Annotation of src/usr.bin/ssh/sshd.8, Revision 1.176.2.1
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: .\"
1.176.2.1! jason 37: .\" $OpenBSD: sshd.8,v 1.181 2002/05/15 21:02:53 markus 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
1.156 markus 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
1.176.2.1! jason 119: .Nm rshd ,
! 120: .Nm rlogind ,
1.2 deraadt 121: and
1.176.2.1! jason 122: .Xr rexecd
1.1 deraadt 123: are disabled (thus completely disabling
1.176.2.1! jason 124: .Xr rlogin
1.1 deraadt 125: and
1.176.2.1! jason 126: .Xr rsh
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
1.167 deraadt 201: .Pa /etc/ssh/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
1.160 stevesk 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
1.160 stevesk 215: host key files are normally not readable by anyone but root).
216: The default is
1.167 deraadt 217: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 218: for protocol version 1, and
1.167 deraadt 219: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 220: and
1.167 deraadt 221: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 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.
1.156 markus 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).
1.158 stevesk 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.
1.157 deraadt 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.
1.170 stevesk 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
1.167 deraadt 319: .Pa /etc/ssh/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
1.176.2.1! jason 335: .Dq no .
1.11 markus 336: .It Cm AllowGroups
1.163 stevesk 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.
1.163 stevesk 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
1.163 stevesk 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.
1.163 stevesk 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
1.161 stevesk 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.
1.173 stevesk 393: By default, no banner is displayed.
1.11 markus 394: .Pp
1.104 deraadt 395: .It Cm ChallengeResponseAuthentication
1.136 markus 396: Specifies whether challenge response authentication is allowed.
397: All authentication styles from
398: .Xr login.conf 5
399: are supported.
1.104 deraadt 400: The default is
401: .Dq yes .
1.122 markus 402: .It Cm Ciphers
403: Specifies the ciphers allowed for protocol version 2.
404: Multiple ciphers must be comma-separated.
405: The default is
1.162 stevesk 406: .Pp
407: .Bd -literal
408: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
409: aes192-cbc,aes256-cbc''
410: .Ed
1.115 beck 411: .It Cm ClientAliveInterval
412: Sets a timeout interval in seconds after which if no data has been received
1.133 itojun 413: from the client,
1.115 beck 414: .Nm
415: will send a message through the encrypted
1.116 stevesk 416: channel to request a response from the client.
417: The default
1.115 beck 418: is 0, indicating that these messages will not be sent to the client.
1.116 stevesk 419: This option applies to protocol version 2 only.
1.115 beck 420: .It Cm ClientAliveCountMax
421: Sets the number of client alive messages (see above) which may be
422: sent without
423: .Nm
424: receiving any messages back from the client. If this threshold is
1.133 itojun 425: reached while client alive messages are being sent,
1.115 beck 426: .Nm
427: will disconnect the client, terminating the session. It is important
1.133 itojun 428: to note that the use of client alive messages is very different from
1.154 markus 429: .Cm KeepAlive
1.116 stevesk 430: (below). The client alive messages are sent through the
1.115 beck 431: encrypted channel and therefore will not be spoofable. The TCP keepalive
1.116 stevesk 432: option enabled by
1.154 markus 433: .Cm KeepAlive
1.147 deraadt 434: is spoofable. The client alive mechanism is valuable when the client or
435: server depend on knowing when a connection has become inactive.
1.116 stevesk 436: .Pp
1.147 deraadt 437: The default value is 3. If
1.116 stevesk 438: .Cm ClientAliveInterval
1.147 deraadt 439: (above) is set to 15, and
1.152 stevesk 440: .Cm ClientAliveCountMax
441: is left at the default, unresponsive ssh clients
1.133 itojun 442: will be disconnected after approximately 45 seconds.
1.11 markus 443: .It Cm DenyGroups
1.163 stevesk 444: This keyword can be followed by a list of group name patterns, separated
1.36 aaron 445: by spaces.
1.163 stevesk 446: Login is disallowed for users whose primary group or supplementary
447: group list matches one of the patterns.
1.11 markus 448: .Ql \&*
449: and
450: .Ql ?
451: can be used as
1.36 aaron 452: wildcards in the patterns.
1.147 deraadt 453: Only group names are valid; a numerical group ID is not recognized.
1.163 stevesk 454: By default, login is allowed for all groups.
1.11 markus 455: .Pp
456: .It Cm DenyUsers
1.163 stevesk 457: This keyword can be followed by a list of user name patterns, separated
1.36 aaron 458: by spaces.
459: Login is disallowed for user names that match one of the patterns.
1.11 markus 460: .Ql \&*
461: and
462: .Ql ?
1.36 aaron 463: can be used as wildcards in the patterns.
1.147 deraadt 464: Only user names are valid; a numerical user ID is not recognized.
1.163 stevesk 465: By default, login is allowed for all users.
1.169 stevesk 466: If the pattern takes the form USER@HOST then USER and HOST
467: are separately checked, restricting logins to particular
468: users from particular hosts.
1.47 markus 469: .It Cm GatewayPorts
470: Specifies whether remote hosts are allowed to connect to ports
471: forwarded for the client.
1.145 stevesk 472: By default,
473: .Nm
474: binds remote port forwardings to the loopback addresss. This
475: prevents other remote hosts from connecting to forwarded ports.
476: .Cm GatewayPorts
477: can be used to specify that
478: .Nm
479: should bind remote port forwardings to the wildcard address,
480: thus allowing remote hosts to connect to forwarded ports.
1.47 markus 481: The argument must be
482: .Dq yes
483: or
484: .Dq no .
485: The default is
486: .Dq no .
1.120 markus 487: .It Cm HostbasedAuthentication
488: Specifies whether rhosts or /etc/hosts.equiv authentication together
489: with successful public key client host authentication is allowed
490: (hostbased authentication).
491: This option is similar to
492: .Cm RhostsRSAAuthentication
493: and applies to protocol version 2 only.
494: The default is
495: .Dq no .
1.2 deraadt 496: .It Cm HostKey
1.160 stevesk 497: Specifies a file containing a private host key
498: used by SSH.
499: The default is
1.167 deraadt 500: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 501: for protocol version 1, and
1.167 deraadt 502: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 503: and
1.167 deraadt 504: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 505: for protocol version 2.
1.9 markus 506: Note that
507: .Nm
1.83 markus 508: will refuse to use a file if it is group/world-accessible.
1.72 markus 509: It is possible to have multiple host key files.
510: .Dq rsa1
511: keys are used for version 1 and
512: .Dq dsa
513: or
514: .Dq rsa
515: are used for version 2 of the SSH protocol.
1.2 deraadt 516: .It Cm IgnoreRhosts
1.34 markus 517: Specifies that
518: .Pa .rhosts
1.40 aaron 519: and
1.34 markus 520: .Pa .shosts
1.120 markus 521: files will not be used in
522: .Cm RhostsAuthentication ,
523: .Cm RhostsRSAAuthentication
524: or
525: .Cm HostbasedAuthentication .
526: .Pp
1.2 deraadt 527: .Pa /etc/hosts.equiv
1.1 deraadt 528: and
1.40 aaron 529: .Pa /etc/shosts.equiv
1.36 aaron 530: are still used.
1.40 aaron 531: The default is
1.34 markus 532: .Dq yes .
1.24 markus 533: .It Cm IgnoreUserKnownHosts
534: Specifies whether
535: .Nm
536: should ignore the user's
537: .Pa $HOME/.ssh/known_hosts
1.45 markus 538: during
1.120 markus 539: .Cm RhostsRSAAuthentication
540: or
541: .Cm HostbasedAuthentication .
1.24 markus 542: The default is
1.2 deraadt 543: .Dq no .
544: .It Cm KeepAlive
1.166 stevesk 545: Specifies whether the system should send TCP keepalive messages to the
1.36 aaron 546: other side.
547: If they are sent, death of the connection or crash of one
548: of the machines will be properly noticed.
549: However, this means that
1.1 deraadt 550: connections will die if the route is down temporarily, and some people
1.36 aaron 551: find it annoying.
1.51 hugh 552: On the other hand, if keepalives are not sent,
1.2 deraadt 553: sessions may hang indefinitely on the server, leaving
554: .Dq ghost
555: users and consuming server resources.
556: .Pp
557: The default is
558: .Dq yes
559: (to send keepalives), and the server will notice
1.166 stevesk 560: if the network goes down or the client host crashes.
1.36 aaron 561: This avoids infinitely hanging sessions.
1.2 deraadt 562: .Pp
563: To disable keepalives, the value should be set to
1.166 stevesk 564: .Dq no .
1.2 deraadt 565: .It Cm KerberosAuthentication
1.36 aaron 566: Specifies whether Kerberos authentication is allowed.
567: This can be in the form of a Kerberos ticket, or if
1.7 markus 568: .Cm PasswordAuthentication
1.1 deraadt 569: is yes, the password provided by the user will be validated through
1.67 aaron 570: the Kerberos KDC.
571: To use this option, the server needs a
1.59 provos 572: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 573: Default is
1.176.2.1! jason 574: .Dq no .
1.2 deraadt 575: .It Cm KerberosOrLocalPasswd
1.1 deraadt 576: If set then if password authentication through Kerberos fails then
577: the password will be validated via any additional local mechanism
1.2 deraadt 578: such as
1.66 markus 579: .Pa /etc/passwd .
1.36 aaron 580: Default is
1.20 dugsong 581: .Dq yes .
1.2 deraadt 582: .It Cm KerberosTgtPassing
1.1 deraadt 583: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 584: Default is
1.3 dugsong 585: .Dq no ,
586: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 587: .It Cm KerberosTicketCleanup
1.7 markus 588: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 589: file on logout.
590: Default is
1.3 dugsong 591: .Dq yes .
1.2 deraadt 592: .It Cm KeyRegenerationInterval
1.120 markus 593: In protocol version 1, the ephemeral server key is automatically regenerated
594: after this many seconds (if it has been used).
1.36 aaron 595: The purpose of regeneration is to prevent
1.1 deraadt 596: decrypting captured sessions by later breaking into the machine and
1.36 aaron 597: stealing the keys.
598: The key is never stored anywhere.
599: If the value is 0, the key is never regenerated.
600: The default is 3600 (seconds).
1.7 markus 601: .It Cm ListenAddress
1.110 stevesk 602: Specifies the local addresses
1.120 markus 603: .Nm
1.7 markus 604: should listen on.
1.110 stevesk 605: The following forms may be used:
606: .Pp
607: .Bl -item -offset indent -compact
608: .It
609: .Cm ListenAddress
1.112 stevesk 610: .Sm off
611: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
612: .Sm on
1.110 stevesk 613: .It
614: .Cm ListenAddress
1.112 stevesk 615: .Sm off
616: .Ar host No | Ar IPv4_addr No : Ar port
617: .Sm on
1.110 stevesk 618: .It
619: .Cm ListenAddress
1.112 stevesk 620: .Sm off
621: .Oo
622: .Ar host No | Ar IPv6_addr Oc : Ar port
623: .Sm on
1.110 stevesk 624: .El
625: .Pp
626: If
1.112 stevesk 627: .Ar port
1.110 stevesk 628: is not specified,
1.120 markus 629: .Nm
1.110 stevesk 630: will listen on the address and all prior
631: .Cm Port
632: options specified. The default is to listen on all local
633: addresses. Multiple
634: .Cm ListenAddress
635: options are permitted. Additionally, any
636: .Cm Port
637: options must precede this option for non port qualified addresses.
1.2 deraadt 638: .It Cm LoginGraceTime
1.1 deraadt 639: The server disconnects after this time if the user has not
1.36 aaron 640: successfully logged in.
641: If the value is 0, there is no time limit.
1.1 deraadt 642: The default is 600 (seconds).
1.23 markus 643: .It Cm LogLevel
644: Gives the verbosity level that is used when logging messages from
645: .Nm sshd .
646: The possible values are:
1.159 stevesk 647: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
648: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
649: and DEBUG3 each specify higher levels of debugging output.
650: Logging with a DEBUG level violates the privacy of users
1.23 markus 651: and is not recommended.
1.93 markus 652: .It Cm MACs
653: Specifies the available MAC (message authentication code) algorithms.
654: The MAC algorithm is used in protocol version 2
655: for data integrity protection.
656: Multiple algorithms must be comma-separated.
657: The default is
1.123 markus 658: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 659: .It Cm MaxStartups
660: Specifies the maximum number of concurrent unauthenticated connections to the
661: .Nm
662: daemon.
663: Additional connections will be dropped until authentication succeeds or the
664: .Cm LoginGraceTime
665: expires for a connection.
666: The default is 10.
1.57 markus 667: .Pp
668: Alternatively, random early drop can be enabled by specifying
669: the three colon separated values
670: .Dq start:rate:full
1.67 aaron 671: (e.g., "10:30:60").
1.57 markus 672: .Nm
1.86 stevesk 673: will refuse connection attempts with a probability of
1.57 markus 674: .Dq rate/100
675: (30%)
676: if there are currently
677: .Dq start
678: (10)
679: unauthenticated connections.
1.86 stevesk 680: The probability increases linearly and all connection attempts
1.57 markus 681: are refused if the number of unauthenticated connections reaches
682: .Dq full
683: (60).
1.2 deraadt 684: .It Cm PasswordAuthentication
1.1 deraadt 685: Specifies whether password authentication is allowed.
1.2 deraadt 686: The default is
687: .Dq yes .
688: .It Cm PermitEmptyPasswords
1.1 deraadt 689: When password authentication is allowed, it specifies whether the
1.36 aaron 690: server allows login to accounts with empty password strings.
691: The default is
1.34 markus 692: .Dq no .
1.2 deraadt 693: .It Cm PermitRootLogin
1.100 stevesk 694: Specifies whether root can login using
1.2 deraadt 695: .Xr ssh 1 .
1.15 markus 696: The argument must be
697: .Dq yes ,
1.94 markus 698: .Dq without-password ,
699: .Dq forced-commands-only
1.15 markus 700: or
701: .Dq no .
1.2 deraadt 702: The default is
703: .Dq yes .
1.94 markus 704: .Pp
705: If this option is set to
1.15 markus 706: .Dq without-password
1.94 markus 707: password authentication is disabled for root.
1.2 deraadt 708: .Pp
1.94 markus 709: If this option is set to
710: .Dq forced-commands-only
711: root login with public key authentication will be allowed,
712: but only if the
1.2 deraadt 713: .Ar command
1.94 markus 714: option has been specified
1.1 deraadt 715: (which may be useful for taking remote backups even if root login is
1.94 markus 716: normally not allowed). All other authentication methods are disabled
717: for root.
1.100 stevesk 718: .Pp
719: If this option is set to
720: .Dq no
721: root is not allowed to login.
1.43 markus 722: .It Cm PidFile
723: Specifies the file that contains the process identifier of the
724: .Nm
725: daemon.
726: The default is
727: .Pa /var/run/sshd.pid .
1.2 deraadt 728: .It Cm Port
1.1 deraadt 729: Specifies the port number that
1.2 deraadt 730: .Nm
1.36 aaron 731: listens on.
732: The default is 22.
1.28 markus 733: Multiple options of this type are permitted.
1.120 markus 734: See also
735: .Cm ListenAddress .
1.108 stevesk 736: .It Cm PrintLastLog
737: Specifies whether
738: .Nm
739: should print the date and time when the user last logged in.
740: The default is
741: .Dq yes .
1.2 deraadt 742: .It Cm PrintMotd
1.1 deraadt 743: Specifies whether
1.2 deraadt 744: .Nm
1.40 aaron 745: should print
1.2 deraadt 746: .Pa /etc/motd
1.36 aaron 747: when a user logs in interactively.
748: (On some systems it is also printed by the shell,
1.2 deraadt 749: .Pa /etc/profile ,
1.36 aaron 750: or equivalent.)
751: The default is
1.2 deraadt 752: .Dq yes .
1.41 markus 753: .It Cm Protocol
754: Specifies the protocol versions
755: .Nm
756: should support.
757: The possible values are
758: .Dq 1
759: and
760: .Dq 2 .
761: Multiple versions must be comma-separated.
762: The default is
1.118 deraadt 763: .Dq 2,1 .
1.104 deraadt 764: .It Cm PubkeyAuthentication
765: Specifies whether public key authentication is allowed.
766: The default is
767: .Dq yes .
768: Note that this option applies to protocol version 2 only.
1.2 deraadt 769: .It Cm RhostsAuthentication
1.1 deraadt 770: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 771: files is sufficient.
772: Normally, this method should not be permitted because it is insecure.
1.7 markus 773: .Cm RhostsRSAAuthentication
774: should be used
1.1 deraadt 775: instead, because it performs RSA-based host authentication in addition
776: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 777: The default is
778: .Dq no .
1.120 markus 779: This option applies to protocol version 1 only.
1.2 deraadt 780: .It Cm RhostsRSAAuthentication
1.1 deraadt 781: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 782: with successful RSA host authentication is allowed.
783: The default is
1.34 markus 784: .Dq no .
1.120 markus 785: This option applies to protocol version 1 only.
1.2 deraadt 786: .It Cm RSAAuthentication
1.36 aaron 787: Specifies whether pure RSA authentication is allowed.
788: The default is
1.2 deraadt 789: .Dq yes .
1.120 markus 790: This option applies to protocol version 1 only.
1.2 deraadt 791: .It Cm ServerKeyBits
1.120 markus 792: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 793: The minimum value is 512, and the default is 768.
1.2 deraadt 794: .It Cm StrictModes
1.12 markus 795: Specifies whether
796: .Nm
797: should check file modes and ownership of the
1.36 aaron 798: user's files and home directory before accepting login.
799: This is normally desirable because novices sometimes accidentally leave their
800: directory or files world-writable.
801: The default is
1.7 markus 802: .Dq yes .
1.54 jakob 803: .It Cm Subsystem
1.67 aaron 804: Configures an external subsystem (e.g., file transfer daemon).
805: Arguments should be a subsystem name and a command to execute upon subsystem
806: request.
1.63 markus 807: The command
808: .Xr sftp-server 8
809: implements the
810: .Dq sftp
811: file transfer subsystem.
1.54 jakob 812: By default no subsystems are defined.
813: Note that this option applies to protocol version 2 only.
1.2 deraadt 814: .It Cm SyslogFacility
1.1 deraadt 815: Gives the facility code that is used when logging messages from
1.2 deraadt 816: .Nm sshd .
1.1 deraadt 817: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 818: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
819: The default is AUTH.
1.10 markus 820: .It Cm UseLogin
821: Specifies whether
822: .Xr login 1
1.53 markus 823: is used for interactive login sessions.
1.127 markus 824: The default is
825: .Dq no .
1.53 markus 826: Note that
827: .Xr login 1
1.58 deraadt 828: is never used for remote command execution.
1.133 itojun 829: Note also, that if this is enabled,
830: .Cm X11Forwarding
1.127 markus 831: will be disabled because
832: .Xr login 1
833: does not know how to handle
1.133 itojun 834: .Xr xauth 1
1.172 provos 835: cookies. If
836: .Cm UsePrivilegeSeparation
837: is specified, it will be disabled after authentication.
838: .It Cm UsePrivilegeSeparation
839: Specifies whether
840: .Nm
841: separated privileges by creating an unprivileged child process
842: to deal with incoming network traffic. After successful authentication,
843: another process will be created that has the privilege of the authenticated
844: user. The goal of privilege separation is to prevent privilege
845: escalation by containing any corruption within the unprivileged processes.
846: The default is
847: .Dq no .
1.165 markus 848: .It Cm VerifyReverseMapping
849: Specifies whether
850: .Nm
851: should try to verify the remote host name and check that
852: the resolved host name for the remote IP address maps back to the
853: very same IP address.
854: The default is
855: .Dq no .
1.6 aaron 856: .It Cm X11DisplayOffset
857: Specifies the first display number available for
858: .Nm sshd Ns 's
1.36 aaron 859: X11 forwarding.
860: This prevents
1.6 aaron 861: .Nm
862: from interfering with real X11 servers.
1.34 markus 863: The default is 10.
1.30 markus 864: .It Cm X11Forwarding
1.36 aaron 865: Specifies whether X11 forwarding is permitted.
866: The default is
1.34 markus 867: .Dq no .
1.30 markus 868: Note that disabling X11 forwarding does not improve security in any
869: way, as users can always install their own forwarders.
1.133 itojun 870: X11 forwarding is automatically disabled if
871: .Cm UseLogin
872: is enabled.
1.164 stevesk 873: .It Cm X11UseLocalhost
874: Specifies whether
875: .Nm
876: should bind the X11 forwarding server to the loopback address or to
877: the wildcard address. By default,
878: .Nm
879: binds the forwarding server to the loopback address and sets the
880: hostname part of the
881: .Ev DISPLAY
882: environment variable to
883: .Dq localhost .
884: This prevents remote hosts from connecting to the fake display.
885: However, some older X11 clients may not function with this
886: configuration.
887: .Cm X11UseLocalhost
888: may be set to
889: .Dq no
890: to specify that the forwarding server should be bound to the wildcard
891: address.
892: The argument must be
893: .Dq yes
894: or
895: .Dq no .
896: The default is
897: .Dq yes .
1.52 markus 898: .It Cm XAuthLocation
899: Specifies the location of the
900: .Xr xauth 1
901: program.
902: The default is
903: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 904: .El
1.124 stevesk 905: .Ss Time Formats
906: .Pp
907: .Nm
908: command-line arguments and configuration file options that specify time
909: may be expressed using a sequence of the form:
910: .Sm off
911: .Ar time Oo Ar qualifier Oc ,
912: .Sm on
913: where
914: .Ar time
915: is a positive integer value and
916: .Ar qualifier
917: is one of the following:
918: .Pp
919: .Bl -tag -width Ds -compact -offset indent
920: .It Cm <none>
921: seconds
922: .It Cm s | Cm S
923: seconds
924: .It Cm m | Cm M
925: minutes
926: .It Cm h | Cm H
927: hours
928: .It Cm d | Cm D
929: days
930: .It Cm w | Cm W
931: weeks
932: .El
933: .Pp
934: Each member of the sequence is added together to calculate
935: the total time value.
936: .Pp
937: Time format examples:
938: .Pp
939: .Bl -tag -width Ds -compact -offset indent
940: .It 600
941: 600 seconds (10 minutes)
942: .It 10m
943: 10 minutes
944: .It 1h30m
945: 1 hour 30 minutes (90 minutes)
946: .El
1.2 deraadt 947: .Sh LOGIN PROCESS
1.1 deraadt 948: When a user successfully logs in,
1.2 deraadt 949: .Nm
1.1 deraadt 950: does the following:
1.2 deraadt 951: .Bl -enum -offset indent
952: .It
1.1 deraadt 953: If the login is on a tty, and no command has been specified,
1.40 aaron 954: prints last login time and
1.2 deraadt 955: .Pa /etc/motd
1.1 deraadt 956: (unless prevented in the configuration file or by
1.2 deraadt 957: .Pa $HOME/.hushlogin ;
958: see the
1.40 aaron 959: .Sx FILES
1.2 deraadt 960: section).
961: .It
1.1 deraadt 962: If the login is on a tty, records login time.
1.2 deraadt 963: .It
964: Checks
965: .Pa /etc/nologin ;
966: if it exists, prints contents and quits
1.1 deraadt 967: (unless root).
1.2 deraadt 968: .It
1.1 deraadt 969: Changes to run with normal user privileges.
1.2 deraadt 970: .It
1.1 deraadt 971: Sets up basic environment.
1.2 deraadt 972: .It
973: Reads
974: .Pa $HOME/.ssh/environment
975: if it exists.
976: .It
1.1 deraadt 977: Changes to user's home directory.
1.2 deraadt 978: .It
979: If
980: .Pa $HOME/.ssh/rc
981: exists, runs it; else if
1.168 deraadt 982: .Pa /etc/ssh/sshrc
1.2 deraadt 983: exists, runs
1.36 aaron 984: it; otherwise runs xauth.
985: The
1.2 deraadt 986: .Dq rc
987: files are given the X11
1.1 deraadt 988: authentication protocol and cookie in standard input.
1.2 deraadt 989: .It
1.1 deraadt 990: Runs user's shell or command.
1.2 deraadt 991: .El
992: .Sh AUTHORIZED_KEYS FILE FORMAT
993: .Pa $HOME/.ssh/authorized_keys
1.130 markus 994: is the default file that lists the public keys that are
995: permitted for RSA authentication in protocol version 1
996: and for public key authentication (PubkeyAuthentication)
997: in protocol version 2.
1.125 markus 998: .Cm AuthorizedKeysFile
999: may be used to specify an alternative file.
1.75 markus 1000: .Pp
1.36 aaron 1001: Each line of the file contains one
1.2 deraadt 1002: key (empty lines and lines starting with a
1003: .Ql #
1004: are ignored as
1.36 aaron 1005: comments).
1.75 markus 1006: Each RSA public key consists of the following fields, separated by
1.36 aaron 1007: spaces: options, bits, exponent, modulus, comment.
1.75 markus 1008: Each protocol version 2 public key consists of:
1009: options, keytype, base64 encoded key, comment.
1010: The options fields
1011: are optional; its presence is determined by whether the line starts
1.1 deraadt 1012: with a number or not (the option field never starts with a number).
1.75 markus 1013: The bits, exponent, modulus and comment fields give the RSA key for
1014: protocol version 1; the
1.1 deraadt 1015: comment field is not used for anything (but may be convenient for the
1016: user to identify the key).
1.75 markus 1017: For protocol version 2 the keytype is
1018: .Dq ssh-dss
1019: or
1020: .Dq ssh-rsa .
1.2 deraadt 1021: .Pp
1.1 deraadt 1022: Note that lines in this file are usually several hundred bytes long
1.36 aaron 1023: (because of the size of the RSA key modulus).
1024: You don't want to type them in; instead, copy the
1.113 itojun 1025: .Pa identity.pub ,
1026: .Pa id_dsa.pub
1.75 markus 1027: or the
1.113 itojun 1028: .Pa id_rsa.pub
1.1 deraadt 1029: file and edit it.
1.175 stevesk 1030: .Pp
1031: .Nm
1032: enforces a minimum RSA key modulus size for protocol 1
1033: and protocol 2 keys of 768 bits.
1.2 deraadt 1034: .Pp
1.58 deraadt 1035: The options (if present) consist of comma-separated option
1.36 aaron 1036: specifications.
1037: No spaces are permitted, except within double quotes.
1.141 stevesk 1038: The following option specifications are supported (note
1039: that option keywords are case-insensitive):
1.2 deraadt 1040: .Bl -tag -width Ds
1041: .It Cm from="pattern-list"
1.1 deraadt 1042: Specifies that in addition to RSA authentication, the canonical name
1043: of the remote host must be present in the comma-separated list of
1.36 aaron 1044: patterns
1045: .Pf ( Ql *
1046: and
1047: .Ql ?
1048: serve as wildcards).
1049: The list may also contain
1050: patterns negated by prefixing them with
1051: .Ql ! ;
1052: if the canonical host name matches a negated pattern, the key is not accepted.
1053: The purpose
1.1 deraadt 1054: of this option is to optionally increase security: RSA authentication
1055: by itself does not trust the network or name servers or anything (but
1056: the key); however, if somebody somehow steals the key, the key
1.36 aaron 1057: permits an intruder to log in from anywhere in the world.
1058: This additional option makes using a stolen key more difficult (name
1.1 deraadt 1059: servers and/or routers would have to be compromised in addition to
1060: just the key).
1.2 deraadt 1061: .It Cm command="command"
1.1 deraadt 1062: Specifies that the command is executed whenever this key is used for
1.36 aaron 1063: authentication.
1064: The command supplied by the user (if any) is ignored.
1.148 markus 1065: The command is run on a pty if the client requests a pty;
1.36 aaron 1066: otherwise it is run without a tty.
1.147 deraadt 1067: If a 8-bit clean channel is required,
1068: one must not request a pty or should specify
1.89 markus 1069: .Cm no-pty .
1.36 aaron 1070: A quote may be included in the command by quoting it with a backslash.
1071: This option might be useful
1072: to restrict certain RSA keys to perform just a specific operation.
1073: An example might be a key that permits remote backups but nothing else.
1.51 hugh 1074: Note that the client may specify TCP/IP and/or X11
1075: forwarding unless they are explicitly prohibited.
1.149 markus 1076: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 1077: .It Cm environment="NAME=value"
1.1 deraadt 1078: Specifies that the string is to be added to the environment when
1.36 aaron 1079: logging in using this key.
1080: Environment variables set this way
1081: override other default environment values.
1082: Multiple options of this type are permitted.
1.155 markus 1083: This option is automatically disabled if
1084: .Cm UseLogin
1085: is enabled.
1.2 deraadt 1086: .It Cm no-port-forwarding
1.1 deraadt 1087: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 1088: Any port forward requests by the client will return an error.
1089: This might be used, e.g., in connection with the
1.2 deraadt 1090: .Cm command
1.1 deraadt 1091: option.
1.2 deraadt 1092: .It Cm no-X11-forwarding
1.1 deraadt 1093: Forbids X11 forwarding when this key is used for authentication.
1094: Any X11 forward requests by the client will return an error.
1.2 deraadt 1095: .It Cm no-agent-forwarding
1.1 deraadt 1096: Forbids authentication agent forwarding when this key is used for
1097: authentication.
1.2 deraadt 1098: .It Cm no-pty
1.1 deraadt 1099: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 1100: .It Cm permitopen="host:port"
1.133 itojun 1101: Limit local
1.107 djm 1102: .Li ``ssh -L''
1.111 stevesk 1103: port forwarding such that it may only connect to the specified host and
1.146 stevesk 1104: port.
1105: IPv6 addresses can be specified with an alternative syntax:
1106: .Ar host/port .
1107: Multiple
1.107 djm 1108: .Cm permitopen
1.133 itojun 1109: options may be applied separated by commas. No pattern matching is
1110: performed on the specified hostnames, they must be literal domains or
1.107 djm 1111: addresses.
1.2 deraadt 1112: .El
1113: .Ss Examples
1.1 deraadt 1114: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1115: .Pp
1.1 deraadt 1116: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1117: .Pp
1.1 deraadt 1118: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 1119: .Pp
1120: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1121: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1122: The
1.167 deraadt 1123: .Pa /etc/ssh/ssh_known_hosts ,
1.40 aaron 1124: and
1.131 markus 1125: .Pa $HOME/.ssh/known_hosts
1.36 aaron 1126: files contain host public keys for all known hosts.
1127: The global file should
1.37 brad 1128: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1129: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1130: its key is added to the per-user file.
1.2 deraadt 1131: .Pp
1.1 deraadt 1132: Each line in these files contains the following fields: hostnames,
1.36 aaron 1133: bits, exponent, modulus, comment.
1134: The fields are separated by spaces.
1.2 deraadt 1135: .Pp
1.1 deraadt 1136: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1137: wildcards); each pattern in turn is matched against the canonical host
1138: name (when authenticating a client) or against the user-supplied
1.36 aaron 1139: name (when authenticating a server).
1140: A pattern may also be preceded by
1.2 deraadt 1141: .Ql !
1142: to indicate negation: if the host name matches a negated
1.1 deraadt 1143: pattern, it is not accepted (by that line) even if it matched another
1144: pattern on the line.
1.2 deraadt 1145: .Pp
1.49 markus 1146: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1147: can be obtained, e.g., from
1.167 deraadt 1148: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 1149: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1150: .Pp
1151: Lines starting with
1152: .Ql #
1153: and empty lines are ignored as comments.
1154: .Pp
1.1 deraadt 1155: When performing host authentication, authentication is accepted if any
1.36 aaron 1156: matching line has the proper key.
1157: It is thus permissible (but not
1.1 deraadt 1158: recommended) to have several lines or different host keys for the same
1.36 aaron 1159: names.
1160: This will inevitably happen when short forms of host names
1161: from different domains are put in the file.
1162: It is possible
1.1 deraadt 1163: that the files contain conflicting information; authentication is
1164: accepted if valid information can be found from either file.
1.2 deraadt 1165: .Pp
1.1 deraadt 1166: Note that the lines in these files are typically hundreds of characters
1167: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1168: Rather, generate them by a script
1.40 aaron 1169: or by taking
1.167 deraadt 1170: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 1171: and adding the host names at the front.
1.2 deraadt 1172: .Ss Examples
1.120 markus 1173: .Bd -literal
1174: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1175: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1176: .Ed
1.2 deraadt 1177: .Sh FILES
1178: .Bl -tag -width Ds
1.167 deraadt 1179: .It Pa /etc/ssh/sshd_config
1.1 deraadt 1180: Contains configuration data for
1.2 deraadt 1181: .Nm sshd .
1.1 deraadt 1182: This file should be writable by root only, but it is recommended
1183: (though not necessary) that it be world-readable.
1.167 deraadt 1184: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 1185: These three files contain the private parts of the host keys.
1.98 deraadt 1186: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1187: accessible to others.
1.14 markus 1188: Note that
1189: .Nm
1190: does not start if this file is group/world-accessible.
1.167 deraadt 1191: .It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
1.120 markus 1192: These three files contain the public parts of the host keys.
1.98 deraadt 1193: These files should be world-readable but writable only by
1.36 aaron 1194: root.
1.98 deraadt 1195: Their contents should match the respective private parts.
1196: These files are not
1197: really used for anything; they are provided for the convenience of
1198: the user so their contents can be copied to known hosts files.
1199: These files are created using
1.7 markus 1200: .Xr ssh-keygen 1 .
1.129 provos 1201: .It Pa /etc/moduli
1.73 provos 1202: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1203: .It Pa /var/run/sshd.pid
1204: Contains the process ID of the
1205: .Nm
1.1 deraadt 1206: listening for connections (if there are several daemons running
1207: concurrently for different ports, this contains the pid of the one
1.36 aaron 1208: started last).
1.58 deraadt 1209: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1210: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 1211: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1212: This file must be readable by root (which may on some machines imply
1213: it being world-readable if the user's home directory resides on an NFS
1214: volume).
1215: It is recommended that it not be accessible by others.
1216: The format of this file is described above.
1217: Users will place the contents of their
1.130 markus 1218: .Pa identity.pub ,
1.44 deraadt 1219: .Pa id_dsa.pub
1.113 itojun 1220: and/or
1221: .Pa id_rsa.pub
1.44 deraadt 1222: files into this file, as described in
1223: .Xr ssh-keygen 1 .
1.167 deraadt 1224: .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1.22 markus 1225: These files are consulted when using rhosts with RSA host
1.131 markus 1226: authentication or protocol version 2 hostbased authentication
1227: to check the public key of the host.
1.36 aaron 1228: The key must be listed in one of these files to be accepted.
1.22 markus 1229: The client uses the same files
1.96 markus 1230: to verify that it is connecting to the correct remote host.
1.36 aaron 1231: These files should be writable only by root/the owner.
1.167 deraadt 1232: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 1233: should be world-readable, and
1234: .Pa $HOME/.ssh/known_hosts
1.120 markus 1235: can but need not be world-readable.
1.6 aaron 1236: .It Pa /etc/nologin
1.40 aaron 1237: If this file exists,
1.2 deraadt 1238: .Nm
1.36 aaron 1239: refuses to let anyone except root log in.
1240: The contents of the file
1.1 deraadt 1241: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1242: refused.
1243: The file should be world-readable.
1.19 dugsong 1244: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 1245: Access controls that should be enforced by tcp-wrappers are defined here.
1246: Further details are described in
1.19 dugsong 1247: .Xr hosts_access 5 .
1.6 aaron 1248: .It Pa $HOME/.rhosts
1.1 deraadt 1249: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1250: line.
1251: The given user on the corresponding host is permitted to log in
1252: without password.
1253: The same file is used by rlogind and rshd.
1.6 aaron 1254: The file must
1.1 deraadt 1255: be writable only by the user; it is recommended that it not be
1256: accessible by others.
1.2 deraadt 1257: .Pp
1.36 aaron 1258: If is also possible to use netgroups in the file.
1259: Either host or user
1.1 deraadt 1260: name may be of the form +@groupname to specify all hosts or all users
1261: in the group.
1.2 deraadt 1262: .It Pa $HOME/.shosts
1263: For ssh,
1264: this file is exactly the same as for
1265: .Pa .rhosts .
1266: However, this file is
1267: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1268: .It Pa /etc/hosts.equiv
1.2 deraadt 1269: This file is used during
1270: .Pa .rhosts
1.36 aaron 1271: authentication.
1272: In the simplest form, this file contains host names, one per line.
1273: Users on
1.1 deraadt 1274: those hosts are permitted to log in without a password, provided they
1.36 aaron 1275: have the same user name on both machines.
1276: The host name may also be
1.1 deraadt 1277: followed by a user name; such users are permitted to log in as
1.2 deraadt 1278: .Em any
1.36 aaron 1279: user on this machine (except root).
1280: Additionally, the syntax
1.2 deraadt 1281: .Dq +@group
1.36 aaron 1282: can be used to specify netgroups.
1283: Negated entries start with
1.2 deraadt 1284: .Ql \&- .
1285: .Pp
1.1 deraadt 1286: If the client host/user is successfully matched in this file, login is
1287: automatically permitted provided the client and server user names are the
1.36 aaron 1288: same.
1289: Additionally, successful RSA host authentication is normally required.
1290: This file must be writable only by root; it is recommended
1.1 deraadt 1291: that it be world-readable.
1.2 deraadt 1292: .Pp
1.6 aaron 1293: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1294: .Pa hosts.equiv .
1.1 deraadt 1295: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1296: .Em anybody ,
1.1 deraadt 1297: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1298: binaries and directories.
1299: Using a user name practically grants the user root access.
1300: The only valid use for user names that I can think
1.1 deraadt 1301: of is in negative entries.
1.2 deraadt 1302: .Pp
1303: Note that this warning also applies to rsh/rlogin.
1304: .It Pa /etc/shosts.equiv
1.1 deraadt 1305: This is processed exactly as
1.2 deraadt 1306: .Pa /etc/hosts.equiv .
1.1 deraadt 1307: However, this file may be useful in environments that want to run both
1.2 deraadt 1308: rsh/rlogin and ssh.
1.6 aaron 1309: .It Pa $HOME/.ssh/environment
1.36 aaron 1310: This file is read into the environment at login (if it exists).
1311: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1312: .Ql # ) ,
1.36 aaron 1313: and assignment lines of the form name=value.
1314: The file should be writable
1.6 aaron 1315: only by the user; it need not be readable by anyone else.
1.2 deraadt 1316: .It Pa $HOME/.ssh/rc
1.1 deraadt 1317: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1318: environment files but before starting the user's shell or command.
1.176 stevesk 1319: It must not produce any output on stdout; stderr must be used
1320: instead.
1321: If X11 forwarding is in use, it will receive the "proto cookie" pair in
1322: its standard input (and
1.2 deraadt 1323: .Ev DISPLAY
1.176 stevesk 1324: in its environment).
1325: The script must call
1.2 deraadt 1326: .Xr xauth 1
1.176 stevesk 1327: because
1328: .Nm
1329: will not run xauth automatically to add X11 cookies.
1.2 deraadt 1330: .Pp
1.1 deraadt 1331: The primary purpose of this file is to run any initialization routines
1332: which may be needed before the user's home directory becomes
1333: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1334: .Pp
1.1 deraadt 1335: This file will probably contain some initialization code followed by
1.120 markus 1336: something similar to:
1337: .Bd -literal
1.176 stevesk 1338: if read proto cookie && [ -n "$DISPLAY" ]; then
1339: if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
1340: # X11UseLocalhost=yes
1341: xauth add unix:`echo $DISPLAY |
1342: cut -c11-` $proto $cookie
1343: else
1344: # X11UseLocalhost=no
1345: xauth add $DISPLAY $proto $cookie
1.120 markus 1346: fi
1.176 stevesk 1347: fi
1.120 markus 1348: .Ed
1.2 deraadt 1349: .Pp
1350: If this file does not exist,
1.167 deraadt 1351: .Pa /etc/ssh/sshrc
1.2 deraadt 1352: is run, and if that
1.176 stevesk 1353: does not exist either, xauth is used to add the cookie.
1.2 deraadt 1354: .Pp
1.1 deraadt 1355: This file should be writable only by the user, and need not be
1356: readable by anyone else.
1.167 deraadt 1357: .It Pa /etc/ssh/sshrc
1.2 deraadt 1358: Like
1359: .Pa $HOME/.ssh/rc .
1360: This can be used to specify
1.36 aaron 1361: machine-specific login-time initializations globally.
1362: This file should be writable only by root, and should be world-readable.
1.56 aaron 1363: .El
1.71 aaron 1364: .Sh AUTHORS
1.84 markus 1365: OpenSSH is a derivative of the original and free
1366: ssh 1.2.12 release by Tatu Ylonen.
1367: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1368: Theo de Raadt and Dug Song
1369: removed many bugs, re-added newer features and
1370: created OpenSSH.
1371: Markus Friedl contributed the support for SSH
1372: protocol versions 1.5 and 2.0.
1.171 provos 1373: Niels Provos and Markus Friedl contributed support
1374: for privilege separation.
1.2 deraadt 1375: .Sh SEE ALSO
1376: .Xr scp 1 ,
1.90 djm 1377: .Xr sftp 1 ,
1.2 deraadt 1378: .Xr ssh 1 ,
1.5 deraadt 1379: .Xr ssh-add 1 ,
1.2 deraadt 1380: .Xr ssh-agent 1 ,
1.5 deraadt 1381: .Xr ssh-keygen 1 ,
1.136 markus 1382: .Xr login.conf 5 ,
1383: .Xr moduli 5 ,
1.128 mpech 1384: .Xr sftp-server 8
1.119 markus 1385: .Rs
1386: .%A T. Ylonen
1387: .%A T. Kivinen
1388: .%A M. Saarinen
1389: .%A T. Rinne
1390: .%A S. Lehtinen
1391: .%T "SSH Protocol Architecture"
1.174 markus 1392: .%N draft-ietf-secsh-architecture-12.txt
1393: .%D January 2002
1.120 markus 1394: .%O work in progress material
1395: .Re
1396: .Rs
1397: .%A M. Friedl
1398: .%A N. Provos
1399: .%A W. A. Simpson
1400: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.174 markus 1401: .%N draft-ietf-secsh-dh-group-exchange-02.txt
1402: .%D January 2002
1.119 markus 1403: .%O work in progress material
1404: .Re
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