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1 | SSHD(8) OpenBSD System Manager's Manual SSHD(8) | ||
2 | |||
3 | NAME | ||
4 | sshd - OpenSSH SSH daemon | ||
5 | |||
6 | SYNOPSIS | ||
7 | sshd [-46DdeiqTt] [-b bits] [-C connection_spec] | ||
8 | [-c host_certificate_file] [-E log_file] [-f config_file] | ||
9 | [-g login_grace_time] [-h host_key_file] [-k key_gen_time] | ||
10 | [-o option] [-p port] [-u len] | ||
11 | |||
12 | DESCRIPTION | ||
13 | sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these | ||
14 | programs replace rlogin(1) and rsh(1), and provide secure encrypted | ||
15 | communications between two untrusted hosts over an insecure network. | ||
16 | |||
17 | sshd listens for connections from clients. It is normally started at | ||
18 | boot from /etc/rc. It forks a new daemon for each incoming connection. | ||
19 | The forked daemons handle key exchange, encryption, authentication, | ||
20 | command execution, and data exchange. | ||
21 | |||
22 | sshd can be configured using command-line options or a configuration file | ||
23 | (by default sshd_config(5)); command-line options override values | ||
24 | specified in the configuration file. sshd rereads its configuration file | ||
25 | when it receives a hangup signal, SIGHUP, by executing itself with the | ||
26 | name and options it was started with, e.g. /usr/sbin/sshd. | ||
27 | |||
28 | The options are as follows: | ||
29 | |||
30 | -4 Forces sshd to use IPv4 addresses only. | ||
31 | |||
32 | -6 Forces sshd to use IPv6 addresses only. | ||
33 | |||
34 | -b bits | ||
35 | Specifies the number of bits in the ephemeral protocol version 1 | ||
36 | server key (default 1024). | ||
37 | |||
38 | -C connection_spec | ||
39 | Specify the connection parameters to use for the -T extended test | ||
40 | mode. If provided, any Match directives in the configuration | ||
41 | file that would apply to the specified user, host, and address | ||
42 | will be set before the configuration is written to standard | ||
43 | output. The connection parameters are supplied as keyword=value | ||
44 | pairs. The keywords are ``user'', ``host'', ``laddr'', | ||
45 | ``lport'', and ``addr''. All are required and may be supplied in | ||
46 | any order, either with multiple -C options or as a comma- | ||
47 | separated list. | ||
48 | |||
49 | -c host_certificate_file | ||
50 | Specifies a path to a certificate file to identify sshd during | ||
51 | key exchange. The certificate file must match a host key file | ||
52 | specified using the -h option or the HostKey configuration | ||
53 | directive. | ||
54 | |||
55 | -D When this option is specified, sshd will not detach and does not | ||
56 | become a daemon. This allows easy monitoring of sshd. | ||
57 | |||
58 | -d Debug mode. The server sends verbose debug output to standard | ||
59 | error, and does not put itself in the background. The server | ||
60 | also will not fork and will only process one connection. This | ||
61 | option is only intended for debugging for the server. Multiple | ||
62 | -d options increase the debugging level. Maximum is 3. | ||
63 | |||
64 | -E log_file | ||
65 | Append debug logs to log_file instead of the system log. | ||
66 | |||
67 | -e Write debug logs to standard error instead of the system log. | ||
68 | |||
69 | -f config_file | ||
70 | Specifies the name of the configuration file. The default is | ||
71 | /etc/ssh/sshd_config. sshd refuses to start if there is no | ||
72 | configuration file. | ||
73 | |||
74 | -g login_grace_time | ||
75 | Gives the grace time for clients to authenticate themselves | ||
76 | (default 120 seconds). If the client fails to authenticate the | ||
77 | user within this many seconds, the server disconnects and exits. | ||
78 | A value of zero indicates no limit. | ||
79 | |||
80 | -h host_key_file | ||
81 | Specifies a file from which a host key is read. This option must | ||
82 | be given if sshd is not run as root (as the normal host key files | ||
83 | are normally not readable by anyone but root). The default is | ||
84 | /etc/ssh/ssh_host_key for protocol version 1, and | ||
85 | /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key. | ||
86 | /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for | ||
87 | protocol version 2. It is possible to have multiple host key | ||
88 | files for the different protocol versions and host key | ||
89 | algorithms. | ||
90 | |||
91 | -i Specifies that sshd is being run from inetd(8). sshd is normally | ||
92 | not run from inetd because it needs to generate the server key | ||
93 | before it can respond to the client, and this may take tens of | ||
94 | seconds. Clients would have to wait too long if the key was | ||
95 | regenerated every time. However, with small key sizes (e.g. 512) | ||
96 | using sshd from inetd may be feasible. | ||
97 | |||
98 | -k key_gen_time | ||
99 | Specifies how often the ephemeral protocol version 1 server key | ||
100 | is regenerated (default 3600 seconds, or one hour). The | ||
101 | motivation for regenerating the key fairly often is that the key | ||
102 | is not stored anywhere, and after about an hour it becomes | ||
103 | impossible to recover the key for decrypting intercepted | ||
104 | communications even if the machine is cracked into or physically | ||
105 | seized. A value of zero indicates that the key will never be | ||
106 | regenerated. | ||
107 | |||
108 | -o option | ||
109 | Can be used to give options in the format used in the | ||
110 | configuration file. This is useful for specifying options for | ||
111 | which there is no separate command-line flag. For full details | ||
112 | of the options, and their values, see sshd_config(5). | ||
113 | |||
114 | -p port | ||
115 | Specifies the port on which the server listens for connections | ||
116 | (default 22). Multiple port options are permitted. Ports | ||
117 | specified in the configuration file with the Port option are | ||
118 | ignored when a command-line port is specified. Ports specified | ||
119 | using the ListenAddress option override command-line ports. | ||
120 | |||
121 | -q Quiet mode. Nothing is sent to the system log. Normally the | ||
122 | beginning, authentication, and termination of each connection is | ||
123 | logged. | ||
124 | |||
125 | -T Extended test mode. Check the validity of the configuration | ||
126 | file, output the effective configuration to stdout and then exit. | ||
127 | Optionally, Match rules may be applied by specifying the | ||
128 | connection parameters using one or more -C options. | ||
129 | |||
130 | -t Test mode. Only check the validity of the configuration file and | ||
131 | sanity of the keys. This is useful for updating sshd reliably as | ||
132 | configuration options may change. | ||
133 | |||
134 | -u len This option is used to specify the size of the field in the utmp | ||
135 | structure that holds the remote host name. If the resolved host | ||
136 | name is longer than len, the dotted decimal value will be used | ||
137 | instead. This allows hosts with very long host names that | ||
138 | overflow this field to still be uniquely identified. Specifying | ||
139 | -u0 indicates that only dotted decimal addresses should be put | ||
140 | into the utmp file. -u0 may also be used to prevent sshd from | ||
141 | making DNS requests unless the authentication mechanism or | ||
142 | configuration requires it. Authentication mechanisms that may | ||
143 | require DNS include RhostsRSAAuthentication, | ||
144 | HostbasedAuthentication, and using a from="pattern-list" option | ||
145 | in a key file. Configuration options that require DNS include | ||
146 | using a USER@HOST pattern in AllowUsers or DenyUsers. | ||
147 | |||
148 | AUTHENTICATION | ||
149 | The OpenSSH SSH daemon supports SSH protocols 1 and 2. The default is to | ||
150 | use protocol 2 only, though this can be changed via the Protocol option | ||
151 | in sshd_config(5). Protocol 2 supports DSA, ECDSA, ED25519 and RSA keys; | ||
152 | protocol 1 only supports RSA keys. For both protocols, each host has a | ||
153 | host-specific key, normally 2048 bits, used to identify the host. | ||
154 | |||
155 | Forward security for protocol 1 is provided through an additional server | ||
156 | key, normally 768 bits, generated when the server starts. This key is | ||
157 | normally regenerated every hour if it has been used, and is never stored | ||
158 | on disk. Whenever a client connects, the daemon responds with its public | ||
159 | host and server keys. The client compares the RSA host key against its | ||
160 | own database to verify that it has not changed. The client then | ||
161 | generates a 256-bit random number. It encrypts this random number using | ||
162 | both the host key and the server key, and sends the encrypted number to | ||
163 | the server. Both sides then use this random number as a session key | ||
164 | which is used to encrypt all further communications in the session. The | ||
165 | rest of the session is encrypted using a conventional cipher, currently | ||
166 | Blowfish or 3DES, with 3DES being used by default. The client selects | ||
167 | the encryption algorithm to use from those offered by the server. | ||
168 | |||
169 | For protocol 2, forward security is provided through a Diffie-Hellman key | ||
170 | agreement. This key agreement results in a shared session key. The rest | ||
171 | of the session is encrypted using a symmetric cipher, currently 128-bit | ||
172 | AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The | ||
173 | client selects the encryption algorithm to use from those offered by the | ||
174 | server. Additionally, session integrity is provided through a | ||
175 | cryptographic message authentication code (hmac-md5, hmac-sha1, umac-64, | ||
176 | umac-128, hmac-ripemd160, hmac-sha2-256 or hmac-sha2-512). | ||
177 | |||
178 | Finally, the server and the client enter an authentication dialog. The | ||
179 | client tries to authenticate itself using host-based authentication, | ||
180 | public key authentication, challenge-response authentication, or password | ||
181 | authentication. | ||
182 | |||
183 | Regardless of the authentication type, the account is checked to ensure | ||
184 | that it is accessible. An account is not accessible if it is locked, | ||
185 | listed in DenyUsers or its group is listed in DenyGroups . The | ||
186 | definition of a locked account is system dependant. Some platforms have | ||
187 | their own account database (eg AIX) and some modify the passwd field ( | ||
188 | `*LK*' on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on | ||
189 | Tru64, a leading `*LOCKED*' on FreeBSD and a leading `!' on most | ||
190 | Linuxes). If there is a requirement to disable password authentication | ||
191 | for the account while allowing still public-key, then the passwd field | ||
192 | should be set to something other than these values (eg `NP' or `*NP*' ). | ||
193 | |||
194 | If the client successfully authenticates itself, a dialog for preparing | ||
195 | the session is entered. At this time the client may request things like | ||
196 | allocating a pseudo-tty, forwarding X11 connections, forwarding TCP | ||
197 | connections, or forwarding the authentication agent connection over the | ||
198 | secure channel. | ||
199 | |||
200 | After this, the client either requests a shell or execution of a command. | ||
201 | The sides then enter session mode. In this mode, either side may send | ||
202 | data at any time, and such data is forwarded to/from the shell or command | ||
203 | on the server side, and the user terminal in the client side. | ||
204 | |||
205 | When the user program terminates and all forwarded X11 and other | ||
206 | connections have been closed, the server sends command exit status to the | ||
207 | client, and both sides exit. | ||
208 | |||
209 | LOGIN PROCESS | ||
210 | When a user successfully logs in, sshd does the following: | ||
211 | |||
212 | 1. If the login is on a tty, and no command has been specified, | ||
213 | prints last login time and /etc/motd (unless prevented in the | ||
214 | configuration file or by ~/.hushlogin; see the FILES section). | ||
215 | |||
216 | 2. If the login is on a tty, records login time. | ||
217 | |||
218 | 3. Checks /etc/nologin; if it exists, prints contents and quits | ||
219 | (unless root). | ||
220 | |||
221 | 4. Changes to run with normal user privileges. | ||
222 | |||
223 | 5. Sets up basic environment. | ||
224 | |||
225 | 6. Reads the file ~/.ssh/environment, if it exists, and users are | ||
226 | allowed to change their environment. See the | ||
227 | PermitUserEnvironment option in sshd_config(5). | ||
228 | |||
229 | 7. Changes to user's home directory. | ||
230 | |||
231 | 8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists, | ||
232 | runs it; otherwise runs xauth. The ``rc'' files are given the | ||
233 | X11 authentication protocol and cookie in standard input. See | ||
234 | SSHRC, below. | ||
235 | |||
236 | 9. Runs user's shell or command. | ||
237 | |||
238 | SSHRC | ||
239 | If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment | ||
240 | files but before starting the user's shell or command. It must not | ||
241 | produce any output on stdout; stderr must be used instead. If X11 | ||
242 | forwarding is in use, it will receive the "proto cookie" pair in its | ||
243 | standard input (and DISPLAY in its environment). The script must call | ||
244 | xauth(1) because sshd will not run xauth automatically to add X11 | ||
245 | cookies. | ||
246 | |||
247 | The primary purpose of this file is to run any initialization routines | ||
248 | which may be needed before the user's home directory becomes accessible; | ||
249 | AFS is a particular example of such an environment. | ||
250 | |||
251 | This file will probably contain some initialization code followed by | ||
252 | something similar to: | ||
253 | |||
254 | if read proto cookie && [ -n "$DISPLAY" ]; then | ||
255 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then | ||
256 | # X11UseLocalhost=yes | ||
257 | echo add unix:`echo $DISPLAY | | ||
258 | cut -c11-` $proto $cookie | ||
259 | else | ||
260 | # X11UseLocalhost=no | ||
261 | echo add $DISPLAY $proto $cookie | ||
262 | fi | xauth -q - | ||
263 | fi | ||
264 | |||
265 | If this file does not exist, /etc/ssh/sshrc is run, and if that does not | ||
266 | exist either, xauth is used to add the cookie. | ||
267 | |||
268 | AUTHORIZED_KEYS FILE FORMAT | ||
269 | AuthorizedKeysFile specifies the files containing public keys for public | ||
270 | key authentication; if none is specified, the default is | ||
271 | ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2. Each line of the | ||
272 | file contains one key (empty lines and lines starting with a `#' are | ||
273 | ignored as comments). Protocol 1 public keys consist of the following | ||
274 | space-separated fields: options, bits, exponent, modulus, comment. | ||
275 | Protocol 2 public key consist of: options, keytype, base64-encoded key, | ||
276 | comment. The options field is optional; its presence is determined by | ||
277 | whether the line starts with a number or not (the options field never | ||
278 | starts with a number). The bits, exponent, modulus, and comment fields | ||
279 | give the RSA key for protocol version 1; the comment field is not used | ||
280 | for anything (but may be convenient for the user to identify the key). | ||
281 | For protocol version 2 the keytype is ``ecdsa-sha2-nistp256'', | ||
282 | ``ecdsa-sha2-nistp384'', ``ecdsa-sha2-nistp521'', ``ssh-ed25519'', | ||
283 | ``ssh-dss'' or ``ssh-rsa''. | ||
284 | |||
285 | Note that lines in this file are usually several hundred bytes long | ||
286 | (because of the size of the public key encoding) up to a limit of 8 | ||
287 | kilobytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16 | ||
288 | kilobits. You don't want to type them in; instead, copy the | ||
289 | identity.pub, id_dsa.pub, id_ecdsa.pub, id_ed25519.pub, or the id_rsa.pub | ||
290 | file and edit it. | ||
291 | |||
292 | sshd enforces a minimum RSA key modulus size for protocol 1 and protocol | ||
293 | 2 keys of 768 bits. | ||
294 | |||
295 | The options (if present) consist of comma-separated option | ||
296 | specifications. No spaces are permitted, except within double quotes. | ||
297 | The following option specifications are supported (note that option | ||
298 | keywords are case-insensitive): | ||
299 | |||
300 | cert-authority | ||
301 | Specifies that the listed key is a certification authority (CA) | ||
302 | that is trusted to validate signed certificates for user | ||
303 | authentication. | ||
304 | |||
305 | Certificates may encode access restrictions similar to these key | ||
306 | options. If both certificate restrictions and key options are | ||
307 | present, the most restrictive union of the two is applied. | ||
308 | |||
309 | command="command" | ||
310 | Specifies that the command is executed whenever this key is used | ||
311 | for authentication. The command supplied by the user (if any) is | ||
312 | ignored. The command is run on a pty if the client requests a | ||
313 | pty; otherwise it is run without a tty. If an 8-bit clean | ||
314 | channel is required, one must not request a pty or should specify | ||
315 | no-pty. A quote may be included in the command by quoting it | ||
316 | with a backslash. This option might be useful to restrict | ||
317 | certain public keys to perform just a specific operation. An | ||
318 | example might be a key that permits remote backups but nothing | ||
319 | else. Note that the client may specify TCP and/or X11 forwarding | ||
320 | unless they are explicitly prohibited. The command originally | ||
321 | supplied by the client is available in the SSH_ORIGINAL_COMMAND | ||
322 | environment variable. Note that this option applies to shell, | ||
323 | command or subsystem execution. Also note that this command may | ||
324 | be superseded by either a sshd_config(5) ForceCommand directive | ||
325 | or a command embedded in a certificate. | ||
326 | |||
327 | environment="NAME=value" | ||
328 | Specifies that the string is to be added to the environment when | ||
329 | logging in using this key. Environment variables set this way | ||
330 | override other default environment values. Multiple options of | ||
331 | this type are permitted. Environment processing is disabled by | ||
332 | default and is controlled via the PermitUserEnvironment option. | ||
333 | This option is automatically disabled if UseLogin is enabled. | ||
334 | |||
335 | from="pattern-list" | ||
336 | Specifies that in addition to public key authentication, either | ||
337 | the canonical name of the remote host or its IP address must be | ||
338 | present in the comma-separated list of patterns. See PATTERNS in | ||
339 | ssh_config(5) for more information on patterns. | ||
340 | |||
341 | In addition to the wildcard matching that may be applied to | ||
342 | hostnames or addresses, a from stanza may match IP addresses | ||
343 | using CIDR address/masklen notation. | ||
344 | |||
345 | The purpose of this option is to optionally increase security: | ||
346 | public key authentication by itself does not trust the network or | ||
347 | name servers or anything (but the key); however, if somebody | ||
348 | somehow steals the key, the key permits an intruder to log in | ||
349 | from anywhere in the world. This additional option makes using a | ||
350 | stolen key more difficult (name servers and/or routers would have | ||
351 | to be compromised in addition to just the key). | ||
352 | |||
353 | no-agent-forwarding | ||
354 | Forbids authentication agent forwarding when this key is used for | ||
355 | authentication. | ||
356 | |||
357 | no-port-forwarding | ||
358 | Forbids TCP forwarding when this key is used for authentication. | ||
359 | Any port forward requests by the client will return an error. | ||
360 | This might be used, e.g. in connection with the command option. | ||
361 | |||
362 | no-pty Prevents tty allocation (a request to allocate a pty will fail). | ||
363 | |||
364 | no-user-rc | ||
365 | Disables execution of ~/.ssh/rc. | ||
366 | |||
367 | no-X11-forwarding | ||
368 | Forbids X11 forwarding when this key is used for authentication. | ||
369 | Any X11 forward requests by the client will return an error. | ||
370 | |||
371 | permitopen="host:port" | ||
372 | Limit local ``ssh -L'' port forwarding such that it may only | ||
373 | connect to the specified host and port. IPv6 addresses can be | ||
374 | specified by enclosing the address in square brackets. Multiple | ||
375 | permitopen options may be applied separated by commas. No | ||
376 | pattern matching is performed on the specified hostnames, they | ||
377 | must be literal domains or addresses. A port specification of * | ||
378 | matches any port. | ||
379 | |||
380 | principals="principals" | ||
381 | On a cert-authority line, specifies allowed principals for | ||
382 | certificate authentication as a comma-separated list. At least | ||
383 | one name from the list must appear in the certificate's list of | ||
384 | principals for the certificate to be accepted. This option is | ||
385 | ignored for keys that are not marked as trusted certificate | ||
386 | signers using the cert-authority option. | ||
387 | |||
388 | tunnel="n" | ||
389 | Force a tun(4) device on the server. Without this option, the | ||
390 | next available device will be used if the client requests a | ||
391 | tunnel. | ||
392 | |||
393 | An example authorized_keys file: | ||
394 | |||
395 | # Comments allowed at start of line | ||
396 | ssh-rsa AAAAB3Nza...LiPk== user@example.net | ||
397 | from="*.sales.example.net,!pc.sales.example.net" ssh-rsa | ||
398 | AAAAB2...19Q== john@example.net | ||
399 | command="dump /home",no-pty,no-port-forwarding ssh-dss | ||
400 | AAAAC3...51R== example.net | ||
401 | permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss | ||
402 | AAAAB5...21S== | ||
403 | tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== | ||
404 | jane@example.net | ||
405 | |||
406 | SSH_KNOWN_HOSTS FILE FORMAT | ||
407 | The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host | ||
408 | public keys for all known hosts. The global file should be prepared by | ||
409 | the administrator (optional), and the per-user file is maintained | ||
410 | automatically: whenever the user connects from an unknown host, its key | ||
411 | is added to the per-user file. | ||
412 | |||
413 | Each line in these files contains the following fields: markers | ||
414 | (optional), hostnames, bits, exponent, modulus, comment. The fields are | ||
415 | separated by spaces. | ||
416 | |||
417 | The marker is optional, but if it is present then it must be one of | ||
418 | ``@cert-authority'', to indicate that the line contains a certification | ||
419 | authority (CA) key, or ``@revoked'', to indicate that the key contained | ||
420 | on the line is revoked and must not ever be accepted. Only one marker | ||
421 | should be used on a key line. | ||
422 | |||
423 | Hostnames is a comma-separated list of patterns (`*' and `?' act as | ||
424 | wildcards); each pattern in turn is matched against the canonical host | ||
425 | name (when authenticating a client) or against the user-supplied name | ||
426 | (when authenticating a server). A pattern may also be preceded by `!' to | ||
427 | indicate negation: if the host name matches a negated pattern, it is not | ||
428 | accepted (by that line) even if it matched another pattern on the line. | ||
429 | A hostname or address may optionally be enclosed within `[' and `]' | ||
430 | brackets then followed by `:' and a non-standard port number. | ||
431 | |||
432 | Alternately, hostnames may be stored in a hashed form which hides host | ||
433 | names and addresses should the file's contents be disclosed. Hashed | ||
434 | hostnames start with a `|' character. Only one hashed hostname may | ||
435 | appear on a single line and none of the above negation or wildcard | ||
436 | operators may be applied. | ||
437 | |||
438 | Bits, exponent, and modulus are taken directly from the RSA host key; | ||
439 | they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The | ||
440 | optional comment field continues to the end of the line, and is not used. | ||
441 | |||
442 | Lines starting with `#' and empty lines are ignored as comments. | ||
443 | |||
444 | When performing host authentication, authentication is accepted if any | ||
445 | matching line has the proper key; either one that matches exactly or, if | ||
446 | the server has presented a certificate for authentication, the key of the | ||
447 | certification authority that signed the certificate. For a key to be | ||
448 | trusted as a certification authority, it must use the ``@cert-authority'' | ||
449 | marker described above. | ||
450 | |||
451 | The known hosts file also provides a facility to mark keys as revoked, | ||
452 | for example when it is known that the associated private key has been | ||
453 | stolen. Revoked keys are specified by including the ``@revoked'' marker | ||
454 | at the beginning of the key line, and are never accepted for | ||
455 | authentication or as certification authorities, but instead will produce | ||
456 | a warning from ssh(1) when they are encountered. | ||
457 | |||
458 | It is permissible (but not recommended) to have several lines or | ||
459 | different host keys for the same names. This will inevitably happen when | ||
460 | short forms of host names from different domains are put in the file. It | ||
461 | is possible that the files contain conflicting information; | ||
462 | authentication is accepted if valid information can be found from either | ||
463 | file. | ||
464 | |||
465 | Note that the lines in these files are typically hundreds of characters | ||
466 | long, and you definitely don't want to type in the host keys by hand. | ||
467 | Rather, generate them by a script, ssh-keyscan(1) or by taking | ||
468 | /etc/ssh/ssh_host_key.pub and adding the host names at the front. | ||
469 | ssh-keygen(1) also offers some basic automated editing for | ||
470 | ~/.ssh/known_hosts including removing hosts matching a host name and | ||
471 | converting all host names to their hashed representations. | ||
472 | |||
473 | An example ssh_known_hosts file: | ||
474 | |||
475 | # Comments allowed at start of line | ||
476 | closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net | ||
477 | cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= | ||
478 | # A hashed hostname | ||
479 | |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa | ||
480 | AAAA1234.....= | ||
481 | # A revoked key | ||
482 | @revoked * ssh-rsa AAAAB5W... | ||
483 | # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org | ||
484 | @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... | ||
485 | |||
486 | FILES | ||
487 | ~/.hushlogin | ||
488 | This file is used to suppress printing the last login time and | ||
489 | /etc/motd, if PrintLastLog and PrintMotd, respectively, are | ||
490 | enabled. It does not suppress printing of the banner specified | ||
491 | by Banner. | ||
492 | |||
493 | ~/.rhosts | ||
494 | This file is used for host-based authentication (see ssh(1) for | ||
495 | more information). On some machines this file may need to be | ||
496 | world-readable if the user's home directory is on an NFS | ||
497 | partition, because sshd reads it as root. Additionally, this | ||
498 | file must be owned by the user, and must not have write | ||
499 | permissions for anyone else. The recommended permission for most | ||
500 | machines is read/write for the user, and not accessible by | ||
501 | others. | ||
502 | |||
503 | ~/.shosts | ||
504 | This file is used in exactly the same way as .rhosts, but allows | ||
505 | host-based authentication without permitting login with | ||
506 | rlogin/rsh. | ||
507 | |||
508 | ~/.ssh/ | ||
509 | This directory is the default location for all user-specific | ||
510 | configuration and authentication information. There is no | ||
511 | general requirement to keep the entire contents of this directory | ||
512 | secret, but the recommended permissions are read/write/execute | ||
513 | for the user, and not accessible by others. | ||
514 | |||
515 | ~/.ssh/authorized_keys | ||
516 | Lists the public keys (DSA, ECDSA, ED25519, RSA) that can be used | ||
517 | for logging in as this user. The format of this file is | ||
518 | described above. The content of the file is not highly | ||
519 | sensitive, but the recommended permissions are read/write for the | ||
520 | user, and not accessible by others. | ||
521 | |||
522 | If this file, the ~/.ssh directory, or the user's home directory | ||
523 | are writable by other users, then the file could be modified or | ||
524 | replaced by unauthorized users. In this case, sshd will not | ||
525 | allow it to be used unless the StrictModes option has been set to | ||
526 | ``no''. | ||
527 | |||
528 | ~/.ssh/environment | ||
529 | This file is read into the environment at login (if it exists). | ||
530 | It can only contain empty lines, comment lines (that start with | ||
531 | `#'), and assignment lines of the form name=value. The file | ||
532 | should be writable only by the user; it need not be readable by | ||
533 | anyone else. Environment processing is disabled by default and | ||
534 | is controlled via the PermitUserEnvironment option. | ||
535 | |||
536 | ~/.ssh/known_hosts | ||
537 | Contains a list of host keys for all hosts the user has logged | ||
538 | into that are not already in the systemwide list of known host | ||
539 | keys. The format of this file is described above. This file | ||
540 | should be writable only by root/the owner and can, but need not | ||
541 | be, world-readable. | ||
542 | |||
543 | ~/.ssh/rc | ||
544 | Contains initialization routines to be run before the user's home | ||
545 | directory becomes accessible. This file should be writable only | ||
546 | by the user, and need not be readable by anyone else. | ||
547 | |||
548 | /etc/hosts.allow | ||
549 | /etc/hosts.deny | ||
550 | Access controls that should be enforced by tcp-wrappers are | ||
551 | defined here. Further details are described in hosts_access(5). | ||
552 | |||
553 | /etc/hosts.equiv | ||
554 | This file is for host-based authentication (see ssh(1)). It | ||
555 | should only be writable by root. | ||
556 | |||
557 | /etc/moduli | ||
558 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group | ||
559 | Exchange". The file format is described in moduli(5). | ||
560 | |||
561 | /etc/motd | ||
562 | See motd(5). | ||
563 | |||
564 | /etc/nologin | ||
565 | If this file exists, sshd refuses to let anyone except root log | ||
566 | in. The contents of the file are displayed to anyone trying to | ||
567 | log in, and non-root connections are refused. The file should be | ||
568 | world-readable. | ||
569 | |||
570 | /etc/shosts.equiv | ||
571 | This file is used in exactly the same way as hosts.equiv, but | ||
572 | allows host-based authentication without permitting login with | ||
573 | rlogin/rsh. | ||
574 | |||
575 | /etc/ssh/ssh_host_key | ||
576 | /etc/ssh/ssh_host_dsa_key | ||
577 | /etc/ssh/ssh_host_ecdsa_key | ||
578 | /etc/ssh/ssh_host_ed25519_key | ||
579 | /etc/ssh/ssh_host_rsa_key | ||
580 | These files contain the private parts of the host keys. These | ||
581 | files should only be owned by root, readable only by root, and | ||
582 | not accessible to others. Note that sshd does not start if these | ||
583 | files are group/world-accessible. | ||
584 | |||
585 | /etc/ssh/ssh_host_key.pub | ||
586 | /etc/ssh/ssh_host_dsa_key.pub | ||
587 | /etc/ssh/ssh_host_ecdsa_key.pub | ||
588 | /etc/ssh/ssh_host_ed25519_key.pub | ||
589 | /etc/ssh/ssh_host_rsa_key.pub | ||
590 | These files contain the public parts of the host keys. These | ||
591 | files should be world-readable but writable only by root. Their | ||
592 | contents should match the respective private parts. These files | ||
593 | are not really used for anything; they are provided for the | ||
594 | convenience of the user so their contents can be copied to known | ||
595 | hosts files. These files are created using ssh-keygen(1). | ||
596 | |||
597 | /etc/ssh/ssh_known_hosts | ||
598 | Systemwide list of known host keys. This file should be prepared | ||
599 | by the system administrator to contain the public host keys of | ||
600 | all machines in the organization. The format of this file is | ||
601 | described above. This file should be writable only by root/the | ||
602 | owner and should be world-readable. | ||
603 | |||
604 | /etc/ssh/sshd_config | ||
605 | Contains configuration data for sshd. The file format and | ||
606 | configuration options are described in sshd_config(5). | ||
607 | |||
608 | /etc/ssh/sshrc | ||
609 | Similar to ~/.ssh/rc, it can be used to specify machine-specific | ||
610 | login-time initializations globally. This file should be | ||
611 | writable only by root, and should be world-readable. | ||
612 | |||
613 | /var/empty | ||
614 | chroot(2) directory used by sshd during privilege separation in | ||
615 | the pre-authentication phase. The directory should not contain | ||
616 | any files and must be owned by root and not group or world- | ||
617 | writable. | ||
618 | |||
619 | /var/run/sshd.pid | ||
620 | Contains the process ID of the sshd listening for connections (if | ||
621 | there are several daemons running concurrently for different | ||
622 | ports, this contains the process ID of the one started last). | ||
623 | The content of this file is not sensitive; it can be world- | ||
624 | readable. | ||
625 | |||
626 | SEE ALSO | ||
627 | scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), | ||
628 | ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5), | ||
629 | sshd_config(5), inetd(8), sftp-server(8) | ||
630 | |||
631 | AUTHORS | ||
632 | OpenSSH is a derivative of the original and free ssh 1.2.12 release by | ||
633 | Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo | ||
634 | de Raadt and Dug Song removed many bugs, re-added newer features and | ||
635 | created OpenSSH. Markus Friedl contributed the support for SSH protocol | ||
636 | versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support | ||
637 | for privilege separation. | ||
638 | |||
639 | CAVEATS | ||
640 | System security is not improved unless rshd, rlogind, and rexecd are | ||
641 | disabled (thus completely disabling rlogin and rsh into the machine). | ||
642 | |||
643 | OpenBSD 5.4 December 7, 2013 OpenBSD 5.4 | ||