Internet-Draft avoid-fragmentation July 2022
Fujiwara & Vixie Expires 5 January 2023 [Page]
Network Working Group
Intended Status:
Best Current Practice
K. Fujiwara
P. Vixie
AWS Security

Fragmentation Avoidance in DNS


EDNS0 enables a DNS server to send large responses using UDP and is widely deployed. Path MTU discovery remains widely undeployed due to security issues, and IP fragmentation has exposed weaknesses in application protocols. Currently, DNS is known to be the largest user of IP fragmentation. It is possible to avoid IP fragmentation in DNS by limiting response size where possible, and signaling the need to upgrade from UDP to TCP transport where necessary. This document proposes to avoid IP fragmentation in DNS.

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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on 5 January 2023.

Table of Contents

1. Introduction

DNS has EDNS0 [RFC6891] mechanism. It enables a DNS server to send large responses using UDP. EDNS0 is now widely deployed, and DNS (over UDP) is said to be the biggest user of IP fragmentation.

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 to set IP_DONTFRAG / IPV6_DONTFRAG in DNS/UDP messages in order to avoid IP fragmentation, and describes how to avoid packet losses due to IP_DONTFRAG / IPV6_DONTFRAG.

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, 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 Classical Path MTU discovery [RFC1191], [RFC8201], Packetization Layer Path MTU discovery [RFC8899] and as well as similar methods.

IP_DONTFRAG option is not defined by any RFCs. It is similar to IPV6_DONTFRAG option defined in [RFC3542]. IP_DONTFRAG option is used on BSD systems to set the Don't Fragment bit [RFC0791] when sending IPv4 packets. On Linux systems this is done via IP_MTU_DISCOVER and IP_PMTUDISC_DO.

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

3. Proposal to avoid IP fragmentation in DNS

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

3.1. Recommendations for UDP responders

  • UDP responders SHOULD send DNS responses with IP_DONTFRAG / IPV6_DONTFRAG [RFC3542] options.
  • If the UDP responder detects immediate error that the UDP packet cannot be sent beyond the path MTU size (EMSGSIZE), the UDP responder MAY recreate response packets fit in path MTU size, or TC bit set.
  • UDP responders MAY probe to discover the real MTU value per destination.
  • UDP responders SHOULD compose UDP responses that result in IP packets that do not exceed the path MTU to the requestor.
  • UDP responders SHOULD limit response size when UDP responders are located on small MTU (<1500) networks.

    The cause and effect of the TC bit is unchanged from EDNS0 [RFC6891].

3.2. Recommendations for UDP requestors

  • UDP requestors SHOULD send DNS requests with IP_DONTFRAG / IPV6_DONTFRAG [RFC3542] options.
  • UDP requestors MAY probe to discover the real MTU value per destination. (Path MTU discovery per destionation) Then, calculate their maximum DNS/UDP payload size as the reported path MTU minus IPv4/IPv6 header size (20 or 40) minus UDP header size (8). If the path MTU discovery failed or is impossible, use the default maximum DNS/UDP payload size 1400.
  • UDP requestors SHOULD use the requestor's payload size as the calculated or the default maximum DNS/UDP payload size.
  • UDP requestors MAY drop fragmented DNS/UDP responses without IP reassembly to avoid cache poisoning attacks.
  • DNS responses may be dropped by IP fragmentation. Upon a timeout, UDP requestors may retry using TCP or UDP, per local policy.

    To avoid name resolution fails, UDP requestors need to retry using TCP, or UDP with smaller maximum DNS/UDP payload size.

4. Incremental deployment

The proposed method supports incremental deployment.

When a full-service resolver implements the proposed method, its stub resolvers (clients) and the authority server network will no longer observe IP fragmentation or reassembly from that server, and will fall back to TCP when necessary.

When an authoritative server implements the proposed method, its full service resolvers (clients) will no longer observe IP fragmentation or reassembly from that server, and will fall back to TCP when necessary.

5. Request to 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,

6. Considerations

6.1. Protocol compliance

In prior research ([Fujiwara2018] and dns-operations mailing list discussions), there are some authoritative servers that ignore EDNS0 requestor's UDP payload size, and return large UDP responses.

It is also well known that there are some authoritative servers that do not support TCP transport.

Such non-compliant behavior cannot become implementation or configuration constraints for the rest of the DNS. If failure is the result, then that failure must be localized to the non-compliant servers.

7. IANA Considerations

This document has no IANA actions.

8. Security Considerations

9. Acknowledgments

The author would like to specifically thank Paul Wouters, Mukund Sivaraman, Tony Finch, Hugo Salgado, Peter van Dijk, Brian Dickson, Puneet Sood and Jim Reid for extensive review and comments.

10. References

10.1. Normative References

Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, , <>.
Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, "Advanced Sockets Application Program Interface (API) for IPv6", RFC 3542, DOI 10.17487/RFC3542, , <>.
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, DOI 10.17487/RFC4035, , <>.
Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., "Path MTU Discovery for IP version 6", STD 87, RFC 8201, DOI 10.17487/RFC8201, , <>.
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, , <>.
Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T. Völker, "Packetization Layer Path MTU Discovery for Datagram Transports", RFC 8899, DOI 10.17487/RFC8899, , <>.
Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., and F. Gont, "IP Fragmentation Considered Fragile", BCP 230, RFC 8900, DOI 10.17487/RFC8900, , <>.

10.2. Informative References

Brandt, M., Dai, T., Klein, A., Shulman, H., and M. Waidner, "Domain Validation++ For MitM-Resilient PKI", Proceedings of the 2018 ACM SIGSAC Conference on Computer and Communications Security , .
"DNS flag day 2020", n.d., <>.
Fujiwara, K., "Measures against cache poisoning attacks using IP fragmentation in DNS", OARC 30 Workshop , .
Herzberg, A. and H. Shulman, "Fragmentation Considered Poisonous", IEEE Conference on Communications and Network Security , .
Hlavacek, T., "IP fragmentation attack on DNS", RIPE 67 Meeting , , <>.
Huston, G. and J. Damas, "Measuring DNS Flag Day 2020", OARC 34 Workshop , .
Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, , <>.
Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS Security (DNSSEC) Hashed Authenticated Denial of Existence", RFC 5155, DOI 10.17487/RFC5155, , <>.
Gont, F., "Security Implications of Predictable Fragment Identification Values", RFC 7739, DOI 10.17487/RFC7739, , <>.
Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, , <>.
Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, , <>.

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 avoid 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 maximum DNS/UDP payload size discussions

There are many discussions for default path MTU size and maximum DNS/UDP payload size.

Appendix C. How to retrieve path MTU value to a destination from applications

Socket options: "IP_MTU (since Linux 2.2) Retrieve the current known path MTU of the current socket. Valid only when the socket has been connected. Returns an integer. Only valid as a getsockopt(2)." (Quoted from Debian GNU Linux manual: ip(7))

"IPV6_MTU getsockopt(): Retrieve the current known path MTU of the current socket. Only valid when the socket has been connected. Returns an integer." (Quoted from Debian GNU Linux manual: ipv6(7))

Section 3.4 of [RFC1122] specifies FIND_MAXSIZES() as one of "INTERNET/TRANSPORT LAYER INTERFACEs".

Appendix D. How to retrieve minimal MTU value to a destination

The Linux tool "tracepath" can be used to measure the path MTU to a destination.

Or, "ping/ping6" command with "-D" Don't Fragment bit set / Disable IPv6 fragmentation options.

Appendix E. Minimal-responses

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

Under the minimal-responses configuration, DNS servers compose response messages using only RRSets corresponding to queries. In case of delegation, DNS servers compose response packets with delegation NS RRSet in authority section and in-domain (in-zone and below-zone) glue in the additional data section. In case of non-existent domain name or non-existent type, the start of authority (SOA RR) will be placed in the Authority Section.

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

Authors' Addresses

Kazunori Fujiwara
Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda, Chiyoda-ku, Tokyo
Paul Vixie
AWS Security
11400 La Honda Road
Woodside, CA, 94062
United States of America