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 1.228 2016/02/20 23:01:46 sobrado Exp $ 37.\" $FreeBSD$ 38.Dd $Mdocdate: February 20 2016 $ 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH SSH client configuration files 44.Sh SYNOPSIS 45.Nm ~/.ssh/config 46.Nm /etc/ssh/ssh_config 47.Sh DESCRIPTION 48.Xr ssh 1 49obtains configuration data from the following sources in 50the following order: 51.Pp 52.Bl -enum -offset indent -compact 53.It 54command-line options 55.It 56user's configuration file 57.Pq Pa ~/.ssh/config 58.It 59system-wide configuration file 60.Pq Pa /etc/ssh/ssh_config 61.El 62.Pp 63For each parameter, the first obtained value 64will be used. 65The configuration files contain sections separated by 66.Dq Host 67specifications, and that section is only applied for hosts that 68match one of the patterns given in the specification. 69The matched host name is usually the one given on the command line 70(see the 71.Cm CanonicalizeHostname 72option for exceptions.) 73.Pp 74Since the first obtained value for each parameter is used, more 75host-specific declarations should be given near the beginning of the 76file, and general defaults at the end. 77.Pp 78The configuration file has the following format: 79.Pp 80Empty lines and lines starting with 81.Ql # 82are comments. 83Otherwise a line is of the format 84.Dq keyword arguments . 85Configuration options may be separated by whitespace or 86optional whitespace and exactly one 87.Ql = ; 88the latter format is useful to avoid the need to quote whitespace 89when specifying configuration options using the 90.Nm ssh , 91.Nm scp , 92and 93.Nm sftp 94.Fl o 95option. 96Arguments may optionally be enclosed in double quotes 97.Pq \&" 98in order to represent arguments containing spaces. 99.Pp 100The possible 101keywords and their meanings are as follows (note that 102keywords are case-insensitive and arguments are case-sensitive): 103.Bl -tag -width Ds 104.It Cm Host 105Restricts the following declarations (up to the next 106.Cm Host 107or 108.Cm Match 109keyword) to be only for those hosts that match one of the patterns 110given after the keyword. 111If more than one pattern is provided, they should be separated by whitespace. 112A single 113.Ql * 114as a pattern can be used to provide global 115defaults for all hosts. 116The host is usually the 117.Ar hostname 118argument given on the command line 119(see the 120.Cm CanonicalizeHostname 121option for exceptions.) 122.Pp 123A pattern entry may be negated by prefixing it with an exclamation mark 124.Pq Sq !\& . 125If a negated entry is matched, then the 126.Cm Host 127entry is ignored, regardless of whether any other patterns on the line 128match. 129Negated matches are therefore useful to provide exceptions for wildcard 130matches. 131.Pp 132See 133.Sx PATTERNS 134for more information on patterns. 135.It Cm Match 136Restricts the following declarations (up to the next 137.Cm Host 138or 139.Cm Match 140keyword) to be used only when the conditions following the 141.Cm Match 142keyword are satisfied. 143Match conditions are specified using one or more criteria 144or the single token 145.Cm all 146which always matches. 147The available criteria keywords are: 148.Cm canonical , 149.Cm exec , 150.Cm host , 151.Cm originalhost , 152.Cm user , 153and 154.Cm localuser . 155The 156.Cm all 157criteria must appear alone or immediately after 158.Cm canonical . 159Other criteria may be combined arbitrarily. 160All criteria but 161.Cm all 162and 163.Cm canonical 164require an argument. 165Criteria may be negated by prepending an exclamation mark 166.Pq Sq !\& . 167.Pp 168The 169.Cm canonical 170keyword matches only when the configuration file is being re-parsed 171after hostname canonicalization (see the 172.Cm CanonicalizeHostname 173option.) 174This may be useful to specify conditions that work with canonical host 175names only. 176The 177.Cm exec 178keyword executes the specified command under the user's shell. 179If the command returns a zero exit status then the condition is considered true. 180Commands containing whitespace characters must be quoted. 181The following character sequences in the command will be expanded prior to 182execution: 183.Ql %L 184will be substituted by the first component of the local host name, 185.Ql %l 186will be substituted by the local host name (including any domain name), 187.Ql %h 188will be substituted by the target host name, 189.Ql %n 190will be substituted by the original target host name 191specified on the command-line, 192.Ql %p 193the destination port, 194.Ql %r 195by the remote login username, and 196.Ql %u 197by the username of the user running 198.Xr ssh 1 . 199.Pp 200The other keywords' criteria must be single entries or comma-separated 201lists and may use the wildcard and negation operators described in the 202.Sx PATTERNS 203section. 204The criteria for the 205.Cm host 206keyword are matched against the target hostname, after any substitution 207by the 208.Cm Hostname 209or 210.Cm CanonicalizeHostname 211options. 212The 213.Cm originalhost 214keyword matches against the hostname as it was specified on the command-line. 215The 216.Cm user 217keyword matches against the target username on the remote host. 218The 219.Cm localuser 220keyword matches against the name of the local user running 221.Xr ssh 1 222(this keyword may be useful in system-wide 223.Nm 224files). 225.It Cm AddKeysToAgent 226Specifies whether keys should be automatically added to a running 227.Xr ssh-agent 1 . 228If this option is set to 229.Dq yes 230and a key is loaded from a file, the key and its passphrase are added to 231the agent with the default lifetime, as if by 232.Xr ssh-add 1 . 233If this option is set to 234.Dq ask , 235.Nm ssh 236will require confirmation using the 237.Ev SSH_ASKPASS 238program before adding a key (see 239.Xr ssh-add 1 240for details). 241If this option is set to 242.Dq confirm , 243each use of the key must be confirmed, as if the 244.Fl c 245option was specified to 246.Xr ssh-add 1 . 247If this option is set to 248.Dq no , 249no keys are added to the agent. 250The argument must be 251.Dq yes , 252.Dq confirm , 253.Dq ask , 254or 255.Dq no . 256The default is 257.Dq no . 258.It Cm AddressFamily 259Specifies which address family to use when connecting. 260Valid arguments are 261.Dq any , 262.Dq inet 263(use IPv4 only), or 264.Dq inet6 265(use IPv6 only). 266The default is 267.Dq any . 268.It Cm BatchMode 269If set to 270.Dq yes , 271passphrase/password querying will be disabled. 272This option is useful in scripts and other batch jobs where no user 273is present to supply the password. 274The argument must be 275.Dq yes 276or 277.Dq no . 278The default is 279.Dq no . 280.It Cm BindAddress 281Use the specified address on the local machine as the source address of 282the connection. 283Only useful on systems with more than one address. 284Note that this option does not work if 285.Cm UsePrivilegedPort 286is set to 287.Dq yes . 288.It Cm CanonicalDomains 289When 290.Cm CanonicalizeHostname 291is enabled, this option specifies the list of domain suffixes in which to 292search for the specified destination host. 293.It Cm CanonicalizeFallbackLocal 294Specifies whether to fail with an error when hostname canonicalization fails. 295The default, 296.Dq yes , 297will attempt to look up the unqualified hostname using the system resolver's 298search rules. 299A value of 300.Dq no 301will cause 302.Xr ssh 1 303to fail instantly if 304.Cm CanonicalizeHostname 305is enabled and the target hostname cannot be found in any of the domains 306specified by 307.Cm CanonicalDomains . 308.It Cm CanonicalizeHostname 309Controls whether explicit hostname canonicalization is performed. 310The default, 311.Dq no , 312is not to perform any name rewriting and let the system resolver handle all 313hostname lookups. 314If set to 315.Dq yes 316then, for connections that do not use a 317.Cm ProxyCommand , 318.Xr ssh 1 319will attempt to canonicalize the hostname specified on the command line 320using the 321.Cm CanonicalDomains 322suffixes and 323.Cm CanonicalizePermittedCNAMEs 324rules. 325If 326.Cm CanonicalizeHostname 327is set to 328.Dq always , 329then canonicalization is applied to proxied connections too. 330.Pp 331If this option is enabled, then the configuration files are processed 332again using the new target name to pick up any new configuration in matching 333.Cm Host 334and 335.Cm Match 336stanzas. 337.It Cm CanonicalizeMaxDots 338Specifies the maximum number of dot characters in a hostname before 339canonicalization is disabled. 340The default, 341.Dq 1 , 342allows a single dot (i.e. hostname.subdomain). 343.It Cm CanonicalizePermittedCNAMEs 344Specifies rules to determine whether CNAMEs should be followed when 345canonicalizing hostnames. 346The rules consist of one or more arguments of 347.Ar source_domain_list : Ns Ar target_domain_list , 348where 349.Ar source_domain_list 350is a pattern-list of domains that may follow CNAMEs in canonicalization, 351and 352.Ar target_domain_list 353is a pattern-list of domains that they may resolve to. 354.Pp 355For example, 356.Dq *.a.example.com:*.b.example.com,*.c.example.com 357will allow hostnames matching 358.Dq *.a.example.com 359to be canonicalized to names in the 360.Dq *.b.example.com 361or 362.Dq *.c.example.com 363domains. 364.It Cm CertificateFile 365Specifies a file from which the user's certificate is read. 366A corresponding private key must be provided separately in order 367to use this certificate either 368from an 369.Cm IdentityFile 370directive or 371.Fl i 372flag to 373.Xr ssh 1 , 374via 375.Xr ssh-agent 1 , 376or via a 377.Cm PKCS11Provider . 378.Pp 379The file name may use the tilde 380syntax to refer to a user's home directory or one of the following 381escape characters: 382.Ql %d 383(local user's home directory), 384.Ql %u 385(local user name), 386.Ql %l 387(local host name), 388.Ql %h 389(remote host name) or 390.Ql %r 391(remote user name). 392.Pp 393It is possible to have multiple certificate files specified in 394configuration files; these certificates will be tried in sequence. 395Multiple 396.Cm CertificateFile 397directives will add to the list of certificates used for 398authentication. 399.It Cm ChallengeResponseAuthentication 400Specifies whether to use challenge-response authentication. 401The argument to this keyword must be 402.Dq yes 403or 404.Dq no . 405The default is 406.Dq yes . 407.It Cm CheckHostIP 408If this flag is set to 409.Dq yes , 410.Xr ssh 1 411will additionally check the host IP address in the 412.Pa known_hosts 413file. 414This allows ssh to detect if a host key changed due to DNS spoofing 415and will add addresses of destination hosts to 416.Pa ~/.ssh/known_hosts 417in the process, regardless of the setting of 418.Cm StrictHostKeyChecking . 419If the option is set to 420.Dq no , 421the check will not be executed. 422The default is 423.Dq no . 424.It Cm Cipher 425Specifies the cipher to use for encrypting the session 426in protocol version 1. 427Currently, 428.Dq blowfish , 429.Dq 3des , 430and 431.Dq des 432are supported. 433.Ar des 434is only supported in the 435.Xr ssh 1 436client for interoperability with legacy protocol 1 implementations 437that do not support the 438.Ar 3des 439cipher. 440Its use is strongly discouraged due to cryptographic weaknesses. 441The default is 442.Dq 3des . 443.It Cm Ciphers 444Specifies the ciphers allowed for protocol version 2 445in order of preference. 446Multiple ciphers must be comma-separated. 447If the specified value begins with a 448.Sq + 449character, then the specified ciphers will be appended to the default set 450instead of replacing them. 451.Pp 452The supported ciphers are: 453.Pp 454.Bl -item -compact -offset indent 455.It 4563des-cbc 457.It 458aes128-cbc 459.It 460aes192-cbc 461.It 462aes256-cbc 463.It 464aes128-ctr 465.It 466aes192-ctr 467.It 468aes256-ctr 469.It 470aes128-gcm@openssh.com 471.It 472aes256-gcm@openssh.com 473.It 474arcfour 475.It 476arcfour128 477.It 478arcfour256 479.It 480blowfish-cbc 481.It 482cast128-cbc 483.It 484chacha20-poly1305@openssh.com 485.El 486.Pp 487The default is: 488.Bd -literal -offset indent 489chacha20-poly1305@openssh.com, 490aes128-ctr,aes192-ctr,aes256-ctr, 491aes128-gcm@openssh.com,aes256-gcm@openssh.com, 492aes128-cbc,aes192-cbc,aes256-cbc,3des-cbc 493.Ed 494.Pp 495The list of available ciphers may also be obtained using the 496.Fl Q 497option of 498.Xr ssh 1 499with an argument of 500.Dq cipher . 501.It Cm ClearAllForwardings 502Specifies that all local, remote, and dynamic port forwardings 503specified in the configuration files or on the command line be 504cleared. 505This option is primarily useful when used from the 506.Xr ssh 1 507command line to clear port forwardings set in 508configuration files, and is automatically set by 509.Xr scp 1 510and 511.Xr sftp 1 . 512The argument must be 513.Dq yes 514or 515.Dq no . 516The default is 517.Dq no . 518.It Cm Compression 519Specifies whether to use compression. 520The argument must be 521.Dq yes 522or 523.Dq no . 524The default is 525.Dq no . 526.It Cm CompressionLevel 527Specifies the compression level to use if compression is enabled. 528The argument must be an integer from 1 (fast) to 9 (slow, best). 529The default level is 6, which is good for most applications. 530The meaning of the values is the same as in 531.Xr gzip 1 . 532Note that this option applies to protocol version 1 only. 533.It Cm ConnectionAttempts 534Specifies the number of tries (one per second) to make before exiting. 535The argument must be an integer. 536This may be useful in scripts if the connection sometimes fails. 537The default is 1. 538.It Cm ConnectTimeout 539Specifies the timeout (in seconds) used when connecting to the 540SSH server, instead of using the default system TCP timeout. 541This value is used only when the target is down or really unreachable, 542not when it refuses the connection. 543.It Cm ControlMaster 544Enables the sharing of multiple sessions over a single network connection. 545When set to 546.Dq yes , 547.Xr ssh 1 548will listen for connections on a control socket specified using the 549.Cm ControlPath 550argument. 551Additional sessions can connect to this socket using the same 552.Cm ControlPath 553with 554.Cm ControlMaster 555set to 556.Dq no 557(the default). 558These sessions will try to reuse the master instance's network connection 559rather than initiating new ones, but will fall back to connecting normally 560if the control socket does not exist, or is not listening. 561.Pp 562Setting this to 563.Dq ask 564will cause ssh 565to listen for control connections, but require confirmation using 566.Xr ssh-askpass 1 . 567If the 568.Cm ControlPath 569cannot be opened, 570ssh will continue without connecting to a master instance. 571.Pp 572X11 and 573.Xr ssh-agent 1 574forwarding is supported over these multiplexed connections, however the 575display and agent forwarded will be the one belonging to the master 576connection i.e. it is not possible to forward multiple displays or agents. 577.Pp 578Two additional options allow for opportunistic multiplexing: try to use a 579master connection but fall back to creating a new one if one does not already 580exist. 581These options are: 582.Dq auto 583and 584.Dq autoask . 585The latter requires confirmation like the 586.Dq ask 587option. 588.It Cm ControlPath 589Specify the path to the control socket used for connection sharing as described 590in the 591.Cm ControlMaster 592section above or the string 593.Dq none 594to disable connection sharing. 595In the path, 596.Ql %L 597will be substituted by the first component of the local host name, 598.Ql %l 599will be substituted by the local host name (including any domain name), 600.Ql %h 601will be substituted by the target host name, 602.Ql %n 603will be substituted by the original target host name 604specified on the command line, 605.Ql %p 606the destination port, 607.Ql %r 608by the remote login username, 609.Ql %u 610by the username and 611.Ql %i 612by the numeric user ID (uid) of the user running 613.Xr ssh 1 , 614and 615.Ql \&%C 616by a hash of the concatenation: %l%h%p%r. 617It is recommended that any 618.Cm ControlPath 619used for opportunistic connection sharing include 620at least %h, %p, and %r (or alternatively %C) and be placed in a directory 621that is not writable by other users. 622This ensures that shared connections are uniquely identified. 623.It Cm ControlPersist 624When used in conjunction with 625.Cm ControlMaster , 626specifies that the master connection should remain open 627in the background (waiting for future client connections) 628after the initial client connection has been closed. 629If set to 630.Dq no , 631then the master connection will not be placed into the background, 632and will close as soon as the initial client connection is closed. 633If set to 634.Dq yes 635or 636.Dq 0 , 637then the master connection will remain in the background indefinitely 638(until killed or closed via a mechanism such as the 639.Xr ssh 1 640.Dq Fl O No exit 641option). 642If set to a time in seconds, or a time in any of the formats documented in 643.Xr sshd_config 5 , 644then the backgrounded master connection will automatically terminate 645after it has remained idle (with no client connections) for the 646specified time. 647.It Cm DynamicForward 648Specifies that a TCP port on the local machine be forwarded 649over the secure channel, and the application 650protocol is then used to determine where to connect to from the 651remote machine. 652.Pp 653The argument must be 654.Sm off 655.Oo Ar bind_address : Oc Ar port . 656.Sm on 657IPv6 addresses can be specified by enclosing addresses in square brackets. 658By default, the local port is bound in accordance with the 659.Cm GatewayPorts 660setting. 661However, an explicit 662.Ar bind_address 663may be used to bind the connection to a specific address. 664The 665.Ar bind_address 666of 667.Dq localhost 668indicates that the listening port be bound for local use only, while an 669empty address or 670.Sq * 671indicates that the port should be available from all interfaces. 672.Pp 673Currently the SOCKS4 and SOCKS5 protocols are supported, and 674.Xr ssh 1 675will act as a SOCKS server. 676Multiple forwardings may be specified, and 677additional forwardings can be given on the command line. 678Only the superuser can forward privileged ports. 679.It Cm EnableSSHKeysign 680Setting this option to 681.Dq yes 682in the global client configuration file 683.Pa /etc/ssh/ssh_config 684enables the use of the helper program 685.Xr ssh-keysign 8 686during 687.Cm HostbasedAuthentication . 688The argument must be 689.Dq yes 690or 691.Dq no . 692The default is 693.Dq no . 694This option should be placed in the non-hostspecific section. 695See 696.Xr ssh-keysign 8 697for more information. 698.It Cm EscapeChar 699Sets the escape character (default: 700.Ql ~ ) . 701The escape character can also 702be set on the command line. 703The argument should be a single character, 704.Ql ^ 705followed by a letter, or 706.Dq none 707to disable the escape 708character entirely (making the connection transparent for binary 709data). 710.It Cm ExitOnForwardFailure 711Specifies whether 712.Xr ssh 1 713should terminate the connection if it cannot set up all requested 714dynamic, tunnel, local, and remote port forwardings, (e.g.\& 715if either end is unable to bind and listen on a specified port). 716Note that 717.Cm ExitOnForwardFailure 718does not apply to connections made over port forwardings and will not, 719for example, cause 720.Xr ssh 1 721to exit if TCP connections to the ultimate forwarding destination fail. 722The argument must be 723.Dq yes 724or 725.Dq no . 726The default is 727.Dq no . 728.It Cm FingerprintHash 729Specifies the hash algorithm used when displaying key fingerprints. 730Valid options are: 731.Dq md5 732and 733.Dq sha256 . 734The default is 735.Dq sha256 . 736.It Cm ForwardAgent 737Specifies whether the connection to the authentication agent (if any) 738will be forwarded to the remote machine. 739The argument must be 740.Dq yes 741or 742.Dq no . 743The default is 744.Dq no . 745.Pp 746Agent forwarding should be enabled with caution. 747Users with the ability to bypass file permissions on the remote host 748(for the agent's Unix-domain socket) 749can access the local agent through the forwarded connection. 750An attacker cannot obtain key material from the agent, 751however they can perform operations on the keys that enable them to 752authenticate using the identities loaded into the agent. 753.It Cm ForwardX11 754Specifies whether X11 connections will be automatically redirected 755over the secure channel and 756.Ev DISPLAY 757set. 758The argument must be 759.Dq yes 760or 761.Dq no . 762The default is 763.Dq no . 764.Pp 765X11 forwarding should be enabled with caution. 766Users with the ability to bypass file permissions on the remote host 767(for the user's X11 authorization database) 768can access the local X11 display through the forwarded connection. 769An attacker may then be able to perform activities such as keystroke monitoring 770if the 771.Cm ForwardX11Trusted 772option is also enabled. 773.It Cm ForwardX11Timeout 774Specify a timeout for untrusted X11 forwarding 775using the format described in the 776TIME FORMATS section of 777.Xr sshd_config 5 . 778X11 connections received by 779.Xr ssh 1 780after this time will be refused. 781The default is to disable untrusted X11 forwarding after twenty minutes has 782elapsed. 783.It Cm ForwardX11Trusted 784If this option is set to 785.Dq yes , 786remote X11 clients will have full access to the original X11 display. 787.Pp 788If this option is set to 789.Dq no , 790remote X11 clients will be considered untrusted and prevented 791from stealing or tampering with data belonging to trusted X11 792clients. 793Furthermore, the 794.Xr xauth 1 795token used for the session will be set to expire after 20 minutes. 796Remote clients will be refused access after this time. 797.Pp 798The default is 799.Dq no . 800.Pp 801See the X11 SECURITY extension specification for full details on 802the restrictions imposed on untrusted clients. 803.It Cm GatewayPorts 804Specifies whether remote hosts are allowed to connect to local 805forwarded ports. 806By default, 807.Xr ssh 1 808binds local port forwardings to the loopback address. 809This prevents other remote hosts from connecting to forwarded ports. 810.Cm GatewayPorts 811can be used to specify that ssh 812should bind local port forwardings to the wildcard address, 813thus allowing remote hosts to connect to forwarded ports. 814The argument must be 815.Dq yes 816or 817.Dq no . 818The default is 819.Dq no . 820.It Cm GlobalKnownHostsFile 821Specifies one or more files to use for the global 822host key database, separated by whitespace. 823The default is 824.Pa /etc/ssh/ssh_known_hosts , 825.Pa /etc/ssh/ssh_known_hosts2 . 826.It Cm GSSAPIAuthentication 827Specifies whether user authentication based on GSSAPI is allowed. 828The default is 829.Dq no . 830.It Cm GSSAPIDelegateCredentials 831Forward (delegate) credentials to the server. 832The default is 833.Dq no . 834.It Cm HashKnownHosts 835Indicates that 836.Xr ssh 1 837should hash host names and addresses when they are added to 838.Pa ~/.ssh/known_hosts . 839These hashed names may be used normally by 840.Xr ssh 1 841and 842.Xr sshd 8 , 843but they do not reveal identifying information should the file's contents 844be disclosed. 845The default is 846.Dq no . 847Note that existing names and addresses in known hosts files 848will not be converted automatically, 849but may be manually hashed using 850.Xr ssh-keygen 1 . 851.It Cm HostbasedAuthentication 852Specifies whether to try rhosts based authentication with public key 853authentication. 854The argument must be 855.Dq yes 856or 857.Dq no . 858The default is 859.Dq no . 860.It Cm HostbasedKeyTypes 861Specifies the key types that will be used for hostbased authentication 862as a comma-separated pattern list. 863Alternately if the specified value begins with a 864.Sq + 865character, then the specified key types will be appended to the default set 866instead of replacing them. 867The default for this option is: 868.Bd -literal -offset 3n 869ecdsa-sha2-nistp256-cert-v01@openssh.com, 870ecdsa-sha2-nistp384-cert-v01@openssh.com, 871ecdsa-sha2-nistp521-cert-v01@openssh.com, 872ssh-ed25519-cert-v01@openssh.com, 873ssh-rsa-cert-v01@openssh.com, 874ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 875ssh-ed25519,ssh-rsa 876.Ed 877.Pp 878The 879.Fl Q 880option of 881.Xr ssh 1 882may be used to list supported key types. 883.It Cm HostKeyAlgorithms 884Specifies the host key algorithms 885that the client wants to use in order of preference. 886Alternately if the specified value begins with a 887.Sq + 888character, then the specified key types will be appended to the default set 889instead of replacing them. 890The default for this option is: 891.Bd -literal -offset 3n 892ecdsa-sha2-nistp256-cert-v01@openssh.com, 893ecdsa-sha2-nistp384-cert-v01@openssh.com, 894ecdsa-sha2-nistp521-cert-v01@openssh.com, 895ssh-ed25519-cert-v01@openssh.com, 896ssh-rsa-cert-v01@openssh.com, 897ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 898ssh-ed25519,ssh-rsa 899.Ed 900.Pp 901If hostkeys are known for the destination host then this default is modified 902to prefer their algorithms. 903.Pp 904The list of available key types may also be obtained using the 905.Fl Q 906option of 907.Xr ssh 1 908with an argument of 909.Dq key . 910.It Cm HostKeyAlias 911Specifies an alias that should be used instead of the 912real host name when looking up or saving the host key 913in the host key database files. 914This option is useful for tunneling SSH connections 915or for multiple servers running on a single host. 916.It Cm HostName 917Specifies the real host name to log into. 918This can be used to specify nicknames or abbreviations for hosts. 919If the hostname contains the character sequence 920.Ql %h , 921then this will be replaced with the host name specified on the command line 922(this is useful for manipulating unqualified names). 923The character sequence 924.Ql %% 925will be replaced by a single 926.Ql % 927character, which may be used when specifying IPv6 link-local addresses. 928.Pp 929The default is the name given on the command line. 930Numeric IP addresses are also permitted (both on the command line and in 931.Cm HostName 932specifications). 933.It Cm IdentitiesOnly 934Specifies that 935.Xr ssh 1 936should only use the authentication identity and certificate files explicitly 937configured in the 938.Nm 939files 940or passed on the 941.Xr ssh 1 942command-line, 943even if 944.Xr ssh-agent 1 945or a 946.Cm PKCS11Provider 947offers more identities. 948The argument to this keyword must be 949.Dq yes 950or 951.Dq no . 952This option is intended for situations where ssh-agent 953offers many different identities. 954The default is 955.Dq no . 956.It Cm IdentityFile 957Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication 958identity is read. 959The default is 960.Pa ~/.ssh/identity 961for protocol version 1, and 962.Pa ~/.ssh/id_dsa , 963.Pa ~/.ssh/id_ecdsa , 964.Pa ~/.ssh/id_ed25519 965and 966.Pa ~/.ssh/id_rsa 967for protocol version 2. 968Additionally, any identities represented by the authentication agent 969will be used for authentication unless 970.Cm IdentitiesOnly 971is set. 972If no certificates have been explicitly specified by 973.Cm CertificateFile , 974.Xr ssh 1 975will try to load certificate information from the filename obtained by 976appending 977.Pa -cert.pub 978to the path of a specified 979.Cm IdentityFile . 980.Pp 981The file name may use the tilde 982syntax to refer to a user's home directory or one of the following 983escape characters: 984.Ql %d 985(local user's home directory), 986.Ql %u 987(local user name), 988.Ql %l 989(local host name), 990.Ql %h 991(remote host name) or 992.Ql %r 993(remote user name). 994.Pp 995It is possible to have 996multiple identity files specified in configuration files; all these 997identities will be tried in sequence. 998Multiple 999.Cm IdentityFile 1000directives will add to the list of identities tried (this behaviour 1001differs from that of other configuration directives). 1002.Pp 1003.Cm IdentityFile 1004may be used in conjunction with 1005.Cm IdentitiesOnly 1006to select which identities in an agent are offered during authentication. 1007.Cm IdentityFile 1008may also be used in conjunction with 1009.Cm CertificateFile 1010in order to provide any certificate also needed for authentication with 1011the identity. 1012.It Cm IgnoreUnknown 1013Specifies a pattern-list of unknown options to be ignored if they are 1014encountered in configuration parsing. 1015This may be used to suppress errors if 1016.Nm 1017contains options that are unrecognised by 1018.Xr ssh 1 . 1019It is recommended that 1020.Cm IgnoreUnknown 1021be listed early in the configuration file as it will not be applied 1022to unknown options that appear before it. 1023.It Cm IPQoS 1024Specifies the IPv4 type-of-service or DSCP class for connections. 1025Accepted values are 1026.Dq af11 , 1027.Dq af12 , 1028.Dq af13 , 1029.Dq af21 , 1030.Dq af22 , 1031.Dq af23 , 1032.Dq af31 , 1033.Dq af32 , 1034.Dq af33 , 1035.Dq af41 , 1036.Dq af42 , 1037.Dq af43 , 1038.Dq cs0 , 1039.Dq cs1 , 1040.Dq cs2 , 1041.Dq cs3 , 1042.Dq cs4 , 1043.Dq cs5 , 1044.Dq cs6 , 1045.Dq cs7 , 1046.Dq ef , 1047.Dq lowdelay , 1048.Dq throughput , 1049.Dq reliability , 1050or a numeric value. 1051This option may take one or two arguments, separated by whitespace. 1052If one argument is specified, it is used as the packet class unconditionally. 1053If two values are specified, the first is automatically selected for 1054interactive sessions and the second for non-interactive sessions. 1055The default is 1056.Dq lowdelay 1057for interactive sessions and 1058.Dq throughput 1059for non-interactive sessions. 1060.It Cm KbdInteractiveAuthentication 1061Specifies whether to use keyboard-interactive authentication. 1062The argument to this keyword must be 1063.Dq yes 1064or 1065.Dq no . 1066The default is 1067.Dq yes . 1068.It Cm KbdInteractiveDevices 1069Specifies the list of methods to use in keyboard-interactive authentication. 1070Multiple method names must be comma-separated. 1071The default is to use the server specified list. 1072The methods available vary depending on what the server supports. 1073For an OpenSSH server, 1074it may be zero or more of: 1075.Dq bsdauth , 1076.Dq pam , 1077and 1078.Dq skey . 1079.It Cm KexAlgorithms 1080Specifies the available KEX (Key Exchange) algorithms. 1081Multiple algorithms must be comma-separated. 1082Alternately if the specified value begins with a 1083.Sq + 1084character, then the specified methods will be appended to the default set 1085instead of replacing them. 1086The default is: 1087.Bd -literal -offset indent 1088curve25519-sha256@libssh.org, 1089ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1090diffie-hellman-group-exchange-sha256, 1091diffie-hellman-group-exchange-sha1, 1092diffie-hellman-group14-sha1 1093.Ed 1094.Pp 1095The list of available key exchange algorithms may also be obtained using the 1096.Fl Q 1097option of 1098.Xr ssh 1 1099with an argument of 1100.Dq kex . 1101.It Cm LocalCommand 1102Specifies a command to execute on the local machine after successfully 1103connecting to the server. 1104The command string extends to the end of the line, and is executed with 1105the user's shell. 1106The following escape character substitutions will be performed: 1107.Ql %d 1108(local user's home directory), 1109.Ql %h 1110(remote host name), 1111.Ql %l 1112(local host name), 1113.Ql %n 1114(host name as provided on the command line), 1115.Ql %p 1116(remote port), 1117.Ql %r 1118(remote user name) or 1119.Ql %u 1120(local user name) or 1121.Ql \&%C 1122by a hash of the concatenation: %l%h%p%r. 1123.Pp 1124The command is run synchronously and does not have access to the 1125session of the 1126.Xr ssh 1 1127that spawned it. 1128It should not be used for interactive commands. 1129.Pp 1130This directive is ignored unless 1131.Cm PermitLocalCommand 1132has been enabled. 1133.It Cm LocalForward 1134Specifies that a TCP port on the local machine be forwarded over 1135the secure channel to the specified host and port from the remote machine. 1136The first argument must be 1137.Sm off 1138.Oo Ar bind_address : Oc Ar port 1139.Sm on 1140and the second argument must be 1141.Ar host : Ns Ar hostport . 1142IPv6 addresses can be specified by enclosing addresses in square brackets. 1143Multiple forwardings may be specified, and additional forwardings can be 1144given on the command line. 1145Only the superuser can forward privileged ports. 1146By default, the local port is bound in accordance with the 1147.Cm GatewayPorts 1148setting. 1149However, an explicit 1150.Ar bind_address 1151may be used to bind the connection to a specific address. 1152The 1153.Ar bind_address 1154of 1155.Dq localhost 1156indicates that the listening port be bound for local use only, while an 1157empty address or 1158.Sq * 1159indicates that the port should be available from all interfaces. 1160.It Cm LogLevel 1161Gives the verbosity level that is used when logging messages from 1162.Xr ssh 1 . 1163The possible values are: 1164QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1165The default is INFO. 1166DEBUG and DEBUG1 are equivalent. 1167DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1168.It Cm MACs 1169Specifies the MAC (message authentication code) algorithms 1170in order of preference. 1171The MAC algorithm is used for data integrity protection. 1172Multiple algorithms must be comma-separated. 1173If the specified value begins with a 1174.Sq + 1175character, then the specified algorithms will be appended to the default set 1176instead of replacing them. 1177.Pp 1178The algorithms that contain 1179.Dq -etm 1180calculate the MAC after encryption (encrypt-then-mac). 1181These are considered safer and their use recommended. 1182.Pp 1183The default is: 1184.Bd -literal -offset indent 1185umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1186hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1187hmac-sha1-etm@openssh.com, 1188umac-64@openssh.com,umac-128@openssh.com, 1189hmac-sha2-256,hmac-sha2-512,hmac-sha1 1190.Ed 1191.Pp 1192The list of available MAC algorithms may also be obtained using the 1193.Fl Q 1194option of 1195.Xr ssh 1 1196with an argument of 1197.Dq mac . 1198.It Cm NoHostAuthenticationForLocalhost 1199This option can be used if the home directory is shared across machines. 1200In this case localhost will refer to a different machine on each of 1201the machines and the user will get many warnings about changed host keys. 1202However, this option disables host authentication for localhost. 1203The argument to this keyword must be 1204.Dq yes 1205or 1206.Dq no . 1207The default is to check the host key for localhost. 1208.It Cm NumberOfPasswordPrompts 1209Specifies the number of password prompts before giving up. 1210The argument to this keyword must be an integer. 1211The default is 3. 1212.It Cm PasswordAuthentication 1213Specifies whether to use password authentication. 1214The argument to this keyword must be 1215.Dq yes 1216or 1217.Dq no . 1218The default is 1219.Dq yes . 1220.It Cm PermitLocalCommand 1221Allow local command execution via the 1222.Ic LocalCommand 1223option or using the 1224.Ic !\& Ns Ar command 1225escape sequence in 1226.Xr ssh 1 . 1227The argument must be 1228.Dq yes 1229or 1230.Dq no . 1231The default is 1232.Dq no . 1233.It Cm PKCS11Provider 1234Specifies which PKCS#11 provider to use. 1235The argument to this keyword is the PKCS#11 shared library 1236.Xr ssh 1 1237should use to communicate with a PKCS#11 token providing the user's 1238private RSA key. 1239.It Cm Port 1240Specifies the port number to connect on the remote host. 1241The default is 22. 1242.It Cm PreferredAuthentications 1243Specifies the order in which the client should try authentication methods. 1244This allows a client to prefer one method (e.g.\& 1245.Cm keyboard-interactive ) 1246over another method (e.g.\& 1247.Cm password ) . 1248The default is: 1249.Bd -literal -offset indent 1250gssapi-with-mic,hostbased,publickey, 1251keyboard-interactive,password 1252.Ed 1253.It Cm Protocol 1254Specifies the protocol versions 1255.Xr ssh 1 1256should support in order of preference. 1257The possible values are 1258.Sq 1 1259and 1260.Sq 2 . 1261Multiple versions must be comma-separated. 1262When this option is set to 1263.Dq 2,1 1264.Nm ssh 1265will try version 2 and fall back to version 1 1266if version 2 is not available. 1267The default is 1268.Sq 2 . 1269Protocol 1 suffers from a number of cryptographic weaknesses and should 1270not be used. 1271It is only offered to support legacy devices. 1272.It Cm ProxyCommand 1273Specifies the command to use to connect to the server. 1274The command 1275string extends to the end of the line, and is executed 1276using the user's shell 1277.Ql exec 1278directive to avoid a lingering shell process. 1279.Pp 1280In the command string, any occurrence of 1281.Ql %h 1282will be substituted by the host name to 1283connect, 1284.Ql %p 1285by the port, and 1286.Ql %r 1287by the remote user name. 1288The command can be basically anything, 1289and should read from its standard input and write to its standard output. 1290It should eventually connect an 1291.Xr sshd 8 1292server running on some machine, or execute 1293.Ic sshd -i 1294somewhere. 1295Host key management will be done using the 1296HostName of the host being connected (defaulting to the name typed by 1297the user). 1298Setting the command to 1299.Dq none 1300disables this option entirely. 1301Note that 1302.Cm CheckHostIP 1303is not available for connects with a proxy command. 1304.Pp 1305This directive is useful in conjunction with 1306.Xr nc 1 1307and its proxy support. 1308For example, the following directive would connect via an HTTP proxy at 1309192.0.2.0: 1310.Bd -literal -offset 3n 1311ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1312.Ed 1313.It Cm ProxyUseFdpass 1314Specifies that 1315.Cm ProxyCommand 1316will pass a connected file descriptor back to 1317.Xr ssh 1 1318instead of continuing to execute and pass data. 1319The default is 1320.Dq no . 1321.It Cm PubkeyAcceptedKeyTypes 1322Specifies the key types that will be used for public key authentication 1323as a comma-separated pattern list. 1324Alternately if the specified value begins with a 1325.Sq + 1326character, then the key types after it will be appended to the default 1327instead of replacing it. 1328The default for this option is: 1329.Bd -literal -offset 3n 1330ecdsa-sha2-nistp256-cert-v01@openssh.com, 1331ecdsa-sha2-nistp384-cert-v01@openssh.com, 1332ecdsa-sha2-nistp521-cert-v01@openssh.com, 1333ssh-ed25519-cert-v01@openssh.com, 1334ssh-rsa-cert-v01@openssh.com, 1335ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1336ssh-ed25519,ssh-rsa 1337.Ed 1338.Pp 1339The 1340.Fl Q 1341option of 1342.Xr ssh 1 1343may be used to list supported key types. 1344.It Cm PubkeyAuthentication 1345Specifies whether to try public key authentication. 1346The argument to this keyword must be 1347.Dq yes 1348or 1349.Dq no . 1350The default is 1351.Dq yes . 1352.It Cm RekeyLimit 1353Specifies the maximum amount of data that may be transmitted before the 1354session key is renegotiated, optionally followed a maximum amount of 1355time that may pass before the session key is renegotiated. 1356The first argument is specified in bytes and may have a suffix of 1357.Sq K , 1358.Sq M , 1359or 1360.Sq G 1361to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1362The default is between 1363.Sq 1G 1364and 1365.Sq 4G , 1366depending on the cipher. 1367The optional second value is specified in seconds and may use any of the 1368units documented in the 1369TIME FORMATS section of 1370.Xr sshd_config 5 . 1371The default value for 1372.Cm RekeyLimit 1373is 1374.Dq default none , 1375which means that rekeying is performed after the cipher's default amount 1376of data has been sent or received and no time based rekeying is done. 1377.It Cm RemoteForward 1378Specifies that a TCP port on the remote machine be forwarded over 1379the secure channel to the specified host and port from the local machine. 1380The first argument must be 1381.Sm off 1382.Oo Ar bind_address : Oc Ar port 1383.Sm on 1384and the second argument must be 1385.Ar host : Ns Ar hostport . 1386IPv6 addresses can be specified by enclosing addresses in square brackets. 1387Multiple forwardings may be specified, and additional 1388forwardings can be given on the command line. 1389Privileged ports can be forwarded only when 1390logging in as root on the remote machine. 1391.Pp 1392If the 1393.Ar port 1394argument is 1395.Ql 0 , 1396the listen port will be dynamically allocated on the server and reported 1397to the client at run time. 1398.Pp 1399If the 1400.Ar bind_address 1401is not specified, the default is to only bind to loopback addresses. 1402If the 1403.Ar bind_address 1404is 1405.Ql * 1406or an empty string, then the forwarding is requested to listen on all 1407interfaces. 1408Specifying a remote 1409.Ar bind_address 1410will only succeed if the server's 1411.Cm GatewayPorts 1412option is enabled (see 1413.Xr sshd_config 5 ) . 1414.It Cm RequestTTY 1415Specifies whether to request a pseudo-tty for the session. 1416The argument may be one of: 1417.Dq no 1418(never request a TTY), 1419.Dq yes 1420(always request a TTY when standard input is a TTY), 1421.Dq force 1422(always request a TTY) or 1423.Dq auto 1424(request a TTY when opening a login session). 1425This option mirrors the 1426.Fl t 1427and 1428.Fl T 1429flags for 1430.Xr ssh 1 . 1431.It Cm RevokedHostKeys 1432Specifies revoked host public keys. 1433Keys listed in this file will be refused for host authentication. 1434Note that if this file does not exist or is not readable, 1435then host authentication will be refused for all hosts. 1436Keys may be specified as a text file, listing one public key per line, or as 1437an OpenSSH Key Revocation List (KRL) as generated by 1438.Xr ssh-keygen 1 . 1439For more information on KRLs, see the KEY REVOCATION LISTS section in 1440.Xr ssh-keygen 1 . 1441.It Cm RhostsRSAAuthentication 1442Specifies whether to try rhosts based authentication with RSA host 1443authentication. 1444The argument must be 1445.Dq yes 1446or 1447.Dq no . 1448The default is 1449.Dq no . 1450This option applies to protocol version 1 only and requires 1451.Xr ssh 1 1452to be setuid root. 1453.It Cm RSAAuthentication 1454Specifies whether to try RSA authentication. 1455The argument to this keyword must be 1456.Dq yes 1457or 1458.Dq no . 1459RSA authentication will only be 1460attempted if the identity file exists, or an authentication agent is 1461running. 1462The default is 1463.Dq yes . 1464Note that this option applies to protocol version 1 only. 1465.It Cm SendEnv 1466Specifies what variables from the local 1467.Xr environ 7 1468should be sent to the server. 1469The server must also support it, and the server must be configured to 1470accept these environment variables. 1471Note that the 1472.Ev TERM 1473environment variable is always sent whenever a 1474pseudo-terminal is requested as it is required by the protocol. 1475Refer to 1476.Cm AcceptEnv 1477in 1478.Xr sshd_config 5 1479for how to configure the server. 1480Variables are specified by name, which may contain wildcard characters. 1481Multiple environment variables may be separated by whitespace or spread 1482across multiple 1483.Cm SendEnv 1484directives. 1485The default is not to send any environment variables. 1486.Pp 1487See 1488.Sx PATTERNS 1489for more information on patterns. 1490.It Cm ServerAliveCountMax 1491Sets the number of server alive messages (see below) which may be 1492sent without 1493.Xr ssh 1 1494receiving any messages back from the server. 1495If this threshold is reached while server alive messages are being sent, 1496ssh will disconnect from the server, terminating the session. 1497It is important to note that the use of server alive messages is very 1498different from 1499.Cm TCPKeepAlive 1500(below). 1501The server alive messages are sent through the encrypted channel 1502and therefore will not be spoofable. 1503The TCP keepalive option enabled by 1504.Cm TCPKeepAlive 1505is spoofable. 1506The server alive mechanism is valuable when the client or 1507server depend on knowing when a connection has become inactive. 1508.Pp 1509The default value is 3. 1510If, for example, 1511.Cm ServerAliveInterval 1512(see below) is set to 15 and 1513.Cm ServerAliveCountMax 1514is left at the default, if the server becomes unresponsive, 1515ssh will disconnect after approximately 45 seconds. 1516.It Cm ServerAliveInterval 1517Sets a timeout interval in seconds after which if no data has been received 1518from the server, 1519.Xr ssh 1 1520will send a message through the encrypted 1521channel to request a response from the server. 1522The default 1523is 0, indicating that these messages will not be sent to the server. 1524.It Cm StreamLocalBindMask 1525Sets the octal file creation mode mask 1526.Pq umask 1527used when creating a Unix-domain socket file for local or remote 1528port forwarding. 1529This option is only used for port forwarding to a Unix-domain socket file. 1530.Pp 1531The default value is 0177, which creates a Unix-domain socket file that is 1532readable and writable only by the owner. 1533Note that not all operating systems honor the file mode on Unix-domain 1534socket files. 1535.It Cm StreamLocalBindUnlink 1536Specifies whether to remove an existing Unix-domain socket file for local 1537or remote port forwarding before creating a new one. 1538If the socket file already exists and 1539.Cm StreamLocalBindUnlink 1540is not enabled, 1541.Nm ssh 1542will be unable to forward the port to the Unix-domain socket file. 1543This option is only used for port forwarding to a Unix-domain socket file. 1544.Pp 1545The argument must be 1546.Dq yes 1547or 1548.Dq no . 1549The default is 1550.Dq no . 1551.It Cm StrictHostKeyChecking 1552If this flag is set to 1553.Dq yes , 1554.Xr ssh 1 1555will never automatically add host keys to the 1556.Pa ~/.ssh/known_hosts 1557file, and refuses to connect to hosts whose host key has changed. 1558This provides maximum protection against trojan horse attacks, 1559though it can be annoying when the 1560.Pa /etc/ssh/ssh_known_hosts 1561file is poorly maintained or when connections to new hosts are 1562frequently made. 1563This option forces the user to manually 1564add all new hosts. 1565If this flag is set to 1566.Dq no , 1567ssh will automatically add new host keys to the 1568user known hosts files. 1569If this flag is set to 1570.Dq ask , 1571new host keys 1572will be added to the user known host files only after the user 1573has confirmed that is what they really want to do, and 1574ssh will refuse to connect to hosts whose host key has changed. 1575The host keys of 1576known hosts will be verified automatically in all cases. 1577The argument must be 1578.Dq yes , 1579.Dq no , 1580or 1581.Dq ask . 1582The default is 1583.Dq ask . 1584.It Cm TCPKeepAlive 1585Specifies whether the system should send TCP keepalive messages to the 1586other side. 1587If they are sent, death of the connection or crash of one 1588of the machines will be properly noticed. 1589However, this means that 1590connections will die if the route is down temporarily, and some people 1591find it annoying. 1592.Pp 1593The default is 1594.Dq yes 1595(to send TCP keepalive messages), and the client will notice 1596if the network goes down or the remote host dies. 1597This is important in scripts, and many users want it too. 1598.Pp 1599To disable TCP keepalive messages, the value should be set to 1600.Dq no . 1601.It Cm Tunnel 1602Request 1603.Xr tun 4 1604device forwarding between the client and the server. 1605The argument must be 1606.Dq yes , 1607.Dq point-to-point 1608(layer 3), 1609.Dq ethernet 1610(layer 2), 1611or 1612.Dq no . 1613Specifying 1614.Dq yes 1615requests the default tunnel mode, which is 1616.Dq point-to-point . 1617The default is 1618.Dq no . 1619.It Cm TunnelDevice 1620Specifies the 1621.Xr tun 4 1622devices to open on the client 1623.Pq Ar local_tun 1624and the server 1625.Pq Ar remote_tun . 1626.Pp 1627The argument must be 1628.Sm off 1629.Ar local_tun Op : Ar remote_tun . 1630.Sm on 1631The devices may be specified by numerical ID or the keyword 1632.Dq any , 1633which uses the next available tunnel device. 1634If 1635.Ar remote_tun 1636is not specified, it defaults to 1637.Dq any . 1638The default is 1639.Dq any:any . 1640.It Cm UpdateHostKeys 1641Specifies whether 1642.Xr ssh 1 1643should accept notifications of additional hostkeys from the server sent 1644after authentication has completed and add them to 1645.Cm UserKnownHostsFile . 1646The argument must be 1647.Dq yes , 1648.Dq no 1649(the default) or 1650.Dq ask . 1651Enabling this option allows learning alternate hostkeys for a server 1652and supports graceful key rotation by allowing a server to send replacement 1653public keys before old ones are removed. 1654Additional hostkeys are only accepted if the key used to authenticate the 1655host was already trusted or explicitly accepted by the user. 1656If 1657.Cm UpdateHostKeys 1658is set to 1659.Dq ask , 1660then the user is asked to confirm the modifications to the known_hosts file. 1661Confirmation is currently incompatible with 1662.Cm ControlPersist , 1663and will be disabled if it is enabled. 1664.Pp 1665Presently, only 1666.Xr sshd 8 1667from OpenSSH 6.8 and greater support the 1668.Dq hostkeys@openssh.com 1669protocol extension used to inform the client of all the server's hostkeys. 1670.It Cm UsePrivilegedPort 1671Specifies whether to use a privileged port for outgoing connections. 1672The argument must be 1673.Dq yes 1674or 1675.Dq no . 1676The default is 1677.Dq no . 1678If set to 1679.Dq yes , 1680.Xr ssh 1 1681must be setuid root. 1682Note that this option must be set to 1683.Dq yes 1684for 1685.Cm RhostsRSAAuthentication 1686with older servers. 1687.It Cm User 1688Specifies the user to log in as. 1689This can be useful when a different user name is used on different machines. 1690This saves the trouble of 1691having to remember to give the user name on the command line. 1692.It Cm UserKnownHostsFile 1693Specifies one or more files to use for the user 1694host key database, separated by whitespace. 1695The default is 1696.Pa ~/.ssh/known_hosts , 1697.Pa ~/.ssh/known_hosts2 . 1698.It Cm VerifyHostKeyDNS 1699Specifies whether to verify the remote key using DNS and SSHFP resource 1700records. 1701If this option is set to 1702.Dq yes , 1703the client will implicitly trust keys that match a secure fingerprint 1704from DNS. 1705Insecure fingerprints will be handled as if this option was set to 1706.Dq ask . 1707If this option is set to 1708.Dq ask , 1709information on fingerprint match will be displayed, but the user will still 1710need to confirm new host keys according to the 1711.Cm StrictHostKeyChecking 1712option. 1713The argument must be 1714.Dq yes , 1715.Dq no , 1716or 1717.Dq ask . 1718The default is 1719.Dq yes 1720if compiled with LDNS and 1721.Dq no 1722otherwise. 1723.Pp 1724See also VERIFYING HOST KEYS in 1725.Xr ssh 1 . 1726.It Cm VersionAddendum 1727Specifies a string to append to the regular version string to identify 1728OS- or site-specific modifications. 1729The default is 1730.Dq FreeBSD-20160310 . 1731The value 1732.Dq none 1733may be used to disable this. 1734.It Cm VisualHostKey 1735If this flag is set to 1736.Dq yes , 1737an ASCII art representation of the remote host key fingerprint is 1738printed in addition to the fingerprint string at login and 1739for unknown host keys. 1740If this flag is set to 1741.Dq no , 1742no fingerprint strings are printed at login and 1743only the fingerprint string will be printed for unknown host keys. 1744The default is 1745.Dq no . 1746.It Cm XAuthLocation 1747Specifies the full pathname of the 1748.Xr xauth 1 1749program. 1750The default is 1751.Pa /usr/local/bin/xauth . 1752.El 1753.Sh PATTERNS 1754A 1755.Em pattern 1756consists of zero or more non-whitespace characters, 1757.Sq * 1758(a wildcard that matches zero or more characters), 1759or 1760.Sq ?\& 1761(a wildcard that matches exactly one character). 1762For example, to specify a set of declarations for any host in the 1763.Dq .co.uk 1764set of domains, 1765the following pattern could be used: 1766.Pp 1767.Dl Host *.co.uk 1768.Pp 1769The following pattern 1770would match any host in the 192.168.0.[0-9] network range: 1771.Pp 1772.Dl Host 192.168.0.? 1773.Pp 1774A 1775.Em pattern-list 1776is a comma-separated list of patterns. 1777Patterns within pattern-lists may be negated 1778by preceding them with an exclamation mark 1779.Pq Sq !\& . 1780For example, 1781to allow a key to be used from anywhere within an organization 1782except from the 1783.Dq dialup 1784pool, 1785the following entry (in authorized_keys) could be used: 1786.Pp 1787.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1788.Sh FILES 1789.Bl -tag -width Ds 1790.It Pa ~/.ssh/config 1791This is the per-user configuration file. 1792The format of this file is described above. 1793This file is used by the SSH client. 1794Because of the potential for abuse, this file must have strict permissions: 1795read/write for the user, and not accessible by others. 1796.It Pa /etc/ssh/ssh_config 1797Systemwide configuration file. 1798This file provides defaults for those 1799values that are not specified in the user's configuration file, and 1800for those users who do not have a configuration file. 1801This file must be world-readable. 1802.El 1803.Sh SEE ALSO 1804.Xr ssh 1 1805.Sh AUTHORS 1806OpenSSH is a derivative of the original and free 1807ssh 1.2.12 release by Tatu Ylonen. 1808Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1809Theo de Raadt and Dug Song 1810removed many bugs, re-added newer features and 1811created OpenSSH. 1812Markus Friedl contributed the support for SSH 1813protocol versions 1.5 and 2.0. 1814