Annotation of src/usr.bin/ssh/sshd.8, Revision 1.123
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,
32: .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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: .\"
1.123 ! markus 37: .\" $OpenBSD: sshd.8,v 1.122 2001/05/19 16:08:43 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 ,
179: by executing itself with the name it was started as, ie.
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
334: .It Cm Banner
335: In some jurisdictions, sending a warning message before authentication
336: may be relevant for getting legal protection.
337: The contents of the specified file are sent to the remote user before
338: authentication is allowed.
339: This option is only available for protocol version 2.
1.11 markus 340: .Pp
1.104 deraadt 341: .It Cm ChallengeResponseAuthentication
342: Specifies whether
343: challenge response
344: authentication is allowed.
345: Currently there is only support for
346: .Xr skey 1
347: authentication.
348: The default is
349: .Dq yes .
1.8 markus 350: .It Cm CheckMail
351: Specifies whether
352: .Nm
353: should check for new mail for interactive logins.
354: The default is
355: .Dq no .
1.122 markus 356: .It Cm Ciphers
357: Specifies the ciphers allowed for protocol version 2.
358: Multiple ciphers must be comma-separated.
359: The default is
360: .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.
1.115 beck 361: .It Cm ClientAliveInterval
362: Sets a timeout interval in seconds after which if no data has been received
363: from the client,
364: .Nm
365: will send a message through the encrypted
1.116 stevesk 366: channel to request a response from the client.
367: The default
1.115 beck 368: is 0, indicating that these messages will not be sent to the client.
1.116 stevesk 369: This option applies to protocol version 2 only.
1.115 beck 370: .It Cm ClientAliveCountMax
371: Sets the number of client alive messages (see above) which may be
372: sent without
373: .Nm
374: receiving any messages back from the client. If this threshold is
375: reached while client alive messages are being sent,
376: .Nm
377: will disconnect the client, terminating the session. It is important
378: to note that the use of client alive messages is very different from
1.116 stevesk 379: .Cm Keepalive
380: (below). The client alive messages are sent through the
1.115 beck 381: encrypted channel and therefore will not be spoofable. The TCP keepalive
1.116 stevesk 382: option enabled by
383: .Cm Keepalive
384: is spoofable. You want to use the client
1.115 beck 385: alive mechanism when you are basing something important on
386: clients having an active connection to the server.
1.116 stevesk 387: .Pp
388: The default value is 3. If you set
389: .Cm ClientAliveInterval
1.115 beck 390: (above) to 15, and leave this value at the default, unresponsive ssh clients
391: will be disconnected after approximately 45 seconds.
1.11 markus 392: .It Cm DenyGroups
393: This keyword can be followed by a number of group names, separated
1.36 aaron 394: by spaces.
1.81 markus 395: Users whose primary group or supplementary group list matches
396: one of the patterns aren't allowed to log in.
1.11 markus 397: .Ql \&*
398: and
399: .Ql ?
400: can be used as
1.36 aaron 401: wildcards in the patterns.
1.58 deraadt 402: Only group names are valid; a numerical group ID isn't recognized.
1.81 markus 403: By default login is allowed regardless of the group list.
1.11 markus 404: .Pp
405: .It Cm DenyUsers
406: This keyword can be followed by a number of user names, separated
1.36 aaron 407: by spaces.
408: Login is disallowed for user names that match one of the patterns.
1.11 markus 409: .Ql \&*
410: and
411: .Ql ?
1.36 aaron 412: can be used as wildcards in the patterns.
1.58 deraadt 413: Only user names are valid; a numerical user ID isn't recognized.
1.36 aaron 414: By default login is allowed regardless of the user name.
1.47 markus 415: .It Cm GatewayPorts
416: Specifies whether remote hosts are allowed to connect to ports
417: forwarded for the client.
418: The argument must be
419: .Dq yes
420: or
421: .Dq no .
422: The default is
423: .Dq no .
1.120 markus 424: .It Cm HostbasedAuthentication
425: Specifies whether rhosts or /etc/hosts.equiv authentication together
426: with successful public key client host authentication is allowed
427: (hostbased authentication).
428: This option is similar to
429: .Cm RhostsRSAAuthentication
430: and applies to protocol version 2 only.
431: The default is
432: .Dq no .
1.2 deraadt 433: .It Cm HostKey
1.72 markus 434: Specifies the file containing the private host keys (default
1.46 markus 435: .Pa /etc/ssh_host_key )
1.72 markus 436: used by SSH protocol versions 1 and 2.
1.9 markus 437: Note that
438: .Nm
1.83 markus 439: will refuse to use a file if it is group/world-accessible.
1.72 markus 440: It is possible to have multiple host key files.
441: .Dq rsa1
442: keys are used for version 1 and
443: .Dq dsa
444: or
445: .Dq rsa
446: are used for version 2 of the SSH protocol.
1.2 deraadt 447: .It Cm IgnoreRhosts
1.34 markus 448: Specifies that
449: .Pa .rhosts
1.40 aaron 450: and
1.34 markus 451: .Pa .shosts
1.120 markus 452: files will not be used in
453: .Cm RhostsAuthentication ,
454: .Cm RhostsRSAAuthentication
455: or
456: .Cm HostbasedAuthentication .
457: .Pp
1.2 deraadt 458: .Pa /etc/hosts.equiv
1.1 deraadt 459: and
1.40 aaron 460: .Pa /etc/shosts.equiv
1.36 aaron 461: are still used.
1.40 aaron 462: The default is
1.34 markus 463: .Dq yes .
1.24 markus 464: .It Cm IgnoreUserKnownHosts
465: Specifies whether
466: .Nm
467: should ignore the user's
468: .Pa $HOME/.ssh/known_hosts
1.45 markus 469: during
1.120 markus 470: .Cm RhostsRSAAuthentication
471: or
472: .Cm HostbasedAuthentication .
1.24 markus 473: The default is
1.2 deraadt 474: .Dq no .
475: .It Cm KeepAlive
1.1 deraadt 476: Specifies whether the system should send keepalive messages to the
1.36 aaron 477: other side.
478: If they are sent, death of the connection or crash of one
479: of the machines will be properly noticed.
480: However, this means that
1.1 deraadt 481: connections will die if the route is down temporarily, and some people
1.36 aaron 482: find it annoying.
1.51 hugh 483: On the other hand, if keepalives are not sent,
1.2 deraadt 484: sessions may hang indefinitely on the server, leaving
485: .Dq ghost
486: users and consuming server resources.
487: .Pp
488: The default is
489: .Dq yes
490: (to send keepalives), and the server will notice
1.36 aaron 491: if the network goes down or the client host reboots.
492: This avoids infinitely hanging sessions.
1.2 deraadt 493: .Pp
494: To disable keepalives, the value should be set to
495: .Dq no
496: in both the server and the client configuration files.
497: .It Cm KerberosAuthentication
1.36 aaron 498: Specifies whether Kerberos authentication is allowed.
499: This can be in the form of a Kerberos ticket, or if
1.7 markus 500: .Cm PasswordAuthentication
1.1 deraadt 501: is yes, the password provided by the user will be validated through
1.67 aaron 502: the Kerberos KDC.
503: To use this option, the server needs a
1.59 provos 504: Kerberos servtab which allows the verification of the KDC's identity.
1.36 aaron 505: Default is
1.60 provos 506: .Dq yes .
1.2 deraadt 507: .It Cm KerberosOrLocalPasswd
1.1 deraadt 508: If set then if password authentication through Kerberos fails then
509: the password will be validated via any additional local mechanism
1.2 deraadt 510: such as
1.66 markus 511: .Pa /etc/passwd .
1.36 aaron 512: Default is
1.20 dugsong 513: .Dq yes .
1.2 deraadt 514: .It Cm KerberosTgtPassing
1.1 deraadt 515: Specifies whether a Kerberos TGT may be forwarded to the server.
1.40 aaron 516: Default is
1.3 dugsong 517: .Dq no ,
518: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 519: .It Cm KerberosTicketCleanup
1.7 markus 520: Specifies whether to automatically destroy the user's ticket cache
1.36 aaron 521: file on logout.
522: Default is
1.3 dugsong 523: .Dq yes .
1.2 deraadt 524: .It Cm KeyRegenerationInterval
1.120 markus 525: In protocol version 1, the ephemeral server key is automatically regenerated
526: after this many seconds (if it has been used).
1.36 aaron 527: The purpose of regeneration is to prevent
1.1 deraadt 528: decrypting captured sessions by later breaking into the machine and
1.36 aaron 529: stealing the keys.
530: The key is never stored anywhere.
531: If the value is 0, the key is never regenerated.
532: The default is 3600 (seconds).
1.7 markus 533: .It Cm ListenAddress
1.110 stevesk 534: Specifies the local addresses
1.120 markus 535: .Nm
1.7 markus 536: should listen on.
1.110 stevesk 537: The following forms may be used:
538: .Pp
539: .Bl -item -offset indent -compact
540: .It
541: .Cm ListenAddress
1.112 stevesk 542: .Sm off
543: .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
544: .Sm on
1.110 stevesk 545: .It
546: .Cm ListenAddress
1.112 stevesk 547: .Sm off
548: .Ar host No | Ar IPv4_addr No : Ar port
549: .Sm on
1.110 stevesk 550: .It
551: .Cm ListenAddress
1.112 stevesk 552: .Sm off
553: .Oo
554: .Ar host No | Ar IPv6_addr Oc : Ar port
555: .Sm on
1.110 stevesk 556: .El
557: .Pp
558: If
1.112 stevesk 559: .Ar port
1.110 stevesk 560: is not specified,
1.120 markus 561: .Nm
1.110 stevesk 562: will listen on the address and all prior
563: .Cm Port
564: options specified. The default is to listen on all local
565: addresses. Multiple
566: .Cm ListenAddress
567: options are permitted. Additionally, any
568: .Cm Port
569: options must precede this option for non port qualified addresses.
1.2 deraadt 570: .It Cm LoginGraceTime
1.1 deraadt 571: The server disconnects after this time if the user has not
1.36 aaron 572: successfully logged in.
573: If the value is 0, there is no time limit.
1.1 deraadt 574: The default is 600 (seconds).
1.23 markus 575: .It Cm LogLevel
576: Gives the verbosity level that is used when logging messages from
577: .Nm sshd .
578: The possible values are:
1.82 markus 579: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
580: The default is INFO.
1.23 markus 581: Logging with level DEBUG violates the privacy of users
582: and is not recommended.
1.93 markus 583: .It Cm MACs
584: Specifies the available MAC (message authentication code) algorithms.
585: The MAC algorithm is used in protocol version 2
586: for data integrity protection.
587: Multiple algorithms must be comma-separated.
588: The default is
1.123 ! markus 589: .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
1.55 markus 590: .It Cm MaxStartups
591: Specifies the maximum number of concurrent unauthenticated connections to the
592: .Nm
593: daemon.
594: Additional connections will be dropped until authentication succeeds or the
595: .Cm LoginGraceTime
596: expires for a connection.
597: The default is 10.
1.57 markus 598: .Pp
599: Alternatively, random early drop can be enabled by specifying
600: the three colon separated values
601: .Dq start:rate:full
1.67 aaron 602: (e.g., "10:30:60").
1.57 markus 603: .Nm
1.86 stevesk 604: will refuse connection attempts with a probability of
1.57 markus 605: .Dq rate/100
606: (30%)
607: if there are currently
608: .Dq start
609: (10)
610: unauthenticated connections.
1.86 stevesk 611: The probability increases linearly and all connection attempts
1.57 markus 612: are refused if the number of unauthenticated connections reaches
613: .Dq full
614: (60).
1.2 deraadt 615: .It Cm PasswordAuthentication
1.1 deraadt 616: Specifies whether password authentication is allowed.
1.2 deraadt 617: The default is
618: .Dq yes .
619: .It Cm PermitEmptyPasswords
1.1 deraadt 620: When password authentication is allowed, it specifies whether the
1.36 aaron 621: server allows login to accounts with empty password strings.
622: The default is
1.34 markus 623: .Dq no .
1.2 deraadt 624: .It Cm PermitRootLogin
1.100 stevesk 625: Specifies whether root can login using
1.2 deraadt 626: .Xr ssh 1 .
1.15 markus 627: The argument must be
628: .Dq yes ,
1.94 markus 629: .Dq without-password ,
630: .Dq forced-commands-only
1.15 markus 631: or
632: .Dq no .
1.2 deraadt 633: The default is
634: .Dq yes .
1.94 markus 635: .Pp
636: If this option is set to
1.15 markus 637: .Dq without-password
1.94 markus 638: password authentication is disabled for root.
1.2 deraadt 639: .Pp
1.94 markus 640: If this option is set to
641: .Dq forced-commands-only
642: root login with public key authentication will be allowed,
643: but only if the
1.2 deraadt 644: .Ar command
1.94 markus 645: option has been specified
1.1 deraadt 646: (which may be useful for taking remote backups even if root login is
1.94 markus 647: normally not allowed). All other authentication methods are disabled
648: for root.
1.100 stevesk 649: .Pp
650: If this option is set to
651: .Dq no
652: root is not allowed to login.
1.43 markus 653: .It Cm PidFile
654: Specifies the file that contains the process identifier of the
655: .Nm
656: daemon.
657: The default is
658: .Pa /var/run/sshd.pid .
1.2 deraadt 659: .It Cm Port
1.1 deraadt 660: Specifies the port number that
1.2 deraadt 661: .Nm
1.36 aaron 662: listens on.
663: The default is 22.
1.28 markus 664: Multiple options of this type are permitted.
1.120 markus 665: See also
666: .Cm ListenAddress .
1.108 stevesk 667: .It Cm PrintLastLog
668: Specifies whether
669: .Nm
670: should print the date and time when the user last logged in.
671: The default is
672: .Dq yes .
1.2 deraadt 673: .It Cm PrintMotd
1.1 deraadt 674: Specifies whether
1.2 deraadt 675: .Nm
1.40 aaron 676: should print
1.2 deraadt 677: .Pa /etc/motd
1.36 aaron 678: when a user logs in interactively.
679: (On some systems it is also printed by the shell,
1.2 deraadt 680: .Pa /etc/profile ,
1.36 aaron 681: or equivalent.)
682: The default is
1.2 deraadt 683: .Dq yes .
1.41 markus 684: .It Cm Protocol
685: Specifies the protocol versions
686: .Nm
687: should support.
688: The possible values are
689: .Dq 1
690: and
691: .Dq 2 .
692: Multiple versions must be comma-separated.
693: The default is
1.118 deraadt 694: .Dq 2,1 .
1.104 deraadt 695: .It Cm PubkeyAuthentication
696: Specifies whether public key authentication is allowed.
697: The default is
698: .Dq yes .
699: Note that this option applies to protocol version 2 only.
1.87 markus 700: .It Cm ReverseMappingCheck
701: Specifies whether
702: .Nm
703: should try to verify the remote host name and check that
704: the resolved host name for the remote IP address maps back to the
705: very same IP address.
706: The default is
707: .Dq no .
1.2 deraadt 708: .It Cm RhostsAuthentication
1.1 deraadt 709: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 710: files is sufficient.
711: Normally, this method should not be permitted because it is insecure.
1.7 markus 712: .Cm RhostsRSAAuthentication
713: should be used
1.1 deraadt 714: instead, because it performs RSA-based host authentication in addition
715: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 716: The default is
717: .Dq no .
1.120 markus 718: This option applies to protocol version 1 only.
1.2 deraadt 719: .It Cm RhostsRSAAuthentication
1.1 deraadt 720: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 721: with successful RSA host authentication is allowed.
722: The default is
1.34 markus 723: .Dq no .
1.120 markus 724: This option applies to protocol version 1 only.
1.2 deraadt 725: .It Cm RSAAuthentication
1.36 aaron 726: Specifies whether pure RSA authentication is allowed.
727: The default is
1.2 deraadt 728: .Dq yes .
1.120 markus 729: This option applies to protocol version 1 only.
1.2 deraadt 730: .It Cm ServerKeyBits
1.120 markus 731: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 732: The minimum value is 512, and the default is 768.
1.2 deraadt 733: .It Cm StrictModes
1.12 markus 734: Specifies whether
735: .Nm
736: should check file modes and ownership of the
1.36 aaron 737: user's files and home directory before accepting login.
738: This is normally desirable because novices sometimes accidentally leave their
739: directory or files world-writable.
740: The default is
1.7 markus 741: .Dq yes .
1.54 jakob 742: .It Cm Subsystem
1.67 aaron 743: Configures an external subsystem (e.g., file transfer daemon).
744: Arguments should be a subsystem name and a command to execute upon subsystem
745: request.
1.63 markus 746: The command
747: .Xr sftp-server 8
748: implements the
749: .Dq sftp
750: file transfer subsystem.
1.54 jakob 751: By default no subsystems are defined.
752: Note that this option applies to protocol version 2 only.
1.2 deraadt 753: .It Cm SyslogFacility
1.1 deraadt 754: Gives the facility code that is used when logging messages from
1.2 deraadt 755: .Nm sshd .
1.1 deraadt 756: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 757: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
758: The default is AUTH.
1.10 markus 759: .It Cm UseLogin
760: Specifies whether
761: .Xr login 1
1.53 markus 762: is used for interactive login sessions.
763: Note that
764: .Xr login 1
1.58 deraadt 765: is never used for remote command execution.
1.36 aaron 766: The default is
1.10 markus 767: .Dq no .
1.6 aaron 768: .It Cm X11DisplayOffset
769: Specifies the first display number available for
770: .Nm sshd Ns 's
1.36 aaron 771: X11 forwarding.
772: This prevents
1.6 aaron 773: .Nm
774: from interfering with real X11 servers.
1.34 markus 775: The default is 10.
1.30 markus 776: .It Cm X11Forwarding
1.36 aaron 777: Specifies whether X11 forwarding is permitted.
778: The default is
1.34 markus 779: .Dq no .
1.30 markus 780: Note that disabling X11 forwarding does not improve security in any
781: way, as users can always install their own forwarders.
1.52 markus 782: .It Cm XAuthLocation
783: Specifies the location of the
784: .Xr xauth 1
785: program.
786: The default is
787: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 788: .El
789: .Sh LOGIN PROCESS
1.1 deraadt 790: When a user successfully logs in,
1.2 deraadt 791: .Nm
1.1 deraadt 792: does the following:
1.2 deraadt 793: .Bl -enum -offset indent
794: .It
1.1 deraadt 795: If the login is on a tty, and no command has been specified,
1.40 aaron 796: prints last login time and
1.2 deraadt 797: .Pa /etc/motd
1.1 deraadt 798: (unless prevented in the configuration file or by
1.2 deraadt 799: .Pa $HOME/.hushlogin ;
800: see the
1.40 aaron 801: .Sx FILES
1.2 deraadt 802: section).
803: .It
1.1 deraadt 804: If the login is on a tty, records login time.
1.2 deraadt 805: .It
806: Checks
807: .Pa /etc/nologin ;
808: if it exists, prints contents and quits
1.1 deraadt 809: (unless root).
1.2 deraadt 810: .It
1.1 deraadt 811: Changes to run with normal user privileges.
1.2 deraadt 812: .It
1.1 deraadt 813: Sets up basic environment.
1.2 deraadt 814: .It
815: Reads
816: .Pa $HOME/.ssh/environment
817: if it exists.
818: .It
1.1 deraadt 819: Changes to user's home directory.
1.2 deraadt 820: .It
821: If
822: .Pa $HOME/.ssh/rc
823: exists, runs it; else if
824: .Pa /etc/sshrc
825: exists, runs
1.36 aaron 826: it; otherwise runs xauth.
827: The
1.2 deraadt 828: .Dq rc
829: files are given the X11
1.1 deraadt 830: authentication protocol and cookie in standard input.
1.2 deraadt 831: .It
1.1 deraadt 832: Runs user's shell or command.
1.2 deraadt 833: .El
834: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 835: The
1.2 deraadt 836: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 837: file lists the RSA keys that are
1.121 stevesk 838: permitted for RSA authentication in protocol version 1.
1.51 hugh 839: Similarly, the
1.44 deraadt 840: .Pa $HOME/.ssh/authorized_keys2
1.75 markus 841: file lists the DSA and RSA keys that are
842: permitted for public key authentication (PubkeyAuthentication)
1.120 markus 843: in protocol version 2.
1.75 markus 844: .Pp
1.36 aaron 845: Each line of the file contains one
1.2 deraadt 846: key (empty lines and lines starting with a
847: .Ql #
848: are ignored as
1.36 aaron 849: comments).
1.75 markus 850: Each RSA public key consists of the following fields, separated by
1.36 aaron 851: spaces: options, bits, exponent, modulus, comment.
1.75 markus 852: Each protocol version 2 public key consists of:
853: options, keytype, base64 encoded key, comment.
854: The options fields
855: are optional; its presence is determined by whether the line starts
1.1 deraadt 856: with a number or not (the option field never starts with a number).
1.75 markus 857: The bits, exponent, modulus and comment fields give the RSA key for
858: protocol version 1; the
1.1 deraadt 859: comment field is not used for anything (but may be convenient for the
860: user to identify the key).
1.75 markus 861: For protocol version 2 the keytype is
862: .Dq ssh-dss
863: or
864: .Dq ssh-rsa .
1.2 deraadt 865: .Pp
1.1 deraadt 866: Note that lines in this file are usually several hundred bytes long
1.36 aaron 867: (because of the size of the RSA key modulus).
868: You don't want to type them in; instead, copy the
1.113 itojun 869: .Pa identity.pub ,
870: .Pa id_dsa.pub
1.75 markus 871: or the
1.113 itojun 872: .Pa id_rsa.pub
1.1 deraadt 873: file and edit it.
1.2 deraadt 874: .Pp
1.58 deraadt 875: The options (if present) consist of comma-separated option
1.36 aaron 876: specifications.
877: No spaces are permitted, except within double quotes.
1.1 deraadt 878: The following option specifications are supported:
1.2 deraadt 879: .Bl -tag -width Ds
880: .It Cm from="pattern-list"
1.1 deraadt 881: Specifies that in addition to RSA authentication, the canonical name
882: of the remote host must be present in the comma-separated list of
1.36 aaron 883: patterns
884: .Pf ( Ql *
885: and
886: .Ql ?
887: serve as wildcards).
888: The list may also contain
889: patterns negated by prefixing them with
890: .Ql ! ;
891: if the canonical host name matches a negated pattern, the key is not accepted.
892: The purpose
1.1 deraadt 893: of this option is to optionally increase security: RSA authentication
894: by itself does not trust the network or name servers or anything (but
895: the key); however, if somebody somehow steals the key, the key
1.36 aaron 896: permits an intruder to log in from anywhere in the world.
897: This additional option makes using a stolen key more difficult (name
1.1 deraadt 898: servers and/or routers would have to be compromised in addition to
899: just the key).
1.2 deraadt 900: .It Cm command="command"
1.1 deraadt 901: Specifies that the command is executed whenever this key is used for
1.36 aaron 902: authentication.
903: The command supplied by the user (if any) is ignored.
1.1 deraadt 904: The command is run on a pty if the connection requests a pty;
1.36 aaron 905: otherwise it is run without a tty.
1.89 markus 906: Note that if you want a 8-bit clean channel,
907: you must not request a pty or should specify
908: .Cm no-pty .
1.36 aaron 909: A quote may be included in the command by quoting it with a backslash.
910: This option might be useful
911: to restrict certain RSA keys to perform just a specific operation.
912: An example might be a key that permits remote backups but nothing else.
1.51 hugh 913: Note that the client may specify TCP/IP and/or X11
914: forwarding unless they are explicitly prohibited.
1.2 deraadt 915: .It Cm environment="NAME=value"
1.1 deraadt 916: Specifies that the string is to be added to the environment when
1.36 aaron 917: logging in using this key.
918: Environment variables set this way
919: override other default environment values.
920: Multiple options of this type are permitted.
1.2 deraadt 921: .It Cm no-port-forwarding
1.1 deraadt 922: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 923: Any port forward requests by the client will return an error.
924: This might be used, e.g., in connection with the
1.2 deraadt 925: .Cm command
1.1 deraadt 926: option.
1.2 deraadt 927: .It Cm no-X11-forwarding
1.1 deraadt 928: Forbids X11 forwarding when this key is used for authentication.
929: Any X11 forward requests by the client will return an error.
1.2 deraadt 930: .It Cm no-agent-forwarding
1.1 deraadt 931: Forbids authentication agent forwarding when this key is used for
932: authentication.
1.2 deraadt 933: .It Cm no-pty
1.1 deraadt 934: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 935: .It Cm permitopen="host:port"
936: Limit local
937: .Li ``ssh -L''
1.111 stevesk 938: port forwarding such that it may only connect to the specified host and
1.107 djm 939: port. Multiple
940: .Cm permitopen
1.111 stevesk 941: options may be applied separated by commas. No pattern matching is
1.107 djm 942: performed on the specified hostnames, they must be literal domains or
943: addresses.
1.2 deraadt 944: .El
945: .Ss Examples
1.1 deraadt 946: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 947: .Pp
1.1 deraadt 948: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 949: .Pp
1.1 deraadt 950: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 951: .Pp
952: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 953: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 954: The
1.44 deraadt 955: .Pa /etc/ssh_known_hosts ,
956: .Pa /etc/ssh_known_hosts2 ,
957: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 958: and
1.44 deraadt 959: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 960: files contain host public keys for all known hosts.
961: The global file should
1.37 brad 962: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 963: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 964: its key is added to the per-user file.
1.2 deraadt 965: .Pp
1.1 deraadt 966: Each line in these files contains the following fields: hostnames,
1.36 aaron 967: bits, exponent, modulus, comment.
968: The fields are separated by spaces.
1.2 deraadt 969: .Pp
1.1 deraadt 970: Hostnames is a comma-separated list of patterns ('*' and '?' act as
971: wildcards); each pattern in turn is matched against the canonical host
972: name (when authenticating a client) or against the user-supplied
1.36 aaron 973: name (when authenticating a server).
974: A pattern may also be preceded by
1.2 deraadt 975: .Ql !
976: to indicate negation: if the host name matches a negated
1.1 deraadt 977: pattern, it is not accepted (by that line) even if it matched another
978: pattern on the line.
1.2 deraadt 979: .Pp
1.49 markus 980: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 981: can be obtained, e.g., from
982: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 983: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 984: .Pp
985: Lines starting with
986: .Ql #
987: and empty lines are ignored as comments.
988: .Pp
1.1 deraadt 989: When performing host authentication, authentication is accepted if any
1.36 aaron 990: matching line has the proper key.
991: It is thus permissible (but not
1.1 deraadt 992: recommended) to have several lines or different host keys for the same
1.36 aaron 993: names.
994: This will inevitably happen when short forms of host names
995: from different domains are put in the file.
996: It is possible
1.1 deraadt 997: that the files contain conflicting information; authentication is
998: accepted if valid information can be found from either file.
1.2 deraadt 999: .Pp
1.1 deraadt 1000: Note that the lines in these files are typically hundreds of characters
1001: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1002: Rather, generate them by a script
1.40 aaron 1003: or by taking
1.2 deraadt 1004: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1005: and adding the host names at the front.
1.2 deraadt 1006: .Ss Examples
1.120 markus 1007: .Bd -literal
1008: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1009: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1010: .Ed
1.2 deraadt 1011: .Sh FILES
1012: .Bl -tag -width Ds
1013: .It Pa /etc/sshd_config
1.1 deraadt 1014: Contains configuration data for
1.2 deraadt 1015: .Nm sshd .
1.1 deraadt 1016: This file should be writable by root only, but it is recommended
1017: (though not necessary) that it be world-readable.
1.98 deraadt 1018: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.120 markus 1019: These three files contain the private parts of the host keys.
1.98 deraadt 1020: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1021: accessible to others.
1.14 markus 1022: Note that
1023: .Nm
1024: does not start if this file is group/world-accessible.
1.98 deraadt 1025: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.120 markus 1026: These three files contain the public parts of the host keys.
1.98 deraadt 1027: These files should be world-readable but writable only by
1.36 aaron 1028: root.
1.98 deraadt 1029: Their contents should match the respective private parts.
1030: These files are not
1031: really used for anything; they are provided for the convenience of
1032: the user so their contents can be copied to known hosts files.
1033: These files are created using
1.7 markus 1034: .Xr ssh-keygen 1 .
1.73 provos 1035: .It Pa /etc/primes
1036: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1037: .It Pa /var/run/sshd.pid
1038: Contains the process ID of the
1039: .Nm
1.1 deraadt 1040: listening for connections (if there are several daemons running
1041: concurrently for different ports, this contains the pid of the one
1.36 aaron 1042: started last).
1.58 deraadt 1043: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1044: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 1045: Lists the RSA keys that can be used to log into the user's account.
1046: This file must be readable by root (which may on some machines imply
1047: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 1048: volume).
1049: It is recommended that it not be accessible by others.
1050: The format of this file is described above.
1.44 deraadt 1051: Users will place the contents of their
1052: .Pa identity.pub
1053: files into this file, as described in
1054: .Xr ssh-keygen 1 .
1055: .It Pa $HOME/.ssh/authorized_keys2
1.120 markus 1056: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1057: This file must be readable by root (which may on some machines imply
1058: it being world-readable if the user's home directory resides on an NFS
1059: volume).
1060: It is recommended that it not be accessible by others.
1061: The format of this file is described above.
1062: Users will place the contents of their
1063: .Pa id_dsa.pub
1.113 itojun 1064: and/or
1065: .Pa id_rsa.pub
1.44 deraadt 1066: files into this file, as described in
1067: .Xr ssh-keygen 1 .
1.22 markus 1068: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1069: These files are consulted when using rhosts with RSA host
1.36 aaron 1070: authentication to check the public key of the host.
1071: The key must be listed in one of these files to be accepted.
1.22 markus 1072: The client uses the same files
1.96 markus 1073: to verify that it is connecting to the correct remote host.
1.36 aaron 1074: These files should be writable only by root/the owner.
1.2 deraadt 1075: .Pa /etc/ssh_known_hosts
1076: should be world-readable, and
1077: .Pa $HOME/.ssh/known_hosts
1078: can but need not be world-readable.
1.120 markus 1079: .It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2"
1080: These files are consulted when using protocol version 2 hostbased
1081: authentication to check the public key of the host.
1082: The key must be listed in one of these files to be accepted.
1083: The client uses the same files
1084: to verify that it is connecting to the correct remote host.
1085: These files should be writable only by root/the owner.
1086: .Pa /etc/ssh_known_hosts2
1087: should be world-readable, and
1088: .Pa $HOME/.ssh/known_hosts2
1089: can but need not be world-readable.
1.6 aaron 1090: .It Pa /etc/nologin
1.40 aaron 1091: If this file exists,
1.2 deraadt 1092: .Nm
1.36 aaron 1093: refuses to let anyone except root log in.
1094: The contents of the file
1.1 deraadt 1095: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1096: refused.
1097: The file should be world-readable.
1.19 dugsong 1098: .It Pa /etc/hosts.allow, /etc/hosts.deny
1099: If compiled with
1100: .Sy LIBWRAP
1101: support, tcp-wrappers access controls may be defined here as described in
1102: .Xr hosts_access 5 .
1.6 aaron 1103: .It Pa $HOME/.rhosts
1.1 deraadt 1104: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1105: line.
1106: The given user on the corresponding host is permitted to log in
1107: without password.
1108: The same file is used by rlogind and rshd.
1.6 aaron 1109: The file must
1.1 deraadt 1110: be writable only by the user; it is recommended that it not be
1111: accessible by others.
1.2 deraadt 1112: .Pp
1.36 aaron 1113: If is also possible to use netgroups in the file.
1114: Either host or user
1.1 deraadt 1115: name may be of the form +@groupname to specify all hosts or all users
1116: in the group.
1.2 deraadt 1117: .It Pa $HOME/.shosts
1118: For ssh,
1119: this file is exactly the same as for
1120: .Pa .rhosts .
1121: However, this file is
1122: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1123: .It Pa /etc/hosts.equiv
1.2 deraadt 1124: This file is used during
1125: .Pa .rhosts
1.36 aaron 1126: authentication.
1127: In the simplest form, this file contains host names, one per line.
1128: Users on
1.1 deraadt 1129: those hosts are permitted to log in without a password, provided they
1.36 aaron 1130: have the same user name on both machines.
1131: The host name may also be
1.1 deraadt 1132: followed by a user name; such users are permitted to log in as
1.2 deraadt 1133: .Em any
1.36 aaron 1134: user on this machine (except root).
1135: Additionally, the syntax
1.2 deraadt 1136: .Dq +@group
1.36 aaron 1137: can be used to specify netgroups.
1138: Negated entries start with
1.2 deraadt 1139: .Ql \&- .
1140: .Pp
1.1 deraadt 1141: If the client host/user is successfully matched in this file, login is
1142: automatically permitted provided the client and server user names are the
1.36 aaron 1143: same.
1144: Additionally, successful RSA host authentication is normally required.
1145: This file must be writable only by root; it is recommended
1.1 deraadt 1146: that it be world-readable.
1.2 deraadt 1147: .Pp
1.6 aaron 1148: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1149: .Pa hosts.equiv .
1.1 deraadt 1150: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1151: .Em anybody ,
1.1 deraadt 1152: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1153: binaries and directories.
1154: Using a user name practically grants the user root access.
1155: The only valid use for user names that I can think
1.1 deraadt 1156: of is in negative entries.
1.2 deraadt 1157: .Pp
1158: Note that this warning also applies to rsh/rlogin.
1159: .It Pa /etc/shosts.equiv
1.1 deraadt 1160: This is processed exactly as
1.2 deraadt 1161: .Pa /etc/hosts.equiv .
1.1 deraadt 1162: However, this file may be useful in environments that want to run both
1.2 deraadt 1163: rsh/rlogin and ssh.
1.6 aaron 1164: .It Pa $HOME/.ssh/environment
1.36 aaron 1165: This file is read into the environment at login (if it exists).
1166: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1167: .Ql # ) ,
1.36 aaron 1168: and assignment lines of the form name=value.
1169: The file should be writable
1.6 aaron 1170: only by the user; it need not be readable by anyone else.
1.2 deraadt 1171: .It Pa $HOME/.ssh/rc
1.1 deraadt 1172: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1173: environment files but before starting the user's shell or command.
1174: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1175: standard input (and
1176: .Ev DISPLAY
1.36 aaron 1177: in environment).
1178: This must call
1.2 deraadt 1179: .Xr xauth 1
1180: in that case.
1181: .Pp
1.1 deraadt 1182: The primary purpose of this file is to run any initialization routines
1183: which may be needed before the user's home directory becomes
1184: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1185: .Pp
1.1 deraadt 1186: This file will probably contain some initialization code followed by
1.120 markus 1187: something similar to:
1188: .Bd -literal
1189: if read proto cookie; then
1190: echo add $DISPLAY $proto $cookie | xauth -q -
1191: fi
1192: .Ed
1.2 deraadt 1193: .Pp
1194: If this file does not exist,
1195: .Pa /etc/sshrc
1196: is run, and if that
1.1 deraadt 1197: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1198: .Pp
1.1 deraadt 1199: This file should be writable only by the user, and need not be
1200: readable by anyone else.
1.2 deraadt 1201: .It Pa /etc/sshrc
1202: Like
1203: .Pa $HOME/.ssh/rc .
1204: This can be used to specify
1.36 aaron 1205: machine-specific login-time initializations globally.
1206: This file should be writable only by root, and should be world-readable.
1.56 aaron 1207: .El
1.71 aaron 1208: .Sh AUTHORS
1.84 markus 1209: OpenSSH is a derivative of the original and free
1210: ssh 1.2.12 release by Tatu Ylonen.
1211: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1212: Theo de Raadt and Dug Song
1213: removed many bugs, re-added newer features and
1214: created OpenSSH.
1215: Markus Friedl contributed the support for SSH
1216: protocol versions 1.5 and 2.0.
1.2 deraadt 1217: .Sh SEE ALSO
1218: .Xr scp 1 ,
1.90 djm 1219: .Xr sftp 1 ,
1.63 markus 1220: .Xr sftp-server 8 ,
1.2 deraadt 1221: .Xr ssh 1 ,
1.5 deraadt 1222: .Xr ssh-add 1 ,
1.2 deraadt 1223: .Xr ssh-agent 1 ,
1.5 deraadt 1224: .Xr ssh-keygen 1 ,
1.38 deraadt 1225: .Xr rlogin 1 ,
1226: .Xr rsh 1
1.119 markus 1227: .Rs
1228: .%A T. Ylonen
1229: .%A T. Kivinen
1230: .%A M. Saarinen
1231: .%A T. Rinne
1232: .%A S. Lehtinen
1233: .%T "SSH Protocol Architecture"
1234: .%N draft-ietf-secsh-architecture-07.txt
1.120 markus 1235: .%D January 2001
1236: .%O work in progress material
1237: .Re
1238: .Rs
1239: .%A M. Friedl
1240: .%A N. Provos
1241: .%A W. A. Simpson
1242: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1243: .%N draft-ietf-secsh-dh-group-exchange-00.txt
1.119 markus 1244: .%D January 2001
1245: .%O work in progress material
1246: .Re
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