Annotation of src/usr.bin/ssh/sshd.8, Revision 1.169
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: .\"
1.169 ! stevesk 37: .\" $OpenBSD: sshd.8,v 1.168 2002/02/10 01:07:05 deraadt 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
1.2 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
1.167 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.
1.74 markus 296: .It Fl D
297: When this option is specified
298: .Nm
299: will not detach and does not become a daemon.
300: This allows easy monitoring of
1.76 markus 301: .Nm sshd .
1.29 markus 302: .It Fl 4
303: Forces
304: .Nm
305: to use IPv4 addresses only.
306: .It Fl 6
307: Forces
308: .Nm
309: to use IPv6 addresses only.
1.2 deraadt 310: .El
311: .Sh CONFIGURATION FILE
312: .Nm
1.40 aaron 313: reads configuration data from
1.167 deraadt 314: .Pa /etc/ssh/sshd_config
1.2 deraadt 315: (or the file specified with
316: .Fl f
1.36 aaron 317: on the command line).
1.141 stevesk 318: The file contains keyword-argument pairs, one per line.
1.36 aaron 319: Lines starting with
1.2 deraadt 320: .Ql #
1.1 deraadt 321: and empty lines are interpreted as comments.
1.2 deraadt 322: .Pp
1.141 stevesk 323: The possible
324: keywords and their meanings are as follows (note that
325: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 326: .Bl -tag -width Ds
327: .It Cm AFSTokenPassing
1.36 aaron 328: Specifies whether an AFS token may be forwarded to the server.
329: Default is
1.2 deraadt 330: .Dq yes .
1.11 markus 331: .It Cm AllowGroups
1.163 stevesk 332: This keyword can be followed by a list of group name patterns, separated
1.36 aaron 333: by spaces.
334: If specified, login is allowed only for users whose primary
1.81 markus 335: group or supplementary group list matches one of the patterns.
1.11 markus 336: .Ql \&*
337: and
338: .Ql ?
339: can be used as
1.36 aaron 340: wildcards in the patterns.
1.147 deraadt 341: Only group names are valid; a numerical group ID is not recognized.
1.163 stevesk 342: By default, login is allowed for all groups.
1.3 dugsong 343: .Pp
1.69 markus 344: .It Cm AllowTcpForwarding
345: Specifies whether TCP forwarding is permitted.
346: The default is
347: .Dq yes .
348: Note that disabling TCP forwarding does not improve security unless
349: users are also denied shell access, as they can always install their
350: own forwarders.
351: .Pp
1.11 markus 352: .It Cm AllowUsers
1.163 stevesk 353: This keyword can be followed by a list of user name patterns, separated
1.36 aaron 354: by spaces.
355: If specified, login is allowed only for users names that
1.11 markus 356: match one of the patterns.
357: .Ql \&*
358: and
359: .Ql ?
360: can be used as
1.36 aaron 361: wildcards in the patterns.
1.147 deraadt 362: Only user names are valid; a numerical user ID is not recognized.
1.163 stevesk 363: By default, login is allowed for all users.
1.135 markus 364: If the pattern takes the form USER@HOST then USER and HOST
1.147 deraadt 365: are separately checked, restricting logins to particular
1.135 markus 366: users from particular hosts.
1.80 markus 367: .Pp
1.125 markus 368: .It Cm AuthorizedKeysFile
1.138 markus 369: Specifies the file that contains the public keys that can be used
370: for user authentication.
1.125 markus 371: .Cm AuthorizedKeysFile
372: may contain tokens of the form %T which are substituted during connection
1.142 stevesk 373: set-up. The following tokens are defined: %% is replaced by a literal '%',
1.125 markus 374: %h is replaced by the home directory of the user being authenticated and
375: %u is replaced by the username of that user.
376: After expansion,
377: .Cm AuthorizedKeysFile
1.126 markus 378: is taken to be an absolute path or one relative to the user's home
1.125 markus 379: directory.
380: The default is
1.161 stevesk 381: .Dq .ssh/authorized_keys .
1.80 markus 382: .It Cm Banner
383: In some jurisdictions, sending a warning message before authentication
384: may be relevant for getting legal protection.
385: The contents of the specified file are sent to the remote user before
386: authentication is allowed.
387: This option is only available for protocol version 2.
1.11 markus 388: .Pp
1.104 deraadt 389: .It Cm ChallengeResponseAuthentication
1.136 markus 390: Specifies whether challenge response authentication is allowed.
391: All authentication styles from
392: .Xr login.conf 5
393: are supported.
1.104 deraadt 394: The default is
395: .Dq yes .
1.122 markus 396: .It Cm Ciphers
397: Specifies the ciphers allowed for protocol version 2.
398: Multiple ciphers must be comma-separated.
399: The default is
1.162 stevesk 400: .Pp
401: .Bd -literal
402: ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
403: aes192-cbc,aes256-cbc''
404: .Ed
1.115 beck 405: .It Cm ClientAliveInterval
406: Sets a timeout interval in seconds after which if no data has been received
1.133 itojun 407: from the client,
1.115 beck 408: .Nm
409: will send a message through the encrypted
1.116 stevesk 410: channel to request a response from the client.
411: The default
1.115 beck 412: is 0, indicating that these messages will not be sent to the client.
1.116 stevesk 413: This option applies to protocol version 2 only.
1.115 beck 414: .It Cm ClientAliveCountMax
415: Sets the number of client alive messages (see above) which may be
416: sent without
417: .Nm
418: receiving any messages back from the client. If this threshold is
1.133 itojun 419: reached while client alive messages are being sent,
1.115 beck 420: .Nm
421: will disconnect the client, terminating the session. It is important
1.133 itojun 422: to note that the use of client alive messages is very different from
1.154 markus 423: .Cm KeepAlive
1.116 stevesk 424: (below). The client alive messages are sent through the
1.115 beck 425: encrypted channel and therefore will not be spoofable. The TCP keepalive
1.116 stevesk 426: option enabled by
1.154 markus 427: .Cm KeepAlive
1.147 deraadt 428: is spoofable. The client alive mechanism is valuable when the client or
429: server depend on knowing when a connection has become inactive.
1.116 stevesk 430: .Pp
1.147 deraadt 431: The default value is 3. If
1.116 stevesk 432: .Cm ClientAliveInterval
1.147 deraadt 433: (above) is set to 15, and
1.152 stevesk 434: .Cm ClientAliveCountMax
435: is left at the default, unresponsive ssh clients
1.133 itojun 436: will be disconnected after approximately 45 seconds.
1.11 markus 437: .It Cm DenyGroups
1.163 stevesk 438: This keyword can be followed by a list of group name patterns, separated
1.36 aaron 439: by spaces.
1.163 stevesk 440: Login is disallowed for users whose primary group or supplementary
441: group list matches one of the patterns.
1.11 markus 442: .Ql \&*
443: and
444: .Ql ?
445: can be used as
1.36 aaron 446: wildcards in the patterns.
1.147 deraadt 447: Only group names are valid; a numerical group ID is not recognized.
1.163 stevesk 448: By default, login is allowed for all groups.
1.11 markus 449: .Pp
450: .It Cm DenyUsers
1.163 stevesk 451: This keyword can be followed by a list of user name patterns, separated
1.36 aaron 452: by spaces.
453: Login is disallowed for user names that match one of the patterns.
1.11 markus 454: .Ql \&*
455: and
456: .Ql ?
1.36 aaron 457: can be used as wildcards in the patterns.
1.147 deraadt 458: Only user names are valid; a numerical user ID is not recognized.
1.163 stevesk 459: By default, login is allowed for all users.
1.169 ! stevesk 460: If the pattern takes the form USER@HOST then USER and HOST
! 461: are separately checked, restricting logins to particular
! 462: users from particular hosts.
1.47 markus 463: .It Cm GatewayPorts
464: Specifies whether remote hosts are allowed to connect to ports
465: forwarded for the client.
1.145 stevesk 466: By default,
467: .Nm
468: binds remote port forwardings to the loopback addresss. This
469: prevents other remote hosts from connecting to forwarded ports.
470: .Cm GatewayPorts
471: can be used to specify that
472: .Nm
473: should bind remote port forwardings to the wildcard address,
474: thus allowing remote hosts to connect to forwarded ports.
1.47 markus 475: The argument must be
476: .Dq yes
477: or
478: .Dq no .
479: The default is
480: .Dq no .
1.120 markus 481: .It Cm HostbasedAuthentication
482: Specifies whether rhosts or /etc/hosts.equiv authentication together
483: with successful public key client host authentication is allowed
484: (hostbased authentication).
485: This option is similar to
486: .Cm RhostsRSAAuthentication
487: and applies to protocol version 2 only.
488: The default is
489: .Dq no .
1.2 deraadt 490: .It Cm HostKey
1.160 stevesk 491: Specifies a file containing a private host key
492: used by SSH.
493: The default is
1.167 deraadt 494: .Pa /etc/ssh/ssh_host_key
1.160 stevesk 495: for protocol version 1, and
1.167 deraadt 496: .Pa /etc/ssh/ssh_host_rsa_key
1.160 stevesk 497: and
1.167 deraadt 498: .Pa /etc/ssh/ssh_host_dsa_key
1.160 stevesk 499: for protocol version 2.
1.9 markus 500: Note that
501: .Nm
1.83 markus 502: will refuse to use a file if it is group/world-accessible.
1.72 markus 503: It is possible to have multiple host key files.
504: .Dq rsa1
505: keys are used for version 1 and
506: .Dq dsa
507: or
508: .Dq rsa
509: are used for version 2 of the SSH protocol.
1.2 deraadt 510: .It Cm IgnoreRhosts
1.34 markus 511: Specifies that
512: .Pa .rhosts
1.40 aaron 513: and
1.34 markus 514: .Pa .shosts
1.120 markus 515: files will not be used in
516: .Cm RhostsAuthentication ,
517: .Cm RhostsRSAAuthentication
518: or
519: .Cm HostbasedAuthentication .
520: .Pp
1.2 deraadt 521: .Pa /etc/hosts.equiv
1.1 deraadt 522: and
1.40 aaron 523: .Pa /etc/shosts.equiv
1.36 aaron 524: are still used.
1.40 aaron 525: The default is
1.34 markus 526: .Dq yes .
1.24 markus 527: .It Cm IgnoreUserKnownHosts
528: Specifies whether
529: .Nm
530: should ignore the user's
531: .Pa $HOME/.ssh/known_hosts
1.45 markus 532: during
1.120 markus 533: .Cm RhostsRSAAuthentication
534: or
535: .Cm HostbasedAuthentication .
1.24 markus 536: The default is
1.2 deraadt 537: .Dq no .
538: .It Cm KeepAlive
1.166 stevesk 539: Specifies whether the system should send TCP keepalive messages to the
1.36 aaron 540: other side.
541: If they are sent, death of the connection or crash of one
542: of the machines will be properly noticed.
543: However, this means that
1.1 deraadt 544: connections will die if the route is down temporarily, and some people
1.36 aaron 545: find it annoying.
1.51 hugh 546: On the other hand, if keepalives are not sent,
1.2 deraadt 547: sessions may hang indefinitely on the server, leaving
548: .Dq ghost
549: users and consuming server resources.
550: .Pp
551: The default is
552: .Dq yes
553: (to send keepalives), and the server will notice
1.166 stevesk 554: if the network goes down or the client host crashes.
1.36 aaron 555: This avoids infinitely hanging sessions.
1.2 deraadt 556: .Pp
557: To disable keepalives, the value should be set to
1.166 stevesk 558: .Dq no .
1.2 deraadt 559: .It Cm KerberosAuthentication
1.36 aaron 560: Specifies whether Kerberos authentication is allowed.
561: This can be in the form of a Kerberos ticket, or if
1.7 markus 562: .Cm PasswordAuthentication
1.1 deraadt 563: is yes, the password provided by the user will be validated through
1.67 aaron 564: the Kerberos KDC.
565: To use this option, the server needs a
1.59 provos 566: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 567: Default is
1.60 provos 568: .Dq yes .
1.2 deraadt 569: .It Cm KerberosOrLocalPasswd
1.1 deraadt 570: If set then if password authentication through Kerberos fails then
571: the password will be validated via any additional local mechanism
1.2 deraadt 572: such as
1.66 markus 573: .Pa /etc/passwd .
1.36 aaron 574: Default is
1.20 dugsong 575: .Dq yes .
1.2 deraadt 576: .It Cm KerberosTgtPassing
1.1 deraadt 577: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 578: Default is
1.3 dugsong 579: .Dq no ,
580: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 581: .It Cm KerberosTicketCleanup
1.7 markus 582: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 583: file on logout.
584: Default is
1.3 dugsong 585: .Dq yes .
1.2 deraadt 586: .It Cm KeyRegenerationInterval
1.120 markus 587: In protocol version 1, the ephemeral server key is automatically regenerated
588: after this many seconds (if it has been used).
1.36 aaron 589: The purpose of regeneration is to prevent
1.1 deraadt 590: decrypting captured sessions by later breaking into the machine and
1.36 aaron 591: stealing the keys.
592: The key is never stored anywhere.
593: If the value is 0, the key is never regenerated.
594: The default is 3600 (seconds).
1.7 markus 595: .It Cm ListenAddress
1.110 stevesk 596: Specifies the local addresses
1.120 markus 597: .Nm
1.7 markus 598: should listen on.
1.110 stevesk 599: The following forms may be used:
600: .Pp
601: .Bl -item -offset indent -compact
602: .It
603: .Cm ListenAddress
1.112 stevesk 604: .Sm off
605: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
606: .Sm on
1.110 stevesk 607: .It
608: .Cm ListenAddress
1.112 stevesk 609: .Sm off
610: .Ar host No | Ar IPv4_addr No : Ar port
611: .Sm on
1.110 stevesk 612: .It
613: .Cm ListenAddress
1.112 stevesk 614: .Sm off
615: .Oo
616: .Ar host No | Ar IPv6_addr Oc : Ar port
617: .Sm on
1.110 stevesk 618: .El
619: .Pp
620: If
1.112 stevesk 621: .Ar port
1.110 stevesk 622: is not specified,
1.120 markus 623: .Nm
1.110 stevesk 624: will listen on the address and all prior
625: .Cm Port
626: options specified. The default is to listen on all local
627: addresses. Multiple
628: .Cm ListenAddress
629: options are permitted. Additionally, any
630: .Cm Port
631: options must precede this option for non port qualified addresses.
1.2 deraadt 632: .It Cm LoginGraceTime
1.1 deraadt 633: The server disconnects after this time if the user has not
1.36 aaron 634: successfully logged in.
635: If the value is 0, there is no time limit.
1.1 deraadt 636: The default is 600 (seconds).
1.23 markus 637: .It Cm LogLevel
638: Gives the verbosity level that is used when logging messages from
639: .Nm sshd .
640: The possible values are:
1.159 stevesk 641: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
642: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
643: and DEBUG3 each specify higher levels of debugging output.
644: Logging with a DEBUG level violates the privacy of users
1.23 markus 645: and is not recommended.
1.93 markus 646: .It Cm MACs
647: Specifies the available MAC (message authentication code) algorithms.
648: The MAC algorithm is used in protocol version 2
649: for data integrity protection.
650: Multiple algorithms must be comma-separated.
651: The default is
1.123 markus 652: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 653: .It Cm MaxStartups
654: Specifies the maximum number of concurrent unauthenticated connections to the
655: .Nm
656: daemon.
657: Additional connections will be dropped until authentication succeeds or the
658: .Cm LoginGraceTime
659: expires for a connection.
660: The default is 10.
1.57 markus 661: .Pp
662: Alternatively, random early drop can be enabled by specifying
663: the three colon separated values
664: .Dq start:rate:full
1.67 aaron 665: (e.g., "10:30:60").
1.57 markus 666: .Nm
1.86 stevesk 667: will refuse connection attempts with a probability of
1.57 markus 668: .Dq rate/100
669: (30%)
670: if there are currently
671: .Dq start
672: (10)
673: unauthenticated connections.
1.86 stevesk 674: The probability increases linearly and all connection attempts
1.57 markus 675: are refused if the number of unauthenticated connections reaches
676: .Dq full
677: (60).
1.2 deraadt 678: .It Cm PasswordAuthentication
1.1 deraadt 679: Specifies whether password authentication is allowed.
1.2 deraadt 680: The default is
681: .Dq yes .
682: .It Cm PermitEmptyPasswords
1.1 deraadt 683: When password authentication is allowed, it specifies whether the
1.36 aaron 684: server allows login to accounts with empty password strings.
685: The default is
1.34 markus 686: .Dq no .
1.2 deraadt 687: .It Cm PermitRootLogin
1.100 stevesk 688: Specifies whether root can login using
1.2 deraadt 689: .Xr ssh 1 .
1.15 markus 690: The argument must be
691: .Dq yes ,
1.94 markus 692: .Dq without-password ,
693: .Dq forced-commands-only
1.15 markus 694: or
695: .Dq no .
1.2 deraadt 696: The default is
697: .Dq yes .
1.94 markus 698: .Pp
699: If this option is set to
1.15 markus 700: .Dq without-password
1.94 markus 701: password authentication is disabled for root.
1.2 deraadt 702: .Pp
1.94 markus 703: If this option is set to
704: .Dq forced-commands-only
705: root login with public key authentication will be allowed,
706: but only if the
1.2 deraadt 707: .Ar command
1.94 markus 708: option has been specified
1.1 deraadt 709: (which may be useful for taking remote backups even if root login is
1.94 markus 710: normally not allowed). All other authentication methods are disabled
711: for root.
1.100 stevesk 712: .Pp
713: If this option is set to
714: .Dq no
715: root is not allowed to login.
1.43 markus 716: .It Cm PidFile
717: Specifies the file that contains the process identifier of the
718: .Nm
719: daemon.
720: The default is
721: .Pa /var/run/sshd.pid .
1.2 deraadt 722: .It Cm Port
1.1 deraadt 723: Specifies the port number that
1.2 deraadt 724: .Nm
1.36 aaron 725: listens on.
726: The default is 22.
1.28 markus 727: Multiple options of this type are permitted.
1.120 markus 728: See also
729: .Cm ListenAddress .
1.108 stevesk 730: .It Cm PrintLastLog
731: Specifies whether
732: .Nm
733: should print the date and time when the user last logged in.
734: The default is
735: .Dq yes .
1.2 deraadt 736: .It Cm PrintMotd
1.1 deraadt 737: Specifies whether
1.2 deraadt 738: .Nm
1.40 aaron 739: should print
1.2 deraadt 740: .Pa /etc/motd
1.36 aaron 741: when a user logs in interactively.
742: (On some systems it is also printed by the shell,
1.2 deraadt 743: .Pa /etc/profile ,
1.36 aaron 744: or equivalent.)
745: The default is
1.2 deraadt 746: .Dq yes .
1.41 markus 747: .It Cm Protocol
748: Specifies the protocol versions
749: .Nm
750: should support.
751: The possible values are
752: .Dq 1
753: and
754: .Dq 2 .
755: Multiple versions must be comma-separated.
756: The default is
1.118 deraadt 757: .Dq 2,1 .
1.104 deraadt 758: .It Cm PubkeyAuthentication
759: Specifies whether public key authentication is allowed.
760: The default is
761: .Dq yes .
762: Note that this option applies to protocol version 2 only.
1.2 deraadt 763: .It Cm RhostsAuthentication
1.1 deraadt 764: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 765: files is sufficient.
766: Normally, this method should not be permitted because it is insecure.
1.7 markus 767: .Cm RhostsRSAAuthentication
768: should be used
1.1 deraadt 769: instead, because it performs RSA-based host authentication in addition
770: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 771: The default is
772: .Dq no .
1.120 markus 773: This option applies to protocol version 1 only.
1.2 deraadt 774: .It Cm RhostsRSAAuthentication
1.1 deraadt 775: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 776: with successful RSA host authentication is allowed.
777: The default is
1.34 markus 778: .Dq no .
1.120 markus 779: This option applies to protocol version 1 only.
1.2 deraadt 780: .It Cm RSAAuthentication
1.36 aaron 781: Specifies whether pure RSA authentication is allowed.
782: The default is
1.2 deraadt 783: .Dq yes .
1.120 markus 784: This option applies to protocol version 1 only.
1.2 deraadt 785: .It Cm ServerKeyBits
1.120 markus 786: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 787: The minimum value is 512, and the default is 768.
1.2 deraadt 788: .It Cm StrictModes
1.12 markus 789: Specifies whether
790: .Nm
791: should check file modes and ownership of the
1.36 aaron 792: user's files and home directory before accepting login.
793: This is normally desirable because novices sometimes accidentally leave their
794: directory or files world-writable.
795: The default is
1.7 markus 796: .Dq yes .
1.54 jakob 797: .It Cm Subsystem
1.67 aaron 798: Configures an external subsystem (e.g., file transfer daemon).
799: Arguments should be a subsystem name and a command to execute upon subsystem
800: request.
1.63 markus 801: The command
802: .Xr sftp-server 8
803: implements the
804: .Dq sftp
805: file transfer subsystem.
1.54 jakob 806: By default no subsystems are defined.
807: Note that this option applies to protocol version 2 only.
1.2 deraadt 808: .It Cm SyslogFacility
1.1 deraadt 809: Gives the facility code that is used when logging messages from
1.2 deraadt 810: .Nm sshd .
1.1 deraadt 811: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 812: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
813: The default is AUTH.
1.10 markus 814: .It Cm UseLogin
815: Specifies whether
816: .Xr login 1
1.53 markus 817: is used for interactive login sessions.
1.127 markus 818: The default is
819: .Dq no .
1.53 markus 820: Note that
821: .Xr login 1
1.58 deraadt 822: is never used for remote command execution.
1.133 itojun 823: Note also, that if this is enabled,
824: .Cm X11Forwarding
1.127 markus 825: will be disabled because
826: .Xr login 1
827: does not know how to handle
1.133 itojun 828: .Xr xauth 1
1.127 markus 829: cookies.
1.165 markus 830: .It Cm VerifyReverseMapping
831: Specifies whether
832: .Nm
833: should try to verify the remote host name and check that
834: the resolved host name for the remote IP address maps back to the
835: very same IP address.
836: The default is
837: .Dq no .
1.6 aaron 838: .It Cm X11DisplayOffset
839: Specifies the first display number available for
840: .Nm sshd Ns 's
1.36 aaron 841: X11 forwarding.
842: This prevents
1.6 aaron 843: .Nm
844: from interfering with real X11 servers.
1.34 markus 845: The default is 10.
1.30 markus 846: .It Cm X11Forwarding
1.36 aaron 847: Specifies whether X11 forwarding is permitted.
848: The default is
1.34 markus 849: .Dq no .
1.30 markus 850: Note that disabling X11 forwarding does not improve security in any
851: way, as users can always install their own forwarders.
1.133 itojun 852: X11 forwarding is automatically disabled if
853: .Cm UseLogin
854: is enabled.
1.164 stevesk 855: .It Cm X11UseLocalhost
856: Specifies whether
857: .Nm
858: should bind the X11 forwarding server to the loopback address or to
859: the wildcard address. By default,
860: .Nm
861: binds the forwarding server to the loopback address and sets the
862: hostname part of the
863: .Ev DISPLAY
864: environment variable to
865: .Dq localhost .
866: This prevents remote hosts from connecting to the fake display.
867: However, some older X11 clients may not function with this
868: configuration.
869: .Cm X11UseLocalhost
870: may be set to
871: .Dq no
872: to specify that the forwarding server should be bound to the wildcard
873: address.
874: The argument must be
875: .Dq yes
876: or
877: .Dq no .
878: The default is
879: .Dq yes .
1.52 markus 880: .It Cm XAuthLocation
881: Specifies the location of the
882: .Xr xauth 1
883: program.
884: The default is
885: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 886: .El
1.124 stevesk 887: .Ss Time Formats
888: .Pp
889: .Nm
890: command-line arguments and configuration file options that specify time
891: may be expressed using a sequence of the form:
892: .Sm off
893: .Ar time Oo Ar qualifier Oc ,
894: .Sm on
895: where
896: .Ar time
897: is a positive integer value and
898: .Ar qualifier
899: is one of the following:
900: .Pp
901: .Bl -tag -width Ds -compact -offset indent
902: .It Cm <none>
903: seconds
904: .It Cm s | Cm S
905: seconds
906: .It Cm m | Cm M
907: minutes
908: .It Cm h | Cm H
909: hours
910: .It Cm d | Cm D
911: days
912: .It Cm w | Cm W
913: weeks
914: .El
915: .Pp
916: Each member of the sequence is added together to calculate
917: the total time value.
918: .Pp
919: Time format examples:
920: .Pp
921: .Bl -tag -width Ds -compact -offset indent
922: .It 600
923: 600 seconds (10 minutes)
924: .It 10m
925: 10 minutes
926: .It 1h30m
927: 1 hour 30 minutes (90 minutes)
928: .El
1.2 deraadt 929: .Sh LOGIN PROCESS
1.1 deraadt 930: When a user successfully logs in,
1.2 deraadt 931: .Nm
1.1 deraadt 932: does the following:
1.2 deraadt 933: .Bl -enum -offset indent
934: .It
1.1 deraadt 935: If the login is on a tty, and no command has been specified,
1.40 aaron 936: prints last login time and
1.2 deraadt 937: .Pa /etc/motd
1.1 deraadt 938: (unless prevented in the configuration file or by
1.2 deraadt 939: .Pa $HOME/.hushlogin ;
940: see the
1.40 aaron 941: .Sx FILES
1.2 deraadt 942: section).
943: .It
1.1 deraadt 944: If the login is on a tty, records login time.
1.2 deraadt 945: .It
946: Checks
947: .Pa /etc/nologin ;
948: if it exists, prints contents and quits
1.1 deraadt 949: (unless root).
1.2 deraadt 950: .It
1.1 deraadt 951: Changes to run with normal user privileges.
1.2 deraadt 952: .It
1.1 deraadt 953: Sets up basic environment.
1.2 deraadt 954: .It
955: Reads
956: .Pa $HOME/.ssh/environment
957: if it exists.
958: .It
1.1 deraadt 959: Changes to user's home directory.
1.2 deraadt 960: .It
961: If
962: .Pa $HOME/.ssh/rc
963: exists, runs it; else if
1.168 deraadt 964: .Pa /etc/ssh/sshrc
1.2 deraadt 965: exists, runs
1.36 aaron 966: it; otherwise runs xauth.
967: The
1.2 deraadt 968: .Dq rc
969: files are given the X11
1.1 deraadt 970: authentication protocol and cookie in standard input.
1.2 deraadt 971: .It
1.1 deraadt 972: Runs user's shell or command.
1.2 deraadt 973: .El
974: .Sh AUTHORIZED_KEYS FILE FORMAT
975: .Pa $HOME/.ssh/authorized_keys
1.130 markus 976: is the default file that lists the public keys that are
977: permitted for RSA authentication in protocol version 1
978: and for public key authentication (PubkeyAuthentication)
979: in protocol version 2.
1.125 markus 980: .Cm AuthorizedKeysFile
981: may be used to specify an alternative file.
1.75 markus 982: .Pp
1.36 aaron 983: Each line of the file contains one
1.2 deraadt 984: key (empty lines and lines starting with a
985: .Ql #
986: are ignored as
1.36 aaron 987: comments).
1.75 markus 988: Each RSA public key consists of the following fields, separated by
1.36 aaron 989: spaces: options, bits, exponent, modulus, comment.
1.75 markus 990: Each protocol version 2 public key consists of:
991: options, keytype, base64 encoded key, comment.
992: The options fields
993: are optional; its presence is determined by whether the line starts
1.1 deraadt 994: with a number or not (the option field never starts with a number).
1.75 markus 995: The bits, exponent, modulus and comment fields give the RSA key for
996: protocol version 1; the
1.1 deraadt 997: comment field is not used for anything (but may be convenient for the
998: user to identify the key).
1.75 markus 999: For protocol version 2 the keytype is
1000: .Dq ssh-dss
1001: or
1002: .Dq ssh-rsa .
1.2 deraadt 1003: .Pp
1.1 deraadt 1004: Note that lines in this file are usually several hundred bytes long
1.36 aaron 1005: (because of the size of the RSA key modulus).
1006: You don't want to type them in; instead, copy the
1.113 itojun 1007: .Pa identity.pub ,
1008: .Pa id_dsa.pub
1.75 markus 1009: or the
1.113 itojun 1010: .Pa id_rsa.pub
1.1 deraadt 1011: file and edit it.
1.2 deraadt 1012: .Pp
1.58 deraadt 1013: The options (if present) consist of comma-separated option
1.36 aaron 1014: specifications.
1015: No spaces are permitted, except within double quotes.
1.141 stevesk 1016: The following option specifications are supported (note
1017: that option keywords are case-insensitive):
1.2 deraadt 1018: .Bl -tag -width Ds
1019: .It Cm from="pattern-list"
1.1 deraadt 1020: Specifies that in addition to RSA authentication, the canonical name
1021: of the remote host must be present in the comma-separated list of
1.36 aaron 1022: patterns
1023: .Pf ( Ql *
1024: and
1025: .Ql ?
1026: serve as wildcards).
1027: The list may also contain
1028: patterns negated by prefixing them with
1029: .Ql ! ;
1030: if the canonical host name matches a negated pattern, the key is not accepted.
1031: The purpose
1.1 deraadt 1032: of this option is to optionally increase security: RSA authentication
1033: by itself does not trust the network or name servers or anything (but
1034: the key); however, if somebody somehow steals the key, the key
1.36 aaron 1035: permits an intruder to log in from anywhere in the world.
1036: This additional option makes using a stolen key more difficult (name
1.1 deraadt 1037: servers and/or routers would have to be compromised in addition to
1038: just the key).
1.2 deraadt 1039: .It Cm command="command"
1.1 deraadt 1040: Specifies that the command is executed whenever this key is used for
1.36 aaron 1041: authentication.
1042: The command supplied by the user (if any) is ignored.
1.148 markus 1043: The command is run on a pty if the client requests a pty;
1.36 aaron 1044: otherwise it is run without a tty.
1.147 deraadt 1045: If a 8-bit clean channel is required,
1046: one must not request a pty or should specify
1.89 markus 1047: .Cm no-pty .
1.36 aaron 1048: A quote may be included in the command by quoting it with a backslash.
1049: This option might be useful
1050: to restrict certain RSA keys to perform just a specific operation.
1051: An example might be a key that permits remote backups but nothing else.
1.51 hugh 1052: Note that the client may specify TCP/IP and/or X11
1053: forwarding unless they are explicitly prohibited.
1.149 markus 1054: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 1055: .It Cm environment="NAME=value"
1.1 deraadt 1056: Specifies that the string is to be added to the environment when
1.36 aaron 1057: logging in using this key.
1058: Environment variables set this way
1059: override other default environment values.
1060: Multiple options of this type are permitted.
1.155 markus 1061: This option is automatically disabled if
1062: .Cm UseLogin
1063: is enabled.
1.2 deraadt 1064: .It Cm no-port-forwarding
1.1 deraadt 1065: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 1066: Any port forward requests by the client will return an error.
1067: This might be used, e.g., in connection with the
1.2 deraadt 1068: .Cm command
1.1 deraadt 1069: option.
1.2 deraadt 1070: .It Cm no-X11-forwarding
1.1 deraadt 1071: Forbids X11 forwarding when this key is used for authentication.
1072: Any X11 forward requests by the client will return an error.
1.2 deraadt 1073: .It Cm no-agent-forwarding
1.1 deraadt 1074: Forbids authentication agent forwarding when this key is used for
1075: authentication.
1.2 deraadt 1076: .It Cm no-pty
1.1 deraadt 1077: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 1078: .It Cm permitopen="host:port"
1.133 itojun 1079: Limit local
1.107 djm 1080: .Li ``ssh -L''
1.111 stevesk 1081: port forwarding such that it may only connect to the specified host and
1.146 stevesk 1082: port.
1083: IPv6 addresses can be specified with an alternative syntax:
1084: .Ar host/port .
1085: Multiple
1.107 djm 1086: .Cm permitopen
1.133 itojun 1087: options may be applied separated by commas. No pattern matching is
1088: performed on the specified hostnames, they must be literal domains or
1.107 djm 1089: addresses.
1.2 deraadt 1090: .El
1091: .Ss Examples
1.1 deraadt 1092: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1093: .Pp
1.1 deraadt 1094: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1095: .Pp
1.1 deraadt 1096: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 1097: .Pp
1098: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1099: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1100: The
1.167 deraadt 1101: .Pa /etc/ssh/ssh_known_hosts ,
1.40 aaron 1102: and
1.131 markus 1103: .Pa $HOME/.ssh/known_hosts
1.36 aaron 1104: files contain host public keys for all known hosts.
1105: The global file should
1.37 brad 1106: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1107: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1108: its key is added to the per-user file.
1.2 deraadt 1109: .Pp
1.1 deraadt 1110: Each line in these files contains the following fields: hostnames,
1.36 aaron 1111: bits, exponent, modulus, comment.
1112: The fields are separated by spaces.
1.2 deraadt 1113: .Pp
1.1 deraadt 1114: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1115: wildcards); each pattern in turn is matched against the canonical host
1116: name (when authenticating a client) or against the user-supplied
1.36 aaron 1117: name (when authenticating a server).
1118: A pattern may also be preceded by
1.2 deraadt 1119: .Ql !
1120: to indicate negation: if the host name matches a negated
1.1 deraadt 1121: pattern, it is not accepted (by that line) even if it matched another
1122: pattern on the line.
1.2 deraadt 1123: .Pp
1.49 markus 1124: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1125: can be obtained, e.g., from
1.167 deraadt 1126: .Pa /etc/ssh/ssh_host_key.pub .
1.1 deraadt 1127: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1128: .Pp
1129: Lines starting with
1130: .Ql #
1131: and empty lines are ignored as comments.
1132: .Pp
1.1 deraadt 1133: When performing host authentication, authentication is accepted if any
1.36 aaron 1134: matching line has the proper key.
1135: It is thus permissible (but not
1.1 deraadt 1136: recommended) to have several lines or different host keys for the same
1.36 aaron 1137: names.
1138: This will inevitably happen when short forms of host names
1139: from different domains are put in the file.
1140: It is possible
1.1 deraadt 1141: that the files contain conflicting information; authentication is
1142: accepted if valid information can be found from either file.
1.2 deraadt 1143: .Pp
1.1 deraadt 1144: Note that the lines in these files are typically hundreds of characters
1145: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1146: Rather, generate them by a script
1.40 aaron 1147: or by taking
1.167 deraadt 1148: .Pa /etc/ssh/ssh_host_key.pub
1.1 deraadt 1149: and adding the host names at the front.
1.2 deraadt 1150: .Ss Examples
1.120 markus 1151: .Bd -literal
1152: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1153: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1154: .Ed
1.2 deraadt 1155: .Sh FILES
1156: .Bl -tag -width Ds
1.167 deraadt 1157: .It Pa /etc/ssh/sshd_config
1.1 deraadt 1158: Contains configuration data for
1.2 deraadt 1159: .Nm sshd .
1.1 deraadt 1160: This file should be writable by root only, but it is recommended
1161: (though not necessary) that it be world-readable.
1.167 deraadt 1162: .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
1.120 markus 1163: These three files contain the private parts of the host keys.
1.98 deraadt 1164: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1165: accessible to others.
1.14 markus 1166: Note that
1167: .Nm
1168: does not start if this file is group/world-accessible.
1.167 deraadt 1169: .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 1170: These three files contain the public parts of the host keys.
1.98 deraadt 1171: These files should be world-readable but writable only by
1.36 aaron 1172: root.
1.98 deraadt 1173: Their contents should match the respective private parts.
1174: These files are not
1175: really used for anything; they are provided for the convenience of
1176: the user so their contents can be copied to known hosts files.
1177: These files are created using
1.7 markus 1178: .Xr ssh-keygen 1 .
1.129 provos 1179: .It Pa /etc/moduli
1.73 provos 1180: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1181: .It Pa /var/run/sshd.pid
1182: Contains the process ID of the
1183: .Nm
1.1 deraadt 1184: listening for connections (if there are several daemons running
1185: concurrently for different ports, this contains the pid of the one
1.36 aaron 1186: started last).
1.58 deraadt 1187: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1188: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 1189: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1190: This file must be readable by root (which may on some machines imply
1191: it being world-readable if the user's home directory resides on an NFS
1192: volume).
1193: It is recommended that it not be accessible by others.
1194: The format of this file is described above.
1195: Users will place the contents of their
1.130 markus 1196: .Pa identity.pub ,
1.44 deraadt 1197: .Pa id_dsa.pub
1.113 itojun 1198: and/or
1199: .Pa id_rsa.pub
1.44 deraadt 1200: files into this file, as described in
1201: .Xr ssh-keygen 1 .
1.167 deraadt 1202: .It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1.22 markus 1203: These files are consulted when using rhosts with RSA host
1.131 markus 1204: authentication or protocol version 2 hostbased authentication
1205: to check the public key of the host.
1.36 aaron 1206: The key must be listed in one of these files to be accepted.
1.22 markus 1207: The client uses the same files
1.96 markus 1208: to verify that it is connecting to the correct remote host.
1.36 aaron 1209: These files should be writable only by root/the owner.
1.167 deraadt 1210: .Pa /etc/ssh/ssh_known_hosts
1.2 deraadt 1211: should be world-readable, and
1212: .Pa $HOME/.ssh/known_hosts
1.120 markus 1213: can but need not be world-readable.
1.6 aaron 1214: .It Pa /etc/nologin
1.40 aaron 1215: If this file exists,
1.2 deraadt 1216: .Nm
1.36 aaron 1217: refuses to let anyone except root log in.
1218: The contents of the file
1.1 deraadt 1219: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1220: refused.
1221: The file should be world-readable.
1.19 dugsong 1222: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 1223: Access controls that should be enforced by tcp-wrappers are defined here.
1224: Further details are described in
1.19 dugsong 1225: .Xr hosts_access 5 .
1.6 aaron 1226: .It Pa $HOME/.rhosts
1.1 deraadt 1227: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1228: line.
1229: The given user on the corresponding host is permitted to log in
1230: without password.
1231: The same file is used by rlogind and rshd.
1.6 aaron 1232: The file must
1.1 deraadt 1233: be writable only by the user; it is recommended that it not be
1234: accessible by others.
1.2 deraadt 1235: .Pp
1.36 aaron 1236: If is also possible to use netgroups in the file.
1237: Either host or user
1.1 deraadt 1238: name may be of the form +@groupname to specify all hosts or all users
1239: in the group.
1.2 deraadt 1240: .It Pa $HOME/.shosts
1241: For ssh,
1242: this file is exactly the same as for
1243: .Pa .rhosts .
1244: However, this file is
1245: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1246: .It Pa /etc/hosts.equiv
1.2 deraadt 1247: This file is used during
1248: .Pa .rhosts
1.36 aaron 1249: authentication.
1250: In the simplest form, this file contains host names, one per line.
1251: Users on
1.1 deraadt 1252: those hosts are permitted to log in without a password, provided they
1.36 aaron 1253: have the same user name on both machines.
1254: The host name may also be
1.1 deraadt 1255: followed by a user name; such users are permitted to log in as
1.2 deraadt 1256: .Em any
1.36 aaron 1257: user on this machine (except root).
1258: Additionally, the syntax
1.2 deraadt 1259: .Dq +@group
1.36 aaron 1260: can be used to specify netgroups.
1261: Negated entries start with
1.2 deraadt 1262: .Ql \&- .
1263: .Pp
1.1 deraadt 1264: If the client host/user is successfully matched in this file, login is
1265: automatically permitted provided the client and server user names are the
1.36 aaron 1266: same.
1267: Additionally, successful RSA host authentication is normally required.
1268: This file must be writable only by root; it is recommended
1.1 deraadt 1269: that it be world-readable.
1.2 deraadt 1270: .Pp
1.6 aaron 1271: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1272: .Pa hosts.equiv .
1.1 deraadt 1273: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1274: .Em anybody ,
1.1 deraadt 1275: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1276: binaries and directories.
1277: Using a user name practically grants the user root access.
1278: The only valid use for user names that I can think
1.1 deraadt 1279: of is in negative entries.
1.2 deraadt 1280: .Pp
1281: Note that this warning also applies to rsh/rlogin.
1282: .It Pa /etc/shosts.equiv
1.1 deraadt 1283: This is processed exactly as
1.2 deraadt 1284: .Pa /etc/hosts.equiv .
1.1 deraadt 1285: However, this file may be useful in environments that want to run both
1.2 deraadt 1286: rsh/rlogin and ssh.
1.6 aaron 1287: .It Pa $HOME/.ssh/environment
1.36 aaron 1288: This file is read into the environment at login (if it exists).
1289: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1290: .Ql # ) ,
1.36 aaron 1291: and assignment lines of the form name=value.
1292: The file should be writable
1.6 aaron 1293: only by the user; it need not be readable by anyone else.
1.2 deraadt 1294: .It Pa $HOME/.ssh/rc
1.1 deraadt 1295: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1296: environment files but before starting the user's shell or command.
1297: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1298: standard input (and
1299: .Ev DISPLAY
1.36 aaron 1300: in environment).
1301: This must call
1.2 deraadt 1302: .Xr xauth 1
1303: in that case.
1304: .Pp
1.1 deraadt 1305: The primary purpose of this file is to run any initialization routines
1306: which may be needed before the user's home directory becomes
1307: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1308: .Pp
1.1 deraadt 1309: This file will probably contain some initialization code followed by
1.120 markus 1310: something similar to:
1311: .Bd -literal
1312: if read proto cookie; then
1313: echo add $DISPLAY $proto $cookie | xauth -q -
1314: fi
1315: .Ed
1.2 deraadt 1316: .Pp
1317: If this file does not exist,
1.167 deraadt 1318: .Pa /etc/ssh/sshrc
1.2 deraadt 1319: is run, and if that
1.1 deraadt 1320: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1321: .Pp
1.1 deraadt 1322: This file should be writable only by the user, and need not be
1323: readable by anyone else.
1.167 deraadt 1324: .It Pa /etc/ssh/sshrc
1.2 deraadt 1325: Like
1326: .Pa $HOME/.ssh/rc .
1327: This can be used to specify
1.36 aaron 1328: machine-specific login-time initializations globally.
1329: This file should be writable only by root, and should be world-readable.
1.56 aaron 1330: .El
1.71 aaron 1331: .Sh AUTHORS
1.84 markus 1332: OpenSSH is a derivative of the original and free
1333: ssh 1.2.12 release by Tatu Ylonen.
1334: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1335: Theo de Raadt and Dug Song
1336: removed many bugs, re-added newer features and
1337: created OpenSSH.
1338: Markus Friedl contributed the support for SSH
1339: protocol versions 1.5 and 2.0.
1.2 deraadt 1340: .Sh SEE ALSO
1341: .Xr scp 1 ,
1.90 djm 1342: .Xr sftp 1 ,
1.2 deraadt 1343: .Xr ssh 1 ,
1.5 deraadt 1344: .Xr ssh-add 1 ,
1.2 deraadt 1345: .Xr ssh-agent 1 ,
1.5 deraadt 1346: .Xr ssh-keygen 1 ,
1.136 markus 1347: .Xr login.conf 5 ,
1348: .Xr moduli 5 ,
1.128 mpech 1349: .Xr sftp-server 8
1.119 markus 1350: .Rs
1351: .%A T. Ylonen
1352: .%A T. Kivinen
1353: .%A M. Saarinen
1354: .%A T. Rinne
1355: .%A S. Lehtinen
1356: .%T "SSH Protocol Architecture"
1.139 markus 1357: .%N draft-ietf-secsh-architecture-09.txt
1358: .%D July 2001
1.120 markus 1359: .%O work in progress material
1360: .Re
1361: .Rs
1362: .%A M. Friedl
1363: .%A N. Provos
1364: .%A W. A. Simpson
1365: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.132 markus 1366: .%N draft-ietf-secsh-dh-group-exchange-01.txt
1367: .%D April 2001
1.119 markus 1368: .%O work in progress material
1369: .Re
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