openssh/sshd_config.5

.\"  -*- nroff -*-
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose.  Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (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_config.5,v 1.22 2003/08/13 08:46:31 markus Exp $
.Dd September 25, 1999
.Dt SSHD_CONFIG 5
.Os
.Sh NAME
.Nm sshd_config
.Nd OpenSSH SSH daemon configuration file
.Sh SYNOPSIS
.Bl -tag -width Ds -compact
.It Pa /etc/ssh/sshd_config
.El
.Sh DESCRIPTION
.Nm sshd
reads configuration data from
.Pa /etc/ssh/sshd_config
(or the file specified with
.Fl f
on the command line).
The file contains keyword-argument pairs, one per line.
Lines starting with
.Ql #
and empty lines are interpreted as comments.
.Pp
The possible
keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are case-sensitive):
.Bl -tag -width Ds
.It Cm AllowGroups
This keyword can be followed by a list of group name patterns, separated
by spaces.
If specified, login is allowed only for users whose primary
group or supplementary group list 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 is not recognized.
By default, login is allowed for all groups.
.Pp
.It Cm AllowTcpForwarding
Specifies whether TCP forwarding is permitted.
The default is
.Dq yes .
Note that disabling TCP forwarding does not improve security unless
users are also denied shell access, as they can always install their
own forwarders.
.Pp
.It Cm AllowUsers
This keyword can be followed by a list of user name patterns, separated
by spaces.
If specified, login is allowed only 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 is not recognized.
By default, login is allowed for all users.
If the pattern takes the form USER@HOST then USER and HOST
are separately checked, restricting logins to particular
users from particular hosts.
.Pp
.It Cm AuthorizedKeysFile
Specifies the file that contains the public keys that can be used
for user authentication.
.Cm AuthorizedKeysFile
may contain tokens of the form %T which are substituted during connection
set-up.
The following tokens are defined: %% is replaced by a literal '%',
%h is replaced by the home directory of the user being authenticated and
%u is replaced by the username of that user.
After expansion,
.Cm AuthorizedKeysFile
is taken to be an absolute path or one relative to the user's home
directory.
The default is
.Dq .ssh/authorized_keys .
.It Cm Banner
In some jurisdictions, sending a warning message before authentication
may be relevant for getting legal protection.
The contents of the specified file are sent to the remote user before
authentication is allowed.
This option is only available for protocol version 2.
By default, no banner is displayed.
.Pp
.It Cm ChallengeResponseAuthentication
Specifies whether challenge response authentication is allowed.
All authentication styles from
.Xr login.conf 5
are supported.
The default is
.Dq yes .
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2.
Multiple ciphers must be comma-separated.
The default is
.Pp
.Bd -literal
  ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
    aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
.Ed
.It Cm ClientAliveInterval
Sets a timeout interval in seconds after which if no data has been received
from the client,
.Nm sshd
will send a message through the encrypted
channel to request a response from the client.
The default
is 0, indicating that these messages will not be sent to the client.
This option applies to protocol version 2 only.
.It Cm ClientAliveCountMax
Sets the number of client alive messages (see above) which may be
sent without
.Nm sshd
receiving any messages back from the client.
If this threshold is reached while client alive messages are being sent,
.Nm sshd
will disconnect the client, terminating the session.
It is important to note that the use of client alive messages is very
different from
.Cm KeepAlive
(below).
The client alive messages are sent through the encrypted channel
and therefore will not be spoofable.
The TCP keepalive option enabled by
.Cm KeepAlive
is spoofable.
The client alive mechanism is valuable when the client or
server depend on knowing when a connection has become inactive.
.Pp
The default value is 3.
If
.Cm ClientAliveInterval
(above) is set to 15, and
.Cm ClientAliveCountMax
is left at the default, unresponsive ssh clients
will be disconnected after approximately 45 seconds.
.It Cm Compression
Specifies whether compression is allowed.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
.It Cm DenyGroups
This keyword can be followed by a list of group name patterns, separated
by spaces.
Login is disallowed for users whose primary group or supplementary
group list 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 is not recognized.
By default, login is allowed for all groups.
.Pp
.It Cm DenyUsers
This keyword can be followed by a list of user name patterns, 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 is not recognized.
By default, login is allowed for all users.
If the pattern takes the form USER@HOST then USER and HOST
are separately checked, restricting logins to particular
users from particular hosts.
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to ports
forwarded for the client.
By default,
.Nm sshd
binds remote port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
.Cm GatewayPorts
can be used to specify that
.Nm sshd
should bind remote port forwardings to the wildcard address,
thus allowing remote hosts to connect to forwarded ports.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful public key client host authentication is allowed
(hostbased authentication).
This option is similar to
.Cm RhostsRSAAuthentication
and applies to protocol version 2 only.
The default is
.Dq no .
.It Cm HostKey
Specifies a file containing a private host key
used by SSH.
The default is
.Pa /etc/ssh/ssh_host_key
for protocol version 1, and
.Pa /etc/ssh/ssh_host_rsa_key
and
.Pa /etc/ssh/ssh_host_dsa_key
for protocol version 2.
Note that
.Nm sshd
will refuse to use a file if it is group/world-accessible.
It is possible to have multiple host key files.
.Dq rsa1
keys are used for version 1 and
.Dq dsa
or
.Dq rsa
are used for version 2 of the SSH protocol.
.It Cm IgnoreRhosts
Specifies that
.Pa .rhosts
and
.Pa .shosts
files will not be used in
.Cm RhostsRSAAuthentication
or
.Cm HostbasedAuthentication .
.Pp
.Pa /etc/hosts.equiv
and
.Pa /etc/shosts.equiv
are still used.
The default is
.Dq yes .
.It Cm IgnoreUserKnownHosts
Specifies whether
.Nm sshd
should ignore the user's
.Pa $HOME/.ssh/known_hosts
during
.Cm RhostsRSAAuthentication
or
.Cm HostbasedAuthentication .
The default is
.Dq no .
.It Cm KeepAlive
Specifies whether the system should send TCP 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 sent,
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 crashes.
This avoids infinitely hanging sessions.
.Pp
To disable keepalives, the value should be set to
.Dq no .
.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.
To use this option, the server needs a
Kerberos servtab which allows the verification of the KDC's identity.
Default is
.Dq no .
.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 .
Default is
.Dq yes .
.It Cm KerberosTgtPassing
Specifies whether a Kerberos TGT may be forwarded to the server.
Default is
.Dq no .
.It Cm KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache
file on logout.
Default is
.Dq yes .
.It Cm KeyRegenerationInterval
In protocol version 1, the ephemeral 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 the local addresses
.Nm sshd
should listen on.
The following forms may be used:
.Pp
.Bl -item -offset indent -compact
.It
.Cm ListenAddress
.Sm off
.Ar host No | Ar IPv4_addr No | Ar IPv6_addr
.Sm on
.It
.Cm ListenAddress
.Sm off
.Ar host No | Ar IPv4_addr No : Ar port
.Sm on
.It
.Cm ListenAddress
.Sm off
.Oo
.Ar host No | Ar IPv6_addr Oc : Ar port
.Sm on
.El
.Pp
If
.Ar port
is not specified,
.Nm sshd
will listen on the address and all prior
.Cm Port
options specified.
The default is to listen on all local addresses.
Multiple
.Cm ListenAddress
options are permitted.
Additionally, any
.Cm Port
options must precede this option for non port qualified 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 120 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, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
The default is INFO.
DEBUG and DEBUG1 are equivalent.
DEBUG2 and DEBUG3 each specify higher levels of debugging output.
Logging with a DEBUG level violates the privacy of users and is not recommended.
.It Cm MACs
Specifies the available MAC (message authentication code) algorithms.
The MAC algorithm is used in protocol version 2
for data integrity protection.
Multiple algorithms must be comma-separated.
The default is
.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
.It Cm MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the
.Nm sshd
daemon.
Additional connections will be dropped until authentication succeeds or the
.Cm LoginGraceTime
expires for a connection.
The default is 10.
.Pp
Alternatively, random early drop can be enabled by specifying
the three colon separated values
.Dq start:rate:full
(e.g., "10:30:60").
.Nm sshd
will refuse connection attempts with a probability of
.Dq rate/100
(30%)
if there are currently
.Dq start
(10)
unauthenticated connections.
The probability increases linearly and all connection attempts
are refused if the number of unauthenticated connections reaches
.Dq full
(60).
.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 root can login using
.Xr ssh 1 .
The argument must be
.Dq yes ,
.Dq without-password ,
.Dq forced-commands-only
or
.Dq no .
The default is
.Dq yes .
.Pp
If this option is set to
.Dq without-password
password authentication is disabled for root.
.Pp
If this option is set to
.Dq forced-commands-only
root login with public key authentication will be allowed,
but only if the
.Ar command
option has been specified
(which may be useful for taking remote backups even if root login is
normally not allowed).
All other authentication methods are disabled for root.
.Pp
If this option is set to
.Dq no
root is not allowed to login.
.It Cm PermitUserEnvironment
Specifies whether
.Pa ~/.ssh/environment
and
.Cm environment=
options in
.Pa ~/.ssh/authorized_keys
are processed by
.Nm sshd .
The default is
.Dq no .
Enabling environment processing may enable users to bypass access
restrictions in some configurations using mechanisms such as
.Ev LD_PRELOAD .
.It Cm PidFile
Specifies the file that contains the process ID of the
.Nm sshd
daemon.
The default is
.Pa /var/run/sshd.pid .
.It Cm Port
Specifies the port number that
.Nm sshd
listens on.
The default is 22.
Multiple options of this type are permitted.
See also
.Cm ListenAddress .
.It Cm PrintLastLog
Specifies whether
.Nm sshd
should print the date and time when the user last logged in.
The default is
.Dq yes .
.It Cm PrintMotd
Specifies whether
.Nm sshd
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 Protocol
Specifies the protocol versions
.Nm sshd
supports.
The possible values are
.Dq 1
and
.Dq 2 .
Multiple versions must be comma-separated.
The default is
.Dq 2,1 .
Note that the order of the protocol list does not indicate preference,
because the client selects among multiple protocol versions offered
by the server.
Specifying
.Dq 2,1
is identical to
.Dq 1,2 .
.It Cm PubkeyAuthentication
Specifies whether public key authentication is allowed.
The default is
.Dq yes .
Note that this option applies to protocol version 2 only.
.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 .
This option applies to protocol version 1 only.
.It Cm RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful RSA host authentication is allowed.
The default is
.Dq no .
This option applies to protocol version 1 only.
.It Cm RSAAuthentication
Specifies whether pure RSA authentication is allowed.
The default is
.Dq yes .
This option applies to protocol version 1 only.
.It Cm ServerKeyBits
Defines the number of bits in the ephemeral protocol version 1 server key.
The minimum value is 512, and the default is 768.
.It Cm StrictModes
Specifies whether
.Nm sshd
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 Subsystem
Configures an external subsystem (e.g., file transfer daemon).
Arguments should be a subsystem name and a command to execute upon subsystem
request.
The command
.Xr sftp-server 8
implements the
.Dq sftp
file transfer subsystem.
By default no subsystems are defined.
Note that this option applies to protocol version 2 only.
.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 UseDNS
Specifies whether
.Nm sshd
should lookup the remote host name and check that
the resolved host name for the remote IP address maps back to the
very same IP address.
The default is
.Dq yes .
.It Cm UseLogin
Specifies whether
.Xr login 1
is used for interactive login sessions.
The default is
.Dq no .
Note that
.Xr login 1
is never used for remote command execution.
Note also, that if this is enabled,
.Cm X11Forwarding
will be disabled because
.Xr login 1
does not know how to handle
.Xr xauth 1
cookies.
If
.Cm UsePrivilegeSeparation
is specified, it will be disabled after authentication.
.It Cm UsePAM
Enables PAM authentication (via challenge-response) and session set up. 
If you enable this, you should probably disable 
.Cm PasswordAuthentication .
If you enable 
.CM UsePAM
then you will not be able to run sshd as a non-root user.
.It Cm UsePrivilegeSeparation
Specifies whether
.Nm sshd
separates privileges by creating an unprivileged child process
to deal with incoming network traffic.
After successful authentication, another process will be created that has
the privilege of the authenticated user.
The goal of privilege separation is to prevent privilege
escalation by containing any corruption within the unprivileged processes.
The default is
.Dq yes .
.It Cm X11DisplayOffset
Specifies the first display number available for
.Nm sshd Ns 's
X11 forwarding.
This prevents
.Nm sshd
from interfering with real X11 servers.
The default is 10.
.It Cm X11Forwarding
Specifies whether X11 forwarding is permitted.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.Pp
When X11 forwarding is enabled, there may be additional exposure to
the server and to client displays if the
.Nm sshd
proxy display is configured to listen on the wildcard address (see
.Cm X11UseLocalhost
below), however this is not the default.
Additionally, the authentication spoofing and authentication data
verification and substitution occur on the client side.
The security risk of using X11 forwarding is that the client's X11
display server may be exposed to attack when the ssh client requests
forwarding (see the warnings for
.Cm ForwardX11
in
.Xr ssh_config 5 ) .
A system administrator may have a stance in which they want to
protect clients that may expose themselves to attack by unwittingly
requesting X11 forwarding, which can warrant a
.Dq no
setting.
.Pp
Note that disabling X11 forwarding does not prevent users from
forwarding X11 traffic, as users can always install their own forwarders.
X11 forwarding is automatically disabled if
.Cm UseLogin
is enabled.
.It Cm X11UseLocalhost
Specifies whether
.Nm sshd
should bind the X11 forwarding server to the loopback address or to
the wildcard address.
By default,
.Nm sshd
binds the forwarding server to the loopback address and sets the
hostname part of the
.Ev DISPLAY
environment variable to
.Dq localhost .
This prevents remote hosts from connecting to the proxy display.
However, some older X11 clients may not function with this
configuration.
.Cm X11UseLocalhost
may be set to
.Dq no
to specify that the forwarding server should be bound to the wildcard
address.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
.It Cm XAuthLocation
Specifies the full pathname of the
.Xr xauth 1
program.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
.Ss Time Formats
.Nm sshd
command-line arguments and configuration file options that specify time
may be expressed using a sequence of the form:
.Sm off
.Ar time Op Ar qualifier ,
.Sm on
where
.Ar time
is a positive integer value and
.Ar qualifier
is one of the following:
.Pp
.Bl -tag -width Ds -compact -offset indent
.It Cm <none>
seconds
.It Cm s | Cm S
seconds
.It Cm m | Cm M
minutes
.It Cm h | Cm H
hours
.It Cm d | Cm D
days
.It Cm w | Cm W
weeks
.El
.Pp
Each member of the sequence is added together to calculate
the total time value.
.Pp
Time format examples:
.Pp
.Bl -tag -width Ds -compact -offset indent
.It 600
600 seconds (10 minutes)
.It 10m
10 minutes
.It 1h30m
1 hour 30 minutes (90 minutes)
.El
.Sh FILES
.Bl -tag -width Ds
.It Pa /etc/ssh/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.
.El
.Sh SEE ALSO
.Xr sshd 8
.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.