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 1.28 2003/12/16 15:49:51 markus Exp $ 38.\" $FreeBSD$ 39.Dd September 25, 1999 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.Bl -tag -width Ds -compact 47.It Pa $HOME/.ssh/config 48.It Pa /etc/ssh/ssh_config 49.El 50.Sh DESCRIPTION 51.Nm ssh 52obtains configuration data from the following sources in 53the following order: 54.Bl -enum -offset indent -compact 55.It 56command-line options 57.It 58user's configuration file 59.Pq Pa $HOME/.ssh/config 60.It 61system-wide configuration file 62.Pq Pa /etc/ssh/ssh_config 63.El 64.Pp 65For each parameter, the first obtained value 66will be used. 67The configuration files contain sections bracketed by 68.Dq Host 69specifications, and that section is only applied for hosts that 70match one of the patterns given in the specification. 71The matched host name is the one given on the command line. 72.Pp 73Since the first obtained value for each parameter is used, more 74host-specific declarations should be given near the beginning of the 75file, and general defaults at the end. 76.Pp 77The configuration file has the following format: 78.Pp 79Empty lines and lines starting with 80.Ql # 81are comments. 82.Pp 83Otherwise a line is of the format 84.Dq keyword arguments . 85Configuration options may be separated by whitespace or 86optional whitespace and exactly one 87.Ql = ; 88the latter format is useful to avoid the need to quote whitespace 89when specifying configuration options using the 90.Nm ssh , 91.Nm scp 92and 93.Nm sftp 94.Fl o 95option. 96.Pp 97The possible 98keywords and their meanings are as follows (note that 99keywords are case-insensitive and arguments are case-sensitive): 100.Bl -tag -width Ds 101.It Cm Host 102Restricts the following declarations (up to the next 103.Cm Host 104keyword) to be only for those hosts that match one of the patterns 105given after the keyword. 106.Ql \&* 107and 108.Ql \&? 109can be used as wildcards in the 110patterns. 111A single 112.Ql \&* 113as a pattern can be used to provide global 114defaults for all hosts. 115The host is the 116.Ar hostname 117argument given on the command line (i.e., the name is not converted to 118a canonicalized host name before matching). 119.It Cm AddressFamily 120Specifies which address family to use when connecting. 121Valid arguments are 122.Dq any , 123.Dq inet 124(Use IPv4 only) or 125.Dq inet6 126(Use IPv6 only.) 127.It Cm BatchMode 128If set to 129.Dq yes , 130passphrase/password querying will be disabled. 131This option is useful in scripts and other batch jobs where no user 132is present to supply the password. 133The argument must be 134.Dq yes 135or 136.Dq no . 137The default is 138.Dq no . 139.It Cm BindAddress 140Specify the interface to transmit from on machines with multiple 141interfaces or aliased addresses. 142Note that this option does not work if 143.Cm UsePrivilegedPort 144is set to 145.Dq yes . 146.It Cm ChallengeResponseAuthentication 147Specifies whether to use challenge response authentication. 148The argument to this keyword must be 149.Dq yes 150or 151.Dq no . 152The default is 153.Dq yes . 154.It Cm CheckHostIP 155If this flag is set to 156.Dq yes , 157ssh will additionally check the host IP address in the 158.Pa known_hosts 159file. 160This allows ssh to detect if a host key changed due to DNS spoofing. 161If the option is set to 162.Dq no , 163the check will not be executed. 164The default is 165.Dq no . 166.It Cm Cipher 167Specifies the cipher to use for encrypting the session 168in protocol version 1. 169Currently, 170.Dq blowfish , 171.Dq 3des , 172and 173.Dq des 174are supported. 175.Ar des 176is only supported in the 177.Nm ssh 178client for interoperability with legacy protocol 1 implementations 179that do not support the 180.Ar 3des 181cipher. 182Its use is strongly discouraged due to cryptographic weaknesses. 183The default is 184.Dq 3des . 185.It Cm Ciphers 186Specifies the ciphers allowed for protocol version 2 187in order of preference. 188Multiple ciphers must be comma-separated. 189The default is 190.Bd -literal 191 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, 192 aes192-cbc,aes256-cbc'' 193.Ed 194.It Cm ClearAllForwardings 195Specifies that all local, remote and dynamic port forwardings 196specified in the configuration files or on the command line be 197cleared. 198This option is primarily useful when used from the 199.Nm ssh 200command line to clear port forwardings set in 201configuration files, and is automatically set by 202.Xr scp 1 203and 204.Xr sftp 1 . 205The argument must be 206.Dq yes 207or 208.Dq no . 209The default is 210.Dq no . 211.It Cm Compression 212Specifies whether to use compression. 213The argument must be 214.Dq yes 215or 216.Dq no . 217The default is 218.Dq no . 219.It Cm CompressionLevel 220Specifies the compression level to use if compression is enabled. 221The argument must be an integer from 1 (fast) to 9 (slow, best). 222The default level is 6, which is good for most applications. 223The meaning of the values is the same as in 224.Xr gzip 1 . 225Note that this option applies to protocol version 1 only. 226.It Cm ConnectionAttempts 227Specifies the number of tries (one per second) to make before exiting. 228The argument must be an integer. 229This may be useful in scripts if the connection sometimes fails. 230The default is 1. 231.It Cm ConnectTimeout 232Specifies the timeout (in seconds) used when connecting to the ssh 233server, instead of using the default system TCP timeout. 234This value is used only when the target is down or really unreachable, 235not when it refuses the connection. 236.It Cm DynamicForward 237Specifies that a TCP/IP port on the local machine be forwarded 238over the secure channel, and the application 239protocol is then used to determine where to connect to from the 240remote machine. 241The argument must be a port number. 242Currently the SOCKS4 and SOCKS5 protocols are supported, and 243.Nm ssh 244will act as a SOCKS server. 245Multiple forwardings may be specified, and 246additional forwardings can be given on the command line. 247Only the superuser can forward privileged ports. 248.It Cm EnableSSHKeysign 249Setting this option to 250.Dq yes 251in the global client configuration file 252.Pa /etc/ssh/ssh_config 253enables the use of the helper program 254.Xr ssh-keysign 8 255during 256.Cm HostbasedAuthentication . 257The argument must be 258.Dq yes 259or 260.Dq no . 261The default is 262.Dq no . 263This option should be placed in the non-hostspecific section. 264See 265.Xr ssh-keysign 8 266for more information. 267.It Cm EscapeChar 268Sets the escape character (default: 269.Ql ~ ) . 270The escape character can also 271be set on the command line. 272The argument should be a single character, 273.Ql ^ 274followed by a letter, or 275.Dq none 276to disable the escape 277character entirely (making the connection transparent for binary 278data). 279.It Cm ForwardAgent 280Specifies whether the connection to the authentication agent (if any) 281will be forwarded to the remote machine. 282The argument must be 283.Dq yes 284or 285.Dq no . 286The default is 287.Dq no . 288.Pp 289Agent forwarding should be enabled with caution. 290Users with the ability to bypass file permissions on the remote host 291(for the agent's Unix-domain socket) 292can access the local agent through the forwarded connection. 293An attacker cannot obtain key material from the agent, 294however they can perform operations on the keys that enable them to 295authenticate using the identities loaded into the agent. 296.It Cm ForwardX11 297Specifies whether X11 connections will be automatically redirected 298over the secure channel and 299.Ev DISPLAY 300set. 301The argument must be 302.Dq yes 303or 304.Dq no . 305The default is 306.Dq no . 307.Pp 308X11 forwarding should be enabled with caution. 309Users with the ability to bypass file permissions on the remote host 310(for the user's X11 authorization database) 311can access the local X11 display through the forwarded connection. 312An attacker may then be able to perform activities such as keystroke monitoring 313if the 314.Cm ForwardX11Trusted 315option is also enabled. 316.It Cm ForwardX11Trusted 317If the this option is set to 318.Dq yes 319then remote X11 clients will have full access to the original X11 display. 320If this option is set to 321.Dq no 322then remote X11 clients will be considered untrusted and prevented 323from stealing or tampering with data belonging to trusted X11 324clients. 325.Pp 326The default is 327.Dq no . 328.Pp 329See the X11 SECURITY extension specification for full details on 330the restrictions imposed on untrusted clients. 331.It Cm GatewayPorts 332Specifies whether remote hosts are allowed to connect to local 333forwarded ports. 334By default, 335.Nm ssh 336binds local port forwardings to the loopback address. 337This prevents other remote hosts from connecting to forwarded ports. 338.Cm GatewayPorts 339can be used to specify that 340.Nm ssh 341should bind local port forwardings to the wildcard address, 342thus allowing remote hosts to connect to forwarded ports. 343The argument must be 344.Dq yes 345or 346.Dq no . 347The default is 348.Dq no . 349.It Cm GlobalKnownHostsFile 350Specifies a file to use for the global 351host key database instead of 352.Pa /etc/ssh/ssh_known_hosts . 353.It Cm GSSAPIAuthentication 354Specifies whether user authentication based on GSSAPI is allowed. 355The default is 356.Dq no . 357Note that this option applies to protocol version 2 only. 358.It Cm GSSAPIDelegateCredentials 359Forward (delegate) credentials to the server. 360The default is 361.Dq no . 362Note that this option applies to protocol version 2 only. 363.It Cm HostbasedAuthentication 364Specifies whether to try rhosts based authentication with public key 365authentication. 366The argument must be 367.Dq yes 368or 369.Dq no . 370The default is 371.Dq no . 372This option applies to protocol version 2 only and 373is similar to 374.Cm RhostsRSAAuthentication . 375.It Cm HostKeyAlgorithms 376Specifies the protocol version 2 host key algorithms 377that the client wants to use in order of preference. 378The default for this option is: 379.Dq ssh-rsa,ssh-dss . 380.It Cm HostKeyAlias 381Specifies an alias that should be used instead of the 382real host name when looking up or saving the host key 383in the host key database files. 384This option is useful for tunneling ssh connections 385or for multiple servers running on a single host. 386.It Cm HostName 387Specifies the real host name to log into. 388This can be used to specify nicknames or abbreviations for hosts. 389Default is the name given on the command line. 390Numeric IP addresses are also permitted (both on the command line and in 391.Cm HostName 392specifications). 393.It Cm IdentityFile 394Specifies a file from which the user's RSA or DSA authentication identity 395is read. 396The default is 397.Pa $HOME/.ssh/identity 398for protocol version 1, and 399.Pa $HOME/.ssh/id_rsa 400and 401.Pa $HOME/.ssh/id_dsa 402for protocol version 2. 403Additionally, any identities represented by the authentication agent 404will be used for authentication. 405The file name may use the tilde 406syntax to refer to a user's home directory. 407It is possible to have 408multiple identity files specified in configuration files; all these 409identities will be tried in sequence. 410.It Cm LocalForward 411Specifies that a TCP/IP port on the local machine be forwarded over 412the secure channel to the specified host and port from the remote machine. 413The first argument must be a port number, and the second must be 414.Ar host:port . 415IPv6 addresses can be specified with an alternative syntax: 416.Ar host/port . 417Multiple forwardings may be specified, and additional 418forwardings can be given on the command line. 419Only the superuser can forward privileged ports. 420.It Cm LogLevel 421Gives the verbosity level that is used when logging messages from 422.Nm ssh . 423The possible values are: 424QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. 425The default is INFO. 426DEBUG and DEBUG1 are equivalent. 427DEBUG2 and DEBUG3 each specify higher levels of verbose output. 428.It Cm MACs 429Specifies the MAC (message authentication code) algorithms 430in order of preference. 431The MAC algorithm is used in protocol version 2 432for data integrity protection. 433Multiple algorithms must be comma-separated. 434The default is 435.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . 436.It Cm NoHostAuthenticationForLocalhost 437This option can be used if the home directory is shared across machines. 438In this case localhost will refer to a different machine on each of 439the machines and the user will get many warnings about changed host keys. 440However, this option disables host authentication for localhost. 441The argument to this keyword must be 442.Dq yes 443or 444.Dq no . 445The default is to check the host key for localhost. 446.It Cm NumberOfPasswordPrompts 447Specifies the number of password prompts before giving up. 448The argument to this keyword must be an integer. 449Default is 3. 450.It Cm PasswordAuthentication 451Specifies whether to use password authentication. 452The argument to this keyword must be 453.Dq yes 454or 455.Dq no . 456The default is 457.Dq yes . 458.It Cm Port 459Specifies the port number to connect on the remote host. 460Default is 22. 461.It Cm PreferredAuthentications 462Specifies the order in which the client should try protocol 2 463authentication methods. 464This allows a client to prefer one method (e.g. 465.Cm keyboard-interactive ) 466over another method (e.g. 467.Cm password ) 468The default for this option is: 469.Dq hostbased,publickey,keyboard-interactive,password . 470.It Cm Protocol 471Specifies the protocol versions 472.Nm ssh 473should support in order of preference. 474The possible values are 475.Dq 1 476and 477.Dq 2 . 478Multiple versions must be comma-separated. 479The default is 480.Dq 2,1 . 481This means that 482.Nm ssh 483tries version 2 and falls back to version 1 484if version 2 is not available. 485.It Cm ProxyCommand 486Specifies the command to use to connect to the server. 487The command 488string extends to the end of the line, and is executed with 489.Pa /bin/sh . 490In the command string, 491.Ql %h 492will be substituted by the host name to 493connect and 494.Ql %p 495by the port. 496The command can be basically anything, 497and should read from its standard input and write to its standard output. 498It should eventually connect an 499.Xr sshd 8 500server running on some machine, or execute 501.Ic sshd -i 502somewhere. 503Host key management will be done using the 504HostName of the host being connected (defaulting to the name typed by 505the user). 506Setting the command to 507.Dq none 508disables this option entirely. 509Note that 510.Cm CheckHostIP 511is not available for connects with a proxy command. 512.Pp 513.It Cm PubkeyAuthentication 514Specifies whether to try public key authentication. 515The argument to this keyword must be 516.Dq yes 517or 518.Dq no . 519The default is 520.Dq yes . 521This option applies to protocol version 2 only. 522.It Cm RemoteForward 523Specifies that a TCP/IP port on the remote machine be forwarded over 524the secure channel to the specified host and port from the local machine. 525The first argument must be a port number, and the second must be 526.Ar host:port . 527IPv6 addresses can be specified with an alternative syntax: 528.Ar host/port . 529Multiple forwardings may be specified, and additional 530forwardings can be given on the command line. 531Only the superuser can forward privileged ports. 532.It Cm RhostsRSAAuthentication 533Specifies whether to try rhosts based authentication with RSA host 534authentication. 535The argument must be 536.Dq yes 537or 538.Dq no . 539The default is 540.Dq no . 541This option applies to protocol version 1 only and requires 542.Nm ssh 543to be setuid root. 544.It Cm RSAAuthentication 545Specifies whether to try RSA authentication. 546The argument to this keyword must be 547.Dq yes 548or 549.Dq no . 550RSA authentication will only be 551attempted if the identity file exists, or an authentication agent is 552running. 553The default is 554.Dq yes . 555Note that this option applies to protocol version 1 only. 556.It Cm ServerAliveInterval 557Sets a timeout interval in seconds after which if no data has been received 558from the server, 559.Nm ssh 560will send a message through the encrypted 561channel to request a response from the server. 562The default 563is 0, indicating that these messages will not be sent to the server. 564This option applies to protocol version 2 only. 565.It Cm ServerAliveCountMax 566Sets the number of server alive messages (see above) which may be 567sent without 568.Nm ssh 569receiving any messages back from the server. 570If this threshold is reached while server alive messages are being sent, 571.Nm ssh 572will disconnect from the server, terminating the session. 573It is important to note that the use of server alive messages is very 574different from 575.Cm TCPKeepAlive 576(below). 577The server alive messages are sent through the encrypted channel 578and therefore will not be spoofable. 579The TCP keepalive option enabled by 580.Cm TCPKeepAlive 581is spoofable. 582The server alive mechanism is valuable when the client or 583server depend on knowing when a connection has become inactive. 584.Pp 585The default value is 3. 586If, for example, 587.Cm ServerAliveInterval 588(above) is set to 15, and 589.Cm ServerAliveCountMax 590is left at the default, if the server becomes unresponsive ssh 591will disconnect after approximately 45 seconds. 592.It Cm SmartcardDevice 593Specifies which smartcard device to use. 594The argument to this keyword is the device 595.Nm ssh 596should use to communicate with a smartcard used for storing the user's 597private RSA key. 598By default, no device is specified and smartcard support is not activated. 599.It Cm StrictHostKeyChecking 600If this flag is set to 601.Dq yes , 602.Nm ssh 603will never automatically add host keys to the 604.Pa $HOME/.ssh/known_hosts 605file, and refuses to connect to hosts whose host key has changed. 606This provides maximum protection against trojan horse attacks, 607however, can be annoying when the 608.Pa /etc/ssh/ssh_known_hosts 609file is poorly maintained, or connections to new hosts are 610frequently made. 611This option forces the user to manually 612add all new hosts. 613If this flag is set to 614.Dq no , 615.Nm ssh 616will automatically add new host keys to the 617user known hosts files. 618If this flag is set to 619.Dq ask , 620new host keys 621will be added to the user known host files only after the user 622has confirmed that is what they really want to do, and 623.Nm ssh 624will refuse to connect to hosts whose host key has changed. 625The host keys of 626known hosts will be verified automatically in all cases. 627The argument must be 628.Dq yes , 629.Dq no 630or 631.Dq ask . 632The default is 633.Dq ask . 634.It Cm TCPKeepAlive 635Specifies whether the system should send TCP keepalive messages to the 636other side. 637If they are sent, death of the connection or crash of one 638of the machines will be properly noticed. 639However, this means that 640connections will die if the route is down temporarily, and some people 641find it annoying. 642.Pp 643The default is 644.Dq yes 645(to send TCP keepalive messages), and the client will notice 646if the network goes down or the remote host dies. 647This is important in scripts, and many users want it too. 648.Pp 649To disable TCP keepalive messages, the value should be set to 650.Dq no . 651.It Cm UsePrivilegedPort 652Specifies whether to use a privileged port for outgoing connections. 653The argument must be 654.Dq yes 655or 656.Dq no . 657The default is 658.Dq no . 659If set to 660.Dq yes 661.Nm ssh 662must be setuid root. 663Note that this option must be set to 664.Dq yes 665for 666.Cm RhostsRSAAuthentication 667with older servers. 668.It Cm User 669Specifies the user to log in as. 670This can be useful when a different user name is used on different machines. 671This saves the trouble of 672having to remember to give the user name on the command line. 673.It Cm UserKnownHostsFile 674Specifies a file to use for the user 675host key database instead of 676.Pa $HOME/.ssh/known_hosts . 677.It Cm VerifyHostKeyDNS 678Specifies whether to verify the remote key using DNS and SSHFP resource 679records. 680If this option is set to 681.Dq yes , 682the client will implicitly trust keys that match a secure fingerprint 683from DNS. 684Insecure fingerprints will be handled as if this option was set to 685.Dq ask . 686If this option is set to 687.Dq ask , 688information on fingerprint match will be displayed, but the user will still 689need to confirm new host keys according to the 690.Cm StrictHostKeyChecking 691option. 692The argument must be 693.Dq yes , 694.Dq no 695or 696.Dq ask . 697The default is 698.Dq no . 699Note that this option applies to protocol version 2 only. 700.It Cm VersionAddendum 701Specifies a string to append to the regular version string to identify 702OS- or site-specific modifications. 703The default is 704.Dq FreeBSD-20040226 . 705.It Cm XAuthLocation 706Specifies the full pathname of the 707.Xr xauth 1 708program. 709The default is 710.Pa /usr/X11R6/bin/xauth . 711.El 712.Sh FILES 713.Bl -tag -width Ds 714.It Pa $HOME/.ssh/config 715This is the per-user configuration file. 716The format of this file is described above. 717This file is used by the 718.Nm ssh 719client. 720This file does not usually contain any sensitive information, 721but the recommended permissions are read/write for the user, and not 722accessible by others. 723.It Pa /etc/ssh/ssh_config 724Systemwide configuration file. 725This file provides defaults for those 726values that are not specified in the user's configuration file, and 727for those users who do not have a configuration file. 728This file must be world-readable. 729.El 730.Sh SEE ALSO 731.Xr ssh 1 732.Sh AUTHORS 733OpenSSH is a derivative of the original and free 734ssh 1.2.12 release by Tatu Ylonen. 735Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 736Theo de Raadt and Dug Song 737removed many bugs, re-added newer features and 738created OpenSSH. 739Markus Friedl contributed the support for SSH 740protocol versions 1.5 and 2.0. 741