1 '\" te
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   7 .TH SSH_CONFIG 4 "Apr 20, 2009"
   8 .SH NAME
   9 ssh_config \- ssh configuration file
  10 .SH SYNOPSIS
  11 .LP
  12 .nf
  13 \fB/etc/ssh/ssh_config\fR
  14 .fi
  15 
  16 .LP
  17 .nf
  18 \fB$HOME/.ssh/config\fR
  19 .fi
  20 
  21 .SH DESCRIPTION
  22 .sp
  23 .LP
  24 The first \fBssh_config\fR path, above, provides the system-wide defaults for
  25 \fBssh\fR(1). The second version is user-specific defaults for \fBssh\fR.
  26 .sp
  27 .LP
  28 \fBssh\fR obtains configuration data from the following sources, in this order:
  29 command line options, user's configuration file (\fB$HOME/.ssh/config\fR), and
  30 system-wide configuration file (\fB/etc/ssh/ssh_config\fR). For each parameter,
  31 the first obtained value is used. The configuration files contain sections
  32 bracketed by \fBHost\fR specifications, and that section is applied only for
  33 hosts that match one of the patterns given in the specification. The matched
  34 host name is the one given on the command line.
  35 .sp
  36 .LP
  37 Since the first obtained value for each parameter is used, host-specific
  38 declarations should be given near the beginning of the file, and general
  39 defaults at the end.
  40 .sp
  41 .LP
  42 The configuration file has the following format and syntax:
  43 .RS +4
  44 .TP
  45 .ie t \(bu
  46 .el o
  47 Empty lines and lines starting with \fB#\fR are comments.
  48 .RE
  49 .RS +4
  50 .TP
  51 .ie t \(bu
  52 .el o
  53 Non-commented lines are of the form:
  54 .sp
  55 .in +2
  56 .nf
  57 \fIkeyword\fR \fIarguments\fR
  58 .fi
  59 .in -2
  60 .sp
  61 
  62 .RE
  63 .RS +4
  64 .TP
  65 .ie t \(bu
  66 .el o
  67 Configuration options can be separated by white space or optional whitespace
  68 and exactly one equal sign. The latter format allows you to avoid the need to
  69 quote white space when specifying configuration options using the \fB-o\fR
  70 option to \fBssh\fR, \fBscp\fR, and \fBsftp\fR.
  71 .RE
  72 .sp
  73 .LP
  74 The possible keywords and their meanings are listed in the following
  75 list.Keywords are case-insensitive and arguments are case-sensitive.
  76 .sp
  77 .ne 2
  78 .na
  79 \fB\fBBatchMode\fR\fR
  80 .ad
  81 .sp .6
  82 .RS 4n
  83 The argument must be \fByes\fR or \fBno\fR. If set to \fByes\fR,
  84 passphrase/password querying is disabled. This option is useful in scripts and
  85 other batch jobs where you have no user to supply the password.
  86 .RE
  87 
  88 .sp
  89 .ne 2
  90 .na
  91 \fB\fBBindAddress\fR\fR
  92 .ad
  93 .sp .6
  94 .RS 4n
  95 Specify the interface to transmit from on machines with multiple interfaces or
  96 aliased addresses. This option does not work if \fBUsePrivilegedPort\fR is set
  97 to \fByes\fR.
  98 .RE
  99 
 100 .sp
 101 .ne 2
 102 .na
 103 \fB\fBCheckHostIP\fR\fR
 104 .ad
 105 .sp .6
 106 .RS 4n
 107 If this flag is set to \fByes\fR, \fBssh\fR additionally checks the host IP
 108 address in the \fBknown_hosts\fR file. This allows \fBssh\fR to detect if a
 109 host key changed due to DNS spoofing. If the option is set to \fBno\fR, the
 110 check is not executed.
 111 .RE
 112 
 113 .sp
 114 .ne 2
 115 .na
 116 \fB\fBCipher\fR\fR
 117 .ad
 118 .sp .6
 119 .RS 4n
 120 Specifies the cipher to use for encrypting the session in protocol version 1.
 121 Only a single cipher can be specified. Currently, \fBblowfish, 3des,\fR and
 122 \fBdes\fR are supported. \fB3des\fR (triple-\fBdes\fR) is an
 123 encrypt-decrypt-encrypt triple with three different keys. It is believed to be
 124 secure. \fBblowfish\fR is a fast block cipher. It appears very secure and is
 125 much faster than \fB3des\fR. \fBdes\fR is only supported in the \fBssh\fR
 126 client for interoperability with legacy protocol 1 implementations that do not
 127 support the \fB3des\fR cipher. Its use is strongly discouraged due to
 128 cryptographic weaknesses. The default is \fB3des\fR.
 129 .RE
 130 
 131 .sp
 132 .ne 2
 133 .na
 134 \fB\fBCiphers\fR\fR
 135 .ad
 136 .sp .6
 137 .RS 4n
 138 Specifies the ciphers allowed for protocol version 2 in order of preference.
 139 Multiple ciphers must be comma separated.
 140 .sp
 141 The default cipher list contains all supported ciphers in this order:
 142 .sp
 143 .in +2
 144 .nf
 145 aes128-ctr, aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, aes128-cbc,
 146 aes192-cbc, aes256-cbc, arcfour, 3des-cbc,blowfish-cbc
 147 .fi
 148 .in -2
 149 .sp
 150 
 151 While CBC modes are not considered as secure as other modes in connection with
 152 the SSH protocol 2 they are present at the back of the default client cipher
 153 list for backward compatibility with SSH servers that do not support other
 154 cipher modes.
 155 .RE
 156 
 157 .sp
 158 .ne 2
 159 .na
 160 \fB\fBClearAllForwardings\fR\fR
 161 .ad
 162 .sp .6
 163 .RS 4n
 164 Specifies that all local, remote, and dynamic port forwardings specified in the
 165 configuration files or on the command line be cleared. This option is primarily
 166 useful when used from the \fBssh\fR command line to clear port forwardings set
 167 in configuration files and is automatically set by \fBscp\fR(1) and
 168 \fBsftp\fR(1). The argument must be \fByes\fR or \fBno\fR. The default is
 169 \fBno\fR.
 170 .RE
 171 
 172 .sp
 173 .ne 2
 174 .na
 175 \fB\fBCompression\fR\fR
 176 .ad
 177 .sp .6
 178 .RS 4n
 179 Specifies whether to use compression. The argument must be \fByes\fR or
 180 \fBno\fR. Defaults to \fBno\fR.
 181 .RE
 182 
 183 .sp
 184 .ne 2
 185 .na
 186 \fB\fBCompressionLevel\fR\fR
 187 .ad
 188 .sp .6
 189 .RS 4n
 190 Specifies the compression level to use if compression is enabled. The argument
 191 must be an integer from 1 (fast) to 9 (slow, best). The default level is 6,
 192 which is good for most applications. This option applies to protocol version 1
 193 only.
 194 .RE
 195 
 196 .sp
 197 .ne 2
 198 .na
 199 \fB\fBConnectionAttempts\fR\fR
 200 .ad
 201 .sp .6
 202 .RS 4n
 203 Specifies the number of tries (one per second) to make before falling back to
 204 \fBrsh\fR or exiting. The argument must be an integer. This can be useful in
 205 scripts if the connection sometimes fails. The default is 1.
 206 .RE
 207 
 208 .sp
 209 .ne 2
 210 .na
 211 \fB\fBConnectTimeout\fR\fR
 212 .ad
 213 .sp .6
 214 .RS 4n
 215 Specifies the timeout (in seconds) used when connecting to the \fBssh\fR
 216 server, instead of using the default system TCP timeout. This value is used
 217 only when the target is down or truly unreachable, not when it refuses the
 218 connection.
 219 .RE
 220 
 221 .sp
 222 .ne 2
 223 .na
 224 \fB\fBDisableBanner\fR\fR
 225 .ad
 226 .sp .6
 227 .RS 4n
 228 If set to \fByes\fR, disables the display of the banner message. If set to
 229 \fBin-exec-mode\fR, disables the display of banner message when in remote
 230 command mode only.
 231 .sp
 232 The default value is \fBno\fR, which means that the banner is displayed unless
 233 the log level is \fBQUIET\fR, \fBFATAL\fR, or \fBERROR\fR. See also the
 234 \fBBanner\fR option in \fBsshd_config\fR(4). This option applies to protocol
 235 version 2 only.
 236 .RE
 237 
 238 .sp
 239 .ne 2
 240 .na
 241 \fB\fBDynamicForward\fR\fR
 242 .ad
 243 .sp .6
 244 .RS 4n
 245 Specifies that a TCP/IP port on the local machine be forwarded over the secure
 246 channel. The application protocol is then used to determine where to connect to
 247 from the remote machine.
 248 .sp
 249 The argument must be \fB[\fR\fIbind_address\fR\fB:]\fR\fIport\fR. IPv6
 250 addresses can be specified by enclosing addresses in square brackets or by
 251 using an alternative syntax: \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR. By
 252 default, the local port is bound in accordance with the \fBGatewayPorts\fR
 253 setting. However, an explicit \fIbind_address\fR can be used to bind the
 254 connection to a specific address. The \fIbind_address\fR of \fBlocalhost\fR
 255 indicates that the listening port be bound for local use only, while an empty
 256 address or \fB*\fR indicates that the port should be available from all
 257 interfaces.
 258 .sp
 259 Currently the \fBSOCKS4\fR and \fBSOCKS5\fR protocols are supported, and
 260 \fBssh\fR acts as a \fBSOCKS\fR server. Multiple forwardings can be specified
 261 and additional forwardings can be specified on the command line. Only a user
 262 with enough privileges can forward privileged ports.
 263 .RE
 264 
 265 .sp
 266 .ne 2
 267 .na
 268 \fB\fBEscapeChar\fR\fR
 269 .ad
 270 .sp .6
 271 .RS 4n
 272 Sets the escape character. The default is tilde (\fB~\fR). The escape character
 273 can also be set on the command line. The argument should be a single character,
 274 \fB^\fR, followed by a letter, or \fBnone\fR to disable the escape character
 275 entirely (making the connection transparent for binary data).
 276 .RE
 277 
 278 .sp
 279 .ne 2
 280 .na
 281 \fB\fBFallBackToRsh\fR\fR
 282 .ad
 283 .sp .6
 284 .RS 4n
 285 Specifies that if connecting with \fBssh\fR fails due to a connection refused
 286 error (there is no \fBsshd\fR(1M) listening on the remote host), \fBrsh\fR(1)
 287 should automatically be used instead (after a suitable warning about the
 288 session being unencrypted). The argument must be \fByes\fR or \fBno\fR.
 289 .RE
 290 
 291 .sp
 292 .ne 2
 293 .na
 294 \fB\fBForwardAgent\fR\fR
 295 .ad
 296 .sp .6
 297 .RS 4n
 298 Specifies whether the connection to the authentication agent (if any) is
 299 forwarded to the remote machine. The argument must be \fByes\fR or \fBno\fR.
 300 The default is \fBno\fR.
 301 .sp
 302 Agent forwarding should be enabled with caution. Users with the ability to
 303 bypass file permissions on the remote host (for the agent's Unix-domain socket)
 304 can access the local agent through the forwarded connection. An attacker cannot
 305 obtain key material from the agent, however he can perform operations on the
 306 keys that enable him to authenticate using the identities loaded into the
 307 agent.
 308 .RE
 309 
 310 .sp
 311 .ne 2
 312 .na
 313 \fB\fBForwardX11\fR\fR
 314 .ad
 315 .sp .6
 316 .RS 4n
 317 Specifies whether X11 connections are automatically redirected over the secure
 318 channel and \fBDISPLAY\fR set. The argument must be \fByes\fR or \fBno\fR. The
 319 default is \fBno\fR.
 320 .sp
 321 X11 forwarding should be enabled with caution. Users with the ability to bypass
 322 file permissions on the remote host (for the user's X authorization database)
 323 can access the local \fBX11\fR display through the forwarded connection. An
 324 attacker might then be able to perform activities such as keystroke monitoring.
 325 See the \fBForwardX11Trusted\fR option for more information how to prevent
 326 this.
 327 .RE
 328 
 329 .sp
 330 .ne 2
 331 .na
 332 \fB\fBForwardX11Trusted\fR\fR
 333 .ad
 334 .sp .6
 335 .RS 4n
 336 If this option is set to \fByes\fR, remote X11 clients have full access to the
 337 original X11 display. This option is set to \fByes\fR by default.
 338 .sp
 339 If this option is set to \fBno\fR, remote X11 clients are considered untrusted
 340 and prevented from stealing or tampering with data belonging to trusted X11
 341 clients. Furthermore, the \fBxauth\fR(1) token used for the session is set to
 342 expire after 20 minutes. Remote clients are refused access after this time.
 343 .sp
 344 See the X11 SECURITY extension specification for full details on the
 345 restrictions imposed on untrusted clients.
 346 .RE
 347 
 348 .sp
 349 .ne 2
 350 .na
 351 \fB\fBGatewayPorts\fR\fR
 352 .ad
 353 .sp .6
 354 .RS 4n
 355 Specifies whether remote hosts are allowed to connect to local forwarded ports.
 356 By default, \fBssh\fR binds local port forwardings to the loopback address.
 357 This prevents other remote hosts from connecting to forwarded ports.
 358 \fBGatewayPorts\fR can be used to specify that \fBssh\fR should bind local port
 359 forwardings to the wildcard address, thus allowing remote hosts to connect to
 360 forwarded ports. The argument must be \fByes\fR or \fBno\fR. The default is
 361 \fBno\fR.
 362 .RE
 363 
 364 .sp
 365 .ne 2
 366 .na
 367 \fB\fBGlobalKnownHostsFile\fR\fR
 368 .ad
 369 .sp .6
 370 .RS 4n
 371 Specifies a file to use instead of \fB/etc/ssh/ssh_known_hosts\fR.
 372 .RE
 373 
 374 .sp
 375 .ne 2
 376 .na
 377 \fB\fBGSSAPIAuthentication\fR\fR
 378 .ad
 379 .sp .6
 380 .RS 4n
 381 Enables/disables GSS-API user authentication. The default is \fByes\fR.
 382 .RE
 383 
 384 .sp
 385 .ne 2
 386 .na
 387 \fB\fBGSSAPIDelegateCredentials\fR\fR
 388 .ad
 389 .sp .6
 390 .RS 4n
 391 Enables/disables GSS-API credential forwarding. The default is \fBno\fR.
 392 .RE
 393 
 394 .sp
 395 .ne 2
 396 .na
 397 \fB\fBGSSAPIKeyExchange\fR\fR
 398 .ad
 399 .sp .6
 400 .RS 4n
 401 Enables/disables GSS-API-authenticated key exchanges. The default is \fByes\fR.
 402 .sp
 403 This option is intended primarily to allow users to disable the use of GSS-API
 404 key exchange for SSHv2 when it would otherwise be selected and then fail (due
 405 to server misconfiguration, for example). SSHv2 key exchange failure always
 406 results in disconnection.
 407 .sp
 408 This option also enables the use of the GSS-API to authenticate the user to the
 409 server after the key exchange. GSS-API key exchange can succeed but the
 410 subsequent authentication using the GSS-API fail if the server does not
 411 authorize the user's GSS principal name to the target user account.
 412 .RE
 413 
 414 .sp
 415 .ne 2
 416 .na
 417 \fB\fBHashKnownHosts\fR\fR
 418 .ad
 419 .sp .6
 420 .RS 4n
 421 Indicates that \fBssh\fR(1), should hash host names and addresses when they are
 422 added to \fB~/.ssh/known_hosts\fR. These hashed names can be used normally by
 423 \fBssh\fR(1) and \fBsshd\fR(1M), but they do not reveal identifying information
 424 should the file's contents be disclosed. The default is \fBno\fR. Existing
 425 names and addresses in known hosts files are not be converted automatically,
 426 but can be manually hashed using \fBssh-keygen\fR(1).
 427 .RE
 428 
 429 .sp
 430 .ne 2
 431 .na
 432 \fB\fBHost\fR\fR
 433 .ad
 434 .sp .6
 435 .RS 4n
 436 Restricts the following declarations (up to the next \fBHost\fR keyword) to be
 437 only for those hosts that match one of the patterns given after the keyword. An
 438 asterisk (\fB*\fR) and a question mark (\fB?\fR) can be used as wildcards in
 439 the patterns. A single asterisk as a pattern can be used to provide global
 440 defaults for all hosts. The host is the host name argument given on the command
 441 line (that is, the name is not converted to a canonicalized host name before
 442 matching).
 443 .RE
 444 
 445 .sp
 446 .ne 2
 447 .na
 448 \fB\fBHostbasedAuthentication\fR\fR
 449 .ad
 450 .sp .6
 451 .RS 4n
 452 Specifies whether to try \fBrhosts\fR-based authentication with public key
 453 authentication. The argument must be \fByes\fR or \fBno\fR. The default is
 454 \fBno\fR. This option applies to protocol version 2 only and is similar to
 455 \fBRhostsRSAAuthentication\fR.
 456 .RE
 457 
 458 .sp
 459 .ne 2
 460 .na
 461 \fB\fBHostKeyAlgorithms\fR\fR
 462 .ad
 463 .sp .6
 464 .RS 4n
 465 Specifies the protocol version 2 host key algorithms that the client wants to
 466 use in order of preference. The default for this option is:
 467 \fBssh-rsa,ssh-dss\fR.
 468 .RE
 469 
 470 .sp
 471 .ne 2
 472 .na
 473 \fB\fBHostKeyAlias\fR\fR
 474 .ad
 475 .sp .6
 476 .RS 4n
 477 Specifies an alias that should be used instead of the real host name when
 478 looking up or saving the host key in the host key database files. This option
 479 is useful for tunneling \fBssh\fR connections or for multiple servers running
 480 on a single host.
 481 .RE
 482 
 483 .sp
 484 .ne 2
 485 .na
 486 \fB\fBHostName\fR\fR
 487 .ad
 488 .sp .6
 489 .RS 4n
 490 Specifies the real host name to log into. This can be used to specify nicknames
 491 or abbreviations for hosts. Default is the name given on the command line.
 492 Numeric IP addresses are also permitted (both on the command line and in
 493 \fBHostName\fR specifications).
 494 .RE
 495 
 496 .sp
 497 .ne 2
 498 .na
 499 \fB\fBIdentityFile\fR\fR
 500 .ad
 501 .sp .6
 502 .RS 4n
 503 Specifies a file from which the user's RSA or DSA authentication identity is
 504 read. The default is \fB$HOME/.ssh/identity\fR for protocol version 1 and
 505 \fB$HOME/.ssh/id_rsa\fR and \fB$HOME/.ssh/id_dsa\fR for protocol version 2.
 506 Additionally, any identities represented by the authentication agent is used
 507 for authentication. The file name can use the tilde syntax to refer to a user's
 508 home directory. It is possible to have multiple identity files specified in
 509 configuration files; all these identities is tried in sequence.
 510 .RE
 511 
 512 .sp
 513 .ne 2
 514 .na
 515 \fB\fBIgnoreIfUnknown\fR\fR
 516 .ad
 517 .sp .6
 518 .RS 4n
 519 Specifies a comma-separated list of \fBssh_config\fR parameters, which, if
 520 unknown to \fBssh\fR(1), are to be ignored by \fBssh\fR.
 521 .sp
 522 This parameter is primarily intended to be used in the per-user
 523 \fBssh_config\fR, \fB~/.ssh/config\fR. While this parameter can also be used in
 524 the system wide \fB/etc/ssh/ssh_config\fR file, it is generally useless as the
 525 capabilities of the \fBssh\fR(1) client on that host should match that file.
 526 .RE
 527 
 528 .sp
 529 .ne 2
 530 .na
 531 \fB\fBKeepAlive\fR\fR
 532 .ad
 533 .sp .6
 534 .RS 4n
 535 Specifies whether the system should send TCP keepalive messages to the other
 536 side. If they are sent, death of the connection or crash of one of the machines
 537 is properly noticed. However, this means that connections die if the route is
 538 down temporarily, which can be a source of annoyance.
 539 .sp
 540 The default is \fByes\fR (to send keepalives), which means the client notices
 541 if the network goes down or the remote host dies. This is important in scripts,
 542 and many users want it too. To disable keepalives, the value should be set to
 543 \fBno\fR in both the server and the client configuration files.
 544 .RE
 545 
 546 .sp
 547 .ne 2
 548 .na
 549 \fB\fBLocalForward\fR\fR
 550 .ad
 551 .sp .6
 552 .RS 4n
 553 Specifies that a TCP/IP port on the local machine be forwarded over the secure
 554 channel to a given \fIhost\fR:\fIport\fR from the remote machine. The first
 555 argument must be \fB[\fR\fIbind_address\fR\fB:]\fR\fIport\fR and the second
 556 must be \fIhost\fR\fB:\fR\fIport\fR. IPv6 addresses can be specified by
 557 enclosing addresses in square brackets or by using an alternative syntax:
 558 \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR and \fIhost\fR\fB/\fR\fIport\fR.
 559 Multiple forwardings can be specified and additional forwardings can be given
 560 on the command line. Only a user with enough privileges can forward privileged
 561 ports. By default, the local port is bound in accordance with the
 562 \fBGatewayPorts\fR setting. However, an explicit \fIbind_address\fR can be used
 563 to bind the connection to a specific address. The \fIbind_address\fR of
 564 \fIlocalhost\fR indicates that the listening port be bound for local use only,
 565 while an empty address or \fB*\fR indicates that the port should be available
 566 from all interfaces.
 567 .RE
 568 
 569 .sp
 570 .ne 2
 571 .na
 572 \fB\fBLogLevel\fR\fR
 573 .ad
 574 .sp .6
 575 .RS 4n
 576 Gives the verbosity level that is used when logging messages from \fBssh\fR.
 577 The possible values are: \fBFATAL\fR, \fBERROR\fR, \fBQUIET\fR, \fBINFO\fR,
 578 \fBVERBOSE\fR, \fBDEBUG\fR, \fBDEBUG1\fR, \fBDEBUG2\fR, and \fBDEBUG3\fR. The
 579 default is \fBINFO\fR. \fBDEBUG\fR and \fBDEBUG1\fR are equivalent.
 580 \fBDEBUG2\fR and \fBDEBUG3\fR each specify higher levels of verbose output.
 581 .RE
 582 
 583 .sp
 584 .ne 2
 585 .na
 586 \fB\fBMACs\fR\fR
 587 .ad
 588 .sp .6
 589 .RS 4n
 590 Specifies the MAC (message authentication code) algorithms in order of
 591 preference. The MAC algorithm is used in protocol version 2 for data integrity
 592 protection. Multiple algorithms must be comma-separated. The default is
 593 \fBhmac-md5,hmac-sha1,hmac-sha1-96,hmac-md5-96\fR.
 594 .RE
 595 
 596 .sp
 597 .ne 2
 598 .na
 599 \fB\fBNoHostAuthenticationForLocalhost\fR\fR
 600 .ad
 601 .sp .6
 602 .RS 4n
 603 This option can be used if the home directory is shared across machines. In
 604 this case \fBlocalhost\fR refers to a different machine on each of the machines
 605 and the user gets many warnings about changed host keys. However, this option
 606 disables host authentication for \fBlocalhost\fR. The argument to this keyword
 607 must be \fByes\fR or \fBno\fR. The default is to check the host key for
 608 \fBlocalhost\fR.
 609 .RE
 610 
 611 .sp
 612 .ne 2
 613 .na
 614 \fB\fBNumberOfPasswordPrompts\fR\fR
 615 .ad
 616 .sp .6
 617 .RS 4n
 618 Specifies the number of attempts before giving up for password and
 619 keyboard-interactive methods. Attempts for each method are counted separately.
 620 The argument to this keyword must be an integer. The default is 3.
 621 .RE
 622 
 623 .sp
 624 .ne 2
 625 .na
 626 \fB\fBPasswordAuthentication\fR\fR
 627 .ad
 628 .sp .6
 629 .RS 4n
 630 Specifies whether to use password authentication. The argument to this keyword
 631 must be \fByes\fR or \fBno\fR. This option applies to both protocol versions 1
 632 and 2. The default is \fByes\fR.
 633 .RE
 634 
 635 .sp
 636 .ne 2
 637 .na
 638 \fB\fBPort\fR\fR
 639 .ad
 640 .sp .6
 641 .RS 4n
 642 Specifies the port number to connect on the remote host. The default is 22.
 643 .RE
 644 
 645 .sp
 646 .ne 2
 647 .na
 648 \fB\fBPreferredAuthentications\fR\fR
 649 .ad
 650 .sp .6
 651 .RS 4n
 652 Specifies the order in which the client should try protocol 2 authentication
 653 methods. This allows a client to prefer one method (for example,
 654 \fBkeyboard-interactive\fR) over another method (for example, \fBpassword\fR).
 655 The default for this option is:
 656 \fBhostbased,publickey,keyboard-interactive,password\fR.
 657 .RE
 658 
 659 .sp
 660 .ne 2
 661 .na
 662 \fB\fBProtocol\fR\fR
 663 .ad
 664 .sp .6
 665 .RS 4n
 666 Specifies the protocol versions \fBssh\fR should support in order of
 667 preference. The possible values are \fB1\fR and \fB2\fR. Multiple versions must
 668 be comma-separated. The default is \fB2,1\fR. This means that \fBssh\fR tries
 669 version 2 and falls back to version 1 if version 2 is not available.
 670 .RE
 671 
 672 .sp
 673 .ne 2
 674 .na
 675 \fB\fBProxyCommand\fR\fR
 676 .ad
 677 .sp .6
 678 .RS 4n
 679 Specifies the command to use to connect to the server. The command string
 680 extends to the end of the line, and is executed with \fB/bin/sh\fR. In the
 681 command string, \fB%h\fR is substituted by the host name to connect and
 682 \fB%p\fR by the port. The string can be any valid command, and should read from
 683 its standard input and write to its standard output. It should eventually
 684 connect an \fBsshd\fR(1M) server running on some machine, or execute \fBsshd\fR
 685 \fB-i\fR somewhere. Host key management is done using the \fBHostName\fR of the
 686 host being connected (defaulting to the name typed by the user).
 687 \fBCheckHostIP\fR is not available for connects with a proxy command.
 688 .RE
 689 
 690 .sp
 691 .ne 2
 692 .na
 693 \fB\fBPubkeyAuthentication\fR\fR
 694 .ad
 695 .sp .6
 696 .RS 4n
 697 Specifies whether to try public key authentication. The argument to this
 698 keyword must be \fByes\fR or \fBno\fR. The default is \fByes\fR. This option
 699 applies to protocol version 2 only.
 700 .RE
 701 
 702 .sp
 703 .ne 2
 704 .na
 705 \fB\fBRekeyLimit\fR\fR
 706 .ad
 707 .sp .6
 708 .RS 4n
 709 Specifies the maximum amount of data that can be transmitted before the session
 710 key is renegotiated. The argument is the number of bytes, with an optional
 711 suffix of \fBK\fR, \fBM\fR, or \fBG\fR to indicate Kilobytes, Megabytes, or
 712 Gigabytes, respectively. The default is between \fB1G\fR and \fB4G\fR,
 713 depending on the cipher. This option applies to protocol version 2 only.
 714 .RE
 715 
 716 .sp
 717 .ne 2
 718 .na
 719 \fB\fBRemoteForward\fR\fR
 720 .ad
 721 .sp .6
 722 .RS 4n
 723 Specifies that a TCP/IP port on the remote machine be forwarded over the secure
 724 channel to a given \fB\fIhost\fR:\fIport\fR\fR from the local machine. The
 725 first argument must be \fB[\fR\fIbind_address\fR\fB:]\fR\fIport\fR and the
 726 second argument must be \fIhost\fR\fB:\fR\fIport\fR. IPv6 addresses can be
 727 specified by enclosing addresses in square brackets or by using an alternative
 728 syntax: \fB[\fR\fIbind_address\fR\fB/]\fR\fIport\fR and
 729 \fIhost\fR\fB/\fR\fIport\fR. You can specify multiple forwardings and give
 730 additional forwardings on the command line. Only a user with enough privileges
 731 can forward privileged ports.
 732 .sp
 733 If the \fIbind_address\fR is not specified, the default is to only bind to
 734 loopback addresses. If the \fIbind_address\fR is \fB*\fR or an empty string,
 735 then the forwarding is requested to listen on all interfaces. Specifying a
 736 remote \fIbind_address\fR only succeeds if the server's \fBGatewayPorts\fR
 737 option is enabled. See \fBsshd_config\fR(4).
 738 .RE
 739 
 740 .sp
 741 .ne 2
 742 .na
 743 \fB\fBRhostsAuthentication\fR\fR
 744 .ad
 745 .sp .6
 746 .RS 4n
 747 Specifies whether to try \fBrhosts\fR-based authentication. This declaration
 748 affects only the client side and has no effect whatsoever on security.
 749 Disabling \fBrhosts\fR authentication can reduce authentication time on slow
 750 connections when \fBrhosts\fR authentication is not used. Most servers do not
 751 permit \fBRhostsAuthentication\fR because it is not secure (see
 752 \fBRhostsRSAAuthentication\fR). The argument to this keyword must be \fByes\fR
 753 or \fBno\fR. This option applies only to the protocol version 1 and requires
 754 that \fBssh\fR be \fBsetuid\fR root and that \fBUsePrivilegedPort\fR be set to
 755 \fByes\fR.
 756 .RE
 757 
 758 .sp
 759 .ne 2
 760 .na
 761 \fB\fBRhostsRSAAuthentication\fR\fR
 762 .ad
 763 .sp .6
 764 .RS 4n
 765 Specifies whether to try \fBrhosts\fR-based authentication with RSA host
 766 authentication. This is the primary authentication method for most sites. The
 767 argument must be \fByes\fR or \fBno\fR. This option applies only to the
 768 protocol version 1 and requires that \fBssh\fR be \fBsetuid\fR root and that
 769 \fBUsePrivilegedPort\fR be set to \fByes\fR.
 770 .RE
 771 
 772 .sp
 773 .ne 2
 774 .na
 775 \fB\fBServerAliveCountMax\fR\fR
 776 .ad
 777 .sp .6
 778 .RS 4n
 779 Sets the number of server alive messages which can be sent without \fBssh\fR(1)
 780 receiving messages back from the server. If this threshold is reached while
 781 server alive messages are being sent, \fBssh\fR disconnects from the server,
 782 terminating the session. The use of server alive messages differs from
 783 \fBTCPKeepAlive\fR. Server alive messages are sent through the encrypted
 784 channel and are not spoofable. The TCP keep alive option enabled by
 785 \fBTCPKeepAlive\fR is spoofable. The server alive mechanism is valuable when
 786 the client or server depend on knowing when a connection has become inactive.
 787 .sp
 788 The default value is 3. If, for example, \fBServerAliveInterval\fR is set to 15
 789 and \fBServerAliveCountMax\fR is left at the default, \fBssh\fR disconnects in
 790 45-60 seconds if the server becomes unresponsive. This option applies to
 791 protocol version 2 only.
 792 .RE
 793 
 794 .sp
 795 .ne 2
 796 .na
 797 \fB\fBServerAliveInterval\fR\fR
 798 .ad
 799 .sp .6
 800 .RS 4n
 801 Sets a timeout interval in seconds after which if no data has been received
 802 from the server, \fBssh\fR(1) sends a message through the encrypted channel to
 803 request a response from the server. The default is 0, indicating that these
 804 messages are not sent to the server. This option applies to protocol version 2
 805 only.
 806 .RE
 807 
 808 .sp
 809 .ne 2
 810 .na
 811 \fB\fBStrictHostKeyChecking\fR\fR
 812 .ad
 813 .sp .6
 814 .RS 4n
 815 If this flag is set to \fByes\fR, \fBssh\fR never automatically adds host keys
 816 to the \fB$HOME/.ssh/known_hosts\fR file, and refuses to connect hosts whose
 817 host key has changed. This provides maximum protection against trojan horse
 818 attacks. However, it can be a source of inconvenience if you do not have good
 819 \fB/etc/ssh/ssh_known_hosts\fR files installed and frequently connect new
 820 hosts. This option forces the user to manually add any new hosts. Normally this
 821 option is disabled, and new hosts are automatically added to the known host
 822 files. The host keys of known hosts are verified automatically in either case.
 823 The argument must be \fByes\fR or \fBno\fR or \fBask\fR. The default is
 824 \fBask\fR.
 825 .RE
 826 
 827 .sp
 828 .ne 2
 829 .na
 830 \fB\fBUseOpenSSLEngine\fR\fR
 831 .ad
 832 .sp .6
 833 .RS 4n
 834 Specifies whether \fBssh\fR should use the OpenSSL PKCS#11 engine for
 835 offloading cryptographic operations to the Cryptographic Framework.
 836 Cryptographic operations are accelerated according to the available installed
 837 plug-ins. When no suitable plug-ins are present this option does not have an
 838 effect. The default is \fByes\fR.
 839 .RE
 840 
 841 .sp
 842 .ne 2
 843 .na
 844 \fB\fBUsePrivilegedPort\fR\fR
 845 .ad
 846 .sp .6
 847 .RS 4n
 848 Specifies whether to use a privileged port for outgoing connections. The
 849 argument must be \fByes\fR or \fBno\fR. The default is \fByes\fR. Setting this
 850 option to \fBno\fR turns off \fBRhostsAuthentication\fR and
 851 \fBRhostsRSAAuthentication\fR. If set to \fByes\fR \fBssh\fR must be
 852 \fBsetuid\fR root. Defaults to \fBno\fR.
 853 .RE
 854 
 855 .sp
 856 .ne 2
 857 .na
 858 \fB\fBUser\fR\fR
 859 .ad
 860 .sp .6
 861 .RS 4n
 862 Specifies the user to log in as. This can be useful if you have different user
 863 names on different machines. This saves you the trouble of having to remember
 864 to enter the user name on the command line.
 865 .RE
 866 
 867 .sp
 868 .ne 2
 869 .na
 870 \fB\fBUserKnownHostsFile\fR\fR
 871 .ad
 872 .sp .6
 873 .RS 4n
 874 Specifies a file to use instead of \fB$HOME/.ssh/known_hosts\fR.
 875 .RE
 876 
 877 .sp
 878 .ne 2
 879 .na
 880 \fB\fBUseRsh\fR\fR
 881 .ad
 882 .sp .6
 883 .RS 4n
 884 Specifies that \fBrlogin\fR or \fBrsh\fR should be used for this host. It is
 885 possible that the host does not support the \fBssh\fR protocol. This causes
 886 \fBssh\fR to immediately execute \fBrsh\fR(1). All other options (except
 887 \fBHostName\fR) are ignored if this has been specified. The argument must be
 888 \fByes\fR or \fBno\fR.
 889 .RE
 890 
 891 .sp
 892 .ne 2
 893 .na
 894 \fB\fBXAuthLocation\fR\fR
 895 .ad
 896 .sp .6
 897 .RS 4n
 898 Specifies the location of the \fBxauth\fR(1) program. The default is
 899 \fB/usr/openwin/bin/xauth\fR.
 900 .RE
 901 
 902 .SH SEE ALSO
 903 .sp
 904 .LP
 905 \fBrsh\fR(1), \fBssh\fR(1), \fBssh-http-proxy-connect\fR(1),
 906 \fBssh-keygen\fR(1), \fBssh-socks5-proxy-connect\fR(1), \fBsshd\fR(1M),
 907 \fBsshd_config\fR(4), \fBkerberos\fR(5)
 908 .sp
 909 .LP
 910 \fIRFC 4252\fR