ssh.0 revision 207311
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 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. See the Ciphers keyword 82 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. Dy- 93 namic port forwardings can also be specified in the configuration 94 file. 95 96 IPv6 addresses can be specified with an alternative syntax: 97 [bind_address/]port or by enclosing the address in square brack- 98 ets. Only the superuser can forward privileged ports. By de- 99 fault, the local port is bound in accordance with the 100 GatewayPorts setting. However, an explicit bind_address may be 101 used to bind the connection to a specific address. The 102 bind_address of ``localhost'' indicates that the listening port 103 be bound for local use only, while an empty address or `*' indi- 104 cates that the port should be available from all interfaces. 105 106 -e escape_char 107 Sets the escape character for sessions with a pty (default: `~'). 108 The escape character is only recognized at the beginning of a 109 line. The escape character followed by a dot (`.') closes the 110 connection; followed by control-Z suspends the connection; and 111 followed by itself sends the escape character once. Setting the 112 character to ``none'' disables any escapes and makes the session 113 fully transparent. 114 115 -F configfile 116 Specifies an alternative per-user configuration file. If a con- 117 figuration file is given on the command line, the system-wide 118 configuration file (/etc/ssh/ssh_config) will be ignored. The 119 default for the per-user configuration file is ~/.ssh/config. 120 121 -f Requests ssh to go to background just before command execution. 122 This is useful if ssh is going to ask for passwords or passphras- 123 es, but the user wants it in the background. This implies -n. 124 The recommended way to start X11 programs at a remote site is 125 with something like ssh -f host xterm. 126 127 If the ExitOnForwardFailure configuration option is set to 128 ``yes'', then a client started with -f will wait for all remote 129 port forwards to be successfully established before placing it- 130 self in the background. 131 132 -g Allows remote hosts to connect to local forwarded ports. 133 134 -I pkcs11 135 Specify the PKCS#11 shared library ssh should use to communicate 136 with a PKCS#11 token providing the user's private RSA key. 137 138 -i identity_file 139 Selects a file from which the identity (private key) for RSA or 140 DSA authentication is read. The default is ~/.ssh/identity for 141 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro- 142 tocol version 2. Identity files may also be specified on a per- 143 host basis in the configuration file. It is possible to have 144 multiple -i options (and multiple identities specified in config- 145 uration files). ssh will also try to load certificate informa- 146 tion from the filename obtained by appending -cert.pub to identi- 147 ty filenames. 148 149 -K Enables GSSAPI-based authentication and forwarding (delegation) 150 of GSSAPI credentials to the server. 151 152 -k Disables forwarding (delegation) of GSSAPI credentials to the 153 server. 154 155 -L [bind_address:]port:host:hostport 156 Specifies that the given port on the local (client) host is to be 157 forwarded to the given host and port on the remote side. This 158 works by allocating a socket to listen to port on the local side, 159 optionally bound to the specified bind_address. Whenever a con- 160 nection is made to this port, the connection is forwarded over 161 the secure channel, and a connection is made to host port 162 hostport from the remote machine. Port forwardings can also be 163 specified in the configuration file. IPv6 addresses can be spec- 164 ified with an alternative syntax: [bind_address/]port/host/host- 165 port or by enclosing the address in square brackets. Only the 166 superuser can forward privileged ports. By default, the local 167 port is bound in accordance with the GatewayPorts setting. How- 168 ever, an explicit bind_address may be used to bind the connection 169 to a specific address. The bind_address of ``localhost'' indi- 170 cates that the listening port be bound for local use only, while 171 an empty address or `*' indicates that the port should be avail- 172 able from all interfaces. 173 174 -l login_name 175 Specifies the user to log in as on the remote machine. This also 176 may be specified on a per-host basis in the configuration file. 177 178 -M Places the ssh client into ``master'' mode for connection shar- 179 ing. Multiple -M options places ssh into ``master'' mode with 180 confirmation required before slave connections are accepted. Re- 181 fer to the description of ControlMaster in ssh_config(5) for de- 182 tails. 183 184 -m mac_spec 185 Additionally, for protocol version 2 a comma-separated list of 186 MAC (message authentication code) algorithms can be specified in 187 order of preference. See the MACs keyword for more information. 188 189 -N Do not execute a remote command. This is useful for just for- 190 warding ports (protocol version 2 only). 191 192 -n Redirects stdin from /dev/null (actually, prevents reading from 193 stdin). This must be used when ssh is run in the background. A 194 common trick is to use this to run X11 programs on a remote ma- 195 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start 196 an emacs on shadows.cs.hut.fi, and the X11 connection will be au- 197 tomatically forwarded over an encrypted channel. The ssh program 198 will be put in the background. (This does not work if ssh needs 199 to ask for a password or passphrase; see also the -f option.) 200 201 -O ctl_cmd 202 Control an active connection multiplexing master process. When 203 the -O option is specified, the ctl_cmd argument is interpreted 204 and passed to the master process. Valid commands are: ``check'' 205 (check that the master process is running) and ``exit'' (request 206 the master to exit). 207 208 -o option 209 Can be used to give options in the format used in the configura- 210 tion file. This is useful for specifying options for which there 211 is no separate command-line flag. For full details of the op- 212 tions listed below, and their possible values, see ssh_config(5). 213 214 AddressFamily 215 BatchMode 216 BindAddress 217 ChallengeResponseAuthentication 218 CheckHostIP 219 Cipher 220 Ciphers 221 ClearAllForwardings 222 Compression 223 CompressionLevel 224 ConnectionAttempts 225 ConnectTimeout 226 ControlMaster 227 ControlPath 228 DynamicForward 229 EscapeChar 230 ExitOnForwardFailure 231 ForwardAgent 232 ForwardX11 233 ForwardX11Trusted 234 GatewayPorts 235 GlobalKnownHostsFile 236 GSSAPIAuthentication 237 GSSAPIDelegateCredentials 238 HashKnownHosts 239 Host 240 HostbasedAuthentication 241 HostKeyAlgorithms 242 HostKeyAlias 243 HostName 244 IdentityFile 245 IdentitiesOnly 246 KbdInteractiveDevices 247 LocalCommand 248 LocalForward 249 LogLevel 250 MACs 251 NoHostAuthenticationForLocalhost 252 NumberOfPasswordPrompts 253 PasswordAuthentication 254 PermitLocalCommand 255 PKCS11Provider 256 Port 257 PreferredAuthentications 258 Protocol 259 ProxyCommand 260 PubkeyAuthentication 261 RekeyLimit 262 RemoteForward 263 RhostsRSAAuthentication 264 RSAAuthentication 265 SendEnv 266 ServerAliveInterval 267 ServerAliveCountMax 268 StrictHostKeyChecking 269 TCPKeepAlive 270 Tunnel 271 TunnelDevice 272 UsePrivilegedPort 273 User 274 UserKnownHostsFile 275 VerifyHostKeyDNS 276 VisualHostKey 277 XAuthLocation 278 279 -p port 280 Port to connect to on the remote host. This can be specified on 281 a per-host basis in the configuration file. 282 283 -q Quiet mode. Causes most warning and diagnostic messages to be 284 suppressed. 285 286 -R [bind_address:]port:host:hostport 287 Specifies that the given port on the remote (server) host is to 288 be forwarded to the given host and port on the local side. This 289 works by allocating a socket to listen to port on the remote 290 side, and whenever a connection is made to this port, the connec- 291 tion is forwarded over the secure channel, and a connection is 292 made to host port hostport from the local machine. 293 294 Port forwardings can also be specified in the configuration file. 295 Privileged ports can be forwarded only when logging in as root on 296 the remote machine. IPv6 addresses can be specified by enclosing 297 the address in square braces or using an alternative syntax: 298 [bind_address/]host/port/hostport. 299 300 By default, the listening socket on the server will be bound to 301 the loopback interface only. This may be overridden by specify- 302 ing a bind_address. An empty bind_address, or the address `*', 303 indicates that the remote socket should listen on all interfaces. 304 Specifying a remote bind_address will only succeed if the serv- 305 er's GatewayPorts option is enabled (see sshd_config(5)). 306 307 If the port argument is `0', the listen port will be dynamically 308 allocated on the server and reported to the client at run time. 309 310 -S ctl_path 311 Specifies the location of a control socket for connection sharing 312 or the string ``none'' to disable connection sharing. Refer to 313 the description of ControlPath and ControlMaster in ssh_config(5) 314 for details. 315 316 -s May be used to request invocation of a subsystem on the remote 317 system. Subsystems are a feature of the SSH2 protocol which fa- 318 cilitate the use of SSH as a secure transport for other applica- 319 tions (eg. sftp(1)). The subsystem is specified as the remote 320 command. 321 322 -T Disable pseudo-tty allocation. 323 324 -t Force pseudo-tty allocation. This can be used to execute arbi- 325 trary screen-based programs on a remote machine, which can be 326 very useful, e.g. when implementing menu services. Multiple -t 327 options force tty allocation, even if ssh has no local tty. 328 329 -V Display the version number and exit. 330 331 -v Verbose mode. Causes ssh to print debugging messages about its 332 progress. This is helpful in debugging connection, authentica- 333 tion, and configuration problems. Multiple -v options increase 334 the verbosity. The maximum is 3. 335 336 -W host:port 337 Requests that standard input and output on the client be forward- 338 ed to host on port over the secure channel. Implies -N, -T, 339 ExitOnForwardFailure and ClearAllForwardings and works with Pro- 340 tocol version 2 only. 341 342 -w local_tun[:remote_tun] 343 Requests tunnel device forwarding with the specified tun(4) de- 344 vices between the client (local_tun) and the server (remote_tun). 345 346 The devices may be specified by numerical ID or the keyword 347 ``any'', which uses the next available tunnel device. If 348 remote_tun is not specified, it defaults to ``any''. See also 349 the Tunnel and TunnelDevice directives in ssh_config(5). If the 350 Tunnel directive is unset, it is set to the default tunnel mode, 351 which is ``point-to-point''. 352 353 -X Enables X11 forwarding. This can also be specified on a per-host 354 basis in a configuration file. 355 356 X11 forwarding should be enabled with caution. Users with the 357 ability to bypass file permissions on the remote host (for the 358 user's X authorization database) can access the local X11 display 359 through the forwarded connection. An attacker may then be able 360 to perform activities such as keystroke monitoring. 361 362 For this reason, X11 forwarding is subjected to X11 SECURITY ex- 363 tension restrictions by default. Please refer to the ssh -Y op- 364 tion and the ForwardX11Trusted directive in ssh_config(5) for 365 more information. 366 367 -x Disables X11 forwarding. 368 369 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not 370 subjected to the X11 SECURITY extension controls. 371 372 -y Send log information using the syslog(3) system module. By de- 373 fault this information is sent to stderr. 374 375 ssh may additionally obtain configuration data from a per-user configura- 376 tion file and a system-wide configuration file. The file format and con- 377 figuration options are described in ssh_config(5). 378 379 ssh exits with the exit status of the remote command or with 255 if an 380 error occurred. 381 382AUTHENTICATION 383 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to 384 use protocol 2 only, though this can be changed via the Protocol option 385 in ssh_config(5) or the -1 and -2 options (see above). Both protocols 386 support similar authentication methods, but protocol 2 is the default 387 since it provides additional mechanisms for confidentiality (the traffic 388 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and integri- 389 ty (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1 lacks a 390 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 A variation on public key authentication is available in the form of cer- 441 tificate authentication: instead of a set of public/private keys, signed 442 certificates are used. This has the advantage that a single trusted cer- 443 tification authority can be used in place of many public/private keys. 444 See the CERTIFICATES section of ssh-keygen(1) for more information. 445 446 The most convenient way to use public key or certificate authentication 447 may be with an authentication agent. See ssh-agent(1) for more informa- 448 tion. 449 450 Challenge-response authentication works as follows: The server sends an 451 arbitrary "challenge" text, and prompts for a response. Protocol 2 al- 452 lows multiple challenges and responses; protocol 1 is restricted to just 453 one challenge/response. Examples of challenge-response authentication 454 include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD 455 systems). 456 457 Finally, if other authentication methods fail, ssh prompts the user for a 458 password. The password is sent to the remote host for checking; however, 459 since all communications are encrypted, the password cannot be seen by 460 someone listening on the network. 461 462 ssh automatically maintains and checks a database containing identifica- 463 tion for all hosts it has ever been used with. Host keys are stored in 464 ~/.ssh/known_hosts in the user's home directory. Additionally, the file 465 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any 466 new hosts are automatically added to the user's file. If a host's iden- 467 tification ever changes, ssh warns about this and disables password au- 468 thentication to prevent server spoofing or man-in-the-middle attacks, 469 which could otherwise be used to circumvent the encryption. The 470 StrictHostKeyChecking option can be used to control logins to machines 471 whose host key is not known or has changed. 472 473 When the user's identity has been accepted by the server, the server ei- 474 ther executes the given command, or logs into the machine and gives the 475 user a normal shell on the remote machine. All communication with the 476 remote command or shell will be automatically encrypted. 477 478 If a pseudo-terminal has been allocated (normal login session), the user 479 may use the escape characters noted below. 480 481 If no pseudo-tty has been allocated, the session is transparent and can 482 be used to reliably transfer binary data. On most systems, setting the 483 escape character to ``none'' will also make the session transparent even 484 if a tty is used. 485 486 The session terminates when the command or shell on the remote machine 487 exits and all X11 and TCP connections have been closed. 488 489ESCAPE CHARACTERS 490 When a pseudo-terminal has been requested, ssh supports a number of func- 491 tions through the use of an escape character. 492 493 A single tilde character can be sent as ~~ or by following the tilde by a 494 character other than those described below. The escape character must 495 always follow a newline to be interpreted as special. The escape charac- 496 ter can be changed in configuration files using the EscapeChar configura- 497 tion directive or on the command line by the -e option. 498 499 The supported escapes (assuming the default `~') are: 500 501 ~. Disconnect. 502 503 ~^Z Background ssh. 504 505 ~# List forwarded connections. 506 507 ~& Background ssh at logout when waiting for forwarded connection / 508 X11 sessions to terminate. 509 510 ~? Display a list of escape characters. 511 512 ~B Send a BREAK to the remote system (only useful for SSH protocol 513 version 2 and if the peer supports it). 514 515 ~C Open command line. Currently this allows the addition of port 516 forwardings using the -L, -R and -D options (see above). It also 517 allows the cancellation of existing remote port-forwardings using 518 -KR[bind_address:]port. !command allows the user to execute a 519 local command if the PermitLocalCommand option is enabled in 520 ssh_config(5). Basic help is available, using the -h option. 521 522 ~R Request rekeying of the connection (only useful for SSH protocol 523 version 2 and if the peer supports it). 524 525TCP FORWARDING 526 Forwarding of arbitrary TCP connections over the secure channel can be 527 specified either on the command line or in a configuration file. One 528 possible application of TCP forwarding is a secure connection to a mail 529 server; another is going through firewalls. 530 531 In the example below, we look at encrypting communication between an IRC 532 client and server, even though the IRC server does not directly support 533 encrypted communications. This works as follows: the user connects to 534 the remote host using ssh, specifying a port to be used to forward con- 535 nections to the remote server. After that it is possible to start the 536 service which is to be encrypted on the client machine, connecting to the 537 same local port, and ssh will encrypt and forward the connection. 538 539 The following example tunnels an IRC session from client machine 540 ``127.0.0.1'' (localhost) to remote server ``server.example.com'': 541 542 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 543 $ irc -c '#users' -p 1234 pinky 127.0.0.1 544 545 This tunnels a connection to IRC server ``server.example.com'', joining 546 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't mat- 547 ter which port is used, as long as it's greater than 1023 (remember, only 548 root can open sockets on privileged ports) and doesn't conflict with any 549 ports already in use. The connection is forwarded to port 6667 on the 550 remote server, since that's the standard port for IRC services. 551 552 The -f option backgrounds ssh and the remote command ``sleep 10'' is 553 specified to allow an amount of time (10 seconds, in the example) to 554 start the service which is to be tunnelled. If no connections are made 555 within the time specified, ssh will exit. 556 557X11 FORWARDING 558 If the ForwardX11 variable is set to ``yes'' (or see the description of 559 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY 560 environment variable is set), the connection to the X11 display is auto- 561 matically forwarded to the remote side in such a way that any X11 pro- 562 grams started from the shell (or command) will go through the encrypted 563 channel, and the connection to the real X server will be made from the 564 local machine. The user should not manually set DISPLAY. Forwarding of 565 X11 connections can be configured on the command line or in configuration 566 files. 567 568 The DISPLAY value set by ssh will point to the server machine, but with a 569 display number greater than zero. This is normal, and happens because 570 ssh creates a ``proxy'' X server on the server machine for forwarding the 571 connections over the encrypted channel. 572 573 ssh will also automatically set up Xauthority data on the server machine. 574 For this purpose, it will generate a random authorization cookie, store 575 it in Xauthority on the server, and verify that any forwarded connections 576 carry this cookie and replace it by the real cookie when the connection 577 is opened. The real authentication cookie is never sent to the server 578 machine (and no cookies are sent in the plain). 579 580 If the ForwardAgent variable is set to ``yes'' (or see the description of 581 the -A and -a options above) and the user is using an authentication 582 agent, the connection to the agent is automatically forwarded to the re- 583 mote side. 584 585VERIFYING HOST KEYS 586 When connecting to a server for the first time, a fingerprint of the 587 server's public key is presented to the user (unless the option 588 StrictHostKeyChecking has been disabled). Fingerprints can be determined 589 using ssh-keygen(1): 590 591 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 592 593 If the fingerprint is already known, it can be matched and the key can be 594 accepted or rejected. Because of the difficulty of comparing host keys 595 just by looking at hex strings, there is also support to compare host 596 keys visually, using random art. By setting the VisualHostKey option to 597 ``yes'', a small ASCII graphic gets displayed on every login to a server, 598 no matter if the session itself is interactive or not. By learning the 599 pattern a known server produces, a user can easily find out that the host 600 key has changed when a completely different pattern is displayed. Be- 601 cause these patterns are not unambiguous however, a pattern that looks 602 similar to the pattern remembered only gives a good probability that the 603 host key is the same, not guaranteed proof. 604 605 To get a listing of the fingerprints along with their random art for all 606 known hosts, the following command line can be used: 607 608 $ ssh-keygen -lv -f ~/.ssh/known_hosts 609 610 If the fingerprint is unknown, an alternative method of verification is 611 available: SSH fingerprints verified by DNS. An additional resource 612 record (RR), SSHFP, is added to a zonefile and the connecting client is 613 able to match the fingerprint with that of the key presented. 614 615 In this example, we are connecting a client to a server, 616 ``host.example.com''. The SSHFP resource records should first be added 617 to the zonefile for host.example.com: 618 619 $ ssh-keygen -r host.example.com. 620 621 The output lines will have to be added to the zonefile. To check that 622 the zone is answering fingerprint queries: 623 624 $ dig -t SSHFP host.example.com 625 626 Finally the client connects: 627 628 $ ssh -o "VerifyHostKeyDNS ask" host.example.com 629 [...] 630 Matching host key fingerprint found in DNS. 631 Are you sure you want to continue connecting (yes/no)? 632 633 See the VerifyHostKeyDNS option in ssh_config(5) for more information. 634 635SSH-BASED VIRTUAL PRIVATE NETWORKS 636 ssh contains support for Virtual Private Network (VPN) tunnelling using 637 the tun(4) network pseudo-device, allowing two networks to be joined se- 638 curely. The sshd_config(5) configuration option PermitTunnel controls 639 whether the server supports this, and at what level (layer 2 or 3 traf- 640 fic). 641 642 The following example would connect client network 10.0.50.0/24 with re- 643 mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 644 to 10.1.1.2, provided that the SSH server running on the gateway to the 645 remote network, at 192.168.1.15, allows it. 646 647 On the client: 648 649 # ssh -f -w 0:1 192.168.1.15 true 650 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 651 # route add 10.0.99.0/24 10.1.1.2 652 653 On the server: 654 655 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 656 # route add 10.0.50.0/24 10.1.1.1 657 658 Client access may be more finely tuned via the /root/.ssh/authorized_keys 659 file (see below) and the PermitRootLogin server option. The following 660 entry would permit connections on tun(4) device 1 from user ``jane'' and 661 on tun device 2 from user ``john'', if PermitRootLogin is set to 662 ``forced-commands-only'': 663 664 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 665 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 666 667 Since an SSH-based setup entails a fair amount of overhead, it may be 668 more suited to temporary setups, such as for wireless VPNs. More perma- 669 nent VPNs are better provided by tools such as ipsecctl(8) and 670 isakmpd(8). 671 672ENVIRONMENT 673 ssh will normally set the following environment variables: 674 675 DISPLAY The DISPLAY variable indicates the location of the 676 X11 server. It is automatically set by ssh to 677 point to a value of the form ``hostname:n'', where 678 ``hostname'' indicates the host where the shell 679 runs, and `n' is an integer >= 1. ssh uses this 680 special value to forward X11 connections over the 681 secure channel. The user should normally not set 682 DISPLAY explicitly, as that will render the X11 683 connection insecure (and will require the user to 684 manually copy any required authorization cookies). 685 686 HOME Set to the path of the user's home directory. 687 688 LOGNAME Synonym for USER; set for compatibility with sys- 689 tems that use this variable. 690 691 MAIL Set to the path of the user's mailbox. 692 693 PATH Set to the default PATH, as specified when compil- 694 ing ssh. 695 696 SSH_ASKPASS If ssh needs a passphrase, it will read the 697 passphrase from the current terminal if it was run 698 from a terminal. If ssh does not have a terminal 699 associated with it but DISPLAY and SSH_ASKPASS are 700 set, it will execute the program specified by 701 SSH_ASKPASS and open an X11 window to read the 702 passphrase. This is particularly useful when call- 703 ing ssh from a .xsession or related script. (Note 704 that on some machines it may be necessary to redi- 705 rect the input from /dev/null to make this work.) 706 707 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to 708 communicate with the agent. 709 710 SSH_CONNECTION Identifies the client and server ends of the con- 711 nection. The variable contains four space-separat- 712 ed values: client IP address, client port number, 713 server IP address, and server port number. 714 715 SSH_ORIGINAL_COMMAND This variable contains the original command line if 716 a forced command is executed. It can be used to 717 extract the original arguments. 718 719 SSH_TTY This is set to the name of the tty (path to the de- 720 vice) associated with the current shell or command. 721 If the current session has no tty, this variable is 722 not set. 723 724 TZ This variable is set to indicate the present time 725 zone if it was set when the daemon was started 726 (i.e. the daemon passes the value on to new connec- 727 tions). 728 729 USER Set to the name of the user logging in. 730 731 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format 732 ``VARNAME=value'' to the environment if the file exists and users are al- 733 lowed to change their environment. For more information, see the 734 PermitUserEnvironment option in sshd_config(5). 735 736FILES 737 ~/.rhosts 738 This file is used for host-based authentication (see above). On 739 some machines this file may need to be world-readable if the us- 740 er's home directory is on an NFS partition, because sshd(8) reads 741 it as root. Additionally, this file must be owned by the user, 742 and must not have write permissions for anyone else. The recom- 743 mended permission for most machines is read/write for the user, 744 and not accessible by others. 745 746 ~/.shosts 747 This file is used in exactly the same way as .rhosts, but allows 748 host-based authentication without permitting login with 749 rlogin/rsh. 750 751 ~/.ssh/ 752 This directory is the default location for all user-specific con- 753 figuration and authentication information. There is no general 754 requirement to keep the entire contents of this directory secret, 755 but the recommended permissions are read/write/execute for the 756 user, and not accessible by others. 757 758 ~/.ssh/authorized_keys 759 Lists the public keys (RSA/DSA) that can be used for logging in 760 as this user. The format of this file is described in the 761 sshd(8) manual page. This file is not highly sensitive, but the 762 recommended permissions are read/write for the user, and not ac- 763 cessible by others. 764 765 ~/.ssh/config 766 This is the per-user configuration file. The file format and 767 configuration options are described in ssh_config(5). Because of 768 the potential for abuse, this file must have strict permissions: 769 read/write for the user, and not accessible by others. 770 771 ~/.ssh/environment 772 Contains additional definitions for environment variables; see 773 ENVIRONMENT, above. 774 775 ~/.ssh/identity 776 ~/.ssh/id_dsa 777 ~/.ssh/id_rsa 778 Contains the private key for authentication. These files contain 779 sensitive data and should be readable by the user but not acces- 780 sible by others (read/write/execute). ssh will simply ignore a 781 private key file if it is accessible by others. It is possible 782 to specify a passphrase when generating the key which will be 783 used to encrypt the sensitive part of this file using 3DES. 784 785 ~/.ssh/identity.pub 786 ~/.ssh/id_dsa.pub 787 ~/.ssh/id_rsa.pub 788 Contains the public key for authentication. These files are not 789 sensitive and can (but need not) be readable by anyone. 790 791 ~/.ssh/known_hosts 792 Contains a list of host keys for all hosts the user has logged 793 into that are not already in the systemwide list of known host 794 keys. See sshd(8) for further details of the format of this 795 file. 796 797 ~/.ssh/rc 798 Commands in this file are executed by ssh when the user logs in, 799 just before the user's shell (or command) is started. See the 800 sshd(8) manual page for more information. 801 802 /etc/hosts.equiv 803 This file is for host-based authentication (see above). It 804 should only be writable by root. 805 806 /etc/shosts.equiv 807 This file is used in exactly the same way as hosts.equiv, but al- 808 lows host-based authentication without permitting login with 809 rlogin/rsh. 810 811 /etc/ssh/ssh_config 812 Systemwide configuration file. The file format and configuration 813 options are described in ssh_config(5). 814 815 /etc/ssh/ssh_host_key 816 /etc/ssh/ssh_host_dsa_key 817 /etc/ssh/ssh_host_rsa_key 818 These three files contain the private parts of the host keys and 819 are used for host-based authentication. If protocol version 1 is 820 used, ssh must be setuid root, since the host key is readable on- 821 ly by root. For protocol version 2, ssh uses ssh-keysign(8) to 822 access the host keys, eliminating the requirement that ssh be se- 823 tuid root when host-based authentication is used. By default ssh 824 is not setuid root. 825 826 /etc/ssh/ssh_known_hosts 827 Systemwide list of known host keys. This file should be prepared 828 by the system administrator to contain the public host keys of 829 all machines in the organization. It should be world-readable. 830 See sshd(8) for further details of the format of this file. 831 832 /etc/ssh/sshrc 833 Commands in this file are executed by ssh when the user logs in, 834 just before the user's shell (or command) is started. See the 835 sshd(8) manual page for more information. 836 837SEE ALSO 838 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), 839 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) 840 841 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006. 842 843 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006. 844 845 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006. 846 847 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006. 848 849 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006. 850 851 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 852 4255, 2006. 853 854 Generic Message Exchange Authentication for the Secure Shell Protocol 855 (SSH), RFC 4256, 2006. 856 857 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006. 858 859 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006. 860 861 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer 862 Protocol, RFC 4345, 2006. 863 864 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer 865 Protocol, RFC 4419, 2006. 866 867 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006. 868 869 A. Perrig and D. Song, Hash Visualization: a New Technique to improve 870 Real-World Security, 1999, International Workshop on Cryptographic 871 Techniques and E-Commerce (CrypTEC '99). 872 873AUTHORS 874 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 875 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 876 de Raadt and Dug Song removed many bugs, re-added newer features and 877 created OpenSSH. Markus Friedl contributed the support for SSH protocol 878 versions 1.5 and 2.0. 879 880OpenBSD 4.7 March 26, 2010 14 881