Annotation of src/usr.bin/ssh/sshd.8, Revision 1.153.2.4
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.64 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.99 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.64 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.64 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.153.2.4! brad 37: .\" $OpenBSD: sshd.8,v 1.153.2.3 2002/03/07 17:37:48 jason Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSHD 8
40: .Os
41: .Sh NAME
42: .Nm sshd
1.120 markus 43: .Nd OpenSSH SSH daemon
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm sshd
1.150 stevesk 46: .Op Fl deiqtD46
1.2 deraadt 47: .Op Fl b Ar bits
48: .Op Fl f Ar config_file
49: .Op Fl g Ar login_grace_time
50: .Op Fl h Ar host_key_file
51: .Op Fl k Ar key_gen_time
1.153.2.3 jason 52: .Op Fl o Ar option
1.2 deraadt 53: .Op Fl p Ar port
1.61 markus 54: .Op Fl u Ar len
1.40 aaron 55: .Sh DESCRIPTION
1.2 deraadt 56: .Nm
1.106 deraadt 57: (SSH Daemon) is the daemon program for
1.2 deraadt 58: .Xr ssh 1 .
1.42 hugh 59: Together these programs replace rlogin and rsh, and
1.1 deraadt 60: provide secure encrypted communications between two untrusted hosts
1.36 aaron 61: over an insecure network.
62: The programs are intended to be as easy to
1.1 deraadt 63: install and use as possible.
1.2 deraadt 64: .Pp
65: .Nm
1.36 aaron 66: is the daemon that listens for connections from clients.
1.40 aaron 67: It is normally started at boot from
1.2 deraadt 68: .Pa /etc/rc .
69: It forks a new
1.36 aaron 70: daemon for each incoming connection.
71: The forked daemons handle
1.1 deraadt 72: key exchange, encryption, authentication, command execution,
73: and data exchange.
1.49 markus 74: This implementation of
75: .Nm
76: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 77: .Nm
1.36 aaron 78: works as follows.
1.49 markus 79: .Pp
80: .Ss SSH protocol version 1
81: .Pp
1.36 aaron 82: Each host has a host-specific RSA key
83: (normally 1024 bits) used to identify the host.
84: Additionally, when
1.1 deraadt 85: the daemon starts, it generates a server RSA key (normally 768 bits).
86: This key is normally regenerated every hour if it has been used, and
87: is never stored on disk.
1.2 deraadt 88: .Pp
1.42 hugh 89: Whenever a client connects the daemon responds with its public
90: host and server keys.
1.36 aaron 91: The client compares the
1.49 markus 92: RSA host key against its own database to verify that it has not changed.
1.36 aaron 93: The client then generates a 256 bit random number.
94: It encrypts this
1.1 deraadt 95: random number using both the host key and the server key, and sends
1.36 aaron 96: the encrypted number to the server.
1.42 hugh 97: Both sides then use this
1.1 deraadt 98: random number as a session key which is used to encrypt all further
1.36 aaron 99: communications in the session.
100: The rest of the session is encrypted
1.42 hugh 101: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 102: being used by default.
1.36 aaron 103: The client selects the encryption algorithm
1.5 deraadt 104: to use from those offered by the server.
1.2 deraadt 105: .Pp
1.36 aaron 106: Next, the server and the client enter an authentication dialog.
107: The client tries to authenticate itself using
1.2 deraadt 108: .Pa .rhosts
109: authentication,
110: .Pa .rhosts
111: authentication combined with RSA host
1.1 deraadt 112: authentication, RSA challenge-response authentication, or password
113: based authentication.
1.2 deraadt 114: .Pp
1.1 deraadt 115: Rhosts authentication is normally disabled
116: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 117: configuration file if desired.
118: System security is not improved unless
1.2 deraadt 119: .Xr rshd 8 ,
120: .Xr rlogind 8 ,
121: and
1.143 stevesk 122: .Xr rexecd 8
1.1 deraadt 123: are disabled (thus completely disabling
1.2 deraadt 124: .Xr rlogin 1
1.1 deraadt 125: and
1.2 deraadt 126: .Xr rsh 1
1.42 hugh 127: into the machine).
1.2 deraadt 128: .Pp
1.49 markus 129: .Ss SSH protocol version 2
130: .Pp
1.58 deraadt 131: Version 2 works similarly:
1.138 markus 132: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 133: However, when the daemon starts, it does not generate a server key.
134: Forward security is provided through a Diffie-Hellman key agreement.
135: This key agreement results in a shared session key.
1.120 markus 136: .Pp
1.103 deraadt 137: The rest of the session is encrypted using a symmetric cipher, currently
1.120 markus 138: 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
1.49 markus 139: The client selects the encryption algorithm
140: to use from those offered by the server.
141: Additionally, session integrity is provided
1.51 hugh 142: through a cryptographic message authentication code
1.49 markus 143: (hmac-sha1 or hmac-md5).
144: .Pp
145: Protocol version 2 provides a public key based
1.120 markus 146: user (PubkeyAuthentication) or
147: client host (HostbasedAuthentication) authentication method,
148: conventional password authentication and challenge response based methods.
1.49 markus 149: .Pp
150: .Ss Command execution and data forwarding
151: .Pp
1.1 deraadt 152: If the client successfully authenticates itself, a dialog for
1.36 aaron 153: preparing the session is entered.
154: At this time the client may request
1.1 deraadt 155: things like allocating a pseudo-tty, forwarding X11 connections,
156: forwarding TCP/IP connections, or forwarding the authentication agent
157: connection over the secure channel.
1.2 deraadt 158: .Pp
1.1 deraadt 159: Finally, the client either requests a shell or execution of a command.
1.36 aaron 160: The sides then enter session mode.
161: In this mode, either side may send
1.1 deraadt 162: data at any time, and such data is forwarded to/from the shell or
163: command on the server side, and the user terminal in the client side.
1.2 deraadt 164: .Pp
1.1 deraadt 165: When the user program terminates and all forwarded X11 and other
166: connections have been closed, the server sends command exit status to
167: the client, and both sides exit.
1.2 deraadt 168: .Pp
169: .Nm
1.1 deraadt 170: can be configured using command-line options or a configuration
1.36 aaron 171: file.
172: Command-line options override values specified in the
1.1 deraadt 173: configuration file.
1.25 markus 174: .Pp
175: .Nm
176: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 177: .Dv SIGHUP ,
1.128 mpech 178: by executing itself with the name it was started as, i.e.,
1.97 deraadt 179: .Pa /usr/sbin/sshd .
1.18 aaron 180: .Pp
181: The options are as follows:
1.2 deraadt 182: .Bl -tag -width Ds
183: .It Fl b Ar bits
1.120 markus 184: Specifies the number of bits in the ephemeral protocol version 1
185: server key (default 768).
1.2 deraadt 186: .It Fl d
1.36 aaron 187: Debug mode.
188: The server sends verbose debug output to the system
189: log, and does not put itself in the background.
190: The server also will not fork and will only process one connection.
191: This option is only intended for debugging for the server.
1.120 markus 192: Multiple -d options increase the debugging level.
1.67 aaron 193: Maximum is 3.
1.120 markus 194: .It Fl e
195: When this option is specified,
196: .Nm
197: will send the output to the standard error instead of the system log.
1.2 deraadt 198: .It Fl f Ar configuration_file
1.36 aaron 199: Specifies the name of the configuration file.
200: The default is
1.153.2.4! brad 201: .Pa /etc/sshd_config .
1.16 markus 202: .Nm
203: refuses to start if there is no configuration file.
1.2 deraadt 204: .It Fl g Ar login_grace_time
1.1 deraadt 205: Gives the grace time for clients to authenticate themselves (default
1.77 markus 206: 600 seconds).
1.36 aaron 207: If the client fails to authenticate the user within
208: this many seconds, the server disconnects and exits.
209: A value of zero indicates no limit.
1.2 deraadt 210: .It Fl h Ar host_key_file
1.153.2.3 jason 211: Specifies a file from which a host key is read.
1.7 markus 212: This option must be given if
213: .Nm
214: is not run as root (as the normal
1.153.2.3 jason 215: host key files are normally not readable by anyone but root).
216: The default is
1.153.2.4! brad 217: .Pa /etc/ssh_host_key
1.153.2.3 jason 218: for protocol version 1, and
1.153.2.4! brad 219: .Pa /etc/ssh_host_rsa_key
1.153.2.3 jason 220: and
1.153.2.4! brad 221: .Pa /etc/ssh_host_dsa_key
1.153.2.3 jason 222: for protocol version 2.
1.75 markus 223: It is possible to have multiple host key files for
1.120 markus 224: the different protocol versions and host key algorithms.
1.2 deraadt 225: .It Fl i
1.7 markus 226: Specifies that
227: .Nm
1.40 aaron 228: is being run from inetd.
1.7 markus 229: .Nm
230: is normally not run
1.1 deraadt 231: from inetd because it needs to generate the server key before it can
1.36 aaron 232: respond to the client, and this may take tens of seconds.
233: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 234: However, with small key sizes (e.g., 512) using
1.7 markus 235: .Nm
236: from inetd may
1.1 deraadt 237: be feasible.
1.2 deraadt 238: .It Fl k Ar key_gen_time
1.120 markus 239: Specifies how often the ephemeral protocol version 1 server key is
240: regenerated (default 3600 seconds, or one hour).
1.36 aaron 241: The motivation for regenerating the key fairly
1.1 deraadt 242: often is that the key is not stored anywhere, and after about an hour,
243: it becomes impossible to recover the key for decrypting intercepted
244: communications even if the machine is cracked into or physically
1.36 aaron 245: seized.
246: A value of zero indicates that the key will never be regenerated.
1.153.2.3 jason 247: .It Fl o Ar option
248: Can be used to give options in the format used in the configuration file.
249: This is useful for specifying options for which there is no separate
250: command-line flag.
1.2 deraadt 251: .It Fl p Ar port
1.1 deraadt 252: Specifies the port on which the server listens for connections
253: (default 22).
1.153.2.3 jason 254: Multiple port options are permitted.
255: Ports specified in the configuration file are ignored when a
256: command-line port is specified.
1.2 deraadt 257: .It Fl q
1.36 aaron 258: Quiet mode.
259: Nothing is sent to the system log.
260: Normally the beginning,
1.1 deraadt 261: authentication, and termination of each connection is logged.
1.137 stevesk 262: .It Fl t
263: Test mode.
264: Only check the validity of the configuration file and sanity of the keys.
1.153.2.3 jason 265: This is useful for updating
1.137 stevesk 266: .Nm
267: reliably as configuration options may change.
1.61 markus 268: .It Fl u Ar len
269: This option is used to specify the size of the field
270: in the
271: .Li utmp
272: structure that holds the remote host name.
273: If the resolved host name is longer than
274: .Ar len ,
275: the dotted decimal value will be used instead.
276: This allows hosts with very long host names that
277: overflow this field to still be uniquely identified.
278: Specifying
279: .Fl u0
280: indicates that only dotted decimal addresses
281: should be put into the
282: .Pa utmp
283: file.
1.144 stevesk 284: .Fl u0
285: is also be used to prevent
286: .Nm
287: from making DNS requests unless the authentication
288: mechanism or configuration requires it.
289: Authentication mechanisms that may require DNS include
290: .Cm RhostsAuthentication ,
291: .Cm RhostsRSAAuthentication ,
292: .Cm HostbasedAuthentication
293: and using a
294: .Cm from="pattern-list"
295: option in a key file.
1.153.2.3 jason 296: Configuration options that require DNS include using a
297: USER@HOST pattern in
298: .Cm AllowUsers
299: or
300: .Cm DenyUsers .
1.74 markus 301: .It Fl D
302: When this option is specified
303: .Nm
304: will not detach and does not become a daemon.
305: This allows easy monitoring of
1.76 markus 306: .Nm sshd .
1.29 markus 307: .It Fl 4
308: Forces
309: .Nm
310: to use IPv4 addresses only.
311: .It Fl 6
312: Forces
313: .Nm
314: to use IPv6 addresses only.
1.2 deraadt 315: .El
316: .Sh CONFIGURATION FILE
317: .Nm
1.40 aaron 318: reads configuration data from
1.153.2.4! brad 319: .Pa /etc/sshd_config
1.2 deraadt 320: (or the file specified with
321: .Fl f
1.36 aaron 322: on the command line).
1.141 stevesk 323: The file contains keyword-argument pairs, one per line.
1.36 aaron 324: Lines starting with
1.2 deraadt 325: .Ql #
1.1 deraadt 326: and empty lines are interpreted as comments.
1.2 deraadt 327: .Pp
1.141 stevesk 328: The possible
329: keywords and their meanings are as follows (note that
330: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 331: .Bl -tag -width Ds
332: .It Cm AFSTokenPassing
1.36 aaron 333: Specifies whether an AFS token may be forwarded to the server.
334: Default is
1.2 deraadt 335: .Dq yes .
1.11 markus 336: .It Cm AllowGroups
1.153.2.3 jason 337: This keyword can be followed by a list of group name patterns, separated
1.36 aaron 338: by spaces.
339: If specified, login is allowed only for users whose primary
1.81 markus 340: group or supplementary group list matches one of the patterns.
1.11 markus 341: .Ql \&*
342: and
343: .Ql ?
344: can be used as
1.36 aaron 345: wildcards in the patterns.
1.147 deraadt 346: Only group names are valid; a numerical group ID is not recognized.
1.153.2.3 jason 347: By default, login is allowed for all groups.
1.3 dugsong 348: .Pp
1.69 markus 349: .It Cm AllowTcpForwarding
350: Specifies whether TCP forwarding is permitted.
351: The default is
352: .Dq yes .
353: Note that disabling TCP forwarding does not improve security unless
354: users are also denied shell access, as they can always install their
355: own forwarders.
356: .Pp
1.11 markus 357: .It Cm AllowUsers
1.153.2.3 jason 358: This keyword can be followed by a list of user name patterns, separated
1.36 aaron 359: by spaces.
360: If specified, login is allowed only for users names that
1.11 markus 361: match one of the patterns.
362: .Ql \&*
363: and
364: .Ql ?
365: can be used as
1.36 aaron 366: wildcards in the patterns.
1.147 deraadt 367: Only user names are valid; a numerical user ID is not recognized.
1.153.2.3 jason 368: By default, login is allowed for all users.
1.135 markus 369: If the pattern takes the form USER@HOST then USER and HOST
1.147 deraadt 370: are separately checked, restricting logins to particular
1.135 markus 371: users from particular hosts.
1.80 markus 372: .Pp
1.125 markus 373: .It Cm AuthorizedKeysFile
1.138 markus 374: Specifies the file that contains the public keys that can be used
375: for user authentication.
1.125 markus 376: .Cm AuthorizedKeysFile
377: may contain tokens of the form %T which are substituted during connection
1.142 stevesk 378: set-up. The following tokens are defined: %% is replaced by a literal '%',
1.125 markus 379: %h is replaced by the home directory of the user being authenticated and
380: %u is replaced by the username of that user.
381: After expansion,
382: .Cm AuthorizedKeysFile
1.126 markus 383: is taken to be an absolute path or one relative to the user's home
1.125 markus 384: directory.
385: The default is
1.153.2.3 jason 386: .Dq .ssh/authorized_keys .
1.80 markus 387: .It Cm Banner
388: In some jurisdictions, sending a warning message before authentication
389: may be relevant for getting legal protection.
390: The contents of the specified file are sent to the remote user before
391: authentication is allowed.
392: This option is only available for protocol version 2.
1.11 markus 393: .Pp
1.104 deraadt 394: .It Cm ChallengeResponseAuthentication
1.136 markus 395: Specifies whether challenge response authentication is allowed.
396: All authentication styles from
397: .Xr login.conf 5
398: are supported.
1.104 deraadt 399: The default is
400: .Dq yes .
1.122 markus 401: .It Cm Ciphers
402: Specifies the ciphers allowed for protocol version 2.
403: Multiple ciphers must be comma-separated.
404: The default is
1.153.2.3 jason 405: .Pp
406: .Bd -literal
407: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
408: aes192-cbc,aes256-cbc''
409: .Ed
1.115 beck 410: .It Cm ClientAliveInterval
411: Sets a timeout interval in seconds after which if no data has been received
1.133 itojun 412: from the client,
1.115 beck 413: .Nm
414: will send a message through the encrypted
1.116 stevesk 415: channel to request a response from the client.
416: The default
1.115 beck 417: is 0, indicating that these messages will not be sent to the client.
1.116 stevesk 418: This option applies to protocol version 2 only.
1.115 beck 419: .It Cm ClientAliveCountMax
420: Sets the number of client alive messages (see above) which may be
421: sent without
422: .Nm
423: receiving any messages back from the client. If this threshold is
1.133 itojun 424: reached while client alive messages are being sent,
1.115 beck 425: .Nm
426: will disconnect the client, terminating the session. It is important
1.133 itojun 427: to note that the use of client alive messages is very different from
1.153.2.1 jason 428: .Cm KeepAlive
1.116 stevesk 429: (below). The client alive messages are sent through the
1.115 beck 430: encrypted channel and therefore will not be spoofable. The TCP keepalive
1.116 stevesk 431: option enabled by
1.153.2.1 jason 432: .Cm KeepAlive
1.147 deraadt 433: is spoofable. The client alive mechanism is valuable when the client or
434: server depend on knowing when a connection has become inactive.
1.116 stevesk 435: .Pp
1.147 deraadt 436: The default value is 3. If
1.116 stevesk 437: .Cm ClientAliveInterval
1.147 deraadt 438: (above) is set to 15, and
1.152 stevesk 439: .Cm ClientAliveCountMax
440: is left at the default, unresponsive ssh clients
1.133 itojun 441: will be disconnected after approximately 45 seconds.
1.11 markus 442: .It Cm DenyGroups
1.153.2.3 jason 443: This keyword can be followed by a list of group name patterns, separated
1.36 aaron 444: by spaces.
1.153.2.3 jason 445: Login is disallowed for users whose primary group or supplementary
446: group list matches one of the patterns.
1.11 markus 447: .Ql \&*
448: and
449: .Ql ?
450: can be used as
1.36 aaron 451: wildcards in the patterns.
1.147 deraadt 452: Only group names are valid; a numerical group ID is not recognized.
1.153.2.3 jason 453: By default, login is allowed for all groups.
1.11 markus 454: .Pp
455: .It Cm DenyUsers
1.153.2.3 jason 456: This keyword can be followed by a list of user name patterns, separated
1.36 aaron 457: by spaces.
458: Login is disallowed for user names that match one of the patterns.
1.11 markus 459: .Ql \&*
460: and
461: .Ql ?
1.36 aaron 462: can be used as wildcards in the patterns.
1.147 deraadt 463: Only user names are valid; a numerical user ID is not recognized.
1.153.2.3 jason 464: By default, login is allowed for all users.
465: If the pattern takes the form USER@HOST then USER and HOST
466: are separately checked, restricting logins to particular
467: users from particular hosts.
1.47 markus 468: .It Cm GatewayPorts
469: Specifies whether remote hosts are allowed to connect to ports
470: forwarded for the client.
1.145 stevesk 471: By default,
472: .Nm
473: binds remote port forwardings to the loopback addresss. This
474: prevents other remote hosts from connecting to forwarded ports.
475: .Cm GatewayPorts
476: can be used to specify that
477: .Nm
478: should bind remote port forwardings to the wildcard address,
479: thus allowing remote hosts to connect to forwarded ports.
1.47 markus 480: The argument must be
481: .Dq yes
482: or
483: .Dq no .
484: The default is
485: .Dq no .
1.120 markus 486: .It Cm HostbasedAuthentication
487: Specifies whether rhosts or /etc/hosts.equiv authentication together
488: with successful public key client host authentication is allowed
489: (hostbased authentication).
490: This option is similar to
491: .Cm RhostsRSAAuthentication
492: and applies to protocol version 2 only.
493: The default is
494: .Dq no .
1.2 deraadt 495: .It Cm HostKey
1.153.2.3 jason 496: Specifies a file containing a private host key
497: used by SSH.
498: The default is
1.153.2.4! brad 499: .Pa /etc/ssh_host_key
1.153.2.3 jason 500: for protocol version 1, and
1.153.2.4! brad 501: .Pa /etc/ssh_host_rsa_key
1.153.2.3 jason 502: and
1.153.2.4! brad 503: .Pa /etc/ssh_host_dsa_key
1.153.2.3 jason 504: for protocol version 2.
1.9 markus 505: Note that
506: .Nm
1.83 markus 507: will refuse to use a file if it is group/world-accessible.
1.72 markus 508: It is possible to have multiple host key files.
509: .Dq rsa1
510: keys are used for version 1 and
511: .Dq dsa
512: or
513: .Dq rsa
514: are used for version 2 of the SSH protocol.
1.2 deraadt 515: .It Cm IgnoreRhosts
1.34 markus 516: Specifies that
517: .Pa .rhosts
1.40 aaron 518: and
1.34 markus 519: .Pa .shosts
1.120 markus 520: files will not be used in
521: .Cm RhostsAuthentication ,
522: .Cm RhostsRSAAuthentication
523: or
524: .Cm HostbasedAuthentication .
525: .Pp
1.2 deraadt 526: .Pa /etc/hosts.equiv
1.1 deraadt 527: and
1.40 aaron 528: .Pa /etc/shosts.equiv
1.36 aaron 529: are still used.
1.40 aaron 530: The default is
1.34 markus 531: .Dq yes .
1.24 markus 532: .It Cm IgnoreUserKnownHosts
533: Specifies whether
534: .Nm
535: should ignore the user's
536: .Pa $HOME/.ssh/known_hosts
1.45 markus 537: during
1.120 markus 538: .Cm RhostsRSAAuthentication
539: or
540: .Cm HostbasedAuthentication .
1.24 markus 541: The default is
1.2 deraadt 542: .Dq no .
543: .It Cm KeepAlive
1.153.2.3 jason 544: Specifies whether the system should send TCP keepalive messages to the
1.36 aaron 545: other side.
546: If they are sent, death of the connection or crash of one
547: of the machines will be properly noticed.
548: However, this means that
1.1 deraadt 549: connections will die if the route is down temporarily, and some people
1.36 aaron 550: find it annoying.
1.51 hugh 551: On the other hand, if keepalives are not sent,
1.2 deraadt 552: sessions may hang indefinitely on the server, leaving
553: .Dq ghost
554: users and consuming server resources.
555: .Pp
556: The default is
557: .Dq yes
558: (to send keepalives), and the server will notice
1.153.2.3 jason 559: if the network goes down or the client host crashes.
1.36 aaron 560: This avoids infinitely hanging sessions.
1.2 deraadt 561: .Pp
562: To disable keepalives, the value should be set to
1.153.2.3 jason 563: .Dq no .
1.2 deraadt 564: .It Cm KerberosAuthentication
1.36 aaron 565: Specifies whether Kerberos authentication is allowed.
566: This can be in the form of a Kerberos ticket, or if
1.7 markus 567: .Cm PasswordAuthentication
1.1 deraadt 568: is yes, the password provided by the user will be validated through
1.67 aaron 569: the Kerberos KDC.
570: To use this option, the server needs a
1.59 provos 571: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 572: Default is
1.60 provos 573: .Dq yes .
1.2 deraadt 574: .It Cm KerberosOrLocalPasswd
1.1 deraadt 575: If set then if password authentication through Kerberos fails then
576: the password will be validated via any additional local mechanism
1.2 deraadt 577: such as
1.66 markus 578: .Pa /etc/passwd .
1.36 aaron 579: Default is
1.20 dugsong 580: .Dq yes .
1.2 deraadt 581: .It Cm KerberosTgtPassing
1.1 deraadt 582: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 583: Default is
1.3 dugsong 584: .Dq no ,
585: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 586: .It Cm KerberosTicketCleanup
1.7 markus 587: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 588: file on logout.
589: Default is
1.3 dugsong 590: .Dq yes .
1.2 deraadt 591: .It Cm KeyRegenerationInterval
1.120 markus 592: In protocol version 1, the ephemeral server key is automatically regenerated
593: after this many seconds (if it has been used).
1.36 aaron 594: The purpose of regeneration is to prevent
1.1 deraadt 595: decrypting captured sessions by later breaking into the machine and
1.36 aaron 596: stealing the keys.
597: The key is never stored anywhere.
598: If the value is 0, the key is never regenerated.
599: The default is 3600 (seconds).
1.7 markus 600: .It Cm ListenAddress
1.110 stevesk 601: Specifies the local addresses
1.120 markus 602: .Nm
1.7 markus 603: should listen on.
1.110 stevesk 604: The following forms may be used:
605: .Pp
606: .Bl -item -offset indent -compact
607: .It
608: .Cm ListenAddress
1.112 stevesk 609: .Sm off
610: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
611: .Sm on
1.110 stevesk 612: .It
613: .Cm ListenAddress
1.112 stevesk 614: .Sm off
615: .Ar host No | Ar IPv4_addr No : Ar port
616: .Sm on
1.110 stevesk 617: .It
618: .Cm ListenAddress
1.112 stevesk 619: .Sm off
620: .Oo
621: .Ar host No | Ar IPv6_addr Oc : Ar port
622: .Sm on
1.110 stevesk 623: .El
624: .Pp
625: If
1.112 stevesk 626: .Ar port
1.110 stevesk 627: is not specified,
1.120 markus 628: .Nm
1.110 stevesk 629: will listen on the address and all prior
630: .Cm Port
631: options specified. The default is to listen on all local
632: addresses. Multiple
633: .Cm ListenAddress
634: options are permitted. Additionally, any
635: .Cm Port
636: options must precede this option for non port qualified addresses.
1.2 deraadt 637: .It Cm LoginGraceTime
1.1 deraadt 638: The server disconnects after this time if the user has not
1.36 aaron 639: successfully logged in.
640: If the value is 0, there is no time limit.
1.1 deraadt 641: The default is 600 (seconds).
1.23 markus 642: .It Cm LogLevel
643: Gives the verbosity level that is used when logging messages from
644: .Nm sshd .
645: The possible values are:
1.153.2.3 jason 646: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
647: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
648: and DEBUG3 each specify higher levels of debugging output.
649: Logging with a DEBUG level violates the privacy of users
1.23 markus 650: and is not recommended.
1.93 markus 651: .It Cm MACs
652: Specifies the available MAC (message authentication code) algorithms.
653: The MAC algorithm is used in protocol version 2
654: for data integrity protection.
655: Multiple algorithms must be comma-separated.
656: The default is
1.123 markus 657: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 658: .It Cm MaxStartups
659: Specifies the maximum number of concurrent unauthenticated connections to the
660: .Nm
661: daemon.
662: Additional connections will be dropped until authentication succeeds or the
663: .Cm LoginGraceTime
664: expires for a connection.
665: The default is 10.
1.57 markus 666: .Pp
667: Alternatively, random early drop can be enabled by specifying
668: the three colon separated values
669: .Dq start:rate:full
1.67 aaron 670: (e.g., "10:30:60").
1.57 markus 671: .Nm
1.86 stevesk 672: will refuse connection attempts with a probability of
1.57 markus 673: .Dq rate/100
674: (30%)
675: if there are currently
676: .Dq start
677: (10)
678: unauthenticated connections.
1.86 stevesk 679: The probability increases linearly and all connection attempts
1.57 markus 680: are refused if the number of unauthenticated connections reaches
681: .Dq full
682: (60).
1.2 deraadt 683: .It Cm PasswordAuthentication
1.1 deraadt 684: Specifies whether password authentication is allowed.
1.2 deraadt 685: The default is
686: .Dq yes .
687: .It Cm PermitEmptyPasswords
1.1 deraadt 688: When password authentication is allowed, it specifies whether the
1.36 aaron 689: server allows login to accounts with empty password strings.
690: The default is
1.34 markus 691: .Dq no .
1.2 deraadt 692: .It Cm PermitRootLogin
1.100 stevesk 693: Specifies whether root can login using
1.2 deraadt 694: .Xr ssh 1 .
1.15 markus 695: The argument must be
696: .Dq yes ,
1.94 markus 697: .Dq without-password ,
698: .Dq forced-commands-only
1.15 markus 699: or
700: .Dq no .
1.2 deraadt 701: The default is
702: .Dq yes .
1.94 markus 703: .Pp
704: If this option is set to
1.15 markus 705: .Dq without-password
1.94 markus 706: password authentication is disabled for root.
1.2 deraadt 707: .Pp
1.94 markus 708: If this option is set to
709: .Dq forced-commands-only
710: root login with public key authentication will be allowed,
711: but only if the
1.2 deraadt 712: .Ar command
1.94 markus 713: option has been specified
1.1 deraadt 714: (which may be useful for taking remote backups even if root login is
1.94 markus 715: normally not allowed). All other authentication methods are disabled
716: for root.
1.100 stevesk 717: .Pp
718: If this option is set to
719: .Dq no
720: root is not allowed to login.
1.43 markus 721: .It Cm PidFile
722: Specifies the file that contains the process identifier of the
723: .Nm
724: daemon.
725: The default is
726: .Pa /var/run/sshd.pid .
1.2 deraadt 727: .It Cm Port
1.1 deraadt 728: Specifies the port number that
1.2 deraadt 729: .Nm
1.36 aaron 730: listens on.
731: The default is 22.
1.28 markus 732: Multiple options of this type are permitted.
1.120 markus 733: See also
734: .Cm ListenAddress .
1.108 stevesk 735: .It Cm PrintLastLog
736: Specifies whether
737: .Nm
738: should print the date and time when the user last logged in.
739: The default is
740: .Dq yes .
1.2 deraadt 741: .It Cm PrintMotd
1.1 deraadt 742: Specifies whether
1.2 deraadt 743: .Nm
1.40 aaron 744: should print
1.2 deraadt 745: .Pa /etc/motd
1.36 aaron 746: when a user logs in interactively.
747: (On some systems it is also printed by the shell,
1.2 deraadt 748: .Pa /etc/profile ,
1.36 aaron 749: or equivalent.)
750: The default is
1.2 deraadt 751: .Dq yes .
1.41 markus 752: .It Cm Protocol
753: Specifies the protocol versions
754: .Nm
755: should support.
756: The possible values are
757: .Dq 1
758: and
759: .Dq 2 .
760: Multiple versions must be comma-separated.
761: The default is
1.118 deraadt 762: .Dq 2,1 .
1.104 deraadt 763: .It Cm PubkeyAuthentication
764: Specifies whether public key authentication is allowed.
765: The default is
766: .Dq yes .
767: Note that this option applies to protocol version 2 only.
1.2 deraadt 768: .It Cm RhostsAuthentication
1.1 deraadt 769: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 770: files is sufficient.
771: Normally, this method should not be permitted because it is insecure.
1.7 markus 772: .Cm RhostsRSAAuthentication
773: should be used
1.1 deraadt 774: instead, because it performs RSA-based host authentication in addition
775: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 776: The default is
777: .Dq no .
1.120 markus 778: This option applies to protocol version 1 only.
1.2 deraadt 779: .It Cm RhostsRSAAuthentication
1.1 deraadt 780: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 781: with successful RSA host authentication is allowed.
782: The default is
1.34 markus 783: .Dq no .
1.120 markus 784: This option applies to protocol version 1 only.
1.2 deraadt 785: .It Cm RSAAuthentication
1.36 aaron 786: Specifies whether pure RSA authentication is allowed.
787: The default is
1.2 deraadt 788: .Dq yes .
1.120 markus 789: This option applies to protocol version 1 only.
1.2 deraadt 790: .It Cm ServerKeyBits
1.120 markus 791: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 792: The minimum value is 512, and the default is 768.
1.2 deraadt 793: .It Cm StrictModes
1.12 markus 794: Specifies whether
795: .Nm
796: should check file modes and ownership of the
1.36 aaron 797: user's files and home directory before accepting login.
798: This is normally desirable because novices sometimes accidentally leave their
799: directory or files world-writable.
800: The default is
1.7 markus 801: .Dq yes .
1.54 jakob 802: .It Cm Subsystem
1.67 aaron 803: Configures an external subsystem (e.g., file transfer daemon).
804: Arguments should be a subsystem name and a command to execute upon subsystem
805: request.
1.63 markus 806: The command
807: .Xr sftp-server 8
808: implements the
809: .Dq sftp
810: file transfer subsystem.
1.54 jakob 811: By default no subsystems are defined.
812: Note that this option applies to protocol version 2 only.
1.2 deraadt 813: .It Cm SyslogFacility
1.1 deraadt 814: Gives the facility code that is used when logging messages from
1.2 deraadt 815: .Nm sshd .
1.1 deraadt 816: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 817: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
818: The default is AUTH.
1.10 markus 819: .It Cm UseLogin
820: Specifies whether
821: .Xr login 1
1.53 markus 822: is used for interactive login sessions.
1.127 markus 823: The default is
824: .Dq no .
1.53 markus 825: Note that
826: .Xr login 1
1.58 deraadt 827: is never used for remote command execution.
1.133 itojun 828: Note also, that if this is enabled,
829: .Cm X11Forwarding
1.127 markus 830: will be disabled because
831: .Xr login 1
832: does not know how to handle
1.133 itojun 833: .Xr xauth 1
1.127 markus 834: cookies.
1.153.2.3 jason 835: .It Cm VerifyReverseMapping
836: Specifies whether
837: .Nm
838: should try to verify the remote host name and check that
839: the resolved host name for the remote IP address maps back to the
840: very same IP address.
841: The default is
842: .Dq no .
1.6 aaron 843: .It Cm X11DisplayOffset
844: Specifies the first display number available for
845: .Nm sshd Ns 's
1.36 aaron 846: X11 forwarding.
847: This prevents
1.6 aaron 848: .Nm
849: from interfering with real X11 servers.
1.34 markus 850: The default is 10.
1.30 markus 851: .It Cm X11Forwarding
1.36 aaron 852: Specifies whether X11 forwarding is permitted.
853: The default is
1.34 markus 854: .Dq no .
1.30 markus 855: Note that disabling X11 forwarding does not improve security in any
856: way, as users can always install their own forwarders.
1.133 itojun 857: X11 forwarding is automatically disabled if
858: .Cm UseLogin
859: is enabled.
1.153.2.3 jason 860: .It Cm X11UseLocalhost
861: Specifies whether
862: .Nm
863: should bind the X11 forwarding server to the loopback address or to
864: the wildcard address. By default,
865: .Nm
866: binds the forwarding server to the loopback address and sets the
867: hostname part of the
868: .Ev DISPLAY
869: environment variable to
870: .Dq localhost .
871: This prevents remote hosts from connecting to the fake display.
872: However, some older X11 clients may not function with this
873: configuration.
874: .Cm X11UseLocalhost
875: may be set to
876: .Dq no
877: to specify that the forwarding server should be bound to the wildcard
878: address.
879: The argument must be
880: .Dq yes
881: or
882: .Dq no .
883: The default is
884: .Dq yes .
1.52 markus 885: .It Cm XAuthLocation
886: Specifies the location of the
887: .Xr xauth 1
888: program.
889: The default is
890: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 891: .El
1.124 stevesk 892: .Ss Time Formats
893: .Pp
894: .Nm
895: command-line arguments and configuration file options that specify time
896: may be expressed using a sequence of the form:
897: .Sm off
898: .Ar time Oo Ar qualifier Oc ,
899: .Sm on
900: where
901: .Ar time
902: is a positive integer value and
903: .Ar qualifier
904: is one of the following:
905: .Pp
906: .Bl -tag -width Ds -compact -offset indent
907: .It Cm <none>
908: seconds
909: .It Cm s | Cm S
910: seconds
911: .It Cm m | Cm M
912: minutes
913: .It Cm h | Cm H
914: hours
915: .It Cm d | Cm D
916: days
917: .It Cm w | Cm W
918: weeks
919: .El
920: .Pp
921: Each member of the sequence is added together to calculate
922: the total time value.
923: .Pp
924: Time format examples:
925: .Pp
926: .Bl -tag -width Ds -compact -offset indent
927: .It 600
928: 600 seconds (10 minutes)
929: .It 10m
930: 10 minutes
931: .It 1h30m
932: 1 hour 30 minutes (90 minutes)
933: .El
1.2 deraadt 934: .Sh LOGIN PROCESS
1.1 deraadt 935: When a user successfully logs in,
1.2 deraadt 936: .Nm
1.1 deraadt 937: does the following:
1.2 deraadt 938: .Bl -enum -offset indent
939: .It
1.1 deraadt 940: If the login is on a tty, and no command has been specified,
1.40 aaron 941: prints last login time and
1.2 deraadt 942: .Pa /etc/motd
1.1 deraadt 943: (unless prevented in the configuration file or by
1.2 deraadt 944: .Pa $HOME/.hushlogin ;
945: see the
1.40 aaron 946: .Sx FILES
1.2 deraadt 947: section).
948: .It
1.1 deraadt 949: If the login is on a tty, records login time.
1.2 deraadt 950: .It
951: Checks
952: .Pa /etc/nologin ;
953: if it exists, prints contents and quits
1.1 deraadt 954: (unless root).
1.2 deraadt 955: .It
1.1 deraadt 956: Changes to run with normal user privileges.
1.2 deraadt 957: .It
1.1 deraadt 958: Sets up basic environment.
1.2 deraadt 959: .It
960: Reads
961: .Pa $HOME/.ssh/environment
962: if it exists.
963: .It
1.1 deraadt 964: Changes to user's home directory.
1.2 deraadt 965: .It
966: If
967: .Pa $HOME/.ssh/rc
968: exists, runs it; else if
1.153.2.4! brad 969: .Pa /etc/sshrc
1.2 deraadt 970: exists, runs
1.36 aaron 971: it; otherwise runs xauth.
972: The
1.2 deraadt 973: .Dq rc
974: files are given the X11
1.1 deraadt 975: authentication protocol and cookie in standard input.
1.2 deraadt 976: .It
1.1 deraadt 977: Runs user's shell or command.
1.2 deraadt 978: .El
979: .Sh AUTHORIZED_KEYS FILE FORMAT
980: .Pa $HOME/.ssh/authorized_keys
1.130 markus 981: is the default file that lists the public keys that are
982: permitted for RSA authentication in protocol version 1
983: and for public key authentication (PubkeyAuthentication)
984: in protocol version 2.
1.125 markus 985: .Cm AuthorizedKeysFile
986: may be used to specify an alternative file.
1.75 markus 987: .Pp
1.36 aaron 988: Each line of the file contains one
1.2 deraadt 989: key (empty lines and lines starting with a
990: .Ql #
991: are ignored as
1.36 aaron 992: comments).
1.75 markus 993: Each RSA public key consists of the following fields, separated by
1.36 aaron 994: spaces: options, bits, exponent, modulus, comment.
1.75 markus 995: Each protocol version 2 public key consists of:
996: options, keytype, base64 encoded key, comment.
997: The options fields
998: are optional; its presence is determined by whether the line starts
1.1 deraadt 999: with a number or not (the option field never starts with a number).
1.75 markus 1000: The bits, exponent, modulus and comment fields give the RSA key for
1001: protocol version 1; the
1.1 deraadt 1002: comment field is not used for anything (but may be convenient for the
1003: user to identify the key).
1.75 markus 1004: For protocol version 2 the keytype is
1005: .Dq ssh-dss
1006: or
1007: .Dq ssh-rsa .
1.2 deraadt 1008: .Pp
1.1 deraadt 1009: Note that lines in this file are usually several hundred bytes long
1.36 aaron 1010: (because of the size of the RSA key modulus).
1011: You don't want to type them in; instead, copy the
1.113 itojun 1012: .Pa identity.pub ,
1013: .Pa id_dsa.pub
1.75 markus 1014: or the
1.113 itojun 1015: .Pa id_rsa.pub
1.1 deraadt 1016: file and edit it.
1.2 deraadt 1017: .Pp
1.58 deraadt 1018: The options (if present) consist of comma-separated option
1.36 aaron 1019: specifications.
1020: No spaces are permitted, except within double quotes.
1.141 stevesk 1021: The following option specifications are supported (note
1022: that option keywords are case-insensitive):
1.2 deraadt 1023: .Bl -tag -width Ds
1024: .It Cm from="pattern-list"
1.1 deraadt 1025: Specifies that in addition to RSA authentication, the canonical name
1026: of the remote host must be present in the comma-separated list of
1.36 aaron 1027: patterns
1028: .Pf ( Ql *
1029: and
1030: .Ql ?
1031: serve as wildcards).
1032: The list may also contain
1033: patterns negated by prefixing them with
1034: .Ql ! ;
1035: if the canonical host name matches a negated pattern, the key is not accepted.
1036: The purpose
1.1 deraadt 1037: of this option is to optionally increase security: RSA authentication
1038: by itself does not trust the network or name servers or anything (but
1039: the key); however, if somebody somehow steals the key, the key
1.36 aaron 1040: permits an intruder to log in from anywhere in the world.
1041: This additional option makes using a stolen key more difficult (name
1.1 deraadt 1042: servers and/or routers would have to be compromised in addition to
1043: just the key).
1.2 deraadt 1044: .It Cm command="command"
1.1 deraadt 1045: Specifies that the command is executed whenever this key is used for
1.36 aaron 1046: authentication.
1047: The command supplied by the user (if any) is ignored.
1.148 markus 1048: The command is run on a pty if the client requests a pty;
1.36 aaron 1049: otherwise it is run without a tty.
1.147 deraadt 1050: If a 8-bit clean channel is required,
1051: one must not request a pty or should specify
1.89 markus 1052: .Cm no-pty .
1.36 aaron 1053: A quote may be included in the command by quoting it with a backslash.
1054: This option might be useful
1055: to restrict certain RSA keys to perform just a specific operation.
1056: An example might be a key that permits remote backups but nothing else.
1.51 hugh 1057: Note that the client may specify TCP/IP and/or X11
1058: forwarding unless they are explicitly prohibited.
1.149 markus 1059: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 1060: .It Cm environment="NAME=value"
1.1 deraadt 1061: Specifies that the string is to be added to the environment when
1.36 aaron 1062: logging in using this key.
1063: Environment variables set this way
1064: override other default environment values.
1065: Multiple options of this type are permitted.
1.153.2.2 jason 1066: This option is automatically disabled if
1067: .Cm UseLogin
1068: is enabled.
1.2 deraadt 1069: .It Cm no-port-forwarding
1.1 deraadt 1070: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 1071: Any port forward requests by the client will return an error.
1072: This might be used, e.g., in connection with the
1.2 deraadt 1073: .Cm command
1.1 deraadt 1074: option.
1.2 deraadt 1075: .It Cm no-X11-forwarding
1.1 deraadt 1076: Forbids X11 forwarding when this key is used for authentication.
1077: Any X11 forward requests by the client will return an error.
1.2 deraadt 1078: .It Cm no-agent-forwarding
1.1 deraadt 1079: Forbids authentication agent forwarding when this key is used for
1080: authentication.
1.2 deraadt 1081: .It Cm no-pty
1.1 deraadt 1082: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 1083: .It Cm permitopen="host:port"
1.133 itojun 1084: Limit local
1.107 djm 1085: .Li ``ssh -L''
1.111 stevesk 1086: port forwarding such that it may only connect to the specified host and
1.146 stevesk 1087: port.
1088: IPv6 addresses can be specified with an alternative syntax:
1089: .Ar host/port .
1090: Multiple
1.107 djm 1091: .Cm permitopen
1.133 itojun 1092: options may be applied separated by commas. No pattern matching is
1093: performed on the specified hostnames, they must be literal domains or
1.107 djm 1094: addresses.
1.2 deraadt 1095: .El
1096: .Ss Examples
1.1 deraadt 1097: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1098: .Pp
1.1 deraadt 1099: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1100: .Pp
1.1 deraadt 1101: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 1102: .Pp
1103: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1104: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1105: The
1.153.2.4! brad 1106: .Pa /etc/ssh_known_hosts ,
1.40 aaron 1107: and
1.131 markus 1108: .Pa $HOME/.ssh/known_hosts
1.36 aaron 1109: files contain host public keys for all known hosts.
1110: The global file should
1.37 brad 1111: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1112: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1113: its key is added to the per-user file.
1.2 deraadt 1114: .Pp
1.1 deraadt 1115: Each line in these files contains the following fields: hostnames,
1.36 aaron 1116: bits, exponent, modulus, comment.
1117: The fields are separated by spaces.
1.2 deraadt 1118: .Pp
1.1 deraadt 1119: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1120: wildcards); each pattern in turn is matched against the canonical host
1121: name (when authenticating a client) or against the user-supplied
1.36 aaron 1122: name (when authenticating a server).
1123: A pattern may also be preceded by
1.2 deraadt 1124: .Ql !
1125: to indicate negation: if the host name matches a negated
1.1 deraadt 1126: pattern, it is not accepted (by that line) even if it matched another
1127: pattern on the line.
1.2 deraadt 1128: .Pp
1.49 markus 1129: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1130: can be obtained, e.g., from
1.153.2.4! brad 1131: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 1132: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1133: .Pp
1134: Lines starting with
1135: .Ql #
1136: and empty lines are ignored as comments.
1137: .Pp
1.1 deraadt 1138: When performing host authentication, authentication is accepted if any
1.36 aaron 1139: matching line has the proper key.
1140: It is thus permissible (but not
1.1 deraadt 1141: recommended) to have several lines or different host keys for the same
1.36 aaron 1142: names.
1143: This will inevitably happen when short forms of host names
1144: from different domains are put in the file.
1145: It is possible
1.1 deraadt 1146: that the files contain conflicting information; authentication is
1147: accepted if valid information can be found from either file.
1.2 deraadt 1148: .Pp
1.1 deraadt 1149: Note that the lines in these files are typically hundreds of characters
1150: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1151: Rather, generate them by a script
1.40 aaron 1152: or by taking
1.153.2.4! brad 1153: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1154: and adding the host names at the front.
1.2 deraadt 1155: .Ss Examples
1.120 markus 1156: .Bd -literal
1157: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1158: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1159: .Ed
1.2 deraadt 1160: .Sh FILES
1161: .Bl -tag -width Ds
1.153.2.4! brad 1162: .It Pa /etc/sshd_config
1.1 deraadt 1163: Contains configuration data for
1.2 deraadt 1164: .Nm sshd .
1.1 deraadt 1165: This file should be writable by root only, but it is recommended
1166: (though not necessary) that it be world-readable.
1.153.2.4! brad 1167: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.120 markus 1168: These three files contain the private parts of the host keys.
1.98 deraadt 1169: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1170: accessible to others.
1.14 markus 1171: Note that
1172: .Nm
1173: does not start if this file is group/world-accessible.
1.153.2.4! brad 1174: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.120 markus 1175: These three files contain the public parts of the host keys.
1.98 deraadt 1176: These files should be world-readable but writable only by
1.36 aaron 1177: root.
1.98 deraadt 1178: Their contents should match the respective private parts.
1179: These files are not
1180: really used for anything; they are provided for the convenience of
1181: the user so their contents can be copied to known hosts files.
1182: These files are created using
1.7 markus 1183: .Xr ssh-keygen 1 .
1.129 provos 1184: .It Pa /etc/moduli
1.73 provos 1185: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1186: .It Pa /var/run/sshd.pid
1187: Contains the process ID of the
1188: .Nm
1.1 deraadt 1189: listening for connections (if there are several daemons running
1190: concurrently for different ports, this contains the pid of the one
1.36 aaron 1191: started last).
1.58 deraadt 1192: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1193: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 1194: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1195: This file must be readable by root (which may on some machines imply
1196: it being world-readable if the user's home directory resides on an NFS
1197: volume).
1198: It is recommended that it not be accessible by others.
1199: The format of this file is described above.
1200: Users will place the contents of their
1.130 markus 1201: .Pa identity.pub ,
1.44 deraadt 1202: .Pa id_dsa.pub
1.113 itojun 1203: and/or
1204: .Pa id_rsa.pub
1.44 deraadt 1205: files into this file, as described in
1206: .Xr ssh-keygen 1 .
1.153.2.4! brad 1207: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1.22 markus 1208: These files are consulted when using rhosts with RSA host
1.131 markus 1209: authentication or protocol version 2 hostbased authentication
1210: to check the public key of the host.
1.36 aaron 1211: The key must be listed in one of these files to be accepted.
1.22 markus 1212: The client uses the same files
1.96 markus 1213: to verify that it is connecting to the correct remote host.
1.36 aaron 1214: These files should be writable only by root/the owner.
1.153.2.4! brad 1215: .Pa /etc/ssh_known_hosts
1.2 deraadt 1216: should be world-readable, and
1217: .Pa $HOME/.ssh/known_hosts
1.120 markus 1218: can but need not be world-readable.
1.6 aaron 1219: .It Pa /etc/nologin
1.40 aaron 1220: If this file exists,
1.2 deraadt 1221: .Nm
1.36 aaron 1222: refuses to let anyone except root log in.
1223: The contents of the file
1.1 deraadt 1224: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1225: refused.
1226: The file should be world-readable.
1.19 dugsong 1227: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 1228: Access controls that should be enforced by tcp-wrappers are defined here.
1229: Further details are described in
1.19 dugsong 1230: .Xr hosts_access 5 .
1.6 aaron 1231: .It Pa $HOME/.rhosts
1.1 deraadt 1232: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1233: line.
1234: The given user on the corresponding host is permitted to log in
1235: without password.
1236: The same file is used by rlogind and rshd.
1.6 aaron 1237: The file must
1.1 deraadt 1238: be writable only by the user; it is recommended that it not be
1239: accessible by others.
1.2 deraadt 1240: .Pp
1.36 aaron 1241: If is also possible to use netgroups in the file.
1242: Either host or user
1.1 deraadt 1243: name may be of the form +@groupname to specify all hosts or all users
1244: in the group.
1.2 deraadt 1245: .It Pa $HOME/.shosts
1246: For ssh,
1247: this file is exactly the same as for
1248: .Pa .rhosts .
1249: However, this file is
1250: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1251: .It Pa /etc/hosts.equiv
1.2 deraadt 1252: This file is used during
1253: .Pa .rhosts
1.36 aaron 1254: authentication.
1255: In the simplest form, this file contains host names, one per line.
1256: Users on
1.1 deraadt 1257: those hosts are permitted to log in without a password, provided they
1.36 aaron 1258: have the same user name on both machines.
1259: The host name may also be
1.1 deraadt 1260: followed by a user name; such users are permitted to log in as
1.2 deraadt 1261: .Em any
1.36 aaron 1262: user on this machine (except root).
1263: Additionally, the syntax
1.2 deraadt 1264: .Dq +@group
1.36 aaron 1265: can be used to specify netgroups.
1266: Negated entries start with
1.2 deraadt 1267: .Ql \&- .
1268: .Pp
1.1 deraadt 1269: If the client host/user is successfully matched in this file, login is
1270: automatically permitted provided the client and server user names are the
1.36 aaron 1271: same.
1272: Additionally, successful RSA host authentication is normally required.
1273: This file must be writable only by root; it is recommended
1.1 deraadt 1274: that it be world-readable.
1.2 deraadt 1275: .Pp
1.6 aaron 1276: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1277: .Pa hosts.equiv .
1.1 deraadt 1278: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1279: .Em anybody ,
1.1 deraadt 1280: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1281: binaries and directories.
1282: Using a user name practically grants the user root access.
1283: The only valid use for user names that I can think
1.1 deraadt 1284: of is in negative entries.
1.2 deraadt 1285: .Pp
1286: Note that this warning also applies to rsh/rlogin.
1287: .It Pa /etc/shosts.equiv
1.1 deraadt 1288: This is processed exactly as
1.2 deraadt 1289: .Pa /etc/hosts.equiv .
1.1 deraadt 1290: However, this file may be useful in environments that want to run both
1.2 deraadt 1291: rsh/rlogin and ssh.
1.6 aaron 1292: .It Pa $HOME/.ssh/environment
1.36 aaron 1293: This file is read into the environment at login (if it exists).
1294: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1295: .Ql # ) ,
1.36 aaron 1296: and assignment lines of the form name=value.
1297: The file should be writable
1.6 aaron 1298: only by the user; it need not be readable by anyone else.
1.2 deraadt 1299: .It Pa $HOME/.ssh/rc
1.1 deraadt 1300: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1301: environment files but before starting the user's shell or command.
1302: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1303: standard input (and
1304: .Ev DISPLAY
1.36 aaron 1305: in environment).
1306: This must call
1.2 deraadt 1307: .Xr xauth 1
1308: in that case.
1309: .Pp
1.1 deraadt 1310: The primary purpose of this file is to run any initialization routines
1311: which may be needed before the user's home directory becomes
1312: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1313: .Pp
1.1 deraadt 1314: This file will probably contain some initialization code followed by
1.120 markus 1315: something similar to:
1316: .Bd -literal
1317: if read proto cookie; then
1318: echo add $DISPLAY $proto $cookie | xauth -q -
1319: fi
1320: .Ed
1.2 deraadt 1321: .Pp
1322: If this file does not exist,
1.153.2.4! brad 1323: .Pa /etc/sshrc
1.2 deraadt 1324: is run, and if that
1.1 deraadt 1325: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1326: .Pp
1.1 deraadt 1327: This file should be writable only by the user, and need not be
1328: readable by anyone else.
1.153.2.4! brad 1329: .It Pa /etc/sshrc
1.2 deraadt 1330: Like
1331: .Pa $HOME/.ssh/rc .
1332: This can be used to specify
1.36 aaron 1333: machine-specific login-time initializations globally.
1334: This file should be writable only by root, and should be world-readable.
1.56 aaron 1335: .El
1.71 aaron 1336: .Sh AUTHORS
1.84 markus 1337: OpenSSH is a derivative of the original and free
1338: ssh 1.2.12 release by Tatu Ylonen.
1339: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1340: Theo de Raadt and Dug Song
1341: removed many bugs, re-added newer features and
1342: created OpenSSH.
1343: Markus Friedl contributed the support for SSH
1344: protocol versions 1.5 and 2.0.
1.2 deraadt 1345: .Sh SEE ALSO
1346: .Xr scp 1 ,
1.90 djm 1347: .Xr sftp 1 ,
1.2 deraadt 1348: .Xr ssh 1 ,
1.5 deraadt 1349: .Xr ssh-add 1 ,
1.2 deraadt 1350: .Xr ssh-agent 1 ,
1.5 deraadt 1351: .Xr ssh-keygen 1 ,
1.136 markus 1352: .Xr login.conf 5 ,
1353: .Xr moduli 5 ,
1.128 mpech 1354: .Xr sftp-server 8
1.119 markus 1355: .Rs
1356: .%A T. Ylonen
1357: .%A T. Kivinen
1358: .%A M. Saarinen
1359: .%A T. Rinne
1360: .%A S. Lehtinen
1361: .%T "SSH Protocol Architecture"
1.139 markus 1362: .%N draft-ietf-secsh-architecture-09.txt
1363: .%D July 2001
1.120 markus 1364: .%O work in progress material
1365: .Re
1366: .Rs
1367: .%A M. Friedl
1368: .%A N. Provos
1369: .%A W. A. Simpson
1370: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.132 markus 1371: .%N draft-ietf-secsh-dh-group-exchange-01.txt
1372: .%D April 2001
1.119 markus 1373: .%O work in progress material
1374: .Re
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