Annotation of src/usr.bin/ssh/sshd.8, Revision 1.162
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,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.162 ! stevesk 37: .\" $OpenBSD: sshd.8,v 1.161 2002/01/05 21:51:56 stevesk 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
1.2 deraadt 201: .Pa /etc/sshd_config .
1.16 markus 202: .Nm
203: refuses to start if there is no configuration file.
1.2 deraadt 204: .It Fl g Ar login_grace_time
1.1 deraadt 205: Gives the grace time for clients to authenticate themselves (default
1.77 markus 206: 600 seconds).
1.36 aaron 207: If the client fails to authenticate the user within
208: this many seconds, the server disconnects and exits.
209: A value of zero indicates no limit.
1.2 deraadt 210: .It Fl h Ar host_key_file
1.160 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
217: .Pa /etc/ssh_host_key
218: for protocol version 1, and
219: .Pa /etc/ssh_host_rsa_key
220: and
221: .Pa /etc/ssh_host_dsa_key
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
1.2 deraadt 314: .Pa /etc/sshd_config
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
1.92 deraadt 332: This keyword can be followed by a list of group names, 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.
1.81 markus 342: By default login is allowed regardless of the group list.
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
1.92 deraadt 353: This keyword can be followed by a list of user names, 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.
1.36 aaron 363: By default login is allowed regardless of the user name.
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
438: This keyword can be followed by a number of group names, separated
1.36 aaron 439: by spaces.
1.81 markus 440: Users whose primary group or supplementary group list matches
441: one of the patterns aren't allowed to log in.
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.
1.81 markus 448: By default login is allowed regardless of the group list.
1.11 markus 449: .Pp
450: .It Cm DenyUsers
451: This keyword can be followed by a number of user names, 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.
1.36 aaron 459: By default login is allowed regardless of the user name.
1.47 markus 460: .It Cm GatewayPorts
461: Specifies whether remote hosts are allowed to connect to ports
462: forwarded for the client.
1.145 stevesk 463: By default,
464: .Nm
465: binds remote port forwardings to the loopback addresss. This
466: prevents other remote hosts from connecting to forwarded ports.
467: .Cm GatewayPorts
468: can be used to specify that
469: .Nm
470: should bind remote port forwardings to the wildcard address,
471: thus allowing remote hosts to connect to forwarded ports.
1.47 markus 472: The argument must be
473: .Dq yes
474: or
475: .Dq no .
476: The default is
477: .Dq no .
1.120 markus 478: .It Cm HostbasedAuthentication
479: Specifies whether rhosts or /etc/hosts.equiv authentication together
480: with successful public key client host authentication is allowed
481: (hostbased authentication).
482: This option is similar to
483: .Cm RhostsRSAAuthentication
484: and applies to protocol version 2 only.
485: The default is
486: .Dq no .
1.2 deraadt 487: .It Cm HostKey
1.160 stevesk 488: Specifies a file containing a private host key
489: used by SSH.
490: The default is
491: .Pa /etc/ssh_host_key
492: for protocol version 1, and
493: .Pa /etc/ssh_host_rsa_key
494: and
495: .Pa /etc/ssh_host_dsa_key
496: for protocol version 2.
1.9 markus 497: Note that
498: .Nm
1.83 markus 499: will refuse to use a file if it is group/world-accessible.
1.72 markus 500: It is possible to have multiple host key files.
501: .Dq rsa1
502: keys are used for version 1 and
503: .Dq dsa
504: or
505: .Dq rsa
506: are used for version 2 of the SSH protocol.
1.2 deraadt 507: .It Cm IgnoreRhosts
1.34 markus 508: Specifies that
509: .Pa .rhosts
1.40 aaron 510: and
1.34 markus 511: .Pa .shosts
1.120 markus 512: files will not be used in
513: .Cm RhostsAuthentication ,
514: .Cm RhostsRSAAuthentication
515: or
516: .Cm HostbasedAuthentication .
517: .Pp
1.2 deraadt 518: .Pa /etc/hosts.equiv
1.1 deraadt 519: and
1.40 aaron 520: .Pa /etc/shosts.equiv
1.36 aaron 521: are still used.
1.40 aaron 522: The default is
1.34 markus 523: .Dq yes .
1.24 markus 524: .It Cm IgnoreUserKnownHosts
525: Specifies whether
526: .Nm
527: should ignore the user's
528: .Pa $HOME/.ssh/known_hosts
1.45 markus 529: during
1.120 markus 530: .Cm RhostsRSAAuthentication
531: or
532: .Cm HostbasedAuthentication .
1.24 markus 533: The default is
1.2 deraadt 534: .Dq no .
535: .It Cm KeepAlive
1.1 deraadt 536: Specifies whether the system should send keepalive messages to the
1.36 aaron 537: other side.
538: If they are sent, death of the connection or crash of one
539: of the machines will be properly noticed.
540: However, this means that
1.1 deraadt 541: connections will die if the route is down temporarily, and some people
1.36 aaron 542: find it annoying.
1.51 hugh 543: On the other hand, if keepalives are not sent,
1.2 deraadt 544: sessions may hang indefinitely on the server, leaving
545: .Dq ghost
546: users and consuming server resources.
547: .Pp
548: The default is
549: .Dq yes
550: (to send keepalives), and the server will notice
1.36 aaron 551: if the network goes down or the client host reboots.
552: This avoids infinitely hanging sessions.
1.2 deraadt 553: .Pp
554: To disable keepalives, the value should be set to
555: .Dq no
556: in both the server and the client configuration files.
557: .It Cm KerberosAuthentication
1.36 aaron 558: Specifies whether Kerberos authentication is allowed.
559: This can be in the form of a Kerberos ticket, or if
1.7 markus 560: .Cm PasswordAuthentication
1.1 deraadt 561: is yes, the password provided by the user will be validated through
1.67 aaron 562: the Kerberos KDC.
563: To use this option, the server needs a
1.59 provos 564: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 565: Default is
1.60 provos 566: .Dq yes .
1.2 deraadt 567: .It Cm KerberosOrLocalPasswd
1.1 deraadt 568: If set then if password authentication through Kerberos fails then
569: the password will be validated via any additional local mechanism
1.2 deraadt 570: such as
1.66 markus 571: .Pa /etc/passwd .
1.36 aaron 572: Default is
1.20 dugsong 573: .Dq yes .
1.2 deraadt 574: .It Cm KerberosTgtPassing
1.1 deraadt 575: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 576: Default is
1.3 dugsong 577: .Dq no ,
578: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 579: .It Cm KerberosTicketCleanup
1.7 markus 580: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 581: file on logout.
582: Default is
1.3 dugsong 583: .Dq yes .
1.2 deraadt 584: .It Cm KeyRegenerationInterval
1.120 markus 585: In protocol version 1, the ephemeral server key is automatically regenerated
586: after this many seconds (if it has been used).
1.36 aaron 587: The purpose of regeneration is to prevent
1.1 deraadt 588: decrypting captured sessions by later breaking into the machine and
1.36 aaron 589: stealing the keys.
590: The key is never stored anywhere.
591: If the value is 0, the key is never regenerated.
592: The default is 3600 (seconds).
1.7 markus 593: .It Cm ListenAddress
1.110 stevesk 594: Specifies the local addresses
1.120 markus 595: .Nm
1.7 markus 596: should listen on.
1.110 stevesk 597: The following forms may be used:
598: .Pp
599: .Bl -item -offset indent -compact
600: .It
601: .Cm ListenAddress
1.112 stevesk 602: .Sm off
603: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
604: .Sm on
1.110 stevesk 605: .It
606: .Cm ListenAddress
1.112 stevesk 607: .Sm off
608: .Ar host No | Ar IPv4_addr No : Ar port
609: .Sm on
1.110 stevesk 610: .It
611: .Cm ListenAddress
1.112 stevesk 612: .Sm off
613: .Oo
614: .Ar host No | Ar IPv6_addr Oc : Ar port
615: .Sm on
1.110 stevesk 616: .El
617: .Pp
618: If
1.112 stevesk 619: .Ar port
1.110 stevesk 620: is not specified,
1.120 markus 621: .Nm
1.110 stevesk 622: will listen on the address and all prior
623: .Cm Port
624: options specified. The default is to listen on all local
625: addresses. Multiple
626: .Cm ListenAddress
627: options are permitted. Additionally, any
628: .Cm Port
629: options must precede this option for non port qualified addresses.
1.2 deraadt 630: .It Cm LoginGraceTime
1.1 deraadt 631: The server disconnects after this time if the user has not
1.36 aaron 632: successfully logged in.
633: If the value is 0, there is no time limit.
1.1 deraadt 634: The default is 600 (seconds).
1.23 markus 635: .It Cm LogLevel
636: Gives the verbosity level that is used when logging messages from
637: .Nm sshd .
638: The possible values are:
1.159 stevesk 639: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
640: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
641: and DEBUG3 each specify higher levels of debugging output.
642: Logging with a DEBUG level violates the privacy of users
1.23 markus 643: and is not recommended.
1.93 markus 644: .It Cm MACs
645: Specifies the available MAC (message authentication code) algorithms.
646: The MAC algorithm is used in protocol version 2
647: for data integrity protection.
648: Multiple algorithms must be comma-separated.
649: The default is
1.123 markus 650: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 651: .It Cm MaxStartups
652: Specifies the maximum number of concurrent unauthenticated connections to the
653: .Nm
654: daemon.
655: Additional connections will be dropped until authentication succeeds or the
656: .Cm LoginGraceTime
657: expires for a connection.
658: The default is 10.
1.57 markus 659: .Pp
660: Alternatively, random early drop can be enabled by specifying
661: the three colon separated values
662: .Dq start:rate:full
1.67 aaron 663: (e.g., "10:30:60").
1.57 markus 664: .Nm
1.86 stevesk 665: will refuse connection attempts with a probability of
1.57 markus 666: .Dq rate/100
667: (30%)
668: if there are currently
669: .Dq start
670: (10)
671: unauthenticated connections.
1.86 stevesk 672: The probability increases linearly and all connection attempts
1.57 markus 673: are refused if the number of unauthenticated connections reaches
674: .Dq full
675: (60).
1.2 deraadt 676: .It Cm PasswordAuthentication
1.1 deraadt 677: Specifies whether password authentication is allowed.
1.2 deraadt 678: The default is
679: .Dq yes .
680: .It Cm PermitEmptyPasswords
1.1 deraadt 681: When password authentication is allowed, it specifies whether the
1.36 aaron 682: server allows login to accounts with empty password strings.
683: The default is
1.34 markus 684: .Dq no .
1.2 deraadt 685: .It Cm PermitRootLogin
1.100 stevesk 686: Specifies whether root can login using
1.2 deraadt 687: .Xr ssh 1 .
1.15 markus 688: The argument must be
689: .Dq yes ,
1.94 markus 690: .Dq without-password ,
691: .Dq forced-commands-only
1.15 markus 692: or
693: .Dq no .
1.2 deraadt 694: The default is
695: .Dq yes .
1.94 markus 696: .Pp
697: If this option is set to
1.15 markus 698: .Dq without-password
1.94 markus 699: password authentication is disabled for root.
1.2 deraadt 700: .Pp
1.94 markus 701: If this option is set to
702: .Dq forced-commands-only
703: root login with public key authentication will be allowed,
704: but only if the
1.2 deraadt 705: .Ar command
1.94 markus 706: option has been specified
1.1 deraadt 707: (which may be useful for taking remote backups even if root login is
1.94 markus 708: normally not allowed). All other authentication methods are disabled
709: for root.
1.100 stevesk 710: .Pp
711: If this option is set to
712: .Dq no
713: root is not allowed to login.
1.43 markus 714: .It Cm PidFile
715: Specifies the file that contains the process identifier of the
716: .Nm
717: daemon.
718: The default is
719: .Pa /var/run/sshd.pid .
1.2 deraadt 720: .It Cm Port
1.1 deraadt 721: Specifies the port number that
1.2 deraadt 722: .Nm
1.36 aaron 723: listens on.
724: The default is 22.
1.28 markus 725: Multiple options of this type are permitted.
1.120 markus 726: See also
727: .Cm ListenAddress .
1.108 stevesk 728: .It Cm PrintLastLog
729: Specifies whether
730: .Nm
731: should print the date and time when the user last logged in.
732: The default is
733: .Dq yes .
1.2 deraadt 734: .It Cm PrintMotd
1.1 deraadt 735: Specifies whether
1.2 deraadt 736: .Nm
1.40 aaron 737: should print
1.2 deraadt 738: .Pa /etc/motd
1.36 aaron 739: when a user logs in interactively.
740: (On some systems it is also printed by the shell,
1.2 deraadt 741: .Pa /etc/profile ,
1.36 aaron 742: or equivalent.)
743: The default is
1.2 deraadt 744: .Dq yes .
1.41 markus 745: .It Cm Protocol
746: Specifies the protocol versions
747: .Nm
748: should support.
749: The possible values are
750: .Dq 1
751: and
752: .Dq 2 .
753: Multiple versions must be comma-separated.
754: The default is
1.118 deraadt 755: .Dq 2,1 .
1.104 deraadt 756: .It Cm PubkeyAuthentication
757: Specifies whether public key authentication is allowed.
758: The default is
759: .Dq yes .
760: Note that this option applies to protocol version 2 only.
1.87 markus 761: .It Cm ReverseMappingCheck
762: Specifies whether
763: .Nm
764: should try to verify the remote host name and check that
765: the resolved host name for the remote IP address maps back to the
766: very same IP address.
767: The default is
768: .Dq no .
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
1.127 markus 835: cookies.
1.6 aaron 836: .It Cm X11DisplayOffset
837: Specifies the first display number available for
838: .Nm sshd Ns 's
1.36 aaron 839: X11 forwarding.
840: This prevents
1.6 aaron 841: .Nm
842: from interfering with real X11 servers.
1.34 markus 843: The default is 10.
1.30 markus 844: .It Cm X11Forwarding
1.36 aaron 845: Specifies whether X11 forwarding is permitted.
846: The default is
1.34 markus 847: .Dq no .
1.30 markus 848: Note that disabling X11 forwarding does not improve security in any
849: way, as users can always install their own forwarders.
1.133 itojun 850: X11 forwarding is automatically disabled if
851: .Cm UseLogin
852: is enabled.
1.52 markus 853: .It Cm XAuthLocation
854: Specifies the location of the
855: .Xr xauth 1
856: program.
857: The default is
858: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 859: .El
1.124 stevesk 860: .Ss Time Formats
861: .Pp
862: .Nm
863: command-line arguments and configuration file options that specify time
864: may be expressed using a sequence of the form:
865: .Sm off
866: .Ar time Oo Ar qualifier Oc ,
867: .Sm on
868: where
869: .Ar time
870: is a positive integer value and
871: .Ar qualifier
872: is one of the following:
873: .Pp
874: .Bl -tag -width Ds -compact -offset indent
875: .It Cm <none>
876: seconds
877: .It Cm s | Cm S
878: seconds
879: .It Cm m | Cm M
880: minutes
881: .It Cm h | Cm H
882: hours
883: .It Cm d | Cm D
884: days
885: .It Cm w | Cm W
886: weeks
887: .El
888: .Pp
889: Each member of the sequence is added together to calculate
890: the total time value.
891: .Pp
892: Time format examples:
893: .Pp
894: .Bl -tag -width Ds -compact -offset indent
895: .It 600
896: 600 seconds (10 minutes)
897: .It 10m
898: 10 minutes
899: .It 1h30m
900: 1 hour 30 minutes (90 minutes)
901: .El
1.2 deraadt 902: .Sh LOGIN PROCESS
1.1 deraadt 903: When a user successfully logs in,
1.2 deraadt 904: .Nm
1.1 deraadt 905: does the following:
1.2 deraadt 906: .Bl -enum -offset indent
907: .It
1.1 deraadt 908: If the login is on a tty, and no command has been specified,
1.40 aaron 909: prints last login time and
1.2 deraadt 910: .Pa /etc/motd
1.1 deraadt 911: (unless prevented in the configuration file or by
1.2 deraadt 912: .Pa $HOME/.hushlogin ;
913: see the
1.40 aaron 914: .Sx FILES
1.2 deraadt 915: section).
916: .It
1.1 deraadt 917: If the login is on a tty, records login time.
1.2 deraadt 918: .It
919: Checks
920: .Pa /etc/nologin ;
921: if it exists, prints contents and quits
1.1 deraadt 922: (unless root).
1.2 deraadt 923: .It
1.1 deraadt 924: Changes to run with normal user privileges.
1.2 deraadt 925: .It
1.1 deraadt 926: Sets up basic environment.
1.2 deraadt 927: .It
928: Reads
929: .Pa $HOME/.ssh/environment
930: if it exists.
931: .It
1.1 deraadt 932: Changes to user's home directory.
1.2 deraadt 933: .It
934: If
935: .Pa $HOME/.ssh/rc
936: exists, runs it; else if
937: .Pa /etc/sshrc
938: exists, runs
1.36 aaron 939: it; otherwise runs xauth.
940: The
1.2 deraadt 941: .Dq rc
942: files are given the X11
1.1 deraadt 943: authentication protocol and cookie in standard input.
1.2 deraadt 944: .It
1.1 deraadt 945: Runs user's shell or command.
1.2 deraadt 946: .El
947: .Sh AUTHORIZED_KEYS FILE FORMAT
948: .Pa $HOME/.ssh/authorized_keys
1.130 markus 949: is the default file that lists the public keys that are
950: permitted for RSA authentication in protocol version 1
951: and for public key authentication (PubkeyAuthentication)
952: in protocol version 2.
1.125 markus 953: .Cm AuthorizedKeysFile
954: may be used to specify an alternative file.
1.75 markus 955: .Pp
1.36 aaron 956: Each line of the file contains one
1.2 deraadt 957: key (empty lines and lines starting with a
958: .Ql #
959: are ignored as
1.36 aaron 960: comments).
1.75 markus 961: Each RSA public key consists of the following fields, separated by
1.36 aaron 962: spaces: options, bits, exponent, modulus, comment.
1.75 markus 963: Each protocol version 2 public key consists of:
964: options, keytype, base64 encoded key, comment.
965: The options fields
966: are optional; its presence is determined by whether the line starts
1.1 deraadt 967: with a number or not (the option field never starts with a number).
1.75 markus 968: The bits, exponent, modulus and comment fields give the RSA key for
969: protocol version 1; the
1.1 deraadt 970: comment field is not used for anything (but may be convenient for the
971: user to identify the key).
1.75 markus 972: For protocol version 2 the keytype is
973: .Dq ssh-dss
974: or
975: .Dq ssh-rsa .
1.2 deraadt 976: .Pp
1.1 deraadt 977: Note that lines in this file are usually several hundred bytes long
1.36 aaron 978: (because of the size of the RSA key modulus).
979: You don't want to type them in; instead, copy the
1.113 itojun 980: .Pa identity.pub ,
981: .Pa id_dsa.pub
1.75 markus 982: or the
1.113 itojun 983: .Pa id_rsa.pub
1.1 deraadt 984: file and edit it.
1.2 deraadt 985: .Pp
1.58 deraadt 986: The options (if present) consist of comma-separated option
1.36 aaron 987: specifications.
988: No spaces are permitted, except within double quotes.
1.141 stevesk 989: The following option specifications are supported (note
990: that option keywords are case-insensitive):
1.2 deraadt 991: .Bl -tag -width Ds
992: .It Cm from="pattern-list"
1.1 deraadt 993: Specifies that in addition to RSA authentication, the canonical name
994: of the remote host must be present in the comma-separated list of
1.36 aaron 995: patterns
996: .Pf ( Ql *
997: and
998: .Ql ?
999: serve as wildcards).
1000: The list may also contain
1001: patterns negated by prefixing them with
1002: .Ql ! ;
1003: if the canonical host name matches a negated pattern, the key is not accepted.
1004: The purpose
1.1 deraadt 1005: of this option is to optionally increase security: RSA authentication
1006: by itself does not trust the network or name servers or anything (but
1007: the key); however, if somebody somehow steals the key, the key
1.36 aaron 1008: permits an intruder to log in from anywhere in the world.
1009: This additional option makes using a stolen key more difficult (name
1.1 deraadt 1010: servers and/or routers would have to be compromised in addition to
1011: just the key).
1.2 deraadt 1012: .It Cm command="command"
1.1 deraadt 1013: Specifies that the command is executed whenever this key is used for
1.36 aaron 1014: authentication.
1015: The command supplied by the user (if any) is ignored.
1.148 markus 1016: The command is run on a pty if the client requests a pty;
1.36 aaron 1017: otherwise it is run without a tty.
1.147 deraadt 1018: If a 8-bit clean channel is required,
1019: one must not request a pty or should specify
1.89 markus 1020: .Cm no-pty .
1.36 aaron 1021: A quote may be included in the command by quoting it with a backslash.
1022: This option might be useful
1023: to restrict certain RSA keys to perform just a specific operation.
1024: An example might be a key that permits remote backups but nothing else.
1.51 hugh 1025: Note that the client may specify TCP/IP and/or X11
1026: forwarding unless they are explicitly prohibited.
1.149 markus 1027: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 1028: .It Cm environment="NAME=value"
1.1 deraadt 1029: Specifies that the string is to be added to the environment when
1.36 aaron 1030: logging in using this key.
1031: Environment variables set this way
1032: override other default environment values.
1033: Multiple options of this type are permitted.
1.155 markus 1034: This option is automatically disabled if
1035: .Cm UseLogin
1036: is enabled.
1.2 deraadt 1037: .It Cm no-port-forwarding
1.1 deraadt 1038: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 1039: Any port forward requests by the client will return an error.
1040: This might be used, e.g., in connection with the
1.2 deraadt 1041: .Cm command
1.1 deraadt 1042: option.
1.2 deraadt 1043: .It Cm no-X11-forwarding
1.1 deraadt 1044: Forbids X11 forwarding when this key is used for authentication.
1045: Any X11 forward requests by the client will return an error.
1.2 deraadt 1046: .It Cm no-agent-forwarding
1.1 deraadt 1047: Forbids authentication agent forwarding when this key is used for
1048: authentication.
1.2 deraadt 1049: .It Cm no-pty
1.1 deraadt 1050: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 1051: .It Cm permitopen="host:port"
1.133 itojun 1052: Limit local
1.107 djm 1053: .Li ``ssh -L''
1.111 stevesk 1054: port forwarding such that it may only connect to the specified host and
1.146 stevesk 1055: port.
1056: IPv6 addresses can be specified with an alternative syntax:
1057: .Ar host/port .
1058: Multiple
1.107 djm 1059: .Cm permitopen
1.133 itojun 1060: options may be applied separated by commas. No pattern matching is
1061: performed on the specified hostnames, they must be literal domains or
1.107 djm 1062: addresses.
1.2 deraadt 1063: .El
1064: .Ss Examples
1.1 deraadt 1065: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1066: .Pp
1.1 deraadt 1067: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1068: .Pp
1.1 deraadt 1069: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 1070: .Pp
1071: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1072: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1073: The
1.44 deraadt 1074: .Pa /etc/ssh_known_hosts ,
1.40 aaron 1075: and
1.131 markus 1076: .Pa $HOME/.ssh/known_hosts
1.36 aaron 1077: files contain host public keys for all known hosts.
1078: The global file should
1.37 brad 1079: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1080: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1081: its key is added to the per-user file.
1.2 deraadt 1082: .Pp
1.1 deraadt 1083: Each line in these files contains the following fields: hostnames,
1.36 aaron 1084: bits, exponent, modulus, comment.
1085: The fields are separated by spaces.
1.2 deraadt 1086: .Pp
1.1 deraadt 1087: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1088: wildcards); each pattern in turn is matched against the canonical host
1089: name (when authenticating a client) or against the user-supplied
1.36 aaron 1090: name (when authenticating a server).
1091: A pattern may also be preceded by
1.2 deraadt 1092: .Ql !
1093: to indicate negation: if the host name matches a negated
1.1 deraadt 1094: pattern, it is not accepted (by that line) even if it matched another
1095: pattern on the line.
1.2 deraadt 1096: .Pp
1.49 markus 1097: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1098: can be obtained, e.g., from
1099: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 1100: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1101: .Pp
1102: Lines starting with
1103: .Ql #
1104: and empty lines are ignored as comments.
1105: .Pp
1.1 deraadt 1106: When performing host authentication, authentication is accepted if any
1.36 aaron 1107: matching line has the proper key.
1108: It is thus permissible (but not
1.1 deraadt 1109: recommended) to have several lines or different host keys for the same
1.36 aaron 1110: names.
1111: This will inevitably happen when short forms of host names
1112: from different domains are put in the file.
1113: It is possible
1.1 deraadt 1114: that the files contain conflicting information; authentication is
1115: accepted if valid information can be found from either file.
1.2 deraadt 1116: .Pp
1.1 deraadt 1117: Note that the lines in these files are typically hundreds of characters
1118: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1119: Rather, generate them by a script
1.40 aaron 1120: or by taking
1.2 deraadt 1121: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1122: and adding the host names at the front.
1.2 deraadt 1123: .Ss Examples
1.120 markus 1124: .Bd -literal
1125: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1126: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1127: .Ed
1.2 deraadt 1128: .Sh FILES
1129: .Bl -tag -width Ds
1130: .It Pa /etc/sshd_config
1.1 deraadt 1131: Contains configuration data for
1.2 deraadt 1132: .Nm sshd .
1.1 deraadt 1133: This file should be writable by root only, but it is recommended
1134: (though not necessary) that it be world-readable.
1.98 deraadt 1135: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.120 markus 1136: These three files contain the private parts of the host keys.
1.98 deraadt 1137: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1138: accessible to others.
1.14 markus 1139: Note that
1140: .Nm
1141: does not start if this file is group/world-accessible.
1.98 deraadt 1142: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.120 markus 1143: These three files contain the public parts of the host keys.
1.98 deraadt 1144: These files should be world-readable but writable only by
1.36 aaron 1145: root.
1.98 deraadt 1146: Their contents should match the respective private parts.
1147: These files are not
1148: really used for anything; they are provided for the convenience of
1149: the user so their contents can be copied to known hosts files.
1150: These files are created using
1.7 markus 1151: .Xr ssh-keygen 1 .
1.129 provos 1152: .It Pa /etc/moduli
1.73 provos 1153: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1154: .It Pa /var/run/sshd.pid
1155: Contains the process ID of the
1156: .Nm
1.1 deraadt 1157: listening for connections (if there are several daemons running
1158: concurrently for different ports, this contains the pid of the one
1.36 aaron 1159: started last).
1.58 deraadt 1160: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1161: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 1162: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1163: This file must be readable by root (which may on some machines imply
1164: it being world-readable if the user's home directory resides on an NFS
1165: volume).
1166: It is recommended that it not be accessible by others.
1167: The format of this file is described above.
1168: Users will place the contents of their
1.130 markus 1169: .Pa identity.pub ,
1.44 deraadt 1170: .Pa id_dsa.pub
1.113 itojun 1171: and/or
1172: .Pa id_rsa.pub
1.44 deraadt 1173: files into this file, as described in
1174: .Xr ssh-keygen 1 .
1.22 markus 1175: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1176: These files are consulted when using rhosts with RSA host
1.131 markus 1177: authentication or protocol version 2 hostbased authentication
1178: to check the public key of the host.
1.36 aaron 1179: The key must be listed in one of these files to be accepted.
1.22 markus 1180: The client uses the same files
1.96 markus 1181: to verify that it is connecting to the correct remote host.
1.36 aaron 1182: These files should be writable only by root/the owner.
1.2 deraadt 1183: .Pa /etc/ssh_known_hosts
1184: should be world-readable, and
1185: .Pa $HOME/.ssh/known_hosts
1.120 markus 1186: can but need not be world-readable.
1.6 aaron 1187: .It Pa /etc/nologin
1.40 aaron 1188: If this file exists,
1.2 deraadt 1189: .Nm
1.36 aaron 1190: refuses to let anyone except root log in.
1191: The contents of the file
1.1 deraadt 1192: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1193: refused.
1194: The file should be world-readable.
1.19 dugsong 1195: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 1196: Access controls that should be enforced by tcp-wrappers are defined here.
1197: Further details are described in
1.19 dugsong 1198: .Xr hosts_access 5 .
1.6 aaron 1199: .It Pa $HOME/.rhosts
1.1 deraadt 1200: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1201: line.
1202: The given user on the corresponding host is permitted to log in
1203: without password.
1204: The same file is used by rlogind and rshd.
1.6 aaron 1205: The file must
1.1 deraadt 1206: be writable only by the user; it is recommended that it not be
1207: accessible by others.
1.2 deraadt 1208: .Pp
1.36 aaron 1209: If is also possible to use netgroups in the file.
1210: Either host or user
1.1 deraadt 1211: name may be of the form +@groupname to specify all hosts or all users
1212: in the group.
1.2 deraadt 1213: .It Pa $HOME/.shosts
1214: For ssh,
1215: this file is exactly the same as for
1216: .Pa .rhosts .
1217: However, this file is
1218: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1219: .It Pa /etc/hosts.equiv
1.2 deraadt 1220: This file is used during
1221: .Pa .rhosts
1.36 aaron 1222: authentication.
1223: In the simplest form, this file contains host names, one per line.
1224: Users on
1.1 deraadt 1225: those hosts are permitted to log in without a password, provided they
1.36 aaron 1226: have the same user name on both machines.
1227: The host name may also be
1.1 deraadt 1228: followed by a user name; such users are permitted to log in as
1.2 deraadt 1229: .Em any
1.36 aaron 1230: user on this machine (except root).
1231: Additionally, the syntax
1.2 deraadt 1232: .Dq +@group
1.36 aaron 1233: can be used to specify netgroups.
1234: Negated entries start with
1.2 deraadt 1235: .Ql \&- .
1236: .Pp
1.1 deraadt 1237: If the client host/user is successfully matched in this file, login is
1238: automatically permitted provided the client and server user names are the
1.36 aaron 1239: same.
1240: Additionally, successful RSA host authentication is normally required.
1241: This file must be writable only by root; it is recommended
1.1 deraadt 1242: that it be world-readable.
1.2 deraadt 1243: .Pp
1.6 aaron 1244: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1245: .Pa hosts.equiv .
1.1 deraadt 1246: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1247: .Em anybody ,
1.1 deraadt 1248: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1249: binaries and directories.
1250: Using a user name practically grants the user root access.
1251: The only valid use for user names that I can think
1.1 deraadt 1252: of is in negative entries.
1.2 deraadt 1253: .Pp
1254: Note that this warning also applies to rsh/rlogin.
1255: .It Pa /etc/shosts.equiv
1.1 deraadt 1256: This is processed exactly as
1.2 deraadt 1257: .Pa /etc/hosts.equiv .
1.1 deraadt 1258: However, this file may be useful in environments that want to run both
1.2 deraadt 1259: rsh/rlogin and ssh.
1.6 aaron 1260: .It Pa $HOME/.ssh/environment
1.36 aaron 1261: This file is read into the environment at login (if it exists).
1262: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1263: .Ql # ) ,
1.36 aaron 1264: and assignment lines of the form name=value.
1265: The file should be writable
1.6 aaron 1266: only by the user; it need not be readable by anyone else.
1.2 deraadt 1267: .It Pa $HOME/.ssh/rc
1.1 deraadt 1268: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1269: environment files but before starting the user's shell or command.
1270: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1271: standard input (and
1272: .Ev DISPLAY
1.36 aaron 1273: in environment).
1274: This must call
1.2 deraadt 1275: .Xr xauth 1
1276: in that case.
1277: .Pp
1.1 deraadt 1278: The primary purpose of this file is to run any initialization routines
1279: which may be needed before the user's home directory becomes
1280: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1281: .Pp
1.1 deraadt 1282: This file will probably contain some initialization code followed by
1.120 markus 1283: something similar to:
1284: .Bd -literal
1285: if read proto cookie; then
1286: echo add $DISPLAY $proto $cookie | xauth -q -
1287: fi
1288: .Ed
1.2 deraadt 1289: .Pp
1290: If this file does not exist,
1291: .Pa /etc/sshrc
1292: is run, and if that
1.1 deraadt 1293: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1294: .Pp
1.1 deraadt 1295: This file should be writable only by the user, and need not be
1296: readable by anyone else.
1.2 deraadt 1297: .It Pa /etc/sshrc
1298: Like
1299: .Pa $HOME/.ssh/rc .
1300: This can be used to specify
1.36 aaron 1301: machine-specific login-time initializations globally.
1302: This file should be writable only by root, and should be world-readable.
1.56 aaron 1303: .El
1.71 aaron 1304: .Sh AUTHORS
1.84 markus 1305: OpenSSH is a derivative of the original and free
1306: ssh 1.2.12 release by Tatu Ylonen.
1307: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1308: Theo de Raadt and Dug Song
1309: removed many bugs, re-added newer features and
1310: created OpenSSH.
1311: Markus Friedl contributed the support for SSH
1312: protocol versions 1.5 and 2.0.
1.2 deraadt 1313: .Sh SEE ALSO
1314: .Xr scp 1 ,
1.90 djm 1315: .Xr sftp 1 ,
1.2 deraadt 1316: .Xr ssh 1 ,
1.5 deraadt 1317: .Xr ssh-add 1 ,
1.2 deraadt 1318: .Xr ssh-agent 1 ,
1.5 deraadt 1319: .Xr ssh-keygen 1 ,
1.136 markus 1320: .Xr login.conf 5 ,
1321: .Xr moduli 5 ,
1.128 mpech 1322: .Xr sftp-server 8
1.119 markus 1323: .Rs
1324: .%A T. Ylonen
1325: .%A T. Kivinen
1326: .%A M. Saarinen
1327: .%A T. Rinne
1328: .%A S. Lehtinen
1329: .%T "SSH Protocol Architecture"
1.139 markus 1330: .%N draft-ietf-secsh-architecture-09.txt
1331: .%D July 2001
1.120 markus 1332: .%O work in progress material
1333: .Re
1334: .Rs
1335: .%A M. Friedl
1336: .%A N. Provos
1337: .%A W. A. Simpson
1338: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.132 markus 1339: .%N draft-ietf-secsh-dh-group-exchange-01.txt
1340: .%D April 2001
1.119 markus 1341: .%O work in progress material
1342: .Re
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