Fragmentation Avoidance in DNS

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Table of Contents

1. Introduction

DNS has an EDNS0 [RFC6891] mechanism. It enables a DNS server to send large responses using UDP. EDNS0 is now widely deployed, and DNS over UDP relies on IP fragmentation when the EDNS buffer size is set to a value larger than the path MTU.¶

Fragmented DNS UDP responses have systemic weaknesses, which expose the requestor to DNS cache poisoning from off-path attackers. (See Appendix A for references and details.)¶

[RFC8900] summarized that IP fragmentation introduces fragility to Internet communication. The transport of DNS messages over UDP should take account of the observations stated in that document.¶

TCP avoids fragmentation using its Maximum Segment Size (MSS) parameter, but each transmitted segment is header-size aware such that the size of the IP and TCP headers is known, as well as the far end's MSS parameter and the interface or path MTU, so that the segment size can be chosen so as to keep the each IP datagram below a target size. This takes advantage of the elasticity of TCP's packetizing process as to how much queued data will fit into the next segment. In contrast, DNS over UDP has little datagram size elasticity and lacks insight into IP header and option size, and so must make more conservative estimates about available UDP payload space.¶

This document proposes that implementations set the "Don't Fragment (DF) bit" [RFC0791] on IPv4 and not use the "Fragment header" [RFC8200] on IPv6 in DNS/UDP messages in order to avoid IP fragmentation. It also describes how to avoid packet losses due to DF bit and small MTU links.¶

A path MTU different from the recommended value could be obtained from static configuration, or server routing hints, or some future discovery protocol; that would be the subject of a future specification and is beyond our scope here.¶

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 BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

"Requestor" refers to the side that sends a request. "Responder" refers to an authoritative server, recursive resolver or other DNS component that responds to questions. (Quoted from EDNS0 [RFC6891])¶

"Path MTU" is the minimum link MTU of all the links in a path between a source node and a destination node. (Quoted from [RFC8201])¶

In this document, the term "Path MTU discovery" includes both Classical Path MTU discovery [RFC1191], [RFC8201], and Packetization Layer Path MTU discovery [RFC8899].¶

Many of the specialized terms used in this document are defined in DNS Terminology [RFC8499].¶

3. Proposal to avoid IP fragmentation in DNS

These recommendations are intended for nodes with global IP addresses on the Internet. Private networks or local networks are out of the scope of this document.¶

The methods to avoid IP fragmentation in DNS are described below:¶

4. Recommendations for zone operators and DNS server operators

Large DNS responses are the result of zone configuration. Zone operators SHOULD seek configurations resulting in small responses. For example,¶

1. Use a smaller number of name servers (13 may be too large)¶
2. Use a smaller number of A/AAAA RRs for a domain name¶
3. Use minimal-responses configuration: Some implementations have a 'minimal responses' configuration option that causes DNS servers to make response packets smaller, containing only mandatory and required data (Appendix C).¶
4. Use a smaller signature / public key size algorithm for DNSSEC. Notably, the signature sizes of ECDSA and EdDSA are smaller than those usually used for RSA.¶

5. Considerations

6. IANA Considerations

This document has no IANA actions.¶

7. Security Considerations

When avoiding fragmentation, a DNS/UDP requestor behind a small-MTU network may experience UDP timeouts which would reduce performance and which may lead to TCP fallback. This would indicate prior reliance upon IP fragmentation, which is universally considered to be harmful to both the performance and stability of applications, endpoints, and gateways. Avoiding IP fragmentation will improve operating conditions overall, and the performance of DNS/TCP has increased and will continue to increase.¶

If a UDP response packet is dropped (for any reason), it increases the attack window for poisoning the requestor's cache.¶

8. Acknowledgments

The author would like to specifically thank Paul Wouters, Mukund Sivaraman, Tony Finch, Hugo Salgado, Peter van Dijk, Brian Dickson, Puneet Sood, Jim Reid, Petr Spacek, Andrew McConachie, Joe Abley, Daisuke Higashi and Joe Touch for extensive review and comments.¶

Appendix A. Weaknesses of IP fragmentation

"Fragmentation Considered Poisonous" [Herzberg2013] proposed effective off-path DNS cache poisoning attack vectors using IP fragmentation. "IP fragmentation attack on DNS" [Hlavacek2013] and "Domain Validation++ For MitM-Resilient PKI" [Brandt2018] proposed that off-path attackers can intervene in path MTU discovery [RFC1191] to perform intentionally fragmented responses from authoritative servers. [RFC7739] stated the security implications of predictable fragment identification values.¶

DNSSEC is a countermeasure against cache poisoning attacks that use IP fragmentation. However, DNS delegation responses are not signed with DNSSEC, and DNSSEC does not have a mechanism to get the correct response if an incorrect delegation is injected. This is a denial-of-service vulnerability that can yield failed name resolutions. If cache poisoning attacks can be avoided, DNSSEC validation failures will be avoided.¶

In Section 3.2 (Message Side Guidelines) of UDP Usage Guidelines [RFC8085] we are told that an application SHOULD NOT send UDP datagrams that result in IP packets that exceed the Maximum Transmission Unit (MTU) along the path to the destination.¶

A DNS message receiver cannot trust fragmented UDP datagrams primarily due to the small amount of entropy provided by UDP port numbers and DNS message identifiers, each of which being only 16 bits in size, and both likely being in the first fragment of a packet, if fragmentation occurs. By comparison, TCP protocol stack controls packet size and avoids IP fragmentation under ICMP NEEDFRAG attacks. In TCP, fragmentation should be avoided for performance reasons, whereas for UDP, fragmentation should be avoided for resiliency and authenticity reasons.¶

Appendix B. Details of requestor's maximum UDP payload size discussions

There are many discussions for default path MTU size and requestor's maximum UDP payload size.¶

• The minimum MTU for an IPv6 interface is 1280 octets (see Section 5 of [RFC8200]). So, we can use it as the default path MTU value for IPv6. The corresponding minimum MTU for an IPv4 interface is 68 (60 + 8) [RFC0791].¶
• Most of the Internet and especially the inner core has an MTU of at least 1500 octets. Maximum DNS/UDP payload size for IPv6 on MTU 1500 ethernet is 1452 (1500 minus 40 (IPv6 header size) minus 8 (UDP header size)). To allow for possible IP options and distant tunnel overhead, the authors' recommendation of default maximum DNS/UDP payload size is 1400.¶
• [RFC4035] defines that "A security-aware name server MUST support the EDNS0 message size extension, MUST support a message size of at least 1220 octets". Then, the smallest number of the maximum DNS/UDP payload size is 1220.¶
• In order to avoid IP fragmentation, [DNSFlagDay2020] proposed that the UDP requestors set the requestor's payload size to 1232, and the UDP responders compose UDP responses so they fit in 1232 octets. The size 1232 is based on an MTU of 1280, which is required by the IPv6 specification [RFC8200], minus 48 octets for the IPv6 and UDP headers.¶
• [Huston2021] analyzed the result of [DNSFlagDay2020] and reported that their measurements suggest that in the interior of the Internet between recursive resolvers and authoritative servers the prevailing MTU is at 1,500 and there is no measurable signal of use of smaller MTUs in this part of the Internet, and proposed that their measurements suggest setting the EDNS0 requestor's UDP payload size to 1472 octets for IPv4, and 1452 octets for IPv6.¶

Appendix C. Minimal-responses

Some implementations have a "minimal responses" configuration setting/option that causes a DNS server to make response packets smaller, containing only mandatory and required data.¶

Under the minimal-responses configuration, a DNS server composes responses containing only nessesary RRs. For delegations, see [I-D.ietf-dnsop-glue-is-not-optional]. In case of a non-existent domain name or non-existent type, the authority section will contain an SOA record and the answer section is empty. (defined in Section 2 of [RFC2308]).¶

Some resource records (MX, SRV, SVCB, HTTTPS) require additional A, AAAA, and SVCB records in the Additional Section defined in [RFC1035], [RFC2782] and [I-D.ietf-dnsop-svcb-https].¶

In addition, if the zone is DNSSEC signed and a query has the DNSSEC OK bit, signatures are added in the answer section, or the corresponding DS RRSet and signatures are added in the authority section. Details are defined in [RFC4035] and [RFC5155].¶

Appendix D. Known Implementations

This section records the status of known implementations of these best practices defined by this specification at the time of publication, and any deviation from the specification.¶

Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors.¶

Authors' Addresses