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 1.338 2020/10/16 14:34:33 jmc Exp $ 37.Dd $Mdocdate: October 16 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 LocalCommand 1125Specifies a command to execute on the local machine after successfully 1126connecting to the server. 1127The command string extends to the end of the line, and is executed with 1128the user's shell. 1129Arguments to 1130.Cm LocalCommand 1131accept the tokens described in the 1132.Sx TOKENS 1133section. 1134.Pp 1135The command is run synchronously and does not have access to the 1136session of the 1137.Xr ssh 1 1138that spawned it. 1139It should not be used for interactive commands. 1140.Pp 1141This directive is ignored unless 1142.Cm PermitLocalCommand 1143has been enabled. 1144.It Cm LocalForward 1145Specifies that a TCP port on the local machine be forwarded over 1146the secure channel to the specified host and port from the remote machine. 1147The first argument specifies the listener and may be 1148.Sm off 1149.Oo Ar bind_address : Oc Ar port 1150.Sm on 1151or a Unix domain socket path. 1152The second argument is the destination and may be 1153.Ar host : Ns Ar hostport 1154or a Unix domain socket path if the remote host supports it. 1155.Pp 1156IPv6 addresses can be specified by enclosing addresses in square brackets. 1157Multiple forwardings may be specified, and additional forwardings can be 1158given on the command line. 1159Only the superuser can forward privileged ports. 1160By default, the local port is bound in accordance with the 1161.Cm GatewayPorts 1162setting. 1163However, an explicit 1164.Ar bind_address 1165may be used to bind the connection to a specific address. 1166The 1167.Ar bind_address 1168of 1169.Cm localhost 1170indicates that the listening port be bound for local use only, while an 1171empty address or 1172.Sq * 1173indicates that the port should be available from all interfaces. 1174Unix domain socket paths may use the tokens described in the 1175.Sx TOKENS 1176section and environment variables as described in the 1177.Sx ENVIRONMENT VARIABLES 1178section. 1179.It Cm LogLevel 1180Gives the verbosity level that is used when logging messages from 1181.Xr ssh 1 . 1182The possible values are: 1183QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1184The default is INFO. 1185DEBUG and DEBUG1 are equivalent. 1186DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1187.It Cm LogVerbose 1188Specify one or more overrides to LogLevel. 1189An override consists of a pattern lists that matches the source file, function 1190and line number to force detailed logging for. 1191For example, an override pattern of: 1192.Bd -literal -offset indent 1193kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1194.Ed 1195.Pp 1196would enable detailed logging for line 1000 of 1197.Pa kex.c , 1198everything in the 1199.Fn kex_exchange_identification 1200function, and all code in the 1201.Pa packet.c 1202file. 1203This option is intended for debugging and no overrides are enabled by default. 1204.It Cm MACs 1205Specifies the MAC (message authentication code) algorithms 1206in order of preference. 1207The MAC algorithm is used for data integrity protection. 1208Multiple algorithms must be comma-separated. 1209If the specified list begins with a 1210.Sq + 1211character, then the specified algorithms will be appended to the default set 1212instead of replacing them. 1213If the specified list begins with a 1214.Sq - 1215character, then the specified algorithms (including wildcards) will be removed 1216from the default set instead of replacing them. 1217If the specified list begins with a 1218.Sq ^ 1219character, then the specified algorithms will be placed at the head of the 1220default set. 1221.Pp 1222The algorithms that contain 1223.Qq -etm 1224calculate the MAC after encryption (encrypt-then-mac). 1225These are considered safer and their use recommended. 1226.Pp 1227The default is: 1228.Bd -literal -offset indent 1229umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1230hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1231hmac-sha1-etm@openssh.com, 1232umac-64@openssh.com,umac-128@openssh.com, 1233hmac-sha2-256,hmac-sha2-512,hmac-sha1 1234.Ed 1235.Pp 1236The list of available MAC algorithms may also be obtained using 1237.Qq ssh -Q mac . 1238.It Cm NoHostAuthenticationForLocalhost 1239Disable host authentication for localhost (loopback addresses). 1240The argument to this keyword must be 1241.Cm yes 1242or 1243.Cm no 1244(the default). 1245.It Cm NumberOfPasswordPrompts 1246Specifies the number of password prompts before giving up. 1247The argument to this keyword must be an integer. 1248The default is 3. 1249.It Cm PasswordAuthentication 1250Specifies whether to use password authentication. 1251The argument to this keyword must be 1252.Cm yes 1253(the default) 1254or 1255.Cm no . 1256.It Cm PermitLocalCommand 1257Allow local command execution via the 1258.Ic LocalCommand 1259option or using the 1260.Ic !\& Ns Ar command 1261escape sequence in 1262.Xr ssh 1 . 1263The argument must be 1264.Cm yes 1265or 1266.Cm no 1267(the default). 1268.It Cm PKCS11Provider 1269Specifies which PKCS#11 provider to use or 1270.Cm none 1271to indicate that no provider should be used (the default). 1272The argument to this keyword is a path to the PKCS#11 shared library 1273.Xr ssh 1 1274should use to communicate with a PKCS#11 token providing keys for user 1275authentication. 1276.It Cm Port 1277Specifies the port number to connect on the remote host. 1278The default is 22. 1279.It Cm PreferredAuthentications 1280Specifies the order in which the client should try authentication methods. 1281This allows a client to prefer one method (e.g.\& 1282.Cm keyboard-interactive ) 1283over another method (e.g.\& 1284.Cm password ) . 1285The default is: 1286.Bd -literal -offset indent 1287gssapi-with-mic,hostbased,publickey, 1288keyboard-interactive,password 1289.Ed 1290.It Cm ProxyCommand 1291Specifies the command to use to connect to the server. 1292The command 1293string extends to the end of the line, and is executed 1294using the user's shell 1295.Ql exec 1296directive to avoid a lingering shell process. 1297.Pp 1298Arguments to 1299.Cm ProxyCommand 1300accept the tokens described in the 1301.Sx TOKENS 1302section. 1303The command can be basically anything, 1304and should read from its standard input and write to its standard output. 1305It should eventually connect an 1306.Xr sshd 8 1307server running on some machine, or execute 1308.Ic sshd -i 1309somewhere. 1310Host key management will be done using the 1311.Cm Hostname 1312of the host being connected (defaulting to the name typed by the user). 1313Setting the command to 1314.Cm none 1315disables this option entirely. 1316Note that 1317.Cm CheckHostIP 1318is not available for connects with a proxy command. 1319.Pp 1320This directive is useful in conjunction with 1321.Xr nc 1 1322and its proxy support. 1323For example, the following directive would connect via an HTTP proxy at 1324192.0.2.0: 1325.Bd -literal -offset 3n 1326ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1327.Ed 1328.It Cm ProxyJump 1329Specifies one or more jump proxies as either 1330.Xo 1331.Sm off 1332.Op Ar user No @ 1333.Ar host 1334.Op : Ns Ar port 1335.Sm on 1336or an ssh URI 1337.Xc . 1338Multiple proxies may be separated by comma characters and will be visited 1339sequentially. 1340Setting this option will cause 1341.Xr ssh 1 1342to connect to the target host by first making a 1343.Xr ssh 1 1344connection to the specified 1345.Cm ProxyJump 1346host and then establishing a 1347TCP forwarding to the ultimate target from there. 1348.Pp 1349Note that this option will compete with the 1350.Cm ProxyCommand 1351option - whichever is specified first will prevent later instances of the 1352other from taking effect. 1353.Pp 1354Note also that the configuration for the destination host (either supplied 1355via the command-line or the configuration file) is not generally applied 1356to jump hosts. 1357.Pa ~/.ssh/config 1358should be used if specific configuration is required for jump hosts. 1359.It Cm ProxyUseFdpass 1360Specifies that 1361.Cm ProxyCommand 1362will pass a connected file descriptor back to 1363.Xr ssh 1 1364instead of continuing to execute and pass data. 1365The default is 1366.Cm no . 1367.It Cm PubkeyAcceptedKeyTypes 1368Specifies the key types that will be used for public key authentication 1369as a comma-separated list of patterns. 1370If the specified list begins with a 1371.Sq + 1372character, then the key types after it will be appended to the default 1373instead of replacing it. 1374If the specified list begins with a 1375.Sq - 1376character, then the specified key types (including wildcards) will be removed 1377from the default set instead of replacing them. 1378If the specified list begins with a 1379.Sq ^ 1380character, then the specified key types will be placed at the head of the 1381default set. 1382The default for this option is: 1383.Bd -literal -offset 3n 1384ssh-ed25519-cert-v01@openssh.com, 1385ecdsa-sha2-nistp256-cert-v01@openssh.com, 1386ecdsa-sha2-nistp384-cert-v01@openssh.com, 1387ecdsa-sha2-nistp521-cert-v01@openssh.com, 1388sk-ssh-ed25519-cert-v01@openssh.com, 1389sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1390rsa-sha2-512-cert-v01@openssh.com, 1391rsa-sha2-256-cert-v01@openssh.com, 1392ssh-rsa-cert-v01@openssh.com, 1393ssh-ed25519, 1394ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1395sk-ssh-ed25519@openssh.com, 1396sk-ecdsa-sha2-nistp256@openssh.com, 1397rsa-sha2-512,rsa-sha2-256,ssh-rsa 1398.Ed 1399.Pp 1400The list of available key types may also be obtained using 1401.Qq ssh -Q PubkeyAcceptedKeyTypes . 1402.It Cm PubkeyAuthentication 1403Specifies whether to try public key authentication. 1404The argument to this keyword must be 1405.Cm yes 1406(the default) 1407or 1408.Cm no . 1409.It Cm RekeyLimit 1410Specifies the maximum amount of data that may be transmitted before the 1411session key is renegotiated, optionally followed a maximum amount of 1412time that may pass before the session key is renegotiated. 1413The first argument is specified in bytes and may have a suffix of 1414.Sq K , 1415.Sq M , 1416or 1417.Sq G 1418to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1419The default is between 1420.Sq 1G 1421and 1422.Sq 4G , 1423depending on the cipher. 1424The optional second value is specified in seconds and may use any of the 1425units documented in the TIME FORMATS section of 1426.Xr sshd_config 5 . 1427The default value for 1428.Cm RekeyLimit 1429is 1430.Cm default none , 1431which means that rekeying is performed after the cipher's default amount 1432of data has been sent or received and no time based rekeying is done. 1433.It Cm RemoteCommand 1434Specifies a command to execute on the remote machine after successfully 1435connecting to the server. 1436The command string extends to the end of the line, and is executed with 1437the user's shell. 1438Arguments to 1439.Cm RemoteCommand 1440accept the tokens described in the 1441.Sx TOKENS 1442section. 1443.It Cm RemoteForward 1444Specifies that a TCP port on the remote machine be forwarded over 1445the secure channel. 1446The remote port may either be forwarded to a specified host and port 1447from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1448client to connect to arbitrary destinations from the local machine. 1449The first argument is the listening specification and may be 1450.Sm off 1451.Oo Ar bind_address : Oc Ar port 1452.Sm on 1453or, if the remote host supports it, a Unix domain socket path. 1454If forwarding to a specific destination then the second argument must be 1455.Ar host : Ns Ar hostport 1456or a Unix domain socket path, 1457otherwise if no destination argument is specified then the remote forwarding 1458will be established as a SOCKS proxy. 1459.Pp 1460IPv6 addresses can be specified by enclosing addresses in square brackets. 1461Multiple forwardings may be specified, and additional 1462forwardings can be given on the command line. 1463Privileged ports can be forwarded only when 1464logging in as root on the remote machine. 1465Unix domain socket paths may use the tokens described in the 1466.Sx TOKENS 1467section and environment variables as described in the 1468.Sx ENVIRONMENT VARIABLES 1469section. 1470.Pp 1471If the 1472.Ar port 1473argument is 0, 1474the listen port will be dynamically allocated on the server and reported 1475to the client at run time. 1476.Pp 1477If the 1478.Ar bind_address 1479is not specified, the default is to only bind to loopback addresses. 1480If the 1481.Ar bind_address 1482is 1483.Ql * 1484or an empty string, then the forwarding is requested to listen on all 1485interfaces. 1486Specifying a remote 1487.Ar bind_address 1488will only succeed if the server's 1489.Cm GatewayPorts 1490option is enabled (see 1491.Xr sshd_config 5 ) . 1492.It Cm RequestTTY 1493Specifies whether to request a pseudo-tty for the session. 1494The argument may be one of: 1495.Cm no 1496(never request a TTY), 1497.Cm yes 1498(always request a TTY when standard input is a TTY), 1499.Cm force 1500(always request a TTY) or 1501.Cm auto 1502(request a TTY when opening a login session). 1503This option mirrors the 1504.Fl t 1505and 1506.Fl T 1507flags for 1508.Xr ssh 1 . 1509.It Cm RevokedHostKeys 1510Specifies revoked host public keys. 1511Keys listed in this file will be refused for host authentication. 1512Note that if this file does not exist or is not readable, 1513then host authentication will be refused for all hosts. 1514Keys may be specified as a text file, listing one public key per line, or as 1515an OpenSSH Key Revocation List (KRL) as generated by 1516.Xr ssh-keygen 1 . 1517For more information on KRLs, see the KEY REVOCATION LISTS section in 1518.Xr ssh-keygen 1 . 1519.It Cm SecurityKeyProvider 1520Specifies a path to a library that will be used when loading any 1521FIDO authenticator-hosted keys, overriding the default of using 1522the built-in USB HID support. 1523.Pp 1524If the specified value begins with a 1525.Sq $ 1526character, then it will be treated as an environment variable containing 1527the path to the library. 1528.It Cm SendEnv 1529Specifies what variables from the local 1530.Xr environ 7 1531should be sent to the server. 1532The server must also support it, and the server must be configured to 1533accept these environment variables. 1534Note that the 1535.Ev TERM 1536environment variable is always sent whenever a 1537pseudo-terminal is requested as it is required by the protocol. 1538Refer to 1539.Cm AcceptEnv 1540in 1541.Xr sshd_config 5 1542for how to configure the server. 1543Variables are specified by name, which may contain wildcard characters. 1544Multiple environment variables may be separated by whitespace or spread 1545across multiple 1546.Cm SendEnv 1547directives. 1548.Pp 1549See 1550.Sx PATTERNS 1551for more information on patterns. 1552.Pp 1553It is possible to clear previously set 1554.Cm SendEnv 1555variable names by prefixing patterns with 1556.Pa - . 1557The default is not to send any environment variables. 1558.It Cm ServerAliveCountMax 1559Sets the number of server alive messages (see below) which may be 1560sent without 1561.Xr ssh 1 1562receiving any messages back from the server. 1563If this threshold is reached while server alive messages are being sent, 1564ssh will disconnect from the server, terminating the session. 1565It is important to note that the use of server alive messages is very 1566different from 1567.Cm TCPKeepAlive 1568(below). 1569The server alive messages are sent through the encrypted channel 1570and therefore will not be spoofable. 1571The TCP keepalive option enabled by 1572.Cm TCPKeepAlive 1573is spoofable. 1574The server alive mechanism is valuable when the client or 1575server depend on knowing when a connection has become unresponsive. 1576.Pp 1577The default value is 3. 1578If, for example, 1579.Cm ServerAliveInterval 1580(see below) is set to 15 and 1581.Cm ServerAliveCountMax 1582is left at the default, if the server becomes unresponsive, 1583ssh will disconnect after approximately 45 seconds. 1584.It Cm ServerAliveInterval 1585Sets a timeout interval in seconds after which if no data has been received 1586from the server, 1587.Xr ssh 1 1588will send a message through the encrypted 1589channel to request a response from the server. 1590The default 1591is 0, indicating that these messages will not be sent to the server. 1592.It Cm SetEnv 1593Directly specify one or more environment variables and their contents to 1594be sent to the server. 1595Similarly to 1596.Cm SendEnv , 1597the server must be prepared to accept the environment variable. 1598.It Cm StreamLocalBindMask 1599Sets the octal file creation mode mask 1600.Pq umask 1601used when creating a Unix-domain socket file for local or remote 1602port forwarding. 1603This option is only used for port forwarding to a Unix-domain socket file. 1604.Pp 1605The default value is 0177, which creates a Unix-domain socket file that is 1606readable and writable only by the owner. 1607Note that not all operating systems honor the file mode on Unix-domain 1608socket files. 1609.It Cm StreamLocalBindUnlink 1610Specifies whether to remove an existing Unix-domain socket file for local 1611or remote port forwarding before creating a new one. 1612If the socket file already exists and 1613.Cm StreamLocalBindUnlink 1614is not enabled, 1615.Nm ssh 1616will be unable to forward the port to the Unix-domain socket file. 1617This option is only used for port forwarding to a Unix-domain socket file. 1618.Pp 1619The argument must be 1620.Cm yes 1621or 1622.Cm no 1623(the default). 1624.It Cm StrictHostKeyChecking 1625If this flag is set to 1626.Cm yes , 1627.Xr ssh 1 1628will never automatically add host keys to the 1629.Pa ~/.ssh/known_hosts 1630file, and refuses to connect to hosts whose host key has changed. 1631This provides maximum protection against man-in-the-middle (MITM) attacks, 1632though it can be annoying when the 1633.Pa /etc/ssh/ssh_known_hosts 1634file is poorly maintained or when connections to new hosts are 1635frequently made. 1636This option forces the user to manually 1637add all new hosts. 1638.Pp 1639If this flag is set to 1640.Dq accept-new 1641then ssh will automatically add new host keys to the user 1642known hosts files, but will not permit connections to hosts with 1643changed host keys. 1644If this flag is set to 1645.Dq no 1646or 1647.Dq off , 1648ssh will automatically add new host keys to the user known hosts files 1649and allow connections to hosts with changed hostkeys to proceed, 1650subject to some restrictions. 1651If this flag is set to 1652.Cm ask 1653(the default), 1654new host keys 1655will be added to the user known host files only after the user 1656has confirmed that is what they really want to do, and 1657ssh will refuse to connect to hosts whose host key has changed. 1658The host keys of 1659known hosts will be verified automatically in all cases. 1660.It Cm SyslogFacility 1661Gives the facility code that is used when logging messages from 1662.Xr ssh 1 . 1663The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1664LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1665The default is USER. 1666.It Cm TCPKeepAlive 1667Specifies whether the system should send TCP keepalive messages to the 1668other side. 1669If they are sent, death of the connection or crash of one 1670of the machines will be properly noticed. 1671However, this means that 1672connections will die if the route is down temporarily, and some people 1673find it annoying. 1674.Pp 1675The default is 1676.Cm yes 1677(to send TCP keepalive messages), and the client will notice 1678if the network goes down or the remote host dies. 1679This is important in scripts, and many users want it too. 1680.Pp 1681To disable TCP keepalive messages, the value should be set to 1682.Cm no . 1683See also 1684.Cm ServerAliveInterval 1685for protocol-level keepalives. 1686.It Cm Tunnel 1687Request 1688.Xr tun 4 1689device forwarding between the client and the server. 1690The argument must be 1691.Cm yes , 1692.Cm point-to-point 1693(layer 3), 1694.Cm ethernet 1695(layer 2), 1696or 1697.Cm no 1698(the default). 1699Specifying 1700.Cm yes 1701requests the default tunnel mode, which is 1702.Cm point-to-point . 1703.It Cm TunnelDevice 1704Specifies the 1705.Xr tun 4 1706devices to open on the client 1707.Pq Ar local_tun 1708and the server 1709.Pq Ar remote_tun . 1710.Pp 1711The argument must be 1712.Sm off 1713.Ar local_tun Op : Ar remote_tun . 1714.Sm on 1715The devices may be specified by numerical ID or the keyword 1716.Cm any , 1717which uses the next available tunnel device. 1718If 1719.Ar remote_tun 1720is not specified, it defaults to 1721.Cm any . 1722The default is 1723.Cm any:any . 1724.It Cm UpdateHostKeys 1725Specifies whether 1726.Xr ssh 1 1727should accept notifications of additional hostkeys from the server sent 1728after authentication has completed and add them to 1729.Cm UserKnownHostsFile . 1730The argument must be 1731.Cm yes , 1732.Cm no 1733or 1734.Cm ask . 1735This option allows learning alternate hostkeys for a server 1736and supports graceful key rotation by allowing a server to send replacement 1737public keys before old ones are removed. 1738.Pp 1739Additional hostkeys are only accepted if the key used to authenticate the 1740host was already trusted or explicitly accepted by the user, the host was 1741authenticated via 1742.Cm UserKnownHostsFile 1743(i.e. not 1744.Cm GlobalKnownHostsFile ) 1745and the host was authenticated using a plain key and not a certificate. 1746.Pp 1747.Cm UpdateHostKeys 1748is enabled by default if the user has not overridden the default 1749.Cm UserKnownHostsFile 1750setting and has not enabled 1751.Cm VerifyHostKeyDNS , 1752otherwise 1753.Cm UpdateHostKeys 1754will be set to 1755.Cm no . 1756.Pp 1757If 1758.Cm UpdateHostKeys 1759is set to 1760.Cm ask , 1761then the user is asked to confirm the modifications to the known_hosts file. 1762Confirmation is currently incompatible with 1763.Cm ControlPersist , 1764and will be disabled if it is enabled. 1765.Pp 1766Presently, only 1767.Xr sshd 8 1768from OpenSSH 6.8 and greater support the 1769.Qq hostkeys@openssh.com 1770protocol extension used to inform the client of all the server's hostkeys. 1771.It Cm User 1772Specifies the user to log in as. 1773This can be useful when a different user name is used on different machines. 1774This saves the trouble of 1775having to remember to give the user name on the command line. 1776.It Cm UserKnownHostsFile 1777Specifies one or more files to use for the user 1778host key database, separated by whitespace. 1779Each filename may use tilde notation to refer to the user's home directory, 1780the tokens described in the 1781.Sx TOKENS 1782section and environment variables as described in the 1783.Sx ENVIRONMENT VARIABLES 1784section. 1785The default is 1786.Pa ~/.ssh/known_hosts , 1787.Pa ~/.ssh/known_hosts2 . 1788.It Cm VerifyHostKeyDNS 1789Specifies whether to verify the remote key using DNS and SSHFP resource 1790records. 1791If this option is set to 1792.Cm yes , 1793the client will implicitly trust keys that match a secure fingerprint 1794from DNS. 1795Insecure fingerprints will be handled as if this option was set to 1796.Cm ask . 1797If this option is set to 1798.Cm ask , 1799information on fingerprint match will be displayed, but the user will still 1800need to confirm new host keys according to the 1801.Cm StrictHostKeyChecking 1802option. 1803The default is 1804.Cm no . 1805.Pp 1806See also 1807.Sx VERIFYING HOST KEYS 1808in 1809.Xr ssh 1 . 1810.It Cm VisualHostKey 1811If this flag is set to 1812.Cm yes , 1813an ASCII art representation of the remote host key fingerprint is 1814printed in addition to the fingerprint string at login and 1815for unknown host keys. 1816If this flag is set to 1817.Cm no 1818(the default), 1819no fingerprint strings are printed at login and 1820only the fingerprint string will be printed for unknown host keys. 1821.It Cm XAuthLocation 1822Specifies the full pathname of the 1823.Xr xauth 1 1824program. 1825The default is 1826.Pa /usr/X11R6/bin/xauth . 1827.El 1828.Sh PATTERNS 1829A 1830.Em pattern 1831consists of zero or more non-whitespace characters, 1832.Sq * 1833(a wildcard that matches zero or more characters), 1834or 1835.Sq ?\& 1836(a wildcard that matches exactly one character). 1837For example, to specify a set of declarations for any host in the 1838.Qq .co.uk 1839set of domains, 1840the following pattern could be used: 1841.Pp 1842.Dl Host *.co.uk 1843.Pp 1844The following pattern 1845would match any host in the 192.168.0.[0-9] network range: 1846.Pp 1847.Dl Host 192.168.0.? 1848.Pp 1849A 1850.Em pattern-list 1851is a comma-separated list of patterns. 1852Patterns within pattern-lists may be negated 1853by preceding them with an exclamation mark 1854.Pq Sq !\& . 1855For example, 1856to allow a key to be used from anywhere within an organization 1857except from the 1858.Qq dialup 1859pool, 1860the following entry (in authorized_keys) could be used: 1861.Pp 1862.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1863.Pp 1864Note that a negated match will never produce a positive result by itself. 1865For example, attempting to match 1866.Qq host3 1867against the following pattern-list will fail: 1868.Pp 1869.Dl from=\&"!host1,!host2\&" 1870.Pp 1871The solution here is to include a term that will yield a positive match, 1872such as a wildcard: 1873.Pp 1874.Dl from=\&"!host1,!host2,*\&" 1875.Sh TOKENS 1876Arguments to some keywords can make use of tokens, 1877which are expanded at runtime: 1878.Pp 1879.Bl -tag -width XXXX -offset indent -compact 1880.It %% 1881A literal 1882.Sq % . 1883.It \&%C 1884Hash of %l%h%p%r. 1885.It %d 1886Local user's home directory. 1887.It %h 1888The remote hostname. 1889.It %i 1890The local user ID. 1891.It %k 1892The host key alias if specified, otherwise the orignal remote hostname given 1893on the command line. 1894.It %L 1895The local hostname. 1896.It %l 1897The local hostname, including the domain name. 1898.It %n 1899The original remote hostname, as given on the command line. 1900.It %p 1901The remote port. 1902.It %r 1903The remote username. 1904.It \&%T 1905The local 1906.Xr tun 4 1907or 1908.Xr tap 4 1909network interface assigned if 1910tunnel forwarding was requested, or 1911.Qq NONE 1912otherwise. 1913.It %u 1914The local username. 1915.El 1916.Pp 1917.Cm CertificateFile , 1918.Cm ControlPath , 1919.Cm IdentityAgent , 1920.Cm IdentityFile , 1921.Cm LocalForward , 1922.Cm Match exec , 1923.Cm RemoteCommand , 1924.Cm RemoteForward , 1925and 1926.Cm UserKnownHostsFile 1927accept the tokens %%, %C, %d, %h, %i, %L, %l, %n, %p, %r, and %u. 1928.Pp 1929.Cm Hostname 1930accepts the tokens %% and %h. 1931.Pp 1932.Cm LocalCommand 1933accepts all tokens. 1934.Pp 1935.Cm ProxyCommand 1936accepts the tokens %%, %h, %n, %p, and %r. 1937.Sh ENVIRONMENT VARIABLES 1938Arguments to some keywords can be expanded at runtime from environment 1939variables on the client by enclosing them in 1940.Ic ${} , 1941for example 1942.Ic ${HOME}/.ssh 1943would refer to the user's .ssh directory. 1944If a specified environment variable does not exist then an error will be 1945returned and the setting for that keyword will be ignored. 1946.Pp 1947The keywords 1948.Cm CertificateFile , 1949.Cm ControlPath , 1950.Cm IdentityAgent , 1951.Cm IdentityFile 1952and 1953.Cm UserKnownHostsFile 1954support environment variables. 1955The keywords 1956.Cm LocalForward 1957and 1958.Cm RemoteForward 1959support environment variables only for Unix domain socket paths. 1960.Sh FILES 1961.Bl -tag -width Ds 1962.It Pa ~/.ssh/config 1963This is the per-user configuration file. 1964The format of this file is described above. 1965This file is used by the SSH client. 1966Because of the potential for abuse, this file must have strict permissions: 1967read/write for the user, and not writable by others. 1968.It Pa /etc/ssh/ssh_config 1969Systemwide configuration file. 1970This file provides defaults for those 1971values that are not specified in the user's configuration file, and 1972for those users who do not have a configuration file. 1973This file must be world-readable. 1974.El 1975.Sh SEE ALSO 1976.Xr ssh 1 1977.Sh AUTHORS 1978.An -nosplit 1979OpenSSH is a derivative of the original and free 1980ssh 1.2.12 release by 1981.An Tatu Ylonen . 1982.An Aaron Campbell , Bob Beck , Markus Friedl , 1983.An Niels Provos , Theo de Raadt 1984and 1985.An Dug Song 1986removed many bugs, re-added newer features and 1987created OpenSSH. 1988.An Markus Friedl 1989contributed the support for SSH protocol versions 1.5 and 2.0. 1990