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+SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5)
+
+NAME
+ sshd_config M-bM-^@M-^S OpenSSH SSH daemon configuration file
+
+DESCRIPTION
+ sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
+ specified with -f on the command line). The file contains keyword-
+ argument pairs, one per line. For each keyword, the first obtained value
+ will be used. Lines starting with M-bM-^@M-^X#M-bM-^@M-^Y 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 M-bM-^@M-^X*M-bM-^@M-^Y and M-bM-^@M-^X?M-bM-^@M-^Y. 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 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 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 directives are processed
+ in the following order: DenyUsers, AllowUsers, DenyGroups, and
+ finally AllowGroups.
+
+ See PATTERNS in ssh_config(5) for more information on patterns.
+
+ AuthenticationMethods
+ Specifies the authentication methods that must be successfully
+ completed for a user to be granted access. This option must be
+ followed by one or more 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)). If a
+ key supplied by AuthorizedKeysCommand does not successfully
+ authenticate and authorize the user then public key
+ authentication continues using the usual AuthorizedKeysFile
+ files. By default, no AuthorizedKeysCommand is run.
+
+ AuthorizedKeysCommandUser
+ Specifies the user under whose account the AuthorizedKeysCommand
+ is run. It is recommended to use a dedicated user that has no
+ other role on the host than running authorized keys commands. If
+ AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser
+ is not, then sshd(8) will refuse to start.
+
+ AuthorizedKeysFile
+ Specifies the file that contains the public keys 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 M-bM-^@M-^X#M-bM-^@M-^Y 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 M-bM-^@M-^S 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:
+
+ ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
+ ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
+
+ Certificates signed using other algorithms will not be accepted
+ for public key or host-based authentication.
+
+ 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
+ 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 value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character,
+ then the specified ciphers will be appended to the default set
+ instead of replacing them. If the specified value begins with a
+ M-bM-^@M-^X-M-bM-^@M-^Y character, then the specified ciphers (including wildcards)
+ will be removed from the default set instead of replacing them.
+
+ The supported ciphers are:
+
+ 3des-cbc
+ aes128-cbc
+ aes192-cbc
+ aes256-cbc
+ aes128-ctr
+ aes192-ctr
+ aes256-ctr
+ aes128-gcm@openssh.com
+ aes256-gcm@openssh.com
+ 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
+ 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 seconds.
+
+ ClientAliveInterval
+ Sets a timeout interval in seconds after which if no data has
+ been received from the client, sshd(8) will send a message
+ through the encrypted channel to request a response from the
+ client. The default is 0, indicating that these messages will
+ not be sent to the client.
+
+ Compression
+ Specifies whether compression is 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 directives are processed in the following order:
+ DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
+
+ See PATTERNS in ssh_config(5) for more information on patterns.
+
+ DenyUsers
+ This keyword can be followed by a list of user name patterns,
+ separated by spaces. Login is disallowed for user names that
+ match one of the patterns. Only user names are valid; a
+ numerical user ID is not recognized. By default, login is
+ allowed for all users. If the pattern takes the form USER@HOST
+ then USER and HOST are separately checked, restricting logins to
+ particular users from particular hosts. HOST criteria may
+ additionally contain addresses to match in CIDR address/masklen
+ format. 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.
+
+ 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.
+
+ HostbasedAcceptedKeyTypes
+ Specifies the key types that will be accepted for hostbased
+ authentication as a list of comma-separated patterns.
+ Alternately if the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character,
+ then the specified key types will be appended to the default set
+ instead of replacing them. If the specified value begins with a
+ M-bM-^@M-^X-M-bM-^@M-^Y 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,
+ rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
+ ssh-rsa-cert-v01@openssh.com,
+ ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
+ ssh-ed25519,rsa-sha2-512,rsa-sha2-256,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_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,
+ rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
+ ssh-rsa-cert-v01@openssh.com,
+ ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
+ ssh-ed25519,rsa-sha2-512,rsa-sha2-256,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 and use only
+ the system-wide known hosts file /etc/ssh/known_hosts. The
+ default is no.
+
+ IPQoS Specifies the IPv4 type-of-service or DSCP class for the
+ connection. Accepted values are af11, af12, af13, af21, af22,
+ af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3,
+ cs4, cs5, cs6, cs7, ef, lowdelay, throughput, reliability, 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.
+ The argument to this keyword must be yes or no. The default is
+ to use whatever value ChallengeResponseAuthentication is set to
+ (by default yes).
+
+ KerberosAuthentication
+ Specifies whether the password provided by the user for
+ PasswordAuthentication will be validated through the Kerberos
+ KDC. To use this option, the server needs a Kerberos servtab
+ which allows the verification of the KDC's identity. The default
+ is no.
+
+ KerberosGetAFSToken
+ If AFS is active and the user has a Kerberos 5 TGT, attempt to
+ acquire an AFS token before accessing the user's home directory.
+ The default is no.
+
+ KerberosOrLocalPasswd
+ If password authentication through Kerberos fails then the
+ password will be validated via any additional local mechanism
+ such as /etc/passwd. The default is yes.
+
+ KerberosTicketCleanup
+ Specifies whether to automatically destroy the user's ticket
+ cache file on logout. The default is yes.
+
+ KexAlgorithms
+ Specifies the available KEX (Key Exchange) algorithms. Multiple
+ algorithms must be comma-separated. Alternately if the specified
+ value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified methods
+ will be appended to the default set instead of replacing them.
+ If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y 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-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
+
+ 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,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 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.
+
+ MACs Specifies the available MAC (message authentication code)
+ algorithms. The MAC algorithm is used for data integrity
+ protection. Multiple algorithms must be comma-separated. If the
+ specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified
+ algorithms will be appended to the default set instead of
+ replacing them. If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y
+ character, 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 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, ForceCommand,
+ GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
+ HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
+ KbdInteractiveAuthentication, KerberosAuthentication, LogLevel,
+ MaxAuthTries, MaxSessions, PasswordAuthentication,
+ PermitEmptyPasswords, PermitListen, PermitOpen, PermitRootLogin,
+ PermitTTY, PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes,
+ 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).
+
+ 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 M-bM-^@M-^X*M-bM-^@M-^Y 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 M-bM-^@M-^\localhostM-bM-^@M-^] if no
+ listen host was specifically requested, and this this name is
+ treated differently to explicit localhost addresses of
+ M-bM-^@M-^\127.0.0.1M-bM-^@M-^] and M-bM-^@M-^\::1M-bM-^@M-^].
+
+ 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 M-bM-^@M-^X*M-bM-^@M-^Y 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, 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.
+
+ 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.
+
+ PubkeyAcceptedKeyTypes
+ Specifies the key types that will be accepted for public key
+ authentication as a list of comma-separated patterns.
+ Alternately if the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character,
+ then the specified key types will be appended to the default set
+ instead of replacing them. If the specified value begins with a
+ M-bM-^@M-^X-M-bM-^@M-^Y 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,
+ rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
+ ssh-rsa-cert-v01@openssh.com,
+ ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
+ ssh-ed25519,rsa-sha2-512,rsa-sha2-256,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 M-bM-^@M-^XKM-bM-^@M-^Y, M-bM-^@M-^XMM-bM-^@M-^Y, or M-bM-^@M-^XGM-bM-^@M-^Y to indicate Kilobytes,
+ Megabytes, or Gigabytes, respectively. The default is between
+ M-bM-^@M-^X1GM-bM-^@M-^Y and M-bM-^@M-^X4GM-bM-^@M-^Y, 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 and 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.
+
+ SetEnv Specifies one or more environment variables to set in child
+ sessions started by sshd(8) as M-bM-^@M-^\NAME=VALUEM-bM-^@M-^]. 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 M-bM-^@M-^X#M-bM-^@M-^Y 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
+ 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
+ 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
+ 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:
+
+ M-bM-^_M-(noneM-bM-^_M-) 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
+ Arguments to some keywords can make use of tokens, which are expanded at
+ runtime:
+
+ %% A literal M-bM-^@M-^X%M-bM-^@M-^Y.
+ %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
+ /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
+ sftp-server(8), sshd(8)
+
+AUTHORS
+ OpenSSH is a derivative of the original and free ssh 1.2.12 release by
+ Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
+ de Raadt and Dug Song removed many bugs, re-added newer features and
+ created OpenSSH. Markus Friedl contributed the support for SSH protocol
+ versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
+ for privilege separation.
+
+OpenBSD 6.4 September 20, 2018 OpenBSD 6.4