1 SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5) 2 3 NAME 4 sshd_config M-bM-^@M-^S OpenSSH SSH daemon configuration file 5 6 SYNOPSIS 7 /etc/ssh/sshd_config 8 9 DESCRIPTION 10 sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file 11 specified with -f on the command line). The file contains keyword- 12 argument pairs, one per line. Lines starting with M-bM-^@M-^X#M-bM-^@M-^Y and empty lines 13 are interpreted as comments. Arguments may optionally be enclosed in 14 double quotes (") in order to represent arguments containing spaces. 15 16 The possible keywords and their meanings are as follows (note that 17 keywords are case-insensitive and arguments are case-sensitive): 18 19 AcceptEnv 20 Specifies what environment variables sent by the client will be 21 copied into the session's environ(7). See SendEnv in 22 ssh_config(5) for how to configure the client. Note that 23 environment passing is only supported for protocol 2, and that 24 the TERM environment variable is always sent whenever the client 25 requests a pseudo-terminal as it is required by the protocol. 26 Variables are specified by name, which may contain the wildcard 27 characters M-bM-^@M-^X*M-bM-^@M-^Y and M-bM-^@M-^X?M-bM-^@M-^Y. Multiple environment variables may be 28 separated by whitespace or spread across multiple AcceptEnv 29 directives. Be warned that some environment variables could be 30 used to bypass restricted user environments. For this reason, 31 care should be taken in the use of this directive. The default 32 is not to accept any environment variables. 33 34 AddressFamily 35 Specifies which address family should be used by sshd(8). Valid 36 arguments are M-bM-^@M-^\anyM-bM-^@M-^], M-bM-^@M-^\inetM-bM-^@M-^] (use IPv4 only), or M-bM-^@M-^\inet6M-bM-^@M-^] (use IPv6 37 only). The default is M-bM-^@M-^\anyM-bM-^@M-^]. 38 39 AllowAgentForwarding 40 Specifies whether ssh-agent(1) forwarding is permitted. The 41 default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that disabling agent forwarding does not 42 improve security unless users are also denied shell access, as 43 they can always install their own forwarders. 44 45 AllowGroups 46 This keyword can be followed by a list of group name patterns, 47 separated by spaces. If specified, login is allowed only for 48 users whose primary group or supplementary group list matches one 49 of the patterns. Only group names are valid; a numerical group 50 ID is not recognized. By default, login is allowed for all 51 groups. The allow/deny directives are processed in the following 52 order: DenyUsers, AllowUsers, DenyGroups, and finally 53 AllowGroups. 54 55 See PATTERNS in ssh_config(5) for more information on patterns. 56 57 AllowTcpForwarding 58 Specifies whether TCP forwarding is permitted. The available 59 options are M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\allM-bM-^@M-^] to allow TCP forwarding, M-bM-^@M-^\noM-bM-^@M-^] to 60 prevent all TCP forwarding, M-bM-^@M-^\localM-bM-^@M-^] to allow local (from the 61 perspective of ssh(1)) forwarding only or M-bM-^@M-^\remoteM-bM-^@M-^] to allow 62 remote forwarding only. The default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that 63 disabling TCP forwarding does not improve security unless users 64 are also denied shell access, as they can always install their 65 own forwarders. 66 67 AllowStreamLocalForwarding 68 Specifies whether StreamLocal (Unix-domain socket) forwarding is 69 permitted. The available options are M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\allM-bM-^@M-^] to allow 70 StreamLocal forwarding, M-bM-^@M-^\noM-bM-^@M-^] to prevent all StreamLocal 71 forwarding, M-bM-^@M-^\localM-bM-^@M-^] to allow local (from the perspective of 72 ssh(1)) forwarding only or M-bM-^@M-^\remoteM-bM-^@M-^] to allow remote forwarding 73 only. The default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that disabling StreamLocal 74 forwarding does not improve security unless users are also denied 75 shell access, as they can always install their own forwarders. 76 77 AllowUsers 78 This keyword can be followed by a list of user name patterns, 79 separated by spaces. If specified, login is allowed only for 80 user names that match one of the patterns. Only user names are 81 valid; a numerical user ID is not recognized. By default, login 82 is allowed for all users. If the pattern takes the form 83 USER@HOST then USER and HOST are separately checked, restricting 84 logins to particular users from particular hosts. The allow/deny 85 directives are processed in the following order: DenyUsers, 86 AllowUsers, DenyGroups, and finally AllowGroups. 87 88 See PATTERNS in ssh_config(5) for more information on patterns. 89 90 AuthenticationMethods 91 Specifies the authentication methods that must be successfully 92 completed for a user to be granted access. This option must be 93 followed by one or more comma-separated lists of authentication 94 method names. Successful authentication requires completion of 95 every method in at least one of these lists. 96 97 For example, an argument of M-bM-^@M-^\publickey,password 98 publickey,keyboard-interactiveM-bM-^@M-^] would require the user to 99 complete public key authentication, followed by either password 100 or keyboard interactive authentication. Only methods that are 101 next in one or more lists are offered at each stage, so for this 102 example, it would not be possible to attempt password or 103 keyboard-interactive authentication before public key. 104 105 For keyboard interactive authentication it is also possible to 106 restrict authentication to a specific device by appending a colon 107 followed by the device identifier M-bM-^@M-^\bsdauthM-bM-^@M-^], M-bM-^@M-^\pamM-bM-^@M-^], or M-bM-^@M-^\skeyM-bM-^@M-^], 108 depending on the server configuration. For example, 109 M-bM-^@M-^\keyboard-interactive:bsdauthM-bM-^@M-^] would restrict keyboard 110 interactive authentication to the M-bM-^@M-^\bsdauthM-bM-^@M-^] device. 111 112 If the M-bM-^@M-^\publickeyM-bM-^@M-^] method is listed more than once, sshd(8) 113 verifies that keys that have been used successfully are not 114 reused for subsequent authentications. For example, an 115 AuthenticationMethods of M-bM-^@M-^\publickey,publickeyM-bM-^@M-^] will require 116 successful authentication using two different public keys. 117 118 This option is only available for SSH protocol 2 and will yield a 119 fatal error if enabled if protocol 1 is also enabled. Note that 120 each authentication method listed should also be explicitly 121 enabled in the configuration. The default is not to require 122 multiple authentication; successful completion of a single 123 authentication method is sufficient. 124 125 AuthorizedKeysCommand 126 Specifies a program to be used to look up the user's public keys. 127 The program must be owned by root, not writable by group or 128 others and specified by an absolute path. 129 130 Arguments to AuthorizedKeysCommand may be provided using the 131 following tokens, which will be expanded at runtime: %% is 132 replaced by a literal '%', %u is replaced by the username being 133 authenticated, %h is replaced by the home directory of the user 134 being authenticated, %t is replaced with the key type offered for 135 authentication, %f is replaced with the fingerprint of the key, 136 and %k is replaced with the key being offered for authentication. 137 If no arguments are specified then the username of the target 138 user will be supplied. 139 140 The program should produce on standard output zero or more lines 141 of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)). If a 142 key supplied by AuthorizedKeysCommand does not successfully 143 authenticate and authorize the user then public key 144 authentication continues using the usual AuthorizedKeysFile 145 files. By default, no AuthorizedKeysCommand is run. 146 147 AuthorizedKeysCommandUser 148 Specifies the user under whose account the AuthorizedKeysCommand 149 is run. It is recommended to use a dedicated user that has no 150 other role on the host than running authorized keys commands. If 151 AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser 152 is not, then sshd(8) will refuse to start. 153 154 AuthorizedKeysFile 155 Specifies the file that contains the public keys that can be used 156 for user authentication. The format is described in the 157 AUTHORIZED_KEYS FILE FORMAT section of sshd(8). 158 AuthorizedKeysFile may contain tokens of the form %T which are 159 substituted during connection setup. The following tokens are 160 defined: %% is replaced by a literal '%', %h is replaced by the 161 home directory of the user being authenticated, and %u is 162 replaced by the username of that user. After expansion, 163 AuthorizedKeysFile is taken to be an absolute path or one 164 relative to the user's home directory. Multiple files may be 165 listed, separated by whitespace. The default is 166 M-bM-^@M-^\.ssh/authorized_keys .ssh/authorized_keys2M-bM-^@M-^]. 167 168 AuthorizedPrincipalsCommand 169 Specifies a program to be used to generate the list of allowed 170 certificate principals as per AuthorizedPrincipalsFile. The 171 program must be owned by root, not writable by group or others 172 and specified by an absolute path. 173 174 Arguments to AuthorizedPrincipalsCommand may be provided using 175 the following tokens, which will be expanded at runtime: %% is 176 replaced by a literal '%', %u is replaced by the username being 177 authenticated and %h is replaced by the home directory of the 178 user being authenticated. 179 180 The program should produce on standard output zero or more lines 181 of AuthorizedPrincipalsFile output. If either 182 AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is 183 specified, then certificates offered by the client for 184 authentication must contain a principal that is listed. By 185 default, no AuthorizedPrincipalsCommand is run. 186 187 AuthorizedPrincipalsCommandUser 188 Specifies the user under whose account the 189 AuthorizedPrincipalsCommand is run. It is recommended to use a 190 dedicated user that has no other role on the host than running 191 authorized principals commands. If AuthorizedPrincipalsCommand 192 is specified but AuthorizedPrincipalsCommandUser is not, then 193 sshd(8) will refuse to start. 194 195 AuthorizedPrincipalsFile 196 Specifies a file that lists principal names that are accepted for 197 certificate authentication. When using certificates signed by a 198 key listed in TrustedUserCAKeys, this file lists names, one of 199 which must appear in the certificate for it to be accepted for 200 authentication. Names are listed one per line preceded by key 201 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)). 202 Empty lines and comments starting with M-bM-^@M-^X#M-bM-^@M-^Y are ignored. 203 204 AuthorizedPrincipalsFile may contain tokens of the form %T which 205 are substituted during connection setup. The following tokens 206 are defined: %% is replaced by a literal '%', %h is replaced by 207 the home directory of the user being authenticated, and %u is 208 replaced by the username of that user. After expansion, 209 AuthorizedPrincipalsFile is taken to be an absolute path or one 210 relative to the user's home directory. 211 212 The default is M-bM-^@M-^\noneM-bM-^@M-^], i.e. not to use a principals file M-bM-^@M-^S in 213 this case, the username of the user must appear in a 214 certificate's principals list for it to be accepted. Note that 215 AuthorizedPrincipalsFile is only used when authentication 216 proceeds using a CA listed in TrustedUserCAKeys and is not 217 consulted for certification authorities trusted via 218 ~/.ssh/authorized_keys, though the principals= key option offers 219 a similar facility (see sshd(8) for details). 220 221 Banner The contents of the specified file are sent to the remote user 222 before authentication is allowed. If the argument is M-bM-^@M-^\noneM-bM-^@M-^] then 223 no banner is displayed. This option is only available for 224 protocol version 2. By default, no banner is displayed. 225 226 ChallengeResponseAuthentication 227 Specifies whether challenge-response authentication is allowed 228 (e.g. via PAM or through authentication styles supported in 229 login.conf(5)) The default is M-bM-^@M-^\yesM-bM-^@M-^]. 230 231 ChrootDirectory 232 Specifies the pathname of a directory to chroot(2) to after 233 authentication. At session startup sshd(8) checks that all 234 components of the pathname are root-owned directories which are 235 not writable by any other user or group. After the chroot, 236 sshd(8) changes the working directory to the user's home 237 directory. 238 239 The pathname may contain the following tokens that are expanded 240 at runtime once the connecting user has been authenticated: %% is 241 replaced by a literal '%', %h is replaced by the home directory 242 of the user being authenticated, and %u is replaced by the 243 username of that user. 244 245 The ChrootDirectory must contain the necessary files and 246 directories to support the user's session. For an interactive 247 session this requires at least a shell, typically sh(1), and 248 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), 249 stderr(4), and tty(4) devices. For file transfer sessions using 250 M-bM-^@M-^\sftpM-bM-^@M-^], no additional configuration of the environment is 251 necessary if the in-process sftp server is used, though sessions 252 which use logging may require /dev/log inside the chroot 253 directory on some operating systems (see sftp-server(8) for 254 details). 255 256 For safety, it is very important that the directory hierarchy be 257 prevented from modification by other processes on the system 258 (especially those outside the jail). Misconfiguration can lead 259 to unsafe environments which sshd(8) cannot detect. 260 261 The default is not to chroot(2). 262 263 Ciphers 264 Specifies the ciphers allowed for protocol version 2. Multiple 265 ciphers must be comma-separated. If the specified value begins 266 with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified ciphers will be appended 267 to the default set instead of replacing them. 268 269 The supported ciphers are: 270 271 3des-cbc 272 aes128-cbc 273 aes192-cbc 274 aes256-cbc 275 aes128-ctr 276 aes192-ctr 277 aes256-ctr 278 aes128-gcm (a] openssh.com 279 aes256-gcm (a] openssh.com 280 arcfour 281 arcfour128 282 arcfour256 283 blowfish-cbc 284 cast128-cbc 285 chacha20-poly1305 (a] openssh.com 286 287 The default is: 288 289 chacha20-poly1305 (a] openssh.com, 290 aes128-ctr,aes192-ctr,aes256-ctr, 291 aes128-gcm (a] openssh.com,aes256-gcm (a] openssh.com 292 293 The list of available ciphers may also be obtained using the -Q 294 option of ssh(1) with an argument of M-bM-^@M-^\cipherM-bM-^@M-^]. 295 296 ClientAliveCountMax 297 Sets the number of client alive messages (see below) which may be 298 sent without sshd(8) receiving any messages back from the client. 299 If this threshold is reached while client alive messages are 300 being sent, sshd will disconnect the client, terminating the 301 session. It is important to note that the use of client alive 302 messages is very different from TCPKeepAlive (below). The client 303 alive messages are sent through the encrypted channel and 304 therefore will not be spoofable. The TCP keepalive option 305 enabled by TCPKeepAlive is spoofable. The client alive mechanism 306 is valuable when the client or server depend on knowing when a 307 connection has become inactive. 308 309 The default value is 3. If ClientAliveInterval (see below) is 310 set to 15, and ClientAliveCountMax is left at the default, 311 unresponsive SSH clients will be disconnected after approximately 312 45 seconds. This option applies to protocol version 2 only. 313 314 ClientAliveInterval 315 Sets a timeout interval in seconds after which if no data has 316 been received from the client, sshd(8) will send a message 317 through the encrypted channel to request a response from the 318 client. The default is 0, indicating that these messages will 319 not be sent to the client. This option applies to protocol 320 version 2 only. 321 322 Compression 323 Specifies whether compression is allowed, or delayed until the 324 user has authenticated successfully. The argument must be M-bM-^@M-^\yesM-bM-^@M-^], 325 M-bM-^@M-^\delayedM-bM-^@M-^], or M-bM-^@M-^\noM-bM-^@M-^]. The default is M-bM-^@M-^\delayedM-bM-^@M-^]. 326 327 DenyGroups 328 This keyword can be followed by a list of group name patterns, 329 separated by spaces. Login is disallowed for users whose primary 330 group or supplementary group list matches one of the patterns. 331 Only group names are valid; a numerical group ID is not 332 recognized. By default, login is allowed for all groups. The 333 allow/deny directives are processed in the following order: 334 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. 335 336 See PATTERNS in ssh_config(5) for more information on patterns. 337 338 DenyUsers 339 This keyword can be followed by a list of user name patterns, 340 separated by spaces. Login is disallowed for user names that 341 match one of the patterns. Only user names are valid; a 342 numerical user ID is not recognized. By default, login is 343 allowed for all users. If the pattern takes the form USER@HOST 344 then USER and HOST are separately checked, restricting logins to 345 particular users from particular hosts. The allow/deny 346 directives are processed in the following order: DenyUsers, 347 AllowUsers, DenyGroups, and finally AllowGroups. 348 349 See PATTERNS in ssh_config(5) for more information on patterns. 350 351 FingerprintHash 352 Specifies the hash algorithm used when logging key fingerprints. 353 Valid options are: M-bM-^@M-^\md5M-bM-^@M-^] and M-bM-^@M-^\sha256M-bM-^@M-^]. The default is M-bM-^@M-^\sha256M-bM-^@M-^]. 354 355 ForceCommand 356 Forces the execution of the command specified by ForceCommand, 357 ignoring any command supplied by the client and ~/.ssh/rc if 358 present. The command is invoked by using the user's login shell 359 with the -c option. This applies to shell, command, or subsystem 360 execution. It is most useful inside a Match block. The command 361 originally supplied by the client is available in the 362 SSH_ORIGINAL_COMMAND environment variable. Specifying a command 363 of M-bM-^@M-^\internal-sftpM-bM-^@M-^] will force the use of an in-process sftp 364 server that requires no support files when used with 365 ChrootDirectory. 366 367 GatewayPorts 368 Specifies whether remote hosts are allowed to connect to ports 369 forwarded for the client. By default, sshd(8) binds remote port 370 forwardings to the loopback address. This prevents other remote 371 hosts from connecting to forwarded ports. GatewayPorts can be 372 used to specify that sshd should allow remote port forwardings to 373 bind to non-loopback addresses, thus allowing other hosts to 374 connect. The argument may be M-bM-^@M-^\noM-bM-^@M-^] to force remote port 375 forwardings to be available to the local host only, M-bM-^@M-^\yesM-bM-^@M-^] to 376 force remote port forwardings to bind to the wildcard address, or 377 M-bM-^@M-^\clientspecifiedM-bM-^@M-^] to allow the client to select the address to 378 which the forwarding is bound. The default is M-bM-^@M-^\noM-bM-^@M-^]. 379 380 GSSAPIAuthentication 381 Specifies whether user authentication based on GSSAPI is allowed. 382 The default is M-bM-^@M-^\noM-bM-^@M-^]. Note that this option applies to protocol 383 version 2 only. 384 385 GSSAPICleanupCredentials 386 Specifies whether to automatically destroy the user's credentials 387 cache on logout. The default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that this option 388 applies to protocol version 2 only. 389 390 GSSAPIStrictAcceptorCheck 391 Determines whether to be strict about the identity of the GSSAPI 392 acceptor a client authenticates against. If set to M-bM-^@M-^\yesM-bM-^@M-^] then 393 the client must authenticate against the host service on the 394 current hostname. If set to M-bM-^@M-^\noM-bM-^@M-^] then the client may 395 authenticate against any service key stored in the machine's 396 default store. This facility is provided to assist with 397 operation on multi homed machines. The default is M-bM-^@M-^\yesM-bM-^@M-^]. 398 399 HostbasedAcceptedKeyTypes 400 Specifies the key types that will be accepted for hostbased 401 authentication as a comma-separated pattern list. Alternately if 402 the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the 403 specified key types will be appended to the default set instead 404 of replacing them. The default for this option is: 405 406 ecdsa-sha2-nistp256-cert-v01 (a] openssh.com, 407 ecdsa-sha2-nistp384-cert-v01 (a] openssh.com, 408 ecdsa-sha2-nistp521-cert-v01 (a] openssh.com, 409 ssh-ed25519-cert-v01 (a] openssh.com, 410 ssh-rsa-cert-v01 (a] openssh.com, 411 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 412 ssh-ed25519,ssh-rsa 413 414 The -Q option of ssh(1) may be used to list supported key types. 415 416 HostbasedAuthentication 417 Specifies whether rhosts or /etc/hosts.equiv authentication 418 together with successful public key client host authentication is 419 allowed (host-based authentication). This option is similar to 420 RhostsRSAAuthentication and applies to protocol version 2 only. 421 The default is M-bM-^@M-^\noM-bM-^@M-^]. 422 423 HostbasedUsesNameFromPacketOnly 424 Specifies whether or not the server will attempt to perform a 425 reverse name lookup when matching the name in the ~/.shosts, 426 ~/.rhosts, and /etc/hosts.equiv files during 427 HostbasedAuthentication. A setting of M-bM-^@M-^\yesM-bM-^@M-^] means that sshd(8) 428 uses the name supplied by the client rather than attempting to 429 resolve the name from the TCP connection itself. The default is 430 M-bM-^@M-^\noM-bM-^@M-^]. 431 432 HostCertificate 433 Specifies a file containing a public host certificate. The 434 certificate's public key must match a private host key already 435 specified by HostKey. The default behaviour of sshd(8) is not to 436 load any certificates. 437 438 HostKey 439 Specifies a file containing a private host key used by SSH. The 440 default is /etc/ssh/ssh_host_key for protocol version 1, and 441 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key, 442 /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for 443 protocol version 2. 444 445 Note that sshd(8) will refuse to use a file if it is group/world- 446 accessible and that the HostKeyAlgorithms option restricts which 447 of the keys are actually used by sshd(8). 448 449 It is possible to have multiple host key files. M-bM-^@M-^\rsa1M-bM-^@M-^] keys are 450 used for version 1 and M-bM-^@M-^\dsaM-bM-^@M-^], M-bM-^@M-^\ecdsaM-bM-^@M-^], M-bM-^@M-^\ed25519M-bM-^@M-^] or M-bM-^@M-^\rsaM-bM-^@M-^] are 451 used for version 2 of the SSH protocol. It is also possible to 452 specify public host key files instead. In this case operations 453 on the private key will be delegated to an ssh-agent(1). 454 455 HostKeyAgent 456 Identifies the UNIX-domain socket used to communicate with an 457 agent that has access to the private host keys. If 458 M-bM-^@M-^\SSH_AUTH_SOCKM-bM-^@M-^] is specified, the location of the socket will be 459 read from the SSH_AUTH_SOCK environment variable. 460 461 HostKeyAlgorithms 462 Specifies the protocol version 2 host key algorithms that the 463 server offers. The default for this option is: 464 465 ecdsa-sha2-nistp256-cert-v01 (a] openssh.com, 466 ecdsa-sha2-nistp384-cert-v01 (a] openssh.com, 467 ecdsa-sha2-nistp521-cert-v01 (a] openssh.com, 468 ssh-ed25519-cert-v01 (a] openssh.com, 469 ssh-rsa-cert-v01 (a] openssh.com, 470 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 471 ssh-ed25519,ssh-rsa 472 473 The list of available key types may also be obtained using the -Q 474 option of ssh(1) with an argument of M-bM-^@M-^\keyM-bM-^@M-^]. 475 476 IgnoreRhosts 477 Specifies that .rhosts and .shosts files will not be used in 478 RhostsRSAAuthentication or HostbasedAuthentication. 479 480 /etc/hosts.equiv and /etc/shosts.equiv are still used. The 481 default is M-bM-^@M-^\yesM-bM-^@M-^]. 482 483 IgnoreUserKnownHosts 484 Specifies whether sshd(8) should ignore the user's 485 ~/.ssh/known_hosts during RhostsRSAAuthentication or 486 HostbasedAuthentication. The default is M-bM-^@M-^\noM-bM-^@M-^]. 487 488 IPQoS Specifies the IPv4 type-of-service or DSCP class for the 489 connection. Accepted values are M-bM-^@M-^\af11M-bM-^@M-^], M-bM-^@M-^\af12M-bM-^@M-^], M-bM-^@M-^\af13M-bM-^@M-^], M-bM-^@M-^\af21M-bM-^@M-^], 490 M-bM-^@M-^\af22M-bM-^@M-^], M-bM-^@M-^\af23M-bM-^@M-^], M-bM-^@M-^\af31M-bM-^@M-^], M-bM-^@M-^\af32M-bM-^@M-^], M-bM-^@M-^\af33M-bM-^@M-^], M-bM-^@M-^\af41M-bM-^@M-^], M-bM-^@M-^\af42M-bM-^@M-^], M-bM-^@M-^\af43M-bM-^@M-^], 491 M-bM-^@M-^\cs0M-bM-^@M-^], M-bM-^@M-^\cs1M-bM-^@M-^], M-bM-^@M-^\cs2M-bM-^@M-^], M-bM-^@M-^\cs3M-bM-^@M-^], M-bM-^@M-^\cs4M-bM-^@M-^], M-bM-^@M-^\cs5M-bM-^@M-^], M-bM-^@M-^\cs6M-bM-^@M-^], M-bM-^@M-^\cs7M-bM-^@M-^], M-bM-^@M-^\efM-bM-^@M-^], 492 M-bM-^@M-^\lowdelayM-bM-^@M-^], M-bM-^@M-^\throughputM-bM-^@M-^], M-bM-^@M-^\reliabilityM-bM-^@M-^], or a numeric value. 493 This option may take one or two arguments, separated by 494 whitespace. If one argument is specified, it is used as the 495 packet class unconditionally. If two values are specified, the 496 first is automatically selected for interactive sessions and the 497 second for non-interactive sessions. The default is M-bM-^@M-^\lowdelayM-bM-^@M-^] 498 for interactive sessions and M-bM-^@M-^\throughputM-bM-^@M-^] for non-interactive 499 sessions. 500 501 KbdInteractiveAuthentication 502 Specifies whether to allow keyboard-interactive authentication. 503 The argument to this keyword must be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The default 504 is to use whatever value ChallengeResponseAuthentication is set 505 to (by default M-bM-^@M-^\yesM-bM-^@M-^]). 506 507 KerberosAuthentication 508 Specifies whether the password provided by the user for 509 PasswordAuthentication will be validated through the Kerberos 510 KDC. To use this option, the server needs a Kerberos servtab 511 which allows the verification of the KDC's identity. The default 512 is M-bM-^@M-^\noM-bM-^@M-^]. 513 514 KerberosGetAFSToken 515 If AFS is active and the user has a Kerberos 5 TGT, attempt to 516 acquire an AFS token before accessing the user's home directory. 517 The default is M-bM-^@M-^\noM-bM-^@M-^]. 518 519 KerberosOrLocalPasswd 520 If password authentication through Kerberos fails then the 521 password will be validated via any additional local mechanism 522 such as /etc/passwd. The default is M-bM-^@M-^\yesM-bM-^@M-^]. 523 524 KerberosTicketCleanup 525 Specifies whether to automatically destroy the user's ticket 526 cache file on logout. The default is M-bM-^@M-^\yesM-bM-^@M-^]. 527 528 KexAlgorithms 529 Specifies the available KEX (Key Exchange) algorithms. Multiple 530 algorithms must be comma-separated. Alternately if the specified 531 value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified methods 532 will be appended to the default set instead of replacing them. 533 The supported algorithms are: 534 535 curve25519-sha256 (a] libssh.org 536 diffie-hellman-group1-sha1 537 diffie-hellman-group14-sha1 538 diffie-hellman-group-exchange-sha1 539 diffie-hellman-group-exchange-sha256 540 ecdh-sha2-nistp256 541 ecdh-sha2-nistp384 542 ecdh-sha2-nistp521 543 544 The default is: 545 546 curve25519-sha256 (a] libssh.org, 547 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 548 diffie-hellman-group-exchange-sha256, 549 diffie-hellman-group14-sha1 550 551 The list of available key exchange algorithms may also be 552 obtained using the -Q option of ssh(1) with an argument of M-bM-^@M-^\kexM-bM-^@M-^]. 553 554 KeyRegenerationInterval 555 In protocol version 1, the ephemeral server key is automatically 556 regenerated after this many seconds (if it has been used). The 557 purpose of regeneration is to prevent decrypting captured 558 sessions by later breaking into the machine and stealing the 559 keys. The key is never stored anywhere. If the value is 0, the 560 key is never regenerated. The default is 3600 (seconds). 561 562 ListenAddress 563 Specifies the local addresses sshd(8) should listen on. The 564 following forms may be used: 565 566 ListenAddress host|IPv4_addr|IPv6_addr 567 ListenAddress host|IPv4_addr:port 568 ListenAddress [host|IPv6_addr]:port 569 570 If port is not specified, sshd will listen on the address and all 571 Port options specified. The default is to listen on all local 572 addresses. Multiple ListenAddress options are permitted. 573 574 LoginGraceTime 575 The server disconnects after this time if the user has not 576 successfully logged in. If the value is 0, there is no time 577 limit. The default is 120 seconds. 578 579 LogLevel 580 Gives the verbosity level that is used when logging messages from 581 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO, 582 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. 583 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify 584 higher levels of debugging output. Logging with a DEBUG level 585 violates the privacy of users and is not recommended. 586 587 MACs Specifies the available MAC (message authentication code) 588 algorithms. The MAC algorithm is used in protocol version 2 for 589 data integrity protection. Multiple algorithms must be comma- 590 separated. If the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, 591 then the specified algorithms will be appended to the default set 592 instead of replacing them. 593 594 The algorithms that contain M-bM-^@M-^\-etmM-bM-^@M-^] calculate the MAC after 595 encryption (encrypt-then-mac). These are considered safer and 596 their use recommended. The supported MACs are: 597 598 hmac-md5 599 hmac-md5-96 600 hmac-ripemd160 601 hmac-sha1 602 hmac-sha1-96 603 hmac-sha2-256 604 hmac-sha2-512 605 umac-64 (a] openssh.com 606 umac-128 (a] openssh.com 607 hmac-md5-etm (a] openssh.com 608 hmac-md5-96-etm (a] openssh.com 609 hmac-ripemd160-etm (a] openssh.com 610 hmac-sha1-etm (a] openssh.com 611 hmac-sha1-96-etm (a] openssh.com 612 hmac-sha2-256-etm (a] openssh.com 613 hmac-sha2-512-etm (a] openssh.com 614 umac-64-etm (a] openssh.com 615 umac-128-etm (a] openssh.com 616 617 The default is: 618 619 umac-64-etm (a] openssh.com,umac-128-etm (a] openssh.com, 620 hmac-sha2-256-etm (a] openssh.com,hmac-sha2-512-etm (a] openssh.com, 621 umac-64 (a] openssh.com,umac-128 (a] openssh.com, 622 hmac-sha2-256,hmac-sha2-512 623 624 The list of available MAC algorithms may also be obtained using 625 the -Q option of ssh(1) with an argument of M-bM-^@M-^\macM-bM-^@M-^]. 626 627 Match Introduces a conditional block. If all of the criteria on the 628 Match line are satisfied, the keywords on the following lines 629 override those set in the global section of the config file, 630 until either another Match line or the end of the file. If a 631 keyword appears in multiple Match blocks that are satisfied, only 632 the first instance of the keyword is applied. 633 634 The arguments to Match are one or more criteria-pattern pairs or 635 the single token All which matches all criteria. The available 636 criteria are User, Group, Host, LocalAddress, LocalPort, and 637 Address. The match patterns may consist of single entries or 638 comma-separated lists and may use the wildcard and negation 639 operators described in the PATTERNS section of ssh_config(5). 640 641 The patterns in an Address criteria may additionally contain 642 addresses to match in CIDR address/masklen format, e.g. 643 M-bM-^@M-^\192.0.2.0/24M-bM-^@M-^] or M-bM-^@M-^\3ffe:ffff::/32M-bM-^@M-^]. Note that the mask length 644 provided must be consistent with the address - it is an error to 645 specify a mask length that is too long for the address or one 646 with bits set in this host portion of the address. For example, 647 M-bM-^@M-^\192.0.2.0/33M-bM-^@M-^] and M-bM-^@M-^\192.0.2.0/8M-bM-^@M-^] respectively. 648 649 Only a subset of keywords may be used on the lines following a 650 Match keyword. Available keywords are AcceptEnv, 651 AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding, 652 AllowTcpForwarding, AllowUsers, AuthenticationMethods, 653 AuthorizedKeysCommand, AuthorizedKeysCommandUser, 654 AuthorizedKeysFile, AuthorizedPrincipalsFile, Banner, 655 ChrootDirectory, DenyGroups, DenyUsers, ForceCommand, 656 GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes, 657 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS, 658 KbdInteractiveAuthentication, KerberosAuthentication, 659 MaxAuthTries, MaxSessions, PasswordAuthentication, 660 PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY, 661 PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes, 662 PubkeyAuthentication, RekeyLimit, RevokedKeys, 663 RhostsRSAAuthentication, RSAAuthentication, StreamLocalBindMask, 664 StreamLocalBindUnlink, TrustedUserCAKeys, X11DisplayOffset, 665 X11Forwarding and X11UseLocalHost. 666 667 MaxAuthTries 668 Specifies the maximum number of authentication attempts permitted 669 per connection. Once the number of failures reaches half this 670 value, additional failures are logged. The default is 6. 671 672 MaxSessions 673 Specifies the maximum number of open sessions permitted per 674 network connection. The default is 10. 675 676 MaxStartups 677 Specifies the maximum number of concurrent unauthenticated 678 connections to the SSH daemon. Additional connections will be 679 dropped until authentication succeeds or the LoginGraceTime 680 expires for a connection. The default is 10:30:100. 681 682 Alternatively, random early drop can be enabled by specifying the 683 three colon separated values M-bM-^@M-^\start:rate:fullM-bM-^@M-^] (e.g. "10:30:60"). 684 sshd(8) will refuse connection attempts with a probability of 685 M-bM-^@M-^\rate/100M-bM-^@M-^] (30%) if there are currently M-bM-^@M-^\startM-bM-^@M-^] (10) 686 unauthenticated connections. The probability increases linearly 687 and all connection attempts are refused if the number of 688 unauthenticated connections reaches M-bM-^@M-^\fullM-bM-^@M-^] (60). 689 690 PasswordAuthentication 691 Specifies whether password authentication is allowed. The 692 default is M-bM-^@M-^\yesM-bM-^@M-^]. 693 694 PermitEmptyPasswords 695 When password authentication is allowed, it specifies whether the 696 server allows login to accounts with empty password strings. The 697 default is M-bM-^@M-^\noM-bM-^@M-^]. 698 699 PermitOpen 700 Specifies the destinations to which TCP port forwarding is 701 permitted. The forwarding specification must be one of the 702 following forms: 703 704 PermitOpen host:port 705 PermitOpen IPv4_addr:port 706 PermitOpen [IPv6_addr]:port 707 708 Multiple forwards may be specified by separating them with 709 whitespace. An argument of M-bM-^@M-^\anyM-bM-^@M-^] can be used to remove all 710 restrictions and permit any forwarding requests. An argument of 711 M-bM-^@M-^\noneM-bM-^@M-^] can be used to prohibit all forwarding requests. By 712 default all port forwarding requests are permitted. 713 714 PermitRootLogin 715 Specifies whether root can log in using ssh(1). The argument 716 must be M-bM-^@M-^\yesM-bM-^@M-^], M-bM-^@M-^\prohibit-passwordM-bM-^@M-^], M-bM-^@M-^\without-passwordM-bM-^@M-^], 717 M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^], or M-bM-^@M-^\noM-bM-^@M-^]. The default is 718 M-bM-^@M-^\prohibit-passwordM-bM-^@M-^]. 719 720 If this option is set to M-bM-^@M-^\prohibit-passwordM-bM-^@M-^] or 721 M-bM-^@M-^\without-passwordM-bM-^@M-^], password and keyboard-interactive 722 authentication are disabled for root. 723 724 If this option is set to M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^], root login with 725 public key authentication will be allowed, but only if the 726 command option has been specified (which may be useful for taking 727 remote backups even if root login is normally not allowed). All 728 other authentication methods are disabled for root. 729 730 If this option is set to M-bM-^@M-^\noM-bM-^@M-^], root is not allowed to log in. 731 732 PermitTunnel 733 Specifies whether tun(4) device forwarding is allowed. The 734 argument must be M-bM-^@M-^\yesM-bM-^@M-^], M-bM-^@M-^\point-to-pointM-bM-^@M-^] (layer 3), M-bM-^@M-^\ethernetM-bM-^@M-^] 735 (layer 2), or M-bM-^@M-^\noM-bM-^@M-^]. Specifying M-bM-^@M-^\yesM-bM-^@M-^] permits both 736 M-bM-^@M-^\point-to-pointM-bM-^@M-^] and M-bM-^@M-^\ethernetM-bM-^@M-^]. The default is M-bM-^@M-^\noM-bM-^@M-^]. 737 738 Independent of this setting, the permissions of the selected 739 tun(4) device must allow access to the user. 740 741 PermitTTY 742 Specifies whether pty(4) allocation is permitted. The default is 743 M-bM-^@M-^\yesM-bM-^@M-^]. 744 745 PermitUserEnvironment 746 Specifies whether ~/.ssh/environment and environment= options in 747 ~/.ssh/authorized_keys are processed by sshd(8). The default is 748 M-bM-^@M-^\noM-bM-^@M-^]. Enabling environment processing may enable users to bypass 749 access restrictions in some configurations using mechanisms such 750 as LD_PRELOAD. 751 752 PermitUserRC 753 Specifies whether any ~/.ssh/rc file is executed. The default is 754 M-bM-^@M-^\yesM-bM-^@M-^]. 755 756 PidFile 757 Specifies the file that contains the process ID of the SSH 758 daemon, or M-bM-^@M-^\noneM-bM-^@M-^] to not write one. The default is 759 /var/run/sshd.pid. 760 761 Port Specifies the port number that sshd(8) listens on. The default 762 is 22. Multiple options of this type are permitted. See also 763 ListenAddress. 764 765 PrintLastLog 766 Specifies whether sshd(8) should print the date and time of the 767 last user login when a user logs in interactively. The default 768 is M-bM-^@M-^\yesM-bM-^@M-^]. 769 770 PrintMotd 771 Specifies whether sshd(8) should print /etc/motd when a user logs 772 in interactively. (On some systems it is also printed by the 773 shell, /etc/profile, or equivalent.) The default is M-bM-^@M-^\yesM-bM-^@M-^]. 774 775 Protocol 776 Specifies the protocol versions sshd(8) supports. The possible 777 values are M-bM-^@M-^X1M-bM-^@M-^Y and M-bM-^@M-^X2M-bM-^@M-^Y. Multiple versions must be comma- 778 separated. The default is M-bM-^@M-^X2M-bM-^@M-^Y. Note that the order of the 779 protocol list does not indicate preference, because the client 780 selects among multiple protocol versions offered by the server. 781 Specifying M-bM-^@M-^\2,1M-bM-^@M-^] is identical to M-bM-^@M-^\1,2M-bM-^@M-^]. 782 783 PubkeyAcceptedKeyTypes 784 Specifies the key types that will be accepted for public key 785 authentication as a comma-separated pattern list. Alternately if 786 the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the 787 specified key types will be appended to the default set instead 788 of replacing them. The default for this option is: 789 790 ecdsa-sha2-nistp256-cert-v01 (a] openssh.com, 791 ecdsa-sha2-nistp384-cert-v01 (a] openssh.com, 792 ecdsa-sha2-nistp521-cert-v01 (a] openssh.com, 793 ssh-ed25519-cert-v01 (a] openssh.com, 794 ssh-rsa-cert-v01 (a] openssh.com, 795 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 796 ssh-ed25519,ssh-rsa 797 798 The -Q option of ssh(1) may be used to list supported key types. 799 800 PubkeyAuthentication 801 Specifies whether public key authentication is allowed. The 802 default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that this option applies to protocol 803 version 2 only. 804 805 RekeyLimit 806 Specifies the maximum amount of data that may be transmitted 807 before the session key is renegotiated, optionally followed a 808 maximum amount of time that may pass before the session key is 809 renegotiated. The first argument is specified in bytes and may 810 have a suffix of M-bM-^@M-^XKM-bM-^@M-^Y, M-bM-^@M-^XMM-bM-^@M-^Y, or M-bM-^@M-^XGM-bM-^@M-^Y to indicate Kilobytes, 811 Megabytes, or Gigabytes, respectively. The default is between 812 M-bM-^@M-^X1GM-bM-^@M-^Y and M-bM-^@M-^X4GM-bM-^@M-^Y, depending on the cipher. The optional second 813 value is specified in seconds and may use any of the units 814 documented in the TIME FORMATS section. The default value for 815 RekeyLimit is M-bM-^@M-^\default noneM-bM-^@M-^], which means that rekeying is 816 performed after the cipher's default amount of data has been sent 817 or received and no time based rekeying is done. This option 818 applies to protocol version 2 only. 819 820 RevokedKeys 821 Specifies revoked public keys file, or M-bM-^@M-^\noneM-bM-^@M-^] to not use one. 822 Keys listed in this file will be refused for public key 823 authentication. Note that if this file is not readable, then 824 public key authentication will be refused for all users. Keys 825 may be specified as a text file, listing one public key per line, 826 or as an OpenSSH Key Revocation List (KRL) as generated by 827 ssh-keygen(1). For more information on KRLs, see the KEY 828 REVOCATION LISTS section in ssh-keygen(1). 829 830 RhostsRSAAuthentication 831 Specifies whether rhosts or /etc/hosts.equiv authentication 832 together with successful RSA host authentication is allowed. The 833 default is M-bM-^@M-^\noM-bM-^@M-^]. This option applies to protocol version 1 only. 834 835 RSAAuthentication 836 Specifies whether pure RSA authentication is allowed. The 837 default is M-bM-^@M-^\yesM-bM-^@M-^]. This option applies to protocol version 1 838 only. 839 840 ServerKeyBits 841 Defines the number of bits in the ephemeral protocol version 1 842 server key. The default and minimum value is 1024. 843 844 StreamLocalBindMask 845 Sets the octal file creation mode mask (umask) used when creating 846 a Unix-domain socket file for local or remote port forwarding. 847 This option is only used for port forwarding to a Unix-domain 848 socket file. 849 850 The default value is 0177, which creates a Unix-domain socket 851 file that is readable and writable only by the owner. Note that 852 not all operating systems honor the file mode on Unix-domain 853 socket files. 854 855 StreamLocalBindUnlink 856 Specifies whether to remove an existing Unix-domain socket file 857 for local or remote port forwarding before creating a new one. 858 If the socket file already exists and StreamLocalBindUnlink is 859 not enabled, sshd will be unable to forward the port to the Unix- 860 domain socket file. This option is only used for port forwarding 861 to a Unix-domain socket file. 862 863 The argument must be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The default is M-bM-^@M-^\noM-bM-^@M-^]. 864 865 StrictModes 866 Specifies whether sshd(8) should check file modes and ownership 867 of the user's files and home directory before accepting login. 868 This is normally desirable because novices sometimes accidentally 869 leave their directory or files world-writable. The default is 870 M-bM-^@M-^\yesM-bM-^@M-^]. Note that this does not apply to ChrootDirectory, whose 871 permissions and ownership are checked unconditionally. 872 873 Subsystem 874 Configures an external subsystem (e.g. file transfer daemon). 875 Arguments should be a subsystem name and a command (with optional 876 arguments) to execute upon subsystem request. 877 878 The command sftp-server(8) implements the M-bM-^@M-^\sftpM-bM-^@M-^] file transfer 879 subsystem. 880 881 Alternately the name M-bM-^@M-^\internal-sftpM-bM-^@M-^] implements an in-process 882 M-bM-^@M-^\sftpM-bM-^@M-^] server. This may simplify configurations using 883 ChrootDirectory to force a different filesystem root on clients. 884 885 By default no subsystems are defined. Note that this option 886 applies to protocol version 2 only. 887 888 SyslogFacility 889 Gives the facility code that is used when logging messages from 890 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0, 891 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The 892 default is AUTH. 893 894 TCPKeepAlive 895 Specifies whether the system should send TCP keepalive messages 896 to the other side. If they are sent, death of the connection or 897 crash of one of the machines will be properly noticed. However, 898 this means that connections will die if the route is down 899 temporarily, and some people find it annoying. On the other 900 hand, if TCP keepalives are not sent, sessions may hang 901 indefinitely on the server, leaving M-bM-^@M-^\ghostM-bM-^@M-^] users and consuming 902 server resources. 903 904 The default is M-bM-^@M-^\yesM-bM-^@M-^] (to send TCP keepalive messages), and the 905 server will notice if the network goes down or the client host 906 crashes. This avoids infinitely hanging sessions. 907 908 To disable TCP keepalive messages, the value should be set to 909 M-bM-^@M-^\noM-bM-^@M-^]. 910 911 TrustedUserCAKeys 912 Specifies a file containing public keys of certificate 913 authorities that are trusted to sign user certificates for 914 authentication, or M-bM-^@M-^\noneM-bM-^@M-^] to not use one. Keys are listed one 915 per line; empty lines and comments starting with M-bM-^@M-^X#M-bM-^@M-^Y are allowed. 916 If a certificate is presented for authentication and has its 917 signing CA key listed in this file, then it may be used for 918 authentication for any user listed in the certificate's 919 principals list. Note that certificates that lack a list of 920 principals will not be permitted for authentication using 921 TrustedUserCAKeys. For more details on certificates, see the 922 CERTIFICATES section in ssh-keygen(1). 923 924 UseDNS Specifies whether sshd(8) should look up the remote host name, 925 and to check that the resolved host name for the remote IP 926 address maps back to the very same IP address. 927 928 If this option is set to M-bM-^@M-^\noM-bM-^@M-^] (the default) then only addresses 929 and not host names may be used in ~/.ssh/known_hosts from and 930 sshd_config Match Host directives. 931 932 UseLogin 933 Specifies whether login(1) is used for interactive login 934 sessions. The default is M-bM-^@M-^\noM-bM-^@M-^]. Note that login(1) is never used 935 for remote command execution. Note also, that if this is 936 enabled, X11Forwarding will be disabled because login(1) does not 937 know how to handle xauth(1) cookies. If UsePrivilegeSeparation 938 is specified, it will be disabled after authentication. 939 940 UsePAM Enables the Pluggable Authentication Module interface. If set to 941 M-bM-^@M-^\yesM-bM-^@M-^] this will enable PAM authentication using 942 ChallengeResponseAuthentication and PasswordAuthentication in 943 addition to PAM account and session module processing for all 944 authentication types. 945 946 Because PAM challenge-response authentication usually serves an 947 equivalent role to password authentication, you should disable 948 either PasswordAuthentication or ChallengeResponseAuthentication. 949 950 If UsePAM is enabled, you will not be able to run sshd(8) as a 951 non-root user. The default is M-bM-^@M-^\noM-bM-^@M-^]. 952 953 UsePrivilegeSeparation 954 Specifies whether sshd(8) separates privileges by creating an 955 unprivileged child process to deal with incoming network traffic. 956 After successful authentication, another process will be created 957 that has the privilege of the authenticated user. The goal of 958 privilege separation is to prevent privilege escalation by 959 containing any corruption within the unprivileged processes. The 960 default is M-bM-^@M-^\yesM-bM-^@M-^]. If UsePrivilegeSeparation is set to M-bM-^@M-^\sandboxM-bM-^@M-^] 961 then the pre-authentication unprivileged process is subject to 962 additional restrictions. 963 964 VersionAddendum 965 Optionally specifies additional text to append to the SSH 966 protocol banner sent by the server upon connection. The default 967 is M-bM-^@M-^\noneM-bM-^@M-^]. 968 969 X11DisplayOffset 970 Specifies the first display number available for sshd(8)'s X11 971 forwarding. This prevents sshd from interfering with real X11 972 servers. The default is 10. 973 974 X11Forwarding 975 Specifies whether X11 forwarding is permitted. The argument must 976 be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The default is M-bM-^@M-^\noM-bM-^@M-^]. 977 978 When X11 forwarding is enabled, there may be additional exposure 979 to the server and to client displays if the sshd(8) proxy display 980 is configured to listen on the wildcard address (see 981 X11UseLocalhost below), though this is not the default. 982 Additionally, the authentication spoofing and authentication data 983 verification and substitution occur on the client side. The 984 security risk of using X11 forwarding is that the client's X11 985 display server may be exposed to attack when the SSH client 986 requests forwarding (see the warnings for ForwardX11 in 987 ssh_config(5)). A system administrator may have a stance in 988 which they want to protect clients that may expose themselves to 989 attack by unwittingly requesting X11 forwarding, which can 990 warrant a M-bM-^@M-^\noM-bM-^@M-^] setting. 991 992 Note that disabling X11 forwarding does not prevent users from 993 forwarding X11 traffic, as users can always install their own 994 forwarders. X11 forwarding is automatically disabled if UseLogin 995 is enabled. 996 997 X11UseLocalhost 998 Specifies whether sshd(8) should bind the X11 forwarding server 999 to the loopback address or to the wildcard address. By default, 1000 sshd binds the forwarding server to the loopback address and sets 1001 the hostname part of the DISPLAY environment variable to 1002 M-bM-^@M-^\localhostM-bM-^@M-^]. This prevents remote hosts from connecting to the 1003 proxy display. However, some older X11 clients may not function 1004 with this configuration. X11UseLocalhost may be set to M-bM-^@M-^\noM-bM-^@M-^] to 1005 specify that the forwarding server should be bound to the 1006 wildcard address. The argument must be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The 1007 default is M-bM-^@M-^\yesM-bM-^@M-^]. 1008 1009 XAuthLocation 1010 Specifies the full pathname of the xauth(1) program, or M-bM-^@M-^\noneM-bM-^@M-^] to 1011 not use one. The default is /usr/X11R6/bin/xauth. 1012 1013 TIME FORMATS 1014 sshd(8) command-line arguments and configuration file options that 1015 specify time may be expressed using a sequence of the form: 1016 time[qualifier], where time is a positive integer value and qualifier is 1017 one of the following: 1018 1019 M-bM-^_M-(noneM-bM-^_M-) seconds 1020 s | S seconds 1021 m | M minutes 1022 h | H hours 1023 d | D days 1024 w | W weeks 1025 1026 Each member of the sequence is added together to calculate the total time 1027 value. 1028 1029 Time format examples: 1030 1031 600 600 seconds (10 minutes) 1032 10m 10 minutes 1033 1h30m 1 hour 30 minutes (90 minutes) 1034 1035 FILES 1036 /etc/ssh/sshd_config 1037 Contains configuration data for sshd(8). This file should be 1038 writable by root only, but it is recommended (though not 1039 necessary) that it be world-readable. 1040 1041 SEE ALSO 1042 sshd(8) 1043 1044 AUTHORS 1045 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 1046 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 1047 de Raadt and Dug Song removed many bugs, re-added newer features and 1048 created OpenSSH. Markus Friedl contributed the support for SSH protocol 1049 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support 1050 for privilege separation. 1051 1052 OpenBSD 5.8 August 14, 2015 OpenBSD 5.8 1053
