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 1.119 2009/02/22 23:50:57 djm Exp $ 38.\" $FreeBSD$ 39.Dd February 22 2009 40.Dt SSH_CONFIG 5 41.Os 42.Sh NAME 43.Nm ssh_config 44.Nd OpenSSH SSH client configuration files 45.Sh SYNOPSIS 46.Nm ~/.ssh/config 47.Nm /etc/ssh/ssh_config 48.Sh DESCRIPTION 49.Xr ssh 1 50obtains configuration data from the following sources in 51the following order: 52.Pp 53.Bl -enum -offset indent -compact 54.It 55command-line options 56.It 57user's configuration file 58.Pq Pa ~/.ssh/config 59.It 60system-wide configuration file 61.Pq Pa /etc/ssh/ssh_config 62.El 63.Pp 64For each parameter, the first obtained value 65will be used. 66The configuration files contain sections separated by 67.Dq Host 68specifications, and that section is only applied for hosts that 69match one of the patterns given in the specification. 70The matched host name is the one given on the command line. 71.Pp 72Since the first obtained value for each parameter is used, more 73host-specific declarations should be given near the beginning of the 74file, and general defaults at the end. 75.Pp 76The configuration file has the following format: 77.Pp 78Empty lines and lines starting with 79.Ql # 80are comments. 81Otherwise a line is of the format 82.Dq keyword arguments . 83Configuration options may be separated by whitespace or 84optional whitespace and exactly one 85.Ql = ; 86the latter format is useful to avoid the need to quote whitespace 87when specifying configuration options using the 88.Nm ssh , 89.Nm scp , 90and 91.Nm sftp 92.Fl o 93option. 94Arguments may optionally be enclosed in double quotes 95.Pq \&" 96in order to represent arguments containing spaces. 97.Pp 98The possible 99keywords and their meanings are as follows (note that 100keywords are case-insensitive and arguments are case-sensitive): 101.Bl -tag -width Ds 102.It Cm Host 103Restricts the following declarations (up to the next 104.Cm Host 105keyword) to be only for those hosts that match one of the patterns 106given after the keyword. 107If more than one pattern is provided, they should be separated by whitespace. 108A single 109.Ql * 110as a pattern can be used to provide global 111defaults for all hosts. 112The host is the 113.Ar hostname 114argument given on the command line (i.e. the name is not converted to 115a canonicalized host name before matching). 116.Pp 117See 118.Sx PATTERNS 119for more information on patterns. 120.It Cm AddressFamily 121Specifies which address family to use when connecting. 122Valid arguments are 123.Dq any , 124.Dq inet 125(use IPv4 only), or 126.Dq inet6 127(use IPv6 only). 128.It Cm BatchMode 129If set to 130.Dq yes , 131passphrase/password querying will be disabled. 132This option is useful in scripts and other batch jobs where no user 133is present to supply the password. 134The argument must be 135.Dq yes 136or 137.Dq no . 138The default is 139.Dq no . 140.It Cm BindAddress 141Use the specified address on the local machine as the source address of 142the connection. 143Only useful on systems with more than one address. 144Note that this option does not work if 145.Cm UsePrivilegedPort 146is set to 147.Dq yes . 148.It Cm ChallengeResponseAuthentication 149Specifies whether to use challenge-response authentication. 150The argument to this keyword must be 151.Dq yes 152or 153.Dq no . 154The default is 155.Dq yes . 156.It Cm CheckHostIP 157If this flag is set to 158.Dq yes , 159.Xr ssh 1 160will additionally check the host IP address in the 161.Pa known_hosts 162file. 163This allows ssh to detect if a host key changed due to DNS spoofing. 164If the option is set to 165.Dq no , 166the check will not be executed. 167The default is 168.Dq no . 169.It Cm Cipher 170Specifies the cipher to use for encrypting the session 171in protocol version 1. 172Currently, 173.Dq blowfish , 174.Dq 3des , 175and 176.Dq des 177are supported. 178.Ar des 179is only supported in the 180.Xr ssh 1 181client for interoperability with legacy protocol 1 implementations 182that do not support the 183.Ar 3des 184cipher. 185Its use is strongly discouraged due to cryptographic weaknesses. 186The default is 187.Dq 3des . 188.It Cm Ciphers 189Specifies the ciphers allowed for protocol version 2 190in order of preference. 191Multiple ciphers must be comma-separated. 192The supported ciphers are 193.Dq 3des-cbc , 194.Dq aes128-cbc , 195.Dq aes192-cbc , 196.Dq aes256-cbc , 197.Dq aes128-ctr , 198.Dq aes192-ctr , 199.Dq aes256-ctr , 200.Dq arcfour128 , 201.Dq arcfour256 , 202.Dq arcfour , 203.Dq blowfish-cbc , 204and 205.Dq cast128-cbc . 206The default is: 207.Bd -literal -offset 3n 208aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 209aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 210aes256-cbc,arcfour 211.Ed 212.It Cm ClearAllForwardings 213Specifies that all local, remote, and dynamic port forwardings 214specified in the configuration files or on the command line be 215cleared. 216This option is primarily useful when used from the 217.Xr ssh 1 218command line to clear port forwardings set in 219configuration files, and is automatically set by 220.Xr scp 1 221and 222.Xr sftp 1 . 223The argument must be 224.Dq yes 225or 226.Dq no . 227The default is 228.Dq no . 229.It Cm Compression 230Specifies whether to use compression. 231The argument must be 232.Dq yes 233or 234.Dq no . 235The default is 236.Dq no . 237.It Cm CompressionLevel 238Specifies the compression level to use if compression is enabled. 239The argument must be an integer from 1 (fast) to 9 (slow, best). 240The default level is 6, which is good for most applications. 241The meaning of the values is the same as in 242.Xr gzip 1 . 243Note that this option applies to protocol version 1 only. 244.It Cm ConnectionAttempts 245Specifies the number of tries (one per second) to make before exiting. 246The argument must be an integer. 247This may be useful in scripts if the connection sometimes fails. 248The default is 1. 249.It Cm ConnectTimeout 250Specifies the timeout (in seconds) used when connecting to the 251SSH server, instead of using the default system TCP timeout. 252This value is used only when the target is down or really unreachable, 253not when it refuses the connection. 254.It Cm ControlMaster 255Enables the sharing of multiple sessions over a single network connection. 256When set to 257.Dq yes , 258.Xr ssh 1 259will listen for connections on a control socket specified using the 260.Cm ControlPath 261argument. 262Additional sessions can connect to this socket using the same 263.Cm ControlPath 264with 265.Cm ControlMaster 266set to 267.Dq no 268(the default). 269These sessions will try to reuse the master instance's network connection 270rather than initiating new ones, but will fall back to connecting normally 271if the control socket does not exist, or is not listening. 272.Pp 273Setting this to 274.Dq ask 275will cause ssh 276to listen for control connections, but require confirmation using the 277.Ev SSH_ASKPASS 278program before they are accepted (see 279.Xr ssh-add 1 280for details). 281If the 282.Cm ControlPath 283cannot be opened, 284ssh will continue without connecting to a master instance. 285.Pp 286X11 and 287.Xr ssh-agent 1 288forwarding is supported over these multiplexed connections, however the 289display and agent forwarded will be the one belonging to the master 290connection i.e. it is not possible to forward multiple displays or agents. 291.Pp 292Two additional options allow for opportunistic multiplexing: try to use a 293master connection but fall back to creating a new one if one does not already 294exist. 295These options are: 296.Dq auto 297and 298.Dq autoask . 299The latter requires confirmation like the 300.Dq ask 301option. 302.It Cm ControlPath 303Specify the path to the control socket used for connection sharing as described 304in the 305.Cm ControlMaster 306section above or the string 307.Dq none 308to disable connection sharing. 309In the path, 310.Ql %l 311will be substituted by the local host name, 312.Ql %h 313will be substituted by the target host name, 314.Ql %p 315the port, and 316.Ql %r 317by the remote login username. 318It is recommended that any 319.Cm ControlPath 320used for opportunistic connection sharing include 321at least %h, %p, and %r. 322This ensures that shared connections are uniquely identified. 323.It Cm DynamicForward 324Specifies that a TCP port on the local machine be forwarded 325over the secure channel, and the application 326protocol is then used to determine where to connect to from the 327remote machine. 328.Pp 329The argument must be 330.Sm off 331.Oo Ar bind_address : Oc Ar port . 332.Sm on 333IPv6 addresses can be specified by enclosing addresses in square brackets or 334by using an alternative syntax: 335.Oo Ar bind_address Ns / Oc Ns Ar port . 336By default, the local port is bound in accordance with the 337.Cm GatewayPorts 338setting. 339However, an explicit 340.Ar bind_address 341may be used to bind the connection to a specific address. 342The 343.Ar bind_address 344of 345.Dq localhost 346indicates that the listening port be bound for local use only, while an 347empty address or 348.Sq * 349indicates that the port should be available from all interfaces. 350.Pp 351Currently the SOCKS4 and SOCKS5 protocols are supported, and 352.Xr ssh 1 353will act as a SOCKS server. 354Multiple forwardings may be specified, and 355additional forwardings can be given on the command line. 356Only the superuser can forward privileged ports. 357.It Cm EnableSSHKeysign 358Setting this option to 359.Dq yes 360in the global client configuration file 361.Pa /etc/ssh/ssh_config 362enables the use of the helper program 363.Xr ssh-keysign 8 364during 365.Cm HostbasedAuthentication . 366The argument must be 367.Dq yes 368or 369.Dq no . 370The default is 371.Dq no . 372This option should be placed in the non-hostspecific section. 373See 374.Xr ssh-keysign 8 375for more information. 376.It Cm EscapeChar 377Sets the escape character (default: 378.Ql ~ ) . 379The escape character can also 380be set on the command line. 381The argument should be a single character, 382.Ql ^ 383followed by a letter, or 384.Dq none 385to disable the escape 386character entirely (making the connection transparent for binary 387data). 388.It Cm ExitOnForwardFailure 389Specifies whether 390.Xr ssh 1 391should terminate the connection if it cannot set up all requested 392dynamic, tunnel, local, and remote port forwardings. 393The argument must be 394.Dq yes 395or 396.Dq no . 397The default is 398.Dq no . 399.It Cm ForwardAgent 400Specifies whether the connection to the authentication agent (if any) 401will be forwarded to the remote machine. 402The argument must be 403.Dq yes 404or 405.Dq no . 406The default is 407.Dq no . 408.Pp 409Agent forwarding should be enabled with caution. 410Users with the ability to bypass file permissions on the remote host 411(for the agent's Unix-domain socket) 412can access the local agent through the forwarded connection. 413An attacker cannot obtain key material from the agent, 414however they can perform operations on the keys that enable them to 415authenticate using the identities loaded into the agent. 416.It Cm ForwardX11 417Specifies whether X11 connections will be automatically redirected 418over the secure channel and 419.Ev DISPLAY 420set. 421The argument must be 422.Dq yes 423or 424.Dq no . 425The default is 426.Dq no . 427.Pp 428X11 forwarding should be enabled with caution. 429Users with the ability to bypass file permissions on the remote host 430(for the user's X11 authorization database) 431can access the local X11 display through the forwarded connection. 432An attacker may then be able to perform activities such as keystroke monitoring 433if the 434.Cm ForwardX11Trusted 435option is also enabled. 436.It Cm ForwardX11Trusted 437If this option is set to 438.Dq yes , 439remote X11 clients will have full access to the original X11 display. 440.Pp 441If this option is set to 442.Dq no , 443remote X11 clients will be considered untrusted and prevented 444from stealing or tampering with data belonging to trusted X11 445clients. 446Furthermore, the 447.Xr xauth 1 448token used for the session will be set to expire after 20 minutes. 449Remote clients will be refused access after this time. 450.Pp 451The default is 452.Dq no . 453.Pp 454See the X11 SECURITY extension specification for full details on 455the restrictions imposed on untrusted clients. 456.It Cm GatewayPorts 457Specifies whether remote hosts are allowed to connect to local 458forwarded ports. 459By default, 460.Xr ssh 1 461binds local port forwardings to the loopback address. 462This prevents other remote hosts from connecting to forwarded ports. 463.Cm GatewayPorts 464can be used to specify that ssh 465should bind local port forwardings to the wildcard address, 466thus allowing remote hosts to connect to forwarded ports. 467The argument must be 468.Dq yes 469or 470.Dq no . 471The default is 472.Dq no . 473.It Cm GlobalKnownHostsFile 474Specifies a file to use for the global 475host key database instead of 476.Pa /etc/ssh/ssh_known_hosts . 477.It Cm GSSAPIAuthentication 478Specifies whether user authentication based on GSSAPI is allowed. 479The default is 480.Dq no . 481Note that this option applies to protocol version 2 only. 482.It Cm GSSAPIDelegateCredentials 483Forward (delegate) credentials to the server. 484The default is 485.Dq no . 486Note that this option applies to protocol version 2 only. 487.It Cm HashKnownHosts 488Indicates that 489.Xr ssh 1 490should hash host names and addresses when they are added to 491.Pa ~/.ssh/known_hosts . 492These hashed names may be used normally by 493.Xr ssh 1 494and 495.Xr sshd 8 , 496but they do not reveal identifying information should the file's contents 497be disclosed. 498The default is 499.Dq no . 500Note that existing names and addresses in known hosts files 501will not be converted automatically, 502but may be manually hashed using 503.Xr ssh-keygen 1 . 504.It Cm HostbasedAuthentication 505Specifies whether to try rhosts based authentication with public key 506authentication. 507The argument must be 508.Dq yes 509or 510.Dq no . 511The default is 512.Dq no . 513This option applies to protocol version 2 only and 514is similar to 515.Cm RhostsRSAAuthentication . 516.It Cm HostKeyAlgorithms 517Specifies the protocol version 2 host key algorithms 518that the client wants to use in order of preference. 519The default for this option is: 520.Dq ssh-rsa,ssh-dss . 521.It Cm HostKeyAlias 522Specifies an alias that should be used instead of the 523real host name when looking up or saving the host key 524in the host key database files. 525This option is useful for tunneling SSH connections 526or for multiple servers running on a single host. 527.It Cm HostName 528Specifies the real host name to log into. 529This can be used to specify nicknames or abbreviations for hosts. 530The default is the name given on the command line. 531Numeric IP addresses are also permitted (both on the command line and in 532.Cm HostName 533specifications). 534.It Cm IdentitiesOnly 535Specifies that 536.Xr ssh 1 537should only use the authentication identity files configured in the 538.Nm 539files, 540even if 541.Xr ssh-agent 1 542offers more identities. 543The argument to this keyword must be 544.Dq yes 545or 546.Dq no . 547This option is intended for situations where ssh-agent 548offers many different identities. 549The default is 550.Dq no . 551.It Cm IdentityFile 552Specifies a file from which the user's RSA or DSA authentication identity 553is read. 554The default is 555.Pa ~/.ssh/identity 556for protocol version 1, and 557.Pa ~/.ssh/id_rsa 558and 559.Pa ~/.ssh/id_dsa 560for protocol version 2. 561Additionally, any identities represented by the authentication agent 562will be used for authentication. 563.Pp 564The file name may use the tilde 565syntax to refer to a user's home directory or one of the following 566escape characters: 567.Ql %d 568(local user's home directory), 569.Ql %u 570(local user name), 571.Ql %l 572(local host name), 573.Ql %h 574(remote host name) or 575.Ql %r 576(remote user name). 577.Pp 578It is possible to have 579multiple identity files specified in configuration files; all these 580identities will be tried in sequence. 581.It Cm KbdInteractiveAuthentication 582Specifies whether to use keyboard-interactive authentication. 583The argument to this keyword must be 584.Dq yes 585or 586.Dq no . 587The default is 588.Dq yes . 589.It Cm KbdInteractiveDevices 590Specifies the list of methods to use in keyboard-interactive authentication. 591Multiple method names must be comma-separated. 592The default is to use the server specified list. 593The methods available vary depending on what the server supports. 594For an OpenSSH server, 595it may be zero or more of: 596.Dq bsdauth , 597.Dq pam , 598and 599.Dq skey . 600.It Cm LocalCommand 601Specifies a command to execute on the local machine after successfully 602connecting to the server. 603The command string extends to the end of the line, and is executed with 604the user's shell. 605The following escape character substitutions will be performed: 606.Ql %d 607(local user's home directory), 608.Ql %h 609(remote host name), 610.Ql %l 611(local host name), 612.Ql %n 613(host name as provided on the command line), 614.Ql %p 615(remote port), 616.Ql %r 617(remote user name) or 618.Ql %u 619(local user name). 620This directive is ignored unless 621.Cm PermitLocalCommand 622has been enabled. 623.It Cm LocalForward 624Specifies that a TCP port on the local machine be forwarded over 625the secure channel to the specified host and port from the remote machine. 626The first argument must be 627.Sm off 628.Oo Ar bind_address : Oc Ar port 629.Sm on 630and the second argument must be 631.Ar host : Ns Ar hostport . 632IPv6 addresses can be specified by enclosing addresses in square brackets or 633by using an alternative syntax: 634.Oo Ar bind_address Ns / Oc Ns Ar port 635and 636.Ar host Ns / Ns Ar hostport . 637Multiple forwardings may be specified, and additional forwardings can be 638given on the command line. 639Only the superuser can forward privileged ports. 640By default, the local port is bound in accordance with the 641.Cm GatewayPorts 642setting. 643However, an explicit 644.Ar bind_address 645may be used to bind the connection to a specific address. 646The 647.Ar bind_address 648of 649.Dq localhost 650indicates that the listening port be bound for local use only, while an 651empty address or 652.Sq * 653indicates that the port should be available from all interfaces. 654.It Cm LogLevel 655Gives the verbosity level that is used when logging messages from 656.Xr ssh 1 . 657The possible values are: 658QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 659The default is INFO. 660DEBUG and DEBUG1 are equivalent. 661DEBUG2 and DEBUG3 each specify higher levels of verbose output. 662.It Cm MACs 663Specifies the MAC (message authentication code) algorithms 664in order of preference. 665The MAC algorithm is used in protocol version 2 666for data integrity protection. 667Multiple algorithms must be comma-separated. 668The default is: 669.Bd -literal -offset indent 670hmac-md5,hmac-sha1,umac-64@openssh.com, 671hmac-ripemd160,hmac-sha1-96,hmac-md5-96 672.Ed 673.It Cm NoHostAuthenticationForLocalhost 674This option can be used if the home directory is shared across machines. 675In this case localhost will refer to a different machine on each of 676the machines and the user will get many warnings about changed host keys. 677However, this option disables host authentication for localhost. 678The argument to this keyword must be 679.Dq yes 680or 681.Dq no . 682The default is to check the host key for localhost. 683.It Cm NumberOfPasswordPrompts 684Specifies the number of password prompts before giving up. 685The argument to this keyword must be an integer. 686The default is 3. 687.It Cm PasswordAuthentication 688Specifies whether to use password authentication. 689The argument to this keyword must be 690.Dq yes 691or 692.Dq no . 693The default is 694.Dq yes . 695.It Cm PermitLocalCommand 696Allow local command execution via the 697.Ic LocalCommand 698option or using the 699.Ic !\& Ns Ar command 700escape sequence in 701.Xr ssh 1 . 702The argument must be 703.Dq yes 704or 705.Dq no . 706The default is 707.Dq no . 708.It Cm Port 709Specifies the port number to connect on the remote host. 710The default is 22. 711.It Cm PreferredAuthentications 712Specifies the order in which the client should try protocol 2 713authentication methods. 714This allows a client to prefer one method (e.g.\& 715.Cm keyboard-interactive ) 716over another method (e.g.\& 717.Cm password ) 718The default for this option is: 719.Do gssapi-with-mic , 720hostbased, 721publickey, 722keyboard-interactive, 723password 724.Dc . 725.It Cm Protocol 726Specifies the protocol versions 727.Xr ssh 1 728should support in order of preference. 729The possible values are 730.Sq 1 731and 732.Sq 2 . 733Multiple versions must be comma-separated. 734The default is 735.Dq 2,1 . 736This means that ssh 737tries version 2 and falls back to version 1 738if version 2 is not available. 739.It Cm ProxyCommand 740Specifies the command to use to connect to the server. 741The command 742string extends to the end of the line, and is executed with 743the user's shell. 744In the command string, 745.Ql %h 746will be substituted by the host name to 747connect and 748.Ql %p 749by the port. 750The command can be basically anything, 751and should read from its standard input and write to its standard output. 752It should eventually connect an 753.Xr sshd 8 754server running on some machine, or execute 755.Ic sshd -i 756somewhere. 757Host key management will be done using the 758HostName of the host being connected (defaulting to the name typed by 759the user). 760Setting the command to 761.Dq none 762disables this option entirely. 763Note that 764.Cm CheckHostIP 765is not available for connects with a proxy command. 766.Pp 767This directive is useful in conjunction with 768.Xr nc 1 769and its proxy support. 770For example, the following directive would connect via an HTTP proxy at 771192.0.2.0: 772.Bd -literal -offset 3n 773ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 774.Ed 775.It Cm PubkeyAuthentication 776Specifies whether to try public key authentication. 777The argument to this keyword must be 778.Dq yes 779or 780.Dq no . 781The default is 782.Dq yes . 783This option applies to protocol version 2 only. 784.It Cm RekeyLimit 785Specifies the maximum amount of data that may be transmitted before the 786session key is renegotiated. 787The argument is the number of bytes, with an optional suffix of 788.Sq K , 789.Sq M , 790or 791.Sq G 792to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 793The default is between 794.Sq 1G 795and 796.Sq 4G , 797depending on the cipher. 798This option applies to protocol version 2 only. 799.It Cm RemoteForward 800Specifies that a TCP port on the remote machine be forwarded over 801the secure channel to the specified host and port from the local machine. 802The first argument must be 803.Sm off 804.Oo Ar bind_address : Oc Ar port 805.Sm on 806and the second argument must be 807.Ar host : Ns Ar hostport . 808IPv6 addresses can be specified by enclosing addresses in square brackets 809or by using an alternative syntax: 810.Oo Ar bind_address Ns / Oc Ns Ar port 811and 812.Ar host Ns / Ns Ar hostport . 813Multiple forwardings may be specified, and additional 814forwardings can be given on the command line. 815Privileged ports can be forwarded only when 816logging in as root on the remote machine. 817.Pp 818If the 819.Ar port 820argument is 821.Ql 0 , 822the listen port will be dynamically allocated on the server and reported 823to the client at run time. 824.Pp 825If the 826.Ar bind_address 827is not specified, the default is to only bind to loopback addresses. 828If the 829.Ar bind_address 830is 831.Ql * 832or an empty string, then the forwarding is requested to listen on all 833interfaces. 834Specifying a remote 835.Ar bind_address 836will only succeed if the server's 837.Cm GatewayPorts 838option is enabled (see 839.Xr sshd_config 5 ) . 840.It Cm RhostsRSAAuthentication 841Specifies whether to try rhosts based authentication with RSA host 842authentication. 843The argument must be 844.Dq yes 845or 846.Dq no . 847The default is 848.Dq no . 849This option applies to protocol version 1 only and requires 850.Xr ssh 1 851to be setuid root. 852.It Cm RSAAuthentication 853Specifies whether to try RSA authentication. 854The argument to this keyword must be 855.Dq yes 856or 857.Dq no . 858RSA authentication will only be 859attempted if the identity file exists, or an authentication agent is 860running. 861The default is 862.Dq yes . 863Note that this option applies to protocol version 1 only. 864.It Cm SendEnv 865Specifies what variables from the local 866.Xr environ 7 867should be sent to the server. 868Note that environment passing is only supported for protocol 2. 869The server must also support it, and the server must be configured to 870accept these environment variables. 871Refer to 872.Cm AcceptEnv 873in 874.Xr sshd_config 5 875for how to configure the server. 876Variables are specified by name, which may contain wildcard characters. 877Multiple environment variables may be separated by whitespace or spread 878across multiple 879.Cm SendEnv 880directives. 881The default is not to send any environment variables. 882.Pp 883See 884.Sx PATTERNS 885for more information on patterns. 886.It Cm ServerAliveCountMax 887Sets the number of server alive messages (see below) which may be 888sent without 889.Xr ssh 1 890receiving any messages back from the server. 891If this threshold is reached while server alive messages are being sent, 892ssh will disconnect from the server, terminating the session. 893It is important to note that the use of server alive messages is very 894different from 895.Cm TCPKeepAlive 896(below). 897The server alive messages are sent through the encrypted channel 898and therefore will not be spoofable. 899The TCP keepalive option enabled by 900.Cm TCPKeepAlive 901is spoofable. 902The server alive mechanism is valuable when the client or 903server depend on knowing when a connection has become inactive. 904.Pp 905The default value is 3. 906If, for example, 907.Cm ServerAliveInterval 908(see below) is set to 15 and 909.Cm ServerAliveCountMax 910is left at the default, if the server becomes unresponsive, 911ssh will disconnect after approximately 45 seconds. 912This option applies to protocol version 2 only. 913.It Cm ServerAliveInterval 914Sets a timeout interval in seconds after which if no data has been received 915from the server, 916.Xr ssh 1 917will send a message through the encrypted 918channel to request a response from the server. 919The default 920is 0, indicating that these messages will not be sent to the server. 921This option applies to protocol version 2 only. 922.It Cm SmartcardDevice 923Specifies which smartcard device to use. 924The argument to this keyword is the device 925.Xr ssh 1 926should use to communicate with a smartcard used for storing the user's 927private RSA key. 928By default, no device is specified and smartcard support is not activated. 929.It Cm StrictHostKeyChecking 930If this flag is set to 931.Dq yes , 932.Xr ssh 1 933will never automatically add host keys to the 934.Pa ~/.ssh/known_hosts 935file, and refuses to connect to hosts whose host key has changed. 936This provides maximum protection against trojan horse attacks, 937though it can be annoying when the 938.Pa /etc/ssh/ssh_known_hosts 939file is poorly maintained or when connections to new hosts are 940frequently made. 941This option forces the user to manually 942add all new hosts. 943If this flag is set to 944.Dq no , 945ssh will automatically add new host keys to the 946user known hosts files. 947If this flag is set to 948.Dq ask , 949new host keys 950will be added to the user known host files only after the user 951has confirmed that is what they really want to do, and 952ssh will refuse to connect to hosts whose host key has changed. 953The host keys of 954known hosts will be verified automatically in all cases. 955The argument must be 956.Dq yes , 957.Dq no , 958or 959.Dq ask . 960The default is 961.Dq ask . 962.It Cm TCPKeepAlive 963Specifies whether the system should send TCP keepalive messages to the 964other side. 965If they are sent, death of the connection or crash of one 966of the machines will be properly noticed. 967However, this means that 968connections will die if the route is down temporarily, and some people 969find it annoying. 970.Pp 971The default is 972.Dq yes 973(to send TCP keepalive messages), and the client will notice 974if the network goes down or the remote host dies. 975This is important in scripts, and many users want it too. 976.Pp 977To disable TCP keepalive messages, the value should be set to 978.Dq no . 979.It Cm Tunnel 980Request 981.Xr tun 4 982device forwarding between the client and the server. 983The argument must be 984.Dq yes , 985.Dq point-to-point 986(layer 3), 987.Dq ethernet 988(layer 2), 989or 990.Dq no . 991Specifying 992.Dq yes 993requests the default tunnel mode, which is 994.Dq point-to-point . 995The default is 996.Dq no . 997.It Cm TunnelDevice 998Specifies the 999.Xr tun 4 1000devices to open on the client 1001.Pq Ar local_tun 1002and the server 1003.Pq Ar remote_tun . 1004.Pp 1005The argument must be 1006.Sm off 1007.Ar local_tun Op : Ar remote_tun . 1008.Sm on 1009The devices may be specified by numerical ID or the keyword 1010.Dq any , 1011which uses the next available tunnel device. 1012If 1013.Ar remote_tun 1014is not specified, it defaults to 1015.Dq any . 1016The default is 1017.Dq any:any . 1018.It Cm UsePrivilegedPort 1019Specifies whether to use a privileged port for outgoing connections. 1020The argument must be 1021.Dq yes 1022or 1023.Dq no . 1024The default is 1025.Dq no . 1026If set to 1027.Dq yes , 1028.Xr ssh 1 1029must be setuid root. 1030Note that this option must be set to 1031.Dq yes 1032for 1033.Cm RhostsRSAAuthentication 1034with older servers. 1035.It Cm User 1036Specifies the user to log in as. 1037This can be useful when a different user name is used on different machines. 1038This saves the trouble of 1039having to remember to give the user name on the command line. 1040.It Cm UserKnownHostsFile 1041Specifies a file to use for the user 1042host key database instead of 1043.Pa ~/.ssh/known_hosts . 1044.It Cm VerifyHostKeyDNS 1045Specifies whether to verify the remote key using DNS and SSHFP resource 1046records. 1047If this option is set to 1048.Dq yes , 1049the client will implicitly trust keys that match a secure fingerprint 1050from DNS. 1051Insecure fingerprints will be handled as if this option was set to 1052.Dq ask . 1053If this option is set to 1054.Dq ask , 1055information on fingerprint match will be displayed, but the user will still 1056need to confirm new host keys according to the 1057.Cm StrictHostKeyChecking 1058option. 1059The argument must be 1060.Dq yes , 1061.Dq no , 1062or 1063.Dq ask . 1064The default is 1065.Dq no . 1066Note that this option applies to protocol version 2 only. 1067.Pp 1068See also 1069.Sx VERIFYING HOST KEYS 1070in 1071.Xr ssh 1 . 1072.It Cm VersionAddendum 1073Specifies a string to append to the regular version string to identify 1074OS- or site-specific modifications. 1075The default is 1076.Dq FreeBSD-20090522 . 1077.It Cm VisualHostKey 1078If this flag is set to 1079.Dq yes , 1080an ASCII art representation of the remote host key fingerprint is 1081printed in addition to the hex fingerprint string at login and 1082for unknown host keys. 1083If this flag is set to 1084.Dq no , 1085no fingerprint strings are printed at login and 1086only the hex fingerprint string will be printed for unknown host keys. 1087The default is 1088.Dq no . 1089.It Cm XAuthLocation 1090Specifies the full pathname of the 1091.Xr xauth 1 1092program. 1093The default is 1094.Pa /usr/local/bin/xauth . 1095.El 1096.Sh PATTERNS 1097A 1098.Em pattern 1099consists of zero or more non-whitespace characters, 1100.Sq * 1101(a wildcard that matches zero or more characters), 1102or 1103.Sq ?\& 1104(a wildcard that matches exactly one character). 1105For example, to specify a set of declarations for any host in the 1106.Dq .co.uk 1107set of domains, 1108the following pattern could be used: 1109.Pp 1110.Dl Host *.co.uk 1111.Pp 1112The following pattern 1113would match any host in the 192.168.0.[0-9] network range: 1114.Pp 1115.Dl Host 192.168.0.? 1116.Pp 1117A 1118.Em pattern-list 1119is a comma-separated list of patterns. 1120Patterns within pattern-lists may be negated 1121by preceding them with an exclamation mark 1122.Pq Sq !\& . 1123For example, 1124to allow a key to be used from anywhere within an organisation 1125except from the 1126.Dq dialup 1127pool, 1128the following entry (in authorized_keys) could be used: 1129.Pp 1130.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1131.Sh FILES 1132.Bl -tag -width Ds 1133.It Pa ~/.ssh/config 1134This is the per-user configuration file. 1135The format of this file is described above. 1136This file is used by the SSH client. 1137Because of the potential for abuse, this file must have strict permissions: 1138read/write for the user, and not accessible by others. 1139.It Pa /etc/ssh/ssh_config 1140Systemwide configuration file. 1141This file provides defaults for those 1142values that are not specified in the user's configuration file, and 1143for those users who do not have a configuration file. 1144This file must be world-readable. 1145.El 1146.Sh SEE ALSO 1147.Xr ssh 1 1148.Sh AUTHORS 1149OpenSSH is a derivative of the original and free 1150ssh 1.2.12 release by Tatu Ylonen. 1151Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1152Theo de Raadt and Dug Song 1153removed many bugs, re-added newer features and 1154created OpenSSH. 1155Markus Friedl contributed the support for SSH 1156protocol versions 1.5 and 2.0. 1157