Annotation of src/usr.bin/ssh/sshd.8, Revision 1.159
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: .\"
1.159 ! stevesk 37: .\" $OpenBSD: sshd.8,v 1.158 2001/12/27 18:22:53 stevesk Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSHD 8
40: .Os
41: .Sh NAME
42: .Nm sshd
1.120 markus 43: .Nd OpenSSH SSH daemon
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm sshd
1.150 stevesk 46: .Op Fl deiqtD46
1.2 deraadt 47: .Op Fl b Ar bits
48: .Op Fl f Ar config_file
49: .Op Fl g Ar login_grace_time
50: .Op Fl h Ar host_key_file
51: .Op Fl k Ar key_gen_time
1.156 markus 52: .Op Fl o Ar option
1.2 deraadt 53: .Op Fl p Ar port
1.61 markus 54: .Op Fl u Ar len
1.40 aaron 55: .Sh DESCRIPTION
1.2 deraadt 56: .Nm
1.106 deraadt 57: (SSH Daemon) is the daemon program for
1.2 deraadt 58: .Xr ssh 1 .
1.42 hugh 59: Together these programs replace rlogin and rsh, and
1.1 deraadt 60: provide secure encrypted communications between two untrusted hosts
1.36 aaron 61: over an insecure network.
62: The programs are intended to be as easy to
1.1 deraadt 63: install and use as possible.
1.2 deraadt 64: .Pp
65: .Nm
1.36 aaron 66: is the daemon that listens for connections from clients.
1.40 aaron 67: It is normally started at boot from
1.2 deraadt 68: .Pa /etc/rc .
69: It forks a new
1.36 aaron 70: daemon for each incoming connection.
71: The forked daemons handle
1.1 deraadt 72: key exchange, encryption, authentication, command execution,
73: and data exchange.
1.49 markus 74: This implementation of
75: .Nm
76: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 77: .Nm
1.36 aaron 78: works as follows.
1.49 markus 79: .Pp
80: .Ss SSH protocol version 1
81: .Pp
1.36 aaron 82: Each host has a host-specific RSA key
83: (normally 1024 bits) used to identify the host.
84: Additionally, when
1.1 deraadt 85: the daemon starts, it generates a server RSA key (normally 768 bits).
86: This key is normally regenerated every hour if it has been used, and
87: is never stored on disk.
1.2 deraadt 88: .Pp
1.42 hugh 89: Whenever a client connects the daemon responds with its public
90: host and server keys.
1.36 aaron 91: The client compares the
1.49 markus 92: RSA host key against its own database to verify that it has not changed.
1.36 aaron 93: The client then generates a 256 bit random number.
94: It encrypts this
1.1 deraadt 95: random number using both the host key and the server key, and sends
1.36 aaron 96: the encrypted number to the server.
1.42 hugh 97: Both sides then use this
1.1 deraadt 98: random number as a session key which is used to encrypt all further
1.36 aaron 99: communications in the session.
100: The rest of the session is encrypted
1.42 hugh 101: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 102: being used by default.
1.36 aaron 103: The client selects the encryption algorithm
1.5 deraadt 104: to use from those offered by the server.
1.2 deraadt 105: .Pp
1.36 aaron 106: Next, the server and the client enter an authentication dialog.
107: The client tries to authenticate itself using
1.2 deraadt 108: .Pa .rhosts
109: authentication,
110: .Pa .rhosts
111: authentication combined with RSA host
1.1 deraadt 112: authentication, RSA challenge-response authentication, or password
113: based authentication.
1.2 deraadt 114: .Pp
1.1 deraadt 115: Rhosts authentication is normally disabled
116: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 117: configuration file if desired.
118: System security is not improved unless
1.2 deraadt 119: .Xr rshd 8 ,
120: .Xr rlogind 8 ,
121: and
1.143 stevesk 122: .Xr rexecd 8
1.1 deraadt 123: are disabled (thus completely disabling
1.2 deraadt 124: .Xr rlogin 1
1.1 deraadt 125: and
1.2 deraadt 126: .Xr rsh 1
1.42 hugh 127: into the machine).
1.2 deraadt 128: .Pp
1.49 markus 129: .Ss SSH protocol version 2
130: .Pp
1.58 deraadt 131: Version 2 works similarly:
1.138 markus 132: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 133: However, when the daemon starts, it does not generate a server key.
134: Forward security is provided through a Diffie-Hellman key agreement.
135: This key agreement results in a shared session key.
1.120 markus 136: .Pp
1.103 deraadt 137: The rest of the session is encrypted using a symmetric cipher, currently
1.120 markus 138: 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
1.49 markus 139: The client selects the encryption algorithm
140: to use from those offered by the server.
141: Additionally, session integrity is provided
1.51 hugh 142: through a cryptographic message authentication code
1.49 markus 143: (hmac-sha1 or hmac-md5).
144: .Pp
145: Protocol version 2 provides a public key based
1.120 markus 146: user (PubkeyAuthentication) or
147: client host (HostbasedAuthentication) authentication method,
148: conventional password authentication and challenge response based methods.
1.49 markus 149: .Pp
150: .Ss Command execution and data forwarding
151: .Pp
1.1 deraadt 152: If the client successfully authenticates itself, a dialog for
1.36 aaron 153: preparing the session is entered.
154: At this time the client may request
1.1 deraadt 155: things like allocating a pseudo-tty, forwarding X11 connections,
156: forwarding TCP/IP connections, or forwarding the authentication agent
157: connection over the secure channel.
1.2 deraadt 158: .Pp
1.1 deraadt 159: Finally, the client either requests a shell or execution of a command.
1.36 aaron 160: The sides then enter session mode.
161: In this mode, either side may send
1.1 deraadt 162: data at any time, and such data is forwarded to/from the shell or
163: command on the server side, and the user terminal in the client side.
1.2 deraadt 164: .Pp
1.1 deraadt 165: When the user program terminates and all forwarded X11 and other
166: connections have been closed, the server sends command exit status to
167: the client, and both sides exit.
1.2 deraadt 168: .Pp
169: .Nm
1.1 deraadt 170: can be configured using command-line options or a configuration
1.36 aaron 171: file.
172: Command-line options override values specified in the
1.1 deraadt 173: configuration file.
1.25 markus 174: .Pp
175: .Nm
176: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 177: .Dv SIGHUP ,
1.128 mpech 178: by executing itself with the name it was started as, i.e.,
1.97 deraadt 179: .Pa /usr/sbin/sshd .
1.18 aaron 180: .Pp
181: The options are as follows:
1.2 deraadt 182: .Bl -tag -width Ds
183: .It Fl b Ar bits
1.120 markus 184: Specifies the number of bits in the ephemeral protocol version 1
185: server key (default 768).
1.2 deraadt 186: .It Fl d
1.36 aaron 187: Debug mode.
188: The server sends verbose debug output to the system
189: log, and does not put itself in the background.
190: The server also will not fork and will only process one connection.
191: This option is only intended for debugging for the server.
1.120 markus 192: Multiple -d options increase the debugging level.
1.67 aaron 193: Maximum is 3.
1.120 markus 194: .It Fl e
195: When this option is specified,
196: .Nm
197: will send the output to the standard error instead of the system log.
1.2 deraadt 198: .It Fl f Ar configuration_file
1.36 aaron 199: Specifies the name of the configuration file.
200: The default is
1.2 deraadt 201: .Pa /etc/sshd_config .
1.16 markus 202: .Nm
203: refuses to start if there is no configuration file.
1.2 deraadt 204: .It Fl g Ar login_grace_time
1.1 deraadt 205: Gives the grace time for clients to authenticate themselves (default
1.77 markus 206: 600 seconds).
1.36 aaron 207: If the client fails to authenticate the user within
208: this many seconds, the server disconnects and exits.
209: A value of zero indicates no limit.
1.2 deraadt 210: .It Fl h Ar host_key_file
1.75 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).
1.158 stevesk 248: Multiple port options are permitted.
249: Ports specified in the configuration file are ignored when a
250: command-line port is specified.
1.2 deraadt 251: .It Fl q
1.36 aaron 252: Quiet mode.
253: Nothing is sent to the system log.
254: Normally the beginning,
1.1 deraadt 255: authentication, and termination of each connection is logged.
1.137 stevesk 256: .It Fl t
257: Test mode.
258: Only check the validity of the configuration file and sanity of the keys.
1.157 deraadt 259: This is useful for updating
1.137 stevesk 260: .Nm
261: reliably as configuration options may change.
1.61 markus 262: .It Fl u Ar len
263: This option is used to specify the size of the field
264: in the
265: .Li utmp
266: structure that holds the remote host name.
267: If the resolved host name is longer than
268: .Ar len ,
269: the dotted decimal value will be used instead.
270: This allows hosts with very long host names that
271: overflow this field to still be uniquely identified.
272: Specifying
273: .Fl u0
274: indicates that only dotted decimal addresses
275: should be put into the
276: .Pa utmp
277: file.
1.144 stevesk 278: .Fl u0
279: is also be used to prevent
280: .Nm
281: from making DNS requests unless the authentication
282: mechanism or configuration requires it.
283: Authentication mechanisms that may require DNS include
284: .Cm RhostsAuthentication ,
285: .Cm RhostsRSAAuthentication ,
286: .Cm HostbasedAuthentication
287: and using a
288: .Cm from="pattern-list"
289: option in a key file.
1.74 markus 290: .It Fl D
291: When this option is specified
292: .Nm
293: will not detach and does not become a daemon.
294: This allows easy monitoring of
1.76 markus 295: .Nm sshd .
1.29 markus 296: .It Fl 4
297: Forces
298: .Nm
299: to use IPv4 addresses only.
300: .It Fl 6
301: Forces
302: .Nm
303: to use IPv6 addresses only.
1.2 deraadt 304: .El
305: .Sh CONFIGURATION FILE
306: .Nm
1.40 aaron 307: reads configuration data from
1.2 deraadt 308: .Pa /etc/sshd_config
309: (or the file specified with
310: .Fl f
1.36 aaron 311: on the command line).
1.141 stevesk 312: The file contains keyword-argument pairs, one per line.
1.36 aaron 313: Lines starting with
1.2 deraadt 314: .Ql #
1.1 deraadt 315: and empty lines are interpreted as comments.
1.2 deraadt 316: .Pp
1.141 stevesk 317: The possible
318: keywords and their meanings are as follows (note that
319: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 320: .Bl -tag -width Ds
321: .It Cm AFSTokenPassing
1.36 aaron 322: Specifies whether an AFS token may be forwarded to the server.
323: Default is
1.2 deraadt 324: .Dq yes .
1.11 markus 325: .It Cm AllowGroups
1.92 deraadt 326: This keyword can be followed by a list of group names, separated
1.36 aaron 327: by spaces.
328: If specified, login is allowed only for users whose primary
1.81 markus 329: group or supplementary group list matches one of the patterns.
1.11 markus 330: .Ql \&*
331: and
332: .Ql ?
333: can be used as
1.36 aaron 334: wildcards in the patterns.
1.147 deraadt 335: Only group names are valid; a numerical group ID is not recognized.
1.81 markus 336: By default login is allowed regardless of the group list.
1.3 dugsong 337: .Pp
1.69 markus 338: .It Cm AllowTcpForwarding
339: Specifies whether TCP forwarding is permitted.
340: The default is
341: .Dq yes .
342: Note that disabling TCP forwarding does not improve security unless
343: users are also denied shell access, as they can always install their
344: own forwarders.
345: .Pp
1.11 markus 346: .It Cm AllowUsers
1.92 deraadt 347: This keyword can be followed by a list of user names, separated
1.36 aaron 348: by spaces.
349: If specified, login is allowed only for users names that
1.11 markus 350: match one of the patterns.
351: .Ql \&*
352: and
353: .Ql ?
354: can be used as
1.36 aaron 355: wildcards in the patterns.
1.147 deraadt 356: Only user names are valid; a numerical user ID is not recognized.
1.36 aaron 357: By default login is allowed regardless of the user name.
1.135 markus 358: If the pattern takes the form USER@HOST then USER and HOST
1.147 deraadt 359: are separately checked, restricting logins to particular
1.135 markus 360: users from particular hosts.
1.80 markus 361: .Pp
1.125 markus 362: .It Cm AuthorizedKeysFile
1.138 markus 363: Specifies the file that contains the public keys that can be used
364: for user authentication.
1.125 markus 365: .Cm AuthorizedKeysFile
366: may contain tokens of the form %T which are substituted during connection
1.142 stevesk 367: set-up. The following tokens are defined: %% is replaced by a literal '%',
1.125 markus 368: %h is replaced by the home directory of the user being authenticated and
369: %u is replaced by the username of that user.
370: After expansion,
371: .Cm AuthorizedKeysFile
1.126 markus 372: is taken to be an absolute path or one relative to the user's home
1.125 markus 373: directory.
374: The default is
375: .Dq .ssh/authorized_keys
1.80 markus 376: .It Cm Banner
377: In some jurisdictions, sending a warning message before authentication
378: may be relevant for getting legal protection.
379: The contents of the specified file are sent to the remote user before
380: authentication is allowed.
381: This option is only available for protocol version 2.
1.11 markus 382: .Pp
1.104 deraadt 383: .It Cm ChallengeResponseAuthentication
1.136 markus 384: Specifies whether challenge response authentication is allowed.
385: All authentication styles from
386: .Xr login.conf 5
387: are supported.
1.104 deraadt 388: The default is
389: .Dq yes .
1.122 markus 390: .It Cm Ciphers
391: Specifies the ciphers allowed for protocol version 2.
392: Multiple ciphers must be comma-separated.
393: The default is
394: .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
1.115 beck 395: .It Cm ClientAliveInterval
396: Sets a timeout interval in seconds after which if no data has been received
1.133 itojun 397: from the client,
1.115 beck 398: .Nm
399: will send a message through the encrypted
1.116 stevesk 400: channel to request a response from the client.
401: The default
1.115 beck 402: is 0, indicating that these messages will not be sent to the client.
1.116 stevesk 403: This option applies to protocol version 2 only.
1.115 beck 404: .It Cm ClientAliveCountMax
405: Sets the number of client alive messages (see above) which may be
406: sent without
407: .Nm
408: receiving any messages back from the client. If this threshold is
1.133 itojun 409: reached while client alive messages are being sent,
1.115 beck 410: .Nm
411: will disconnect the client, terminating the session. It is important
1.133 itojun 412: to note that the use of client alive messages is very different from
1.154 markus 413: .Cm KeepAlive
1.116 stevesk 414: (below). The client alive messages are sent through the
1.115 beck 415: encrypted channel and therefore will not be spoofable. The TCP keepalive
1.116 stevesk 416: option enabled by
1.154 markus 417: .Cm KeepAlive
1.147 deraadt 418: is spoofable. The client alive mechanism is valuable when the client or
419: server depend on knowing when a connection has become inactive.
1.116 stevesk 420: .Pp
1.147 deraadt 421: The default value is 3. If
1.116 stevesk 422: .Cm ClientAliveInterval
1.147 deraadt 423: (above) is set to 15, and
1.152 stevesk 424: .Cm ClientAliveCountMax
425: is left at the default, unresponsive ssh clients
1.133 itojun 426: will be disconnected after approximately 45 seconds.
1.11 markus 427: .It Cm DenyGroups
428: This keyword can be followed by a number of group names, separated
1.36 aaron 429: by spaces.
1.81 markus 430: Users whose primary group or supplementary group list matches
431: one of the patterns aren't allowed to log in.
1.11 markus 432: .Ql \&*
433: and
434: .Ql ?
435: can be used as
1.36 aaron 436: wildcards in the patterns.
1.147 deraadt 437: Only group names are valid; a numerical group ID is not recognized.
1.81 markus 438: By default login is allowed regardless of the group list.
1.11 markus 439: .Pp
440: .It Cm DenyUsers
441: This keyword can be followed by a number of user names, separated
1.36 aaron 442: by spaces.
443: Login is disallowed for user names that match one of the patterns.
1.11 markus 444: .Ql \&*
445: and
446: .Ql ?
1.36 aaron 447: can be used as wildcards in the patterns.
1.147 deraadt 448: Only user names are valid; a numerical user ID is not recognized.
1.36 aaron 449: By default login is allowed regardless of the user name.
1.47 markus 450: .It Cm GatewayPorts
451: Specifies whether remote hosts are allowed to connect to ports
452: forwarded for the client.
1.145 stevesk 453: By default,
454: .Nm
455: binds remote port forwardings to the loopback addresss. This
456: prevents other remote hosts from connecting to forwarded ports.
457: .Cm GatewayPorts
458: can be used to specify that
459: .Nm
460: should bind remote port forwardings to the wildcard address,
461: thus allowing remote hosts to connect to forwarded ports.
1.47 markus 462: The argument must be
463: .Dq yes
464: or
465: .Dq no .
466: The default is
467: .Dq no .
1.120 markus 468: .It Cm HostbasedAuthentication
469: Specifies whether rhosts or /etc/hosts.equiv authentication together
470: with successful public key client host authentication is allowed
471: (hostbased authentication).
472: This option is similar to
473: .Cm RhostsRSAAuthentication
474: and applies to protocol version 2 only.
475: The default is
476: .Dq no .
1.2 deraadt 477: .It Cm HostKey
1.72 markus 478: Specifies the file containing the private host keys (default
1.46 markus 479: .Pa /etc/ssh_host_key )
1.72 markus 480: used by SSH protocol versions 1 and 2.
1.9 markus 481: Note that
482: .Nm
1.83 markus 483: will refuse to use a file if it is group/world-accessible.
1.72 markus 484: It is possible to have multiple host key files.
485: .Dq rsa1
486: keys are used for version 1 and
487: .Dq dsa
488: or
489: .Dq rsa
490: are used for version 2 of the SSH protocol.
1.2 deraadt 491: .It Cm IgnoreRhosts
1.34 markus 492: Specifies that
493: .Pa .rhosts
1.40 aaron 494: and
1.34 markus 495: .Pa .shosts
1.120 markus 496: files will not be used in
497: .Cm RhostsAuthentication ,
498: .Cm RhostsRSAAuthentication
499: or
500: .Cm HostbasedAuthentication .
501: .Pp
1.2 deraadt 502: .Pa /etc/hosts.equiv
1.1 deraadt 503: and
1.40 aaron 504: .Pa /etc/shosts.equiv
1.36 aaron 505: are still used.
1.40 aaron 506: The default is
1.34 markus 507: .Dq yes .
1.24 markus 508: .It Cm IgnoreUserKnownHosts
509: Specifies whether
510: .Nm
511: should ignore the user's
512: .Pa $HOME/.ssh/known_hosts
1.45 markus 513: during
1.120 markus 514: .Cm RhostsRSAAuthentication
515: or
516: .Cm HostbasedAuthentication .
1.24 markus 517: The default is
1.2 deraadt 518: .Dq no .
519: .It Cm KeepAlive
1.1 deraadt 520: Specifies whether the system should send keepalive messages to the
1.36 aaron 521: other side.
522: If they are sent, death of the connection or crash of one
523: of the machines will be properly noticed.
524: However, this means that
1.1 deraadt 525: connections will die if the route is down temporarily, and some people
1.36 aaron 526: find it annoying.
1.51 hugh 527: On the other hand, if keepalives are not sent,
1.2 deraadt 528: sessions may hang indefinitely on the server, leaving
529: .Dq ghost
530: users and consuming server resources.
531: .Pp
532: The default is
533: .Dq yes
534: (to send keepalives), and the server will notice
1.36 aaron 535: if the network goes down or the client host reboots.
536: This avoids infinitely hanging sessions.
1.2 deraadt 537: .Pp
538: To disable keepalives, the value should be set to
539: .Dq no
540: in both the server and the client configuration files.
541: .It Cm KerberosAuthentication
1.36 aaron 542: Specifies whether Kerberos authentication is allowed.
543: This can be in the form of a Kerberos ticket, or if
1.7 markus 544: .Cm PasswordAuthentication
1.1 deraadt 545: is yes, the password provided by the user will be validated through
1.67 aaron 546: the Kerberos KDC.
547: To use this option, the server needs a
1.59 provos 548: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 549: Default is
1.60 provos 550: .Dq yes .
1.2 deraadt 551: .It Cm KerberosOrLocalPasswd
1.1 deraadt 552: If set then if password authentication through Kerberos fails then
553: the password will be validated via any additional local mechanism
1.2 deraadt 554: such as
1.66 markus 555: .Pa /etc/passwd .
1.36 aaron 556: Default is
1.20 dugsong 557: .Dq yes .
1.2 deraadt 558: .It Cm KerberosTgtPassing
1.1 deraadt 559: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 560: Default is
1.3 dugsong 561: .Dq no ,
562: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 563: .It Cm KerberosTicketCleanup
1.7 markus 564: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 565: file on logout.
566: Default is
1.3 dugsong 567: .Dq yes .
1.2 deraadt 568: .It Cm KeyRegenerationInterval
1.120 markus 569: In protocol version 1, the ephemeral server key is automatically regenerated
570: after this many seconds (if it has been used).
1.36 aaron 571: The purpose of regeneration is to prevent
1.1 deraadt 572: decrypting captured sessions by later breaking into the machine and
1.36 aaron 573: stealing the keys.
574: The key is never stored anywhere.
575: If the value is 0, the key is never regenerated.
576: The default is 3600 (seconds).
1.7 markus 577: .It Cm ListenAddress
1.110 stevesk 578: Specifies the local addresses
1.120 markus 579: .Nm
1.7 markus 580: should listen on.
1.110 stevesk 581: The following forms may be used:
582: .Pp
583: .Bl -item -offset indent -compact
584: .It
585: .Cm ListenAddress
1.112 stevesk 586: .Sm off
587: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
588: .Sm on
1.110 stevesk 589: .It
590: .Cm ListenAddress
1.112 stevesk 591: .Sm off
592: .Ar host No | Ar IPv4_addr No : Ar port
593: .Sm on
1.110 stevesk 594: .It
595: .Cm ListenAddress
1.112 stevesk 596: .Sm off
597: .Oo
598: .Ar host No | Ar IPv6_addr Oc : Ar port
599: .Sm on
1.110 stevesk 600: .El
601: .Pp
602: If
1.112 stevesk 603: .Ar port
1.110 stevesk 604: is not specified,
1.120 markus 605: .Nm
1.110 stevesk 606: will listen on the address and all prior
607: .Cm Port
608: options specified. The default is to listen on all local
609: addresses. Multiple
610: .Cm ListenAddress
611: options are permitted. Additionally, any
612: .Cm Port
613: options must precede this option for non port qualified addresses.
1.2 deraadt 614: .It Cm LoginGraceTime
1.1 deraadt 615: The server disconnects after this time if the user has not
1.36 aaron 616: successfully logged in.
617: If the value is 0, there is no time limit.
1.1 deraadt 618: The default is 600 (seconds).
1.23 markus 619: .It Cm LogLevel
620: Gives the verbosity level that is used when logging messages from
621: .Nm sshd .
622: The possible values are:
1.159 ! stevesk 623: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
! 624: The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2
! 625: and DEBUG3 each specify higher levels of debugging output.
! 626: Logging with a DEBUG level violates the privacy of users
1.23 markus 627: and is not recommended.
1.93 markus 628: .It Cm MACs
629: Specifies the available MAC (message authentication code) algorithms.
630: The MAC algorithm is used in protocol version 2
631: for data integrity protection.
632: Multiple algorithms must be comma-separated.
633: The default is
1.123 markus 634: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 635: .It Cm MaxStartups
636: Specifies the maximum number of concurrent unauthenticated connections to the
637: .Nm
638: daemon.
639: Additional connections will be dropped until authentication succeeds or the
640: .Cm LoginGraceTime
641: expires for a connection.
642: The default is 10.
1.57 markus 643: .Pp
644: Alternatively, random early drop can be enabled by specifying
645: the three colon separated values
646: .Dq start:rate:full
1.67 aaron 647: (e.g., "10:30:60").
1.57 markus 648: .Nm
1.86 stevesk 649: will refuse connection attempts with a probability of
1.57 markus 650: .Dq rate/100
651: (30%)
652: if there are currently
653: .Dq start
654: (10)
655: unauthenticated connections.
1.86 stevesk 656: The probability increases linearly and all connection attempts
1.57 markus 657: are refused if the number of unauthenticated connections reaches
658: .Dq full
659: (60).
1.2 deraadt 660: .It Cm PasswordAuthentication
1.1 deraadt 661: Specifies whether password authentication is allowed.
1.2 deraadt 662: The default is
663: .Dq yes .
664: .It Cm PermitEmptyPasswords
1.1 deraadt 665: When password authentication is allowed, it specifies whether the
1.36 aaron 666: server allows login to accounts with empty password strings.
667: The default is
1.34 markus 668: .Dq no .
1.2 deraadt 669: .It Cm PermitRootLogin
1.100 stevesk 670: Specifies whether root can login using
1.2 deraadt 671: .Xr ssh 1 .
1.15 markus 672: The argument must be
673: .Dq yes ,
1.94 markus 674: .Dq without-password ,
675: .Dq forced-commands-only
1.15 markus 676: or
677: .Dq no .
1.2 deraadt 678: The default is
679: .Dq yes .
1.94 markus 680: .Pp
681: If this option is set to
1.15 markus 682: .Dq without-password
1.94 markus 683: password authentication is disabled for root.
1.2 deraadt 684: .Pp
1.94 markus 685: If this option is set to
686: .Dq forced-commands-only
687: root login with public key authentication will be allowed,
688: but only if the
1.2 deraadt 689: .Ar command
1.94 markus 690: option has been specified
1.1 deraadt 691: (which may be useful for taking remote backups even if root login is
1.94 markus 692: normally not allowed). All other authentication methods are disabled
693: for root.
1.100 stevesk 694: .Pp
695: If this option is set to
696: .Dq no
697: root is not allowed to login.
1.43 markus 698: .It Cm PidFile
699: Specifies the file that contains the process identifier of the
700: .Nm
701: daemon.
702: The default is
703: .Pa /var/run/sshd.pid .
1.2 deraadt 704: .It Cm Port
1.1 deraadt 705: Specifies the port number that
1.2 deraadt 706: .Nm
1.36 aaron 707: listens on.
708: The default is 22.
1.28 markus 709: Multiple options of this type are permitted.
1.120 markus 710: See also
711: .Cm ListenAddress .
1.108 stevesk 712: .It Cm PrintLastLog
713: Specifies whether
714: .Nm
715: should print the date and time when the user last logged in.
716: The default is
717: .Dq yes .
1.2 deraadt 718: .It Cm PrintMotd
1.1 deraadt 719: Specifies whether
1.2 deraadt 720: .Nm
1.40 aaron 721: should print
1.2 deraadt 722: .Pa /etc/motd
1.36 aaron 723: when a user logs in interactively.
724: (On some systems it is also printed by the shell,
1.2 deraadt 725: .Pa /etc/profile ,
1.36 aaron 726: or equivalent.)
727: The default is
1.2 deraadt 728: .Dq yes .
1.41 markus 729: .It Cm Protocol
730: Specifies the protocol versions
731: .Nm
732: should support.
733: The possible values are
734: .Dq 1
735: and
736: .Dq 2 .
737: Multiple versions must be comma-separated.
738: The default is
1.118 deraadt 739: .Dq 2,1 .
1.104 deraadt 740: .It Cm PubkeyAuthentication
741: Specifies whether public key authentication is allowed.
742: The default is
743: .Dq yes .
744: Note that this option applies to protocol version 2 only.
1.87 markus 745: .It Cm ReverseMappingCheck
746: Specifies whether
747: .Nm
748: should try to verify the remote host name and check that
749: the resolved host name for the remote IP address maps back to the
750: very same IP address.
751: The default is
752: .Dq no .
1.2 deraadt 753: .It Cm RhostsAuthentication
1.1 deraadt 754: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 755: files is sufficient.
756: Normally, this method should not be permitted because it is insecure.
1.7 markus 757: .Cm RhostsRSAAuthentication
758: should be used
1.1 deraadt 759: instead, because it performs RSA-based host authentication in addition
760: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 761: The default is
762: .Dq no .
1.120 markus 763: This option applies to protocol version 1 only.
1.2 deraadt 764: .It Cm RhostsRSAAuthentication
1.1 deraadt 765: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 766: with successful RSA host authentication is allowed.
767: The default is
1.34 markus 768: .Dq no .
1.120 markus 769: This option applies to protocol version 1 only.
1.2 deraadt 770: .It Cm RSAAuthentication
1.36 aaron 771: Specifies whether pure RSA authentication is allowed.
772: The default is
1.2 deraadt 773: .Dq yes .
1.120 markus 774: This option applies to protocol version 1 only.
1.2 deraadt 775: .It Cm ServerKeyBits
1.120 markus 776: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 777: The minimum value is 512, and the default is 768.
1.2 deraadt 778: .It Cm StrictModes
1.12 markus 779: Specifies whether
780: .Nm
781: should check file modes and ownership of the
1.36 aaron 782: user's files and home directory before accepting login.
783: This is normally desirable because novices sometimes accidentally leave their
784: directory or files world-writable.
785: The default is
1.7 markus 786: .Dq yes .
1.54 jakob 787: .It Cm Subsystem
1.67 aaron 788: Configures an external subsystem (e.g., file transfer daemon).
789: Arguments should be a subsystem name and a command to execute upon subsystem
790: request.
1.63 markus 791: The command
792: .Xr sftp-server 8
793: implements the
794: .Dq sftp
795: file transfer subsystem.
1.54 jakob 796: By default no subsystems are defined.
797: Note that this option applies to protocol version 2 only.
1.2 deraadt 798: .It Cm SyslogFacility
1.1 deraadt 799: Gives the facility code that is used when logging messages from
1.2 deraadt 800: .Nm sshd .
1.1 deraadt 801: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 802: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
803: The default is AUTH.
1.10 markus 804: .It Cm UseLogin
805: Specifies whether
806: .Xr login 1
1.53 markus 807: is used for interactive login sessions.
1.127 markus 808: The default is
809: .Dq no .
1.53 markus 810: Note that
811: .Xr login 1
1.58 deraadt 812: is never used for remote command execution.
1.133 itojun 813: Note also, that if this is enabled,
814: .Cm X11Forwarding
1.127 markus 815: will be disabled because
816: .Xr login 1
817: does not know how to handle
1.133 itojun 818: .Xr xauth 1
1.127 markus 819: cookies.
1.6 aaron 820: .It Cm X11DisplayOffset
821: Specifies the first display number available for
822: .Nm sshd Ns 's
1.36 aaron 823: X11 forwarding.
824: This prevents
1.6 aaron 825: .Nm
826: from interfering with real X11 servers.
1.34 markus 827: The default is 10.
1.30 markus 828: .It Cm X11Forwarding
1.36 aaron 829: Specifies whether X11 forwarding is permitted.
830: The default is
1.34 markus 831: .Dq no .
1.30 markus 832: Note that disabling X11 forwarding does not improve security in any
833: way, as users can always install their own forwarders.
1.133 itojun 834: X11 forwarding is automatically disabled if
835: .Cm UseLogin
836: is enabled.
1.52 markus 837: .It Cm XAuthLocation
838: Specifies the location of the
839: .Xr xauth 1
840: program.
841: The default is
842: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 843: .El
1.124 stevesk 844: .Ss Time Formats
845: .Pp
846: .Nm
847: command-line arguments and configuration file options that specify time
848: may be expressed using a sequence of the form:
849: .Sm off
850: .Ar time Oo Ar qualifier Oc ,
851: .Sm on
852: where
853: .Ar time
854: is a positive integer value and
855: .Ar qualifier
856: is one of the following:
857: .Pp
858: .Bl -tag -width Ds -compact -offset indent
859: .It Cm <none>
860: seconds
861: .It Cm s | Cm S
862: seconds
863: .It Cm m | Cm M
864: minutes
865: .It Cm h | Cm H
866: hours
867: .It Cm d | Cm D
868: days
869: .It Cm w | Cm W
870: weeks
871: .El
872: .Pp
873: Each member of the sequence is added together to calculate
874: the total time value.
875: .Pp
876: Time format examples:
877: .Pp
878: .Bl -tag -width Ds -compact -offset indent
879: .It 600
880: 600 seconds (10 minutes)
881: .It 10m
882: 10 minutes
883: .It 1h30m
884: 1 hour 30 minutes (90 minutes)
885: .El
1.2 deraadt 886: .Sh LOGIN PROCESS
1.1 deraadt 887: When a user successfully logs in,
1.2 deraadt 888: .Nm
1.1 deraadt 889: does the following:
1.2 deraadt 890: .Bl -enum -offset indent
891: .It
1.1 deraadt 892: If the login is on a tty, and no command has been specified,
1.40 aaron 893: prints last login time and
1.2 deraadt 894: .Pa /etc/motd
1.1 deraadt 895: (unless prevented in the configuration file or by
1.2 deraadt 896: .Pa $HOME/.hushlogin ;
897: see the
1.40 aaron 898: .Sx FILES
1.2 deraadt 899: section).
900: .It
1.1 deraadt 901: If the login is on a tty, records login time.
1.2 deraadt 902: .It
903: Checks
904: .Pa /etc/nologin ;
905: if it exists, prints contents and quits
1.1 deraadt 906: (unless root).
1.2 deraadt 907: .It
1.1 deraadt 908: Changes to run with normal user privileges.
1.2 deraadt 909: .It
1.1 deraadt 910: Sets up basic environment.
1.2 deraadt 911: .It
912: Reads
913: .Pa $HOME/.ssh/environment
914: if it exists.
915: .It
1.1 deraadt 916: Changes to user's home directory.
1.2 deraadt 917: .It
918: If
919: .Pa $HOME/.ssh/rc
920: exists, runs it; else if
921: .Pa /etc/sshrc
922: exists, runs
1.36 aaron 923: it; otherwise runs xauth.
924: The
1.2 deraadt 925: .Dq rc
926: files are given the X11
1.1 deraadt 927: authentication protocol and cookie in standard input.
1.2 deraadt 928: .It
1.1 deraadt 929: Runs user's shell or command.
1.2 deraadt 930: .El
931: .Sh AUTHORIZED_KEYS FILE FORMAT
932: .Pa $HOME/.ssh/authorized_keys
1.130 markus 933: is the default file that lists the public keys that are
934: permitted for RSA authentication in protocol version 1
935: and for public key authentication (PubkeyAuthentication)
936: in protocol version 2.
1.125 markus 937: .Cm AuthorizedKeysFile
938: may be used to specify an alternative file.
1.75 markus 939: .Pp
1.36 aaron 940: Each line of the file contains one
1.2 deraadt 941: key (empty lines and lines starting with a
942: .Ql #
943: are ignored as
1.36 aaron 944: comments).
1.75 markus 945: Each RSA public key consists of the following fields, separated by
1.36 aaron 946: spaces: options, bits, exponent, modulus, comment.
1.75 markus 947: Each protocol version 2 public key consists of:
948: options, keytype, base64 encoded key, comment.
949: The options fields
950: are optional; its presence is determined by whether the line starts
1.1 deraadt 951: with a number or not (the option field never starts with a number).
1.75 markus 952: The bits, exponent, modulus and comment fields give the RSA key for
953: protocol version 1; the
1.1 deraadt 954: comment field is not used for anything (but may be convenient for the
955: user to identify the key).
1.75 markus 956: For protocol version 2 the keytype is
957: .Dq ssh-dss
958: or
959: .Dq ssh-rsa .
1.2 deraadt 960: .Pp
1.1 deraadt 961: Note that lines in this file are usually several hundred bytes long
1.36 aaron 962: (because of the size of the RSA key modulus).
963: You don't want to type them in; instead, copy the
1.113 itojun 964: .Pa identity.pub ,
965: .Pa id_dsa.pub
1.75 markus 966: or the
1.113 itojun 967: .Pa id_rsa.pub
1.1 deraadt 968: file and edit it.
1.2 deraadt 969: .Pp
1.58 deraadt 970: The options (if present) consist of comma-separated option
1.36 aaron 971: specifications.
972: No spaces are permitted, except within double quotes.
1.141 stevesk 973: The following option specifications are supported (note
974: that option keywords are case-insensitive):
1.2 deraadt 975: .Bl -tag -width Ds
976: .It Cm from="pattern-list"
1.1 deraadt 977: Specifies that in addition to RSA authentication, the canonical name
978: of the remote host must be present in the comma-separated list of
1.36 aaron 979: patterns
980: .Pf ( Ql *
981: and
982: .Ql ?
983: serve as wildcards).
984: The list may also contain
985: patterns negated by prefixing them with
986: .Ql ! ;
987: if the canonical host name matches a negated pattern, the key is not accepted.
988: The purpose
1.1 deraadt 989: of this option is to optionally increase security: RSA authentication
990: by itself does not trust the network or name servers or anything (but
991: the key); however, if somebody somehow steals the key, the key
1.36 aaron 992: permits an intruder to log in from anywhere in the world.
993: This additional option makes using a stolen key more difficult (name
1.1 deraadt 994: servers and/or routers would have to be compromised in addition to
995: just the key).
1.2 deraadt 996: .It Cm command="command"
1.1 deraadt 997: Specifies that the command is executed whenever this key is used for
1.36 aaron 998: authentication.
999: The command supplied by the user (if any) is ignored.
1.148 markus 1000: The command is run on a pty if the client requests a pty;
1.36 aaron 1001: otherwise it is run without a tty.
1.147 deraadt 1002: If a 8-bit clean channel is required,
1003: one must not request a pty or should specify
1.89 markus 1004: .Cm no-pty .
1.36 aaron 1005: A quote may be included in the command by quoting it with a backslash.
1006: This option might be useful
1007: to restrict certain RSA keys to perform just a specific operation.
1008: An example might be a key that permits remote backups but nothing else.
1.51 hugh 1009: Note that the client may specify TCP/IP and/or X11
1010: forwarding unless they are explicitly prohibited.
1.149 markus 1011: Note that this option applies to shell, command or subsystem execution.
1.2 deraadt 1012: .It Cm environment="NAME=value"
1.1 deraadt 1013: Specifies that the string is to be added to the environment when
1.36 aaron 1014: logging in using this key.
1015: Environment variables set this way
1016: override other default environment values.
1017: Multiple options of this type are permitted.
1.155 markus 1018: This option is automatically disabled if
1019: .Cm UseLogin
1020: is enabled.
1.2 deraadt 1021: .It Cm no-port-forwarding
1.1 deraadt 1022: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 1023: Any port forward requests by the client will return an error.
1024: This might be used, e.g., in connection with the
1.2 deraadt 1025: .Cm command
1.1 deraadt 1026: option.
1.2 deraadt 1027: .It Cm no-X11-forwarding
1.1 deraadt 1028: Forbids X11 forwarding when this key is used for authentication.
1029: Any X11 forward requests by the client will return an error.
1.2 deraadt 1030: .It Cm no-agent-forwarding
1.1 deraadt 1031: Forbids authentication agent forwarding when this key is used for
1032: authentication.
1.2 deraadt 1033: .It Cm no-pty
1.1 deraadt 1034: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 1035: .It Cm permitopen="host:port"
1.133 itojun 1036: Limit local
1.107 djm 1037: .Li ``ssh -L''
1.111 stevesk 1038: port forwarding such that it may only connect to the specified host and
1.146 stevesk 1039: port.
1040: IPv6 addresses can be specified with an alternative syntax:
1041: .Ar host/port .
1042: Multiple
1.107 djm 1043: .Cm permitopen
1.133 itojun 1044: options may be applied separated by commas. No pattern matching is
1045: performed on the specified hostnames, they must be literal domains or
1.107 djm 1046: addresses.
1.2 deraadt 1047: .El
1048: .Ss Examples
1.1 deraadt 1049: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1050: .Pp
1.1 deraadt 1051: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1052: .Pp
1.1 deraadt 1053: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 1054: .Pp
1055: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1056: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1057: The
1.44 deraadt 1058: .Pa /etc/ssh_known_hosts ,
1.40 aaron 1059: and
1.131 markus 1060: .Pa $HOME/.ssh/known_hosts
1.36 aaron 1061: files contain host public keys for all known hosts.
1062: The global file should
1.37 brad 1063: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1064: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1065: its key is added to the per-user file.
1.2 deraadt 1066: .Pp
1.1 deraadt 1067: Each line in these files contains the following fields: hostnames,
1.36 aaron 1068: bits, exponent, modulus, comment.
1069: The fields are separated by spaces.
1.2 deraadt 1070: .Pp
1.1 deraadt 1071: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1072: wildcards); each pattern in turn is matched against the canonical host
1073: name (when authenticating a client) or against the user-supplied
1.36 aaron 1074: name (when authenticating a server).
1075: A pattern may also be preceded by
1.2 deraadt 1076: .Ql !
1077: to indicate negation: if the host name matches a negated
1.1 deraadt 1078: pattern, it is not accepted (by that line) even if it matched another
1079: pattern on the line.
1.2 deraadt 1080: .Pp
1.49 markus 1081: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1082: can be obtained, e.g., from
1083: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 1084: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1085: .Pp
1086: Lines starting with
1087: .Ql #
1088: and empty lines are ignored as comments.
1089: .Pp
1.1 deraadt 1090: When performing host authentication, authentication is accepted if any
1.36 aaron 1091: matching line has the proper key.
1092: It is thus permissible (but not
1.1 deraadt 1093: recommended) to have several lines or different host keys for the same
1.36 aaron 1094: names.
1095: This will inevitably happen when short forms of host names
1096: from different domains are put in the file.
1097: It is possible
1.1 deraadt 1098: that the files contain conflicting information; authentication is
1099: accepted if valid information can be found from either file.
1.2 deraadt 1100: .Pp
1.1 deraadt 1101: Note that the lines in these files are typically hundreds of characters
1102: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1103: Rather, generate them by a script
1.40 aaron 1104: or by taking
1.2 deraadt 1105: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1106: and adding the host names at the front.
1.2 deraadt 1107: .Ss Examples
1.120 markus 1108: .Bd -literal
1109: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1110: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1111: .Ed
1.2 deraadt 1112: .Sh FILES
1113: .Bl -tag -width Ds
1114: .It Pa /etc/sshd_config
1.1 deraadt 1115: Contains configuration data for
1.2 deraadt 1116: .Nm sshd .
1.1 deraadt 1117: This file should be writable by root only, but it is recommended
1118: (though not necessary) that it be world-readable.
1.98 deraadt 1119: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.120 markus 1120: These three files contain the private parts of the host keys.
1.98 deraadt 1121: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1122: accessible to others.
1.14 markus 1123: Note that
1124: .Nm
1125: does not start if this file is group/world-accessible.
1.98 deraadt 1126: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.120 markus 1127: These three files contain the public parts of the host keys.
1.98 deraadt 1128: These files should be world-readable but writable only by
1.36 aaron 1129: root.
1.98 deraadt 1130: Their contents should match the respective private parts.
1131: These files are not
1132: really used for anything; they are provided for the convenience of
1133: the user so their contents can be copied to known hosts files.
1134: These files are created using
1.7 markus 1135: .Xr ssh-keygen 1 .
1.129 provos 1136: .It Pa /etc/moduli
1.73 provos 1137: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1138: .It Pa /var/run/sshd.pid
1139: Contains the process ID of the
1140: .Nm
1.1 deraadt 1141: listening for connections (if there are several daemons running
1142: concurrently for different ports, this contains the pid of the one
1.36 aaron 1143: started last).
1.58 deraadt 1144: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1145: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 1146: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1147: This file must be readable by root (which may on some machines imply
1148: it being world-readable if the user's home directory resides on an NFS
1149: volume).
1150: It is recommended that it not be accessible by others.
1151: The format of this file is described above.
1152: Users will place the contents of their
1.130 markus 1153: .Pa identity.pub ,
1.44 deraadt 1154: .Pa id_dsa.pub
1.113 itojun 1155: and/or
1156: .Pa id_rsa.pub
1.44 deraadt 1157: files into this file, as described in
1158: .Xr ssh-keygen 1 .
1.22 markus 1159: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1160: These files are consulted when using rhosts with RSA host
1.131 markus 1161: authentication or protocol version 2 hostbased authentication
1162: to check the public key of the host.
1.36 aaron 1163: The key must be listed in one of these files to be accepted.
1.22 markus 1164: The client uses the same files
1.96 markus 1165: to verify that it is connecting to the correct remote host.
1.36 aaron 1166: These files should be writable only by root/the owner.
1.2 deraadt 1167: .Pa /etc/ssh_known_hosts
1168: should be world-readable, and
1169: .Pa $HOME/.ssh/known_hosts
1.120 markus 1170: can but need not be world-readable.
1.6 aaron 1171: .It Pa /etc/nologin
1.40 aaron 1172: If this file exists,
1.2 deraadt 1173: .Nm
1.36 aaron 1174: refuses to let anyone except root log in.
1175: The contents of the file
1.1 deraadt 1176: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1177: refused.
1178: The file should be world-readable.
1.19 dugsong 1179: .It Pa /etc/hosts.allow, /etc/hosts.deny
1.153 camield 1180: Access controls that should be enforced by tcp-wrappers are defined here.
1181: Further details are described in
1.19 dugsong 1182: .Xr hosts_access 5 .
1.6 aaron 1183: .It Pa $HOME/.rhosts
1.1 deraadt 1184: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1185: line.
1186: The given user on the corresponding host is permitted to log in
1187: without password.
1188: The same file is used by rlogind and rshd.
1.6 aaron 1189: The file must
1.1 deraadt 1190: be writable only by the user; it is recommended that it not be
1191: accessible by others.
1.2 deraadt 1192: .Pp
1.36 aaron 1193: If is also possible to use netgroups in the file.
1194: Either host or user
1.1 deraadt 1195: name may be of the form +@groupname to specify all hosts or all users
1196: in the group.
1.2 deraadt 1197: .It Pa $HOME/.shosts
1198: For ssh,
1199: this file is exactly the same as for
1200: .Pa .rhosts .
1201: However, this file is
1202: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1203: .It Pa /etc/hosts.equiv
1.2 deraadt 1204: This file is used during
1205: .Pa .rhosts
1.36 aaron 1206: authentication.
1207: In the simplest form, this file contains host names, one per line.
1208: Users on
1.1 deraadt 1209: those hosts are permitted to log in without a password, provided they
1.36 aaron 1210: have the same user name on both machines.
1211: The host name may also be
1.1 deraadt 1212: followed by a user name; such users are permitted to log in as
1.2 deraadt 1213: .Em any
1.36 aaron 1214: user on this machine (except root).
1215: Additionally, the syntax
1.2 deraadt 1216: .Dq +@group
1.36 aaron 1217: can be used to specify netgroups.
1218: Negated entries start with
1.2 deraadt 1219: .Ql \&- .
1220: .Pp
1.1 deraadt 1221: If the client host/user is successfully matched in this file, login is
1222: automatically permitted provided the client and server user names are the
1.36 aaron 1223: same.
1224: Additionally, successful RSA host authentication is normally required.
1225: This file must be writable only by root; it is recommended
1.1 deraadt 1226: that it be world-readable.
1.2 deraadt 1227: .Pp
1.6 aaron 1228: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1229: .Pa hosts.equiv .
1.1 deraadt 1230: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1231: .Em anybody ,
1.1 deraadt 1232: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1233: binaries and directories.
1234: Using a user name practically grants the user root access.
1235: The only valid use for user names that I can think
1.1 deraadt 1236: of is in negative entries.
1.2 deraadt 1237: .Pp
1238: Note that this warning also applies to rsh/rlogin.
1239: .It Pa /etc/shosts.equiv
1.1 deraadt 1240: This is processed exactly as
1.2 deraadt 1241: .Pa /etc/hosts.equiv .
1.1 deraadt 1242: However, this file may be useful in environments that want to run both
1.2 deraadt 1243: rsh/rlogin and ssh.
1.6 aaron 1244: .It Pa $HOME/.ssh/environment
1.36 aaron 1245: This file is read into the environment at login (if it exists).
1246: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1247: .Ql # ) ,
1.36 aaron 1248: and assignment lines of the form name=value.
1249: The file should be writable
1.6 aaron 1250: only by the user; it need not be readable by anyone else.
1.2 deraadt 1251: .It Pa $HOME/.ssh/rc
1.1 deraadt 1252: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1253: environment files but before starting the user's shell or command.
1254: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1255: standard input (and
1256: .Ev DISPLAY
1.36 aaron 1257: in environment).
1258: This must call
1.2 deraadt 1259: .Xr xauth 1
1260: in that case.
1261: .Pp
1.1 deraadt 1262: The primary purpose of this file is to run any initialization routines
1263: which may be needed before the user's home directory becomes
1264: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1265: .Pp
1.1 deraadt 1266: This file will probably contain some initialization code followed by
1.120 markus 1267: something similar to:
1268: .Bd -literal
1269: if read proto cookie; then
1270: echo add $DISPLAY $proto $cookie | xauth -q -
1271: fi
1272: .Ed
1.2 deraadt 1273: .Pp
1274: If this file does not exist,
1275: .Pa /etc/sshrc
1276: is run, and if that
1.1 deraadt 1277: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1278: .Pp
1.1 deraadt 1279: This file should be writable only by the user, and need not be
1280: readable by anyone else.
1.2 deraadt 1281: .It Pa /etc/sshrc
1282: Like
1283: .Pa $HOME/.ssh/rc .
1284: This can be used to specify
1.36 aaron 1285: machine-specific login-time initializations globally.
1286: This file should be writable only by root, and should be world-readable.
1.56 aaron 1287: .El
1.71 aaron 1288: .Sh AUTHORS
1.84 markus 1289: OpenSSH is a derivative of the original and free
1290: ssh 1.2.12 release by Tatu Ylonen.
1291: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1292: Theo de Raadt and Dug Song
1293: removed many bugs, re-added newer features and
1294: created OpenSSH.
1295: Markus Friedl contributed the support for SSH
1296: protocol versions 1.5 and 2.0.
1.2 deraadt 1297: .Sh SEE ALSO
1298: .Xr scp 1 ,
1.90 djm 1299: .Xr sftp 1 ,
1.2 deraadt 1300: .Xr ssh 1 ,
1.5 deraadt 1301: .Xr ssh-add 1 ,
1.2 deraadt 1302: .Xr ssh-agent 1 ,
1.5 deraadt 1303: .Xr ssh-keygen 1 ,
1.136 markus 1304: .Xr login.conf 5 ,
1305: .Xr moduli 5 ,
1.128 mpech 1306: .Xr sftp-server 8
1.119 markus 1307: .Rs
1308: .%A T. Ylonen
1309: .%A T. Kivinen
1310: .%A M. Saarinen
1311: .%A T. Rinne
1312: .%A S. Lehtinen
1313: .%T "SSH Protocol Architecture"
1.139 markus 1314: .%N draft-ietf-secsh-architecture-09.txt
1315: .%D July 2001
1.120 markus 1316: .%O work in progress material
1317: .Re
1318: .Rs
1319: .%A M. Friedl
1320: .%A N. Provos
1321: .%A W. A. Simpson
1322: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.132 markus 1323: .%N draft-ietf-secsh-dh-group-exchange-01.txt
1324: .%D April 2001
1.119 markus 1325: .%O work in progress material
1326: .Re
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