SSHD_CONFIG(5) UNIX System V (September 25, 1999) SSHD_CONFIG(5)
NAME
sshd_config - OpenSSH SSH daemon configuration file
SYNOPSIS
/etc/openssh/sshd_config
DESCRIPTION
sshd reads configuration data from /etc/openssh/sshd_config
(or the file specified with -f on the command line). The
file contains keyword-argument pairs, one per line. Lines
starting with `#' and empty lines are interpreted as
comments.
The possible keywords and their meanings are as follows
(note that keywords are case-insensitive and arguments are
case-sensitive):
AcceptEnv
Specifies what environment variables sent by the client
will be copied into the session's environ(7). See
SendEnv in ssh_config(5) for how to configure the
client. Note that environment passing is only
supported for protocol 2. Variables are specified by
name, which may contain the wildcard characters `*' and
`?'. Multiple environment variables may be separated
by whitespace or spread across multiple AcceptEnv
directives. Be warned that some environment variables
could be used to bypass restricted user environments.
For this reason, care should be taken in the use of
this directive. The default is not to accept any
environment variables.
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.
`*' and `?' 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.
AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The
default is ``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.
AllowUsers
This keyword can be followed by a list of user name
patterns, separated by spaces. If specified, login is
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allowed only for user names that match one of the
patterns. `*' and `?' 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.
AuthorizedKeysFile
Specifies the file that contains the public keys that
can be used for user authentication.
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,
AuthorizedKeysFile is taken to be an absolute path or
one relative to the user's home directory. The default
is ``.ssh/authorized_keys''.
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.
ChallengeResponseAuthentication
Specifies whether challenge response authentication is
allowed. All authentication styles from login.conf(5)
are supported. The default is ``yes''.
Ciphers
Specifies the ciphers allowed for protocol version 2.
Multiple ciphers must be comma-separated. The
supported ciphers are ``3des-cbc'', ``aes128-cbc'',
``aes192-cbc'', ``aes256-cbc'', ``aes128-ctr'',
``aes192-ctr'', ``aes256-ctr'', ``arcfour'',
``blowfish-cbc'', and ``cast128-cbc''. The default is
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-
cbc,arcfour,
aes192-cbc,aes256-cbc,aes128-ctr,aes192-
ctr,aes256-ctr''
ClientAliveInterval
Sets a timeout interval in seconds after which if no
data has been received from the client, sshd will send
a message through the encrypted channel to request a
response from the client. The default is 0, indicating
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that these messages will not be sent to the client.
This option applies to protocol version 2 only.
ClientAliveCountMax
Sets the number of client alive messages (see above)
which may be sent without sshd receiving any messages
back from the client. If this threshold is reached
while client alive messages are being sent, sshd will
disconnect the client, terminating the session. It is
important to note that the use of client alive messages
is very different from TCPKeepAlive (below). The
client alive messages are sent through the encrypted
channel and therefore will not be spoofable. The TCP
keepalive option enabled by TCPKeepAlive is spoofable.
The client alive mechanism is valuable when the client
or server depend on knowing when a connection has
become inactive.
The default value is 3. If ClientAliveInterval (above)
is set to 15, and ClientAliveCountMax is left at the
default, unresponsive ssh clients will be disconnected
after approximately 45 seconds.
Compression
Specifies whether compression is allowed. The argument
must be ``yes'' or ``no''. The default is ``yes''.
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. `*' and `?' 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.
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. `*' and `?'
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.
GatewayPorts
Specifies whether remote hosts are allowed to connect
to ports forwarded for the client. By default, sshd
binds remote port forwardings to the loopback address.
This prevents other remote hosts from connecting to
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forwarded ports. GatewayPorts can be used to specify
that sshd should bind remote port forwardings to the
wildcard address, thus allowing remote hosts to connect
to forwarded ports. The argument must be ``yes'' or
``no''. The default is ``no''.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI
is allowed. The default is ``no''. Note that this
option applies to protocol version 2 only.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's
credentials cache on logout. The default is ``yes''.
Note that this option applies to protocol version 2
only.
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
RhostsRSAAuthentication and applies to protocol version
2 only. The default is ``no''.
HostKey
Specifies a file containing a private host key used by
SSH. The default is /etc/openssh/ssh_host_key for
protocol version 1, and /etc/openssh/ssh_host_rsa_key
and /etc/openssh/ssh_host_dsa_key for protocol version
2. Note that sshd will refuse to use a file if it is
group/world-accessible. It is possible to have
multiple host key files. ``rsa1'' keys are used for
version 1 and ``dsa'' or ``rsa'' are used for version 2
of the SSH protocol.
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be
used in RhostsRSAAuthentication or
HostbasedAuthentication.
/etc/hosts.equiv and /etc/openssh/shosts.equiv are
still used. The default is ``yes''.
IgnoreUserKnownHosts
Specifies whether sshd should ignore the user's
$HOME/.ssh/known_hosts during RhostsRSAAuthentication
or HostbasedAuthentication. The default is ``no''.
KerberosAuthentication
Specifies whether the password provided by the user for
PasswordAuthentication will be validated through the
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Kerberos KDC. To use this option, the server needs a
Kerberos servtab which allows the verification of the
KDC's identity. Default is ``no''.
KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT,
attempt to aquire an AFS token before accessing the
user's home directory. Default is ``no''.
KerberosOrLocalPasswd
If set then if password authentication through Kerberos
fails then the password will be validated via any
additional local mechanism such as /etc/passwd.
Default is ``yes''.
KerberosTicketCleanup
Specifies whether to automatically destroy the user's
ticket cache file on logout. Default is ``yes''.
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).
ListenAddress
Specifies the local addresses sshd should listen on.
The following forms may be used:
ListenAddress host|IPv4_addr|IPv6_addr
ListenAddress host|IPv4_addr:port
ListenAddress [host|IPv6_addr]:port
If port is not specified, sshd will listen on the
address and all prior Port options specified. The
default is to listen on all local addresses. Multiple
ListenAddress options are permitted. Additionally, any
Port options must precede this option for non port
qualified addresses.
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.
LogLevel
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Gives the verbosity level that is used when logging
messages from 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.
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 ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-
sha1-96,hmac-md5-96''.
MaxAuthTries
Specifies the maximum number of authentication attempts
permitted per connection. Once the number of failures
reaches half this value, additional failures are
logged. The default is 6.
MaxStartups
Specifies the maximum number of concurrent
unauthenticated connections to the sshd daemon.
Additional connections will be dropped until
authentication succeeds or the LoginGraceTime expires
for a connection. The default is 10.
Alternatively, random early drop can be enabled by
specifying the three colon separated values
``start:rate:full'' (e.g., "10:30:60"). sshd will
refuse connection attempts with a probability of
``rate/100'' (30%) if there are currently ``start''
(10) unauthenticated connections. The probability
increases linearly and all connection attempts are
refused if the number of unauthenticated connections
reaches ``full'' (60).
PasswordAuthentication
Specifies whether password authentication is allowed.
The default is ``yes''.
PermitEmptyPasswords
When password authentication is allowed, it specifies
whether the server allows login to accounts with empty
password strings. The default is ``no''.
PermitRootLogin
Specifies whether root can login using ssh(1). The
argument must be ``yes'', ``without-password'',
``forced-commands-only'' or ``no''. The default is
``yes''.
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If this option is set to ``without-password'' password
authentication is disabled for root. Note that other
authentication methods (e.g., keyboard-interactive/PAM)
may still allow root to login using a password.
If this option is set to ``forced-commands-only'' root
login with public key authentication will be allowed,
but only if the 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.
If this option is set to ``no'' root is not allowed to
login.
PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment=
options in ~/.ssh/authorized_keys are processed by
sshd. The default is ``no''. Enabling environment
processing may enable users to bypass access
restrictions in some configurations using mechanisms
such as LD_PRELOAD.
PidFile
Specifies the file that contains the process ID of the
sshd daemon. The default is /etc/openssh/sshd.pid.
Port Specifies the port number that sshd listens on. The
default is 22. Multiple options of this type are
permitted. See also ListenAddress.
PrintLastLog
Specifies whether sshd should print the date and time
when the user last logged in. The default is ``yes''.
PrintMotd
Specifies whether sshd should print /etc/motd when a
user logs in interactively. (On some systems it is
also printed by the shell, /etc/profile, or
equivalent.) The default is ``yes''.
Protocol
Specifies the protocol versions sshd supports. The
possible values are ``1'' and ``2''. Multiple versions
must be comma-separated. The default is ``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
``2,1'' is identical to ``1,2''.
PubkeyAuthentication
Specifies whether public key authentication is allowed.
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The default is ``yes''. Note that this option applies
to protocol version 2 only.
RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv
authentication together with successful RSA host
authentication is allowed. The default is ``no''.
This option applies to protocol version 1 only.
RSAAuthentication
Specifies whether pure RSA authentication is allowed.
The default is ``yes''. This option applies to
protocol version 1 only.
ServerKeyBits
Defines the number of bits in the ephemeral protocol
version 1 server key. The minimum value is 512, and
the default is 768.
StrictModes
Specifies whether 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 ``yes''.
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
sftp-server(8) implements the ``sftp'' file transfer
subsystem. By default no subsystems are defined. Note
that this option applies to protocol version 2 only.
SyslogFacility
Gives the facility code that is used when logging
messages from sshd. The possible values are: DAEMON,
USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4,
LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
TCPKeepAlive
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 TCP
keepalives are not sent, sessions may hang indefinitely
on the server, leaving ``ghost'' users and consuming
server resources.
The default is ``yes'' (to send TCP keepalive
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messages), and the server will notice if the network
goes down or the client host crashes. This avoids
infinitely hanging sessions.
To disable TCP keepalive messages, the value should be
set to ``no''.
UseDNS
Specifies whether 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 ``yes''.
UseLogin
Specifies whether login(1) is used for interactive
login sessions. The default is ``no''. Note that
login(1) is never used for remote command execution.
Note also, that if this is enabled, X11Forwarding will
be disabled because login(1) does not know how to
handle xauth(1) cookies. If UsePrivilegeSeparation is
specified, it will be disabled after authentication.
UsePAM
Enables the Pluggable Authentication Module interface.
If set to ``yes'' this will enable PAM authentication
using ChallengeResponseAuthentication and PAM account
and session module processing for all authentication
types.
Because PAM challenge-response authentication usually
serves an equivalent role to password authentication,
you should disable either PasswordAuthentication or
ChallengeResponseAuthentication.
If UsePAM is enabled, you will not be able to run
sshd(8) as a non-root user. The default is ``no''.
UsePrivilegeSeparation
Specifies whether 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 ``yes''.
X11DisplayOffset
Specifies the first display number available for sshd's
X11 forwarding. This prevents sshd from interfering
with real X11 servers. The default is 10.
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X11Forwarding
Specifies whether X11 forwarding is permitted. The
argument must be ``yes'' or ``no''. The default is
``no''.
When X11 forwarding is enabled, there may be additional
exposure to the server and to client displays if the
sshd proxy display is configured to listen on the
wildcard address (see 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 ForwardX11 in 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 ``no'' setting.
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 UseLogin is enabled.
X11UseLocalhost
Specifies whether sshd should bind the X11 forwarding
server to the loopback address or to the wildcard
address. By default, sshd binds the forwarding server
to the loopback address and sets the hostname part of
the DISPLAY environment variable to ``localhost''.
This prevents remote hosts from connecting to the proxy
display. However, some older X11 clients may not
function with this configuration. X11UseLocalhost may
be set to ``no'' to specify that the forwarding server
should be bound to the wildcard address. The argument
must be ``yes'' or ``no''. The default is ``yes''.
XAuthLocation
Specifies the full pathname of the xauth(1) program.
The default is /usr/bin/X11/xauth.
Time Formats
sshd command-line arguments and configuration file options
that specify time may be expressed using a sequence of the
form: time[qualifier,] where time is a positive integer
value and qualifier is one of the following:
<none>
seconds
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s | S
seconds
m | M
minutes
h | H
hours
d | D
days
w | W
weeks
Each member of the sequence is added together to
calculate the total time value.
Time format examples:
600 600 seconds (10 minutes)
10m 10 minutes
1h30m
1 hour 30 minutes (90 minutes)
FILES
/etc/openssh/sshd_config
Contains configuration data for sshd. This file should
be writable by root only, but it is recommended (though
not necessary) that it be world-readable.
SEE ALSO
sshd(8)
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.
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