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 1.369 2022/02/15 05:13:36 djm Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: February 15 2022 $ 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH client configuration file 44.Sh DESCRIPTION 45.Xr ssh 1 46obtains configuration data from the following sources in 47the following order: 48.Pp 49.Bl -enum -offset indent -compact 50.It 51command-line options 52.It 53user's configuration file 54.Pq Pa ~/.ssh/config 55.It 56system-wide configuration file 57.Pq Pa /etc/ssh/ssh_config 58.El 59.Pp 60For each parameter, the first obtained value 61will be used. 62The configuration files contain sections separated by 63.Cm Host 64specifications, and that section is only applied for hosts that 65match one of the patterns given in the specification. 66The matched host name is usually the one given on the command line 67(see the 68.Cm CanonicalizeHostname 69option for exceptions). 70.Pp 71Since the first obtained value for each parameter is used, more 72host-specific declarations should be given near the beginning of the 73file, and general defaults at the end. 74.Pp 75The file contains keyword-argument pairs, one per line. 76Lines starting with 77.Ql # 78and empty lines are interpreted as comments. 79Arguments may optionally be enclosed in double quotes 80.Pq \&" 81in order to represent arguments containing spaces. 82Configuration options may be separated by whitespace or 83optional whitespace and exactly one 84.Ql = ; 85the latter format is useful to avoid the need to quote whitespace 86when specifying configuration options using the 87.Nm ssh , 88.Nm scp , 89and 90.Nm sftp 91.Fl o 92option. 93.Pp 94The possible 95keywords and their meanings are as follows (note that 96keywords are case-insensitive and arguments are case-sensitive): 97.Bl -tag -width Ds 98.It Cm Host 99Restricts the following declarations (up to the next 100.Cm Host 101or 102.Cm Match 103keyword) to be only for those hosts that match one of the patterns 104given after the keyword. 105If more than one pattern is provided, they should be separated by whitespace. 106A single 107.Ql * 108as a pattern can be used to provide global 109defaults for all hosts. 110The host is usually the 111.Ar hostname 112argument given on the command line 113(see the 114.Cm CanonicalizeHostname 115keyword for exceptions). 116.Pp 117A pattern entry may be negated by prefixing it with an exclamation mark 118.Pq Sq !\& . 119If a negated entry is matched, then the 120.Cm Host 121entry is ignored, regardless of whether any other patterns on the line 122match. 123Negated matches are therefore useful to provide exceptions for wildcard 124matches. 125.Pp 126See 127.Sx PATTERNS 128for more information on patterns. 129.It Cm Match 130Restricts the following declarations (up to the next 131.Cm Host 132or 133.Cm Match 134keyword) to be used only when the conditions following the 135.Cm Match 136keyword are satisfied. 137Match conditions are specified using one or more criteria 138or the single token 139.Cm all 140which always matches. 141The available criteria keywords are: 142.Cm canonical , 143.Cm final , 144.Cm exec , 145.Cm host , 146.Cm originalhost , 147.Cm user , 148and 149.Cm localuser . 150The 151.Cm all 152criteria must appear alone or immediately after 153.Cm canonical 154or 155.Cm final . 156Other criteria may be combined arbitrarily. 157All criteria but 158.Cm all , 159.Cm canonical , 160and 161.Cm final 162require an argument. 163Criteria may be negated by prepending an exclamation mark 164.Pq Sq !\& . 165.Pp 166The 167.Cm canonical 168keyword matches only when the configuration file is being re-parsed 169after hostname canonicalization (see the 170.Cm CanonicalizeHostname 171option). 172This may be useful to specify conditions that work with canonical host 173names only. 174.Pp 175The 176.Cm final 177keyword requests that the configuration be re-parsed (regardless of whether 178.Cm CanonicalizeHostname 179is enabled), and matches only during this final pass. 180If 181.Cm CanonicalizeHostname 182is enabled, then 183.Cm canonical 184and 185.Cm final 186match during the same pass. 187.Pp 188The 189.Cm exec 190keyword executes the specified command under the user's shell. 191If the command returns a zero exit status then the condition is considered true. 192Commands containing whitespace characters must be quoted. 193Arguments to 194.Cm exec 195accept the tokens described in the 196.Sx TOKENS 197section. 198.Pp 199The other keywords' criteria must be single entries or comma-separated 200lists and may use the wildcard and negation operators described in the 201.Sx PATTERNS 202section. 203The criteria for the 204.Cm host 205keyword are matched against the target hostname, after any substitution 206by the 207.Cm Hostname 208or 209.Cm CanonicalizeHostname 210options. 211The 212.Cm originalhost 213keyword matches against the hostname as it was specified on the command-line. 214The 215.Cm user 216keyword matches against the target username on the remote host. 217The 218.Cm localuser 219keyword matches against the name of the local user running 220.Xr ssh 1 221(this keyword may be useful in system-wide 222.Nm 223files). 224.It Cm AddKeysToAgent 225Specifies whether keys should be automatically added to a running 226.Xr ssh-agent 1 . 227If this option is set to 228.Cm yes 229and a key is loaded from a file, the key and its passphrase are added to 230the agent with the default lifetime, as if by 231.Xr ssh-add 1 . 232If this option is set to 233.Cm ask , 234.Xr ssh 1 235will require confirmation using the 236.Ev SSH_ASKPASS 237program before adding a key (see 238.Xr ssh-add 1 239for details). 240If this option is set to 241.Cm confirm , 242each use of the key must be confirmed, as if the 243.Fl c 244option was specified to 245.Xr ssh-add 1 . 246If this option is set to 247.Cm no , 248no keys are added to the agent. 249Alternately, this option may be specified as a time interval 250using the format described in the 251.Sx TIME FORMATS 252section of 253.Xr sshd_config 5 254to specify the key's lifetime in 255.Xr ssh-agent 1 , 256after which it will automatically be removed. 257The argument must be 258.Cm no 259(the default), 260.Cm yes , 261.Cm confirm 262(optionally followed by a time interval), 263.Cm ask 264or a time interval. 265.It Cm AddressFamily 266Specifies which address family to use when connecting. 267Valid arguments are 268.Cm any 269(the default), 270.Cm inet 271(use IPv4 only), or 272.Cm inet6 273(use IPv6 only). 274.It Cm BatchMode 275If set to 276.Cm yes , 277user interaction such as password prompts and host key confirmation requests 278will be disabled. 279This option is useful in scripts and other batch jobs where no user 280is present to interact with 281.Xr ssh 1 . 282The argument must be 283.Cm yes 284or 285.Cm no 286(the default). 287.It Cm BindAddress 288Use the specified address on the local machine as the source address of 289the connection. 290Only useful on systems with more than one address. 291.It Cm BindInterface 292Use the address of the specified interface on the local machine as the 293source address of the connection. 294.It Cm CanonicalDomains 295When 296.Cm CanonicalizeHostname 297is enabled, this option specifies the list of domain suffixes in which to 298search for the specified destination host. 299.It Cm CanonicalizeFallbackLocal 300Specifies whether to fail with an error when hostname canonicalization fails. 301The default, 302.Cm yes , 303will attempt to look up the unqualified hostname using the system resolver's 304search rules. 305A value of 306.Cm no 307will cause 308.Xr ssh 1 309to fail instantly if 310.Cm CanonicalizeHostname 311is enabled and the target hostname cannot be found in any of the domains 312specified by 313.Cm CanonicalDomains . 314.It Cm CanonicalizeHostname 315Controls whether explicit hostname canonicalization is performed. 316The default, 317.Cm no , 318is not to perform any name rewriting and let the system resolver handle all 319hostname lookups. 320If set to 321.Cm yes 322then, for connections that do not use a 323.Cm ProxyCommand 324or 325.Cm ProxyJump , 326.Xr ssh 1 327will attempt to canonicalize the hostname specified on the command line 328using the 329.Cm CanonicalDomains 330suffixes and 331.Cm CanonicalizePermittedCNAMEs 332rules. 333If 334.Cm CanonicalizeHostname 335is set to 336.Cm always , 337then canonicalization is applied to proxied connections too. 338.Pp 339If this option is enabled, then the configuration files are processed 340again using the new target name to pick up any new configuration in matching 341.Cm Host 342and 343.Cm Match 344stanzas. 345A value of 346.Cm none 347disables the use of a 348.Cm ProxyJump 349host. 350.It Cm CanonicalizeMaxDots 351Specifies the maximum number of dot characters in a hostname before 352canonicalization is disabled. 353The default, 1, 354allows a single dot (i.e. hostname.subdomain). 355.It Cm CanonicalizePermittedCNAMEs 356Specifies rules to determine whether CNAMEs should be followed when 357canonicalizing hostnames. 358The rules consist of one or more arguments of 359.Ar source_domain_list : Ns Ar target_domain_list , 360where 361.Ar source_domain_list 362is a pattern-list of domains that may follow CNAMEs in canonicalization, 363and 364.Ar target_domain_list 365is a pattern-list of domains that they may resolve to. 366.Pp 367For example, 368.Qq *.a.example.com:*.b.example.com,*.c.example.com 369will allow hostnames matching 370.Qq *.a.example.com 371to be canonicalized to names in the 372.Qq *.b.example.com 373or 374.Qq *.c.example.com 375domains. 376.Pp 377A single argument of 378.Qq none 379causes no CNAMEs to be considered for canonicalization. 380This is the default behaviour. 381.It Cm CASignatureAlgorithms 382Specifies which algorithms are allowed for signing of certificates 383by certificate authorities (CAs). 384The default is: 385.Bd -literal -offset indent 386ssh-ed25519,ecdsa-sha2-nistp256, 387ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 388sk-ssh-ed25519@openssh.com, 389sk-ecdsa-sha2-nistp256@openssh.com, 390rsa-sha2-512,rsa-sha2-256 391.Ed 392.Pp 393If the specified list begins with a 394.Sq + 395character, then the specified algorithms will be appended to the default set 396instead of replacing them. 397If the specified list begins with a 398.Sq - 399character, then the specified algorithms (including wildcards) will be removed 400from the default set instead of replacing them. 401.Pp 402.Xr ssh 1 403will not accept host certificates signed using algorithms other than those 404specified. 405.It Cm CertificateFile 406Specifies a file from which the user's certificate is read. 407A corresponding private key must be provided separately in order 408to use this certificate either 409from an 410.Cm IdentityFile 411directive or 412.Fl i 413flag to 414.Xr ssh 1 , 415via 416.Xr ssh-agent 1 , 417or via a 418.Cm PKCS11Provider 419or 420.Cm SecurityKeyProvider . 421.Pp 422Arguments to 423.Cm CertificateFile 424may use the tilde syntax to refer to a user's home directory, 425the tokens described in the 426.Sx TOKENS 427section and environment variables as described in the 428.Sx ENVIRONMENT VARIABLES 429section. 430.Pp 431It is possible to have multiple certificate files specified in 432configuration files; these certificates will be tried in sequence. 433Multiple 434.Cm CertificateFile 435directives will add to the list of certificates used for 436authentication. 437.It Cm CheckHostIP 438If set to 439.Cm yes , 440.Xr ssh 1 441will additionally check the host IP address in the 442.Pa known_hosts 443file. 444This allows it to detect if a host key changed due to DNS spoofing 445and will add addresses of destination hosts to 446.Pa ~/.ssh/known_hosts 447in the process, regardless of the setting of 448.Cm StrictHostKeyChecking . 449If the option is set to 450.Cm no 451(the default), 452the check will not be executed. 453The default is 454.Cm no . 455.It Cm Ciphers 456Specifies the ciphers allowed and their order of preference. 457Multiple ciphers must be comma-separated. 458If the specified list begins with a 459.Sq + 460character, then the specified ciphers will be appended to the default set 461instead of replacing them. 462If the specified list begins with a 463.Sq - 464character, then the specified ciphers (including wildcards) will be removed 465from the default set instead of replacing them. 466If the specified list begins with a 467.Sq ^ 468character, then the specified ciphers will be placed at the head of the 469default set. 470.Pp 471The supported ciphers are: 472.Bd -literal -offset indent 4733des-cbc 474aes128-cbc 475aes192-cbc 476aes256-cbc 477aes128-ctr 478aes192-ctr 479aes256-ctr 480aes128-gcm@openssh.com 481aes256-gcm@openssh.com 482chacha20-poly1305@openssh.com 483.Ed 484.Pp 485The default is: 486.Bd -literal -offset indent 487chacha20-poly1305@openssh.com, 488aes128-ctr,aes192-ctr,aes256-ctr, 489aes128-gcm@openssh.com,aes256-gcm@openssh.com 490.Ed 491.Pp 492The list of available ciphers may also be obtained using 493.Qq ssh -Q cipher . 494.It Cm ClearAllForwardings 495Specifies that all local, remote, and dynamic port forwardings 496specified in the configuration files or on the command line be 497cleared. 498This option is primarily useful when used from the 499.Xr ssh 1 500command line to clear port forwardings set in 501configuration files, and is automatically set by 502.Xr scp 1 503and 504.Xr sftp 1 . 505The argument must be 506.Cm yes 507or 508.Cm no 509(the default). 510.It Cm Compression 511Specifies whether to use compression. 512The argument must be 513.Cm yes 514or 515.Cm no 516(the default). 517.It Cm ConnectionAttempts 518Specifies the number of tries (one per second) to make before exiting. 519The argument must be an integer. 520This may be useful in scripts if the connection sometimes fails. 521The default is 1. 522.It Cm ConnectTimeout 523Specifies the timeout (in seconds) used when connecting to the 524SSH server, instead of using the default system TCP timeout. 525This timeout is applied both to establishing the connection and to performing 526the initial SSH protocol handshake and key exchange. 527.It Cm ControlMaster 528Enables the sharing of multiple sessions over a single network connection. 529When set to 530.Cm yes , 531.Xr ssh 1 532will listen for connections on a control socket specified using the 533.Cm ControlPath 534argument. 535Additional sessions can connect to this socket using the same 536.Cm ControlPath 537with 538.Cm ControlMaster 539set to 540.Cm no 541(the default). 542These sessions will try to reuse the master instance's network connection 543rather than initiating new ones, but will fall back to connecting normally 544if the control socket does not exist, or is not listening. 545.Pp 546Setting this to 547.Cm ask 548will cause 549.Xr ssh 1 550to listen for control connections, but require confirmation using 551.Xr ssh-askpass 1 . 552If the 553.Cm ControlPath 554cannot be opened, 555.Xr ssh 1 556will continue without connecting to a master instance. 557.Pp 558X11 and 559.Xr ssh-agent 1 560forwarding is supported over these multiplexed connections, however the 561display and agent forwarded will be the one belonging to the master 562connection i.e. it is not possible to forward multiple displays or agents. 563.Pp 564Two additional options allow for opportunistic multiplexing: try to use a 565master connection but fall back to creating a new one if one does not already 566exist. 567These options are: 568.Cm auto 569and 570.Cm autoask . 571The latter requires confirmation like the 572.Cm ask 573option. 574.It Cm ControlPath 575Specify the path to the control socket used for connection sharing as described 576in the 577.Cm ControlMaster 578section above or the string 579.Cm none 580to disable connection sharing. 581Arguments to 582.Cm ControlPath 583may use the tilde syntax to refer to a user's home directory, 584the tokens described in the 585.Sx TOKENS 586section and environment variables as described in the 587.Sx ENVIRONMENT VARIABLES 588section. 589It is recommended that any 590.Cm ControlPath 591used for opportunistic connection sharing include 592at least %h, %p, and %r (or alternatively %C) and be placed in a directory 593that is not writable by other users. 594This ensures that shared connections are uniquely identified. 595.It Cm ControlPersist 596When used in conjunction with 597.Cm ControlMaster , 598specifies that the master connection should remain open 599in the background (waiting for future client connections) 600after the initial client connection has been closed. 601If set to 602.Cm no 603(the default), 604then the master connection will not be placed into the background, 605and will close as soon as the initial client connection is closed. 606If set to 607.Cm yes 608or 0, 609then the master connection will remain in the background indefinitely 610(until killed or closed via a mechanism such as the 611.Qq ssh -O exit ) . 612If set to a time in seconds, or a time in any of the formats documented in 613.Xr sshd_config 5 , 614then the backgrounded master connection will automatically terminate 615after it has remained idle (with no client connections) for the 616specified time. 617.It Cm DynamicForward 618Specifies that a TCP port on the local machine be forwarded 619over the secure channel, and the application 620protocol is then used to determine where to connect to from the 621remote machine. 622.Pp 623The argument must be 624.Sm off 625.Oo Ar bind_address : Oc Ar port . 626.Sm on 627IPv6 addresses can be specified by enclosing addresses in square brackets. 628By default, the local port is bound in accordance with the 629.Cm GatewayPorts 630setting. 631However, an explicit 632.Ar bind_address 633may be used to bind the connection to a specific address. 634The 635.Ar bind_address 636of 637.Cm localhost 638indicates that the listening port be bound for local use only, while an 639empty address or 640.Sq * 641indicates that the port should be available from all interfaces. 642.Pp 643Currently the SOCKS4 and SOCKS5 protocols are supported, and 644.Xr ssh 1 645will act as a SOCKS server. 646Multiple forwardings may be specified, and 647additional forwardings can be given on the command line. 648Only the superuser can forward privileged ports. 649.It Cm EnableSSHKeysign 650Setting this option to 651.Cm yes 652in the global client configuration file 653.Pa /etc/ssh/ssh_config 654enables the use of the helper program 655.Xr ssh-keysign 8 656during 657.Cm HostbasedAuthentication . 658The argument must be 659.Cm yes 660or 661.Cm no 662(the default). 663This option should be placed in the non-hostspecific section. 664See 665.Xr ssh-keysign 8 666for more information. 667.It Cm EscapeChar 668Sets the escape character (default: 669.Ql ~ ) . 670The escape character can also 671be set on the command line. 672The argument should be a single character, 673.Ql ^ 674followed by a letter, or 675.Cm none 676to disable the escape 677character entirely (making the connection transparent for binary 678data). 679.It Cm ExitOnForwardFailure 680Specifies whether 681.Xr ssh 1 682should terminate the connection if it cannot set up all requested 683dynamic, tunnel, local, and remote port forwardings, (e.g.\& 684if either end is unable to bind and listen on a specified port). 685Note that 686.Cm ExitOnForwardFailure 687does not apply to connections made over port forwardings and will not, 688for example, cause 689.Xr ssh 1 690to exit if TCP connections to the ultimate forwarding destination fail. 691The argument must be 692.Cm yes 693or 694.Cm no 695(the default). 696.It Cm FingerprintHash 697Specifies the hash algorithm used when displaying key fingerprints. 698Valid options are: 699.Cm md5 700and 701.Cm sha256 702(the default). 703.It Cm ForkAfterAuthentication 704Requests 705.Nm ssh 706to go to background just before command execution. 707This is useful if 708.Nm ssh 709is going to ask for passwords or passphrases, but the user 710wants it in the background. 711This implies the 712.Cm StdinNull 713configuration option being set to 714.Dq yes . 715The recommended way to start X11 programs at a remote site is with 716something like 717.Ic ssh -f host xterm , 718which is the same as 719.Ic ssh host xterm 720if the 721.Cm ForkAfterAuthentication 722configuration option is set to 723.Dq yes . 724.Pp 725If the 726.Cm ExitOnForwardFailure 727configuration option is set to 728.Dq yes , 729then a client started with the 730.Cm ForkAfterAuthentication 731configuration option being set to 732.Dq yes 733will wait for all remote port forwards to be successfully established 734before placing itself in the background. 735The argument to this keyword must be 736.Cm yes 737(same as the 738.Fl f 739option) or 740.Cm no 741(the default). 742.It Cm ForwardAgent 743Specifies whether the connection to the authentication agent (if any) 744will be forwarded to the remote machine. 745The argument may be 746.Cm yes , 747.Cm no 748(the default), 749an explicit path to an agent socket or the name of an environment variable 750(beginning with 751.Sq $ ) 752in which to find the path. 753.Pp 754Agent forwarding should be enabled with caution. 755Users with the ability to bypass file permissions on the remote host 756(for the agent's Unix-domain socket) 757can access the local agent through the forwarded connection. 758An attacker cannot obtain key material from the agent, 759however they can perform operations on the keys that enable them to 760authenticate using the identities loaded into the agent. 761.It Cm ForwardX11 762Specifies whether X11 connections will be automatically redirected 763over the secure channel and 764.Ev DISPLAY 765set. 766The argument must be 767.Cm yes 768or 769.Cm no 770(the default). 771.Pp 772X11 forwarding should be enabled with caution. 773Users with the ability to bypass file permissions on the remote host 774(for the user's X11 authorization database) 775can access the local X11 display through the forwarded connection. 776An attacker may then be able to perform activities such as keystroke monitoring 777if the 778.Cm ForwardX11Trusted 779option is also enabled. 780.It Cm ForwardX11Timeout 781Specify a timeout for untrusted X11 forwarding 782using the format described in the 783.Sx TIME FORMATS 784section of 785.Xr sshd_config 5 . 786X11 connections received by 787.Xr ssh 1 788after this time will be refused. 789Setting 790.Cm ForwardX11Timeout 791to zero will disable the timeout and permit X11 forwarding for the life 792of the connection. 793The default is to disable untrusted X11 forwarding after twenty minutes has 794elapsed. 795.It Cm ForwardX11Trusted 796If this option is set to 797.Cm yes , 798remote X11 clients will have full access to the original X11 display. 799.Pp 800If this option is set to 801.Cm no 802(the default), 803remote X11 clients will be considered untrusted and prevented 804from stealing or tampering with data belonging to trusted X11 805clients. 806Furthermore, the 807.Xr xauth 1 808token used for the session will be set to expire after 20 minutes. 809Remote clients will be refused access after this time. 810.Pp 811See the X11 SECURITY extension specification for full details on 812the restrictions imposed on untrusted clients. 813.It Cm GatewayPorts 814Specifies whether remote hosts are allowed to connect to local 815forwarded ports. 816By default, 817.Xr ssh 1 818binds local port forwardings to the loopback address. 819This prevents other remote hosts from connecting to forwarded ports. 820.Cm GatewayPorts 821can be used to specify that ssh 822should bind local port forwardings to the wildcard address, 823thus allowing remote hosts to connect to forwarded ports. 824The argument must be 825.Cm yes 826or 827.Cm no 828(the default). 829.It Cm GlobalKnownHostsFile 830Specifies one or more files to use for the global 831host key database, separated by whitespace. 832The default is 833.Pa /etc/ssh/ssh_known_hosts , 834.Pa /etc/ssh/ssh_known_hosts2 . 835.It Cm GSSAPIAuthentication 836Specifies whether user authentication based on GSSAPI is allowed. 837The default is 838.Cm no . 839.It Cm GSSAPIDelegateCredentials 840Forward (delegate) credentials to the server. 841The default is 842.Cm no . 843.It Cm HashKnownHosts 844Indicates that 845.Xr ssh 1 846should hash host names and addresses when they are added to 847.Pa ~/.ssh/known_hosts . 848These hashed names may be used normally by 849.Xr ssh 1 850and 851.Xr sshd 8 , 852but they do not visually reveal identifying information if the 853file's contents are disclosed. 854The default is 855.Cm no . 856Note that existing names and addresses in known hosts files 857will not be converted automatically, 858but may be manually hashed using 859.Xr ssh-keygen 1 . 860.It Cm HostbasedAcceptedAlgorithms 861Specifies the signature algorithms that will be used for hostbased 862authentication as a comma-separated list of patterns. 863Alternately if the specified list begins with a 864.Sq + 865character, then the specified signature algorithms will be appended 866to the default set instead of replacing them. 867If the specified list begins with a 868.Sq - 869character, then the specified signature algorithms (including wildcards) 870will be removed from the default set instead of replacing them. 871If the specified list begins with a 872.Sq ^ 873character, then the specified signature algorithms will be placed 874at the head of the default set. 875The default for this option is: 876.Bd -literal -offset 3n 877ssh-ed25519-cert-v01@openssh.com, 878ecdsa-sha2-nistp256-cert-v01@openssh.com, 879ecdsa-sha2-nistp384-cert-v01@openssh.com, 880ecdsa-sha2-nistp521-cert-v01@openssh.com, 881sk-ssh-ed25519-cert-v01@openssh.com, 882sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 883rsa-sha2-512-cert-v01@openssh.com, 884rsa-sha2-256-cert-v01@openssh.com, 885ssh-ed25519, 886ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 887sk-ssh-ed25519@openssh.com, 888sk-ecdsa-sha2-nistp256@openssh.com, 889rsa-sha2-512,rsa-sha2-256 890.Ed 891.Pp 892The 893.Fl Q 894option of 895.Xr ssh 1 896may be used to list supported signature algorithms. 897This was formerly named HostbasedKeyTypes. 898.It Cm HostbasedAuthentication 899Specifies whether to try rhosts based authentication with public key 900authentication. 901The argument must be 902.Cm yes 903or 904.Cm no 905(the default). 906.It Cm HostKeyAlgorithms 907Specifies the host key signature algorithms 908that the client wants to use in order of preference. 909Alternately if the specified list begins with a 910.Sq + 911character, then the specified signature algorithms will be appended to 912the default set instead of replacing them. 913If the specified list begins with a 914.Sq - 915character, then the specified signature algorithms (including wildcards) 916will be removed from the default set instead of replacing them. 917If the specified list begins with a 918.Sq ^ 919character, then the specified signature algorithms will be placed 920at the head of the default set. 921The default for this option is: 922.Bd -literal -offset 3n 923ssh-ed25519-cert-v01@openssh.com, 924ecdsa-sha2-nistp256-cert-v01@openssh.com, 925ecdsa-sha2-nistp384-cert-v01@openssh.com, 926ecdsa-sha2-nistp521-cert-v01@openssh.com, 927sk-ssh-ed25519-cert-v01@openssh.com, 928sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 929rsa-sha2-512-cert-v01@openssh.com, 930rsa-sha2-256-cert-v01@openssh.com, 931ssh-ed25519, 932ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 933sk-ecdsa-sha2-nistp256@openssh.com, 934sk-ssh-ed25519@openssh.com, 935rsa-sha2-512,rsa-sha2-256 936.Ed 937.Pp 938If hostkeys are known for the destination host then this default is modified 939to prefer their algorithms. 940.Pp 941The list of available signature algorithms may also be obtained using 942.Qq ssh -Q HostKeyAlgorithms . 943.It Cm HostKeyAlias 944Specifies an alias that should be used instead of the 945real host name when looking up or saving the host key 946in the host key database files and when validating host certificates. 947This option is useful for tunneling SSH connections 948or for multiple servers running on a single host. 949.It Cm Hostname 950Specifies the real host name to log into. 951This can be used to specify nicknames or abbreviations for hosts. 952Arguments to 953.Cm Hostname 954accept the tokens described in the 955.Sx TOKENS 956section. 957Numeric IP addresses are also permitted (both on the command line and in 958.Cm Hostname 959specifications). 960The default is the name given on the command line. 961.It Cm IdentitiesOnly 962Specifies that 963.Xr ssh 1 964should only use the configured authentication identity and certificate files 965(either the default files, or those explicitly configured in the 966.Nm 967files 968or passed on the 969.Xr ssh 1 970command-line), 971even if 972.Xr ssh-agent 1 973or a 974.Cm PKCS11Provider 975or 976.Cm SecurityKeyProvider 977offers more identities. 978The argument to this keyword must be 979.Cm yes 980or 981.Cm no 982(the default). 983This option is intended for situations where ssh-agent 984offers many different identities. 985.It Cm IdentityAgent 986Specifies the 987.Ux Ns -domain 988socket used to communicate with the authentication agent. 989.Pp 990This option overrides the 991.Ev SSH_AUTH_SOCK 992environment variable and can be used to select a specific agent. 993Setting the socket name to 994.Cm none 995disables the use of an authentication agent. 996If the string 997.Qq SSH_AUTH_SOCK 998is specified, the location of the socket will be read from the 999.Ev SSH_AUTH_SOCK 1000environment variable. 1001Otherwise if the specified value begins with a 1002.Sq $ 1003character, then it will be treated as an environment variable containing 1004the location of the socket. 1005.Pp 1006Arguments to 1007.Cm IdentityAgent 1008may use the tilde syntax to refer to a user's home directory, 1009the tokens described in the 1010.Sx TOKENS 1011section and environment variables as described in the 1012.Sx ENVIRONMENT VARIABLES 1013section. 1014.It Cm IdentityFile 1015Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, 1016Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read. 1017The default is 1018.Pa ~/.ssh/id_rsa , 1019.Pa ~/.ssh/id_ecdsa , 1020.Pa ~/.ssh/id_ecdsa_sk , 1021.Pa ~/.ssh/id_ed25519 , 1022.Pa ~/.ssh/id_ed25519_sk 1023and 1024.Pa ~/.ssh/id_dsa . 1025Additionally, any identities represented by the authentication agent 1026will be used for authentication unless 1027.Cm IdentitiesOnly 1028is set. 1029If no certificates have been explicitly specified by 1030.Cm CertificateFile , 1031.Xr ssh 1 1032will try to load certificate information from the filename obtained by 1033appending 1034.Pa -cert.pub 1035to the path of a specified 1036.Cm IdentityFile . 1037.Pp 1038Arguments to 1039.Cm IdentityFile 1040may use the tilde syntax to refer to a user's home directory 1041or the tokens described in the 1042.Sx TOKENS 1043section. 1044.Pp 1045It is possible to have 1046multiple identity files specified in configuration files; all these 1047identities will be tried in sequence. 1048Multiple 1049.Cm IdentityFile 1050directives will add to the list of identities tried (this behaviour 1051differs from that of other configuration directives). 1052.Pp 1053.Cm IdentityFile 1054may be used in conjunction with 1055.Cm IdentitiesOnly 1056to select which identities in an agent are offered during authentication. 1057.Cm IdentityFile 1058may also be used in conjunction with 1059.Cm CertificateFile 1060in order to provide any certificate also needed for authentication with 1061the identity. 1062.It Cm IgnoreUnknown 1063Specifies a pattern-list of unknown options to be ignored if they are 1064encountered in configuration parsing. 1065This may be used to suppress errors if 1066.Nm 1067contains options that are unrecognised by 1068.Xr ssh 1 . 1069It is recommended that 1070.Cm IgnoreUnknown 1071be listed early in the configuration file as it will not be applied 1072to unknown options that appear before it. 1073.It Cm Include 1074Include the specified configuration file(s). 1075Multiple pathnames may be specified and each pathname may contain 1076.Xr glob 7 1077wildcards and, for user configurations, shell-like 1078.Sq ~ 1079references to user home directories. 1080Wildcards will be expanded and processed in lexical order. 1081Files without absolute paths are assumed to be in 1082.Pa ~/.ssh 1083if included in a user configuration file or 1084.Pa /etc/ssh 1085if included from the system configuration file. 1086.Cm Include 1087directive may appear inside a 1088.Cm Match 1089or 1090.Cm Host 1091block 1092to perform conditional inclusion. 1093.It Cm IPQoS 1094Specifies the IPv4 type-of-service or DSCP class for connections. 1095Accepted values are 1096.Cm af11 , 1097.Cm af12 , 1098.Cm af13 , 1099.Cm af21 , 1100.Cm af22 , 1101.Cm af23 , 1102.Cm af31 , 1103.Cm af32 , 1104.Cm af33 , 1105.Cm af41 , 1106.Cm af42 , 1107.Cm af43 , 1108.Cm cs0 , 1109.Cm cs1 , 1110.Cm cs2 , 1111.Cm cs3 , 1112.Cm cs4 , 1113.Cm cs5 , 1114.Cm cs6 , 1115.Cm cs7 , 1116.Cm ef , 1117.Cm le , 1118.Cm lowdelay , 1119.Cm throughput , 1120.Cm reliability , 1121a numeric value, or 1122.Cm none 1123to use the operating system default. 1124This option may take one or two arguments, separated by whitespace. 1125If one argument is specified, it is used as the packet class unconditionally. 1126If two values are specified, the first is automatically selected for 1127interactive sessions and the second for non-interactive sessions. 1128The default is 1129.Cm af21 1130(Low-Latency Data) 1131for interactive sessions and 1132.Cm cs1 1133(Lower Effort) 1134for non-interactive sessions. 1135.It Cm KbdInteractiveAuthentication 1136Specifies whether to use keyboard-interactive authentication. 1137The argument to this keyword must be 1138.Cm yes 1139(the default) 1140or 1141.Cm no . 1142.Cm ChallengeResponseAuthentication 1143is a deprecated alias for this. 1144.It Cm KbdInteractiveDevices 1145Specifies the list of methods to use in keyboard-interactive authentication. 1146Multiple method names must be comma-separated. 1147The default is to use the server specified list. 1148The methods available vary depending on what the server supports. 1149For an OpenSSH server, 1150it may be zero or more of: 1151.Cm bsdauth 1152and 1153.Cm pam . 1154.It Cm KexAlgorithms 1155Specifies the available KEX (Key Exchange) algorithms. 1156Multiple algorithms must be comma-separated. 1157If the specified list begins with a 1158.Sq + 1159character, then the specified algorithms will be appended to the default set 1160instead of replacing them. 1161If the specified list begins with a 1162.Sq - 1163character, then the specified algorithms (including wildcards) will be removed 1164from the default set instead of replacing them. 1165If the specified list begins with a 1166.Sq ^ 1167character, then the specified algorithms will be placed at the head of the 1168default set. 1169The default is: 1170.Bd -literal -offset indent 1171curve25519-sha256,curve25519-sha256@libssh.org, 1172ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1173sntrup761x25519-sha512@openssh.com, 1174diffie-hellman-group-exchange-sha256, 1175diffie-hellman-group16-sha512, 1176diffie-hellman-group18-sha512, 1177diffie-hellman-group14-sha256 1178.Ed 1179.Pp 1180The list of available key exchange algorithms may also be obtained using 1181.Qq ssh -Q kex . 1182.It Cm KnownHostsCommand 1183Specifies a command to use to obtain a list of host keys, in addition to 1184those listed in 1185.Cm UserKnownHostsFile 1186and 1187.Cm GlobalKnownHostsFile . 1188This command is executed after the files have been read. 1189It may write host key lines to standard output in identical format to the 1190usual files (described in the 1191.Sx VERIFYING HOST KEYS 1192section in 1193.Xr ssh 1 ) . 1194Arguments to 1195.Cm KnownHostsCommand 1196accept the tokens described in the 1197.Sx TOKENS 1198section. 1199The command may be invoked multiple times per connection: once when preparing 1200the preference list of host key algorithms to use, again to obtain the 1201host key for the requested host name and, if 1202.Cm CheckHostIP 1203is enabled, one more time to obtain the host key matching the server's 1204address. 1205If the command exits abnormally or returns a non-zero exit status then the 1206connection is terminated. 1207.It Cm LocalCommand 1208Specifies a command to execute on the local machine after successfully 1209connecting to the server. 1210The command string extends to the end of the line, and is executed with 1211the user's shell. 1212Arguments to 1213.Cm LocalCommand 1214accept the tokens described in the 1215.Sx TOKENS 1216section. 1217.Pp 1218The command is run synchronously and does not have access to the 1219session of the 1220.Xr ssh 1 1221that spawned it. 1222It should not be used for interactive commands. 1223.Pp 1224This directive is ignored unless 1225.Cm PermitLocalCommand 1226has been enabled. 1227.It Cm LocalForward 1228Specifies that a TCP port on the local machine be forwarded over 1229the secure channel to the specified host and port from the remote machine. 1230The first argument specifies the listener and may be 1231.Sm off 1232.Oo Ar bind_address : Oc Ar port 1233.Sm on 1234or a Unix domain socket path. 1235The second argument is the destination and may be 1236.Ar host : Ns Ar hostport 1237or a Unix domain socket path if the remote host supports it. 1238.Pp 1239IPv6 addresses can be specified by enclosing addresses in square brackets. 1240Multiple forwardings may be specified, and additional forwardings can be 1241given on the command line. 1242Only the superuser can forward privileged ports. 1243By default, the local port is bound in accordance with the 1244.Cm GatewayPorts 1245setting. 1246However, an explicit 1247.Ar bind_address 1248may be used to bind the connection to a specific address. 1249The 1250.Ar bind_address 1251of 1252.Cm localhost 1253indicates that the listening port be bound for local use only, while an 1254empty address or 1255.Sq * 1256indicates that the port should be available from all interfaces. 1257Unix domain socket paths may use the tokens described in the 1258.Sx TOKENS 1259section and environment variables as described in the 1260.Sx ENVIRONMENT VARIABLES 1261section. 1262.It Cm LogLevel 1263Gives the verbosity level that is used when logging messages from 1264.Xr ssh 1 . 1265The possible values are: 1266QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1267The default is INFO. 1268DEBUG and DEBUG1 are equivalent. 1269DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1270.It Cm LogVerbose 1271Specify one or more overrides to LogLevel. 1272An override consists of a pattern lists that matches the source file, function 1273and line number to force detailed logging for. 1274For example, an override pattern of: 1275.Bd -literal -offset indent 1276kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* 1277.Ed 1278.Pp 1279would enable detailed logging for line 1000 of 1280.Pa kex.c , 1281everything in the 1282.Fn kex_exchange_identification 1283function, and all code in the 1284.Pa packet.c 1285file. 1286This option is intended for debugging and no overrides are enabled by default. 1287.It Cm MACs 1288Specifies the MAC (message authentication code) algorithms 1289in order of preference. 1290The MAC algorithm is used for data integrity protection. 1291Multiple algorithms must be comma-separated. 1292If the specified list begins with a 1293.Sq + 1294character, then the specified algorithms will be appended to the default set 1295instead of replacing them. 1296If the specified list begins with a 1297.Sq - 1298character, then the specified algorithms (including wildcards) will be removed 1299from the default set instead of replacing them. 1300If the specified list begins with a 1301.Sq ^ 1302character, then the specified algorithms will be placed at the head of the 1303default set. 1304.Pp 1305The algorithms that contain 1306.Qq -etm 1307calculate the MAC after encryption (encrypt-then-mac). 1308These are considered safer and their use recommended. 1309.Pp 1310The default is: 1311.Bd -literal -offset indent 1312umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1313hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1314hmac-sha1-etm@openssh.com, 1315umac-64@openssh.com,umac-128@openssh.com, 1316hmac-sha2-256,hmac-sha2-512,hmac-sha1 1317.Ed 1318.Pp 1319The list of available MAC algorithms may also be obtained using 1320.Qq ssh -Q mac . 1321.It Cm NoHostAuthenticationForLocalhost 1322Disable host authentication for localhost (loopback addresses). 1323The argument to this keyword must be 1324.Cm yes 1325or 1326.Cm no 1327(the default). 1328.It Cm NumberOfPasswordPrompts 1329Specifies the number of password prompts before giving up. 1330The argument to this keyword must be an integer. 1331The default is 3. 1332.It Cm PasswordAuthentication 1333Specifies whether to use password authentication. 1334The argument to this keyword must be 1335.Cm yes 1336(the default) 1337or 1338.Cm no . 1339.It Cm PermitLocalCommand 1340Allow local command execution via the 1341.Ic LocalCommand 1342option or using the 1343.Ic !\& Ns Ar command 1344escape sequence in 1345.Xr ssh 1 . 1346The argument must be 1347.Cm yes 1348or 1349.Cm no 1350(the default). 1351.It Cm PermitRemoteOpen 1352Specifies the destinations to which remote TCP port forwarding is permitted when 1353.Cm RemoteForward 1354is used as a SOCKS proxy. 1355The forwarding specification must be one of the following forms: 1356.Pp 1357.Bl -item -offset indent -compact 1358.It 1359.Cm PermitRemoteOpen 1360.Sm off 1361.Ar host : port 1362.Sm on 1363.It 1364.Cm PermitRemoteOpen 1365.Sm off 1366.Ar IPv4_addr : port 1367.Sm on 1368.It 1369.Cm PermitRemoteOpen 1370.Sm off 1371.Ar \&[ IPv6_addr \&] : port 1372.Sm on 1373.El 1374.Pp 1375Multiple forwards may be specified by separating them with whitespace. 1376An argument of 1377.Cm any 1378can be used to remove all restrictions and permit any forwarding requests. 1379An argument of 1380.Cm none 1381can be used to prohibit all forwarding requests. 1382The wildcard 1383.Sq * 1384can be used for host or port to allow all hosts or ports respectively. 1385Otherwise, no pattern matching or address lookups are performed on supplied 1386names. 1387.It Cm PKCS11Provider 1388Specifies which PKCS#11 provider to use or 1389.Cm none 1390to indicate that no provider should be used (the default). 1391The argument to this keyword is a path to the PKCS#11 shared library 1392.Xr ssh 1 1393should use to communicate with a PKCS#11 token providing keys for user 1394authentication. 1395.It Cm Port 1396Specifies the port number to connect on the remote host. 1397The default is 22. 1398.It Cm PreferredAuthentications 1399Specifies the order in which the client should try authentication methods. 1400This allows a client to prefer one method (e.g.\& 1401.Cm keyboard-interactive ) 1402over another method (e.g.\& 1403.Cm password ) . 1404The default is: 1405.Bd -literal -offset indent 1406gssapi-with-mic,hostbased,publickey, 1407keyboard-interactive,password 1408.Ed 1409.It Cm ProxyCommand 1410Specifies the command to use to connect to the server. 1411The command 1412string extends to the end of the line, and is executed 1413using the user's shell 1414.Ql exec 1415directive to avoid a lingering shell process. 1416.Pp 1417Arguments to 1418.Cm ProxyCommand 1419accept the tokens described in the 1420.Sx TOKENS 1421section. 1422The command can be basically anything, 1423and should read from its standard input and write to its standard output. 1424It should eventually connect an 1425.Xr sshd 8 1426server running on some machine, or execute 1427.Ic sshd -i 1428somewhere. 1429Host key management will be done using the 1430.Cm Hostname 1431of the host being connected (defaulting to the name typed by the user). 1432Setting the command to 1433.Cm none 1434disables this option entirely. 1435Note that 1436.Cm CheckHostIP 1437is not available for connects with a proxy command. 1438.Pp 1439This directive is useful in conjunction with 1440.Xr nc 1 1441and its proxy support. 1442For example, the following directive would connect via an HTTP proxy at 1443192.0.2.0: 1444.Bd -literal -offset 3n 1445ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1446.Ed 1447.It Cm ProxyJump 1448Specifies one or more jump proxies as either 1449.Xo 1450.Sm off 1451.Op Ar user No @ 1452.Ar host 1453.Op : Ns Ar port 1454.Sm on 1455or an ssh URI 1456.Xc . 1457Multiple proxies may be separated by comma characters and will be visited 1458sequentially. 1459Setting this option will cause 1460.Xr ssh 1 1461to connect to the target host by first making a 1462.Xr ssh 1 1463connection to the specified 1464.Cm ProxyJump 1465host and then establishing a 1466TCP forwarding to the ultimate target from there. 1467Setting the host to 1468.Cm none 1469disables this option entirely. 1470.Pp 1471Note that this option will compete with the 1472.Cm ProxyCommand 1473option - whichever is specified first will prevent later instances of the 1474other from taking effect. 1475.Pp 1476Note also that the configuration for the destination host (either supplied 1477via the command-line or the configuration file) is not generally applied 1478to jump hosts. 1479.Pa ~/.ssh/config 1480should be used if specific configuration is required for jump hosts. 1481.It Cm ProxyUseFdpass 1482Specifies that 1483.Cm ProxyCommand 1484will pass a connected file descriptor back to 1485.Xr ssh 1 1486instead of continuing to execute and pass data. 1487The default is 1488.Cm no . 1489.It Cm PubkeyAcceptedAlgorithms 1490Specifies the signature algorithms that will be used for public key 1491authentication as a comma-separated list of patterns. 1492If the specified list begins with a 1493.Sq + 1494character, then the algorithms after it will be appended to the default 1495instead of replacing it. 1496If the specified list begins with a 1497.Sq - 1498character, then the specified algorithms (including wildcards) will be removed 1499from the default set instead of replacing them. 1500If the specified list begins with a 1501.Sq ^ 1502character, then the specified algorithms will be placed at the head of the 1503default set. 1504The default for this option is: 1505.Bd -literal -offset 3n 1506ssh-ed25519-cert-v01@openssh.com, 1507ecdsa-sha2-nistp256-cert-v01@openssh.com, 1508ecdsa-sha2-nistp384-cert-v01@openssh.com, 1509ecdsa-sha2-nistp521-cert-v01@openssh.com, 1510sk-ssh-ed25519-cert-v01@openssh.com, 1511sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, 1512rsa-sha2-512-cert-v01@openssh.com, 1513rsa-sha2-256-cert-v01@openssh.com, 1514ssh-ed25519, 1515ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1516sk-ssh-ed25519@openssh.com, 1517sk-ecdsa-sha2-nistp256@openssh.com, 1518rsa-sha2-512,rsa-sha2-256 1519.Ed 1520.Pp 1521The list of available signature algorithms may also be obtained using 1522.Qq ssh -Q PubkeyAcceptedAlgorithms . 1523.It Cm PubkeyAuthentication 1524Specifies whether to try public key authentication. 1525The argument to this keyword must be 1526.Cm yes 1527(the default), 1528.Cm no , 1529.Cm unbound 1530or 1531.Cm host-bound . 1532The final two options enable public key authentication while respectively 1533disabling or enabling the OpenSSH host-bound authentication protocol 1534extension required for restricted 1535.Xr ssh-agent 1 1536forwarding. 1537.It Cm RekeyLimit 1538Specifies the maximum amount of data that may be transmitted before the 1539session key is renegotiated, optionally followed by a maximum amount of 1540time that may pass before the session key is renegotiated. 1541The first argument is specified in bytes and may have a suffix of 1542.Sq K , 1543.Sq M , 1544or 1545.Sq G 1546to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1547The default is between 1548.Sq 1G 1549and 1550.Sq 4G , 1551depending on the cipher. 1552The optional second value is specified in seconds and may use any of the 1553units documented in the TIME FORMATS section of 1554.Xr sshd_config 5 . 1555The default value for 1556.Cm RekeyLimit 1557is 1558.Cm default none , 1559which means that rekeying is performed after the cipher's default amount 1560of data has been sent or received and no time based rekeying is done. 1561.It Cm RemoteCommand 1562Specifies a command to execute on the remote machine after successfully 1563connecting to the server. 1564The command string extends to the end of the line, and is executed with 1565the user's shell. 1566Arguments to 1567.Cm RemoteCommand 1568accept the tokens described in the 1569.Sx TOKENS 1570section. 1571.It Cm RemoteForward 1572Specifies that a TCP port on the remote machine be forwarded over 1573the secure channel. 1574The remote port may either be forwarded to a specified host and port 1575from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1576client to connect to arbitrary destinations from the local machine. 1577The first argument is the listening specification and may be 1578.Sm off 1579.Oo Ar bind_address : Oc Ar port 1580.Sm on 1581or, if the remote host supports it, a Unix domain socket path. 1582If forwarding to a specific destination then the second argument must be 1583.Ar host : Ns Ar hostport 1584or a Unix domain socket path, 1585otherwise if no destination argument is specified then the remote forwarding 1586will be established as a SOCKS proxy. 1587When acting as a SOCKS proxy the destination of the connection can be 1588restricted by 1589.Cm PermitRemoteOpen . 1590.Pp 1591IPv6 addresses can be specified by enclosing addresses in square brackets. 1592Multiple forwardings may be specified, and additional 1593forwardings can be given on the command line. 1594Privileged ports can be forwarded only when 1595logging in as root on the remote machine. 1596Unix domain socket paths may use the tokens described in the 1597.Sx TOKENS 1598section and environment variables as described in the 1599.Sx ENVIRONMENT VARIABLES 1600section. 1601.Pp 1602If the 1603.Ar port 1604argument is 0, 1605the listen port will be dynamically allocated on the server and reported 1606to the client at run time. 1607.Pp 1608If the 1609.Ar bind_address 1610is not specified, the default is to only bind to loopback addresses. 1611If the 1612.Ar bind_address 1613is 1614.Ql * 1615or an empty string, then the forwarding is requested to listen on all 1616interfaces. 1617Specifying a remote 1618.Ar bind_address 1619will only succeed if the server's 1620.Cm GatewayPorts 1621option is enabled (see 1622.Xr sshd_config 5 ) . 1623.It Cm RequestTTY 1624Specifies whether to request a pseudo-tty for the session. 1625The argument may be one of: 1626.Cm no 1627(never request a TTY), 1628.Cm yes 1629(always request a TTY when standard input is a TTY), 1630.Cm force 1631(always request a TTY) or 1632.Cm auto 1633(request a TTY when opening a login session). 1634This option mirrors the 1635.Fl t 1636and 1637.Fl T 1638flags for 1639.Xr ssh 1 . 1640.It Cm RevokedHostKeys 1641Specifies revoked host public keys. 1642Keys listed in this file will be refused for host authentication. 1643Note that if this file does not exist or is not readable, 1644then host authentication will be refused for all hosts. 1645Keys may be specified as a text file, listing one public key per line, or as 1646an OpenSSH Key Revocation List (KRL) as generated by 1647.Xr ssh-keygen 1 . 1648For more information on KRLs, see the KEY REVOCATION LISTS section in 1649.Xr ssh-keygen 1 . 1650.It Cm SecurityKeyProvider 1651Specifies a path to a library that will be used when loading any 1652FIDO authenticator-hosted keys, overriding the default of using 1653the built-in USB HID support. 1654.Pp 1655If the specified value begins with a 1656.Sq $ 1657character, then it will be treated as an environment variable containing 1658the path to the library. 1659.It Cm SendEnv 1660Specifies what variables from the local 1661.Xr environ 7 1662should be sent to the server. 1663The server must also support it, and the server must be configured to 1664accept these environment variables. 1665Note that the 1666.Ev TERM 1667environment variable is always sent whenever a 1668pseudo-terminal is requested as it is required by the protocol. 1669Refer to 1670.Cm AcceptEnv 1671in 1672.Xr sshd_config 5 1673for how to configure the server. 1674Variables are specified by name, which may contain wildcard characters. 1675Multiple environment variables may be separated by whitespace or spread 1676across multiple 1677.Cm SendEnv 1678directives. 1679.Pp 1680See 1681.Sx PATTERNS 1682for more information on patterns. 1683.Pp 1684It is possible to clear previously set 1685.Cm SendEnv 1686variable names by prefixing patterns with 1687.Pa - . 1688The default is not to send any environment variables. 1689.It Cm ServerAliveCountMax 1690Sets the number of server alive messages (see below) which may be 1691sent without 1692.Xr ssh 1 1693receiving any messages back from the server. 1694If this threshold is reached while server alive messages are being sent, 1695ssh will disconnect from the server, terminating the session. 1696It is important to note that the use of server alive messages is very 1697different from 1698.Cm TCPKeepAlive 1699(below). 1700The server alive messages are sent through the encrypted channel 1701and therefore will not be spoofable. 1702The TCP keepalive option enabled by 1703.Cm TCPKeepAlive 1704is spoofable. 1705The server alive mechanism is valuable when the client or 1706server depend on knowing when a connection has become unresponsive. 1707.Pp 1708The default value is 3. 1709If, for example, 1710.Cm ServerAliveInterval 1711(see below) is set to 15 and 1712.Cm ServerAliveCountMax 1713is left at the default, if the server becomes unresponsive, 1714ssh will disconnect after approximately 45 seconds. 1715.It Cm ServerAliveInterval 1716Sets a timeout interval in seconds after which if no data has been received 1717from the server, 1718.Xr ssh 1 1719will send a message through the encrypted 1720channel to request a response from the server. 1721The default 1722is 0, indicating that these messages will not be sent to the server. 1723.It Cm SessionType 1724May be used to either request invocation of a subsystem on the remote system, 1725or to prevent the execution of a remote command at all. 1726The latter is useful for just forwarding ports. 1727The argument to this keyword must be 1728.Cm none 1729(same as the 1730.Fl N 1731option), 1732.Cm subsystem 1733(same as the 1734.Fl s 1735option) or 1736.Cm default 1737(shell or command execution). 1738.It Cm SetEnv 1739Directly specify one or more environment variables and their contents to 1740be sent to the server. 1741Similarly to 1742.Cm SendEnv , 1743with the exception of the 1744.Ev TERM 1745variable, the server must be prepared to accept the environment variable. 1746.It Cm StdinNull 1747Redirects stdin from 1748.Pa /dev/null 1749(actually, prevents reading from stdin). 1750Either this or the equivalent 1751.Fl n 1752option must be used when 1753.Nm ssh 1754is run in the background. 1755The argument to this keyword must be 1756.Cm yes 1757(same as the 1758.Fl n 1759option) or 1760.Cm no 1761(the default). 1762.It Cm StreamLocalBindMask 1763Sets the octal file creation mode mask 1764.Pq umask 1765used when creating a Unix-domain socket file for local or remote 1766port forwarding. 1767This option is only used for port forwarding to a Unix-domain socket file. 1768.Pp 1769The default value is 0177, which creates a Unix-domain socket file that is 1770readable and writable only by the owner. 1771Note that not all operating systems honor the file mode on Unix-domain 1772socket files. 1773.It Cm StreamLocalBindUnlink 1774Specifies whether to remove an existing Unix-domain socket file for local 1775or remote port forwarding before creating a new one. 1776If the socket file already exists and 1777.Cm StreamLocalBindUnlink 1778is not enabled, 1779.Nm ssh 1780will be unable to forward the port to the Unix-domain socket file. 1781This option is only used for port forwarding to a Unix-domain socket file. 1782.Pp 1783The argument must be 1784.Cm yes 1785or 1786.Cm no 1787(the default). 1788.It Cm StrictHostKeyChecking 1789If this flag is set to 1790.Cm yes , 1791.Xr ssh 1 1792will never automatically add host keys to the 1793.Pa ~/.ssh/known_hosts 1794file, and refuses to connect to hosts whose host key has changed. 1795This provides maximum protection against man-in-the-middle (MITM) attacks, 1796though it can be annoying when the 1797.Pa /etc/ssh/ssh_known_hosts 1798file is poorly maintained or when connections to new hosts are 1799frequently made. 1800This option forces the user to manually 1801add all new hosts. 1802.Pp 1803If this flag is set to 1804.Cm accept-new 1805then ssh will automatically add new host keys to the user's 1806.Pa known_hosts 1807file, but will not permit connections to hosts with 1808changed host keys. 1809If this flag is set to 1810.Cm no 1811or 1812.Cm off , 1813ssh will automatically add new host keys to the user known hosts files 1814and allow connections to hosts with changed hostkeys to proceed, 1815subject to some restrictions. 1816If this flag is set to 1817.Cm ask 1818(the default), 1819new host keys 1820will be added to the user known host files only after the user 1821has confirmed that is what they really want to do, and 1822ssh will refuse to connect to hosts whose host key has changed. 1823The host keys of 1824known hosts will be verified automatically in all cases. 1825.It Cm SyslogFacility 1826Gives the facility code that is used when logging messages from 1827.Xr ssh 1 . 1828The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1829LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1830The default is USER. 1831.It Cm TCPKeepAlive 1832Specifies whether the system should send TCP keepalive messages to the 1833other side. 1834If they are sent, death of the connection or crash of one 1835of the machines will be properly noticed. 1836However, this means that 1837connections will die if the route is down temporarily, and some people 1838find it annoying. 1839.Pp 1840The default is 1841.Cm yes 1842(to send TCP keepalive messages), and the client will notice 1843if the network goes down or the remote host dies. 1844This is important in scripts, and many users want it too. 1845.Pp 1846To disable TCP keepalive messages, the value should be set to 1847.Cm no . 1848See also 1849.Cm ServerAliveInterval 1850for protocol-level keepalives. 1851.It Cm Tunnel 1852Request 1853.Xr tun 4 1854device forwarding between the client and the server. 1855The argument must be 1856.Cm yes , 1857.Cm point-to-point 1858(layer 3), 1859.Cm ethernet 1860(layer 2), 1861or 1862.Cm no 1863(the default). 1864Specifying 1865.Cm yes 1866requests the default tunnel mode, which is 1867.Cm point-to-point . 1868.It Cm TunnelDevice 1869Specifies the 1870.Xr tun 4 1871devices to open on the client 1872.Pq Ar local_tun 1873and the server 1874.Pq Ar remote_tun . 1875.Pp 1876The argument must be 1877.Sm off 1878.Ar local_tun Op : Ar remote_tun . 1879.Sm on 1880The devices may be specified by numerical ID or the keyword 1881.Cm any , 1882which uses the next available tunnel device. 1883If 1884.Ar remote_tun 1885is not specified, it defaults to 1886.Cm any . 1887The default is 1888.Cm any:any . 1889.It Cm UpdateHostKeys 1890Specifies whether 1891.Xr ssh 1 1892should accept notifications of additional hostkeys from the server sent 1893after authentication has completed and add them to 1894.Cm UserKnownHostsFile . 1895The argument must be 1896.Cm yes , 1897.Cm no 1898or 1899.Cm ask . 1900This option allows learning alternate hostkeys for a server 1901and supports graceful key rotation by allowing a server to send replacement 1902public keys before old ones are removed. 1903.Pp 1904Additional hostkeys are only accepted if the key used to authenticate the 1905host was already trusted or explicitly accepted by the user, the host was 1906authenticated via 1907.Cm UserKnownHostsFile 1908(i.e. not 1909.Cm GlobalKnownHostsFile ) 1910and the host was authenticated using a plain key and not a certificate. 1911.Pp 1912.Cm UpdateHostKeys 1913is enabled by default if the user has not overridden the default 1914.Cm UserKnownHostsFile 1915setting and has not enabled 1916.Cm VerifyHostKeyDNS , 1917otherwise 1918.Cm UpdateHostKeys 1919will be set to 1920.Cm no . 1921.Pp 1922If 1923.Cm UpdateHostKeys 1924is set to 1925.Cm ask , 1926then the user is asked to confirm the modifications to the known_hosts file. 1927Confirmation is currently incompatible with 1928.Cm ControlPersist , 1929and will be disabled if it is enabled. 1930.Pp 1931Presently, only 1932.Xr sshd 8 1933from OpenSSH 6.8 and greater support the 1934.Qq hostkeys@openssh.com 1935protocol extension used to inform the client of all the server's hostkeys. 1936.It Cm User 1937Specifies the user to log in as. 1938This can be useful when a different user name is used on different machines. 1939This saves the trouble of 1940having to remember to give the user name on the command line. 1941.It Cm UserKnownHostsFile 1942Specifies one or more files to use for the user 1943host key database, separated by whitespace. 1944Each filename may use tilde notation to refer to the user's home directory, 1945the tokens described in the 1946.Sx TOKENS 1947section and environment variables as described in the 1948.Sx ENVIRONMENT VARIABLES 1949section. 1950The default is 1951.Pa ~/.ssh/known_hosts , 1952.Pa ~/.ssh/known_hosts2 . 1953.It Cm VerifyHostKeyDNS 1954Specifies whether to verify the remote key using DNS and SSHFP resource 1955records. 1956If this option is set to 1957.Cm yes , 1958the client will implicitly trust keys that match a secure fingerprint 1959from DNS. 1960Insecure fingerprints will be handled as if this option was set to 1961.Cm ask . 1962If this option is set to 1963.Cm ask , 1964information on fingerprint match will be displayed, but the user will still 1965need to confirm new host keys according to the 1966.Cm StrictHostKeyChecking 1967option. 1968The default is 1969.Cm yes 1970if compiled with LDNS and 1971.Cm no 1972otherwise. 1973.Pp 1974See also 1975.Sx VERIFYING HOST KEYS 1976in 1977.Xr ssh 1 . 1978.It Cm VersionAddendum 1979Specifies a string to append to the regular version string to identify 1980OS- or site-specific modifications. 1981The default is 1982.Dq FreeBSD-20220413 . 1983The value 1984.Cm none 1985may be used to disable this. 1986.It Cm VisualHostKey 1987If this flag is set to 1988.Cm yes , 1989an ASCII art representation of the remote host key fingerprint is 1990printed in addition to the fingerprint string at login and 1991for unknown host keys. 1992If this flag is set to 1993.Cm no 1994(the default), 1995no fingerprint strings are printed at login and 1996only the fingerprint string will be printed for unknown host keys. 1997.It Cm XAuthLocation 1998Specifies the full pathname of the 1999.Xr xauth 1 2000program. 2001The default is 2002.Pa /usr/local/bin/xauth . 2003.El 2004.Sh PATTERNS 2005A 2006.Em pattern 2007consists of zero or more non-whitespace characters, 2008.Sq * 2009(a wildcard that matches zero or more characters), 2010or 2011.Sq ?\& 2012(a wildcard that matches exactly one character). 2013For example, to specify a set of declarations for any host in the 2014.Qq .co.uk 2015set of domains, 2016the following pattern could be used: 2017.Pp 2018.Dl Host *.co.uk 2019.Pp 2020The following pattern 2021would match any host in the 192.168.0.[0-9] network range: 2022.Pp 2023.Dl Host 192.168.0.? 2024.Pp 2025A 2026.Em pattern-list 2027is a comma-separated list of patterns. 2028Patterns within pattern-lists may be negated 2029by preceding them with an exclamation mark 2030.Pq Sq !\& . 2031For example, 2032to allow a key to be used from anywhere within an organization 2033except from the 2034.Qq dialup 2035pool, 2036the following entry (in authorized_keys) could be used: 2037.Pp 2038.Dl from=\&"!*.dialup.example.com,*.example.com\&" 2039.Pp 2040Note that a negated match will never produce a positive result by itself. 2041For example, attempting to match 2042.Qq host3 2043against the following pattern-list will fail: 2044.Pp 2045.Dl from=\&"!host1,!host2\&" 2046.Pp 2047The solution here is to include a term that will yield a positive match, 2048such as a wildcard: 2049.Pp 2050.Dl from=\&"!host1,!host2,*\&" 2051.Sh TOKENS 2052Arguments to some keywords can make use of tokens, 2053which are expanded at runtime: 2054.Pp 2055.Bl -tag -width XXXX -offset indent -compact 2056.It %% 2057A literal 2058.Sq % . 2059.It \&%C 2060Hash of %l%h%p%r. 2061.It %d 2062Local user's home directory. 2063.It %f 2064The fingerprint of the server's host key. 2065.It %H 2066The 2067.Pa known_hosts 2068hostname or address that is being searched for. 2069.It %h 2070The remote hostname. 2071.It \%%I 2072A string describing the reason for a 2073.Cm KnownHostsCommand 2074execution: either 2075.Cm ADDRESS 2076when looking up a host by address (only when 2077.Cm CheckHostIP 2078is enabled), 2079.Cm HOSTNAME 2080when searching by hostname, or 2081.Cm ORDER 2082when preparing the host key algorithm preference list to use for the 2083destination host. 2084.It %i 2085The local user ID. 2086.It %K 2087The base64 encoded host key. 2088.It %k 2089The host key alias if specified, otherwise the original remote hostname given 2090on the command line. 2091.It %L 2092The local hostname. 2093.It %l 2094The local hostname, including the domain name. 2095.It %n 2096The original remote hostname, as given on the command line. 2097.It %p 2098The remote port. 2099.It %r 2100The remote username. 2101.It \&%T 2102The local 2103.Xr tun 4 2104or 2105.Xr tap 4 2106network interface assigned if 2107tunnel forwarding was requested, or 2108.Qq NONE 2109otherwise. 2110.It %t 2111The type of the server host key, e.g. 2112.Cm ssh-ed25519 . 2113.It %u 2114The local username. 2115.El 2116.Pp 2117.Cm CertificateFile , 2118.Cm ControlPath , 2119.Cm IdentityAgent , 2120.Cm IdentityFile , 2121.Cm KnownHostsCommand , 2122.Cm LocalForward , 2123.Cm Match exec , 2124.Cm RemoteCommand , 2125.Cm RemoteForward , 2126and 2127.Cm UserKnownHostsFile 2128accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u. 2129.Pp 2130.Cm KnownHostsCommand 2131additionally accepts the tokens %f, %H, %I, %K and %t. 2132.Pp 2133.Cm Hostname 2134accepts the tokens %% and %h. 2135.Pp 2136.Cm LocalCommand 2137accepts all tokens. 2138.Pp 2139.Cm ProxyCommand 2140accepts the tokens %%, %h, %n, %p, and %r. 2141.Sh ENVIRONMENT VARIABLES 2142Arguments to some keywords can be expanded at runtime from environment 2143variables on the client by enclosing them in 2144.Ic ${} , 2145for example 2146.Ic ${HOME}/.ssh 2147would refer to the user's .ssh directory. 2148If a specified environment variable does not exist then an error will be 2149returned and the setting for that keyword will be ignored. 2150.Pp 2151The keywords 2152.Cm CertificateFile , 2153.Cm ControlPath , 2154.Cm IdentityAgent , 2155.Cm IdentityFile , 2156.Cm KnownHostsCommand , 2157and 2158.Cm UserKnownHostsFile 2159support environment variables. 2160The keywords 2161.Cm LocalForward 2162and 2163.Cm RemoteForward 2164support environment variables only for Unix domain socket paths. 2165.Sh FILES 2166.Bl -tag -width Ds 2167.It Pa ~/.ssh/config 2168This is the per-user configuration file. 2169The format of this file is described above. 2170This file is used by the SSH client. 2171Because of the potential for abuse, this file must have strict permissions: 2172read/write for the user, and not writable by others. 2173.It Pa /etc/ssh/ssh_config 2174Systemwide configuration file. 2175This file provides defaults for those 2176values that are not specified in the user's configuration file, and 2177for those users who do not have a configuration file. 2178This file must be world-readable. 2179.El 2180.Sh SEE ALSO 2181.Xr ssh 1 2182.Sh AUTHORS 2183.An -nosplit 2184OpenSSH is a derivative of the original and free 2185ssh 1.2.12 release by 2186.An Tatu Ylonen . 2187.An Aaron Campbell , Bob Beck , Markus Friedl , 2188.An Niels Provos , Theo de Raadt 2189and 2190.An Dug Song 2191removed many bugs, re-added newer features and 2192created OpenSSH. 2193.An Markus Friedl 2194contributed the support for SSH protocol versions 1.5 and 2.0. 2195