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):
AFSTokenPassing [Toc] [Back]
Specifies whether an AFS token may be forwarded to the server.
Default is ``no''.
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.
Specifically, in FreeBSD, this controls the use of PAM (see
pam(3)) for authentication. Note that this affects the effectiveness
of the PasswordAuthentication and PermitRootLogin variables.
The default is ``yes''.
Ciphers [Toc] [Back]
Specifies the ciphers allowed for protocol version 2. Multiple
ciphers must be comma-separated. The default is
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc''
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 KeepAlive (below). The client alive messages
are sent through the encrypted channel and therefore will not be
spoofable. The TCP keepalive option enabled by KeepAlive 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''.
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_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
RhostsAuthentication, RhostsRSAAuthentication or
HostbasedAuthentication.
/etc/hosts.equiv and /etc/ssh/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''.
KeepAlive [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
keepalives are not sent, sessions may hang indefinitely on the
server, leaving ``ghost'' users and consuming server resources.
The default is ``yes'' (to send keepalives), and the server will
notice if the network goes down or the client host crashes. This
avoids infinitely hanging sessions.
To disable keepalives, the value should be set to ``no''.
KerberosAuthentication [Toc] [Back]
Specifies whether Kerberos authentication is allowed. This can
be in the form of a Kerberos ticket, or if 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 ``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''.
KerberosTgtPassing [Toc] [Back]
Specifies whether a Kerberos TGT may be forwarded to the server.
Default is ``no'', as this only works when the Kerberos KDC is
actually an AFS kaserver.
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,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''.
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''. Note that if ChallengeResponseAuthentication
is ``yes'', and the PAM authentication policy for sshd includes
pam_unix(8), password authentication will be allowed through the
challenge-response mechanism regardless of the value of
PasswordAuthentication.
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-commands-only'' or
``no''. The default is ``no''. Note that if
ChallengeResponseAuthentication is ``yes'', the root user may be
allowed in with its password even if PermitRootLogin is set to
``without-password''.
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.
RhostsAuthentication [Toc] [Back]
Specifies whether authentication using rhosts or /etc/hosts.equiv
files is sufficient. Normally, this method should not be permitted
because it is insecure. 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 ``no''. This option applies to protocol version 1
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.
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. If UsePrivilegeSeparation
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''.
VerifyReverseMapping [Toc] [Back]
Specifies whether sshd should try to verify 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 ``no''.
VersionAddendum [Toc] [Back]
Specifies a string to append to the regular version string to
identify OS- or site-specific modifications. The default is
``FreeBSD-20030924''.
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 ``yes''.
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 [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.
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.
sshd(8)
FreeBSD 5.2.1 September 25, 1999 FreeBSD 5.2.1 [ Back ] |
