1.\" -*- nroff -*- 2.\" 3.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 5.\" All rights reserved 6.\" 7.\" As far as I am concerned, the code I have written for this software 8.\" can be used freely for any purpose. Any derived versions of this 9.\" software must be clearly marked as such, and if the derived work is 10.\" incompatible with the protocol description in the RFC file, it must be 11.\" called by a name other than "ssh" or "Secure Shell". 12.\" 13.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $OpenBSD: sshd_config.5,v 1.106 2009/04/21 15:13:17 stevesk Exp $ 38.\" $FreeBSD$ 39.Dd April 21 2009 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 123.Sx PATTERNS 124in 125.Xr ssh_config 5 126for more information on patterns. 127.It Cm AllowTcpForwarding 128Specifies whether TCP forwarding is permitted. 129The default is 130.Dq yes . 131Note that disabling TCP forwarding does not improve security unless 132users are also denied shell access, as they can always install their 133own forwarders. 134.It Cm AllowUsers 135This keyword can be followed by a list of user name patterns, separated 136by spaces. 137If specified, login is allowed only for user names that 138match one of the patterns. 139Only user names are valid; a numerical user ID is not recognized. 140By default, login is allowed for all users. 141If the pattern takes the form USER@HOST then USER and HOST 142are separately checked, restricting logins to particular 143users from particular hosts. 144The allow/deny directives are processed in the following order: 145.Cm DenyUsers , 146.Cm AllowUsers , 147.Cm DenyGroups , 148and finally 149.Cm AllowGroups . 150.Pp 151See 152.Sx PATTERNS 153in 154.Xr ssh_config 5 155for more information on patterns. 156.It Cm AuthorizedKeysFile 157Specifies the file that contains the public keys that can be used 158for user authentication. 159.Cm AuthorizedKeysFile 160may contain tokens of the form %T which are substituted during connection 161setup. 162The following tokens are defined: %% is replaced by a literal '%', 163%h is replaced by the home directory of the user being authenticated, and 164%u is replaced by the username of that user. 165After expansion, 166.Cm AuthorizedKeysFile 167is taken to be an absolute path or one relative to the user's home 168directory. 169The default is 170.Dq .ssh/authorized_keys . 171.It Cm Banner 172The contents of the specified file are sent to the remote user before 173authentication is allowed. 174If the argument is 175.Dq none 176then no banner is displayed. 177This option is only available for protocol version 2. 178By default, no banner is displayed. 179.It Cm ChallengeResponseAuthentication 180Specifies whether challenge-response authentication is allowed (e.g. via 181PAM or though authentication styles supported in 182.Xr login.conf 5 ) 183The default is 184.Dq yes . 185.It Cm ChrootDirectory 186Specifies a path to 187.Xr chroot 2 188to after authentication. 189This path, and all its components, must be root-owned directories that are 190not writable by any other user or group. 191After the chroot, 192.Xr sshd 8 193changes the working directory to the user's home directory. 194.Pp 195The path may contain the following tokens that are expanded at runtime once 196the connecting user has been authenticated: %% is replaced by a literal '%', 197%h is replaced by the home directory of the user being authenticated, and 198%u is replaced by the username of that user. 199.Pp 200The 201.Cm ChrootDirectory 202must contain the necessary files and directories to support the 203user's session. 204For an interactive session this requires at least a shell, typically 205.Xr sh 1 , 206and basic 207.Pa /dev 208nodes such as 209.Xr null 4 , 210.Xr zero 4 , 211.Xr stdin 4 , 212.Xr stdout 4 , 213.Xr stderr 4 , 214.Xr arandom 4 215and 216.Xr tty 4 217devices. 218For file transfer sessions using 219.Dq sftp , 220no additional configuration of the environment is necessary if the 221in-process sftp server is used, 222though sessions which use logging do require 223.Pa /dev/log 224inside the chroot directory (see 225.Xr sftp-server 8 226for details). 227.Pp 228The default is not to 229.Xr chroot 2 . 230.It Cm Ciphers 231Specifies the ciphers allowed for protocol version 2. 232Multiple ciphers must be comma-separated. 233The supported ciphers are 234.Dq 3des-cbc , 235.Dq aes128-cbc , 236.Dq aes192-cbc , 237.Dq aes256-cbc , 238.Dq aes128-ctr , 239.Dq aes192-ctr , 240.Dq aes256-ctr , 241.Dq arcfour128 , 242.Dq arcfour256 , 243.Dq arcfour , 244.Dq blowfish-cbc , 245and 246.Dq cast128-cbc . 247The default is: 248.Bd -literal -offset 3n 249aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 250aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 251aes256-cbc,arcfour 252.Ed 253.It Cm ClientAliveCountMax 254Sets the number of client alive messages (see below) which may be 255sent without 256.Xr sshd 8 257receiving any messages back from the client. 258If this threshold is reached while client alive messages are being sent, 259sshd will disconnect the client, terminating the session. 260It is important to note that the use of client alive messages is very 261different from 262.Cm TCPKeepAlive 263(below). 264The client alive messages are sent through the encrypted channel 265and therefore will not be spoofable. 266The TCP keepalive option enabled by 267.Cm TCPKeepAlive 268is spoofable. 269The client alive mechanism is valuable when the client or 270server depend on knowing when a connection has become inactive. 271.Pp 272The default value is 3. 273If 274.Cm ClientAliveInterval 275(see below) is set to 15, and 276.Cm ClientAliveCountMax 277is left at the default, unresponsive SSH clients 278will be disconnected after approximately 45 seconds. 279This option applies to protocol version 2 only. 280.It Cm ClientAliveInterval 281Sets a timeout interval in seconds after which if no data has been received 282from the client, 283.Xr sshd 8 284will send a message through the encrypted 285channel to request a response from the client. 286The default 287is 0, indicating that these messages will not be sent to the client. 288This option applies to protocol version 2 only. 289.It Cm Compression 290Specifies whether compression is allowed, or delayed until 291the user has authenticated successfully. 292The argument must be 293.Dq yes , 294.Dq delayed , 295or 296.Dq no . 297The default is 298.Dq delayed . 299.It Cm DenyGroups 300This keyword can be followed by a list of group name patterns, separated 301by spaces. 302Login is disallowed for users whose primary group or supplementary 303group list matches one of the patterns. 304Only group names are valid; a numerical group ID is not recognized. 305By default, login is allowed for all groups. 306The allow/deny directives are processed in the following order: 307.Cm DenyUsers , 308.Cm AllowUsers , 309.Cm DenyGroups , 310and finally 311.Cm AllowGroups . 312.Pp 313See 314.Sx PATTERNS 315in 316.Xr ssh_config 5 317for more information on patterns. 318.It Cm DenyUsers 319This keyword can be followed by a list of user name patterns, separated 320by spaces. 321Login is disallowed for user names that match one of the patterns. 322Only user names are valid; a numerical user ID is not recognized. 323By default, login is allowed for all users. 324If the pattern takes the form USER@HOST then USER and HOST 325are separately checked, restricting logins to particular 326users from particular hosts. 327The allow/deny directives are processed in the following order: 328.Cm DenyUsers , 329.Cm AllowUsers , 330.Cm DenyGroups , 331and finally 332.Cm AllowGroups . 333.Pp 334See 335.Sx PATTERNS 336in 337.Xr ssh_config 5 338for more information on patterns. 339.It Cm ForceCommand 340Forces the execution of the command specified by 341.Cm ForceCommand , 342ignoring any command supplied by the client and 343.Pa ~/.ssh/rc 344if present. 345The command is invoked by using the user's login shell with the -c option. 346This applies to shell, command, or subsystem execution. 347It is most useful inside a 348.Cm Match 349block. 350The command originally supplied by the client is available in the 351.Ev SSH_ORIGINAL_COMMAND 352environment variable. 353Specifying a command of 354.Dq internal-sftp 355will force the use of an in-process sftp server that requires no support 356files when used with 357.Cm ChrootDirectory . 358.It Cm GatewayPorts 359Specifies whether remote hosts are allowed to connect to ports 360forwarded for the client. 361By default, 362.Xr sshd 8 363binds remote port forwardings to the loopback address. 364This prevents other remote hosts from connecting to forwarded ports. 365.Cm GatewayPorts 366can be used to specify that sshd 367should allow remote port forwardings to bind to non-loopback addresses, thus 368allowing other hosts to connect. 369The argument may be 370.Dq no 371to force remote port forwardings to be available to the local host only, 372.Dq yes 373to force remote port forwardings to bind to the wildcard address, or 374.Dq clientspecified 375to allow the client to select the address to which the forwarding is bound. 376The default is 377.Dq no . 378.It Cm GSSAPIAuthentication 379Specifies whether user authentication based on GSSAPI is allowed. 380The default is 381.Dq no . 382Note that this option applies to protocol version 2 only. 383.It Cm GSSAPICleanupCredentials 384Specifies whether to automatically destroy the user's credentials cache 385on logout. 386The default is 387.Dq yes . 388Note that this option applies to protocol version 2 only. 389.It Cm HostbasedAuthentication 390Specifies whether rhosts or /etc/hosts.equiv authentication together 391with successful public key client host authentication is allowed 392(host-based authentication). 393This option is similar to 394.Cm RhostsRSAAuthentication 395and applies to protocol version 2 only. 396The default is 397.Dq no . 398.It Cm HostbasedUsesNameFromPacketOnly 399Specifies whether or not the server will attempt to perform a reverse 400name lookup when matching the name in the 401.Pa ~/.shosts , 402.Pa ~/.rhosts , 403and 404.Pa /etc/hosts.equiv 405files during 406.Cm HostbasedAuthentication . 407A setting of 408.Dq yes 409means that 410.Xr sshd 8 411uses the name supplied by the client rather than 412attempting to resolve the name from the TCP connection itself. 413The default is 414.Dq no . 415.It Cm HostKey 416Specifies a file containing a private host key 417used by SSH. 418The default is 419.Pa /etc/ssh/ssh_host_key 420for protocol version 1, and 421.Pa /etc/ssh/ssh_host_rsa_key 422and 423.Pa /etc/ssh/ssh_host_dsa_key 424for protocol version 2. 425Note that 426.Xr sshd 8 427will refuse to use a file if it is group/world-accessible. 428It is possible to have multiple host key files. 429.Dq rsa1 430keys are used for version 1 and 431.Dq dsa 432or 433.Dq rsa 434are used for version 2 of the SSH protocol. 435.It Cm IgnoreRhosts 436Specifies that 437.Pa .rhosts 438and 439.Pa .shosts 440files will not be used in 441.Cm RhostsRSAAuthentication 442or 443.Cm HostbasedAuthentication . 444.Pp 445.Pa /etc/hosts.equiv 446and 447.Pa /etc/ssh/shosts.equiv 448are still used. 449The default is 450.Dq yes . 451.It Cm IgnoreUserKnownHosts 452Specifies whether 453.Xr sshd 8 454should ignore the user's 455.Pa ~/.ssh/known_hosts 456during 457.Cm RhostsRSAAuthentication 458or 459.Cm HostbasedAuthentication . 460The default is 461.Dq no . 462.It Cm KerberosAuthentication 463Specifies whether the password provided by the user for 464.Cm PasswordAuthentication 465will be validated through the Kerberos KDC. 466To use this option, the server needs a 467Kerberos servtab which allows the verification of the KDC's identity. 468The default is 469.Dq no . 470.It Cm KerberosGetAFSToken 471If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 472an AFS token before accessing the user's home directory. 473The default is 474.Dq no . 475.It Cm KerberosOrLocalPasswd 476If password authentication through Kerberos fails then 477the password will be validated via any additional local mechanism 478such as 479.Pa /etc/passwd . 480The default is 481.Dq yes . 482.It Cm KerberosTicketCleanup 483Specifies whether to automatically destroy the user's ticket cache 484file on logout. 485The default is 486.Dq yes . 487.It Cm KeyRegenerationInterval 488In protocol version 1, the ephemeral server key is automatically regenerated 489after this many seconds (if it has been used). 490The purpose of regeneration is to prevent 491decrypting captured sessions by later breaking into the machine and 492stealing the keys. 493The key is never stored anywhere. 494If the value is 0, the key is never regenerated. 495The default is 3600 (seconds). 496.It Cm ListenAddress 497Specifies the local addresses 498.Xr sshd 8 499should listen on. 500The following forms may be used: 501.Pp 502.Bl -item -offset indent -compact 503.It 504.Cm ListenAddress 505.Sm off 506.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 507.Sm on 508.It 509.Cm ListenAddress 510.Sm off 511.Ar host No | Ar IPv4_addr No : Ar port 512.Sm on 513.It 514.Cm ListenAddress 515.Sm off 516.Oo 517.Ar host No | Ar IPv6_addr Oc : Ar port 518.Sm on 519.El 520.Pp 521If 522.Ar port 523is not specified, 524sshd will listen on the address and all prior 525.Cm Port 526options specified. 527The default is to listen on all local addresses. 528Multiple 529.Cm ListenAddress 530options are permitted. 531Additionally, any 532.Cm Port 533options must precede this option for non-port qualified addresses. 534.It Cm LoginGraceTime 535The server disconnects after this time if the user has not 536successfully logged in. 537If the value is 0, there is no time limit. 538The default is 120 seconds. 539.It Cm LogLevel 540Gives the verbosity level that is used when logging messages from 541.Xr sshd 8 . 542The possible values are: 543QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 544The default is INFO. 545DEBUG and DEBUG1 are equivalent. 546DEBUG2 and DEBUG3 each specify higher levels of debugging output. 547Logging with a DEBUG level violates the privacy of users and is not recommended. 548.It Cm MACs 549Specifies the available MAC (message authentication code) algorithms. 550The MAC algorithm is used in protocol version 2 551for data integrity protection. 552Multiple algorithms must be comma-separated. 553The default is: 554.Bd -literal -offset indent 555hmac-md5,hmac-sha1,umac-64@openssh.com, 556hmac-ripemd160,hmac-sha1-96,hmac-md5-96 557.Ed 558.It Cm Match 559Introduces a conditional block. 560If all of the criteria on the 561.Cm Match 562line are satisfied, the keywords on the following lines override those 563set in the global section of the config file, until either another 564.Cm Match 565line or the end of the file. 566.Pp 567The arguments to 568.Cm Match 569are one or more criteria-pattern pairs. 570The available criteria are 571.Cm User , 572.Cm Group , 573.Cm Host , 574and 575.Cm Address . 576The match patterns may consist of single entries or comma-separated 577lists and may use the wildcard and negation operators described in the 578.Sx PATTERNS 579section of 580.Xr ssh_config 5 . 581.Pp 582The patterns in an 583.Cm Address 584criteria may additionally contain addresses to match in CIDR 585address/masklen format, e.g.\& 586.Dq 192.0.2.0/24 587or 588.Dq 3ffe:ffff::/32 . 589Note that the mask length provided must be consistent with the address - 590it is an error to specify a mask length that is too long for the address 591or one with bits set in this host portion of the address. 592For example, 593.Dq 192.0.2.0/33 594and 595.Dq 192.0.2.0/8 596respectively. 597.Pp 598Only a subset of keywords may be used on the lines following a 599.Cm Match 600keyword. 601Available keywords are 602.Cm AllowAgentForwarding , 603.Cm AllowTcpForwarding , 604.Cm Banner , 605.Cm ChrootDirectory , 606.Cm ForceCommand , 607.Cm GatewayPorts , 608.Cm GSSAPIAuthentication , 609.Cm HostbasedAuthentication , 610.Cm KbdInteractiveAuthentication , 611.Cm KerberosAuthentication , 612.Cm MaxAuthTries , 613.Cm MaxSessions , 614.Cm PasswordAuthentication , 615.Cm PermitEmptyPasswords , 616.Cm PermitOpen , 617.Cm PermitRootLogin , 618.Cm RhostsRSAAuthentication , 619.Cm RSAAuthentication , 620.Cm X11DisplayOffset , 621.Cm X11Forwarding 622and 623.Cm X11UseLocalHost . 624.It Cm MaxAuthTries 625Specifies the maximum number of authentication attempts permitted per 626connection. 627Once the number of failures reaches half this value, 628additional failures are logged. 629The default is 6. 630.It Cm MaxSessions 631Specifies the maximum number of open sessions permitted per network connection. 632The default is 10. 633.It Cm MaxStartups 634Specifies the maximum number of concurrent unauthenticated connections to the 635SSH daemon. 636Additional connections will be dropped until authentication succeeds or the 637.Cm LoginGraceTime 638expires for a connection. 639The default is 10. 640.Pp 641Alternatively, random early drop can be enabled by specifying 642the three colon separated values 643.Dq start:rate:full 644(e.g. "10:30:60"). 645.Xr sshd 8 646will refuse connection attempts with a probability of 647.Dq rate/100 648(30%) 649if there are currently 650.Dq start 651(10) 652unauthenticated connections. 653The probability increases linearly and all connection attempts 654are refused if the number of unauthenticated connections reaches 655.Dq full 656(60). 657.It Cm PasswordAuthentication 658Specifies whether password authentication is allowed. 659See also 660.Cm UsePAM . 661The default is 662.Dq no . 663.It Cm PermitEmptyPasswords 664When password authentication is allowed, it specifies whether the 665server allows login to accounts with empty password strings. 666The default is 667.Dq no . 668.It Cm PermitOpen 669Specifies the destinations to which TCP port forwarding is permitted. 670The forwarding specification must be one of the following forms: 671.Pp 672.Bl -item -offset indent -compact 673.It 674.Cm PermitOpen 675.Sm off 676.Ar host : port 677.Sm on 678.It 679.Cm PermitOpen 680.Sm off 681.Ar IPv4_addr : port 682.Sm on 683.It 684.Cm PermitOpen 685.Sm off 686.Ar \&[ IPv6_addr \&] : port 687.Sm on 688.El 689.Pp 690Multiple forwards may be specified by separating them with whitespace. 691An argument of 692.Dq any 693can be used to remove all restrictions and permit any forwarding requests. 694By default all port forwarding requests are permitted. 695.It Cm PermitRootLogin 696Specifies whether root can log in using 697.Xr ssh 1 . 698The argument must be 699.Dq yes , 700.Dq without-password , 701.Dq forced-commands-only , 702or 703.Dq no . 704The default is 705.Dq no . 706Note that if 707.Cm ChallengeResponseAuthentication 708is 709.Dq yes , 710the root user may be allowed in with its password even if 711.Cm PermitRootLogin is set to 712.Dq without-password . 713.Pp 714If this option is set to 715.Dq without-password , 716password authentication is disabled for root. 717.Pp 718If this option is set to 719.Dq forced-commands-only , 720root login with public key authentication will be allowed, 721but only if the 722.Ar command 723option has been specified 724(which may be useful for taking remote backups even if root login is 725normally not allowed). 726All other authentication methods are disabled for root. 727.Pp 728If this option is set to 729.Dq no , 730root is not allowed to log in. 731.It Cm PermitTunnel 732Specifies whether 733.Xr tun 4 734device forwarding is allowed. 735The argument must be 736.Dq yes , 737.Dq point-to-point 738(layer 3), 739.Dq ethernet 740(layer 2), or 741.Dq no . 742Specifying 743.Dq yes 744permits both 745.Dq point-to-point 746and 747.Dq ethernet . 748The default is 749.Dq no . 750.It Cm PermitUserEnvironment 751Specifies whether 752.Pa ~/.ssh/environment 753and 754.Cm environment= 755options in 756.Pa ~/.ssh/authorized_keys 757are processed by 758.Xr sshd 8 . 759The default is 760.Dq no . 761Enabling environment processing may enable users to bypass access 762restrictions in some configurations using mechanisms such as 763.Ev LD_PRELOAD . 764.It Cm PidFile 765Specifies the file that contains the process ID of the 766SSH daemon. 767The default is 768.Pa /var/run/sshd.pid . 769.It Cm Port 770Specifies the port number that 771.Xr sshd 8 772listens on. 773The default is 22. 774Multiple options of this type are permitted. 775See also 776.Cm ListenAddress . 777.It Cm PrintLastLog 778Specifies whether 779.Xr sshd 8 780should print the date and time of the last user login when a user logs 781in interactively. 782The default is 783.Dq yes . 784.It Cm PrintMotd 785Specifies whether 786.Xr sshd 8 787should print 788.Pa /etc/motd 789when a user logs in interactively. 790(On some systems it is also printed by the shell, 791.Pa /etc/profile , 792or equivalent.) 793The default is 794.Dq yes . 795.It Cm Protocol 796Specifies the protocol versions 797.Xr sshd 8 798supports. 799The possible values are 800.Sq 1 801and 802.Sq 2 . 803Multiple versions must be comma-separated. 804The default is 805.Dq 2 . 806Note that the order of the protocol list does not indicate preference, 807because the client selects among multiple protocol versions offered 808by the server. 809Specifying 810.Dq 2,1 811is identical to 812.Dq 1,2 . 813.It Cm PubkeyAuthentication 814Specifies whether public key authentication is allowed. 815The default is 816.Dq yes . 817Note that this option applies to protocol version 2 only. 818.It Cm RhostsRSAAuthentication 819Specifies whether rhosts or 820.Pa /etc/hosts.equiv 821authentication together 822with successful RSA host authentication is allowed. 823The default is 824.Dq no . 825This option applies to protocol version 1 only. 826.It Cm RSAAuthentication 827Specifies whether pure RSA authentication is allowed. 828The default is 829.Dq yes . 830This option applies to protocol version 1 only. 831.It Cm ServerKeyBits 832Defines the number of bits in the ephemeral protocol version 1 server key. 833The minimum value is 512, and the default is 1024. 834.It Cm StrictModes 835Specifies whether 836.Xr sshd 8 837should check file modes and ownership of the 838user's files and home directory before accepting login. 839This is normally desirable because novices sometimes accidentally leave their 840directory or files world-writable. 841The default is 842.Dq yes . 843.It Cm Subsystem 844Configures an external subsystem (e.g. file transfer daemon). 845Arguments should be a subsystem name and a command (with optional arguments) 846to execute upon subsystem request. 847.Pp 848The command 849.Xr sftp-server 8 850implements the 851.Dq sftp 852file transfer subsystem. 853.Pp 854Alternately the name 855.Dq internal-sftp 856implements an in-process 857.Dq sftp 858server. 859This may simplify configurations using 860.Cm ChrootDirectory 861to force a different filesystem root on clients. 862.Pp 863By default no subsystems are defined. 864Note that this option applies to protocol version 2 only. 865.It Cm SyslogFacility 866Gives the facility code that is used when logging messages from 867.Xr sshd 8 . 868The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 869LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 870The default is AUTH. 871.It Cm TCPKeepAlive 872Specifies whether the system should send TCP keepalive messages to the 873other side. 874If they are sent, death of the connection or crash of one 875of the machines will be properly noticed. 876However, this means that 877connections will die if the route is down temporarily, and some people 878find it annoying. 879On the other hand, if TCP keepalives are not sent, 880sessions may hang indefinitely on the server, leaving 881.Dq ghost 882users and consuming server resources. 883.Pp 884The default is 885.Dq yes 886(to send TCP keepalive messages), and the server will notice 887if the network goes down or the client host crashes. 888This avoids infinitely hanging sessions. 889.Pp 890To disable TCP keepalive messages, the value should be set to 891.Dq no . 892.It Cm UseDNS 893Specifies whether 894.Xr sshd 8 895should look up the remote host name and check that 896the resolved host name for the remote IP address maps back to the 897very same IP address. 898The default is 899.Dq yes . 900.It Cm UseLogin 901Specifies whether 902.Xr login 1 903is used for interactive login sessions. 904The default is 905.Dq no . 906Note that 907.Xr login 1 908is never used for remote command execution. 909Note also, that if this is enabled, 910.Cm X11Forwarding 911will be disabled because 912.Xr login 1 913does not know how to handle 914.Xr xauth 1 915cookies. 916If 917.Cm UsePrivilegeSeparation 918is specified, it will be disabled after authentication. 919.It Cm UsePAM 920Enables the Pluggable Authentication Module interface. 921If set to 922.Dq yes 923this will enable PAM authentication using 924.Cm ChallengeResponseAuthentication 925and 926.Cm PasswordAuthentication 927in addition to PAM account and session module processing for all 928authentication types. 929.Pp 930Because PAM challenge-response authentication usually serves an equivalent 931role to password authentication, you should disable either 932.Cm PasswordAuthentication 933or 934.Cm ChallengeResponseAuthentication. 935.Pp 936If 937.Cm UsePAM 938is enabled, you will not be able to run 939.Xr sshd 8 940as a non-root user. 941The default is 942.Dq yes . 943.It Cm UsePrivilegeSeparation 944Specifies whether 945.Xr sshd 8 946separates privileges by creating an unprivileged child process 947to deal with incoming network traffic. 948After successful authentication, another process will be created that has 949the privilege of the authenticated user. 950The goal of privilege separation is to prevent privilege 951escalation by containing any corruption within the unprivileged processes. 952The default is 953.Dq yes . 954.It Cm VersionAddendum 955Specifies a string to append to the regular version string to identify 956OS- or site-specific modifications. 957The default is 958.Dq FreeBSD-20091001 . 959.It Cm X11DisplayOffset 960Specifies the first display number available for 961.Xr sshd 8 Ns 's 962X11 forwarding. 963This prevents sshd from interfering with real X11 servers. 964The default is 10. 965.It Cm X11Forwarding 966Specifies whether X11 forwarding is permitted. 967The argument must be 968.Dq yes 969or 970.Dq no . 971The default is 972.Dq yes . 973.Pp 974When X11 forwarding is enabled, there may be additional exposure to 975the server and to client displays if the 976.Xr sshd 8 977proxy display is configured to listen on the wildcard address (see 978.Cm X11UseLocalhost 979below), though this is not the default. 980Additionally, the authentication spoofing and authentication data 981verification and substitution occur on the client side. 982The security risk of using X11 forwarding is that the client's X11 983display server may be exposed to attack when the SSH client requests 984forwarding (see the warnings for 985.Cm ForwardX11 986in 987.Xr ssh_config 5 ) . 988A system administrator may have a stance in which they want to 989protect clients that may expose themselves to attack by unwittingly 990requesting X11 forwarding, which can warrant a 991.Dq no 992setting. 993.Pp 994Note that disabling X11 forwarding does not prevent users from 995forwarding X11 traffic, as users can always install their own forwarders. 996X11 forwarding is automatically disabled if 997.Cm UseLogin 998is enabled. 999.It Cm X11UseLocalhost 1000Specifies whether 1001.Xr sshd 8 1002should bind the X11 forwarding server to the loopback address or to 1003the wildcard address. 1004By default, 1005sshd binds the forwarding server to the loopback address and sets the 1006hostname part of the 1007.Ev DISPLAY 1008environment variable to 1009.Dq localhost . 1010This prevents remote hosts from connecting to the proxy display. 1011However, some older X11 clients may not function with this 1012configuration. 1013.Cm X11UseLocalhost 1014may be set to 1015.Dq no 1016to specify that the forwarding server should be bound to the wildcard 1017address. 1018The argument must be 1019.Dq yes 1020or 1021.Dq no . 1022The default is 1023.Dq yes . 1024.It Cm XAuthLocation 1025Specifies the full pathname of the 1026.Xr xauth 1 1027program. 1028The default is 1029.Pa /usr/local/bin/xauth . 1030.El 1031.Sh TIME FORMATS 1032.Xr sshd 8 1033command-line arguments and configuration file options that specify time 1034may be expressed using a sequence of the form: 1035.Sm off 1036.Ar time Op Ar qualifier , 1037.Sm on 1038where 1039.Ar time 1040is a positive integer value and 1041.Ar qualifier 1042is one of the following: 1043.Pp 1044.Bl -tag -width Ds -compact -offset indent 1045.It Aq Cm none 1046seconds 1047.It Cm s | Cm S 1048seconds 1049.It Cm m | Cm M 1050minutes 1051.It Cm h | Cm H 1052hours 1053.It Cm d | Cm D 1054days 1055.It Cm w | Cm W 1056weeks 1057.El 1058.Pp 1059Each member of the sequence is added together to calculate 1060the total time value. 1061.Pp 1062Time format examples: 1063.Pp 1064.Bl -tag -width Ds -compact -offset indent 1065.It 600 1066600 seconds (10 minutes) 1067.It 10m 106810 minutes 1069.It 1h30m 10701 hour 30 minutes (90 minutes) 1071.El 1072.Sh FILES 1073.Bl -tag -width Ds 1074.It Pa /etc/ssh/sshd_config 1075Contains configuration data for 1076.Xr sshd 8 . 1077This file should be writable by root only, but it is recommended 1078(though not necessary) that it be world-readable. 1079.El 1080.Sh SEE ALSO 1081.Xr sshd 8 1082.Sh AUTHORS 1083OpenSSH is a derivative of the original and free 1084ssh 1.2.12 release by Tatu Ylonen. 1085Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1086Theo de Raadt and Dug Song 1087removed many bugs, re-added newer features and 1088created OpenSSH. 1089Markus Friedl contributed the support for SSH 1090protocol versions 1.5 and 2.0. 1091Niels Provos and Markus Friedl contributed support 1092for privilege separation. 1093