1.\" 2.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 3.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 4.\" All rights reserved 5.\" 6.\" As far as I am concerned, the code I have written for this software 7.\" can be used freely for any purpose. Any derived versions of this 8.\" software must be clearly marked as such, and if the derived work is 9.\" incompatible with the protocol description in the RFC file, it must be 10.\" called by a name other than "ssh" or "Secure Shell". 11.\" 12.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 13.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 14.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 15.\" 16.\" Redistribution and use in source and binary forms, with or without 17.\" modification, are permitted provided that the following conditions 18.\" are met: 19.\" 1. Redistributions of source code must retain the above copyright 20.\" notice, this list of conditions and the following disclaimer. 21.\" 2. Redistributions in binary form must reproduce the above copyright 22.\" notice, this list of conditions and the following disclaimer in the 23.\" documentation and/or other materials provided with the distribution. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 35.\" 36.\" $OpenBSD: sshd_config.5,v 1.194 2015/02/20 23:46:01 djm Exp $ 37.Dd $Mdocdate: February 20 2015 $ 38.Dt SSHD_CONFIG 5 39.Os 40.Sh NAME 41.Nm sshd_config 42.Nd OpenSSH SSH daemon configuration file 43.Sh SYNOPSIS 44.Nm /etc/ssh/sshd_config 45.Sh DESCRIPTION 46.Xr sshd 8 47reads configuration data from 48.Pa /etc/ssh/sshd_config 49(or the file specified with 50.Fl f 51on the command line). 52The file contains keyword-argument pairs, one per line. 53Lines starting with 54.Ql # 55and empty lines are interpreted as comments. 56Arguments may optionally be enclosed in double quotes 57.Pq \&" 58in order to represent arguments containing spaces. 59.Pp 60The possible 61keywords and their meanings are as follows (note that 62keywords are case-insensitive and arguments are case-sensitive): 63.Bl -tag -width Ds 64.It Cm AcceptEnv 65Specifies what environment variables sent by the client will be copied into 66the session's 67.Xr environ 7 . 68See 69.Cm SendEnv 70in 71.Xr ssh_config 5 72for how to configure the client. 73Note that environment passing is only supported for protocol 2. 74Variables are specified by name, which may contain the wildcard characters 75.Ql * 76and 77.Ql \&? . 78Multiple environment variables may be separated by whitespace or spread 79across multiple 80.Cm AcceptEnv 81directives. 82Be warned that some environment variables could be used to bypass restricted 83user environments. 84For this reason, care should be taken in the use of this directive. 85The default is not to accept any environment variables. 86.It Cm AddressFamily 87Specifies which address family should be used by 88.Xr sshd 8 . 89Valid arguments are 90.Dq any , 91.Dq inet 92(use IPv4 only), or 93.Dq inet6 94(use IPv6 only). 95The default is 96.Dq any . 97.It Cm AllowAgentForwarding 98Specifies whether 99.Xr ssh-agent 1 100forwarding is permitted. 101The default is 102.Dq yes . 103Note that disabling agent forwarding does not improve security 104unless users are also denied shell access, as they can always install 105their own forwarders. 106.It Cm AllowGroups 107This keyword can be followed by a list of group name patterns, separated 108by spaces. 109If specified, login is allowed only for users whose primary 110group or supplementary group list matches one of the patterns. 111Only group names are valid; a numerical group ID is not recognized. 112By default, login is allowed for all groups. 113The allow/deny directives are processed in the following order: 114.Cm DenyUsers , 115.Cm AllowUsers , 116.Cm DenyGroups , 117and finally 118.Cm AllowGroups . 119.Pp 120See PATTERNS in 121.Xr ssh_config 5 122for more information on patterns. 123.It Cm AllowTcpForwarding 124Specifies whether TCP forwarding is permitted. 125The available options are 126.Dq yes 127or 128.Dq all 129to allow TCP forwarding, 130.Dq no 131to prevent all TCP forwarding, 132.Dq local 133to allow local (from the perspective of 134.Xr ssh 1 ) 135forwarding only or 136.Dq remote 137to allow remote forwarding only. 138The default is 139.Dq yes . 140Note that disabling TCP forwarding does not improve security unless 141users are also denied shell access, as they can always install their 142own forwarders. 143.It Cm AllowStreamLocalForwarding 144Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. 145The available options are 146.Dq yes 147or 148.Dq all 149to allow StreamLocal forwarding, 150.Dq no 151to prevent all StreamLocal forwarding, 152.Dq local 153to allow local (from the perspective of 154.Xr ssh 1 ) 155forwarding only or 156.Dq remote 157to allow remote forwarding only. 158The default is 159.Dq yes . 160Note that disabling StreamLocal forwarding does not improve security unless 161users are also denied shell access, as they can always install their 162own forwarders. 163.It Cm AllowUsers 164This keyword can be followed by a list of user name patterns, separated 165by spaces. 166If specified, login is allowed only for user names that 167match one of the patterns. 168Only user names are valid; a numerical user ID is not recognized. 169By default, login is allowed for all users. 170If the pattern takes the form USER@HOST then USER and HOST 171are separately checked, restricting logins to particular 172users from particular hosts. 173The allow/deny directives are processed in the following order: 174.Cm DenyUsers , 175.Cm AllowUsers , 176.Cm DenyGroups , 177and finally 178.Cm AllowGroups . 179.Pp 180See PATTERNS in 181.Xr ssh_config 5 182for more information on patterns. 183.It Cm AuthenticationMethods 184Specifies the authentication methods that must be successfully completed 185for a user to be granted access. 186This option must be followed by one or more comma-separated lists of 187authentication method names. 188Successful authentication requires completion of every method in at least 189one of these lists. 190.Pp 191For example, an argument of 192.Dq publickey,password publickey,keyboard-interactive 193would require the user to complete public key authentication, followed by 194either password or keyboard interactive authentication. 195Only methods that are next in one or more lists are offered at each stage, 196so for this example, it would not be possible to attempt password or 197keyboard-interactive authentication before public key. 198.Pp 199For keyboard interactive authentication it is also possible to 200restrict authentication to a specific device by appending a 201colon followed by the device identifier 202.Dq bsdauth , 203.Dq pam , 204or 205.Dq skey , 206depending on the server configuration. 207For example, 208.Dq keyboard-interactive:bsdauth 209would restrict keyboard interactive authentication to the 210.Dq bsdauth 211device. 212.Pp 213If the 214.Dq publickey 215method is listed more than once, 216.Xr sshd 8 217verifies that keys that have been used successfully are not reused for 218subsequent authentications. 219For example, an 220.Cm AuthenticationMethods 221of 222.Dq publickey,publickey 223will require successful authentication using two different public keys. 224.Pp 225This option is only available for SSH protocol 2 and will yield a fatal 226error if enabled if protocol 1 is also enabled. 227Note that each authentication method listed should also be explicitly enabled 228in the configuration. 229The default is not to require multiple authentication; successful completion 230of a single authentication method is sufficient. 231.It Cm AuthorizedKeysCommand 232Specifies a program to be used to look up the user's public keys. 233The program must be owned by root and not writable by group or others. 234It will be invoked with a single argument of the username 235being authenticated, and should produce on standard output zero or 236more lines of authorized_keys output (see AUTHORIZED_KEYS in 237.Xr sshd 8 ) . 238If a key supplied by AuthorizedKeysCommand does not successfully authenticate 239and authorize the user then public key authentication continues using the usual 240.Cm AuthorizedKeysFile 241files. 242By default, no AuthorizedKeysCommand is run. 243.It Cm AuthorizedKeysCommandUser 244Specifies the user under whose account the AuthorizedKeysCommand is run. 245It is recommended to use a dedicated user that has no other role on the host 246than running authorized keys commands. 247If 248.Cm AuthorizedKeysCommand 249is specified but 250.Cm AuthorizedKeysCommandUser 251is not, then 252.Xr sshd 8 253will refuse to start. 254.It Cm AuthorizedKeysFile 255Specifies the file that contains the public keys that can be used 256for user authentication. 257The format is described in the 258AUTHORIZED_KEYS FILE FORMAT 259section of 260.Xr sshd 8 . 261.Cm AuthorizedKeysFile 262may contain tokens of the form %T which are substituted during connection 263setup. 264The following tokens are defined: %% is replaced by a literal '%', 265%h is replaced by the home directory of the user being authenticated, and 266%u is replaced by the username of that user. 267After expansion, 268.Cm AuthorizedKeysFile 269is taken to be an absolute path or one relative to the user's home 270directory. 271Multiple files may be listed, separated by whitespace. 272The default is 273.Dq .ssh/authorized_keys .ssh/authorized_keys2 . 274.It Cm AuthorizedPrincipalsFile 275Specifies a file that lists principal names that are accepted for 276certificate authentication. 277When using certificates signed by a key listed in 278.Cm TrustedUserCAKeys , 279this file lists names, one of which must appear in the certificate for it 280to be accepted for authentication. 281Names are listed one per line preceded by key options (as described 282in AUTHORIZED_KEYS FILE FORMAT in 283.Xr sshd 8 ) . 284Empty lines and comments starting with 285.Ql # 286are ignored. 287.Pp 288.Cm AuthorizedPrincipalsFile 289may contain tokens of the form %T which are substituted during connection 290setup. 291The following tokens are defined: %% is replaced by a literal '%', 292%h is replaced by the home directory of the user being authenticated, and 293%u is replaced by the username of that user. 294After expansion, 295.Cm AuthorizedPrincipalsFile 296is taken to be an absolute path or one relative to the user's home 297directory. 298.Pp 299The default is 300.Dq none , 301i.e. not to use a principals file \(en in this case, the username 302of the user must appear in a certificate's principals list for it to be 303accepted. 304Note that 305.Cm AuthorizedPrincipalsFile 306is only used when authentication proceeds using a CA listed in 307.Cm TrustedUserCAKeys 308and is not consulted for certification authorities trusted via 309.Pa ~/.ssh/authorized_keys , 310though the 311.Cm principals= 312key option offers a similar facility (see 313.Xr sshd 8 314for details). 315.It Cm Banner 316The contents of the specified file are sent to the remote user before 317authentication is allowed. 318If the argument is 319.Dq none 320then no banner is displayed. 321This option is only available for protocol version 2. 322By default, no banner is displayed. 323.It Cm ChallengeResponseAuthentication 324Specifies whether challenge-response authentication is allowed (e.g. via 325PAM or through authentication styles supported in 326.Xr login.conf 5 ) 327The default is 328.Dq yes . 329.It Cm ChrootDirectory 330Specifies the pathname of a directory to 331.Xr chroot 2 332to after authentication. 333At session startup 334.Xr sshd 8 335checks that all components of the pathname are root-owned directories 336which are not writable by any other user or group. 337After the chroot, 338.Xr sshd 8 339changes the working directory to the user's home directory. 340.Pp 341The pathname may contain the following tokens that are expanded at runtime once 342the connecting user has been authenticated: %% is replaced by a literal '%', 343%h is replaced by the home directory of the user being authenticated, and 344%u is replaced by the username of that user. 345.Pp 346The 347.Cm ChrootDirectory 348must contain the necessary files and directories to support the 349user's session. 350For an interactive session this requires at least a shell, typically 351.Xr sh 1 , 352and basic 353.Pa /dev 354nodes such as 355.Xr null 4 , 356.Xr zero 4 , 357.Xr stdin 4 , 358.Xr stdout 4 , 359.Xr stderr 4 , 360and 361.Xr tty 4 362devices. 363For file transfer sessions using 364.Dq sftp , 365no additional configuration of the environment is necessary if the 366in-process sftp server is used, 367though sessions which use logging may require 368.Pa /dev/log 369inside the chroot directory on some operating systems (see 370.Xr sftp-server 8 371for details). 372.Pp 373For safety, it is very important that the directory hierarchy be 374prevented from modification by other processes on the system (especially 375those outside the jail). 376Misconfiguration can lead to unsafe environments which 377.Xr sshd 8 378cannot detect. 379.Pp 380The default is not to 381.Xr chroot 2 . 382.It Cm Ciphers 383Specifies the ciphers allowed for protocol version 2. 384Multiple ciphers must be comma-separated. 385The supported ciphers are: 386.Pp 387.Bl -item -compact -offset indent 388.It 3893des-cbc 390.It 391aes128-cbc 392.It 393aes192-cbc 394.It 395aes256-cbc 396.It 397aes128-ctr 398.It 399aes192-ctr 400.It 401aes256-ctr 402.It 403aes128-gcm@openssh.com 404.It 405aes256-gcm@openssh.com 406.It 407arcfour 408.It 409arcfour128 410.It 411arcfour256 412.It 413blowfish-cbc 414.It 415cast128-cbc 416.It 417chacha20-poly1305@openssh.com 418.El 419.Pp 420The default is: 421.Bd -literal -offset indent 422aes128-ctr,aes192-ctr,aes256-ctr, 423aes128-gcm@openssh.com,aes256-gcm@openssh.com, 424chacha20-poly1305@openssh.com 425.Ed 426.Pp 427The list of available ciphers may also be obtained using the 428.Fl Q 429option of 430.Xr ssh 1 431with an argument of 432.Dq cipher . 433.It Cm ClientAliveCountMax 434Sets the number of client alive messages (see below) which may be 435sent without 436.Xr sshd 8 437receiving any messages back from the client. 438If this threshold is reached while client alive messages are being sent, 439sshd will disconnect the client, terminating the session. 440It is important to note that the use of client alive messages is very 441different from 442.Cm TCPKeepAlive 443(below). 444The client alive messages are sent through the encrypted channel 445and therefore will not be spoofable. 446The TCP keepalive option enabled by 447.Cm TCPKeepAlive 448is spoofable. 449The client alive mechanism is valuable when the client or 450server depend on knowing when a connection has become inactive. 451.Pp 452The default value is 3. 453If 454.Cm ClientAliveInterval 455(see below) is set to 15, and 456.Cm ClientAliveCountMax 457is left at the default, unresponsive SSH clients 458will be disconnected after approximately 45 seconds. 459This option applies to protocol version 2 only. 460.It Cm ClientAliveInterval 461Sets a timeout interval in seconds after which if no data has been received 462from the client, 463.Xr sshd 8 464will send a message through the encrypted 465channel to request a response from the client. 466The default 467is 0, indicating that these messages will not be sent to the client. 468This option applies to protocol version 2 only. 469.It Cm Compression 470Specifies whether compression is allowed, or delayed until 471the user has authenticated successfully. 472The argument must be 473.Dq yes , 474.Dq delayed , 475or 476.Dq no . 477The default is 478.Dq delayed . 479.It Cm DenyGroups 480This keyword can be followed by a list of group name patterns, separated 481by spaces. 482Login is disallowed for users whose primary group or supplementary 483group list matches one of the patterns. 484Only group names are valid; a numerical group ID is not recognized. 485By default, login is allowed for all groups. 486The allow/deny directives are processed in the following order: 487.Cm DenyUsers , 488.Cm AllowUsers , 489.Cm DenyGroups , 490and finally 491.Cm AllowGroups . 492.Pp 493See PATTERNS in 494.Xr ssh_config 5 495for more information on patterns. 496.It Cm DenyUsers 497This keyword can be followed by a list of user name patterns, separated 498by spaces. 499Login is disallowed for user names that match one of the patterns. 500Only user names are valid; a numerical user ID is not recognized. 501By default, login is allowed for all users. 502If the pattern takes the form USER@HOST then USER and HOST 503are separately checked, restricting logins to particular 504users from particular hosts. 505The allow/deny directives are processed in the following order: 506.Cm DenyUsers , 507.Cm AllowUsers , 508.Cm DenyGroups , 509and finally 510.Cm AllowGroups . 511.Pp 512See PATTERNS in 513.Xr ssh_config 5 514for more information on patterns. 515.It Cm FingerprintHash 516Specifies the hash algorithm used when logging key fingerprints. 517Valid options are: 518.Dq md5 519and 520.Dq sha256 . 521The default is 522.Dq sha256 . 523.It Cm ForceCommand 524Forces the execution of the command specified by 525.Cm ForceCommand , 526ignoring any command supplied by the client and 527.Pa ~/.ssh/rc 528if present. 529The command is invoked by using the user's login shell with the -c option. 530This applies to shell, command, or subsystem execution. 531It is most useful inside a 532.Cm Match 533block. 534The command originally supplied by the client is available in the 535.Ev SSH_ORIGINAL_COMMAND 536environment variable. 537Specifying a command of 538.Dq internal-sftp 539will force the use of an in-process sftp server that requires no support 540files when used with 541.Cm ChrootDirectory . 542.It Cm GatewayPorts 543Specifies whether remote hosts are allowed to connect to ports 544forwarded for the client. 545By default, 546.Xr sshd 8 547binds remote port forwardings to the loopback address. 548This prevents other remote hosts from connecting to forwarded ports. 549.Cm GatewayPorts 550can be used to specify that sshd 551should allow remote port forwardings to bind to non-loopback addresses, thus 552allowing other hosts to connect. 553The argument may be 554.Dq no 555to force remote port forwardings to be available to the local host only, 556.Dq yes 557to force remote port forwardings to bind to the wildcard address, or 558.Dq clientspecified 559to allow the client to select the address to which the forwarding is bound. 560The default is 561.Dq no . 562.It Cm GSSAPIAuthentication 563Specifies whether user authentication based on GSSAPI is allowed. 564The default is 565.Dq no . 566Note that this option applies to protocol version 2 only. 567.It Cm GSSAPICleanupCredentials 568Specifies whether to automatically destroy the user's credentials cache 569on logout. 570The default is 571.Dq yes . 572Note that this option applies to protocol version 2 only. 573.It Cm HostbasedAcceptedKeyTypes 574Specifies the key types that will be accepted for hostbased authentication 575as a comma-separated pattern list. 576The default 577.Dq * 578will allow all key types. 579The 580.Fl Q 581option of 582.Xr ssh 1 583may be used to list supported key types. 584.It Cm HostbasedAuthentication 585Specifies whether rhosts or /etc/hosts.equiv authentication together 586with successful public key client host authentication is allowed 587(host-based authentication). 588This option is similar to 589.Cm RhostsRSAAuthentication 590and applies to protocol version 2 only. 591The default is 592.Dq no . 593.It Cm HostbasedUsesNameFromPacketOnly 594Specifies whether or not the server will attempt to perform a reverse 595name lookup when matching the name in the 596.Pa ~/.shosts , 597.Pa ~/.rhosts , 598and 599.Pa /etc/hosts.equiv 600files during 601.Cm HostbasedAuthentication . 602A setting of 603.Dq yes 604means that 605.Xr sshd 8 606uses the name supplied by the client rather than 607attempting to resolve the name from the TCP connection itself. 608The default is 609.Dq no . 610.It Cm HostCertificate 611Specifies a file containing a public host certificate. 612The certificate's public key must match a private host key already specified 613by 614.Cm HostKey . 615The default behaviour of 616.Xr sshd 8 617is not to load any certificates. 618.It Cm HostKey 619Specifies a file containing a private host key 620used by SSH. 621The default is 622.Pa /etc/ssh/ssh_host_key 623for protocol version 1, and 624.Pa /etc/ssh/ssh_host_dsa_key , 625.Pa /etc/ssh/ssh_host_ecdsa_key , 626.Pa /etc/ssh/ssh_host_ed25519_key 627and 628.Pa /etc/ssh/ssh_host_rsa_key 629for protocol version 2. 630Note that 631.Xr sshd 8 632will refuse to use a file if it is group/world-accessible. 633It is possible to have multiple host key files. 634.Dq rsa1 635keys are used for version 1 and 636.Dq dsa , 637.Dq ecdsa , 638.Dq ed25519 639or 640.Dq rsa 641are used for version 2 of the SSH protocol. 642It is also possible to specify public host key files instead. 643In this case operations on the private key will be delegated 644to an 645.Xr ssh-agent 1 . 646.It Cm HostKeyAgent 647Identifies the UNIX-domain socket used to communicate 648with an agent that has access to the private host keys. 649If 650.Dq SSH_AUTH_SOCK 651is specified, the location of the socket will be read from the 652.Ev SSH_AUTH_SOCK 653environment variable. 654.It Cm IgnoreRhosts 655Specifies that 656.Pa .rhosts 657and 658.Pa .shosts 659files will not be used in 660.Cm RhostsRSAAuthentication 661or 662.Cm HostbasedAuthentication . 663.Pp 664.Pa /etc/hosts.equiv 665and 666.Pa /etc/shosts.equiv 667are still used. 668The default is 669.Dq yes . 670.It Cm IgnoreUserKnownHosts 671Specifies whether 672.Xr sshd 8 673should ignore the user's 674.Pa ~/.ssh/known_hosts 675during 676.Cm RhostsRSAAuthentication 677or 678.Cm HostbasedAuthentication . 679The default is 680.Dq no . 681.It Cm IPQoS 682Specifies the IPv4 type-of-service or DSCP class for the connection. 683Accepted values are 684.Dq af11 , 685.Dq af12 , 686.Dq af13 , 687.Dq af21 , 688.Dq af22 , 689.Dq af23 , 690.Dq af31 , 691.Dq af32 , 692.Dq af33 , 693.Dq af41 , 694.Dq af42 , 695.Dq af43 , 696.Dq cs0 , 697.Dq cs1 , 698.Dq cs2 , 699.Dq cs3 , 700.Dq cs4 , 701.Dq cs5 , 702.Dq cs6 , 703.Dq cs7 , 704.Dq ef , 705.Dq lowdelay , 706.Dq throughput , 707.Dq reliability , 708or a numeric value. 709This option may take one or two arguments, separated by whitespace. 710If one argument is specified, it is used as the packet class unconditionally. 711If two values are specified, the first is automatically selected for 712interactive sessions and the second for non-interactive sessions. 713The default is 714.Dq lowdelay 715for interactive sessions and 716.Dq throughput 717for non-interactive sessions. 718.It Cm KbdInteractiveAuthentication 719Specifies whether to allow keyboard-interactive authentication. 720The argument to this keyword must be 721.Dq yes 722or 723.Dq no . 724The default is to use whatever value 725.Cm ChallengeResponseAuthentication 726is set to 727(by default 728.Dq yes ) . 729.It Cm KerberosAuthentication 730Specifies whether the password provided by the user for 731.Cm PasswordAuthentication 732will be validated through the Kerberos KDC. 733To use this option, the server needs a 734Kerberos servtab which allows the verification of the KDC's identity. 735The default is 736.Dq no . 737.It Cm KerberosGetAFSToken 738If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 739an AFS token before accessing the user's home directory. 740The default is 741.Dq no . 742.It Cm KerberosOrLocalPasswd 743If password authentication through Kerberos fails then 744the password will be validated via any additional local mechanism 745such as 746.Pa /etc/passwd . 747The default is 748.Dq yes . 749.It Cm KerberosTicketCleanup 750Specifies whether to automatically destroy the user's ticket cache 751file on logout. 752The default is 753.Dq yes . 754.It Cm KexAlgorithms 755Specifies the available KEX (Key Exchange) algorithms. 756Multiple algorithms must be comma-separated. 757The supported algorithms are: 758.Pp 759.Bl -item -compact -offset indent 760.It 761curve25519-sha256@libssh.org 762.It 763diffie-hellman-group1-sha1 764.It 765diffie-hellman-group14-sha1 766.It 767diffie-hellman-group-exchange-sha1 768.It 769diffie-hellman-group-exchange-sha256 770.It 771ecdh-sha2-nistp256 772.It 773ecdh-sha2-nistp384 774.It 775ecdh-sha2-nistp521 776.El 777.Pp 778The default is: 779.Bd -literal -offset indent 780curve25519-sha256@libssh.org, 781ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 782diffie-hellman-group-exchange-sha256, 783diffie-hellman-group14-sha1 784.Ed 785.Pp 786The list of available key exchange algorithms may also be obtained using the 787.Fl Q 788option of 789.Xr ssh 1 790with an argument of 791.Dq kex . 792.It Cm KeyRegenerationInterval 793In protocol version 1, the ephemeral server key is automatically regenerated 794after this many seconds (if it has been used). 795The purpose of regeneration is to prevent 796decrypting captured sessions by later breaking into the machine and 797stealing the keys. 798The key is never stored anywhere. 799If the value is 0, the key is never regenerated. 800The default is 3600 (seconds). 801.It Cm ListenAddress 802Specifies the local addresses 803.Xr sshd 8 804should listen on. 805The following forms may be used: 806.Pp 807.Bl -item -offset indent -compact 808.It 809.Cm ListenAddress 810.Sm off 811.Ar host | Ar IPv4_addr | Ar IPv6_addr 812.Sm on 813.It 814.Cm ListenAddress 815.Sm off 816.Ar host | Ar IPv4_addr : Ar port 817.Sm on 818.It 819.Cm ListenAddress 820.Sm off 821.Oo 822.Ar host | Ar IPv6_addr Oc : Ar port 823.Sm on 824.El 825.Pp 826If 827.Ar port 828is not specified, 829sshd will listen on the address and all prior 830.Cm Port 831options specified. 832The default is to listen on all local addresses. 833Multiple 834.Cm ListenAddress 835options are permitted. 836Additionally, any 837.Cm Port 838options must precede this option for non-port qualified addresses. 839.It Cm LoginGraceTime 840The server disconnects after this time if the user has not 841successfully logged in. 842If the value is 0, there is no time limit. 843The default is 120 seconds. 844.It Cm LogLevel 845Gives the verbosity level that is used when logging messages from 846.Xr sshd 8 . 847The possible values are: 848QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 849The default is INFO. 850DEBUG and DEBUG1 are equivalent. 851DEBUG2 and DEBUG3 each specify higher levels of debugging output. 852Logging with a DEBUG level violates the privacy of users and is not recommended. 853.It Cm MACs 854Specifies the available MAC (message authentication code) algorithms. 855The MAC algorithm is used in protocol version 2 856for data integrity protection. 857Multiple algorithms must be comma-separated. 858The algorithms that contain 859.Dq -etm 860calculate the MAC after encryption (encrypt-then-mac). 861These are considered safer and their use recommended. 862The supported MACs are: 863.Pp 864.Bl -item -compact -offset indent 865.It 866hmac-md5 867.It 868hmac-md5-96 869.It 870hmac-ripemd160 871.It 872hmac-sha1 873.It 874hmac-sha1-96 875.It 876hmac-sha2-256 877.It 878hmac-sha2-512 879.It 880umac-64@openssh.com 881.It 882umac-128@openssh.com 883.It 884hmac-md5-etm@openssh.com 885.It 886hmac-md5-96-etm@openssh.com 887.It 888hmac-ripemd160-etm@openssh.com 889.It 890hmac-sha1-etm@openssh.com 891.It 892hmac-sha1-96-etm@openssh.com 893.It 894hmac-sha2-256-etm@openssh.com 895.It 896hmac-sha2-512-etm@openssh.com 897.It 898umac-64-etm@openssh.com 899.It 900umac-128-etm@openssh.com 901.El 902.Pp 903The default is: 904.Bd -literal -offset indent 905umac-64-etm@openssh.com,umac-128-etm@openssh.com, 906hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 907umac-64@openssh.com,umac-128@openssh.com, 908hmac-sha2-256,hmac-sha2-512 909.Ed 910.Pp 911The list of available MAC algorithms may also be obtained using the 912.Fl Q 913option of 914.Xr ssh 1 915with an argument of 916.Dq mac . 917.It Cm Match 918Introduces a conditional block. 919If all of the criteria on the 920.Cm Match 921line are satisfied, the keywords on the following lines override those 922set in the global section of the config file, until either another 923.Cm Match 924line or the end of the file. 925If a keyword appears in multiple 926.Cm Match 927blocks that are satisfied, only the first instance of the keyword is 928applied. 929.Pp 930The arguments to 931.Cm Match 932are one or more criteria-pattern pairs or the single token 933.Cm All 934which matches all criteria. 935The available criteria are 936.Cm User , 937.Cm Group , 938.Cm Host , 939.Cm LocalAddress , 940.Cm LocalPort , 941and 942.Cm Address . 943The match patterns may consist of single entries or comma-separated 944lists and may use the wildcard and negation operators described in the 945PATTERNS section of 946.Xr ssh_config 5 . 947.Pp 948The patterns in an 949.Cm Address 950criteria may additionally contain addresses to match in CIDR 951address/masklen format, e.g.\& 952.Dq 192.0.2.0/24 953or 954.Dq 3ffe:ffff::/32 . 955Note that the mask length provided must be consistent with the address - 956it is an error to specify a mask length that is too long for the address 957or one with bits set in this host portion of the address. 958For example, 959.Dq 192.0.2.0/33 960and 961.Dq 192.0.2.0/8 962respectively. 963.Pp 964Only a subset of keywords may be used on the lines following a 965.Cm Match 966keyword. 967Available keywords are 968.Cm AcceptEnv , 969.Cm AllowAgentForwarding , 970.Cm AllowGroups , 971.Cm AllowStreamLocalForwarding , 972.Cm AllowTcpForwarding , 973.Cm AllowUsers , 974.Cm AuthenticationMethods , 975.Cm AuthorizedKeysCommand , 976.Cm AuthorizedKeysCommandUser , 977.Cm AuthorizedKeysFile , 978.Cm AuthorizedPrincipalsFile , 979.Cm Banner , 980.Cm ChrootDirectory , 981.Cm DenyGroups , 982.Cm DenyUsers , 983.Cm ForceCommand , 984.Cm GatewayPorts , 985.Cm GSSAPIAuthentication , 986.Cm HostbasedAcceptedKeyTypes , 987.Cm HostbasedAuthentication , 988.Cm HostbasedUsesNameFromPacketOnly , 989.Cm IPQoS , 990.Cm KbdInteractiveAuthentication , 991.Cm KerberosAuthentication , 992.Cm MaxAuthTries , 993.Cm MaxSessions , 994.Cm PasswordAuthentication , 995.Cm PermitEmptyPasswords , 996.Cm PermitOpen , 997.Cm PermitRootLogin , 998.Cm PermitTTY , 999.Cm PermitTunnel , 1000.Cm PermitUserRC , 1001.Cm PubkeyAcceptedKeyTypes , 1002.Cm PubkeyAuthentication , 1003.Cm RekeyLimit , 1004.Cm RevokedKeys , 1005.Cm RhostsRSAAuthentication , 1006.Cm RSAAuthentication , 1007.Cm StreamLocalBindMask , 1008.Cm StreamLocalBindUnlink , 1009.Cm TrustedUserCAKeys , 1010.Cm X11DisplayOffset , 1011.Cm X11Forwarding 1012and 1013.Cm X11UseLocalHost . 1014.It Cm MaxAuthTries 1015Specifies the maximum number of authentication attempts permitted per 1016connection. 1017Once the number of failures reaches half this value, 1018additional failures are logged. 1019The default is 6. 1020.It Cm MaxSessions 1021Specifies the maximum number of open sessions permitted per network connection. 1022The default is 10. 1023.It Cm MaxStartups 1024Specifies the maximum number of concurrent unauthenticated connections to the 1025SSH daemon. 1026Additional connections will be dropped until authentication succeeds or the 1027.Cm LoginGraceTime 1028expires for a connection. 1029The default is 10:30:100. 1030.Pp 1031Alternatively, random early drop can be enabled by specifying 1032the three colon separated values 1033.Dq start:rate:full 1034(e.g. "10:30:60"). 1035.Xr sshd 8 1036will refuse connection attempts with a probability of 1037.Dq rate/100 1038(30%) 1039if there are currently 1040.Dq start 1041(10) 1042unauthenticated connections. 1043The probability increases linearly and all connection attempts 1044are refused if the number of unauthenticated connections reaches 1045.Dq full 1046(60). 1047.It Cm PasswordAuthentication 1048Specifies whether password authentication is allowed. 1049The default is 1050.Dq yes . 1051.It Cm PermitEmptyPasswords 1052When password authentication is allowed, it specifies whether the 1053server allows login to accounts with empty password strings. 1054The default is 1055.Dq no . 1056.It Cm PermitOpen 1057Specifies the destinations to which TCP port forwarding is permitted. 1058The forwarding specification must be one of the following forms: 1059.Pp 1060.Bl -item -offset indent -compact 1061.It 1062.Cm PermitOpen 1063.Sm off 1064.Ar host : port 1065.Sm on 1066.It 1067.Cm PermitOpen 1068.Sm off 1069.Ar IPv4_addr : port 1070.Sm on 1071.It 1072.Cm PermitOpen 1073.Sm off 1074.Ar \&[ IPv6_addr \&] : port 1075.Sm on 1076.El 1077.Pp 1078Multiple forwards may be specified by separating them with whitespace. 1079An argument of 1080.Dq any 1081can be used to remove all restrictions and permit any forwarding requests. 1082An argument of 1083.Dq none 1084can be used to prohibit all forwarding requests. 1085By default all port forwarding requests are permitted. 1086.It Cm PermitRootLogin 1087Specifies whether root can log in using 1088.Xr ssh 1 . 1089The argument must be 1090.Dq yes , 1091.Dq without-password , 1092.Dq forced-commands-only , 1093or 1094.Dq no . 1095The default is 1096.Dq yes . 1097.Pp 1098If this option is set to 1099.Dq without-password , 1100password authentication is disabled for root. 1101.Pp 1102If this option is set to 1103.Dq forced-commands-only , 1104root login with public key authentication will be allowed, 1105but only if the 1106.Ar command 1107option has been specified 1108(which may be useful for taking remote backups even if root login is 1109normally not allowed). 1110All other authentication methods are disabled for root. 1111.Pp 1112If this option is set to 1113.Dq no , 1114root is not allowed to log in. 1115.It Cm PermitTunnel 1116Specifies whether 1117.Xr tun 4 1118device forwarding is allowed. 1119The argument must be 1120.Dq yes , 1121.Dq point-to-point 1122(layer 3), 1123.Dq ethernet 1124(layer 2), or 1125.Dq no . 1126Specifying 1127.Dq yes 1128permits both 1129.Dq point-to-point 1130and 1131.Dq ethernet . 1132The default is 1133.Dq no . 1134.Pp 1135Independent of this setting, the permissions of the selected 1136.Xr tun 4 1137device must allow access to the user. 1138.It Cm PermitTTY 1139Specifies whether 1140.Xr pty 4 1141allocation is permitted. 1142The default is 1143.Dq yes . 1144.It Cm PermitUserEnvironment 1145Specifies whether 1146.Pa ~/.ssh/environment 1147and 1148.Cm environment= 1149options in 1150.Pa ~/.ssh/authorized_keys 1151are processed by 1152.Xr sshd 8 . 1153The default is 1154.Dq no . 1155Enabling environment processing may enable users to bypass access 1156restrictions in some configurations using mechanisms such as 1157.Ev LD_PRELOAD . 1158.It Cm PermitUserRC 1159Specifies whether any 1160.Pa ~/.ssh/rc 1161file is executed. 1162The default is 1163.Dq yes . 1164.It Cm PidFile 1165Specifies the file that contains the process ID of the 1166SSH daemon. 1167The default is 1168.Pa /var/run/sshd.pid . 1169.It Cm Port 1170Specifies the port number that 1171.Xr sshd 8 1172listens on. 1173The default is 22. 1174Multiple options of this type are permitted. 1175See also 1176.Cm ListenAddress . 1177.It Cm PrintLastLog 1178Specifies whether 1179.Xr sshd 8 1180should print the date and time of the last user login when a user logs 1181in interactively. 1182The default is 1183.Dq yes . 1184.It Cm PrintMotd 1185Specifies whether 1186.Xr sshd 8 1187should print 1188.Pa /etc/motd 1189when a user logs in interactively. 1190(On some systems it is also printed by the shell, 1191.Pa /etc/profile , 1192or equivalent.) 1193The default is 1194.Dq yes . 1195.It Cm Protocol 1196Specifies the protocol versions 1197.Xr sshd 8 1198supports. 1199The possible values are 1200.Sq 1 1201and 1202.Sq 2 . 1203Multiple versions must be comma-separated. 1204The default is 1205.Sq 2 . 1206Note that the order of the protocol list does not indicate preference, 1207because the client selects among multiple protocol versions offered 1208by the server. 1209Specifying 1210.Dq 2,1 1211is identical to 1212.Dq 1,2 . 1213.It Cm PubkeyAcceptedKeyTypes 1214Specifies the key types that will be accepted for public key authentication 1215as a comma-separated pattern list. 1216The default 1217.Dq * 1218will allow all key types. 1219The 1220.Fl Q 1221option of 1222.Xr ssh 1 1223may be used to list supported key types. 1224.It Cm PubkeyAuthentication 1225Specifies whether public key authentication is allowed. 1226The default is 1227.Dq yes . 1228Note that this option applies to protocol version 2 only. 1229.It Cm RekeyLimit 1230Specifies the maximum amount of data that may be transmitted before the 1231session key is renegotiated, optionally followed a maximum amount of 1232time that may pass before the session key is renegotiated. 1233The first argument is specified in bytes and may have a suffix of 1234.Sq K , 1235.Sq M , 1236or 1237.Sq G 1238to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1239The default is between 1240.Sq 1G 1241and 1242.Sq 4G , 1243depending on the cipher. 1244The optional second value is specified in seconds and may use any of the 1245units documented in the 1246.Sx TIME FORMATS 1247section. 1248The default value for 1249.Cm RekeyLimit 1250is 1251.Dq default none , 1252which means that rekeying is performed after the cipher's default amount 1253of data has been sent or received and no time based rekeying is done. 1254This option applies to protocol version 2 only. 1255.It Cm RevokedKeys 1256Specifies revoked public keys. 1257Keys listed in this file will be refused for public key authentication. 1258Note that if this file is not readable, then public key authentication will 1259be refused for all users. 1260Keys may be specified as a text file, listing one public key per line, or as 1261an OpenSSH Key Revocation List (KRL) as generated by 1262.Xr ssh-keygen 1 . 1263For more information on KRLs, see the KEY REVOCATION LISTS section in 1264.Xr ssh-keygen 1 . 1265.It Cm RhostsRSAAuthentication 1266Specifies whether rhosts or /etc/hosts.equiv authentication together 1267with successful RSA host authentication is allowed. 1268The default is 1269.Dq no . 1270This option applies to protocol version 1 only. 1271.It Cm RSAAuthentication 1272Specifies whether pure RSA authentication is allowed. 1273The default is 1274.Dq yes . 1275This option applies to protocol version 1 only. 1276.It Cm ServerKeyBits 1277Defines the number of bits in the ephemeral protocol version 1 server key. 1278The minimum value is 512, and the default is 1024. 1279.It Cm StreamLocalBindMask 1280Sets the octal file creation mode mask 1281.Pq umask 1282used when creating a Unix-domain socket file for local or remote 1283port forwarding. 1284This option is only used for port forwarding to a Unix-domain socket file. 1285.Pp 1286The default value is 0177, which creates a Unix-domain socket file that is 1287readable and writable only by the owner. 1288Note that not all operating systems honor the file mode on Unix-domain 1289socket files. 1290.It Cm StreamLocalBindUnlink 1291Specifies whether to remove an existing Unix-domain socket file for local 1292or remote port forwarding before creating a new one. 1293If the socket file already exists and 1294.Cm StreamLocalBindUnlink 1295is not enabled, 1296.Nm sshd 1297will be unable to forward the port to the Unix-domain socket file. 1298This option is only used for port forwarding to a Unix-domain socket file. 1299.Pp 1300The argument must be 1301.Dq yes 1302or 1303.Dq no . 1304The default is 1305.Dq no . 1306.It Cm StrictModes 1307Specifies whether 1308.Xr sshd 8 1309should check file modes and ownership of the 1310user's files and home directory before accepting login. 1311This is normally desirable because novices sometimes accidentally leave their 1312directory or files world-writable. 1313The default is 1314.Dq yes . 1315Note that this does not apply to 1316.Cm ChrootDirectory , 1317whose permissions and ownership are checked unconditionally. 1318.It Cm Subsystem 1319Configures an external subsystem (e.g. file transfer daemon). 1320Arguments should be a subsystem name and a command (with optional arguments) 1321to execute upon subsystem request. 1322.Pp 1323The command 1324.Xr sftp-server 8 1325implements the 1326.Dq sftp 1327file transfer subsystem. 1328.Pp 1329Alternately the name 1330.Dq internal-sftp 1331implements an in-process 1332.Dq sftp 1333server. 1334This may simplify configurations using 1335.Cm ChrootDirectory 1336to force a different filesystem root on clients. 1337.Pp 1338By default no subsystems are defined. 1339Note that this option applies to protocol version 2 only. 1340.It Cm SyslogFacility 1341Gives the facility code that is used when logging messages from 1342.Xr sshd 8 . 1343The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1344LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1345The default is AUTH. 1346.It Cm TCPKeepAlive 1347Specifies whether the system should send TCP keepalive messages to the 1348other side. 1349If they are sent, death of the connection or crash of one 1350of the machines will be properly noticed. 1351However, this means that 1352connections will die if the route is down temporarily, and some people 1353find it annoying. 1354On the other hand, if TCP keepalives are not sent, 1355sessions may hang indefinitely on the server, leaving 1356.Dq ghost 1357users and consuming server resources. 1358.Pp 1359The default is 1360.Dq yes 1361(to send TCP keepalive messages), and the server will notice 1362if the network goes down or the client host crashes. 1363This avoids infinitely hanging sessions. 1364.Pp 1365To disable TCP keepalive messages, the value should be set to 1366.Dq no . 1367.It Cm TrustedUserCAKeys 1368Specifies a file containing public keys of certificate authorities that are 1369trusted to sign user certificates for authentication. 1370Keys are listed one per line; empty lines and comments starting with 1371.Ql # 1372are allowed. 1373If a certificate is presented for authentication and has its signing CA key 1374listed in this file, then it may be used for authentication for any user 1375listed in the certificate's principals list. 1376Note that certificates that lack a list of principals will not be permitted 1377for authentication using 1378.Cm TrustedUserCAKeys . 1379For more details on certificates, see the CERTIFICATES section in 1380.Xr ssh-keygen 1 . 1381.It Cm UseDNS 1382Specifies whether 1383.Xr sshd 8 1384should look up the remote host name and check that 1385the resolved host name for the remote IP address maps back to the 1386very same IP address. 1387The default is 1388.Dq no . 1389.It Cm UseLogin 1390Specifies whether 1391.Xr login 1 1392is used for interactive login sessions. 1393The default is 1394.Dq no . 1395Note that 1396.Xr login 1 1397is never used for remote command execution. 1398Note also, that if this is enabled, 1399.Cm X11Forwarding 1400will be disabled because 1401.Xr login 1 1402does not know how to handle 1403.Xr xauth 1 1404cookies. 1405If 1406.Cm UsePrivilegeSeparation 1407is specified, it will be disabled after authentication. 1408.It Cm UsePAM 1409Enables the Pluggable Authentication Module interface. 1410If set to 1411.Dq yes 1412this will enable PAM authentication using 1413.Cm ChallengeResponseAuthentication 1414and 1415.Cm PasswordAuthentication 1416in addition to PAM account and session module processing for all 1417authentication types. 1418.Pp 1419Because PAM challenge-response authentication usually serves an equivalent 1420role to password authentication, you should disable either 1421.Cm PasswordAuthentication 1422or 1423.Cm ChallengeResponseAuthentication. 1424.Pp 1425If 1426.Cm UsePAM 1427is enabled, you will not be able to run 1428.Xr sshd 8 1429as a non-root user. 1430The default is 1431.Dq no . 1432.It Cm UsePrivilegeSeparation 1433Specifies whether 1434.Xr sshd 8 1435separates privileges by creating an unprivileged child process 1436to deal with incoming network traffic. 1437After successful authentication, another process will be created that has 1438the privilege of the authenticated user. 1439The goal of privilege separation is to prevent privilege 1440escalation by containing any corruption within the unprivileged processes. 1441The default is 1442.Dq yes . 1443If 1444.Cm UsePrivilegeSeparation 1445is set to 1446.Dq sandbox 1447then the pre-authentication unprivileged process is subject to additional 1448restrictions. 1449.It Cm VersionAddendum 1450Optionally specifies additional text to append to the SSH protocol banner 1451sent by the server upon connection. 1452The default is 1453.Dq none . 1454.It Cm X11DisplayOffset 1455Specifies the first display number available for 1456.Xr sshd 8 Ns 's 1457X11 forwarding. 1458This prevents sshd from interfering with real X11 servers. 1459The default is 10. 1460.It Cm X11Forwarding 1461Specifies whether X11 forwarding is permitted. 1462The argument must be 1463.Dq yes 1464or 1465.Dq no . 1466The default is 1467.Dq no . 1468.Pp 1469When X11 forwarding is enabled, there may be additional exposure to 1470the server and to client displays if the 1471.Xr sshd 8 1472proxy display is configured to listen on the wildcard address (see 1473.Cm X11UseLocalhost 1474below), though this is not the default. 1475Additionally, the authentication spoofing and authentication data 1476verification and substitution occur on the client side. 1477The security risk of using X11 forwarding is that the client's X11 1478display server may be exposed to attack when the SSH client requests 1479forwarding (see the warnings for 1480.Cm ForwardX11 1481in 1482.Xr ssh_config 5 ) . 1483A system administrator may have a stance in which they want to 1484protect clients that may expose themselves to attack by unwittingly 1485requesting X11 forwarding, which can warrant a 1486.Dq no 1487setting. 1488.Pp 1489Note that disabling X11 forwarding does not prevent users from 1490forwarding X11 traffic, as users can always install their own forwarders. 1491X11 forwarding is automatically disabled if 1492.Cm UseLogin 1493is enabled. 1494.It Cm X11UseLocalhost 1495Specifies whether 1496.Xr sshd 8 1497should bind the X11 forwarding server to the loopback address or to 1498the wildcard address. 1499By default, 1500sshd binds the forwarding server to the loopback address and sets the 1501hostname part of the 1502.Ev DISPLAY 1503environment variable to 1504.Dq localhost . 1505This prevents remote hosts from connecting to the proxy display. 1506However, some older X11 clients may not function with this 1507configuration. 1508.Cm X11UseLocalhost 1509may be set to 1510.Dq no 1511to specify that the forwarding server should be bound to the wildcard 1512address. 1513The argument must be 1514.Dq yes 1515or 1516.Dq no . 1517The default is 1518.Dq yes . 1519.It Cm XAuthLocation 1520Specifies the full pathname of the 1521.Xr xauth 1 1522program. 1523The default is 1524.Pa /usr/X11R6/bin/xauth . 1525.El 1526.Sh TIME FORMATS 1527.Xr sshd 8 1528command-line arguments and configuration file options that specify time 1529may be expressed using a sequence of the form: 1530.Sm off 1531.Ar time Op Ar qualifier , 1532.Sm on 1533where 1534.Ar time 1535is a positive integer value and 1536.Ar qualifier 1537is one of the following: 1538.Pp 1539.Bl -tag -width Ds -compact -offset indent 1540.It Aq Cm none 1541seconds 1542.It Cm s | Cm S 1543seconds 1544.It Cm m | Cm M 1545minutes 1546.It Cm h | Cm H 1547hours 1548.It Cm d | Cm D 1549days 1550.It Cm w | Cm W 1551weeks 1552.El 1553.Pp 1554Each member of the sequence is added together to calculate 1555the total time value. 1556.Pp 1557Time format examples: 1558.Pp 1559.Bl -tag -width Ds -compact -offset indent 1560.It 600 1561600 seconds (10 minutes) 1562.It 10m 156310 minutes 1564.It 1h30m 15651 hour 30 minutes (90 minutes) 1566.El 1567.Sh FILES 1568.Bl -tag -width Ds 1569.It Pa /etc/ssh/sshd_config 1570Contains configuration data for 1571.Xr sshd 8 . 1572This file should be writable by root only, but it is recommended 1573(though not necessary) that it be world-readable. 1574.El 1575.Sh SEE ALSO 1576.Xr sshd 8 1577.Sh AUTHORS 1578OpenSSH is a derivative of the original and free 1579ssh 1.2.12 release by Tatu Ylonen. 1580Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1581Theo de Raadt and Dug Song 1582removed many bugs, re-added newer features and 1583created OpenSSH. 1584Markus Friedl contributed the support for SSH 1585protocol versions 1.5 and 2.0. 1586Niels Provos and Markus Friedl contributed support 1587for privilege separation. 1588