1 SSH(1) OpenBSD Reference Manual SSH(1) 2 3 NAME 4 ssh - OpenSSH SSH client (remote login program) 5 6 SYNOPSIS 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 14 DESCRIPTION 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), ``exit'' (request the 206 master to exit), and ``stop'' (request the master to stop 207 accepting further multiplexing requests). 208 209 -o option 210 Can be used to give options in the format used in the 211 configuration file. This is useful for specifying options for 212 which there is no separate command-line flag. For full details 213 of the options listed below, and their possible values, see 214 ssh_config(5). 215 216 AddressFamily 217 BatchMode 218 BindAddress 219 ChallengeResponseAuthentication 220 CheckHostIP 221 Cipher 222 Ciphers 223 ClearAllForwardings 224 Compression 225 CompressionLevel 226 ConnectionAttempts 227 ConnectTimeout 228 ControlMaster 229 ControlPath 230 DynamicForward 231 EscapeChar 232 ExitOnForwardFailure 233 ForwardAgent 234 ForwardX11 235 ForwardX11Trusted 236 GatewayPorts 237 GlobalKnownHostsFile 238 GSSAPIAuthentication 239 GSSAPIDelegateCredentials 240 HashKnownHosts 241 Host 242 HostbasedAuthentication 243 HostKeyAlgorithms 244 HostKeyAlias 245 HostName 246 IdentityFile 247 IdentitiesOnly 248 IPQoS 249 KbdInteractiveDevices 250 KexAlgorithms 251 LocalCommand 252 LocalForward 253 LogLevel 254 MACs 255 NoHostAuthenticationForLocalhost 256 NumberOfPasswordPrompts 257 PasswordAuthentication 258 PermitLocalCommand 259 PKCS11Provider 260 Port 261 PreferredAuthentications 262 Protocol 263 ProxyCommand 264 PubkeyAuthentication 265 RekeyLimit 266 RemoteForward 267 RequestTTY 268 RhostsRSAAuthentication 269 RSAAuthentication 270 SendEnv 271 ServerAliveInterval 272 ServerAliveCountMax 273 StrictHostKeyChecking 274 TCPKeepAlive 275 Tunnel 276 TunnelDevice 277 UsePrivilegedPort 278 User 279 UserKnownHostsFile 280 VerifyHostKeyDNS 281 VisualHostKey 282 XAuthLocation 283 284 -p port 285 Port to connect to on the remote host. This can be specified on 286 a per-host basis in the configuration file. 287 288 -q Quiet mode. Causes most warning and diagnostic messages to be 289 suppressed. 290 291 -R [bind_address:]port:host:hostport 292 Specifies that the given port on the remote (server) host is to 293 be forwarded to the given host and port on the local side. This 294 works by allocating a socket to listen to port on the remote 295 side, and whenever a connection is made to this port, the 296 connection is forwarded over the secure channel, and a connection 297 is made to host port hostport from the local machine. 298 299 Port forwardings can also be specified in the configuration file. 300 Privileged ports can be forwarded only when logging in as root on 301 the remote machine. IPv6 addresses can be specified by enclosing 302 the address in square braces. 303 304 By default, the listening socket on the server will be bound to 305 the loopback interface only. This may be overridden by 306 specifying a bind_address. An empty bind_address, or the address 307 `*', indicates that the remote socket should listen on all 308 interfaces. Specifying a remote bind_address will only succeed 309 if the server's GatewayPorts option is enabled (see 310 sshd_config(5)). 311 312 If the port argument is `0', the listen port will be dynamically 313 allocated on the server and reported to the client at run time. 314 When used together with -O forward the allocated port will be 315 printed to the standard output. 316 317 -S ctl_path 318 Specifies the location of a control socket for connection 319 sharing, or the string ``none'' to disable connection sharing. 320 Refer to the description of ControlPath and ControlMaster in 321 ssh_config(5) for details. 322 323 -s May be used to request invocation of a subsystem on the remote 324 system. Subsystems are a feature of the SSH2 protocol which 325 facilitate the use of SSH as a secure transport for other 326 applications (eg. sftp(1)). The subsystem is specified as the 327 remote command. 328 329 -T Disable pseudo-tty allocation. 330 331 -t Force pseudo-tty allocation. This can be used to execute 332 arbitrary screen-based programs on a remote machine, which can be 333 very useful, e.g. when implementing menu services. Multiple -t 334 options force tty allocation, even if ssh has no local tty. 335 336 -V Display the version number and exit. 337 338 -v Verbose mode. Causes ssh to print debugging messages about its 339 progress. This is helpful in debugging connection, 340 authentication, and configuration problems. Multiple -v options 341 increase the verbosity. The maximum is 3. 342 343 -W host:port 344 Requests that standard input and output on the client be 345 forwarded to host on port over the secure channel. Implies -N, 346 -T, ExitOnForwardFailure and ClearAllForwardings and works with 347 Protocol version 2 only. 348 349 -w local_tun[:remote_tun] 350 Requests tunnel device forwarding with the specified tun(4) 351 devices between the client (local_tun) and the server 352 (remote_tun). 353 354 The devices may be specified by numerical ID or the keyword 355 ``any'', which uses the next available tunnel device. If 356 remote_tun is not specified, it defaults to ``any''. See also 357 the Tunnel and TunnelDevice directives in ssh_config(5). If the 358 Tunnel directive is unset, it is set to the default tunnel mode, 359 which is ``point-to-point''. 360 361 -X Enables X11 forwarding. This can also be specified on a per-host 362 basis in a configuration file. 363 364 X11 forwarding should be enabled with caution. Users with the 365 ability to bypass file permissions on the remote host (for the 366 user's X authorization database) can access the local X11 display 367 through the forwarded connection. An attacker may then be able 368 to perform activities such as keystroke monitoring. 369 370 For this reason, X11 forwarding is subjected to X11 SECURITY 371 extension restrictions by default. Please refer to the ssh -Y 372 option and the ForwardX11Trusted directive in ssh_config(5) for 373 more information. 374 375 -x Disables X11 forwarding. 376 377 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not 378 subjected to the X11 SECURITY extension controls. 379 380 -y Send log information using the syslog(3) system module. By 381 default this information is sent to stderr. 382 383 ssh may additionally obtain configuration data from a per-user 384 configuration file and a system-wide configuration file. The file format 385 and configuration options are described in ssh_config(5). 386 387 AUTHENTICATION 388 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to 389 use protocol 2 only, though this can be changed via the Protocol option 390 in ssh_config(5) or the -1 and -2 options (see above). Both protocols 391 support similar authentication methods, but protocol 2 is the default 392 since it provides additional mechanisms for confidentiality (the traffic 393 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and 394 integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64, 395 hmac-ripemd160). Protocol 1 lacks a strong mechanism for ensuring the 396 integrity of the connection. 397 398 The methods available for authentication are: GSSAPI-based 399 authentication, host-based authentication, public key authentication, 400 challenge-response authentication, and password authentication. 401 Authentication methods are tried in the order specified above, though 402 protocol 2 has a configuration option to change the default order: 403 PreferredAuthentications. 404 405 Host-based authentication works as follows: If the machine the user logs 406 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote 407 machine, and the user names are the same on both sides, or if the files 408 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote 409 machine and contain a line containing the name of the client machine and 410 the name of the user on that machine, the user is considered for login. 411 Additionally, the server must be able to verify the client's host key 412 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, 413 below) for login to be permitted. This authentication method closes 414 security holes due to IP spoofing, DNS spoofing, and routing spoofing. 415 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the 416 rlogin/rsh protocol in general, are inherently insecure and should be 417 disabled if security is desired.] 418 419 Public key authentication works as follows: The scheme is based on 420 public-key cryptography, using cryptosystems where encryption and 421 decryption are done using separate keys, and it is unfeasible to derive 422 the decryption key from the encryption key. The idea is that each user 423 creates a public/private key pair for authentication purposes. The 424 server knows the public key, and only the user knows the private key. 425 ssh implements public key authentication protocol automatically, using 426 one of the DSA, ECDSA or RSA algorithms. Protocol 1 is restricted to 427 using only RSA keys, but protocol 2 may use any. The HISTORY section of 428 ssl(8) contains a brief discussion of the DSA and RSA algorithms. 429 430 The file ~/.ssh/authorized_keys lists the public keys that are permitted 431 for logging in. When the user logs in, the ssh program tells the server 432 which key pair it would like to use for authentication. The client 433 proves that it has access to the private key and the server checks that 434 the corresponding public key is authorized to accept the account. 435 436 The user creates his/her key pair by running ssh-keygen(1). This stores 437 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 438 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa (protocol 2 439 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), 440 ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2 441 ECDSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home 442 directory. The user should then copy the public key to 443 ~/.ssh/authorized_keys in his/her home directory on the remote machine. 444 The authorized_keys file corresponds to the conventional ~/.rhosts file, 445 and has one key per line, though the lines can be very long. After this, 446 the user can log in without giving the password. 447 448 A variation on public key authentication is available in the form of 449 certificate authentication: instead of a set of public/private keys, 450 signed certificates are used. This has the advantage that a single 451 trusted certification authority can be used in place of many 452 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for 453 more information. 454 455 The most convenient way to use public key or certificate authentication 456 may be with an authentication agent. See ssh-agent(1) for more 457 information. 458 459 Challenge-response authentication works as follows: The server sends an 460 arbitrary "challenge" text, and prompts for a response. Protocol 2 461 allows multiple challenges and responses; protocol 1 is restricted to 462 just one challenge/response. Examples of challenge-response 463 authentication include BSD Authentication (see login.conf(5)) and PAM 464 (some non-OpenBSD systems). 465 466 Finally, if other authentication methods fail, ssh prompts the user for a 467 password. The password is sent to the remote host for checking; however, 468 since all communications are encrypted, the password cannot be seen by 469 someone listening on the network. 470 471 ssh automatically maintains and checks a database containing 472 identification for all hosts it has ever been used with. Host keys are 473 stored in ~/.ssh/known_hosts in the user's home directory. Additionally, 474 the file /etc/ssh/ssh_known_hosts is automatically checked for known 475 hosts. Any new hosts are automatically added to the user's file. If a 476 host's identification ever changes, ssh warns about this and disables 477 password authentication to prevent server spoofing or man-in-the-middle 478 attacks, which could otherwise be used to circumvent the encryption. The 479 StrictHostKeyChecking option can be used to control logins to machines 480 whose host key is not known or has changed. 481 482 When the user's identity has been accepted by the server, the server 483 either executes the given command, or logs into the machine and gives the 484 user a normal shell on the remote machine. All communication with the 485 remote command or shell will be automatically encrypted. 486 487 If a pseudo-terminal has been allocated (normal login session), the user 488 may use the escape characters noted below. 489 490 If no pseudo-tty has been allocated, the session is transparent and can 491 be used to reliably transfer binary data. On most systems, setting the 492 escape character to ``none'' will also make the session transparent even 493 if a tty is used. 494 495 The session terminates when the command or shell on the remote machine 496 exits and all X11 and TCP connections have been closed. 497 498 ESCAPE CHARACTERS 499 When a pseudo-terminal has been requested, ssh supports a number of 500 functions through the use of an escape character. 501 502 A single tilde character can be sent as ~~ or by following the tilde by a 503 character other than those described below. The escape character must 504 always follow a newline to be interpreted as special. The escape 505 character can be changed in configuration files using the EscapeChar 506 configuration directive or on the command line by the -e option. 507 508 The supported escapes (assuming the default `~') are: 509 510 ~. Disconnect. 511 512 ~^Z Background ssh. 513 514 ~# List forwarded connections. 515 516 ~& Background ssh at logout when waiting for forwarded connection / 517 X11 sessions to terminate. 518 519 ~? Display a list of escape characters. 520 521 ~B Send a BREAK to the remote system (only useful for SSH protocol 522 version 2 and if the peer supports it). 523 524 ~C Open command line. Currently this allows the addition of port 525 forwardings using the -L, -R and -D options (see above). It also 526 allows the cancellation of existing remote port-forwardings using 527 -KR[bind_address:]port. !command allows the user to execute a 528 local command if the PermitLocalCommand option is enabled in 529 ssh_config(5). Basic help is available, using the -h option. 530 531 ~R Request rekeying of the connection (only useful for SSH protocol 532 version 2 and if the peer supports it). 533 534 TCP FORWARDING 535 Forwarding of arbitrary TCP connections over the secure channel can be 536 specified either on the command line or in a configuration file. One 537 possible application of TCP forwarding is a secure connection to a mail 538 server; another is going through firewalls. 539 540 In the example below, we look at encrypting communication between an IRC 541 client and server, even though the IRC server does not directly support 542 encrypted communications. This works as follows: the user connects to 543 the remote host using ssh, specifying a port to be used to forward 544 connections to the remote server. After that it is possible to start the 545 service which is to be encrypted on the client machine, connecting to the 546 same local port, and ssh will encrypt and forward the connection. 547 548 The following example tunnels an IRC session from client machine 549 ``127.0.0.1'' (localhost) to remote server ``server.example.com'': 550 551 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 552 $ irc -c '#users' -p 1234 pinky 127.0.0.1 553 554 This tunnels a connection to IRC server ``server.example.com'', joining 555 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't 556 matter which port is used, as long as it's greater than 1023 (remember, 557 only root can open sockets on privileged ports) and doesn't conflict with 558 any ports already in use. The connection is forwarded to port 6667 on 559 the remote server, since that's the standard port for IRC services. 560 561 The -f option backgrounds ssh and the remote command ``sleep 10'' is 562 specified to allow an amount of time (10 seconds, in the example) to 563 start the service which is to be tunnelled. If no connections are made 564 within the time specified, ssh will exit. 565 566 X11 FORWARDING 567 If the ForwardX11 variable is set to ``yes'' (or see the description of 568 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY 569 environment variable is set), the connection to the X11 display is 570 automatically forwarded to the remote side in such a way that any X11 571 programs started from the shell (or command) will go through the 572 encrypted channel, and the connection to the real X server will be made 573 from the local machine. The user should not manually set DISPLAY. 574 Forwarding of X11 connections can be configured on the command line or in 575 configuration files. 576 577 The DISPLAY value set by ssh will point to the server machine, but with a 578 display number greater than zero. This is normal, and happens because 579 ssh creates a ``proxy'' X server on the server machine for forwarding the 580 connections over the encrypted channel. 581 582 ssh will also automatically set up Xauthority data on the server machine. 583 For this purpose, it will generate a random authorization cookie, store 584 it in Xauthority on the server, and verify that any forwarded connections 585 carry this cookie and replace it by the real cookie when the connection 586 is opened. The real authentication cookie is never sent to the server 587 machine (and no cookies are sent in the plain). 588 589 If the ForwardAgent variable is set to ``yes'' (or see the description of 590 the -A and -a options above) and the user is using an authentication 591 agent, the connection to the agent is automatically forwarded to the 592 remote side. 593 594 VERIFYING HOST KEYS 595 When connecting to a server for the first time, a fingerprint of the 596 server's public key is presented to the user (unless the option 597 StrictHostKeyChecking has been disabled). Fingerprints can be determined 598 using ssh-keygen(1): 599 600 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 601 602 If the fingerprint is already known, it can be matched and the key can be 603 accepted or rejected. Because of the difficulty of comparing host keys 604 just by looking at hex strings, there is also support to compare host 605 keys visually, using random art. By setting the VisualHostKey option to 606 ``yes'', a small ASCII graphic gets displayed on every login to a server, 607 no matter if the session itself is interactive or not. By learning the 608 pattern a known server produces, a user can easily find out that the host 609 key has changed when a completely different pattern is displayed. 610 Because these patterns are not unambiguous however, a pattern that looks 611 similar to the pattern remembered only gives a good probability that the 612 host key is the same, not guaranteed proof. 613 614 To get a listing of the fingerprints along with their random art for all 615 known hosts, the following command line can be used: 616 617 $ ssh-keygen -lv -f ~/.ssh/known_hosts 618 619 If the fingerprint is unknown, an alternative method of verification is 620 available: SSH fingerprints verified by DNS. An additional resource 621 record (RR), SSHFP, is added to a zonefile and the connecting client is 622 able to match the fingerprint with that of the key presented. 623 624 In this example, we are connecting a client to a server, 625 ``host.example.com''. The SSHFP resource records should first be added 626 to the zonefile for host.example.com: 627 628 $ ssh-keygen -r host.example.com. 629 630 The output lines will have to be added to the zonefile. To check that 631 the zone is answering fingerprint queries: 632 633 $ dig -t SSHFP host.example.com 634 635 Finally the client connects: 636 637 $ ssh -o "VerifyHostKeyDNS ask" host.example.com 638 [...] 639 Matching host key fingerprint found in DNS. 640 Are you sure you want to continue connecting (yes/no)? 641 642 See the VerifyHostKeyDNS option in ssh_config(5) for more information. 643 644 SSH-BASED VIRTUAL PRIVATE NETWORKS 645 ssh contains support for Virtual Private Network (VPN) tunnelling using 646 the tun(4) network pseudo-device, allowing two networks to be joined 647 securely. The sshd_config(5) configuration option PermitTunnel controls 648 whether the server supports this, and at what level (layer 2 or 3 649 traffic). 650 651 The following example would connect client network 10.0.50.0/24 with 652 remote network 10.0.99.0/24 using a point-to-point connection from 653 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway 654 to the remote network, at 192.168.1.15, allows it. 655 656 On the client: 657 658 # ssh -f -w 0:1 192.168.1.15 true 659 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 660 # route add 10.0.99.0/24 10.1.1.2 661 662 On the server: 663 664 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 665 # route add 10.0.50.0/24 10.1.1.1 666 667 Client access may be more finely tuned via the /root/.ssh/authorized_keys 668 file (see below) and the PermitRootLogin server option. The following 669 entry would permit connections on tun(4) device 1 from user ``jane'' and 670 on tun device 2 from user ``john'', if PermitRootLogin is set to 671 ``forced-commands-only'': 672 673 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 674 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 675 676 Since an SSH-based setup entails a fair amount of overhead, it may be 677 more suited to temporary setups, such as for wireless VPNs. More 678 permanent VPNs are better provided by tools such as ipsecctl(8) and 679 isakmpd(8). 680 681 ENVIRONMENT 682 ssh will normally set the following environment variables: 683 684 DISPLAY The DISPLAY variable indicates the location of the 685 X11 server. It is automatically set by ssh to 686 point to a value of the form ``hostname:n'', where 687 ``hostname'' indicates the host where the shell 688 runs, and `n' is an integer >= 1. ssh uses this 689 special value to forward X11 connections over the 690 secure channel. The user should normally not set 691 DISPLAY explicitly, as that will render the X11 692 connection insecure (and will require the user to 693 manually copy any required authorization cookies). 694 695 HOME Set to the path of the user's home directory. 696 697 LOGNAME Synonym for USER; set for compatibility with 698 systems that use this variable. 699 700 MAIL Set to the path of the user's mailbox. 701 702 PATH Set to the default PATH, as specified when 703 compiling ssh. 704 705 SSH_ASKPASS If ssh needs a passphrase, it will read the 706 passphrase from the current terminal if it was run 707 from a terminal. If ssh does not have a terminal 708 associated with it but DISPLAY and SSH_ASKPASS are 709 set, it will execute the program specified by 710 SSH_ASKPASS and open an X11 window to read the 711 passphrase. This is particularly useful when 712 calling ssh from a .xsession or related script. 713 (Note that on some machines it may be necessary to 714 redirect the input from /dev/null to make this 715 work.) 716 717 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to 718 communicate with the agent. 719 720 SSH_CONNECTION Identifies the client and server ends of the 721 connection. The variable contains four space- 722 separated values: client IP address, client port 723 number, server IP address, and server port number. 724 725 SSH_ORIGINAL_COMMAND This variable contains the original command line if 726 a forced command is executed. It can be used to 727 extract the original arguments. 728 729 SSH_TTY This is set to the name of the tty (path to the 730 device) associated with the current shell or 731 command. If the current session has no tty, this 732 variable is not set. 733 734 TZ This variable is set to indicate the present time 735 zone if it was set when the daemon was started 736 (i.e. the daemon passes the value on to new 737 connections). 738 739 USER Set to the name of the user logging in. 740 741 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format 742 ``VARNAME=value'' to the environment if the file exists and users are 743 allowed to change their environment. For more information, see the 744 PermitUserEnvironment option in sshd_config(5). 745 746 FILES 747 ~/.rhosts 748 This file is used for host-based authentication (see above). On 749 some machines this file may need to be world-readable if the 750 user's home directory is on an NFS partition, because sshd(8) 751 reads it as root. Additionally, this file must be owned by the 752 user, and must not have write permissions for anyone else. The 753 recommended permission for most machines is read/write for the 754 user, and not accessible by others. 755 756 ~/.shosts 757 This file is used in exactly the same way as .rhosts, but allows 758 host-based authentication without permitting login with 759 rlogin/rsh. 760 761 ~/.ssh/ 762 This directory is the default location for all user-specific 763 configuration and authentication information. There is no 764 general requirement to keep the entire contents of this directory 765 secret, but the recommended permissions are read/write/execute 766 for the user, and not accessible by others. 767 768 ~/.ssh/authorized_keys 769 Lists the public keys (DSA/ECDSA/RSA) that can be used for 770 logging in as this user. The format of this file is described in 771 the sshd(8) manual page. This file is not highly sensitive, but 772 the recommended permissions are read/write for the user, and not 773 accessible by others. 774 775 ~/.ssh/config 776 This is the per-user configuration file. The file format and 777 configuration options are described in ssh_config(5). Because of 778 the potential for abuse, this file must have strict permissions: 779 read/write for the user, and not accessible by others. 780 781 ~/.ssh/environment 782 Contains additional definitions for environment variables; see 783 ENVIRONMENT, above. 784 785 ~/.ssh/identity 786 ~/.ssh/id_dsa 787 ~/.ssh/id_ecdsa 788 ~/.ssh/id_rsa 789 Contains the private key for authentication. These files contain 790 sensitive data and should be readable by the user but not 791 accessible by others (read/write/execute). ssh will simply 792 ignore a private key file if it is accessible by others. It is 793 possible to specify a passphrase when generating the key which 794 will be used to encrypt the sensitive part of this file using 795 3DES. 796 797 ~/.ssh/identity.pub 798 ~/.ssh/id_dsa.pub 799 ~/.ssh/id_ecdsa.pub 800 ~/.ssh/id_rsa.pub 801 Contains the public key for authentication. These files are not 802 sensitive and can (but need not) be readable by anyone. 803 804 ~/.ssh/known_hosts 805 Contains a list of host keys for all hosts the user has logged 806 into that are not already in the systemwide list of known host 807 keys. See sshd(8) for further details of the format of this 808 file. 809 810 ~/.ssh/rc 811 Commands in this file are executed by ssh when the user logs in, 812 just before the user's shell (or command) is started. See the 813 sshd(8) manual page for more information. 814 815 /etc/hosts.equiv 816 This file is for host-based authentication (see above). It 817 should only be writable by root. 818 819 /etc/shosts.equiv 820 This file is used in exactly the same way as hosts.equiv, but 821 allows host-based authentication without permitting login with 822 rlogin/rsh. 823 824 /etc/ssh/ssh_config 825 Systemwide configuration file. The file format and configuration 826 options are described in ssh_config(5). 827 828 /etc/ssh/ssh_host_key 829 /etc/ssh/ssh_host_dsa_key 830 /etc/ssh/ssh_host_ecdsa_key 831 /etc/ssh/ssh_host_rsa_key 832 These three files contain the private parts of the host keys and 833 are used for host-based authentication. If protocol version 1 is 834 used, ssh must be setuid root, since the host key is readable 835 only by root. For protocol version 2, ssh uses ssh-keysign(8) to 836 access the host keys, eliminating the requirement that ssh be 837 setuid root when host-based authentication is used. By default 838 ssh is not setuid root. 839 840 /etc/ssh/ssh_known_hosts 841 Systemwide list of known host keys. This file should be prepared 842 by the system administrator to contain the public host keys of 843 all machines in the organization. It should be world-readable. 844 See sshd(8) for further details of the format of this file. 845 846 /etc/ssh/sshrc 847 Commands in this file are executed by ssh when the user logs in, 848 just before the user's shell (or command) is started. See the 849 sshd(8) manual page for more information. 850 851 EXIT STATUS 852 ssh exits with the exit status of the remote command or with 255 if an 853 error occurred. 854 855 SEE ALSO 856 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), 857 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) 858 859 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006. 860 861 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006. 862 863 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006. 864 865 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006. 866 867 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006. 868 869 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 870 4255, 2006. 871 872 Generic Message Exchange Authentication for the Secure Shell Protocol 873 (SSH), RFC 4256, 2006. 874 875 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006. 876 877 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006. 878 879 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer 880 Protocol, RFC 4345, 2006. 881 882 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer 883 Protocol, RFC 4419, 2006. 884 885 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006. 886 887 Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, 888 RFC 5656, 2009. 889 890 A. Perrig and D. Song, Hash Visualization: a New Technique to improve 891 Real-World Security, 1999, International Workshop on Cryptographic 892 Techniques and E-Commerce (CrypTEC '99). 893 894 AUTHORS 895 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 896 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 897 de Raadt and Dug Song removed many bugs, re-added newer features and 898 created OpenSSH. Markus Friedl contributed the support for SSH protocol 899 versions 1.5 and 2.0. 900 901 OpenBSD 5.0 August 2, 2011 OpenBSD 5.0 902
