Annotation of src/usr.bin/ssh/sshd.8, Revision 1.122
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: .\"
1.122 ! markus 37: .\" $OpenBSD: sshd.8,v 1.121 2001/05/04 14:21:56 stevesk Exp $
1.2 deraadt 38: .Dd September 25, 1999
39: .Dt SSHD 8
40: .Os
41: .Sh NAME
42: .Nm sshd
1.120 markus 43: .Nd OpenSSH SSH daemon
1.2 deraadt 44: .Sh SYNOPSIS
45: .Nm sshd
1.114 lebel 46: .Op Fl deiqD46
1.2 deraadt 47: .Op Fl b Ar bits
48: .Op Fl f Ar config_file
49: .Op Fl g Ar login_grace_time
50: .Op Fl h Ar host_key_file
51: .Op Fl k Ar key_gen_time
52: .Op Fl p Ar port
1.61 markus 53: .Op Fl u Ar len
1.26 markus 54: .Op Fl V Ar client_protocol_id
1.40 aaron 55: .Sh DESCRIPTION
1.2 deraadt 56: .Nm
1.106 deraadt 57: (SSH Daemon) is the daemon program for
1.2 deraadt 58: .Xr ssh 1 .
1.42 hugh 59: Together these programs replace rlogin and rsh, and
1.1 deraadt 60: provide secure encrypted communications between two untrusted hosts
1.36 aaron 61: over an insecure network.
62: The programs are intended to be as easy to
1.1 deraadt 63: install and use as possible.
1.2 deraadt 64: .Pp
65: .Nm
1.36 aaron 66: is the daemon that listens for connections from clients.
1.40 aaron 67: It is normally started at boot from
1.2 deraadt 68: .Pa /etc/rc .
69: It forks a new
1.36 aaron 70: daemon for each incoming connection.
71: The forked daemons handle
1.1 deraadt 72: key exchange, encryption, authentication, command execution,
73: and data exchange.
1.49 markus 74: This implementation of
75: .Nm
76: supports both SSH protocol version 1 and 2 simultaneously.
1.2 deraadt 77: .Nm
1.36 aaron 78: works as follows.
1.49 markus 79: .Pp
80: .Ss SSH protocol version 1
81: .Pp
1.36 aaron 82: Each host has a host-specific RSA key
83: (normally 1024 bits) used to identify the host.
84: Additionally, when
1.1 deraadt 85: the daemon starts, it generates a server RSA key (normally 768 bits).
86: This key is normally regenerated every hour if it has been used, and
87: is never stored on disk.
1.2 deraadt 88: .Pp
1.42 hugh 89: Whenever a client connects the daemon responds with its public
90: host and server keys.
1.36 aaron 91: The client compares the
1.49 markus 92: RSA host key against its own database to verify that it has not changed.
1.36 aaron 93: The client then generates a 256 bit random number.
94: It encrypts this
1.1 deraadt 95: random number using both the host key and the server key, and sends
1.36 aaron 96: the encrypted number to the server.
1.42 hugh 97: Both sides then use this
1.1 deraadt 98: random number as a session key which is used to encrypt all further
1.36 aaron 99: communications in the session.
100: The rest of the session is encrypted
1.42 hugh 101: using a conventional cipher, currently Blowfish or 3DES, with 3DES
1.39 deraadt 102: being used by default.
1.36 aaron 103: The client selects the encryption algorithm
1.5 deraadt 104: to use from those offered by the server.
1.2 deraadt 105: .Pp
1.36 aaron 106: Next, the server and the client enter an authentication dialog.
107: The client tries to authenticate itself using
1.2 deraadt 108: .Pa .rhosts
109: authentication,
110: .Pa .rhosts
111: authentication combined with RSA host
1.1 deraadt 112: authentication, RSA challenge-response authentication, or password
113: based authentication.
1.2 deraadt 114: .Pp
1.1 deraadt 115: Rhosts authentication is normally disabled
116: because it is fundamentally insecure, but can be enabled in the server
1.36 aaron 117: configuration file if desired.
118: System security is not improved unless
1.2 deraadt 119: .Xr rshd 8 ,
120: .Xr rlogind 8 ,
121: .Xr rexecd 8 ,
122: and
123: .Xr rexd 8
1.1 deraadt 124: are disabled (thus completely disabling
1.2 deraadt 125: .Xr rlogin 1
1.1 deraadt 126: and
1.2 deraadt 127: .Xr rsh 1
1.42 hugh 128: into the machine).
1.2 deraadt 129: .Pp
1.49 markus 130: .Ss SSH protocol version 2
131: .Pp
1.58 deraadt 132: Version 2 works similarly:
1.49 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
589: .Pp
590: .Bd -literal
1.105 deraadt 591: ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,
1.93 markus 592: hmac-sha1-96,hmac-md5-96''
593: .Ed
1.55 markus 594: .It Cm MaxStartups
595: Specifies the maximum number of concurrent unauthenticated connections to the
596: .Nm
597: daemon.
598: Additional connections will be dropped until authentication succeeds or the
599: .Cm LoginGraceTime
600: expires for a connection.
601: The default is 10.
1.57 markus 602: .Pp
603: Alternatively, random early drop can be enabled by specifying
604: the three colon separated values
605: .Dq start:rate:full
1.67 aaron 606: (e.g., "10:30:60").
1.57 markus 607: .Nm
1.86 stevesk 608: will refuse connection attempts with a probability of
1.57 markus 609: .Dq rate/100
610: (30%)
611: if there are currently
612: .Dq start
613: (10)
614: unauthenticated connections.
1.86 stevesk 615: The probability increases linearly and all connection attempts
1.57 markus 616: are refused if the number of unauthenticated connections reaches
617: .Dq full
618: (60).
1.2 deraadt 619: .It Cm PasswordAuthentication
1.1 deraadt 620: Specifies whether password authentication is allowed.
1.2 deraadt 621: The default is
622: .Dq yes .
623: .It Cm PermitEmptyPasswords
1.1 deraadt 624: When password authentication is allowed, it specifies whether the
1.36 aaron 625: server allows login to accounts with empty password strings.
626: The default is
1.34 markus 627: .Dq no .
1.2 deraadt 628: .It Cm PermitRootLogin
1.100 stevesk 629: Specifies whether root can login using
1.2 deraadt 630: .Xr ssh 1 .
1.15 markus 631: The argument must be
632: .Dq yes ,
1.94 markus 633: .Dq without-password ,
634: .Dq forced-commands-only
1.15 markus 635: or
636: .Dq no .
1.2 deraadt 637: The default is
638: .Dq yes .
1.94 markus 639: .Pp
640: If this option is set to
1.15 markus 641: .Dq without-password
1.94 markus 642: password authentication is disabled for root.
1.2 deraadt 643: .Pp
1.94 markus 644: If this option is set to
645: .Dq forced-commands-only
646: root login with public key authentication will be allowed,
647: but only if the
1.2 deraadt 648: .Ar command
1.94 markus 649: option has been specified
1.1 deraadt 650: (which may be useful for taking remote backups even if root login is
1.94 markus 651: normally not allowed). All other authentication methods are disabled
652: for root.
1.100 stevesk 653: .Pp
654: If this option is set to
655: .Dq no
656: root is not allowed to login.
1.43 markus 657: .It Cm PidFile
658: Specifies the file that contains the process identifier of the
659: .Nm
660: daemon.
661: The default is
662: .Pa /var/run/sshd.pid .
1.2 deraadt 663: .It Cm Port
1.1 deraadt 664: Specifies the port number that
1.2 deraadt 665: .Nm
1.36 aaron 666: listens on.
667: The default is 22.
1.28 markus 668: Multiple options of this type are permitted.
1.120 markus 669: See also
670: .Cm ListenAddress .
1.108 stevesk 671: .It Cm PrintLastLog
672: Specifies whether
673: .Nm
674: should print the date and time when the user last logged in.
675: The default is
676: .Dq yes .
1.2 deraadt 677: .It Cm PrintMotd
1.1 deraadt 678: Specifies whether
1.2 deraadt 679: .Nm
1.40 aaron 680: should print
1.2 deraadt 681: .Pa /etc/motd
1.36 aaron 682: when a user logs in interactively.
683: (On some systems it is also printed by the shell,
1.2 deraadt 684: .Pa /etc/profile ,
1.36 aaron 685: or equivalent.)
686: The default is
1.2 deraadt 687: .Dq yes .
1.41 markus 688: .It Cm Protocol
689: Specifies the protocol versions
690: .Nm
691: should support.
692: The possible values are
693: .Dq 1
694: and
695: .Dq 2 .
696: Multiple versions must be comma-separated.
697: The default is
1.118 deraadt 698: .Dq 2,1 .
1.104 deraadt 699: .It Cm PubkeyAuthentication
700: Specifies whether public key authentication is allowed.
701: The default is
702: .Dq yes .
703: Note that this option applies to protocol version 2 only.
1.87 markus 704: .It Cm ReverseMappingCheck
705: Specifies whether
706: .Nm
707: should try to verify the remote host name and check that
708: the resolved host name for the remote IP address maps back to the
709: very same IP address.
710: The default is
711: .Dq no .
1.2 deraadt 712: .It Cm RhostsAuthentication
1.1 deraadt 713: Specifies whether authentication using rhosts or /etc/hosts.equiv
1.36 aaron 714: files is sufficient.
715: Normally, this method should not be permitted because it is insecure.
1.7 markus 716: .Cm RhostsRSAAuthentication
717: should be used
1.1 deraadt 718: instead, because it performs RSA-based host authentication in addition
719: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 720: The default is
721: .Dq no .
1.120 markus 722: This option applies to protocol version 1 only.
1.2 deraadt 723: .It Cm RhostsRSAAuthentication
1.1 deraadt 724: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.36 aaron 725: with successful RSA host authentication is allowed.
726: The default is
1.34 markus 727: .Dq no .
1.120 markus 728: This option applies to protocol version 1 only.
1.2 deraadt 729: .It Cm RSAAuthentication
1.36 aaron 730: Specifies whether pure RSA authentication is allowed.
731: The default is
1.2 deraadt 732: .Dq yes .
1.120 markus 733: This option applies to protocol version 1 only.
1.2 deraadt 734: .It Cm ServerKeyBits
1.120 markus 735: Defines the number of bits in the ephemeral protocol version 1 server key.
1.36 aaron 736: The minimum value is 512, and the default is 768.
1.2 deraadt 737: .It Cm StrictModes
1.12 markus 738: Specifies whether
739: .Nm
740: should check file modes and ownership of the
1.36 aaron 741: user's files and home directory before accepting login.
742: This is normally desirable because novices sometimes accidentally leave their
743: directory or files world-writable.
744: The default is
1.7 markus 745: .Dq yes .
1.54 jakob 746: .It Cm Subsystem
1.67 aaron 747: Configures an external subsystem (e.g., file transfer daemon).
748: Arguments should be a subsystem name and a command to execute upon subsystem
749: request.
1.63 markus 750: The command
751: .Xr sftp-server 8
752: implements the
753: .Dq sftp
754: file transfer subsystem.
1.54 jakob 755: By default no subsystems are defined.
756: Note that this option applies to protocol version 2 only.
1.2 deraadt 757: .It Cm SyslogFacility
1.1 deraadt 758: Gives the facility code that is used when logging messages from
1.2 deraadt 759: .Nm sshd .
1.1 deraadt 760: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.36 aaron 761: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
762: The default is AUTH.
1.10 markus 763: .It Cm UseLogin
764: Specifies whether
765: .Xr login 1
1.53 markus 766: is used for interactive login sessions.
767: Note that
768: .Xr login 1
1.58 deraadt 769: is never used for remote command execution.
1.36 aaron 770: The default is
1.10 markus 771: .Dq no .
1.6 aaron 772: .It Cm X11DisplayOffset
773: Specifies the first display number available for
774: .Nm sshd Ns 's
1.36 aaron 775: X11 forwarding.
776: This prevents
1.6 aaron 777: .Nm
778: from interfering with real X11 servers.
1.34 markus 779: The default is 10.
1.30 markus 780: .It Cm X11Forwarding
1.36 aaron 781: Specifies whether X11 forwarding is permitted.
782: The default is
1.34 markus 783: .Dq no .
1.30 markus 784: Note that disabling X11 forwarding does not improve security in any
785: way, as users can always install their own forwarders.
1.52 markus 786: .It Cm XAuthLocation
787: Specifies the location of the
788: .Xr xauth 1
789: program.
790: The default is
791: .Pa /usr/X11R6/bin/xauth .
1.2 deraadt 792: .El
793: .Sh LOGIN PROCESS
1.1 deraadt 794: When a user successfully logs in,
1.2 deraadt 795: .Nm
1.1 deraadt 796: does the following:
1.2 deraadt 797: .Bl -enum -offset indent
798: .It
1.1 deraadt 799: If the login is on a tty, and no command has been specified,
1.40 aaron 800: prints last login time and
1.2 deraadt 801: .Pa /etc/motd
1.1 deraadt 802: (unless prevented in the configuration file or by
1.2 deraadt 803: .Pa $HOME/.hushlogin ;
804: see the
1.40 aaron 805: .Sx FILES
1.2 deraadt 806: section).
807: .It
1.1 deraadt 808: If the login is on a tty, records login time.
1.2 deraadt 809: .It
810: Checks
811: .Pa /etc/nologin ;
812: if it exists, prints contents and quits
1.1 deraadt 813: (unless root).
1.2 deraadt 814: .It
1.1 deraadt 815: Changes to run with normal user privileges.
1.2 deraadt 816: .It
1.1 deraadt 817: Sets up basic environment.
1.2 deraadt 818: .It
819: Reads
820: .Pa $HOME/.ssh/environment
821: if it exists.
822: .It
1.1 deraadt 823: Changes to user's home directory.
1.2 deraadt 824: .It
825: If
826: .Pa $HOME/.ssh/rc
827: exists, runs it; else if
828: .Pa /etc/sshrc
829: exists, runs
1.36 aaron 830: it; otherwise runs xauth.
831: The
1.2 deraadt 832: .Dq rc
833: files are given the X11
1.1 deraadt 834: authentication protocol and cookie in standard input.
1.2 deraadt 835: .It
1.1 deraadt 836: Runs user's shell or command.
1.2 deraadt 837: .El
838: .Sh AUTHORIZED_KEYS FILE FORMAT
1.40 aaron 839: The
1.2 deraadt 840: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 841: file lists the RSA keys that are
1.121 stevesk 842: permitted for RSA authentication in protocol version 1.
1.51 hugh 843: Similarly, the
1.44 deraadt 844: .Pa $HOME/.ssh/authorized_keys2
1.75 markus 845: file lists the DSA and RSA keys that are
846: permitted for public key authentication (PubkeyAuthentication)
1.120 markus 847: in protocol version 2.
1.75 markus 848: .Pp
1.36 aaron 849: Each line of the file contains one
1.2 deraadt 850: key (empty lines and lines starting with a
851: .Ql #
852: are ignored as
1.36 aaron 853: comments).
1.75 markus 854: Each RSA public key consists of the following fields, separated by
1.36 aaron 855: spaces: options, bits, exponent, modulus, comment.
1.75 markus 856: Each protocol version 2 public key consists of:
857: options, keytype, base64 encoded key, comment.
858: The options fields
859: are optional; its presence is determined by whether the line starts
1.1 deraadt 860: with a number or not (the option field never starts with a number).
1.75 markus 861: The bits, exponent, modulus and comment fields give the RSA key for
862: protocol version 1; the
1.1 deraadt 863: comment field is not used for anything (but may be convenient for the
864: user to identify the key).
1.75 markus 865: For protocol version 2 the keytype is
866: .Dq ssh-dss
867: or
868: .Dq ssh-rsa .
1.2 deraadt 869: .Pp
1.1 deraadt 870: Note that lines in this file are usually several hundred bytes long
1.36 aaron 871: (because of the size of the RSA key modulus).
872: You don't want to type them in; instead, copy the
1.113 itojun 873: .Pa identity.pub ,
874: .Pa id_dsa.pub
1.75 markus 875: or the
1.113 itojun 876: .Pa id_rsa.pub
1.1 deraadt 877: file and edit it.
1.2 deraadt 878: .Pp
1.58 deraadt 879: The options (if present) consist of comma-separated option
1.36 aaron 880: specifications.
881: No spaces are permitted, except within double quotes.
1.1 deraadt 882: The following option specifications are supported:
1.2 deraadt 883: .Bl -tag -width Ds
884: .It Cm from="pattern-list"
1.1 deraadt 885: Specifies that in addition to RSA authentication, the canonical name
886: of the remote host must be present in the comma-separated list of
1.36 aaron 887: patterns
888: .Pf ( Ql *
889: and
890: .Ql ?
891: serve as wildcards).
892: The list may also contain
893: patterns negated by prefixing them with
894: .Ql ! ;
895: if the canonical host name matches a negated pattern, the key is not accepted.
896: The purpose
1.1 deraadt 897: of this option is to optionally increase security: RSA authentication
898: by itself does not trust the network or name servers or anything (but
899: the key); however, if somebody somehow steals the key, the key
1.36 aaron 900: permits an intruder to log in from anywhere in the world.
901: This additional option makes using a stolen key more difficult (name
1.1 deraadt 902: servers and/or routers would have to be compromised in addition to
903: just the key).
1.2 deraadt 904: .It Cm command="command"
1.1 deraadt 905: Specifies that the command is executed whenever this key is used for
1.36 aaron 906: authentication.
907: The command supplied by the user (if any) is ignored.
1.1 deraadt 908: The command is run on a pty if the connection requests a pty;
1.36 aaron 909: otherwise it is run without a tty.
1.89 markus 910: Note that if you want a 8-bit clean channel,
911: you must not request a pty or should specify
912: .Cm no-pty .
1.36 aaron 913: A quote may be included in the command by quoting it with a backslash.
914: This option might be useful
915: to restrict certain RSA keys to perform just a specific operation.
916: An example might be a key that permits remote backups but nothing else.
1.51 hugh 917: Note that the client may specify TCP/IP and/or X11
918: forwarding unless they are explicitly prohibited.
1.2 deraadt 919: .It Cm environment="NAME=value"
1.1 deraadt 920: Specifies that the string is to be added to the environment when
1.36 aaron 921: logging in using this key.
922: Environment variables set this way
923: override other default environment values.
924: Multiple options of this type are permitted.
1.2 deraadt 925: .It Cm no-port-forwarding
1.1 deraadt 926: Forbids TCP/IP forwarding when this key is used for authentication.
1.36 aaron 927: Any port forward requests by the client will return an error.
928: This might be used, e.g., in connection with the
1.2 deraadt 929: .Cm command
1.1 deraadt 930: option.
1.2 deraadt 931: .It Cm no-X11-forwarding
1.1 deraadt 932: Forbids X11 forwarding when this key is used for authentication.
933: Any X11 forward requests by the client will return an error.
1.2 deraadt 934: .It Cm no-agent-forwarding
1.1 deraadt 935: Forbids authentication agent forwarding when this key is used for
936: authentication.
1.2 deraadt 937: .It Cm no-pty
1.1 deraadt 938: Prevents tty allocation (a request to allocate a pty will fail).
1.107 djm 939: .It Cm permitopen="host:port"
940: Limit local
941: .Li ``ssh -L''
1.111 stevesk 942: port forwarding such that it may only connect to the specified host and
1.107 djm 943: port. Multiple
944: .Cm permitopen
1.111 stevesk 945: options may be applied separated by commas. No pattern matching is
1.107 djm 946: performed on the specified hostnames, they must be literal domains or
947: addresses.
1.2 deraadt 948: .El
949: .Ss Examples
1.1 deraadt 950: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 951: .Pp
1.1 deraadt 952: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 953: .Pp
1.1 deraadt 954: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.107 djm 955: .Pp
956: permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
1.2 deraadt 957: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.40 aaron 958: The
1.44 deraadt 959: .Pa /etc/ssh_known_hosts ,
960: .Pa /etc/ssh_known_hosts2 ,
961: .Pa $HOME/.ssh/known_hosts ,
1.40 aaron 962: and
1.44 deraadt 963: .Pa $HOME/.ssh/known_hosts2
1.36 aaron 964: files contain host public keys for all known hosts.
965: The global file should
1.37 brad 966: be prepared by the administrator (optional), and the per-user file is
1.58 deraadt 967: maintained automatically: whenever the user connects from an unknown host
1.36 aaron 968: its key is added to the per-user file.
1.2 deraadt 969: .Pp
1.1 deraadt 970: Each line in these files contains the following fields: hostnames,
1.36 aaron 971: bits, exponent, modulus, comment.
972: The fields are separated by spaces.
1.2 deraadt 973: .Pp
1.1 deraadt 974: Hostnames is a comma-separated list of patterns ('*' and '?' act as
975: wildcards); each pattern in turn is matched against the canonical host
976: name (when authenticating a client) or against the user-supplied
1.36 aaron 977: name (when authenticating a server).
978: A pattern may also be preceded by
1.2 deraadt 979: .Ql !
980: to indicate negation: if the host name matches a negated
1.1 deraadt 981: pattern, it is not accepted (by that line) even if it matched another
982: pattern on the line.
1.2 deraadt 983: .Pp
1.49 markus 984: Bits, exponent, and modulus are taken directly from the RSA host key; they
1.2 deraadt 985: can be obtained, e.g., from
986: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 987: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 988: .Pp
989: Lines starting with
990: .Ql #
991: and empty lines are ignored as comments.
992: .Pp
1.1 deraadt 993: When performing host authentication, authentication is accepted if any
1.36 aaron 994: matching line has the proper key.
995: It is thus permissible (but not
1.1 deraadt 996: recommended) to have several lines or different host keys for the same
1.36 aaron 997: names.
998: This will inevitably happen when short forms of host names
999: from different domains are put in the file.
1000: It is possible
1.1 deraadt 1001: that the files contain conflicting information; authentication is
1002: accepted if valid information can be found from either file.
1.2 deraadt 1003: .Pp
1.1 deraadt 1004: Note that the lines in these files are typically hundreds of characters
1005: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 1006: Rather, generate them by a script
1.40 aaron 1007: or by taking
1.2 deraadt 1008: .Pa /etc/ssh_host_key.pub
1.1 deraadt 1009: and adding the host names at the front.
1.2 deraadt 1010: .Ss Examples
1.120 markus 1011: .Bd -literal
1012: closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1013: cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
1014: .Ed
1.2 deraadt 1015: .Sh FILES
1016: .Bl -tag -width Ds
1017: .It Pa /etc/sshd_config
1.1 deraadt 1018: Contains configuration data for
1.2 deraadt 1019: .Nm sshd .
1.1 deraadt 1020: This file should be writable by root only, but it is recommended
1021: (though not necessary) that it be world-readable.
1.98 deraadt 1022: .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
1.120 markus 1023: These three files contain the private parts of the host keys.
1.98 deraadt 1024: These files should only be owned by root, readable only by root, and not
1.1 deraadt 1025: accessible to others.
1.14 markus 1026: Note that
1027: .Nm
1028: does not start if this file is group/world-accessible.
1.98 deraadt 1029: .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/ssh_host_rsa_key.pub
1.120 markus 1030: These three files contain the public parts of the host keys.
1.98 deraadt 1031: These files should be world-readable but writable only by
1.36 aaron 1032: root.
1.98 deraadt 1033: Their contents should match the respective private parts.
1034: These files are not
1035: really used for anything; they are provided for the convenience of
1036: the user so their contents can be copied to known hosts files.
1037: These files are created using
1.7 markus 1038: .Xr ssh-keygen 1 .
1.73 provos 1039: .It Pa /etc/primes
1040: Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
1.2 deraadt 1041: .It Pa /var/run/sshd.pid
1042: Contains the process ID of the
1043: .Nm
1.1 deraadt 1044: listening for connections (if there are several daemons running
1045: concurrently for different ports, this contains the pid of the one
1.36 aaron 1046: started last).
1.58 deraadt 1047: The content of this file is not sensitive; it can be world-readable.
1.2 deraadt 1048: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 1049: Lists the RSA keys that can be used to log into the user's account.
1050: This file must be readable by root (which may on some machines imply
1051: it being world-readable if the user's home directory resides on an NFS
1.36 aaron 1052: volume).
1053: It is recommended that it not be accessible by others.
1054: The format of this file is described above.
1.44 deraadt 1055: Users will place the contents of their
1056: .Pa identity.pub
1057: files into this file, as described in
1058: .Xr ssh-keygen 1 .
1059: .It Pa $HOME/.ssh/authorized_keys2
1.120 markus 1060: Lists the public keys (RSA or DSA) that can be used to log into the user's account.
1.44 deraadt 1061: This file must be readable by root (which may on some machines imply
1062: it being world-readable if the user's home directory resides on an NFS
1063: volume).
1064: It is recommended that it not be accessible by others.
1065: The format of this file is described above.
1066: Users will place the contents of their
1067: .Pa id_dsa.pub
1.113 itojun 1068: and/or
1069: .Pa id_rsa.pub
1.44 deraadt 1070: files into this file, as described in
1071: .Xr ssh-keygen 1 .
1.22 markus 1072: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1073: These files are consulted when using rhosts with RSA host
1.36 aaron 1074: authentication to check the public key of the host.
1075: The key must be listed in one of these files to be accepted.
1.22 markus 1076: The client uses the same files
1.96 markus 1077: to verify that it is connecting to the correct remote host.
1.36 aaron 1078: These files should be writable only by root/the owner.
1.2 deraadt 1079: .Pa /etc/ssh_known_hosts
1080: should be world-readable, and
1081: .Pa $HOME/.ssh/known_hosts
1082: can but need not be world-readable.
1.120 markus 1083: .It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2"
1084: These files are consulted when using protocol version 2 hostbased
1085: authentication to check the public key of the host.
1086: The key must be listed in one of these files to be accepted.
1087: The client uses the same files
1088: to verify that it is connecting to the correct remote host.
1089: These files should be writable only by root/the owner.
1090: .Pa /etc/ssh_known_hosts2
1091: should be world-readable, and
1092: .Pa $HOME/.ssh/known_hosts2
1093: can but need not be world-readable.
1.6 aaron 1094: .It Pa /etc/nologin
1.40 aaron 1095: If this file exists,
1.2 deraadt 1096: .Nm
1.36 aaron 1097: refuses to let anyone except root log in.
1098: The contents of the file
1.1 deraadt 1099: are displayed to anyone trying to log in, and non-root connections are
1.36 aaron 1100: refused.
1101: The file should be world-readable.
1.19 dugsong 1102: .It Pa /etc/hosts.allow, /etc/hosts.deny
1103: If compiled with
1104: .Sy LIBWRAP
1105: support, tcp-wrappers access controls may be defined here as described in
1106: .Xr hosts_access 5 .
1.6 aaron 1107: .It Pa $HOME/.rhosts
1.1 deraadt 1108: This file contains host-username pairs, separated by a space, one per
1.36 aaron 1109: line.
1110: The given user on the corresponding host is permitted to log in
1111: without password.
1112: The same file is used by rlogind and rshd.
1.6 aaron 1113: The file must
1.1 deraadt 1114: be writable only by the user; it is recommended that it not be
1115: accessible by others.
1.2 deraadt 1116: .Pp
1.36 aaron 1117: If is also possible to use netgroups in the file.
1118: Either host or user
1.1 deraadt 1119: name may be of the form +@groupname to specify all hosts or all users
1120: in the group.
1.2 deraadt 1121: .It Pa $HOME/.shosts
1122: For ssh,
1123: this file is exactly the same as for
1124: .Pa .rhosts .
1125: However, this file is
1126: not used by rlogin and rshd, so using this permits access using SSH only.
1.58 deraadt 1127: .It Pa /etc/hosts.equiv
1.2 deraadt 1128: This file is used during
1129: .Pa .rhosts
1.36 aaron 1130: authentication.
1131: In the simplest form, this file contains host names, one per line.
1132: Users on
1.1 deraadt 1133: those hosts are permitted to log in without a password, provided they
1.36 aaron 1134: have the same user name on both machines.
1135: The host name may also be
1.1 deraadt 1136: followed by a user name; such users are permitted to log in as
1.2 deraadt 1137: .Em any
1.36 aaron 1138: user on this machine (except root).
1139: Additionally, the syntax
1.2 deraadt 1140: .Dq +@group
1.36 aaron 1141: can be used to specify netgroups.
1142: Negated entries start with
1.2 deraadt 1143: .Ql \&- .
1144: .Pp
1.1 deraadt 1145: If the client host/user is successfully matched in this file, login is
1146: automatically permitted provided the client and server user names are the
1.36 aaron 1147: same.
1148: Additionally, successful RSA host authentication is normally required.
1149: This file must be writable only by root; it is recommended
1.1 deraadt 1150: that it be world-readable.
1.2 deraadt 1151: .Pp
1.6 aaron 1152: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 1153: .Pa hosts.equiv .
1.1 deraadt 1154: Beware that it really means that the named user(s) can log in as
1.2 deraadt 1155: .Em anybody ,
1.1 deraadt 1156: which includes bin, daemon, adm, and other accounts that own critical
1.36 aaron 1157: binaries and directories.
1158: Using a user name practically grants the user root access.
1159: The only valid use for user names that I can think
1.1 deraadt 1160: of is in negative entries.
1.2 deraadt 1161: .Pp
1162: Note that this warning also applies to rsh/rlogin.
1163: .It Pa /etc/shosts.equiv
1.1 deraadt 1164: This is processed exactly as
1.2 deraadt 1165: .Pa /etc/hosts.equiv .
1.1 deraadt 1166: However, this file may be useful in environments that want to run both
1.2 deraadt 1167: rsh/rlogin and ssh.
1.6 aaron 1168: .It Pa $HOME/.ssh/environment
1.36 aaron 1169: This file is read into the environment at login (if it exists).
1170: It can only contain empty lines, comment lines (that start with
1.2 deraadt 1171: .Ql # ) ,
1.36 aaron 1172: and assignment lines of the form name=value.
1173: The file should be writable
1.6 aaron 1174: only by the user; it need not be readable by anyone else.
1.2 deraadt 1175: .It Pa $HOME/.ssh/rc
1.1 deraadt 1176: If this file exists, it is run with /bin/sh after reading the
1.36 aaron 1177: environment files but before starting the user's shell or command.
1178: If X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 1179: standard input (and
1180: .Ev DISPLAY
1.36 aaron 1181: in environment).
1182: This must call
1.2 deraadt 1183: .Xr xauth 1
1184: in that case.
1185: .Pp
1.1 deraadt 1186: The primary purpose of this file is to run any initialization routines
1187: which may be needed before the user's home directory becomes
1188: accessible; AFS is a particular example of such an environment.
1.2 deraadt 1189: .Pp
1.1 deraadt 1190: This file will probably contain some initialization code followed by
1.120 markus 1191: something similar to:
1192: .Bd -literal
1193: if read proto cookie; then
1194: echo add $DISPLAY $proto $cookie | xauth -q -
1195: fi
1196: .Ed
1.2 deraadt 1197: .Pp
1198: If this file does not exist,
1199: .Pa /etc/sshrc
1200: is run, and if that
1.1 deraadt 1201: does not exist either, xauth is used to store the cookie.
1.2 deraadt 1202: .Pp
1.1 deraadt 1203: This file should be writable only by the user, and need not be
1204: readable by anyone else.
1.2 deraadt 1205: .It Pa /etc/sshrc
1206: Like
1207: .Pa $HOME/.ssh/rc .
1208: This can be used to specify
1.36 aaron 1209: machine-specific login-time initializations globally.
1210: This file should be writable only by root, and should be world-readable.
1.56 aaron 1211: .El
1.71 aaron 1212: .Sh AUTHORS
1.84 markus 1213: OpenSSH is a derivative of the original and free
1214: ssh 1.2.12 release by Tatu Ylonen.
1215: Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1216: Theo de Raadt and Dug Song
1217: removed many bugs, re-added newer features and
1218: created OpenSSH.
1219: Markus Friedl contributed the support for SSH
1220: protocol versions 1.5 and 2.0.
1.2 deraadt 1221: .Sh SEE ALSO
1222: .Xr scp 1 ,
1.90 djm 1223: .Xr sftp 1 ,
1.63 markus 1224: .Xr sftp-server 8 ,
1.2 deraadt 1225: .Xr ssh 1 ,
1.5 deraadt 1226: .Xr ssh-add 1 ,
1.2 deraadt 1227: .Xr ssh-agent 1 ,
1.5 deraadt 1228: .Xr ssh-keygen 1 ,
1.38 deraadt 1229: .Xr rlogin 1 ,
1230: .Xr rsh 1
1.119 markus 1231: .Rs
1232: .%A T. Ylonen
1233: .%A T. Kivinen
1234: .%A M. Saarinen
1235: .%A T. Rinne
1236: .%A S. Lehtinen
1237: .%T "SSH Protocol Architecture"
1238: .%N draft-ietf-secsh-architecture-07.txt
1.120 markus 1239: .%D January 2001
1240: .%O work in progress material
1241: .Re
1242: .Rs
1243: .%A M. Friedl
1244: .%A N. Provos
1245: .%A W. A. Simpson
1246: .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
1247: .%N draft-ietf-secsh-dh-group-exchange-00.txt
1.119 markus 1248: .%D January 2001
1249: .%O work in progress material
1250: .Re
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