.Op Ar command
.Pp
.Nm ssh
-.Op Fl afgknqtvxCPX46
-.Op Fl c Ar blowfish | 3des
+.Op Fl afgknqtvxACNPTX246
+.Op Fl c Ar cipher_spec
.Op Fl e Ar escape_char
.Op Fl i Ar identity_file
.Op Fl l Ar login_name
connects and logs into the specified
.Ar hostname .
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:
+.Pp
+.Ss SSH protocol version 1
.Pp
First, if the machine the user logs in from is listed in
.Pa /etc/hosts.equiv
.Pa hosts.equiv
method combined with RSA-based host authentication.
It means that if the login would be permitted by
-.Pa \&.rhosts ,
-.Pa \&.shosts ,
+.Pa $HOME/.rhosts ,
+.Pa $HOME/.shosts ,
.Pa /etc/hosts.equiv ,
or
.Pa /etc/shosts.equiv ,
spoofing, DNS spoofing and routing spoofing.
[Note to the administrator:
.Pa /etc/hosts.equiv ,
-.Pa \&.rhosts ,
+.Pa $HOME/.rhosts ,
and the rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.]
.Pp
The user creates his/her RSA key pair by running
.Xr ssh-keygen 1 .
This stores the private key in
-.Pa \&.ssh/identity
+.Pa $HOME/.ssh/identity
and the public key in
-.Pa \&.ssh/identity.pub
+.Pa $HOME/.ssh/identity.pub
in the user's home directory.
The user should then copy the
.Pa identity.pub
to
-.Pa \&.ssh/authorized_keys
+.Pa $HOME/.ssh/authorized_keys
in his/her home directory on the remote machine (the
.Pa authorized_keys
file corresponds to the conventional
-.Pa \&.rhosts
+.Pa $HOME/.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.
host for checking; however, since all communications are encrypted,
the password cannot be seen by someone listening on the network.
.Pp
+.Ss SSH protocol version 2
+.Pp
+When a user connects using the protocol version 2
+different authentication methods are available:
+At first, the client attempts to authenticate using the public key method.
+If this method fails password authentication is tried.
+.Pp
+The public key method is similar to RSA authentication described
+in the previous section except that the DSA algorithm is used
+instead of the patented RSA algorithm.
+The client uses his private DSA key
+.Pa $HOME/.ssh/id_dsa
+to sign the session identifier and sends the result to the server.
+The server checks whether the matching public key is listed in
+.Pa $HOME/.ssh/authorized_keys2
+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 for proving the user's identity.
+This protocol 2 implementation does not yet support Kerberos or
+S/Key authentication.
+.Pp
+Protocol 2 provides additional mechanisms for confidentiality
+(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
+and integrity (hmac-sha1, hmac-md5).
+Note that protocol 1 lacks a strong mechanism for ensuring the
+integrity of the connection.
+.Pp
+.Ss Login session and remote execution
+.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.
of
.Nm ssh .
.Pp
+.Ss X11 and TCP forwarding
+.Pp
If the user is using X11 (the
.Ev DISPLAY
environment variable is set), the connection to the X11 display is
One possible application of TCP/IP forwarding is a secure connection to an
electronic purse; another is going trough firewalls.
.Pp
+.Ss Server authentication
+.Pp
.Nm
-automatically maintains and checks a database containing RSA-based
+automatically maintains and checks a database containing
identifications for all hosts it has ever been used with.
-The database is stored in
-.Pa \&.ssh/known_hosts
+RSA host keys are stored in
+.Pa $HOME/.ssh/known_hosts
+and
+DSA host keys are stored in
+.Pa $HOME/.ssh/known_hosts2
in the user's home directory.
-Additionally, the file
+Additionally, the files
.Pa /etc/ssh_known_hosts
-is automatically checked for known hosts.
+and
+.Pa /etc/ssh_known_hosts2
+are automatically checked for known hosts.
Any new hosts are automatically added to the user's file.
If a host's identification
ever changes,
.Bl -tag -width Ds
.It Fl a
Disables forwarding of the authentication agent connection.
-This may also be specified on a per-host basis in the configuration file.
+.It Fl A
+Enables forwarding of the authentication agent connection.
+This can also be specified on a per-host basis in a configuration file.
.It Fl c Ar blowfish|3des
Selects the cipher to use for encrypting the session.
.Ar 3des
(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
It is presumably more secure than the
.Ar des
-cipher which is no longer supported in ssh.
+cipher which is no longer supported in
+.Nm ssh .
.Ar blowfish
is a fast block cipher, it appears very secure and is much faster than
.Ar 3des .
+.It Fl c Ar "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"
+Additionally, for protocol version 2 a comma-separated list of ciphers can
+be specified in order of preference. Protocol version 2 supports
+3DES, Blowfish and CAST128 in CBC mode and Arcfour.
.It Fl e Ar ch|^ch|none
Sets the escape character for sessions with a pty (default:
.Ql ~ ) .
Selects the file from which the identity (private key) for
RSA authentication is read.
Default is
-.Pa \&.ssh/identity
+.Pa $HOME/.ssh/identity
in the user's home directory.
Identity files may also be specified on
a per-host basis in the configuration file.
needs to ask for a password or passphrase; see also the
.Fl f
option.)
+.It Fl N
+Do not execute a remote command.
+This is usefull if you just want to forward ports
+(protocol version 2 only).
.It Fl o Ar option
Can be used to give options in the format used in the config file.
This is useful for specifying options for which there is no separate
This can be used to execute arbitrary
screen-based programs on a remote machine, which can be very useful,
e.g., when implementing menu services.
+.It Fl T
+Disable pseudo-tty allocation (protocol version 2 only).
.It Fl v
Verbose mode.
Causes
challenges, if the user entered "s/key" as password.
.It Fl x
Disables X11 forwarding.
-This can also be specified on a per-host basis in a configuration file.
.It Fl X
Enables X11 forwarding.
+This can also be specified on a per-host basis in a configuration file.
.It Fl C
Requests compression of all data (including stdin, stdout, stderr, and
data for forwarded X11 and TCP/IP connections).
Port forwardings can also be specified in the configuration file.
Privileged ports can be forwarded only when
logging in as root on the remote machine.
+.It Fl 2
+Forces
+.Nm
+to try protocol version 2 only.
.It Fl 4
Forces
.Nm
in order of preference.
Multiple ciphers must be comma-separated.
The default is
-.Dq blowfish-cbc,3des-cbc,arcfour,cast128-cbc .
+.Dq 3des-cbc,blowfish-cbc,arcfour,cast128-cbc .
.It Cm Compression
Specifies whether to use compression.
The argument must be
back to rsh or exiting.
The argument must be an integer.
This may be useful in scripts if the connection sometimes fails.
+.It Cm DSAAuthentication
+Specifies whether to try DSA authentication.
+The argument to this keyword must be
+.Dq yes
+or
+.Dq no .
+DSA authentication will only be
+attempted if a DSA identity file exists.
+Note that this option applies to protocol version 2 only.
.It Cm EscapeChar
Sets the escape character (default:
.Ql ~ ) .
.Dq yes
or
.Dq no .
+The default is
+.Dq no .
.It Cm ForwardX11
Specifies whether X11 connections will be automatically redirected
over the secure channel and
.It Cm IdentityFile
Specifies the file from which the user's RSA authentication identity
is read (default
-.Pa .ssh/identity
+.Pa $HOME/.ssh/identity
in the user's home directory).
Additionally, any identities represented by the authentication agent
will be used for authentication.
It is possible to have
multiple identity files specified in configuration files; all these
identities will be tried in sequence.
+.It Cm IdentityFile2
+Specifies the file from which the user's DSA authentication identity
+is read (default
+.Pa $HOME/.ssh/id_dsa
+in the user's home directory).
+The file name may use the tilde
+syntax to refer to a user's home directory.
+It is possible to have
+multiple identity files specified in configuration files; all these
+identities will be tried in sequence.
.It Cm KeepAlive
Specifies whether the system should send keepalive messages to the
other side.
.Dq yes
or
.Dq no .
+Note that this option applies to both protocol version 1 and 2.
.It Cm Port
Specifies the port number to connect on the remote host.
Default is 22.
.Dq 2 .
Multiple versions must be comma-separated.
The default is
-.Dq 1 .
+.Dq 1,2 .
+This means that
+.Nm
+tries version 1 and falls back to version 2
+if version 1 is not available.
.It Cm ProxyCommand
Specifies the command to use to connect to the server.
The command
RSA authentication will only be
attempted if the identity file exists, or an authentication agent is
running.
+Note that this option applies to protocol version 1 only.
.It Cm SkeyAuthentication
Specifies whether to use
.Xr skey 1
.Nm
ssh will never automatically add host keys to the
.Pa $HOME/.ssh/known_hosts
-file, and refuses to connect hosts whose host key has changed.
+and
+.Pa $HOME/.ssh/known_hosts2
+files, and refuses to connect hosts whose host key has changed.
This provides maximum protection against trojan horse attacks.
However, it can be somewhat annoying if you don't have good
.Pa /etc/ssh_known_hosts
+and
+.Pa /etc/ssh_known_hosts2
files installed and frequently
connect new hosts.
Basically this option forces the user to manually
.Pa /etc/ssh_known_hosts ) .
See
.Xr sshd 8 .
-.It Pa $HOME/.ssh/identity
-Contains the RSA authentication identity of the user.
-This file
-contains sensitive data and should be readable by the user but not
+.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
+Contains the RSA and the DSA authentication identity of the user.
+These files
+contain sensitive data and should be readable by the user but not
accessible by others (read/write/execute).
Note that
.Nm
-ignores this file if it is accessible by others.
+ignores a private key file if it is accessible by others.
It is possible to specify a passphrase when
generating the key; the passphrase will be used to encrypt the
sensitive part of this file using 3DES.
-.It Pa $HOME/.ssh/identity.pub
+.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
Contains the public key for authentication (public part of the
identity file in human-readable form).
-The contents of this file should be added to
+The contents of the
+.Pa $HOME/.ssh/identity.pub
+file should be added to
.Pa $HOME/.ssh/authorized_keys
on all machines
where you wish to log in using RSA authentication.
-This file is not
+The contents of the
+.Pa $HOME/.ssh/id_dsa.pub
+file should be added to
+.Pa $HOME/.ssh/authorized_keys2
+on all machines
+where you wish to log in using DSA authentication.
+These files are not
sensitive and can (but need not) be readable by anyone.
-This file is
-never used automatically and is not necessary; it is only provided for
+These files are
+never used automatically and are not necessary; they is only provided for
the convenience of the user.
.It Pa $HOME/.ssh/config
This is the per-user configuration file.
spaces).
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_known_hosts
+.It Pa $HOME/.ssh/authorized_keys2
+Lists the DSA keys that can be used for logging in as this user.
+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_known_hosts, /etc/ssh_known_hosts2
Systemwide list of known host keys.
-This file should be prepared by the
+.Pa /etc/ssh_known_hosts
+contains RSA and
+.Pa /etc/ssh_known_hosts2
+contains DSA keys.
+These files 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.
.Pa $HOME/.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 inxi
+will automatically add the host key to
.Pa $HOME/.ssh/known_hosts .
.It Pa $HOME/.shosts
This file is used exactly the same way as
Rapidly after the
1.2.12 release, newer versions of the original ssh bore successively
more restrictive licenses, and thus demand for a free version was born.
+.Pp
This version of OpenSSH
.Bl -bullet
.It
-has all components of a restrictive nature (i.e., patents, see
-.Xr ssl 8 )
+has all components of a restrictive nature (i.e., patents)
directly removed from the source code; any licensed or patented components
are chosen from
external libraries.
.It
-has been updated to support ssh protocol 1.5, making it compatible with
-all other ssh protocol 1 clients and servers.
+has been updated to support SSH protocol 1.5 and 2, making it compatible with
+all other SSH clients and servers.
.It
contains added support for
.Xr kerberos 8
.Xr skey 1 .
.El
.Pp
-The libraries described in
-.Xr ssl 8
-are required for proper operation.
-.Pp
OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
Niels Provos, Theo de Raadt, and Dug Song.
+.Pp
+The support for SSH protocol 2 was written by Markus Friedl.
.Sh SEE ALSO
.Xr rlogin 1 ,
.Xr rsh 1 ,
.Xr ssh-keygen 1 ,
.Xr telnet 1 ,
.Xr sshd 8 ,
-.Xr ssl 8
andersk / openssh.git / blobdiff