.2 deraadt 567: .It Cm IdentityFile
1.1 deraadt 568: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 569: is read (default
570: .Pa .ssh/identity
571: in the user's home directory).
1.1 deraadt 572: Additionally, any identities represented by the authentication agent
573: will be used for authentication. The file name may use the tilde
574: syntax to refer to a user's home directory. It is possible to have
575: multiple identity files specified in configuration files; all these
576: identities will be tried in sequence.
1.2 deraadt 577: .It Cm KeepAlive
1.1 deraadt 578: Specifies whether the system should send keepalive messages to the
579: other side. If they are sent, death of the connection or crash of one
580: of the machines will be properly noticed. However, this means that
581: connections will die if the route is down temporarily, and some people
582: find it annoying.
1.2 deraadt 583: .Pp
584: The default is
585: .Dq yes
586: (to send keepalives), and the client will notice
1.1 deraadt 587: if the network goes down or the remote host dies. This is important
588: in scripts, and many users want it too.
1.2 deraadt 589: .Pp
590: To disable keepalives, the value should be set to
591: .Dq no
592: in both the server and the client configuration files.
593: .It Cm KerberosAuthentication
1.4 dugsong 594: Specifies whether Kerberos authentication will be used. The argument to
595: this keyword must be
596: .Dq yes
597: or
598: .Dq no .
1.2 deraadt 599: .It Cm KerberosTgtPassing
1.4 dugsong 600: Specifies whether a Kerberos TGT will be forwarded to the server. This
601: will only work if the Kerberos server is actually an AFS kaserver. The
602: argument to this keyword must be
603: .Dq yes
604: or
605: .Dq no .
1.2 deraadt 606: .It Cm LocalForward
1.1 deraadt 607: Specifies that a TCP/IP port on the local machine be forwarded over
608: the secure channel to given host:port from the remote machine. The
609: first argument must be a port number, and the second must be
610: host:port. Multiple forwardings may be specified, and additional
611: forwardings can be given on the command line. Only the root can
612: forward privileged ports.
1.2 deraadt 613: .It Cm PasswordAuthentication
1.1 deraadt 614: Specifies whether to use password authentication. The argument to
615: this keyword must be
1.2 deraadt 616: .Dq yes
617: or
618: .Dq no .
1.24 markus 619: .It Cm LogLevel
620: Gives the verbosity level that is used when logging messages from
621: .Nm ssh .
622: The possible values are:
623: QUIET, FATAL, ERROR, INFO, CHAT and DEBUG.
624: The default is INFO.
1.14 dugsong 625: .It Cm NumberOfPasswordPrompts
626: Specifies the number of password prompts before giving up. The
627: argument to this keyword must be an integer. Default is 3.
1.2 deraadt 628: .It Cm Port
1.1 deraadt 629: Specifies the port number to connect on the remote host. Default is
630: 22.
1.2 deraadt 631: .It Cm ProxyCommand
1.1 deraadt 632: Specifies the command to use to connect to the server. The command
633: string extends to the end of the line, and is executed with /bin/sh.
634: In the command string, %h will be substituted by the host name to
635: connect and %p by the port. The command can be basically anything,
636: and should read from its stdin and write to its stdout. It should
637: eventually connect an
1.2 deraadt 638: .Xr sshd 8
1.1 deraadt 639: server running on some machine, or execute
1.2 deraadt 640: .Ic sshd -i
641: somewhere. Host key management will be done using the
1.1 deraadt 642: HostName of the host being connected (defaulting to the name typed by
643: the user).
1.29 markus 644: Note that
645: .Cm CheckHostIP
646: is not available for connects with a proxy command.
1.2 deraadt 647: .Pp
648: .It Cm RemoteForward
1.1 deraadt 649: Specifies that a TCP/IP port on the remote machine be forwarded over
650: the secure channel to given host:port from the local machine. The
651: first argument must be a port number, and the second must be
652: host:port. Multiple forwardings may be specified, and additional
653: forwardings can be given on the command line. Only the root can
654: forward privileged ports.
1.2 deraadt 655: .It Cm RhostsAuthentication
1.1 deraadt 656: Specifies whether to try rhosts based authentication. Note that this
657: declaration only affects the client side and has no effect whatsoever
658: on security. Disabling rhosts authentication may reduce
659: authentication time on slow connections when rhosts authentication is
660: not used. Most servers do not permit RhostsAuthentication because it
661: is not secure (see RhostsRSAAuthentication). The argument to this
662: keyword must be
1.2 deraadt 663: .Dq yes
664: or
665: .Dq no .
666: .It Cm RhostsRSAAuthentication
1.1 deraadt 667: Specifies whether to try rhosts based authentication with RSA host
668: authentication. This is the primary authentication method for most
669: sites. The argument must be
1.2 deraadt 670: .Dq yes
671: or
672: .Dq no .
673: .It Cm RSAAuthentication
1.1 deraadt 674: Specifies whether to try RSA authentication. The argument to this
675: keyword must be
1.2 deraadt 676: .Dq yes
677: or
678: .Dq no .
1.1 deraadt 679: RSA authentication will only be
680: attempted if the identity file exists, or an authentication agent is
681: running.
1.27 markus 682: .It Cm SkeyAuthentication
683: Specifies whether to use
684: .Xr skey 1
685: authentication. The argument to
686: this keyword must be
687: .Dq yes
688: or
689: .Dq no .
690: The default is
691: .Dq no .
1.13 provos 692: .It Cm CheckHostIP
693: If this flag is set to
694: .Dq yes ,
695: ssh will additionally check the host ip address in the
696: .Pa known_hosts
697: file. This allows ssh to detect if a host key changed due to DNS spoofing.
698: If the option is set to
699: .Dq no ,
700: the check will not be executed.
1.2 deraadt 701: .It Cm StrictHostKeyChecking
702: If this flag is set to
703: .Dq yes ,
704: .Nm
1.1 deraadt 705: ssh will never automatically add host keys to the
1.2 deraadt 706: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 707: file, and refuses to connect hosts whose host key has changed. This
708: provides maximum protection against trojan horse attacks. However, it
709: can be somewhat annoying if you don't have good
1.2 deraadt 710: .Pa /etc/ssh_known_hosts
1.1 deraadt 711: files installed and frequently
712: connect new hosts. Basically this option forces the user to manually
713: add any new hosts. Normally this option is disabled, and new hosts
714: will automatically be added to the known host files. The host keys of
715: known hosts will be verified automatically in either case. The
716: argument must be
1.2 deraadt 717: .Dq yes
718: or
719: .Dq no .
720: .It Cm User
1.1 deraadt 721: Specifies the user to log in as. This can be useful if you have a
722: different user name in different machines. This saves the trouble of
723: having to remember to give the user name on the command line.
1.2 deraadt 724: .It Cm UserKnownHostsFile
725: Specifies a file to use instead of
726: .Pa $HOME/.ssh/known_hosts .
1.16 markus 727: .It Cm UsePrivilegedPort
728: Specifies whether to use a privileged port for outgoing connections.
729: The argument must be
730: .Dq yes
731: or
732: .Dq no .
733: The default is
734: .Dq yes .
735: Note that setting this option to
736: .Dq no
1.30 provos 737: turns off
1.16 markus 738: .Cm RhostsAuthentication
739: and
740: .Cm RhostsRSAAuthentication .
1.2 deraadt 741: .It Cm UseRsh
1.1 deraadt 742: Specifies that rlogin/rsh should be used for this host. It is
743: possible that the host does not at all support the
1.2 deraadt 744: .Nm
1.1 deraadt 745: protocol. This causes
1.2 deraadt 746: .Nm
1.1 deraadt 747: to immediately exec
1.2 deraadt 748: .Xr rsh 1 .
1.1 deraadt 749: All other options (except
1.2 deraadt 750: .Cm HostName )
1.1 deraadt 751: are ignored if this has been specified. The argument must be
1.2 deraadt 752: .Dq yes
753: or
754: .Dq no .
755: .Sh ENVIRONMENT
756: .Nm
1.1 deraadt 757: will normally set the following environment variables:
1.2 deraadt 758: .Bl -tag -width Ds
759: .It Ev DISPLAY
760: The
761: .Ev DISPLAY
762: variable indicates the location of the X11 server. It is
1.1 deraadt 763: automatically set by
1.2 deraadt 764: .Nm
765: to point to a value of the form
766: .Dq hostname:n
767: where hostname indicates
1.1 deraadt 768: the host where the shell runs, and n is an integer >= 1. Ssh uses
769: this special value to forward X11 connections over the secure
770: channel. The user should normally not set DISPLAY explicitly, as that
771: will render the X11 connection insecure (and will require the user to
772: manually copy any required authorization cookies).
1.2 deraadt 773: .It Ev HOME
1.1 deraadt 774: Set to the path of the user's home directory.
1.2 deraadt 775: .It Ev LOGNAME
776: Synonym for
1.12 aaron 777: .Ev USER ;
778: set for compatibility with systems that use this variable.
1.2 deraadt 779: .It Ev MAIL
1.1 deraadt 780: Set to point the user's mailbox.
1.2 deraadt 781: .It Ev PATH
782: Set to the default
783: .Ev PATH ,
784: as specified when compiling
1.12 aaron 785: .Nm ssh .
1.18 markus 786: .It Ev SSH_AUTH_SOCK
1.17 markus 787: indicates the path of a unix-domain socket used to communicate with the
788: agent.
1.2 deraadt 789: .It Ev SSH_CLIENT
1.1 deraadt 790: Identifies the client end of the connection. The variable contains
791: three space-separated values: client ip-address, client port number,
792: and server port number.
1.2 deraadt 793: .It Ev SSH_TTY
1.1 deraadt 794: This is set to the name of the tty (path to the device) associated
795: with the current shell or command. If the current session has no tty,
796: this variable is not set.
1.2 deraadt 797: .It Ev TZ
1.1 deraadt 798: The timezone variable is set to indicate the present timezone if it
799: was set when the daemon was started (e.i., the daemon passes the value
800: on to new connections).
1.2 deraadt 801: .It Ev USER
1.1 deraadt 802: Set to the name of the user logging in.
1.2 deraadt 803: .El
804: .Pp
1.1 deraadt 805: Additionally,
1.2 deraadt 806: .Nm
1.1 deraadt 807: reads
1.2 deraadt 808: .Pa $HOME/.ssh/environment ,
809: and adds lines of the format
810: .Dq VARNAME=value
1.12 aaron 811: to the environment.
1.2 deraadt 812: .Sh FILES
813: .Bl -tag -width $HOME/.ssh/known_hosts
814: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 815: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 816: in
817: .Pa /etc/ssh_known_hosts ) .
818: See
819: .Xr sshd 8 .
820: .It Pa $HOME/.ssh/identity
1.1 deraadt 821: Contains the RSA authentication identity of the user. This file
822: contains sensitive data and should be readable by the user but not
1.15 markus 823: accessible by others (read/write/execute).
824: Note that
825: .Nm
826: ignores this file if it is accessible by others.
827: It is possible to specify a passphrase when
1.1 deraadt 828: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 829: sensitive part of this file using 3DES.
1.2 deraadt 830: .It Pa $HOME/.ssh/identity.pub
1.1 deraadt 831: Contains the public key for authentication (public part of the
832: identity file in human-readable form). The contents of this file
1.2 deraadt 833: should be added to
834: .Pa $HOME/.ssh/authorized_keys
835: on all machines
1.1 deraadt 836: where you wish to log in using RSA authentication. This file is not
837: sensitive and can (but need not) be readable by anyone. This file is
838: never used automatically and is not necessary; it is only provided for
839: the convenience of the user.
1.2 deraadt 840: .It Pa $HOME/.ssh/config
1.1 deraadt 841: This is the per-user configuration file. The format of this file is
842: described above. This file is used by the
1.2 deraadt 843: .Nm
1.1 deraadt 844: client. This file does not usually contain any sensitive information,
845: but the recommended permissions are read/write for the user, and not
846: accessible by others.
1.2 deraadt 847: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 848: Lists the RSA keys that can be used for logging in as this user. The
849: format of this file is described in the
1.2 deraadt 850: .Xr sshd 8
1.1 deraadt 851: manual page. In the simplest form the format is the same as the .pub
852: identity files (that is, each line contains the number of bits in
853: modulus, public exponent, modulus, and comment fields, separated by
854: spaces). This file is not highly sensitive, but the recommended
855: permissions are read/write for the user, and not accessible by others.
1.2 deraadt 856: .It Pa /etc/ssh_known_hosts
1.1 deraadt 857: Systemwide list of known host keys. This file should be prepared by the
858: system administrator to contain the public host keys of all machines in the
859: organization. This file should be world-readable. This file contains
860: public keys, one per line, in the following format (fields separated
861: by spaces): system name, number of bits in modulus, public exponent,
862: modulus, and optional comment field. When different names are used
863: for the same machine, all such names should be listed, separated by
864: commas. The format is described on the
1.2 deraadt 865: .Xr sshd 8
1.1 deraadt 866: manual page.
1.2 deraadt 867: .Pp
1.1 deraadt 868: The canonical system name (as returned by name servers) is used by
1.2 deraadt 869: .Xr sshd 8
1.1 deraadt 870: to verify the client host when logging in; other names are needed because
1.2 deraadt 871: .Nm
1.1 deraadt 872: does not convert the user-supplied name to a canonical name before
873: checking the key, because someone with access to the name servers
874: would then be able to fool host authentication.
1.2 deraadt 875: .It Pa /etc/ssh_config
1.1 deraadt 876: Systemwide configuration file. This file provides defaults for those
877: values that are not specified in the user's configuration file, and
878: for those users who do not have a configuration file. This file must
879: be world-readable.
1.2 deraadt 880: .It Pa $HOME/.rhosts
881: This file is used in
882: .Pa \&.rhosts
883: authentication to list the
1.1 deraadt 884: host/user pairs that are permitted to log in. (Note that this file is
885: also used by rlogin and rsh, which makes using this file insecure.)
886: Each line of the file contains a host name (in the canonical form
887: returned by name servers), and then a user name on that host,
888: separated by a space. One some machines this file may need to be
889: world-readable if the user's home directory is on a NFS partition,
890: because
1.2 deraadt 891: .Xr sshd 8
1.1 deraadt 892: reads it as root. Additionally, this file must be owned by the user,
893: and must not have write permissions for anyone else. The recommended
894: permission for most machines is read/write for the user, and not
895: accessible by others.
1.2 deraadt 896: .Pp
1.1 deraadt 897: Note that by default
1.2 deraadt 898: .Xr sshd 8
1.1 deraadt 899: will be installed so that it requires successful RSA host
900: authentication before permitting \s+2.\s0rhosts authentication. If your
901: server machine does not have the client's host key in
1.2 deraadt 902: .Pa /etc/ssh_known_hosts ,
903: you can store it in
904: .Pa $HOME/.ssh/known_hosts .
905: The easiest way to do this is to
1.1 deraadt 906: connect back to the client from the server machine using ssh; this
1.2 deraadt 907: will automatically add the host key inxi
908: .Pa $HOME/.ssh/known_hosts .
909: .It Pa $HOME/.shosts
910: This file is used exactly the same way as
911: .Pa \&.rhosts .
912: The purpose for
1.1 deraadt 913: having this file is to be able to use rhosts authentication with
1.2 deraadt 914: .Nm
915: without permitting login with
916: .Xr rlogin 1
917: or
918: .Xr rsh 1 .
919: .It Pa /etc/hosts.equiv
920: This file is used during
921: .Pa \&.rhosts authentication. It contains
1.1 deraadt 922: canonical hosts names, one per line (the full format is described on
923: the
1.2 deraadt 924: .Xr sshd 8
1.1 deraadt 925: manual page). If the client host is found in this file, login is
926: automatically permitted provided client and server user names are the
927: same. Additionally, successful RSA host authentication is normally
928: required. This file should only be writable by root.
1.2 deraadt 929: .It Pa /etc/shosts.equiv
1.1 deraadt 930: This file is processed exactly as
1.2 deraadt 931: .Pa /etc/hosts.equiv .
1.1 deraadt 932: This file may be useful to permit logins using
1.2 deraadt 933: .Nm
1.1 deraadt 934: but not using rsh/rlogin.
1.2 deraadt 935: .It Pa /etc/sshrc
1.1 deraadt 936: Commands in this file are executed by
1.2 deraadt 937: .Nm
1.1 deraadt 938: when the user logs in just before the user's shell (or command) is started.
939: See the
1.2 deraadt 940: .Xr sshd 8
1.1 deraadt 941: manual page for more information.
1.2 deraadt 942: .It Pa $HOME/.ssh/rc
1.1 deraadt 943: Commands in this file are executed by
1.2 deraadt 944: .Nm
1.1 deraadt 945: when the user logs in just before the user's shell (or command) is
946: started.
947: See the
1.2 deraadt 948: .Xr sshd 8
1.1 deraadt 949: manual page for more information.
1.31 markus 950: .It Pa $HOME/.ssh/environment
951: Contains additional definitions for environment variables, see section
952: .Sx ENVIRONMENT
953: above.
1.5 deraadt 954: .It Pa libcrypto.so.X.1
955: A version of this library which includes support for the RSA algorithm
956: is required for proper operation.
1.2 deraadt 957: .Sh AUTHOR
1.1 deraadt 958: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 959: .Pp
960: Issues can be found from the SSH WWW home page:
961: .Pp
962: .Dl http://www.cs.hut.fi/ssh
1.5 deraadt 963: .Pp
1.20 provos 964: OpenSSH
1.21 deraadt 965: is a derivative of the original (free) ssh 1.2.12 release, but with bugs
966: removed and newer features re-added. Rapidly after the 1.2.12 release,
967: newer versions bore successively more restrictive licenses. This version
968: of OpenSSH
1.20 provos 969: .Bl -bullet
970: .It
1.21 deraadt 971: has all components of a restrictive nature (ie. patents, see
972: .Xr ssl 8 )
973: directly removed from the source code; any licensed or patented components
974: are chosen from
975: external libraries.
1.20 provos 976: .It
1.21 deraadt 977: has been updated to support ssh protocol 1.5.
1.20 provos 978: .It
979: contains added support for
980: .Xr kerberos 8
981: authentication and ticket passing.
982: .It
1.21 deraadt 983: supports one-time password authentication with
1.20 provos 984: .Xr skey 1 .
985: .El
986: .Pp
987: The libraries described in
1.5 deraadt 988: .Xr ssl 8
989: are required for proper operation.
1.25 provos 990: .Pp
1.26 aaron 991: OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
1.25 provos 992: Niels Provos, Theo de Raadt, and Dug Song.
1.2 deraadt 993: .Sh SEE ALSO
994: .Xr rlogin 1 ,
995: .Xr rsh 1 ,
996: .Xr scp 1 ,
997: .Xr ssh-add 1 ,
998: .Xr ssh-agent 1 ,
999: .Xr ssh-keygen 1 ,
1000: .Xr telnet 1 ,
1.5 deraadt 1001: .Xr sshd 8 ,
1002: .Xr ssl 8
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