5 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
7 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
8 .\" All rights reserved
10 .\" Created: Sat Apr 22 21:55:14 1995 ylo
14 .Dd September 25, 1999
19 .Nd secure shell daemon
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
31 (Secure Shell Daemon) is the daemon program for
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.
39 is the daemon that listens for connections from clients. It is
40 normally started at boot from
43 daemon for each incoming connection. The forked daemons handle
44 key exchange, encryption, authentication, command execution,
48 works as follows. Each host has a host-specific RSA key
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.
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
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.
66 Next, the server and the client enter an authentication dialog. The
67 client tries to authenticate itself using
71 authentication combined with RSA host
72 authentication, RSA challenge-response authentication, or password
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
83 are disabled (thus completely disabling
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.
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.
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.
105 can be configured using command-line options or a configuration
106 file. Command-line options override values specified in the
109 The options are as follows:
112 Specifies the number of bits in the server key (default 768).
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.
119 .It Fl f Ar configuration_file
120 Specifies the name of the configuration file. The default is
121 .Pa /etc/openssh/sshd_config .
123 refuses to start if there is no configuration file.
124 .It Fl g Ar login_grace_time
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
129 .It Fl h Ar host_key_file
130 Specifies the file from which the host key is read (default
131 .Pa /etc/openssh/ssh_host_key ) .
132 This option must be given if
134 is not run as root (as the normal
135 host file is normally not readable by anyone but root).
139 is being run from inetd.
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.
145 However, with small key sizes (e.g. 512) using
149 .It Fl k Ar key_gen_time
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.
157 Specifies the port on which the server listens for connections
160 Quiet mode. Nothing is sent to the system log. Normally the beginning,
161 authentication, and termination of each connection is logged.
163 Do not print an error message if RSA support is missing.
165 .Sh CONFIGURATION FILE
167 reads configuration data from
168 .Pa /etc/openssh/sshd_config
169 (or the file specified with
171 on the command line). The file
172 contains keyword-value pairs, one per line. Lines starting with
174 and empty lines are interpreted as comments.
176 The following keywords are possible.
178 .It Cm AFSTokenPassing
179 Specifies whether an AFS token may be forwarded to the server. Default is
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.
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
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.
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
208 should check for new mail for interactive logins.
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.
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
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
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
235 .It Cm FascistLogging
236 Specifies whether to use verbose logging. Verbose logging violates
237 the privacy of users and is not recommended. The argument must be
244 Specifies the file containing the private host key (default
245 .Pa /etc/openssh/ssh_host_key ) .
248 does not start if this file is group/world-accessible.
250 Specifies that rhosts and shosts files will not be used in
254 .Pa /etc/openssh/shosts.equiv
255 are still used. The default is
258 Specifies whether the system should send keepalive messages to the
259 other side. If they are sent, death of the connection or crash of one
260 of the machines will be properly noticed. However, this means that
261 connections will die if the route is down temporarily, and some people
262 find it annoying. On the other hand, if keepalives are not send,
263 sessions may hang indefinitely on the server, leaving
265 users and consuming server resources.
269 (to send keepalives), and the server will notice
270 if the network goes down or the client host reboots. This avoids
271 infinitely hanging sessions.
273 To disable keepalives, the value should be set to
275 in both the server and the client configuration files.
276 .It Cm KerberosAuthentication
277 Specifies whether Kerberos authentication is allowed. This can
278 be in the form of a Kerberos ticket, or if
279 .Cm PasswordAuthentication
280 is yes, the password provided by the user will be validated through
281 the Kerberos KDC. Default is
283 .It Cm KerberosOrLocalPasswd
284 If set then if password authentication through Kerberos fails then
285 the password will be validated via any additional local mechanism
288 or SecurID. Default is
290 .It Cm KerberosTgtPassing
291 Specifies whether a Kerberos TGT may be forwarded to the server.
294 as this only works when the Kerberos KDC is actually an AFS kaserver.
295 .It Cm KerberosTicketCleanup
296 Specifies whether to automatically destroy the user's ticket cache
297 file on logout. Default is
299 .It Cm KeyRegenerationInterval
300 The server key is automatically regenerated after this many seconds
301 (if it has been used). The purpose of regeneration is to prevent
302 decrypting captured sessions by later breaking into the machine and
303 stealing the keys. The key is never stored anywhere. If the value is
304 0, the key is never regenerated. The default is 3600
307 Specifies what local address
310 The default is to listen to all local addresses.
311 .It Cm LoginGraceTime
312 The server disconnects after this time if the user has not
313 successfully logged in. If the value is 0, there is no time limit.
314 The default is 600 (seconds).
315 .It Cm PasswordAuthentication
316 Specifies whether password authentication is allowed.
319 .It Cm PermitEmptyPasswords
320 When password authentication is allowed, it specifies whether the
321 server allows login to accounts with empty password strings. The default
324 .It Cm PermitRootLogin
325 Specifies whether the root can log in using
334 If this options is set to
336 only password authentication is disabled for root.
338 Root login with RSA authentication when the
341 specified will be allowed regardless of the value of this setting
342 (which may be useful for taking remote backups even if root login is
343 normally not allowed).
345 Specifies the port number that
347 listens on. The default is 22.
353 when a user logs in interactively. (On some systems it is also
354 printed by the shell,
356 or equivalent.) The default is
359 Specifies whether the system runs in quiet mode. In quiet mode,
360 nothing is logged in the system log, except fatal errors. The default
364 Obsolete. Random number generation uses other techniques.
365 .It Cm RhostsAuthentication
366 Specifies whether authentication using rhosts or /etc/hosts.equiv
367 files is sufficient. Normally, this method should not be permitted
368 because it is insecure.
369 .Cm RhostsRSAAuthentication
371 instead, because it performs RSA-based host authentication in addition
372 to normal rhosts or /etc/hosts.equiv authentication.
375 .It Cm RhostsRSAAuthentication
376 Specifies whether rhosts or /etc/hosts.equiv authentication together
377 with successful RSA host authentication is allowed. The default is
379 .It Cm RSAAuthentication
380 Specifies whether pure RSA authentication is allowed. The default is
383 Defines the number of bits in the server key. The minimum value is
384 512, and the default is 768.
385 .It Cm SkeyAuthentication
388 authentication is allowed. The default is
390 Note that s/key authentication is enabled only if
391 .Cm PasswordAuthentication
396 should check file modes and ownership of the
397 user's files and home directory before accepting login. This
398 is normally desirable because novices sometimes accidentally leave their
399 directory or files world-writable. The default is
401 .It Cm SyslogFacility
402 Gives the facility code that is used when logging messages from
404 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
405 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
409 is used. The default is
412 Specifies whether X11 forwarding is permitted. The default is
414 Note that disabling X11 forwarding does not improve security in any
415 way, as users can always install their own forwarders.
416 .It Cm X11DisplayOffset
417 Specifies the first display number available for
419 X11 forwarding. This prevents
421 from interfering with real X11 servers.
424 When a user successfully logs in,
427 .Bl -enum -offset indent
429 If the login is on a tty, and no command has been specified,
430 prints last login time and
432 (unless prevented in the configuration file or by
433 .Pa $HOME/.hushlogin ;
438 If the login is on a tty, records login time.
442 if it exists, prints contents and quits
445 Changes to run with normal user privileges.
447 Sets up basic environment.
450 .Pa $HOME/.ssh/environment
453 Changes to user's home directory.
457 exists, runs it; else if
458 .Pa /etc/openssh/sshrc
460 it; otherwise runs xauth. The
462 files are given the X11
463 authentication protocol and cookie in standard input.
465 Runs user's shell or command.
467 .Sh AUTHORIZED_KEYS FILE FORMAT
469 .Pa $HOME/.ssh/authorized_keys
470 file lists the RSA keys that are
471 permitted for RSA authentication. Each line of the file contains one
472 key (empty lines and lines starting with a
475 comments). Each line consists of the following fields, separated by
476 spaces: options, bits, exponent, modulus, comment. The options field
477 is optional; its presence is determined by whether the line starts
478 with a number or not (the option field never starts with a number).
479 The bits, exponent, modulus and comment fields give the RSA key; the
480 comment field is not used for anything (but may be convenient for the
481 user to identify the key).
483 Note that lines in this file are usually several hundred bytes long
484 (because of the size of the RSA key modulus). You don't want to type
485 them in; instead, copy the
489 The options (if present) consists of comma-separated option
490 specifications. No spaces are permitted, except within double quotes.
491 The following option specifications are supported:
493 .It Cm from="pattern-list"
494 Specifies that in addition to RSA authentication, the canonical name
495 of the remote host must be present in the comma-separated list of
496 patterns ('*' and '?' serve as wildcards). The list may also contain
497 patterns negated by prefixing them with '!'; if the canonical host
498 name matches a negated pattern, the key is not accepted. The purpose
499 of this option is to optionally increase security: RSA authentication
500 by itself does not trust the network or name servers or anything (but
501 the key); however, if somebody somehow steals the key, the key
502 permits an intruder to log in from anywhere in the world. This
503 additional option makes using a stolen key more difficult (name
504 servers and/or routers would have to be compromised in addition to
506 .It Cm command="command"
507 Specifies that the command is executed whenever this key is used for
508 authentication. The command supplied by the user (if any) is ignored.
509 The command is run on a pty if the connection requests a pty;
510 otherwise it is run without a tty. A quote may be included in the
511 command by quoting it with a backslash. This option might be useful
512 to restrict certain RSA keys to perform just a specific operation. An
513 example might be a key that permits remote backups but nothing
514 else. Notice that the client may specify TCP/IP and/or X11
515 forwardings unless they are explicitly prohibited.
516 .It Cm environment="NAME=value"
517 Specifies that the string is to be added to the environment when
518 logging in using this key. Environment variables set this way
519 override other default environment values. Multiple options of this
521 .It Cm no-port-forwarding
522 Forbids TCP/IP forwarding when this key is used for authentication.
523 Any port forward requests by the client will return an error. This
524 might be used, e.g., in connection with the
527 .It Cm no-X11-forwarding
528 Forbids X11 forwarding when this key is used for authentication.
529 Any X11 forward requests by the client will return an error.
530 .It Cm no-agent-forwarding
531 Forbids authentication agent forwarding when this key is used for
534 Prevents tty allocation (a request to allocate a pty will fail).
537 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
539 from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
541 command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
542 .Sh SSH_KNOWN_HOSTS FILE FORMAT
544 .Pa /etc/openssh/ssh_known_hosts
546 .Pa $HOME/.ssh/known_hosts
547 files contain host public keys for all known hosts. The global file should
548 be prepared by the admistrator (optional), and the per-user file is
549 maintained automatically: whenever the user connects an unknown host
550 its key is added to the per-user file.
552 Each line in these files contains the following fields: hostnames,
553 bits, exponent, modulus, comment. The fields are separated by spaces.
555 Hostnames is a comma-separated list of patterns ('*' and '?' act as
556 wildcards); each pattern in turn is matched against the canonical host
557 name (when authenticating a client) or against the user-supplied
558 name (when authenticating a server). A pattern may also be preceded
561 to indicate negation: if the host name matches a negated
562 pattern, it is not accepted (by that line) even if it matched another
565 Bits, exponent, and modulus are taken directly from the host key; they
566 can be obtained, e.g., from
567 .Pa /etc/openssh/ssh_host_key.pub .
568 The optional comment field continues to the end of the line, and is not used.
572 and empty lines are ignored as comments.
574 When performing host authentication, authentication is accepted if any
575 matching line has the proper key. It is thus permissible (but not
576 recommended) to have several lines or different host keys for the same
577 names. This will inevitably happen when short forms of host names
578 from different domains are put in the file. It is possible
579 that the files contain conflicting information; authentication is
580 accepted if valid information can be found from either file.
582 Note that the lines in these files are typically hundreds of characters
583 long, and you definitely don't want to type in the host keys by hand.
584 Rather, generate them by a script
586 .Pa /etc/openssh/ssh_host_key.pub
587 and adding the host names at the front.
589 closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
592 .It Pa /etc/openssh/sshd_config
593 Contains configuration data for
595 This file should be writable by root only, but it is recommended
596 (though not necessary) that it be world-readable.
597 .It Pa /etc/openssh/ssh_host_key
598 Contains the private part of the host key.
599 This file should only be owned by root, readable only by root, and not
600 accessible to others.
603 does not start if this file is group/world-accessible.
604 .It Pa /etc/openssh/ssh_host_key.pub
605 Contains the public part of the host key.
606 This file should be world-readable but writable only by
607 root. Its contents should match the private part. This file is not
608 really used for anything; it is only provided for the convenience of
609 the user so its contents can be copied to known hosts files.
610 These two files are created using
612 .It Pa /var/run/sshd.pid
613 Contains the process ID of the
615 listening for connections (if there are several daemons running
616 concurrently for different ports, this contains the pid of the one
617 started last). The contents of this file are not sensitive; it can be
619 .It Pa $HOME/.ssh/authorized_keys
620 Lists the RSA keys that can be used to log into the user's account.
621 This file must be readable by root (which may on some machines imply
622 it being world-readable if the user's home directory resides on an NFS
623 volume). It is recommended that it not be accessible by others. The
624 format of this file is described above.
625 .It Pa /etc/openssh/ssh_known_hosts
626 This file is consulted when using rhosts with RSA host
627 authentication to check the public key of the host. The key must be
628 listed in this file to be accepted.
629 .It Pa $HOME/.ssh/known_hosts
630 The client uses this file
632 .Pa /etc/openssh/ssh_known_hosts
633 to verify that the remote host is the one we intended to
634 connect. These files should be writable only by root/the owner.
635 .Pa /etc/openssh/ssh_known_hosts
636 should be world-readable, and
637 .Pa $HOME/.ssh/known_hosts
638 can but need not be world-readable.
642 refuses to let anyone except root log in. The contents of the file
643 are displayed to anyone trying to log in, and non-root connections are
644 refused. The file should be world-readable.
645 .It Pa /etc/hosts.allow, /etc/hosts.deny
648 support, tcp-wrappers access controls may be defined here as described in
651 This file contains host-username pairs, separated by a space, one per
652 line. The given user on the corresponding host is permitted to log in
653 without password. The same file is used by rlogind and rshd.
655 be writable only by the user; it is recommended that it not be
656 accessible by others.
658 If is also possible to use netgroups in the file. Either host or user
659 name may be of the form +@groupname to specify all hosts or all users
663 this file is exactly the same as for
665 However, this file is
666 not used by rlogin and rshd, so using this permits access using SSH only.
668 This file is used during
670 authentication. In the
671 simplest form, this file contains host names, one per line. Users on
672 those hosts are permitted to log in without a password, provided they
673 have the same user name on both machines. The host name may also be
674 followed by a user name; such users are permitted to log in as
676 user on this machine (except root). Additionally, the syntax
678 can be used to specify netgroups. Negated entries start with
681 If the client host/user is successfully matched in this file, login is
682 automatically permitted provided the client and server user names are the
683 same. Additionally, successful RSA host authentication is normally
684 required. This file must be writable only by root; it is recommended
685 that it be world-readable.
687 .Sy "Warning: It is almost never a good idea to use user names in"
689 Beware that it really means that the named user(s) can log in as
691 which includes bin, daemon, adm, and other accounts that own critical
692 binaries and directories. Using a user name practically grants the
693 user root access. The only valid use for user names that I can think
694 of is in negative entries.
696 Note that this warning also applies to rsh/rlogin.
697 .It Pa /etc/openssh/shosts.equiv
698 This is processed exactly as
699 .Pa /etc/hosts.equiv .
700 However, this file may be useful in environments that want to run both
702 .It Pa $HOME/.ssh/environment
703 This file is read into the environment at login (if it exists). It
704 can only contain empty lines, comment lines (that start with
706 and assignment lines of the form name=value. The file should be writable
707 only by the user; it need not be readable by anyone else.
709 If this file exists, it is run with /bin/sh after reading the
710 environment files but before starting the user's shell or command. If
711 X11 spoofing is in use, this will receive the "proto cookie" pair in
714 in environment). This must call
718 The primary purpose of this file is to run any initialization routines
719 which may be needed before the user's home directory becomes
720 accessible; AFS is a particular example of such an environment.
722 This file will probably contain some initialization code followed by
723 something similar to: "if read proto cookie; then echo add $DISPLAY
724 $proto $cookie | xauth -q -; fi".
726 If this file does not exist,
727 .Pa /etc/openssh/sshrc
729 does not exist either, xauth is used to store the cookie.
731 This file should be writable only by the user, and need not be
732 readable by anyone else.
733 .It Pa /etc/openssh/sshrc
736 This can be used to specify
737 machine-specific login-time initializations globally. This file
738 should be writable only by root, and should be world-readable.
740 Tatu Ylonen <ylo@cs.hut.fi>
742 Information about new releases, mailing lists, and other related
743 issues can be found from the SSH WWW home page:
745 .Dl http://www.cs.hut.fi/ssh.
748 is a derivative of the original (free) ssh 1.2.12 release, but with bugs
749 removed and newer features re-added. Rapidly after the 1.2.12 release,
750 newer versions bore successively more restrictive licenses. This version
754 has all components of a restrictive nature (ie. patents, see
756 directly removed from the source code; any licensed or patented components
760 has been updated to support ssh protocol 1.5.
762 contains added support for
764 authentication and ticket passing.
766 supports one-time password authentication with
770 The libraries described in
772 are required for proper operation.