ntp_acc(5) File Formats Manual ntp_acc(5)
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 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 184.108.40.206 mask 255.255.0.0 # allow campus access
restrict 220.127.116.11 mask 255.255.0.0 none # allow ECE and CIS access
restrict 18.104.22.168 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 malicious 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
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
Specify the minimum average interpacket spacing (minimum average headway time) in log2 s with default 3.
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 argument can be
a valid host DNS name. 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 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 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 ser-
vice. 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, 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 many-
cast 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 con-
trol message protocol which is intended for use by remote event logging programs.
notrust Deny packets that are not cryptographically authenticated. 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 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 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).
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