RFC 9715

IP Fragmentation Avoidance in DNS over UDP

📅 published January 2025, from ietf

RFC 9715 (Part 1 of 3) 💖

Title Block 🎀

Okay bestie, here's the cast list ✨ This is RFC 9715, called IP Fragmentation Avoidance in DNS over UDP 🦋 It's by K. Fujiwara from JPRS and P. Vixie from AWS Security. It's Informational (not a Standards Track thing, so it's a how-to-think-about-it doc, not a must-do rulebook). It's dated January 2025 and its ISSN is 2070-1721. The title block has no Obsoletes or Updates line, so it doesn't claim to replace or patch any other RFC 💅

Abstract ✨

Okay so DNS has this super popular feature called EDNS(0) 🧚 It lets the DNS receiver say "hey, I can take big UDP messages!" and that lets the server send big UDP replies. Big DNS/UDP messages are more likely to get fragmented (chopped up into pieces), and IP fragmentation has caused problems for application protocols 😭 You can dodge fragmentation in DNS by keeping responses small when you can, and by signaling that the conversation should move from UDP to TCP when you can't. This doc is all about those techniques 🌸

Status of This Memo 🌈

Quick heads up: this doc is not an Internet Standards Track spec. It's published for informational purposes, so it's more of a guide than a law 📚

This doc is a product of the Internet Engineering Task Force (IETF). It totally represents the consensus of the IETF community 🫶 It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). Not every IESG-approved doc is a candidate for any level of Internet Standard, which is a whole thing you can read about in Section 2 of RFC 7841 🙈

For the current status, any errata, and how to send feedback, check out https://www.rfc-editor.org/info/rfc9715 🔥

Copyright Notice 💜

This doc is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as they stand on the publication date. Please read these carefully, because they spell out your rights and restrictions for this doc 🧾 Code Components pulled out of this doc have to include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions, and they come with no warranty, as described in the Revised BSD License ⭐

Table of Contents 🍓

1.  Introduction
2.  Terminology
3.  How to Avoid IP Fragmentation in DNS
  3.1.  Proposed Recommendations for UDP Responders
  3.2.  Proposed Recommendations for UDP Requestors
4.  Proposed Recommendations for DNS Operators
5.  Protocol Compliance Considerations
6.  IANA Considerations
7.  Security Considerations
  7.1.  On-Path Fragmentation on IPv4
  7.2.  Small MTU Network
  7.3.  Weaknesses of IP Fragmentation
  7.4.  DNS Security Protections
  7.5.  Possible Actions for Resolver Operators
8.  References
  8.1.  Normative References
  8.2.  Informative References
Appendix A.  Details of Requestor's Maximum UDP Payload Size
        Discussions
Appendix B.  Minimal Responses
Appendix C.  Known Implementations
  C.1.  BIND 9
  C.2.  Knot DNS and Knot Resolver
  C.3.  PowerDNS Authoritative Server, PowerDNS Recursor, and
          PowerDNS dnsdist
  C.4.  PowerDNS Authoritative Server
  C.5.  Unbound
  Acknowledgments
Authors' Addresses

1. Introduction 🦋

Okay, so this doc was originally meant to be a Best Current Practice (BCP), but some of the recommendations haven't gotten real-world experience yet because of operating system and socket option limitations 🙃 So it's Informational for now. The plan is that as operating systems and implementations evolve, we'll get more experience and publish an updated version as a Best Current Practice later on ✨

DNS has an EDNS(0) mechanism [RFC6891]. The super popular EDNS(0) feature lets a DNS receiver say what size UDP messages it can handle, which means a DNS server can send big UDP responses 💅 But DNS over UDP invites IP fragmentation whenever a packet is bigger than the Maximum Transmission Unit (MTU) of some network along the path 😬

Fragmented DNS UDP responses have systemic weaknesses. They expose the requestor to DNS cache poisoning from off-path attackers (see Section 7.3 for the references and details). Not me being scared, but this is a real problem 💀

[RFC8900] says IP fragmentation makes Internet communication fragile. DNS message transport over UDP should take the observations in that doc into account 🧚

TCP avoids fragmentation by splitting data into packets that are smaller than or equal to the Maximum Segment Size (MSS). For every segment that gets sent, the size of the IP and TCP headers is known, so the IP packet size can be picked to stay within the estimated MTU and the MSS. This works because TCP's packetizing is elastic, depending on how much queued data fits into the next segment 🌊 DNS over UDP has basically no datagram size elasticity and doesn't know the IP header and option sizes, so we have to make more conservative guesses about how much UDP payload space is available 🫠

[RFC7766] says all general-purpose DNS implementations MUST support both UDP and TCP transport 🔥

DNS transaction security [RFC8945] [RFC2931] does protect against the security risks of fragmentation, and it protects delegation responses too 🛡️ But [RFC8945] has limited applicability because of its key distribution requirements, and there's little if any deployment of [RFC2931] 😔

This doc describes a bunch of techniques for avoiding IP fragmentation of UDP packets in DNS. It's mainly for DNS use on the global Internet 🌍

On the flip side, a path MTU that's different from the recommended value might come from static configuration, server routing hints, or a future discovery protocol. Addressing that is outside the scope of this doc, though it might get its own specs down the road 🌸

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 🎀

The definitions of "requestor" and "responder" come from [RFC6891]:

"Requestor" refers to the side that sends a request.  "Responder"
refers to an authoritative, recursive resolver or other DNS
component that responds to questions.

The definition of "path MTU" comes from [RFC8201]:

path MTU [is] the minimum link MTU of all the links in a path
between a source node and a destination node.

In this doc, the term "Path MTU Discovery" covers both Classical Path MTU Discovery [RFC1191] [RFC8201] and Packetization Layer Path MTU Discovery [RFC8899] 🧠

A lot of the specialized terms in this doc are defined in "DNS Terminology" [RFC9499] 💜

3. How to Avoid IP Fragmentation in DNS 🧚

These recommendations are for nodes with global IP addresses on the Internet. Private networks and local networks are out of scope for this doc 🙅‍♀️

Here's how to avoid IP fragmentation in DNS:

3.1. Proposed Recommendations for UDP Responders 🍓

R1. UDP responders should not use IPv6 fragmentation [RFC8200] 💅

R2. UDP responders should configure their systems to prevent fragmentation of UDP packets when sending replies, provided it can be done safely. The ways to do this are different across operating systems 🖥️

For BSD-like operating systems, the IP Don't Fragment (DF) flag bit [RFC0791] can be used to prevent fragmentation. Linux systems, on the other hand, don't expose a direct API for this and need the Path MTU socket options (IP_MTU_DISCOVER) to manage fragmentation settings. It's really important to know that turning on IPv4 Path MTU Discovery for UDP in current Linux versions is considered harmful and dangerous 🚨 For more details, see Appendix C.

R3. UDP responders should compose response packets that fit in the minimum of the offered requestor's maximum UDP payload size [RFC6891], the interface MTU, the network MTU value configured by the knowledge of the network operators, and the RECOMMENDED maximum DNS/UDP payload size 1400. For more details, see Appendix A ✨

R4. If the UDP responder detects an immediate error indicating that the UDP packet exceeds the path MTU size, the UDP responder may recreate response packets that fit in the path MTU size or with the TC bit set 🔄

The cause and effect of the TC bit are unchanged [RFC1035] 🫶

3.2. Proposed Recommendations for UDP Requestors 🌸

R5. UDP requestors should limit the requestor's maximum UDP payload size to fit in the minimum of the interface MTU, the network MTU value configured by the network operators, and the RECOMMENDED maximum DNS/UDP payload size 1400. A smaller limit may be allowed. For more details, see Appendix A 🎀

R6. UDP requestors should drop fragmented DNS/UDP responses without IP reassembly to avoid cache poisoning attacks (at the firewall function) 🛡️

R7. DNS responses may be dropped by IP fragmentation. It is recommended that requestors eventually try alternative transport protocols 🦋

7.2. Small MTU Network 🌸

okay so when you're avoiding fragmentation, a DNS/UDP requestor behind a small MTU network can run into UDP timeouts 😭. That makes things slower and can push you over to TCP fallback. Honestly, that's a sign it was leaning on IP fragmentation before, and IP fragmentation is considered harmful to the performance and stability of applications, endpoints, and gateways 💀. Avoiding it makes operating conditions better overall, and DNS/TCP performance has already improved and will keep getting better ✨.

If a UDP response packet gets dropped in transit, all the way up to and including the initiator's own network stack, it widens the attack window for poisoning the requestor's cache 🙈.

7.3. Weaknesses of IP Fragmentation 🦋

so "Fragmentation Considered Poisonous" [Herzberg2013] is the big one that lays out effective off-path DNS cache poisoning attacks that use IP fragmentation 👀. "IP fragmentation attack on DNS" [Hlavacek2013] and "Domain Validation++ For MitM-Resilient PKI" [Brandt2018] show that off-path attackers can mess with Path MTU Discovery [RFC1191] to get authoritative servers to produce fragmented responses 😤. And [RFC7739] explains the security implications of predictable fragment identification values, which is a whole other can of worms 🪱.

Section 3.2 of [RFC8085] says 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" 📏.

Here's the thing: a DNS message receiver cannot trust fragmented UDP datagrams, mainly because UDP port numbers and DNS message identifiers only give a small amount of entropy. Each one is just 16 bits, and both are likely to sit in the first fragment if fragmentation happens 🥺. The TCP stack, by comparison, controls packet size and avoids IP fragmentation, even under ICMP NEEDFRAG attacks 🛡️. For TCP, avoiding fragmentation is about performance, but for UDP it's about resiliency and authenticity, which is a much bigger deal 💅.

7.4. DNS Security Protections 🔐

DNSSEC is a countermeasure against cache poisoning attacks that use IP fragmentation, so it's on our side 🫶. BUT, and this is a big but, DNS delegation responses are not signed with DNSSEC, and DNSSEC has no mechanism for getting the correct response if an incorrect delegation gets injected 😬. That makes it a denial-of-service vulnerability that can yield failed name resolutions. If cache poisoning attacks can be avoided, though, DNSSEC validation failures will be avoided too ✨.

7.5. Possible Actions for Resolver Operators 🛠️

Because this document is published as Informational rather than a Best Current Practice, this section lays out steps that resolver operators can take to avoid vulnerabilities related to IP fragmentation 📝.

To avoid those vulnerabilities, implement R5 and R6 💖.

Specifically, configure the firewall functions protecting the full-service resolver to discard incoming DNS response packets with a non-zero Fragment Offset (FO) or a More Fragments (MF) flag bit of 1 on IPv4, and discard packets with IPv6 Fragment Headers 🧱. (If the resolver's IP address isn't dedicated to the DNS resolver and it uses UDP communication that relies on IP Fragmentation for purposes other than DNS, discard only the first fragment that contains the UDP header from port 53 🎯.)

The most recent resolver software is believed to implement R7 🌈.

Even if R7 isn't implemented, the worst case is a name resolution error, which stops attacks from leading to malicious sites 🙅‍♀️💜.

8. References 📚

8.1. Normative References

[RFC1035]  Mockapetris, P., "Domain names - implementation and
           specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
           November 1987, <https://www.rfc-editor.org/info/rfc1035>.

[RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
           DOI 10.17487/RFC1191, November 1990,
           <https://www.rfc-editor.org/info/rfc1191>.

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119,
           DOI 10.17487/RFC2119, March 1997,
           <https://www.rfc-editor.org/info/rfc2119>.

[RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
           ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
           2000, <https://www.rfc-editor.org/info/rfc2931>.

[RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
           for DNS (EDNS(0))", STD 75, RFC 6891,
           DOI 10.17487/RFC6891, April 2013,
           <https://www.rfc-editor.org/info/rfc6891>.

[RFC7739]  Gont, F., "Security Implications of Predictable Fragment
           Identification Values", RFC 7739, DOI 10.17487/RFC7739,
           February 2016, <https://www.rfc-editor.org/info/rfc7739>.

[RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
           D. Wessels, "DNS Transport over TCP - Implementation
           Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
           <https://www.rfc-editor.org/info/rfc7766>.

[RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
           Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
           March 2017, <https://www.rfc-editor.org/info/rfc8085>.

[RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
           2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
           May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", STD 86, RFC 8200,
           DOI 10.17487/RFC8200, July 2017,
           <https://www.rfc-editor.org/info/rfc8200>.

[RFC8201]  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, July 2017,
           <https://www.rfc-editor.org/info/rfc8201>.

[RFC8899]  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,
           September 2020, <https://www.rfc-editor.org/info/rfc8899>.

[RFC8945]  Dupont, F., Morris, S., Vixie, P., Eastlake 3rd, D.,
           Gudmundsson, O., and B. Wellington, "Secret Key
           Transaction Authentication for DNS (TSIG)", STD 93,
           RFC 8945, DOI 10.17487/RFC8945, November 2020,
           <https://www.rfc-editor.org/info/rfc8945>.

[RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
           RFC 9499, DOI 10.17487/RFC9499, March 2024,
           <https://www.rfc-editor.org/info/rfc9499>.

8.2. Informative References

[Brandt2018]
           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, pp. 2060-2076,
           DOI 10.1145/3243734.3243790, October 2018,
           <https://dl.acm.org/doi/10.1145/3243734.3243790>.

[DNSFlagDay2020]
           "DNS flag day 2020", <https://dnsflagday.net/2020/>.

[Fujiwara2018]
           Fujiwara, K., "Measures against DNS cache poisoning
           attacks using IP fragmentation", OARC 30 Workshop, 2019,
           <https://indico.dns-
           oarc.net/event/31/contributions/692/attachments/660/1115/
           fujiwara-5.pdf>.

[Herzberg2013]
           Herzberg, A. and H. Shulman, "Fragmentation Considered
           Poisonous, or: One-domain-to-rule-them-all.org", IEEE
           Conference on Communications and Network Security (CNS),
           DOI 10.1109/CNS.2013.6682711, 2013,
           <https://ieeexplore.ieee.org/document/6682711>.

[Hlavacek2013]
           Hlavacek, T., "IP fragmentation attack on DNS", RIPE 67
           Meeting, 2013, <https://ripe67.ripe.net/
           presentations/240-ipfragattack.pdf>.

[Huston2021]
           Huston, G. and J. Damas, "Measuring DNS Flag Day 2020",
           OARC 34 Workshop, February 2021, <https://indico.dns-
           oarc.net/event/37/contributions/806/
           attachments/782/1366/2021-02-04-dns-flag.pdf>.

[RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
           DOI 10.17487/RFC0791, September 1981,
           <https://www.rfc-editor.org/info/rfc791>.

[RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
           NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
           <https://www.rfc-editor.org/info/rfc2308>.

[RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
           RFC 2671, DOI 10.17487/RFC2671, August 1999,
           <https://www.rfc-editor.org/info/rfc2671>.

[RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
           specifying the location of services (DNS SRV)", RFC 2782,
           DOI 10.17487/RFC2782, February 2000,
           <https://www.rfc-editor.org/info/rfc2782>.

[RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
           Rose, "Protocol Modifications for the DNS Security
           Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
           <https://www.rfc-editor.org/info/rfc4035>.

[RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
           Security (DNSSEC) Hashed Authenticated Denial of
           Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
           <https://www.rfc-editor.org/info/rfc5155>.

[RFC8900]  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, September 2020,
           <https://www.rfc-editor.org/info/rfc8900>.

[RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
           and Parameter Specification via the DNS (SVCB and HTTPS
           Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
           November 2023, <https://www.rfc-editor.org/info/rfc9460>.

[RFC9471]  Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS
           Glue Requirements in Referral Responses", RFC 9471,
           DOI 10.17487/RFC9471, September 2023,
           <https://www.rfc-editor.org/info/rfc9471>.

Appendix A. Details of Requestor's Maximum UDP Payload Size Discussions 💬✨

Okay bestie, this appendix is basically the group chat where everyone argues about how big a UDP payload is allowed to be 🍓 There are many discussions about default path MTU size and a requestor's maximum UDP payload size, so let's go through the receipts 🧾

The minimum MTU for an IPv6 interface is 1280 octets (see Section 5 of [RFC8200]) 🌸 So that's what can be used as the default path MTU value for IPv6. Over on IPv4, the corresponding minimum MTU for an interface is 68 (60 + 8) [RFC0791] 🦋
[RFC4035] says "A security-aware name server MUST support the EDNS0 ([RFC2671]) message size extension, [and it] MUST support a message size of at least 1220 octets" 👑 So the smallest possible maximum DNS/UDP payload size is 1220.
To avoid IP fragmentation, [DNSFlagDay2020] proposes that UDP requestors set the requestor's payload size to 1232, and that 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. Math ate, no crumbs 🧚
Most of the Internet, especially the inner core, has an MTU of at least 1500 octets 🌈 Maximum DNS/UDP payload size for IPv6 on an 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 recommendation of default maximum DNS/UDP payload size is 1400 ✨
[Huston2021] analyzes the result of [DNSFlagDay2020] and reports that their measurements suggest that in the interior of the Internet, between recursive resolvers and authoritative servers, the prevailing MTU is 1500. There is no measurable signal of use of smaller MTUs in this part of the Internet 🙈 They propose that their measurements suggest setting the EDNS(0) requestor's UDP payload size to 1472 octets for IPv4 and 1452 octets for IPv6.

So after all that drama, this document recommends a value of 1400, with smaller values also allowed 💖

Appendix B. 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 necessary Resource Records (RRs) 💅 For delegations, see [RFC9471]. 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 (see Section 2 of [RFC2308]) 🙈

Some resource records (MX, SRV, SVCB, and HTTPS) require additional A, AAAA, and Service Binding (SVCB) records in the Additional section defined in [RFC1035], [RFC2782], and [RFC9460] 🧚

Also, 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 C. Known Implementations 🌸

This section records the status of known implementations of the proposed recommendations described in Section 3 📋

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

C.1. BIND 9 👑

BIND 9 does not implement R1 and R2 🙈

BIND 9 on Linux sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with a fallback to IP_PMTUDISC_DONT.

When BIND 9 is on systems with IP_DONTFRAG (such as FreeBSD), IP_DONTFRAG is disabled.

Accepting Path MTU Discovery for UDP is considered harmful and dangerous 😭 BIND 9's settings avoid attacks to Path MTU Discovery 🛡️

For R3, BIND 9 will honor the requestor's size up to the configured limit (max-udp-size). The UDP response packet is bound to be between 512 and 4096 bytes, with the default set to 1232. BIND 9 supports the requestor's size up to the configured limit (max-udp-size) ✨

In the case of R4 and the send fails with EMSGSIZE, BIND 9 sets the TC bit and tries to send a minimal answer again 🔥

For R5, BIND 9 uses the edns-buf-size option, with the default of 1232 💅

For R7, after two UDP timeouts, BIND 9 will fall back to TCP 🦋

C.2. Knot DNS and Knot Resolver 🌈

Both Knot servers set IP_PMTUDISC_OMIT to avoid path MTU spoofing. The UDP size limit is 1232 by default 🫶

Fragments are ignored if they arrive over a Linux XDP interface 🧚

TCP is attempted after repeated UDP timeouts ⭐

Minimal responses are returned and are currently not configurable 💜

Smaller signatures are used, with ecdsap256sha256 as the default 🍓

C.3. PowerDNS Authoritative Server, PowerDNS Recursor, and PowerDNS dnsdist 💖

Use IP_PMTUDISC_OMIT with a fallback to IP_PMTUDISC_DONT.
The default EDNS buffer size of 1232; no probing for smaller sizes.
There is no handling of EMSGSIZE.
Recursor: UDP timeouts do not cause a switch to TCP, but "spoofing near misses" may 👀

C.4. PowerDNS Authoritative Server 🎀

The default DNSSEC algorithm is 13.
Responses are minimal; this is not configurable.

C.5. Unbound 🦋

Unbound sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with fallback to IP_PMTUDISC_DONT. It also disables IP_DONTFRAG on systems that have it, but not on Apple systems. On systems that support it, Unbound sets IPV6_USE_MIN_MTU, with a fallback to IPV6_MTU at 1280, with a fallback to IPV6_USER_MTU. It also sets IPV6_MTU_DISCOVER to IPV6_PMTUDISC_OMIT, with a fallback to IPV6_PMTUDISC_DONT 🌸

Unbound requests a UDP size of 1232 from peers, by default. The requestor's size is limited to a max of 1232 💅

After some timeouts, Unbound retries with a smaller size, if applicable, or at size 1232 for IPv6 and 1472 for IPv4. This does not cause any negative effects due to the "flag day" [DNSFlagDay2020] change to 1232 🙌

Unbound has the "minimal responses" configuration option, set default on 👑

Acknowledgments ✨

The authors 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, Joe Touch, Wouter Wijngaards, Vladimir Cunat, Benno Overeinder, and Štěpán Němec for their extensive reviews and comments 🫶💜

Authors' Addresses 🦋

Kazunori Fujiwara
Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F
3-8-1 Nishi-Kanda, Chiyoda-ku, Tokyo
101-0065
Japan
Phone: +81 3 5215 8451
Email: fujiwara@jprs.co.jp


Paul Vixie
AWS Security
11400 La Honda Road
Woodside, CA 94062
United States of America
Phone: +1 650 393 3994
Email: paul@redbarn.org

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