Annotation of src/usr.bin/ssh/sshd.8, Revision 1.70.2.7
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: .\"
1.70.2.3 jason 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,
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35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.70.2.7! miod 37: .\" $OpenBSD: sshd.8,v 1.70.2.6 2001/11/15 00:15:00 miod Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSHD 8
40: .Os
41: .Sh NAME
42: .Nm sshd
1.70.2.4 jason 43: .Nd OpenSSH SSH daemon
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm sshd
1.70.2.5 miod 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
52: .Op Fl p Ar port
1.61 markus 53: .Op Fl u Ar len
1.40 aaron 54: .Sh DESCRIPTION
1.2 deraadt 55: .Nm
1.70.2.3 jason 56: (SSH Daemon) is the daemon program for
1.2 deraadt 57: .Xr ssh 1 .
1.42 hugh 58: Together these programs replace rlogin and rsh, and
1.1 deraadt 59: provide secure encrypted communications between two untrusted hosts
1.36 aaron 60: over an insecure network.
61: The programs are intended to be as easy to
1.1 deraadt 62: install and use as possible.
1.2 deraadt 63: .Pp
64: .Nm
1.36 aaron 65: is the daemon that listens for connections from clients.
1.40 aaron 66: It is normally started at boot from
1.2 deraadt 67: .Pa /etc/rc .
68: It forks a new
1.36 aaron 69: daemon for each incoming connection.
70: The forked daemons handle
1.1 deraadt 71: key exchange, encryption, authentication, command execution,
72: and data exchange.
1.49 markus 73: This implementation of
74: .Nm
75: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 76: .Nm
1.36 aaron 77: works as follows.
1.49 markus 78: .Pp
79: .Ss SSH protocol version 1
80: .Pp
1.36 aaron 81: Each host has a host-specific RSA key
82: (normally 1024 bits) used to identify the host.
83: Additionally, when
1.1 deraadt 84: the daemon starts, it generates a server RSA key (normally 768 bits).
85: This key is normally regenerated every hour if it has been used, and
86: is never stored on disk.
1.2 deraadt 87: .Pp
1.42 hugh 88: Whenever a client connects the daemon responds with its public
89: host and server keys.
1.36 aaron 90: The client compares the
1.49 markus 91: RSA host key against its own database to verify that it has not changed.
1.36 aaron 92: The client then generates a 256 bit random number.
93: It encrypts this
1.1 deraadt 94: random number using both the host key and the server key, and sends
1.36 aaron 95: the encrypted number to the server.
1.42 hugh 96: Both sides then use this
1.1 deraadt 97: random number as a session key which is used to encrypt all further
1.36 aaron 98: communications in the session.
99: The rest of the session is encrypted
1.42 hugh 100: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 101: being used by default.
1.36 aaron 102: The client selects the encryption algorithm
1.5 deraadt 103: to use from those offered by the server.
1.2 deraadt 104: .Pp
1.36 aaron 105: Next, the server and the client enter an authentication dialog.
106: The client tries to authenticate itself using
1.2 deraadt 107: .Pa .rhosts
108: authentication,
109: .Pa .rhosts
110: authentication combined with RSA host
1.1 deraadt 111: authentication, RSA challenge-response authentication, or password
112: based authentication.
1.2 deraadt 113: .Pp
1.1 deraadt 114: Rhosts authentication is normally disabled
115: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 116: configuration file if desired.
117: System security is not improved unless
1.2 deraadt 118: .Xr rshd 8 ,
119: .Xr rlogind 8 ,
120: and
1.70.2.5 miod 121: .Xr rexecd 8
1.1 deraadt 122: are disabled (thus completely disabling
1.2 deraadt 123: .Xr rlogin 1
1.1 deraadt 124: and
1.2 deraadt 125: .Xr rsh 1
1.42 hugh 126: into the machine).
1.2 deraadt 127: .Pp
1.49 markus 128: .Ss SSH protocol version 2
129: .Pp
1.58 deraadt 130: Version 2 works similarly:
1.70.2.5 miod 131: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 132: However, when the daemon starts, it does not generate a server key.
133: Forward security is provided through a Diffie-Hellman key agreement.
134: This key agreement results in a shared session key.
1.70.2.4 jason 135: .Pp
1.70.2.3 jason 136: The rest of the session is encrypted using a symmetric cipher, currently
1.70.2.4 jason 137: 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
1.49 markus 138: The client selects the encryption algorithm
139: to use from those offered by the server.
140: Additionally, session integrity is provided
1.51 hugh 141: through a cryptographic message authentication code
1.49 markus 142: (hmac-sha1 or hmac-md5).
143: .Pp
144: Protocol version 2 provides a public key based
1.70.2.4 jason 145: user (PubkeyAuthentication) or
146: client host (HostbasedAuthentication) authentication method,
147: conventional password authentication and challenge response based methods.
1.49 markus 148: .Pp
149: .Ss Command execution and data forwarding
150: .Pp
1.1 deraadt 151: If the client successfully authenticates itself, a dialog for
1.36 aaron 152: preparing the session is entered.
153: At this time the client may request
1.1 deraadt 154: things like allocating a pseudo-tty, forwarding X11 connections,
155: forwarding TCP/IP connections, or forwarding the authentication agent
156: connection over the secure channel.
1.2 deraadt 157: .Pp
1.1 deraadt 158: Finally, the client either requests a shell or execution of a command.
1.36 aaron 159: The sides then enter session mode.
160: In this mode, either side may send
1.1 deraadt 161: data at any time, and such data is forwarded to/from the shell or
162: command on the server side, and the user terminal in the client side.
1.2 deraadt 163: .Pp
1.1 deraadt 164: When the user program terminates and all forwarded X11 and other
165: connections have been closed, the server sends command exit status to
166: the client, and both sides exit.
1.2 deraadt 167: .Pp
168: .Nm
1.1 deraadt 169: can be configured using command-line options or a configuration
1.36 aaron 170: file.
171: Command-line options override values specified in the
1.1 deraadt 172: configuration file.
1.25 markus 173: .Pp
174: .Nm
175: rereads its configuration file when it receives a hangup signal,
1.70.2.3 jason 176: .Dv SIGHUP ,
1.70.2.5 miod 177: by executing itself with the name it was started as, i.e.,
1.70.2.3 jason 178: .Pa /usr/sbin/sshd .
1.18 aaron 179: .Pp
180: The options are as follows:
1.2 deraadt 181: .Bl -tag -width Ds
182: .It Fl b Ar bits
1.70.2.4 jason 183: Specifies the number of bits in the ephemeral protocol version 1
184: server key (default 768).
1.2 deraadt 185: .It Fl d
1.36 aaron 186: Debug mode.
187: The server sends verbose debug output to the system
188: log, and does not put itself in the background.
189: The server also will not fork and will only process one connection.
190: This option is only intended for debugging for the server.
1.70.2.4 jason 191: Multiple -d options increase the debugging level.
1.67 aaron 192: Maximum is 3.
1.70.2.4 jason 193: .It Fl e
194: When this option is specified,
195: .Nm
196: will send the output to the standard error instead of the system log.
1.2 deraadt 197: .It Fl f Ar configuration_file
1.36 aaron 198: Specifies the name of the configuration file.
199: The default is
1.2 deraadt 200: .Pa /etc/sshd_config .
1.16 markus 201: .Nm
202: refuses to start if there is no configuration file.
1.2 deraadt 203: .It Fl g Ar login_grace_time
1.1 deraadt 204: Gives the grace time for clients to authenticate themselves (default
1.70.2.1 jason 205: 600 seconds).
1.36 aaron 206: If the client fails to authenticate the user within
207: this many seconds, the server disconnects and exits.
208: A value of zero indicates no limit.
1.2 deraadt 209: .It Fl h Ar host_key_file
1.70.2.1 jason 210: Specifies the file from which the host key is read (default
1.2 deraadt 211: .Pa /etc/ssh_host_key ) .
1.7 markus 212: This option must be given if
213: .Nm
214: is not run as root (as the normal
1.1 deraadt 215: host file is normally not readable by anyone but root).
1.70.2.1 jason 216: It is possible to have multiple host key files for
1.70.2.4 jason 217: the different protocol versions and host key algorithms.
1.2 deraadt 218: .It Fl i
1.7 markus 219: Specifies that
220: .Nm
1.40 aaron 221: is being run from inetd.
1.7 markus 222: .Nm
223: is normally not run
1.1 deraadt 224: from inetd because it needs to generate the server key before it can
1.36 aaron 225: respond to the client, and this may take tens of seconds.
226: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 227: However, with small key sizes (e.g., 512) using
1.7 markus 228: .Nm
229: from inetd may
1.1 deraadt 230: be feasible.
1.2 deraadt 231: .It Fl k Ar key_gen_time
1.70.2.4 jason 232: Specifies how often the ephemeral protocol version 1 server key is
233: regenerated (default 3600 seconds, or one hour).
1.36 aaron 234: The motivation for regenerating the key fairly
1.1 deraadt 235: often is that the key is not stored anywhere, and after about an hour,
236: it becomes impossible to recover the key for decrypting intercepted
237: communications even if the machine is cracked into or physically
1.36 aaron 238: seized.
239: A value of zero indicates that the key will never be regenerated.
1.2 deraadt 240: .It Fl p Ar port
1.1 deraadt 241: Specifies the port on which the server listens for connections
242: (default 22).
1.2 deraadt 243: .It Fl q
1.36 aaron 244: Quiet mode.
245: Nothing is sent to the system log.
246: Normally the beginning,
1.1 deraadt 247: authentication, and termination of each connection is logged.
1.70.2.5 miod 248: .It Fl t
249: Test mode.
250: Only check the validity of the configuration file and sanity of the keys.
251: This is useful for updating
252: .Nm
253: reliably as configuration options may change.
1.61 markus 254: .It Fl u Ar len
255: This option is used to specify the size of the field
256: in the
257: .Li utmp
258: structure that holds the remote host name.
259: If the resolved host name is longer than
260: .Ar len ,
261: the dotted decimal value will be used instead.
262: This allows hosts with very long host names that
263: overflow this field to still be uniquely identified.
264: Specifying
265: .Fl u0
266: indicates that only dotted decimal addresses
267: should be put into the
268: .Pa utmp
269: file.
1.70.2.5 miod 270: .Fl u0
271: is also be used to prevent
272: .Nm
273: from making DNS requests unless the authentication
274: mechanism or configuration requires it.
275: Authentication mechanisms that may require DNS include
276: .Cm RhostsAuthentication ,
277: .Cm RhostsRSAAuthentication ,
278: .Cm HostbasedAuthentication
279: and using a
280: .Cm from="pattern-list"
281: option in a key file.
1.70.2.1 jason 282: .It Fl D
283: When this option is specified
284: .Nm
285: will not detach and does not become a daemon.
286: This allows easy monitoring of
287: .Nm sshd .
1.29 markus 288: .It Fl 4
289: Forces
290: .Nm
291: to use IPv4 addresses only.
292: .It Fl 6
293: Forces
294: .Nm
295: to use IPv6 addresses only.
1.2 deraadt 296: .El
297: .Sh CONFIGURATION FILE
298: .Nm
1.40 aaron 299: reads configuration data from
1.2 deraadt 300: .Pa /etc/sshd_config
301: (or the file specified with
302: .Fl f
1.36 aaron 303: on the command line).
1.70.2.5 miod 304: The file contains keyword-argument pairs, one per line.
1.36 aaron 305: Lines starting with
1.2 deraadt 306: .Ql #
1.1 deraadt 307: and empty lines are interpreted as comments.
1.2 deraadt 308: .Pp
1.70.2.5 miod 309: The possible
310: keywords and their meanings are as follows (note that
311: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 312: .Bl -tag -width Ds
313: .It Cm AFSTokenPassing
1.36 aaron 314: Specifies whether an AFS token may be forwarded to the server.
315: Default is
1.2 deraadt 316: .Dq yes .
1.11 markus 317: .It Cm AllowGroups
1.70.2.1 jason 318: This keyword can be followed by a list of group names, separated
1.36 aaron 319: by spaces.
320: If specified, login is allowed only for users whose primary
1.70.2.1 jason 321: group or supplementary group list matches one of the patterns.
1.11 markus 322: .Ql \&*
323: and
324: .Ql ?
325: can be used as
1.36 aaron 326: wildcards in the patterns.
1.70.2.5 miod 327: Only group names are valid; a numerical group ID is not recognized.
1.70.2.1 jason 328: By default login is allowed regardless of the group list.
1.3 dugsong 329: .Pp
1.69 markus 330: .It Cm AllowTcpForwarding
331: Specifies whether TCP forwarding is permitted.
332: The default is
333: .Dq yes .
334: Note that disabling TCP forwarding does not improve security unless
335: users are also denied shell access, as they can always install their
336: own forwarders.
337: .Pp
1.11 markus 338: .It Cm AllowUsers
1.70.2.1 jason 339: This keyword can be followed by a list of user names, separated
1.36 aaron 340: by spaces.
341: If specified, login is allowed only for users names that
1.11 markus 342: match one of the patterns.
343: .Ql \&*
344: and
345: .Ql ?
346: can be used as
1.36 aaron 347: wildcards in the patterns.
1.70.2.5 miod 348: Only user names are valid; a numerical user ID is not recognized.
1.36 aaron 349: By default login is allowed regardless of the user name.
1.70.2.5 miod 350: If the pattern takes the form USER@HOST then USER and HOST
351: are separately checked, restricting logins to particular
352: users from particular hosts.
353: .Pp
354: .It Cm AuthorizedKeysFile
355: Specifies the file that contains the public keys that can be used
356: for user authentication.
357: .Cm AuthorizedKeysFile
358: may contain tokens of the form %T which are substituted during connection
359: set-up. The following tokens are defined: %% is replaced by a literal '%',
360: %h is replaced by the home directory of the user being authenticated and
361: %u is replaced by the username of that user.
362: After expansion,
363: .Cm AuthorizedKeysFile
364: is taken to be an absolute path or one relative to the user's home
365: directory.
366: The default is
367: .Dq .ssh/authorized_keys
1.70.2.1 jason 368: .It Cm Banner
369: In some jurisdictions, sending a warning message before authentication
370: may be relevant for getting legal protection.
371: The contents of the specified file are sent to the remote user before
372: authentication is allowed.
373: This option is only available for protocol version 2.
374: .Pp
1.70.2.3 jason 375: .It Cm ChallengeResponseAuthentication
1.70.2.5 miod 376: Specifies whether challenge response authentication is allowed.
377: All authentication styles from
378: .Xr login.conf 5
379: are supported.
1.70.2.3 jason 380: The default is
381: .Dq yes .
1.41 markus 382: .It Cm Ciphers
383: Specifies the ciphers allowed for protocol version 2.
384: Multiple ciphers must be comma-separated.
385: The default is
1.70.2.3 jason 386: .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
1.70.2.4 jason 387: .It Cm ClientAliveInterval
388: Sets a timeout interval in seconds after which if no data has been received
1.70.2.5 miod 389: from the client,
1.70.2.4 jason 390: .Nm
391: will send a message through the encrypted
392: channel to request a response from the client.
393: The default
394: is 0, indicating that these messages will not be sent to the client.
395: This option applies to protocol version 2 only.
396: .It Cm ClientAliveCountMax
397: Sets the number of client alive messages (see above) which may be
398: sent without
399: .Nm
400: receiving any messages back from the client. If this threshold is
1.70.2.5 miod 401: reached while client alive messages are being sent,
1.70.2.4 jason 402: .Nm
403: will disconnect the client, terminating the session. It is important
1.70.2.5 miod 404: to note that the use of client alive messages is very different from
1.70.2.7! miod 405: .Cm KeepAlive
1.70.2.4 jason 406: (below). The client alive messages are sent through the
407: encrypted channel and therefore will not be spoofable. The TCP keepalive
408: option enabled by
1.70.2.7! miod 409: .Cm KeepAlive
1.70.2.5 miod 410: is spoofable. The client alive mechanism is valuable when the client or
411: server depend on knowing when a connection has become inactive.
1.70.2.4 jason 412: .Pp
1.70.2.5 miod 413: The default value is 3. If
1.70.2.4 jason 414: .Cm ClientAliveInterval
1.70.2.5 miod 415: (above) is set to 15, and
416: .Cm ClientAliveCountMax
417: is left at the default, unresponsive ssh clients
418: will be disconnected after approximately 45 seconds.
1.11 markus 419: .It Cm DenyGroups
420: This keyword can be followed by a number of group names, separated
1.36 aaron 421: by spaces.
1.70.2.1 jason 422: Users whose primary group or supplementary group list matches
423: one of the patterns aren't allowed to log in.
1.11 markus 424: .Ql \&*
425: and
426: .Ql ?
427: can be used as
1.36 aaron 428: wildcards in the patterns.
1.70.2.5 miod 429: Only group names are valid; a numerical group ID is not recognized.
1.70.2.1 jason 430: By default login is allowed regardless of the group list.
1.11 markus 431: .Pp
432: .It Cm DenyUsers
433: This keyword can be followed by a number of user names, separated
1.36 aaron 434: by spaces.
435: Login is disallowed for user names that match one of the patterns.
1.11 markus 436: .Ql \&*
437: and
438: .Ql ?
1.36 aaron 439: can be used as wildcards in the patterns.
1.70.2.5 miod 440: Only user names are valid; a numerical user ID is not recognized.
1.36 aaron 441: By default login is allowed regardless of the user name.
1.47 markus 442: .It Cm GatewayPorts
443: Specifies whether remote hosts are allowed to connect to ports
444: forwarded for the client.
1.70.2.5 miod 445: By default,
446: .Nm
447: binds remote port forwardings to the loopback addresss. This
448: prevents other remote hosts from connecting to forwarded ports.
449: .Cm GatewayPorts
450: can be used to specify that
451: .Nm
452: should bind remote port forwardings to the wildcard address,
453: thus allowing remote hosts to connect to forwarded ports.
1.47 markus 454: The argument must be
455: .Dq yes
456: or
457: .Dq no .
458: The default is
459: .Dq no .
1.70.2.4 jason 460: .It Cm HostbasedAuthentication
461: Specifies whether rhosts or /etc/hosts.equiv authentication together
462: with successful public key client host authentication is allowed
463: (hostbased authentication).
464: This option is similar to
465: .Cm RhostsRSAAuthentication
466: and applies to protocol version 2 only.
467: The default is
468: .Dq no .
1.2 deraadt 469: .It Cm HostKey
1.70.2.1 jason 470: Specifies the file containing the private host keys (default
1.46 markus 471: .Pa /etc/ssh_host_key )
1.70.2.1 jason 472: used by SSH protocol versions 1 and 2.
1.9 markus 473: Note that
474: .Nm
1.70.2.1 jason 475: will refuse to use a file if it is group/world-accessible.
476: It is possible to have multiple host key files.
477: .Dq rsa1
478: keys are used for version 1 and
479: .Dq dsa
480: or
481: .Dq rsa
482: are used for version 2 of the SSH protocol.
1.2 deraadt 483: .It Cm IgnoreRhosts
1.34 markus 484: Specifies that
485: .Pa .rhosts
1.40 aaron 486: and
1.34 markus 487: .Pa .shosts
1.70.2.4 jason 488: files will not be used in
489: .Cm RhostsAuthentication ,
490: .Cm RhostsRSAAuthentication
491: or
492: .Cm HostbasedAuthentication .
493: .Pp
1.2 deraadt 494: .Pa /etc/hosts.equiv
1.1 deraadt 495: and
1.40 aaron 496: .Pa /etc/shosts.equiv
1.36 aaron 497: are still used.
1.40 aaron 498: The default is
1.34 markus 499: .Dq yes .
1.24 markus 500: .It Cm IgnoreUserKnownHosts
501: Specifies whether
502: .Nm
503: should ignore the user's
504: .Pa $HOME/.ssh/known_hosts
1.45 markus 505: during
1.70.2.4 jason 506: .Cm RhostsRSAAuthentication
507: or
508: .Cm HostbasedAuthentication .
1.24 markus 509: The default is
1.2 deraadt 510: .Dq no .
511: .It Cm KeepAlive
1.1 deraadt 512: Specifies whether the system should send keepalive messages to the
1.36 aaron 513: other side.
514: If they are sent, death of the connection or crash of one
515: of the machines will be properly noticed.
516: However, this means that
1.1 deraadt 517: connections will die if the route is down temporarily, and some people
1.36 aaron 518: find it annoying.
1.51 hugh 519: On the other hand, if keepalives are not sent,
1.2 deraadt 520: sessions may hang indefinitely on the server, leaving
521: .Dq ghost
522: users and consuming server resources.
523: .Pp
524: The default is
525: .Dq yes
526: (to send keepalives), and the server will notice
1.36 aaron 527: if the network goes down or the client host reboots.
528: This avoids infinitely hanging sessions.
1.2 deraadt 529: .Pp
530: To disable keepalives, the value should be set to
531: .Dq no
532: in both the server and the client configuration files.
533: .It Cm KerberosAuthentication
1.36 aaron 534: Specifies whether Kerberos authentication is allowed.
535: This can be in the form of a Kerberos ticket, or if
1.7 markus 536: .Cm PasswordAuthentication
1.1 deraadt 537: is yes, the password provided by the user will be validated through
1.67 aaron 538: the Kerberos KDC.
539: To use this option, the server needs a
1.59 provos 540: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 541: Default is
1.60 provos 542: .Dq yes .
1.2 deraadt 543: .It Cm KerberosOrLocalPasswd
1.1 deraadt 544: If set then if password authentication through Kerberos fails then
545: the password will be validated via any additional local mechanism
1.2 deraadt 546: such as
1.66 markus 547: .Pa /etc/passwd .
1.36 aaron 548: Default is
1.20 dugsong 549: .Dq yes .
1.2 deraadt 550: .It Cm KerberosTgtPassing
1.1 deraadt 551: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 552: Default is
1.3 dugsong 553: .Dq no ,
554: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 555: .It Cm KerberosTicketCleanup
1.7 markus 556: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 557: file on logout.
558: Default is
1.3 dugsong 559: .Dq yes .
1.2 deraadt 560: .It Cm KeyRegenerationInterval
1.70.2.4 jason 561: In protocol version 1, the ephemeral server key is automatically regenerated
562: after this many seconds (if it has been used).
1.36 aaron 563: The purpose of regeneration is to prevent
1.1 deraadt 564: decrypting captured sessions by later breaking into the machine and
1.36 aaron 565: stealing the keys.
566: The key is never stored anywhere.
567: If the value is 0, the key is never regenerated.
568: The default is 3600 (seconds).
1.7 markus 569: .It Cm ListenAddress
1.70.2.4 jason 570: Specifies the local addresses
1.7 markus 571: .Nm
572: should listen on.
1.70.2.4 jason 573: The following forms may be used:
574: .Pp
575: .Bl -item -offset indent -compact
576: .It
577: .Cm ListenAddress
578: .Sm off
579: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
580: .Sm on
581: .It
582: .Cm ListenAddress
583: .Sm off
584: .Ar host No | Ar IPv4_addr No : Ar port
585: .Sm on
586: .It
587: .Cm ListenAddress
588: .Sm off
589: .Oo
590: .Ar host No | Ar IPv6_addr Oc : Ar port
591: .Sm on
592: .El
593: .Pp
594: If
595: .Ar port
596: is not specified,
597: .Nm
598: will listen on the address and all prior
599: .Cm Port
600: options specified. The default is to listen on all local
601: addresses. Multiple
602: .Cm ListenAddress
603: options are permitted. Additionally, any
604: .Cm Port
605: options must precede this option for non port qualified addresses.
1.2 deraadt 606: .It Cm LoginGraceTime
1.1 deraadt 607: The server disconnects after this time if the user has not
1.36 aaron 608: successfully logged in.
609: If the value is 0, there is no time limit.
1.1 deraadt 610: The default is 600 (seconds).
1.23 markus 611: .It Cm LogLevel
612: Gives the verbosity level that is used when logging messages from
613: .Nm sshd .
614: The possible values are:
1.27 markus 615: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
1.23 markus 616: The default is INFO.
617: Logging with level DEBUG violates the privacy of users
618: and is not recommended.
1.70.2.1 jason 619: .It Cm MACs
620: Specifies the available MAC (message authentication code) algorithms.
621: The MAC algorithm is used in protocol version 2
622: for data integrity protection.
623: Multiple algorithms must be comma-separated.
624: The default is
1.70.2.5 miod 625: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 626: .It Cm MaxStartups
627: Specifies the maximum number of concurrent unauthenticated connections to the
628: .Nm
629: daemon.
630: Additional connections will be dropped until authentication succeeds or the
631: .Cm LoginGraceTime
632: expires for a connection.
633: The default is 10.
1.57 markus 634: .Pp
635: Alternatively, random early drop can be enabled by specifying
636: the three colon separated values
637: .Dq start:rate:full
1.67 aaron 638: (e.g., "10:30:60").
1.57 markus 639: .Nm
1.70.2.1 jason 640: will refuse connection attempts with a probability of
1.57 markus 641: .Dq rate/100
642: (30%)
643: if there are currently
644: .Dq start
645: (10)
646: unauthenticated connections.
1.70.2.1 jason 647: The probability increases linearly and all connection attempts
1.57 markus 648: are refused if the number of unauthenticated connections reaches
649: .Dq full
650: (60).
1.2 deraadt 651: .It Cm PasswordAuthentication
1.1 deraadt 652: Specifies whether password authentication is allowed.
1.2 deraadt 653: The default is
654: .Dq yes .
655: .It Cm PermitEmptyPasswords
1.1 deraadt 656: When password authentication is allowed, it specifies whether the
1.36 aaron 657: server allows login to accounts with empty password strings.
658: The default is
1.34 markus 659: .Dq no .
1.2 deraadt 660: .It Cm PermitRootLogin
1.70.2.3 jason 661: Specifies whether root can login using
1.2 deraadt 662: .Xr ssh 1 .
1.15 markus 663: The argument must be
664: .Dq yes ,
1.70.2.1 jason 665: .Dq without-password ,
666: .Dq forced-commands-only
1.15 markus 667: or
668: .Dq no .
1.2 deraadt 669: The default is
670: .Dq yes .
1.70.2.1 jason 671: .Pp
672: If this option is set to
1.15 markus 673: .Dq without-password
1.70.2.1 jason 674: password authentication is disabled for root.
1.2 deraadt 675: .Pp
1.70.2.1 jason 676: If this option is set to
677: .Dq forced-commands-only
678: root login with public key authentication will be allowed,
679: but only if the
1.2 deraadt 680: .Ar command
1.70.2.1 jason 681: option has been specified
1.1 deraadt 682: (which may be useful for taking remote backups even if root login is
1.70.2.1 jason 683: normally not allowed). All other authentication methods are disabled
684: for root.
1.70.2.3 jason 685: .Pp
686: If this option is set to
687: .Dq no
688: root is not allowed to login.
1.43 markus 689: .It Cm PidFile
690: Specifies the file that contains the process identifier of the
691: .Nm
692: daemon.
693: The default is
694: .Pa /var/run/sshd.pid .
1.2 deraadt 695: .It Cm Port
1.1 deraadt 696: Specifies the port number that
1.2 deraadt 697: .Nm
1.36 aaron 698: listens on.
699: The default is 22.
1.28 markus 700: Multiple options of this type are permitted.
1.70.2.4 jason 701: See also
702: .Cm ListenAddress .
703: .It Cm PrintLastLog
704: Specifies whether
705: .Nm
706: should print the date and time when the user last logged in.
707: The default is
708: .Dq yes .
1.2 deraadt 709: .It Cm PrintMotd
1.1 deraadt 710: Specifies whether
1.2 deraadt 711: .Nm
1.40 aaron 712: should print
1.2 deraadt 713: .Pa /etc/motd
1.36 aaron 714: when a user logs in interactively.
715: (On some systems it is also printed by the shell,
1.2 deraadt 716: .Pa /etc/profile ,
1.36 aaron 717: or equivalent.)
718: The default is
1.2 deraadt 719: .Dq yes .
1.41 markus 720: .It Cm Protocol
721: Specifies the protocol versions
722: .Nm
723: should support.
724: The possible values are
725: .Dq 1
726: and
727: .Dq 2 .
728: Multiple versions must be comma-separated.
729: The default is
1.70.2.4 jason 730: .Dq 2,1 .
1.70.2.3 jason 731: .It Cm PubkeyAuthentication
732: Specifies whether public key authentication is allowed.
733: The default is
734: .Dq yes .
735: Note that this option applies to protocol version 2 only.
1.70.2.1 jason 736: .It Cm ReverseMappingCheck
737: Specifies whether
738: .Nm
739: should try to verify the remote host name and check that
740: the resolved host name for the remote IP address maps back to the
741: very same IP address.
742: The default is
743: .Dq no .
1.2 deraadt 744: .It Cm RhostsAuthentication
1.1 deraadt 745: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 746: files is sufficient.
747: Normally, this method should not be permitted because it is insecure.
1.7 markus 748: .Cm RhostsRSAAuthentication
749: should be used
1.1 deraadt 750: instead, because it performs RSA-based host authentication in addition
751: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 752: The default is
753: .Dq no .
1.70.2.4 jason 754: This option applies to protocol version 1 only.
1.2 deraadt 755: .It Cm RhostsRSAAuthentication
1.1 deraadt 756: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 757: with successful RSA host authentication is allowed.
758: The default is
1.34 markus 759: .Dq no .
1.70.2.4 jason 760: This option applies to protocol version 1 only.
1.2 deraadt 761: .It Cm RSAAuthentication
1.36 aaron 762: Specifies whether pure RSA authentication is allowed.
763: The default is
1.2 deraadt 764: .Dq yes .
1.70.2.4 jason 765: This option applies to protocol version 1 only.
1.2 deraadt 766: .It Cm ServerKeyBits
1.70.2.4 jason 767: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 768: The minimum value is 512, and the default is 768.
1.2 deraadt 769: .It Cm StrictModes
1.12 markus 770: Specifies whether
771: .Nm
772: should check file modes and ownership of the
1.36 aaron 773: user's files and home directory before accepting login.
774: This is normally desirable because novices sometimes accidentally leave their
775: directory or files world-writable.
776: The default is
1.7 markus 777: .Dq yes .
1.54 jakob 778: .It Cm Subsystem
1.67 aaron 779: Configures an external subsystem (e.g., file transfer daemon).
780: Arguments should be a subsystem name and a command to execute upon subsystem
781: request.
1.63 markus 782: The command
783: .Xr sftp-server 8
784: implements the
785: .Dq sftp
786: file transfer subsystem.
1.54 jakob 787: By default no subsystems are defined.
788: Note that this option applies to protocol version 2 only.
1.2 deraadt 789: .It Cm SyslogFacility
1.1 deraadt 790: Gives the facility code that is used when logging messages from
1.2 deraadt 791: .Nm sshd .
1.1 deraadt 792: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 793: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
794: The default is AUTH.
1.10 markus 795: .It Cm UseLogin
796: Specifies whether
797: .Xr login 1
1.53 markus 798: is used for interactive login sessions.
1.70.2.5 miod 799: The default is
800: .Dq no .
1.53 markus 801: Note that
802: .Xr login 1
1.58 deraadt 803: is never used for remote command execution.
1.70.2.5 miod 804: Note also, that if this is enabled,
805: .Cm X11Forwarding
806: will be disabled because
807: .Xr login 1
808: does not know how to handle
809: .Xr xauth 1
810: cookies.
1.6 aaron 811: .It Cm X11DisplayOffset
812: Specifies the first display number available for
813: .Nm sshd Ns 's
1.36 aaron 814: X11 forwarding.
815: This prevents
1.6 aaron 816: .Nm
817: from interfering with real X11 servers.
1.34 markus 818: The default is 10.
1.30 markus 819: .It Cm X11Forwarding
1.36 aaron 820: Specifies whether X11 forwarding is permitted.
821: The default is
1.34 markus 822: .Dq no .
1.30 markus 823: Note that disabling X11 forwarding does not improve security in any
824: way, as users can always install their own forwarders.
1.70.2.5 miod 825: X11 forwarding is automatically disabled if
826: .Cm UseLogin
827: is enabled.
1.52 markus 828: .It Cm XAuthLocation
829: Specifies the location of the
830: .Xr xauth 1
831: program.
832: The default is
833: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 834: .El
1.70.2.5 miod 835: .Ss Time Formats
836: .Pp
837: .Nm
838: command-line arguments and configuration file options that specify time
839: may be expressed using a sequence of the form:
840: .Sm off
841: .Ar time Oo Ar qualifier Oc ,
842: .Sm on
843: where
844: .Ar time
845: is a positive integer value and
846: .Ar qualifier
847: is one of the following:
848: .Pp
849: .Bl -tag -width Ds -compact -offset indent
850: .It Cm <none>
851: seconds
852: .It Cm s | Cm S
853: seconds
854: .It Cm m | Cm M
855: minutes
856: .It Cm h | Cm H
857: hours
858: .It Cm d | Cm D
859: days
860: .It Cm w | Cm W
861: weeks
862: .El
863: .Pp
864: Each member of the sequence is added together to calculate
865: the total time value.
866: .Pp
867: Time format examples:
868: .Pp
869: .Bl -tag -width Ds -compact -offset indent
870: .It 600
871: 600 seconds (10 minutes)
872: .It 10m
873: 10 minutes
874: .It 1h30m
875: 1 hour 30 minutes (90 minutes)
876: .El
1.2 deraadt 877: .Sh LOGIN PROCESS
1.1 deraadt 878: When a user successfully logs in,
1.2 deraadt 879: .Nm
1.1 deraadt 880: does the following:
1.2 deraadt 881: .Bl -enum -offset indent
882: .It
1.1 deraadt 883: If the login is on a tty, and no command has been specified,
1.40 aaron 884: prints last login time and
1.2 deraadt 885: .Pa /etc/motd
1.1 deraadt 886: (unless prevented in the configuration file or by
1.2 deraadt 887: .Pa $HOME/.hushlogin ;
888: see the
1.40 aaron 889: .Sx FILES
1.2 deraadt 890: section).
891: .It
1.1 deraadt 892: If the login is on a tty, records login time.
1.2 deraadt 893: .It
894: Checks
895: .Pa /etc/nologin ;
896: if it exists, prints contents and quits
1.1 deraadt 897: (unless root).
1.2 deraadt 898: .It
1.1 deraadt 899: Changes to run with normal user privileges.
1.2 deraadt 900: .It
1.1 deraadt 901: Sets up basic environment.
1.2 deraadt 902: .It
903: Reads
904: .Pa $HOME/.ssh/environment
905: if it exists.
906: .It
1.1 deraadt 907: Changes to user's home directory.
1.2 deraadt 908: .It
909: If
910: .Pa $HOME/.ssh/rc
911: exists, runs it; else if
912: .Pa /etc/sshrc
913: exists, runs
1.36 aaron 914: it; otherwise runs xauth.
915: The
1.2 deraadt 916: .Dq rc
917: files are given the X11
1.1 deraadt 918: authentication protocol and cookie in standard input.
1.2 deraadt 919: .It
1.1 deraadt 920: Runs user's shell or command.
1.2 deraadt 921: .El
922: .Sh AUTHORIZED_KEYS FILE FORMAT
923: .Pa $HOME/.ssh/authorized_keys
1.70.2.5 miod 924: is the default file that lists the public keys that are
1.70.2.4 jason 925: permitted for RSA authentication in protocol version 1
1.70.2.5 miod 926: and for public key authentication (PubkeyAuthentication)
1.70.2.4 jason 927: in protocol version 2.
1.70.2.5 miod 928: .Cm AuthorizedKeysFile
929: may be used to specify an alternative file.
1.70.2.1 jason 930: .Pp
1.36 aaron 931: Each line of the file contains one
1.2 deraadt 932: key (empty lines and lines starting with a
933: .Ql #
934: are ignored as
1.36 aaron 935: comments).
1.70.2.1 jason 936: Each RSA public key consists of the following fields, separated by
1.36 aaron 937: spaces: options, bits, exponent, modulus, comment.
1.70.2.1 jason 938: Each protocol version 2 public key consists of:
939: options, keytype, base64 encoded key, comment.
940: The options fields
941: are optional; its presence is determined by whether the line starts
1.1 deraadt 942: with a number or not (the option field never starts with a number).
1.70.2.1 jason 943: The bits, exponent, modulus and comment fields give the RSA key for
944: protocol version 1; the
1.1 deraadt 945: comment field is not used for anything (but may be convenient for the
946: user to identify the key).
1.70.2.1 jason 947: For protocol version 2 the keytype is
948: .Dq ssh-dss
949: or
950: .Dq ssh-rsa .
1.2 deraadt 951: .Pp
1.1 deraadt 952: Note that lines in this file are usually several hundred bytes long
1.36 aaron 953: (because of the size of the RSA key modulus).
954: You don't want to type them in; instead, copy the
1.70.2.4 jason 955: .Pa identity.pub ,
1.70.2.1 jason 956: .Pa id_dsa.pub
1.70.2.4 jason 957: or the
958: .Pa id_rsa.pub
1.1 deraadt 959: file and edit it.
1.2 deraadt 960: .Pp
1.58 deraadt 961: The options (if present) consist of comma-separated option
1.36 aaron 962: specifications.
963: No spaces are permitted, except within double quotes.
1.70.2.5 miod 964: The following option specifications are supported (note
965: that option keywords are case-insensitive):
1.2 deraadt 966: .Bl -tag -width Ds
967: .It Cm from="pattern-list"
1.1 deraadt 968: Specifies that in addition to RSA authentication, the canonical name
969: of the remote host must be present in the comma-separated list of
1.36 aaron 970: patterns
971: .Pf ( Ql *
972: and
973: .Ql ?
974: serve as wildcards).
975: The list may also contain
976: patterns negated by prefixing them with
977: .Ql ! ;
978: if the canonical host name matches a negated pattern, the key is not accepted.
979: The purpose
1.1 deraadt 980: of this option is to optionally increase security: RSA authentication
981: by itself does not trust the network or name servers or anything (but
982: the key); however, if somebody somehow steals the key, the key
1.36 aaron 983: permits an intruder to log in from anywhere in the world.
984: This additional option makes using a stolen key more difficult (name
1.1 deraadt 985: servers and/or routers would have to be compromised in addition to
986: just the key).
1.2 deraadt 987: .It Cm command="command"
1.1 deraadt 988: Specifies that the command is executed whenever this key is used for
1.36 aaron 989: authentication.
990: The command supplied by the user (if any) is ignored.
1.70.2.5 miod 991: The command is run on a pty if the client requests a pty;
1.36 aaron 992: otherwise it is run without a tty.
1.70.2.5 miod 993: If a 8-bit clean channel is required,
994: one must not request a pty or should specify
1.70.2.1 jason 995: .Cm no-pty .
1.36 aaron 996: A quote may be included in the command by quoting it with a backslash.
997: This option might be useful
998: to restrict certain RSA keys to perform just a specific operation.
999: An example might be a key that permits remote backups but nothing else.
1.51 hugh 1000: Note that the client may specify TCP/IP and/or X11
1001: forwarding unless they are explicitly prohibited.
1.70.2.5 miod 1002: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 1003: .It Cm environment="NAME=value"
1.1 deraadt 1004: Specifies that the string is to be added to the environment when
1.36 aaron 1005: logging in using this key.
1006: Environment variables set this way
1007: override other default environment values.
1008: Multiple options of this type are permitted.
1.2 deraadt 1009: .It Cm no-port-forwarding
1.1 deraadt 1010: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 1011: Any port forward requests by the client will return an error.
1012: This might be used, e.g., in connection with the
1.2 deraadt 1013: .Cm command
1.1 deraadt 1014: option.
1.2 deraadt 1015: .It Cm no-X11-forwarding
1.1 deraadt 1016: Forbids X11 forwarding when this key is used for authentication.
1017: Any X11 forward requests by the client will return an error.
1.2 deraadt 1018: .It Cm no-agent-forwarding
1.1 deraadt 1019: Forbids authentication agent forwarding when this key is used for
1020: authentication.
1.2 deraadt 1021: .It Cm no-pty
1.1 deraadt 1022: Prevents tty allocation (a request to allocate a pty will fail).
1.70.2.3 jason 1023: .It Cm permitopen="host:port"
1.70.2.5 miod 1024: Limit local
1.70.2.3 jason 1025: .Li ``ssh -L''
1.70.2.4 jason 1026: port forwarding such that it may only connect to the specified host and
1.70.2.5 miod 1027: port.
1028: IPv6 addresses can be specified with an alternative syntax:
1029: .Ar host/port .
1030: Multiple
1.70.2.3 jason 1031: .Cm permitopen
1.70.2.5 miod 1032: options may be applied separated by commas. No pattern matching is
1033: performed on the specified hostnames, they must be literal domains or
1.70.2.3 jason 1034: addresses.
1.2 deraadt 1035: .El
1036: .Ss Examples
1.1 deraadt 1037: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1038: .Pp
1.1 deraadt 1039: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1040: .Pp
1.1 deraadt 1041: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.70.2.3 jason 1042: .Pp
1043: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1044: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1045: The
1.44 deraadt 1046: .Pa /etc/ssh_known_hosts ,
1.40 aaron 1047: and
1.70.2.5 miod 1048: .Pa $HOME/.ssh/known_hosts
1.36 aaron 1049: files contain host public keys for all known hosts.
1050: The global file should
1.37 brad 1051: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1052: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1053: its key is added to the per-user file.
1.2 deraadt 1054: .Pp
1.1 deraadt 1055: Each line in these files contains the following fields: hostnames,
1.36 aaron 1056: bits, exponent, modulus, comment.
1057: The fields are separated by spaces.
1.2 deraadt 1058: .Pp
1.1 deraadt 1059: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1060: wildcards); each pattern in turn is matched against the canonical host
1061: name (when authenticating a client) or against the user-supplied
1.36 aaron 1062: name (when authenticating a server).
1063: A pattern may also be preceded by
1.2 deraadt 1064: .Ql !
1065: to indicate negation: if the host name matches a negated
1.1 deraadt 1066: pattern, it is not accepted (by that line) even if it matched another
1067: pattern on the line.
1.2 deraadt 1068: .Pp
1.49 markus 1069: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1070: can be obtained, e.g., from
1071: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 1072: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1073: .Pp
1074: Lines starting with
1075: .Ql #
1076: and empty lines are ignored as comments.
1077: .Pp
1.1 deraadt 1078: When performing host authentication, authentication is accepted if any
1.36 aaron 1079: matching line has the proper key.
1080: It is thus permissible (but not
1.1 deraadt 1081: recommended) to have several lines or different host keys for the same
1.36 aaron 1082: names.
1083: This will inevitably happen when short forms of host names
1084: from different domains are put in the file.
1085: It is possible
1.1 deraadt 1086: that the files contain conflicting information; authentication is
1087: accepted if valid information can be found from either file.
1.2 deraadt 1088: .Pp
1.1 deraadt 1089: Note that the lines in these files are typically hundreds of characters
1090: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1091: Rather, generate them by a script
1.40 aaron 1092: or by taking
1.2 deraadt 1093: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1094: and adding the host names at the front.
1.2 deraadt 1095: .Ss Examples
1.70.2.4 jason 1096: .Bd -literal
1097: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1098: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1099: .Ed
1.2 deraadt 1100: .Sh FILES
1101: .Bl -tag -width Ds
1102: .It Pa /etc/sshd_config
1.1 deraadt 1103: Contains configuration data for
1.2 deraadt 1104: .Nm sshd .
1.1 deraadt 1105: This file should be writable by root only, but it is recommended
1106: (though not necessary) that it be world-readable.
1.70.2.3 jason 1107: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.70.2.4 jason 1108: These three files contain the private parts of the host keys.
1.70.2.3 jason 1109: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1110: accessible to others.
1.14 markus 1111: Note that
1112: .Nm
1113: does not start if this file is group/world-accessible.
1.70.2.3 jason 1114: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.70.2.4 jason 1115: These three files contain the public parts of the host keys.
1.70.2.3 jason 1116: These files should be world-readable but writable only by
1.36 aaron 1117: root.
1.70.2.3 jason 1118: Their contents should match the respective private parts.
1119: These files are not
1120: really used for anything; they are provided for the convenience of
1121: the user so their contents can be copied to known hosts files.
1122: These files are created using
1.7 markus 1123: .Xr ssh-keygen 1 .
1.70.2.5 miod 1124: .It Pa /etc/moduli
1.70.2.1 jason 1125: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1126: .It Pa /var/run/sshd.pid
1127: Contains the process ID of the
1128: .Nm
1.1 deraadt 1129: listening for connections (if there are several daemons running
1130: concurrently for different ports, this contains the pid of the one
1.36 aaron 1131: started last).
1.58 deraadt 1132: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1133: .It Pa $HOME/.ssh/authorized_keys
1.70.2.4 jason 1134: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1135: This file must be readable by root (which may on some machines imply
1136: it being world-readable if the user's home directory resides on an NFS
1137: volume).
1138: It is recommended that it not be accessible by others.
1139: The format of this file is described above.
1140: Users will place the contents of their
1.70.2.5 miod 1141: .Pa identity.pub ,
1.44 deraadt 1142: .Pa id_dsa.pub
1.70.2.4 jason 1143: and/or
1144: .Pa id_rsa.pub
1.44 deraadt 1145: files into this file, as described in
1146: .Xr ssh-keygen 1 .
1.22 markus 1147: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1148: These files are consulted when using rhosts with RSA host
1.70.2.5 miod 1149: authentication or protocol version 2 hostbased authentication
1150: to check the public key of the host.
1.36 aaron 1151: The key must be listed in one of these files to be accepted.
1.22 markus 1152: The client uses the same files
1.70.2.3 jason 1153: to verify that it is connecting to the correct remote host.
1.36 aaron 1154: These files should be writable only by root/the owner.
1.2 deraadt 1155: .Pa /etc/ssh_known_hosts
1156: should be world-readable, and
1157: .Pa $HOME/.ssh/known_hosts
1158: can but need not be world-readable.
1.6 aaron 1159: .It Pa /etc/nologin
1.40 aaron 1160: If this file exists,
1.2 deraadt 1161: .Nm
1.36 aaron 1162: refuses to let anyone except root log in.
1163: The contents of the file
1.1 deraadt 1164: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1165: refused.
1166: The file should be world-readable.
1.19 dugsong 1167: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.70.2.6 miod 1168: Access controls that should be enforced by tcp-wrappers are defined here.
1169: Further details are described in
1.19 dugsong 1170: .Xr hosts_access 5 .
1.6 aaron 1171: .It Pa $HOME/.rhosts
1.1 deraadt 1172: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1173: line.
1174: The given user on the corresponding host is permitted to log in
1175: without password.
1176: The same file is used by rlogind and rshd.
1.6 aaron 1177: The file must
1.1 deraadt 1178: be writable only by the user; it is recommended that it not be
1179: accessible by others.
1.2 deraadt 1180: .Pp
1.36 aaron 1181: If is also possible to use netgroups in the file.
1182: Either host or user
1.1 deraadt 1183: name may be of the form +@groupname to specify all hosts or all users
1184: in the group.
1.2 deraadt 1185: .It Pa $HOME/.shosts
1186: For ssh,
1187: this file is exactly the same as for
1188: .Pa .rhosts .
1189: However, this file is
1190: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1191: .It Pa /etc/hosts.equiv
1.2 deraadt 1192: This file is used during
1193: .Pa .rhosts
1.36 aaron 1194: authentication.
1195: In the simplest form, this file contains host names, one per line.
1196: Users on
1.1 deraadt 1197: those hosts are permitted to log in without a password, provided they
1.36 aaron 1198: have the same user name on both machines.
1199: The host name may also be
1.1 deraadt 1200: followed by a user name; such users are permitted to log in as
1.2 deraadt 1201: .Em any
1.36 aaron 1202: user on this machine (except root).
1203: Additionally, the syntax
1.2 deraadt 1204: .Dq +@group
1.36 aaron 1205: can be used to specify netgroups.
1206: Negated entries start with
1.2 deraadt 1207: .Ql \&- .
1208: .Pp
1.1 deraadt 1209: If the client host/user is successfully matched in this file, login is
1210: automatically permitted provided the client and server user names are the
1.36 aaron 1211: same.
1212: Additionally, successful RSA host authentication is normally required.
1213: This file must be writable only by root; it is recommended
1.1 deraadt 1214: that it be world-readable.
1.2 deraadt 1215: .Pp
1.6 aaron 1216: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1217: .Pa hosts.equiv .
1.1 deraadt 1218: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1219: .Em anybody ,
1.1 deraadt 1220: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1221: binaries and directories.
1222: Using a user name practically grants the user root access.
1223: The only valid use for user names that I can think
1.1 deraadt 1224: of is in negative entries.
1.2 deraadt 1225: .Pp
1226: Note that this warning also applies to rsh/rlogin.
1227: .It Pa /etc/shosts.equiv
1.1 deraadt 1228: This is processed exactly as
1.2 deraadt 1229: .Pa /etc/hosts.equiv .
1.1 deraadt 1230: However, this file may be useful in environments that want to run both
1.2 deraadt 1231: rsh/rlogin and ssh.
1.6 aaron 1232: .It Pa $HOME/.ssh/environment
1.36 aaron 1233: This file is read into the environment at login (if it exists).
1234: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1235: .Ql # ) ,
1.36 aaron 1236: and assignment lines of the form name=value.
1237: The file should be writable
1.6 aaron 1238: only by the user; it need not be readable by anyone else.
1.2 deraadt 1239: .It Pa $HOME/.ssh/rc
1.1 deraadt 1240: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1241: environment files but before starting the user's shell or command.
1242: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1243: standard input (and
1244: .Ev DISPLAY
1.36 aaron 1245: in environment).
1246: This must call
1.2 deraadt 1247: .Xr xauth 1
1248: in that case.
1249: .Pp
1.1 deraadt 1250: The primary purpose of this file is to run any initialization routines
1251: which may be needed before the user's home directory becomes
1252: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1253: .Pp
1.1 deraadt 1254: This file will probably contain some initialization code followed by
1.70.2.4 jason 1255: something similar to:
1256: .Bd -literal
1257: if read proto cookie; then
1258: echo add $DISPLAY $proto $cookie | xauth -q -
1259: fi
1260: .Ed
1.2 deraadt 1261: .Pp
1262: If this file does not exist,
1263: .Pa /etc/sshrc
1264: is run, and if that
1.1 deraadt 1265: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1266: .Pp
1.1 deraadt 1267: This file should be writable only by the user, and need not be
1268: readable by anyone else.
1.2 deraadt 1269: .It Pa /etc/sshrc
1270: Like
1271: .Pa $HOME/.ssh/rc .
1272: This can be used to specify
1.36 aaron 1273: machine-specific login-time initializations globally.
1274: This file should be writable only by root, and should be world-readable.
1.56 aaron 1275: .El
1.70.2.1 jason 1276: .Sh AUTHORS
1277: OpenSSH is a derivative of the original and free
1278: ssh 1.2.12 release by Tatu Ylonen.
1279: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1280: Theo de Raadt and Dug Song
1281: removed many bugs, re-added newer features and
1282: created OpenSSH.
1283: Markus Friedl contributed the support for SSH
1284: protocol versions 1.5 and 2.0.
1.2 deraadt 1285: .Sh SEE ALSO
1286: .Xr scp 1 ,
1.70.2.1 jason 1287: .Xr sftp 1 ,
1.2 deraadt 1288: .Xr ssh 1 ,
1.5 deraadt 1289: .Xr ssh-add 1 ,
1.2 deraadt 1290: .Xr ssh-agent 1 ,
1.5 deraadt 1291: .Xr ssh-keygen 1 ,
1.70.2.5 miod 1292: .Xr login.conf 5 ,
1293: .Xr moduli 5 ,
1294: .Xr sftp-server 8
1.70.2.4 jason 1295: .Rs
1296: .%A T. Ylonen
1297: .%A T. Kivinen
1298: .%A M. Saarinen
1299: .%A T. Rinne
1300: .%A S. Lehtinen
1301: .%T "SSH Protocol Architecture"
1.70.2.5 miod 1302: .%N draft-ietf-secsh-architecture-09.txt
1303: .%D July 2001
1.70.2.4 jason 1304: .%O work in progress material
1305: .Re
1306: .Rs
1307: .%A M. Friedl
1308: .%A N. Provos
1309: .%A W. A. Simpson
1310: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.70.2.5 miod 1311: .%N draft-ietf-secsh-dh-group-exchange-01.txt
1312: .%D April 2001
1.70.2.4 jason 1313: .%O work in progress material
1314: .Re
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