Access Control Options

Access Control Support

ntpd implements a general purpose address-and-mask based restriction list. The list is sorted by address and by mask, and the list is searched in this order for matches, with the last match found defining the restriction flags associated with the incoming packets. The source address of incoming packets is used for the match, with the 32-bit address being and'ed with the mask associated with the restriction entry and then compared with the entry's address (which has also been and'ed with the mask) to look for a match. Additional information and examples can be found in the Notes on Configuring NTP and Setting up a NTP Subnet page.

The restriction facility was implemented in conformance with the access policies for the original NSFnet backbone time servers. While this facility may be otherwise useful for keeping unwanted or broken remote time servers from affecting your own, it should not be considered an alternative to the standard NTP authentication facility. Source address based restrictions are easily circumvented by a determined cracker.

Access Control Commands

restrict numeric_address [ mask numeric_mask ] [ flag ] [ ... ]
The numeric_address argument, expressed in dotted-quad form, is the address of an host or network. The mask argument, also expressed in dotted-quad form, defaults to, meaning that the numeric_address is treated as the address of an individual host. A default entry (address, mask is always included and, given the sort algorithm, is always the first entry in the list. Note that, while numeric_address is normally given in dotted-quad format, the text string default, with no mask option, may be used to indicate the default entry.

In the current implementation, flag always restricts access, i.e., an entry with no flags indicates that free access to the server is to be given. The flags are not orthogonal, in that more restrictive flags will often make less restrictive ones redundant. The flags can generally be classed into two catagories, those which restrict time service and those which restrict informational queries and attempts to do run-time reconfiguration of the server. One or more of the following flags may be specified:
Ignore all packets from hosts which match this entry. If this flag is specified neither queries nor time server polls will be responded to.

Ignore all NTP mode 6 and 7 packets (i.e. information queries and configuration requests) from the source. Time service is not affected.

Ignore all NTP mode 6 and 7 packets which attempt to modify the state of the server (i.e. run time reconfiguration). Queries which return information are permitted.

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

Declare traps set by matching hosts to be low priority. The number of traps a server can maintain 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 assignment algorithm by allowing low priority traps to be overridden by later requests for normal priority traps.

Ignore NTP packets whose mode is other than 6 or 7. In effect, time service is denied, though queries may still be permitted.

Provide stateless time service to polling hosts, but do not allocate peer memory resources to these hosts even if they otherwise might be considered useful as future synchronization partners.

Treat these hosts normally in other respects, but never use them as synchronization sources.

These hosts are subject to limitation of number of clients from the same net. Net in this context refers to the IP notion of net (class A, class B, class C, etc.). Only the first client_limit hosts that have shown up at the server and that have been active during the last client_limit_period seconds are accepted. Requests from other clients from the same net are rejected. Only time request packets are taken into account. Query packets sent by the ntpq and ntpdc programs are not subject to these limits. A history of clients is kept using the monitoring capability of ntpd. Thus, monitoring is always active as long as there is a restriction entry with the limited flag.

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 standard NTP UDP port (123). Both ntpport and non-ntpport may be specified. The ntpport is considered more specific and is sorted later in the list.

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 associated with the default entry (i.e., everything besides your own NTP server is unrestricted).

clientlimit limit
Set the client_limit variable, which limits the number of simultaneous access-controlled clients. The default value for this variable is 3.

clientperiod period
Set the client_limit_period variable, which specifies the number of seconds after which a client is considered inactive and thus no longer is counted for client limit restriction. The default value for this variable is 3600 seconds.

David L. Mills (