1SSHD(8) BSD System Manager's Manual SSHD(8)
2
4 sshd — OpenSSH SSH daemon
5
7 sshd [-46Ddeiqt] [-b bits] [-f config_file] [-g login_grace_time]
8 [-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]
9
11 sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these
12 programs replace rlogin and rsh, and provide secure encrypted communica‐
13 tions between two untrusted hosts over an insecure network.
14 sshd listens for connections from clients. It is normally started at
16 boot from /etc/rc. It forks a new daemon for each incoming connection.
17 The forked daemons handle key exchange, encryption, authentication, com‐
18 mand execution, and data exchange.
19 sshd can be configured using command-line options or a configuration file
21 (by default sshd_config(5)); command-line options override values speci‐
22 fied in the configuration file. sshd rereads its configuration file when
23 it receives a hangup signal, SIGHUP, by executing itself with the name
24 and options it was started with, e.g. /usr/sbin/sshd.
25 The options are as follows:
27 -4 Forces sshd to use IPv4 addresses only.
29 -6 Forces sshd to use IPv6 addresses only.
31 -b bits
33 Specifies the number of bits in the ephemeral protocol version 1
34 server key (default 768).
35 -D When this option is specified, sshd will not detach and does not
37 become a daemon. This allows easy monitoring of sshd.
38 -d Debug mode. The server sends verbose debug output to the system
40 log, and does not put itself in the background. The server also
41 will not fork and will only process one connection. This option
42 is only intended for debugging for the server. Multiple -d
43 options increase the debugging level. Maximum is 3.
44 -e When this option is specified, sshd will send the output to the
46 standard error instead of the system log.
47 -f configuration_file
49 Specifies the name of the configuration file. The default is
50 /etc/ssh/sshd_config. sshd refuses to start if there is no con‐
51 figuration file.
52 -g login_grace_time
54 Gives the grace time for clients to authenticate themselves
55 (default 120 seconds). If the client fails to authenticate the
56 user within this many seconds, the server disconnects and exits.
57 A value of zero indicates no limit.
58 -h host_key_file
60 Specifies a file from which a host key is read. This option must
61 be given if sshd is not run as root (as the normal host key files
62 are normally not readable by anyone but root). The default is
63 /etc/ssh/ssh_host_key for protocol version 1, and
64 /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro‐
65 tocol version 2. It is possible to have multiple host key files
66 for the different protocol versions and host key algorithms.
67 -i Specifies that sshd is being run from inetd(8). sshd is normally
69 not run from inetd because it needs to generate the server key
70 before it can respond to the client, and this may take tens of
71 seconds. Clients would have to wait too long if the key was
72 regenerated every time. However, with small key sizes (e.g. 512)
73 using sshd from inetd may be feasible.
74 -k key_gen_time
76 Specifies how often the ephemeral protocol version 1 server key
77 is regenerated (default 3600 seconds, or one hour). The motiva‐
78 tion for regenerating the key fairly often is that the key is not
79 stored anywhere, and after about an hour it becomes impossible to
80 recover the key for decrypting intercepted communications even if
81 the machine is cracked into or physically seized. A value of
82 zero indicates that the key will never be regenerated.
83 -o option
85 Can be used to give options in the format used in the configura‐
86 tion file. This is useful for specifying options for which there
87 is no separate command-line flag. For full details of the
88 options, and their values, see sshd_config(5).
89 -p port
91 Specifies the port on which the server listens for connections
92 (default 22). Multiple port options are permitted. Ports speci‐
93 fied in the configuration file with the Port option are ignored
94 when a command-line port is specified. Ports specified using the
95 ListenAddress option override command-line ports.
96 -q Quiet mode. Nothing is sent to the system log. Normally the
98 beginning, authentication, and termination of each connection is
99 logged.
100 -t Test mode. Only check the validity of the configuration file and
102 sanity of the keys. This is useful for updating sshd reliably as
103 configuration options may change.
104 -u len This option is used to specify the size of the field in the utmp
106 structure that holds the remote host name. If the resolved host
107 name is longer than len, the dotted decimal value will be used
108 instead. This allows hosts with very long host names that over‐
109 flow this field to still be uniquely identified. Specifying -u0
110 indicates that only dotted decimal addresses should be put into
111 the utmp file. -u0 may also be used to prevent sshd from making
112 DNS requests unless the authentication mechanism or configuration
113 requires it. Authentication mechanisms that may require DNS
114 include RhostsRSAAuthentication, HostbasedAuthentication, and
115 using a from="pattern-list" option in a key file. Configuration
116 options that require DNS include using a USER@HOST pattern in
117 AllowUsers or DenyUsers.
118
120 The OpenSSH SSH daemon supports SSH protocols 1 and 2. Both protocols
121 are supported by default, though this can be changed via the Protocol
122 option in sshd_config(5). Protocol 2 supports both RSA and DSA keys;
123 protocol 1 only supports RSA keys. For both protocols, each host has a
124 host-specific key, normally 2048 bits, used to identify the host.
125 Forward security for protocol 1 is provided through an additional server
127 key, normally 768 bits, generated when the server starts. This key is
128 normally regenerated every hour if it has been used, and is never stored
129 on disk. Whenever a client connects, the daemon responds with its public
130 host and server keys. The client compares the RSA host key against its
131 own database to verify that it has not changed. The client then gener‐
132 ates a 256-bit random number. It encrypts this random number using both
133 the host key and the server key, and sends the encrypted number to the
134 server. Both sides then use this random number as a session key which is
135 used to encrypt all further communications in the session. The rest of
136 the session is encrypted using a conventional cipher, currently Blowfish
137 or 3DES, with 3DES being used by default. The client selects the encryp‐
138 tion algorithm to use from those offered by the server.
139 For protocol 2, forward security is provided through a Diffie-Hellman key
141 agreement. This key agreement results in a shared session key. The rest
142 of the session is encrypted using a symmetric cipher, currently 128-bit
143 AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The
144 client selects the encryption algorithm to use from those offered by the
145 server. Additionally, session integrity is provided through a crypto‐
146 graphic message authentication code (hmac-sha1 or hmac-md5).
147 Finally, the server and the client enter an authentication dialog. The
149 client tries to authenticate itself using host-based authentication, pub‐
150 lic key authentication, challenge-response authentication, or password
151 authentication.
152 Regardless of the authentication type, the account is checked to ensure
154 that it is accessible. An account is not accessible if it is locked,
155 listed in DenyUsers or its group is listed in DenyGroups . The defini‐
156 tion of a locked account is system dependant. Some platforms have their
157 own account database (eg AIX) and some modify the passwd field ( ‘*LK*’
158 on Solaris and UnixWare, ‘*’ on HP-UX, containing ‘Nologin’ on Tru64, a
159 leading ‘*LOCKED*’ on FreeBSD and a leading ‘!!’ on Linux). If there is
160 a requirement to disable password authentication for the account while
161 allowing still public-key, then the passwd field should be set to some‐
162 thing other than these values (eg ‘NP’ or ‘*NP*’ ).
163 If the client successfully authenticates itself, a dialog for preparing
165 the session is entered. At this time the client may request things like
166 allocating a pseudo-tty, forwarding X11 connections, forwarding TCP con‐
167 nections, or forwarding the authentication agent connection over the
168 secure channel.
169 After this, the client either requests a shell or execution of a command.
171 The sides then enter session mode. In this mode, either side may send
172 data at any time, and such data is forwarded to/from the shell or command
173 on the server side, and the user terminal in the client side.
174 When the user program terminates and all forwarded X11 and other connec‐
176 tions have been closed, the server sends command exit status to the
177 client, and both sides exit.
178
180 When a user successfully logs in, sshd does the following:
181 1. If the login is on a tty, and no command has been specified,
183 prints last login time and /etc/motd (unless prevented in the
184 configuration file or by ~/.hushlogin; see the FILES section).
185 2. If the login is on a tty, records login time.
187 3. Checks /etc/nologin; if it exists, prints contents and quits
189 (unless root).
190 4. Changes to run with normal user privileges.
192 5. Sets up basic environment.
194 6. Reads the file ~/.ssh/environment, if it exists, and users are
196 allowed to change their environment. See the
197 PermitUserEnvironment option in sshd_config(5).
198 7. Changes to user's home directory.
200 8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists,
202 runs it; otherwise runs xauth. The “rc” files are given the
203 X11 authentication protocol and cookie in standard input. See
204 SSHRC, below.
205 9. Runs user's shell or command.
207
209 If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
210 files but before starting the user's shell or command. It must not pro‐
211 duce any output on stdout; stderr must be used instead. If X11 forward‐
212 ing is in use, it will receive the "proto cookie" pair in its standard
213 input (and DISPLAY in its environment). The script must call xauth(1)
214 because sshd will not run xauth automatically to add X11 cookies.
215 The primary purpose of this file is to run any initialization routines
217 which may be needed before the user's home directory becomes accessible;
218 AFS is a particular example of such an environment.
219 This file will probably contain some initialization code followed by
221 something similar to:
222 if read proto cookie && [ -n "$DISPLAY" ]; then
224 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
225 # X11UseLocalhost=yes
226 echo add unix:`echo $DISPLAY |
227 cut -c11-` $proto $cookie
228 else
229 # X11UseLocalhost=no
230 echo add $DISPLAY $proto $cookie
231 fi | xauth -q -
232 fi
233 If this file does not exist, /etc/ssh/sshrc is run, and if that does not
235 exist either, xauth is used to add the cookie.
236
238 AuthorizedKeysFile specifies the file containing public keys for public
239 key authentication; if none is specified, the default is
240 ~/.ssh/authorized_keys. Each line of the file contains one key (empty
241 lines and lines starting with a ‘#’ are ignored as comments). Protocol 1
242 public keys consist of the following space-separated fields: options,
243 bits, exponent, modulus, comment. Protocol 2 public key consist of:
244 options, keytype, base64-encoded key, comment. The options field is
245 optional; its presence is determined by whether the line starts with a
246 number or not (the options field never starts with a number). The bits,
247 exponent, modulus, and comment fields give the RSA key for protocol ver‐
248 sion 1; the comment field is not used for anything (but may be convenient
249 for the user to identify the key). For protocol version 2 the keytype is
250 “ssh-dss” or “ssh-rsa”.
251 Note that lines in this file are usually several hundred bytes long
253 (because of the size of the public key encoding) up to a limit of 8 kilo‐
254 bytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
255 kilobits. You don't want to type them in; instead, copy the
256 identity.pub, id_dsa.pub, or the id_rsa.pub file and edit it.
257 sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
259 2 keys of 768 bits.
260 The options (if present) consist of comma-separated option specifica‐
262 tions. No spaces are permitted, except within double quotes. The fol‐
263 lowing option specifications are supported (note that option keywords are
264 case-insensitive):
265 command="command"
267 Specifies that the command is executed whenever this key is used
268 for authentication. The command supplied by the user (if any) is
269 ignored. The command is run on a pty if the client requests a
270 pty; otherwise it is run without a tty. If an 8-bit clean chan‐
271 nel is required, one must not request a pty or should specify
272 no-pty. A quote may be included in the command by quoting it
273 with a backslash. This option might be useful to restrict cer‐
274 tain public keys to perform just a specific operation. An exam‐
275 ple might be a key that permits remote backups but nothing else.
276 Note that the client may specify TCP and/or X11 forwarding unless
277 they are explicitly prohibited. The command originally supplied
278 by the client is available in the SSH_ORIGINAL_COMMAND environ‐
279 ment variable. Note that this option applies to shell, command
280 or subsystem execution.
281 environment="NAME=value"
283 Specifies that the string is to be added to the environment when
284 logging in using this key. Environment variables set this way
285 override other default environment values. Multiple options of
286 this type are permitted. Environment processing is disabled by
287 default and is controlled via the PermitUserEnvironment option.
288 This option is automatically disabled if UseLogin is enabled.
289 from="pattern-list"
291 Specifies that in addition to public key authentication, the
292 canonical name of the remote host must be present in the comma-
293 separated list of patterns. The purpose of this option is to
294 optionally increase security: public key authentication by itself
295 does not trust the network or name servers or anything (but the
296 key); however, if somebody somehow steals the key, the key per‐
297 mits an intruder to log in from anywhere in the world. This
298 additional option makes using a stolen key more difficult (name
299 servers and/or routers would have to be compromised in addition
300 to just the key).
301 See PATTERNS in ssh_config(5) for more information on patterns.
303 no-agent-forwarding
305 Forbids authentication agent forwarding when this key is used for
306 authentication.
307 no-port-forwarding
309 Forbids TCP forwarding when this key is used for authentication.
310 Any port forward requests by the client will return an error.
311 This might be used, e.g. in connection with the command option.
312 no-pty Prevents tty allocation (a request to allocate a pty will fail).
314 no-X11-forwarding
316 Forbids X11 forwarding when this key is used for authentication.
317 Any X11 forward requests by the client will return an error.
318 permitopen="host:port"
320 Limit local ``ssh -L'' port forwarding such that it may only con‐
321 nect to the specified host and port. IPv6 addresses can be spec‐
322 ified with an alternative syntax: host/port. Multiple permitopen
323 options may be applied separated by commas. No pattern matching
324 is performed on the specified hostnames, they must be literal
325 domains or addresses.
326 tunnel="n"
328 Force a tun(4) device on the server. Without this option, the
329 next available device will be used if the client requests a tun‐
330 nel.
331 An example authorized_keys file:
333 # Comments allowed at start of line
335 ssh-rsa AAAAB3Nza...LiPk== user@example.net
336 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
337 AAAAB2...19Q== john@example.net
338 command="dump /home",no-pty,no-port-forwarding ssh-dss
339 AAAAC3...51R== example.net
340 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
341 AAAAB5...21S==
342 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
343 jane@example.net
344
346 The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
347 public keys for all known hosts. The global file should be prepared by
348 the administrator (optional), and the per-user file is maintained auto‐
349 matically: whenever the user connects from an unknown host, its key is
350 added to the per-user file.
351 Each line in these files contains the following fields: hostnames, bits,
353 exponent, modulus, comment. The fields are separated by spaces.
354 Hostnames is a comma-separated list of patterns (‘*’ and ‘?’ act as wild‐
356 cards); each pattern in turn is matched against the canonical host name
357 (when authenticating a client) or against the user-supplied name (when
358 authenticating a server). A pattern may also be preceded by ‘!’ to indi‐
359 cate negation: if the host name matches a negated pattern, it is not
360 accepted (by that line) even if it matched another pattern on the line.
361 A hostname or address may optionally be enclosed within ‘[’ and ‘]’
362 brackets then followed by ‘:’ and a non-standard port number.
363 Alternately, hostnames may be stored in a hashed form which hides host
365 names and addresses should the file's contents be disclosed. Hashed
366 hostnames start with a ‘|’ character. Only one hashed hostname may
367 appear on a single line and none of the above negation or wildcard opera‐
368 tors may be applied.
369 Bits, exponent, and modulus are taken directly from the RSA host key;
371 they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The
372 optional comment field continues to the end of the line, and is not used.
373 Lines starting with ‘#’ and empty lines are ignored as comments.
375 When performing host authentication, authentication is accepted if any
377 matching line has the proper key. It is thus permissible (but not recom‐
378 mended) to have several lines or different host keys for the same names.
379 This will inevitably happen when short forms of host names from different
380 domains are put in the file. It is possible that the files contain con‐
381 flicting information; authentication is accepted if valid information can
382 be found from either file.
383 Note that the lines in these files are typically hundreds of characters
385 long, and you definitely don't want to type in the host keys by hand.
386 Rather, generate them by a script or by taking /etc/ssh/ssh_host_key.pub
387 and adding the host names at the front.
388 An example ssh_known_hosts file:
390 # Comments allowed at start of line
392 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
393 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
394 # A hashed hostname
395 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
396 AAAA1234.....=
397
399 ~/.hushlogin
400 This file is used to suppress printing the last login time and
401 /etc/motd, if PrintLastLog and PrintMotd, respectively, are
402 enabled. It does not suppress printing of the banner specified
403 by Banner.
404 ~/.rhosts
406 This file is used for host-based authentication (see ssh(1) for
407 more information). On some machines this file may need to be
408 world-readable if the user's home directory is on an NFS parti‐
409 tion, because sshd reads it as root. Additionally, this file
410 must be owned by the user, and must not have write permissions
411 for anyone else. The recommended permission for most machines is
412 read/write for the user, and not accessible by others.
413 ~/.shosts
415 This file is used in exactly the same way as .rhosts, but allows
416 host-based authentication without permitting login with
417 rlogin/rsh.
418 ~/.ssh/authorized_keys
420 Lists the public keys (RSA/DSA) that can be used for logging in
421 as this user. The format of this file is described above. The
422 content of the file is not highly sensitive, but the recommended
423 permissions are read/write for the user, and not accessible by
424 others.
425 If this file, the ~/.ssh directory, or the user's home directory
427 are writable by other users, then the file could be modified or
428 replaced by unauthorized users. In this case, sshd will not
429 allow it to be used unless the StrictModes option has been set to
430 “no”. The recommended permissions can be set by executing “chmod
431 go-w ~/ ~/.ssh ~/.ssh/authorized_keys”.
432 ~/.ssh/environment
434 This file is read into the environment at login (if it exists).
435 It can only contain empty lines, comment lines (that start with
436 ‘#’), and assignment lines of the form name=value. The file
437 should be writable only by the user; it need not be readable by
438 anyone else. Environment processing is disabled by default and
439 is controlled via the PermitUserEnvironment option.
440 ~/.ssh/known_hosts
442 Contains a list of host keys for all hosts the user has logged
443 into that are not already in the systemwide list of known host
444 keys. The format of this file is described above. This file
445 should be writable only by root/the owner and can, but need not
446 be, world-readable.
447 ~/.ssh/rc
449 Contains initialization routines to be run before the user's home
450 directory becomes accessible. This file should be writable only
451 by the user, and need not be readable by anyone else.
452 /etc/hosts.allow
454 /etc/hosts.deny
455 Access controls that should be enforced by tcp-wrappers are
456 defined here. Further details are described in hosts_access(5).
457 /etc/hosts.equiv
459 This file is for host-based authentication (see ssh(1)). It
460 should only be writable by root.
461 /etc/moduli
463 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
464 Exchange". The file format is described in moduli(5).
465 /etc/motd
467 See motd(5).
468 /etc/nologin
470 If this file exists, sshd refuses to let anyone except root log
471 in. The contents of the file are displayed to anyone trying to
472 log in, and non-root connections are refused. The file should be
473 world-readable.
474 /etc/ssh/shosts.equiv
476 This file is used in exactly the same way as hosts.equiv, but
477 allows host-based authentication without permitting login with
478 rlogin/rsh.
479 /etc/ssh/ssh_known_hosts
481 Systemwide list of known host keys. This file should be prepared
482 by the system administrator to contain the public host keys of
483 all machines in the organization. The format of this file is
484 described above. This file should be writable only by root/the
485 owner and should be world-readable.
486 /etc/ssh/ssh_host_key
488 /etc/ssh/ssh_host_dsa_key
489 /etc/ssh/ssh_host_rsa_key
490 These three files contain the private parts of the host keys.
491 These files should only be owned by root, readable only by root,
492 and not accessible to others. Note that sshd does not start if
493 these files are group/world-accessible.
494 /etc/ssh/ssh_host_key.pub
496 /etc/ssh/ssh_host_dsa_key.pub
497 /etc/ssh/ssh_host_rsa_key.pub
498 These three files contain the public parts of the host keys.
499 These files should be world-readable but writable only by root.
500 Their contents should match the respective private parts. These
501 files are not really used for anything; they are provided for the
502 convenience of the user so their contents can be copied to known
503 hosts files. These files are created using ssh-keygen(1).
504 /etc/ssh/sshd_config
506 Contains configuration data for sshd. The file format and con‐
507 figuration options are described in sshd_config(5).
508 /etc/ssh/sshrc
510 Similar to ~/.ssh/rc, it can be used to specify machine-specific
511 login-time initializations globally. This file should be
512 writable only by root, and should be world-readable.
513 /var/empty/sshd
515 chroot(2) directory used by sshd during privilege separation in
516 the pre-authentication phase. The directory should not contain
517 any files and must be owned by root and not group or world-
518 writable.
519 /var/run/sshd.pid
521 Contains the process ID of the sshd listening for connections (if
522 there are several daemons running concurrently for different
523 ports, this contains the process ID of the one started last).
524 The content of this file is not sensitive; it can be world-read‐
525 able.
526
528 scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
529 chroot(2), hosts_access(5), login.conf(5), moduli(5), sshd_config(5),
530 inetd(8), sftp-server(8)
531
533 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
534 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
535 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
536 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
537 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
538 for privilege separation.
539
541 System security is not improved unless rshd, rlogind, and rexecd are dis‐
542 abled (thus completely disabling rlogin and rsh into the machine).
543BSD September 25, 1999 BSD