rfc9715.original   rfc9715.txt 
Network Working Group K. Fujiwara Internet Engineering Task Force (IETF) K. Fujiwara
Internet-Draft JPRS Request for Comments: 9715 JPRS
Intended status: Informational P. Vixie Category: Informational P. Vixie
Expires: 30 March 2025 AWS Security ISSN: 2070-1721 AWS Security
26 September 2024 January 2025
IP Fragmentation Avoidance in DNS over UDP IP Fragmentation Avoidance in DNS over UDP
draft-ietf-dnsop-avoid-fragmentation-20
Abstract Abstract
The widely deployed EDNS0 feature in the DNS enables a DNS receiver The widely deployed Extension Mechanisms for DNS (EDNS0) feature in
to indicate its received UDP message size capacity, which supports the DNS enables a DNS receiver to indicate its received UDP message
the sending of large UDP responses by a DNS server. Large DNS/UDP size capacity, which supports the sending of large UDP responses by a
messages are more likely to be fragmented and IP fragmentation has DNS server. Large DNS/UDP messages are more likely to be fragmented,
exposed weaknesses in application protocols. It is possible to avoid and IP fragmentation has exposed weaknesses in application protocols.
IP fragmentation in DNS by limiting the response size where possible, It is possible to avoid IP fragmentation in DNS by limiting the
and signaling the need to upgrade from UDP to TCP transport where response size where possible and signaling the need to upgrade from
necessary. This document describes techniques to avoid IP UDP to TCP transport where necessary. This document describes
fragmentation in DNS. techniques to avoid IP fragmentation in DNS.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
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 https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
This Internet-Draft will expire on 30 March 2025. 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/rfc9715.
Copyright Notice Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology
3. How to avoid IP fragmentation in DNS . . . . . . . . . . . . 4 3. How to Avoid IP Fragmentation in DNS
3.1. Proposed Recommendations for UDP responders . . . . . . . 4 3.1. Proposed Recommendations for UDP Responders
3.2. Proposed Recommendations for UDP requestors . . . . . . . 5 3.2. Proposed Recommendations for UDP Requestors
4. Proposed Recommendations for DNS operators . . . . . . . . . 5 4. Proposed Recommendations for DNS Operators
5. Protocol compliance considerations . . . . . . . . . . . . . 6 5. Protocol Compliance Considerations
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations
7.1. On-path fragmentation on IPv4 . . . . . . . . . . . . . . 6 7.1. On-Path Fragmentation on IPv4
7.2. Small MTU network . . . . . . . . . . . . . . . . . . . . 6 7.2. Small MTU Network
7.3. Weaknesses of IP fragmentation . . . . . . . . . . . . . 7 7.3. Weaknesses of IP Fragmentation
7.4. DNS Security Protections . . . . . . . . . . . . . . . . 7 7.4. DNS Security Protections
7.5. Possible actions for resolver operators . . . . . . . . . 7 7.5. Possible Actions for Resolver Operators
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 8. References
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References
9.1. Normative References . . . . . . . . . . . . . . . . . . 8 8.2. Informative References
9.2. Informative References . . . . . . . . . . . . . . . . . 9 Appendix A. Details of Requestor's Maximum UDP Payload Size
Appendix A. Details of requestor's maximum UDP payload size Discussions
discussions . . . . . . . . . . . . . . . . . . . . . . . 11 Appendix B. Minimal Responses
Appendix B. Minimal-responses . . . . . . . . . . . . . . . . . 12 Appendix C. Known Implementations
Appendix C. Known Implementations . . . . . . . . . . . . . . . 12 C.1. BIND 9
C.1. BIND 9 . . . . . . . . . . . . . . . . . . . . . . . . . 12 C.2. Knot DNS and Knot Resolver
C.2. Knot DNS and Knot Resolver . . . . . . . . . . . . . . . 13 C.3. PowerDNS Authoritative Server, PowerDNS Recursor, and
C.3. PowerDNS Authoritative Server, PowerDNS Recursor, PowerDNS PowerDNS dnsdist
dnsdist . . . . . . . . . . . . . . . . . . . . . . . . . 13 C.4. PowerDNS Authoritative Server
C.4. PowerDNS Authoritative Server . . . . . . . . . . . . . . 14 C.5. Unbound
C.5. Unbound . . . . . . . . . . . . . . . . . . . . . . . . . 14 Acknowledgments
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses
1. Introduction 1. Introduction
This document was originally intended to be a BCP, but due to This document was originally intended to be a Best Current Practice,
operating system and socket option limitations, some of the but due to operating system and socket option limitations, some of
recommendations have not yet gained real-world experience and the recommendations have not yet gained real-world experience;
therefore the document is published as Informational. It is hoped therefore, this document is Informational. It is expected that, as
and expected that, as operating systems and implementations evolve, operating systems and implementations evolve, we will gain more
we will gain more experience with the recommendations, and plan to experience with the recommendations and will publish an updated
publish an updated document as a Best Current Practice. document as a Best Current Practice in the future.
DNS has an EDNS0 [RFC6891] mechanism. The widely deployed EDNS0 DNS has an EDNS0 mechanism [RFC6891]. The widely deployed EDNS0
feature in the DNS enables a DNS receiver to indicate its received feature in the DNS enables a DNS receiver to indicate its received
UDP message size capacity which supports the sending of large UDP UDP message size capacity, which supports the sending of large UDP
responses by a DNS server. DNS over UDP invites IP fragmentation responses by a DNS server. DNS over UDP invites IP fragmentation
when a packet is larger than the MTU of some network in the packet's when a packet is larger than the Maximum Transmission Unit (MTU) of
path. some network in the packet's path.
Fragmented DNS UDP responses have systemic weaknesses, which expose Fragmented DNS UDP responses have systemic weaknesses, which expose
the requestor to DNS cache poisoning from off-path attackers. (See the requestor to DNS cache poisoning from off-path attackers (see
Section 7.3 for references and details.) Section 7.3 for references and details).
[RFC8900] states that IP fragmentation introduces fragility to [RFC8900] states that IP fragmentation introduces fragility to
Internet communication. The transport of DNS messages over UDP Internet communication. The transport of DNS messages over UDP
should take account of the observations stated in that document. should take account of the observations stated in that document.
TCP avoids fragmentation by segmenting data into packets that are TCP avoids fragmentation by segmenting data into packets that are
smaller than or equal to the Maximum Segment Size (MSS). For each smaller than or equal to the Maximum Segment Size (MSS). For each
transmitted segment, the size of the IP and TCP headers is known, and transmitted segment, the size of the IP and TCP headers is known, and
the IP packet size can be chosen to keep it within the estimated MTU the IP packet size can be chosen to keep it within the estimated MTU
and the other end's MSS. This takes advantage of the elasticity of and the other end's MSS. This takes advantage of the elasticity of
TCP's packetizing process as to how much queued data will fit into 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 the next segment. In contrast, DNS over UDP has little datagram size
elasticity and lacks insight into IP header and option size, so we elasticity and lacks insight into IP header and option size, so we
must make more conservative estimates about available UDP payload must make more conservative estimates about available UDP payload
space. space.
[RFC7766] states that all general-purpose DNS implementations MUST [RFC7766] states that all general-purpose DNS implementations MUST
support both UDP and TCP transport. support both UDP and TCP transport.
DNS transaction security [RFC8945] [RFC2931] does protect against the DNS transaction security [RFC8945] [RFC2931] does protect against the
security risks of fragmentation, including protecting delegation security risks of fragmentation, and it protects delegation
responses. But [RFC8945] has limited applicability due to key responses. But [RFC8945] has limited applicability due to key
distribution requirements and there is little if any deployment of distribution requirements, and there is little if any deployment of
[RFC2931]. [RFC2931].
This document describes various techniques to avoid IP fragmentation This document describes various techniques to avoid IP fragmentation
of UDP packets in DNS. This document is primarily applicable to DNS of UDP packets in DNS. This document is primarily applicable to DNS
use on the global Internet. use on the global Internet.
In contrast, a path MTU that deviates from the recommended value In contrast, a path MTU that deviates from the recommended value
might be obtained through static configuration, server routing hints, might be obtained through static configuration, server routing hints,
or a future discovery protocol. However, addressing this falls or a future discovery protocol. However, addressing this falls
outside the scope of this document and may be the subject of future outside the scope of this document and may be the subject of future
specifications. specifications.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
"Requestor" refers to the side that sends a request. "Responder" The definitions of "requestor" and "responder" are per [RFC6891]:
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 | "Requestor" refers to the side that sends a request. "Responder"
a source node and a destination node. (Quoted from [RFC8201]) | refers to an authoritative, recursive resolver or other DNS
| component that responds to questions.
In this document, the term "Path MTU discovery" includes both The definition of "path MTU" is per [RFC8201]:
Classical Path MTU discovery [RFC1191], [RFC8201], and Packetization
Layer Path MTU discovery [RFC8899]. | path MTU [is] the minimum link MTU of all the links in a path
| between a source node and a destination node.
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 Many of the specialized terms used in this document are defined in
DNS Terminology [RFC8499]. "DNS Terminology" [RFC9499].
3. How to avoid IP fragmentation in DNS 3. How to Avoid IP Fragmentation in DNS
These recommendations are intended for nodes with global IP addresses These recommendations are intended for nodes with global IP addresses
on the Internet. Private networks or local networks are out of the on the Internet. Private networks or local networks are out of the
scope of this document. scope of this document.
The methods to avoid IP fragmentation in DNS are described below: The methods to avoid IP fragmentation in DNS are described below:
3.1. Proposed Recommendations for UDP responders 3.1. Proposed Recommendations for UDP Responders
R1. UDP responders should not use IPv6 fragmentation [RFC8200]. R1. UDP responders should not use IPv6 fragmentation [RFC8200].
R2. UDP responders should configure their systems to prevent R2. UDP responders should configure their systems to prevent
fragmentation of UDP packets when sending replies, provided it can be fragmentation of UDP packets when sending replies, provided it
done safely. The mechanisms to achieve this vary across different can be done safely. The mechanisms to achieve this vary
operating systems. across different operating systems.
For BSD-like operating systems, the IP "Don't Fragment flag (DF) bit" For BSD-like operating systems, the IP Don't Fragment flag
[RFC0791] can be used to prevent fragmentation. In contrast, Linux (DF) bit [RFC0791] can be used to prevent fragmentation. In
systems do not expose a direct API for this purpose and require the contrast, Linux systems do not expose a direct API for this
use of Path MTU socket options (IP_MTU_DISCOVER) to manage purpose and require the use of Path MTU socket options
fragmentation settings. However, it is important to note that (IP_MTU_DISCOVER) to manage fragmentation settings. However,
enabling IPv4 Path MTU Discovery for UDP in current Linux versions is it is important to note that enabling IPv4 Path MTU Discovery
considered harmful and dangerous. For more details, refer to for UDP in current Linux versions is considered harmful and
Appendix C. dangerous. For more details, see Appendix C.
R3. UDP responders should compose response packets that fit in the R3. UDP responders should compose response packets that fit in the
minimum of the offered requestor's maximum UDP payload size minimum of the offered requestor's maximum UDP payload size
[RFC6891], the interface MTU, the network MTU value configured by the [RFC6891], the interface MTU, the network MTU value configured
knowledge of the network operators, and the RECOMMENDED maximum DNS/ by the knowledge of the network operators, and the RECOMMENDED
UDP payload size 1400. (See Appendix A for more information.) maximum DNS/UDP payload size 1400. For more details, see
Appendix A.
R4. If the UDP responder detects an immediate error indicating that R4. If the UDP responder detects an immediate error indicating
the UDP packet exceeds the path MTU size, the UDP responder may that the UDP packet exceeds the path MTU size, the UDP
recreate response packets that fit in the path MTU size, or with the responder may recreate response packets that fit in the path
TC bit set. MTU size or with the TC bit set.
The cause and effect of the TC bit are unchanged [RFC1035]. The cause and effect of the TC bit are unchanged [RFC1035].
3.2. Proposed Recommendations for UDP requestors 3.2. Proposed Recommendations for UDP Requestors
R5. UDP requestors should limit the requestor's maximum UDP payload R5. UDP requestors should limit the requestor's maximum UDP
size to fit in the minimum of the interface MTU, the network MTU payload size to fit in the minimum of the interface MTU, the
value configured by the network operators, and the RECOMMENDED network MTU value configured by the network operators, and the
maximum DNS/UDP payload size 1400. A smaller limit may be allowed. RECOMMENDED maximum DNS/UDP payload size 1400. A smaller
(See Appendix A for more information.) limit may be allowed. For more details, see Appendix A.
R6. UDP requestors should/may drop fragmented DNS/UDP responses R6. UDP requestors should/may drop fragmented DNS/UDP responses
without IP reassembly to avoid cache poisoning attacks (at firewall without IP reassembly to avoid cache poisoning attacks (at the
function). firewall function).
R7. DNS responses may be dropped by IP fragmentation. Requestors R7. DNS responses may be dropped by IP fragmentation. It is
are recommended to try alternative transport protocols eventually. recommended that requestors eventually try alternative
transport protocols.
4. Proposed Recommendations for DNS operators 4. Proposed Recommendations for DNS Operators
Large DNS responses are typically the result of zone configuration. Large DNS responses are typically the result of zone configuration.
People who publish information in the DNS should seek configurations People who publish information in the DNS should seek configurations
resulting in small responses. For example, resulting in small responses. For example:
R8. Use a smaller number of name servers. R8. Use a smaller number of name servers.
R9. Use a smaller number of A/AAAA RRs for a domain name. R9. Use a smaller number of A/AAAA RRs for a domain name.
R10. Use minimal-responses configuration: Some implementations have R10. Use minimal-responses configuration: Some implementations have
a 'minimal responses' configuration option that causes DNS servers to a 'minimal responses' configuration option that causes DNS
make response packets smaller, containing only mandatory and required servers to make response packets smaller by containing only
data (Appendix B). mandatory and required data (Appendix B).
R11. Use a smaller signature / public key size algorithm for DNSSEC. R11. Use a smaller signature / public key size algorithm for
Notably, the signature sizes of ECDSA and EdDSA are smaller than DNSSEC. Notably, the signature sizes of the Elliptic Curve
those of equivalent cryptographic strength using RSA. Digital Signature Algorithm (ECDSA) and Edwards-curve Digital
Signature Algorithm (EdDSA) are smaller than those of
equivalent cryptographic strength using RSA.
It is difficult to determine a specific upper limit for R8, R9, and It is difficult to determine a specific upper limit for R8, R9, and
R11, but it is sufficient if all responses from the DNS servers are R11, but it is sufficient if all responses from the DNS servers are
below the size of R3 and R5. below the size of R3 and R5.
5. Protocol compliance considerations 5. Protocol Compliance Considerations
Some authoritative servers deviate from the DNS standard as follows: Some authoritative servers deviate from the DNS standard as follows:
* Some authoritative servers ignore the EDNS0 requestor's maximum * Some authoritative servers ignore the EDNS0 requestor's maximum
UDP payload size and return large UDP responses. [Fujiwara2018] UDP payload size and return large UDP responses [Fujiwara2018].
* Some authoritative servers do not support TCP transport. * Some authoritative servers do not support TCP transport.
Such non-compliant behavior cannot become implementation or Such non-compliant behavior cannot become implementation or
configuration constraints for the rest of the DNS. If failure is the configuration constraints for the rest of the DNS. If failure is the
result, then that failure must be localized to the non-compliant result, then that failure must be localized to the non-compliant
servers. servers.
6. IANA Considerations 6. IANA Considerations
This document requests no IANA actions. This document has no IANA actions.
7. Security Considerations 7. Security Considerations
7.1. On-path fragmentation on IPv4 7.1. On-Path Fragmentation on IPv4
If the Don't Fragment (DF) bit is not set, on-path fragmentation may If the Don't Fragment (DF) bit is not set, on-path fragmentation may
happen on IPv4, and lead to vulnerabilities, as shown in Section 7.3. happen on IPv4, and it can lead to vulnerabilities as shown in
To avoid this, recommendation R6 needs to be used to discard the Section 7.3. To avoid this, recommendation R6 needs to be used to
fragmented responses and retry by TCP. discard the fragmented responses and retry using TCP.
7.2. Small MTU network 7.2. Small MTU Network
When avoiding fragmentation, a DNS/UDP requestor behind a small MTU When avoiding fragmentation, a DNS/UDP requestor behind a small MTU
network may experience UDP timeouts, which would reduce performance network may experience UDP timeouts, which would reduce performance
and which may lead to TCP fallback. This would indicate prior and may lead to TCP fallback. This would indicate prior reliance
reliance upon IP fragmentation, which is considered to be harmful to upon IP fragmentation, which is considered to be harmful to both the
both the performance and stability of applications, endpoints, and performance and stability of applications, endpoints, and gateways.
gateways. Avoiding IP fragmentation will improve operating Avoiding IP fragmentation will improve operating conditions overall,
conditions overall, and the performance of DNS/TCP has increased and and the performance of DNS/TCP has increased and will continue to
will continue to increase. increase.
If a UDP response packet is dropped in transit, up to and including If a UDP response packet is dropped in transit, up to and including
the network stack of the initiator, it increases the attack window the network stack of the initiator, it increases the attack window
for poisoning the requestor's cache. for poisoning the requestor's cache.
7.3. Weaknesses of IP fragmentation 7.3. Weaknesses of IP Fragmentation
"Fragmentation Considered Poisonous" [Herzberg2013] noted effective "Fragmentation Considered Poisonous" [Herzberg2013] notes effective
off-path DNS cache poisoning attack vectors using IP fragmentation. off-path DNS cache poisoning attack vectors using IP fragmentation.
"IP fragmentation attack on DNS" [Hlavacek2013] and "Domain "IP fragmentation attack on DNS" [Hlavacek2013] and "Domain
Validation++ For MitM-Resilient PKI" [Brandt2018] noted that off-path Validation++ For MitM-Resilient PKI" [Brandt2018] note that off-path
attackers can intervene in the path MTU discovery [RFC1191] to cause attackers can intervene in the Path MTU Discovery [RFC1191] to cause
authoritative servers to produce fragmented responses. [RFC7739] authoritative servers to produce fragmented responses. [RFC7739]
stated the security implications of predictable fragment states the security implications of predictable fragment
identification values. identification values.
In Section 3.2 (Message Side Guidelines) of UDP Usage Guidelines In Section 3.2 ("Message Size Guidelines") of "UDP Usage Guidelines"
[RFC8085] we are told that an application SHOULD NOT send UDP [RFC8085], we are told that an application SHOULD NOT send UDP
datagrams that result in IP packets that exceed the Maximum datagrams that result in IP packets that exceed the MTU along the
Transmission Unit (MTU) along the path to the destination. path to the destination.
A DNS message receiver cannot trust fragmented UDP datagrams A DNS message receiver cannot trust fragmented UDP datagrams
primarily due to the small amount of entropy provided by UDP port primarily due to the small amount of entropy provided by UDP port
numbers and DNS message identifiers, each of which being only 16 bits numbers and DNS message identifiers, each of which is only 16 bits in
in size, and both likely being in the first fragment of a packet if size, and both are likely to be in the first fragment of a packet if
fragmentation occurs. By comparison, the TCP protocol stack controls fragmentation occurs. By comparison, the TCP protocol stack controls
packet size and avoids IP fragmentation under ICMP NEEDFRAG attacks. packet size and avoids IP fragmentation under ICMP NEEDFRAG attacks.
In TCP, fragmentation should be avoided for performance reasons, In TCP, fragmentation should be avoided for performance reasons,
whereas for UDP, fragmentation should be avoided for resiliency and whereas for UDP, fragmentation should be avoided for resiliency and
authenticity reasons. authenticity reasons.
7.4. DNS Security Protections 7.4. DNS Security Protections
DNSSEC is a countermeasure against cache poisoning attacks that use DNSSEC is a countermeasure against cache poisoning attacks that use
IP fragmentation. However, DNS delegation responses are not signed IP fragmentation. However, DNS delegation responses are not signed
with DNSSEC, and DNSSEC does not have a mechanism to get the correct with DNSSEC, and DNSSEC does not have a mechanism to get the correct
response if an incorrect delegation is injected. This is a denial- response if an incorrect delegation is injected. This is a denial-
of-service vulnerability that can yield failed name resolutions. If of-service vulnerability that can yield failed name resolutions. If
cache poisoning attacks can be avoided, DNSSEC validation failures cache poisoning attacks can be avoided, DNSSEC validation failures
will be avoided. will be avoided.
7.5. Possible actions for resolver operators 7.5. Possible Actions for Resolver Operators
Because this document is published as an "Informational" document Because this document is published as Informational rather than a
rather than a "Best Current Practice," this section presents steps Best Current Practice, this section presents steps that resolver
that resolver operators can take to avoid vulnerabilities related to operators can take to avoid vulnerabilities related to IP
IP fragmentation. fragmentation.
To avoid vulnerabilities related to IP fragmentation, implement R5 To avoid vulnerabilities related to IP fragmentation, implement R5
and R6. and R6.
Specifically, configure the firewall functions protecting the full- Specifically, configure the firewall functions protecting the full-
service resolver to discard incoming DNS response packets with a non- service resolver to discard incoming DNS response packets with a non-
zero Fragment offset or a More Fragments (MF) bit of 1 on IPv4, and zero Fragment Offset (FO) or a More Fragments (MF) bit of 1 on IPv4,
discard packets with IPv6 Fragment Headers. (If the resolver's IP and discard packets with IPv6 Fragment Headers. (If the resolver's
address is not dedicated to the DNS resolver and uses UDP IP address is not dedicated to the DNS resolver and uses UDP
communication that relies on IP Fragmentation for purposes other than communication that relies on IP Fragmentation for purposes other than
DNS, discard only the first fragment that contains the UDP header DNS, discard only the first fragment that contains the UDP header
from port 53.) from port 53.)
The most recent resolver software is believed to implement R7. The most recent resolver software is believed to implement R7.
Even if R7 is not implemented, it will only result in a name Even if R7 is not implemented, it will only result in a name
resolution error, preventing attacks from leading to malicious sites. resolution error, preventing attacks from leading to malicious sites.
8. Acknowledgments 8. References
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, Joe Touch, Wouter Wijngaards, Vladimir Cunat, Benno
Overeinder and Štěpán Němec for extensive review and comments.
9. References
9.1. Normative References 8.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/rfc/rfc1035>. November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990, DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/rfc/rfc1191>. <https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
2000, <https://www.rfc-editor.org/rfc/rfc2931>. 2000, <https://www.rfc-editor.org/info/rfc2931>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013, DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/rfc/rfc6891>. <https://www.rfc-editor.org/info/rfc6891>.
[RFC7739] Gont, F., "Security Implications of Predictable Fragment [RFC7739] Gont, F., "Security Implications of Predictable Fragment
Identification Values", RFC 7739, DOI 10.17487/RFC7739, Identification Values", RFC 7739, DOI 10.17487/RFC7739,
February 2016, <https://www.rfc-editor.org/rfc/rfc7739>. February 2016, <https://www.rfc-editor.org/info/rfc7739>.
[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and [RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016, Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/rfc/rfc7766>. <https://www.rfc-editor.org/info/rfc7766>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/rfc/rfc8085>. March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200, (IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017, DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/rfc/rfc8200>. <https://www.rfc-editor.org/info/rfc8200>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201, "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017, DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/rfc/rfc8201>. <https://www.rfc-editor.org/info/rfc8201>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 8499, DOI 10.17487/RFC8499, January
2019, <https://www.rfc-editor.org/rfc/rfc8499>.
[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T. [RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899, Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/rfc/rfc8899>. September 2020, <https://www.rfc-editor.org/info/rfc8899>.
[RFC8945] Dupont, F., Morris, S., Vixie, P., Eastlake 3rd, D., [RFC8945] Dupont, F., Morris, S., Vixie, P., Eastlake 3rd, D.,
Gudmundsson, O., and B. Wellington, "Secret Key Gudmundsson, O., and B. Wellington, "Secret Key
Transaction Authentication for DNS (TSIG)", STD 93, Transaction Authentication for DNS (TSIG)", STD 93,
RFC 8945, DOI 10.17487/RFC8945, November 2020, RFC 8945, DOI 10.17487/RFC8945, November 2020,
<https://www.rfc-editor.org/rfc/rfc8945>. <https://www.rfc-editor.org/info/rfc8945>.
9.2. Informative References [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] [Brandt2018]
Brandt, M., Dai, T., Klein, A., Shulman, H., and M. Brandt, M., Dai, T., Klein, A., Shulman, H., and M.
Waidner, "Domain Validation++ For MitM-Resilient PKI", Waidner, "Domain Validation++ For MitM-Resilient PKI",
Proceedings of the 2018 ACM SIGSAC Conference on Computer Proceedings of the 2018 ACM SIGSAC Conference on Computer
and Communications Security , 2018. and Communications Security, pp. 2060-2076,
DOI 10.1145/3243734.3243790, October 2018,
<https://doi.org/10.1145/3243734.3243790>.
[DNSFlagDay2020] [DNSFlagDay2020]
"DNS flag day 2020", n.d., <https://dnsflagday.net/2020/>. "DNS flag day 2020", <https://dnsflagday.net/2020/>.
[Fujiwara2018] [Fujiwara2018]
Fujiwara, K., "Measures against cache poisoning attacks Fujiwara, K., "Measures against DNS cache poisoning
using IP fragmentation in DNS", OARC 30 Workshop , 2019. attacks using IP fragmentation", OARC 30 Workshop, 2019.
[Herzberg2013] [Herzberg2013]
Herzberg, A. and H. Shulman, "Fragmentation Considered Herzberg, A. and H. Shulman, "Fragmentation Considered
Poisonous", IEEE Conference on Communications and Network Poisonous, or: One-domain-to-rule-them-all.org", IEEE
Security , 2013. Conference on Communications and Network Security (CNS),
DOI 10.1109/CNS.2013.6682711, 2013,
<https://doi.org/10.1109/CNS.2013.6682711>.
[Hlavacek2013] [Hlavacek2013]
Hlavacek, T., "IP fragmentation attack on DNS", RIPE 67 Hlavacek, T., "IP fragmentation attack on DNS", RIPE 67
Meeting , 2013, <https://ripe67.ripe.net/ Meeting, 2013, <https://ripe67.ripe.net/
presentations/240-ipfragattack.pdf>. presentations/240-ipfragattack.pdf>.
[Huston2021] [Huston2021]
Huston, G. and J. Damas, "Measuring DNS Flag Day 2020", Huston, G. and J. Damas, "Measuring DNS Flag Day 2020",
OARC 34 Workshop , February 2021. OARC 34 Workshop, February 2021.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981, DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/rfc/rfc791>. <https://www.rfc-editor.org/info/rfc791>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/rfc/rfc2308>. <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 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782, specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000, DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/rfc/rfc2782>. <https://www.rfc-editor.org/info/rfc2782>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/rfc/rfc4035>. <https://www.rfc-editor.org/info/rfc4035>.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008, Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<https://www.rfc-editor.org/rfc/rfc5155>. <https://www.rfc-editor.org/info/rfc5155>.
[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., [RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile", and F. Gont, "IP Fragmentation Considered Fragile",
BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020, BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
<https://www.rfc-editor.org/rfc/rfc8900>. <https://www.rfc-editor.org/info/rfc8900>.
[RFC9460] Schwartz, B., Bishop, M., and E. Nygren, "Service Binding [RFC9460] Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
and Parameter Specification via the DNS (SVCB and HTTPS and Parameter Specification via the DNS (SVCB and HTTPS
Resource Records)", RFC 9460, DOI 10.17487/RFC9460, Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
November 2023, <https://www.rfc-editor.org/rfc/rfc9460>. November 2023, <https://www.rfc-editor.org/info/rfc9460>.
[RFC9471] Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS [RFC9471] Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS
Glue Requirements in Referral Responses", RFC 9471, Glue Requirements in Referral Responses", RFC 9471,
DOI 10.17487/RFC9471, September 2023, DOI 10.17487/RFC9471, September 2023,
<https://www.rfc-editor.org/rfc/rfc9471>. <https://www.rfc-editor.org/info/rfc9471>.
Appendix A. Details of requestor's maximum UDP payload size discussions Appendix A. Details of Requestor's Maximum UDP Payload Size Discussions
There are many discussions for default path MTU size and requestor's There are many discussions about default path MTU size and a
maximum UDP payload size. requestor's maximum UDP payload size.
* The minimum MTU for an IPv6 interface is 1280 octets (see * The minimum MTU for an IPv6 interface is 1280 octets (see
Section 5 of [RFC8200]). So, we can use it as the default path Section 5 of [RFC8200]). So, it can be used as the default path
MTU value for IPv6. The corresponding minimum MTU for an IPv4 MTU value for IPv6. The corresponding minimum MTU for an IPv4
interface is 68 (60 + 8) [RFC0791]. interface is 68 (60 + 8) [RFC0791].
* [RFC4035] defines that "A security-aware name server MUST support * [RFC4035] states that "A security-aware name server MUST support
the EDNS0 message size extension, MUST support a message size of the EDNS0 ([RFC2671]) message size extension, [and it] MUST
at least 1220 octets". Then, the smallest number of the maximum support a message size of at least 1220 octets". Then, the
DNS/UDP payload size is 1220. smallest number of the maximum DNS/UDP payload size is 1220.
* In order to avoid IP fragmentation, [DNSFlagDay2020] proposed that * In order to avoid IP fragmentation, [DNSFlagDay2020] proposes that
the UDP requestors set the requestor's payload size to 1232, and UDP requestors set the requestor's payload size to 1232 and UDP
the UDP responders compose UDP responses so they fit in 1232 responders compose UDP responses so they fit in 1232 octets. The
octets. The size 1232 is based on an MTU of 1280, which is size 1232 is based on an MTU of 1280, which is required by the
required by the IPv6 specification [RFC8200], minus 48 octets for IPv6 specification [RFC8200], minus 48 octets for the IPv6 and UDP
the IPv6 and UDP headers. headers.
* Most of the Internet and especially the inner core has an MTU of * Most of the Internet, especially the inner core, has an MTU of at
at least 1500 octets. Maximum DNS/UDP payload size for IPv6 on least 1500 octets. Maximum DNS/UDP payload size for IPv6 on an
MTU 1500 ethernet is 1452 (1500 minus 40 (IPv6 header size) minus MTU 1500 Ethernet is 1452 (1500 minus 40 (IPv6 header size) minus
8 (UDP header size)). To allow for possible IP options and 8 (UDP header size)). To allow for possible IP options and
distant tunnel overhead, the recommendation of default maximum distant tunnel overhead, the recommendation of default maximum
DNS/UDP payload size is 1400. DNS/UDP payload size is 1400.
* [Huston2021] analyzed the result of [DNSFlagDay2020] and reported * [Huston2021] analyzes the result of [DNSFlagDay2020] and reports
that their measurements suggest that in the interior of the that their measurements suggest that in the interior of the
Internet between recursive resolvers and authoritative servers the Internet between recursive resolvers and authoritative servers,
prevailing MTU is 1500 and there is no measurable signal of use of the prevailing MTU is 1500 and there is no measurable signal of
smaller MTUs in this part of the Internet, and proposed that their use of smaller MTUs in this part of the Internet. They propose
measurements suggest setting the EDNS0 requestor's UDP payload that their measurements suggest setting the EDNS0 requestor's UDP
size to 1472 octets for IPv4, and 1452 octets for IPv6. payload size to 1472 octets for IPv4 and 1452 octets for IPv6.
As a result of discussions, this document decided to recommend a As a result of these discussions, this document recommends a value of
value of 1400, with smaller values also allowed. 1400, with smaller values also allowed.
Appendix B. Minimal-responses Appendix B. Minimal Responses
Some implementations have a "minimal responses" configuration Some implementations have a "minimal responses" configuration
setting/option that causes a DNS server to make response packets setting/option that causes a DNS server to make response packets
smaller, containing only mandatory and required data. smaller, containing only mandatory and required data.
Under the minimal-responses configuration, a DNS server composes Under the minimal-responses configuration, a DNS server composes
responses containing only necessary RRs. For delegations, see responses containing only necessary Resource Records (RRs). For
[RFC9471]. In case of a non-existent domain name or non-existent delegations, see [RFC9471]. In case of a non-existent domain name or
type, the authority section will contain an SOA record and the answer non-existent type, the authority section will contain an SOA record,
section is empty. (defined in Section 2 of [RFC2308]). and the answer section is empty (see Section 2 of [RFC2308]).
Some resource records (MX, SRV, SVCB, HTTPS) require additional A, Some resource records (MX, SRV, SVCB, and HTTPS) require additional
AAAA, and SVCB records in the Additional Section defined in A, AAAA, and Service Binding (SVCB) records in the Additional section
[RFC1035], [RFC2782] and [RFC9460]. defined in [RFC1035], [RFC2782], and [RFC9460].
In addition, if the zone is DNSSEC signed and a query has the DNSSEC 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 OK bit, signatures are added in the answer section, or the
corresponding DS RRSet and signatures are added in the authority corresponding DS RRSet and signatures are added in the authority
section. Details are defined in [RFC4035] and [RFC5155]. section. Details are defined in [RFC4035] and [RFC5155].
Appendix C. Known Implementations Appendix C. Known Implementations
This section records the status of known implementations of these This section records the status of known implementations of the best
best practices defined by this specification at the time of practices defined by this specification at the time of publication
publication, and any deviation from the specification. and any deviation from the specification.
Please note that the listing of any individual implementation here Please note that the listing of any individual implementation here
does not imply endorsement by the IETF. Furthermore, no effort has does not imply endorsement by the IETF. Furthermore, no effort has
been spent to verify the information presented here that was supplied been made to verify the information that was supplied by IETF
by IETF contributors. contributors and presented here.
C.1. BIND 9 C.1. BIND 9
BIND 9 does not implement the recommendations 1 and 2 in Section 3.1. BIND 9 does not implement recommendations 1 and 2 in Section 3.1.
BIND 9 on Linux sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with a BIND 9 on Linux sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with a
fallback to IP_PMTUDISC_DONT. fallback to IP_PMTUDISC_DONT.
BIND 9 on systems with IP_DONTFRAG (such as FreeBSD), IP_DONTFRAG is BIND 9 on systems with IP_DONTFRAG (such as FreeBSD), IP_DONTFRAG is
disabled. disabled.
Accepting PATH MTU Discovery for UDP is considered harmful and Accepting Path MTU Discovery for UDP is considered harmful and
dangerous. BIND 9's settings avoid attacks to path MTU discovery. dangerous. BIND 9's settings avoid attacks to Path MTU Discovery.
For recommendation 3, BIND 9 will honor the requestor's size up to For recommendation 3, BIND 9 will honor the requestor's size up to
the configured limit (max-udp-size). The UDP response packet is 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. 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- BIND 9 supports the requestor's size up to the configured limit (max-
udp-size). udp-size).
In the case of recommendation 4, and the send fails with EMSGSIZE, In the case of recommendation 4 and the send fails with EMSGSIZE,
BIND 9 set the TC bit and try to send a minimal answer again. BIND 9 sets the TC bit and tries to send a minimal answer again.
In the first recommendation of Section 3.2, BIND 9 uses the edns-buf- In the first recommendation of Section 3.2, BIND 9 uses the edns-buf-
size option, with the default of 1232. size option, with the default of 1232.
BIND 9 does implement recommendation 2 of Section 3.2. BIND 9 does implement recommendation 2 (Section 3.2).
For recommendation 3, after two UDP timeouts, BIND 9 will fall back For recommendation 3, after two UDP timeouts, BIND 9 will fall back
to TCP. to TCP.
C.2. Knot DNS and Knot Resolver C.2. Knot DNS and Knot Resolver
Both Knot servers set IP_PMTUDISC_OMIT to avoid path MTU spoofing. Both Knot servers set IP_PMTUDISC_OMIT to avoid path MTU spoofing.
UDP size limit is 1232 by default. The UDP size limit is 1232 by default.
Fragments are ignored if they arrive over an XDP interface. Fragments are ignored if they arrive over an XDP interface.
TCP is attempted after repeated UDP timeouts. TCP is attempted after repeated UDP timeouts.
Minimal responses are returned and are currently not configurable. Minimal responses are returned and are currently not configurable.
Smaller signatures are used, with ecdsap256sha256 as the default. Smaller signatures are used, with ecdsap256sha256 as the default.
C.3. PowerDNS Authoritative Server, PowerDNS Recursor, PowerDNS dnsdist C.3. PowerDNS Authoritative Server, PowerDNS Recursor, and PowerDNS
dnsdist
* IP_PMTUDISC_OMIT with fallback to IP_PMTUDISC_DONT * IP_PMTUDISC_OMIT with a fallback to IP_PMTUDISC_DONT
* default EDNS buffer size of 1232, no probing for smaller sizes * default EDNS buffer size of 1232; no probing for smaller sizes
* no handling of EMSGSIZE * no handling of EMSGSIZE
* Recursor: UDP timeouts do not cause a switch to TCP. "Spoofing
* Recursor: UDP timeouts do not cause a switch to TCP; "Spoofing
nearmisses" do. nearmisses" do.
C.4. PowerDNS Authoritative Server C.4. PowerDNS Authoritative Server
* the default DNSSEC algorithm is 13 * The default DNSSEC algorithm is 13.
* responses are minimal, this is not configurable * Responses are minimal; this is not configurable.
C.5. Unbound C.5. Unbound
Unbound sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with fallback to Unbound sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with fallback to
IP_PMTUDISC_DONT. It also disables IP_DONTFRAG on systems that have IP_PMTUDISC_DONT. It also disables IP_DONTFRAG on systems that have
it, but not on Apple systems. On systems that support it Unbound 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 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 fallback to IPV6_USER_MTU. It also sets IPV6_MTU_DISCOVER to
IPV6_PMTUDISC_OMIT with a fallback to IPV6_PMTUDISC_DONT. IPV6_PMTUDISC_OMIT, with a fallback to IPV6_PMTUDISC_DONT.
Unbound requests UDP size 1232 from peers, by default. The Unbound requests a UDP size of 1232 from peers, by default. The
requestors size is limited to a max of 1232. requestor's size is limited to a max of 1232.
After some timeouts, Unbound retries with a smaller size, if that is After some timeouts, Unbound retries with a smaller size, if that is
smaller, at size 1232 for IPv6 and 1472 for IPv4. This does not do smaller, at size 1232 for IPv6 and 1472 for IPv4. This does not do
anything since the flag day change to 1232. anything since the flag day change to 1232.
Unbound has minimal responses as an option, default on. Unbound has minimal responses as an option, 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 Authors' Addresses
Kazunori Fujiwara Kazunori Fujiwara
Japan Registry Services Co., Ltd. Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda, Chiyoda-ku, Tokyo Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda, Chiyoda-ku, Tokyo
101-0065 101-0065
Japan Japan
Phone: +81 3 5215 8451 Phone: +81 3 5215 8451
Email: fujiwara@jprs.co.jp Email: fujiwara@jprs.co.jp
Paul Vixie Paul Vixie
AWS Security AWS Security
11400 La Honda Road 11400 La Honda Road
Woodside, CA, 94062 Woodside, CA 94062
United States of America United States of America
Phone: +1 650 393 3994 Phone: +1 650 393 3994
Email: paul@redbarn.org Email: paul@redbarn.org
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