sshd_config

SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5)

NAME

   sshd_config  OpenSSH 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.  Unless noted
   otherwise, for each keyword, the first obtained value will be used.  Lines starting with #’ and empty lines are interpreted as comments.  Arguments may optionally be enclosed in dou‐
   ble quotes (") in order to represent arguments containing spaces.

   Note that the Debian openssh-server package sets several options as standard in /etc/ssh/sshd_config which are not the default in sshd(8):

         •   Include /etc/ssh/sshd_config.d/*.conf
         •   KbdInteractiveAuthentication no
         •   X11Forwarding yes
         •   PrintMotd no
         •   AcceptEnv LANG LC_* COLORTERM NO_COLOR
         •   Subsystem sftp /usr/lib/openssh/sftp-server
         •   UsePAM yes

   /etc/ssh/sshd_config.d/*.conf files are included at the start of the configuration file, so options set there will override those in /etc/ssh/sshd_config.

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

   AcceptEnv
           Specifies what environment variables sent by the client will be copied into the session's environ(7).  See SendEnv and SetEnv in ssh_config(5) for how to configure the client.
           The TERM environment variable is always accepted whenever the client requests a pseudo-terminal as it is required by the protocol.  Variables are specified by name, which  may
           contain  the  wildcard  characters  ‘*’ and ‘?’.  Multiple environment variables may be separated by whitespace or spread across multiple AcceptEnv directives.  Be warned that
           some environment variables could be used to bypass restricted user environments.  For this reason, care should be taken in the use of this directive.  The default  is  not  to
           accept any environment variables.

   AddressFamily
           Specifies which address family should be used by sshd(8).  Valid arguments are any (the default), inet (use IPv4 only), or inet6 (use IPv6 only).

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

   AllowGroups
           This  keyword  can be followed by a list of group name patterns, separated by spaces.  If specified, login is allowed only for users whose primary group or supplementary group
           list matches one of the patterns.  Only group names are valid; a numerical group ID is not recognized.  By default, login is allowed for all groups.  The allow/deny groups di‐
           rectives are processed in the following order: DenyGroups, AllowGroups.

           See PATTERNS in ssh_config(5) for more information on patterns.  This keyword may appear multiple times in sshd_config with each instance appending to the list.

   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 sepa‐
           rately  checked,  restricting logins to particular users from particular hosts.  HOST criteria may additionally contain addresses to match in CIDR address/masklen format.  The
           allow/deny users directives are processed in the following order: DenyUsers, AllowUsers.

           See PATTERNS in ssh_config(5) for more information on patterns.  This keyword may appear multiple times in sshd_config with each instance appending to the list.

   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 interac‐
           tive  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.  Argu‐
           ments to AuthorizedKeysCommand accept the tokens described in the “TOKENS” section.  If no arguments are specified then the username of the target user is used.

           The  program  should  produce on standard output zero or more lines of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)).  AuthorizedKeysCommand is tried after the usual
           AuthorizedKeysFile files and will not be executed if a matching key is found there.  By default, no AuthorizedKeysCommand is run.

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

   AuthorizedKeysFile
           Specifies  the  file  that contains the public keys used for user authentication.  The format is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8).  Arguments to
           AuthorizedKeysFile may include wildcards and 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 speci‐
           fied 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 au‐
           thorized 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 may include wildcards and accept the tokens described in the “TOKENS” section.  After expansion, AuthorizedPrincipalsFile is taken to  be
           an  absolute path or one relative to the user's home directory.  The default is none, i.e. not to use a principals file – in this case, the username of the user must appear in
           a certificate's principals list for it to be accepted.

           Note that AuthorizedPrincipalsFile is only used when authentication proceeds using a CA listed in TrustedUserCAKeys and is not consulted for certification authorities  trusted
           via ~/.ssh/authorized_keys, though the principals= key option offers a similar facility (see sshd(8) for details).

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

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

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

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

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

   ChannelTimeout
           Specifies  whether  and  how  quickly  sshd(8)  should close inactive channels.  Timeouts are specified as one or more “type=interval” pairs separated by whitespace, where the
           “type” must be the special keyword “global” or a channel type name from the list below, optionally containing wildcard characters.

           The timeout value “interval” is specified in seconds or may use any of the units documented in the “TIME FORMATS” section.  For example, “session=5m” would  cause  interactive
           sessions to terminate after five minutes of inactivity.  Specifying a zero value disables the inactivity timeout.

           The  special timeout “global” applies to all active channels, taken together.  Traffic on any active channel will reset the timeout, but when the timeout expires then all open
           channels will be closed.  Note that this global timeout is not matched by wildcards and must be specified explicitly.

           The available channel type names include:

           agent-connection
                   Open connections to ssh-agent(1).

           direct-tcpip, direct-streamlocal@openssh.com
                   Open TCP or Unix socket (respectively) connections that have been established from a ssh(1) local forwarding, i.e. LocalForward or DynamicForward.

           forwarded-tcpip, forwarded-streamlocal@openssh.com
                   Open TCP or Unix socket (respectively) connections that have been established to a sshd(8) listening on behalf of a ssh(1) remote forwarding, i.e. RemoteForward.

           session
                   The interactive main session, including shell session, command execution, scp(1), sftp(1), etc.

           tun-connection
                   Open TunnelForward connections.

           x11-connection
                   Open X11 forwarding sessions.

           Note that in all the above cases, terminating an inactive session does not guarantee to remove all resources associated with the session, e.g. shell processes or  X11  clients
           relating to the session may continue to execute.

           Moreover,  terminating  an  inactive channel or session does not necessarily close the SSH connection, nor does it prevent a client from requesting another channel of the same
           type.  In particular, expiring an inactive forwarding session does not prevent another identical forwarding from being subsequently created.

           The default is not to expire channels of any type for inactivity.

   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 group or others.  After the chroot, sshd(8) changes the working directory to the user's home directory.  Arguments to ChrootDirectory accept the to‐
           kens 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 sys‐
           tems (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).  Misconfigu‐
           ration can lead to unsafe environments which sshd(8) cannot detect.

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

   Ciphers
           Specifies the ciphers allowed.  Multiple ciphers must be comma-separated.  If the specified list begins with a ‘+’ character, then the specified ciphers will  be  appended  to
           the  default  set  instead of replacing them.  If the specified list begins with a ‘-’ character, then the specified ciphers (including wildcards) will be removed from the de‐
           fault set instead of replacing them.  If the specified list begins with a ‘^’ character, then the specified ciphers will be placed at the head of the default set.

           The supported ciphers are:

                 3des-cbc
                 aes128-cbc
                 aes192-cbc
                 aes256-cbc
                 aes128-ctr
                 aes192-ctr
                 aes256-ctr
                 aes128-gcm@openssh.com
                 aes256-gcm@openssh.com
                 chacha20-poly1305@openssh.com

           The default is:

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

           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  mes‐
           sages  are  being  sent,  sshd  will  disconnect  the  client,  terminating  the session.  It is important to note that the use of client alive messages is very different from
           TCPKeepAlive.  The client alive messages are sent through the encrypted channel and therefore will not be spoofable.  The TCP  keepalive  option  enabled  by  TCPKeepAlive  is
           spoofable.  The client alive mechanism is valuable when the client or server depend on knowing when a connection has become unresponsive.

           The default value is 3.  If ClientAliveInterval is set to 15, and ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected after approximately
           45 seconds.  Setting a zero ClientAliveCountMax disables connection termination.

   ClientAliveInterval
           Sets  a  timeout  interval in seconds after which if no data has been received from the client, sshd(8) will send a message through the encrypted channel to request a response
           from the client.  The default is 0, indicating that these messages will not be sent to the client.

   Compression
           Specifies whether compression is enabled after the user has authenticated successfully.  The argument must be yes, delayed (a legacy synonym for yes) or no.   The  default  is
           yes.

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

   DenyGroups
           This  keyword can be followed by a list of group name patterns, separated by spaces.  Login is disallowed for users whose primary group or supplementary group list matches one
           of the patterns.  Only group names are valid; a numerical group ID is not recognized.  By default, login is allowed for all  groups.   The  allow/deny  groups  directives  are
           processed in the following order: DenyGroups, AllowGroups.

           See PATTERNS in ssh_config(5) for more information on patterns.  This keyword may appear multiple times in sshd_config with each instance appending to the list.

   DenyUsers
           This  keyword  can  be  followed by a list of user name patterns, separated by spaces.  Login is disallowed for user names that match one of the patterns.  Only user names are
           valid; a numerical user ID is not recognized.  By default, login is allowed for all users.  If the pattern takes the form USER@HOST then USER and HOST are separately  checked,
           restricting  logins to particular users from particular hosts.  HOST criteria may additionally contain addresses to match in CIDR address/masklen format.  The allow/deny users
           directives are processed in the following order: DenyUsers, AllowUsers.

           See PATTERNS in ssh_config(5) for more information on patterns.  This keyword may appear multiple times in sshd_config with each instance appending to the list.

   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 pre‐
           vents  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 ad‐
           dresses, 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  for‐
           wardings 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.

   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”.

           For this to work GSSAPIKeyExchange needs to be enabled in the server and also used by the client.

   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-,
              gss-group14-sha256-,
              gss-group16-sha512-,
              gss-nistp256-sha256-,
              gss-curve25519-sha256-

           The  default is “gss-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,gss-curve25519-sha256-,gss-gex-sha1-,gss-group14-sha1-”.  This option only applies to connections
           using GSSAPI.

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

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

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

   HostbasedAuthentication
           Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public key client host authentication is allowed (host-based authentication).  The default
           is no.

   HostbasedUsesNameFromPacketOnly
           Specifies  whether  or  not  the  server  will  attempt  to perform a reverse name lookup when matching the name in the ~/.shosts, ~/.rhosts, and /etc/hosts.equiv files during
           HostbasedAuthentication.  A setting of yes means that sshd(8) uses the name supplied by the client rather than attempting to resolve the name from the TCP  connection  itself.
           The default is no.

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

   HostKey
           Specifies a file containing a private host key used by SSH.  The defaults are /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key.

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

           It is possible to have multiple host key files.  It is also possible to specify public host key files instead.  In this case operations on the private key will be delegated to
           an ssh-agent(1).

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

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

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

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

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

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

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

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

   IPQoS   Specifies the IPv4 type-of-service or DSCP class for the connection.  Accepted values are af11, af12, af13, af21, af22, af23, af31, af32, af33, af41,  af42,  af43,  cs0,  cs1,
           cs2,  cs3,  cs4, cs5, cs6, cs7, ef, le, lowdelay, throughput, reliability, a numeric value, or none to use the operating system default.  This option may take one or two argu‐
           ments, 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  se‐
           lected 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 default is yes.  The argument to this keyword must be yes or no.  ChallengeResponseAuthentication is a
           deprecated alias for this.

   KerberosAuthentication
           Specifies whether the password provided by the user for PasswordAuthentication will be validated through the Kerberos KDC.  To use this option, the  server  needs  a  Kerberos
           servtab which allows the verification of the KDC's identity.  The default is no.

   KerberosGetAFSToken
           If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token before accessing the user's home directory.  The default is no.

   KerberosOrLocalPasswd
           If password authentication through Kerberos fails then the password will be validated via any additional local mechanism such as /etc/passwd.  The default is yes.

   KerberosTicketCleanup
           Specifies whether to automatically destroy the user's ticket cache file on logout.  The default is yes.

   KexAlgorithms
           Specifies  the  permitted KEX (Key Exchange) algorithms that the server will offer to clients.  The ordering of this list is not important, as the client specifies the prefer‐
           ence order.  Multiple algorithms must be comma-separated.

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

           The 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
                 mlkem768x25519-sha256
                 sntrup761x25519-sha512
                 sntrup761x25519-sha512@openssh.com

           The default is:

                 mlkem768x25519-sha256,
                 sntrup761x25519-sha512,sntrup761x25519-sha512@openssh.com,
                 curve25519-sha256,curve25519-sha256@libssh.org,
                 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521

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

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

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

           The optional rdomain qualifier requests sshd(8) listen in an explicit routing domain.  If port is not specified, sshd will listen on the address and all  Port  options  speci‐
           fied.  The default is to listen on all local addresses on the current default routing domain.  Multiple ListenAddress options are permitted.

           On Linux, routing domains are implemented using Virtual Routing and Forwarding domains (VRFs); for more information, see ip-vrf(8).

   LoginGraceTime
           The server disconnects after this time if the user has not successfully logged in.  If the value is 0, there is no time limit.  The default is 120 seconds.

   LogLevel
           Gives  the  verbosity  level  that is used when logging messages from sshd(8).  The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
           The default is INFO.  DEBUG and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify higher levels of debugging output.  Logging with a DEBUG level violates the  privacy  of
           users and is not recommended.

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

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

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

   MACs    Specifies the available MAC (message authentication code) algorithms.  The MAC algorithm is used for data integrity protection.  Multiple algorithms must  be  comma-separated.
           If  the  specified list begins with a ‘+’ character, then the specified algorithms will be appended to the default set instead of replacing them.  If the specified list begins
           with a ‘-’ character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them.  If the specified list begins with  a
           ‘^’ character, then the specified algorithms will be placed at the head of the default set.

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

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

           The default is:

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

           The list of available MAC algorithms may also be obtained using "ssh -Q mac".

   Match   Introduces  a  conditional  block.  If all of the criteria on the Match line are satisfied, the keywords on the following lines override those set in the global section of the
           config file, until either another Match line or the end of the file.  If a keyword appears in multiple Match blocks that are satisfied, only the first instance of the  keyword
           is applied.

           The arguments to Match are one or more criteria-pattern pairs or one of the single token criteria: All, which matches all criteria, or Invalid-User, which matches when the re‐
           quested  user-name does not match any known account.  The available criteria are User, Group, Host, LocalAddress, LocalPort, Version, RDomain, and Address (with RDomain repre‐
           senting the routing domain on which the connection was received; see ip-vrf(8)).

           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.

           The Version keyword matches against the version string of sshd(8), for example “OpenSSH_10.0”.

           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,  CASignatureAlgorithms,   ChannelTimeout,   ChrootDirectory,   ClientAliveCountMax,   ClientAliveInterval,
           DenyGroups,   DenyUsers,   DisableForwarding,   ExposeAuthInfo,   ForceCommand,   GatewayPorts,   GSSAPIAuthentication,  HostbasedAcceptedAlgorithms,  HostbasedAuthentication,
           HostbasedUsesNameFromPacketOnly, IgnoreRhosts, Include, IPQoS,  KbdInteractiveAuthentication,  KerberosAuthentication,  LogLevel,  MaxAuthTries,  MaxSessions,  PAMServiceName,
           PasswordAuthentication, PermitEmptyPasswords, PermitListen, PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel, PermitUserRC, PubkeyAcceptedAlgorithms, PubkeyAuthentication,
           PubkeyAuthOptions, RefuseConnection, RekeyLimit, RevokedKeys, SetEnv, StreamLocalBindMask, StreamLocalBindUnlink, TrustedUserCAKeys, UnusedConnectionTimeout, X11DisplayOffset,
           X11Forwarding and X11UseLocalhost.

   MaxAuthTries
           Specifies  the  maximum  number of authentication attempts permitted per connection.  Once the number of failures reaches half this value, additional failures are logged.  The
           default is 6.

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

   MaxStartups
           Specifies the maximum number of concurrent unauthenticated connections to the SSH daemon.  Additional  connections  will  be  dropped  until  authentication  succeeds  or  the
           LoginGraceTime expires for a connection.  The default is 10:30:100.

           Alternatively, random early drop can be enabled by specifying the three colon separated values start:rate:full (e.g. "10:30:60").  sshd(8) will refuse connection attempts with
           a  probability  of rate/100 (30%) if there are currently start (10) unauthenticated connections.  The probability increases linearly and all connection attempts are refused if
           the number of unauthenticated connections reaches full (60).

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

   PAMServiceName
           Specifies the service name used for Pluggable Authentication Modules (PAM) authentication, authorisation and session controls when UsePAM is enabled.  The default is sshd.

   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 argu‐
           ment of none can be used to prohibit all listen requests.  The host name may contain wildcards as described in the PATTERNS section in ssh_config(5).   The  wildcard  ‘*’  can
           also  be  used  in place of a port number to allow all ports.  By default all port forwarding listen requests are permitted.  Note that the GatewayPorts option may further re‐
           strict which addresses may be listened on.  Note also that ssh(1) will request a listen host of “localhost” if no listen host was specifically  requested,  and  this  name  is
           treated differently to explicit localhost addresses of “127.0.0.1” and “::1”.

   PermitOpen
           Specifies the destinations to which TCP port forwarding is permitted.  The forwarding specification must be one of the following forms:

                 PermitOpen host:port
                 PermitOpen IPv4_addr:port
                 PermitOpen [IPv6_addr]:port

           Multiple forwards may be specified by separating them with whitespace.  An argument of any can be used to remove all restrictions and permit any forwarding requests.  An argu‐
           ment  of  none can be used to prohibit all forwarding requests.  The wildcard ‘*’ can be used for host or port to allow all hosts or ports respectively.  Otherwise, no pattern
           matching or address lookups are performed on supplied names.  By default all port forwarding requests are permitted.

   PermitRootLogin
           Specifies whether root can log in using ssh(1).  The argument must be yes, prohibit-password, forced-commands-only, or no.  The default is prohibit-password.

           If this option is set to prohibit-password (or its deprecated alias, without-password), password and keyboard-interactive authentication are disabled for root.

           If this option is set to forced-commands-only, root login with public key authentication will be allowed, but only if the command option has been specified (which may be  use‐
           ful for taking remote backups even if root login is normally not allowed).  All other authentication methods are disabled for root.

           If this option is set to no, root is not allowed to log in.

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

   PermitTunnel
           Specifies  whether  tun(4)  device  forwarding  is  allowed.   The  argument  must  be  yes,  point-to-point (layer 3), ethernet (layer 2), or no.  Specifying yes permits both
           point-to-point and ethernet.  The default is no.

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

   PermitUserEnvironment
           Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are processed by sshd(8).  Valid options are yes, no or a pattern-list specifying which
           environment variable names to accept (for example "LANG,LC_*").  The default is no.  Enabling environment processing may enable users to bypass  access  restrictions  in  some
           configurations using mechanisms such as LD_PRELOAD.

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

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

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

   PerSourcePenalties
           Controls  penalties for various conditions that may represent attacks on sshd(8).  If a penalty is enforced against a client then its source address and any others in the same
           network, as defined by PerSourceNetBlockSize, will be refused connection for a period.

           A penalty doesn't affect concurrent connections in progress, but multiple penalties from the same source from concurrent connections will accumulate up  to  a  maximum.   Con‐
           versely, penalties are not applied until a minimum threshold time has been accumulated.

           Penalties are enabled by default with the default settings listed below but may disabled using the no keyword.  The defaults may be overridden by specifying one or more of the
           keywords below, separated by whitespace.  All keywords accept arguments, e.g. "crash:2m".

           crash:duration
                   Specifies how long to refuse clients that cause a crash of sshd(8) (default: 90s).

           authfail:duration
                   Specifies how long to refuse clients that disconnect after making one or more unsuccessful authentication attempts (default: 5s).

           refuseconnection:duration
                   Specifies how long to refuse clients that were administratively prohibited connection via the RefuseConnection option (default: 10s).

           noauth:duration
                   Specifies how long to refuse clients that disconnect without attempting authentication (default: 1s).  This timeout should be used cautiously otherwise it may penalise
                   legitimate scanning tools such as ssh-keyscan(1).

           grace-exceeded:duration
                   Specifies how long to refuse clients that fail to authenticate after LoginGraceTime (default: 10s).

           max:duration
                   Specifies the maximum time a particular source address range will be refused access for (default: 10m).  Repeated penalties will accumulate up to this maximum.

           min:duration
                   Specifies the minimum penalty that must accrue before enforcement begins (default: 15s).

           max-sources4:number, max-sources6:number
                   Specifies the maximum number of client IPv4 and IPv6 address ranges to track for penalties (default: 65536 for both).

           overflow:mode
                   Controls  how  the  server behaves when max-sources4 or max-sources6 is exceeded.  There are two operating modes: deny-all, which denies all incoming connections other
                   than those exempted via PerSourcePenaltyExemptList until a penalty expires, and permissive, which allows new connections by removing existing penalties early (default:
                   permissive).  Note that client penalties below the min threshold count against the total number of tracked penalties.  IPv4 and IPv6 addresses are tracked  separately,
                   so an overflow in one will not affect the other.

           overflow6:mode
                   Allows specifying a different overflow mode for IPv6 addresses.  The default it to use the same overflow mode as was specified for IPv4.

   PerSourcePenaltyExemptList
           Specifies  a  comma-separated list of addresses to exempt from penalties.  This list may contain wildcards and CIDR address/masklen ranges.  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.  The default is not to exempt any addresses.

   PidFile
           Specifies the file that contains the process ID of the SSH daemon, or none to not write one.  The default is /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 de‐
           fault is yes.

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

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

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

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

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

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

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

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

   RefuseConnection
           Indicates  that  sshd(8)  should  unconditionally  terminate  the connection.  Additionally, a refuseconnection penalty may be recorded against the source of the connection if
           PerSourcePenalties are enabled.  This option is only really useful in a Match block.

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

   RequiredRSASize
           Specifies  the  minimum RSA key size (in bits) that sshd(8) will accept.  User and host-based authentication keys smaller than this limit will be refused.  The default is 1024
           bits.  Note that this limit may only be raised from the default.

   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).

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

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

   SshdAuthPath
           Overrides  the  default  path to the sshd-auth binary that is invoked to complete user authentication.  The default is /usr/lib/openssh/sshd-auth.  This option is intended for
           use by tests.

   SshdSessionPath
           Overrides the default path to the sshd-session binary that is invoked to handle each connection.  The default is /usr/lib/openssh/sshd-session.  This option  is  intended  for
           use by tests.

   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.   It  accepts the same command line arguments as sftp-server and even though it is in-process, settings such as LogLevel or SyslogFacility do not apply to it and must
           be set explicitly via command line arguments.

           By default no subsystems are defined.

   SyslogFacility
           Gives the facility code that is used when logging messages from sshd(8).  The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5,  LOCAL6,
           LOCAL7.  The default is AUTH.

   TCPKeepAlive
           Specifies  whether the system should send TCP keepalive messages to the other side.  If they are sent, death of the connection or crash of one of the machines will be properly
           noticed.  However, this means that connections will die if the route is down temporarily, and some people find it annoying.  On the other hand, if TCP keepalives are not sent,
           sessions may hang indefinitely on the server, leaving "ghost" users and consuming server resources.

           The default is yes (to send TCP keepalive messages), and the server will notice if the network goes down or the client host crashes.  This avoids infinitely hanging sessions.

           To disable TCP keepalive messages, the value should be set to no.

           This option was formerly called KeepAlive.

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

   UnusedConnectionTimeout
           Specifies  whether and how quickly sshd(8) should close client connections with no open channels.  Open channels include active shell, command execution or subsystem sessions,
           connected network, socket, agent or X11 forwardings.  Forwarding listeners, such as those from the ssh(1) -R flag, are not considered as open channels and do not  prevent  the
           timeout.  The timeout value is specified in seconds or may use any of the units documented in the “TIME FORMATS” section.

           Note  that  this timeout starts when the client connection completes user authentication but before the client has an opportunity to open any channels.  Caution should be used
           when using short timeout values, as they may not provide sufficient time for the client to request and open its channels before terminating the connection.

           The default none is to never expire connections for having no open channels.  This option may be useful in conjunction with ChannelTimeout.

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

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

   UsePAM  Enables the Pluggable Authentication Module interface.  If set to yes this will enable PAM authentication using KbdInteractiveAuthentication and PasswordAuthentication in  ad‐
           dition to PAM account and session module processing for all authentication types.

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

           If UsePAM is enabled, you will not be able to run sshd(8) as a non-root user.  The default is no.

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

   X11DisplayOffset
           Specifies the first display number available for sshd(8)'s X11 forwarding.  This prevents sshd from interfering with real X11 servers.  The default is 10.

   X11Forwarding
           Specifies whether X11 forwarding is permitted.  The argument must be yes or no.  The default is no.

           When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the sshd(8) proxy display is configured to listen on the wildcard  ad
           dress  (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 warn
           ings 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  re
           questing 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 loop
           back address and sets the hostname part of the DISPLAY environment variable to localhost.  This prevents remote hosts from connecting to  the  proxy  display.   However,  some
           older  X11  clients  may not function with this configuration.  X11UseLocalhost may be set to no to specify that the forwarding server should be bound to the wildcard address.
           The argument must be yes or no.  The default is yes.

   XAuthLocation
           Specifies the full pathname of the xauth(1) program, or none to not use one.  The default is /usr/bin/xauth.

TIME FORMATS

   sshd(8) command-line arguments and configuration file options that specify time may be expressed using a sequence of the form: time[qualifier], where time is a positive integer  value
   and qualifier is one of the following:

         none  seconds
         s | S   seconds
         m | M   minutes
         h | H   hours
         d | D   days
         w | W   weeks

   Each member of the sequence is added together to calculate the total time value.

   Time format examples:

         600     600 seconds (10 minutes)
         10m     10 minutes
         1h30m   1 hour 30 minutes (90 minutes)

TOKENS

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

         %%    A literal %.
         %C    Identifies the connection endpoints, containing four space-separated values: client address, client port number, server address, and server port number.
         %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 %%, %C, %D, %f, %h, %k, %t, %U, and %u.

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

   AuthorizedPrincipalsCommand accepts the tokens %%, %C, %D, %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.

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.

Debian February 15, 2025 SSHD_CONFIG(5)