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ntp_acc(5)                                                          ntp_acc(5)

       ntp_acc - Access Control Options

       The  ntpd daemon implements a general purpose access control list (ACL) containing address/match entries sorted
       first by increasing address values and then by increasing mask values. A match occurs when the bitwise  AND  of
       the  mask  and  the  packet source address is equal to the bitwise AND of the mask and address in the list. The
       list is searched in order with the last match found defining the restriction flags associated with the entry.

       An example may clarify how it works. Our campus has two class-B networks, 128.4 for the ECE and CIS departments
       and  128.175  for the rest of campus. Let's assume (not true!) that subnet 128.4.1 homes critical services like
       class rosters and spread sheets. A suitable ACL might be

       restrict default nopeer                      # deny new associations
       restrict mask        # allow campus access
       restrict mask none     # allow ECE and CIS access
       restrict mask notrust # require authentication on subnet 1
       restrict                            # allow access

       While this facility may be useful for keeping unwanted, broken or malicious clients  from  congesting  innocent
       servers,  it should not be considered an alternative to the NTP authentication facilities. Source address based
       restrictions are easily circumvented by a determined cracker.

       discard [ average avg ][ minimum min ] [ monitor prob ]
               Set the parameters of the rate control facility which protects the server from  client  abuse.  If  the
               limited flag is present in the ACL, packets that violate these limits are discarded. If in addition the
               kod restriction is present, a kiss-o'-death packet is returned.

               average avg
                       Specify the minimum average interpacket spacing (minimum average headway time) in log2  s  with
                       default 3.

               minimum min
                       Specify the minimum interpacket spacing (guard time) in log2 s with default 1.

               monitor Specify the probability of discard for packets that overflow the rate-control window. This is a
                       performance optimization for servers with aggregate arrivals of  1000  packets  per  second  or

       restrict address [mask mask] [flag][...]
               The  address argument expressed in dotted-quad form is the address of a host or network. Alternatively,
               the address argument can be a valid host DNS name, but it must be resolvable at the time when  ntpd  is
               started  and  if it's resolved to multiple addresses, only the first address will be added to the list.
               The mask argument expressed in dotted-quad form defaults to, meaning that  the  address
               is  treated  as  the  address of an individual host. A default entry (address, mask is
               always included and is always the first entry in the list. Note that the text string default,  with  no
               mask  option,  may  be used to indicate the default entry.  Some flags have the effect to deny service,
               some have the effect to enable service and some are conditioned by other  flags.  The  flags.  are  not
               orthogonal,  in  that more restrictive flags will often make less restrictive ones redundant. The flags
               that deny service are classed in two categories, those  that  restrict  time  service  and  those  that
               restrict  informational  queries and attempts to do run-time reconfiguration of the server. One or more
               of the following flags may be specified:

               flake   Discard received NTP packets with probability 0.1; that is, on average drop one packet in  ten.
                       This  is for testing and amusement. The name comes from Bob Braden's flakeway, which once did a
                       similar thing for early Internet testing.

               ignore  Deny packets of all kinds, including ntpq and ntpdc queries.

               kod     Send a kiss-o'-death (KoD) packet if the limited flag is present and a packet violates the rate
                       limits  established  by  the  discard command. KoD packets are themselves rate limited for each
                       source address separately. If this flag is not present, packets that violate  the  rate  limits
                       are discarded.

               limited Deny  time  service  if the packet violates the rate limits established by the discard command.
                       This does not apply to ntpq and ntpdc queries.

                       Declare traps set by matching hosts to be low priority. The number of traps a server can  main-
                       tain  is  limited  (the  current limit is 3). Traps are usually assigned on a first come, first
                       served basis, with later trap requestors being denied service. This flag modifies  the  assign-
                       ment  algorithm  by  allowing  low priority traps to be overridden by later requests for normal
                       priority traps.

               mssntp  Enable Microsoft Windows MS-SNTP authentication using Active Directory services.  Note:  Poten-
                       tial users should be aware that these services involve a TCP connection to another process that
                       could potentially block, denying services to other users. Therefore, this flag should  be  used
                       only for a dedicated server with no clients other than MS-SNTP.

                       Deny  ntpq  and  ntpdc  queries which attempt to modify the state of the server (i.e., run time
                       reconfiguration). Queries which return information are permitted.

               noquery Deny ntpq and ntpdc queries. Time service is not affected.

               nopeer  Deny packets that might mobilize an association unless authenticated. This includes  broadcast,
                       symmetric-active and manycast server packets when a configured association does not exist. Note
                       that this flag does not apply to packets that do not attempt to mobilize an association.

               noserve Deny all packets except ntpq and ntpdc queries.

               notrap  Decline to provide mode 6 control message trap service to matching hosts. The trap service is a
                       subsystem  of the ntpdc control message protocol which is intended for use by remote event log-
                       ging programs.

               notrust Deny packets that are not cryptographically authenticated. Note carefully how this flag  inter-
                       acts  with the auth option of the enable and disable commands. If auth is enabled, which is the
                       default, authentication is required for all packets that might mobilize an association. If auth
                       is  disabled,  but  the notrust flag is not present, an association can be mobilized whether or
                       not authenticated. If auth is disabled, but the notrust  flag  is  present,  authentication  is
                       required only for the specified address/mask range.


                       This  is  actually  a  match  algorithm  modifier, rather than a restriction flag. Its presence
                       causes the restriction entry to be matched only if the source port in the packet is  the  stan-
                       dard  NTP UDP port (123). Both ntpport and non-ntpport may be specified. The ntpport is consid-
                       ered more specific and is sorted later in the list.

               version Deny packets that do not match the current NTP version.

       Default restriction list entries with the flags ignore,  ntpport,  for  each  of  the  local  host's  interface
       addresses  are  inserted  into the table at startup to prevent the server from attempting to synchronize to its
       own time. A default entry is also always present, though if it is otherwise unconfigured; no flags are  associ-
       ated with the default entry (i.e., everything besides your own NTP server is unrestricted).


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