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 1.325 2020/04/11 20:20:09 jmc Exp $ 37.Dd $Mdocdate: April 11 2020 $ 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. 248The argument must be 249.Cm yes , 250.Cm confirm , 251.Cm ask , 252or 253.Cm no 254(the default). 255.It Cm AddressFamily 256Specifies which address family to use when connecting. 257Valid arguments are 258.Cm any 259(the default), 260.Cm inet 261(use IPv4 only), or 262.Cm inet6 263(use IPv6 only). 264.It Cm BatchMode 265If set to 266.Cm yes , 267user interaction such as password prompts and host key confirmation requests 268will be disabled. 269This option is useful in scripts and other batch jobs where no user 270is present to interact with 271.Xr ssh 1 . 272The argument must be 273.Cm yes 274or 275.Cm no 276(the default). 277.It Cm BindAddress 278Use the specified address on the local machine as the source address of 279the connection. 280Only useful on systems with more than one address. 281.It Cm BindInterface 282Use the address of the specified interface on the local machine as the 283source address of the connection. 284.It Cm CanonicalDomains 285When 286.Cm CanonicalizeHostname 287is enabled, this option specifies the list of domain suffixes in which to 288search for the specified destination host. 289.It Cm CanonicalizeFallbackLocal 290Specifies whether to fail with an error when hostname canonicalization fails. 291The default, 292.Cm yes , 293will attempt to look up the unqualified hostname using the system resolver's 294search rules. 295A value of 296.Cm no 297will cause 298.Xr ssh 1 299to fail instantly if 300.Cm CanonicalizeHostname 301is enabled and the target hostname cannot be found in any of the domains 302specified by 303.Cm CanonicalDomains . 304.It Cm CanonicalizeHostname 305Controls whether explicit hostname canonicalization is performed. 306The default, 307.Cm no , 308is not to perform any name rewriting and let the system resolver handle all 309hostname lookups. 310If set to 311.Cm yes 312then, for connections that do not use a 313.Cm ProxyCommand 314or 315.Cm ProxyJump , 316.Xr ssh 1 317will attempt to canonicalize the hostname specified on the command line 318using the 319.Cm CanonicalDomains 320suffixes and 321.Cm CanonicalizePermittedCNAMEs 322rules. 323If 324.Cm CanonicalizeHostname 325is set to 326.Cm always , 327then canonicalization is applied to proxied connections too. 328.Pp 329If this option is enabled, then the configuration files are processed 330again using the new target name to pick up any new configuration in matching 331.Cm Host 332and 333.Cm Match 334stanzas. 335.It Cm CanonicalizeMaxDots 336Specifies the maximum number of dot characters in a hostname before 337canonicalization is disabled. 338The default, 1, 339allows a single dot (i.e. hostname.subdomain). 340.It Cm CanonicalizePermittedCNAMEs 341Specifies rules to determine whether CNAMEs should be followed when 342canonicalizing hostnames. 343The rules consist of one or more arguments of 344.Ar source_domain_list : Ns Ar target_domain_list , 345where 346.Ar source_domain_list 347is a pattern-list of domains that may follow CNAMEs in canonicalization, 348and 349.Ar target_domain_list 350is a pattern-list of domains that they may resolve to. 351.Pp 352For example, 353.Qq *.a.example.com:*.b.example.com,*.c.example.com 354will allow hostnames matching 355.Qq *.a.example.com 356to be canonicalized to names in the 357.Qq *.b.example.com 358or 359.Qq *.c.example.com 360domains. 361.It Cm CASignatureAlgorithms 362Specifies which algorithms are allowed for signing of certificates 363by certificate authorities (CAs). 364The default is: 365.Bd -literal -offset indent 366ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 367ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 368.Ed 369.Pp 370.Xr ssh 1 371will not accept host certificates signed using algorithms other than those 372specified. 373.It Cm CertificateFile 374Specifies a file from which the user's certificate is read. 375A corresponding private key must be provided separately in order 376to use this certificate either 377from an 378.Cm IdentityFile 379directive or 380.Fl i 381flag to 382.Xr ssh 1 , 383via 384.Xr ssh-agent 1 , 385or via a 386.Cm PKCS11Provider 387or 388.Cm SecurityKeyProvider . 389.Pp 390Arguments to 391.Cm CertificateFile 392may use the tilde syntax to refer to a user's home directory 393or the tokens described in the 394.Sx TOKENS 395section. 396.Pp 397It is possible to have multiple certificate files specified in 398configuration files; these certificates will be tried in sequence. 399Multiple 400.Cm CertificateFile 401directives will add to the list of certificates used for 402authentication. 403.It Cm ChallengeResponseAuthentication 404Specifies whether to use challenge-response authentication. 405The argument to this keyword must be 406.Cm yes 407(the default) 408or 409.Cm no . 410.It Cm CheckHostIP 411If set to 412.Cm yes 413(the default), 414.Xr ssh 1 415will additionally check the host IP address in the 416.Pa known_hosts 417file. 418This allows it to detect if a host key changed due to DNS spoofing 419and will add addresses of destination hosts to 420.Pa ~/.ssh/known_hosts 421in the process, regardless of the setting of 422.Cm StrictHostKeyChecking . 423If the option is set to 424.Cm no , 425the check will not be executed. 426.It Cm Ciphers 427Specifies the ciphers allowed and their order of preference. 428Multiple ciphers must be comma-separated. 429If the specified list begins with a 430.Sq + 431character, then the specified ciphers will be appended to the default set 432instead of replacing them. 433If the specified list begins with a 434.Sq - 435character, then the specified ciphers (including wildcards) will be removed 436from the default set instead of replacing them. 437If the specified list begins with a 438.Sq ^ 439character, then the specified ciphers will be placed at the head of the 440default set. 441.Pp 442The supported ciphers are: 443.Bd -literal -offset indent 4443des-cbc 445aes128-cbc 446aes192-cbc 447aes256-cbc 448aes128-ctr 449aes192-ctr 450aes256-ctr 451aes128-gcm@openssh.com 452aes256-gcm@openssh.com 453chacha20-poly1305@openssh.com 454.Ed 455.Pp 456The default is: 457.Bd -literal -offset indent 458chacha20-poly1305@openssh.com, 459aes128-ctr,aes192-ctr,aes256-ctr, 460aes128-gcm@openssh.com,aes256-gcm@openssh.com 461.Ed 462.Pp 463The list of available ciphers may also be obtained using 464.Qq ssh -Q cipher . 465.It Cm ClearAllForwardings 466Specifies that all local, remote, and dynamic port forwardings 467specified in the configuration files or on the command line be 468cleared. 469This option is primarily useful when used from the 470.Xr ssh 1 471command line to clear port forwardings set in 472configuration files, and is automatically set by 473.Xr scp 1 474and 475.Xr sftp 1 . 476The argument must be 477.Cm yes 478or 479.Cm no 480(the default). 481.It Cm Compression 482Specifies whether to use compression. 483The argument must be 484.Cm yes 485or 486.Cm no 487(the default). 488.It Cm ConnectionAttempts 489Specifies the number of tries (one per second) to make before exiting. 490The argument must be an integer. 491This may be useful in scripts if the connection sometimes fails. 492The default is 1. 493.It Cm ConnectTimeout 494Specifies the timeout (in seconds) used when connecting to the 495SSH server, instead of using the default system TCP timeout. 496This timeout is applied both to establishing the connection and to performing 497the initial SSH protocol handshake and key exchange. 498.It Cm ControlMaster 499Enables the sharing of multiple sessions over a single network connection. 500When set to 501.Cm yes , 502.Xr ssh 1 503will listen for connections on a control socket specified using the 504.Cm ControlPath 505argument. 506Additional sessions can connect to this socket using the same 507.Cm ControlPath 508with 509.Cm ControlMaster 510set to 511.Cm no 512(the default). 513These sessions will try to reuse the master instance's network connection 514rather than initiating new ones, but will fall back to connecting normally 515if the control socket does not exist, or is not listening. 516.Pp 517Setting this to 518.Cm ask 519will cause 520.Xr ssh 1 521to listen for control connections, but require confirmation using 522.Xr ssh-askpass 1 . 523If the 524.Cm ControlPath 525cannot be opened, 526.Xr ssh 1 527will continue without connecting to a master instance. 528.Pp 529X11 and 530.Xr ssh-agent 1 531forwarding is supported over these multiplexed connections, however the 532display and agent forwarded will be the one belonging to the master 533connection i.e. it is not possible to forward multiple displays or agents. 534.Pp 535Two additional options allow for opportunistic multiplexing: try to use a 536master connection but fall back to creating a new one if one does not already 537exist. 538These options are: 539.Cm auto 540and 541.Cm autoask . 542The latter requires confirmation like the 543.Cm ask 544option. 545.It Cm ControlPath 546Specify the path to the control socket used for connection sharing as described 547in the 548.Cm ControlMaster 549section above or the string 550.Cm none 551to disable connection sharing. 552Arguments to 553.Cm ControlPath 554may use the tilde syntax to refer to a user's home directory 555or the tokens described in the 556.Sx TOKENS 557section. 558It is recommended that any 559.Cm ControlPath 560used for opportunistic connection sharing include 561at least %h, %p, and %r (or alternatively %C) and be placed in a directory 562that is not writable by other users. 563This ensures that shared connections are uniquely identified. 564.It Cm ControlPersist 565When used in conjunction with 566.Cm ControlMaster , 567specifies that the master connection should remain open 568in the background (waiting for future client connections) 569after the initial client connection has been closed. 570If set to 571.Cm no 572(the default), 573then the master connection will not be placed into the background, 574and will close as soon as the initial client connection is closed. 575If set to 576.Cm yes 577or 0, 578then the master connection will remain in the background indefinitely 579(until killed or closed via a mechanism such as the 580.Qq ssh -O exit ) . 581If set to a time in seconds, or a time in any of the formats documented in 582.Xr sshd_config 5 , 583then the backgrounded master connection will automatically terminate 584after it has remained idle (with no client connections) for the 585specified time. 586.It Cm DynamicForward 587Specifies that a TCP port on the local machine be forwarded 588over the secure channel, and the application 589protocol is then used to determine where to connect to from the 590remote machine. 591.Pp 592The argument must be 593.Sm off 594.Oo Ar bind_address : Oc Ar port . 595.Sm on 596IPv6 addresses can be specified by enclosing addresses in square brackets. 597By default, the local port is bound in accordance with the 598.Cm GatewayPorts 599setting. 600However, an explicit 601.Ar bind_address 602may be used to bind the connection to a specific address. 603The 604.Ar bind_address 605of 606.Cm localhost 607indicates that the listening port be bound for local use only, while an 608empty address or 609.Sq * 610indicates that the port should be available from all interfaces. 611.Pp 612Currently the SOCKS4 and SOCKS5 protocols are supported, and 613.Xr ssh 1 614will act as a SOCKS server. 615Multiple forwardings may be specified, and 616additional forwardings can be given on the command line. 617Only the superuser can forward privileged ports. 618.It Cm EnableSSHKeysign 619Setting this option to 620.Cm yes 621in the global client configuration file 622.Pa /etc/ssh/ssh_config 623enables the use of the helper program 624.Xr ssh-keysign 8 625during 626.Cm HostbasedAuthentication . 627The argument must be 628.Cm yes 629or 630.Cm no 631(the default). 632This option should be placed in the non-hostspecific section. 633See 634.Xr ssh-keysign 8 635for more information. 636.It Cm EscapeChar 637Sets the escape character (default: 638.Ql ~ ) . 639The escape character can also 640be set on the command line. 641The argument should be a single character, 642.Ql ^ 643followed by a letter, or 644.Cm none 645to disable the escape 646character entirely (making the connection transparent for binary 647data). 648.It Cm ExitOnForwardFailure 649Specifies whether 650.Xr ssh 1 651should terminate the connection if it cannot set up all requested 652dynamic, tunnel, local, and remote port forwardings, (e.g.\& 653if either end is unable to bind and listen on a specified port). 654Note that 655.Cm ExitOnForwardFailure 656does not apply to connections made over port forwardings and will not, 657for example, cause 658.Xr ssh 1 659to exit if TCP connections to the ultimate forwarding destination fail. 660The argument must be 661.Cm yes 662or 663.Cm no 664(the default). 665.It Cm FingerprintHash 666Specifies the hash algorithm used when displaying key fingerprints. 667Valid options are: 668.Cm md5 669and 670.Cm sha256 671(the default). 672.It Cm ForwardAgent 673Specifies whether the connection to the authentication agent (if any) 674will be forwarded to the remote machine. 675The argument may be 676.Cm yes , 677.Cm no 678(the default), 679an explicit path to an agent socket or the name of an environment variable 680(beginning with 681.Sq $ ) 682in which to find the path. 683.Pp 684Agent forwarding should be enabled with caution. 685Users with the ability to bypass file permissions on the remote host 686(for the agent's Unix-domain socket) 687can access the local agent through the forwarded connection. 688An attacker cannot obtain key material from the agent, 689however they can perform operations on the keys that enable them to 690authenticate using the identities loaded into the agent. 691.It Cm ForwardX11 692Specifies whether X11 connections will be automatically redirected 693over the secure channel and 694.Ev DISPLAY 695set. 696The argument must be 697.Cm yes 698or 699.Cm no 700(the default). 701.Pp 702X11 forwarding should be enabled with caution. 703Users with the ability to bypass file permissions on the remote host 704(for the user's X11 authorization database) 705can access the local X11 display through the forwarded connection. 706An attacker may then be able to perform activities such as keystroke monitoring 707if the 708.Cm ForwardX11Trusted 709option is also enabled. 710.It Cm ForwardX11Timeout 711Specify a timeout for untrusted X11 forwarding 712using the format described in the 713.Sx TIME FORMATS 714section of 715.Xr sshd_config 5 . 716X11 connections received by 717.Xr ssh 1 718after this time will be refused. 719Setting 720.Cm ForwardX11Timeout 721to zero will disable the timeout and permit X11 forwarding for the life 722of the connection. 723The default is to disable untrusted X11 forwarding after twenty minutes has 724elapsed. 725.It Cm ForwardX11Trusted 726If this option is set to 727.Cm yes , 728remote X11 clients will have full access to the original X11 display. 729.Pp 730If this option is set to 731.Cm no 732(the default), 733remote X11 clients will be considered untrusted and prevented 734from stealing or tampering with data belonging to trusted X11 735clients. 736Furthermore, the 737.Xr xauth 1 738token used for the session will be set to expire after 20 minutes. 739Remote clients will be refused access after this time. 740.Pp 741See the X11 SECURITY extension specification for full details on 742the restrictions imposed on untrusted clients. 743.It Cm GatewayPorts 744Specifies whether remote hosts are allowed to connect to local 745forwarded ports. 746By default, 747.Xr ssh 1 748binds local port forwardings to the loopback address. 749This prevents other remote hosts from connecting to forwarded ports. 750.Cm GatewayPorts 751can be used to specify that ssh 752should bind local port forwardings to the wildcard address, 753thus allowing remote hosts to connect to forwarded ports. 754The argument must be 755.Cm yes 756or 757.Cm no 758(the default). 759.It Cm GlobalKnownHostsFile 760Specifies one or more files to use for the global 761host key database, separated by whitespace. 762The default is 763.Pa /etc/ssh/ssh_known_hosts , 764.Pa /etc/ssh/ssh_known_hosts2 . 765.It Cm GSSAPIAuthentication 766Specifies whether user authentication based on GSSAPI is allowed. 767The default is 768.Cm no . 769.It Cm GSSAPIDelegateCredentials 770Forward (delegate) credentials to the server. 771The default is 772.Cm no . 773.It Cm HashKnownHosts 774Indicates that 775.Xr ssh 1 776should hash host names and addresses when they are added to 777.Pa ~/.ssh/known_hosts . 778These hashed names may be used normally by 779.Xr ssh 1 780and 781.Xr sshd 8 , 782but they do not visually reveal identifying information if the 783file's contents are disclosed. 784The default is 785.Cm no . 786Note that existing names and addresses in known hosts files 787will not be converted automatically, 788but may be manually hashed using 789.Xr ssh-keygen 1 . 790.It Cm HostbasedAuthentication 791Specifies whether to try rhosts based authentication with public key 792authentication. 793The argument must be 794.Cm yes 795or 796.Cm no 797(the default). 798.It Cm HostbasedKeyTypes 799Specifies the key types that will be used for hostbased authentication 800as a comma-separated list of patterns. 801Alternately if the specified list begins with a 802.Sq + 803character, then the specified key types will be appended to the default set 804instead of replacing them. 805If the specified list begins with a 806.Sq - 807character, then the specified key types (including wildcards) will be removed 808from the default set instead of replacing them. 809If the specified list begins with a 810.Sq ^ 811character, then the specified key types will be placed at the head of the 812default set. 813The default for this option is: 814.Bd -literal -offset 3n 815ecdsa-sha2-nistp256-cert-v01@openssh.com, 816ecdsa-sha2-nistp384-cert-v01@openssh.com, 817ecdsa-sha2-nistp521-cert-v01@openssh.com, 818sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 819ssh-ed25519-cert-v01@openssh.com, 820sk-ssh-ed25519-cert-v01@openssh.com, 821rsa-sha2-512-cert-v01@openssh.com, 822rsa-sha2-256-cert-v01@openssh.com, 823ssh-rsa-cert-v01@openssh.com, 824ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 825sk-ecdsa-sha2-nistp256@openssh.com, 826ssh-ed25519,sk-ssh-ed25519@openssh.com, 827rsa-sha2-512,rsa-sha2-256,ssh-rsa 828.Ed 829.Pp 830The 831.Fl Q 832option of 833.Xr ssh 1 834may be used to list supported key types. 835.It Cm HostKeyAlgorithms 836Specifies the host key algorithms 837that the client wants to use in order of preference. 838Alternately if the specified list begins with a 839.Sq + 840character, then the specified key types will be appended to the default set 841instead of replacing them. 842If the specified list begins with a 843.Sq - 844character, then the specified key types (including wildcards) will be removed 845from the default set instead of replacing them. 846If the specified list begins with a 847.Sq ^ 848character, then the specified key types will be placed at the head of the 849default set. 850The default for this option is: 851.Bd -literal -offset 3n 852ecdsa-sha2-nistp256-cert-v01@openssh.com, 853ecdsa-sha2-nistp384-cert-v01@openssh.com, 854ecdsa-sha2-nistp521-cert-v01@openssh.com, 855sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 856ssh-ed25519-cert-v01@openssh.com, 857sk-ssh-ed25519-cert-v01@openssh.com, 858rsa-sha2-512-cert-v01@openssh.com, 859rsa-sha2-256-cert-v01@openssh.com, 860ssh-rsa-cert-v01@openssh.com, 861ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 862sk-ecdsa-sha2-nistp256@openssh.com, 863ssh-ed25519,sk-ssh-ed25519@openssh.com, 864rsa-sha2-512,rsa-sha2-256,ssh-rsa 865.Ed 866.Pp 867If hostkeys are known for the destination host then this default is modified 868to prefer their algorithms. 869.Pp 870The list of available key types may also be obtained using 871.Qq ssh -Q HostKeyAlgorithms . 872.It Cm HostKeyAlias 873Specifies an alias that should be used instead of the 874real host name when looking up or saving the host key 875in the host key database files and when validating host certificates. 876This option is useful for tunneling SSH connections 877or for multiple servers running on a single host. 878.It Cm Hostname 879Specifies the real host name to log into. 880This can be used to specify nicknames or abbreviations for hosts. 881Arguments to 882.Cm Hostname 883accept the tokens described in the 884.Sx TOKENS 885section. 886Numeric IP addresses are also permitted (both on the command line and in 887.Cm Hostname 888specifications). 889The default is the name given on the command line. 890.It Cm IdentitiesOnly 891Specifies that 892.Xr ssh 1 893should only use the configured authentication identity and certificate files 894(either the default files, or those explicitly configured in the 895.Nm 896files 897or passed on the 898.Xr ssh 1 899command-line), 900even if 901.Xr ssh-agent 1 902or a 903.Cm PKCS11Provider 904or 905.Cm SecurityKeyProvider 906offers more identities. 907The argument to this keyword must be 908.Cm yes 909or 910.Cm no 911(the default). 912This option is intended for situations where ssh-agent 913offers many different identities. 914.It Cm IdentityAgent 915Specifies the 916.Ux Ns -domain 917socket used to communicate with the authentication agent. 918.Pp 919This option overrides the 920.Ev SSH_AUTH_SOCK 921environment variable and can be used to select a specific agent. 922Setting the socket name to 923.Cm none 924disables the use of an authentication agent. 925If the string 926.Qq SSH_AUTH_SOCK 927is specified, the location of the socket will be read from the 928.Ev SSH_AUTH_SOCK 929environment variable. 930Otherwise if the specified value begins with a 931.Sq $ 932character, then it will be treated as an environment variable containing 933the location of the socket. 934.Pp 935Arguments to 936.Cm IdentityAgent 937may use the tilde syntax to refer to a user's home directory 938or the tokens described in the 939.Sx TOKENS 940section. 941.It Cm IdentityFile 942Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 943Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 944The default is 945.Pa ~/.ssh/id_dsa , 946.Pa ~/.ssh/id_ecdsa , 947.Pa ~/.ssh/id_ecdsa_sk , 948.Pa ~/.ssh/id_ed25519 , 949.Pa ~/.ssh/id_ed25519_sk 950and 951.Pa ~/.ssh/id_rsa . 952Additionally, any identities represented by the authentication agent 953will be used for authentication unless 954.Cm IdentitiesOnly 955is set. 956If no certificates have been explicitly specified by 957.Cm CertificateFile , 958.Xr ssh 1 959will try to load certificate information from the filename obtained by 960appending 961.Pa -cert.pub 962to the path of a specified 963.Cm IdentityFile . 964.Pp 965Arguments to 966.Cm IdentityFile 967may use the tilde syntax to refer to a user's home directory 968or the tokens described in the 969.Sx TOKENS 970section. 971.Pp 972It is possible to have 973multiple identity files specified in configuration files; all these 974identities will be tried in sequence. 975Multiple 976.Cm IdentityFile 977directives will add to the list of identities tried (this behaviour 978differs from that of other configuration directives). 979.Pp 980.Cm IdentityFile 981may be used in conjunction with 982.Cm IdentitiesOnly 983to select which identities in an agent are offered during authentication. 984.Cm IdentityFile 985may also be used in conjunction with 986.Cm CertificateFile 987in order to provide any certificate also needed for authentication with 988the identity. 989.It Cm IgnoreUnknown 990Specifies a pattern-list of unknown options to be ignored if they are 991encountered in configuration parsing. 992This may be used to suppress errors if 993.Nm 994contains options that are unrecognised by 995.Xr ssh 1 . 996It is recommended that 997.Cm IgnoreUnknown 998be listed early in the configuration file as it will not be applied 999to unknown options that appear before it. 1000.It Cm Include 1001Include the specified configuration file(s). 1002Multiple pathnames may be specified and each pathname may contain 1003.Xr glob 7 1004wildcards and, for user configurations, shell-like 1005.Sq ~ 1006references to user home directories. 1007Files without absolute paths are assumed to be in 1008.Pa ~/.ssh 1009if included in a user configuration file or 1010.Pa /etc/ssh 1011if included from the system configuration file. 1012.Cm Include 1013directive may appear inside a 1014.Cm Match 1015or 1016.Cm Host 1017block 1018to perform conditional inclusion. 1019.It Cm IPQoS 1020Specifies the IPv4 type-of-service or DSCP class for connections. 1021Accepted values are 1022.Cm af11 , 1023.Cm af12 , 1024.Cm af13 , 1025.Cm af21 , 1026.Cm af22 , 1027.Cm af23 , 1028.Cm af31 , 1029.Cm af32 , 1030.Cm af33 , 1031.Cm af41 , 1032.Cm af42 , 1033.Cm af43 , 1034.Cm cs0 , 1035.Cm cs1 , 1036.Cm cs2 , 1037.Cm cs3 , 1038.Cm cs4 , 1039.Cm cs5 , 1040.Cm cs6 , 1041.Cm cs7 , 1042.Cm ef , 1043.Cm le , 1044.Cm lowdelay , 1045.Cm throughput , 1046.Cm reliability , 1047a numeric value, or 1048.Cm none 1049to use the operating system default. 1050This option may take one or two arguments, separated by whitespace. 1051If one argument is specified, it is used as the packet class unconditionally. 1052If two values are specified, the first is automatically selected for 1053interactive sessions and the second for non-interactive sessions. 1054The default is 1055.Cm af21 1056(Low-Latency Data) 1057for interactive sessions and 1058.Cm cs1 1059(Lower Effort) 1060for non-interactive sessions. 1061.It Cm KbdInteractiveAuthentication 1062Specifies whether to use keyboard-interactive authentication. 1063The argument to this keyword must be 1064.Cm yes 1065(the default) 1066or 1067.Cm no . 1068.It Cm KbdInteractiveDevices 1069Specifies the list of methods to use in keyboard-interactive authentication. 1070Multiple method names must be comma-separated. 1071The default is to use the server specified list. 1072The methods available vary depending on what the server supports. 1073For an OpenSSH server, 1074it may be zero or more of: 1075.Cm bsdauth 1076and 1077.Cm pam . 1078.It Cm KexAlgorithms 1079Specifies the available KEX (Key Exchange) algorithms. 1080Multiple algorithms must be comma-separated. 1081If the specified list begins with a 1082.Sq + 1083character, then the specified methods will be appended to the default set 1084instead of replacing them. 1085If the specified list begins with a 1086.Sq - 1087character, then the specified methods (including wildcards) will be removed 1088from the default set instead of replacing them. 1089If the specified list begins with a 1090.Sq ^ 1091character, then the specified methods will be placed at the head of the 1092default set. 1093The default is: 1094.Bd -literal -offset indent 1095curve25519-sha256,curve25519-sha256@libssh.org, 1096ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1097diffie-hellman-group-exchange-sha256, 1098diffie-hellman-group16-sha512, 1099diffie-hellman-group18-sha512, 1100diffie-hellman-group14-sha256 1101.Ed 1102.Pp 1103The list of available key exchange algorithms may also be obtained using 1104.Qq ssh -Q kex . 1105.It Cm LocalCommand 1106Specifies a command to execute on the local machine after successfully 1107connecting to the server. 1108The command string extends to the end of the line, and is executed with 1109the user's shell. 1110Arguments to 1111.Cm LocalCommand 1112accept the tokens described in the 1113.Sx TOKENS 1114section. 1115.Pp 1116The command is run synchronously and does not have access to the 1117session of the 1118.Xr ssh 1 1119that spawned it. 1120It should not be used for interactive commands. 1121.Pp 1122This directive is ignored unless 1123.Cm PermitLocalCommand 1124has been enabled. 1125.It Cm LocalForward 1126Specifies that a TCP port on the local machine be forwarded over 1127the secure channel to the specified host and port from the remote machine. 1128The first argument specifies the listener and may be 1129.Sm off 1130.Oo Ar bind_address : Oc Ar port 1131.Sm on 1132or a Unix domain socket path. 1133The second argument is the destination and may be 1134.Ar host : Ns Ar hostport 1135or a Unix domain socket path if the remote host supports it. 1136.Pp 1137IPv6 addresses can be specified by enclosing addresses in square brackets. 1138Multiple forwardings may be specified, and additional forwardings can be 1139given on the command line. 1140Only the superuser can forward privileged ports. 1141By default, the local port is bound in accordance with the 1142.Cm GatewayPorts 1143setting. 1144However, an explicit 1145.Ar bind_address 1146may be used to bind the connection to a specific address. 1147The 1148.Ar bind_address 1149of 1150.Cm localhost 1151indicates that the listening port be bound for local use only, while an 1152empty address or 1153.Sq * 1154indicates that the port should be available from all interfaces. 1155Unix domain socket paths accept the tokens described in the 1156.Sx TOKENS 1157section. 1158.It Cm LogLevel 1159Gives the verbosity level that is used when logging messages from 1160.Xr ssh 1 . 1161The possible values are: 1162QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1163The default is INFO. 1164DEBUG and DEBUG1 are equivalent. 1165DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1166.It Cm MACs 1167Specifies the MAC (message authentication code) algorithms 1168in order of preference. 1169The MAC algorithm is used for data integrity protection. 1170Multiple algorithms must be comma-separated. 1171If the specified list begins with a 1172.Sq + 1173character, then the specified algorithms will be appended to the default set 1174instead of replacing them. 1175If the specified list begins with a 1176.Sq - 1177character, then the specified algorithms (including wildcards) will be removed 1178from the default set instead of replacing them. 1179If the specified list begins with a 1180.Sq ^ 1181character, then the specified algorithms will be placed at the head of the 1182default set. 1183.Pp 1184The algorithms that contain 1185.Qq -etm 1186calculate the MAC after encryption (encrypt-then-mac). 1187These are considered safer and their use recommended. 1188.Pp 1189The default is: 1190.Bd -literal -offset indent 1191umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1192hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1193hmac-sha1-etm@openssh.com, 1194umac-64@openssh.com,umac-128@openssh.com, 1195hmac-sha2-256,hmac-sha2-512,hmac-sha1 1196.Ed 1197.Pp 1198The list of available MAC algorithms may also be obtained using 1199.Qq ssh -Q mac . 1200.It Cm NoHostAuthenticationForLocalhost 1201Disable host authentication for localhost (loopback addresses). 1202The argument to this keyword must be 1203.Cm yes 1204or 1205.Cm no 1206(the default). 1207.It Cm NumberOfPasswordPrompts 1208Specifies the number of password prompts before giving up. 1209The argument to this keyword must be an integer. 1210The default is 3. 1211.It Cm PasswordAuthentication 1212Specifies whether to use password authentication. 1213The argument to this keyword must be 1214.Cm yes 1215(the default) 1216or 1217.Cm no . 1218.It Cm PermitLocalCommand 1219Allow local command execution via the 1220.Ic LocalCommand 1221option or using the 1222.Ic !\& Ns Ar command 1223escape sequence in 1224.Xr ssh 1 . 1225The argument must be 1226.Cm yes 1227or 1228.Cm no 1229(the default). 1230.It Cm PKCS11Provider 1231Specifies which PKCS#11 provider to use or 1232.Cm none 1233to indicate that no provider should be used (the default). 1234The argument to this keyword is a path to the PKCS#11 shared library 1235.Xr ssh 1 1236should use to communicate with a PKCS#11 token providing keys for user 1237authentication. 1238.It Cm Port 1239Specifies the port number to connect on the remote host. 1240The default is 22. 1241.It Cm PreferredAuthentications 1242Specifies the order in which the client should try authentication methods. 1243This allows a client to prefer one method (e.g.\& 1244.Cm keyboard-interactive ) 1245over another method (e.g.\& 1246.Cm password ) . 1247The default is: 1248.Bd -literal -offset indent 1249gssapi-with-mic,hostbased,publickey, 1250keyboard-interactive,password 1251.Ed 1252.It Cm ProxyCommand 1253Specifies the command to use to connect to the server. 1254The command 1255string extends to the end of the line, and is executed 1256using the user's shell 1257.Ql exec 1258directive to avoid a lingering shell process. 1259.Pp 1260Arguments to 1261.Cm ProxyCommand 1262accept the tokens described in the 1263.Sx TOKENS 1264section. 1265The command can be basically anything, 1266and should read from its standard input and write to its standard output. 1267It should eventually connect an 1268.Xr sshd 8 1269server running on some machine, or execute 1270.Ic sshd -i 1271somewhere. 1272Host key management will be done using the 1273.Cm Hostname 1274of the host being connected (defaulting to the name typed by the user). 1275Setting the command to 1276.Cm none 1277disables this option entirely. 1278Note that 1279.Cm CheckHostIP 1280is not available for connects with a proxy command. 1281.Pp 1282This directive is useful in conjunction with 1283.Xr nc 1 1284and its proxy support. 1285For example, the following directive would connect via an HTTP proxy at 1286192.0.2.0: 1287.Bd -literal -offset 3n 1288ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1289.Ed 1290.It Cm ProxyJump 1291Specifies one or more jump proxies as either 1292.Xo 1293.Sm off 1294.Op Ar user No @ 1295.Ar host 1296.Op : Ns Ar port 1297.Sm on 1298or an ssh URI 1299.Xc . 1300Multiple proxies may be separated by comma characters and will be visited 1301sequentially. 1302Setting this option will cause 1303.Xr ssh 1 1304to connect to the target host by first making a 1305.Xr ssh 1 1306connection to the specified 1307.Cm ProxyJump 1308host and then establishing a 1309TCP forwarding to the ultimate target from there. 1310.Pp 1311Note that this option will compete with the 1312.Cm ProxyCommand 1313option - whichever is specified first will prevent later instances of the 1314other from taking effect. 1315.Pp 1316Note also that the configuration for the destination host (either supplied 1317via the command-line or the configuration file) is not generally applied 1318to jump hosts. 1319.Pa ~/.ssh/config 1320should be used if specific configuration is required for jump hosts. 1321.It Cm ProxyUseFdpass 1322Specifies that 1323.Cm ProxyCommand 1324will pass a connected file descriptor back to 1325.Xr ssh 1 1326instead of continuing to execute and pass data. 1327The default is 1328.Cm no . 1329.It Cm PubkeyAcceptedKeyTypes 1330Specifies the key types that will be used for public key authentication 1331as a comma-separated list of patterns. 1332If the specified list begins with a 1333.Sq + 1334character, then the key types after it will be appended to the default 1335instead of replacing it. 1336If the specified list begins with a 1337.Sq - 1338character, then the specified key types (including wildcards) will be removed 1339from the default set instead of replacing them. 1340If the specified list begins with a 1341.Sq ^ 1342character, then the specified key types will be placed at the head of the 1343default set. 1344The default for this option is: 1345.Bd -literal -offset 3n 1346ecdsa-sha2-nistp256-cert-v01@openssh.com, 1347ecdsa-sha2-nistp384-cert-v01@openssh.com, 1348ecdsa-sha2-nistp521-cert-v01@openssh.com, 1349sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1350ssh-ed25519-cert-v01@openssh.com, 1351sk-ssh-ed25519-cert-v01@openssh.com, 1352rsa-sha2-512-cert-v01@openssh.com, 1353rsa-sha2-256-cert-v01@openssh.com, 1354ssh-rsa-cert-v01@openssh.com, 1355ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1356sk-ecdsa-sha2-nistp256@openssh.com, 1357ssh-ed25519,sk-ssh-ed25519@openssh.com, 1358rsa-sha2-512,rsa-sha2-256,ssh-rsa 1359.Ed 1360.Pp 1361The list of available key types may also be obtained using 1362.Qq ssh -Q PubkeyAcceptedKeyTypes . 1363.It Cm PubkeyAuthentication 1364Specifies whether to try public key authentication. 1365The argument to this keyword must be 1366.Cm yes 1367(the default) 1368or 1369.Cm no . 1370.It Cm RekeyLimit 1371Specifies the maximum amount of data that may be transmitted before the 1372session key is renegotiated, optionally followed a maximum amount of 1373time that may pass before the session key is renegotiated. 1374The first argument is specified in bytes and may have a suffix of 1375.Sq K , 1376.Sq M , 1377or 1378.Sq G 1379to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1380The default is between 1381.Sq 1G 1382and 1383.Sq 4G , 1384depending on the cipher. 1385The optional second value is specified in seconds and may use any of the 1386units documented in the TIME FORMATS section of 1387.Xr sshd_config 5 . 1388The default value for 1389.Cm RekeyLimit 1390is 1391.Cm default none , 1392which means that rekeying is performed after the cipher's default amount 1393of data has been sent or received and no time based rekeying is done. 1394.It Cm RemoteCommand 1395Specifies a command to execute on the remote machine after successfully 1396connecting to the server. 1397The command string extends to the end of the line, and is executed with 1398the user's shell. 1399Arguments to 1400.Cm RemoteCommand 1401accept the tokens described in the 1402.Sx TOKENS 1403section. 1404.It Cm RemoteForward 1405Specifies that a TCP port on the remote machine be forwarded over 1406the secure channel. 1407The remote port may either be forwarded to a specified host and port 1408from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1409client to connect to arbitrary destinations from the local machine. 1410The first argument is the listening specification and may be 1411.Sm off 1412.Oo Ar bind_address : Oc Ar port 1413.Sm on 1414or, if the remote host supports it, a Unix domain socket path. 1415If forwarding to a specific destination then the second argument must be 1416.Ar host : Ns Ar hostport 1417or a Unix domain socket path, 1418otherwise if no destination argument is specified then the remote forwarding 1419will be established as a SOCKS proxy. 1420.Pp 1421IPv6 addresses can be specified by enclosing addresses in square brackets. 1422Multiple forwardings may be specified, and additional 1423forwardings can be given on the command line. 1424Privileged ports can be forwarded only when 1425logging in as root on the remote machine. 1426Unix domain socket paths accept the tokens described in the 1427.Sx TOKENS 1428section. 1429.Pp 1430If the 1431.Ar port 1432argument is 0, 1433the listen port will be dynamically allocated on the server and reported 1434to the client at run time. 1435.Pp 1436If the 1437.Ar bind_address 1438is not specified, the default is to only bind to loopback addresses. 1439If the 1440.Ar bind_address 1441is 1442.Ql * 1443or an empty string, then the forwarding is requested to listen on all 1444interfaces. 1445Specifying a remote 1446.Ar bind_address 1447will only succeed if the server's 1448.Cm GatewayPorts 1449option is enabled (see 1450.Xr sshd_config 5 ) . 1451.It Cm RequestTTY 1452Specifies whether to request a pseudo-tty for the session. 1453The argument may be one of: 1454.Cm no 1455(never request a TTY), 1456.Cm yes 1457(always request a TTY when standard input is a TTY), 1458.Cm force 1459(always request a TTY) or 1460.Cm auto 1461(request a TTY when opening a login session). 1462This option mirrors the 1463.Fl t 1464and 1465.Fl T 1466flags for 1467.Xr ssh 1 . 1468.It Cm RevokedHostKeys 1469Specifies revoked host public keys. 1470Keys listed in this file will be refused for host authentication. 1471Note that if this file does not exist or is not readable, 1472then host authentication will be refused for all hosts. 1473Keys may be specified as a text file, listing one public key per line, or as 1474an OpenSSH Key Revocation List (KRL) as generated by 1475.Xr ssh-keygen 1 . 1476For more information on KRLs, see the KEY REVOCATION LISTS section in 1477.Xr ssh-keygen 1 . 1478.It Cm SecurityKeyProvider 1479Specifies a path to a library that will be used when loading any 1480FIDO authenticator-hosted keys, overriding the default of using 1481the built-in USB HID support. 1482.Pp 1483If the specified value begins with a 1484.Sq $ 1485character, then it will be treated as an environment variable containing 1486the path to the library. 1487.It Cm SendEnv 1488Specifies what variables from the local 1489.Xr environ 7 1490should be sent to the server. 1491The server must also support it, and the server must be configured to 1492accept these environment variables. 1493Note that the 1494.Ev TERM 1495environment variable is always sent whenever a 1496pseudo-terminal is requested as it is required by the protocol. 1497Refer to 1498.Cm AcceptEnv 1499in 1500.Xr sshd_config 5 1501for how to configure the server. 1502Variables are specified by name, which may contain wildcard characters. 1503Multiple environment variables may be separated by whitespace or spread 1504across multiple 1505.Cm SendEnv 1506directives. 1507.Pp 1508See 1509.Sx PATTERNS 1510for more information on patterns. 1511.Pp 1512It is possible to clear previously set 1513.Cm SendEnv 1514variable names by prefixing patterns with 1515.Pa - . 1516The default is not to send any environment variables. 1517.It Cm ServerAliveCountMax 1518Sets the number of server alive messages (see below) which may be 1519sent without 1520.Xr ssh 1 1521receiving any messages back from the server. 1522If this threshold is reached while server alive messages are being sent, 1523ssh will disconnect from the server, terminating the session. 1524It is important to note that the use of server alive messages is very 1525different from 1526.Cm TCPKeepAlive 1527(below). 1528The server alive messages are sent through the encrypted channel 1529and therefore will not be spoofable. 1530The TCP keepalive option enabled by 1531.Cm TCPKeepAlive 1532is spoofable. 1533The server alive mechanism is valuable when the client or 1534server depend on knowing when a connection has become unresponsive. 1535.Pp 1536The default value is 3. 1537If, for example, 1538.Cm ServerAliveInterval 1539(see below) is set to 15 and 1540.Cm ServerAliveCountMax 1541is left at the default, if the server becomes unresponsive, 1542ssh will disconnect after approximately 45 seconds. 1543.It Cm ServerAliveInterval 1544Sets a timeout interval in seconds after which if no data has been received 1545from the server, 1546.Xr ssh 1 1547will send a message through the encrypted 1548channel to request a response from the server. 1549The default 1550is 0, indicating that these messages will not be sent to the server. 1551.It Cm SetEnv 1552Directly specify one or more environment variables and their contents to 1553be sent to the server. 1554Similarly to 1555.Cm SendEnv , 1556the server must be prepared to accept the environment variable. 1557.It Cm StreamLocalBindMask 1558Sets the octal file creation mode mask 1559.Pq umask 1560used when creating a Unix-domain socket file for local or remote 1561port forwarding. 1562This option is only used for port forwarding to a Unix-domain socket file. 1563.Pp 1564The default value is 0177, which creates a Unix-domain socket file that is 1565readable and writable only by the owner. 1566Note that not all operating systems honor the file mode on Unix-domain 1567socket files. 1568.It Cm StreamLocalBindUnlink 1569Specifies whether to remove an existing Unix-domain socket file for local 1570or remote port forwarding before creating a new one. 1571If the socket file already exists and 1572.Cm StreamLocalBindUnlink 1573is not enabled, 1574.Nm ssh 1575will be unable to forward the port to the Unix-domain socket file. 1576This option is only used for port forwarding to a Unix-domain socket file. 1577.Pp 1578The argument must be 1579.Cm yes 1580or 1581.Cm no 1582(the default). 1583.It Cm StrictHostKeyChecking 1584If this flag is set to 1585.Cm yes , 1586.Xr ssh 1 1587will never automatically add host keys to the 1588.Pa ~/.ssh/known_hosts 1589file, and refuses to connect to hosts whose host key has changed. 1590This provides maximum protection against man-in-the-middle (MITM) attacks, 1591though it can be annoying when the 1592.Pa /etc/ssh/ssh_known_hosts 1593file is poorly maintained or when connections to new hosts are 1594frequently made. 1595This option forces the user to manually 1596add all new hosts. 1597.Pp 1598If this flag is set to 1599.Dq accept-new 1600then ssh will automatically add new host keys to the user 1601known hosts files, but will not permit connections to hosts with 1602changed host keys. 1603If this flag is set to 1604.Dq no 1605or 1606.Dq off , 1607ssh will automatically add new host keys to the user known hosts files 1608and allow connections to hosts with changed hostkeys to proceed, 1609subject to some restrictions. 1610If this flag is set to 1611.Cm ask 1612(the default), 1613new host keys 1614will be added to the user known host files only after the user 1615has confirmed that is what they really want to do, and 1616ssh will refuse to connect to hosts whose host key has changed. 1617The host keys of 1618known hosts will be verified automatically in all cases. 1619.It Cm SyslogFacility 1620Gives the facility code that is used when logging messages from 1621.Xr ssh 1 . 1622The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1623LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1624The default is USER. 1625.It Cm TCPKeepAlive 1626Specifies whether the system should send TCP keepalive messages to the 1627other side. 1628If they are sent, death of the connection or crash of one 1629of the machines will be properly noticed. 1630However, this means that 1631connections will die if the route is down temporarily, and some people 1632find it annoying. 1633.Pp 1634The default is 1635.Cm yes 1636(to send TCP keepalive messages), and the client will notice 1637if the network goes down or the remote host dies. 1638This is important in scripts, and many users want it too. 1639.Pp 1640To disable TCP keepalive messages, the value should be set to 1641.Cm no . 1642See also 1643.Cm ServerAliveInterval 1644for protocol-level keepalives. 1645.It Cm Tunnel 1646Request 1647.Xr tun 4 1648device forwarding between the client and the server. 1649The argument must be 1650.Cm yes , 1651.Cm point-to-point 1652(layer 3), 1653.Cm ethernet 1654(layer 2), 1655or 1656.Cm no 1657(the default). 1658Specifying 1659.Cm yes 1660requests the default tunnel mode, which is 1661.Cm point-to-point . 1662.It Cm TunnelDevice 1663Specifies the 1664.Xr tun 4 1665devices to open on the client 1666.Pq Ar local_tun 1667and the server 1668.Pq Ar remote_tun . 1669.Pp 1670The argument must be 1671.Sm off 1672.Ar local_tun Op : Ar remote_tun . 1673.Sm on 1674The devices may be specified by numerical ID or the keyword 1675.Cm any , 1676which uses the next available tunnel device. 1677If 1678.Ar remote_tun 1679is not specified, it defaults to 1680.Cm any . 1681The default is 1682.Cm any:any . 1683.It Cm UpdateHostKeys 1684Specifies whether 1685.Xr ssh 1 1686should accept notifications of additional hostkeys from the server sent 1687after authentication has completed and add them to 1688.Cm UserKnownHostsFile . 1689The argument must be 1690.Cm yes , 1691.Cm no 1692or 1693.Cm ask . 1694This option allows learning alternate hostkeys for a server 1695and supports graceful key rotation by allowing a server to send replacement 1696public keys before old ones are removed. 1697Additional hostkeys are only accepted if the key used to authenticate the 1698host was already trusted or explicitly accepted by the user. 1699.Pp 1700.Cm UpdateHostKeys 1701is enabled by default if the user has not overridden the default 1702.Cm UserKnownHostsFile 1703setting, otherwise 1704.Cm UpdateHostKeys 1705will be set to 1706.Cm ask . 1707.Pp 1708If 1709.Cm UpdateHostKeys 1710is set to 1711.Cm ask , 1712then the user is asked to confirm the modifications to the known_hosts file. 1713Confirmation is currently incompatible with 1714.Cm ControlPersist , 1715and will be disabled if it is enabled. 1716.Pp 1717Presently, only 1718.Xr sshd 8 1719from OpenSSH 6.8 and greater support the 1720.Qq hostkeys@openssh.com 1721protocol extension used to inform the client of all the server's hostkeys. 1722.It Cm User 1723Specifies the user to log in as. 1724This can be useful when a different user name is used on different machines. 1725This saves the trouble of 1726having to remember to give the user name on the command line. 1727.It Cm UserKnownHostsFile 1728Specifies one or more files to use for the user 1729host key database, separated by whitespace. 1730The default is 1731.Pa ~/.ssh/known_hosts , 1732.Pa ~/.ssh/known_hosts2 . 1733.It Cm VerifyHostKeyDNS 1734Specifies whether to verify the remote key using DNS and SSHFP resource 1735records. 1736If this option is set to 1737.Cm yes , 1738the client will implicitly trust keys that match a secure fingerprint 1739from DNS. 1740Insecure fingerprints will be handled as if this option was set to 1741.Cm ask . 1742If this option is set to 1743.Cm ask , 1744information on fingerprint match will be displayed, but the user will still 1745need to confirm new host keys according to the 1746.Cm StrictHostKeyChecking 1747option. 1748The default is 1749.Cm no . 1750.Pp 1751See also 1752.Sx VERIFYING HOST KEYS 1753in 1754.Xr ssh 1 . 1755.It Cm VisualHostKey 1756If this flag is set to 1757.Cm yes , 1758an ASCII art representation of the remote host key fingerprint is 1759printed in addition to the fingerprint string at login and 1760for unknown host keys. 1761If this flag is set to 1762.Cm no 1763(the default), 1764no fingerprint strings are printed at login and 1765only the fingerprint string will be printed for unknown host keys. 1766.It Cm XAuthLocation 1767Specifies the full pathname of the 1768.Xr xauth 1 1769program. 1770The default is 1771.Pa /usr/X11R6/bin/xauth . 1772.El 1773.Sh PATTERNS 1774A 1775.Em pattern 1776consists of zero or more non-whitespace characters, 1777.Sq * 1778(a wildcard that matches zero or more characters), 1779or 1780.Sq ?\& 1781(a wildcard that matches exactly one character). 1782For example, to specify a set of declarations for any host in the 1783.Qq .co.uk 1784set of domains, 1785the following pattern could be used: 1786.Pp 1787.Dl Host *.co.uk 1788.Pp 1789The following pattern 1790would match any host in the 192.168.0.[0-9] network range: 1791.Pp 1792.Dl Host 192.168.0.? 1793.Pp 1794A 1795.Em pattern-list 1796is a comma-separated list of patterns. 1797Patterns within pattern-lists may be negated 1798by preceding them with an exclamation mark 1799.Pq Sq !\& . 1800For example, 1801to allow a key to be used from anywhere within an organization 1802except from the 1803.Qq dialup 1804pool, 1805the following entry (in authorized_keys) could be used: 1806.Pp 1807.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1808.Pp 1809Note that a negated match will never produce a positive result by itself. 1810For example, attempting to match 1811.Qq host3 1812against the following pattern-list will fail: 1813.Pp 1814.Dl from=\&"!host1,!host2\&" 1815.Pp 1816The solution here is to include a term that will yield a positive match, 1817such as a wildcard: 1818.Pp 1819.Dl from=\&"!host1,!host2,*\&" 1820.Sh TOKENS 1821Arguments to some keywords can make use of tokens, 1822which are expanded at runtime: 1823.Pp 1824.Bl -tag -width XXXX -offset indent -compact 1825.It %% 1826A literal 1827.Sq % . 1828.It \&%C 1829Hash of %l%h%p%r. 1830.It %d 1831Local user's home directory. 1832.It %h 1833The remote hostname. 1834.It %i 1835The local user ID. 1836.It %L 1837The local hostname. 1838.It %l 1839The local hostname, including the domain name. 1840.It %n 1841The original remote hostname, as given on the command line. 1842.It %p 1843The remote port. 1844.It %r 1845The remote username. 1846.It \&%T 1847The local 1848.Xr tun 4 1849or 1850.Xr tap 4 1851network interface assigned if 1852tunnel forwarding was requested, or 1853.Qq NONE 1854otherwise. 1855.It %u 1856The local username. 1857.El 1858.Pp 1859.Cm CertificateFile , 1860.Cm ControlPath , 1861.Cm IdentityAgent , 1862.Cm IdentityFile , 1863.Cm LocalForward , 1864.Cm Match exec , 1865.Cm RemoteCommand , 1866and 1867.Cm RemoteForward 1868accept the tokens %%, %C, %d, %h, %i, %L, %l, %n, %p, %r, and %u. 1869.Pp 1870.Cm Hostname 1871accepts the tokens %% and %h. 1872.Pp 1873.Cm LocalCommand 1874accepts all tokens. 1875.Pp 1876.Cm ProxyCommand 1877accepts the tokens %%, %h, %n, %p, and %r. 1878.Sh FILES 1879.Bl -tag -width Ds 1880.It Pa ~/.ssh/config 1881This is the per-user configuration file. 1882The format of this file is described above. 1883This file is used by the SSH client. 1884Because of the potential for abuse, this file must have strict permissions: 1885read/write for the user, and not writable by others. 1886.It Pa /etc/ssh/ssh_config 1887Systemwide configuration file. 1888This file provides defaults for those 1889values that are not specified in the user's configuration file, and 1890for those users who do not have a configuration file. 1891This file must be world-readable. 1892.El 1893.Sh SEE ALSO 1894.Xr ssh 1 1895.Sh AUTHORS 1896.An -nosplit 1897OpenSSH is a derivative of the original and free 1898ssh 1.2.12 release by 1899.An Tatu Ylonen . 1900.An Aaron Campbell , Bob Beck , Markus Friedl , 1901.An Niels Provos , Theo de Raadt 1902and 1903.An Dug Song 1904removed many bugs, re-added newer features and 1905created OpenSSH. 1906.An Markus Friedl 1907contributed the support for SSH protocol versions 1.5 and 2.0. 1908