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):
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 envi-
ronment 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. 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. `*' 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.
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 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 ses-
sion. It is important to note that the use of client alive mes-
sages is very different from TCPKeepAlive (below). The client
alive messages are sent through the encrypted channel and there-
fore will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The client alive mechanism is valu-
able when the client or server depend on knowing when a connec-
tion 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.
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.
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
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
used to specify that sshd should allow remote port forwardings to
bind to non-loopback addresses, thus allowing other hosts to con-
nect. The argument may be ``no'' to force remote port forward-
ings 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.
GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
key exchange doesn't rely on ssh keys to verify host identity.
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/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.
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''.
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 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
three colon separated values ``start:rate:full'' (e.g.,
"10:30:60"). sshd will refuse connection attempts with a proba-
bility of ``rate/100'' (30%) if there are currently ``start''
(10) unauthenticated connections. The probability increases lin-
early 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 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 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 log in.
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 of the last
user login when a user logs in interactively. The default is
``yes''.
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 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 tempo-
rarily, 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
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 mod-
ule 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 con-
taining any corruption within the unprivileged processes. The
default is ``yes''.
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
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
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/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.
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 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.
BSD September 25, 1999 BSD
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