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Diff for /src/usr.bin/ssh/ssh.1 between version 1.209 and 1.209.2.3

version 1.209, 2005/07/06 09:33:05 version 1.209.2.3, 2006/11/08 00:44:05
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 .Nd OpenSSH SSH client (remote login program)  .Nd OpenSSH SSH client (remote login program)
 .Sh SYNOPSIS  .Sh SYNOPSIS
 .Nm ssh  .Nm ssh
 .Bk -words  
 .Op Fl 1246AaCfgkMNnqsTtVvXxY  .Op Fl 1246AaCfgkMNnqsTtVvXxY
 .Op Fl b Ar bind_address  .Op Fl b Ar bind_address
 .Op Fl c Ar cipher_spec  .Op Fl c Ar cipher_spec
 .Op Fl D Ar port  .Oo Fl D\ \&
   .Sm off
   .Oo Ar bind_address : Oc
   .Ar port
   .Sm on
   .Oc
 .Op Fl e Ar escape_char  .Op Fl e Ar escape_char
 .Op Fl F Ar configfile  .Op Fl F Ar configfile
   .Bk -words
 .Op Fl i Ar identity_file  .Op Fl i Ar identity_file
   .Ek
 .Oo Fl L\ \&  .Oo Fl L\ \&
 .Sm off  .Sm off
 .Oo Ar bind_address : Oc  .Oo Ar bind_address : Oc
 .Ar port : host : hostport  .Ar port : host : hostport
 .Sm on  .Sm on
 .Oc  .Oc
   .Bk -words
 .Op Fl l Ar login_name  .Op Fl l Ar login_name
   .Ek
 .Op Fl m Ar mac_spec  .Op Fl m Ar mac_spec
 .Op Fl O Ar ctl_cmd  .Op Fl O Ar ctl_cmd
 .Op Fl o Ar option  .Op Fl o Ar option
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 .Sm on  .Sm on
 .Oc  .Oc
 .Op Fl S Ar ctl_path  .Op Fl S Ar ctl_path
   .Bk -words
   .Oo Fl w Ar local_tun Ns
   .Op : Ns Ar remote_tun Oc
 .Oo Ar user Ns @ Oc Ns Ar hostname  .Oo Ar user Ns @ Oc Ns Ar hostname
 .Op Ar command  .Op Ar command
 .Ek  .Ek
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 It is intended to replace rlogin and rsh,  It is intended to replace rlogin and rsh,
 and provide secure encrypted communications between  and provide secure encrypted communications between
 two untrusted hosts over an insecure network.  two untrusted hosts over an insecure network.
 X11 connections and arbitrary TCP/IP ports  X11 connections and arbitrary TCP ports
 can also be forwarded over the secure channel.  can also be forwarded over the secure channel.
 .Pp  .Pp
 .Nm  .Nm
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 name).  name).
 The user must prove  The user must prove
 his/her identity to the remote machine using one of several methods  his/her identity to the remote machine using one of several methods
 depending on the protocol version used.  depending on the protocol version used (see below).
 .Pp  .Pp
 If  If
 .Ar command  .Ar command
 is specified,  is specified,
 .Ar command  it 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 .  
 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
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 Only useful on systems with more than one address.  Only useful on systems with more than one address.
 .It Fl C  .It Fl C
 Requests compression of all data (including stdin, stdout, stderr, and  Requests compression of all data (including stdin, stdout, stderr, and
 data for forwarded X11 and TCP/IP connections).  data for forwarded X11 and TCP connections).
 The compression algorithm is the same used by  The compression algorithm is the same used by
 .Xr gzip 1 ,  .Xr gzip 1 ,
 and the  and the
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 Selects the cipher specification for encrypting the session.  Selects the cipher specification for encrypting the session.
 .Pp  .Pp
 Protocol version 1 allows specification of a single cipher.  Protocol version 1 allows specification of a single cipher.
 The suported values are  The supported values are
 .Dq 3des ,  .Dq 3des ,
 .Dq blowfish  .Dq blowfish ,
 and  and
 .Dq des .  .Dq des .
 .Ar 3des  .Ar 3des
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 The default is  The default is
 .Dq 3des .  .Dq 3des .
 .Pp  .Pp
 For protocol version 2  For protocol version 2,
 .Ar cipher_spec  .Ar cipher_spec
 is a comma-separated list of ciphers  is a comma-separated list of ciphers
 listed in order of preference.  listed in order of preference.
 The supported ciphers are  The supported ciphers are:
 .Dq 3des-cbc ,  3des-cbc,
 .Dq aes128-cbc ,  aes128-cbc,
 .Dq aes192-cbc ,  aes192-cbc,
 .Dq aes256-cbc ,  aes256-cbc,
 .Dq aes128-ctr ,  aes128-ctr,
 .Dq aes192-ctr ,  aes192-ctr,
 .Dq aes256-ctr ,  aes256-ctr,
 .Dq arcfour128 ,  arcfour128,
 .Dq arcfour256 ,  arcfour256,
 .Dq arcfour ,  arcfour,
 .Dq blowfish-cbc ,  blowfish-cbc,
 and  and
 .Dq cast128-cbc .  cast128-cbc.
 The default is  The default is:
 .Bd -literal  .Bd -literal -offset indent
   ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,  aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
     arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,  arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
     aes192-ctr,aes256-ctr''  aes192-ctr,aes256-ctr
 .Ed  .Ed
 .It Fl D Ar port  .It Fl D Xo
   .Sm off
   .Oo Ar bind_address : Oc
   .Ar port
   .Sm on
   .Xc
 Specifies a local  Specifies a local
 .Dq dynamic  .Dq dynamic
 application-level port forwarding.  application-level port forwarding.
 This works by allocating a socket to listen to  This works by allocating a socket to listen to
 .Ar port  .Ar port
 on the local side, and whenever a connection is made to this port, the  on the local side, optionally bound to the specified
   .Ar bind_address .
   Whenever a connection is made to this port, the
 connection is forwarded over the secure channel, and the application  connection is forwarded over the secure channel, and the application
 protocol is then used to determine where to connect to from the  protocol is then used to determine where to connect to from the
 remote machine.  remote machine.
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 will act as a SOCKS server.  will act as a SOCKS server.
 Only root can forward privileged ports.  Only root can forward privileged ports.
 Dynamic port forwardings can also be specified in the configuration file.  Dynamic port forwardings can also be specified in the configuration file.
 .It Fl e Ar ch | ^ch | none  .Pp
   IPv6 addresses can be specified with an alternative syntax:
   .Sm off
   .Xo
   .Op Ar bind_address No /
   .Ar port
   .Xc
   .Sm on
   or by enclosing the address in square brackets.
   Only the superuser can forward privileged ports.
   By default, the local port is bound in accordance with the
   .Cm GatewayPorts
   setting.
   However, an explicit
   .Ar bind_address
   may be used to bind the connection to a specific address.
   The
   .Ar bind_address
   of
   .Dq localhost
   indicates that the listening port be bound for local use only, while an
   empty address or
   .Sq *
   indicates that the port should be available from all interfaces.
   .It Fl e Ar escape_char
 Sets the escape character for sessions with a pty (default:  Sets the escape character for sessions with a pty (default:
 .Ql ~ ) .  .Ql ~ ) .
 The escape character is only recognized at the beginning of a line.  The escape character is only recognized at the beginning of a line.
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 .It Fl g  .It Fl g
 Allows remote hosts to connect to local forwarded ports.  Allows remote hosts to connect to local forwarded ports.
 .It Fl I Ar smartcard_device  .It Fl I Ar smartcard_device
 Specifies which smartcard device to use.  Specify the device
 The argument is the device  
 .Nm  .Nm
 should use to communicate with a smartcard used for storing the user's  should use to communicate with a smartcard used for storing the user's
 private RSA key.  private RSA key.
   This option is only available if support for smartcard devices
   is compiled in (default is no support).
 .It Fl i Ar identity_file  .It Fl i Ar identity_file
 Selects a file from which the identity (private key) for  Selects a file from which the identity (private key) for
 RSA or DSA authentication is read.  RSA or DSA authentication is read.
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 client into  client into
 .Dq master  .Dq master
 mode for connection sharing.  mode for connection sharing.
   Multiple
   .Fl M
   options places
   .Nm
   into
   .Dq master
   mode with confirmation required before slave connections are accepted.
 Refer to the description of  Refer to the description of
 .Cm ControlMaster  .Cm ControlMaster
 in  in
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 .It ControlPath  .It ControlPath
 .It DynamicForward  .It DynamicForward
 .It EscapeChar  .It EscapeChar
   .It ExitOnForwardFailure
 .It ForwardAgent  .It ForwardAgent
 .It ForwardX11  .It ForwardX11
 .It ForwardX11Trusted  .It ForwardX11Trusted
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 .It IdentityFile  .It IdentityFile
 .It IdentitiesOnly  .It IdentitiesOnly
 .It KbdInteractiveDevices  .It KbdInteractiveDevices
   .It LocalCommand
 .It LocalForward  .It LocalForward
 .It LogLevel  .It LogLevel
 .It MACs  .It MACs
 .It NoHostAuthenticationForLocalhost  .It NoHostAuthenticationForLocalhost
 .It NumberOfPasswordPrompts  .It NumberOfPasswordPrompts
 .It PasswordAuthentication  .It PasswordAuthentication
   .It PermitLocalCommand
 .It Port  .It Port
 .It PreferredAuthentications  .It PreferredAuthentications
 .It Protocol  .It Protocol
 .It ProxyCommand  .It ProxyCommand
 .It PubkeyAuthentication  .It PubkeyAuthentication
   .It RekeyLimit
 .It RemoteForward  .It RemoteForward
 .It RhostsRSAAuthentication  .It RhostsRSAAuthentication
 .It RSAAuthentication  .It RSAAuthentication
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 .It SmartcardDevice  .It SmartcardDevice
 .It StrictHostKeyChecking  .It StrictHostKeyChecking
 .It TCPKeepAlive  .It TCPKeepAlive
   .It Tunnel
   .It TunnelDevice
 .It UsePrivilegedPort  .It UsePrivilegedPort
 .It User  .It User
 .It UserKnownHostsFile  .It UserKnownHostsFile
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 Force pseudo-tty allocation.  Force pseudo-tty allocation.
 This can be used to execute arbitrary  This can be used to execute arbitrary
 screen-based programs on a remote machine, which can be very useful,  screen-based programs on a remote machine, which can be very useful,
 e.g., when implementing menu services.  e.g. when implementing menu services.
 Multiple  Multiple
 .Fl t  .Fl t
 options force tty allocation, even if  options force tty allocation, even if
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 .Fl v  .Fl v
 options increase the verbosity.  options increase the verbosity.
 The maximum is 3.  The maximum is 3.
   .It Fl w Xo
   .Ar local_tun Ns Op : Ns Ar remote_tun
   .Xc
   Requests
   tunnel
   device forwarding with the specified
   .Xr tun 4
   devices between the client
   .Pq Ar local_tun
   and the server
   .Pq Ar remote_tun .
   .Pp
   The devices may be specified by numerical ID or the keyword
   .Dq any ,
   which uses the next available tunnel device.
   If
   .Ar remote_tun
   is not specified, it defaults to
   .Dq any .
   See also the
   .Cm Tunnel
   and
   .Cm TunnelDevice
   directives in
   .Xr ssh_config 5 .
   If the
   .Cm Tunnel
   directive is unset, it is set to the default tunnel mode, which is
   .Dq point-to-point .
 .It Fl X  .It Fl X
 Enables X11 forwarding.  Enables X11 forwarding.
 This can also be specified on a per-host basis in a configuration file.  This can also be specified on a per-host basis in a configuration file.
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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  .Pp
 .Nm  .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 .
   .Pp
   .Nm
   exits with the exit status of the remote command or with 255
   if an error occurred.
   .Sh AUTHENTICATION
   The OpenSSH SSH client supports SSH protocols 1 and 2.
   Protocol 2 is the default, with
   .Nm
   falling back to protocol 1 if it detects protocol 2 is unsupported.
   These settings may be altered using the
   .Cm Protocol
   option in
   .Xr ssh_config 5 ,
   or enforced using the
   .Fl 1
   and
   .Fl 2
   options (see above).
   Both protocols support similar authentication methods,
   but protocol 2 is preferred since
   it provides additional mechanisms for confidentiality
   (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
   and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
   Protocol 1 lacks a strong mechanism for ensuring the
   integrity of the connection.
   .Pp
   The methods available for authentication are:
   GSSAPI-based authentication,
   host-based authentication,
   public key authentication,
   challenge-response authentication,
   and password authentication.
   Authentication methods are tried in the order specified above,
   though protocol 2 has a configuration option to change the default order:
   .Cm PreferredAuthentications .
   .Pp
   Host-based authentication works as follows:
   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 login.
   Additionally, the server
   .Em must
   be able to verify the client's
   host key (see the description of
   .Pa /etc/ssh/ssh_known_hosts
   and
   .Pa ~/.ssh/known_hosts ,
   below)
   for login to be 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
   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 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 knows the public key, and only the user knows the private key.
   .Nm
   implements public key authentication protocol automatically,
   using either the RSA or DSA algorithms.
   Protocol 1 is restricted to using only RSA keys,
   but protocol 2 may use either.
   The
   .Sx HISTORY
   section of
   .Xr ssl 8
   contains a brief discussion of the two algorithms.
   .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 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.
   .Pp
   The user creates his/her key pair by running
   .Xr ssh-keygen 1 .
   This stores the private key in
   .Pa ~/.ssh/identity
   (protocol 1),
   .Pa ~/.ssh/id_dsa
   (protocol 2 DSA),
   or
   .Pa ~/.ssh/id_rsa
   (protocol 2 RSA)
   and stores the public key in
   .Pa ~/.ssh/identity.pub
   (protocol 1),
   .Pa ~/.ssh/id_dsa.pub
   (protocol 2 DSA),
   or
   .Pa ~/.ssh/id_rsa.pub
   (protocol 2 RSA)
   in the user's home directory.
   The user should then copy the public key
   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 public key authentication may be with an
   authentication agent.
   See
   .Xr ssh-agent 1
   for more information.
   .Pp
   Challenge-response authentication works as follows:
   The server sends an arbitrary
   .Qq challenge
   text, and prompts for a response.
   Protocol 2 allows multiple challenges and responses;
   protocol 1 is restricted to just one challenge/response.
   Examples of challenge-response authentication include
   BSD Authentication (see
   .Xr login.conf 5 )
   and PAM (some non-OpenBSD systems).
   .Pp
   Finally, 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.
   .Pp
   .Nm
   automatically maintains and checks a database containing
   identification 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
   server spoofing or man-in-the-middle attacks,
   which could otherwise be used to circumvent the encryption.
   The
   .Cm StrictHostKeyChecking
   option can be used to control logins to machines whose
   host key is not known or has changed.
   .Pp
   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 connections have been closed.
   .Sh 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 .
   .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 above).
   It also allows the cancellation of existing remote port-forwardings
   using
   .Sm off
   .Fl KR Oo Ar bind_address : Oc Ar port .
   .Sm on
   .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
   .Sh TCP FORWARDING
   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 forwarding is a secure connection to a
   mail server; another is going through firewalls.
   .Pp
   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 works as follows:
   the user connects to the remote host using
   .Nm ,
   specifying a port to be used to forward connections
   to the remote server.
   After that it is possible to start the service which is to be encrypted
   on the client machine,
   connecting to the same local port,
   and
   .Nm
   will encrypt and forward the connection.
   .Pp
   The following example tunnels an IRC session from client machine
   .Dq 127.0.0.1
   (localhost)
   to remote server
   .Dq server.example.com :
   .Bd -literal -offset 4n
   $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
   $ irc -c '#users' -p 1234 pinky 127.0.0.1
   .Ed
   .Pp
   This tunnels a connection to IRC server
   .Dq server.example.com ,
   joining channel
   .Dq #users ,
   nickname
   .Dq pinky ,
   using port 1234.
   It doesn't matter which port is used,
   as long as it's 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 remote server,
   since that's the standard port for IRC services.
   .Pp
   The
   .Fl f
   option backgrounds
   .Nm
   and the remote command
   .Dq sleep 10
   is specified to allow an amount of time
   (10 seconds, in the example)
   to start the service which is to be tunnelled.
   If no connections are made within the time specified,
   .Nm
   will exit.
   .Sh X11 FORWARDING
   If the
   .Cm ForwardX11
   variable is set to
   .Dq yes
   (or see the description of the
   .Fl X ,
   .Fl x ,
   and
   .Fl Y
   options above)
   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 above) and
   the user is using an authentication agent, the connection to the agent
   is automatically forwarded to the remote side.
   .Sh VERIFYING HOST KEYS
   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
   .Cm StrictHostKeyChecking
   has been disabled).
   Fingerprints can be determined using
   .Xr ssh-keygen 1 :
   .Pp
   .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
   .Pp
   If the fingerprint is already known,
   it can be matched and verified,
   and the key can be accepted.
   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 zonefile
   and the connecting client is able to match the fingerprint
   with that of the key presented.
   .Pp
   In this example, we are connecting a client to a server,
   .Dq host.example.com .
   The SSHFP resource records should first be added to the zonefile for
   host.example.com:
   .Bd -literal -offset indent
   $ ssh-keygen -r host.example.com.
   .Ed
   .Pp
   The output lines will have to be added to the zonefile.
   To check that the zone is answering fingerprint queries:
   .Pp
   .Dl $ dig -t SSHFP host.example.com
   .Pp
   Finally the client connects:
   .Bd -literal -offset indent
   $ ssh -o "VerifyHostKeyDNS ask" host.example.com
   [...]
   Matching host key fingerprint found in DNS.
   Are you sure you want to continue connecting (yes/no)?
   .Ed
   .Pp
   See the
   .Cm VerifyHostKeyDNS
   option in
   .Xr ssh_config 5
   for more information.
   .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
   .Nm
   contains support for Virtual Private Network (VPN) tunnelling
   using the
   .Xr tun 4
   network pseudo-device,
   allowing two networks to be joined securely.
   The
   .Xr sshd_config 5
   configuration option
   .Cm PermitTunnel
   controls whether the server supports this,
   and at what level (layer 2 or 3 traffic).
   .Pp
   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 SSH server running on the gateway to the remote network,
   at 192.168.1.15, allows it.
   .Pp
   On the client:
   .Bd -literal -offset indent
   # ssh -f -w 0:1 192.168.1.15 true
   # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
   # route add 10.0.99.0/24 10.1.1.2
   .Ed
   .Pp
   On the server:
   .Bd -literal -offset indent
   # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
   # route add 10.0.50.0/24 10.1.1.1
   .Ed
   .Pp
   Client access may be more finely tuned via the
   .Pa /root/.ssh/authorized_keys
   file (see below) and the
   .Cm PermitRootLogin
   server option.
   The following entry would permit connections on
   .Xr tun 4
   device 1 from user
   .Dq jane
   and on tun device 2 from user
   .Dq john ,
   if
   .Cm PermitRootLogin
   is set to
   .Dq forced-commands-only :
   .Bd -literal -offset 2n
   tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
   tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
   .Ed
   .Pp
   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 tools such as
   .Xr ipsecctl 8
   and
   .Xr isakmpd 8 .
 .Sh ENVIRONMENT  .Sh ENVIRONMENT
 .Nm  .Nm
 will normally set the following environment variables:  will normally set the following environment variables:
 .Bl -tag -width LOGNAME  .Bl -tag -width "SSH_ORIGINAL_COMMAND"
 .It Ev DISPLAY  .It Ev DISPLAY
 The  The
 .Ev DISPLAY  .Ev DISPLAY
Line 872 
Line 1133 
 It is automatically set by  It is automatically set by
 .Nm  .Nm
 to point to a value of the form  to point to a value of the form
 .Dq hostname:n  .Dq hostname:n ,
 where hostname indicates  where
 the host where the shell runs, and n is an integer \*(Ge 1.  .Dq hostname
   indicates the host where the shell runs, and
   .Sq n
   is an integer \*(Ge 1.
 .Nm  .Nm
 uses this special value to forward X11 connections over the secure  uses this special value to forward X11 connections over the secure
 channel.  channel.
Line 895 
Line 1159 
 Set to the default  Set to the default
 .Ev PATH ,  .Ev PATH ,
 as specified when compiling  as specified when compiling
 .Nm ssh .  .Nm .
 .It Ev SSH_ASKPASS  .It Ev SSH_ASKPASS
 If  If
 .Nm  .Nm
Line 920 
Line 1184 
 .Pa /dev/null  .Pa /dev/null
 to make this work.)  to make this work.)
 .It Ev SSH_AUTH_SOCK  .It Ev SSH_AUTH_SOCK
 Identifies the path of a unix-domain socket used to communicate with the  Identifies the path of a
 agent.  .Ux Ns -domain
   socket used to communicate with the agent.
 .It Ev SSH_CONNECTION  .It Ev SSH_CONNECTION
 Identifies the client and server ends of the connection.  Identifies the client and server ends of the connection.
 The variable contains  The variable contains
 four space-separated values: client ip-address, client port number,  four space-separated values: client IP address, client port number,
 server ip-address and server port number.  server IP address, and server port number.
 .It Ev SSH_ORIGINAL_COMMAND  .It Ev SSH_ORIGINAL_COMMAND
 The variable contains the original command line if a forced command  This variable contains the original command line if a forced command
 is executed.  is executed.
 It can be used to extract the original arguments.  It can be used to extract the original arguments.
 .It Ev SSH_TTY  .It Ev SSH_TTY
Line 937 
Line 1202 
 If the current session has no tty,  If the current session has no tty,
 this variable is not set.  this variable is not set.
 .It Ev TZ  .It Ev TZ
 The timezone variable is set to indicate the present timezone if it  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  was set when the daemon was started (i.e. the daemon passes the value
 on to new connections).  on to new connections).
 .It Ev USER  .It Ev USER
 Set to the name of the user logging in.  Set to the name of the user logging in.
Line 950 
Line 1215 
 .Pa ~/.ssh/environment ,  .Pa ~/.ssh/environment ,
 and adds lines of the format  and adds lines of the format
 .Dq VARNAME=value  .Dq VARNAME=value
 to the environment if the file exists and if users are allowed to  to the environment if the file exists and users are allowed to
 change their environment.  change their environment.
 For more information, see the  For more information, see the
 .Cm PermitUserEnvironment  .Cm PermitUserEnvironment
 option in  option in
 .Xr sshd_config 5 .  .Xr sshd_config 5 .
 .Sh FILES  .Sh FILES
 .Bl -tag -width Ds  .Bl -tag -width Ds -compact
 .It Pa ~/.ssh/known_hosts  .It ~/.rhosts
 Records host keys for all hosts the user has logged into that are not  This file is used for host-based authentication (see above).
 in  On some machines this file may need to be
 .Pa /etc/ssh/ssh_known_hosts .  world-readable if the user's home directory is on an NFS partition,
 See  because
 .Xr sshd 8 .  .Xr sshd 8
 .It Pa ~/.ssh/identity, ~/.ssh/id_dsa, ~/.ssh/id_rsa  reads it as root.
 Contains the authentication identity of the user.  Additionally, this file must be owned by the user,
 They are for protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.  and must not have write permissions for anyone else.
   The recommended
   permission for most machines is read/write for the user, and not
   accessible by others.
   .Pp
   .It ~/.shosts
   This file is used in exactly the same way as
   .Pa .rhosts ,
   but allows host-based authentication without permitting login with
   rlogin/rsh.
   .Pp
   .It ~/.ssh/authorized_keys
   Lists the public keys (RSA/DSA) that can be used for logging in as this user.
   The format of this file is described in the
   .Xr sshd 8
   manual page.
   This file is not highly sensitive, but the recommended
   permissions are read/write for the user, and not accessible by others.
   .Pp
   .It ~/.ssh/config
   This is the per-user configuration file.
   The file format and configuration options are described in
   .Xr ssh_config 5 .
   Because of the potential for abuse, this file must have strict permissions:
   read/write for the user, and not accessible by others.
   .Pp
   .It ~/.ssh/environment
   Contains additional definitions for environment variables; see
   .Sx ENVIRONMENT ,
   above.
   .Pp
   .It ~/.ssh/identity
   .It ~/.ssh/id_dsa
   .It ~/.ssh/id_rsa
   Contains the private key for authentication.
 These files  These files
 contain sensitive data and should be readable by the user but not  contain sensitive data and should be readable by the user but not
 accessible by others (read/write/execute).  accessible by others (read/write/execute).
 Note that  
 .Nm  .Nm
 ignores a private key file if it is accessible by others.  will simply ignore a private key file if it is accessible by others.
 It is possible to specify a passphrase when  It is possible to specify a passphrase when
 generating the key; the passphrase will be used to encrypt the  generating the key which will be used to encrypt the
 sensitive part of this file using 3DES.  sensitive part of this file using 3DES.
 .It Pa ~/.ssh/identity.pub, ~/.ssh/id_dsa.pub, ~/.ssh/id_rsa.pub  .Pp
 Contains the public key for authentication (public part of the  .It ~/.ssh/identity.pub
 identity file in human-readable form).  .It ~/.ssh/id_dsa.pub
 The contents of the  .It ~/.ssh/id_rsa.pub
 .Pa ~/.ssh/identity.pub  Contains the public key for authentication.
 file should be added to the file  
 .Pa ~/.ssh/authorized_keys  
 on all machines  
 where the user wishes to log in using protocol version 1 RSA authentication.  
 The contents of the  
 .Pa ~/.ssh/id_dsa.pub  
 and  
 .Pa ~/.ssh/id_rsa.pub  
 file should be added to  
 .Pa ~/.ssh/authorized_keys  
 on all machines  
 where the user wishes to log in using protocol version 2 DSA/RSA authentication.  
 These files are not  These files are not
 sensitive and can (but need not) be readable by anyone.  sensitive and can (but need not) be readable by anyone.
 These files are  
 never used automatically and are not necessary; they are only provided for  
 the convenience of the user.  
 .It Pa ~/.ssh/config  
 This is the per-user configuration file.  
 The file format and configuration options are described in  
 .Xr ssh_config 5 .  
 Because of the potential for abuse, this file must have strict permissions:  
 read/write for the user, and not accessible by others.  
 .It Pa ~/.ssh/authorized_keys  
 Lists the public keys (RSA/DSA) that can be used for logging in as this user.  
 The format of this file is described in the  
 .Xr sshd 8  
 manual page.  
 In the simplest form the format is the same as the  
 .Pa .pub  
 identity files.  
 This file is not highly sensitive, but the recommended  
 permissions are read/write for the user, and not accessible by others.  
 .It Pa /etc/ssh/ssh_known_hosts  
 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.  
 This file should be world-readable.  
 This file contains  
 public keys, one per line, in the following format (fields separated  
 by spaces): system name, public key and optional comment field.  
 When different names are used  
 for the same machine, all such names should be listed, separated by  
 commas.  
 The format is described in the  
 .Xr sshd 8  
 manual page.  
 .Pp  .Pp
 The canonical system name (as returned by name servers) is used by  .It ~/.ssh/known_hosts
   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
 .Xr sshd 8  .Xr sshd 8
 to verify the client host when logging in; other names are needed because  for further details of the format of this file.
   .Pp
   .It ~/.ssh/rc
   Commands in this file are executed by
 .Nm  .Nm
 does not convert the user-supplied name to a canonical name before  when the user logs in, just before the user's shell (or command) is
 checking the key, because someone with access to the name servers  started.
 would then be able to fool host authentication.  See the
   .Xr sshd 8
   manual page for more information.
   .Pp
   .It /etc/hosts.equiv
   This file is for host-based authentication (see above).
   It should only be writable by root.
   .Pp
   .It /etc/shosts.equiv
   This file is used in exactly the same way as
   .Pa hosts.equiv ,
   but allows host-based authentication without permitting login with
   rlogin/rsh.
   .Pp
 .It Pa /etc/ssh/ssh_config  .It Pa /etc/ssh/ssh_config
 Systemwide configuration file.  Systemwide 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 .
 .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key  .Pp
   .It /etc/ssh/ssh_host_key
   .It /etc/ssh/ssh_host_dsa_key
   .It /etc/ssh/ssh_host_rsa_key
 These three files contain the private parts of the host keys  These three files contain the private parts of the host keys
 and are used for  and are used for host-based authentication.
 .Cm RhostsRSAAuthentication  If protocol version 1 is used,
 and  
 .Cm HostbasedAuthentication .  
 If the protocol version 1  
 .Cm RhostsRSAAuthentication  
 method is used,  
 .Nm  .Nm
 must be setuid root, since the host key is readable only by root.  must be setuid root, since the host key is readable only by root.
 For protocol version 2,  For protocol version 2,
 .Nm  .Nm
 uses  uses
 .Xr ssh-keysign 8  .Xr ssh-keysign 8
 to access the host keys for  to access the host keys,
 .Cm HostbasedAuthentication .  eliminating the requirement that
 This eliminates the requirement that  
 .Nm  .Nm
 be setuid root when that authentication method is used.  be setuid root when host-based authentication is used.
 By default  By default
 .Nm  .Nm
 is not setuid root.  is not setuid root.
 .It Pa ~/.rhosts  
 This file is used in  
 .Cm RhostsRSAAuthentication  
 and  
 .Cm HostbasedAuthentication  
 authentication to list the  
 host/user pairs that are permitted to log in.  
 (Note that this file is  
 also used by rlogin and rsh, which makes using this file insecure.)  
 Each line of the file contains a host name (in the canonical form  
 returned by name servers), and then a user name on that host,  
 separated by a space.  
 On some machines this file may need to be  
 world-readable if the user's home directory is on a NFS partition,  
 because  
 .Xr 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  
 permission for most machines is read/write for the user, and not  
 accessible by others.  
 .Pp  .Pp
 Note that  .It /etc/ssh/ssh_known_hosts
   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 should be world-readable.
   See
 .Xr sshd 8  .Xr sshd 8
 allows authentication only in combination with client host key  for further details of the format of this file.
 authentication before permitting log in.  .Pp
 If the server machine does not have the client's host key in  .It /etc/ssh/sshrc
 .Pa /etc/ssh/ssh_known_hosts ,  
 it can be stored in  
 .Pa ~/.ssh/known_hosts .  
 The easiest way to do this is to  
 connect back to the client from the server machine using ssh; this  
 will automatically add the host key to  
 .Pa ~/.ssh/known_hosts .  
 .It Pa ~/.shosts  
 This file is used exactly the same way as  
 .Pa .rhosts .  
 The purpose for  
 having this file is to be able to use  
 .Cm RhostsRSAAuthentication  
 and  
 .Cm HostbasedAuthentication  
 authentication without permitting login with  
 .Xr rlogin  
 or  
 .Xr rsh 1 .  
 .It Pa /etc/hosts.equiv  
 This file is used during  
 .Cm RhostsRSAAuthentication  
 and  
 .Cm HostbasedAuthentication  
 authentication.  
 It contains  
 canonical hosts names, one per line (the full format is described in the  
 .Xr sshd 8  
 manual page).  
 If the client host is found in this file, login is  
 automatically permitted provided client and server user names are the  
 same.  
 Additionally, successful client host key authentication is required.  
 This file should only be writable by root.  
 .It Pa /etc/shosts.equiv  
 This file is processed exactly as  
 .Pa /etc/hosts.equiv .  
 This file may be useful to permit logins using  
 .Nm  
 but not using rsh/rlogin.  
 .It Pa /etc/ssh/sshrc  
 Commands in this file are executed by  Commands in this file are executed by
 .Nm  .Nm
 when the user logs in just before the user's shell (or command) is started.  when the user logs in, just before the user's shell (or command) is started.
 See the  See the
 .Xr sshd 8  .Xr sshd 8
 manual page for more information.  manual page for more information.
 .It Pa ~/.ssh/rc  
 Commands in this file are executed by  
 .Nm  
 when the user logs in just before the user's shell (or command) is  
 started.  
 See the  
 .Xr sshd 8  
 manual page for more information.  
 .It Pa ~/.ssh/environment  
 Contains additional definitions for environment variables, see section  
 .Sx ENVIRONMENT  
 above.  
 .El  .El
 .Sh DIAGNOSTICS  
 .Nm  
 exits with the exit status of the remote command or with 255  
 if an error occurred.  
 .Sh SEE ALSO  .Sh SEE ALSO
 .Xr gzip 1 ,  
 .Xr rsh 1 ,  
 .Xr scp 1 ,  .Xr scp 1 ,
 .Xr sftp 1 ,  .Xr sftp 1 ,
 .Xr ssh-add 1 ,  .Xr ssh-add 1 ,
 .Xr ssh-agent 1 ,  .Xr ssh-agent 1 ,
 .Xr ssh-keygen 1 ,  .Xr ssh-keygen 1 ,
 .Xr telnet 1 ,  .Xr ssh-keyscan 1 ,
   .Xr tun 4 ,
 .Xr hosts.equiv 5 ,  .Xr hosts.equiv 5 ,
 .Xr ssh_config 5 ,  .Xr ssh_config 5 ,
 .Xr ssh-keysign 8 ,  .Xr ssh-keysign 8 ,
 .Xr sshd 8  .Xr sshd 8
 .Rs  .Rs
 .%A T. Ylonen  .%R RFC 4250
 .%A T. Kivinen  .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
 .%A M. Saarinen  .%D 2006
 .%A T. Rinne  .Re
 .%A S. Lehtinen  .Rs
 .%T "SSH Protocol Architecture"  .%R RFC 4251
 .%N draft-ietf-secsh-architecture-12.txt  .%T "The Secure Shell (SSH) Protocol Architecture"
 .%D January 2002  .%D 2006
 .%O work in progress material  .Re
   .Rs
   .%R RFC 4252
   .%T "The Secure Shell (SSH) Authentication Protocol"
   .%D 2006
   .Re
   .Rs
   .%R RFC 4253
   .%T "The Secure Shell (SSH) Transport Layer Protocol"
   .%D 2006
   .Re
   .Rs
   .%R RFC 4254
   .%T "The Secure Shell (SSH) Connection Protocol"
   .%D 2006
   .Re
   .Rs
   .%R RFC 4255
   .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
   .%D 2006
   .Re
   .Rs
   .%R RFC 4256
   .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
   .%D 2006
   .Re
   .Rs
   .%R RFC 4335
   .%T "The Secure Shell (SSH) Session Channel Break Extension"
   .%D 2006
   .Re
   .Rs
   .%R RFC 4344
   .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
   .%D 2006
   .Re
   .Rs
   .%R RFC 4345
   .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
   .%D 2006
   .Re
   .Rs
   .%R RFC 4419
   .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
   .%D 2006
 .Re  .Re
 .Sh AUTHORS  .Sh AUTHORS
 OpenSSH is a derivative of the original and free  OpenSSH is a derivative of the original and free
Legend:
Removed from v.1.209  
changed lines
  Added in v.1.209.2.3