Annotation of src/usr.bin/ssh/sshd.8, Revision 1.23
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: .\"
1.23 ! markus 12: .\" $Id: sshd.8,v 1.22 1999/11/09 23:09:58 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
247: .Dq no .
248: .It Cm KeepAlive
1.1 deraadt 249: Specifies whether the system should send keepalive messages to the
250: other side. If they are sent, death of the connection or crash of one
251: of the machines will be properly noticed. However, this means that
252: connections will die if the route is down temporarily, and some people
253: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 254: sessions may hang indefinitely on the server, leaving
255: .Dq ghost
256: users and consuming server resources.
257: .Pp
258: The default is
259: .Dq yes
260: (to send keepalives), and the server will notice
1.1 deraadt 261: if the network goes down or the client host reboots. This avoids
262: infinitely hanging sessions.
1.2 deraadt 263: .Pp
264: To disable keepalives, the value should be set to
265: .Dq no
266: in both the server and the client configuration files.
267: .It Cm KerberosAuthentication
1.1 deraadt 268: Specifies whether Kerberos authentication is allowed. This can
1.7 markus 269: be in the form of a Kerberos ticket, or if
270: .Cm PasswordAuthentication
1.1 deraadt 271: is yes, the password provided by the user will be validated through
1.3 dugsong 272: the Kerberos KDC. Default is
273: .Dq yes .
1.2 deraadt 274: .It Cm KerberosOrLocalPasswd
1.1 deraadt 275: If set then if password authentication through Kerberos fails then
276: the password will be validated via any additional local mechanism
1.2 deraadt 277: such as
278: .Pa /etc/passwd
279: or SecurID. Default is
1.20 dugsong 280: .Dq yes .
1.2 deraadt 281: .It Cm KerberosTgtPassing
1.1 deraadt 282: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 283: Default is
284: .Dq no ,
285: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 286: .It Cm KerberosTicketCleanup
1.7 markus 287: Specifies whether to automatically destroy the user's ticket cache
288: file on logout. Default is
1.3 dugsong 289: .Dq yes .
1.2 deraadt 290: .It Cm KeyRegenerationInterval
1.1 deraadt 291: The server key is automatically regenerated after this many seconds
292: (if it has been used). The purpose of regeneration is to prevent
293: decrypting captured sessions by later breaking into the machine and
294: stealing the keys. The key is never stored anywhere. If the value is
295: 0, the key is never regenerated. The default is 3600
296: (seconds).
1.7 markus 297: .It Cm ListenAddress
298: Specifies what local address
299: .Nm
300: should listen on.
301: The default is to listen to all local addresses.
1.2 deraadt 302: .It Cm LoginGraceTime
1.1 deraadt 303: The server disconnects after this time if the user has not
304: successfully logged in. If the value is 0, there is no time limit.
305: The default is 600 (seconds).
1.23 ! markus 306: .It Cm LogLevel
! 307: Gives the verbosity level that is used when logging messages from
! 308: .Nm sshd .
! 309: The possible values are:
! 310: QUIET, FATAL, ERROR, INFO, CHAT and DEBUG.
! 311: The default is INFO.
! 312: Logging with level DEBUG violates the privacy of users
! 313: and is not recommended.
1.2 deraadt 314: .It Cm PasswordAuthentication
1.1 deraadt 315: Specifies whether password authentication is allowed.
1.2 deraadt 316: The default is
317: .Dq yes .
318: .It Cm PermitEmptyPasswords
1.1 deraadt 319: When password authentication is allowed, it specifies whether the
320: server allows login to accounts with empty password strings. The default
1.2 deraadt 321: is
322: .Dq yes .
323: .It Cm PermitRootLogin
1.1 deraadt 324: Specifies whether the root can log in using
1.2 deraadt 325: .Xr ssh 1 .
1.15 markus 326: The argument must be
327: .Dq yes ,
328: .Dq without-password
329: or
330: .Dq no .
1.2 deraadt 331: The default is
332: .Dq yes .
1.15 markus 333: If this options is set to
334: .Dq without-password
335: only password authentication is disabled for root.
1.2 deraadt 336: .Pp
337: Root login with RSA authentication when the
338: .Ar command
339: option has been
1.1 deraadt 340: specified will be allowed regardless of the value of this setting
341: (which may be useful for taking remote backups even if root login is
342: normally not allowed).
1.2 deraadt 343: .It Cm Port
1.1 deraadt 344: Specifies the port number that
1.2 deraadt 345: .Nm
1.1 deraadt 346: listens on. The default is 22.
1.2 deraadt 347: .It Cm PrintMotd
1.1 deraadt 348: Specifies whether
1.2 deraadt 349: .Nm
1.1 deraadt 350: should print
1.2 deraadt 351: .Pa /etc/motd
1.1 deraadt 352: when a user logs in interactively. (On some systems it is also
1.2 deraadt 353: printed by the shell,
354: .Pa /etc/profile ,
355: or equivalent.) The default is
356: .Dq yes .
357: .It Cm RandomSeed
1.4 deraadt 358: Obsolete. Random number generation uses other techniques.
1.2 deraadt 359: .It Cm RhostsAuthentication
1.1 deraadt 360: Specifies whether authentication using rhosts or /etc/hosts.equiv
361: files is sufficient. Normally, this method should not be permitted
1.7 markus 362: because it is insecure.
363: .Cm RhostsRSAAuthentication
364: should be used
1.1 deraadt 365: instead, because it performs RSA-based host authentication in addition
366: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 367: The default is
368: .Dq no .
369: .It Cm RhostsRSAAuthentication
1.1 deraadt 370: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 371: with successful RSA host authentication is allowed. The default is
372: .Dq yes .
373: .It Cm RSAAuthentication
374: Specifies whether pure RSA authentication is allowed. The default is
375: .Dq yes .
376: .It Cm ServerKeyBits
1.1 deraadt 377: Defines the number of bits in the server key. The minimum value is
378: 512, and the default is 768.
1.7 markus 379: .It Cm SkeyAuthentication
380: Specifies whether
381: .Xr skey 1
382: authentication is allowed. The default is
383: .Dq yes .
384: Note that s/key authentication is enabled only if
385: .Cm PasswordAuthentication
386: is allowed, too.
1.2 deraadt 387: .It Cm StrictModes
1.12 markus 388: Specifies whether
389: .Nm
390: should check file modes and ownership of the
391: user's files and home directory before accepting login. This
1.1 deraadt 392: is normally desirable because novices sometimes accidentally leave their
1.7 markus 393: directory or files world-writable. The default is
394: .Dq yes .
1.2 deraadt 395: .It Cm SyslogFacility
1.1 deraadt 396: Gives the facility code that is used when logging messages from
1.2 deraadt 397: .Nm sshd .
1.1 deraadt 398: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1.21 markus 399: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
1.10 markus 400: .It Cm UseLogin
401: Specifies whether
402: .Xr login 1
403: is used. The default is
404: .Dq no .
1.2 deraadt 405: .It Cm X11Forwarding
406: Specifies whether X11 forwarding is permitted. The default is
407: .Dq yes .
1.1 deraadt 408: Note that disabling X11 forwarding does not improve security in any
409: way, as users can always install their own forwarders.
1.6 aaron 410: .It Cm X11DisplayOffset
411: Specifies the first display number available for
412: .Nm sshd Ns 's
413: X11 forwarding. This prevents
414: .Nm
415: from interfering with real X11 servers.
1.2 deraadt 416: .El
417: .Sh LOGIN PROCESS
1.1 deraadt 418: When a user successfully logs in,
1.2 deraadt 419: .Nm
1.1 deraadt 420: does the following:
1.2 deraadt 421: .Bl -enum -offset indent
422: .It
1.1 deraadt 423: If the login is on a tty, and no command has been specified,
424: prints last login time and
1.2 deraadt 425: .Pa /etc/motd
1.1 deraadt 426: (unless prevented in the configuration file or by
1.2 deraadt 427: .Pa $HOME/.hushlogin ;
428: see the
429: .Sx FILES
430: section).
431: .It
1.1 deraadt 432: If the login is on a tty, records login time.
1.2 deraadt 433: .It
434: Checks
435: .Pa /etc/nologin ;
436: if it exists, prints contents and quits
1.1 deraadt 437: (unless root).
1.2 deraadt 438: .It
1.1 deraadt 439: Changes to run with normal user privileges.
1.2 deraadt 440: .It
1.1 deraadt 441: Sets up basic environment.
1.2 deraadt 442: .It
443: Reads
444: .Pa $HOME/.ssh/environment
445: if it exists.
446: .It
1.1 deraadt 447: Changes to user's home directory.
1.2 deraadt 448: .It
449: If
450: .Pa $HOME/.ssh/rc
451: exists, runs it; else if
452: .Pa /etc/sshrc
453: exists, runs
454: it; otherwise runs xauth. The
455: .Dq rc
456: files are given the X11
1.1 deraadt 457: authentication protocol and cookie in standard input.
1.2 deraadt 458: .It
1.1 deraadt 459: Runs user's shell or command.
1.2 deraadt 460: .El
461: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 462: The
1.2 deraadt 463: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 464: file lists the RSA keys that are
465: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 466: key (empty lines and lines starting with a
467: .Ql #
468: are ignored as
1.1 deraadt 469: comments). Each line consists of the following fields, separated by
470: spaces: options, bits, exponent, modulus, comment. The options field
471: is optional; its presence is determined by whether the line starts
472: with a number or not (the option field never starts with a number).
473: The bits, exponent, modulus and comment fields give the RSA key; the
474: comment field is not used for anything (but may be convenient for the
475: user to identify the key).
1.2 deraadt 476: .Pp
1.1 deraadt 477: Note that lines in this file are usually several hundred bytes long
478: (because of the size of the RSA key modulus). You don't want to type
479: them in; instead, copy the
1.2 deraadt 480: .Pa identity.pub
1.1 deraadt 481: file and edit it.
1.2 deraadt 482: .Pp
1.1 deraadt 483: The options (if present) consists of comma-separated option
484: specifications. No spaces are permitted, except within double quotes.
485: The following option specifications are supported:
1.2 deraadt 486: .Bl -tag -width Ds
487: .It Cm from="pattern-list"
1.1 deraadt 488: Specifies that in addition to RSA authentication, the canonical name
489: of the remote host must be present in the comma-separated list of
490: patterns ('*' and '?' serve as wildcards). The list may also contain
491: patterns negated by prefixing them with '!'; if the canonical host
492: name matches a negated pattern, the key is not accepted. The purpose
493: of this option is to optionally increase security: RSA authentication
494: by itself does not trust the network or name servers or anything (but
495: the key); however, if somebody somehow steals the key, the key
496: permits an intruder to log in from anywhere in the world. This
497: additional option makes using a stolen key more difficult (name
498: servers and/or routers would have to be compromised in addition to
499: just the key).
1.2 deraadt 500: .It Cm command="command"
1.1 deraadt 501: Specifies that the command is executed whenever this key is used for
502: authentication. The command supplied by the user (if any) is ignored.
503: The command is run on a pty if the connection requests a pty;
504: otherwise it is run without a tty. A quote may be included in the
505: command by quoting it with a backslash. This option might be useful
506: to restrict certain RSA keys to perform just a specific operation. An
507: example might be a key that permits remote backups but nothing
508: else. Notice that the client may specify TCP/IP and/or X11
509: forwardings unless they are explicitly prohibited.
1.2 deraadt 510: .It Cm environment="NAME=value"
1.1 deraadt 511: Specifies that the string is to be added to the environment when
512: logging in using this key. Environment variables set this way
513: override other default environment values. Multiple options of this
514: type are permitted.
1.2 deraadt 515: .It Cm no-port-forwarding
1.1 deraadt 516: Forbids TCP/IP forwarding when this key is used for authentication.
517: Any port forward requests by the client will return an error. This
1.2 deraadt 518: might be used, e.g., in connection with the
519: .Cm command
1.1 deraadt 520: option.
1.2 deraadt 521: .It Cm no-X11-forwarding
1.1 deraadt 522: Forbids X11 forwarding when this key is used for authentication.
523: Any X11 forward requests by the client will return an error.
1.2 deraadt 524: .It Cm no-agent-forwarding
1.1 deraadt 525: Forbids authentication agent forwarding when this key is used for
526: authentication.
1.2 deraadt 527: .It Cm no-pty
1.1 deraadt 528: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 529: .El
530: .Ss Examples
1.1 deraadt 531: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 532: .Pp
1.1 deraadt 533: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 534: .Pp
1.1 deraadt 535: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 536: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 537: The
1.2 deraadt 538: .Pa /etc/ssh_known_hosts
1.1 deraadt 539: and
1.2 deraadt 540: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 541: files contain host public keys for all known hosts. The global file should
542: be prepared by the admistrator (optional), and the per-user file is
543: maintained automatically: whenever the user connects an unknown host
1.6 aaron 544: its key is added to the per-user file.
1.2 deraadt 545: .Pp
1.1 deraadt 546: Each line in these files contains the following fields: hostnames,
547: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 548: .Pp
1.1 deraadt 549: Hostnames is a comma-separated list of patterns ('*' and '?' act as
550: wildcards); each pattern in turn is matched against the canonical host
551: name (when authenticating a client) or against the user-supplied
552: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 553: by
554: .Ql !
555: to indicate negation: if the host name matches a negated
1.1 deraadt 556: pattern, it is not accepted (by that line) even if it matched another
557: pattern on the line.
1.2 deraadt 558: .Pp
1.1 deraadt 559: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 560: can be obtained, e.g., from
561: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 562: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 563: .Pp
564: Lines starting with
565: .Ql #
566: and empty lines are ignored as comments.
567: .Pp
1.1 deraadt 568: When performing host authentication, authentication is accepted if any
569: matching line has the proper key. It is thus permissible (but not
570: recommended) to have several lines or different host keys for the same
571: names. This will inevitably happen when short forms of host names
572: from different domains are put in the file. It is possible
573: that the files contain conflicting information; authentication is
574: accepted if valid information can be found from either file.
1.2 deraadt 575: .Pp
1.1 deraadt 576: Note that the lines in these files are typically hundreds of characters
577: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 578: Rather, generate them by a script
1.1 deraadt 579: or by taking
1.2 deraadt 580: .Pa /etc/ssh_host_key.pub
1.1 deraadt 581: and adding the host names at the front.
1.2 deraadt 582: .Ss Examples
1.1 deraadt 583: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 584: .Sh FILES
585: .Bl -tag -width Ds
586: .It Pa /etc/sshd_config
1.1 deraadt 587: Contains configuration data for
1.2 deraadt 588: .Nm sshd .
1.1 deraadt 589: This file should be writable by root only, but it is recommended
590: (though not necessary) that it be world-readable.
1.2 deraadt 591: .It Pa /etc/ssh_host_key
1.7 markus 592: Contains the private part of the host key.
1.1 deraadt 593: This file should only be owned by root, readable only by root, and not
594: accessible to others.
1.14 markus 595: Note that
596: .Nm
597: does not start if this file is group/world-accessible.
1.2 deraadt 598: .It Pa /etc/ssh_host_key.pub
1.7 markus 599: Contains the public part of the host key.
600: This file should be world-readable but writable only by
1.1 deraadt 601: root. Its contents should match the private part. This file is not
602: really used for anything; it is only provided for the convenience of
603: the user so its contents can be copied to known hosts files.
1.7 markus 604: These two files are created using
605: .Xr ssh-keygen 1 .
1.2 deraadt 606: .It Pa /var/run/sshd.pid
607: Contains the process ID of the
608: .Nm
1.1 deraadt 609: listening for connections (if there are several daemons running
610: concurrently for different ports, this contains the pid of the one
611: started last). The contents of this file are not sensitive; it can be
612: world-readable.
1.2 deraadt 613: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 614: Lists the RSA keys that can be used to log into the user's account.
615: This file must be readable by root (which may on some machines imply
616: it being world-readable if the user's home directory resides on an NFS
617: volume). It is recommended that it not be accessible by others. The
618: format of this file is described above.
1.22 markus 619: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
620: These files are consulted when using rhosts with RSA host
1.1 deraadt 621: authentication to check the public key of the host. The key must be
1.22 markus 622: listed in one of these files to be accepted.
623: The client uses the same files
1.6 aaron 624: to verify that the remote host is the one we intended to
625: connect. These files should be writable only by root/the owner.
1.2 deraadt 626: .Pa /etc/ssh_known_hosts
627: should be world-readable, and
628: .Pa $HOME/.ssh/known_hosts
629: can but need not be world-readable.
1.6 aaron 630: .It Pa /etc/nologin
1.1 deraadt 631: If this file exists,
1.2 deraadt 632: .Nm
1.1 deraadt 633: refuses to let anyone except root log in. The contents of the file
634: are displayed to anyone trying to log in, and non-root connections are
635: refused. The file should be world-readable.
1.19 dugsong 636: .It Pa /etc/hosts.allow, /etc/hosts.deny
637: If compiled with
638: .Sy LIBWRAP
639: support, tcp-wrappers access controls may be defined here as described in
640: .Xr hosts_access 5 .
1.6 aaron 641: .It Pa $HOME/.rhosts
1.1 deraadt 642: This file contains host-username pairs, separated by a space, one per
643: line. The given user on the corresponding host is permitted to log in
644: without password. The same file is used by rlogind and rshd.
1.6 aaron 645: The file must
1.1 deraadt 646: be writable only by the user; it is recommended that it not be
647: accessible by others.
1.2 deraadt 648: .Pp
1.1 deraadt 649: If is also possible to use netgroups in the file. Either host or user
650: name may be of the form +@groupname to specify all hosts or all users
651: in the group.
1.2 deraadt 652: .It Pa $HOME/.shosts
653: For ssh,
654: this file is exactly the same as for
655: .Pa .rhosts .
656: However, this file is
657: not used by rlogin and rshd, so using this permits access using SSH only.
658: .Pa /etc/hosts.equiv
659: This file is used during
660: .Pa .rhosts
661: authentication. In the
1.1 deraadt 662: simplest form, this file contains host names, one per line. Users on
663: those hosts are permitted to log in without a password, provided they
664: have the same user name on both machines. The host name may also be
665: followed by a user name; such users are permitted to log in as
1.2 deraadt 666: .Em any
667: user on this machine (except root). Additionally, the syntax
668: .Dq +@group
669: can be used to specify netgroups. Negated entries start with
670: .Ql \&- .
671: .Pp
1.1 deraadt 672: If the client host/user is successfully matched in this file, login is
673: automatically permitted provided the client and server user names are the
674: same. Additionally, successful RSA host authentication is normally
675: required. This file must be writable only by root; it is recommended
676: that it be world-readable.
1.2 deraadt 677: .Pp
1.6 aaron 678: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 679: .Pa hosts.equiv .
1.1 deraadt 680: Beware that it really means that the named user(s) can log in as
1.2 deraadt 681: .Em anybody ,
1.1 deraadt 682: which includes bin, daemon, adm, and other accounts that own critical
683: binaries and directories. Using a user name practically grants the
684: user root access. The only valid use for user names that I can think
685: of is in negative entries.
1.2 deraadt 686: .Pp
687: Note that this warning also applies to rsh/rlogin.
688: .It Pa /etc/shosts.equiv
1.1 deraadt 689: This is processed exactly as
1.2 deraadt 690: .Pa /etc/hosts.equiv .
1.1 deraadt 691: However, this file may be useful in environments that want to run both
1.2 deraadt 692: rsh/rlogin and ssh.
1.6 aaron 693: .It Pa $HOME/.ssh/environment
1.1 deraadt 694: This file is read into the environment at login (if it exists). It
1.2 deraadt 695: can only contain empty lines, comment lines (that start with
696: .Ql # ) ,
1.6 aaron 697: and assignment lines of the form name=value. The file should be writable
698: only by the user; it need not be readable by anyone else.
1.2 deraadt 699: .It Pa $HOME/.ssh/rc
1.1 deraadt 700: If this file exists, it is run with /bin/sh after reading the
701: environment files but before starting the user's shell or command. If
702: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 703: standard input (and
704: .Ev DISPLAY
705: in environment). This must call
706: .Xr xauth 1
707: in that case.
708: .Pp
1.1 deraadt 709: The primary purpose of this file is to run any initialization routines
710: which may be needed before the user's home directory becomes
711: accessible; AFS is a particular example of such an environment.
1.2 deraadt 712: .Pp
1.1 deraadt 713: This file will probably contain some initialization code followed by
714: something similar to: "if read proto cookie; then echo add $DISPLAY
715: $proto $cookie | xauth -q -; fi".
1.2 deraadt 716: .Pp
717: If this file does not exist,
718: .Pa /etc/sshrc
719: is run, and if that
1.1 deraadt 720: does not exist either, xauth is used to store the cookie.
1.2 deraadt 721: .Pp
1.1 deraadt 722: This file should be writable only by the user, and need not be
723: readable by anyone else.
1.2 deraadt 724: .It Pa /etc/sshrc
725: Like
726: .Pa $HOME/.ssh/rc .
727: This can be used to specify
1.1 deraadt 728: machine-specific login-time initializations globally. This file
729: should be writable only by root, and should be world-readable.
1.2 deraadt 730: .Sh AUTHOR
1.1 deraadt 731: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 732: .Pp
1.1 deraadt 733: Information about new releases, mailing lists, and other related
1.2 deraadt 734: issues can be found from the SSH WWW home page:
735: .Pp
736: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 737: .Pp
1.17 deraadt 738: OpenSSH
739: is a derivative of the original (free) ssh 1.2.12 release, but with bugs
740: removed and newer features re-added. Rapidly after the 1.2.12 release,
741: newer versions bore successively more restrictive licenses. This version
742: of OpenSSH
743: .Bl -bullet
744: .It
745: has all components of a restrictive nature (ie. patents, see
746: .Xr ssl 8 )
747: directly removed from the source code; any licensed or patented components
748: are chosen from
749: external libraries.
750: .It
751: has been updated to support ssh protocol 1.5.
752: .It
753: contains added support for
754: .Xr kerberos 8
755: authentication and ticket passing.
756: .It
757: supports one-time password authentication with
758: .Xr skey 1 .
759: .El
760: .Pp
761: The libraries described in
1.5 deraadt 762: .Xr ssl 8
763: are required for proper operation.
1.2 deraadt 764: .Sh SEE ALSO
765: .Xr rlogin 1 ,
766: .Xr rsh 1 ,
767: .Xr scp 1 ,
768: .Xr ssh 1 ,
1.5 deraadt 769: .Xr ssh-add 1 ,
1.2 deraadt 770: .Xr ssh-agent 1 ,
1.5 deraadt 771: .Xr ssh-keygen 1 ,
772: .Xr ssl 8
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