sshd_config.0 revision 180750
1180740SdesSSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5) 2180740Sdes 3180740SdesNAME 4180740Sdes sshd_config - OpenSSH SSH daemon configuration file 5180740Sdes 6180740SdesSYNOPSIS 7180740Sdes /etc/ssh/sshd_config 8180740Sdes 9180740SdesDESCRIPTION 10180740Sdes sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file 11180740Sdes specified with -f on the command line). The file contains keyword-argu- 12180740Sdes ment pairs, one per line. Lines starting with `#' and empty lines are 13180740Sdes interpreted as comments. Arguments may optionally be enclosed in double 14180740Sdes quotes (") in order to represent arguments containing spaces. 15180740Sdes 16180740Sdes The possible keywords and their meanings are as follows (note that key- 17180740Sdes words are case-insensitive and arguments are case-sensitive): 18180740Sdes 19180740Sdes AcceptEnv 20180740Sdes Specifies what environment variables sent by the client will be 21180740Sdes copied into the session's environ(7). See SendEnv in 22180740Sdes ssh_config(5) for how to configure the client. Note that envi- 23180740Sdes ronment passing is only supported for protocol 2. Variables are 24180740Sdes specified by name, which may contain the wildcard characters `*' 25180740Sdes and `?'. Multiple environment variables may be separated by 26180740Sdes whitespace or spread across multiple AcceptEnv directives. Be 27180740Sdes warned that some environment variables could be used to bypass 28180740Sdes restricted user environments. For this reason, care should be 29180740Sdes taken in the use of this directive. The default is not to accept 30180740Sdes any environment variables. 31180740Sdes 32180740Sdes AddressFamily 33180740Sdes Specifies which address family should be used by sshd(8). Valid 34180740Sdes arguments are ``any'', ``inet'' (use IPv4 only), or ``inet6'' 35180740Sdes (use IPv6 only). The default is ``any''. 36180740Sdes 37180750Sdes AllowAgentForwarding 38180750Sdes Specifies whether ssh-agent(1) forwarding is permitted. The de- 39180750Sdes fault is ``yes''. Note that disabling agent forwarding does not 40180750Sdes improve security unless users are also denied shell access, as 41180750Sdes they can always install their own forwarders. 42180750Sdes 43180740Sdes AllowGroups 44180740Sdes This keyword can be followed by a list of group name patterns, 45180740Sdes separated by spaces. If specified, login is allowed only for 46180740Sdes users whose primary group or supplementary group list matches one 47180740Sdes of the patterns. Only group names are valid; a numerical group 48180740Sdes ID is not recognized. By default, login is allowed for all 49180740Sdes groups. The allow/deny directives are processed in the following 50180740Sdes order: DenyUsers, AllowUsers, DenyGroups, and finally 51180740Sdes AllowGroups. 52180740Sdes 53180740Sdes See PATTERNS in ssh_config(5) for more information on patterns. 54180740Sdes 55180740Sdes AllowTcpForwarding 56180740Sdes Specifies whether TCP forwarding is permitted. The default is 57180740Sdes ``yes''. Note that disabling TCP forwarding does not improve se- 58180740Sdes curity unless users are also denied shell access, as they can al- 59180740Sdes ways install their own forwarders. 60180740Sdes 61180740Sdes AllowUsers 62180740Sdes This keyword can be followed by a list of user name patterns, 63180740Sdes separated by spaces. If specified, login is allowed only for us- 64180740Sdes er names that match one of the patterns. Only user names are 65180740Sdes valid; a numerical user ID is not recognized. By default, login 66180740Sdes is allowed for all users. If the pattern takes the form US- 67180740Sdes ER@HOST then USER and HOST are separately checked, restricting 68180740Sdes logins to particular users from particular hosts. The allow/deny 69180740Sdes directives are processed in the following order: DenyUsers, 70180740Sdes AllowUsers, DenyGroups, and finally AllowGroups. 71180740Sdes 72180740Sdes See PATTERNS in ssh_config(5) for more information on patterns. 73180740Sdes 74180740Sdes AuthorizedKeysFile 75180740Sdes Specifies the file that contains the public keys that can be used 76180740Sdes for user authentication. AuthorizedKeysFile may contain tokens 77180740Sdes of the form %T which are substituted during connection setup. 78180740Sdes The following tokens are defined: %% is replaced by a literal 79180740Sdes '%', %h is replaced by the home directory of the user being au- 80180740Sdes thenticated, and %u is replaced by the username of that user. 81180740Sdes After expansion, AuthorizedKeysFile is taken to be an absolute 82180740Sdes path or one relative to the user's home directory. The default 83180740Sdes is ``.ssh/authorized_keys''. 84180740Sdes 85180746Sdes Banner The contents of the specified file are sent to the remote user 86180746Sdes before authentication is allowed. If the argument is ``none'' 87180746Sdes then no banner is displayed. This option is only available for 88180740Sdes protocol version 2. By default, no banner is displayed. 89180740Sdes 90180740Sdes ChallengeResponseAuthentication 91180740Sdes Specifies whether challenge-response authentication is allowed. 92180740Sdes All authentication styles from login.conf(5) are supported. The 93180740Sdes default is ``yes''. 94180740Sdes 95180746Sdes ChrootDirectory 96180746Sdes Specifies a path to chroot(2) to after authentication. This 97180746Sdes path, and all its components, must be root-owned directories that 98180746Sdes are not writable by any other user or group. 99180746Sdes 100180746Sdes The path may contain the following tokens that are expanded at 101180746Sdes runtime once the connecting user has been authenticated: %% is 102180746Sdes replaced by a literal '%', %h is replaced by the home directory 103180746Sdes of the user being authenticated, and %u is replaced by the user- 104180746Sdes name of that user. 105180746Sdes 106180746Sdes The ChrootDirectory must contain the necessary files and directo- 107180746Sdes ries to support the users' session. For an interactive session 108180746Sdes this requires at least a shell, typically sh(1), and basic /dev 109180746Sdes nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), 110180746Sdes arandom(4) and tty(4) devices. For file transfer sessions using 111180746Sdes ``sftp'', no additional configuration of the environment is nec- 112180746Sdes essary if the in-process sftp server is used (see Subsystem for 113180746Sdes details). 114180746Sdes 115180746Sdes The default is not to chroot(2). 116180746Sdes 117180740Sdes Ciphers 118180740Sdes Specifies the ciphers allowed for protocol version 2. Multiple 119180740Sdes ciphers must be comma-separated. The supported ciphers are 120180740Sdes ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'', 121180740Sdes ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour128'', 122180740Sdes ``arcfour256'', ``arcfour'', ``blowfish-cbc'', and 123180740Sdes ``cast128-cbc''. The default is: 124180740Sdes 125180740Sdes aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, 126180740Sdes arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, 127180740Sdes aes192-ctr,aes256-ctr 128180740Sdes 129180740Sdes ClientAliveCountMax 130180740Sdes Sets the number of client alive messages (see below) which may be 131180740Sdes sent without sshd(8) receiving any messages back from the client. 132180740Sdes If this threshold is reached while client alive messages are be- 133180740Sdes ing sent, sshd will disconnect the client, terminating the ses- 134180740Sdes sion. It is important to note that the use of client alive mes- 135180740Sdes sages is very different from TCPKeepAlive (below). The client 136180740Sdes alive messages are sent through the encrypted channel and there- 137180740Sdes fore will not be spoofable. The TCP keepalive option enabled by 138180740Sdes TCPKeepAlive is spoofable. The client alive mechanism is valu- 139180740Sdes able when the client or server depend on knowing when a connec- 140180740Sdes tion has become inactive. 141180740Sdes 142180740Sdes The default value is 3. If ClientAliveInterval (see below) is 143180740Sdes set to 15, and ClientAliveCountMax is left at the default, unre- 144180740Sdes sponsive SSH clients will be disconnected after approximately 45 145180740Sdes seconds. This option applies to protocol version 2 only. 146180740Sdes 147180740Sdes ClientAliveInterval 148180740Sdes Sets a timeout interval in seconds after which if no data has 149180740Sdes been received from the client, sshd(8) will send a message 150180740Sdes through the encrypted channel to request a response from the 151180740Sdes client. The default is 0, indicating that these messages will 152180740Sdes not be sent to the client. This option applies to protocol ver- 153180740Sdes sion 2 only. 154180740Sdes 155180740Sdes Compression 156180740Sdes Specifies whether compression is allowed, or delayed until the 157180740Sdes user has authenticated successfully. The argument must be 158180740Sdes ``yes'', ``delayed'', or ``no''. The default is ``delayed''. 159180740Sdes 160180740Sdes DenyGroups 161180740Sdes This keyword can be followed by a list of group name patterns, 162180740Sdes separated by spaces. Login is disallowed for users whose primary 163180740Sdes group or supplementary group list matches one of the patterns. 164180740Sdes Only group names are valid; a numerical group ID is not recog- 165180740Sdes nized. By default, login is allowed for all groups. The al- 166180740Sdes low/deny directives are processed in the following order: 167180740Sdes DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. 168180740Sdes 169180740Sdes See PATTERNS in ssh_config(5) for more information on patterns. 170180740Sdes 171180740Sdes DenyUsers 172180740Sdes This keyword can be followed by a list of user name patterns, 173180740Sdes separated by spaces. Login is disallowed for user names that 174180740Sdes match one of the patterns. Only user names are valid; a numeri- 175180740Sdes cal user ID is not recognized. By default, login is allowed for 176180740Sdes all users. If the pattern takes the form USER@HOST then USER and 177180740Sdes HOST are separately checked, restricting logins to particular 178180740Sdes users from particular hosts. The allow/deny directives are pro- 179180740Sdes cessed in the following order: DenyUsers, AllowUsers, DenyGroups, 180180740Sdes and finally AllowGroups. 181180740Sdes 182180740Sdes See PATTERNS in ssh_config(5) for more information on patterns. 183180740Sdes 184180740Sdes ForceCommand 185180740Sdes Forces the execution of the command specified by ForceCommand, 186180746Sdes ignoring any command supplied by the client and ~/.ssh/rc if pre- 187180746Sdes sent. The command is invoked by using the user's login shell 188180746Sdes with the -c option. This applies to shell, command, or subsystem 189180746Sdes execution. It is most useful inside a Match block. The command 190180746Sdes originally supplied by the client is available in the 191180746Sdes SSH_ORIGINAL_COMMAND environment variable. Specifying a command 192180746Sdes of ``internal-sftp'' will force the use of an in-process sftp 193180746Sdes server that requires no support files when used with 194180746Sdes ChrootDirectory. 195180740Sdes 196180740Sdes GatewayPorts 197180740Sdes Specifies whether remote hosts are allowed to connect to ports 198180740Sdes forwarded for the client. By default, sshd(8) binds remote port 199180740Sdes forwardings to the loopback address. This prevents other remote 200180740Sdes hosts from connecting to forwarded ports. GatewayPorts can be 201180740Sdes used to specify that sshd should allow remote port forwardings to 202180740Sdes bind to non-loopback addresses, thus allowing other hosts to con- 203180740Sdes nect. The argument may be ``no'' to force remote port forward- 204180740Sdes ings to be available to the local host only, ``yes'' to force re- 205180740Sdes mote port forwardings to bind to the wildcard address, or 206180740Sdes ``clientspecified'' to allow the client to select the address to 207180740Sdes which the forwarding is bound. The default is ``no''. 208180740Sdes 209180740Sdes GSSAPIAuthentication 210180740Sdes Specifies whether user authentication based on GSSAPI is allowed. 211180740Sdes The default is ``no''. Note that this option applies to protocol 212180740Sdes version 2 only. 213180740Sdes 214180740Sdes GSSAPICleanupCredentials 215180740Sdes Specifies whether to automatically destroy the user's credentials 216180740Sdes cache on logout. The default is ``yes''. Note that this option 217180740Sdes applies to protocol version 2 only. 218180740Sdes 219180740Sdes HostbasedAuthentication 220180740Sdes Specifies whether rhosts or /etc/hosts.equiv authentication to- 221180740Sdes gether with successful public key client host authentication is 222180740Sdes allowed (host-based authentication). This option is similar to 223180740Sdes RhostsRSAAuthentication and applies to protocol version 2 only. 224180740Sdes The default is ``no''. 225180740Sdes 226180740Sdes HostbasedUsesNameFromPacketOnly 227180740Sdes Specifies whether or not the server will attempt to perform a re- 228180740Sdes verse name lookup when matching the name in the ~/.shosts, 229180740Sdes ~/.rhosts, and /etc/hosts.equiv files during 230180740Sdes HostbasedAuthentication. A setting of ``yes'' means that sshd(8) 231180740Sdes uses the name supplied by the client rather than attempting to 232180740Sdes resolve the name from the TCP connection itself. The default is 233180740Sdes ``no''. 234180740Sdes 235180740Sdes HostKey 236180740Sdes Specifies a file containing a private host key used by SSH. The 237180740Sdes default is /etc/ssh/ssh_host_key for protocol version 1, and 238180740Sdes /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro- 239180740Sdes tocol version 2. Note that sshd(8) will refuse to use a file if 240180740Sdes it is group/world-accessible. It is possible to have multiple 241180740Sdes host key files. ``rsa1'' keys are used for version 1 and ``dsa'' 242180740Sdes or ``rsa'' are used for version 2 of the SSH protocol. 243180740Sdes 244180740Sdes IgnoreRhosts 245180740Sdes Specifies that .rhosts and .shosts files will not be used in 246180740Sdes RhostsRSAAuthentication or HostbasedAuthentication. 247180740Sdes 248180740Sdes /etc/hosts.equiv and /etc/shosts.equiv are still used. The de- 249180740Sdes fault is ``yes''. 250180740Sdes 251180740Sdes IgnoreUserKnownHosts 252180740Sdes Specifies whether sshd(8) should ignore the user's 253180740Sdes ~/.ssh/known_hosts during RhostsRSAAuthentication or 254180740Sdes HostbasedAuthentication. The default is ``no''. 255180740Sdes 256180740Sdes KerberosAuthentication 257180740Sdes Specifies whether the password provided by the user for 258180740Sdes PasswordAuthentication will be validated through the Kerberos 259180740Sdes KDC. To use this option, the server needs a Kerberos servtab 260180740Sdes which allows the verification of the KDC's identity. The default 261180740Sdes is ``no''. 262180740Sdes 263180740Sdes KerberosGetAFSToken 264180740Sdes If AFS is active and the user has a Kerberos 5 TGT, attempt to 265180740Sdes acquire an AFS token before accessing the user's home directory. 266180740Sdes The default is ``no''. 267180740Sdes 268180740Sdes KerberosOrLocalPasswd 269180740Sdes If password authentication through Kerberos fails then the pass- 270180740Sdes word will be validated via any additional local mechanism such as 271180740Sdes /etc/passwd. The default is ``yes''. 272180740Sdes 273180740Sdes KerberosTicketCleanup 274180740Sdes Specifies whether to automatically destroy the user's ticket 275180740Sdes cache file on logout. The default is ``yes''. 276180740Sdes 277180740Sdes KeyRegenerationInterval 278180740Sdes In protocol version 1, the ephemeral server key is automatically 279180740Sdes regenerated after this many seconds (if it has been used). The 280180740Sdes purpose of regeneration is to prevent decrypting captured ses- 281180740Sdes sions by later breaking into the machine and stealing the keys. 282180740Sdes The key is never stored anywhere. If the value is 0, the key is 283180740Sdes never regenerated. The default is 3600 (seconds). 284180740Sdes 285180740Sdes ListenAddress 286180740Sdes Specifies the local addresses sshd(8) should listen on. The fol- 287180740Sdes lowing forms may be used: 288180740Sdes 289180740Sdes ListenAddress host|IPv4_addr|IPv6_addr 290180740Sdes ListenAddress host|IPv4_addr:port 291180740Sdes ListenAddress [host|IPv6_addr]:port 292180740Sdes 293180740Sdes If port is not specified, sshd will listen on the address and all 294180740Sdes prior Port options specified. The default is to listen on all 295180740Sdes local addresses. Multiple ListenAddress options are permitted. 296180740Sdes Additionally, any Port options must precede this option for non- 297180740Sdes port qualified addresses. 298180740Sdes 299180740Sdes LoginGraceTime 300180740Sdes The server disconnects after this time if the user has not suc- 301180740Sdes cessfully logged in. If the value is 0, there is no time limit. 302180740Sdes The default is 120 seconds. 303180740Sdes 304180740Sdes LogLevel 305180740Sdes Gives the verbosity level that is used when logging messages from 306180740Sdes sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO, 307180740Sdes VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. 308180740Sdes DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify 309180740Sdes higher levels of debugging output. Logging with a DEBUG level 310180740Sdes violates the privacy of users and is not recommended. 311180740Sdes 312180740Sdes MACs Specifies the available MAC (message authentication code) algo- 313180740Sdes rithms. The MAC algorithm is used in protocol version 2 for data 314180740Sdes integrity protection. Multiple algorithms must be comma-separat- 315180744Sdes ed. The default is: 316180740Sdes 317180744Sdes hmac-md5,hmac-sha1,umac-64@openssh.com, 318180744Sdes hmac-ripemd160,hmac-sha1-96,hmac-md5-96 319180744Sdes 320180740Sdes Match Introduces a conditional block. If all of the criteria on the 321180740Sdes Match line are satisfied, the keywords on the following lines 322180740Sdes override those set in the global section of the config file, un- 323180750Sdes til either another Match line or the end of the file. 324180750Sdes 325180750Sdes The arguments to Match are one or more criteria-pattern pairs. 326180750Sdes The available criteria are User, Group, Host, and Address. The 327180750Sdes match patterns may consist of single entries or comma-separated 328180750Sdes lists and may use the wildcard and negation operators described 329180750Sdes in the PATTERNS section of ssh_config(5). 330180750Sdes 331180750Sdes The patterns in an Address criteria may additionally contain ad- 332180750Sdes dresses to match in CIDR address/masklen format, e.g. 333180750Sdes ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask 334180750Sdes length provided must be consistent with the address - it is an 335180750Sdes error to specify a mask length that is too long for the address 336180750Sdes or one with bits set in this host portion of the address. For 337180750Sdes example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively. 338180750Sdes 339180750Sdes Only a subset of keywords may be used on the lines following a 340180750Sdes Match keyword. Available keywords are AllowTcpForwarding, 341180750Sdes Banner, ChrootDirectory, ForceCommand, GatewayPorts, 342180750Sdes GSSAPIAuthentication, HostbasedAuthentication, 343180740Sdes KbdInteractiveAuthentication, KerberosAuthentication, 344180750Sdes MaxAuthTries, MaxSessions, PasswordAuthentication, PermitOpen, 345180750Sdes PermitRootLogin, RhostsRSAAuthentication, RSAAuthentication, 346180750Sdes X11DisplayOffset, X11Forwarding, and X11UseLocalHost. 347180740Sdes 348180740Sdes MaxAuthTries 349180740Sdes Specifies the maximum number of authentication attempts permitted 350180740Sdes per connection. Once the number of failures reaches half this 351180740Sdes value, additional failures are logged. The default is 6. 352180740Sdes 353180750Sdes MaxSessions 354180750Sdes Specifies the maximum number of open sessions permitted per net- 355180750Sdes work connection. The default is 10. 356180750Sdes 357180740Sdes MaxStartups 358180740Sdes Specifies the maximum number of concurrent unauthenticated con- 359180740Sdes nections to the SSH daemon. Additional connections will be 360180740Sdes dropped until authentication succeeds or the LoginGraceTime ex- 361180740Sdes pires for a connection. The default is 10. 362180740Sdes 363180740Sdes Alternatively, random early drop can be enabled by specifying the 364180740Sdes three colon separated values ``start:rate:full'' (e.g. 365180740Sdes "10:30:60"). sshd(8) will refuse connection attempts with a 366180740Sdes probability of ``rate/100'' (30%) if there are currently 367180740Sdes ``start'' (10) unauthenticated connections. The probability in- 368180740Sdes creases linearly and all connection attempts are refused if the 369180740Sdes number of unauthenticated connections reaches ``full'' (60). 370180740Sdes 371180740Sdes PasswordAuthentication 372180740Sdes Specifies whether password authentication is allowed. The de- 373180740Sdes fault is ``yes''. 374180740Sdes 375180740Sdes PermitEmptyPasswords 376180740Sdes When password authentication is allowed, it specifies whether the 377180740Sdes server allows login to accounts with empty password strings. The 378180740Sdes default is ``no''. 379180740Sdes 380180740Sdes PermitOpen 381180740Sdes Specifies the destinations to which TCP port forwarding is per- 382180740Sdes mitted. The forwarding specification must be one of the follow- 383180740Sdes ing forms: 384180740Sdes 385180740Sdes PermitOpen host:port 386180740Sdes PermitOpen IPv4_addr:port 387180740Sdes PermitOpen [IPv6_addr]:port 388180740Sdes 389180740Sdes Multiple forwards may be specified by separating them with 390180740Sdes whitespace. An argument of ``any'' can be used to remove all re- 391180740Sdes strictions and permit any forwarding requests. By default all 392180740Sdes port forwarding requests are permitted. 393180740Sdes 394180740Sdes PermitRootLogin 395180740Sdes Specifies whether root can log in using ssh(1). The argument 396180740Sdes must be ``yes'', ``without-password'', ``forced-commands-only'', 397180740Sdes or ``no''. The default is ``yes''. 398180740Sdes 399180740Sdes If this option is set to ``without-password'', password authenti- 400180740Sdes cation is disabled for root. 401180740Sdes 402180740Sdes If this option is set to ``forced-commands-only'', root login 403180740Sdes with public key authentication will be allowed, but only if the 404180740Sdes command option has been specified (which may be useful for taking 405180740Sdes remote backups even if root login is normally not allowed). All 406180740Sdes other authentication methods are disabled for root. 407180740Sdes 408180740Sdes If this option is set to ``no'', root is not allowed to log in. 409180740Sdes 410180740Sdes PermitTunnel 411180740Sdes Specifies whether tun(4) device forwarding is allowed. The argu- 412180740Sdes ment must be ``yes'', ``point-to-point'' (layer 3), ``ethernet'' 413180740Sdes (layer 2), or ``no''. Specifying ``yes'' permits both ``point- 414180740Sdes to-point'' and ``ethernet''. The default is ``no''. 415180740Sdes 416180740Sdes PermitUserEnvironment 417180740Sdes Specifies whether ~/.ssh/environment and environment= options in 418180740Sdes ~/.ssh/authorized_keys are processed by sshd(8). The default is 419180740Sdes ``no''. Enabling environment processing may enable users to by- 420180740Sdes pass access restrictions in some configurations using mechanisms 421180740Sdes such as LD_PRELOAD. 422180740Sdes 423180740Sdes PidFile 424180740Sdes Specifies the file that contains the process ID of the SSH dae- 425180740Sdes mon. The default is /var/run/sshd.pid. 426180740Sdes 427180740Sdes Port Specifies the port number that sshd(8) listens on. The default 428180740Sdes is 22. Multiple options of this type are permitted. See also 429180740Sdes ListenAddress. 430180740Sdes 431180740Sdes PrintLastLog 432180740Sdes Specifies whether sshd(8) should print the date and time of the 433180740Sdes last user login when a user logs in interactively. The default 434180740Sdes is ``yes''. 435180740Sdes 436180740Sdes PrintMotd 437180740Sdes Specifies whether sshd(8) should print /etc/motd when a user logs 438180740Sdes in interactively. (On some systems it is also printed by the 439180740Sdes shell, /etc/profile, or equivalent.) The default is ``yes''. 440180740Sdes 441180740Sdes Protocol 442180740Sdes Specifies the protocol versions sshd(8) supports. The possible 443180740Sdes values are `1' and `2'. Multiple versions must be comma-separat- 444180740Sdes ed. The default is ``2,1''. Note that the order of the protocol 445180740Sdes list does not indicate preference, because the client selects 446180740Sdes among multiple protocol versions offered by the server. Specify- 447180740Sdes ing ``2,1'' is identical to ``1,2''. 448180740Sdes 449180740Sdes PubkeyAuthentication 450180740Sdes Specifies whether public key authentication is allowed. The de- 451180740Sdes fault is ``yes''. Note that this option applies to protocol ver- 452180740Sdes sion 2 only. 453180740Sdes 454180740Sdes RhostsRSAAuthentication 455180740Sdes Specifies whether rhosts or /etc/hosts.equiv authentication to- 456180740Sdes gether with successful RSA host authentication is allowed. The 457180740Sdes default is ``no''. This option applies to protocol version 1 on- 458180740Sdes ly. 459180740Sdes 460180740Sdes RSAAuthentication 461180740Sdes Specifies whether pure RSA authentication is allowed. The de- 462180740Sdes fault is ``yes''. This option applies to protocol version 1 on- 463180740Sdes ly. 464180740Sdes 465180740Sdes ServerKeyBits 466180740Sdes Defines the number of bits in the ephemeral protocol version 1 467180750Sdes server key. The minimum value is 512, and the default is 1024. 468180740Sdes 469180740Sdes StrictModes 470180740Sdes Specifies whether sshd(8) should check file modes and ownership 471180740Sdes of the user's files and home directory before accepting login. 472180740Sdes This is normally desirable because novices sometimes accidentally 473180740Sdes leave their directory or files world-writable. The default is 474180740Sdes ``yes''. 475180740Sdes 476180740Sdes Subsystem 477180740Sdes Configures an external subsystem (e.g. file transfer daemon). 478180740Sdes Arguments should be a subsystem name and a command (with optional 479180746Sdes arguments) to execute upon subsystem request. 480180746Sdes 481180746Sdes The command sftp-server(8) implements the ``sftp'' file transfer 482180746Sdes subsystem. 483180746Sdes 484180746Sdes Alternately the name ``internal-sftp'' implements an in-process 485180746Sdes ``sftp'' server. This may simplify configurations using 486180746Sdes ChrootDirectory to force a different filesystem root on clients. 487180746Sdes 488180740Sdes By default no subsystems are defined. Note that this option ap- 489180740Sdes plies to protocol version 2 only. 490180740Sdes 491180740Sdes SyslogFacility 492180740Sdes Gives the facility code that is used when logging messages from 493180740Sdes sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0, 494180740Sdes LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The de- 495180740Sdes fault is AUTH. 496180740Sdes 497180740Sdes TCPKeepAlive 498180740Sdes Specifies whether the system should send TCP keepalive messages 499180740Sdes to the other side. If they are sent, death of the connection or 500180740Sdes crash of one of the machines will be properly noticed. However, 501180740Sdes this means that connections will die if the route is down tem- 502180740Sdes porarily, and some people find it annoying. On the other hand, 503180740Sdes if TCP keepalives are not sent, sessions may hang indefinitely on 504180740Sdes the server, leaving ``ghost'' users and consuming server re- 505180740Sdes sources. 506180740Sdes 507180740Sdes The default is ``yes'' (to send TCP keepalive messages), and the 508180740Sdes server will notice if the network goes down or the client host 509180740Sdes crashes. This avoids infinitely hanging sessions. 510180740Sdes 511180740Sdes To disable TCP keepalive messages, the value should be set to 512180740Sdes ``no''. 513180740Sdes 514180740Sdes UseDNS Specifies whether sshd(8) should look up the remote host name and 515180740Sdes check that the resolved host name for the remote IP address maps 516180740Sdes back to the very same IP address. The default is ``yes''. 517180740Sdes 518180740Sdes UseLogin 519180740Sdes Specifies whether login(1) is used for interactive login ses- 520180740Sdes sions. The default is ``no''. Note that login(1) is never used 521180740Sdes for remote command execution. Note also, that if this is en- 522180740Sdes abled, X11Forwarding will be disabled because login(1) does not 523180740Sdes know how to handle xauth(1) cookies. If UsePrivilegeSeparation 524180740Sdes is specified, it will be disabled after authentication. 525180740Sdes 526180740Sdes UsePAM Enables the Pluggable Authentication Module interface. If set to 527180740Sdes ``yes'' this will enable PAM authentication using 528180740Sdes ChallengeResponseAuthentication and PasswordAuthentication in ad- 529180740Sdes dition to PAM account and session module processing for all au- 530180740Sdes thentication types. 531180740Sdes 532180740Sdes Because PAM challenge-response authentication usually serves an 533180740Sdes equivalent role to password authentication, you should disable 534180740Sdes either PasswordAuthentication or ChallengeResponseAuthentication. 535180740Sdes 536180740Sdes If UsePAM is enabled, you will not be able to run sshd(8) as a 537180740Sdes non-root user. The default is ``no''. 538180740Sdes 539180740Sdes UsePrivilegeSeparation 540180740Sdes Specifies whether sshd(8) separates privileges by creating an un- 541180740Sdes privileged child process to deal with incoming network traffic. 542180740Sdes After successful authentication, another process will be created 543180740Sdes that has the privilege of the authenticated user. The goal of 544180740Sdes privilege separation is to prevent privilege escalation by con- 545180740Sdes taining any corruption within the unprivileged processes. The 546180740Sdes default is ``yes''. 547180740Sdes 548180740Sdes X11DisplayOffset 549180740Sdes Specifies the first display number available for sshd(8)'s X11 550180740Sdes forwarding. This prevents sshd from interfering with real X11 551180740Sdes servers. The default is 10. 552180740Sdes 553180740Sdes X11Forwarding 554180740Sdes Specifies whether X11 forwarding is permitted. The argument must 555180740Sdes be ``yes'' or ``no''. The default is ``no''. 556180740Sdes 557180740Sdes When X11 forwarding is enabled, there may be additional exposure 558180740Sdes to the server and to client displays if the sshd(8) proxy display 559180740Sdes is configured to listen on the wildcard address (see 560180740Sdes X11UseLocalhost below), though this is not the default. Addi- 561180740Sdes tionally, the authentication spoofing and authentication data 562180740Sdes verification and substitution occur on the client side. The se- 563180740Sdes curity risk of using X11 forwarding is that the client's X11 dis- 564180740Sdes play server may be exposed to attack when the SSH client requests 565180740Sdes forwarding (see the warnings for ForwardX11 in ssh_config(5)). A 566180740Sdes system administrator may have a stance in which they want to pro- 567180740Sdes tect clients that may expose themselves to attack by unwittingly 568180740Sdes requesting X11 forwarding, which can warrant a ``no'' setting. 569180740Sdes 570180740Sdes Note that disabling X11 forwarding does not prevent users from 571180740Sdes forwarding X11 traffic, as users can always install their own 572180740Sdes forwarders. X11 forwarding is automatically disabled if UseLogin 573180740Sdes is enabled. 574180740Sdes 575180740Sdes X11UseLocalhost 576180740Sdes Specifies whether sshd(8) should bind the X11 forwarding server 577180740Sdes to the loopback address or to the wildcard address. By default, 578180740Sdes sshd binds the forwarding server to the loopback address and sets 579180740Sdes the hostname part of the DISPLAY environment variable to 580180740Sdes ``localhost''. This prevents remote hosts from connecting to the 581180740Sdes proxy display. However, some older X11 clients may not function 582180740Sdes with this configuration. X11UseLocalhost may be set to ``no'' to 583180740Sdes specify that the forwarding server should be bound to the wild- 584180740Sdes card address. The argument must be ``yes'' or ``no''. The de- 585180740Sdes fault is ``yes''. 586180740Sdes 587180740Sdes XAuthLocation 588180740Sdes Specifies the full pathname of the xauth(1) program. The default 589180740Sdes is /usr/X11R6/bin/xauth. 590180740Sdes 591180740SdesTIME FORMATS 592180740Sdes sshd(8) command-line arguments and configuration file options that speci- 593180740Sdes fy time may be expressed using a sequence of the form: time[qualifier], 594180740Sdes where time is a positive integer value and qualifier is one of the fol- 595180740Sdes lowing: 596180740Sdes 597180740Sdes <none> seconds 598180740Sdes s | S seconds 599180740Sdes m | M minutes 600180740Sdes h | H hours 601180740Sdes d | D days 602180740Sdes w | W weeks 603180740Sdes 604180740Sdes Each member of the sequence is added together to calculate the total time 605180740Sdes value. 606180740Sdes 607180740Sdes Time format examples: 608180740Sdes 609180740Sdes 600 600 seconds (10 minutes) 610180740Sdes 10m 10 minutes 611180740Sdes 1h30m 1 hour 30 minutes (90 minutes) 612180740Sdes 613180740SdesFILES 614180740Sdes /etc/ssh/sshd_config 615180740Sdes Contains configuration data for sshd(8). This file should be 616180740Sdes writable by root only, but it is recommended (though not neces- 617180740Sdes sary) that it be world-readable. 618180740Sdes 619180740SdesSEE ALSO 620180740Sdes sshd(8) 621180740Sdes 622180740SdesAUTHORS 623180740Sdes OpenSSH is a derivative of the original and free ssh 1.2.12 release by 624180740Sdes Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 625180740Sdes de Raadt and Dug Song removed many bugs, re-added newer features and cre- 626180740Sdes ated OpenSSH. Markus Friedl contributed the support for SSH protocol 627180740Sdes versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support 628180740Sdes for privilege separation. 629180740Sdes 630180750SdesOpenBSD 4.4 July 2, 2008 10 631