Annotation of src/usr.bin/ssh/sshd.8, Revision 1.157
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.64 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.99 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.64 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.64 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.157 ! deraadt 37: .\" $OpenBSD: sshd.8,v 1.156 2001/12/06 13:30:06 markus Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSHD 8
40: .Os
41: .Sh NAME
42: .Nm sshd
1.120 markus 43: .Nd OpenSSH SSH daemon
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm sshd
1.150 stevesk 46: .Op Fl deiqtD46
1.2 deraadt 47: .Op Fl b Ar bits
48: .Op Fl f Ar config_file
49: .Op Fl g Ar login_grace_time
50: .Op Fl h Ar host_key_file
51: .Op Fl k Ar key_gen_time
1.156 markus 52: .Op Fl o Ar option
1.2 deraadt 53: .Op Fl p Ar port
1.61 markus 54: .Op Fl u Ar len
1.40 aaron 55: .Sh DESCRIPTION
1.2 deraadt 56: .Nm
1.106 deraadt 57: (SSH Daemon) is the daemon program for
1.2 deraadt 58: .Xr ssh 1 .
1.42 hugh 59: Together these programs replace rlogin and rsh, and
1.1 deraadt 60: provide secure encrypted communications between two untrusted hosts
1.36 aaron 61: over an insecure network.
62: The programs are intended to be as easy to
1.1 deraadt 63: install and use as possible.
1.2 deraadt 64: .Pp
65: .Nm
1.36 aaron 66: is the daemon that listens for connections from clients.
1.40 aaron 67: It is normally started at boot from
1.2 deraadt 68: .Pa /etc/rc .
69: It forks a new
1.36 aaron 70: daemon for each incoming connection.
71: The forked daemons handle
1.1 deraadt 72: key exchange, encryption, authentication, command execution,
73: and data exchange.
1.49 markus 74: This implementation of
75: .Nm
76: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 77: .Nm
1.36 aaron 78: works as follows.
1.49 markus 79: .Pp
80: .Ss SSH protocol version 1
81: .Pp
1.36 aaron 82: Each host has a host-specific RSA key
83: (normally 1024 bits) used to identify the host.
84: Additionally, when
1.1 deraadt 85: the daemon starts, it generates a server RSA key (normally 768 bits).
86: This key is normally regenerated every hour if it has been used, and
87: is never stored on disk.
1.2 deraadt 88: .Pp
1.42 hugh 89: Whenever a client connects the daemon responds with its public
90: host and server keys.
1.36 aaron 91: The client compares the
1.49 markus 92: RSA host key against its own database to verify that it has not changed.
1.36 aaron 93: The client then generates a 256 bit random number.
94: It encrypts this
1.1 deraadt 95: random number using both the host key and the server key, and sends
1.36 aaron 96: the encrypted number to the server.
1.42 hugh 97: Both sides then use this
1.1 deraadt 98: random number as a session key which is used to encrypt all further
1.36 aaron 99: communications in the session.
100: The rest of the session is encrypted
1.42 hugh 101: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 102: being used by default.
1.36 aaron 103: The client selects the encryption algorithm
1.5 deraadt 104: to use from those offered by the server.
1.2 deraadt 105: .Pp
1.36 aaron 106: Next, the server and the client enter an authentication dialog.
107: The client tries to authenticate itself using
1.2 deraadt 108: .Pa .rhosts
109: authentication,
110: .Pa .rhosts
111: authentication combined with RSA host
1.1 deraadt 112: authentication, RSA challenge-response authentication, or password
113: based authentication.
1.2 deraadt 114: .Pp
1.1 deraadt 115: Rhosts authentication is normally disabled
116: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 117: configuration file if desired.
118: System security is not improved unless
1.2 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
1.75 markus 211: Specifies the file from which the host key is read (default
1.2 deraadt 212: .Pa /etc/ssh_host_key ) .
1.7 markus 213: This option must be given if
214: .Nm
215: is not run as root (as the normal
1.1 deraadt 216: host file is normally not readable by anyone but root).
1.75 markus 217: It is possible to have multiple host key files for
1.120 markus 218: the different protocol versions and host key algorithms.
1.2 deraadt 219: .It Fl i
1.7 markus 220: Specifies that
221: .Nm
1.40 aaron 222: is being run from inetd.
1.7 markus 223: .Nm
224: is normally not run
1.1 deraadt 225: from inetd because it needs to generate the server key before it can
1.36 aaron 226: respond to the client, and this may take tens of seconds.
227: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 228: However, with small key sizes (e.g., 512) using
1.7 markus 229: .Nm
230: from inetd may
1.1 deraadt 231: be feasible.
1.2 deraadt 232: .It Fl k Ar key_gen_time
1.120 markus 233: Specifies how often the ephemeral protocol version 1 server key is
234: regenerated (default 3600 seconds, or one hour).
1.36 aaron 235: The motivation for regenerating the key fairly
1.1 deraadt 236: often is that the key is not stored anywhere, and after about an hour,
237: it becomes impossible to recover the key for decrypting intercepted
238: communications even if the machine is cracked into or physically
1.36 aaron 239: seized.
240: A value of zero indicates that the key will never be regenerated.
1.156 markus 241: .It Fl o Ar option
242: Can be used to give options in the format used in the configuration file.
243: This is useful for specifying options for which there is no separate
244: command-line flag.
1.2 deraadt 245: .It Fl p Ar port
1.1 deraadt 246: Specifies the port on which the server listens for connections
247: (default 22).
1.2 deraadt 248: .It Fl q
1.36 aaron 249: Quiet mode.
250: Nothing is sent to the system log.
251: Normally the beginning,
1.1 deraadt 252: authentication, and termination of each connection is logged.
1.137 stevesk 253: .It Fl t
254: Test mode.
255: Only check the validity of the configuration file and sanity of the keys.
1.157 ! deraadt 256: This is useful for updating
1.137 stevesk 257: .Nm
258: reliably as configuration options may change.
1.61 markus 259: .It Fl u Ar len
260: This option is used to specify the size of the field
261: in the
262: .Li utmp
263: structure that holds the remote host name.
264: If the resolved host name is longer than
265: .Ar len ,
266: the dotted decimal value will be used instead.
267: This allows hosts with very long host names that
268: overflow this field to still be uniquely identified.
269: Specifying
270: .Fl u0
271: indicates that only dotted decimal addresses
272: should be put into the
273: .Pa utmp
274: file.
1.144 stevesk 275: .Fl u0
276: is also be used to prevent
277: .Nm
278: from making DNS requests unless the authentication
279: mechanism or configuration requires it.
280: Authentication mechanisms that may require DNS include
281: .Cm RhostsAuthentication ,
282: .Cm RhostsRSAAuthentication ,
283: .Cm HostbasedAuthentication
284: and using a
285: .Cm from="pattern-list"
286: option in a key file.
1.74 markus 287: .It Fl D
288: When this option is specified
289: .Nm
290: will not detach and does not become a daemon.
291: This allows easy monitoring of
1.76 markus 292: .Nm sshd .
1.29 markus 293: .It Fl 4
294: Forces
295: .Nm
296: to use IPv4 addresses only.
297: .It Fl 6
298: Forces
299: .Nm
300: to use IPv6 addresses only.
1.2 deraadt 301: .El
302: .Sh CONFIGURATION FILE
303: .Nm
1.40 aaron 304: reads configuration data from
1.2 deraadt 305: .Pa /etc/sshd_config
306: (or the file specified with
307: .Fl f
1.36 aaron 308: on the command line).
1.141 stevesk 309: The file contains keyword-argument pairs, one per line.
1.36 aaron 310: Lines starting with
1.2 deraadt 311: .Ql #
1.1 deraadt 312: and empty lines are interpreted as comments.
1.2 deraadt 313: .Pp
1.141 stevesk 314: The possible
315: keywords and their meanings are as follows (note that
316: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 317: .Bl -tag -width Ds
318: .It Cm AFSTokenPassing
1.36 aaron 319: Specifies whether an AFS token may be forwarded to the server.
320: Default is
1.2 deraadt 321: .Dq yes .
1.11 markus 322: .It Cm AllowGroups
1.92 deraadt 323: This keyword can be followed by a list of group names, separated
1.36 aaron 324: by spaces.
325: If specified, login is allowed only for users whose primary
1.81 markus 326: group or supplementary group list matches one of the patterns.
1.11 markus 327: .Ql \&*
328: and
329: .Ql ?
330: can be used as
1.36 aaron 331: wildcards in the patterns.
1.147 deraadt 332: Only group names are valid; a numerical group ID is not recognized.
1.81 markus 333: By default login is allowed regardless of the group list.
1.3 dugsong 334: .Pp
1.69 markus 335: .It Cm AllowTcpForwarding
336: Specifies whether TCP forwarding is permitted.
337: The default is
338: .Dq yes .
339: Note that disabling TCP forwarding does not improve security unless
340: users are also denied shell access, as they can always install their
341: own forwarders.
342: .Pp
1.11 markus 343: .It Cm AllowUsers
1.92 deraadt 344: This keyword can be followed by a list of user names, separated
1.36 aaron 345: by spaces.
346: If specified, login is allowed only for users names that
1.11 markus 347: match one of the patterns.
348: .Ql \&*
349: and
350: .Ql ?
351: can be used as
1.36 aaron 352: wildcards in the patterns.
1.147 deraadt 353: Only user names are valid; a numerical user ID is not recognized.
1.36 aaron 354: By default login is allowed regardless of the user name.
1.135 markus 355: If the pattern takes the form USER@HOST then USER and HOST
1.147 deraadt 356: are separately checked, restricting logins to particular
1.135 markus 357: users from particular hosts.
1.80 markus 358: .Pp
1.125 markus 359: .It Cm AuthorizedKeysFile
1.138 markus 360: Specifies the file that contains the public keys that can be used
361: for user authentication.
1.125 markus 362: .Cm AuthorizedKeysFile
363: may contain tokens of the form %T which are substituted during connection
1.142 stevesk 364: set-up. The following tokens are defined: %% is replaced by a literal '%',
1.125 markus 365: %h is replaced by the home directory of the user being authenticated and
366: %u is replaced by the username of that user.
367: After expansion,
368: .Cm AuthorizedKeysFile
1.126 markus 369: is taken to be an absolute path or one relative to the user's home
1.125 markus 370: directory.
371: The default is
372: .Dq .ssh/authorized_keys
1.80 markus 373: .It Cm Banner
374: In some jurisdictions, sending a warning message before authentication
375: may be relevant for getting legal protection.
376: The contents of the specified file are sent to the remote user before
377: authentication is allowed.
378: This option is only available for protocol version 2.
1.11 markus 379: .Pp
1.104 deraadt 380: .It Cm ChallengeResponseAuthentication
1.136 markus 381: Specifies whether challenge response authentication is allowed.
382: All authentication styles from
383: .Xr login.conf 5
384: are supported.
1.104 deraadt 385: The default is
386: .Dq yes .
1.122 markus 387: .It Cm Ciphers
388: Specifies the ciphers allowed for protocol version 2.
389: Multiple ciphers must be comma-separated.
390: The default is
391: .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
1.115 beck 392: .It Cm ClientAliveInterval
393: Sets a timeout interval in seconds after which if no data has been received
1.133 itojun 394: from the client,
1.115 beck 395: .Nm
396: will send a message through the encrypted
1.116 stevesk 397: channel to request a response from the client.
398: The default
1.115 beck 399: is 0, indicating that these messages will not be sent to the client.
1.116 stevesk 400: This option applies to protocol version 2 only.
1.115 beck 401: .It Cm ClientAliveCountMax
402: Sets the number of client alive messages (see above) which may be
403: sent without
404: .Nm
405: receiving any messages back from the client. If this threshold is
1.133 itojun 406: reached while client alive messages are being sent,
1.115 beck 407: .Nm
408: will disconnect the client, terminating the session. It is important
1.133 itojun 409: to note that the use of client alive messages is very different from
1.154 markus 410: .Cm KeepAlive
1.116 stevesk 411: (below). The client alive messages are sent through the
1.115 beck 412: encrypted channel and therefore will not be spoofable. The TCP keepalive
1.116 stevesk 413: option enabled by
1.154 markus 414: .Cm KeepAlive
1.147 deraadt 415: is spoofable. The client alive mechanism is valuable when the client or
416: server depend on knowing when a connection has become inactive.
1.116 stevesk 417: .Pp
1.147 deraadt 418: The default value is 3. If
1.116 stevesk 419: .Cm ClientAliveInterval
1.147 deraadt 420: (above) is set to 15, and
1.152 stevesk 421: .Cm ClientAliveCountMax
422: is left at the default, unresponsive ssh clients
1.133 itojun 423: will be disconnected after approximately 45 seconds.
1.11 markus 424: .It Cm DenyGroups
425: This keyword can be followed by a number of group names, separated
1.36 aaron 426: by spaces.
1.81 markus 427: Users whose primary group or supplementary group list matches
428: one of the patterns aren't allowed to log in.
1.11 markus 429: .Ql \&*
430: and
431: .Ql ?
432: can be used as
1.36 aaron 433: wildcards in the patterns.
1.147 deraadt 434: Only group names are valid; a numerical group ID is not recognized.
1.81 markus 435: By default login is allowed regardless of the group list.
1.11 markus 436: .Pp
437: .It Cm DenyUsers
438: This keyword can be followed by a number of user names, separated
1.36 aaron 439: by spaces.
440: Login is disallowed for user names that match one of the patterns.
1.11 markus 441: .Ql \&*
442: and
443: .Ql ?
1.36 aaron 444: can be used as wildcards in the patterns.
1.147 deraadt 445: Only user names are valid; a numerical user ID is not recognized.
1.36 aaron 446: By default login is allowed regardless of the user name.
1.47 markus 447: .It Cm GatewayPorts
448: Specifies whether remote hosts are allowed to connect to ports
449: forwarded for the client.
1.145 stevesk 450: By default,
451: .Nm
452: binds remote port forwardings to the loopback addresss. This
453: prevents other remote hosts from connecting to forwarded ports.
454: .Cm GatewayPorts
455: can be used to specify that
456: .Nm
457: should bind remote port forwardings to the wildcard address,
458: thus allowing remote hosts to connect to forwarded ports.
1.47 markus 459: The argument must be
460: .Dq yes
461: or
462: .Dq no .
463: The default is
464: .Dq no .
1.120 markus 465: .It Cm HostbasedAuthentication
466: Specifies whether rhosts or /etc/hosts.equiv authentication together
467: with successful public key client host authentication is allowed
468: (hostbased authentication).
469: This option is similar to
470: .Cm RhostsRSAAuthentication
471: and applies to protocol version 2 only.
472: The default is
473: .Dq no .
1.2 deraadt 474: .It Cm HostKey
1.72 markus 475: Specifies the file containing the private host keys (default
1.46 markus 476: .Pa /etc/ssh_host_key )
1.72 markus 477: used by SSH protocol versions 1 and 2.
1.9 markus 478: Note that
479: .Nm
1.83 markus 480: will refuse to use a file if it is group/world-accessible.
1.72 markus 481: It is possible to have multiple host key files.
482: .Dq rsa1
483: keys are used for version 1 and
484: .Dq dsa
485: or
486: .Dq rsa
487: are used for version 2 of the SSH protocol.
1.2 deraadt 488: .It Cm IgnoreRhosts
1.34 markus 489: Specifies that
490: .Pa .rhosts
1.40 aaron 491: and
1.34 markus 492: .Pa .shosts
1.120 markus 493: files will not be used in
494: .Cm RhostsAuthentication ,
495: .Cm RhostsRSAAuthentication
496: or
497: .Cm HostbasedAuthentication .
498: .Pp
1.2 deraadt 499: .Pa /etc/hosts.equiv
1.1 deraadt 500: and
1.40 aaron 501: .Pa /etc/shosts.equiv
1.36 aaron 502: are still used.
1.40 aaron 503: The default is
1.34 markus 504: .Dq yes .
1.24 markus 505: .It Cm IgnoreUserKnownHosts
506: Specifies whether
507: .Nm
508: should ignore the user's
509: .Pa $HOME/.ssh/known_hosts
1.45 markus 510: during
1.120 markus 511: .Cm RhostsRSAAuthentication
512: or
513: .Cm HostbasedAuthentication .
1.24 markus 514: The default is
1.2 deraadt 515: .Dq no .
516: .It Cm KeepAlive
1.1 deraadt 517: Specifies whether the system should send keepalive messages to the
1.36 aaron 518: other side.
519: If they are sent, death of the connection or crash of one
520: of the machines will be properly noticed.
521: However, this means that
1.1 deraadt 522: connections will die if the route is down temporarily, and some people
1.36 aaron 523: find it annoying.
1.51 hugh 524: On the other hand, if keepalives are not sent,
1.2 deraadt 525: sessions may hang indefinitely on the server, leaving
526: .Dq ghost
527: users and consuming server resources.
528: .Pp
529: The default is
530: .Dq yes
531: (to send keepalives), and the server will notice
1.36 aaron 532: if the network goes down or the client host reboots.
533: This avoids infinitely hanging sessions.
1.2 deraadt 534: .Pp
535: To disable keepalives, the value should be set to
536: .Dq no
537: in both the server and the client configuration files.
538: .It Cm KerberosAuthentication
1.36 aaron 539: Specifies whether Kerberos authentication is allowed.
540: This can be in the form of a Kerberos ticket, or if
1.7 markus 541: .Cm PasswordAuthentication
1.1 deraadt 542: is yes, the password provided by the user will be validated through
1.67 aaron 543: the Kerberos KDC.
544: To use this option, the server needs a
1.59 provos 545: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 546: Default is
1.60 provos 547: .Dq yes .
1.2 deraadt 548: .It Cm KerberosOrLocalPasswd
1.1 deraadt 549: If set then if password authentication through Kerberos fails then
550: the password will be validated via any additional local mechanism
1.2 deraadt 551: such as
1.66 markus 552: .Pa /etc/passwd .
1.36 aaron 553: Default is
1.20 dugsong 554: .Dq yes .
1.2 deraadt 555: .It Cm KerberosTgtPassing
1.1 deraadt 556: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 557: Default is
1.3 dugsong 558: .Dq no ,
559: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 560: .It Cm KerberosTicketCleanup
1.7 markus 561: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 562: file on logout.
563: Default is
1.3 dugsong 564: .Dq yes .
1.2 deraadt 565: .It Cm KeyRegenerationInterval
1.120 markus 566: In protocol version 1, the ephemeral server key is automatically regenerated
567: after this many seconds (if it has been used).
1.36 aaron 568: The purpose of regeneration is to prevent
1.1 deraadt 569: decrypting captured sessions by later breaking into the machine and
1.36 aaron 570: stealing the keys.
571: The key is never stored anywhere.
572: If the value is 0, the key is never regenerated.
573: The default is 3600 (seconds).
1.7 markus 574: .It Cm ListenAddress
1.110 stevesk 575: Specifies the local addresses
1.120 markus 576: .Nm
1.7 markus 577: should listen on.
1.110 stevesk 578: The following forms may be used:
579: .Pp
580: .Bl -item -offset indent -compact
581: .It
582: .Cm ListenAddress
1.112 stevesk 583: .Sm off
584: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
585: .Sm on
1.110 stevesk 586: .It
587: .Cm ListenAddress
1.112 stevesk 588: .Sm off
589: .Ar host No | Ar IPv4_addr No : Ar port
590: .Sm on
1.110 stevesk 591: .It
592: .Cm ListenAddress
1.112 stevesk 593: .Sm off
594: .Oo
595: .Ar host No | Ar IPv6_addr Oc : Ar port
596: .Sm on
1.110 stevesk 597: .El
598: .Pp
599: If
1.112 stevesk 600: .Ar port
1.110 stevesk 601: is not specified,
1.120 markus 602: .Nm
1.110 stevesk 603: will listen on the address and all prior
604: .Cm Port
605: options specified. The default is to listen on all local
606: addresses. Multiple
607: .Cm ListenAddress
608: options are permitted. Additionally, any
609: .Cm Port
610: options must precede this option for non port qualified addresses.
1.2 deraadt 611: .It Cm LoginGraceTime
1.1 deraadt 612: The server disconnects after this time if the user has not
1.36 aaron 613: successfully logged in.
614: If the value is 0, there is no time limit.
1.1 deraadt 615: The default is 600 (seconds).
1.23 markus 616: .It Cm LogLevel
617: Gives the verbosity level that is used when logging messages from
618: .Nm sshd .
619: The possible values are:
1.82 markus 620: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
621: The default is INFO.
1.23 markus 622: Logging with level DEBUG violates the privacy of users
623: and is not recommended.
1.93 markus 624: .It Cm MACs
625: Specifies the available MAC (message authentication code) algorithms.
626: The MAC algorithm is used in protocol version 2
627: for data integrity protection.
628: Multiple algorithms must be comma-separated.
629: The default is
1.123 markus 630: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 631: .It Cm MaxStartups
632: Specifies the maximum number of concurrent unauthenticated connections to the
633: .Nm
634: daemon.
635: Additional connections will be dropped until authentication succeeds or the
636: .Cm LoginGraceTime
637: expires for a connection.
638: The default is 10.
1.57 markus 639: .Pp
640: Alternatively, random early drop can be enabled by specifying
641: the three colon separated values
642: .Dq start:rate:full
1.67 aaron 643: (e.g., "10:30:60").
1.57 markus 644: .Nm
1.86 stevesk 645: will refuse connection attempts with a probability of
1.57 markus 646: .Dq rate/100
647: (30%)
648: if there are currently
649: .Dq start
650: (10)
651: unauthenticated connections.
1.86 stevesk 652: The probability increases linearly and all connection attempts
1.57 markus 653: are refused if the number of unauthenticated connections reaches
654: .Dq full
655: (60).
1.2 deraadt 656: .It Cm PasswordAuthentication
1.1 deraadt 657: Specifies whether password authentication is allowed.
1.2 deraadt 658: The default is
659: .Dq yes .
660: .It Cm PermitEmptyPasswords
1.1 deraadt 661: When password authentication is allowed, it specifies whether the
1.36 aaron 662: server allows login to accounts with empty password strings.
663: The default is
1.34 markus 664: .Dq no .
1.2 deraadt 665: .It Cm PermitRootLogin
1.100 stevesk 666: Specifies whether root can login using
1.2 deraadt 667: .Xr ssh 1 .
1.15 markus 668: The argument must be
669: .Dq yes ,
1.94 markus 670: .Dq without-password ,
671: .Dq forced-commands-only
1.15 markus 672: or
673: .Dq no .
1.2 deraadt 674: The default is
675: .Dq yes .
1.94 markus 676: .Pp
677: If this option is set to
1.15 markus 678: .Dq without-password
1.94 markus 679: password authentication is disabled for root.
1.2 deraadt 680: .Pp
1.94 markus 681: If this option is set to
682: .Dq forced-commands-only
683: root login with public key authentication will be allowed,
684: but only if the
1.2 deraadt 685: .Ar command
1.94 markus 686: option has been specified
1.1 deraadt 687: (which may be useful for taking remote backups even if root login is
1.94 markus 688: normally not allowed). All other authentication methods are disabled
689: for root.
1.100 stevesk 690: .Pp
691: If this option is set to
692: .Dq no
693: root is not allowed to login.
1.43 markus 694: .It Cm PidFile
695: Specifies the file that contains the process identifier of the
696: .Nm
697: daemon.
698: The default is
699: .Pa /var/run/sshd.pid .
1.2 deraadt 700: .It Cm Port
1.1 deraadt 701: Specifies the port number that
1.2 deraadt 702: .Nm
1.36 aaron 703: listens on.
704: The default is 22.
1.28 markus 705: Multiple options of this type are permitted.
1.120 markus 706: See also
707: .Cm ListenAddress .
1.108 stevesk 708: .It Cm PrintLastLog
709: Specifies whether
710: .Nm
711: should print the date and time when the user last logged in.
712: The default is
713: .Dq yes .
1.2 deraadt 714: .It Cm PrintMotd
1.1 deraadt 715: Specifies whether
1.2 deraadt 716: .Nm
1.40 aaron 717: should print
1.2 deraadt 718: .Pa /etc/motd
1.36 aaron 719: when a user logs in interactively.
720: (On some systems it is also printed by the shell,
1.2 deraadt 721: .Pa /etc/profile ,
1.36 aaron 722: or equivalent.)
723: The default is
1.2 deraadt 724: .Dq yes .
1.41 markus 725: .It Cm Protocol
726: Specifies the protocol versions
727: .Nm
728: should support.
729: The possible values are
730: .Dq 1
731: and
732: .Dq 2 .
733: Multiple versions must be comma-separated.
734: The default is
1.118 deraadt 735: .Dq 2,1 .
1.104 deraadt 736: .It Cm PubkeyAuthentication
737: Specifies whether public key authentication is allowed.
738: The default is
739: .Dq yes .
740: Note that this option applies to protocol version 2 only.
1.87 markus 741: .It Cm ReverseMappingCheck
742: Specifies whether
743: .Nm
744: should try to verify the remote host name and check that
745: the resolved host name for the remote IP address maps back to the
746: very same IP address.
747: The default is
748: .Dq no .
1.2 deraadt 749: .It Cm RhostsAuthentication
1.1 deraadt 750: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 751: files is sufficient.
752: Normally, this method should not be permitted because it is insecure.
1.7 markus 753: .Cm RhostsRSAAuthentication
754: should be used
1.1 deraadt 755: instead, because it performs RSA-based host authentication in addition
756: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 757: The default is
758: .Dq no .
1.120 markus 759: This option applies to protocol version 1 only.
1.2 deraadt 760: .It Cm RhostsRSAAuthentication
1.1 deraadt 761: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 762: with successful RSA host authentication is allowed.
763: The default is
1.34 markus 764: .Dq no .
1.120 markus 765: This option applies to protocol version 1 only.
1.2 deraadt 766: .It Cm RSAAuthentication
1.36 aaron 767: Specifies whether pure RSA authentication is allowed.
768: The default is
1.2 deraadt 769: .Dq yes .
1.120 markus 770: This option applies to protocol version 1 only.
1.2 deraadt 771: .It Cm ServerKeyBits
1.120 markus 772: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 773: The minimum value is 512, and the default is 768.
1.2 deraadt 774: .It Cm StrictModes
1.12 markus 775: Specifies whether
776: .Nm
777: should check file modes and ownership of the
1.36 aaron 778: user's files and home directory before accepting login.
779: This is normally desirable because novices sometimes accidentally leave their
780: directory or files world-writable.
781: The default is
1.7 markus 782: .Dq yes .
1.54 jakob 783: .It Cm Subsystem
1.67 aaron 784: Configures an external subsystem (e.g., file transfer daemon).
785: Arguments should be a subsystem name and a command to execute upon subsystem
786: request.
1.63 markus 787: The command
788: .Xr sftp-server 8
789: implements the
790: .Dq sftp
791: file transfer subsystem.
1.54 jakob 792: By default no subsystems are defined.
793: Note that this option applies to protocol version 2 only.
1.2 deraadt 794: .It Cm SyslogFacility
1.1 deraadt 795: Gives the facility code that is used when logging messages from
1.2 deraadt 796: .Nm sshd .
1.1 deraadt 797: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 798: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
799: The default is AUTH.
1.10 markus 800: .It Cm UseLogin
801: Specifies whether
802: .Xr login 1
1.53 markus 803: is used for interactive login sessions.
1.127 markus 804: The default is
805: .Dq no .
1.53 markus 806: Note that
807: .Xr login 1
1.58 deraadt 808: is never used for remote command execution.
1.133 itojun 809: Note also, that if this is enabled,
810: .Cm X11Forwarding
1.127 markus 811: will be disabled because
812: .Xr login 1
813: does not know how to handle
1.133 itojun 814: .Xr xauth 1
1.127 markus 815: cookies.
1.6 aaron 816: .It Cm X11DisplayOffset
817: Specifies the first display number available for
818: .Nm sshd Ns 's
1.36 aaron 819: X11 forwarding.
820: This prevents
1.6 aaron 821: .Nm
822: from interfering with real X11 servers.
1.34 markus 823: The default is 10.
1.30 markus 824: .It Cm X11Forwarding
1.36 aaron 825: Specifies whether X11 forwarding is permitted.
826: The default is
1.34 markus 827: .Dq no .
1.30 markus 828: Note that disabling X11 forwarding does not improve security in any
829: way, as users can always install their own forwarders.
1.133 itojun 830: X11 forwarding is automatically disabled if
831: .Cm UseLogin
832: is enabled.
1.52 markus 833: .It Cm XAuthLocation
834: Specifies the location of the
835: .Xr xauth 1
836: program.
837: The default is
838: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 839: .El
1.124 stevesk 840: .Ss Time Formats
841: .Pp
842: .Nm
843: command-line arguments and configuration file options that specify time
844: may be expressed using a sequence of the form:
845: .Sm off
846: .Ar time Oo Ar qualifier Oc ,
847: .Sm on
848: where
849: .Ar time
850: is a positive integer value and
851: .Ar qualifier
852: is one of the following:
853: .Pp
854: .Bl -tag -width Ds -compact -offset indent
855: .It Cm <none>
856: seconds
857: .It Cm s | Cm S
858: seconds
859: .It Cm m | Cm M
860: minutes
861: .It Cm h | Cm H
862: hours
863: .It Cm d | Cm D
864: days
865: .It Cm w | Cm W
866: weeks
867: .El
868: .Pp
869: Each member of the sequence is added together to calculate
870: the total time value.
871: .Pp
872: Time format examples:
873: .Pp
874: .Bl -tag -width Ds -compact -offset indent
875: .It 600
876: 600 seconds (10 minutes)
877: .It 10m
878: 10 minutes
879: .It 1h30m
880: 1 hour 30 minutes (90 minutes)
881: .El
1.2 deraadt 882: .Sh LOGIN PROCESS
1.1 deraadt 883: When a user successfully logs in,
1.2 deraadt 884: .Nm
1.1 deraadt 885: does the following:
1.2 deraadt 886: .Bl -enum -offset indent
887: .It
1.1 deraadt 888: If the login is on a tty, and no command has been specified,
1.40 aaron 889: prints last login time and
1.2 deraadt 890: .Pa /etc/motd
1.1 deraadt 891: (unless prevented in the configuration file or by
1.2 deraadt 892: .Pa $HOME/.hushlogin ;
893: see the
1.40 aaron 894: .Sx FILES
1.2 deraadt 895: section).
896: .It
1.1 deraadt 897: If the login is on a tty, records login time.
1.2 deraadt 898: .It
899: Checks
900: .Pa /etc/nologin ;
901: if it exists, prints contents and quits
1.1 deraadt 902: (unless root).
1.2 deraadt 903: .It
1.1 deraadt 904: Changes to run with normal user privileges.
1.2 deraadt 905: .It
1.1 deraadt 906: Sets up basic environment.
1.2 deraadt 907: .It
908: Reads
909: .Pa $HOME/.ssh/environment
910: if it exists.
911: .It
1.1 deraadt 912: Changes to user's home directory.
1.2 deraadt 913: .It
914: If
915: .Pa $HOME/.ssh/rc
916: exists, runs it; else if
917: .Pa /etc/sshrc
918: exists, runs
1.36 aaron 919: it; otherwise runs xauth.
920: The
1.2 deraadt 921: .Dq rc
922: files are given the X11
1.1 deraadt 923: authentication protocol and cookie in standard input.
1.2 deraadt 924: .It
1.1 deraadt 925: Runs user's shell or command.
1.2 deraadt 926: .El
927: .Sh AUTHORIZED_KEYS FILE FORMAT
928: .Pa $HOME/.ssh/authorized_keys
1.130 markus 929: is the default file that lists the public keys that are
930: permitted for RSA authentication in protocol version 1
931: and for public key authentication (PubkeyAuthentication)
932: in protocol version 2.
1.125 markus 933: .Cm AuthorizedKeysFile
934: may be used to specify an alternative file.
1.75 markus 935: .Pp
1.36 aaron 936: Each line of the file contains one
1.2 deraadt 937: key (empty lines and lines starting with a
938: .Ql #
939: are ignored as
1.36 aaron 940: comments).
1.75 markus 941: Each RSA public key consists of the following fields, separated by
1.36 aaron 942: spaces: options, bits, exponent, modulus, comment.
1.75 markus 943: Each protocol version 2 public key consists of:
944: options, keytype, base64 encoded key, comment.
945: The options fields
946: are optional; its presence is determined by whether the line starts
1.1 deraadt 947: with a number or not (the option field never starts with a number).
1.75 markus 948: The bits, exponent, modulus and comment fields give the RSA key for
949: protocol version 1; the
1.1 deraadt 950: comment field is not used for anything (but may be convenient for the
951: user to identify the key).
1.75 markus 952: For protocol version 2 the keytype is
953: .Dq ssh-dss
954: or
955: .Dq ssh-rsa .
1.2 deraadt 956: .Pp
1.1 deraadt 957: Note that lines in this file are usually several hundred bytes long
1.36 aaron 958: (because of the size of the RSA key modulus).
959: You don't want to type them in; instead, copy the
1.113 itojun 960: .Pa identity.pub ,
961: .Pa id_dsa.pub
1.75 markus 962: or the
1.113 itojun 963: .Pa id_rsa.pub
1.1 deraadt 964: file and edit it.
1.2 deraadt 965: .Pp
1.58 deraadt 966: The options (if present) consist of comma-separated option
1.36 aaron 967: specifications.
968: No spaces are permitted, except within double quotes.
1.141 stevesk 969: The following option specifications are supported (note
970: that option keywords are case-insensitive):
1.2 deraadt 971: .Bl -tag -width Ds
972: .It Cm from="pattern-list"
1.1 deraadt 973: Specifies that in addition to RSA authentication, the canonical name
974: of the remote host must be present in the comma-separated list of
1.36 aaron 975: patterns
976: .Pf ( Ql *
977: and
978: .Ql ?
979: serve as wildcards).
980: The list may also contain
981: patterns negated by prefixing them with
982: .Ql ! ;
983: if the canonical host name matches a negated pattern, the key is not accepted.
984: The purpose
1.1 deraadt 985: of this option is to optionally increase security: RSA authentication
986: by itself does not trust the network or name servers or anything (but
987: the key); however, if somebody somehow steals the key, the key
1.36 aaron 988: permits an intruder to log in from anywhere in the world.
989: This additional option makes using a stolen key more difficult (name
1.1 deraadt 990: servers and/or routers would have to be compromised in addition to
991: just the key).
1.2 deraadt 992: .It Cm command="command"
1.1 deraadt 993: Specifies that the command is executed whenever this key is used for
1.36 aaron 994: authentication.
995: The command supplied by the user (if any) is ignored.
1.148 markus 996: The command is run on a pty if the client requests a pty;
1.36 aaron 997: otherwise it is run without a tty.
1.147 deraadt 998: If a 8-bit clean channel is required,
999: one must not request a pty or should specify
1.89 markus 1000: .Cm no-pty .
1.36 aaron 1001: A quote may be included in the command by quoting it with a backslash.
1002: This option might be useful
1003: to restrict certain RSA keys to perform just a specific operation.
1004: An example might be a key that permits remote backups but nothing else.
1.51 hugh 1005: Note that the client may specify TCP/IP and/or X11
1006: forwarding unless they are explicitly prohibited.
1.149 markus 1007: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 1008: .It Cm environment="NAME=value"
1.1 deraadt 1009: Specifies that the string is to be added to the environment when
1.36 aaron 1010: logging in using this key.
1011: Environment variables set this way
1012: override other default environment values.
1013: Multiple options of this type are permitted.
1.155 markus 1014: This option is automatically disabled if
1015: .Cm UseLogin
1016: is enabled.
1.2 deraadt 1017: .It Cm no-port-forwarding
1.1 deraadt 1018: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 1019: Any port forward requests by the client will return an error.
1020: This might be used, e.g., in connection with the
1.2 deraadt 1021: .Cm command
1.1 deraadt 1022: option.
1.2 deraadt 1023: .It Cm no-X11-forwarding
1.1 deraadt 1024: Forbids X11 forwarding when this key is used for authentication.
1025: Any X11 forward requests by the client will return an error.
1.2 deraadt 1026: .It Cm no-agent-forwarding
1.1 deraadt 1027: Forbids authentication agent forwarding when this key is used for
1028: authentication.
1.2 deraadt 1029: .It Cm no-pty
1.1 deraadt 1030: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 1031: .It Cm permitopen="host:port"
1.133 itojun 1032: Limit local
1.107 djm 1033: .Li ``ssh -L''
1.111 stevesk 1034: port forwarding such that it may only connect to the specified host and
1.146 stevesk 1035: port.
1036: IPv6 addresses can be specified with an alternative syntax:
1037: .Ar host/port .
1038: Multiple
1.107 djm 1039: .Cm permitopen
1.133 itojun 1040: options may be applied separated by commas. No pattern matching is
1041: performed on the specified hostnames, they must be literal domains or
1.107 djm 1042: addresses.
1.2 deraadt 1043: .El
1044: .Ss Examples
1.1 deraadt 1045: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1046: .Pp
1.1 deraadt 1047: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1048: .Pp
1.1 deraadt 1049: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 1050: .Pp
1051: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1052: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1053: The
1.44 deraadt 1054: .Pa /etc/ssh_known_hosts ,
1.40 aaron 1055: and
1.131 markus 1056: .Pa $HOME/.ssh/known_hosts
1.36 aaron 1057: files contain host public keys for all known hosts.
1058: The global file should
1.37 brad 1059: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1060: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1061: its key is added to the per-user file.
1.2 deraadt 1062: .Pp
1.1 deraadt 1063: Each line in these files contains the following fields: hostnames,
1.36 aaron 1064: bits, exponent, modulus, comment.
1065: The fields are separated by spaces.
1.2 deraadt 1066: .Pp
1.1 deraadt 1067: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1068: wildcards); each pattern in turn is matched against the canonical host
1069: name (when authenticating a client) or against the user-supplied
1.36 aaron 1070: name (when authenticating a server).
1071: A pattern may also be preceded by
1.2 deraadt 1072: .Ql !
1073: to indicate negation: if the host name matches a negated
1.1 deraadt 1074: pattern, it is not accepted (by that line) even if it matched another
1075: pattern on the line.
1.2 deraadt 1076: .Pp
1.49 markus 1077: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1078: can be obtained, e.g., from
1079: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 1080: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1081: .Pp
1082: Lines starting with
1083: .Ql #
1084: and empty lines are ignored as comments.
1085: .Pp
1.1 deraadt 1086: When performing host authentication, authentication is accepted if any
1.36 aaron 1087: matching line has the proper key.
1088: It is thus permissible (but not
1.1 deraadt 1089: recommended) to have several lines or different host keys for the same
1.36 aaron 1090: names.
1091: This will inevitably happen when short forms of host names
1092: from different domains are put in the file.
1093: It is possible
1.1 deraadt 1094: that the files contain conflicting information; authentication is
1095: accepted if valid information can be found from either file.
1.2 deraadt 1096: .Pp
1.1 deraadt 1097: Note that the lines in these files are typically hundreds of characters
1098: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1099: Rather, generate them by a script
1.40 aaron 1100: or by taking
1.2 deraadt 1101: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1102: and adding the host names at the front.
1.2 deraadt 1103: .Ss Examples
1.120 markus 1104: .Bd -literal
1105: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1106: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1107: .Ed
1.2 deraadt 1108: .Sh FILES
1109: .Bl -tag -width Ds
1110: .It Pa /etc/sshd_config
1.1 deraadt 1111: Contains configuration data for
1.2 deraadt 1112: .Nm sshd .
1.1 deraadt 1113: This file should be writable by root only, but it is recommended
1114: (though not necessary) that it be world-readable.
1.98 deraadt 1115: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.120 markus 1116: These three files contain the private parts of the host keys.
1.98 deraadt 1117: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1118: accessible to others.
1.14 markus 1119: Note that
1120: .Nm
1121: does not start if this file is group/world-accessible.
1.98 deraadt 1122: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.120 markus 1123: These three files contain the public parts of the host keys.
1.98 deraadt 1124: These files should be world-readable but writable only by
1.36 aaron 1125: root.
1.98 deraadt 1126: Their contents should match the respective private parts.
1127: These files are not
1128: really used for anything; they are provided for the convenience of
1129: the user so their contents can be copied to known hosts files.
1130: These files are created using
1.7 markus 1131: .Xr ssh-keygen 1 .
1.129 provos 1132: .It Pa /etc/moduli
1.73 provos 1133: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1134: .It Pa /var/run/sshd.pid
1135: Contains the process ID of the
1136: .Nm
1.1 deraadt 1137: listening for connections (if there are several daemons running
1138: concurrently for different ports, this contains the pid of the one
1.36 aaron 1139: started last).
1.58 deraadt 1140: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1141: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 1142: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1143: This file must be readable by root (which may on some machines imply
1144: it being world-readable if the user's home directory resides on an NFS
1145: volume).
1146: It is recommended that it not be accessible by others.
1147: The format of this file is described above.
1148: Users will place the contents of their
1.130 markus 1149: .Pa identity.pub ,
1.44 deraadt 1150: .Pa id_dsa.pub
1.113 itojun 1151: and/or
1152: .Pa id_rsa.pub
1.44 deraadt 1153: files into this file, as described in
1154: .Xr ssh-keygen 1 .
1.22 markus 1155: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1156: These files are consulted when using rhosts with RSA host
1.131 markus 1157: authentication or protocol version 2 hostbased authentication
1158: to check the public key of the host.
1.36 aaron 1159: The key must be listed in one of these files to be accepted.
1.22 markus 1160: The client uses the same files
1.96 markus 1161: to verify that it is connecting to the correct remote host.
1.36 aaron 1162: These files should be writable only by root/the owner.
1.2 deraadt 1163: .Pa /etc/ssh_known_hosts
1164: should be world-readable, and
1165: .Pa $HOME/.ssh/known_hosts
1.120 markus 1166: can but need not be world-readable.
1.6 aaron 1167: .It Pa /etc/nologin
1.40 aaron 1168: If this file exists,
1.2 deraadt 1169: .Nm
1.36 aaron 1170: refuses to let anyone except root log in.
1171: The contents of the file
1.1 deraadt 1172: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1173: refused.
1174: The file should be world-readable.
1.19 dugsong 1175: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 1176: Access controls that should be enforced by tcp-wrappers are defined here.
1177: Further details are described in
1.19 dugsong 1178: .Xr hosts_access 5 .
1.6 aaron 1179: .It Pa $HOME/.rhosts
1.1 deraadt 1180: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1181: line.
1182: The given user on the corresponding host is permitted to log in
1183: without password.
1184: The same file is used by rlogind and rshd.
1.6 aaron 1185: The file must
1.1 deraadt 1186: be writable only by the user; it is recommended that it not be
1187: accessible by others.
1.2 deraadt 1188: .Pp
1.36 aaron 1189: If is also possible to use netgroups in the file.
1190: Either host or user
1.1 deraadt 1191: name may be of the form +@groupname to specify all hosts or all users
1192: in the group.
1.2 deraadt 1193: .It Pa $HOME/.shosts
1194: For ssh,
1195: this file is exactly the same as for
1196: .Pa .rhosts .
1197: However, this file is
1198: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1199: .It Pa /etc/hosts.equiv
1.2 deraadt 1200: This file is used during
1201: .Pa .rhosts
1.36 aaron 1202: authentication.
1203: In the simplest form, this file contains host names, one per line.
1204: Users on
1.1 deraadt 1205: those hosts are permitted to log in without a password, provided they
1.36 aaron 1206: have the same user name on both machines.
1207: The host name may also be
1.1 deraadt 1208: followed by a user name; such users are permitted to log in as
1.2 deraadt 1209: .Em any
1.36 aaron 1210: user on this machine (except root).
1211: Additionally, the syntax
1.2 deraadt 1212: .Dq +@group
1.36 aaron 1213: can be used to specify netgroups.
1214: Negated entries start with
1.2 deraadt 1215: .Ql \&- .
1216: .Pp
1.1 deraadt 1217: If the client host/user is successfully matched in this file, login is
1218: automatically permitted provided the client and server user names are the
1.36 aaron 1219: same.
1220: Additionally, successful RSA host authentication is normally required.
1221: This file must be writable only by root; it is recommended
1.1 deraadt 1222: that it be world-readable.
1.2 deraadt 1223: .Pp
1.6 aaron 1224: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1225: .Pa hosts.equiv .
1.1 deraadt 1226: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1227: .Em anybody ,
1.1 deraadt 1228: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1229: binaries and directories.
1230: Using a user name practically grants the user root access.
1231: The only valid use for user names that I can think
1.1 deraadt 1232: of is in negative entries.
1.2 deraadt 1233: .Pp
1234: Note that this warning also applies to rsh/rlogin.
1235: .It Pa /etc/shosts.equiv
1.1 deraadt 1236: This is processed exactly as
1.2 deraadt 1237: .Pa /etc/hosts.equiv .
1.1 deraadt 1238: However, this file may be useful in environments that want to run both
1.2 deraadt 1239: rsh/rlogin and ssh.
1.6 aaron 1240: .It Pa $HOME/.ssh/environment
1.36 aaron 1241: This file is read into the environment at login (if it exists).
1242: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1243: .Ql # ) ,
1.36 aaron 1244: and assignment lines of the form name=value.
1245: The file should be writable
1.6 aaron 1246: only by the user; it need not be readable by anyone else.
1.2 deraadt 1247: .It Pa $HOME/.ssh/rc
1.1 deraadt 1248: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1249: environment files but before starting the user's shell or command.
1250: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1251: standard input (and
1252: .Ev DISPLAY
1.36 aaron 1253: in environment).
1254: This must call
1.2 deraadt 1255: .Xr xauth 1
1256: in that case.
1257: .Pp
1.1 deraadt 1258: The primary purpose of this file is to run any initialization routines
1259: which may be needed before the user's home directory becomes
1260: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1261: .Pp
1.1 deraadt 1262: This file will probably contain some initialization code followed by
1.120 markus 1263: something similar to:
1264: .Bd -literal
1265: if read proto cookie; then
1266: echo add $DISPLAY $proto $cookie | xauth -q -
1267: fi
1268: .Ed
1.2 deraadt 1269: .Pp
1270: If this file does not exist,
1271: .Pa /etc/sshrc
1272: is run, and if that
1.1 deraadt 1273: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1274: .Pp
1.1 deraadt 1275: This file should be writable only by the user, and need not be
1276: readable by anyone else.
1.2 deraadt 1277: .It Pa /etc/sshrc
1278: Like
1279: .Pa $HOME/.ssh/rc .
1280: This can be used to specify
1.36 aaron 1281: machine-specific login-time initializations globally.
1282: This file should be writable only by root, and should be world-readable.
1.56 aaron 1283: .El
1.71 aaron 1284: .Sh AUTHORS
1.84 markus 1285: OpenSSH is a derivative of the original and free
1286: ssh 1.2.12 release by Tatu Ylonen.
1287: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1288: Theo de Raadt and Dug Song
1289: removed many bugs, re-added newer features and
1290: created OpenSSH.
1291: Markus Friedl contributed the support for SSH
1292: protocol versions 1.5 and 2.0.
1.2 deraadt 1293: .Sh SEE ALSO
1294: .Xr scp 1 ,
1.90 djm 1295: .Xr sftp 1 ,
1.2 deraadt 1296: .Xr ssh 1 ,
1.5 deraadt 1297: .Xr ssh-add 1 ,
1.2 deraadt 1298: .Xr ssh-agent 1 ,
1.5 deraadt 1299: .Xr ssh-keygen 1 ,
1.136 markus 1300: .Xr login.conf 5 ,
1301: .Xr moduli 5 ,
1.128 mpech 1302: .Xr sftp-server 8
1.119 markus 1303: .Rs
1304: .%A T. Ylonen
1305: .%A T. Kivinen
1306: .%A M. Saarinen
1307: .%A T. Rinne
1308: .%A S. Lehtinen
1309: .%T "SSH Protocol Architecture"
1.139 markus 1310: .%N draft-ietf-secsh-architecture-09.txt
1311: .%D July 2001
1.120 markus 1312: .%O work in progress material
1313: .Re
1314: .Rs
1315: .%A M. Friedl
1316: .%A N. Provos
1317: .%A W. A. Simpson
1318: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.132 markus 1319: .%N draft-ietf-secsh-dh-group-exchange-01.txt
1320: .%D April 2001
1.119 markus 1321: .%O work in progress material
1322: .Re
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