The forked daemons handle
key exchange, encryption, authentication, command execution,
and data exchange.
-.Pp
+This implementation of
+.Nm
+supports both SSH protocol version 1 and 2 simultaneously.
.Nm
works as follows.
+.Pp
+.Ss SSH protocol version 1
+.Pp
Each host has a host-specific RSA key
(normally 1024 bits) used to identify the host.
Additionally, when
Whenever a client connects the daemon responds with its public
host and server keys.
The client compares the
-host key against its own database to verify that it has not changed.
+RSA host key against its own database to verify that it has not changed.
The client then generates a 256 bit random number.
It encrypts this
random number using both the host key and the server key, and sends
.Xr rsh 1
into the machine).
.Pp
+.Ss SSH protocol version 2
+.Pp
+Version 2 works similar:
+Each host has a host-specific DSA key used to identify the host.
+However, when the daemon starts, it does not generate a server key.
+Forward security is provided through a Diffie-Hellman key agreement.
+This key agreement results in a shared session key.
+The rest of the session is encrypted
+using a symmetric cipher, currently
+Blowfish, 3DES or CAST128 in CBC mode or Arcfour.
+The client selects the encryption algorithm
+to use from those offered by the server.
+Additionally, session integrity is provided
+through a crytographic message authentication code
+(hmac-sha1 or hmac-md5).
+.Pp
+Protocol version 2 provides a public key based
+user authentication method (DSAAuthentication)
+and conventional password authentication.
+.Pp
+.Ss Command execution and data forwarding
+.Pp
If the client successfully authenticates itself, a dialog for
preparing the session is entered.
At this time the client may request
this many seconds, the server disconnects and exits.
A value of zero indicates no limit.
.It Fl h Ar host_key_file
-Specifies the file from which the host key is read (default
+Specifies the file from which the RSA host key is read (default
.Pa /etc/ssh_host_key ) .
This option must be given if
.Nm
can be used as wildcards in the patterns.
Only user names are valid, a numerical user ID isn't recognized.
By default login is allowed regardless of the user name.
+.It Cm DSAAuthentication
+Specifies whether DSA authentication is allowed.
+The default is
+.Dq yes .
+Note that this option applies to protocol version 2 only.
+.It Cm GatewayPorts
+Specifies whether remote hosts are allowed to connect to ports
+forwarded for the client.
+The argument must be
+.Dq yes
+or
+.Dq no .
+The default is
+.Dq no .
+.It Cm HostDsaKey
+Specifies the file containing the private DSA host key (default
+.Pa /etc/ssh_host_dsa_key )
+used by SSH protocol 2.0.
+Note that
+.Nm
+disables protcol 2.0 if this file is group/world-accessible.
.It Cm HostKey
-Specifies the file containing the private host key (default
-.Pa /etc/ssh_host_key ) .
+Specifies the file containing the private RSA host key (default
+.Pa /etc/ssh_host_key )
+used by SSH protocols 1.3 and 1.5.
Note that
.Nm
-does not start if this file is group/world-accessible.
+disables protcols 1.3 and 1.5 if this file is group/world-accessible.
.It Cm IgnoreRhosts
Specifies that
.Pa .rhosts
Specifies whether password authentication is allowed.
The default is
.Dq yes .
+Note that this option applies to both protocol version 1 and 2.
.It Cm PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings.
Specifies whether pure RSA authentication is allowed.
The default is
.Dq yes .
+Note that this option applies to protocol version 1 only.
.It Cm ServerKeyBits
Defines the number of bits in the server key.
The minimum value is 512, and the default is 768.
The
.Pa $HOME/.ssh/authorized_keys
file lists the RSA keys that are
-permitted for RSA authentication.
+permitted for RSA authentication in SSH protocols 1.3 and 1.5
+Similarily, the
+.Pa $HOME/.ssh/authorized_keys2
+file lists the DSA keys that are
+permitted for DSA authentication in SSH protocol 2.0.
Each line of the file contains one
key (empty lines and lines starting with a
.Ql #
command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
-.Pa /etc/ssh_known_hosts
+.Pa /etc/ssh_known_hosts ,
+.Pa /etc/ssh_known_hosts2 ,
+.Pa $HOME/.ssh/known_hosts ,
and
-.Pa $HOME/.ssh/known_hosts
+.Pa $HOME/.ssh/known_hosts2
files contain host public keys for all known hosts.
The global file should
be prepared by the administrator (optional), and the per-user file is
pattern, it is not accepted (by that line) even if it matched another
pattern on the line.
.Pp
-Bits, exponent, and modulus are taken directly from the host key; they
+Bits, exponent, and modulus are taken directly from the RSA host key; they
can be obtained, e.g., from
.Pa /etc/ssh_host_key.pub .
The optional comment field continues to the end of the line, and is not used.
volume).
It is recommended that it not be accessible by others.
The format of this file is described above.
+Users will place the contents of their
+.Pa identity.pub
+files into this file, as described in
+.Xr ssh-keygen 1 .
+.It Pa $HOME/.ssh/authorized_keys2
+Lists the DSA keys that can be used to log into the user's account.
+This file must be readable by root (which may on some machines imply
+it being world-readable if the user's home directory resides on an NFS
+volume).
+It is recommended that it not be accessible by others.
+The format of this file is described above.
+Users will place the contents of their
+.Pa id_dsa.pub
+files into this file, as described in
+.Xr ssh-keygen 1 .
.It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
These files are consulted when using rhosts with RSA host
authentication to check the public key of the host.
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
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
supports one-time password authentication with
.Xr skey 1 .
.El
+.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 scp 1 ,
.Xr ssh 1 ,
andersk / openssh.git / blobdiff