rfc9715v1.txt		rfc9715.txt
	skipping to change at line 12 ¶		skipping to change at line 12 ¶
	Internet Engineering Task Force (IETF) K. Fujiwara		Internet Engineering Task Force (IETF) K. Fujiwara
	Request for Comments: 9715 JPRS		Request for Comments: 9715 JPRS
	Category: Informational P. Vixie		Category: Informational P. Vixie
	ISSN: 2070-1721 AWS Security		ISSN: 2070-1721 AWS Security
	January 2025		January 2025
	IP Fragmentation Avoidance in DNS over UDP		IP Fragmentation Avoidance in DNS over UDP
	Abstract		Abstract
	The widely deployed Extension Mechanisms for DNS (EDNS0) feature in		The widely deployed Extension Mechanisms for DNS (EDNS(0)) feature in
	the DNS enables a DNS receiver to indicate its received UDP message		the DNS enables a DNS receiver to indicate its received UDP message
	size capacity, which supports the sending of large UDP responses by a		size capacity, which supports the sending of large UDP responses by a
	DNS server. Large DNS/UDP messages are more likely to be fragmented,		DNS server. Large DNS/UDP messages are more likely to be fragmented,
	and IP fragmentation has exposed weaknesses in application protocols.		and IP fragmentation has exposed weaknesses in application protocols.
	It is possible to avoid IP fragmentation in DNS by limiting the		It is possible to avoid IP fragmentation in DNS by limiting the
	response size where possible and signaling the need to upgrade from		response size where possible and signaling the need to upgrade from
	UDP to TCP transport where necessary. This document describes		UDP to TCP transport where necessary. This document describes
	techniques to avoid IP fragmentation in DNS.		techniques to avoid IP fragmentation in DNS.
	Status of This Memo		Status of This Memo
	skipping to change at line 95 ¶		skipping to change at line 95 ¶
	1. Introduction		1. Introduction
	This document was originally intended to be a Best Current Practice,		This document was originally intended to be a Best Current Practice,
	but due to operating system and socket option limitations, some of		but due to operating system and socket option limitations, some of
	the recommendations have not yet gained real-world experience;		the recommendations have not yet gained real-world experience;
	therefore, this document is Informational. It is expected that, as		therefore, this document is Informational. It is expected that, as
	operating systems and implementations evolve, we will gain more		operating systems and implementations evolve, we will gain more
	experience with the recommendations and will publish an updated		experience with the recommendations and will publish an updated
	document as a Best Current Practice in the future.		document as a Best Current Practice in the future.
	DNS has an EDNS0 mechanism [RFC6891]. The widely deployed EDNS0		DNS has an EDNS(0) mechanism [RFC6891]. The widely deployed EDNS(0)
	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 Maximum Transmission Unit (MTU) of		when a packet is larger than the Maximum Transmission Unit (MTU) of
	some network in the packet's 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 MSS. This takes advantage of the elasticity of the TCP's
	TCP's packetizing process as to how much queued data will fit into		packetizing process, depending on 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, and it protects delegation		security risks of fragmentation, and it protects delegation
	skipping to change at line 183 ¶		skipping to change at line 183 ¶
	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		fragmentation of UDP packets when sending replies, provided it
	can be done safely. The mechanisms to achieve this vary		can be done safely. The mechanisms to achieve this vary
	across different operating systems.		across different operating systems.
	For BSD-like operating systems, the IP Don't Fragment flag		For BSD-like operating systems, the IP Don't Fragment (DF)
	(DF) bit [RFC0791] can be used to prevent fragmentation. In		flag bit [RFC0791] can be used to prevent fragmentation. In
	contrast, Linux systems do not expose a direct API for this		contrast, Linux systems do not expose a direct API for this
	purpose and require the use of Path MTU socket options		purpose and require the use of Path MTU socket options
	(IP_MTU_DISCOVER) to manage fragmentation settings. However,		(IP_MTU_DISCOVER) to manage fragmentation settings. However,
	it is important to note that enabling IPv4 Path MTU Discovery		it is important to note that enabling IPv4 Path MTU Discovery
	for UDP in current Linux versions is considered harmful and		for UDP in current Linux versions is considered harmful and
	dangerous. For more details, see 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		[RFC6891], the interface MTU, the network MTU value configured
	skipping to change at line 214 ¶		skipping to change at line 214 ¶
	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		R5. UDP requestors should limit the requestor's maximum UDP
	payload size to fit in the minimum of the interface MTU, the		payload size to fit in the minimum of the interface MTU, the
	network MTU value configured by the network operators, and the		network MTU value configured by the network operators, and the
	RECOMMENDED maximum DNS/UDP payload size 1400. A smaller		RECOMMENDED maximum DNS/UDP payload size 1400. A smaller
	limit may be allowed. For more details, see Appendix A.		limit may be allowed. For more details, see Appendix A.
	R6. UDP requestors should/may drop fragmented DNS/UDP responses		R6. UDP requestors should drop fragmented DNS/UDP responses
	without IP reassembly to avoid cache poisoning attacks (at the		without IP reassembly to avoid cache poisoning attacks (at the
	firewall function).		firewall function).
	R7. DNS responses may be dropped by IP fragmentation. It is		R7. DNS responses may be dropped by IP fragmentation. It is
	recommended that requestors eventually try alternative		recommended that requestors eventually try alternative
	transport protocols.		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.
	skipping to change at line 251 ¶		skipping to change at line 251 ¶
	equivalent cryptographic strength using RSA.		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 EDNS(0) 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 has 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) flag bit is not set, on-path fragmentation
	happen on IPv4, and it can lead to vulnerabilities as shown in		may happen on IPv4, and it can lead to vulnerabilities as shown in
	Section 7.3. To avoid this, recommendation R6 needs to be used to		Section 7.3. To avoid this, R6 needs to be used to discard the
	discard the fragmented responses and retry using TCP.		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 may lead to TCP fallback. This would indicate prior reliance		and may lead to TCP fallback. This would indicate prior reliance
	upon IP fragmentation, which is considered to be harmful to both the		upon IP fragmentation, which is considered to be harmful to both the
	performance and stability of applications, endpoints, and gateways.		performance and stability of applications, endpoints, and gateways.
	Avoiding IP fragmentation will improve operating conditions overall,		Avoiding IP fragmentation will improve operating conditions overall,
	and the performance of DNS/TCP has increased and will continue to		and the performance of DNS/TCP has increased and will continue to
	skipping to change at line 300 ¶		skipping to change at line 300 ¶
	"Fragmentation Considered Poisonous" [Herzberg2013] notes 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] note 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]
	states the security implications of predictable fragment		states the security implications of predictable fragment
	identification values.		identification values.
	In Section 3.2 ("Message Size Guidelines") of "UDP Usage Guidelines"		Section 3.2 of [RFC8085] states that "an application SHOULD NOT send
	[RFC8085], we are told that an application SHOULD NOT send UDP		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 is only 16 bits in		numbers and DNS message identifiers, each of which is only 16 bits in
	size, and both are likely to be 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.
	skipping to change at line 337 ¶		skipping to change at line 336 ¶
	Because this document is published as Informational rather than a		Because this document is published as Informational rather than a
	Best Current Practice, this section presents steps that resolver		Best Current Practice, this section presents steps that resolver
	operators can take to avoid vulnerabilities related to IP		operators can take to avoid vulnerabilities related to 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 (FO) or a More Fragments (MF) bit of 1 on IPv4,		zero Fragment Offset (FO) or a More Fragments (MF) flag bit of 1 on
	and discard packets with IPv6 Fragment Headers. (If the resolver's		IPv4, and discard packets with IPv6 Fragment Headers. (If the
	IP address is not dedicated to the DNS resolver and uses UDP		resolver's IP address is not dedicated to the DNS resolver and uses
	communication that relies on IP Fragmentation for purposes other than		UDP communication that relies on IP Fragmentation for purposes other
	DNS, discard only the first fragment that contains the UDP header		than DNS, discard only the first fragment that contains the UDP
	from port 53.)		header 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. References		8. References
	8.1. Normative References		8.1. Normative References
	skipping to change at line 425 ¶		skipping to change at line 424 ¶
	<https://www.rfc-editor.org/info/rfc9499>.		<https://www.rfc-editor.org/info/rfc9499>.
	8.2. Informative References		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, pp. 2060-2076,		and Communications Security, pp. 2060-2076,
	DOI 10.1145/3243734.3243790, October 2018,		DOI 10.1145/3243734.3243790, October 2018,
	<https://doi.org/10.1145/3243734.3243790>.		<https://dl.acm.org/doi/10.1145/3243734.3243790>.
	[DNSFlagDay2020]		[DNSFlagDay2020]
	"DNS flag day 2020", <https://dnsflagday.net/2020/>.		"DNS flag day 2020", <https://dnsflagday.net/2020/>.
	[Fujiwara2018]		[Fujiwara2018]
	Fujiwara, K., "Measures against DNS cache poisoning		Fujiwara, K., "Measures against DNS cache poisoning
	attacks using IP fragmentation", OARC 30 Workshop, 2019.		attacks using IP fragmentation", OARC 30 Workshop, 2019,
			<https://indico.dns-
			oarc.net/event/31/contributions/692/attachments/660/1115/
			fujiwara-5.pdf>.
	[Herzberg2013]		[Herzberg2013]
	Herzberg, A. and H. Shulman, "Fragmentation Considered		Herzberg, A. and H. Shulman, "Fragmentation Considered
	Poisonous, or: One-domain-to-rule-them-all.org", IEEE		Poisonous, or: One-domain-to-rule-them-all.org", IEEE
	Conference on Communications and Network Security (CNS),		Conference on Communications and Network Security (CNS),
	DOI 10.1109/CNS.2013.6682711, 2013,		DOI 10.1109/CNS.2013.6682711, 2013,
	<https://doi.org/10.1109/CNS.2013.6682711>.		<https://ieeexplore.ieee.org/document/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, <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,		[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/info/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/info/rfc2308>.		<https://www.rfc-editor.org/info/rfc2308>.
	[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",		[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
	skipping to change at line 526 ¶		skipping to change at line 530 ¶
	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] analyzes the result of [DNSFlagDay2020] and reports		* [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,		Internet between recursive resolvers and authoritative servers,
	the prevailing MTU is 1500 and there is no measurable signal of		the prevailing MTU is 1500 and there is no measurable signal of
	use of smaller MTUs in this part of the Internet. They propose		use of smaller MTUs in this part of the Internet. They propose
	that their measurements suggest setting the EDNS0 requestor's UDP		that their measurements suggest setting the EDNS(0) requestor's
	payload size to 1472 octets for IPv4 and 1452 octets for IPv6.		UDP payload size to 1472 octets for IPv4 and 1452 octets for IPv6.
	As a result of these discussions, this document recommends a value of		As a result of these discussions, this document recommends a 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.
	skipping to change at line 555 ¶		skipping to change at line 559 ¶
	A, AAAA, and Service Binding (SVCB) records in the Additional section		A, AAAA, and Service Binding (SVCB) records in the Additional section
	defined in [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 the best		This section records the status of known implementations of the
	practices defined by this specification at the time of publication		proposed recommendations described in Section 3.
	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 made to verify the information that was supplied by IETF		been made to verify the information that was supplied by IETF
	contributors and presented here.		contributors and presented here.
	C.1. BIND 9		C.1. BIND 9
	BIND 9 does not implement recommendations 1 and 2 in Section 3.1.		BIND 9 does not implement R1 and R2.
	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		When BIND 9 is on systems with IP_DONTFRAG (such as FreeBSD),
	disabled.		IP_DONTFRAG is 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 R3, BIND 9 will honor the requestor's size up to the configured
	the configured limit (max-udp-size). The UDP response packet is		limit (max-udp-size). The UDP response packet is bound to be between
	bound to be between 512 and 4096 bytes, with the default set to 1232.		512 and 4096 bytes, with the default set to 1232. BIND 9 supports
	BIND 9 supports the requestor's size up to the configured limit (max-		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 R4 and the send fails with EMSGSIZE, BIND 9 sets the
	BIND 9 sets the TC bit and tries to send a minimal answer again.		TC bit and tries to send a minimal answer again.
	In the first recommendation of Section 3.2, BIND 9 uses the edns-buf-		For R5, BIND 9 uses the edns-buf-size option, with the default of
	size option, with the default of 1232.		1232.
	BIND 9 does implement recommendation 2 (Section 3.2).		BIND 9 does implement R6.
	For recommendation 3, after two UDP timeouts, BIND 9 will fall back		For R7, 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.
	The 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 a Linux 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, and PowerDNS		C.3. PowerDNS Authoritative Server, PowerDNS Recursor, and PowerDNS
	dnsdist		dnsdist
	* IP_PMTUDISC_OMIT with a fallback to IP_PMTUDISC_DONT		* Use IP_PMTUDISC_OMIT with a fallback to IP_PMTUDISC_DONT.
	* default EDNS buffer size of 1232; no probing for smaller sizes		* The default EDNS buffer size of 1232; no probing for smaller
			sizes.
	* no handling of EMSGSIZE		* There is 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, but "spoofing
	nearmisses" do.		near misses" may.
	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 a UDP size of 1232 from peers, by default. The		Unbound requests a UDP size of 1232 from peers, by default. The
	requestor's 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
	smaller, at size 1232 for IPv6 and 1472 for IPv4. This does not do		applicable, or at size 1232 for IPv6 and 1472 for IPv4. This does
	anything since the flag day change to 1232.		not cause any negative effects due to the "flag day" [DNSFlagDay2020]
			change to 1232.
	Unbound has minimal responses as an option, default on.		Unbound has the "minimal responses" configuration option; set default
			on.
	Acknowledgments		Acknowledgments
	The authors would like to specifically thank Paul Wouters, Mukund		The authors would like to specifically thank Paul Wouters, Mukund
	Sivaraman, Tony Finch, Hugo Salgado, Peter van Dijk, Brian Dickson,		Sivaraman, Tony Finch, Hugo Salgado, Peter van Dijk, Brian Dickson,
	Puneet Sood, Jim Reid, Petr Spacek, Andrew McConachie, Joe Abley,		Puneet Sood, Jim Reid, Petr Spacek, Andrew McConachie, Joe Abley,
	Daisuke Higashi, Joe Touch, Wouter Wijngaards, Vladimir Cunat, Benno		Daisuke Higashi, Joe Touch, Wouter Wijngaards, Vladimir Cunat, Benno
	Overeinder, and Štěpán Němec for their extensive reviews and		Overeinder, and Štěpán Němec for their extensive reviews and
	comments.		comments.
End of changes. 29 change blocks.
	56 lines changed or deleted		60 lines changed or added
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