ssh.0 revision 180741
1SSH(1) OpenBSD Reference Manual SSH(1) 2 3NAME 4 ssh - OpenSSH SSH client (remote login program) 5 6SYNOPSIS 7 ssh [-1246AaCfgkMNnqsTtVvXxY] [-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 -g Allows remote hosts to connect to local forwarded ports. 134 135 -I smartcard_device 136 Specify the device ssh should use to communicate with a smartcard 137 used for storing the user's private RSA key. This option is only 138 available if support for smartcard devices is compiled in (de- 139 fault is no support). 140 141 -i identity_file 142 Selects a file from which the identity (private key) for RSA or 143 DSA authentication is read. The default is ~/.ssh/identity for 144 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro- 145 tocol version 2. Identity files may also be specified on a per- 146 host basis in the configuration file. It is possible to have 147 multiple -i options (and multiple identities specified in config- 148 uration files). 149 150 -k Disables forwarding (delegation) of GSSAPI credentials to the 151 server. 152 153 -L [bind_address:]port:host:hostport 154 Specifies that the given port on the local (client) host is to be 155 forwarded to the given host and port on the remote side. This 156 works by allocating a socket to listen to port on the local side, 157 optionally bound to the specified bind_address. Whenever a con- 158 nection is made to this port, the connection is forwarded over 159 the secure channel, and a connection is made to host port 160 hostport from the remote machine. Port forwardings can also be 161 specified in the configuration file. IPv6 addresses can be spec- 162 ified with an alternative syntax: [bind_address/]port/host/host- 163 port or 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. How- 166 ever, an explicit bind_address may be used to bind the connection 167 to a specific address. The bind_address of ``localhost'' indi- 168 cates that the listening port be bound for local use only, while 169 an empty address or `*' indicates that the port should be avail- 170 able 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 shar- 177 ing. Multiple -M options places ssh into ``master'' mode with 178 confirmation required before slave connections are accepted. Re- 179 fer to the description of ControlMaster in ssh_config(5) for de- 180 tails. 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 for- 188 warding 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 ma- 193 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start 194 an emacs on shadows.cs.hut.fi, and the X11 connection will be au- 195 tomatically forwarded over an encrypted channel. The ssh program 196 will be put in the background. (This does not work if ssh needs 197 to ask for a password or passphrase; see also the -f option.) 198 199 -O ctl_cmd 200 Control an active connection multiplexing master process. When 201 the -O option is specified, the ctl_cmd argument is interpreted 202 and passed to the master process. Valid commands are: ``check'' 203 (check that the master process is running) and ``exit'' (request 204 the master to exit). 205 206 -o option 207 Can be used to give options in the format used in the configura- 208 tion file. This is useful for specifying options for which there 209 is no separate command-line flag. For full details of the op- 210 tions listed below, and their possible values, see ssh_config(5). 211 212 AddressFamily 213 BatchMode 214 BindAddress 215 ChallengeResponseAuthentication 216 CheckHostIP 217 Cipher 218 Ciphers 219 ClearAllForwardings 220 Compression 221 CompressionLevel 222 ConnectionAttempts 223 ConnectTimeout 224 ControlMaster 225 ControlPath 226 DynamicForward 227 EscapeChar 228 ExitOnForwardFailure 229 ForwardAgent 230 ForwardX11 231 ForwardX11Trusted 232 GatewayPorts 233 GlobalKnownHostsFile 234 GSSAPIAuthentication 235 GSSAPIDelegateCredentials 236 HashKnownHosts 237 Host 238 HostbasedAuthentication 239 HostKeyAlgorithms 240 HostKeyAlias 241 HostName 242 IdentityFile 243 IdentitiesOnly 244 KbdInteractiveDevices 245 LocalCommand 246 LocalForward 247 LogLevel 248 MACs 249 NoHostAuthenticationForLocalhost 250 NumberOfPasswordPrompts 251 PasswordAuthentication 252 PermitLocalCommand 253 Port 254 PreferredAuthentications 255 Protocol 256 ProxyCommand 257 PubkeyAuthentication 258 RekeyLimit 259 RemoteForward 260 RhostsRSAAuthentication 261 RSAAuthentication 262 SendEnv 263 ServerAliveInterval 264 ServerAliveCountMax 265 SmartcardDevice 266 StrictHostKeyChecking 267 TCPKeepAlive 268 Tunnel 269 TunnelDevice 270 UsePrivilegedPort 271 User 272 UserKnownHostsFile 273 VerifyHostKeyDNS 274 XAuthLocation 275 276 -p port 277 Port to connect to on the remote host. This can be specified on 278 a per-host basis in the configuration file. 279 280 -q Quiet mode. Causes all warning and diagnostic messages to be 281 suppressed. 282 283 -R [bind_address:]port:host:hostport 284 Specifies that the given port on the remote (server) host is to 285 be forwarded to the given host and port on the local side. This 286 works by allocating a socket to listen to port on the remote 287 side, and whenever a connection is made to this port, the connec- 288 tion is forwarded over the secure channel, and a connection is 289 made to host port hostport from the local machine. 290 291 Port forwardings can also be specified in the configuration file. 292 Privileged ports can be forwarded only when logging in as root on 293 the remote machine. IPv6 addresses can be specified by enclosing 294 the address in square braces or using an alternative syntax: 295 [bind_address/]host/port/hostport. 296 297 By default, the listening socket on the server will be bound to 298 the loopback interface only. This may be overriden by specifying 299 a bind_address. An empty bind_address, or the address `*', indi- 300 cates that the remote socket should listen on all interfaces. 301 Specifying a remote bind_address will only succeed if the serv- 302 er's GatewayPorts option is enabled (see sshd_config(5)). 303 304 -S ctl_path 305 Specifies the location of a control socket for connection shar- 306 ing. Refer to the description of ControlPath and ControlMaster 307 in ssh_config(5) for details. 308 309 -s May be used to request invocation of a subsystem on the remote 310 system. Subsystems are a feature of the SSH2 protocol which fa- 311 cilitate the use of SSH as a secure transport for other applica- 312 tions (eg. sftp(1)). The subsystem is specified as the remote 313 command. 314 315 -T Disable pseudo-tty allocation. 316 317 -t Force pseudo-tty allocation. This can be used to execute arbi- 318 trary screen-based programs on a remote machine, which can be 319 very useful, e.g. when implementing menu services. Multiple -t 320 options force tty allocation, even if ssh has no local tty. 321 322 -V Display the version number and exit. 323 324 -v Verbose mode. Causes ssh to print debugging messages about its 325 progress. This is helpful in debugging connection, authentica- 326 tion, and configuration problems. Multiple -v options increase 327 the verbosity. The maximum is 3. 328 329 -w local_tun[:remote_tun] 330 Requests tunnel device forwarding with the specified tun(4) de- 331 vices between the client (local_tun) and the server (remote_tun). 332 333 The devices may be specified by numerical ID or the keyword 334 ``any'', which uses the next available tunnel device. If 335 remote_tun is not specified, it defaults to ``any''. See also 336 the Tunnel and TunnelDevice directives in ssh_config(5). If the 337 Tunnel directive is unset, it is set to the default tunnel mode, 338 which is ``point-to-point''. 339 340 -X Enables X11 forwarding. This can also be specified on a per-host 341 basis in a configuration file. 342 343 X11 forwarding should be enabled with caution. Users with the 344 ability to bypass file permissions on the remote host (for the 345 user's X authorization database) can access the local X11 display 346 through the forwarded connection. An attacker may then be able 347 to perform activities such as keystroke monitoring. 348 349 For this reason, X11 forwarding is subjected to X11 SECURITY ex- 350 tension restrictions by default. Please refer to the ssh -Y op- 351 tion and the ForwardX11Trusted directive in ssh_config(5) for 352 more information. 353 354 -x Disables X11 forwarding. 355 356 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not 357 subjected to the X11 SECURITY extension controls. 358 359 ssh may additionally obtain configuration data from a per-user configura- 360 tion file and a system-wide configuration file. The file format and con- 361 figuration options are described in ssh_config(5). 362 363 ssh exits with the exit status of the remote command or with 255 if an 364 error occurred. 365 366AUTHENTICATION 367 The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the 368 default, with ssh falling back to protocol 1 if it detects protocol 2 is 369 unsupported. These settings may be altered using the Protocol option in 370 ssh_config(5), or enforced using the -1 and -2 options (see above). Both 371 protocols support similar authentication methods, but protocol 2 is pre- 372 ferred since it provides additional mechanisms for confidentiality (the 373 traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and 374 integrity (hmac-md5, hmac-sha1, hmac-ripemd160). Protocol 1 lacks a 375 strong mechanism for ensuring the integrity of the connection. 376 377 The methods available for authentication are: GSSAPI-based authentica- 378 tion, host-based authentication, public key authentication, challenge-re- 379 sponse authentication, and password authentication. Authentication meth- 380 ods are tried in the order specified above, though protocol 2 has a con- 381 figuration option to change the default order: PreferredAuthentications. 382 383 Host-based authentication works as follows: If the machine the user logs 384 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote 385 machine, and the user names are the same on both sides, or if the files 386 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote 387 machine and contain a line containing the name of the client machine and 388 the name of the user on that machine, the user is considered for login. 389 Additionally, the server must be able to verify the client's host key 390 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, 391 below) for login to be permitted. This authentication method closes se- 392 curity holes due to IP spoofing, DNS spoofing, and routing spoofing. 393 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the 394 rlogin/rsh protocol in general, are inherently insecure and should be 395 disabled if security is desired.] 396 397 Public key authentication works as follows: The scheme is based on pub- 398 lic-key cryptography, using cryptosystems where encryption and decryption 399 are done using separate keys, and it is unfeasible to derive the decryp- 400 tion key from the encryption key. The idea is that each user creates a 401 public/private key pair for authentication purposes. The server knows 402 the public key, and only the user knows the private key. ssh implements 403 public key authentication protocol automatically, using either the RSA or 404 DSA algorithms. Protocol 1 is restricted to using only RSA keys, but 405 protocol 2 may use either. The HISTORY section of ssl(8) contains a 406 brief discussion of the two algorithms. 407 408 The file ~/.ssh/authorized_keys lists the public keys that are permitted 409 for logging in. When the user logs in, the ssh program tells the server 410 which key pair it would like to use for authentication. The client 411 proves that it has access to the private key and the server checks that 412 the corresponding public key is authorized to accept the account. 413 414 The user creates his/her key pair by running ssh-keygen(1). This stores 415 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol 416 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in 417 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or 418 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The us- 419 er should then copy the public key to ~/.ssh/authorized_keys in his/her 420 home directory on the remote machine. The authorized_keys file corre- 421 sponds to the conventional ~/.rhosts file, and has one key per line, 422 though the lines can be very long. After this, the user can log in with- 423 out giving the password. 424 425 The most convenient way to use public key authentication may be with an 426 authentication agent. See ssh-agent(1) for more information. 427 428 Challenge-response authentication works as follows: The server sends an 429 arbitrary "challenge" text, and prompts for a response. Protocol 2 al- 430 lows multiple challenges and responses; protocol 1 is restricted to just 431 one challenge/response. Examples of challenge-response authentication 432 include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD 433 systems). 434 435 Finally, if other authentication methods fail, ssh prompts the user for a 436 password. The password is sent to the remote host for checking; however, 437 since all communications are encrypted, the password cannot be seen by 438 someone listening on the network. 439 440 ssh automatically maintains and checks a database containing identifica- 441 tion for all hosts it has ever been used with. Host keys are stored in 442 ~/.ssh/known_hosts in the user's home directory. Additionally, the file 443 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any 444 new hosts are automatically added to the user's file. If a host's iden- 445 tification ever changes, ssh warns about this and disables password au- 446 thentication to prevent server spoofing or man-in-the-middle attacks, 447 which could otherwise be used to circumvent the encryption. The 448 StrictHostKeyChecking option can be used to control logins to machines 449 whose host key is not known or has changed. 450 451 When the user's identity has been accepted by the server, the server ei- 452 ther executes the given command, or logs into the machine and gives the 453 user a normal shell on the remote machine. All communication with the 454 remote command or shell will be automatically encrypted. 455 456 If a pseudo-terminal has been allocated (normal login session), the user 457 may use the escape characters noted below. 458 459 If no pseudo-tty has been allocated, the session is transparent and can 460 be used to reliably transfer binary data. On most systems, setting the 461 escape character to ``none'' will also make the session transparent even 462 if a tty is used. 463 464 The session terminates when the command or shell on the remote machine 465 exits and all X11 and TCP connections have been closed. 466 467ESCAPE CHARACTERS 468 When a pseudo-terminal has been requested, ssh supports a number of func- 469 tions through the use of an escape character. 470 471 A single tilde character can be sent as ~~ or by following the tilde by a 472 character other than those described below. The escape character must 473 always follow a newline to be interpreted as special. The escape charac- 474 ter can be changed in configuration files using the EscapeChar configura- 475 tion directive or on the command line by the -e option. 476 477 The supported escapes (assuming the default `~') are: 478 479 ~. Disconnect. 480 481 ~^Z Background ssh. 482 483 ~# List forwarded connections. 484 485 ~& Background ssh at logout when waiting for forwarded connection / 486 X11 sessions to terminate. 487 488 ~? Display a list of escape characters. 489 490 ~B Send a BREAK to the remote system (only useful for SSH protocol 491 version 2 and if the peer supports it). 492 493 ~C Open command line. Currently this allows the addition of port 494 forwardings using the -L and -R options (see above). It also al- 495 lows the cancellation of existing remote port-forwardings using 496 -KR[bind_address:]port. !command allows the user to execute a 497 local command if the PermitLocalCommand option is enabled in 498 ssh_config(5). Basic help is available, using the -h option. 499 500 ~R Request rekeying of the connection (only useful for SSH protocol 501 version 2 and if the peer supports it). 502 503TCP FORWARDING 504 Forwarding of arbitrary TCP connections over the secure channel can be 505 specified either on the command line or in a configuration file. One 506 possible application of TCP forwarding is a secure connection to a mail 507 server; another is going through firewalls. 508 509 In the example below, we look at encrypting communication between an IRC 510 client and server, even though the IRC server does not directly support 511 encrypted communications. This works as follows: the user connects to 512 the remote host using ssh, specifying a port to be used to forward con- 513 nections to the remote server. After that it is possible to start the 514 service which is to be encrypted on the client machine, connecting to the 515 same local port, and ssh will encrypt and forward the connection. 516 517 The following example tunnels an IRC session from client machine 518 ``127.0.0.1'' (localhost) to remote server ``server.example.com'': 519 520 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 521 $ irc -c '#users' -p 1234 pinky 127.0.0.1 522 523 This tunnels a connection to IRC server ``server.example.com'', joining 524 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't mat- 525 ter which port is used, as long as it's greater than 1023 (remember, only 526 root can open sockets on privileged ports) and doesn't conflict with any 527 ports already in use. The connection is forwarded to port 6667 on the 528 remote server, since that's the standard port for IRC services. 529 530 The -f option backgrounds ssh and the remote command ``sleep 10'' is 531 specified to allow an amount of time (10 seconds, in the example) to 532 start the service which is to be tunnelled. If no connections are made 533 within the time specified, ssh will exit. 534 535X11 FORWARDING 536 If the ForwardX11 variable is set to ``yes'' (or see the description of 537 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY 538 environment variable is set), the connection to the X11 display is auto- 539 matically forwarded to the remote side in such a way that any X11 pro- 540 grams started from the shell (or command) will go through the encrypted 541 channel, and the connection to the real X server will be made from the 542 local machine. The user should not manually set DISPLAY. Forwarding of 543 X11 connections can be configured on the command line or in configuration 544 files. 545 546 The DISPLAY value set by ssh will point to the server machine, but with a 547 display number greater than zero. This is normal, and happens because 548 ssh creates a ``proxy'' X server on the server machine for forwarding the 549 connections over the encrypted channel. 550 551 ssh will also automatically set up Xauthority data on the server machine. 552 For this purpose, it will generate a random authorization cookie, store 553 it in Xauthority on the server, and verify that any forwarded connections 554 carry this cookie and replace it by the real cookie when the connection 555 is opened. The real authentication cookie is never sent to the server 556 machine (and no cookies are sent in the plain). 557 558 If the ForwardAgent variable is set to ``yes'' (or see the description of 559 the -A and -a options above) and the user is using an authentication 560 agent, the connection to the agent is automatically forwarded to the re- 561 mote side. 562 563VERIFYING HOST KEYS 564 When connecting to a server for the first time, a fingerprint of the 565 server's public key is presented to the user (unless the option 566 StrictHostKeyChecking has been disabled). Fingerprints can be determined 567 using ssh-keygen(1): 568 569 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key 570 571 If the fingerprint is already known, it can be matched and verified, and 572 the key can be accepted. If the fingerprint is unknown, an alternative 573 method of verification is available: SSH fingerprints verified by DNS. 574 An additional resource record (RR), SSHFP, is added to a zonefile and the 575 connecting client is able to match the fingerprint with that of the key 576 presented. 577 578 In this example, we are connecting a client to a server, 579 ``host.example.com''. The SSHFP resource records should first be added 580 to the zonefile for host.example.com: 581 582 $ ssh-keygen -r host.example.com. 583 584 The output lines will have to be added to the zonefile. To check that 585 the zone is answering fingerprint queries: 586 587 $ dig -t SSHFP host.example.com 588 589 Finally the client connects: 590 591 $ ssh -o "VerifyHostKeyDNS ask" host.example.com 592 [...] 593 Matching host key fingerprint found in DNS. 594 Are you sure you want to continue connecting (yes/no)? 595 596 See the VerifyHostKeyDNS option in ssh_config(5) for more information. 597 598SSH-BASED VIRTUAL PRIVATE NETWORKS 599 ssh contains support for Virtual Private Network (VPN) tunnelling using 600 the tun(4) network pseudo-device, allowing two networks to be joined se- 601 curely. The sshd_config(5) configuration option PermitTunnel controls 602 whether the server supports this, and at what level (layer 2 or 3 traf- 603 fic). 604 605 The following example would connect client network 10.0.50.0/24 with re- 606 mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 607 to 10.1.1.2, provided that the SSH server running on the gateway to the 608 remote network, at 192.168.1.15, allows it. 609 610 On the client: 611 612 # ssh -f -w 0:1 192.168.1.15 true 613 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 614 # route add 10.0.99.0/24 10.1.1.2 615 616 On the server: 617 618 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 619 # route add 10.0.50.0/24 10.1.1.1 620 621 Client access may be more finely tuned via the /root/.ssh/authorized_keys 622 file (see below) and the PermitRootLogin server option. The following 623 entry would permit connections on tun(4) device 1 from user ``jane'' and 624 on tun device 2 from user ``john'', if PermitRootLogin is set to 625 ``forced-commands-only'': 626 627 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane 628 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john 629 630 Since an SSH-based setup entails a fair amount of overhead, it may be 631 more suited to temporary setups, such as for wireless VPNs. More perma- 632 nent VPNs are better provided by tools such as ipsecctl(8) and 633 isakmpd(8). 634 635ENVIRONMENT 636 ssh will normally set the following environment variables: 637 638 DISPLAY The DISPLAY variable indicates the location of the 639 X11 server. It is automatically set by ssh to 640 point to a value of the form ``hostname:n'', where 641 ``hostname'' indicates the host where the shell 642 runs, and `n' is an integer >= 1. ssh uses this 643 special value to forward X11 connections over the 644 secure channel. The user should normally not set 645 DISPLAY explicitly, as that will render the X11 646 connection insecure (and will require the user to 647 manually copy any required authorization cookies). 648 649 HOME Set to the path of the user's home directory. 650 651 LOGNAME Synonym for USER; set for compatibility with sys- 652 tems that use this variable. 653 654 MAIL Set to the path of the user's mailbox. 655 656 PATH Set to the default PATH, as specified when compil- 657 ing ssh. 658 659 SSH_ASKPASS If ssh needs a passphrase, it will read the 660 passphrase from the current terminal if it was run 661 from a terminal. If ssh does not have a terminal 662 associated with it but DISPLAY and SSH_ASKPASS are 663 set, it will execute the program specified by 664 SSH_ASKPASS and open an X11 window to read the 665 passphrase. This is particularly useful when call- 666 ing ssh from a .xsession or related script. (Note 667 that on some machines it may be necessary to redi- 668 rect the input from /dev/null to make this work.) 669 670 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to 671 communicate with the agent. 672 673 SSH_CONNECTION Identifies the client and server ends of the con- 674 nection. The variable contains four space-separat- 675 ed values: client IP address, client port number, 676 server IP address, and server port number. 677 678 SSH_ORIGINAL_COMMAND This variable contains the original command line if 679 a forced command is executed. It can be used to 680 extract the original arguments. 681 682 SSH_TTY This is set to the name of the tty (path to the de- 683 vice) associated with the current shell or command. 684 If the current session has no tty, this variable is 685 not set. 686 687 TZ This variable is set to indicate the present time 688 zone if it was set when the daemon was started 689 (i.e. the daemon passes the value on to new connec- 690 tions). 691 692 USER Set to the name of the user logging in. 693 694 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format 695 ``VARNAME=value'' to the environment if the file exists and users are al- 696 lowed to change their environment. For more information, see the 697 PermitUserEnvironment option in sshd_config(5). 698 699FILES 700 ~/.rhosts 701 This file is used for host-based authentication (see above). On 702 some machines this file may need to be world-readable if the us- 703 er's home directory is on an NFS partition, because sshd(8) reads 704 it as root. Additionally, this file must be owned by the user, 705 and must not have write permissions for anyone else. The recom- 706 mended permission for most machines is read/write for the user, 707 and not accessible by others. 708 709 ~/.shosts 710 This file is used in exactly the same way as .rhosts, but allows 711 host-based authentication without permitting login with 712 rlogin/rsh. 713 714 ~/.ssh/authorized_keys 715 Lists the public keys (RSA/DSA) that can be used for logging in 716 as this user. The format of this file is described in the 717 sshd(8) manual page. This file is not highly sensitive, but the 718 recommended permissions are read/write for the user, and not ac- 719 cessible by others. 720 721 ~/.ssh/config 722 This is the per-user configuration file. The file format and 723 configuration options are described in ssh_config(5). Because of 724 the potential for abuse, this file must have strict permissions: 725 read/write for the user, and not accessible by others. 726 727 ~/.ssh/environment 728 Contains additional definitions for environment variables; see 729 ENVIRONMENT, above. 730 731 ~/.ssh/identity 732 ~/.ssh/id_dsa 733 ~/.ssh/id_rsa 734 Contains the private key for authentication. These files contain 735 sensitive data and should be readable by the user but not acces- 736 sible by others (read/write/execute). ssh will simply ignore a 737 private key file if it is accessible by others. It is possible 738 to specify a passphrase when generating the key which will be 739 used to encrypt the sensitive part of this file using 3DES. 740 741 ~/.ssh/identity.pub 742 ~/.ssh/id_dsa.pub 743 ~/.ssh/id_rsa.pub 744 Contains the public key for authentication. These files are not 745 sensitive and can (but need not) be readable by anyone. 746 747 ~/.ssh/known_hosts 748 Contains a list of host keys for all hosts the user has logged 749 into that are not already in the systemwide list of known host 750 keys. See sshd(8) for further details of the format of this 751 file. 752 753 ~/.ssh/rc 754 Commands in this file are executed by ssh when the user logs in, 755 just before the user's shell (or command) is started. See the 756 sshd(8) manual page for more information. 757 758 /etc/hosts.equiv 759 This file is for host-based authentication (see above). It 760 should only be writable by root. 761 762 /etc/shosts.equiv 763 This file is used in exactly the same way as hosts.equiv, but al- 764 lows host-based authentication without permitting login with 765 rlogin/rsh. 766 767 /etc/ssh/ssh_config 768 Systemwide configuration file. The file format and configuration 769 options are described in ssh_config(5). 770 771 /etc/ssh/ssh_host_key 772 /etc/ssh/ssh_host_dsa_key 773 /etc/ssh/ssh_host_rsa_key 774 These three files contain the private parts of the host keys and 775 are used for host-based authentication. If protocol version 1 is 776 used, ssh must be setuid root, since the host key is readable on- 777 ly by root. For protocol version 2, ssh uses ssh-keysign(8) to 778 access the host keys, eliminating the requirement that ssh be se- 779 tuid root when host-based authentication is used. By default ssh 780 is not setuid root. 781 782 /etc/ssh/ssh_known_hosts 783 Systemwide list of known host keys. This file should be prepared 784 by the system administrator to contain the public host keys of 785 all machines in the organization. It should be world-readable. 786 See sshd(8) for further details of the format of this file. 787 788 /etc/ssh/sshrc 789 Commands in this file are executed by ssh when the user logs in, 790 just before the user's shell (or command) is started. See the 791 sshd(8) manual page for more information. 792 793SEE ALSO 794 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), 795 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) 796 797 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006. 798 799 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006. 800 801 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006. 802 803 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006. 804 805 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006. 806 807 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 808 4255, 2006. 809 810 Generic Message Exchange Authentication for the Secure Shell Protocol 811 (SSH), RFC 4256, 2006. 812 813 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006. 814 815 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006. 816 817 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer 818 Protocol, RFC 4345, 2006. 819 820 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer 821 Protocol, RFC 4419, 2006. 822 823 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006. 824 825AUTHORS 826 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 827 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 828 de Raadt and Dug Song removed many bugs, re-added newer features and 829 created OpenSSH. Markus Friedl contributed the support for SSH protocol 830 versions 1.5 and 2.0. 831 832OpenBSD 4.1 September 25, 1999 13 833