Annotation of src/usr.bin/ssh/sshd.8, Revision 1.137
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,
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35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.137 ! stevesk 37: .\" $OpenBSD: sshd.8,v 1.136 2001/07/22 22:24:16 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
1.114 lebel 46: .Op Fl deiqD46
1.2 deraadt 47: .Op Fl b Ar bits
48: .Op Fl f Ar config_file
49: .Op Fl g Ar login_grace_time
50: .Op Fl h Ar host_key_file
51: .Op Fl k Ar key_gen_time
52: .Op Fl p Ar port
1.61 markus 53: .Op Fl u Ar len
1.26 markus 54: .Op Fl V Ar client_protocol_id
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: .Xr rexecd 8 ,
122: and
123: .Xr rexd 8
1.1 deraadt 124: are disabled (thus completely disabling
1.2 deraadt 125: .Xr rlogin 1
1.1 deraadt 126: and
1.2 deraadt 127: .Xr rsh 1
1.42 hugh 128: into the machine).
1.2 deraadt 129: .Pp
1.49 markus 130: .Ss SSH protocol version 2
131: .Pp
1.58 deraadt 132: Version 2 works similarly:
1.49 markus 133: Each host has a host-specific DSA key used to identify the host.
134: However, when the daemon starts, it does not generate a server key.
135: Forward security is provided through a Diffie-Hellman key agreement.
136: This key agreement results in a shared session key.
1.120 markus 137: .Pp
1.103 deraadt 138: The rest of the session is encrypted using a symmetric cipher, currently
1.120 markus 139: 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
1.49 markus 140: The client selects the encryption algorithm
141: to use from those offered by the server.
142: Additionally, session integrity is provided
1.51 hugh 143: through a cryptographic message authentication code
1.49 markus 144: (hmac-sha1 or hmac-md5).
145: .Pp
146: Protocol version 2 provides a public key based
1.120 markus 147: user (PubkeyAuthentication) or
148: client host (HostbasedAuthentication) authentication method,
149: conventional password authentication and challenge response based methods.
1.49 markus 150: .Pp
151: .Ss Command execution and data forwarding
152: .Pp
1.1 deraadt 153: If the client successfully authenticates itself, a dialog for
1.36 aaron 154: preparing the session is entered.
155: At this time the client may request
1.1 deraadt 156: things like allocating a pseudo-tty, forwarding X11 connections,
157: forwarding TCP/IP connections, or forwarding the authentication agent
158: connection over the secure channel.
1.2 deraadt 159: .Pp
1.1 deraadt 160: Finally, the client either requests a shell or execution of a command.
1.36 aaron 161: The sides then enter session mode.
162: In this mode, either side may send
1.1 deraadt 163: data at any time, and such data is forwarded to/from the shell or
164: command on the server side, and the user terminal in the client side.
1.2 deraadt 165: .Pp
1.1 deraadt 166: When the user program terminates and all forwarded X11 and other
167: connections have been closed, the server sends command exit status to
168: the client, and both sides exit.
1.2 deraadt 169: .Pp
170: .Nm
1.1 deraadt 171: can be configured using command-line options or a configuration
1.36 aaron 172: file.
173: Command-line options override values specified in the
1.1 deraadt 174: configuration file.
1.25 markus 175: .Pp
176: .Nm
177: rereads its configuration file when it receives a hangup signal,
1.97 deraadt 178: .Dv SIGHUP ,
1.128 mpech 179: by executing itself with the name it was started as, i.e.,
1.97 deraadt 180: .Pa /usr/sbin/sshd .
1.18 aaron 181: .Pp
182: The options are as follows:
1.2 deraadt 183: .Bl -tag -width Ds
184: .It Fl b Ar bits
1.120 markus 185: Specifies the number of bits in the ephemeral protocol version 1
186: server key (default 768).
1.2 deraadt 187: .It Fl d
1.36 aaron 188: Debug mode.
189: The server sends verbose debug output to the system
190: log, and does not put itself in the background.
191: The server also will not fork and will only process one connection.
192: This option is only intended for debugging for the server.
1.120 markus 193: Multiple -d options increase the debugging level.
1.67 aaron 194: Maximum is 3.
1.120 markus 195: .It Fl e
196: When this option is specified,
197: .Nm
198: will send the output to the standard error instead of the system log.
1.2 deraadt 199: .It Fl f Ar configuration_file
1.36 aaron 200: Specifies the name of the configuration file.
201: The default is
1.2 deraadt 202: .Pa /etc/sshd_config .
1.16 markus 203: .Nm
204: refuses to start if there is no configuration file.
1.2 deraadt 205: .It Fl g Ar login_grace_time
1.1 deraadt 206: Gives the grace time for clients to authenticate themselves (default
1.77 markus 207: 600 seconds).
1.36 aaron 208: If the client fails to authenticate the user within
209: this many seconds, the server disconnects and exits.
210: A value of zero indicates no limit.
1.2 deraadt 211: .It Fl h Ar host_key_file
1.75 markus 212: Specifies the file from which the host key is read (default
1.2 deraadt 213: .Pa /etc/ssh_host_key ) .
1.7 markus 214: This option must be given if
215: .Nm
216: is not run as root (as the normal
1.1 deraadt 217: host file is normally not readable by anyone but root).
1.75 markus 218: It is possible to have multiple host key files for
1.120 markus 219: the different protocol versions and host key algorithms.
1.2 deraadt 220: .It Fl i
1.7 markus 221: Specifies that
222: .Nm
1.40 aaron 223: is being run from inetd.
1.7 markus 224: .Nm
225: is normally not run
1.1 deraadt 226: from inetd because it needs to generate the server key before it can
1.36 aaron 227: respond to the client, and this may take tens of seconds.
228: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 229: However, with small key sizes (e.g., 512) using
1.7 markus 230: .Nm
231: from inetd may
1.1 deraadt 232: be feasible.
1.2 deraadt 233: .It Fl k Ar key_gen_time
1.120 markus 234: Specifies how often the ephemeral protocol version 1 server key is
235: regenerated (default 3600 seconds, or one hour).
1.36 aaron 236: The motivation for regenerating the key fairly
1.1 deraadt 237: often is that the key is not stored anywhere, and after about an hour,
238: it becomes impossible to recover the key for decrypting intercepted
239: communications even if the machine is cracked into or physically
1.36 aaron 240: seized.
241: A value of zero indicates that the key will never be regenerated.
1.2 deraadt 242: .It Fl p Ar port
1.1 deraadt 243: Specifies the port on which the server listens for connections
244: (default 22).
1.2 deraadt 245: .It Fl q
1.36 aaron 246: Quiet mode.
247: Nothing is sent to the system log.
248: Normally the beginning,
1.1 deraadt 249: authentication, and termination of each connection is logged.
1.137 ! stevesk 250: .It Fl t
! 251: Test mode.
! 252: Only check the validity of the configuration file and sanity of the keys.
! 253: This is useful for updating
! 254: .Nm
! 255: reliably as configuration options may change.
1.61 markus 256: .It Fl u Ar len
257: This option is used to specify the size of the field
258: in the
259: .Li utmp
260: structure that holds the remote host name.
261: If the resolved host name is longer than
262: .Ar len ,
263: the dotted decimal value will be used instead.
264: This allows hosts with very long host names that
265: overflow this field to still be uniquely identified.
266: Specifying
267: .Fl u0
268: indicates that only dotted decimal addresses
269: should be put into the
270: .Pa utmp
271: file.
1.74 markus 272: .It Fl D
273: When this option is specified
274: .Nm
275: will not detach and does not become a daemon.
276: This allows easy monitoring of
1.76 markus 277: .Nm sshd .
1.29 markus 278: .It Fl 4
279: Forces
280: .Nm
281: to use IPv4 addresses only.
282: .It Fl 6
283: Forces
284: .Nm
285: to use IPv6 addresses only.
1.2 deraadt 286: .El
287: .Sh CONFIGURATION FILE
288: .Nm
1.40 aaron 289: reads configuration data from
1.2 deraadt 290: .Pa /etc/sshd_config
291: (or the file specified with
292: .Fl f
1.36 aaron 293: on the command line).
294: The file contains keyword-value pairs, one per line.
295: Lines starting with
1.2 deraadt 296: .Ql #
1.1 deraadt 297: and empty lines are interpreted as comments.
1.2 deraadt 298: .Pp
1.1 deraadt 299: The following keywords are possible.
1.2 deraadt 300: .Bl -tag -width Ds
301: .It Cm AFSTokenPassing
1.36 aaron 302: Specifies whether an AFS token may be forwarded to the server.
303: Default is
1.2 deraadt 304: .Dq yes .
1.11 markus 305: .It Cm AllowGroups
1.92 deraadt 306: This keyword can be followed by a list of group names, separated
1.36 aaron 307: by spaces.
308: If specified, login is allowed only for users whose primary
1.81 markus 309: group or supplementary group list matches one of the patterns.
1.11 markus 310: .Ql \&*
311: and
312: .Ql ?
313: can be used as
1.36 aaron 314: wildcards in the patterns.
1.58 deraadt 315: Only group names are valid; a numerical group ID isn't recognized.
1.81 markus 316: By default login is allowed regardless of the group list.
1.3 dugsong 317: .Pp
1.69 markus 318: .It Cm AllowTcpForwarding
319: Specifies whether TCP forwarding is permitted.
320: The default is
321: .Dq yes .
322: Note that disabling TCP forwarding does not improve security unless
323: users are also denied shell access, as they can always install their
324: own forwarders.
325: .Pp
1.11 markus 326: .It Cm AllowUsers
1.92 deraadt 327: This keyword can be followed by a list of user names, separated
1.36 aaron 328: by spaces.
329: If specified, login is allowed only for users names that
1.11 markus 330: match one of the patterns.
331: .Ql \&*
332: and
333: .Ql ?
334: can be used as
1.36 aaron 335: wildcards in the patterns.
1.58 deraadt 336: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 337: By default login is allowed regardless of the user name.
1.135 markus 338: If the pattern takes the form USER@HOST then USER and HOST
339: are separately checked, allowing you to restrict logins to particular
340: users from particular hosts.
1.80 markus 341: .Pp
1.125 markus 342: .It Cm AuthorizedKeysFile
343: Specifies the file that contains the public RSA keys that can be used
344: for RSA authentication in protocol version 1.
345: .Cm AuthorizedKeysFile
346: may contain tokens of the form %T which are substituted during connection
347: set-up. The following tokens are defined; %% is replaces by a literal '%',
348: %h is replaced by the home directory of the user being authenticated and
349: %u is replaced by the username of that user.
350: After expansion,
351: .Cm AuthorizedKeysFile
1.126 markus 352: is taken to be an absolute path or one relative to the user's home
1.125 markus 353: directory.
354: The default is
355: .Dq .ssh/authorized_keys
1.80 markus 356: .It Cm Banner
357: In some jurisdictions, sending a warning message before authentication
358: may be relevant for getting legal protection.
359: The contents of the specified file are sent to the remote user before
360: authentication is allowed.
361: This option is only available for protocol version 2.
1.11 markus 362: .Pp
1.104 deraadt 363: .It Cm ChallengeResponseAuthentication
1.136 markus 364: Specifies whether challenge response authentication is allowed.
365: All authentication styles from
366: .Xr login.conf 5
367: are supported.
1.104 deraadt 368: The default is
369: .Dq yes .
1.8 markus 370: .It Cm CheckMail
371: Specifies whether
372: .Nm
373: should check for new mail for interactive logins.
374: The default is
375: .Dq no .
1.122 markus 376: .It Cm Ciphers
377: Specifies the ciphers allowed for protocol version 2.
378: Multiple ciphers must be comma-separated.
379: The default is
380: .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
1.115 beck 381: .It Cm ClientAliveInterval
382: Sets a timeout interval in seconds after which if no data has been received
1.133 itojun 383: from the client,
1.115 beck 384: .Nm
385: will send a message through the encrypted
1.116 stevesk 386: channel to request a response from the client.
387: The default
1.115 beck 388: is 0, indicating that these messages will not be sent to the client.
1.116 stevesk 389: This option applies to protocol version 2 only.
1.115 beck 390: .It Cm ClientAliveCountMax
391: Sets the number of client alive messages (see above) which may be
392: sent without
393: .Nm
394: receiving any messages back from the client. If this threshold is
1.133 itojun 395: reached while client alive messages are being sent,
1.115 beck 396: .Nm
397: will disconnect the client, terminating the session. It is important
1.133 itojun 398: to note that the use of client alive messages is very different from
1.116 stevesk 399: .Cm Keepalive
400: (below). The client alive messages are sent through the
1.115 beck 401: encrypted channel and therefore will not be spoofable. The TCP keepalive
1.116 stevesk 402: option enabled by
403: .Cm Keepalive
404: is spoofable. You want to use the client
1.115 beck 405: alive mechanism when you are basing something important on
406: clients having an active connection to the server.
1.116 stevesk 407: .Pp
408: The default value is 3. If you set
409: .Cm ClientAliveInterval
1.115 beck 410: (above) to 15, and leave this value at the default, unresponsive ssh clients
1.133 itojun 411: will be disconnected after approximately 45 seconds.
1.11 markus 412: .It Cm DenyGroups
413: This keyword can be followed by a number of group names, separated
1.36 aaron 414: by spaces.
1.81 markus 415: Users whose primary group or supplementary group list matches
416: one of the patterns aren't allowed to log in.
1.11 markus 417: .Ql \&*
418: and
419: .Ql ?
420: can be used as
1.36 aaron 421: wildcards in the patterns.
1.58 deraadt 422: Only group names are valid; a numerical group ID isn't recognized.
1.81 markus 423: By default login is allowed regardless of the group list.
1.11 markus 424: .Pp
425: .It Cm DenyUsers
426: This keyword can be followed by a number of user names, separated
1.36 aaron 427: by spaces.
428: Login is disallowed for user names that match one of the patterns.
1.11 markus 429: .Ql \&*
430: and
431: .Ql ?
1.36 aaron 432: can be used as wildcards in the patterns.
1.58 deraadt 433: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 434: By default login is allowed regardless of the user name.
1.47 markus 435: .It Cm GatewayPorts
436: Specifies whether remote hosts are allowed to connect to ports
437: forwarded for the client.
438: The argument must be
439: .Dq yes
440: or
441: .Dq no .
442: The default is
443: .Dq no .
1.120 markus 444: .It Cm HostbasedAuthentication
445: Specifies whether rhosts or /etc/hosts.equiv authentication together
446: with successful public key client host authentication is allowed
447: (hostbased authentication).
448: This option is similar to
449: .Cm RhostsRSAAuthentication
450: and applies to protocol version 2 only.
451: The default is
452: .Dq no .
1.2 deraadt 453: .It Cm HostKey
1.72 markus 454: Specifies the file containing the private host keys (default
1.46 markus 455: .Pa /etc/ssh_host_key )
1.72 markus 456: used by SSH protocol versions 1 and 2.
1.9 markus 457: Note that
458: .Nm
1.83 markus 459: will refuse to use a file if it is group/world-accessible.
1.72 markus 460: It is possible to have multiple host key files.
461: .Dq rsa1
462: keys are used for version 1 and
463: .Dq dsa
464: or
465: .Dq rsa
466: are used for version 2 of the SSH protocol.
1.2 deraadt 467: .It Cm IgnoreRhosts
1.34 markus 468: Specifies that
469: .Pa .rhosts
1.40 aaron 470: and
1.34 markus 471: .Pa .shosts
1.120 markus 472: files will not be used in
473: .Cm RhostsAuthentication ,
474: .Cm RhostsRSAAuthentication
475: or
476: .Cm HostbasedAuthentication .
477: .Pp
1.2 deraadt 478: .Pa /etc/hosts.equiv
1.1 deraadt 479: and
1.40 aaron 480: .Pa /etc/shosts.equiv
1.36 aaron 481: are still used.
1.40 aaron 482: The default is
1.34 markus 483: .Dq yes .
1.24 markus 484: .It Cm IgnoreUserKnownHosts
485: Specifies whether
486: .Nm
487: should ignore the user's
488: .Pa $HOME/.ssh/known_hosts
1.45 markus 489: during
1.120 markus 490: .Cm RhostsRSAAuthentication
491: or
492: .Cm HostbasedAuthentication .
1.24 markus 493: The default is
1.2 deraadt 494: .Dq no .
495: .It Cm KeepAlive
1.1 deraadt 496: Specifies whether the system should send keepalive messages to the
1.36 aaron 497: other side.
498: If they are sent, death of the connection or crash of one
499: of the machines will be properly noticed.
500: However, this means that
1.1 deraadt 501: connections will die if the route is down temporarily, and some people
1.36 aaron 502: find it annoying.
1.51 hugh 503: On the other hand, if keepalives are not sent,
1.2 deraadt 504: sessions may hang indefinitely on the server, leaving
505: .Dq ghost
506: users and consuming server resources.
507: .Pp
508: The default is
509: .Dq yes
510: (to send keepalives), and the server will notice
1.36 aaron 511: if the network goes down or the client host reboots.
512: This avoids infinitely hanging sessions.
1.2 deraadt 513: .Pp
514: To disable keepalives, the value should be set to
515: .Dq no
516: in both the server and the client configuration files.
517: .It Cm KerberosAuthentication
1.36 aaron 518: Specifies whether Kerberos authentication is allowed.
519: This can be in the form of a Kerberos ticket, or if
1.7 markus 520: .Cm PasswordAuthentication
1.1 deraadt 521: is yes, the password provided by the user will be validated through
1.67 aaron 522: the Kerberos KDC.
523: To use this option, the server needs a
1.59 provos 524: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 525: Default is
1.60 provos 526: .Dq yes .
1.2 deraadt 527: .It Cm KerberosOrLocalPasswd
1.1 deraadt 528: If set then if password authentication through Kerberos fails then
529: the password will be validated via any additional local mechanism
1.2 deraadt 530: such as
1.66 markus 531: .Pa /etc/passwd .
1.36 aaron 532: Default is
1.20 dugsong 533: .Dq yes .
1.2 deraadt 534: .It Cm KerberosTgtPassing
1.1 deraadt 535: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 536: Default is
1.3 dugsong 537: .Dq no ,
538: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 539: .It Cm KerberosTicketCleanup
1.7 markus 540: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 541: file on logout.
542: Default is
1.3 dugsong 543: .Dq yes .
1.2 deraadt 544: .It Cm KeyRegenerationInterval
1.120 markus 545: In protocol version 1, the ephemeral server key is automatically regenerated
546: after this many seconds (if it has been used).
1.36 aaron 547: The purpose of regeneration is to prevent
1.1 deraadt 548: decrypting captured sessions by later breaking into the machine and
1.36 aaron 549: stealing the keys.
550: The key is never stored anywhere.
551: If the value is 0, the key is never regenerated.
552: The default is 3600 (seconds).
1.7 markus 553: .It Cm ListenAddress
1.110 stevesk 554: Specifies the local addresses
1.120 markus 555: .Nm
1.7 markus 556: should listen on.
1.110 stevesk 557: The following forms may be used:
558: .Pp
559: .Bl -item -offset indent -compact
560: .It
561: .Cm ListenAddress
1.112 stevesk 562: .Sm off
563: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
564: .Sm on
1.110 stevesk 565: .It
566: .Cm ListenAddress
1.112 stevesk 567: .Sm off
568: .Ar host No | Ar IPv4_addr No : Ar port
569: .Sm on
1.110 stevesk 570: .It
571: .Cm ListenAddress
1.112 stevesk 572: .Sm off
573: .Oo
574: .Ar host No | Ar IPv6_addr Oc : Ar port
575: .Sm on
1.110 stevesk 576: .El
577: .Pp
578: If
1.112 stevesk 579: .Ar port
1.110 stevesk 580: is not specified,
1.120 markus 581: .Nm
1.110 stevesk 582: will listen on the address and all prior
583: .Cm Port
584: options specified. The default is to listen on all local
585: addresses. Multiple
586: .Cm ListenAddress
587: options are permitted. Additionally, any
588: .Cm Port
589: options must precede this option for non port qualified addresses.
1.2 deraadt 590: .It Cm LoginGraceTime
1.1 deraadt 591: The server disconnects after this time if the user has not
1.36 aaron 592: successfully logged in.
593: If the value is 0, there is no time limit.
1.1 deraadt 594: The default is 600 (seconds).
1.23 markus 595: .It Cm LogLevel
596: Gives the verbosity level that is used when logging messages from
597: .Nm sshd .
598: The possible values are:
1.82 markus 599: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
600: The default is INFO.
1.23 markus 601: Logging with level DEBUG violates the privacy of users
602: and is not recommended.
1.93 markus 603: .It Cm MACs
604: Specifies the available MAC (message authentication code) algorithms.
605: The MAC algorithm is used in protocol version 2
606: for data integrity protection.
607: Multiple algorithms must be comma-separated.
608: The default is
1.123 markus 609: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 610: .It Cm MaxStartups
611: Specifies the maximum number of concurrent unauthenticated connections to the
612: .Nm
613: daemon.
614: Additional connections will be dropped until authentication succeeds or the
615: .Cm LoginGraceTime
616: expires for a connection.
617: The default is 10.
1.57 markus 618: .Pp
619: Alternatively, random early drop can be enabled by specifying
620: the three colon separated values
621: .Dq start:rate:full
1.67 aaron 622: (e.g., "10:30:60").
1.57 markus 623: .Nm
1.86 stevesk 624: will refuse connection attempts with a probability of
1.57 markus 625: .Dq rate/100
626: (30%)
627: if there are currently
628: .Dq start
629: (10)
630: unauthenticated connections.
1.86 stevesk 631: The probability increases linearly and all connection attempts
1.57 markus 632: are refused if the number of unauthenticated connections reaches
633: .Dq full
634: (60).
1.2 deraadt 635: .It Cm PasswordAuthentication
1.1 deraadt 636: Specifies whether password authentication is allowed.
1.2 deraadt 637: The default is
638: .Dq yes .
639: .It Cm PermitEmptyPasswords
1.1 deraadt 640: When password authentication is allowed, it specifies whether the
1.36 aaron 641: server allows login to accounts with empty password strings.
642: The default is
1.34 markus 643: .Dq no .
1.2 deraadt 644: .It Cm PermitRootLogin
1.100 stevesk 645: Specifies whether root can login using
1.2 deraadt 646: .Xr ssh 1 .
1.15 markus 647: The argument must be
648: .Dq yes ,
1.94 markus 649: .Dq without-password ,
650: .Dq forced-commands-only
1.15 markus 651: or
652: .Dq no .
1.2 deraadt 653: The default is
654: .Dq yes .
1.94 markus 655: .Pp
656: If this option is set to
1.15 markus 657: .Dq without-password
1.94 markus 658: password authentication is disabled for root.
1.2 deraadt 659: .Pp
1.94 markus 660: If this option is set to
661: .Dq forced-commands-only
662: root login with public key authentication will be allowed,
663: but only if the
1.2 deraadt 664: .Ar command
1.94 markus 665: option has been specified
1.1 deraadt 666: (which may be useful for taking remote backups even if root login is
1.94 markus 667: normally not allowed). All other authentication methods are disabled
668: for root.
1.100 stevesk 669: .Pp
670: If this option is set to
671: .Dq no
672: root is not allowed to login.
1.43 markus 673: .It Cm PidFile
674: Specifies the file that contains the process identifier of the
675: .Nm
676: daemon.
677: The default is
678: .Pa /var/run/sshd.pid .
1.2 deraadt 679: .It Cm Port
1.1 deraadt 680: Specifies the port number that
1.2 deraadt 681: .Nm
1.36 aaron 682: listens on.
683: The default is 22.
1.28 markus 684: Multiple options of this type are permitted.
1.120 markus 685: See also
686: .Cm ListenAddress .
1.108 stevesk 687: .It Cm PrintLastLog
688: Specifies whether
689: .Nm
690: should print the date and time when the user last logged in.
691: The default is
692: .Dq yes .
1.2 deraadt 693: .It Cm PrintMotd
1.1 deraadt 694: Specifies whether
1.2 deraadt 695: .Nm
1.40 aaron 696: should print
1.2 deraadt 697: .Pa /etc/motd
1.36 aaron 698: when a user logs in interactively.
699: (On some systems it is also printed by the shell,
1.2 deraadt 700: .Pa /etc/profile ,
1.36 aaron 701: or equivalent.)
702: The default is
1.2 deraadt 703: .Dq yes .
1.41 markus 704: .It Cm Protocol
705: Specifies the protocol versions
706: .Nm
707: should support.
708: The possible values are
709: .Dq 1
710: and
711: .Dq 2 .
712: Multiple versions must be comma-separated.
713: The default is
1.118 deraadt 714: .Dq 2,1 .
1.104 deraadt 715: .It Cm PubkeyAuthentication
716: Specifies whether public key authentication is allowed.
717: The default is
718: .Dq yes .
719: Note that this option applies to protocol version 2 only.
1.87 markus 720: .It Cm ReverseMappingCheck
721: Specifies whether
722: .Nm
723: should try to verify the remote host name and check that
724: the resolved host name for the remote IP address maps back to the
725: very same IP address.
726: The default is
727: .Dq no .
1.2 deraadt 728: .It Cm RhostsAuthentication
1.1 deraadt 729: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 730: files is sufficient.
731: Normally, this method should not be permitted because it is insecure.
1.7 markus 732: .Cm RhostsRSAAuthentication
733: should be used
1.1 deraadt 734: instead, because it performs RSA-based host authentication in addition
735: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 736: The default is
737: .Dq no .
1.120 markus 738: This option applies to protocol version 1 only.
1.2 deraadt 739: .It Cm RhostsRSAAuthentication
1.1 deraadt 740: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 741: with successful RSA host authentication is allowed.
742: The default is
1.34 markus 743: .Dq no .
1.120 markus 744: This option applies to protocol version 1 only.
1.2 deraadt 745: .It Cm RSAAuthentication
1.36 aaron 746: Specifies whether pure RSA authentication is allowed.
747: The default is
1.2 deraadt 748: .Dq yes .
1.120 markus 749: This option applies to protocol version 1 only.
1.2 deraadt 750: .It Cm ServerKeyBits
1.120 markus 751: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 752: The minimum value is 512, and the default is 768.
1.2 deraadt 753: .It Cm StrictModes
1.12 markus 754: Specifies whether
755: .Nm
756: should check file modes and ownership of the
1.36 aaron 757: user's files and home directory before accepting login.
758: This is normally desirable because novices sometimes accidentally leave their
759: directory or files world-writable.
760: The default is
1.7 markus 761: .Dq yes .
1.54 jakob 762: .It Cm Subsystem
1.67 aaron 763: Configures an external subsystem (e.g., file transfer daemon).
764: Arguments should be a subsystem name and a command to execute upon subsystem
765: request.
1.63 markus 766: The command
767: .Xr sftp-server 8
768: implements the
769: .Dq sftp
770: file transfer subsystem.
1.54 jakob 771: By default no subsystems are defined.
772: Note that this option applies to protocol version 2 only.
1.2 deraadt 773: .It Cm SyslogFacility
1.1 deraadt 774: Gives the facility code that is used when logging messages from
1.2 deraadt 775: .Nm sshd .
1.1 deraadt 776: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 777: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
778: The default is AUTH.
1.10 markus 779: .It Cm UseLogin
780: Specifies whether
781: .Xr login 1
1.53 markus 782: is used for interactive login sessions.
1.127 markus 783: The default is
784: .Dq no .
1.53 markus 785: Note that
786: .Xr login 1
1.58 deraadt 787: is never used for remote command execution.
1.133 itojun 788: Note also, that if this is enabled,
789: .Cm X11Forwarding
1.127 markus 790: will be disabled because
791: .Xr login 1
792: does not know how to handle
1.133 itojun 793: .Xr xauth 1
1.127 markus 794: cookies.
1.6 aaron 795: .It Cm X11DisplayOffset
796: Specifies the first display number available for
797: .Nm sshd Ns 's
1.36 aaron 798: X11 forwarding.
799: This prevents
1.6 aaron 800: .Nm
801: from interfering with real X11 servers.
1.34 markus 802: The default is 10.
1.30 markus 803: .It Cm X11Forwarding
1.36 aaron 804: Specifies whether X11 forwarding is permitted.
805: The default is
1.34 markus 806: .Dq no .
1.30 markus 807: Note that disabling X11 forwarding does not improve security in any
808: way, as users can always install their own forwarders.
1.133 itojun 809: X11 forwarding is automatically disabled if
810: .Cm UseLogin
811: is enabled.
1.52 markus 812: .It Cm XAuthLocation
813: Specifies the location of the
814: .Xr xauth 1
815: program.
816: The default is
817: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 818: .El
1.124 stevesk 819: .Ss Time Formats
820: .Pp
821: .Nm
822: command-line arguments and configuration file options that specify time
823: may be expressed using a sequence of the form:
824: .Sm off
825: .Ar time Oo Ar qualifier Oc ,
826: .Sm on
827: where
828: .Ar time
829: is a positive integer value and
830: .Ar qualifier
831: is one of the following:
832: .Pp
833: .Bl -tag -width Ds -compact -offset indent
834: .It Cm <none>
835: seconds
836: .It Cm s | Cm S
837: seconds
838: .It Cm m | Cm M
839: minutes
840: .It Cm h | Cm H
841: hours
842: .It Cm d | Cm D
843: days
844: .It Cm w | Cm W
845: weeks
846: .El
847: .Pp
848: Each member of the sequence is added together to calculate
849: the total time value.
850: .Pp
851: Time format examples:
852: .Pp
853: .Bl -tag -width Ds -compact -offset indent
854: .It 600
855: 600 seconds (10 minutes)
856: .It 10m
857: 10 minutes
858: .It 1h30m
859: 1 hour 30 minutes (90 minutes)
860: .El
1.2 deraadt 861: .Sh LOGIN PROCESS
1.1 deraadt 862: When a user successfully logs in,
1.2 deraadt 863: .Nm
1.1 deraadt 864: does the following:
1.2 deraadt 865: .Bl -enum -offset indent
866: .It
1.1 deraadt 867: If the login is on a tty, and no command has been specified,
1.40 aaron 868: prints last login time and
1.2 deraadt 869: .Pa /etc/motd
1.1 deraadt 870: (unless prevented in the configuration file or by
1.2 deraadt 871: .Pa $HOME/.hushlogin ;
872: see the
1.40 aaron 873: .Sx FILES
1.2 deraadt 874: section).
875: .It
1.1 deraadt 876: If the login is on a tty, records login time.
1.2 deraadt 877: .It
878: Checks
879: .Pa /etc/nologin ;
880: if it exists, prints contents and quits
1.1 deraadt 881: (unless root).
1.2 deraadt 882: .It
1.1 deraadt 883: Changes to run with normal user privileges.
1.2 deraadt 884: .It
1.1 deraadt 885: Sets up basic environment.
1.2 deraadt 886: .It
887: Reads
888: .Pa $HOME/.ssh/environment
889: if it exists.
890: .It
1.1 deraadt 891: Changes to user's home directory.
1.2 deraadt 892: .It
893: If
894: .Pa $HOME/.ssh/rc
895: exists, runs it; else if
896: .Pa /etc/sshrc
897: exists, runs
1.36 aaron 898: it; otherwise runs xauth.
899: The
1.2 deraadt 900: .Dq rc
901: files are given the X11
1.1 deraadt 902: authentication protocol and cookie in standard input.
1.2 deraadt 903: .It
1.1 deraadt 904: Runs user's shell or command.
1.2 deraadt 905: .El
906: .Sh AUTHORIZED_KEYS FILE FORMAT
907: .Pa $HOME/.ssh/authorized_keys
1.130 markus 908: is the default file that lists the public keys that are
909: permitted for RSA authentication in protocol version 1
910: and for public key authentication (PubkeyAuthentication)
911: in protocol version 2.
1.125 markus 912: .Cm AuthorizedKeysFile
913: may be used to specify an alternative file.
1.75 markus 914: .Pp
1.36 aaron 915: Each line of the file contains one
1.2 deraadt 916: key (empty lines and lines starting with a
917: .Ql #
918: are ignored as
1.36 aaron 919: comments).
1.75 markus 920: Each RSA public key consists of the following fields, separated by
1.36 aaron 921: spaces: options, bits, exponent, modulus, comment.
1.75 markus 922: Each protocol version 2 public key consists of:
923: options, keytype, base64 encoded key, comment.
924: The options fields
925: are optional; its presence is determined by whether the line starts
1.1 deraadt 926: with a number or not (the option field never starts with a number).
1.75 markus 927: The bits, exponent, modulus and comment fields give the RSA key for
928: protocol version 1; the
1.1 deraadt 929: comment field is not used for anything (but may be convenient for the
930: user to identify the key).
1.75 markus 931: For protocol version 2 the keytype is
932: .Dq ssh-dss
933: or
934: .Dq ssh-rsa .
1.2 deraadt 935: .Pp
1.1 deraadt 936: Note that lines in this file are usually several hundred bytes long
1.36 aaron 937: (because of the size of the RSA key modulus).
938: You don't want to type them in; instead, copy the
1.113 itojun 939: .Pa identity.pub ,
940: .Pa id_dsa.pub
1.75 markus 941: or the
1.113 itojun 942: .Pa id_rsa.pub
1.1 deraadt 943: file and edit it.
1.2 deraadt 944: .Pp
1.58 deraadt 945: The options (if present) consist of comma-separated option
1.36 aaron 946: specifications.
947: No spaces are permitted, except within double quotes.
1.1 deraadt 948: The following option specifications are supported:
1.2 deraadt 949: .Bl -tag -width Ds
950: .It Cm from="pattern-list"
1.1 deraadt 951: Specifies that in addition to RSA authentication, the canonical name
952: of the remote host must be present in the comma-separated list of
1.36 aaron 953: patterns
954: .Pf ( Ql *
955: and
956: .Ql ?
957: serve as wildcards).
958: The list may also contain
959: patterns negated by prefixing them with
960: .Ql ! ;
961: if the canonical host name matches a negated pattern, the key is not accepted.
962: The purpose
1.1 deraadt 963: of this option is to optionally increase security: RSA authentication
964: by itself does not trust the network or name servers or anything (but
965: the key); however, if somebody somehow steals the key, the key
1.36 aaron 966: permits an intruder to log in from anywhere in the world.
967: This additional option makes using a stolen key more difficult (name
1.1 deraadt 968: servers and/or routers would have to be compromised in addition to
969: just the key).
1.2 deraadt 970: .It Cm command="command"
1.1 deraadt 971: Specifies that the command is executed whenever this key is used for
1.36 aaron 972: authentication.
973: The command supplied by the user (if any) is ignored.
1.1 deraadt 974: The command is run on a pty if the connection requests a pty;
1.36 aaron 975: otherwise it is run without a tty.
1.89 markus 976: Note that if you want a 8-bit clean channel,
977: you must not request a pty or should specify
978: .Cm no-pty .
1.36 aaron 979: A quote may be included in the command by quoting it with a backslash.
980: This option might be useful
981: to restrict certain RSA keys to perform just a specific operation.
982: An example might be a key that permits remote backups but nothing else.
1.51 hugh 983: Note that the client may specify TCP/IP and/or X11
984: forwarding unless they are explicitly prohibited.
1.2 deraadt 985: .It Cm environment="NAME=value"
1.1 deraadt 986: Specifies that the string is to be added to the environment when
1.36 aaron 987: logging in using this key.
988: Environment variables set this way
989: override other default environment values.
990: Multiple options of this type are permitted.
1.2 deraadt 991: .It Cm no-port-forwarding
1.1 deraadt 992: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 993: Any port forward requests by the client will return an error.
994: This might be used, e.g., in connection with the
1.2 deraadt 995: .Cm command
1.1 deraadt 996: option.
1.2 deraadt 997: .It Cm no-X11-forwarding
1.1 deraadt 998: Forbids X11 forwarding when this key is used for authentication.
999: Any X11 forward requests by the client will return an error.
1.2 deraadt 1000: .It Cm no-agent-forwarding
1.1 deraadt 1001: Forbids authentication agent forwarding when this key is used for
1002: authentication.
1.2 deraadt 1003: .It Cm no-pty
1.1 deraadt 1004: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 1005: .It Cm permitopen="host:port"
1.133 itojun 1006: Limit local
1.107 djm 1007: .Li ``ssh -L''
1.111 stevesk 1008: port forwarding such that it may only connect to the specified host and
1.133 itojun 1009: port. Multiple
1.107 djm 1010: .Cm permitopen
1.133 itojun 1011: options may be applied separated by commas. No pattern matching is
1012: performed on the specified hostnames, they must be literal domains or
1.107 djm 1013: addresses.
1.2 deraadt 1014: .El
1015: .Ss Examples
1.1 deraadt 1016: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1017: .Pp
1.1 deraadt 1018: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1019: .Pp
1.1 deraadt 1020: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 1021: .Pp
1022: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1023: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1024: The
1.44 deraadt 1025: .Pa /etc/ssh_known_hosts ,
1.40 aaron 1026: and
1.131 markus 1027: .Pa $HOME/.ssh/known_hosts
1.36 aaron 1028: files contain host public keys for all known hosts.
1029: The global file should
1.37 brad 1030: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1031: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1032: its key is added to the per-user file.
1.2 deraadt 1033: .Pp
1.1 deraadt 1034: Each line in these files contains the following fields: hostnames,
1.36 aaron 1035: bits, exponent, modulus, comment.
1036: The fields are separated by spaces.
1.2 deraadt 1037: .Pp
1.1 deraadt 1038: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1039: wildcards); each pattern in turn is matched against the canonical host
1040: name (when authenticating a client) or against the user-supplied
1.36 aaron 1041: name (when authenticating a server).
1042: A pattern may also be preceded by
1.2 deraadt 1043: .Ql !
1044: to indicate negation: if the host name matches a negated
1.1 deraadt 1045: pattern, it is not accepted (by that line) even if it matched another
1046: pattern on the line.
1.2 deraadt 1047: .Pp
1.49 markus 1048: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1049: can be obtained, e.g., from
1050: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 1051: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1052: .Pp
1053: Lines starting with
1054: .Ql #
1055: and empty lines are ignored as comments.
1056: .Pp
1.1 deraadt 1057: When performing host authentication, authentication is accepted if any
1.36 aaron 1058: matching line has the proper key.
1059: It is thus permissible (but not
1.1 deraadt 1060: recommended) to have several lines or different host keys for the same
1.36 aaron 1061: names.
1062: This will inevitably happen when short forms of host names
1063: from different domains are put in the file.
1064: It is possible
1.1 deraadt 1065: that the files contain conflicting information; authentication is
1066: accepted if valid information can be found from either file.
1.2 deraadt 1067: .Pp
1.1 deraadt 1068: Note that the lines in these files are typically hundreds of characters
1069: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1070: Rather, generate them by a script
1.40 aaron 1071: or by taking
1.2 deraadt 1072: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1073: and adding the host names at the front.
1.2 deraadt 1074: .Ss Examples
1.120 markus 1075: .Bd -literal
1076: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1077: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1078: .Ed
1.2 deraadt 1079: .Sh FILES
1080: .Bl -tag -width Ds
1081: .It Pa /etc/sshd_config
1.1 deraadt 1082: Contains configuration data for
1.2 deraadt 1083: .Nm sshd .
1.1 deraadt 1084: This file should be writable by root only, but it is recommended
1085: (though not necessary) that it be world-readable.
1.98 deraadt 1086: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.120 markus 1087: These three files contain the private parts of the host keys.
1.98 deraadt 1088: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1089: accessible to others.
1.14 markus 1090: Note that
1091: .Nm
1092: does not start if this file is group/world-accessible.
1.98 deraadt 1093: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.120 markus 1094: These three files contain the public parts of the host keys.
1.98 deraadt 1095: These files should be world-readable but writable only by
1.36 aaron 1096: root.
1.98 deraadt 1097: Their contents should match the respective private parts.
1098: These files are not
1099: really used for anything; they are provided for the convenience of
1100: the user so their contents can be copied to known hosts files.
1101: These files are created using
1.7 markus 1102: .Xr ssh-keygen 1 .
1.129 provos 1103: .It Pa /etc/moduli
1.73 provos 1104: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1105: .It Pa /var/run/sshd.pid
1106: Contains the process ID of the
1107: .Nm
1.1 deraadt 1108: listening for connections (if there are several daemons running
1109: concurrently for different ports, this contains the pid of the one
1.36 aaron 1110: started last).
1.58 deraadt 1111: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1112: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 1113: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1114: This file must be readable by root (which may on some machines imply
1115: it being world-readable if the user's home directory resides on an NFS
1116: volume).
1117: It is recommended that it not be accessible by others.
1118: The format of this file is described above.
1119: Users will place the contents of their
1.130 markus 1120: .Pa identity.pub ,
1.44 deraadt 1121: .Pa id_dsa.pub
1.113 itojun 1122: and/or
1123: .Pa id_rsa.pub
1.44 deraadt 1124: files into this file, as described in
1125: .Xr ssh-keygen 1 .
1.22 markus 1126: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1127: These files are consulted when using rhosts with RSA host
1.131 markus 1128: authentication or protocol version 2 hostbased authentication
1129: to check the public key of the host.
1.36 aaron 1130: The key must be listed in one of these files to be accepted.
1.22 markus 1131: The client uses the same files
1.96 markus 1132: to verify that it is connecting to the correct remote host.
1.36 aaron 1133: These files should be writable only by root/the owner.
1.2 deraadt 1134: .Pa /etc/ssh_known_hosts
1135: should be world-readable, and
1136: .Pa $HOME/.ssh/known_hosts
1.120 markus 1137: can but need not be world-readable.
1.6 aaron 1138: .It Pa /etc/nologin
1.40 aaron 1139: If this file exists,
1.2 deraadt 1140: .Nm
1.36 aaron 1141: refuses to let anyone except root log in.
1142: The contents of the file
1.1 deraadt 1143: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1144: refused.
1145: The file should be world-readable.
1.19 dugsong 1146: .It Pa /etc/hosts.allow, /etc/hosts.deny
1147: If compiled with
1148: .Sy LIBWRAP
1149: support, tcp-wrappers access controls may be defined here as described in
1150: .Xr hosts_access 5 .
1.6 aaron 1151: .It Pa $HOME/.rhosts
1.1 deraadt 1152: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1153: line.
1154: The given user on the corresponding host is permitted to log in
1155: without password.
1156: The same file is used by rlogind and rshd.
1.6 aaron 1157: The file must
1.1 deraadt 1158: be writable only by the user; it is recommended that it not be
1159: accessible by others.
1.2 deraadt 1160: .Pp
1.36 aaron 1161: If is also possible to use netgroups in the file.
1162: Either host or user
1.1 deraadt 1163: name may be of the form +@groupname to specify all hosts or all users
1164: in the group.
1.2 deraadt 1165: .It Pa $HOME/.shosts
1166: For ssh,
1167: this file is exactly the same as for
1168: .Pa .rhosts .
1169: However, this file is
1170: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1171: .It Pa /etc/hosts.equiv
1.2 deraadt 1172: This file is used during
1173: .Pa .rhosts
1.36 aaron 1174: authentication.
1175: In the simplest form, this file contains host names, one per line.
1176: Users on
1.1 deraadt 1177: those hosts are permitted to log in without a password, provided they
1.36 aaron 1178: have the same user name on both machines.
1179: The host name may also be
1.1 deraadt 1180: followed by a user name; such users are permitted to log in as
1.2 deraadt 1181: .Em any
1.36 aaron 1182: user on this machine (except root).
1183: Additionally, the syntax
1.2 deraadt 1184: .Dq +@group
1.36 aaron 1185: can be used to specify netgroups.
1186: Negated entries start with
1.2 deraadt 1187: .Ql \&- .
1188: .Pp
1.1 deraadt 1189: If the client host/user is successfully matched in this file, login is
1190: automatically permitted provided the client and server user names are the
1.36 aaron 1191: same.
1192: Additionally, successful RSA host authentication is normally required.
1193: This file must be writable only by root; it is recommended
1.1 deraadt 1194: that it be world-readable.
1.2 deraadt 1195: .Pp
1.6 aaron 1196: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1197: .Pa hosts.equiv .
1.1 deraadt 1198: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1199: .Em anybody ,
1.1 deraadt 1200: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1201: binaries and directories.
1202: Using a user name practically grants the user root access.
1203: The only valid use for user names that I can think
1.1 deraadt 1204: of is in negative entries.
1.2 deraadt 1205: .Pp
1206: Note that this warning also applies to rsh/rlogin.
1207: .It Pa /etc/shosts.equiv
1.1 deraadt 1208: This is processed exactly as
1.2 deraadt 1209: .Pa /etc/hosts.equiv .
1.1 deraadt 1210: However, this file may be useful in environments that want to run both
1.2 deraadt 1211: rsh/rlogin and ssh.
1.6 aaron 1212: .It Pa $HOME/.ssh/environment
1.36 aaron 1213: This file is read into the environment at login (if it exists).
1214: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1215: .Ql # ) ,
1.36 aaron 1216: and assignment lines of the form name=value.
1217: The file should be writable
1.6 aaron 1218: only by the user; it need not be readable by anyone else.
1.2 deraadt 1219: .It Pa $HOME/.ssh/rc
1.1 deraadt 1220: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1221: environment files but before starting the user's shell or command.
1222: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1223: standard input (and
1224: .Ev DISPLAY
1.36 aaron 1225: in environment).
1226: This must call
1.2 deraadt 1227: .Xr xauth 1
1228: in that case.
1229: .Pp
1.1 deraadt 1230: The primary purpose of this file is to run any initialization routines
1231: which may be needed before the user's home directory becomes
1232: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1233: .Pp
1.1 deraadt 1234: This file will probably contain some initialization code followed by
1.120 markus 1235: something similar to:
1236: .Bd -literal
1237: if read proto cookie; then
1238: echo add $DISPLAY $proto $cookie | xauth -q -
1239: fi
1240: .Ed
1.2 deraadt 1241: .Pp
1242: If this file does not exist,
1243: .Pa /etc/sshrc
1244: is run, and if that
1.1 deraadt 1245: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1246: .Pp
1.1 deraadt 1247: This file should be writable only by the user, and need not be
1248: readable by anyone else.
1.2 deraadt 1249: .It Pa /etc/sshrc
1250: Like
1251: .Pa $HOME/.ssh/rc .
1252: This can be used to specify
1.36 aaron 1253: machine-specific login-time initializations globally.
1254: This file should be writable only by root, and should be world-readable.
1.56 aaron 1255: .El
1.71 aaron 1256: .Sh AUTHORS
1.84 markus 1257: OpenSSH is a derivative of the original and free
1258: ssh 1.2.12 release by Tatu Ylonen.
1259: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1260: Theo de Raadt and Dug Song
1261: removed many bugs, re-added newer features and
1262: created OpenSSH.
1263: Markus Friedl contributed the support for SSH
1264: protocol versions 1.5 and 2.0.
1.2 deraadt 1265: .Sh SEE ALSO
1266: .Xr scp 1 ,
1.90 djm 1267: .Xr sftp 1 ,
1.2 deraadt 1268: .Xr ssh 1 ,
1.5 deraadt 1269: .Xr ssh-add 1 ,
1.2 deraadt 1270: .Xr ssh-agent 1 ,
1.5 deraadt 1271: .Xr ssh-keygen 1 ,
1.136 markus 1272: .Xr login.conf 5 ,
1273: .Xr moduli 5 ,
1.128 mpech 1274: .Xr sftp-server 8
1.119 markus 1275: .Rs
1276: .%A T. Ylonen
1277: .%A T. Kivinen
1278: .%A M. Saarinen
1279: .%A T. Rinne
1280: .%A S. Lehtinen
1281: .%T "SSH Protocol Architecture"
1282: .%N draft-ietf-secsh-architecture-07.txt
1.120 markus 1283: .%D January 2001
1284: .%O work in progress material
1285: .Re
1286: .Rs
1287: .%A M. Friedl
1288: .%A N. Provos
1289: .%A W. A. Simpson
1290: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.132 markus 1291: .%N draft-ietf-secsh-dh-group-exchange-01.txt
1292: .%D April 2001
1.119 markus 1293: .%O work in progress material
1294: .Re
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