RFC 9109: Network Time Protocol Version 4: Port Randomization

Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9109.¶

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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1. Introduction

The Network Time Protocol (NTP) is one of the oldest Internet protocols and is currently specified in [RFC5905]. 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 [NTP-VULN]. 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 Section 9.1 of [RFC5905]). 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 [NIST-NTP] 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 [RFC6056] already recommends the randomization of transport-protocol ephemeral ports. This document aligns NTP with the recommendation in BCP 156 [RFC6056] by formally updating [RFC5905] such that port randomization is employed for those NTP modes for which the use of the NTP well-known port is not needed.¶

2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Considerations about Port Randomization in NTP

The following subsections analyze a number of considerations about transport-protocol ephemeral port randomization when applied to NTP.¶

4. Update to RFC 5905

The following text from Section 9.1 (Peer Process Variables) of [RFC5905]:¶

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 [RFC6056]. 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 [RFC0792] [RFC4443] 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 [RFC6056], 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).¶

5. IANA Considerations

This document has no IANA actions.¶

6. Security Considerations

The security implications of predictable numeric identifiers [PEARG-NUMERIC-IDS] (and of predictable transport-protocol port numbers [RFC6056] 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., [NTP-DATA-MINIMIZATION]). 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 [RFC5905], 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 [VULN-REPORT] in the U.S. National Vulnerability Database (NVD).¶

Acknowledgments

The authors would like to thank (in alphabetical order) Ivan Arce, Roman Danyliw, Dhruv Dhody, Lars Eggert, Todd Glassey, Blake Hudson, Benjamin Kaduk, Erik Kline, Watson Ladd, Aanchal Malhotra, Danny Mayer, Gary E. Miller, Bjorn Mork, Hal Murray, Francesca Palombini, Tomoyuki Sahara, Zaheduzzaman Sarker, Dieter Sibold, Steven Sommars, Jean St-Laurent, Kristof Teichel, Brian Trammell, Éric Vyncke, Ulrich Windl, and Dan Wing for providing valuable comments on earlier draft versions of this document.¶

Watson Ladd raised the problem of DDoS mitigation when the NTP well-known port is employed as the client port (discussed in Section 3.3 of this document).¶

The authors would like to thank Harlan Stenn for answering questions about a popular NTP implementation (see <https://www.nwtime.org>).¶

Fernando Gont would like to thank Nelida Garcia and Jorge Oscar Gont for their love and support.¶

Authors' Addresses