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 1.96 2008/07/02 02:24:18 djm Exp $ 38.\" $FreeBSD$ 39.Dd July 2 2008 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. 181See also 182.Cm UsePAM . 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. 191.Pp 192The path may contain the following tokens that are expanded at runtime once 193the connecting user has been authenticated: %% is replaced by a literal '%', 194%h is replaced by the home directory of the user being authenticated, and 195%u is replaced by the username of that user. 196.Pp 197The 198.Cm ChrootDirectory 199must contain the necessary files and directories to support the 200users' session. 201For an interactive session this requires at least a shell, typically 202.Xr sh 1 , 203and basic 204.Pa /dev 205nodes such as 206.Xr null 4 , 207.Xr zero 4 , 208.Xr stdin 4 , 209.Xr stdout 4 , 210.Xr stderr 4 , 211.Xr arandom 4 212and 213.Xr tty 4 214devices. 215For file transfer sessions using 216.Dq sftp , 217no additional configuration of the environment is necessary if the 218in-process sftp server is used (see 219.Cm Subsystem 220for details). 221.Pp 222The default is not to 223.Xr chroot 2 . 224.It Cm Ciphers 225Specifies the ciphers allowed for protocol version 2. 226Multiple ciphers must be comma-separated. 227The supported ciphers are 228.Dq 3des-cbc , 229.Dq aes128-cbc , 230.Dq aes192-cbc , 231.Dq aes256-cbc , 232.Dq aes128-ctr , 233.Dq aes192-ctr , 234.Dq aes256-ctr , 235.Dq arcfour128 , 236.Dq arcfour256 , 237.Dq arcfour , 238.Dq blowfish-cbc , 239and 240.Dq cast128-cbc . 241The default is: 242.Bd -literal -offset 3n 243aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, 244arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, 245aes192-ctr,aes256-ctr 246.Ed 247.It Cm ClientAliveCountMax 248Sets the number of client alive messages (see below) which may be 249sent without 250.Xr sshd 8 251receiving any messages back from the client. 252If this threshold is reached while client alive messages are being sent, 253sshd will disconnect the client, terminating the session. 254It is important to note that the use of client alive messages is very 255different from 256.Cm TCPKeepAlive 257(below). 258The client alive messages are sent through the encrypted channel 259and therefore will not be spoofable. 260The TCP keepalive option enabled by 261.Cm TCPKeepAlive 262is spoofable. 263The client alive mechanism is valuable when the client or 264server depend on knowing when a connection has become inactive. 265.Pp 266The default value is 3. 267If 268.Cm ClientAliveInterval 269(see below) is set to 15, and 270.Cm ClientAliveCountMax 271is left at the default, unresponsive SSH clients 272will be disconnected after approximately 45 seconds. 273This option applies to protocol version 2 only. 274.It Cm ClientAliveInterval 275Sets a timeout interval in seconds after which if no data has been received 276from the client, 277.Xr sshd 8 278will send a message through the encrypted 279channel to request a response from the client. 280The default 281is 0, indicating that these messages will not be sent to the client. 282This option applies to protocol version 2 only. 283.It Cm Compression 284Specifies whether compression is allowed, or delayed until 285the user has authenticated successfully. 286The argument must be 287.Dq yes , 288.Dq delayed , 289or 290.Dq no . 291The default is 292.Dq delayed . 293.It Cm DenyGroups 294This keyword can be followed by a list of group name patterns, separated 295by spaces. 296Login is disallowed for users whose primary group or supplementary 297group list matches one of the patterns. 298Only group names are valid; a numerical group ID is not recognized. 299By default, login is allowed for all groups. 300The allow/deny directives are processed in the following order: 301.Cm DenyUsers , 302.Cm AllowUsers , 303.Cm DenyGroups , 304and finally 305.Cm AllowGroups . 306.Pp 307See 308.Sx PATTERNS 309in 310.Xr ssh_config 5 311for more information on patterns. 312.It Cm DenyUsers 313This keyword can be followed by a list of user name patterns, separated 314by spaces. 315Login is disallowed for user names that match one of the patterns. 316Only user names are valid; a numerical user ID is not recognized. 317By default, login is allowed for all users. 318If the pattern takes the form USER@HOST then USER and HOST 319are separately checked, restricting logins to particular 320users from particular hosts. 321The allow/deny directives are processed in the following order: 322.Cm DenyUsers , 323.Cm AllowUsers , 324.Cm DenyGroups , 325and finally 326.Cm AllowGroups . 327.Pp 328See 329.Sx PATTERNS 330in 331.Xr ssh_config 5 332for more information on patterns. 333.It Cm ForceCommand 334Forces the execution of the command specified by 335.Cm ForceCommand , 336ignoring any command supplied by the client and 337.Pa ~/.ssh/rc 338if present. 339The command is invoked by using the user's login shell with the -c option. 340This applies to shell, command, or subsystem execution. 341It is most useful inside a 342.Cm Match 343block. 344The command originally supplied by the client is available in the 345.Ev SSH_ORIGINAL_COMMAND 346environment variable. 347Specifying a command of 348.Dq internal-sftp 349will force the use of an in-process sftp server that requires no support 350files when used with 351.Cm ChrootDirectory . 352.It Cm GatewayPorts 353Specifies whether remote hosts are allowed to connect to ports 354forwarded for the client. 355By default, 356.Xr sshd 8 357binds remote port forwardings to the loopback address. 358This prevents other remote hosts from connecting to forwarded ports. 359.Cm GatewayPorts 360can be used to specify that sshd 361should allow remote port forwardings to bind to non-loopback addresses, thus 362allowing other hosts to connect. 363The argument may be 364.Dq no 365to force remote port forwardings to be available to the local host only, 366.Dq yes 367to force remote port forwardings to bind to the wildcard address, or 368.Dq clientspecified 369to allow the client to select the address to which the forwarding is bound. 370The default is 371.Dq no . 372.It Cm GSSAPIAuthentication 373Specifies whether user authentication based on GSSAPI is allowed. 374The default is 375.Dq no . 376Note that this option applies to protocol version 2 only. 377.It Cm GSSAPICleanupCredentials 378Specifies whether to automatically destroy the user's credentials cache 379on logout. 380The default is 381.Dq yes . 382Note that this option applies to protocol version 2 only. 383.It Cm HostbasedAuthentication 384Specifies whether rhosts or /etc/hosts.equiv authentication together 385with successful public key client host authentication is allowed 386(host-based authentication). 387This option is similar to 388.Cm RhostsRSAAuthentication 389and applies to protocol version 2 only. 390The default is 391.Dq no . 392.It Cm HostbasedUsesNameFromPacketOnly 393Specifies whether or not the server will attempt to perform a reverse 394name lookup when matching the name in the 395.Pa ~/.shosts , 396.Pa ~/.rhosts , 397and 398.Pa /etc/hosts.equiv 399files during 400.Cm HostbasedAuthentication . 401A setting of 402.Dq yes 403means that 404.Xr sshd 8 405uses the name supplied by the client rather than 406attempting to resolve the name from the TCP connection itself. 407The default is 408.Dq no . 409.It Cm HostKey 410Specifies a file containing a private host key 411used by SSH. 412The default is 413.Pa /etc/ssh/ssh_host_key 414for protocol version 1, and 415.Pa /etc/ssh/ssh_host_rsa_key 416and 417.Pa /etc/ssh/ssh_host_dsa_key 418for protocol version 2. 419Note that 420.Xr sshd 8 421will refuse to use a file if it is group/world-accessible. 422It is possible to have multiple host key files. 423.Dq rsa1 424keys are used for version 1 and 425.Dq dsa 426or 427.Dq rsa 428are used for version 2 of the SSH protocol. 429.It Cm IgnoreRhosts 430Specifies that 431.Pa .rhosts 432and 433.Pa .shosts 434files will not be used in 435.Cm RhostsRSAAuthentication 436or 437.Cm HostbasedAuthentication . 438.Pp 439.Pa /etc/hosts.equiv 440and 441.Pa /etc/ssh/shosts.equiv 442are still used. 443The default is 444.Dq yes . 445.It Cm IgnoreUserKnownHosts 446Specifies whether 447.Xr sshd 8 448should ignore the user's 449.Pa ~/.ssh/known_hosts 450during 451.Cm RhostsRSAAuthentication 452or 453.Cm HostbasedAuthentication . 454The default is 455.Dq no . 456.It Cm KerberosAuthentication 457Specifies whether the password provided by the user for 458.Cm PasswordAuthentication 459will be validated through the Kerberos KDC. 460To use this option, the server needs a 461Kerberos servtab which allows the verification of the KDC's identity. 462The default is 463.Dq no . 464.It Cm KerberosGetAFSToken 465If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 466an AFS token before accessing the user's home directory. 467The default is 468.Dq no . 469.It Cm KerberosOrLocalPasswd 470If password authentication through Kerberos fails then 471the password will be validated via any additional local mechanism 472such as 473.Pa /etc/passwd . 474The default is 475.Dq yes . 476.It Cm KerberosTicketCleanup 477Specifies whether to automatically destroy the user's ticket cache 478file on logout. 479The default is 480.Dq yes . 481.It Cm KeyRegenerationInterval 482In protocol version 1, the ephemeral server key is automatically regenerated 483after this many seconds (if it has been used). 484The purpose of regeneration is to prevent 485decrypting captured sessions by later breaking into the machine and 486stealing the keys. 487The key is never stored anywhere. 488If the value is 0, the key is never regenerated. 489The default is 3600 (seconds). 490.It Cm ListenAddress 491Specifies the local addresses 492.Xr sshd 8 493should listen on. 494The following forms may be used: 495.Pp 496.Bl -item -offset indent -compact 497.It 498.Cm ListenAddress 499.Sm off 500.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 501.Sm on 502.It 503.Cm ListenAddress 504.Sm off 505.Ar host No | Ar IPv4_addr No : Ar port 506.Sm on 507.It 508.Cm ListenAddress 509.Sm off 510.Oo 511.Ar host No | Ar IPv6_addr Oc : Ar port 512.Sm on 513.El 514.Pp 515If 516.Ar port 517is not specified, 518sshd will listen on the address and all prior 519.Cm Port 520options specified. 521The default is to listen on all local addresses. 522Multiple 523.Cm ListenAddress 524options are permitted. 525Additionally, any 526.Cm Port 527options must precede this option for non-port qualified addresses. 528.It Cm LoginGraceTime 529The server disconnects after this time if the user has not 530successfully logged in. 531If the value is 0, there is no time limit. 532The default is 120 seconds. 533.It Cm LogLevel 534Gives the verbosity level that is used when logging messages from 535.Xr sshd 8 . 536The possible values are: 537QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 538The default is INFO. 539DEBUG and DEBUG1 are equivalent. 540DEBUG2 and DEBUG3 each specify higher levels of debugging output. 541Logging with a DEBUG level violates the privacy of users and is not recommended. 542.It Cm MACs 543Specifies the available MAC (message authentication code) algorithms. 544The MAC algorithm is used in protocol version 2 545for data integrity protection. 546Multiple algorithms must be comma-separated. 547The default is: 548.Bd -literal -offset indent 549hmac-md5,hmac-sha1,umac-64@openssh.com, 550hmac-ripemd160,hmac-sha1-96,hmac-md5-96 551.Ed 552.It Cm Match 553Introduces a conditional block. 554If all of the criteria on the 555.Cm Match 556line are satisfied, the keywords on the following lines override those 557set in the global section of the config file, until either another 558.Cm Match 559line or the end of the file. 560.Pp 561The arguments to 562.Cm Match 563are one or more criteria-pattern pairs. 564The available criteria are 565.Cm User , 566.Cm Group , 567.Cm Host , 568and 569.Cm Address . 570The match patterns may consist of single entries or comma-separated 571lists and may use the wildcard and negation operators described in the 572.Sx PATTERNS 573section of 574.Xr ssh_config 5 . 575.Pp 576The patterns in an 577.Cm Address 578criteria may additionally contain addresses to match in CIDR 579address/masklen format, e.g.\& 580.Dq 192.0.2.0/24 581or 582.Dq 3ffe:ffff::/32 . 583Note that the mask length provided must be consistent with the address - 584it is an error to specify a mask length that is too long for the address 585or one with bits set in this host portion of the address. 586For example, 587.Dq 192.0.2.0/33 588and 589.Dq 192.0.2.0/8 590respectively. 591.Pp 592Only a subset of keywords may be used on the lines following a 593.Cm Match 594keyword. 595Available keywords are 596.Cm AllowTcpForwarding , 597.Cm Banner , 598.Cm ChrootDirectory , 599.Cm ForceCommand , 600.Cm GatewayPorts , 601.Cm GSSAPIAuthentication , 602.Cm HostbasedAuthentication , 603.Cm KbdInteractiveAuthentication , 604.Cm KerberosAuthentication , 605.Cm MaxAuthTries , 606.Cm MaxSessions , 607.Cm PasswordAuthentication , 608.Cm PermitOpen , 609.Cm PermitRootLogin , 610.Cm RhostsRSAAuthentication , 611.Cm RSAAuthentication , 612.Cm X11DisplayOffset , 613.Cm X11Forwarding , 614and 615.Cm X11UseLocalHost . 616.It Cm MaxAuthTries 617Specifies the maximum number of authentication attempts permitted per 618connection. 619Once the number of failures reaches half this value, 620additional failures are logged. 621The default is 6. 622.It Cm MaxSessions 623Specifies the maximum number of open sessions permitted per network connection. 624The default is 10. 625.It Cm MaxStartups 626Specifies the maximum number of concurrent unauthenticated connections to the 627SSH daemon. 628Additional connections will be dropped until authentication succeeds or the 629.Cm LoginGraceTime 630expires for a connection. 631The default is 10. 632.Pp 633Alternatively, random early drop can be enabled by specifying 634the three colon separated values 635.Dq start:rate:full 636(e.g. "10:30:60"). 637.Xr sshd 8 638will refuse connection attempts with a probability of 639.Dq rate/100 640(30%) 641if there are currently 642.Dq start 643(10) 644unauthenticated connections. 645The probability increases linearly and all connection attempts 646are refused if the number of unauthenticated connections reaches 647.Dq full 648(60). 649.It Cm PasswordAuthentication 650Specifies whether password authentication is allowed. 651See also 652.Cm UsePAM . 653The default is 654.Dq no . 655.It Cm PermitEmptyPasswords 656When password authentication is allowed, it specifies whether the 657server allows login to accounts with empty password strings. 658The default is 659.Dq no . 660.It Cm PermitOpen 661Specifies the destinations to which TCP port forwarding is permitted. 662The forwarding specification must be one of the following forms: 663.Pp 664.Bl -item -offset indent -compact 665.It 666.Cm PermitOpen 667.Sm off 668.Ar host : port 669.Sm on 670.It 671.Cm PermitOpen 672.Sm off 673.Ar IPv4_addr : port 674.Sm on 675.It 676.Cm PermitOpen 677.Sm off 678.Ar \&[ IPv6_addr \&] : port 679.Sm on 680.El 681.Pp 682Multiple forwards may be specified by separating them with whitespace. 683An argument of 684.Dq any 685can be used to remove all restrictions and permit any forwarding requests. 686By default all port forwarding requests are permitted. 687.It Cm PermitRootLogin 688Specifies whether root can log in using 689.Xr ssh 1 . 690The argument must be 691.Dq yes , 692.Dq without-password , 693.Dq forced-commands-only , 694or 695.Dq no . 696The default is 697.Dq no . 698Note that if 699.Cm ChallengeResponseAuthentication 700is 701.Dq yes , 702the root user may be allowed in with its password even if 703.Cm PermitRootLogin is set to 704.Dq without-password . 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 . 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 812.Pa /etc/hosts.equiv 813authentication together 814with successful RSA host authentication is allowed. 815The default is 816.Dq no . 817This option applies to protocol version 1 only. 818.It Cm RSAAuthentication 819Specifies whether pure RSA authentication is allowed. 820The default is 821.Dq yes . 822This option applies to protocol version 1 only. 823.It Cm ServerKeyBits 824Defines the number of bits in the ephemeral protocol version 1 server key. 825The minimum value is 512, and the default is 1024. 826.It Cm StrictModes 827Specifies whether 828.Xr sshd 8 829should check file modes and ownership of the 830user's files and home directory before accepting login. 831This is normally desirable because novices sometimes accidentally leave their 832directory or files world-writable. 833The default is 834.Dq yes . 835.It Cm Subsystem 836Configures an external subsystem (e.g. file transfer daemon). 837Arguments should be a subsystem name and a command (with optional arguments) 838to execute upon subsystem request. 839.Pp 840The command 841.Xr sftp-server 8 842implements the 843.Dq sftp 844file transfer subsystem. 845.Pp 846Alternately the name 847.Dq internal-sftp 848implements an in-process 849.Dq sftp 850server. 851This may simplify configurations using 852.Cm ChrootDirectory 853to force a different filesystem root on clients. 854.Pp 855By default no subsystems are defined. 856Note that this option applies to protocol version 2 only. 857.It Cm SyslogFacility 858Gives the facility code that is used when logging messages from 859.Xr sshd 8 . 860The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 861LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 862The default is AUTH. 863.It Cm TCPKeepAlive 864Specifies whether the system should send TCP keepalive messages to the 865other side. 866If they are sent, death of the connection or crash of one 867of the machines will be properly noticed. 868However, this means that 869connections will die if the route is down temporarily, and some people 870find it annoying. 871On the other hand, if TCP keepalives are not sent, 872sessions may hang indefinitely on the server, leaving 873.Dq ghost 874users and consuming server resources. 875.Pp 876The default is 877.Dq yes 878(to send TCP keepalive messages), and the server will notice 879if the network goes down or the client host crashes. 880This avoids infinitely hanging sessions. 881.Pp 882To disable TCP keepalive messages, the value should be set to 883.Dq no . 884.It Cm UseDNS 885Specifies whether 886.Xr sshd 8 887should look up the remote host name and check that 888the resolved host name for the remote IP address maps back to the 889very same IP address. 890The default is 891.Dq yes . 892.It Cm UseLogin 893Specifies whether 894.Xr login 1 895is used for interactive login sessions. 896The default is 897.Dq no . 898Note that 899.Xr login 1 900is never used for remote command execution. 901Note also, that if this is enabled, 902.Cm X11Forwarding 903will be disabled because 904.Xr login 1 905does not know how to handle 906.Xr xauth 1 907cookies. 908If 909.Cm UsePrivilegeSeparation 910is specified, it will be disabled after authentication. 911.It Cm UsePAM 912Enables the Pluggable Authentication Module interface. 913If set to 914.Dq yes 915this will enable PAM authentication using 916.Cm ChallengeResponseAuthentication 917and 918.Cm PasswordAuthentication 919in addition to PAM account and session module processing for all 920authentication types. 921.Pp 922Because PAM challenge-response authentication usually serves an equivalent 923role to password authentication, you should disable either 924.Cm PasswordAuthentication 925or 926.Cm ChallengeResponseAuthentication. 927.Pp 928If 929.Cm UsePAM 930is enabled, you will not be able to run 931.Xr sshd 8 932as a non-root user. 933The default is 934.Dq yes . 935.It Cm UsePrivilegeSeparation 936Specifies whether 937.Xr sshd 8 938separates privileges by creating an unprivileged child process 939to deal with incoming network traffic. 940After successful authentication, another process will be created that has 941the privilege of the authenticated user. 942The goal of privilege separation is to prevent privilege 943escalation by containing any corruption within the unprivileged processes. 944The default is 945.Dq yes . 946.It Cm VersionAddendum 947Specifies a string to append to the regular version string to identify 948OS- or site-specific modifications. 949The default is 950.Dq FreeBSD-20080801 . 951.It Cm X11DisplayOffset 952Specifies the first display number available for 953.Xr sshd 8 Ns 's 954X11 forwarding. 955This prevents sshd from interfering with real X11 servers. 956The default is 10. 957.It Cm X11Forwarding 958Specifies whether X11 forwarding is permitted. 959The argument must be 960.Dq yes 961or 962.Dq no . 963The default is 964.Dq yes . 965.Pp 966When X11 forwarding is enabled, there may be additional exposure to 967the server and to client displays if the 968.Xr sshd 8 969proxy display is configured to listen on the wildcard address (see 970.Cm X11UseLocalhost 971below), though this is not the default. 972Additionally, the authentication spoofing and authentication data 973verification and substitution occur on the client side. 974The security risk of using X11 forwarding is that the client's X11 975display server may be exposed to attack when the SSH client requests 976forwarding (see the warnings for 977.Cm ForwardX11 978in 979.Xr ssh_config 5 ) . 980A system administrator may have a stance in which they want to 981protect clients that may expose themselves to attack by unwittingly 982requesting X11 forwarding, which can warrant a 983.Dq no 984setting. 985.Pp 986Note that disabling X11 forwarding does not prevent users from 987forwarding X11 traffic, as users can always install their own forwarders. 988X11 forwarding is automatically disabled if 989.Cm UseLogin 990is enabled. 991.It Cm X11UseLocalhost 992Specifies whether 993.Xr sshd 8 994should bind the X11 forwarding server to the loopback address or to 995the wildcard address. 996By default, 997sshd binds the forwarding server to the loopback address and sets the 998hostname part of the 999.Ev DISPLAY 1000environment variable to 1001.Dq localhost . 1002This prevents remote hosts from connecting to the proxy display. 1003However, some older X11 clients may not function with this 1004configuration. 1005.Cm X11UseLocalhost 1006may be set to 1007.Dq no 1008to specify that the forwarding server should be bound to the wildcard 1009address. 1010The argument must be 1011.Dq yes 1012or 1013.Dq no . 1014The default is 1015.Dq yes . 1016.It Cm XAuthLocation 1017Specifies the full pathname of the 1018.Xr xauth 1 1019program. 1020The default is 1021.Pa /usr/local/bin/xauth . 1022.El 1023.Sh TIME FORMATS 1024.Xr sshd 8 1025command-line arguments and configuration file options that specify time 1026may be expressed using a sequence of the form: 1027.Sm off 1028.Ar time Op Ar qualifier , 1029.Sm on 1030where 1031.Ar time 1032is a positive integer value and 1033.Ar qualifier 1034is one of the following: 1035.Pp 1036.Bl -tag -width Ds -compact -offset indent 1037.It Aq Cm none 1038seconds 1039.It Cm s | Cm S 1040seconds 1041.It Cm m | Cm M 1042minutes 1043.It Cm h | Cm H 1044hours 1045.It Cm d | Cm D 1046days 1047.It Cm w | Cm W 1048weeks 1049.El 1050.Pp 1051Each member of the sequence is added together to calculate 1052the total time value. 1053.Pp 1054Time format examples: 1055.Pp 1056.Bl -tag -width Ds -compact -offset indent 1057.It 600 1058600 seconds (10 minutes) 1059.It 10m 106010 minutes 1061.It 1h30m 10621 hour 30 minutes (90 minutes) 1063.El 1064.Sh FILES 1065.Bl -tag -width Ds 1066.It Pa /etc/ssh/sshd_config 1067Contains configuration data for 1068.Xr sshd 8 . 1069This file should be writable by root only, but it is recommended 1070(though not necessary) that it be world-readable. 1071.El 1072.Sh SEE ALSO 1073.Xr sshd 8 1074.Sh AUTHORS 1075OpenSSH is a derivative of the original and free 1076ssh 1.2.12 release by Tatu Ylonen. 1077Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1078Theo de Raadt and Dug Song 1079removed many bugs, re-added newer features and 1080created OpenSSH. 1081Markus Friedl contributed the support for SSH 1082protocol versions 1.5 and 2.0. 1083Niels Provos and Markus Friedl contributed support 1084for privilege separation. 1085