Annotation of src/usr.bin/ssh/sshd.8, Revision 1.128
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5: .\" All rights reserved
6: .\"
1.64 deraadt 7: .\" As far as I am concerned, the code I have written for this software
8: .\" can be used freely for any purpose. Any derived versions of this
9: .\" software must be clearly marked as such, and if the derived work is
10: .\" incompatible with the protocol description in the RFC file, it must be
11: .\" called by a name other than "ssh" or "Secure Shell".
12: .\"
1.99 deraadt 13: .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14: .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15: .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
1.64 deraadt 16: .\"
17: .\" Redistribution and use in source and binary forms, with or without
18: .\" modification, are permitted provided that the following conditions
19: .\" are met:
20: .\" 1. Redistributions of source code must retain the above copyright
21: .\" notice, this list of conditions and the following disclaimer.
22: .\" 2. Redistributions in binary form must reproduce the above copyright
23: .\" notice, this list of conditions and the following disclaimer in the
24: .\" documentation and/or other materials provided with the distribution.
1.1 deraadt 25: .\"
1.64 deraadt 26: .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27: .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28: .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29: .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30: .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31: .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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35: .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1.1 deraadt 36: .\"
1.128 ! mpech 37: .\" $OpenBSD: sshd.8,v 1.127 2001/06/19 14:09:45 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: .Pp
188: .It Fl d
1.36 aaron 189: Debug mode.
190: The server sends verbose debug output to the system
191: log, and does not put itself in the background.
192: The server also will not fork and will only process one connection.
193: This option is only intended for debugging for the server.
1.120 markus 194: Multiple -d options increase the debugging level.
1.67 aaron 195: Maximum is 3.
1.120 markus 196: .It Fl e
197: When this option is specified,
198: .Nm
199: will send the output to the standard error instead of the system log.
1.2 deraadt 200: .It Fl f Ar configuration_file
1.36 aaron 201: Specifies the name of the configuration file.
202: The default is
1.2 deraadt 203: .Pa /etc/sshd_config .
1.16 markus 204: .Nm
205: refuses to start if there is no configuration file.
1.2 deraadt 206: .It Fl g Ar login_grace_time
1.1 deraadt 207: Gives the grace time for clients to authenticate themselves (default
1.77 markus 208: 600 seconds).
1.36 aaron 209: If the client fails to authenticate the user within
210: this many seconds, the server disconnects and exits.
211: A value of zero indicates no limit.
1.2 deraadt 212: .It Fl h Ar host_key_file
1.75 markus 213: Specifies the file from which the host key is read (default
1.2 deraadt 214: .Pa /etc/ssh_host_key ) .
1.7 markus 215: This option must be given if
216: .Nm
217: is not run as root (as the normal
1.1 deraadt 218: host file is normally not readable by anyone but root).
1.75 markus 219: It is possible to have multiple host key files for
1.120 markus 220: the different protocol versions and host key algorithms.
1.2 deraadt 221: .It Fl i
1.7 markus 222: Specifies that
223: .Nm
1.40 aaron 224: is being run from inetd.
1.7 markus 225: .Nm
226: is normally not run
1.1 deraadt 227: from inetd because it needs to generate the server key before it can
1.36 aaron 228: respond to the client, and this may take tens of seconds.
229: Clients would have to wait too long if the key was regenerated every time.
1.35 aaron 230: However, with small key sizes (e.g., 512) using
1.7 markus 231: .Nm
232: from inetd may
1.1 deraadt 233: be feasible.
1.2 deraadt 234: .It Fl k Ar key_gen_time
1.120 markus 235: Specifies how often the ephemeral protocol version 1 server key is
236: regenerated (default 3600 seconds, or one hour).
1.36 aaron 237: The motivation for regenerating the key fairly
1.1 deraadt 238: often is that the key is not stored anywhere, and after about an hour,
239: it becomes impossible to recover the key for decrypting intercepted
240: communications even if the machine is cracked into or physically
1.36 aaron 241: seized.
242: A value of zero indicates that the key will never be regenerated.
1.2 deraadt 243: .It Fl p Ar port
1.1 deraadt 244: Specifies the port on which the server listens for connections
245: (default 22).
1.2 deraadt 246: .It Fl q
1.36 aaron 247: Quiet mode.
248: Nothing is sent to the system log.
249: Normally the beginning,
1.1 deraadt 250: authentication, and termination of each connection is logged.
1.61 markus 251: .It Fl u Ar len
252: This option is used to specify the size of the field
253: in the
254: .Li utmp
255: structure that holds the remote host name.
256: If the resolved host name is longer than
257: .Ar len ,
258: the dotted decimal value will be used instead.
259: This allows hosts with very long host names that
260: overflow this field to still be uniquely identified.
261: Specifying
262: .Fl u0
263: indicates that only dotted decimal addresses
264: should be put into the
265: .Pa utmp
266: file.
1.74 markus 267: .It Fl D
268: When this option is specified
269: .Nm
270: will not detach and does not become a daemon.
271: This allows easy monitoring of
1.76 markus 272: .Nm sshd .
1.29 markus 273: .It Fl 4
274: Forces
275: .Nm
276: to use IPv4 addresses only.
277: .It Fl 6
278: Forces
279: .Nm
280: to use IPv6 addresses only.
1.2 deraadt 281: .El
282: .Sh CONFIGURATION FILE
283: .Nm
1.40 aaron 284: reads configuration data from
1.2 deraadt 285: .Pa /etc/sshd_config
286: (or the file specified with
287: .Fl f
1.36 aaron 288: on the command line).
289: The file contains keyword-value pairs, one per line.
290: Lines starting with
1.2 deraadt 291: .Ql #
1.1 deraadt 292: and empty lines are interpreted as comments.
1.2 deraadt 293: .Pp
1.1 deraadt 294: The following keywords are possible.
1.2 deraadt 295: .Bl -tag -width Ds
296: .It Cm AFSTokenPassing
1.36 aaron 297: Specifies whether an AFS token may be forwarded to the server.
298: Default is
1.2 deraadt 299: .Dq yes .
1.11 markus 300: .It Cm AllowGroups
1.92 deraadt 301: This keyword can be followed by a list of group names, separated
1.36 aaron 302: by spaces.
303: If specified, login is allowed only for users whose primary
1.81 markus 304: group or supplementary group list matches one of the patterns.
1.11 markus 305: .Ql \&*
306: and
307: .Ql ?
308: can be used as
1.36 aaron 309: wildcards in the patterns.
1.58 deraadt 310: Only group names are valid; a numerical group ID isn't recognized.
1.81 markus 311: By default login is allowed regardless of the group list.
1.3 dugsong 312: .Pp
1.69 markus 313: .It Cm AllowTcpForwarding
314: Specifies whether TCP forwarding is permitted.
315: The default is
316: .Dq yes .
317: Note that disabling TCP forwarding does not improve security unless
318: users are also denied shell access, as they can always install their
319: own forwarders.
320: .Pp
1.11 markus 321: .It Cm AllowUsers
1.92 deraadt 322: This keyword can be followed by a list of user names, separated
1.36 aaron 323: by spaces.
324: If specified, login is allowed only for users names that
1.11 markus 325: match one of the patterns.
326: .Ql \&*
327: and
328: .Ql ?
329: can be used as
1.36 aaron 330: wildcards in the patterns.
1.58 deraadt 331: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 332: By default login is allowed regardless of the user name.
1.80 markus 333: .Pp
1.125 markus 334: .It Cm AuthorizedKeysFile
335: Specifies the file that contains the public RSA keys that can be used
336: for RSA authentication in protocol version 1.
337: .Cm AuthorizedKeysFile
338: may contain tokens of the form %T which are substituted during connection
339: set-up. The following tokens are defined; %% is replaces by a literal '%',
340: %h is replaced by the home directory of the user being authenticated and
341: %u is replaced by the username of that user.
342: After expansion,
343: .Cm AuthorizedKeysFile
1.126 markus 344: is taken to be an absolute path or one relative to the user's home
1.125 markus 345: directory.
346: The default is
347: .Dq .ssh/authorized_keys
348: .It Cm AuthorizedKeysFile2
349: Specifies the file that contains the public keys that can be used
350: for public key authentication in protocol version 2.
351: .Cm AuthorizedKeysFile2
352: may contain tokens of the form %T which are substituted during connection
353: set-up. The following tokens are defined; %% is replaces by a literal '%',
354: %h is replaced by the home directory of the user being authenticated and
355: %u is replaced by the username of that user.
356: After expansion,
357: .Cm AuthorizedKeysFile2
1.126 markus 358: is taken to be an absolute path or one relative to the user's home
1.125 markus 359: directory.
360: The default is
361: .Dq .ssh/authorized_keys2
1.80 markus 362: .It Cm Banner
363: In some jurisdictions, sending a warning message before authentication
364: may be relevant for getting legal protection.
365: The contents of the specified file are sent to the remote user before
366: authentication is allowed.
367: This option is only available for protocol version 2.
1.11 markus 368: .Pp
1.104 deraadt 369: .It Cm ChallengeResponseAuthentication
370: Specifies whether
371: challenge response
372: authentication is allowed.
373: Currently there is only support for
374: .Xr skey 1
375: authentication.
376: The default is
377: .Dq yes .
1.8 markus 378: .It Cm CheckMail
379: Specifies whether
380: .Nm
381: should check for new mail for interactive logins.
382: The default is
383: .Dq no .
1.122 markus 384: .It Cm Ciphers
385: Specifies the ciphers allowed for protocol version 2.
386: Multiple ciphers must be comma-separated.
387: The default is
388: .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
1.115 beck 389: .It Cm ClientAliveInterval
390: Sets a timeout interval in seconds after which if no data has been received
391: from the client,
392: .Nm
393: will send a message through the encrypted
1.116 stevesk 394: channel to request a response from the client.
395: The default
1.115 beck 396: is 0, indicating that these messages will not be sent to the client.
1.116 stevesk 397: This option applies to protocol version 2 only.
1.115 beck 398: .It Cm ClientAliveCountMax
399: Sets the number of client alive messages (see above) which may be
400: sent without
401: .Nm
402: receiving any messages back from the client. If this threshold is
403: reached while client alive messages are being sent,
404: .Nm
405: will disconnect the client, terminating the session. It is important
406: to note that the use of client alive messages is very different from
1.116 stevesk 407: .Cm Keepalive
408: (below). The client alive messages are sent through the
1.115 beck 409: encrypted channel and therefore will not be spoofable. The TCP keepalive
1.116 stevesk 410: option enabled by
411: .Cm Keepalive
412: is spoofable. You want to use the client
1.115 beck 413: alive mechanism when you are basing something important on
414: clients having an active connection to the server.
1.116 stevesk 415: .Pp
416: The default value is 3. If you set
417: .Cm ClientAliveInterval
1.115 beck 418: (above) to 15, and leave this value at the default, unresponsive ssh clients
419: will be disconnected after approximately 45 seconds.
1.11 markus 420: .It Cm DenyGroups
421: This keyword can be followed by a number of group names, separated
1.36 aaron 422: by spaces.
1.81 markus 423: Users whose primary group or supplementary group list matches
424: one of the patterns aren't allowed to log in.
1.11 markus 425: .Ql \&*
426: and
427: .Ql ?
428: can be used as
1.36 aaron 429: wildcards in the patterns.
1.58 deraadt 430: Only group names are valid; a numerical group ID isn't recognized.
1.81 markus 431: By default login is allowed regardless of the group list.
1.11 markus 432: .Pp
433: .It Cm DenyUsers
434: This keyword can be followed by a number of user names, separated
1.36 aaron 435: by spaces.
436: Login is disallowed for user names that match one of the patterns.
1.11 markus 437: .Ql \&*
438: and
439: .Ql ?
1.36 aaron 440: can be used as wildcards in the patterns.
1.58 deraadt 441: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 442: By default login is allowed regardless of the user name.
1.47 markus 443: .It Cm GatewayPorts
444: Specifies whether remote hosts are allowed to connect to ports
445: forwarded for the client.
446: The argument must be
447: .Dq yes
448: or
449: .Dq no .
450: The default is
451: .Dq no .
1.120 markus 452: .It Cm HostbasedAuthentication
453: Specifies whether rhosts or /etc/hosts.equiv authentication together
454: with successful public key client host authentication is allowed
455: (hostbased authentication).
456: This option is similar to
457: .Cm RhostsRSAAuthentication
458: and applies to protocol version 2 only.
459: The default is
460: .Dq no .
1.2 deraadt 461: .It Cm HostKey
1.72 markus 462: Specifies the file containing the private host keys (default
1.46 markus 463: .Pa /etc/ssh_host_key )
1.72 markus 464: used by SSH protocol versions 1 and 2.
1.9 markus 465: Note that
466: .Nm
1.83 markus 467: will refuse to use a file if it is group/world-accessible.
1.72 markus 468: It is possible to have multiple host key files.
469: .Dq rsa1
470: keys are used for version 1 and
471: .Dq dsa
472: or
473: .Dq rsa
474: are used for version 2 of the SSH protocol.
1.2 deraadt 475: .It Cm IgnoreRhosts
1.34 markus 476: Specifies that
477: .Pa .rhosts
1.40 aaron 478: and
1.34 markus 479: .Pa .shosts
1.120 markus 480: files will not be used in
481: .Cm RhostsAuthentication ,
482: .Cm RhostsRSAAuthentication
483: or
484: .Cm HostbasedAuthentication .
485: .Pp
1.2 deraadt 486: .Pa /etc/hosts.equiv
1.1 deraadt 487: and
1.40 aaron 488: .Pa /etc/shosts.equiv
1.36 aaron 489: are still used.
1.40 aaron 490: The default is
1.34 markus 491: .Dq yes .
1.24 markus 492: .It Cm IgnoreUserKnownHosts
493: Specifies whether
494: .Nm
495: should ignore the user's
496: .Pa $HOME/.ssh/known_hosts
1.45 markus 497: during
1.120 markus 498: .Cm RhostsRSAAuthentication
499: or
500: .Cm HostbasedAuthentication .
1.24 markus 501: The default is
1.2 deraadt 502: .Dq no .
503: .It Cm KeepAlive
1.1 deraadt 504: Specifies whether the system should send keepalive messages to the
1.36 aaron 505: other side.
506: If they are sent, death of the connection or crash of one
507: of the machines will be properly noticed.
508: However, this means that
1.1 deraadt 509: connections will die if the route is down temporarily, and some people
1.36 aaron 510: find it annoying.
1.51 hugh 511: On the other hand, if keepalives are not sent,
1.2 deraadt 512: sessions may hang indefinitely on the server, leaving
513: .Dq ghost
514: users and consuming server resources.
515: .Pp
516: The default is
517: .Dq yes
518: (to send keepalives), and the server will notice
1.36 aaron 519: if the network goes down or the client host reboots.
520: This avoids infinitely hanging sessions.
1.2 deraadt 521: .Pp
522: To disable keepalives, the value should be set to
523: .Dq no
524: in both the server and the client configuration files.
525: .It Cm KerberosAuthentication
1.36 aaron 526: Specifies whether Kerberos authentication is allowed.
527: This can be in the form of a Kerberos ticket, or if
1.7 markus 528: .Cm PasswordAuthentication
1.1 deraadt 529: is yes, the password provided by the user will be validated through
1.67 aaron 530: the Kerberos KDC.
531: To use this option, the server needs a
1.59 provos 532: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 533: Default is
1.60 provos 534: .Dq yes .
1.2 deraadt 535: .It Cm KerberosOrLocalPasswd
1.1 deraadt 536: If set then if password authentication through Kerberos fails then
537: the password will be validated via any additional local mechanism
1.2 deraadt 538: such as
1.66 markus 539: .Pa /etc/passwd .
1.36 aaron 540: Default is
1.20 dugsong 541: .Dq yes .
1.2 deraadt 542: .It Cm KerberosTgtPassing
1.1 deraadt 543: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 544: Default is
1.3 dugsong 545: .Dq no ,
546: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 547: .It Cm KerberosTicketCleanup
1.7 markus 548: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 549: file on logout.
550: Default is
1.3 dugsong 551: .Dq yes .
1.2 deraadt 552: .It Cm KeyRegenerationInterval
1.120 markus 553: In protocol version 1, the ephemeral server key is automatically regenerated
554: after this many seconds (if it has been used).
1.36 aaron 555: The purpose of regeneration is to prevent
1.1 deraadt 556: decrypting captured sessions by later breaking into the machine and
1.36 aaron 557: stealing the keys.
558: The key is never stored anywhere.
559: If the value is 0, the key is never regenerated.
560: The default is 3600 (seconds).
1.7 markus 561: .It Cm ListenAddress
1.110 stevesk 562: Specifies the local addresses
1.120 markus 563: .Nm
1.7 markus 564: should listen on.
1.110 stevesk 565: The following forms may be used:
566: .Pp
567: .Bl -item -offset indent -compact
568: .It
569: .Cm ListenAddress
1.112 stevesk 570: .Sm off
571: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
572: .Sm on
1.110 stevesk 573: .It
574: .Cm ListenAddress
1.112 stevesk 575: .Sm off
576: .Ar host No | Ar IPv4_addr No : Ar port
577: .Sm on
1.110 stevesk 578: .It
579: .Cm ListenAddress
1.112 stevesk 580: .Sm off
581: .Oo
582: .Ar host No | Ar IPv6_addr Oc : Ar port
583: .Sm on
1.110 stevesk 584: .El
585: .Pp
586: If
1.112 stevesk 587: .Ar port
1.110 stevesk 588: is not specified,
1.120 markus 589: .Nm
1.110 stevesk 590: will listen on the address and all prior
591: .Cm Port
592: options specified. The default is to listen on all local
593: addresses. Multiple
594: .Cm ListenAddress
595: options are permitted. Additionally, any
596: .Cm Port
597: options must precede this option for non port qualified addresses.
1.2 deraadt 598: .It Cm LoginGraceTime
1.1 deraadt 599: The server disconnects after this time if the user has not
1.36 aaron 600: successfully logged in.
601: If the value is 0, there is no time limit.
1.1 deraadt 602: The default is 600 (seconds).
1.23 markus 603: .It Cm LogLevel
604: Gives the verbosity level that is used when logging messages from
605: .Nm sshd .
606: The possible values are:
1.82 markus 607: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
608: The default is INFO.
1.23 markus 609: Logging with level DEBUG violates the privacy of users
610: and is not recommended.
1.93 markus 611: .It Cm MACs
612: Specifies the available MAC (message authentication code) algorithms.
613: The MAC algorithm is used in protocol version 2
614: for data integrity protection.
615: Multiple algorithms must be comma-separated.
616: The default is
1.123 markus 617: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 618: .It Cm MaxStartups
619: Specifies the maximum number of concurrent unauthenticated connections to the
620: .Nm
621: daemon.
622: Additional connections will be dropped until authentication succeeds or the
623: .Cm LoginGraceTime
624: expires for a connection.
625: The default is 10.
1.57 markus 626: .Pp
627: Alternatively, random early drop can be enabled by specifying
628: the three colon separated values
629: .Dq start:rate:full
1.67 aaron 630: (e.g., "10:30:60").
1.57 markus 631: .Nm
1.86 stevesk 632: will refuse connection attempts with a probability of
1.57 markus 633: .Dq rate/100
634: (30%)
635: if there are currently
636: .Dq start
637: (10)
638: unauthenticated connections.
1.86 stevesk 639: The probability increases linearly and all connection attempts
1.57 markus 640: are refused if the number of unauthenticated connections reaches
641: .Dq full
642: (60).
1.2 deraadt 643: .It Cm PasswordAuthentication
1.1 deraadt 644: Specifies whether password authentication is allowed.
1.2 deraadt 645: The default is
646: .Dq yes .
647: .It Cm PermitEmptyPasswords
1.1 deraadt 648: When password authentication is allowed, it specifies whether the
1.36 aaron 649: server allows login to accounts with empty password strings.
650: The default is
1.34 markus 651: .Dq no .
1.2 deraadt 652: .It Cm PermitRootLogin
1.100 stevesk 653: Specifies whether root can login using
1.2 deraadt 654: .Xr ssh 1 .
1.15 markus 655: The argument must be
656: .Dq yes ,
1.94 markus 657: .Dq without-password ,
658: .Dq forced-commands-only
1.15 markus 659: or
660: .Dq no .
1.2 deraadt 661: The default is
662: .Dq yes .
1.94 markus 663: .Pp
664: If this option is set to
1.15 markus 665: .Dq without-password
1.94 markus 666: password authentication is disabled for root.
1.2 deraadt 667: .Pp
1.94 markus 668: If this option is set to
669: .Dq forced-commands-only
670: root login with public key authentication will be allowed,
671: but only if the
1.2 deraadt 672: .Ar command
1.94 markus 673: option has been specified
1.1 deraadt 674: (which may be useful for taking remote backups even if root login is
1.94 markus 675: normally not allowed). All other authentication methods are disabled
676: for root.
1.100 stevesk 677: .Pp
678: If this option is set to
679: .Dq no
680: root is not allowed to login.
1.43 markus 681: .It Cm PidFile
682: Specifies the file that contains the process identifier of the
683: .Nm
684: daemon.
685: The default is
686: .Pa /var/run/sshd.pid .
1.2 deraadt 687: .It Cm Port
1.1 deraadt 688: Specifies the port number that
1.2 deraadt 689: .Nm
1.36 aaron 690: listens on.
691: The default is 22.
1.28 markus 692: Multiple options of this type are permitted.
1.120 markus 693: See also
694: .Cm ListenAddress .
1.108 stevesk 695: .It Cm PrintLastLog
696: Specifies whether
697: .Nm
698: should print the date and time when the user last logged in.
699: The default is
700: .Dq yes .
1.2 deraadt 701: .It Cm PrintMotd
1.1 deraadt 702: Specifies whether
1.2 deraadt 703: .Nm
1.40 aaron 704: should print
1.2 deraadt 705: .Pa /etc/motd
1.36 aaron 706: when a user logs in interactively.
707: (On some systems it is also printed by the shell,
1.2 deraadt 708: .Pa /etc/profile ,
1.36 aaron 709: or equivalent.)
710: The default is
1.2 deraadt 711: .Dq yes .
1.41 markus 712: .It Cm Protocol
713: Specifies the protocol versions
714: .Nm
715: should support.
716: The possible values are
717: .Dq 1
718: and
719: .Dq 2 .
720: Multiple versions must be comma-separated.
721: The default is
1.118 deraadt 722: .Dq 2,1 .
1.104 deraadt 723: .It Cm PubkeyAuthentication
724: Specifies whether public key authentication is allowed.
725: The default is
726: .Dq yes .
727: Note that this option applies to protocol version 2 only.
1.87 markus 728: .It Cm ReverseMappingCheck
729: Specifies whether
730: .Nm
731: should try to verify the remote host name and check that
732: the resolved host name for the remote IP address maps back to the
733: very same IP address.
734: The default is
735: .Dq no .
1.2 deraadt 736: .It Cm RhostsAuthentication
1.1 deraadt 737: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 738: files is sufficient.
739: Normally, this method should not be permitted because it is insecure.
1.7 markus 740: .Cm RhostsRSAAuthentication
741: should be used
1.1 deraadt 742: instead, because it performs RSA-based host authentication in addition
743: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 744: The default is
745: .Dq no .
1.120 markus 746: This option applies to protocol version 1 only.
1.2 deraadt 747: .It Cm RhostsRSAAuthentication
1.1 deraadt 748: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 749: with successful RSA host authentication is allowed.
750: The default is
1.34 markus 751: .Dq no .
1.120 markus 752: This option applies to protocol version 1 only.
1.2 deraadt 753: .It Cm RSAAuthentication
1.36 aaron 754: Specifies whether pure RSA authentication is allowed.
755: The default is
1.2 deraadt 756: .Dq yes .
1.120 markus 757: This option applies to protocol version 1 only.
1.2 deraadt 758: .It Cm ServerKeyBits
1.120 markus 759: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 760: The minimum value is 512, and the default is 768.
1.2 deraadt 761: .It Cm StrictModes
1.12 markus 762: Specifies whether
763: .Nm
764: should check file modes and ownership of the
1.36 aaron 765: user's files and home directory before accepting login.
766: This is normally desirable because novices sometimes accidentally leave their
767: directory or files world-writable.
768: The default is
1.7 markus 769: .Dq yes .
1.54 jakob 770: .It Cm Subsystem
1.67 aaron 771: Configures an external subsystem (e.g., file transfer daemon).
772: Arguments should be a subsystem name and a command to execute upon subsystem
773: request.
1.63 markus 774: The command
775: .Xr sftp-server 8
776: implements the
777: .Dq sftp
778: file transfer subsystem.
1.54 jakob 779: By default no subsystems are defined.
780: Note that this option applies to protocol version 2 only.
1.2 deraadt 781: .It Cm SyslogFacility
1.1 deraadt 782: Gives the facility code that is used when logging messages from
1.2 deraadt 783: .Nm sshd .
1.1 deraadt 784: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 785: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
786: The default is AUTH.
1.10 markus 787: .It Cm UseLogin
788: Specifies whether
789: .Xr login 1
1.53 markus 790: is used for interactive login sessions.
1.127 markus 791: The default is
792: .Dq no .
1.53 markus 793: Note that
794: .Xr login 1
1.58 deraadt 795: is never used for remote command execution.
1.127 markus 796: Note also, that if this is enabled,
797: .Cm X11Forwarding
798: will be disabled because
799: .Xr login 1
800: does not know how to handle
801: .Xr xauth 1
802: cookies.
1.6 aaron 803: .It Cm X11DisplayOffset
804: Specifies the first display number available for
805: .Nm sshd Ns 's
1.36 aaron 806: X11 forwarding.
807: This prevents
1.6 aaron 808: .Nm
809: from interfering with real X11 servers.
1.34 markus 810: The default is 10.
1.30 markus 811: .It Cm X11Forwarding
1.36 aaron 812: Specifies whether X11 forwarding is permitted.
813: The default is
1.34 markus 814: .Dq no .
1.30 markus 815: Note that disabling X11 forwarding does not improve security in any
816: way, as users can always install their own forwarders.
1.127 markus 817: X11 forwarding is automatically disabled if
818: .Cm UseLogin
819: is enabled.
1.52 markus 820: .It Cm XAuthLocation
821: Specifies the location of the
822: .Xr xauth 1
823: program.
824: The default is
825: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 826: .El
1.124 stevesk 827: .Ss Time Formats
828: .Pp
829: .Nm
830: command-line arguments and configuration file options that specify time
831: may be expressed using a sequence of the form:
832: .Sm off
833: .Ar time Oo Ar qualifier Oc ,
834: .Sm on
835: where
836: .Ar time
837: is a positive integer value and
838: .Ar qualifier
839: is one of the following:
840: .Pp
841: .Bl -tag -width Ds -compact -offset indent
842: .It Cm <none>
843: seconds
844: .It Cm s | Cm S
845: seconds
846: .It Cm m | Cm M
847: minutes
848: .It Cm h | Cm H
849: hours
850: .It Cm d | Cm D
851: days
852: .It Cm w | Cm W
853: weeks
854: .El
855: .Pp
856: Each member of the sequence is added together to calculate
857: the total time value.
858: .Pp
859: Time format examples:
860: .Pp
861: .Bl -tag -width Ds -compact -offset indent
862: .It 600
863: 600 seconds (10 minutes)
864: .It 10m
865: 10 minutes
866: .It 1h30m
867: 1 hour 30 minutes (90 minutes)
868: .El
1.2 deraadt 869: .Sh LOGIN PROCESS
1.1 deraadt 870: When a user successfully logs in,
1.2 deraadt 871: .Nm
1.1 deraadt 872: does the following:
1.2 deraadt 873: .Bl -enum -offset indent
874: .It
1.1 deraadt 875: If the login is on a tty, and no command has been specified,
1.40 aaron 876: prints last login time and
1.2 deraadt 877: .Pa /etc/motd
1.1 deraadt 878: (unless prevented in the configuration file or by
1.2 deraadt 879: .Pa $HOME/.hushlogin ;
880: see the
1.40 aaron 881: .Sx FILES
1.2 deraadt 882: section).
883: .It
1.1 deraadt 884: If the login is on a tty, records login time.
1.2 deraadt 885: .It
886: Checks
887: .Pa /etc/nologin ;
888: if it exists, prints contents and quits
1.1 deraadt 889: (unless root).
1.2 deraadt 890: .It
1.1 deraadt 891: Changes to run with normal user privileges.
1.2 deraadt 892: .It
1.1 deraadt 893: Sets up basic environment.
1.2 deraadt 894: .It
895: Reads
896: .Pa $HOME/.ssh/environment
897: if it exists.
898: .It
1.1 deraadt 899: Changes to user's home directory.
1.2 deraadt 900: .It
901: If
902: .Pa $HOME/.ssh/rc
903: exists, runs it; else if
904: .Pa /etc/sshrc
905: exists, runs
1.36 aaron 906: it; otherwise runs xauth.
907: The
1.2 deraadt 908: .Dq rc
909: files are given the X11
1.1 deraadt 910: authentication protocol and cookie in standard input.
1.2 deraadt 911: .It
1.1 deraadt 912: Runs user's shell or command.
1.2 deraadt 913: .El
914: .Sh AUTHORIZED_KEYS FILE FORMAT
915: .Pa $HOME/.ssh/authorized_keys
1.125 markus 916: is the default file that lists the RSA keys that are
1.121 stevesk 917: permitted for RSA authentication in protocol version 1.
1.125 markus 918: .Cm AuthorizedKeysFile
919: may be used to specify an alternative file.
920: Similarly,
1.44 deraadt 921: .Pa $HOME/.ssh/authorized_keys2
1.125 markus 922: is the default file that lists the DSA and RSA keys that are
1.75 markus 923: permitted for public key authentication (PubkeyAuthentication)
1.120 markus 924: in protocol version 2.
1.125 markus 925: .Cm AuthorizedKeysFile2
926: may be used to specify an alternative file.
1.75 markus 927: .Pp
1.36 aaron 928: Each line of the file contains one
1.2 deraadt 929: key (empty lines and lines starting with a
930: .Ql #
931: are ignored as
1.36 aaron 932: comments).
1.75 markus 933: Each RSA public key consists of the following fields, separated by
1.36 aaron 934: spaces: options, bits, exponent, modulus, comment.
1.75 markus 935: Each protocol version 2 public key consists of:
936: options, keytype, base64 encoded key, comment.
937: The options fields
938: are optional; its presence is determined by whether the line starts
1.1 deraadt 939: with a number or not (the option field never starts with a number).
1.75 markus 940: The bits, exponent, modulus and comment fields give the RSA key for
941: protocol version 1; the
1.1 deraadt 942: comment field is not used for anything (but may be convenient for the
943: user to identify the key).
1.75 markus 944: For protocol version 2 the keytype is
945: .Dq ssh-dss
946: or
947: .Dq ssh-rsa .
1.2 deraadt 948: .Pp
1.1 deraadt 949: Note that lines in this file are usually several hundred bytes long
1.36 aaron 950: (because of the size of the RSA key modulus).
951: You don't want to type them in; instead, copy the
1.113 itojun 952: .Pa identity.pub ,
953: .Pa id_dsa.pub
1.75 markus 954: or the
1.113 itojun 955: .Pa id_rsa.pub
1.1 deraadt 956: file and edit it.
1.2 deraadt 957: .Pp
1.58 deraadt 958: The options (if present) consist of comma-separated option
1.36 aaron 959: specifications.
960: No spaces are permitted, except within double quotes.
1.1 deraadt 961: The following option specifications are supported:
1.2 deraadt 962: .Bl -tag -width Ds
963: .It Cm from="pattern-list"
1.1 deraadt 964: Specifies that in addition to RSA authentication, the canonical name
965: of the remote host must be present in the comma-separated list of
1.36 aaron 966: patterns
967: .Pf ( Ql *
968: and
969: .Ql ?
970: serve as wildcards).
971: The list may also contain
972: patterns negated by prefixing them with
973: .Ql ! ;
974: if the canonical host name matches a negated pattern, the key is not accepted.
975: The purpose
1.1 deraadt 976: of this option is to optionally increase security: RSA authentication
977: by itself does not trust the network or name servers or anything (but
978: the key); however, if somebody somehow steals the key, the key
1.36 aaron 979: permits an intruder to log in from anywhere in the world.
980: This additional option makes using a stolen key more difficult (name
1.1 deraadt 981: servers and/or routers would have to be compromised in addition to
982: just the key).
1.2 deraadt 983: .It Cm command="command"
1.1 deraadt 984: Specifies that the command is executed whenever this key is used for
1.36 aaron 985: authentication.
986: The command supplied by the user (if any) is ignored.
1.1 deraadt 987: The command is run on a pty if the connection requests a pty;
1.36 aaron 988: otherwise it is run without a tty.
1.89 markus 989: Note that if you want a 8-bit clean channel,
990: you must not request a pty or should specify
991: .Cm no-pty .
1.36 aaron 992: A quote may be included in the command by quoting it with a backslash.
993: This option might be useful
994: to restrict certain RSA keys to perform just a specific operation.
995: An example might be a key that permits remote backups but nothing else.
1.51 hugh 996: Note that the client may specify TCP/IP and/or X11
997: forwarding unless they are explicitly prohibited.
1.2 deraadt 998: .It Cm environment="NAME=value"
1.1 deraadt 999: Specifies that the string is to be added to the environment when
1.36 aaron 1000: logging in using this key.
1001: Environment variables set this way
1002: override other default environment values.
1003: Multiple options of this type are permitted.
1.2 deraadt 1004: .It Cm no-port-forwarding
1.1 deraadt 1005: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 1006: Any port forward requests by the client will return an error.
1007: This might be used, e.g., in connection with the
1.2 deraadt 1008: .Cm command
1.1 deraadt 1009: option.
1.2 deraadt 1010: .It Cm no-X11-forwarding
1.1 deraadt 1011: Forbids X11 forwarding when this key is used for authentication.
1012: Any X11 forward requests by the client will return an error.
1.2 deraadt 1013: .It Cm no-agent-forwarding
1.1 deraadt 1014: Forbids authentication agent forwarding when this key is used for
1015: authentication.
1.2 deraadt 1016: .It Cm no-pty
1.1 deraadt 1017: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 1018: .It Cm permitopen="host:port"
1019: Limit local
1020: .Li ``ssh -L''
1.111 stevesk 1021: port forwarding such that it may only connect to the specified host and
1.107 djm 1022: port. Multiple
1023: .Cm permitopen
1.111 stevesk 1024: options may be applied separated by commas. No pattern matching is
1.107 djm 1025: performed on the specified hostnames, they must be literal domains or
1026: addresses.
1.2 deraadt 1027: .El
1028: .Ss Examples
1.1 deraadt 1029: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 1030: .Pp
1.1 deraadt 1031: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 1032: .Pp
1.1 deraadt 1033: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 1034: .Pp
1035: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 1036: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 1037: The
1.44 deraadt 1038: .Pa /etc/ssh_known_hosts ,
1039: .Pa /etc/ssh_known_hosts2 ,
1040: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 1041: and
1.44 deraadt 1042: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 1043: files contain host public keys for all known hosts.
1044: The global file should
1.37 brad 1045: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 1046: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 1047: its key is added to the per-user file.
1.2 deraadt 1048: .Pp
1.1 deraadt 1049: Each line in these files contains the following fields: hostnames,
1.36 aaron 1050: bits, exponent, modulus, comment.
1051: The fields are separated by spaces.
1.2 deraadt 1052: .Pp
1.1 deraadt 1053: Hostnames is a comma-separated list of patterns ('*' and '?' act as
1054: wildcards); each pattern in turn is matched against the canonical host
1055: name (when authenticating a client) or against the user-supplied
1.36 aaron 1056: name (when authenticating a server).
1057: A pattern may also be preceded by
1.2 deraadt 1058: .Ql !
1059: to indicate negation: if the host name matches a negated
1.1 deraadt 1060: pattern, it is not accepted (by that line) even if it matched another
1061: pattern on the line.
1.2 deraadt 1062: .Pp
1.49 markus 1063: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 1064: can be obtained, e.g., from
1065: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 1066: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 1067: .Pp
1068: Lines starting with
1069: .Ql #
1070: and empty lines are ignored as comments.
1071: .Pp
1.1 deraadt 1072: When performing host authentication, authentication is accepted if any
1.36 aaron 1073: matching line has the proper key.
1074: It is thus permissible (but not
1.1 deraadt 1075: recommended) to have several lines or different host keys for the same
1.36 aaron 1076: names.
1077: This will inevitably happen when short forms of host names
1078: from different domains are put in the file.
1079: It is possible
1.1 deraadt 1080: that the files contain conflicting information; authentication is
1081: accepted if valid information can be found from either file.
1.2 deraadt 1082: .Pp
1.1 deraadt 1083: Note that the lines in these files are typically hundreds of characters
1084: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1085: Rather, generate them by a script
1.40 aaron 1086: or by taking
1.2 deraadt 1087: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1088: and adding the host names at the front.
1.2 deraadt 1089: .Ss Examples
1.120 markus 1090: .Bd -literal
1091: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1092: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1093: .Ed
1.2 deraadt 1094: .Sh FILES
1095: .Bl -tag -width Ds
1096: .It Pa /etc/sshd_config
1.1 deraadt 1097: Contains configuration data for
1.2 deraadt 1098: .Nm sshd .
1.1 deraadt 1099: This file should be writable by root only, but it is recommended
1100: (though not necessary) that it be world-readable.
1.98 deraadt 1101: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.120 markus 1102: These three files contain the private parts of the host keys.
1.98 deraadt 1103: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1104: accessible to others.
1.14 markus 1105: Note that
1106: .Nm
1107: does not start if this file is group/world-accessible.
1.98 deraadt 1108: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.120 markus 1109: These three files contain the public parts of the host keys.
1.98 deraadt 1110: These files should be world-readable but writable only by
1.36 aaron 1111: root.
1.98 deraadt 1112: Their contents should match the respective private parts.
1113: These files are not
1114: really used for anything; they are provided for the convenience of
1115: the user so their contents can be copied to known hosts files.
1116: These files are created using
1.7 markus 1117: .Xr ssh-keygen 1 .
1.73 provos 1118: .It Pa /etc/primes
1119: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1120: .It Pa /var/run/sshd.pid
1121: Contains the process ID of the
1122: .Nm
1.1 deraadt 1123: listening for connections (if there are several daemons running
1124: concurrently for different ports, this contains the pid of the one
1.36 aaron 1125: started last).
1.58 deraadt 1126: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1127: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 1128: Lists the RSA keys that can be used to log into the user's account.
1129: This file must be readable by root (which may on some machines imply
1130: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 1131: volume).
1132: It is recommended that it not be accessible by others.
1133: The format of this file is described above.
1.44 deraadt 1134: Users will place the contents of their
1135: .Pa identity.pub
1136: files into this file, as described in
1137: .Xr ssh-keygen 1 .
1138: .It Pa $HOME/.ssh/authorized_keys2
1.120 markus 1139: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1140: This file must be readable by root (which may on some machines imply
1141: it being world-readable if the user's home directory resides on an NFS
1142: volume).
1143: It is recommended that it not be accessible by others.
1144: The format of this file is described above.
1145: Users will place the contents of their
1146: .Pa id_dsa.pub
1.113 itojun 1147: and/or
1148: .Pa id_rsa.pub
1.44 deraadt 1149: files into this file, as described in
1150: .Xr ssh-keygen 1 .
1.22 markus 1151: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1152: These files are consulted when using rhosts with RSA host
1.36 aaron 1153: authentication to check the public key of the host.
1154: The key must be listed in one of these files to be accepted.
1.22 markus 1155: The client uses the same files
1.96 markus 1156: to verify that it is connecting to the correct remote host.
1.36 aaron 1157: These files should be writable only by root/the owner.
1.2 deraadt 1158: .Pa /etc/ssh_known_hosts
1159: should be world-readable, and
1160: .Pa $HOME/.ssh/known_hosts
1161: can but need not be world-readable.
1.120 markus 1162: .It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2"
1163: These files are consulted when using protocol version 2 hostbased
1164: authentication to check the public key of the host.
1165: The key must be listed in one of these files to be accepted.
1166: The client uses the same files
1167: to verify that it is connecting to the correct remote host.
1168: These files should be writable only by root/the owner.
1169: .Pa /etc/ssh_known_hosts2
1170: should be world-readable, and
1171: .Pa $HOME/.ssh/known_hosts2
1172: can but need not be world-readable.
1.6 aaron 1173: .It Pa /etc/nologin
1.40 aaron 1174: If this file exists,
1.2 deraadt 1175: .Nm
1.36 aaron 1176: refuses to let anyone except root log in.
1177: The contents of the file
1.1 deraadt 1178: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1179: refused.
1180: The file should be world-readable.
1.19 dugsong 1181: .It Pa /etc/hosts.allow, /etc/hosts.deny
1182: If compiled with
1183: .Sy LIBWRAP
1184: support, tcp-wrappers access controls may be defined here as described in
1185: .Xr hosts_access 5 .
1.6 aaron 1186: .It Pa $HOME/.rhosts
1.1 deraadt 1187: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1188: line.
1189: The given user on the corresponding host is permitted to log in
1190: without password.
1191: The same file is used by rlogind and rshd.
1.6 aaron 1192: The file must
1.1 deraadt 1193: be writable only by the user; it is recommended that it not be
1194: accessible by others.
1.2 deraadt 1195: .Pp
1.36 aaron 1196: If is also possible to use netgroups in the file.
1197: Either host or user
1.1 deraadt 1198: name may be of the form +@groupname to specify all hosts or all users
1199: in the group.
1.2 deraadt 1200: .It Pa $HOME/.shosts
1201: For ssh,
1202: this file is exactly the same as for
1203: .Pa .rhosts .
1204: However, this file is
1205: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1206: .It Pa /etc/hosts.equiv
1.2 deraadt 1207: This file is used during
1208: .Pa .rhosts
1.36 aaron 1209: authentication.
1210: In the simplest form, this file contains host names, one per line.
1211: Users on
1.1 deraadt 1212: those hosts are permitted to log in without a password, provided they
1.36 aaron 1213: have the same user name on both machines.
1214: The host name may also be
1.1 deraadt 1215: followed by a user name; such users are permitted to log in as
1.2 deraadt 1216: .Em any
1.36 aaron 1217: user on this machine (except root).
1218: Additionally, the syntax
1.2 deraadt 1219: .Dq +@group
1.36 aaron 1220: can be used to specify netgroups.
1221: Negated entries start with
1.2 deraadt 1222: .Ql \&- .
1223: .Pp
1.1 deraadt 1224: If the client host/user is successfully matched in this file, login is
1225: automatically permitted provided the client and server user names are the
1.36 aaron 1226: same.
1227: Additionally, successful RSA host authentication is normally required.
1228: This file must be writable only by root; it is recommended
1.1 deraadt 1229: that it be world-readable.
1.2 deraadt 1230: .Pp
1.6 aaron 1231: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1232: .Pa hosts.equiv .
1.1 deraadt 1233: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1234: .Em anybody ,
1.1 deraadt 1235: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1236: binaries and directories.
1237: Using a user name practically grants the user root access.
1238: The only valid use for user names that I can think
1.1 deraadt 1239: of is in negative entries.
1.2 deraadt 1240: .Pp
1241: Note that this warning also applies to rsh/rlogin.
1242: .It Pa /etc/shosts.equiv
1.1 deraadt 1243: This is processed exactly as
1.2 deraadt 1244: .Pa /etc/hosts.equiv .
1.1 deraadt 1245: However, this file may be useful in environments that want to run both
1.2 deraadt 1246: rsh/rlogin and ssh.
1.6 aaron 1247: .It Pa $HOME/.ssh/environment
1.36 aaron 1248: This file is read into the environment at login (if it exists).
1249: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1250: .Ql # ) ,
1.36 aaron 1251: and assignment lines of the form name=value.
1252: The file should be writable
1.6 aaron 1253: only by the user; it need not be readable by anyone else.
1.2 deraadt 1254: .It Pa $HOME/.ssh/rc
1.1 deraadt 1255: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1256: environment files but before starting the user's shell or command.
1257: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1258: standard input (and
1259: .Ev DISPLAY
1.36 aaron 1260: in environment).
1261: This must call
1.2 deraadt 1262: .Xr xauth 1
1263: in that case.
1264: .Pp
1.1 deraadt 1265: The primary purpose of this file is to run any initialization routines
1266: which may be needed before the user's home directory becomes
1267: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1268: .Pp
1.1 deraadt 1269: This file will probably contain some initialization code followed by
1.120 markus 1270: something similar to:
1271: .Bd -literal
1272: if read proto cookie; then
1273: echo add $DISPLAY $proto $cookie | xauth -q -
1274: fi
1275: .Ed
1.2 deraadt 1276: .Pp
1277: If this file does not exist,
1278: .Pa /etc/sshrc
1279: is run, and if that
1.1 deraadt 1280: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1281: .Pp
1.1 deraadt 1282: This file should be writable only by the user, and need not be
1283: readable by anyone else.
1.2 deraadt 1284: .It Pa /etc/sshrc
1285: Like
1286: .Pa $HOME/.ssh/rc .
1287: This can be used to specify
1.36 aaron 1288: machine-specific login-time initializations globally.
1289: This file should be writable only by root, and should be world-readable.
1.56 aaron 1290: .El
1.71 aaron 1291: .Sh AUTHORS
1.84 markus 1292: OpenSSH is a derivative of the original and free
1293: ssh 1.2.12 release by Tatu Ylonen.
1294: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1295: Theo de Raadt and Dug Song
1296: removed many bugs, re-added newer features and
1297: created OpenSSH.
1298: Markus Friedl contributed the support for SSH
1299: protocol versions 1.5 and 2.0.
1.2 deraadt 1300: .Sh SEE ALSO
1.128 ! mpech 1301: .Xr rlogin 1 ,
! 1302: .Xr rsh 1 ,
1.2 deraadt 1303: .Xr scp 1 ,
1.90 djm 1304: .Xr sftp 1 ,
1.2 deraadt 1305: .Xr ssh 1 ,
1.5 deraadt 1306: .Xr ssh-add 1 ,
1.2 deraadt 1307: .Xr ssh-agent 1 ,
1.5 deraadt 1308: .Xr ssh-keygen 1 ,
1.128 ! mpech 1309: .Xr sftp-server 8
1.119 markus 1310: .Rs
1311: .%A T. Ylonen
1312: .%A T. Kivinen
1313: .%A M. Saarinen
1314: .%A T. Rinne
1315: .%A S. Lehtinen
1316: .%T "SSH Protocol Architecture"
1317: .%N draft-ietf-secsh-architecture-07.txt
1.120 markus 1318: .%D January 2001
1319: .%O work in progress material
1320: .Re
1321: .Rs
1322: .%A M. Friedl
1323: .%A N. Provos
1324: .%A W. A. Simpson
1325: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1326: .%N draft-ietf-secsh-dh-group-exchange-00.txt
1.119 markus 1327: .%D January 2001
1328: .%O work in progress material
1329: .Re
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