Annotation of src/usr.bin/ssh/sshd.8, Revision 1.5
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: .\"
1.5 ! deraadt 12: .\" $Id: sshd.8,v 1.4 1999/10/01 22:23:48 deraadt Exp $
1.1 deraadt 13: .\"
1.2 deraadt 14: .Dd September 25, 1999
15: .Dt SSHD 8
16: .Os
17: .Sh NAME
18: .Nm sshd
19: .Nd secure shell daemon
20: .Sh SYNOPSIS
21: .Nm sshd
22: .Op Fl diq
23: .Op Fl b Ar bits
24: .Op Fl f Ar config_file
25: .Op Fl g Ar login_grace_time
26: .Op Fl h Ar host_key_file
27: .Op Fl k Ar key_gen_time
28: .Op Fl p Ar port
29: .Sh DESCRIPTION
30: .Nm
1.1 deraadt 31: (Secure Shell Daemon) is the daemon program for
1.2 deraadt 32: .Xr ssh 1 .
1.1 deraadt 33: Together these programs replace rlogin and rsh programs, and
34: provide secure encrypted communications between two untrusted hosts
35: over an insecure network. The programs are intended to be as easy to
36: install and use as possible.
1.2 deraadt 37: .Pp
38: .Nm
1.1 deraadt 39: is the daemon that listens for connections from clients. It is
40: normally started at boot from
1.2 deraadt 41: .Pa /etc/rc .
42: It forks a new
1.1 deraadt 43: daemon for each incoming connection. The forked daemons handle
44: key exchange, encryption, authentication, command execution,
45: and data exchange.
1.2 deraadt 46: .Pp
47: .Nm
48: works as follows. Each host has a host-specific RSA key
1.1 deraadt 49: (normally 1024 bits) used to identify the host. Additionally, when
50: the daemon starts, it generates a server RSA key (normally 768 bits).
51: This key is normally regenerated every hour if it has been used, and
52: is never stored on disk.
1.2 deraadt 53: .Pp
1.1 deraadt 54: Whenever a client connects the daemon, the daemon sends its host
55: and server public keys to the client. The client compares the
56: host key against its own database to verify that it has not changed.
57: The client then generates a 256 bit random number. It encrypts this
58: random number using both the host key and the server key, and sends
59: the encrypted number to the server. Both sides then start to use this
60: random number as a session key which is used to encrypt all further
61: communications in the session. The rest of the session is encrypted
1.5 ! deraadt 62: using a conventional cipher, currently Blowfish and 3DES, with 3DES
! 63: being is used by default. The client selects the encryption algorithm
! 64: to use from those offered by the server.
1.2 deraadt 65: .Pp
1.1 deraadt 66: Next, the server and the client enter an authentication dialog. The
1.2 deraadt 67: client tries to authenticate itself using
68: .Pa .rhosts
69: authentication,
70: .Pa .rhosts
71: authentication combined with RSA host
1.1 deraadt 72: authentication, RSA challenge-response authentication, or password
73: based authentication.
1.2 deraadt 74: .Pp
1.1 deraadt 75: Rhosts authentication is normally disabled
76: because it is fundamentally insecure, but can be enabled in the server
77: configuration file if desired. System security is not improved unless
1.2 deraadt 78: .Xr rshd 8 ,
79: .Xr rlogind 8 ,
80: .Xr rexecd 8 ,
81: and
82: .Xr rexd 8
1.1 deraadt 83: are disabled (thus completely disabling
1.2 deraadt 84: .Xr rlogin 1
1.1 deraadt 85: and
1.2 deraadt 86: .Xr rsh 1
1.1 deraadt 87: into that machine).
1.2 deraadt 88: .Pp
1.1 deraadt 89: If the client successfully authenticates itself, a dialog for
90: preparing the session is entered. At this time the client may request
91: things like allocating a pseudo-tty, forwarding X11 connections,
92: forwarding TCP/IP connections, or forwarding the authentication agent
93: connection over the secure channel.
1.2 deraadt 94: .Pp
1.1 deraadt 95: Finally, the client either requests a shell or execution of a command.
96: The sides then enter session mode. In this mode, either side may send
97: data at any time, and such data is forwarded to/from the shell or
98: command on the server side, and the user terminal in the client side.
1.2 deraadt 99: .Pp
1.1 deraadt 100: When the user program terminates and all forwarded X11 and other
101: connections have been closed, the server sends command exit status to
102: the client, and both sides exit.
1.2 deraadt 103: .Pp
104: .Nm
1.1 deraadt 105: can be configured using command-line options or a configuration
106: file. Command-line options override values specified in the
107: configuration file.
1.2 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 ) .
1.1 deraadt 129: This option must be given if sshd is not run as root (as the normal
130: host file is normally not readable by anyone but root).
1.2 deraadt 131: .It Fl i
1.1 deraadt 132: Specifies that sshd is being run from inetd. Sshd is normally not run
133: from inetd because it needs to generate the server key before it can
134: respond to the client, and this may take tens of seconds. Clients
135: would have to wait too long if the key was regenerated every time.
136: However, with small key sizes (e.g. 512) using sshd from inetd may
137: be feasible.
1.2 deraadt 138: .It Fl k Ar key_gen_time
1.1 deraadt 139: Specifies how often the server key is regenerated (default 3600
140: seconds, or one hour). The motivation for regenerating the key fairly
141: often is that the key is not stored anywhere, and after about an hour,
142: it becomes impossible to recover the key for decrypting intercepted
143: communications even if the machine is cracked into or physically
144: seized. A value of zero indicates that the key will never be regenerated.
1.2 deraadt 145: .It Fl p Ar port
1.1 deraadt 146: Specifies the port on which the server listens for connections
147: (default 22).
1.2 deraadt 148: .It Fl q
1.1 deraadt 149: Quiet mode. Nothing is sent to the system log. Normally the beginning,
150: authentication, and termination of each connection is logged.
1.2 deraadt 151: .El
152: .Sh CONFIGURATION FILE
153: .Nm
1.1 deraadt 154: reads configuration data from
1.2 deraadt 155: .Pa /etc/sshd_config
156: (or the file specified with
157: .Fl f
158: on the command line). The file
159: contains keyword-value pairs, one per line. Lines starting with
160: .Ql #
1.1 deraadt 161: and empty lines are interpreted as comments.
1.2 deraadt 162: .Pp
1.1 deraadt 163: The following keywords are possible.
1.2 deraadt 164: .Bl -tag -width Ds
165: .It Cm AFSTokenPassing
1.3 dugsong 166: Specifies whether an AFS token may be forwarded to the server. Default is
1.2 deraadt 167: .Dq yes .
1.3 dugsong 168: .Pp
1.2 deraadt 169: .It Cm AllowHosts
1.1 deraadt 170: This keyword can be followed by any number of host name patterns,
171: separated by spaces. If specified, login is allowed only from hosts
1.2 deraadt 172: whose name matches one of the patterns.
173: .Ql \&*
174: and
175: .Ql ?
176: can be used as
1.1 deraadt 177: wildcards in the patterns. Normal name servers are used to map the
178: client's host into a canonical host name. If the name cannot be
179: mapped, its IP-address is used as the host name. By default all hosts
180: are allowed to connect.
1.2 deraadt 181: .Pp
1.1 deraadt 182: Note that
1.2 deraadt 183: .Nm
184: can also be configured to use tcp_wrappers using the
185: .Fl -with-libwrap
1.1 deraadt 186: compile-time configuration option.
1.2 deraadt 187: .It Cm DenyHosts
1.1 deraadt 188: This keyword can be followed by any number of host name patterns,
189: separated by spaces. If specified, login is disallowed from the hosts
190: whose name matches any of the patterns.
1.2 deraadt 191: .It Cm FascistLogging
1.1 deraadt 192: Specifies whether to use verbose logging. Verbose logging violates
193: the privacy of users and is not recommended. The argument must be
1.2 deraadt 194: .Dq yes
195: or
196: .Dq no .
197: The default is
198: .Dq no .
199: .It Cm HostKey
1.1 deraadt 200: Specifies the file containing the private host key (default
1.2 deraadt 201: .Pa /etc/ssh_host_key ) .
202: .It Cm IgnoreRhosts
1.1 deraadt 203: Specifies that rhosts and shosts files will not be used in
204: authentication.
1.2 deraadt 205: .Pa /etc/hosts.equiv
1.1 deraadt 206: and
1.2 deraadt 207: .Pa /etc/shosts.equiv
208: are still used. The default is
209: .Dq no .
210: .It Cm KeepAlive
1.1 deraadt 211: Specifies whether the system should send keepalive messages to the
212: other side. If they are sent, death of the connection or crash of one
213: of the machines will be properly noticed. However, this means that
214: connections will die if the route is down temporarily, and some people
215: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 216: sessions may hang indefinitely on the server, leaving
217: .Dq ghost
218: users and consuming server resources.
219: .Pp
220: The default is
221: .Dq yes
222: (to send keepalives), and the server will notice
1.1 deraadt 223: if the network goes down or the client host reboots. This avoids
224: infinitely hanging sessions.
1.2 deraadt 225: .Pp
226: To disable keepalives, the value should be set to
227: .Dq no
228: in both the server and the client configuration files.
229: .It Cm KerberosAuthentication
1.1 deraadt 230: Specifies whether Kerberos authentication is allowed. This can
231: be in the form of a Kerberos ticket, or if PasswordAuthentication
232: is yes, the password provided by the user will be validated through
1.3 dugsong 233: the Kerberos KDC. Default is
234: .Dq yes .
1.2 deraadt 235: .It Cm KerberosOrLocalPasswd
1.1 deraadt 236: If set then if password authentication through Kerberos fails then
237: the password will be validated via any additional local mechanism
1.2 deraadt 238: such as
239: .Pa /etc/passwd
240: or SecurID. Default is
241: .Dq no .
242: .It Cm KerberosTgtPassing
1.1 deraadt 243: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 244: Default is
245: .Dq no ,
246: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 247: .It Cm KerberosTicketCleanup
1.1 deraadt 248: Specifies whether to automatically destroy the user's
1.3 dugsong 249: ticket cache file on logout. Default is
250: .Dq yes .
1.2 deraadt 251: .It Cm KeyRegenerationInterval
1.1 deraadt 252: The server key is automatically regenerated after this many seconds
253: (if it has been used). The purpose of regeneration is to prevent
254: decrypting captured sessions by later breaking into the machine and
255: stealing the keys. The key is never stored anywhere. If the value is
256: 0, the key is never regenerated. The default is 3600
257: (seconds).
1.2 deraadt 258: .It Cm LoginGraceTime
1.1 deraadt 259: The server disconnects after this time if the user has not
260: successfully logged in. If the value is 0, there is no time limit.
261: The default is 600 (seconds).
1.2 deraadt 262: .It Cm PasswordAuthentication
1.1 deraadt 263: Specifies whether password authentication is allowed.
1.2 deraadt 264: The default is
265: .Dq yes .
266: .It Cm PermitEmptyPasswords
1.1 deraadt 267: When password authentication is allowed, it specifies whether the
268: server allows login to accounts with empty password strings. The default
1.2 deraadt 269: is
270: .Dq yes .
271: .It Cm PermitRootLogin
1.1 deraadt 272: Specifies whether the root can log in using
1.2 deraadt 273: .Xr ssh 1 .
274: The default is
275: .Dq yes .
276: .Pp
277: Root login with RSA authentication when the
278: .Ar command
279: option has been
1.1 deraadt 280: specified will be allowed regardless of the value of this setting
281: (which may be useful for taking remote backups even if root login is
282: normally not allowed).
1.2 deraadt 283: .It Cm Port
1.1 deraadt 284: Specifies the port number that
1.2 deraadt 285: .Nm
1.1 deraadt 286: listens on. The default is 22.
1.2 deraadt 287: .It Cm PrintMotd
1.1 deraadt 288: Specifies whether
1.2 deraadt 289: .Nm
1.1 deraadt 290: should print
1.2 deraadt 291: .Pa /etc/motd
1.1 deraadt 292: when a user logs in interactively. (On some systems it is also
1.2 deraadt 293: printed by the shell,
294: .Pa /etc/profile ,
295: or equivalent.) The default is
296: .Dq yes .
297: .It Cm QuietMode
1.1 deraadt 298: Specifies whether the system runs in quiet mode. In quiet mode,
299: nothing is logged in the system log, except fatal errors. The default
1.2 deraadt 300: is
301: .Dq no .
302: .It Cm RandomSeed
1.4 deraadt 303: Obsolete. Random number generation uses other techniques.
1.2 deraadt 304: .It Cm RhostsAuthentication
1.1 deraadt 305: Specifies whether authentication using rhosts or /etc/hosts.equiv
306: files is sufficient. Normally, this method should not be permitted
307: because it is insecure. RhostsRSAAuthentication should be used
308: instead, because it performs RSA-based host authentication in addition
309: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 310: The default is
311: .Dq no .
312: .It Cm RhostsRSAAuthentication
1.1 deraadt 313: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 314: with successful RSA host authentication is allowed. The default is
315: .Dq yes .
316: .It Cm RSAAuthentication
317: Specifies whether pure RSA authentication is allowed. The default is
318: .Dq yes .
319: .It Cm ServerKeyBits
1.1 deraadt 320: Defines the number of bits in the server key. The minimum value is
321: 512, and the default is 768.
1.2 deraadt 322: .It Cm StrictModes
323: Specifies whether SSH should check file modes and ownership of the
1.1 deraadt 324: user's home directory and rhosts files before accepting login. This
325: is normally desirable because novices sometimes accidentally leave their
326: directory or files world-writable. The default is "yes".
1.2 deraadt 327: .It Cm SyslogFacility
1.1 deraadt 328: Gives the facility code that is used when logging messages from
1.2 deraadt 329: .Nm sshd .
1.1 deraadt 330: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
331: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.
1.2 deraadt 332: .It Cm X11Forwarding
333: Specifies whether X11 forwarding is permitted. The default is
334: .Dq yes .
1.1 deraadt 335: Note that disabling X11 forwarding does not improve security in any
336: way, as users can always install their own forwarders.
1.2 deraadt 337: .El
338: .Sh LOGIN PROCESS
1.1 deraadt 339: When a user successfully logs in,
1.2 deraadt 340: .Nm
1.1 deraadt 341: does the following:
1.2 deraadt 342: .Bl -enum -offset indent
343: .It
1.1 deraadt 344: If the login is on a tty, and no command has been specified,
345: prints last login time and
1.2 deraadt 346: .Pa /etc/motd
1.1 deraadt 347: (unless prevented in the configuration file or by
1.2 deraadt 348: .Pa $HOME/.hushlogin ;
349: see the
350: .Sx FILES
351: section).
352: .It
1.1 deraadt 353: If the login is on a tty, records login time.
1.2 deraadt 354: .It
355: Checks
356: .Pa /etc/nologin ;
357: if it exists, prints contents and quits
1.1 deraadt 358: (unless root).
1.2 deraadt 359: .It
1.1 deraadt 360: Changes to run with normal user privileges.
1.2 deraadt 361: .It
1.1 deraadt 362: Sets up basic environment.
1.2 deraadt 363: .It
364: Reads
365: .Pa /etc/environment
366: if it exists.
367: .It
368: Reads
369: .Pa $HOME/.ssh/environment
370: if it exists.
371: .It
1.1 deraadt 372: Changes to user's home directory.
1.2 deraadt 373: .It
374: If
375: .Pa $HOME/.ssh/rc
376: exists, runs it; else if
377: .Pa /etc/sshrc
378: exists, runs
379: it; otherwise runs xauth. The
380: .Dq rc
381: files are given the X11
1.1 deraadt 382: authentication protocol and cookie in standard input.
1.2 deraadt 383: .It
1.1 deraadt 384: Runs user's shell or command.
1.2 deraadt 385: .El
386: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 387: The
1.2 deraadt 388: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 389: file lists the RSA keys that are
390: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 391: key (empty lines and lines starting with a
392: .Ql #
393: are ignored as
1.1 deraadt 394: comments). Each line consists of the following fields, separated by
395: spaces: options, bits, exponent, modulus, comment. The options field
396: is optional; its presence is determined by whether the line starts
397: with a number or not (the option field never starts with a number).
398: The bits, exponent, modulus and comment fields give the RSA key; the
399: comment field is not used for anything (but may be convenient for the
400: user to identify the key).
1.2 deraadt 401: .Pp
1.1 deraadt 402: Note that lines in this file are usually several hundred bytes long
403: (because of the size of the RSA key modulus). You don't want to type
404: them in; instead, copy the
1.2 deraadt 405: .Pa identity.pub
1.1 deraadt 406: file and edit it.
1.2 deraadt 407: .Pp
1.1 deraadt 408: The options (if present) consists of comma-separated option
409: specifications. No spaces are permitted, except within double quotes.
410: The following option specifications are supported:
1.2 deraadt 411: .Bl -tag -width Ds
412: .It Cm from="pattern-list"
1.1 deraadt 413: Specifies that in addition to RSA authentication, the canonical name
414: of the remote host must be present in the comma-separated list of
415: patterns ('*' and '?' serve as wildcards). The list may also contain
416: patterns negated by prefixing them with '!'; if the canonical host
417: name matches a negated pattern, the key is not accepted. The purpose
418: of this option is to optionally increase security: RSA authentication
419: by itself does not trust the network or name servers or anything (but
420: the key); however, if somebody somehow steals the key, the key
421: permits an intruder to log in from anywhere in the world. This
422: additional option makes using a stolen key more difficult (name
423: servers and/or routers would have to be compromised in addition to
424: just the key).
1.2 deraadt 425: .It Cm command="command"
1.1 deraadt 426: Specifies that the command is executed whenever this key is used for
427: authentication. The command supplied by the user (if any) is ignored.
428: The command is run on a pty if the connection requests a pty;
429: otherwise it is run without a tty. A quote may be included in the
430: command by quoting it with a backslash. This option might be useful
431: to restrict certain RSA keys to perform just a specific operation. An
432: example might be a key that permits remote backups but nothing
433: else. Notice that the client may specify TCP/IP and/or X11
434: forwardings unless they are explicitly prohibited.
1.2 deraadt 435: .It Cm environment="NAME=value"
1.1 deraadt 436: Specifies that the string is to be added to the environment when
437: logging in using this key. Environment variables set this way
438: override other default environment values. Multiple options of this
439: type are permitted.
1.2 deraadt 440: .It Cm no-port-forwarding
1.1 deraadt 441: Forbids TCP/IP forwarding when this key is used for authentication.
442: Any port forward requests by the client will return an error. This
1.2 deraadt 443: might be used, e.g., in connection with the
444: .Cm command
1.1 deraadt 445: option.
1.2 deraadt 446: .It Cm no-X11-forwarding
1.1 deraadt 447: Forbids X11 forwarding when this key is used for authentication.
448: Any X11 forward requests by the client will return an error.
1.2 deraadt 449: .It Cm no-agent-forwarding
1.1 deraadt 450: Forbids authentication agent forwarding when this key is used for
451: authentication.
1.2 deraadt 452: .It Cm no-pty
1.1 deraadt 453: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 454: .El
455: .Ss Examples
1.1 deraadt 456: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 457: .Pp
1.1 deraadt 458: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 459: .Pp
1.1 deraadt 460: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 461: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 462: The
1.2 deraadt 463: .Pa /etc/ssh_known_hosts
1.1 deraadt 464: and
1.2 deraadt 465: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 466: files contain host public keys for all known hosts. The global file should
467: be prepared by the admistrator (optional), and the per-user file is
468: maintained automatically: whenever the user connects an unknown host
469: its key is added to the per-user file. The recommended way to create
1.2 deraadt 470: .Pa /etc/ssh_known_hosts
1.1 deraadt 471: is to use the
1.2 deraadt 472: .Xr make-ssh-known-hosts 1
1.1 deraadt 473: command.
1.2 deraadt 474: .Pp
1.1 deraadt 475: Each line in these files contains the following fields: hostnames,
476: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 477: .Pp
1.1 deraadt 478: Hostnames is a comma-separated list of patterns ('*' and '?' act as
479: wildcards); each pattern in turn is matched against the canonical host
480: name (when authenticating a client) or against the user-supplied
481: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 482: by
483: .Ql !
484: to indicate negation: if the host name matches a negated
1.1 deraadt 485: pattern, it is not accepted (by that line) even if it matched another
486: pattern on the line.
1.2 deraadt 487: .Pp
1.1 deraadt 488: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 489: can be obtained, e.g., from
490: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 491: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 492: .Pp
493: Lines starting with
494: .Ql #
495: and empty lines are ignored as comments.
496: .Pp
1.1 deraadt 497: When performing host authentication, authentication is accepted if any
498: matching line has the proper key. It is thus permissible (but not
499: recommended) to have several lines or different host keys for the same
500: names. This will inevitably happen when short forms of host names
501: from different domains are put in the file. It is possible
502: that the files contain conflicting information; authentication is
503: accepted if valid information can be found from either file.
1.2 deraadt 504: .Pp
1.1 deraadt 505: Note that the lines in these files are typically hundreds of characters
506: long, and you definitely don't want to type in the host keys by hand.
507: Rather, generate them by a script (see
1.2 deraadt 508: .Xr make-ssh-known-hosts 1 )
1.1 deraadt 509: or by taking
1.2 deraadt 510: .Pa /etc/ssh_host_key.pub
1.1 deraadt 511: and adding the host names at the front.
1.2 deraadt 512: .Ss Examples
1.1 deraadt 513: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 514: .Sh FILES
515: .Bl -tag -width Ds
516: .It Pa /etc/sshd_config
1.1 deraadt 517: Contains configuration data for
1.2 deraadt 518: .Nm sshd .
1.1 deraadt 519: This file should be writable by root only, but it is recommended
520: (though not necessary) that it be world-readable.
1.2 deraadt 521: .It Pa /etc/ssh_host_key
1.1 deraadt 522: Contains the private part of the host key. This file is normally
523: created automatically by "make install", but can also be created
524: manually using
1.2 deraadt 525: .Xr ssh-keygen 1 .
1.1 deraadt 526: This file should only be owned by root, readable only by root, and not
527: accessible to others.
1.2 deraadt 528: .It Pa /etc/ssh_host_key.pub
1.1 deraadt 529: Contains the public part of the host key. This file is normally
530: created automatically by "make install", but can also be created
531: manually. This file should be world-readable but writable only by
532: root. Its contents should match the private part. This file is not
533: really used for anything; it is only provided for the convenience of
534: the user so its contents can be copied to known hosts files.
1.2 deraadt 535: .It Pa /var/run/sshd.pid
536: Contains the process ID of the
537: .Nm
1.1 deraadt 538: listening for connections (if there are several daemons running
539: concurrently for different ports, this contains the pid of the one
540: started last). The contents of this file are not sensitive; it can be
541: world-readable.
1.2 deraadt 542: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 543: Lists the RSA keys that can be used to log into the user's account.
544: This file must be readable by root (which may on some machines imply
545: it being world-readable if the user's home directory resides on an NFS
546: volume). It is recommended that it not be accessible by others. The
547: format of this file is described above.
1.2 deraadt 548: .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
1.1 deraadt 549: These files are consulted when using rhosts with RSA host
550: authentication to check the public key of the host. The key must be
551: listed in one of these files to be accepted. (The client uses the
552: same files to verify that the remote host is the one we intended to
553: connect.) These files should be writable only by root/the owner.
1.2 deraadt 554: .Pa /etc/ssh_known_hosts
555: should be world-readable, and
556: .Pa $HOME/.ssh/known_hosts
557: can but need not be world-readable.
558: .Pa /etc/nologin
1.1 deraadt 559: If this file exists,
1.2 deraadt 560: .Nm
1.1 deraadt 561: refuses to let anyone except root log in. The contents of the file
562: are displayed to anyone trying to log in, and non-root connections are
563: refused. The file should be world-readable.
1.2 deraadt 564: .Pa $HOME/.rhosts
1.1 deraadt 565: This file contains host-username pairs, separated by a space, one per
566: line. The given user on the corresponding host is permitted to log in
567: without password. The same file is used by rlogind and rshd.
1.2 deraadt 568: Ssh differs from rlogind
1.1 deraadt 569: and rshd in that it requires RSA host authentication in addition to
570: validating the host name retrieved from domain name servers (unless
1.2 deraadt 571: compiled with the
572: .Fl -with-rhosts
573: configuration option). The file must
1.1 deraadt 574: be writable only by the user; it is recommended that it not be
575: accessible by others.
1.2 deraadt 576: .Pp
1.1 deraadt 577: If is also possible to use netgroups in the file. Either host or user
578: name may be of the form +@groupname to specify all hosts or all users
579: in the group.
1.2 deraadt 580: .It Pa $HOME/.shosts
581: For ssh,
582: this file is exactly the same as for
583: .Pa .rhosts .
584: However, this file is
585: not used by rlogin and rshd, so using this permits access using SSH only.
586: .Pa /etc/hosts.equiv
587: This file is used during
588: .Pa .rhosts
589: authentication. In the
1.1 deraadt 590: simplest form, this file contains host names, one per line. Users on
591: those hosts are permitted to log in without a password, provided they
592: have the same user name on both machines. The host name may also be
593: followed by a user name; such users are permitted to log in as
1.2 deraadt 594: .Em any
595: user on this machine (except root). Additionally, the syntax
596: .Dq +@group
597: can be used to specify netgroups. Negated entries start with
598: .Ql \&- .
599: .Pp
1.1 deraadt 600: If the client host/user is successfully matched in this file, login is
601: automatically permitted provided the client and server user names are the
602: same. Additionally, successful RSA host authentication is normally
603: required. This file must be writable only by root; it is recommended
604: that it be world-readable.
1.2 deraadt 605: .Pp
606: .Sy Warning: It is almost never a good idea to use user names in
607: .Pa hosts.equiv .
1.1 deraadt 608: Beware that it really means that the named user(s) can log in as
1.2 deraadt 609: .Em anybody ,
1.1 deraadt 610: which includes bin, daemon, adm, and other accounts that own critical
611: binaries and directories. Using a user name practically grants the
612: user root access. The only valid use for user names that I can think
613: of is in negative entries.
1.2 deraadt 614: .Pp
615: Note that this warning also applies to rsh/rlogin.
616: .It Pa /etc/shosts.equiv
1.1 deraadt 617: This is processed exactly as
1.2 deraadt 618: .Pa /etc/hosts.equiv .
1.1 deraadt 619: However, this file may be useful in environments that want to run both
1.2 deraadt 620: rsh/rlogin and ssh.
621: .It Pa /etc/environment
1.1 deraadt 622: This file is read into the environment at login (if it exists). It
1.2 deraadt 623: can only contain empty lines, comment lines (that start with
624: .Ql # ) ,
625: and assignment lines of the form name=value. This file is processed in
1.1 deraadt 626: all environments (normal rsh/rlogin only process it on AIX and
627: potentially some other systems). The file should be writable only by
628: root, and should be world-readable.
1.2 deraadt 629: .It Pa $HOME/.ssh/environment
1.1 deraadt 630: This file is read into the environment after /etc/environment. It has
631: the same format. The file should be writable only by the user; it
632: need not be readable by anyone else.
1.2 deraadt 633: .It Pa $HOME/.ssh/rc
1.1 deraadt 634: If this file exists, it is run with /bin/sh after reading the
635: environment files but before starting the user's shell or command. If
636: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 637: standard input (and
638: .Ev DISPLAY
639: in environment). This must call
640: .Xr xauth 1
641: in that case.
642: .Pp
1.1 deraadt 643: The primary purpose of this file is to run any initialization routines
644: which may be needed before the user's home directory becomes
645: accessible; AFS is a particular example of such an environment.
1.2 deraadt 646: .Pp
1.1 deraadt 647: This file will probably contain some initialization code followed by
648: something similar to: "if read proto cookie; then echo add $DISPLAY
649: $proto $cookie | xauth -q -; fi".
1.2 deraadt 650: .Pp
651: If this file does not exist,
652: .Pa /etc/sshrc
653: is run, and if that
1.1 deraadt 654: does not exist either, xauth is used to store the cookie.
1.2 deraadt 655: .Pp
1.1 deraadt 656: This file should be writable only by the user, and need not be
657: readable by anyone else.
1.2 deraadt 658: .It Pa /etc/sshrc
659: Like
660: .Pa $HOME/.ssh/rc .
661: This can be used to specify
1.1 deraadt 662: machine-specific login-time initializations globally. This file
663: should be writable only by root, and should be world-readable.
1.2 deraadt 664: .Sh AUTHOR
1.1 deraadt 665: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 666: .Pp
1.1 deraadt 667: Information about new releases, mailing lists, and other related
1.2 deraadt 668: issues can be found from the SSH WWW home page:
669: .Pp
670: .Dl http://www.cs.hut.fi/ssh.
1.5 ! deraadt 671: .Pp
! 672: This version of
! 673: .Nm
! 674: is a derivative of the original 1.2.12 release, but with bugs removed and
! 675: newer features re-added. Rapidly after the 1.2.12 release, newer versions
! 676: bore successively more restrictive licenses. In this version, all components
! 677: of a restrictive nature (ie. patents) have been directly removed from the
! 678: source code; any licensed or patented components are chosen from external
! 679: libraries. The libraries described in
! 680: .Xr ssl 8
! 681: are required for proper operation.
1.2 deraadt 682: .Sh SEE ALSO
683: .Xr rlogin 1 ,
684: .Xr rsh 1 ,
685: .Xr scp 1 ,
686: .Xr ssh 1 ,
1.5 ! deraadt 687: .Xr ssh-add 1 ,
1.2 deraadt 688: .Xr ssh-agent 1 ,
1.5 ! deraadt 689: .Xr ssh-keygen 1 ,
! 690: .Xr ssl 8
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