ssh.0 revision 248613
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 escape_char] [-F configfile] [-I pkcs11] 9 [-i identity_file] [-L [bind_address:]port:host:hostport] 10 [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] 11 [-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port] 12 [-w local_tun[:remote_tun]] [user@]hostname [command] 13 14DESCRIPTION 15 ssh (SSH client) is a program for logging into a remote machine and for 16 executing commands on a remote machine. It is intended to replace rlogin 17 and rsh, and provide secure encrypted communications between two 18 untrusted hosts over an insecure network. X11 connections and arbitrary 19 TCP ports can also be forwarded over the secure channel. 20 21 ssh connects and logs into the specified hostname (with optional user 22 name). The user must prove his/her identity to the remote machine using 23 one of several methods depending on the protocol version used (see 24 below). 25 26 If command is specified, it is executed on the remote host instead of a 27 login shell. 28 29 The options are as follows: 30 31 -1 Forces ssh to try protocol version 1 only. 32 33 -2 Forces ssh to try protocol version 2 only. 34 35 -4 Forces ssh to use IPv4 addresses only. 36 37 -6 Forces ssh to use IPv6 addresses only. 38 39 -A Enables forwarding of the authentication agent connection. This 40 can also be specified on a per-host basis in a configuration 41 file. 42 43 Agent forwarding should be enabled with caution. Users with the 44 ability to bypass file permissions on the remote host (for the 45 agent's UNIX-domain socket) can access the local agent through 46 the forwarded connection. An attacker cannot obtain key material 47 from the agent, however they can perform operations on the keys 48 that enable them to authenticate using the identities loaded into 49 the agent. 50 51 -a Disables forwarding of the authentication agent connection. 52 53 -b bind_address 54 Use bind_address on the local machine as the source address of 55 the connection. Only useful on systems with more than one 56 address. 57 58 -C Requests compression of all data (including stdin, stdout, 59 stderr, and data for forwarded X11 and TCP connections). The 60 compression algorithm is the same used by gzip(1), and the 61 ``level'' can be controlled by the CompressionLevel option for 62 protocol version 1. Compression is desirable on modem lines and 63 other slow connections, but will only slow down things on fast 64 networks. The default value can be set on a host-by-host basis 65 in the configuration files; see the Compression option. 66 67 -c cipher_spec 68 Selects the cipher specification for encrypting the session. 69 70 Protocol version 1 allows specification of a single cipher. The 71 supported values are ``3des'', ``blowfish'', and ``des''. 3des 72 (triple-des) is an encrypt-decrypt-encrypt triple with three 73 different keys. It is believed to be secure. blowfish is a fast 74 block cipher; it appears very secure and is much faster than 75 3des. des is only supported in the ssh client for 76 interoperability with legacy protocol 1 implementations that do 77 not support the 3des cipher. Its use is strongly discouraged due 78 to cryptographic weaknesses. The default is ``3des''. 79 80 For protocol version 2, cipher_spec is a comma-separated list of 81 ciphers listed in order of preference. See the Ciphers keyword 82 in ssh_config(5) for more information. 83 84 -D [bind_address:]port 85 Specifies a local ``dynamic'' application-level port forwarding. 86 This works by allocating a socket to listen to port on the local 87 side, optionally bound to the specified bind_address. Whenever a 88 connection is made to this port, the connection is forwarded over 89 the secure channel, and the application protocol is then used to 90 determine where to connect to from the remote machine. Currently 91 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act 92 as a SOCKS server. Only root can forward privileged ports. 93 Dynamic port forwardings can also be specified in the 94 configuration file. 95 96 IPv6 addresses can be specified by enclosing the address in 97 square brackets. Only the superuser can forward privileged 98 ports. By default, the local port is bound in accordance with 99 the GatewayPorts setting. However, an explicit bind_address may 100 be used to bind the connection to a specific address. The 101 bind_address of ``localhost'' indicates that the listening port 102 be bound for local use only, while an empty address or `*' 103 indicates that the port should be available from all interfaces. 104 105 -e escape_char 106 Sets the escape character for sessions with a pty (default: `~'). 107 The escape character is only recognized at the beginning of a 108 line. The escape character followed by a dot (`.') closes the 109 connection; followed by control-Z suspends the connection; and 110 followed by itself sends the escape character once. Setting the 111 character to ``none'' disables any escapes and makes the session 112 fully transparent. 113 114 -F configfile 115 Specifies an alternative per-user configuration file. If a 116 configuration file is given on the command line, the system-wide 117 configuration file (/etc/ssh/ssh_config) will be ignored. The 118 default for the per-user configuration file is ~/.ssh/config. 119 120 -f Requests ssh to go to background just before command execution. 121 This is useful if ssh is going to ask for passwords or 122 passphrases, but the user wants it in the background. This 123 implies -n. The recommended way to start X11 programs at a 124 remote site is with something like ssh -f host xterm. 125 126 If the ExitOnForwardFailure configuration option is set to 127 ``yes'', then a client started with -f will wait for all remote 128 port forwards to be successfully established before placing 129 itself in the background. 130 131 -g Allows remote hosts to connect to local forwarded ports. 132 133 -I pkcs11 134 Specify the PKCS#11 shared library ssh should use to communicate 135 with a PKCS#11 token providing the user's private RSA key. 136 137 -i identity_file 138 Selects a file from which the identity (private key) for public 139 key authentication is read. The default is ~/.ssh/identity for 140 protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and 141 ~/.ssh/id_rsa for protocol version 2. Identity files may also be 142 specified on a per-host basis in the configuration file. It is 143 possible to have multiple -i options (and multiple identities 144 specified in configuration files). ssh will also try to load 145 certificate information from the filename obtained by appending 146 -cert.pub to identity filenames. 147 148 -K Enables GSSAPI-based authentication and forwarding (delegation) 149 of GSSAPI credentials to the server. 150 151 -k Disables forwarding (delegation) of GSSAPI credentials to the 152 server. 153 154 -L [bind_address:]port:host:hostport 155 Specifies that the given port on the local (client) host is to be 156 forwarded to the given host and port on the remote side. This 157 works by allocating a socket to listen to port on the local side, 158 optionally bound to the specified bind_address. Whenever a 159 connection is made to this port, the connection is forwarded over 160 the secure channel, and a connection is made to host port 161 hostport from the remote machine. Port forwardings can also be 162 specified in the configuration file. IPv6 addresses can be 163 specified by enclosing the address in square brackets. Only the 164 superuser can forward privileged ports. By default, the local 165 port is bound in accordance with the GatewayPorts setting. 166 However, an explicit bind_address may be used to bind the 167 connection to a specific address. The bind_address of 168 ``localhost'' indicates that the listening port be bound for 169 local use only, while an empty address or `*' indicates that the 170 port should be available from all interfaces. 171 172 -l login_name 173 Specifies the user to log in as on the remote machine. This also 174 may be specified on a per-host basis in the configuration file. 175 176 -M Places the ssh client into ``master'' mode for connection 177 sharing. Multiple -M options places ssh into ``master'' mode 178 with confirmation required before slave connections are accepted. 179 Refer to the description of ControlMaster in ssh_config(5) for 180 details. 181 182 -m mac_spec 183 Additionally, for protocol version 2 a comma-separated list of 184 MAC (message authentication code) algorithms can be specified in 185 order of preference. See the MACs keyword for more information. 186 187 -N Do not execute a remote command. This is useful for just 188 forwarding ports (protocol version 2 only). 189 190 -n Redirects stdin from /dev/null (actually, prevents reading from 191 stdin). This must be used when ssh is run in the background. A 192 common trick is to use this to run X11 programs on a remote 193 machine. For example, ssh -n shadows.cs.hut.fi emacs & will 194 start an emacs on shadows.cs.hut.fi, and the X11 connection will 195 be automatically forwarded over an encrypted channel. The ssh 196 program will be put in the background. (This does not work if 197 ssh needs to ask for a password or passphrase; see also the -f 198 option.) 199 200 -O ctl_cmd 201 Control an active connection multiplexing master process. When 202 the -O option is specified, the ctl_cmd argument is interpreted 203 and passed to the master process. Valid commands are: ``check'' 204 (check that the master process is running), ``forward'' (request 205 forwardings without command execution), ``cancel'' (cancel 206 forwardings), ``exit'' (request the master to exit), and ``stop'' 207 (request the master to stop accepting further multiplexing 208 requests). 209 210 -o option 211 Can be used to give options in the format used in the 212 configuration file. This is useful for specifying options for 213 which there is no separate command-line flag. For full details 214 of the options listed below, and their possible values, see 215 ssh_config(5). 216 217 AddressFamily 218 BatchMode 219 BindAddress 220 ChallengeResponseAuthentication 221 CheckHostIP 222 Cipher 223 Ciphers 224 ClearAllForwardings 225 Compression 226 CompressionLevel 227 ConnectionAttempts 228 ConnectTimeout 229 ControlMaster 230 ControlPath 231 ControlPersist 232 DynamicForward 233 EscapeChar 234 ExitOnForwardFailure 235 ForwardAgent 236 ForwardX11 237 ForwardX11Timeout 238 ForwardX11Trusted 239 GatewayPorts 240 GlobalKnownHostsFile 241 GSSAPIAuthentication 242 GSSAPIDelegateCredentials 243 HashKnownHosts 244 Host 245 HostbasedAuthentication 246 HostKeyAlgorithms 247 HostKeyAlias 248 HostName 249 IdentityFile 250 IdentitiesOnly 251 IPQoS 252 KbdInteractiveAuthentication 253 KbdInteractiveDevices 254 KexAlgorithms 255 LocalCommand 256 LocalForward 257 LogLevel 258 MACs 259 NoHostAuthenticationForLocalhost 260 NumberOfPasswordPrompts 261 PasswordAuthentication 262 PermitLocalCommand 263 PKCS11Provider 264 Port 265 PreferredAuthentications 266 Protocol 267 ProxyCommand 268 PubkeyAuthentication 269 RekeyLimit 270 RemoteForward 271 RequestTTY 272 RhostsRSAAuthentication 273 RSAAuthentication 274 SendEnv 275 ServerAliveInterval 276 ServerAliveCountMax 277 StrictHostKeyChecking 278 TCPKeepAlive 279 Tunnel 280 TunnelDevice 281 UsePrivilegedPort 282 User 283 UserKnownHostsFile 284 VerifyHostKeyDNS 285 VisualHostKey 286 XAuthLocation 287 288 -p port 289 Port to connect to on the remote host. This can be specified on 290 a per-host basis in the configuration file. 291 292 -q Quiet mode. Causes most warning and diagnostic messages to be 293 suppressed. 294 295 -R [bind_address:]port:host:hostport 296 Specifies that the given port on the remote (server) host is to 297 be forwarded to the given host and port on the local side. This 298 works by allocating a socket to listen to port on the remote 299 side, and whenever a connection is made to this port, the 300 connection is forwarded over the secure channel, and a connection 301 is made to host port hostport from the local machine. 302 303 Port forwardings can also be specified in the configuration file. 304 Privileged ports can be forwarded only when logging in as root on 305 the remote machine. IPv6 addresses can be specified by enclosing 306 the address in square brackets. 307 308 By default, the listening socket on the server will be bound to 309 the loopback interface only. This may be overridden by 310 specifying a bind_address. An empty bind_address, or the address 311 `*', indicates that the remote socket should listen on all 312 interfaces. Specifying a remote bind_address will only succeed 313 if the server's GatewayPorts option is enabled (see 314 sshd_config(5)). 315 316 If the port argument is `0', the listen port will be dynamically 317 allocated on the server and reported to the client at run time. 318 When used together with -O forward the allocated port will be 319 printed to the standard output. 320 321 -S ctl_path 322 Specifies the location of a control socket for connection 323 sharing, or the string ``none'' to disable connection sharing. 324 Refer to the description of ControlPath and ControlMaster in 325 ssh_config(5) for details. 326 327 -s May be used to request invocation of a subsystem on the remote 328 system. Subsystems are a feature of the SSH2 protocol which 329 facilitate the use of SSH as a secure transport for other 330 applications (eg. sftp(1)). The subsystem is specified as the 331 remote command. 332 333 -T Disable pseudo-tty allocation. 334 335 -t Force pseudo-tty allocation. This can be used to execute 336 arbitrary screen-based programs on a remote machine, which can be 337 very useful, e.g. when implementing menu services. Multiple -t 338 options force tty allocation, even if ssh has no local tty. 339 340 -V Display the version number and exit. 341 342 -v Verbose mode. Causes ssh to print debugging messages about its 343 progress. This is helpful in debugging connection, 344 authentication, and configuration problems. Multiple -v options 345 increase the verbosity. The maximum is 3. 346 347 -W host:port 348 Requests that standard input and output on the client be 349 forwarded to host on port over the secure channel. Implies -N, 350 -T, ExitOnForwardFailure and ClearAllForwardings. Works with 351 Protocol version 2 only. 352 353 -w local_tun[:remote_tun] 354 Requests tunnel device forwarding with the specified tun(4) 355 devices between the client (local_tun) and the server 356 (remote_tun). 357 358 The devices may be specified by numerical ID or the keyword 359 ``any'', which uses the next available tunnel device. If 360 remote_tun is not specified, it defaults to ``any''. See also 361 the Tunnel and TunnelDevice directives in ssh_config(5). If the 362 Tunnel directive is unset, it is set to the default tunnel mode, 363 which is ``point-to-point''. 364 365 -X Enables X11 forwarding. This can also be specified on a per-host 366 basis in a configuration file. 367 368 X11 forwarding should be enabled with caution. Users with the 369 ability to bypass file permissions on the remote host (for the 370 user's X authorization database) can access the local X11 display 371 through the forwarded connection. An attacker may then be able 372 to perform activities such as keystroke monitoring. 373 374 For this reason, X11 forwarding is subjected to X11 SECURITY 375 extension restrictions by default. Please refer to the ssh -Y 376 option and the ForwardX11Trusted directive in ssh_config(5) for 377 more information. 378 379 -x Disables X11 forwarding. 380 381 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not 382 subjected to the X11 SECURITY extension controls. 383 384 -y Send log information using the syslog(3) system module. By 385 default this information is sent to stderr. 386 387 ssh may additionally obtain configuration data from a per-user 388 configuration file and a system-wide configuration file. The file format 389 and configuration options are described in ssh_config(5). 390 391AUTHENTICATION 392 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to 393 use protocol 2 only, though this can be changed via the Protocol option 394 in ssh_config(5) or the -1 and -2 options (see above). Both protocols 395 support similar authentication methods, but protocol 2 is the default 396 since it provides additional mechanisms for confidentiality (the traffic 397 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and 398 integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64, 399 umac-128, hmac-ripemd160). Protocol 1 lacks a strong mechanism for 400 ensuring the integrity of the connection. 401 402 The methods available for authentication are: GSSAPI-based 403 authentication, host-based authentication, public key authentication, 404 challenge-response authentication, and password authentication. 405 Authentication methods are tried in the order specified above, though 406 protocol 2 has a configuration option to change the default order: 407 PreferredAuthentications. 408 409 Host-based authentication works as follows: If the machine the user logs 410 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote 411 machine, and the user names are the same on both sides, or if the files 412 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote 413 machine and contain a line containing the name of the client machine and 414 the name of the user on that machine, the user is considered for login. 415 Additionally, the server must be able to verify the client's host key 416 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, 417 below) for login to be permitted. This authentication method closes 418 security holes due to IP spoofing, DNS spoofing, and routing spoofing. 419 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the 420 rlogin/rsh protocol in general, are inherently insecure and should be 421 disabled if security is desired.] 422 423 Public key authentication works as follows: The scheme is based on 424 public-key cryptography, using cryptosystems where encryption and 425 decryption are done using separate keys, and it is unfeasible to derive 426 the decryption key from the encryption key. The idea is that each user 427 creates a public/private key pair for authentication purposes. The 428 server knows the public key, and only the user knows the private key. 429 ssh implements public key authentication protocol automatically, using 430 one of the DSA, ECDSA or RSA algorithms. Protocol 1 is restricted to 431 using only RSA keys, but protocol 2 may use any. The HISTORY section of 432 ssl(8) contains a brief discussion of the DSA and RSA algorithms. 433 434 The file ~/.ssh/authorized_keys lists the public keys that are permitted 435 for logging in. When the user logs in, the ssh program tells the server 436 which key pair it would like to use for authentication. The client 437 proves that it has access to the private key and the server checks that 438 the corresponding public key is authorized to accept the account. 439 440 The user creates his/her key pair by running ssh-keygen(1). This stores 441 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 442 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa (protocol 2 443 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), 444 ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2 445 ECDSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home 446 directory. The user should then copy the public key to 447 ~/.ssh/authorized_keys in his/her home directory on the remote machine. 448 The authorized_keys file corresponds to the conventional ~/.rhosts file, 449 and has one key per line, though the lines can be very long. After this, 450 the user can log in without giving the password. 451 452 A variation on public key authentication is available in the form of 453 certificate authentication: instead of a set of public/private keys, 454 signed certificates are used. This has the advantage that a single 455 trusted certification authority can be used in place of many 456 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for 457 more information. 458 459 The most convenient way to use public key or certificate authentication 460 may be with an authentication agent. See ssh-agent(1) for more 461 information. 462 463 Challenge-response authentication works as follows: The server sends an 464 arbitrary "challenge" text, and prompts for a response. Protocol 2 465 allows multiple challenges and responses; protocol 1 is restricted to 466 just one challenge/response. Examples of challenge-response 467 authentication include BSD Authentication (see login.conf(5)) and PAM 468 (some non-OpenBSD systems). 469 470 Finally, if other authentication methods fail, ssh prompts the user for a 471 password. The password is sent to the remote host for checking; however, 472 since all communications are encrypted, the password cannot be seen by 473 someone listening on the network. 474 475 ssh automatically maintains and checks a database containing 476 identification for all hosts it has ever been used with. Host keys are 477 stored in ~/.ssh/known_hosts in the user's home directory. Additionally, 478 the file /etc/ssh/ssh_known_hosts is automatically checked for known 479 hosts. Any new hosts are automatically added to the user's file. If a 480 host's identification ever changes, ssh warns about this and disables 481 password authentication to prevent server spoofing or man-in-the-middle 482 attacks, which could otherwise be used to circumvent the encryption. The 483 StrictHostKeyChecking option can be used to control logins to machines 484 whose host key is not known or has changed. 485 486 When the user's identity has been accepted by the server, the server 487 either executes the given command, or logs into the machine and gives the 488 user a normal shell on the remote machine. All communication with the 489 remote command or shell will be automatically encrypted. 490 491 If a pseudo-terminal has been allocated (normal login session), the user 492 may use the escape characters noted below. 493 494 If no pseudo-tty has been allocated, the session is transparent and can 495 be used to reliably transfer binary data. On most systems, setting the 496 escape character to ``none'' will also make the session transparent even 497 if a tty is used. 498 499 The session terminates when the command or shell on the remote machine 500 exits and all X11 and TCP connections have been closed. 501 502ESCAPE CHARACTERS 503 When a pseudo-terminal has been requested, ssh supports a number of 504 functions through the use of an escape character. 505 506 A single tilde character can be sent as ~~ or by following the tilde by a 507 character other than those described below. The escape character must 508 always follow a newline to be interpreted as special. The escape 509 character can be changed in configuration files using the EscapeChar 510 configuration directive or on the command line by the -e option. 511 512 The supported escapes (assuming the default `~') are: 513 514 ~. Disconnect. 515 516 ~^Z Background ssh. 517 518 ~# List forwarded connections. 519 520 ~& Background ssh at logout when waiting for forwarded connection / 521 X11 sessions to terminate. 522 523 ~? Display a list of escape characters. 524 525 ~B Send a BREAK to the remote system (only useful for SSH protocol 526 version 2 and if the peer supports it). 527 528 ~C Open command line. Currently this allows the addition of port 529 forwardings using the -L, -R and -D options (see above). It also 530 allows the cancellation of existing port-forwardings with 531 -KL[bind_address:]port for local, -KR[bind_address:]port for 532 remote and -KD[bind_address:]port for dynamic port-forwardings. 533 !command allows the user to execute a local command if the 534 PermitLocalCommand option is enabled in ssh_config(5). Basic 535 help is available, using the -h option. 536 537 ~R Request rekeying of the connection (only useful for SSH protocol 538 version 2 and if the peer supports it). 539 540 ~V Decrease the verbosity (LogLevel) when errors are being written 541 to stderr. 542 543 ~v Increase the verbosity (LogLevel) when errors are being written 544 to stderr. 545 546TCP FORWARDING 547 Forwarding of arbitrary TCP connections over the secure channel can be 548 specified either on the command line or in a configuration file. One 549 possible application of TCP forwarding is a secure connection to a mail 550 server; another is going through firewalls. 551 552 In the example below, we look at encrypting communication between an IRC 553 client and server, even though the IRC server does not directly support 554 encrypted communications. This works as follows: the user connects to 555 the remote host using ssh, specifying a port to be used to forward 556 connections to the remote server. After that it is possible to start the 557 service which is to be encrypted on the client machine, connecting to the 558 same local port, and ssh will encrypt and forward the connection. 559 560 The following example tunnels an IRC session from client machine 561 ``127.0.0.1'' (localhost) to remote server ``server.example.com'': 562 563 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 564 $ irc -c '#users' -p 1234 pinky 127.0.0.1 565 566 This tunnels a connection to IRC server ``server.example.com'', joining 567 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't 568 matter which port is used, as long as it's greater than 1023 (remember, 569 only root can open sockets on privileged ports) and doesn't conflict with 570 any ports already in use. The connection is forwarded to port 6667 on 571 the remote server, since that's the standard port for IRC services. 572 573 The -f option backgrounds ssh and the remote command ``sleep 10'' is 574 specified to allow an amount of time (10 seconds, in the example) to 575 start the service which is to be tunnelled. If no connections are made 576 within the time specified, ssh will exit. 577 578X11 FORWARDING 579 If the ForwardX11 variable is set to ``yes'' (or see the description of 580 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY 581 environment variable is set), the connection to the X11 display is 582 automatically forwarded to the remote side in such a way that any X11 583 programs started from the shell (or command) will go through the 584 encrypted channel, and the connection to the real X server will be made 585 from the local machine. The user should not manually set DISPLAY. 586 Forwarding of X11 connections can be configured on the command line or in 587 configuration files. 588 589 The DISPLAY value set by ssh will point to the server machine, but with a 590 display number greater than zero. This is normal, and happens because 591 ssh creates a ``proxy'' X server on the server machine for forwarding the 592 connections over the encrypted channel. 593 594 ssh will also automatically set up Xauthority data on the server machine. 595 For this purpose, it will generate a random authorization cookie, store 596 it in Xauthority on the server, and verify that any forwarded connections 597 carry this cookie and replace it by the real cookie when the connection 598 is opened. The real authentication cookie is never sent to the server 599 machine (and no cookies are sent in the plain). 600 601 If the ForwardAgent variable is set to ``yes'' (or see the description of 602 the -A and -a options above) and the user is using an authentication 603 agent, the connection to the agent is automatically forwarded to the 604 remote side. 605 606VERIFYING HOST KEYS 607 When connecting to a server for the first time, a fingerprint of the 608 server's public key is presented to the user (unless the option 609 StrictHostKeyChecking has been disabled). Fingerprints can be determined 610 using ssh-keygen(1): 611 612 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 613 614 If the fingerprint is already known, it can be matched and the key can be 615 accepted or rejected. Because of the difficulty of comparing host keys 616 just by looking at hex strings, there is also support to compare host 617 keys visually, using random art. By setting the VisualHostKey option to 618 ``yes'', a small ASCII graphic gets displayed on every login to a server, 619 no matter if the session itself is interactive or not. By learning the 620 pattern a known server produces, a user can easily find out that the host 621 key has changed when a completely different pattern is displayed. 622 Because these patterns are not unambiguous however, a pattern that looks 623 similar to the pattern remembered only gives a good probability that the 624 host key is the same, not guaranteed proof. 625 626 To get a listing of the fingerprints along with their random art for all 627 known hosts, the following command line can be used: 628 629 $ ssh-keygen -lv -f ~/.ssh/known_hosts 630 631 If the fingerprint is unknown, an alternative method of verification is 632 available: SSH fingerprints verified by DNS. An additional resource 633 record (RR), SSHFP, is added to a zonefile and the connecting client is 634 able to match the fingerprint with that of the key presented. 635 636 In this example, we are connecting a client to a server, 637 ``host.example.com''. The SSHFP resource records should first be added 638 to the zonefile for host.example.com: 639 640 $ ssh-keygen -r host.example.com. 641 642 The output lines will have to be added to the zonefile. To check that 643 the zone is answering fingerprint queries: 644 645 $ dig -t SSHFP host.example.com 646 647 Finally the client connects: 648 649 $ ssh -o "VerifyHostKeyDNS ask" host.example.com 650 [...] 651 Matching host key fingerprint found in DNS. 652 Are you sure you want to continue connecting (yes/no)? 653 654 See the VerifyHostKeyDNS option in ssh_config(5) for more information. 655 656SSH-BASED VIRTUAL PRIVATE NETWORKS 657 ssh contains support for Virtual Private Network (VPN) tunnelling using 658 the tun(4) network pseudo-device, allowing two networks to be joined 659 securely. The sshd_config(5) configuration option PermitTunnel controls 660 whether the server supports this, and at what level (layer 2 or 3 661 traffic). 662 663 The following example would connect client network 10.0.50.0/24 with 664 remote network 10.0.99.0/24 using a point-to-point connection from 665 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway 666 to the remote network, at 192.168.1.15, allows it. 667 668 On the client: 669 670 # ssh -f -w 0:1 192.168.1.15 true 671 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 672 # route add 10.0.99.0/24 10.1.1.2 673 674 On the server: 675 676 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 677 # route add 10.0.50.0/24 10.1.1.1 678 679 Client access may be more finely tuned via the /root/.ssh/authorized_keys 680 file (see below) and the PermitRootLogin server option. The following 681 entry would permit connections on tun(4) device 1 from user ``jane'' and 682 on tun device 2 from user ``john'', if PermitRootLogin is set to 683 ``forced-commands-only'': 684 685 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 686 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 687 688 Since an SSH-based setup entails a fair amount of overhead, it may be 689 more suited to temporary setups, such as for wireless VPNs. More 690 permanent VPNs are better provided by tools such as ipsecctl(8) and 691 isakmpd(8). 692 693ENVIRONMENT 694 ssh will normally set the following environment variables: 695 696 DISPLAY The DISPLAY variable indicates the location of the 697 X11 server. It is automatically set by ssh to 698 point to a value of the form ``hostname:n'', where 699 ``hostname'' indicates the host where the shell 700 runs, and `n' is an integer >= 1. ssh uses this 701 special value to forward X11 connections over the 702 secure channel. The user should normally not set 703 DISPLAY explicitly, as that will render the X11 704 connection insecure (and will require the user to 705 manually copy any required authorization cookies). 706 707 HOME Set to the path of the user's home directory. 708 709 LOGNAME Synonym for USER; set for compatibility with 710 systems that use this variable. 711 712 MAIL Set to the path of the user's mailbox. 713 714 PATH Set to the default PATH, as specified when 715 compiling ssh. 716 717 SSH_ASKPASS If ssh needs a passphrase, it will read the 718 passphrase from the current terminal if it was run 719 from a terminal. If ssh does not have a terminal 720 associated with it but DISPLAY and SSH_ASKPASS are 721 set, it will execute the program specified by 722 SSH_ASKPASS and open an X11 window to read the 723 passphrase. This is particularly useful when 724 calling ssh from a .xsession or related script. 725 (Note that on some machines it may be necessary to 726 redirect the input from /dev/null to make this 727 work.) 728 729 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to 730 communicate with the agent. 731 732 SSH_CONNECTION Identifies the client and server ends of the 733 connection. The variable contains four space- 734 separated values: client IP address, client port 735 number, server IP address, and server port number. 736 737 SSH_ORIGINAL_COMMAND This variable contains the original command line if 738 a forced command is executed. It can be used to 739 extract the original arguments. 740 741 SSH_TTY This is set to the name of the tty (path to the 742 device) associated with the current shell or 743 command. If the current session has no tty, this 744 variable is not set. 745 746 TZ This variable is set to indicate the present time 747 zone if it was set when the daemon was started 748 (i.e. the daemon passes the value on to new 749 connections). 750 751 USER Set to the name of the user logging in. 752 753 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format 754 ``VARNAME=value'' to the environment if the file exists and users are 755 allowed to change their environment. For more information, see the 756 PermitUserEnvironment option in sshd_config(5). 757 758FILES 759 ~/.rhosts 760 This file is used for host-based authentication (see above). On 761 some machines this file may need to be world-readable if the 762 user's home directory is on an NFS partition, because sshd(8) 763 reads it as root. Additionally, this file must be owned by the 764 user, and must not have write permissions for anyone else. The 765 recommended permission for most machines is read/write for the 766 user, and not accessible by others. 767 768 ~/.shosts 769 This file is used in exactly the same way as .rhosts, but allows 770 host-based authentication without permitting login with 771 rlogin/rsh. 772 773 ~/.ssh/ 774 This directory is the default location for all user-specific 775 configuration and authentication information. There is no 776 general requirement to keep the entire contents of this directory 777 secret, but the recommended permissions are read/write/execute 778 for the user, and not accessible by others. 779 780 ~/.ssh/authorized_keys 781 Lists the public keys (DSA/ECDSA/RSA) that can be used for 782 logging in as this user. The format of this file is described in 783 the sshd(8) manual page. This file is not highly sensitive, but 784 the recommended permissions are read/write for the user, and not 785 accessible by others. 786 787 ~/.ssh/config 788 This is the per-user configuration file. The file format and 789 configuration options are described in ssh_config(5). Because of 790 the potential for abuse, this file must have strict permissions: 791 read/write for the user, and not accessible by others. 792 793 ~/.ssh/environment 794 Contains additional definitions for environment variables; see 795 ENVIRONMENT, above. 796 797 ~/.ssh/identity 798 ~/.ssh/id_dsa 799 ~/.ssh/id_ecdsa 800 ~/.ssh/id_rsa 801 Contains the private key for authentication. These files contain 802 sensitive data and should be readable by the user but not 803 accessible by others (read/write/execute). ssh will simply 804 ignore a private key file if it is accessible by others. It is 805 possible to specify a passphrase when generating the key which 806 will be used to encrypt the sensitive part of this file using 807 3DES. 808 809 ~/.ssh/identity.pub 810 ~/.ssh/id_dsa.pub 811 ~/.ssh/id_ecdsa.pub 812 ~/.ssh/id_rsa.pub 813 Contains the public key for authentication. These files are not 814 sensitive and can (but need not) be readable by anyone. 815 816 ~/.ssh/known_hosts 817 Contains a list of host keys for all hosts the user has logged 818 into that are not already in the systemwide list of known host 819 keys. See sshd(8) for further details of the format of this 820 file. 821 822 ~/.ssh/rc 823 Commands in this file are executed by ssh when the user logs in, 824 just before the user's shell (or command) is started. See the 825 sshd(8) manual page for more information. 826 827 /etc/hosts.equiv 828 This file is for host-based authentication (see above). It 829 should only be writable by root. 830 831 /etc/shosts.equiv 832 This file is used in exactly the same way as hosts.equiv, but 833 allows host-based authentication without permitting login with 834 rlogin/rsh. 835 836 /etc/ssh/ssh_config 837 Systemwide configuration file. The file format and configuration 838 options are described in ssh_config(5). 839 840 /etc/ssh/ssh_host_key 841 /etc/ssh/ssh_host_dsa_key 842 /etc/ssh/ssh_host_ecdsa_key 843 /etc/ssh/ssh_host_rsa_key 844 These files contain the private parts of the host keys and are 845 used for host-based authentication. If protocol version 1 is 846 used, ssh must be setuid root, since the host key is readable 847 only by root. For protocol version 2, ssh uses ssh-keysign(8) to 848 access the host keys, eliminating the requirement that ssh be 849 setuid root when host-based authentication is used. By default 850 ssh is not setuid root. 851 852 /etc/ssh/ssh_known_hosts 853 Systemwide list of known host keys. This file should be prepared 854 by the system administrator to contain the public host keys of 855 all machines in the organization. It should be world-readable. 856 See sshd(8) for further details of the format of this file. 857 858 /etc/ssh/sshrc 859 Commands in this file are executed by ssh when the user logs in, 860 just before the user's shell (or command) is started. See the 861 sshd(8) manual page for more information. 862 863EXIT STATUS 864 ssh exits with the exit status of the remote command or with 255 if an 865 error occurred. 866 867SEE ALSO 868 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), 869 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) 870 871STANDARDS 872 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned 873 Numbers, RFC 4250, January 2006. 874 875 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, 876 RFC 4251, January 2006. 877 878 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, 879 RFC 4252, January 2006. 880 881 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer 882 Protocol, RFC 4253, January 2006. 883 884 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 885 4254, January 2006. 886 887 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell 888 (SSH) Key Fingerprints, RFC 4255, January 2006. 889 890 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the 891 Secure Shell Protocol (SSH), RFC 4256, January 2006. 892 893 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break 894 Extension, RFC 4335, January 2006. 895 896 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport 897 Layer Encryption Modes, RFC 4344, January 2006. 898 899 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport 900 Layer Protocol, RFC 4345, January 2006. 901 902 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for 903 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006. 904 905 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File 906 Format, RFC 4716, November 2006. 907 908 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the 909 Secure Shell Transport Layer, RFC 5656, December 2009. 910 911 A. Perrig and D. Song, Hash Visualization: a New Technique to improve 912 Real-World Security, 1999, International Workshop on Cryptographic 913 Techniques and E-Commerce (CrypTEC '99). 914 915AUTHORS 916 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 917 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 918 de Raadt and Dug Song removed many bugs, re-added newer features and 919 created OpenSSH. Markus Friedl contributed the support for SSH protocol 920 versions 1.5 and 2.0. 921 922OpenBSD 5.3 October 4, 2012 OpenBSD 5.3 923