Provided by: openssh-server_8.2p1-4ubuntu0.13_amd64 bug

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

       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
                ChallengeResponseAuthentication no
                X11Forwarding yes
                PrintMotd no
                AcceptEnv LANG LC_*
                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 directives are processed in
               the following order: DenyGroups, AllowGroups.

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

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

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

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

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

       AuthenticationMethods
               Specifies the authentication methods that must be successfully completed for a user to be granted
               access.   This  option  must  be  followed by one or more lists of comma-separated authentication
               method names, or by the single string any to indicate the  default  behaviour  of  accepting  any
               single  authentication  method.   If  the  default  is overridden, then successful authentication
               requires completion of every method in at least one of these lists.

               For example,  "publickey,password  publickey,keyboard-interactive"  would  require  the  user  to
               complete  public  key  authentication,  followed  by  either  password  or  keyboard  interactive
               authentication.  Only methods that are next in one or more lists are offered at  each  stage,  so
               for  this  example  it  would  not  be  possible  to  attempt  password  or  keyboard-interactive
               authentication before public key.

               For keyboard interactive authentication it is also  possible  to  restrict  authentication  to  a
               specific device by appending a colon followed by the device identifier bsdauth or pam.  depending
               on the server configuration.  For example, "keyboard-interactive:bsdauth" would restrict keyboard
               interactive authentication to the bsdauth device.

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

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

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

       AuthorizedKeysCommand
               Specifies  a  program to be used to look up the user's public keys.  The program must be owned by
               root, not writable by  group  or  others  and  specified  by  an  absolute  path.   Arguments  to
               AuthorizedKeysCommand  accept  the tokens described in the “TOKENS” section.  If no arguments are
               specified then the username of the target user is used.

               The program should produce on standard output zero or more lines of authorized_keys  output  (see
               “AUTHORIZED_KEYS”  in sshd(8)).  If a key supplied by AuthorizedKeysCommand does not successfully
               authenticate and authorize the user then public key  authentication  continues  using  the  usual
               AuthorizedKeysFile files.  By default, no AuthorizedKeysCommand is run.

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

       AuthorizedKeysFile
               Specifies the file that contains the public keys used for user  authentication.   The  format  is
               described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8).  Arguments to AuthorizedKeysFile
               accept  the  tokens  described  in  the “TOKENS” section.  After expansion, AuthorizedKeysFile is
               taken to be an absolute path or one relative to the user's home directory.  Multiple files may be
               listed, separated by whitespace.  Alternately this option may be set to none to skip checking for
               user keys in files.  The default is ".ssh/authorized_keys .ssh/authorized_keys2".

       AuthorizedPrincipalsCommand
               Specifies a program to be used to generate the list of  allowed  certificate  principals  as  per
               AuthorizedPrincipalsFile.  The program must be owned by root, not writable by group or others and
               specified  by  an  absolute  path.   Arguments  to  AuthorizedPrincipalsCommand accept the tokens
               described in the “TOKENS” section.  If no arguments are specified then the username of the target
               user is used.

               The program should produce on standard output zero  or  more  lines  of  AuthorizedPrincipalsFile
               output.   If  either  AuthorizedPrincipalsCommand  or AuthorizedPrincipalsFile is specified, then
               certificates offered by the client for authentication must contain a principal  that  is  listed.
               By default, no AuthorizedPrincipalsCommand is run.

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

       AuthorizedPrincipalsFile
               Specifies  a  file  that  lists principal names that are accepted for certificate authentication.
               When using certificates signed by a key listed in TrustedUserCAKeys, this file lists  names,  one
               of  which  must  appear  in  the certificate for it to be accepted for authentication.  Names are
               listed one per line preceded by key options (as described in  “AUTHORIZED_KEYS  FILE  FORMAT”  in
               sshd(8)).  Empty lines and comments starting with ‘#’ are ignored.

               Arguments to AuthorizedPrincipalsFile accept the tokens described in the “TOKENS” section.  After
               expansion, AuthorizedPrincipalsFile is taken to be an absolute path or one relative to the user's
               home  directory.   The  default  is  none,  i.e. not to use a principals file – in this case, the
               username of the user must appear in a certificate's principals list for it to be accepted.

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

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

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

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

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

       ChallengeResponseAuthentication
               Specifies  whether  challenge-response  authentication is allowed (e.g. via PAM).  The default is
               yes.

       ChrootDirectory
               Specifies the pathname of a directory to chroot(2) to after authentication.  At  session  startup
               sshd(8)  checks  that  all  components  of  the pathname are root-owned directories which are not
               writable by any other user or group.  After the chroot, sshd(8) changes the working directory  to
               the  user's  home  directory.   Arguments  to  ChrootDirectory accept the tokens described in the
               “TOKENS” section.

               The ChrootDirectory must contain the necessary  files  and  directories  to  support  the  user's
               session.   For  an interactive session this requires at least a shell, typically sh(1), and basic
               /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),  and  tty(4)  devices.   For
               file  transfer sessions using SFTP no additional configuration of the environment is necessary if
               the in-process sftp-server is used, though sessions which use logging may require /dev/log inside
               the chroot directory on some operating systems (see sftp-server(8) for details).

               For safety, it is very important that the directory hierarchy be prevented from  modification  by
               other  processes on the system (especially those outside the jail).  Misconfiguration can lead to
               unsafe environments which sshd(8) cannot detect.

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

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

               The supported ciphers are:

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

               The default is:

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

               The list of available ciphers may also be obtained using "ssh -Q cipher".

       ClientAliveCountMax
               Sets the number of client alive messages which may be sent without sshd(8) receiving any messages
               back  from  the client.  If this threshold is reached while client alive messages are being sent,
               sshd will disconnect the client, terminating the session.  It is important to note that  the  use
               of client alive messages is very different from TCPKeepAlive.  The client alive messages are sent
               through  the  encrypted  channel  and  therefore will not be spoofable.  The TCP keepalive option
               enabled by TCPKeepAlive is spoofable.  The client alive mechanism is valuable when the client  or
               server depend on knowing when a connection has become unresponsive.

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

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

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

       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.

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

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

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

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

       FingerprintHash
               Specifies the hash algorithm used when logging key fingerprints.   Valid  options  are:  md5  and
               sha256.  The default is sha256.

       ForceCommand
               Forces  the  execution of the command specified by ForceCommand, ignoring any command supplied by
               the client and ~/.ssh/rc if present.  The command is invoked by using the user's login shell with
               the -c option.  This applies to shell, command, or subsystem execution.  It is most useful inside
               a  Match  block.   The  command  originally  supplied  by  the  client  is   available   in   the
               SSH_ORIGINAL_COMMAND  environment variable.  Specifying a command of internal-sftp will force the
               use of an in-process SFTP server that requires no support files when used  with  ChrootDirectory.
               The default is none.

       GatewayPorts
               Specifies  whether  remote  hosts  are  allowed to connect to ports forwarded for the client.  By
               default, sshd(8) binds remote port forwardings to the  loopback  address.   This  prevents  other
               remote  hosts  from connecting to forwarded ports.  GatewayPorts can be used to specify that sshd
               should allow remote port forwardings to bind to non-loopback addresses, thus allowing other hosts
               to connect.  The argument may be no to force remote port forwardings to be available to the local
               host  only,  yes  to  force  remote  port  forwardings  to  bind  to  the  wildcard  address,  or
               clientspecified  to allow the client to select the address to which the forwarding is bound.  The
               default is no.

       GSSAPIAuthentication
               Specifies whether user authentication based on GSSAPI is allowed.  The default is no.

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

       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-gex-sha1-,gss-group14-sha1-”.  This option only applies to protocol version 2
               connections using GSSAPI.

       HostbasedAcceptedKeyTypes
               Specifies  the  key  types 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  key  types  will  be  appended  to  the default set instead of replacing them.  If the
               specified list begins with a ‘-’ character, then the specified key  types  (including  wildcards)
               will  be  removed  from  the default set instead of replacing them.  If the specified list begins
               with a ‘^’ character, then the specified key types will be placed at the head of the default set.
               The default for this option is:

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

               The list of available key types may also be obtained using "ssh -Q 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 algorithms that the server offers.  The default for this option is:

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

               The list of available key types may also be obtained using "ssh -Q HostKeyAlgorithms".

       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  and use only the system-wide known hosts file /etc/ssh/known_hosts.  The
               default is “no”.

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

       IPQoS   Specifies the IPv4 type-of-service or DSCP class for the connection.  Accepted values  are  af11,
               af12,  af13,  af21, af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5,
               cs6, cs7, ef, le, lowdelay, throughput,  reliability,  a  numeric  value,  or  none  to  use  the
               operating  system  default.   This option may take one or two arguments, separated by whitespace.
               If one argument is specified, it is used as the packet class unconditionally.  If two values  are
               specified,  the  first is automatically selected for interactive sessions and the second for non-
               interactive sessions.  The default is 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 password will  be  validated  via  any
               additional local mechanism such as /etc/passwd.  The default is yes.

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

       KexAlgorithms
               Specifies the available KEX (Key  Exchange)  algorithms.   Multiple  algorithms  must  be  comma-
               separated.   Alternately  if  the  specified list begins with a ‘+’ character, then the specified
               methods will be appended to the default set instead of replacing them.   If  the  specified  list
               begins  with  a  ‘-’  character, then the specified methods (including wildcards) will be removed
               from the default set instead of replacing  them.   If  the  specified  list  begins  with  a  ‘^’
               character,  then  the  specified  methods  will  be  placed  at the head of the default set.  The
               supported algorithms are:

                     curve25519-sha256
                     curve25519-sha256@libssh.org
                     diffie-hellman-group1-sha1
                     diffie-hellman-group14-sha1
                     diffie-hellman-group14-sha256
                     diffie-hellman-group16-sha512
                     diffie-hellman-group18-sha512
                     diffie-hellman-group-exchange-sha1
                     diffie-hellman-group-exchange-sha256
                     ecdh-sha2-nistp256
                     ecdh-sha2-nistp384
                     ecdh-sha2-nistp521
                     sntrup4591761x25519-sha512@tinyssh.org

               The default is:

                     curve25519-sha256,curve25519-sha256@libssh.org,
                     ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                     diffie-hellman-group-exchange-sha256,
                     diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
                     diffie-hellman-group14-sha256

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

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

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

               The optional rdomain qualifier requests sshd(8) listen in an explicit routing domain.  If port is
               not specified, sshd will listen on the address and all Port options specified.  The default is to
               listen on all local addresses on the current  default  routing  domain.   Multiple  ListenAddress
               options are permitted.  For more information on routing domains, see rdomain(4).

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

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

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

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

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

               The default is:

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

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

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

               The  arguments  to  Match  are  one  or more criteria-pattern pairs or the single token All which
               matches all criteria.  The available criteria are User,  Group,  Host,  LocalAddress,  LocalPort,
               RDomain,  and  Address  (with  RDomain  representing  the  rdomain(4) on which the connection was
               received).

               The match patterns may consist of single  entries  or  comma-separated  lists  and  may  use  the
               wildcard and negation operators described in the “PATTERNS” section of ssh_config(5).

               The  patterns  in  an  Address  criteria  may  additionally  contain  addresses  to match in CIDR
               address/masklen format, such as  192.0.2.0/24  or  2001:db8::/32.   Note  that  the  mask  length
               provided  must  be  consistent with the address - it is an error to specify a mask length that is
               too long for the address or one with bits set in this host portion of the address.  For  example,
               192.0.2.0/33 and 192.0.2.0/8, respectively.

               Only a subset of keywords may be used on the lines following a Match keyword.  Available keywords
               are AcceptEnv, AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding, AllowTcpForwarding,
               AllowUsers,      AuthenticationMethods,     AuthorizedKeysCommand,     AuthorizedKeysCommandUser,
               AuthorizedKeysFile,         AuthorizedPrincipalsCommand,         AuthorizedPrincipalsCommandUser,
               AuthorizedPrincipalsFile,   Banner,  ChrootDirectory,  ClientAliveCountMax,  ClientAliveInterval,
               DenyGroups,       DenyUsers,       ForceCommand,       GatewayPorts,        GSSAPIAuthentication,
               HostbasedAcceptedKeyTypes,   HostbasedAuthentication,  HostbasedUsesNameFromPacketOnly,  Include,
               IPQoS, KbdInteractiveAuthentication, KerberosAuthentication, LogLevel, MaxAuthTries, MaxSessions,
               PasswordAuthentication,   PermitEmptyPasswords,   PermitListen,   PermitOpen,    PermitRootLogin,
               PermitTTY,  PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication, RekeyLimit,
               RevokedKeys,  RDomain,  SetEnv,  StreamLocalBindMask,  StreamLocalBindUnlink,  TrustedUserCAKeys,
               X11DisplayOffset, X11Forwarding and X11UseLocalhost.

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

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

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

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

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

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

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

                     PermitListen port
                     PermitListen host:port

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

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

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

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

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

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

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

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

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

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

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

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

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

       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 default is yes.

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

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

               The list of available key types may also be obtained using "ssh -Q PubkeyAcceptedKeyTypes".

       PubkeyAuthOptions
               Sets one or more public key authentication options.  Two option keywords are currently supported:
               none (the default; indicating no additional options are enabled) and touch-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.   This  option  has  no  effect  for  other, non-
               authenticator public key types.

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

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

       RevokedKeys
               Specifies revoked public keys file, or none to not use one.  Keys listed in  this  file  will  be
               refused  for  public key authentication.  Note that if this file is not readable, then public key
               authentication will be refused for all users.  Keys may be specified as a text file, listing  one
               public  key  per  line, or as an OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
               For more information on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).

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

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

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

       StreamLocalBindMask
               Sets the octal file creation mode mask (umask) used when creating a Unix-domain socket  file  for
               local  or  remote port forwarding.  This option is only used for port forwarding to a Unix-domain
               socket file.

               The default value is 0177, which creates a Unix-domain socket file that is readable and  writable
               only by the owner.  Note that not all operating systems honor the file mode on Unix-domain socket
               files.

       StreamLocalBindUnlink
               Specifies  whether  to  remove  an  existing  Unix-domain  socket  file  for local or remote port
               forwarding  before  creating  a   new   one.    If   the   socket   file   already   exists   and
               StreamLocalBindUnlink  is not enabled, sshd will be unable to forward the port to the Unix-domain
               socket file.  This option is only used for port forwarding to a Unix-domain socket file.

               The argument must be yes or no.  The default is no.

       StrictModes
               Specifies whether sshd(8) should check file modes and ownership of  the  user's  files  and  home
               directory  before  accepting  login.   This  is  normally  desirable  because  novices  sometimes
               accidentally leave their directory or files world-writable.  The default is yes.  Note that  this
               does not apply to ChrootDirectory, whose permissions and ownership are checked unconditionally.

       Subsystem
               Configures  an  external  subsystem (e.g. file transfer daemon).  Arguments should be a subsystem
               name and a command (with optional arguments) to execute upon subsystem request.

               The command sftp-server implements the SFTP file transfer subsystem.

               Alternately the name internal-sftp implements an  in-process  SFTP  server.   This  may  simplify
               configurations using ChrootDirectory to force a different filesystem root on clients.

               By default no subsystems are defined.

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

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

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

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

               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 authentication using  TrustedUserCAKeys.
               For more details on certificates, see the CERTIFICATES section in ssh-keygen(1).

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

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

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

               Because  PAM  challenge-response  authentication  usually  serves  an equivalent role to password
               authentication,      you      should      disable      either      PasswordAuthentication      or
               ChallengeResponseAuthentication.

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

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

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

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

               When  X11  forwarding  is  enabled,  there may be additional exposure to the server and to client
               displays if the sshd(8) proxy display is configured  to  listen  on  the  wildcard  address  (see
               X11UseLocalhost),  though this is not the default.  Additionally, the authentication spoofing and
               authentication data verification and substitution occur on the client side.  The security risk of
               using X11 forwarding is that the client's X11 display server may be exposed to  attack  when  the
               SSH  client  requests  forwarding  (see  the warnings for ForwardX11 in ssh_config(5)).  A system
               administrator may have a stance in which they want to protect clients that may expose  themselves
               to attack by unwittingly requesting X11 forwarding, which can warrant a no setting.

               Note  that  disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users
               can always install their own forwarders.

       X11UseLocalhost
               Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address or to the
               wildcard address.  By default, sshd binds the forwarding server to the loopback address and  sets
               the  hostname  part of the DISPLAY environment variable to localhost.  This prevents remote hosts
               from connecting to the proxy display.  However, some older X11 clients may not function with this
               configuration.  X11UseLocalhost may be set to no to specify that the forwarding server should  be
               bound to the wildcard address.  The argument must be yes or no.  The default is yes.

       XAuthLocation
               Specifies  the  full  pathname  of  the xauth(1) program, or none to not use one.  The default is
               /usr/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 ‘%’.
             %D    The routing domain in which the incoming connection was received.
             %F    The fingerprint of the CA key.
             %f    The fingerprint of the key or certificate.
             %h    The home directory of the user.
             %i    The key ID in the certificate.
             %K    The base64-encoded CA key.
             %k    The base64-encoded key or certificate for authentication.
             %s    The serial number of the certificate.
             %T    The type of the CA key.
             %t    The key or certificate type.
             %U    The numeric user ID of the target user.
             %u    The username.

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

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

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

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

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

       RoutingDomain accepts the token %D.

FILES

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

SEE ALSO

       sftp-server(8), sshd(8)

AUTHORS

       OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.  Aaron Campbell,  Bob
       Beck,  Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features
       and created OpenSSH.  Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.  Niels
       Provos and Markus Friedl contributed support for privilege separation.

Debian                                          February 7, 2020                                  SSHD_CONFIG(5)