.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
-.\" $OpenBSD: sshd.8,v 1.186 2002/06/22 16:45:29 stevesk Exp $
+.\" $OpenBSD: sshd.8,v 1.200 2003/10/08 08:27:36 jmc Exp $
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Nd OpenSSH SSH daemon
.Sh SYNOPSIS
.Nm sshd
-.Op Fl deiqtD46
+.Bk -words
+.Op Fl 46Ddeiqt
.Op Fl b Ar bits
.Op Fl f Ar config_file
.Op Fl g Ar login_grace_time
.Op Fl o Ar option
.Op Fl p Ar port
.Op Fl u Ar len
+.Ek
.Sh DESCRIPTION
.Nm
(SSH Daemon) is the daemon program for
.Nm
supports both SSH protocol version 1 and 2 simultaneously.
.Nm
-works as follows.
-.Pp
+works as follows:
.Ss SSH protocol version 1
-.Pp
Each host has a host-specific RSA key
(normally 1024 bits) used to identify the host.
Additionally, when
This key is normally regenerated every hour if it has been used, and
is never stored on disk.
.Pp
-Whenever a client connects the daemon responds with its public
+Whenever a client connects, the daemon responds with its public
host and server keys.
The client compares the
RSA host key against its own database to verify that it has not changed.
-The client then generates a 256 bit random number.
+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
the encrypted number to the server.
.Pp
Next, the server and the client enter an authentication dialog.
The client tries to authenticate itself using
-.Pa .rhosts
+.Em .rhosts
authentication,
-.Pa .rhosts
+.Em .rhosts
authentication combined with RSA host
authentication, RSA challenge-response authentication, or password
based authentication.
.Pp
-Rhosts authentication is normally disabled
+Regardless of the authentication type, the account is checked to
+ensure that it is accessible. An account is not accessible if it is
+locked, listed in
+.Cm DenyUsers
+or its group is listed in
+.Cm DenyGroups
+\&. The definition of a locked account is system dependant. Some platforms
+have their own account database (eg AIX) and some modify the passwd field (
+.Ql \&*LK\&*
+on Solaris,
+.Ql \&*
+on HP-UX, containing
+.Ql Nologin
+on Tru64 and a leading
+.Ql \&!!
+on Linux). If there is a requirement to disable password authentication
+for the account while allowing still public-key, then the passwd field
+should be set to something other than these values (eg
+.Ql NP
+or
+.Ql \&*NP\&*
+).
+.Pp
+.Em rhosts
+authentication is normally disabled
because it is fundamentally insecure, but can be enabled in the server
configuration file if desired.
System security is not improved unless
.Nm rshd ,
.Nm rlogind ,
and
-.Xr rexecd
+.Nm rexecd
are disabled (thus completely disabling
.Xr rlogin
and
.Xr rsh
into the machine).
-.Pp
.Ss SSH protocol version 2
-.Pp
Version 2 works similarly:
Each host has a host-specific key (RSA or DSA) used to identify the host.
However, when the daemon starts, it does not generate a server key.
This key agreement results in a shared session key.
.Pp
The rest of the session is encrypted using a symmetric cipher, currently
-128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
+128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.
The client selects the encryption algorithm
to use from those offered by the server.
Additionally, session integrity is provided
user (PubkeyAuthentication) or
client host (HostbasedAuthentication) authentication method,
conventional password authentication and challenge response based methods.
-.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
the client, and both sides exit.
.Pp
.Nm
-can be configured using command-line options or a configuration
-file.
+can be configured using command-line options or a configuration file
+(by default
+.Xr sshd_config 5 ) .
Command-line options override values specified in the
configuration file.
.Pp
.Pp
The options are as follows:
.Bl -tag -width Ds
+.It Fl 4
+Forces
+.Nm
+to use IPv4 addresses only.
+.It Fl 6
+Forces
+.Nm
+to use IPv6 addresses only.
.It Fl b Ar bits
Specifies the number of bits in the ephemeral protocol version 1
server key (default 768).
+.It Fl D
+When this option is specified,
+.Nm
+will not detach and does not become a daemon.
+This allows easy monitoring of
+.Nm sshd .
.It Fl d
Debug mode.
The server sends verbose debug output to the system
log, and does not put itself in the background.
The server also will not fork and will only process one connection.
This option is only intended for debugging for the server.
-Multiple -d options increase the debugging level.
+Multiple
+.Fl d
+options increase the debugging level.
Maximum is 3.
.It Fl e
When this option is specified,
refuses to start if there is no configuration file.
.It Fl g Ar login_grace_time
Gives the grace time for clients to authenticate themselves (default
-600 seconds).
+120 seconds).
If the client fails to authenticate the user within
this many seconds, the server disconnects and exits.
A value of zero indicates no limit.
.It Fl i
Specifies that
.Nm
-is being run from inetd.
+is being run from
+.Xr inetd 8 .
.Nm
is normally not run
from inetd because it needs to generate the server key before it can
Specifies how often the ephemeral protocol version 1 server key is
regenerated (default 3600 seconds, or one hour).
The motivation for regenerating the key fairly
-often is that the key is not stored anywhere, and after about an hour,
+often is that the key is not stored anywhere, and after about an hour
it becomes impossible to recover the key for decrypting intercepted
communications even if the machine is cracked into or physically
seized.
Can be used to give options in the format used in the configuration file.
This is useful for specifying options for which there is no separate
command-line flag.
+For full details of the options, and their values, see
+.Xr sshd_config 5 .
.It Fl p Ar port
Specifies the port on which the server listens for connections
(default 22).
.Pa utmp
file.
.Fl u0
-is also be used to prevent
+may also be used to prevent
.Nm
from making DNS requests unless the authentication
mechanism or configuration requires it.
Authentication mechanisms that may require DNS include
-.Cm RhostsAuthentication ,
.Cm RhostsRSAAuthentication ,
.Cm HostbasedAuthentication
and using a
.Cm AllowUsers
or
.Cm DenyUsers .
-.It Fl D
-When this option is specified
-.Nm
-will not detach and does not become a daemon.
-This allows easy monitoring of
-.Nm sshd .
-.It Fl 4
-Forces
-.Nm
-to use IPv4 addresses only.
-.It Fl 6
-Forces
-.Nm
-to use IPv6 addresses only.
.El
.Sh CONFIGURATION FILE
.Nm
.It
Sets up basic environment.
.It
-Reads
-.Pa $HOME/.ssh/environment
-if it exists.
+Reads the file
+.Pa $HOME/.ssh/environment ,
+if it exists, and users are allowed to change their environment.
+See the
+.Cm PermitUserEnvironment
+option in
+.Xr sshd_config 5 .
.It
Changes to user's home directory.
.It
spaces: options, bits, exponent, modulus, comment.
Each protocol version 2 public key consists of:
options, keytype, base64 encoded key, comment.
-The options fields
-are optional; its presence is determined by whether the line starts
-with a number or not (the option field never starts with a number).
+The options field
+is optional; its presence is determined by whether the line starts
+with a number or not (the options field never starts with a number).
The bits, exponent, modulus and comment fields give the RSA key for
protocol version 1; the
comment field is not used for anything (but may be convenient for the
.Dq ssh-rsa .
.Pp
Note that lines in this file are usually several hundred bytes long
-(because of the size of the RSA key modulus).
+(because of the size of the public key encoding).
You don't want to type them in; instead, copy the
.Pa identity.pub ,
.Pa id_dsa.pub
that option keywords are case-insensitive):
.Bl -tag -width Ds
.It Cm from="pattern-list"
-Specifies that in addition to RSA authentication, the canonical name
+Specifies that in addition to public key authentication, the canonical name
of the remote host must be present in the comma-separated list of
patterns
-.Pf ( Ql *
+.Pf ( Ql \&*
and
-.Ql ?
+.Ql \&?
serve as wildcards).
The list may also contain
patterns negated by prefixing them with
-.Ql ! ;
+.Ql \&! ;
if the canonical host name matches a negated pattern, the key is not accepted.
The purpose
-of this option is to optionally increase security: RSA authentication
+of this option is to optionally increase security: public key authentication
by itself does not trust the network or name servers or anything (but
the key); however, if somebody somehow steals the key, the key
permits an intruder to log in from anywhere in the world.
The command supplied by the user (if any) is ignored.
The command is run on a pty if the client requests a pty;
otherwise it is run without a tty.
-If a 8-bit clean channel is required,
+If an 8-bit clean channel is required,
one must not request a pty or should specify
.Cm no-pty .
A quote may be included in the command by quoting it with a backslash.
This option might be useful
-to restrict certain RSA keys to perform just a specific operation.
+to restrict certain public keys to perform just a specific operation.
An example might be a key that permits remote backups but nothing else.
Note that the client may specify TCP/IP and/or X11
forwarding unless they are explicitly prohibited.
Environment variables set this way
override other default environment values.
Multiple options of this type are permitted.
+Environment processing is disabled by default and is
+controlled via the
+.Cm PermitUserEnvironment
+option.
This option is automatically disabled if
.Cm UseLogin
is enabled.
port forwarding such that it may only connect to the specified host and
port.
IPv6 addresses can be specified with an alternative syntax:
-.Ar host/port .
+.Ar host Ns / Ns Ar port .
Multiple
.Cm permitopen
-options may be applied separated by commas. No pattern matching is
-performed on the specified hostnames, they must be literal domains or
-addresses.
+options may be applied separated by commas.
+No pattern matching is performed on the specified hostnames,
+they must be literal domains or addresses.
.El
.Ss Examples
-1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
+1024 33 12121...312314325 ylo@foo.bar
.Pp
-from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
+from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
.Pp
-command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
+command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
.Pp
-permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
+permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
-.Pa /etc/ssh/ssh_known_hosts ,
+.Pa /etc/ssh/ssh_known_hosts
and
.Pa $HOME/.ssh/known_hosts
files contain host public keys for all known hosts.
bits, exponent, modulus, comment.
The fields are separated by spaces.
.Pp
-Hostnames is a comma-separated list of patterns ('*' and '?' act as
+Hostnames is a comma-separated list of patterns
+.Pf ( Ql \&*
+and
+.Ql \&?
+act as
wildcards); each pattern in turn is matched against the canonical host
name (when authenticating a client) or against the user-supplied
name (when authenticating a server).
A pattern may also be preceded by
-.Ql !
+.Ql \&!
to indicate negation: if the host name matches a negated
pattern, it is not accepted (by that line) even if it matched another
pattern on the line.
and adding the host names at the front.
.Ss Examples
.Bd -literal
-closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
+closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
.Ed
.Sh FILES
.Xr ssh-keygen 1 .
.It Pa /etc/moduli
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
+The file format is described in
+.Xr moduli 5 .
.It Pa /var/empty
.Xr chroot 2
directory used by
.Pa id_rsa.pub
files into this file, as described in
.Xr ssh-keygen 1 .
-.It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
+.It Pa "/etc/ssh/ssh_known_hosts", "$HOME/.ssh/known_hosts"
These files are consulted when using rhosts with RSA host
authentication or protocol version 2 hostbased authentication
to check the public key of the host.
.Pa /etc/ssh/ssh_known_hosts
should be world-readable, and
.Pa $HOME/.ssh/known_hosts
-can but need not be world-readable.
+can, but need not be, world-readable.
.It Pa /etc/nologin
If this file exists,
.Nm
This file contains host-username pairs, separated by a space, one per
line.
The given user on the corresponding host is permitted to log in
-without password.
+without a password.
The same file is used by rlogind and rshd.
The file must
be writable only by the user; it is recommended that it not be
accessible by others.
.Pp
-If is also possible to use netgroups in the file.
+It is also possible to use netgroups in the file.
Either host or user
name may be of the form +@groupname to specify all hosts or all users
in the group.
not used by rlogin and rshd, so using this permits access using SSH only.
.It Pa /etc/hosts.equiv
This file is used during
-.Pa .rhosts
+.Em rhosts
authentication.
In the simplest form, this file contains host names, one per line.
Users on
and assignment lines of the form name=value.
The file should be writable
only by the user; it need not be readable by anyone else.
+Environment processing is disabled by default and is
+controlled via the
+.Cm PermitUserEnvironment
+option.
.It Pa $HOME/.ssh/rc
-If this file exists, it is run with /bin/sh after reading the
+If this file exists, it is run with
+.Pa /bin/sh
+after reading the
environment files but before starting the user's shell or command.
It must not produce any output on stdout; stderr must be used
instead.
if read proto cookie && [ -n "$DISPLAY" ]; then
if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
# X11UseLocalhost=yes
- xauth add unix:`echo $DISPLAY |
+ echo add unix:`echo $DISPLAY |
cut -c11-` $proto $cookie
else
# X11UseLocalhost=no
- xauth add $DISPLAY $proto $cookie
- fi
+ echo add $DISPLAY $proto $cookie
+ fi | xauth -q -
fi
.Ed
.Pp
machine-specific login-time initializations globally.
This file should be writable only by root, and should be world-readable.
.El
-.Sh AUTHORS
-OpenSSH is a derivative of the original and free
-ssh 1.2.12 release by Tatu Ylonen.
-Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
-Theo de Raadt and Dug Song
-removed many bugs, re-added newer features and
-created OpenSSH.
-Markus Friedl contributed the support for SSH
-protocol versions 1.5 and 2.0.
-Niels Provos and Markus Friedl contributed support
-for privilege separation.
.Sh SEE ALSO
.Xr scp 1 ,
.Xr sftp 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
+.Xr chroot 2 ,
+.Xr hosts_access 5 ,
.Xr login.conf 5 ,
.Xr moduli 5 ,
.Xr sshd_config 5 ,
+.Xr inetd 8 ,
.Xr sftp-server 8
.Rs
.%A T. Ylonen
.%D January 2002
.%O work in progress material
.Re
+.Sh AUTHORS
+OpenSSH is a derivative of the original and free
+ssh 1.2.12 release by Tatu Ylonen.
+Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
+Theo de Raadt and Dug Song
+removed many bugs, re-added newer features and
+created OpenSSH.
+Markus Friedl contributed the support for SSH
+protocol versions 1.5 and 2.0.
+Niels Provos and Markus Friedl contributed support
+for privilege separation.
andersk / gssapi-openssh.git / blobdiff