sshd.0 revision 214979
1180740SdesSSHD(8) OpenBSD System Manager's Manual SSHD(8) 2180740Sdes 3180740SdesNAME 4180740Sdes sshd - OpenSSH SSH daemon 5180740Sdes 6180740SdesSYNOPSIS 7204861Sdes sshd [-46DdeiqTt] [-b bits] [-C connection_spec] 8204861Sdes [-c host_certificate_file] [-f config_file] [-g login_grace_time] 9204861Sdes [-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len] 10180740Sdes 11180740SdesDESCRIPTION 12180740Sdes sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these 13214979Sdes programs replace rlogin(1) and rsh(1), and provide secure encrypted 14214979Sdes communications between two untrusted hosts over an insecure network. 15180740Sdes 16180740Sdes sshd listens for connections from clients. It is normally started at 17180740Sdes boot from /etc/rc. It forks a new daemon for each incoming connection. 18214979Sdes The forked daemons handle key exchange, encryption, authentication, 19214979Sdes command execution, and data exchange. 20180740Sdes 21180740Sdes sshd can be configured using command-line options or a configuration file 22214979Sdes (by default sshd_config(5)); command-line options override values 23214979Sdes specified in the configuration file. sshd rereads its configuration file 24214979Sdes when it receives a hangup signal, SIGHUP, by executing itself with the 25214979Sdes name and options it was started with, e.g. /usr/sbin/sshd. 26180740Sdes 27180740Sdes The options are as follows: 28180740Sdes 29180740Sdes -4 Forces sshd to use IPv4 addresses only. 30180740Sdes 31180740Sdes -6 Forces sshd to use IPv6 addresses only. 32180740Sdes 33180740Sdes -b bits 34180740Sdes Specifies the number of bits in the ephemeral protocol version 1 35180750Sdes server key (default 1024). 36180740Sdes 37180750Sdes -C connection_spec 38180750Sdes Specify the connection parameters to use for the -T extended test 39180750Sdes mode. If provided, any Match directives in the configuration 40180750Sdes file that would apply to the specified user, host, and address 41214979Sdes will be set before the configuration is written to standard 42214979Sdes output. The connection parameters are supplied as keyword=value 43180750Sdes pairs. The keywords are ``user'', ``host'', and ``addr''. All 44214979Sdes are required and may be supplied in any order, either with 45214979Sdes multiple -C options or as a comma-separated list. 46180750Sdes 47204861Sdes -c host_certificate_file 48204861Sdes Specifies a path to a certificate file to identify sshd during 49204861Sdes key exchange. The certificate file must match a host key file 50214979Sdes specified using the -h option or the HostKey configuration 51214979Sdes directive. 52204861Sdes 53180740Sdes -D When this option is specified, sshd will not detach and does not 54180740Sdes become a daemon. This allows easy monitoring of sshd. 55180740Sdes 56204861Sdes -d Debug mode. The server sends verbose debug output to standard 57214979Sdes error, and does not put itself in the background. The server 58214979Sdes also will not fork and will only process one connection. This 59214979Sdes option is only intended for debugging for the server. Multiple 60214979Sdes -d options increase the debugging level. Maximum is 3. 61180740Sdes 62180740Sdes -e When this option is specified, sshd will send the output to the 63180740Sdes standard error instead of the system log. 64180740Sdes 65180744Sdes -f config_file 66180740Sdes Specifies the name of the configuration file. The default is 67214979Sdes /etc/ssh/sshd_config. sshd refuses to start if there is no 68214979Sdes configuration file. 69180740Sdes 70180740Sdes -g login_grace_time 71214979Sdes Gives the grace time for clients to authenticate themselves 72214979Sdes (default 120 seconds). If the client fails to authenticate the 73214979Sdes user within this many seconds, the server disconnects and exits. 74214979Sdes A value of zero indicates no limit. 75180740Sdes 76180740Sdes -h host_key_file 77180740Sdes Specifies a file from which a host key is read. This option must 78180740Sdes be given if sshd is not run as root (as the normal host key files 79180740Sdes are normally not readable by anyone but root). The default is 80180740Sdes /etc/ssh/ssh_host_key for protocol version 1, and 81214979Sdes /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for 82214979Sdes protocol version 2. It is possible to have multiple host key 83214979Sdes files for the different protocol versions and host key 84214979Sdes algorithms. 85180740Sdes 86180740Sdes -i Specifies that sshd is being run from inetd(8). sshd is normally 87180740Sdes not run from inetd because it needs to generate the server key 88180740Sdes before it can respond to the client, and this may take tens of 89214979Sdes seconds. Clients would have to wait too long if the key was 90214979Sdes regenerated every time. However, with small key sizes (e.g. 512) 91180740Sdes using sshd from inetd may be feasible. 92180740Sdes 93180740Sdes -k key_gen_time 94180740Sdes Specifies how often the ephemeral protocol version 1 server key 95214979Sdes is regenerated (default 3600 seconds, or one hour). The 96214979Sdes motivation for regenerating the key fairly often is that the key 97214979Sdes is not stored anywhere, and after about an hour it becomes 98214979Sdes impossible to recover the key for decrypting intercepted 99214979Sdes communications even if the machine is cracked into or physically 100214979Sdes seized. A value of zero indicates that the key will never be 101214979Sdes regenerated. 102180740Sdes 103180740Sdes -o option 104214979Sdes Can be used to give options in the format used in the 105214979Sdes configuration file. This is useful for specifying options for 106214979Sdes which there is no separate command-line flag. For full details 107214979Sdes of the options, and their values, see sshd_config(5). 108180740Sdes 109180740Sdes -p port 110180740Sdes Specifies the port on which the server listens for connections 111214979Sdes (default 22). Multiple port options are permitted. Ports 112214979Sdes specified in the configuration file with the Port option are 113214979Sdes ignored when a command-line port is specified. Ports specified 114214979Sdes using the ListenAddress option override command-line ports. 115180740Sdes 116214979Sdes -q Quiet mode. Nothing is sent to the system log. Normally the 117214979Sdes beginning, authentication, and termination of each connection is 118180740Sdes logged. 119180740Sdes 120180750Sdes -T Extended test mode. Check the validity of the configuration 121180750Sdes file, output the effective configuration to stdout and then exit. 122214979Sdes Optionally, Match rules may be applied by specifying the 123214979Sdes connection parameters using one or more -C options. 124180750Sdes 125180740Sdes -t Test mode. Only check the validity of the configuration file and 126180740Sdes sanity of the keys. This is useful for updating sshd reliably as 127180740Sdes configuration options may change. 128180740Sdes 129180740Sdes -u len This option is used to specify the size of the field in the utmp 130180740Sdes structure that holds the remote host name. If the resolved host 131180740Sdes name is longer than len, the dotted decimal value will be used 132214979Sdes instead. This allows hosts with very long host names that 133214979Sdes overflow this field to still be uniquely identified. Specifying 134214979Sdes -u0 indicates that only dotted decimal addresses should be put 135214979Sdes into the utmp file. -u0 may also be used to prevent sshd from 136214979Sdes making DNS requests unless the authentication mechanism or 137214979Sdes configuration requires it. Authentication mechanisms that may 138214979Sdes require DNS include RhostsRSAAuthentication, 139214979Sdes HostbasedAuthentication, and using a from="pattern-list" option 140214979Sdes in a key file. Configuration options that require DNS include 141214979Sdes using a USER@HOST pattern in AllowUsers or DenyUsers. 142180740Sdes 143180740SdesAUTHENTICATION 144204861Sdes The OpenSSH SSH daemon supports SSH protocols 1 and 2. The default is to 145204861Sdes use protocol 2 only, though this can be changed via the Protocol option 146204861Sdes in sshd_config(5). Protocol 2 supports both RSA and DSA keys; protocol 1 147214979Sdes only supports RSA keys. For both protocols, each host has a host- 148214979Sdes specific key, normally 2048 bits, used to identify the host. 149180740Sdes 150180740Sdes Forward security for protocol 1 is provided through an additional server 151180740Sdes key, normally 768 bits, generated when the server starts. This key is 152180740Sdes normally regenerated every hour if it has been used, and is never stored 153180740Sdes on disk. Whenever a client connects, the daemon responds with its public 154180740Sdes host and server keys. The client compares the RSA host key against its 155214979Sdes own database to verify that it has not changed. The client then 156214979Sdes generates a 256-bit random number. It encrypts this random number using 157214979Sdes both the host key and the server key, and sends the encrypted number to 158214979Sdes the server. Both sides then use this random number as a session key 159214979Sdes which is used to encrypt all further communications in the session. The 160214979Sdes rest of the session is encrypted using a conventional cipher, currently 161214979Sdes Blowfish or 3DES, with 3DES being used by default. The client selects 162214979Sdes the encryption algorithm to use from those offered by the server. 163180740Sdes 164180740Sdes For protocol 2, forward security is provided through a Diffie-Hellman key 165180740Sdes agreement. This key agreement results in a shared session key. The rest 166180740Sdes of the session is encrypted using a symmetric cipher, currently 128-bit 167180740Sdes AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The 168180740Sdes client selects the encryption algorithm to use from those offered by the 169214979Sdes server. Additionally, session integrity is provided through a 170214979Sdes cryptographic message authentication code (hmac-md5, hmac-sha1, umac-64 171214979Sdes or hmac-ripemd160). 172180740Sdes 173180740Sdes Finally, the server and the client enter an authentication dialog. The 174214979Sdes client tries to authenticate itself using host-based authentication, 175214979Sdes public key authentication, challenge-response authentication, or password 176180740Sdes authentication. 177180740Sdes 178180740Sdes Regardless of the authentication type, the account is checked to ensure 179180740Sdes that it is accessible. An account is not accessible if it is locked, 180214979Sdes listed in DenyUsers or its group is listed in DenyGroups . The 181214979Sdes definition of a locked account is system dependant. Some platforms have 182214979Sdes their own account database (eg AIX) and some modify the passwd field ( 183214979Sdes `*LK*' on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on 184214979Sdes Tru64, a leading `*LOCKED*' on FreeBSD and a leading `!' on most 185214979Sdes Linuxes). If there is a requirement to disable password authentication 186214979Sdes for the account while allowing still public-key, then the passwd field 187214979Sdes should be set to something other than these values (eg `NP' or `*NP*' ). 188180740Sdes 189180740Sdes If the client successfully authenticates itself, a dialog for preparing 190180740Sdes the session is entered. At this time the client may request things like 191214979Sdes allocating a pseudo-tty, forwarding X11 connections, forwarding TCP 192214979Sdes connections, or forwarding the authentication agent connection over the 193214979Sdes secure channel. 194180740Sdes 195180740Sdes After this, the client either requests a shell or execution of a command. 196180740Sdes The sides then enter session mode. In this mode, either side may send 197180740Sdes data at any time, and such data is forwarded to/from the shell or command 198180740Sdes on the server side, and the user terminal in the client side. 199180740Sdes 200214979Sdes When the user program terminates and all forwarded X11 and other 201214979Sdes connections have been closed, the server sends command exit status to the 202180740Sdes client, and both sides exit. 203180740Sdes 204180740SdesLOGIN PROCESS 205180740Sdes When a user successfully logs in, sshd does the following: 206180740Sdes 207180740Sdes 1. If the login is on a tty, and no command has been specified, 208180740Sdes prints last login time and /etc/motd (unless prevented in the 209180740Sdes configuration file or by ~/.hushlogin; see the FILES section). 210180740Sdes 211180740Sdes 2. If the login is on a tty, records login time. 212180740Sdes 213180740Sdes 3. Checks /etc/nologin; if it exists, prints contents and quits 214180740Sdes (unless root). 215180740Sdes 216180740Sdes 4. Changes to run with normal user privileges. 217180740Sdes 218180740Sdes 5. Sets up basic environment. 219180740Sdes 220180740Sdes 6. Reads the file ~/.ssh/environment, if it exists, and users are 221180740Sdes allowed to change their environment. See the 222180740Sdes PermitUserEnvironment option in sshd_config(5). 223180740Sdes 224180740Sdes 7. Changes to user's home directory. 225180740Sdes 226180740Sdes 8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists, 227180740Sdes runs it; otherwise runs xauth. The ``rc'' files are given the 228180740Sdes X11 authentication protocol and cookie in standard input. See 229180740Sdes SSHRC, below. 230180740Sdes 231180740Sdes 9. Runs user's shell or command. 232180740Sdes 233180740SdesSSHRC 234180740Sdes If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment 235214979Sdes files but before starting the user's shell or command. It must not 236214979Sdes produce any output on stdout; stderr must be used instead. If X11 237214979Sdes forwarding is in use, it will receive the "proto cookie" pair in its 238214979Sdes standard input (and DISPLAY in its environment). The script must call 239214979Sdes xauth(1) because sshd will not run xauth automatically to add X11 240214979Sdes cookies. 241180740Sdes 242180740Sdes The primary purpose of this file is to run any initialization routines 243180740Sdes which may be needed before the user's home directory becomes accessible; 244180740Sdes AFS is a particular example of such an environment. 245180740Sdes 246180740Sdes This file will probably contain some initialization code followed by 247180740Sdes something similar to: 248180740Sdes 249180740Sdes if read proto cookie && [ -n "$DISPLAY" ]; then 250180740Sdes if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then 251180740Sdes # X11UseLocalhost=yes 252180740Sdes echo add unix:`echo $DISPLAY | 253180740Sdes cut -c11-` $proto $cookie 254180740Sdes else 255180740Sdes # X11UseLocalhost=no 256180740Sdes echo add $DISPLAY $proto $cookie 257180740Sdes fi | xauth -q - 258180740Sdes fi 259180740Sdes 260180740Sdes If this file does not exist, /etc/ssh/sshrc is run, and if that does not 261180740Sdes exist either, xauth is used to add the cookie. 262180740Sdes 263180740SdesAUTHORIZED_KEYS FILE FORMAT 264180740Sdes AuthorizedKeysFile specifies the file containing public keys for public 265180740Sdes key authentication; if none is specified, the default is 266180740Sdes ~/.ssh/authorized_keys. Each line of the file contains one key (empty 267180740Sdes lines and lines starting with a `#' are ignored as comments). Protocol 1 268180740Sdes public keys consist of the following space-separated fields: options, 269214979Sdes bits, exponent, modulus, comment. Protocol 2 public key consist of: 270214979Sdes options, keytype, base64-encoded key, comment. The options field is 271214979Sdes optional; its presence is determined by whether the line starts with a 272214979Sdes number or not (the options field never starts with a number). The bits, 273214979Sdes exponent, modulus, and comment fields give the RSA key for protocol 274214979Sdes version 1; the comment field is not used for anything (but may be 275214979Sdes convenient for the user to identify the key). For protocol version 2 the 276214979Sdes keytype is ``ssh-dss'' or ``ssh-rsa''. 277180740Sdes 278214979Sdes Note that lines in this file are usually several hundred bytes long 279214979Sdes (because of the size of the public key encoding) up to a limit of 8 280214979Sdes kilobytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16 281180740Sdes kilobits. You don't want to type them in; instead, copy the 282180740Sdes identity.pub, id_dsa.pub, or the id_rsa.pub file and edit it. 283180740Sdes 284180740Sdes sshd enforces a minimum RSA key modulus size for protocol 1 and protocol 285180740Sdes 2 keys of 768 bits. 286180740Sdes 287214979Sdes The options (if present) consist of comma-separated option 288214979Sdes specifications. No spaces are permitted, except within double quotes. 289214979Sdes The following option specifications are supported (note that option 290214979Sdes keywords are case-insensitive): 291180740Sdes 292204861Sdes cert-authority 293204861Sdes Specifies that the listed key is a certification authority (CA) 294214979Sdes that is trusted to validate signed certificates for user 295214979Sdes authentication. 296204861Sdes 297204861Sdes Certificates may encode access restrictions similar to these key 298204861Sdes options. If both certificate restrictions and key options are 299204861Sdes present, the most restrictive union of the two is applied. 300204861Sdes 301180740Sdes command="command" 302180740Sdes Specifies that the command is executed whenever this key is used 303180740Sdes for authentication. The command supplied by the user (if any) is 304180740Sdes ignored. The command is run on a pty if the client requests a 305214979Sdes pty; otherwise it is run without a tty. If an 8-bit clean 306214979Sdes channel is required, one must not request a pty or should specify 307214979Sdes no-pty. A quote may be included in the command by quoting it 308214979Sdes with a backslash. This option might be useful to restrict 309214979Sdes certain public keys to perform just a specific operation. An 310214979Sdes example might be a key that permits remote backups but nothing 311214979Sdes else. Note that the client may specify TCP and/or X11 forwarding 312214979Sdes unless they are explicitly prohibited. The command originally 313214979Sdes supplied by the client is available in the SSH_ORIGINAL_COMMAND 314214979Sdes environment variable. Note that this option applies to shell, 315214979Sdes command or subsystem execution. Also note that this command may 316214979Sdes be superseded by either a sshd_config(5) ForceCommand directive 317214979Sdes or a command embedded in a certificate. 318180740Sdes 319180740Sdes environment="NAME=value" 320180740Sdes Specifies that the string is to be added to the environment when 321180740Sdes logging in using this key. Environment variables set this way 322180740Sdes override other default environment values. Multiple options of 323180740Sdes this type are permitted. Environment processing is disabled by 324180740Sdes default and is controlled via the PermitUserEnvironment option. 325180740Sdes This option is automatically disabled if UseLogin is enabled. 326180740Sdes 327180740Sdes from="pattern-list" 328180750Sdes Specifies that in addition to public key authentication, either 329180750Sdes the canonical name of the remote host or its IP address must be 330180750Sdes present in the comma-separated list of patterns. See PATTERNS in 331180750Sdes ssh_config(5) for more information on patterns. 332180740Sdes 333214979Sdes In addition to the wildcard matching that may be applied to 334214979Sdes hostnames or addresses, a from stanza may match IP addresses 335214979Sdes using CIDR address/masklen notation. 336180740Sdes 337180750Sdes The purpose of this option is to optionally increase security: 338180750Sdes public key authentication by itself does not trust the network or 339180750Sdes name servers or anything (but the key); however, if somebody 340180750Sdes somehow steals the key, the key permits an intruder to log in 341180750Sdes from anywhere in the world. This additional option makes using a 342180750Sdes stolen key more difficult (name servers and/or routers would have 343180750Sdes to be compromised in addition to just the key). 344180750Sdes 345180740Sdes no-agent-forwarding 346180740Sdes Forbids authentication agent forwarding when this key is used for 347180740Sdes authentication. 348180740Sdes 349180740Sdes no-port-forwarding 350180740Sdes Forbids TCP forwarding when this key is used for authentication. 351180740Sdes Any port forward requests by the client will return an error. 352180740Sdes This might be used, e.g. in connection with the command option. 353180740Sdes 354180740Sdes no-pty Prevents tty allocation (a request to allocate a pty will fail). 355180740Sdes 356180746Sdes no-user-rc 357180746Sdes Disables execution of ~/.ssh/rc. 358180746Sdes 359180740Sdes no-X11-forwarding 360180740Sdes Forbids X11 forwarding when this key is used for authentication. 361180740Sdes Any X11 forward requests by the client will return an error. 362180740Sdes 363180740Sdes permitopen="host:port" 364214979Sdes Limit local ``ssh -L'' port forwarding such that it may only 365214979Sdes connect to the specified host and port. IPv6 addresses can be 366214979Sdes specified by enclosing the address in square brackets. Multiple 367214979Sdes permitopen options may be applied separated by commas. No 368214979Sdes pattern matching is performed on the specified hostnames, they 369214979Sdes must be literal domains or addresses. 370180740Sdes 371214979Sdes principals="principals" 372214979Sdes On a cert-authority line, specifies allowed principals for 373214979Sdes certificate authentication as a comma-separated list. At least 374214979Sdes one name from the list must appear in the certificate's list of 375214979Sdes principals for the certificate to be accepted. This option is 376214979Sdes ignored for keys that are not marked as trusted certificate 377214979Sdes signers using the cert-authority option. 378214979Sdes 379180740Sdes tunnel="n" 380180740Sdes Force a tun(4) device on the server. Without this option, the 381214979Sdes next available device will be used if the client requests a 382214979Sdes tunnel. 383180740Sdes 384180740Sdes An example authorized_keys file: 385180740Sdes 386180740Sdes # Comments allowed at start of line 387180740Sdes ssh-rsa AAAAB3Nza...LiPk== user@example.net 388180740Sdes from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 389180740Sdes AAAAB2...19Q== john@example.net 390180740Sdes command="dump /home",no-pty,no-port-forwarding ssh-dss 391180740Sdes AAAAC3...51R== example.net 392180740Sdes permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 393180740Sdes AAAAB5...21S== 394180740Sdes tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 395180740Sdes jane@example.net 396180740Sdes 397180740SdesSSH_KNOWN_HOSTS FILE FORMAT 398180740Sdes The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host 399180740Sdes public keys for all known hosts. The global file should be prepared by 400214979Sdes the administrator (optional), and the per-user file is maintained 401214979Sdes automatically: whenever the user connects from an unknown host, its key 402214979Sdes is added to the per-user file. 403180740Sdes 404214979Sdes Each line in these files contains the following fields: markers 405214979Sdes (optional), hostnames, bits, exponent, modulus, comment. The fields are 406214979Sdes separated by spaces. 407180740Sdes 408204861Sdes The marker is optional, but if it is present then it must be one of 409204861Sdes ``@cert-authority'', to indicate that the line contains a certification 410204861Sdes authority (CA) key, or ``@revoked'', to indicate that the key contained 411204861Sdes on the line is revoked and must not ever be accepted. Only one marker 412204861Sdes should be used on a key line. 413204861Sdes 414214979Sdes Hostnames is a comma-separated list of patterns (`*' and `?' act as 415214979Sdes wildcards); each pattern in turn is matched against the canonical host 416214979Sdes name (when authenticating a client) or against the user-supplied name 417214979Sdes (when authenticating a server). A pattern may also be preceded by `!' to 418214979Sdes indicate negation: if the host name matches a negated pattern, it is not 419214979Sdes accepted (by that line) even if it matched another pattern on the line. 420214979Sdes A hostname or address may optionally be enclosed within `[' and `]' 421214979Sdes brackets then followed by `:' and a non-standard port number. 422180740Sdes 423180740Sdes Alternately, hostnames may be stored in a hashed form which hides host 424180740Sdes names and addresses should the file's contents be disclosed. Hashed 425214979Sdes hostnames start with a `|' character. Only one hashed hostname may 426214979Sdes appear on a single line and none of the above negation or wildcard 427214979Sdes operators may be applied. 428180740Sdes 429180740Sdes Bits, exponent, and modulus are taken directly from the RSA host key; 430180740Sdes they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The 431180740Sdes optional comment field continues to the end of the line, and is not used. 432180740Sdes 433180740Sdes Lines starting with `#' and empty lines are ignored as comments. 434180740Sdes 435180740Sdes When performing host authentication, authentication is accepted if any 436204861Sdes matching line has the proper key; either one that matches exactly or, if 437204861Sdes the server has presented a certificate for authentication, the key of the 438204861Sdes certification authority that signed the certificate. For a key to be 439204861Sdes trusted as a certification authority, it must use the ``@cert-authority'' 440204861Sdes marker described above. 441180740Sdes 442204861Sdes The known hosts file also provides a facility to mark keys as revoked, 443204861Sdes for example when it is known that the associated private key has been 444204861Sdes stolen. Revoked keys are specified by including the ``@revoked'' marker 445214979Sdes at the beginning of the key line, and are never accepted for 446214979Sdes authentication or as certification authorities, but instead will produce 447214979Sdes a warning from ssh(1) when they are encountered. 448204861Sdes 449214979Sdes It is permissible (but not recommended) to have several lines or 450214979Sdes different host keys for the same names. This will inevitably happen when 451214979Sdes short forms of host names from different domains are put in the file. It 452214979Sdes is possible that the files contain conflicting information; 453214979Sdes authentication is accepted if valid information can be found from either 454214979Sdes file. 455204861Sdes 456180740Sdes Note that the lines in these files are typically hundreds of characters 457180740Sdes long, and you definitely don't want to type in the host keys by hand. 458204861Sdes Rather, generate them by a script, ssh-keyscan(1) or by taking 459204861Sdes /etc/ssh/ssh_host_key.pub and adding the host names at the front. 460204861Sdes ssh-keygen(1) also offers some basic automated editing for 461214979Sdes ~/.ssh/known_hosts including removing hosts matching a host name and 462214979Sdes converting all host names to their hashed representations. 463180740Sdes 464180740Sdes An example ssh_known_hosts file: 465180740Sdes 466180740Sdes # Comments allowed at start of line 467180740Sdes closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 468180740Sdes cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 469180740Sdes # A hashed hostname 470180740Sdes |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 471180740Sdes AAAA1234.....= 472204861Sdes # A revoked key 473204861Sdes @revoked * ssh-rsa AAAAB5W... 474204861Sdes # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 475204861Sdes @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 476180740Sdes 477180740SdesFILES 478180740Sdes ~/.hushlogin 479180740Sdes This file is used to suppress printing the last login time and 480214979Sdes /etc/motd, if PrintLastLog and PrintMotd, respectively, are 481214979Sdes enabled. It does not suppress printing of the banner specified 482214979Sdes by Banner. 483180740Sdes 484180740Sdes ~/.rhosts 485180740Sdes This file is used for host-based authentication (see ssh(1) for 486180740Sdes more information). On some machines this file may need to be 487214979Sdes world-readable if the user's home directory is on an NFS 488214979Sdes partition, because sshd reads it as root. Additionally, this 489214979Sdes file must be owned by the user, and must not have write 490214979Sdes permissions for anyone else. The recommended permission for most 491214979Sdes machines is read/write for the user, and not accessible by 492214979Sdes others. 493180740Sdes 494180740Sdes ~/.shosts 495180740Sdes This file is used in exactly the same way as .rhosts, but allows 496180740Sdes host-based authentication without permitting login with 497180740Sdes rlogin/rsh. 498180740Sdes 499180746Sdes ~/.ssh/ 500214979Sdes This directory is the default location for all user-specific 501214979Sdes configuration and authentication information. There is no 502214979Sdes general requirement to keep the entire contents of this directory 503214979Sdes secret, but the recommended permissions are read/write/execute 504214979Sdes for the user, and not accessible by others. 505180746Sdes 506180740Sdes ~/.ssh/authorized_keys 507180740Sdes Lists the public keys (RSA/DSA) that can be used for logging in 508180740Sdes as this user. The format of this file is described above. The 509180740Sdes content of the file is not highly sensitive, but the recommended 510180740Sdes permissions are read/write for the user, and not accessible by 511180740Sdes others. 512180740Sdes 513180740Sdes If this file, the ~/.ssh directory, or the user's home directory 514180740Sdes are writable by other users, then the file could be modified or 515214979Sdes replaced by unauthorized users. In this case, sshd will not 516214979Sdes allow it to be used unless the StrictModes option has been set to 517189006Sdes ``no''. 518180740Sdes 519180740Sdes ~/.ssh/environment 520180740Sdes This file is read into the environment at login (if it exists). 521180740Sdes It can only contain empty lines, comment lines (that start with 522180740Sdes `#'), and assignment lines of the form name=value. The file 523180740Sdes should be writable only by the user; it need not be readable by 524180740Sdes anyone else. Environment processing is disabled by default and 525180740Sdes is controlled via the PermitUserEnvironment option. 526180740Sdes 527180740Sdes ~/.ssh/known_hosts 528180740Sdes Contains a list of host keys for all hosts the user has logged 529180740Sdes into that are not already in the systemwide list of known host 530180740Sdes keys. The format of this file is described above. This file 531180740Sdes should be writable only by root/the owner and can, but need not 532180740Sdes be, world-readable. 533180740Sdes 534180740Sdes ~/.ssh/rc 535180740Sdes Contains initialization routines to be run before the user's home 536180740Sdes directory becomes accessible. This file should be writable only 537180740Sdes by the user, and need not be readable by anyone else. 538180740Sdes 539180740Sdes /etc/hosts.allow 540180740Sdes /etc/hosts.deny 541214979Sdes Access controls that should be enforced by tcp-wrappers are 542214979Sdes defined here. Further details are described in hosts_access(5). 543180740Sdes 544180740Sdes /etc/hosts.equiv 545180740Sdes This file is for host-based authentication (see ssh(1)). It 546180740Sdes should only be writable by root. 547180740Sdes 548180740Sdes /etc/moduli 549180740Sdes Contains Diffie-Hellman groups used for the "Diffie-Hellman Group 550180740Sdes Exchange". The file format is described in moduli(5). 551180740Sdes 552180740Sdes /etc/motd 553180740Sdes See motd(5). 554180740Sdes 555180740Sdes /etc/nologin 556180740Sdes If this file exists, sshd refuses to let anyone except root log 557180740Sdes in. The contents of the file are displayed to anyone trying to 558180740Sdes log in, and non-root connections are refused. The file should be 559180740Sdes world-readable. 560180740Sdes 561180740Sdes /etc/shosts.equiv 562214979Sdes This file is used in exactly the same way as hosts.equiv, but 563214979Sdes allows host-based authentication without permitting login with 564180740Sdes rlogin/rsh. 565180740Sdes 566180740Sdes /etc/ssh/ssh_host_key 567180740Sdes /etc/ssh/ssh_host_dsa_key 568180740Sdes /etc/ssh/ssh_host_rsa_key 569180740Sdes These three files contain the private parts of the host keys. 570180740Sdes These files should only be owned by root, readable only by root, 571180740Sdes and not accessible to others. Note that sshd does not start if 572180740Sdes these files are group/world-accessible. 573180740Sdes 574180740Sdes /etc/ssh/ssh_host_key.pub 575180740Sdes /etc/ssh/ssh_host_dsa_key.pub 576180740Sdes /etc/ssh/ssh_host_rsa_key.pub 577180740Sdes These three files contain the public parts of the host keys. 578180740Sdes These files should be world-readable but writable only by root. 579180740Sdes Their contents should match the respective private parts. These 580180740Sdes files are not really used for anything; they are provided for the 581180740Sdes convenience of the user so their contents can be copied to known 582180740Sdes hosts files. These files are created using ssh-keygen(1). 583180740Sdes 584180744Sdes /etc/ssh/ssh_known_hosts 585180744Sdes Systemwide list of known host keys. This file should be prepared 586180744Sdes by the system administrator to contain the public host keys of 587214979Sdes all machines in the organization. The format of this file is 588214979Sdes described above. This file should be writable only by root/the 589180744Sdes owner and should be world-readable. 590180744Sdes 591180740Sdes /etc/ssh/sshd_config 592214979Sdes Contains configuration data for sshd. The file format and 593214979Sdes configuration options are described in sshd_config(5). 594180740Sdes 595180740Sdes /etc/ssh/sshrc 596180740Sdes Similar to ~/.ssh/rc, it can be used to specify machine-specific 597180740Sdes login-time initializations globally. This file should be 598180740Sdes writable only by root, and should be world-readable. 599180740Sdes 600180740Sdes /var/empty 601180740Sdes chroot(2) directory used by sshd during privilege separation in 602180740Sdes the pre-authentication phase. The directory should not contain 603180740Sdes any files and must be owned by root and not group or world- 604180740Sdes writable. 605180740Sdes 606180740Sdes /var/run/sshd.pid 607180740Sdes Contains the process ID of the sshd listening for connections (if 608180740Sdes there are several daemons running concurrently for different 609180740Sdes ports, this contains the process ID of the one started last). 610214979Sdes The content of this file is not sensitive; it can be world- 611214979Sdes readable. 612180740Sdes 613180740SdesSEE ALSO 614180740Sdes scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), 615180744Sdes ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5), 616180744Sdes sshd_config(5), inetd(8), sftp-server(8) 617180740Sdes 618180740SdesAUTHORS 619180740Sdes OpenSSH is a derivative of the original and free ssh 1.2.12 release by 620180740Sdes Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 621214979Sdes de Raadt and Dug Song removed many bugs, re-added newer features and 622214979Sdes created OpenSSH. Markus Friedl contributed the support for SSH protocol 623180740Sdes versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support 624180740Sdes for privilege separation. 625180740Sdes 626180740SdesCAVEATS 627214979Sdes System security is not improved unless rshd, rlogind, and rexecd are 628214979Sdes disabled (thus completely disabling rlogin and rsh into the machine). 629180740Sdes 630214979SdesOpenBSD 4.8 August 4, 2010 OpenBSD 4.8 631