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 [-Q cipher | cipher-auth | mac | kex | key] 13 [-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port] 14 [-w local_tun[:remote_tun]] [user@]hostname [command] 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, 146 ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2. 147 Identity files may also be specified on a per-host basis in the 148 configuration file. It is possible to have multiple -i options 149 (and multiple identities specified in configuration files). ssh 150 will also try to load certificate information from the filename 151 obtained by appending -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 CanonicalDomains 226 CanonicalizeFallbackLocal 227 CanonicalizeHostname 228 CanonicalizeMaxDots 229 CanonicalizePermittedCNAMEs 230 ChallengeResponseAuthentication 231 CheckHostIP 232 Cipher 233 Ciphers 234 ClearAllForwardings 235 Compression 236 CompressionLevel 237 ConnectionAttempts 238 ConnectTimeout 239 ControlMaster 240 ControlPath 241 ControlPersist 242 DynamicForward 243 EscapeChar 244 ExitOnForwardFailure 245 ForwardAgent 246 ForwardX11 247 ForwardX11Timeout 248 ForwardX11Trusted 249 GatewayPorts 250 GlobalKnownHostsFile 251 GSSAPIAuthentication 252 GSSAPIDelegateCredentials 253 HashKnownHosts 254 Host 255 HostbasedAuthentication 256 HostKeyAlgorithms 257 HostKeyAlias 258 HostName 259 IdentityFile 260 IdentitiesOnly 261 IPQoS 262 KbdInteractiveAuthentication 263 KbdInteractiveDevices 264 KexAlgorithms 265 LocalCommand 266 LocalForward 267 LogLevel 268 MACs 269 Match 270 NoHostAuthenticationForLocalhost 271 NumberOfPasswordPrompts 272 PasswordAuthentication 273 PermitLocalCommand 274 PKCS11Provider 275 Port 276 PreferredAuthentications 277 Protocol 278 ProxyCommand 279 ProxyUseFdpass 280 PubkeyAuthentication 281 RekeyLimit 282 RemoteForward 283 RequestTTY 284 RhostsRSAAuthentication 285 RSAAuthentication 286 SendEnv 287 ServerAliveInterval 288 ServerAliveCountMax 289 StrictHostKeyChecking 290 TCPKeepAlive 291 Tunnel 292 TunnelDevice 293 UsePrivilegedPort 294 User 295 UserKnownHostsFile 296 VerifyHostKeyDNS 297 VisualHostKey 298 XAuthLocation 299 300 -p port 301 Port to connect to on the remote host. This can be specified on 302 a per-host basis in the configuration file. 303 304 -Q cipher | cipher-auth | mac | kex | key 305 Queries ssh for the algorithms supported for the specified 306 version 2. The available features are: cipher (supported 307 symmetric ciphers), cipher-auth (supported symmetric ciphers that 308 support authenticated encryption), mac (supported message 309 integrity codes), kex (key exchange algorithms), key (key types). 310 311 -q Quiet mode. Causes most warning and diagnostic messages to be 312 suppressed. 313 314 -R [bind_address:]port:host:hostport 315 Specifies that the given port on the remote (server) host is to 316 be forwarded to the given host and port on the local side. This 317 works by allocating a socket to listen to port on the remote 318 side, and whenever a connection is made to this port, the 319 connection is forwarded over the secure channel, and a connection 320 is made to host port hostport from the local machine. 321 322 Port forwardings can also be specified in the configuration file. 323 Privileged ports can be forwarded only when logging in as root on 324 the remote machine. IPv6 addresses can be specified by enclosing 325 the address in square brackets. 326 327 By default, the listening socket on the server will be bound to 328 the loopback interface only. This may be overridden by 329 specifying a bind_address. An empty bind_address, or the address 330 `*', indicates that the remote socket should listen on all 331 interfaces. Specifying a remote bind_address will only succeed 332 if the server's GatewayPorts option is enabled (see 333 sshd_config(5)). 334 335 If the port argument is `0', the listen port will be dynamically 336 allocated on the server and reported to the client at run time. 337 When used together with -O forward the allocated port will be 338 printed to the standard output. 339 340 -S ctl_path 341 Specifies the location of a control socket for connection 342 sharing, or the string ``none'' to disable connection sharing. 343 Refer to the description of ControlPath and ControlMaster in 344 ssh_config(5) for details. 345 346 -s May be used to request invocation of a subsystem on the remote 347 system. Subsystems are a feature of the SSH2 protocol which 348 facilitate the use of SSH as a secure transport for other 349 applications (eg. sftp(1)). The subsystem is specified as the 350 remote command. 351 352 -T Disable pseudo-tty allocation. 353 354 -t Force pseudo-tty allocation. This can be used to execute 355 arbitrary screen-based programs on a remote machine, which can be 356 very useful, e.g. when implementing menu services. Multiple -t 357 options force tty allocation, even if ssh has no local tty. 358 359 -V Display the version number and exit. 360 361 -v Verbose mode. Causes ssh to print debugging messages about its 362 progress. This is helpful in debugging connection, 363 authentication, and configuration problems. Multiple -v options 364 increase the verbosity. The maximum is 3. 365 366 -W host:port 367 Requests that standard input and output on the client be 368 forwarded to host on port over the secure channel. Implies -N, 369 -T, ExitOnForwardFailure and ClearAllForwardings. Works with 370 Protocol version 2 only. 371 372 -w local_tun[:remote_tun] 373 Requests tunnel device forwarding with the specified tun(4) 374 devices between the client (local_tun) and the server 375 (remote_tun). 376 377 The devices may be specified by numerical ID or the keyword 378 ``any'', which uses the next available tunnel device. If 379 remote_tun is not specified, it defaults to ``any''. See also 380 the Tunnel and TunnelDevice directives in ssh_config(5). If the 381 Tunnel directive is unset, it is set to the default tunnel mode, 382 which is ``point-to-point''. 383 384 -X Enables X11 forwarding. This can also be specified on a per-host 385 basis in a configuration file. 386 387 X11 forwarding should be enabled with caution. Users with the 388 ability to bypass file permissions on the remote host (for the 389 user's X authorization database) can access the local X11 display 390 through the forwarded connection. An attacker may then be able 391 to perform activities such as keystroke monitoring. 392 393 For this reason, X11 forwarding is subjected to X11 SECURITY 394 extension restrictions by default. Please refer to the ssh -Y 395 option and the ForwardX11Trusted directive in ssh_config(5) for 396 more information. 397 398 -x Disables X11 forwarding. 399 400 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not 401 subjected to the X11 SECURITY extension controls. 402 403 -y Send log information using the syslog(3) system module. By 404 default this information is sent to stderr. 405 406 ssh may additionally obtain configuration data from a per-user 407 configuration file and a system-wide configuration file. The file format 408 and configuration options are described in ssh_config(5). 409 410AUTHENTICATION 411 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to 412 use protocol 2 only, though this can be changed via the Protocol option 413 in ssh_config(5) or the -1 and -2 options (see above). Both protocols 414 support similar authentication methods, but protocol 2 is the default 415 since it provides additional mechanisms for confidentiality (the traffic 416 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and 417 integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64, 418 umac-128, hmac-ripemd160). Protocol 1 lacks a strong mechanism for 419 ensuring the integrity of the connection. 420 421 The methods available for authentication are: GSSAPI-based 422 authentication, host-based authentication, public key authentication, 423 challenge-response authentication, and password authentication. 424 Authentication methods are tried in the order specified above, though 425 protocol 2 has a configuration option to change the default order: 426 PreferredAuthentications. 427 428 Host-based authentication works as follows: If the machine the user logs 429 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote 430 machine, and the user names are the same on both sides, or if the files 431 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote 432 machine and contain a line containing the name of the client machine and 433 the name of the user on that machine, the user is considered for login. 434 Additionally, the server must be able to verify the client's host key 435 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, 436 below) for login to be permitted. This authentication method closes 437 security holes due to IP spoofing, DNS spoofing, and routing spoofing. 438 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the 439 rlogin/rsh protocol in general, are inherently insecure and should be 440 disabled if security is desired.] 441 442 Public key authentication works as follows: The scheme is based on 443 public-key cryptography, using cryptosystems where encryption and 444 decryption are done using separate keys, and it is unfeasible to derive 445 the decryption key from the encryption key. The idea is that each user 446 creates a public/private key pair for authentication purposes. The 447 server knows the public key, and only the user knows the private key. 448 ssh implements public key authentication protocol automatically, using 449 one of the DSA, ECDSA, ED25519 or RSA algorithms. Protocol 1 is 450 restricted to using only RSA keys, but protocol 2 may use any. The 451 HISTORY section of ssl(8) contains a brief discussion of the DSA and RSA 452 algorithms. 453 454 The file ~/.ssh/authorized_keys lists the public keys that are permitted 455 for logging in. When the user logs in, the ssh program tells the server 456 which key pair it would like to use for authentication. The client 457 proves that it has access to the private key and the server checks that 458 the corresponding public key is authorized to accept the account. 459 460 The user creates his/her key pair by running ssh-keygen(1). This stores 461 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 462 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), ~/.ssh/id_ed25519 (protocol 2 463 ED25519), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in 464 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), 465 ~/.ssh/id_ecdsa.pub (protocol 2 ECDSA), ~/.ssh/id_ed25519.pub (protocol 2 466 ED25519), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home 467 directory. The user should then copy the public key to 468 ~/.ssh/authorized_keys in his/her home directory on the remote machine. 469 The authorized_keys file corresponds to the conventional ~/.rhosts file, 470 and has one key per line, though the lines can be very long. After this, 471 the user can log in without giving the password. 472 473 A variation on public key authentication is available in the form of 474 certificate authentication: instead of a set of public/private keys, 475 signed certificates are used. This has the advantage that a single 476 trusted certification authority can be used in place of many 477 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for 478 more information. 479 480 The most convenient way to use public key or certificate authentication 481 may be with an authentication agent. See ssh-agent(1) for more 482 information. 483 484 Challenge-response authentication works as follows: The server sends an 485 arbitrary "challenge" text, and prompts for a response. Protocol 2 486 allows multiple challenges and responses; protocol 1 is restricted to 487 just one challenge/response. Examples of challenge-response 488 authentication include BSD Authentication (see login.conf(5)) and PAM 489 (some non-OpenBSD systems). 490 491 Finally, if other authentication methods fail, ssh prompts the user for a 492 password. The password is sent to the remote host for checking; however, 493 since all communications are encrypted, the password cannot be seen by 494 someone listening on the network. 495 496 ssh automatically maintains and checks a database containing 497 identification for all hosts it has ever been used with. Host keys are 498 stored in ~/.ssh/known_hosts in the user's home directory. Additionally, 499 the file /etc/ssh/ssh_known_hosts is automatically checked for known 500 hosts. Any new hosts are automatically added to the user's file. If a 501 host's identification ever changes, ssh warns about this and disables 502 password authentication to prevent server spoofing or man-in-the-middle 503 attacks, which could otherwise be used to circumvent the encryption. The 504 StrictHostKeyChecking option can be used to control logins to machines 505 whose host key is not known or has changed. 506 507 When the user's identity has been accepted by the server, the server 508 either executes the given command, or logs into the machine and gives the 509 user a normal shell on the remote machine. All communication with the 510 remote command or shell will be automatically encrypted. 511 512 If a pseudo-terminal has been allocated (normal login session), the user 513 may use the escape characters noted below. 514 515 If no pseudo-tty has been allocated, the session is transparent and can 516 be used to reliably transfer binary data. On most systems, setting the 517 escape character to ``none'' will also make the session transparent even 518 if a tty is used. 519 520 The session terminates when the command or shell on the remote machine 521 exits and all X11 and TCP connections have been closed. 522 523ESCAPE CHARACTERS 524 When a pseudo-terminal has been requested, ssh supports a number of 525 functions through the use of an escape character. 526 527 A single tilde character can be sent as ~~ or by following the tilde by a 528 character other than those described below. The escape character must 529 always follow a newline to be interpreted as special. The escape 530 character can be changed in configuration files using the EscapeChar 531 configuration directive or on the command line by the -e option. 532 533 The supported escapes (assuming the default `~') are: 534 535 ~. Disconnect. 536 537 ~^Z Background ssh. 538 539 ~# List forwarded connections. 540 541 ~& Background ssh at logout when waiting for forwarded connection / 542 X11 sessions to terminate. 543 544 ~? Display a list of escape characters. 545 546 ~B Send a BREAK to the remote system (only useful for SSH protocol 547 version 2 and if the peer supports it). 548 549 ~C Open command line. Currently this allows the addition of port 550 forwardings using the -L, -R and -D options (see above). It also 551 allows the cancellation of existing port-forwardings with 552 -KL[bind_address:]port for local, -KR[bind_address:]port for 553 remote and -KD[bind_address:]port for dynamic port-forwardings. 554 !command allows the user to execute a local command if the 555 PermitLocalCommand option is enabled in ssh_config(5). Basic 556 help is available, using the -h option. 557 558 ~R Request rekeying of the connection (only useful for SSH protocol 559 version 2 and if the peer supports it). 560 561 ~V Decrease the verbosity (LogLevel) when errors are being written 562 to stderr. 563 564 ~v Increase the verbosity (LogLevel) when errors are being written 565 to stderr. 566 567TCP FORWARDING 568 Forwarding of arbitrary TCP connections over the secure channel can be 569 specified either on the command line or in a configuration file. One 570 possible application of TCP forwarding is a secure connection to a mail 571 server; another is going through firewalls. 572 573 In the example below, we look at encrypting communication between an IRC 574 client and server, even though the IRC server does not directly support 575 encrypted communications. This works as follows: the user connects to 576 the remote host using ssh, specifying a port to be used to forward 577 connections to the remote server. After that it is possible to start the 578 service which is to be encrypted on the client machine, connecting to the 579 same local port, and ssh will encrypt and forward the connection. 580 581 The following example tunnels an IRC session from client machine 582 ``127.0.0.1'' (localhost) to remote server ``server.example.com'': 583 584 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 585 $ irc -c '#users' -p 1234 pinky 127.0.0.1 586 587 This tunnels a connection to IRC server ``server.example.com'', joining 588 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't 589 matter which port is used, as long as it's greater than 1023 (remember, 590 only root can open sockets on privileged ports) and doesn't conflict with 591 any ports already in use. The connection is forwarded to port 6667 on 592 the remote server, since that's the standard port for IRC services. 593 594 The -f option backgrounds ssh and the remote command ``sleep 10'' is 595 specified to allow an amount of time (10 seconds, in the example) to 596 start the service which is to be tunnelled. If no connections are made 597 within the time specified, ssh will exit. 598 599X11 FORWARDING 600 If the ForwardX11 variable is set to ``yes'' (or see the description of 601 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY 602 environment variable is set), the connection to the X11 display is 603 automatically forwarded to the remote side in such a way that any X11 604 programs started from the shell (or command) will go through the 605 encrypted channel, and the connection to the real X server will be made 606 from the local machine. The user should not manually set DISPLAY. 607 Forwarding of X11 connections can be configured on the command line or in 608 configuration files. 609 610 The DISPLAY value set by ssh will point to the server machine, but with a 611 display number greater than zero. This is normal, and happens because 612 ssh creates a ``proxy'' X server on the server machine for forwarding the 613 connections over the encrypted channel. 614 615 ssh will also automatically set up Xauthority data on the server machine. 616 For this purpose, it will generate a random authorization cookie, store 617 it in Xauthority on the server, and verify that any forwarded connections 618 carry this cookie and replace it by the real cookie when the connection 619 is opened. The real authentication cookie is never sent to the server 620 machine (and no cookies are sent in the plain). 621 622 If the ForwardAgent variable is set to ``yes'' (or see the description of 623 the -A and -a options above) and the user is using an authentication 624 agent, the connection to the agent is automatically forwarded to the 625 remote side. 626 627VERIFYING HOST KEYS 628 When connecting to a server for the first time, a fingerprint of the 629 server's public key is presented to the user (unless the option 630 StrictHostKeyChecking has been disabled). Fingerprints can be determined 631 using ssh-keygen(1): 632 633 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 634 635 If the fingerprint is already known, it can be matched and the key can be 636 accepted or rejected. Because of the difficulty of comparing host keys 637 just by looking at hex strings, there is also support to compare host 638 keys visually, using random art. By setting the VisualHostKey option to 639 ``yes'', a small ASCII graphic gets displayed on every login to a server, 640 no matter if the session itself is interactive or not. By learning the 641 pattern a known server produces, a user can easily find out that the host 642 key has changed when a completely different pattern is displayed. 643 Because these patterns are not unambiguous however, a pattern that looks 644 similar to the pattern remembered only gives a good probability that the 645 host key is the same, not guaranteed proof. 646 647 To get a listing of the fingerprints along with their random art for all 648 known hosts, the following command line can be used: 649 650 $ ssh-keygen -lv -f ~/.ssh/known_hosts 651 652 If the fingerprint is unknown, an alternative method of verification is 653 available: SSH fingerprints verified by DNS. An additional resource 654 record (RR), SSHFP, is added to a zonefile and the connecting client is 655 able to match the fingerprint with that of the key presented. 656 657 In this example, we are connecting a client to a server, 658 ``host.example.com''. The SSHFP resource records should first be added 659 to the zonefile for host.example.com: 660 661 $ ssh-keygen -r host.example.com. 662 663 The output lines will have to be added to the zonefile. To check that 664 the zone is answering fingerprint queries: 665 666 $ dig -t SSHFP host.example.com 667 668 Finally the client connects: 669 670 $ ssh -o "VerifyHostKeyDNS ask" host.example.com 671 [...] 672 Matching host key fingerprint found in DNS. 673 Are you sure you want to continue connecting (yes/no)? 674 675 See the VerifyHostKeyDNS option in ssh_config(5) for more information. 676 677SSH-BASED VIRTUAL PRIVATE NETWORKS 678 ssh contains support for Virtual Private Network (VPN) tunnelling using 679 the tun(4) network pseudo-device, allowing two networks to be joined 680 securely. The sshd_config(5) configuration option PermitTunnel controls 681 whether the server supports this, and at what level (layer 2 or 3 682 traffic). 683 684 The following example would connect client network 10.0.50.0/24 with 685 remote network 10.0.99.0/24 using a point-to-point connection from 686 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway 687 to the remote network, at 192.168.1.15, allows it. 688 689 On the client: 690 691 # ssh -f -w 0:1 192.168.1.15 true 692 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 693 # route add 10.0.99.0/24 10.1.1.2 694 695 On the server: 696 697 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 698 # route add 10.0.50.0/24 10.1.1.1 699 700 Client access may be more finely tuned via the /root/.ssh/authorized_keys 701 file (see below) and the PermitRootLogin server option. The following 702 entry would permit connections on tun(4) device 1 from user ``jane'' and 703 on tun device 2 from user ``john'', if PermitRootLogin is set to 704 ``forced-commands-only'': 705 706 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 707 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 708 709 Since an SSH-based setup entails a fair amount of overhead, it may be 710 more suited to temporary setups, such as for wireless VPNs. More 711 permanent VPNs are better provided by tools such as ipsecctl(8) and 712 isakmpd(8). 713 714ENVIRONMENT 715 ssh will normally set the following environment variables: 716 717 DISPLAY The DISPLAY variable indicates the location of the 718 X11 server. It is automatically set by ssh to 719 point to a value of the form ``hostname:n'', where 720 ``hostname'' indicates the host where the shell 721 runs, and `n' is an integer >= 1. ssh uses this 722 special value to forward X11 connections over the 723 secure channel. The user should normally not set 724 DISPLAY explicitly, as that will render the X11 725 connection insecure (and will require the user to 726 manually copy any required authorization cookies). 727 728 HOME Set to the path of the user's home directory. 729 730 LOGNAME Synonym for USER; set for compatibility with 731 systems that use this variable. 732 733 MAIL Set to the path of the user's mailbox. 734 735 PATH Set to the default PATH, as specified when 736 compiling ssh. 737 738 SSH_ASKPASS If ssh needs a passphrase, it will read the 739 passphrase from the current terminal if it was run 740 from a terminal. If ssh does not have a terminal 741 associated with it but DISPLAY and SSH_ASKPASS are 742 set, it will execute the program specified by 743 SSH_ASKPASS and open an X11 window to read the 744 passphrase. This is particularly useful when 745 calling ssh from a .xsession or related script. 746 (Note that on some machines it may be necessary to 747 redirect the input from /dev/null to make this 748 work.) 749 750 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to 751 communicate with the agent. 752 753 SSH_CONNECTION Identifies the client and server ends of the 754 connection. The variable contains four space- 755 separated values: client IP address, client port 756 number, server IP address, and server port number. 757 758 SSH_ORIGINAL_COMMAND This variable contains the original command line if 759 a forced command is executed. It can be used to 760 extract the original arguments. 761 762 SSH_TTY This is set to the name of the tty (path to the 763 device) associated with the current shell or 764 command. If the current session has no tty, this 765 variable is not set. 766 767 TZ This variable is set to indicate the present time 768 zone if it was set when the daemon was started 769 (i.e. the daemon passes the value on to new 770 connections). 771 772 USER Set to the name of the user logging in. 773 774 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format 775 ``VARNAME=value'' to the environment if the file exists and users are 776 allowed to change their environment. For more information, see the 777 PermitUserEnvironment option in sshd_config(5). 778 779FILES 780 ~/.rhosts 781 This file is used for host-based authentication (see above). On 782 some machines this file may need to be world-readable if the 783 user's home directory is on an NFS partition, because sshd(8) 784 reads it as root. Additionally, this file must be owned by the 785 user, and must not have write permissions for anyone else. The 786 recommended permission for most machines is read/write for the 787 user, and not accessible by others. 788 789 ~/.shosts 790 This file is used in exactly the same way as .rhosts, but allows 791 host-based authentication without permitting login with 792 rlogin/rsh. 793 794 ~/.ssh/ 795 This directory is the default location for all user-specific 796 configuration and authentication information. There is no 797 general requirement to keep the entire contents of this directory 798 secret, but the recommended permissions are read/write/execute 799 for the user, and not accessible by others. 800 801 ~/.ssh/authorized_keys 802 Lists the public keys (DSA, ECDSA, ED25519, RSA) that can be used 803 for logging in as this user. The format of this file is 804 described in the sshd(8) manual page. This file is not highly 805 sensitive, but the recommended permissions are read/write for the 806 user, and not accessible by others. 807 808 ~/.ssh/config 809 This is the per-user configuration file. The file format and 810 configuration options are described in ssh_config(5). Because of 811 the potential for abuse, this file must have strict permissions: 812 read/write for the user, and not writable by others. 813 814 ~/.ssh/environment 815 Contains additional definitions for environment variables; see 816 ENVIRONMENT, above. 817 818 ~/.ssh/identity 819 ~/.ssh/id_dsa 820 ~/.ssh/id_ecdsa 821 ~/.ssh/id_ed25519 822 ~/.ssh/id_rsa 823 Contains the private key for authentication. These files contain 824 sensitive data and should be readable by the user but not 825 accessible by others (read/write/execute). ssh will simply 826 ignore a private key file if it is accessible by others. It is 827 possible to specify a passphrase when generating the key which 828 will be used to encrypt the sensitive part of this file using 829 3DES. 830 831 ~/.ssh/identity.pub 832 ~/.ssh/id_dsa.pub 833 ~/.ssh/id_ecdsa.pub 834 ~/.ssh/id_ed25519.pub 835 ~/.ssh/id_rsa.pub 836 Contains the public key for authentication. These files are not 837 sensitive and can (but need not) be readable by anyone. 838 839 ~/.ssh/known_hosts 840 Contains a list of host keys for all hosts the user has logged 841 into that are not already in the systemwide list of known host 842 keys. See sshd(8) for further details of the format of this 843 file. 844 845 ~/.ssh/rc 846 Commands in this file are executed by ssh when the user logs in, 847 just before the user's shell (or command) is started. See the 848 sshd(8) manual page for more information. 849 850 /etc/hosts.equiv 851 This file is for host-based authentication (see above). It 852 should only be writable by root. 853 854 /etc/shosts.equiv 855 This file is used in exactly the same way as hosts.equiv, but 856 allows host-based authentication without permitting login with 857 rlogin/rsh. 858 859 /etc/ssh/ssh_config 860 Systemwide configuration file. The file format and configuration 861 options are described in ssh_config(5). 862 863 /etc/ssh/ssh_host_key 864 /etc/ssh/ssh_host_dsa_key 865 /etc/ssh/ssh_host_ecdsa_key 866 /etc/ssh/ssh_host_ed25519_key 867 /etc/ssh/ssh_host_rsa_key 868 These files contain the private parts of the host keys and are 869 used for host-based authentication. If protocol version 1 is 870 used, ssh must be setuid root, since the host key is readable 871 only by root. For protocol version 2, ssh uses ssh-keysign(8) to 872 access the host keys, eliminating the requirement that ssh be 873 setuid root when host-based authentication is used. By default 874 ssh is not setuid root. 875 876 /etc/ssh/ssh_known_hosts 877 Systemwide list of known host keys. This file should be prepared 878 by the system administrator to contain the public host keys of 879 all machines in the organization. It should be world-readable. 880 See sshd(8) for further details of the format of this file. 881 882 /etc/ssh/sshrc 883 Commands in this file are executed by ssh when the user logs in, 884 just before the user's shell (or command) is started. See the 885 sshd(8) manual page for more information. 886 887EXIT STATUS 888 ssh exits with the exit status of the remote command or with 255 if an 889 error occurred. 890 891SEE ALSO 892 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), 893 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) 894 895STANDARDS 896 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned 897 Numbers, RFC 4250, January 2006. 898 899 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, 900 RFC 4251, January 2006. 901 902 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, 903 RFC 4252, January 2006. 904 905 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer 906 Protocol, RFC 4253, January 2006. 907 908 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 909 4254, January 2006. 910 911 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell 912 (SSH) Key Fingerprints, RFC 4255, January 2006. 913 914 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the 915 Secure Shell Protocol (SSH), RFC 4256, January 2006. 916 917 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break 918 Extension, RFC 4335, January 2006. 919 920 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport 921 Layer Encryption Modes, RFC 4344, January 2006. 922 923 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport 924 Layer Protocol, RFC 4345, January 2006. 925 926 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for 927 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006. 928 929 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File 930 Format, RFC 4716, November 2006. 931 932 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the 933 Secure Shell Transport Layer, RFC 5656, December 2009. 934 935 A. Perrig and D. Song, Hash Visualization: a New Technique to improve 936 Real-World Security, 1999, International Workshop on Cryptographic 937 Techniques and E-Commerce (CrypTEC '99). 938 939AUTHORS 940 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 941 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 942 de Raadt and Dug Song removed many bugs, re-added newer features and 943 created OpenSSH. Markus Friedl contributed the support for SSH protocol 944 versions 1.5 and 2.0. 945 946OpenBSD 5.5 December 7, 2013 OpenBSD 5.5 947