1.\" $NetBSD: sshd_config.5,v 1.5 2010/04/29 18:14:09 wiz Exp $ 2.\" -*- nroff -*- 3.\" 4.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 5.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 6.\" All rights reserved 7.\" 8.\" As far as I am concerned, the code I have written for this software 9.\" can be used freely for any purpose. Any derived versions of this 10.\" software must be clearly marked as such, and if the derived work is 11.\" incompatible with the protocol description in the RFC file, it must be 12.\" called by a name other than "ssh" or "Secure Shell". 13.\" 14.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 15.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 16.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 17.\" 18.\" Redistribution and use in source and binary forms, with or without 19.\" modification, are permitted provided that the following conditions 20.\" are met: 21.\" 1. Redistributions of source code must retain the above copyright 22.\" notice, this list of conditions and the following disclaimer. 23.\" 2. Redistributions in binary form must reproduce the above copyright 24.\" notice, this list of conditions and the following disclaimer in the 25.\" documentation and/or other materials provided with the distribution. 26.\" 27.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 28.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 29.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 30.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 31.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 32.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 33.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 34.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 35.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 36.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 37.\" 38.\" $OpenBSD: sshd_config.5,v 1.106 2009/04/21 15:13:17 stevesk Exp $ 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. 181All authentication styles from 182.Xr login.conf 5 183are supported. 184The default is 185.Dq yes . 186.It Cm ChrootDirectory 187Specifies a path to 188.Xr chroot 2 189to after authentication. 190This path, and all its components, must be root-owned directories that are 191not writable by any other user or group. 192After the chroot, 193.Xr sshd 8 194changes the working directory to the user's home directory. 195.Pp 196The path may contain the following tokens that are expanded at runtime once 197the connecting user has been authenticated: %% is replaced by a literal '%', 198%h is replaced by the home directory of the user being authenticated, and 199%u is replaced by the username of that user. 200.Pp 201The 202.Cm ChrootDirectory 203must contain the necessary files and directories to support the 204user's session. 205For an interactive session this requires at least a shell, typically 206.Xr sh 1 , 207and basic 208.Pa /dev 209nodes such as 210.Xr null 4 , 211.Xr zero 4 , 212.Xr stdin 4 , 213.Xr stdout 4 , 214.Xr stderr 4 , 215.Xr arandom 4 216and 217.Xr tty 4 218devices. 219For file transfer sessions using 220.Dq sftp , 221no additional configuration of the environment is necessary if the 222in-process sftp server is used, 223though sessions which use logging do require 224.Pa /dev/log 225inside the chroot directory (see 226.Xr sftp-server 8 227for details). 228.Pp 229The default is not to 230.Xr chroot 2 . 231.It Cm Ciphers 232Specifies the ciphers allowed for protocol version 2. 233Multiple ciphers must be comma-separated. 234The supported ciphers are 235.Dq 3des-cbc , 236.Dq aes128-cbc , 237.Dq aes192-cbc , 238.Dq aes256-cbc , 239.Dq aes128-ctr , 240.Dq aes192-ctr , 241.Dq aes256-ctr , 242.Dq arcfour128 , 243.Dq arcfour256 , 244.Dq arcfour , 245.Dq blowfish-cbc , 246and 247.Dq cast128-cbc . 248The default is: 249.Bd -literal -offset 3n 250aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 251aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 252aes256-cbc,arcfour 253.Ed 254.It Cm ClientAliveCountMax 255Sets the number of client alive messages (see below) which may be 256sent without 257.Xr sshd 8 258receiving any messages back from the client. 259If this threshold is reached while client alive messages are being sent, 260sshd will disconnect the client, terminating the session. 261It is important to note that the use of client alive messages is very 262different from 263.Cm TCPKeepAlive 264(below). 265The client alive messages are sent through the encrypted channel 266and therefore will not be spoofable. 267The TCP keepalive option enabled by 268.Cm TCPKeepAlive 269is spoofable. 270The client alive mechanism is valuable when the client or 271server depend on knowing when a connection has become inactive. 272.Pp 273The default value is 3. 274If 275.Cm ClientAliveInterval 276(see below) is set to 15, and 277.Cm ClientAliveCountMax 278is left at the default, unresponsive SSH clients 279will be disconnected after approximately 45 seconds. 280This option applies to protocol version 2 only. 281.It Cm ClientAliveInterval 282Sets a timeout interval in seconds after which if no data has been received 283from the client, 284.Xr sshd 8 285will send a message through the encrypted 286channel to request a response from the client. 287The default 288is 0, indicating that these messages will not be sent to the client. 289This option applies to protocol version 2 only. 290.It Cm Compression 291Specifies whether compression is allowed, or delayed until 292the user has authenticated successfully. 293The argument must be 294.Dq yes , 295.Dq delayed , 296or 297.Dq no . 298The default is 299.Dq delayed . 300.It Cm DenyGroups 301This keyword can be followed by a list of group name patterns, separated 302by spaces. 303Login is disallowed for users whose primary group or supplementary 304group list matches one of the patterns. 305Only group names are valid; a numerical group ID is not recognized. 306By default, login is allowed for all groups. 307The allow/deny directives are processed in the following order: 308.Cm DenyUsers , 309.Cm AllowUsers , 310.Cm DenyGroups , 311and finally 312.Cm AllowGroups . 313.Pp 314See 315.Sx PATTERNS 316in 317.Xr ssh_config 5 318for more information on patterns. 319.It Cm DenyUsers 320This keyword can be followed by a list of user name patterns, separated 321by spaces. 322Login is disallowed for user names that match one of the patterns. 323Only user names are valid; a numerical user ID is not recognized. 324By default, login is allowed for all users. 325If the pattern takes the form USER@HOST then USER and HOST 326are separately checked, restricting logins to particular 327users from particular hosts. 328The allow/deny directives are processed in the following order: 329.Cm DenyUsers , 330.Cm AllowUsers , 331.Cm DenyGroups , 332and finally 333.Cm AllowGroups . 334.Pp 335See 336.Sx PATTERNS 337in 338.Xr ssh_config 5 339for more information on patterns. 340.It Cm ForceCommand 341Forces the execution of the command specified by 342.Cm ForceCommand , 343ignoring any command supplied by the client and 344.Pa ~/.ssh/rc 345if present. 346The command is invoked by using the user's login shell with the -c option. 347This applies to shell, command, or subsystem execution. 348It is most useful inside a 349.Cm Match 350block. 351The command originally supplied by the client is available in the 352.Ev SSH_ORIGINAL_COMMAND 353environment variable. 354Specifying a command of 355.Dq internal-sftp 356will force the use of an in-process sftp server that requires no support 357files when used with 358.Cm ChrootDirectory . 359.It Cm GatewayPorts 360Specifies whether remote hosts are allowed to connect to ports 361forwarded for the client. 362By default, 363.Xr sshd 8 364binds remote port forwardings to the loopback address. 365This prevents other remote hosts from connecting to forwarded ports. 366.Cm GatewayPorts 367can be used to specify that sshd 368should allow remote port forwardings to bind to non-loopback addresses, thus 369allowing other hosts to connect. 370The argument may be 371.Dq no 372to force remote port forwardings to be available to the local host only, 373.Dq yes 374to force remote port forwardings to bind to the wildcard address, or 375.Dq clientspecified 376to allow the client to select the address to which the forwarding is bound. 377The default is 378.Dq no . 379.It Cm GSSAPIAuthentication 380Specifies whether user authentication based on GSSAPI is allowed. 381The default is 382.Dq no . 383Note that this option applies to protocol version 2 only. 384.It Cm GSSAPICleanupCredentials 385Specifies whether to automatically destroy the user's credentials cache 386on logout. 387The default is 388.Dq yes . 389Note that this option applies to protocol version 2 only. 390.It Cm HostbasedAuthentication 391Specifies whether rhosts or /etc/hosts.equiv authentication together 392with successful public key client host authentication is allowed 393(host-based authentication). 394This option is similar to 395.Cm RhostsRSAAuthentication 396and applies to protocol version 2 only. 397The default is 398.Dq no . 399.It Cm HostbasedUsesNameFromPacketOnly 400Specifies whether or not the server will attempt to perform a reverse 401name lookup when matching the name in the 402.Pa ~/.shosts , 403.Pa ~/.rhosts , 404and 405.Pa /etc/hosts.equiv 406files during 407.Cm HostbasedAuthentication . 408A setting of 409.Dq yes 410means that 411.Xr sshd 8 412uses the name supplied by the client rather than 413attempting to resolve the name from the TCP connection itself. 414The default is 415.Dq no . 416.It Cm HostKey 417Specifies a file containing a private host key 418used by SSH. 419The default is 420.Pa /etc/ssh/ssh_host_key 421for protocol version 1, and 422.Pa /etc/ssh/ssh_host_rsa_key 423and 424.Pa /etc/ssh/ssh_host_dsa_key 425for protocol version 2. 426Note that 427.Xr sshd 8 428will refuse to use a file if it is group/world-accessible. 429It is possible to have multiple host key files. 430.Dq rsa1 431keys are used for version 1 and 432.Dq dsa 433or 434.Dq rsa 435are used for version 2 of the SSH protocol. 436.It Cm IgnoreRhosts 437Specifies that 438.Pa .rhosts 439and 440.Pa .shosts 441files will not be used in 442.Cm RhostsRSAAuthentication 443or 444.Cm HostbasedAuthentication . 445.Pp 446.Pa /etc/hosts.equiv 447and 448.Pa /etc/shosts.equiv 449are still used. 450The default is 451.Dq yes . 452.It Cm IgnoreUserKnownHosts 453Specifies whether 454.Xr sshd 8 455should ignore the user's 456.Pa ~/.ssh/known_hosts 457during 458.Cm RhostsRSAAuthentication 459or 460.Cm HostbasedAuthentication . 461The default is 462.Dq no . 463.It Cm KerberosAuthentication 464Specifies whether the password provided by the user for 465.Cm PasswordAuthentication 466will be validated through the Kerberos KDC. 467To use this option, the server needs a 468Kerberos servtab which allows the verification of the KDC's identity. 469The default is 470.Dq no . 471.It Cm KerberosGetAFSToken 472If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 473an AFS token before accessing the user's home directory. 474The default is 475.Dq no . 476.It Cm KerberosOrLocalPasswd 477If password authentication through Kerberos fails then 478the password will be validated via any additional local mechanism 479such as 480.Pa /etc/passwd . 481The default is 482.Dq yes . 483.It Cm KerberosTicketCleanup 484Specifies whether to automatically destroy the user's ticket cache 485file on logout. 486The default is 487.Dq yes . 488.It Cm KeyRegenerationInterval 489In protocol version 1, the ephemeral server key is automatically regenerated 490after this many seconds (if it has been used). 491The purpose of regeneration is to prevent 492decrypting captured sessions by later breaking into the machine and 493stealing the keys. 494The key is never stored anywhere. 495If the value is 0, the key is never regenerated. 496The default is 3600 (seconds). 497.It Cm ListenAddress 498Specifies the local addresses 499.Xr sshd 8 500should listen on. 501The following forms may be used: 502.Pp 503.Bl -item -offset indent -compact 504.It 505.Cm ListenAddress 506.Sm off 507.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 508.Sm on 509.It 510.Cm ListenAddress 511.Sm off 512.Ar host No | Ar IPv4_addr No : Ar port 513.Sm on 514.It 515.Cm ListenAddress 516.Sm off 517.Oo 518.Ar host No | Ar IPv6_addr Oc : Ar port 519.Sm on 520.El 521.Pp 522If 523.Ar port 524is not specified, 525sshd will listen on the address and all prior 526.Cm Port 527options specified. 528The default is to listen on all local addresses. 529Multiple 530.Cm ListenAddress 531options are permitted. 532Additionally, any 533.Cm Port 534options must precede this option for non-port qualified addresses. 535.It Cm LoginGraceTime 536The server disconnects after this time if the user has not 537successfully logged in. 538If the value is 0, there is no time limit. 539The default is 120 seconds. 540.It Cm LogLevel 541Gives the verbosity level that is used when logging messages from 542.Xr sshd 8 . 543The possible values are: 544QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 545The default is INFO. 546DEBUG and DEBUG1 are equivalent. 547DEBUG2 and DEBUG3 each specify higher levels of debugging output. 548Logging with a DEBUG level violates the privacy of users and is not recommended. 549.It Cm MACs 550Specifies the available MAC (message authentication code) algorithms. 551The MAC algorithm is used in protocol version 2 552for data integrity protection. 553Multiple algorithms must be comma-separated. 554The default is: 555.Bd -literal -offset indent 556hmac-md5,hmac-sha1, 557hmac-ripemd160,hmac-sha1-96,hmac-md5-96 558.Ed 559.It Cm Match 560Introduces a conditional block. 561If all of the criteria on the 562.Cm Match 563line are satisfied, the keywords on the following lines override those 564set in the global section of the config file, until either another 565.Cm Match 566line or the end of the file. 567.Pp 568The arguments to 569.Cm Match 570are one or more criteria-pattern pairs. 571The available criteria are 572.Cm User , 573.Cm Group , 574.Cm Host , 575and 576.Cm Address . 577The match patterns may consist of single entries or comma-separated 578lists and may use the wildcard and negation operators described in the 579.Sx PATTERNS 580section of 581.Xr ssh_config 5 . 582.Pp 583The patterns in an 584.Cm Address 585criteria may additionally contain addresses to match in CIDR 586address/masklen format, e.g.\& 587.Dq 192.0.2.0/24 588or 589.Dq 3ffe:ffff::/32 . 590Note that the mask length provided must be consistent with the address - 591it is an error to specify a mask length that is too long for the address 592or one with bits set in this host portion of the address. 593For example, 594.Dq 192.0.2.0/33 595and 596.Dq 192.0.2.0/8 597respectively. 598.Pp 599Only a subset of keywords may be used on the lines following a 600.Cm Match 601keyword. 602Available keywords are 603.Cm AllowAgentForwarding , 604.Cm AllowTcpForwarding , 605.Cm Banner , 606.Cm ChrootDirectory , 607.Cm ForceCommand , 608.Cm GatewayPorts , 609.Cm GSSAPIAuthentication , 610.Cm HostbasedAuthentication , 611.Cm KbdInteractiveAuthentication , 612.Cm KerberosAuthentication , 613.Cm MaxAuthTries , 614.Cm MaxSessions , 615.Cm PasswordAuthentication , 616.Cm PermitEmptyPasswords , 617.Cm PermitOpen , 618.Cm PermitRootLogin , 619.Cm RhostsRSAAuthentication , 620.Cm RSAAuthentication , 621.Cm X11DisplayOffset , 622.Cm X11Forwarding 623and 624.Cm X11UseLocalHost . 625.It Cm MaxAuthTries 626Specifies the maximum number of authentication attempts permitted per 627connection. 628Once the number of failures reaches half this value, 629additional failures are logged. 630The default is 6. 631.It Cm MaxSessions 632Specifies the maximum number of open sessions permitted per network connection. 633The default is 10. 634.It Cm MaxStartups 635Specifies the maximum number of concurrent unauthenticated connections to the 636SSH daemon. 637Additional connections will be dropped until authentication succeeds or the 638.Cm LoginGraceTime 639expires for a connection. 640The default is 10. 641.Pp 642Alternatively, random early drop can be enabled by specifying 643the three colon separated values 644.Dq start:rate:full 645(e.g. "10:30:60"). 646.Xr sshd 8 647will refuse connection attempts with a probability of 648.Dq rate/100 649(30%) 650if there are currently 651.Dq start 652(10) 653unauthenticated connections. 654The probability increases linearly and all connection attempts 655are refused if the number of unauthenticated connections reaches 656.Dq full 657(60). 658.It Cm PasswordAuthentication 659Specifies whether password authentication is allowed. 660The default is 661.Dq yes . 662.It Cm PermitEmptyPasswords 663When password authentication is allowed, it specifies whether the 664server allows login to accounts with empty password strings. 665The default is 666.Dq no . 667.It Cm PermitOpen 668Specifies the destinations to which TCP port forwarding is permitted. 669The forwarding specification must be one of the following forms: 670.Pp 671.Bl -item -offset indent -compact 672.It 673.Cm PermitOpen 674.Sm off 675.Ar host : port 676.Sm on 677.It 678.Cm PermitOpen 679.Sm off 680.Ar IPv4_addr : port 681.Sm on 682.It 683.Cm PermitOpen 684.Sm off 685.Ar \&[ IPv6_addr \&] : port 686.Sm on 687.El 688.Pp 689Multiple forwards may be specified by separating them with whitespace. 690An argument of 691.Dq any 692can be used to remove all restrictions and permit any forwarding requests. 693By default all port forwarding requests are permitted. 694.It Cm PermitRootLogin 695Specifies whether root can log in using 696.Xr ssh 1 . 697The argument must be 698.Dq yes , 699.Dq without-password , 700.Dq forced-commands-only , 701or 702.Dq no . 703The default is 704.Dq no . 705.Pp 706If this option is set to 707.Dq without-password , 708password authentication is disabled for root. 709.Pp 710If this option is set to 711.Dq forced-commands-only , 712root login with public key authentication will be allowed, 713but only if the 714.Ar command 715option has been specified 716(which may be useful for taking remote backups even if root login is 717normally not allowed). 718All other authentication methods are disabled for root. 719.Pp 720If this option is set to 721.Dq no , 722root is not allowed to log in. 723.It Cm PermitTunnel 724Specifies whether 725.Xr tun 4 726device forwarding is allowed. 727The argument must be 728.Dq yes , 729.Dq point-to-point 730(layer 3), 731.Dq ethernet 732(layer 2), or 733.Dq no . 734Specifying 735.Dq yes 736permits both 737.Dq point-to-point 738and 739.Dq ethernet . 740The default is 741.Dq no . 742.It Cm PermitUserEnvironment 743Specifies whether 744.Pa ~/.ssh/environment 745and 746.Cm environment= 747options in 748.Pa ~/.ssh/authorized_keys 749are processed by 750.Xr sshd 8 . 751The default is 752.Dq no . 753Enabling environment processing may enable users to bypass access 754restrictions in some configurations using mechanisms such as 755.Ev LD_PRELOAD . 756.It Cm PidFile 757Specifies the file that contains the process ID of the 758SSH daemon. 759The default is 760.Pa /var/run/sshd.pid . 761.It Cm Port 762Specifies the port number that 763.Xr sshd 8 764listens on. 765The default is 22. 766Multiple options of this type are permitted. 767See also 768.Cm ListenAddress . 769.It Cm PrintLastLog 770Specifies whether 771.Xr sshd 8 772should print the date and time of the last user login when a user logs 773in interactively. 774The default is 775.Dq yes . 776.It Cm PrintMotd 777Specifies whether 778.Xr sshd 8 779should print 780.Pa /etc/motd 781when a user logs in interactively. 782(On some systems it is also printed by the shell, 783.Pa /etc/profile , 784or equivalent.) 785The default is 786.Dq yes . 787.It Cm Protocol 788Specifies the protocol versions 789.Xr sshd 8 790supports. 791The possible values are 792.Sq 1 793and 794.Sq 2 . 795Multiple versions must be comma-separated. 796The default is 797.Dq 2,1 . 798Note that the order of the protocol list does not indicate preference, 799because the client selects among multiple protocol versions offered 800by the server. 801Specifying 802.Dq 2,1 803is identical to 804.Dq 1,2 . 805.It Cm PubkeyAuthentication 806Specifies whether public key authentication is allowed. 807The default is 808.Dq yes . 809Note that this option applies to protocol version 2 only. 810.It Cm RhostsRSAAuthentication 811Specifies whether rhosts or /etc/hosts.equiv authentication together 812with successful RSA host authentication is allowed. 813The default is 814.Dq no . 815This option applies to protocol version 1 only. 816.It Cm RSAAuthentication 817Specifies whether pure RSA authentication is allowed. 818The default is 819.Dq yes . 820This option applies to protocol version 1 only. 821.It Cm ServerKeyBits 822Defines the number of bits in the ephemeral protocol version 1 server key. 823The minimum value is 512, and the default is 1024. 824.It Cm StrictModes 825Specifies whether 826.Xr sshd 8 827should check file modes and ownership of the 828user's files and home directory before accepting login. 829This is normally desirable because novices sometimes accidentally leave their 830directory or files world-writable. 831The default is 832.Dq yes . 833.It Cm Subsystem 834Configures an external subsystem (e.g. file transfer daemon). 835Arguments should be a subsystem name and a command (with optional arguments) 836to execute upon subsystem request. 837.Pp 838The command 839.Xr sftp-server 8 840implements the 841.Dq sftp 842file transfer subsystem. 843.Pp 844Alternately the name 845.Dq internal-sftp 846implements an in-process 847.Dq sftp 848server. 849This may simplify configurations using 850.Cm ChrootDirectory 851to force a different filesystem root on clients. 852.Pp 853By default no subsystems are defined. 854Note that this option applies to protocol version 2 only. 855.It Cm SyslogFacility 856Gives the facility code that is used when logging messages from 857.Xr sshd 8 . 858The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 859LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 860The default is AUTH. 861.It Cm TCPKeepAlive 862Specifies whether the system should send TCP keepalive messages to the 863other side. 864If they are sent, death of the connection or crash of one 865of the machines will be properly noticed. 866However, this means that 867connections will die if the route is down temporarily, and some people 868find it annoying. 869On the other hand, if TCP keepalives are not sent, 870sessions may hang indefinitely on the server, leaving 871.Dq ghost 872users and consuming server resources. 873.Pp 874The default is 875.Dq yes 876(to send TCP keepalive messages), and the server will notice 877if the network goes down or the client host crashes. 878This avoids infinitely hanging sessions. 879.Pp 880To disable TCP keepalive messages, the value should be set to 881.Dq no . 882.It Cm UseDNS 883Specifies whether 884.Xr sshd 8 885should look up the remote host name and check that 886the resolved host name for the remote IP address maps back to the 887very same IP address. 888The default is 889.Dq yes . 890.It Cm UseLogin 891Specifies whether 892.Xr login 1 893is used for interactive login sessions. 894The default is 895.Dq no . 896Note that 897.Xr login 1 898is never used for remote command execution. 899Note also, that if this is enabled, 900.Cm X11Forwarding 901will be disabled because 902.Xr login 1 903does not know how to handle 904.Xr xauth 1 905cookies. 906If 907.Cm UsePrivilegeSeparation 908is specified, it will be disabled after authentication. 909.It Cm UsePrivilegeSeparation 910Specifies whether 911.Xr sshd 8 912separates privileges by creating an unprivileged child process 913to deal with incoming network traffic. 914After successful authentication, another process will be created that has 915the privilege of the authenticated user. 916The goal of privilege separation is to prevent privilege 917escalation by containing any corruption within the unprivileged processes. 918The default is 919.Dq yes . 920.It Cm X11DisplayOffset 921Specifies the first display number available for 922.Xr sshd 8 Ns 's 923X11 forwarding. 924This prevents sshd from interfering with real X11 servers. 925The default is 10. 926.It Cm X11Forwarding 927Specifies whether X11 forwarding is permitted. 928The argument must be 929.Dq yes 930or 931.Dq no . 932The default is 933.Dq no . 934.Pp 935When X11 forwarding is enabled, there may be additional exposure to 936the server and to client displays if the 937.Xr sshd 8 938proxy display is configured to listen on the wildcard address (see 939.Cm X11UseLocalhost 940below), though this is not the default. 941Additionally, the authentication spoofing and authentication data 942verification and substitution occur on the client side. 943The security risk of using X11 forwarding is that the client's X11 944display server may be exposed to attack when the SSH client requests 945forwarding (see the warnings for 946.Cm ForwardX11 947in 948.Xr ssh_config 5 ) . 949A system administrator may have a stance in which they want to 950protect clients that may expose themselves to attack by unwittingly 951requesting X11 forwarding, which can warrant a 952.Dq no 953setting. 954.Pp 955Note that disabling X11 forwarding does not prevent users from 956forwarding X11 traffic, as users can always install their own forwarders. 957X11 forwarding is automatically disabled if 958.Cm UseLogin 959is enabled. 960.It Cm X11UseLocalhost 961Specifies whether 962.Xr sshd 8 963should bind the X11 forwarding server to the loopback address or to 964the wildcard address. 965By default, 966sshd binds the forwarding server to the loopback address and sets the 967hostname part of the 968.Ev DISPLAY 969environment variable to 970.Dq localhost . 971This prevents remote hosts from connecting to the proxy display. 972However, some older X11 clients may not function with this 973configuration. 974.Cm X11UseLocalhost 975may be set to 976.Dq no 977to specify that the forwarding server should be bound to the wildcard 978address. 979The argument must be 980.Dq yes 981or 982.Dq no . 983The default is 984.Dq yes . 985.It Cm XAuthLocation 986Specifies the full pathname of the 987.Xr xauth 1 988program. 989The default is 990.Pa /usr/X11R6/bin/xauth . 991.El 992.Sh TIME FORMATS 993.Xr sshd 8 994command-line arguments and configuration file options that specify time 995may be expressed using a sequence of the form: 996.Sm off 997.Ar time Op Ar qualifier , 998.Sm on 999where 1000.Ar time 1001is a positive integer value and 1002.Ar qualifier 1003is one of the following: 1004.Pp 1005.Bl -tag -width Ds -compact -offset indent 1006.It Aq Cm none 1007seconds 1008.It Cm s | Cm S 1009seconds 1010.It Cm m | Cm M 1011minutes 1012.It Cm h | Cm H 1013hours 1014.It Cm d | Cm D 1015days 1016.It Cm w | Cm W 1017weeks 1018.El 1019.Pp 1020Each member of the sequence is added together to calculate 1021the total time value. 1022.Pp 1023Time format examples: 1024.Pp 1025.Bl -tag -width Ds -compact -offset indent 1026.It 600 1027600 seconds (10 minutes) 1028.It 10m 102910 minutes 1030.It 1h30m 10311 hour 30 minutes (90 minutes) 1032.El 1033.Sh FILES 1034.Bl -tag -width Ds 1035.It Pa /etc/ssh/sshd_config 1036Contains configuration data for 1037.Xr sshd 8 . 1038This file should be writable by root only, but it is recommended 1039(though not necessary) that it be world-readable. 1040.El 1041.Sh SEE ALSO 1042.Xr sshd 8 1043.Sh AUTHORS 1044OpenSSH is a derivative of the original and free 1045ssh 1.2.12 release by Tatu Ylonen. 1046Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1047Theo de Raadt and Dug Song 1048removed many bugs, re-added newer features and 1049created OpenSSH. 1050Markus Friedl contributed the support for SSH 1051protocol versions 1.5 and 2.0. 1052Niels Provos and Markus Friedl contributed support 1053for privilege separation. 1054