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SSHD_CONFIG(5)             BSD File Formats Manual            SSHD_CONFIG(5)

NAME         top

     sshd_config — OpenSSH SSH daemon configuration file

DESCRIPTION         top

     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.

     The possible keywords and their meanings are as follows (note that key‐
     words are case-insensitive and arguments are case-sensitive):

     AcceptEnv
             Specifies what environment variables sent by the client will be
             copied into the session's environ(7).  See SendEnv in
             ssh_config(5) for how to configure the client.  The TERM envi‐
             ronment 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 charac‐
             ters ‘*’ and ‘?’.  Multiple environment variables may be sepa‐
             rated by whitespace or spread across multiple AcceptEnv direc‐
             tives.  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 (the default), inet (use IPv4 only), or
             inet6 (use IPv6 only).

     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.

     AllowStreamLocalForwarding
             Specifies whether StreamLocal (Unix-domain socket) forwarding
             is permitted.  The available options are yes (the default) or
             all to allow StreamLocal forwarding, no to prevent all Stream‐
             Local forwarding, local to allow local (from the perspective of
             ssh(1)) forwarding only or remote to allow remote forwarding
             only.  Note that disabling StreamLocal forwarding does not
             improve security unless users are also denied shell access, as
             they can always install their own forwarders.

     AllowTcpForwarding
             Specifies whether TCP forwarding is permitted.  The available
             options are yes (the default) 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.  Note that disabling TCP forwarding
             does not improve security unless users are also denied shell
             access, as they can always install their own forwarders.

     AllowUsers
             This keyword can be followed by a list of user name patterns,
             separated by spaces.  If specified, login is allowed only for
             user names that match one of the patterns.  Only user names are
             valid; a numerical user ID is not recognized.  By default,
             login is allowed for all users.  If the pattern takes the form
             USER@HOST then USER and HOST are separately checked, restrict‐
             ing logins to particular users from particular hosts.  HOST
             criteria may additionally contain addresses to match in CIDR
             address/masklen format.  The allow/deny directives are pro‐
             cessed 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, or by the single string any to indicate the
             default behaviour of accepting any single authentication
             method.  If the default is overridden, then successful authen‐
             tication requires completion of every method in at least one of
             these lists.

             For example, "publickey,password
             publickey,keyboard-interactive" would require the user to com‐
             plete 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 interac‐
             tive authentication to the bsdauth device.

             If the publickey method is listed more than once, sshd(8) veri‐
             fies that keys that have been used successfully are not reused
             for subsequent authentications.  For example,
             "publickey,publickey" requires successful authentication using
             two different public keys.

             Note that each authentication method listed should also be
             explicitly enabled in the configuration.

             The available authentication methods are: "gssapi-with-mic",
             "hostbased", "keyboard-interactive", "none" (used for access to
             password-less accounts when PermitEmptyPassword is enabled),
             "password" and "publickey".

     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 accept the tokens described in the TOKENS
             section.  If no arguments are specified then the username of
             the target user is used.

             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 dedi‐
             cated user that has no other role on the host than running
             authorized keys commands.  If AuthorizedKeysCommand is speci‐
             fied but AuthorizedKeysCommandUser is not, then sshd(8) will
             refuse to start.

     AuthorizedKeysFile
             Specifies the file that contains the public keys used for user
             authentication.  The format is described in the AUTHORIZED_KEYS
             FILE FORMAT section of sshd(8).  Arguments to
             AuthorizedKeysFile accept the tokens described in the TOKENS
             section.  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.  Alter‐
             nately 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 accept the tokens described in the
             TOKENS section.  If no arguments are specified then the user‐
             name of the target user is used.

             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 authen‐
             tication 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.

             Arguments to AuthorizedPrincipalsFile accept the tokens
             described in the TOKENS section.  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 authenti‐
             cation 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 or through authentication styles supported in
             login.conf(5)) 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 direc‐
             tory.  Arguments to ChrootDirectory accept the tokens described
             in the TOKENS section.

             The ChrootDirectory must contain the necessary files and direc‐
             tories to support the user's session.  For an interactive ses‐
             sion 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 ses‐
             sions 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.  If the specified value begins with
             a ‘-’ character, then the specified ciphers (including wild‐
             cards) will be removed from the default set instead of replac‐
             ing 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
                   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 "ssh
             -Q cipher".

     ClientAliveCountMax
             Sets the number of client alive messages 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.  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 valu‐
             able when the client or server depend on knowing when a connec‐
             tion has become inactive.

             The default value is 3.  If ClientAliveInterval is set to 15,
             and ClientAliveCountMax is left at the default, unresponsive
             SSH clients will be disconnected after approximately 45 sec‐
             onds.

     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 enabled after the user has
             authenticated successfully.  The argument must be yes, delayed
             (a legacy synonym for yes) or no.  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 pri‐
             mary group or supplementary group list matches one of the pat‐
             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 numer‐
             ical 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.  HOST criteria may
             additionally contain addresses to match in CIDR address/masklen
             format.  The allow/deny directives are processed in the follow‐
             ing order: DenyUsers, AllowUsers, DenyGroups, and finally
             AllowGroups.

             See PATTERNS in ssh_config(5) for more information on patterns.

     DisableForwarding
             Disables all forwarding features, including X11, ssh-agent(1),
             TCP and StreamLocal.  This option overrides all other forward‐
             ing-related options and may simplify restricted configurations.

     ExposeAuthInfo
             Writes a temporary file containing a list of authentication
             methods and public credentials (e.g. keys) used to authenticate
             the user.  The location of the file is exposed to the user ses‐
             sion through the SSH_USER_AUTH environment variable.  The
             default is no.

     FingerprintHash
             Specifies the hash algorithm used when logging key finger‐
             prints.  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 for‐
             wardings 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.

     GSSAPICleanupCredentials
             Specifies whether to automatically destroy the user's creden‐
             tials cache on logout.  The default is yes.

     GSSAPIStrictAcceptorCheck
             Determines whether to be strict about the identity of the GSS‐
             API 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 authen‐
             ticate 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.

     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.  If the specified value begins with a ‘-’
             character, then the specified key types (including wildcards)
             will be removed from 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 list of available key types may also be obtained using "ssh
             -Q key".

     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 defaults are /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.

             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.  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 the string
             "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 "ssh
             -Q key".

     IgnoreRhosts
             Specifies that .rhosts and .shosts files will not be used in
             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 HostbasedAuthentication.  The default
             is no.

     IPQoS   Specifies the IPv4 type-of-service or DSCP class for the con‐
             nection.  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, a
             numeric value, or none to use the operating system default.
             This option may take one or two arguments, separated by white‐
             space.  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 sec‐
             ond for non-interactive sessions.  The default is lowdelay for
             interactive sessions and throughput for non-interactive ses‐
             sions.

     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 direc‐
             tory.  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.  Multi‐
             ple 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 replac‐
             ing them.  If the specified value begins with a ‘-’ character,
             then the specified methods (including wildcards) will be
             removed from the default set instead of replacing them.  The
             supported algorithms are:

                   curve25519-sha256
                   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,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 "ssh -Q kex".

     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 suc‐
             cessfully logged in.  If the value is 0, there is no time
             limit.  The default is 120 seconds.

     LogLevel
             Gives the verbosity level that is used when logging messages
             from sshd(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 recom‐
             mended.

     MACs    Specifies the available MAC (message authentication code) algo‐
             rithms.  The MAC algorithm is used for data integrity protec‐
             tion.  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.  If the specified value begins with a ‘-’ char‐
             acter, then the specified algorithms (including wildcards) will
             be removed from 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-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-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
             "ssh -Q 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 avail‐
             able 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, such as
             192.0.2.0/24 or 2001:db8::/32.  Note that the mask length pro‐
             vided 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 exam‐
             ple, 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, ClientAliveCountMax,
             ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand,
             GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
             HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
             IPQoS, KbdInteractiveAuthentication, KerberosAuthentication,
             LogLevel, MaxAuthTries, MaxSessions, PasswordAuthentication,
             PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
             PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes,
             PubkeyAuthentication, RekeyLimit, RevokedKeys,
             StreamLocalBindMask, StreamLocalBindUnlink, TrustedUserCAKeys,
             X11DisplayOffset, X11Forwarding and X11UseLocalHost.

     MaxAuthTries
             Specifies the maximum number of authentication attempts permit‐
             ted 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.  Multi‐
             ple sessions may be established by clients that support connec‐
             tion multiplexing.  Setting MaxSessions to 1 will effectively
             disable session multiplexing, whereas setting it to 0 will pre‐
             vent all shell, login and subsystem sessions while still per‐
             mitting forwarding.  The default is 10.

     MaxStartups
             Specifies the maximum number of concurrent unauthenticated con‐
             nections 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 lin‐
             early and all connection attempts are refused if the number of
             unauthenticated connections reaches full (60).

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

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

     PermitOpen
             Specifies the destinations to which TCP port forwarding is per‐
             mitted.  The forwarding specification must be one of the fol‐
             lowing 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.  The
             wildcard ‘*’ can be used for host or port to allow all hosts or
             ports, respectively.  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 tak‐
             ing 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.

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

     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.

     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 dae‐
             mon, 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.

     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.  If the specified value begins with a ‘-’
             character, then the specified key types (including wildcards)
             will be removed from 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 list of available key types may also be obtained using "ssh
             -Q key".

     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 doc‐
             umented in the TIME FORMATS section.  The default value for
             RekeyLimit is default none, which means that rekeying is per‐
             formed 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 authen‐
             tication.  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 REVO‐
             CATION LISTS section in ssh-keygen(1).

     StreamLocalBindMask
             Sets the octal file creation mode mask (umask) used when creat‐
             ing a Unix-domain socket file for local or remote port forward‐
             ing.  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 acciden‐
             tally 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 implements the SFTP file transfer sub‐
             system.

             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.  How‐
             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 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.

     TrustedUserCAKeys
             Specifies a file containing public keys of certificate authori‐
             ties that are trusted to sign user certificates for authentica‐
             tion, 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 authentica‐
             tion 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/authorized_keys from
             and sshd_config Match Host directives.

     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.

     VersionAddendum
             Optionally specifies additional text to append to the SSH pro‐
             tocol 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 expo‐
             sure to the server and to client displays if the sshd(8) proxy
             display is configured to listen on the wildcard address (see
             X11UseLocalhost), though this is not the default.  Addition‐
             ally, the authentication spoofing and authentication data veri‐
             fication and substitution occur on the client side.  The secu‐
             rity risk of using X11 forwarding is that the client's X11 dis‐
             play 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.

     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 con‐
             necting 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/X11R6/bin/xauth.

TIME FORMATS         top

     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)

TOKENS         top

     Arguments to some keywords can make use of tokens, which are expanded
     at runtime:

           %%    A literal ‘%’.
           %F    The fingerprint of the CA key.
           %f    The fingerprint of the key or certificate.
           %h    The home directory of the user.
           %i    The key ID in the certificate.
           %K    The base64-encoded CA key.
           %k    The base64-encoded key or certificate for authentication.
           %s    The serial number of the certificate.
           %T    The type of the CA key.
           %t    The key or certificate type.
           %u    The username.

     AuthorizedKeysCommand accepts the tokens %%, %f, %h, %k, %t, and %u.

     AuthorizedKeysFile accepts the tokens %%, %h, and %u.

     AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %h, %i, %K,
     %k, %s, %T, %t, and %u.

     AuthorizedPrincipalsFile accepts the tokens %%, %h, and %u.

     ChrootDirectory accepts the tokens %%, %h, and %u.

FILES         top

     /etc/ssh/sshd_config
             Contains configuration data for sshd(8).  This file should be
             writable by root only, but it is recommended (though not neces‐
             sary) that it be world-readable.

SEE ALSO         top

     sftp-server(8), sshd(8)

AUTHORS         top

     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 contrib‐
     uted support for privilege separation.

COLOPHON         top

     This page is part of the openssh (Portable OpenSSH) project.  Informa‐
     tion about the project can be found at
     http://www.openssh.com/portable.html.  If you have a bug report for
     this manual page, see ⟨http://www.openssh.com/report.html⟩.  This page
     was obtained from the tarball openssh-7.6p1.tar.gz fetched from
     ⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
     2017-11-25.  If you discover any rendering problems in this HTML ver‐
     sion of the page, or you believe there is a better or more up-to-date
     source for the page, or you have corrections or improvements to the
     information in this COLOPHON (which is not part of the original manual
     page), send a mail to man-pages@man7.org

BSD                          September 27, 2017                          BSD