ssh.0 revision 189006
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] 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] 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 untrust- 18 ed hosts over an insecure network. X11 connections and arbitrary TCP 19 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 be- 24 low). 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 ad- 56 dress. 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 dif- 73 ferent 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 interoperabil- 76 ity with legacy protocol 1 implementations that do not support 77 the 3des cipher. Its use is strongly discouraged due to crypto- 78 graphic 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. The supported ciphers 82 are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, 83 aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blow- 84 fish-cbc, and cast128-cbc. The default is: 85 86 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, 87 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, 88 aes192-ctr,aes256-ctr 89 90 -D [bind_address:]port 91 Specifies a local ``dynamic'' application-level port forwarding. 92 This works by allocating a socket to listen to port on the local 93 side, optionally bound to the specified bind_address. Whenever a 94 connection is made to this port, the connection is forwarded over 95 the secure channel, and the application protocol is then used to 96 determine where to connect to from the remote machine. Currently 97 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act 98 as a SOCKS server. Only root can forward privileged ports. Dy- 99 namic port forwardings can also be specified in the configuration 100 file. 101 102 IPv6 addresses can be specified with an alternative syntax: 103 [bind_address/]port or by enclosing the address in square brack- 104 ets. Only the superuser can forward privileged ports. By de- 105 fault, the local port is bound in accordance with the 106 GatewayPorts setting. However, an explicit bind_address may be 107 used to bind the connection to a specific address. The 108 bind_address of ``localhost'' indicates that the listening port 109 be bound for local use only, while an empty address or `*' indi- 110 cates that the port should be available from all interfaces. 111 112 -e escape_char 113 Sets the escape character for sessions with a pty (default: `~'). 114 The escape character is only recognized at the beginning of a 115 line. The escape character followed by a dot (`.') closes the 116 connection; followed by control-Z suspends the connection; and 117 followed by itself sends the escape character once. Setting the 118 character to ``none'' disables any escapes and makes the session 119 fully transparent. 120 121 -F configfile 122 Specifies an alternative per-user configuration file. If a con- 123 figuration file is given on the command line, the system-wide 124 configuration file (/etc/ssh/ssh_config) will be ignored. The 125 default for the per-user configuration file is ~/.ssh/config. 126 127 -f Requests ssh to go to background just before command execution. 128 This is useful if ssh is going to ask for passwords or passphras- 129 es, but the user wants it in the background. This implies -n. 130 The recommended way to start X11 programs at a remote site is 131 with something like ssh -f host xterm. 132 133 If the ExitOnForwardFailure configuration option is set to 134 ``yes'', then a client started with -f will wait for all remote 135 port forwards to be successfully established before placing it- 136 self in the background. 137 138 -g Allows remote hosts to connect to local forwarded ports. 139 140 -I smartcard_device 141 Specify the device ssh should use to communicate with a smartcard 142 used for storing the user's private RSA key. This option is only 143 available if support for smartcard devices is compiled in (de- 144 fault is no support). 145 146 -i identity_file 147 Selects a file from which the identity (private key) for RSA or 148 DSA authentication is read. The default is ~/.ssh/identity for 149 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro- 150 tocol version 2. Identity files may also be specified on a per- 151 host basis in the configuration file. It is possible to have 152 multiple -i options (and multiple identities specified in config- 153 uration files). 154 155 -K Enables GSSAPI-based authentication and forwarding (delegation) 156 of GSSAPI credentials to the server. 157 158 -k Disables forwarding (delegation) of GSSAPI credentials to the 159 server. 160 161 -L [bind_address:]port:host:hostport 162 Specifies that the given port on the local (client) host is to be 163 forwarded to the given host and port on the remote side. This 164 works by allocating a socket to listen to port on the local side, 165 optionally bound to the specified bind_address. Whenever a con- 166 nection is made to this port, the connection is forwarded over 167 the secure channel, and a connection is made to host port 168 hostport from the remote machine. Port forwardings can also be 169 specified in the configuration file. IPv6 addresses can be spec- 170 ified with an alternative syntax: [bind_address/]port/host/host- 171 port or by enclosing the address in square brackets. Only the 172 superuser can forward privileged ports. By default, the local 173 port is bound in accordance with the GatewayPorts setting. How- 174 ever, an explicit bind_address may be used to bind the connection 175 to a specific address. The bind_address of ``localhost'' indi- 176 cates that the listening port be bound for local use only, while 177 an empty address or `*' indicates that the port should be avail- 178 able from all interfaces. 179 180 -l login_name 181 Specifies the user to log in as on the remote machine. This also 182 may be specified on a per-host basis in the configuration file. 183 184 -M Places the ssh client into ``master'' mode for connection shar- 185 ing. Multiple -M options places ssh into ``master'' mode with 186 confirmation required before slave connections are accepted. Re- 187 fer to the description of ControlMaster in ssh_config(5) for de- 188 tails. 189 190 -m mac_spec 191 Additionally, for protocol version 2 a comma-separated list of 192 MAC (message authentication code) algorithms can be specified in 193 order of preference. See the MACs keyword for more information. 194 195 -N Do not execute a remote command. This is useful for just for- 196 warding ports (protocol version 2 only). 197 198 -n Redirects stdin from /dev/null (actually, prevents reading from 199 stdin). This must be used when ssh is run in the background. A 200 common trick is to use this to run X11 programs on a remote ma- 201 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start 202 an emacs on shadows.cs.hut.fi, and the X11 connection will be au- 203 tomatically forwarded over an encrypted channel. The ssh program 204 will be put in the background. (This does not work if ssh needs 205 to ask for a password or passphrase; see also the -f option.) 206 207 -O ctl_cmd 208 Control an active connection multiplexing master process. When 209 the -O option is specified, the ctl_cmd argument is interpreted 210 and passed to the master process. Valid commands are: ``check'' 211 (check that the master process is running) and ``exit'' (request 212 the master to exit). 213 214 -o option 215 Can be used to give options in the format used in the configura- 216 tion file. This is useful for specifying options for which there 217 is no separate command-line flag. For full details of the op- 218 tions listed below, and their possible values, see ssh_config(5). 219 220 AddressFamily 221 BatchMode 222 BindAddress 223 ChallengeResponseAuthentication 224 CheckHostIP 225 Cipher 226 Ciphers 227 ClearAllForwardings 228 Compression 229 CompressionLevel 230 ConnectionAttempts 231 ConnectTimeout 232 ControlMaster 233 ControlPath 234 DynamicForward 235 EscapeChar 236 ExitOnForwardFailure 237 ForwardAgent 238 ForwardX11 239 ForwardX11Trusted 240 GatewayPorts 241 GlobalKnownHostsFile 242 GSSAPIAuthentication 243 GSSAPIDelegateCredentials 244 HashKnownHosts 245 Host 246 HostbasedAuthentication 247 HostKeyAlgorithms 248 HostKeyAlias 249 HostName 250 IdentityFile 251 IdentitiesOnly 252 KbdInteractiveDevices 253 LocalCommand 254 LocalForward 255 LogLevel 256 MACs 257 NoHostAuthenticationForLocalhost 258 NumberOfPasswordPrompts 259 PasswordAuthentication 260 PermitLocalCommand 261 Port 262 PreferredAuthentications 263 Protocol 264 ProxyCommand 265 PubkeyAuthentication 266 RekeyLimit 267 RemoteForward 268 RhostsRSAAuthentication 269 RSAAuthentication 270 SendEnv 271 ServerAliveInterval 272 ServerAliveCountMax 273 SmartcardDevice 274 StrictHostKeyChecking 275 TCPKeepAlive 276 Tunnel 277 TunnelDevice 278 UsePrivilegedPort 279 User 280 UserKnownHostsFile 281 VerifyHostKeyDNS 282 VisualHostKey 283 XAuthLocation 284 285 -p port 286 Port to connect to on the remote host. This can be specified on 287 a per-host basis in the configuration file. 288 289 -q Quiet mode. Causes most warning and diagnostic messages to be 290 suppressed. 291 292 -R [bind_address:]port:host:hostport 293 Specifies that the given port on the remote (server) host is to 294 be forwarded to the given host and port on the local side. This 295 works by allocating a socket to listen to port on the remote 296 side, and whenever a connection is made to this port, the connec- 297 tion is forwarded over the secure channel, and a connection is 298 made to host port hostport from the local machine. 299 300 Port forwardings can also be specified in the configuration file. 301 Privileged ports can be forwarded only when logging in as root on 302 the remote machine. IPv6 addresses can be specified by enclosing 303 the address in square braces or using an alternative syntax: 304 [bind_address/]host/port/hostport. 305 306 By default, the listening socket on the server will be bound to 307 the loopback interface only. This may be overridden by specify- 308 ing a bind_address. An empty bind_address, or the address `*', 309 indicates that the remote socket should listen on all interfaces. 310 Specifying a remote bind_address will only succeed if the serv- 311 er's GatewayPorts option is enabled (see sshd_config(5)). 312 313 If the port argument is `0', the listen port will be dynamically 314 allocated on the server and reported to the client at run time. 315 316 -S ctl_path 317 Specifies the location of a control socket for connection shar- 318 ing. Refer to the description of ControlPath and ControlMaster 319 in ssh_config(5) for details. 320 321 -s May be used to request invocation of a subsystem on the remote 322 system. Subsystems are a feature of the SSH2 protocol which fa- 323 cilitate the use of SSH as a secure transport for other applica- 324 tions (eg. sftp(1)). The subsystem is specified as the remote 325 command. 326 327 -T Disable pseudo-tty allocation. 328 329 -t Force pseudo-tty allocation. This can be used to execute arbi- 330 trary screen-based programs on a remote machine, which can be 331 very useful, e.g. when implementing menu services. Multiple -t 332 options force tty allocation, even if ssh has no local tty. 333 334 -V Display the version number and exit. 335 336 -v Verbose mode. Causes ssh to print debugging messages about its 337 progress. This is helpful in debugging connection, authentica- 338 tion, and configuration problems. Multiple -v options increase 339 the verbosity. The maximum is 3. 340 341 -w local_tun[:remote_tun] 342 Requests tunnel device forwarding with the specified tun(4) de- 343 vices between the client (local_tun) and the server (remote_tun). 344 345 The devices may be specified by numerical ID or the keyword 346 ``any'', which uses the next available tunnel device. If 347 remote_tun is not specified, it defaults to ``any''. See also 348 the Tunnel and TunnelDevice directives in ssh_config(5). If the 349 Tunnel directive is unset, it is set to the default tunnel mode, 350 which is ``point-to-point''. 351 352 -X Enables X11 forwarding. This can also be specified on a per-host 353 basis in a configuration file. 354 355 X11 forwarding should be enabled with caution. Users with the 356 ability to bypass file permissions on the remote host (for the 357 user's X authorization database) can access the local X11 display 358 through the forwarded connection. An attacker may then be able 359 to perform activities such as keystroke monitoring. 360 361 For this reason, X11 forwarding is subjected to X11 SECURITY ex- 362 tension restrictions by default. Please refer to the ssh -Y op- 363 tion and the ForwardX11Trusted directive in ssh_config(5) for 364 more information. 365 366 -x Disables X11 forwarding. 367 368 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not 369 subjected to the X11 SECURITY extension controls. 370 371 -y Send log information using the syslog(3) system module. By de- 372 fault this information is sent to stderr. 373 374 ssh may additionally obtain configuration data from a per-user configura- 375 tion file and a system-wide configuration file. The file format and con- 376 figuration options are described in ssh_config(5). 377 378 ssh exits with the exit status of the remote command or with 255 if an 379 error occurred. 380 381AUTHENTICATION 382 The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the 383 default, with ssh falling back to protocol 1 if it detects protocol 2 is 384 unsupported. These settings may be altered using the Protocol option in 385 ssh_config(5), or enforced using the -1 and -2 options (see above). Both 386 protocols support similar authentication methods, but protocol 2 is pre- 387 ferred since it provides additional mechanisms for confidentiality (the 388 traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and 389 integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1 390 lacks a strong mechanism for ensuring the integrity of the connection. 391 392 The methods available for authentication are: GSSAPI-based authentica- 393 tion, host-based authentication, public key authentication, challenge-re- 394 sponse authentication, and password authentication. Authentication meth- 395 ods are tried in the order specified above, though protocol 2 has a con- 396 figuration option to change the default order: PreferredAuthentications. 397 398 Host-based authentication works as follows: If the machine the user logs 399 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote 400 machine, and the user names are the same on both sides, or if the files 401 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote 402 machine and contain a line containing the name of the client machine and 403 the name of the user on that machine, the user is considered for login. 404 Additionally, the server must be able to verify the client's host key 405 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, 406 below) for login to be permitted. This authentication method closes se- 407 curity holes due to IP spoofing, DNS spoofing, and routing spoofing. 408 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the 409 rlogin/rsh protocol in general, are inherently insecure and should be 410 disabled if security is desired.] 411 412 Public key authentication works as follows: The scheme is based on pub- 413 lic-key cryptography, using cryptosystems where encryption and decryption 414 are done using separate keys, and it is unfeasible to derive the decryp- 415 tion key from the encryption key. The idea is that each user creates a 416 public/private key pair for authentication purposes. The server knows 417 the public key, and only the user knows the private key. ssh implements 418 public key authentication protocol automatically, using either the RSA or 419 DSA algorithms. Protocol 1 is restricted to using only RSA keys, but 420 protocol 2 may use either. The HISTORY section of ssl(8) contains a 421 brief discussion of the two algorithms. 422 423 The file ~/.ssh/authorized_keys lists the public keys that are permitted 424 for logging in. When the user logs in, the ssh program tells the server 425 which key pair it would like to use for authentication. The client 426 proves that it has access to the private key and the server checks that 427 the corresponding public key is authorized to accept the account. 428 429 The user creates his/her key pair by running ssh-keygen(1). This stores 430 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 431 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in 432 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or 433 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The us- 434 er should then copy the public key to ~/.ssh/authorized_keys in his/her 435 home directory on the remote machine. The authorized_keys file corre- 436 sponds to the conventional ~/.rhosts file, and has one key per line, 437 though the lines can be very long. After this, the user can log in with- 438 out giving the password. 439 440 The most convenient way to use public key authentication may be with an 441 authentication agent. See ssh-agent(1) for more information. 442 443 Challenge-response authentication works as follows: The server sends an 444 arbitrary "challenge" text, and prompts for a response. Protocol 2 al- 445 lows multiple challenges and responses; protocol 1 is restricted to just 446 one challenge/response. Examples of challenge-response authentication 447 include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD 448 systems). 449 450 Finally, if other authentication methods fail, ssh prompts the user for a 451 password. The password is sent to the remote host for checking; however, 452 since all communications are encrypted, the password cannot be seen by 453 someone listening on the network. 454 455 ssh automatically maintains and checks a database containing identifica- 456 tion for all hosts it has ever been used with. Host keys are stored in 457 ~/.ssh/known_hosts in the user's home directory. Additionally, the file 458 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any 459 new hosts are automatically added to the user's file. If a host's iden- 460 tification ever changes, ssh warns about this and disables password au- 461 thentication to prevent server spoofing or man-in-the-middle attacks, 462 which could otherwise be used to circumvent the encryption. The 463 StrictHostKeyChecking option can be used to control logins to machines 464 whose host key is not known or has changed. 465 466 When the user's identity has been accepted by the server, the server ei- 467 ther executes the given command, or logs into the machine and gives the 468 user a normal shell on the remote machine. All communication with the 469 remote command or shell will be automatically encrypted. 470 471 If a pseudo-terminal has been allocated (normal login session), the user 472 may use the escape characters noted below. 473 474 If no pseudo-tty has been allocated, the session is transparent and can 475 be used to reliably transfer binary data. On most systems, setting the 476 escape character to ``none'' will also make the session transparent even 477 if a tty is used. 478 479 The session terminates when the command or shell on the remote machine 480 exits and all X11 and TCP connections have been closed. 481 482ESCAPE CHARACTERS 483 When a pseudo-terminal has been requested, ssh supports a number of func- 484 tions through the use of an escape character. 485 486 A single tilde character can be sent as ~~ or by following the tilde by a 487 character other than those described below. The escape character must 488 always follow a newline to be interpreted as special. The escape charac- 489 ter can be changed in configuration files using the EscapeChar configura- 490 tion directive or on the command line by the -e option. 491 492 The supported escapes (assuming the default `~') are: 493 494 ~. Disconnect. 495 496 ~^Z Background ssh. 497 498 ~# List forwarded connections. 499 500 ~& Background ssh at logout when waiting for forwarded connection / 501 X11 sessions to terminate. 502 503 ~? Display a list of escape characters. 504 505 ~B Send a BREAK to the remote system (only useful for SSH protocol 506 version 2 and if the peer supports it). 507 508 ~C Open command line. Currently this allows the addition of port 509 forwardings using the -L, -R and -D options (see above). It also 510 allows the cancellation of existing remote port-forwardings using 511 -KR[bind_address:]port. !command allows the user to execute a 512 local command if the PermitLocalCommand option is enabled in 513 ssh_config(5). Basic help is available, using the -h option. 514 515 ~R Request rekeying of the connection (only useful for SSH protocol 516 version 2 and if the peer supports it). 517 518TCP FORWARDING 519 Forwarding of arbitrary TCP connections over the secure channel can be 520 specified either on the command line or in a configuration file. One 521 possible application of TCP forwarding is a secure connection to a mail 522 server; another is going through firewalls. 523 524 In the example below, we look at encrypting communication between an IRC 525 client and server, even though the IRC server does not directly support 526 encrypted communications. This works as follows: the user connects to 527 the remote host using ssh, specifying a port to be used to forward con- 528 nections to the remote server. After that it is possible to start the 529 service which is to be encrypted on the client machine, connecting to the 530 same local port, and ssh will encrypt and forward the connection. 531 532 The following example tunnels an IRC session from client machine 533 ``127.0.0.1'' (localhost) to remote server ``server.example.com'': 534 535 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 536 $ irc -c '#users' -p 1234 pinky 127.0.0.1 537 538 This tunnels a connection to IRC server ``server.example.com'', joining 539 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't mat- 540 ter which port is used, as long as it's greater than 1023 (remember, only 541 root can open sockets on privileged ports) and doesn't conflict with any 542 ports already in use. The connection is forwarded to port 6667 on the 543 remote server, since that's the standard port for IRC services. 544 545 The -f option backgrounds ssh and the remote command ``sleep 10'' is 546 specified to allow an amount of time (10 seconds, in the example) to 547 start the service which is to be tunnelled. If no connections are made 548 within the time specified, ssh will exit. 549 550X11 FORWARDING 551 If the ForwardX11 variable is set to ``yes'' (or see the description of 552 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY 553 environment variable is set), the connection to the X11 display is auto- 554 matically forwarded to the remote side in such a way that any X11 pro- 555 grams started from the shell (or command) will go through the encrypted 556 channel, and the connection to the real X server will be made from the 557 local machine. The user should not manually set DISPLAY. Forwarding of 558 X11 connections can be configured on the command line or in configuration 559 files. 560 561 The DISPLAY value set by ssh will point to the server machine, but with a 562 display number greater than zero. This is normal, and happens because 563 ssh creates a ``proxy'' X server on the server machine for forwarding the 564 connections over the encrypted channel. 565 566 ssh will also automatically set up Xauthority data on the server machine. 567 For this purpose, it will generate a random authorization cookie, store 568 it in Xauthority on the server, and verify that any forwarded connections 569 carry this cookie and replace it by the real cookie when the connection 570 is opened. The real authentication cookie is never sent to the server 571 machine (and no cookies are sent in the plain). 572 573 If the ForwardAgent variable is set to ``yes'' (or see the description of 574 the -A and -a options above) and the user is using an authentication 575 agent, the connection to the agent is automatically forwarded to the re- 576 mote side. 577 578VERIFYING HOST KEYS 579 When connecting to a server for the first time, a fingerprint of the 580 server's public key is presented to the user (unless the option 581 StrictHostKeyChecking has been disabled). Fingerprints can be determined 582 using ssh-keygen(1): 583 584 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 585 586 If the fingerprint is already known, it can be matched and the key can be 587 accepted or rejected. Because of the difficulty of comparing host keys 588 just by looking at hex strings, there is also support to compare host 589 keys visually, using random art. By setting the VisualHostKey option to 590 ``yes'', a small ASCII graphic gets displayed on every login to a server, 591 no matter if the session itself is interactive or not. By learning the 592 pattern a known server produces, a user can easily find out that the host 593 key has changed when a completely different pattern is displayed. Be- 594 cause these patterns are not unambiguous however, a pattern that looks 595 similar to the pattern remembered only gives a good probability that the 596 host key is the same, not guaranteed proof. 597 598 To get a listing of the fingerprints along with their random art for all 599 known hosts, the following command line can be used: 600 601 $ ssh-keygen -lv -f ~/.ssh/known_hosts 602 603 If the fingerprint is unknown, an alternative method of verification is 604 available: SSH fingerprints verified by DNS. An additional resource 605 record (RR), SSHFP, is added to a zonefile and the connecting client is 606 able to match the fingerprint with that of the key presented. 607 608 In this example, we are connecting a client to a server, 609 ``host.example.com''. The SSHFP resource records should first be added 610 to the zonefile for host.example.com: 611 612 $ ssh-keygen -r host.example.com. 613 614 The output lines will have to be added to the zonefile. To check that 615 the zone is answering fingerprint queries: 616 617 $ dig -t SSHFP host.example.com 618 619 Finally the client connects: 620 621 $ ssh -o "VerifyHostKeyDNS ask" host.example.com 622 [...] 623 Matching host key fingerprint found in DNS. 624 Are you sure you want to continue connecting (yes/no)? 625 626 See the VerifyHostKeyDNS option in ssh_config(5) for more information. 627 628SSH-BASED VIRTUAL PRIVATE NETWORKS 629 ssh contains support for Virtual Private Network (VPN) tunnelling using 630 the tun(4) network pseudo-device, allowing two networks to be joined se- 631 curely. The sshd_config(5) configuration option PermitTunnel controls 632 whether the server supports this, and at what level (layer 2 or 3 traf- 633 fic). 634 635 The following example would connect client network 10.0.50.0/24 with re- 636 mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 637 to 10.1.1.2, provided that the SSH server running on the gateway to the 638 remote network, at 192.168.1.15, allows it. 639 640 On the client: 641 642 # ssh -f -w 0:1 192.168.1.15 true 643 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 644 # route add 10.0.99.0/24 10.1.1.2 645 646 On the server: 647 648 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 649 # route add 10.0.50.0/24 10.1.1.1 650 651 Client access may be more finely tuned via the /root/.ssh/authorized_keys 652 file (see below) and the PermitRootLogin server option. The following 653 entry would permit connections on tun(4) device 1 from user ``jane'' and 654 on tun device 2 from user ``john'', if PermitRootLogin is set to 655 ``forced-commands-only'': 656 657 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 658 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 659 660 Since an SSH-based setup entails a fair amount of overhead, it may be 661 more suited to temporary setups, such as for wireless VPNs. More perma- 662 nent VPNs are better provided by tools such as ipsecctl(8) and 663 isakmpd(8). 664 665ENVIRONMENT 666 ssh will normally set the following environment variables: 667 668 DISPLAY The DISPLAY variable indicates the location of the 669 X11 server. It is automatically set by ssh to 670 point to a value of the form ``hostname:n'', where 671 ``hostname'' indicates the host where the shell 672 runs, and `n' is an integer >= 1. ssh uses this 673 special value to forward X11 connections over the 674 secure channel. The user should normally not set 675 DISPLAY explicitly, as that will render the X11 676 connection insecure (and will require the user to 677 manually copy any required authorization cookies). 678 679 HOME Set to the path of the user's home directory. 680 681 LOGNAME Synonym for USER; set for compatibility with sys- 682 tems that use this variable. 683 684 MAIL Set to the path of the user's mailbox. 685 686 PATH Set to the default PATH, as specified when compil- 687 ing ssh. 688 689 SSH_ASKPASS If ssh needs a passphrase, it will read the 690 passphrase from the current terminal if it was run 691 from a terminal. If ssh does not have a terminal 692 associated with it but DISPLAY and SSH_ASKPASS are 693 set, it will execute the program specified by 694 SSH_ASKPASS and open an X11 window to read the 695 passphrase. This is particularly useful when call- 696 ing ssh from a .xsession or related script. (Note 697 that on some machines it may be necessary to redi- 698 rect the input from /dev/null to make this work.) 699 700 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to 701 communicate with the agent. 702 703 SSH_CONNECTION Identifies the client and server ends of the con- 704 nection. The variable contains four space-separat- 705 ed values: client IP address, client port number, 706 server IP address, and server port number. 707 708 SSH_ORIGINAL_COMMAND This variable contains the original command line if 709 a forced command is executed. It can be used to 710 extract the original arguments. 711 712 SSH_TTY This is set to the name of the tty (path to the de- 713 vice) associated with the current shell or command. 714 If the current session has no tty, this variable is 715 not set. 716 717 TZ This variable is set to indicate the present time 718 zone if it was set when the daemon was started 719 (i.e. the daemon passes the value on to new connec- 720 tions). 721 722 USER Set to the name of the user logging in. 723 724 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format 725 ``VARNAME=value'' to the environment if the file exists and users are al- 726 lowed to change their environment. For more information, see the 727 PermitUserEnvironment option in sshd_config(5). 728 729FILES 730 ~/.rhosts 731 This file is used for host-based authentication (see above). On 732 some machines this file may need to be world-readable if the us- 733 er's home directory is on an NFS partition, because sshd(8) reads 734 it as root. Additionally, this file must be owned by the user, 735 and must not have write permissions for anyone else. The recom- 736 mended permission for most machines is read/write for the user, 737 and not accessible by others. 738 739 ~/.shosts 740 This file is used in exactly the same way as .rhosts, but allows 741 host-based authentication without permitting login with 742 rlogin/rsh. 743 744 ~/.ssh/ 745 This directory is the default location for all user-specific con- 746 figuration and authentication information. There is no general 747 requirement to keep the entire contents of this directory secret, 748 but the recommended permissions are read/write/execute for the 749 user, and not accessible by others. 750 751 ~/.ssh/authorized_keys 752 Lists the public keys (RSA/DSA) that can be used for logging in 753 as this user. The format of this file is described in the 754 sshd(8) manual page. This file is not highly sensitive, but the 755 recommended permissions are read/write for the user, and not ac- 756 cessible by others. 757 758 ~/.ssh/config 759 This is the per-user configuration file. The file format and 760 configuration options are described in ssh_config(5). Because of 761 the potential for abuse, this file must have strict permissions: 762 read/write for the user, and not accessible by others. 763 764 ~/.ssh/environment 765 Contains additional definitions for environment variables; see 766 ENVIRONMENT, above. 767 768 ~/.ssh/identity 769 ~/.ssh/id_dsa 770 ~/.ssh/id_rsa 771 Contains the private key for authentication. These files contain 772 sensitive data and should be readable by the user but not acces- 773 sible by others (read/write/execute). ssh will simply ignore a 774 private key file if it is accessible by others. It is possible 775 to specify a passphrase when generating the key which will be 776 used to encrypt the sensitive part of this file using 3DES. 777 778 ~/.ssh/identity.pub 779 ~/.ssh/id_dsa.pub 780 ~/.ssh/id_rsa.pub 781 Contains the public key for authentication. These files are not 782 sensitive and can (but need not) be readable by anyone. 783 784 ~/.ssh/known_hosts 785 Contains a list of host keys for all hosts the user has logged 786 into that are not already in the systemwide list of known host 787 keys. See sshd(8) for further details of the format of this 788 file. 789 790 ~/.ssh/rc 791 Commands in this file are executed by ssh when the user logs in, 792 just before the user's shell (or command) is started. See the 793 sshd(8) manual page for more information. 794 795 /etc/hosts.equiv 796 This file is for host-based authentication (see above). It 797 should only be writable by root. 798 799 /etc/shosts.equiv 800 This file is used in exactly the same way as hosts.equiv, but al- 801 lows host-based authentication without permitting login with 802 rlogin/rsh. 803 804 /etc/ssh/ssh_config 805 Systemwide configuration file. The file format and configuration 806 options are described in ssh_config(5). 807 808 /etc/ssh/ssh_host_key 809 /etc/ssh/ssh_host_dsa_key 810 /etc/ssh/ssh_host_rsa_key 811 These three files contain the private parts of the host keys and 812 are used for host-based authentication. If protocol version 1 is 813 used, ssh must be setuid root, since the host key is readable on- 814 ly by root. For protocol version 2, ssh uses ssh-keysign(8) to 815 access the host keys, eliminating the requirement that ssh be se- 816 tuid root when host-based authentication is used. By default ssh 817 is not setuid root. 818 819 /etc/ssh/ssh_known_hosts 820 Systemwide list of known host keys. This file should be prepared 821 by the system administrator to contain the public host keys of 822 all machines in the organization. It should be world-readable. 823 See sshd(8) for further details of the format of this file. 824 825 /etc/ssh/sshrc 826 Commands in this file are executed by ssh when the user logs in, 827 just before the user's shell (or command) is started. See the 828 sshd(8) manual page for more information. 829 830SEE ALSO 831 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), 832 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) 833 834 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006. 835 836 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006. 837 838 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006. 839 840 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006. 841 842 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006. 843 844 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 845 4255, 2006. 846 847 Generic Message Exchange Authentication for the Secure Shell Protocol 848 (SSH), RFC 4256, 2006. 849 850 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006. 851 852 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006. 853 854 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer 855 Protocol, RFC 4345, 2006. 856 857 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer 858 Protocol, RFC 4419, 2006. 859 860 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006. 861 862 A. Perrig and D. Song, Hash Visualization: a New Technique to improve 863 Real-World Security, 1999, International Workshop on Cryptographic 864 Techniques and E-Commerce (CrypTEC '99). 865 866AUTHORS 867 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 868 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 869 de Raadt and Dug Song removed many bugs, re-added newer features and 870 created OpenSSH. Markus Friedl contributed the support for SSH protocol 871 versions 1.5 and 2.0. 872 873OpenBSD 4.5 February 12, 2009 14 874