Provided by: openssh-server_7.2p2-4_amd64 bug

NAME

     sshd_config — OpenSSH SSH daemon configuration file

SYNOPSIS

     /etc/ssh/sshd_config

DESCRIPTION

     sshd(8) reads configuration data from /etc/ssh/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.

     Note that the Debian openssh-server package sets several options as standard in
     /etc/ssh/sshd_config which are not the default in sshd(8).  The exact list depends on
     whether the package was installed fresh or upgraded from various possible previous versions,
     but includes at least the following:

           ·   ChallengeResponseAuthentication no
           ·   X11Forwarding yes
           ·   PrintMotd no
           ·   AcceptEnv LANG LC_*
           ·   Subsystem sftp /usr/lib/openssh/sftp-server
           ·   UsePAM yes

     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.
             The TERM environment variable is always sent whenever the client requests a pseudo-
             terminal as it is required by the protocol.  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”.

     AllowAgentForwarding
             Specifies whether ssh-agent(1) forwarding is permitted.  The default is “yes”.  Note
             that disabling agent forwarding does not improve security unless users are also
             denied shell access, as they can always install their own forwarders.

     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 available options are “yes” or
             “all” to allow TCP forwarding, “no” to prevent all TCP forwarding, “local” to allow
             local (from the perspective of ssh(1)) forwarding only or “remote” to allow remote
             forwarding only.  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.

     AllowStreamLocalForwarding
             Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted.  The
             available options are “yes” or “all” to allow StreamLocal forwarding, “no” to
             prevent all StreamLocal forwarding, “local” to allow local (from the perspective of
             ssh(1)) forwarding only or “remote” to allow remote forwarding only.  The default is
             “yes”.  Note that disabling StreamLocal 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.

     AuthenticationMethods
             Specifies the authentication methods that must be successfully completed for a user
             to be granted access.  This option must be followed by one or more comma-separated
             lists of authentication method names.  Successful authentication requires completion
             of every method in at least one of these lists.

             For example, an argument of “publickey,password publickey,keyboard-interactive”
             would require the user to complete public key authentication, followed by either
             password or keyboard interactive authentication.  Only methods that are next in one
             or more lists are offered at each stage, so for this example, it would not be
             possible to attempt password or keyboard-interactive authentication before public
             key.

             For keyboard interactive authentication it is also possible to restrict
             authentication to a specific device by appending a colon followed by the device
             identifier “bsdauth”, “pam”, or “skey”, depending on the server configuration.  For
             example, “keyboard-interactive:bsdauth” would restrict keyboard interactive
             authentication to the “bsdauth” device.

             If the “publickey” method is listed more than once, sshd(8) verifies that keys that
             have been used successfully are not reused for subsequent authentications.  For
             example, an AuthenticationMethods of “publickey,publickey” will require successful
             authentication using two different public keys.

             This option will yield a fatal error if enabled if protocol 1 is also enabled.  Note
             that each authentication method listed should also be explicitly enabled in the
             configuration.  The default is not to require multiple authentication; successful
             completion of a single authentication method is sufficient.

     AuthorizedKeysCommand
             Specifies a program to be used to look up the user's public keys.  The program must
             be owned by root, not writable by group or others and specified by an absolute path.

             Arguments to AuthorizedKeysCommand may be provided using the following tokens, which
             will be expanded at runtime: %% is replaced by a literal '%', %u is replaced by the
             username being authenticated, %h is replaced by the home directory of the user being
             authenticated, %t is replaced with the key type offered for authentication, %f is
             replaced with the fingerprint of the key, and %k is replaced with the key being
             offered for authentication.  If no arguments are specified then the username of the
             target user will be supplied.

             The program should produce on standard output zero or more lines of authorized_keys
             output (see AUTHORIZED_KEYS in sshd(8)).  If a key supplied by AuthorizedKeysCommand
             does not successfully authenticate and authorize the user then public key
             authentication continues using the usual AuthorizedKeysFile files.  By default, no
             AuthorizedKeysCommand is run.

     AuthorizedKeysCommandUser
             Specifies the user under whose account the AuthorizedKeysCommand is run.  It is
             recommended to use a dedicated user that has no other role on the host than running
             authorized keys commands.  If AuthorizedKeysCommand is specified but
             AuthorizedKeysCommandUser is not, then sshd(8) will refuse to start.

     AuthorizedKeysFile
             Specifies the file that contains the public keys that can be used for user
             authentication.  The format is described in the AUTHORIZED_KEYS FILE FORMAT section
             of sshd(8).  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.  Multiple files may be listed, separated by whitespace.  Alternately
             this option may be set to “none” to skip checking for user keys in files.  The
             default is “.ssh/authorized_keys .ssh/authorized_keys2”.

     AuthorizedPrincipalsCommand
             Specifies a program to be used to generate the list of allowed certificate
             principals as per AuthorizedPrincipalsFile.  The program must be owned by root, not
             writable by group or others and specified by an absolute path.

             Arguments to AuthorizedPrincipalsCommand may be provided using the following tokens,
             which will be expanded at runtime: %% is replaced by a literal '%', %u is replaced
             by the username being authenticated and %h is replaced by the home directory of the
             user being authenticated.

             The program should produce on standard output zero or more lines of
             AuthorizedPrincipalsFile output.  If either AuthorizedPrincipalsCommand or
             AuthorizedPrincipalsFile is specified, then certificates offered by the client for
             authentication must contain a principal that is listed.  By default, no
             AuthorizedPrincipalsCommand is run.

     AuthorizedPrincipalsCommandUser
             Specifies the user under whose account the AuthorizedPrincipalsCommand is run.  It
             is recommended to use a dedicated user that has no other role on the host than
             running authorized principals commands.  If AuthorizedPrincipalsCommand is specified
             but AuthorizedPrincipalsCommandUser is not, then sshd(8) will refuse to start.

     AuthorizedPrincipalsFile
             Specifies a file that lists principal names that are accepted for certificate
             authentication.  When using certificates signed by a key listed in
             TrustedUserCAKeys, this file lists names, one of which must appear in the
             certificate for it to be accepted for authentication.  Names are listed one per line
             preceded by key options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
             Empty lines and comments starting with ‘#’ are ignored.

             AuthorizedPrincipalsFile 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,
             AuthorizedPrincipalsFile is taken to be an absolute path or one relative to the
             user's home directory.

             The default is “none”, i.e. not to use a principals file – in this case, the
             username of the user must appear in a certificate's principals list for it to be
             accepted.  Note that AuthorizedPrincipalsFile is only used when authentication
             proceeds using a CA listed in TrustedUserCAKeys and is not consulted for
             certification authorities trusted via ~/.ssh/authorized_keys, though the principals=
             key option offers a similar facility (see sshd(8) for details).

     Banner  The contents of the specified file are sent to the remote user before authentication
             is allowed.  If the argument is “none” then no banner is displayed.  By default, no
             banner is displayed.

     ChallengeResponseAuthentication
             Specifies whether challenge-response authentication is allowed (e.g. via PAM).  The
             default is “yes”.

     ChrootDirectory
             Specifies the pathname of a directory to chroot(2) to after authentication.  At
             session startup sshd(8) checks that all components of the pathname are root-owned
             directories which are not writable by any other user or group.  After the chroot,
             sshd(8) changes the working directory to the user's home directory.

             The pathname may contain the following tokens that are expanded at runtime once the
             connecting user has been authenticated: %% 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.

             The ChrootDirectory must contain the necessary files and directories to support the
             user's session.  For an interactive session this requires at least a shell,
             typically sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4),
             stderr(4), and tty(4) devices.  For file transfer sessions using “sftp”, no
             additional configuration of the environment is necessary if the in-process sftp
             server is used, though sessions which use logging may require /dev/log inside the
             chroot directory on some operating systems (see sftp-server(8) for details).

             For safety, it is very important that the directory hierarchy be prevented from
             modification by other processes on the system (especially those outside the jail).
             Misconfiguration can lead to unsafe environments which sshd(8) cannot detect.

             The default is “none”, indicating not to chroot(2).

     Ciphers
             Specifies the ciphers allowed.  Multiple ciphers must be comma-separated.  If the
             specified value begins with a ‘+’ character, then the specified ciphers will be
             appended to the default set instead of replacing them.

             The supported ciphers are:

                   3des-cbc
                   aes128-cbc
                   aes192-cbc
                   aes256-cbc
                   aes128-ctr
                   aes192-ctr
                   aes256-ctr
                   aes128-gcm@openssh.com
                   aes256-gcm@openssh.com
                   arcfour
                   arcfour128
                   arcfour256
                   blowfish-cbc
                   cast128-cbc
                   chacha20-poly1305@openssh.com

             The default is:

                   chacha20-poly1305@openssh.com,
                   aes128-ctr,aes192-ctr,aes256-ctr,
                   aes128-gcm@openssh.com,aes256-gcm@openssh.com

             The list of available ciphers may also be obtained using the -Q option of ssh(1)
             with an argument of “cipher”.

     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.

     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.

     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”.

     DebianBanner
             Specifies whether the distribution-specified extra version suffix is included during
             initial protocol handshake.  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.  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.

     FingerprintHash
             Specifies the hash algorithm used when logging key fingerprints.  Valid options are:
             “md5” and “sha256”.  The default is “sha256”.

     ForceCommand
             Forces the execution of the command specified by ForceCommand, ignoring any command
             supplied by the client and ~/.ssh/rc if present.  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.  Specifying a command of “internal-sftp” will force the use of
             an in-process sftp server that requires no support files when used with
             ChrootDirectory.  The default is “none”.

     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”.

     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”.

     GSSAPICleanupCredentials
             Specifies whether to automatically destroy the user's credentials cache on logout.
             The default is “yes”.

     GSSAPIStrictAcceptorCheck
             Determines whether to be strict about the identity of the GSSAPI acceptor a client
             authenticates against.  If set to “yes” then the client must authenticate against
             the host service on the current hostname.  If set to “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”.

     GSSAPIStoreCredentialsOnRekey
             Controls whether the user's GSSAPI credentials should be updated following a
             successful connection rekeying. This option can be used to accepted renewed or
             updated credentials from a compatible client. The default is “no”.

     HostbasedAcceptedKeyTypes
             Specifies the key types that will be accepted for hostbased authentication as a
             comma-separated pattern list.  Alternately if the specified value begins with a ‘+’
             character, then the specified key types will be appended to the default set instead
             of replacing them.  The default for this option is:

                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                ssh-ed25519-cert-v01@openssh.com,
                ssh-rsa-cert-v01@openssh.com,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                ssh-ed25519,ssh-rsa

             The -Q option of ssh(1) may be used to list supported key types.

     HostbasedAuthentication
             Specifies whether rhosts or /etc/hosts.equiv authentication together with successful
             public key client host authentication is allowed (host-based authentication).  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”.

     HostCertificate
             Specifies a file containing a public host certificate.  The certificate's public key
             must match a private host key already specified by HostKey.  The default behaviour
             of sshd(8) is not to load any certificates.

     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_dsa_key,
             /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and
             /etc/ssh/ssh_host_rsa_key for protocol version 2.

             Note that sshd(8) will refuse to use a file if it is group/world-accessible and that
             the HostKeyAlgorithms option restricts which of the keys are actually used by
             sshd(8).

             It is possible to have multiple host key files.  “rsa1” keys are used for version 1
             and “dsa”, “ecdsa”, “ed25519” or “rsa” are used for version 2 of the SSH protocol.
             It is also possible to specify public host key files instead.  In this case
             operations on the private key will be delegated to an ssh-agent(1).

     HostKeyAgent
             Identifies the UNIX-domain socket used to communicate with an agent that has access
             to the private host keys.  If “SSH_AUTH_SOCK” is specified, the location of the
             socket will be read from the SSH_AUTH_SOCK environment variable.

     HostKeyAlgorithms
             Specifies the host key algorithms that the server offers.  The default for this
             option is:

                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                ssh-ed25519-cert-v01@openssh.com,
                ssh-rsa-cert-v01@openssh.com,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                ssh-ed25519,ssh-rsa

             The list of available key types may also be obtained using the -Q option of ssh(1)
             with an argument of “key”.

     IgnoreRhosts
             Specifies that .rhosts and .shosts files will not be used in RhostsRSAAuthentication
             or HostbasedAuthentication.

             /etc/hosts.equiv and /etc/ssh/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”.

     IPQoS   Specifies the IPv4 type-of-service or DSCP class for the connection.  Accepted
             values are “af11”, “af12”, “af13”, “af21”, “af22”, “af23”, “af31”, “af32”, “af33”,
             “af41”, “af42”, “af43”, “cs0”, “cs1”, “cs2”, “cs3”, “cs4”, “cs5”, “cs6”, “cs7”,
             “ef”, “lowdelay”, “throughput”, “reliability”, or a numeric value.  This option may
             take one or two arguments, separated by whitespace.  If one argument is specified,
             it is used as the packet class unconditionally.  If two values are specified, the
             first is automatically selected for interactive sessions and the second for non-
             interactive sessions.  The default is “lowdelay” for interactive sessions and
             “throughput” for non-interactive sessions.

     KbdInteractiveAuthentication
             Specifies whether to allow keyboard-interactive authentication.  The argument to
             this keyword must be “yes” or “no”.  The default is to use whatever value
             ChallengeResponseAuthentication is set to (by default “yes”).

     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”.

     KexAlgorithms
             Specifies the available KEX (Key Exchange) algorithms.  Multiple algorithms must be
             comma-separated.  Alternately if the specified value begins with a ‘+’ character,
             then the specified methods will be appended to the default set instead of replacing
             them.  The supported algorithms are:

                   curve25519-sha256@libssh.org
                   diffie-hellman-group1-sha1
                   diffie-hellman-group14-sha1
                   diffie-hellman-group-exchange-sha1
                   diffie-hellman-group-exchange-sha256
                   ecdh-sha2-nistp256
                   ecdh-sha2-nistp384
                   ecdh-sha2-nistp521

             The default is:

                   curve25519-sha256@libssh.org,
                   ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                   diffie-hellman-group-exchange-sha256,
                   diffie-hellman-group14-sha1

             The list of available key exchange algorithms may also be obtained using the -Q
             option of ssh(1) with an argument of “kex”.

     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 Port options
             specified.  The default is to listen on all local addresses.  Multiple ListenAddress
             options are permitted.

     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 for data integrity protection.  Multiple algorithms must be comma-
             separated.  If the specified value begins with a ‘+’ character, then the specified
             algorithms will be appended to the default set instead of replacing them.

             The algorithms that contain “-etm” calculate the MAC after encryption (encrypt-then-
             mac).  These are considered safer and their use recommended.  The supported MACs
             are:

                   hmac-md5
                   hmac-md5-96
                   hmac-ripemd160
                   hmac-sha1
                   hmac-sha1-96
                   hmac-sha2-256
                   hmac-sha2-512
                   umac-64@openssh.com
                   umac-128@openssh.com
                   hmac-md5-etm@openssh.com
                   hmac-md5-96-etm@openssh.com
                   hmac-ripemd160-etm@openssh.com
                   hmac-sha1-etm@openssh.com
                   hmac-sha1-96-etm@openssh.com
                   hmac-sha2-256-etm@openssh.com
                   hmac-sha2-512-etm@openssh.com
                   umac-64-etm@openssh.com
                   umac-128-etm@openssh.com

             The default is:

                   umac-64-etm@openssh.com,umac-128-etm@openssh.com,
                   hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
                   hmac-sha1-etm@openssh.com,
                   umac-64@openssh.com,umac-128@openssh.com,
                   hmac-sha2-256,hmac-sha2-512,hmac-sha1

             The list of available MAC algorithms may also be obtained using the -Q option of
             ssh(1) with an argument of “mac”.

     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.
             If a keyword appears in multiple Match blocks that are satisfied, only the first
             instance of the keyword is applied.

             The arguments to Match are one or more criteria-pattern pairs or the single token
             All which matches all criteria.  The available criteria are User, Group, Host,
             LocalAddress, LocalPort, and Address.  The match patterns may consist of single
             entries or comma-separated lists and may use the wildcard and negation operators
             described in the PATTERNS section of ssh_config(5).

             The patterns in an Address criteria may additionally contain addresses to match in
             CIDR address/masklen format, e.g. “192.0.2.0/24” or “3ffe:ffff::/32”.  Note that the
             mask length provided must be consistent with the address - it is an error to specify
             a mask length that is too long for the address or one with bits set in this host
             portion of the address.  For example, “192.0.2.0/33” and “192.0.2.0/8” respectively.

             Only a subset of keywords may be used on the lines following a Match keyword.
             Available keywords are AcceptEnv, AllowAgentForwarding, AllowGroups,
             AllowStreamLocalForwarding, AllowTcpForwarding, AllowUsers, AuthenticationMethods,
             AuthorizedKeysCommand, AuthorizedKeysCommandUser, AuthorizedKeysFile,
             AuthorizedPrincipalsCommand, AuthorizedPrincipalsCommandUser,
             AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups, DenyUsers,
             ForceCommand, GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
             HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
             KbdInteractiveAuthentication, KerberosAuthentication, MaxAuthTries, MaxSessions,
             PasswordAuthentication, PermitEmptyPasswords, PermitOpen, PermitRootLogin,
             PermitTTY, PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication,
             RekeyLimit, RevokedKeys, RhostsRSAAuthentication, RSAAuthentication,
             StreamLocalBindMask, StreamLocalBindUnlink, TrustedUserCAKeys, 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.

     MaxSessions
             Specifies the maximum number of open shell, login or subsystem (e.g. sftp) sessions
             permitted per network connection.  Multiple sessions may be established by clients
             that support connection multiplexing.  Setting MaxSessions to 1 will effectively
             disable session multiplexing, whereas setting it to 0 will prevent all shell, login
             and subsystem sessions while still permitting forwarding.  The default is 10.

     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 10:30:100.

             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:

                   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.
             An argument of “none” can be used to prohibit all 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”,
             “prohibit-password”, “without-password”, “forced-commands-only”, or “no”.  The
             default is “prohibit-password”.

             If this option is set to “prohibit-password” or “without-password”, password and
             keyboard-interactive authentication are 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”.

             Independent of this setting, the permissions of the selected tun(4) device must
             allow access to the user.

     PermitTTY
             Specifies whether pty(4) allocation is permitted.  The default is “yes”.

     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.

     PermitUserRC
             Specifies whether any ~/.ssh/rc file is executed.  The default is “yes”.

     PidFile
             Specifies the file that contains the process ID of the SSH daemon, or “none” to not
             write one.  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’.  Protocol 1
             suffers from a number of cryptographic weaknesses and should not be used.  It is
             only offered to support legacy devices.

             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”.

     PubkeyAcceptedKeyTypes
             Specifies the key types that will be accepted for public key authentication as a
             comma-separated pattern list.  Alternately if the specified value begins with a ‘+’
             character, then the specified key types will be appended to the default set instead
             of replacing them.  The default for this option is:

                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                ssh-ed25519-cert-v01@openssh.com,
                ssh-rsa-cert-v01@openssh.com,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                ssh-ed25519,ssh-rsa

             The -Q option of ssh(1) may be used to list supported key types.

     PubkeyAuthentication
             Specifies whether public key authentication is allowed.  The default is “yes”.

     RekeyLimit
             Specifies the maximum amount of data that may be transmitted before the session key
             is renegotiated, optionally followed a maximum amount of time that may pass before
             the session key is renegotiated.  The first argument is specified in bytes and may
             have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or Gigabytes,
             respectively.  The default is between ‘1G’ and ‘4G’, depending on the cipher.  The
             optional second value is specified in seconds and may use any of the units
             documented in the TIME FORMATS section.  The default value for RekeyLimit is
             “default none”, which means that rekeying is performed after the cipher's default
             amount of data has been sent or received and no time based rekeying is done.

     RevokedKeys
             Specifies revoked public keys file, or “none” to not use one.  Keys listed in this
             file will be refused for public key authentication.  Note that if this file is not
             readable, then public key authentication will be refused for all users.  Keys may be
             specified as a text file, listing one public key per line, or as an OpenSSH Key
             Revocation List (KRL) as generated by ssh-keygen(1).  For more information on KRLs,
             see the KEY REVOCATION LISTS section in ssh-keygen(1).

     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
             default and minimum value is 1024.

     StreamLocalBindMask
             Sets the octal file creation mode mask (umask) used when creating a Unix-domain
             socket file for local or remote port forwarding.  This option is only used for port
             forwarding to a Unix-domain socket file.

             The default value is 0177, which creates a Unix-domain socket file that is readable
             and writable only by the owner.  Note that not all operating systems honor the file
             mode on Unix-domain socket files.

     StreamLocalBindUnlink
             Specifies whether to remove an existing Unix-domain socket file for local or remote
             port forwarding before creating a new one.  If the socket file already exists and
             StreamLocalBindUnlink is not enabled, sshd will be unable to forward the port to the
             Unix-domain socket file.  This option is only used for port forwarding to a Unix-
             domain socket file.

             The argument must be “yes” or “no”.  The default is “no”.

     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”.  Note that this does not apply to ChrootDirectory, whose
             permissions and ownership are checked unconditionally.

     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.

             Alternately the name “internal-sftp” implements an in-process “sftp” server.  This
             may simplify configurations using ChrootDirectory to force a different filesystem
             root on clients.

             By default no subsystems are defined.

     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”.

             This option was formerly called KeepAlive.

     TrustedUserCAKeys
             Specifies a file containing public keys of certificate authorities that are trusted
             to sign user certificates for authentication, or “none” to not use one.  Keys are
             listed one per line; empty lines and comments starting with ‘#’ are allowed.  If a
             certificate is presented for authentication and has its signing CA key listed in
             this file, then it may be used for authentication for any user listed in the
             certificate's principals list.  Note that certificates that lack a list of
             principals will not be permitted for authentication using TrustedUserCAKeys.  For
             more details on certificates, see the CERTIFICATES section in ssh-keygen(1).

     UseDNS  Specifies whether sshd(8) should look up the remote host name, and to check that the
             resolved host name for the remote IP address maps back to the very same IP address.

             If this option is set to “no” (the default) then only addresses and not host names
             may be used in ~/.ssh/known_hosts from and sshd_config Match Host directives.

     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 argument must be “yes”, “no”,
             or “sandbox”.  If UsePrivilegeSeparation is set to “sandbox” then the pre-
             authentication unprivileged process is subject to additional restrictions.  The
             default is “sandbox”.

     VersionAddendum
             Optionally specifies additional text to append to the SSH protocol banner sent by
             the server upon connection.  The default is “none”.

     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, or “none” to not use one.  The
             default is /usr/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/ssh/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.