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.\" $FreeBSD$ 39.Dd August 4, 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 ControlPersist 324When used in conjunction with 325.Cm ControlMaster , 326specifies that the master connection should remain open 327in the background (waiting for future client connections) 328after the initial client connection has been closed. 329If set to 330.Dq no , 331then the master connection will not be placed into the background, 332and will close as soon as the initial client connection is closed. 333If set to 334.Dq yes , 335then the master connection will remain in the background indefinitely 336(until killed or closed via a mechanism such as the 337.Xr ssh 1 338.Dq Fl O No exit 339option). 340If set to a time in seconds, or a time in any of the formats documented in 341.Xr sshd_config 5 , 342then the backgrounded master connection will automatically terminate 343after it has remained idle (with no client connections) for the 344specified time. 345.It Cm DynamicForward 346Specifies that a TCP port on the local machine be forwarded 347over the secure channel, and the application 348protocol is then used to determine where to connect to from the 349remote machine. 350.Pp 351The argument must be 352.Sm off 353.Oo Ar bind_address : Oc Ar port . 354.Sm on 355IPv6 addresses can be specified by enclosing addresses in square brackets. 356By default, the local port is bound in accordance with the 357.Cm GatewayPorts 358setting. 359However, an explicit 360.Ar bind_address 361may be used to bind the connection to a specific address. 362The 363.Ar bind_address 364of 365.Dq localhost 366indicates that the listening port be bound for local use only, while an 367empty address or 368.Sq * 369indicates that the port should be available from all interfaces. 370.Pp 371Currently the SOCKS4 and SOCKS5 protocols are supported, and 372.Xr ssh 1 373will act as a SOCKS server. 374Multiple forwardings may be specified, and 375additional forwardings can be given on the command line. 376Only the superuser can forward privileged ports. 377.It Cm EnableSSHKeysign 378Setting this option to 379.Dq yes 380in the global client configuration file 381.Pa /etc/ssh/ssh_config 382enables the use of the helper program 383.Xr ssh-keysign 8 384during 385.Cm HostbasedAuthentication . 386The argument must be 387.Dq yes 388or 389.Dq no . 390The default is 391.Dq no . 392This option should be placed in the non-hostspecific section. 393See 394.Xr ssh-keysign 8 395for more information. 396.It Cm EscapeChar 397Sets the escape character (default: 398.Ql ~ ) . 399The escape character can also 400be set on the command line. 401The argument should be a single character, 402.Ql ^ 403followed by a letter, or 404.Dq none 405to disable the escape 406character entirely (making the connection transparent for binary 407data). 408.It Cm ExitOnForwardFailure 409Specifies whether 410.Xr ssh 1 411should terminate the connection if it cannot set up all requested 412dynamic, tunnel, local, and remote port forwardings. 413The argument must be 414.Dq yes 415or 416.Dq no . 417The default is 418.Dq no . 419.It Cm ForwardAgent 420Specifies whether the connection to the authentication agent (if any) 421will be forwarded to the remote machine. 422The argument must be 423.Dq yes 424or 425.Dq no . 426The default is 427.Dq no . 428.Pp 429Agent forwarding should be enabled with caution. 430Users with the ability to bypass file permissions on the remote host 431(for the agent's Unix-domain socket) 432can access the local agent through the forwarded connection. 433An attacker cannot obtain key material from the agent, 434however they can perform operations on the keys that enable them to 435authenticate using the identities loaded into the agent. 436.It Cm ForwardX11 437Specifies whether X11 connections will be automatically redirected 438over the secure channel and 439.Ev DISPLAY 440set. 441The argument must be 442.Dq yes 443or 444.Dq no . 445The default is 446.Dq no . 447.Pp 448X11 forwarding should be enabled with caution. 449Users with the ability to bypass file permissions on the remote host 450(for the user's X11 authorization database) 451can access the local X11 display through the forwarded connection. 452An attacker may then be able to perform activities such as keystroke monitoring 453if the 454.Cm ForwardX11Trusted 455option is also enabled. 456.It Cm ForwardX11Timeout 457Specify a timeout for untrusted X11 forwarding 458using the format described in the 459.Sx TIME FORMATS 460section of 461.Xr sshd_config 5 . 462X11 connections received by 463.Xr ssh 1 464after this time will be refused. 465The default is to disable untrusted X11 forwarding after twenty minutes has 466elapsed. 467.It Cm ForwardX11Trusted 468If this option is set to 469.Dq yes , 470remote X11 clients will have full access to the original X11 display. 471.Pp 472If this option is set to 473.Dq no , 474remote X11 clients will be considered untrusted and prevented 475from stealing or tampering with data belonging to trusted X11 476clients. 477Furthermore, the 478.Xr xauth 1 479token used for the session will be set to expire after 20 minutes. 480Remote clients will be refused access after this time. 481.Pp 482The default is 483.Dq no . 484.Pp 485See the X11 SECURITY extension specification for full details on 486the restrictions imposed on untrusted clients. 487.It Cm GatewayPorts 488Specifies whether remote hosts are allowed to connect to local 489forwarded ports. 490By default, 491.Xr ssh 1 492binds local port forwardings to the loopback address. 493This prevents other remote hosts from connecting to forwarded ports. 494.Cm GatewayPorts 495can be used to specify that ssh 496should bind local port forwardings to the wildcard address, 497thus allowing remote hosts to connect to forwarded ports. 498The argument must be 499.Dq yes 500or 501.Dq no . 502The default is 503.Dq no . 504.It Cm GlobalKnownHostsFile 505Specifies a file to use for the global 506host key database instead of 507.Pa /etc/ssh/ssh_known_hosts . 508.It Cm GSSAPIAuthentication 509Specifies whether user authentication based on GSSAPI is allowed. 510The default is 511.Dq no . 512Note that this option applies to protocol version 2 only. 513.It Cm GSSAPIDelegateCredentials 514Forward (delegate) credentials to the server. 515The default is 516.Dq no . 517Note that this option applies to protocol version 2 only. 518.It Cm HashKnownHosts 519Indicates that 520.Xr ssh 1 521should hash host names and addresses when they are added to 522.Pa ~/.ssh/known_hosts . 523These hashed names may be used normally by 524.Xr ssh 1 525and 526.Xr sshd 8 , 527but they do not reveal identifying information should the file's contents 528be disclosed. 529The default is 530.Dq no . 531Note that existing names and addresses in known hosts files 532will not be converted automatically, 533but may be manually hashed using 534.Xr ssh-keygen 1 . 535.It Cm HostbasedAuthentication 536Specifies whether to try rhosts based authentication with public key 537authentication. 538The argument must be 539.Dq yes 540or 541.Dq no . 542The default is 543.Dq no . 544This option applies to protocol version 2 only and 545is similar to 546.Cm RhostsRSAAuthentication . 547.It Cm HostKeyAlgorithms 548Specifies the protocol version 2 host key algorithms 549that the client wants to use in order of preference. 550The default for this option is: 551.Dq ssh-rsa,ssh-dss . 552.It Cm HostKeyAlias 553Specifies an alias that should be used instead of the 554real host name when looking up or saving the host key 555in the host key database files. 556This option is useful for tunneling SSH connections 557or for multiple servers running on a single host. 558.It Cm HostName 559Specifies the real host name to log into. 560This can be used to specify nicknames or abbreviations for hosts. 561If the hostname contains the character sequence 562.Ql %h , 563then this will be replaced with the host name specified on the commandline 564(this is useful for manipulating unqualified names). 565The default is the name given on the command line. 566Numeric IP addresses are also permitted (both on the command line and in 567.Cm HostName 568specifications). 569.It Cm IdentitiesOnly 570Specifies that 571.Xr ssh 1 572should only use the authentication identity files configured in the 573.Nm 574files, 575even if 576.Xr ssh-agent 1 577offers more identities. 578The argument to this keyword must be 579.Dq yes 580or 581.Dq no . 582This option is intended for situations where ssh-agent 583offers many different identities. 584The default is 585.Dq no . 586.It Cm IdentityFile 587Specifies a file from which the user's RSA or DSA authentication identity 588is read. 589The default is 590.Pa ~/.ssh/identity 591for protocol version 1, and 592.Pa ~/.ssh/id_rsa 593and 594.Pa ~/.ssh/id_dsa 595for protocol version 2. 596Additionally, any identities represented by the authentication agent 597will be used for authentication. 598.Xr ssh 1 599will try to load certificate information from the filename obtained by 600appending 601.Pa -cert.pub 602to the path of a specified 603.Cm IdentityFile . 604.Pp 605The file name may use the tilde 606syntax to refer to a user's home directory or one of the following 607escape characters: 608.Ql %d 609(local user's home directory), 610.Ql %u 611(local user name), 612.Ql %l 613(local host name), 614.Ql %h 615(remote host name) or 616.Ql %r 617(remote user name). 618.Pp 619It is possible to have 620multiple identity files specified in configuration files; all these 621identities will be tried in sequence. 622.It Cm KbdInteractiveAuthentication 623Specifies whether to use keyboard-interactive authentication. 624The argument to this keyword must be 625.Dq yes 626or 627.Dq no . 628The default is 629.Dq yes . 630.It Cm KbdInteractiveDevices 631Specifies the list of methods to use in keyboard-interactive authentication. 632Multiple method names must be comma-separated. 633The default is to use the server specified list. 634The methods available vary depending on what the server supports. 635For an OpenSSH server, 636it may be zero or more of: 637.Dq bsdauth , 638.Dq pam , 639and 640.Dq skey . 641.It Cm LocalCommand 642Specifies a command to execute on the local machine after successfully 643connecting to the server. 644The command string extends to the end of the line, and is executed with 645the user's shell. 646The following escape character substitutions will be performed: 647.Ql %d 648(local user's home directory), 649.Ql %h 650(remote host name), 651.Ql %l 652(local host name), 653.Ql %n 654(host name as provided on the command line), 655.Ql %p 656(remote port), 657.Ql %r 658(remote user name) or 659.Ql %u 660(local user name). 661.Pp 662The command is run synchronously and does not have access to the 663session of the 664.Xr ssh 1 665that spawned it. 666It should not be used for interactive commands. 667.Pp 668This directive is ignored unless 669.Cm PermitLocalCommand 670has been enabled. 671.It Cm LocalForward 672Specifies that a TCP port on the local machine be forwarded over 673the secure channel to the specified host and port from the remote machine. 674The first argument must be 675.Sm off 676.Oo Ar bind_address : Oc Ar port 677.Sm on 678and the second argument must be 679.Ar host : Ns Ar hostport . 680IPv6 addresses can be specified by enclosing addresses in square brackets. 681Multiple forwardings may be specified, and additional forwardings can be 682given on the command line. 683Only the superuser can forward privileged ports. 684By default, the local port is bound in accordance with the 685.Cm GatewayPorts 686setting. 687However, an explicit 688.Ar bind_address 689may be used to bind the connection to a specific address. 690The 691.Ar bind_address 692of 693.Dq localhost 694indicates that the listening port be bound for local use only, while an 695empty address or 696.Sq * 697indicates that the port should be available from all interfaces. 698.It Cm LogLevel 699Gives the verbosity level that is used when logging messages from 700.Xr ssh 1 . 701The possible values are: 702QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. 703The default is INFO. 704DEBUG and DEBUG1 are equivalent. 705DEBUG2 and DEBUG3 each specify higher levels of verbose output. 706.It Cm MACs 707Specifies the MAC (message authentication code) algorithms 708in order of preference. 709The MAC algorithm is used in protocol version 2 710for data integrity protection. 711Multiple algorithms must be comma-separated. 712The default is: 713.Bd -literal -offset indent 714hmac-md5,hmac-sha1,umac-64@openssh.com, 715hmac-ripemd160,hmac-sha1-96,hmac-md5-96 716.Ed 717.It Cm NoHostAuthenticationForLocalhost 718This option can be used if the home directory is shared across machines. 719In this case localhost will refer to a different machine on each of 720the machines and the user will get many warnings about changed host keys. 721However, this option disables host authentication for localhost. 722The argument to this keyword must be 723.Dq yes 724or 725.Dq no . 726The default is to check the host key for localhost. 727.It Cm NumberOfPasswordPrompts 728Specifies the number of password prompts before giving up. 729The argument to this keyword must be an integer. 730The default is 3. 731.It Cm PasswordAuthentication 732Specifies whether to use password authentication. 733The argument to this keyword must be 734.Dq yes 735or 736.Dq no . 737The default is 738.Dq yes . 739.It Cm PermitLocalCommand 740Allow local command execution via the 741.Ic LocalCommand 742option or using the 743.Ic !\& Ns Ar command 744escape sequence in 745.Xr ssh 1 . 746The argument must be 747.Dq yes 748or 749.Dq no . 750The default is 751.Dq no . 752.It Cm PKCS11Provider 753Specifies which PKCS#11 provider to use. 754The argument to this keyword is the PKCS#11 shared libary 755.Xr ssh 1 756should use to communicate with a PKCS#11 token providing the user's 757private RSA key. 758.It Cm Port 759Specifies the port number to connect on the remote host. 760The default is 22. 761.It Cm PreferredAuthentications 762Specifies the order in which the client should try protocol 2 763authentication methods. 764This allows a client to prefer one method (e.g.\& 765.Cm keyboard-interactive ) 766over another method (e.g.\& 767.Cm password ) . 768The default is: 769.Bd -literal -offset indent 770gssapi-with-mic,hostbased,publickey, 771keyboard-interactive,password 772.Ed 773.It Cm Protocol 774Specifies the protocol versions 775.Xr ssh 1 776should support in order of preference. 777The possible values are 778.Sq 1 779and 780.Sq 2 . 781Multiple versions must be comma-separated. 782When this option is set to 783.Dq 2,1 784.Nm ssh 785will try version 2 and fall back to version 1 786if version 2 is not available. 787The default is 788.Sq 2 . 789.It Cm ProxyCommand 790Specifies the command to use to connect to the server. 791The command 792string extends to the end of the line, and is executed with 793the user's shell. 794In the command string, any occurrence of 795.Ql %h 796will be substituted by the host name to 797connect, 798.Ql %p 799by the port, and 800.Ql %r 801by the remote user name. 802The command can be basically anything, 803and should read from its standard input and write to its standard output. 804It should eventually connect an 805.Xr sshd 8 806server running on some machine, or execute 807.Ic sshd -i 808somewhere. 809Host key management will be done using the 810HostName of the host being connected (defaulting to the name typed by 811the user). 812Setting the command to 813.Dq none 814disables this option entirely. 815Note that 816.Cm CheckHostIP 817is not available for connects with a proxy command. 818.Pp 819This directive is useful in conjunction with 820.Xr nc 1 821and its proxy support. 822For example, the following directive would connect via an HTTP proxy at 823192.0.2.0: 824.Bd -literal -offset 3n 825ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p 826.Ed 827.It Cm PubkeyAuthentication 828Specifies whether to try public key authentication. 829The argument to this keyword must be 830.Dq yes 831or 832.Dq no . 833The default is 834.Dq yes . 835This option applies to protocol version 2 only. 836.It Cm RekeyLimit 837Specifies the maximum amount of data that may be transmitted before the 838session key is renegotiated. 839The argument is the number of bytes, with an optional suffix of 840.Sq K , 841.Sq M , 842or 843.Sq G 844to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 845The default is between 846.Sq 1G 847and 848.Sq 4G , 849depending on the cipher. 850This option applies to protocol version 2 only. 851.It Cm RemoteForward 852Specifies that a TCP port on the remote machine be forwarded over 853the secure channel to the specified host and port from the local machine. 854The first argument must be 855.Sm off 856.Oo Ar bind_address : Oc Ar port 857.Sm on 858and the second argument must be 859.Ar host : Ns Ar hostport . 860IPv6 addresses can be specified by enclosing addresses in square brackets. 861Multiple forwardings may be specified, and additional 862forwardings can be given on the command line. 863Privileged ports can be forwarded only when 864logging in as root on the remote machine. 865.Pp 866If the 867.Ar port 868argument is 869.Ql 0 , 870the listen port will be dynamically allocated on the server and reported 871to the client at run time. 872.Pp 873If the 874.Ar bind_address 875is not specified, the default is to only bind to loopback addresses. 876If the 877.Ar bind_address 878is 879.Ql * 880or an empty string, then the forwarding is requested to listen on all 881interfaces. 882Specifying a remote 883.Ar bind_address 884will only succeed if the server's 885.Cm GatewayPorts 886option is enabled (see 887.Xr sshd_config 5 ) . 888.It Cm RhostsRSAAuthentication 889Specifies whether to try rhosts based authentication with RSA host 890authentication. 891The argument must be 892.Dq yes 893or 894.Dq no . 895The default is 896.Dq no . 897This option applies to protocol version 1 only and requires 898.Xr ssh 1 899to be setuid root. 900.It Cm RSAAuthentication 901Specifies whether to try RSA authentication. 902The argument to this keyword must be 903.Dq yes 904or 905.Dq no . 906RSA authentication will only be 907attempted if the identity file exists, or an authentication agent is 908running. 909The default is 910.Dq yes . 911Note that this option applies to protocol version 1 only. 912.It Cm SendEnv 913Specifies what variables from the local 914.Xr environ 7 915should be sent to the server. 916Note that environment passing is only supported for protocol 2. 917The server must also support it, and the server must be configured to 918accept these environment variables. 919Refer to 920.Cm AcceptEnv 921in 922.Xr sshd_config 5 923for how to configure the server. 924Variables are specified by name, which may contain wildcard characters. 925Multiple environment variables may be separated by whitespace or spread 926across multiple 927.Cm SendEnv 928directives. 929The default is not to send any environment variables. 930.Pp 931See 932.Sx PATTERNS 933for more information on patterns. 934.It Cm ServerAliveCountMax 935Sets the number of server alive messages (see below) which may be 936sent without 937.Xr ssh 1 938receiving any messages back from the server. 939If this threshold is reached while server alive messages are being sent, 940ssh will disconnect from the server, terminating the session. 941It is important to note that the use of server alive messages is very 942different from 943.Cm TCPKeepAlive 944(below). 945The server alive messages are sent through the encrypted channel 946and therefore will not be spoofable. 947The TCP keepalive option enabled by 948.Cm TCPKeepAlive 949is spoofable. 950The server alive mechanism is valuable when the client or 951server depend on knowing when a connection has become inactive. 952.Pp 953The default value is 3. 954If, for example, 955.Cm ServerAliveInterval 956(see below) is set to 15 and 957.Cm ServerAliveCountMax 958is left at the default, if the server becomes unresponsive, 959ssh will disconnect after approximately 45 seconds. 960This option applies to protocol version 2 only. 961.It Cm ServerAliveInterval 962Sets a timeout interval in seconds after which if no data has been received 963from the server, 964.Xr ssh 1 965will send a message through the encrypted 966channel to request a response from the server. 967The default 968is 0, indicating that these messages will not be sent to the server. 969This option applies to protocol version 2 only. 970.It Cm StrictHostKeyChecking 971If this flag is set to 972.Dq yes , 973.Xr ssh 1 974will never automatically add host keys to the 975.Pa ~/.ssh/known_hosts 976file, and refuses to connect to hosts whose host key has changed. 977This provides maximum protection against trojan horse attacks, 978though it can be annoying when the 979.Pa /etc/ssh/ssh_known_hosts 980file is poorly maintained or when connections to new hosts are 981frequently made. 982This option forces the user to manually 983add all new hosts. 984If this flag is set to 985.Dq no , 986ssh will automatically add new host keys to the 987user known hosts files. 988If this flag is set to 989.Dq ask , 990new host keys 991will be added to the user known host files only after the user 992has confirmed that is what they really want to do, and 993ssh will refuse to connect to hosts whose host key has changed. 994The host keys of 995known hosts will be verified automatically in all cases. 996The argument must be 997.Dq yes , 998.Dq no , 999or 1000.Dq ask . 1001The default is 1002.Dq ask . 1003.It Cm TCPKeepAlive 1004Specifies whether the system should send TCP keepalive messages to the 1005other side. 1006If they are sent, death of the connection or crash of one 1007of the machines will be properly noticed. 1008However, this means that 1009connections will die if the route is down temporarily, and some people 1010find it annoying. 1011.Pp 1012The default is 1013.Dq yes 1014(to send TCP keepalive messages), and the client will notice 1015if the network goes down or the remote host dies. 1016This is important in scripts, and many users want it too. 1017.Pp 1018To disable TCP keepalive messages, the value should be set to 1019.Dq no . 1020.It Cm Tunnel 1021Request 1022.Xr tun 4 1023device forwarding between the client and the server. 1024The argument must be 1025.Dq yes , 1026.Dq point-to-point 1027(layer 3), 1028.Dq ethernet 1029(layer 2), 1030or 1031.Dq no . 1032Specifying 1033.Dq yes 1034requests the default tunnel mode, which is 1035.Dq point-to-point . 1036The default is 1037.Dq no . 1038.It Cm TunnelDevice 1039Specifies the 1040.Xr tun 4 1041devices to open on the client 1042.Pq Ar local_tun 1043and the server 1044.Pq Ar remote_tun . 1045.Pp 1046The argument must be 1047.Sm off 1048.Ar local_tun Op : Ar remote_tun . 1049.Sm on 1050The devices may be specified by numerical ID or the keyword 1051.Dq any , 1052which uses the next available tunnel device. 1053If 1054.Ar remote_tun 1055is not specified, it defaults to 1056.Dq any . 1057The default is 1058.Dq any:any . 1059.It Cm UsePrivilegedPort 1060Specifies whether to use a privileged port for outgoing connections. 1061The argument must be 1062.Dq yes 1063or 1064.Dq no . 1065The default is 1066.Dq no . 1067If set to 1068.Dq yes , 1069.Xr ssh 1 1070must be setuid root. 1071Note that this option must be set to 1072.Dq yes 1073for 1074.Cm RhostsRSAAuthentication 1075with older servers. 1076.It Cm User 1077Specifies the user to log in as. 1078This can be useful when a different user name is used on different machines. 1079This saves the trouble of 1080having to remember to give the user name on the command line. 1081.It Cm UserKnownHostsFile 1082Specifies a file to use for the user 1083host key database instead of 1084.Pa ~/.ssh/known_hosts . 1085.It Cm VerifyHostKeyDNS 1086Specifies whether to verify the remote key using DNS and SSHFP resource 1087records. 1088If this option is set to 1089.Dq yes , 1090the client will implicitly trust keys that match a secure fingerprint 1091from DNS. 1092Insecure fingerprints will be handled as if this option was set to 1093.Dq ask . 1094If this option is set to 1095.Dq ask , 1096information on fingerprint match will be displayed, but the user will still 1097need to confirm new host keys according to the 1098.Cm StrictHostKeyChecking 1099option. 1100The argument must be 1101.Dq yes , 1102.Dq no , 1103or 1104.Dq ask . 1105The default is 1106.Dq no . 1107Note that this option applies to protocol version 2 only. 1108.Pp 1109See also 1110.Sx VERIFYING HOST KEYS 1111in 1112.Xr ssh 1 . 1113.It Cm VersionAddendum 1114Specifies a string to append to the regular version string to identify 1115OS- or site-specific modifications. 1116The default is 1117.Dq FreeBSD-20101111 . 1118.It Cm VisualHostKey 1119If this flag is set to 1120.Dq yes , 1121an ASCII art representation of the remote host key fingerprint is 1122printed in addition to the hex fingerprint string at login and 1123for unknown host keys. 1124If this flag is set to 1125.Dq no , 1126no fingerprint strings are printed at login and 1127only the hex fingerprint string will be printed for unknown host keys. 1128The default is 1129.Dq no . 1130.It Cm XAuthLocation 1131Specifies the full pathname of the 1132.Xr xauth 1 1133program. 1134The default is 1135.Pa /usr/local/bin/xauth . 1136.El 1137.Sh PATTERNS 1138A 1139.Em pattern 1140consists of zero or more non-whitespace characters, 1141.Sq * 1142(a wildcard that matches zero or more characters), 1143or 1144.Sq ?\& 1145(a wildcard that matches exactly one character). 1146For example, to specify a set of declarations for any host in the 1147.Dq .co.uk 1148set of domains, 1149the following pattern could be used: 1150.Pp 1151.Dl Host *.co.uk 1152.Pp 1153The following pattern 1154would match any host in the 192.168.0.[0-9] network range: 1155.Pp 1156.Dl Host 192.168.0.? 1157.Pp 1158A 1159.Em pattern-list 1160is a comma-separated list of patterns. 1161Patterns within pattern-lists may be negated 1162by preceding them with an exclamation mark 1163.Pq Sq !\& . 1164For example, 1165to allow a key to be used from anywhere within an organisation 1166except from the 1167.Dq dialup 1168pool, 1169the following entry (in authorized_keys) could be used: 1170.Pp 1171.Dl from=\&"!*.dialup.example.com,*.example.com\&" 1172.Sh FILES 1173.Bl -tag -width Ds 1174.It Pa ~/.ssh/config 1175This is the per-user configuration file. 1176The format of this file is described above. 1177This file is used by the SSH client. 1178Because of the potential for abuse, this file must have strict permissions: 1179read/write for the user, and not accessible by others. 1180.It Pa /etc/ssh/ssh_config 1181Systemwide configuration file. 1182This file provides defaults for those 1183values that are not specified in the user's configuration file, and 1184for those users who do not have a configuration file. 1185This file must be world-readable. 1186.El 1187.Sh SEE ALSO 1188.Xr ssh 1 1189.Sh AUTHORS 1190OpenSSH is a derivative of the original and free 1191ssh 1.2.12 release by Tatu Ylonen. 1192Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 1193Theo de Raadt and Dug Song 1194removed many bugs, re-added newer features and 1195created OpenSSH. 1196Markus Friedl contributed the support for SSH 1197protocol versions 1.5 and 2.0. 1198