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Linux命令:ssh

ssh介绍

ssh用法

ssh帮助

1 SSH(1) BSD General Commands Manual SSH(1)
2
3 NAME
4 ssh — OpenSSH SSH client (remote login program)
5
6 SYNOPSIS
7 ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
8 [-J [user@]host[:port]] [-L address] [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address] [-S ctl_path] [-W host:port]
9 [-w local_tun[:remote_tun]] [user@]hostname [command]
10
11 DESCRIPTION
12 ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. It is intended to provide secure encrypted communications between
13 two untrusted hosts over an insecure network. X11 connections, arbitrary TCP ports and UNIX-domain sockets can also be forwarded over the secure channel.
14
15 ssh connects and logs into the specified hostname (with optional user name). The user must prove his/her identity to the remote machine using one of several methods (see below).
16
17 If command is specified, it is executed on the remote host instead of a login shell.
18
19 The options are as follows:
20
21 -1 Forces ssh to try protocol version 1 only.
22
23 -2 Forces ssh to try protocol version 2 only.
24
25 -4 Forces ssh to use IPv4 addresses only.
26
27 -6 Forces ssh to use IPv6 addresses only.
28
29 -A Enables forwarding of the authentication agent connection. This can also be specified on a per-host basis in a configuration file.
30
31 Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's UNIX-domain socket) can access the
32 local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to
33 authenticate using the identities loaded into the agent.
34
35 -a Disables forwarding of the authentication agent connection.
36
37 -b bind_address
38 Use bind_address on the local machine as the source address of the connection. Only useful on systems with more than one address.
39
40 -C Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11, TCP and UNIX-domain connections). The compression algorithm is the same
41 used by gzip(1), and the “level” can be controlled by the CompressionLevel option for protocol version 1. Compression is desirable on modem lines and other slow connec‐
42 tions, but will only slow down things on fast networks. The default value can be set on a host-by-host basis in the configuration files; see the Compression option.
43
44 -c cipher_spec
45 Selects the cipher specification for encrypting the session.
46
47 Protocol version 1 allows specification of a single cipher. The supported values are “3des”, “blowfish”, and “des”. For protocol version 2, cipher_spec is a comma-sepa‐
48 rated list of ciphers listed in order of preference. See the Ciphers keyword in ssh_config(5) for more information.
49
50 -D [bind_address:]port
51 Specifies a local “dynamic” application-level port forwarding. This works by allocating a socket to listen to port on the local side, optionally bound to the specified
52 bind_address. Whenever a connection is made to this port, the connection is forwarded over the secure channel, and the application protocol is then used to determine
53 where to connect to from the remote machine. Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as a SOCKS server. Only root can forward privi‐
54 leged ports. Dynamic port forwardings can also be specified in the configuration file.
55
56 IPv6 addresses can be specified by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in
57 accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of “localhost”
58 indicates that the listening port be bound for local use only, while an empty address or ‘*’ indicates that the port should be available from all interfaces.
59
60 -E log_file
61 Append debug logs to log_file instead of standard error.
62
63 -e escape_char
64 Sets the escape character for sessions with a pty (default: ‘~’). The escape character is only recognized at the beginning of a line. The escape character followed by a
65 dot (‘.’) closes the connection; followed by control-Z suspends the connection; and followed by itself sends the escape character once. Setting the character to “none”
66 disables any escapes and makes the session fully transparent.
67
68 -F configfile
69 Specifies an alternative per-user configuration file. If a configuration file is given on the command line, the system-wide configuration file (/etc/ssh/ssh_config) will
70 be ignored. The default for the per-user configuration file is ~/.ssh/config.
71
72 -f Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the back‐
73 ground. This implies -n. The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm.
74
75 If the ExitOnForwardFailure configuration option is set to “yes”, then a client started with -f will wait for all remote port forwards to be successfully established
76 before placing itself in the background.
77
78 -G Causes ssh to print its configuration after evaluating Host and Match blocks and exit.
79
80 -g Allows remote hosts to connect to local forwarded ports. If used on a multiplexed connection, then this option must be specified on the master process.
81
82 -I pkcs11
83 Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing the user's private RSA key.
84
85 -i identity_file
86 Selects a file from which the identity (private key) for public key authentication is read. The default is ~/.ssh/identity for protocol version 1, and ~/.ssh/id_dsa,
87 ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2. Identity files may also be specified on a per-host basis in the configuration file. It is
88 possible to have multiple -i options (and multiple identities specified in configuration files). If no certificates have been explicitly specified by the CertificateFile
89 directive, ssh will also try to load certificate information from the filename obtained by appending -cert.pub to identity filenames.
90
91 -J [user@]host[:port]
92 Connect to the target host by first making a ssh connection to the jump host and then establishing a TCP forwarding to the ultimate destination from there. Multiple jump
93 hops may be specified separated by comma characters. This is a shortcut to specify a ProxyJump configuration directive.
94
95 -K Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to the server.
96
97 -k Disables forwarding (delegation) of GSSAPI credentials to the server.
98
99 -L [bind_address:]port:host:hostport
100 -L [bind_address:]port:remote_socket
101 -L local_socket:host:hostport
102 -L local_socket:remote_socket
103 Specifies that connections to the given TCP port or Unix socket on the local (client) host are to be forwarded to the given host and port, or Unix socket, on the remote
104 side. This works by allocating a socket to listen to either a TCP port on the local side, optionally bound to the specified bind_address, or to a Unix socket. Whenever
105 a connection is made to the local port or socket, the connection is forwarded over the secure channel, and a connection is made to either host port hostport, or the Unix
106 socket remote_socket, from the remote machine.
107
108 Port forwardings can also be specified in the configuration file. Only the superuser can forward privileged ports. IPv6 addresses can be specified by enclosing the
109 address in square brackets.
110
111 By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific
112 address. The bind_address of “localhost” indicates that the listening port be bound for local use only, while an empty address or ‘*’ indicates that the port should be
113 available from all interfaces.
114
115 -l login_name
116 Specifies the user to log in as on the remote machine. This also may be specified on a per-host basis in the configuration file.
117
118 -M Places the ssh client into “master” mode for connection sharing. Multiple -M options places ssh into “master” mode with confirmation required before slave connections
119 are accepted. Refer to the description of ControlMaster in ssh_config(5) for details.
120
121 -m mac_spec
122 A comma-separated list of MAC (message authentication code) algorithms, specified in order of preference. See the MACs keyword for more information.
123
124 -N Do not execute a remote command. This is useful for just forwarding ports.
125
126 -n Redirects stdin from /dev/null (actually, prevents reading from stdin). This must be used when ssh is run in the background. A common trick is to use this to run X11
127 programs on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs & will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically for‐
128 warded over an encrypted channel. The ssh program will be put in the background. (This does not work if ssh needs to ask for a password or passphrase; see also the -f
129 option.)
130
131 -O ctl_cmd
132 Control an active connection multiplexing master process. When the -O option is specified, the ctl_cmd argument is interpreted and passed to the master process. Valid
133 commands are: “check” (check that the master process is running), “forward” (request forwardings without command execution), “cancel” (cancel forwardings), “exit”
134 (request the master to exit), and “stop” (request the master to stop accepting further multiplexing requests).
135
136 -o option
137 Can be used to give options in the format used in the configuration file. This is useful for specifying options for which there is no separate command-line flag. For
138 full details of the options listed below, and their possible values, see ssh_config(5).
139
140 AddKeysToAgent
141 AddressFamily
142 BatchMode
143 BindAddress
144 CanonicalDomains
145 CanonicalizeFallbackLocal
146 CanonicalizeHostname
147 CanonicalizeMaxDots
148 CanonicalizePermittedCNAMEs
149 CertificateFile
150 ChallengeResponseAuthentication
151 CheckHostIP
152 Cipher
153 Ciphers
154 ClearAllForwardings
155 Compression
156 CompressionLevel
157 ConnectionAttempts
158 ConnectTimeout
159 ControlMaster
160 ControlPath
161 ControlPersist
162 DynamicForward
163 EscapeChar
164 ExitOnForwardFailure
165 FingerprintHash
166 ForwardAgent
167 ForwardX11
168 ForwardX11Timeout
169 ForwardX11Trusted
170 GatewayPorts
171 GlobalKnownHostsFile
172 GSSAPIAuthentication
173 GSSAPIKeyExchange
174 GSSAPIClientIdentity
175 GSSAPIDelegateCredentials
176 GSSAPIRenewalForcesRekey
177 GSSAPITrustDNS
178 GSSAPIKexAlgorithms
179 HashKnownHosts
180 Host
181 HostbasedAuthentication
182 HostbasedKeyTypes
183 HostKeyAlgorithms
184 HostKeyAlias
185 HostName
186 IdentitiesOnly
187 IdentityAgent
188 IdentityFile
189 Include
190 IPQoS
191 KbdInteractiveAuthentication
192 KbdInteractiveDevices
193 KexAlgorithms
194 LocalCommand
195 LocalForward
196 LogLevel
197 MACs
198 Match
199 NoHostAuthenticationForLocalhost
200 NumberOfPasswordPrompts
201 PasswordAuthentication
202 PermitLocalCommand
203 PKCS11Provider
204 Port
205 PreferredAuthentications
206 Protocol
207 ProxyCommand
208 ProxyJump
209 ProxyUseFdpass
210 PubkeyAcceptedKeyTypes
211 PubkeyAuthentication
212 RekeyLimit
213 RemoteForward
214 RequestTTY
215 RhostsRSAAuthentication
216 RSAAuthentication
217 SendEnv
218 ServerAliveInterval
219 ServerAliveCountMax
220 StreamLocalBindMask
221 StreamLocalBindUnlink
222 StrictHostKeyChecking
223 TCPKeepAlive
224 Tunnel
225 TunnelDevice
226 UpdateHostKeys
227 UsePrivilegedPort
228 User
229 UserKnownHostsFile
230 VerifyHostKeyDNS
231 VisualHostKey
232 XAuthLocation
233
234 -p port
235 Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file.
236
237 -Q query_option
238 Queries ssh for the algorithms supported for the specified version 2. The available features are: cipher (supported symmetric ciphers), cipher-auth (supported symmetric
239 ciphers that support authenticated encryption), mac (supported message integrity codes), kex (key exchange algorithms), key (key types), key-cert (certificate key types),
240 key-plain (non-certificate key types), and protocol-version (supported SSH protocol versions).
241
242 -q Quiet mode. Causes most warning and diagnostic messages to be suppressed.
243
244 -R [bind_address:]port:host:hostport
245 -R [bind_address:]port:local_socket
246 -R remote_socket:host:hostport
247 -R remote_socket:local_socket
248 Specifies that connections to the given TCP port or Unix socket on the remote (server) host are to be forwarded to the given host and port, or Unix socket, on the local
249 side. This works by allocating a socket to listen to either a TCP port or to a Unix socket on the remote side. Whenever a connection is made to this port or Unix
250 socket, the connection is forwarded over the secure channel, and a connection is made to either host port hostport, or local_socket, from the local machine.
251
252 Port forwardings can also be specified in the configuration file. Privileged ports can be forwarded only when logging in as root on the remote machine. IPv6 addresses
253 can be specified by enclosing the address in square brackets.
254
255 By default, TCP listening sockets on the server will be bound to the loopback interface only. This may be overridden by specifying a bind_address. An empty
256 bind_address, or the address ‘*’, indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the server's
257 GatewayPorts option is enabled (see sshd_config(5)).
258
259 If the port argument is ‘0’, the listen port will be dynamically allocated on the server and reported to the client at run time. When used together with -O forward the
260 allocated port will be printed to the standard output.
261
262 -S ctl_path
263 Specifies the location of a control socket for connection sharing, or the string “none” to disable connection sharing. Refer to the description of ControlPath and
264 ControlMaster in ssh_config(5) for details.
265
266 -s May be used to request invocation of a subsystem on the remote system. Subsystems facilitate the use of SSH as a secure transport for other applications (e.g. sftp(1)).
267 The subsystem is specified as the remote command.
268
269 -T Disable pseudo-terminal allocation.
270
271 -t Force pseudo-terminal allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu
272 services. Multiple -t options force tty allocation, even if ssh has no local tty.
273
274 -V Display the version number and exit.
275
276 -v Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple
277 -v options increase the verbosity. The maximum is 3.
278
279 -W host:port
280 Requests that standard input and output on the client be forwarded to host on port over the secure channel. Implies -N, -T, ExitOnForwardFailure and ClearAllForwardings,
281 though these can be overridden in the configuration file or using -o command line options.
282
283 -w local_tun[:remote_tun]
284 Requests tunnel device forwarding with the specified tun(4) devices between the client (local_tun) and the server (remote_tun).
285
286 The devices may be specified by numerical ID or the keyword “any”, which uses the next available tunnel device. If remote_tun is not specified, it defaults to “any”.
287 See also the Tunnel and TunnelDevice directives in ssh_config(5). If the Tunnel directive is unset, it is set to the default tunnel mode, which is “point-to-point”.
288
289 -X Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.
290
291 X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X authorization database) can access
292 the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring.
293
294 For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by default. Please refer to the ssh -Y option and the ForwardX11Trusted directive in
295 ssh_config(5) for more information.
296
297 -x Disables X11 forwarding.
298
299 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls.
300
301 -y Send log information using the syslog(3) system module. By default this information is sent to stderr.
302
303 ssh may additionally obtain configuration data from a per-user configuration file and a system-wide configuration file. The file format and configuration options are described
304 in ssh_config(5).
305
306 AUTHENTICATION
307 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to use protocol 2 only, though this can be changed via the Protocol option in ssh_config(5) or the -1 and
308 -2 options (see above). Protocol 1 should not be used and is only offered to support legacy devices. It suffers from a number of cryptographic weaknesses and doesn't support
309 many of the advanced features available for protocol 2.
310
311 The methods available for authentication are: GSSAPI-based authentication, host-based authentication, public key authentication, challenge-response authentication, and password
312 authentication. Authentication methods are tried in the order specified above, though PreferredAuthentications can be used to change the default order.
313
314 Host-based authentication works as follows: If the machine the user logs in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the remote machine, and the user names
315 are the same on both sides, or if the files ~/.rhosts or ~/.shosts exist in the user's home directory on the remote machine and contain a line containing the name of the client
316 machine and the name of the user on that machine, the user is considered for login. Additionally, the server must be able to verify the client's host key (see the description of
317 /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be permitted. This authentication method closes security holes due to IP spoofing, DNS spoofing, and routing
318 spoofing. [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is
319 desired.]
320
321 Public key authentication works as follows: The scheme is based on public-key cryptography, using cryptosystems where encryption and decryption are done using separate keys, and
322 it is unfeasible to derive the decryption key from the encryption key. The idea is that each user creates a public/private key pair for authentication purposes. The server
323 knows the public key, and only the user knows the private key. ssh implements public key authentication protocol automatically, using one of the DSA, ECDSA, Ed25519 or RSA algo‐
324 rithms. The HISTORY section of ssl(8) contains a brief discussion of the DSA and RSA algorithms.
325
326 The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the ssh program tells the server which key pair it would like to
327 use for authentication. The client proves that it has access to the private key and the server checks that the corresponding public key is authorized to accept the account.
328
329 The user creates his/her key pair by running ssh-keygen(1). This stores the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
330 ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
331 ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's home directory. The user should then copy the public key to ~/.ssh/authorized_keys in his/her home
332 directory on the remote machine. The authorized_keys file corresponds to the conventional ~/.rhosts file, and has one key per line, though the lines can be very long. After
333 this, the user can log in without giving the password.
334
335 A variation on public key authentication is available in the form of certificate authentication: instead of a set of public/private keys, signed certificates are used. This has
336 the advantage that a single trusted certification authority can be used in place of many public/private keys. See the CERTIFICATES section of ssh-keygen(1) for more information.
337
338 The most convenient way to use public key or certificate authentication may be with an authentication agent. See ssh-agent(1) and (optionally) the AddKeysToAgent directive in
339 ssh_config(5) for more information.
340
341 Challenge-response authentication works as follows: The server sends an arbitrary "challenge" text, and prompts for a response. Examples of challenge-response authentication
342 include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD systems).
343
344 Finally, if other authentication methods fail, ssh prompts the user for a password. The password is sent to the remote host for checking; however, since all communications are
345 encrypted, the password cannot be seen by someone listening on the network.
346
347 ssh automatically maintains and checks a database containing identification for all hosts it has ever been used with. Host keys are stored in ~/.ssh/known_hosts in the user's
348 home directory. Additionally, the file /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any new hosts are automatically added to the user's file. If a host's
349 identification ever changes, ssh warns about this and disables password authentication to prevent server spoofing or man-in-the-middle attacks, which could otherwise be used to
350 circumvent the encryption. The StrictHostKeyChecking option can be used to control logins to machines whose host key is not known or has changed.
351
352 When the user's identity has been accepted by the server, the server either executes the given command in a non-interactive session or, if no command has been specified, logs
353 into the machine and gives the user a normal shell as an interactive session. All communication with the remote command or shell will be automatically encrypted.
354
355 If an interactive session is requested ssh by default will only request a pseudo-terminal (pty) for interactive sessions when the client has one. The flags -T and -t can be used
356 to override this behaviour.
357
358 If a pseudo-terminal has been allocated the user may use the escape characters noted below.
359
360 If no pseudo-terminal has been allocated, the session is transparent and can be used to reliably transfer binary data. On most systems, setting the escape character to “none”
361 will also make the session transparent even if a tty is used.
362
363 The session terminates when the command or shell on the remote machine exits and all X11 and TCP connections have been closed.
364
365 ESCAPE CHARACTERS
366 When a pseudo-terminal has been requested, ssh supports a number of functions through the use of an escape character.
367
368 A single tilde character can be sent as ~~ or by following the tilde by a character other than those described below. The escape character must always follow a newline to be
369 interpreted as special. The escape character can be changed in configuration files using the EscapeChar configuration directive or on the command line by the -e option.
370
371 The supported escapes (assuming the default ‘~’) are:
372
373 ~. Disconnect.
374
375 ~^Z Background ssh.
376
377 ~# List forwarded connections.
378
379 ~& Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate.
380
381 ~? Display a list of escape characters.
382
383 ~B Send a BREAK to the remote system (only useful if the peer supports it).
384
385 ~C Open command line. Currently this allows the addition of port forwardings using the -L, -R and -D options (see above). It also allows the cancellation of existing port-
386 forwardings with -KL[bind_address:]port for local, -KR[bind_address:]port for remote and -KD[bind_address:]port for dynamic port-forwardings. !command allows the user to
387 execute a local command if the PermitLocalCommand option is enabled in ssh_config(5). Basic help is available, using the -h option.
388
389 ~R Request rekeying of the connection (only useful if the peer supports it).
390
391 ~V Decrease the verbosity (LogLevel) when errors are being written to stderr.
392
393 ~v Increase the verbosity (LogLevel) when errors are being written to stderr.
394
395 TCP FORWARDING
396 Forwarding of arbitrary TCP connections over the secure channel can be specified either on the command line or in a configuration file. One possible application of TCP forward‐
397 ing is a secure connection to a mail server; another is going through firewalls.
398
399 In the example below, we look at encrypting communication between an IRC client and server, even though the IRC server does not directly support encrypted communications. This
400 works as follows: the user connects to the remote host using ssh, specifying a port to be used to forward connections to the remote server. After that it is possible to start
401 the service which is to be encrypted on the client machine, connecting to the same local port, and ssh will encrypt and forward the connection.
402
403 The following example tunnels an IRC session from client machine “127.0.0.1” (localhost) to remote server “server.example.com”:
404
405 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
406 $ irc -c '#users' -p 1234 pinky 127.0.0.1
407
408 This tunnels a connection to IRC server “server.example.com”, joining channel “#users”, nickname “pinky”, using port 1234. It doesn't matter which port is used, as long as it's
409 greater than 1023 (remember, only root can open sockets on privileged ports) and doesn't conflict with any ports already in use. The connection is forwarded to port 6667 on the
410 remote server, since that's the standard port for IRC services.
411
412 The -f option backgrounds ssh and the remote command “sleep 10” is specified to allow an amount of time (10 seconds, in the example) to start the service which is to be tun‐
413 nelled. If no connections are made within the time specified, ssh will exit.
414
415 X11 FORWARDING
416 If the ForwardX11 variable is set to “yes” (or see the description of the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY environment variable is set), the
417 connection to the X11 display is automatically forwarded to the remote side in such a way that any X11 programs started from the shell (or command) will go through the encrypted
418 channel, and the connection to the real X server will be made from the local machine. The user should not manually set DISPLAY. Forwarding of X11 connections can be configured
419 on the command line or in configuration files.
420
421 The DISPLAY value set by ssh will point to the server machine, but with a display number greater than zero. This is normal, and happens because ssh creates a “proxy” X server on
422 the server machine for forwarding the connections over the encrypted channel.
423
424 ssh will also automatically set up Xauthority data on the server machine. For this purpose, it will generate a random authorization cookie, store it in Xauthority on the server,
425 and verify that any forwarded connections carry this cookie and replace it by the real cookie when the connection is opened. The real authentication cookie is never sent to the
426 server machine (and no cookies are sent in the plain).
427
428 If the ForwardAgent variable is set to “yes” (or see the description of the -A and -a options above) and the user is using an authentication agent, the connection to the agent is
429 automatically forwarded to the remote side.
430
431 VERIFYING HOST KEYS
432 When connecting to a server for the first time, a fingerprint of the server's public key is presented to the user (unless the option StrictHostKeyChecking has been disabled).
433 Fingerprints can be determined using ssh-keygen(1):
434
435 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
436
437 If the fingerprint is already known, it can be matched and the key can be accepted or rejected. If only legacy (MD5) fingerprints for the server are available, the ssh-keygen(1)
438 -E option may be used to downgrade the fingerprint algorithm to match.
439
440 Because of the difficulty of comparing host keys just by looking at fingerprint strings, there is also support to compare host keys visually, using random art. By setting the
441 VisualHostKey option to “yes”, a small ASCII graphic gets displayed on every login to a server, no matter if the session itself is interactive or not. By learning the pattern a
442 known server produces, a user can easily find out that the host key has changed when a completely different pattern is displayed. Because these patterns are not unambiguous how‐
443 ever, a pattern that looks similar to the pattern remembered only gives a good probability that the host key is the same, not guaranteed proof.
444
445 To get a listing of the fingerprints along with their random art for all known hosts, the following command line can be used:
446
447 $ ssh-keygen -lv -f ~/.ssh/known_hosts
448
449 If the fingerprint is unknown, an alternative method of verification is available: SSH fingerprints verified by DNS. An additional resource record (RR), SSHFP, is added to a
450 zonefile and the connecting client is able to match the fingerprint with that of the key presented.
451
452 In this example, we are connecting a client to a server, “host.example.com”. The SSHFP resource records should first be added to the zonefile for host.example.com:
453
454 $ ssh-keygen -r host.example.com.
455
456 The output lines will have to be added to the zonefile. To check that the zone is answering fingerprint queries:
457
458 $ dig -t SSHFP host.example.com
459
460 Finally the client connects:
461
462 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
463 [...]
464 Matching host key fingerprint found in DNS.
465 Are you sure you want to continue connecting (yes/no)?
466
467 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
468
469 SSH-BASED VIRTUAL PRIVATE NETWORKS
470 ssh contains support for Virtual Private Network (VPN) tunnelling using the tun(4) network pseudo-device, allowing two networks to be joined securely. The sshd_config(5) config‐
471 uration option PermitTunnel controls whether the server supports this, and at what level (layer 2 or 3 traffic).
472
473 The following example would connect client network 10.0.50.0/24 with remote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1 to 10.1.1.2, provided that the
474 SSH server running on the gateway to the remote network, at 192.168.1.15, allows it.
475
476 On the client:
477
478 # ssh -f -w 0:1 192.168.1.15 true
479 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
480 # route add 10.0.99.0/24 10.1.1.2
481
482 On the server:
483
484 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
485 # route add 10.0.50.0/24 10.1.1.1
486
487 Client access may be more finely tuned via the /root/.ssh/authorized_keys file (see below) and the PermitRootLogin server option. The following entry would permit connections on
488 tun(4) device 1 from user “jane” and on tun device 2 from user “john”, if PermitRootLogin is set to “forced-commands-only”:
489
490 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
491 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
492
493 Since an SSH-based setup entails a fair amount of overhead, it may be more suited to temporary setups, such as for wireless VPNs. More permanent VPNs are better provided by
494 tools such as ipsecctl(8) and isakmpd(8).
495
496 ENVIRONMENT
497 ssh will normally set the following environment variables:
498
499 DISPLAY The DISPLAY variable indicates the location of the X11 server. It is automatically set by ssh to point to a value of the form “hostname:n”, where
500 “hostname” indicates the host where the shell runs, and ‘n’ is an integer ≥ 1. ssh uses this special value to forward X11 connections over the secure chan‐
501 nel. The user should normally not set DISPLAY explicitly, as that will render the X11 connection insecure (and will require the user to manually copy any
502 required authorization cookies).
503
504 HOME Set to the path of the user's home directory.
505
506 LOGNAME Synonym for USER; set for compatibility with systems that use this variable.
507
508 MAIL Set to the path of the user's mailbox.
509
510 PATH Set to the default PATH, as specified when compiling ssh.
511
512 SSH_ASKPASS If ssh needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If ssh does not have a terminal associated
513 with it but DISPLAY and SSH_ASKPASS are set, it will execute the program specified by SSH_ASKPASS and open an X11 window to read the passphrase. This is
514 particularly useful when calling ssh from a .xsession or related script. (Note that on some machines it may be necessary to redirect the input from
515 /dev/null to make this work.)
516
517 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to communicate with the agent.
518
519 SSH_CONNECTION Identifies the client and server ends of the connection. The variable contains four space-separated values: client IP address, client port number, server
520 IP address, and server port number.
521
522 SSH_ORIGINAL_COMMAND This variable contains the original command line if a forced command is executed. It can be used to extract the original arguments.
523
524 SSH_USER_AUTH This variable contains, for SSH2 only, a comma-separated list of authentication methods that were successfuly used to authenticate. When possible, these
525 methods are extended with detailed information on the credential used.
526
527 SSH_TTY This is set to the name of the tty (path to the device) associated with the current shell or command. If the current session has no tty, this variable is
528 not set.
529
530 TZ This variable is set to indicate the present time zone if it was set when the daemon was started (i.e. the daemon passes the value on to new connections).
531
532 USER Set to the name of the user logging in.
533
534 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format “VARNAME=value” to the environment if the file exists and users are allowed to change their environment.
535 For more information, see the PermitUserEnvironment option in sshd_config(5).
536
537 ENVIRONMENT
538 SSH_USE_STRONG_RNG
539 The reseeding of the OpenSSL random generator is usually done from /dev/urandom. If the SSH_USE_STRONG_RNG environment variable is set to value other than 0 the OpenSSL
540 random generator is reseeded from /dev/random. The number of bytes read is defined by the SSH_USE_STRONG_RNG value. Minimum is 14 bytes. This setting is not recom‐
541 mended on the computers without the hardware random generator because insufficient entropy causes the connection to be blocked until enough entropy is available.
542
543 FILES
544 ~/.rhosts
545 This file is used for host-based authentication (see above). On some machines this file may need to be world-readable if the user's home directory is on an NFS parti‐
546 tion, because sshd(8) reads it as root. Additionally, this file must be owned by the user, and must not have write permissions for anyone else. The recommended permis‐
547 sion for most machines is read/write for the user, and not accessible by others.
548
549 ~/.shosts
550 This file is used in exactly the same way as .rhosts, but allows host-based authentication without permitting login with rlogin/rsh.
551
552 ~/.ssh/
553 This directory is the default location for all user-specific configuration and authentication information. There is no general requirement to keep the entire contents of
554 this directory secret, but the recommended permissions are read/write/execute for the user, and not accessible by others.
555
556 ~/.ssh/authorized_keys
557 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used for logging in as this user. The format of this file is described in the sshd(8) manual page. This
558 file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others.
559
560 ~/.ssh/config
561 This is the per-user configuration file. The file format and configuration options are described in ssh_config(5). Because of the potential for abuse, this file must
562 have strict permissions: read/write for the user, and not writable by others.
563
564 ~/.ssh/environment
565 Contains additional definitions for environment variables; see ENVIRONMENT, above.
566
567 ~/.ssh/identity
568 ~/.ssh/id_dsa
569 ~/.ssh/id_ecdsa
570 ~/.ssh/id_ed25519
571 ~/.ssh/id_rsa
572 Contains the private key for authentication. These files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute).
573 ssh will simply ignore a private key file if it is accessible by others. It is possible to specify a passphrase when generating the key which will be used to encrypt the
574 sensitive part of this file using 3DES.
575
576 ~/.ssh/identity.pub
577 ~/.ssh/id_dsa.pub
578 ~/.ssh/id_ecdsa.pub
579 ~/.ssh/id_ed25519.pub
580 ~/.ssh/id_rsa.pub
581 Contains the public key for authentication. These files are not sensitive and can (but need not) be readable by anyone.
582
583 ~/.ssh/known_hosts
584 Contains a list of host keys for all hosts the user has logged into that are not already in the systemwide list of known host keys. See sshd(8) for further details of
585 the format of this file.
586
587 ~/.ssh/rc
588 Commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started. See the sshd(8) manual page for more information.
589
590 /etc/hosts.equiv
591 This file is for host-based authentication (see above). It should only be writable by root.
592
593 /etc/ssh/shosts.equiv
594 This file is used in exactly the same way as hosts.equiv, but allows host-based authentication without permitting login with rlogin/rsh.
595
596 /etc/ssh/ssh_config
597 Systemwide configuration file. The file format and configuration options are described in ssh_config(5).
598
599 /etc/ssh/ssh_host_key
600 /etc/ssh/ssh_host_dsa_key
601 /etc/ssh/ssh_host_ecdsa_key
602 /etc/ssh/ssh_host_ed25519_key
603 /etc/ssh/ssh_host_rsa_key
604 These files contain the private parts of the host keys and are used for host-based authentication.
605
606 /etc/ssh/ssh_known_hosts
607 Systemwide list of known host keys. This file should be prepared by the system administrator to contain the public host keys of all machines in the organization. It
608 should be world-readable. See sshd(8) for further details of the format of this file.
609
610 /etc/ssh/sshrc
611 Commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started. See the sshd(8) manual page for more information.
612
613 EXIT STATUS
614 ssh exits with the exit status of the remote command or with 255 if an error occurred.
615
616 IPV6
617 IPv6 address can be used everywhere where IPv4 address. In all entries must be the IPv6 address enclosed in square brackets. Note: The square brackets are metacharacters for the
618 shell and must be escaped in shell.
619
620 SEE ALSO
621 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
622
623 STANDARDS
624 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, January 2006.
625
626 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture, RFC 4251, January 2006.
627
628 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol, RFC 4252, January 2006.
629
630 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, January 2006.
631
632 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC 4254, January 2006.
633
634 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC 4255, January 2006.
635
636 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the Secure Shell Protocol (SSH), RFC 4256, January 2006.
637
638 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, January 2006.
639
640 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, January 2006.
641
642 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol, RFC 4345, January 2006.
643
644 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
645
646 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File Format, RFC 4716, November 2006.
647
648 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, RFC 5656, December 2009.
649
650 A. Perrig and D. Song, Hash Visualization: a New Technique to improve Real-World Security, 1999, International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99).
651
652 AUTHORS
653 OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed
654 many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.
655
656 BSD July 16, 2016 BSD

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