path: root/usr.bin/ssh/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 1.10 1999/10/11 21:07:37 markus 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.
.Sh OPTIONS
.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 .
.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.
.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 .
.Pp
.It Cm AllowHosts
This keyword can be followed by any number of host name patterns,
separated by spaces.  If specified, login is allowed only from hosts
whose name matches one of the patterns.
.Ql \&*
and
.Ql ?
can be used as
wildcards in the patterns.  Normal name servers are used to map the
client's host into a canonical host name.  If the name cannot be
mapped, its IP-address is used as the host name.  By default all hosts
are allowed to connect.
.Pp
Note that
.Nm
can also be configured to use tcp_wrappers using the
.Sy LIBWARP
compile-time option.
.It Cm CheckMail
Specifies whether
.Nm
should check for new mail for interactive logins.
The default is
.Dq no .
.It Cm DenyHosts
This keyword can be followed by any number of host name patterns,
separated by spaces.  If specified, login is disallowed from the hosts
whose name matches any of the patterns.
.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 the private host key 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 no .
.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 default is
.Dq yes .
.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 SSH should check file modes and ownership of the
user's home directory and rhosts files 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 DAEMON.
.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 key
permits an intruder to log in from anywhere in the world.  This
additional option makes using a stolen key more difficult (name
servers and/or routers would have to be compromised in addition to
just the key).
.It Cm command="command"
Specifies that the command is executed whenever this key is used for
authentication.  The command supplied by the user (if any) is ignored.
The command is run on a pty if the connection requests a pty;
otherwise it is run without a tty.  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.  An
example might be a key that permits remote backups but nothing
else.  Notice that the client may specify TCP/IP and/or X11
forwardings unless they are explicitly prohibited.
.It Cm environment="NAME=value"
Specifies that the string is to be added to the environment when
logging in using this key.  Environment variables set this way
override other default environment values.  Multiple options of this
type are permitted.
.It Cm no-port-forwarding
Forbids TCP/IP forwarding when this key is used for authentication.
Any port forward requests by the client will return an error.  This
might be used, e.g., in connection with the
.Cm command
option.
.It Cm no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication.
Any X11 forward requests by the client will return an error.
.It Cm no-agent-forwarding
Forbids authentication agent forwarding when this key is used for
authentication.
.It Cm no-pty
Prevents tty allocation (a request to allocate a pty will fail).
.El
.Ss Examples
1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
.Pp
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
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The 
.Pa /etc/ssh_known_hosts
and 
.Pa $HOME/.ssh/known_hosts
files contain host public keys for all known hosts.  The global file should
be prepared by the admistrator (optional), and the per-user file is
maintained automatically: whenever the user connects an unknown host
its key is added to the per-user file.  
.Pp
Each line in these files contains the following fields: hostnames,
bits, exponent, modulus, comment.  The fields are separated by spaces.
.Pp
Hostnames is a comma-separated list of patterns ('*' and '?' 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 !
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.
.Pp
Bits, exponent, and modulus are taken directly from the 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.
.Pp
Lines starting with
.Ql #
and empty lines are ignored as comments.
.Pp
When performing host authentication, authentication is accepted if any
matching line has the proper key.  It is thus permissible (but not
recommended) to have several lines or different host keys for the same
names.  This will inevitably happen when short forms of host names
from different domains are put in the file.  It is possible
that the files contain conflicting information; authentication is
accepted if valid information can be found from either file.
.Pp
Note that the lines in these files are typically hundreds of characters
long, and you definitely don't want to type in the host keys by hand.
Rather, generate them by a script
or by taking 
.Pa /etc/ssh_host_key.pub
and adding the host names at the front.
.Ss Examples
closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
.Sh FILES
.Bl -tag -width Ds
.It Pa /etc/sshd_config
Contains configuration data for
.Nm sshd .
This file should be writable by root only, but it is recommended
(though not necessary) that it be world-readable.
.It Pa /etc/ssh_host_key
Contains the private part of the host key.
This file should only be owned by root, readable only by root, and not
accessible to others.
.It Pa /etc/ssh_host_key.pub
Contains the public part of the host key.
This file should be world-readable but writable only by
root.  Its contents should match the private part.  This file is not
really used for anything; it is only provided for the convenience of
the user so its contents can be copied to known hosts files.
These two files are created using
.Xr ssh-keygen 1 .
.It Pa /var/run/sshd.pid
Contains the process ID of the
.Nm
listening for connections (if there are several daemons running
concurrently for different ports, this contains the pid of the one
started last).  The contents of this file are not sensitive; it can be
world-readable.
.It Pa $HOME/.ssh/authorized_keys
Lists the RSA 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.
.It Pa /etc/ssh_known_hosts
This file is consulted when using rhosts with RSA host
authentication to check the public key of the host.  The key must be
listed in this file to be accepted.
.It Pa $HOME/.ssh/known_hosts
The client uses this file
and
.Pa /etc/ssh_known_hosts
to verify that the remote host is the one we intended to
connect. These files should be writable only by root/the owner.
.Pa /etc/ssh_known_hosts
should be world-readable, and
.Pa $HOME/.ssh/known_hosts
can but need not be world-readable.
.It Pa /etc/nologin
If this file exists, 
.Nm
refuses to let anyone except root log in.  The contents of the file
are displayed to anyone trying to log in, and non-root connections are
refused.  The file should be world-readable.
.It Pa $HOME/.rhosts
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.  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.  Either host or user
name may be of the form +@groupname to specify all hosts or all users
in the group.
.It Pa $HOME/.shosts
For ssh,
this file is exactly the same as for
.Pa .rhosts .
However, this file is
not used by rlogin and rshd, so using this permits access using SSH only.
.Pa /etc/hosts.equiv
This file is used during
.Pa .rhosts
authentication.  In the
simplest form, this file contains host names, one per line.  Users on
those hosts are permitted to log in without a password, provided they
have the same user name on both machines.  The host name may also be
followed by a user name; such users are permitted to log in as
.Em any
user on this machine (except root).  Additionally, the syntax
.Dq +@group
can be used to specify netgroups.  Negated entries start with
.Ql \&- .
.Pp
If the client host/user is successfully matched in this file, login is
automatically permitted provided the client and server user names are the
same.  Additionally, successful RSA host authentication is normally
required.  This file must be writable only by root; it is recommended
that it be world-readable.
.Pp
.Sy "Warning: It is almost never a good idea to use user names in"
.Pa hosts.equiv .
Beware that it really means that the named user(s) can log in as
.Em anybody ,
which includes bin, daemon, adm, and other accounts that own critical
binaries and directories.  Using a user name practically grants the
user root access.  The only valid use for user names that I can think
of is in negative entries.
.Pp
Note that this warning also applies to rsh/rlogin.
.It Pa /etc/shosts.equiv
This is processed exactly as
.Pa /etc/hosts.equiv .
However, this file may be useful in environments that want to run both
rsh/rlogin and ssh.
.It Pa $HOME/.ssh/environment
This file is read into the environment at login (if it exists).  It
can only contain empty lines, comment lines (that start with
.Ql # ) ,
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.
.It Pa $HOME/.ssh/rc
If this file exists, it is run with /bin/sh after reading the
environment files but before starting the user's shell or command.  If
X11 spoofing is in use, this will receive the "proto cookie" pair in
standard input (and
.Ev DISPLAY
in environment).  This must call
.Xr xauth 1
in that case.
.Pp
The primary purpose of this file is to run any initialization routines
which may be needed before the user's home directory becomes
accessible; AFS is a particular example of such an environment.
.Pp
This file will probably contain some initialization code followed by
something similar to: "if read proto cookie; then echo add $DISPLAY
$proto $cookie | xauth -q -; fi".
.Pp
If this file does not exist,
.Pa /etc/sshrc
is run, and if that
does not exist either, xauth is used to store the cookie.
.Pp
This file should be writable only by the user, and need not be
readable by anyone else.
.It Pa /etc/sshrc
Like
.Pa $HOME/.ssh/rc .
This can be used to specify
machine-specific login-time initializations globally.  This file
should be writable only by root, and should be world-readable.
.Sh AUTHOR
Tatu Ylonen <ylo@cs.hut.fi>
.Pp
Information about new releases, mailing lists, and other related
issues can be found from the SSH WWW home page:
.Pp
.Dl http://www.cs.hut.fi/ssh.
.Pp
This version of
.Nm
is a derivative of the original 1.2.12 release, but with bugs removed and
newer features re-added.   Rapidly after the 1.2.12 release, newer versions
bore successively more restrictive licenses.  In this version, all components
of a restrictive nature (ie. patents) have been directly removed from the
source code; any licensed or patented components are chosen from external
libraries.  The libraries described in
.Xr ssl 8
are required for proper operation.
.Sh SEE ALSO
.Xr rlogin 1 ,
.Xr rsh 1 ,
.Xr scp 1 ,
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
.Xr ssl 8