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1SSH(1) BSD General Commands Manual SSH(1) 1SSH(1) OpenBSD Reference Manual SSH(1)
2 2
3NAME 3NAME
4 ssh - OpenSSH SSH client (remote login program) 4 ssh - OpenSSH SSH client (remote login program)
5 5
6SYNOPSIS 6SYNOPSIS
7 ssh [-l login_name] hostname | user@hostname [command] 7 ssh [-1246AaCfgkNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] [-D port]
8 8 [-e escape_char] [-F configfile] [-i identity_file]
9 ssh [-afgknqstvxACNTVX1246] [-b bind_address] [-c cipher_spec] 9 [-L port:host:hostport] [-l login_name] [-m mac_spec] [-o option]
10 [-e escape_char] [-i identity_file] [-l login_name] [-m mac_spec] 10 [-p port] [-R port:host:hostport] [user@]hostname [command]
11 [-o option] [-p port] [-F configfile] [-L port:host:hostport]
12 [-R port:host:hostport] [-D port] hostname | user@hostname [command]
13 11
14DESCRIPTION 12DESCRIPTION
15 ssh (SSH client) is a program for logging into a remote machine and for 13 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 14 executing commands on a remote machine. It is intended to replace rlogin
17 and rsh, and provide secure encrypted communications between two 15 and rsh, and provide secure encrypted communications between two untrust-
18 untrusted hosts over an insecure network. X11 connections and arbitrary 16 ed hosts over an insecure network. X11 connections and arbitrary TCP/IP
19 TCP/IP ports can also be forwarded over the secure channel. 17 ports can also be forwarded over the secure channel.
18
19 ssh connects and logs into the specified hostname (with optional user
20 name). The user must prove his/her identity to the remote machine using
21 one of several methods depending on the protocol version used.
20 22
21 ssh connects and logs into the specified hostname. The user must prove 23 If command is specified, command is executed on the remote host instead
22 his/her identity to the remote machine using one of several methods 24 of a login shell.
23 depending on the protocol version used:
24 25
25 SSH protocol version 1 26 SSH protocol version 1
26 First, if the machine the user logs in from is listed in /etc/hosts.equiv 27 First, if the machine the user logs in from is listed in /etc/hosts.equiv
27 or /etc/shosts.equiv on the remote machine, and the user names are the 28 or /etc/shosts.equiv on the remote machine, and the user names are the
28 same on both sides, the user is immediately permitted to log in. Second, 29 same on both sides, the user is immediately permitted to log in. Second,
29 if .rhosts or .shosts exists in the userM-bM-^@M-^Ys home directory on the remote 30 if .rhosts or .shosts exists in the user's home directory on the remote
30 machine and contains a line containing the name of the client machine and 31 machine and contains a line containing the name of the client machine and
31 the name of the user on that machine, the user is permitted to log in. 32 the name of the user on that machine, the user is permitted to log in.
32 This form of authentication alone is normally not allowed by the server 33 This form of authentication alone is normally not allowed by the server
@@ -35,9 +36,9 @@ DESCRIPTION
35 The second authentication method is the rhosts or hosts.equiv method com- 36 The second authentication method is the rhosts or hosts.equiv method com-
36 bined with RSA-based host authentication. It means that if the login 37 bined with RSA-based host authentication. It means that if the login
37 would be permitted by $HOME/.rhosts, $HOME/.shosts, /etc/hosts.equiv, or 38 would be permitted by $HOME/.rhosts, $HOME/.shosts, /etc/hosts.equiv, or
38 /etc/shosts.equiv, and if additionally the server can verify the clientM-bM-^@M-^Ys 39 /etc/shosts.equiv, and if additionally the server can verify the client's
39 host key (see /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts in the 40 host key (see /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts in the
40 FILES section), only then login is permitted. This authentication method 41 FILES section), only then is login permitted. This authentication method
41 closes security holes due to IP spoofing, DNS spoofing and routing spoof- 42 closes security holes due to IP spoofing, DNS spoofing and routing spoof-
42 ing. [Note to the administrator: /etc/hosts.equiv, $HOME/.rhosts, and 43 ing. [Note to the administrator: /etc/hosts.equiv, $HOME/.rhosts, and
43 the rlogin/rsh protocol in general, are inherently insecure and should be 44 the rlogin/rsh protocol in general, are inherently insecure and should be
@@ -49,21 +50,22 @@ DESCRIPTION
49 not possible to derive the decryption key from the encryption key. RSA 50 not possible to derive the decryption key from the encryption key. RSA
50 is one such system. The idea is that each user creates a public/private 51 is one such system. The idea is that each user creates a public/private
51 key pair for authentication purposes. The server knows the public key, 52 key pair for authentication purposes. The server knows the public key,
52 and only the user knows the private key. The file 53 and only the user knows the private key.
53 $HOME/.ssh/authorized_keys lists the public keys that are permitted for 54
54 logging in. When the user logs in, the ssh program tells the server 55 The file $HOME/.ssh/authorized_keys lists the public keys that are per-
55 which key pair it would like to use for authentication. The server 56 mitted for logging in. When the user logs in, the ssh program tells the
56 checks if this key is permitted, and if so, sends the user (actually the 57 server which key pair it would like to use for authentication. The serv-
57 ssh program running on behalf of the user) a challenge, a random number, 58 er checks if this key is permitted, and if so, sends the user (actually
58 encrypted by the userM-bM-^@M-^Ys public key. The challenge can only be decrypted 59 the ssh program running on behalf of the user) a challenge, a random num-
59 using the proper private key. The userM-bM-^@M-^Ys client then decrypts the chal- 60 ber, encrypted by the user's public key. The challenge can only be de-
60 lenge using the private key, proving that he/she knows the private key 61 crypted using the proper private key. The user's client then decrypts
61 but without disclosing it to the server. 62 the challenge using the private key, proving that he/she knows the pri-
63 vate key but without disclosing it to the server.
62 64
63 ssh implements the RSA authentication protocol automatically. The user 65 ssh implements the RSA authentication protocol automatically. The user
64 creates his/her RSA key pair by running ssh-keygen(1). This stores the 66 creates his/her RSA key pair by running ssh-keygen(1). This stores the
65 private key in $HOME/.ssh/identity and the public key in 67 private key in $HOME/.ssh/identity and stores the public key in
66 $HOME/.ssh/identity.pub in the userM-bM-^@M-^Ys home directory. The user should 68 $HOME/.ssh/identity.pub in the user's home directory. The user should
67 then copy the identity.pub to $HOME/.ssh/authorized_keys in his/her home 69 then copy the identity.pub to $HOME/.ssh/authorized_keys in his/her home
68 directory on the remote machine (the authorized_keys file corresponds to 70 directory on the remote machine (the authorized_keys file corresponds to
69 the conventional $HOME/.rhosts file, and has one key per line, though the 71 the conventional $HOME/.rhosts file, and has one key per line, though the
@@ -80,11 +82,11 @@ DESCRIPTION
80 someone listening on the network. 82 someone listening on the network.
81 83
82 SSH protocol version 2 84 SSH protocol version 2
83 When a user connects using protocol version 2 similar authentication 85 When a user connects using protocol version 2, similar authentication
84 methods are available. Using the default values for 86 methods are available. Using the default values for
85 PreferredAuthentications, the client will try to authenticate first using 87 PreferredAuthentications, the client will try to authenticate first using
86 the hostbased method; if this method fails public key authentication is 88 the hostbased method; if this method fails, public key authentication is
87 attempted, and finally if this method fails keyboard-interactive and 89 attempted, and finally if this method fails, keyboard-interactive and
88 password authentication are tried. 90 password authentication are tried.
89 91
90 The public key method is similar to RSA authentication described in the 92 The public key method is similar to RSA authentication described in the
@@ -94,11 +96,11 @@ DESCRIPTION
94 server checks whether the matching public key is listed in 96 server checks whether the matching public key is listed in
95 $HOME/.ssh/authorized_keys and grants access if both the key is found and 97 $HOME/.ssh/authorized_keys and grants access if both the key is found and
96 the signature is correct. The session identifier is derived from a 98 the signature is correct. The session identifier is derived from a
97 shared Diffie-Hellman value and is only known to the client and the 99 shared Diffie-Hellman value and is only known to the client and the serv-
98 server. 100 er.
99 101
100 If public key authentication fails or is not available a password can be 102 If public key authentication fails or is not available, a password can be
101 sent encrypted to the remote host for proving the userM-bM-^@M-^Ys identity. 103 sent encrypted to the remote host to prove the user's identity.
102 104
103 Additionally, ssh supports hostbased or challenge response authentica- 105 Additionally, ssh supports hostbased or challenge response authentica-
104 tion. 106 tion.
@@ -109,25 +111,25 @@ DESCRIPTION
109 ensuring the integrity of the connection. 111 ensuring the integrity of the connection.
110 112
111 Login session and remote execution 113 Login session and remote execution
112 When the userM-bM-^@M-^Ys identity has been accepted by the server, the server 114 When the user's identity has been accepted by the server, the server ei-
113 either executes the given command, or logs into the machine and gives the 115 ther executes the given command, or logs into the machine and gives the
114 user a normal shell on the remote machine. All communication with the 116 user a normal shell on the remote machine. All communication with the
115 remote command or shell will be automatically encrypted. 117 remote command or shell will be automatically encrypted.
116 118
117 If a pseudo-terminal has been allocated (normal login session), the user 119 If a pseudo-terminal has been allocated (normal login session), the user
118 may use the escape characters noted below. 120 may use the escape characters noted below.
119 121
120 If no pseudo tty has been allocated, the session is transparent and can 122 If no pseudo-tty has been allocated, the session is transparent and can
121 be used to reliably transfer binary data. On most systems, setting the 123 be used to reliably transfer binary data. On most systems, setting the
122 escape character to M-bM-^@M-^\noneM-bM-^@M-^] will also make the session transparent even if 124 escape character to ``none'' will also make the session transparent even
123 a tty is used. 125 if a tty is used.
124 126
125 The session terminates when the command or shell on the remote machine 127 The session terminates when the command or shell on the remote machine
126 exits and all X11 and TCP/IP connections have been closed. The exit sta- 128 exits and all X11 and TCP/IP connections have been closed. The exit sta-
127 tus of the remote program is returned as the exit status of ssh. 129 tus of the remote program is returned as the exit status of ssh.
128 130
129 Escape Characters 131 Escape Characters
130 When a pseudo terminal has been requested, ssh supports a number of func- 132 When a pseudo-terminal has been requested, ssh supports a number of func-
131 tions through the use of an escape character. 133 tions through the use of an escape character.
132 134
133 A single tilde character can be sent as ~~ or by following the tilde by a 135 A single tilde character can be sent as ~~ or by following the tilde by a
@@ -136,42 +138,42 @@ DESCRIPTION
136 ter can be changed in configuration files using the EscapeChar configura- 138 ter can be changed in configuration files using the EscapeChar configura-
137 tion directive or on the command line by the -e option. 139 tion directive or on the command line by the -e option.
138 140
139 The supported escapes (assuming the default M-bM-^@M-^X~M-bM-^@M-^Y) are: 141 The supported escapes (assuming the default `~') are:
140 142
141 ~. Disconnect 143 ~. Disconnect.
142 144
143 ~^Z Background ssh 145 ~^Z Background ssh.
144 146
145 ~# List forwarded connections 147 ~# List forwarded connections.
146 148
147 ~& Background ssh at logout when waiting for forwarded connection / 149 ~& Background ssh at logout when waiting for forwarded connection /
148 X11 sessions to terminate 150 X11 sessions to terminate.
149 151
150 ~? Display a list of escape characters 152 ~? Display a list of escape characters.
151 153
152 ~B Send a BREAK to the remote system (only useful for SSH protocol 154 ~B Send a BREAK to the remote system (only useful for SSH protocol
153 version 2 and if the peer supports it) 155 version 2 and if the peer supports it).
154 156
155 ~C Open command line (only useful for adding port forwardings using 157 ~C Open command line (only useful for adding port forwardings using
156 the -L and -R options) 158 the -L and -R options).
157 159
158 ~R Request rekeying of the connection (only useful for SSH protocol 160 ~R Request rekeying of the connection (only useful for SSH protocol
159 version 2 and if the peer supports it) 161 version 2 and if the peer supports it).
160 162
161 X11 and TCP forwarding 163 X11 and TCP forwarding
162 If the ForwardX11 variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or, see the description of 164 If the ForwardX11 variable is set to ``yes'' (or see the description of
163 the -X and -x options described later) and the user is using X11 (the 165 the -X and -x options described later) and the user is using X11 (the
164 DISPLAY environment variable is set), the connection to the X11 display 166 DISPLAY environment variable is set), the connection to the X11 display
165 is automatically forwarded to the remote side in such a way that any X11 167 is automatically forwarded to the remote side in such a way that any X11
166 programs started from the shell (or command) will go through the 168 programs started from the shell (or command) will go through the encrypt-
167 encrypted channel, and the connection to the real X server will be made 169 ed channel, and the connection to the real X server will be made from the
168 from the local machine. The user should not manually set DISPLAY. For- 170 local machine. The user should not manually set DISPLAY. Forwarding of
169 warding of X11 connections can be configured on the command line or in 171 X11 connections can be configured on the command line or in configuration
170 configuration files. 172 files.
171 173
172 The DISPLAY value set by ssh will point to the server machine, but with a 174 The DISPLAY value set by ssh will point to the server machine, but with a
173 display number greater than zero. This is normal, and happens because 175 display number greater than zero. This is normal, and happens because
174 ssh creates a M-bM-^@M-^\proxyM-bM-^@M-^] X server on the server machine for forwarding the 176 ssh creates a ``proxy'' X server on the server machine for forwarding the
175 connections over the encrypted channel. 177 connections over the encrypted channel.
176 178
177 ssh will also automatically set up Xauthority data on the server machine. 179 ssh will also automatically set up Xauthority data on the server machine.
@@ -181,7 +183,7 @@ DESCRIPTION
181 is opened. The real authentication cookie is never sent to the server 183 is opened. The real authentication cookie is never sent to the server
182 machine (and no cookies are sent in the plain). 184 machine (and no cookies are sent in the plain).
183 185
184 If the ForwardAgent variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or, see the description of 186 If the ForwardAgent variable is set to ``yes'' (or see the description of
185 the -A and -a options described later) and the user is using an authenti- 187 the -A and -a options described later) and the user is using an authenti-
186 cation agent, the connection to the agent is automatically forwarded to 188 cation agent, the connection to the agent is automatically forwarded to
187 the remote side. 189 the remote side.
@@ -194,11 +196,11 @@ DESCRIPTION
194 Server authentication 196 Server authentication
195 ssh automatically maintains and checks a database containing identifica- 197 ssh automatically maintains and checks a database containing identifica-
196 tions for all hosts it has ever been used with. Host keys are stored in 198 tions for all hosts it has ever been used with. Host keys are stored in
197 $HOME/.ssh/known_hosts in the userM-bM-^@M-^Ys home directory. Additionally, the 199 $HOME/.ssh/known_hosts in the user's home directory. Additionally, the
198 file /etc/ssh/ssh_known_hosts is automatically checked for known hosts. 200 file /etc/ssh/ssh_known_hosts is automatically checked for known hosts.
199 Any new hosts are automatically added to the userM-bM-^@M-^Ys file. If a hostM-bM-^@M-^Ys 201 Any new hosts are automatically added to the user's file. If a host's
200 identification ever changes, ssh warns about this and disables password 202 identification ever changes, ssh warns about this and disables password
201 authentication to prevent a trojan horse from getting the userM-bM-^@M-^Ys pass- 203 authentication to prevent a trojan horse from getting the user's pass-
202 word. Another purpose of this mechanism is to prevent man-in-the-middle 204 word. Another purpose of this mechanism is to prevent man-in-the-middle
203 attacks which could otherwise be used to circumvent the encryption. The 205 attacks which could otherwise be used to circumvent the encryption. The
204 StrictHostKeyChecking option can be used to prevent logins to machines 206 StrictHostKeyChecking option can be used to prevent logins to machines
@@ -206,7 +208,13 @@ DESCRIPTION
206 208
207 The options are as follows: 209 The options are as follows:
208 210
209 -a Disables forwarding of the authentication agent connection. 211 -1 Forces ssh to try protocol version 1 only.
212
213 -2 Forces ssh to try protocol version 2 only.
214
215 -4 Forces ssh to use IPv4 addresses only.
216
217 -6 Forces ssh to use IPv6 addresses only.
210 218
211 -A Enables forwarding of the authentication agent connection. This 219 -A Enables forwarding of the authentication agent connection. This
212 can also be specified on a per-host basis in a configuration 220 can also be specified on a per-host basis in a configuration
@@ -214,21 +222,32 @@ DESCRIPTION
214 222
215 Agent forwarding should be enabled with caution. Users with the 223 Agent forwarding should be enabled with caution. Users with the
216 ability to bypass file permissions on the remote host (for the 224 ability to bypass file permissions on the remote host (for the
217 agentM-bM-^@M-^Ys Unix-domain socket) can access the local agent through 225 agent's Unix-domain socket) can access the local agent through
218 the forwarded connection. An attacker cannot obtain key material 226 the forwarded connection. An attacker cannot obtain key material
219 from the agent, however they can perform operations on the keys 227 from the agent, however they can perform operations on the keys
220 that enable them to authenticate using the identities loaded into 228 that enable them to authenticate using the identities loaded into
221 the agent. 229 the agent.
222 230
231 -a Disables forwarding of the authentication agent connection.
232
223 -b bind_address 233 -b bind_address
224 Specify the interface to transmit from on machines with multiple 234 Specify the interface to transmit from on machines with multiple
225 interfaces or aliased addresses. 235 interfaces or aliased addresses.
226 236
227 -c blowfish|3des|des 237 -C Requests compression of all data (including stdin, stdout,
238 stderr, and data for forwarded X11 and TCP/IP connections). The
239 compression algorithm is the same used by gzip(1), and the
240 ``level'' can be controlled by the CompressionLevel option for
241 protocol version 1. Compression is desirable on modem lines and
242 other slow connections, but will only slow down things on fast
243 networks. The default value can be set on a host-by-host basis
244 in the configuration files; see the Compression option.
245
246 -c blowfish | 3des | des
228 Selects the cipher to use for encrypting the session. 3des is 247 Selects the cipher to use for encrypting the session. 3des is
229 used by default. It is believed to be secure. 3des (triple-des) 248 used by default. It is believed to be secure. 3des (triple-des)
230 is an encrypt-decrypt-encrypt triple with three different keys. 249 is an encrypt-decrypt-encrypt triple with three different keys.
231 blowfish is a fast block cipher, it appears very secure and is 250 blowfish is a fast block cipher; it appears very secure and is
232 much faster than 3des. des is only supported in the ssh client 251 much faster than 3des. des is only supported in the ssh client
233 for interoperability with legacy protocol 1 implementations that 252 for interoperability with legacy protocol 1 implementations that
234 do not support the 3des cipher. Its use is strongly discouraged 253 do not support the 3des cipher. Its use is strongly discouraged
@@ -239,39 +258,67 @@ DESCRIPTION
239 ciphers can be specified in order of preference. See Ciphers for 258 ciphers can be specified in order of preference. See Ciphers for
240 more information. 259 more information.
241 260
242 -e ch|^ch|none 261 -D port
243 Sets the escape character for sessions with a pty (default: M-bM-^@M-^X~M-bM-^@M-^Y). 262 Specifies a local ``dynamic'' application-level port forwarding.
263 This works by allocating a socket to listen to port on the local
264 side, and whenever a connection is made to this port, the connec-
265 tion is forwarded over the secure channel, and the application
266 protocol is then used to determine where to connect to from the
267 remote machine. Currently the SOCKS4 and SOCKS5 protocols are
268 supported, and ssh will act as a SOCKS server. Only root can
269 forward privileged ports. Dynamic port forwardings can also be
270 specified in the configuration file.
271
272 -e ch | ^ch | none
273 Sets the escape character for sessions with a pty (default: `~').
244 The escape character is only recognized at the beginning of a 274 The escape character is only recognized at the beginning of a
245 line. The escape character followed by a dot (M-bM-^@M-^X.M-bM-^@M-^Y) closes the 275 line. The escape character followed by a dot (`.') closes the
246 connection, followed by control-Z suspends the connection, and 276 connection; followed by control-Z suspends the connection; and
247 followed by itself sends the escape character once. Setting the 277 followed by itself sends the escape character once. Setting the
248 character to M-bM-^@M-^\noneM-bM-^@M-^] disables any escapes and makes the session 278 character to ``none'' disables any escapes and makes the session
249 fully transparent. 279 fully transparent.
250 280
281 -F configfile
282 Specifies an alternative per-user configuration file. If a con-
283 figuration file is given on the command line, the system-wide
284 configuration file (/etc/ssh/ssh_config) will be ignored. The
285 default for the per-user configuration file is $HOME/.ssh/config.
286
251 -f Requests ssh to go to background just before command execution. 287 -f Requests ssh to go to background just before command execution.
252 This is useful if ssh is going to ask for passwords or 288 This is useful if ssh is going to ask for passwords or passphras-
253 passphrases, but the user wants it in the background. This 289 es, but the user wants it in the background. This implies -n.
254 implies -n. The recommended way to start X11 programs at a 290 The recommended way to start X11 programs at a remote site is
255 remote site is with something like ssh -f host xterm. 291 with something like ssh -f host xterm.
256 292
257 -g Allows remote hosts to connect to local forwarded ports. 293 -g Allows remote hosts to connect to local forwarded ports.
258 294
295 -I smartcard_device
296 Specifies which smartcard device to use. The argument is the de-
297 vice ssh should use to communicate with a smartcard used for
298 storing the user's private RSA key.
299
259 -i identity_file 300 -i identity_file
260 Selects a file from which the identity (private key) for RSA or 301 Selects a file from which the identity (private key) for RSA or
261 DSA authentication is read. The default is $HOME/.ssh/identity 302 DSA authentication is read. The default is $HOME/.ssh/identity
262 for protocol version 1, and $HOME/.ssh/id_rsa and 303 for protocol version 1, and $HOME/.ssh/id_rsa and
263 $HOME/.ssh/id_dsa for protocol version 2. Identity files may 304 $HOME/.ssh/id_dsa for protocol version 2. Identity files may al-
264 also be specified on a per-host basis in the configuration file. 305 so be specified on a per-host basis in the configuration file.
265 It is possible to have multiple -i options (and multiple identi- 306 It is possible to have multiple -i options (and multiple identi-
266 ties specified in configuration files). 307 ties specified in configuration files).
267 308
268 -I smartcard_device 309 -k Disables forwarding (delegation) of GSSAPI credentials to the
269 Specifies which smartcard device to use. The argument is the 310 server.
270 device ssh should use to communicate with a smartcard used for
271 storing the userM-bM-^@M-^Ys private RSA key.
272 311
273 -k Disables forwarding of Kerberos tickets. This may also be speci- 312 -L port:host:hostport
274 fied on a per-host basis in the configuration file. 313 Specifies that the given port on the local (client) host is to be
314 forwarded to the given host and port on the remote side. This
315 works by allocating a socket to listen to port on the local side,
316 and whenever a connection is made to this port, the connection is
317 forwarded over the secure channel, and a connection is made to
318 host port hostport from the remote machine. Port forwardings can
319 also be specified in the configuration file. Only root can for-
320 ward privileged ports. IPv6 addresses can be specified with an
321 alternative syntax: port/host/hostport.
275 322
276 -l login_name 323 -l login_name
277 Specifies the user to log in as on the remote machine. This also 324 Specifies the user to log in as on the remote machine. This also
@@ -282,23 +329,75 @@ DESCRIPTION
282 MAC (message authentication code) algorithms can be specified in 329 MAC (message authentication code) algorithms can be specified in
283 order of preference. See the MACs keyword for more information. 330 order of preference. See the MACs keyword for more information.
284 331
285 -n Redirects stdin from /dev/null (actually, prevents reading from
286 stdin). This must be used when ssh is run in the background. A
287 common trick is to use this to run X11 programs on a remote
288 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
289 start an emacs on shadows.cs.hut.fi, and the X11 connection will
290 be automatically forwarded over an encrypted channel. The ssh
291 program will be put in the background. (This does not work if
292 ssh needs to ask for a password or passphrase; see also the -f
293 option.)
294
295 -N Do not execute a remote command. This is useful for just for- 332 -N Do not execute a remote command. This is useful for just for-
296 warding ports (protocol version 2 only). 333 warding ports (protocol version 2 only).
297 334
335 -n Redirects stdin from /dev/null (actually, prevents reading from
336 stdin). This must be used when ssh is run in the background. A
337 common trick is to use this to run X11 programs on a remote ma-
338 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start
339 an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
340 tomatically forwarded over an encrypted channel. The ssh program
341 will be put in the background. (This does not work if ssh needs
342 to ask for a password or passphrase; see also the -f option.)
343
298 -o option 344 -o option
299 Can be used to give options in the format used in the configura- 345 Can be used to give options in the format used in the configura-
300 tion file. This is useful for specifying options for which there 346 tion file. This is useful for specifying options for which there
301 is no separate command-line flag. 347 is no separate command-line flag. For full details of the op-
348 tions listed below, and their possible values, see ssh_config(5).
349
350 AddressFamily
351 BatchMode
352 BindAddress
353 ChallengeResponseAuthentication
354 CheckHostIP
355 Cipher
356 Ciphers
357 ClearAllForwardings
358 Compression
359 CompressionLevel
360 ConnectionAttempts
361 ConnectionTimeout
362 DynamicForward
363 EscapeChar
364 ForwardAgent
365 ForwardX11
366 ForwardX11Trusted
367 GatewayPorts
368 GlobalKnownHostsFile
369 GSSAPIAuthentication
370 GSSAPIDelegateCredentials
371 Host
372 HostbasedAuthentication
373 HostKeyAlgorithms
374 HostKeyAlias
375 HostName
376 IdentityFile
377 LocalForward
378 LogLevel
379 MACs
380 NoHostAuthenticationForLocalhost
381 NumberOfPasswordPrompts
382 PasswordAuthentication
383 Port
384 PreferredAuthentications
385 Protocol
386 ProxyCommand
387 PubkeyAuthentication
388 RemoteForward
389 RhostsRSAAuthentication
390 RSAAuthentication
391 ServerAliveInterval
392 ServerAliveCountMax
393 SmartcardDevice
394 StrictHostKeyChecking
395 TCPKeepAlive
396 UsePrivilegedPort
397 User
398 UserKnownHostsFile
399 VerifyHostKeyDNS
400 XAuthLocation
302 401
303 -p port 402 -p port
304 Port to connect to on the remote host. This can be specified on 403 Port to connect to on the remote host. This can be specified on
@@ -307,93 +406,50 @@ DESCRIPTION
307 -q Quiet mode. Causes all warning and diagnostic messages to be 406 -q Quiet mode. Causes all warning and diagnostic messages to be
308 suppressed. 407 suppressed.
309 408
409 -R port:host:hostport
410 Specifies that the given port on the remote (server) host is to
411 be forwarded to the given host and port on the local side. This
412 works by allocating a socket to listen to port on the remote
413 side, and whenever a connection is made to this port, the connec-
414 tion is forwarded over the secure channel, and a connection is
415 made to host port hostport from the local machine. Port forward-
416 ings can also be specified in the configuration file. Privileged
417 ports can be forwarded only when logging in as root on the remote
418 machine. IPv6 addresses can be specified with an alternative
419 syntax: port/host/hostport.
420
310 -s May be used to request invocation of a subsystem on the remote 421 -s May be used to request invocation of a subsystem on the remote
311 system. Subsystems are a feature of the SSH2 protocol which 422 system. Subsystems are a feature of the SSH2 protocol which fa-
312 facilitate the use of SSH as a secure transport for other appli- 423 cilitate the use of SSH as a secure transport for other applica-
313 cations (eg. sftp). The subsystem is specified as the remote 424 tions (eg. sftp(1)). The subsystem is specified as the remote
314 command. 425 command.
315 426
427 -T Disable pseudo-tty allocation.
428
316 -t Force pseudo-tty allocation. This can be used to execute arbi- 429 -t Force pseudo-tty allocation. This can be used to execute arbi-
317 trary screen-based programs on a remote machine, which can be 430 trary screen-based programs on a remote machine, which can be
318 very useful, e.g., when implementing menu services. Multiple -t 431 very useful, e.g., when implementing menu services. Multiple -t
319 options force tty allocation, even if ssh has no local tty. 432 options force tty allocation, even if ssh has no local tty.
320 433
321 -T Disable pseudo-tty allocation. 434 -V Display the version number and exit.
322 435
323 -v Verbose mode. Causes ssh to print debugging messages about its 436 -v Verbose mode. Causes ssh to print debugging messages about its
324 progress. This is helpful in debugging connection, authentica- 437 progress. This is helpful in debugging connection, authentica-
325 tion, and configuration problems. Multiple -v options increase 438 tion, and configuration problems. Multiple -v options increase
326 the verbosity. The maximum is 3. 439 the verbosity. The maximum is 3.
327 440
328 -V Display the version number and exit.
329
330 -x Disables X11 forwarding.
331
332 -X Enables X11 forwarding. This can also be specified on a per-host 441 -X Enables X11 forwarding. This can also be specified on a per-host
333 basis in a configuration file. 442 basis in a configuration file.
334 443
335 X11 forwarding should be enabled with caution. Users with the 444 X11 forwarding should be enabled with caution. Users with the
336 ability to bypass file permissions on the remote host (for the 445 ability to bypass file permissions on the remote host (for the
337 userM-bM-^@M-^Ys X authorization database) can access the local X11 display 446 user's X authorization database) can access the local X11 display
338 through the forwarded connection. An attacker may then be able 447 through the forwarded connection. An attacker may then be able
339 to perform activities such as keystroke monitoring. 448 to perform activities such as keystroke monitoring.
340 449
341 -C Requests compression of all data (including stdin, stdout, 450 -x Disables X11 forwarding.
342 stderr, and data for forwarded X11 and TCP/IP connections). The
343 compression algorithm is the same used by gzip(1), and the
344 M-bM-^@M-^\levelM-bM-^@M-^] can be controlled by the CompressionLevel option for pro-
345 tocol version 1. Compression is desirable on modem lines and
346 other slow connections, but will only slow down things on fast
347 networks. The default value can be set on a host-by-host basis
348 in the configuration files; see the Compression option.
349
350 -F configfile
351 Specifies an alternative per-user configuration file. If a con-
352 figuration file is given on the command line, the system-wide
353 configuration file (/etc/ssh/ssh_config) will be ignored. The
354 default for the per-user configuration file is $HOME/.ssh/config.
355
356 -L port:host:hostport
357 Specifies that the given port on the local (client) host is to be
358 forwarded to the given host and port on the remote side. This
359 works by allocating a socket to listen to port on the local side,
360 and whenever a connection is made to this port, the connection is
361 forwarded over the secure channel, and a connection is made to
362 host port hostport from the remote machine. Port forwardings can
363 also be specified in the configuration file. Only root can for-
364 ward privileged ports. IPv6 addresses can be specified with an
365 alternative syntax: port/host/hostport
366
367 -R port:host:hostport
368 Specifies that the given port on the remote (server) host is to
369 be forwarded to the given host and port on the local side. This
370 works by allocating a socket to listen to port on the remote
371 side, and whenever a connection is made to this port, the connec-
372 tion is forwarded over the secure channel, and a connection is
373 made to host port hostport from the local machine. Port forward-
374 ings can also be specified in the configuration file. Privileged
375 ports can be forwarded only when logging in as root on the remote
376 machine. IPv6 addresses can be specified with an alternative
377 syntax: port/host/hostport
378
379 -D port
380 Specifies a local M-bM-^@M-^\dynamicM-bM-^@M-^] application-level port forwarding.
381 This works by allocating a socket to listen to port on the local
382 side, and whenever a connection is made to this port, the connec-
383 tion is forwarded over the secure channel, and the application
384 protocol is then used to determine where to connect to from the
385 remote machine. Currently the SOCKS4 and SOCKS5 protocols are
386 supported, and ssh will act as a SOCKS server. Only root can
387 forward privileged ports. Dynamic port forwardings can also be
388 specified in the configuration file.
389
390 -1 Forces ssh to try protocol version 1 only.
391
392 -2 Forces ssh to try protocol version 2 only.
393
394 -4 Forces ssh to use IPv4 addresses only.
395 451
396 -6 Forces ssh to use IPv6 addresses only. 452 -Y Enables trusted X11 forwarding.
397 453
398CONFIGURATION FILES 454CONFIGURATION FILES
399 ssh may additionally obtain configuration data from a per-user configura- 455 ssh may additionally obtain configuration data from a per-user configura-
@@ -403,65 +459,63 @@ CONFIGURATION FILES
403ENVIRONMENT 459ENVIRONMENT
404 ssh will normally set the following environment variables: 460 ssh will normally set the following environment variables:
405 461
406 DISPLAY 462 DISPLAY The DISPLAY variable indicates the location of the X11 server.
407 The DISPLAY variable indicates the location of the X11 server. 463 It is automatically set by ssh to point to a value of the form
408 It is automatically set by ssh to point to a value of the form 464 ``hostname:n'' where hostname indicates the host where the shell
409 M-bM-^@M-^\hostname:nM-bM-^@M-^] where hostname indicates the host where the shell 465 runs, and n is an integer >= 1. ssh uses this special value to
410 runs, and n is an integer >= 1. ssh uses this special value to 466 forward X11 connections over the secure channel. The user
411 forward X11 connections over the secure channel. The user should 467 should normally not set DISPLAY explicitly, as that will render
412 normally not set DISPLAY explicitly, as that will render the X11 468 the X11 connection insecure (and will require the user to manu-
413 connection insecure (and will require the user to manually copy 469 ally copy any required authorization cookies).
414 any required authorization cookies).
415 470
416 HOME Set to the path of the userM-bM-^@M-^Ys home directory. 471 HOME Set to the path of the user's home directory.
417 472
418 LOGNAME 473 LOGNAME Synonym for USER; set for compatibility with systems that use
419 Synonym for USER; set for compatibility with systems that use 474 this variable.
420 this variable.
421 475
422 MAIL Set to the path of the userM-bM-^@M-^Ys mailbox. 476 MAIL Set to the path of the user's mailbox.
423 477
424 PATH Set to the default PATH, as specified when compiling ssh. 478 PATH Set to the default PATH, as specified when compiling ssh.
425 479
426 SSH_ASKPASS 480 SSH_ASKPASS
427 If ssh needs a passphrase, it will read the passphrase from the 481 If ssh needs a passphrase, it will read the passphrase from the
428 current terminal if it was run from a terminal. If ssh does not 482 current terminal if it was run from a terminal. If ssh does not
429 have a terminal associated with it but DISPLAY and SSH_ASKPASS 483 have a terminal associated with it but DISPLAY and SSH_ASKPASS
430 are set, it will execute the program specified by SSH_ASKPASS and 484 are set, it will execute the program specified by SSH_ASKPASS
431 open an X11 window to read the passphrase. This is particularly 485 and open an X11 window to read the passphrase. This is particu-
432 useful when calling ssh from a .Xsession or related script. 486 larly useful when calling ssh from a .Xsession or related
433 (Note that on some machines it may be necessary to redirect the 487 script. (Note that on some machines it may be necessary to
434 input from /dev/null to make this work.) 488 redirect the input from /dev/null to make this work.)
435 489
436 SSH_AUTH_SOCK 490 SSH_AUTH_SOCK
437 Identifies the path of a unix-domain socket used to communicate 491 Identifies the path of a unix-domain socket used to communicate
438 with the agent. 492 with the agent.
439 493
440 SSH_CONNECTION 494 SSH_CONNECTION
441 Identifies the client and server ends of the connection. The 495 Identifies the client and server ends of the connection. The
442 variable contains four space-separated values: client ip-address, 496 variable contains four space-separated values: client ip-ad-
443 client port number, server ip-address and server port number. 497 dress, client port number, server ip-address and server port
498 number.
444 499
445 SSH_ORIGINAL_COMMAND 500 SSH_ORIGINAL_COMMAND
446 The variable contains the original command line if a forced com- 501 The variable contains the original command line if a forced com-
447 mand is executed. It can be used to extract the original argu- 502 mand is executed. It can be used to extract the original argu-
448 ments. 503 ments.
449 504
450 SSH_TTY 505 SSH_TTY This is set to the name of the tty (path to the device) associ-
451 This is set to the name of the tty (path to the device) associ- 506 ated with the current shell or command. If the current session
452 ated with the current shell or command. If the current session 507 has no tty, this variable is not set.
453 has no tty, this variable is not set.
454 508
455 TZ The timezone variable is set to indicate the present timezone if 509 TZ The timezone variable is set to indicate the present timezone if
456 it was set when the daemon was started (i.e., the daemon passes 510 it was set when the daemon was started (i.e., the daemon passes
457 the value on to new connections). 511 the value on to new connections).
458 512
459 USER Set to the name of the user logging in. 513 USER Set to the name of the user logging in.
460 514
461 Additionally, ssh reads $HOME/.ssh/environment, and adds lines of the 515 Additionally, ssh reads $HOME/.ssh/environment, and adds lines of the
462 format M-bM-^@M-^\VARNAME=valueM-bM-^@M-^] to the environment if the file exists and if users 516 format ``VARNAME=value'' to the environment if the file exists and if
463 are allowed to change their environment. See the PermitUserEnvironment 517 users are allowed to change their environment. For more information, see
464 option in sshd_config(5). 518 the PermitUserEnvironment option in sshd_config(5).
465 519
466FILES 520FILES
467 $HOME/.ssh/known_hosts 521 $HOME/.ssh/known_hosts
@@ -481,7 +535,7 @@ FILES
481 $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub 535 $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
482 Contains the public key for authentication (public part of the 536 Contains the public key for authentication (public part of the
483 identity file in human-readable form). The contents of the 537 identity file in human-readable form). The contents of the
484 $HOME/.ssh/identity.pub file should be added to 538 $HOME/.ssh/identity.pub file should be added to the file
485 $HOME/.ssh/authorized_keys on all machines where the user wishes 539 $HOME/.ssh/authorized_keys on all machines where the user wishes
486 to log in using protocol version 1 RSA authentication. The con- 540 to log in using protocol version 1 RSA authentication. The con-
487 tents of the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file 541 tents of the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file
@@ -512,15 +566,15 @@ FILES
512 following format (fields separated by spaces): system name, pub- 566 following format (fields separated by spaces): system name, pub-
513 lic key and optional comment field. When different names are 567 lic key and optional comment field. When different names are
514 used for the same machine, all such names should be listed, sepa- 568 used for the same machine, all such names should be listed, sepa-
515 rated by commas. The format is described on the sshd(8) manual 569 rated by commas. The format is described in the sshd(8) manual
516 page. 570 page.
517 571
518 The canonical system name (as returned by name servers) is used 572 The canonical system name (as returned by name servers) is used
519 by sshd(8) to verify the client host when logging in; other names 573 by sshd(8) to verify the client host when logging in; other names
520 are needed because ssh does not convert the user-supplied name to 574 are needed because ssh does not convert the user-supplied name to
521 a canonical name before checking the key, because someone with 575 a canonical name before checking the key, because someone with
522 access to the name servers would then be able to fool host 576 access to the name servers would then be able to fool host au-
523 authentication. 577 thentication.
524 578
525 /etc/ssh/ssh_config 579 /etc/ssh/ssh_config
526 Systemwide configuration file. The file format and configuration 580 Systemwide configuration file. The file format and configuration
@@ -538,22 +592,22 @@ FILES
538 method is used. By default ssh is not setuid root. 592 method is used. By default ssh is not setuid root.
539 593
540 $HOME/.rhosts 594 $HOME/.rhosts
541 This file is used in .rhosts authentication to list the host/user 595 This file is used in rhosts authentication to list the host/user
542 pairs that are permitted to log in. (Note that this file is also 596 pairs that are permitted to log in. (Note that this file is also
543 used by rlogin and rsh, which makes using this file insecure.) 597 used by rlogin and rsh, which makes using this file insecure.)
544 Each line of the file contains a host name (in the canonical form 598 Each line of the file contains a host name (in the canonical form
545 returned by name servers), and then a user name on that host, 599 returned by name servers), and then a user name on that host,
546 separated by a space. On some machines this file may need to be 600 separated by a space. On some machines this file may need to be
547 world-readable if the userM-bM-^@M-^Ys home directory is on a NFS parti- 601 world-readable if the user's home directory is on a NFS parti-
548 tion, because sshd(8) reads it as root. Additionally, this file 602 tion, because sshd(8) reads it as root. Additionally, this file
549 must be owned by the user, and must not have write permissions 603 must be owned by the user, and must not have write permissions
550 for anyone else. The recommended permission for most machines is 604 for anyone else. The recommended permission for most machines is
551 read/write for the user, and not accessible by others. 605 read/write for the user, and not accessible by others.
552 606
553 Note that by default sshd(8) will be installed so that it 607 Note that by default sshd(8) will be installed so that it re-
554 requires successful RSA host authentication before permitting 608 quires successful RSA host authentication before permitting
555 .rhosts authentication. If the server machine does not have the 609 rhosts authentication. If the server machine does not have the
556 clientM-bM-^@M-^Ys host key in /etc/ssh/ssh_known_hosts, it can be stored 610 client's host key in /etc/ssh/ssh_known_hosts, it can be stored
557 in $HOME/.ssh/known_hosts. The easiest way to do this is to con- 611 in $HOME/.ssh/known_hosts. The easiest way to do this is to con-
558 nect back to the client from the server machine using ssh; this 612 nect back to the client from the server machine using ssh; this
559 will automatically add the host key to $HOME/.ssh/known_hosts. 613 will automatically add the host key to $HOME/.ssh/known_hosts.
@@ -564,12 +618,12 @@ FILES
564 with ssh without permitting login with rlogin or rsh(1). 618 with ssh without permitting login with rlogin or rsh(1).
565 619
566 /etc/hosts.equiv 620 /etc/hosts.equiv
567 This file is used during .rhosts authentication. It contains 621 This file is used during rhosts authentication. It contains
568 canonical hosts names, one per line (the full format is described 622 canonical hosts names, one per line (the full format is described
569 on the sshd(8) manual page). If the client host is found in this 623 in the sshd(8) manual page). If the client host is found in this
570 file, login is automatically permitted provided client and server 624 file, login is automatically permitted provided client and server
571 user names are the same. Additionally, successful RSA host 625 user names are the same. Additionally, successful RSA host au-
572 authentication is normally required. This file should only be 626 thentication is normally required. This file should only be
573 writable by root. 627 writable by root.
574 628
575 /etc/shosts.equiv 629 /etc/shosts.equiv
@@ -579,12 +633,12 @@ FILES
579 633
580 /etc/ssh/sshrc 634 /etc/ssh/sshrc
581 Commands in this file are executed by ssh when the user logs in 635 Commands in this file are executed by ssh when the user logs in
582 just before the userM-bM-^@M-^Ys shell (or command) is started. See the 636 just before the user's shell (or command) is started. See the
583 sshd(8) manual page for more information. 637 sshd(8) manual page for more information.
584 638
585 $HOME/.ssh/rc 639 $HOME/.ssh/rc
586 Commands in this file are executed by ssh when the user logs in 640 Commands in this file are executed by ssh when the user logs in
587 just before the userM-bM-^@M-^Ys shell (or command) is started. See the 641 just before the user's shell (or command) is started. See the
588 sshd(8) manual page for more information. 642 sshd(8) manual page for more information.
589 643
590 $HOME/.ssh/environment 644 $HOME/.ssh/environment
@@ -596,8 +650,9 @@ DIAGNOSTICS
596 error occurred. 650 error occurred.
597 651
598SEE ALSO 652SEE ALSO
599 rsh(1), scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), 653 gzip(1), rsh(1), scp(1), sftp(1), ssh-add(1), ssh-agent(1),
600 telnet(1), ssh_config(5), ssh-keysign(8), sshd(8) 654 ssh-keygen(1), telnet(1), hosts.equiv(5), ssh_config(5), ssh-keysign(8),
655 sshd(8)
601 656
602 T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH 657 T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
603 Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January 658 Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January
@@ -606,8 +661,8 @@ SEE ALSO
606AUTHORS 661AUTHORS
607 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 662 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
608 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 663 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
609 de Raadt and Dug Song removed many bugs, re-added newer features and cre- 664 de Raadt and Dug Song removed many bugs, re-added newer features and
610 ated OpenSSH. Markus Friedl contributed the support for SSH protocol 665 created OpenSSH. Markus Friedl contributed the support for SSH protocol
611 versions 1.5 and 2.0. 666 versions 1.5 and 2.0.
612 667
613BSD September 25, 1999 BSD 668OpenBSD 3.4 September 25, 1999 11
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