DESCRIPTION
sshd reads configuration data from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The file contains keyword-argu-
ment 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 key-
words are case-insensitive and arguments are case-sensitive):
AFSTokenPassing
Specifies whether an AFS token may be forwarded to the server.
Default is ``no''.
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 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 authenti-
cation may be relevant for getting legal protection. The con-
tents of the specified file are sent to the remote user before
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 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 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 inac-
tive.
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 forwarded ports. GatewayPorts can be
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 pro-
tocol 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.
GssapiAuthentication
Specifies whether authentication based on GSSAPI may be used,
either using the result of a successful key exchange, or using
GSSAPI user authentication. The default is ``yes''. Note that
this option applies to protocol version 2 only.
GssapiKeyExchange
Specifies whether key exchange based on GSSAPI may be used. When
using GSSAPI key exchange the server need not have a host key.
The default is ``yes''. Note that this option applies to proto-
col version 2 only.
GssapiUseSessionCredCache
Specifies whether a unique credentials cache name should be gen-
erated per session for storing delegated credentials. The
default is ``yes''. Note that this option applies to protocol
version 2 only.
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be used in
RhostsAuthentication, RhostsRSAAuthentication or
HostbasedAuthentication.
/etc/hosts.equiv and /etc/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''.
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 tempo-
rarily, 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.
KerberosTgtPassing
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
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 ses-
sions 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 follow-
ing 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 suc-
cessfully logged in. If the value is 0, there is no time limit.
The default is 120 seconds.
LogLevel
Gives the verbosity level that is used when logging messages from
sshd. The possible values are: QUIET, FATAL, ERROR, INFO, VER-
BOSE, 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) algo-
rithms. The MAC algorithm is used in protocol version 2 for data
integrity protection. Multiple algorithms must be comma-sepa-
rated. The default is
``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''.
MaxStartups
Specifies whether PAM challenge response authentication is
allowed. This allows the use of most PAM challenge response
authentication modules, but it will allow password authentication
regardless of whether PasswordAuthentication is enabled.
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''.
If this option is set to ``without-password'' password authenti-
cation 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
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 mecha-
nisms such as LD_PRELOAD.
PidFile
Specifies the file that contains the process ID of the sshd dae-
mon. 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
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,
RhostsAuthentication
Specifies whether authentication using rhosts or /etc/hosts.equiv
files is sufficient. Normally, this method should not be permit-
ted 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
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.
UseLogin
Specifies whether login(1) is used for interactive login ses-
sions. 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
maps back to the very same IP address. The default is ``no''.
X11DisplayOffset
Specifies the first display number available for sshd's X11 for-
warding. This prevents sshd from interfering with real X11
servers. The default is 10.
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 adminis-
trator 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 wild-
card 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/X11R6/bin/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 fol-
lowing:
<none> seconds
FILES
/etc/ssh/sshd_config
Contains configuration data for sshd. This file should be
writable by root only, but it is recommended (though not neces-
sary) that it be world-readable.
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 cre-
ated 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.
SEE ALSO
sshd(8)
BSD September 25, 1999 BSD
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