Annotation of src/usr.bin/ssh/ssh.1, Revision 1.15
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: .\"
1.15 ! markus 12: .\" $Id: ssh.1,v 1.14 1999/10/06 20:07:42 dugsong 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
19: .Nd secure shell client (remote login program)
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
1.4 dugsong 27: .Op Fl agknqtvxXC
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
1.1 deraadt 96: in the
1.2 deraadt 97: .Sx FILES
1.1 deraadt 98: section), only then login is
99: permitted. This authentication method closes security holes due to IP
100: spoofing, DNS spoofing and routing spoofing. [Note to the
101: administrator:
1.2 deraadt 102: .Pa /etc/hosts.equiv ,
103: .Pa \&.rhosts ,
1.1 deraadt 104: and the rlogin/rsh protocol in general, are inherently insecure and should be
105: disabled if security is desired.]
1.2 deraadt 106: .Pp
1.1 deraadt 107: As a third authentication method,
1.2 deraadt 108: .Nm
1.1 deraadt 109: supports RSA based authentication.
110: The scheme is based on public-key cryptography: there are cryptosystems
111: where encryption and decryption are done using separate keys, and it
112: is not possible to derive the decryption key from the encryption key.
113: RSA is one such system. The idea is that each user creates a public/private
114: key pair for authentication purposes. The
115: server knows the public key, and only the user knows the private key.
116: The file
1.2 deraadt 117: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 118: lists the public keys that are permitted for logging
119: in. When the user logs in, the
1.2 deraadt 120: .Nm
1.1 deraadt 121: program tells the server which key pair it would like to use for
122: authentication. The server checks if this key is permitted, and if
123: so, sends the user (actually the
1.2 deraadt 124: .Nm
1.1 deraadt 125: program running on behalf of the user) a challenge, a random number,
126: encrypted by the user's public key. The challenge can only be
127: decrypted using the proper private key. The user's client then decrypts the
128: challenge using the private key, proving that he/she knows the private
129: key but without disclosing it to the server.
1.2 deraadt 130: .Pp
131: .Nm
1.1 deraadt 132: implements the RSA authentication protocol automatically. The user
133: creates his/her RSA key pair by running
1.2 deraadt 134: .Xr ssh-keygen 1 .
1.1 deraadt 135: This stores the private key in
1.2 deraadt 136: .Pa \&.ssh/identity
1.1 deraadt 137: and the public key in
1.2 deraadt 138: .Pa \&.ssh/identity.pub
1.1 deraadt 139: in the user's home directory. The user should then
140: copy the
1.2 deraadt 141: .Pa identity.pub
1.1 deraadt 142: to
1.2 deraadt 143: .Pa \&.ssh/authorized_keys
1.1 deraadt 144: in his/her home directory on the remote machine (the
1.2 deraadt 145: .Pa authorized_keys
1.1 deraadt 146: file corresponds to the conventional
1.2 deraadt 147: .Pa \&.rhosts
1.1 deraadt 148: file, and has one key
149: per line, though the lines can be very long). After this, the user
150: can log in without giving the password. RSA authentication is much
151: more secure than rhosts authentication.
1.2 deraadt 152: .Pp
1.1 deraadt 153: The most convenient way to use RSA authentication may be with an
154: authentication agent. See
1.2 deraadt 155: .Xr ssh-agent 1
1.1 deraadt 156: for more information.
1.2 deraadt 157: .Pp
1.1 deraadt 158: If other authentication methods fail,
1.2 deraadt 159: .Nm
1.1 deraadt 160: prompts the user for a password. The password is sent to the remote
161: host for checking; however, since all communications are encrypted,
162: the password cannot be seen by someone listening on the network.
1.2 deraadt 163: .Pp
1.1 deraadt 164: When the user's identity has been accepted by the server, the server
165: either executes the given command, or logs into the machine and gives
166: the user a normal shell on the remote machine. All communication with
167: the remote command or shell will be automatically encrypted.
1.2 deraadt 168: .Pp
1.1 deraadt 169: If a pseudo-terminal has been allocated (normal login session), the
1.2 deraadt 170: user can disconnect with
171: .Ic ~. ,
172: and suspend
173: .Nm
174: with
175: .Ic ~^Z .
176: All forwarded connections can be listed with
177: .Ic ~#
178: and if
1.1 deraadt 179: the session blocks waiting for forwarded X11 or TCP/IP
1.2 deraadt 180: connections to terminate, it can be backgrounded with
181: .Ic ~&
182: (this should not be used while the user shell is active, as it can cause the
183: shell to hang). All available escapes can be listed with
184: .Ic ~? .
185: .Pp
186: A single tilde character can be sent as
187: .Ic ~~
188: (or by following the tilde by a character other than those described above).
1.1 deraadt 189: The escape character must always follow a newline to be interpreted as
190: special. The escape character can be changed in configuration files
191: or on the command line.
1.2 deraadt 192: .Pp
1.1 deraadt 193: If no pseudo tty has been allocated, the
194: session is transparent and can be used to reliably transfer binary
1.2 deraadt 195: data. On most systems, setting the escape character to
196: .Dq none
197: will also make the session transparent even if a tty is used.
198: .Pp
1.1 deraadt 199: The session terminates when the command or shell in on the remote
200: machine exists and all X11 and TCP/IP connections have been closed.
201: The exit status of the remote program is returned as the exit status
202: of
1.2 deraadt 203: .Nm ssh .
204: .Pp
1.1 deraadt 205: If the user is using X11 (the
1.2 deraadt 206: .Ev DISPLAY
1.1 deraadt 207: environment variable is set), the connection to the X11 display is
208: automatically forwarded to the remote side in such a way that any X11
209: programs started from the shell (or command) will go through the
210: encrypted channel, and the connection to the real X server will be made
211: from the local machine. The user should not manually set
1.2 deraadt 212: .Ev DISPLAY .
1.1 deraadt 213: Forwarding of X11 connections can be
214: configured on the command line or in configuration files.
1.2 deraadt 215: .Pp
216: The
217: .Ev DISPLAY
218: value set by
219: .Nm
1.1 deraadt 220: will point to the server machine, but with a display number greater
221: than zero. This is normal, and happens because
1.2 deraadt 222: .Nm
223: creates a
224: .Dq proxy
225: X server on the server machine for forwarding the
1.1 deraadt 226: connections over the encrypted channel.
1.2 deraadt 227: .Pp
228: .Nm
1.1 deraadt 229: will also automatically set up Xauthority data on the server machine.
230: For this purpose, it will generate a random authorization cookie,
231: store it in Xauthority on the server, and verify that any forwarded
232: connections carry this cookie and replace it by the real cookie when
233: the connection is opened. The real authentication cookie is never
234: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 235: .Pp
1.1 deraadt 236: If the user is using an authentication agent, the connection to the agent
237: is automatically forwarded to the remote side unless disabled on
238: command line or in a configuration file.
1.2 deraadt 239: .Pp
1.1 deraadt 240: Forwarding of arbitrary TCP/IP connections over the secure channel can
241: be specified either on command line or in a configuration file. One
242: possible application of TCP/IP forwarding is a secure connection to an
243: electronic purse; another is going trough firewalls.
1.2 deraadt 244: .Pp
245: .Nm
1.1 deraadt 246: automatically maintains and checks a database containing RSA-based
247: identifications for all hosts it has ever been used with. The
248: database is stored in
1.2 deraadt 249: .Pa \&.ssh/known_hosts
1.1 deraadt 250: in the user's home directory. Additionally, the file
1.2 deraadt 251: .Pa /etc/ssh_known_hosts
1.1 deraadt 252: is automatically checked for known hosts. Any new hosts are
253: automatically added to the user's file. If a host's identification
254: ever changes,
1.2 deraadt 255: .Nm
1.1 deraadt 256: warns about this and disables password authentication to prevent a
257: trojan horse from getting the user's password. Another purpose of
258: this mechanism is to prevent man-in-the-middle attacks which could
259: otherwise be used to circumvent the encryption. The
1.2 deraadt 260: .Cm StrictHostKeyChecking
1.1 deraadt 261: option (see below) can be used to prevent logins to machines whose
262: host key is not known or has changed.
1.2 deraadt 263: .Sh OPTIONS
264: .Bl -tag -width Ds
1.4 dugsong 265: .It Fl a
266: Disables forwarding of the authentication agent connection. This may
267: also be specified on a per-host basis in the configuration file.
1.6 deraadt 268: .It Fl c Ar blowfish|3des
1.1 deraadt 269: Selects the cipher to use for encrypting the session.
1.2 deraadt 270: .Ar 3des
1.1 deraadt 271: is used by default. It is believed to be secure.
1.5 deraadt 272: .Ar 3des
273: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
274: It is presumably more secure than the
1.2 deraadt 275: .Ar des
1.5 deraadt 276: cipher which is no longer supported in ssh.
277: .Ar blowfish
278: is a fast block cipher, it appears very secure and is much faster than
279: .Ar 3des .
1.2 deraadt 280: .It Fl e Ar ch|^ch|none
281: Sets the escape character for sessions with a pty (default:
282: .Ql ~ ) .
283: The escape character is only recognized at the beginning of a line. The
284: escape character followed by a dot
285: .Pq Ql \&.
286: closes the connection, followed
1.1 deraadt 287: by control-Z suspends the connection, and followed by itself sends the
1.2 deraadt 288: escape character once. Setting the character to
289: .Dq none
290: disables any escapes and makes the session fully transparent.
291: .It Fl f
292: Requests
293: .Nm
294: to go to background after authentication. This is useful
295: if
296: .Nm
297: is going to ask for passwords or passphrases, but the user
1.1 deraadt 298: wants it in the background. This implies
1.2 deraadt 299: .Fl n .
1.1 deraadt 300: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 301: something like
302: .Ic ssh -f host xterm .
303: .It Fl i Ar identity_file
1.1 deraadt 304: Selects the file from which the identity (private key) for
1.2 deraadt 305: RSA authentication is read. Default is
306: .Pa \&.ssh/identity
1.1 deraadt 307: in the user's home directory. Identity files may also be specified on
308: a per-host basis in the configuration file. It is possible to have
1.2 deraadt 309: multiple
310: .Fl i
311: options (and multiple identities specified in
1.1 deraadt 312: configuration files).
1.3 deraadt 313: .It Fl g
314: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 315: .It Fl k
1.4 dugsong 316: Disables forwarding of Kerberos tickets and AFS tokens. This may
1.1 deraadt 317: also be specified on a per-host basis in the configuration file.
1.2 deraadt 318: .It Fl l Ar login_name
1.1 deraadt 319: Specifies the user to log in as on the remote machine. This may also
320: be specified on a per-host basis in the configuration file.
1.2 deraadt 321: .It Fl n
322: Redirects stdin from
323: .Pa /dev/null
324: (actually, prevents reading from stdin).
1.1 deraadt 325: This must be used when
1.2 deraadt 326: .Nm
1.1 deraadt 327: is run in the background. A common trick is to use this to run X11
1.2 deraadt 328: programs in a remote machine. For example,
329: .Ic ssh -n shadows.cs.hut.fi emacs &
330: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 331: connection will be automatically forwarded over an encrypted channel.
332: The
1.2 deraadt 333: .Nm
1.1 deraadt 334: program will be put in the background.
335: (This does not work if
1.2 deraadt 336: .Nm
337: needs to ask for a password or passphrase; see also the
338: .Fl f
339: option.)
340: .It Fl o Ar option
1.1 deraadt 341: Can be used to give options in the format used in the config file.
342: This is useful for specifying options for which there is no separate
343: command-line flag. The option has the same format as a line in the
344: configuration file.
1.2 deraadt 345: .It Fl p Ar port
1.1 deraadt 346: Port to connect to on the remote host. This can be specified on a
347: per-host basis in the configuration file.
1.2 deraadt 348: .It Fl q
1.1 deraadt 349: Quiet mode. Causes all warning and diagnostic messages to be
350: suppressed. Only fatal errors are displayed.
1.2 deraadt 351: .It Fl t
1.1 deraadt 352: Force pseudo-tty allocation. This can be used to execute arbitary
353: screen-based programs on a remote machine, which can be very useful
354: e.g. when implementing menu services.
1.2 deraadt 355: .It Fl v
1.1 deraadt 356: Verbose mode. Causes
1.2 deraadt 357: .Nm
1.1 deraadt 358: to print debugging messages about its progress. This is helpful in
359: debugging connection, authentication, and configuration problems.
1.2 deraadt 360: .It Fl x
1.1 deraadt 361: Disables X11 forwarding. This can also be specified on a per-host
362: basis in a configuration file.
1.2 deraadt 363: .It Fl X
1.1 deraadt 364: Enables X11 forwarding.
1.2 deraadt 365: .It Fl C
1.1 deraadt 366: Requests compression of all data (including stdin, stdout, stderr, and
367: data for forwarded X11 and TCP/IP connections). The compression
1.2 deraadt 368: algorithm is the same used by gzip, and the
369: .Dq level
370: can be controlled by the
371: .Cm CompressionLevel
1.1 deraadt 372: option (see below). Compression is desirable on modem lines and other
373: slow connections, but will only slow down things on fast networks.
374: The default value can be set on a host-by-host basis in the
375: configuration files; see the
1.2 deraadt 376: .Cm Compress
1.1 deraadt 377: option below.
1.2 deraadt 378: .It Fl L Ar port:host:hostport
1.1 deraadt 379: Specifies that the given port on the local (client) host is to be
380: forwarded to the given host and port on the remote side. This works
381: by allocating a socket to listen to
1.2 deraadt 382: .Ar port
1.1 deraadt 383: on the local side, and whenever a connection is made to this port, the
384: connection is forwarded over the secure channel, and a connection is
385: made to
1.2 deraadt 386: .Ar host:hostport
1.1 deraadt 387: from the remote machine. Port forwardings can also be specified in the
388: configuration file. Only root can forward privileged ports.
1.2 deraadt 389: .It Fl R Ar port:host:hostport
1.1 deraadt 390: Specifies that the given port on the remote (server) host is to be
391: forwarded to the given host and port on the local side. This works
392: by allocating a socket to listen to
1.2 deraadt 393: .Ar port
1.1 deraadt 394: on the remote side, and whenever a connection is made to this port, the
395: connection is forwarded over the secure channel, and a connection is
396: made to
1.2 deraadt 397: .Ar host:hostport
1.1 deraadt 398: from the local machine. Port forwardings can also be specified in the
399: configuration file. Privileged ports can be forwarded only when
400: logging in as root on the remote machine.
1.2 deraadt 401: .El
402: .Sh CONFIGURATION FILES
403: .Nm
1.1 deraadt 404: obtains configuration data from the following sources (in this order):
405: command line options, user's configuration file
1.2 deraadt 406: .Pq Pa $HOME/.ssh/config ,
407: and system-wide configuration file
408: .Pq Pa /etc/ssh_config .
409: For each parameter, the first obtained value
1.1 deraadt 410: will be used. The configuration files contain sections bracketed by
411: "Host" specifications, and that section is only applied for hosts that
412: match one of the patterns given in the specification. The matched
413: host name is the one given on the command line.
1.2 deraadt 414: .Pp
1.1 deraadt 415: Since the first obtained value for each parameter is used, more
416: host-specific declarations should be given near the beginning of the
417: file, and general defaults at the end.
1.2 deraadt 418: .Pp
1.1 deraadt 419: The configuration file has the following format:
1.2 deraadt 420: .Pp
421: Empty lines and lines starting with
422: .Ql #
423: are comments.
424: .Pp
425: Otherwise a line is of the format
426: .Dq keyword arguments .
427: The possible
1.1 deraadt 428: keywords and their meanings are as follows (note that the
429: configuration files are case-sensitive):
1.2 deraadt 430: .Bl -tag -width Ds
431: .It Cm Host
1.1 deraadt 432: Restricts the following declarations (up to the next
1.2 deraadt 433: .Cm Host
1.1 deraadt 434: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 435: given after the keyword.
436: .Ql \&*
437: and
438: .Ql ?
439: can be used as wildcards in the
440: patterns. A single
441: .Ql \&*
442: as a pattern can be used to provide global
1.1 deraadt 443: defaults for all hosts. The host is the
1.2 deraadt 444: .Ar hostname
1.1 deraadt 445: argument given on the command line (i.e., the name is not converted to
446: a canonicalized host name before matching).
1.2 deraadt 447: .It Cm AFSTokenPassing
1.1 deraadt 448: Specifies whether to pass AFS tokens to remote host. The argument to
449: this keyword must be
1.2 deraadt 450: .Dq yes
451: or
452: .Dq no .
453: .It Cm BatchMode
454: If set to
455: .Dq yes ,
456: passphrase/password querying will be disabled. This
1.1 deraadt 457: option is useful in scripts and other batch jobs where you have no
458: user to supply the password. The argument must be
1.2 deraadt 459: .Dq yes
460: or
461: .Dq no .
462: .It Cm Cipher
1.1 deraadt 463: Specifies the cipher to use for encrypting the session. Currently,
1.2 deraadt 464: .Dq blowfish ,
1.1 deraadt 465: and
1.10 provos 466: .Dq 3des
1.2 deraadt 467: are supported. The default is
468: .Dq 3des .
469: .It Cm Compression
1.1 deraadt 470: Specifies whether to use compression. The argument must be
1.2 deraadt 471: .Dq yes
472: or
473: .Dq no .
474: .It Cm CompressionLevel
1.1 deraadt 475: Specifies the compression level to use if compression is enable. The
476: argument must be an integer from 1 (fast) to 9 (slow, best). The
477: default level is 6, which is good for most applications. The meaning
478: of the values is the same as in GNU GZIP.
1.2 deraadt 479: .It Cm ConnectionAttempts
1.1 deraadt 480: Specifies the number of tries (one per second) to make before falling
481: back to rsh or exiting. The argument must be an integer. This may be
482: useful in scripts if the connection sometimes fails.
1.2 deraadt 483: .It Cm EscapeChar
484: Sets the escape character (default:
485: .Ql ~ ) .
486: The escape character can also
1.1 deraadt 487: be set on the command line. The argument should be a single
1.2 deraadt 488: character,
489: .Ql ^
490: followed by a letter, or
491: .Dq none
492: to disable the escape
1.1 deraadt 493: character entirely (making the connection transparent for binary
494: data).
1.2 deraadt 495: .It Cm FallBackToRsh
1.1 deraadt 496: Specifies that if connecting via
1.2 deraadt 497: .Nm
1.1 deraadt 498: fails due to a connection refused error (there is no
1.2 deraadt 499: .Xr sshd 8
1.1 deraadt 500: listening on the remote host),
1.2 deraadt 501: .Xr rsh 1
1.1 deraadt 502: should automatically be used instead (after a suitable warning about
503: the session being unencrypted). The argument must be
1.2 deraadt 504: .Dq yes
505: or
506: .Dq no .
507: .It Cm ForwardAgent
1.1 deraadt 508: Specifies whether the connection to the authentication agent (if any)
509: will be forwarded to the remote machine. The argument must be
1.2 deraadt 510: .Dq yes
511: or
512: .Dq no .
513: .It Cm ForwardX11
1.1 deraadt 514: Specifies whether X11 connections will be automatically redirected
515: over the secure channel and
1.2 deraadt 516: .Ev DISPLAY
1.1 deraadt 517: set. The argument must be
1.2 deraadt 518: .Dq yes
519: or
1.3 deraadt 520: .Dq no .
521: .It Cm GatewayPorts
522: Specifies whether remote hosts are allowed to connect to local
523: forwarded ports.
524: The argument must be
525: .Dq yes
526: or
527: .Dq no .
528: The default is
1.2 deraadt 529: .Dq no .
530: .It Cm GlobalKnownHostsFile
1.1 deraadt 531: Specifies a file to use instead of
1.2 deraadt 532: .Pa /etc/ssh_known_hosts .
533: .It Cm HostName
1.1 deraadt 534: Specifies the real host name to log into. This can be used to specify
535: nicnames or abbreviations for hosts. Default is the name given on the
536: command line. Numeric IP addresses are also permitted (both on the
537: command line and in
1.2 deraadt 538: .Cm HostName
1.1 deraadt 539: specifications).
1.2 deraadt 540: .It Cm IdentityFile
1.1 deraadt 541: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 542: is read (default
543: .Pa .ssh/identity
544: in the user's home directory).
1.1 deraadt 545: Additionally, any identities represented by the authentication agent
546: will be used for authentication. The file name may use the tilde
547: syntax to refer to a user's home directory. It is possible to have
548: multiple identity files specified in configuration files; all these
549: identities will be tried in sequence.
1.2 deraadt 550: .It Cm KeepAlive
1.1 deraadt 551: Specifies whether the system should send keepalive messages to the
552: other side. If they are sent, death of the connection or crash of one
553: of the machines will be properly noticed. However, this means that
554: connections will die if the route is down temporarily, and some people
555: find it annoying.
1.2 deraadt 556: .Pp
557: The default is
558: .Dq yes
559: (to send keepalives), and the client will notice
1.1 deraadt 560: if the network goes down or the remote host dies. This is important
561: in scripts, and many users want it too.
1.2 deraadt 562: .Pp
563: To disable keepalives, the value should be set to
564: .Dq no
565: in both the server and the client configuration files.
566: .It Cm KerberosAuthentication
1.4 dugsong 567: Specifies whether Kerberos authentication will be used. The argument to
568: this keyword must be
569: .Dq yes
570: or
571: .Dq no .
1.2 deraadt 572: .It Cm KerberosTgtPassing
1.4 dugsong 573: Specifies whether a Kerberos TGT will be forwarded to the server. This
574: will only work if the Kerberos server is actually an AFS kaserver. The
575: argument to this keyword must be
576: .Dq yes
577: or
578: .Dq no .
1.2 deraadt 579: .It Cm LocalForward
1.1 deraadt 580: Specifies that a TCP/IP port on the local machine be forwarded over
581: the secure channel to given host:port from the remote machine. The
582: first argument must be a port number, and the second must be
583: host:port. Multiple forwardings may be specified, and additional
584: forwardings can be given on the command line. Only the root can
585: forward privileged ports.
1.2 deraadt 586: .It Cm PasswordAuthentication
1.1 deraadt 587: Specifies whether to use password authentication. The argument to
588: this keyword must be
1.2 deraadt 589: .Dq yes
590: or
591: .Dq no .
1.14 dugsong 592: .It Cm NumberOfPasswordPrompts
593: Specifies the number of password prompts before giving up. The
594: argument to this keyword must be an integer. Default is 3.
1.2 deraadt 595: .It Cm Port
1.1 deraadt 596: Specifies the port number to connect on the remote host. Default is
597: 22.
1.2 deraadt 598: .It Cm ProxyCommand
1.1 deraadt 599: Specifies the command to use to connect to the server. The command
600: string extends to the end of the line, and is executed with /bin/sh.
601: In the command string, %h will be substituted by the host name to
602: connect and %p by the port. The command can be basically anything,
603: and should read from its stdin and write to its stdout. It should
604: eventually connect an
1.2 deraadt 605: .Xr sshd 8
1.1 deraadt 606: server running on some machine, or execute
1.2 deraadt 607: .Ic sshd -i
608: somewhere. Host key management will be done using the
1.1 deraadt 609: HostName of the host being connected (defaulting to the name typed by
610: the user).
1.2 deraadt 611: .Pp
612: .It Cm RemoteForward
1.1 deraadt 613: Specifies that a TCP/IP port on the remote machine be forwarded over
614: the secure channel to given host:port from the local machine. The
615: first argument must be a port number, and the second must be
616: host:port. Multiple forwardings may be specified, and additional
617: forwardings can be given on the command line. Only the root can
618: forward privileged ports.
1.2 deraadt 619: .It Cm RhostsAuthentication
1.1 deraadt 620: Specifies whether to try rhosts based authentication. Note that this
621: declaration only affects the client side and has no effect whatsoever
622: on security. Disabling rhosts authentication may reduce
623: authentication time on slow connections when rhosts authentication is
624: not used. Most servers do not permit RhostsAuthentication because it
625: is not secure (see RhostsRSAAuthentication). The argument to this
626: keyword must be
1.2 deraadt 627: .Dq yes
628: or
629: .Dq no .
630: .It Cm RhostsRSAAuthentication
1.1 deraadt 631: Specifies whether to try rhosts based authentication with RSA host
632: authentication. This is the primary authentication method for most
633: sites. The argument must be
1.2 deraadt 634: .Dq yes
635: or
636: .Dq no .
637: .It Cm RSAAuthentication
1.1 deraadt 638: Specifies whether to try RSA authentication. The argument to this
639: keyword must be
1.2 deraadt 640: .Dq yes
641: or
642: .Dq no .
1.1 deraadt 643: RSA authentication will only be
644: attempted if the identity file exists, or an authentication agent is
645: running.
1.13 provos 646: .It Cm CheckHostIP
647: If this flag is set to
648: .Dq yes ,
649: ssh will additionally check the host ip address in the
650: .Pa known_hosts
651: file. This allows ssh to detect if a host key changed due to DNS spoofing.
652: If the option is set to
653: .Dq no ,
654: the check will not be executed.
1.2 deraadt 655: .It Cm StrictHostKeyChecking
656: If this flag is set to
657: .Dq yes ,
658: .Nm
1.1 deraadt 659: ssh will never automatically add host keys to the
1.2 deraadt 660: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 661: file, and refuses to connect hosts whose host key has changed. This
662: provides maximum protection against trojan horse attacks. However, it
663: can be somewhat annoying if you don't have good
1.2 deraadt 664: .Pa /etc/ssh_known_hosts
1.1 deraadt 665: files installed and frequently
666: connect new hosts. Basically this option forces the user to manually
667: add any new hosts. Normally this option is disabled, and new hosts
668: will automatically be added to the known host files. The host keys of
669: known hosts will be verified automatically in either case. The
670: argument must be
1.2 deraadt 671: .Dq yes
672: or
673: .Dq no .
674: .It Cm User
1.1 deraadt 675: Specifies the user to log in as. This can be useful if you have a
676: different user name in different machines. This saves the trouble of
677: having to remember to give the user name on the command line.
1.2 deraadt 678: .It Cm UserKnownHostsFile
679: Specifies a file to use instead of
680: .Pa $HOME/.ssh/known_hosts .
681: .It Cm UseRsh
1.1 deraadt 682: Specifies that rlogin/rsh should be used for this host. It is
683: possible that the host does not at all support the
1.2 deraadt 684: .Nm
1.1 deraadt 685: protocol. This causes
1.2 deraadt 686: .Nm
1.1 deraadt 687: to immediately exec
1.2 deraadt 688: .Xr rsh 1 .
1.1 deraadt 689: All other options (except
1.2 deraadt 690: .Cm HostName )
1.1 deraadt 691: are ignored if this has been specified. The argument must be
1.2 deraadt 692: .Dq yes
693: or
694: .Dq no .
695: .Sh ENVIRONMENT
696: .Nm
1.1 deraadt 697: will normally set the following environment variables:
1.2 deraadt 698: .Bl -tag -width Ds
699: .It Ev DISPLAY
700: The
701: .Ev DISPLAY
702: variable indicates the location of the X11 server. It is
1.1 deraadt 703: automatically set by
1.2 deraadt 704: .Nm
705: to point to a value of the form
706: .Dq hostname:n
707: where hostname indicates
1.1 deraadt 708: the host where the shell runs, and n is an integer >= 1. Ssh uses
709: this special value to forward X11 connections over the secure
710: channel. The user should normally not set DISPLAY explicitly, as that
711: will render the X11 connection insecure (and will require the user to
712: manually copy any required authorization cookies).
1.2 deraadt 713: .It Ev HOME
1.1 deraadt 714: Set to the path of the user's home directory.
1.2 deraadt 715: .It Ev LOGNAME
716: Synonym for
1.12 aaron 717: .Ev USER ;
718: set for compatibility with systems that use this variable.
1.2 deraadt 719: .It Ev MAIL
1.1 deraadt 720: Set to point the user's mailbox.
1.2 deraadt 721: .It Ev PATH
722: Set to the default
723: .Ev PATH ,
724: as specified when compiling
1.12 aaron 725: .Nm ssh .
1.2 deraadt 726: .It Ev SSH_AUTHENTICATION_FD
1.1 deraadt 727: This is set to an integer value if you are using the authentication
728: agent and a connection to it has been forwarded. The value indicates
729: a file descriptor number used for communicating with the agent. On
730: some systems,
1.2 deraadt 731: .Ev SSH_AUTHENTICATION_SOCKET
1.1 deraadt 732: may be used instead to
733: indicate the path of a unix-domain socket used to communicate with the
734: agent (this method is less secure, and is only used on systems that
735: don't support the first method).
1.2 deraadt 736: .It Ev SSH_CLIENT
1.1 deraadt 737: Identifies the client end of the connection. The variable contains
738: three space-separated values: client ip-address, client port number,
739: and server port number.
1.2 deraadt 740: .It Ev SSH_TTY
1.1 deraadt 741: This is set to the name of the tty (path to the device) associated
742: with the current shell or command. If the current session has no tty,
743: this variable is not set.
1.2 deraadt 744: .It Ev TZ
1.1 deraadt 745: The timezone variable is set to indicate the present timezone if it
746: was set when the daemon was started (e.i., the daemon passes the value
747: on to new connections).
1.2 deraadt 748: .It Ev USER
1.1 deraadt 749: Set to the name of the user logging in.
1.2 deraadt 750: .El
751: .Pp
1.1 deraadt 752: Additionally,
1.2 deraadt 753: .Nm
1.1 deraadt 754: reads
1.2 deraadt 755: .Pa $HOME/.ssh/environment ,
756: and adds lines of the format
757: .Dq VARNAME=value
1.12 aaron 758: to the environment.
1.2 deraadt 759: .Sh FILES
760: .Bl -tag -width $HOME/.ssh/known_hosts
761: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 762: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 763: in
764: .Pa /etc/ssh_known_hosts ) .
765: See
766: .Xr sshd 8 .
767: .It Pa $HOME/.ssh/random_seed
1.1 deraadt 768: Used for seeding the random number generator. This file contains
769: sensitive data and should read/write for the user and not accessible
770: for others. This file is created the first time the program is run
771: and updated automatically. The user should never need to read or
772: modify this file.
1.2 deraadt 773: .It Pa $HOME/.ssh/identity
1.1 deraadt 774: Contains the RSA authentication identity of the user. This file
775: contains sensitive data and should be readable by the user but not
1.15 ! markus 776: accessible by others (read/write/execute).
! 777: Note that
! 778: .Nm
! 779: ignores this file if it is accessible by others.
! 780: It is possible to specify a passphrase when
1.1 deraadt 781: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 782: sensitive part of this file using 3DES.
1.2 deraadt 783: .It Pa $HOME/.ssh/identity.pub
1.1 deraadt 784: Contains the public key for authentication (public part of the
785: identity file in human-readable form). The contents of this file
1.2 deraadt 786: should be added to
787: .Pa $HOME/.ssh/authorized_keys
788: on all machines
1.1 deraadt 789: where you wish to log in using RSA authentication. This file is not
790: sensitive and can (but need not) be readable by anyone. This file is
791: never used automatically and is not necessary; it is only provided for
792: the convenience of the user.
1.2 deraadt 793: .It Pa $HOME/.ssh/config
1.1 deraadt 794: This is the per-user configuration file. The format of this file is
795: described above. This file is used by the
1.2 deraadt 796: .Nm
1.1 deraadt 797: client. This file does not usually contain any sensitive information,
798: but the recommended permissions are read/write for the user, and not
799: accessible by others.
1.2 deraadt 800: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 801: Lists the RSA keys that can be used for logging in as this user. The
802: format of this file is described in the
1.2 deraadt 803: .Xr sshd 8
1.1 deraadt 804: manual page. In the simplest form the format is the same as the .pub
805: identity files (that is, each line contains the number of bits in
806: modulus, public exponent, modulus, and comment fields, separated by
807: spaces). This file is not highly sensitive, but the recommended
808: permissions are read/write for the user, and not accessible by others.
1.2 deraadt 809: .It Pa /etc/ssh_known_hosts
1.1 deraadt 810: Systemwide list of known host keys. This file should be prepared by the
811: system administrator to contain the public host keys of all machines in the
812: organization. This file should be world-readable. This file contains
813: public keys, one per line, in the following format (fields separated
814: by spaces): system name, number of bits in modulus, public exponent,
815: modulus, and optional comment field. When different names are used
816: for the same machine, all such names should be listed, separated by
817: commas. The format is described on the
1.2 deraadt 818: .Xr sshd 8
1.1 deraadt 819: manual page.
1.2 deraadt 820: .Pp
1.1 deraadt 821: The canonical system name (as returned by name servers) is used by
1.2 deraadt 822: .Xr sshd 8
1.1 deraadt 823: to verify the client host when logging in; other names are needed because
1.2 deraadt 824: .Nm
1.1 deraadt 825: does not convert the user-supplied name to a canonical name before
826: checking the key, because someone with access to the name servers
827: would then be able to fool host authentication.
1.2 deraadt 828: .It Pa /etc/ssh_config
1.1 deraadt 829: Systemwide configuration file. This file provides defaults for those
830: values that are not specified in the user's configuration file, and
831: for those users who do not have a configuration file. This file must
832: be world-readable.
1.2 deraadt 833: .It Pa $HOME/.rhosts
834: This file is used in
835: .Pa \&.rhosts
836: authentication to list the
1.1 deraadt 837: host/user pairs that are permitted to log in. (Note that this file is
838: also used by rlogin and rsh, which makes using this file insecure.)
839: Each line of the file contains a host name (in the canonical form
840: returned by name servers), and then a user name on that host,
841: separated by a space. One some machines this file may need to be
842: world-readable if the user's home directory is on a NFS partition,
843: because
1.2 deraadt 844: .Xr sshd 8
1.1 deraadt 845: reads it as root. Additionally, this file must be owned by the user,
846: and must not have write permissions for anyone else. The recommended
847: permission for most machines is read/write for the user, and not
848: accessible by others.
1.2 deraadt 849: .Pp
1.1 deraadt 850: Note that by default
1.2 deraadt 851: .Xr sshd 8
1.1 deraadt 852: will be installed so that it requires successful RSA host
853: authentication before permitting \s+2.\s0rhosts authentication. If your
854: server machine does not have the client's host key in
1.2 deraadt 855: .Pa /etc/ssh_known_hosts ,
856: you can store it in
857: .Pa $HOME/.ssh/known_hosts .
858: The easiest way to do this is to
1.1 deraadt 859: connect back to the client from the server machine using ssh; this
1.2 deraadt 860: will automatically add the host key inxi
861: .Pa $HOME/.ssh/known_hosts .
862: .It Pa $HOME/.shosts
863: This file is used exactly the same way as
864: .Pa \&.rhosts .
865: The purpose for
1.1 deraadt 866: having this file is to be able to use rhosts authentication with
1.2 deraadt 867: .Nm
868: without permitting login with
869: .Xr rlogin 1
870: or
871: .Xr rsh 1 .
872: .It Pa /etc/hosts.equiv
873: This file is used during
874: .Pa \&.rhosts authentication. It contains
1.1 deraadt 875: canonical hosts names, one per line (the full format is described on
876: the
1.2 deraadt 877: .Xr sshd 8
1.1 deraadt 878: manual page). If the client host is found in this file, login is
879: automatically permitted provided client and server user names are the
880: same. Additionally, successful RSA host authentication is normally
881: required. This file should only be writable by root.
1.2 deraadt 882: .It Pa /etc/shosts.equiv
1.1 deraadt 883: This file is processed exactly as
1.2 deraadt 884: .Pa /etc/hosts.equiv .
1.1 deraadt 885: This file may be useful to permit logins using
1.2 deraadt 886: .Nm
1.1 deraadt 887: but not using rsh/rlogin.
1.2 deraadt 888: .It Pa /etc/sshrc
1.1 deraadt 889: Commands in this file are executed by
1.2 deraadt 890: .Nm
1.1 deraadt 891: when the user logs in just before the user's shell (or command) is started.
892: See the
1.2 deraadt 893: .Xr sshd 8
1.1 deraadt 894: manual page for more information.
1.2 deraadt 895: .It Pa $HOME/.ssh/rc
1.1 deraadt 896: Commands in this file are executed by
1.2 deraadt 897: .Nm
1.1 deraadt 898: when the user logs in just before the user's shell (or command) is
899: started.
900: See the
1.2 deraadt 901: .Xr sshd 8
1.1 deraadt 902: manual page for more information.
1.5 deraadt 903: .It Pa libcrypto.so.X.1
904: A version of this library which includes support for the RSA algorithm
905: is required for proper operation.
1.2 deraadt 906: .Sh AUTHOR
1.1 deraadt 907: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 908: .Pp
909: Issues can be found from the SSH WWW home page:
910: .Pp
911: .Dl http://www.cs.hut.fi/ssh
1.5 deraadt 912: .Pp
913: This version of
914: .Nm
1.7 deraadt 915: is a derivative of the original 1.2.12 release, but with bugs removed and
1.9 deraadt 916: newer features re-added. Rapidly after the 1.2.12 release, newer versions
917: bore successively more restrictive licenses. In this version, all components
1.5 deraadt 918: of a restrictive nature (ie. patents) have been directly removed from the
919: source code; any licensed or patented components are chosen from external
920: libraries. The libraries described in
921: .Xr ssl 8
922: are required for proper operation.
1.2 deraadt 923: .Sh SEE ALSO
924: .Xr rlogin 1 ,
925: .Xr rsh 1 ,
926: .Xr scp 1 ,
927: .Xr ssh-add 1 ,
928: .Xr ssh-agent 1 ,
929: .Xr ssh-keygen 1 ,
930: .Xr telnet 1 ,
1.5 deraadt 931: .Xr sshd 8 ,
932: .Xr ssl 8
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