sshd.0 revision 189006
1SSHD(8) OpenBSD System Manager's Manual SSHD(8) 2 3NAME 4 sshd - OpenSSH SSH daemon 5 6SYNOPSIS 7 sshd [-46DdeiqTt] [-b bits] [-C connection_spec] [-f config_file] 8 [-g login_grace_time] [-h host_key_file] [-k key_gen_time] 9 [-o option] [-p port] [-u len] 10 11DESCRIPTION 12 sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these 13 programs replace rlogin(1) and rsh(1), and provide secure encrypted com- 14 munications between two untrusted hosts over an insecure network. 15 16 sshd listens for connections from clients. It is normally started at 17 boot from /etc/rc. It forks a new daemon for each incoming connection. 18 The forked daemons handle key exchange, encryption, authentication, com- 19 mand execution, and data exchange. 20 21 sshd can be configured using command-line options or a configuration file 22 (by default sshd_config(5)); command-line options override values speci- 23 fied in the configuration file. sshd rereads its configuration file when 24 it receives a hangup signal, SIGHUP, by executing itself with the name 25 and options it was started with, e.g. /usr/sbin/sshd. 26 27 The options are as follows: 28 29 -4 Forces sshd to use IPv4 addresses only. 30 31 -6 Forces sshd to use IPv6 addresses only. 32 33 -b bits 34 Specifies the number of bits in the ephemeral protocol version 1 35 server key (default 1024). 36 37 -C connection_spec 38 Specify the connection parameters to use for the -T extended test 39 mode. If provided, any Match directives in the configuration 40 file that would apply to the specified user, host, and address 41 will be set before the configuration is written to standard out- 42 put. The connection parameters are supplied as keyword=value 43 pairs. The keywords are ``user'', ``host'', and ``addr''. All 44 are required and may be supplied in any order, either with multi- 45 ple -C options or as a comma-separated list. 46 47 -D When this option is specified, sshd will not detach and does not 48 become a daemon. This allows easy monitoring of sshd. 49 50 -d Debug mode. The server sends verbose debug output to the system 51 log, and does not put itself in the background. The server also 52 will not fork and will only process one connection. This option 53 is only intended for debugging for the server. Multiple -d op- 54 tions increase the debugging level. Maximum is 3. 55 56 -e When this option is specified, sshd will send the output to the 57 standard error instead of the system log. 58 59 -f config_file 60 Specifies the name of the configuration file. The default is 61 /etc/ssh/sshd_config. sshd refuses to start if there is no con- 62 figuration file. 63 64 -g login_grace_time 65 Gives the grace time for clients to authenticate themselves (de- 66 fault 120 seconds). If the client fails to authenticate the user 67 within this many seconds, the server disconnects and exits. A 68 value of zero indicates no limit. 69 70 -h host_key_file 71 Specifies a file from which a host key is read. This option must 72 be given if sshd is not run as root (as the normal host key files 73 are normally not readable by anyone but root). The default is 74 /etc/ssh/ssh_host_key for protocol version 1, and 75 /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro- 76 tocol version 2. It is possible to have multiple host key files 77 for the different protocol versions and host key algorithms. 78 79 -i Specifies that sshd is being run from inetd(8). sshd is normally 80 not run from inetd because it needs to generate the server key 81 before it can respond to the client, and this may take tens of 82 seconds. Clients would have to wait too long if the key was re- 83 generated every time. However, with small key sizes (e.g. 512) 84 using sshd from inetd may be feasible. 85 86 -k key_gen_time 87 Specifies how often the ephemeral protocol version 1 server key 88 is regenerated (default 3600 seconds, or one hour). The motiva- 89 tion for regenerating the key fairly often is that the key is not 90 stored anywhere, and after about an hour it becomes impossible to 91 recover the key for decrypting intercepted communications even if 92 the machine is cracked into or physically seized. A value of ze- 93 ro indicates that the key will never be regenerated. 94 95 -o option 96 Can be used to give options in the format used in the configura- 97 tion file. This is useful for specifying options for which there 98 is no separate command-line flag. For full details of the op- 99 tions, and their values, see sshd_config(5). 100 101 -p port 102 Specifies the port on which the server listens for connections 103 (default 22). Multiple port options are permitted. Ports speci- 104 fied in the configuration file with the Port option are ignored 105 when a command-line port is specified. Ports specified using the 106 ListenAddress option override command-line ports. 107 108 -q Quiet mode. Nothing is sent to the system log. Normally the be- 109 ginning, authentication, and termination of each connection is 110 logged. 111 112 -T Extended test mode. Check the validity of the configuration 113 file, output the effective configuration to stdout and then exit. 114 Optionally, Match rules may be applied by specifying the connec- 115 tion parameters using one or more -C options. 116 117 -t Test mode. Only check the validity of the configuration file and 118 sanity of the keys. This is useful for updating sshd reliably as 119 configuration options may change. 120 121 -u len This option is used to specify the size of the field in the utmp 122 structure that holds the remote host name. If the resolved host 123 name is longer than len, the dotted decimal value will be used 124 instead. This allows hosts with very long host names that over- 125 flow this field to still be uniquely identified. Specifying -u0 126 indicates that only dotted decimal addresses should be put into 127 the utmp file. -u0 may also be used to prevent sshd from making 128 DNS requests unless the authentication mechanism or configuration 129 requires it. Authentication mechanisms that may require DNS in- 130 clude RhostsRSAAuthentication, HostbasedAuthentication, and using 131 a from="pattern-list" option in a key file. Configuration op- 132 tions that require DNS include using a USER@HOST pattern in 133 AllowUsers or DenyUsers. 134 135AUTHENTICATION 136 The OpenSSH SSH daemon supports SSH protocols 1 and 2. Both protocols 137 are supported by default, though this can be changed via the Protocol op- 138 tion in sshd_config(5). Protocol 2 supports both RSA and DSA keys; pro- 139 tocol 1 only supports RSA keys. For both protocols, each host has a 140 host-specific key, normally 2048 bits, used to identify the host. 141 142 Forward security for protocol 1 is provided through an additional server 143 key, normally 768 bits, generated when the server starts. This key is 144 normally regenerated every hour if it has been used, and is never stored 145 on disk. Whenever a client connects, the daemon responds with its public 146 host and server keys. The client compares the RSA host key against its 147 own database to verify that it has not changed. The client then gener- 148 ates a 256-bit random number. It encrypts this random number using both 149 the host key and the server key, and sends the encrypted number to the 150 server. Both sides then use this random number as a session key which is 151 used to encrypt all further communications in the session. The rest of 152 the session is encrypted using a conventional cipher, currently Blowfish 153 or 3DES, with 3DES being used by default. The client selects the encryp- 154 tion algorithm to use from those offered by the server. 155 156 For protocol 2, forward security is provided through a Diffie-Hellman key 157 agreement. This key agreement results in a shared session key. The rest 158 of the session is encrypted using a symmetric cipher, currently 128-bit 159 AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The 160 client selects the encryption algorithm to use from those offered by the 161 server. Additionally, session integrity is provided through a crypto- 162 graphic message authentication code (hmac-md5, hmac-sha1, umac-64 or 163 hmac-ripemd160). 164 165 Finally, the server and the client enter an authentication dialog. The 166 client tries to authenticate itself using host-based authentication, pub- 167 lic key authentication, challenge-response authentication, or password 168 authentication. 169 170 Regardless of the authentication type, the account is checked to ensure 171 that it is accessible. An account is not accessible if it is locked, 172 listed in DenyUsers or its group is listed in DenyGroups . The defini- 173 tion of a locked account is system dependant. Some platforms have their 174 own account database (eg AIX) and some modify the passwd field ( `*LK*' 175 on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on Tru64, a 176 leading `*LOCKED*' on FreeBSD and a leading `!' on most Linuxes). If 177 there is a requirement to disable password authentication for the account 178 while allowing still public-key, then the passwd field should be set to 179 something other than these values (eg `NP' or `*NP*' ). 180 181 If the client successfully authenticates itself, a dialog for preparing 182 the session is entered. At this time the client may request things like 183 allocating a pseudo-tty, forwarding X11 connections, forwarding TCP con- 184 nections, or forwarding the authentication agent connection over the se- 185 cure channel. 186 187 After this, the client either requests a shell or execution of a command. 188 The sides then enter session mode. In this mode, either side may send 189 data at any time, and such data is forwarded to/from the shell or command 190 on the server side, and the user terminal in the client side. 191 192 When the user program terminates and all forwarded X11 and other connec- 193 tions have been closed, the server sends command exit status to the 194 client, and both sides exit. 195 196LOGIN PROCESS 197 When a user successfully logs in, sshd does the following: 198 199 1. If the login is on a tty, and no command has been specified, 200 prints last login time and /etc/motd (unless prevented in the 201 configuration file or by ~/.hushlogin; see the FILES section). 202 203 2. If the login is on a tty, records login time. 204 205 3. Checks /etc/nologin; if it exists, prints contents and quits 206 (unless root). 207 208 4. Changes to run with normal user privileges. 209 210 5. Sets up basic environment. 211 212 6. Reads the file ~/.ssh/environment, if it exists, and users are 213 allowed to change their environment. See the 214 PermitUserEnvironment option in sshd_config(5). 215 216 7. Changes to user's home directory. 217 218 8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists, 219 runs it; otherwise runs xauth. The ``rc'' files are given the 220 X11 authentication protocol and cookie in standard input. See 221 SSHRC, below. 222 223 9. Runs user's shell or command. 224 225SSHRC 226 If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment 227 files but before starting the user's shell or command. It must not pro- 228 duce any output on stdout; stderr must be used instead. If X11 forward- 229 ing is in use, it will receive the "proto cookie" pair in its standard 230 input (and DISPLAY in its environment). The script must call xauth(1) 231 because sshd will not run xauth automatically to add X11 cookies. 232 233 The primary purpose of this file is to run any initialization routines 234 which may be needed before the user's home directory becomes accessible; 235 AFS is a particular example of such an environment. 236 237 This file will probably contain some initialization code followed by 238 something similar to: 239 240 if read proto cookie && [ -n "$DISPLAY" ]; then 241 if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 242 # X11UseLocalhost=yes 243 echo add unix:`echo $DISPLAY | 244 cut -c11-` $proto $cookie 245 else 246 # X11UseLocalhost=no 247 echo add $DISPLAY $proto $cookie 248 fi | xauth -q - 249 fi 250 251 If this file does not exist, /etc/ssh/sshrc is run, and if that does not 252 exist either, xauth is used to add the cookie. 253 254AUTHORIZED_KEYS FILE FORMAT 255 AuthorizedKeysFile specifies the file containing public keys for public 256 key authentication; if none is specified, the default is 257 ~/.ssh/authorized_keys. Each line of the file contains one key (empty 258 lines and lines starting with a `#' are ignored as comments). Protocol 1 259 public keys consist of the following space-separated fields: options, 260 bits, exponent, modulus, comment. Protocol 2 public key consist of: op- 261 tions, keytype, base64-encoded key, comment. The options field is op- 262 tional; its presence is determined by whether the line starts with a num- 263 ber or not (the options field never starts with a number). The bits, ex- 264 ponent, modulus, and comment fields give the RSA key for protocol version 265 1; the comment field is not used for anything (but may be convenient for 266 the user to identify the key). For protocol version 2 the keytype is 267 ``ssh-dss'' or ``ssh-rsa''. 268 269 Note that lines in this file are usually several hundred bytes long (be- 270 cause of the size of the public key encoding) up to a limit of 8 kilo- 271 bytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16 272 kilobits. You don't want to type them in; instead, copy the 273 identity.pub, id_dsa.pub, or the id_rsa.pub file and edit it. 274 275 sshd enforces a minimum RSA key modulus size for protocol 1 and protocol 276 2 keys of 768 bits. 277 278 The options (if present) consist of comma-separated option specifica- 279 tions. No spaces are permitted, except within double quotes. The fol- 280 lowing option specifications are supported (note that option keywords are 281 case-insensitive): 282 283 command="command" 284 Specifies that the command is executed whenever this key is used 285 for authentication. The command supplied by the user (if any) is 286 ignored. The command is run on a pty if the client requests a 287 pty; otherwise it is run without a tty. If an 8-bit clean chan- 288 nel is required, one must not request a pty or should specify no- 289 pty. A quote may be included in the command by quoting it with a 290 backslash. This option might be useful to restrict certain pub- 291 lic keys to perform just a specific operation. An example might 292 be a key that permits remote backups but nothing else. Note that 293 the client may specify TCP and/or X11 forwarding unless they are 294 explicitly prohibited. The command originally supplied by the 295 client is available in the SSH_ORIGINAL_COMMAND environment vari- 296 able. Note that this option applies to shell, command or subsys- 297 tem execution. 298 299 environment="NAME=value" 300 Specifies that the string is to be added to the environment when 301 logging in using this key. Environment variables set this way 302 override other default environment values. Multiple options of 303 this type are permitted. Environment processing is disabled by 304 default and is controlled via the PermitUserEnvironment option. 305 This option is automatically disabled if UseLogin is enabled. 306 307 from="pattern-list" 308 Specifies that in addition to public key authentication, either 309 the canonical name of the remote host or its IP address must be 310 present in the comma-separated list of patterns. See PATTERNS in 311 ssh_config(5) for more information on patterns. 312 313 In addition to the wildcard matching that may be applied to host- 314 names or addresses, a from stanza may match IP addressess using 315 CIDR address/masklen notation. 316 317 The purpose of this option is to optionally increase security: 318 public key authentication by itself does not trust the network or 319 name servers or anything (but the key); however, if somebody 320 somehow steals the key, the key permits an intruder to log in 321 from anywhere in the world. This additional option makes using a 322 stolen key more difficult (name servers and/or routers would have 323 to be compromised in addition to just the key). 324 325 no-agent-forwarding 326 Forbids authentication agent forwarding when this key is used for 327 authentication. 328 329 no-port-forwarding 330 Forbids TCP forwarding when this key is used for authentication. 331 Any port forward requests by the client will return an error. 332 This might be used, e.g. in connection with the command option. 333 334 no-pty Prevents tty allocation (a request to allocate a pty will fail). 335 336 no-user-rc 337 Disables execution of ~/.ssh/rc. 338 339 no-X11-forwarding 340 Forbids X11 forwarding when this key is used for authentication. 341 Any X11 forward requests by the client will return an error. 342 343 permitopen="host:port" 344 Limit local ``ssh -L'' port forwarding such that it may only con- 345 nect to the specified host and port. IPv6 addresses can be spec- 346 ified with an alternative syntax: host/port. Multiple permitopen 347 options may be applied separated by commas. No pattern matching 348 is performed on the specified hostnames, they must be literal do- 349 mains or addresses. 350 351 tunnel="n" 352 Force a tun(4) device on the server. Without this option, the 353 next available device will be used if the client requests a tun- 354 nel. 355 356 An example authorized_keys file: 357 358 # Comments allowed at start of line 359 ssh-rsa AAAAB3Nza...LiPk== user@example.net 360 from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 361 AAAAB2...19Q== john@example.net 362 command="dump /home",no-pty,no-port-forwarding ssh-dss 363 AAAAC3...51R== example.net 364 permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 365 AAAAB5...21S== 366 tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 367 jane@example.net 368 369SSH_KNOWN_HOSTS FILE FORMAT 370 The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host 371 public keys for all known hosts. The global file should be prepared by 372 the administrator (optional), and the per-user file is maintained auto- 373 matically: whenever the user connects from an unknown host, its key is 374 added to the per-user file. 375 376 Each line in these files contains the following fields: hostnames, bits, 377 exponent, modulus, comment. The fields are separated by spaces. 378 379 Hostnames is a comma-separated list of patterns (`*' and `?' act as wild- 380 cards); each pattern in turn is matched against the canonical host name 381 (when authenticating a client) or against the user-supplied name (when 382 authenticating a server). A pattern may also be preceded by `!' to indi- 383 cate negation: if the host name matches a negated pattern, it is not ac- 384 cepted (by that line) even if it matched another pattern on the line. A 385 hostname or address may optionally be enclosed within `[' and `]' brack- 386 ets then followed by `:' and a non-standard port number. 387 388 Alternately, hostnames may be stored in a hashed form which hides host 389 names and addresses should the file's contents be disclosed. Hashed 390 hostnames start with a `|' character. Only one hashed hostname may ap- 391 pear on a single line and none of the above negation or wildcard opera- 392 tors may be applied. 393 394 Bits, exponent, and modulus are taken directly from the RSA host key; 395 they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The 396 optional comment field continues to the end of the line, and is not used. 397 398 Lines starting with `#' and empty lines are ignored as comments. 399 400 When performing host authentication, authentication is accepted if any 401 matching line has the proper key. It is thus permissible (but not recom- 402 mended) to have several lines or different host keys for the same names. 403 This will inevitably happen when short forms of host names from different 404 domains are put in the file. It is possible that the files contain con- 405 flicting information; authentication is accepted if valid information can 406 be found from either file. 407 408 Note that the lines in these files are typically hundreds of characters 409 long, and you definitely don't want to type in the host keys by hand. 410 Rather, generate them by a script or by taking /etc/ssh/ssh_host_key.pub 411 and adding the host names at the front. 412 413 An example ssh_known_hosts file: 414 415 # Comments allowed at start of line 416 closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 417 cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 418 # A hashed hostname 419 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 420 AAAA1234.....= 421 422FILES 423 ~/.hushlogin 424 This file is used to suppress printing the last login time and 425 /etc/motd, if PrintLastLog and PrintMotd, respectively, are en- 426 abled. It does not suppress printing of the banner specified by 427 Banner. 428 429 ~/.rhosts 430 This file is used for host-based authentication (see ssh(1) for 431 more information). On some machines this file may need to be 432 world-readable if the user's home directory is on an NFS parti- 433 tion, because sshd reads it as root. Additionally, this file 434 must be owned by the user, and must not have write permissions 435 for anyone else. The recommended permission for most machines is 436 read/write for the user, and not accessible by others. 437 438 ~/.shosts 439 This file is used in exactly the same way as .rhosts, but allows 440 host-based authentication without permitting login with 441 rlogin/rsh. 442 443 ~/.ssh/ 444 This directory is the default location for all user-specific con- 445 figuration and authentication information. There is no general 446 requirement to keep the entire contents of this directory secret, 447 but the recommended permissions are read/write/execute for the 448 user, and not accessible by others. 449 450 ~/.ssh/authorized_keys 451 Lists the public keys (RSA/DSA) that can be used for logging in 452 as this user. The format of this file is described above. The 453 content of the file is not highly sensitive, but the recommended 454 permissions are read/write for the user, and not accessible by 455 others. 456 457 If this file, the ~/.ssh directory, or the user's home directory 458 are writable by other users, then the file could be modified or 459 replaced by unauthorized users. In this case, sshd will not al- 460 low it to be used unless the StrictModes option has been set to 461 ``no''. 462 463 ~/.ssh/environment 464 This file is read into the environment at login (if it exists). 465 It can only contain empty lines, comment lines (that start with 466 `#'), and assignment lines of the form name=value. The file 467 should be writable only by the user; it need not be readable by 468 anyone else. Environment processing is disabled by default and 469 is controlled via the PermitUserEnvironment option. 470 471 ~/.ssh/known_hosts 472 Contains a list of host keys for all hosts the user has logged 473 into that are not already in the systemwide list of known host 474 keys. The format of this file is described above. This file 475 should be writable only by root/the owner and can, but need not 476 be, world-readable. 477 478 ~/.ssh/rc 479 Contains initialization routines to be run before the user's home 480 directory becomes accessible. This file should be writable only 481 by the user, and need not be readable by anyone else. 482 483 /etc/hosts.allow 484 /etc/hosts.deny 485 Access controls that should be enforced by tcp-wrappers are de- 486 fined here. Further details are described in hosts_access(5). 487 488 /etc/hosts.equiv 489 This file is for host-based authentication (see ssh(1)). It 490 should only be writable by root. 491 492 /etc/moduli 493 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group 494 Exchange". The file format is described in moduli(5). 495 496 /etc/motd 497 See motd(5). 498 499 /etc/nologin 500 If this file exists, sshd refuses to let anyone except root log 501 in. The contents of the file are displayed to anyone trying to 502 log in, and non-root connections are refused. The file should be 503 world-readable. 504 505 /etc/shosts.equiv 506 This file is used in exactly the same way as hosts.equiv, but al- 507 lows host-based authentication without permitting login with 508 rlogin/rsh. 509 510 /etc/ssh/ssh_host_key 511 /etc/ssh/ssh_host_dsa_key 512 /etc/ssh/ssh_host_rsa_key 513 These three files contain the private parts of the host keys. 514 These files should only be owned by root, readable only by root, 515 and not accessible to others. Note that sshd does not start if 516 these files are group/world-accessible. 517 518 /etc/ssh/ssh_host_key.pub 519 /etc/ssh/ssh_host_dsa_key.pub 520 /etc/ssh/ssh_host_rsa_key.pub 521 These three files contain the public parts of the host keys. 522 These files should be world-readable but writable only by root. 523 Their contents should match the respective private parts. These 524 files are not really used for anything; they are provided for the 525 convenience of the user so their contents can be copied to known 526 hosts files. These files are created using ssh-keygen(1). 527 528 /etc/ssh/ssh_known_hosts 529 Systemwide list of known host keys. This file should be prepared 530 by the system administrator to contain the public host keys of 531 all machines in the organization. The format of this file is de- 532 scribed above. This file should be writable only by root/the 533 owner and should be world-readable. 534 535 /etc/ssh/sshd_config 536 Contains configuration data for sshd. The file format and con- 537 figuration options are described in sshd_config(5). 538 539 /etc/ssh/sshrc 540 Similar to ~/.ssh/rc, it can be used to specify machine-specific 541 login-time initializations globally. This file should be 542 writable only by root, and should be world-readable. 543 544 /var/empty 545 chroot(2) directory used by sshd during privilege separation in 546 the pre-authentication phase. The directory should not contain 547 any files and must be owned by root and not group or world- 548 writable. 549 550 /var/run/sshd.pid 551 Contains the process ID of the sshd listening for connections (if 552 there are several daemons running concurrently for different 553 ports, this contains the process ID of the one started last). 554 The content of this file is not sensitive; it can be world-read- 555 able. 556 557SEE ALSO 558 scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), 559 ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5), 560 sshd_config(5), inetd(8), sftp-server(8) 561 562AUTHORS 563 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 564 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 565 de Raadt and Dug Song removed many bugs, re-added newer features and cre- 566 ated OpenSSH. Markus Friedl contributed the support for SSH protocol 567 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support 568 for privilege separation. 569 570CAVEATS 571 System security is not improved unless rshd, rlogind, and rexecd are dis- 572 abled (thus completely disabling rlogin and rsh into the machine). 573 574OpenBSD 4.5 October 3, 2008 9 575