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