sshd_config(5) — Linux manual page

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

NAME         top

     sshd_config — OpenSSH 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.  For each keyword, the first
     obtained value will be used.  Lines starting with ‘#’ and empty
     lines are interpreted as comments.  Arguments may optionally be
     enclosed in double quotes (") in order to represent arguments
     containing spaces.

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

     AcceptEnv
             Specifies what environment variables sent by the client
             will be copied into the session's environ(7).  See SendEnv
             and SetEnv in ssh_config(5) for how to configure the
             client.  The TERM environment variable is always accepted
             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 (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
             groups directives are processed in the following order:
             DenyGroups, 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 StreamLocal 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, restricting logins to particular
             users from particular hosts.  HOST criteria may
             additionally contain addresses to match in CIDR
             address/masklen format.  The allow/deny users directives
             are processed in the following order: DenyUsers,
             AllowUsers.

             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 lists of comma-
             separated 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 authentication requires
             completion of every method in at least one of these lists.

             For example, "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
             or pam.  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,
             "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
             PermitEmptyPasswords 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)).  AuthorizedKeysCommand is tried after the usual
             AuthorizedKeysFile files and will not be executed if a
             matching key is found there.  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 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.  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
             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 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.

             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
             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.

     CASignatureAlgorithms
             Specifies which algorithms are allowed for signing of
             certificates by certificate authorities (CAs).  The default
             is:

                   ssh-ed25519,ecdsa-sha2-nistp256,
                   ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                   sk-ssh-ed25519@openssh.com,
                   sk-ecdsa-sha2-nistp256@openssh.com,
                   rsa-sha2-512,rsa-sha2-256

             If the specified list begins with a ‘+’ character, then the
             specified algorithms will be appended to the default set
             instead of replacing them.  If the specified list begins
             with a ‘-’ character, then the specified algorithms
             (including wildcards) will be removed from the default set
             instead of replacing them.

             Certificates signed using other algorithms will not be
             accepted for public key or host-based authentication.

     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.  Arguments to ChrootDirectory accept the
             tokens described in the TOKENS section.

             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 list begins with a ‘+’
             character, then the specified ciphers will be appended to
             the default set instead of replacing them.  If the
             specified list begins with a ‘-’ character, then the
             specified ciphers (including wildcards) will be removed
             from the default set instead of replacing them.  If the
             specified list begins with a ‘^’ character, then the
             specified ciphers will be placed at the head of the default
             set.

             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 valuable when the
             client or server depend on knowing when a connection has
             become unresponsive.

             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 seconds.  Setting a zero
             ClientAliveCountMax disables connection termination.

     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 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 groups directives
             are processed in the following order: DenyGroups,
             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.  HOST criteria may additionally contain
             addresses to match in CIDR address/masklen format.  The
             allow/deny users directives are processed in the following
             order: DenyUsers, AllowUsers.

             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 forwarding-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 session through the SSH_USER_AUTH environment
             variable.  The default is no.

     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.

     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.

     HostbasedAcceptedAlgorithms
             Specifies the signature algorithms that will be accepted
             for hostbased authentication as a list of comma-separated
             patterns.  Alternately if the specified list begins with a
             ‘+’ character, then the specified signature algorithms will
             be appended to the default set instead of replacing them.
             If the specified list begins with a ‘-’ character, then the
             specified signature algorithms (including wildcards) will
             be removed from the default set instead of replacing them.
             If the specified list begins with a ‘^’ character, then the
             specified signature algorithms will be placed at the head
             of the default set.  The default for this option is:

                ssh-ed25519-cert-v01@openssh.com,
                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                sk-ssh-ed25519-cert-v01@openssh.com,
                sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
                rsa-sha2-512-cert-v01@openssh.com,
                rsa-sha2-256-cert-v01@openssh.com,
                ssh-rsa-cert-v01@openssh.com,
                ssh-ed25519,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                sk-ssh-ed25519@openssh.com,
                sk-ecdsa-sha2-nistp256@openssh.com,
                rsa-sha2-512,rsa-sha2-256,ssh-rsa

             The list of available signature algorithms may also be
             obtained using "ssh -Q HostbasedAcceptedAlgorithms".  This
             was formerly named HostbasedAcceptedKeyTypes.

     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_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 signature algorithms that the server
             offers.  The default for this option is:

                ssh-ed25519-cert-v01@openssh.com,
                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                sk-ssh-ed25519-cert-v01@openssh.com,
                sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
                rsa-sha2-512-cert-v01@openssh.com,
                rsa-sha2-256-cert-v01@openssh.com,
                ssh-rsa-cert-v01@openssh.com,
                ssh-ed25519,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                sk-ssh-ed25519@openssh.com,
                sk-ecdsa-sha2-nistp256@openssh.com,
                rsa-sha2-512,rsa-sha2-256,ssh-rsa

             The list of available signature algorithms may also be
             obtained using "ssh -Q HostKeyAlgorithms".

     IgnoreRhosts
             Specifies whether to ignore per-user .rhosts and .shosts
             files during HostbasedAuthentication.  The system-wide
             /etc/hosts.equiv and /etc/shosts.equiv are still used
             regardless of this setting.

             Accepted values are yes (the default) to ignore all per-
             user files, shosts-only to allow the use of .shosts but to
             ignore .rhosts or no to allow both .shosts and rhosts.

     IgnoreUserKnownHosts
             Specifies whether sshd(8) should ignore the user's
             ~/.ssh/known_hosts during HostbasedAuthentication and use
             only the system-wide known hosts file /etc/ssh/known_hosts.
             The default is “no”.

     Include
             Include the specified configuration file(s).  Multiple
             pathnames may be specified and each pathname may contain
             glob(7) wildcards that will be expanded and processed in
             lexical order.  Files without absolute paths are assumed to
             be in /etc/ssh.  An Include directive may appear inside a
             Match block to perform conditional inclusion.

     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, le, lowdelay, throughput,
             reliability, a numeric value, or none to use the operating
             system default.  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 af21 (Low-Latency Data) for
             interactive sessions and cs1 (Lower Effort) for non-
             interactive sessions.

     KbdInteractiveAuthentication
             Specifies whether to allow keyboard-interactive
             authentication.  All authentication styles from
             login.conf(5) are supported.  The default is yes.  The
             argument to this keyword must be yes or no.
             ChallengeResponseAuthentication is a deprecated alias for
             this.

     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 list begins with a ‘+’ character, then the
             specified methods will be appended to the default set
             instead of replacing them.  If the specified list begins
             with a ‘-’ character, then the specified methods (including
             wildcards) will be removed from the default set instead of
             replacing them.  If the specified list begins with a ‘^’
             character, then the specified methods will be placed at the
             head of the default set.  The supported algorithms are:

                   curve25519-sha256
                   curve25519-sha256@libssh.org
                   diffie-hellman-group1-sha1
                   diffie-hellman-group14-sha1
                   diffie-hellman-group14-sha256
                   diffie-hellman-group16-sha512
                   diffie-hellman-group18-sha512
                   diffie-hellman-group-exchange-sha1
                   diffie-hellman-group-exchange-sha256
                   ecdh-sha2-nistp256
                   ecdh-sha2-nistp384
                   ecdh-sha2-nistp521
                   sntrup761x25519-sha512@openssh.com

             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-group16-sha512,diffie-hellman-group18-sha512,
                   diffie-hellman-group14-sha256

             The list of available key exchange algorithms may also be
             obtained using "ssh -Q KexAlgorithms".

     ListenAddress
             Specifies the local addresses sshd(8) should listen on.
             The following forms may be used:

                   ListenAddress hostname|address [rdomain domain]
                   ListenAddress hostname:port [rdomain domain]
                   ListenAddress IPv4_address:port [rdomain domain]
                   ListenAddress [hostname|address]:port [rdomain
                   domain]

             The optional rdomain qualifier requests sshd(8) listen in
             an explicit routing domain.  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 on the
             current default routing domain.  Multiple ListenAddress
             options are permitted.  For more information on routing
             domains, see rdomain(4).

     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.

     LogVerbose
             Specify one or more overrides to LogLevel.  An override
             consists of a pattern lists that matches the source file,
             function and line number to force detailed logging for.
             For example, an override pattern of:

                   kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*

             would enable detailed logging for line 1000 of kex.c,
             everything in the kex_exchange_identification() function,
             and all code in the packet.c file.  This option is intended
             for debugging and no overrides are enabled by default.

     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 list begins with a ‘+’ character, then the
             specified algorithms will be appended to the default set
             instead of replacing them.  If the specified list begins
             with a ‘-’ character, then the specified algorithms
             (including wildcards) will be removed from the default set
             instead of replacing them.  If the specified list begins
             with a ‘^’ character, then the specified algorithms will be
             placed at the head of the default set.

             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 available criteria are User, Group, Host, LocalAddress,
             LocalPort, RDomain, and Address (with RDomain representing
             the rdomain(4) on which the connection was received).

             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 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, ClientAliveCountMax,
             ClientAliveInterval, DenyGroups, DenyUsers,
             DisableForwarding, ForceCommand, GatewayPorts,
             GSSAPIAuthentication, HostbasedAcceptedAlgorithms,
             HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
             IgnoreRhosts, Include, IPQoS, KbdInteractiveAuthentication,
             KerberosAuthentication, LogLevel, MaxAuthTries,
             MaxSessions, PasswordAuthentication, PermitEmptyPasswords,
             PermitListen, PermitOpen, PermitRootLogin, PermitTTY,
             PermitTunnel, PermitUserRC, PubkeyAcceptedAlgorithms,
             PubkeyAuthentication, RekeyLimit, RevokedKeys, RDomain,
             SetEnv, 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).

     ModuliFile
             Specifies the moduli(5) file that contains the Diffie-
             Hellman groups used for the
             “diffie-hellman-group-exchange-sha1” and
             “diffie-hellman-group-exchange-sha256” key exchange
             methods.  The default is /etc/moduli.

     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.

     PermitListen
             Specifies the addresses/ports on which a remote TCP port
             forwarding may listen.  The listen specification must be
             one of the following forms:

                   PermitListen port
                   PermitListen host:port

             Multiple permissions may be specified by separating them
             with whitespace.  An argument of any can be used to remove
             all restrictions and permit any listen requests.  An
             argument of none can be used to prohibit all listen
             requests.  The host name may contain wildcards as described
             in the PATTERNS section in ssh_config(5).  The wildcard ‘*’
             can also be used in place of a port number to allow all
             ports.  By default all port forwarding listen requests are
             permitted.  Note that the GatewayPorts option may further
             restrict which addresses may be listened on.  Note also
             that ssh(1) will request a listen host of “localhost” if no
             listen host was specifically requested, and this name is
             treated differently to explicit localhost addresses of
             “127.0.0.1” and “::1”.

     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.  The wildcard ‘*’ can be used for host or port to
             allow all hosts or ports respectively.  Otherwise, no
             pattern matching or address lookups are performed on
             supplied names.  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,
             forced-commands-only, or no.  The default is
             prohibit-password.

             If this option is set to prohibit-password (or its
             deprecated alias, 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.

     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).
             Valid options are yes, no or a pattern-list specifying
             which environment variable names to accept (for example
             "LANG,LC_*").  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.

     PerSourceMaxStartups
             Specifies the number of unauthenticated connections allowed
             from a given source address, or “none” if there is no
             limit.  This limit is applied in addition to MaxStartups,
             whichever is lower.  The default is none.

     PerSourceNetBlockSize
             Specifies the number of bits of source address that are
             grouped together for the purposes of applying
             PerSourceMaxStartups limits.  Values for IPv4 and
             optionally IPv6 may be specified, separated by a colon.
             The default is 32:128, which means each address is
             considered individually.

     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.

     PubkeyAcceptedAlgorithms
             Specifies the signature algorithms that will be accepted
             for public key authentication as a list of comma-separated
             patterns.  Alternately if the specified list begins with a
             ‘+’ character, then the specified algorithms will be
             appended to the default set instead of replacing them.  If
             the specified list begins with a ‘-’ character, then the
             specified algorithms (including wildcards) will be removed
             from the default set instead of replacing them.  If the
             specified list begins with a ‘^’ character, then the
             specified algorithms will be placed at the head of the
             default set.  The default for this option is:

                ssh-ed25519-cert-v01@openssh.com,
                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                sk-ssh-ed25519-cert-v01@openssh.com,
                sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
                rsa-sha2-512-cert-v01@openssh.com,
                rsa-sha2-256-cert-v01@openssh.com,
                ssh-rsa-cert-v01@openssh.com,
                ssh-ed25519,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                sk-ssh-ed25519@openssh.com,
                sk-ecdsa-sha2-nistp256@openssh.com,
                rsa-sha2-512,rsa-sha2-256,ssh-rsa

             The list of available signature algorithms may also be
             obtained using "ssh -Q PubkeyAcceptedAlgorithms".

     PubkeyAuthOptions
             Sets one or more public key authentication options.  The
             supported keywords are: none (the default; indicating no
             additional options are enabled), touch-required and
             verify-required.

             The touch-required option causes public key authentication
             using a FIDO authenticator algorithm (i.e. ecdsa-sk or
             ed25519-sk) to always require the signature to attest that
             a physically present user explicitly confirmed the
             authentication (usually by touching the authenticator).  By
             default, sshd(8) requires user presence unless overridden
             with an authorized_keys option.  The touch-required flag
             disables this override.

             The verify-required option requires a FIDO key signature
             attest that the user was verified, e.g. via a PIN.

             Neither the touch-required or verify-required options have
             any effect for other, non-FIDO, public 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 by 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).

     RDomain
             Specifies an explicit routing domain that is applied after
             authentication has completed.  The user session, as well as
             any forwarded or listening IP sockets, will be bound to
             this rdomain(4).  If the routing domain is set to %D, then
             the domain in which the incoming connection was received
             will be applied.

     SecurityKeyProvider
             Specifies a path to a library that will be used when
             loading FIDO authenticator-hosted keys, overriding the
             default of using the built-in USB HID support.

     SetEnv  Specifies one or more environment variables to set in child
             sessions started by sshd(8) as “NAME=VALUE”.  The
             environment value may be quoted (e.g. if it contains
             whitespace characters).  Environment variables set by
             SetEnv override the default environment and any variables
             specified by the user via AcceptEnv or
             PermitUserEnvironment.

     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 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.

     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/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
             KbdInteractiveAuthentication and PasswordAuthentication in
             addition to PAM account and session module processing for
             all authentication types.

             Because PAM keyboard-interactive authentication usually
             serves an equivalent role to password authentication, you
             should disable either PasswordAuthentication or
             KbdInteractiveAuthentication.

             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
             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), 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.

     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/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 ‘%’.
           %D    The routing domain in which the incoming connection was
                 received.
           %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 numeric user ID of the target user.
           %u    The username.

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

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

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

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

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

     RoutingDomain accepts the token %D.

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
             necessary) 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 contributed support for privilege separation.

COLOPHON         top

     This page is part of the openssh (Portable OpenSSH) project.
     Information 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-8.7p1.tar.gz fetched
     from ⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
     2021-08-27.  If you discover any rendering problems in this HTML
     version 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                          August 12, 2021                         BSD

Pages that refer to this page: userdbctl(1)