Internet DRAFT - draft-ietf-ntp-mode-6-cmds
draft-ietf-ntp-mode-6-cmds
Network Working Group B. Haberman, Ed.
Internet-Draft JHU
Intended status: Historic February 2022
Expires: 19 August 2022
Control Messages Protocol for Use with Network Time Protocol Version 4
draft-ietf-ntp-mode-6-cmds-11
Abstract
This document describes the structure of the control messages that
were historically used with the Network Time Protocol before the
advent of more modern control and management approaches. These
control messages have been used to monitor and control the Network
Time Protocol application running on any IP network attached
computer. The information in this document was originally described
in Appendix B of RFC 1305. The goal of this document is to provide
an updated description of the control messages described in RFC 1305
in order to conform with the updated Network Time Protocol
specification documented in RFC 5905.
The publication of this document is not meant to encourage the
development and deployment of these control messages. This document
is only providing a current reference for these control messages
given the current status of RFC 1305.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 5 August 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Control Message Overview . . . . . . . . . . . . . . . . 3
1.2. Remote Facility Message Overview . . . . . . . . . . . . 5
2. NTP Control Message Format . . . . . . . . . . . . . . . . . 5
3. Status Words . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. System Status Word . . . . . . . . . . . . . . . . . . . 8
3.2. Peer Status Word . . . . . . . . . . . . . . . . . . . . 10
3.3. Clock Status Word . . . . . . . . . . . . . . . . . . . . 12
3.4. Error Status Word . . . . . . . . . . . . . . . . . . . . 12
4. Commands . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. NTP Remote Facility Message Format . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
RFC 1305 [RFC1305] described a set of control messages for use within
the Network Time Protocol (NTP) when a comprehensive network
management solution was not available. The definitions of these
control messages were not promulgated to RFC 5905 [RFC5905] when NTP
version 4 was documented. These messages were intended for use only
in systems where no other management facilities were available or
appropriate, such as in dedicated-function bus peripherals. Support
for these messages is not required in order to conform to RFC 5905
[RFC5905]. The control messages are described here as a current
reference for use with an RFC 5905 implementation of NTP.
The publication of this document is not meant to encourage the
development and deployment of these control messages. This document
is only providing a current reference for these control messages
given the current status of RFC 1305.
1.1. Control Message Overview
The NTP Mode 6 control messages are used by NTP management programs
(e.g., ntpq) when a more robust network management facility (e.g.,
SNMP) is not available. These control messages provide rudimentary
control and monitoring functions to manage a running instance of an
NTP server. These commands are not designed to be used for
communication between instances of running NTP servers.
The NTP Control Message has the value 6 specified in the mode field
of the first octet of the NTP header and is formatted as shown in
Figure 1. The format of the data field is specific to each command
or response; however, in most cases the format is designed to be
constructed and viewed by humans and so is coded in free-form ASCII.
This facilitates the specification and implementation of simple
management tools in the absence of fully evolved network-management
facilities. As in ordinary NTP messages, the authenticator field
follows the data field. If the authenticator is used the data field
is zero-padded to a 32-bit boundary, but the padding bits are not
considered part of the data field and are not included in the field
count.
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IP hosts are not required to reassemble datagrams over a certain size
(576 octets for IPv4 [RFC0791] and 1280 octets for IPv6 [RFC2460]);
however, some commands or responses may involve more data than will
fit into a single datagram. Accordingly, a simple reassembly feature
is included in which each octet of the message data is numbered
starting with zero. As each fragment is transmitted the number of
its first octet is inserted in the offset field and the number of
octets is inserted in the count field. The more-data (M) bit is set
in all fragments except the last.
Most control functions involve sending a command and receiving a
response, perhaps involving several fragments. The sender chooses a
distinct, nonzero sequence number and sets the status field and "R"
and "E" bits to zero. The responder interprets the opcode and
additional information in the data field, updates the status field,
sets the "R" bit to one and returns the three 32-bit words of the
header along with additional information in the data field. In case
of invalid message format or contents the responder inserts a code in
the status field, sets the "R" and "E" bits to one and, optionally,
inserts a diagnostic message in the data field.
Some commands read or write system variables (e.g., s.offset) and
peer variables (e.g., p.stratum) for an association identified in the
command. Others read or write variables associated with a radio
clock or other device directly connected to a source of primary
synchronization information. To identify which type of variable and
association the Association ID is used. System variables are
indicated by the identifier zero. As each association is mobilized a
unique, nonzero identifier is created for it. These identifiers are
used in a cyclic fashion, so that the chance of using an old
identifier which matches a newly created association is remote. A
management entity can request a list of current identifiers and
subsequently use them to read and write variables for each
association. An attempt to use an expired identifier results in an
exception response, following which the list can be requested again.
Some exception events, such as when a peer becomes reachable or
unreachable, occur spontaneously and are not necessarily associated
with a command. An implementation may elect to save the event
information for later retrieval or to send an asynchronous response
(called a trap) or both. In case of a trap the IP address and port
number is determined by a previous command and the sequence field is
set as described below. Current status and summary information for
the latest exception event is returned in all normal responses. Bits
in the status field indicate whether an exception has occurred since
the last response and whether more than one exception has occurred.
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Commands need not necessarily be sent by an NTP peer, so ordinary
access-control procedures may not apply; however, the optional mask/
match mechanism suggested in Section Section 6 elsewhere in this
document provides the capability to control access by mode number, so
this could be used to limit access for control messages (mode 6) to
selected address ranges.
1.2. Remote Facility Message Overview
The original development of the NTP daemon included a remote facility
for monitoring and configuration. This facility used mode 7 commands
to communicate with the NTP daemon. This document illustrates the
mode 7 packet format only. The commands embedded in the mode 7
messages are implementation specific and not standardized in any way.
The mode 7 message format is described in Appendix A.
2. NTP Control Message Format
The format of the NTP Control Message header, which immediately
follows the UDP header, is shown in Figure 1. Following is a
description of its fields.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|LI | VN |Mode |R|E|M| OpCode | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset | Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Data (up to 468 bytes) /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Authenticator (optional, 20 or 24 bits) /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: NTP Control Message Header
Leap Indicator (LI): This is a two-bit integer that is set to b00 for
control message requests and responses. The Leap Indicator value
used at this position in most NTP modes is in the System Status Word
provided in some control message responses.
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Version Number (VN): This is a three-bit integer indicating a minimum
NTP version number. NTP servers do not respond to control messages
with an unrecognized version number. Requests may intentionally use
a lower version number to enable interoperability with earlier
versions of NTP. Responses carry the same version as the
corresponding request.
Mode: This is a three-bit integer indicating the mode. The value 6
indicates an NTP control message.
Response Bit (R): Set to zero for commands, one for responses.
Error Bit (E): Set to zero for normal response, one for error
response.
More Bit (M): Set to zero for last fragment, one for all others.
Operation Code (OpCode): This is a five-bit integer specifying the
command function. Values currently defined include the following:
+-------+--------------------------------------------------+
| Code | Meaning |
+-------+--------------------------------------------------+
| 0 | reserved |
| 1 | read status command/response |
| 2 | read variables command/response |
| 3 | write variables command/response |
| 4 | read clock variables command/response |
| 5 | write clock variables command/response |
| 6 | set trap address/port command/response |
| 7 | trap response |
| 8 | runtime configuration command/response |
| 9 | export configuration to file command/response |
| 10 | retrieve remote address stats command/response |
| 11 | retrieve ordered list command/response |
| 12 | request client-specific nonce command/response |
| 13-30 | reserved |
| 31 | unset trap address/port command/response |
+-------+--------------------------------------------------+
Sequence Number: This is a 16-bit integer indicating the sequence
number of the command or response. Each request uses a different
sequence number. Each response carries the same sequence number as
its corresponding request. For asynchronous trap responses, the
responder increments the sequence number by one for each response,
allowing trap receivers to detect missing trap responses. The
sequence number of each fragment of a multiple-datagram response
carries the same sequence number, copied from the request.
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Status: This is a 16-bit code indicating the current status of the
system, peer or clock, with values coded as described in following
sections.
Association ID: This is a 16-bit unsigned integer identifying a valid
association, or zero for the system clock.
Offset: This is a 16-bit unsigned integer indicating the offset, in
octets, of the first octet in the data area. The offset is set to
zero in requests. Responses spanning multiple datagrams use a
positive offset in all but the first datagram.
Count: This is a 16-bit unsigned integer indicating the length of the
data field, in octets.
Data: This contains the message data for the command or response.
The maximum number of data octets is 468.
Padding (optional): Contains zero to three octets with value zero, as
needed to ensure the overall control message size is a multiple of 4
octets.
Authenticator (optional): When the NTP authentication mechanism is
implemented, this contains the authenticator information defined in
Appendix C of [RFC1305].
3. Status Words
Status words indicate the present status of the system, associations
and clock. They are designed to be interpreted by network-monitoring
programs and are in one of four 16-bit formats shown in Figure 2 and
described in this section. System and peer status words are
associated with responses for all commands except the read clock
variables, write clock variables and set trap address/port commands.
The association identifier zero specifies the system status word,
while a nonzero identifier specifies a particular peer association.
The status word returned in response to read clock variables and
write clock variables commands indicates the state of the clock
hardware and decoding software. A special error status word is used
to report malformed command fields or invalid values.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LI| Clock Src | Count | Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
System Status Word
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | SEL | Count | Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Peer Status Word
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Clock Status | Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Radio Status Word
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Status Word
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Count | Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Clock Status Word
Figure 2: Status Word Formats
3.1. System Status Word
The system status word appears in the status field of the response to
a read status or read variables command with a zero association
identifier. The format of the system status word is as follows:
Leap Indicator (LI): This is a two-bit code warning of an impending
leap second to be inserted/deleted in the last minute of the current
day, with bit 0 and bit 1, respectively, coded as follows:
+------+------------------------------------------------------------+
| LI | Meaning |
+------+------------------------------------------------------------+
| 00 | no warning |
| 01 | insert second after 23:59:59 of the current day |
| 10 | delete second 23:59:59 of the current day |
| 11 | unsynchronized |
+------+------------------------------------------------------------+
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Clock Source (Clock Src): This is a six-bit integer indicating the
current synchronization source, with values coded as follows:
+-------+-----------------------------------------------------------+
| Code | Meaning |
+-------+-----------------------------------------------------------+
| 0 | unspecified or unknown |
| 1 | Calibrated atomic clock (e.g., PPS, HP 5061) |
| 2 | VLF (band 4) or LF (band 5) radio (e.g., OMEGA,, WWVB) |
| 3 | HF (band 7) radio (e.g., CHU, MSF, WWV/H) |
| 4 | UHF (band 9) satellite (e.g., GOES, GPS) |
| 5 | local net (e.g., DCN, TSP, DTS) |
| 6 | UDP/NTP |
| 7 | UDP/TIME |
| 8 | eyeball-and-wristwatch |
| 9 | telephone modem (e.g., NIST) |
| 10-63 | reserved |
+-------+-----------------------------------------------------------+
System Event Counter (Count): This is a four-bit integer indicating
the number of system events occurring since the last time the System
Event Code changed. Upon reaching 15, subsequent events with the
same code are not counted.
System Event Code (Code): This is a four-bit integer identifying the
latest system exception event, with new values overwriting previous
values, and coded as follows:
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+------+---------------------------------------------------------+
| Code | Meaning |
+------+---------------------------------------------------------+
| 0 | unspecified |
| 1 | frequency correction (drift) file not available |
| 2 | frequency correction started (frequency stepped) |
| 3 | spike detected and ignored, starting stepout timer |
| 4 | frequency training started |
| 5 | clock synchronized |
| 6 | system restart |
| 7 | panic stop (required step greater than panic threshold) |
| 8 | no system peer |
| 9 | leap second insertion/deletion armed for the |
| | of the current month |
| 10 | leap second disarmed |
| 11 | leap second inserted or deleted |
| 12 | clock stepped (stepout timer expired) |
| 13 | kernel loop discipline status changed |
| 14 | leapseconds table loaded from file |
| 15 | leapseconds table outdated, updated file needed |
+------+---------------------------------------------------------+
3.2. Peer Status Word
A peer status word is returned in the status field of a response to a
read status, read variables or write variables command and appears
also in the list of association identifiers and status words returned
by a read status command with a zero association identifier. The
format of a peer status word is as follows:
Peer Status (Status): This is a five-bit code indicating the status
of the peer determined by the packet procedure, with bits assigned as
follows:
+-------------+---------------------------------------------------+
| Peer Status | Meaning |
| bit | |
+-------------+---------------------------------------------------+
| 0 | configured (peer.config) |
| 1 | authentication enabled (peer.authenable) |
| 2 | authentication okay (peer.authentic) |
| 3 | reachability okay (peer.reach != 0) |
| 4 | broadcast association |
+-------------+---------------------------------------------------+
Peer Selection (SEL): This is a three-bit integer indicating the
status of the peer determined by the clock-selection procedure, with
values coded as follows:
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+-----+-------------------------------------------------------------+
| Sel | Meaning |
+-----+-------------------------------------------------------------+
| 0 | rejected |
| 1 | discarded by intersection algorithm |
| 2 | discarded by table overflow (not currently used) |
| 3 | discarded by the cluster algorithm |
| 4 | included by the combine algorithm |
| 5 | backup source (with more than sys.maxclock survivors) |
| 6 | system peer (synchronization source) |
| 7 | PPS (pulse per second) peer |
+-----+-------------------------------------------------------------+
Peer Event Counter (Count): This is a four-bit integer indicating the
number of peer exception events that occurred since the last time the
peer event code changed. Upon reaching 15, subsequent events with
the same code are not counted.
Peer Event Code (Code): This is a four-bit integer identifying the
latest peer exception event, with new values overwriting previous
values, and coded as follows:
+-------+--------------------------------------------------------+
| Peer | |
| Event | Meaning |
| Code | |
+-------+--------------------------------------------------------+
| 0 | unspecified |
| 1 | association mobilized |
| 2 | association demobilized |
| 3 | peer unreachable (peer.reach was nonzero now zero) |
| 4 | peer reachable (peer.reach was zero now nonzero) |
| 5 | association restarted or timed out |
| 6 | no reply (only used with one-shot clock set command) |
| 7 | peer rate limit exceeded (kiss code RATE received) |
| 8 | access denied (kiss code DENY received) |
| 9 | leap second insertion/deletion at month's end armed |
| | by peer vote |
| 10 | became system peer (sys.peer) |
| 11 | reference clock event (see clock status word) |
| 12 | authentication failed |
| 13 | popcorn spike suppressed by peer clock filter register |
| 14 | entering interleaved mode |
| 15 | recovered from interleave error |
+-------+--------------------------------------------------------+
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3.3. Clock Status Word
There are two ways a reference clock can be attached to a NTP service
host, as a dedicated device managed by the operating system and as a
synthetic peer managed by NTP. As in the read status command, the
association identifier is used to identify which one, zero for the
system clock and nonzero for a peer clock. Only one system clock is
supported by the protocol, although many peer clocks can be
supported. A system or peer clock status word appears in the status
field of the response to a read clock variables or write clock
variables command. This word can be considered an extension of the
system status word or the peer status word as appropriate. The
format of the clock status word is as follows:
Reserved: An eight-bit integer that is ignored by requesters and
zeroed by responders.
Count: This is a four-bit integer indicating the number of clock
events that occurred since the last time the clock event code
changed. Upon reaching 15, subsequent events with the same code are
not counted.
Clock Code (Code): This is a four-bit integer indicating the current
clock status, with values coded as follows:
+--------------+--------------------------------------------------+
| Clock Status | Meaning |
+--------------+--------------------------------------------------+
| 0 | clock operating within nominals |
| 1 | reply timeout |
| 2 | bad reply format |
| 3 | hardware or software fault |
| 4 | propagation failure |
| 5 | bad date format or value |
| 6 | bad time format or value |
| 7-15 | reserved |
+--------------+--------------------------------------------------+
3.4. Error Status Word
An error status word is returned in the status field of an error
response as the result of invalid message format or contents. Its
presence is indicated when the E (error) bit is set along with the
response (R) bit in the response. It consists of an eight-bit
integer coded as follows:
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+--------------+--------------------------------------------------+
| Error Status | Meaning |
+--------------+--------------------------------------------------+
| 0 | unspecified |
| 1 | authentication failure |
| 2 | invalid message length or format |
| 3 | invalid opcode |
| 4 | unknown association identifier |
| 5 | unknown variable name |
| 6 | invalid variable value |
| 7 | administratively prohibited |
| 8-255 | reserved |
+--------------+--------------------------------------------------+
4. Commands
Commands consist of the header and optional data field shown in
Figure 1. When present, the data field contains a list of
identifiers or assignments in the form
<<identifier>>[=<<value>>],<<identifier>>[=<<value>>],... where
<<identifier>> is the ASCII name of a system or peer variable such as
the ones specified in RFC 5905 and <<value>> is expressed as a
decimal, hexadecimal or string constant in the syntax of the C
programming language. Where no ambiguity exists, the "sys." or
"peer." prefixes can be suppressed. Whitespace (ASCII nonprinting
format effectors) can be added to improve readability for simple
monitoring programs that do not reformat the data field. Internet
addresses are represented as follows: IPv4 addresses are written in
the form [n.n.n.n], where n is in decimal notation and the brackets
are optional; IPv6 addresses are formulated based on the guidelines
defined in [RFC5952]. Timestamps, including reference, originate,
receive and transmit values, as well as the logical clock, are
represented in units of seconds and fractions, preferably in
hexadecimal notation. Delay, offset, dispersion and distance values
are represented in units of milliseconds and fractions, preferably in
decimal notation. All other values are represented as-is, preferably
in decimal notation.
Implementations may define variables other than those described in
RFC 5905. Called extramural variables, these are distinguished by
the inclusion of some character type other than alphanumeric or "."
in the name. For those commands that return a list of assignments in
the response data field, if the command data field is empty, it is
expected that all available variables defined in RFC 5905 will be
included in the response. For the read commands, if the command data
field is nonempty, an implementation may choose to process this field
to individually select which variables are to be returned.
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Commands are interpreted as follows:
Read Status (1): The command data field is empty or contains a list
of identifiers separated by commas. The command operates in two ways
depending on the value of the association identifier. If this
identifier is nonzero, the response includes the peer identifier and
status word. Optionally, the response data field may contain other
information, such as described in the Read Variables command. If the
association identifier is zero, the response includes the system
identifier (0) and status word, while the data field contains a list
of binary-coded pairs <<association identifier>> <<status word>>, one
for each currently defined association.
Read Variables (2): The command data field is empty or contains a
list of identifiers separated by commas. If the association
identifier is nonzero, the response includes the requested peer
identifier and status word, while the data field contains a list of
peer variables and values as described above. If the association
identifier is zero, the data field contains a list of system
variables. If a peer has been selected as the synchronization
source, the response includes the peer identifier and status word;
otherwise, the response includes the system identifier (0) and status
word.
Write Variables (3): The command data field contains a list of
assignments as described above. The variables are updated as
indicated. The response is as described for the Read Variables
command.
Read Clock Variables (4): The command data field is empty or contains
a list of identifiers separated by commas. The association
identifier selects the system clock variables or peer clock variables
in the same way as in the Read Variables command. The response
includes the requested clock identifier and status word and the data
field contains a list of clock variables and values, including the
last timecode message received from the clock.
Write Clock Variables (5): The command data field contains a list of
assignments as described above. The clock variables are updated as
indicated. The response is as described for the Read Clock Variables
command.
Set Trap Address/Port (6): The command association identifier, status
and data fields are ignored. The address and port number for
subsequent trap messages are taken from the source address and port
of the control message itself. The initial trap counter for trap
response messages is taken from the sequence field of the command.
The response association identifier, status and data fields are not
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significant. Implementations should include sanity timeouts which
prevent trap transmissions if the monitoring program does not renew
this information after a lengthy interval.
Trap Response (7): This message is sent when a system, peer or clock
exception event occurs. The opcode field is 7 and the R bit is set.
The trap counter is incremented by one for each trap sent and the
sequence field set to that value. The trap message is sent using the
IP address and port fields established by the set trap address/port
command. If a system trap the association identifier field is set to
zero and the status field contains the system status word. If a peer
trap the association identifier field is set to that peer and the
status field contains the peer status word. Optional ASCII-coded
information can be included in the data field.
Configure (8): The command data is parsed and applied as if supplied
in the daemon configuration file.
Save Configuration (9): Write a snapshot of the current configuration
to the file name supplied as the command data. Further, the command
is refused unless a directory in which to store the resulting files
has been explicitly configured by the operator.
Read Most Recently Used (MRU) list (10): Retrieves records of
recently seen remote addresses and associated statistics. This
command supports all of the state variables defined in Section 9 of
[RFC5905]. Command data consists of name=value pairs controlling the
selection of records, as well as a requestor-specific nonce
previously retrieved using this command or opcode 12, Request Nonce.
The response consists of name=value pairs where some names can appear
multiple times using a dot followed by a zero-based index to
distinguish them, and to associate elements of the same record with
the same index. A new nonce is provided with each successful
response.
Read ordered list (11): Retrieves a list ordered by IP address (IPv4
information precedes IPv6 information). If the command data is empty
or the seven characters "ifstats", the associated statistics, status
and counters for each local address are returned. If the command
data is the characters "addr_restrictions" then the set of IPv4
remote address restrictions followed by the set of IPv6 remote
address restrictions (access control lists) are returned. Other
command data returns error code 5 (unknown variable name). Similar
to Read MRU, response information uses zero-based indexes as part of
the variable name preceding the equals sign and value, where each
index relates information for a single address or network. This
opcode requires authentication.
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Request Nonce (12): Retrieves a 96-bit nonce specific to the
requesting remote address, which is valid for a limited period.
Command data is not used in the request. The nonce consists of a
64-bit NTP timestamp and 32 bits of hash derived from that timestamp,
the remote address, and salt known only to the server which varies
between daemon runs. Inclusion of the nonce by a management agent
demonstrates to the server that the agent can receive datagrams sent
to the source address of the request, making source address
"spoofing" more difficult in a similar way as TCP's three-way
handshake.
Unset Trap (31): Removes the requesting remote address and port from
the list of trap receivers. Command data is not used in the request.
If the address and port are not in the list of trap receivers, the
error code is 4, bad association.
5. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
6. Security Considerations
A number of security vulnerabilities have been identified with these
control messages.
NTP's control query interface allows reading and writing of system,
peer, and clock variables remotely from arbitrary IP addresses using
commands mentioned in Section 4. Traditionally, overwriting these
variables, but not reading them, requires authentication by default.
However, this document argues that an NTP host must authenticate all
control queries and not just ones that overwrite these variables.
Alternatively, the host can use an access control list to explicitly
list IP addresses that are allowed to control query the clients.
These access controls are required for the following reasons:
* NTP as a Distributed Denial-of-Service (DDoS) vector. NTP timing
query and response packets (modes 1-2, 3-4, 5) are usually short
in size. However, some NTP control queries generate a very long
packet in response to a short query. As such, there is a history
of use of NTP's control queries, which exhibit such behavior, to
perform DDoS attacks. These off-path attacks exploit the large
size of NTP control queries to cause UDP-based amplification
attacks (e.g., mode 7 monlist command generates a very long packet
in response to a small query [CVE-DOS]). These attacks only use
NTP as a vector for DoS attacks on other protocols, but do not
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affect the time service on the NTP host itself. To limit the
sources of these malicious commands, NTP server operators are
recommended to deploy ingress filtering [RFC3704].
* Time-shifting attacks through information leakage/overwriting.
NTP hosts save important system and peer state variables. An off-
path attacker who can read these variables remotely can leverage
the information leaked by these control queries to perform time-
shifting and DoS attacks on NTP clients. These attacks do affect
time synchronization on the NTP hosts. For instance,
- In the client/server mode, the client stores its local time
when it sends the query to the server in its xmt peer variable.
This variable is used to perform TEST2 to non-cryptographically
authenticate the server, i.e., if the origin timestamp field in
the corresponding server response packet matches the xmt peer
variable, then the client accepts the packet. An off-path
attacker, with the ability to read this variable can easily
spoof server response packets for the client, which will pass
TEST2, and can deny service or shift time on the NTP client.
The specific attack is described in [CVE-SPOOF].
- The client also stores its local time when the server response
is received in its rec peer variable. This variable is used
for authentication in interleaved-pivot mode. An off-path
attacker with the ability to read this state variable can
easily shift time on the client by passing this test. This
attack is described in [CVE-SHIFT].
* Fast-Scanning. NTP mode 6 control messages are usually small UDP
packets. Fast-scanning tools like ZMap can be used to spray the
entire (potentially reachable) Internet with these messages within
hours to identify vulnerable hosts. To make things worse, these
attacks can be extremely low-rate, only requiring a control query
for reconnaissance and a spoofed response to shift time on
vulnerable clients.
* The mode 6 and 7 messages are vulnerable to replay attacks
[CVE-Replay]. If an attacker observes mode 6/7 packets that
modify the configuration of the server in any way, the attacker
can apply the same change at any time later simply by sending the
packets to the server again. The use of the nonce (Request Nonce
command) provides limited protection against replay attacks.
NTP best practices recommend configuring NTP with the no-query
parameter. The no-query parameter blocks access to all remote
control queries. However, sometimes the hosts do not want to block
all queries and want to give access for certain control queries
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remotely. This could be for the purpose of remote management and
configuration of the hosts in certain scenarios. Such hosts tend to
use firewalls or other middleboxes to blacklist certain queries
within the network.
Significantly fewer hosts respond to mode 7 monlist queries as
compared to other control queries because it is a well-known and
exploited control query. These queries are likely blocked using
blacklists on firewalls and middleboxes rather than the no-query
option on NTP hosts. The remaining control queries that can be
exploited likely remain out of the blacklist because they are
undocumented in the current NTP specification [RFC5905].
This document describes all of the mode 6 control queries allowed by
NTP and can help administrators make informed decisions on security
measures to protect NTP devices from harmful queries and likely make
those systems less vulnerable. The use of the legacy mode 6
interface is NOT RECOMMENDED.Regardless of which mode 6 commands an
administrator may elect to allow, remote access to this facility
needs to be protected from unauthorized access (e.g., strict ACLs).
Additionally, the legacy interface for mode 6 commands SHOULD NOT be
utilized in new deployments or implementation of NTP.
7. Contributors
Dr. David Mills specified the vast majority of the mode 6 commands
during the development of RFC 1305 [RFC1305] and deserves the credit
for their existence and use.
8. Acknowledgements
Tim Plunkett created the original version of this document. Aanchal
Malhotra provided the initial version of the Security Considerations
section.
Karen O'Donoghue, David Hart, Harlan Stenn, and Philip Chimento
deserve credit for portions of this document due to their earlier
efforts to document these commands.
Miroshav Lichvar, Ulrich Windl, Dieter Sibold, J Ignacio Alvarez-
Hamelin, and Alex Campbell provided valuable comments on various
versions of this document.
9. References
9.1. Normative References
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[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation and Analysis", RFC 1305,
DOI 10.17487/RFC1305, March 1992,
<https://www.rfc-editor.org/info/rfc1305>.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <https://www.rfc-editor.org/info/rfc3704>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
9.2. Informative References
[CVE-DOS] NIST National Vulnerability Database, "CVE-2013-5211,
https://nvd.nist.gov/vuln/detail/CVE-2013-5211", 2 January
2014.
[CVE-Replay]
NIST National Vulnerability Database, "CVE-2015-8140,
https://nvd.nist.gov/vuln/detail/CVE-2015-8140", 30
January 2015.
[CVE-SHIFT]
NIST National Vulnerability Database, "CVE-2016-1548,
https://nvd.nist.gov/vuln/detail/CVE-2016-1548", 6 January
2017.
[CVE-SPOOF]
NIST National Vulnerability Database, "CVE-2015-8139,
https://nvd.nist.gov/vuln/detail/CVE-2015-8139", 30
January 2017.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
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Appendix A. NTP Remote Facility Message Format
The format of the NTP Remote Facility Message header, which
immediately follows the UDP header, is shown in Figure 3. Following
is a description of its fields. Bit positions marked as zero are
reserved and should always be transmitted as zero.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|M| VN |Mode |A| Sequence | Implementation| Req Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Err | Count | MBZ | Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Data (up to 500 bytes) /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encryption KeyID (when A bit set) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Message Authentication Code (when A bit set) /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: NTP Remote Facility Message Header
Response Bit (R) : Set to 0 if the packet is a request. Set to 1 if
the packet is a response.
More Bit (M) : Set to 0 if this is the last packet in a response,
otherwise set to 1 in responses requiring more than one packet.
Version Number (VN) : Set to the version number of the NTP daemon.
Mode : Set to 7 for Remote Facility messages.
Authenticated Bit (A) : If set to 1, this packet contains
authentication information.
Sequence : For a multi-packet response, this field contains the
sequence number of this packet. Packets in a multi-packet response
are numbered starting with 0. The More Bit is set to 1 for all
packets but the last.
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Implementation : The version number of the implementation that
defined the request code used in this message. An implementation
number of 0 is used for a Request Code supported by all versions of
the NTP daemon. The value 255 is reserved for future extensions.
Request Code (Req Code) : An implementation-specific code which
specifies the operation being requested. A Request Code definition
includes the format and semantics of the data included in the packet.
Error (Err) : Set to 0 for a request. For a response, this field
contains an error code relating to the request. If the Error is non-
zero, the operation requested wasn't performed.
0 - no error
1 - incompatible implementation number
2 - unimplemented request code
3 - format error
4 - no data available
7 - authentication failure
Count : The number of data items in the packet. Range is 0 to 500.
Must Be Zero (MBZ) : A reserved field set to 0 in requests and
responses.
Size : The size of each data item in the packet. Range is 0 to 500.
Data : A variable-sized field containing request/response data. For
requests and responses, the size in octets must be greater than or
equal to the product of the number of data items (Count) and the size
of a data item (Size). For requests, the data area is exactly 40
octets in length. For responses, the data area will range from 0 to
500 octets, inclusive.
Encryption KeyID : A 32-bit unsigned integer used to designate the
key used for the Message Authentication Code. This field is included
only when the A bit is set to 1.
Message Authentication Code : An optional Message Authentication Code
defined by the version of the NTP daemon indicated in the
Implementation field. This field is included only when the A bit is
set to 1.
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Author's Address
Brian Haberman (editor)
JHU
Email: brian@innovationslab.net
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