Appendix - Sample sshd_config File

The following is a sample sshd_config file that contains descriptions for many of the options available for OpenSSH. Many of these descriptions were taken directly from the man page for sshd.

Note  

Some of the settings specified below may be machine- or installation-specific, so please read through the descriptions for each option to make sure you understand each of the settings before you implement them.

# Specifies whether an AFS token may be forwarded to the server.
 # Default is "yes".
 #AFSTokenPassing

 # 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.
 AllowGroups *

 # 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.
 AllowTcpForwarding yes

 # This keyword can be followed by a list of user name patterns,
 # separated by spaces. If specified, login is allowed only for
 # users 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.
 AllowUsers *

 # 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".
 AuthorizedKeysFile %h/.ssh/authorized_keys

 # 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.
 Banner /etc/issue

 # Specifies whether challenge response authentication is allowed.
 # All authentication styles from login.conf(5) are supported. The
 # default is "yes".
 ChallengeResponseAuthentication no

 # 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"
 Ciphers aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc

 # 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.
 ClientAliveInterval 0

 # 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.
 ClientAliveCountMax 3

 # 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.
 #DenyGroups

 # 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.
 #DenyUsers

 # Specifies whether remote hosts are allowed to connect to ports
 # forwarded for the client. By default, sshd binds remote port
 # forwardings to the loopback addresss. 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".
 GatewayPorts no

 # 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".
 HostbasedAuthentication no

 # Specifies a file containing a private host key used by SSH. The
 # default is /etc/ssh/sshuser_key for protocol version 1, and
 # /etc/ssh/sshuser_rsa_key and /etc/ssh/sshuser_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.
 HostKey /etc/ssh/sshuser_rsa_key
 HostKey /etc/ssh/sshuser_dsa_key

 # 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".
 IgnoreRhosts yes

 # Specifies whether sshd should ignore the user's
 # $HOME/.ssh/known_hosts during RhostsRSAAuthentication or
 # HostbasedAuthentication. The default is "no".
 IgnoreUserKnownHosts no

 # 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".
 KeepAlive yes

 # 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 "yes".
 #KerberosAuthentication

 # 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".
 #KerberosOrLocalPasswd

 # 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.
 #KerberosTgtPassing

 # Specifies whether to automatically destroy the user's ticket
 # cache file on logout. Default is "yes".
 #KerberosTicketCleanup

 # 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).
 #KeyRegenerationInterval

 # 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 disabled. The
 # default is "no".
 PAMAuthenticationViaKbdInt no
 # Specifies the port number that sshd listens on. The default is
 # 22. Multiple options of this type are permitted. See also
 # ListenAddress.
 Port 22

 # Specifies the local addresses sshd should listen on. The
 # following forms may be used:
 # ListenAddress hostIPv4_addrIPv6_addr
 # ListenAddress hostIPv4_addr:port
 # ListenAddress [hostIPv6_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.
 ListenAddress 0.0.0.0

 # 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 600 (seconds).
 LoginGraceTime 60

 # 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.
 LogLevel INFO

 # 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".
 MACs hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96

 # 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).
 MaxStartups 10

 # Specifies whether password authentication is allowed. The
 # default is "yes".
 PasswordAuthentication yes

 # When password authentication is allowed, it specifies whether the
 # server allows login to accounts with empty password strings. The
 # default is "no".
 PermitEmptyPasswords no

 # 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
 # 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.
 PermitRootLogin forced-commands-only

 # Specifies the file that contains the process identifier of the
 # sshd daemon. The default is /var/run/sshd.pid.
 PidFile /etc/ssh/sshd.pid

 # Specifies whether sshd should print the date and time when the
 # user last logged in. The default is "yes".
 PrintLastLog yes

 # 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".
 PrintMotd yes

 # Specifies the protocol versions sshd should support. The
 # possible values are "1" and "2". Multiple versions must be
 # comma-separated. The default is "2,1".
 Protocol 2

 # Specifies whether public key authentication is allowed. The
 # default is "yes". Note that this option applies to protocol
 # version 2 only.
 PubkeyAuthentication yes

 # 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.
 #RhostsAuthentication

 # 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.
 #RhostsRSAAuthentication

 # Specifies whether pure RSA authentication is allowed. The
 # default is "yes". This option applies to protocol version 1
 # only.
 #RSAAuthentication

 # Defines the number of bits in the ephemeral protocol version 1
 # server key. The minimum value is 512, and the default is 768.
 #ServerKeyBits

 # 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".
 StrictModes yes

 # 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.Subsystem sftp /opt/corp/local/openSSH/sbin/sftp-server

 # 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.
 SyslogFacility AUTH

 # 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.
 UseLogin no

 # 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".
 VerifyReverseMapping no

 # Specifies the first display number available for sshd's X11
 # forwarding. This prevents sshd from interfering with real X11
 # servers. The default is 10.
 X11DisplayOffset 10

 # Specifies whether X11 forwarding is permitted. The default is
 # "no". Note that disabling X11 forwarding does not improve
 # security in any way, as users can always install their own
 # forwarders. X11 forwarding is automatically disabled if UseLogin
 # is enabled.
 X11Forwarding no

 # 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
 # fake 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".
 X11UseLocalhost yes

 # Specifies the location of the xauth(1) program. The default is
 # /usr/bin/X11/xauth.
 XAuthLocation /usr/bin/X11/xauth