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 1.1.1.1 1999/10/27 03:42:46 damien Exp $
.\"
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Sh NAME
.Nm sshd
.Nd secure shell daemon
.Sh SYNOPSIS
.Nm sshd
.Op Fl diq
.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
.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 is 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
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.
.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 allowed 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.
.Pp
.It Cm FascistLogging
Specifies whether to use verbose logging.  Verbose logging violates
the privacy of users and is not recommended.  The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.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 rhosts and shosts files will not be used in
authentication.
.Pa /etc/hosts.equiv
and
.Pa /etc/shosts.equiv 
are still used.  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.
.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 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 yes .
.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.
.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 QuietMode
Specifies whether the system runs in quiet mode.  In quiet mode,
nothing is logged in the system log, except fatal errors.  The default
is
.Dq no .
.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 yes .
.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 Cm UseLogin
Specifies whether
.Xr login 1
is used. The default is
.Dq no .
.It Cm X11Forwarding
Specifies whether X11 forwarding is permitted.  The default is
.Dq yes .
Note that disabling X11 forwarding does not improve security in any
way, as users can always install their own forwarders.
.It Cm X11DisplayOffset
Specifies the first display number available for
.Nm sshd Ns 's
X11 forwarding.  This prevents
.Nm
from interfering with real X11 servers.
.El
.Sh LOGIN PROCESS
When a user successfully logs in,
.Nm
does the following:
.Bl -enum -offset indent
.It
If the login is on a tty, and no command has been specified,
prints last login time and 
.Pa /etc/motd
(unless prevented in the configuration file or by
.Pa $HOME/.hushlogin ;
see the
.Sx FILES 
section).
.It
If the login is on a tty, records login time.
.It
Checks
.Pa /etc/nologin ;
if it exists, prints contents and quits
(unless root).
.It
Changes to run with normal user privileges.
.It
Sets up basic environment.
.It
Reads
.Pa $HOME/.ssh/environment
if it exists.
.It
Changes to user's home directory.
.It
If
.Pa $HOME/.ssh/rc
exists, runs it; else if
.Pa /etc/sshrc
exists, runs
it; otherwise runs xauth.  The
.Dq rc
files are given the X11
authentication protocol and cookie in standard input.
.It
Runs user's shell or command.
.El
.Sh AUTHORIZED_KEYS FILE FORMAT
The 
.Pa $HOME/.ssh/authorized_keys
file lists the RSA keys that are
permitted for RSA authentication.  Each line of the file contains one
key (empty lines and lines starting with a
.Ql #
are ignored as
comments).  Each line consists of the following fields, separated by
spaces: options, bits, exponent, modulus, comment.  The options field
is optional; its presence is determined by whether the line starts
with a number or not (the option field never starts with a number).
The bits, exponent, modulus and comment fields give the RSA key; the
comment field is not used for anything (but may be convenient for the
user to identify the key).
.Pp
Note that lines in this file are usually several hundred bytes long
(because of the size of the RSA key modulus).  You don't want to type
them in; instead, copy the 
.Pa identity.pub
file and edit it.
.Pp
The options (if present) consists of comma-separated option
specifications.  No spaces are permitted, except within double quotes.
The following option specifications are supported:
.Bl -tag -width Ds
.It Cm from="pattern-list"
Specifies that in addition to RSA authentication, the canonical name
of the remote host must be present in the comma-separated list of
patterns ('*' and '?' serve as wildcards).  The list may also contain
patterns negated by prefixing them with '!'; 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
by itself does not trust the network or name servers or anything (but
the key); however, if somebody somehow steals the key, the