Access Control Options
from Pogo, Walt Kelly
The skunk watches for intruders and sprays.
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
Access Control Commands
- restrict numeric_address [mask numeric_mask]
- 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 255.255.255.255, meaning that the
numeric_address is treated as the address of an
individual host. A default entry (address 0.0.0.0, mask
0.0.0.0) 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
- Ignore all NTP mode 6 and 7 packets (i.e. information queries and
configuration requests) from the source. Time service is not
- 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
- 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
- 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
- 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 <email@example.com>