1.\" 2.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 3.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 4.\" All rights reserved 5.\" 6.\" As far as I am concerned, the code I have written for this software 7.\" can be used freely for any purpose. Any derived versions of this 8.\" software must be clearly marked as such, and if the derived work is 9.\" incompatible with the protocol description in the RFC file, it must be 10.\" called by a name other than "ssh" or "Secure Shell". 11.\" 12.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 13.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 14.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 15.\" 16.\" Redistribution and use in source and binary forms, with or without 17.\" modification, are permitted provided that the following conditions 18.\" are met: 19.\" 1. Redistributions of source code must retain the above copyright 20.\" notice, this list of conditions and the following disclaimer. 21.\" 2. Redistributions in binary form must reproduce the above copyright 22.\" notice, this list of conditions and the following disclaimer in the 23.\" documentation and/or other materials provided with the distribution. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 26.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 27.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 28.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 29.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 30.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 31.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 32.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 33.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 34.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 35.\" 36.\" $OpenBSD: sshd_config.5,v 1.156 2013/02/06 00:20:42 dtucker Exp $ 37.\" $FreeBSD$ 38.Dd February 6, 2013 39.Dt SSHD_CONFIG 5 40.Os 41.Sh NAME 42.Nm sshd_config 43.Nd OpenSSH SSH daemon configuration file 44.Sh SYNOPSIS 45.Nm /etc/ssh/sshd_config 46.Sh DESCRIPTION 47.Xr sshd 8 48reads configuration data from 49.Pa /etc/ssh/sshd_config 50(or the file specified with 51.Fl f 52on the command line). 53The file contains keyword-argument pairs, one per line. 54Lines starting with 55.Ql # 56and empty lines are interpreted as comments. 57Arguments may optionally be enclosed in double quotes 58.Pq \&" 59in order to represent arguments containing spaces. 60.Pp 61The possible 62keywords and their meanings are as follows (note that 63keywords are case-insensitive and arguments are case-sensitive): 64.Bl -tag -width Ds 65.It Cm AcceptEnv 66Specifies what environment variables sent by the client will be copied into 67the session's 68.Xr environ 7 . 69See 70.Cm SendEnv 71in 72.Xr ssh_config 5 73for how to configure the client. 74Note that environment passing is only supported for protocol 2. 75Variables are specified by name, which may contain the wildcard characters 76.Ql * 77and 78.Ql \&? . 79Multiple environment variables may be separated by whitespace or spread 80across multiple 81.Cm AcceptEnv 82directives. 83Be warned that some environment variables could be used to bypass restricted 84user environments. 85For this reason, care should be taken in the use of this directive. 86The default is not to accept any environment variables. 87.It Cm AddressFamily 88Specifies which address family should be used by 89.Xr sshd 8 . 90Valid arguments are 91.Dq any , 92.Dq inet 93(use IPv4 only), or 94.Dq inet6 95(use IPv6 only). 96The default is 97.Dq any . 98.It Cm AllowAgentForwarding 99Specifies whether 100.Xr ssh-agent 1 101forwarding is permitted. 102The default is 103.Dq yes . 104Note that disabling agent forwarding does not improve security 105unless users are also denied shell access, as they can always install 106their own forwarders. 107.It Cm AllowGroups 108This keyword can be followed by a list of group name patterns, separated 109by spaces. 110If specified, login is allowed only for users whose primary 111group or supplementary group list matches one of the patterns. 112Only group names are valid; a numerical group ID is not recognized. 113By default, login is allowed for all groups. 114The allow/deny directives are processed in the following order: 115.Cm DenyUsers , 116.Cm AllowUsers , 117.Cm DenyGroups , 118and finally 119.Cm AllowGroups . 120.Pp 121See 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 available options are 129.Dq yes 130or 131.Dq all 132to allow TCP forwarding, 133.Dq no 134to prevent all TCP forwarding, 135.Dq local 136to allow local (from the perspective of 137.Xr ssh 1 ) 138forwarding only or 139.Dq remote 140to allow remote forwarding only. 141The default is 142.Dq yes . 143Note that disabling TCP forwarding does not improve security unless 144users are also denied shell access, as they can always install their 145own forwarders. 146.It Cm AllowUsers 147This keyword can be followed by a list of user name patterns, separated 148by spaces. 149If specified, login is allowed only for user names that 150match one of the patterns. 151Only user names are valid; a numerical user ID is not recognized. 152By default, login is allowed for all users. 153If the pattern takes the form USER@HOST then USER and HOST 154are separately checked, restricting logins to particular 155users from particular hosts. 156The allow/deny directives are processed in the following order: 157.Cm DenyUsers , 158.Cm AllowUsers , 159.Cm DenyGroups , 160and finally 161.Cm AllowGroups . 162.Pp 163See 164.Sx PATTERNS 165in 166.Xr ssh_config 5 167for more information on patterns. 168.It Cm AuthenticationMethods 169Specifies the authentication methods that must be successfully completed 170for a user to be granted access. 171This option must be followed by one or more comma-separated lists of 172authentication method names. 173Successful authentication requires completion of every method in at least 174one of these lists. 175.Pp 176For example, an argument of 177.Dq publickey,password publickey,keyboard-interactive 178would require the user to complete public key authentication, followed by 179either password or keyboard interactive authentication. 180Only methods that are next in one or more lists are offered at each stage, 181so for this example, it would not be possible to attempt password or 182keyboard-interactive authentication before public key. 183.Pp 184This option is only available for SSH protocol 2 and will yield a fatal 185error if enabled if protocol 1 is also enabled. 186Note that each authentication method listed should also be explicitly enabled 187in the configuration. 188The default is not to require multiple authentication; successful completion 189of a single authentication method is sufficient. 190.It Cm AuthorizedKeysCommand 191Specifies a program to be used to look up the user's public keys. 192The program will be invoked with a single argument of the username 193being authenticated, and should produce on standard output zero or 194more lines of authorized_keys output (see 195.Sx AUTHORIZED_KEYS 196in 197.Xr sshd 8 ) . 198If a key supplied by AuthorizedKeysCommand does not successfully authenticate 199and authorize the user then public key authentication continues using the usual 200.Cm AuthorizedKeysFile 201files. 202By default, no AuthorizedKeysCommand is run. 203.It Cm AuthorizedKeysCommandUser 204Specifies the user under whose account the AuthorizedKeysCommand is run. 205It is recommended to use a dedicated user that has no other role on the host 206than running authorized keys commands. 207.It Cm AuthorizedKeysFile 208Specifies the file that contains the public keys that can be used 209for user authentication. 210The format is described in the 211.Sx AUTHORIZED_KEYS FILE FORMAT 212section of 213.Xr sshd 8 . 214.Cm AuthorizedKeysFile 215may contain tokens of the form %T which are substituted during connection 216setup. 217The following tokens are defined: %% is replaced by a literal '%', 218%h is replaced by the home directory of the user being authenticated, and 219%u is replaced by the username of that user. 220After expansion, 221.Cm AuthorizedKeysFile 222is taken to be an absolute path or one relative to the user's home 223directory. 224Multiple files may be listed, separated by whitespace. 225The default is 226.Dq .ssh/authorized_keys .ssh/authorized_keys2 . 227.It Cm AuthorizedPrincipalsFile 228Specifies a file that lists principal names that are accepted for 229certificate authentication. 230When using certificates signed by a key listed in 231.Cm TrustedUserCAKeys , 232this file lists names, one of which must appear in the certificate for it 233to be accepted for authentication. 234Names are listed one per line preceded by key options (as described 235in 236.Sx AUTHORIZED_KEYS FILE FORMAT 237in 238.Xr sshd 8 ) . 239Empty lines and comments starting with 240.Ql # 241are ignored. 242.Pp 243.Cm AuthorizedPrincipalsFile 244may contain tokens of the form %T which are substituted during connection 245setup. 246The following tokens are defined: %% is replaced by a literal '%', 247%h is replaced by the home directory of the user being authenticated, and 248%u is replaced by the username of that user. 249After expansion, 250.Cm AuthorizedPrincipalsFile 251is taken to be an absolute path or one relative to the user's home 252directory. 253.Pp 254The default is 255.Dq none , 256i.e. not to use a principals file \(en in this case, the username 257of the user must appear in a certificate's principals list for it to be 258accepted. 259Note that 260.Cm AuthorizedPrincipalsFile 261is only used when authentication proceeds using a CA listed in 262.Cm TrustedUserCAKeys 263and is not consulted for certification authorities trusted via 264.Pa ~/.ssh/authorized_keys , 265though the 266.Cm principals= 267key option offers a similar facility (see 268.Xr sshd 8 269for details). 270.It Cm Banner 271The contents of the specified file are sent to the remote user before 272authentication is allowed. 273If the argument is 274.Dq none 275then no banner is displayed. 276This option is only available for protocol version 2. 277By default, no banner is displayed. 278.It Cm ChallengeResponseAuthentication 279Specifies whether challenge-response authentication is allowed (e.g. via 280PAM or though authentication styles supported in 281.Xr login.conf 5 ) 282The default is 283.Dq yes . 284.It Cm ChrootDirectory 285Specifies the pathname of a directory to 286.Xr chroot 2 287to after authentication. 288All components of the pathname must be root-owned directories that are 289not writable by any other user or group. 290After the chroot, 291.Xr sshd 8 292changes the working directory to the user's home directory. 293.Pp 294The pathname may contain the following tokens that are expanded at runtime once 295the connecting user has been authenticated: %% is replaced by a literal '%', 296%h is replaced by the home directory of the user being authenticated, and 297%u is replaced by the username of that user. 298.Pp 299The 300.Cm ChrootDirectory 301must contain the necessary files and directories to support the 302user's session. 303For an interactive session this requires at least a shell, typically 304.Xr sh 1 , 305and basic 306.Pa /dev 307nodes such as 308.Xr null 4 , 309.Xr zero 4 , 310.Xr stdin 4 , 311.Xr stdout 4 , 312.Xr stderr 4 , 313.Xr arandom 4 314and 315.Xr tty 4 316devices. 317For file transfer sessions using 318.Dq sftp , 319no additional configuration of the environment is necessary if the 320in-process sftp server is used, 321though sessions which use logging do require 322.Pa /dev/log 323inside the chroot directory (see 324.Xr sftp-server 8 325for details). 326.Pp 327The default is not to 328.Xr chroot 2 . 329.It Cm Ciphers 330Specifies the ciphers allowed for protocol version 2. 331Multiple ciphers must be comma-separated. 332The supported ciphers are 333.Dq 3des-cbc , 334.Dq aes128-cbc , 335.Dq aes192-cbc , 336.Dq aes256-cbc , 337.Dq aes128-ctr , 338.Dq aes192-ctr , 339.Dq aes256-ctr , 340.Dq aes128-gcm@openssh.com , 341.Dq aes256-gcm@openssh.com , 342.Dq arcfour128 , 343.Dq arcfour256 , 344.Dq arcfour , 345.Dq blowfish-cbc , 346and 347.Dq cast128-cbc . 348The default is: 349.Bd -literal -offset 3n 350aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 351aes128-gcm@openssh.com,aes256-gcm@openssh.com, 352aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 353aes256-cbc,arcfour 354.Ed 355.It Cm ClientAliveCountMax 356Sets the number of client alive messages (see below) which may be 357sent without 358.Xr sshd 8 359receiving any messages back from the client. 360If this threshold is reached while client alive messages are being sent, 361sshd will disconnect the client, terminating the session. 362It is important to note that the use of client alive messages is very 363different from 364.Cm TCPKeepAlive 365(below). 366The client alive messages are sent through the encrypted channel 367and therefore will not be spoofable. 368The TCP keepalive option enabled by 369.Cm TCPKeepAlive 370is spoofable. 371The client alive mechanism is valuable when the client or 372server depend on knowing when a connection has become inactive. 373.Pp 374The default value is 3. 375If 376.Cm ClientAliveInterval 377(see below) is set to 15, and 378.Cm ClientAliveCountMax 379is left at the default, unresponsive SSH clients 380will be disconnected after approximately 45 seconds. 381This option applies to protocol version 2 only. 382.It Cm ClientAliveInterval 383Sets a timeout interval in seconds after which if no data has been received 384from the client, 385.Xr sshd 8 386will send a message through the encrypted 387channel to request a response from the client. 388The default 389is 0, indicating that these messages will not be sent to the client. 390This option applies to protocol version 2 only. 391.It Cm Compression 392Specifies whether compression is allowed, or delayed until 393the user has authenticated successfully. 394The argument must be 395.Dq yes , 396.Dq delayed , 397or 398.Dq no . 399The default is 400.Dq delayed . 401.It Cm DenyGroups 402This keyword can be followed by a list of group name patterns, separated 403by spaces. 404Login is disallowed for users whose primary group or supplementary 405group list matches one of the patterns. 406Only group names are valid; a numerical group ID is not recognized. 407By default, login is allowed for all groups. 408The allow/deny directives are processed in the following order: 409.Cm DenyUsers , 410.Cm AllowUsers , 411.Cm DenyGroups , 412and finally 413.Cm AllowGroups . 414.Pp 415See 416.Sx PATTERNS 417in 418.Xr ssh_config 5 419for more information on patterns. 420.It Cm DenyUsers 421This keyword can be followed by a list of user name patterns, separated 422by spaces. 423Login is disallowed for user names that match one of the patterns. 424Only user names are valid; a numerical user ID is not recognized. 425By default, login is allowed for all users. 426If the pattern takes the form USER@HOST then USER and HOST 427are separately checked, restricting logins to particular 428users from particular hosts. 429The allow/deny directives are processed in the following order: 430.Cm DenyUsers , 431.Cm AllowUsers , 432.Cm DenyGroups , 433and finally 434.Cm AllowGroups . 435.Pp 436See 437.Sx PATTERNS 438in 439.Xr ssh_config 5 440for more information on patterns. 441.It Cm ForceCommand 442Forces the execution of the command specified by 443.Cm ForceCommand , 444ignoring any command supplied by the client and 445.Pa ~/.ssh/rc 446if present. 447The command is invoked by using the user's login shell with the -c option. 448This applies to shell, command, or subsystem execution. 449It is most useful inside a 450.Cm Match 451block. 452The command originally supplied by the client is available in the 453.Ev SSH_ORIGINAL_COMMAND 454environment variable. 455Specifying a command of 456.Dq internal-sftp 457will force the use of an in-process sftp server that requires no support 458files when used with 459.Cm ChrootDirectory . 460.It Cm GatewayPorts 461Specifies whether remote hosts are allowed to connect to ports 462forwarded for the client. 463By default, 464.Xr sshd 8 465binds remote port forwardings to the loopback address. 466This prevents other remote hosts from connecting to forwarded ports. 467.Cm GatewayPorts 468can be used to specify that sshd 469should allow remote port forwardings to bind to non-loopback addresses, thus 470allowing other hosts to connect. 471The argument may be 472.Dq no 473to force remote port forwardings to be available to the local host only, 474.Dq yes 475to force remote port forwardings to bind to the wildcard address, or 476.Dq clientspecified 477to allow the client to select the address to which the forwarding is bound. 478The default is 479.Dq no . 480.It Cm GSSAPIAuthentication 481Specifies whether user authentication based on GSSAPI is allowed. 482The default is 483.Dq no . 484Note that this option applies to protocol version 2 only. 485.It Cm GSSAPICleanupCredentials 486Specifies whether to automatically destroy the user's credentials cache 487on logout. 488The default is 489.Dq yes . 490Note that this option applies to protocol version 2 only. 491.It Cm HostbasedAuthentication 492Specifies whether rhosts or /etc/hosts.equiv authentication together 493with successful public key client host authentication is allowed 494(host-based authentication). 495This option is similar to 496.Cm RhostsRSAAuthentication 497and applies to protocol version 2 only. 498The default is 499.Dq no . 500.It Cm HostbasedUsesNameFromPacketOnly 501Specifies whether or not the server will attempt to perform a reverse 502name lookup when matching the name in the 503.Pa ~/.shosts , 504.Pa ~/.rhosts , 505and 506.Pa /etc/hosts.equiv 507files during 508.Cm HostbasedAuthentication . 509A setting of 510.Dq yes 511means that 512.Xr sshd 8 513uses the name supplied by the client rather than 514attempting to resolve the name from the TCP connection itself. 515The default is 516.Dq no . 517.It Cm HostCertificate 518Specifies a file containing a public host certificate. 519The certificate's public key must match a private host key already specified 520by 521.Cm HostKey . 522The default behaviour of 523.Xr sshd 8 524is not to load any certificates. 525.It Cm HostKey 526Specifies a file containing a private host key 527used by SSH. 528The default is 529.Pa /etc/ssh/ssh_host_key 530for protocol version 1, and 531.Pa /etc/ssh/ssh_host_dsa_key , 532.Pa /etc/ssh/ssh_host_ecdsa_key 533and 534.Pa /etc/ssh/ssh_host_rsa_key 535for protocol version 2. 536Note that 537.Xr sshd 8 538will refuse to use a file if it is group/world-accessible. 539It is possible to have multiple host key files. 540.Dq rsa1 541keys are used for version 1 and 542.Dq dsa , 543.Dq ecdsa 544or 545.Dq rsa 546are used for version 2 of the SSH protocol. 547.It Cm IgnoreRhosts 548Specifies that 549.Pa .rhosts 550and 551.Pa .shosts 552files will not be used in 553.Cm RhostsRSAAuthentication 554or 555.Cm HostbasedAuthentication . 556.Pp 557.Pa /etc/hosts.equiv 558and 559.Pa /etc/ssh/shosts.equiv 560are still used. 561The default is 562.Dq yes . 563.It Cm IgnoreUserKnownHosts 564Specifies whether 565.Xr sshd 8 566should ignore the user's 567.Pa ~/.ssh/known_hosts 568during 569.Cm RhostsRSAAuthentication 570or 571.Cm HostbasedAuthentication . 572The default is 573.Dq no . 574.It Cm IPQoS 575Specifies the IPv4 type-of-service or DSCP class for the connection. 576Accepted values are 577.Dq af11 , 578.Dq af12 , 579.Dq af13 , 580.Dq af21 , 581.Dq af22 , 582.Dq af23 , 583.Dq af31 , 584.Dq af32 , 585.Dq af33 , 586.Dq af41 , 587.Dq af42 , 588.Dq af43 , 589.Dq cs0 , 590.Dq cs1 , 591.Dq cs2 , 592.Dq cs3 , 593.Dq cs4 , 594.Dq cs5 , 595.Dq cs6 , 596.Dq cs7 , 597.Dq ef , 598.Dq lowdelay , 599.Dq throughput , 600.Dq reliability , 601or a numeric value. 602This option may take one or two arguments, separated by whitespace. 603If one argument is specified, it is used as the packet class unconditionally. 604If two values are specified, the first is automatically selected for 605interactive sessions and the second for non-interactive sessions. 606The default is 607.Dq lowdelay 608for interactive sessions and 609.Dq throughput 610for non-interactive sessions. 611.It Cm KerberosAuthentication 612Specifies whether the password provided by the user for 613.Cm PasswordAuthentication 614will be validated through the Kerberos KDC. 615To use this option, the server needs a 616Kerberos servtab which allows the verification of the KDC's identity. 617The default is 618.Dq no . 619.It Cm KerberosGetAFSToken 620If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire 621an AFS token before accessing the user's home directory. 622The default is 623.Dq no . 624.It Cm KerberosOrLocalPasswd 625If password authentication through Kerberos fails then 626the password will be validated via any additional local mechanism 627such as 628.Pa /etc/passwd . 629The default is 630.Dq yes . 631.It Cm KerberosTicketCleanup 632Specifies whether to automatically destroy the user's ticket cache 633file on logout. 634The default is 635.Dq yes . 636.It Cm KexAlgorithms 637Specifies the available KEX (Key Exchange) algorithms. 638Multiple algorithms must be comma-separated. 639The default is 640.Dq ecdh-sha2-nistp256 , 641.Dq ecdh-sha2-nistp384 , 642.Dq ecdh-sha2-nistp521 , 643.Dq diffie-hellman-group-exchange-sha256 , 644.Dq diffie-hellman-group-exchange-sha1 , 645.Dq diffie-hellman-group14-sha1 , 646.Dq diffie-hellman-group1-sha1 . 647.It Cm KeyRegenerationInterval 648In protocol version 1, the ephemeral server key is automatically regenerated 649after this many seconds (if it has been used). 650The purpose of regeneration is to prevent 651decrypting captured sessions by later breaking into the machine and 652stealing the keys. 653The key is never stored anywhere. 654If the value is 0, the key is never regenerated. 655The default is 3600 (seconds). 656.It Cm ListenAddress 657Specifies the local addresses 658.Xr sshd 8 659should listen on. 660The following forms may be used: 661.Pp 662.Bl -item -offset indent -compact 663.It 664.Cm ListenAddress 665.Sm off 666.Ar host No | Ar IPv4_addr No | Ar IPv6_addr 667.Sm on 668.It 669.Cm ListenAddress 670.Sm off 671.Ar host No | Ar IPv4_addr No : Ar port 672.Sm on 673.It 674.Cm ListenAddress 675.Sm off 676.Oo 677.Ar host No | Ar IPv6_addr Oc : Ar port 678.Sm on 679.El 680.Pp 681If 682.Ar port 683is not specified, 684sshd will listen on the address and all prior 685.Cm Port 686options specified. 687The default is to listen on all local addresses. 688Multiple 689.Cm ListenAddress 690options are permitted. 691Additionally, any 692.Cm Port 693options must precede this option for non-port qualified addresses. 694.It Cm LoginGraceTime 695The server disconnects after this time if the user has not 696successfully logged in. 697If the value is 0, there is no time limit. 698The default is 120 seconds. 699.It Cm LogLevel 700Gives the verbosity level that is used when logging messages from 701.Xr sshd 8 . 702The possible values are: 703QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 704The default is INFO. 705DEBUG and DEBUG1 are equivalent. 706DEBUG2 and DEBUG3 each specify higher levels of debugging output. 707Logging with a DEBUG level violates the privacy of users and is not recommended. 708.It Cm MACs 709Specifies the available MAC (message authentication code) algorithms. 710The MAC algorithm is used in protocol version 2 711for data integrity protection. 712Multiple algorithms must be comma-separated. 713The algorithms that contain 714.Dq -etm 715calculate the MAC after encryption (encrypt-then-mac). 716These are considered safer and their use recommended. 717The default is: 718.Bd -literal -offset indent 719hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com, 720umac-64-etm@openssh.com,umac-128-etm@openssh.com, 721hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 722hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com, 723hmac-md5-96-etm@openssh.com, 724hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com, 725hmac-sha2-256,hmac-sha2-512,hmac-ripemd160, 726hmac-sha1-96,hmac-md5-96 727.Ed 728.It Cm Match 729Introduces a conditional block. 730If all of the criteria on the 731.Cm Match 732line are satisfied, the keywords on the following lines override those 733set in the global section of the config file, until either another 734.Cm Match 735line or the end of the file. 736.Pp 737The arguments to 738.Cm Match 739are one or more criteria-pattern pairs. 740The available criteria are 741.Cm User , 742.Cm Group , 743.Cm Host , 744.Cm LocalAddress , 745.Cm LocalPort , 746and 747.Cm Address . 748The match patterns may consist of single entries or comma-separated 749lists and may use the wildcard and negation operators described in the 750.Sx PATTERNS 751section of 752.Xr ssh_config 5 . 753.Pp 754The patterns in an 755.Cm Address 756criteria may additionally contain addresses to match in CIDR 757address/masklen format, e.g.\& 758.Dq 192.0.2.0/24 759or 760.Dq 3ffe:ffff::/32 . 761Note that the mask length provided must be consistent with the address - 762it is an error to specify a mask length that is too long for the address 763or one with bits set in this host portion of the address. 764For example, 765.Dq 192.0.2.0/33 766and 767.Dq 192.0.2.0/8 768respectively. 769.Pp 770Only a subset of keywords may be used on the lines following a 771.Cm Match 772keyword. 773Available keywords are 774.Cm AcceptEnv , 775.Cm AllowAgentForwarding , 776.Cm AllowGroups , 777.Cm AllowTcpForwarding , 778.Cm AllowUsers , 779.Cm AuthenticationMethods , 780.Cm AuthorizedKeysCommand , 781.Cm AuthorizedKeysCommandUser , 782.Cm AuthorizedKeysFile , 783.Cm AuthorizedPrincipalsFile , 784.Cm Banner , 785.Cm ChrootDirectory , 786.Cm DenyGroups , 787.Cm DenyUsers , 788.Cm ForceCommand , 789.Cm GatewayPorts , 790.Cm GSSAPIAuthentication , 791.Cm HostbasedAuthentication , 792.Cm HostbasedUsesNameFromPacketOnly , 793.Cm KbdInteractiveAuthentication , 794.Cm KerberosAuthentication , 795.Cm MaxAuthTries , 796.Cm MaxSessions , 797.Cm PasswordAuthentication , 798.Cm PermitEmptyPasswords , 799.Cm PermitOpen , 800.Cm PermitRootLogin , 801.Cm PermitTunnel , 802.Cm PubkeyAuthentication , 803.Cm RhostsRSAAuthentication , 804.Cm RSAAuthentication , 805.Cm X11DisplayOffset , 806.Cm X11Forwarding 807and 808.Cm X11UseLocalHost . 809.It Cm MaxAuthTries 810Specifies the maximum number of authentication attempts permitted per 811connection. 812Once the number of failures reaches half this value, 813additional failures are logged. 814The default is 6. 815.It Cm MaxSessions 816Specifies the maximum number of open sessions permitted per network connection. 817The default is 10. 818.It Cm MaxStartups 819Specifies the maximum number of concurrent unauthenticated connections to the 820SSH daemon. 821Additional connections will be dropped until authentication succeeds or the 822.Cm LoginGraceTime 823expires for a connection. 824The default is 10:30:100. 825.Pp 826Alternatively, random early drop can be enabled by specifying 827the three colon separated values 828.Dq start:rate:full 829(e.g. "10:30:60"). 830.Xr sshd 8 831will refuse connection attempts with a probability of 832.Dq rate/100 833(30%) 834if there are currently 835.Dq start 836(10) 837unauthenticated connections. 838The probability increases linearly and all connection attempts 839are refused if the number of unauthenticated connections reaches 840.Dq full 841(60). 842.It Cm PasswordAuthentication 843Specifies whether password authentication is allowed. 844See also 845.Cm UsePAM . 846The default is 847.Dq no . 848.It Cm PermitEmptyPasswords 849When password authentication is allowed, it specifies whether the 850server allows login to accounts with empty password strings. 851The default is 852.Dq no . 853.It Cm PermitOpen 854Specifies the destinations to which TCP port forwarding is permitted. 855The forwarding specification must be one of the following forms: 856.Pp 857.Bl -item -offset indent -compact 858.It 859.Cm PermitOpen 860.Sm off 861.Ar host : port 862.Sm on 863.It 864.Cm PermitOpen 865.Sm off 866.Ar IPv4_addr : port 867.Sm on 868.It 869.Cm PermitOpen 870.Sm off 871.Ar \&[ IPv6_addr \&] : port 872.Sm on 873.El 874.Pp 875Multiple forwards may be specified by separating them with whitespace. 876An argument of 877.Dq any 878can be used to remove all restrictions and permit any forwarding requests. 879An argument of 880.Dq none 881can be used to prohibit all forwarding requests. 882By default all port forwarding requests are permitted. 883.It Cm PermitRootLogin 884Specifies whether root can log in using 885.Xr ssh 1 . 886The argument must be 887.Dq yes , 888.Dq without-password , 889.Dq forced-commands-only , 890or 891.Dq no . 892The default is 893.Dq no . 894Note that if 895.Cm ChallengeResponseAuthentication 896is 897.Dq yes , 898the root user may be allowed in with its password even if 899.Cm PermitRootLogin is set to 900.Dq without-password . 901.Pp 902If this option is set to 903.Dq without-password , 904password authentication is disabled for root. 905.Pp 906If this option is set to 907.Dq forced-commands-only , 908root login with public key authentication will be allowed, 909but only if the 910.Ar command 911option has been specified 912(which may be useful for taking remote backups even if root login is 913normally not allowed). 914All other authentication methods are disabled for root. 915.Pp 916If this option is set to 917.Dq no , 918root is not allowed to log in. 919.It Cm PermitTunnel 920Specifies whether 921.Xr tun 4 922device forwarding is allowed. 923The argument must be 924.Dq yes , 925.Dq point-to-point 926(layer 3), 927.Dq ethernet 928(layer 2), or 929.Dq no . 930Specifying 931.Dq yes 932permits both 933.Dq point-to-point 934and 935.Dq ethernet . 936The default is 937.Dq no . 938.It Cm PermitUserEnvironment 939Specifies whether 940.Pa ~/.ssh/environment 941and 942.Cm environment= 943options in 944.Pa ~/.ssh/authorized_keys 945are processed by 946.Xr sshd 8 . 947The default is 948.Dq no . 949Enabling environment processing may enable users to bypass access 950restrictions in some configurations using mechanisms such as 951.Ev LD_PRELOAD . 952.It Cm PidFile 953Specifies the file that contains the process ID of the 954SSH daemon. 955The default is 956.Pa /var/run/sshd.pid . 957.It Cm Port 958Specifies the port number that 959.Xr sshd 8 960listens on. 961The default is 22. 962Multiple options of this type are permitted. 963See also 964.Cm ListenAddress . 965.It Cm PrintLastLog 966Specifies whether 967.Xr sshd 8 968should print the date and time of the last user login when a user logs 969in interactively. 970The default is 971.Dq yes . 972.It Cm PrintMotd 973Specifies whether 974.Xr sshd 8 975should print 976.Pa /etc/motd 977when a user logs in interactively. 978(On some systems it is also printed by the shell, 979.Pa /etc/profile , 980or equivalent.) 981The default is 982.Dq yes . 983.It Cm Protocol 984Specifies the protocol versions 985.Xr sshd 8 986supports. 987The possible values are 988.Sq 1 989and 990.Sq 2 . 991Multiple versions must be comma-separated. 992The default is 993.Sq 2 . 994Note that the order of the protocol list does not indicate preference, 995because the client selects among multiple protocol versions offered 996by the server. 997Specifying 998.Dq 2,1 999is identical to 1000.Dq 1,2 . 1001.It Cm PubkeyAuthentication 1002Specifies whether public key authentication is allowed. 1003The default is 1004.Dq yes . 1005Note that this option applies to protocol version 2 only. 1006.It Cm RevokedKeys 1007Specifies revoked public keys. 1008Keys listed in this file will be refused for public key authentication. 1009Note that if this file is not readable, then public key authentication will 1010be refused for all users. 1011Keys may be specified as a text file, listing one public key per line, or as 1012an OpenSSH Key Revocation List (KRL) as generated by 1013.Xr ssh-keygen 1 . 1014For more information on KRLs, see the 1015.Sx KEY REVOCATION LISTS 1016section in 1017.Xr ssh-keygen 1 . 1018.It Cm RhostsRSAAuthentication 1019Specifies whether rhosts or 1020.Pa /etc/hosts.equiv 1021authentication together 1022with successful RSA host authentication is allowed. 1023The default is 1024.Dq no . 1025This option applies to protocol version 1 only. 1026.It Cm RSAAuthentication 1027Specifies whether pure RSA authentication is allowed. 1028The default is 1029.Dq yes . 1030This option applies to protocol version 1 only. 1031.It Cm ServerKeyBits 1032Defines the number of bits in the ephemeral protocol version 1 server key. 1033The minimum value is 512, and the default is 1024. 1034.It Cm StrictModes 1035Specifies whether 1036.Xr sshd 8 1037should check file modes and ownership of the 1038user's files and home directory before accepting login. 1039This is normally desirable because novices sometimes accidentally leave their 1040directory or files world-writable. 1041The default is 1042.Dq yes . 1043Note that this does not apply to 1044.Cm ChrootDirectory , 1045whose permissions and ownership are checked unconditionally. 1046.It Cm Subsystem 1047Configures an external subsystem (e.g. file transfer daemon). 1048Arguments should be a subsystem name and a command (with optional arguments) 1049to execute upon subsystem request. 1050.Pp 1051The command 1052.Xr sftp-server 8 1053implements the 1054.Dq sftp 1055file transfer subsystem. 1056.Pp 1057Alternately the name 1058.Dq internal-sftp 1059implements an in-process 1060.Dq sftp 1061server. 1062This may simplify configurations using 1063.Cm ChrootDirectory 1064to force a different filesystem root on clients. 1065.Pp 1066By default no subsystems are defined. 1067Note that this option applies to protocol version 2 only. 1068.It Cm SyslogFacility 1069Gives the facility code that is used when logging messages from 1070.Xr sshd 8 . 1071The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1072LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1073The default is AUTH. 1074.It Cm TCPKeepAlive 1075Specifies whether the system should send TCP keepalive messages to the 1076other side. 1077If they are sent, death of the connection or crash of one 1078of the machines will be properly noticed. 1079However, this means that 1080connections will die if the route is down temporarily, and some people 1081find it annoying. 1082On the other hand, if TCP keepalives are not sent, 1083sessions may hang indefinitely on the server, leaving 1084.Dq ghost 1085users and consuming server resources. 1086.Pp 1087The default is 1088.Dq yes 1089(to send TCP keepalive messages), and the server will notice 1090if the network goes down or the client host crashes. 1091This avoids infinitely hanging sessions. 1092.Pp 1093To disable TCP keepalive messages, the value should be set to 1094.Dq no . 1095.It Cm TrustedUserCAKeys 1096Specifies a file containing public keys of certificate authorities that are 1097trusted to sign user certificates for authentication. 1098Keys are listed one per line; empty lines and comments starting with 1099.Ql # 1100are allowed. 1101If a certificate is presented for authentication and has its signing CA key 1102listed in this file, then it may be used for authentication for any user 1103listed in the certificate's principals list. 1104Note that certificates that lack a list of principals will not be permitted 1105for authentication using 1106.Cm TrustedUserCAKeys . 1107For more details on certificates, see the 1108.Sx CERTIFICATES 1109section in 1110.Xr ssh-keygen 1 . 1111.It Cm UseDNS 1112Specifies whether 1113.Xr sshd 8 1114should look up the remote host name and check that 1115the resolved host name for the remote IP address maps back to the 1116very same IP address. 1117The default is 1118.Dq yes . 1119.It Cm UseLogin 1120Specifies whether 1121.Xr login 1 1122is used for interactive login sessions. 1123The default is 1124.Dq no . 1125Note that 1126.Xr login 1 1127is never used for remote command execution. 1128Note also, that if this is enabled, 1129.Cm X11Forwarding 1130will be disabled because 1131.Xr login 1 1132does not know how to handle 1133.Xr xauth 1 1134cookies. 1135If 1136.Cm UsePrivilegeSeparation 1137is specified, it will be disabled after authentication. 1138.It Cm UsePAM 1139Enables the Pluggable Authentication Module interface. 1140If set to 1141.Dq yes 1142this will enable PAM authentication using 1143.Cm ChallengeResponseAuthentication 1144and 1145.Cm PasswordAuthentication 1146in addition to PAM account and session module processing for all 1147authentication types. 1148.Pp 1149Because PAM challenge-response authentication usually serves an equivalent 1150role to password authentication, you should disable either 1151.Cm PasswordAuthentication 1152or 1153.Cm ChallengeResponseAuthentication. 1154.Pp 1155If 1156.Cm UsePAM 1157is enabled, you will not be able to run 1158.Xr sshd 8 1159as a non-root user. 1160The default is 1161.Dq yes . 1162.It Cm UsePrivilegeSeparation 1163Specifies whether 1164.Xr sshd 8 1165separates privileges by creating an unprivileged child process 1166to deal with incoming network traffic. 1167After successful authentication, another process will be created that has 1168the privilege of the authenticated user. 1169The goal of privilege separation is to prevent privilege 1170escalation by containing any corruption within the unprivileged processes. 1171The default is 1172.Dq yes . 1173If 1174.Cm UsePrivilegeSeparation 1175is set to 1176.Dq sandbox 1177then the pre-authentication unprivileged process is subject to additional 1178restrictions. 1179.It Cm VersionAddendum 1180Optionally specifies additional text to append to the SSH protocol banner 1181sent by the server upon connection. 1182The default is 1183.Dq FreeBSD-20130515 . 1184.It Cm X11DisplayOffset 1185Specifies the first display number available for 1186.Xr sshd 8 Ns 's 1187X11 forwarding. 1188This prevents sshd from interfering with real X11 servers. 1189The default is 10. 1190.It Cm X11Forwarding 1191Specifies whether X11 forwarding is permitted. 1192The argument must be 1193.Dq yes 1194or 1195.Dq no . 1196The default is 1197.Dq yes . 1198.Pp 1199When X11 forwarding is enabled, there may be additional exposure to 1200the server and to client displays if the 1201.Xr sshd 8 1202proxy display is configured to listen on the wildcard address (see 1203.Cm X11UseLocalhost 1204below), though this is not the default. 1205Additionally, the authentication spoofing and authentication data 1206verification and substitution occur on the client side. 1207The security risk of using X11 forwarding is that the client's X11 1208display server may be exposed to attack when the SSH client requests 1209forwarding (see the warnings for 1210.Cm ForwardX11 1211in 1212.Xr ssh_config 5 ) . 1213A system administrator may have a stance in which they want to 1214protect clients that may expose themselves to attack by unwittingly 1215requesting X11 forwarding, which can warrant a 1216.Dq no 1217setting. 1218.Pp 1219Note that disabling X11 forwarding does not prevent users from 1220forwarding X11 traffic, as users can always install their own forwarders. 1221X11 forwarding is automatically disabled if 1222.Cm UseLogin 1223is enabled. 1224.It Cm X11UseLocalhost 1225Specifies whether 1226.Xr sshd 8 1227should bind the X11 forwarding server to the loopback address or to 1228the wildcard address. 1229By default, 1230sshd binds the forwarding server to the loopback address and sets the 1231hostname part of the 1232.Ev DISPLAY 1233environment variable to 1234.Dq localhost . 1235This prevents remote hosts from connecting to the proxy display. 1236However, some older X11 clients may not function with this 1237configuration. 1238.Cm X11UseLocalhost 1239may be set to 1240.Dq no 1241to specify that the forwarding server should be bound to the wildcard 1242address. 1243The argument must be 1244.Dq yes 1245or 1246.Dq no . 1247The default is 1248.Dq yes . 1249.It Cm XAuthLocation 1250Specifies the full pathname of the 1251.Xr xauth 1 1252program. 1253The default is 1254.Pa /usr/local/bin/xauth . 1255.El 1256.Sh TIME FORMATS 1257.Xr sshd 8 1258command-line arguments and configuration file options that specify time 1259may be expressed using a sequence of the form: 1260.Sm off 1261.Ar time Op Ar qualifier , 1262.Sm on 1263where 1264.Ar time 1265is a positive integer value and 1266.Ar qualifier 1267is one of the following: 1268.Pp 1269.Bl -tag -width Ds -compact -offset indent 1270.It Aq Cm none 1271seconds 1272.It Cm s | Cm S 1273seconds 1274.It Cm m | Cm M 1275minutes 1276.It Cm h | Cm H 1277hours 1278.It Cm d | Cm D 1279days 1280.It Cm w | Cm W 1281weeks 1282.El 1283.Pp 1284Each member of the sequence is added together to calculate 1285the total time value. 1286.Pp 1287Time format examples: 1288.Pp 1289.Bl -tag -width Ds -compact -offset indent 1290.It 600 1291600 seconds (10 minutes) 1292.It 10m 129310 minutes 1294.It 1h30m 12951 hour 30 minutes (90 minutes) 1296.El 1297.Sh FILES 1298.Bl -tag -width Ds 1299.It Pa /etc/ssh/sshd_config 1300Contains configuration data for 1301.Xr sshd 8 . 1302This file should be writable by root only, but it is recommended 1303(though not necessary) that it be world-readable. 1304.El 1305.Sh SEE ALSO 1306.Xr sshd 8 1307.Sh AUTHORS 1308OpenSSH is a derivative of the original and free 1309ssh 1.2.12 release by Tatu Ylonen. 1310Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1311Theo de Raadt and Dug Song 1312removed many bugs, re-added newer features and 1313created OpenSSH. 1314Markus Friedl contributed the support for SSH 1315protocol versions 1.5 and 2.0. 1316Niels Provos and Markus Friedl contributed support 1317for privilege separation. 1318