Annotation of src/usr.bin/ssh/sshd.8, Revision 1.10
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: .\"
1.10 ! markus 12: .\" $Id: sshd.8,v 1.9 1999/10/11 20:40:08 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.
1.2 deraadt 108: .Sh OPTIONS
109: .Bl -tag -width Ds
110: .It Fl b Ar bits
1.1 deraadt 111: Specifies the number of bits in the server key (default 768).
1.2 deraadt 112: .Pp
113: .It Fl d
1.1 deraadt 114: Debug mode. The server sends verbose debug output to the system
115: log, and does not put itself in the background. The server also will
116: not fork and will only process one connection. This option is only
117: intended for debugging for the server.
1.2 deraadt 118: .It Fl f Ar configuration_file
1.1 deraadt 119: Specifies the name of the configuration file. The default is
1.2 deraadt 120: .Pa /etc/sshd_config .
121: .It Fl g Ar login_grace_time
1.1 deraadt 122: Gives the grace time for clients to authenticate themselves (default
123: 300 seconds). If the client fails to authenticate the user within
124: this many seconds, the server disconnects and exits. A value of zero
125: indicates no limit.
1.2 deraadt 126: .It Fl h Ar host_key_file
1.1 deraadt 127: Specifies the file from which the host key is read (default
1.2 deraadt 128: .Pa /etc/ssh_host_key ) .
1.7 markus 129: This option must be given if
130: .Nm
131: is not run as root (as the normal
1.1 deraadt 132: host file is normally not readable by anyone but root).
1.2 deraadt 133: .It Fl i
1.7 markus 134: Specifies that
135: .Nm
136: is being run from inetd.
137: .Nm
138: is normally not run
1.1 deraadt 139: from inetd because it needs to generate the server key before it can
140: respond to the client, and this may take tens of seconds. Clients
141: would have to wait too long if the key was regenerated every time.
1.7 markus 142: However, with small key sizes (e.g. 512) using
143: .Nm
144: from inetd may
1.1 deraadt 145: be feasible.
1.2 deraadt 146: .It Fl k Ar key_gen_time
1.1 deraadt 147: Specifies how often the server key is regenerated (default 3600
148: seconds, or one hour). The motivation for regenerating the key fairly
149: often is that the key is not stored anywhere, and after about an hour,
150: it becomes impossible to recover the key for decrypting intercepted
151: communications even if the machine is cracked into or physically
152: seized. A value of zero indicates that the key will never be regenerated.
1.2 deraadt 153: .It Fl p Ar port
1.1 deraadt 154: Specifies the port on which the server listens for connections
155: (default 22).
1.2 deraadt 156: .It Fl q
1.1 deraadt 157: Quiet mode. Nothing is sent to the system log. Normally the beginning,
158: authentication, and termination of each connection is logged.
1.2 deraadt 159: .El
160: .Sh CONFIGURATION FILE
161: .Nm
1.1 deraadt 162: reads configuration data from
1.2 deraadt 163: .Pa /etc/sshd_config
164: (or the file specified with
165: .Fl f
166: on the command line). The file
167: contains keyword-value pairs, one per line. Lines starting with
168: .Ql #
1.1 deraadt 169: and empty lines are interpreted as comments.
1.2 deraadt 170: .Pp
1.1 deraadt 171: The following keywords are possible.
1.2 deraadt 172: .Bl -tag -width Ds
173: .It Cm AFSTokenPassing
1.3 dugsong 174: Specifies whether an AFS token may be forwarded to the server. Default is
1.2 deraadt 175: .Dq yes .
1.3 dugsong 176: .Pp
1.2 deraadt 177: .It Cm AllowHosts
1.1 deraadt 178: This keyword can be followed by any number of host name patterns,
179: separated by spaces. If specified, login is allowed only from hosts
1.2 deraadt 180: whose name matches one of the patterns.
181: .Ql \&*
182: and
183: .Ql ?
184: can be used as
1.1 deraadt 185: wildcards in the patterns. Normal name servers are used to map the
186: client's host into a canonical host name. If the name cannot be
187: mapped, its IP-address is used as the host name. By default all hosts
188: are allowed to connect.
1.2 deraadt 189: .Pp
1.1 deraadt 190: Note that
1.2 deraadt 191: .Nm
192: can also be configured to use tcp_wrappers using the
1.7 markus 193: .Sy LIBWARP
1.6 aaron 194: compile-time option.
1.8 markus 195: .It Cm CheckMail
196: Specifies whether
197: .Nm
198: should check for new mail for interactive logins.
199: The default is
200: .Dq no .
1.2 deraadt 201: .It Cm DenyHosts
1.1 deraadt 202: This keyword can be followed by any number of host name patterns,
203: separated by spaces. If specified, login is disallowed from the hosts
204: whose name matches any of the patterns.
1.2 deraadt 205: .It Cm FascistLogging
1.1 deraadt 206: Specifies whether to use verbose logging. Verbose logging violates
207: the privacy of users and is not recommended. The argument must be
1.2 deraadt 208: .Dq yes
209: or
210: .Dq no .
211: The default is
212: .Dq no .
213: .It Cm HostKey
1.1 deraadt 214: Specifies the file containing the private host key (default
1.2 deraadt 215: .Pa /etc/ssh_host_key ) .
1.9 markus 216: Note that
217: .Nm
218: does not start if the private host key file is group/world-accessible.
1.2 deraadt 219: .It Cm IgnoreRhosts
1.1 deraadt 220: Specifies that rhosts and shosts files will not be used in
221: authentication.
1.2 deraadt 222: .Pa /etc/hosts.equiv
1.1 deraadt 223: and
1.2 deraadt 224: .Pa /etc/shosts.equiv
225: are still used. The default is
226: .Dq no .
227: .It Cm KeepAlive
1.1 deraadt 228: Specifies whether the system should send keepalive messages to the
229: other side. If they are sent, death of the connection or crash of one
230: of the machines will be properly noticed. However, this means that
231: connections will die if the route is down temporarily, and some people
232: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 233: sessions may hang indefinitely on the server, leaving
234: .Dq ghost
235: users and consuming server resources.
236: .Pp
237: The default is
238: .Dq yes
239: (to send keepalives), and the server will notice
1.1 deraadt 240: if the network goes down or the client host reboots. This avoids
241: infinitely hanging sessions.
1.2 deraadt 242: .Pp
243: To disable keepalives, the value should be set to
244: .Dq no
245: in both the server and the client configuration files.
246: .It Cm KerberosAuthentication
1.1 deraadt 247: Specifies whether Kerberos authentication is allowed. This can
1.7 markus 248: be in the form of a Kerberos ticket, or if
249: .Cm PasswordAuthentication
1.1 deraadt 250: is yes, the password provided by the user will be validated through
1.3 dugsong 251: the Kerberos KDC. Default is
252: .Dq yes .
1.2 deraadt 253: .It Cm KerberosOrLocalPasswd
1.1 deraadt 254: If set then if password authentication through Kerberos fails then
255: the password will be validated via any additional local mechanism
1.2 deraadt 256: such as
257: .Pa /etc/passwd
258: or SecurID. Default is
259: .Dq no .
260: .It Cm KerberosTgtPassing
1.1 deraadt 261: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 262: Default is
263: .Dq no ,
264: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 265: .It Cm KerberosTicketCleanup
1.7 markus 266: Specifies whether to automatically destroy the user's ticket cache
267: file on logout. Default is
1.3 dugsong 268: .Dq yes .
1.2 deraadt 269: .It Cm KeyRegenerationInterval
1.1 deraadt 270: The server key is automatically regenerated after this many seconds
271: (if it has been used). The purpose of regeneration is to prevent
272: decrypting captured sessions by later breaking into the machine and
273: stealing the keys. The key is never stored anywhere. If the value is
274: 0, the key is never regenerated. The default is 3600
275: (seconds).
1.7 markus 276: .It Cm ListenAddress
277: Specifies what local address
278: .Nm
279: should listen on.
280: The default is to listen to all local addresses.
1.2 deraadt 281: .It Cm LoginGraceTime
1.1 deraadt 282: The server disconnects after this time if the user has not
283: successfully logged in. If the value is 0, there is no time limit.
284: The default is 600 (seconds).
1.2 deraadt 285: .It Cm PasswordAuthentication
1.1 deraadt 286: Specifies whether password authentication is allowed.
1.2 deraadt 287: The default is
288: .Dq yes .
289: .It Cm PermitEmptyPasswords
1.1 deraadt 290: When password authentication is allowed, it specifies whether the
291: server allows login to accounts with empty password strings. The default
1.2 deraadt 292: is
293: .Dq yes .
294: .It Cm PermitRootLogin
1.1 deraadt 295: Specifies whether the root can log in using
1.2 deraadt 296: .Xr ssh 1 .
297: The default is
298: .Dq yes .
299: .Pp
300: Root login with RSA authentication when the
301: .Ar command
302: option has been
1.1 deraadt 303: specified will be allowed regardless of the value of this setting
304: (which may be useful for taking remote backups even if root login is
305: normally not allowed).
1.2 deraadt 306: .It Cm Port
1.1 deraadt 307: Specifies the port number that
1.2 deraadt 308: .Nm
1.1 deraadt 309: listens on. The default is 22.
1.2 deraadt 310: .It Cm PrintMotd
1.1 deraadt 311: Specifies whether
1.2 deraadt 312: .Nm
1.1 deraadt 313: should print
1.2 deraadt 314: .Pa /etc/motd
1.1 deraadt 315: when a user logs in interactively. (On some systems it is also
1.2 deraadt 316: printed by the shell,
317: .Pa /etc/profile ,
318: or equivalent.) The default is
319: .Dq yes .
320: .It Cm QuietMode
1.1 deraadt 321: Specifies whether the system runs in quiet mode. In quiet mode,
322: nothing is logged in the system log, except fatal errors. The default
1.2 deraadt 323: is
324: .Dq no .
325: .It Cm RandomSeed
1.4 deraadt 326: Obsolete. Random number generation uses other techniques.
1.2 deraadt 327: .It Cm RhostsAuthentication
1.1 deraadt 328: Specifies whether authentication using rhosts or /etc/hosts.equiv
329: files is sufficient. Normally, this method should not be permitted
1.7 markus 330: because it is insecure.
331: .Cm RhostsRSAAuthentication
332: should be used
1.1 deraadt 333: instead, because it performs RSA-based host authentication in addition
334: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 335: The default is
336: .Dq no .
337: .It Cm RhostsRSAAuthentication
1.1 deraadt 338: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 339: with successful RSA host authentication is allowed. The default is
340: .Dq yes .
341: .It Cm RSAAuthentication
342: Specifies whether pure RSA authentication is allowed. The default is
343: .Dq yes .
344: .It Cm ServerKeyBits
1.1 deraadt 345: Defines the number of bits in the server key. The minimum value is
346: 512, and the default is 768.
1.7 markus 347: .It Cm SkeyAuthentication
348: Specifies whether
349: .Xr skey 1
350: authentication is allowed. The default is
351: .Dq yes .
352: Note that s/key authentication is enabled only if
353: .Cm PasswordAuthentication
354: is allowed, too.
1.2 deraadt 355: .It Cm StrictModes
356: Specifies whether SSH should check file modes and ownership of the
1.1 deraadt 357: user's home directory and rhosts files before accepting login. This
358: is normally desirable because novices sometimes accidentally leave their
1.7 markus 359: directory or files world-writable. The default is
360: .Dq yes .
1.2 deraadt 361: .It Cm SyslogFacility
1.1 deraadt 362: Gives the facility code that is used when logging messages from
1.2 deraadt 363: .Nm sshd .
1.1 deraadt 364: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
365: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.
1.10 ! markus 366: .It Cm UseLogin
! 367: Specifies whether
! 368: .Xr login 1
! 369: is used. The default is
! 370: .Dq no .
1.2 deraadt 371: .It Cm X11Forwarding
372: Specifies whether X11 forwarding is permitted. The default is
373: .Dq yes .
1.1 deraadt 374: Note that disabling X11 forwarding does not improve security in any
375: way, as users can always install their own forwarders.
1.6 aaron 376: .It Cm X11DisplayOffset
377: Specifies the first display number available for
378: .Nm sshd Ns 's
379: X11 forwarding. This prevents
380: .Nm
381: from interfering with real X11 servers.
1.2 deraadt 382: .El
383: .Sh LOGIN PROCESS
1.1 deraadt 384: When a user successfully logs in,
1.2 deraadt 385: .Nm
1.1 deraadt 386: does the following:
1.2 deraadt 387: .Bl -enum -offset indent
388: .It
1.1 deraadt 389: If the login is on a tty, and no command has been specified,
390: prints last login time and
1.2 deraadt 391: .Pa /etc/motd
1.1 deraadt 392: (unless prevented in the configuration file or by
1.2 deraadt 393: .Pa $HOME/.hushlogin ;
394: see the
395: .Sx FILES
396: section).
397: .It
1.1 deraadt 398: If the login is on a tty, records login time.
1.2 deraadt 399: .It
400: Checks
401: .Pa /etc/nologin ;
402: if it exists, prints contents and quits
1.1 deraadt 403: (unless root).
1.2 deraadt 404: .It
1.1 deraadt 405: Changes to run with normal user privileges.
1.2 deraadt 406: .It
1.1 deraadt 407: Sets up basic environment.
1.2 deraadt 408: .It
409: Reads
410: .Pa $HOME/.ssh/environment
411: if it exists.
412: .It
1.1 deraadt 413: Changes to user's home directory.
1.2 deraadt 414: .It
415: If
416: .Pa $HOME/.ssh/rc
417: exists, runs it; else if
418: .Pa /etc/sshrc
419: exists, runs
420: it; otherwise runs xauth. The
421: .Dq rc
422: files are given the X11
1.1 deraadt 423: authentication protocol and cookie in standard input.
1.2 deraadt 424: .It
1.1 deraadt 425: Runs user's shell or command.
1.2 deraadt 426: .El
427: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 428: The
1.2 deraadt 429: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 430: file lists the RSA keys that are
431: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 432: key (empty lines and lines starting with a
433: .Ql #
434: are ignored as
1.1 deraadt 435: comments). Each line consists of the following fields, separated by
436: spaces: options, bits, exponent, modulus, comment. The options field
437: is optional; its presence is determined by whether the line starts
438: with a number or not (the option field never starts with a number).
439: The bits, exponent, modulus and comment fields give the RSA key; the
440: comment field is not used for anything (but may be convenient for the
441: user to identify the key).
1.2 deraadt 442: .Pp
1.1 deraadt 443: Note that lines in this file are usually several hundred bytes long
444: (because of the size of the RSA key modulus). You don't want to type
445: them in; instead, copy the
1.2 deraadt 446: .Pa identity.pub
1.1 deraadt 447: file and edit it.
1.2 deraadt 448: .Pp
1.1 deraadt 449: The options (if present) consists of comma-separated option
450: specifications. No spaces are permitted, except within double quotes.
451: The following option specifications are supported:
1.2 deraadt 452: .Bl -tag -width Ds
453: .It Cm from="pattern-list"
1.1 deraadt 454: Specifies that in addition to RSA authentication, the canonical name
455: of the remote host must be present in the comma-separated list of
456: patterns ('*' and '?' serve as wildcards). The list may also contain
457: patterns negated by prefixing them with '!'; if the canonical host
458: name matches a negated pattern, the key is not accepted. The purpose
459: of this option is to optionally increase security: RSA authentication
460: by itself does not trust the network or name servers or anything (but
461: the key); however, if somebody somehow steals the key, the key
462: permits an intruder to log in from anywhere in the world. This
463: additional option makes using a stolen key more difficult (name
464: servers and/or routers would have to be compromised in addition to
465: just the key).
1.2 deraadt 466: .It Cm command="command"
1.1 deraadt 467: Specifies that the command is executed whenever this key is used for
468: authentication. The command supplied by the user (if any) is ignored.
469: The command is run on a pty if the connection requests a pty;
470: otherwise it is run without a tty. A quote may be included in the
471: command by quoting it with a backslash. This option might be useful
472: to restrict certain RSA keys to perform just a specific operation. An
473: example might be a key that permits remote backups but nothing
474: else. Notice that the client may specify TCP/IP and/or X11
475: forwardings unless they are explicitly prohibited.
1.2 deraadt 476: .It Cm environment="NAME=value"
1.1 deraadt 477: Specifies that the string is to be added to the environment when
478: logging in using this key. Environment variables set this way
479: override other default environment values. Multiple options of this
480: type are permitted.
1.2 deraadt 481: .It Cm no-port-forwarding
1.1 deraadt 482: Forbids TCP/IP forwarding when this key is used for authentication.
483: Any port forward requests by the client will return an error. This
1.2 deraadt 484: might be used, e.g., in connection with the
485: .Cm command
1.1 deraadt 486: option.
1.2 deraadt 487: .It Cm no-X11-forwarding
1.1 deraadt 488: Forbids X11 forwarding when this key is used for authentication.
489: Any X11 forward requests by the client will return an error.
1.2 deraadt 490: .It Cm no-agent-forwarding
1.1 deraadt 491: Forbids authentication agent forwarding when this key is used for
492: authentication.
1.2 deraadt 493: .It Cm no-pty
1.1 deraadt 494: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 495: .El
496: .Ss Examples
1.1 deraadt 497: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 498: .Pp
1.1 deraadt 499: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 500: .Pp
1.1 deraadt 501: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 502: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 503: The
1.2 deraadt 504: .Pa /etc/ssh_known_hosts
1.1 deraadt 505: and
1.2 deraadt 506: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 507: files contain host public keys for all known hosts. The global file should
508: be prepared by the admistrator (optional), and the per-user file is
509: maintained automatically: whenever the user connects an unknown host
1.6 aaron 510: its key is added to the per-user file.
1.2 deraadt 511: .Pp
1.1 deraadt 512: Each line in these files contains the following fields: hostnames,
513: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 514: .Pp
1.1 deraadt 515: Hostnames is a comma-separated list of patterns ('*' and '?' act as
516: wildcards); each pattern in turn is matched against the canonical host
517: name (when authenticating a client) or against the user-supplied
518: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 519: by
520: .Ql !
521: to indicate negation: if the host name matches a negated
1.1 deraadt 522: pattern, it is not accepted (by that line) even if it matched another
523: pattern on the line.
1.2 deraadt 524: .Pp
1.1 deraadt 525: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 526: can be obtained, e.g., from
527: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 528: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 529: .Pp
530: Lines starting with
531: .Ql #
532: and empty lines are ignored as comments.
533: .Pp
1.1 deraadt 534: When performing host authentication, authentication is accepted if any
535: matching line has the proper key. It is thus permissible (but not
536: recommended) to have several lines or different host keys for the same
537: names. This will inevitably happen when short forms of host names
538: from different domains are put in the file. It is possible
539: that the files contain conflicting information; authentication is
540: accepted if valid information can be found from either file.
1.2 deraadt 541: .Pp
1.1 deraadt 542: Note that the lines in these files are typically hundreds of characters
543: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 544: Rather, generate them by a script
1.1 deraadt 545: or by taking
1.2 deraadt 546: .Pa /etc/ssh_host_key.pub
1.1 deraadt 547: and adding the host names at the front.
1.2 deraadt 548: .Ss Examples
1.1 deraadt 549: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 550: .Sh FILES
551: .Bl -tag -width Ds
552: .It Pa /etc/sshd_config
1.1 deraadt 553: Contains configuration data for
1.2 deraadt 554: .Nm sshd .
1.1 deraadt 555: This file should be writable by root only, but it is recommended
556: (though not necessary) that it be world-readable.
1.2 deraadt 557: .It Pa /etc/ssh_host_key
1.7 markus 558: Contains the private part of the host key.
1.1 deraadt 559: This file should only be owned by root, readable only by root, and not
560: accessible to others.
1.2 deraadt 561: .It Pa /etc/ssh_host_key.pub
1.7 markus 562: Contains the public part of the host key.
563: This file should be world-readable but writable only by
1.1 deraadt 564: root. Its contents should match the private part. This file is not
565: really used for anything; it is only provided for the convenience of
566: the user so its contents can be copied to known hosts files.
1.7 markus 567: These two files are created using
568: .Xr ssh-keygen 1 .
1.2 deraadt 569: .It Pa /var/run/sshd.pid
570: Contains the process ID of the
571: .Nm
1.1 deraadt 572: listening for connections (if there are several daemons running
573: concurrently for different ports, this contains the pid of the one
574: started last). The contents of this file are not sensitive; it can be
575: world-readable.
1.2 deraadt 576: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 577: Lists the RSA keys that can be used to log into the user's account.
578: This file must be readable by root (which may on some machines imply
579: it being world-readable if the user's home directory resides on an NFS
580: volume). It is recommended that it not be accessible by others. The
581: format of this file is described above.
1.6 aaron 582: .It Pa /etc/ssh_known_hosts
583: This file is consulted when using rhosts with RSA host
1.1 deraadt 584: authentication to check the public key of the host. The key must be
1.6 aaron 585: listed in this file to be accepted.
586: .It Pa $HOME/.ssh/known_hosts
587: The client uses this file
588: and
589: .Pa /etc/ssh_known_hosts
590: to verify that the remote host is the one we intended to
591: connect. These files should be writable only by root/the owner.
1.2 deraadt 592: .Pa /etc/ssh_known_hosts
593: should be world-readable, and
594: .Pa $HOME/.ssh/known_hosts
595: can but need not be world-readable.
1.6 aaron 596: .It Pa /etc/nologin
1.1 deraadt 597: If this file exists,
1.2 deraadt 598: .Nm
1.1 deraadt 599: refuses to let anyone except root log in. The contents of the file
600: are displayed to anyone trying to log in, and non-root connections are
601: refused. The file should be world-readable.
1.6 aaron 602: .It Pa $HOME/.rhosts
1.1 deraadt 603: This file contains host-username pairs, separated by a space, one per
604: line. The given user on the corresponding host is permitted to log in
605: without password. The same file is used by rlogind and rshd.
1.6 aaron 606: The file must
1.1 deraadt 607: be writable only by the user; it is recommended that it not be
608: accessible by others.
1.2 deraadt 609: .Pp
1.1 deraadt 610: If is also possible to use netgroups in the file. Either host or user
611: name may be of the form +@groupname to specify all hosts or all users
612: in the group.
1.2 deraadt 613: .It Pa $HOME/.shosts
614: For ssh,
615: this file is exactly the same as for
616: .Pa .rhosts .
617: However, this file is
618: not used by rlogin and rshd, so using this permits access using SSH only.
619: .Pa /etc/hosts.equiv
620: This file is used during
621: .Pa .rhosts
622: authentication. In the
1.1 deraadt 623: simplest form, this file contains host names, one per line. Users on
624: those hosts are permitted to log in without a password, provided they
625: have the same user name on both machines. The host name may also be
626: followed by a user name; such users are permitted to log in as
1.2 deraadt 627: .Em any
628: user on this machine (except root). Additionally, the syntax
629: .Dq +@group
630: can be used to specify netgroups. Negated entries start with
631: .Ql \&- .
632: .Pp
1.1 deraadt 633: If the client host/user is successfully matched in this file, login is
634: automatically permitted provided the client and server user names are the
635: same. Additionally, successful RSA host authentication is normally
636: required. This file must be writable only by root; it is recommended
637: that it be world-readable.
1.2 deraadt 638: .Pp
1.6 aaron 639: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 640: .Pa hosts.equiv .
1.1 deraadt 641: Beware that it really means that the named user(s) can log in as
1.2 deraadt 642: .Em anybody ,
1.1 deraadt 643: which includes bin, daemon, adm, and other accounts that own critical
644: binaries and directories. Using a user name practically grants the
645: user root access. The only valid use for user names that I can think
646: of is in negative entries.
1.2 deraadt 647: .Pp
648: Note that this warning also applies to rsh/rlogin.
649: .It Pa /etc/shosts.equiv
1.1 deraadt 650: This is processed exactly as
1.2 deraadt 651: .Pa /etc/hosts.equiv .
1.1 deraadt 652: However, this file may be useful in environments that want to run both
1.2 deraadt 653: rsh/rlogin and ssh.
1.6 aaron 654: .It Pa $HOME/.ssh/environment
1.1 deraadt 655: This file is read into the environment at login (if it exists). It
1.2 deraadt 656: can only contain empty lines, comment lines (that start with
657: .Ql # ) ,
1.6 aaron 658: and assignment lines of the form name=value. The file should be writable
659: only by the user; it need not be readable by anyone else.
1.2 deraadt 660: .It Pa $HOME/.ssh/rc
1.1 deraadt 661: If this file exists, it is run with /bin/sh after reading the
662: environment files but before starting the user's shell or command. If
663: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 664: standard input (and
665: .Ev DISPLAY
666: in environment). This must call
667: .Xr xauth 1
668: in that case.
669: .Pp
1.1 deraadt 670: The primary purpose of this file is to run any initialization routines
671: which may be needed before the user's home directory becomes
672: accessible; AFS is a particular example of such an environment.
1.2 deraadt 673: .Pp
1.1 deraadt 674: This file will probably contain some initialization code followed by
675: something similar to: "if read proto cookie; then echo add $DISPLAY
676: $proto $cookie | xauth -q -; fi".
1.2 deraadt 677: .Pp
678: If this file does not exist,
679: .Pa /etc/sshrc
680: is run, and if that
1.1 deraadt 681: does not exist either, xauth is used to store the cookie.
1.2 deraadt 682: .Pp
1.1 deraadt 683: This file should be writable only by the user, and need not be
684: readable by anyone else.
1.2 deraadt 685: .It Pa /etc/sshrc
686: Like
687: .Pa $HOME/.ssh/rc .
688: This can be used to specify
1.1 deraadt 689: machine-specific login-time initializations globally. This file
690: should be writable only by root, and should be world-readable.
1.2 deraadt 691: .Sh AUTHOR
1.1 deraadt 692: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 693: .Pp
1.1 deraadt 694: Information about new releases, mailing lists, and other related
1.2 deraadt 695: issues can be found from the SSH WWW home page:
696: .Pp
697: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 698: .Pp
699: This version of
700: .Nm
701: is a derivative of the original 1.2.12 release, but with bugs removed and
702: newer features re-added. Rapidly after the 1.2.12 release, newer versions
703: bore successively more restrictive licenses. In this version, all components
704: of a restrictive nature (ie. patents) have been directly removed from the
705: source code; any licensed or patented components are chosen from external
706: libraries. The libraries described in
707: .Xr ssl 8
708: are required for proper operation.
1.2 deraadt 709: .Sh SEE ALSO
710: .Xr rlogin 1 ,
711: .Xr rsh 1 ,
712: .Xr scp 1 ,
713: .Xr ssh 1 ,
1.5 deraadt 714: .Xr ssh-add 1 ,
1.2 deraadt 715: .Xr ssh-agent 1 ,
1.5 deraadt 716: .Xr ssh-keygen 1 ,
717: .Xr ssl 8
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