path: root/sshd.8

.\"  -*- nroff -*-
.\"
.\" sshd.8.in
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\"
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" Created: Sat Apr 22 21:55:14 1995 ylo
.\"
.\" $Id: sshd.8,v 1.16 2000/04/01 01:09:27 damien Exp $
.\"
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Sh NAME
.Nm sshd
.Nd secure shell daemon
.Sh SYNOPSIS
.Nm sshd
.Op Fl diqQ46
.Op Fl b Ar bits
.Op Fl f Ar config_file
.Op Fl g Ar login_grace_time
.Op Fl h Ar host_key_file
.Op Fl k Ar key_gen_time
.Op Fl p Ar port
.Op Fl V Ar client_protocol_id
.Sh DESCRIPTION 
.Nm
(Secure Shell Daemon) is the daemon program for 
.Xr ssh 1 .
Together these programs replace rlogin and rsh programs, and
provide secure encrypted communications between two untrusted hosts
over an insecure network.
The programs are intended to be as easy to
install and use as possible.
.Pp
.Nm
is the daemon that listens for connections from clients.
It is normally started at boot from 
.Pa /etc/rc .
It forks a new
daemon for each incoming connection.
The forked daemons handle
key exchange, encryption, authentication, command execution,
and data exchange.
.Pp
.Nm
works as follows.
Each host has a host-specific RSA key
(normally 1024 bits) used to identify the host.
Additionally, when
the daemon starts, it generates a server RSA key (normally 768 bits).
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, the daemon sends its host
and server public keys to the client.
The client compares the
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
the encrypted number to the server.
Both sides then start to use this
random number as a session key which is used to encrypt all further
communications in the session.
The rest of the session is encrypted
using a conventional cipher, currently Blowfish and 3DES, with 3DES
being used by default.
The client selects the encryption algorithm
to use from those offered by the server.
.Pp
Next, the server and the client enter an authentication dialog.
The client tries to authenticate itself using
.Pa .rhosts
authentication,
.Pa .rhosts
authentication combined with RSA host
authentication, RSA challenge-response authentication, or password
based authentication.
.Pp
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
.Xr rshd 8 ,
.Xr rlogind 8 ,
.Xr rexecd 8 ,
and
.Xr rexd 8
are disabled (thus completely disabling
.Xr rlogin 1
and
.Xr rsh 1
into that machine).
.Pp
If the client successfully authenticates itself, a dialog for
preparing the session is entered.
At this time the client may request
things like allocating a pseudo-tty, forwarding X11 connections,
forwarding TCP/IP connections, or forwarding the authentication agent
connection over the secure channel.
.Pp
Finally, the client either requests a shell or execution of a command.
The sides then enter session mode.
In this mode, either side may send
data at any time, and such data is forwarded to/from the shell or
command on the server side, and the user terminal in the client side.
.Pp
When the user program terminates and all forwarded X11 and other
connections have been closed, the server sends command exit status to
the client, and both sides exit.
.Pp
.Nm
can be configured using command-line options or a configuration
file.
Command-line options override values specified in the
configuration file.
.Pp
.Nm
rereads its configuration file when it receives a hangup signal,
.Dv SIGHUP .
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl b Ar bits
Specifies the number of bits in the server key (default 768).
.Pp
.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.
.It Fl f Ar configuration_file
Specifies the name of the configuration file.
The default is
.Pa /etc/sshd_config .
.Nm
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
300 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 h Ar host_key_file
Specifies the file from which the host key is read (default
.Pa /etc/ssh_host_key ) .
This option must be given if
.Nm
is not run as root (as the normal
host file is normally not readable by anyone but root).
.It Fl i
Specifies that
.Nm
is being run from inetd. 
.Nm
is normally not run
from inetd because it needs to generate the server key before it can
respond to the client, and this may take tens of seconds.
Clients would have to wait too long if the key was regenerated every time.
However, with small key sizes (e.g., 512) using
.Nm
from inetd may
be feasible.
.It Fl k Ar key_gen_time
Specifies how often the 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,
it becomes impossible to recover the key for decrypting intercepted
communications even if the machine is cracked into or physically
seized.
A value of zero indicates that the key will never be regenerated.
.It Fl p Ar port
Specifies the port on which the server listens for connections
(default 22).
.It Fl q
Quiet mode.
Nothing is sent to the system log.
Normally the beginning,
authentication, and termination of each connection is logged.
.It Fl Q
Do not print an error message if RSA support is missing.
.It Fl V Ar client_protocol_id
SSH2 compatibility mode.
When this options is specified
.Nm
assumes the client has sent the given version string
and skips the
Protocol Version Identification Exchange.
.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
reads configuration data from 
.Pa /etc/sshd_config
(or the file specified with
.Fl f
on the command line).
The file contains keyword-value pairs, one per line.
Lines starting with
.Ql #
and empty lines are interpreted as comments.
.Pp
The following keywords are possible.
.Bl -tag -width Ds
.It Cm AFSTokenPassing
Specifies whether an AFS token may be forwarded to the server.
Default is
.Dq yes .
.It Cm AllowGroups
This keyword can be followed by a number of group names, separated
by spaces.
If specified, login is allowed only for users whose primary
group matches one of the patterns.
.Ql \&*
and
.Ql ?
can be used as
wildcards in the patterns.
Only group names are valid, a numerical group ID isn't recognized.
By default login is allowed regardless of the primary group.
.Pp
.It Cm AllowUsers
This keyword can be followed by a number of user names, separated
by spaces.
If specified, login is allowed only for users names that
match one of the patterns.
.Ql \&*
and
.Ql ?
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.
.Pp
.It Cm CheckMail
Specifies whether
.Nm
should check for new mail for interactive logins.
The default is
.Dq no .
.It Cm DenyGroups
This keyword can be followed by a number of group names, separated
by spaces.
Users whose primary group matches one of the patterns
aren't allowed to log in.
.Ql \&*
and
.Ql ?
can be used as
wildcards in the patterns.
Only group names are valid, a numerical group ID isn't recognized.
By default login is allowed regardless of the primary group.
.Pp
.It Cm DenyUsers
This keyword can be followed by a number of user names, separated
by spaces.
Login is disallowed for user names that match one of the patterns.
.Ql \&*
and
.Ql ?
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 HostKey
Specifies the file containing the private host key (default
.Pa /etc/ssh_host_key ) .
Note that
.Nm
does not start if this file is group/world-accessible.
.It Cm IgnoreRhosts
Specifies that
.Pa .rhosts
and 
.Pa .shosts
files will not be used in authentication.
.Pa /etc/hosts.equiv
and
.Pa /etc/shosts.equiv 
are still used.
The default is 
.Dq yes .
.It Cm IgnoreUserKnownHosts
Specifies whether
.Nm
should ignore the user's
.Pa $HOME/.ssh/known_hosts
during
.Cm RhostsRSAAuthentication .
The default is
.Dq no .
.It Cm KeepAlive
Specifies whether the system should send keepalive messages to the
other side.
If they are sent, death of the connection or crash of one
of the machines will be properly noticed.
However, this means that
connections will die if the route is down temporarily, and some people
find it annoying.
On the other hand, if keepalives are not send,
sessions may hang indefinitely on the server, leaving
.Dq ghost
users and consuming server resources.
.Pp
The default is
.Dq yes
(to send keepalives), and the server will notice
if the network goes down or the client host reboots.
This avoids infinitely hanging sessions.
.Pp
To disable keepalives, the value should be set to
.Dq no
in both the server and the client configuration files.
.It Cm KerberosAuthentication
Specifies whether Kerberos authentication is allowed.
This can be in the form of a Kerberos ticket, or if
.Cm PasswordAuthentication
is yes, the password provided by the user will be validated through
the Kerberos KDC.
Default is
.Dq yes .
.It Cm KerberosOrLocalPasswd
If set then if password authentication through Kerberos fails then
the password will be validated via any additional local mechanism
such as
.Pa /etc/passwd
or SecurID.
Default is
.Dq yes .
.It Cm KerberosTgtPassing
Specifies whether a Kerberos TGT may be forwarded to the server.
Default is 
.Dq no ,
as this only works when the Kerberos KDC is actually an AFS kaserver.
.It Cm KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache
file on logout.
Default is
.Dq yes .
.It Cm KeyRegenerationInterval
The server key is automatically regenerated after this many seconds
(if it has been used).
The purpose of regeneration is to prevent
decrypting captured sessions by later breaking into the machine and
stealing the keys.
The key is never stored anywhere.
If the value is 0, the key is never regenerated.
The default is 3600 (seconds).
.It Cm ListenAddress
Specifies what local address
.Nm
should listen on.
The default is to listen to all local addresses.
Multiple options of this type are permitted.
Additionally, the
.Cm Ports
options must precede this option.
.It Cm LoginGraceTime
The server disconnects after this time if the user has not
successfully logged in.
If the value is 0, there is no time limit.
The default is 600 (seconds).
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Nm sshd .
The possible values are:
QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
The default is INFO.
Logging with level DEBUG violates the privacy of users
and is not recommended.
.It Cm PasswordAuthentication
Specifies whether password authentication is allowed.
The default is
.Dq yes .
.It Cm PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings.
The default is
.Dq no .
.It Cm PermitRootLogin
Specifies whether the root can log in using
.Xr ssh 1 .
The argument must be
.Dq yes ,
.Dq without-password
or
.Dq no .
The default is
.Dq yes .
If this options is set to
.Dq without-password
only password authentication is disabled for root.
.Pp
Root login with RSA authentication when the
.Ar command
option has been
specified will be allowed regardless of the value of this setting
(which may be useful for taking remote backups even if root login is
normally not allowed).
.It Cm Port
Specifies the port number that
.Nm
listens on.
The default is 22.
Multiple options of this type are permitted.
.It Cm PrintMotd
Specifies whether
.Nm
should print 
.Pa /etc/motd
when a user logs in interactively.
(On some systems it is also printed by the shell,
.Pa /etc/profile ,
or equivalent.)
The default is
.Dq yes .
.It Cm RandomSeed
Obsolete.
Random number generation uses other techniques.
.It Cm RhostsAuthentication
Specifies whether authentication using rhosts or /etc/hosts.equiv
files is sufficient.
Normally, this method should not be permitted because it is insecure.
.Cm RhostsRSAAuthentication
should be used
instead, because it performs RSA-based host authentication in addition
to normal rhosts or /etc/hosts.equiv authentication.
The default is
.Dq no .
.It Cm RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful RSA host authentication is allowed.
The default is
.Dq no .
.It Cm RSAAuthentication
Specifies whether pure RSA authentication is allowed.
The default is
.Dq yes .
.It Cm ServerKeyBits
Defines the number of bits in the server key.
The minimum value is 512, and the default is 768.
.It Cm SkeyAuthentication
Specifies whether
.Xr skey 1 
authentication is allowed.
The default is
.Dq yes .
Note that s/key authentication is enabled only if
.Cm PasswordAuthentication
is allowed, too.
.It Cm StrictModes
Specifies whether
.Nm
should check file modes and ownership of the
user's files and home directory before accepting login.
This is normally desirable because novices sometimes accidentally leave their
directory or files world-writable.
The default is
.Dq yes .
.It Cm SyslogFacility
Gives the facility code that is used when logging messages from
.Nm sshd .
The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
The default is AUTH.
.It