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 1.371 2022/03/31 17:58:44 naddy Exp $ 37.Dd $Mdocdate: March 31 2022 $ 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. 344A value of 345.Cm none 346disables the use of a 347.Cm ProxyJump 348host. 349.It Cm CanonicalizeMaxDots 350Specifies the maximum number of dot characters in a hostname before 351canonicalization is disabled. 352The default, 1, 353allows a single dot (i.e. hostname.subdomain). 354.It Cm CanonicalizePermittedCNAMEs 355Specifies rules to determine whether CNAMEs should be followed when 356canonicalizing hostnames. 357The rules consist of one or more arguments of 358.Ar source_domain_list : Ns Ar target_domain_list , 359where 360.Ar source_domain_list 361is a pattern-list of domains that may follow CNAMEs in canonicalization, 362and 363.Ar target_domain_list 364is a pattern-list of domains that they may resolve to. 365.Pp 366For example, 367.Qq *.a.example.com:*.b.example.com,*.c.example.com 368will allow hostnames matching 369.Qq *.a.example.com 370to be canonicalized to names in the 371.Qq *.b.example.com 372or 373.Qq *.c.example.com 374domains. 375.Pp 376A single argument of 377.Qq none 378causes no CNAMEs to be considered for canonicalization. 379This is the default behaviour. 380.It Cm CASignatureAlgorithms 381Specifies which algorithms are allowed for signing of certificates 382by certificate authorities (CAs). 383The default is: 384.Bd -literal -offset indent 385ssh-ed25519,ecdsa-sha2-nistp256, 386ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 387sk-ssh-ed25519@openssh.com, 388sk-ecdsa-sha2-nistp256@openssh.com, 389rsa-sha2-512,rsa-sha2-256 390.Ed 391.Pp 392If the specified list begins with a 393.Sq + 394character, then the specified algorithms will be appended to the default set 395instead of replacing them. 396If the specified list begins with a 397.Sq - 398character, then the specified algorithms (including wildcards) will be removed 399from the default set instead of replacing them. 400.Pp 401.Xr ssh 1 402will not accept host certificates signed using algorithms other than those 403specified. 404.It Cm CertificateFile 405Specifies a file from which the user's certificate is read. 406A corresponding private key must be provided separately in order 407to use this certificate either 408from an 409.Cm IdentityFile 410directive or 411.Fl i 412flag to 413.Xr ssh 1 , 414via 415.Xr ssh-agent 1 , 416or via a 417.Cm PKCS11Provider 418or 419.Cm SecurityKeyProvider . 420.Pp 421Arguments to 422.Cm CertificateFile 423may use the tilde syntax to refer to a user's home directory, 424the tokens described in the 425.Sx TOKENS 426section and environment variables as described in the 427.Sx ENVIRONMENT VARIABLES 428section. 429.Pp 430It is possible to have multiple certificate files specified in 431configuration files; these certificates will be tried in sequence. 432Multiple 433.Cm CertificateFile 434directives will add to the list of certificates used for 435authentication. 436.It Cm CheckHostIP 437If set to 438.Cm yes , 439.Xr ssh 1 440will additionally check the host IP address in the 441.Pa known_hosts 442file. 443This allows it to detect if a host key changed due to DNS spoofing 444and will add addresses of destination hosts to 445.Pa ~/.ssh/known_hosts 446in the process, regardless of the setting of 447.Cm StrictHostKeyChecking . 448If the option is set to 449.Cm no 450(the default), 451the check will not be executed. 452The default is 453.Cm no . 454.It Cm Ciphers 455Specifies the ciphers allowed and their order of preference. 456Multiple ciphers must be comma-separated. 457If the specified list begins with a 458.Sq + 459character, then the specified ciphers will be appended to the default set 460instead of replacing them. 461If the specified list begins with a 462.Sq - 463character, then the specified ciphers (including wildcards) will be removed 464from the default set instead of replacing them. 465If the specified list begins with a 466.Sq ^ 467character, then the specified ciphers will be placed at the head of the 468default set. 469.Pp 470The supported ciphers are: 471.Bd -literal -offset indent 4723des-cbc 473aes128-cbc 474aes192-cbc 475aes256-cbc 476aes128-ctr 477aes192-ctr 478aes256-ctr 479aes128-gcm@openssh.com 480aes256-gcm@openssh.com 481chacha20-poly1305@openssh.com 482.Ed 483.Pp 484The default is: 485.Bd -literal -offset indent 486chacha20-poly1305@openssh.com, 487aes128-ctr,aes192-ctr,aes256-ctr, 488aes128-gcm@openssh.com,aes256-gcm@openssh.com 489.Ed 490.Pp 491The list of available ciphers may also be obtained using 492.Qq ssh -Q cipher . 493.It Cm ClearAllForwardings 494Specifies that all local, remote, and dynamic port forwardings 495specified in the configuration files or on the command line be 496cleared. 497This option is primarily useful when used from the 498.Xr ssh 1 499command line to clear port forwardings set in 500configuration files, and is automatically set by 501.Xr scp 1 502and 503.Xr sftp 1 . 504The argument must be 505.Cm yes 506or 507.Cm no 508(the default). 509.It Cm Compression 510Specifies whether to use compression. 511The argument must be 512.Cm yes 513or 514.Cm no 515(the default). 516.It Cm ConnectionAttempts 517Specifies the number of tries (one per second) to make before exiting. 518The argument must be an integer. 519This may be useful in scripts if the connection sometimes fails. 520The default is 1. 521.It Cm ConnectTimeout 522Specifies the timeout (in seconds) used when connecting to the 523SSH server, instead of using the default system TCP timeout. 524This timeout is applied both to establishing the connection and to performing 525the initial SSH protocol handshake and key exchange. 526.It Cm ControlMaster 527Enables the sharing of multiple sessions over a single network connection. 528When set to 529.Cm yes , 530.Xr ssh 1 531will listen for connections on a control socket specified using the 532.Cm ControlPath 533argument. 534Additional sessions can connect to this socket using the same 535.Cm ControlPath 536with 537.Cm ControlMaster 538set to 539.Cm no 540(the default). 541These sessions will try to reuse the master instance's network connection 542rather than initiating new ones, but will fall back to connecting normally 543if the control socket does not exist, or is not listening. 544.Pp 545Setting this to 546.Cm ask 547will cause 548.Xr ssh 1 549to listen for control connections, but require confirmation using 550.Xr ssh-askpass 1 . 551If the 552.Cm ControlPath 553cannot be opened, 554.Xr ssh 1 555will continue without connecting to a master instance. 556.Pp 557X11 and 558.Xr ssh-agent 1 559forwarding is supported over these multiplexed connections, however the 560display and agent forwarded will be the one belonging to the master 561connection i.e. it is not possible to forward multiple displays or agents. 562.Pp 563Two additional options allow for opportunistic multiplexing: try to use a 564master connection but fall back to creating a new one if one does not already 565exist. 566These options are: 567.Cm auto 568and 569.Cm autoask . 570The latter requires confirmation like the 571.Cm ask 572option. 573.It Cm ControlPath 574Specify the path to the control socket used for connection sharing as described 575in the 576.Cm ControlMaster 577section above or the string 578.Cm none 579to disable connection sharing. 580Arguments to 581.Cm ControlPath 582may use the tilde syntax to refer to a user's home directory, 583the tokens described in the 584.Sx TOKENS 585section and environment variables as described in the 586.Sx ENVIRONMENT VARIABLES 587section. 588It is recommended that any 589.Cm ControlPath 590used for opportunistic connection sharing include 591at least %h, %p, and %r (or alternatively %C) and be placed in a directory 592that is not writable by other users. 593This ensures that shared connections are uniquely identified. 594.It Cm ControlPersist 595When used in conjunction with 596.Cm ControlMaster , 597specifies that the master connection should remain open 598in the background (waiting for future client connections) 599after the initial client connection has been closed. 600If set to 601.Cm no 602(the default), 603then the master connection will not be placed into the background, 604and will close as soon as the initial client connection is closed. 605If set to 606.Cm yes 607or 0, 608then the master connection will remain in the background indefinitely 609(until killed or closed via a mechanism such as the 610.Qq ssh -O exit ) . 611If set to a time in seconds, or a time in any of the formats documented in 612.Xr sshd_config 5 , 613then the backgrounded master connection will automatically terminate 614after it has remained idle (with no client connections) for the 615specified time. 616.It Cm DynamicForward 617Specifies that a TCP port on the local machine be forwarded 618over the secure channel, and the application 619protocol is then used to determine where to connect to from the 620remote machine. 621.Pp 622The argument must be 623.Sm off 624.Oo Ar bind_address : Oc Ar port . 625.Sm on 626IPv6 addresses can be specified by enclosing addresses in square brackets. 627By default, the local port is bound in accordance with the 628.Cm GatewayPorts 629setting. 630However, an explicit 631.Ar bind_address 632may be used to bind the connection to a specific address. 633The 634.Ar bind_address 635of 636.Cm localhost 637indicates that the listening port be bound for local use only, while an 638empty address or 639.Sq * 640indicates that the port should be available from all interfaces. 641.Pp 642Currently the SOCKS4 and SOCKS5 protocols are supported, and 643.Xr ssh 1 644will act as a SOCKS server. 645Multiple forwardings may be specified, and 646additional forwardings can be given on the command line. 647Only the superuser can forward privileged ports. 648.It Cm EnableSSHKeysign 649Setting this option to 650.Cm yes 651in the global client configuration file 652.Pa /etc/ssh/ssh_config 653enables the use of the helper program 654.Xr ssh-keysign 8 655during 656.Cm HostbasedAuthentication . 657The argument must be 658.Cm yes 659or 660.Cm no 661(the default). 662This option should be placed in the non-hostspecific section. 663See 664.Xr ssh-keysign 8 665for more information. 666.It Cm EscapeChar 667Sets the escape character (default: 668.Ql ~ ) . 669The escape character can also 670be set on the command line. 671The argument should be a single character, 672.Ql ^ 673followed by a letter, or 674.Cm none 675to disable the escape 676character entirely (making the connection transparent for binary 677data). 678.It Cm ExitOnForwardFailure 679Specifies whether 680.Xr ssh 1 681should terminate the connection if it cannot set up all requested 682dynamic, tunnel, local, and remote port forwardings, (e.g.\& 683if either end is unable to bind and listen on a specified port). 684Note that 685.Cm ExitOnForwardFailure 686does not apply to connections made over port forwardings and will not, 687for example, cause 688.Xr ssh 1 689to exit if TCP connections to the ultimate forwarding destination fail. 690The argument must be 691.Cm yes 692or 693.Cm no 694(the default). 695.It Cm FingerprintHash 696Specifies the hash algorithm used when displaying key fingerprints. 697Valid options are: 698.Cm md5 699and 700.Cm sha256 701(the default). 702.It Cm ForkAfterAuthentication 703Requests 704.Nm ssh 705to go to background just before command execution. 706This is useful if 707.Nm ssh 708is going to ask for passwords or passphrases, but the user 709wants it in the background. 710This implies the 711.Cm StdinNull 712configuration option being set to 713.Dq yes . 714The recommended way to start X11 programs at a remote site is with 715something like 716.Ic ssh -f host xterm , 717which is the same as 718.Ic ssh host xterm 719if the 720.Cm ForkAfterAuthentication 721configuration option is set to 722.Dq yes . 723.Pp 724If the 725.Cm ExitOnForwardFailure 726configuration option is set to 727.Dq yes , 728then a client started with the 729.Cm ForkAfterAuthentication 730configuration option being set to 731.Dq yes 732will wait for all remote port forwards to be successfully established 733before placing itself in the background. 734The argument to this keyword must be 735.Cm yes 736(same as the 737.Fl f 738option) or 739.Cm no 740(the default). 741.It Cm ForwardAgent 742Specifies whether the connection to the authentication agent (if any) 743will be forwarded to the remote machine. 744The argument may be 745.Cm yes , 746.Cm no 747(the default), 748an explicit path to an agent socket or the name of an environment variable 749(beginning with 750.Sq $ ) 751in which to find the path. 752.Pp 753Agent forwarding should be enabled with caution. 754Users with the ability to bypass file permissions on the remote host 755(for the agent's Unix-domain socket) 756can access the local agent through the forwarded connection. 757An attacker cannot obtain key material from the agent, 758however they can perform operations on the keys that enable them to 759authenticate using the identities loaded into the agent. 760.It Cm ForwardX11 761Specifies whether X11 connections will be automatically redirected 762over the secure channel and 763.Ev DISPLAY 764set. 765The argument must be 766.Cm yes 767or 768.Cm no 769(the default). 770.Pp 771X11 forwarding should be enabled with caution. 772Users with the ability to bypass file permissions on the remote host 773(for the user's X11 authorization database) 774can access the local X11 display through the forwarded connection. 775An attacker may then be able to perform activities such as keystroke monitoring 776if the 777.Cm ForwardX11Trusted 778option is also enabled. 779.It Cm ForwardX11Timeout 780Specify a timeout for untrusted X11 forwarding 781using the format described in the 782.Sx TIME FORMATS 783section of 784.Xr sshd_config 5 . 785X11 connections received by 786.Xr ssh 1 787after this time will be refused. 788Setting 789.Cm ForwardX11Timeout 790to zero will disable the timeout and permit X11 forwarding for the life 791of the connection. 792The default is to disable untrusted X11 forwarding after twenty minutes has 793elapsed. 794.It Cm ForwardX11Trusted 795If this option is set to 796.Cm yes , 797remote X11 clients will have full access to the original X11 display. 798.Pp 799If this option is set to 800.Cm no 801(the default), 802remote X11 clients will be considered untrusted and prevented 803from stealing or tampering with data belonging to trusted X11 804clients. 805Furthermore, the 806.Xr xauth 1 807token used for the session will be set to expire after 20 minutes. 808Remote clients will be refused access after this time. 809.Pp 810See the X11 SECURITY extension specification for full details on 811the restrictions imposed on untrusted clients. 812.It Cm GatewayPorts 813Specifies whether remote hosts are allowed to connect to local 814forwarded ports. 815By default, 816.Xr ssh 1 817binds local port forwardings to the loopback address. 818This prevents other remote hosts from connecting to forwarded ports. 819.Cm GatewayPorts 820can be used to specify that ssh 821should bind local port forwardings to the wildcard address, 822thus allowing remote hosts to connect to forwarded ports. 823The argument must be 824.Cm yes 825or 826.Cm no 827(the default). 828.It Cm GlobalKnownHostsFile 829Specifies one or more files to use for the global 830host key database, separated by whitespace. 831The default is 832.Pa /etc/ssh/ssh_known_hosts , 833.Pa /etc/ssh/ssh_known_hosts2 . 834.It Cm GSSAPIAuthentication 835Specifies whether user authentication based on GSSAPI is allowed. 836The default is 837.Cm no . 838.It Cm GSSAPIDelegateCredentials 839Forward (delegate) credentials to the server. 840The default is 841.Cm no . 842.It Cm HashKnownHosts 843Indicates that 844.Xr ssh 1 845should hash host names and addresses when they are added to 846.Pa ~/.ssh/known_hosts . 847These hashed names may be used normally by 848.Xr ssh 1 849and 850.Xr sshd 8 , 851but they do not visually reveal identifying information if the 852file's contents are disclosed. 853The default is 854.Cm no . 855Note that existing names and addresses in known hosts files 856will not be converted automatically, 857but may be manually hashed using 858.Xr ssh-keygen 1 . 859.It Cm HostbasedAcceptedAlgorithms 860Specifies the signature algorithms that will be used for hostbased 861authentication as a comma-separated list of patterns. 862Alternately if the specified list begins with a 863.Sq + 864character, then the specified signature algorithms will be appended 865to the default set instead of replacing them. 866If the specified list begins with a 867.Sq - 868character, then the specified signature algorithms (including wildcards) 869will be removed from the default set instead of replacing them. 870If the specified list begins with a 871.Sq ^ 872character, then the specified signature algorithms will be placed 873at the head of the default set. 874The default for this option is: 875.Bd -literal -offset 3n 876ssh-ed25519-cert-v01@openssh.com, 877ecdsa-sha2-nistp256-cert-v01@openssh.com, 878ecdsa-sha2-nistp384-cert-v01@openssh.com, 879ecdsa-sha2-nistp521-cert-v01@openssh.com, 880sk-ssh-ed25519-cert-v01@openssh.com, 881sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 882rsa-sha2-512-cert-v01@openssh.com, 883rsa-sha2-256-cert-v01@openssh.com, 884ssh-ed25519, 885ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 886sk-ssh-ed25519@openssh.com, 887sk-ecdsa-sha2-nistp256@openssh.com, 888rsa-sha2-512,rsa-sha2-256 889.Ed 890.Pp 891The 892.Fl Q 893option of 894.Xr ssh 1 895may be used to list supported signature algorithms. 896This was formerly named HostbasedKeyTypes. 897.It Cm HostbasedAuthentication 898Specifies whether to try rhosts based authentication with public key 899authentication. 900The argument must be 901.Cm yes 902or 903.Cm no 904(the default). 905.It Cm HostKeyAlgorithms 906Specifies the host key signature algorithms 907that the client wants to use in order of preference. 908Alternately if the specified list begins with a 909.Sq + 910character, then the specified signature algorithms will be appended to 911the default set instead of replacing them. 912If the specified list begins with a 913.Sq - 914character, then the specified signature algorithms (including wildcards) 915will be removed from the default set instead of replacing them. 916If the specified list begins with a 917.Sq ^ 918character, then the specified signature algorithms will be placed 919at the head of the default set. 920The default for this option is: 921.Bd -literal -offset 3n 922ssh-ed25519-cert-v01@openssh.com, 923ecdsa-sha2-nistp256-cert-v01@openssh.com, 924ecdsa-sha2-nistp384-cert-v01@openssh.com, 925ecdsa-sha2-nistp521-cert-v01@openssh.com, 926sk-ssh-ed25519-cert-v01@openssh.com, 927sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 928rsa-sha2-512-cert-v01@openssh.com, 929rsa-sha2-256-cert-v01@openssh.com, 930ssh-ed25519, 931ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 932sk-ecdsa-sha2-nistp256@openssh.com, 933sk-ssh-ed25519@openssh.com, 934rsa-sha2-512,rsa-sha2-256 935.Ed 936.Pp 937If hostkeys are known for the destination host then this default is modified 938to prefer their algorithms. 939.Pp 940The list of available signature algorithms may also be obtained using 941.Qq ssh -Q HostKeyAlgorithms . 942.It Cm HostKeyAlias 943Specifies an alias that should be used instead of the 944real host name when looking up or saving the host key 945in the host key database files and when validating host certificates. 946This option is useful for tunneling SSH connections 947or for multiple servers running on a single host. 948.It Cm Hostname 949Specifies the real host name to log into. 950This can be used to specify nicknames or abbreviations for hosts. 951Arguments to 952.Cm Hostname 953accept the tokens described in the 954.Sx TOKENS 955section. 956Numeric IP addresses are also permitted (both on the command line and in 957.Cm Hostname 958specifications). 959The default is the name given on the command line. 960.It Cm IdentitiesOnly 961Specifies that 962.Xr ssh 1 963should only use the configured authentication identity and certificate files 964(either the default files, or those explicitly configured in the 965.Nm 966files 967or passed on the 968.Xr ssh 1 969command-line), 970even if 971.Xr ssh-agent 1 972or a 973.Cm PKCS11Provider 974or 975.Cm SecurityKeyProvider 976offers more identities. 977The argument to this keyword must be 978.Cm yes 979or 980.Cm no 981(the default). 982This option is intended for situations where ssh-agent 983offers many different identities. 984.It Cm IdentityAgent 985Specifies the 986.Ux Ns -domain 987socket used to communicate with the authentication agent. 988.Pp 989This option overrides the 990.Ev SSH_AUTH_SOCK 991environment variable and can be used to select a specific agent. 992Setting the socket name to 993.Cm none 994disables the use of an authentication agent. 995If the string 996.Qq SSH_AUTH_SOCK 997is specified, the location of the socket will be read from the 998.Ev SSH_AUTH_SOCK 999environment variable. 1000Otherwise if the specified value begins with a 1001.Sq $ 1002character, then it will be treated as an environment variable containing 1003the location of the socket. 1004.Pp 1005Arguments to 1006.Cm IdentityAgent 1007may use the tilde syntax to refer to a user's home directory, 1008the tokens described in the 1009.Sx TOKENS 1010section and environment variables as described in the 1011.Sx ENVIRONMENT VARIABLES 1012section. 1013.It Cm IdentityFile 1014Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1015Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1016The default is 1017.Pa ~/.ssh/id_rsa , 1018.Pa ~/.ssh/id_ecdsa , 1019.Pa ~/.ssh/id_ecdsa_sk , 1020.Pa ~/.ssh/id_ed25519 , 1021.Pa ~/.ssh/id_ed25519_sk 1022and 1023.Pa ~/.ssh/id_dsa . 1024Additionally, any identities represented by the authentication agent 1025will be used for authentication unless 1026.Cm IdentitiesOnly 1027is set. 1028If no certificates have been explicitly specified by 1029.Cm CertificateFile , 1030.Xr ssh 1 1031will try to load certificate information from the filename obtained by 1032appending 1033.Pa -cert.pub 1034to the path of a specified 1035.Cm IdentityFile . 1036.Pp 1037Arguments to 1038.Cm IdentityFile 1039may use the tilde syntax to refer to a user's home directory 1040or the tokens described in the 1041.Sx TOKENS 1042section. 1043.Pp 1044It is possible to have 1045multiple identity files specified in configuration files; all these 1046identities will be tried in sequence. 1047Multiple 1048.Cm IdentityFile 1049directives will add to the list of identities tried (this behaviour 1050differs from that of other configuration directives). 1051.Pp 1052.Cm IdentityFile 1053may be used in conjunction with 1054.Cm IdentitiesOnly 1055to select which identities in an agent are offered during authentication. 1056.Cm IdentityFile 1057may also be used in conjunction with 1058.Cm CertificateFile 1059in order to provide any certificate also needed for authentication with 1060the identity. 1061.It Cm IgnoreUnknown 1062Specifies a pattern-list of unknown options to be ignored if they are 1063encountered in configuration parsing. 1064This may be used to suppress errors if 1065.Nm 1066contains options that are unrecognised by 1067.Xr ssh 1 . 1068It is recommended that 1069.Cm IgnoreUnknown 1070be listed early in the configuration file as it will not be applied 1071to unknown options that appear before it. 1072.It Cm Include 1073Include the specified configuration file(s). 1074Multiple pathnames may be specified and each pathname may contain 1075.Xr glob 7 1076wildcards and, for user configurations, shell-like 1077.Sq ~ 1078references to user home directories. 1079Wildcards will be expanded and processed in lexical order. 1080Files without absolute paths are assumed to be in 1081.Pa ~/.ssh 1082if included in a user configuration file or 1083.Pa /etc/ssh 1084if included from the system configuration file. 1085.Cm Include 1086directive may appear inside a 1087.Cm Match 1088or 1089.Cm Host 1090block 1091to perform conditional inclusion. 1092.It Cm IPQoS 1093Specifies the IPv4 type-of-service or DSCP class for connections. 1094Accepted values are 1095.Cm af11 , 1096.Cm af12 , 1097.Cm af13 , 1098.Cm af21 , 1099.Cm af22 , 1100.Cm af23 , 1101.Cm af31 , 1102.Cm af32 , 1103.Cm af33 , 1104.Cm af41 , 1105.Cm af42 , 1106.Cm af43 , 1107.Cm cs0 , 1108.Cm cs1 , 1109.Cm cs2 , 1110.Cm cs3 , 1111.Cm cs4 , 1112.Cm cs5 , 1113.Cm cs6 , 1114.Cm cs7 , 1115.Cm ef , 1116.Cm le , 1117.Cm lowdelay , 1118.Cm throughput , 1119.Cm reliability , 1120a numeric value, or 1121.Cm none 1122to use the operating system default. 1123This option may take one or two arguments, separated by whitespace. 1124If one argument is specified, it is used as the packet class unconditionally. 1125If two values are specified, the first is automatically selected for 1126interactive sessions and the second for non-interactive sessions. 1127The default is 1128.Cm af21 1129(Low-Latency Data) 1130for interactive sessions and 1131.Cm cs1 1132(Lower Effort) 1133for non-interactive sessions. 1134.It Cm KbdInteractiveAuthentication 1135Specifies whether to use keyboard-interactive authentication. 1136The argument to this keyword must be 1137.Cm yes 1138(the default) 1139or 1140.Cm no . 1141.Cm ChallengeResponseAuthentication 1142is a deprecated alias for this. 1143.It Cm KbdInteractiveDevices 1144Specifies the list of methods to use in keyboard-interactive authentication. 1145Multiple method names must be comma-separated. 1146The default is to use the server specified list. 1147The methods available vary depending on what the server supports. 1148For an OpenSSH server, 1149it may be zero or more of: 1150.Cm bsdauth 1151and 1152.Cm pam . 1153.It Cm KexAlgorithms 1154Specifies the available KEX (Key Exchange) algorithms. 1155Multiple algorithms must be comma-separated. 1156If the specified list begins with a 1157.Sq + 1158character, then the specified algorithms will be appended to the default set 1159instead of replacing them. 1160If the specified list begins with a 1161.Sq - 1162character, then the specified algorithms (including wildcards) will be removed 1163from the default set instead of replacing them. 1164If the specified list begins with a 1165.Sq ^ 1166character, then the specified algorithms will be placed at the head of the 1167default set. 1168The default is: 1169.Bd -literal -offset indent 1170sntrup761x25519-sha512@openssh.com, 1171curve25519-sha256,curve25519-sha256@libssh.org, 1172ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1173diffie-hellman-group-exchange-sha256, 1174diffie-hellman-group16-sha512, 1175diffie-hellman-group18-sha512, 1176diffie-hellman-group14-sha256 1177.Ed 1178.Pp 1179The list of available key exchange algorithms may also be obtained using 1180.Qq ssh -Q kex . 1181.It Cm KnownHostsCommand 1182Specifies a command to use to obtain a list of host keys, in addition to 1183those listed in 1184.Cm UserKnownHostsFile 1185and 1186.Cm GlobalKnownHostsFile . 1187This command is executed after the files have been read. 1188It may write host key lines to standard output in identical format to the 1189usual files (described in the 1190.Sx VERIFYING HOST KEYS 1191section in 1192.Xr ssh 1 ) . 1193Arguments to 1194.Cm KnownHostsCommand 1195accept the tokens described in the 1196.Sx TOKENS 1197section. 1198The command may be invoked multiple times per connection: once when preparing 1199the preference list of host key algorithms to use, again to obtain the 1200host key for the requested host name and, if 1201.Cm CheckHostIP 1202is enabled, one more time to obtain the host key matching the server's 1203address. 1204If the command exits abnormally or returns a non-zero exit status then the 1205connection is terminated. 1206.It Cm LocalCommand 1207Specifies a command to execute on the local machine after successfully 1208connecting to the server. 1209The command string extends to the end of the line, and is executed with 1210the user's shell. 1211Arguments to 1212.Cm LocalCommand 1213accept the tokens described in the 1214.Sx TOKENS 1215section. 1216.Pp 1217The command is run synchronously and does not have access to the 1218session of the 1219.Xr ssh 1 1220that spawned it. 1221It should not be used for interactive commands. 1222.Pp 1223This directive is ignored unless 1224.Cm PermitLocalCommand 1225has been enabled. 1226.It Cm LocalForward 1227Specifies that a TCP port on the local machine be forwarded over 1228the secure channel to the specified host and port from the remote machine. 1229The first argument specifies the listener and may be 1230.Sm off 1231.Oo Ar bind_address : Oc Ar port 1232.Sm on 1233or a Unix domain socket path. 1234The second argument is the destination and may be 1235.Ar host : Ns Ar hostport 1236or a Unix domain socket path if the remote host supports it. 1237.Pp 1238IPv6 addresses can be specified by enclosing addresses in square brackets. 1239Multiple forwardings may be specified, and additional forwardings can be 1240given on the command line. 1241Only the superuser can forward privileged ports. 1242By default, the local port is bound in accordance with the 1243.Cm GatewayPorts 1244setting. 1245However, an explicit 1246.Ar bind_address 1247may be used to bind the connection to a specific address. 1248The 1249.Ar bind_address 1250of 1251.Cm localhost 1252indicates that the listening port be bound for local use only, while an 1253empty address or 1254.Sq * 1255indicates that the port should be available from all interfaces. 1256Unix domain socket paths may use the tokens described in the 1257.Sx TOKENS 1258section and environment variables as described in the 1259.Sx ENVIRONMENT VARIABLES 1260section. 1261.It Cm LogLevel 1262Gives the verbosity level that is used when logging messages from 1263.Xr ssh 1 . 1264The possible values are: 1265QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1266The default is INFO. 1267DEBUG and DEBUG1 are equivalent. 1268DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1269.It Cm LogVerbose 1270Specify one or more overrides to LogLevel. 1271An override consists of a pattern lists that matches the source file, function 1272and line number to force detailed logging for. 1273For example, an override pattern of: 1274.Bd -literal -offset indent 1275kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1276.Ed 1277.Pp 1278would enable detailed logging for line 1000 of 1279.Pa kex.c , 1280everything in the 1281.Fn kex_exchange_identification 1282function, and all code in the 1283.Pa packet.c 1284file. 1285This option is intended for debugging and no overrides are enabled by default. 1286.It Cm MACs 1287Specifies the MAC (message authentication code) algorithms 1288in order of preference. 1289The MAC algorithm is used for data integrity protection. 1290Multiple algorithms must be comma-separated. 1291If the specified list begins with a 1292.Sq + 1293character, then the specified algorithms will be appended to the default set 1294instead of replacing them. 1295If the specified list begins with a 1296.Sq - 1297character, then the specified algorithms (including wildcards) will be removed 1298from the default set instead of replacing them. 1299If the specified list begins with a 1300.Sq ^ 1301character, then the specified algorithms will be placed at the head of the 1302default set. 1303.Pp 1304The algorithms that contain 1305.Qq -etm 1306calculate the MAC after encryption (encrypt-then-mac). 1307These are considered safer and their use recommended. 1308.Pp 1309The default is: 1310.Bd -literal -offset indent 1311umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1312hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1313hmac-sha1-etm@openssh.com, 1314umac-64@openssh.com,umac-128@openssh.com, 1315hmac-sha2-256,hmac-sha2-512,hmac-sha1 1316.Ed 1317.Pp 1318The list of available MAC algorithms may also be obtained using 1319.Qq ssh -Q mac . 1320.It Cm NoHostAuthenticationForLocalhost 1321Disable host authentication for localhost (loopback addresses). 1322The argument to this keyword must be 1323.Cm yes 1324or 1325.Cm no 1326(the default). 1327.It Cm NumberOfPasswordPrompts 1328Specifies the number of password prompts before giving up. 1329The argument to this keyword must be an integer. 1330The default is 3. 1331.It Cm PasswordAuthentication 1332Specifies whether to use password authentication. 1333The argument to this keyword must be 1334.Cm yes 1335(the default) 1336or 1337.Cm no . 1338.It Cm PermitLocalCommand 1339Allow local command execution via the 1340.Ic LocalCommand 1341option or using the 1342.Ic !\& Ns Ar command 1343escape sequence in 1344.Xr ssh 1 . 1345The argument must be 1346.Cm yes 1347or 1348.Cm no 1349(the default). 1350.It Cm PermitRemoteOpen 1351Specifies the destinations to which remote TCP port forwarding is permitted when 1352.Cm RemoteForward 1353is used as a SOCKS proxy. 1354The forwarding specification must be one of the following forms: 1355.Pp 1356.Bl -item -offset indent -compact 1357.It 1358.Cm PermitRemoteOpen 1359.Sm off 1360.Ar host : port 1361.Sm on 1362.It 1363.Cm PermitRemoteOpen 1364.Sm off 1365.Ar IPv4_addr : port 1366.Sm on 1367.It 1368.Cm PermitRemoteOpen 1369.Sm off 1370.Ar \&[ IPv6_addr \&] : port 1371.Sm on 1372.El 1373.Pp 1374Multiple forwards may be specified by separating them with whitespace. 1375An argument of 1376.Cm any 1377can be used to remove all restrictions and permit any forwarding requests. 1378An argument of 1379.Cm none 1380can be used to prohibit all forwarding requests. 1381The wildcard 1382.Sq * 1383can be used for host or port to allow all hosts or ports respectively. 1384Otherwise, no pattern matching or address lookups are performed on supplied 1385names. 1386.It Cm PKCS11Provider 1387Specifies which PKCS#11 provider to use or 1388.Cm none 1389to indicate that no provider should be used (the default). 1390The argument to this keyword is a path to the PKCS#11 shared library 1391.Xr ssh 1 1392should use to communicate with a PKCS#11 token providing keys for user 1393authentication. 1394.It Cm Port 1395Specifies the port number to connect on the remote host. 1396The default is 22. 1397.It Cm PreferredAuthentications 1398Specifies the order in which the client should try authentication methods. 1399This allows a client to prefer one method (e.g.\& 1400.Cm keyboard-interactive ) 1401over another method (e.g.\& 1402.Cm password ) . 1403The default is: 1404.Bd -literal -offset indent 1405gssapi-with-mic,hostbased,publickey, 1406keyboard-interactive,password 1407.Ed 1408.It Cm ProxyCommand 1409Specifies the command to use to connect to the server. 1410The command 1411string extends to the end of the line, and is executed 1412using the user's shell 1413.Ql exec 1414directive to avoid a lingering shell process. 1415.Pp 1416Arguments to 1417.Cm ProxyCommand 1418accept the tokens described in the 1419.Sx TOKENS 1420section. 1421The command can be basically anything, 1422and should read from its standard input and write to its standard output. 1423It should eventually connect an 1424.Xr sshd 8 1425server running on some machine, or execute 1426.Ic sshd -i 1427somewhere. 1428Host key management will be done using the 1429.Cm Hostname 1430of the host being connected (defaulting to the name typed by the user). 1431Setting the command to 1432.Cm none 1433disables this option entirely. 1434Note that 1435.Cm CheckHostIP 1436is not available for connects with a proxy command. 1437.Pp 1438This directive is useful in conjunction with 1439.Xr nc 1 1440and its proxy support. 1441For example, the following directive would connect via an HTTP proxy at 1442192.0.2.0: 1443.Bd -literal -offset 3n 1444ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1445.Ed 1446.It Cm ProxyJump 1447Specifies one or more jump proxies as either 1448.Xo 1449.Sm off 1450.Op Ar user No @ 1451.Ar host 1452.Op : Ns Ar port 1453.Sm on 1454or an ssh URI 1455.Xc . 1456Multiple proxies may be separated by comma characters and will be visited 1457sequentially. 1458Setting this option will cause 1459.Xr ssh 1 1460to connect to the target host by first making a 1461.Xr ssh 1 1462connection to the specified 1463.Cm ProxyJump 1464host and then establishing a 1465TCP forwarding to the ultimate target from there. 1466Setting the host to 1467.Cm none 1468disables this option entirely. 1469.Pp 1470Note that this option will compete with the 1471.Cm ProxyCommand 1472option - whichever is specified first will prevent later instances of the 1473other from taking effect. 1474.Pp 1475Note also that the configuration for the destination host (either supplied 1476via the command-line or the configuration file) is not generally applied 1477to jump hosts. 1478.Pa ~/.ssh/config 1479should be used if specific configuration is required for jump hosts. 1480.It Cm ProxyUseFdpass 1481Specifies that 1482.Cm ProxyCommand 1483will pass a connected file descriptor back to 1484.Xr ssh 1 1485instead of continuing to execute and pass data. 1486The default is 1487.Cm no . 1488.It Cm PubkeyAcceptedAlgorithms 1489Specifies the signature algorithms that will be used for public key 1490authentication as a comma-separated list of patterns. 1491If the specified list begins with a 1492.Sq + 1493character, then the algorithms after it will be appended to the default 1494instead of replacing it. 1495If the specified list begins with a 1496.Sq - 1497character, then the specified algorithms (including wildcards) will be removed 1498from the default set instead of replacing them. 1499If the specified list begins with a 1500.Sq ^ 1501character, then the specified algorithms will be placed at the head of the 1502default set. 1503The default for this option is: 1504.Bd -literal -offset 3n 1505ssh-ed25519-cert-v01@openssh.com, 1506ecdsa-sha2-nistp256-cert-v01@openssh.com, 1507ecdsa-sha2-nistp384-cert-v01@openssh.com, 1508ecdsa-sha2-nistp521-cert-v01@openssh.com, 1509sk-ssh-ed25519-cert-v01@openssh.com, 1510sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1511rsa-sha2-512-cert-v01@openssh.com, 1512rsa-sha2-256-cert-v01@openssh.com, 1513ssh-ed25519, 1514ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1515sk-ssh-ed25519@openssh.com, 1516sk-ecdsa-sha2-nistp256@openssh.com, 1517rsa-sha2-512,rsa-sha2-256 1518.Ed 1519.Pp 1520The list of available signature algorithms may also be obtained using 1521.Qq ssh -Q PubkeyAcceptedAlgorithms . 1522.It Cm PubkeyAuthentication 1523Specifies whether to try public key authentication. 1524The argument to this keyword must be 1525.Cm yes 1526(the default), 1527.Cm no , 1528.Cm unbound 1529or 1530.Cm host-bound . 1531The final two options enable public key authentication while respectively 1532disabling or enabling the OpenSSH host-bound authentication protocol 1533extension required for restricted 1534.Xr ssh-agent 1 1535forwarding. 1536.It Cm RekeyLimit 1537Specifies the maximum amount of data that may be transmitted before the 1538session key is renegotiated, optionally followed by a maximum amount of 1539time that may pass before the session key is renegotiated. 1540The first argument is specified in bytes and may have a suffix of 1541.Sq K , 1542.Sq M , 1543or 1544.Sq G 1545to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1546The default is between 1547.Sq 1G 1548and 1549.Sq 4G , 1550depending on the cipher. 1551The optional second value is specified in seconds and may use any of the 1552units documented in the TIME FORMATS section of 1553.Xr sshd_config 5 . 1554The default value for 1555.Cm RekeyLimit 1556is 1557.Cm default none , 1558which means that rekeying is performed after the cipher's default amount 1559of data has been sent or received and no time based rekeying is done. 1560.It Cm RemoteCommand 1561Specifies a command to execute on the remote machine after successfully 1562connecting to the server. 1563The command string extends to the end of the line, and is executed with 1564the user's shell. 1565Arguments to 1566.Cm RemoteCommand 1567accept the tokens described in the 1568.Sx TOKENS 1569section. 1570.It Cm RemoteForward 1571Specifies that a TCP port on the remote machine be forwarded over 1572the secure channel. 1573The remote port may either be forwarded to a specified host and port 1574from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1575client to connect to arbitrary destinations from the local machine. 1576The first argument is the listening specification and may be 1577.Sm off 1578.Oo Ar bind_address : Oc Ar port 1579.Sm on 1580or, if the remote host supports it, a Unix domain socket path. 1581If forwarding to a specific destination then the second argument must be 1582.Ar host : Ns Ar hostport 1583or a Unix domain socket path, 1584otherwise if no destination argument is specified then the remote forwarding 1585will be established as a SOCKS proxy. 1586When acting as a SOCKS proxy, the destination of the connection can be 1587restricted by 1588.Cm PermitRemoteOpen . 1589.Pp 1590IPv6 addresses can be specified by enclosing addresses in square brackets. 1591Multiple forwardings may be specified, and additional 1592forwardings can be given on the command line. 1593Privileged ports can be forwarded only when 1594logging in as root on the remote machine. 1595Unix domain socket paths may use the tokens described in the 1596.Sx TOKENS 1597section and environment variables as described in the 1598.Sx ENVIRONMENT VARIABLES 1599section. 1600.Pp 1601If the 1602.Ar port 1603argument is 0, 1604the listen port will be dynamically allocated on the server and reported 1605to the client at run time. 1606.Pp 1607If the 1608.Ar bind_address 1609is not specified, the default is to only bind to loopback addresses. 1610If the 1611.Ar bind_address 1612is 1613.Ql * 1614or an empty string, then the forwarding is requested to listen on all 1615interfaces. 1616Specifying a remote 1617.Ar bind_address 1618will only succeed if the server's 1619.Cm GatewayPorts 1620option is enabled (see 1621.Xr sshd_config 5 ) . 1622.It Cm RequestTTY 1623Specifies whether to request a pseudo-tty for the session. 1624The argument may be one of: 1625.Cm no 1626(never request a TTY), 1627.Cm yes 1628(always request a TTY when standard input is a TTY), 1629.Cm force 1630(always request a TTY) or 1631.Cm auto 1632(request a TTY when opening a login session). 1633This option mirrors the 1634.Fl t 1635and 1636.Fl T 1637flags for 1638.Xr ssh 1 . 1639.It Cm RevokedHostKeys 1640Specifies revoked host public keys. 1641Keys listed in this file will be refused for host authentication. 1642Note that if this file does not exist or is not readable, 1643then host authentication will be refused for all hosts. 1644Keys may be specified as a text file, listing one public key per line, or as 1645an OpenSSH Key Revocation List (KRL) as generated by 1646.Xr ssh-keygen 1 . 1647For more information on KRLs, see the KEY REVOCATION LISTS section in 1648.Xr ssh-keygen 1 . 1649.It Cm SecurityKeyProvider 1650Specifies a path to a library that will be used when loading any 1651FIDO authenticator-hosted keys, overriding the default of using 1652the built-in USB HID support. 1653.Pp 1654If the specified value begins with a 1655.Sq $ 1656character, then it will be treated as an environment variable containing 1657the path to the library. 1658.It Cm SendEnv 1659Specifies what variables from the local 1660.Xr environ 7 1661should be sent to the server. 1662The server must also support it, and the server must be configured to 1663accept these environment variables. 1664Note that the 1665.Ev TERM 1666environment variable is always sent whenever a 1667pseudo-terminal is requested as it is required by the protocol. 1668Refer to 1669.Cm AcceptEnv 1670in 1671.Xr sshd_config 5 1672for how to configure the server. 1673Variables are specified by name, which may contain wildcard characters. 1674Multiple environment variables may be separated by whitespace or spread 1675across multiple 1676.Cm SendEnv 1677directives. 1678.Pp 1679See 1680.Sx PATTERNS 1681for more information on patterns. 1682.Pp 1683It is possible to clear previously set 1684.Cm SendEnv 1685variable names by prefixing patterns with 1686.Pa - . 1687The default is not to send any environment variables. 1688.It Cm ServerAliveCountMax 1689Sets the number of server alive messages (see below) which may be 1690sent without 1691.Xr ssh 1 1692receiving any messages back from the server. 1693If this threshold is reached while server alive messages are being sent, 1694ssh will disconnect from the server, terminating the session. 1695It is important to note that the use of server alive messages is very 1696different from 1697.Cm TCPKeepAlive 1698(below). 1699The server alive messages are sent through the encrypted channel 1700and therefore will not be spoofable. 1701The TCP keepalive option enabled by 1702.Cm TCPKeepAlive 1703is spoofable. 1704The server alive mechanism is valuable when the client or 1705server depend on knowing when a connection has become unresponsive. 1706.Pp 1707The default value is 3. 1708If, for example, 1709.Cm ServerAliveInterval 1710(see below) is set to 15 and 1711.Cm ServerAliveCountMax 1712is left at the default, if the server becomes unresponsive, 1713ssh will disconnect after approximately 45 seconds. 1714.It Cm ServerAliveInterval 1715Sets a timeout interval in seconds after which if no data has been received 1716from the server, 1717.Xr ssh 1 1718will send a message through the encrypted 1719channel to request a response from the server. 1720The default 1721is 0, indicating that these messages will not be sent to the server. 1722.It Cm SessionType 1723May be used to either request invocation of a subsystem on the remote system, 1724or to prevent the execution of a remote command at all. 1725The latter is useful for just forwarding ports. 1726The argument to this keyword must be 1727.Cm none 1728(same as the 1729.Fl N 1730option), 1731.Cm subsystem 1732(same as the 1733.Fl s 1734option) or 1735.Cm default 1736(shell or command execution). 1737.It Cm SetEnv 1738Directly specify one or more environment variables and their contents to 1739be sent to the server. 1740Similarly to 1741.Cm SendEnv , 1742with the exception of the 1743.Ev TERM 1744variable, the server must be prepared to accept the environment variable. 1745.It Cm StdinNull 1746Redirects stdin from 1747.Pa /dev/null 1748(actually, prevents reading from stdin). 1749Either this or the equivalent 1750.Fl n 1751option must be used when 1752.Nm ssh 1753is run in the background. 1754The argument to this keyword must be 1755.Cm yes 1756(same as the 1757.Fl n 1758option) or 1759.Cm no 1760(the default). 1761.It Cm StreamLocalBindMask 1762Sets the octal file creation mode mask 1763.Pq umask 1764used when creating a Unix-domain socket file for local or remote 1765port forwarding. 1766This option is only used for port forwarding to a Unix-domain socket file. 1767.Pp 1768The default value is 0177, which creates a Unix-domain socket file that is 1769readable and writable only by the owner. 1770Note that not all operating systems honor the file mode on Unix-domain 1771socket files. 1772.It Cm StreamLocalBindUnlink 1773Specifies whether to remove an existing Unix-domain socket file for local 1774or remote port forwarding before creating a new one. 1775If the socket file already exists and 1776.Cm StreamLocalBindUnlink 1777is not enabled, 1778.Nm ssh 1779will be unable to forward the port to the Unix-domain socket file. 1780This option is only used for port forwarding to a Unix-domain socket file. 1781.Pp 1782The argument must be 1783.Cm yes 1784or 1785.Cm no 1786(the default). 1787.It Cm StrictHostKeyChecking 1788If this flag is set to 1789.Cm yes , 1790.Xr ssh 1 1791will never automatically add host keys to the 1792.Pa ~/.ssh/known_hosts 1793file, and refuses to connect to hosts whose host key has changed. 1794This provides maximum protection against man-in-the-middle (MITM) attacks, 1795though it can be annoying when the 1796.Pa /etc/ssh/ssh_known_hosts 1797file is poorly maintained or when connections to new hosts are 1798frequently made. 1799This option forces the user to manually 1800add all new hosts. 1801.Pp 1802If this flag is set to 1803.Cm accept-new 1804then ssh will automatically add new host keys to the user's 1805.Pa known_hosts 1806file, but will not permit connections to hosts with 1807changed host keys. 1808If this flag is set to 1809.Cm no 1810or 1811.Cm off , 1812ssh will automatically add new host keys to the user known hosts files 1813and allow connections to hosts with changed hostkeys to proceed, 1814subject to some restrictions. 1815If this flag is set to 1816.Cm ask 1817(the default), 1818new host keys 1819will be added to the user known host files only after the user 1820has confirmed that is what they really want to do, and 1821ssh will refuse to connect to hosts whose host key has changed. 1822The host keys of 1823known hosts will be verified automatically in all cases. 1824.It Cm SyslogFacility 1825Gives the facility code that is used when logging messages from 1826.Xr ssh 1 . 1827The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1828LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1829The default is USER. 1830.It Cm TCPKeepAlive 1831Specifies whether the system should send TCP keepalive messages to the 1832other side. 1833If they are sent, death of the connection or crash of one 1834of the machines will be properly noticed. 1835However, this means that 1836connections will die if the route is down temporarily, and some people 1837find it annoying. 1838.Pp 1839The default is 1840.Cm yes 1841(to send TCP keepalive messages), and the client will notice 1842if the network goes down or the remote host dies. 1843This is important in scripts, and many users want it too. 1844.Pp 1845To disable TCP keepalive messages, the value should be set to 1846.Cm no . 1847See also 1848.Cm ServerAliveInterval 1849for protocol-level keepalives. 1850.It Cm Tunnel 1851Request 1852.Xr tun 4 1853device forwarding between the client and the server. 1854The argument must be 1855.Cm yes , 1856.Cm point-to-point 1857(layer 3), 1858.Cm ethernet 1859(layer 2), 1860or 1861.Cm no 1862(the default). 1863Specifying 1864.Cm yes 1865requests the default tunnel mode, which is 1866.Cm point-to-point . 1867.It Cm TunnelDevice 1868Specifies the 1869.Xr tun 4 1870devices to open on the client 1871.Pq Ar local_tun 1872and the server 1873.Pq Ar remote_tun . 1874.Pp 1875The argument must be 1876.Sm off 1877.Ar local_tun Op : Ar remote_tun . 1878.Sm on 1879The devices may be specified by numerical ID or the keyword 1880.Cm any , 1881which uses the next available tunnel device. 1882If 1883.Ar remote_tun 1884is not specified, it defaults to 1885.Cm any . 1886The default is 1887.Cm any:any . 1888.It Cm UpdateHostKeys 1889Specifies whether 1890.Xr ssh 1 1891should accept notifications of additional hostkeys from the server sent 1892after authentication has completed and add them to 1893.Cm UserKnownHostsFile . 1894The argument must be 1895.Cm yes , 1896.Cm no 1897or 1898.Cm ask . 1899This option allows learning alternate hostkeys for a server 1900and supports graceful key rotation by allowing a server to send replacement 1901public keys before old ones are removed. 1902.Pp 1903Additional hostkeys are only accepted if the key used to authenticate the 1904host was already trusted or explicitly accepted by the user, the host was 1905authenticated via 1906.Cm UserKnownHostsFile 1907(i.e. not 1908.Cm GlobalKnownHostsFile ) 1909and the host was authenticated using a plain key and not a certificate. 1910.Pp 1911.Cm UpdateHostKeys 1912is enabled by default if the user has not overridden the default 1913.Cm UserKnownHostsFile 1914setting and has not enabled 1915.Cm VerifyHostKeyDNS , 1916otherwise 1917.Cm UpdateHostKeys 1918will be set to 1919.Cm no . 1920.Pp 1921If 1922.Cm UpdateHostKeys 1923is set to 1924.Cm ask , 1925then the user is asked to confirm the modifications to the known_hosts file. 1926Confirmation is currently incompatible with 1927.Cm ControlPersist , 1928and will be disabled if it is enabled. 1929.Pp 1930Presently, only 1931.Xr sshd 8 1932from OpenSSH 6.8 and greater support the 1933.Qq hostkeys@openssh.com 1934protocol extension used to inform the client of all the server's hostkeys. 1935.It Cm User 1936Specifies the user to log in as. 1937This can be useful when a different user name is used on different machines. 1938This saves the trouble of 1939having to remember to give the user name on the command line. 1940.It Cm UserKnownHostsFile 1941Specifies one or more files to use for the user 1942host key database, separated by whitespace. 1943Each filename may use tilde notation to refer to the user's home directory, 1944the tokens described in the 1945.Sx TOKENS 1946section and environment variables as described in the 1947.Sx ENVIRONMENT VARIABLES 1948section. 1949The default is 1950.Pa ~/.ssh/known_hosts , 1951.Pa ~/.ssh/known_hosts2 . 1952.It Cm VerifyHostKeyDNS 1953Specifies whether to verify the remote key using DNS and SSHFP resource 1954records. 1955If this option is set to 1956.Cm yes , 1957the client will implicitly trust keys that match a secure fingerprint 1958from DNS. 1959Insecure fingerprints will be handled as if this option was set to 1960.Cm ask . 1961If this option is set to 1962.Cm ask , 1963information on fingerprint match will be displayed, but the user will still 1964need to confirm new host keys according to the 1965.Cm StrictHostKeyChecking 1966option. 1967The default is 1968.Cm yes 1969if compiled with LDNS and 1970.Cm no 1971otherwise. 1972.Pp 1973See also 1974.Sx VERIFYING HOST KEYS 1975in 1976.Xr ssh 1 . 1977.It Cm VersionAddendum 1978Specifies a string to append to the regular version string to identify 1979OS- or site-specific modifications. 1980The default is 1981.Dq FreeBSD-20220415 . 1982The value 1983.Cm none 1984may be used to disable this. 1985.It Cm VisualHostKey 1986If this flag is set to 1987.Cm yes , 1988an ASCII art representation of the remote host key fingerprint is 1989printed in addition to the fingerprint string at login and 1990for unknown host keys. 1991If this flag is set to 1992.Cm no 1993(the default), 1994no fingerprint strings are printed at login and 1995only the fingerprint string will be printed for unknown host keys. 1996.It Cm XAuthLocation 1997Specifies the full pathname of the 1998.Xr xauth 1 1999program. 2000The default is 2001.Pa /usr/local/bin/xauth . 2002.El 2003.Sh PATTERNS 2004A 2005.Em pattern 2006consists of zero or more non-whitespace characters, 2007.Sq * 2008(a wildcard that matches zero or more characters), 2009or 2010.Sq ?\& 2011(a wildcard that matches exactly one character). 2012For example, to specify a set of declarations for any host in the 2013.Qq .co.uk 2014set of domains, 2015the following pattern could be used: 2016.Pp 2017.Dl Host *.co.uk 2018.Pp 2019The following pattern 2020would match any host in the 192.168.0.[0-9] network range: 2021.Pp 2022.Dl Host 192.168.0.? 2023.Pp 2024A 2025.Em pattern-list 2026is a comma-separated list of patterns. 2027Patterns within pattern-lists may be negated 2028by preceding them with an exclamation mark 2029.Pq Sq !\& . 2030For example, 2031to allow a key to be used from anywhere within an organization 2032except from the 2033.Qq dialup 2034pool, 2035the following entry (in authorized_keys) could be used: 2036.Pp 2037.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2038.Pp 2039Note that a negated match will never produce a positive result by itself. 2040For example, attempting to match 2041.Qq host3 2042against the following pattern-list will fail: 2043.Pp 2044.Dl from=\&"!host1,!host2\&" 2045.Pp 2046The solution here is to include a term that will yield a positive match, 2047such as a wildcard: 2048.Pp 2049.Dl from=\&"!host1,!host2,*\&" 2050.Sh TOKENS 2051Arguments to some keywords can make use of tokens, 2052which are expanded at runtime: 2053.Pp 2054.Bl -tag -width XXXX -offset indent -compact 2055.It %% 2056A literal 2057.Sq % . 2058.It \&%C 2059Hash of %l%h%p%r. 2060.It %d 2061Local user's home directory. 2062.It %f 2063The fingerprint of the server's host key. 2064.It %H 2065The 2066.Pa known_hosts 2067hostname or address that is being searched for. 2068.It %h 2069The remote hostname. 2070.It \%%I 2071A string describing the reason for a 2072.Cm KnownHostsCommand 2073execution: either 2074.Cm ADDRESS 2075when looking up a host by address (only when 2076.Cm CheckHostIP 2077is enabled), 2078.Cm HOSTNAME 2079when searching by hostname, or 2080.Cm ORDER 2081when preparing the host key algorithm preference list to use for the 2082destination host. 2083.It %i 2084The local user ID. 2085.It %K 2086The base64 encoded host key. 2087.It %k 2088The host key alias if specified, otherwise the original remote hostname given 2089on the command line. 2090.It %L 2091The local hostname. 2092.It %l 2093The local hostname, including the domain name. 2094.It %n 2095The original remote hostname, as given on the command line. 2096.It %p 2097The remote port. 2098.It %r 2099The remote username. 2100.It \&%T 2101The local 2102.Xr tun 4 2103or 2104.Xr tap 4 2105network interface assigned if 2106tunnel forwarding was requested, or 2107.Qq NONE 2108otherwise. 2109.It %t 2110The type of the server host key, e.g. 2111.Cm ssh-ed25519 . 2112.It %u 2113The local username. 2114.El 2115.Pp 2116.Cm CertificateFile , 2117.Cm ControlPath , 2118.Cm IdentityAgent , 2119.Cm IdentityFile , 2120.Cm KnownHostsCommand , 2121.Cm LocalForward , 2122.Cm Match exec , 2123.Cm RemoteCommand , 2124.Cm RemoteForward , 2125and 2126.Cm UserKnownHostsFile 2127accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2128.Pp 2129.Cm KnownHostsCommand 2130additionally accepts the tokens %f, %H, %I, %K and %t. 2131.Pp 2132.Cm Hostname 2133accepts the tokens %% and %h. 2134.Pp 2135.Cm LocalCommand 2136accepts all tokens. 2137.Pp 2138.Cm ProxyCommand 2139accepts the tokens %%, %h, %n, %p, and %r. 2140.Sh ENVIRONMENT VARIABLES 2141Arguments to some keywords can be expanded at runtime from environment 2142variables on the client by enclosing them in 2143.Ic ${} , 2144for example 2145.Ic ${HOME}/.ssh 2146would refer to the user's .ssh directory. 2147If a specified environment variable does not exist then an error will be 2148returned and the setting for that keyword will be ignored. 2149.Pp 2150The keywords 2151.Cm CertificateFile , 2152.Cm ControlPath , 2153.Cm IdentityAgent , 2154.Cm IdentityFile , 2155.Cm KnownHostsCommand , 2156and 2157.Cm UserKnownHostsFile 2158support environment variables. 2159The keywords 2160.Cm LocalForward 2161and 2162.Cm RemoteForward 2163support environment variables only for Unix domain socket paths. 2164.Sh FILES 2165.Bl -tag -width Ds 2166.It Pa ~/.ssh/config 2167This is the per-user configuration file. 2168The format of this file is described above. 2169This file is used by the SSH client. 2170Because of the potential for abuse, this file must have strict permissions: 2171read/write for the user, and not writable by others. 2172.It Pa /etc/ssh/ssh_config 2173Systemwide configuration file. 2174This file provides defaults for those 2175values that are not specified in the user's configuration file, and 2176for those users who do not have a configuration file. 2177This file must be world-readable. 2178.El 2179.Sh SEE ALSO 2180.Xr ssh 1 2181.Sh AUTHORS 2182.An -nosplit 2183OpenSSH is a derivative of the original and free 2184ssh 1.2.12 release by 2185.An Tatu Ylonen . 2186.An Aaron Campbell , Bob Beck , Markus Friedl , 2187.An Niels Provos , Theo de Raadt 2188and 2189.An Dug Song 2190removed many bugs, re-added newer features and 2191created OpenSSH. 2192.An Markus Friedl 2193contributed the support for SSH protocol versions 1.5 and 2.0. 2194