Annotation of src/usr.bin/ssh/ssh.1, Revision 1.3
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: .\"
1.3 ! deraadt 12: .\" $Id: ssh.1,v 1.2 1999/09/26 22:30:06 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
23: .Ar hostname
24: .Op Ar command
25: .Pp
26: .Nm ssh
1.3 ! deraadt 27: .Op Fl gknqtvxXC
1.2 deraadt 28: .Op Fl c Ar blowfish|idea|des|3des|none
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
36: .Ar hostname
37: .Op Ar command
38: .Sh DESCRIPTION
39: .Nm
1.1 deraadt 40: (Secure Shell) a program for logging into a remote machine and for
41: executing commands in a remote machine. It is intended to replace
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
253: .It Fl c Ar blowfish|idea|des|3des|none
1.1 deraadt 254: Selects the cipher to use for encrypting the session.
1.2 deraadt 255: .Ar 3des
1.1 deraadt 256: is used by default. It is believed to be secure.
1.2 deraadt 257: .Ar des
1.1 deraadt 258: is the data encryption standard, but is breakable by
259: governments, large corporations, and major criminal organizations.
1.2 deraadt 260: .Ar 3des
261: (triple-des) is encrypt-decrypt-encrypt triple with three different keys.
262: It is presumably more secure than DES.
263: .Ar none
1.1 deraadt 264: disables encryption entirely; it is only intended for debugging, and
265: it renders the connection insecure.
1.2 deraadt 266: .It Fl e Ar ch|^ch|none
267: Sets the escape character for sessions with a pty (default:
268: .Ql ~ ) .
269: The escape character is only recognized at the beginning of a line. The
270: escape character followed by a dot
271: .Pq Ql \&.
272: closes the connection, followed
1.1 deraadt 273: by control-Z suspends the connection, and followed by itself sends the
1.2 deraadt 274: escape character once. Setting the character to
275: .Dq none
276: disables any escapes and makes the session fully transparent.
277: .It Fl f
278: Requests
279: .Nm
280: to go to background after authentication. This is useful
281: if
282: .Nm
283: is going to ask for passwords or passphrases, but the user
1.1 deraadt 284: wants it in the background. This implies
1.2 deraadt 285: .Fl n .
1.1 deraadt 286: The recommended way to start X11 programs at a remote site is with
1.2 deraadt 287: something like
288: .Ic ssh -f host xterm .
289: .It Fl i Ar identity_file
1.1 deraadt 290: Selects the file from which the identity (private key) for
1.2 deraadt 291: RSA authentication is read. Default is
292: .Pa \&.ssh/identity
1.1 deraadt 293: in the user's home directory. Identity files may also be specified on
294: a per-host basis in the configuration file. It is possible to have
1.2 deraadt 295: multiple
296: .Fl i
297: options (and multiple identities specified in
1.1 deraadt 298: configuration files).
1.3 ! deraadt 299: .It Fl g
! 300: Allows remote hosts to connect to local forwarded ports.
1.2 deraadt 301: .It Fl k
1.1 deraadt 302: Disables forwarding of Kerberos tickets / AFS tokens. This may
303: also be specified on a per-host basis in the configuration file.
1.2 deraadt 304: .It Fl l Ar login_name
1.1 deraadt 305: Specifies the user to log in as on the remote machine. This may also
306: be specified on a per-host basis in the configuration file.
1.2 deraadt 307: .It Fl n
308: Redirects stdin from
309: .Pa /dev/null
310: (actually, prevents reading from stdin).
1.1 deraadt 311: This must be used when
1.2 deraadt 312: .Nm
1.1 deraadt 313: is run in the background. A common trick is to use this to run X11
1.2 deraadt 314: programs in a remote machine. For example,
315: .Ic ssh -n shadows.cs.hut.fi emacs &
316: will start an emacs on shadows.cs.hut.fi, and the X11
1.1 deraadt 317: connection will be automatically forwarded over an encrypted channel.
318: The
1.2 deraadt 319: .Nm
1.1 deraadt 320: program will be put in the background.
321: (This does not work if
1.2 deraadt 322: .Nm
323: needs to ask for a password or passphrase; see also the
324: .Fl f
325: option.)
326: .It Fl o Ar option
1.1 deraadt 327: Can be used to give options in the format used in the config file.
328: This is useful for specifying options for which there is no separate
329: command-line flag. The option has the same format as a line in the
330: configuration file.
1.2 deraadt 331: .It Fl p Ar port
1.1 deraadt 332: Port to connect to on the remote host. This can be specified on a
333: per-host basis in the configuration file.
1.2 deraadt 334: .It Fl q
1.1 deraadt 335: Quiet mode. Causes all warning and diagnostic messages to be
336: suppressed. Only fatal errors are displayed.
1.2 deraadt 337: .It Fl t
1.1 deraadt 338: Force pseudo-tty allocation. This can be used to execute arbitary
339: screen-based programs on a remote machine, which can be very useful
340: e.g. when implementing menu services.
1.2 deraadt 341: .It Fl v
1.1 deraadt 342: Verbose mode. Causes
1.2 deraadt 343: .Nm
1.1 deraadt 344: to print debugging messages about its progress. This is helpful in
345: debugging connection, authentication, and configuration problems.
1.2 deraadt 346: .It Fl x
1.1 deraadt 347: Disables X11 forwarding. This can also be specified on a per-host
348: basis in a configuration file.
1.2 deraadt 349: .It Fl X
1.1 deraadt 350: Enables X11 forwarding.
1.2 deraadt 351: .It Fl C
1.1 deraadt 352: Requests compression of all data (including stdin, stdout, stderr, and
353: data for forwarded X11 and TCP/IP connections). The compression
1.2 deraadt 354: algorithm is the same used by gzip, and the
355: .Dq level
356: can be controlled by the
357: .Cm CompressionLevel
1.1 deraadt 358: option (see below). Compression is desirable on modem lines and other
359: slow connections, but will only slow down things on fast networks.
360: The default value can be set on a host-by-host basis in the
361: configuration files; see the
1.2 deraadt 362: .Cm Compress
1.1 deraadt 363: option below.
1.2 deraadt 364: .It Fl L Ar port:host:hostport
1.1 deraadt 365: Specifies that the given port on the local (client) host is to be
366: forwarded to the given host and port on the remote side. This works
367: by allocating a socket to listen to
1.2 deraadt 368: .Ar port
1.1 deraadt 369: on the local side, and whenever a connection is made to this port, the
370: connection is forwarded over the secure channel, and a connection is
371: made to
1.2 deraadt 372: .Ar host:hostport
1.1 deraadt 373: from the remote machine. Port forwardings can also be specified in the
374: configuration file. Only root can forward privileged ports.
1.2 deraadt 375: .It Fl R Ar port:host:hostport
1.1 deraadt 376: Specifies that the given port on the remote (server) host is to be
377: forwarded to the given host and port on the local side. This works
378: by allocating a socket to listen to
1.2 deraadt 379: .Ar port
1.1 deraadt 380: on the remote side, and whenever a connection is made to this port, the
381: connection is forwarded over the secure channel, and a connection is
382: made to
1.2 deraadt 383: .Ar host:hostport
1.1 deraadt 384: from the local machine. Port forwardings can also be specified in the
385: configuration file. Privileged ports can be forwarded only when
386: logging in as root on the remote machine.
1.2 deraadt 387: .El
388: .Sh CONFIGURATION FILES
389: .Nm
1.1 deraadt 390: obtains configuration data from the following sources (in this order):
391: command line options, user's configuration file
1.2 deraadt 392: .Pq Pa $HOME/.ssh/config ,
393: and system-wide configuration file
394: .Pq Pa /etc/ssh_config .
395: For each parameter, the first obtained value
1.1 deraadt 396: will be used. The configuration files contain sections bracketed by
397: "Host" specifications, and that section is only applied for hosts that
398: match one of the patterns given in the specification. The matched
399: host name is the one given on the command line.
1.2 deraadt 400: .Pp
1.1 deraadt 401: Since the first obtained value for each parameter is used, more
402: host-specific declarations should be given near the beginning of the
403: file, and general defaults at the end.
1.2 deraadt 404: .Pp
1.1 deraadt 405: The configuration file has the following format:
1.2 deraadt 406: .Pp
407: Empty lines and lines starting with
408: .Ql #
409: are comments.
410: .Pp
411: Otherwise a line is of the format
412: .Dq keyword arguments .
413: The possible
1.1 deraadt 414: keywords and their meanings are as follows (note that the
415: configuration files are case-sensitive):
1.2 deraadt 416: .Bl -tag -width Ds
417: .It Cm Host
1.1 deraadt 418: Restricts the following declarations (up to the next
1.2 deraadt 419: .Cm Host
1.1 deraadt 420: keyword) to be only for those hosts that match one of the patterns
1.2 deraadt 421: given after the keyword.
422: .Ql \&*
423: and
424: .Ql ?
425: can be used as wildcards in the
426: patterns. A single
427: .Ql \&*
428: as a pattern can be used to provide global
1.1 deraadt 429: defaults for all hosts. The host is the
1.2 deraadt 430: .Ar hostname
1.1 deraadt 431: argument given on the command line (i.e., the name is not converted to
432: a canonicalized host name before matching).
1.2 deraadt 433: .It Cm AFSTokenPassing
1.1 deraadt 434: Specifies whether to pass AFS tokens to remote host. The argument to
435: this keyword must be
1.2 deraadt 436: .Dq yes
437: or
438: .Dq no .
439: .It Cm BatchMode
440: If set to
441: .Dq yes ,
442: passphrase/password querying will be disabled. This
1.1 deraadt 443: option is useful in scripts and other batch jobs where you have no
444: user to supply the password. The argument must be
1.2 deraadt 445: .Dq yes
446: or
447: .Dq no .
448: .It Cm Cipher
1.1 deraadt 449: Specifies the cipher to use for encrypting the session. Currently,
1.2 deraadt 450: .Dq blowfish ,
451: .Dq idea ,
452: .Dq des ,
453: .Dq 3des ,
1.1 deraadt 454: and
1.2 deraadt 455: .Dq none
456: are supported. The default is
457: .Dq 3des .
458: Using
459: .Dq none
460: (no encryption) is intended only for debugging, and will render the connection
461: insecure.
462: .It Cm Compression
1.1 deraadt 463: Specifies whether to use compression. The argument must be
1.2 deraadt 464: .Dq yes
465: or
466: .Dq no .
467: .It Cm CompressionLevel
1.1 deraadt 468: Specifies the compression level to use if compression is enable. The
469: argument must be an integer from 1 (fast) to 9 (slow, best). The
470: default level is 6, which is good for most applications. The meaning
471: of the values is the same as in GNU GZIP.
1.2 deraadt 472: .It Cm ConnectionAttempts
1.1 deraadt 473: Specifies the number of tries (one per second) to make before falling
474: back to rsh or exiting. The argument must be an integer. This may be
475: useful in scripts if the connection sometimes fails.
1.2 deraadt 476: .It Cm EscapeChar
477: Sets the escape character (default:
478: .Ql ~ ) .
479: The escape character can also
1.1 deraadt 480: be set on the command line. The argument should be a single
1.2 deraadt 481: character,
482: .Ql ^
483: followed by a letter, or
484: .Dq none
485: to disable the escape
1.1 deraadt 486: character entirely (making the connection transparent for binary
487: data).
1.2 deraadt 488: .It Cm FallBackToRsh
1.1 deraadt 489: Specifies that if connecting via
1.2 deraadt 490: .Nm
1.1 deraadt 491: fails due to a connection refused error (there is no
1.2 deraadt 492: .Xr sshd 8
1.1 deraadt 493: listening on the remote host),
1.2 deraadt 494: .Xr rsh 1
1.1 deraadt 495: should automatically be used instead (after a suitable warning about
496: the session being unencrypted). The argument must be
1.2 deraadt 497: .Dq yes
498: or
499: .Dq no .
500: .It Cm ForwardAgent
1.1 deraadt 501: Specifies whether the connection to the authentication agent (if any)
502: will be forwarded to the remote machine. The argument must be
1.2 deraadt 503: .Dq yes
504: or
505: .Dq no .
506: .It Cm ForwardX11
1.1 deraadt 507: Specifies whether X11 connections will be automatically redirected
508: over the secure channel and
1.2 deraadt 509: .Ev DISPLAY
1.1 deraadt 510: set. The argument must be
1.2 deraadt 511: .Dq yes
512: or
1.3 ! deraadt 513: .Dq no .
! 514: .It Cm GatewayPorts
! 515: Specifies whether remote hosts are allowed to connect to local
! 516: forwarded ports.
! 517: The argument must be
! 518: .Dq yes
! 519: or
! 520: .Dq no .
! 521: The default is
1.2 deraadt 522: .Dq no .
523: .It Cm GlobalKnownHostsFile
1.1 deraadt 524: Specifies a file to use instead of
1.2 deraadt 525: .Pa /etc/ssh_known_hosts .
526: .It Cm HostName
1.1 deraadt 527: Specifies the real host name to log into. This can be used to specify
528: nicnames or abbreviations for hosts. Default is the name given on the
529: command line. Numeric IP addresses are also permitted (both on the
530: command line and in
1.2 deraadt 531: .Cm HostName
1.1 deraadt 532: specifications).
1.2 deraadt 533: .It Cm IdentityFile
1.1 deraadt 534: Specifies the file from which the user's RSA authentication identity
1.2 deraadt 535: is read (default
536: .Pa .ssh/identity
537: in the user's home directory).
1.1 deraadt 538: Additionally, any identities represented by the authentication agent
539: will be used for authentication. The file name may use the tilde
540: syntax to refer to a user's home directory. It is possible to have
541: multiple identity files specified in configuration files; all these
542: identities will be tried in sequence.
1.2 deraadt 543: .It Cm KeepAlive
1.1 deraadt 544: Specifies whether the system should send keepalive messages to the
545: other side. If they are sent, death of the connection or crash of one
546: of the machines will be properly noticed. However, this means that
547: connections will die if the route is down temporarily, and some people
548: find it annoying.
1.2 deraadt 549: .Pp
550: The default is
551: .Dq yes
552: (to send keepalives), and the client will notice
1.1 deraadt 553: if the network goes down or the remote host dies. This is important
554: in scripts, and many users want it too.
1.2 deraadt 555: .Pp
556: To disable keepalives, the value should be set to
557: .Dq no
558: in both the server and the client configuration files.
559: .It Cm KerberosAuthentication
1.1 deraadt 560: Specifies whether Kerberos authentication will be used.
1.2 deraadt 561: .It Cm KerberosTgtPassing
1.1 deraadt 562: Specifies whether a Kerberos TGT will be forwarded to the server.
563: Note that TGT forwarding is normally not enabled in the server.
1.2 deraadt 564: .It Cm LocalForward
1.1 deraadt 565: Specifies that a TCP/IP port on the local machine be forwarded over
566: the secure channel to given host:port from the remote machine. The
567: first argument must be a port number, and the second must be
568: host:port. Multiple forwardings may be specified, and additional
569: forwardings can be given on the command line. Only the root can
570: forward privileged ports.
1.2 deraadt 571: .It Cm PasswordAuthentication
1.1 deraadt 572: Specifies whether to use password authentication. The argument to
573: this keyword must be
1.2 deraadt 574: .Dq yes
575: or
576: .Dq no .
577: .It Cm Port
1.1 deraadt 578: Specifies the port number to connect on the remote host. Default is
579: 22.
1.2 deraadt 580: .It Cm ProxyCommand
1.1 deraadt 581: Specifies the command to use to connect to the server. The command
582: string extends to the end of the line, and is executed with /bin/sh.
583: In the command string, %h will be substituted by the host name to
584: connect and %p by the port. The command can be basically anything,
585: and should read from its stdin and write to its stdout. It should
586: eventually connect an
1.2 deraadt 587: .Xr sshd 8
1.1 deraadt 588: server running on some machine, or execute
1.2 deraadt 589: .Ic sshd -i
590: somewhere. Host key management will be done using the
1.1 deraadt 591: HostName of the host being connected (defaulting to the name typed by
592: the user).
1.2 deraadt 593: .Pp
1.1 deraadt 594: Note that
1.2 deraadt 595: .Nm
1.1 deraadt 596: can also be configured to support the SOCKS system using the
597: --with-socks compile-time configuration option.
1.2 deraadt 598: .It Cm RemoteForward
1.1 deraadt 599: Specifies that a TCP/IP port on the remote machine be forwarded over
600: the secure channel to given host:port from the local machine. The
601: first argument must be a port number, and the second must be
602: host:port. Multiple forwardings may be specified, and additional
603: forwardings can be given on the command line. Only the root can
604: forward privileged ports.
1.2 deraadt 605: .It Cm RhostsAuthentication
1.1 deraadt 606: Specifies whether to try rhosts based authentication. Note that this
607: declaration only affects the client side and has no effect whatsoever
608: on security. Disabling rhosts authentication may reduce
609: authentication time on slow connections when rhosts authentication is
610: not used. Most servers do not permit RhostsAuthentication because it
611: is not secure (see RhostsRSAAuthentication). The argument to this
612: keyword must be
1.2 deraadt 613: .Dq yes
614: or
615: .Dq no .
616: .It Cm RhostsRSAAuthentication
1.1 deraadt 617: Specifies whether to try rhosts based authentication with RSA host
618: authentication. This is the primary authentication method for most
619: sites. The argument must be
1.2 deraadt 620: .Dq yes
621: or
622: .Dq no .
623: .It Cm RSAAuthentication
1.1 deraadt 624: Specifies whether to try RSA authentication. The argument to this
625: keyword must be
1.2 deraadt 626: .Dq yes
627: or
628: .Dq no .
1.1 deraadt 629: RSA authentication will only be
630: attempted if the identity file exists, or an authentication agent is
631: running.
1.2 deraadt 632: .It Cm StrictHostKeyChecking
633: If this flag is set to
634: .Dq yes ,
635: .Nm
1.1 deraadt 636: ssh will never automatically add host keys to the
1.2 deraadt 637: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 638: file, and refuses to connect hosts whose host key has changed. This
639: provides maximum protection against trojan horse attacks. However, it
640: can be somewhat annoying if you don't have good
1.2 deraadt 641: .Pa /etc/ssh_known_hosts
1.1 deraadt 642: files installed and frequently
643: connect new hosts. Basically this option forces the user to manually
644: add any new hosts. Normally this option is disabled, and new hosts
645: will automatically be added to the known host files. The host keys of
646: known hosts will be verified automatically in either case. The
647: argument must be
1.2 deraadt 648: .Dq yes
649: or
650: .Dq no .
651: .It Cm User
1.1 deraadt 652: Specifies the user to log in as. This can be useful if you have a
653: different user name in different machines. This saves the trouble of
654: having to remember to give the user name on the command line.
1.2 deraadt 655: .It Cm UserKnownHostsFile
656: Specifies a file to use instead of
657: .Pa $HOME/.ssh/known_hosts .
658: .It Cm UseRsh
1.1 deraadt 659: Specifies that rlogin/rsh should be used for this host. It is
660: possible that the host does not at all support the
1.2 deraadt 661: .Nm
1.1 deraadt 662: protocol. This causes
1.2 deraadt 663: .Nm
1.1 deraadt 664: to immediately exec
1.2 deraadt 665: .Xr rsh 1 .
1.1 deraadt 666: All other options (except
1.2 deraadt 667: .Cm HostName )
1.1 deraadt 668: are ignored if this has been specified. The argument must be
1.2 deraadt 669: .Dq yes
670: or
671: .Dq no .
672: .Sh ENVIRONMENT
673: .Nm
1.1 deraadt 674: will normally set the following environment variables:
1.2 deraadt 675: .Bl -tag -width Ds
676: .It Ev DISPLAY
677: The
678: .Ev DISPLAY
679: variable indicates the location of the X11 server. It is
1.1 deraadt 680: automatically set by
1.2 deraadt 681: .Nm
682: to point to a value of the form
683: .Dq hostname:n
684: where hostname indicates
1.1 deraadt 685: the host where the shell runs, and n is an integer >= 1. Ssh uses
686: this special value to forward X11 connections over the secure
687: channel. The user should normally not set DISPLAY explicitly, as that
688: will render the X11 connection insecure (and will require the user to
689: manually copy any required authorization cookies).
1.2 deraadt 690: .It Ev HOME
1.1 deraadt 691: Set to the path of the user's home directory.
1.2 deraadt 692: .It Ev LOGNAME
693: Synonym for
694: .Ev USER ; set for compatibility with systems that use
1.1 deraadt 695: this variable.
1.2 deraadt 696: .It Ev MAIL
1.1 deraadt 697: Set to point the user's mailbox.
1.2 deraadt 698: .It Ev PATH
699: Set to the default
700: .Ev PATH ,
701: as specified when compiling
702: .Nm
1.1 deraadt 703: or, on some systems,
1.2 deraadt 704: .Pa /etc/environment
1.1 deraadt 705: or
1.2 deraadt 706: .Pa /etc/default/login .
707: .It Ev SSH_AUTHENTICATION_FD
1.1 deraadt 708: This is set to an integer value if you are using the authentication
709: agent and a connection to it has been forwarded. The value indicates
710: a file descriptor number used for communicating with the agent. On
711: some systems,
1.2 deraadt 712: .Ev SSH_AUTHENTICATION_SOCKET
1.1 deraadt 713: may be used instead to
714: indicate the path of a unix-domain socket used to communicate with the
715: agent (this method is less secure, and is only used on systems that
716: don't support the first method).
1.2 deraadt 717: .It Ev SSH_CLIENT
1.1 deraadt 718: Identifies the client end of the connection. The variable contains
719: three space-separated values: client ip-address, client port number,
720: and server port number.
1.2 deraadt 721: .It Ev SSH_TTY
1.1 deraadt 722: This is set to the name of the tty (path to the device) associated
723: with the current shell or command. If the current session has no tty,
724: this variable is not set.
1.2 deraadt 725: .It Ev TZ
1.1 deraadt 726: The timezone variable is set to indicate the present timezone if it
727: was set when the daemon was started (e.i., the daemon passes the value
728: on to new connections).
1.2 deraadt 729: .It Ev USER
1.1 deraadt 730: Set to the name of the user logging in.
1.2 deraadt 731: .El
732: .Pp
1.1 deraadt 733: Additionally,
1.2 deraadt 734: .Nm
1.1 deraadt 735: reads
1.2 deraadt 736: .Pa /etc/environment
1.1 deraadt 737: and
1.2 deraadt 738: .Pa $HOME/.ssh/environment ,
739: and adds lines of the format
740: .Dq VARNAME=value
1.1 deraadt 741: to the environment. Some systems may have
742: still additional mechanisms for setting up the environment, such as
1.2 deraadt 743: .Pa /etc/default/login
1.1 deraadt 744: on Solaris.
1.2 deraadt 745: .Sh FILES
746: .Bl -tag -width $HOME/.ssh/known_hosts
747: .It Pa $HOME/.ssh/known_hosts
1.1 deraadt 748: Records host keys for all hosts the user has logged into (that are not
1.2 deraadt 749: in
750: .Pa /etc/ssh_known_hosts ) .
751: See
752: .Xr sshd 8 .
753: .It Pa $HOME/.ssh/random_seed
1.1 deraadt 754: Used for seeding the random number generator. This file contains
755: sensitive data and should read/write for the user and not accessible
756: for others. This file is created the first time the program is run
757: and updated automatically. The user should never need to read or
758: modify this file.
1.2 deraadt 759: .It Pa $HOME/.ssh/identity
1.1 deraadt 760: Contains the RSA authentication identity of the user. This file
761: contains sensitive data and should be readable by the user but not
762: accessible by others. It is possible to specify a passphrase when
763: generating the key; the passphrase will be used to encrypt the
1.2 deraadt 764: sensitive part of this file using IDEA.
765: .It Pa $HOME/.ssh/identity.pub
1.1 deraadt 766: Contains the public key for authentication (public part of the
767: identity file in human-readable form). The contents of this file
1.2 deraadt 768: should be added to
769: .Pa $HOME/.ssh/authorized_keys
770: on all machines
1.1 deraadt 771: where you wish to log in using RSA authentication. This file is not
772: sensitive and can (but need not) be readable by anyone. This file is
773: never used automatically and is not necessary; it is only provided for
774: the convenience of the user.
1.2 deraadt 775: .It Pa $HOME/.ssh/config
1.1 deraadt 776: This is the per-user configuration file. The format of this file is
777: described above. This file is used by the
1.2 deraadt 778: .Nm
1.1 deraadt 779: client. This file does not usually contain any sensitive information,
780: but the recommended permissions are read/write for the user, and not
781: accessible by others.
1.2 deraadt 782: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 783: Lists the RSA keys that can be used for logging in as this user. The
784: format of this file is described in the
1.2 deraadt 785: .Xr sshd 8
1.1 deraadt 786: manual page. In the simplest form the format is the same as the .pub
787: identity files (that is, each line contains the number of bits in
788: modulus, public exponent, modulus, and comment fields, separated by
789: spaces). This file is not highly sensitive, but the recommended
790: permissions are read/write for the user, and not accessible by others.
1.2 deraadt 791: .It Pa /etc/ssh_known_hosts
1.1 deraadt 792: Systemwide list of known host keys. This file should be prepared by the
793: system administrator to contain the public host keys of all machines in the
794: organization. This file should be world-readable. This file contains
795: public keys, one per line, in the following format (fields separated
796: by spaces): system name, number of bits in modulus, public exponent,
797: modulus, and optional comment field. When different names are used
798: for the same machine, all such names should be listed, separated by
799: commas. The format is described on the
1.2 deraadt 800: .Xr sshd 8
1.1 deraadt 801: manual page.
1.2 deraadt 802: .Pp
1.1 deraadt 803: The canonical system name (as returned by name servers) is used by
1.2 deraadt 804: .Xr sshd 8
1.1 deraadt 805: to verify the client host when logging in; other names are needed because
1.2 deraadt 806: .Nm
1.1 deraadt 807: does not convert the user-supplied name to a canonical name before
808: checking the key, because someone with access to the name servers
809: would then be able to fool host authentication.
1.2 deraadt 810: .It Pa /etc/ssh_config
1.1 deraadt 811: Systemwide configuration file. This file provides defaults for those
812: values that are not specified in the user's configuration file, and
813: for those users who do not have a configuration file. This file must
814: be world-readable.
1.2 deraadt 815: .It Pa $HOME/.rhosts
816: This file is used in
817: .Pa \&.rhosts
818: authentication to list the
1.1 deraadt 819: host/user pairs that are permitted to log in. (Note that this file is
820: also used by rlogin and rsh, which makes using this file insecure.)
821: Each line of the file contains a host name (in the canonical form
822: returned by name servers), and then a user name on that host,
823: separated by a space. One some machines this file may need to be
824: world-readable if the user's home directory is on a NFS partition,
825: because
1.2 deraadt 826: .Xr sshd 8
1.1 deraadt 827: reads it as root. Additionally, this file must be owned by the user,
828: and must not have write permissions for anyone else. The recommended
829: permission for most machines is read/write for the user, and not
830: accessible by others.
1.2 deraadt 831: .Pp
1.1 deraadt 832: Note that by default
1.2 deraadt 833: .Xr sshd 8
1.1 deraadt 834: will be installed so that it requires successful RSA host
835: authentication before permitting \s+2.\s0rhosts authentication. If your
836: server machine does not have the client's host key in
1.2 deraadt 837: .Pa /etc/ssh_known_hosts ,
838: you can store it in
839: .Pa $HOME/.ssh/known_hosts .
840: The easiest way to do this is to
1.1 deraadt 841: connect back to the client from the server machine using ssh; this
1.2 deraadt 842: will automatically add the host key inxi
843: .Pa $HOME/.ssh/known_hosts .
844: .It Pa $HOME/.shosts
845: This file is used exactly the same way as
846: .Pa \&.rhosts .
847: The purpose for
1.1 deraadt 848: having this file is to be able to use rhosts authentication with
1.2 deraadt 849: .Nm
850: without permitting login with
851: .Xr rlogin 1
852: or
853: .Xr rsh 1 .
854: .It Pa /etc/hosts.equiv
855: This file is used during
856: .Pa \&.rhosts authentication. It contains
1.1 deraadt 857: canonical hosts names, one per line (the full format is described on
858: the
1.2 deraadt 859: .Xr sshd 8
1.1 deraadt 860: manual page). If the client host is found in this file, login is
861: automatically permitted provided client and server user names are the
862: same. Additionally, successful RSA host authentication is normally
863: required. This file should only be writable by root.
1.2 deraadt 864: .It Pa /etc/shosts.equiv
1.1 deraadt 865: This file is processed exactly as
1.2 deraadt 866: .Pa /etc/hosts.equiv .
1.1 deraadt 867: This file may be useful to permit logins using
1.2 deraadt 868: .Nm
1.1 deraadt 869: but not using rsh/rlogin.
1.2 deraadt 870: .It Pa /etc/sshrc
1.1 deraadt 871: Commands in this file are executed by
1.2 deraadt 872: .Nm
1.1 deraadt 873: when the user logs in just before the user's shell (or command) is started.
874: See the
1.2 deraadt 875: .Xr sshd 8
1.1 deraadt 876: manual page for more information.
1.2 deraadt 877: .It Pa $HOME/.ssh/rc
1.1 deraadt 878: Commands in this file are executed by
1.2 deraadt 879: .Nm
1.1 deraadt 880: when the user logs in just before the user's shell (or command) is
881: started.
882: See the
1.2 deraadt 883: .Xr sshd 8
1.1 deraadt 884: manual page for more information.
1.2 deraadt 885: .Sh INSTALLATION
886: .Nm
1.1 deraadt 887: is normally installed as suid root. It needs root privileges only for
888: rhosts authentication (rhosts authentication requires that the
889: connection must come from a privileged port, and allocating such a
890: port requires root privileges). It also needs to be able to read
1.2 deraadt 891: .Pa /etc/ssh_host_key
892: to perform RSA
1.1 deraadt 893: host authentication. It is possible to use
1.2 deraadt 894: .Nm
1.1 deraadt 895: without root privileges, but rhosts authentication will then be
896: disabled.
1.2 deraadt 897: .Nm
1.1 deraadt 898: drops any extra privileges immediately after the connection to the
899: remote host has been made.
1.2 deraadt 900: .Pp
1.1 deraadt 901: Considerable work has been put into making
1.2 deraadt 902: .Xr sshd 8
1.1 deraadt 903: secure. However, if you find a security problem, please report it
904: immediately to <ssh-bugs@cs.hut.fi>.
1.2 deraadt 905: .Sh AUTHOR
1.1 deraadt 906: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 907: .Pp
908: Issues can be found from the SSH WWW home page:
909: .Pp
910: .Dl http://www.cs.hut.fi/ssh
911: .Sh SEE ALSO
912: .Xr make-ssh-known-hosts 1 ,
913: .Xr rlogin 1 ,
914: .Xr rsh 1 ,
915: .Xr scp 1 ,
916: .Xr ssh-add 1 ,
917: .Xr ssh-agent 1 ,
918: .Xr ssh-keygen 1 ,
919: .Xr telnet 1 ,
920: .Xr sshd 8
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