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 1.16 2003/07/22 13:35:22 markus Exp $ 38.Dd September 25, 1999 39.Dt SSH_CONFIG 5 40.Os 41.Sh NAME 42.Nm ssh_config 43.Nd OpenSSH SSH client configuration files 44.Sh SYNOPSIS 45.Bl -tag -width Ds -compact 46.It Pa $HOME/.ssh/config 47.It Pa /etc/ssh/ssh_config 48.El 49.Sh DESCRIPTION 50.Nm ssh 51obtains configuration data from the following sources in 52the following order: 53.Bl -enum -offset indent -compact 54.It 55command-line options 56.It 57user's configuration file 58.Pq Pa $HOME/.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 bracketed 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. 81.Pp 82Otherwise a line is of the format 83.Dq keyword arguments . 84Configuration options may be separated by whitespace or 85optional whitespace and exactly one 86.Ql = ; 87the latter format is useful to avoid the need to quote whitespace 88when specifying configuration options using the 89.Nm ssh , 90.Nm scp 91and 92.Nm sftp 93.Fl o 94option. 95.Pp 96The possible 97keywords and their meanings are as follows (note that 98keywords are case-insensitive and arguments are case-sensitive): 99.Bl -tag -width Ds 100.It Cm Host 101Restricts the following declarations (up to the next 102.Cm Host 103keyword) to be only for those hosts that match one of the patterns 104given after the keyword. 105.Ql \&* 106and 107.Ql ? 108can be used as wildcards in the 109patterns. 110A single 111.Ql \&* 112as a pattern can be used to provide global 113defaults for all hosts. 114The host is the 115.Ar hostname 116argument given on the command line (i.e., the name is not converted to 117a canonicalized host name before matching). 118.It Cm AddressFamily 119Specifies which address family to use when connecting. 120Valid arguments are 121.Dq any , 122.Dq inet 123(Use IPv4 only) or 124.Dq inet6 125(Use IPv6 only.) 126.It Cm BatchMode 127If set to 128.Dq yes , 129passphrase/password querying will be disabled. 130This option is useful in scripts and other batch jobs where no user 131is present to supply the password. 132The argument must be 133.Dq yes 134or 135.Dq no . 136The default is 137.Dq no . 138.It Cm BindAddress 139Specify the interface to transmit from on machines with multiple 140interfaces or aliased addresses. 141Note that this option does not work if 142.Cm UsePrivilegedPort 143is set to 144.Dq yes . 145.It Cm ChallengeResponseAuthentication 146Specifies whether to use challenge response authentication. 147The argument to this keyword must be 148.Dq yes 149or 150.Dq no . 151The default is 152.Dq yes . 153.It Cm CheckHostIP 154If this flag is set to 155.Dq yes , 156ssh will additionally check the host IP address in the 157.Pa known_hosts 158file. 159This allows ssh to detect if a host key changed due to DNS spoofing. 160If the option is set to 161.Dq no , 162the check will not be executed. 163The default is 164.Dq yes . 165.It Cm Cipher 166Specifies the cipher to use for encrypting the session 167in protocol version 1. 168Currently, 169.Dq blowfish , 170.Dq 3des , 171and 172.Dq des 173are supported. 174.Ar des 175is only supported in the 176.Nm ssh 177client for interoperability with legacy protocol 1 implementations 178that do not support the 179.Ar 3des 180cipher. 181Its use is strongly discouraged due to cryptographic weaknesses. 182The default is 183.Dq 3des . 184.It Cm Ciphers 185Specifies the ciphers allowed for protocol version 2 186in order of preference. 187Multiple ciphers must be comma-separated. 188The default is 189.Pp 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 . 263See 264.Xr ssh-keysign 8 265for more information. 266.It Cm EscapeChar 267Sets the escape character (default: 268.Ql ~ ) . 269The escape character can also 270be set on the command line. 271The argument should be a single character, 272.Ql ^ 273followed by a letter, or 274.Dq none 275to disable the escape 276character entirely (making the connection transparent for binary 277data). 278.It Cm ForwardAgent 279Specifies whether the connection to the authentication agent (if any) 280will be forwarded to the remote machine. 281The argument must be 282.Dq yes 283or 284.Dq no . 285The default is 286.Dq no . 287.Pp 288Agent forwarding should be enabled with caution. 289Users with the ability to bypass file permissions on the remote host 290(for the agent's Unix-domain socket) 291can access the local agent through the forwarded connection. 292An attacker cannot obtain key material from the agent, 293however they can perform operations on the keys that enable them to 294authenticate using the identities loaded into the agent. 295.It Cm ForwardX11 296Specifies whether X11 connections will be automatically redirected 297over the secure channel and 298.Ev DISPLAY 299set. 300The argument must be 301.Dq yes 302or 303.Dq no . 304The default is 305.Dq no . 306.Pp 307X11 forwarding should be enabled with caution. 308Users with the ability to bypass file permissions on the remote host 309(for the user's X authorization database) 310can access the local X11 display through the forwarded connection. 311An attacker may then be able to perform activities such as keystroke monitoring. 312.It Cm GatewayPorts 313Specifies whether remote hosts are allowed to connect to local 314forwarded ports. 315By default, 316.Nm ssh 317binds local port forwardings to the loopback address. 318This prevents other remote hosts from connecting to forwarded ports. 319.Cm GatewayPorts 320can be used to specify that 321.Nm ssh 322should bind local port forwardings to the wildcard address, 323thus allowing remote hosts to connect to forwarded ports. 324The argument must be 325.Dq yes 326or 327.Dq no . 328The default is 329.Dq no . 330.It Cm GlobalKnownHostsFile 331Specifies a file to use for the global 332host key database instead of 333.Pa /etc/ssh/ssh_known_hosts . 334.It Cm HostbasedAuthentication 335Specifies whether to try rhosts based authentication with public key 336authentication. 337The argument must be 338.Dq yes 339or 340.Dq no . 341The default is 342.Dq no . 343This option applies to protocol version 2 only and 344is similar to 345.Cm RhostsRSAAuthentication . 346.It Cm HostKeyAlgorithms 347Specifies the protocol version 2 host key algorithms 348that the client wants to use in order of preference. 349The default for this option is: 350.Dq ssh-rsa,ssh-dss . 351.It Cm HostKeyAlias 352Specifies an alias that should be used instead of the 353real host name when looking up or saving the host key 354in the host key database files. 355This option is useful for tunneling ssh connections 356or for multiple servers running on a single host. 357.It Cm HostName 358Specifies the real host name to log into. 359This can be used to specify nicknames or abbreviations for hosts. 360Default is the name given on the command line. 361Numeric IP addresses are also permitted (both on the command line and in 362.Cm HostName 363specifications). 364.It Cm IdentityFile 365Specifies a file from which the user's RSA or DSA authentication identity 366is read. 367The default is 368.Pa $HOME/.ssh/identity 369for protocol version 1, and 370.Pa $HOME/.ssh/id_rsa 371and 372.Pa $HOME/.ssh/id_dsa 373for protocol version 2. 374Additionally, any identities represented by the authentication agent 375will be used for authentication. 376The file name may use the tilde 377syntax to refer to a user's home directory. 378It is possible to have 379multiple identity files specified in configuration files; all these 380identities will be tried in sequence. 381.It Cm KeepAlive 382Specifies whether the system should send TCP keepalive messages to the 383other side. 384If they are sent, death of the connection or crash of one 385of the machines will be properly noticed. 386However, this means that 387connections will die if the route is down temporarily, and some people 388find it annoying. 389.Pp 390The default is 391.Dq yes 392(to send keepalives), and the client will notice 393if the network goes down or the remote host dies. 394This is important in scripts, and many users want it too. 395.Pp 396To disable keepalives, the value should be set to 397.Dq no . 398.It Cm KerberosAuthentication 399Specifies whether Kerberos authentication will be used. 400The argument to this keyword must be 401.Dq yes 402or 403.Dq no . 404.It Cm KerberosTgtPassing 405Specifies whether a Kerberos TGT will be forwarded to the server. 406The argument to this keyword must be 407.Dq yes 408or 409.Dq no . 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 RhostsAuthentication 533Specifies whether to try rhosts based authentication. 534Note that this 535declaration only affects the client side and has no effect whatsoever 536on security. 537Most servers do not permit RhostsAuthentication because it 538is not secure (see 539.Cm RhostsRSAAuthentication ) . 540The argument to this keyword must be 541.Dq yes 542or 543.Dq no . 544The default is 545.Dq no . 546This option applies to protocol version 1 only and requires 547.Nm ssh 548to be setuid root and 549.Cm UsePrivilegedPort 550to be set to 551.Dq yes . 552.It Cm RhostsRSAAuthentication 553Specifies whether to try rhosts based authentication with RSA host 554authentication. 555The argument must be 556.Dq yes 557or 558.Dq no . 559The default is 560.Dq no . 561This option applies to protocol version 1 only and requires 562.Nm ssh 563to be setuid root. 564.It Cm RSAAuthentication 565Specifies whether to try RSA authentication. 566The argument to this keyword must be 567.Dq yes 568or 569.Dq no . 570RSA authentication will only be 571attempted if the identity file exists, or an authentication agent is 572running. 573The default is 574.Dq yes . 575Note that this option applies to protocol version 1 only. 576.It Cm SmartcardDevice 577Specifies which smartcard device to use. 578The argument to this keyword is the device 579.Nm ssh 580should use to communicate with a smartcard used for storing the user's 581private RSA key. 582By default, no device is specified and smartcard support is not activated. 583.It Cm StrictHostKeyChecking 584If this flag is set to 585.Dq yes , 586.Nm ssh 587will never automatically add host keys to the 588.Pa $HOME/.ssh/known_hosts 589file, and refuses to connect to hosts whose host key has changed. 590This provides maximum protection against trojan horse attacks, 591however, can be annoying when the 592.Pa /etc/ssh/ssh_known_hosts 593file is poorly maintained, or connections to new hosts are 594frequently made. 595This option forces the user to manually 596add all new hosts. 597If this flag is set to 598.Dq no , 599.Nm ssh 600will automatically add new host keys to the 601user known hosts files. 602If this flag is set to 603.Dq ask , 604new host keys 605will be added to the user known host files only after the user 606has confirmed that is what they really want to do, and 607.Nm ssh 608will refuse to connect to hosts whose host key has changed. 609The host keys of 610known hosts will be verified automatically in all cases. 611The argument must be 612.Dq yes , 613.Dq no 614or 615.Dq ask . 616The default is 617.Dq ask . 618.It Cm UsePrivilegedPort 619Specifies whether to use a privileged port for outgoing connections. 620The argument must be 621.Dq yes 622or 623.Dq no . 624The default is 625.Dq no . 626If set to 627.Dq yes 628.Nm ssh 629must be setuid root. 630Note that this option must be set to 631.Dq yes 632if 633.Cm RhostsAuthentication 634and 635.Cm RhostsRSAAuthentication 636authentications are needed with older servers. 637.It Cm User 638Specifies the user to log in as. 639This can be useful when a different user name is used on different machines. 640This saves the trouble of 641having to remember to give the user name on the command line. 642.It Cm UserKnownHostsFile 643Specifies a file to use for the user 644host key database instead of 645.Pa $HOME/.ssh/known_hosts . 646.It Cm VerifyHostKeyDNS 647Specifies whether to verify the remote key using DNS and SSHFP resource 648records. 649The default is 650.Dq no . 651Note that this option applies to protocol version 2 only. 652.It Cm XAuthLocation 653Specifies the full pathname of the 654.Xr xauth 1 655program. 656The default is 657.Pa /usr/X11R6/bin/xauth . 658.El 659.Sh FILES 660.Bl -tag -width Ds 661.It Pa $HOME/.ssh/config 662This is the per-user configuration file. 663The format of this file is described above. 664This file is used by the 665.Nm ssh 666client. 667This file does not usually contain any sensitive information, 668but the recommended permissions are read/write for the user, and not 669accessible by others. 670.It Pa /etc/ssh/ssh_config 671Systemwide configuration file. 672This file provides defaults for those 673values that are not specified in the user's configuration file, and 674for those users who do not have a configuration file. 675This file must be world-readable. 676.El 677.Sh SEE ALSO 678.Xr ssh 1 679.Sh AUTHORS 680OpenSSH is a derivative of the original and free 681ssh 1.2.12 release by Tatu Ylonen. 682Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, 683Theo de Raadt and Dug Song 684removed many bugs, re-added newer features and 685created OpenSSH. 686Markus Friedl contributed the support for SSH 687protocol versions 1.5 and 2.0. 688