sshd_config(5) - phpMan

SSHD_CONFIG(5)              BSD File Formats Manual             SSHD_CONFIG(5)
NAME
     sshd_config -- OpenSSH SSH daemon configuration file
SYNOPSIS
     /etc/ssh/sshd_config
DESCRIPTION
     sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
     specified with -f on the command line).  The file contains keyword-argu-
     ment pairs, one per line.  Lines starting with `#' and empty lines are
     interpreted as comments.  Arguments may optionally be enclosed in double
     quotes (") in order to represent arguments containing spaces.
     The possible keywords and their meanings are as follows (note that key-
     words are case-insensitive and arguments are case-sensitive):
     AcceptEnv
             Specifies what environment variables sent by the client will be
             copied into the session's environ(7).  See SendEnv in
             ssh_config(5) for how to configure the client.  The TERM environ-
             ment variable is always sent whenever the client requests a
             pseudo-terminal as it is required by the protocol.  Variables are
             specified by name, which may contain the wildcard characters `*'
             and `?'.  Multiple environment variables may be separated by
             whitespace or spread across multiple AcceptEnv directives.  Be
             warned that some environment variables could be used to bypass
             restricted user environments.  For this reason, care should be
             taken in the use of this directive.  The default is not to accept
             any environment variables.
     AddressFamily
             Specifies which address family should be used by sshd(8).  Valid
             arguments are any (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.  All of the specified user and group tests must suc-
             ceed, before user is allowed to log in.
             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 for-
             warding, 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.  All of the specified user and group tests
             must succeed, before user is allowed to log in.
             See PATTERNS in ssh_config(5) for more information on patterns.
     AuthenticationMethods
             Specifies the authentication methods that must be successfully
             completed for a user to be granted access.  This option must be
             followed by one or more comma-separated lists of authentication
             method names, or by the single string any to indicate the default
             behaviour of accepting any single authentication method.  If the
             default is overridden, then successful 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 authentica-
             tion.  Only methods that are next in one or more lists are
             offered at each stage, so for this example it would not be possi-
             ble to attempt password or keyboard-interactive authentication
             before public key.
             For keyboard interactive authentication it is also possible to
             restrict authentication to a specific device by appending a colon
             followed by the device identifier bsdauth, pam, or skey, depend-
             ing on the server configuration.  For example,
             "keyboard-interactive:bsdauth" would restrict keyboard interac-
             tive authentication to the bsdauth device.
             If the publickey method is listed more than once, sshd(8) veri-
             fies that keys that have been used successfully are not reused
             for subsequent authentications.  For example,
             "publickey,publickey" requires successful authentication using
             two different public keys.
             Note that each authentication method listed should also be
             explicitly enabled in the configuration.
     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 oth-
             ers 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 authentica-
             tion 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 expan-
             sion, 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 pro-
             gram 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 speci-
             fied, 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 princi-
             pals 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 authentica-
             tion proceeds using a CA listed in TrustedUserCAKeys and is not
             consulted for certification authorities trusted via
             ~/.ssh/authorized_keys, though the principals= key option offers
             a similar facility (see sshd(8) for details).
     Banner  The contents of the specified file are sent to the remote user
             before authentication is allowed.  If the argument is none then
             no banner is displayed.  By default, no banner is displayed.
     ChallengeResponseAuthentication
             Specifies whether challenge-response authentication is allowed
             (e.g. via PAM or through authentication styles supported in
             login.conf(5)) The default is yes.
     ChrootDirectory
             Specifies the pathname of a directory to chroot(2) to after
             authentication.  At session startup sshd(8) checks that all com-
             ponents 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 directo-
             ries 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 operat-
             ing systems (see sftp-server(8) for details).
             For safety, it is very important that the directory hierarchy be
             prevented from modification by other processes on the system
             (especially those outside the jail).  Misconfiguration can lead
             to unsafe environments which sshd(8) cannot detect.
             The default is none, indicating not to chroot(2).
     Ciphers
             Specifies the ciphers allowed.  Multiple ciphers must be comma-
             separated.  If the specified value begins with a `+' character,
             then the specified ciphers will be appended to the default set
             instead of replacing them.
             The supported ciphers are:
                   3des-cbc
                   aes128-cbc
                   aes192-cbc
                   aes256-cbc
                   aes128-ctr
                   aes192-ctr
                   aes256-ctr
                   aes128-gcm AT openssh.com
                   aes256-gcm AT openssh.com
                   arcfour
                   arcfour128
                   arcfour256
                   blowfish-cbc
                   cast128-cbc
                   chacha20-poly1305 AT openssh.com
             The default is:
                   chacha20-poly1305 AT openssh.com,
                   aes128-ctr,aes192-ctr,aes256-ctr,
                   aes128-gcm AT openssh.com,aes256-gcm AT openssh.com,
                   aes128-cbc,aes192-cbc,aes256-cbc,
                   blowfish-cbc,cast128-cbc,3des-cbc
             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 with-
             out 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 spoofa-
             ble.  The TCP keepalive option enabled by TCPKeepAlive is spoofa-
             ble.  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 recog-
             nized.  By default, login is allowed for all groups.  The
             allow/deny directives are processed in the following order:
             DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.  All
             of the specified user and group tests must succeed, before user
             is allowed to log in.
             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 numeri-
             cal 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 con-
             tain addresses to match in CIDR address/masklen format.  The
             allow/deny directives are processed in the following order:
             DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.  All
             of the specified user and group tests must succeed, before user
             is allowed to log in.
             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.
     ExposeAuthenticationMethods
             When using SSH2, this option controls the exposure of the list of
             successful authentication methods to PAM during the authentica-
             tion and to the shell environment via the SSH_USER_AUTH variable.
             See the description of this variable for more details.  Valid
             options are: never (Do not expose successful authentication meth-
             ods), pam-only (Only expose them to PAM during authentication,
             not afterwards), pam-and-env (Expose them to PAM and keep them in
             the shell environment).  The default is never.
     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 con-
             nect.  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.
     GSSAPIKeyExchange
             Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
             key exchange doesn't rely on ssh keys to verify host identity.
             The default is ``no''.
     GSSAPIEnablek5users
             Specifies whether to look at .k5users file for GSSAPI authentica-
             tion access control. Further details are described in ksu(1).
             The default is no.
     GSSAPIStrictAcceptorCheck
             Determines whether to be strict about the identity of the GSSAPI
             acceptor a client authenticates against.  If set to yes then the
             client must authenticate against the host service on the current
             hostname.  If set to no then the client may authenticate against
             any service key stored in the machine's default store.  This
             facility is provided to assist with operation on multi homed
             machines.  The default is yes.
     GSSAPIStoreCredentialsOnRekey
             Controls whether the user's GSSAPI credentials should be updated
             following a successful connection rekeying. This option can be
             used to accepted renewed or updated credentials from a compatible
             client. The default is ``no''.
     GSSAPIKexAlgorithms
             The list of key exchange algorithms that are accepted by GSSAPI
             key exchange. Possible values are
                gss-gex-sha1-,
                gss-group1-sha1-,
                gss-group14-sha1-
             The default is
             ``gss-gex-sha1-,gss-group1-sha1-,gss-group14-sha1-''.  This
             option only applies to protocol version 2 connections using GSS-
             API.
     HostbasedAcceptedKeyTypes
             Specifies the key types that will be accepted for hostbased
             authentication as a comma-separated pattern list.  Alternately if
             the specified value begins with a `+' character, then the speci-
             fied key types will be appended to the default set instead of
             replacing them.  The default for this option is:
                ecdsa-sha2-nistp256-cert-v01 AT openssh.com,
                ecdsa-sha2-nistp384-cert-v01 AT openssh.com,
                ecdsa-sha2-nistp521-cert-v01 AT openssh.com,
                ssh-ed25519-cert-v01 AT openssh.com,
                ssh-rsa-cert-v01 AT openssh.com,
                ssh-dss-cert-v01 AT openssh.com,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                ssh-ed25519,ssh-rsa,ssh-dss
             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 cer-
             tificate's public key must match a private host key already spec-
             ified by HostKey.  The default behaviour of sshd(8) is not to
             load any certificates.
     HostKey
             Specifies a file containing a private host key used by SSH.  The
             defaults are /etc/ssh/ssh_host_dsa_key,
             /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and
             /etc/ssh/ssh_host_rsa_key.
             Note that sshd(8) will refuse to use a file if it is group/world-
             accessible and that the HostKeyAlgorithms option restricts which
             of the keys are actually used by sshd(8).
             It is possible to have multiple host key files.  It is also pos-
             sible 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 AT openssh.com,
                ecdsa-sha2-nistp384-cert-v01 AT openssh.com,
                ecdsa-sha2-nistp521-cert-v01 AT openssh.com,
                ssh-ed25519-cert-v01 AT openssh.com,
                ssh-rsa-cert-v01 AT openssh.com,
                ssh-dss-cert-v01 AT openssh.com,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                ssh-ed25519,ssh-rsa,ssh-dss
             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/ssh/shosts.equiv are still used.  The
             default is yes.
     IgnoreUserKnownHosts
             Specifies whether sshd(8) should ignore the user's
             ~/.ssh/known_hosts during HostbasedAuthentication.  The default
             is no.
     IPQoS   Specifies the IPv4 type-of-service or DSCP class for the connec-
             tion.  Accepted values are af11, af12, af13, af21, af22, af23,
             af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5,
             cs6, cs7, ef, lowdelay, throughput, reliability, or a numeric
             value.  This option may take one or two arguments, separated by
             whitespace.  If one argument is specified, it is used as the
             packet class unconditionally.  If two values are specified, the
             first is automatically selected for interactive sessions and the
             second for non-interactive sessions.  The default is lowdelay for
             interactive sessions and throughput for non-interactive sessions.
     KbdInteractiveAuthentication
             Specifies whether to allow keyboard-interactive authentication.
             The argument to this keyword must be yes or no.  The default is
             to use whatever value ChallengeResponseAuthentication is set to
             (by default yes).
     KerberosAuthentication
             Specifies whether the password provided by the user for
             PasswordAuthentication will be validated through the Kerberos
             KDC.  To use this option, the server needs a Kerberos servtab
             which allows the verification of the KDC's identity.  The default
             is no.
     KerberosGetAFSToken
             If AFS is active and the user has a Kerberos 5 TGT, attempt to
             acquire an AFS token before accessing the user's home directory.
             The default is no.
     KerberosOrLocalPasswd
             If password authentication through Kerberos fails then the pass-
             word 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.
     KerberosUseKuserok
             Specifies whether to look at .k5login file for user's aliases.
             The default is yes.
     KexAlgorithms
             Specifies the available KEX (Key Exchange) algorithms.  Multiple
             algorithms must be comma-separated.  Alternately if the specified
             value begins with a `+' character, then the specified methods
             will be appended to the default set instead of replacing them.
             The supported algorithms are:
                   curve25519-sha256
                   curve25519-sha256 AT libssh.org
                   diffie-hellman-group1-sha1
                   diffie-hellman-group14-sha1
                   diffie-hellman-group-exchange-sha1
                   diffie-hellman-group-exchange-sha256
                   ecdh-sha2-nistp256
                   ecdh-sha2-nistp384
                   ecdh-sha2-nistp521
             The default is:
                   curve25519-sha256,curve25519-sha256 AT libssh.org,
                   ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                   diffie-hellman-group-exchange-sha256,
                   diffie-hellman-group14-sha1,
                   diffie-hellman-group1-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 fol-
             lowing forms may be used:
                   ListenAddress host|IPv4_addr|IPv6_addr
                   ListenAddress host|IPv4_addr:port
                   ListenAddress [host|IPv6_addr]:port
             If port is not specified, sshd will listen on the address and all
             Port options specified.  The default is to listen on all local
             addresses.  Multiple ListenAddress options are permitted.
     LoginGraceTime
             The server disconnects after this time if the user has not suc-
             cessfully logged in.  If the value is 0, there is no time limit.
             The default is 120 seconds.
     LogLevel
             Gives the verbosity level that is used when logging messages from
             sshd(8).  The possible values are: QUIET, FATAL, ERROR, INFO,
             VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.  The default is INFO.
             DEBUG and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify
             higher levels of debugging output.  Logging with a DEBUG level
             violates the privacy of users and is not recommended.
     MACs    Specifies the available MAC (message authentication code) algo-
             rithms.  The MAC algorithm is used for data integrity protection.
             Multiple algorithms must be comma-separated.  If the specified
             value begins with a `+' character, then the specified algorithms
             will be appended to the default set instead of replacing them.
             The algorithms that contain "-etm" calculate the MAC after
             encryption (encrypt-then-mac).  These are considered safer and
             their use recommended.  The supported MACs are:
                   hmac-md5
                   hmac-md5-96
                   hmac-ripemd160
                   hmac-sha1
                   hmac-sha1-96
                   hmac-sha2-256
                   hmac-sha2-512
                   umac-64 AT openssh.com
                   umac-128 AT openssh.com
                   hmac-md5-etm AT openssh.com
                   hmac-md5-96-etm AT openssh.com
                   hmac-ripemd160-etm AT openssh.com
                   hmac-sha1-etm AT openssh.com
                   hmac-sha1-96-etm AT openssh.com
                   hmac-sha2-256-etm AT openssh.com
                   hmac-sha2-512-etm AT openssh.com
                   umac-64-etm AT openssh.com
                   umac-128-etm AT openssh.com
             The default is:
                   umac-64-etm AT openssh.com,umac-128-etm AT openssh.com,
                   hmac-sha2-256-etm AT openssh.com,hmac-sha2-512-etm AT openssh.com,
                   hmac-sha1-etm AT openssh.com,
                   umac-64 AT openssh.com,umac-128 AT openssh.com,
                   hmac-sha2-256,hmac-sha2-512,hmac-sha1,
                   hmac-sha1-etm AT openssh.com
             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, and
             Address.  The match patterns may consist of single entries or
             comma-separated lists and may use the wildcard and negation oper-
             ators described in the PATTERNS section of ssh_config(5).
             The patterns in an Address criteria may additionally contain
             addresses to match in CIDR address/masklen format, such as
             192.0.2.0/24 or 2001:db8::/32.  Note that the mask length pro-
             vided must be consistent with the address - it is an error to
             specify a mask length that is too long for the address or one
             with bits set in this host portion of the address.  For 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,
             KerberosUseKuserok, MaxAuthTries, MaxSessions,
             PasswordAuthentication, PermitEmptyPasswords, PermitOpen,
             PermitRootLogin, PermitTTY, PermitTunnel, PermitUserRC,
             PubkeyAcceptedKeyTypes, PubkeyAuthentication, RekeyLimit,
             RevokedKeys, StreamLocalBindMask, StreamLocalBindUnlink,
             TrustedUserCAKeys, X11DisplayOffset, X11MaxDisplays,
             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 for-
             warding.  The default is 10.
     MaxStartups
             Specifies the maximum number of concurrent unauthenticated con-
             nections to the SSH daemon.  Additional connections will be
             dropped until authentication succeeds or the LoginGraceTime
             expires for a connection.  The default is 10:30:100.
             Alternatively, random early drop can be enabled by specifying the
             three colon separated values start:rate:full (e.g. "10:30:60").
             sshd(8) will refuse connection attempts with a probability of
             rate/100 (30%) if there are currently start (10) unauthenticated
             connections.  The probability increases linearly and all connec-
             tion attempts are refused if the number of unauthenticated con-
             nections reaches full (60).
     PasswordAuthentication
             Specifies whether password authentication is allowed.  The
             default is yes.
     PermitEmptyPasswords
             When password authentication is allowed, it specifies whether the
             server allows login to accounts with empty password strings.  The
             default is no.
     PermitOpen
             Specifies the destinations to which TCP port forwarding is per-
             mitted.  The forwarding specification must be one of the follow-
             ing forms:
                   PermitOpen host:port
                   PermitOpen IPv4_addr:port
                   PermitOpen [IPv6_addr]:port
             Multiple forwards may be specified by separating them with white-
             space.  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, respec-
             tively.  By default all port forwarding requests are permitted.
     PermitRootLogin
             Specifies whether root can log in using ssh(1).  The argument
             must be yes, prohibit-password, without-password,
             forced-commands-only, or no.  The default is yes.
             If this option is set to prohibit-password or without-password,
             password and keyboard-interactive authentication are disabled for
             root.
             If this option is set to forced-commands-only, root login with
             public key authentication will be allowed, but only if the
             command option has been specified (which may be useful for taking
             remote backups even if root login is normally not allowed).  All
             other authentication methods are disabled for root.
             If this option is set to no, root is not allowed to log in.
     PermitTTY
             Specifies whether pty(4) allocation is permitted.  The default is
             yes.
     PermitTunnel
             Specifies whether tun(4) device forwarding is allowed.  The argu-
             ment must be yes, point-to-point (layer 3), ethernet (layer 2),
             or no.  Specifying yes permits both point-to-point and ethernet.
             The default is no.
             Independent of this setting, the permissions of the selected
             tun(4) device must allow access to the user.
     PermitUserEnvironment
             Specifies whether ~/.ssh/environment and environment= options in
             ~/.ssh/authorized_keys are processed by sshd(8).  The default is
             no.  Enabling environment processing may enable users to bypass
             access restrictions in some configurations using mechanisms such
             as LD_PRELOAD.
     PermitUserRC
             Specifies whether any ~/.ssh/rc file is executed.  The default is
             yes.
     PidFile
             Specifies the file that contains the process ID of the SSH dae-
             mon, or none to not write one.  The default is /var/run/sshd.pid.
     Port    Specifies the port number that sshd(8) listens on.  The default
             is 22.  Multiple options of this type are permitted.  See also
             ListenAddress.
     PrintLastLog
             Specifies whether sshd(8) should print the date and time of the
             last user login when a user logs in interactively.  The default
             is yes.
     PrintMotd
             Specifies whether sshd(8) should print /etc/motd when a user logs
             in interactively.  (On some systems it is also printed by the
             shell, /etc/profile, or equivalent.)  The default is yes.
     PubkeyAcceptedKeyTypes
             Specifies the key types that will be accepted for public key
             authentication as a comma-separated pattern list.  Alternately if
             the specified value begins with a `+' character, then the speci-
             fied key types will be appended to the default set instead of
             replacing them.  The default for this option is:
                ecdsa-sha2-nistp256-cert-v01 AT openssh.com,
                ecdsa-sha2-nistp384-cert-v01 AT openssh.com,
                ecdsa-sha2-nistp521-cert-v01 AT openssh.com,
                ssh-ed25519-cert-v01 AT openssh.com,
                ssh-rsa-cert-v01 AT openssh.com,
                ssh-dss-cert-v01 AT openssh.com,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                ssh-ed25519,ssh-rsa,ssh-dss
             The list of available key types may also be obtained using "ssh
             -Q key".
     PubkeyAuthentication
             Specifies whether public key authentication is allowed.  The
             default is yes.
     RekeyLimit
             Specifies the maximum amount of data that may be transmitted
             before the session key is renegotiated, optionally followed a
             maximum amount of time that may pass before the session key is
             renegotiated.  The first argument is specified in bytes and may
             have a suffix of `K', `M', or `G' to indicate Kilobytes,
             Megabytes, or Gigabytes, respectively.  The default is between
             `1G' and `4G', depending on the cipher.  The optional second
             value is specified in seconds and may use any of the units docu-
             mented in the TIME FORMATS section.  The default value for
             RekeyLimit is default none, which means that rekeying is per-
             formed after the cipher's default amount of data has been sent or
             received and no time based rekeying is done.
     RevokedKeys
             Specifies revoked public keys file, or none to not use one.  Keys
             listed in this file will be refused for public key authentica-
             tion.  Note that if this file is not readable, then public key
             authentication will be refused for all users.  Keys may be speci-
             fied 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 sec-
             tion in ssh-keygen(1).
     ShowPatchLevel
             Specifies whether sshd will display the patch level of the binary
             in the identification string.  The patch level is set at compile-
             time.  The default is ``no''.  This option applies to protocol
             version 1 only.
     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 subsys-
             tem.
             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, AUTHPRIV,
             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 tempo-
             rarily, 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 authori-
             ties that are trusted to sign user certificates for authentica-
             tion, or none to not use one.  Keys are listed one per line;
             empty lines and comments starting with `#' are allowed.  If a
             certificate is presented for authentication and has its signing
             CA key listed in this file, then it may be used for authentica-
             tion for any user listed in the certificate's principals list.
             Note that certificates that lack a list of principals will not be
             permitted for authentication using TrustedUserCAKeys.  For more
             details on certificates, see the CERTIFICATES section in
             ssh-keygen(1).
     UseDNS  Specifies whether sshd(8) should look up the remote host name,
             and to check that the resolved host name for the remote IP
             address maps back to the very same IP address.  The default is
             ``yes''.
             If this option is set to no 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.
     UsePrivilegeSeparation
             Specifies whether sshd(8) separates privileges by creating an
             unprivileged child process to deal with incoming network traffic.
             After successful authentication, another process will be created
             that has the privilege of the authenticated user.  The goal of
             privilege separation is to prevent privilege escalation by con-
             taining any corruption within the unprivileged processes.  The
             argument must be yes, no, or sandbox.  If UsePrivilegeSeparation
             is set to sandbox then the pre-authentication unprivileged
             process is subject to additional restrictions.  The default is
             sandbox.
     VersionAddendum
             Optionally specifies additional text to append to the SSH proto-
             col 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.
     X11MaxDisplays
             Specifies the maximum number of displays available for sshd(8)'s
             X11 forwarding.  This prevents sshd from exhausting local ports.
             The default is 1000.
     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 adminis-
             trator 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 wild-
             card address.  The argument must be yes or no.  The default is
             yes.
     XAuthLocation
             Specifies the full pathname of the xauth(1) program, or none to
             not use one.  The default is /usr/bin/xauth.
TIME FORMATS
     sshd(8) command-line arguments and configuration file options that spec-
     ify 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 fol-
     lowing:
           <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
     Arguments to some keywords can make use of tokens, which are expanded at
     runtime:
           %%    A literal `%'.
           %F    The fingerprint of the CA key.
           %f    The fingerprint of the key or certificate.
           %h    The home directory of the user.
           %i    The key ID in the certificate.
           %K    The base64-encoded CA key.
           %k    The base64-encoded key or certificate for authentication.
           %s    The serial number of the certificate.
           %T    The type of the CA key.
           %t    The key or certificate type.
           %u    The username.
     AuthorizedKeysCommand accepts the tokens %%, %f, %h, %t, and %u.
     AuthorizedKeysFile accepts the tokens %%, %h, and %u.
     AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %K, %k, %h,
     %i, %s, %T, %t, and %u.
     AuthorizedPrincipalsFile accepts the tokens %%, %h, and %u.
     ChrootDirectory accepts the tokens %%, %h, and %u.
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 neces-
             sary) 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 cre-
     ated 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.
BSD                              May 25, 2020                              BSD