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. Variables 24 are specified by name, which may contain the wildcard characters 25 M-bM-^@M-^X*M-bM-^@M-^Y and M-bM-^@M-^X?M-bM-^@M-^Y. Multiple environment variables may be separated by 26 whitespace or spread across multiple AcceptEnv directives. Be 27 warned that some environment variables could be used to bypass 28 restricted user environments. For this reason, care should be 29 taken in the use of this directive. The default is not to accept 30 any environment variables. 31 32 AddressFamily 33 Specifies which address family should be used by sshd(8). Valid 34 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 35 only). The default is M-bM-^@M-^\anyM-bM-^@M-^]. 36 37 AllowAgentForwarding 38 Specifies whether ssh-agent(1) forwarding is permitted. The 39 default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that disabling agent forwarding does not 40 improve security unless users are also denied shell access, as 41 they can always install their own forwarders. 42 43 AllowGroups 44 This keyword can be followed by a list of group name patterns, 45 separated by spaces. If specified, login is allowed only for 46 users whose primary group or supplementary group list matches one 47 of the patterns. Only group names are valid; a numerical group 48 ID is not recognized. By default, login is allowed for all 49 groups. The allow/deny directives are processed in the following 50 order: DenyUsers, AllowUsers, DenyGroups, and finally 51 AllowGroups. 52 53 See PATTERNS in ssh_config(5) for more information on patterns. 54 55 AllowTcpForwarding 56 Specifies whether TCP forwarding is permitted. The available 57 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 58 prevent all TCP forwarding, M-bM-^@M-^\localM-bM-^@M-^] to allow local (from the 59 perspective of ssh(1)) forwarding only or M-bM-^@M-^\remoteM-bM-^@M-^] to allow 60 remote forwarding only. The default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that 61 disabling TCP forwarding does not improve security unless users 62 are also denied shell access, as they can always install their 63 own forwarders. 64 65 AllowStreamLocalForwarding 66 Specifies whether StreamLocal (Unix-domain socket) forwarding is 67 permitted. The available options are M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\allM-bM-^@M-^] to allow 68 StreamLocal forwarding, M-bM-^@M-^\noM-bM-^@M-^] to prevent all StreamLocal 69 forwarding, M-bM-^@M-^\localM-bM-^@M-^] to allow local (from the perspective of 70 ssh(1)) forwarding only or M-bM-^@M-^\remoteM-bM-^@M-^] to allow remote forwarding 71 only. The default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that disabling StreamLocal 72 forwarding does not improve security unless users are also denied 73 shell access, as they can always install their own forwarders. 74 75 AllowUsers 76 This keyword can be followed by a list of user name patterns, 77 separated by spaces. If specified, login is allowed only for 78 user names that match one of the patterns. Only user names are 79 valid; a numerical user ID is not recognized. By default, login 80 is allowed for all users. If the pattern takes the form 81 USER@HOST then USER and HOST are separately checked, restricting 82 logins to particular users from particular hosts. The allow/deny 83 directives are processed in the following order: DenyUsers, 84 AllowUsers, DenyGroups, and finally AllowGroups. 85 86 See PATTERNS in ssh_config(5) for more information on patterns. 87 88 AuthenticationMethods 89 Specifies the authentication methods that must be successfully 90 completed for a user to be granted access. This option must be 91 followed by one or more comma-separated lists of authentication 92 method names. Successful authentication requires completion of 93 every method in at least one of these lists. 94 95 For example, an argument of M-bM-^@M-^\publickey,password 96 publickey,keyboard-interactiveM-bM-^@M-^] would require the user to 97 complete public key authentication, followed by either password 98 or keyboard interactive authentication. Only methods that are 99 next in one or more lists are offered at each stage, so for this 100 example, it would not be possible to attempt password or 101 keyboard-interactive authentication before public key. 102 103 For keyboard interactive authentication it is also possible to 104 restrict authentication to a specific device by appending a colon 105 followed by the device identifier M-bM-^@M-^\bsdauthM-bM-^@M-^], M-bM-^@M-^\pamM-bM-^@M-^], or M-bM-^@M-^\skeyM-bM-^@M-^], 106 depending on the server configuration. For example, 107 M-bM-^@M-^\keyboard-interactive:bsdauthM-bM-^@M-^] would restrict keyboard 108 interactive authentication to the M-bM-^@M-^\bsdauthM-bM-^@M-^] device. 109 110 If the M-bM-^@M-^\publickeyM-bM-^@M-^] method is listed more than once, sshd(8) 111 verifies that keys that have been used successfully are not 112 reused for subsequent authentications. For example, an 113 AuthenticationMethods of M-bM-^@M-^\publickey,publickeyM-bM-^@M-^] will require 114 successful authentication using two different public keys. 115 116 This option is only available for SSH protocol 2 and will yield a 117 fatal error if enabled if protocol 1 is also enabled. Note that 118 each authentication method listed should also be explicitly 119 enabled in the configuration. The default is not to require 120 multiple authentication; successful completion of a single 121 authentication method is sufficient. 122 123 AuthorizedKeysCommand 124 Specifies a program to be used to look up the user's public keys. 125 The program must be owned by root and not writable by group or 126 others. It will be invoked with a single argument of the 127 username being authenticated, and should produce on standard 128 output zero or more lines of authorized_keys output (see 129 AUTHORIZED_KEYS in sshd(8)). If a key supplied by 130 AuthorizedKeysCommand does not successfully authenticate and 131 authorize the user then public key authentication continues using 132 the usual AuthorizedKeysFile files. By default, no 133 AuthorizedKeysCommand is run. 134 135 AuthorizedKeysCommandUser 136 Specifies the user under whose account the AuthorizedKeysCommand 137 is run. It is recommended to use a dedicated user that has no 138 other role on the host than running authorized keys commands. If 139 AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser 140 is not, then sshd(8) will refuse to start. 141 142 AuthorizedKeysFile 143 Specifies the file that contains the public keys that can be used 144 for user authentication. The format is described in the 145 AUTHORIZED_KEYS FILE FORMAT section of sshd(8). 146 AuthorizedKeysFile may contain tokens of the form %T which are 147 substituted during connection setup. The following tokens are 148 defined: %% is replaced by a literal '%', %h is replaced by the 149 home directory of the user being authenticated, and %u is 150 replaced by the username of that user. After expansion, 151 AuthorizedKeysFile is taken to be an absolute path or one 152 relative to the user's home directory. Multiple files may be 153 listed, separated by whitespace. The default is 154 M-bM-^@M-^\.ssh/authorized_keys .ssh/authorized_keys2M-bM-^@M-^]. 155 156 AuthorizedPrincipalsFile 157 Specifies a file that lists principal names that are accepted for 158 certificate authentication. When using certificates signed by a 159 key listed in TrustedUserCAKeys, this file lists names, one of 160 which must appear in the certificate for it to be accepted for 161 authentication. Names are listed one per line preceded by key 162 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)). 163 Empty lines and comments starting with M-bM-^@M-^X#M-bM-^@M-^Y are ignored. 164 165 AuthorizedPrincipalsFile may contain tokens of the form %T which 166 are substituted during connection setup. The following tokens 167 are defined: %% is replaced by a literal '%', %h is replaced by 168 the home directory of the user being authenticated, and %u is 169 replaced by the username of that user. After expansion, 170 AuthorizedPrincipalsFile is taken to be an absolute path or one 171 relative to the user's home directory. 172 173 The default is M-bM-^@M-^\noneM-bM-^@M-^], i.e. not to use a principals file M-bM-^@M-^S in 174 this case, the username of the user must appear in a 175 certificate's principals list for it to be accepted. Note that 176 AuthorizedPrincipalsFile is only used when authentication 177 proceeds using a CA listed in TrustedUserCAKeys and is not 178 consulted for certification authorities trusted via 179 ~/.ssh/authorized_keys, though the principals= key option offers 180 a similar facility (see sshd(8) for details). 181 182 Banner The contents of the specified file are sent to the remote user 183 before authentication is allowed. If the argument is M-bM-^@M-^\noneM-bM-^@M-^] then 184 no banner is displayed. This option is only available for 185 protocol version 2. By default, no banner is displayed. 186 187 ChallengeResponseAuthentication 188 Specifies whether challenge-response authentication is allowed 189 (e.g. via PAM or through authentication styles supported in 190 login.conf(5)) The default is M-bM-^@M-^\yesM-bM-^@M-^]. 191 192 ChrootDirectory 193 Specifies the pathname of a directory to chroot(2) to after 194 authentication. At session startup sshd(8) checks that all 195 components of the pathname are root-owned directories which are 196 not writable by any other user or group. After the chroot, 197 sshd(8) changes the working directory to the user's home 198 directory. 199 200 The pathname may contain the following tokens that are expanded 201 at runtime once the connecting user has been authenticated: %% is 202 replaced by a literal '%', %h is replaced by the home directory 203 of the user being authenticated, and %u is replaced by the 204 username of that user. 205 206 The ChrootDirectory must contain the necessary files and 207 directories to support the user's session. For an interactive 208 session this requires at least a shell, typically sh(1), and 209 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), 210 stderr(4), and tty(4) devices. For file transfer sessions using 211 M-bM-^@M-^\sftpM-bM-^@M-^], no additional configuration of the environment is 212 necessary if the in-process sftp server is used, though sessions 213 which use logging may require /dev/log inside the chroot 214 directory on some operating systems (see sftp-server(8) for 215 details). 216 217 For safety, it is very important that the directory hierarchy be 218 prevented from modification by other processes on the system 219 (especially those outside the jail). Misconfiguration can lead 220 to unsafe environments which sshd(8) cannot detect. 221 222 The default is not to chroot(2). 223 224 Ciphers 225 Specifies the ciphers allowed for protocol version 2. Multiple 226 ciphers must be comma-separated. The supported ciphers are: 227 228 3des-cbc 229 aes128-cbc 230 aes192-cbc 231 aes256-cbc 232 aes128-ctr 233 aes192-ctr 234 aes256-ctr 235 aes128-gcm (a] openssh.com 236 aes256-gcm (a] openssh.com 237 arcfour 238 arcfour128 239 arcfour256 240 blowfish-cbc 241 cast128-cbc 242 chacha20-poly1305 (a] openssh.com 243 244 The default is: 245 246 aes128-ctr,aes192-ctr,aes256-ctr, 247 aes128-gcm (a] openssh.com,aes256-gcm (a] openssh.com, 248 chacha20-poly1305 (a] openssh.com 249 250 The list of available ciphers may also be obtained using the -Q 251 option of ssh(1) with an argument of M-bM-^@M-^\cipherM-bM-^@M-^]. 252 253 ClientAliveCountMax 254 Sets the number of client alive messages (see below) which may be 255 sent without sshd(8) receiving any messages back from the client. 256 If this threshold is reached while client alive messages are 257 being sent, sshd will disconnect the client, terminating the 258 session. It is important to note that the use of client alive 259 messages is very different from TCPKeepAlive (below). The client 260 alive messages are sent through the encrypted channel and 261 therefore will not be spoofable. The TCP keepalive option 262 enabled by TCPKeepAlive is spoofable. The client alive mechanism 263 is valuable when the client or server depend on knowing when a 264 connection has become inactive. 265 266 The default value is 3. If ClientAliveInterval (see below) is 267 set to 15, and ClientAliveCountMax is left at the default, 268 unresponsive SSH clients will be disconnected after approximately 269 45 seconds. This option applies to protocol version 2 only. 270 271 ClientAliveInterval 272 Sets a timeout interval in seconds after which if no data has 273 been received from the client, sshd(8) will send a message 274 through the encrypted channel to request a response from the 275 client. The default is 0, indicating that these messages will 276 not be sent to the client. This option applies to protocol 277 version 2 only. 278 279 Compression 280 Specifies whether compression is allowed, or delayed until the 281 user has authenticated successfully. The argument must be M-bM-^@M-^\yesM-bM-^@M-^], 282 M-bM-^@M-^\delayedM-bM-^@M-^], or M-bM-^@M-^\noM-bM-^@M-^]. The default is M-bM-^@M-^\delayedM-bM-^@M-^]. 283 284 DenyGroups 285 This keyword can be followed by a list of group name patterns, 286 separated by spaces. Login is disallowed for users whose primary 287 group or supplementary group list matches one of the patterns. 288 Only group names are valid; a numerical group ID is not 289 recognized. By default, login is allowed for all groups. The 290 allow/deny directives are processed in the following order: 291 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. 292 293 See PATTERNS in ssh_config(5) for more information on patterns. 294 295 DenyUsers 296 This keyword can be followed by a list of user name patterns, 297 separated by spaces. Login is disallowed for user names that 298 match one of the patterns. Only user names are valid; a 299 numerical user ID is not recognized. By default, login is 300 allowed for all users. If the pattern takes the form USER@HOST 301 then USER and HOST are separately checked, restricting logins to 302 particular users from particular hosts. The allow/deny 303 directives are processed in the following order: DenyUsers, 304 AllowUsers, DenyGroups, and finally AllowGroups. 305 306 See PATTERNS in ssh_config(5) for more information on patterns. 307 308 FingerprintHash 309 Specifies the hash algorithm used when logging key fingerprints. 310 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-^]. 311 312 ForceCommand 313 Forces the execution of the command specified by ForceCommand, 314 ignoring any command supplied by the client and ~/.ssh/rc if 315 present. The command is invoked by using the user's login shell 316 with the -c option. This applies to shell, command, or subsystem 317 execution. It is most useful inside a Match block. The command 318 originally supplied by the client is available in the 319 SSH_ORIGINAL_COMMAND environment variable. Specifying a command 320 of M-bM-^@M-^\internal-sftpM-bM-^@M-^] will force the use of an in-process sftp 321 server that requires no support files when used with 322 ChrootDirectory. 323 324 GatewayPorts 325 Specifies whether remote hosts are allowed to connect to ports 326 forwarded for the client. By default, sshd(8) binds remote port 327 forwardings to the loopback address. This prevents other remote 328 hosts from connecting to forwarded ports. GatewayPorts can be 329 used to specify that sshd should allow remote port forwardings to 330 bind to non-loopback addresses, thus allowing other hosts to 331 connect. The argument may be M-bM-^@M-^\noM-bM-^@M-^] to force remote port 332 forwardings to be available to the local host only, M-bM-^@M-^\yesM-bM-^@M-^] to 333 force remote port forwardings to bind to the wildcard address, or 334 M-bM-^@M-^\clientspecifiedM-bM-^@M-^] to allow the client to select the address to 335 which the forwarding is bound. The default is M-bM-^@M-^\noM-bM-^@M-^]. 336 337 GSSAPIAuthentication 338 Specifies whether user authentication based on GSSAPI is allowed. 339 The default is M-bM-^@M-^\noM-bM-^@M-^]. Note that this option applies to protocol 340 version 2 only. 341 342 GSSAPICleanupCredentials 343 Specifies whether to automatically destroy the user's credentials 344 cache on logout. The default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that this option 345 applies to protocol version 2 only. 346 347 HostbasedAcceptedKeyTypes 348 Specifies the key types that will be accepted for hostbased 349 authentication as a comma-separated pattern list. The default 350 M-bM-^@M-^\*M-bM-^@M-^] will allow all key types. The -Q option of ssh(1) may be 351 used to list supported key types. 352 353 HostbasedAuthentication 354 Specifies whether rhosts or /etc/hosts.equiv authentication 355 together with successful public key client host authentication is 356 allowed (host-based authentication). This option is similar to 357 RhostsRSAAuthentication and applies to protocol version 2 only. 358 The default is M-bM-^@M-^\noM-bM-^@M-^]. 359 360 HostbasedUsesNameFromPacketOnly 361 Specifies whether or not the server will attempt to perform a 362 reverse name lookup when matching the name in the ~/.shosts, 363 ~/.rhosts, and /etc/hosts.equiv files during 364 HostbasedAuthentication. A setting of M-bM-^@M-^\yesM-bM-^@M-^] means that sshd(8) 365 uses the name supplied by the client rather than attempting to 366 resolve the name from the TCP connection itself. The default is 367 M-bM-^@M-^\noM-bM-^@M-^]. 368 369 HostCertificate 370 Specifies a file containing a public host certificate. The 371 certificate's public key must match a private host key already 372 specified by HostKey. The default behaviour of sshd(8) is not to 373 load any certificates. 374 375 HostKey 376 Specifies a file containing a private host key used by SSH. The 377 default is /etc/ssh/ssh_host_key for protocol version 1, and 378 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key, 379 /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for 380 protocol version 2. Note that sshd(8) will refuse to use a file 381 if it is group/world-accessible. It is possible to have multiple 382 host key files. M-bM-^@M-^\rsa1M-bM-^@M-^] keys are used for version 1 and M-bM-^@M-^\dsaM-bM-^@M-^], 383 M-bM-^@M-^\ecdsaM-bM-^@M-^], M-bM-^@M-^\ed25519M-bM-^@M-^] or M-bM-^@M-^\rsaM-bM-^@M-^] are used for version 2 of the SSH 384 protocol. It is also possible to specify public host key files 385 instead. In this case operations on the private key will be 386 delegated to an ssh-agent(1). 387 388 HostKeyAgent 389 Identifies the UNIX-domain socket used to communicate with an 390 agent that has access to the private host keys. If 391 M-bM-^@M-^\SSH_AUTH_SOCKM-bM-^@M-^] is specified, the location of the socket will be 392 read from the SSH_AUTH_SOCK environment variable. 393 394 IgnoreRhosts 395 Specifies that .rhosts and .shosts files will not be used in 396 RhostsRSAAuthentication or HostbasedAuthentication. 397 398 /etc/hosts.equiv and /etc/shosts.equiv are still used. The 399 default is M-bM-^@M-^\yesM-bM-^@M-^]. 400 401 IgnoreUserKnownHosts 402 Specifies whether sshd(8) should ignore the user's 403 ~/.ssh/known_hosts during RhostsRSAAuthentication or 404 HostbasedAuthentication. The default is M-bM-^@M-^\noM-bM-^@M-^]. 405 406 IPQoS Specifies the IPv4 type-of-service or DSCP class for the 407 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-^], 408 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-^], 409 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-^], 410 M-bM-^@M-^\lowdelayM-bM-^@M-^], M-bM-^@M-^\throughputM-bM-^@M-^], M-bM-^@M-^\reliabilityM-bM-^@M-^], or a numeric value. 411 This option may take one or two arguments, separated by 412 whitespace. If one argument is specified, it is used as the 413 packet class unconditionally. If two values are specified, the 414 first is automatically selected for interactive sessions and the 415 second for non-interactive sessions. The default is M-bM-^@M-^\lowdelayM-bM-^@M-^] 416 for interactive sessions and M-bM-^@M-^\throughputM-bM-^@M-^] for non-interactive 417 sessions. 418 419 KbdInteractiveAuthentication 420 Specifies whether to allow keyboard-interactive authentication. 421 The argument to this keyword must be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The default 422 is to use whatever value ChallengeResponseAuthentication is set 423 to (by default M-bM-^@M-^\yesM-bM-^@M-^]). 424 425 KerberosAuthentication 426 Specifies whether the password provided by the user for 427 PasswordAuthentication will be validated through the Kerberos 428 KDC. To use this option, the server needs a Kerberos servtab 429 which allows the verification of the KDC's identity. The default 430 is M-bM-^@M-^\noM-bM-^@M-^]. 431 432 KerberosGetAFSToken 433 If AFS is active and the user has a Kerberos 5 TGT, attempt to 434 acquire an AFS token before accessing the user's home directory. 435 The default is M-bM-^@M-^\noM-bM-^@M-^]. 436 437 KerberosOrLocalPasswd 438 If password authentication through Kerberos fails then the 439 password will be validated via any additional local mechanism 440 such as /etc/passwd. The default is M-bM-^@M-^\yesM-bM-^@M-^]. 441 442 KerberosTicketCleanup 443 Specifies whether to automatically destroy the user's ticket 444 cache file on logout. The default is M-bM-^@M-^\yesM-bM-^@M-^]. 445 446 KexAlgorithms 447 Specifies the available KEX (Key Exchange) algorithms. Multiple 448 algorithms must be comma-separated. The supported algorithms 449 are: 450 451 curve25519-sha256 (a] libssh.org 452 diffie-hellman-group1-sha1 453 diffie-hellman-group14-sha1 454 diffie-hellman-group-exchange-sha1 455 diffie-hellman-group-exchange-sha256 456 ecdh-sha2-nistp256 457 ecdh-sha2-nistp384 458 ecdh-sha2-nistp521 459 460 The default is: 461 462 curve25519-sha256 (a] libssh.org, 463 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 464 diffie-hellman-group-exchange-sha256, 465 diffie-hellman-group14-sha1 466 467 The list of available key exchange algorithms may also be 468 obtained using the -Q option of ssh(1) with an argument of M-bM-^@M-^\kexM-bM-^@M-^]. 469 470 KeyRegenerationInterval 471 In protocol version 1, the ephemeral server key is automatically 472 regenerated after this many seconds (if it has been used). The 473 purpose of regeneration is to prevent decrypting captured 474 sessions by later breaking into the machine and stealing the 475 keys. The key is never stored anywhere. If the value is 0, the 476 key is never regenerated. The default is 3600 (seconds). 477 478 ListenAddress 479 Specifies the local addresses sshd(8) should listen on. The 480 following forms may be used: 481 482 ListenAddress host|IPv4_addr|IPv6_addr 483 ListenAddress host|IPv4_addr:port 484 ListenAddress [host|IPv6_addr]:port 485 486 If port is not specified, sshd will listen on the address and all 487 prior Port options specified. The default is to listen on all 488 local addresses. Multiple ListenAddress options are permitted. 489 Additionally, any Port options must precede this option for non- 490 port qualified addresses. 491 492 LoginGraceTime 493 The server disconnects after this time if the user has not 494 successfully logged in. If the value is 0, there is no time 495 limit. The default is 120 seconds. 496 497 LogLevel 498 Gives the verbosity level that is used when logging messages from 499 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO, 500 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. 501 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify 502 higher levels of debugging output. Logging with a DEBUG level 503 violates the privacy of users and is not recommended. 504 505 MACs Specifies the available MAC (message authentication code) 506 algorithms. The MAC algorithm is used in protocol version 2 for 507 data integrity protection. Multiple algorithms must be comma- 508 separated. The algorithms that contain M-bM-^@M-^\-etmM-bM-^@M-^] calculate the MAC 509 after encryption (encrypt-then-mac). These are considered safer 510 and their use recommended. The supported MACs are: 511 512 hmac-md5 513 hmac-md5-96 514 hmac-ripemd160 515 hmac-sha1 516 hmac-sha1-96 517 hmac-sha2-256 518 hmac-sha2-512 519 umac-64 (a] openssh.com 520 umac-128 (a] openssh.com 521 hmac-md5-etm (a] openssh.com 522 hmac-md5-96-etm (a] openssh.com 523 hmac-ripemd160-etm (a] openssh.com 524 hmac-sha1-etm (a] openssh.com 525 hmac-sha1-96-etm (a] openssh.com 526 hmac-sha2-256-etm (a] openssh.com 527 hmac-sha2-512-etm (a] openssh.com 528 umac-64-etm (a] openssh.com 529 umac-128-etm (a] openssh.com 530 531 The default is: 532 533 umac-64-etm (a] openssh.com,umac-128-etm (a] openssh.com, 534 hmac-sha2-256-etm (a] openssh.com,hmac-sha2-512-etm (a] openssh.com, 535 umac-64 (a] openssh.com,umac-128 (a] openssh.com, 536 hmac-sha2-256,hmac-sha2-512 537 538 The list of available MAC algorithms may also be obtained using 539 the -Q option of ssh(1) with an argument of M-bM-^@M-^\macM-bM-^@M-^]. 540 541 Match Introduces a conditional block. If all of the criteria on the 542 Match line are satisfied, the keywords on the following lines 543 override those set in the global section of the config file, 544 until either another Match line or the end of the file. If a 545 keyword appears in multiple Match blocks that are satisfied, only 546 the first instance of the keyword is applied. 547 548 The arguments to Match are one or more criteria-pattern pairs or 549 the single token All which matches all criteria. The available 550 criteria are User, Group, Host, LocalAddress, LocalPort, and 551 Address. The match patterns may consist of single entries or 552 comma-separated lists and may use the wildcard and negation 553 operators described in the PATTERNS section of ssh_config(5). 554 555 The patterns in an Address criteria may additionally contain 556 addresses to match in CIDR address/masklen format, e.g. 557 M-bM-^@M-^\192.0.2.0/24M-bM-^@M-^] or M-bM-^@M-^\3ffe:ffff::/32M-bM-^@M-^]. Note that the mask length 558 provided must be consistent with the address - it is an error to 559 specify a mask length that is too long for the address or one 560 with bits set in this host portion of the address. For example, 561 M-bM-^@M-^\192.0.2.0/33M-bM-^@M-^] and M-bM-^@M-^\192.0.2.0/8M-bM-^@M-^] respectively. 562 563 Only a subset of keywords may be used on the lines following a 564 Match keyword. Available keywords are AcceptEnv, 565 AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding, 566 AllowTcpForwarding, AllowUsers, AuthenticationMethods, 567 AuthorizedKeysCommand, AuthorizedKeysCommandUser, 568 AuthorizedKeysFile, AuthorizedPrincipalsFile, Banner, 569 ChrootDirectory, DenyGroups, DenyUsers, ForceCommand, 570 GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes, 571 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS, 572 KbdInteractiveAuthentication, KerberosAuthentication, 573 MaxAuthTries, MaxSessions, PasswordAuthentication, 574 PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY, 575 PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes, 576 PubkeyAuthentication, RekeyLimit, RevokedKeys, 577 RhostsRSAAuthentication, RSAAuthentication, StreamLocalBindMask, 578 StreamLocalBindUnlink, TrustedUserCAKeys, X11DisplayOffset, 579 X11Forwarding and X11UseLocalHost. 580 581 MaxAuthTries 582 Specifies the maximum number of authentication attempts permitted 583 per connection. Once the number of failures reaches half this 584 value, additional failures are logged. The default is 6. 585 586 MaxSessions 587 Specifies the maximum number of open sessions permitted per 588 network connection. The default is 10. 589 590 MaxStartups 591 Specifies the maximum number of concurrent unauthenticated 592 connections to the SSH daemon. Additional connections will be 593 dropped until authentication succeeds or the LoginGraceTime 594 expires for a connection. The default is 10:30:100. 595 596 Alternatively, random early drop can be enabled by specifying the 597 three colon separated values M-bM-^@M-^\start:rate:fullM-bM-^@M-^] (e.g. "10:30:60"). 598 sshd(8) will refuse connection attempts with a probability of 599 M-bM-^@M-^\rate/100M-bM-^@M-^] (30%) if there are currently M-bM-^@M-^\startM-bM-^@M-^] (10) 600 unauthenticated connections. The probability increases linearly 601 and all connection attempts are refused if the number of 602 unauthenticated connections reaches M-bM-^@M-^\fullM-bM-^@M-^] (60). 603 604 PasswordAuthentication 605 Specifies whether password authentication is allowed. The 606 default is M-bM-^@M-^\yesM-bM-^@M-^]. 607 608 PermitEmptyPasswords 609 When password authentication is allowed, it specifies whether the 610 server allows login to accounts with empty password strings. The 611 default is M-bM-^@M-^\noM-bM-^@M-^]. 612 613 PermitOpen 614 Specifies the destinations to which TCP port forwarding is 615 permitted. The forwarding specification must be one of the 616 following forms: 617 618 PermitOpen host:port 619 PermitOpen IPv4_addr:port 620 PermitOpen [IPv6_addr]:port 621 622 Multiple forwards may be specified by separating them with 623 whitespace. An argument of M-bM-^@M-^\anyM-bM-^@M-^] can be used to remove all 624 restrictions and permit any forwarding requests. An argument of 625 M-bM-^@M-^\noneM-bM-^@M-^] can be used to prohibit all forwarding requests. By 626 default all port forwarding requests are permitted. 627 628 PermitRootLogin 629 Specifies whether root can log in using ssh(1). The argument 630 must be M-bM-^@M-^\yesM-bM-^@M-^], M-bM-^@M-^\without-passwordM-bM-^@M-^], M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^], or 631 M-bM-^@M-^\noM-bM-^@M-^]. The default is M-bM-^@M-^\yesM-bM-^@M-^]. 632 633 If this option is set to M-bM-^@M-^\without-passwordM-bM-^@M-^], password 634 authentication is disabled for root. 635 636 If this option is set to M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^], root login with 637 public key authentication will be allowed, but only if the 638 command option has been specified (which may be useful for taking 639 remote backups even if root login is normally not allowed). All 640 other authentication methods are disabled for root. 641 642 If this option is set to M-bM-^@M-^\noM-bM-^@M-^], root is not allowed to log in. 643 644 PermitTunnel 645 Specifies whether tun(4) device forwarding is allowed. The 646 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-^] 647 (layer 2), or M-bM-^@M-^\noM-bM-^@M-^]. Specifying M-bM-^@M-^\yesM-bM-^@M-^] permits both 648 M-bM-^@M-^\point-to-pointM-bM-^@M-^] and M-bM-^@M-^\ethernetM-bM-^@M-^]. The default is M-bM-^@M-^\noM-bM-^@M-^]. 649 650 Independent of this setting, the permissions of the selected 651 tun(4) device must allow access to the user. 652 653 PermitTTY 654 Specifies whether pty(4) allocation is permitted. The default is 655 M-bM-^@M-^\yesM-bM-^@M-^]. 656 657 PermitUserEnvironment 658 Specifies whether ~/.ssh/environment and environment= options in 659 ~/.ssh/authorized_keys are processed by sshd(8). The default is 660 M-bM-^@M-^\noM-bM-^@M-^]. Enabling environment processing may enable users to bypass 661 access restrictions in some configurations using mechanisms such 662 as LD_PRELOAD. 663 664 PermitUserRC 665 Specifies whether any ~/.ssh/rc file is executed. The default is 666 M-bM-^@M-^\yesM-bM-^@M-^]. 667 668 PidFile 669 Specifies the file that contains the process ID of the SSH 670 daemon. The default is /var/run/sshd.pid. 671 672 Port Specifies the port number that sshd(8) listens on. The default 673 is 22. Multiple options of this type are permitted. See also 674 ListenAddress. 675 676 PrintLastLog 677 Specifies whether sshd(8) should print the date and time of the 678 last user login when a user logs in interactively. The default 679 is M-bM-^@M-^\yesM-bM-^@M-^]. 680 681 PrintMotd 682 Specifies whether sshd(8) should print /etc/motd when a user logs 683 in interactively. (On some systems it is also printed by the 684 shell, /etc/profile, or equivalent.) The default is M-bM-^@M-^\yesM-bM-^@M-^]. 685 686 Protocol 687 Specifies the protocol versions sshd(8) supports. The possible 688 values are M-bM-^@M-^X1M-bM-^@M-^Y and M-bM-^@M-^X2M-bM-^@M-^Y. Multiple versions must be comma- 689 separated. The default is M-bM-^@M-^X2M-bM-^@M-^Y. Note that the order of the 690 protocol list does not indicate preference, because the client 691 selects among multiple protocol versions offered by the server. 692 Specifying M-bM-^@M-^\2,1M-bM-^@M-^] is identical to M-bM-^@M-^\1,2M-bM-^@M-^]. 693 694 PubkeyAcceptedKeyTypes 695 Specifies the key types that will be accepted for public key 696 authentication as a comma-separated pattern list. The default 697 M-bM-^@M-^\*M-bM-^@M-^] will allow all key types. The -Q option of ssh(1) may be 698 used to list supported key types. 699 700 PubkeyAuthentication 701 Specifies whether public key authentication is allowed. The 702 default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that this option applies to protocol 703 version 2 only. 704 705 RekeyLimit 706 Specifies the maximum amount of data that may be transmitted 707 before the session key is renegotiated, optionally followed a 708 maximum amount of time that may pass before the session key is 709 renegotiated. The first argument is specified in bytes and may 710 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, 711 Megabytes, or Gigabytes, respectively. The default is between 712 M-bM-^@M-^X1GM-bM-^@M-^Y and M-bM-^@M-^X4GM-bM-^@M-^Y, depending on the cipher. The optional second 713 value is specified in seconds and may use any of the units 714 documented in the TIME FORMATS section. The default value for 715 RekeyLimit is M-bM-^@M-^\default noneM-bM-^@M-^], which means that rekeying is 716 performed after the cipher's default amount of data has been sent 717 or received and no time based rekeying is done. This option 718 applies to protocol version 2 only. 719 720 RevokedKeys 721 Specifies revoked public keys. Keys listed in this file will be 722 refused for public key authentication. Note that if this file is 723 not readable, then public key authentication will be refused for 724 all users. Keys may be specified as a text file, listing one 725 public key per line, or as an OpenSSH Key Revocation List (KRL) 726 as generated by ssh-keygen(1). For more information on KRLs, see 727 the KEY REVOCATION LISTS section in ssh-keygen(1). 728 729 RhostsRSAAuthentication 730 Specifies whether rhosts or /etc/hosts.equiv authentication 731 together with successful RSA host authentication is allowed. The 732 default is M-bM-^@M-^\noM-bM-^@M-^]. This option applies to protocol version 1 only. 733 734 RSAAuthentication 735 Specifies whether pure RSA authentication is allowed. The 736 default is M-bM-^@M-^\yesM-bM-^@M-^]. This option applies to protocol version 1 737 only. 738 739 ServerKeyBits 740 Defines the number of bits in the ephemeral protocol version 1 741 server key. The minimum value is 512, and the default is 1024. 742 743 StreamLocalBindMask 744 Sets the octal file creation mode mask (umask) used when creating 745 a Unix-domain socket file for local or remote port forwarding. 746 This option is only used for port forwarding to a Unix-domain 747 socket file. 748 749 The default value is 0177, which creates a Unix-domain socket 750 file that is readable and writable only by the owner. Note that 751 not all operating systems honor the file mode on Unix-domain 752 socket files. 753 754 StreamLocalBindUnlink 755 Specifies whether to remove an existing Unix-domain socket file 756 for local or remote port forwarding before creating a new one. 757 If the socket file already exists and StreamLocalBindUnlink is 758 not enabled, sshd will be unable to forward the port to the Unix- 759 domain socket file. This option is only used for port forwarding 760 to a Unix-domain socket file. 761 762 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-^]. 763 764 StrictModes 765 Specifies whether sshd(8) should check file modes and ownership 766 of the user's files and home directory before accepting login. 767 This is normally desirable because novices sometimes accidentally 768 leave their directory or files world-writable. The default is 769 M-bM-^@M-^\yesM-bM-^@M-^]. Note that this does not apply to ChrootDirectory, whose 770 permissions and ownership are checked unconditionally. 771 772 Subsystem 773 Configures an external subsystem (e.g. file transfer daemon). 774 Arguments should be a subsystem name and a command (with optional 775 arguments) to execute upon subsystem request. 776 777 The command sftp-server(8) implements the M-bM-^@M-^\sftpM-bM-^@M-^] file transfer 778 subsystem. 779 780 Alternately the name M-bM-^@M-^\internal-sftpM-bM-^@M-^] implements an in-process 781 M-bM-^@M-^\sftpM-bM-^@M-^] server. This may simplify configurations using 782 ChrootDirectory to force a different filesystem root on clients. 783 784 By default no subsystems are defined. Note that this option 785 applies to protocol version 2 only. 786 787 SyslogFacility 788 Gives the facility code that is used when logging messages from 789 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0, 790 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The 791 default is AUTH. 792 793 TCPKeepAlive 794 Specifies whether the system should send TCP keepalive messages 795 to the other side. If they are sent, death of the connection or 796 crash of one of the machines will be properly noticed. However, 797 this means that connections will die if the route is down 798 temporarily, and some people find it annoying. On the other 799 hand, if TCP keepalives are not sent, sessions may hang 800 indefinitely on the server, leaving M-bM-^@M-^\ghostM-bM-^@M-^] users and consuming 801 server resources. 802 803 The default is M-bM-^@M-^\yesM-bM-^@M-^] (to send TCP keepalive messages), and the 804 server will notice if the network goes down or the client host 805 crashes. This avoids infinitely hanging sessions. 806 807 To disable TCP keepalive messages, the value should be set to 808 M-bM-^@M-^\noM-bM-^@M-^]. 809 810 TrustedUserCAKeys 811 Specifies a file containing public keys of certificate 812 authorities that are trusted to sign user certificates for 813 authentication. Keys are listed one per line; empty lines and 814 comments starting with M-bM-^@M-^X#M-bM-^@M-^Y are allowed. If a certificate is 815 presented for authentication and has its signing CA key listed in 816 this file, then it may be used for authentication for any user 817 listed in the certificate's principals list. Note that 818 certificates that lack a list of principals will not be permitted 819 for authentication using TrustedUserCAKeys. For more details on 820 certificates, see the CERTIFICATES section in ssh-keygen(1). 821 822 UseDNS Specifies whether sshd(8) should look up the remote host name and 823 check that the resolved host name for the remote IP address maps 824 back to the very same IP address. The default is M-bM-^@M-^\noM-bM-^@M-^]. 825 826 UseLogin 827 Specifies whether login(1) is used for interactive login 828 sessions. The default is M-bM-^@M-^\noM-bM-^@M-^]. Note that login(1) is never used 829 for remote command execution. Note also, that if this is 830 enabled, X11Forwarding will be disabled because login(1) does not 831 know how to handle xauth(1) cookies. If UsePrivilegeSeparation 832 is specified, it will be disabled after authentication. 833 834 UsePAM Enables the Pluggable Authentication Module interface. If set to 835 M-bM-^@M-^\yesM-bM-^@M-^] this will enable PAM authentication using 836 ChallengeResponseAuthentication and PasswordAuthentication in 837 addition to PAM account and session module processing for all 838 authentication types. 839 840 Because PAM challenge-response authentication usually serves an 841 equivalent role to password authentication, you should disable 842 either PasswordAuthentication or ChallengeResponseAuthentication. 843 844 If UsePAM is enabled, you will not be able to run sshd(8) as a 845 non-root user. The default is M-bM-^@M-^\noM-bM-^@M-^]. 846 847 UsePrivilegeSeparation 848 Specifies whether sshd(8) separates privileges by creating an 849 unprivileged child process to deal with incoming network traffic. 850 After successful authentication, another process will be created 851 that has the privilege of the authenticated user. The goal of 852 privilege separation is to prevent privilege escalation by 853 containing any corruption within the unprivileged processes. The 854 default is M-bM-^@M-^\yesM-bM-^@M-^]. If UsePrivilegeSeparation is set to M-bM-^@M-^\sandboxM-bM-^@M-^] 855 then the pre-authentication unprivileged process is subject to 856 additional restrictions. 857 858 VersionAddendum 859 Optionally specifies additional text to append to the SSH 860 protocol banner sent by the server upon connection. The default 861 is M-bM-^@M-^\noneM-bM-^@M-^]. 862 863 X11DisplayOffset 864 Specifies the first display number available for sshd(8)'s X11 865 forwarding. This prevents sshd from interfering with real X11 866 servers. The default is 10. 867 868 X11Forwarding 869 Specifies whether X11 forwarding is permitted. The argument must 870 be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The default is M-bM-^@M-^\noM-bM-^@M-^]. 871 872 When X11 forwarding is enabled, there may be additional exposure 873 to the server and to client displays if the sshd(8) proxy display 874 is configured to listen on the wildcard address (see 875 X11UseLocalhost below), though this is not the default. 876 Additionally, the authentication spoofing and authentication data 877 verification and substitution occur on the client side. The 878 security risk of using X11 forwarding is that the client's X11 879 display server may be exposed to attack when the SSH client 880 requests forwarding (see the warnings for ForwardX11 in 881 ssh_config(5)). A system administrator may have a stance in 882 which they want to protect clients that may expose themselves to 883 attack by unwittingly requesting X11 forwarding, which can 884 warrant a M-bM-^@M-^\noM-bM-^@M-^] setting. 885 886 Note that disabling X11 forwarding does not prevent users from 887 forwarding X11 traffic, as users can always install their own 888 forwarders. X11 forwarding is automatically disabled if UseLogin 889 is enabled. 890 891 X11UseLocalhost 892 Specifies whether sshd(8) should bind the X11 forwarding server 893 to the loopback address or to the wildcard address. By default, 894 sshd binds the forwarding server to the loopback address and sets 895 the hostname part of the DISPLAY environment variable to 896 M-bM-^@M-^\localhostM-bM-^@M-^]. This prevents remote hosts from connecting to the 897 proxy display. However, some older X11 clients may not function 898 with this configuration. X11UseLocalhost may be set to M-bM-^@M-^\noM-bM-^@M-^] to 899 specify that the forwarding server should be bound to the 900 wildcard address. The argument must be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The 901 default is M-bM-^@M-^\yesM-bM-^@M-^]. 902 903 XAuthLocation 904 Specifies the full pathname of the xauth(1) program. The default 905 is /usr/X11R6/bin/xauth. 906 907 TIME FORMATS 908 sshd(8) command-line arguments and configuration file options that 909 specify time may be expressed using a sequence of the form: 910 time[qualifier], where time is a positive integer value and qualifier is 911 one of the following: 912 913 M-bM-^_M-(noneM-bM-^_M-) seconds 914 s | S seconds 915 m | M minutes 916 h | H hours 917 d | D days 918 w | W weeks 919 920 Each member of the sequence is added together to calculate the total time 921 value. 922 923 Time format examples: 924 925 600 600 seconds (10 minutes) 926 10m 10 minutes 927 1h30m 1 hour 30 minutes (90 minutes) 928 929 FILES 930 /etc/ssh/sshd_config 931 Contains configuration data for sshd(8). This file should be 932 writable by root only, but it is recommended (though not 933 necessary) that it be world-readable. 934 935 SEE ALSO 936 sshd(8) 937 938 AUTHORS 939 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 940 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 941 de Raadt and Dug Song removed many bugs, re-added newer features and 942 created OpenSSH. Markus Friedl contributed the support for SSH protocol 943 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support 944 for privilege separation. 945 946 OpenBSD 5.7 February 20, 2015 OpenBSD 5.7 947
