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: ssh_config.5,v 1.353 2021/04/04 11:36:56 jmc Exp $ 37.Dd $Mdocdate: April 4 2021 $ 38.Dt SSH_CONFIG 5 39.Os 40.Sh NAME 41.Nm ssh_config 42.Nd OpenSSH client configuration file 43.Sh DESCRIPTION 44.Xr ssh 1 45obtains configuration data from the following sources in 46the following order: 47.Pp 48.Bl -enum -offset indent -compact 49.It 50command-line options 51.It 52user's configuration file 53.Pq Pa ~/.ssh/config 54.It 55system-wide configuration file 56.Pq Pa /etc/ssh/ssh_config 57.El 58.Pp 59For each parameter, the first obtained value 60will be used. 61The configuration files contain sections separated by 62.Cm Host 63specifications, and that section is only applied for hosts that 64match one of the patterns given in the specification. 65The matched host name is usually the one given on the command line 66(see the 67.Cm CanonicalizeHostname 68option for exceptions). 69.Pp 70Since the first obtained value for each parameter is used, more 71host-specific declarations should be given near the beginning of the 72file, and general defaults at the end. 73.Pp 74The file contains keyword-argument pairs, one per line. 75Lines starting with 76.Ql # 77and empty lines are interpreted as comments. 78Arguments may optionally be enclosed in double quotes 79.Pq \&" 80in order to represent arguments containing spaces. 81Configuration options may be separated by whitespace or 82optional whitespace and exactly one 83.Ql = ; 84the latter format is useful to avoid the need to quote whitespace 85when specifying configuration options using the 86.Nm ssh , 87.Nm scp , 88and 89.Nm sftp 90.Fl o 91option. 92.Pp 93The possible 94keywords and their meanings are as follows (note that 95keywords are case-insensitive and arguments are case-sensitive): 96.Bl -tag -width Ds 97.It Cm Host 98Restricts the following declarations (up to the next 99.Cm Host 100or 101.Cm Match 102keyword) to be only for those hosts that match one of the patterns 103given after the keyword. 104If more than one pattern is provided, they should be separated by whitespace. 105A single 106.Ql * 107as a pattern can be used to provide global 108defaults for all hosts. 109The host is usually the 110.Ar hostname 111argument given on the command line 112(see the 113.Cm CanonicalizeHostname 114keyword for exceptions). 115.Pp 116A pattern entry may be negated by prefixing it with an exclamation mark 117.Pq Sq !\& . 118If a negated entry is matched, then the 119.Cm Host 120entry is ignored, regardless of whether any other patterns on the line 121match. 122Negated matches are therefore useful to provide exceptions for wildcard 123matches. 124.Pp 125See 126.Sx PATTERNS 127for more information on patterns. 128.It Cm Match 129Restricts the following declarations (up to the next 130.Cm Host 131or 132.Cm Match 133keyword) to be used only when the conditions following the 134.Cm Match 135keyword are satisfied. 136Match conditions are specified using one or more criteria 137or the single token 138.Cm all 139which always matches. 140The available criteria keywords are: 141.Cm canonical , 142.Cm final , 143.Cm exec , 144.Cm host , 145.Cm originalhost , 146.Cm user , 147and 148.Cm localuser . 149The 150.Cm all 151criteria must appear alone or immediately after 152.Cm canonical 153or 154.Cm final . 155Other criteria may be combined arbitrarily. 156All criteria but 157.Cm all , 158.Cm canonical , 159and 160.Cm final 161require an argument. 162Criteria may be negated by prepending an exclamation mark 163.Pq Sq !\& . 164.Pp 165The 166.Cm canonical 167keyword matches only when the configuration file is being re-parsed 168after hostname canonicalization (see the 169.Cm CanonicalizeHostname 170option). 171This may be useful to specify conditions that work with canonical host 172names only. 173.Pp 174The 175.Cm final 176keyword requests that the configuration be re-parsed (regardless of whether 177.Cm CanonicalizeHostname 178is enabled), and matches only during this final pass. 179If 180.Cm CanonicalizeHostname 181is enabled, then 182.Cm canonical 183and 184.Cm final 185match during the same pass. 186.Pp 187The 188.Cm exec 189keyword executes the specified command under the user's shell. 190If the command returns a zero exit status then the condition is considered true. 191Commands containing whitespace characters must be quoted. 192Arguments to 193.Cm exec 194accept the tokens described in the 195.Sx TOKENS 196section. 197.Pp 198The other keywords' criteria must be single entries or comma-separated 199lists and may use the wildcard and negation operators described in the 200.Sx PATTERNS 201section. 202The criteria for the 203.Cm host 204keyword are matched against the target hostname, after any substitution 205by the 206.Cm Hostname 207or 208.Cm CanonicalizeHostname 209options. 210The 211.Cm originalhost 212keyword matches against the hostname as it was specified on the command-line. 213The 214.Cm user 215keyword matches against the target username on the remote host. 216The 217.Cm localuser 218keyword matches against the name of the local user running 219.Xr ssh 1 220(this keyword may be useful in system-wide 221.Nm 222files). 223.It Cm AddKeysToAgent 224Specifies whether keys should be automatically added to a running 225.Xr ssh-agent 1 . 226If this option is set to 227.Cm yes 228and a key is loaded from a file, the key and its passphrase are added to 229the agent with the default lifetime, as if by 230.Xr ssh-add 1 . 231If this option is set to 232.Cm ask , 233.Xr ssh 1 234will require confirmation using the 235.Ev SSH_ASKPASS 236program before adding a key (see 237.Xr ssh-add 1 238for details). 239If this option is set to 240.Cm confirm , 241each use of the key must be confirmed, as if the 242.Fl c 243option was specified to 244.Xr ssh-add 1 . 245If this option is set to 246.Cm no , 247no keys are added to the agent. 248Alternately, this option may be specified as a time interval 249using the format described in the 250.Sx TIME FORMATS 251section of 252.Xr sshd_config 5 253to specify the key's lifetime in 254.Xr ssh-agent 1 , 255after which it will automatically be removed. 256The argument must be 257.Cm no 258(the default), 259.Cm yes , 260.Cm confirm 261(optionally followed by a time interval), 262.Cm ask 263or a time interval. 264.It Cm AddressFamily 265Specifies which address family to use when connecting. 266Valid arguments are 267.Cm any 268(the default), 269.Cm inet 270(use IPv4 only), or 271.Cm inet6 272(use IPv6 only). 273.It Cm BatchMode 274If set to 275.Cm yes , 276user interaction such as password prompts and host key confirmation requests 277will be disabled. 278This option is useful in scripts and other batch jobs where no user 279is present to interact with 280.Xr ssh 1 . 281The argument must be 282.Cm yes 283or 284.Cm no 285(the default). 286.It Cm BindAddress 287Use the specified address on the local machine as the source address of 288the connection. 289Only useful on systems with more than one address. 290.It Cm BindInterface 291Use the address of the specified interface on the local machine as the 292source address of the connection. 293.It Cm CanonicalDomains 294When 295.Cm CanonicalizeHostname 296is enabled, this option specifies the list of domain suffixes in which to 297search for the specified destination host. 298.It Cm CanonicalizeFallbackLocal 299Specifies whether to fail with an error when hostname canonicalization fails. 300The default, 301.Cm yes , 302will attempt to look up the unqualified hostname using the system resolver's 303search rules. 304A value of 305.Cm no 306will cause 307.Xr ssh 1 308to fail instantly if 309.Cm CanonicalizeHostname 310is enabled and the target hostname cannot be found in any of the domains 311specified by 312.Cm CanonicalDomains . 313.It Cm CanonicalizeHostname 314Controls whether explicit hostname canonicalization is performed. 315The default, 316.Cm no , 317is not to perform any name rewriting and let the system resolver handle all 318hostname lookups. 319If set to 320.Cm yes 321then, for connections that do not use a 322.Cm ProxyCommand 323or 324.Cm ProxyJump , 325.Xr ssh 1 326will attempt to canonicalize the hostname specified on the command line 327using the 328.Cm CanonicalDomains 329suffixes and 330.Cm CanonicalizePermittedCNAMEs 331rules. 332If 333.Cm CanonicalizeHostname 334is set to 335.Cm always , 336then canonicalization is applied to proxied connections too. 337.Pp 338If this option is enabled, then the configuration files are processed 339again using the new target name to pick up any new configuration in matching 340.Cm Host 341and 342.Cm Match 343stanzas. 344.It Cm CanonicalizeMaxDots 345Specifies the maximum number of dot characters in a hostname before 346canonicalization is disabled. 347The default, 1, 348allows a single dot (i.e. hostname.subdomain). 349.It Cm CanonicalizePermittedCNAMEs 350Specifies rules to determine whether CNAMEs should be followed when 351canonicalizing hostnames. 352The rules consist of one or more arguments of 353.Ar source_domain_list : Ns Ar target_domain_list , 354where 355.Ar source_domain_list 356is a pattern-list of domains that may follow CNAMEs in canonicalization, 357and 358.Ar target_domain_list 359is a pattern-list of domains that they may resolve to. 360.Pp 361For example, 362.Qq *.a.example.com:*.b.example.com,*.c.example.com 363will allow hostnames matching 364.Qq *.a.example.com 365to be canonicalized to names in the 366.Qq *.b.example.com 367or 368.Qq *.c.example.com 369domains. 370.It Cm CASignatureAlgorithms 371Specifies which algorithms are allowed for signing of certificates 372by certificate authorities (CAs). 373The default is: 374.Bd -literal -offset indent 375ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 376sk-ssh-ed25519@openssh.com,sk-ecdsa-sha2-nistp256@openssh.com, 377rsa-sha2-512,rsa-sha2-256 378.Ed 379.Pp 380.Xr ssh 1 381will not accept host certificates signed using algorithms other than those 382specified. 383.It Cm CertificateFile 384Specifies a file from which the user's certificate is read. 385A corresponding private key must be provided separately in order 386to use this certificate either 387from an 388.Cm IdentityFile 389directive or 390.Fl i 391flag to 392.Xr ssh 1 , 393via 394.Xr ssh-agent 1 , 395or via a 396.Cm PKCS11Provider 397or 398.Cm SecurityKeyProvider . 399.Pp 400Arguments to 401.Cm CertificateFile 402may use the tilde syntax to refer to a user's home directory, 403the tokens described in the 404.Sx TOKENS 405section and environment variables as described in the 406.Sx ENVIRONMENT VARIABLES 407section. 408.Pp 409It is possible to have multiple certificate files specified in 410configuration files; these certificates will be tried in sequence. 411Multiple 412.Cm CertificateFile 413directives will add to the list of certificates used for 414authentication. 415.It Cm ChallengeResponseAuthentication 416Specifies whether to use challenge-response authentication. 417The argument to this keyword must be 418.Cm yes 419(the default) 420or 421.Cm no . 422.It Cm CheckHostIP 423If set to 424.Cm yes 425.Xr ssh 1 426will additionally check the host IP address in the 427.Pa known_hosts 428file. 429This allows it to detect if a host key changed due to DNS spoofing 430and will add addresses of destination hosts to 431.Pa ~/.ssh/known_hosts 432in the process, regardless of the setting of 433.Cm StrictHostKeyChecking . 434If the option is set to 435.Cm no 436(the default), 437the check will not be executed. 438.It Cm Ciphers 439Specifies the ciphers allowed and their order of preference. 440Multiple ciphers must be comma-separated. 441If the specified list begins with a 442.Sq + 443character, then the specified ciphers will be appended to the default set 444instead of replacing them. 445If the specified list begins with a 446.Sq - 447character, then the specified ciphers (including wildcards) will be removed 448from the default set instead of replacing them. 449If the specified list begins with a 450.Sq ^ 451character, then the specified ciphers will be placed at the head of the 452default set. 453.Pp 454The supported ciphers are: 455.Bd -literal -offset indent 4563des-cbc 457aes128-cbc 458aes192-cbc 459aes256-cbc 460aes128-ctr 461aes192-ctr 462aes256-ctr 463aes128-gcm@openssh.com 464aes256-gcm@openssh.com 465chacha20-poly1305@openssh.com 466.Ed 467.Pp 468The default is: 469.Bd -literal -offset indent 470chacha20-poly1305@openssh.com, 471aes128-ctr,aes192-ctr,aes256-ctr, 472aes128-gcm@openssh.com,aes256-gcm@openssh.com 473.Ed 474.Pp 475The list of available ciphers may also be obtained using 476.Qq ssh -Q cipher . 477.It Cm ClearAllForwardings 478Specifies that all local, remote, and dynamic port forwardings 479specified in the configuration files or on the command line be 480cleared. 481This option is primarily useful when used from the 482.Xr ssh 1 483command line to clear port forwardings set in 484configuration files, and is automatically set by 485.Xr scp 1 486and 487.Xr sftp 1 . 488The argument must be 489.Cm yes 490or 491.Cm no 492(the default). 493.It Cm Compression 494Specifies whether to use compression. 495The argument must be 496.Cm yes 497or 498.Cm no 499(the default). 500.It Cm ConnectionAttempts 501Specifies the number of tries (one per second) to make before exiting. 502The argument must be an integer. 503This may be useful in scripts if the connection sometimes fails. 504The default is 1. 505.It Cm ConnectTimeout 506Specifies the timeout (in seconds) used when connecting to the 507SSH server, instead of using the default system TCP timeout. 508This timeout is applied both to establishing the connection and to performing 509the initial SSH protocol handshake and key exchange. 510.It Cm ControlMaster 511Enables the sharing of multiple sessions over a single network connection. 512When set to 513.Cm yes , 514.Xr ssh 1 515will listen for connections on a control socket specified using the 516.Cm ControlPath 517argument. 518Additional sessions can connect to this socket using the same 519.Cm ControlPath 520with 521.Cm ControlMaster 522set to 523.Cm no 524(the default). 525These sessions will try to reuse the master instance's network connection 526rather than initiating new ones, but will fall back to connecting normally 527if the control socket does not exist, or is not listening. 528.Pp 529Setting this to 530.Cm ask 531will cause 532.Xr ssh 1 533to listen for control connections, but require confirmation using 534.Xr ssh-askpass 1 . 535If the 536.Cm ControlPath 537cannot be opened, 538.Xr ssh 1 539will continue without connecting to a master instance. 540.Pp 541X11 and 542.Xr ssh-agent 1 543forwarding is supported over these multiplexed connections, however the 544display and agent forwarded will be the one belonging to the master 545connection i.e. it is not possible to forward multiple displays or agents. 546.Pp 547Two additional options allow for opportunistic multiplexing: try to use a 548master connection but fall back to creating a new one if one does not already 549exist. 550These options are: 551.Cm auto 552and 553.Cm autoask . 554The latter requires confirmation like the 555.Cm ask 556option. 557.It Cm ControlPath 558Specify the path to the control socket used for connection sharing as described 559in the 560.Cm ControlMaster 561section above or the string 562.Cm none 563to disable connection sharing. 564Arguments to 565.Cm ControlPath 566may use the tilde syntax to refer to a user's home directory, 567the tokens described in the 568.Sx TOKENS 569section and environment variables as described in the 570.Sx ENVIRONMENT VARIABLES 571section. 572It is recommended that any 573.Cm ControlPath 574used for opportunistic connection sharing include 575at least %h, %p, and %r (or alternatively %C) and be placed in a directory 576that is not writable by other users. 577This ensures that shared connections are uniquely identified. 578.It Cm ControlPersist 579When used in conjunction with 580.Cm ControlMaster , 581specifies that the master connection should remain open 582in the background (waiting for future client connections) 583after the initial client connection has been closed. 584If set to 585.Cm no 586(the default), 587then the master connection will not be placed into the background, 588and will close as soon as the initial client connection is closed. 589If set to 590.Cm yes 591or 0, 592then the master connection will remain in the background indefinitely 593(until killed or closed via a mechanism such as the 594.Qq ssh -O exit ) . 595If set to a time in seconds, or a time in any of the formats documented in 596.Xr sshd_config 5 , 597then the backgrounded master connection will automatically terminate 598after it has remained idle (with no client connections) for the 599specified time. 600.It Cm DynamicForward 601Specifies that a TCP port on the local machine be forwarded 602over the secure channel, and the application 603protocol is then used to determine where to connect to from the 604remote machine. 605.Pp 606The argument must be 607.Sm off 608.Oo Ar bind_address : Oc Ar port . 609.Sm on 610IPv6 addresses can be specified by enclosing addresses in square brackets. 611By default, the local port is bound in accordance with the 612.Cm GatewayPorts 613setting. 614However, an explicit 615.Ar bind_address 616may be used to bind the connection to a specific address. 617The 618.Ar bind_address 619of 620.Cm localhost 621indicates that the listening port be bound for local use only, while an 622empty address or 623.Sq * 624indicates that the port should be available from all interfaces. 625.Pp 626Currently the SOCKS4 and SOCKS5 protocols are supported, and 627.Xr ssh 1 628will act as a SOCKS server. 629Multiple forwardings may be specified, and 630additional forwardings can be given on the command line. 631Only the superuser can forward privileged ports. 632.It Cm EnableSSHKeysign 633Setting this option to 634.Cm yes 635in the global client configuration file 636.Pa /etc/ssh/ssh_config 637enables the use of the helper program 638.Xr ssh-keysign 8 639during 640.Cm HostbasedAuthentication . 641The argument must be 642.Cm yes 643or 644.Cm no 645(the default). 646This option should be placed in the non-hostspecific section. 647See 648.Xr ssh-keysign 8 649for more information. 650.It Cm EscapeChar 651Sets the escape character (default: 652.Ql ~ ) . 653The escape character can also 654be set on the command line. 655The argument should be a single character, 656.Ql ^ 657followed by a letter, or 658.Cm none 659to disable the escape 660character entirely (making the connection transparent for binary 661data). 662.It Cm ExitOnForwardFailure 663Specifies whether 664.Xr ssh 1 665should terminate the connection if it cannot set up all requested 666dynamic, tunnel, local, and remote port forwardings, (e.g.\& 667if either end is unable to bind and listen on a specified port). 668Note that 669.Cm ExitOnForwardFailure 670does not apply to connections made over port forwardings and will not, 671for example, cause 672.Xr ssh 1 673to exit if TCP connections to the ultimate forwarding destination fail. 674The argument must be 675.Cm yes 676or 677.Cm no 678(the default). 679.It Cm FingerprintHash 680Specifies the hash algorithm used when displaying key fingerprints. 681Valid options are: 682.Cm md5 683and 684.Cm sha256 685(the default). 686.It Cm ForwardAgent 687Specifies whether the connection to the authentication agent (if any) 688will be forwarded to the remote machine. 689The argument may be 690.Cm yes , 691.Cm no 692(the default), 693an explicit path to an agent socket or the name of an environment variable 694(beginning with 695.Sq $ ) 696in which to find the path. 697.Pp 698Agent forwarding should be enabled with caution. 699Users with the ability to bypass file permissions on the remote host 700(for the agent's Unix-domain socket) 701can access the local agent through the forwarded connection. 702An attacker cannot obtain key material from the agent, 703however they can perform operations on the keys that enable them to 704authenticate using the identities loaded into the agent. 705.It Cm ForwardX11 706Specifies whether X11 connections will be automatically redirected 707over the secure channel and 708.Ev DISPLAY 709set. 710The argument must be 711.Cm yes 712or 713.Cm no 714(the default). 715.Pp 716X11 forwarding should be enabled with caution. 717Users with the ability to bypass file permissions on the remote host 718(for the user's X11 authorization database) 719can access the local X11 display through the forwarded connection. 720An attacker may then be able to perform activities such as keystroke monitoring 721if the 722.Cm ForwardX11Trusted 723option is also enabled. 724.It Cm ForwardX11Timeout 725Specify a timeout for untrusted X11 forwarding 726using the format described in the 727.Sx TIME FORMATS 728section of 729.Xr sshd_config 5 . 730X11 connections received by 731.Xr ssh 1 732after this time will be refused. 733Setting 734.Cm ForwardX11Timeout 735to zero will disable the timeout and permit X11 forwarding for the life 736of the connection. 737The default is to disable untrusted X11 forwarding after twenty minutes has 738elapsed. 739.It Cm ForwardX11Trusted 740If this option is set to 741.Cm yes , 742remote X11 clients will have full access to the original X11 display. 743.Pp 744If this option is set to 745.Cm no 746(the default), 747remote X11 clients will be considered untrusted and prevented 748from stealing or tampering with data belonging to trusted X11 749clients. 750Furthermore, the 751.Xr xauth 1 752token used for the session will be set to expire after 20 minutes. 753Remote clients will be refused access after this time. 754.Pp 755See the X11 SECURITY extension specification for full details on 756the restrictions imposed on untrusted clients. 757.It Cm GatewayPorts 758Specifies whether remote hosts are allowed to connect to local 759forwarded ports. 760By default, 761.Xr ssh 1 762binds local port forwardings to the loopback address. 763This prevents other remote hosts from connecting to forwarded ports. 764.Cm GatewayPorts 765can be used to specify that ssh 766should bind local port forwardings to the wildcard address, 767thus allowing remote hosts to connect to forwarded ports. 768The argument must be 769.Cm yes 770or 771.Cm no 772(the default). 773.It Cm GlobalKnownHostsFile 774Specifies one or more files to use for the global 775host key database, separated by whitespace. 776The default is 777.Pa /etc/ssh/ssh_known_hosts , 778.Pa /etc/ssh/ssh_known_hosts2 . 779.It Cm GSSAPIAuthentication 780Specifies whether user authentication based on GSSAPI is allowed. 781The default is 782.Cm no . 783.It Cm GSSAPIDelegateCredentials 784Forward (delegate) credentials to the server. 785The default is 786.Cm no . 787.It Cm HashKnownHosts 788Indicates that 789.Xr ssh 1 790should hash host names and addresses when they are added to 791.Pa ~/.ssh/known_hosts . 792These hashed names may be used normally by 793.Xr ssh 1 794and 795.Xr sshd 8 , 796but they do not visually reveal identifying information if the 797file's contents are disclosed. 798The default is 799.Cm no . 800Note that existing names and addresses in known hosts files 801will not be converted automatically, 802but may be manually hashed using 803.Xr ssh-keygen 1 . 804.It Cm HostbasedAcceptedAlgorithms 805Specifies the signature algorithms that will be used for hostbased 806authentication as a comma-separated list of patterns. 807Alternately if the specified list begins with a 808.Sq + 809character, then the specified signature algorithms will be appended 810to the default set instead of replacing them. 811If the specified list begins with a 812.Sq - 813character, then the specified signature algorithms (including wildcards) 814will be removed from the default set instead of replacing them. 815If the specified list begins with a 816.Sq ^ 817character, then the specified signature algorithms will be placed 818at the head of the default set. 819The default for this option is: 820.Bd -literal -offset 3n 821ssh-ed25519-cert-v01@openssh.com, 822ecdsa-sha2-nistp256-cert-v01@openssh.com, 823ecdsa-sha2-nistp384-cert-v01@openssh.com, 824ecdsa-sha2-nistp521-cert-v01@openssh.com, 825sk-ssh-ed25519-cert-v01@openssh.com, 826sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 827rsa-sha2-512-cert-v01@openssh.com, 828rsa-sha2-256-cert-v01@openssh.com, 829ssh-rsa-cert-v01@openssh.com, 830ssh-ed25519, 831ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 832sk-ssh-ed25519@openssh.com, 833sk-ecdsa-sha2-nistp256@openssh.com, 834rsa-sha2-512,rsa-sha2-256,ssh-rsa 835.Ed 836.Pp 837The 838.Fl Q 839option of 840.Xr ssh 1 841may be used to list supported signature algorithms. 842This was formerly named HostbasedKeyTypes. 843.It Cm HostbasedAuthentication 844Specifies whether to try rhosts based authentication with public key 845authentication. 846The argument must be 847.Cm yes 848or 849.Cm no 850(the default). 851.It Cm HostKeyAlgorithms 852Specifies the host key signature algorithms 853that the client wants to use in order of preference. 854Alternately if the specified list begins with a 855.Sq + 856character, then the specified signature algorithms will be appended to 857the default set instead of replacing them. 858If the specified list begins with a 859.Sq - 860character, then the specified signature algorithms (including wildcards) 861will be removed from the default set instead of replacing them. 862If the specified list begins with a 863.Sq ^ 864character, then the specified signature algorithms will be placed 865at the head of the default set. 866The default for this option is: 867.Bd -literal -offset 3n 868ssh-ed25519-cert-v01@openssh.com, 869ecdsa-sha2-nistp256-cert-v01@openssh.com, 870ecdsa-sha2-nistp384-cert-v01@openssh.com, 871ecdsa-sha2-nistp521-cert-v01@openssh.com, 872sk-ssh-ed25519-cert-v01@openssh.com, 873sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 874rsa-sha2-512-cert-v01@openssh.com, 875rsa-sha2-256-cert-v01@openssh.com, 876ssh-rsa-cert-v01@openssh.com, 877ssh-ed25519, 878ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 879sk-ecdsa-sha2-nistp256@openssh.com, 880sk-ssh-ed25519@openssh.com, 881rsa-sha2-512,rsa-sha2-256,ssh-rsa 882.Ed 883.Pp 884If hostkeys are known for the destination host then this default is modified 885to prefer their algorithms. 886.Pp 887The list of available signature algorithms may also be obtained using 888.Qq ssh -Q HostKeyAlgorithms . 889.It Cm HostKeyAlias 890Specifies an alias that should be used instead of the 891real host name when looking up or saving the host key 892in the host key database files and when validating host certificates. 893This option is useful for tunneling SSH connections 894or for multiple servers running on a single host. 895.It Cm Hostname 896Specifies the real host name to log into. 897This can be used to specify nicknames or abbreviations for hosts. 898Arguments to 899.Cm Hostname 900accept the tokens described in the 901.Sx TOKENS 902section. 903Numeric IP addresses are also permitted (both on the command line and in 904.Cm Hostname 905specifications). 906The default is the name given on the command line. 907.It Cm IdentitiesOnly 908Specifies that 909.Xr ssh 1 910should only use the configured authentication identity and certificate files 911(either the default files, or those explicitly configured in the 912.Nm 913files 914or passed on the 915.Xr ssh 1 916command-line), 917even if 918.Xr ssh-agent 1 919or a 920.Cm PKCS11Provider 921or 922.Cm SecurityKeyProvider 923offers more identities. 924The argument to this keyword must be 925.Cm yes 926or 927.Cm no 928(the default). 929This option is intended for situations where ssh-agent 930offers many different identities. 931.It Cm IdentityAgent 932Specifies the 933.Ux Ns -domain 934socket used to communicate with the authentication agent. 935.Pp 936This option overrides the 937.Ev SSH_AUTH_SOCK 938environment variable and can be used to select a specific agent. 939Setting the socket name to 940.Cm none 941disables the use of an authentication agent. 942If the string 943.Qq SSH_AUTH_SOCK 944is specified, the location of the socket will be read from the 945.Ev SSH_AUTH_SOCK 946environment variable. 947Otherwise if the specified value begins with a 948.Sq $ 949character, then it will be treated as an environment variable containing 950the location of the socket. 951.Pp 952Arguments to 953.Cm IdentityAgent 954may use the tilde syntax to refer to a user's home directory, 955the tokens described in the 956.Sx TOKENS 957section and environment variables as described in the 958.Sx ENVIRONMENT VARIABLES 959section. 960.It Cm IdentityFile 961Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 962Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 963The default is 964.Pa ~/.ssh/id_dsa , 965.Pa ~/.ssh/id_ecdsa , 966.Pa ~/.ssh/id_ecdsa_sk , 967.Pa ~/.ssh/id_ed25519 , 968.Pa ~/.ssh/id_ed25519_sk 969and 970.Pa ~/.ssh/id_rsa . 971Additionally, any identities represented by the authentication agent 972will be used for authentication unless 973.Cm IdentitiesOnly 974is set. 975If no certificates have been explicitly specified by 976.Cm CertificateFile , 977.Xr ssh 1 978will try to load certificate information from the filename obtained by 979appending 980.Pa -cert.pub 981to the path of a specified 982.Cm IdentityFile . 983.Pp 984Arguments to 985.Cm IdentityFile 986may use the tilde syntax to refer to a user's home directory 987or the tokens described in the 988.Sx TOKENS 989section. 990.Pp 991It is possible to have 992multiple identity files specified in configuration files; all these 993identities will be tried in sequence. 994Multiple 995.Cm IdentityFile 996directives will add to the list of identities tried (this behaviour 997differs from that of other configuration directives). 998.Pp 999.Cm IdentityFile 1000may be used in conjunction with 1001.Cm IdentitiesOnly 1002to select which identities in an agent are offered during authentication. 1003.Cm IdentityFile 1004may also be used in conjunction with 1005.Cm CertificateFile 1006in order to provide any certificate also needed for authentication with 1007the identity. 1008.It Cm IgnoreUnknown 1009Specifies a pattern-list of unknown options to be ignored if they are 1010encountered in configuration parsing. 1011This may be used to suppress errors if 1012.Nm 1013contains options that are unrecognised by 1014.Xr ssh 1 . 1015It is recommended that 1016.Cm IgnoreUnknown 1017be listed early in the configuration file as it will not be applied 1018to unknown options that appear before it. 1019.It Cm Include 1020Include the specified configuration file(s). 1021Multiple pathnames may be specified and each pathname may contain 1022.Xr glob 7 1023wildcards and, for user configurations, shell-like 1024.Sq ~ 1025references to user home directories. 1026Wildcards will be expanded and processed in lexical order. 1027Files without absolute paths are assumed to be in 1028.Pa ~/.ssh 1029if included in a user configuration file or 1030.Pa /etc/ssh 1031if included from the system configuration file. 1032.Cm Include 1033directive may appear inside a 1034.Cm Match 1035or 1036.Cm Host 1037block 1038to perform conditional inclusion. 1039.It Cm IPQoS 1040Specifies the IPv4 type-of-service or DSCP class for connections. 1041Accepted values are 1042.Cm af11 , 1043.Cm af12 , 1044.Cm af13 , 1045.Cm af21 , 1046.Cm af22 , 1047.Cm af23 , 1048.Cm af31 , 1049.Cm af32 , 1050.Cm af33 , 1051.Cm af41 , 1052.Cm af42 , 1053.Cm af43 , 1054.Cm cs0 , 1055.Cm cs1 , 1056.Cm cs2 , 1057.Cm cs3 , 1058.Cm cs4 , 1059.Cm cs5 , 1060.Cm cs6 , 1061.Cm cs7 , 1062.Cm ef , 1063.Cm le , 1064.Cm lowdelay , 1065.Cm throughput , 1066.Cm reliability , 1067a numeric value, or 1068.Cm none 1069to use the operating system default. 1070This option may take one or two arguments, separated by whitespace. 1071If one argument is specified, it is used as the packet class unconditionally. 1072If two values are specified, the first is automatically selected for 1073interactive sessions and the second for non-interactive sessions. 1074The default is 1075.Cm af21 1076(Low-Latency Data) 1077for interactive sessions and 1078.Cm cs1 1079(Lower Effort) 1080for non-interactive sessions. 1081.It Cm KbdInteractiveAuthentication 1082Specifies whether to use keyboard-interactive authentication. 1083The argument to this keyword must be 1084.Cm yes 1085(the default) 1086or 1087.Cm no . 1088.It Cm KbdInteractiveDevices 1089Specifies the list of methods to use in keyboard-interactive authentication. 1090Multiple method names must be comma-separated. 1091The default is to use the server specified list. 1092The methods available vary depending on what the server supports. 1093For an OpenSSH server, 1094it may be zero or more of: 1095.Cm bsdauth , 1096.Cm pam , 1097and 1098.Cm skey . 1099.It Cm KexAlgorithms 1100Specifies the available KEX (Key Exchange) algorithms. 1101Multiple algorithms must be comma-separated. 1102If the specified list begins with a 1103.Sq + 1104character, then the specified methods will be appended to the default set 1105instead of replacing them. 1106If the specified list begins with a 1107.Sq - 1108character, then the specified methods (including wildcards) will be removed 1109from the default set instead of replacing them. 1110If the specified list begins with a 1111.Sq ^ 1112character, then the specified methods will be placed at the head of the 1113default set. 1114The default is: 1115.Bd -literal -offset indent 1116curve25519-sha256,curve25519-sha256@libssh.org, 1117ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1118diffie-hellman-group-exchange-sha256, 1119diffie-hellman-group16-sha512, 1120diffie-hellman-group18-sha512, 1121diffie-hellman-group14-sha256 1122.Ed 1123.Pp 1124The list of available key exchange algorithms may also be obtained using 1125.Qq ssh -Q kex . 1126.It Cm KnownHostsCommand 1127Specifies a command to use to obtain a list of host keys, in addition to 1128those listed in 1129.Cm UserKnownHostsFile 1130and 1131.Cm GlobalKnownHostsFile . 1132This command is executed after the files have been read. 1133It may write host key lines to standard output in identical format to the 1134usual files (described in the 1135.Sx VERIFYING HOST KEYS 1136section in 1137.Xr ssh 1 ) . 1138Arguments to 1139.Cm KnownHostsCommand 1140accept the tokens described in the 1141.Sx TOKENS 1142section. 1143The command may be invoked multiple times per connection: once when preparing 1144the preference list of host key algorithms to use, again to obtain the 1145host key for the requested host name and, if 1146.Cm CheckHostIP 1147is enabled, one more time to obtain the host key matching the server's 1148address. 1149If the command exits abnormally or returns a non-zero exit status then the 1150connection is terminated. 1151.It Cm LocalCommand 1152Specifies a command to execute on the local machine after successfully 1153connecting to the server. 1154The command string extends to the end of the line, and is executed with 1155the user's shell. 1156Arguments to 1157.Cm LocalCommand 1158accept the tokens described in the 1159.Sx TOKENS 1160section. 1161.Pp 1162The command is run synchronously and does not have access to the 1163session of the 1164.Xr ssh 1 1165that spawned it. 1166It should not be used for interactive commands. 1167.Pp 1168This directive is ignored unless 1169.Cm PermitLocalCommand 1170has been enabled. 1171.It Cm LocalForward 1172Specifies that a TCP port on the local machine be forwarded over 1173the secure channel to the specified host and port from the remote machine. 1174The first argument specifies the listener and may be 1175.Sm off 1176.Oo Ar bind_address : Oc Ar port 1177.Sm on 1178or a Unix domain socket path. 1179The second argument is the destination and may be 1180.Ar host : Ns Ar hostport 1181or a Unix domain socket path if the remote host supports it. 1182.Pp 1183IPv6 addresses can be specified by enclosing addresses in square brackets. 1184Multiple forwardings may be specified, and additional forwardings can be 1185given on the command line. 1186Only the superuser can forward privileged ports. 1187By default, the local port is bound in accordance with the 1188.Cm GatewayPorts 1189setting. 1190However, an explicit 1191.Ar bind_address 1192may be used to bind the connection to a specific address. 1193The 1194.Ar bind_address 1195of 1196.Cm localhost 1197indicates that the listening port be bound for local use only, while an 1198empty address or 1199.Sq * 1200indicates that the port should be available from all interfaces. 1201Unix domain socket paths may use the tokens described in the 1202.Sx TOKENS 1203section and environment variables as described in the 1204.Sx ENVIRONMENT VARIABLES 1205section. 1206.It Cm LogLevel 1207Gives the verbosity level that is used when logging messages from 1208.Xr ssh 1 . 1209The possible values are: 1210QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1211The default is INFO. 1212DEBUG and DEBUG1 are equivalent. 1213DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1214.It Cm LogVerbose 1215Specify one or more overrides to LogLevel. 1216An override consists of a pattern lists that matches the source file, function 1217and line number to force detailed logging for. 1218For example, an override pattern of: 1219.Bd -literal -offset indent 1220kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1221.Ed 1222.Pp 1223would enable detailed logging for line 1000 of 1224.Pa kex.c , 1225everything in the 1226.Fn kex_exchange_identification 1227function, and all code in the 1228.Pa packet.c 1229file. 1230This option is intended for debugging and no overrides are enabled by default. 1231.It Cm MACs 1232Specifies the MAC (message authentication code) algorithms 1233in order of preference. 1234The MAC algorithm is used for data integrity protection. 1235Multiple algorithms must be comma-separated. 1236If the specified list begins with a 1237.Sq + 1238character, then the specified algorithms will be appended to the default set 1239instead of replacing them. 1240If the specified list begins with a 1241.Sq - 1242character, then the specified algorithms (including wildcards) will be removed 1243from the default set instead of replacing them. 1244If the specified list begins with a 1245.Sq ^ 1246character, then the specified algorithms will be placed at the head of the 1247default set. 1248.Pp 1249The algorithms that contain 1250.Qq -etm 1251calculate the MAC after encryption (encrypt-then-mac). 1252These are considered safer and their use recommended. 1253.Pp 1254The default is: 1255.Bd -literal -offset indent 1256umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1257hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1258hmac-sha1-etm@openssh.com, 1259umac-64@openssh.com,umac-128@openssh.com, 1260hmac-sha2-256,hmac-sha2-512,hmac-sha1 1261.Ed 1262.Pp 1263The list of available MAC algorithms may also be obtained using 1264.Qq ssh -Q mac . 1265.It Cm NoHostAuthenticationForLocalhost 1266Disable host authentication for localhost (loopback addresses). 1267The argument to this keyword must be 1268.Cm yes 1269or 1270.Cm no 1271(the default). 1272.It Cm NumberOfPasswordPrompts 1273Specifies the number of password prompts before giving up. 1274The argument to this keyword must be an integer. 1275The default is 3. 1276.It Cm PasswordAuthentication 1277Specifies whether to use password authentication. 1278The argument to this keyword must be 1279.Cm yes 1280(the default) 1281or 1282.Cm no . 1283.It Cm PermitLocalCommand 1284Allow local command execution via the 1285.Ic LocalCommand 1286option or using the 1287.Ic !\& Ns Ar command 1288escape sequence in 1289.Xr ssh 1 . 1290The argument must be 1291.Cm yes 1292or 1293.Cm no 1294(the default). 1295.It Cm PermitRemoteOpen 1296Specifies the destinations to which remote TCP port forwarding is permitted when 1297.Cm RemoteForward 1298is used as a SOCKS proxy. 1299The forwarding specification must be one of the following forms: 1300.Pp 1301.Bl -item -offset indent -compact 1302.It 1303.Cm PermitRemoteOpen 1304.Sm off 1305.Ar host : port 1306.Sm on 1307.It 1308.Cm PermitRemoteOpen 1309.Sm off 1310.Ar IPv4_addr : port 1311.Sm on 1312.It 1313.Cm PermitRemoteOpen 1314.Sm off 1315.Ar \&[ IPv6_addr \&] : port 1316.Sm on 1317.El 1318.Pp 1319Multiple forwards may be specified by separating them with whitespace. 1320An argument of 1321.Cm any 1322can be used to remove all restrictions and permit any forwarding requests. 1323An argument of 1324.Cm none 1325can be used to prohibit all forwarding requests. 1326The wildcard 1327.Sq * 1328can be used for host or port to allow all hosts or ports respectively. 1329Otherwise, no pattern matching or address lookups are performed on supplied 1330names. 1331.It Cm PKCS11Provider 1332Specifies which PKCS#11 provider to use or 1333.Cm none 1334to indicate that no provider should be used (the default). 1335The argument to this keyword is a path to the PKCS#11 shared library 1336.Xr ssh 1 1337should use to communicate with a PKCS#11 token providing keys for user 1338authentication. 1339.It Cm Port 1340Specifies the port number to connect on the remote host. 1341The default is 22. 1342.It Cm PreferredAuthentications 1343Specifies the order in which the client should try authentication methods. 1344This allows a client to prefer one method (e.g.\& 1345.Cm keyboard-interactive ) 1346over another method (e.g.\& 1347.Cm password ) . 1348The default is: 1349.Bd -literal -offset indent 1350gssapi-with-mic,hostbased,publickey, 1351keyboard-interactive,password 1352.Ed 1353.It Cm ProxyCommand 1354Specifies the command to use to connect to the server. 1355The command 1356string extends to the end of the line, and is executed 1357using the user's shell 1358.Ql exec 1359directive to avoid a lingering shell process. 1360.Pp 1361Arguments to 1362.Cm ProxyCommand 1363accept the tokens described in the 1364.Sx TOKENS 1365section. 1366The command can be basically anything, 1367and should read from its standard input and write to its standard output. 1368It should eventually connect an 1369.Xr sshd 8 1370server running on some machine, or execute 1371.Ic sshd -i 1372somewhere. 1373Host key management will be done using the 1374.Cm Hostname 1375of the host being connected (defaulting to the name typed by the user). 1376Setting the command to 1377.Cm none 1378disables this option entirely. 1379Note that 1380.Cm CheckHostIP 1381is not available for connects with a proxy command. 1382.Pp 1383This directive is useful in conjunction with 1384.Xr nc 1 1385and its proxy support. 1386For example, the following directive would connect via an HTTP proxy at 1387192.0.2.0: 1388.Bd -literal -offset 3n 1389ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1390.Ed 1391.It Cm ProxyJump 1392Specifies one or more jump proxies as either 1393.Xo 1394.Sm off 1395.Op Ar user No @ 1396.Ar host 1397.Op : Ns Ar port 1398.Sm on 1399or an ssh URI 1400.Xc . 1401Multiple proxies may be separated by comma characters and will be visited 1402sequentially. 1403Setting this option will cause 1404.Xr ssh 1 1405to connect to the target host by first making a 1406.Xr ssh 1 1407connection to the specified 1408.Cm ProxyJump 1409host and then establishing a 1410TCP forwarding to the ultimate target from there. 1411Setting the host to 1412.Cm none 1413disables this option entirely. 1414.Pp 1415Note that this option will compete with the 1416.Cm ProxyCommand 1417option - whichever is specified first will prevent later instances of the 1418other from taking effect. 1419.Pp 1420Note also that the configuration for the destination host (either supplied 1421via the command-line or the configuration file) is not generally applied 1422to jump hosts. 1423.Pa ~/.ssh/config 1424should be used if specific configuration is required for jump hosts. 1425.It Cm ProxyUseFdpass 1426Specifies that 1427.Cm ProxyCommand 1428will pass a connected file descriptor back to 1429.Xr ssh 1 1430instead of continuing to execute and pass data. 1431The default is 1432.Cm no . 1433.It Cm PubkeyAcceptedAlgorithms 1434Specifies the signature algorithms that will be used for public key 1435authentication as a comma-separated list of patterns. 1436If the specified list begins with a 1437.Sq + 1438character, then the algorithms after it will be appended to the default 1439instead of replacing it. 1440If the specified list begins with a 1441.Sq - 1442character, then the specified algorithms (including wildcards) will be removed 1443from the default set instead of replacing them. 1444If the specified list begins with a 1445.Sq ^ 1446character, then the specified algorithms will be placed at the head of the 1447default set. 1448The default for this option is: 1449.Bd -literal -offset 3n 1450ssh-ed25519-cert-v01@openssh.com, 1451ecdsa-sha2-nistp256-cert-v01@openssh.com, 1452ecdsa-sha2-nistp384-cert-v01@openssh.com, 1453ecdsa-sha2-nistp521-cert-v01@openssh.com, 1454sk-ssh-ed25519-cert-v01@openssh.com, 1455sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1456rsa-sha2-512-cert-v01@openssh.com, 1457rsa-sha2-256-cert-v01@openssh.com, 1458ssh-rsa-cert-v01@openssh.com, 1459ssh-ed25519, 1460ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1461sk-ssh-ed25519@openssh.com, 1462sk-ecdsa-sha2-nistp256@openssh.com, 1463rsa-sha2-512,rsa-sha2-256,ssh-rsa 1464.Ed 1465.Pp 1466The list of available signature algorithms may also be obtained using 1467.Qq ssh -Q PubkeyAcceptedAlgorithms . 1468.It Cm PubkeyAuthentication 1469Specifies whether to try public key authentication. 1470The argument to this keyword must be 1471.Cm yes 1472(the default) 1473or 1474.Cm no . 1475.It Cm RekeyLimit 1476Specifies the maximum amount of data that may be transmitted before the 1477session key is renegotiated, optionally followed by a maximum amount of 1478time that may pass before the session key is renegotiated. 1479The first argument is specified in bytes and may have a suffix of 1480.Sq K , 1481.Sq M , 1482or 1483.Sq G 1484to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1485The default is between 1486.Sq 1G 1487and 1488.Sq 4G , 1489depending on the cipher. 1490The optional second value is specified in seconds and may use any of the 1491units documented in the TIME FORMATS section of 1492.Xr sshd_config 5 . 1493The default value for 1494.Cm RekeyLimit 1495is 1496.Cm default none , 1497which means that rekeying is performed after the cipher's default amount 1498of data has been sent or received and no time based rekeying is done. 1499.It Cm RemoteCommand 1500Specifies a command to execute on the remote machine after successfully 1501connecting to the server. 1502The command string extends to the end of the line, and is executed with 1503the user's shell. 1504Arguments to 1505.Cm RemoteCommand 1506accept the tokens described in the 1507.Sx TOKENS 1508section. 1509.It Cm RemoteForward 1510Specifies that a TCP port on the remote machine be forwarded over 1511the secure channel. 1512The remote port may either be forwarded to a specified host and port 1513from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1514client to connect to arbitrary destinations from the local machine. 1515The first argument is the listening specification and may be 1516.Sm off 1517.Oo Ar bind_address : Oc Ar port 1518.Sm on 1519or, if the remote host supports it, a Unix domain socket path. 1520If forwarding to a specific destination then the second argument must be 1521.Ar host : Ns Ar hostport 1522or a Unix domain socket path, 1523otherwise if no destination argument is specified then the remote forwarding 1524will be established as a SOCKS proxy. 1525When acting as a SOCKS proxy the destination of the connection can be 1526restricted by 1527.Cm PermitRemoteOpen . 1528.Pp 1529IPv6 addresses can be specified by enclosing addresses in square brackets. 1530Multiple forwardings may be specified, and additional 1531forwardings can be given on the command line. 1532Privileged ports can be forwarded only when 1533logging in as root on the remote machine. 1534Unix domain socket paths may use the tokens described in the 1535.Sx TOKENS 1536section and environment variables as described in the 1537.Sx ENVIRONMENT VARIABLES 1538section. 1539.Pp 1540If the 1541.Ar port 1542argument is 0, 1543the listen port will be dynamically allocated on the server and reported 1544to the client at run time. 1545.Pp 1546If the 1547.Ar bind_address 1548is not specified, the default is to only bind to loopback addresses. 1549If the 1550.Ar bind_address 1551is 1552.Ql * 1553or an empty string, then the forwarding is requested to listen on all 1554interfaces. 1555Specifying a remote 1556.Ar bind_address 1557will only succeed if the server's 1558.Cm GatewayPorts 1559option is enabled (see 1560.Xr sshd_config 5 ) . 1561.It Cm RequestTTY 1562Specifies whether to request a pseudo-tty for the session. 1563The argument may be one of: 1564.Cm no 1565(never request a TTY), 1566.Cm yes 1567(always request a TTY when standard input is a TTY), 1568.Cm force 1569(always request a TTY) or 1570.Cm auto 1571(request a TTY when opening a login session). 1572This option mirrors the 1573.Fl t 1574and 1575.Fl T 1576flags for 1577.Xr ssh 1 . 1578.It Cm RevokedHostKeys 1579Specifies revoked host public keys. 1580Keys listed in this file will be refused for host authentication. 1581Note that if this file does not exist or is not readable, 1582then host authentication will be refused for all hosts. 1583Keys may be specified as a text file, listing one public key per line, or as 1584an OpenSSH Key Revocation List (KRL) as generated by 1585.Xr ssh-keygen 1 . 1586For more information on KRLs, see the KEY REVOCATION LISTS section in 1587.Xr ssh-keygen 1 . 1588.It Cm SecurityKeyProvider 1589Specifies a path to a library that will be used when loading any 1590FIDO authenticator-hosted keys, overriding the default of using 1591the built-in USB HID support. 1592.Pp 1593If the specified value begins with a 1594.Sq $ 1595character, then it will be treated as an environment variable containing 1596the path to the library. 1597.It Cm SendEnv 1598Specifies what variables from the local 1599.Xr environ 7 1600should be sent to the server. 1601The server must also support it, and the server must be configured to 1602accept these environment variables. 1603Note that the 1604.Ev TERM 1605environment variable is always sent whenever a 1606pseudo-terminal is requested as it is required by the protocol. 1607Refer to 1608.Cm AcceptEnv 1609in 1610.Xr sshd_config 5 1611for how to configure the server. 1612Variables are specified by name, which may contain wildcard characters. 1613Multiple environment variables may be separated by whitespace or spread 1614across multiple 1615.Cm SendEnv 1616directives. 1617.Pp 1618See 1619.Sx PATTERNS 1620for more information on patterns. 1621.Pp 1622It is possible to clear previously set 1623.Cm SendEnv 1624variable names by prefixing patterns with 1625.Pa - . 1626The default is not to send any environment variables. 1627.It Cm ServerAliveCountMax 1628Sets the number of server alive messages (see below) which may be 1629sent without 1630.Xr ssh 1 1631receiving any messages back from the server. 1632If this threshold is reached while server alive messages are being sent, 1633ssh will disconnect from the server, terminating the session. 1634It is important to note that the use of server alive messages is very 1635different from 1636.Cm TCPKeepAlive 1637(below). 1638The server alive messages are sent through the encrypted channel 1639and therefore will not be spoofable. 1640The TCP keepalive option enabled by 1641.Cm TCPKeepAlive 1642is spoofable. 1643The server alive mechanism is valuable when the client or 1644server depend on knowing when a connection has become unresponsive. 1645.Pp 1646The default value is 3. 1647If, for example, 1648.Cm ServerAliveInterval 1649(see below) is set to 15 and 1650.Cm ServerAliveCountMax 1651is left at the default, if the server becomes unresponsive, 1652ssh will disconnect after approximately 45 seconds. 1653.It Cm ServerAliveInterval 1654Sets a timeout interval in seconds after which if no data has been received 1655from the server, 1656.Xr ssh 1 1657will send a message through the encrypted 1658channel to request a response from the server. 1659The default 1660is 0, indicating that these messages will not be sent to the server. 1661.It Cm SetEnv 1662Directly specify one or more environment variables and their contents to 1663be sent to the server. 1664Similarly to 1665.Cm SendEnv , 1666the server must be prepared to accept the environment variable. 1667.It Cm StreamLocalBindMask 1668Sets the octal file creation mode mask 1669.Pq umask 1670used when creating a Unix-domain socket file for local or remote 1671port forwarding. 1672This option is only used for port forwarding to a Unix-domain socket file. 1673.Pp 1674The default value is 0177, which creates a Unix-domain socket file that is 1675readable and writable only by the owner. 1676Note that not all operating systems honor the file mode on Unix-domain 1677socket files. 1678.It Cm StreamLocalBindUnlink 1679Specifies whether to remove an existing Unix-domain socket file for local 1680or remote port forwarding before creating a new one. 1681If the socket file already exists and 1682.Cm StreamLocalBindUnlink 1683is not enabled, 1684.Nm ssh 1685will be unable to forward the port to the Unix-domain socket file. 1686This option is only used for port forwarding to a Unix-domain socket file. 1687.Pp 1688The argument must be 1689.Cm yes 1690or 1691.Cm no 1692(the default). 1693.It Cm StrictHostKeyChecking 1694If this flag is set to 1695.Cm yes , 1696.Xr ssh 1 1697will never automatically add host keys to the 1698.Pa ~/.ssh/known_hosts 1699file, and refuses to connect to hosts whose host key has changed. 1700This provides maximum protection against man-in-the-middle (MITM) attacks, 1701though it can be annoying when the 1702.Pa /etc/ssh/ssh_known_hosts 1703file is poorly maintained or when connections to new hosts are 1704frequently made. 1705This option forces the user to manually 1706add all new hosts. 1707.Pp 1708If this flag is set to 1709.Dq accept-new 1710then ssh will automatically add new host keys to the user 1711known hosts files, but will not permit connections to hosts with 1712changed host keys. 1713If this flag is set to 1714.Dq no 1715or 1716.Dq off , 1717ssh will automatically add new host keys to the user known hosts files 1718and allow connections to hosts with changed hostkeys to proceed, 1719subject to some restrictions. 1720If this flag is set to 1721.Cm ask 1722(the default), 1723new host keys 1724will be added to the user known host files only after the user 1725has confirmed that is what they really want to do, and 1726ssh will refuse to connect to hosts whose host key has changed. 1727The host keys of 1728known hosts will be verified automatically in all cases. 1729.It Cm SyslogFacility 1730Gives the facility code that is used when logging messages from 1731.Xr ssh 1 . 1732The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1733LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1734The default is USER. 1735.It Cm TCPKeepAlive 1736Specifies whether the system should send TCP keepalive messages to the 1737other side. 1738If they are sent, death of the connection or crash of one 1739of the machines will be properly noticed. 1740However, this means that 1741connections will die if the route is down temporarily, and some people 1742find it annoying. 1743.Pp 1744The default is 1745.Cm yes 1746(to send TCP keepalive messages), and the client will notice 1747if the network goes down or the remote host dies. 1748This is important in scripts, and many users want it too. 1749.Pp 1750To disable TCP keepalive messages, the value should be set to 1751.Cm no . 1752See also 1753.Cm ServerAliveInterval 1754for protocol-level keepalives. 1755.It Cm Tunnel 1756Request 1757.Xr tun 4 1758device forwarding between the client and the server. 1759The argument must be 1760.Cm yes , 1761.Cm point-to-point 1762(layer 3), 1763.Cm ethernet 1764(layer 2), 1765or 1766.Cm no 1767(the default). 1768Specifying 1769.Cm yes 1770requests the default tunnel mode, which is 1771.Cm point-to-point . 1772.It Cm TunnelDevice 1773Specifies the 1774.Xr tun 4 1775devices to open on the client 1776.Pq Ar local_tun 1777and the server 1778.Pq Ar remote_tun . 1779.Pp 1780The argument must be 1781.Sm off 1782.Ar local_tun Op : Ar remote_tun . 1783.Sm on 1784The devices may be specified by numerical ID or the keyword 1785.Cm any , 1786which uses the next available tunnel device. 1787If 1788.Ar remote_tun 1789is not specified, it defaults to 1790.Cm any . 1791The default is 1792.Cm any:any . 1793.It Cm UpdateHostKeys 1794Specifies whether 1795.Xr ssh 1 1796should accept notifications of additional hostkeys from the server sent 1797after authentication has completed and add them to 1798.Cm UserKnownHostsFile . 1799The argument must be 1800.Cm yes , 1801.Cm no 1802or 1803.Cm ask . 1804This option allows learning alternate hostkeys for a server 1805and supports graceful key rotation by allowing a server to send replacement 1806public keys before old ones are removed. 1807.Pp 1808Additional hostkeys are only accepted if the key used to authenticate the 1809host was already trusted or explicitly accepted by the user, the host was 1810authenticated via 1811.Cm UserKnownHostsFile 1812(i.e. not 1813.Cm GlobalKnownHostsFile ) 1814and the host was authenticated using a plain key and not a certificate. 1815.Pp 1816.Cm UpdateHostKeys 1817is enabled by default if the user has not overridden the default 1818.Cm UserKnownHostsFile 1819setting and has not enabled 1820.Cm VerifyHostKeyDNS , 1821otherwise 1822.Cm UpdateHostKeys 1823will be set to 1824.Cm no . 1825.Pp 1826If 1827.Cm UpdateHostKeys 1828is set to 1829.Cm ask , 1830then the user is asked to confirm the modifications to the known_hosts file. 1831Confirmation is currently incompatible with 1832.Cm ControlPersist , 1833and will be disabled if it is enabled. 1834.Pp 1835Presently, only 1836.Xr sshd 8 1837from OpenSSH 6.8 and greater support the 1838.Qq hostkeys@openssh.com 1839protocol extension used to inform the client of all the server's hostkeys. 1840.It Cm User 1841Specifies the user to log in as. 1842This can be useful when a different user name is used on different machines. 1843This saves the trouble of 1844having to remember to give the user name on the command line. 1845.It Cm UserKnownHostsFile 1846Specifies one or more files to use for the user 1847host key database, separated by whitespace. 1848Each filename may use tilde notation to refer to the user's home directory, 1849the tokens described in the 1850.Sx TOKENS 1851section and environment variables as described in the 1852.Sx ENVIRONMENT VARIABLES 1853section. 1854The default is 1855.Pa ~/.ssh/known_hosts , 1856.Pa ~/.ssh/known_hosts2 . 1857.It Cm VerifyHostKeyDNS 1858Specifies whether to verify the remote key using DNS and SSHFP resource 1859records. 1860If this option is set to 1861.Cm yes , 1862the client will implicitly trust keys that match a secure fingerprint 1863from DNS. 1864Insecure fingerprints will be handled as if this option was set to 1865.Cm ask . 1866If this option is set to 1867.Cm ask , 1868information on fingerprint match will be displayed, but the user will still 1869need to confirm new host keys according to the 1870.Cm StrictHostKeyChecking 1871option. 1872The default is 1873.Cm no . 1874.Pp 1875See also 1876.Sx VERIFYING HOST KEYS 1877in 1878.Xr ssh 1 . 1879.It Cm VisualHostKey 1880If this flag is set to 1881.Cm yes , 1882an ASCII art representation of the remote host key fingerprint is 1883printed in addition to the fingerprint string at login and 1884for unknown host keys. 1885If this flag is set to 1886.Cm no 1887(the default), 1888no fingerprint strings are printed at login and 1889only the fingerprint string will be printed for unknown host keys. 1890.It Cm XAuthLocation 1891Specifies the full pathname of the 1892.Xr xauth 1 1893program. 1894The default is 1895.Pa /usr/X11R6/bin/xauth . 1896.El 1897.Sh PATTERNS 1898A 1899.Em pattern 1900consists of zero or more non-whitespace characters, 1901.Sq * 1902(a wildcard that matches zero or more characters), 1903or 1904.Sq ?\& 1905(a wildcard that matches exactly one character). 1906For example, to specify a set of declarations for any host in the 1907.Qq .co.uk 1908set of domains, 1909the following pattern could be used: 1910.Pp 1911.Dl Host *.co.uk 1912.Pp 1913The following pattern 1914would match any host in the 192.168.0.[0-9] network range: 1915.Pp 1916.Dl Host 192.168.0.? 1917.Pp 1918A 1919.Em pattern-list 1920is a comma-separated list of patterns. 1921Patterns within pattern-lists may be negated 1922by preceding them with an exclamation mark 1923.Pq Sq !\& . 1924For example, 1925to allow a key to be used from anywhere within an organization 1926except from the 1927.Qq dialup 1928pool, 1929the following entry (in authorized_keys) could be used: 1930.Pp 1931.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1932.Pp 1933Note that a negated match will never produce a positive result by itself. 1934For example, attempting to match 1935.Qq host3 1936against the following pattern-list will fail: 1937.Pp 1938.Dl from=\&"!host1,!host2\&" 1939.Pp 1940The solution here is to include a term that will yield a positive match, 1941such as a wildcard: 1942.Pp 1943.Dl from=\&"!host1,!host2,*\&" 1944.Sh TOKENS 1945Arguments to some keywords can make use of tokens, 1946which are expanded at runtime: 1947.Pp 1948.Bl -tag -width XXXX -offset indent -compact 1949.It %% 1950A literal 1951.Sq % . 1952.It \&%C 1953Hash of %l%h%p%r. 1954.It %d 1955Local user's home directory. 1956.It %f 1957The fingerprint of the server's host key. 1958.It %H 1959The 1960.Pa known_hosts 1961hostname or address that is being searched for. 1962.It %h 1963The remote hostname. 1964.It \%%I 1965A string describing the reason for a 1966.Cm KnownHostsCommand 1967execution: either 1968.Cm ADDRESS 1969when looking up a host by address (only when 1970.Cm CheckHostIP 1971is enabled), 1972.Cm HOSTNAME 1973when searching by hostname, or 1974.Cm ORDER 1975when preparing the host key algorithm preference list to use for the 1976destination host. 1977.It %i 1978The local user ID. 1979.It %K 1980The base64 encoded host key. 1981.It %k 1982The host key alias if specified, otherwise the original remote hostname given 1983on the command line. 1984.It %L 1985The local hostname. 1986.It %l 1987The local hostname, including the domain name. 1988.It %n 1989The original remote hostname, as given on the command line. 1990.It %p 1991The remote port. 1992.It %r 1993The remote username. 1994.It \&%T 1995The local 1996.Xr tun 4 1997or 1998.Xr tap 4 1999network interface assigned if 2000tunnel forwarding was requested, or 2001.Qq NONE 2002otherwise. 2003.It %t 2004The type of the server host key, e.g. 2005.Cm ssh-ed25519 2006.It %u 2007The local username. 2008.El 2009.Pp 2010.Cm CertificateFile , 2011.Cm ControlPath , 2012.Cm IdentityAgent , 2013.Cm IdentityFile , 2014.Cm KnownHostsCommand , 2015.Cm LocalForward , 2016.Cm Match exec , 2017.Cm RemoteCommand , 2018.Cm RemoteForward , 2019and 2020.Cm UserKnownHostsFile 2021accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2022.Pp 2023.Cm KnownHostsCommand 2024additionally accepts the tokens %f, %H, %I, %K and %t. 2025.Pp 2026.Cm Hostname 2027accepts the tokens %% and %h. 2028.Pp 2029.Cm LocalCommand 2030accepts all tokens. 2031.Pp 2032.Cm ProxyCommand 2033accepts the tokens %%, %h, %n, %p, and %r. 2034.Sh ENVIRONMENT VARIABLES 2035Arguments to some keywords can be expanded at runtime from environment 2036variables on the client by enclosing them in 2037.Ic ${} , 2038for example 2039.Ic ${HOME}/.ssh 2040would refer to the user's .ssh directory. 2041If a specified environment variable does not exist then an error will be 2042returned and the setting for that keyword will be ignored. 2043.Pp 2044The keywords 2045.Cm CertificateFile , 2046.Cm ControlPath , 2047.Cm IdentityAgent , 2048.Cm IdentityFile , 2049.Cm KnownHostsCommand , 2050and 2051.Cm UserKnownHostsFile 2052support environment variables. 2053The keywords 2054.Cm LocalForward 2055and 2056.Cm RemoteForward 2057support environment variables only for Unix domain socket paths. 2058.Sh FILES 2059.Bl -tag -width Ds 2060.It Pa ~/.ssh/config 2061This is the per-user configuration file. 2062The format of this file is described above. 2063This file is used by the SSH client. 2064Because of the potential for abuse, this file must have strict permissions: 2065read/write for the user, and not writable by others. 2066.It Pa /etc/ssh/ssh_config 2067Systemwide configuration file. 2068This file provides defaults for those 2069values that are not specified in the user's configuration file, and 2070for those users who do not have a configuration file. 2071This file must be world-readable. 2072.El 2073.Sh SEE ALSO 2074.Xr ssh 1 2075.Sh AUTHORS 2076.An -nosplit 2077OpenSSH is a derivative of the original and free 2078ssh 1.2.12 release by 2079.An Tatu Ylonen . 2080.An Aaron Campbell , Bob Beck , Markus Friedl , 2081.An Niels Provos , Theo de Raadt 2082and 2083.An Dug Song 2084removed many bugs, re-added newer features and 2085created OpenSSH. 2086.An Markus Friedl 2087contributed the support for SSH protocol versions 1.5 and 2.0. 2088