Annotation of src/usr.bin/ssh/sshd.8, Revision 1.142
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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33: .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34: .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.142 ! stevesk 37: .\" $OpenBSD: sshd.8,v 1.141 2001/08/16 20:14:57 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
1.114 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:
1.138 markus 133: Each host has a host-specific key (RSA or DSA) used to identify the host.
1.49 markus 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).
1.141 stevesk 294: The file contains keyword-argument pairs, one per line.
1.36 aaron 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
1.141 stevesk 299: The possible
300: keywords and their meanings are as follows (note that
301: keywords are case-insensitive and arguments are case-sensitive):
1.2 deraadt 302: .Bl -tag -width Ds
303: .It Cm AFSTokenPassing
1.36 aaron 304: Specifies whether an AFS token may be forwarded to the server.
305: Default is
1.2 deraadt 306: .Dq yes .
1.11 markus 307: .It Cm AllowGroups
1.92 deraadt 308: This keyword can be followed by a list of group names, separated
1.36 aaron 309: by spaces.
310: If specified, login is allowed only for users whose primary
1.81 markus 311: group or supplementary group list matches one of the patterns.
1.11 markus 312: .Ql \&*
313: and
314: .Ql ?
315: can be used as
1.36 aaron 316: wildcards in the patterns.
1.58 deraadt 317: Only group names are valid; a numerical group ID isn't recognized.
1.81 markus 318: By default login is allowed regardless of the group list.
1.3 dugsong 319: .Pp
1.69 markus 320: .It Cm AllowTcpForwarding
321: Specifies whether TCP forwarding is permitted.
322: The default is
323: .Dq yes .
324: Note that disabling TCP forwarding does not improve security unless
325: users are also denied shell access, as they can always install their
326: own forwarders.
327: .Pp
1.11 markus 328: .It Cm AllowUsers
1.92 deraadt 329: This keyword can be followed by a list of user names, separated
1.36 aaron 330: by spaces.
331: If specified, login is allowed only for users names that
1.11 markus 332: match one of the patterns.
333: .Ql \&*
334: and
335: .Ql ?
336: can be used as
1.36 aaron 337: wildcards in the patterns.
1.58 deraadt 338: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 339: By default login is allowed regardless of the user name.
1.135 markus 340: If the pattern takes the form USER@HOST then USER and HOST
341: are separately checked, allowing you to restrict logins to particular
342: users from particular hosts.
1.80 markus 343: .Pp
1.125 markus 344: .It Cm AuthorizedKeysFile
1.138 markus 345: Specifies the file that contains the public keys that can be used
346: for user authentication.
1.125 markus 347: .Cm AuthorizedKeysFile
348: may contain tokens of the form %T which are substituted during connection
1.142 ! stevesk 349: set-up. The following tokens are defined: %% is replaced by a literal '%',
1.125 markus 350: %h is replaced by the home directory of the user being authenticated and
351: %u is replaced by the username of that user.
352: After expansion,
353: .Cm AuthorizedKeysFile
1.126 markus 354: is taken to be an absolute path or one relative to the user's home
1.125 markus 355: directory.
356: The default is
357: .Dq .ssh/authorized_keys
1.80 markus 358: .It Cm Banner
359: In some jurisdictions, sending a warning message before authentication
360: may be relevant for getting legal protection.
361: The contents of the specified file are sent to the remote user before
362: authentication is allowed.
363: This option is only available for protocol version 2.
1.11 markus 364: .Pp
1.104 deraadt 365: .It Cm ChallengeResponseAuthentication
1.136 markus 366: Specifies whether challenge response authentication is allowed.
367: All authentication styles from
368: .Xr login.conf 5
369: are supported.
1.104 deraadt 370: The default is
371: .Dq yes .
1.122 markus 372: .It Cm Ciphers
373: Specifies the ciphers allowed for protocol version 2.
374: Multiple ciphers must be comma-separated.
375: The default is
376: .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
1.115 beck 377: .It Cm ClientAliveInterval
378: Sets a timeout interval in seconds after which if no data has been received
1.133 itojun 379: from the client,
1.115 beck 380: .Nm
381: will send a message through the encrypted
1.116 stevesk 382: channel to request a response from the client.
383: The default
1.115 beck 384: is 0, indicating that these messages will not be sent to the client.
1.116 stevesk 385: This option applies to protocol version 2 only.
1.115 beck 386: .It Cm ClientAliveCountMax
387: Sets the number of client alive messages (see above) which may be
388: sent without
389: .Nm
390: receiving any messages back from the client. If this threshold is
1.133 itojun 391: reached while client alive messages are being sent,
1.115 beck 392: .Nm
393: will disconnect the client, terminating the session. It is important
1.133 itojun 394: to note that the use of client alive messages is very different from
1.116 stevesk 395: .Cm Keepalive
396: (below). The client alive messages are sent through the
1.115 beck 397: encrypted channel and therefore will not be spoofable. The TCP keepalive
1.116 stevesk 398: option enabled by
399: .Cm Keepalive
400: is spoofable. You want to use the client
1.115 beck 401: alive mechanism when you are basing something important on
402: clients having an active connection to the server.
1.116 stevesk 403: .Pp
404: The default value is 3. If you set
405: .Cm ClientAliveInterval
1.115 beck 406: (above) to 15, and leave this value at the default, unresponsive ssh clients
1.133 itojun 407: will be disconnected after approximately 45 seconds.
1.11 markus 408: .It Cm DenyGroups
409: This keyword can be followed by a number of group names, separated
1.36 aaron 410: by spaces.
1.81 markus 411: Users whose primary group or supplementary group list matches
412: one of the patterns aren't allowed to log in.
1.11 markus 413: .Ql \&*
414: and
415: .Ql ?
416: can be used as
1.36 aaron 417: wildcards in the patterns.
1.58 deraadt 418: Only group names are valid; a numerical group ID isn't recognized.
1.81 markus 419: By default login is allowed regardless of the group list.
1.11 markus 420: .Pp
421: .It Cm DenyUsers
422: This keyword can be followed by a number of user names, separated
1.36 aaron 423: by spaces.
424: Login is disallowed for user names that match one of the patterns.
1.11 markus 425: .Ql \&*
426: and
427: .Ql ?
1.36 aaron 428: can be used as wildcards in the patterns.
1.58 deraadt 429: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 430: By default login is allowed regardless of the user name.
1.47 markus 431: .It Cm GatewayPorts
432: Specifies whether remote hosts are allowed to connect to ports
433: forwarded for the client.
434: The argument must be
435: .Dq yes
436: or
437: .Dq no .
438: The default is
439: .Dq no .
1.120 markus 440: .It Cm HostbasedAuthentication
441: Specifies whether rhosts or /etc/hosts.equiv authentication together
442: with successful public key client host authentication is allowed
443: (hostbased authentication).
444: This option is similar to
445: .Cm RhostsRSAAuthentication
446: and applies to protocol version 2 only.
447: The default is
448: .Dq no .
1.2 deraadt 449: .It Cm HostKey
1.72 markus 450: Specifies the file containing the private host keys (default
1.46 markus 451: .Pa /etc/ssh_host_key )
1.72 markus 452: used by SSH protocol versions 1 and 2.
1.9 markus 453: Note that
454: .Nm
1.83 markus 455: will refuse to use a file if it is group/world-accessible.
1.72 markus 456: It is possible to have multiple host key files.
457: .Dq rsa1
458: keys are used for version 1 and
459: .Dq dsa
460: or
461: .Dq rsa
462: are used for version 2 of the SSH protocol.
1.2 deraadt 463: .It Cm IgnoreRhosts
1.34 markus 464: Specifies that
465: .Pa .rhosts
1.40 aaron 466: and
1.34 markus 467: .Pa .shosts
1.120 markus 468: files will not be used in
469: .Cm RhostsAuthentication ,
470: .Cm RhostsRSAAuthentication
471: or
472: .Cm HostbasedAuthentication .
473: .Pp
1.2 deraadt 474: .Pa /etc/hosts.equiv
1.1 deraadt 475: and
1.40 aaron 476: .Pa /etc/shosts.equiv
1.36 aaron 477: are still used.
1.40 aaron 478: The default is
1.34 markus 479: .Dq yes .
1.24 markus 480: .It Cm IgnoreUserKnownHosts
481: Specifies whether
482: .Nm
483: should ignore the user's
484: .Pa $HOME/.ssh/known_hosts
1.45 markus 485: during
1.120 markus 486: .Cm RhostsRSAAuthentication
487: or
488: .Cm HostbasedAuthentication .
1.24 markus 489: The default is
1.2 deraadt 490: .Dq no .
491: .It Cm KeepAlive
1.1 deraadt 492: Specifies whether the system should send keepalive messages to the
1.36 aaron 493: other side.
494: If they are sent, death of the connection or crash of one
495: of the machines will be properly noticed.
496: However, this means that
1.1 deraadt 497: connections will die if the route is down temporarily, and some people
1.36 aaron 498: find it annoying.
1.51 hugh 499: On the other hand, if keepalives are not sent,
1.2 deraadt 500: sessions may hang indefinitely on the server, leaving
501: .Dq ghost
502: users and consuming server resources.
503: .Pp
504: The default is
505: .Dq yes
506: (to send keepalives), and the server will notice
1.36 aaron 507: if the network goes down or the client host reboots.
508: This avoids infinitely hanging sessions.
1.2 deraadt 509: .Pp
510: To disable keepalives, the value should be set to
511: .Dq no
512: in both the server and the client configuration files.
513: .It Cm KerberosAuthentication
1.36 aaron 514: Specifies whether Kerberos authentication is allowed.
515: This can be in the form of a Kerberos ticket, or if
1.7 markus 516: .Cm PasswordAuthentication
1.1 deraadt 517: is yes, the password provided by the user will be validated through
1.67 aaron 518: the Kerberos KDC.
519: To use this option, the server needs a
1.59 provos 520: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 521: Default is
1.60 provos 522: .Dq yes .
1.2 deraadt 523: .It Cm KerberosOrLocalPasswd
1.1 deraadt 524: If set then if password authentication through Kerberos fails then
525: the password will be validated via any additional local mechanism
1.2 deraadt 526: such as
1.66 markus 527: .Pa /etc/passwd .
1.36 aaron 528: Default is
1.20 dugsong 529: .Dq yes .
1.2 deraadt 530: .It Cm KerberosTgtPassing
1.1 deraadt 531: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 532: Default is
1.3 dugsong 533: .Dq no ,
534: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 535: .It Cm KerberosTicketCleanup
1.7 markus 536: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 537: file on logout.
538: Default is
1.3 dugsong 539: .Dq yes .
1.2 deraadt 540: .It Cm KeyRegenerationInterval
1.120 markus 541: In protocol version 1, the ephemeral server key is automatically regenerated
542: after this many seconds (if it has been used).
1.36 aaron 543: The purpose of regeneration is to prevent
1.1 deraadt 544: decrypting captured sessions by later breaking into the machine and
1.36 aaron 545: stealing the keys.
546: The key is never stored anywhere.
547: If the value is 0, the key is never regenerated.
548: The default is 3600 (seconds).
1.7 markus 549: .It Cm ListenAddress
1.110 stevesk 550: Specifies the local addresses
1.120 markus 551: .Nm
1.7 markus 552: should listen on.
1.110 stevesk 553: The following forms may be used:
554: .Pp
555: .Bl -item -offset indent -compact
556: .It
557: .Cm ListenAddress
1.112 stevesk 558: .Sm off
559: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
560: .Sm on
1.110 stevesk 561: .It
562: .Cm ListenAddress
1.112 stevesk 563: .Sm off
564: .Ar host No | Ar IPv4_addr No : Ar port
565: .Sm on
1.110 stevesk 566: .It
567: .Cm ListenAddress
1.112 stevesk 568: .Sm off
569: .Oo
570: .Ar host No | Ar IPv6_addr Oc : Ar port
571: .Sm on
1.110 stevesk 572: .El
573: .Pp
574: If
1.112 stevesk 575: .Ar port
1.110 stevesk 576: is not specified,
1.120 markus 577: .Nm
1.110 stevesk 578: will listen on the address and all prior
579: .Cm Port
580: options specified. The default is to listen on all local
581: addresses. Multiple
582: .Cm ListenAddress
583: options are permitted. Additionally, any
584: .Cm Port
585: options must precede this option for non port qualified addresses.
1.2 deraadt 586: .It Cm LoginGraceTime
1.1 deraadt 587: The server disconnects after this time if the user has not
1.36 aaron 588: successfully logged in.
589: If the value is 0, there is no time limit.
1.1 deraadt 590: The default is 600 (seconds).
1.23 markus 591: .It Cm LogLevel
592: Gives the verbosity level that is used when logging messages from
593: .Nm sshd .
594: The possible values are:
1.82 markus 595: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
596: The default is INFO.
1.23 markus 597: Logging with level DEBUG violates the privacy of users
598: and is not recommended.
1.93 markus 599: .It Cm MACs
600: Specifies the available MAC (message authentication code) algorithms.
601: The MAC algorithm is used in protocol version 2
602: for data integrity protection.
603: Multiple algorithms must be comma-separated.
604: The default is
1.123 markus 605: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 606: .It Cm MaxStartups
607: Specifies the maximum number of concurrent unauthenticated connections to the
608: .Nm
609: daemon.
610: Additional connections will be dropped until authentication succeeds or the
611: .Cm LoginGraceTime
612: expires for a connection.
613: The default is 10.
1.57 markus 614: .Pp
615: Alternatively, random early drop can be enabled by specifying
616: the three colon separated values
617: .Dq start:rate:full
1.67 aaron 618: (e.g., "10:30:60").
1.57 markus 619: .Nm
1.86 stevesk 620: will refuse connection attempts with a probability of
1.57 markus 621: .Dq rate/100
622: (30%)
623: if there are currently
624: .Dq start
625: (10)
626: unauthenticated connections.
1.86 stevesk 627: The probability increases linearly and all connection attempts
1.57 markus 628: are refused if the number of unauthenticated connections reaches
629: .Dq full
630: (60).
1.2 deraadt 631: .It Cm PasswordAuthentication
1.1 deraadt 632: Specifies whether password authentication is allowed.
1.2 deraadt 633: The default is
634: .Dq yes .
635: .It Cm PermitEmptyPasswords
1.1 deraadt 636: When password authentication is allowed, it specifies whether the
1.36 aaron 637: server allows login to accounts with empty password strings.
638: The default is
1.34 markus 639: .Dq no .
1.2 deraadt 640: .It Cm PermitRootLogin
1.100 stevesk 641: Specifies whether root can login using
1.2 deraadt 642: .Xr ssh 1 .
1.15 markus 643: The argument must be
644: .Dq yes ,
1.94 markus 645: .Dq without-password ,
646: .Dq forced-commands-only
1.15 markus 647: or
648: .Dq no .
1.2 deraadt 649: The default is
650: .Dq yes .
1.94 markus 651: .Pp
652: If this option is set to
1.15 markus 653: .Dq without-password
1.94 markus 654: password authentication is disabled for root.
1.2 deraadt 655: .Pp
1.94 markus 656: If this option is set to
657: .Dq forced-commands-only
658: root login with public key authentication will be allowed,
659: but only if the
1.2 deraadt 660: .Ar command
1.94 markus 661: option has been specified
1.1 deraadt 662: (which may be useful for taking remote backups even if root login is
1.94 markus 663: normally not allowed). All other authentication methods are disabled
664: for root.
1.100 stevesk 665: .Pp
666: If this option is set to
667: .Dq no
668: root is not allowed to login.
1.43 markus 669: .It Cm PidFile
670: Specifies the file that contains the process identifier of the
671: .Nm
672: daemon.
673: The default is
674: .Pa /var/run/sshd.pid .
1.2 deraadt 675: .It Cm Port
1.1 deraadt 676: Specifies the port number that
1.2 deraadt 677: .Nm
1.36 aaron 678: listens on.
679: The default is 22.
1.28 markus 680: Multiple options of this type are permitted.
1.120 markus 681: See also
682: .Cm ListenAddress .
1.108 stevesk 683: .It Cm PrintLastLog
684: Specifies whether
685: .Nm
686: should print the date and time when the user last logged in.
687: The default is
688: .Dq yes .
1.2 deraadt 689: .It Cm PrintMotd
1.1 deraadt 690: Specifies whether
1.2 deraadt 691: .Nm
1.40 aaron 692: should print
1.2 deraadt 693: .Pa /etc/motd
1.36 aaron 694: when a user logs in interactively.
695: (On some systems it is also printed by the shell,
1.2 deraadt 696: .Pa /etc/profile ,
1.36 aaron 697: or equivalent.)
698: The default is
1.2 deraadt 699: .Dq yes .
1.41 markus 700: .It Cm Protocol
701: Specifies the protocol versions
702: .Nm
703: should support.
704: The possible values are
705: .Dq 1
706: and
707: .Dq 2 .
708: Multiple versions must be comma-separated.
709: The default is
1.118 deraadt 710: .Dq 2,1 .
1.104 deraadt 711: .It Cm PubkeyAuthentication
712: Specifies whether public key authentication is allowed.
713: The default is
714: .Dq yes .
715: Note that this option applies to protocol version 2 only.
1.87 markus 716: .It Cm ReverseMappingCheck
717: Specifies whether
718: .Nm
719: should try to verify the remote host name and check that
720: the resolved host name for the remote IP address maps back to the
721: very same IP address.
722: The default is
723: .Dq no .
1.2 deraadt 724: .It Cm RhostsAuthentication
1.1 deraadt 725: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 726: files is sufficient.
727: Normally, this method should not be permitted because it is insecure.
1.7 markus 728: .Cm RhostsRSAAuthentication
729: should be used
1.1 deraadt 730: instead, because it performs RSA-based host authentication in addition
731: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 732: The default is
733: .Dq no .
1.120 markus 734: This option applies to protocol version 1 only.
1.2 deraadt 735: .It Cm RhostsRSAAuthentication
1.1 deraadt 736: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 737: with successful RSA host authentication is allowed.
738: The default is
1.34 markus 739: .Dq no .
1.120 markus 740: This option applies to protocol version 1 only.
1.2 deraadt 741: .It Cm RSAAuthentication
1.36 aaron 742: Specifies whether pure RSA authentication is allowed.
743: The default is
1.2 deraadt 744: .Dq yes .
1.120 markus 745: This option applies to protocol version 1 only.
1.2 deraadt 746: .It Cm ServerKeyBits
1.120 markus 747: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 748: The minimum value is 512, and the default is 768.
1.2 deraadt 749: .It Cm StrictModes
1.12 markus 750: Specifies whether
751: .Nm
752: should check file modes and ownership of the
1.36 aaron 753: user's files and home directory before accepting login.
754: This is normally desirable because novices sometimes accidentally leave their
755: directory or files world-writable.
756: The default is
1.7 markus 757: .Dq yes .
1.54 jakob 758: .It Cm Subsystem
1.67 aaron 759: Configures an external subsystem (e.g., file transfer daemon).
760: Arguments should be a subsystem name and a command to execute upon subsystem
761: request.
1.63 markus 762: The command
763: .Xr sftp-server 8
764: implements the
765: .Dq sftp
766: file transfer subsystem.
1.54 jakob 767: By default no subsystems are defined.
768: Note that this option applies to protocol version 2 only.
1.2 deraadt 769: .It Cm SyslogFacility
1.1 deraadt 770: Gives the facility code that is used when logging messages from
1.2 deraadt 771: .Nm sshd .
1.1 deraadt 772: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 773: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
774: The default is AUTH.
1.10 markus 775: .It Cm UseLogin
776: Specifies whether
777: .Xr login 1
1.53 markus 778: is used for interactive login sessions.
1.127 markus 779: The default is
780: .Dq no .
1.53 markus 781: Note that
782: .Xr login 1
1.58 deraadt 783: is never used for remote command execution.
1.133 itojun 784: Note also, that if this is enabled,
785: .Cm X11Forwarding
1.127 markus 786: will be disabled because
787: .Xr login 1
788: does not know how to handle
1.133 itojun 789: .Xr xauth 1
1.127 markus 790: cookies.
1.6 aaron 791: .It Cm X11DisplayOffset
792: Specifies the first display number available for
793: .Nm sshd Ns 's
1.36 aaron 794: X11 forwarding.
795: This prevents
1.6 aaron 796: .Nm
797: from interfering with real X11 servers.
1.34 markus 798: The default is 10.
1.30 markus 799: .It Cm X11Forwarding
1.36 aaron 800: Specifies whether X11 forwarding is permitted.
801: The default is
1.34 markus 802: .Dq no .
1.30 markus 803: Note that disabling X11 forwarding does not improve security in any
804: way, as users can always install their own forwarders.
1.133 itojun 805: X11 forwarding is automatically disabled if
806: .Cm UseLogin
807: is enabled.
1.52 markus 808: .It Cm XAuthLocation
809: Specifies the location of the
810: .Xr xauth 1
811: program.
812: The default is
813: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 814: .El
1.124 stevesk 815: .Ss Time Formats
816: .Pp
817: .Nm
818: command-line arguments and configuration file options that specify time
819: may be expressed using a sequence of the form:
820: .Sm off
821: .Ar time Oo Ar qualifier Oc ,
822: .Sm on
823: where
824: .Ar time
825: is a positive integer value and
826: .Ar qualifier
827: is one of the following:
828: .Pp
829: .Bl -tag -width Ds -compact -offset indent
830: .It Cm <none>
831: seconds
832: .It Cm s | Cm S
833: seconds
834: .It Cm m | Cm M
835: minutes
836: .It Cm h | Cm H
837: hours
838: .It Cm d | Cm D
839: days
840: .It Cm w | Cm W
841: weeks
842: .El
843: .Pp
844: Each member of the sequence is added together to calculate
845: the total time value.
846: .Pp
847: Time format examples:
848: .Pp
849: .Bl -tag -width Ds -compact -offset indent
850: .It 600
851: 600 seconds (10 minutes)
852: .It 10m
853: 10 minutes
854: .It 1h30m
855: 1 hour 30 minutes (90 minutes)
856: .El
1.2 deraadt 857: .Sh LOGIN PROCESS
1.1 deraadt 858: When a user successfully logs in,
1.2 deraadt 859: .Nm
1.1 deraadt 860: does the following:
1.2 deraadt 861: .Bl -enum -offset indent
862: .It
1.1 deraadt 863: If the login is on a tty, and no command has been specified,
1.40 aaron 864: prints last login time and
1.2 deraadt 865: .Pa /etc/motd
1.1 deraadt 866: (unless prevented in the configuration file or by
1.2 deraadt 867: .Pa $HOME/.hushlogin ;
868: see the
1.40 aaron 869: .Sx FILES
1.2 deraadt 870: section).
871: .It
1.1 deraadt 872: If the login is on a tty, records login time.
1.2 deraadt 873: .It
874: Checks
875: .Pa /etc/nologin ;
876: if it exists, prints contents and quits
1.1 deraadt 877: (unless root).
1.2 deraadt 878: .It
1.1 deraadt 879: Changes to run with normal user privileges.
1.2 deraadt 880: .It
1.1 deraadt 881: Sets up basic environment.
1.2 deraadt 882: .It
883: Reads
884: .Pa $HOME/.ssh/environment
885: if it exists.
886: .It
1.1 deraadt 887: Changes to user's home directory.
1.2 deraadt 888: .It
889: If
890: .Pa $HOME/.ssh/rc
891: exists, runs it; else if
892: .Pa /etc/sshrc
893: exists, runs
1.36 aaron 894: it; otherwise runs xauth.
895: The
1.2 deraadt 896: .Dq rc
897: files are given the X11
1.1 deraadt 898: authentication protocol and cookie in standard input.
1.2 deraadt 899: .It
1.1 deraadt 900: Runs user's shell or command.
1.2 deraadt 901: .El
902: .Sh AUTHORIZED_KEYS FILE FORMAT
903: .Pa $HOME/.ssh/authorized_keys
1.130 markus 904: is the default file that lists the public keys that are
905: permitted for RSA authentication in protocol version 1
906: and for public key authentication (PubkeyAuthentication)
907: in protocol version 2.
1.125 markus 908: .Cm AuthorizedKeysFile
909: may be used to specify an alternative file.
1.75 markus 910: .Pp
1.36 aaron 911: Each line of the file contains one
1.2 deraadt 912: key (empty lines and lines starting with a
913: .Ql #
914: are ignored as
1.36 aaron 915: comments).
1.75 markus 916: Each RSA public key consists of the following fields, separated by
1.36 aaron 917: spaces: options, bits, exponent, modulus, comment.
1.75 markus 918: Each protocol version 2 public key consists of:
919: options, keytype, base64 encoded key, comment.
920: The options fields
921: are optional; its presence is determined by whether the line starts
1.1 deraadt 922: with a number or not (the option field never starts with a number).
1.75 markus 923: The bits, exponent, modulus and comment fields give the RSA key for
924: protocol version 1; the
1.1 deraadt 925: comment field is not used for anything (but may be convenient for the
926: user to identify the key).
1.75 markus 927: For protocol version 2 the keytype is
928: .Dq ssh-dss
929: or
930: .Dq ssh-rsa .
1.2 deraadt 931: .Pp
1.1 deraadt 932: Note that lines in this file are usually several hundred bytes long
1.36 aaron 933: (because of the size of the RSA key modulus).
934: You don't want to type them in; instead, copy the
1.113 itojun 935: .Pa identity.pub ,
936: .Pa id_dsa.pub
1.75 markus 937: or the
1.113 itojun 938: .Pa id_rsa.pub
1.1 deraadt 939: file and edit it.
1.2 deraadt 940: .Pp
1.58 deraadt 941: The options (if present) consist of comma-separated option
1.36 aaron 942: specifications.
943: No spaces are permitted, except within double quotes.
1.141 stevesk 944: The following option specifications are supported (note
945: that option keywords are case-insensitive):
1.2 deraadt 946: .Bl -tag -width Ds
947: .It Cm from="pattern-list"
1.1 deraadt 948: Specifies that in addition to RSA authentication, the canonical name
949: of the remote host must be present in the comma-separated list of
1.36 aaron 950: patterns
951: .Pf ( Ql *
952: and
953: .Ql ?
954: serve as wildcards).
955: The list may also contain
956: patterns negated by prefixing them with
957: .Ql ! ;
958: if the canonical host name matches a negated pattern, the key is not accepted.
959: The purpose
1.1 deraadt 960: of this option is to optionally increase security: RSA authentication
961: by itself does not trust the network or name servers or anything (but
962: the key); however, if somebody somehow steals the key, the key
1.36 aaron 963: permits an intruder to log in from anywhere in the world.
964: This additional option makes using a stolen key more difficult (name
1.1 deraadt 965: servers and/or routers would have to be compromised in addition to
966: just the key).
1.2 deraadt 967: .It Cm command="command"
1.1 deraadt 968: Specifies that the command is executed whenever this key is used for
1.36 aaron 969: authentication.
970: The command supplied by the user (if any) is ignored.
1.1 deraadt 971: The command is run on a pty if the connection requests a pty;
1.36 aaron 972: otherwise it is run without a tty.
1.89 markus 973: Note that if you want a 8-bit clean channel,
974: you must not request a pty or should specify
975: .Cm no-pty .
1.36 aaron 976: A quote may be included in the command by quoting it with a backslash.
977: This option might be useful
978: to restrict certain RSA keys to perform just a specific operation.
979: An example might be a key that permits remote backups but nothing else.
1.51 hugh 980: Note that the client may specify TCP/IP and/or X11
981: forwarding unless they are explicitly prohibited.
1.2 deraadt 982: .It Cm environment="NAME=value"
1.1 deraadt 983: Specifies that the string is to be added to the environment when
1.36 aaron 984: logging in using this key.
985: Environment variables set this way
986: override other default environment values.
987: Multiple options of this type are permitted.
1.2 deraadt 988: .It Cm no-port-forwarding
1.1 deraadt 989: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 990: Any port forward requests by the client will return an error.
991: This might be used, e.g., in connection with the
1.2 deraadt 992: .Cm command
1.1 deraadt 993: option.
1.2 deraadt 994: .It Cm no-X11-forwarding
1.1 deraadt 995: Forbids X11 forwarding when this key is used for authentication.
996: Any X11 forward requests by the client will return an error.
1.2 deraadt 997: .It Cm no-agent-forwarding
1.1 deraadt 998: Forbids authentication agent forwarding when this key is used for
999: authentication.
1.2 deraadt 1000: .It Cm no-pty
1.1 deraadt 1001: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 1002: .It Cm permitopen="host:port"
1.133 itojun 1003: Limit local
1.107 djm 1004: .Li ``ssh -L''
1.111 stevesk 1005: port forwarding such that it may only connect to the specified host and
1.133 itojun 1006: port. Multiple
1.107 djm 1007: .Cm permitopen
1.133 itojun 1008: options may be applied separated by commas. No pattern matching is
1009: performed on the specified hostnames, they must be literal domains or
1.107 djm 1010: addresses.
1.2 deraadt 1011: .El
1012: .Ss Examples
1.1 deraadt 1013: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1014: .Pp
1.1 deraadt 1015: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1016: .Pp
1.1 deraadt 1017: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 1018: .Pp
1019: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1020: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1021: The
1.44 deraadt 1022: .Pa /etc/ssh_known_hosts ,
1.40 aaron 1023: and
1.131 markus 1024: .Pa $HOME/.ssh/known_hosts
1.36 aaron 1025: files contain host public keys for all known hosts.
1026: The global file should
1.37 brad 1027: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1028: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1029: its key is added to the per-user file.
1.2 deraadt 1030: .Pp
1.1 deraadt 1031: Each line in these files contains the following fields: hostnames,
1.36 aaron 1032: bits, exponent, modulus, comment.
1033: The fields are separated by spaces.
1.2 deraadt 1034: .Pp
1.1 deraadt 1035: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1036: wildcards); each pattern in turn is matched against the canonical host
1037: name (when authenticating a client) or against the user-supplied
1.36 aaron 1038: name (when authenticating a server).
1039: A pattern may also be preceded by
1.2 deraadt 1040: .Ql !
1041: to indicate negation: if the host name matches a negated
1.1 deraadt 1042: pattern, it is not accepted (by that line) even if it matched another
1043: pattern on the line.
1.2 deraadt 1044: .Pp
1.49 markus 1045: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1046: can be obtained, e.g., from
1047: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 1048: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1049: .Pp
1050: Lines starting with
1051: .Ql #
1052: and empty lines are ignored as comments.
1053: .Pp
1.1 deraadt 1054: When performing host authentication, authentication is accepted if any
1.36 aaron 1055: matching line has the proper key.
1056: It is thus permissible (but not
1.1 deraadt 1057: recommended) to have several lines or different host keys for the same
1.36 aaron 1058: names.
1059: This will inevitably happen when short forms of host names
1060: from different domains are put in the file.
1061: It is possible
1.1 deraadt 1062: that the files contain conflicting information; authentication is
1063: accepted if valid information can be found from either file.
1.2 deraadt 1064: .Pp
1.1 deraadt 1065: Note that the lines in these files are typically hundreds of characters
1066: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1067: Rather, generate them by a script
1.40 aaron 1068: or by taking
1.2 deraadt 1069: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1070: and adding the host names at the front.
1.2 deraadt 1071: .Ss Examples
1.120 markus 1072: .Bd -literal
1073: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1074: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1075: .Ed
1.2 deraadt 1076: .Sh FILES
1077: .Bl -tag -width Ds
1078: .It Pa /etc/sshd_config
1.1 deraadt 1079: Contains configuration data for
1.2 deraadt 1080: .Nm sshd .
1.1 deraadt 1081: This file should be writable by root only, but it is recommended
1082: (though not necessary) that it be world-readable.
1.98 deraadt 1083: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.120 markus 1084: These three files contain the private parts of the host keys.
1.98 deraadt 1085: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1086: accessible to others.
1.14 markus 1087: Note that
1088: .Nm
1089: does not start if this file is group/world-accessible.
1.98 deraadt 1090: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.120 markus 1091: These three files contain the public parts of the host keys.
1.98 deraadt 1092: These files should be world-readable but writable only by
1.36 aaron 1093: root.
1.98 deraadt 1094: Their contents should match the respective private parts.
1095: These files are not
1096: really used for anything; they are provided for the convenience of
1097: the user so their contents can be copied to known hosts files.
1098: These files are created using
1.7 markus 1099: .Xr ssh-keygen 1 .
1.129 provos 1100: .It Pa /etc/moduli
1.73 provos 1101: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1102: .It Pa /var/run/sshd.pid
1103: Contains the process ID of the
1104: .Nm
1.1 deraadt 1105: listening for connections (if there are several daemons running
1106: concurrently for different ports, this contains the pid of the one
1.36 aaron 1107: started last).
1.58 deraadt 1108: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1109: .It Pa $HOME/.ssh/authorized_keys
1.120 markus 1110: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1111: This file must be readable by root (which may on some machines imply
1112: it being world-readable if the user's home directory resides on an NFS
1113: volume).
1114: It is recommended that it not be accessible by others.
1115: The format of this file is described above.
1116: Users will place the contents of their
1.130 markus 1117: .Pa identity.pub ,
1.44 deraadt 1118: .Pa id_dsa.pub
1.113 itojun 1119: and/or
1120: .Pa id_rsa.pub
1.44 deraadt 1121: files into this file, as described in
1122: .Xr ssh-keygen 1 .
1.22 markus 1123: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1124: These files are consulted when using rhosts with RSA host
1.131 markus 1125: authentication or protocol version 2 hostbased authentication
1126: to check the public key of the host.
1.36 aaron 1127: The key must be listed in one of these files to be accepted.
1.22 markus 1128: The client uses the same files
1.96 markus 1129: to verify that it is connecting to the correct remote host.
1.36 aaron 1130: These files should be writable only by root/the owner.
1.2 deraadt 1131: .Pa /etc/ssh_known_hosts
1132: should be world-readable, and
1133: .Pa $HOME/.ssh/known_hosts
1.120 markus 1134: can but need not be world-readable.
1.6 aaron 1135: .It Pa /etc/nologin
1.40 aaron 1136: If this file exists,
1.2 deraadt 1137: .Nm
1.36 aaron 1138: refuses to let anyone except root log in.
1139: The contents of the file
1.1 deraadt 1140: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1141: refused.
1142: The file should be world-readable.
1.19 dugsong 1143: .It Pa /etc/hosts.allow, /etc/hosts.deny
1144: If compiled with
1145: .Sy LIBWRAP
1146: support, tcp-wrappers access controls may be defined here as described in
1147: .Xr hosts_access 5 .
1.6 aaron 1148: .It Pa $HOME/.rhosts
1.1 deraadt 1149: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1150: line.
1151: The given user on the corresponding host is permitted to log in
1152: without password.
1153: The same file is used by rlogind and rshd.
1.6 aaron 1154: The file must
1.1 deraadt 1155: be writable only by the user; it is recommended that it not be
1156: accessible by others.
1.2 deraadt 1157: .Pp
1.36 aaron 1158: If is also possible to use netgroups in the file.
1159: Either host or user
1.1 deraadt 1160: name may be of the form +@groupname to specify all hosts or all users
1161: in the group.
1.2 deraadt 1162: .It Pa $HOME/.shosts
1163: For ssh,
1164: this file is exactly the same as for
1165: .Pa .rhosts .
1166: However, this file is
1167: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1168: .It Pa /etc/hosts.equiv
1.2 deraadt 1169: This file is used during
1170: .Pa .rhosts
1.36 aaron 1171: authentication.
1172: In the simplest form, this file contains host names, one per line.
1173: Users on
1.1 deraadt 1174: those hosts are permitted to log in without a password, provided they
1.36 aaron 1175: have the same user name on both machines.
1176: The host name may also be
1.1 deraadt 1177: followed by a user name; such users are permitted to log in as
1.2 deraadt 1178: .Em any
1.36 aaron 1179: user on this machine (except root).
1180: Additionally, the syntax
1.2 deraadt 1181: .Dq +@group
1.36 aaron 1182: can be used to specify netgroups.
1183: Negated entries start with
1.2 deraadt 1184: .Ql \&- .
1185: .Pp
1.1 deraadt 1186: If the client host/user is successfully matched in this file, login is
1187: automatically permitted provided the client and server user names are the
1.36 aaron 1188: same.
1189: Additionally, successful RSA host authentication is normally required.
1190: This file must be writable only by root; it is recommended
1.1 deraadt 1191: that it be world-readable.
1.2 deraadt 1192: .Pp
1.6 aaron 1193: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1194: .Pa hosts.equiv .
1.1 deraadt 1195: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1196: .Em anybody ,
1.1 deraadt 1197: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1198: binaries and directories.
1199: Using a user name practically grants the user root access.
1200: The only valid use for user names that I can think
1.1 deraadt 1201: of is in negative entries.
1.2 deraadt 1202: .Pp
1203: Note that this warning also applies to rsh/rlogin.
1204: .It Pa /etc/shosts.equiv
1.1 deraadt 1205: This is processed exactly as
1.2 deraadt 1206: .Pa /etc/hosts.equiv .
1.1 deraadt 1207: However, this file may be useful in environments that want to run both
1.2 deraadt 1208: rsh/rlogin and ssh.
1.6 aaron 1209: .It Pa $HOME/.ssh/environment
1.36 aaron 1210: This file is read into the environment at login (if it exists).
1211: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1212: .Ql # ) ,
1.36 aaron 1213: and assignment lines of the form name=value.
1214: The file should be writable
1.6 aaron 1215: only by the user; it need not be readable by anyone else.
1.2 deraadt 1216: .It Pa $HOME/.ssh/rc
1.1 deraadt 1217: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1218: environment files but before starting the user's shell or command.
1219: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1220: standard input (and
1221: .Ev DISPLAY
1.36 aaron 1222: in environment).
1223: This must call
1.2 deraadt 1224: .Xr xauth 1
1225: in that case.
1226: .Pp
1.1 deraadt 1227: The primary purpose of this file is to run any initialization routines
1228: which may be needed before the user's home directory becomes
1229: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1230: .Pp
1.1 deraadt 1231: This file will probably contain some initialization code followed by
1.120 markus 1232: something similar to:
1233: .Bd -literal
1234: if read proto cookie; then
1235: echo add $DISPLAY $proto $cookie | xauth -q -
1236: fi
1237: .Ed
1.2 deraadt 1238: .Pp
1239: If this file does not exist,
1240: .Pa /etc/sshrc
1241: is run, and if that
1.1 deraadt 1242: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1243: .Pp
1.1 deraadt 1244: This file should be writable only by the user, and need not be
1245: readable by anyone else.
1.2 deraadt 1246: .It Pa /etc/sshrc
1247: Like
1248: .Pa $HOME/.ssh/rc .
1249: This can be used to specify
1.36 aaron 1250: machine-specific login-time initializations globally.
1251: This file should be writable only by root, and should be world-readable.
1.56 aaron 1252: .El
1.71 aaron 1253: .Sh AUTHORS
1.84 markus 1254: OpenSSH is a derivative of the original and free
1255: ssh 1.2.12 release by Tatu Ylonen.
1256: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1257: Theo de Raadt and Dug Song
1258: removed many bugs, re-added newer features and
1259: created OpenSSH.
1260: Markus Friedl contributed the support for SSH
1261: protocol versions 1.5 and 2.0.
1.2 deraadt 1262: .Sh SEE ALSO
1263: .Xr scp 1 ,
1.90 djm 1264: .Xr sftp 1 ,
1.2 deraadt 1265: .Xr ssh 1 ,
1.5 deraadt 1266: .Xr ssh-add 1 ,
1.2 deraadt 1267: .Xr ssh-agent 1 ,
1.5 deraadt 1268: .Xr ssh-keygen 1 ,
1.136 markus 1269: .Xr login.conf 5 ,
1270: .Xr moduli 5 ,
1.128 mpech 1271: .Xr sftp-server 8
1.119 markus 1272: .Rs
1273: .%A T. Ylonen
1274: .%A T. Kivinen
1275: .%A M. Saarinen
1276: .%A T. Rinne
1277: .%A S. Lehtinen
1278: .%T "SSH Protocol Architecture"
1.139 markus 1279: .%N draft-ietf-secsh-architecture-09.txt
1280: .%D July 2001
1.120 markus 1281: .%O work in progress material
1282: .Re
1283: .Rs
1284: .%A M. Friedl
1285: .%A N. Provos
1286: .%A W. A. Simpson
1287: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1.132 markus 1288: .%N draft-ietf-secsh-dh-group-exchange-01.txt
1289: .%D April 2001
1.119 markus 1290: .%O work in progress material
1291: .Re
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