Annotation of src/usr.bin/ssh/ssh.1, Revision 1.10
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: .\"
1.10 ! provos 12: .\" $Id: ssh.1,v 1.9 1999/10/02 13:10:26 deraadt 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
1.6 deraadt 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
34: .Op Fl L Ar port:host:hostport
35: .Op Fl R Ar port:host:hostport
1.5 deraadt 36: .Op Ar hostname | user@hostname
1.2 deraadt 37: .Op Ar command
38: .Sh DESCRIPTION
39: .Nm
1.5 deraadt 40: (Secure Shell) is a program for logging into a remote machine and for
41: executing commands on a remote machine. It is intended to replace
1.1 deraadt 42: rlogin and rsh, and provide secure encrypted communications between
43: two untrusted hosts over an insecure network. X11 connections and
44: arbitrary TCP/IP ports can also be forwarded over the secure channel.
1.2 deraadt 45: .Pp
46: .Nm
1.1 deraadt 47: connects and logs into the specified
1.2 deraadt 48: .Ar hostname .
1.1 deraadt 49: The user must prove
50: his/her identity to the remote machine using one of several methods.
1.2 deraadt 51: .Pp
1.1 deraadt 52: First, if the machine the user logs in from is listed in
1.2 deraadt 53: .Pa /etc/hosts.equiv
1.1 deraadt 54: or
1.2 deraadt 55: .Pa /etc/shosts.equiv
1.1 deraadt 56: on the remote machine, and the user names are
57: the same on both sides, the user is immediately permitted to log in.
58: Second, if
1.2 deraadt 59: .Pa \&.rhosts
1.1 deraadt 60: or
1.2 deraadt 61: .Pa \&.shosts
1.1 deraadt 62: exists in the user's home directory on the
63: remote machine and contains a line containing the name of the client
64: machine and the name of the user on that machine, the user is
65: permitted to log in. This form of authentication alone is normally not
66: allowed by the server because it is not secure.
1.2 deraadt 67: .Pp
1.1 deraadt 68: The second (and primary) authentication method is the
1.2 deraadt 69: .Pa rhosts
1.1 deraadt 70: or
1.2 deraadt 71: .Pa hosts.equiv
1.1 deraadt 72: method combined with RSA-based host authentication. It
73: means that if the login would be permitted by
1.2 deraadt 74: .Pa \&.rhosts ,
75: .Pa \&.shosts ,
76: .Pa /etc/hosts.equiv ,
1.1 deraadt 77: or
1.2 deraadt 78: .Pa /etc/shosts.equiv ,
1.1 deraadt 79: and additionally it can verify the client's
80: host key (see
1.2 deraadt 81: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 82: and
1.2 deraadt 83: .Pa /etc/ssh_known_hosts
1.1 deraadt 84: in the
1.2 deraadt 85: .Sx FILES
1.1 deraadt 86: section), only then login is
87: permitted. This authentication method closes security holes due to IP
88: spoofing, DNS spoofing and routing spoofing. [Note to the
89: administrator:
1.2 deraadt 90: .Pa /etc/hosts.equiv ,
91: .Pa \&.rhosts ,
1.1 deraadt 92: and the rlogin/rsh protocol in general, are inherently insecure and should be
93: disabled if security is desired.]
1.2 deraadt 94: .Pp
1.1 deraadt 95: As a third authentication method,
1.2 deraadt 96: .Nm
1.1 deraadt 97: supports RSA based authentication.
98: The scheme is based on public-key cryptography: there are cryptosystems
99: where encryption and decryption are done using separate keys, and it
100: is not possible to derive the decryption key from the encryption key.
101: RSA is one such system. The idea is that each user creates a public/private
102: key pair for authentication purposes. The
103: server knows the public key, and only the user knows the private key.
104: The file
1.2 deraadt 105: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 106: lists the public keys that are permitted for logging
107: in. When the user logs in, the
1.2 deraadt 108: .Nm
1.1 deraadt 109: program tells the server which key pair it would like to use for
110: authentication. The server checks if this key is permitted, and if
111: so, sends the user (actually the
1.2 deraadt 112: .Nm
1.1 deraadt 113: program running on behalf of the user) a challenge, a random number,
114: encrypted by the user's public key. The challenge can only be
115: decrypted using the proper private key. The user's client then decrypts the
116: challenge using the private key, proving that he/she knows the private
117: key but without disclosing it to the server.
1.2 deraadt 118: .Pp
119: .Nm
1.1 deraadt 120: implements the RSA authentication protocol automatically. The user
121: creates his/her RSA key pair by running
1.2 deraadt 122: .Xr ssh-keygen 1 .
1.1 deraadt 123: This stores the private key in
1.2 deraadt 124: .Pa \&.ssh/identity
1.1 deraadt 125: and the public key in
1.2 deraadt 126: .Pa \&.ssh/identity.pub
1.1 deraadt 127: in the user's home directory. The user should then
128: copy the
1.2 deraadt 129: .Pa identity.pub
1.1 deraadt 130: to
1.2 deraadt 131: .Pa \&.ssh/authorized_keys
1.1 deraadt 132: in his/her home directory on the remote machine (the
1.2 deraadt 133: .Pa authorized_keys
1.1 deraadt 134: file corresponds to the conventional
1.2 deraadt 135: .Pa \&.rhosts
1.1 deraadt 136: file, and has one key
137: per line, though the lines can be very long). After this, the user
138: can log in without giving the password. RSA authentication is much
139: more secure than rhosts authentication.
1.2 deraadt 140: .Pp
1.1 deraadt 141: The most convenient way to use RSA authentication may be with an
142: authentication agent. See
1.2 deraadt 143: .Xr ssh-agent 1
1.1 deraadt 144: for more information.
1.2 deraadt 145: .Pp
1.1 deraadt 146: If other authentication methods fail,
1.2 deraadt 147: .Nm
1.1 deraadt 148: prompts the user for a password. The password is sent to the remote
149: host for checking; however, since all communications are encrypted,
150: the password cannot be seen by someone listening on the network.
1.2 deraadt 151: .Pp
1.1 deraadt 152: When the user's identity has been accepted by the server, the server
153: either executes the given command, or logs into the machine and gives
154: the user a normal shell on the remote machine. All communication with
155: the remote command or shell will be automatically encrypted.
1.2 deraadt 156: .Pp
1.1 deraadt 157: If a pseudo-terminal has been allocated (normal login session), the
1.2 deraadt 158: user can disconnect with
159: .Ic ~. ,
160: and suspend
161: .Nm
162: with
163: .Ic ~^Z .
164: All forwarded connections can be listed with
165: .Ic ~#
166: and if
1.1 deraadt 167: the session blocks waiting for forwarded X11 or TCP/IP
1.2 deraadt 168: connections to terminate, it can be backgrounded with
169: .Ic ~&
170: (this should not be used while the user shell is active, as it can cause the
171: shell to hang). All available escapes can be listed with
172: .Ic ~? .
173: .Pp
174: A single tilde character can be sent as
175: .Ic ~~
176: (or by following the tilde by a character other than those described above).
1.1 deraadt 177: The escape character must always follow a newline to be interpreted as
178: special. The escape character can be changed in configuration files
179: or on the command line.
1.2 deraadt 180: .Pp
1.1 deraadt 181: If no pseudo tty has been allocated, the
182: session is transparent and can be used to reliably transfer binary
1.2 deraadt 183: data. On most systems, setting the escape character to
184: .Dq none
185: will also make the session transparent even if a tty is used.
186: .Pp
1.1 deraadt 187: The session terminates when the command or shell in on the remote
188: machine exists and all X11 and TCP/IP connections have been closed.
189: The exit status of the remote program is returned as the exit status
190: of
1.2 deraadt 191: .Nm ssh .
192: .Pp
1.1 deraadt 193: If the user is using X11 (the
1.2 deraadt 194: .Ev DISPLAY
1.1 deraadt 195: environment variable is set), the connection to the X11 display is
196: automatically forwarded to the remote side in such a way that any X11
197: programs started from the shell (or command) will go through the
198: encrypted channel, and the connection to the real X server will be made
199: from the local machine. The user should not manually set
1.2 deraadt 200: .Ev DISPLAY .
1.1 deraadt 201: Forwarding of X11 connections can be
202: configured on the command line or in configuration files.
1.2 deraadt 203: .Pp
204: The
205: .Ev DISPLAY
206: value set by
207: .Nm
1.1 deraadt 208: will point to the server machine, but with a display number greater
209: than zero. This is normal, and happens because
1.2 deraadt 210: .Nm
211: creates a
212: .Dq proxy
213: X server on the server machine for forwarding the
1.1 deraadt 214: connections over the encrypted channel.
1.2 deraadt 215: .Pp
216: .Nm
1.1 deraadt 217: will also automatically set up Xauthority data on the server machine.
218: For this purpose, it will generate a random authorization cookie,
219: store it in Xauthority on the server, and verify that any forwarded
220: connections carry this cookie and replace it by the real cookie when
221: the connection is opened. The real authentication cookie is never
222: sent to the server machine (and no cookies are sent in the plain).
1.2 deraadt 223: .Pp
1.1 deraadt 224: If the user is using an authentication agent, the connection to the agent
225: is automatically forwarded to the remote side unless disabled on
226: command line or in a configuration file.
1.2 deraadt 227: .Pp
1.1 deraadt 228: Forwarding of arbitrary TCP/IP connections over the secure channel can
229: be specified either on command line or in a configuration file. One
230: possible application of TCP/IP forwarding is a secure connection to an
231: electronic purse; another is going trough firewalls.
1.2 deraadt 232: .Pp
233: .Nm
1.1 deraadt 234: automatically maintains and checks a database containing RSA-based
235: identifications for all hosts it has ever been used with. The
236: database is stored in
1.2 deraadt 237: .Pa \&.ssh/known_hosts
1.1 deraadt 238: in the user's home directory. Additionally, the file
1.2 deraadt 239: .Pa /etc/ssh_known_hosts
1.1 deraadt 240: is automatically checked for known hosts. Any new hosts are
241: automatically added to the user's file. If a host's identification
242: ever changes,
1.2 deraadt 243: .Nm
1.1 deraadt 244: warns about this and disables password authentication to prevent a
245: trojan horse from getting the user's password. Another purpose of
246: this mechanism is to prevent man-in-the-middle attacks which could
247: otherwise be used to circumvent the encryption. The
1.2 deraadt 248: .Cm StrictHostKeyChecking
1.1 deraadt 249: option (see below) can be used to prevent logins to machines whose
250: host key is not known or has changed.
1.2 deraadt 251: .Sh OPTIONS
252: .Bl -tag -width Ds
1.4 dugsong 253: .It Fl a
254: Disables forwarding of the authentication agent connection. This may
255: also be specified on a per-host basis in the configuration file.
1.6 deraadt 256: .It Fl c Ar blowfish|3des
1.1 deraadt 257: Selects the cipher to use for encrypting the session.
1.2 deraadt 258: .Ar 3des
1.1 deraadt 259: is used by default. It is believed to be secure.
1.5 deraadt 260: .Ar 3des
261: (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
262: It is presumably more secure than the
1.2 deraadt 263: .Ar des
1.5 deraadt 264: cipher which is no longer supported in ssh.
265: .Ar blowfish
266: is a fast block cipher, it appears very secure and is much faster than
267: .Ar 3des .
1.2 deraadt 268: .It Fl e Ar ch|^ch|none
269: Sets the escape character for sessions with a pty (default:
270: .Ql ~ ) .
271: The escape character is only recognized at the beginning of a line. The
272: escape character followed by a dot
273: .Pq Ql \&.
274: closes the connection, followed
1.1 deraadt 275: by control-Z suspends the connection, and followed by itself sends the
1.2 deraadt 276: escape character once. Setting the character to
277: .Dq none
278: disables any escapes and makes the session fully transparent.
279: .It Fl f
280: Requests
281: .Nm
282: to go to background after authentication. This is useful
283: if
284: .Nm
285: is going to ask for passwords or passphrases, but the user
1.1 deraadt 286: wants it in the background. This implies
1.2 deraadt 287: .Fl n .
1.1 deraadt 288: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 289: something like
290: .Ic ssh -f host xterm .
291: .It Fl i Ar identity_file
1.1 deraadt 292: Selects the file from which the identity (private key) for
1.2 deraadt 293: RSA authentication is read. Default is
294: .Pa \&.ssh/identity
1.1 deraadt 295: in the user's home directory. Identity files may also be specified on
296: a per-host basis in the configuration file. It is possible to have
1.2 deraadt 297: multiple
298: .Fl i
299: options (and multiple identities specified in
1.1 deraadt 300: configuration files).
1.3 deraadt 301: .It Fl g
302: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 303: .It Fl k
1.4 dugsong 304: Disables forwarding of Kerberos tickets and AFS tokens. This may
1.1 deraadt 305: also be specified on a per-host basis in the configuration file.
1.2 deraadt 306: .It Fl l Ar login_name
1.1 deraadt 307: Specifies the user to log in as on the remote machine. This may also
308: be specified on a per-host basis in the configuration file.
1.2 deraadt 309: .It Fl n
310: Redirects stdin from
311: .Pa /dev/null
312: (actually, prevents reading from stdin).
1.1 deraadt 313: This must be used when
1.2 deraadt 314: .Nm
1.1 deraadt 315: is run in the background. A common trick is to use this to run X11
1.2 deraadt 316: programs in a remote machine. For example,
317: .Ic ssh -n shadows.cs.hut.fi emacs &
318: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 319: connection will be automatically forwarded over an encrypted channel.
320: The
1.2 deraadt 321: .Nm
1.1 deraadt 322: program will be put in the background.
323: (This does not work if
1.2 deraadt 324: .Nm
325: needs to ask for a password or passphrase; see also the
326: .Fl f
327: option.)
328: .It Fl o Ar option
1.1 deraadt 329: Can be used to give options in the format used in the config file.
330: This is useful for specifying options for which there is no separate
331: command-line flag. The option has the same format as a line in the
332: configuration file.
1.2 deraadt 333: .It Fl p Ar port
1.1 deraadt 334: Port to connect to on the remote host. This can be specified on a
335: per-host basis in the configuration file.
1.2 deraadt 336: .It Fl q
1.1 deraadt 337: Quiet mode. Causes all warning and diagnostic messages to be
338: suppressed. Only fatal errors are displayed.
1.2 deraadt 339: .It Fl t
1.1 deraadt 340: Force pseudo-tty allocation. This can be used to execute arbitary
341: screen-based programs on a remote machine, which can be very useful
342: e.g. when implementing menu services.
1.2 deraadt 343: .It Fl v
1.1 deraadt 344: Verbose mode. Causes
1.2 deraadt 345: .Nm
1.1 deraadt 346: to print debugging messages about its progress. This is helpful in
347: debugging connection, authentication, and configuration problems.
1.2 deraadt 348: .It Fl x
1.1 deraadt 349: Disables X11 forwarding. This can also be specified on a per-host
350: basis in a configuration file.
1.2 deraadt 351: .It Fl X
1.1 deraadt 352: Enables X11 forwarding.
1.2 deraadt 353: .It Fl C
1.1 deraadt 354: Requests compression of all data (including stdin, stdout, stderr, and
355: data for forwarded X11 and TCP/IP connections). The compression
1.2 deraadt 356: algorithm is the same used by gzip, and the
357: .Dq level
358: can be controlled by the
359: .Cm CompressionLevel
1.1 deraadt 360: option (see below). Compression is desirable on modem lines and other
361: slow connections, but will only slow down things on fast networks.
362: The default value can be set on a host-by-host basis in the
363: configuration files; see the
1.2 deraadt 364: .Cm Compress
1.1 deraadt 365: option below.
1.2 deraadt 366: .It Fl L Ar port:host:hostport
1.1 deraadt 367: Specifies that the given port on the local (client) host is to be
368: forwarded to the given host and port on the remote side. This works
369: by allocating a socket to listen to
1.2 deraadt 370: .Ar port
1.1 deraadt 371: on the local side, and whenever a connection is made to this port, the
372: connection is forwarded over the secure channel, and a connection is
373: made to
1.2 deraadt 374: .Ar host:hostport
1.1 deraadt 375: from the remote machine. Port forwardings can also be specified in the
376: configuration file. Only root can forward privileged ports.
1.2 deraadt 377: .It Fl R Ar port:host:hostport
1.1 deraadt 378: Specifies that the given port on the remote (server) host is to be
379: forwarded to the given host and port on the local side. This works
380: by allocating a socket to listen to
1.2 deraadt 381: .Ar port
1.1 deraadt 382: on the remote side, and whenever a connection is made to this port, the
383: connection is forwarded over the secure channel, and a connection is
384: made to
1.2 deraadt 385: .Ar host:hostport
1.1 deraadt 386: from the local machine. Port forwardings can also be specified in the
387: configuration file. Privileged ports can be forwarded only when
388: logging in as root on the remote machine.
1.2 deraadt 389: .El
390: .Sh CONFIGURATION FILES
391: .Nm
1.1 deraadt 392: obtains configuration data from the following sources (in this order):
393: command line options, user's configuration file
1.2 deraadt 394: .Pq Pa $HOME/.ssh/config ,
395: and system-wide configuration file
396: .Pq Pa /etc/ssh_config .
397: For each parameter, the first obtained value
1.1 deraadt 398: will be used. The configuration files contain sections bracketed by
399: "Host" specifications, and that section is only applied for hosts that
400: match one of the patterns given in the specification. The matched
401: host name is the one given on the command line.
1.2 deraadt 402: .Pp
1.1 deraadt 403: Since the first obtained value for each parameter is used, more
404: host-specific declarations should be given near the beginning of the
405: file, and general defaults at the end.
1.2 deraadt 406: .Pp
1.1 deraadt 407: The configuration file has the following format:
1.2 deraadt 408: .Pp
409: Empty lines and lines starting with
410: .Ql #
411: are comments.
412: .Pp
413: Otherwise a line is of the format
414: .Dq keyword arguments .
415: The possible
1.1 deraadt 416: keywords and their meanings are as follows (note that the
417: configuration files are case-sensitive):
1.2 deraadt 418: .Bl -tag -width Ds
419: .It Cm Host
1.1 deraadt 420: Restricts the following declarations (up to the next
1.2 deraadt 421: .Cm Host
1.1 deraadt 422: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 423: given after the keyword.
424: .Ql \&*
425: and
426: .Ql ?
427: can be used as wildcards in the
428: patterns. A single
429: .Ql \&*
430: as a pattern can be used to provide global
1.1 deraadt 431: defaults for all hosts. The host is the
1.2 deraadt 432: .Ar hostname
1.1 deraadt 433: argument given on the command line (i.e., the name is not converted to
434: a canonicalized host name before matching).
1.2 deraadt 435: .It Cm AFSTokenPassing
1.1 deraadt 436: Specifies whether to pass AFS tokens to remote host. The argument to
437: this keyword must be
1.2 deraadt 438: .Dq yes
439: or
440: .Dq no .
441: .It Cm BatchMode
442: If set to
443: .Dq yes ,
444: passphrase/password querying will be disabled. This
1.1 deraadt 445: option is useful in scripts and other batch jobs where you have no
446: user to supply the password. The argument must be
1.2 deraadt 447: .Dq yes
448: or
449: .Dq no .
450: .It Cm Cipher
1.1 deraadt 451: Specifies the cipher to use for encrypting the session. Currently,
1.2 deraadt 452: .Dq blowfish ,
1.1 deraadt 453: and
1.10 ! provos 454: .Dq 3des
1.2 deraadt 455: are supported. The default is
456: .Dq 3des .
457: .It Cm Compression
1.1 deraadt 458: Specifies whether to use compression. The argument must be
1.2 deraadt 459: .Dq yes
460: or
461: .Dq no .
462: .It Cm CompressionLevel
1.1 deraadt 463: Specifies the compression level to use if compression is enable. The
464: argument must be an integer from 1 (fast) to 9 (slow, best). The
465: default level is 6, which is good for most applications. The meaning
466: of the values is the same as in GNU GZIP.
1.2 deraadt 467: .It Cm ConnectionAttempts
1.1 deraadt 468: Specifies the number of tries (one per second) to make before falling
469: back to rsh or exiting. The argument must be an integer. This may be
470: useful in scripts if the connection sometimes fails.
1.2 deraadt 471: .It Cm EscapeChar
472: Sets the escape character (default:
473: .Ql ~ ) .
474: The escape character can also
1.1 deraadt 475: be set on the command line. The argument should be a single
1.2 deraadt 476: character,
477: .Ql ^
478: followed by a letter, or
479: .Dq none
480: to disable the escape
1.1 deraadt 481: character entirely (making the connection transparent for binary
482: data).
1.2 deraadt 483: .It Cm FallBackToRsh
1.1 deraadt 484: Specifies that if connecting via
1.2 deraadt 485: .Nm
1.1 deraadt 486: fails due to a connection refused error (there is no
1.2 deraadt 487: .Xr sshd 8
1.1 deraadt 488: listening on the remote host),
1.2 deraadt 489: .Xr rsh 1
1.1 deraadt 490: should automatically be used instead (after a suitable warning about
491: the session being unencrypted). The argument must be
1.2 deraadt 492: .Dq yes
493: or
494: .Dq no .
495: .It Cm ForwardAgent
1.1 deraadt 496: Specifies whether the connection to the authentication agent (if any)
497: will be forwarded to the remote machine. The argument must be
1.2 deraadt 498: .Dq yes
499: or
500: .Dq no .
501: .It Cm ForwardX11
1.1 deraadt 502: Specifies whether X11 connections will be automatically redirected
503: over the secure channel and
1.2 deraadt 504: .Ev DISPLAY
1.1 deraadt 505: set. The argument must be
1.2 deraadt 506: .Dq yes
507: or
1.3 deraadt 508: .Dq no .
509: .It Cm GatewayPorts
510: Specifies whether remote hosts are allowed to connect to local
511: forwarded ports.
512: The argument must be
513: .Dq yes
514: or
515: .Dq no .
516: The default is
1.2 deraadt 517: .Dq no .
518: .It Cm GlobalKnownHostsFile
1.1 deraadt 519: Specifies a file to use instead of
1.2 deraadt 520: .Pa /etc/ssh_known_hosts .
521: .It Cm HostName
1.1 deraadt 522: Specifies the real host name to log into. This can be used to specify
523: nicnames or abbreviations for hosts. Default is the name given on the
524: command line. Numeric IP addresses are also permitted (both on the
525: command line and in
1.2 deraadt 526: .Cm HostName
1.1 deraadt 527: specifications).
1.2 deraadt 528: .It Cm IdentityFile
1.1 deraadt 529: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 530: is read (default
531: .Pa .ssh/identity
532: in the user's home directory).
1.1 deraadt 533: Additionally, any identities represented by the authentication agent
534: will be used for authentication. The file name may use the tilde
535: syntax to refer to a user's home directory. It is possible to have
536: multiple identity files specified in configuration files; all these
537: identities will be tried in sequence.
1.2 deraadt 538: .It Cm KeepAlive
1.1 deraadt 539: Specifies whether the system should send keepalive messages to the
540: other side. If they are sent, death of the connection or crash of one
541: of the machines will be properly noticed. However, this means that
542: connections will die if the route is down temporarily, and some people
543: find it annoying.
1.2 deraadt 544: .Pp
545: The default is
546: .Dq yes
547: (to send keepalives), and the client will notice
1.1 deraadt 548: if the network goes down or the remote host dies. This is important
549: in scripts, and many users want it too.
1.2 deraadt 550: .Pp
551: To disable keepalives, the value should be set to
552: .Dq no
553: in both the server and the client configuration files.
554: .It Cm KerberosAuthentication
1.4 dugsong 555: Specifies whether Kerberos authentication will be used. The argument to
556: this keyword must be
557: .Dq yes
558: or
559: .Dq no .
1.2 deraadt 560: .It Cm KerberosTgtPassing
1.4 dugsong 561: Specifies whether a Kerberos TGT will be forwarded to the server. This
562: will only work if the Kerberos server is actually an AFS kaserver. The
563: argument to this keyword must be
564: .Dq yes
565: or
566: .Dq no .
1.2 deraadt 567: .It Cm LocalForward
1.1 deraadt 568: Specifies that a TCP/IP port on the local machine be forwarded over
569: the secure channel to given host:port from the remote machine. The
570: first argument must be a port number, and the second must be
571: host:port. Multiple forwardings may be specified, and additional
572: forwardings can be given on the command line. Only the root can
573: forward privileged ports.
1.2 deraadt 574: .It Cm PasswordAuthentication
1.1 deraadt 575: Specifies whether to use password authentication. The argument to
576: this keyword must be
1.2 deraadt 577: .Dq yes
578: or
579: .Dq no .
580: .It Cm Port
1.1 deraadt 581: Specifies the port number to connect on the remote host. Default is
582: 22.
1.2 deraadt 583: .It Cm ProxyCommand
1.1 deraadt 584: Specifies the command to use to connect to the server. The command
585: string extends to the end of the line, and is executed with /bin/sh.
586: In the command string, %h will be substituted by the host name to
587: connect and %p by the port. The command can be basically anything,
588: and should read from its stdin and write to its stdout. It should
589: eventually connect an
1.2 deraadt 590: .Xr sshd 8
1.1 deraadt 591: server running on some machine, or execute
1.2 deraadt 592: .Ic sshd -i
593: somewhere. Host key management will be done using the
1.1 deraadt 594: HostName of the host being connected (defaulting to the name typed by
595: the user).
1.2 deraadt 596: .Pp
1.1 deraadt 597: Note that
1.2 deraadt 598: .Nm
1.1 deraadt 599: can also be configured to support the SOCKS system using the
600: --with-socks compile-time configuration option.
1.2 deraadt 601: .It Cm RemoteForward
1.1 deraadt 602: Specifies that a TCP/IP port on the remote machine be forwarded over
603: the secure channel to given host:port from the local machine. The
604: first argument must be a port number, and the second must be
605: host:port. Multiple forwardings may be specified, and additional
606: forwardings can be given on the command line. Only the root can
607: forward privileged ports.
1.2 deraadt 608: .It Cm RhostsAuthentication
1.1 deraadt 609: Specifies whether to try rhosts based authentication. Note that this
610: declaration only affects the client side and has no effect whatsoever
611: on security. Disabling rhosts authentication may reduce
612: authentication time on slow connections when rhosts authentication is
613: not used. Most servers do not permit RhostsAuthentication because it
614: is not secure (see RhostsRSAAuthentication). The argument to this
615: keyword must be
1.2 deraadt 616: .Dq yes
617: or
618: .Dq no .
619: .It Cm RhostsRSAAuthentication
1.1 deraadt 620: Specifies whether to try rhosts based authentication with RSA host
621: authentication. This is the primary authentication method for most
622: sites. The argument must be
1.2 deraadt 623: .Dq yes
624: or
625: .Dq no .
626: .It Cm RSAAuthentication
1.1 deraadt 627: Specifies whether to try RSA authentication. The argument to this
628: keyword must be
1.2 deraadt 629: .Dq yes
630: or
631: .Dq no .
1.1 deraadt 632: RSA authentication will only be
633: attempted if the identity file exists, or an authentication agent is
634: running.
1.2 deraadt 635: .It Cm StrictHostKeyChecking
636: If this flag is set to
637: .Dq yes ,
638: .Nm
1.1 deraadt 639: ssh will never automatically add host keys to the
1.2 deraadt 640: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 641: file, and refuses to connect hosts whose host key has changed. This
642: provides maximum protection against trojan horse attacks. However, it
643: can be somewhat annoying if you don't have good
1.2 deraadt 644: .Pa /etc/ssh_known_hosts
1.1 deraadt 645: files installed and frequently
646: connect new hosts. Basically this option forces the user to manually
647: add any new hosts. Normally this option is disabled, and new hosts
648: will automatically be added to the known host files. The host keys of
649: known hosts will be verified automatically in either case. The
650: argument must be
1.2 deraadt 651: .Dq yes
652: or
653: .Dq no .
654: .It Cm User
1.1 deraadt 655: Specifies the user to log in as. This can be useful if you have a
656: different user name in different machines. This saves the trouble of
657: having to remember to give the user name on the command line.
1.2 deraadt 658: .It Cm UserKnownHostsFile
659: Specifies a file to use instead of
660: .Pa $HOME/.ssh/known_hosts .
661: .It Cm UseRsh
1.1 deraadt 662: Specifies that rlogin/rsh should be used for this host. It is
663: possible that the host does not at all support the
1.2 deraadt 664: .Nm
1.1 deraadt 665: protocol. This causes
1.2 deraadt 666: .Nm
1.1 deraadt 667: to immediately exec
1.2 deraadt 668: .Xr rsh 1 .
1.1 deraadt 669: All other options (except
1.2 deraadt 670: .Cm HostName )
1.1 deraadt 671: are ignored if this has been specified. The argument must be
1.2 deraadt 672: .Dq yes
673: or
674: .Dq no .
675: .Sh ENVIRONMENT
676: .Nm
1.1 deraadt 677: will normally set the following environment variables:
1.2 deraadt 678: .Bl -tag -width Ds
679: .It Ev DISPLAY
680: The
681: .Ev DISPLAY
682: variable indicates the location of the X11 server. It is
1.1 deraadt 683: automatically set by
1.2 deraadt 684: .Nm
685: to point to a value of the form
686: .Dq hostname:n
687: where hostname indicates
1.1 deraadt 688: the host where the shell runs, and n is an integer >= 1. Ssh uses
689: this special value to forward X11 connections over the secure
690: channel. The user should normally not set DISPLAY explicitly, as that
691: will render the X11 connection insecure (and will require the user to
692: manually copy any required authorization cookies).
1.2 deraadt 693: .It Ev HOME
1.1 deraadt 694: Set to the path of the user's home directory.
1.2 deraadt 695: .It Ev LOGNAME
696: Synonym for
697: .Ev USER ; set for compatibility with systems that use
1.1 deraadt 698: this variable.
1.2 deraadt 699: .It Ev MAIL
1.1 deraadt 700: Set to point the user's mailbox.
1.2 deraadt 701: .It Ev PATH
702: Set to the default
703: .Ev PATH ,
704: as specified when compiling
705: .Nm
1.1 deraadt 706: or, on some systems,
1.2 deraadt 707: .Pa /etc/environment
1.1 deraadt 708: or
1.2 deraadt 709: .Pa /etc/default/login .
710: .It Ev SSH_AUTHENTICATION_FD
1.1 deraadt 711: This is set to an integer value if you are using the authentication
712: agent and a connection to it has been forwarded. The value indicates
713: a file descriptor number used for communicating with the agent. On
714: some systems,
1.2 deraadt 715: .Ev SSH_AUTHENTICATION_SOCKET
1.1 deraadt 716: may be used instead to
717: indicate the path of a unix-domain socket used to communicate with the
718: agent (this method is less secure, and is only used on systems that
719: don't support the first method).
1.2 deraadt 720: .It Ev SSH_CLIENT
1.1 deraadt 721: Identifies the client end of the connection. The variable contains
722: three space-separated values: client ip-address, client port number,
723: and server port number.
1.2 deraadt 724: .It Ev SSH_TTY
1.1 deraadt 725: This is set to the name of the tty (path to the device) associated
726: with the current shell or command. If the current session has no tty,
727: this variable is not set.
1.2 deraadt 728: .It Ev TZ
1.1 deraadt 729: The timezone variable is set to indicate the present timezone if it
730: was set when the daemon was started (e.i., the daemon passes the value
731: on to new connections).
1.2 deraadt 732: .It Ev USER
1.1 deraadt 733: Set to the name of the user logging in.
1.2 deraadt 734: .El
735: .Pp
1.1 deraadt 736: Additionally,
1.2 deraadt 737: .Nm
1.1 deraadt 738: reads
1.2 deraadt 739: .Pa /etc/environment
1.1 deraadt 740: and
1.2 deraadt 741: .Pa $HOME/.ssh/environment ,
742: and adds lines of the format
743: .Dq VARNAME=value
1.1 deraadt 744: to the environment. Some systems may have
745: still additional mechanisms for setting up the environment, such as
1.2 deraadt 746: .Pa /etc/default/login
1.1 deraadt 747: on Solaris.
1.2 deraadt 748: .Sh FILES
749: .Bl -tag -width $HOME/.ssh/known_hosts
750: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 751: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 752: in
753: .Pa /etc/ssh_known_hosts ) .
754: See
755: .Xr sshd 8 .
756: .It Pa $HOME/.ssh/random_seed
1.1 deraadt 757: Used for seeding the random number generator. This file contains
758: sensitive data and should read/write for the user and not accessible
759: for others. This file is created the first time the program is run
760: and updated automatically. The user should never need to read or
761: modify this file.
1.2 deraadt 762: .It Pa $HOME/.ssh/identity
1.1 deraadt 763: Contains the RSA authentication identity of the user. This file
764: contains sensitive data and should be readable by the user but not
765: accessible by others. It is possible to specify a passphrase when
766: generating the key; the passphrase will be used to encrypt the
1.8 deraadt 767: sensitive part of this file using 3DES.
1.2 deraadt 768: .It Pa $HOME/.ssh/identity.pub
1.1 deraadt 769: Contains the public key for authentication (public part of the
770: identity file in human-readable form). The contents of this file
1.2 deraadt 771: should be added to
772: .Pa $HOME/.ssh/authorized_keys
773: on all machines
1.1 deraadt 774: where you wish to log in using RSA authentication. This file is not
775: sensitive and can (but need not) be readable by anyone. This file is
776: never used automatically and is not necessary; it is only provided for
777: the convenience of the user.
1.2 deraadt 778: .It Pa $HOME/.ssh/config
1.1 deraadt 779: This is the per-user configuration file. The format of this file is
780: described above. This file is used by the
1.2 deraadt 781: .Nm
1.1 deraadt 782: client. This file does not usually contain any sensitive information,
783: but the recommended permissions are read/write for the user, and not
784: accessible by others.
1.2 deraadt 785: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 786: Lists the RSA keys that can be used for logging in as this user. The
787: format of this file is described in the
1.2 deraadt 788: .Xr sshd 8
1.1 deraadt 789: manual page. In the simplest form the format is the same as the .pub
790: identity files (that is, each line contains the number of bits in
791: modulus, public exponent, modulus, and comment fields, separated by
792: spaces). This file is not highly sensitive, but the recommended
793: permissions are read/write for the user, and not accessible by others.
1.2 deraadt 794: .It Pa /etc/ssh_known_hosts
1.1 deraadt 795: Systemwide list of known host keys. This file should be prepared by the
796: system administrator to contain the public host keys of all machines in the
797: organization. This file should be world-readable. This file contains
798: public keys, one per line, in the following format (fields separated
799: by spaces): system name, number of bits in modulus, public exponent,
800: modulus, and optional comment field. When different names are used
801: for the same machine, all such names should be listed, separated by
802: commas. The format is described on the
1.2 deraadt 803: .Xr sshd 8
1.1 deraadt 804: manual page.
1.2 deraadt 805: .Pp
1.1 deraadt 806: The canonical system name (as returned by name servers) is used by
1.2 deraadt 807: .Xr sshd 8
1.1 deraadt 808: to verify the client host when logging in; other names are needed because
1.2 deraadt 809: .Nm
1.1 deraadt 810: does not convert the user-supplied name to a canonical name before
811: checking the key, because someone with access to the name servers
812: would then be able to fool host authentication.
1.2 deraadt 813: .It Pa /etc/ssh_config
1.1 deraadt 814: Systemwide configuration file. This file provides defaults for those
815: values that are not specified in the user's configuration file, and
816: for those users who do not have a configuration file. This file must
817: be world-readable.
1.2 deraadt 818: .It Pa $HOME/.rhosts
819: This file is used in
820: .Pa \&.rhosts
821: authentication to list the
1.1 deraadt 822: host/user pairs that are permitted to log in. (Note that this file is
823: also used by rlogin and rsh, which makes using this file insecure.)
824: Each line of the file contains a host name (in the canonical form
825: returned by name servers), and then a user name on that host,
826: separated by a space. One some machines this file may need to be
827: world-readable if the user's home directory is on a NFS partition,
828: because
1.2 deraadt 829: .Xr sshd 8
1.1 deraadt 830: reads it as root. Additionally, this file must be owned by the user,
831: and must not have write permissions for anyone else. The recommended
832: permission for most machines is read/write for the user, and not
833: accessible by others.
1.2 deraadt 834: .Pp
1.1 deraadt 835: Note that by default
1.2 deraadt 836: .Xr sshd 8
1.1 deraadt 837: will be installed so that it requires successful RSA host
838: authentication before permitting \s+2.\s0rhosts authentication. If your
839: server machine does not have the client's host key in
1.2 deraadt 840: .Pa /etc/ssh_known_hosts ,
841: you can store it in
842: .Pa $HOME/.ssh/known_hosts .
843: The easiest way to do this is to
1.1 deraadt 844: connect back to the client from the server machine using ssh; this
1.2 deraadt 845: will automatically add the host key inxi
846: .Pa $HOME/.ssh/known_hosts .
847: .It Pa $HOME/.shosts
848: This file is used exactly the same way as
849: .Pa \&.rhosts .
850: The purpose for
1.1 deraadt 851: having this file is to be able to use rhosts authentication with
1.2 deraadt 852: .Nm
853: without permitting login with
854: .Xr rlogin 1
855: or
856: .Xr rsh 1 .
857: .It Pa /etc/hosts.equiv
858: This file is used during
859: .Pa \&.rhosts authentication. It contains
1.1 deraadt 860: canonical hosts names, one per line (the full format is described on
861: the
1.2 deraadt 862: .Xr sshd 8
1.1 deraadt 863: manual page). If the client host is found in this file, login is
864: automatically permitted provided client and server user names are the
865: same. Additionally, successful RSA host authentication is normally
866: required. This file should only be writable by root.
1.2 deraadt 867: .It Pa /etc/shosts.equiv
1.1 deraadt 868: This file is processed exactly as
1.2 deraadt 869: .Pa /etc/hosts.equiv .
1.1 deraadt 870: This file may be useful to permit logins using
1.2 deraadt 871: .Nm
1.1 deraadt 872: but not using rsh/rlogin.
1.2 deraadt 873: .It Pa /etc/sshrc
1.1 deraadt 874: Commands in this file are executed by
1.2 deraadt 875: .Nm
1.1 deraadt 876: when the user logs in just before the user's shell (or command) is started.
877: See the
1.2 deraadt 878: .Xr sshd 8
1.1 deraadt 879: manual page for more information.
1.2 deraadt 880: .It Pa $HOME/.ssh/rc
1.1 deraadt 881: Commands in this file are executed by
1.2 deraadt 882: .Nm
1.1 deraadt 883: when the user logs in just before the user's shell (or command) is
884: started.
885: See the
1.2 deraadt 886: .Xr sshd 8
1.1 deraadt 887: manual page for more information.
1.5 deraadt 888: .It Pa libcrypto.so.X.1
889: A version of this library which includes support for the RSA algorithm
890: is required for proper operation.
1.2 deraadt 891: .Sh AUTHOR
1.1 deraadt 892: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 893: .Pp
894: Issues can be found from the SSH WWW home page:
895: .Pp
896: .Dl http://www.cs.hut.fi/ssh
1.5 deraadt 897: .Pp
898: This version of
899: .Nm
1.7 deraadt 900: is a derivative of the original 1.2.12 release, but with bugs removed and
1.9 deraadt 901: newer features re-added. Rapidly after the 1.2.12 release, newer versions
902: bore successively more restrictive licenses. In this version, all components
1.5 deraadt 903: of a restrictive nature (ie. patents) have been directly removed from the
904: source code; any licensed or patented components are chosen from external
905: libraries. The libraries described in
906: .Xr ssl 8
907: are required for proper operation.
1.2 deraadt 908: .Sh SEE ALSO
909: .Xr rlogin 1 ,
910: .Xr rsh 1 ,
911: .Xr scp 1 ,
912: .Xr ssh-add 1 ,
913: .Xr ssh-agent 1 ,
914: .Xr ssh-keygen 1 ,
915: .Xr telnet 1 ,
1.5 deraadt 916: .Xr sshd 8 ,
917: .Xr ssl 8