Network Time Protocol Version 4: Port RandomizationSI6 NetworksEvaristo Carriego 2644Haedo, Provincia de Buenos Aires1706Argentina+54 11 4650 8472fgont@si6networks.comhttps://www.si6networks.comSI6 NetworksEvaristo Carriego 2644Haedo, Provincia de Buenos Aires1706Argentina+54 11 4650 8472ggont@si6networks.comhttps://www.si6networks.comRed HatPurkynova 115Brno612 00Czech Republicmlichvar@redhat.comNetwork Time Protocol (ntp) Working Groupsecuritytransport protocols
The Network Time Protocol (NTP) can operate in several modes. Some of these
modes are based on the receipt of unsolicited packets and therefore
require the use of a well-known port as the local port. However, in
the case of NTP modes where the use of a well-known port is not required,
employing such a well-known port unnecessarily facilitates the ability of
attackers to perform blind/off-path attacks. This document formally
updates RFC 5905, recommending the use of transport-protocol ephemeral port
randomization for those modes where use of the NTP well-known port is not
required.Introduction
The Network Time Protocol (NTP) is one of the oldest Internet
protocols and is currently specified in . Since its original
implementation, standardization, and deployment, a number of
vulnerabilities have been found both in the NTP specification and in
some of its implementations . Some of these
vulnerabilities allow for blind/off-path attacks, where an attacker
can send forged packets to one or both NTP peers to achieve Denial
of Service (DoS), time shifts, or other undesirable outcomes. Many
of these attacks require the attacker to guess or know at least a
target NTP association, typically identified by the tuple {srcaddr,
srcport, dstaddr, dstport, keyid} (see ).
Some of these parameters may be known or easily guessed.
NTP can operate in several modes. Some of these modes rely on the
ability of nodes to receive unsolicited packets and therefore
require the use of the NTP well-known port (123). However, for modes
where the use of a well-known port is not required, employing the
NTP well-known port unnecessarily facilitates the ability of attackers
to perform blind/off-path attacks (since knowledge of the port
numbers is typically required for such attacks). A recent study
that analyzes the port numbers employed by NTP clients
suggests that numerous NTP clients employ the NTP well-known port as their local port, or select predictable ephemeral
port numbers, thus unnecessarily facilitating the ability of
attackers to perform blind/off-path attacks against NTP.
BCP 156 already recommends the randomization of transport-protocol ephemeral ports. This document aligns NTP with the
recommendation in BCP 156 by formally updating
such that port randomization is employed for those NTP modes for
which the use of the NTP well-known port is not needed.Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 when, and only when, they appear in all
capitals, as shown here.Considerations about Port Randomization in NTP
The following subsections analyze a number of considerations about
transport-protocol ephemeral port randomization when applied to NTP.Mitigation against Off-Path Attacks
There has been a fair share of work in the area of blind/off-path
attacks against transport protocols and upper-layer protocols, such
as and . Whether the target of the attack is a
transport-protocol instance (e.g., TCP connection) or an upper-layer
protocol instance (e.g., an application-protocol instance), the
attacker is required to know or guess the five-tuple {Protocol, IP
Source Address, IP Destination Address, Source Port, Destination
Port} that identifies the target transport-protocol instance or the
transport-protocol instance employed by the target upper-layer
protocol instance. Therefore, increasing the difficulty of guessing
this five-tuple helps mitigate blind/off-path attacks.
As a result of these considerations, transport-protocol ephemeral
port randomization is a best current practice (BCP 156) that helps
mitigate off-path attacks at the transport layer. This document
aligns the NTP specification with the existing best current
practice on transport-protocol ephemeral port selection, irrespective of other
techniques that may (and should) be implemented for mitigating off-path attacks.
We note that transport-protocol ephemeral port randomization is a
transport-layer mitigation against blind/off-path attacks and does
not preclude (nor is it precluded by) other possible mitigations for
off-path attacks that might be implemented at other layers (e.g.,
). For instance, some of the
aforementioned mitigations may be ineffective against some off-path
attacks or may benefit from the additional entropy
provided by port randomization .Effects on Path Selection
Intermediate systems implementing the Equal-Cost Multipath (ECMP)
algorithm may select the outgoing link by computing a hash over a
number of values, including the transport-protocol source port.
Thus, as discussed in , the selected client port may have
an influence on the measured offset and delay.
If the source port is changed with each request, packets in different
exchanges will be more likely to take different paths, which could
cause the measurements to be less stable and have a negative impact
on the stability of the clock.
Network paths to/from a given server are less likely to change between
requests if port randomization is applied on a per-association basis. This
approach minimizes the impact on the stability of NTP measurements,
but it may cause different clients in the same network synchronized to the
same NTP server to have a significant stable offset between their clocks.
This is due to their NTP exchanges consistently taking different paths with
different asymmetry in the network delay. recommends that NTP implementations randomize the ephemeral
port number of client/server associations. The choice of whether to
randomize the port number on a per-association or a per-request basis
is left to the implementation.Filtering of NTP Traffic
In a number of scenarios (such as when mitigating DDoS attacks), a
network operator may want to differentiate between NTP requests sent
by clients and NTP responses sent by NTP servers. If an
implementation employs the NTP well-known port for the client port, requests/responses cannot be readily differentiated by
inspecting the source and destination port numbers.
Implementation of port randomization for nonsymmetrical modes allows for
simple differentiation of NTP requests and responses and for the
enforcement of security policies that may be valuable for the mitigation of
DDoS attacks, when all NTP clients in a given network employ port randomization.Effect on NAPT Devices
Some NAPT devices will reportedly not translate the source port of a
packet when a system port number (i.e., a port number in the range
0-1023) is employed. In networks where such NAPT devices
are employed, use of the NTP well-known port for the client port may
limit the number of hosts that may successfully employ NTP client
implementations at any given time.
In the case of NAPT devices that will translate the source port even
when a system port is employed, packets reaching the external realm
of the NAPT will not employ the NTP well-known port as the source
port, as a result of the port translation function being performed by the
NAPT device.Update to RFC 5905
The following text from Section
Peer
Process Variables of :
dstport:
UDP port number of the client, ordinarily the NTP port
number PORT (123) assigned by the IANA. This becomes the source
port number in packets sent from this association.
is replaced with:
dstport:
UDP port number of the client. In the case of broadcast
server mode (5) and symmetric modes (1 and 2), it SHOULD contain
the NTP port number PORT (123) assigned by IANA. In the
client mode (3), it SHOULD contain a randomized port number, as
specified in . The value in this variable becomes the
source port number of packets sent from this association. The
randomized port number SHOULD NOT be shared with other
associations, to avoid revealing the randomized port to other
associations.
If a client implementation performs transport-protocol ephemeral port randomization
on a per-request basis, it SHOULD close the corresponding socket/port
after each request/response exchange. In order to prevent duplicate
or delayed server packets from eliciting ICMP port unreachable error
messages at the client, the client MAY wait for more responses from
the server for a specific period of time (e.g., 3 seconds) before
closing the UDP socket/port.
NOTES:Randomizing the ephemeral port number on a per-request basis
will better mitigate blind/off-path attacks, particularly if
the socket/port is closed after each request/response exchange,
as recommended above. The choice of whether to randomize the
ephemeral port number on a per-request or a per-association
basis is left to the implementation, and it should consider the
possible effects on path selection along with its possible
impact on time measurement.
On most current operating systems, which implement ephemeral
port randomization , an NTP client may normally rely
on the operating system to perform ephemeral port
randomization. For example, NTP implementations using POSIX
sockets may achieve ephemeral port randomization by not
binding the socket with the bind() function or binding it to
port 0, which has a special meaning of "any port". Using the connect() function for the socket will make the port inaccessible
by other systems (that is, only packets from the specified remote socket will be
received by the application).
IANA Considerations
This document has no IANA actions.Security Considerations
The security implications of predictable numeric identifiers
(and of predictable
transport-protocol port numbers in particular) have been
known for a long time now. However, the NTP specification has
traditionally followed a pattern of employing common settings even
when not strictly necessary, which at times has resulted in negative
security and privacy implications (see, e.g.,
). The use of the NTP well-known
port (123) for the srcport and dstport variables is not required for
all operating modes. Such unnecessary usage comes at the expense of
reducing the amount of work required for an attacker to successfully
perform blind/off-path attacks against NTP. Therefore, this document
formally updates , recommending the use of transport-protocol port randomization when use of the NTP well-known port is
not required.
This issue has been assigned CVE-2019-11331 in the U.S.
National Vulnerability Database (NVD).ReferencesNormative ReferencesInformative ReferencesNTP Client Data MinimizationUsage Analysis of the NIST Internet Time ServiceJournal of Research of the National Institute of Standards and Technology, Volume 121Challenges in Time Transfer using the Network Time Protocol (NTP)Proceedings of the 48th Annual Precise Time and Time Interval Systems and Applications Meeting, pp. 271-290Attacking the Network Time ProtocolNDSS '16The Security of NTP's Datagram ProtocolCryptology ePrint Archive Report 2016/1006Network Time FoundationCVE-2019-1133The MITRE CorporationNational Vulnerability DatabaseAcknowledgments
The authors would like to thank (in alphabetical order) ,
, , , , ,
, , , , , , , , ,
, , , ,
, , , , , and for providing valuable comments on earlier draft versions of this document. raised the problem of DDoS mitigation when the NTP well-known
port is employed as the client port (discussed in of this document).
The authors would like to thank for answering questions
about a popular NTP implementation (see ). would like to thank and for
their love and support.