SSHD_CONFIG(5) BSD File Formats Manual SSHD_CONFIG(5)
NAME
sshd_config -- OpenSSH SSH daemon configuration file
SYNOPSIS
/etc/sshd_config
DESCRIPTION
sshd(8) reads configuration data from /etc/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. Arguments may optionally be enclosed in double quotes (") in order
to represent arguments containing spaces.
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 environ-ment environment
ment passing is only supported for protocol 2. Variables are specified by name, which may con-tain contain
tain 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 speci-fied, specified,
fied, 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 fol-lowing following
lowing 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 speci-fied, specified,
fied, 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 get-ting getting
ting 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. The default is ``yes''.
Ciphers
Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-sepa-rated. comma-separated.
rated. 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) receiv-ing receiving
ing any messages back from the client. If this threshold is reached while client alive mes-sages messages
sages are being sent, sshd will disconnect the client, terminating the session. It is impor-tant important
tant 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 success-fully. successfully.
fully. 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 pat-terns. patterns.
terns. 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 numeri-cal numerical
cal 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 par-ticular particular
ticular 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 environ-ment environment
ment 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 forward-ing forwarding
ing 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.
GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the GSSAPI acceptor a client authenti-cates authenticates
cates against. If ``yes'' then the client must authenticate against the host service on the
current hostname. If ``no'' then the client may authenticate against any service key stored in
the machine's default store. This facility is provided to assist with operation on multi homed
machines. The default is ``yes''. Note that this option applies only to protocol version 2
GSSAPI connections, and setting it to ``no'' may only work with recent Kerberos GSSAPI
libraries.
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_host_key
for protocol version 1, and /etc/ssh_host_rsa_key and /etc/ssh_host_dsa_key for protocol ver-sion version
sion 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 vali-dated validated
dated 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:
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 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 debug-ging debugging
ging 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. commaseparated.
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 proba-bility probability
bility 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 specifi-cation specification
cation must be one of the following forms:
PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port
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'. Multi-ple Multiple
ple versions must be comma-separated. The default is ``2,1''. Note that the order of the pro-tocol protocol
tocol 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 acci-dentally accidentally
dentally 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. How-ever, However,
ever, 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 indefi-nitely indefinitely
nitely 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 net-work network
work 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 war-rant warrant
rant 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.
TIME FORMATS
sshd(8) 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)
FILES
/etc/sshd_config
Contains configuration data for sshd(8). This file should be writable by root only, but it is
recommended (though not necessary) 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 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.
BSD September 25, 1999 BSD
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