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 1.362 2021/08/12 23:59:25 djm Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: August 12 2021 $ 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH client configuration file 44.Sh DESCRIPTION 45.Xr ssh 1 46obtains configuration data from the following sources in 47the following order: 48.Pp 49.Bl -enum -offset indent -compact 50.It 51command-line options 52.It 53user's configuration file 54.Pq Pa ~/.ssh/config 55.It 56system-wide configuration file 57.Pq Pa /etc/ssh/ssh_config 58.El 59.Pp 60For each parameter, the first obtained value 61will be used. 62The configuration files contain sections separated by 63.Cm Host 64specifications, and that section is only applied for hosts that 65match one of the patterns given in the specification. 66The matched host name is usually the one given on the command line 67(see the 68.Cm CanonicalizeHostname 69option for exceptions). 70.Pp 71Since the first obtained value for each parameter is used, more 72host-specific declarations should be given near the beginning of the 73file, and general defaults at the end. 74.Pp 75The file contains keyword-argument pairs, one per line. 76Lines starting with 77.Ql # 78and empty lines are interpreted as comments. 79Arguments may optionally be enclosed in double quotes 80.Pq \&" 81in order to represent arguments containing spaces. 82Configuration options may be separated by whitespace or 83optional whitespace and exactly one 84.Ql = ; 85the latter format is useful to avoid the need to quote whitespace 86when specifying configuration options using the 87.Nm ssh , 88.Nm scp , 89and 90.Nm sftp 91.Fl o 92option. 93.Pp 94The possible 95keywords and their meanings are as follows (note that 96keywords are case-insensitive and arguments are case-sensitive): 97.Bl -tag -width Ds 98.It Cm Host 99Restricts the following declarations (up to the next 100.Cm Host 101or 102.Cm Match 103keyword) to be only for those hosts that match one of the patterns 104given after the keyword. 105If more than one pattern is provided, they should be separated by whitespace. 106A single 107.Ql * 108as a pattern can be used to provide global 109defaults for all hosts. 110The host is usually the 111.Ar hostname 112argument given on the command line 113(see the 114.Cm CanonicalizeHostname 115keyword for exceptions). 116.Pp 117A pattern entry may be negated by prefixing it with an exclamation mark 118.Pq Sq !\& . 119If a negated entry is matched, then the 120.Cm Host 121entry is ignored, regardless of whether any other patterns on the line 122match. 123Negated matches are therefore useful to provide exceptions for wildcard 124matches. 125.Pp 126See 127.Sx PATTERNS 128for more information on patterns. 129.It Cm Match 130Restricts the following declarations (up to the next 131.Cm Host 132or 133.Cm Match 134keyword) to be used only when the conditions following the 135.Cm Match 136keyword are satisfied. 137Match conditions are specified using one or more criteria 138or the single token 139.Cm all 140which always matches. 141The available criteria keywords are: 142.Cm canonical , 143.Cm final , 144.Cm exec , 145.Cm host , 146.Cm originalhost , 147.Cm user , 148and 149.Cm localuser . 150The 151.Cm all 152criteria must appear alone or immediately after 153.Cm canonical 154or 155.Cm final . 156Other criteria may be combined arbitrarily. 157All criteria but 158.Cm all , 159.Cm canonical , 160and 161.Cm final 162require an argument. 163Criteria may be negated by prepending an exclamation mark 164.Pq Sq !\& . 165.Pp 166The 167.Cm canonical 168keyword matches only when the configuration file is being re-parsed 169after hostname canonicalization (see the 170.Cm CanonicalizeHostname 171option). 172This may be useful to specify conditions that work with canonical host 173names only. 174.Pp 175The 176.Cm final 177keyword requests that the configuration be re-parsed (regardless of whether 178.Cm CanonicalizeHostname 179is enabled), and matches only during this final pass. 180If 181.Cm CanonicalizeHostname 182is enabled, then 183.Cm canonical 184and 185.Cm final 186match during the same pass. 187.Pp 188The 189.Cm exec 190keyword executes the specified command under the user's shell. 191If the command returns a zero exit status then the condition is considered true. 192Commands containing whitespace characters must be quoted. 193Arguments to 194.Cm exec 195accept the tokens described in the 196.Sx TOKENS 197section. 198.Pp 199The other keywords' criteria must be single entries or comma-separated 200lists and may use the wildcard and negation operators described in the 201.Sx PATTERNS 202section. 203The criteria for the 204.Cm host 205keyword are matched against the target hostname, after any substitution 206by the 207.Cm Hostname 208or 209.Cm CanonicalizeHostname 210options. 211The 212.Cm originalhost 213keyword matches against the hostname as it was specified on the command-line. 214The 215.Cm user 216keyword matches against the target username on the remote host. 217The 218.Cm localuser 219keyword matches against the name of the local user running 220.Xr ssh 1 221(this keyword may be useful in system-wide 222.Nm 223files). 224.It Cm AddKeysToAgent 225Specifies whether keys should be automatically added to a running 226.Xr ssh-agent 1 . 227If this option is set to 228.Cm yes 229and a key is loaded from a file, the key and its passphrase are added to 230the agent with the default lifetime, as if by 231.Xr ssh-add 1 . 232If this option is set to 233.Cm ask , 234.Xr ssh 1 235will require confirmation using the 236.Ev SSH_ASKPASS 237program before adding a key (see 238.Xr ssh-add 1 239for details). 240If this option is set to 241.Cm confirm , 242each use of the key must be confirmed, as if the 243.Fl c 244option was specified to 245.Xr ssh-add 1 . 246If this option is set to 247.Cm no , 248no keys are added to the agent. 249Alternately, this option may be specified as a time interval 250using the format described in the 251.Sx TIME FORMATS 252section of 253.Xr sshd_config 5 254to specify the key's lifetime in 255.Xr ssh-agent 1 , 256after which it will automatically be removed. 257The argument must be 258.Cm no 259(the default), 260.Cm yes , 261.Cm confirm 262(optionally followed by a time interval), 263.Cm ask 264or a time interval. 265.It Cm AddressFamily 266Specifies which address family to use when connecting. 267Valid arguments are 268.Cm any 269(the default), 270.Cm inet 271(use IPv4 only), or 272.Cm inet6 273(use IPv6 only). 274.It Cm BatchMode 275If set to 276.Cm yes , 277user interaction such as password prompts and host key confirmation requests 278will be disabled. 279This option is useful in scripts and other batch jobs where no user 280is present to interact with 281.Xr ssh 1 . 282The argument must be 283.Cm yes 284or 285.Cm no 286(the default). 287.It Cm BindAddress 288Use the specified address on the local machine as the source address of 289the connection. 290Only useful on systems with more than one address. 291.It Cm BindInterface 292Use the address of the specified interface on the local machine as the 293source address of the connection. 294.It Cm CanonicalDomains 295When 296.Cm CanonicalizeHostname 297is enabled, this option specifies the list of domain suffixes in which to 298search for the specified destination host. 299.It Cm CanonicalizeFallbackLocal 300Specifies whether to fail with an error when hostname canonicalization fails. 301The default, 302.Cm yes , 303will attempt to look up the unqualified hostname using the system resolver's 304search rules. 305A value of 306.Cm no 307will cause 308.Xr ssh 1 309to fail instantly if 310.Cm CanonicalizeHostname 311is enabled and the target hostname cannot be found in any of the domains 312specified by 313.Cm CanonicalDomains . 314.It Cm CanonicalizeHostname 315Controls whether explicit hostname canonicalization is performed. 316The default, 317.Cm no , 318is not to perform any name rewriting and let the system resolver handle all 319hostname lookups. 320If set to 321.Cm yes 322then, for connections that do not use a 323.Cm ProxyCommand 324or 325.Cm ProxyJump , 326.Xr ssh 1 327will attempt to canonicalize the hostname specified on the command line 328using the 329.Cm CanonicalDomains 330suffixes and 331.Cm CanonicalizePermittedCNAMEs 332rules. 333If 334.Cm CanonicalizeHostname 335is set to 336.Cm always , 337then canonicalization is applied to proxied connections too. 338.Pp 339If this option is enabled, then the configuration files are processed 340again using the new target name to pick up any new configuration in matching 341.Cm Host 342and 343.Cm Match 344stanzas. 345A value of 346.Cm none 347disables the use of a 348.Cm ProxyJump 349host. 350.It Cm CanonicalizeMaxDots 351Specifies the maximum number of dot characters in a hostname before 352canonicalization is disabled. 353The default, 1, 354allows a single dot (i.e. hostname.subdomain). 355.It Cm CanonicalizePermittedCNAMEs 356Specifies rules to determine whether CNAMEs should be followed when 357canonicalizing hostnames. 358The rules consist of one or more arguments of 359.Ar source_domain_list : Ns Ar target_domain_list , 360where 361.Ar source_domain_list 362is a pattern-list of domains that may follow CNAMEs in canonicalization, 363and 364.Ar target_domain_list 365is a pattern-list of domains that they may resolve to. 366.Pp 367For example, 368.Qq *.a.example.com:*.b.example.com,*.c.example.com 369will allow hostnames matching 370.Qq *.a.example.com 371to be canonicalized to names in the 372.Qq *.b.example.com 373or 374.Qq *.c.example.com 375domains. 376.It Cm CASignatureAlgorithms 377Specifies which algorithms are allowed for signing of certificates 378by certificate authorities (CAs). 379The default is: 380.Bd -literal -offset indent 381ssh-ed25519,ecdsa-sha2-nistp256, 382ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 383sk-ssh-ed25519@openssh.com, 384sk-ecdsa-sha2-nistp256@openssh.com, 385rsa-sha2-512,rsa-sha2-256 386.Ed 387.Pp 388If the specified list begins with a 389.Sq + 390character, then the specified algorithms will be appended to the default set 391instead of replacing them. 392If the specified list begins with a 393.Sq - 394character, then the specified algorithms (including wildcards) will be removed 395from the default set instead of replacing them. 396.Pp 397.Xr ssh 1 398will not accept host certificates signed using algorithms other than those 399specified. 400.It Cm CertificateFile 401Specifies a file from which the user's certificate is read. 402A corresponding private key must be provided separately in order 403to use this certificate either 404from an 405.Cm IdentityFile 406directive or 407.Fl i 408flag to 409.Xr ssh 1 , 410via 411.Xr ssh-agent 1 , 412or via a 413.Cm PKCS11Provider 414or 415.Cm SecurityKeyProvider . 416.Pp 417Arguments to 418.Cm CertificateFile 419may use the tilde syntax to refer to a user's home directory, 420the tokens described in the 421.Sx TOKENS 422section and environment variables as described in the 423.Sx ENVIRONMENT VARIABLES 424section. 425.Pp 426It is possible to have multiple certificate files specified in 427configuration files; these certificates will be tried in sequence. 428Multiple 429.Cm CertificateFile 430directives will add to the list of certificates used for 431authentication. 432.It Cm CheckHostIP 433If set to 434.Cm yes , 435.Xr ssh 1 436will additionally check the host IP address in the 437.Pa known_hosts 438file. 439This allows it to detect if a host key changed due to DNS spoofing 440and will add addresses of destination hosts to 441.Pa ~/.ssh/known_hosts 442in the process, regardless of the setting of 443.Cm StrictHostKeyChecking . 444If the option is set to 445.Cm no 446(the default), 447the check will not be executed. 448The default is 449.Cm no . 450.It Cm Ciphers 451Specifies the ciphers allowed and their order of preference. 452Multiple ciphers must be comma-separated. 453If the specified list begins with a 454.Sq + 455character, then the specified ciphers will be appended to the default set 456instead of replacing them. 457If the specified list begins with a 458.Sq - 459character, then the specified ciphers (including wildcards) will be removed 460from the default set instead of replacing them. 461If the specified list begins with a 462.Sq ^ 463character, then the specified ciphers will be placed at the head of the 464default set. 465.Pp 466The supported ciphers are: 467.Bd -literal -offset indent 4683des-cbc 469aes128-cbc 470aes192-cbc 471aes256-cbc 472aes128-ctr 473aes192-ctr 474aes256-ctr 475aes128-gcm@openssh.com 476aes256-gcm@openssh.com 477chacha20-poly1305@openssh.com 478.Ed 479.Pp 480The default is: 481.Bd -literal -offset indent 482chacha20-poly1305@openssh.com, 483aes128-ctr,aes192-ctr,aes256-ctr, 484aes128-gcm@openssh.com,aes256-gcm@openssh.com 485.Ed 486.Pp 487The list of available ciphers may also be obtained using 488.Qq ssh -Q cipher . 489.It Cm ClearAllForwardings 490Specifies that all local, remote, and dynamic port forwardings 491specified in the configuration files or on the command line be 492cleared. 493This option is primarily useful when used from the 494.Xr ssh 1 495command line to clear port forwardings set in 496configuration files, and is automatically set by 497.Xr scp 1 498and 499.Xr sftp 1 . 500The argument must be 501.Cm yes 502or 503.Cm no 504(the default). 505.It Cm Compression 506Specifies whether to use compression. 507The argument must be 508.Cm yes 509or 510.Cm no 511(the default). 512.It Cm ConnectionAttempts 513Specifies the number of tries (one per second) to make before exiting. 514The argument must be an integer. 515This may be useful in scripts if the connection sometimes fails. 516The default is 1. 517.It Cm ConnectTimeout 518Specifies the timeout (in seconds) used when connecting to the 519SSH server, instead of using the default system TCP timeout. 520This timeout is applied both to establishing the connection and to performing 521the initial SSH protocol handshake and key exchange. 522.It Cm ControlMaster 523Enables the sharing of multiple sessions over a single network connection. 524When set to 525.Cm yes , 526.Xr ssh 1 527will listen for connections on a control socket specified using the 528.Cm ControlPath 529argument. 530Additional sessions can connect to this socket using the same 531.Cm ControlPath 532with 533.Cm ControlMaster 534set to 535.Cm no 536(the default). 537These sessions will try to reuse the master instance's network connection 538rather than initiating new ones, but will fall back to connecting normally 539if the control socket does not exist, or is not listening. 540.Pp 541Setting this to 542.Cm ask 543will cause 544.Xr ssh 1 545to listen for control connections, but require confirmation using 546.Xr ssh-askpass 1 . 547If the 548.Cm ControlPath 549cannot be opened, 550.Xr ssh 1 551will continue without connecting to a master instance. 552.Pp 553X11 and 554.Xr ssh-agent 1 555forwarding is supported over these multiplexed connections, however the 556display and agent forwarded will be the one belonging to the master 557connection i.e. it is not possible to forward multiple displays or agents. 558.Pp 559Two additional options allow for opportunistic multiplexing: try to use a 560master connection but fall back to creating a new one if one does not already 561exist. 562These options are: 563.Cm auto 564and 565.Cm autoask . 566The latter requires confirmation like the 567.Cm ask 568option. 569.It Cm ControlPath 570Specify the path to the control socket used for connection sharing as described 571in the 572.Cm ControlMaster 573section above or the string 574.Cm none 575to disable connection sharing. 576Arguments to 577.Cm ControlPath 578may use the tilde syntax to refer to a user's home directory, 579the tokens described in the 580.Sx TOKENS 581section and environment variables as described in the 582.Sx ENVIRONMENT VARIABLES 583section. 584It is recommended that any 585.Cm ControlPath 586used for opportunistic connection sharing include 587at least %h, %p, and %r (or alternatively %C) and be placed in a directory 588that is not writable by other users. 589This ensures that shared connections are uniquely identified. 590.It Cm ControlPersist 591When used in conjunction with 592.Cm ControlMaster , 593specifies that the master connection should remain open 594in the background (waiting for future client connections) 595after the initial client connection has been closed. 596If set to 597.Cm no 598(the default), 599then the master connection will not be placed into the background, 600and will close as soon as the initial client connection is closed. 601If set to 602.Cm yes 603or 0, 604then the master connection will remain in the background indefinitely 605(until killed or closed via a mechanism such as the 606.Qq ssh -O exit ) . 607If set to a time in seconds, or a time in any of the formats documented in 608.Xr sshd_config 5 , 609then the backgrounded master connection will automatically terminate 610after it has remained idle (with no client connections) for the 611specified time. 612.It Cm DynamicForward 613Specifies that a TCP port on the local machine be forwarded 614over the secure channel, and the application 615protocol is then used to determine where to connect to from the 616remote machine. 617.Pp 618The argument must be 619.Sm off 620.Oo Ar bind_address : Oc Ar port . 621.Sm on 622IPv6 addresses can be specified by enclosing addresses in square brackets. 623By default, the local port is bound in accordance with the 624.Cm GatewayPorts 625setting. 626However, an explicit 627.Ar bind_address 628may be used to bind the connection to a specific address. 629The 630.Ar bind_address 631of 632.Cm localhost 633indicates that the listening port be bound for local use only, while an 634empty address or 635.Sq * 636indicates that the port should be available from all interfaces. 637.Pp 638Currently the SOCKS4 and SOCKS5 protocols are supported, and 639.Xr ssh 1 640will act as a SOCKS server. 641Multiple forwardings may be specified, and 642additional forwardings can be given on the command line. 643Only the superuser can forward privileged ports. 644.It Cm EnableSSHKeysign 645Setting this option to 646.Cm yes 647in the global client configuration file 648.Pa /etc/ssh/ssh_config 649enables the use of the helper program 650.Xr ssh-keysign 8 651during 652.Cm HostbasedAuthentication . 653The argument must be 654.Cm yes 655or 656.Cm no 657(the default). 658This option should be placed in the non-hostspecific section. 659See 660.Xr ssh-keysign 8 661for more information. 662.It Cm EscapeChar 663Sets the escape character (default: 664.Ql ~ ) . 665The escape character can also 666be set on the command line. 667The argument should be a single character, 668.Ql ^ 669followed by a letter, or 670.Cm none 671to disable the escape 672character entirely (making the connection transparent for binary 673data). 674.It Cm ExitOnForwardFailure 675Specifies whether 676.Xr ssh 1 677should terminate the connection if it cannot set up all requested 678dynamic, tunnel, local, and remote port forwardings, (e.g.\& 679if either end is unable to bind and listen on a specified port). 680Note that 681.Cm ExitOnForwardFailure 682does not apply to connections made over port forwardings and will not, 683for example, cause 684.Xr ssh 1 685to exit if TCP connections to the ultimate forwarding destination fail. 686The argument must be 687.Cm yes 688or 689.Cm no 690(the default). 691.It Cm FingerprintHash 692Specifies the hash algorithm used when displaying key fingerprints. 693Valid options are: 694.Cm md5 695and 696.Cm sha256 697(the default). 698.It Cm ForkAfterAuthentication 699Requests 700.Nm ssh 701to go to background just before command execution. 702This is useful if 703.Nm ssh 704is going to ask for passwords or passphrases, but the user 705wants it in the background. 706This implies the 707.Cm StdinNull 708configuration option being set to 709.Dq yes . 710The recommended way to start X11 programs at a remote site is with 711something like 712.Ic ssh -f host xterm , 713which is the same as 714.Ic ssh host xterm 715if the 716.Cm ForkAfterAuthentication 717configuration option is set to 718.Dq yes . 719.Pp 720If the 721.Cm ExitOnForwardFailure 722configuration option is set to 723.Dq yes , 724then a client started with the 725.Cm ForkAfterAuthentication 726configuration option being set to 727.Dq yes 728will wait for all remote port forwards to be successfully established 729before placing itself in the background. 730The argument to this keyword must be 731.Cm yes 732(same as the 733.Fl f 734option) or 735.Cm no 736(the default). 737.It Cm ForwardAgent 738Specifies whether the connection to the authentication agent (if any) 739will be forwarded to the remote machine. 740The argument may be 741.Cm yes , 742.Cm no 743(the default), 744an explicit path to an agent socket or the name of an environment variable 745(beginning with 746.Sq $ ) 747in which to find the path. 748.Pp 749Agent forwarding should be enabled with caution. 750Users with the ability to bypass file permissions on the remote host 751(for the agent's Unix-domain socket) 752can access the local agent through the forwarded connection. 753An attacker cannot obtain key material from the agent, 754however they can perform operations on the keys that enable them to 755authenticate using the identities loaded into the agent. 756.It Cm ForwardX11 757Specifies whether X11 connections will be automatically redirected 758over the secure channel and 759.Ev DISPLAY 760set. 761The argument must be 762.Cm yes 763or 764.Cm no 765(the default). 766.Pp 767X11 forwarding should be enabled with caution. 768Users with the ability to bypass file permissions on the remote host 769(for the user's X11 authorization database) 770can access the local X11 display through the forwarded connection. 771An attacker may then be able to perform activities such as keystroke monitoring 772if the 773.Cm ForwardX11Trusted 774option is also enabled. 775.It Cm ForwardX11Timeout 776Specify a timeout for untrusted X11 forwarding 777using the format described in the 778.Sx TIME FORMATS 779section of 780.Xr sshd_config 5 . 781X11 connections received by 782.Xr ssh 1 783after this time will be refused. 784Setting 785.Cm ForwardX11Timeout 786to zero will disable the timeout and permit X11 forwarding for the life 787of the connection. 788The default is to disable untrusted X11 forwarding after twenty minutes has 789elapsed. 790.It Cm ForwardX11Trusted 791If this option is set to 792.Cm yes , 793remote X11 clients will have full access to the original X11 display. 794.Pp 795If this option is set to 796.Cm no 797(the default), 798remote X11 clients will be considered untrusted and prevented 799from stealing or tampering with data belonging to trusted X11 800clients. 801Furthermore, the 802.Xr xauth 1 803token used for the session will be set to expire after 20 minutes. 804Remote clients will be refused access after this time. 805.Pp 806See the X11 SECURITY extension specification for full details on 807the restrictions imposed on untrusted clients. 808.It Cm GatewayPorts 809Specifies whether remote hosts are allowed to connect to local 810forwarded ports. 811By default, 812.Xr ssh 1 813binds local port forwardings to the loopback address. 814This prevents other remote hosts from connecting to forwarded ports. 815.Cm GatewayPorts 816can be used to specify that ssh 817should bind local port forwardings to the wildcard address, 818thus allowing remote hosts to connect to forwarded ports. 819The argument must be 820.Cm yes 821or 822.Cm no 823(the default). 824.It Cm GlobalKnownHostsFile 825Specifies one or more files to use for the global 826host key database, separated by whitespace. 827The default is 828.Pa /etc/ssh/ssh_known_hosts , 829.Pa /etc/ssh/ssh_known_hosts2 . 830.It Cm GSSAPIAuthentication 831Specifies whether user authentication based on GSSAPI is allowed. 832The default is 833.Cm no . 834.It Cm GSSAPIDelegateCredentials 835Forward (delegate) credentials to the server. 836The default is 837.Cm no . 838.It Cm HashKnownHosts 839Indicates that 840.Xr ssh 1 841should hash host names and addresses when they are added to 842.Pa ~/.ssh/known_hosts . 843These hashed names may be used normally by 844.Xr ssh 1 845and 846.Xr sshd 8 , 847but they do not visually reveal identifying information if the 848file's contents are disclosed. 849The default is 850.Cm no . 851Note that existing names and addresses in known hosts files 852will not be converted automatically, 853but may be manually hashed using 854.Xr ssh-keygen 1 . 855.It Cm HostbasedAcceptedAlgorithms 856Specifies the signature algorithms that will be used for hostbased 857authentication as a comma-separated list of patterns. 858Alternately if the specified list begins with a 859.Sq + 860character, then the specified signature algorithms will be appended 861to the default set instead of replacing them. 862If the specified list begins with a 863.Sq - 864character, then the specified signature algorithms (including wildcards) 865will be removed from the default set instead of replacing them. 866If the specified list begins with a 867.Sq ^ 868character, then the specified signature algorithms will be placed 869at the head of the default set. 870The default for this option is: 871.Bd -literal -offset 3n 872ssh-ed25519-cert-v01@openssh.com, 873ecdsa-sha2-nistp256-cert-v01@openssh.com, 874ecdsa-sha2-nistp384-cert-v01@openssh.com, 875ecdsa-sha2-nistp521-cert-v01@openssh.com, 876sk-ssh-ed25519-cert-v01@openssh.com, 877sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 878rsa-sha2-512-cert-v01@openssh.com, 879rsa-sha2-256-cert-v01@openssh.com, 880ssh-rsa-cert-v01@openssh.com, 881ssh-ed25519, 882ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 883sk-ssh-ed25519@openssh.com, 884sk-ecdsa-sha2-nistp256@openssh.com, 885rsa-sha2-512,rsa-sha2-256,ssh-rsa 886.Ed 887.Pp 888The 889.Fl Q 890option of 891.Xr ssh 1 892may be used to list supported signature algorithms. 893This was formerly named HostbasedKeyTypes. 894.It Cm HostbasedAuthentication 895Specifies whether to try rhosts based authentication with public key 896authentication. 897The argument must be 898.Cm yes 899or 900.Cm no 901(the default). 902.It Cm HostKeyAlgorithms 903Specifies the host key signature algorithms 904that the client wants to use in order of preference. 905Alternately if the specified list begins with a 906.Sq + 907character, then the specified signature algorithms will be appended to 908the default set instead of replacing them. 909If the specified list begins with a 910.Sq - 911character, then the specified signature algorithms (including wildcards) 912will be removed from the default set instead of replacing them. 913If the specified list begins with a 914.Sq ^ 915character, then the specified signature algorithms will be placed 916at the head of the default set. 917The default for this option is: 918.Bd -literal -offset 3n 919ssh-ed25519-cert-v01@openssh.com, 920ecdsa-sha2-nistp256-cert-v01@openssh.com, 921ecdsa-sha2-nistp384-cert-v01@openssh.com, 922ecdsa-sha2-nistp521-cert-v01@openssh.com, 923sk-ssh-ed25519-cert-v01@openssh.com, 924sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 925rsa-sha2-512-cert-v01@openssh.com, 926rsa-sha2-256-cert-v01@openssh.com, 927ssh-rsa-cert-v01@openssh.com, 928ssh-ed25519, 929ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 930sk-ecdsa-sha2-nistp256@openssh.com, 931sk-ssh-ed25519@openssh.com, 932rsa-sha2-512,rsa-sha2-256,ssh-rsa 933.Ed 934.Pp 935If hostkeys are known for the destination host then this default is modified 936to prefer their algorithms. 937.Pp 938The list of available signature algorithms may also be obtained using 939.Qq ssh -Q HostKeyAlgorithms . 940.It Cm HostKeyAlias 941Specifies an alias that should be used instead of the 942real host name when looking up or saving the host key 943in the host key database files and when validating host certificates. 944This option is useful for tunneling SSH connections 945or for multiple servers running on a single host. 946.It Cm Hostname 947Specifies the real host name to log into. 948This can be used to specify nicknames or abbreviations for hosts. 949Arguments to 950.Cm Hostname 951accept the tokens described in the 952.Sx TOKENS 953section. 954Numeric IP addresses are also permitted (both on the command line and in 955.Cm Hostname 956specifications). 957The default is the name given on the command line. 958.It Cm IdentitiesOnly 959Specifies that 960.Xr ssh 1 961should only use the configured authentication identity and certificate files 962(either the default files, or those explicitly configured in the 963.Nm 964files 965or passed on the 966.Xr ssh 1 967command-line), 968even if 969.Xr ssh-agent 1 970or a 971.Cm PKCS11Provider 972or 973.Cm SecurityKeyProvider 974offers more identities. 975The argument to this keyword must be 976.Cm yes 977or 978.Cm no 979(the default). 980This option is intended for situations where ssh-agent 981offers many different identities. 982.It Cm IdentityAgent 983Specifies the 984.Ux Ns -domain 985socket used to communicate with the authentication agent. 986.Pp 987This option overrides the 988.Ev SSH_AUTH_SOCK 989environment variable and can be used to select a specific agent. 990Setting the socket name to 991.Cm none 992disables the use of an authentication agent. 993If the string 994.Qq SSH_AUTH_SOCK 995is specified, the location of the socket will be read from the 996.Ev SSH_AUTH_SOCK 997environment variable. 998Otherwise if the specified value begins with a 999.Sq $ 1000character, then it will be treated as an environment variable containing 1001the location of the socket. 1002.Pp 1003Arguments to 1004.Cm IdentityAgent 1005may use the tilde syntax to refer to a user's home directory, 1006the tokens described in the 1007.Sx TOKENS 1008section and environment variables as described in the 1009.Sx ENVIRONMENT VARIABLES 1010section. 1011.It Cm IdentityFile 1012Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1013Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1014The default is 1015.Pa ~/.ssh/id_dsa , 1016.Pa ~/.ssh/id_ecdsa , 1017.Pa ~/.ssh/id_ecdsa_sk , 1018.Pa ~/.ssh/id_ed25519 , 1019.Pa ~/.ssh/id_ed25519_sk 1020and 1021.Pa ~/.ssh/id_rsa . 1022Additionally, any identities represented by the authentication agent 1023will be used for authentication unless 1024.Cm IdentitiesOnly 1025is set. 1026If no certificates have been explicitly specified by 1027.Cm CertificateFile , 1028.Xr ssh 1 1029will try to load certificate information from the filename obtained by 1030appending 1031.Pa -cert.pub 1032to the path of a specified 1033.Cm IdentityFile . 1034.Pp 1035Arguments to 1036.Cm IdentityFile 1037may use the tilde syntax to refer to a user's home directory 1038or the tokens described in the 1039.Sx TOKENS 1040section. 1041.Pp 1042It is possible to have 1043multiple identity files specified in configuration files; all these 1044identities will be tried in sequence. 1045Multiple 1046.Cm IdentityFile 1047directives will add to the list of identities tried (this behaviour 1048differs from that of other configuration directives). 1049.Pp 1050.Cm IdentityFile 1051may be used in conjunction with 1052.Cm IdentitiesOnly 1053to select which identities in an agent are offered during authentication. 1054.Cm IdentityFile 1055may also be used in conjunction with 1056.Cm CertificateFile 1057in order to provide any certificate also needed for authentication with 1058the identity. 1059.It Cm IgnoreUnknown 1060Specifies a pattern-list of unknown options to be ignored if they are 1061encountered in configuration parsing. 1062This may be used to suppress errors if 1063.Nm 1064contains options that are unrecognised by 1065.Xr ssh 1 . 1066It is recommended that 1067.Cm IgnoreUnknown 1068be listed early in the configuration file as it will not be applied 1069to unknown options that appear before it. 1070.It Cm Include 1071Include the specified configuration file(s). 1072Multiple pathnames may be specified and each pathname may contain 1073.Xr glob 7 1074wildcards and, for user configurations, shell-like 1075.Sq ~ 1076references to user home directories. 1077Wildcards will be expanded and processed in lexical order. 1078Files without absolute paths are assumed to be in 1079.Pa ~/.ssh 1080if included in a user configuration file or 1081.Pa /etc/ssh 1082if included from the system configuration file. 1083.Cm Include 1084directive may appear inside a 1085.Cm Match 1086or 1087.Cm Host 1088block 1089to perform conditional inclusion. 1090.It Cm IPQoS 1091Specifies the IPv4 type-of-service or DSCP class for connections. 1092Accepted values are 1093.Cm af11 , 1094.Cm af12 , 1095.Cm af13 , 1096.Cm af21 , 1097.Cm af22 , 1098.Cm af23 , 1099.Cm af31 , 1100.Cm af32 , 1101.Cm af33 , 1102.Cm af41 , 1103.Cm af42 , 1104.Cm af43 , 1105.Cm cs0 , 1106.Cm cs1 , 1107.Cm cs2 , 1108.Cm cs3 , 1109.Cm cs4 , 1110.Cm cs5 , 1111.Cm cs6 , 1112.Cm cs7 , 1113.Cm ef , 1114.Cm le , 1115.Cm lowdelay , 1116.Cm throughput , 1117.Cm reliability , 1118a numeric value, or 1119.Cm none 1120to use the operating system default. 1121This option may take one or two arguments, separated by whitespace. 1122If one argument is specified, it is used as the packet class unconditionally. 1123If two values are specified, the first is automatically selected for 1124interactive sessions and the second for non-interactive sessions. 1125The default is 1126.Cm af21 1127(Low-Latency Data) 1128for interactive sessions and 1129.Cm cs1 1130(Lower Effort) 1131for non-interactive sessions. 1132.It Cm KbdInteractiveAuthentication 1133Specifies whether to use keyboard-interactive authentication. 1134The argument to this keyword must be 1135.Cm yes 1136(the default) 1137or 1138.Cm no . 1139.Cm ChallengeResponseAuthentication 1140is a deprecated alias for this. 1141.It Cm KbdInteractiveDevices 1142Specifies the list of methods to use in keyboard-interactive authentication. 1143Multiple method names must be comma-separated. 1144The default is to use the server specified list. 1145The methods available vary depending on what the server supports. 1146For an OpenSSH server, 1147it may be zero or more of: 1148.Cm bsdauth 1149and 1150.Cm pam . 1151.It Cm KexAlgorithms 1152Specifies the available KEX (Key Exchange) algorithms. 1153Multiple algorithms must be comma-separated. 1154If the specified list begins with a 1155.Sq + 1156character, then the specified methods will be appended to the default set 1157instead of replacing them. 1158If the specified list begins with a 1159.Sq - 1160character, then the specified methods (including wildcards) will be removed 1161from the default set instead of replacing them. 1162If the specified list begins with a 1163.Sq ^ 1164character, then the specified methods will be placed at the head of the 1165default set. 1166The default is: 1167.Bd -literal -offset indent 1168curve25519-sha256,curve25519-sha256@libssh.org, 1169ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1170diffie-hellman-group-exchange-sha256, 1171diffie-hellman-group16-sha512, 1172diffie-hellman-group18-sha512, 1173diffie-hellman-group14-sha256 1174.Ed 1175.Pp 1176The list of available key exchange algorithms may also be obtained using 1177.Qq ssh -Q kex . 1178.It Cm KnownHostsCommand 1179Specifies a command to use to obtain a list of host keys, in addition to 1180those listed in 1181.Cm UserKnownHostsFile 1182and 1183.Cm GlobalKnownHostsFile . 1184This command is executed after the files have been read. 1185It may write host key lines to standard output in identical format to the 1186usual files (described in the 1187.Sx VERIFYING HOST KEYS 1188section in 1189.Xr ssh 1 ) . 1190Arguments to 1191.Cm KnownHostsCommand 1192accept the tokens described in the 1193.Sx TOKENS 1194section. 1195The command may be invoked multiple times per connection: once when preparing 1196the preference list of host key algorithms to use, again to obtain the 1197host key for the requested host name and, if 1198.Cm CheckHostIP 1199is enabled, one more time to obtain the host key matching the server's 1200address. 1201If the command exits abnormally or returns a non-zero exit status then the 1202connection is terminated. 1203.It Cm LocalCommand 1204Specifies a command to execute on the local machine after successfully 1205connecting to the server. 1206The command string extends to the end of the line, and is executed with 1207the user's shell. 1208Arguments to 1209.Cm LocalCommand 1210accept the tokens described in the 1211.Sx TOKENS 1212section. 1213.Pp 1214The command is run synchronously and does not have access to the 1215session of the 1216.Xr ssh 1 1217that spawned it. 1218It should not be used for interactive commands. 1219.Pp 1220This directive is ignored unless 1221.Cm PermitLocalCommand 1222has been enabled. 1223.It Cm LocalForward 1224Specifies that a TCP port on the local machine be forwarded over 1225the secure channel to the specified host and port from the remote machine. 1226The first argument specifies the listener and may be 1227.Sm off 1228.Oo Ar bind_address : Oc Ar port 1229.Sm on 1230or a Unix domain socket path. 1231The second argument is the destination and may be 1232.Ar host : Ns Ar hostport 1233or a Unix domain socket path if the remote host supports it. 1234.Pp 1235IPv6 addresses can be specified by enclosing addresses in square brackets. 1236Multiple forwardings may be specified, and additional forwardings can be 1237given on the command line. 1238Only the superuser can forward privileged ports. 1239By default, the local port is bound in accordance with the 1240.Cm GatewayPorts 1241setting. 1242However, an explicit 1243.Ar bind_address 1244may be used to bind the connection to a specific address. 1245The 1246.Ar bind_address 1247of 1248.Cm localhost 1249indicates that the listening port be bound for local use only, while an 1250empty address or 1251.Sq * 1252indicates that the port should be available from all interfaces. 1253Unix domain socket paths may use the tokens described in the 1254.Sx TOKENS 1255section and environment variables as described in the 1256.Sx ENVIRONMENT VARIABLES 1257section. 1258.It Cm LogLevel 1259Gives the verbosity level that is used when logging messages from 1260.Xr ssh 1 . 1261The possible values are: 1262QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1263The default is INFO. 1264DEBUG and DEBUG1 are equivalent. 1265DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1266.It Cm LogVerbose 1267Specify one or more overrides to LogLevel. 1268An override consists of a pattern lists that matches the source file, function 1269and line number to force detailed logging for. 1270For example, an override pattern of: 1271.Bd -literal -offset indent 1272kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1273.Ed 1274.Pp 1275would enable detailed logging for line 1000 of 1276.Pa kex.c , 1277everything in the 1278.Fn kex_exchange_identification 1279function, and all code in the 1280.Pa packet.c 1281file. 1282This option is intended for debugging and no overrides are enabled by default. 1283.It Cm MACs 1284Specifies the MAC (message authentication code) algorithms 1285in order of preference. 1286The MAC algorithm is used for data integrity protection. 1287Multiple algorithms must be comma-separated. 1288If the specified list begins with a 1289.Sq + 1290character, then the specified algorithms will be appended to the default set 1291instead of replacing them. 1292If the specified list begins with a 1293.Sq - 1294character, then the specified algorithms (including wildcards) will be removed 1295from the default set instead of replacing them. 1296If the specified list begins with a 1297.Sq ^ 1298character, then the specified algorithms will be placed at the head of the 1299default set. 1300.Pp 1301The algorithms that contain 1302.Qq -etm 1303calculate the MAC after encryption (encrypt-then-mac). 1304These are considered safer and their use recommended. 1305.Pp 1306The default is: 1307.Bd -literal -offset indent 1308umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1309hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1310hmac-sha1-etm@openssh.com, 1311umac-64@openssh.com,umac-128@openssh.com, 1312hmac-sha2-256,hmac-sha2-512,hmac-sha1 1313.Ed 1314.Pp 1315The list of available MAC algorithms may also be obtained using 1316.Qq ssh -Q mac . 1317.It Cm NoHostAuthenticationForLocalhost 1318Disable host authentication for localhost (loopback addresses). 1319The argument to this keyword must be 1320.Cm yes 1321or 1322.Cm no 1323(the default). 1324.It Cm NumberOfPasswordPrompts 1325Specifies the number of password prompts before giving up. 1326The argument to this keyword must be an integer. 1327The default is 3. 1328.It Cm PasswordAuthentication 1329Specifies whether to use password authentication. 1330The argument to this keyword must be 1331.Cm yes 1332(the default) 1333or 1334.Cm no . 1335.It Cm PermitLocalCommand 1336Allow local command execution via the 1337.Ic LocalCommand 1338option or using the 1339.Ic !\& Ns Ar command 1340escape sequence in 1341.Xr ssh 1 . 1342The argument must be 1343.Cm yes 1344or 1345.Cm no 1346(the default). 1347.It Cm PermitRemoteOpen 1348Specifies the destinations to which remote TCP port forwarding is permitted when 1349.Cm RemoteForward 1350is used as a SOCKS proxy. 1351The forwarding specification must be one of the following forms: 1352.Pp 1353.Bl -item -offset indent -compact 1354.It 1355.Cm PermitRemoteOpen 1356.Sm off 1357.Ar host : port 1358.Sm on 1359.It 1360.Cm PermitRemoteOpen 1361.Sm off 1362.Ar IPv4_addr : port 1363.Sm on 1364.It 1365.Cm PermitRemoteOpen 1366.Sm off 1367.Ar \&[ IPv6_addr \&] : port 1368.Sm on 1369.El 1370.Pp 1371Multiple forwards may be specified by separating them with whitespace. 1372An argument of 1373.Cm any 1374can be used to remove all restrictions and permit any forwarding requests. 1375An argument of 1376.Cm none 1377can be used to prohibit all forwarding requests. 1378The wildcard 1379.Sq * 1380can be used for host or port to allow all hosts or ports respectively. 1381Otherwise, no pattern matching or address lookups are performed on supplied 1382names. 1383.It Cm PKCS11Provider 1384Specifies which PKCS#11 provider to use or 1385.Cm none 1386to indicate that no provider should be used (the default). 1387The argument to this keyword is a path to the PKCS#11 shared library 1388.Xr ssh 1 1389should use to communicate with a PKCS#11 token providing keys for user 1390authentication. 1391.It Cm Port 1392Specifies the port number to connect on the remote host. 1393The default is 22. 1394.It Cm PreferredAuthentications 1395Specifies the order in which the client should try authentication methods. 1396This allows a client to prefer one method (e.g.\& 1397.Cm keyboard-interactive ) 1398over another method (e.g.\& 1399.Cm password ) . 1400The default is: 1401.Bd -literal -offset indent 1402gssapi-with-mic,hostbased,publickey, 1403keyboard-interactive,password 1404.Ed 1405.It Cm ProxyCommand 1406Specifies the command to use to connect to the server. 1407The command 1408string extends to the end of the line, and is executed 1409using the user's shell 1410.Ql exec 1411directive to avoid a lingering shell process. 1412.Pp 1413Arguments to 1414.Cm ProxyCommand 1415accept the tokens described in the 1416.Sx TOKENS 1417section. 1418The command can be basically anything, 1419and should read from its standard input and write to its standard output. 1420It should eventually connect an 1421.Xr sshd 8 1422server running on some machine, or execute 1423.Ic sshd -i 1424somewhere. 1425Host key management will be done using the 1426.Cm Hostname 1427of the host being connected (defaulting to the name typed by the user). 1428Setting the command to 1429.Cm none 1430disables this option entirely. 1431Note that 1432.Cm CheckHostIP 1433is not available for connects with a proxy command. 1434.Pp 1435This directive is useful in conjunction with 1436.Xr nc 1 1437and its proxy support. 1438For example, the following directive would connect via an HTTP proxy at 1439192.0.2.0: 1440.Bd -literal -offset 3n 1441ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1442.Ed 1443.It Cm ProxyJump 1444Specifies one or more jump proxies as either 1445.Xo 1446.Sm off 1447.Op Ar user No @ 1448.Ar host 1449.Op : Ns Ar port 1450.Sm on 1451or an ssh URI 1452.Xc . 1453Multiple proxies may be separated by comma characters and will be visited 1454sequentially. 1455Setting this option will cause 1456.Xr ssh 1 1457to connect to the target host by first making a 1458.Xr ssh 1 1459connection to the specified 1460.Cm ProxyJump 1461host and then establishing a 1462TCP forwarding to the ultimate target from there. 1463Setting the host to 1464.Cm none 1465disables this option entirely. 1466.Pp 1467Note that this option will compete with the 1468.Cm ProxyCommand 1469option - whichever is specified first will prevent later instances of the 1470other from taking effect. 1471.Pp 1472Note also that the configuration for the destination host (either supplied 1473via the command-line or the configuration file) is not generally applied 1474to jump hosts. 1475.Pa ~/.ssh/config 1476should be used if specific configuration is required for jump hosts. 1477.It Cm ProxyUseFdpass 1478Specifies that 1479.Cm ProxyCommand 1480will pass a connected file descriptor back to 1481.Xr ssh 1 1482instead of continuing to execute and pass data. 1483The default is 1484.Cm no . 1485.It Cm PubkeyAcceptedAlgorithms 1486Specifies the signature algorithms that will be used for public key 1487authentication as a comma-separated list of patterns. 1488If the specified list begins with a 1489.Sq + 1490character, then the algorithms after it will be appended to the default 1491instead of replacing it. 1492If the specified list begins with a 1493.Sq - 1494character, then the specified algorithms (including wildcards) will be removed 1495from the default set instead of replacing them. 1496If the specified list begins with a 1497.Sq ^ 1498character, then the specified algorithms will be placed at the head of the 1499default set. 1500The default for this option is: 1501.Bd -literal -offset 3n 1502ssh-ed25519-cert-v01@openssh.com, 1503ecdsa-sha2-nistp256-cert-v01@openssh.com, 1504ecdsa-sha2-nistp384-cert-v01@openssh.com, 1505ecdsa-sha2-nistp521-cert-v01@openssh.com, 1506sk-ssh-ed25519-cert-v01@openssh.com, 1507sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1508rsa-sha2-512-cert-v01@openssh.com, 1509rsa-sha2-256-cert-v01@openssh.com, 1510ssh-rsa-cert-v01@openssh.com, 1511ssh-ed25519, 1512ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1513sk-ssh-ed25519@openssh.com, 1514sk-ecdsa-sha2-nistp256@openssh.com, 1515rsa-sha2-512,rsa-sha2-256,ssh-rsa 1516.Ed 1517.Pp 1518The list of available signature algorithms may also be obtained using 1519.Qq ssh -Q PubkeyAcceptedAlgorithms . 1520.It Cm PubkeyAuthentication 1521Specifies whether to try public key authentication. 1522The argument to this keyword must be 1523.Cm yes 1524(the default) 1525or 1526.Cm no . 1527.It Cm RekeyLimit 1528Specifies the maximum amount of data that may be transmitted before the 1529session key is renegotiated, optionally followed by a maximum amount of 1530time that may pass before the session key is renegotiated. 1531The first argument is specified in bytes and may have a suffix of 1532.Sq K , 1533.Sq M , 1534or 1535.Sq G 1536to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1537The default is between 1538.Sq 1G 1539and 1540.Sq 4G , 1541depending on the cipher. 1542The optional second value is specified in seconds and may use any of the 1543units documented in the TIME FORMATS section of 1544.Xr sshd_config 5 . 1545The default value for 1546.Cm RekeyLimit 1547is 1548.Cm default none , 1549which means that rekeying is performed after the cipher's default amount 1550of data has been sent or received and no time based rekeying is done. 1551.It Cm RemoteCommand 1552Specifies a command to execute on the remote machine after successfully 1553connecting to the server. 1554The command string extends to the end of the line, and is executed with 1555the user's shell. 1556Arguments to 1557.Cm RemoteCommand 1558accept the tokens described in the 1559.Sx TOKENS 1560section. 1561.It Cm RemoteForward 1562Specifies that a TCP port on the remote machine be forwarded over 1563the secure channel. 1564The remote port may either be forwarded to a specified host and port 1565from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1566client to connect to arbitrary destinations from the local machine. 1567The first argument is the listening specification and may be 1568.Sm off 1569.Oo Ar bind_address : Oc Ar port 1570.Sm on 1571or, if the remote host supports it, a Unix domain socket path. 1572If forwarding to a specific destination then the second argument must be 1573.Ar host : Ns Ar hostport 1574or a Unix domain socket path, 1575otherwise if no destination argument is specified then the remote forwarding 1576will be established as a SOCKS proxy. 1577When acting as a SOCKS proxy the destination of the connection can be 1578restricted by 1579.Cm PermitRemoteOpen . 1580.Pp 1581IPv6 addresses can be specified by enclosing addresses in square brackets. 1582Multiple forwardings may be specified, and additional 1583forwardings can be given on the command line. 1584Privileged ports can be forwarded only when 1585logging in as root on the remote machine. 1586Unix domain socket paths may use the tokens described in the 1587.Sx TOKENS 1588section and environment variables as described in the 1589.Sx ENVIRONMENT VARIABLES 1590section. 1591.Pp 1592If the 1593.Ar port 1594argument is 0, 1595the listen port will be dynamically allocated on the server and reported 1596to the client at run time. 1597.Pp 1598If the 1599.Ar bind_address 1600is not specified, the default is to only bind to loopback addresses. 1601If the 1602.Ar bind_address 1603is 1604.Ql * 1605or an empty string, then the forwarding is requested to listen on all 1606interfaces. 1607Specifying a remote 1608.Ar bind_address 1609will only succeed if the server's 1610.Cm GatewayPorts 1611option is enabled (see 1612.Xr sshd_config 5 ) . 1613.It Cm RequestTTY 1614Specifies whether to request a pseudo-tty for the session. 1615The argument may be one of: 1616.Cm no 1617(never request a TTY), 1618.Cm yes 1619(always request a TTY when standard input is a TTY), 1620.Cm force 1621(always request a TTY) or 1622.Cm auto 1623(request a TTY when opening a login session). 1624This option mirrors the 1625.Fl t 1626and 1627.Fl T 1628flags for 1629.Xr ssh 1 . 1630.It Cm RevokedHostKeys 1631Specifies revoked host public keys. 1632Keys listed in this file will be refused for host authentication. 1633Note that if this file does not exist or is not readable, 1634then host authentication will be refused for all hosts. 1635Keys may be specified as a text file, listing one public key per line, or as 1636an OpenSSH Key Revocation List (KRL) as generated by 1637.Xr ssh-keygen 1 . 1638For more information on KRLs, see the KEY REVOCATION LISTS section in 1639.Xr ssh-keygen 1 . 1640.It Cm SecurityKeyProvider 1641Specifies a path to a library that will be used when loading any 1642FIDO authenticator-hosted keys, overriding the default of using 1643the built-in USB HID support. 1644.Pp 1645If the specified value begins with a 1646.Sq $ 1647character, then it will be treated as an environment variable containing 1648the path to the library. 1649.It Cm SendEnv 1650Specifies what variables from the local 1651.Xr environ 7 1652should be sent to the server. 1653The server must also support it, and the server must be configured to 1654accept these environment variables. 1655Note that the 1656.Ev TERM 1657environment variable is always sent whenever a 1658pseudo-terminal is requested as it is required by the protocol. 1659Refer to 1660.Cm AcceptEnv 1661in 1662.Xr sshd_config 5 1663for how to configure the server. 1664Variables are specified by name, which may contain wildcard characters. 1665Multiple environment variables may be separated by whitespace or spread 1666across multiple 1667.Cm SendEnv 1668directives. 1669.Pp 1670See 1671.Sx PATTERNS 1672for more information on patterns. 1673.Pp 1674It is possible to clear previously set 1675.Cm SendEnv 1676variable names by prefixing patterns with 1677.Pa - . 1678The default is not to send any environment variables. 1679.It Cm ServerAliveCountMax 1680Sets the number of server alive messages (see below) which may be 1681sent without 1682.Xr ssh 1 1683receiving any messages back from the server. 1684If this threshold is reached while server alive messages are being sent, 1685ssh will disconnect from the server, terminating the session. 1686It is important to note that the use of server alive messages is very 1687different from 1688.Cm TCPKeepAlive 1689(below). 1690The server alive messages are sent through the encrypted channel 1691and therefore will not be spoofable. 1692The TCP keepalive option enabled by 1693.Cm TCPKeepAlive 1694is spoofable. 1695The server alive mechanism is valuable when the client or 1696server depend on knowing when a connection has become unresponsive. 1697.Pp 1698The default value is 3. 1699If, for example, 1700.Cm ServerAliveInterval 1701(see below) is set to 15 and 1702.Cm ServerAliveCountMax 1703is left at the default, if the server becomes unresponsive, 1704ssh will disconnect after approximately 45 seconds. 1705.It Cm ServerAliveInterval 1706Sets a timeout interval in seconds after which if no data has been received 1707from the server, 1708.Xr ssh 1 1709will send a message through the encrypted 1710channel to request a response from the server. 1711The default 1712is 0, indicating that these messages will not be sent to the server. 1713.It Cm SessionType 1714May be used to either request invocation of a subsystem on the remote system, 1715or to prevent the execution of a remote command at all. 1716The latter is useful for just forwarding ports. 1717The argument to this keyword must be 1718.Cm none 1719(same as the 1720.Fl N 1721option), 1722.Cm subsystem 1723(same as the 1724.Fl s 1725option) or 1726.Cm default 1727(shell or command execution). 1728.It Cm SetEnv 1729Directly specify one or more environment variables and their contents to 1730be sent to the server. 1731Similarly to 1732.Cm SendEnv , 1733with the exception of the 1734.Ev TERM 1735variable, the server must be prepared to accept the environment variable. 1736.It Cm StdinNull 1737Redirects stdin from 1738.Pa /dev/null 1739(actually, prevents reading from stdin). 1740Either this or the equivalent 1741.Fl n 1742option must be used when 1743.Nm ssh 1744is run in the background. 1745The argument to this keyword must be 1746.Cm yes 1747(same as the 1748.Fl n 1749option) or 1750.Cm no 1751(the default). 1752.It Cm StreamLocalBindMask 1753Sets the octal file creation mode mask 1754.Pq umask 1755used when creating a Unix-domain socket file for local or remote 1756port forwarding. 1757This option is only used for port forwarding to a Unix-domain socket file. 1758.Pp 1759The default value is 0177, which creates a Unix-domain socket file that is 1760readable and writable only by the owner. 1761Note that not all operating systems honor the file mode on Unix-domain 1762socket files. 1763.It Cm StreamLocalBindUnlink 1764Specifies whether to remove an existing Unix-domain socket file for local 1765or remote port forwarding before creating a new one. 1766If the socket file already exists and 1767.Cm StreamLocalBindUnlink 1768is not enabled, 1769.Nm ssh 1770will be unable to forward the port to the Unix-domain socket file. 1771This option is only used for port forwarding to a Unix-domain socket file. 1772.Pp 1773The argument must be 1774.Cm yes 1775or 1776.Cm no 1777(the default). 1778.It Cm StrictHostKeyChecking 1779If this flag is set to 1780.Cm yes , 1781.Xr ssh 1 1782will never automatically add host keys to the 1783.Pa ~/.ssh/known_hosts 1784file, and refuses to connect to hosts whose host key has changed. 1785This provides maximum protection against man-in-the-middle (MITM) attacks, 1786though it can be annoying when the 1787.Pa /etc/ssh/ssh_known_hosts 1788file is poorly maintained or when connections to new hosts are 1789frequently made. 1790This option forces the user to manually 1791add all new hosts. 1792.Pp 1793If this flag is set to 1794.Dq accept-new 1795then ssh will automatically add new host keys to the user's 1796.Pa known_hosts 1797file, but will not permit connections to hosts with 1798changed host keys. 1799If this flag is set to 1800.Dq no 1801or 1802.Dq off , 1803ssh will automatically add new host keys to the user known hosts files 1804and allow connections to hosts with changed hostkeys to proceed, 1805subject to some restrictions. 1806If this flag is set to 1807.Cm ask 1808(the default), 1809new host keys 1810will be added to the user known host files only after the user 1811has confirmed that is what they really want to do, and 1812ssh will refuse to connect to hosts whose host key has changed. 1813The host keys of 1814known hosts will be verified automatically in all cases. 1815.It Cm SyslogFacility 1816Gives the facility code that is used when logging messages from 1817.Xr ssh 1 . 1818The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1819LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1820The default is USER. 1821.It Cm TCPKeepAlive 1822Specifies whether the system should send TCP keepalive messages to the 1823other side. 1824If they are sent, death of the connection or crash of one 1825of the machines will be properly noticed. 1826However, this means that 1827connections will die if the route is down temporarily, and some people 1828find it annoying. 1829.Pp 1830The default is 1831.Cm yes 1832(to send TCP keepalive messages), and the client will notice 1833if the network goes down or the remote host dies. 1834This is important in scripts, and many users want it too. 1835.Pp 1836To disable TCP keepalive messages, the value should be set to 1837.Cm no . 1838See also 1839.Cm ServerAliveInterval 1840for protocol-level keepalives. 1841.It Cm Tunnel 1842Request 1843.Xr tun 4 1844device forwarding between the client and the server. 1845The argument must be 1846.Cm yes , 1847.Cm point-to-point 1848(layer 3), 1849.Cm ethernet 1850(layer 2), 1851or 1852.Cm no 1853(the default). 1854Specifying 1855.Cm yes 1856requests the default tunnel mode, which is 1857.Cm point-to-point . 1858.It Cm TunnelDevice 1859Specifies the 1860.Xr tun 4 1861devices to open on the client 1862.Pq Ar local_tun 1863and the server 1864.Pq Ar remote_tun . 1865.Pp 1866The argument must be 1867.Sm off 1868.Ar local_tun Op : Ar remote_tun . 1869.Sm on 1870The devices may be specified by numerical ID or the keyword 1871.Cm any , 1872which uses the next available tunnel device. 1873If 1874.Ar remote_tun 1875is not specified, it defaults to 1876.Cm any . 1877The default is 1878.Cm any:any . 1879.It Cm UpdateHostKeys 1880Specifies whether 1881.Xr ssh 1 1882should accept notifications of additional hostkeys from the server sent 1883after authentication has completed and add them to 1884.Cm UserKnownHostsFile . 1885The argument must be 1886.Cm yes , 1887.Cm no 1888or 1889.Cm ask . 1890This option allows learning alternate hostkeys for a server 1891and supports graceful key rotation by allowing a server to send replacement 1892public keys before old ones are removed. 1893.Pp 1894Additional hostkeys are only accepted if the key used to authenticate the 1895host was already trusted or explicitly accepted by the user, the host was 1896authenticated via 1897.Cm UserKnownHostsFile 1898(i.e. not 1899.Cm GlobalKnownHostsFile ) 1900and the host was authenticated using a plain key and not a certificate. 1901.Pp 1902.Cm UpdateHostKeys 1903is enabled by default if the user has not overridden the default 1904.Cm UserKnownHostsFile 1905setting and has not enabled 1906.Cm VerifyHostKeyDNS , 1907otherwise 1908.Cm UpdateHostKeys 1909will be set to 1910.Cm no . 1911.Pp 1912If 1913.Cm UpdateHostKeys 1914is set to 1915.Cm ask , 1916then the user is asked to confirm the modifications to the known_hosts file. 1917Confirmation is currently incompatible with 1918.Cm ControlPersist , 1919and will be disabled if it is enabled. 1920.Pp 1921Presently, only 1922.Xr sshd 8 1923from OpenSSH 6.8 and greater support the 1924.Qq hostkeys@openssh.com 1925protocol extension used to inform the client of all the server's hostkeys. 1926.It Cm User 1927Specifies the user to log in as. 1928This can be useful when a different user name is used on different machines. 1929This saves the trouble of 1930having to remember to give the user name on the command line. 1931.It Cm UserKnownHostsFile 1932Specifies one or more files to use for the user 1933host key database, separated by whitespace. 1934Each filename may use tilde notation to refer to the user's home directory, 1935the tokens described in the 1936.Sx TOKENS 1937section and environment variables as described in the 1938.Sx ENVIRONMENT VARIABLES 1939section. 1940The default is 1941.Pa ~/.ssh/known_hosts , 1942.Pa ~/.ssh/known_hosts2 . 1943.It Cm VerifyHostKeyDNS 1944Specifies whether to verify the remote key using DNS and SSHFP resource 1945records. 1946If this option is set to 1947.Cm yes , 1948the client will implicitly trust keys that match a secure fingerprint 1949from DNS. 1950Insecure fingerprints will be handled as if this option was set to 1951.Cm ask . 1952If this option is set to 1953.Cm ask , 1954information on fingerprint match will be displayed, but the user will still 1955need to confirm new host keys according to the 1956.Cm StrictHostKeyChecking 1957option. 1958The default is 1959.Cm yes 1960if compiled with LDNS and 1961.Cm no 1962otherwise. 1963.Pp 1964See also 1965.Sx VERIFYING HOST KEYS 1966in 1967.Xr ssh 1 . 1968.It Cm VersionAddendum 1969Specifies a string to append to the regular version string to identify 1970OS- or site-specific modifications. 1971The default is 1972.Dq FreeBSD-20210907 . 1973The value 1974.Cm none 1975may be used to disable this. 1976.It Cm VisualHostKey 1977If this flag is set to 1978.Cm yes , 1979an ASCII art representation of the remote host key fingerprint is 1980printed in addition to the fingerprint string at login and 1981for unknown host keys. 1982If this flag is set to 1983.Cm no 1984(the default), 1985no fingerprint strings are printed at login and 1986only the fingerprint string will be printed for unknown host keys. 1987.It Cm XAuthLocation 1988Specifies the full pathname of the 1989.Xr xauth 1 1990program. 1991The default is 1992.Pa /usr/local/bin/xauth . 1993.El 1994.Sh PATTERNS 1995A 1996.Em pattern 1997consists of zero or more non-whitespace characters, 1998.Sq * 1999(a wildcard that matches zero or more characters), 2000or 2001.Sq ?\& 2002(a wildcard that matches exactly one character). 2003For example, to specify a set of declarations for any host in the 2004.Qq .co.uk 2005set of domains, 2006the following pattern could be used: 2007.Pp 2008.Dl Host *.co.uk 2009.Pp 2010The following pattern 2011would match any host in the 192.168.0.[0-9] network range: 2012.Pp 2013.Dl Host 192.168.0.? 2014.Pp 2015A 2016.Em pattern-list 2017is a comma-separated list of patterns. 2018Patterns within pattern-lists may be negated 2019by preceding them with an exclamation mark 2020.Pq Sq !\& . 2021For example, 2022to allow a key to be used from anywhere within an organization 2023except from the 2024.Qq dialup 2025pool, 2026the following entry (in authorized_keys) could be used: 2027.Pp 2028.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2029.Pp 2030Note that a negated match will never produce a positive result by itself. 2031For example, attempting to match 2032.Qq host3 2033against the following pattern-list will fail: 2034.Pp 2035.Dl from=\&"!host1,!host2\&" 2036.Pp 2037The solution here is to include a term that will yield a positive match, 2038such as a wildcard: 2039.Pp 2040.Dl from=\&"!host1,!host2,*\&" 2041.Sh TOKENS 2042Arguments to some keywords can make use of tokens, 2043which are expanded at runtime: 2044.Pp 2045.Bl -tag -width XXXX -offset indent -compact 2046.It %% 2047A literal 2048.Sq % . 2049.It \&%C 2050Hash of %l%h%p%r. 2051.It %d 2052Local user's home directory. 2053.It %f 2054The fingerprint of the server's host key. 2055.It %H 2056The 2057.Pa known_hosts 2058hostname or address that is being searched for. 2059.It %h 2060The remote hostname. 2061.It \%%I 2062A string describing the reason for a 2063.Cm KnownHostsCommand 2064execution: either 2065.Cm ADDRESS 2066when looking up a host by address (only when 2067.Cm CheckHostIP 2068is enabled), 2069.Cm HOSTNAME 2070when searching by hostname, or 2071.Cm ORDER 2072when preparing the host key algorithm preference list to use for the 2073destination host. 2074.It %i 2075The local user ID. 2076.It %K 2077The base64 encoded host key. 2078.It %k 2079The host key alias if specified, otherwise the original remote hostname given 2080on the command line. 2081.It %L 2082The local hostname. 2083.It %l 2084The local hostname, including the domain name. 2085.It %n 2086The original remote hostname, as given on the command line. 2087.It %p 2088The remote port. 2089.It %r 2090The remote username. 2091.It \&%T 2092The local 2093.Xr tun 4 2094or 2095.Xr tap 4 2096network interface assigned if 2097tunnel forwarding was requested, or 2098.Qq NONE 2099otherwise. 2100.It %t 2101The type of the server host key, e.g. 2102.Cm ssh-ed25519 . 2103.It %u 2104The local username. 2105.El 2106.Pp 2107.Cm CertificateFile , 2108.Cm ControlPath , 2109.Cm IdentityAgent , 2110.Cm IdentityFile , 2111.Cm KnownHostsCommand , 2112.Cm LocalForward , 2113.Cm Match exec , 2114.Cm RemoteCommand , 2115.Cm RemoteForward , 2116and 2117.Cm UserKnownHostsFile 2118accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2119.Pp 2120.Cm KnownHostsCommand 2121additionally accepts the tokens %f, %H, %I, %K and %t. 2122.Pp 2123.Cm Hostname 2124accepts the tokens %% and %h. 2125.Pp 2126.Cm LocalCommand 2127accepts all tokens. 2128.Pp 2129.Cm ProxyCommand 2130accepts the tokens %%, %h, %n, %p, and %r. 2131.Sh ENVIRONMENT VARIABLES 2132Arguments to some keywords can be expanded at runtime from environment 2133variables on the client by enclosing them in 2134.Ic ${} , 2135for example 2136.Ic ${HOME}/.ssh 2137would refer to the user's .ssh directory. 2138If a specified environment variable does not exist then an error will be 2139returned and the setting for that keyword will be ignored. 2140.Pp 2141The keywords 2142.Cm CertificateFile , 2143.Cm ControlPath , 2144.Cm IdentityAgent , 2145.Cm IdentityFile , 2146.Cm KnownHostsCommand , 2147and 2148.Cm UserKnownHostsFile 2149support environment variables. 2150The keywords 2151.Cm LocalForward 2152and 2153.Cm RemoteForward 2154support environment variables only for Unix domain socket paths. 2155.Sh FILES 2156.Bl -tag -width Ds 2157.It Pa ~/.ssh/config 2158This is the per-user configuration file. 2159The format of this file is described above. 2160This file is used by the SSH client. 2161Because of the potential for abuse, this file must have strict permissions: 2162read/write for the user, and not writable by others. 2163.It Pa /etc/ssh/ssh_config 2164Systemwide configuration file. 2165This file provides defaults for those 2166values that are not specified in the user's configuration file, and 2167for those users who do not have a configuration file. 2168This file must be world-readable. 2169.El 2170.Sh SEE ALSO 2171.Xr ssh 1 2172.Sh AUTHORS 2173.An -nosplit 2174OpenSSH is a derivative of the original and free 2175ssh 1.2.12 release by 2176.An Tatu Ylonen . 2177.An Aaron Campbell , Bob Beck , Markus Friedl , 2178.An Niels Provos , Theo de Raadt 2179and 2180.An Dug Song 2181removed many bugs, re-added newer features and 2182created OpenSSH. 2183.An Markus Friedl 2184contributed the support for SSH protocol versions 1.5 and 2.0. 2185