1ntp_acc(5) File Formats Manual ntp_acc(5)
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6 ntp_acc - Access Control Options
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10 The ntpd daemon implements a general purpose access control list (ACL)
11 containing address/match entries sorted first by increasing address
12 values and then by increasing mask values. A match occurs when the bit‐
13 wise AND of the mask and the packet source address is equal to the bit‐
14 wise AND of the mask and address in the list. The list is searched in
15 order with the last match found defining the restriction flags associ‐
16 ated with the entry.
17 An example may clarify how it works. Our campus has two class-B net‐
19 works, 128.4 for the ECE and CIS departments and 128.175 for the rest
20 of campus. Let's assume (not true!) that subnet 128.4.1 homes critical
21 services like class rosters and spread sheets. A suitable ACL might be
22 restrict default nopeer # deny new associations
24 restrict 128.175.0.0 mask 255.255.0.0 # allow campus access
25 restrict 128.4.0.0 mask 255.255.0.0 none # allow ECE and CIS access
26 restrict 128.4.1.0 mask 255.255.255.0 notrust # require authentication on subnet 1
27 restrict time.nist.gov # allow access
28 While this facility may be useful for keeping unwanted, broken or mali‐
30 cious clients from congesting innocent servers, it should not be con‐
31 sidered an alternative to the NTP authentication facilities. Source
32 address based restrictions are easily circumvented by a determined
33 cracker.
34
37 discard [ average avg ][ minimum min ] [ monitor prob ]
38 Set the parameters of the rate control facility which protects
39 the server from client abuse. If the limited flag is present in
40 the ACL, packets that violate these limits are discarded. If in
41 addition the kod restriction is present, a kiss-o'-death packet
42 is returned.
43 average avg
46 Specify the minimum average interpacket spacing (mini‐
47 mum average headway time) in log2 s with default 3.
48 minimum min
50 Specify the minimum interpacket spacing (guard time) in
51 log2 s with default 1.
52 monitor Specify the probability of discard for packets that
54 overflow the rate-control window. This is a performance
55 optimization for servers with aggregate arrivals of
56 1000 packets per second or more.
57 restrict address [mask mask] [flag][...]
60 The address argument expressed in dotted-quad form is the
61 address of a host or network. Alternatively, the address argu‐
62 ment can be a valid host DNS name. The mask argument expressed
63 in dotted-quad form defaults to 255.255.255.255, meaning that
64 the address is treated as the address of an individual host. A
65 default entry (address 0.0.0.0, mask 0.0.0.0) is always
66 included and is always the first entry in the list. Note that
67 the text string default, with no mask option, may be used to
68 indicate the default entry. Some flags have the effect to deny
69 service, some have the effect to enable service and some are
70 conditioned by other flags. The flags. are not orthogonal, in
71 that more restrictive flags will often make less restrictive
72 ones redundant. The flags that deny service are classed in two
73 categories, those that restrict time service and those that
74 restrict informational queries and attempts to do run-time
75 reconfiguration of the server. One or more of the following
76 flags may be specified:
77 flake Discard received NTP packets with probability 0.1; that
80 is, on average drop one packet in ten. This is for
81 testing and amusement. The name comes from Bob Braden's
82 flakeway, which once did a similar thing for early
83 Internet testing.
84 ignore Deny packets of all kinds, including ntpq and ntpdc
86 queries.
87 kod Send a kiss-o'-death (KoD) packet if the limited flag
89 is present and a packet violates the rate limits estab‐
90 lished by the discard command. KoD packets are them‐
91 selves rate limited for each source address separately.
92 If this flag is not present, packets that violate the
93 rate limits are discarded.
94 limited Deny time service if the packet violates the rate lim‐
96 its established by the discard command. This does not
97 apply to ntpq and ntpdc queries.
98 lowpriotrap
100 Declare traps set by matching hosts to be low priority.
101 The number of traps a server can maintain is limited
102 (the current limit is 3). Traps are usually assigned on
103 a first come, first served basis, with later trap
104 requestors being denied service. This flag modifies the
105 assignment algorithm by allowing low priority traps to
106 be overridden by later requests for normal priority
107 traps.
108 mssntp Enable Microsoft Windows MS-SNTP authentication using
110 Active Directory services. Note: Potential users should
111 be aware that these services involve a TCP connection
112 to another process that could potentially block, deny‐
113 ing services to other users. Therefore, this flag
114 should be used only for a dedicated server with no
115 clients other than MS-SNTP.
116 nomodify
118 Deny ntpq and ntpdc queries which attempt to modify the
119 state of the server (i.e., run time reconfiguration).
120 Queries which return information are permitted.
121 noquery Deny ntpq and ntpdc queries. Time service is not
123 affected.
124 nopeer Deny packets that might mobilize an association unless
126 authenticated. This includes broadcast, symmetric-
127 active and manycast server packets when a configured
128 association does not exist. Note that this flag does
129 not apply to packets that do not attempt to mobilize an
130 association.
131 noserve Deny all packets except ntpq and ntpdc queries.
133 notrap Decline to provide mode 6 control message trap service
135 to matching hosts. The trap service is a subsystem of
136 the ntpdc control message protocol which is intended
137 for use by remote event logging programs.
138 notrust Deny packets that are not cryptographically authenti‐
140 cated. Note carefully how this flag interacts with the
141 auth option of the enable and disable commands. If auth
142 is enabled, which is the default, authentication is
143 required for all packets that might mobilize an associ‐
144 ation. If auth is disabled, but the notrust flag is not
145 present, an association can be mobilized whether or not
146 authenticated. If auth is disabled, but the notrust
147 flag is present, authentication is required only for
148 the specified address/mask range.
149 ntpport
151 non-ntpport
153 This is actually a match algorithm modifier, rather
154 than a restriction flag. Its presence causes the
155 restriction entry to be matched only if the source port
156 in the packet is the standard NTP UDP port (123). Both
157 ntpport and non-ntpport may be specified. The ntpport
158 is considered more specific and is sorted later in the
159 list.
160 version Deny packets that do not match the current NTP version.
162 Default restriction list entries with the flags ignore, ntpport, for
164 each of the local host's interface addresses are inserted into the ta‐
165 ble at startup to prevent the server from attempting to synchronize to
166 its own time. A default entry is also always present, though if it is
167 otherwise unconfigured; no flags are associated with the default entry
168 (i.e., everything besides your own NTP server is unrestricted).
169
172 ntp.conf(5)
173 HTML documentation in ntp-doc package.
175 This file was automatically generated from HTML source.
177 ntp_acc(5)