1.\" $NetBSD: sshd_config.5,v 1.15 2013/11/08 19:18:25 christos Exp $ 2.\" -*- nroff -*- 3.\" 4.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 5.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 6.\" All rights reserved 7.\" 8.\" As far as I am concerned, the code I have written for this software 9.\" can be used freely for any purpose. Any derived versions of this 10.\" software must be clearly marked as such, and if the derived work is 11.\" incompatible with the protocol description in the RFC file, it must be 12.\" called by a name other than "ssh" or "Secure Shell". 13.\" 14.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 15.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 16.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 17.\" 18.\" Redistribution and use in source and binary forms, with or without 19.\" modification, are permitted provided that the following conditions 20.\" are met: 21.\" 1. Redistributions of source code must retain the above copyright 22.\" notice, this list of conditions and the following disclaimer. 23.\" 2. Redistributions in binary form must reproduce the above copyright 24.\" notice, this list of conditions and the following disclaimer in the 25.\" documentation and/or other materials provided with the distribution. 26.\" 27.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 28.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 29.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 30.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 31.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 32.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 33.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 34.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 35.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 36.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 37.\" 38.\" $OpenBSD: sshd_config.5,v 1.162 2013/07/19 07:37:48 markus Exp $ 39.Dd July 19 2013 40.Dt SSHD_CONFIG 5 41.Os 42.Sh NAME 43.Nm sshd_config 44.Nd OpenSSH SSH daemon configuration file 45.Sh SYNOPSIS 46.Nm /etc/ssh/sshd_config 47.Sh DESCRIPTION 48.Xr sshd 8 49reads configuration data from 50.Pa /etc/ssh/sshd_config 51(or the file specified with 52.Fl f 53on the command line). 54The file contains keyword-argument pairs, one per line. 55Lines starting with 56.Ql # 57and empty lines are interpreted as comments. 58Arguments may optionally be enclosed in double quotes 59.Pq \&" 60in order to represent arguments containing spaces. 61.Pp 62The possible 63keywords and their meanings are as follows (note that 64keywords are case-insensitive and arguments are case-sensitive): 65.Bl -tag -width Ds 66.It Cm AcceptEnv 67Specifies what environment variables sent by the client will be copied into 68the session's 69.Xr environ 7 . 70See 71.Cm SendEnv 72in 73.Xr ssh_config 5 74for how to configure the client. 75Note that environment passing is only supported for protocol 2. 76Variables are specified by name, which may contain the wildcard characters 77.Ql * 78and 79.Ql \&? . 80Multiple environment variables may be separated by whitespace or spread 81across multiple 82.Cm AcceptEnv 83directives. 84Be warned that some environment variables could be used to bypass restricted 85user environments. 86For this reason, care should be taken in the use of this directive. 87The default is not to accept any environment variables. 88.It Cm AddressFamily 89Specifies which address family should be used by 90.Xr sshd 8 . 91Valid arguments are 92.Dq any , 93.Dq inet 94(use IPv4 only), or 95.Dq inet6 96(use IPv6 only). 97The default is 98.Dq any . 99.It Cm AllowAgentForwarding 100Specifies whether 101.Xr ssh-agent 1 102forwarding is permitted. 103The default is 104.Dq yes . 105Note that disabling agent forwarding does not improve security 106unless users are also denied shell access, as they can always install 107their own forwarders. 108.It Cm AllowGroups 109This keyword can be followed by a list of group name patterns, separated 110by spaces. 111If specified, login is allowed only for users whose primary 112group or supplementary group list matches one of the patterns. 113Only group names are valid; a numerical group ID is not recognized. 114By default, login is allowed for all groups. 115The allow/deny directives are processed in the following order: 116.Cm DenyUsers , 117.Cm AllowUsers , 118.Cm DenyGroups , 119and finally 120.Cm AllowGroups . 121.Pp 122See PATTERNS in 123.Xr ssh_config 5 124for more information on patterns. 125.It Cm AllowTcpForwarding 126Specifies whether TCP forwarding is permitted. 127The available options are 128.Dq yes 129or 130.Dq all 131to allow TCP forwarding, 132.Dq no 133to prevent all TCP forwarding, 134.Dq local 135to allow local (from the perspective of 136.Xr ssh 1 ) 137forwarding only or 138.Dq remote 139to allow remote forwarding only. 140The default is 141.Dq yes . 142Note that disabling TCP forwarding does not improve security unless 143users are also denied shell access, as they can always install their 144own forwarders. 145.It Cm AllowUsers 146This keyword can be followed by a list of user name patterns, separated 147by spaces. 148If specified, login is allowed only for user names that 149match one of the patterns. 150Only user names are valid; a numerical user ID is not recognized. 151By default, login is allowed for all users. 152If the pattern takes the form USER@HOST then USER and HOST 153are separately checked, restricting logins to particular 154users from particular hosts. 155The allow/deny directives are processed in the following order: 156.Cm DenyUsers , 157.Cm AllowUsers , 158.Cm DenyGroups , 159and finally 160.Cm AllowGroups . 161.Pp 162See PATTERNS in 163.Xr ssh_config 5 164for more information on patterns. 165.It Cm AuthenticationMethods 166Specifies the authentication methods that must be successfully completed 167for a user to be granted access. 168This option must be followed by one or more comma-separated lists of 169authentication method names. 170Successful authentication requires completion of every method in at least 171one of these lists. 172.Pp 173For example, an argument of 174.Dq publickey,password publickey,keyboard-interactive 175would require the user to complete public key authentication, followed by 176either password or keyboard interactive authentication. 177Only methods that are next in one or more lists are offered at each stage, 178so for this example, it would not be possible to attempt password or 179keyboard-interactive authentication before public key. 180.Pp 181For keyboard interactive authentication it is also possible to 182restrict authentication to a specific device by appending a 183colon followed by the device identifier 184.Dq bsdauth , 185.Dq pam , 186or 187.Dq skey , 188depending on the server configuration. 189For example, 190.Dq keyboard-interactive:bsdauth 191would restrict keyboard interactive authentication to the 192.Dq bsdauth 193device. 194.Pp 195This option is only available for SSH protocol 2 and will yield a fatal 196error if enabled if protocol 1 is also enabled. 197Note that each authentication method listed should also be explicitly enabled 198in the configuration. 199The default is not to require multiple authentication; successful completion 200of a single authentication method is sufficient. 201.It Cm AuthorizedKeysCommand 202Specifies a program to be used to look up the user's public keys. 203The program must be owned by root and not writable by group or others. 204It will be invoked with a single argument of the username 205being authenticated, and should produce on standard output zero or 206more lines of authorized_keys output (see AUTHORIZED_KEYS in 207.Xr sshd 8 ) . 208If a key supplied by AuthorizedKeysCommand does not successfully authenticate 209and authorize the user then public key authentication continues using the usual 210.Cm AuthorizedKeysFile 211files. 212By default, no AuthorizedKeysCommand is run. 213.It Cm AuthorizedKeysCommandUser 214Specifies the user under whose account the AuthorizedKeysCommand is run. 215It is recommended to use a dedicated user that has no other role on the host 216than running authorized keys commands. 217.It Cm AuthorizedKeysFile 218Specifies the file that contains the public keys that can be used 219for user authentication. 220The format is described in the 221AUTHORIZED_KEYS FILE FORMAT 222section of 223.Xr sshd 8 . 224.Cm AuthorizedKeysFile 225may contain tokens of the form %T which are substituted during connection 226setup. 227The following tokens are defined: %% is replaced by a literal '%', 228%h is replaced by the home directory of the user being authenticated, and 229%u is replaced by the username of that user. 230After expansion, 231.Cm AuthorizedKeysFile 232is taken to be an absolute path or one relative to the user's home 233directory. 234Multiple files may be listed, separated by whitespace. 235The default is 236.Dq .ssh/authorized_keys .ssh/authorized_keys2 . 237.It Cm AuthorizedPrincipalsFile 238Specifies a file that lists principal names that are accepted for 239certificate authentication. 240When using certificates signed by a key listed in 241.Cm TrustedUserCAKeys , 242this file lists names, one of which must appear in the certificate for it 243to be accepted for authentication. 244Names are listed one per line preceded by key options (as described 245in AUTHORIZED_KEYS FILE FORMAT in 246.Xr sshd 8 ) . 247Empty lines and comments starting with 248.Ql # 249are ignored. 250.Pp 251.Cm AuthorizedPrincipalsFile 252may contain tokens of the form %T which are substituted during connection 253setup. 254The following tokens are defined: %% is replaced by a literal '%', 255%h is replaced by the home directory of the user being authenticated, and 256%u is replaced by the username of that user. 257After expansion, 258.Cm AuthorizedPrincipalsFile 259is taken to be an absolute path or one relative to the user's home 260directory. 261.Pp 262The default is 263.Dq none , 264i.e. not to use a principals file \(en in this case, the username 265of the user must appear in a certificate's principals list for it to be 266accepted. 267Note that 268.Cm AuthorizedPrincipalsFile 269is only used when authentication proceeds using a CA listed in 270.Cm TrustedUserCAKeys 271and is not consulted for certification authorities trusted via 272.Pa ~/.ssh/authorized_keys , 273though the 274.Cm principals= 275key option offers a similar facility (see 276.Xr sshd 8 277for details). 278.It Cm Banner 279The contents of the specified file are sent to the remote user before 280authentication is allowed. 281If the argument is 282.Dq none 283then no banner is displayed. 284This option is only available for protocol version 2. 285By default, no banner is displayed. 286.It Cm ChallengeResponseAuthentication 287Specifies whether challenge-response authentication is allowed. 288All authentication styles from 289.Xr login.conf 5 290are supported. 291The default is 292.Dq yes . 293.It Cm ChrootDirectory 294Specifies the pathname of a directory to 295.Xr chroot 2 296to after authentication. 297All components of the pathname must be root-owned directories that are 298not writable by any other user or group. 299After the chroot, 300.Xr sshd 8 301changes the working directory to the user's home directory. 302.Pp 303The pathname may contain the following tokens that are expanded at runtime once 304the connecting user has been authenticated: %% is replaced by a literal '%', 305%h is replaced by the home directory of the user being authenticated, and 306%u is replaced by the username of that user. 307.Pp 308The 309.Cm ChrootDirectory 310must contain the necessary files and directories to support the 311user's session. 312For an interactive session this requires at least a shell, typically 313.Xr sh 1 , 314and basic 315.Pa /dev 316nodes such as 317.Xr null 4 , 318.Xr zero 4 , 319.Xr stdin 4 , 320.Xr stdout 4 , 321.Xr stderr 4 , 322.Xr arandom 4 323and 324.Xr tty 4 325devices. 326For file transfer sessions using 327.Dq sftp , 328no additional configuration of the environment is necessary if the 329in-process sftp server is used, 330though sessions which use logging do require 331.Pa /dev/log 332inside the chroot directory (see 333.Xr sftp-server 8 334for details). 335.Pp 336The default is not to 337.Xr chroot 2 . 338.It Cm Ciphers 339Specifies the ciphers allowed for protocol version 2. 340Multiple ciphers must be comma-separated. 341The supported ciphers are 342.Dq 3des-cbc , 343.Dq aes128-cbc , 344.Dq aes192-cbc , 345.Dq aes256-cbc , 346.Dq aes128-ctr , 347.Dq aes192-ctr , 348.Dq aes256-ctr , 349.Dq aes128-gcm@openssh.com , 350.Dq aes256-gcm@openssh.com , 351.Dq arcfour128 , 352.Dq arcfour256 , 353.Dq arcfour , 354.Dq blowfish-cbc , 355and 356.Dq cast128-cbc . 357The default is: 358.Bd -literal -offset 3n 359aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 360aes128-gcm@openssh.com,aes256-gcm@openssh.com, 361aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 362aes256-cbc,arcfour 363.Ed 364.It Cm ClientAliveCountMax 365Sets the number of client alive messages (see below) which may be 366sent without 367.Xr sshd 8 368receiving any messages back from the client. 369If this threshold is reached while client alive messages are being sent, 370sshd will disconnect the client, terminating the session. 371It is important to note that the use of client alive messages is very 372different from 373.Cm TCPKeepAlive 374(below). 375The client alive messages are sent through the encrypted channel 376and therefore will not be spoofable. 377The TCP keepalive option enabled by 378.Cm TCPKeepAlive 379is spoofable. 380The client alive mechanism is valuable when the client or 381server depend on knowing when a connection has become inactive. 382.Pp 383The default value is 3. 384If 385.Cm ClientAliveInterval 386(see below) is set to 15, and 387.Cm ClientAliveCountMax 388is left at the default, unresponsive SSH clients 389will be disconnected after approximately 45 seconds. 390This option applies to protocol version 2 only. 391.It Cm ClientAliveInterval 392Sets a timeout interval in seconds after which if no data has been received 393from the client, 394.Xr sshd 8 395will send a message through the encrypted 396channel to request a response from the client. 397The default 398is 0, indicating that these messages will not be sent to the client. 399This option applies to protocol version 2 only. 400.It Cm Compression 401Specifies whether compression is allowed, or delayed until 402the user has authenticated successfully. 403The argument must be 404.Dq yes , 405.Dq delayed , 406or 407.Dq no . 408The default is 409.Dq delayed . 410.It Cm DenyGroups 411This keyword can be followed by a list of group name patterns, separated 412by spaces. 413Login is disallowed for users whose primary group or supplementary 414group list matches one of the patterns. 415Only group names are valid; a numerical group ID is not recognized. 416By default, login is allowed for all groups. 417The allow/deny directives are processed in the following order: 418.Cm DenyUsers , 419.Cm AllowUsers , 420.Cm DenyGroups , 421and finally 422.Cm AllowGroups . 423.Pp 424See PATTERNS in 425.Xr ssh_config 5 426for more information on patterns. 427.It Cm DenyUsers 428This keyword can be followed by a list of user name patterns, separated 429by spaces. 430Login is disallowed for user names that match one of the patterns. 431Only user names are valid; a numerical user ID is not recognized. 432By default, login is allowed for all users. 433If the pattern takes the form USER@HOST then USER and HOST 434are separately checked, restricting logins to particular 435users from particular hosts. 436The allow/deny directives are processed in the following order: 437.Cm DenyUsers , 438.Cm AllowUsers , 439.Cm DenyGroups , 440and finally 441.Cm AllowGroups . 442.Pp 443See PATTERNS in 444.Xr ssh_config 5 445for more information on patterns. 446.It Cm ForceCommand 447Forces the execution of the command specified by 448.Cm ForceCommand , 449ignoring any command supplied by the client and 450.Pa ~/.ssh/rc 451if present. 452The command is invoked by using the user's login shell with the -c option. 453This applies to shell, command, or subsystem execution. 454It is most useful inside a 455.Cm Match 456block. 457The command originally supplied by the client is available in the 458.Ev SSH_ORIGINAL_COMMAND 459environment variable. 460Specifying a command of 461.Dq internal-sftp 462will force the use of an in-process sftp server that requires no support 463files when used with 464.Cm ChrootDirectory . 465.It Cm GatewayPorts 466Specifies whether remote hosts are allowed to connect to ports 467forwarded for the client. 468By default, 469.Xr sshd 8 470binds remote port forwardings to the loopback address. 471This prevents other remote hosts from connecting to forwarded ports. 472.Cm GatewayPorts 473can be used to specify that sshd 474should allow remote port forwardings to bind to non-loopback addresses, thus 475allowing other hosts to connect. 476The argument may be 477.Dq no 478to force remote port forwardings to be available to the local host only, 479.Dq yes 480to force remote port forwardings to bind to the wildcard address, or 481.Dq clientspecified 482to allow the client to select the address to which the forwarding is bound. 483The default is 484.Dq no . 485.It Cm GSSAPIAuthentication 486Specifies whether user authentication based on GSSAPI is allowed. 487The default is 488.Dq no . 489Note that this option applies to protocol version 2 only. 490.It Cm GSSAPICleanupCredentials 491Specifies whether to automatically destroy the user's credentials cache 492on logout. 493The default is 494.Dq yes . 495Note that this option applies to protocol version 2 only. 496.It Cm HostbasedAuthentication 497Specifies whether rhosts or /etc/hosts.equiv authentication together 498with successful public key client host authentication is allowed 499(host-based authentication). 500This option is similar to 501.Cm RhostsRSAAuthentication 502and applies to protocol version 2 only. 503The default is 504.Dq no . 505.It Cm HostbasedUsesNameFromPacketOnly 506Specifies whether or not the server will attempt to perform a reverse 507name lookup when matching the name in the 508.Pa ~/.shosts , 509.Pa ~/.rhosts , 510and 511.Pa /etc/hosts.equiv 512files during 513.Cm HostbasedAuthentication . 514A setting of 515.Dq yes 516means that 517.Xr sshd 8 518uses the name supplied by the client rather than 519attempting to resolve the name from the TCP connection itself. 520The default is 521.Dq no . 522.It Cm HostCertificate 523Specifies a file containing a public host certificate. 524The certificate's public key must match a private host key already specified 525by 526.Cm HostKey . 527The default behaviour of 528.Xr sshd 8 529is not to load any certificates. 530.It Cm HostKey 531Specifies a file containing a private host key 532used by SSH. 533The default is 534.Pa /etc/ssh/ssh_host_key 535for protocol version 1, and 536.Pa /etc/ssh/ssh_host_dsa_key , 537.Pa /etc/ssh/ssh_host_ecdsa_key 538and 539.Pa /etc/ssh/ssh_host_rsa_key 540for protocol version 2. 541Note that 542.Xr sshd 8 543will refuse to use a file if it is group/world-accessible. 544It is possible to have multiple host key files. 545.Dq rsa1 546keys are used for version 1 and 547.Dq dsa , 548.Dq ecdsa 549or 550.Dq rsa 551are used for version 2 of the SSH protocol. 552It is also possible to specify public host key files instead. 553In this case operations on the private key will be delegated 554to an 555.Xr ssh-agent 1 . 556.It Cm HostKeyAgent 557Identifies the UNIX-domain socket used to communicate 558with an agent that has access to the private host keys. 559If 560.Dq SSH_AUTH_SOCK 561is specified, the location of the socket will be read from the 562.Ev SSH_AUTH_SOCK 563environment variable. 564.It Cm IgnoreRhosts 565Specifies that 566.Pa .rhosts 567and 568.Pa .shosts 569files will not be used in 570.Cm RhostsRSAAuthentication 571or 572.Cm HostbasedAuthentication . 573.Pp 574.Pa /etc/hosts.equiv 575and 576.Pa /etc/shosts.equiv 577are still used. 578The default is 579.Dq yes . 580.It Cm IgnoreUserKnownHosts 581Specifies whether 582.Xr sshd 8 583should ignore the user's 584.Pa ~/.ssh/known_hosts 585during 586.Cm RhostsRSAAuthentication 587or 588.Cm HostbasedAuthentication . 589The default is 590.Dq no . 591.It Cm IPQoS 592Specifies the IPv4 type-of-service or DSCP class for the connection. 593Accepted values are 594.Dq af11 , 595.Dq af12 , 596.Dq af13 , 597.Dq af21 , 598.Dq af22 , 599.Dq af23 , 600.Dq af31 , 601.Dq af32 , 602.Dq af33 , 603.Dq af41 , 604.Dq af42 , 605.Dq af43 , 606.Dq cs0 , 607.Dq cs1 , 608.Dq cs2 , 609.Dq cs3 , 610.Dq cs4 , 611.Dq cs5 , 612.Dq cs6 , 613.Dq cs7 , 614.Dq ef , 615.Dq lowdelay , 616.Dq throughput , 617.Dq reliability , 618or a numeric value. 619This option may take one or two arguments, separated by whitespace. 620If one argument is specified, it is used as the packet class unconditionally. 621If two values are specified, the first is automatically selected for 622interactive sessions and the second for non-interactive sessions. 623The default is 624.Dq lowdelay 625for interactive sessions and 626.Dq throughput 627for non-interactive sessions. 628.It Cm KerberosAuthentication 629Specifies whether the password provided by the user for 630.Cm PasswordAuthentication 631will be validated through the Kerberos KDC. 632To use this option, the server needs a 633Kerberos servtab which allows the verification of the KDC's identity. 634The default is 635.Dq no . 636.It Cm KerberosGetAFSToken 637If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 638an AFS token before accessing the user's home directory. 639The default is 640.Dq no . 641.It Cm KerberosOrLocalPasswd 642If password authentication through Kerberos fails then 643the password will be validated via any additional local mechanism 644such as 645.Pa /etc/passwd . 646The default is 647.Dq yes . 648.It Cm KerberosTicketCleanup 649Specifies whether to automatically destroy the user's ticket cache 650file on logout. 651The default is 652.Dq yes . 653.It Cm KexAlgorithms 654Specifies the available KEX (Key Exchange) algorithms. 655Multiple algorithms must be comma-separated. 656The default is 657.Dq ecdh-sha2-nistp256 , 658.Dq ecdh-sha2-nistp384 , 659.Dq ecdh-sha2-nistp521 , 660.Dq diffie-hellman-group-exchange-sha256 , 661.Dq diffie-hellman-group-exchange-sha1 , 662.Dq diffie-hellman-group14-sha1 , 663.Dq diffie-hellman-group1-sha1 . 664.It Cm KeyRegenerationInterval 665In protocol version 1, the ephemeral server key is automatically regenerated 666after this many seconds (if it has been used). 667The purpose of regeneration is to prevent 668decrypting captured sessions by later breaking into the machine and 669stealing the keys. 670The key is never stored anywhere. 671If the value is 0, the key is never regenerated. 672The default is 3600 (seconds). 673.It Cm ListenAddress 674Specifies the local addresses 675.Xr sshd 8 676should listen on. 677The following forms may be used: 678.Pp 679.Bl -item -offset indent -compact 680.It 681.Cm ListenAddress 682.Sm off 683.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 684.Sm on 685.It 686.Cm ListenAddress 687.Sm off 688.Ar host No | Ar IPv4_addr No : Ar port 689.Sm on 690.It 691.Cm ListenAddress 692.Sm off 693.Oo 694.Ar host No | Ar IPv6_addr Oc : Ar port 695.Sm on 696.El 697.Pp 698If 699.Ar port 700is not specified, 701sshd will listen on the address and all prior 702.Cm Port 703options specified. 704The default is to listen on all local addresses. 705Multiple 706.Cm ListenAddress 707options are permitted. 708Additionally, any 709.Cm Port 710options must precede this option for non-port qualified addresses. 711.It Cm LoginGraceTime 712The server disconnects after this time if the user has not 713successfully logged in. 714If the value is 0, there is no time limit. 715The default is 120 seconds but the default 716.Pa /etc/ssh/sshd_config 717overwrites it to 600 seconds. 718.It Cm LogLevel 719Gives the verbosity level that is used when logging messages from 720.Xr sshd 8 . 721The possible values are: 722QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 723The default is INFO. 724DEBUG and DEBUG1 are equivalent. 725DEBUG2 and DEBUG3 each specify higher levels of debugging output. 726Logging with a DEBUG level violates the privacy of users and is not recommended. 727.It Cm MACs 728Specifies the available MAC (message authentication code) algorithms. 729The MAC algorithm is used in protocol version 2 730for data integrity protection. 731Multiple algorithms must be comma-separated. 732The algorithms that contain 733.Dq -etm 734calculate the MAC after encryption (encrypt-then-mac). 735These are considered safer and their use recommended. 736The default is: 737.Bd -literal -offset indent 738hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com, 739umac-64-etm@openssh.com,umac-128-etm@openssh.com, 740hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 741hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com, 742hmac-md5-96-etm@openssh.com, 743hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com, 744hmac-sha2-256,hmac-sha2-512,hmac-ripemd160, 745hmac-sha1-96,hmac-md5-96 746.Ed 747.It Cm Match 748Introduces a conditional block. 749If all of the criteria on the 750.Cm Match 751line are satisfied, the keywords on the following lines override those 752set in the global section of the config file, until either another 753.Cm Match 754line or the end of the file. 755.Pp 756The arguments to 757.Cm Match 758are one or more criteria-pattern pairs. 759The available criteria are 760.Cm User , 761.Cm Group , 762.Cm Host , 763.Cm LocalAddress , 764.Cm LocalPort , 765and 766.Cm Address . 767The match patterns may consist of single entries or comma-separated 768lists and may use the wildcard and negation operators described in the 769PATTERNS section of 770.Xr ssh_config 5 . 771.Pp 772The patterns in an 773.Cm Address 774criteria may additionally contain addresses to match in CIDR 775address/masklen format, e.g.\& 776.Dq 192.0.2.0/24 777or 778.Dq 3ffe:ffff::/32 . 779Note that the mask length provided must be consistent with the address - 780it is an error to specify a mask length that is too long for the address 781or one with bits set in this host portion of the address. 782For example, 783.Dq 192.0.2.0/33 784and 785.Dq 192.0.2.0/8 786respectively. 787.Pp 788Only a subset of keywords may be used on the lines following a 789.Cm Match 790keyword. 791Available keywords are 792.Cm AcceptEnv , 793.Cm AllowAgentForwarding , 794.Cm AllowGroups , 795.Cm AllowTcpForwarding , 796.Cm AllowUsers , 797.Cm AuthenticationMethods , 798.Cm AuthorizedKeysCommand , 799.Cm AuthorizedKeysCommandUser , 800.Cm AuthorizedKeysFile , 801.Cm AuthorizedPrincipalsFile , 802.Cm Banner , 803.Cm ChrootDirectory , 804.Cm DenyGroups , 805.Cm DenyUsers , 806.Cm ForceCommand , 807.Cm GatewayPorts , 808.Cm GSSAPIAuthentication , 809.Cm HostbasedAuthentication , 810.Cm HostbasedUsesNameFromPacketOnly , 811.Cm KbdInteractiveAuthentication , 812.Cm KerberosAuthentication , 813.Cm MaxAuthTries , 814.Cm MaxSessions , 815.Cm PasswordAuthentication , 816.Cm PermitEmptyPasswords , 817.Cm PermitOpen , 818.Cm PermitRootLogin , 819.Cm PermitTunnel , 820.Cm PubkeyAuthentication , 821.Cm RekeyLimit , 822.Cm RhostsRSAAuthentication , 823.Cm RSAAuthentication , 824.Cm X11DisplayOffset , 825.Cm X11Forwarding 826and 827.Cm X11UseLocalHost . 828.It Cm MaxAuthTries 829Specifies the maximum number of authentication attempts permitted per 830connection. 831Once the number of failures reaches half this value, 832additional failures are logged. 833The default is 6. 834.It Cm MaxSessions 835Specifies the maximum number of open sessions permitted per network connection. 836The default is 10. 837.It Cm MaxStartups 838Specifies the maximum number of concurrent unauthenticated connections to the 839SSH daemon. 840Additional connections will be dropped until authentication succeeds or the 841.Cm LoginGraceTime 842expires for a connection. 843The default is 10:30:100. 844.Pp 845Alternatively, random early drop can be enabled by specifying 846the three colon separated values 847.Dq start:rate:full 848(e.g. "10:30:60"). 849.Xr sshd 8 850will refuse connection attempts with a probability of 851.Dq rate/100 852(30%) 853if there are currently 854.Dq start 855(10) 856unauthenticated connections. 857The probability increases linearly and all connection attempts 858are refused if the number of unauthenticated connections reaches 859.Dq full 860(60). 861.It Cm PasswordAuthentication 862Specifies whether password authentication is allowed. 863The default is 864.Dq yes . 865.It Cm PermitEmptyPasswords 866When password authentication is allowed, it specifies whether the 867server allows login to accounts with empty password strings. 868The default is 869.Dq no . 870.It Cm PermitOpen 871Specifies the destinations to which TCP port forwarding is permitted. 872The forwarding specification must be one of the following forms: 873.Pp 874.Bl -item -offset indent -compact 875.It 876.Cm PermitOpen 877.Sm off 878.Ar host : port 879.Sm on 880.It 881.Cm PermitOpen 882.Sm off 883.Ar IPv4_addr : port 884.Sm on 885.It 886.Cm PermitOpen 887.Sm off 888.Ar \&[ IPv6_addr \&] : port 889.Sm on 890.El 891.Pp 892Multiple forwards may be specified by separating them with whitespace. 893An argument of 894.Dq any 895can be used to remove all restrictions and permit any forwarding requests. 896An argument of 897.Dq none 898can be used to prohibit all forwarding requests. 899By default all port forwarding requests are permitted. 900.It Cm PermitRootLogin 901Specifies whether root can log in using 902.Xr ssh 1 . 903The argument must be 904.Dq yes , 905.Dq without-password , 906.Dq forced-commands-only , 907or 908.Dq no . 909The default is 910.Dq no . 911.Pp 912If this option is set to 913.Dq without-password , 914password authentication is disabled for root. 915.Pp 916If this option is set to 917.Dq forced-commands-only , 918root login with public key authentication will be allowed, 919but only if the 920.Ar command 921option has been specified 922(which may be useful for taking remote backups even if root login is 923normally not allowed). 924All other authentication methods are disabled for root. 925.Pp 926If this option is set to 927.Dq no , 928root is not allowed to log in. 929.It Cm PermitTunnel 930Specifies whether 931.Xr tun 4 932device forwarding is allowed. 933The argument must be 934.Dq yes , 935.Dq point-to-point 936(layer 3), 937.Dq ethernet 938(layer 2), or 939.Dq no . 940Specifying 941.Dq yes 942permits both 943.Dq point-to-point 944and 945.Dq ethernet . 946The default is 947.Dq no . 948.It Cm PermitUserEnvironment 949Specifies whether 950.Pa ~/.ssh/environment 951and 952.Cm environment= 953options in 954.Pa ~/.ssh/authorized_keys 955are processed by 956.Xr sshd 8 . 957The default is 958.Dq no . 959Enabling environment processing may enable users to bypass access 960restrictions in some configurations using mechanisms such as 961.Ev LD_PRELOAD . 962.It Cm PidFile 963Specifies the file that contains the process ID of the 964SSH daemon. 965The default is 966.Pa /var/run/sshd.pid . 967.It Cm Port 968Specifies the port number that 969.Xr sshd 8 970listens on. 971The default is 22. 972Multiple options of this type are permitted. 973See also 974.Cm ListenAddress . 975.It Cm PrintLastLog 976Specifies whether 977.Xr sshd 8 978should print the date and time of the last user login when a user logs 979in interactively. 980The default is 981.Dq yes . 982.It Cm PrintMotd 983Specifies whether 984.Xr sshd 8 985should print 986.Pa /etc/motd 987when a user logs in interactively. 988(On some systems it is also printed by the shell, 989.Pa /etc/profile , 990or equivalent.) 991The default is 992.Dq yes . 993.It Cm Protocol 994Specifies the protocol versions 995.Xr sshd 8 996supports. 997The possible values are 998.Sq 1 999and 1000.Sq 2 . 1001Multiple versions must be comma-separated. 1002The default is 1003.Sq 2 . 1004Note that the order of the protocol list does not indicate preference, 1005because the client selects among multiple protocol versions offered 1006by the server. 1007Specifying 1008.Dq 2,1 1009is identical to 1010.Dq 1,2 . 1011.It Cm PubkeyAuthentication 1012Specifies whether public key authentication is allowed. 1013The default is 1014.Dq yes . 1015Note that this option applies to protocol version 2 only. 1016.It Cm RekeyLimit 1017Specifies the maximum amount of data that may be transmitted before the 1018session key is renegotiated, optionally followed a maximum amount of 1019time that may pass before the session key is renegotiated. 1020The first argument is specified in bytes and may have a suffix of 1021.Sq K , 1022.Sq M , 1023or 1024.Sq G 1025to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1026The default is between 1027.Sq 1G 1028and 1029.Sq 4G , 1030depending on the cipher. 1031The optional second value is specified in seconds and may use any of the 1032units documented in the 1033.Sx TIME FORMATS 1034section. 1035The default value for 1036.Cm RekeyLimit 1037is 1038.Dq default none , 1039which means that rekeying is performed after the cipher's default amount 1040of data has been sent or received and no time based rekeying is done. 1041This option applies to protocol version 2 only. 1042.It Cm RevokedKeys 1043Specifies revoked public keys. 1044Keys listed in this file will be refused for public key authentication. 1045Note that if this file is not readable, then public key authentication will 1046be refused for all users. 1047Keys may be specified as a text file, listing one public key per line, or as 1048an OpenSSH Key Revocation List (KRL) as generated by 1049.Xr ssh-keygen 1 . 1050For more information on KRLs, see the KEY REVOCATION LISTS section in 1051.Xr ssh-keygen 1 . 1052.It Cm RhostsRSAAuthentication 1053Specifies whether rhosts or /etc/hosts.equiv authentication together 1054with successful RSA host authentication is allowed. 1055The default is 1056.Dq no . 1057This option applies to protocol version 1 only. 1058.It Cm RSAAuthentication 1059Specifies whether pure RSA authentication is allowed. 1060The default is 1061.Dq yes . 1062This option applies to protocol version 1 only. 1063.It Cm ServerKeyBits 1064Defines the number of bits in the ephemeral protocol version 1 server key. 1065The minimum value is 512, and the default is 1024. 1066.It Cm StrictModes 1067Specifies whether 1068.Xr sshd 8 1069should check file modes and ownership of the 1070user's files and home directory before accepting login. 1071This is normally desirable because novices sometimes accidentally leave their 1072directory or files world-writable. 1073The default is 1074.Dq yes . 1075Note that this does not apply to 1076.Cm ChrootDirectory , 1077whose permissions and ownership are checked unconditionally. 1078.It Cm Subsystem 1079Configures an external subsystem (e.g. file transfer daemon). 1080Arguments should be a subsystem name and a command (with optional arguments) 1081to execute upon subsystem request. 1082.Pp 1083The command 1084.Xr sftp-server 8 1085implements the 1086.Dq sftp 1087file transfer subsystem. 1088.Pp 1089Alternately the name 1090.Dq internal-sftp 1091implements an in-process 1092.Dq sftp 1093server. 1094This may simplify configurations using 1095.Cm ChrootDirectory 1096to force a different filesystem root on clients. 1097.Pp 1098By default no subsystems are defined. 1099Note that this option applies to protocol version 2 only. 1100.It Cm SyslogFacility 1101Gives the facility code that is used when logging messages from 1102.Xr sshd 8 . 1103The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1104LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1105The default is AUTH. 1106.It Cm TCPKeepAlive 1107Specifies whether the system should send TCP keepalive messages to the 1108other side. 1109If they are sent, death of the connection or crash of one 1110of the machines will be properly noticed. 1111However, this means that 1112connections will die if the route is down temporarily, and some people 1113find it annoying. 1114On the other hand, if TCP keepalives are not sent, 1115sessions may hang indefinitely on the server, leaving 1116.Dq ghost 1117users and consuming server resources. 1118.Pp 1119The default is 1120.Dq yes 1121(to send TCP keepalive messages), and the server will notice 1122if the network goes down or the client host crashes. 1123This avoids infinitely hanging sessions. 1124.Pp 1125To disable TCP keepalive messages, the value should be set to 1126.Dq no . 1127.It Cm TrustedUserCAKeys 1128Specifies a file containing public keys of certificate authorities that are 1129trusted to sign user certificates for authentication. 1130Keys are listed one per line; empty lines and comments starting with 1131.Ql # 1132are allowed. 1133If a certificate is presented for authentication and has its signing CA key 1134listed in this file, then it may be used for authentication for any user 1135listed in the certificate's principals list. 1136Note that certificates that lack a list of principals will not be permitted 1137for authentication using 1138.Cm TrustedUserCAKeys . 1139For more details on certificates, see the CERTIFICATES section in 1140.Xr ssh-keygen 1 . 1141.It Cm UseDNS 1142Specifies whether 1143.Xr sshd 8 1144should look up the remote host name and check that 1145the resolved host name for the remote IP address maps back to the 1146very same IP address. 1147The default is 1148.Dq yes . 1149.It Cm UseLogin 1150Specifies whether 1151.Xr login 1 1152is used for interactive login sessions. 1153The default is 1154.Dq no . 1155Note that 1156.Xr login 1 1157is never used for remote command execution. 1158Note also, that if this is enabled, 1159.Cm X11Forwarding 1160will be disabled because 1161.Xr login 1 1162does not know how to handle 1163.Xr xauth 1 1164cookies. 1165If 1166.Cm UsePrivilegeSeparation 1167is specified, it will be disabled after authentication. 1168.It Cm UsePrivilegeSeparation 1169Specifies whether 1170.Xr sshd 8 1171separates privileges by creating an unprivileged child process 1172to deal with incoming network traffic. 1173After successful authentication, another process will be created that has 1174the privilege of the authenticated user. 1175The goal of privilege separation is to prevent privilege 1176escalation by containing any corruption within the unprivileged processes. 1177The default is 1178.Dq yes . 1179If 1180.Cm UsePrivilegeSeparation 1181is set to 1182.Dq sandbox 1183then the pre-authentication unprivileged process is subject to additional 1184restrictions. 1185.It Cm VersionAddendum 1186Optionally specifies additional text to append to the SSH protocol banner 1187sent by the server upon connection. 1188The default is 1189.Dq none . 1190.It Cm X11DisplayOffset 1191Specifies the first display number available for 1192.Xr sshd 8 Ns 's 1193X11 forwarding. 1194This prevents sshd from interfering with real X11 servers. 1195The default is 10. 1196.It Cm X11Forwarding 1197Specifies whether X11 forwarding is permitted. 1198The argument must be 1199.Dq yes 1200or 1201.Dq no . 1202The default is 1203.Dq no . 1204.Pp 1205When X11 forwarding is enabled, there may be additional exposure to 1206the server and to client displays if the 1207.Xr sshd 8 1208proxy display is configured to listen on the wildcard address (see 1209.Cm X11UseLocalhost 1210below), though this is not the default. 1211Additionally, the authentication spoofing and authentication data 1212verification and substitution occur on the client side. 1213The security risk of using X11 forwarding is that the client's X11 1214display server may be exposed to attack when the SSH client requests 1215forwarding (see the warnings for 1216.Cm ForwardX11 1217in 1218.Xr ssh_config 5 ) . 1219A system administrator may have a stance in which they want to 1220protect clients that may expose themselves to attack by unwittingly 1221requesting X11 forwarding, which can warrant a 1222.Dq no 1223setting. 1224.Pp 1225Note that disabling X11 forwarding does not prevent users from 1226forwarding X11 traffic, as users can always install their own forwarders. 1227X11 forwarding is automatically disabled if 1228.Cm UseLogin 1229is enabled. 1230.It Cm X11UseLocalhost 1231Specifies whether 1232.Xr sshd 8 1233should bind the X11 forwarding server to the loopback address or to 1234the wildcard address. 1235By default, 1236sshd binds the forwarding server to the loopback address and sets the 1237hostname part of the 1238.Ev DISPLAY 1239environment variable to 1240.Dq localhost . 1241This prevents remote hosts from connecting to the proxy display. 1242However, some older X11 clients may not function with this 1243configuration. 1244.Cm X11UseLocalhost 1245may be set to 1246.Dq no 1247to specify that the forwarding server should be bound to the wildcard 1248address. 1249The argument must be 1250.Dq yes 1251or 1252.Dq no . 1253The default is 1254.Dq yes . 1255.It Cm XAuthLocation 1256Specifies the full pathname of the 1257.Xr xauth 1 1258program. 1259The default is 1260.Pa /usr/X11R6/bin/xauth . 1261.It Cm UseLPK 1262Specifies whether LDAP public key retrieval must be used or not. It allow 1263an easy centralisation of public keys within an LDAP directory. The argument must be 1264.Dq yes 1265or 1266.Dq no . 1267.It Cm LpkLdapConf 1268Specifies whether LDAP Public keys should parse the specified ldap.conf file 1269instead of sshd_config Tokens. The argument must be a valid path to an ldap.conf 1270file like 1271.Pa /etc/ldap.conf 1272.It Cm LpkServers 1273Specifies LDAP one or more [:space:] separated server's url the following form may be used: 1274.Pp 1275LpkServers ldaps://127.0.0.1 ldap://127.0.0.2 ldap://127.0.0.3 1276.It Cm LpkUserDN 1277Specifies the LDAP user DN. 1278.Pp 1279LpkUserDN ou=users,dc=phear,dc=org 1280.It Cm LpkGroupDN 1281Specifies the LDAP groups DN. 1282.Pp 1283LpkGroupDN ou=groups,dc=phear,dc=org 1284.It Cm LpkBindDN 1285Specifies the LDAP bind DN to use if necessary. 1286.Pp 1287LpkBindDN cn=Manager,dc=phear,dc=org 1288.It Cm LpkBindPw 1289Specifies the LDAP bind credential. 1290.Pp 1291LpkBindPw secret 1292.It Cm LpkServerGroup 1293Specifies one or more [:space:] separated group the server is part of. 1294.Pp 1295LpkServerGroup unix mail prod 1296.It Cm LpkFilter 1297Specifies an additional LDAP filter to use for finding SSH keys 1298.Pp 1299LpkFilter (hostAccess=master.phear.org) 1300.It Cm LpkForceTLS 1301Specifies if the LDAP server connection must be tried, forced or not used. The argument must be 1302.Dq yes 1303or 1304.Dq no 1305or 1306.Dq try . 1307.It Cm LpkSearchTimelimit 1308Specifies the search time limit before the search is considered over. value is 1309in seconds. 1310.Pp 1311LpkSearchTimelimit 3 1312.It Cm LpkBindTimelimit 1313Specifies the bind time limit before the connection is considered dead. value is 1314in seconds. 1315.Pp 1316LpkBindTimelimit 3 1317.El 1318.Sh TIME FORMATS 1319.Xr sshd 8 1320command-line arguments and configuration file options that specify time 1321may be expressed using a sequence of the form: 1322.Sm off 1323.Ar time Op Ar qualifier , 1324.Sm on 1325where 1326.Ar time 1327is a positive integer value and 1328.Ar qualifier 1329is one of the following: 1330.Pp 1331.Bl -tag -width Ds -compact -offset indent 1332.It Aq Cm none 1333seconds 1334.It Cm s | Cm S 1335seconds 1336.It Cm m | Cm M 1337minutes 1338.It Cm h | Cm H 1339hours 1340.It Cm d | Cm D 1341days 1342.It Cm w | Cm W 1343weeks 1344.El 1345.Pp 1346Each member of the sequence is added together to calculate 1347the total time value. 1348.Pp 1349Time format examples: 1350.Pp 1351.Bl -tag -width Ds -compact -offset indent 1352.It 600 1353600 seconds (10 minutes) 1354.It 10m 135510 minutes 1356.It 1h30m 13571 hour 30 minutes (90 minutes) 1358.El 1359.Sh FILES 1360.Bl -tag -width Ds 1361.It Pa /etc/ssh/sshd_config 1362Contains configuration data for 1363.Xr sshd 8 . 1364This file should be writable by root only, but it is recommended 1365(though not necessary) that it be world-readable. 1366.El 1367.Sh SEE ALSO 1368.Xr sshd 8 1369.Sh AUTHORS 1370OpenSSH is a derivative of the original and free 1371ssh 1.2.12 release by Tatu Ylonen. 1372Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1373Theo de Raadt and Dug Song 1374removed many bugs, re-added newer features and 1375created OpenSSH. 1376Markus Friedl contributed the support for SSH 1377protocol versions 1.5 and 2.0. 1378Niels Provos and Markus Friedl contributed support 1379for privilege separation. 1380