1.\" -*- nroff -*- 2.\" 3.\" Author: Tatu Ylonen <ylo@cs.hut.fi> 4.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland 5.\" All rights reserved 6.\" 7.\" As far as I am concerned, the code I have written for this software 8.\" can be used freely for any purpose. Any derived versions of this 9.\" software must be clearly marked as such, and if the derived work is 10.\" incompatible with the protocol description in the RFC file, it must be 11.\" called by a name other than "ssh" or "Secure Shell". 12.\" 13.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. 14.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. 15.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. 16.\" 17.\" Redistribution and use in source and binary forms, with or without 18.\" modification, are permitted provided that the following conditions 19.\" are met: 20.\" 1. Redistributions of source code must retain the above copyright 21.\" notice, this list of conditions and the following disclaimer. 22.\" 2. Redistributions in binary form must reproduce the above copyright 23.\" notice, this list of conditions and the following disclaimer in the 24.\" documentation and/or other materials provided with the distribution. 25.\" 26.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $OpenBSD: ssh_config.5,v 1.138 2010/08/04 05:37:01 djm Exp $ 38.Dd $Mdocdate: August 4 2010 $ 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH SSH client configuration files 44.Sh SYNOPSIS 45.Nm ~/.ssh/config 46.Nm /etc/ssh/ssh_config 47.Sh DESCRIPTION 48.Xr ssh 1 49obtains configuration data from the following sources in 50the following order: 51.Pp 52.Bl -enum -offset indent -compact 53.It 54command-line options 55.It 56user's configuration file 57.Pq Pa ~/.ssh/config 58.It 59system-wide configuration file 60.Pq Pa /etc/ssh/ssh_config 61.El 62.Pp 63For each parameter, the first obtained value 64will be used. 65The configuration files contain sections separated by 66.Dq Host 67specifications, and that section is only applied for hosts that 68match one of the patterns given in the specification. 69The matched host name is the one given on the command line. 70.Pp 71Since the first obtained value for each parameter is used, more 72host-specific declarations should be given near the beginning of the 73file, and general defaults at the end. 74.Pp 75The configuration file has the following format: 76.Pp 77Empty lines and lines starting with 78.Ql # 79are comments. 80Otherwise a line is of the format 81.Dq keyword arguments . 82Configuration options may be separated by whitespace or 83optional whitespace and exactly one 84.Ql = ; 85the latter format is useful to avoid the need to quote whitespace 86when specifying configuration options using the 87.Nm ssh , 88.Nm scp , 89and 90.Nm sftp 91.Fl o 92option. 93Arguments may optionally be enclosed in double quotes 94.Pq \&" 95in order to represent arguments containing spaces. 96.Pp 97The possible 98keywords and their meanings are as follows (note that 99keywords are case-insensitive and arguments are case-sensitive): 100.Bl -tag -width Ds 101.It Cm Host 102Restricts the following declarations (up to the next 103.Cm Host 104keyword) to be only for those hosts that match one of the patterns 105given after the keyword. 106If more than one pattern is provided, they should be separated by whitespace. 107A single 108.Ql * 109as a pattern can be used to provide global 110defaults for all hosts. 111The host is the 112.Ar hostname 113argument given on the command line (i.e. the name is not converted to 114a canonicalized host name before matching). 115.Pp 116See 117.Sx PATTERNS 118for more information on patterns. 119.It Cm AddressFamily 120Specifies which address family to use when connecting. 121Valid arguments are 122.Dq any , 123.Dq inet 124(use IPv4 only), or 125.Dq inet6 126(use IPv6 only). 127.It Cm BatchMode 128If set to 129.Dq yes , 130passphrase/password querying will be disabled. 131This option is useful in scripts and other batch jobs where no user 132is present to supply the password. 133The argument must be 134.Dq yes 135or 136.Dq no . 137The default is 138.Dq no . 139.It Cm BindAddress 140Use the specified address on the local machine as the source address of 141the connection. 142Only useful on systems with more than one address. 143Note that this option does not work if 144.Cm UsePrivilegedPort 145is set to 146.Dq yes . 147.It Cm ChallengeResponseAuthentication 148Specifies whether to use challenge-response authentication. 149The argument to this keyword must be 150.Dq yes 151or 152.Dq no . 153The default is 154.Dq yes . 155.It Cm CheckHostIP 156If this flag is set to 157.Dq yes , 158.Xr ssh 1 159will additionally check the host IP address in the 160.Pa known_hosts 161file. 162This allows ssh to detect if a host key changed due to DNS spoofing. 163If the option is set to 164.Dq no , 165the check will not be executed. 166The default is 167.Dq no . 168.It Cm Cipher 169Specifies the cipher to use for encrypting the session 170in protocol version 1. 171Currently, 172.Dq blowfish , 173.Dq 3des , 174and 175.Dq des 176are supported. 177.Ar des 178is only supported in the 179.Xr ssh 1 180client for interoperability with legacy protocol 1 implementations 181that do not support the 182.Ar 3des 183cipher. 184Its use is strongly discouraged due to cryptographic weaknesses. 185The default is 186.Dq 3des . 187.It Cm Ciphers 188Specifies the ciphers allowed for protocol version 2 189in order of preference. 190Multiple ciphers must be comma-separated. 191The supported ciphers are 192.Dq 3des-cbc , 193.Dq aes128-cbc , 194.Dq aes192-cbc , 195.Dq aes256-cbc , 196.Dq aes128-ctr , 197.Dq aes192-ctr , 198.Dq aes256-ctr , 199.Dq arcfour128 , 200.Dq arcfour256 , 201.Dq arcfour , 202.Dq blowfish-cbc , 203and 204.Dq cast128-cbc . 205The default is: 206.Bd -literal -offset 3n 207aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 208aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 209aes256-cbc,arcfour 210.Ed 211.It Cm ClearAllForwardings 212Specifies that all local, remote, and dynamic port forwardings 213specified in the configuration files or on the command line be 214cleared. 215This option is primarily useful when used from the 216.Xr ssh 1 217command line to clear port forwardings set in 218configuration files, and is automatically set by 219.Xr scp 1 220and 221.Xr sftp 1 . 222The argument must be 223.Dq yes 224or 225.Dq no . 226The default is 227.Dq no . 228.It Cm Compression 229Specifies whether to use compression. 230The argument must be 231.Dq yes 232or 233.Dq no . 234The default is 235.Dq no . 236.It Cm CompressionLevel 237Specifies the compression level to use if compression is enabled. 238The argument must be an integer from 1 (fast) to 9 (slow, best). 239The default level is 6, which is good for most applications. 240The meaning of the values is the same as in 241.Xr gzip 1 . 242Note that this option applies to protocol version 1 only. 243.It Cm ConnectionAttempts 244Specifies the number of tries (one per second) to make before exiting. 245The argument must be an integer. 246This may be useful in scripts if the connection sometimes fails. 247The default is 1. 248.It Cm ConnectTimeout 249Specifies the timeout (in seconds) used when connecting to the 250SSH server, instead of using the default system TCP timeout. 251This value is used only when the target is down or really unreachable, 252not when it refuses the connection. 253.It Cm ControlMaster 254Enables the sharing of multiple sessions over a single network connection. 255When set to 256.Dq yes , 257.Xr ssh 1 258will listen for connections on a control socket specified using the 259.Cm ControlPath 260argument. 261Additional sessions can connect to this socket using the same 262.Cm ControlPath 263with 264.Cm ControlMaster 265set to 266.Dq no 267(the default). 268These sessions will try to reuse the master instance's network connection 269rather than initiating new ones, but will fall back to connecting normally 270if the control socket does not exist, or is not listening. 271.Pp 272Setting this to 273.Dq ask 274will cause ssh 275to listen for control connections, but require confirmation using the 276.Ev SSH_ASKPASS 277program before they are accepted (see 278.Xr ssh-add 1 279for details). 280If the 281.Cm ControlPath 282cannot be opened, 283ssh will continue without connecting to a master instance. 284.Pp 285X11 and 286.Xr ssh-agent 1 287forwarding is supported over these multiplexed connections, however the 288display and agent forwarded will be the one belonging to the master 289connection i.e. it is not possible to forward multiple displays or agents. 290.Pp 291Two additional options allow for opportunistic multiplexing: try to use a 292master connection but fall back to creating a new one if one does not already 293exist. 294These options are: 295.Dq auto 296and 297.Dq autoask . 298The latter requires confirmation like the 299.Dq ask 300option. 301.It Cm ControlPath 302Specify the path to the control socket used for connection sharing as described 303in the 304.Cm ControlMaster 305section above or the string 306.Dq none 307to disable connection sharing. 308In the path, 309.Ql %l 310will be substituted by the local host name, 311.Ql %h 312will be substituted by the target host name, 313.Ql %p 314the port, and 315.Ql %r 316by the remote login username. 317It is recommended that any 318.Cm ControlPath 319used for opportunistic connection sharing include 320at least %h, %p, and %r. 321This ensures that shared connections are uniquely identified. 322.It Cm ControlPersist 323When used in conjunction with 324.Cm ControlMaster , 325specifies that the master connection should remain open 326in the background (waiting for future client connections) 327after the initial client connection has been closed. 328If set to 329.Dq no , 330then the master connection will not be placed into the background, 331and will close as soon as the initial client connection is closed. 332If set to 333.Dq yes , 334then the master connection will remain in the background indefinitely 335(until killed or closed via a mechanism such as the 336.Xr ssh 1 337.Dq Fl O No exit 338option). 339If set to a time in seconds, or a time in any of the formats documented in 340.Xr sshd_config 5 , 341then the backgrounded master connection will automatically terminate 342after it has remained idle (with no client connections) for the 343specified time. 344.It Cm DynamicForward 345Specifies that a TCP port on the local machine be forwarded 346over the secure channel, and the application 347protocol is then used to determine where to connect to from the 348remote machine. 349.Pp 350The argument must be 351.Sm off 352.Oo Ar bind_address : Oc Ar port . 353.Sm on 354IPv6 addresses can be specified by enclosing addresses in square brackets. 355By default, the local port is bound in accordance with the 356.Cm GatewayPorts 357setting. 358However, an explicit 359.Ar bind_address 360may be used to bind the connection to a specific address. 361The 362.Ar bind_address 363of 364.Dq localhost 365indicates that the listening port be bound for local use only, while an 366empty address or 367.Sq * 368indicates that the port should be available from all interfaces. 369.Pp 370Currently the SOCKS4 and SOCKS5 protocols are supported, and 371.Xr ssh 1 372will act as a SOCKS server. 373Multiple forwardings may be specified, and 374additional forwardings can be given on the command line. 375Only the superuser can forward privileged ports. 376.It Cm EnableSSHKeysign 377Setting this option to 378.Dq yes 379in the global client configuration file 380.Pa /etc/ssh/ssh_config 381enables the use of the helper program 382.Xr ssh-keysign 8 383during 384.Cm HostbasedAuthentication . 385The argument must be 386.Dq yes 387or 388.Dq no . 389The default is 390.Dq no . 391This option should be placed in the non-hostspecific section. 392See 393.Xr ssh-keysign 8 394for more information. 395.It Cm EscapeChar 396Sets the escape character (default: 397.Ql ~ ) . 398The escape character can also 399be set on the command line. 400The argument should be a single character, 401.Ql ^ 402followed by a letter, or 403.Dq none 404to disable the escape 405character entirely (making the connection transparent for binary 406data). 407.It Cm ExitOnForwardFailure 408Specifies whether 409.Xr ssh 1 410should terminate the connection if it cannot set up all requested 411dynamic, tunnel, local, and remote port forwardings. 412The argument must be 413.Dq yes 414or 415.Dq no . 416The default is 417.Dq no . 418.It Cm ForwardAgent 419Specifies whether the connection to the authentication agent (if any) 420will be forwarded to the remote machine. 421The argument must be 422.Dq yes 423or 424.Dq no . 425The default is 426.Dq no . 427.Pp 428Agent forwarding should be enabled with caution. 429Users with the ability to bypass file permissions on the remote host 430(for the agent's Unix-domain socket) 431can access the local agent through the forwarded connection. 432An attacker cannot obtain key material from the agent, 433however they can perform operations on the keys that enable them to 434authenticate using the identities loaded into the agent. 435.It Cm ForwardX11 436Specifies whether X11 connections will be automatically redirected 437over the secure channel and 438.Ev DISPLAY 439set. 440The argument must be 441.Dq yes 442or 443.Dq no . 444The default is 445.Dq no . 446.Pp 447X11 forwarding should be enabled with caution. 448Users with the ability to bypass file permissions on the remote host 449(for the user's X11 authorization database) 450can access the local X11 display through the forwarded connection. 451An attacker may then be able to perform activities such as keystroke monitoring 452if the 453.Cm ForwardX11Trusted 454option is also enabled. 455.It Cm ForwardX11Timeout 456Specify a timeout for untrusted X11 forwarding 457using the format described in the 458.Sx TIME FORMATS 459section of 460.Xr sshd_config 5 . 461X11 connections received by 462.Xr ssh 1 463after this time will be refused. 464The default is to disable untrusted X11 forwarding after twenty minutes has 465elapsed. 466.It Cm ForwardX11Trusted 467If this option is set to 468.Dq yes , 469remote X11 clients will have full access to the original X11 display. 470.Pp 471If this option is set to 472.Dq no , 473remote X11 clients will be considered untrusted and prevented 474from stealing or tampering with data belonging to trusted X11 475clients. 476Furthermore, the 477.Xr xauth 1 478token used for the session will be set to expire after 20 minutes. 479Remote clients will be refused access after this time. 480.Pp 481The default is 482.Dq no . 483.Pp 484See the X11 SECURITY extension specification for full details on 485the restrictions imposed on untrusted clients. 486.It Cm GatewayPorts 487Specifies whether remote hosts are allowed to connect to local 488forwarded ports. 489By default, 490.Xr ssh 1 491binds local port forwardings to the loopback address. 492This prevents other remote hosts from connecting to forwarded ports. 493.Cm GatewayPorts 494can be used to specify that ssh 495should bind local port forwardings to the wildcard address, 496thus allowing remote hosts to connect to forwarded ports. 497The argument must be 498.Dq yes 499or 500.Dq no . 501The default is 502.Dq no . 503.It Cm GlobalKnownHostsFile 504Specifies a file to use for the global 505host key database instead of 506.Pa /etc/ssh/ssh_known_hosts . 507.It Cm GSSAPIAuthentication 508Specifies whether user authentication based on GSSAPI is allowed. 509The default is 510.Dq no . 511Note that this option applies to protocol version 2 only. 512.It Cm GSSAPIDelegateCredentials 513Forward (delegate) credentials to the server. 514The default is 515.Dq no . 516Note that this option applies to protocol version 2 only. 517.It Cm HashKnownHosts 518Indicates that 519.Xr ssh 1 520should hash host names and addresses when they are added to 521.Pa ~/.ssh/known_hosts . 522These hashed names may be used normally by 523.Xr ssh 1 524and 525.Xr sshd 8 , 526but they do not reveal identifying information should the file's contents 527be disclosed. 528The default is 529.Dq no . 530Note that existing names and addresses in known hosts files 531will not be converted automatically, 532but may be manually hashed using 533.Xr ssh-keygen 1 . 534.It Cm HostbasedAuthentication 535Specifies whether to try rhosts based authentication with public key 536authentication. 537The argument must be 538.Dq yes 539or 540.Dq no . 541The default is 542.Dq no . 543This option applies to protocol version 2 only and 544is similar to 545.Cm RhostsRSAAuthentication . 546.It Cm HostKeyAlgorithms 547Specifies the protocol version 2 host key algorithms 548that the client wants to use in order of preference. 549The default for this option is: 550.Dq ssh-rsa,ssh-dss . 551.It Cm HostKeyAlias 552Specifies an alias that should be used instead of the 553real host name when looking up or saving the host key 554in the host key database files. 555This option is useful for tunneling SSH connections 556or for multiple servers running on a single host. 557.It Cm HostName 558Specifies the real host name to log into. 559This can be used to specify nicknames or abbreviations for hosts. 560If the hostname contains the character sequence 561.Ql %h , 562then this will be replaced with the host name specified on the commandline 563(this is useful for manipulating unqualified names). 564The default is the name given on the command line. 565Numeric IP addresses are also permitted (both on the command line and in 566.Cm HostName 567specifications). 568.It Cm IdentitiesOnly 569Specifies that 570.Xr ssh 1 571should only use the authentication identity files configured in the 572.Nm 573files, 574even if 575.Xr ssh-agent 1 576offers more identities. 577The argument to this keyword must be 578.Dq yes 579or 580.Dq no . 581This option is intended for situations where ssh-agent 582offers many different identities. 583The default is 584.Dq no . 585.It Cm IdentityFile 586Specifies a file from which the user's RSA or DSA authentication identity 587is read. 588The default is 589.Pa ~/.ssh/identity 590for protocol version 1, and 591.Pa ~/.ssh/id_rsa 592and 593.Pa ~/.ssh/id_dsa 594for protocol version 2. 595Additionally, any identities represented by the authentication agent 596will be used for authentication. 597.Xr ssh 1 598will try to load certificate information from the filename obtained by 599appending 600.Pa -cert.pub 601to the path of a specified 602.Cm IdentityFile . 603.Pp 604The file name may use the tilde 605syntax to refer to a user's home directory or one of the following 606escape characters: 607.Ql %d 608(local user's home directory), 609.Ql %u 610(local user name), 611.Ql %l 612(local host name), 613.Ql %h 614(remote host name) or 615.Ql %r 616(remote user name). 617.Pp 618It is possible to have 619multiple identity files specified in configuration files; all these 620identities will be tried in sequence. 621.It Cm KbdInteractiveAuthentication 622Specifies whether to use keyboard-interactive authentication. 623The argument to this keyword must be 624.Dq yes 625or 626.Dq no . 627The default is 628.Dq yes . 629.It Cm KbdInteractiveDevices 630Specifies the list of methods to use in keyboard-interactive authentication. 631Multiple method names must be comma-separated. 632The default is to use the server specified list. 633The methods available vary depending on what the server supports. 634For an OpenSSH server, 635it may be zero or more of: 636.Dq bsdauth , 637.Dq pam , 638and 639.Dq skey . 640.It Cm LocalCommand 641Specifies a command to execute on the local machine after successfully 642connecting to the server. 643The command string extends to the end of the line, and is executed with 644the user's shell. 645The following escape character substitutions will be performed: 646.Ql %d 647(local user's home directory), 648.Ql %h 649(remote host name), 650.Ql %l 651(local host name), 652.Ql %n 653(host name as provided on the command line), 654.Ql %p 655(remote port), 656.Ql %r 657(remote user name) or 658.Ql %u 659(local user name). 660.Pp 661The command is run synchronously and does not have access to the 662session of the 663.Xr ssh 1 664that spawned it. 665It should not be used for interactive commands. 666.Pp 667This directive is ignored unless 668.Cm PermitLocalCommand 669has been enabled. 670.It Cm LocalForward 671Specifies that a TCP port on the local machine be forwarded over 672the secure channel to the specified host and port from the remote machine. 673The first argument must be 674.Sm off 675.Oo Ar bind_address : Oc Ar port 676.Sm on 677and the second argument must be 678.Ar host : Ns Ar hostport . 679IPv6 addresses can be specified by enclosing addresses in square brackets. 680Multiple forwardings may be specified, and additional forwardings can be 681given on the command line. 682Only the superuser can forward privileged ports. 683By default, the local port is bound in accordance with the 684.Cm GatewayPorts 685setting. 686However, an explicit 687.Ar bind_address 688may be used to bind the connection to a specific address. 689The 690.Ar bind_address 691of 692.Dq localhost 693indicates that the listening port be bound for local use only, while an 694empty address or 695.Sq * 696indicates that the port should be available from all interfaces. 697.It Cm LogLevel 698Gives the verbosity level that is used when logging messages from 699.Xr ssh 1 . 700The possible values are: 701QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 702The default is INFO. 703DEBUG and DEBUG1 are equivalent. 704DEBUG2 and DEBUG3 each specify higher levels of verbose output. 705.It Cm MACs 706Specifies the MAC (message authentication code) algorithms 707in order of preference. 708The MAC algorithm is used in protocol version 2 709for data integrity protection. 710Multiple algorithms must be comma-separated. 711The default is: 712.Bd -literal -offset indent 713hmac-md5,hmac-sha1,umac-64@openssh.com, 714hmac-ripemd160,hmac-sha1-96,hmac-md5-96 715.Ed 716.It Cm NoHostAuthenticationForLocalhost 717This option can be used if the home directory is shared across machines. 718In this case localhost will refer to a different machine on each of 719the machines and the user will get many warnings about changed host keys. 720However, this option disables host authentication for localhost. 721The argument to this keyword must be 722.Dq yes 723or 724.Dq no . 725The default is to check the host key for localhost. 726.It Cm NumberOfPasswordPrompts 727Specifies the number of password prompts before giving up. 728The argument to this keyword must be an integer. 729The default is 3. 730.It Cm PasswordAuthentication 731Specifies whether to use password authentication. 732The argument to this keyword must be 733.Dq yes 734or 735.Dq no . 736The default is 737.Dq yes . 738.It Cm PermitLocalCommand 739Allow local command execution via the 740.Ic LocalCommand 741option or using the 742.Ic !\& Ns Ar command 743escape sequence in 744.Xr ssh 1 . 745The argument must be 746.Dq yes 747or 748.Dq no . 749The default is 750.Dq no . 751.It Cm PKCS11Provider 752Specifies which PKCS#11 provider to use. 753The argument to this keyword is the PKCS#11 shared libary 754.Xr ssh 1 755should use to communicate with a PKCS#11 token providing the user's 756private RSA key. 757.It Cm Port 758Specifies the port number to connect on the remote host. 759The default is 22. 760.It Cm PreferredAuthentications 761Specifies the order in which the client should try protocol 2 762authentication methods. 763This allows a client to prefer one method (e.g.\& 764.Cm keyboard-interactive ) 765over another method (e.g.\& 766.Cm password ) . 767The default is: 768.Bd -literal -offset indent 769gssapi-with-mic,hostbased,publickey, 770keyboard-interactive,password 771.Ed 772.It Cm Protocol 773Specifies the protocol versions 774.Xr ssh 1 775should support in order of preference. 776The possible values are 777.Sq 1 778and 779.Sq 2 . 780Multiple versions must be comma-separated. 781When this option is set to 782.Dq 2,1 783.Nm ssh 784will try version 2 and fall back to version 1 785if version 2 is not available. 786The default is 787.Sq 2 . 788.It Cm ProxyCommand 789Specifies the command to use to connect to the server. 790The command 791string extends to the end of the line, and is executed with 792the user's shell. 793In the command string, any occurrence of 794.Ql %h 795will be substituted by the host name to 796connect, 797.Ql %p 798by the port, and 799.Ql %r 800by the remote user name. 801The command can be basically anything, 802and should read from its standard input and write to its standard output. 803It should eventually connect an 804.Xr sshd 8 805server running on some machine, or execute 806.Ic sshd -i 807somewhere. 808Host key management will be done using the 809HostName of the host being connected (defaulting to the name typed by 810the user). 811Setting the command to 812.Dq none 813disables this option entirely. 814Note that 815.Cm CheckHostIP 816is not available for connects with a proxy command. 817.Pp 818This directive is useful in conjunction with 819.Xr nc 1 820and its proxy support. 821For example, the following directive would connect via an HTTP proxy at 822192.0.2.0: 823.Bd -literal -offset 3n 824ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 825.Ed 826.It Cm PubkeyAuthentication 827Specifies whether to try public key authentication. 828The argument to this keyword must be 829.Dq yes 830or 831.Dq no . 832The default is 833.Dq yes . 834This option applies to protocol version 2 only. 835.It Cm RekeyLimit 836Specifies the maximum amount of data that may be transmitted before the 837session key is renegotiated. 838The argument is the number of bytes, with an optional suffix of 839.Sq K , 840.Sq M , 841or 842.Sq G 843to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 844The default is between 845.Sq 1G 846and 847.Sq 4G , 848depending on the cipher. 849This option applies to protocol version 2 only. 850.It Cm RemoteForward 851Specifies that a TCP port on the remote machine be forwarded over 852the secure channel to the specified host and port from the local machine. 853The first argument must be 854.Sm off 855.Oo Ar bind_address : Oc Ar port 856.Sm on 857and the second argument must be 858.Ar host : Ns Ar hostport . 859IPv6 addresses can be specified by enclosing addresses in square brackets. 860Multiple forwardings may be specified, and additional 861forwardings can be given on the command line. 862Privileged ports can be forwarded only when 863logging in as root on the remote machine. 864.Pp 865If the 866.Ar port 867argument is 868.Ql 0 , 869the listen port will be dynamically allocated on the server and reported 870to the client at run time. 871.Pp 872If the 873.Ar bind_address 874is not specified, the default is to only bind to loopback addresses. 875If the 876.Ar bind_address 877is 878.Ql * 879or an empty string, then the forwarding is requested to listen on all 880interfaces. 881Specifying a remote 882.Ar bind_address 883will only succeed if the server's 884.Cm GatewayPorts 885option is enabled (see 886.Xr sshd_config 5 ) . 887.It Cm RhostsRSAAuthentication 888Specifies whether to try rhosts based authentication with RSA host 889authentication. 890The argument must be 891.Dq yes 892or 893.Dq no . 894The default is 895.Dq no . 896This option applies to protocol version 1 only and requires 897.Xr ssh 1 898to be setuid root. 899.It Cm RSAAuthentication 900Specifies whether to try RSA authentication. 901The argument to this keyword must be 902.Dq yes 903or 904.Dq no . 905RSA authentication will only be 906attempted if the identity file exists, or an authentication agent is 907running. 908The default is 909.Dq yes . 910Note that this option applies to protocol version 1 only. 911.It Cm SendEnv 912Specifies what variables from the local 913.Xr environ 7 914should be sent to the server. 915Note that environment passing is only supported for protocol 2. 916The server must also support it, and the server must be configured to 917accept these environment variables. 918Refer to 919.Cm AcceptEnv 920in 921.Xr sshd_config 5 922for how to configure the server. 923Variables are specified by name, which may contain wildcard characters. 924Multiple environment variables may be separated by whitespace or spread 925across multiple 926.Cm SendEnv 927directives. 928The default is not to send any environment variables. 929.Pp 930See 931.Sx PATTERNS 932for more information on patterns. 933.It Cm ServerAliveCountMax 934Sets the number of server alive messages (see below) which may be 935sent without 936.Xr ssh 1 937receiving any messages back from the server. 938If this threshold is reached while server alive messages are being sent, 939ssh will disconnect from the server, terminating the session. 940It is important to note that the use of server alive messages is very 941different from 942.Cm TCPKeepAlive 943(below). 944The server alive messages are sent through the encrypted channel 945and therefore will not be spoofable. 946The TCP keepalive option enabled by 947.Cm TCPKeepAlive 948is spoofable. 949The server alive mechanism is valuable when the client or 950server depend on knowing when a connection has become inactive. 951.Pp 952The default value is 3. 953If, for example, 954.Cm ServerAliveInterval 955(see below) is set to 15 and 956.Cm ServerAliveCountMax 957is left at the default, if the server becomes unresponsive, 958ssh will disconnect after approximately 45 seconds. 959This option applies to protocol version 2 only. 960.It Cm ServerAliveInterval 961Sets a timeout interval in seconds after which if no data has been received 962from the server, 963.Xr ssh 1 964will send a message through the encrypted 965channel to request a response from the server. 966The default 967is 0, indicating that these messages will not be sent to the server. 968This option applies to protocol version 2 only. 969.It Cm StrictHostKeyChecking 970If this flag is set to 971.Dq yes , 972.Xr ssh 1 973will never automatically add host keys to the 974.Pa ~/.ssh/known_hosts 975file, and refuses to connect to hosts whose host key has changed. 976This provides maximum protection against trojan horse attacks, 977though it can be annoying when the 978.Pa /etc/ssh/ssh_known_hosts 979file is poorly maintained or when connections to new hosts are 980frequently made. 981This option forces the user to manually 982add all new hosts. 983If this flag is set to 984.Dq no , 985ssh will automatically add new host keys to the 986user known hosts files. 987If this flag is set to 988.Dq ask , 989new host keys 990will be added to the user known host files only after the user 991has confirmed that is what they really want to do, and 992ssh will refuse to connect to hosts whose host key has changed. 993The host keys of 994known hosts will be verified automatically in all cases. 995The argument must be 996.Dq yes , 997.Dq no , 998or 999.Dq ask . 1000The default is 1001.Dq ask . 1002.It Cm TCPKeepAlive 1003Specifies whether the system should send TCP keepalive messages to the 1004other side. 1005If they are sent, death of the connection or crash of one 1006of the machines will be properly noticed. 1007However, this means that 1008connections will die if the route is down temporarily, and some people 1009find it annoying. 1010.Pp 1011The default is 1012.Dq yes 1013(to send TCP keepalive messages), and the client will notice 1014if the network goes down or the remote host dies. 1015This is important in scripts, and many users want it too. 1016.Pp 1017To disable TCP keepalive messages, the value should be set to 1018.Dq no . 1019.It Cm Tunnel 1020Request 1021.Xr tun 4 1022device forwarding between the client and the server. 1023The argument must be 1024.Dq yes , 1025.Dq point-to-point 1026(layer 3), 1027.Dq ethernet 1028(layer 2), 1029or 1030.Dq no . 1031Specifying 1032.Dq yes 1033requests the default tunnel mode, which is 1034.Dq point-to-point . 1035The default is 1036.Dq no . 1037.It Cm TunnelDevice 1038Specifies the 1039.Xr tun 4 1040devices to open on the client 1041.Pq Ar local_tun 1042and the server 1043.Pq Ar remote_tun . 1044.Pp 1045The argument must be 1046.Sm off 1047.Ar local_tun Op : Ar remote_tun . 1048.Sm on 1049The devices may be specified by numerical ID or the keyword 1050.Dq any , 1051which uses the next available tunnel device. 1052If 1053.Ar remote_tun 1054is not specified, it defaults to 1055.Dq any . 1056The default is 1057.Dq any:any . 1058.It Cm UsePrivilegedPort 1059Specifies whether to use a privileged port for outgoing connections. 1060The argument must be 1061.Dq yes 1062or 1063.Dq no . 1064The default is 1065.Dq no . 1066If set to 1067.Dq yes , 1068.Xr ssh 1 1069must be setuid root. 1070Note that this option must be set to 1071.Dq yes 1072for 1073.Cm RhostsRSAAuthentication 1074with older servers. 1075.It Cm User 1076Specifies the user to log in as. 1077This can be useful when a different user name is used on different machines. 1078This saves the trouble of 1079having to remember to give the user name on the command line. 1080.It Cm UserKnownHostsFile 1081Specifies a file to use for the user 1082host key database instead of 1083.Pa ~/.ssh/known_hosts . 1084.It Cm VerifyHostKeyDNS 1085Specifies whether to verify the remote key using DNS and SSHFP resource 1086records. 1087If this option is set to 1088.Dq yes , 1089the client will implicitly trust keys that match a secure fingerprint 1090from DNS. 1091Insecure fingerprints will be handled as if this option was set to 1092.Dq ask . 1093If this option is set to 1094.Dq ask , 1095information on fingerprint match will be displayed, but the user will still 1096need to confirm new host keys according to the 1097.Cm StrictHostKeyChecking 1098option. 1099The argument must be 1100.Dq yes , 1101.Dq no , 1102or 1103.Dq ask . 1104The default is 1105.Dq no . 1106Note that this option applies to protocol version 2 only. 1107.Pp 1108See also 1109.Sx VERIFYING HOST KEYS 1110in 1111.Xr ssh 1 . 1112.It Cm VersionAddendum 1113Specifies a string to append to the regular version string to identify 1114OS- or site-specific modifications. 1115The default is 1116.Dq DragonFly-20090620 . 1117.It Cm VisualHostKey 1118If this flag is set to 1119.Dq yes , 1120an ASCII art representation of the remote host key fingerprint is 1121printed in addition to the hex fingerprint string at login and 1122for unknown host keys. 1123If this flag is set to 1124.Dq no , 1125no fingerprint strings are printed at login and 1126only the hex fingerprint string will be printed for unknown host keys. 1127The default is 1128.Dq no . 1129.It Cm XAuthLocation 1130Specifies the full pathname of the 1131.Xr xauth 1 1132program. 1133The default is 1134.Pa /usr/X11R6/bin/xauth . 1135.El 1136.Sh PATTERNS 1137A 1138.Em pattern 1139consists of zero or more non-whitespace characters, 1140.Sq * 1141(a wildcard that matches zero or more characters), 1142or 1143.Sq ?\& 1144(a wildcard that matches exactly one character). 1145For example, to specify a set of declarations for any host in the 1146.Dq .co.uk 1147set of domains, 1148the following pattern could be used: 1149.Pp 1150.Dl Host *.co.uk 1151.Pp 1152The following pattern 1153would match any host in the 192.168.0.[0-9] network range: 1154.Pp 1155.Dl Host 192.168.0.? 1156.Pp 1157A 1158.Em pattern-list 1159is a comma-separated list of patterns. 1160Patterns within pattern-lists may be negated 1161by preceding them with an exclamation mark 1162.Pq Sq !\& . 1163For example, 1164to allow a key to be used from anywhere within an organisation 1165except from the 1166.Dq dialup 1167pool, 1168the following entry (in authorized_keys) could be used: 1169.Pp 1170.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1171.Sh FILES 1172.Bl -tag -width Ds 1173.It Pa ~/.ssh/config 1174This is the per-user configuration file. 1175The format of this file is described above. 1176This file is used by the SSH client. 1177Because of the potential for abuse, this file must have strict permissions: 1178read/write for the user, and not accessible by others. 1179.It Pa /etc/ssh/ssh_config 1180Systemwide configuration file. 1181This file provides defaults for those 1182values that are not specified in the user's configuration file, and 1183for those users who do not have a configuration file. 1184This file must be world-readable. 1185.El 1186.Sh SEE ALSO 1187.Xr ssh 1 1188.Sh AUTHORS 1189OpenSSH is a derivative of the original and free 1190ssh 1.2.12 release by Tatu Ylonen. 1191Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1192Theo de Raadt and Dug Song 1193removed many bugs, re-added newer features and 1194created OpenSSH. 1195Markus Friedl contributed the support for SSH 1196protocol versions 1.5 and 2.0. 1197