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 1.292 2019/03/01 02:16:47 djm Exp $ 37.Dd $Mdocdate: March 1 2019 $ 38.Dt SSH_CONFIG 5 39.Os 40.Sh NAME 41.Nm ssh_config 42.Nd OpenSSH SSH client configuration files 43.Sh DESCRIPTION 44.Xr ssh 1 45obtains configuration data from the following sources in 46the following order: 47.Pp 48.Bl -enum -offset indent -compact 49.It 50command-line options 51.It 52user's configuration file 53.Pq Pa ~/.ssh/config 54.It 55system-wide configuration file 56.Pq Pa /etc/ssh/ssh_config 57.El 58.Pp 59For each parameter, the first obtained value 60will be used. 61The configuration files contain sections separated by 62.Cm Host 63specifications, and that section is only applied for hosts that 64match one of the patterns given in the specification. 65The matched host name is usually the one given on the command line 66(see the 67.Cm CanonicalizeHostname 68option for exceptions). 69.Pp 70Since the first obtained value for each parameter is used, more 71host-specific declarations should be given near the beginning of the 72file, and general defaults at the end. 73.Pp 74The file contains keyword-argument pairs, one per line. 75Lines starting with 76.Ql # 77and empty lines are interpreted as comments. 78Arguments may optionally be enclosed in double quotes 79.Pq \&" 80in order to represent arguments containing spaces. 81Configuration options may be separated by whitespace or 82optional whitespace and exactly one 83.Ql = ; 84the latter format is useful to avoid the need to quote whitespace 85when specifying configuration options using the 86.Nm ssh , 87.Nm scp , 88and 89.Nm sftp 90.Fl o 91option. 92.Pp 93The possible 94keywords and their meanings are as follows (note that 95keywords are case-insensitive and arguments are case-sensitive): 96.Bl -tag -width Ds 97.It Cm Host 98Restricts the following declarations (up to the next 99.Cm Host 100or 101.Cm Match 102keyword) to be only for those hosts that match one of the patterns 103given after the keyword. 104If more than one pattern is provided, they should be separated by whitespace. 105A single 106.Ql * 107as a pattern can be used to provide global 108defaults for all hosts. 109The host is usually the 110.Ar hostname 111argument given on the command line 112(see the 113.Cm CanonicalizeHostname 114keyword for exceptions). 115.Pp 116A pattern entry may be negated by prefixing it with an exclamation mark 117.Pq Sq !\& . 118If a negated entry is matched, then the 119.Cm Host 120entry is ignored, regardless of whether any other patterns on the line 121match. 122Negated matches are therefore useful to provide exceptions for wildcard 123matches. 124.Pp 125See 126.Sx PATTERNS 127for more information on patterns. 128.It Cm Match 129Restricts the following declarations (up to the next 130.Cm Host 131or 132.Cm Match 133keyword) to be used only when the conditions following the 134.Cm Match 135keyword are satisfied. 136Match conditions are specified using one or more criteria 137or the single token 138.Cm all 139which always matches. 140The available criteria keywords are: 141.Cm canonical , 142.Cm final , 143.Cm exec , 144.Cm host , 145.Cm originalhost , 146.Cm user , 147and 148.Cm localuser . 149The 150.Cm all 151criteria must appear alone or immediately after 152.Cm canonical 153or 154.Cm final . 155Other criteria may be combined arbitrarily. 156All criteria but 157.Cm all , 158.Cm canonical , 159and 160.Cm final 161require an argument. 162Criteria may be negated by prepending an exclamation mark 163.Pq Sq !\& . 164.Pp 165The 166.Cm canonical 167keyword matches only when the configuration file is being re-parsed 168after hostname canonicalization (see the 169.Cm CanonicalizeHostname 170option). 171This may be useful to specify conditions that work with canonical host 172names only. 173.Pp 174The 175.Cm final 176keyword requests that the configuration be re-parsed (regardless of whether 177.Cm CanonicalizeHostname 178is enabled), and matches only during this final pass. 179If 180.Cm CanonicalizeHostname 181is enabled, then 182.Cm canonical 183and 184.Cm final 185match during the same pass. 186.Pp 187The 188.Cm exec 189keyword executes the specified command under the user's shell. 190If the command returns a zero exit status then the condition is considered true. 191Commands containing whitespace characters must be quoted. 192Arguments to 193.Cm exec 194accept the tokens described in the 195.Sx TOKENS 196section. 197.Pp 198The other keywords' criteria must be single entries or comma-separated 199lists and may use the wildcard and negation operators described in the 200.Sx PATTERNS 201section. 202The criteria for the 203.Cm host 204keyword are matched against the target hostname, after any substitution 205by the 206.Cm Hostname 207or 208.Cm CanonicalizeHostname 209options. 210The 211.Cm originalhost 212keyword matches against the hostname as it was specified on the command-line. 213The 214.Cm user 215keyword matches against the target username on the remote host. 216The 217.Cm localuser 218keyword matches against the name of the local user running 219.Xr ssh 1 220(this keyword may be useful in system-wide 221.Nm 222files). 223.It Cm AddKeysToAgent 224Specifies whether keys should be automatically added to a running 225.Xr ssh-agent 1 . 226If this option is set to 227.Cm yes 228and a key is loaded from a file, the key and its passphrase are added to 229the agent with the default lifetime, as if by 230.Xr ssh-add 1 . 231If this option is set to 232.Cm ask , 233.Xr ssh 1 234will require confirmation using the 235.Ev SSH_ASKPASS 236program before adding a key (see 237.Xr ssh-add 1 238for details). 239If this option is set to 240.Cm confirm , 241each use of the key must be confirmed, as if the 242.Fl c 243option was specified to 244.Xr ssh-add 1 . 245If this option is set to 246.Cm no , 247no keys are added to the agent. 248The argument must be 249.Cm yes , 250.Cm confirm , 251.Cm ask , 252or 253.Cm no 254(the default). 255.It Cm AddressFamily 256Specifies which address family to use when connecting. 257Valid arguments are 258.Cm any 259(the default), 260.Cm inet 261(use IPv4 only), or 262.Cm inet6 263(use IPv6 only). 264.It Cm BatchMode 265If set to 266.Cm yes , 267passphrase/password querying will be disabled. 268This option is useful in scripts and other batch jobs where no user 269is present to supply the password. 270The argument must be 271.Cm yes 272or 273.Cm no 274(the default). 275.It Cm BindAddress 276Use the specified address on the local machine as the source address of 277the connection. 278Only useful on systems with more than one address. 279.It Cm BindInterface 280Use the address of the specified interface on the local machine as the 281source address of the connection. 282.It Cm CanonicalDomains 283When 284.Cm CanonicalizeHostname 285is enabled, this option specifies the list of domain suffixes in which to 286search for the specified destination host. 287.It Cm CanonicalizeFallbackLocal 288Specifies whether to fail with an error when hostname canonicalization fails. 289The default, 290.Cm yes , 291will attempt to look up the unqualified hostname using the system resolver's 292search rules. 293A value of 294.Cm no 295will cause 296.Xr ssh 1 297to fail instantly if 298.Cm CanonicalizeHostname 299is enabled and the target hostname cannot be found in any of the domains 300specified by 301.Cm CanonicalDomains . 302.It Cm CanonicalizeHostname 303Controls whether explicit hostname canonicalization is performed. 304The default, 305.Cm no , 306is not to perform any name rewriting and let the system resolver handle all 307hostname lookups. 308If set to 309.Cm yes 310then, for connections that do not use a 311.Cm ProxyCommand 312or 313.Cm ProxyJump , 314.Xr ssh 1 315will attempt to canonicalize the hostname specified on the command line 316using the 317.Cm CanonicalDomains 318suffixes and 319.Cm CanonicalizePermittedCNAMEs 320rules. 321If 322.Cm CanonicalizeHostname 323is set to 324.Cm always , 325then canonicalization is applied to proxied connections too. 326.Pp 327If this option is enabled, then the configuration files are processed 328again using the new target name to pick up any new configuration in matching 329.Cm Host 330and 331.Cm Match 332stanzas. 333.It Cm CanonicalizeMaxDots 334Specifies the maximum number of dot characters in a hostname before 335canonicalization is disabled. 336The default, 1, 337allows a single dot (i.e. hostname.subdomain). 338.It Cm CanonicalizePermittedCNAMEs 339Specifies rules to determine whether CNAMEs should be followed when 340canonicalizing hostnames. 341The rules consist of one or more arguments of 342.Ar source_domain_list : Ns Ar target_domain_list , 343where 344.Ar source_domain_list 345is a pattern-list of domains that may follow CNAMEs in canonicalization, 346and 347.Ar target_domain_list 348is a pattern-list of domains that they may resolve to. 349.Pp 350For example, 351.Qq *.a.example.com:*.b.example.com,*.c.example.com 352will allow hostnames matching 353.Qq *.a.example.com 354to be canonicalized to names in the 355.Qq *.b.example.com 356or 357.Qq *.c.example.com 358domains. 359.It Cm CASignatureAlgorithms 360Specifies which algorithms are allowed for signing of certificates 361by certificate authorities (CAs). 362The default is: 363.Bd -literal -offset indent 364ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 365ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 366.Ed 367.Pp 368.Xr ssh 1 369will not accept host certificates signed using algorithms other than those 370specified. 371.It Cm CertificateFile 372Specifies a file from which the user's certificate is read. 373A corresponding private key must be provided separately in order 374to use this certificate either 375from an 376.Cm IdentityFile 377directive or 378.Fl i 379flag to 380.Xr ssh 1 , 381via 382.Xr ssh-agent 1 , 383or via a 384.Cm PKCS11Provider . 385.Pp 386Arguments to 387.Cm CertificateFile 388may use the tilde syntax to refer to a user's home directory 389or the tokens described in the 390.Sx TOKENS 391section. 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.Cm yes 403(the default) 404or 405.Cm no . 406.It Cm CheckHostIP 407If set to 408.Cm yes 409(the default), 410.Xr ssh 1 411will additionally check the host IP address in the 412.Pa known_hosts 413file. 414This allows it 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.Cm no , 421the check will not be executed. 422.It Cm Ciphers 423Specifies the ciphers allowed and their order of preference. 424Multiple ciphers must be comma-separated. 425If the specified value begins with a 426.Sq + 427character, then the specified ciphers will be appended to the default set 428instead of replacing them. 429If the specified value begins with a 430.Sq - 431character, then the specified ciphers (including wildcards) will be removed 432from the default set instead of replacing them. 433.Pp 434The supported ciphers are: 435.Bd -literal -offset indent 4363des-cbc 437aes128-cbc 438aes192-cbc 439aes256-cbc 440aes128-ctr 441aes192-ctr 442aes256-ctr 443aes128-gcm@openssh.com 444aes256-gcm@openssh.com 445chacha20-poly1305@openssh.com 446.Ed 447.Pp 448The default is: 449.Bd -literal -offset indent 450chacha20-poly1305@openssh.com, 451aes128-ctr,aes192-ctr,aes256-ctr, 452aes128-gcm@openssh.com,aes256-gcm@openssh.com 453.Ed 454.Pp 455The list of available ciphers may also be obtained using 456.Qq ssh -Q cipher . 457.It Cm ClearAllForwardings 458Specifies that all local, remote, and dynamic port forwardings 459specified in the configuration files or on the command line be 460cleared. 461This option is primarily useful when used from the 462.Xr ssh 1 463command line to clear port forwardings set in 464configuration files, and is automatically set by 465.Xr scp 1 466and 467.Xr sftp 1 . 468The argument must be 469.Cm yes 470or 471.Cm no 472(the default). 473.It Cm Compression 474Specifies whether to use compression. 475The argument must be 476.Cm yes 477or 478.Cm no 479(the default). 480.It Cm ConnectionAttempts 481Specifies the number of tries (one per second) to make before exiting. 482The argument must be an integer. 483This may be useful in scripts if the connection sometimes fails. 484The default is 1. 485.It Cm ConnectTimeout 486Specifies the timeout (in seconds) used when connecting to the 487SSH server, instead of using the default system TCP timeout. 488This value is used only when the target is down or really unreachable, 489not when it refuses the connection. 490.It Cm ControlMaster 491Enables the sharing of multiple sessions over a single network connection. 492When set to 493.Cm yes , 494.Xr ssh 1 495will listen for connections on a control socket specified using the 496.Cm ControlPath 497argument. 498Additional sessions can connect to this socket using the same 499.Cm ControlPath 500with 501.Cm ControlMaster 502set to 503.Cm no 504(the default). 505These sessions will try to reuse the master instance's network connection 506rather than initiating new ones, but will fall back to connecting normally 507if the control socket does not exist, or is not listening. 508.Pp 509Setting this to 510.Cm ask 511will cause 512.Xr ssh 1 513to listen for control connections, but require confirmation using 514.Xr ssh-askpass 1 . 515If the 516.Cm ControlPath 517cannot be opened, 518.Xr ssh 1 519will continue without connecting to a master instance. 520.Pp 521X11 and 522.Xr ssh-agent 1 523forwarding is supported over these multiplexed connections, however the 524display and agent forwarded will be the one belonging to the master 525connection i.e. it is not possible to forward multiple displays or agents. 526.Pp 527Two additional options allow for opportunistic multiplexing: try to use a 528master connection but fall back to creating a new one if one does not already 529exist. 530These options are: 531.Cm auto 532and 533.Cm autoask . 534The latter requires confirmation like the 535.Cm ask 536option. 537.It Cm ControlPath 538Specify the path to the control socket used for connection sharing as described 539in the 540.Cm ControlMaster 541section above or the string 542.Cm none 543to disable connection sharing. 544Arguments to 545.Cm ControlPath 546may use the tilde syntax to refer to a user's home directory 547or the tokens described in the 548.Sx TOKENS 549section. 550It is recommended that any 551.Cm ControlPath 552used for opportunistic connection sharing include 553at least %h, %p, and %r (or alternatively %C) and be placed in a directory 554that is not writable by other users. 555This ensures that shared connections are uniquely identified. 556.It Cm ControlPersist 557When used in conjunction with 558.Cm ControlMaster , 559specifies that the master connection should remain open 560in the background (waiting for future client connections) 561after the initial client connection has been closed. 562If set to 563.Cm no , 564then the master connection will not be placed into the background, 565and will close as soon as the initial client connection is closed. 566If set to 567.Cm yes 568or 0, 569then the master connection will remain in the background indefinitely 570(until killed or closed via a mechanism such as the 571.Qq ssh -O exit ) . 572If set to a time in seconds, or a time in any of the formats documented in 573.Xr sshd_config 5 , 574then the backgrounded master connection will automatically terminate 575after it has remained idle (with no client connections) for the 576specified time. 577.It Cm DynamicForward 578Specifies that a TCP port on the local machine be forwarded 579over the secure channel, and the application 580protocol is then used to determine where to connect to from the 581remote machine. 582.Pp 583The argument must be 584.Sm off 585.Oo Ar bind_address : Oc Ar port . 586.Sm on 587IPv6 addresses can be specified by enclosing addresses in square brackets. 588By default, the local port is bound in accordance with the 589.Cm GatewayPorts 590setting. 591However, an explicit 592.Ar bind_address 593may be used to bind the connection to a specific address. 594The 595.Ar bind_address 596of 597.Cm localhost 598indicates that the listening port be bound for local use only, while an 599empty address or 600.Sq * 601indicates that the port should be available from all interfaces. 602.Pp 603Currently the SOCKS4 and SOCKS5 protocols are supported, and 604.Xr ssh 1 605will act as a SOCKS server. 606Multiple forwardings may be specified, and 607additional forwardings can be given on the command line. 608Only the superuser can forward privileged ports. 609.It Cm EnableSSHKeysign 610Setting this option to 611.Cm yes 612in the global client configuration file 613.Pa /etc/ssh/ssh_config 614enables the use of the helper program 615.Xr ssh-keysign 8 616during 617.Cm HostbasedAuthentication . 618The argument must be 619.Cm yes 620or 621.Cm no 622(the default). 623This option should be placed in the non-hostspecific section. 624See 625.Xr ssh-keysign 8 626for more information. 627.It Cm EscapeChar 628Sets the escape character (default: 629.Ql ~ ) . 630The escape character can also 631be set on the command line. 632The argument should be a single character, 633.Ql ^ 634followed by a letter, or 635.Cm none 636to disable the escape 637character entirely (making the connection transparent for binary 638data). 639.It Cm ExitOnForwardFailure 640Specifies whether 641.Xr ssh 1 642should terminate the connection if it cannot set up all requested 643dynamic, tunnel, local, and remote port forwardings, (e.g.\& 644if either end is unable to bind and listen on a specified port). 645Note that 646.Cm ExitOnForwardFailure 647does not apply to connections made over port forwardings and will not, 648for example, cause 649.Xr ssh 1 650to exit if TCP connections to the ultimate forwarding destination fail. 651The argument must be 652.Cm yes 653or 654.Cm no 655(the default). 656.It Cm FingerprintHash 657Specifies the hash algorithm used when displaying key fingerprints. 658Valid options are: 659.Cm md5 660and 661.Cm sha256 662(the default). 663.It Cm ForwardAgent 664Specifies whether the connection to the authentication agent (if any) 665will be forwarded to the remote machine. 666The argument must be 667.Cm yes 668or 669.Cm no 670(the default). 671.Pp 672Agent forwarding should be enabled with caution. 673Users with the ability to bypass file permissions on the remote host 674(for the agent's Unix-domain socket) 675can access the local agent through the forwarded connection. 676An attacker cannot obtain key material from the agent, 677however they can perform operations on the keys that enable them to 678authenticate using the identities loaded into the agent. 679.It Cm ForwardX11 680Specifies whether X11 connections will be automatically redirected 681over the secure channel and 682.Ev DISPLAY 683set. 684The argument must be 685.Cm yes 686or 687.Cm no 688(the default). 689.Pp 690X11 forwarding should be enabled with caution. 691Users with the ability to bypass file permissions on the remote host 692(for the user's X11 authorization database) 693can access the local X11 display through the forwarded connection. 694An attacker may then be able to perform activities such as keystroke monitoring 695if the 696.Cm ForwardX11Trusted 697option is also enabled. 698.It Cm ForwardX11Timeout 699Specify a timeout for untrusted X11 forwarding 700using the format described in the 701.Sx TIME FORMATS 702section of 703.Xr sshd_config 5 . 704X11 connections received by 705.Xr ssh 1 706after this time will be refused. 707Setting 708.Cm ForwardX11Timeout 709to zero will disable the timeout and permit X11 forwarding for the life 710of the connection. 711The default is to disable untrusted X11 forwarding after twenty minutes has 712elapsed. 713.It Cm ForwardX11Trusted 714If this option is set to 715.Cm yes , 716remote X11 clients will have full access to the original X11 display. 717.Pp 718If this option is set to 719.Cm no 720(the default), 721remote X11 clients will be considered untrusted and prevented 722from stealing or tampering with data belonging to trusted X11 723clients. 724Furthermore, the 725.Xr xauth 1 726token used for the session will be set to expire after 20 minutes. 727Remote clients will be refused access after this time. 728.Pp 729See the X11 SECURITY extension specification for full details on 730the restrictions imposed on untrusted clients. 731.It Cm GatewayPorts 732Specifies whether remote hosts are allowed to connect to local 733forwarded ports. 734By default, 735.Xr ssh 1 736binds local port forwardings to the loopback address. 737This prevents other remote hosts from connecting to forwarded ports. 738.Cm GatewayPorts 739can be used to specify that ssh 740should bind local port forwardings to the wildcard address, 741thus allowing remote hosts to connect to forwarded ports. 742The argument must be 743.Cm yes 744or 745.Cm no 746(the default). 747.It Cm GlobalKnownHostsFile 748Specifies one or more files to use for the global 749host key database, separated by whitespace. 750The default is 751.Pa /etc/ssh/ssh_known_hosts , 752.Pa /etc/ssh/ssh_known_hosts2 . 753.It Cm GSSAPIAuthentication 754Specifies whether user authentication based on GSSAPI is allowed. 755The default is 756.Cm no . 757.It Cm GSSAPIDelegateCredentials 758Forward (delegate) credentials to the server. 759The default is 760.Cm no . 761.It Cm HashKnownHosts 762Indicates that 763.Xr ssh 1 764should hash host names and addresses when they are added to 765.Pa ~/.ssh/known_hosts . 766These hashed names may be used normally by 767.Xr ssh 1 768and 769.Xr sshd 8 , 770but they do not reveal identifying information should the file's contents 771be disclosed. 772The default is 773.Cm no . 774Note that existing names and addresses in known hosts files 775will not be converted automatically, 776but may be manually hashed using 777.Xr ssh-keygen 1 . 778.It Cm HostbasedAuthentication 779Specifies whether to try rhosts based authentication with public key 780authentication. 781The argument must be 782.Cm yes 783or 784.Cm no 785(the default). 786.It Cm HostbasedKeyTypes 787Specifies the key types that will be used for hostbased authentication 788as a comma-separated list of patterns. 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. 793If the specified value begins with a 794.Sq - 795character, then the specified key types (including wildcards) will be removed 796from the default set instead of replacing them. 797The default for this option is: 798.Bd -literal -offset 3n 799ecdsa-sha2-nistp256-cert-v01@openssh.com, 800ecdsa-sha2-nistp384-cert-v01@openssh.com, 801ecdsa-sha2-nistp521-cert-v01@openssh.com, 802ssh-ed25519-cert-v01@openssh.com, 803rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 804ssh-rsa-cert-v01@openssh.com, 805ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 806ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 807.Ed 808.Pp 809The 810.Fl Q 811option of 812.Xr ssh 1 813may be used to list supported key types. 814.It Cm HostKeyAlgorithms 815Specifies the host key algorithms 816that the client wants to use in order of preference. 817Alternately if the specified value begins with a 818.Sq + 819character, then the specified key types will be appended to the default set 820instead of replacing them. 821If the specified value begins with a 822.Sq - 823character, then the specified key types (including wildcards) will be removed 824from the default set instead of replacing them. 825The default for this option is: 826.Bd -literal -offset 3n 827ecdsa-sha2-nistp256-cert-v01@openssh.com, 828ecdsa-sha2-nistp384-cert-v01@openssh.com, 829ecdsa-sha2-nistp521-cert-v01@openssh.com, 830ssh-ed25519-cert-v01@openssh.com, 831rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 832ssh-rsa-cert-v01@openssh.com, 833ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 834ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 835.Ed 836.Pp 837If hostkeys are known for the destination host then this default is modified 838to prefer their algorithms. 839.Pp 840The list of available key types may also be obtained using 841.Qq ssh -Q key . 842.It Cm HostKeyAlias 843Specifies an alias that should be used instead of the 844real host name when looking up or saving the host key 845in the host key database files and when validating host certificates. 846This option is useful for tunneling SSH connections 847or for multiple servers running on a single host. 848.It Cm HostName 849Specifies the real host name to log into. 850This can be used to specify nicknames or abbreviations for hosts. 851Arguments to 852.Cm HostName 853accept the tokens described in the 854.Sx TOKENS 855section. 856Numeric IP addresses are also permitted (both on the command line and in 857.Cm HostName 858specifications). 859The default is the name given on the command line. 860.It Cm IdentitiesOnly 861Specifies that 862.Xr ssh 1 863should only use the authentication identity and certificate files explicitly 864configured in the 865.Nm 866files 867or passed on the 868.Xr ssh 1 869command-line, 870even if 871.Xr ssh-agent 1 872or a 873.Cm PKCS11Provider 874offers more identities. 875The argument to this keyword must be 876.Cm yes 877or 878.Cm no 879(the default). 880This option is intended for situations where ssh-agent 881offers many different identities. 882.It Cm IdentityAgent 883Specifies the 884.Ux Ns -domain 885socket used to communicate with the authentication agent. 886.Pp 887This option overrides the 888.Ev SSH_AUTH_SOCK 889environment variable and can be used to select a specific agent. 890Setting the socket name to 891.Cm none 892disables the use of an authentication agent. 893If the string 894.Qq SSH_AUTH_SOCK 895is specified, the location of the socket will be read from the 896.Ev SSH_AUTH_SOCK 897environment variable. 898Otherwise if the specified value begins with a 899.Sq $ 900character, then it will be treated as an environment variable containing 901the location of the socket. 902.Pp 903Arguments to 904.Cm IdentityAgent 905may use the tilde syntax to refer to a user's home directory 906or the tokens described in the 907.Sx TOKENS 908section. 909.It Cm IdentityFile 910Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication 911identity is read. 912The default is 913.Pa ~/.ssh/id_dsa , 914.Pa ~/.ssh/id_ecdsa , 915.Pa ~/.ssh/id_ed25519 916and 917.Pa ~/.ssh/id_rsa . 918Additionally, any identities represented by the authentication agent 919will be used for authentication unless 920.Cm IdentitiesOnly 921is set. 922If no certificates have been explicitly specified by 923.Cm CertificateFile , 924.Xr ssh 1 925will try to load certificate information from the filename obtained by 926appending 927.Pa -cert.pub 928to the path of a specified 929.Cm IdentityFile . 930.Pp 931Arguments to 932.Cm IdentityFile 933may use the tilde syntax to refer to a user's home directory 934or the tokens described in the 935.Sx TOKENS 936section. 937.Pp 938It is possible to have 939multiple identity files specified in configuration files; all these 940identities will be tried in sequence. 941Multiple 942.Cm IdentityFile 943directives will add to the list of identities tried (this behaviour 944differs from that of other configuration directives). 945.Pp 946.Cm IdentityFile 947may be used in conjunction with 948.Cm IdentitiesOnly 949to select which identities in an agent are offered during authentication. 950.Cm IdentityFile 951may also be used in conjunction with 952.Cm CertificateFile 953in order to provide any certificate also needed for authentication with 954the identity. 955.It Cm IgnoreUnknown 956Specifies a pattern-list of unknown options to be ignored if they are 957encountered in configuration parsing. 958This may be used to suppress errors if 959.Nm 960contains options that are unrecognised by 961.Xr ssh 1 . 962It is recommended that 963.Cm IgnoreUnknown 964be listed early in the configuration file as it will not be applied 965to unknown options that appear before it. 966.It Cm Include 967Include the specified configuration file(s). 968Multiple pathnames may be specified and each pathname may contain 969.Xr glob 7 970wildcards and, for user configurations, shell-like 971.Sq ~ 972references to user home directories. 973Files without absolute paths are assumed to be in 974.Pa ~/.ssh 975if included in a user configuration file or 976.Pa /etc/ssh 977if included from the system configuration file. 978.Cm Include 979directive may appear inside a 980.Cm Match 981or 982.Cm Host 983block 984to perform conditional inclusion. 985.It Cm IPQoS 986Specifies the IPv4 type-of-service or DSCP class for connections. 987Accepted values are 988.Cm af11 , 989.Cm af12 , 990.Cm af13 , 991.Cm af21 , 992.Cm af22 , 993.Cm af23 , 994.Cm af31 , 995.Cm af32 , 996.Cm af33 , 997.Cm af41 , 998.Cm af42 , 999.Cm af43 , 1000.Cm cs0 , 1001.Cm cs1 , 1002.Cm cs2 , 1003.Cm cs3 , 1004.Cm cs4 , 1005.Cm cs5 , 1006.Cm cs6 , 1007.Cm cs7 , 1008.Cm ef , 1009.Cm lowdelay , 1010.Cm throughput , 1011.Cm reliability , 1012a numeric value, or 1013.Cm none 1014to use the operating system default. 1015This option may take one or two arguments, separated by whitespace. 1016If one argument is specified, it is used as the packet class unconditionally. 1017If two values are specified, the first is automatically selected for 1018interactive sessions and the second for non-interactive sessions. 1019The default is 1020.Cm af21 1021(Low-Latency Data) 1022for interactive sessions and 1023.Cm cs1 1024(Lower Effort) 1025for non-interactive sessions. 1026.It Cm KbdInteractiveAuthentication 1027Specifies whether to use keyboard-interactive authentication. 1028The argument to this keyword must be 1029.Cm yes 1030(the default) 1031or 1032.Cm no . 1033.It Cm KbdInteractiveDevices 1034Specifies the list of methods to use in keyboard-interactive authentication. 1035Multiple method names must be comma-separated. 1036The default is to use the server specified list. 1037The methods available vary depending on what the server supports. 1038For an OpenSSH server, 1039it may be zero or more of: 1040.Cm bsdauth , 1041.Cm pam , 1042and 1043.Cm skey . 1044.It Cm KexAlgorithms 1045Specifies the available KEX (Key Exchange) algorithms. 1046Multiple algorithms must be comma-separated. 1047Alternately if the specified value begins with a 1048.Sq + 1049character, then the specified methods will be appended to the default set 1050instead of replacing them. 1051If the specified value begins with a 1052.Sq - 1053character, then the specified methods (including wildcards) will be removed 1054from the default set instead of replacing them. 1055The default is: 1056.Bd -literal -offset indent 1057curve25519-sha256,curve25519-sha256@libssh.org, 1058ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, 1059diffie-hellman-group-exchange-sha256, 1060diffie-hellman-group16-sha512, 1061diffie-hellman-group18-sha512, 1062diffie-hellman-group14-sha256, 1063diffie-hellman-group14-sha1 1064.Ed 1065.Pp 1066The list of available key exchange algorithms may also be obtained using 1067.Qq ssh -Q kex . 1068.It Cm LocalCommand 1069Specifies a command to execute on the local machine after successfully 1070connecting to the server. 1071The command string extends to the end of the line, and is executed with 1072the user's shell. 1073Arguments to 1074.Cm LocalCommand 1075accept the tokens described in the 1076.Sx TOKENS 1077section. 1078.Pp 1079The command is run synchronously and does not have access to the 1080session of the 1081.Xr ssh 1 1082that spawned it. 1083It should not be used for interactive commands. 1084.Pp 1085This directive is ignored unless 1086.Cm PermitLocalCommand 1087has been enabled. 1088.It Cm LocalForward 1089Specifies that a TCP port on the local machine be forwarded over 1090the secure channel to the specified host and port from the remote machine. 1091The first argument must be 1092.Sm off 1093.Oo Ar bind_address : Oc Ar port 1094.Sm on 1095and the second argument must be 1096.Ar host : Ns Ar hostport . 1097IPv6 addresses can be specified by enclosing addresses in square brackets. 1098Multiple forwardings may be specified, and additional forwardings can be 1099given on the command line. 1100Only the superuser can forward privileged ports. 1101By default, the local port is bound in accordance with the 1102.Cm GatewayPorts 1103setting. 1104However, an explicit 1105.Ar bind_address 1106may be used to bind the connection to a specific address. 1107The 1108.Ar bind_address 1109of 1110.Cm localhost 1111indicates that the listening port be bound for local use only, while an 1112empty address or 1113.Sq * 1114indicates that the port should be available from all interfaces. 1115.It Cm LogLevel 1116Gives the verbosity level that is used when logging messages from 1117.Xr ssh 1 . 1118The possible values are: 1119QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 1120The default is INFO. 1121DEBUG and DEBUG1 are equivalent. 1122DEBUG2 and DEBUG3 each specify higher levels of verbose output. 1123.It Cm MACs 1124Specifies the MAC (message authentication code) algorithms 1125in order of preference. 1126The MAC algorithm is used for data integrity protection. 1127Multiple algorithms must be comma-separated. 1128If the specified value begins with a 1129.Sq + 1130character, then the specified algorithms will be appended to the default set 1131instead of replacing them. 1132If the specified value begins with a 1133.Sq - 1134character, then the specified algorithms (including wildcards) will be removed 1135from the default set instead of replacing them. 1136.Pp 1137The algorithms that contain 1138.Qq -etm 1139calculate the MAC after encryption (encrypt-then-mac). 1140These are considered safer and their use recommended. 1141.Pp 1142The default is: 1143.Bd -literal -offset indent 1144umac-64-etm@openssh.com,umac-128-etm@openssh.com, 1145hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 1146hmac-sha1-etm@openssh.com, 1147umac-64@openssh.com,umac-128@openssh.com, 1148hmac-sha2-256,hmac-sha2-512,hmac-sha1 1149.Ed 1150.Pp 1151The list of available MAC algorithms may also be obtained using 1152.Qq ssh -Q mac . 1153.It Cm NoHostAuthenticationForLocalhost 1154Disable host authentication for localhost (loopback addresses). 1155The argument to this keyword must be 1156.Cm yes 1157or 1158.Cm no 1159(the default). 1160.It Cm NumberOfPasswordPrompts 1161Specifies the number of password prompts before giving up. 1162The argument to this keyword must be an integer. 1163The default is 3. 1164.It Cm PasswordAuthentication 1165Specifies whether to use password authentication. 1166The argument to this keyword must be 1167.Cm yes 1168(the default) 1169or 1170.Cm no . 1171.It Cm PermitLocalCommand 1172Allow local command execution via the 1173.Ic LocalCommand 1174option or using the 1175.Ic !\& Ns Ar command 1176escape sequence in 1177.Xr ssh 1 . 1178The argument must be 1179.Cm yes 1180or 1181.Cm no 1182(the default). 1183.It Cm PKCS11Provider 1184Specifies which PKCS#11 provider to use or 1185.Cm none 1186to indicate that no provider should be used (the default). 1187The argument to this keyword is a path to the PKCS#11 shared library 1188.Xr ssh 1 1189should use to communicate with a PKCS#11 token providing keys for user 1190authentication. 1191.It Cm Port 1192Specifies the port number to connect on the remote host. 1193The default is 22. 1194.It Cm PreferredAuthentications 1195Specifies the order in which the client should try authentication methods. 1196This allows a client to prefer one method (e.g.\& 1197.Cm keyboard-interactive ) 1198over another method (e.g.\& 1199.Cm password ) . 1200The default is: 1201.Bd -literal -offset indent 1202gssapi-with-mic,hostbased,publickey, 1203keyboard-interactive,password 1204.Ed 1205.It Cm ProxyCommand 1206Specifies the command to use to connect to the server. 1207The command 1208string extends to the end of the line, and is executed 1209using the user's shell 1210.Ql exec 1211directive to avoid a lingering shell process. 1212.Pp 1213Arguments to 1214.Cm ProxyCommand 1215accept the tokens described in the 1216.Sx TOKENS 1217section. 1218The command can be basically anything, 1219and should read from its standard input and write to its standard output. 1220It should eventually connect an 1221.Xr sshd 8 1222server running on some machine, or execute 1223.Ic sshd -i 1224somewhere. 1225Host key management will be done using the 1226HostName of the host being connected (defaulting to the name typed by 1227the user). 1228Setting the command to 1229.Cm none 1230disables this option entirely. 1231Note that 1232.Cm CheckHostIP 1233is not available for connects with a proxy command. 1234.Pp 1235This directive is useful in conjunction with 1236.Xr nc 1 1237and its proxy support. 1238For example, the following directive would connect via an HTTP proxy at 1239192.0.2.0: 1240.Bd -literal -offset 3n 1241ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 1242.Ed 1243.It Cm ProxyJump 1244Specifies one or more jump proxies as either 1245.Xo 1246.Sm off 1247.Op Ar user No @ 1248.Ar host 1249.Op : Ns Ar port 1250.Sm on 1251or an ssh URI 1252.Xc . 1253Multiple proxies may be separated by comma characters and will be visited 1254sequentially. 1255Setting this option will cause 1256.Xr ssh 1 1257to connect to the target host by first making a 1258.Xr ssh 1 1259connection to the specified 1260.Cm ProxyJump 1261host and then establishing a 1262TCP forwarding to the ultimate target from there. 1263.Pp 1264Note that this option will compete with the 1265.Cm ProxyCommand 1266option - whichever is specified first will prevent later instances of the 1267other from taking effect. 1268.Pp 1269Note also that the configuration for the destination host (either supplied 1270via the command-line or the configuration file) is not generally applied 1271to jump hosts. 1272.Pa ~/.ssh/config 1273should be used if specific configuration is required for jump hosts. 1274.It Cm ProxyUseFdpass 1275Specifies that 1276.Cm ProxyCommand 1277will pass a connected file descriptor back to 1278.Xr ssh 1 1279instead of continuing to execute and pass data. 1280The default is 1281.Cm no . 1282.It Cm PubkeyAcceptedKeyTypes 1283Specifies the key types that will be used for public key authentication 1284as a comma-separated list of patterns. 1285Alternately if the specified value begins with a 1286.Sq + 1287character, then the key types after it will be appended to the default 1288instead of replacing it. 1289If the specified value begins with a 1290.Sq - 1291character, then the specified key types (including wildcards) will be removed 1292from the default set instead of replacing them. 1293The default for this option is: 1294.Bd -literal -offset 3n 1295ecdsa-sha2-nistp256-cert-v01@openssh.com, 1296ecdsa-sha2-nistp384-cert-v01@openssh.com, 1297ecdsa-sha2-nistp521-cert-v01@openssh.com, 1298ssh-ed25519-cert-v01@openssh.com, 1299rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, 1300ssh-rsa-cert-v01@openssh.com, 1301ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, 1302ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa 1303.Ed 1304.Pp 1305The list of available key types may also be obtained using 1306.Qq ssh -Q key . 1307.It Cm PubkeyAuthentication 1308Specifies whether to try public key authentication. 1309The argument to this keyword must be 1310.Cm yes 1311(the default) 1312or 1313.Cm no . 1314.It Cm RekeyLimit 1315Specifies the maximum amount of data that may be transmitted before the 1316session key is renegotiated, optionally followed a maximum amount of 1317time that may pass before the session key is renegotiated. 1318The first argument is specified in bytes and may have a suffix of 1319.Sq K , 1320.Sq M , 1321or 1322.Sq G 1323to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 1324The default is between 1325.Sq 1G 1326and 1327.Sq 4G , 1328depending on the cipher. 1329The optional second value is specified in seconds and may use any of the 1330units documented in the 1331.Sx TIME FORMATS 1332section of 1333.Xr sshd_config 5 . 1334The default value for 1335.Cm RekeyLimit 1336is 1337.Cm default none , 1338which means that rekeying is performed after the cipher's default amount 1339of data has been sent or received and no time based rekeying is done. 1340.It Cm RemoteCommand 1341Specifies a command to execute on the remote machine after successfully 1342connecting to the server. 1343The command string extends to the end of the line, and is executed with 1344the user's shell. 1345Arguments to 1346.Cm RemoteCommand 1347accept the tokens described in the 1348.Sx TOKENS 1349section. 1350.It Cm RemoteForward 1351Specifies that a TCP port on the remote machine be forwarded over 1352the secure channel. 1353The remote port may either be forwarded to a specified host and port 1354from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote 1355client to connect to arbitrary destinations from the local machine. 1356The first argument must be 1357.Sm off 1358.Oo Ar bind_address : Oc Ar port 1359.Sm on 1360If forwarding to a specific destination then the second argument must be 1361.Ar host : Ns Ar hostport , 1362otherwise if no destination argument is specified then the remote forwarding 1363will be established as a SOCKS proxy. 1364.Pp 1365IPv6 addresses can be specified by enclosing addresses in square brackets. 1366Multiple forwardings may be specified, and additional 1367forwardings can be given on the command line. 1368Privileged ports can be forwarded only when 1369logging in as root on the remote machine. 1370.Pp 1371If the 1372.Ar port 1373argument is 0, 1374the listen port will be dynamically allocated on the server and reported 1375to the client at run time. 1376.Pp 1377If the 1378.Ar bind_address 1379is not specified, the default is to only bind to loopback addresses. 1380If the 1381.Ar bind_address 1382is 1383.Ql * 1384or an empty string, then the forwarding is requested to listen on all 1385interfaces. 1386Specifying a remote 1387.Ar bind_address 1388will only succeed if the server's 1389.Cm GatewayPorts 1390option is enabled (see 1391.Xr sshd_config 5 ) . 1392.It Cm RequestTTY 1393Specifies whether to request a pseudo-tty for the session. 1394The argument may be one of: 1395.Cm no 1396(never request a TTY), 1397.Cm yes 1398(always request a TTY when standard input is a TTY), 1399.Cm force 1400(always request a TTY) or 1401.Cm auto 1402(request a TTY when opening a login session). 1403This option mirrors the 1404.Fl t 1405and 1406.Fl T 1407flags for 1408.Xr ssh 1 . 1409.It Cm RevokedHostKeys 1410Specifies revoked host public keys. 1411Keys listed in this file will be refused for host authentication. 1412Note that if this file does not exist or is not readable, 1413then host authentication will be refused for all hosts. 1414Keys may be specified as a text file, listing one public key per line, or as 1415an OpenSSH Key Revocation List (KRL) as generated by 1416.Xr ssh-keygen 1 . 1417For more information on KRLs, see the KEY REVOCATION LISTS section in 1418.Xr ssh-keygen 1 . 1419.It Cm SendEnv 1420Specifies what variables from the local 1421.Xr environ 7 1422should be sent to the server. 1423The server must also support it, and the server must be configured to 1424accept these environment variables. 1425Note that the 1426.Ev TERM 1427environment variable is always sent whenever a 1428pseudo-terminal is requested as it is required by the protocol. 1429Refer to 1430.Cm AcceptEnv 1431in 1432.Xr sshd_config 5 1433for how to configure the server. 1434Variables are specified by name, which may contain wildcard characters. 1435Multiple environment variables may be separated by whitespace or spread 1436across multiple 1437.Cm SendEnv 1438directives. 1439.Pp 1440See 1441.Sx PATTERNS 1442for more information on patterns. 1443.Pp 1444It is possible to clear previously set 1445.Cm SendEnv 1446variable names by prefixing patterns with 1447.Pa - . 1448The default is not to send any environment variables. 1449.It Cm ServerAliveCountMax 1450Sets the number of server alive messages (see below) which may be 1451sent without 1452.Xr ssh 1 1453receiving any messages back from the server. 1454If this threshold is reached while server alive messages are being sent, 1455ssh will disconnect from the server, terminating the session. 1456It is important to note that the use of server alive messages is very 1457different from 1458.Cm TCPKeepAlive 1459(below). 1460The server alive messages are sent through the encrypted channel 1461and therefore will not be spoofable. 1462The TCP keepalive option enabled by 1463.Cm TCPKeepAlive 1464is spoofable. 1465The server alive mechanism is valuable when the client or 1466server depend on knowing when a connection has become inactive. 1467.Pp 1468The default value is 3. 1469If, for example, 1470.Cm ServerAliveInterval 1471(see below) is set to 15 and 1472.Cm ServerAliveCountMax 1473is left at the default, if the server becomes unresponsive, 1474ssh will disconnect after approximately 45 seconds. 1475.It Cm ServerAliveInterval 1476Sets a timeout interval in seconds after which if no data has been received 1477from the server, 1478.Xr ssh 1 1479will send a message through the encrypted 1480channel to request a response from the server. 1481The default 1482is 0, indicating that these messages will not be sent to the server. 1483.It Cm SetEnv 1484Directly specify one or more environment variables and their contents to 1485be sent to the server. 1486Similarly to 1487.Cm SendEnv , 1488the server must be prepared to accept the environment variable. 1489.It Cm StreamLocalBindMask 1490Sets the octal file creation mode mask 1491.Pq umask 1492used when creating a Unix-domain socket file for local or remote 1493port forwarding. 1494This option is only used for port forwarding to a Unix-domain socket file. 1495.Pp 1496The default value is 0177, which creates a Unix-domain socket file that is 1497readable and writable only by the owner. 1498Note that not all operating systems honor the file mode on Unix-domain 1499socket files. 1500.It Cm StreamLocalBindUnlink 1501Specifies whether to remove an existing Unix-domain socket file for local 1502or remote port forwarding before creating a new one. 1503If the socket file already exists and 1504.Cm StreamLocalBindUnlink 1505is not enabled, 1506.Nm ssh 1507will be unable to forward the port to the Unix-domain socket file. 1508This option is only used for port forwarding to a Unix-domain socket file. 1509.Pp 1510The argument must be 1511.Cm yes 1512or 1513.Cm no 1514(the default). 1515.It Cm StrictHostKeyChecking 1516If this flag is set to 1517.Cm yes , 1518.Xr ssh 1 1519will never automatically add host keys to the 1520.Pa ~/.ssh/known_hosts 1521file, and refuses to connect to hosts whose host key has changed. 1522This provides maximum protection against man-in-the-middle (MITM) attacks, 1523though it can be annoying when the 1524.Pa /etc/ssh/ssh_known_hosts 1525file is poorly maintained or when connections to new hosts are 1526frequently made. 1527This option forces the user to manually 1528add all new hosts. 1529.Pp 1530If this flag is set to 1531.Dq accept-new 1532then ssh will automatically add new host keys to the user 1533known hosts files, but will not permit connections to hosts with 1534changed host keys. 1535If this flag is set to 1536.Dq no 1537or 1538.Dq off , 1539ssh will automatically add new host keys to the user known hosts files 1540and allow connections to hosts with changed hostkeys to proceed, 1541subject to some restrictions. 1542If this flag is set to 1543.Cm ask 1544(the default), 1545new host keys 1546will be added to the user known host files only after the user 1547has confirmed that is what they really want to do, and 1548ssh will refuse to connect to hosts whose host key has changed. 1549The host keys of 1550known hosts will be verified automatically in all cases. 1551.It Cm SyslogFacility 1552Gives the facility code that is used when logging messages from 1553.Xr ssh 1 . 1554The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, 1555LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. 1556The default is USER. 1557.It Cm TCPKeepAlive 1558Specifies whether the system should send TCP keepalive messages to the 1559other side. 1560If they are sent, death of the connection or crash of one 1561of the machines will be properly noticed. 1562However, this means that 1563connections will die if the route is down temporarily, and some people 1564find it annoying. 1565.Pp 1566The default is 1567.Cm yes 1568(to send TCP keepalive messages), and the client will notice 1569if the network goes down or the remote host dies. 1570This is important in scripts, and many users want it too. 1571.Pp 1572To disable TCP keepalive messages, the value should be set to 1573.Cm no . 1574See also 1575.Cm ServerAliveInterval 1576for protocol-level keepalives. 1577.It Cm Tunnel 1578Request 1579.Xr tun 4 1580device forwarding between the client and the server. 1581The argument must be 1582.Cm yes , 1583.Cm point-to-point 1584(layer 3), 1585.Cm ethernet 1586(layer 2), 1587or 1588.Cm no 1589(the default). 1590Specifying 1591.Cm yes 1592requests the default tunnel mode, which is 1593.Cm point-to-point . 1594.It Cm TunnelDevice 1595Specifies the 1596.Xr tun 4 1597devices to open on the client 1598.Pq Ar local_tun 1599and the server 1600.Pq Ar remote_tun . 1601.Pp 1602The argument must be 1603.Sm off 1604.Ar local_tun Op : Ar remote_tun . 1605.Sm on 1606The devices may be specified by numerical ID or the keyword 1607.Cm any , 1608which uses the next available tunnel device. 1609If 1610.Ar remote_tun 1611is not specified, it defaults to 1612.Cm any . 1613The default is 1614.Cm any:any . 1615.It Cm UpdateHostKeys 1616Specifies whether 1617.Xr ssh 1 1618should accept notifications of additional hostkeys from the server sent 1619after authentication has completed and add them to 1620.Cm UserKnownHostsFile . 1621The argument must be 1622.Cm yes , 1623.Cm no 1624(the default) or 1625.Cm ask . 1626Enabling this option allows learning alternate hostkeys for a server 1627and supports graceful key rotation by allowing a server to send replacement 1628public keys before old ones are removed. 1629Additional hostkeys are only accepted if the key used to authenticate the 1630host was already trusted or explicitly accepted by the user. 1631If 1632.Cm UpdateHostKeys 1633is set to 1634.Cm ask , 1635then the user is asked to confirm the modifications to the known_hosts file. 1636Confirmation is currently incompatible with 1637.Cm ControlPersist , 1638and will be disabled if it is enabled. 1639.Pp 1640Presently, only 1641.Xr sshd 8 1642from OpenSSH 6.8 and greater support the 1643.Qq hostkeys@openssh.com 1644protocol extension used to inform the client of all the server's hostkeys. 1645.It Cm User 1646Specifies the user to log in as. 1647This can be useful when a different user name is used on different machines. 1648This saves the trouble of 1649having to remember to give the user name on the command line. 1650.It Cm UserKnownHostsFile 1651Specifies one or more files to use for the user 1652host key database, separated by whitespace. 1653The default is 1654.Pa ~/.ssh/known_hosts , 1655.Pa ~/.ssh/known_hosts2 . 1656.It Cm VerifyHostKeyDNS 1657Specifies whether to verify the remote key using DNS and SSHFP resource 1658records. 1659If this option is set to 1660.Cm yes , 1661the client will implicitly trust keys that match a secure fingerprint 1662from DNS. 1663Insecure fingerprints will be handled as if this option was set to 1664.Cm ask . 1665If this option is set to 1666.Cm ask , 1667information on fingerprint match will be displayed, but the user will still 1668need to confirm new host keys according to the 1669.Cm StrictHostKeyChecking 1670option. 1671The default is 1672.Cm no . 1673.Pp 1674See also 1675.Sx VERIFYING HOST KEYS 1676in 1677.Xr ssh 1 . 1678.It Cm VisualHostKey 1679If this flag is set to 1680.Cm yes , 1681an ASCII art representation of the remote host key fingerprint is 1682printed in addition to the fingerprint string at login and 1683for unknown host keys. 1684If this flag is set to 1685.Cm no 1686(the default), 1687no fingerprint strings are printed at login and 1688only the fingerprint string will be printed for unknown host keys. 1689.It Cm XAuthLocation 1690Specifies the full pathname of the 1691.Xr xauth 1 1692program. 1693The default is 1694.Pa /usr/X11R6/bin/xauth . 1695.El 1696.Sh PATTERNS 1697A 1698.Em pattern 1699consists of zero or more non-whitespace characters, 1700.Sq * 1701(a wildcard that matches zero or more characters), 1702or 1703.Sq ?\& 1704(a wildcard that matches exactly one character). 1705For example, to specify a set of declarations for any host in the 1706.Qq .co.uk 1707set of domains, 1708the following pattern could be used: 1709.Pp 1710.Dl Host *.co.uk 1711.Pp 1712The following pattern 1713would match any host in the 192.168.0.[0-9] network range: 1714.Pp 1715.Dl Host 192.168.0.? 1716.Pp 1717A 1718.Em pattern-list 1719is a comma-separated list of patterns. 1720Patterns within pattern-lists may be negated 1721by preceding them with an exclamation mark 1722.Pq Sq !\& . 1723For example, 1724to allow a key to be used from anywhere within an organization 1725except from the 1726.Qq dialup 1727pool, 1728the following entry (in authorized_keys) could be used: 1729.Pp 1730.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1731.Pp 1732Note that a negated match will never produce a positive result by itself. 1733For example, attempting to match 1734.Qq host3 1735against the following pattern-list will fail: 1736.Pp 1737.Dl from=\&"!host1,!host2\&" 1738.Pp 1739The solution here is to include a term that will yield a positive match, 1740such as a wildcard: 1741.Pp 1742.Dl from=\&"!host1,!host2,*\&" 1743.Sh TOKENS 1744Arguments to some keywords can make use of tokens, 1745which are expanded at runtime: 1746.Pp 1747.Bl -tag -width XXXX -offset indent -compact 1748.It %% 1749A literal 1750.Sq % . 1751.It \&%C 1752Hash of %l%h%p%r. 1753.It %d 1754Local user's home directory. 1755.It %h 1756The remote hostname. 1757.It %i 1758The local user ID. 1759.It %L 1760The local hostname. 1761.It %l 1762The local hostname, including the domain name. 1763.It %n 1764The original remote hostname, as given on the command line. 1765.It %p 1766The remote port. 1767.It %r 1768The remote username. 1769.It \&%T 1770The local 1771.Xr tun 4 1772or 1773.Xr tap 4 1774network interface assigned if 1775tunnel forwarding was requested, or 1776.Qq NONE 1777otherwise. 1778.It %u 1779The local username. 1780.El 1781.Pp 1782.Cm Match exec 1783accepts the tokens %%, %h, %i, %L, %l, %n, %p, %r, and %u. 1784.Pp 1785.Cm CertificateFile 1786accepts the tokens %%, %d, %h, %i, %l, %r, and %u. 1787.Pp 1788.Cm ControlPath 1789accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u. 1790.Pp 1791.Cm HostName 1792accepts the tokens %% and %h. 1793.Pp 1794.Cm IdentityAgent 1795and 1796.Cm IdentityFile 1797accept the tokens %%, %d, %h, %i, %l, %r, and %u. 1798.Pp 1799.Cm LocalCommand 1800accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, %T, and %u. 1801.Pp 1802.Cm ProxyCommand 1803accepts the tokens %%, %h, %p, and %r. 1804.Pp 1805.Cm RemoteCommand 1806accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, and %u. 1807.Sh FILES 1808.Bl -tag -width Ds 1809.It Pa ~/.ssh/config 1810This is the per-user configuration file. 1811The format of this file is described above. 1812This file is used by the SSH client. 1813Because of the potential for abuse, this file must have strict permissions: 1814read/write for the user, and not writable by others. 1815.It Pa /etc/ssh/ssh_config 1816Systemwide configuration file. 1817This file provides defaults for those 1818values that are not specified in the user's configuration file, and 1819for those users who do not have a configuration file. 1820This file must be world-readable. 1821.El 1822.Sh SEE ALSO 1823.Xr ssh 1 1824.Sh AUTHORS 1825.An -nosplit 1826OpenSSH is a derivative of the original and free 1827ssh 1.2.12 release by 1828.An Tatu Ylonen . 1829.An Aaron Campbell , Bob Beck , Markus Friedl , 1830.An Niels Provos , Theo de Raadt 1831and 1832.An Dug Song 1833removed many bugs, re-added newer features and 1834created OpenSSH. 1835.An Markus Friedl 1836contributed the support for SSH protocol versions 1.5 and 2.0. 1837