ssh.0 revision 256281
1SSH(1) OpenBSD Reference Manual SSH(1) 2 3NAME 4 ssh - OpenSSH SSH client (remote login program) 5 6SYNOPSIS 7 ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] 8 [-D [bind_address:]port] [-E log_file] [-e escape_char] 9 [-F configfile] [-I pkcs11] [-i identity_file] 10 [-L [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec] 11 [-O ctl_cmd] [-o option] [-p port] 12 [-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port] 13 [-w local_tun[:remote_tun]] [user@]hostname [command] 14 ssh -Q protocol_feature 15 16DESCRIPTION 17 ssh (SSH client) is a program for logging into a remote machine and for 18 executing commands on a remote machine. It is intended to replace rlogin 19 and rsh, and provide secure encrypted communications between two 20 untrusted hosts over an insecure network. X11 connections and arbitrary 21 TCP ports can also be forwarded over the secure channel. 22 23 ssh connects and logs into the specified hostname (with optional user 24 name). The user must prove his/her identity to the remote machine using 25 one of several methods depending on the protocol version used (see 26 below). 27 28 If command is specified, it is executed on the remote host instead of a 29 login shell. 30 31 The options are as follows: 32 33 -1 Forces ssh to try protocol version 1 only. 34 35 -2 Forces ssh to try protocol version 2 only. 36 37 -4 Forces ssh to use IPv4 addresses only. 38 39 -6 Forces ssh to use IPv6 addresses only. 40 41 -A Enables forwarding of the authentication agent connection. This 42 can also be specified on a per-host basis in a configuration 43 file. 44 45 Agent forwarding should be enabled with caution. Users with the 46 ability to bypass file permissions on the remote host (for the 47 agent's UNIX-domain socket) can access the local agent through 48 the forwarded connection. An attacker cannot obtain key material 49 from the agent, however they can perform operations on the keys 50 that enable them to authenticate using the identities loaded into 51 the agent. 52 53 -a Disables forwarding of the authentication agent connection. 54 55 -b bind_address 56 Use bind_address on the local machine as the source address of 57 the connection. Only useful on systems with more than one 58 address. 59 60 -C Requests compression of all data (including stdin, stdout, 61 stderr, and data for forwarded X11 and TCP connections). The 62 compression algorithm is the same used by gzip(1), and the 63 ``level'' can be controlled by the CompressionLevel option for 64 protocol version 1. Compression is desirable on modem lines and 65 other slow connections, but will only slow down things on fast 66 networks. The default value can be set on a host-by-host basis 67 in the configuration files; see the Compression option. 68 69 -c cipher_spec 70 Selects the cipher specification for encrypting the session. 71 72 Protocol version 1 allows specification of a single cipher. The 73 supported values are ``3des'', ``blowfish'', and ``des''. 3des 74 (triple-des) is an encrypt-decrypt-encrypt triple with three 75 different keys. It is believed to be secure. blowfish is a fast 76 block cipher; it appears very secure and is much faster than 77 3des. des is only supported in the ssh client for 78 interoperability with legacy protocol 1 implementations that do 79 not support the 3des cipher. Its use is strongly discouraged due 80 to cryptographic weaknesses. The default is ``3des''. 81 82 For protocol version 2, cipher_spec is a comma-separated list of 83 ciphers listed in order of preference. See the Ciphers keyword 84 in ssh_config(5) for more information. 85 86 -D [bind_address:]port 87 Specifies a local ``dynamic'' application-level port forwarding. 88 This works by allocating a socket to listen to port on the local 89 side, optionally bound to the specified bind_address. Whenever a 90 connection is made to this port, the connection is forwarded over 91 the secure channel, and the application protocol is then used to 92 determine where to connect to from the remote machine. Currently 93 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act 94 as a SOCKS server. Only root can forward privileged ports. 95 Dynamic port forwardings can also be specified in the 96 configuration file. 97 98 IPv6 addresses can be specified by enclosing the address in 99 square brackets. Only the superuser can forward privileged 100 ports. By default, the local port is bound in accordance with 101 the GatewayPorts setting. However, an explicit bind_address may 102 be used to bind the connection to a specific address. The 103 bind_address of ``localhost'' indicates that the listening port 104 be bound for local use only, while an empty address or `*' 105 indicates that the port should be available from all interfaces. 106 107 -E log_file 108 Append debug logs to log_file instead of standard error. 109 110 -e escape_char 111 Sets the escape character for sessions with a pty (default: `~'). 112 The escape character is only recognized at the beginning of a 113 line. The escape character followed by a dot (`.') closes the 114 connection; followed by control-Z suspends the connection; and 115 followed by itself sends the escape character once. Setting the 116 character to ``none'' disables any escapes and makes the session 117 fully transparent. 118 119 -F configfile 120 Specifies an alternative per-user configuration file. If a 121 configuration file is given on the command line, the system-wide 122 configuration file (/etc/ssh/ssh_config) will be ignored. The 123 default for the per-user configuration file is ~/.ssh/config. 124 125 -f Requests ssh to go to background just before command execution. 126 This is useful if ssh is going to ask for passwords or 127 passphrases, but the user wants it in the background. This 128 implies -n. The recommended way to start X11 programs at a 129 remote site is with something like ssh -f host xterm. 130 131 If the ExitOnForwardFailure configuration option is set to 132 ``yes'', then a client started with -f will wait for all remote 133 port forwards to be successfully established before placing 134 itself in the background. 135 136 -g Allows remote hosts to connect to local forwarded ports. 137 138 -I pkcs11 139 Specify the PKCS#11 shared library ssh should use to communicate 140 with a PKCS#11 token providing the user's private RSA key. 141 142 -i identity_file 143 Selects a file from which the identity (private key) for public 144 key authentication is read. The default is ~/.ssh/identity for 145 protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and 146 ~/.ssh/id_rsa for protocol version 2. Identity files may also be 147 specified on a per-host basis in the configuration file. It is 148 possible to have multiple -i options (and multiple identities 149 specified in configuration files). ssh will also try to load 150 certificate information from the filename obtained by appending 151 -cert.pub to identity filenames. 152 153 -K Enables GSSAPI-based authentication and forwarding (delegation) 154 of GSSAPI credentials to the server. 155 156 -k Disables forwarding (delegation) of GSSAPI credentials to the 157 server. 158 159 -L [bind_address:]port:host:hostport 160 Specifies that the given port on the local (client) host is to be 161 forwarded to the given host and port on the remote side. This 162 works by allocating a socket to listen to port on the local side, 163 optionally bound to the specified bind_address. Whenever a 164 connection is made to this port, the connection is forwarded over 165 the secure channel, and a connection is made to host port 166 hostport from the remote machine. Port forwardings can also be 167 specified in the configuration file. IPv6 addresses can be 168 specified by enclosing the address in square brackets. Only the 169 superuser can forward privileged ports. By default, the local 170 port is bound in accordance with the GatewayPorts setting. 171 However, an explicit bind_address may be used to bind the 172 connection to a specific address. The bind_address of 173 ``localhost'' indicates that the listening port be bound for 174 local use only, while an empty address or `*' indicates that the 175 port should be available from all interfaces. 176 177 -l login_name 178 Specifies the user to log in as on the remote machine. This also 179 may be specified on a per-host basis in the configuration file. 180 181 -M Places the ssh client into ``master'' mode for connection 182 sharing. Multiple -M options places ssh into ``master'' mode 183 with confirmation required before slave connections are accepted. 184 Refer to the description of ControlMaster in ssh_config(5) for 185 details. 186 187 -m mac_spec 188 Additionally, for protocol version 2 a comma-separated list of 189 MAC (message authentication code) algorithms can be specified in 190 order of preference. See the MACs keyword for more information. 191 192 -N Do not execute a remote command. This is useful for just 193 forwarding ports (protocol version 2 only). 194 195 -n Redirects stdin from /dev/null (actually, prevents reading from 196 stdin). This must be used when ssh is run in the background. A 197 common trick is to use this to run X11 programs on a remote 198 machine. For example, ssh -n shadows.cs.hut.fi emacs & will 199 start an emacs on shadows.cs.hut.fi, and the X11 connection will 200 be automatically forwarded over an encrypted channel. The ssh 201 program will be put in the background. (This does not work if 202 ssh needs to ask for a password or passphrase; see also the -f 203 option.) 204 205 -O ctl_cmd 206 Control an active connection multiplexing master process. When 207 the -O option is specified, the ctl_cmd argument is interpreted 208 and passed to the master process. Valid commands are: ``check'' 209 (check that the master process is running), ``forward'' (request 210 forwardings without command execution), ``cancel'' (cancel 211 forwardings), ``exit'' (request the master to exit), and ``stop'' 212 (request the master to stop accepting further multiplexing 213 requests). 214 215 -o option 216 Can be used to give options in the format used in the 217 configuration file. This is useful for specifying options for 218 which there is no separate command-line flag. For full details 219 of the options listed below, and their possible values, see 220 ssh_config(5). 221 222 AddressFamily 223 BatchMode 224 BindAddress 225 ChallengeResponseAuthentication 226 CheckHostIP 227 Cipher 228 Ciphers 229 ClearAllForwardings 230 Compression 231 CompressionLevel 232 ConnectionAttempts 233 ConnectTimeout 234 ControlMaster 235 ControlPath 236 ControlPersist 237 DynamicForward 238 EscapeChar 239 ExitOnForwardFailure 240 ForwardAgent 241 ForwardX11 242 ForwardX11Timeout 243 ForwardX11Trusted 244 GatewayPorts 245 GlobalKnownHostsFile 246 GSSAPIAuthentication 247 GSSAPIDelegateCredentials 248 HashKnownHosts 249 Host 250 HostbasedAuthentication 251 HostKeyAlgorithms 252 HostKeyAlias 253 HostName 254 IdentityFile 255 IdentitiesOnly 256 IPQoS 257 KbdInteractiveAuthentication 258 KbdInteractiveDevices 259 KexAlgorithms 260 LocalCommand 261 LocalForward 262 LogLevel 263 MACs 264 NoHostAuthenticationForLocalhost 265 NumberOfPasswordPrompts 266 PasswordAuthentication 267 PermitLocalCommand 268 PKCS11Provider 269 Port 270 PreferredAuthentications 271 Protocol 272 ProxyCommand 273 PubkeyAuthentication 274 RekeyLimit 275 RemoteForward 276 RequestTTY 277 RhostsRSAAuthentication 278 RSAAuthentication 279 SendEnv 280 ServerAliveInterval 281 ServerAliveCountMax 282 StrictHostKeyChecking 283 TCPKeepAlive 284 Tunnel 285 TunnelDevice 286 UsePrivilegedPort 287 User 288 UserKnownHostsFile 289 VerifyHostKeyDNS 290 VisualHostKey 291 XAuthLocation 292 293 -p port 294 Port to connect to on the remote host. This can be specified on 295 a per-host basis in the configuration file. 296 297 -Q protocol_feature 298 Queries ssh for the algorithms supported for the specified 299 version 2 protocol_feature. The queriable features are: 300 ``cipher'' (supported symmetric ciphers), ``MAC'' (supported 301 message integrity codes), ``KEX'' (key exchange algorithms), 302 ``key'' (key types). Protocol features are treated case- 303 insensitively. 304 305 -q Quiet mode. Causes most warning and diagnostic messages to be 306 suppressed. 307 308 -R [bind_address:]port:host:hostport 309 Specifies that the given port on the remote (server) host is to 310 be forwarded to the given host and port on the local side. This 311 works by allocating a socket to listen to port on the remote 312 side, and whenever a connection is made to this port, the 313 connection is forwarded over the secure channel, and a connection 314 is made to host port hostport from the local machine. 315 316 Port forwardings can also be specified in the configuration file. 317 Privileged ports can be forwarded only when logging in as root on 318 the remote machine. IPv6 addresses can be specified by enclosing 319 the address in square brackets. 320 321 By default, the listening socket on the server will be bound to 322 the loopback interface only. This may be overridden by 323 specifying a bind_address. An empty bind_address, or the address 324 `*', indicates that the remote socket should listen on all 325 interfaces. Specifying a remote bind_address will only succeed 326 if the server's GatewayPorts option is enabled (see 327 sshd_config(5)). 328 329 If the port argument is `0', the listen port will be dynamically 330 allocated on the server and reported to the client at run time. 331 When used together with -O forward the allocated port will be 332 printed to the standard output. 333 334 -S ctl_path 335 Specifies the location of a control socket for connection 336 sharing, or the string ``none'' to disable connection sharing. 337 Refer to the description of ControlPath and ControlMaster in 338 ssh_config(5) for details. 339 340 -s May be used to request invocation of a subsystem on the remote 341 system. Subsystems are a feature of the SSH2 protocol which 342 facilitate the use of SSH as a secure transport for other 343 applications (eg. sftp(1)). The subsystem is specified as the 344 remote command. 345 346 -T Disable pseudo-tty allocation. 347 348 -t Force pseudo-tty allocation. This can be used to execute 349 arbitrary screen-based programs on a remote machine, which can be 350 very useful, e.g. when implementing menu services. Multiple -t 351 options force tty allocation, even if ssh has no local tty. 352 353 -V Display the version number and exit. 354 355 -v Verbose mode. Causes ssh to print debugging messages about its 356 progress. This is helpful in debugging connection, 357 authentication, and configuration problems. Multiple -v options 358 increase the verbosity. The maximum is 3. 359 360 -W host:port 361 Requests that standard input and output on the client be 362 forwarded to host on port over the secure channel. Implies -N, 363 -T, ExitOnForwardFailure and ClearAllForwardings. Works with 364 Protocol version 2 only. 365 366 -w local_tun[:remote_tun] 367 Requests tunnel device forwarding with the specified tun(4) 368 devices between the client (local_tun) and the server 369 (remote_tun). 370 371 The devices may be specified by numerical ID or the keyword 372 ``any'', which uses the next available tunnel device. If 373 remote_tun is not specified, it defaults to ``any''. See also 374 the Tunnel and TunnelDevice directives in ssh_config(5). If the 375 Tunnel directive is unset, it is set to the default tunnel mode, 376 which is ``point-to-point''. 377 378 -X Enables X11 forwarding. This can also be specified on a per-host 379 basis in a configuration file. 380 381 X11 forwarding should be enabled with caution. Users with the 382 ability to bypass file permissions on the remote host (for the 383 user's X authorization database) can access the local X11 display 384 through the forwarded connection. An attacker may then be able 385 to perform activities such as keystroke monitoring. 386 387 For this reason, X11 forwarding is subjected to X11 SECURITY 388 extension restrictions by default. Please refer to the ssh -Y 389 option and the ForwardX11Trusted directive in ssh_config(5) for 390 more information. 391 392 -x Disables X11 forwarding. 393 394 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not 395 subjected to the X11 SECURITY extension controls. 396 397 -y Send log information using the syslog(3) system module. By 398 default this information is sent to stderr. 399 400 ssh may additionally obtain configuration data from a per-user 401 configuration file and a system-wide configuration file. The file format 402 and configuration options are described in ssh_config(5). 403 404AUTHENTICATION 405 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to 406 use protocol 2 only, though this can be changed via the Protocol option 407 in ssh_config(5) or the -1 and -2 options (see above). Both protocols 408 support similar authentication methods, but protocol 2 is the default 409 since it provides additional mechanisms for confidentiality (the traffic 410 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and 411 integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64, 412 umac-128, hmac-ripemd160). Protocol 1 lacks a strong mechanism for 413 ensuring the integrity of the connection. 414 415 The methods available for authentication are: GSSAPI-based 416 authentication, host-based authentication, public key authentication, 417 challenge-response authentication, and password authentication. 418 Authentication methods are tried in the order specified above, though 419 protocol 2 has a configuration option to change the default order: 420 PreferredAuthentications. 421 422 Host-based authentication works as follows: If the machine the user logs 423 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote 424 machine, and the user names are the same on both sides, or if the files 425 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote 426 machine and contain a line containing the name of the client machine and 427 the name of the user on that machine, the user is considered for login. 428 Additionally, the server must be able to verify the client's host key 429 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, 430 below) for login to be permitted. This authentication method closes 431 security holes due to IP spoofing, DNS spoofing, and routing spoofing. 432 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the 433 rlogin/rsh protocol in general, are inherently insecure and should be 434 disabled if security is desired.] 435 436 Public key authentication works as follows: The scheme is based on 437 public-key cryptography, using cryptosystems where encryption and 438 decryption are done using separate keys, and it is unfeasible to derive 439 the decryption key from the encryption key. The idea is that each user 440 creates a public/private key pair for authentication purposes. The 441 server knows the public key, and only the user knows the private key. 442 ssh implements public key authentication protocol automatically, using 443 one of the DSA, ECDSA or RSA algorithms. Protocol 1 is restricted to 444 using only RSA keys, but protocol 2 may use any. The HISTORY section of 445 ssl(8) contains a brief discussion of the DSA and RSA algorithms. 446 447 The file ~/.ssh/authorized_keys lists the public keys that are permitted 448 for logging in. When the user logs in, the ssh program tells the server 449 which key pair it would like to use for authentication. The client 450 proves that it has access to the private key and the server checks that 451 the corresponding public key is authorized to accept the account. 452 453 The user creates his/her key pair by running ssh-keygen(1). This stores 454 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 455 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa (protocol 2 456 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), 457 ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2 458 ECDSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home 459 directory. The user should then copy the public key to 460 ~/.ssh/authorized_keys in his/her home directory on the remote machine. 461 The authorized_keys file corresponds to the conventional ~/.rhosts file, 462 and has one key per line, though the lines can be very long. After this, 463 the user can log in without giving the password. 464 465 A variation on public key authentication is available in the form of 466 certificate authentication: instead of a set of public/private keys, 467 signed certificates are used. This has the advantage that a single 468 trusted certification authority can be used in place of many 469 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for 470 more information. 471 472 The most convenient way to use public key or certificate authentication 473 may be with an authentication agent. See ssh-agent(1) for more 474 information. 475 476 Challenge-response authentication works as follows: The server sends an 477 arbitrary "challenge" text, and prompts for a response. Protocol 2 478 allows multiple challenges and responses; protocol 1 is restricted to 479 just one challenge/response. Examples of challenge-response 480 authentication include BSD Authentication (see login.conf(5)) and PAM 481 (some non-OpenBSD systems). 482 483 Finally, if other authentication methods fail, ssh prompts the user for a 484 password. The password is sent to the remote host for checking; however, 485 since all communications are encrypted, the password cannot be seen by 486 someone listening on the network. 487 488 ssh automatically maintains and checks a database containing 489 identification for all hosts it has ever been used with. Host keys are 490 stored in ~/.ssh/known_hosts in the user's home directory. Additionally, 491 the file /etc/ssh/ssh_known_hosts is automatically checked for known 492 hosts. Any new hosts are automatically added to the user's file. If a 493 host's identification ever changes, ssh warns about this and disables 494 password authentication to prevent server spoofing or man-in-the-middle 495 attacks, which could otherwise be used to circumvent the encryption. The 496 StrictHostKeyChecking option can be used to control logins to machines 497 whose host key is not known or has changed. 498 499 When the user's identity has been accepted by the server, the server 500 either executes the given command, or logs into the machine and gives the 501 user a normal shell on the remote machine. All communication with the 502 remote command or shell will be automatically encrypted. 503 504 If a pseudo-terminal has been allocated (normal login session), the user 505 may use the escape characters noted below. 506 507 If no pseudo-tty has been allocated, the session is transparent and can 508 be used to reliably transfer binary data. On most systems, setting the 509 escape character to ``none'' will also make the session transparent even 510 if a tty is used. 511 512 The session terminates when the command or shell on the remote machine 513 exits and all X11 and TCP connections have been closed. 514 515ESCAPE CHARACTERS 516 When a pseudo-terminal has been requested, ssh supports a number of 517 functions through the use of an escape character. 518 519 A single tilde character can be sent as ~~ or by following the tilde by a 520 character other than those described below. The escape character must 521 always follow a newline to be interpreted as special. The escape 522 character can be changed in configuration files using the EscapeChar 523 configuration directive or on the command line by the -e option. 524 525 The supported escapes (assuming the default `~') are: 526 527 ~. Disconnect. 528 529 ~^Z Background ssh. 530 531 ~# List forwarded connections. 532 533 ~& Background ssh at logout when waiting for forwarded connection / 534 X11 sessions to terminate. 535 536 ~? Display a list of escape characters. 537 538 ~B Send a BREAK to the remote system (only useful for SSH protocol 539 version 2 and if the peer supports it). 540 541 ~C Open command line. Currently this allows the addition of port 542 forwardings using the -L, -R and -D options (see above). It also 543 allows the cancellation of existing port-forwardings with 544 -KL[bind_address:]port for local, -KR[bind_address:]port for 545 remote and -KD[bind_address:]port for dynamic port-forwardings. 546 !command allows the user to execute a local command if the 547 PermitLocalCommand option is enabled in ssh_config(5). Basic 548 help is available, using the -h option. 549 550 ~R Request rekeying of the connection (only useful for SSH protocol 551 version 2 and if the peer supports it). 552 553 ~V Decrease the verbosity (LogLevel) when errors are being written 554 to stderr. 555 556 ~v Increase the verbosity (LogLevel) when errors are being written 557 to stderr. 558 559TCP FORWARDING 560 Forwarding of arbitrary TCP connections over the secure channel can be 561 specified either on the command line or in a configuration file. One 562 possible application of TCP forwarding is a secure connection to a mail 563 server; another is going through firewalls. 564 565 In the example below, we look at encrypting communication between an IRC 566 client and server, even though the IRC server does not directly support 567 encrypted communications. This works as follows: the user connects to 568 the remote host using ssh, specifying a port to be used to forward 569 connections to the remote server. After that it is possible to start the 570 service which is to be encrypted on the client machine, connecting to the 571 same local port, and ssh will encrypt and forward the connection. 572 573 The following example tunnels an IRC session from client machine 574 ``127.0.0.1'' (localhost) to remote server ``server.example.com'': 575 576 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 577 $ irc -c '#users' -p 1234 pinky 127.0.0.1 578 579 This tunnels a connection to IRC server ``server.example.com'', joining 580 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't 581 matter which port is used, as long as it's greater than 1023 (remember, 582 only root can open sockets on privileged ports) and doesn't conflict with 583 any ports already in use. The connection is forwarded to port 6667 on 584 the remote server, since that's the standard port for IRC services. 585 586 The -f option backgrounds ssh and the remote command ``sleep 10'' is 587 specified to allow an amount of time (10 seconds, in the example) to 588 start the service which is to be tunnelled. If no connections are made 589 within the time specified, ssh will exit. 590 591X11 FORWARDING 592 If the ForwardX11 variable is set to ``yes'' (or see the description of 593 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY 594 environment variable is set), the connection to the X11 display is 595 automatically forwarded to the remote side in such a way that any X11 596 programs started from the shell (or command) will go through the 597 encrypted channel, and the connection to the real X server will be made 598 from the local machine. The user should not manually set DISPLAY. 599 Forwarding of X11 connections can be configured on the command line or in 600 configuration files. 601 602 The DISPLAY value set by ssh will point to the server machine, but with a 603 display number greater than zero. This is normal, and happens because 604 ssh creates a ``proxy'' X server on the server machine for forwarding the 605 connections over the encrypted channel. 606 607 ssh will also automatically set up Xauthority data on the server machine. 608 For this purpose, it will generate a random authorization cookie, store 609 it in Xauthority on the server, and verify that any forwarded connections 610 carry this cookie and replace it by the real cookie when the connection 611 is opened. The real authentication cookie is never sent to the server 612 machine (and no cookies are sent in the plain). 613 614 If the ForwardAgent variable is set to ``yes'' (or see the description of 615 the -A and -a options above) and the user is using an authentication 616 agent, the connection to the agent is automatically forwarded to the 617 remote side. 618 619VERIFYING HOST KEYS 620 When connecting to a server for the first time, a fingerprint of the 621 server's public key is presented to the user (unless the option 622 StrictHostKeyChecking has been disabled). Fingerprints can be determined 623 using ssh-keygen(1): 624 625 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 626 627 If the fingerprint is already known, it can be matched and the key can be 628 accepted or rejected. Because of the difficulty of comparing host keys 629 just by looking at hex strings, there is also support to compare host 630 keys visually, using random art. By setting the VisualHostKey option to 631 ``yes'', a small ASCII graphic gets displayed on every login to a server, 632 no matter if the session itself is interactive or not. By learning the 633 pattern a known server produces, a user can easily find out that the host 634 key has changed when a completely different pattern is displayed. 635 Because these patterns are not unambiguous however, a pattern that looks 636 similar to the pattern remembered only gives a good probability that the 637 host key is the same, not guaranteed proof. 638 639 To get a listing of the fingerprints along with their random art for all 640 known hosts, the following command line can be used: 641 642 $ ssh-keygen -lv -f ~/.ssh/known_hosts 643 644 If the fingerprint is unknown, an alternative method of verification is 645 available: SSH fingerprints verified by DNS. An additional resource 646 record (RR), SSHFP, is added to a zonefile and the connecting client is 647 able to match the fingerprint with that of the key presented. 648 649 In this example, we are connecting a client to a server, 650 ``host.example.com''. The SSHFP resource records should first be added 651 to the zonefile for host.example.com: 652 653 $ ssh-keygen -r host.example.com. 654 655 The output lines will have to be added to the zonefile. To check that 656 the zone is answering fingerprint queries: 657 658 $ dig -t SSHFP host.example.com 659 660 Finally the client connects: 661 662 $ ssh -o "VerifyHostKeyDNS ask" host.example.com 663 [...] 664 Matching host key fingerprint found in DNS. 665 Are you sure you want to continue connecting (yes/no)? 666 667 See the VerifyHostKeyDNS option in ssh_config(5) for more information. 668 669SSH-BASED VIRTUAL PRIVATE NETWORKS 670 ssh contains support for Virtual Private Network (VPN) tunnelling using 671 the tun(4) network pseudo-device, allowing two networks to be joined 672 securely. The sshd_config(5) configuration option PermitTunnel controls 673 whether the server supports this, and at what level (layer 2 or 3 674 traffic). 675 676 The following example would connect client network 10.0.50.0/24 with 677 remote network 10.0.99.0/24 using a point-to-point connection from 678 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway 679 to the remote network, at 192.168.1.15, allows it. 680 681 On the client: 682 683 # ssh -f -w 0:1 192.168.1.15 true 684 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 685 # route add 10.0.99.0/24 10.1.1.2 686 687 On the server: 688 689 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 690 # route add 10.0.50.0/24 10.1.1.1 691 692 Client access may be more finely tuned via the /root/.ssh/authorized_keys 693 file (see below) and the PermitRootLogin server option. The following 694 entry would permit connections on tun(4) device 1 from user ``jane'' and 695 on tun device 2 from user ``john'', if PermitRootLogin is set to 696 ``forced-commands-only'': 697 698 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 699 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 700 701 Since an SSH-based setup entails a fair amount of overhead, it may be 702 more suited to temporary setups, such as for wireless VPNs. More 703 permanent VPNs are better provided by tools such as ipsecctl(8) and 704 isakmpd(8). 705 706ENVIRONMENT 707 ssh will normally set the following environment variables: 708 709 DISPLAY The DISPLAY variable indicates the location of the 710 X11 server. It is automatically set by ssh to 711 point to a value of the form ``hostname:n'', where 712 ``hostname'' indicates the host where the shell 713 runs, and `n' is an integer >= 1. ssh uses this 714 special value to forward X11 connections over the 715 secure channel. The user should normally not set 716 DISPLAY explicitly, as that will render the X11 717 connection insecure (and will require the user to 718 manually copy any required authorization cookies). 719 720 HOME Set to the path of the user's home directory. 721 722 LOGNAME Synonym for USER; set for compatibility with 723 systems that use this variable. 724 725 MAIL Set to the path of the user's mailbox. 726 727 PATH Set to the default PATH, as specified when 728 compiling ssh. 729 730 SSH_ASKPASS If ssh needs a passphrase, it will read the 731 passphrase from the current terminal if it was run 732 from a terminal. If ssh does not have a terminal 733 associated with it but DISPLAY and SSH_ASKPASS are 734 set, it will execute the program specified by 735 SSH_ASKPASS and open an X11 window to read the 736 passphrase. This is particularly useful when 737 calling ssh from a .xsession or related script. 738 (Note that on some machines it may be necessary to 739 redirect the input from /dev/null to make this 740 work.) 741 742 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to 743 communicate with the agent. 744 745 SSH_CONNECTION Identifies the client and server ends of the 746 connection. The variable contains four space- 747 separated values: client IP address, client port 748 number, server IP address, and server port number. 749 750 SSH_ORIGINAL_COMMAND This variable contains the original command line if 751 a forced command is executed. It can be used to 752 extract the original arguments. 753 754 SSH_TTY This is set to the name of the tty (path to the 755 device) associated with the current shell or 756 command. If the current session has no tty, this 757 variable is not set. 758 759 TZ This variable is set to indicate the present time 760 zone if it was set when the daemon was started 761 (i.e. the daemon passes the value on to new 762 connections). 763 764 USER Set to the name of the user logging in. 765 766 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format 767 ``VARNAME=value'' to the environment if the file exists and users are 768 allowed to change their environment. For more information, see the 769 PermitUserEnvironment option in sshd_config(5). 770 771FILES 772 ~/.rhosts 773 This file is used for host-based authentication (see above). On 774 some machines this file may need to be world-readable if the 775 user's home directory is on an NFS partition, because sshd(8) 776 reads it as root. Additionally, this file must be owned by the 777 user, and must not have write permissions for anyone else. The 778 recommended permission for most machines is read/write for the 779 user, and not accessible by others. 780 781 ~/.shosts 782 This file is used in exactly the same way as .rhosts, but allows 783 host-based authentication without permitting login with 784 rlogin/rsh. 785 786 ~/.ssh/ 787 This directory is the default location for all user-specific 788 configuration and authentication information. There is no 789 general requirement to keep the entire contents of this directory 790 secret, but the recommended permissions are read/write/execute 791 for the user, and not accessible by others. 792 793 ~/.ssh/authorized_keys 794 Lists the public keys (DSA/ECDSA/RSA) that can be used for 795 logging in as this user. The format of this file is described in 796 the sshd(8) manual page. This file is not highly sensitive, but 797 the recommended permissions are read/write for the user, and not 798 accessible by others. 799 800 ~/.ssh/config 801 This is the per-user configuration file. The file format and 802 configuration options are described in ssh_config(5). Because of 803 the potential for abuse, this file must have strict permissions: 804 read/write for the user, and not writable by others. 805 806 ~/.ssh/environment 807 Contains additional definitions for environment variables; see 808 ENVIRONMENT, above. 809 810 ~/.ssh/identity 811 ~/.ssh/id_dsa 812 ~/.ssh/id_ecdsa 813 ~/.ssh/id_rsa 814 Contains the private key for authentication. These files contain 815 sensitive data and should be readable by the user but not 816 accessible by others (read/write/execute). ssh will simply 817 ignore a private key file if it is accessible by others. It is 818 possible to specify a passphrase when generating the key which 819 will be used to encrypt the sensitive part of this file using 820 3DES. 821 822 ~/.ssh/identity.pub 823 ~/.ssh/id_dsa.pub 824 ~/.ssh/id_ecdsa.pub 825 ~/.ssh/id_rsa.pub 826 Contains the public key for authentication. These files are not 827 sensitive and can (but need not) be readable by anyone. 828 829 ~/.ssh/known_hosts 830 Contains a list of host keys for all hosts the user has logged 831 into that are not already in the systemwide list of known host 832 keys. See sshd(8) for further details of the format of this 833 file. 834 835 ~/.ssh/rc 836 Commands in this file are executed by ssh when the user logs in, 837 just before the user's shell (or command) is started. See the 838 sshd(8) manual page for more information. 839 840 /etc/hosts.equiv 841 This file is for host-based authentication (see above). It 842 should only be writable by root. 843 844 /etc/shosts.equiv 845 This file is used in exactly the same way as hosts.equiv, but 846 allows host-based authentication without permitting login with 847 rlogin/rsh. 848 849 /etc/ssh/ssh_config 850 Systemwide configuration file. The file format and configuration 851 options are described in ssh_config(5). 852 853 /etc/ssh/ssh_host_key 854 /etc/ssh/ssh_host_dsa_key 855 /etc/ssh/ssh_host_ecdsa_key 856 /etc/ssh/ssh_host_rsa_key 857 These files contain the private parts of the host keys and are 858 used for host-based authentication. If protocol version 1 is 859 used, ssh must be setuid root, since the host key is readable 860 only by root. For protocol version 2, ssh uses ssh-keysign(8) to 861 access the host keys, eliminating the requirement that ssh be 862 setuid root when host-based authentication is used. By default 863 ssh is not setuid root. 864 865 /etc/ssh/ssh_known_hosts 866 Systemwide list of known host keys. This file should be prepared 867 by the system administrator to contain the public host keys of 868 all machines in the organization. It should be world-readable. 869 See sshd(8) for further details of the format of this file. 870 871 /etc/ssh/sshrc 872 Commands in this file are executed by ssh when the user logs in, 873 just before the user's shell (or command) is started. See the 874 sshd(8) manual page for more information. 875 876EXIT STATUS 877 ssh exits with the exit status of the remote command or with 255 if an 878 error occurred. 879 880SEE ALSO 881 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), 882 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) 883 884STANDARDS 885 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned 886 Numbers, RFC 4250, January 2006. 887 888 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, 889 RFC 4251, January 2006. 890 891 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, 892 RFC 4252, January 2006. 893 894 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer 895 Protocol, RFC 4253, January 2006. 896 897 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 898 4254, January 2006. 899 900 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell 901 (SSH) Key Fingerprints, RFC 4255, January 2006. 902 903 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the 904 Secure Shell Protocol (SSH), RFC 4256, January 2006. 905 906 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break 907 Extension, RFC 4335, January 2006. 908 909 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport 910 Layer Encryption Modes, RFC 4344, January 2006. 911 912 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport 913 Layer Protocol, RFC 4345, January 2006. 914 915 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for 916 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006. 917 918 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File 919 Format, RFC 4716, November 2006. 920 921 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the 922 Secure Shell Transport Layer, RFC 5656, December 2009. 923 924 A. Perrig and D. Song, Hash Visualization: a New Technique to improve 925 Real-World Security, 1999, International Workshop on Cryptographic 926 Techniques and E-Commerce (CrypTEC '99). 927 928AUTHORS 929 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 930 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 931 de Raadt and Dug Song removed many bugs, re-added newer features and 932 created OpenSSH. Markus Friedl contributed the support for SSH protocol 933 versions 1.5 and 2.0. 934 935OpenBSD 5.4 July 18, 2013 OpenBSD 5.4 936