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 1.130 2010/03/26 01:06:13 dtucker Exp $ 38.\" $FreeBSD$ 39.Dd March 26, 2010 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.Xr ssh 1 564will try to load certificate information from the filename obtained by 565appending 566.Pa -cert.pub 567to the path of a specified 568.Cm IdentityFile . 569.Pp 570The file name may use the tilde 571syntax to refer to a user's home directory or one of the following 572escape characters: 573.Ql %d 574(local user's home directory), 575.Ql %u 576(local user name), 577.Ql %l 578(local host name), 579.Ql %h 580(remote host name) or 581.Ql %r 582(remote user name). 583.Pp 584It is possible to have 585multiple identity files specified in configuration files; all these 586identities will be tried in sequence. 587.It Cm KbdInteractiveAuthentication 588Specifies whether to use keyboard-interactive authentication. 589The argument to this keyword must be 590.Dq yes 591or 592.Dq no . 593The default is 594.Dq yes . 595.It Cm KbdInteractiveDevices 596Specifies the list of methods to use in keyboard-interactive authentication. 597Multiple method names must be comma-separated. 598The default is to use the server specified list. 599The methods available vary depending on what the server supports. 600For an OpenSSH server, 601it may be zero or more of: 602.Dq bsdauth , 603.Dq pam , 604and 605.Dq skey . 606.It Cm LocalCommand 607Specifies a command to execute on the local machine after successfully 608connecting to the server. 609The command string extends to the end of the line, and is executed with 610the user's shell. 611The following escape character substitutions will be performed: 612.Ql %d 613(local user's home directory), 614.Ql %h 615(remote host name), 616.Ql %l 617(local host name), 618.Ql %n 619(host name as provided on the command line), 620.Ql %p 621(remote port), 622.Ql %r 623(remote user name) or 624.Ql %u 625(local user name). 626.Pp 627The command is run synchronously and does not have access to the 628session of the 629.Xr ssh 1 630that spawned it. 631It should not be used for interactive commands. 632.Pp 633This directive is ignored unless 634.Cm PermitLocalCommand 635has been enabled. 636.It Cm LocalForward 637Specifies that a TCP port on the local machine be forwarded over 638the secure channel to the specified host and port from the remote machine. 639The first argument must be 640.Sm off 641.Oo Ar bind_address : Oc Ar port 642.Sm on 643and the second argument must be 644.Ar host : Ns Ar hostport . 645IPv6 addresses can be specified by enclosing addresses in square brackets or 646by using an alternative syntax: 647.Oo Ar bind_address Ns / Oc Ns Ar port 648and 649.Ar host Ns / Ns Ar hostport . 650Multiple forwardings may be specified, and additional forwardings can be 651given on the command line. 652Only the superuser can forward privileged ports. 653By default, the local port is bound in accordance with the 654.Cm GatewayPorts 655setting. 656However, an explicit 657.Ar bind_address 658may be used to bind the connection to a specific address. 659The 660.Ar bind_address 661of 662.Dq localhost 663indicates that the listening port be bound for local use only, while an 664empty address or 665.Sq * 666indicates that the port should be available from all interfaces. 667.It Cm LogLevel 668Gives the verbosity level that is used when logging messages from 669.Xr ssh 1 . 670The possible values are: 671QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 672The default is INFO. 673DEBUG and DEBUG1 are equivalent. 674DEBUG2 and DEBUG3 each specify higher levels of verbose output. 675.It Cm MACs 676Specifies the MAC (message authentication code) algorithms 677in order of preference. 678The MAC algorithm is used in protocol version 2 679for data integrity protection. 680Multiple algorithms must be comma-separated. 681The default is: 682.Bd -literal -offset indent 683hmac-md5,hmac-sha1,umac-64@openssh.com, 684hmac-ripemd160,hmac-sha1-96,hmac-md5-96 685.Ed 686.It Cm NoHostAuthenticationForLocalhost 687This option can be used if the home directory is shared across machines. 688In this case localhost will refer to a different machine on each of 689the machines and the user will get many warnings about changed host keys. 690However, this option disables host authentication for localhost. 691The argument to this keyword must be 692.Dq yes 693or 694.Dq no . 695The default is to check the host key for localhost. 696.It Cm NumberOfPasswordPrompts 697Specifies the number of password prompts before giving up. 698The argument to this keyword must be an integer. 699The default is 3. 700.It Cm PasswordAuthentication 701Specifies whether to use password authentication. 702The argument to this keyword must be 703.Dq yes 704or 705.Dq no . 706The default is 707.Dq yes . 708.It Cm PermitLocalCommand 709Allow local command execution via the 710.Ic LocalCommand 711option or using the 712.Ic !\& Ns Ar command 713escape sequence in 714.Xr ssh 1 . 715The argument must be 716.Dq yes 717or 718.Dq no . 719The default is 720.Dq no . 721.It Cm PKCS11Provider 722Specifies which PKCS#11 provider to use. 723The argument to this keyword is the PKCS#11 shared libary 724.Xr ssh 1 725should use to communicate with a PKCS#11 token providing the user's 726private RSA key. 727.It Cm Port 728Specifies the port number to connect on the remote host. 729The default is 22. 730.It Cm PreferredAuthentications 731Specifies the order in which the client should try protocol 2 732authentication methods. 733This allows a client to prefer one method (e.g.\& 734.Cm keyboard-interactive ) 735over another method (e.g.\& 736.Cm password ) 737The default for this option is: 738.Do gssapi-with-mic,hostbased,publickey,keyboard-interactive,password 739.Dc . 740.It Cm Protocol 741Specifies the protocol versions 742.Xr ssh 1 743should support in order of preference. 744The possible values are 745.Sq 1 746and 747.Sq 2 . 748Multiple versions must be comma-separated. 749When this option is set to 750.Dq 2,1 751.Nm ssh 752will try version 2 and fall back to version 1 753if version 2 is not available. 754The default is 755.Sq 2 . 756.It Cm ProxyCommand 757Specifies the command to use to connect to the server. 758The command 759string extends to the end of the line, and is executed with 760the user's shell. 761In the command string, 762.Ql %h 763will be substituted by the host name to 764connect and 765.Ql %p 766by the port. 767The command can be basically anything, 768and should read from its standard input and write to its standard output. 769It should eventually connect an 770.Xr sshd 8 771server running on some machine, or execute 772.Ic sshd -i 773somewhere. 774Host key management will be done using the 775HostName of the host being connected (defaulting to the name typed by 776the user). 777Setting the command to 778.Dq none 779disables this option entirely. 780Note that 781.Cm CheckHostIP 782is not available for connects with a proxy command. 783.Pp 784This directive is useful in conjunction with 785.Xr nc 1 786and its proxy support. 787For example, the following directive would connect via an HTTP proxy at 788192.0.2.0: 789.Bd -literal -offset 3n 790ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 791.Ed 792.It Cm PubkeyAuthentication 793Specifies whether to try public key authentication. 794The argument to this keyword must be 795.Dq yes 796or 797.Dq no . 798The default is 799.Dq yes . 800This option applies to protocol version 2 only. 801.It Cm RekeyLimit 802Specifies the maximum amount of data that may be transmitted before the 803session key is renegotiated. 804The argument is the number of bytes, with an optional suffix of 805.Sq K , 806.Sq M , 807or 808.Sq G 809to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 810The default is between 811.Sq 1G 812and 813.Sq 4G , 814depending on the cipher. 815This option applies to protocol version 2 only. 816.It Cm RemoteForward 817Specifies that a TCP port on the remote machine be forwarded over 818the secure channel to the specified host and port from the local machine. 819The first argument must be 820.Sm off 821.Oo Ar bind_address : Oc Ar port 822.Sm on 823and the second argument must be 824.Ar host : Ns Ar hostport . 825IPv6 addresses can be specified by enclosing addresses in square brackets 826or by using an alternative syntax: 827.Oo Ar bind_address Ns / Oc Ns Ar port 828and 829.Ar host Ns / Ns Ar hostport . 830Multiple forwardings may be specified, and additional 831forwardings can be given on the command line. 832Privileged ports can be forwarded only when 833logging in as root on the remote machine. 834.Pp 835If the 836.Ar port 837argument is 838.Ql 0 , 839the listen port will be dynamically allocated on the server and reported 840to the client at run time. 841.Pp 842If the 843.Ar bind_address 844is not specified, the default is to only bind to loopback addresses. 845If the 846.Ar bind_address 847is 848.Ql * 849or an empty string, then the forwarding is requested to listen on all 850interfaces. 851Specifying a remote 852.Ar bind_address 853will only succeed if the server's 854.Cm GatewayPorts 855option is enabled (see 856.Xr sshd_config 5 ) . 857.It Cm RhostsRSAAuthentication 858Specifies whether to try rhosts based authentication with RSA host 859authentication. 860The argument must be 861.Dq yes 862or 863.Dq no . 864The default is 865.Dq no . 866This option applies to protocol version 1 only and requires 867.Xr ssh 1 868to be setuid root. 869.It Cm RSAAuthentication 870Specifies whether to try RSA authentication. 871The argument to this keyword must be 872.Dq yes 873or 874.Dq no . 875RSA authentication will only be 876attempted if the identity file exists, or an authentication agent is 877running. 878The default is 879.Dq yes . 880Note that this option applies to protocol version 1 only. 881.It Cm SendEnv 882Specifies what variables from the local 883.Xr environ 7 884should be sent to the server. 885Note that environment passing is only supported for protocol 2. 886The server must also support it, and the server must be configured to 887accept these environment variables. 888Refer to 889.Cm AcceptEnv 890in 891.Xr sshd_config 5 892for how to configure the server. 893Variables are specified by name, which may contain wildcard characters. 894Multiple environment variables may be separated by whitespace or spread 895across multiple 896.Cm SendEnv 897directives. 898The default is not to send any environment variables. 899.Pp 900See 901.Sx PATTERNS 902for more information on patterns. 903.It Cm ServerAliveCountMax 904Sets the number of server alive messages (see below) which may be 905sent without 906.Xr ssh 1 907receiving any messages back from the server. 908If this threshold is reached while server alive messages are being sent, 909ssh will disconnect from the server, terminating the session. 910It is important to note that the use of server alive messages is very 911different from 912.Cm TCPKeepAlive 913(below). 914The server alive messages are sent through the encrypted channel 915and therefore will not be spoofable. 916The TCP keepalive option enabled by 917.Cm TCPKeepAlive 918is spoofable. 919The server alive mechanism is valuable when the client or 920server depend on knowing when a connection has become inactive. 921.Pp 922The default value is 3. 923If, for example, 924.Cm ServerAliveInterval 925(see below) is set to 15 and 926.Cm ServerAliveCountMax 927is left at the default, if the server becomes unresponsive, 928ssh will disconnect after approximately 45 seconds. 929This option applies to protocol version 2 only. 930.It Cm ServerAliveInterval 931Sets a timeout interval in seconds after which if no data has been received 932from the server, 933.Xr ssh 1 934will send a message through the encrypted 935channel to request a response from the server. 936The default 937is 0, indicating that these messages will not be sent to the server. 938This option applies to protocol version 2 only. 939.It Cm StrictHostKeyChecking 940If this flag is set to 941.Dq yes , 942.Xr ssh 1 943will never automatically add host keys to the 944.Pa ~/.ssh/known_hosts 945file, and refuses to connect to hosts whose host key has changed. 946This provides maximum protection against trojan horse attacks, 947though it can be annoying when the 948.Pa /etc/ssh/ssh_known_hosts 949file is poorly maintained or when connections to new hosts are 950frequently made. 951This option forces the user to manually 952add all new hosts. 953If this flag is set to 954.Dq no , 955ssh will automatically add new host keys to the 956user known hosts files. 957If this flag is set to 958.Dq ask , 959new host keys 960will be added to the user known host files only after the user 961has confirmed that is what they really want to do, and 962ssh will refuse to connect to hosts whose host key has changed. 963The host keys of 964known hosts will be verified automatically in all cases. 965The argument must be 966.Dq yes , 967.Dq no , 968or 969.Dq ask . 970The default is 971.Dq ask . 972.It Cm TCPKeepAlive 973Specifies whether the system should send TCP keepalive messages to the 974other side. 975If they are sent, death of the connection or crash of one 976of the machines will be properly noticed. 977However, this means that 978connections will die if the route is down temporarily, and some people 979find it annoying. 980.Pp 981The default is 982.Dq yes 983(to send TCP keepalive messages), and the client will notice 984if the network goes down or the remote host dies. 985This is important in scripts, and many users want it too. 986.Pp 987To disable TCP keepalive messages, the value should be set to 988.Dq no . 989.It Cm Tunnel 990Request 991.Xr tun 4 992device forwarding between the client and the server. 993The argument must be 994.Dq yes , 995.Dq point-to-point 996(layer 3), 997.Dq ethernet 998(layer 2), 999or 1000.Dq no . 1001Specifying 1002.Dq yes 1003requests the default tunnel mode, which is 1004.Dq point-to-point . 1005The default is 1006.Dq no . 1007.It Cm TunnelDevice 1008Specifies the 1009.Xr tun 4 1010devices to open on the client 1011.Pq Ar local_tun 1012and the server 1013.Pq Ar remote_tun . 1014.Pp 1015The argument must be 1016.Sm off 1017.Ar local_tun Op : Ar remote_tun . 1018.Sm on 1019The devices may be specified by numerical ID or the keyword 1020.Dq any , 1021which uses the next available tunnel device. 1022If 1023.Ar remote_tun 1024is not specified, it defaults to 1025.Dq any . 1026The default is 1027.Dq any:any . 1028.It Cm UsePrivilegedPort 1029Specifies whether to use a privileged port for outgoing connections. 1030The argument must be 1031.Dq yes 1032or 1033.Dq no . 1034The default is 1035.Dq no . 1036If set to 1037.Dq yes , 1038.Xr ssh 1 1039must be setuid root. 1040Note that this option must be set to 1041.Dq yes 1042for 1043.Cm RhostsRSAAuthentication 1044with older servers. 1045.It Cm User 1046Specifies the user to log in as. 1047This can be useful when a different user name is used on different machines. 1048This saves the trouble of 1049having to remember to give the user name on the command line. 1050.It Cm UserKnownHostsFile 1051Specifies a file to use for the user 1052host key database instead of 1053.Pa ~/.ssh/known_hosts . 1054.It Cm VerifyHostKeyDNS 1055Specifies whether to verify the remote key using DNS and SSHFP resource 1056records. 1057If this option is set to 1058.Dq yes , 1059the client will implicitly trust keys that match a secure fingerprint 1060from DNS. 1061Insecure fingerprints will be handled as if this option was set to 1062.Dq ask . 1063If this option is set to 1064.Dq ask , 1065information on fingerprint match will be displayed, but the user will still 1066need to confirm new host keys according to the 1067.Cm StrictHostKeyChecking 1068option. 1069The argument must be 1070.Dq yes , 1071.Dq no , 1072or 1073.Dq ask . 1074The default is 1075.Dq no . 1076Note that this option applies to protocol version 2 only. 1077.Pp 1078See also 1079.Sx VERIFYING HOST KEYS 1080in 1081.Xr ssh 1 . 1082.It Cm VersionAddendum 1083Specifies a string to append to the regular version string to identify 1084OS- or site-specific modifications. 1085The default is 1086.Dq FreeBSD-20100428 . 1087.It Cm VisualHostKey 1088If this flag is set to 1089.Dq yes , 1090an ASCII art representation of the remote host key fingerprint is 1091printed in addition to the hex fingerprint string at login and 1092for unknown host keys. 1093If this flag is set to 1094.Dq no , 1095no fingerprint strings are printed at login and 1096only the hex fingerprint string will be printed for unknown host keys. 1097The default is 1098.Dq no . 1099.It Cm XAuthLocation 1100Specifies the full pathname of the 1101.Xr xauth 1 1102program. 1103The default is 1104.Pa /usr/local/bin/xauth . 1105.El 1106.Sh PATTERNS 1107A 1108.Em pattern 1109consists of zero or more non-whitespace characters, 1110.Sq * 1111(a wildcard that matches zero or more characters), 1112or 1113.Sq ?\& 1114(a wildcard that matches exactly one character). 1115For example, to specify a set of declarations for any host in the 1116.Dq .co.uk 1117set of domains, 1118the following pattern could be used: 1119.Pp 1120.Dl Host *.co.uk 1121.Pp 1122The following pattern 1123would match any host in the 192.168.0.[0-9] network range: 1124.Pp 1125.Dl Host 192.168.0.? 1126.Pp 1127A 1128.Em pattern-list 1129is a comma-separated list of patterns. 1130Patterns within pattern-lists may be negated 1131by preceding them with an exclamation mark 1132.Pq Sq !\& . 1133For example, 1134to allow a key to be used from anywhere within an organisation 1135except from the 1136.Dq dialup 1137pool, 1138the following entry (in authorized_keys) could be used: 1139.Pp 1140.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1141.Sh FILES 1142.Bl -tag -width Ds 1143.It Pa ~/.ssh/config 1144This is the per-user configuration file. 1145The format of this file is described above. 1146This file is used by the SSH client. 1147Because of the potential for abuse, this file must have strict permissions: 1148read/write for the user, and not accessible by others. 1149.It Pa /etc/ssh/ssh_config 1150Systemwide configuration file. 1151This file provides defaults for those 1152values that are not specified in the user's configuration file, and 1153for those users who do not have a configuration file. 1154This file must be world-readable. 1155.El 1156.Sh SEE ALSO 1157.Xr ssh 1 1158.Sh AUTHORS 1159OpenSSH is a derivative of the original and free 1160ssh 1.2.12 release by Tatu Ylonen. 1161Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1162Theo de Raadt and Dug Song 1163removed many bugs, re-added newer features and 1164created OpenSSH. 1165Markus Friedl contributed the support for SSH 1166protocol versions 1.5 and 2.0. 1167