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 1.345 2021/01/26 15:40:17 naddy Exp $ 37.Dd $Mdocdate: January 26 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, 376ecdsa-sha2-nistp521,rsa-sha2-512,rsa-sha2-256,ssh-rsa 377.Ed 378.Pp 379.Xr ssh 1 380will not accept host certificates signed using algorithms other than those 381specified. 382.It Cm CertificateFile 383Specifies a file from which the user's certificate is read. 384A corresponding private key must be provided separately in order 385to use this certificate either 386from an 387.Cm IdentityFile 388directive or 389.Fl i 390flag to 391.Xr ssh 1 , 392via 393.Xr ssh-agent 1 , 394or via a 395.Cm PKCS11Provider 396or 397.Cm SecurityKeyProvider . 398.Pp 399Arguments to 400.Cm CertificateFile 401may use the tilde syntax to refer to a user's home directory, 402the tokens described in the 403.Sx TOKENS 404section and environment variables as described in the 405.Sx ENVIRONMENT VARIABLES 406section. 407.Pp 408It is possible to have multiple certificate files specified in 409configuration files; these certificates will be tried in sequence. 410Multiple 411.Cm CertificateFile 412directives will add to the list of certificates used for 413authentication. 414.It Cm ChallengeResponseAuthentication 415Specifies whether to use challenge-response authentication. 416The argument to this keyword must be 417.Cm yes 418(the default) 419or 420.Cm no . 421.It Cm CheckHostIP 422If set to 423.Cm yes 424.Xr ssh 1 425will additionally check the host IP address in the 426.Pa known_hosts 427file. 428This allows it to detect if a host key changed due to DNS spoofing 429and will add addresses of destination hosts to 430.Pa ~/.ssh/known_hosts 431in the process, regardless of the setting of 432.Cm StrictHostKeyChecking . 433If the option is set to 434.Cm no 435(the default), 436the check will not be executed. 437.It Cm Ciphers 438Specifies the ciphers allowed and their order of preference. 439Multiple ciphers must be comma-separated. 440If the specified list begins with a 441.Sq + 442character, then the specified ciphers will be appended to the default set 443instead of replacing them. 444If the specified list begins with a 445.Sq - 446character, then the specified ciphers (including wildcards) will be removed 447from the default set instead of replacing them. 448If the specified list begins with a 449.Sq ^ 450character, then the specified ciphers will be placed at the head of the 451default set. 452.Pp 453The supported ciphers are: 454.Bd -literal -offset indent 4553des-cbc 456aes128-cbc 457aes192-cbc 458aes256-cbc 459aes128-ctr 460aes192-ctr 461aes256-ctr 462aes128-gcm@openssh.com 463aes256-gcm@openssh.com 464chacha20-poly1305@openssh.com 465.Ed 466.Pp 467The default is: 468.Bd -literal -offset indent 469chacha20-poly1305@openssh.com, 470aes128-ctr,aes192-ctr,aes256-ctr, 471aes128-gcm@openssh.com,aes256-gcm@openssh.com 472.Ed 473.Pp 474The list of available ciphers may also be obtained using 475.Qq ssh -Q cipher . 476.It Cm ClearAllForwardings 477Specifies that all local, remote, and dynamic port forwardings 478specified in the configuration files or on the command line be 479cleared. 480This option is primarily useful when used from the 481.Xr ssh 1 482command line to clear port forwardings set in 483configuration files, and is automatically set by 484.Xr scp 1 485and 486.Xr sftp 1 . 487The argument must be 488.Cm yes 489or 490.Cm no 491(the default). 492.It Cm Compression 493Specifies whether to use compression. 494The argument must be 495.Cm yes 496or 497.Cm no 498(the default). 499.It Cm ConnectionAttempts 500Specifies the number of tries (one per second) to make before exiting. 501The argument must be an integer. 502This may be useful in scripts if the connection sometimes fails. 503The default is 1. 504.It Cm ConnectTimeout 505Specifies the timeout (in seconds) used when connecting to the 506SSH server, instead of using the default system TCP timeout. 507This timeout is applied both to establishing the connection and to performing 508the initial SSH protocol handshake and key exchange. 509.It Cm ControlMaster 510Enables the sharing of multiple sessions over a single network connection. 511When set to 512.Cm yes , 513.Xr ssh 1 514will listen for connections on a control socket specified using the 515.Cm ControlPath 516argument. 517Additional sessions can connect to this socket using the same 518.Cm ControlPath 519with 520.Cm ControlMaster 521set to 522.Cm no 523(the default). 524These sessions will try to reuse the master instance's network connection 525rather than initiating new ones, but will fall back to connecting normally 526if the control socket does not exist, or is not listening. 527.Pp 528Setting this to 529.Cm ask 530will cause 531.Xr ssh 1 532to listen for control connections, but require confirmation using 533.Xr ssh-askpass 1 . 534If the 535.Cm ControlPath 536cannot be opened, 537.Xr ssh 1 538will continue without connecting to a master instance. 539.Pp 540X11 and 541.Xr ssh-agent 1 542forwarding is supported over these multiplexed connections, however the 543display and agent forwarded will be the one belonging to the master 544connection i.e. it is not possible to forward multiple displays or agents. 545.Pp 546Two additional options allow for opportunistic multiplexing: try to use a 547master connection but fall back to creating a new one if one does not already 548exist. 549These options are: 550.Cm auto 551and 552.Cm autoask . 553The latter requires confirmation like the 554.Cm ask 555option. 556.It Cm ControlPath 557Specify the path to the control socket used for connection sharing as described 558in the 559.Cm ControlMaster 560section above or the string 561.Cm none 562to disable connection sharing. 563Arguments to 564.Cm ControlPath 565may use the tilde syntax to refer to a user's home directory, 566the tokens described in the 567.Sx TOKENS 568section and environment variables as described in the 569.Sx ENVIRONMENT VARIABLES 570section. 571It is recommended that any 572.Cm ControlPath 573used for opportunistic connection sharing include 574at least %h, %p, and %r (or alternatively %C) and be placed in a directory 575that is not writable by other users. 576This ensures that shared connections are uniquely identified. 577.It Cm ControlPersist 578When used in conjunction with 579.Cm ControlMaster , 580specifies that the master connection should remain open 581in the background (waiting for future client connections) 582after the initial client connection has been closed. 583If set to 584.Cm no 585(the default), 586then the master connection will not be placed into the background, 587and will close as soon as the initial client connection is closed. 588If set to 589.Cm yes 590or 0, 591then the master connection will remain in the background indefinitely 592(until killed or closed via a mechanism such as the 593.Qq ssh -O exit ) . 594If set to a time in seconds, or a time in any of the formats documented in 595.Xr sshd_config 5 , 596then the backgrounded master connection will automatically terminate 597after it has remained idle (with no client connections) for the 598specified time. 599.It Cm DynamicForward 600Specifies that a TCP port on the local machine be forwarded 601over the secure channel, and the application 602protocol is then used to determine where to connect to from the 603remote machine. 604.Pp 605The argument must be 606.Sm off 607.Oo Ar bind_address : Oc Ar port . 608.Sm on 609IPv6 addresses can be specified by enclosing addresses in square brackets. 610By default, the local port is bound in accordance with the 611.Cm GatewayPorts 612setting. 613However, an explicit 614.Ar bind_address 615may be used to bind the connection to a specific address. 616The 617.Ar bind_address 618of 619.Cm localhost 620indicates that the listening port be bound for local use only, while an 621empty address or 622.Sq * 623indicates that the port should be available from all interfaces. 624.Pp 625Currently the SOCKS4 and SOCKS5 protocols are supported, and 626.Xr ssh 1 627will act as a SOCKS server. 628Multiple forwardings may be specified, and 629additional forwardings can be given on the command line. 630Only the superuser can forward privileged ports. 631.It Cm EnableSSHKeysign 632Setting this option to 633.Cm yes 634in the global client configuration file 635.Pa /etc/ssh/ssh_config 636enables the use of the helper program 637.Xr ssh-keysign 8 638during 639.Cm HostbasedAuthentication . 640The argument must be 641.Cm yes 642or 643.Cm no 644(the default). 645This option should be placed in the non-hostspecific section. 646See 647.Xr ssh-keysign 8 648for more information. 649.It Cm EscapeChar 650Sets the escape character (default: 651.Ql ~ ) . 652The escape character can also 653be set on the command line. 654The argument should be a single character, 655.Ql ^ 656followed by a letter, or 657.Cm none 658to disable the escape 659character entirely (making the connection transparent for binary 660data). 661.It Cm ExitOnForwardFailure 662Specifies whether 663.Xr ssh 1 664should terminate the connection if it cannot set up all requested 665dynamic, tunnel, local, and remote port forwardings, (e.g.\& 666if either end is unable to bind and listen on a specified port). 667Note that 668.Cm ExitOnForwardFailure 669does not apply to connections made over port forwardings and will not, 670for example, cause 671.Xr ssh 1 672to exit if TCP connections to the ultimate forwarding destination fail. 673The argument must be 674.Cm yes 675or 676.Cm no 677(the default). 678.It Cm FingerprintHash 679Specifies the hash algorithm used when displaying key fingerprints. 680Valid options are: 681.Cm md5 682and 683.Cm sha256 684(the default). 685.It Cm ForwardAgent 686Specifies whether the connection to the authentication agent (if any) 687will be forwarded to the remote machine. 688The argument may be 689.Cm yes , 690.Cm no 691(the default), 692an explicit path to an agent socket or the name of an environment variable 693(beginning with 694.Sq $ ) 695in which to find the path. 696.Pp 697Agent forwarding should be enabled with caution. 698Users with the ability to bypass file permissions on the remote host 699(for the agent's Unix-domain socket) 700can access the local agent through the forwarded connection. 701An attacker cannot obtain key material from the agent, 702however they can perform operations on the keys that enable them to 703authenticate using the identities loaded into the agent. 704.It Cm ForwardX11 705Specifies whether X11 connections will be automatically redirected 706over the secure channel and 707.Ev DISPLAY 708set. 709The argument must be 710.Cm yes 711or 712.Cm no 713(the default). 714.Pp 715X11 forwarding should be enabled with caution. 716Users with the ability to bypass file permissions on the remote host 717(for the user's X11 authorization database) 718can access the local X11 display through the forwarded connection. 719An attacker may then be able to perform activities such as keystroke monitoring 720if the 721.Cm ForwardX11Trusted 722option is also enabled. 723.It Cm ForwardX11Timeout 724Specify a timeout for untrusted X11 forwarding 725using the format described in the 726.Sx TIME FORMATS 727section of 728.Xr sshd_config 5 . 729X11 connections received by 730.Xr ssh 1 731after this time will be refused. 732Setting 733.Cm ForwardX11Timeout 734to zero will disable the timeout and permit X11 forwarding for the life 735of the connection. 736The default is to disable untrusted X11 forwarding after twenty minutes has 737elapsed. 738.It Cm ForwardX11Trusted 739If this option is set to 740.Cm yes , 741remote X11 clients will have full access to the original X11 display. 742.Pp 743If this option is set to 744.Cm no 745(the default), 746remote X11 clients will be considered untrusted and prevented 747from stealing or tampering with data belonging to trusted X11 748clients. 749Furthermore, the 750.Xr xauth 1 751token used for the session will be set to expire after 20 minutes. 752Remote clients will be refused access after this time. 753.Pp 754See the X11 SECURITY extension specification for full details on 755the restrictions imposed on untrusted clients. 756.It Cm GatewayPorts 757Specifies whether remote hosts are allowed to connect to local 758forwarded ports. 759By default, 760.Xr ssh 1 761binds local port forwardings to the loopback address. 762This prevents other remote hosts from connecting to forwarded ports. 763.Cm GatewayPorts 764can be used to specify that ssh 765should bind local port forwardings to the wildcard address, 766thus allowing remote hosts to connect to forwarded ports. 767The argument must be 768.Cm yes 769or 770.Cm no 771(the default). 772.It Cm GlobalKnownHostsFile 773Specifies one or more files to use for the global 774host key database, separated by whitespace. 775The default is 776.Pa /etc/ssh/ssh_known_hosts , 777.Pa /etc/ssh/ssh_known_hosts2 . 778.It Cm GSSAPIAuthentication 779Specifies whether user authentication based on GSSAPI is allowed. 780The default is 781.Cm no . 782.It Cm GSSAPIDelegateCredentials 783Forward (delegate) credentials to the server. 784The default is 785.Cm no . 786.It Cm HashKnownHosts 787Indicates that 788.Xr ssh 1 789should hash host names and addresses when they are added to 790.Pa ~/.ssh/known_hosts . 791These hashed names may be used normally by 792.Xr ssh 1 793and 794.Xr sshd 8 , 795but they do not visually reveal identifying information if the 796file's contents are disclosed. 797The default is 798.Cm no . 799Note that existing names and addresses in known hosts files 800will not be converted automatically, 801but may be manually hashed using 802.Xr ssh-keygen 1 . 803.It Cm HostbasedAcceptedAlgorithms 804Specifies the key types that will be used for hostbased authentication 805as a comma-separated list of patterns. 806Alternately if the specified list begins with a 807.Sq + 808character, then the specified key types will be appended to the default set 809instead of replacing them. 810If the specified list begins with a 811.Sq - 812character, then the specified key types (including wildcards) will be removed 813from the default set instead of replacing them. 814If the specified list begins with a 815.Sq ^ 816character, then the specified key types will be placed at the head of the 817default set. 818The default for this option is: 819.Bd -literal -offset 3n 820ssh-ed25519-cert-v01@openssh.com, 821ecdsa-sha2-nistp256-cert-v01@openssh.com, 822ecdsa-sha2-nistp384-cert-v01@openssh.com, 823ecdsa-sha2-nistp521-cert-v01@openssh.com, 824sk-ssh-ed25519-cert-v01@openssh.com, 825sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 826rsa-sha2-512-cert-v01@openssh.com, 827rsa-sha2-256-cert-v01@openssh.com, 828ssh-rsa-cert-v01@openssh.com, 829ssh-ed25519, 830ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 831sk-ssh-ed25519@openssh.com, 832sk-ecdsa-sha2-nistp256@openssh.com, 833rsa-sha2-512,rsa-sha2-256,ssh-rsa 834.Ed 835.Pp 836The 837.Fl Q 838option of 839.Xr ssh 1 840may be used to list supported key types. 841This was formerly named HostbasedKeyTypes. 842.It Cm HostbasedAuthentication 843Specifies whether to try rhosts based authentication with public key 844authentication. 845The argument must be 846.Cm yes 847or 848.Cm no 849(the default). 850.It Cm HostKeyAlgorithms 851Specifies the host key algorithms 852that the client wants to use in order of preference. 853Alternately if the specified list begins with a 854.Sq + 855character, then the specified key types will be appended to the default set 856instead of replacing them. 857If the specified list begins with a 858.Sq - 859character, then the specified key types (including wildcards) will be removed 860from the default set instead of replacing them. 861If the specified list begins with a 862.Sq ^ 863character, then the specified key types will be placed at the head of the 864default set. 865The default for this option is: 866.Bd -literal -offset 3n 867ssh-ed25519-cert-v01@openssh.com, 868ecdsa-sha2-nistp256-cert-v01@openssh.com, 869ecdsa-sha2-nistp384-cert-v01@openssh.com, 870ecdsa-sha2-nistp521-cert-v01@openssh.com, 871sk-ssh-ed25519-cert-v01@openssh.com, 872sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 873rsa-sha2-512-cert-v01@openssh.com, 874rsa-sha2-256-cert-v01@openssh.com, 875ssh-rsa-cert-v01@openssh.com, 876ssh-ed25519, 877ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 878sk-ecdsa-sha2-nistp256@openssh.com, 879sk-ssh-ed25519@openssh.com, 880rsa-sha2-512,rsa-sha2-256,ssh-rsa 881.Ed 882.Pp 883If hostkeys are known for the destination host then this default is modified 884to prefer their algorithms. 885.Pp 886The list of available key types may also be obtained using 887.Qq ssh -Q HostKeyAlgorithms . 888.It Cm HostKeyAlias 889Specifies an alias that should be used instead of the 890real host name when looking up or saving the host key 891in the host key database files and when validating host certificates. 892This option is useful for tunneling SSH connections 893or for multiple servers running on a single host. 894.It Cm Hostname 895Specifies the real host name to log into. 896This can be used to specify nicknames or abbreviations for hosts. 897Arguments to 898.Cm Hostname 899accept the tokens described in the 900.Sx TOKENS 901section. 902Numeric IP addresses are also permitted (both on the command line and in 903.Cm Hostname 904specifications). 905The default is the name given on the command line. 906.It Cm IdentitiesOnly 907Specifies that 908.Xr ssh 1 909should only use the configured authentication identity and certificate files 910(either the default files, or those explicitly configured in the 911.Nm 912files 913or passed on the 914.Xr ssh 1 915command-line), 916even if 917.Xr ssh-agent 1 918or a 919.Cm PKCS11Provider 920or 921.Cm SecurityKeyProvider 922offers more identities. 923The argument to this keyword must be 924.Cm yes 925or 926.Cm no 927(the default). 928This option is intended for situations where ssh-agent 929offers many different identities. 930.It Cm IdentityAgent 931Specifies the 932.Ux Ns -domain 933socket used to communicate with the authentication agent. 934.Pp 935This option overrides the 936.Ev SSH_AUTH_SOCK 937environment variable and can be used to select a specific agent. 938Setting the socket name to 939.Cm none 940disables the use of an authentication agent. 941If the string 942.Qq SSH_AUTH_SOCK 943is specified, the location of the socket will be read from the 944.Ev SSH_AUTH_SOCK 945environment variable. 946Otherwise if the specified value begins with a 947.Sq $ 948character, then it will be treated as an environment variable containing 949the location of the socket. 950.Pp 951Arguments to 952.Cm IdentityAgent 953may use the tilde syntax to refer to a user's home directory, 954the tokens described in the 955.Sx TOKENS 956section and environment variables as described in the 957.Sx ENVIRONMENT VARIABLES 958section. 959.It Cm IdentityFile 960Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 961Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 962The default is 963.Pa ~/.ssh/id_dsa , 964.Pa ~/.ssh/id_ecdsa , 965.Pa ~/.ssh/id_ecdsa_sk , 966.Pa ~/.ssh/id_ed25519 , 967.Pa ~/.ssh/id_ed25519_sk 968and 969.Pa ~/.ssh/id_rsa . 970Additionally, any identities represented by the authentication agent 971will be used for authentication unless 972.Cm IdentitiesOnly 973is set. 974If no certificates have been explicitly specified by 975.Cm CertificateFile , 976.Xr ssh 1 977will try to load certificate information from the filename obtained by 978appending 979.Pa -cert.pub 980to the path of a specified 981.Cm IdentityFile . 982.Pp 983Arguments to 984.Cm IdentityFile 985may use the tilde syntax to refer to a user's home directory 986or the tokens described in the 987.Sx TOKENS 988section. 989.Pp 990It is possible to have 991multiple identity files specified in configuration files; all these 992identities will be tried in sequence. 993Multiple 994.Cm IdentityFile 995directives will add to the list of identities tried (this behaviour 996differs from that of other configuration directives). 997.Pp 998.Cm IdentityFile 999may be used in conjunction with 1000.Cm IdentitiesOnly 1001to select which identities in an agent are offered during authentication. 1002.Cm IdentityFile 1003may also be used in conjunction with 1004.Cm CertificateFile 1005in order to provide any certificate also needed for authentication with 1006the identity. 1007.It Cm IgnoreUnknown 1008Specifies a pattern-list of unknown options to be ignored if they are 1009encountered in configuration parsing. 1010This may be used to suppress errors if 1011.Nm 1012contains options that are unrecognised by 1013.Xr ssh 1 . 1014It is recommended that 1015.Cm IgnoreUnknown 1016be listed early in the configuration file as it will not be applied 1017to unknown options that appear before it. 1018.It Cm Include 1019Include the specified configuration file(s). 1020Multiple pathnames may be specified and each pathname may contain 1021.Xr glob 7 1022wildcards and, for user configurations, shell-like 1023.Sq ~ 1024references to user home directories. 1025Wildcards will be expanded and processed in lexical order. 1026Files without absolute paths are assumed to be in 1027.Pa ~/.ssh 1028if included in a user configuration file or 1029.Pa /etc/ssh 1030if included from the system configuration file. 1031.Cm Include 1032directive may appear inside a 1033.Cm Match 1034or 1035.Cm Host 1036block 1037to perform conditional inclusion. 1038.It Cm IPQoS 1039Specifies the IPv4 type-of-service or DSCP class for connections. 1040Accepted values are 1041.Cm af11 , 1042.Cm af12 , 1043.Cm af13 , 1044.Cm af21 , 1045.Cm af22 , 1046.Cm af23 , 1047.Cm af31 , 1048.Cm af32 , 1049.Cm af33 , 1050.Cm af41 , 1051.Cm af42 , 1052.Cm af43 , 1053.Cm cs0 , 1054.Cm cs1 , 1055.Cm cs2 , 1056.Cm cs3 , 1057.Cm cs4 , 1058.Cm cs5 , 1059.Cm cs6 , 1060.Cm cs7 , 1061.Cm ef , 1062.Cm le , 1063.Cm lowdelay , 1064.Cm throughput , 1065.Cm reliability , 1066a numeric value, or 1067.Cm none 1068to use the operating system default. 1069This option may take one or two arguments, separated by whitespace. 1070If one argument is specified, it is used as the packet class unconditionally. 1071If two values are specified, the first is automatically selected for 1072interactive sessions and the second for non-interactive sessions. 1073The default is 1074.Cm af21 1075(Low-Latency Data) 1076for interactive sessions and 1077.Cm cs1 1078(Lower Effort) 1079for non-interactive sessions. 1080.It Cm KbdInteractiveAuthentication 1081Specifies whether to use keyboard-interactive authentication. 1082The argument to this keyword must be 1083.Cm yes 1084(the default) 1085or 1086.Cm no . 1087.It Cm KbdInteractiveDevices 1088Specifies the list of methods to use in keyboard-interactive authentication. 1089Multiple method names must be comma-separated. 1090The default is to use the server specified list. 1091The methods available vary depending on what the server supports. 1092For an OpenSSH server, 1093it may be zero or more of: 1094.Cm bsdauth , 1095.Cm pam , 1096and 1097.Cm skey . 1098.It Cm KexAlgorithms 1099Specifies the available KEX (Key Exchange) algorithms. 1100Multiple algorithms must be comma-separated. 1101If the specified list begins with a 1102.Sq + 1103character, then the specified methods will be appended to the default set 1104instead of replacing them. 1105If the specified list begins with a 1106.Sq - 1107character, then the specified methods (including wildcards) will be removed 1108from the default set instead of replacing them. 1109If the specified list begins with a 1110.Sq ^ 1111character, then the specified methods will be placed at the head of the 1112default set. 1113The default is: 1114.Bd -literal -offset indent 1115curve25519-sha256,curve25519-sha256@libssh.org, 1116ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1117diffie-hellman-group-exchange-sha256, 1118diffie-hellman-group16-sha512, 1119diffie-hellman-group18-sha512, 1120diffie-hellman-group14-sha256 1121.Ed 1122.Pp 1123The list of available key exchange algorithms may also be obtained using 1124.Qq ssh -Q kex . 1125.It Cm KnownHostsCommand 1126Specifies a command to use to obtain a list of host keys, in addition to 1127those listed in 1128.Cm UserKnownHostsFile 1129and 1130.Cm GlobalKnownHostsFile . 1131This command is executed after the files have been read. 1132It may write host key lines to standard output in identical format to the 1133usual files (described in the 1134.Sx VERIFYING HOST KEYS 1135section in 1136.Xr ssh 1 ) . 1137Arguments to 1138.Cm KnownHostsCommand 1139accept the tokens described in the 1140.Sx TOKENS 1141section. 1142The command may be invoked multiple times per connection: once when preparing 1143the preference list of host key algorithms to use, again to obtain the 1144host key for the requested host name and, if 1145.Cm CheckHostIP 1146is enabled, one more time to obtain the host key matching the server's 1147address. 1148If the command exits abnormally or returns a non-zero exit status then the 1149connection is terminated. 1150.It Cm LocalCommand 1151Specifies a command to execute on the local machine after successfully 1152connecting to the server. 1153The command string extends to the end of the line, and is executed with 1154the user's shell. 1155Arguments to 1156.Cm LocalCommand 1157accept the tokens described in the 1158.Sx TOKENS 1159section. 1160.Pp 1161The command is run synchronously and does not have access to the 1162session of the 1163.Xr ssh 1 1164that spawned it. 1165It should not be used for interactive commands. 1166.Pp 1167This directive is ignored unless 1168.Cm PermitLocalCommand 1169has been enabled. 1170.It Cm LocalForward 1171Specifies that a TCP port on the local machine be forwarded over 1172the secure channel to the specified host and port from the remote machine. 1173The first argument specifies the listener and may be 1174.Sm off 1175.Oo Ar bind_address : Oc Ar port 1176.Sm on 1177or a Unix domain socket path. 1178The second argument is the destination and may be 1179.Ar host : Ns Ar hostport 1180or a Unix domain socket path if the remote host supports it. 1181.Pp 1182IPv6 addresses can be specified by enclosing addresses in square brackets. 1183Multiple forwardings may be specified, and additional forwardings can be 1184given on the command line. 1185Only the superuser can forward privileged ports. 1186By default, the local port is bound in accordance with the 1187.Cm GatewayPorts 1188setting. 1189However, an explicit 1190.Ar bind_address 1191may be used to bind the connection to a specific address. 1192The 1193.Ar bind_address 1194of 1195.Cm localhost 1196indicates that the listening port be bound for local use only, while an 1197empty address or 1198.Sq * 1199indicates that the port should be available from all interfaces. 1200Unix domain socket paths may use the tokens described in the 1201.Sx TOKENS 1202section and environment variables as described in the 1203.Sx ENVIRONMENT VARIABLES 1204section. 1205.It Cm LogLevel 1206Gives the verbosity level that is used when logging messages from 1207.Xr ssh 1 . 1208The possible values are: 1209QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1210The default is INFO. 1211DEBUG and DEBUG1 are equivalent. 1212DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1213.It Cm LogVerbose 1214Specify one or more overrides to LogLevel. 1215An override consists of a pattern lists that matches the source file, function 1216and line number to force detailed logging for. 1217For example, an override pattern of: 1218.Bd -literal -offset indent 1219kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1220.Ed 1221.Pp 1222would enable detailed logging for line 1000 of 1223.Pa kex.c , 1224everything in the 1225.Fn kex_exchange_identification 1226function, and all code in the 1227.Pa packet.c 1228file. 1229This option is intended for debugging and no overrides are enabled by default. 1230.It Cm MACs 1231Specifies the MAC (message authentication code) algorithms 1232in order of preference. 1233The MAC algorithm is used for data integrity protection. 1234Multiple algorithms must be comma-separated. 1235If the specified list begins with a 1236.Sq + 1237character, then the specified algorithms will be appended to the default set 1238instead of replacing them. 1239If the specified list begins with a 1240.Sq - 1241character, then the specified algorithms (including wildcards) will be removed 1242from the default set instead of replacing them. 1243If the specified list begins with a 1244.Sq ^ 1245character, then the specified algorithms will be placed at the head of the 1246default set. 1247.Pp 1248The algorithms that contain 1249.Qq -etm 1250calculate the MAC after encryption (encrypt-then-mac). 1251These are considered safer and their use recommended. 1252.Pp 1253The default is: 1254.Bd -literal -offset indent 1255umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1256hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1257hmac-sha1-etm@openssh.com, 1258umac-64@openssh.com,umac-128@openssh.com, 1259hmac-sha2-256,hmac-sha2-512,hmac-sha1 1260.Ed 1261.Pp 1262The list of available MAC algorithms may also be obtained using 1263.Qq ssh -Q mac . 1264.It Cm NoHostAuthenticationForLocalhost 1265Disable host authentication for localhost (loopback addresses). 1266The argument to this keyword must be 1267.Cm yes 1268or 1269.Cm no 1270(the default). 1271.It Cm NumberOfPasswordPrompts 1272Specifies the number of password prompts before giving up. 1273The argument to this keyword must be an integer. 1274The default is 3. 1275.It Cm PasswordAuthentication 1276Specifies whether to use password authentication. 1277The argument to this keyword must be 1278.Cm yes 1279(the default) 1280or 1281.Cm no . 1282.It Cm PermitLocalCommand 1283Allow local command execution via the 1284.Ic LocalCommand 1285option or using the 1286.Ic !\& Ns Ar command 1287escape sequence in 1288.Xr ssh 1 . 1289The argument must be 1290.Cm yes 1291or 1292.Cm no 1293(the default). 1294.It Cm PKCS11Provider 1295Specifies which PKCS#11 provider to use or 1296.Cm none 1297to indicate that no provider should be used (the default). 1298The argument to this keyword is a path to the PKCS#11 shared library 1299.Xr ssh 1 1300should use to communicate with a PKCS#11 token providing keys for user 1301authentication. 1302.It Cm Port 1303Specifies the port number to connect on the remote host. 1304The default is 22. 1305.It Cm PreferredAuthentications 1306Specifies the order in which the client should try authentication methods. 1307This allows a client to prefer one method (e.g.\& 1308.Cm keyboard-interactive ) 1309over another method (e.g.\& 1310.Cm password ) . 1311The default is: 1312.Bd -literal -offset indent 1313gssapi-with-mic,hostbased,publickey, 1314keyboard-interactive,password 1315.Ed 1316.It Cm ProxyCommand 1317Specifies the command to use to connect to the server. 1318The command 1319string extends to the end of the line, and is executed 1320using the user's shell 1321.Ql exec 1322directive to avoid a lingering shell process. 1323.Pp 1324Arguments to 1325.Cm ProxyCommand 1326accept the tokens described in the 1327.Sx TOKENS 1328section. 1329The command can be basically anything, 1330and should read from its standard input and write to its standard output. 1331It should eventually connect an 1332.Xr sshd 8 1333server running on some machine, or execute 1334.Ic sshd -i 1335somewhere. 1336Host key management will be done using the 1337.Cm Hostname 1338of the host being connected (defaulting to the name typed by the user). 1339Setting the command to 1340.Cm none 1341disables this option entirely. 1342Note that 1343.Cm CheckHostIP 1344is not available for connects with a proxy command. 1345.Pp 1346This directive is useful in conjunction with 1347.Xr nc 1 1348and its proxy support. 1349For example, the following directive would connect via an HTTP proxy at 1350192.0.2.0: 1351.Bd -literal -offset 3n 1352ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1353.Ed 1354.It Cm ProxyJump 1355Specifies one or more jump proxies as either 1356.Xo 1357.Sm off 1358.Op Ar user No @ 1359.Ar host 1360.Op : Ns Ar port 1361.Sm on 1362or an ssh URI 1363.Xc . 1364Multiple proxies may be separated by comma characters and will be visited 1365sequentially. 1366Setting this option will cause 1367.Xr ssh 1 1368to connect to the target host by first making a 1369.Xr ssh 1 1370connection to the specified 1371.Cm ProxyJump 1372host and then establishing a 1373TCP forwarding to the ultimate target from there. 1374.Pp 1375Note that this option will compete with the 1376.Cm ProxyCommand 1377option - whichever is specified first will prevent later instances of the 1378other from taking effect. 1379.Pp 1380Note also that the configuration for the destination host (either supplied 1381via the command-line or the configuration file) is not generally applied 1382to jump hosts. 1383.Pa ~/.ssh/config 1384should be used if specific configuration is required for jump hosts. 1385.It Cm ProxyUseFdpass 1386Specifies that 1387.Cm ProxyCommand 1388will pass a connected file descriptor back to 1389.Xr ssh 1 1390instead of continuing to execute and pass data. 1391The default is 1392.Cm no . 1393.It Cm PubkeyAcceptedAlgorithms 1394Specifies the signature algorithms that will be used for public key 1395authentication as a comma-separated list of patterns. 1396If the specified list begins with a 1397.Sq + 1398character, then the algorithms after it will be appended to the default 1399instead of replacing it. 1400If the specified list begins with a 1401.Sq - 1402character, then the specified algorithms (including wildcards) will be removed 1403from the default set instead of replacing them. 1404If the specified list begins with a 1405.Sq ^ 1406character, then the specified algorithms will be placed at the head of the 1407default set. 1408The default for this option is: 1409.Bd -literal -offset 3n 1410ssh-ed25519-cert-v01@openssh.com, 1411ecdsa-sha2-nistp256-cert-v01@openssh.com, 1412ecdsa-sha2-nistp384-cert-v01@openssh.com, 1413ecdsa-sha2-nistp521-cert-v01@openssh.com, 1414sk-ssh-ed25519-cert-v01@openssh.com, 1415sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1416rsa-sha2-512-cert-v01@openssh.com, 1417rsa-sha2-256-cert-v01@openssh.com, 1418ssh-rsa-cert-v01@openssh.com, 1419ssh-ed25519, 1420ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1421sk-ssh-ed25519@openssh.com, 1422sk-ecdsa-sha2-nistp256@openssh.com, 1423rsa-sha2-512,rsa-sha2-256,ssh-rsa 1424.Ed 1425.Pp 1426The list of available key types may also be obtained using 1427.Qq ssh -Q PubkeyAcceptedAlgorithms . 1428.It Cm PubkeyAuthentication 1429Specifies whether to try public key authentication. 1430The argument to this keyword must be 1431.Cm yes 1432(the default) 1433or 1434.Cm no . 1435.It Cm RekeyLimit 1436Specifies the maximum amount of data that may be transmitted before the 1437session key is renegotiated, optionally followed by a maximum amount of 1438time that may pass before the session key is renegotiated. 1439The first argument is specified in bytes and may have a suffix of 1440.Sq K , 1441.Sq M , 1442or 1443.Sq G 1444to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1445The default is between 1446.Sq 1G 1447and 1448.Sq 4G , 1449depending on the cipher. 1450The optional second value is specified in seconds and may use any of the 1451units documented in the TIME FORMATS section of 1452.Xr sshd_config 5 . 1453The default value for 1454.Cm RekeyLimit 1455is 1456.Cm default none , 1457which means that rekeying is performed after the cipher's default amount 1458of data has been sent or received and no time based rekeying is done. 1459.It Cm RemoteCommand 1460Specifies a command to execute on the remote machine after successfully 1461connecting to the server. 1462The command string extends to the end of the line, and is executed with 1463the user's shell. 1464Arguments to 1465.Cm RemoteCommand 1466accept the tokens described in the 1467.Sx TOKENS 1468section. 1469.It Cm RemoteForward 1470Specifies that a TCP port on the remote machine be forwarded over 1471the secure channel. 1472The remote port may either be forwarded to a specified host and port 1473from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1474client to connect to arbitrary destinations from the local machine. 1475The first argument is the listening specification and may be 1476.Sm off 1477.Oo Ar bind_address : Oc Ar port 1478.Sm on 1479or, if the remote host supports it, a Unix domain socket path. 1480If forwarding to a specific destination then the second argument must be 1481.Ar host : Ns Ar hostport 1482or a Unix domain socket path, 1483otherwise if no destination argument is specified then the remote forwarding 1484will be established as a SOCKS proxy. 1485.Pp 1486IPv6 addresses can be specified by enclosing addresses in square brackets. 1487Multiple forwardings may be specified, and additional 1488forwardings can be given on the command line. 1489Privileged ports can be forwarded only when 1490logging in as root on the remote machine. 1491Unix domain socket paths may use the tokens described in the 1492.Sx TOKENS 1493section and environment variables as described in the 1494.Sx ENVIRONMENT VARIABLES 1495section. 1496.Pp 1497If the 1498.Ar port 1499argument is 0, 1500the listen port will be dynamically allocated on the server and reported 1501to the client at run time. 1502.Pp 1503If the 1504.Ar bind_address 1505is not specified, the default is to only bind to loopback addresses. 1506If the 1507.Ar bind_address 1508is 1509.Ql * 1510or an empty string, then the forwarding is requested to listen on all 1511interfaces. 1512Specifying a remote 1513.Ar bind_address 1514will only succeed if the server's 1515.Cm GatewayPorts 1516option is enabled (see 1517.Xr sshd_config 5 ) . 1518.It Cm RequestTTY 1519Specifies whether to request a pseudo-tty for the session. 1520The argument may be one of: 1521.Cm no 1522(never request a TTY), 1523.Cm yes 1524(always request a TTY when standard input is a TTY), 1525.Cm force 1526(always request a TTY) or 1527.Cm auto 1528(request a TTY when opening a login session). 1529This option mirrors the 1530.Fl t 1531and 1532.Fl T 1533flags for 1534.Xr ssh 1 . 1535.It Cm RevokedHostKeys 1536Specifies revoked host public keys. 1537Keys listed in this file will be refused for host authentication. 1538Note that if this file does not exist or is not readable, 1539then host authentication will be refused for all hosts. 1540Keys may be specified as a text file, listing one public key per line, or as 1541an OpenSSH Key Revocation List (KRL) as generated by 1542.Xr ssh-keygen 1 . 1543For more information on KRLs, see the KEY REVOCATION LISTS section in 1544.Xr ssh-keygen 1 . 1545.It Cm SecurityKeyProvider 1546Specifies a path to a library that will be used when loading any 1547FIDO authenticator-hosted keys, overriding the default of using 1548the built-in USB HID support. 1549.Pp 1550If the specified value begins with a 1551.Sq $ 1552character, then it will be treated as an environment variable containing 1553the path to the library. 1554.It Cm SendEnv 1555Specifies what variables from the local 1556.Xr environ 7 1557should be sent to the server. 1558The server must also support it, and the server must be configured to 1559accept these environment variables. 1560Note that the 1561.Ev TERM 1562environment variable is always sent whenever a 1563pseudo-terminal is requested as it is required by the protocol. 1564Refer to 1565.Cm AcceptEnv 1566in 1567.Xr sshd_config 5 1568for how to configure the server. 1569Variables are specified by name, which may contain wildcard characters. 1570Multiple environment variables may be separated by whitespace or spread 1571across multiple 1572.Cm SendEnv 1573directives. 1574.Pp 1575See 1576.Sx PATTERNS 1577for more information on patterns. 1578.Pp 1579It is possible to clear previously set 1580.Cm SendEnv 1581variable names by prefixing patterns with 1582.Pa - . 1583The default is not to send any environment variables. 1584.It Cm ServerAliveCountMax 1585Sets the number of server alive messages (see below) which may be 1586sent without 1587.Xr ssh 1 1588receiving any messages back from the server. 1589If this threshold is reached while server alive messages are being sent, 1590ssh will disconnect from the server, terminating the session. 1591It is important to note that the use of server alive messages is very 1592different from 1593.Cm TCPKeepAlive 1594(below). 1595The server alive messages are sent through the encrypted channel 1596and therefore will not be spoofable. 1597The TCP keepalive option enabled by 1598.Cm TCPKeepAlive 1599is spoofable. 1600The server alive mechanism is valuable when the client or 1601server depend on knowing when a connection has become unresponsive. 1602.Pp 1603The default value is 3. 1604If, for example, 1605.Cm ServerAliveInterval 1606(see below) is set to 15 and 1607.Cm ServerAliveCountMax 1608is left at the default, if the server becomes unresponsive, 1609ssh will disconnect after approximately 45 seconds. 1610.It Cm ServerAliveInterval 1611Sets a timeout interval in seconds after which if no data has been received 1612from the server, 1613.Xr ssh 1 1614will send a message through the encrypted 1615channel to request a response from the server. 1616The default 1617is 0, indicating that these messages will not be sent to the server. 1618.It Cm SetEnv 1619Directly specify one or more environment variables and their contents to 1620be sent to the server. 1621Similarly to 1622.Cm SendEnv , 1623the server must be prepared to accept the environment variable. 1624.It Cm StreamLocalBindMask 1625Sets the octal file creation mode mask 1626.Pq umask 1627used when creating a Unix-domain socket file for local or remote 1628port forwarding. 1629This option is only used for port forwarding to a Unix-domain socket file. 1630.Pp 1631The default value is 0177, which creates a Unix-domain socket file that is 1632readable and writable only by the owner. 1633Note that not all operating systems honor the file mode on Unix-domain 1634socket files. 1635.It Cm StreamLocalBindUnlink 1636Specifies whether to remove an existing Unix-domain socket file for local 1637or remote port forwarding before creating a new one. 1638If the socket file already exists and 1639.Cm StreamLocalBindUnlink 1640is not enabled, 1641.Nm ssh 1642will be unable to forward the port to the Unix-domain socket file. 1643This option is only used for port forwarding to a Unix-domain socket file. 1644.Pp 1645The argument must be 1646.Cm yes 1647or 1648.Cm no 1649(the default). 1650.It Cm StrictHostKeyChecking 1651If this flag is set to 1652.Cm yes , 1653.Xr ssh 1 1654will never automatically add host keys to the 1655.Pa ~/.ssh/known_hosts 1656file, and refuses to connect to hosts whose host key has changed. 1657This provides maximum protection against man-in-the-middle (MITM) attacks, 1658though it can be annoying when the 1659.Pa /etc/ssh/ssh_known_hosts 1660file is poorly maintained or when connections to new hosts are 1661frequently made. 1662This option forces the user to manually 1663add all new hosts. 1664.Pp 1665If this flag is set to 1666.Dq accept-new 1667then ssh will automatically add new host keys to the user 1668known hosts files, but will not permit connections to hosts with 1669changed host keys. 1670If this flag is set to 1671.Dq no 1672or 1673.Dq off , 1674ssh will automatically add new host keys to the user known hosts files 1675and allow connections to hosts with changed hostkeys to proceed, 1676subject to some restrictions. 1677If this flag is set to 1678.Cm ask 1679(the default), 1680new host keys 1681will be added to the user known host files only after the user 1682has confirmed that is what they really want to do, and 1683ssh will refuse to connect to hosts whose host key has changed. 1684The host keys of 1685known hosts will be verified automatically in all cases. 1686.It Cm SyslogFacility 1687Gives the facility code that is used when logging messages from 1688.Xr ssh 1 . 1689The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1690LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1691The default is USER. 1692.It Cm TCPKeepAlive 1693Specifies whether the system should send TCP keepalive messages to the 1694other side. 1695If they are sent, death of the connection or crash of one 1696of the machines will be properly noticed. 1697However, this means that 1698connections will die if the route is down temporarily, and some people 1699find it annoying. 1700.Pp 1701The default is 1702.Cm yes 1703(to send TCP keepalive messages), and the client will notice 1704if the network goes down or the remote host dies. 1705This is important in scripts, and many users want it too. 1706.Pp 1707To disable TCP keepalive messages, the value should be set to 1708.Cm no . 1709See also 1710.Cm ServerAliveInterval 1711for protocol-level keepalives. 1712.It Cm Tunnel 1713Request 1714.Xr tun 4 1715device forwarding between the client and the server. 1716The argument must be 1717.Cm yes , 1718.Cm point-to-point 1719(layer 3), 1720.Cm ethernet 1721(layer 2), 1722or 1723.Cm no 1724(the default). 1725Specifying 1726.Cm yes 1727requests the default tunnel mode, which is 1728.Cm point-to-point . 1729.It Cm TunnelDevice 1730Specifies the 1731.Xr tun 4 1732devices to open on the client 1733.Pq Ar local_tun 1734and the server 1735.Pq Ar remote_tun . 1736.Pp 1737The argument must be 1738.Sm off 1739.Ar local_tun Op : Ar remote_tun . 1740.Sm on 1741The devices may be specified by numerical ID or the keyword 1742.Cm any , 1743which uses the next available tunnel device. 1744If 1745.Ar remote_tun 1746is not specified, it defaults to 1747.Cm any . 1748The default is 1749.Cm any:any . 1750.It Cm UpdateHostKeys 1751Specifies whether 1752.Xr ssh 1 1753should accept notifications of additional hostkeys from the server sent 1754after authentication has completed and add them to 1755.Cm UserKnownHostsFile . 1756The argument must be 1757.Cm yes , 1758.Cm no 1759or 1760.Cm ask . 1761This option allows learning alternate hostkeys for a server 1762and supports graceful key rotation by allowing a server to send replacement 1763public keys before old ones are removed. 1764.Pp 1765Additional hostkeys are only accepted if the key used to authenticate the 1766host was already trusted or explicitly accepted by the user, the host was 1767authenticated via 1768.Cm UserKnownHostsFile 1769(i.e. not 1770.Cm GlobalKnownHostsFile ) 1771and the host was authenticated using a plain key and not a certificate. 1772.Pp 1773.Cm UpdateHostKeys 1774is enabled by default if the user has not overridden the default 1775.Cm UserKnownHostsFile 1776setting and has not enabled 1777.Cm VerifyHostKeyDNS , 1778otherwise 1779.Cm UpdateHostKeys 1780will be set to 1781.Cm no . 1782.Pp 1783If 1784.Cm UpdateHostKeys 1785is set to 1786.Cm ask , 1787then the user is asked to confirm the modifications to the known_hosts file. 1788Confirmation is currently incompatible with 1789.Cm ControlPersist , 1790and will be disabled if it is enabled. 1791.Pp 1792Presently, only 1793.Xr sshd 8 1794from OpenSSH 6.8 and greater support the 1795.Qq hostkeys@openssh.com 1796protocol extension used to inform the client of all the server's hostkeys. 1797.It Cm User 1798Specifies the user to log in as. 1799This can be useful when a different user name is used on different machines. 1800This saves the trouble of 1801having to remember to give the user name on the command line. 1802.It Cm UserKnownHostsFile 1803Specifies one or more files to use for the user 1804host key database, separated by whitespace. 1805Each filename may use tilde notation to refer to the user's home directory, 1806the tokens described in the 1807.Sx TOKENS 1808section and environment variables as described in the 1809.Sx ENVIRONMENT VARIABLES 1810section. 1811The default is 1812.Pa ~/.ssh/known_hosts , 1813.Pa ~/.ssh/known_hosts2 . 1814.It Cm VerifyHostKeyDNS 1815Specifies whether to verify the remote key using DNS and SSHFP resource 1816records. 1817If this option is set to 1818.Cm yes , 1819the client will implicitly trust keys that match a secure fingerprint 1820from DNS. 1821Insecure fingerprints will be handled as if this option was set to 1822.Cm ask . 1823If this option is set to 1824.Cm ask , 1825information on fingerprint match will be displayed, but the user will still 1826need to confirm new host keys according to the 1827.Cm StrictHostKeyChecking 1828option. 1829The default is 1830.Cm no . 1831.Pp 1832See also 1833.Sx VERIFYING HOST KEYS 1834in 1835.Xr ssh 1 . 1836.It Cm VisualHostKey 1837If this flag is set to 1838.Cm yes , 1839an ASCII art representation of the remote host key fingerprint is 1840printed in addition to the fingerprint string at login and 1841for unknown host keys. 1842If this flag is set to 1843.Cm no 1844(the default), 1845no fingerprint strings are printed at login and 1846only the fingerprint string will be printed for unknown host keys. 1847.It Cm XAuthLocation 1848Specifies the full pathname of the 1849.Xr xauth 1 1850program. 1851The default is 1852.Pa /usr/X11R6/bin/xauth . 1853.El 1854.Sh PATTERNS 1855A 1856.Em pattern 1857consists of zero or more non-whitespace characters, 1858.Sq * 1859(a wildcard that matches zero or more characters), 1860or 1861.Sq ?\& 1862(a wildcard that matches exactly one character). 1863For example, to specify a set of declarations for any host in the 1864.Qq .co.uk 1865set of domains, 1866the following pattern could be used: 1867.Pp 1868.Dl Host *.co.uk 1869.Pp 1870The following pattern 1871would match any host in the 192.168.0.[0-9] network range: 1872.Pp 1873.Dl Host 192.168.0.? 1874.Pp 1875A 1876.Em pattern-list 1877is a comma-separated list of patterns. 1878Patterns within pattern-lists may be negated 1879by preceding them with an exclamation mark 1880.Pq Sq !\& . 1881For example, 1882to allow a key to be used from anywhere within an organization 1883except from the 1884.Qq dialup 1885pool, 1886the following entry (in authorized_keys) could be used: 1887.Pp 1888.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1889.Pp 1890Note that a negated match will never produce a positive result by itself. 1891For example, attempting to match 1892.Qq host3 1893against the following pattern-list will fail: 1894.Pp 1895.Dl from=\&"!host1,!host2\&" 1896.Pp 1897The solution here is to include a term that will yield a positive match, 1898such as a wildcard: 1899.Pp 1900.Dl from=\&"!host1,!host2,*\&" 1901.Sh TOKENS 1902Arguments to some keywords can make use of tokens, 1903which are expanded at runtime: 1904.Pp 1905.Bl -tag -width XXXX -offset indent -compact 1906.It %% 1907A literal 1908.Sq % . 1909.It \&%C 1910Hash of %l%h%p%r. 1911.It %d 1912Local user's home directory. 1913.It %f 1914The fingerprint of the server's host key. 1915.It %H 1916The 1917.Pa known_hosts 1918hostname or address that is being searched for. 1919.It %h 1920The remote hostname. 1921.It \%%I 1922A string describing the reason for a 1923.Cm KnownHostsCommand 1924execution: either 1925.Cm ADDRESS 1926when looking up a host by address (only when 1927.Cm CheckHostIP 1928is enabled), 1929.Cm HOSTNAME 1930when searching by hostname, or 1931.Cm ORDER 1932when preparing the host key algorithm preference list to use for the 1933destination host. 1934.It %i 1935The local user ID. 1936.It %K 1937The base64 encoded host key. 1938.It %k 1939The host key alias if specified, otherwise the orignal remote hostname given 1940on the command line. 1941.It %L 1942The local hostname. 1943.It %l 1944The local hostname, including the domain name. 1945.It %n 1946The original remote hostname, as given on the command line. 1947.It %p 1948The remote port. 1949.It %r 1950The remote username. 1951.It \&%T 1952The local 1953.Xr tun 4 1954or 1955.Xr tap 4 1956network interface assigned if 1957tunnel forwarding was requested, or 1958.Qq NONE 1959otherwise. 1960.It %t 1961The type of the server host key, e.g. 1962.Cm ssh-ed25519 1963.It %u 1964The local username. 1965.El 1966.Pp 1967.Cm CertificateFile , 1968.Cm ControlPath , 1969.Cm IdentityAgent , 1970.Cm IdentityFile , 1971.Cm KnownHostsCommand , 1972.Cm LocalForward , 1973.Cm Match exec , 1974.Cm RemoteCommand , 1975.Cm RemoteForward , 1976and 1977.Cm UserKnownHostsFile 1978accept the tokens %%, %C, %d, %h, %i, %L, %l, %n, %p, %r, and %u. 1979.Pp 1980.Cm KnownHostsCommand 1981additionally accepts the tokens %f, %H, %I, %K and %t. 1982.Pp 1983.Cm Hostname 1984accepts the tokens %% and %h. 1985.Pp 1986.Cm LocalCommand 1987accepts all tokens. 1988.Pp 1989.Cm ProxyCommand 1990accepts the tokens %%, %h, %n, %p, and %r. 1991.Sh ENVIRONMENT VARIABLES 1992Arguments to some keywords can be expanded at runtime from environment 1993variables on the client by enclosing them in 1994.Ic ${} , 1995for example 1996.Ic ${HOME}/.ssh 1997would refer to the user's .ssh directory. 1998If a specified environment variable does not exist then an error will be 1999returned and the setting for that keyword will be ignored. 2000.Pp 2001The keywords 2002.Cm CertificateFile , 2003.Cm ControlPath , 2004.Cm IdentityAgent , 2005.Cm IdentityFile 2006.Cm KnownHostsCommand , 2007and 2008.Cm UserKnownHostsFile 2009support environment variables. 2010The keywords 2011.Cm LocalForward 2012and 2013.Cm RemoteForward 2014support environment variables only for Unix domain socket paths. 2015.Sh FILES 2016.Bl -tag -width Ds 2017.It Pa ~/.ssh/config 2018This is the per-user configuration file. 2019The format of this file is described above. 2020This file is used by the SSH client. 2021Because of the potential for abuse, this file must have strict permissions: 2022read/write for the user, and not writable by others. 2023.It Pa /etc/ssh/ssh_config 2024Systemwide configuration file. 2025This file provides defaults for those 2026values that are not specified in the user's configuration file, and 2027for those users who do not have a configuration file. 2028This file must be world-readable. 2029.El 2030.Sh SEE ALSO 2031.Xr ssh 1 2032.Sh AUTHORS 2033.An -nosplit 2034OpenSSH is a derivative of the original and free 2035ssh 1.2.12 release by 2036.An Tatu Ylonen . 2037.An Aaron Campbell , Bob Beck , Markus Friedl , 2038.An Niels Provos , Theo de Raadt 2039and 2040.An Dug Song 2041removed many bugs, re-added newer features and 2042created OpenSSH. 2043.An Markus Friedl 2044contributed the support for SSH protocol versions 1.5 and 2.0. 2045