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 1.215 2015/08/14 15:32:41 jmc Exp $ 37.Dd $Mdocdate: August 14 2015 $ 38.Dt SSH_CONFIG 5 39.Os 40.Sh NAME 41.Nm ssh_config 42.Nd OpenSSH SSH client configuration files 43.Sh SYNOPSIS 44.Nm ~/.ssh/config 45.Nm /etc/ssh/ssh_config 46.Sh DESCRIPTION 47.Xr ssh 1 48obtains configuration data from the following sources in 49the following order: 50.Pp 51.Bl -enum -offset indent -compact 52.It 53command-line options 54.It 55user's configuration file 56.Pq Pa ~/.ssh/config 57.It 58system-wide configuration file 59.Pq Pa /etc/ssh/ssh_config 60.El 61.Pp 62For each parameter, the first obtained value 63will be used. 64The configuration files contain sections separated by 65.Dq Host 66specifications, and that section is only applied for hosts that 67match one of the patterns given in the specification. 68The matched host name is usually the one given on the command line 69(see the 70.Cm CanonicalizeHostname 71option for exceptions.) 72.Pp 73Since the first obtained value for each parameter is used, more 74host-specific declarations should be given near the beginning of the 75file, and general defaults at the end. 76.Pp 77The configuration file has the following format: 78.Pp 79Empty lines and lines starting with 80.Ql # 81are comments. 82Otherwise a line is of the format 83.Dq keyword arguments . 84Configuration options may be separated by whitespace or 85optional whitespace and exactly one 86.Ql = ; 87the latter format is useful to avoid the need to quote whitespace 88when specifying configuration options using the 89.Nm ssh , 90.Nm scp , 91and 92.Nm sftp 93.Fl o 94option. 95Arguments may optionally be enclosed in double quotes 96.Pq \&" 97in order to represent arguments containing spaces. 98.Pp 99The possible 100keywords and their meanings are as follows (note that 101keywords are case-insensitive and arguments are case-sensitive): 102.Bl -tag -width Ds 103.It Cm Host 104Restricts the following declarations (up to the next 105.Cm Host 106or 107.Cm Match 108keyword) to be only for those hosts that match one of the patterns 109given after the keyword. 110If more than one pattern is provided, they should be separated by whitespace. 111A single 112.Ql * 113as a pattern can be used to provide global 114defaults for all hosts. 115The host is usually the 116.Ar hostname 117argument given on the command line 118(see the 119.Cm CanonicalizeHostname 120option for exceptions.) 121.Pp 122A pattern entry may be negated by prefixing it with an exclamation mark 123.Pq Sq !\& . 124If a negated entry is matched, then the 125.Cm Host 126entry is ignored, regardless of whether any other patterns on the line 127match. 128Negated matches are therefore useful to provide exceptions for wildcard 129matches. 130.Pp 131See 132.Sx PATTERNS 133for more information on patterns. 134.It Cm Match 135Restricts the following declarations (up to the next 136.Cm Host 137or 138.Cm Match 139keyword) to be used only when the conditions following the 140.Cm Match 141keyword are satisfied. 142Match conditions are specified using one or more critera 143or the single token 144.Cm all 145which always matches. 146The available criteria keywords are: 147.Cm canonical , 148.Cm exec , 149.Cm host , 150.Cm originalhost , 151.Cm user , 152and 153.Cm localuser . 154The 155.Cm all 156criteria must appear alone or immediately after 157.Cm canonical . 158Other criteria may be combined arbitrarily. 159All criteria but 160.Cm all 161and 162.Cm canonical 163require an argument. 164Criteria may be negated by prepending an exclamation mark 165.Pq Sq !\& . 166.Pp 167The 168.Cm canonical 169keyword matches only when the configuration file is being re-parsed 170after hostname canonicalization (see the 171.Cm CanonicalizeHostname 172option.) 173This may be useful to specify conditions that work with canonical host 174names only. 175The 176.Cm exec 177keyword executes the specified command under the user's shell. 178If the command returns a zero exit status then the condition is considered true. 179Commands containing whitespace characters must be quoted. 180The following character sequences in the command will be expanded prior to 181execution: 182.Ql %L 183will be substituted by the first component of the local host name, 184.Ql %l 185will be substituted by the local host name (including any domain name), 186.Ql %h 187will be substituted by the target host name, 188.Ql %n 189will be substituted by the original target host name 190specified on the command-line, 191.Ql %p 192the destination port, 193.Ql %r 194by the remote login username, and 195.Ql %u 196by the username of the user running 197.Xr ssh 1 . 198.Pp 199The other keywords' criteria must be single entries or comma-separated 200lists and may use the wildcard and negation operators described in the 201.Sx PATTERNS 202section. 203The criteria for the 204.Cm host 205keyword are matched against the target hostname, after any substitution 206by the 207.Cm Hostname 208or 209.Cm CanonicalizeHostname 210options. 211The 212.Cm originalhost 213keyword matches against the hostname as it was specified on the command-line. 214The 215.Cm user 216keyword matches against the target username on the remote host. 217The 218.Cm localuser 219keyword matches against the name of the local user running 220.Xr ssh 1 221(this keyword may be useful in system-wide 222.Nm 223files). 224.It Cm AddressFamily 225Specifies which address family to use when connecting. 226Valid arguments are 227.Dq any , 228.Dq inet 229(use IPv4 only), or 230.Dq inet6 231(use IPv6 only). 232.It Cm BatchMode 233If set to 234.Dq yes , 235passphrase/password querying will be disabled. 236This option is useful in scripts and other batch jobs where no user 237is present to supply the password. 238The argument must be 239.Dq yes 240or 241.Dq no . 242The default is 243.Dq no . 244.It Cm BindAddress 245Use the specified address on the local machine as the source address of 246the connection. 247Only useful on systems with more than one address. 248Note that this option does not work if 249.Cm UsePrivilegedPort 250is set to 251.Dq yes . 252.It Cm CanonicalDomains 253When 254.Cm CanonicalizeHostname 255is enabled, this option specifies the list of domain suffixes in which to 256search for the specified destination host. 257.It Cm CanonicalizeFallbackLocal 258Specifies whether to fail with an error when hostname canonicalization fails. 259The default, 260.Dq yes , 261will attempt to look up the unqualified hostname using the system resolver's 262search rules. 263A value of 264.Dq no 265will cause 266.Xr ssh 1 267to fail instantly if 268.Cm CanonicalizeHostname 269is enabled and the target hostname cannot be found in any of the domains 270specified by 271.Cm CanonicalDomains . 272.It Cm CanonicalizeHostname 273Controls whether explicit hostname canonicalization is performed. 274The default, 275.Dq no , 276is not to perform any name rewriting and let the system resolver handle all 277hostname lookups. 278If set to 279.Dq yes 280then, for connections that do not use a 281.Cm ProxyCommand , 282.Xr ssh 1 283will attempt to canonicalize the hostname specified on the command line 284using the 285.Cm CanonicalDomains 286suffixes and 287.Cm CanonicalizePermittedCNAMEs 288rules. 289If 290.Cm CanonicalizeHostname 291is set to 292.Dq always , 293then canonicalization is applied to proxied connections too. 294.Pp 295If this option is enabled, then the configuration files are processed 296again using the new target name to pick up any new configuration in matching 297.Cm Host 298and 299.Cm Match 300stanzas. 301.It Cm CanonicalizeMaxDots 302Specifies the maximum number of dot characters in a hostname before 303canonicalization is disabled. 304The default, 305.Dq 1 , 306allows a single dot (i.e. hostname.subdomain). 307.It Cm CanonicalizePermittedCNAMEs 308Specifies rules to determine whether CNAMEs should be followed when 309canonicalizing hostnames. 310The rules consist of one or more arguments of 311.Ar source_domain_list : Ns Ar target_domain_list , 312where 313.Ar source_domain_list 314is a pattern-list of domains that may follow CNAMEs in canonicalization, 315and 316.Ar target_domain_list 317is a pattern-list of domains that they may resolve to. 318.Pp 319For example, 320.Dq *.a.example.com:*.b.example.com,*.c.example.com 321will allow hostnames matching 322.Dq *.a.example.com 323to be canonicalized to names in the 324.Dq *.b.example.com 325or 326.Dq *.c.example.com 327domains. 328.It Cm ChallengeResponseAuthentication 329Specifies whether to use challenge-response authentication. 330The argument to this keyword must be 331.Dq yes 332or 333.Dq no . 334The default is 335.Dq yes . 336.It Cm CheckHostIP 337If this flag is set to 338.Dq yes , 339.Xr ssh 1 340will additionally check the host IP address in the 341.Pa known_hosts 342file. 343This allows ssh to detect if a host key changed due to DNS spoofing 344and will add addresses of destination hosts to 345.Pa ~/.ssh/known_hosts 346in the process, regardless of the setting of 347.Cm StrictHostKeyChecking . 348If the option is set to 349.Dq no , 350the check will not be executed. 351The default is 352.Dq yes . 353.It Cm Cipher 354Specifies the cipher to use for encrypting the session 355in protocol version 1. 356Currently, 357.Dq blowfish , 358.Dq 3des , 359and 360.Dq des 361are supported. 362.Ar des 363is only supported in the 364.Xr ssh 1 365client for interoperability with legacy protocol 1 implementations 366that do not support the 367.Ar 3des 368cipher. 369Its use is strongly discouraged due to cryptographic weaknesses. 370The default is 371.Dq 3des . 372.It Cm Ciphers 373Specifies the ciphers allowed for protocol version 2 374in order of preference. 375Multiple ciphers must be comma-separated. 376If the specified value begins with a 377.Sq + 378character, then the specified ciphers will be appended to the default set 379instead of replacing them. 380.Pp 381The supported ciphers are: 382.Pp 383.Bl -item -compact -offset indent 384.It 3853des-cbc 386.It 387aes128-cbc 388.It 389aes192-cbc 390.It 391aes256-cbc 392.It 393aes128-ctr 394.It 395aes192-ctr 396.It 397aes256-ctr 398.It 399aes128-gcm@openssh.com 400.It 401aes256-gcm@openssh.com 402.It 403arcfour 404.It 405arcfour128 406.It 407arcfour256 408.It 409blowfish-cbc 410.It 411cast128-cbc 412.It 413chacha20-poly1305@openssh.com 414.El 415.Pp 416The default is: 417.Bd -literal -offset indent 418chacha20-poly1305@openssh.com, 419aes128-ctr,aes192-ctr,aes256-ctr, 420aes128-gcm@openssh.com,aes256-gcm@openssh.com, 421arcfour256,arcfour128, 422aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc, 423aes192-cbc,aes256-cbc,arcfour 424.Ed 425.Pp 426The list of available ciphers may also be obtained using the 427.Fl Q 428option of 429.Xr ssh 1 430with an argument of 431.Dq cipher . 432.It Cm ClearAllForwardings 433Specifies that all local, remote, and dynamic port forwardings 434specified in the configuration files or on the command line be 435cleared. 436This option is primarily useful when used from the 437.Xr ssh 1 438command line to clear port forwardings set in 439configuration files, and is automatically set by 440.Xr scp 1 441and 442.Xr sftp 1 . 443The argument must be 444.Dq yes 445or 446.Dq no . 447The default is 448.Dq no . 449.It Cm Compression 450Specifies whether to use compression. 451The argument must be 452.Dq yes 453or 454.Dq no . 455The default is 456.Dq no . 457.It Cm CompressionLevel 458Specifies the compression level to use if compression is enabled. 459The argument must be an integer from 1 (fast) to 9 (slow, best). 460The default level is 6, which is good for most applications. 461The meaning of the values is the same as in 462.Xr gzip 1 . 463Note that this option applies to protocol version 1 only. 464.It Cm ConnectionAttempts 465Specifies the number of tries (one per second) to make before exiting. 466The argument must be an integer. 467This may be useful in scripts if the connection sometimes fails. 468The default is 1. 469.It Cm ConnectTimeout 470Specifies the timeout (in seconds) used when connecting to the 471SSH server, instead of using the default system TCP timeout. 472This value is used only when the target is down or really unreachable, 473not when it refuses the connection. 474.It Cm ControlMaster 475Enables the sharing of multiple sessions over a single network connection. 476When set to 477.Dq yes , 478.Xr ssh 1 479will listen for connections on a control socket specified using the 480.Cm ControlPath 481argument. 482Additional sessions can connect to this socket using the same 483.Cm ControlPath 484with 485.Cm ControlMaster 486set to 487.Dq no 488(the default). 489These sessions will try to reuse the master instance's network connection 490rather than initiating new ones, but will fall back to connecting normally 491if the control socket does not exist, or is not listening. 492.Pp 493Setting this to 494.Dq ask 495will cause ssh 496to listen for control connections, but require confirmation using 497.Xr ssh-askpass 1 . 498If the 499.Cm ControlPath 500cannot be opened, 501ssh will continue without connecting to a master instance. 502.Pp 503X11 and 504.Xr ssh-agent 1 505forwarding is supported over these multiplexed connections, however the 506display and agent forwarded will be the one belonging to the master 507connection i.e. it is not possible to forward multiple displays or agents. 508.Pp 509Two additional options allow for opportunistic multiplexing: try to use a 510master connection but fall back to creating a new one if one does not already 511exist. 512These options are: 513.Dq auto 514and 515.Dq autoask . 516The latter requires confirmation like the 517.Dq ask 518option. 519.It Cm ControlPath 520Specify the path to the control socket used for connection sharing as described 521in the 522.Cm ControlMaster 523section above or the string 524.Dq none 525to disable connection sharing. 526In the path, 527.Ql %L 528will be substituted by the first component of the local host name, 529.Ql %l 530will be substituted by the local host name (including any domain name), 531.Ql %h 532will be substituted by the target host name, 533.Ql %n 534will be substituted by the original target host name 535specified on the command line, 536.Ql %p 537the destination port, 538.Ql %r 539by the remote login username, 540.Ql %u 541by the username of the user running 542.Xr ssh 1 , and 543.Ql \&%C 544by a hash of the concatenation: %l%h%p%r. 545It is recommended that any 546.Cm ControlPath 547used for opportunistic connection sharing include 548at least %h, %p, and %r (or alternatively %C) and be placed in a directory 549that is not writable by other users. 550This ensures that shared connections are uniquely identified. 551.It Cm ControlPersist 552When used in conjunction with 553.Cm ControlMaster , 554specifies that the master connection should remain open 555in the background (waiting for future client connections) 556after the initial client connection has been closed. 557If set to 558.Dq no , 559then the master connection will not be placed into the background, 560and will close as soon as the initial client connection is closed. 561If set to 562.Dq yes 563or 564.Dq 0 , 565then the master connection will remain in the background indefinitely 566(until killed or closed via a mechanism such as the 567.Xr ssh 1 568.Dq Fl O No exit 569option). 570If set to a time in seconds, or a time in any of the formats documented in 571.Xr sshd_config 5 , 572then the backgrounded master connection will automatically terminate 573after it has remained idle (with no client connections) for the 574specified time. 575.It Cm DynamicForward 576Specifies that a TCP port on the local machine be forwarded 577over the secure channel, and the application 578protocol is then used to determine where to connect to from the 579remote machine. 580.Pp 581The argument must be 582.Sm off 583.Oo Ar bind_address : Oc Ar port . 584.Sm on 585IPv6 addresses can be specified by enclosing addresses in square brackets. 586By default, the local port is bound in accordance with the 587.Cm GatewayPorts 588setting. 589However, an explicit 590.Ar bind_address 591may be used to bind the connection to a specific address. 592The 593.Ar bind_address 594of 595.Dq localhost 596indicates that the listening port be bound for local use only, while an 597empty address or 598.Sq * 599indicates that the port should be available from all interfaces. 600.Pp 601Currently the SOCKS4 and SOCKS5 protocols are supported, and 602.Xr ssh 1 603will act as a SOCKS server. 604Multiple forwardings may be specified, and 605additional forwardings can be given on the command line. 606Only the superuser can forward privileged ports. 607.It Cm EnableSSHKeysign 608Setting this option to 609.Dq yes 610in the global client configuration file 611.Pa /etc/ssh/ssh_config 612enables the use of the helper program 613.Xr ssh-keysign 8 614during 615.Cm HostbasedAuthentication . 616The argument must be 617.Dq yes 618or 619.Dq no . 620The default is 621.Dq no . 622This option should be placed in the non-hostspecific section. 623See 624.Xr ssh-keysign 8 625for more information. 626.It Cm EscapeChar 627Sets the escape character (default: 628.Ql ~ ) . 629The escape character can also 630be set on the command line. 631The argument should be a single character, 632.Ql ^ 633followed by a letter, or 634.Dq none 635to disable the escape 636character entirely (making the connection transparent for binary 637data). 638.It Cm ExitOnForwardFailure 639Specifies whether 640.Xr ssh 1 641should terminate the connection if it cannot set up all requested 642dynamic, tunnel, local, and remote port forwardings. 643The argument must be 644.Dq yes 645or 646.Dq no . 647The default is 648.Dq no . 649.It Cm FingerprintHash 650Specifies the hash algorithm used when displaying key fingerprints. 651Valid options are: 652.Dq md5 653and 654.Dq sha256 . 655The default is 656.Dq sha256 . 657.It Cm ForwardAgent 658Specifies whether the connection to the authentication agent (if any) 659will be forwarded to the remote machine. 660The argument must be 661.Dq yes 662or 663.Dq no . 664The default is 665.Dq no . 666.Pp 667Agent forwarding should be enabled with caution. 668Users with the ability to bypass file permissions on the remote host 669(for the agent's Unix-domain socket) 670can access the local agent through the forwarded connection. 671An attacker cannot obtain key material from the agent, 672however they can perform operations on the keys that enable them to 673authenticate using the identities loaded into the agent. 674.It Cm ForwardX11 675Specifies whether X11 connections will be automatically redirected 676over the secure channel and 677.Ev DISPLAY 678set. 679The argument must be 680.Dq yes 681or 682.Dq no . 683The default is 684.Dq no . 685.Pp 686X11 forwarding should be enabled with caution. 687Users with the ability to bypass file permissions on the remote host 688(for the user's X11 authorization database) 689can access the local X11 display through the forwarded connection. 690An attacker may then be able to perform activities such as keystroke monitoring 691if the 692.Cm ForwardX11Trusted 693option is also enabled. 694.It Cm ForwardX11Timeout 695Specify a timeout for untrusted X11 forwarding 696using the format described in the 697TIME FORMATS section of 698.Xr sshd_config 5 . 699X11 connections received by 700.Xr ssh 1 701after this time will be refused. 702The default is to disable untrusted X11 forwarding after twenty minutes has 703elapsed. 704.It Cm ForwardX11Trusted 705If this option is set to 706.Dq yes , 707remote X11 clients will have full access to the original X11 display. 708.Pp 709If this option is set to 710.Dq no , 711remote X11 clients will be considered untrusted and prevented 712from stealing or tampering with data belonging to trusted X11 713clients. 714Furthermore, the 715.Xr xauth 1 716token used for the session will be set to expire after 20 minutes. 717Remote clients will be refused access after this time. 718.Pp 719The default is 720.Dq no . 721.Pp 722See the X11 SECURITY extension specification for full details on 723the restrictions imposed on untrusted clients. 724.It Cm GatewayPorts 725Specifies whether remote hosts are allowed to connect to local 726forwarded ports. 727By default, 728.Xr ssh 1 729binds local port forwardings to the loopback address. 730This prevents other remote hosts from connecting to forwarded ports. 731.Cm GatewayPorts 732can be used to specify that ssh 733should bind local port forwardings to the wildcard address, 734thus allowing remote hosts to connect to forwarded ports. 735The argument must be 736.Dq yes 737or 738.Dq no . 739The default is 740.Dq no . 741.It Cm GlobalKnownHostsFile 742Specifies one or more files to use for the global 743host key database, separated by whitespace. 744The default is 745.Pa /etc/ssh/ssh_known_hosts , 746.Pa /etc/ssh/ssh_known_hosts2 . 747.It Cm GSSAPIAuthentication 748Specifies whether user authentication based on GSSAPI is allowed. 749The default is 750.Dq no . 751Note that this option applies to protocol version 2 only. 752.It Cm GSSAPIDelegateCredentials 753Forward (delegate) credentials to the server. 754The default is 755.Dq no . 756Note that this option applies to protocol version 2 only. 757.It Cm HashKnownHosts 758Indicates that 759.Xr ssh 1 760should hash host names and addresses when they are added to 761.Pa ~/.ssh/known_hosts . 762These hashed names may be used normally by 763.Xr ssh 1 764and 765.Xr sshd 8 , 766but they do not reveal identifying information should the file's contents 767be disclosed. 768The default is 769.Dq no . 770Note that existing names and addresses in known hosts files 771will not be converted automatically, 772but may be manually hashed using 773.Xr ssh-keygen 1 . 774.It Cm HostbasedAuthentication 775Specifies whether to try rhosts based authentication with public key 776authentication. 777The argument must be 778.Dq yes 779or 780.Dq no . 781The default is 782.Dq no . 783This option applies to protocol version 2 only and 784is similar to 785.Cm RhostsRSAAuthentication . 786.It Cm HostbasedKeyTypes 787Specifies the key types that will be used for hostbased authentication 788as a comma-separated pattern list. 789Alternately if the specified value begins with a 790.Sq + 791character, then the specified key types will be appended to the default set 792instead of replacing them. 793The default for this option is: 794.Bd -literal -offset 3n 795ecdsa-sha2-nistp256-cert-v01@openssh.com, 796ecdsa-sha2-nistp384-cert-v01@openssh.com, 797ecdsa-sha2-nistp521-cert-v01@openssh.com, 798ssh-ed25519-cert-v01@openssh.com, 799ssh-rsa-cert-v01@openssh.com, 800ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 801ssh-ed25519,ssh-rsa 802.Ed 803.Pp 804The 805.Fl Q 806option of 807.Xr ssh 1 808may be used to list supported key types. 809.It Cm HostKeyAlgorithms 810Specifies the protocol version 2 host key algorithms 811that the client wants to use in order of preference. 812Alternately if the specified value begins with a 813.Sq + 814character, then the specified key types will be appended to the default set 815instead of replacing them. 816The default for this option is: 817.Bd -literal -offset 3n 818ecdsa-sha2-nistp256-cert-v01@openssh.com, 819ecdsa-sha2-nistp384-cert-v01@openssh.com, 820ecdsa-sha2-nistp521-cert-v01@openssh.com, 821ssh-ed25519-cert-v01@openssh.com, 822ssh-rsa-cert-v01@openssh.com, 823ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 824ssh-ed25519,ssh-rsa 825.Ed 826.Pp 827If hostkeys are known for the destination host then this default is modified 828to prefer their algorithms. 829.Pp 830The list of available key types may also be obtained using the 831.Fl Q 832option of 833.Xr ssh 1 834with an argument of 835.Dq key . 836.It Cm HostKeyAlias 837Specifies an alias that should be used instead of the 838real host name when looking up or saving the host key 839in the host key database files. 840This option is useful for tunneling SSH connections 841or for multiple servers running on a single host. 842.It Cm HostName 843Specifies the real host name to log into. 844This can be used to specify nicknames or abbreviations for hosts. 845If the hostname contains the character sequence 846.Ql %h , 847then this will be replaced with the host name specified on the command line 848(this is useful for manipulating unqualified names). 849The character sequence 850.Ql %% 851will be replaced by a single 852.Ql % 853character, which may be used when specifying IPv6 link-local addresses. 854.Pp 855The default is the name given on the command line. 856Numeric IP addresses are also permitted (both on the command line and in 857.Cm HostName 858specifications). 859.It Cm IdentitiesOnly 860Specifies that 861.Xr ssh 1 862should only use the authentication identity files configured in the 863.Nm 864files, 865even if 866.Xr ssh-agent 1 867or a 868.Cm PKCS11Provider 869offers more identities. 870The argument to this keyword must be 871.Dq yes 872or 873.Dq no . 874This option is intended for situations where ssh-agent 875offers many different identities. 876The default is 877.Dq no . 878.It Cm IdentityFile 879Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication 880identity is read. 881The default is 882.Pa ~/.ssh/identity 883for protocol version 1, and 884.Pa ~/.ssh/id_dsa , 885.Pa ~/.ssh/id_ecdsa , 886.Pa ~/.ssh/id_ed25519 887and 888.Pa ~/.ssh/id_rsa 889for protocol version 2. 890Additionally, any identities represented by the authentication agent 891will be used for authentication unless 892.Cm IdentitiesOnly 893is set. 894.Xr ssh 1 895will try to load certificate information from the filename obtained by 896appending 897.Pa -cert.pub 898to the path of a specified 899.Cm IdentityFile . 900.Pp 901The file name may use the tilde 902syntax to refer to a user's home directory or one of the following 903escape characters: 904.Ql %d 905(local user's home directory), 906.Ql %u 907(local user name), 908.Ql %l 909(local host name), 910.Ql %h 911(remote host name) or 912.Ql %r 913(remote user name). 914.Pp 915It is possible to have 916multiple identity files specified in configuration files; all these 917identities will be tried in sequence. 918Multiple 919.Cm IdentityFile 920directives will add to the list of identities tried (this behaviour 921differs from that of other configuration directives). 922.Pp 923.Cm IdentityFile 924may be used in conjunction with 925.Cm IdentitiesOnly 926to select which identities in an agent are offered during authentication. 927.It Cm IgnoreUnknown 928Specifies a pattern-list of unknown options to be ignored if they are 929encountered in configuration parsing. 930This may be used to suppress errors if 931.Nm 932contains options that are unrecognised by 933.Xr ssh 1 . 934It is recommended that 935.Cm IgnoreUnknown 936be listed early in the configuration file as it will not be applied 937to unknown options that appear before it. 938.It Cm IPQoS 939Specifies the IPv4 type-of-service or DSCP class for connections. 940Accepted values are 941.Dq af11 , 942.Dq af12 , 943.Dq af13 , 944.Dq af21 , 945.Dq af22 , 946.Dq af23 , 947.Dq af31 , 948.Dq af32 , 949.Dq af33 , 950.Dq af41 , 951.Dq af42 , 952.Dq af43 , 953.Dq cs0 , 954.Dq cs1 , 955.Dq cs2 , 956.Dq cs3 , 957.Dq cs4 , 958.Dq cs5 , 959.Dq cs6 , 960.Dq cs7 , 961.Dq ef , 962.Dq lowdelay , 963.Dq throughput , 964.Dq reliability , 965or a numeric value. 966This option may take one or two arguments, separated by whitespace. 967If one argument is specified, it is used as the packet class unconditionally. 968If two values are specified, the first is automatically selected for 969interactive sessions and the second for non-interactive sessions. 970The default is 971.Dq lowdelay 972for interactive sessions and 973.Dq throughput 974for non-interactive sessions. 975.It Cm KbdInteractiveAuthentication 976Specifies whether to use keyboard-interactive authentication. 977The argument to this keyword must be 978.Dq yes 979or 980.Dq no . 981The default is 982.Dq yes . 983.It Cm KbdInteractiveDevices 984Specifies the list of methods to use in keyboard-interactive authentication. 985Multiple method names must be comma-separated. 986The default is to use the server specified list. 987The methods available vary depending on what the server supports. 988For an OpenSSH server, 989it may be zero or more of: 990.Dq bsdauth , 991.Dq pam , 992and 993.Dq skey . 994.It Cm KexAlgorithms 995Specifies the available KEX (Key Exchange) algorithms. 996Multiple algorithms must be comma-separated. 997Alternately if the specified value begins with a 998.Sq + 999character, then the specified methods will be appended to the default set 1000instead of replacing them. 1001The default is: 1002.Bd -literal -offset indent 1003curve25519-sha256@libssh.org, 1004ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1005diffie-hellman-group-exchange-sha256, 1006diffie-hellman-group-exchange-sha1, 1007diffie-hellman-group14-sha1 1008.Ed 1009.Pp 1010The list of available key exchange algorithms may also be obtained using the 1011.Fl Q 1012option of 1013.Xr ssh 1 1014with an argument of 1015.Dq kex . 1016.It Cm LocalCommand 1017Specifies a command to execute on the local machine after successfully 1018connecting to the server. 1019The command string extends to the end of the line, and is executed with 1020the user's shell. 1021The following escape character substitutions will be performed: 1022.Ql %d 1023(local user's home directory), 1024.Ql %h 1025(remote host name), 1026.Ql %l 1027(local host name), 1028.Ql %n 1029(host name as provided on the command line), 1030.Ql %p 1031(remote port), 1032.Ql %r 1033(remote user name) or 1034.Ql %u 1035(local user name) or 1036.Ql \&%C 1037by a hash of the concatenation: %l%h%p%r. 1038.Pp 1039The command is run synchronously and does not have access to the 1040session of the 1041.Xr ssh 1 1042that spawned it. 1043It should not be used for interactive commands. 1044.Pp 1045This directive is ignored unless 1046.Cm PermitLocalCommand 1047has been enabled. 1048.It Cm LocalForward 1049Specifies that a TCP port on the local machine be forwarded over 1050the secure channel to the specified host and port from the remote machine. 1051The first argument must be 1052.Sm off 1053.Oo Ar bind_address : Oc Ar port 1054.Sm on 1055and the second argument must be 1056.Ar host : Ns Ar hostport . 1057IPv6 addresses can be specified by enclosing addresses in square brackets. 1058Multiple forwardings may be specified, and additional forwardings can be 1059given on the command line. 1060Only the superuser can forward privileged ports. 1061By default, the local port is bound in accordance with the 1062.Cm GatewayPorts 1063setting. 1064However, an explicit 1065.Ar bind_address 1066may be used to bind the connection to a specific address. 1067The 1068.Ar bind_address 1069of 1070.Dq localhost 1071indicates that the listening port be bound for local use only, while an 1072empty address or 1073.Sq * 1074indicates that the port should be available from all interfaces. 1075.It Cm LogLevel 1076Gives the verbosity level that is used when logging messages from 1077.Xr ssh 1 . 1078The possible values are: 1079QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1080The default is INFO. 1081DEBUG and DEBUG1 are equivalent. 1082DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1083.It Cm MACs 1084Specifies the MAC (message authentication code) algorithms 1085in order of preference. 1086The MAC algorithm is used in protocol version 2 1087for data integrity protection. 1088Multiple algorithms must be comma-separated. 1089If the specified value begins with a 1090.Sq + 1091character, then the specified algorithms will be appended to the default set 1092instead of replacing them. 1093.Pp 1094The algorithms that contain 1095.Dq -etm 1096calculate the MAC after encryption (encrypt-then-mac). 1097These are considered safer and their use recommended. 1098.Pp 1099The default is: 1100.Bd -literal -offset indent 1101umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1102hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1103umac-64@openssh.com,umac-128@openssh.com, 1104hmac-sha2-256,hmac-sha2-512, 1105hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com, 1106hmac-ripemd160-etm@openssh.com, 1107hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com, 1108hmac-md5,hmac-sha1,hmac-ripemd160, 1109hmac-sha1-96,hmac-md5-96 1110.Ed 1111.Pp 1112The list of available MAC algorithms may also be obtained using the 1113.Fl Q 1114option of 1115.Xr ssh 1 1116with an argument of 1117.Dq mac . 1118.It Cm NoHostAuthenticationForLocalhost 1119This option can be used if the home directory is shared across machines. 1120In this case localhost will refer to a different machine on each of 1121the machines and the user will get many warnings about changed host keys. 1122However, this option disables host authentication for localhost. 1123The argument to this keyword must be 1124.Dq yes 1125or 1126.Dq no . 1127The default is to check the host key for localhost. 1128.It Cm NumberOfPasswordPrompts 1129Specifies the number of password prompts before giving up. 1130The argument to this keyword must be an integer. 1131The default is 3. 1132.It Cm PasswordAuthentication 1133Specifies whether to use password authentication. 1134The argument to this keyword must be 1135.Dq yes 1136or 1137.Dq no . 1138The default is 1139.Dq yes . 1140.It Cm PermitLocalCommand 1141Allow local command execution via the 1142.Ic LocalCommand 1143option or using the 1144.Ic !\& Ns Ar command 1145escape sequence in 1146.Xr ssh 1 . 1147The argument must be 1148.Dq yes 1149or 1150.Dq no . 1151The default is 1152.Dq no . 1153.It Cm PKCS11Provider 1154Specifies which PKCS#11 provider to use. 1155The argument to this keyword is the PKCS#11 shared library 1156.Xr ssh 1 1157should use to communicate with a PKCS#11 token providing the user's 1158private RSA key. 1159.It Cm Port 1160Specifies the port number to connect on the remote host. 1161The default is 22. 1162.It Cm PreferredAuthentications 1163Specifies the order in which the client should try protocol 2 1164authentication methods. 1165This allows a client to prefer one method (e.g.\& 1166.Cm keyboard-interactive ) 1167over another method (e.g.\& 1168.Cm password ) . 1169The default is: 1170.Bd -literal -offset indent 1171gssapi-with-mic,hostbased,publickey, 1172keyboard-interactive,password 1173.Ed 1174.It Cm Protocol 1175Specifies the protocol versions 1176.Xr ssh 1 1177should support in order of preference. 1178The possible values are 1179.Sq 1 1180and 1181.Sq 2 . 1182Multiple versions must be comma-separated. 1183When this option is set to 1184.Dq 2,1 1185.Nm ssh 1186will try version 2 and fall back to version 1 1187if version 2 is not available. 1188The default is 1189.Sq 2 . 1190.It Cm ProxyCommand 1191Specifies the command to use to connect to the server. 1192The command 1193string extends to the end of the line, and is executed 1194using the user's shell 1195.Ql exec 1196directive to avoid a lingering shell process. 1197.Pp 1198In the command string, any occurrence of 1199.Ql %h 1200will be substituted by the host name to 1201connect, 1202.Ql %p 1203by the port, and 1204.Ql %r 1205by the remote user name. 1206The command can be basically anything, 1207and should read from its standard input and write to its standard output. 1208It should eventually connect an 1209.Xr sshd 8 1210server running on some machine, or execute 1211.Ic sshd -i 1212somewhere. 1213Host key management will be done using the 1214HostName of the host being connected (defaulting to the name typed by 1215the user). 1216Setting the command to 1217.Dq none 1218disables this option entirely. 1219Note that 1220.Cm CheckHostIP 1221is not available for connects with a proxy command. 1222.Pp 1223This directive is useful in conjunction with 1224.Xr nc 1 1225and its proxy support. 1226For example, the following directive would connect via an HTTP proxy at 1227192.0.2.0: 1228.Bd -literal -offset 3n 1229ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1230.Ed 1231.It Cm ProxyUseFdpass 1232Specifies that 1233.Cm ProxyCommand 1234will pass a connected file descriptor back to 1235.Xr ssh 1 1236instead of continuing to execute and pass data. 1237The default is 1238.Dq no . 1239.It Cm PubkeyAcceptedKeyTypes 1240Specifies the key types that will be used for public key authentication 1241as a comma-separated pattern list. 1242Alternately if the specified value begins with a 1243.Sq + 1244character, then the key types after it will be appended to the default 1245instead of replacing it. 1246The default for this option is: 1247.Bd -literal -offset 3n 1248ecdsa-sha2-nistp256-cert-v01@openssh.com, 1249ecdsa-sha2-nistp384-cert-v01@openssh.com, 1250ecdsa-sha2-nistp521-cert-v01@openssh.com, 1251ssh-ed25519-cert-v01@openssh.com, 1252ssh-rsa-cert-v01@openssh.com, 1253ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1254ssh-ed25519,ssh-rsa 1255.Ed 1256.Pp 1257The 1258.Fl Q 1259option of 1260.Xr ssh 1 1261may be used to list supported key types. 1262.It Cm PubkeyAuthentication 1263Specifies whether to try public key authentication. 1264The argument to this keyword must be 1265.Dq yes 1266or 1267.Dq no . 1268The default is 1269.Dq yes . 1270This option applies to protocol version 2 only. 1271.It Cm RekeyLimit 1272Specifies the maximum amount of data that may be transmitted before the 1273session key is renegotiated, optionally followed a maximum amount of 1274time that may pass before the session key is renegotiated. 1275The first argument is specified in bytes and may have a suffix of 1276.Sq K , 1277.Sq M , 1278or 1279.Sq G 1280to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1281The default is between 1282.Sq 1G 1283and 1284.Sq 4G , 1285depending on the cipher. 1286The optional second value is specified in seconds and may use any of the 1287units documented in the 1288TIME FORMATS section of 1289.Xr sshd_config 5 . 1290The default value for 1291.Cm RekeyLimit 1292is 1293.Dq default none , 1294which means that rekeying is performed after the cipher's default amount 1295of data has been sent or received and no time based rekeying is done. 1296This option applies to protocol version 2 only. 1297.It Cm RemoteForward 1298Specifies that a TCP port on the remote machine be forwarded over 1299the secure channel to the specified host and port from the local machine. 1300The first argument must be 1301.Sm off 1302.Oo Ar bind_address : Oc Ar port 1303.Sm on 1304and the second argument must be 1305.Ar host : Ns Ar hostport . 1306IPv6 addresses can be specified by enclosing addresses in square brackets. 1307Multiple forwardings may be specified, and additional 1308forwardings can be given on the command line. 1309Privileged ports can be forwarded only when 1310logging in as root on the remote machine. 1311.Pp 1312If the 1313.Ar port 1314argument is 1315.Ql 0 , 1316the listen port will be dynamically allocated on the server and reported 1317to the client at run time. 1318.Pp 1319If the 1320.Ar bind_address 1321is not specified, the default is to only bind to loopback addresses. 1322If the 1323.Ar bind_address 1324is 1325.Ql * 1326or an empty string, then the forwarding is requested to listen on all 1327interfaces. 1328Specifying a remote 1329.Ar bind_address 1330will only succeed if the server's 1331.Cm GatewayPorts 1332option is enabled (see 1333.Xr sshd_config 5 ) . 1334.It Cm RequestTTY 1335Specifies whether to request a pseudo-tty for the session. 1336The argument may be one of: 1337.Dq no 1338(never request a TTY), 1339.Dq yes 1340(always request a TTY when standard input is a TTY), 1341.Dq force 1342(always request a TTY) or 1343.Dq auto 1344(request a TTY when opening a login session). 1345This option mirrors the 1346.Fl t 1347and 1348.Fl T 1349flags for 1350.Xr ssh 1 . 1351.It Cm RevokedHostKeys 1352Specifies revoked host public keys. 1353Keys listed in this file will be refused for host authentication. 1354Note that if this file does not exist or is not readable, 1355then host authentication will be refused for all hosts. 1356Keys may be specified as a text file, listing one public key per line, or as 1357an OpenSSH Key Revocation List (KRL) as generated by 1358.Xr ssh-keygen 1 . 1359For more information on KRLs, see the KEY REVOCATION LISTS section in 1360.Xr ssh-keygen 1 . 1361.It Cm RhostsRSAAuthentication 1362Specifies whether to try rhosts based authentication with RSA host 1363authentication. 1364The argument must be 1365.Dq yes 1366or 1367.Dq no . 1368The default is 1369.Dq no . 1370This option applies to protocol version 1 only and requires 1371.Xr ssh 1 1372to be setuid root. 1373.It Cm RSAAuthentication 1374Specifies whether to try RSA authentication. 1375The argument to this keyword must be 1376.Dq yes 1377or 1378.Dq no . 1379RSA authentication will only be 1380attempted if the identity file exists, or an authentication agent is 1381running. 1382The default is 1383.Dq yes . 1384Note that this option applies to protocol version 1 only. 1385.It Cm SendEnv 1386Specifies what variables from the local 1387.Xr environ 7 1388should be sent to the server. 1389Note that environment passing is only supported for protocol 2. 1390The server must also support it, and the server must be configured to 1391accept these environment variables. 1392Note that the 1393.Ev TERM 1394environment variable is always sent whenever a 1395pseudo-terminal is requested as it is required by the protocol. 1396Refer to 1397.Cm AcceptEnv 1398in 1399.Xr sshd_config 5 1400for how to configure the server. 1401Variables are specified by name, which may contain wildcard characters. 1402Multiple environment variables may be separated by whitespace or spread 1403across multiple 1404.Cm SendEnv 1405directives. 1406The default is not to send any environment variables. 1407.Pp 1408See 1409.Sx PATTERNS 1410for more information on patterns. 1411.It Cm ServerAliveCountMax 1412Sets the number of server alive messages (see below) which may be 1413sent without 1414.Xr ssh 1 1415receiving any messages back from the server. 1416If this threshold is reached while server alive messages are being sent, 1417ssh will disconnect from the server, terminating the session. 1418It is important to note that the use of server alive messages is very 1419different from 1420.Cm TCPKeepAlive 1421(below). 1422The server alive messages are sent through the encrypted channel 1423and therefore will not be spoofable. 1424The TCP keepalive option enabled by 1425.Cm TCPKeepAlive 1426is spoofable. 1427The server alive mechanism is valuable when the client or 1428server depend on knowing when a connection has become inactive. 1429.Pp 1430The default value is 3. 1431If, for example, 1432.Cm ServerAliveInterval 1433(see below) is set to 15 and 1434.Cm ServerAliveCountMax 1435is left at the default, if the server becomes unresponsive, 1436ssh will disconnect after approximately 45 seconds. 1437This option applies to protocol version 2 only. 1438.It Cm ServerAliveInterval 1439Sets a timeout interval in seconds after which if no data has been received 1440from the server, 1441.Xr ssh 1 1442will send a message through the encrypted 1443channel to request a response from the server. 1444The default 1445is 0, indicating that these messages will not be sent to the server. 1446This option applies to protocol version 2 only. 1447.It Cm StreamLocalBindMask 1448Sets the octal file creation mode mask 1449.Pq umask 1450used when creating a Unix-domain socket file for local or remote 1451port forwarding. 1452This option is only used for port forwarding to a Unix-domain socket file. 1453.Pp 1454The default value is 0177, which creates a Unix-domain socket file that is 1455readable and writable only by the owner. 1456Note that not all operating systems honor the file mode on Unix-domain 1457socket files. 1458.It Cm StreamLocalBindUnlink 1459Specifies whether to remove an existing Unix-domain socket file for local 1460or remote port forwarding before creating a new one. 1461If the socket file already exists and 1462.Cm StreamLocalBindUnlink 1463is not enabled, 1464.Nm ssh 1465will be unable to forward the port to the Unix-domain socket file. 1466This option is only used for port forwarding to a Unix-domain socket file. 1467.Pp 1468The argument must be 1469.Dq yes 1470or 1471.Dq no . 1472The default is 1473.Dq no . 1474.It Cm StrictHostKeyChecking 1475If this flag is set to 1476.Dq yes , 1477.Xr ssh 1 1478will never automatically add host keys to the 1479.Pa ~/.ssh/known_hosts 1480file, and refuses to connect to hosts whose host key has changed. 1481This provides maximum protection against trojan horse attacks, 1482though it can be annoying when the 1483.Pa /etc/ssh/ssh_known_hosts 1484file is poorly maintained or when connections to new hosts are 1485frequently made. 1486This option forces the user to manually 1487add all new hosts. 1488If this flag is set to 1489.Dq no , 1490ssh will automatically add new host keys to the 1491user known hosts files. 1492If this flag is set to 1493.Dq ask , 1494new host keys 1495will be added to the user known host files only after the user 1496has confirmed that is what they really want to do, and 1497ssh will refuse to connect to hosts whose host key has changed. 1498The host keys of 1499known hosts will be verified automatically in all cases. 1500The argument must be 1501.Dq yes , 1502.Dq no , 1503or 1504.Dq ask . 1505The default is 1506.Dq ask . 1507.It Cm TCPKeepAlive 1508Specifies whether the system should send TCP keepalive messages to the 1509other side. 1510If they are sent, death of the connection or crash of one 1511of the machines will be properly noticed. 1512However, this means that 1513connections will die if the route is down temporarily, and some people 1514find it annoying. 1515.Pp 1516The default is 1517.Dq yes 1518(to send TCP keepalive messages), and the client will notice 1519if the network goes down or the remote host dies. 1520This is important in scripts, and many users want it too. 1521.Pp 1522To disable TCP keepalive messages, the value should be set to 1523.Dq no . 1524.It Cm Tunnel 1525Request 1526.Xr tun 4 1527device forwarding between the client and the server. 1528The argument must be 1529.Dq yes , 1530.Dq point-to-point 1531(layer 3), 1532.Dq ethernet 1533(layer 2), 1534or 1535.Dq no . 1536Specifying 1537.Dq yes 1538requests the default tunnel mode, which is 1539.Dq point-to-point . 1540The default is 1541.Dq no . 1542.It Cm TunnelDevice 1543Specifies the 1544.Xr tun 4 1545devices to open on the client 1546.Pq Ar local_tun 1547and the server 1548.Pq Ar remote_tun . 1549.Pp 1550The argument must be 1551.Sm off 1552.Ar local_tun Op : Ar remote_tun . 1553.Sm on 1554The devices may be specified by numerical ID or the keyword 1555.Dq any , 1556which uses the next available tunnel device. 1557If 1558.Ar remote_tun 1559is not specified, it defaults to 1560.Dq any . 1561The default is 1562.Dq any:any . 1563.It Cm UpdateHostKeys 1564Specifies whether 1565.Xr ssh 1 1566should accept notifications of additional hostkeys from the server sent 1567after authentication has completed and add them to 1568.Cm UserKnownHostsFile . 1569The argument must be 1570.Dq yes , 1571.Dq no 1572(the default) or 1573.Dq ask . 1574Enabling this option allows learning alternate hostkeys for a server 1575and supports graceful key rotation by allowing a server to send replacement 1576public keys before old ones are removed. 1577Additional hostkeys are only accepted if the key used to authenticate the 1578host was already trusted or explicity accepted by the user. 1579If 1580.Cm UpdateHostKeys 1581is set to 1582.Dq ask , 1583then the user is asked to confirm the modifications to the known_hosts file. 1584Confirmation is currently incompatible with 1585.Cm ControlPersist , 1586and will be disabled if it is enabled. 1587.Pp 1588Presently, only 1589.Xr sshd 8 1590from OpenSSH 6.8 and greater support the 1591.Dq hostkeys@openssh.com 1592protocol extension used to inform the client of all the server's hostkeys. 1593.It Cm UsePrivilegedPort 1594Specifies whether to use a privileged port for outgoing connections. 1595The argument must be 1596.Dq yes 1597or 1598.Dq no . 1599The default is 1600.Dq no . 1601If set to 1602.Dq yes , 1603.Xr ssh 1 1604must be setuid root. 1605Note that this option must be set to 1606.Dq yes 1607for 1608.Cm RhostsRSAAuthentication 1609with older servers. 1610.It Cm User 1611Specifies the user to log in as. 1612This can be useful when a different user name is used on different machines. 1613This saves the trouble of 1614having to remember to give the user name on the command line. 1615.It Cm UserKnownHostsFile 1616Specifies one or more files to use for the user 1617host key database, separated by whitespace. 1618The default is 1619.Pa ~/.ssh/known_hosts , 1620.Pa ~/.ssh/known_hosts2 . 1621.It Cm VerifyHostKeyDNS 1622Specifies whether to verify the remote key using DNS and SSHFP resource 1623records. 1624If this option is set to 1625.Dq yes , 1626the client will implicitly trust keys that match a secure fingerprint 1627from DNS. 1628Insecure fingerprints will be handled as if this option was set to 1629.Dq ask . 1630If this option is set to 1631.Dq ask , 1632information on fingerprint match will be displayed, but the user will still 1633need to confirm new host keys according to the 1634.Cm StrictHostKeyChecking 1635option. 1636The argument must be 1637.Dq yes , 1638.Dq no , 1639or 1640.Dq ask . 1641The default is 1642.Dq no . 1643Note that this option applies to protocol version 2 only. 1644.Pp 1645See also VERIFYING HOST KEYS in 1646.Xr ssh 1 . 1647.It Cm VisualHostKey 1648If this flag is set to 1649.Dq yes , 1650an ASCII art representation of the remote host key fingerprint is 1651printed in addition to the fingerprint string at login and 1652for unknown host keys. 1653If this flag is set to 1654.Dq no , 1655no fingerprint strings are printed at login and 1656only the fingerprint string will be printed for unknown host keys. 1657The default is 1658.Dq no . 1659.It Cm XAuthLocation 1660Specifies the full pathname of the 1661.Xr xauth 1 1662program. 1663The default is 1664.Pa /usr/X11R6/bin/xauth . 1665.El 1666.Sh PATTERNS 1667A 1668.Em pattern 1669consists of zero or more non-whitespace characters, 1670.Sq * 1671(a wildcard that matches zero or more characters), 1672or 1673.Sq ?\& 1674(a wildcard that matches exactly one character). 1675For example, to specify a set of declarations for any host in the 1676.Dq .co.uk 1677set of domains, 1678the following pattern could be used: 1679.Pp 1680.Dl Host *.co.uk 1681.Pp 1682The following pattern 1683would match any host in the 192.168.0.[0-9] network range: 1684.Pp 1685.Dl Host 192.168.0.? 1686.Pp 1687A 1688.Em pattern-list 1689is a comma-separated list of patterns. 1690Patterns within pattern-lists may be negated 1691by preceding them with an exclamation mark 1692.Pq Sq !\& . 1693For example, 1694to allow a key to be used from anywhere within an organization 1695except from the 1696.Dq dialup 1697pool, 1698the following entry (in authorized_keys) could be used: 1699.Pp 1700.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1701.Sh FILES 1702.Bl -tag -width Ds 1703.It Pa ~/.ssh/config 1704This is the per-user configuration file. 1705The format of this file is described above. 1706This file is used by the SSH client. 1707Because of the potential for abuse, this file must have strict permissions: 1708read/write for the user, and not accessible by others. 1709.It Pa /etc/ssh/ssh_config 1710Systemwide configuration file. 1711This file provides defaults for those 1712values that are not specified in the user's configuration file, and 1713for those users who do not have a configuration file. 1714This file must be world-readable. 1715.El 1716.Sh SEE ALSO 1717.Xr ssh 1 1718.Sh AUTHORS 1719OpenSSH is a derivative of the original and free 1720ssh 1.2.12 release by Tatu Ylonen. 1721Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1722Theo de Raadt and Dug Song 1723removed many bugs, re-added newer features and 1724created OpenSSH. 1725Markus Friedl contributed the support for SSH 1726protocol versions 1.5 and 2.0. 1727