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, 874ssh-dss-cert-v01@openssh.com, 875ecdsa-sha2-nistp256,ecdsa-sha2-nistp384, 876ecdsa-sha2-nistp521,ssh-ed25519, 877ssh-rsa,ssh-dss 878.Ed 879.Pp 880The 881.Fl Q 882option of 883.Xr ssh 1 884may be used to list supported key types. 885.It Cm HostKeyAlgorithms 886Specifies the host key algorithms 887that the client wants to use in order of preference. 888Alternately if the specified value begins with a 889.Sq + 890character, then the specified key types will be appended to the default set 891instead of replacing them. 892The default for this option is: 893.Bd -literal -offset 3n 894ecdsa-sha2-nistp256-cert-v01@openssh.com, 895ecdsa-sha2-nistp384-cert-v01@openssh.com, 896ecdsa-sha2-nistp521-cert-v01@openssh.com, 897ssh-ed25519-cert-v01@openssh.com, 898ssh-rsa-cert-v01@openssh.com, 899ssh-dss-cert-v01@openssh.com, 900ecdsa-sha2-nistp256,ecdsa-sha2-nistp384, 901ecdsa-sha2-nistp521,ssh-ed25519, 902ssh-rsa,ssh-dss 903.Ed 904.Pp 905If hostkeys are known for the destination host then this default is modified 906to prefer their algorithms. 907.Pp 908The list of available key types may also be obtained using the 909.Fl Q 910option of 911.Xr ssh 1 912with an argument of 913.Dq key . 914.It Cm HostKeyAlias 915Specifies an alias that should be used instead of the 916real host name when looking up or saving the host key 917in the host key database files. 918This option is useful for tunneling SSH connections 919or for multiple servers running on a single host. 920.It Cm HostName 921Specifies the real host name to log into. 922This can be used to specify nicknames or abbreviations for hosts. 923If the hostname contains the character sequence 924.Ql %h , 925then this will be replaced with the host name specified on the command line 926(this is useful for manipulating unqualified names). 927The character sequence 928.Ql %% 929will be replaced by a single 930.Ql % 931character, which may be used when specifying IPv6 link-local addresses. 932.Pp 933The default is the name given on the command line. 934Numeric IP addresses are also permitted (both on the command line and in 935.Cm HostName 936specifications). 937.It Cm IdentitiesOnly 938Specifies that 939.Xr ssh 1 940should only use the authentication identity and certificate files explicitly 941configured in the 942.Nm 943files 944or passed on the 945.Xr ssh 1 946command-line, 947even if 948.Xr ssh-agent 1 949or a 950.Cm PKCS11Provider 951offers more identities. 952The argument to this keyword must be 953.Dq yes 954or 955.Dq no . 956This option is intended for situations where ssh-agent 957offers many different identities. 958The default is 959.Dq no . 960.It Cm IdentityFile 961Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication 962identity is read. 963The default is 964.Pa ~/.ssh/identity 965for protocol version 1, and 966.Pa ~/.ssh/id_dsa , 967.Pa ~/.ssh/id_ecdsa , 968.Pa ~/.ssh/id_ed25519 969and 970.Pa ~/.ssh/id_rsa 971for protocol version 2. 972Additionally, any identities represented by the authentication agent 973will be used for authentication unless 974.Cm IdentitiesOnly 975is set. 976If no certificates have been explicitly specified by 977.Cm CertificateFile , 978.Xr ssh 1 979will try to load certificate information from the filename obtained by 980appending 981.Pa -cert.pub 982to the path of a specified 983.Cm IdentityFile . 984.Pp 985The file name may use the tilde 986syntax to refer to a user's home directory or one of the following 987escape characters: 988.Ql %d 989(local user's home directory), 990.Ql %u 991(local user name), 992.Ql %l 993(local host name), 994.Ql %h 995(remote host name) or 996.Ql %r 997(remote user name). 998.Pp 999It is possible to have 1000multiple identity files specified in configuration files; all these 1001identities will be tried in sequence. 1002Multiple 1003.Cm IdentityFile 1004directives will add to the list of identities tried (this behaviour 1005differs from that of other configuration directives). 1006.Pp 1007.Cm IdentityFile 1008may be used in conjunction with 1009.Cm IdentitiesOnly 1010to select which identities in an agent are offered during authentication. 1011.Cm IdentityFile 1012may also be used in conjunction with 1013.Cm CertificateFile 1014in order to provide any certificate also needed for authentication with 1015the identity. 1016.It Cm IgnoreUnknown 1017Specifies a pattern-list of unknown options to be ignored if they are 1018encountered in configuration parsing. 1019This may be used to suppress errors if 1020.Nm 1021contains options that are unrecognised by 1022.Xr ssh 1 . 1023It is recommended that 1024.Cm IgnoreUnknown 1025be listed early in the configuration file as it will not be applied 1026to unknown options that appear before it. 1027.It Cm IPQoS 1028Specifies the IPv4 type-of-service or DSCP class for connections. 1029Accepted values are 1030.Dq af11 , 1031.Dq af12 , 1032.Dq af13 , 1033.Dq af21 , 1034.Dq af22 , 1035.Dq af23 , 1036.Dq af31 , 1037.Dq af32 , 1038.Dq af33 , 1039.Dq af41 , 1040.Dq af42 , 1041.Dq af43 , 1042.Dq cs0 , 1043.Dq cs1 , 1044.Dq cs2 , 1045.Dq cs3 , 1046.Dq cs4 , 1047.Dq cs5 , 1048.Dq cs6 , 1049.Dq cs7 , 1050.Dq ef , 1051.Dq lowdelay , 1052.Dq throughput , 1053.Dq reliability , 1054or a numeric value. 1055This option may take one or two arguments, separated by whitespace. 1056If one argument is specified, it is used as the packet class unconditionally. 1057If two values are specified, the first is automatically selected for 1058interactive sessions and the second for non-interactive sessions. 1059The default is 1060.Dq lowdelay 1061for interactive sessions and 1062.Dq throughput 1063for non-interactive sessions. 1064.It Cm KbdInteractiveAuthentication 1065Specifies whether to use keyboard-interactive authentication. 1066The argument to this keyword must be 1067.Dq yes 1068or 1069.Dq no . 1070The default is 1071.Dq yes . 1072.It Cm KbdInteractiveDevices 1073Specifies the list of methods to use in keyboard-interactive authentication. 1074Multiple method names must be comma-separated. 1075The default is to use the server specified list. 1076The methods available vary depending on what the server supports. 1077For an OpenSSH server, 1078it may be zero or more of: 1079.Dq bsdauth , 1080.Dq pam , 1081and 1082.Dq skey . 1083.It Cm KexAlgorithms 1084Specifies the available KEX (Key Exchange) algorithms. 1085Multiple algorithms must be comma-separated. 1086Alternately if the specified value begins with a 1087.Sq + 1088character, then the specified methods will be appended to the default set 1089instead of replacing them. 1090The default is: 1091.Bd -literal -offset indent 1092curve25519-sha256@libssh.org, 1093ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1094diffie-hellman-group-exchange-sha256, 1095diffie-hellman-group-exchange-sha1, 1096diffie-hellman-group14-sha1 1097.Ed 1098.Pp 1099The list of available key exchange algorithms may also be obtained using the 1100.Fl Q 1101option of 1102.Xr ssh 1 1103with an argument of 1104.Dq kex . 1105.It Cm LocalCommand 1106Specifies a command to execute on the local machine after successfully 1107connecting to the server. 1108The command string extends to the end of the line, and is executed with 1109the user's shell. 1110The following escape character substitutions will be performed: 1111.Ql %d 1112(local user's home directory), 1113.Ql %h 1114(remote host name), 1115.Ql %l 1116(local host name), 1117.Ql %n 1118(host name as provided on the command line), 1119.Ql %p 1120(remote port), 1121.Ql %r 1122(remote user name) or 1123.Ql %u 1124(local user name) or 1125.Ql \&%C 1126by a hash of the concatenation: %l%h%p%r. 1127.Pp 1128The command is run synchronously and does not have access to the 1129session of the 1130.Xr ssh 1 1131that spawned it. 1132It should not be used for interactive commands. 1133.Pp 1134This directive is ignored unless 1135.Cm PermitLocalCommand 1136has been enabled. 1137.It Cm LocalForward 1138Specifies that a TCP port on the local machine be forwarded over 1139the secure channel to the specified host and port from the remote machine. 1140The first argument must be 1141.Sm off 1142.Oo Ar bind_address : Oc Ar port 1143.Sm on 1144and the second argument must be 1145.Ar host : Ns Ar hostport . 1146IPv6 addresses can be specified by enclosing addresses in square brackets. 1147Multiple forwardings may be specified, and additional forwardings can be 1148given on the command line. 1149Only the superuser can forward privileged ports. 1150By default, the local port is bound in accordance with the 1151.Cm GatewayPorts 1152setting. 1153However, an explicit 1154.Ar bind_address 1155may be used to bind the connection to a specific address. 1156The 1157.Ar bind_address 1158of 1159.Dq localhost 1160indicates that the listening port be bound for local use only, while an 1161empty address or 1162.Sq * 1163indicates that the port should be available from all interfaces. 1164.It Cm LogLevel 1165Gives the verbosity level that is used when logging messages from 1166.Xr ssh 1 . 1167The possible values are: 1168QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1169The default is INFO. 1170DEBUG and DEBUG1 are equivalent. 1171DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1172.It Cm MACs 1173Specifies the MAC (message authentication code) algorithms 1174in order of preference. 1175The MAC algorithm is used for data integrity protection. 1176Multiple algorithms must be comma-separated. 1177If the specified value begins with a 1178.Sq + 1179character, then the specified algorithms will be appended to the default set 1180instead of replacing them. 1181.Pp 1182The algorithms that contain 1183.Dq -etm 1184calculate the MAC after encryption (encrypt-then-mac). 1185These are considered safer and their use recommended. 1186.Pp 1187The default is: 1188.Bd -literal -offset indent 1189umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1190hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1191hmac-sha1-etm@openssh.com, 1192umac-64@openssh.com,umac-128@openssh.com, 1193hmac-sha2-256,hmac-sha2-512,hmac-sha1 1194.Ed 1195.Pp 1196The list of available MAC algorithms may also be obtained using the 1197.Fl Q 1198option of 1199.Xr ssh 1 1200with an argument of 1201.Dq mac . 1202.It Cm NoHostAuthenticationForLocalhost 1203This option can be used if the home directory is shared across machines. 1204In this case localhost will refer to a different machine on each of 1205the machines and the user will get many warnings about changed host keys. 1206However, this option disables host authentication for localhost. 1207The argument to this keyword must be 1208.Dq yes 1209or 1210.Dq no . 1211The default is to check the host key for localhost. 1212.It Cm NumberOfPasswordPrompts 1213Specifies the number of password prompts before giving up. 1214The argument to this keyword must be an integer. 1215The default is 3. 1216.It Cm PasswordAuthentication 1217Specifies whether to use password authentication. 1218The argument to this keyword must be 1219.Dq yes 1220or 1221.Dq no . 1222The default is 1223.Dq yes . 1224.It Cm PermitLocalCommand 1225Allow local command execution via the 1226.Ic LocalCommand 1227option or using the 1228.Ic !\& Ns Ar command 1229escape sequence in 1230.Xr ssh 1 . 1231The argument must be 1232.Dq yes 1233or 1234.Dq no . 1235The default is 1236.Dq no . 1237.It Cm PKCS11Provider 1238Specifies which PKCS#11 provider to use. 1239The argument to this keyword is the PKCS#11 shared library 1240.Xr ssh 1 1241should use to communicate with a PKCS#11 token providing the user's 1242private RSA key. 1243.It Cm Port 1244Specifies the port number to connect on the remote host. 1245The default is 22. 1246.It Cm PreferredAuthentications 1247Specifies the order in which the client should try authentication methods. 1248This allows a client to prefer one method (e.g.\& 1249.Cm keyboard-interactive ) 1250over another method (e.g.\& 1251.Cm password ) . 1252The default is: 1253.Bd -literal -offset indent 1254gssapi-with-mic,hostbased,publickey, 1255keyboard-interactive,password 1256.Ed 1257.It Cm Protocol 1258Specifies the protocol versions 1259.Xr ssh 1 1260should support in order of preference. 1261The possible values are 1262.Sq 1 1263and 1264.Sq 2 . 1265Multiple versions must be comma-separated. 1266When this option is set to 1267.Dq 2,1 1268.Nm ssh 1269will try version 2 and fall back to version 1 1270if version 2 is not available. 1271The default is 1272.Sq 2 . 1273Protocol 1 suffers from a number of cryptographic weaknesses and should 1274not be used. 1275It is only offered to support legacy devices. 1276.It Cm ProxyCommand 1277Specifies the command to use to connect to the server. 1278The command 1279string extends to the end of the line, and is executed 1280using the user's shell 1281.Ql exec 1282directive to avoid a lingering shell process. 1283.Pp 1284In the command string, any occurrence of 1285.Ql %h 1286will be substituted by the host name to 1287connect, 1288.Ql %p 1289by the port, and 1290.Ql %r 1291by the remote user name. 1292The command can be basically anything, 1293and should read from its standard input and write to its standard output. 1294It should eventually connect an 1295.Xr sshd 8 1296server running on some machine, or execute 1297.Ic sshd -i 1298somewhere. 1299Host key management will be done using the 1300HostName of the host being connected (defaulting to the name typed by 1301the user). 1302Setting the command to 1303.Dq none 1304disables this option entirely. 1305Note that 1306.Cm CheckHostIP 1307is not available for connects with a proxy command. 1308.Pp 1309This directive is useful in conjunction with 1310.Xr nc 1 1311and its proxy support. 1312For example, the following directive would connect via an HTTP proxy at 1313192.0.2.0: 1314.Bd -literal -offset 3n 1315ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1316.Ed 1317.It Cm ProxyUseFdpass 1318Specifies that 1319.Cm ProxyCommand 1320will pass a connected file descriptor back to 1321.Xr ssh 1 1322instead of continuing to execute and pass data. 1323The default is 1324.Dq no . 1325.It Cm PubkeyAcceptedKeyTypes 1326Specifies the key types that will be used for public key authentication 1327as a comma-separated pattern list. 1328Alternately if the specified value begins with a 1329.Sq + 1330character, then the key types after it will be appended to the default 1331instead of replacing it. 1332The default for this option is: 1333.Bd -literal -offset 3n 1334ecdsa-sha2-nistp256-cert-v01@openssh.com, 1335ecdsa-sha2-nistp384-cert-v01@openssh.com, 1336ecdsa-sha2-nistp521-cert-v01@openssh.com, 1337ssh-ed25519-cert-v01@openssh.com, 1338ssh-rsa-cert-v01@openssh.com, 1339ssh-dss-cert-v01@openssh.com, 1340ecdsa-sha2-nistp256,ecdsa-sha2-nistp384, 1341ecdsa-sha2-nistp521,ssh-ed25519, 1342ssh-rsa,ssh-dss 1343.Ed 1344.Pp 1345The 1346.Fl Q 1347option of 1348.Xr ssh 1 1349may be used to list supported key types. 1350.It Cm PubkeyAuthentication 1351Specifies whether to try public key authentication. 1352The argument to this keyword must be 1353.Dq yes 1354or 1355.Dq no . 1356The default is 1357.Dq yes . 1358.It Cm RekeyLimit 1359Specifies the maximum amount of data that may be transmitted before the 1360session key is renegotiated, optionally followed a maximum amount of 1361time that may pass before the session key is renegotiated. 1362The first argument is specified in bytes and may have a suffix of 1363.Sq K , 1364.Sq M , 1365or 1366.Sq G 1367to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1368The default is between 1369.Sq 1G 1370and 1371.Sq 4G , 1372depending on the cipher. 1373The optional second value is specified in seconds and may use any of the 1374units documented in the 1375TIME FORMATS section of 1376.Xr sshd_config 5 . 1377The default value for 1378.Cm RekeyLimit 1379is 1380.Dq default none , 1381which means that rekeying is performed after the cipher's default amount 1382of data has been sent or received and no time based rekeying is done. 1383.It Cm RemoteForward 1384Specifies that a TCP port on the remote machine be forwarded over 1385the secure channel to the specified host and port from the local machine. 1386The first argument must be 1387.Sm off 1388.Oo Ar bind_address : Oc Ar port 1389.Sm on 1390and the second argument must be 1391.Ar host : Ns Ar hostport . 1392IPv6 addresses can be specified by enclosing addresses in square brackets. 1393Multiple forwardings may be specified, and additional 1394forwardings can be given on the command line. 1395Privileged ports can be forwarded only when 1396logging in as root on the remote machine. 1397.Pp 1398If the 1399.Ar port 1400argument is 1401.Ql 0 , 1402the listen port will be dynamically allocated on the server and reported 1403to the client at run time. 1404.Pp 1405If the 1406.Ar bind_address 1407is not specified, the default is to only bind to loopback addresses. 1408If the 1409.Ar bind_address 1410is 1411.Ql * 1412or an empty string, then the forwarding is requested to listen on all 1413interfaces. 1414Specifying a remote 1415.Ar bind_address 1416will only succeed if the server's 1417.Cm GatewayPorts 1418option is enabled (see 1419.Xr sshd_config 5 ) . 1420.It Cm RequestTTY 1421Specifies whether to request a pseudo-tty for the session. 1422The argument may be one of: 1423.Dq no 1424(never request a TTY), 1425.Dq yes 1426(always request a TTY when standard input is a TTY), 1427.Dq force 1428(always request a TTY) or 1429.Dq auto 1430(request a TTY when opening a login session). 1431This option mirrors the 1432.Fl t 1433and 1434.Fl T 1435flags for 1436.Xr ssh 1 . 1437.It Cm RevokedHostKeys 1438Specifies revoked host public keys. 1439Keys listed in this file will be refused for host authentication. 1440Note that if this file does not exist or is not readable, 1441then host authentication will be refused for all hosts. 1442Keys may be specified as a text file, listing one public key per line, or as 1443an OpenSSH Key Revocation List (KRL) as generated by 1444.Xr ssh-keygen 1 . 1445For more information on KRLs, see the KEY REVOCATION LISTS section in 1446.Xr ssh-keygen 1 . 1447.It Cm RhostsRSAAuthentication 1448Specifies whether to try rhosts based authentication with RSA host 1449authentication. 1450The argument must be 1451.Dq yes 1452or 1453.Dq no . 1454The default is 1455.Dq no . 1456This option applies to protocol version 1 only and requires 1457.Xr ssh 1 1458to be setuid root. 1459.It Cm RSAAuthentication 1460Specifies whether to try RSA authentication. 1461The argument to this keyword must be 1462.Dq yes 1463or 1464.Dq no . 1465RSA authentication will only be 1466attempted if the identity file exists, or an authentication agent is 1467running. 1468The default is 1469.Dq yes . 1470Note that this option applies to protocol version 1 only. 1471.It Cm SendEnv 1472Specifies what variables from the local 1473.Xr environ 7 1474should be sent to the server. 1475The server must also support it, and the server must be configured to 1476accept these environment variables. 1477Note that the 1478.Ev TERM 1479environment variable is always sent whenever a 1480pseudo-terminal is requested as it is required by the protocol. 1481Refer to 1482.Cm AcceptEnv 1483in 1484.Xr sshd_config 5 1485for how to configure the server. 1486Variables are specified by name, which may contain wildcard characters. 1487Multiple environment variables may be separated by whitespace or spread 1488across multiple 1489.Cm SendEnv 1490directives. 1491The default is not to send any environment variables. 1492.Pp 1493See 1494.Sx PATTERNS 1495for more information on patterns. 1496.It Cm ServerAliveCountMax 1497Sets the number of server alive messages (see below) which may be 1498sent without 1499.Xr ssh 1 1500receiving any messages back from the server. 1501If this threshold is reached while server alive messages are being sent, 1502ssh will disconnect from the server, terminating the session. 1503It is important to note that the use of server alive messages is very 1504different from 1505.Cm TCPKeepAlive 1506(below). 1507The server alive messages are sent through the encrypted channel 1508and therefore will not be spoofable. 1509The TCP keepalive option enabled by 1510.Cm TCPKeepAlive 1511is spoofable. 1512The server alive mechanism is valuable when the client or 1513server depend on knowing when a connection has become inactive. 1514.Pp 1515The default value is 3. 1516If, for example, 1517.Cm ServerAliveInterval 1518(see below) is set to 15 and 1519.Cm ServerAliveCountMax 1520is left at the default, if the server becomes unresponsive, 1521ssh will disconnect after approximately 45 seconds. 1522.It Cm ServerAliveInterval 1523Sets a timeout interval in seconds after which if no data has been received 1524from the server, 1525.Xr ssh 1 1526will send a message through the encrypted 1527channel to request a response from the server. 1528The default 1529is 0, indicating that these messages will not be sent to the server. 1530.It Cm StreamLocalBindMask 1531Sets the octal file creation mode mask 1532.Pq umask 1533used when creating a Unix-domain socket file for local or remote 1534port forwarding. 1535This option is only used for port forwarding to a Unix-domain socket file. 1536.Pp 1537The default value is 0177, which creates a Unix-domain socket file that is 1538readable and writable only by the owner. 1539Note that not all operating systems honor the file mode on Unix-domain 1540socket files. 1541.It Cm StreamLocalBindUnlink 1542Specifies whether to remove an existing Unix-domain socket file for local 1543or remote port forwarding before creating a new one. 1544If the socket file already exists and 1545.Cm StreamLocalBindUnlink 1546is not enabled, 1547.Nm ssh 1548will be unable to forward the port to the Unix-domain socket file. 1549This option is only used for port forwarding to a Unix-domain socket file. 1550.Pp 1551The argument must be 1552.Dq yes 1553or 1554.Dq no . 1555The default is 1556.Dq no . 1557.It Cm StrictHostKeyChecking 1558If this flag is set to 1559.Dq yes , 1560.Xr ssh 1 1561will never automatically add host keys to the 1562.Pa ~/.ssh/known_hosts 1563file, and refuses to connect to hosts whose host key has changed. 1564This provides maximum protection against trojan horse attacks, 1565though it can be annoying when the 1566.Pa /etc/ssh/ssh_known_hosts 1567file is poorly maintained or when connections to new hosts are 1568frequently made. 1569This option forces the user to manually 1570add all new hosts. 1571If this flag is set to 1572.Dq no , 1573ssh will automatically add new host keys to the 1574user known hosts files. 1575If this flag is set to 1576.Dq ask , 1577new host keys 1578will be added to the user known host files only after the user 1579has confirmed that is what they really want to do, and 1580ssh will refuse to connect to hosts whose host key has changed. 1581The host keys of 1582known hosts will be verified automatically in all cases. 1583The argument must be 1584.Dq yes , 1585.Dq no , 1586or 1587.Dq ask . 1588The default is 1589.Dq ask . 1590.It Cm TCPKeepAlive 1591Specifies whether the system should send TCP keepalive messages to the 1592other side. 1593If they are sent, death of the connection or crash of one 1594of the machines will be properly noticed. 1595However, this means that 1596connections will die if the route is down temporarily, and some people 1597find it annoying. 1598.Pp 1599The default is 1600.Dq yes 1601(to send TCP keepalive messages), and the client will notice 1602if the network goes down or the remote host dies. 1603This is important in scripts, and many users want it too. 1604.Pp 1605To disable TCP keepalive messages, the value should be set to 1606.Dq no . 1607.It Cm Tunnel 1608Request 1609.Xr tun 4 1610device forwarding between the client and the server. 1611The argument must be 1612.Dq yes , 1613.Dq point-to-point 1614(layer 3), 1615.Dq ethernet 1616(layer 2), 1617or 1618.Dq no . 1619Specifying 1620.Dq yes 1621requests the default tunnel mode, which is 1622.Dq point-to-point . 1623The default is 1624.Dq no . 1625.It Cm TunnelDevice 1626Specifies the 1627.Xr tun 4 1628devices to open on the client 1629.Pq Ar local_tun 1630and the server 1631.Pq Ar remote_tun . 1632.Pp 1633The argument must be 1634.Sm off 1635.Ar local_tun Op : Ar remote_tun . 1636.Sm on 1637The devices may be specified by numerical ID or the keyword 1638.Dq any , 1639which uses the next available tunnel device. 1640If 1641.Ar remote_tun 1642is not specified, it defaults to 1643.Dq any . 1644The default is 1645.Dq any:any . 1646.It Cm UpdateHostKeys 1647Specifies whether 1648.Xr ssh 1 1649should accept notifications of additional hostkeys from the server sent 1650after authentication has completed and add them to 1651.Cm UserKnownHostsFile . 1652The argument must be 1653.Dq yes , 1654.Dq no 1655(the default) or 1656.Dq ask . 1657Enabling this option allows learning alternate hostkeys for a server 1658and supports graceful key rotation by allowing a server to send replacement 1659public keys before old ones are removed. 1660Additional hostkeys are only accepted if the key used to authenticate the 1661host was already trusted or explicitly accepted by the user. 1662If 1663.Cm UpdateHostKeys 1664is set to 1665.Dq ask , 1666then the user is asked to confirm the modifications to the known_hosts file. 1667Confirmation is currently incompatible with 1668.Cm ControlPersist , 1669and will be disabled if it is enabled. 1670.Pp 1671Presently, only 1672.Xr sshd 8 1673from OpenSSH 6.8 and greater support the 1674.Dq hostkeys@openssh.com 1675protocol extension used to inform the client of all the server's hostkeys. 1676.It Cm UsePrivilegedPort 1677Specifies whether to use a privileged port for outgoing connections. 1678The argument must be 1679.Dq yes 1680or 1681.Dq no . 1682The default is 1683.Dq no . 1684If set to 1685.Dq yes , 1686.Xr ssh 1 1687must be setuid root. 1688Note that this option must be set to 1689.Dq yes 1690for 1691.Cm RhostsRSAAuthentication 1692with older servers. 1693.It Cm User 1694Specifies the user to log in as. 1695This can be useful when a different user name is used on different machines. 1696This saves the trouble of 1697having to remember to give the user name on the command line. 1698.It Cm UserKnownHostsFile 1699Specifies one or more files to use for the user 1700host key database, separated by whitespace. 1701The default is 1702.Pa ~/.ssh/known_hosts , 1703.Pa ~/.ssh/known_hosts2 . 1704.It Cm VerifyHostKeyDNS 1705Specifies whether to verify the remote key using DNS and SSHFP resource 1706records. 1707If this option is set to 1708.Dq yes , 1709the client will implicitly trust keys that match a secure fingerprint 1710from DNS. 1711Insecure fingerprints will be handled as if this option was set to 1712.Dq ask . 1713If this option is set to 1714.Dq ask , 1715information on fingerprint match will be displayed, but the user will still 1716need to confirm new host keys according to the 1717.Cm StrictHostKeyChecking 1718option. 1719The argument must be 1720.Dq yes , 1721.Dq no , 1722or 1723.Dq ask . 1724The default is 1725.Dq yes 1726if compiled with LDNS and 1727.Dq no 1728otherwise. 1729.Pp 1730See also VERIFYING HOST KEYS in 1731.Xr ssh 1 . 1732.It Cm VersionAddendum 1733Specifies a string to append to the regular version string to identify 1734OS- or site-specific modifications. 1735The default is 1736.Dq FreeBSD-20160310 . 1737The value 1738.Dq none 1739may be used to disable this. 1740.It Cm VisualHostKey 1741If this flag is set to 1742.Dq yes , 1743an ASCII art representation of the remote host key fingerprint is 1744printed in addition to the fingerprint string at login and 1745for unknown host keys. 1746If this flag is set to 1747.Dq no , 1748no fingerprint strings are printed at login and 1749only the fingerprint string will be printed for unknown host keys. 1750The default is 1751.Dq no . 1752.It Cm XAuthLocation 1753Specifies the full pathname of the 1754.Xr xauth 1 1755program. 1756The default is 1757.Pa /usr/local/bin/xauth . 1758.El 1759.Sh PATTERNS 1760A 1761.Em pattern 1762consists of zero or more non-whitespace characters, 1763.Sq * 1764(a wildcard that matches zero or more characters), 1765or 1766.Sq ?\& 1767(a wildcard that matches exactly one character). 1768For example, to specify a set of declarations for any host in the 1769.Dq .co.uk 1770set of domains, 1771the following pattern could be used: 1772.Pp 1773.Dl Host *.co.uk 1774.Pp 1775The following pattern 1776would match any host in the 192.168.0.[0-9] network range: 1777.Pp 1778.Dl Host 192.168.0.? 1779.Pp 1780A 1781.Em pattern-list 1782is a comma-separated list of patterns. 1783Patterns within pattern-lists may be negated 1784by preceding them with an exclamation mark 1785.Pq Sq !\& . 1786For example, 1787to allow a key to be used from anywhere within an organization 1788except from the 1789.Dq dialup 1790pool, 1791the following entry (in authorized_keys) could be used: 1792.Pp 1793.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1794.Sh FILES 1795.Bl -tag -width Ds 1796.It Pa ~/.ssh/config 1797This is the per-user configuration file. 1798The format of this file is described above. 1799This file is used by the SSH client. 1800Because of the potential for abuse, this file must have strict permissions: 1801read/write for the user, and not accessible by others. 1802.It Pa /etc/ssh/ssh_config 1803Systemwide configuration file. 1804This file provides defaults for those 1805values that are not specified in the user's configuration file, and 1806for those users who do not have a configuration file. 1807This file must be world-readable. 1808.El 1809.Sh SEE ALSO 1810.Xr ssh 1 1811.Sh AUTHORS 1812OpenSSH is a derivative of the original and free 1813ssh 1.2.12 release by Tatu Ylonen. 1814Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1815Theo de Raadt and Dug Song 1816removed many bugs, re-added newer features and 1817created OpenSSH. 1818Markus Friedl contributed the support for SSH 1819protocol versions 1.5 and 2.0. 1820