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.Dd $Mdocdate: April 21 2009 $ 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 122.Sx PATTERNS 123in 124.Xr ssh_config 5 125for more information on patterns. 126.It Cm AllowTcpForwarding 127Specifies whether TCP forwarding is permitted. 128The default is 129.Dq yes . 130Note that disabling TCP forwarding does not improve security unless 131users are also denied shell access, as they can always install their 132own forwarders. 133.It Cm AllowUsers 134This keyword can be followed by a list of user name patterns, separated 135by spaces. 136If specified, login is allowed only for user names that 137match one of the patterns. 138Only user names are valid; a numerical user ID is not recognized. 139By default, login is allowed for all users. 140If the pattern takes the form USER@HOST then USER and HOST 141are separately checked, restricting logins to particular 142users from particular hosts. 143The allow/deny directives are processed in the following order: 144.Cm DenyUsers , 145.Cm AllowUsers , 146.Cm DenyGroups , 147and finally 148.Cm AllowGroups . 149.Pp 150See 151.Sx PATTERNS 152in 153.Xr ssh_config 5 154for more information on patterns. 155.It Cm AuthorizedKeysFile 156Specifies the file that contains the public keys that can be used 157for user authentication. 158.Cm AuthorizedKeysFile 159may contain tokens of the form %T which are substituted during connection 160setup. 161The following tokens are defined: %% is replaced by a literal '%', 162%h is replaced by the home directory of the user being authenticated, and 163%u is replaced by the username of that user. 164After expansion, 165.Cm AuthorizedKeysFile 166is taken to be an absolute path or one relative to the user's home 167directory. 168The default is 169.Dq .ssh/authorized_keys . 170.It Cm Banner 171The contents of the specified file are sent to the remote user before 172authentication is allowed. 173If the argument is 174.Dq none 175then no banner is displayed. 176This option is only available for protocol version 2. 177By default, no banner is displayed. 178.It Cm ChallengeResponseAuthentication 179Specifies whether challenge-response authentication is allowed. 180All authentication styles from 181.Xr login.conf 5 182are supported. 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/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. 659The default is 660.Dq yes . 661.It Cm PermitEmptyPasswords 662When password authentication is allowed, it specifies whether the 663server allows login to accounts with empty password strings. 664The default is 665.Dq no . 666.It Cm PermitOpen 667Specifies the destinations to which TCP port forwarding is permitted. 668The forwarding specification must be one of the following forms: 669.Pp 670.Bl -item -offset indent -compact 671.It 672.Cm PermitOpen 673.Sm off 674.Ar host : port 675.Sm on 676.It 677.Cm PermitOpen 678.Sm off 679.Ar IPv4_addr : port 680.Sm on 681.It 682.Cm PermitOpen 683.Sm off 684.Ar \&[ IPv6_addr \&] : port 685.Sm on 686.El 687.Pp 688Multiple forwards may be specified by separating them with whitespace. 689An argument of 690.Dq any 691can be used to remove all restrictions and permit any forwarding requests. 692By default all port forwarding requests are permitted. 693.It Cm PermitRootLogin 694Specifies whether root can log in using 695.Xr ssh 1 . 696The argument must be 697.Dq yes , 698.Dq without-password , 699.Dq forced-commands-only , 700or 701.Dq no . 702The default is 703.Dq yes . 704.Pp 705If this option is set to 706.Dq without-password , 707password authentication is disabled for root. 708.Pp 709If this option is set to 710.Dq forced-commands-only , 711root login with public key authentication will be allowed, 712but only if the 713.Ar command 714option has been specified 715(which may be useful for taking remote backups even if root login is 716normally not allowed). 717All other authentication methods are disabled for root. 718.Pp 719If this option is set to 720.Dq no , 721root is not allowed to log in. 722.It Cm PermitTunnel 723Specifies whether 724.Xr tun 4 725device forwarding is allowed. 726The argument must be 727.Dq yes , 728.Dq point-to-point 729(layer 3), 730.Dq ethernet 731(layer 2), or 732.Dq no . 733Specifying 734.Dq yes 735permits both 736.Dq point-to-point 737and 738.Dq ethernet . 739The default is 740.Dq no . 741.It Cm PermitUserEnvironment 742Specifies whether 743.Pa ~/.ssh/environment 744and 745.Cm environment= 746options in 747.Pa ~/.ssh/authorized_keys 748are processed by 749.Xr sshd 8 . 750The default is 751.Dq no . 752Enabling environment processing may enable users to bypass access 753restrictions in some configurations using mechanisms such as 754.Ev LD_PRELOAD . 755.It Cm PidFile 756Specifies the file that contains the process ID of the 757SSH daemon. 758The default is 759.Pa /var/run/sshd.pid . 760.It Cm Port 761Specifies the port number that 762.Xr sshd 8 763listens on. 764The default is 22. 765Multiple options of this type are permitted. 766See also 767.Cm ListenAddress . 768.It Cm PrintLastLog 769Specifies whether 770.Xr sshd 8 771should print the date and time of the last user login when a user logs 772in interactively. 773The default is 774.Dq yes . 775.It Cm PrintMotd 776Specifies whether 777.Xr sshd 8 778should print 779.Pa /etc/motd 780when a user logs in interactively. 781(On some systems it is also printed by the shell, 782.Pa /etc/profile , 783or equivalent.) 784The default is 785.Dq yes . 786.It Cm Protocol 787Specifies the protocol versions 788.Xr sshd 8 789supports. 790The possible values are 791.Sq 1 792and 793.Sq 2 . 794Multiple versions must be comma-separated. 795The default is 796.Dq 2,1 . 797Note that the order of the protocol list does not indicate preference, 798because the client selects among multiple protocol versions offered 799by the server. 800Specifying 801.Dq 2,1 802is identical to 803.Dq 1,2 . 804.It Cm PubkeyAuthentication 805Specifies whether public key authentication is allowed. 806The default is 807.Dq yes . 808Note that this option applies to protocol version 2 only. 809.It Cm RhostsRSAAuthentication 810Specifies whether rhosts or /etc/hosts.equiv authentication together 811with successful RSA host authentication is allowed. 812The default is 813.Dq no . 814This option applies to protocol version 1 only. 815.It Cm RSAAuthentication 816Specifies whether pure RSA authentication is allowed. 817The default is 818.Dq yes . 819This option applies to protocol version 1 only. 820.It Cm ServerKeyBits 821Defines the number of bits in the ephemeral protocol version 1 server key. 822The minimum value is 512, and the default is 1024. 823.It Cm StrictModes 824Specifies whether 825.Xr sshd 8 826should check file modes and ownership of the 827user's files and home directory before accepting login. 828This is normally desirable because novices sometimes accidentally leave their 829directory or files world-writable. 830The default is 831.Dq yes . 832.It Cm Subsystem 833Configures an external subsystem (e.g. file transfer daemon). 834Arguments should be a subsystem name and a command (with optional arguments) 835to execute upon subsystem request. 836.Pp 837The command 838.Xr sftp-server 8 839implements the 840.Dq sftp 841file transfer subsystem. 842.Pp 843Alternately the name 844.Dq internal-sftp 845implements an in-process 846.Dq sftp 847server. 848This may simplify configurations using 849.Cm ChrootDirectory 850to force a different filesystem root on clients. 851.Pp 852By default no subsystems are defined. 853Note that this option applies to protocol version 2 only. 854.It Cm SyslogFacility 855Gives the facility code that is used when logging messages from 856.Xr sshd 8 . 857The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 858LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 859The default is AUTH. 860.It Cm TCPKeepAlive 861Specifies whether the system should send TCP keepalive messages to the 862other side. 863If they are sent, death of the connection or crash of one 864of the machines will be properly noticed. 865However, this means that 866connections will die if the route is down temporarily, and some people 867find it annoying. 868On the other hand, if TCP keepalives are not sent, 869sessions may hang indefinitely on the server, leaving 870.Dq ghost 871users and consuming server resources. 872.Pp 873The default is 874.Dq yes 875(to send TCP keepalive messages), and the server will notice 876if the network goes down or the client host crashes. 877This avoids infinitely hanging sessions. 878.Pp 879To disable TCP keepalive messages, the value should be set to 880.Dq no . 881.It Cm UseDNS 882Specifies whether 883.Xr sshd 8 884should look up the remote host name and check that 885the resolved host name for the remote IP address maps back to the 886very same IP address. 887The default is 888.Dq yes . 889.It Cm UseLogin 890Specifies whether 891.Xr login 1 892is used for interactive login sessions. 893The default is 894.Dq no . 895Note that 896.Xr login 1 897is never used for remote command execution. 898Note also, that if this is enabled, 899.Cm X11Forwarding 900will be disabled because 901.Xr login 1 902does not know how to handle 903.Xr xauth 1 904cookies. 905If 906.Cm UsePrivilegeSeparation 907is specified, it will be disabled after authentication. 908.It Cm UsePrivilegeSeparation 909Specifies whether 910.Xr sshd 8 911separates privileges by creating an unprivileged child process 912to deal with incoming network traffic. 913After successful authentication, another process will be created that has 914the privilege of the authenticated user. 915The goal of privilege separation is to prevent privilege 916escalation by containing any corruption within the unprivileged processes. 917The default is 918.Dq yes . 919.It Cm X11DisplayOffset 920Specifies the first display number available for 921.Xr sshd 8 Ns 's 922X11 forwarding. 923This prevents sshd from interfering with real X11 servers. 924The default is 10. 925.It Cm X11Forwarding 926Specifies whether X11 forwarding is permitted. 927The argument must be 928.Dq yes 929or 930.Dq no . 931The default is 932.Dq no . 933.Pp 934When X11 forwarding is enabled, there may be additional exposure to 935the server and to client displays if the 936.Xr sshd 8 937proxy display is configured to listen on the wildcard address (see 938.Cm X11UseLocalhost 939below), though this is not the default. 940Additionally, the authentication spoofing and authentication data 941verification and substitution occur on the client side. 942The security risk of using X11 forwarding is that the client's X11 943display server may be exposed to attack when the SSH client requests 944forwarding (see the warnings for 945.Cm ForwardX11 946in 947.Xr ssh_config 5 ) . 948A system administrator may have a stance in which they want to 949protect clients that may expose themselves to attack by unwittingly 950requesting X11 forwarding, which can warrant a 951.Dq no 952setting. 953.Pp 954Note that disabling X11 forwarding does not prevent users from 955forwarding X11 traffic, as users can always install their own forwarders. 956X11 forwarding is automatically disabled if 957.Cm UseLogin 958is enabled. 959.It Cm X11UseLocalhost 960Specifies whether 961.Xr sshd 8 962should bind the X11 forwarding server to the loopback address or to 963the wildcard address. 964By default, 965sshd binds the forwarding server to the loopback address and sets the 966hostname part of the 967.Ev DISPLAY 968environment variable to 969.Dq localhost . 970This prevents remote hosts from connecting to the proxy display. 971However, some older X11 clients may not function with this 972configuration. 973.Cm X11UseLocalhost 974may be set to 975.Dq no 976to specify that the forwarding server should be bound to the wildcard 977address. 978The argument must be 979.Dq yes 980or 981.Dq no . 982The default is 983.Dq yes . 984.It Cm XAuthLocation 985Specifies the full pathname of the 986.Xr xauth 1 987program. 988The default is 989.Pa /usr/X11R6/bin/xauth . 990.El 991.Sh TIME FORMATS 992.Xr sshd 8 993command-line arguments and configuration file options that specify time 994may be expressed using a sequence of the form: 995.Sm off 996.Ar time Op Ar qualifier , 997.Sm on 998where 999.Ar time 1000is a positive integer value and 1001.Ar qualifier 1002is one of the following: 1003.Pp 1004.Bl -tag -width Ds -compact -offset indent 1005.It Aq Cm none 1006seconds 1007.It Cm s | Cm S 1008seconds 1009.It Cm m | Cm M 1010minutes 1011.It Cm h | Cm H 1012hours 1013.It Cm d | Cm D 1014days 1015.It Cm w | Cm W 1016weeks 1017.El 1018.Pp 1019Each member of the sequence is added together to calculate 1020the total time value. 1021.Pp 1022Time format examples: 1023.Pp 1024.Bl -tag -width Ds -compact -offset indent 1025.It 600 1026600 seconds (10 minutes) 1027.It 10m 102810 minutes 1029.It 1h30m 10301 hour 30 minutes (90 minutes) 1031.El 1032.Sh FILES 1033.Bl -tag -width Ds 1034.It Pa /etc/ssh/sshd_config 1035Contains configuration data for 1036.Xr sshd 8 . 1037This file should be writable by root only, but it is recommended 1038(though not necessary) that it be world-readable. 1039.El 1040.Sh SEE ALSO 1041.Xr sshd 8 1042.Sh AUTHORS 1043OpenSSH is a derivative of the original and free 1044ssh 1.2.12 release by Tatu Ylonen. 1045Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1046Theo de Raadt and Dug Song 1047removed many bugs, re-added newer features and 1048created OpenSSH. 1049Markus Friedl contributed the support for SSH 1050protocol versions 1.5 and 2.0. 1051Niels Provos and Markus Friedl contributed support 1052for privilege separation. 1053