Annotation of src/usr.bin/ssh/sshd.8, Revision 1.24
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: .\"
1.24 ! markus 12: .\" $Id: sshd.8,v 1.23 1999/11/10 23:36:45 markus 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
1.11 markus 193: .It Cm AllowUsers
194: This keyword can be followed by a number of user names, separated
195: by spaces. If specified, login is allowed only for users names that
196: match one of the patterns.
197: .Ql \&*
198: and
199: .Ql ?
200: can be used as
201: wildcards in the patterns. Only user names are valid, a numerical user
202: id isn't recognized. By default login is allowed regardless of
203: the user name.
204: .Pp
1.8 markus 205: .It Cm CheckMail
206: Specifies whether
207: .Nm
208: should check for new mail for interactive logins.
209: The default is
210: .Dq no .
1.11 markus 211: .It Cm DenyGroups
212: This keyword can be followed by a number of group names, separated
213: by spaces. Users whose primary group matches one of the patterns
214: aren't allowed to log in.
215: .Ql \&*
216: and
217: .Ql ?
218: can be used as
219: wildcards in the patterns. Only group names are valid, a numerical group
220: id isn't recognized. By default login is allowed regardless of
221: the primary group.
222: .Pp
223: .It Cm DenyUsers
224: This keyword can be followed by a number of user names, separated
225: by spaces. Login is allowed disallowed for user names that match
226: one of the patterns.
227: .Ql \&*
228: and
229: .Ql ?
230: can be used as
231: wildcards in the patterns. Only user names are valid, a numerical user
232: id isn't recognized. By default login is allowed regardless of
233: the user name.
1.2 deraadt 234: .It Cm HostKey
1.1 deraadt 235: Specifies the file containing the private host key (default
1.2 deraadt 236: .Pa /etc/ssh_host_key ) .
1.9 markus 237: Note that
238: .Nm
1.14 markus 239: does not start if this file is group/world-accessible.
1.2 deraadt 240: .It Cm IgnoreRhosts
1.1 deraadt 241: Specifies that rhosts and shosts files will not be used in
242: authentication.
1.2 deraadt 243: .Pa /etc/hosts.equiv
1.1 deraadt 244: and
1.2 deraadt 245: .Pa /etc/shosts.equiv
246: are still used. The default is
1.24 ! markus 247: .Dq no .
! 248: .It Cm IgnoreUserKnownHosts
! 249: Specifies whether
! 250: .Nm
! 251: should ignore the user's
! 252: .Pa $HOME/.ssh/known_hosts
! 253: during
! 254: .Cm RhostsRSAAuthentication .
! 255: The default is
1.2 deraadt 256: .Dq no .
257: .It Cm KeepAlive
1.1 deraadt 258: Specifies whether the system should send keepalive messages to the
259: other side. If they are sent, death of the connection or crash of one
260: of the machines will be properly noticed. However, this means that
261: connections will die if the route is down temporarily, and some people
262: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 263: sessions may hang indefinitely on the server, leaving
264: .Dq ghost
265: users and consuming server resources.
266: .Pp
267: The default is
268: .Dq yes
269: (to send keepalives), and the server will notice
1.1 deraadt 270: if the network goes down or the client host reboots. This avoids
271: infinitely hanging sessions.
1.2 deraadt 272: .Pp
273: To disable keepalives, the value should be set to
274: .Dq no
275: in both the server and the client configuration files.
276: .It Cm KerberosAuthentication
1.1 deraadt 277: Specifies whether Kerberos authentication is allowed. This can
1.7 markus 278: be in the form of a Kerberos ticket, or if
279: .Cm PasswordAuthentication
1.1 deraadt 280: is yes, the password provided by the user will be validated through
1.3 dugsong 281: the Kerberos KDC. Default is
282: .Dq yes .
1.2 deraadt 283: .It Cm KerberosOrLocalPasswd
1.1 deraadt 284: If set then if password authentication through Kerberos fails then
285: the password will be validated via any additional local mechanism
1.2 deraadt 286: such as
287: .Pa /etc/passwd
288: or SecurID. Default is
1.20 dugsong 289: .Dq yes .
1.2 deraadt 290: .It Cm KerberosTgtPassing
1.1 deraadt 291: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 292: Default is
293: .Dq no ,
294: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 295: .It Cm KerberosTicketCleanup
1.7 markus 296: Specifies whether to automatically destroy the user's ticket cache
297: file on logout. Default is
1.3 dugsong 298: .Dq yes .
1.2 deraadt 299: .It Cm KeyRegenerationInterval
1.1 deraadt 300: The server key is automatically regenerated after this many seconds
301: (if it has been used). The purpose of regeneration is to prevent
302: decrypting captured sessions by later breaking into the machine and
303: stealing the keys. The key is never stored anywhere. If the value is
304: 0, the key is never regenerated. The default is 3600
305: (seconds).
1.7 markus 306: .It Cm ListenAddress
307: Specifies what local address
308: .Nm
309: should listen on.
310: The default is to listen to all local addresses.
1.2 deraadt 311: .It Cm LoginGraceTime
1.1 deraadt 312: The server disconnects after this time if the user has not
313: successfully logged in. If the value is 0, there is no time limit.
314: The default is 600 (seconds).
1.23 markus 315: .It Cm LogLevel
316: Gives the verbosity level that is used when logging messages from
317: .Nm sshd .
318: The possible values are:
319: QUIET, FATAL, ERROR, INFO, CHAT and DEBUG.
320: The default is INFO.
321: Logging with level DEBUG violates the privacy of users
322: and is not recommended.
1.2 deraadt 323: .It Cm PasswordAuthentication
1.1 deraadt 324: Specifies whether password authentication is allowed.
1.2 deraadt 325: The default is
326: .Dq yes .
327: .It Cm PermitEmptyPasswords
1.1 deraadt 328: When password authentication is allowed, it specifies whether the
329: server allows login to accounts with empty password strings. The default
1.2 deraadt 330: is
331: .Dq yes .
332: .It Cm PermitRootLogin
1.1 deraadt 333: Specifies whether the root can log in using
1.2 deraadt 334: .Xr ssh 1 .
1.15 markus 335: The argument must be
336: .Dq yes ,
337: .Dq without-password
338: or
339: .Dq no .
1.2 deraadt 340: The default is
341: .Dq yes .
1.15 markus 342: If this options is set to
343: .Dq without-password
344: only password authentication is disabled for root.
1.2 deraadt 345: .Pp
346: Root login with RSA authentication when the
347: .Ar command
348: option has been
1.1 deraadt 349: specified will be allowed regardless of the value of this setting
350: (which may be useful for taking remote backups even if root login is
351: normally not allowed).
1.2 deraadt 352: .It Cm Port
1.1 deraadt 353: Specifies the port number that
1.2 deraadt 354: .Nm
1.1 deraadt 355: listens on. The default is 22.
1.2 deraadt 356: .It Cm PrintMotd
1.1 deraadt 357: Specifies whether
1.2 deraadt 358: .Nm
1.1 deraadt 359: should print
1.2 deraadt 360: .Pa /etc/motd
1.1 deraadt 361: when a user logs in interactively. (On some systems it is also
1.2 deraadt 362: printed by the shell,
363: .Pa /etc/profile ,
364: or equivalent.) The default is
365: .Dq yes .
366: .It Cm RandomSeed
1.4 deraadt 367: Obsolete. Random number generation uses other techniques.
1.2 deraadt 368: .It Cm RhostsAuthentication
1.1 deraadt 369: Specifies whether authentication using rhosts or /etc/hosts.equiv
370: files is sufficient. Normally, this method should not be permitted
1.7 markus 371: because it is insecure.
372: .Cm RhostsRSAAuthentication
373: should be used
1.1 deraadt 374: instead, because it performs RSA-based host authentication in addition
375: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 376: The default is
377: .Dq no .
378: .It Cm RhostsRSAAuthentication
1.1 deraadt 379: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 380: with successful RSA host authentication is allowed. The default is
381: .Dq yes .
382: .It Cm RSAAuthentication
383: Specifies whether pure RSA authentication is allowed. The default is
384: .Dq yes .
385: .It Cm ServerKeyBits
1.1 deraadt 386: Defines the number of bits in the server key. The minimum value is
387: 512, and the default is 768.
1.7 markus 388: .It Cm SkeyAuthentication
389: Specifies whether
390: .Xr skey 1
391: authentication is allowed. The default is
392: .Dq yes .
393: Note that s/key authentication is enabled only if
394: .Cm PasswordAuthentication
395: is allowed, too.
1.2 deraadt 396: .It Cm StrictModes
1.12 markus 397: Specifies whether
398: .Nm
399: should check file modes and ownership of the
400: user's files and home directory before accepting login. This
1.1 deraadt 401: is normally desirable because novices sometimes accidentally leave their
1.7 markus 402: directory or files world-writable. The default is
403: .Dq yes .
1.2 deraadt 404: .It Cm SyslogFacility
1.1 deraadt 405: Gives the facility code that is used when logging messages from
1.2 deraadt 406: .Nm sshd .
1.1 deraadt 407: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.21 markus 408: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
1.10 markus 409: .It Cm UseLogin
410: Specifies whether
411: .Xr login 1
412: is used. The default is
413: .Dq no .
1.2 deraadt 414: .It Cm X11Forwarding
415: Specifies whether X11 forwarding is permitted. The default is
416: .Dq yes .
1.1 deraadt 417: Note that disabling X11 forwarding does not improve security in any
418: way, as users can always install their own forwarders.
1.6 aaron 419: .It Cm X11DisplayOffset
420: Specifies the first display number available for
421: .Nm sshd Ns 's
422: X11 forwarding. This prevents
423: .Nm
424: from interfering with real X11 servers.
1.2 deraadt 425: .El
426: .Sh LOGIN PROCESS
1.1 deraadt 427: When a user successfully logs in,
1.2 deraadt 428: .Nm
1.1 deraadt 429: does the following:
1.2 deraadt 430: .Bl -enum -offset indent
431: .It
1.1 deraadt 432: If the login is on a tty, and no command has been specified,
433: prints last login time and
1.2 deraadt 434: .Pa /etc/motd
1.1 deraadt 435: (unless prevented in the configuration file or by
1.2 deraadt 436: .Pa $HOME/.hushlogin ;
437: see the
438: .Sx FILES
439: section).
440: .It
1.1 deraadt 441: If the login is on a tty, records login time.
1.2 deraadt 442: .It
443: Checks
444: .Pa /etc/nologin ;
445: if it exists, prints contents and quits
1.1 deraadt 446: (unless root).
1.2 deraadt 447: .It
1.1 deraadt 448: Changes to run with normal user privileges.
1.2 deraadt 449: .It
1.1 deraadt 450: Sets up basic environment.
1.2 deraadt 451: .It
452: Reads
453: .Pa $HOME/.ssh/environment
454: if it exists.
455: .It
1.1 deraadt 456: Changes to user's home directory.
1.2 deraadt 457: .It
458: If
459: .Pa $HOME/.ssh/rc
460: exists, runs it; else if
461: .Pa /etc/sshrc
462: exists, runs
463: it; otherwise runs xauth. The
464: .Dq rc
465: files are given the X11
1.1 deraadt 466: authentication protocol and cookie in standard input.
1.2 deraadt 467: .It
1.1 deraadt 468: Runs user's shell or command.
1.2 deraadt 469: .El
470: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 471: The
1.2 deraadt 472: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 473: file lists the RSA keys that are
474: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 475: key (empty lines and lines starting with a
476: .Ql #
477: are ignored as
1.1 deraadt 478: comments). Each line consists of the following fields, separated by
479: spaces: options, bits, exponent, modulus, comment. The options field
480: is optional; its presence is determined by whether the line starts
481: with a number or not (the option field never starts with a number).
482: The bits, exponent, modulus and comment fields give the RSA key; the
483: comment field is not used for anything (but may be convenient for the
484: user to identify the key).
1.2 deraadt 485: .Pp
1.1 deraadt 486: Note that lines in this file are usually several hundred bytes long
487: (because of the size of the RSA key modulus). You don't want to type
488: them in; instead, copy the
1.2 deraadt 489: .Pa identity.pub
1.1 deraadt 490: file and edit it.
1.2 deraadt 491: .Pp
1.1 deraadt 492: The options (if present) consists of comma-separated option
493: specifications. No spaces are permitted, except within double quotes.
494: The following option specifications are supported:
1.2 deraadt 495: .Bl -tag -width Ds
496: .It Cm from="pattern-list"
1.1 deraadt 497: Specifies that in addition to RSA authentication, the canonical name
498: of the remote host must be present in the comma-separated list of
499: patterns ('*' and '?' serve as wildcards). The list may also contain
500: patterns negated by prefixing them with '!'; if the canonical host
501: name matches a negated pattern, the key is not accepted. The purpose
502: of this option is to optionally increase security: RSA authentication
503: by itself does not trust the network or name servers or anything (but
504: the key); however, if somebody somehow steals the key, the key
505: permits an intruder to log in from anywhere in the world. This
506: additional option makes using a stolen key more difficult (name
507: servers and/or routers would have to be compromised in addition to
508: just the key).
1.2 deraadt 509: .It Cm command="command"
1.1 deraadt 510: Specifies that the command is executed whenever this key is used for
511: authentication. The command supplied by the user (if any) is ignored.
512: The command is run on a pty if the connection requests a pty;
513: otherwise it is run without a tty. A quote may be included in the
514: command by quoting it with a backslash. This option might be useful
515: to restrict certain RSA keys to perform just a specific operation. An
516: example might be a key that permits remote backups but nothing
517: else. Notice that the client may specify TCP/IP and/or X11
518: forwardings unless they are explicitly prohibited.
1.2 deraadt 519: .It Cm environment="NAME=value"
1.1 deraadt 520: Specifies that the string is to be added to the environment when
521: logging in using this key. Environment variables set this way
522: override other default environment values. Multiple options of this
523: type are permitted.
1.2 deraadt 524: .It Cm no-port-forwarding
1.1 deraadt 525: Forbids TCP/IP forwarding when this key is used for authentication.
526: Any port forward requests by the client will return an error. This
1.2 deraadt 527: might be used, e.g., in connection with the
528: .Cm command
1.1 deraadt 529: option.
1.2 deraadt 530: .It Cm no-X11-forwarding
1.1 deraadt 531: Forbids X11 forwarding when this key is used for authentication.
532: Any X11 forward requests by the client will return an error.
1.2 deraadt 533: .It Cm no-agent-forwarding
1.1 deraadt 534: Forbids authentication agent forwarding when this key is used for
535: authentication.
1.2 deraadt 536: .It Cm no-pty
1.1 deraadt 537: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 538: .El
539: .Ss Examples
1.1 deraadt 540: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 541: .Pp
1.1 deraadt 542: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 543: .Pp
1.1 deraadt 544: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 545: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 546: The
1.2 deraadt 547: .Pa /etc/ssh_known_hosts
1.1 deraadt 548: and
1.2 deraadt 549: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 550: files contain host public keys for all known hosts. The global file should
551: be prepared by the admistrator (optional), and the per-user file is
552: maintained automatically: whenever the user connects an unknown host
1.6 aaron 553: its key is added to the per-user file.
1.2 deraadt 554: .Pp
1.1 deraadt 555: Each line in these files contains the following fields: hostnames,
556: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 557: .Pp
1.1 deraadt 558: Hostnames is a comma-separated list of patterns ('*' and '?' act as
559: wildcards); each pattern in turn is matched against the canonical host
560: name (when authenticating a client) or against the user-supplied
561: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 562: by
563: .Ql !
564: to indicate negation: if the host name matches a negated
1.1 deraadt 565: pattern, it is not accepted (by that line) even if it matched another
566: pattern on the line.
1.2 deraadt 567: .Pp
1.1 deraadt 568: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 569: can be obtained, e.g., from
570: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 571: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 572: .Pp
573: Lines starting with
574: .Ql #
575: and empty lines are ignored as comments.
576: .Pp
1.1 deraadt 577: When performing host authentication, authentication is accepted if any
578: matching line has the proper key. It is thus permissible (but not
579: recommended) to have several lines or different host keys for the same
580: names. This will inevitably happen when short forms of host names
581: from different domains are put in the file. It is possible
582: that the files contain conflicting information; authentication is
583: accepted if valid information can be found from either file.
1.2 deraadt 584: .Pp
1.1 deraadt 585: Note that the lines in these files are typically hundreds of characters
586: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 587: Rather, generate them by a script
1.1 deraadt 588: or by taking
1.2 deraadt 589: .Pa /etc/ssh_host_key.pub
1.1 deraadt 590: and adding the host names at the front.
1.2 deraadt 591: .Ss Examples
1.1 deraadt 592: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 593: .Sh FILES
594: .Bl -tag -width Ds
595: .It Pa /etc/sshd_config
1.1 deraadt 596: Contains configuration data for
1.2 deraadt 597: .Nm sshd .
1.1 deraadt 598: This file should be writable by root only, but it is recommended
599: (though not necessary) that it be world-readable.
1.2 deraadt 600: .It Pa /etc/ssh_host_key
1.7 markus 601: Contains the private part of the host key.
1.1 deraadt 602: This file should only be owned by root, readable only by root, and not
603: accessible to others.
1.14 markus 604: Note that
605: .Nm
606: does not start if this file is group/world-accessible.
1.2 deraadt 607: .It Pa /etc/ssh_host_key.pub
1.7 markus 608: Contains the public part of the host key.
609: This file should be world-readable but writable only by
1.1 deraadt 610: root. Its contents should match the private part. This file is not
611: really used for anything; it is only provided for the convenience of
612: the user so its contents can be copied to known hosts files.
1.7 markus 613: These two files are created using
614: .Xr ssh-keygen 1 .
1.2 deraadt 615: .It Pa /var/run/sshd.pid
616: Contains the process ID of the
617: .Nm
1.1 deraadt 618: listening for connections (if there are several daemons running
619: concurrently for different ports, this contains the pid of the one
620: started last). The contents of this file are not sensitive; it can be
621: world-readable.
1.2 deraadt 622: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 623: Lists the RSA keys that can be used to log into the user's account.
624: This file must be readable by root (which may on some machines imply
625: it being world-readable if the user's home directory resides on an NFS
626: volume). It is recommended that it not be accessible by others. The
627: format of this file is described above.
1.22 markus 628: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
629: These files are consulted when using rhosts with RSA host
1.1 deraadt 630: authentication to check the public key of the host. The key must be
1.22 markus 631: listed in one of these files to be accepted.
632: The client uses the same files
1.6 aaron 633: to verify that the remote host is the one we intended to
634: connect. These files should be writable only by root/the owner.
1.2 deraadt 635: .Pa /etc/ssh_known_hosts
636: should be world-readable, and
637: .Pa $HOME/.ssh/known_hosts
638: can but need not be world-readable.
1.6 aaron 639: .It Pa /etc/nologin
1.1 deraadt 640: If this file exists,
1.2 deraadt 641: .Nm
1.1 deraadt 642: refuses to let anyone except root log in. The contents of the file
643: are displayed to anyone trying to log in, and non-root connections are
644: refused. The file should be world-readable.
1.19 dugsong 645: .It Pa /etc/hosts.allow, /etc/hosts.deny
646: If compiled with
647: .Sy LIBWRAP
648: support, tcp-wrappers access controls may be defined here as described in
649: .Xr hosts_access 5 .
1.6 aaron 650: .It Pa $HOME/.rhosts
1.1 deraadt 651: This file contains host-username pairs, separated by a space, one per
652: line. The given user on the corresponding host is permitted to log in
653: without password. The same file is used by rlogind and rshd.
1.6 aaron 654: The file must
1.1 deraadt 655: be writable only by the user; it is recommended that it not be
656: accessible by others.
1.2 deraadt 657: .Pp
1.1 deraadt 658: If is also possible to use netgroups in the file. Either host or user
659: name may be of the form +@groupname to specify all hosts or all users
660: in the group.
1.2 deraadt 661: .It Pa $HOME/.shosts
662: For ssh,
663: this file is exactly the same as for
664: .Pa .rhosts .
665: However, this file is
666: not used by rlogin and rshd, so using this permits access using SSH only.
667: .Pa /etc/hosts.equiv
668: This file is used during
669: .Pa .rhosts
670: authentication. In the
1.1 deraadt 671: simplest form, this file contains host names, one per line. Users on
672: those hosts are permitted to log in without a password, provided they
673: have the same user name on both machines. The host name may also be
674: followed by a user name; such users are permitted to log in as
1.2 deraadt 675: .Em any
676: user on this machine (except root). Additionally, the syntax
677: .Dq +@group
678: can be used to specify netgroups. Negated entries start with
679: .Ql \&- .
680: .Pp
1.1 deraadt 681: If the client host/user is successfully matched in this file, login is
682: automatically permitted provided the client and server user names are the
683: same. Additionally, successful RSA host authentication is normally
684: required. This file must be writable only by root; it is recommended
685: that it be world-readable.
1.2 deraadt 686: .Pp
1.6 aaron 687: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 688: .Pa hosts.equiv .
1.1 deraadt 689: Beware that it really means that the named user(s) can log in as
1.2 deraadt 690: .Em anybody ,
1.1 deraadt 691: which includes bin, daemon, adm, and other accounts that own critical
692: binaries and directories. Using a user name practically grants the
693: user root access. The only valid use for user names that I can think
694: of is in negative entries.
1.2 deraadt 695: .Pp
696: Note that this warning also applies to rsh/rlogin.
697: .It Pa /etc/shosts.equiv
1.1 deraadt 698: This is processed exactly as
1.2 deraadt 699: .Pa /etc/hosts.equiv .
1.1 deraadt 700: However, this file may be useful in environments that want to run both
1.2 deraadt 701: rsh/rlogin and ssh.
1.6 aaron 702: .It Pa $HOME/.ssh/environment
1.1 deraadt 703: This file is read into the environment at login (if it exists). It
1.2 deraadt 704: can only contain empty lines, comment lines (that start with
705: .Ql # ) ,
1.6 aaron 706: and assignment lines of the form name=value. The file should be writable
707: only by the user; it need not be readable by anyone else.
1.2 deraadt 708: .It Pa $HOME/.ssh/rc
1.1 deraadt 709: If this file exists, it is run with /bin/sh after reading the
710: environment files but before starting the user's shell or command. If
711: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 712: standard input (and
713: .Ev DISPLAY
714: in environment). This must call
715: .Xr xauth 1
716: in that case.
717: .Pp
1.1 deraadt 718: The primary purpose of this file is to run any initialization routines
719: which may be needed before the user's home directory becomes
720: accessible; AFS is a particular example of such an environment.
1.2 deraadt 721: .Pp
1.1 deraadt 722: This file will probably contain some initialization code followed by
723: something similar to: "if read proto cookie; then echo add $DISPLAY
724: $proto $cookie | xauth -q -; fi".
1.2 deraadt 725: .Pp
726: If this file does not exist,
727: .Pa /etc/sshrc
728: is run, and if that
1.1 deraadt 729: does not exist either, xauth is used to store the cookie.
1.2 deraadt 730: .Pp
1.1 deraadt 731: This file should be writable only by the user, and need not be
732: readable by anyone else.
1.2 deraadt 733: .It Pa /etc/sshrc
734: Like
735: .Pa $HOME/.ssh/rc .
736: This can be used to specify
1.1 deraadt 737: machine-specific login-time initializations globally. This file
738: should be writable only by root, and should be world-readable.
1.2 deraadt 739: .Sh AUTHOR
1.1 deraadt 740: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 741: .Pp
1.1 deraadt 742: Information about new releases, mailing lists, and other related
1.2 deraadt 743: issues can be found from the SSH WWW home page:
744: .Pp
745: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 746: .Pp
1.17 deraadt 747: OpenSSH
748: is a derivative of the original (free) ssh 1.2.12 release, but with bugs
749: removed and newer features re-added. Rapidly after the 1.2.12 release,
750: newer versions bore successively more restrictive licenses. This version
751: of OpenSSH
752: .Bl -bullet
753: .It
754: has all components of a restrictive nature (ie. patents, see
755: .Xr ssl 8 )
756: directly removed from the source code; any licensed or patented components
757: are chosen from
758: external libraries.
759: .It
760: has been updated to support ssh protocol 1.5.
761: .It
762: contains added support for
763: .Xr kerberos 8
764: authentication and ticket passing.
765: .It
766: supports one-time password authentication with
767: .Xr skey 1 .
768: .El
769: .Pp
770: The libraries described in
1.5 deraadt 771: .Xr ssl 8
772: are required for proper operation.
1.2 deraadt 773: .Sh SEE ALSO
774: .Xr rlogin 1 ,
775: .Xr rsh 1 ,
776: .Xr scp 1 ,
777: .Xr ssh 1 ,
1.5 deraadt 778: .Xr ssh-add 1 ,
1.2 deraadt 779: .Xr ssh-agent 1 ,
1.5 deraadt 780: .Xr ssh-keygen 1 ,
781: .Xr ssl 8
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