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Diff for /src/usr.bin/ssh/ssh.1 between version 1.217 and 1.218

version 1.217, 2005/12/08 14:59:44 version 1.218, 2005/12/16 18:07:08
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 is specified,  is specified,
 .Ar command  .Ar command
 is executed on the remote host instead of a login shell.  is executed on the remote host instead of a login shell.
 .Ss SSH protocol version 1  
 The first authentication method is the  
 .Em rhosts  
 or  
 .Em hosts.equiv  
 method combined with RSA-based host authentication.  
 If the machine the user logs in from is listed in  
 .Pa /etc/hosts.equiv  
 or  
 .Pa /etc/shosts.equiv  
 on the remote machine, and the user names are  
 the same on both sides, or if the files  
 .Pa ~/.rhosts  
 or  
 .Pa ~/.shosts  
 exist in the user's home directory on the  
 remote machine and contain a line containing the name of the client  
 machine and the name of the user on that machine, the user is  
 considered for log in.  
 Additionally, if the server can verify the client's  
 host key (see  
 .Pa /etc/ssh/ssh_known_hosts  
 and  
 .Pa ~/.ssh/known_hosts  
 in the  
 .Sx FILES  
 section), only then is login permitted.  
 This authentication method closes security holes due to IP  
 spoofing, DNS spoofing and routing spoofing.  
 [Note to the administrator:  
 .Pa /etc/hosts.equiv ,  
 .Pa ~/.rhosts ,  
 and the rlogin/rsh protocol in general, are inherently insecure and should be  
 disabled if security is desired.]  
 .Pp  .Pp
 As a second authentication method,  
 .Nm  
 supports RSA based authentication.  
 The scheme is based on public-key cryptography: there are cryptosystems  
 where encryption and decryption are done using separate keys, and it  
 is not possible to derive the decryption key from the encryption key.  
 RSA is one such system.  
 The idea is that each user creates a public/private  
 key pair for authentication purposes.  
 The server knows the public key, and only the user knows the private key.  
 .Pp  
 The file  
 .Pa ~/.ssh/authorized_keys  
 lists the public keys that are permitted for logging in.  
 When the user logs in, the  
 .Nm  
 program tells the server which key pair it would like to use for  
 authentication.  
 The server checks if this key is permitted, and if so,  
 sends the user (actually the  
 .Nm  
 program running on behalf of the user) a challenge, a random number,  
 encrypted by the user's public key.  
 The challenge can only be decrypted using the proper private key.  
 The user's client then decrypts the challenge using the private key,  
 proving that he/she knows the private key  
 but without disclosing it to the server.  
 .Pp  
 .Nm  
 implements the RSA authentication protocol automatically.  
 The user creates his/her RSA key pair by running  
 .Xr ssh-keygen 1 .  
 This stores the private key in  
 .Pa ~/.ssh/identity  
 and stores the public key in  
 .Pa ~/.ssh/identity.pub  
 in the user's home directory.  
 The user should then copy the  
 .Pa identity.pub  
 to  
 .Pa ~/.ssh/authorized_keys  
 in his/her home directory on the remote machine (the  
 .Pa authorized_keys  
 file corresponds to the conventional  
 .Pa ~/.rhosts  
 file, and has one key  
 per line, though the lines can be very long).  
 After this, the user can log in without giving the password.  
 .Pp  
 The most convenient way to use RSA authentication may be with an  
 authentication agent.  
 See  
 .Xr ssh-agent 1  
 for more information.  
 .Pp  
 If other authentication methods fail,  
 .Nm  
 prompts the user for a password.  
 The password is sent to the remote  
 host for checking; however, since all communications are encrypted,  
 the password cannot be seen by someone listening on the network.  
 .Ss SSH protocol version 2  
 When a user connects using protocol version 2,  
 similar authentication methods are available.  
 Using the default values for  
 .Cm PreferredAuthentications ,  
 the client will try to authenticate first using the hostbased method;  
 if this method fails, public key authentication is attempted,  
 and finally if this method fails, keyboard-interactive and  
 password authentication are tried.  
 .Pp  
 The public key method is similar to RSA authentication described  
 in the previous section and allows the RSA or DSA algorithm to be used:  
 The client uses his private key,  
 .Pa ~/.ssh/id_dsa  
 or  
 .Pa ~/.ssh/id_rsa ,  
 to sign the session identifier and sends the result to the server.  
 The server checks whether the matching public key is listed in  
 .Pa ~/.ssh/authorized_keys  
 and grants access if both the key is found and the signature is correct.  
 The session identifier is derived from a shared Diffie-Hellman value  
 and is only known to the client and the server.  
 .Pp  
 If public key authentication fails or is not available, a password  
 can be sent encrypted to the remote host to prove the user's identity.  
 .Pp  
 Additionally,  
 .Nm  
 supports hostbased or challenge response authentication.  
 .Pp  
 Protocol 2 provides additional mechanisms for confidentiality  
 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour)  
 and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).  
 Note that protocol 1 lacks a strong mechanism for ensuring the  
 integrity of the connection.  
 .Ss Login session and remote execution  
 When the user's identity has been accepted by the server, the server  
 either executes the given command, or logs into the machine and gives  
 the user a normal shell on the remote machine.  
 All communication with  
 the remote command or shell will be automatically encrypted.  
 .Pp  
 If a pseudo-terminal has been allocated (normal login session), the  
 user may use the escape characters noted below.  
 .Pp  
 If no pseudo-tty has been allocated,  
 the session is transparent and can be used to reliably transfer binary data.  
 On most systems, setting the escape character to  
 .Dq none  
 will also make the session transparent even if a tty is used.  
 .Pp  
 The session terminates when the command or shell on the remote  
 machine exits and all X11 and TCP/IP connections have been closed.  
 The exit status of the remote program is returned as the exit status of  
 .Nm ssh .  
 .Ss Escape Characters  
 When a pseudo-terminal has been requested,  
 .Nm  
 supports a number of functions through the use of an escape character.  
 .Pp  
 A single tilde character can be sent as  
 .Ic ~~  
 or by following the tilde by a character other than those described below.  
 The escape character must always follow a newline to be interpreted as  
 special.  
 The escape character can be changed in configuration files using the  
 .Cm EscapeChar  
 configuration directive or on the command line by the  
 .Fl e  
 option.  
 .Pp  
 The supported escapes (assuming the default  
 .Ql ~ )  
 are:  
 .Bl -tag -width Ds  
 .It Cm ~.  
 Disconnect.  
 .It Cm ~^Z  
 Background  
 .Nm ssh .  
 .It Cm ~#  
 List forwarded connections.  
 .It Cm ~&  
 Background  
 .Nm  
 at logout when waiting for forwarded connection / X11 sessions to terminate.  
 .It Cm ~?  
 Display a list of escape characters.  
 .It Cm ~B  
 Send a BREAK to the remote system  
 (only useful for SSH protocol version 2 and if the peer supports it).  
 .It Cm ~C  
 Open command line.  
 Currently this allows the addition of port forwardings using the  
 .Fl L  
 and  
 .Fl R  
 options (see below).  
 It also allows the cancellation of existing remote port-forwardings  
 using  
 .Fl KR Ar hostport .  
 .Ic !\& Ns Ar command  
 allows the user to execute a local command if the  
 .Ic PermitLocalCommand  
 option is enabled in  
 .Xr ssh_config 5 .  
 Basic help is available, using the  
 .Fl h  
 option.  
 .It Cm ~R  
 Request rekeying of the connection  
 (only useful for SSH protocol version 2 and if the peer supports it).  
 .El  
 .Ss X11 and TCP forwarding  
 If the  
 .Cm ForwardX11  
 variable is set to  
 .Dq yes  
 (or see the description of the  
 .Fl X  
 and  
 .Fl x  
 options described later)  
 and the user is using X11 (the  
 .Ev DISPLAY  
 environment variable is set), the 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 channel, and the connection to the real X server will be made  
 from the local machine.  
 The user should not manually set  
 .Ev DISPLAY .  
 Forwarding of X11 connections can be  
 configured on the command line or in configuration files.  
 .Pp  
 The  
 .Ev DISPLAY  
 value set by  
 .Nm  
 will point to the server machine, but with a display number greater than zero.  
 This is normal, and happens because  
 .Nm  
 creates a  
 .Dq proxy  
 X server on the server machine for forwarding the  
 connections over the encrypted channel.  
 .Pp  
 .Nm  
 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, 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 server machine (and no cookies are sent in the plain).  
 .Pp  
 If the  
 .Cm ForwardAgent  
 variable is set to  
 .Dq yes  
 (or see the description of the  
 .Fl A  
 and  
 .Fl a  
 options described later) and  
 the user is using an authentication agent, the connection to the agent  
 is automatically forwarded to the remote side.  
 .Pp  
 Forwarding of arbitrary TCP/IP connections over the secure channel can  
 be specified either on the command line or in a configuration file.  
 One possible application of TCP/IP forwarding is a secure connection to an  
 electronic purse; another is going through firewalls.  
 .Ss Server authentication  
 .Nm  
 automatically maintains and checks a database containing  
 identifications for all hosts it has ever been used with.  
 Host keys are stored in  
 .Pa ~/.ssh/known_hosts  
 in the user's home directory.  
 Additionally, the file  
 .Pa /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 identification ever changes,  
 .Nm  
 warns about this and disables password authentication to prevent a  
 trojan horse from getting the user's password.  
 Another purpose of this mechanism is to prevent man-in-the-middle attacks  
 which could otherwise be used to circumvent the encryption.  
 The  
 .Cm StrictHostKeyChecking  
 option can be used to prevent logins to machines whose  
 host key is not known or has changed.  
 .Pp  
 .Nm  
 can be configured to verify host identification using fingerprint resource  
 records (SSHFP) published in DNS.  
 The  
 .Cm VerifyHostKeyDNS  
 option can be used to control how DNS lookups are performed.  
 SSHFP resource records can be generated using  
 .Xr ssh-keygen 1 .  
 .Pp  
 The options are as follows:  The options are as follows:
 .Bl -tag -width Ds  .Bl -tag -width Ds
 .It Fl 1  .It Fl 1
Line 912 
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 Trusted X11 forwardings are not subjected to the X11 SECURITY extension  Trusted X11 forwardings are not subjected to the X11 SECURITY extension
 controls.  controls.
 .El  .El
 .Sh CONFIGURATION FILES  .Ss SSH protocol version 1
   The first authentication method is the
   .Em rhosts
   or
   .Em hosts.equiv
   method combined with RSA-based host authentication.
   If the machine the user logs in from is listed in
   .Pa /etc/hosts.equiv
   or
   .Pa /etc/shosts.equiv
   on the remote machine, and the user names are
   the same on both sides, or if the files
   .Pa ~/.rhosts
   or
   .Pa ~/.shosts
   exist in the user's home directory on the
   remote machine and contain a line containing the name of the client
   machine and the name of the user on that machine, the user is
   considered for log in.
   Additionally, if the server can verify the client's
   host key (see
   .Pa /etc/ssh/ssh_known_hosts
   and
   .Pa ~/.ssh/known_hosts
   in the
   .Sx FILES
   section), only then is login permitted.
   This authentication method closes security holes due to IP
   spoofing, DNS spoofing and routing spoofing.
   [Note to the administrator:
   .Pa /etc/hosts.equiv ,
   .Pa ~/.rhosts ,
   and the rlogin/rsh protocol in general, are inherently insecure and should be
   disabled if security is desired.]
   .Pp
   As a second authentication method,
 .Nm  .Nm
   supports RSA based authentication.
   The scheme is based on public-key cryptography: there are cryptosystems
   where encryption and decryption are done using separate keys, and it
   is not possible to derive the decryption key from the encryption key.
   RSA is one such system.
   The idea is that each user creates a public/private
   key pair for authentication purposes.
   The server knows the public key, and only the user knows the private key.
   .Pp
   The file
   .Pa ~/.ssh/authorized_keys
   lists the public keys that are permitted for logging in.
   When the user logs in, the
   .Nm
   program tells the server which key pair it would like to use for
   authentication.
   The server checks if this key is permitted, and if so,
   sends the user (actually the
   .Nm
   program running on behalf of the user) a challenge, a random number,
   encrypted by the user's public key.
   The challenge can only be decrypted using the proper private key.
   The user's client then decrypts the challenge using the private key,
   proving that he/she knows the private key
   but without disclosing it to the server.
   .Pp
   .Nm
   implements the RSA authentication protocol automatically.
   The user creates his/her RSA key pair by running
   .Xr ssh-keygen 1 .
   This stores the private key in
   .Pa ~/.ssh/identity
   and stores the public key in
   .Pa ~/.ssh/identity.pub
   in the user's home directory.
   The user should then copy the
   .Pa identity.pub
   to
   .Pa ~/.ssh/authorized_keys
   in his/her home directory on the remote machine (the
   .Pa authorized_keys
   file corresponds to the conventional
   .Pa ~/.rhosts
   file, and has one key
   per line, though the lines can be very long).
   After this, the user can log in without giving the password.
   .Pp
   The most convenient way to use RSA authentication may be with an
   authentication agent.
   See
   .Xr ssh-agent 1
   for more information.
   .Pp
   If other authentication methods fail,
   .Nm
   prompts the user for a password.
   The password is sent to the remote
   host for checking; however, since all communications are encrypted,
   the password cannot be seen by someone listening on the network.
   .Ss SSH protocol version 2
   When a user connects using protocol version 2,
   similar authentication methods are available.
   Using the default values for
   .Cm PreferredAuthentications ,
   the client will try to authenticate first using the hostbased method;
   if this method fails, public key authentication is attempted,
   and finally if this method fails, keyboard-interactive and
   password authentication are tried.
   .Pp
   The public key method is similar to RSA authentication described
   in the previous section and allows the RSA or DSA algorithm to be used:
   The client uses his private key,
   .Pa ~/.ssh/id_dsa
   or
   .Pa ~/.ssh/id_rsa ,
   to sign the session identifier and sends the result to the server.
   The server checks whether the matching public key is listed in
   .Pa ~/.ssh/authorized_keys
   and grants access if both the key is found and the signature is correct.
   The session identifier is derived from a shared Diffie-Hellman value
   and is only known to the client and the server.
   .Pp
   If public key authentication fails or is not available, a password
   can be sent encrypted to the remote host to prove the user's identity.
   .Pp
   Additionally,
   .Nm
   supports hostbased or challenge response authentication.
   .Pp
   Protocol 2 provides additional mechanisms for confidentiality
   (the traffic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour)
   and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
   Note that protocol 1 lacks a strong mechanism for ensuring the
   integrity of the connection.
   .Ss Login session and remote execution
   When the user's identity has been accepted by the server, the server
   either executes the given command, or logs into the machine and gives
   the user a normal shell on the remote machine.
   All communication with
   the remote command or shell will be automatically encrypted.
   .Pp
   If a pseudo-terminal has been allocated (normal login session), the
   user may use the escape characters noted below.
   .Pp
   If no pseudo-tty has been allocated,
   the session is transparent and can be used to reliably transfer binary data.
   On most systems, setting the escape character to
   .Dq none
   will also make the session transparent even if a tty is used.
   .Pp
   The session terminates when the command or shell on the remote
   machine exits and all X11 and TCP/IP connections have been closed.
   The exit status of the remote program is returned as the exit status of
   .Nm ssh .
   .Pp
   .Nm
 may additionally obtain configuration data from  may additionally obtain configuration data from
 a per-user configuration file and a system-wide configuration file.  a per-user configuration file and a system-wide configuration file.
 The file format and configuration options are described in  The file format and configuration options are described in
 .Xr ssh_config 5 .  .Xr ssh_config 5 .
   .Ss Escape Characters
   When a pseudo-terminal has been requested,
   .Nm
   supports a number of functions through the use of an escape character.
   .Pp
   A single tilde character can be sent as
   .Ic ~~
   or by following the tilde by a character other than those described below.
   The escape character must always follow a newline to be interpreted as
   special.
   The escape character can be changed in configuration files using the
   .Cm EscapeChar
   configuration directive or on the command line by the
   .Fl e
   option.
   .Pp
   The supported escapes (assuming the default
   .Ql ~ )
   are:
   .Bl -tag -width Ds
   .It Cm ~.
   Disconnect.
   .It Cm ~^Z
   Background
   .Nm ssh .
   .It Cm ~#
   List forwarded connections.
   .It Cm ~&
   Background
   .Nm
   at logout when waiting for forwarded connection / X11 sessions to terminate.
   .It Cm ~?
   Display a list of escape characters.
   .It Cm ~B
   Send a BREAK to the remote system
   (only useful for SSH protocol version 2 and if the peer supports it).
   .It Cm ~C
   Open command line.
   Currently this allows the addition of port forwardings using the
   .Fl L
   and
   .Fl R
   options (see below).
   It also allows the cancellation of existing remote port-forwardings
   using
   .Fl KR Ar hostport .
   .Ic !\& Ns Ar command
   allows the user to execute a local command if the
   .Ic PermitLocalCommand
   option is enabled in
   .Xr ssh_config 5 .
   Basic help is available, using the
   .Fl h
   option.
   .It Cm ~R
   Request rekeying of the connection
   (only useful for SSH protocol version 2 and if the peer supports it).
   .El
   .Ss X11 and TCP forwarding
   If the
   .Cm ForwardX11
   variable is set to
   .Dq yes
   (or see the description of the
   .Fl X
   and
   .Fl x
   options described later)
   and the user is using X11 (the
   .Ev DISPLAY
   environment variable is set), the 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 channel, and the connection to the real X server will be made
   from the local machine.
   The user should not manually set
   .Ev DISPLAY .
   Forwarding of X11 connections can be
   configured on the command line or in configuration files.
   .Pp
   The
   .Ev DISPLAY
   value set by
   .Nm
   will point to the server machine, but with a display number greater than zero.
   This is normal, and happens because
   .Nm
   creates a
   .Dq proxy
   X server on the server machine for forwarding the
   connections over the encrypted channel.
   .Pp
   .Nm
   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, 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 server machine (and no cookies are sent in the plain).
   .Pp
   If the
   .Cm ForwardAgent
   variable is set to
   .Dq yes
   (or see the description of the
   .Fl A
   and
   .Fl a
   options described later) and
   the user is using an authentication agent, the connection to the agent
   is automatically forwarded to the remote side.
   .Pp
   Forwarding of arbitrary TCP/IP connections over the secure channel can
   be specified either on the command line or in a configuration file.
   One possible application of TCP/IP forwarding is a secure connection to an
   electronic purse; another is going through firewalls.
   .Ss Server authentication
   .Nm
   automatically maintains and checks a database containing
   identifications for all hosts it has ever been used with.
   Host keys are stored in
   .Pa ~/.ssh/known_hosts
   in the user's home directory.
   Additionally, the file
   .Pa /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 identification ever changes,
   .Nm
   warns about this and disables password authentication to prevent a
   trojan horse from getting the user's password.
   Another purpose of this mechanism is to prevent man-in-the-middle attacks
   which could otherwise be used to circumvent the encryption.
   The
   .Cm StrictHostKeyChecking
   option can be used to prevent logins to machines whose
   host key is not known or has changed.
   .Pp
   .Nm
   can be configured to verify host identification using fingerprint resource
   records (SSHFP) published in DNS.
   The
   .Cm VerifyHostKeyDNS
   option can be used to control how DNS lookups are performed.
   SSHFP resource records can be generated using
   .Xr ssh-keygen 1 .
 .Sh ENVIRONMENT  .Sh ENVIRONMENT
 .Nm  .Nm
 will normally set the following environment variables:  will normally set the following environment variables:
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