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 1.340 2020/12/22 07:40:26 jmc Exp $ 37.Dd $Mdocdate: December 22 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. 248Alternately, this option may be specified as a time interval 249using the format described in the 250.Sx TIME FORMATS 251section of 252.Xr sshd_config 5 253to specify the key's lifetime in 254.Xr ssh-agent 1 , 255after which it will automatically be removed. 256The argument must be 257.Cm no 258(the default), 259.Cm yes , 260.Cm confirm 261(optionally followed by a time interval), 262.Cm ask 263or a time interval. 264.It Cm AddressFamily 265Specifies which address family to use when connecting. 266Valid arguments are 267.Cm any 268(the default), 269.Cm inet 270(use IPv4 only), or 271.Cm inet6 272(use IPv6 only). 273.It Cm BatchMode 274If set to 275.Cm yes , 276user interaction such as password prompts and host key confirmation requests 277will be disabled. 278This option is useful in scripts and other batch jobs where no user 279is present to interact with 280.Xr ssh 1 . 281The argument must be 282.Cm yes 283or 284.Cm no 285(the default). 286.It Cm BindAddress 287Use the specified address on the local machine as the source address of 288the connection. 289Only useful on systems with more than one address. 290.It Cm BindInterface 291Use the address of the specified interface on the local machine as the 292source address of the connection. 293.It Cm CanonicalDomains 294When 295.Cm CanonicalizeHostname 296is enabled, this option specifies the list of domain suffixes in which to 297search for the specified destination host. 298.It Cm CanonicalizeFallbackLocal 299Specifies whether to fail with an error when hostname canonicalization fails. 300The default, 301.Cm yes , 302will attempt to look up the unqualified hostname using the system resolver's 303search rules. 304A value of 305.Cm no 306will cause 307.Xr ssh 1 308to fail instantly if 309.Cm CanonicalizeHostname 310is enabled and the target hostname cannot be found in any of the domains 311specified by 312.Cm CanonicalDomains . 313.It Cm CanonicalizeHostname 314Controls whether explicit hostname canonicalization is performed. 315The default, 316.Cm no , 317is not to perform any name rewriting and let the system resolver handle all 318hostname lookups. 319If set to 320.Cm yes 321then, for connections that do not use a 322.Cm ProxyCommand 323or 324.Cm ProxyJump , 325.Xr ssh 1 326will attempt to canonicalize the hostname specified on the command line 327using the 328.Cm CanonicalDomains 329suffixes and 330.Cm CanonicalizePermittedCNAMEs 331rules. 332If 333.Cm CanonicalizeHostname 334is set to 335.Cm always , 336then canonicalization is applied to proxied connections too. 337.Pp 338If this option is enabled, then the configuration files are processed 339again using the new target name to pick up any new configuration in matching 340.Cm Host 341and 342.Cm Match 343stanzas. 344.It Cm CanonicalizeMaxDots 345Specifies the maximum number of dot characters in a hostname before 346canonicalization is disabled. 347The default, 1, 348allows a single dot (i.e. hostname.subdomain). 349.It Cm CanonicalizePermittedCNAMEs 350Specifies rules to determine whether CNAMEs should be followed when 351canonicalizing hostnames. 352The rules consist of one or more arguments of 353.Ar source_domain_list : Ns Ar target_domain_list , 354where 355.Ar source_domain_list 356is a pattern-list of domains that may follow CNAMEs in canonicalization, 357and 358.Ar target_domain_list 359is a pattern-list of domains that they may resolve to. 360.Pp 361For example, 362.Qq *.a.example.com:*.b.example.com,*.c.example.com 363will allow hostnames matching 364.Qq *.a.example.com 365to be canonicalized to names in the 366.Qq *.b.example.com 367or 368.Qq *.c.example.com 369domains. 370.It Cm CASignatureAlgorithms 371Specifies which algorithms are allowed for signing of certificates 372by certificate authorities (CAs). 373The default is: 374.Bd -literal -offset indent 375ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384, 376ecdsa-sha2-nistp521,rsa-sha2-512,rsa-sha2-256,ssh-rsa 377.Ed 378.Pp 379.Xr ssh 1 380will not accept host certificates signed using algorithms other than those 381specified. 382.It Cm CertificateFile 383Specifies a file from which the user's certificate is read. 384A corresponding private key must be provided separately in order 385to use this certificate either 386from an 387.Cm IdentityFile 388directive or 389.Fl i 390flag to 391.Xr ssh 1 , 392via 393.Xr ssh-agent 1 , 394or via a 395.Cm PKCS11Provider 396or 397.Cm SecurityKeyProvider . 398.Pp 399Arguments to 400.Cm CertificateFile 401may use the tilde syntax to refer to a user's home directory, 402the tokens described in the 403.Sx TOKENS 404section and environment variables as described in the 405.Sx ENVIRONMENT VARIABLES 406section. 407.Pp 408It is possible to have multiple certificate files specified in 409configuration files; these certificates will be tried in sequence. 410Multiple 411.Cm CertificateFile 412directives will add to the list of certificates used for 413authentication. 414.It Cm ChallengeResponseAuthentication 415Specifies whether to use challenge-response authentication. 416The argument to this keyword must be 417.Cm yes 418(the default) 419or 420.Cm no . 421.It Cm CheckHostIP 422If set to 423.Cm yes 424(the default), 425.Xr ssh 1 426will additionally check the host IP address in the 427.Pa known_hosts 428file. 429This allows it to detect if a host key changed due to DNS spoofing 430and will add addresses of destination hosts to 431.Pa ~/.ssh/known_hosts 432in the process, regardless of the setting of 433.Cm StrictHostKeyChecking . 434If the option is set to 435.Cm no , 436the check will not be executed. 437.It Cm Ciphers 438Specifies the ciphers allowed and their order of preference. 439Multiple ciphers must be comma-separated. 440If the specified list begins with a 441.Sq + 442character, then the specified ciphers will be appended to the default set 443instead of replacing them. 444If the specified list begins with a 445.Sq - 446character, then the specified ciphers (including wildcards) will be removed 447from the default set instead of replacing them. 448If the specified list begins with a 449.Sq ^ 450character, then the specified ciphers will be placed at the head of the 451default set. 452.Pp 453The supported ciphers are: 454.Bd -literal -offset indent 4553des-cbc 456aes128-cbc 457aes192-cbc 458aes256-cbc 459aes128-ctr 460aes192-ctr 461aes256-ctr 462aes128-gcm@openssh.com 463aes256-gcm@openssh.com 464chacha20-poly1305@openssh.com 465.Ed 466.Pp 467The default is: 468.Bd -literal -offset indent 469chacha20-poly1305@openssh.com, 470aes128-ctr,aes192-ctr,aes256-ctr, 471aes128-gcm@openssh.com,aes256-gcm@openssh.com 472.Ed 473.Pp 474The list of available ciphers may also be obtained using 475.Qq ssh -Q cipher . 476.It Cm ClearAllForwardings 477Specifies that all local, remote, and dynamic port forwardings 478specified in the configuration files or on the command line be 479cleared. 480This option is primarily useful when used from the 481.Xr ssh 1 482command line to clear port forwardings set in 483configuration files, and is automatically set by 484.Xr scp 1 485and 486.Xr sftp 1 . 487The argument must be 488.Cm yes 489or 490.Cm no 491(the default). 492.It Cm Compression 493Specifies whether to use compression. 494The argument must be 495.Cm yes 496or 497.Cm no 498(the default). 499.It Cm ConnectionAttempts 500Specifies the number of tries (one per second) to make before exiting. 501The argument must be an integer. 502This may be useful in scripts if the connection sometimes fails. 503The default is 1. 504.It Cm ConnectTimeout 505Specifies the timeout (in seconds) used when connecting to the 506SSH server, instead of using the default system TCP timeout. 507This timeout is applied both to establishing the connection and to performing 508the initial SSH protocol handshake and key exchange. 509.It Cm ControlMaster 510Enables the sharing of multiple sessions over a single network connection. 511When set to 512.Cm yes , 513.Xr ssh 1 514will listen for connections on a control socket specified using the 515.Cm ControlPath 516argument. 517Additional sessions can connect to this socket using the same 518.Cm ControlPath 519with 520.Cm ControlMaster 521set to 522.Cm no 523(the default). 524These sessions will try to reuse the master instance's network connection 525rather than initiating new ones, but will fall back to connecting normally 526if the control socket does not exist, or is not listening. 527.Pp 528Setting this to 529.Cm ask 530will cause 531.Xr ssh 1 532to listen for control connections, but require confirmation using 533.Xr ssh-askpass 1 . 534If the 535.Cm ControlPath 536cannot be opened, 537.Xr ssh 1 538will continue without connecting to a master instance. 539.Pp 540X11 and 541.Xr ssh-agent 1 542forwarding is supported over these multiplexed connections, however the 543display and agent forwarded will be the one belonging to the master 544connection i.e. it is not possible to forward multiple displays or agents. 545.Pp 546Two additional options allow for opportunistic multiplexing: try to use a 547master connection but fall back to creating a new one if one does not already 548exist. 549These options are: 550.Cm auto 551and 552.Cm autoask . 553The latter requires confirmation like the 554.Cm ask 555option. 556.It Cm ControlPath 557Specify the path to the control socket used for connection sharing as described 558in the 559.Cm ControlMaster 560section above or the string 561.Cm none 562to disable connection sharing. 563Arguments to 564.Cm ControlPath 565may use the tilde syntax to refer to a user's home directory, 566the tokens described in the 567.Sx TOKENS 568section and environment variables as described in the 569.Sx ENVIRONMENT VARIABLES 570section. 571It is recommended that any 572.Cm ControlPath 573used for opportunistic connection sharing include 574at least %h, %p, and %r (or alternatively %C) and be placed in a directory 575that is not writable by other users. 576This ensures that shared connections are uniquely identified. 577.It Cm ControlPersist 578When used in conjunction with 579.Cm ControlMaster , 580specifies that the master connection should remain open 581in the background (waiting for future client connections) 582after the initial client connection has been closed. 583If set to 584.Cm no 585(the default), 586then the master connection will not be placed into the background, 587and will close as soon as the initial client connection is closed. 588If set to 589.Cm yes 590or 0, 591then the master connection will remain in the background indefinitely 592(until killed or closed via a mechanism such as the 593.Qq ssh -O exit ) . 594If set to a time in seconds, or a time in any of the formats documented in 595.Xr sshd_config 5 , 596then the backgrounded master connection will automatically terminate 597after it has remained idle (with no client connections) for the 598specified time. 599.It Cm DynamicForward 600Specifies that a TCP port on the local machine be forwarded 601over the secure channel, and the application 602protocol is then used to determine where to connect to from the 603remote machine. 604.Pp 605The argument must be 606.Sm off 607.Oo Ar bind_address : Oc Ar port . 608.Sm on 609IPv6 addresses can be specified by enclosing addresses in square brackets. 610By default, the local port is bound in accordance with the 611.Cm GatewayPorts 612setting. 613However, an explicit 614.Ar bind_address 615may be used to bind the connection to a specific address. 616The 617.Ar bind_address 618of 619.Cm localhost 620indicates that the listening port be bound for local use only, while an 621empty address or 622.Sq * 623indicates that the port should be available from all interfaces. 624.Pp 625Currently the SOCKS4 and SOCKS5 protocols are supported, and 626.Xr ssh 1 627will act as a SOCKS server. 628Multiple forwardings may be specified, and 629additional forwardings can be given on the command line. 630Only the superuser can forward privileged ports. 631.It Cm EnableSSHKeysign 632Setting this option to 633.Cm yes 634in the global client configuration file 635.Pa /etc/ssh/ssh_config 636enables the use of the helper program 637.Xr ssh-keysign 8 638during 639.Cm HostbasedAuthentication . 640The argument must be 641.Cm yes 642or 643.Cm no 644(the default). 645This option should be placed in the non-hostspecific section. 646See 647.Xr ssh-keysign 8 648for more information. 649.It Cm EscapeChar 650Sets the escape character (default: 651.Ql ~ ) . 652The escape character can also 653be set on the command line. 654The argument should be a single character, 655.Ql ^ 656followed by a letter, or 657.Cm none 658to disable the escape 659character entirely (making the connection transparent for binary 660data). 661.It Cm ExitOnForwardFailure 662Specifies whether 663.Xr ssh 1 664should terminate the connection if it cannot set up all requested 665dynamic, tunnel, local, and remote port forwardings, (e.g.\& 666if either end is unable to bind and listen on a specified port). 667Note that 668.Cm ExitOnForwardFailure 669does not apply to connections made over port forwardings and will not, 670for example, cause 671.Xr ssh 1 672to exit if TCP connections to the ultimate forwarding destination fail. 673The argument must be 674.Cm yes 675or 676.Cm no 677(the default). 678.It Cm FingerprintHash 679Specifies the hash algorithm used when displaying key fingerprints. 680Valid options are: 681.Cm md5 682and 683.Cm sha256 684(the default). 685.It Cm ForwardAgent 686Specifies whether the connection to the authentication agent (if any) 687will be forwarded to the remote machine. 688The argument may be 689.Cm yes , 690.Cm no 691(the default), 692an explicit path to an agent socket or the name of an environment variable 693(beginning with 694.Sq $ ) 695in which to find the path. 696.Pp 697Agent forwarding should be enabled with caution. 698Users with the ability to bypass file permissions on the remote host 699(for the agent's Unix-domain socket) 700can access the local agent through the forwarded connection. 701An attacker cannot obtain key material from the agent, 702however they can perform operations on the keys that enable them to 703authenticate using the identities loaded into the agent. 704.It Cm ForwardX11 705Specifies whether X11 connections will be automatically redirected 706over the secure channel and 707.Ev DISPLAY 708set. 709The argument must be 710.Cm yes 711or 712.Cm no 713(the default). 714.Pp 715X11 forwarding should be enabled with caution. 716Users with the ability to bypass file permissions on the remote host 717(for the user's X11 authorization database) 718can access the local X11 display through the forwarded connection. 719An attacker may then be able to perform activities such as keystroke monitoring 720if the 721.Cm ForwardX11Trusted 722option is also enabled. 723.It Cm ForwardX11Timeout 724Specify a timeout for untrusted X11 forwarding 725using the format described in the 726.Sx TIME FORMATS 727section of 728.Xr sshd_config 5 . 729X11 connections received by 730.Xr ssh 1 731after this time will be refused. 732Setting 733.Cm ForwardX11Timeout 734to zero will disable the timeout and permit X11 forwarding for the life 735of the connection. 736The default is to disable untrusted X11 forwarding after twenty minutes has 737elapsed. 738.It Cm ForwardX11Trusted 739If this option is set to 740.Cm yes , 741remote X11 clients will have full access to the original X11 display. 742.Pp 743If this option is set to 744.Cm no 745(the default), 746remote X11 clients will be considered untrusted and prevented 747from stealing or tampering with data belonging to trusted X11 748clients. 749Furthermore, the 750.Xr xauth 1 751token used for the session will be set to expire after 20 minutes. 752Remote clients will be refused access after this time. 753.Pp 754See the X11 SECURITY extension specification for full details on 755the restrictions imposed on untrusted clients. 756.It Cm GatewayPorts 757Specifies whether remote hosts are allowed to connect to local 758forwarded ports. 759By default, 760.Xr ssh 1 761binds local port forwardings to the loopback address. 762This prevents other remote hosts from connecting to forwarded ports. 763.Cm GatewayPorts 764can be used to specify that ssh 765should bind local port forwardings to the wildcard address, 766thus allowing remote hosts to connect to forwarded ports. 767The argument must be 768.Cm yes 769or 770.Cm no 771(the default). 772.It Cm GlobalKnownHostsFile 773Specifies one or more files to use for the global 774host key database, separated by whitespace. 775The default is 776.Pa /etc/ssh/ssh_known_hosts , 777.Pa /etc/ssh/ssh_known_hosts2 . 778.It Cm GSSAPIAuthentication 779Specifies whether user authentication based on GSSAPI is allowed. 780The default is 781.Cm no . 782.It Cm GSSAPIDelegateCredentials 783Forward (delegate) credentials to the server. 784The default is 785.Cm no . 786.It Cm HashKnownHosts 787Indicates that 788.Xr ssh 1 789should hash host names and addresses when they are added to 790.Pa ~/.ssh/known_hosts . 791These hashed names may be used normally by 792.Xr ssh 1 793and 794.Xr sshd 8 , 795but they do not visually reveal identifying information if the 796file's contents are disclosed. 797The default is 798.Cm no . 799Note that existing names and addresses in known hosts files 800will not be converted automatically, 801but may be manually hashed using 802.Xr ssh-keygen 1 . 803.It Cm HostbasedAuthentication 804Specifies whether to try rhosts based authentication with public key 805authentication. 806The argument must be 807.Cm yes 808or 809.Cm no 810(the default). 811.It Cm HostbasedKeyTypes 812Specifies the key types that will be used for hostbased authentication 813as a comma-separated list of patterns. 814Alternately if the specified list begins with a 815.Sq + 816character, then the specified key types will be appended to the default set 817instead of replacing them. 818If the specified list begins with a 819.Sq - 820character, then the specified key types (including wildcards) will be removed 821from the default set instead of replacing them. 822If the specified list begins with a 823.Sq ^ 824character, then the specified key types will be placed at the head of the 825default set. 826The default for this option is: 827.Bd -literal -offset 3n 828ssh-ed25519-cert-v01@openssh.com, 829ecdsa-sha2-nistp256-cert-v01@openssh.com, 830ecdsa-sha2-nistp384-cert-v01@openssh.com, 831ecdsa-sha2-nistp521-cert-v01@openssh.com, 832sk-ssh-ed25519-cert-v01@openssh.com, 833sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 834rsa-sha2-512-cert-v01@openssh.com, 835rsa-sha2-256-cert-v01@openssh.com, 836ssh-rsa-cert-v01@openssh.com, 837ssh-ed25519, 838ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 839sk-ssh-ed25519@openssh.com, 840sk-ecdsa-sha2-nistp256@openssh.com, 841rsa-sha2-512,rsa-sha2-256,ssh-rsa 842.Ed 843.Pp 844The 845.Fl Q 846option of 847.Xr ssh 1 848may be used to list supported key types. 849.It Cm HostKeyAlgorithms 850Specifies the host key algorithms 851that the client wants to use in order of preference. 852Alternately if the specified list begins with a 853.Sq + 854character, then the specified key types will be appended to the default set 855instead of replacing them. 856If the specified list begins with a 857.Sq - 858character, then the specified key types (including wildcards) will be removed 859from the default set instead of replacing them. 860If the specified list begins with a 861.Sq ^ 862character, then the specified key types will be placed at the head of the 863default set. 864The default for this option is: 865.Bd -literal -offset 3n 866ssh-ed25519-cert-v01@openssh.com, 867ecdsa-sha2-nistp256-cert-v01@openssh.com, 868ecdsa-sha2-nistp384-cert-v01@openssh.com, 869ecdsa-sha2-nistp521-cert-v01@openssh.com, 870sk-ssh-ed25519-cert-v01@openssh.com, 871sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 872rsa-sha2-512-cert-v01@openssh.com, 873rsa-sha2-256-cert-v01@openssh.com, 874ssh-rsa-cert-v01@openssh.com, 875ssh-ed25519, 876ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 877sk-ecdsa-sha2-nistp256@openssh.com, 878sk-ssh-ed25519@openssh.com, 879rsa-sha2-512,rsa-sha2-256,ssh-rsa 880.Ed 881.Pp 882If hostkeys are known for the destination host then this default is modified 883to prefer their algorithms. 884.Pp 885The list of available key types may also be obtained using 886.Qq ssh -Q HostKeyAlgorithms . 887.It Cm HostKeyAlias 888Specifies an alias that should be used instead of the 889real host name when looking up or saving the host key 890in the host key database files and when validating host certificates. 891This option is useful for tunneling SSH connections 892or for multiple servers running on a single host. 893.It Cm Hostname 894Specifies the real host name to log into. 895This can be used to specify nicknames or abbreviations for hosts. 896Arguments to 897.Cm Hostname 898accept the tokens described in the 899.Sx TOKENS 900section. 901Numeric IP addresses are also permitted (both on the command line and in 902.Cm Hostname 903specifications). 904The default is the name given on the command line. 905.It Cm IdentitiesOnly 906Specifies that 907.Xr ssh 1 908should only use the configured authentication identity and certificate files 909(either the default files, or those explicitly configured in the 910.Nm 911files 912or passed on the 913.Xr ssh 1 914command-line), 915even if 916.Xr ssh-agent 1 917or a 918.Cm PKCS11Provider 919or 920.Cm SecurityKeyProvider 921offers more identities. 922The argument to this keyword must be 923.Cm yes 924or 925.Cm no 926(the default). 927This option is intended for situations where ssh-agent 928offers many different identities. 929.It Cm IdentityAgent 930Specifies the 931.Ux Ns -domain 932socket used to communicate with the authentication agent. 933.Pp 934This option overrides the 935.Ev SSH_AUTH_SOCK 936environment variable and can be used to select a specific agent. 937Setting the socket name to 938.Cm none 939disables the use of an authentication agent. 940If the string 941.Qq SSH_AUTH_SOCK 942is specified, the location of the socket will be read from the 943.Ev SSH_AUTH_SOCK 944environment variable. 945Otherwise if the specified value begins with a 946.Sq $ 947character, then it will be treated as an environment variable containing 948the location of the socket. 949.Pp 950Arguments to 951.Cm IdentityAgent 952may use the tilde syntax to refer to a user's home directory, 953the tokens described in the 954.Sx TOKENS 955section and environment variables as described in the 956.Sx ENVIRONMENT VARIABLES 957section. 958.It Cm IdentityFile 959Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 960Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 961The default is 962.Pa ~/.ssh/id_dsa , 963.Pa ~/.ssh/id_ecdsa , 964.Pa ~/.ssh/id_ecdsa_sk , 965.Pa ~/.ssh/id_ed25519 , 966.Pa ~/.ssh/id_ed25519_sk 967and 968.Pa ~/.ssh/id_rsa . 969Additionally, any identities represented by the authentication agent 970will be used for authentication unless 971.Cm IdentitiesOnly 972is set. 973If no certificates have been explicitly specified by 974.Cm CertificateFile , 975.Xr ssh 1 976will try to load certificate information from the filename obtained by 977appending 978.Pa -cert.pub 979to the path of a specified 980.Cm IdentityFile . 981.Pp 982Arguments to 983.Cm IdentityFile 984may use the tilde syntax to refer to a user's home directory 985or the tokens described in the 986.Sx TOKENS 987section. 988.Pp 989It is possible to have 990multiple identity files specified in configuration files; all these 991identities will be tried in sequence. 992Multiple 993.Cm IdentityFile 994directives will add to the list of identities tried (this behaviour 995differs from that of other configuration directives). 996.Pp 997.Cm IdentityFile 998may be used in conjunction with 999.Cm IdentitiesOnly 1000to select which identities in an agent are offered during authentication. 1001.Cm IdentityFile 1002may also be used in conjunction with 1003.Cm CertificateFile 1004in order to provide any certificate also needed for authentication with 1005the identity. 1006.It Cm IgnoreUnknown 1007Specifies a pattern-list of unknown options to be ignored if they are 1008encountered in configuration parsing. 1009This may be used to suppress errors if 1010.Nm 1011contains options that are unrecognised by 1012.Xr ssh 1 . 1013It is recommended that 1014.Cm IgnoreUnknown 1015be listed early in the configuration file as it will not be applied 1016to unknown options that appear before it. 1017.It Cm Include 1018Include the specified configuration file(s). 1019Multiple pathnames may be specified and each pathname may contain 1020.Xr glob 7 1021wildcards and, for user configurations, shell-like 1022.Sq ~ 1023references to user home directories. 1024Wildcards will be expanded and processed in lexical order. 1025Files without absolute paths are assumed to be in 1026.Pa ~/.ssh 1027if included in a user configuration file or 1028.Pa /etc/ssh 1029if included from the system configuration file. 1030.Cm Include 1031directive may appear inside a 1032.Cm Match 1033or 1034.Cm Host 1035block 1036to perform conditional inclusion. 1037.It Cm IPQoS 1038Specifies the IPv4 type-of-service or DSCP class for connections. 1039Accepted values are 1040.Cm af11 , 1041.Cm af12 , 1042.Cm af13 , 1043.Cm af21 , 1044.Cm af22 , 1045.Cm af23 , 1046.Cm af31 , 1047.Cm af32 , 1048.Cm af33 , 1049.Cm af41 , 1050.Cm af42 , 1051.Cm af43 , 1052.Cm cs0 , 1053.Cm cs1 , 1054.Cm cs2 , 1055.Cm cs3 , 1056.Cm cs4 , 1057.Cm cs5 , 1058.Cm cs6 , 1059.Cm cs7 , 1060.Cm ef , 1061.Cm le , 1062.Cm lowdelay , 1063.Cm throughput , 1064.Cm reliability , 1065a numeric value, or 1066.Cm none 1067to use the operating system default. 1068This option may take one or two arguments, separated by whitespace. 1069If one argument is specified, it is used as the packet class unconditionally. 1070If two values are specified, the first is automatically selected for 1071interactive sessions and the second for non-interactive sessions. 1072The default is 1073.Cm af21 1074(Low-Latency Data) 1075for interactive sessions and 1076.Cm cs1 1077(Lower Effort) 1078for non-interactive sessions. 1079.It Cm KbdInteractiveAuthentication 1080Specifies whether to use keyboard-interactive authentication. 1081The argument to this keyword must be 1082.Cm yes 1083(the default) 1084or 1085.Cm no . 1086.It Cm KbdInteractiveDevices 1087Specifies the list of methods to use in keyboard-interactive authentication. 1088Multiple method names must be comma-separated. 1089The default is to use the server specified list. 1090The methods available vary depending on what the server supports. 1091For an OpenSSH server, 1092it may be zero or more of: 1093.Cm bsdauth , 1094.Cm pam , 1095and 1096.Cm skey . 1097.It Cm KexAlgorithms 1098Specifies the available KEX (Key Exchange) algorithms. 1099Multiple algorithms must be comma-separated. 1100If the specified list begins with a 1101.Sq + 1102character, then the specified methods will be appended to the default set 1103instead of replacing them. 1104If the specified list begins with a 1105.Sq - 1106character, then the specified methods (including wildcards) will be removed 1107from the default set instead of replacing them. 1108If the specified list begins with a 1109.Sq ^ 1110character, then the specified methods will be placed at the head of the 1111default set. 1112The default is: 1113.Bd -literal -offset indent 1114curve25519-sha256,curve25519-sha256@libssh.org, 1115ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1116diffie-hellman-group-exchange-sha256, 1117diffie-hellman-group16-sha512, 1118diffie-hellman-group18-sha512, 1119diffie-hellman-group14-sha256 1120.Ed 1121.Pp 1122The list of available key exchange algorithms may also be obtained using 1123.Qq ssh -Q kex . 1124.It Cm KnownHostsCommand 1125Specifies a command to use to obtain a list of host keys, in addition to 1126those listed in 1127.Cm UserKnownHostsFile 1128and 1129.Cm GlobalKnownHostsFile . 1130This command is executed after the files have been read. 1131It may write host key lines to standard output in identical format to the 1132usual files (described in the 1133.Sx VERIFYING HOST KEYS 1134section in 1135.Xr ssh 1 ) . 1136Arguments to 1137.Cm KnownHostsCommand 1138accept the tokens described in the 1139.Sx TOKENS 1140section. 1141The command may be invoked multiple times per connection: once when preparing 1142the preference list of host key algorithms to use, again to obtain the 1143host key for the requested host name and, if 1144.Cm CheckHostIP 1145is enabled, one more time to obtain the host key matching the server's 1146address. 1147If the command exits abnormally or returns a non-zero exit status then the 1148connection is terminated. 1149.It Cm LocalCommand 1150Specifies a command to execute on the local machine after successfully 1151connecting to the server. 1152The command string extends to the end of the line, and is executed with 1153the user's shell. 1154Arguments to 1155.Cm LocalCommand 1156accept the tokens described in the 1157.Sx TOKENS 1158section. 1159.Pp 1160The command is run synchronously and does not have access to the 1161session of the 1162.Xr ssh 1 1163that spawned it. 1164It should not be used for interactive commands. 1165.Pp 1166This directive is ignored unless 1167.Cm PermitLocalCommand 1168has been enabled. 1169.It Cm LocalForward 1170Specifies that a TCP port on the local machine be forwarded over 1171the secure channel to the specified host and port from the remote machine. 1172The first argument specifies the listener and may be 1173.Sm off 1174.Oo Ar bind_address : Oc Ar port 1175.Sm on 1176or a Unix domain socket path. 1177The second argument is the destination and may be 1178.Ar host : Ns Ar hostport 1179or a Unix domain socket path if the remote host supports it. 1180.Pp 1181IPv6 addresses can be specified by enclosing addresses in square brackets. 1182Multiple forwardings may be specified, and additional forwardings can be 1183given on the command line. 1184Only the superuser can forward privileged ports. 1185By default, the local port is bound in accordance with the 1186.Cm GatewayPorts 1187setting. 1188However, an explicit 1189.Ar bind_address 1190may be used to bind the connection to a specific address. 1191The 1192.Ar bind_address 1193of 1194.Cm localhost 1195indicates that the listening port be bound for local use only, while an 1196empty address or 1197.Sq * 1198indicates that the port should be available from all interfaces. 1199Unix domain socket paths may use the tokens described in the 1200.Sx TOKENS 1201section and environment variables as described in the 1202.Sx ENVIRONMENT VARIABLES 1203section. 1204.It Cm LogLevel 1205Gives the verbosity level that is used when logging messages from 1206.Xr ssh 1 . 1207The possible values are: 1208QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1209The default is INFO. 1210DEBUG and DEBUG1 are equivalent. 1211DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1212.It Cm LogVerbose 1213Specify one or more overrides to LogLevel. 1214An override consists of a pattern lists that matches the source file, function 1215and line number to force detailed logging for. 1216For example, an override pattern of: 1217.Bd -literal -offset indent 1218kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1219.Ed 1220.Pp 1221would enable detailed logging for line 1000 of 1222.Pa kex.c , 1223everything in the 1224.Fn kex_exchange_identification 1225function, and all code in the 1226.Pa packet.c 1227file. 1228This option is intended for debugging and no overrides are enabled by default. 1229.It Cm MACs 1230Specifies the MAC (message authentication code) algorithms 1231in order of preference. 1232The MAC algorithm is used for data integrity protection. 1233Multiple algorithms must be comma-separated. 1234If the specified list begins with a 1235.Sq + 1236character, then the specified algorithms will be appended to the default set 1237instead of replacing them. 1238If the specified list begins with a 1239.Sq - 1240character, then the specified algorithms (including wildcards) will be removed 1241from the default set instead of replacing them. 1242If the specified list begins with a 1243.Sq ^ 1244character, then the specified algorithms will be placed at the head of the 1245default set. 1246.Pp 1247The algorithms that contain 1248.Qq -etm 1249calculate the MAC after encryption (encrypt-then-mac). 1250These are considered safer and their use recommended. 1251.Pp 1252The default is: 1253.Bd -literal -offset indent 1254umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1255hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1256hmac-sha1-etm@openssh.com, 1257umac-64@openssh.com,umac-128@openssh.com, 1258hmac-sha2-256,hmac-sha2-512,hmac-sha1 1259.Ed 1260.Pp 1261The list of available MAC algorithms may also be obtained using 1262.Qq ssh -Q mac . 1263.It Cm NoHostAuthenticationForLocalhost 1264Disable host authentication for localhost (loopback addresses). 1265The argument to this keyword must be 1266.Cm yes 1267or 1268.Cm no 1269(the default). 1270.It Cm NumberOfPasswordPrompts 1271Specifies the number of password prompts before giving up. 1272The argument to this keyword must be an integer. 1273The default is 3. 1274.It Cm PasswordAuthentication 1275Specifies whether to use password authentication. 1276The argument to this keyword must be 1277.Cm yes 1278(the default) 1279or 1280.Cm no . 1281.It Cm PermitLocalCommand 1282Allow local command execution via the 1283.Ic LocalCommand 1284option or using the 1285.Ic !\& Ns Ar command 1286escape sequence in 1287.Xr ssh 1 . 1288The argument must be 1289.Cm yes 1290or 1291.Cm no 1292(the default). 1293.It Cm PKCS11Provider 1294Specifies which PKCS#11 provider to use or 1295.Cm none 1296to indicate that no provider should be used (the default). 1297The argument to this keyword is a path to the PKCS#11 shared library 1298.Xr ssh 1 1299should use to communicate with a PKCS#11 token providing keys for user 1300authentication. 1301.It Cm Port 1302Specifies the port number to connect on the remote host. 1303The default is 22. 1304.It Cm PreferredAuthentications 1305Specifies the order in which the client should try authentication methods. 1306This allows a client to prefer one method (e.g.\& 1307.Cm keyboard-interactive ) 1308over another method (e.g.\& 1309.Cm password ) . 1310The default is: 1311.Bd -literal -offset indent 1312gssapi-with-mic,hostbased,publickey, 1313keyboard-interactive,password 1314.Ed 1315.It Cm ProxyCommand 1316Specifies the command to use to connect to the server. 1317The command 1318string extends to the end of the line, and is executed 1319using the user's shell 1320.Ql exec 1321directive to avoid a lingering shell process. 1322.Pp 1323Arguments to 1324.Cm ProxyCommand 1325accept the tokens described in the 1326.Sx TOKENS 1327section. 1328The command can be basically anything, 1329and should read from its standard input and write to its standard output. 1330It should eventually connect an 1331.Xr sshd 8 1332server running on some machine, or execute 1333.Ic sshd -i 1334somewhere. 1335Host key management will be done using the 1336.Cm Hostname 1337of the host being connected (defaulting to the name typed by the user). 1338Setting the command to 1339.Cm none 1340disables this option entirely. 1341Note that 1342.Cm CheckHostIP 1343is not available for connects with a proxy command. 1344.Pp 1345This directive is useful in conjunction with 1346.Xr nc 1 1347and its proxy support. 1348For example, the following directive would connect via an HTTP proxy at 1349192.0.2.0: 1350.Bd -literal -offset 3n 1351ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1352.Ed 1353.It Cm ProxyJump 1354Specifies one or more jump proxies as either 1355.Xo 1356.Sm off 1357.Op Ar user No @ 1358.Ar host 1359.Op : Ns Ar port 1360.Sm on 1361or an ssh URI 1362.Xc . 1363Multiple proxies may be separated by comma characters and will be visited 1364sequentially. 1365Setting this option will cause 1366.Xr ssh 1 1367to connect to the target host by first making a 1368.Xr ssh 1 1369connection to the specified 1370.Cm ProxyJump 1371host and then establishing a 1372TCP forwarding to the ultimate target from there. 1373.Pp 1374Note that this option will compete with the 1375.Cm ProxyCommand 1376option - whichever is specified first will prevent later instances of the 1377other from taking effect. 1378.Pp 1379Note also that the configuration for the destination host (either supplied 1380via the command-line or the configuration file) is not generally applied 1381to jump hosts. 1382.Pa ~/.ssh/config 1383should be used if specific configuration is required for jump hosts. 1384.It Cm ProxyUseFdpass 1385Specifies that 1386.Cm ProxyCommand 1387will pass a connected file descriptor back to 1388.Xr ssh 1 1389instead of continuing to execute and pass data. 1390The default is 1391.Cm no . 1392.It Cm PubkeyAcceptedKeyTypes 1393Specifies the key types that will be used for public key authentication 1394as a comma-separated list of patterns. 1395If the specified list begins with a 1396.Sq + 1397character, then the key types after it will be appended to the default 1398instead of replacing it. 1399If the specified list begins with a 1400.Sq - 1401character, then the specified key types (including wildcards) will be removed 1402from the default set instead of replacing them. 1403If the specified list begins with a 1404.Sq ^ 1405character, then the specified key types will be placed at the head of the 1406default set. 1407The default for this option is: 1408.Bd -literal -offset 3n 1409ssh-ed25519-cert-v01@openssh.com, 1410ecdsa-sha2-nistp256-cert-v01@openssh.com, 1411ecdsa-sha2-nistp384-cert-v01@openssh.com, 1412ecdsa-sha2-nistp521-cert-v01@openssh.com, 1413sk-ssh-ed25519-cert-v01@openssh.com, 1414sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1415rsa-sha2-512-cert-v01@openssh.com, 1416rsa-sha2-256-cert-v01@openssh.com, 1417ssh-rsa-cert-v01@openssh.com, 1418ssh-ed25519, 1419ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1420sk-ssh-ed25519@openssh.com, 1421sk-ecdsa-sha2-nistp256@openssh.com, 1422rsa-sha2-512,rsa-sha2-256,ssh-rsa 1423.Ed 1424.Pp 1425The list of available key types may also be obtained using 1426.Qq ssh -Q PubkeyAcceptedKeyTypes . 1427.It Cm PubkeyAuthentication 1428Specifies whether to try public key authentication. 1429The argument to this keyword must be 1430.Cm yes 1431(the default) 1432or 1433.Cm no . 1434.It Cm RekeyLimit 1435Specifies the maximum amount of data that may be transmitted before the 1436session key is renegotiated, optionally followed a maximum amount of 1437time that may pass before the session key is renegotiated. 1438The first argument is specified in bytes and may have a suffix of 1439.Sq K , 1440.Sq M , 1441or 1442.Sq G 1443to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1444The default is between 1445.Sq 1G 1446and 1447.Sq 4G , 1448depending on the cipher. 1449The optional second value is specified in seconds and may use any of the 1450units documented in the TIME FORMATS section of 1451.Xr sshd_config 5 . 1452The default value for 1453.Cm RekeyLimit 1454is 1455.Cm default none , 1456which means that rekeying is performed after the cipher's default amount 1457of data has been sent or received and no time based rekeying is done. 1458.It Cm RemoteCommand 1459Specifies a command to execute on the remote machine after successfully 1460connecting to the server. 1461The command string extends to the end of the line, and is executed with 1462the user's shell. 1463Arguments to 1464.Cm RemoteCommand 1465accept the tokens described in the 1466.Sx TOKENS 1467section. 1468.It Cm RemoteForward 1469Specifies that a TCP port on the remote machine be forwarded over 1470the secure channel. 1471The remote port may either be forwarded to a specified host and port 1472from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1473client to connect to arbitrary destinations from the local machine. 1474The first argument is the listening specification and may be 1475.Sm off 1476.Oo Ar bind_address : Oc Ar port 1477.Sm on 1478or, if the remote host supports it, a Unix domain socket path. 1479If forwarding to a specific destination then the second argument must be 1480.Ar host : Ns Ar hostport 1481or a Unix domain socket path, 1482otherwise if no destination argument is specified then the remote forwarding 1483will be established as a SOCKS proxy. 1484.Pp 1485IPv6 addresses can be specified by enclosing addresses in square brackets. 1486Multiple forwardings may be specified, and additional 1487forwardings can be given on the command line. 1488Privileged ports can be forwarded only when 1489logging in as root on the remote machine. 1490Unix domain socket paths may use the tokens described in the 1491.Sx TOKENS 1492section and environment variables as described in the 1493.Sx ENVIRONMENT VARIABLES 1494section. 1495.Pp 1496If the 1497.Ar port 1498argument is 0, 1499the listen port will be dynamically allocated on the server and reported 1500to the client at run time. 1501.Pp 1502If the 1503.Ar bind_address 1504is not specified, the default is to only bind to loopback addresses. 1505If the 1506.Ar bind_address 1507is 1508.Ql * 1509or an empty string, then the forwarding is requested to listen on all 1510interfaces. 1511Specifying a remote 1512.Ar bind_address 1513will only succeed if the server's 1514.Cm GatewayPorts 1515option is enabled (see 1516.Xr sshd_config 5 ) . 1517.It Cm RequestTTY 1518Specifies whether to request a pseudo-tty for the session. 1519The argument may be one of: 1520.Cm no 1521(never request a TTY), 1522.Cm yes 1523(always request a TTY when standard input is a TTY), 1524.Cm force 1525(always request a TTY) or 1526.Cm auto 1527(request a TTY when opening a login session). 1528This option mirrors the 1529.Fl t 1530and 1531.Fl T 1532flags for 1533.Xr ssh 1 . 1534.It Cm RevokedHostKeys 1535Specifies revoked host public keys. 1536Keys listed in this file will be refused for host authentication. 1537Note that if this file does not exist or is not readable, 1538then host authentication will be refused for all hosts. 1539Keys may be specified as a text file, listing one public key per line, or as 1540an OpenSSH Key Revocation List (KRL) as generated by 1541.Xr ssh-keygen 1 . 1542For more information on KRLs, see the KEY REVOCATION LISTS section in 1543.Xr ssh-keygen 1 . 1544.It Cm SecurityKeyProvider 1545Specifies a path to a library that will be used when loading any 1546FIDO authenticator-hosted keys, overriding the default of using 1547the built-in USB HID support. 1548.Pp 1549If the specified value begins with a 1550.Sq $ 1551character, then it will be treated as an environment variable containing 1552the path to the library. 1553.It Cm SendEnv 1554Specifies what variables from the local 1555.Xr environ 7 1556should be sent to the server. 1557The server must also support it, and the server must be configured to 1558accept these environment variables. 1559Note that the 1560.Ev TERM 1561environment variable is always sent whenever a 1562pseudo-terminal is requested as it is required by the protocol. 1563Refer to 1564.Cm AcceptEnv 1565in 1566.Xr sshd_config 5 1567for how to configure the server. 1568Variables are specified by name, which may contain wildcard characters. 1569Multiple environment variables may be separated by whitespace or spread 1570across multiple 1571.Cm SendEnv 1572directives. 1573.Pp 1574See 1575.Sx PATTERNS 1576for more information on patterns. 1577.Pp 1578It is possible to clear previously set 1579.Cm SendEnv 1580variable names by prefixing patterns with 1581.Pa - . 1582The default is not to send any environment variables. 1583.It Cm ServerAliveCountMax 1584Sets the number of server alive messages (see below) which may be 1585sent without 1586.Xr ssh 1 1587receiving any messages back from the server. 1588If this threshold is reached while server alive messages are being sent, 1589ssh will disconnect from the server, terminating the session. 1590It is important to note that the use of server alive messages is very 1591different from 1592.Cm TCPKeepAlive 1593(below). 1594The server alive messages are sent through the encrypted channel 1595and therefore will not be spoofable. 1596The TCP keepalive option enabled by 1597.Cm TCPKeepAlive 1598is spoofable. 1599The server alive mechanism is valuable when the client or 1600server depend on knowing when a connection has become unresponsive. 1601.Pp 1602The default value is 3. 1603If, for example, 1604.Cm ServerAliveInterval 1605(see below) is set to 15 and 1606.Cm ServerAliveCountMax 1607is left at the default, if the server becomes unresponsive, 1608ssh will disconnect after approximately 45 seconds. 1609.It Cm ServerAliveInterval 1610Sets a timeout interval in seconds after which if no data has been received 1611from the server, 1612.Xr ssh 1 1613will send a message through the encrypted 1614channel to request a response from the server. 1615The default 1616is 0, indicating that these messages will not be sent to the server. 1617.It Cm SetEnv 1618Directly specify one or more environment variables and their contents to 1619be sent to the server. 1620Similarly to 1621.Cm SendEnv , 1622the server must be prepared to accept the environment variable. 1623.It Cm StreamLocalBindMask 1624Sets the octal file creation mode mask 1625.Pq umask 1626used when creating a Unix-domain socket file for local or remote 1627port forwarding. 1628This option is only used for port forwarding to a Unix-domain socket file. 1629.Pp 1630The default value is 0177, which creates a Unix-domain socket file that is 1631readable and writable only by the owner. 1632Note that not all operating systems honor the file mode on Unix-domain 1633socket files. 1634.It Cm StreamLocalBindUnlink 1635Specifies whether to remove an existing Unix-domain socket file for local 1636or remote port forwarding before creating a new one. 1637If the socket file already exists and 1638.Cm StreamLocalBindUnlink 1639is not enabled, 1640.Nm ssh 1641will be unable to forward the port to the Unix-domain socket file. 1642This option is only used for port forwarding to a Unix-domain socket file. 1643.Pp 1644The argument must be 1645.Cm yes 1646or 1647.Cm no 1648(the default). 1649.It Cm StrictHostKeyChecking 1650If this flag is set to 1651.Cm yes , 1652.Xr ssh 1 1653will never automatically add host keys to the 1654.Pa ~/.ssh/known_hosts 1655file, and refuses to connect to hosts whose host key has changed. 1656This provides maximum protection against man-in-the-middle (MITM) attacks, 1657though it can be annoying when the 1658.Pa /etc/ssh/ssh_known_hosts 1659file is poorly maintained or when connections to new hosts are 1660frequently made. 1661This option forces the user to manually 1662add all new hosts. 1663.Pp 1664If this flag is set to 1665.Dq accept-new 1666then ssh will automatically add new host keys to the user 1667known hosts files, but will not permit connections to hosts with 1668changed host keys. 1669If this flag is set to 1670.Dq no 1671or 1672.Dq off , 1673ssh will automatically add new host keys to the user known hosts files 1674and allow connections to hosts with changed hostkeys to proceed, 1675subject to some restrictions. 1676If this flag is set to 1677.Cm ask 1678(the default), 1679new host keys 1680will be added to the user known host files only after the user 1681has confirmed that is what they really want to do, and 1682ssh will refuse to connect to hosts whose host key has changed. 1683The host keys of 1684known hosts will be verified automatically in all cases. 1685.It Cm SyslogFacility 1686Gives the facility code that is used when logging messages from 1687.Xr ssh 1 . 1688The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1689LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1690The default is USER. 1691.It Cm TCPKeepAlive 1692Specifies whether the system should send TCP keepalive messages to the 1693other side. 1694If they are sent, death of the connection or crash of one 1695of the machines will be properly noticed. 1696However, this means that 1697connections will die if the route is down temporarily, and some people 1698find it annoying. 1699.Pp 1700The default is 1701.Cm yes 1702(to send TCP keepalive messages), and the client will notice 1703if the network goes down or the remote host dies. 1704This is important in scripts, and many users want it too. 1705.Pp 1706To disable TCP keepalive messages, the value should be set to 1707.Cm no . 1708See also 1709.Cm ServerAliveInterval 1710for protocol-level keepalives. 1711.It Cm Tunnel 1712Request 1713.Xr tun 4 1714device forwarding between the client and the server. 1715The argument must be 1716.Cm yes , 1717.Cm point-to-point 1718(layer 3), 1719.Cm ethernet 1720(layer 2), 1721or 1722.Cm no 1723(the default). 1724Specifying 1725.Cm yes 1726requests the default tunnel mode, which is 1727.Cm point-to-point . 1728.It Cm TunnelDevice 1729Specifies the 1730.Xr tun 4 1731devices to open on the client 1732.Pq Ar local_tun 1733and the server 1734.Pq Ar remote_tun . 1735.Pp 1736The argument must be 1737.Sm off 1738.Ar local_tun Op : Ar remote_tun . 1739.Sm on 1740The devices may be specified by numerical ID or the keyword 1741.Cm any , 1742which uses the next available tunnel device. 1743If 1744.Ar remote_tun 1745is not specified, it defaults to 1746.Cm any . 1747The default is 1748.Cm any:any . 1749.It Cm UpdateHostKeys 1750Specifies whether 1751.Xr ssh 1 1752should accept notifications of additional hostkeys from the server sent 1753after authentication has completed and add them to 1754.Cm UserKnownHostsFile . 1755The argument must be 1756.Cm yes , 1757.Cm no 1758or 1759.Cm ask . 1760This option allows learning alternate hostkeys for a server 1761and supports graceful key rotation by allowing a server to send replacement 1762public keys before old ones are removed. 1763.Pp 1764Additional hostkeys are only accepted if the key used to authenticate the 1765host was already trusted or explicitly accepted by the user, the host was 1766authenticated via 1767.Cm UserKnownHostsFile 1768(i.e. not 1769.Cm GlobalKnownHostsFile ) 1770and the host was authenticated using a plain key and not a certificate. 1771.Pp 1772.Cm UpdateHostKeys 1773is enabled by default if the user has not overridden the default 1774.Cm UserKnownHostsFile 1775setting and has not enabled 1776.Cm VerifyHostKeyDNS , 1777otherwise 1778.Cm UpdateHostKeys 1779will be set to 1780.Cm no . 1781.Pp 1782If 1783.Cm UpdateHostKeys 1784is set to 1785.Cm ask , 1786then the user is asked to confirm the modifications to the known_hosts file. 1787Confirmation is currently incompatible with 1788.Cm ControlPersist , 1789and will be disabled if it is enabled. 1790.Pp 1791Presently, only 1792.Xr sshd 8 1793from OpenSSH 6.8 and greater support the 1794.Qq hostkeys@openssh.com 1795protocol extension used to inform the client of all the server's hostkeys. 1796.It Cm User 1797Specifies the user to log in as. 1798This can be useful when a different user name is used on different machines. 1799This saves the trouble of 1800having to remember to give the user name on the command line. 1801.It Cm UserKnownHostsFile 1802Specifies one or more files to use for the user 1803host key database, separated by whitespace. 1804Each filename may use tilde notation to refer to the user's home directory, 1805the tokens described in the 1806.Sx TOKENS 1807section and environment variables as described in the 1808.Sx ENVIRONMENT VARIABLES 1809section. 1810The default is 1811.Pa ~/.ssh/known_hosts , 1812.Pa ~/.ssh/known_hosts2 . 1813.It Cm VerifyHostKeyDNS 1814Specifies whether to verify the remote key using DNS and SSHFP resource 1815records. 1816If this option is set to 1817.Cm yes , 1818the client will implicitly trust keys that match a secure fingerprint 1819from DNS. 1820Insecure fingerprints will be handled as if this option was set to 1821.Cm ask . 1822If this option is set to 1823.Cm ask , 1824information on fingerprint match will be displayed, but the user will still 1825need to confirm new host keys according to the 1826.Cm StrictHostKeyChecking 1827option. 1828The default is 1829.Cm no . 1830.Pp 1831See also 1832.Sx VERIFYING HOST KEYS 1833in 1834.Xr ssh 1 . 1835.It Cm VisualHostKey 1836If this flag is set to 1837.Cm yes , 1838an ASCII art representation of the remote host key fingerprint is 1839printed in addition to the fingerprint string at login and 1840for unknown host keys. 1841If this flag is set to 1842.Cm no 1843(the default), 1844no fingerprint strings are printed at login and 1845only the fingerprint string will be printed for unknown host keys. 1846.It Cm XAuthLocation 1847Specifies the full pathname of the 1848.Xr xauth 1 1849program. 1850The default is 1851.Pa /usr/X11R6/bin/xauth . 1852.El 1853.Sh PATTERNS 1854A 1855.Em pattern 1856consists of zero or more non-whitespace characters, 1857.Sq * 1858(a wildcard that matches zero or more characters), 1859or 1860.Sq ?\& 1861(a wildcard that matches exactly one character). 1862For example, to specify a set of declarations for any host in the 1863.Qq .co.uk 1864set of domains, 1865the following pattern could be used: 1866.Pp 1867.Dl Host *.co.uk 1868.Pp 1869The following pattern 1870would match any host in the 192.168.0.[0-9] network range: 1871.Pp 1872.Dl Host 192.168.0.? 1873.Pp 1874A 1875.Em pattern-list 1876is a comma-separated list of patterns. 1877Patterns within pattern-lists may be negated 1878by preceding them with an exclamation mark 1879.Pq Sq !\& . 1880For example, 1881to allow a key to be used from anywhere within an organization 1882except from the 1883.Qq dialup 1884pool, 1885the following entry (in authorized_keys) could be used: 1886.Pp 1887.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1888.Pp 1889Note that a negated match will never produce a positive result by itself. 1890For example, attempting to match 1891.Qq host3 1892against the following pattern-list will fail: 1893.Pp 1894.Dl from=\&"!host1,!host2\&" 1895.Pp 1896The solution here is to include a term that will yield a positive match, 1897such as a wildcard: 1898.Pp 1899.Dl from=\&"!host1,!host2,*\&" 1900.Sh TOKENS 1901Arguments to some keywords can make use of tokens, 1902which are expanded at runtime: 1903.Pp 1904.Bl -tag -width XXXX -offset indent -compact 1905.It %% 1906A literal 1907.Sq % . 1908.It \&%C 1909Hash of %l%h%p%r. 1910.It %d 1911Local user's home directory. 1912.It %f 1913The fingerprint of the server's host key. 1914.It %H 1915The 1916.Pa known_hosts 1917hostname or address that is being searched for. 1918.It %h 1919The remote hostname. 1920.It \%%I 1921A string describing the reason for a 1922.Cm KnownHostsCommand 1923execution: either 1924.Cm ADDRESS 1925when looking up a host by address (only when 1926.Cm CheckHostIP 1927is enabled), 1928.Cm HOSTNAME 1929when searching by hostname, or 1930.Cm ORDER 1931when preparing the host key algorithm preference list to use for the 1932destination host. 1933.It %i 1934The local user ID. 1935.It %K 1936The base64 encoded host key. 1937.It %k 1938The host key alias if specified, otherwise the orignal remote hostname given 1939on the command line. 1940.It %L 1941The local hostname. 1942.It %l 1943The local hostname, including the domain name. 1944.It %n 1945The original remote hostname, as given on the command line. 1946.It %p 1947The remote port. 1948.It %r 1949The remote username. 1950.It \&%T 1951The local 1952.Xr tun 4 1953or 1954.Xr tap 4 1955network interface assigned if 1956tunnel forwarding was requested, or 1957.Qq NONE 1958otherwise. 1959.It %t 1960The type of the server host key, e.g. 1961.Cm ssh-ed25519 1962.It %u 1963The local username. 1964.El 1965.Pp 1966.Cm CertificateFile , 1967.Cm ControlPath , 1968.Cm IdentityAgent , 1969.Cm IdentityFile , 1970.Cm KnownHostsCommand , 1971.Cm LocalForward , 1972.Cm Match exec , 1973.Cm RemoteCommand , 1974.Cm RemoteForward , 1975and 1976.Cm UserKnownHostsFile 1977accept the tokens %%, %C, %d, %h, %i, %L, %l, %n, %p, %r, and %u. 1978.Pp 1979.Cm KnownHostsCommand 1980additionally accepts the tokens %f, %H, %I, %K and %t. 1981.Pp 1982.Cm Hostname 1983accepts the tokens %% and %h. 1984.Pp 1985.Cm LocalCommand 1986accepts all tokens. 1987.Pp 1988.Cm ProxyCommand 1989accepts the tokens %%, %h, %n, %p, and %r. 1990.Sh ENVIRONMENT VARIABLES 1991Arguments to some keywords can be expanded at runtime from environment 1992variables on the client by enclosing them in 1993.Ic ${} , 1994for example 1995.Ic ${HOME}/.ssh 1996would refer to the user's .ssh directory. 1997If a specified environment variable does not exist then an error will be 1998returned and the setting for that keyword will be ignored. 1999.Pp 2000The keywords 2001.Cm CertificateFile , 2002.Cm ControlPath , 2003.Cm IdentityAgent , 2004.Cm IdentityFile 2005.Cm KnownHostsCommand , 2006and 2007.Cm UserKnownHostsFile 2008support environment variables. 2009The keywords 2010.Cm LocalForward 2011and 2012.Cm RemoteForward 2013support environment variables only for Unix domain socket paths. 2014.Sh FILES 2015.Bl -tag -width Ds 2016.It Pa ~/.ssh/config 2017This is the per-user configuration file. 2018The format of this file is described above. 2019This file is used by the SSH client. 2020Because of the potential for abuse, this file must have strict permissions: 2021read/write for the user, and not writable by others. 2022.It Pa /etc/ssh/ssh_config 2023Systemwide configuration file. 2024This file provides defaults for those 2025values that are not specified in the user's configuration file, and 2026for those users who do not have a configuration file. 2027This file must be world-readable. 2028.El 2029.Sh SEE ALSO 2030.Xr ssh 1 2031.Sh AUTHORS 2032.An -nosplit 2033OpenSSH is a derivative of the original and free 2034ssh 1.2.12 release by 2035.An Tatu Ylonen . 2036.An Aaron Campbell , Bob Beck , Markus Friedl , 2037.An Niels Provos , Theo de Raadt 2038and 2039.An Dug Song 2040removed many bugs, re-added newer features and 2041created OpenSSH. 2042.An Markus Friedl 2043contributed the support for SSH protocol versions 1.5 and 2.0. 2044