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.
AddressFamily- Specifies which address family should be used by
sshd(8). Valid arguments are “any”,
“inet” (use IPv4 only), or “inet6” (use IPv6
only). The default is “any”.
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. Only group
names are valid; a numerical group ID is not recognized. By default, login
is allowed for all groups. The allow/deny directives are processed in the
following order:
DenyUsers,
AllowUsers, DenyGroups,
and finally AllowGroups.
See PATTERNS in
ssh_config(5) for more information on patterns.
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. 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. The
allow/deny directives are processed in the following order:
DenyUsers, AllowUsers,
DenyGroups, and finally
AllowGroups.
See PATTERNS in
ssh_config(5) for more information on patterns.
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 setup. 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”, “arcfour128”,
“arcfour256”, “arcfour”,
“blowfish-cbc”, and “cast128-cbc”. The default
is:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr
ClientAliveCountMax- Sets the number of client alive messages (see below) which may be sent
without sshd(8) 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 (see below) is set to 15,
and ClientAliveCountMax is left at the default,
unresponsive SSH clients will be disconnected after approximately 45
seconds. This option applies to protocol version 2 only.
ClientAliveInterval- Sets a timeout interval in seconds after which if no data has been
received from the client, sshd(8) 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.
Compression- Specifies whether compression is allowed, or delayed until the user has
authenticated successfully. The argument must be “yes”,
“delayed”, or “no”. The default is
“delayed”.
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. Only group names are
valid; a numerical group ID is not recognized. By default, login is
allowed for all groups. The allow/deny directives are processed in the
following order:
DenyUsers,
AllowUsers, DenyGroups,
and finally AllowGroups.
See PATTERNS in
ssh_config(5) for more information on patterns.
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.
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. The allow/deny directives are
processed in the following order:
DenyUsers,
AllowUsers, DenyGroups,
and finally AllowGroups.
See PATTERNS in
ssh_config(5) for more information on patterns.
ForceCommand- Forces the execution of the command specified by
ForceCommand, ignoring any command supplied by the
client. The command is invoked by using the user's login shell with the -c
option. This applies to shell, command, or subsystem execution. It is most
useful inside a Match block. The command
originally supplied by the client is available in the
SSH_ORIGINAL_COMMAND environment variable.
GatewayPorts- Specifies whether remote hosts are allowed to connect to ports forwarded
for the client. By default, sshd(8) 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 allow remote port forwardings to bind to
non-loopback addresses, thus allowing other hosts to connect. The argument
may be “no” to force remote port forwardings to be available
to the local host only, “yes” to force remote port
forwardings to bind to the wildcard address, or
“clientspecified” to allow the client to select the address
to which the forwarding is bound. 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 (host-based
authentication). This option is similar to
RhostsRSAAuthentication and applies to protocol
version 2 only. The default is “no”.
HostbasedUsesNameFromPacketOnly- Specifies whether or not the server will attempt to perform a reverse name
lookup when matching the name in the ~/.shosts,
~/.rhosts, and
/etc/hosts.equiv files during
HostbasedAuthentication. A setting of
“yes” means that sshd(8) uses the name
supplied by the client rather than attempting to resolve the name from the
TCP connection itself. The default is “no”.
HostKey- 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(8) 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/shosts.equiv are still used. The default is
“yes”.
IgnoreUserKnownHosts- Specifies whether sshd(8) should ignore the user's
~/.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 Kerberos KDC. To use this option, the server needs a Kerberos servtab
which allows the verification of the KDC's identity. The default is
“no”.
KerberosGetAFSToken- If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an
AFS token before accessing the user's home directory. The default is
“no”.
KerberosOrLocalPasswd- If password authentication through Kerberos fails then the password will
be validated via any additional local mechanism such as
/etc/passwd. The default is
“yes”.
KerberosTicketCleanup- Specifies whether to automatically destroy the user's ticket cache file on
logout. The 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(8) should listen on.
The following forms may be used:
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- Gives the verbosity level that is used when logging messages from
sshd(8). 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”.
Match- Introduces a conditional block. If all of the criteria on the
Match line are satisfied, the keywords on the
following lines override those set in the global section of the config
file, until either another Match line or the end
of the file. The arguments to Match are one or
more criteria-pattern pairs. The available criteria are
User, Group,
Host, and Address. Only a
subset of keywords may be used on the lines following a
Match keyword. Available keywords are
AllowTcpForwarding,
ForceCommand,
GatewayPorts, PermitOpen,
X11DisplayOffset,
X11Forwarding, and
X11UseLocalHost.
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 SSH 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(8) 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”.
PermitOpen- Specifies the destinations to which TCP port forwarding is permitted. The
forwarding specification must be one of the following forms:
Multiple forwards may be specified by separating them with
whitespace. An argument of “any” can be used to remove all
restrictions and permit any forwarding requests. By default all port
forwarding requests are permitted.
PermitRootLogin- Specifies whether root can log in 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 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 log in.
PermitTunnel- Specifies whether tun(4) device forwarding is allowed.
The argument must be “yes”, “point-to-point”
(layer 3), “ethernet” (layer 2), or “no”.
Specifying “yes” permits both “point-to-point”
and “ethernet”. The default is “no”.
PermitUserEnvironment- Specifies whether ~/.ssh/environment and
environment= options in
~/.ssh/authorized_keys are processed by
sshd(8). 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 SSH daemon. The
default is /var/run/sshd.pid.
Port- Specifies the port number that sshd(8) listens on. The
default is 22. Multiple options of this type are permitted. See also
ListenAddress.
PrintLastLog- Specifies whether sshd(8) should print the date and time
of the last user login when a user logs in interactively. The default is
“yes”.
PrintMotd- Specifies whether sshd(8) 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(8) 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. 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(8) 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 (with optional arguments) 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(8). 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
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(8) should look up 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
PasswordAuthentication in addition to 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(8) 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(8)'s X11 forwarding. 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(8) proxy display is configured to listen on the
wildcard address (see X11UseLocalhost below),
though 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(8) 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/X11R6/bin/xauth.