Annotation of src/usr.bin/ssh/sshd.8, Revision 1.18
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" sshd.8.in
4: .\"
5: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
6: .\"
7: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
8: .\" All rights reserved
9: .\"
10: .\" Created: Sat Apr 22 21:55:14 1995 ylo
11: .\"
1.18 ! aaron 12: .\" $Id: sshd.8,v 1.17 1999/10/17 00:31:06 deraadt Exp $
1.1 deraadt 13: .\"
1.2 deraadt 14: .Dd September 25, 1999
15: .Dt SSHD 8
16: .Os
17: .Sh NAME
18: .Nm sshd
19: .Nd secure shell daemon
20: .Sh SYNOPSIS
21: .Nm sshd
22: .Op Fl diq
23: .Op Fl b Ar bits
24: .Op Fl f Ar config_file
25: .Op Fl g Ar login_grace_time
26: .Op Fl h Ar host_key_file
27: .Op Fl k Ar key_gen_time
28: .Op Fl p Ar port
29: .Sh DESCRIPTION
30: .Nm
1.1 deraadt 31: (Secure Shell Daemon) is the daemon program for
1.2 deraadt 32: .Xr ssh 1 .
1.1 deraadt 33: Together these programs replace rlogin and rsh programs, and
34: provide secure encrypted communications between two untrusted hosts
35: over an insecure network. The programs are intended to be as easy to
36: install and use as possible.
1.2 deraadt 37: .Pp
38: .Nm
1.1 deraadt 39: is the daemon that listens for connections from clients. It is
40: normally started at boot from
1.2 deraadt 41: .Pa /etc/rc .
42: It forks a new
1.1 deraadt 43: daemon for each incoming connection. The forked daemons handle
44: key exchange, encryption, authentication, command execution,
45: and data exchange.
1.2 deraadt 46: .Pp
47: .Nm
48: works as follows. Each host has a host-specific RSA key
1.1 deraadt 49: (normally 1024 bits) used to identify the host. Additionally, when
50: the daemon starts, it generates a server RSA key (normally 768 bits).
51: This key is normally regenerated every hour if it has been used, and
52: is never stored on disk.
1.2 deraadt 53: .Pp
1.1 deraadt 54: Whenever a client connects the daemon, the daemon sends its host
55: and server public keys to the client. The client compares the
56: host key against its own database to verify that it has not changed.
57: The client then generates a 256 bit random number. It encrypts this
58: random number using both the host key and the server key, and sends
59: the encrypted number to the server. Both sides then start to use this
60: random number as a session key which is used to encrypt all further
61: communications in the session. The rest of the session is encrypted
1.5 deraadt 62: using a conventional cipher, currently Blowfish and 3DES, with 3DES
63: being is used by default. The client selects the encryption algorithm
64: to use from those offered by the server.
1.2 deraadt 65: .Pp
1.1 deraadt 66: Next, the server and the client enter an authentication dialog. The
1.2 deraadt 67: client tries to authenticate itself using
68: .Pa .rhosts
69: authentication,
70: .Pa .rhosts
71: authentication combined with RSA host
1.1 deraadt 72: authentication, RSA challenge-response authentication, or password
73: based authentication.
1.2 deraadt 74: .Pp
1.1 deraadt 75: Rhosts authentication is normally disabled
76: because it is fundamentally insecure, but can be enabled in the server
77: configuration file if desired. System security is not improved unless
1.2 deraadt 78: .Xr rshd 8 ,
79: .Xr rlogind 8 ,
80: .Xr rexecd 8 ,
81: and
82: .Xr rexd 8
1.1 deraadt 83: are disabled (thus completely disabling
1.2 deraadt 84: .Xr rlogin 1
1.1 deraadt 85: and
1.2 deraadt 86: .Xr rsh 1
1.1 deraadt 87: into that machine).
1.2 deraadt 88: .Pp
1.1 deraadt 89: If the client successfully authenticates itself, a dialog for
90: preparing the session is entered. At this time the client may request
91: things like allocating a pseudo-tty, forwarding X11 connections,
92: forwarding TCP/IP connections, or forwarding the authentication agent
93: connection over the secure channel.
1.2 deraadt 94: .Pp
1.1 deraadt 95: Finally, the client either requests a shell or execution of a command.
96: The sides then enter session mode. In this mode, either side may send
97: data at any time, and such data is forwarded to/from the shell or
98: command on the server side, and the user terminal in the client side.
1.2 deraadt 99: .Pp
1.1 deraadt 100: When the user program terminates and all forwarded X11 and other
101: connections have been closed, the server sends command exit status to
102: the client, and both sides exit.
1.2 deraadt 103: .Pp
104: .Nm
1.1 deraadt 105: can be configured using command-line options or a configuration
106: file. Command-line options override values specified in the
107: configuration file.
1.18 ! aaron 108: .Pp
! 109: The options are as follows:
1.2 deraadt 110: .Bl -tag -width Ds
111: .It Fl b Ar bits
1.1 deraadt 112: Specifies the number of bits in the server key (default 768).
1.2 deraadt 113: .Pp
114: .It Fl d
1.1 deraadt 115: Debug mode. The server sends verbose debug output to the system
116: log, and does not put itself in the background. The server also will
117: not fork and will only process one connection. This option is only
118: intended for debugging for the server.
1.2 deraadt 119: .It Fl f Ar configuration_file
1.1 deraadt 120: Specifies the name of the configuration file. The default is
1.2 deraadt 121: .Pa /etc/sshd_config .
1.16 markus 122: .Nm
123: refuses to start if there is no configuration file.
1.2 deraadt 124: .It Fl g Ar login_grace_time
1.1 deraadt 125: Gives the grace time for clients to authenticate themselves (default
126: 300 seconds). If the client fails to authenticate the user within
127: this many seconds, the server disconnects and exits. A value of zero
128: indicates no limit.
1.2 deraadt 129: .It Fl h Ar host_key_file
1.1 deraadt 130: Specifies the file from which the host key is read (default
1.2 deraadt 131: .Pa /etc/ssh_host_key ) .
1.7 markus 132: This option must be given if
133: .Nm
134: is not run as root (as the normal
1.1 deraadt 135: host file is normally not readable by anyone but root).
1.2 deraadt 136: .It Fl i
1.7 markus 137: Specifies that
138: .Nm
139: is being run from inetd.
140: .Nm
141: is normally not run
1.1 deraadt 142: from inetd because it needs to generate the server key before it can
143: respond to the client, and this may take tens of seconds. Clients
144: would have to wait too long if the key was regenerated every time.
1.7 markus 145: However, with small key sizes (e.g. 512) using
146: .Nm
147: from inetd may
1.1 deraadt 148: be feasible.
1.2 deraadt 149: .It Fl k Ar key_gen_time
1.1 deraadt 150: Specifies how often the server key is regenerated (default 3600
151: seconds, or one hour). The motivation for regenerating the key fairly
152: often is that the key is not stored anywhere, and after about an hour,
153: it becomes impossible to recover the key for decrypting intercepted
154: communications even if the machine is cracked into or physically
155: seized. A value of zero indicates that the key will never be regenerated.
1.2 deraadt 156: .It Fl p Ar port
1.1 deraadt 157: Specifies the port on which the server listens for connections
158: (default 22).
1.2 deraadt 159: .It Fl q
1.1 deraadt 160: Quiet mode. Nothing is sent to the system log. Normally the beginning,
161: authentication, and termination of each connection is logged.
1.18 ! aaron 162: .It Fl Q
! 163: Do not print an error message if RSA support is missing.
1.2 deraadt 164: .El
165: .Sh CONFIGURATION FILE
166: .Nm
1.1 deraadt 167: reads configuration data from
1.2 deraadt 168: .Pa /etc/sshd_config
169: (or the file specified with
170: .Fl f
171: on the command line). The file
172: contains keyword-value pairs, one per line. Lines starting with
173: .Ql #
1.1 deraadt 174: and empty lines are interpreted as comments.
1.2 deraadt 175: .Pp
1.1 deraadt 176: The following keywords are possible.
1.2 deraadt 177: .Bl -tag -width Ds
178: .It Cm AFSTokenPassing
1.3 dugsong 179: Specifies whether an AFS token may be forwarded to the server. Default is
1.2 deraadt 180: .Dq yes .
1.11 markus 181: .It Cm AllowGroups
182: This keyword can be followed by a number of group names, separated
183: by spaces. If specified, login is allowed only for users whose primary
184: group matches one of the patterns.
185: .Ql \&*
186: and
187: .Ql ?
188: can be used as
189: wildcards in the patterns. Only group names are valid, a numerical group
190: id isn't recognized. By default login is allowed regardless of
191: the primary group.
1.3 dugsong 192: .Pp
1.2 deraadt 193: .It Cm AllowHosts
1.1 deraadt 194: This keyword can be followed by any number of host name patterns,
195: separated by spaces. If specified, login is allowed only from hosts
1.2 deraadt 196: whose name matches one of the patterns.
197: .Ql \&*
198: and
199: .Ql ?
200: can be used as
1.1 deraadt 201: wildcards in the patterns. Normal name servers are used to map the
202: client's host into a canonical host name. If the name cannot be
203: mapped, its IP-address is used as the host name. By default all hosts
204: are allowed to connect.
1.2 deraadt 205: .Pp
1.1 deraadt 206: Note that
1.2 deraadt 207: .Nm
208: can also be configured to use tcp_wrappers using the
1.7 markus 209: .Sy LIBWARP
1.6 aaron 210: compile-time option.
1.11 markus 211: .It Cm AllowUsers
212: This keyword can be followed by a number of user names, separated
213: by spaces. If specified, login is allowed only for users names that
214: match one of the patterns.
215: .Ql \&*
216: and
217: .Ql ?
218: can be used as
219: wildcards in the patterns. Only user names are valid, a numerical user
220: id isn't recognized. By default login is allowed regardless of
221: the user name.
222: .Pp
1.8 markus 223: .It Cm CheckMail
224: Specifies whether
225: .Nm
226: should check for new mail for interactive logins.
227: The default is
228: .Dq no .
1.11 markus 229: .It Cm DenyGroups
230: This keyword can be followed by a number of group names, separated
231: by spaces. Users whose primary group matches one of the patterns
232: aren't allowed to log in.
233: .Ql \&*
234: and
235: .Ql ?
236: can be used as
237: wildcards in the patterns. Only group names are valid, a numerical group
238: id isn't recognized. By default login is allowed regardless of
239: the primary group.
240: .Pp
1.2 deraadt 241: .It Cm DenyHosts
1.1 deraadt 242: This keyword can be followed by any number of host name patterns,
243: separated by spaces. If specified, login is disallowed from the hosts
244: whose name matches any of the patterns.
1.11 markus 245: .It Cm DenyUsers
246: This keyword can be followed by a number of user names, separated
247: by spaces. Login is allowed disallowed for user names that match
248: one of the patterns.
249: .Ql \&*
250: and
251: .Ql ?
252: can be used as
253: wildcards in the patterns. Only user names are valid, a numerical user
254: id isn't recognized. By default login is allowed regardless of
255: the user name.
256: .Pp
1.2 deraadt 257: .It Cm FascistLogging
1.1 deraadt 258: Specifies whether to use verbose logging. Verbose logging violates
259: the privacy of users and is not recommended. The argument must be
1.2 deraadt 260: .Dq yes
261: or
262: .Dq no .
263: The default is
264: .Dq no .
265: .It Cm HostKey
1.1 deraadt 266: Specifies the file containing the private host key (default
1.2 deraadt 267: .Pa /etc/ssh_host_key ) .
1.9 markus 268: Note that
269: .Nm
1.14 markus 270: does not start if this file is group/world-accessible.
1.2 deraadt 271: .It Cm IgnoreRhosts
1.1 deraadt 272: Specifies that rhosts and shosts files will not be used in
273: authentication.
1.2 deraadt 274: .Pa /etc/hosts.equiv
1.1 deraadt 275: and
1.2 deraadt 276: .Pa /etc/shosts.equiv
277: are still used. The default is
278: .Dq no .
279: .It Cm KeepAlive
1.1 deraadt 280: Specifies whether the system should send keepalive messages to the
281: other side. If they are sent, death of the connection or crash of one
282: of the machines will be properly noticed. However, this means that
283: connections will die if the route is down temporarily, and some people
284: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 285: sessions may hang indefinitely on the server, leaving
286: .Dq ghost
287: users and consuming server resources.
288: .Pp
289: The default is
290: .Dq yes
291: (to send keepalives), and the server will notice
1.1 deraadt 292: if the network goes down or the client host reboots. This avoids
293: infinitely hanging sessions.
1.2 deraadt 294: .Pp
295: To disable keepalives, the value should be set to
296: .Dq no
297: in both the server and the client configuration files.
298: .It Cm KerberosAuthentication
1.1 deraadt 299: Specifies whether Kerberos authentication is allowed. This can
1.7 markus 300: be in the form of a Kerberos ticket, or if
301: .Cm PasswordAuthentication
1.1 deraadt 302: is yes, the password provided by the user will be validated through
1.3 dugsong 303: the Kerberos KDC. Default is
304: .Dq yes .
1.2 deraadt 305: .It Cm KerberosOrLocalPasswd
1.1 deraadt 306: If set then if password authentication through Kerberos fails then
307: the password will be validated via any additional local mechanism
1.2 deraadt 308: such as
309: .Pa /etc/passwd
310: or SecurID. Default is
311: .Dq no .
312: .It Cm KerberosTgtPassing
1.1 deraadt 313: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 314: Default is
315: .Dq no ,
316: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 317: .It Cm KerberosTicketCleanup
1.7 markus 318: Specifies whether to automatically destroy the user's ticket cache
319: file on logout. Default is
1.3 dugsong 320: .Dq yes .
1.2 deraadt 321: .It Cm KeyRegenerationInterval
1.1 deraadt 322: The server key is automatically regenerated after this many seconds
323: (if it has been used). The purpose of regeneration is to prevent
324: decrypting captured sessions by later breaking into the machine and
325: stealing the keys. The key is never stored anywhere. If the value is
326: 0, the key is never regenerated. The default is 3600
327: (seconds).
1.7 markus 328: .It Cm ListenAddress
329: Specifies what local address
330: .Nm
331: should listen on.
332: The default is to listen to all local addresses.
1.2 deraadt 333: .It Cm LoginGraceTime
1.1 deraadt 334: The server disconnects after this time if the user has not
335: successfully logged in. If the value is 0, there is no time limit.
336: The default is 600 (seconds).
1.2 deraadt 337: .It Cm PasswordAuthentication
1.1 deraadt 338: Specifies whether password authentication is allowed.
1.2 deraadt 339: The default is
340: .Dq yes .
341: .It Cm PermitEmptyPasswords
1.1 deraadt 342: When password authentication is allowed, it specifies whether the
343: server allows login to accounts with empty password strings. The default
1.2 deraadt 344: is
345: .Dq yes .
346: .It Cm PermitRootLogin
1.1 deraadt 347: Specifies whether the root can log in using
1.2 deraadt 348: .Xr ssh 1 .
1.15 markus 349: The argument must be
350: .Dq yes ,
351: .Dq without-password
352: or
353: .Dq no .
1.2 deraadt 354: The default is
355: .Dq yes .
1.15 markus 356: If this options is set to
357: .Dq without-password
358: only password authentication is disabled for root.
1.2 deraadt 359: .Pp
360: Root login with RSA authentication when the
361: .Ar command
362: option has been
1.1 deraadt 363: specified will be allowed regardless of the value of this setting
364: (which may be useful for taking remote backups even if root login is
365: normally not allowed).
1.2 deraadt 366: .It Cm Port
1.1 deraadt 367: Specifies the port number that
1.2 deraadt 368: .Nm
1.1 deraadt 369: listens on. The default is 22.
1.2 deraadt 370: .It Cm PrintMotd
1.1 deraadt 371: Specifies whether
1.2 deraadt 372: .Nm
1.1 deraadt 373: should print
1.2 deraadt 374: .Pa /etc/motd
1.1 deraadt 375: when a user logs in interactively. (On some systems it is also
1.2 deraadt 376: printed by the shell,
377: .Pa /etc/profile ,
378: or equivalent.) The default is
379: .Dq yes .
380: .It Cm QuietMode
1.1 deraadt 381: Specifies whether the system runs in quiet mode. In quiet mode,
382: nothing is logged in the system log, except fatal errors. The default
1.2 deraadt 383: is
384: .Dq no .
385: .It Cm RandomSeed
1.4 deraadt 386: Obsolete. Random number generation uses other techniques.
1.2 deraadt 387: .It Cm RhostsAuthentication
1.1 deraadt 388: Specifies whether authentication using rhosts or /etc/hosts.equiv
389: files is sufficient. Normally, this method should not be permitted
1.7 markus 390: because it is insecure.
391: .Cm RhostsRSAAuthentication
392: should be used
1.1 deraadt 393: instead, because it performs RSA-based host authentication in addition
394: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 395: The default is
396: .Dq no .
397: .It Cm RhostsRSAAuthentication
1.1 deraadt 398: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 399: with successful RSA host authentication is allowed. The default is
400: .Dq yes .
401: .It Cm RSAAuthentication
402: Specifies whether pure RSA authentication is allowed. The default is
403: .Dq yes .
404: .It Cm ServerKeyBits
1.1 deraadt 405: Defines the number of bits in the server key. The minimum value is
406: 512, and the default is 768.
1.13 markus 407: .It Cm SilentDeny
408: Specifies whether
409: .Nm
410: should log and complain to denied hosts or close the
411: connection silently.
412: The default is
413: .Dq no .
1.7 markus 414: .It Cm SkeyAuthentication
415: Specifies whether
416: .Xr skey 1
417: authentication is allowed. The default is
418: .Dq yes .
419: Note that s/key authentication is enabled only if
420: .Cm PasswordAuthentication
421: is allowed, too.
1.2 deraadt 422: .It Cm StrictModes
1.12 markus 423: Specifies whether
424: .Nm
425: should check file modes and ownership of the
426: user's files and home directory before accepting login. This
1.1 deraadt 427: is normally desirable because novices sometimes accidentally leave their
1.7 markus 428: directory or files world-writable. The default is
429: .Dq yes .
1.2 deraadt 430: .It Cm SyslogFacility
1.1 deraadt 431: Gives the facility code that is used when logging messages from
1.2 deraadt 432: .Nm sshd .
1.1 deraadt 433: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
434: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.
1.10 markus 435: .It Cm UseLogin
436: Specifies whether
437: .Xr login 1
438: is used. The default is
439: .Dq no .
1.2 deraadt 440: .It Cm X11Forwarding
441: Specifies whether X11 forwarding is permitted. The default is
442: .Dq yes .
1.1 deraadt 443: Note that disabling X11 forwarding does not improve security in any
444: way, as users can always install their own forwarders.
1.6 aaron 445: .It Cm X11DisplayOffset
446: Specifies the first display number available for
447: .Nm sshd Ns 's
448: X11 forwarding. This prevents
449: .Nm
450: from interfering with real X11 servers.
1.2 deraadt 451: .El
452: .Sh LOGIN PROCESS
1.1 deraadt 453: When a user successfully logs in,
1.2 deraadt 454: .Nm
1.1 deraadt 455: does the following:
1.2 deraadt 456: .Bl -enum -offset indent
457: .It
1.1 deraadt 458: If the login is on a tty, and no command has been specified,
459: prints last login time and
1.2 deraadt 460: .Pa /etc/motd
1.1 deraadt 461: (unless prevented in the configuration file or by
1.2 deraadt 462: .Pa $HOME/.hushlogin ;
463: see the
464: .Sx FILES
465: section).
466: .It
1.1 deraadt 467: If the login is on a tty, records login time.
1.2 deraadt 468: .It
469: Checks
470: .Pa /etc/nologin ;
471: if it exists, prints contents and quits
1.1 deraadt 472: (unless root).
1.2 deraadt 473: .It
1.1 deraadt 474: Changes to run with normal user privileges.
1.2 deraadt 475: .It
1.1 deraadt 476: Sets up basic environment.
1.2 deraadt 477: .It
478: Reads
479: .Pa $HOME/.ssh/environment
480: if it exists.
481: .It
1.1 deraadt 482: Changes to user's home directory.
1.2 deraadt 483: .It
484: If
485: .Pa $HOME/.ssh/rc
486: exists, runs it; else if
487: .Pa /etc/sshrc
488: exists, runs
489: it; otherwise runs xauth. The
490: .Dq rc
491: files are given the X11
1.1 deraadt 492: authentication protocol and cookie in standard input.
1.2 deraadt 493: .It
1.1 deraadt 494: Runs user's shell or command.
1.2 deraadt 495: .El
496: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 497: The
1.2 deraadt 498: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 499: file lists the RSA keys that are
500: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 501: key (empty lines and lines starting with a
502: .Ql #
503: are ignored as
1.1 deraadt 504: comments). Each line consists of the following fields, separated by
505: spaces: options, bits, exponent, modulus, comment. The options field
506: is optional; its presence is determined by whether the line starts
507: with a number or not (the option field never starts with a number).
508: The bits, exponent, modulus and comment fields give the RSA key; the
509: comment field is not used for anything (but may be convenient for the
510: user to identify the key).
1.2 deraadt 511: .Pp
1.1 deraadt 512: Note that lines in this file are usually several hundred bytes long
513: (because of the size of the RSA key modulus). You don't want to type
514: them in; instead, copy the
1.2 deraadt 515: .Pa identity.pub
1.1 deraadt 516: file and edit it.
1.2 deraadt 517: .Pp
1.1 deraadt 518: The options (if present) consists of comma-separated option
519: specifications. No spaces are permitted, except within double quotes.
520: The following option specifications are supported:
1.2 deraadt 521: .Bl -tag -width Ds
522: .It Cm from="pattern-list"
1.1 deraadt 523: Specifies that in addition to RSA authentication, the canonical name
524: of the remote host must be present in the comma-separated list of
525: patterns ('*' and '?' serve as wildcards). The list may also contain
526: patterns negated by prefixing them with '!'; if the canonical host
527: name matches a negated pattern, the key is not accepted. The purpose
528: of this option is to optionally increase security: RSA authentication
529: by itself does not trust the network or name servers or anything (but
530: the key); however, if somebody somehow steals the key, the key
531: permits an intruder to log in from anywhere in the world. This
532: additional option makes using a stolen key more difficult (name
533: servers and/or routers would have to be compromised in addition to
534: just the key).
1.2 deraadt 535: .It Cm command="command"
1.1 deraadt 536: Specifies that the command is executed whenever this key is used for
537: authentication. The command supplied by the user (if any) is ignored.
538: The command is run on a pty if the connection requests a pty;
539: otherwise it is run without a tty. A quote may be included in the
540: command by quoting it with a backslash. This option might be useful
541: to restrict certain RSA keys to perform just a specific operation. An
542: example might be a key that permits remote backups but nothing
543: else. Notice that the client may specify TCP/IP and/or X11
544: forwardings unless they are explicitly prohibited.
1.2 deraadt 545: .It Cm environment="NAME=value"
1.1 deraadt 546: Specifies that the string is to be added to the environment when
547: logging in using this key. Environment variables set this way
548: override other default environment values. Multiple options of this
549: type are permitted.
1.2 deraadt 550: .It Cm no-port-forwarding
1.1 deraadt 551: Forbids TCP/IP forwarding when this key is used for authentication.
552: Any port forward requests by the client will return an error. This
1.2 deraadt 553: might be used, e.g., in connection with the
554: .Cm command
1.1 deraadt 555: option.
1.2 deraadt 556: .It Cm no-X11-forwarding
1.1 deraadt 557: Forbids X11 forwarding when this key is used for authentication.
558: Any X11 forward requests by the client will return an error.
1.2 deraadt 559: .It Cm no-agent-forwarding
1.1 deraadt 560: Forbids authentication agent forwarding when this key is used for
561: authentication.
1.2 deraadt 562: .It Cm no-pty
1.1 deraadt 563: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 564: .El
565: .Ss Examples
1.1 deraadt 566: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 567: .Pp
1.1 deraadt 568: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 569: .Pp
1.1 deraadt 570: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 571: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 572: The
1.2 deraadt 573: .Pa /etc/ssh_known_hosts
1.1 deraadt 574: and
1.2 deraadt 575: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 576: files contain host public keys for all known hosts. The global file should
577: be prepared by the admistrator (optional), and the per-user file is
578: maintained automatically: whenever the user connects an unknown host
1.6 aaron 579: its key is added to the per-user file.
1.2 deraadt 580: .Pp
1.1 deraadt 581: Each line in these files contains the following fields: hostnames,
582: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 583: .Pp
1.1 deraadt 584: Hostnames is a comma-separated list of patterns ('*' and '?' act as
585: wildcards); each pattern in turn is matched against the canonical host
586: name (when authenticating a client) or against the user-supplied
587: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 588: by
589: .Ql !
590: to indicate negation: if the host name matches a negated
1.1 deraadt 591: pattern, it is not accepted (by that line) even if it matched another
592: pattern on the line.
1.2 deraadt 593: .Pp
1.1 deraadt 594: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 595: can be obtained, e.g., from
596: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 597: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 598: .Pp
599: Lines starting with
600: .Ql #
601: and empty lines are ignored as comments.
602: .Pp
1.1 deraadt 603: When performing host authentication, authentication is accepted if any
604: matching line has the proper key. It is thus permissible (but not
605: recommended) to have several lines or different host keys for the same
606: names. This will inevitably happen when short forms of host names
607: from different domains are put in the file. It is possible
608: that the files contain conflicting information; authentication is
609: accepted if valid information can be found from either file.
1.2 deraadt 610: .Pp
1.1 deraadt 611: Note that the lines in these files are typically hundreds of characters
612: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 613: Rather, generate them by a script
1.1 deraadt 614: or by taking
1.2 deraadt 615: .Pa /etc/ssh_host_key.pub
1.1 deraadt 616: and adding the host names at the front.
1.2 deraadt 617: .Ss Examples
1.1 deraadt 618: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 619: .Sh FILES
620: .Bl -tag -width Ds
621: .It Pa /etc/sshd_config
1.1 deraadt 622: Contains configuration data for
1.2 deraadt 623: .Nm sshd .
1.1 deraadt 624: This file should be writable by root only, but it is recommended
625: (though not necessary) that it be world-readable.
1.2 deraadt 626: .It Pa /etc/ssh_host_key
1.7 markus 627: Contains the private part of the host key.
1.1 deraadt 628: This file should only be owned by root, readable only by root, and not
629: accessible to others.
1.14 markus 630: Note that
631: .Nm
632: does not start if this file is group/world-accessible.
1.2 deraadt 633: .It Pa /etc/ssh_host_key.pub
1.7 markus 634: Contains the public part of the host key.
635: This file should be world-readable but writable only by
1.1 deraadt 636: root. Its contents should match the private part. This file is not
637: really used for anything; it is only provided for the convenience of
638: the user so its contents can be copied to known hosts files.
1.7 markus 639: These two files are created using
640: .Xr ssh-keygen 1 .
1.2 deraadt 641: .It Pa /var/run/sshd.pid
642: Contains the process ID of the
643: .Nm
1.1 deraadt 644: listening for connections (if there are several daemons running
645: concurrently for different ports, this contains the pid of the one
646: started last). The contents of this file are not sensitive; it can be
647: world-readable.
1.2 deraadt 648: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 649: Lists the RSA keys that can be used to log into the user's account.
650: This file must be readable by root (which may on some machines imply
651: it being world-readable if the user's home directory resides on an NFS
652: volume). It is recommended that it not be accessible by others. The
653: format of this file is described above.
1.6 aaron 654: .It Pa /etc/ssh_known_hosts
655: This file is consulted when using rhosts with RSA host
1.1 deraadt 656: authentication to check the public key of the host. The key must be
1.6 aaron 657: listed in this file to be accepted.
658: .It Pa $HOME/.ssh/known_hosts
659: The client uses this file
660: and
661: .Pa /etc/ssh_known_hosts
662: to verify that the remote host is the one we intended to
663: connect. These files should be writable only by root/the owner.
1.2 deraadt 664: .Pa /etc/ssh_known_hosts
665: should be world-readable, and
666: .Pa $HOME/.ssh/known_hosts
667: can but need not be world-readable.
1.6 aaron 668: .It Pa /etc/nologin
1.1 deraadt 669: If this file exists,
1.2 deraadt 670: .Nm
1.1 deraadt 671: refuses to let anyone except root log in. The contents of the file
672: are displayed to anyone trying to log in, and non-root connections are
673: refused. The file should be world-readable.
1.6 aaron 674: .It Pa $HOME/.rhosts
1.1 deraadt 675: This file contains host-username pairs, separated by a space, one per
676: line. The given user on the corresponding host is permitted to log in
677: without password. The same file is used by rlogind and rshd.
1.6 aaron 678: The file must
1.1 deraadt 679: be writable only by the user; it is recommended that it not be
680: accessible by others.
1.2 deraadt 681: .Pp
1.1 deraadt 682: If is also possible to use netgroups in the file. Either host or user
683: name may be of the form +@groupname to specify all hosts or all users
684: in the group.
1.2 deraadt 685: .It Pa $HOME/.shosts
686: For ssh,
687: this file is exactly the same as for
688: .Pa .rhosts .
689: However, this file is
690: not used by rlogin and rshd, so using this permits access using SSH only.
691: .Pa /etc/hosts.equiv
692: This file is used during
693: .Pa .rhosts
694: authentication. In the
1.1 deraadt 695: simplest form, this file contains host names, one per line. Users on
696: those hosts are permitted to log in without a password, provided they
697: have the same user name on both machines. The host name may also be
698: followed by a user name; such users are permitted to log in as
1.2 deraadt 699: .Em any
700: user on this machine (except root). Additionally, the syntax
701: .Dq +@group
702: can be used to specify netgroups. Negated entries start with
703: .Ql \&- .
704: .Pp
1.1 deraadt 705: If the client host/user is successfully matched in this file, login is
706: automatically permitted provided the client and server user names are the
707: same. Additionally, successful RSA host authentication is normally
708: required. This file must be writable only by root; it is recommended
709: that it be world-readable.
1.2 deraadt 710: .Pp
1.6 aaron 711: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 712: .Pa hosts.equiv .
1.1 deraadt 713: Beware that it really means that the named user(s) can log in as
1.2 deraadt 714: .Em anybody ,
1.1 deraadt 715: which includes bin, daemon, adm, and other accounts that own critical
716: binaries and directories. Using a user name practically grants the
717: user root access. The only valid use for user names that I can think
718: of is in negative entries.
1.2 deraadt 719: .Pp
720: Note that this warning also applies to rsh/rlogin.
721: .It Pa /etc/shosts.equiv
1.1 deraadt 722: This is processed exactly as
1.2 deraadt 723: .Pa /etc/hosts.equiv .
1.1 deraadt 724: However, this file may be useful in environments that want to run both
1.2 deraadt 725: rsh/rlogin and ssh.
1.6 aaron 726: .It Pa $HOME/.ssh/environment
1.1 deraadt 727: This file is read into the environment at login (if it exists). It
1.2 deraadt 728: can only contain empty lines, comment lines (that start with
729: .Ql # ) ,
1.6 aaron 730: and assignment lines of the form name=value. The file should be writable
731: only by the user; it need not be readable by anyone else.
1.2 deraadt 732: .It Pa $HOME/.ssh/rc
1.1 deraadt 733: If this file exists, it is run with /bin/sh after reading the
734: environment files but before starting the user's shell or command. If
735: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 736: standard input (and
737: .Ev DISPLAY
738: in environment). This must call
739: .Xr xauth 1
740: in that case.
741: .Pp
1.1 deraadt 742: The primary purpose of this file is to run any initialization routines
743: which may be needed before the user's home directory becomes
744: accessible; AFS is a particular example of such an environment.
1.2 deraadt 745: .Pp
1.1 deraadt 746: This file will probably contain some initialization code followed by
747: something similar to: "if read proto cookie; then echo add $DISPLAY
748: $proto $cookie | xauth -q -; fi".
1.2 deraadt 749: .Pp
750: If this file does not exist,
751: .Pa /etc/sshrc
752: is run, and if that
1.1 deraadt 753: does not exist either, xauth is used to store the cookie.
1.2 deraadt 754: .Pp
1.1 deraadt 755: This file should be writable only by the user, and need not be
756: readable by anyone else.
1.2 deraadt 757: .It Pa /etc/sshrc
758: Like
759: .Pa $HOME/.ssh/rc .
760: This can be used to specify
1.1 deraadt 761: machine-specific login-time initializations globally. This file
762: should be writable only by root, and should be world-readable.
1.2 deraadt 763: .Sh AUTHOR
1.1 deraadt 764: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 765: .Pp
1.1 deraadt 766: Information about new releases, mailing lists, and other related
1.2 deraadt 767: issues can be found from the SSH WWW home page:
768: .Pp
769: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 770: .Pp
1.17 deraadt 771: OpenSSH
772: is a derivative of the original (free) ssh 1.2.12 release, but with bugs
773: removed and newer features re-added. Rapidly after the 1.2.12 release,
774: newer versions bore successively more restrictive licenses. This version
775: of OpenSSH
776: .Bl -bullet
777: .It
778: has all components of a restrictive nature (ie. patents, see
779: .Xr ssl 8 )
780: directly removed from the source code; any licensed or patented components
781: are chosen from
782: external libraries.
783: .It
784: has been updated to support ssh protocol 1.5.
785: .It
786: contains added support for
787: .Xr kerberos 8
788: authentication and ticket passing.
789: .It
790: supports one-time password authentication with
791: .Xr skey 1 .
792: .El
793: .Pp
794: The libraries described in
1.5 deraadt 795: .Xr ssl 8
796: are required for proper operation.
1.2 deraadt 797: .Sh SEE ALSO
798: .Xr rlogin 1 ,
799: .Xr rsh 1 ,
800: .Xr scp 1 ,
801: .Xr ssh 1 ,
1.5 deraadt 802: .Xr ssh-add 1 ,
1.2 deraadt 803: .Xr ssh-agent 1 ,
1.5 deraadt 804: .Xr ssh-keygen 1 ,
805: .Xr ssl 8
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