ntp_acc(5) File Formats Manual ntp_acc(5)
NAME
ntp_acc - Access Control Options
ACCESS CONTROL SUPPORT
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 bit-
wise AND of the mask and the packet source address is equal to the bit-
wise AND of the mask and address in the list. The list is searched in
order with the last match found defining the restriction flags associ-
ated with the entry.
An example may clarify how it works. Our campus has two class-B net-
works, 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 128.175.0.0 mask 255.255.0.0 # allow campus access
restrict 128.4.0.0 mask 255.255.0.0 none # allow ECE and CIS access
restrict 128.4.1.0 mask 255.255.255.0 notrust # require authentication on subnet 1
restrict time.nist.gov # allow access
While this facility may be useful for keeping unwanted, broken or mali-
cious clients from congesting innocent servers, it should not be con-
sidered an alternative to the NTP authentication facilities. Source
address based restrictions are easily circumvented by a determined
cracker.
ACCESS CONTROL COMMANDS
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 (mini-
mum 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 more.
restrict address [mask mask] [flag][...]
The address argument expressed in dotted-quad form is the
address of a host or network. Alternatively, the address argu-
ment 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
255.255.255.255, meaning that the 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 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 informa-
tional queries and attempts to do run-time reconfiguration of
the server. One or more of the following flags may be speci-
fied:
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 estab-
lished by the discard command. KoD packets are them-
selves 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 lim-
its established by the discard command. This does not
apply to ntpq and ntpdc queries.
lowpriotrap
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.
mssntp Enable Microsoft Windows MS-SNTP authentication using
Active Directory services. Note: Potential users should
be aware that these services involve a TCP connection
to another process that could potentially block, deny-
ing services to other users. Therefore, this flag
should be used only for a dedicated server with no
clients other than MS-SNTP.
nomodify
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 logging programs.
notrust Deny packets that are not cryptographically authenti-
cated. Note carefully how this flag interacts 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 associ-
ation. 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.
ntpport
non-ntpport
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.
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 ta-
ble 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).
SEE ALSO
ntp.conf(5)
The official HTML documentation.
This file was automatically generated from HTML source.
ntp_acc(5)
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