IP Fragmentation Avoidance in DNS over UDP
draft-ietf-dnsop-avoid-fragmentation-20
| Document | Type | Active Internet-Draft (dnsop WG) | |
|---|---|---|---|
| Authors | Kazunori Fujiwara , Paul A. Vixie | ||
| Last updated | 2024-09-30 (Latest revision 2024-09-26) | ||
| Replaces | draft-fujiwara-dnsop-avoid-fragmentation | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Tim Wicinski | ||
| Shepherd write-up | Show Last changed 2023-10-10 | ||
| IESG | IESG state | RFC Ed Queue | |
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draft-ietf-dnsop-avoid-fragmentation-20
Network Working Group K. Fujiwara
Internet-Draft JPRS
Intended status: Informational P. Vixie
Expires: 30 March 2025 AWS Security
26 September 2024
IP Fragmentation Avoidance in DNS over UDP
draft-ietf-dnsop-avoid-fragmentation-20
Abstract
The widely deployed EDNS0 feature in the DNS enables a DNS receiver
to indicate its received UDP message size capacity, which supports
the sending of large UDP responses by a DNS server. Large DNS/UDP
messages are more likely to be fragmented and IP fragmentation has
exposed weaknesses in application protocols. It is possible to avoid
IP fragmentation in DNS by limiting the response size where possible,
and signaling the need to upgrade from UDP to TCP transport where
necessary. This document describes techniques to avoid IP
fragmentation in DNS.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 30 March 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. How to avoid IP fragmentation in DNS . . . . . . . . . . . . 4
3.1. Proposed Recommendations for UDP responders . . . . . . . 4
3.2. Proposed Recommendations for UDP requestors . . . . . . . 5
4. Proposed Recommendations for DNS operators . . . . . . . . . 5
5. Protocol compliance considerations . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7.1. On-path fragmentation on IPv4 . . . . . . . . . . . . . . 6
7.2. Small MTU network . . . . . . . . . . . . . . . . . . . . 6
7.3. Weaknesses of IP fragmentation . . . . . . . . . . . . . 7
7.4. DNS Security Protections . . . . . . . . . . . . . . . . 7
7.5. Possible actions for resolver operators . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Details of requestor's maximum UDP payload size
discussions . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Minimal-responses . . . . . . . . . . . . . . . . . 12
Appendix C. Known Implementations . . . . . . . . . . . . . . . 12
C.1. BIND 9 . . . . . . . . . . . . . . . . . . . . . . . . . 12
C.2. Knot DNS and Knot Resolver . . . . . . . . . . . . . . . 13
C.3. PowerDNS Authoritative Server, PowerDNS Recursor, PowerDNS
dnsdist . . . . . . . . . . . . . . . . . . . . . . . . . 13
C.4. PowerDNS Authoritative Server . . . . . . . . . . . . . . 14
C.5. Unbound . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
This document was originally intended to be a BCP, but due to
operating system and socket option limitations, some of the
recommendations have not yet gained real-world experience and
therefore the document is published as Informational. It is hoped
and expected that, as operating systems and implementations evolve,
we will gain more experience with the recommendations, and plan to
publish an updated document as a Best Current Practice.
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DNS has an EDNS0 [RFC6891] mechanism. The widely deployed EDNS0
feature in the DNS enables a DNS receiver to indicate its received
UDP message size capacity which supports the sending of large UDP
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
path.
Fragmented DNS UDP responses have systemic weaknesses, which expose
the requestor to DNS cache poisoning from off-path attackers. (See
Section 7.3 for references and details.)
[RFC8900] states that IP fragmentation introduces fragility to
Internet communication. The transport of DNS messages over UDP
should take account of the observations stated in that document.
TCP avoids fragmentation by segmenting data into packets that are
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
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
TCP's packetizing process as to how much queued data will fit into
the next segment. In contrast, DNS over UDP has little datagram size
elasticity and lacks insight into IP header and option size, so we
must make more conservative estimates about available UDP payload
space.
[RFC7766] states that 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, including protecting delegation
responses. But [RFC8945] has limited applicability due to key
distribution requirements and there is little if any deployment of
[RFC2931].
This document describes various techniques to avoid IP fragmentation
of UDP packets in DNS. This document is primarily applicable to DNS
use on the global Internet.
In contrast, a path MTU that deviates from the recommended value
might be obtained through static configuration, server routing hints,
or a future discovery protocol. However, addressing this falls
outside the scope of this document and may be the subject of future
specifications.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
"Requestor" refers to the side that sends a request. "Responder"
refers to an authoritative server, recursive resolver or other DNS
component that responds to questions. (Quoted from EDNS0 [RFC6891])
"Path MTU" is the minimum link MTU of all the links in a path between
a source node and a destination node. (Quoted from [RFC8201])
In this document, the term "Path MTU discovery" includes both
Classical Path MTU discovery [RFC1191], [RFC8201], and Packetization
Layer Path MTU discovery [RFC8899].
Many of the specialized terms used in this document are defined in
DNS Terminology [RFC8499].
3. How to avoid IP fragmentation in DNS
These recommendations are intended for nodes with global IP addresses
on the Internet. Private networks or local networks are out of the
scope of this document.
The methods to avoid IP fragmentation in DNS are described below:
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 mechanisms to achieve this vary across different
operating systems.
For BSD-like operating systems, the IP "Don't Fragment flag (DF) bit"
[RFC0791] can be used to prevent fragmentation. In contrast, Linux
systems do not expose a direct API for this purpose and require the
use of Path MTU socket options (IP_MTU_DISCOVER) to manage
fragmentation settings. However, it is important to note that
enabling IPv4 Path MTU Discovery for UDP in current Linux versions is
considered harmful and dangerous. For more details, refer to
Appendix C.
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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. (See Appendix A for more information.)
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.
(See Appendix A for more information.)
R6. UDP requestors should/may drop fragmented DNS/UDP responses
without IP reassembly to avoid cache poisoning attacks (at firewall
function).
R7. DNS responses may be dropped by IP fragmentation. Requestors
are recommended to try alternative transport protocols eventually.
4. Proposed Recommendations for DNS operators
Large DNS responses are typically the result of zone configuration.
People who publish information in the DNS should seek configurations
resulting in small responses. For example,
R8. Use a smaller number of name servers.
R9. Use a smaller number of A/AAAA RRs for a domain name.
R10. Use minimal-responses configuration: Some implementations have
a 'minimal responses' configuration option that causes DNS servers to
make response packets smaller, containing only mandatory and required
data (Appendix B).
R11. Use a smaller signature / public key size algorithm for DNSSEC.
Notably, the signature sizes of ECDSA and EdDSA are smaller than
those of equivalent cryptographic strength using RSA.
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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
below the size of R3 and R5.
5. Protocol compliance considerations
Some authoritative servers deviate from the DNS standard as follows:
* Some authoritative servers ignore the EDNS0 requestor's maximum
UDP payload size and return large UDP responses. [Fujiwara2018]
* Some authoritative servers do not support TCP transport.
Such non-compliant behavior cannot become implementation or
configuration constraints for the rest of the DNS. If failure is the
result, then that failure must be localized to the non-compliant
servers.
6. IANA Considerations
This document requests no IANA actions.
7. Security Considerations
7.1. On-path fragmentation on IPv4
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.
To avoid this, recommendation R6 needs to be used to discard the
fragmented responses and retry by TCP.
7.2. Small MTU network
When avoiding fragmentation, a DNS/UDP requestor behind a small MTU
network may experience UDP timeouts, which would reduce performance
and which may lead to TCP fallback. This would indicate prior
reliance upon IP fragmentation, which is considered to be harmful to
both the performance and stability of applications, endpoints, and
gateways. Avoiding IP fragmentation will improve operating
conditions overall, and the performance of DNS/TCP has increased and
will continue to increase.
If a UDP response packet is dropped in transit, up to and including
the network stack of the initiator, it increases the attack window
for poisoning the requestor's cache.
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7.3. Weaknesses of IP fragmentation
"Fragmentation Considered Poisonous" [Herzberg2013] noted effective
off-path DNS cache poisoning attack vectors using IP fragmentation.
"IP fragmentation attack on DNS" [Hlavacek2013] and "Domain
Validation++ For MitM-Resilient PKI" [Brandt2018] noted that off-path
attackers can intervene in the path MTU discovery [RFC1191] to cause
authoritative servers to produce fragmented responses. [RFC7739]
stated the security implications of predictable fragment
identification values.
In Section 3.2 (Message Side Guidelines) of UDP Usage Guidelines
[RFC8085] we are told that an application SHOULD NOT send UDP
datagrams that result in IP packets that exceed the Maximum
Transmission Unit (MTU) along the path to the destination.
A DNS message receiver cannot trust fragmented UDP datagrams
primarily due to the small amount of entropy provided by UDP port
numbers and DNS message identifiers, each of which being only 16 bits
in size, and both likely being in the first fragment of a packet if
fragmentation occurs. By comparison, the TCP protocol stack controls
packet size and avoids IP fragmentation under ICMP NEEDFRAG attacks.
In TCP, fragmentation should be avoided for performance reasons,
whereas for UDP, fragmentation should be avoided for resiliency and
authenticity reasons.
7.4. DNS Security Protections
DNSSEC is a countermeasure against cache poisoning attacks that use
IP fragmentation. However, DNS delegation responses are not signed
with DNSSEC, and DNSSEC does not have a mechanism to get the correct
response if an incorrect delegation is injected. This is a denial-
of-service vulnerability that can yield failed name resolutions. If
cache poisoning attacks can be avoided, DNSSEC validation failures
will be avoided.
7.5. Possible actions for resolver operators
Because this document is published as an "Informational" document
rather than a "Best Current Practice," this section presents steps
that resolver operators can take to avoid vulnerabilities related to
IP fragmentation.
To avoid vulnerabilities related to IP fragmentation, implement R5
and R6.
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Specifically, configure the firewall functions protecting the full-
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
discard packets with IPv6 Fragment Headers. (If the resolver's IP
address is not dedicated to the DNS resolver and 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 is not implemented, it will only result in a name
resolution error, preventing attacks from leading to malicious sites.
8. Acknowledgments
The author would like to specifically thank Paul Wouters, Mukund
Sivaraman, Tony Finch, Hugo Salgado, Peter van Dijk, Brian Dickson,
Puneet Sood, Jim Reid, Petr Spacek, Andrew McConachie, Joe Abley,
Daisuke Higashi, Joe Touch, Wouter Wijngaards, Vladimir Cunat, Benno
Overeinder and Štěpán Němec for extensive review and comments.
9. References
9.1. 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/rfc/rfc1035>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/rfc/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/rfc/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/rfc/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/rfc/rfc6891>.
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[RFC7739] Gont, F., "Security Implications of Predictable Fragment
Identification Values", RFC 7739, DOI 10.17487/RFC7739,
February 2016, <https://www.rfc-editor.org/rfc/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/rfc/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/rfc/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/rfc/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/rfc/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/rfc/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.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/rfc/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/rfc/rfc8945>.
9.2. Informative References
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[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 , 2018.
[DNSFlagDay2020]
"DNS flag day 2020", n.d., <https://dnsflagday.net/2020/>.
[Fujiwara2018]
Fujiwara, K., "Measures against cache poisoning attacks
using IP fragmentation in DNS", OARC 30 Workshop , 2019.
[Herzberg2013]
Herzberg, A. and H. Shulman, "Fragmentation Considered
Poisonous", IEEE Conference on Communications and Network
Security , 2013.
[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.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/rfc/rfc791>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/rfc/rfc2308>.
[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/rfc/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/rfc/rfc4035>.
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[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/rfc/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/rfc/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/rfc/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/rfc/rfc9471>.
Appendix A. Details of requestor's maximum UDP payload size discussions
There are many discussions for default path MTU size and requestor's
maximum UDP payload size.
* The minimum MTU for an IPv6 interface is 1280 octets (see
Section 5 of [RFC8200]). So, we can use it as the default path
MTU value for IPv6. The corresponding minimum MTU for an IPv4
interface is 68 (60 + 8) [RFC0791].
* [RFC4035] defines that "A security-aware name server MUST support
the EDNS0 message size extension, MUST support a message size of
at least 1220 octets". Then, the smallest number of the maximum
DNS/UDP payload size is 1220.
* In order to avoid IP fragmentation, [DNSFlagDay2020] proposed that
the UDP requestors set the requestor's payload size to 1232, and
the UDP responders compose UDP responses so they fit in 1232
octets. The size 1232 is based on an MTU of 1280, which is
required by the IPv6 specification [RFC8200], minus 48 octets for
the IPv6 and UDP headers.
* Most of the Internet and especially the inner core has an MTU of
at least 1500 octets. Maximum DNS/UDP payload size for IPv6 on
MTU 1500 ethernet is 1452 (1500 minus 40 (IPv6 header size) minus
8 (UDP header size)). To allow for possible IP options and
distant tunnel overhead, the recommendation of default maximum
DNS/UDP payload size is 1400.
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* [Huston2021] analyzed the result of [DNSFlagDay2020] and reported
that their measurements suggest that in the interior of the
Internet between recursive resolvers and authoritative servers the
prevailing MTU is 1500 and there is no measurable signal of use of
smaller MTUs in this part of the Internet, and proposed that their
measurements suggest setting the EDNS0 requestor's UDP payload
size to 1472 octets for IPv4, and 1452 octets for IPv6.
As a result of discussions, this document decided to recommend 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 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. (defined in Section 2 of [RFC2308]).
Some resource records (MX, SRV, SVCB, HTTPS) require additional A,
AAAA, and SVCB records in the Additional Section defined in
[RFC1035], [RFC2782] and [RFC9460].
In addition, if the zone is DNSSEC signed and a query has the DNSSEC
OK bit, signatures are added in the answer section, or the
corresponding DS RRSet and signatures are added in the authority
section. Details are defined in [RFC4035] and [RFC5155].
Appendix C. Known Implementations
This section records the status of known implementations of these
best practices defined by this specification at the time of
publication, and any deviation from the specification.
Please note that the listing of any individual implementation here
does not imply endorsement by the IETF. Furthermore, no effort has
been spent to verify the information presented here that was supplied
by IETF contributors.
C.1. BIND 9
BIND 9 does not implement the recommendations 1 and 2 in Section 3.1.
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BIND 9 on Linux sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with a
fallback to IP_PMTUDISC_DONT.
BIND 9 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 recommendation 3, 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 recommendation 4, and the send fails with EMSGSIZE,
BIND 9 set the TC bit and try to send a minimal answer again.
In the first recommendation of Section 3.2, BIND 9 uses the edns-buf-
size option, with the default of 1232.
BIND 9 does implement recommendation 2 of Section 3.2.
For recommendation 3, 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.
UDP size limit is 1232 by default.
Fragments are ignored if they arrive over an 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, PowerDNS dnsdist
* IP_PMTUDISC_OMIT with fallback to IP_PMTUDISC_DONT
* default EDNS buffer size of 1232, no probing for smaller sizes
* no handling of EMSGSIZE
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* Recursor: UDP timeouts do not cause a switch to TCP. "Spoofing
nearmisses" do.
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 UDP size 1232 from peers, by default. The
requestors size is limited to a max of 1232.
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
anything since the flag day change to 1232.
Unbound has minimal responses as an option, default on.
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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