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 1.170 2013/12/08 09:53:27 dtucker Exp $ 37.\" $FreeBSD$ 38.Dd December 8, 2013 39.Dt SSHD_CONFIG 5 40.Os 41.Sh NAME 42.Nm sshd_config 43.Nd OpenSSH SSH daemon configuration file 44.Sh SYNOPSIS 45.Nm /etc/ssh/sshd_config 46.Sh DESCRIPTION 47.Xr sshd 8 48reads configuration data from 49.Pa /etc/ssh/sshd_config 50(or the file specified with 51.Fl f 52on the command line). 53The file contains keyword-argument pairs, one per line. 54Lines starting with 55.Ql # 56and empty lines are interpreted as comments. 57Arguments may optionally be enclosed in double quotes 58.Pq \&" 59in order to represent arguments containing spaces. 60.Pp 61The possible 62keywords and their meanings are as follows (note that 63keywords are case-insensitive and arguments are case-sensitive): 64.Bl -tag -width Ds 65.It Cm AcceptEnv 66Specifies what environment variables sent by the client will be copied into 67the session's 68.Xr environ 7 . 69See 70.Cm SendEnv 71in 72.Xr ssh_config 5 73for how to configure the client. 74Note that environment passing is only supported for protocol 2. 75Variables are specified by name, which may contain the wildcard characters 76.Ql * 77and 78.Ql \&? . 79Multiple environment variables may be separated by whitespace or spread 80across multiple 81.Cm AcceptEnv 82directives. 83Be warned that some environment variables could be used to bypass restricted 84user environments. 85For this reason, care should be taken in the use of this directive. 86The default is not to accept any environment variables. 87.It Cm AddressFamily 88Specifies which address family should be used by 89.Xr sshd 8 . 90Valid arguments are 91.Dq any , 92.Dq inet 93(use IPv4 only), or 94.Dq inet6 95(use IPv6 only). 96The default is 97.Dq any . 98.It Cm AllowAgentForwarding 99Specifies whether 100.Xr ssh-agent 1 101forwarding is permitted. 102The default is 103.Dq yes . 104Note that disabling agent forwarding does not improve security 105unless users are also denied shell access, as they can always install 106their own forwarders. 107.It Cm AllowGroups 108This keyword can be followed by a list of group name patterns, separated 109by spaces. 110If specified, login is allowed only for users whose primary 111group or supplementary group list matches one of the patterns. 112Only group names are valid; a numerical group ID is not recognized. 113By default, login is allowed for all groups. 114The allow/deny directives are processed in the following order: 115.Cm DenyUsers , 116.Cm AllowUsers , 117.Cm DenyGroups , 118and finally 119.Cm AllowGroups . 120.Pp 121See PATTERNS in 122.Xr ssh_config 5 123for more information on patterns. 124.It Cm AllowTcpForwarding 125Specifies whether TCP forwarding is permitted. 126The available options are 127.Dq yes 128or 129.Dq all 130to allow TCP forwarding, 131.Dq no 132to prevent all TCP forwarding, 133.Dq local 134to allow local (from the perspective of 135.Xr ssh 1 ) 136forwarding only or 137.Dq remote 138to allow remote forwarding only. 139The default is 140.Dq yes . 141Note that disabling TCP forwarding does not improve security unless 142users are also denied shell access, as they can always install their 143own forwarders. 144.It Cm AllowUsers 145This keyword can be followed by a list of user name patterns, separated 146by spaces. 147If specified, login is allowed only for user names that 148match one of the patterns. 149Only user names are valid; a numerical user ID is not recognized. 150By default, login is allowed for all users. 151If the pattern takes the form USER@HOST then USER and HOST 152are separately checked, restricting logins to particular 153users from particular hosts. 154The allow/deny directives are processed in the following order: 155.Cm DenyUsers , 156.Cm AllowUsers , 157.Cm DenyGroups , 158and finally 159.Cm AllowGroups . 160.Pp 161See PATTERNS in 162.Xr ssh_config 5 163for more information on patterns. 164.It Cm AuthenticationMethods 165Specifies the authentication methods that must be successfully completed 166for a user to be granted access. 167This option must be followed by one or more comma-separated lists of 168authentication method names. 169Successful authentication requires completion of every method in at least 170one of these lists. 171.Pp 172For example, an argument of 173.Dq publickey,password publickey,keyboard-interactive 174would require the user to complete public key authentication, followed by 175either password or keyboard interactive authentication. 176Only methods that are next in one or more lists are offered at each stage, 177so for this example, it would not be possible to attempt password or 178keyboard-interactive authentication before public key. 179.Pp 180For keyboard interactive authentication it is also possible to 181restrict authentication to a specific device by appending a 182colon followed by the device identifier 183.Dq bsdauth , 184.Dq pam , 185or 186.Dq skey , 187depending on the server configuration. 188For example, 189.Dq keyboard-interactive:bsdauth 190would restrict keyboard interactive authentication to the 191.Dq bsdauth 192device. 193.Pp 194This option is only available for SSH protocol 2 and will yield a fatal 195error if enabled if protocol 1 is also enabled. 196Note that each authentication method listed should also be explicitly enabled 197in the configuration. 198The default is not to require multiple authentication; successful completion 199of a single authentication method is sufficient. 200.It Cm AuthorizedKeysCommand 201Specifies a program to be used to look up the user's public keys. 202The program must be owned by root and not writable by group or others. 203It will be invoked with a single argument of the username 204being authenticated, and should produce on standard output zero or 205more lines of authorized_keys output (see AUTHORIZED_KEYS in 206.Xr sshd 8 ) . 207If a key supplied by AuthorizedKeysCommand does not successfully authenticate 208and authorize the user then public key authentication continues using the usual 209.Cm AuthorizedKeysFile 210files. 211By default, no AuthorizedKeysCommand is run. 212.It Cm AuthorizedKeysCommandUser 213Specifies the user under whose account the AuthorizedKeysCommand is run. 214It is recommended to use a dedicated user that has no other role on the host 215than running authorized keys commands. 216.It Cm AuthorizedKeysFile 217Specifies the file that contains the public keys that can be used 218for user authentication. 219The format is described in the 220AUTHORIZED_KEYS FILE FORMAT 221section of 222.Xr sshd 8 . 223.Cm AuthorizedKeysFile 224may contain tokens of the form %T which are substituted during connection 225setup. 226The following tokens are defined: %% is replaced by a literal '%', 227%h is replaced by the home directory of the user being authenticated, and 228%u is replaced by the username of that user. 229After expansion, 230.Cm AuthorizedKeysFile 231is taken to be an absolute path or one relative to the user's home 232directory. 233Multiple files may be listed, separated by whitespace. 234The default is 235.Dq .ssh/authorized_keys .ssh/authorized_keys2 . 236.It Cm AuthorizedPrincipalsFile 237Specifies a file that lists principal names that are accepted for 238certificate authentication. 239When using certificates signed by a key listed in 240.Cm TrustedUserCAKeys , 241this file lists names, one of which must appear in the certificate for it 242to be accepted for authentication. 243Names are listed one per line preceded by key options (as described 244in AUTHORIZED_KEYS FILE FORMAT in 245.Xr sshd 8 ) . 246Empty lines and comments starting with 247.Ql # 248are ignored. 249.Pp 250.Cm AuthorizedPrincipalsFile 251may contain tokens of the form %T which are substituted during connection 252setup. 253The following tokens are defined: %% is replaced by a literal '%', 254%h is replaced by the home directory of the user being authenticated, and 255%u is replaced by the username of that user. 256After expansion, 257.Cm AuthorizedPrincipalsFile 258is taken to be an absolute path or one relative to the user's home 259directory. 260.Pp 261The default is 262.Dq none , 263i.e. not to use a principals file \(en in this case, the username 264of the user must appear in a certificate's principals list for it to be 265accepted. 266Note that 267.Cm AuthorizedPrincipalsFile 268is only used when authentication proceeds using a CA listed in 269.Cm TrustedUserCAKeys 270and is not consulted for certification authorities trusted via 271.Pa ~/.ssh/authorized_keys , 272though the 273.Cm principals= 274key option offers a similar facility (see 275.Xr sshd 8 276for details). 277.It Cm Banner 278The contents of the specified file are sent to the remote user before 279authentication is allowed. 280If the argument is 281.Dq none 282then no banner is displayed. 283This option is only available for protocol version 2. 284By default, no banner is displayed. 285.It Cm ChallengeResponseAuthentication 286Specifies whether challenge-response authentication is allowed (e.g. via 287PAM or though authentication styles supported in 288.Xr login.conf 5 ) 289The default is 290.Dq yes . 291.It Cm ChrootDirectory 292Specifies the pathname of a directory to 293.Xr chroot 2 294to after authentication. 295All components of the pathname must be root-owned directories that are 296not writable by any other user or group. 297After the chroot, 298.Xr sshd 8 299changes the working directory to the user's home directory. 300.Pp 301The pathname may contain the following tokens that are expanded at runtime once 302the connecting user has been authenticated: %% is replaced by a literal '%', 303%h is replaced by the home directory of the user being authenticated, and 304%u is replaced by the username of that user. 305.Pp 306The 307.Cm ChrootDirectory 308must contain the necessary files and directories to support the 309user's session. 310For an interactive session this requires at least a shell, typically 311.Xr sh 1 , 312and basic 313.Pa /dev 314nodes such as 315.Xr null 4 , 316.Xr zero 4 , 317.Xr stdin 4 , 318.Xr stdout 4 , 319.Xr stderr 4 , 320.Xr arandom 4 321and 322.Xr tty 4 323devices. 324For file transfer sessions using 325.Dq sftp , 326no additional configuration of the environment is necessary if the 327in-process sftp server is used, 328though sessions which use logging do require 329.Pa /dev/log 330inside the chroot directory (see 331.Xr sftp-server 8 332for details). 333.Pp 334The default is not to 335.Xr chroot 2 . 336.It Cm Ciphers 337Specifies the ciphers allowed for protocol version 2. 338Multiple ciphers must be comma-separated. 339The supported ciphers are: 340.Pp 341.Dq 3des-cbc , 342.Dq aes128-cbc , 343.Dq aes192-cbc , 344.Dq aes256-cbc , 345.Dq aes128-ctr , 346.Dq aes192-ctr , 347.Dq aes256-ctr , 348.Dq aes128-gcm@openssh.com , 349.Dq aes256-gcm@openssh.com , 350.Dq arcfour128 , 351.Dq arcfour256 , 352.Dq arcfour , 353.Dq blowfish-cbc , 354.Dq cast128-cbc , 355and 356.Dq chacha20-poly1305@openssh.com . 357.Pp 358The default is: 359.Bd -literal -offset 3n 360aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 361aes128-gcm@openssh.com,aes256-gcm@openssh.com, 362chacha20-poly1305@openssh.com, 363aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 364aes256-cbc,arcfour 365.Ed 366.Pp 367The list of available ciphers may also be obtained using the 368.Fl Q 369option of 370.Xr ssh 1 . 371.It Cm ClientAliveCountMax 372Sets the number of client alive messages (see below) which may be 373sent without 374.Xr sshd 8 375receiving any messages back from the client. 376If this threshold is reached while client alive messages are being sent, 377sshd will disconnect the client, terminating the session. 378It is important to note that the use of client alive messages is very 379different from 380.Cm TCPKeepAlive 381(below). 382The client alive messages are sent through the encrypted channel 383and therefore will not be spoofable. 384The TCP keepalive option enabled by 385.Cm TCPKeepAlive 386is spoofable. 387The client alive mechanism is valuable when the client or 388server depend on knowing when a connection has become inactive. 389.Pp 390The default value is 3. 391If 392.Cm ClientAliveInterval 393(see below) is set to 15, and 394.Cm ClientAliveCountMax 395is left at the default, unresponsive SSH clients 396will be disconnected after approximately 45 seconds. 397This option applies to protocol version 2 only. 398.It Cm ClientAliveInterval 399Sets a timeout interval in seconds after which if no data has been received 400from the client, 401.Xr sshd 8 402will send a message through the encrypted 403channel to request a response from the client. 404The default 405is 0, indicating that these messages will not be sent to the client. 406This option applies to protocol version 2 only. 407.It Cm Compression 408Specifies whether compression is allowed, or delayed until 409the user has authenticated successfully. 410The argument must be 411.Dq yes , 412.Dq delayed , 413or 414.Dq no . 415The default is 416.Dq delayed . 417.It Cm DenyGroups 418This keyword can be followed by a list of group name patterns, separated 419by spaces. 420Login is disallowed for users whose primary group or supplementary 421group list matches one of the patterns. 422Only group names are valid; a numerical group ID is not recognized. 423By default, login is allowed for all groups. 424The allow/deny directives are processed in the following order: 425.Cm DenyUsers , 426.Cm AllowUsers , 427.Cm DenyGroups , 428and finally 429.Cm AllowGroups . 430.Pp 431See PATTERNS in 432.Xr ssh_config 5 433for more information on patterns. 434.It Cm DenyUsers 435This keyword can be followed by a list of user name patterns, separated 436by spaces. 437Login is disallowed for user names that match one of the patterns. 438Only user names are valid; a numerical user ID is not recognized. 439By default, login is allowed for all users. 440If the pattern takes the form USER@HOST then USER and HOST 441are separately checked, restricting logins to particular 442users from particular hosts. 443The allow/deny directives are processed in the following order: 444.Cm DenyUsers , 445.Cm AllowUsers , 446.Cm DenyGroups , 447and finally 448.Cm AllowGroups . 449.Pp 450See PATTERNS in 451.Xr ssh_config 5 452for more information on patterns. 453.It Cm ForceCommand 454Forces the execution of the command specified by 455.Cm ForceCommand , 456ignoring any command supplied by the client and 457.Pa ~/.ssh/rc 458if present. 459The command is invoked by using the user's login shell with the -c option. 460This applies to shell, command, or subsystem execution. 461It is most useful inside a 462.Cm Match 463block. 464The command originally supplied by the client is available in the 465.Ev SSH_ORIGINAL_COMMAND 466environment variable. 467Specifying a command of 468.Dq internal-sftp 469will force the use of an in-process sftp server that requires no support 470files when used with 471.Cm ChrootDirectory . 472.It Cm GatewayPorts 473Specifies whether remote hosts are allowed to connect to ports 474forwarded for the client. 475By default, 476.Xr sshd 8 477binds remote port forwardings to the loopback address. 478This prevents other remote hosts from connecting to forwarded ports. 479.Cm GatewayPorts 480can be used to specify that sshd 481should allow remote port forwardings to bind to non-loopback addresses, thus 482allowing other hosts to connect. 483The argument may be 484.Dq no 485to force remote port forwardings to be available to the local host only, 486.Dq yes 487to force remote port forwardings to bind to the wildcard address, or 488.Dq clientspecified 489to allow the client to select the address to which the forwarding is bound. 490The default is 491.Dq no . 492.It Cm GSSAPIAuthentication 493Specifies whether user authentication based on GSSAPI is allowed. 494The default is 495.Dq no . 496Note that this option applies to protocol version 2 only. 497.It Cm GSSAPICleanupCredentials 498Specifies whether to automatically destroy the user's credentials cache 499on logout. 500The default is 501.Dq yes . 502Note that this option applies to protocol version 2 only. 503.It Cm HostbasedAuthentication 504Specifies whether rhosts or /etc/hosts.equiv authentication together 505with successful public key client host authentication is allowed 506(host-based authentication). 507This option is similar to 508.Cm RhostsRSAAuthentication 509and applies to protocol version 2 only. 510The default is 511.Dq no . 512.It Cm HostbasedUsesNameFromPacketOnly 513Specifies whether or not the server will attempt to perform a reverse 514name lookup when matching the name in the 515.Pa ~/.shosts , 516.Pa ~/.rhosts , 517and 518.Pa /etc/hosts.equiv 519files during 520.Cm HostbasedAuthentication . 521A setting of 522.Dq yes 523means that 524.Xr sshd 8 525uses the name supplied by the client rather than 526attempting to resolve the name from the TCP connection itself. 527The default is 528.Dq no . 529.It Cm HostCertificate 530Specifies a file containing a public host certificate. 531The certificate's public key must match a private host key already specified 532by 533.Cm HostKey . 534The default behaviour of 535.Xr sshd 8 536is not to load any certificates. 537.It Cm HostKey 538Specifies a file containing a private host key 539used by SSH. 540The default is 541.Pa /etc/ssh/ssh_host_key 542for protocol version 1, and 543.Pa /etc/ssh/ssh_host_dsa_key , 544.Pa /etc/ssh/ssh_host_ecdsa_key , 545.Pa /etc/ssh/ssh_host_ed25519_key 546and 547.Pa /etc/ssh/ssh_host_rsa_key 548for protocol version 2. 549Note that 550.Xr sshd 8 551will refuse to use a file if it is group/world-accessible. 552It is possible to have multiple host key files. 553.Dq rsa1 554keys are used for version 1 and 555.Dq dsa , 556.Dq ecdsa , 557.Dq ed25519 558or 559.Dq rsa 560are used for version 2 of the SSH protocol. 561It is also possible to specify public host key files instead. 562In this case operations on the private key will be delegated 563to an 564.Xr ssh-agent 1 . 565.It Cm HostKeyAgent 566Identifies the UNIX-domain socket used to communicate 567with an agent that has access to the private host keys. 568If 569.Dq SSH_AUTH_SOCK 570is specified, the location of the socket will be read from the 571.Ev SSH_AUTH_SOCK 572environment variable. 573.It Cm IgnoreRhosts 574Specifies that 575.Pa .rhosts 576and 577.Pa .shosts 578files will not be used in 579.Cm RhostsRSAAuthentication 580or 581.Cm HostbasedAuthentication . 582.Pp 583.Pa /etc/hosts.equiv 584and 585.Pa /etc/ssh/shosts.equiv 586are still used. 587The default is 588.Dq yes . 589.It Cm IgnoreUserKnownHosts 590Specifies whether 591.Xr sshd 8 592should ignore the user's 593.Pa ~/.ssh/known_hosts 594during 595.Cm RhostsRSAAuthentication 596or 597.Cm HostbasedAuthentication . 598The default is 599.Dq no . 600.It Cm IPQoS 601Specifies the IPv4 type-of-service or DSCP class for the connection. 602Accepted values are 603.Dq af11 , 604.Dq af12 , 605.Dq af13 , 606.Dq af21 , 607.Dq af22 , 608.Dq af23 , 609.Dq af31 , 610.Dq af32 , 611.Dq af33 , 612.Dq af41 , 613.Dq af42 , 614.Dq af43 , 615.Dq cs0 , 616.Dq cs1 , 617.Dq cs2 , 618.Dq cs3 , 619.Dq cs4 , 620.Dq cs5 , 621.Dq cs6 , 622.Dq cs7 , 623.Dq ef , 624.Dq lowdelay , 625.Dq throughput , 626.Dq reliability , 627or a numeric value. 628This option may take one or two arguments, separated by whitespace. 629If one argument is specified, it is used as the packet class unconditionally. 630If two values are specified, the first is automatically selected for 631interactive sessions and the second for non-interactive sessions. 632The default is 633.Dq lowdelay 634for interactive sessions and 635.Dq throughput 636for non-interactive sessions. 637.It Cm KerberosAuthentication 638Specifies whether the password provided by the user for 639.Cm PasswordAuthentication 640will be validated through the Kerberos KDC. 641To use this option, the server needs a 642Kerberos servtab which allows the verification of the KDC's identity. 643The default is 644.Dq no . 645.It Cm KerberosGetAFSToken 646If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 647an AFS token before accessing the user's home directory. 648The default is 649.Dq no . 650.It Cm KerberosOrLocalPasswd 651If password authentication through Kerberos fails then 652the password will be validated via any additional local mechanism 653such as 654.Pa /etc/passwd . 655The default is 656.Dq yes . 657.It Cm KerberosTicketCleanup 658Specifies whether to automatically destroy the user's ticket cache 659file on logout. 660The default is 661.Dq yes . 662.It Cm KexAlgorithms 663Specifies the available KEX (Key Exchange) algorithms. 664Multiple algorithms must be comma-separated. 665The default is 666.Bd -literal -offset indent 667curve25519-sha256@libssh.org, 668ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 669diffie-hellman-group-exchange-sha256, 670diffie-hellman-group-exchange-sha1, 671diffie-hellman-group14-sha1, 672diffie-hellman-group1-sha1 673.Ed 674.It Cm KeyRegenerationInterval 675In protocol version 1, the ephemeral server key is automatically regenerated 676after this many seconds (if it has been used). 677The purpose of regeneration is to prevent 678decrypting captured sessions by later breaking into the machine and 679stealing the keys. 680The key is never stored anywhere. 681If the value is 0, the key is never regenerated. 682The default is 3600 (seconds). 683.It Cm ListenAddress 684Specifies the local addresses 685.Xr sshd 8 686should listen on. 687The following forms may be used: 688.Pp 689.Bl -item -offset indent -compact 690.It 691.Cm ListenAddress 692.Sm off 693.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 694.Sm on 695.It 696.Cm ListenAddress 697.Sm off 698.Ar host No | Ar IPv4_addr No : Ar port 699.Sm on 700.It 701.Cm ListenAddress 702.Sm off 703.Oo 704.Ar host No | Ar IPv6_addr Oc : Ar port 705.Sm on 706.El 707.Pp 708If 709.Ar port 710is not specified, 711sshd will listen on the address and all prior 712.Cm Port 713options specified. 714The default is to listen on all local addresses. 715Multiple 716.Cm ListenAddress 717options are permitted. 718Additionally, any 719.Cm Port 720options must precede this option for non-port qualified addresses. 721.It Cm LoginGraceTime 722The server disconnects after this time if the user has not 723successfully logged in. 724If the value is 0, there is no time limit. 725The default is 120 seconds. 726.It Cm LogLevel 727Gives the verbosity level that is used when logging messages from 728.Xr sshd 8 . 729The possible values are: 730QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 731The default is INFO. 732DEBUG and DEBUG1 are equivalent. 733DEBUG2 and DEBUG3 each specify higher levels of debugging output. 734Logging with a DEBUG level violates the privacy of users and is not recommended. 735.It Cm MACs 736Specifies the available MAC (message authentication code) algorithms. 737The MAC algorithm is used in protocol version 2 738for data integrity protection. 739Multiple algorithms must be comma-separated. 740The algorithms that contain 741.Dq -etm 742calculate the MAC after encryption (encrypt-then-mac). 743These are considered safer and their use recommended. 744The default is: 745.Bd -literal -offset indent 746hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com, 747umac-64-etm@openssh.com,umac-128-etm@openssh.com, 748hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 749hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com, 750hmac-md5-96-etm@openssh.com, 751hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com, 752hmac-sha2-256,hmac-sha2-512,hmac-ripemd160, 753hmac-sha1-96,hmac-md5-96 754.Ed 755.It Cm Match 756Introduces a conditional block. 757If all of the criteria on the 758.Cm Match 759line are satisfied, the keywords on the following lines override those 760set in the global section of the config file, until either another 761.Cm Match 762line or the end of the file. 763.Pp 764The arguments to 765.Cm Match 766are one or more criteria-pattern pairs or the single token 767.Cm All 768which matches all criteria. 769The available criteria are 770.Cm User , 771.Cm Group , 772.Cm Host , 773.Cm LocalAddress , 774.Cm LocalPort , 775and 776.Cm Address . 777The match patterns may consist of single entries or comma-separated 778lists and may use the wildcard and negation operators described in the 779PATTERNS section of 780.Xr ssh_config 5 . 781.Pp 782The patterns in an 783.Cm Address 784criteria may additionally contain addresses to match in CIDR 785address/masklen format, e.g.\& 786.Dq 192.0.2.0/24 787or 788.Dq 3ffe:ffff::/32 . 789Note that the mask length provided must be consistent with the address - 790it is an error to specify a mask length that is too long for the address 791or one with bits set in this host portion of the address. 792For example, 793.Dq 192.0.2.0/33 794and 795.Dq 192.0.2.0/8 796respectively. 797.Pp 798Only a subset of keywords may be used on the lines following a 799.Cm Match 800keyword. 801Available keywords are 802.Cm AcceptEnv , 803.Cm AllowAgentForwarding , 804.Cm AllowGroups , 805.Cm AllowTcpForwarding , 806.Cm AllowUsers , 807.Cm AuthenticationMethods , 808.Cm AuthorizedKeysCommand , 809.Cm AuthorizedKeysCommandUser , 810.Cm AuthorizedKeysFile , 811.Cm AuthorizedPrincipalsFile , 812.Cm Banner , 813.Cm ChrootDirectory , 814.Cm DenyGroups , 815.Cm DenyUsers , 816.Cm ForceCommand , 817.Cm GatewayPorts , 818.Cm GSSAPIAuthentication , 819.Cm HostbasedAuthentication , 820.Cm HostbasedUsesNameFromPacketOnly , 821.Cm KbdInteractiveAuthentication , 822.Cm KerberosAuthentication , 823.Cm MaxAuthTries , 824.Cm MaxSessions , 825.Cm PasswordAuthentication , 826.Cm PermitEmptyPasswords , 827.Cm PermitOpen , 828.Cm PermitRootLogin , 829.Cm PermitTTY , 830.Cm PermitTunnel , 831.Cm PubkeyAuthentication , 832.Cm RekeyLimit , 833.Cm RhostsRSAAuthentication , 834.Cm RSAAuthentication , 835.Cm X11DisplayOffset , 836.Cm X11Forwarding 837and 838.Cm X11UseLocalHost . 839.It Cm MaxAuthTries 840Specifies the maximum number of authentication attempts permitted per 841connection. 842Once the number of failures reaches half this value, 843additional failures are logged. 844The default is 6. 845.It Cm MaxSessions 846Specifies the maximum number of open sessions permitted per network connection. 847The default is 10. 848.It Cm MaxStartups 849Specifies the maximum number of concurrent unauthenticated connections to the 850SSH daemon. 851Additional connections will be dropped until authentication succeeds or the 852.Cm LoginGraceTime 853expires for a connection. 854The default is 10:30:100. 855.Pp 856Alternatively, random early drop can be enabled by specifying 857the three colon separated values 858.Dq start:rate:full 859(e.g. "10:30:60"). 860.Xr sshd 8 861will refuse connection attempts with a probability of 862.Dq rate/100 863(30%) 864if there are currently 865.Dq start 866(10) 867unauthenticated connections. 868The probability increases linearly and all connection attempts 869are refused if the number of unauthenticated connections reaches 870.Dq full 871(60). 872.It Cm PasswordAuthentication 873Specifies whether password authentication is allowed. 874See also 875.Cm UsePAM . 876The default is 877.Dq no . 878.It Cm PermitEmptyPasswords 879When password authentication is allowed, it specifies whether the 880server allows login to accounts with empty password strings. 881The default is 882.Dq no . 883.It Cm PermitOpen 884Specifies the destinations to which TCP port forwarding is permitted. 885The forwarding specification must be one of the following forms: 886.Pp 887.Bl -item -offset indent -compact 888.It 889.Cm PermitOpen 890.Sm off 891.Ar host : port 892.Sm on 893.It 894.Cm PermitOpen 895.Sm off 896.Ar IPv4_addr : port 897.Sm on 898.It 899.Cm PermitOpen 900.Sm off 901.Ar \&[ IPv6_addr \&] : port 902.Sm on 903.El 904.Pp 905Multiple forwards may be specified by separating them with whitespace. 906An argument of 907.Dq any 908can be used to remove all restrictions and permit any forwarding requests. 909An argument of 910.Dq none 911can be used to prohibit all forwarding requests. 912By default all port forwarding requests are permitted. 913.It Cm PermitRootLogin 914Specifies whether root can log in using 915.Xr ssh 1 . 916The argument must be 917.Dq yes , 918.Dq without-password , 919.Dq forced-commands-only , 920or 921.Dq no . 922The default is 923.Dq no . 924Note that if 925.Cm ChallengeResponseAuthentication 926is 927.Dq yes , 928the root user may be allowed in with its password even if 929.Cm PermitRootLogin is set to 930.Dq without-password . 931.Pp 932If this option is set to 933.Dq without-password , 934password authentication is disabled for root. 935.Pp 936If this option is set to 937.Dq forced-commands-only , 938root login with public key authentication will be allowed, 939but only if the 940.Ar command 941option has been specified 942(which may be useful for taking remote backups even if root login is 943normally not allowed). 944All other authentication methods are disabled for root. 945.Pp 946If this option is set to 947.Dq no , 948root is not allowed to log in. 949.It Cm PermitTunnel 950Specifies whether 951.Xr tun 4 952device forwarding is allowed. 953The argument must be 954.Dq yes , 955.Dq point-to-point 956(layer 3), 957.Dq ethernet 958(layer 2), or 959.Dq no . 960Specifying 961.Dq yes 962permits both 963.Dq point-to-point 964and 965.Dq ethernet . 966The default is 967.Dq no . 968.It Cm PermitTTY 969Specifies whether 970.Xr pty 4 971allocation is permitted. 972The default is 973.Dq yes . 974.It Cm PermitUserEnvironment 975Specifies whether 976.Pa ~/.ssh/environment 977and 978.Cm environment= 979options in 980.Pa ~/.ssh/authorized_keys 981are processed by 982.Xr sshd 8 . 983The default is 984.Dq no . 985Enabling environment processing may enable users to bypass access 986restrictions in some configurations using mechanisms such as 987.Ev LD_PRELOAD . 988.It Cm PidFile 989Specifies the file that contains the process ID of the 990SSH daemon. 991The default is 992.Pa /var/run/sshd.pid . 993.It Cm Port 994Specifies the port number that 995.Xr sshd 8 996listens on. 997The default is 22. 998Multiple options of this type are permitted. 999See also 1000.Cm ListenAddress . 1001.It Cm PrintLastLog 1002Specifies whether 1003.Xr sshd 8 1004should print the date and time of the last user login when a user logs 1005in interactively. 1006The default is 1007.Dq yes . 1008.It Cm PrintMotd 1009Specifies whether 1010.Xr sshd 8 1011should print 1012.Pa /etc/motd 1013when a user logs in interactively. 1014(On some systems it is also printed by the shell, 1015.Pa /etc/profile , 1016or equivalent.) 1017The default is 1018.Dq yes . 1019.It Cm Protocol 1020Specifies the protocol versions 1021.Xr sshd 8 1022supports. 1023The possible values are 1024.Sq 1 1025and 1026.Sq 2 . 1027Multiple versions must be comma-separated. 1028The default is 1029.Sq 2 . 1030Note that the order of the protocol list does not indicate preference, 1031because the client selects among multiple protocol versions offered 1032by the server. 1033Specifying 1034.Dq 2,1 1035is identical to 1036.Dq 1,2 . 1037.It Cm PubkeyAuthentication 1038Specifies whether public key authentication is allowed. 1039The default is 1040.Dq yes . 1041Note that this option applies to protocol version 2 only. 1042.It Cm RekeyLimit 1043Specifies the maximum amount of data that may be transmitted before the 1044session key is renegotiated, optionally followed a maximum amount of 1045time that may pass before the session key is renegotiated. 1046The first argument is specified in bytes and may have a suffix of 1047.Sq K , 1048.Sq M , 1049or 1050.Sq G 1051to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1052The default is between 1053.Sq 1G 1054and 1055.Sq 4G , 1056depending on the cipher. 1057The optional second value is specified in seconds and may use any of the 1058units documented in the 1059.Sx TIME FORMATS 1060section. 1061The default value for 1062.Cm RekeyLimit 1063is 1064.Dq default none , 1065which means that rekeying is performed after the cipher's default amount 1066of data has been sent or received and no time based rekeying is done. 1067This option applies to protocol version 2 only. 1068.It Cm RevokedKeys 1069Specifies revoked public keys. 1070Keys listed in this file will be refused for public key authentication. 1071Note that if this file is not readable, then public key authentication will 1072be refused for all users. 1073Keys may be specified as a text file, listing one public key per line, or as 1074an OpenSSH Key Revocation List (KRL) as generated by 1075.Xr ssh-keygen 1 . 1076For more information on KRLs, see the KEY REVOCATION LISTS section in 1077.Xr ssh-keygen 1 . 1078.It Cm RhostsRSAAuthentication 1079Specifies whether rhosts or 1080.Pa /etc/hosts.equiv 1081authentication together 1082with successful RSA host authentication is allowed. 1083The default is 1084.Dq no . 1085This option applies to protocol version 1 only. 1086.It Cm RSAAuthentication 1087Specifies whether pure RSA authentication is allowed. 1088The default is 1089.Dq yes . 1090This option applies to protocol version 1 only. 1091.It Cm ServerKeyBits 1092Defines the number of bits in the ephemeral protocol version 1 server key. 1093The minimum value is 512, and the default is 1024. 1094.It Cm StrictModes 1095Specifies whether 1096.Xr sshd 8 1097should check file modes and ownership of the 1098user's files and home directory before accepting login. 1099This is normally desirable because novices sometimes accidentally leave their 1100directory or files world-writable. 1101The default is 1102.Dq yes . 1103Note that this does not apply to 1104.Cm ChrootDirectory , 1105whose permissions and ownership are checked unconditionally. 1106.It Cm Subsystem 1107Configures an external subsystem (e.g. file transfer daemon). 1108Arguments should be a subsystem name and a command (with optional arguments) 1109to execute upon subsystem request. 1110.Pp 1111The command 1112.Xr sftp-server 8 1113implements the 1114.Dq sftp 1115file transfer subsystem. 1116.Pp 1117Alternately the name 1118.Dq internal-sftp 1119implements an in-process 1120.Dq sftp 1121server. 1122This may simplify configurations using 1123.Cm ChrootDirectory 1124to force a different filesystem root on clients. 1125.Pp 1126By default no subsystems are defined. 1127Note that this option applies to protocol version 2 only. 1128.It Cm SyslogFacility 1129Gives the facility code that is used when logging messages from 1130.Xr sshd 8 . 1131The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1132LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1133The default is AUTH. 1134.It Cm TCPKeepAlive 1135Specifies whether the system should send TCP keepalive messages to the 1136other side. 1137If they are sent, death of the connection or crash of one 1138of the machines will be properly noticed. 1139However, this means that 1140connections will die if the route is down temporarily, and some people 1141find it annoying. 1142On the other hand, if TCP keepalives are not sent, 1143sessions may hang indefinitely on the server, leaving 1144.Dq ghost 1145users and consuming server resources. 1146.Pp 1147The default is 1148.Dq yes 1149(to send TCP keepalive messages), and the server will notice 1150if the network goes down or the client host crashes. 1151This avoids infinitely hanging sessions. 1152.Pp 1153To disable TCP keepalive messages, the value should be set to 1154.Dq no . 1155.It Cm TrustedUserCAKeys 1156Specifies a file containing public keys of certificate authorities that are 1157trusted to sign user certificates for authentication. 1158Keys are listed one per line; empty lines and comments starting with 1159.Ql # 1160are allowed. 1161If a certificate is presented for authentication and has its signing CA key 1162listed in this file, then it may be used for authentication for any user 1163listed in the certificate's principals list. 1164Note that certificates that lack a list of principals will not be permitted 1165for authentication using 1166.Cm TrustedUserCAKeys . 1167For more details on certificates, see the CERTIFICATES section in 1168.Xr ssh-keygen 1 . 1169.It Cm UseDNS 1170Specifies whether 1171.Xr sshd 8 1172should look up the remote host name and check that 1173the resolved host name for the remote IP address maps back to the 1174very same IP address. 1175The default is 1176.Dq yes . 1177.It Cm UseLogin 1178Specifies whether 1179.Xr login 1 1180is used for interactive login sessions. 1181The default is 1182.Dq no . 1183Note that 1184.Xr login 1 1185is never used for remote command execution. 1186Note also, that if this is enabled, 1187.Cm X11Forwarding 1188will be disabled because 1189.Xr login 1 1190does not know how to handle 1191.Xr xauth 1 1192cookies. 1193If 1194.Cm UsePrivilegeSeparation 1195is specified, it will be disabled after authentication. 1196.It Cm UsePAM 1197Enables the Pluggable Authentication Module interface. 1198If set to 1199.Dq yes 1200this will enable PAM authentication using 1201.Cm ChallengeResponseAuthentication 1202and 1203.Cm PasswordAuthentication 1204in addition to PAM account and session module processing for all 1205authentication types. 1206.Pp 1207Because PAM challenge-response authentication usually serves an equivalent 1208role to password authentication, you should disable either 1209.Cm PasswordAuthentication 1210or 1211.Cm ChallengeResponseAuthentication. 1212.Pp 1213If 1214.Cm UsePAM 1215is enabled, you will not be able to run 1216.Xr sshd 8 1217as a non-root user. 1218The default is 1219.Dq yes . 1220.It Cm UsePrivilegeSeparation 1221Specifies whether 1222.Xr sshd 8 1223separates privileges by creating an unprivileged child process 1224to deal with incoming network traffic. 1225After successful authentication, another process will be created that has 1226the privilege of the authenticated user. 1227The goal of privilege separation is to prevent privilege 1228escalation by containing any corruption within the unprivileged processes. 1229The default is 1230.Dq sandbox . 1231If 1232.Cm UsePrivilegeSeparation 1233is set to 1234.Dq sandbox 1235then the pre-authentication unprivileged process is subject to additional 1236restrictions. 1237.It Cm VersionAddendum 1238Optionally specifies additional text to append to the SSH protocol banner 1239sent by the server upon connection. 1240The default is 1241.Dq FreeBSD-20140130 . 1242.It Cm X11DisplayOffset 1243Specifies the first display number available for 1244.Xr sshd 8 Ns 's 1245X11 forwarding. 1246This prevents sshd from interfering with real X11 servers. 1247The default is 10. 1248.It Cm X11Forwarding 1249Specifies whether X11 forwarding is permitted. 1250The argument must be 1251.Dq yes 1252or 1253.Dq no . 1254The default is 1255.Dq yes . 1256.Pp 1257When X11 forwarding is enabled, there may be additional exposure to 1258the server and to client displays if the 1259.Xr sshd 8 1260proxy display is configured to listen on the wildcard address (see 1261.Cm X11UseLocalhost 1262below), though this is not the default. 1263Additionally, the authentication spoofing and authentication data 1264verification and substitution occur on the client side. 1265The security risk of using X11 forwarding is that the client's X11 1266display server may be exposed to attack when the SSH client requests 1267forwarding (see the warnings for 1268.Cm ForwardX11 1269in 1270.Xr ssh_config 5 ) . 1271A system administrator may have a stance in which they want to 1272protect clients that may expose themselves to attack by unwittingly 1273requesting X11 forwarding, which can warrant a 1274.Dq no 1275setting. 1276.Pp 1277Note that disabling X11 forwarding does not prevent users from 1278forwarding X11 traffic, as users can always install their own forwarders. 1279X11 forwarding is automatically disabled if 1280.Cm UseLogin 1281is enabled. 1282.It Cm X11UseLocalhost 1283Specifies whether 1284.Xr sshd 8 1285should bind the X11 forwarding server to the loopback address or to 1286the wildcard address. 1287By default, 1288sshd binds the forwarding server to the loopback address and sets the 1289hostname part of the 1290.Ev DISPLAY 1291environment variable to 1292.Dq localhost . 1293This prevents remote hosts from connecting to the proxy display. 1294However, some older X11 clients may not function with this 1295configuration. 1296.Cm X11UseLocalhost 1297may be set to 1298.Dq no 1299to specify that the forwarding server should be bound to the wildcard 1300address. 1301The argument must be 1302.Dq yes 1303or 1304.Dq no . 1305The default is 1306.Dq yes . 1307.It Cm XAuthLocation 1308Specifies the full pathname of the 1309.Xr xauth 1 1310program. 1311The default is 1312.Pa /usr/local/bin/xauth . 1313.El 1314.Sh TIME FORMATS 1315.Xr sshd 8 1316command-line arguments and configuration file options that specify time 1317may be expressed using a sequence of the form: 1318.Sm off 1319.Ar time Op Ar qualifier , 1320.Sm on 1321where 1322.Ar time 1323is a positive integer value and 1324.Ar qualifier 1325is one of the following: 1326.Pp 1327.Bl -tag -width Ds -compact -offset indent 1328.It Aq Cm none 1329seconds 1330.It Cm s | Cm S 1331seconds 1332.It Cm m | Cm M 1333minutes 1334.It Cm h | Cm H 1335hours 1336.It Cm d | Cm D 1337days 1338.It Cm w | Cm W 1339weeks 1340.El 1341.Pp 1342Each member of the sequence is added together to calculate 1343the total time value. 1344.Pp 1345Time format examples: 1346.Pp 1347.Bl -tag -width Ds -compact -offset indent 1348.It 600 1349600 seconds (10 minutes) 1350.It 10m 135110 minutes 1352.It 1h30m 13531 hour 30 minutes (90 minutes) 1354.El 1355.Sh FILES 1356.Bl -tag -width Ds 1357.It Pa /etc/ssh/sshd_config 1358Contains configuration data for 1359.Xr sshd 8 . 1360This file should be writable by root only, but it is recommended 1361(though not necessary) that it be world-readable. 1362.El 1363.Sh SEE ALSO 1364.Xr sshd 8 1365.Sh AUTHORS 1366OpenSSH is a derivative of the original and free 1367ssh 1.2.12 release by Tatu Ylonen. 1368Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1369Theo de Raadt and Dug Song 1370removed many bugs, re-added newer features and 1371created OpenSSH. 1372Markus Friedl contributed the support for SSH 1373protocol versions 1.5 and 2.0. 1374Niels Provos and Markus Friedl contributed support 1375for privilege separation. 1376