sshd_config - OpenSSH SSH daemon configuration file
/etc/ssh/sshd_config
sshd reads configuration data from /etc/ssh/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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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. `*' 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 [Toc] [Back]
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 [Toc] [Back]
Specifies whether challenge response authentication
is allowed.
All authentication styles from login.conf(5) are
supported. The
default is ``yes''.
Ciphers [Toc] [Back]
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 [Toc] [Back]
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 that these messages will
not be sent to
the client. This option applies to protocol version
2 only.
ClientAliveCountMax [Toc] [Back]
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 [Toc] [Back]
Specifies whether compression is allowed. The argument must be
``yes'' or ``no''. The default is ``yes''.
DenyGroups [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 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 [Toc] [Back]
Specifies whether user authentication based on GSSAPI is allowed.
The default is ``no''. Note that this option applies to protocol
version 2 only.
GSSAPICleanupCredentials [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
Specifies a file containing a private host key used
by SSH. The
default is /etc/ssh/ssh_host_key for protocol version 1, and
/etc/ssh/ssh_host_rsa_key and
/etc/ssh/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 [Toc] [Back]
Specifies that .rhosts and .shosts files will not be
used in
RhostsRSAAuthentication or HostbasedAuthentication.
/etc/hosts.equiv and /etc/shosts.equiv are still
used. The default
is ``yes''.
IgnoreUserKnownHosts [Toc] [Back]
Specifies whether sshd should ignore the user's
$HOME/.ssh/known_hosts during
RhostsRSAAuthentication or
HostbasedAuthentication. The default is ``no''.
KerberosAuthentication [Toc] [Back]
Specifies whether the password provided by the user
for
PasswordAuthentication 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
``no''.
KerberosGetAFSToken [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
Specifies whether to automatically destroy the user's ticket
cache file on logout. Default is ``yes''.
KeyRegenerationInterval [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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,hmacripemd160,hmacsha1-96,hmac-md5-96''.
MaxAuthTries [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
Specifies whether password authentication is allowed. The default
is ``yes''.
PermitEmptyPasswords [Toc] [Back]
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password
strings. The
default is ``no''.
PermitRootLogin [Toc] [Back]
Specifies whether root can login using ssh(1). The
argument must
be ``yes'', ``without-password'', ``forced-commandsonly'' or
``no''. The default is ``yes''.
If this option is set to ``without-password'' password authentication
is disabled for root.
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 [Toc] [Back]
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 [Toc] [Back]
Specifies the file that contains the process ID of
the sshd daemon.
The default is /var/run/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 [Toc] [Back]
Specifies whether sshd should print the date and
time when the
user last logged in. The default is ``yes''.
PrintMotd [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
Specifies whether public key authentication is allowed. The default
is ``yes''. Note that this option applies to
protocol version
2 only.
RhostsRSAAuthentication [Toc] [Back]
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 [Toc] [Back]
Specifies whether pure RSA authentication is allowed. The default
is ``yes''. This option applies to protocol
version 1 only.
ServerKeyBits [Toc] [Back]
Defines the number of bits in the ephemeral protocol
version 1
server key. The minimum value is 512, and the default is 768.
StrictModes [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 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 [Toc] [Back]
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. IfUsePrivilegeSeparation [Toc] [Back] is specified, it will be disabled after authentication.
UsePrivilegeSeparation [Toc] [Back]
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 [Toc] [Back]
Specifies the first display number available for
sshd's X11 forwarding.
This prevents sshd from interfering with
real X11
servers. The default is 10.
X11Forwarding [Toc] [Back]
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 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 [Toc] [Back]
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 [Toc] [Back]
Specifies the full pathname of the xauth(1) program.
The default
is /usr/X11R6/bin/xauth.
Time Formats [Toc] [Back]
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
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)
/etc/ssh/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.
sshd(8)
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.
OpenBSD 3.6 September 25, 1999
[ Back ] |
