sshd.0 revision 239844
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 81218767Sdes /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and 82218767Sdes /etc/ssh/ssh_host_rsa_key for protocol version 2. It is possible 83218767Sdes to have multiple host key files for the different protocol 84218767Sdes versions and host key 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 146218767Sdes in sshd_config(5). Protocol 2 supports DSA, ECDSA and RSA keys; protocol 147218767Sdes 1 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 170225825Sdes cryptographic message authentication code (hmac-md5, hmac-sha1, umac-64, 171225825Sdes hmac-ripemd160, hmac-sha2-256 or hmac-sha2-512). 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 264225825Sdes AuthorizedKeysFile specifies the files containing public keys for public 265180740Sdes key authentication; if none is specified, the default is 266225825Sdes ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2. Each line of the 267225825Sdes file contains one key (empty lines and lines starting with a `#' are 268225825Sdes ignored as comments). Protocol 1 public keys consist of the following 269225825Sdes space-separated fields: options, bits, exponent, modulus, comment. 270225825Sdes Protocol 2 public key consist of: options, keytype, base64-encoded key, 271225825Sdes comment. The options field is optional; its presence is determined by 272225825Sdes whether the line starts with a number or not (the options field never 273225825Sdes starts with a number). The bits, exponent, modulus, and comment fields 274225825Sdes give the RSA key for protocol version 1; the comment field is not used 275225825Sdes for anything (but may be convenient for the user to identify the key). 276225825Sdes For protocol version 2 the keytype is ``ecdsa-sha2-nistp256'', 277225825Sdes ``ecdsa-sha2-nistp384'', ``ecdsa-sha2-nistp521'', ``ssh-dss'' or 278225825Sdes ``ssh-rsa''. 279180740Sdes 280214979Sdes Note that lines in this file are usually several hundred bytes long 281214979Sdes (because of the size of the public key encoding) up to a limit of 8 282214979Sdes kilobytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16 283180740Sdes kilobits. You don't want to type them in; instead, copy the 284218767Sdes identity.pub, id_dsa.pub, id_ecdsa.pub, or the id_rsa.pub file and edit 285218767Sdes it. 286180740Sdes 287180740Sdes sshd enforces a minimum RSA key modulus size for protocol 1 and protocol 288180740Sdes 2 keys of 768 bits. 289180740Sdes 290214979Sdes The options (if present) consist of comma-separated option 291214979Sdes specifications. No spaces are permitted, except within double quotes. 292214979Sdes The following option specifications are supported (note that option 293214979Sdes keywords are case-insensitive): 294180740Sdes 295204861Sdes cert-authority 296204861Sdes Specifies that the listed key is a certification authority (CA) 297214979Sdes that is trusted to validate signed certificates for user 298214979Sdes authentication. 299204861Sdes 300204861Sdes Certificates may encode access restrictions similar to these key 301204861Sdes options. If both certificate restrictions and key options are 302204861Sdes present, the most restrictive union of the two is applied. 303204861Sdes 304180740Sdes command="command" 305180740Sdes Specifies that the command is executed whenever this key is used 306180740Sdes for authentication. The command supplied by the user (if any) is 307180740Sdes ignored. The command is run on a pty if the client requests a 308214979Sdes pty; otherwise it is run without a tty. If an 8-bit clean 309214979Sdes channel is required, one must not request a pty or should specify 310214979Sdes no-pty. A quote may be included in the command by quoting it 311214979Sdes with a backslash. This option might be useful to restrict 312214979Sdes certain public keys to perform just a specific operation. An 313214979Sdes example might be a key that permits remote backups but nothing 314214979Sdes else. Note that the client may specify TCP and/or X11 forwarding 315214979Sdes unless they are explicitly prohibited. The command originally 316214979Sdes supplied by the client is available in the SSH_ORIGINAL_COMMAND 317214979Sdes environment variable. Note that this option applies to shell, 318214979Sdes command or subsystem execution. Also note that this command may 319214979Sdes be superseded by either a sshd_config(5) ForceCommand directive 320214979Sdes or a command embedded in a certificate. 321180740Sdes 322180740Sdes environment="NAME=value" 323180740Sdes Specifies that the string is to be added to the environment when 324180740Sdes logging in using this key. Environment variables set this way 325180740Sdes override other default environment values. Multiple options of 326180740Sdes this type are permitted. Environment processing is disabled by 327180740Sdes default and is controlled via the PermitUserEnvironment option. 328180740Sdes This option is automatically disabled if UseLogin is enabled. 329180740Sdes 330180740Sdes from="pattern-list" 331180750Sdes Specifies that in addition to public key authentication, either 332180750Sdes the canonical name of the remote host or its IP address must be 333180750Sdes present in the comma-separated list of patterns. See PATTERNS in 334180750Sdes ssh_config(5) for more information on patterns. 335180740Sdes 336214979Sdes In addition to the wildcard matching that may be applied to 337214979Sdes hostnames or addresses, a from stanza may match IP addresses 338214979Sdes using CIDR address/masklen notation. 339180740Sdes 340180750Sdes The purpose of this option is to optionally increase security: 341180750Sdes public key authentication by itself does not trust the network or 342180750Sdes name servers or anything (but the key); however, if somebody 343180750Sdes somehow steals the key, the key permits an intruder to log in 344180750Sdes from anywhere in the world. This additional option makes using a 345180750Sdes stolen key more difficult (name servers and/or routers would have 346180750Sdes to be compromised in addition to just the key). 347180750Sdes 348180740Sdes no-agent-forwarding 349180740Sdes Forbids authentication agent forwarding when this key is used for 350180740Sdes authentication. 351180740Sdes 352180740Sdes no-port-forwarding 353180740Sdes Forbids TCP forwarding when this key is used for authentication. 354180740Sdes Any port forward requests by the client will return an error. 355180740Sdes This might be used, e.g. in connection with the command option. 356180740Sdes 357180740Sdes no-pty Prevents tty allocation (a request to allocate a pty will fail). 358180740Sdes 359180746Sdes no-user-rc 360180746Sdes Disables execution of ~/.ssh/rc. 361180746Sdes 362180740Sdes no-X11-forwarding 363180740Sdes Forbids X11 forwarding when this key is used for authentication. 364180740Sdes Any X11 forward requests by the client will return an error. 365180740Sdes 366180740Sdes permitopen="host:port" 367214979Sdes Limit local ``ssh -L'' port forwarding such that it may only 368214979Sdes connect to the specified host and port. IPv6 addresses can be 369214979Sdes specified by enclosing the address in square brackets. Multiple 370214979Sdes permitopen options may be applied separated by commas. No 371214979Sdes pattern matching is performed on the specified hostnames, they 372239844Sdes must be literal domains or addresses. A port specification of * 373239844Sdes matches any port. 374180740Sdes 375214979Sdes principals="principals" 376214979Sdes On a cert-authority line, specifies allowed principals for 377214979Sdes certificate authentication as a comma-separated list. At least 378214979Sdes one name from the list must appear in the certificate's list of 379214979Sdes principals for the certificate to be accepted. This option is 380214979Sdes ignored for keys that are not marked as trusted certificate 381214979Sdes signers using the cert-authority option. 382214979Sdes 383180740Sdes tunnel="n" 384180740Sdes Force a tun(4) device on the server. Without this option, the 385214979Sdes next available device will be used if the client requests a 386214979Sdes tunnel. 387180740Sdes 388180740Sdes An example authorized_keys file: 389180740Sdes 390180740Sdes # Comments allowed at start of line 391180740Sdes ssh-rsa AAAAB3Nza...LiPk== user@example.net 392180740Sdes from="*.sales.example.net,!pc.sales.example.net" ssh-rsa 393180740Sdes AAAAB2...19Q== john@example.net 394180740Sdes command="dump /home",no-pty,no-port-forwarding ssh-dss 395180740Sdes AAAAC3...51R== example.net 396180740Sdes permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss 397180740Sdes AAAAB5...21S== 398180740Sdes tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== 399180740Sdes jane@example.net 400180740Sdes 401180740SdesSSH_KNOWN_HOSTS FILE FORMAT 402180740Sdes The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host 403180740Sdes public keys for all known hosts. The global file should be prepared by 404214979Sdes the administrator (optional), and the per-user file is maintained 405214979Sdes automatically: whenever the user connects from an unknown host, its key 406214979Sdes is added to the per-user file. 407180740Sdes 408214979Sdes Each line in these files contains the following fields: markers 409214979Sdes (optional), hostnames, bits, exponent, modulus, comment. The fields are 410214979Sdes separated by spaces. 411180740Sdes 412204861Sdes The marker is optional, but if it is present then it must be one of 413204861Sdes ``@cert-authority'', to indicate that the line contains a certification 414204861Sdes authority (CA) key, or ``@revoked'', to indicate that the key contained 415204861Sdes on the line is revoked and must not ever be accepted. Only one marker 416204861Sdes should be used on a key line. 417204861Sdes 418214979Sdes Hostnames is a comma-separated list of patterns (`*' and `?' act as 419214979Sdes wildcards); each pattern in turn is matched against the canonical host 420214979Sdes name (when authenticating a client) or against the user-supplied name 421214979Sdes (when authenticating a server). A pattern may also be preceded by `!' to 422214979Sdes indicate negation: if the host name matches a negated pattern, it is not 423214979Sdes accepted (by that line) even if it matched another pattern on the line. 424214979Sdes A hostname or address may optionally be enclosed within `[' and `]' 425214979Sdes brackets then followed by `:' and a non-standard port number. 426180740Sdes 427180740Sdes Alternately, hostnames may be stored in a hashed form which hides host 428180740Sdes names and addresses should the file's contents be disclosed. Hashed 429214979Sdes hostnames start with a `|' character. Only one hashed hostname may 430214979Sdes appear on a single line and none of the above negation or wildcard 431214979Sdes operators may be applied. 432180740Sdes 433180740Sdes Bits, exponent, and modulus are taken directly from the RSA host key; 434180740Sdes they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The 435180740Sdes optional comment field continues to the end of the line, and is not used. 436180740Sdes 437180740Sdes Lines starting with `#' and empty lines are ignored as comments. 438180740Sdes 439180740Sdes When performing host authentication, authentication is accepted if any 440204861Sdes matching line has the proper key; either one that matches exactly or, if 441204861Sdes the server has presented a certificate for authentication, the key of the 442204861Sdes certification authority that signed the certificate. For a key to be 443204861Sdes trusted as a certification authority, it must use the ``@cert-authority'' 444204861Sdes marker described above. 445180740Sdes 446204861Sdes The known hosts file also provides a facility to mark keys as revoked, 447204861Sdes for example when it is known that the associated private key has been 448204861Sdes stolen. Revoked keys are specified by including the ``@revoked'' marker 449214979Sdes at the beginning of the key line, and are never accepted for 450214979Sdes authentication or as certification authorities, but instead will produce 451214979Sdes a warning from ssh(1) when they are encountered. 452204861Sdes 453214979Sdes It is permissible (but not recommended) to have several lines or 454214979Sdes different host keys for the same names. This will inevitably happen when 455214979Sdes short forms of host names from different domains are put in the file. It 456214979Sdes is possible that the files contain conflicting information; 457214979Sdes authentication is accepted if valid information can be found from either 458214979Sdes file. 459204861Sdes 460180740Sdes Note that the lines in these files are typically hundreds of characters 461180740Sdes long, and you definitely don't want to type in the host keys by hand. 462204861Sdes Rather, generate them by a script, ssh-keyscan(1) or by taking 463204861Sdes /etc/ssh/ssh_host_key.pub and adding the host names at the front. 464204861Sdes ssh-keygen(1) also offers some basic automated editing for 465214979Sdes ~/.ssh/known_hosts including removing hosts matching a host name and 466214979Sdes converting all host names to their hashed representations. 467180740Sdes 468180740Sdes An example ssh_known_hosts file: 469180740Sdes 470180740Sdes # Comments allowed at start of line 471180740Sdes closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net 472180740Sdes cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= 473180740Sdes # A hashed hostname 474180740Sdes |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa 475180740Sdes AAAA1234.....= 476204861Sdes # A revoked key 477204861Sdes @revoked * ssh-rsa AAAAB5W... 478204861Sdes # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org 479204861Sdes @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... 480180740Sdes 481180740SdesFILES 482180740Sdes ~/.hushlogin 483180740Sdes This file is used to suppress printing the last login time and 484214979Sdes /etc/motd, if PrintLastLog and PrintMotd, respectively, are 485214979Sdes enabled. It does not suppress printing of the banner specified 486214979Sdes by Banner. 487180740Sdes 488180740Sdes ~/.rhosts 489180740Sdes This file is used for host-based authentication (see ssh(1) for 490180740Sdes more information). On some machines this file may need to be 491214979Sdes world-readable if the user's home directory is on an NFS 492214979Sdes partition, because sshd reads it as root. Additionally, this 493214979Sdes file must be owned by the user, and must not have write 494214979Sdes permissions for anyone else. The recommended permission for most 495214979Sdes machines is read/write for the user, and not accessible by 496214979Sdes others. 497180740Sdes 498180740Sdes ~/.shosts 499180740Sdes This file is used in exactly the same way as .rhosts, but allows 500180740Sdes host-based authentication without permitting login with 501180740Sdes rlogin/rsh. 502180740Sdes 503180746Sdes ~/.ssh/ 504214979Sdes This directory is the default location for all user-specific 505214979Sdes configuration and authentication information. There is no 506214979Sdes general requirement to keep the entire contents of this directory 507214979Sdes secret, but the recommended permissions are read/write/execute 508214979Sdes for the user, and not accessible by others. 509180746Sdes 510180740Sdes ~/.ssh/authorized_keys 511218767Sdes Lists the public keys (DSA/ECDSA/RSA) that can be used for 512218767Sdes logging in as this user. The format of this file is described 513218767Sdes above. The content of the file is not highly sensitive, but the 514218767Sdes recommended permissions are read/write for the user, and not 515218767Sdes accessible by others. 516180740Sdes 517180740Sdes If this file, the ~/.ssh directory, or the user's home directory 518180740Sdes are writable by other users, then the file could be modified or 519214979Sdes replaced by unauthorized users. In this case, sshd will not 520214979Sdes allow it to be used unless the StrictModes option has been set to 521189006Sdes ``no''. 522180740Sdes 523180740Sdes ~/.ssh/environment 524180740Sdes This file is read into the environment at login (if it exists). 525180740Sdes It can only contain empty lines, comment lines (that start with 526180740Sdes `#'), and assignment lines of the form name=value. The file 527180740Sdes should be writable only by the user; it need not be readable by 528180740Sdes anyone else. Environment processing is disabled by default and 529180740Sdes is controlled via the PermitUserEnvironment option. 530180740Sdes 531180740Sdes ~/.ssh/known_hosts 532180740Sdes Contains a list of host keys for all hosts the user has logged 533180740Sdes into that are not already in the systemwide list of known host 534180740Sdes keys. The format of this file is described above. This file 535180740Sdes should be writable only by root/the owner and can, but need not 536180740Sdes be, world-readable. 537180740Sdes 538180740Sdes ~/.ssh/rc 539180740Sdes Contains initialization routines to be run before the user's home 540180740Sdes directory becomes accessible. This file should be writable only 541180740Sdes by the user, and need not be readable by anyone else. 542180740Sdes 543180740Sdes /etc/hosts.allow 544180740Sdes /etc/hosts.deny 545214979Sdes Access controls that should be enforced by tcp-wrappers are 546214979Sdes defined here. Further details are described in hosts_access(5). 547180740Sdes 548180740Sdes /etc/hosts.equiv 549180740Sdes This file is for host-based authentication (see ssh(1)). It 550180740Sdes should only be writable by root. 551180740Sdes 552180740Sdes /etc/moduli 553180740Sdes Contains Diffie-Hellman groups used for the "Diffie-Hellman Group 554180740Sdes Exchange". The file format is described in moduli(5). 555180740Sdes 556180740Sdes /etc/motd 557180740Sdes See motd(5). 558180740Sdes 559180740Sdes /etc/nologin 560180740Sdes If this file exists, sshd refuses to let anyone except root log 561180740Sdes in. The contents of the file are displayed to anyone trying to 562180740Sdes log in, and non-root connections are refused. The file should be 563180740Sdes world-readable. 564180740Sdes 565180740Sdes /etc/shosts.equiv 566214979Sdes This file is used in exactly the same way as hosts.equiv, but 567214979Sdes allows host-based authentication without permitting login with 568180740Sdes rlogin/rsh. 569180740Sdes 570180740Sdes /etc/ssh/ssh_host_key 571180740Sdes /etc/ssh/ssh_host_dsa_key 572218767Sdes /etc/ssh/ssh_host_ecdsa_key 573180740Sdes /etc/ssh/ssh_host_rsa_key 574180740Sdes These three files contain the private parts of the host keys. 575180740Sdes These files should only be owned by root, readable only by root, 576180740Sdes and not accessible to others. Note that sshd does not start if 577180740Sdes these files are group/world-accessible. 578180740Sdes 579180740Sdes /etc/ssh/ssh_host_key.pub 580180740Sdes /etc/ssh/ssh_host_dsa_key.pub 581218767Sdes /etc/ssh/ssh_host_ecdsa_key.pub 582180740Sdes /etc/ssh/ssh_host_rsa_key.pub 583180740Sdes These three files contain the public parts of the host keys. 584180740Sdes These files should be world-readable but writable only by root. 585180740Sdes Their contents should match the respective private parts. These 586180740Sdes files are not really used for anything; they are provided for the 587180740Sdes convenience of the user so their contents can be copied to known 588180740Sdes hosts files. These files are created using ssh-keygen(1). 589180740Sdes 590180744Sdes /etc/ssh/ssh_known_hosts 591180744Sdes Systemwide list of known host keys. This file should be prepared 592180744Sdes by the system administrator to contain the public host keys of 593214979Sdes all machines in the organization. The format of this file is 594214979Sdes described above. This file should be writable only by root/the 595180744Sdes owner and should be world-readable. 596180744Sdes 597180740Sdes /etc/ssh/sshd_config 598214979Sdes Contains configuration data for sshd. The file format and 599214979Sdes configuration options are described in sshd_config(5). 600180740Sdes 601180740Sdes /etc/ssh/sshrc 602180740Sdes Similar to ~/.ssh/rc, it can be used to specify machine-specific 603180740Sdes login-time initializations globally. This file should be 604180740Sdes writable only by root, and should be world-readable. 605180740Sdes 606180740Sdes /var/empty 607180740Sdes chroot(2) directory used by sshd during privilege separation in 608180740Sdes the pre-authentication phase. The directory should not contain 609180740Sdes any files and must be owned by root and not group or world- 610180740Sdes writable. 611180740Sdes 612180740Sdes /var/run/sshd.pid 613180740Sdes Contains the process ID of the sshd listening for connections (if 614180740Sdes there are several daemons running concurrently for different 615180740Sdes ports, this contains the process ID of the one started last). 616214979Sdes The content of this file is not sensitive; it can be world- 617214979Sdes readable. 618180740Sdes 619180740SdesSEE ALSO 620180740Sdes scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), 621180744Sdes ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5), 622180744Sdes sshd_config(5), inetd(8), sftp-server(8) 623180740Sdes 624180740SdesAUTHORS 625180740Sdes OpenSSH is a derivative of the original and free ssh 1.2.12 release by 626180740Sdes Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 627214979Sdes de Raadt and Dug Song removed many bugs, re-added newer features and 628214979Sdes created OpenSSH. Markus Friedl contributed the support for SSH protocol 629180740Sdes versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support 630180740Sdes for privilege separation. 631180740Sdes 632180740SdesCAVEATS 633214979Sdes System security is not improved unless rshd, rlogind, and rexecd are 634214979Sdes disabled (thus completely disabling rlogin and rsh into the machine). 635180740Sdes 636239844SdesOpenBSD 5.0 September 23, 2011 OpenBSD 5.0 637