ssh.0 revision 214979
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 RSA or 139 DSA authentication is read. The default is ~/.ssh/identity for 140 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for 141 protocol version 2. Identity files may also be specified on a 142 per-host basis in the configuration file. It is possible to have 143 multiple -i options (and multiple identities specified in 144 configuration files). ssh will also try to load certificate 145 information from the filename obtained by appending -cert.pub to 146 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) and ``exit'' (request the 206 master to exit). 207 208 -o option 209 Can be used to give options in the format used in the 210 configuration file. This is useful for specifying options for 211 which there is no separate command-line flag. For full details 212 of the options listed below, and their possible values, see 213 ssh_config(5). 214 215 AddressFamily 216 BatchMode 217 BindAddress 218 ChallengeResponseAuthentication 219 CheckHostIP 220 Cipher 221 Ciphers 222 ClearAllForwardings 223 Compression 224 CompressionLevel 225 ConnectionAttempts 226 ConnectTimeout 227 ControlMaster 228 ControlPath 229 DynamicForward 230 EscapeChar 231 ExitOnForwardFailure 232 ForwardAgent 233 ForwardX11 234 ForwardX11Trusted 235 GatewayPorts 236 GlobalKnownHostsFile 237 GSSAPIAuthentication 238 GSSAPIDelegateCredentials 239 HashKnownHosts 240 Host 241 HostbasedAuthentication 242 HostKeyAlgorithms 243 HostKeyAlias 244 HostName 245 IdentityFile 246 IdentitiesOnly 247 KbdInteractiveDevices 248 LocalCommand 249 LocalForward 250 LogLevel 251 MACs 252 NoHostAuthenticationForLocalhost 253 NumberOfPasswordPrompts 254 PasswordAuthentication 255 PermitLocalCommand 256 PKCS11Provider 257 Port 258 PreferredAuthentications 259 Protocol 260 ProxyCommand 261 PubkeyAuthentication 262 RekeyLimit 263 RemoteForward 264 RhostsRSAAuthentication 265 RSAAuthentication 266 SendEnv 267 ServerAliveInterval 268 ServerAliveCountMax 269 StrictHostKeyChecking 270 TCPKeepAlive 271 Tunnel 272 TunnelDevice 273 UsePrivilegedPort 274 User 275 UserKnownHostsFile 276 VerifyHostKeyDNS 277 VisualHostKey 278 XAuthLocation 279 280 -p port 281 Port to connect to on the remote host. This can be specified on 282 a per-host basis in the configuration file. 283 284 -q Quiet mode. Causes most warning and diagnostic messages to be 285 suppressed. 286 287 -R [bind_address:]port:host:hostport 288 Specifies that the given port on the remote (server) host is to 289 be forwarded to the given host and port on the local side. This 290 works by allocating a socket to listen to port on the remote 291 side, and whenever a connection is made to this port, the 292 connection is forwarded over the secure channel, and a connection 293 is made to host port hostport from the local machine. 294 295 Port forwardings can also be specified in the configuration file. 296 Privileged ports can be forwarded only when logging in as root on 297 the remote machine. IPv6 addresses can be specified by enclosing 298 the address in square braces. 299 300 By default, the listening socket on the server will be bound to 301 the loopback interface only. This may be overridden by 302 specifying a bind_address. An empty bind_address, or the address 303 `*', indicates that the remote socket should listen on all 304 interfaces. Specifying a remote bind_address will only succeed 305 if the server's GatewayPorts option is enabled (see 306 sshd_config(5)). 307 308 If the port argument is `0', the listen port will be dynamically 309 allocated on the server and reported to the client at run time. 310 When used together with -O forward the allocated port will be 311 printed to the standard output. 312 313 -S ctl_path 314 Specifies the location of a control socket for connection 315 sharing, or the string ``none'' to disable connection sharing. 316 Refer to the description of ControlPath and ControlMaster in 317 ssh_config(5) for details. 318 319 -s May be used to request invocation of a subsystem on the remote 320 system. Subsystems are a feature of the SSH2 protocol which 321 facilitate the use of SSH as a secure transport for other 322 applications (eg. sftp(1)). The subsystem is specified as the 323 remote command. 324 325 -T Disable pseudo-tty allocation. 326 327 -t Force pseudo-tty allocation. This can be used to execute 328 arbitrary screen-based programs on a remote machine, which can be 329 very useful, e.g. when implementing menu services. Multiple -t 330 options force tty allocation, even if ssh has no local tty. 331 332 -V Display the version number and exit. 333 334 -v Verbose mode. Causes ssh to print debugging messages about its 335 progress. This is helpful in debugging connection, 336 authentication, and configuration problems. Multiple -v options 337 increase the verbosity. The maximum is 3. 338 339 -W host:port 340 Requests that standard input and output on the client be 341 forwarded to host on port over the secure channel. Implies -N, 342 -T, ExitOnForwardFailure and ClearAllForwardings and works with 343 Protocol version 2 only. 344 345 -w local_tun[:remote_tun] 346 Requests tunnel device forwarding with the specified tun(4) 347 devices between the client (local_tun) and the server 348 (remote_tun). 349 350 The devices may be specified by numerical ID or the keyword 351 ``any'', which uses the next available tunnel device. If 352 remote_tun is not specified, it defaults to ``any''. See also 353 the Tunnel and TunnelDevice directives in ssh_config(5). If the 354 Tunnel directive is unset, it is set to the default tunnel mode, 355 which is ``point-to-point''. 356 357 -X Enables X11 forwarding. This can also be specified on a per-host 358 basis in a configuration file. 359 360 X11 forwarding should be enabled with caution. Users with the 361 ability to bypass file permissions on the remote host (for the 362 user's X authorization database) can access the local X11 display 363 through the forwarded connection. An attacker may then be able 364 to perform activities such as keystroke monitoring. 365 366 For this reason, X11 forwarding is subjected to X11 SECURITY 367 extension restrictions by default. Please refer to the ssh -Y 368 option and the ForwardX11Trusted directive in ssh_config(5) for 369 more information. 370 371 -x Disables X11 forwarding. 372 373 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not 374 subjected to the X11 SECURITY extension controls. 375 376 -y Send log information using the syslog(3) system module. By 377 default this information is sent to stderr. 378 379 ssh may additionally obtain configuration data from a per-user 380 configuration file and a system-wide configuration file. The file format 381 and configuration options are described in ssh_config(5). 382 383 ssh exits with the exit status of the remote command or with 255 if an 384 error occurred. 385 386AUTHENTICATION 387 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to 388 use protocol 2 only, though this can be changed via the Protocol option 389 in ssh_config(5) or the -1 and -2 options (see above). Both protocols 390 support similar authentication methods, but protocol 2 is the default 391 since it provides additional mechanisms for confidentiality (the traffic 392 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and 393 integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1 394 lacks a strong mechanism for ensuring the integrity of the connection. 395 396 The methods available for authentication are: GSSAPI-based 397 authentication, host-based authentication, public key authentication, 398 challenge-response authentication, and password authentication. 399 Authentication methods are tried in the order specified above, though 400 protocol 2 has a configuration option to change the default order: 401 PreferredAuthentications. 402 403 Host-based authentication works as follows: If the machine the user logs 404 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote 405 machine, and the user names are the same on both sides, or if the files 406 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote 407 machine and contain a line containing the name of the client machine and 408 the name of the user on that machine, the user is considered for login. 409 Additionally, the server must be able to verify the client's host key 410 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, 411 below) for login to be permitted. This authentication method closes 412 security holes due to IP spoofing, DNS spoofing, and routing spoofing. 413 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the 414 rlogin/rsh protocol in general, are inherently insecure and should be 415 disabled if security is desired.] 416 417 Public key authentication works as follows: The scheme is based on 418 public-key cryptography, using cryptosystems where encryption and 419 decryption are done using separate keys, and it is unfeasible to derive 420 the decryption key from the encryption key. The idea is that each user 421 creates a public/private key pair for authentication purposes. The 422 server knows the public key, and only the user knows the private key. 423 ssh implements public key authentication protocol automatically, using 424 either the RSA or DSA algorithms. Protocol 1 is restricted to using only 425 RSA keys, but protocol 2 may use either. The HISTORY section of ssl(8) 426 contains a brief discussion of the two algorithms. 427 428 The file ~/.ssh/authorized_keys lists the public keys that are permitted 429 for logging in. When the user logs in, the ssh program tells the server 430 which key pair it would like to use for authentication. The client 431 proves that it has access to the private key and the server checks that 432 the corresponding public key is authorized to accept the account. 433 434 The user creates his/her key pair by running ssh-keygen(1). This stores 435 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 436 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in 437 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or 438 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The 439 user should then copy the public key to ~/.ssh/authorized_keys in his/her 440 home directory on the remote machine. The authorized_keys file 441 corresponds to the conventional ~/.rhosts file, and has one key per line, 442 though the lines can be very long. After this, the user can log in 443 without giving the password. 444 445 A variation on public key authentication is available in the form of 446 certificate authentication: instead of a set of public/private keys, 447 signed certificates are used. This has the advantage that a single 448 trusted certification authority can be used in place of many 449 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for 450 more information. 451 452 The most convenient way to use public key or certificate authentication 453 may be with an authentication agent. See ssh-agent(1) for more 454 information. 455 456 Challenge-response authentication works as follows: The server sends an 457 arbitrary "challenge" text, and prompts for a response. Protocol 2 458 allows multiple challenges and responses; protocol 1 is restricted to 459 just one challenge/response. Examples of challenge-response 460 authentication include BSD Authentication (see login.conf(5)) and PAM 461 (some non-OpenBSD systems). 462 463 Finally, if other authentication methods fail, ssh prompts the user for a 464 password. The password is sent to the remote host for checking; however, 465 since all communications are encrypted, the password cannot be seen by 466 someone listening on the network. 467 468 ssh automatically maintains and checks a database containing 469 identification for all hosts it has ever been used with. Host keys are 470 stored in ~/.ssh/known_hosts in the user's home directory. Additionally, 471 the file /etc/ssh/ssh_known_hosts is automatically checked for known 472 hosts. Any new hosts are automatically added to the user's file. If a 473 host's identification ever changes, ssh warns about this and disables 474 password authentication to prevent server spoofing or man-in-the-middle 475 attacks, which could otherwise be used to circumvent the encryption. The 476 StrictHostKeyChecking option can be used to control logins to machines 477 whose host key is not known or has changed. 478 479 When the user's identity has been accepted by the server, the server 480 either executes the given command, or logs into the machine and gives the 481 user a normal shell on the remote machine. All communication with the 482 remote command or shell will be automatically encrypted. 483 484 If a pseudo-terminal has been allocated (normal login session), the user 485 may use the escape characters noted below. 486 487 If no pseudo-tty has been allocated, the session is transparent and can 488 be used to reliably transfer binary data. On most systems, setting the 489 escape character to ``none'' will also make the session transparent even 490 if a tty is used. 491 492 The session terminates when the command or shell on the remote machine 493 exits and all X11 and TCP connections have been closed. 494 495ESCAPE CHARACTERS 496 When a pseudo-terminal has been requested, ssh supports a number of 497 functions through the use of an escape character. 498 499 A single tilde character can be sent as ~~ or by following the tilde by a 500 character other than those described below. The escape character must 501 always follow a newline to be interpreted as special. The escape 502 character can be changed in configuration files using the EscapeChar 503 configuration directive or on the command line by the -e option. 504 505 The supported escapes (assuming the default `~') are: 506 507 ~. Disconnect. 508 509 ~^Z Background ssh. 510 511 ~# List forwarded connections. 512 513 ~& Background ssh at logout when waiting for forwarded connection / 514 X11 sessions to terminate. 515 516 ~? Display a list of escape characters. 517 518 ~B Send a BREAK to the remote system (only useful for SSH protocol 519 version 2 and if the peer supports it). 520 521 ~C Open command line. Currently this allows the addition of port 522 forwardings using the -L, -R and -D options (see above). It also 523 allows the cancellation of existing remote port-forwardings using 524 -KR[bind_address:]port. !command allows the user to execute a 525 local command if the PermitLocalCommand option is enabled in 526 ssh_config(5). Basic help is available, using the -h option. 527 528 ~R Request rekeying of the connection (only useful for SSH protocol 529 version 2 and if the peer supports it). 530 531TCP FORWARDING 532 Forwarding of arbitrary TCP connections over the secure channel can be 533 specified either on the command line or in a configuration file. One 534 possible application of TCP forwarding is a secure connection to a mail 535 server; another is going through firewalls. 536 537 In the example below, we look at encrypting communication between an IRC 538 client and server, even though the IRC server does not directly support 539 encrypted communications. This works as follows: the user connects to 540 the remote host using ssh, specifying a port to be used to forward 541 connections to the remote server. After that it is possible to start the 542 service which is to be encrypted on the client machine, connecting to the 543 same local port, and ssh will encrypt and forward the connection. 544 545 The following example tunnels an IRC session from client machine 546 ``127.0.0.1'' (localhost) to remote server ``server.example.com'': 547 548 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 549 $ irc -c '#users' -p 1234 pinky 127.0.0.1 550 551 This tunnels a connection to IRC server ``server.example.com'', joining 552 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't 553 matter which port is used, as long as it's greater than 1023 (remember, 554 only root can open sockets on privileged ports) and doesn't conflict with 555 any ports already in use. The connection is forwarded to port 6667 on 556 the remote server, since that's the standard port for IRC services. 557 558 The -f option backgrounds ssh and the remote command ``sleep 10'' is 559 specified to allow an amount of time (10 seconds, in the example) to 560 start the service which is to be tunnelled. If no connections are made 561 within the time specified, ssh will exit. 562 563X11 FORWARDING 564 If the ForwardX11 variable is set to ``yes'' (or see the description of 565 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY 566 environment variable is set), the connection to the X11 display is 567 automatically forwarded to the remote side in such a way that any X11 568 programs started from the shell (or command) will go through the 569 encrypted channel, and the connection to the real X server will be made 570 from the local machine. The user should not manually set DISPLAY. 571 Forwarding of X11 connections can be configured on the command line or in 572 configuration files. 573 574 The DISPLAY value set by ssh will point to the server machine, but with a 575 display number greater than zero. This is normal, and happens because 576 ssh creates a ``proxy'' X server on the server machine for forwarding the 577 connections over the encrypted channel. 578 579 ssh will also automatically set up Xauthority data on the server machine. 580 For this purpose, it will generate a random authorization cookie, store 581 it in Xauthority on the server, and verify that any forwarded connections 582 carry this cookie and replace it by the real cookie when the connection 583 is opened. The real authentication cookie is never sent to the server 584 machine (and no cookies are sent in the plain). 585 586 If the ForwardAgent variable is set to ``yes'' (or see the description of 587 the -A and -a options above) and the user is using an authentication 588 agent, the connection to the agent is automatically forwarded to the 589 remote side. 590 591VERIFYING HOST KEYS 592 When connecting to a server for the first time, a fingerprint of the 593 server's public key is presented to the user (unless the option 594 StrictHostKeyChecking has been disabled). Fingerprints can be determined 595 using ssh-keygen(1): 596 597 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 598 599 If the fingerprint is already known, it can be matched and the key can be 600 accepted or rejected. Because of the difficulty of comparing host keys 601 just by looking at hex strings, there is also support to compare host 602 keys visually, using random art. By setting the VisualHostKey option to 603 ``yes'', a small ASCII graphic gets displayed on every login to a server, 604 no matter if the session itself is interactive or not. By learning the 605 pattern a known server produces, a user can easily find out that the host 606 key has changed when a completely different pattern is displayed. 607 Because these patterns are not unambiguous however, a pattern that looks 608 similar to the pattern remembered only gives a good probability that the 609 host key is the same, not guaranteed proof. 610 611 To get a listing of the fingerprints along with their random art for all 612 known hosts, the following command line can be used: 613 614 $ ssh-keygen -lv -f ~/.ssh/known_hosts 615 616 If the fingerprint is unknown, an alternative method of verification is 617 available: SSH fingerprints verified by DNS. An additional resource 618 record (RR), SSHFP, is added to a zonefile and the connecting client is 619 able to match the fingerprint with that of the key presented. 620 621 In this example, we are connecting a client to a server, 622 ``host.example.com''. The SSHFP resource records should first be added 623 to the zonefile for host.example.com: 624 625 $ ssh-keygen -r host.example.com. 626 627 The output lines will have to be added to the zonefile. To check that 628 the zone is answering fingerprint queries: 629 630 $ dig -t SSHFP host.example.com 631 632 Finally the client connects: 633 634 $ ssh -o "VerifyHostKeyDNS ask" host.example.com 635 [...] 636 Matching host key fingerprint found in DNS. 637 Are you sure you want to continue connecting (yes/no)? 638 639 See the VerifyHostKeyDNS option in ssh_config(5) for more information. 640 641SSH-BASED VIRTUAL PRIVATE NETWORKS 642 ssh contains support for Virtual Private Network (VPN) tunnelling using 643 the tun(4) network pseudo-device, allowing two networks to be joined 644 securely. The sshd_config(5) configuration option PermitTunnel controls 645 whether the server supports this, and at what level (layer 2 or 3 646 traffic). 647 648 The following example would connect client network 10.0.50.0/24 with 649 remote network 10.0.99.0/24 using a point-to-point connection from 650 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway 651 to the remote network, at 192.168.1.15, allows it. 652 653 On the client: 654 655 # ssh -f -w 0:1 192.168.1.15 true 656 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 657 # route add 10.0.99.0/24 10.1.1.2 658 659 On the server: 660 661 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 662 # route add 10.0.50.0/24 10.1.1.1 663 664 Client access may be more finely tuned via the /root/.ssh/authorized_keys 665 file (see below) and the PermitRootLogin server option. The following 666 entry would permit connections on tun(4) device 1 from user ``jane'' and 667 on tun device 2 from user ``john'', if PermitRootLogin is set to 668 ``forced-commands-only'': 669 670 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 671 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 672 673 Since an SSH-based setup entails a fair amount of overhead, it may be 674 more suited to temporary setups, such as for wireless VPNs. More 675 permanent VPNs are better provided by tools such as ipsecctl(8) and 676 isakmpd(8). 677 678ENVIRONMENT 679 ssh will normally set the following environment variables: 680 681 DISPLAY The DISPLAY variable indicates the location of the 682 X11 server. It is automatically set by ssh to 683 point to a value of the form ``hostname:n'', where 684 ``hostname'' indicates the host where the shell 685 runs, and `n' is an integer >= 1. ssh uses this 686 special value to forward X11 connections over the 687 secure channel. The user should normally not set 688 DISPLAY explicitly, as that will render the X11 689 connection insecure (and will require the user to 690 manually copy any required authorization cookies). 691 692 HOME Set to the path of the user's home directory. 693 694 LOGNAME Synonym for USER; set for compatibility with 695 systems that use this variable. 696 697 MAIL Set to the path of the user's mailbox. 698 699 PATH Set to the default PATH, as specified when 700 compiling ssh. 701 702 SSH_ASKPASS If ssh needs a passphrase, it will read the 703 passphrase from the current terminal if it was run 704 from a terminal. If ssh does not have a terminal 705 associated with it but DISPLAY and SSH_ASKPASS are 706 set, it will execute the program specified by 707 SSH_ASKPASS and open an X11 window to read the 708 passphrase. This is particularly useful when 709 calling ssh from a .xsession or related script. 710 (Note that on some machines it may be necessary to 711 redirect the input from /dev/null to make this 712 work.) 713 714 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to 715 communicate with the agent. 716 717 SSH_CONNECTION Identifies the client and server ends of the 718 connection. The variable contains four space- 719 separated values: client IP address, client port 720 number, server IP address, and server port number. 721 722 SSH_ORIGINAL_COMMAND This variable contains the original command line if 723 a forced command is executed. It can be used to 724 extract the original arguments. 725 726 SSH_TTY This is set to the name of the tty (path to the 727 device) associated with the current shell or 728 command. If the current session has no tty, this 729 variable is not set. 730 731 TZ This variable is set to indicate the present time 732 zone if it was set when the daemon was started 733 (i.e. the daemon passes the value on to new 734 connections). 735 736 USER Set to the name of the user logging in. 737 738 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format 739 ``VARNAME=value'' to the environment if the file exists and users are 740 allowed to change their environment. For more information, see the 741 PermitUserEnvironment option in sshd_config(5). 742 743FILES 744 ~/.rhosts 745 This file is used for host-based authentication (see above). On 746 some machines this file may need to be world-readable if the 747 user's home directory is on an NFS partition, because sshd(8) 748 reads it as root. Additionally, this file must be owned by the 749 user, and must not have write permissions for anyone else. The 750 recommended permission for most machines is read/write for the 751 user, and not accessible by others. 752 753 ~/.shosts 754 This file is used in exactly the same way as .rhosts, but allows 755 host-based authentication without permitting login with 756 rlogin/rsh. 757 758 ~/.ssh/ 759 This directory is the default location for all user-specific 760 configuration and authentication information. There is no 761 general requirement to keep the entire contents of this directory 762 secret, but the recommended permissions are read/write/execute 763 for the user, and not accessible by others. 764 765 ~/.ssh/authorized_keys 766 Lists the public keys (RSA/DSA) that can be used for logging in 767 as this user. The format of this file is described in the 768 sshd(8) manual page. This file is not highly sensitive, but the 769 recommended permissions are read/write for the user, and not 770 accessible by others. 771 772 ~/.ssh/config 773 This is the per-user configuration file. The file format and 774 configuration options are described in ssh_config(5). Because of 775 the potential for abuse, this file must have strict permissions: 776 read/write for the user, and not accessible by others. 777 778 ~/.ssh/environment 779 Contains additional definitions for environment variables; see 780 ENVIRONMENT, above. 781 782 ~/.ssh/identity 783 ~/.ssh/id_dsa 784 ~/.ssh/id_rsa 785 Contains the private key for authentication. These files contain 786 sensitive data and should be readable by the user but not 787 accessible by others (read/write/execute). ssh will simply 788 ignore a private key file if it is accessible by others. It is 789 possible to specify a passphrase when generating the key which 790 will be used to encrypt the sensitive part of this file using 791 3DES. 792 793 ~/.ssh/identity.pub 794 ~/.ssh/id_dsa.pub 795 ~/.ssh/id_rsa.pub 796 Contains the public key for authentication. These files are not 797 sensitive and can (but need not) be readable by anyone. 798 799 ~/.ssh/known_hosts 800 Contains a list of host keys for all hosts the user has logged 801 into that are not already in the systemwide list of known host 802 keys. See sshd(8) for further details of the format of this 803 file. 804 805 ~/.ssh/rc 806 Commands in this file are executed by ssh when the user logs in, 807 just before the user's shell (or command) is started. See the 808 sshd(8) manual page for more information. 809 810 /etc/hosts.equiv 811 This file is for host-based authentication (see above). It 812 should only be writable by root. 813 814 /etc/shosts.equiv 815 This file is used in exactly the same way as hosts.equiv, but 816 allows host-based authentication without permitting login with 817 rlogin/rsh. 818 819 /etc/ssh/ssh_config 820 Systemwide configuration file. The file format and configuration 821 options are described in ssh_config(5). 822 823 /etc/ssh/ssh_host_key 824 /etc/ssh/ssh_host_dsa_key 825 /etc/ssh/ssh_host_rsa_key 826 These three files contain the private parts of the host keys and 827 are used for host-based authentication. If protocol version 1 is 828 used, ssh must be setuid root, since the host key is readable 829 only by root. For protocol version 2, ssh uses ssh-keysign(8) to 830 access the host keys, eliminating the requirement that ssh be 831 setuid root when host-based authentication is used. By default 832 ssh is not setuid root. 833 834 /etc/ssh/ssh_known_hosts 835 Systemwide list of known host keys. This file should be prepared 836 by the system administrator to contain the public host keys of 837 all machines in the organization. It should be world-readable. 838 See sshd(8) for further details of the format of this file. 839 840 /etc/ssh/sshrc 841 Commands in this file are executed by ssh when the user logs in, 842 just before the user's shell (or command) is started. See the 843 sshd(8) manual page for more information. 844 845SEE ALSO 846 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), 847 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) 848 849 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006. 850 851 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006. 852 853 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006. 854 855 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006. 856 857 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006. 858 859 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 860 4255, 2006. 861 862 Generic Message Exchange Authentication for the Secure Shell Protocol 863 (SSH), RFC 4256, 2006. 864 865 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006. 866 867 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006. 868 869 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer 870 Protocol, RFC 4345, 2006. 871 872 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer 873 Protocol, RFC 4419, 2006. 874 875 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006. 876 877 A. Perrig, D. Song, Hash Visualization: a New Technique to improve 878 Real-World Security, 1999, International Workshop on Cryptographic 879 Techniques and E-Commerce (CrypTEC '99). 880 881AUTHORS 882 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 883 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 884 de Raadt and Dug Song removed many bugs, re-added newer features and 885 created OpenSSH. Markus Friedl contributed the support for SSH protocol 886 versions 1.5 and 2.0. 887 888OpenBSD 4.8 August 4, 2010 OpenBSD 4.8 889