Network Working Group K. Fujiwara
Internet-Draft JPRS
Intended status: Best Current Practice P. Vixie
Expires: 24 June 2023 AWS Security
21 December 2022
Fragmentation Avoidance in DNS
draft-ietf-dnsop-avoid-fragmentation-10
Abstract
EDNS0 enables a DNS server to send large responses using UDP and is
widely deployed. Large DNS/UDP responses are fragmented, and IP
fragmentation has exposed weaknesses in application protocols. It is
possible to avoid IP fragmentation in DNS by limiting response size
where possible, and signaling the need to upgrade from UDP to TCP
transport where necessary. This document proposes to avoid IP
fragmentation in DNS.
Status of This Memo
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proposal to avoid IP fragmentation in DNS . . . . . . . . . . 3
3.1. Recommendations for UDP responders . . . . . . . . . . . 3
3.2. Recommendations for UDP requestors . . . . . . . . . . . 4
4. Request to zone operators and DNS server operators . . . . . 4
5. Considerations . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Protocol compliance . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Weaknesses of IP fragmentation . . . . . . . . . . . 8
Appendix B. Details of requestor's maximum UDP payload size
discussions . . . . . . . . . . . . . . . . . . . . . . . 8
Appendix C. Minimal-responses . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
DNS has an EDNS0 [RFC6891] mechanism. It enables a DNS server to
send large responses using UDP. EDNS0 is now widely deployed, and
DNS (over UDP) is said to be the biggest user of IP fragmentation.
Fragmented DNS UDP responses have systemic weaknesses, which expose
the requestor to DNS cache poisoning from off-path attackers. (See
Appendix A for references and details.)
[RFC8900] summarized that IP fragmentation introduces fragility to
Internet communication. The transport of DNS messages over UDP
should take account of the observations stated in that document.
TCP avoids fragmentation using its Maximum Segment Size (MSS)
parameter, but each transmitted segment is header-size aware such
that the size of the IP and TCP headers is known, as well as the far
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end's MSS parameter and the interface or path MTU, so that the
segment size can be chosen so as to keep the each IP datagram below a
target size. This takes advantage of the elasticity of TCP's
packetizing process as to how much queued data will fit into the next
segment. In contrast, DNS over UDP has little datagram size
elasticity and lacks insight into IP header and option size, and so
must make more conservative estimates about available UDP payload
space.
This document proposes to set the "Don't Fragment flag (DF) bit"
[RFC0791] on IPv4 and not to use "Fragment header" [RFC8200] on IPv6
in DNS/UDP messages in order to avoid IP fragmentation, and describes
how to avoid packet losses due to DF bit and small MTU links.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
"Requestor" refers to the side that sends a request. "Responder"
refers to an authoritative, recursive resolver or other DNS component
that responds to questions. (Quoted from EDNS0 [RFC6891])
"Path MTU" is the minimum link MTU of all the links in a path between
a source node and a destination node. (Quoted from [RFC8201])
In this document, the term "Path MTU discovery" includes 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. Proposal 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. Recommendations for UDP responders
* UDP responders SHOULD send DNS responses without "Fragment header"
[RFC8200] on IPv6.
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* UDP responders RECOMMENDED to set IP "Don't Fragment flag (DF)
bit" [RFC0791] on IPv4.
* UDP responders SHOULD compose response packets fit in path MTU
discovery results (if available) to measure path MTU discovery
attacks, interface MTU and the requestor's maximum UDP payload
size [RFC6891].
* If the UDP responder detects an immediate error that the UDP
packet cannot be sent beyond the path MTU size (EMSGSIZE), the UDP
responder MAY recreate response packets fit in path MTU size, or
TC bit set.
* UDP responders SHOULD limit response size when UDP responders are
located on small MTU (<1500) networks.
The cause and effect of the TC bit are unchanged from EDNS0
[RFC6891].
3.2. Recommendations for UDP requestors
* UDP requestors SHOULD limit the requestor's maximum UDP payload
size to 1400 or smaller size. [ UDP requestors MAY set the
requestor's maximum UDP payload size as 1232. ]
* UDP requestors MAY perform "Path MTU discovery" per destination to
use the requestor's maximum UDP payload size larger than 1400.
Then, calculate their requestors' maximum UDP payload size as the
reported path MTU minus IPv4/IPv6 header size (20/40) minus UDP
header size (8).
* UDP requestors MAY drop fragmented DNS/UDP responses without IP
reassembly to avoid cache poisoning attacks.
* DNS responses may be dropped by IP fragmentation. Upon a timeout,
to avoid name resolution fails, UDP requestors MAY retry using TCP
or UDP with a smaller requestor's maximum UDP payload size per
local policy.
4. Request to zone operators and DNS server operators
Large DNS responses are the result of zone configuration. Zone
operators SHOULD seek configurations resulting in small responses.
For example,
* Use a smaller number of name servers (13 may be too large)
* Use a smaller number of A/AAAA RRs for a domain name
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* Use 'minimal-responses' configuration: Some implementations have a
'minimal responses' configuration that causes DNS servers to make
response packets smaller, containing only mandatory and required
data (Appendix C).
* Use a smaller signature / public key size algorithm for DNSSEC.
Notably, the signature size of ECDSA or EdDSA is smaller than RSA.
5. Considerations
5.1. Protocol compliance
In prior research ([Fujiwara2018] and dns-operations mailing list
discussions), there are some authoritative servers that ignore the
EDNS0 requestor's maximum UDP payload size, and return large UDP
responses.
It is also well known that 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 has no IANA actions.
7. Security Considerations
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 universally 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.
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, Peter van Dijk, Andrew
McConachie, Joe Abley, Daisuke Higashi and Joe Touch for extensive
review and comments.
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9. References
9.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
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[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
9.2. Informative References
[Brandt2018]
Brandt, M., Dai, T., Klein, A., Shulman, H., and M.
Waidner, "Domain Validation++ For MitM-Resilient PKI",
Proceedings of the 2018 ACM SIGSAC Conference on Computer
and Communications Security , 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.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<https://www.rfc-editor.org/info/rfc5155>.
[RFC7739] Gont, F., "Security Implications of Predictable Fragment
Identification Values", RFC 7739, DOI 10.17487/RFC7739,
February 2016, <https://www.rfc-editor.org/info/rfc7739>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
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[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile",
BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
<https://www.rfc-editor.org/info/rfc8900>.
Appendix A. Weaknesses of IP fragmentation
"Fragmentation Considered Poisonous" [Herzberg2013] proposed
effective off-path DNS cache poisoning attack vectors using IP
fragmentation. "IP fragmentation attack on DNS" [Hlavacek2013] and
"Domain Validation++ For MitM-Resilient PKI" [Brandt2018] proposed
that off-path attackers can intervene in path MTU discovery [RFC1191]
to perform intentionally fragmented responses from authoritative
servers. [RFC7739] stated the security implications of predictable
fragment identification values.
DNSSEC is a countermeasure against cache poisoning attacks that use
IP fragmentation. However, DNS delegation responses are not signed
with DNSSEC, and DNSSEC does not have a mechanism to get the correct
response if an incorrect delegation is injected. This is a denial-
of-service vulnerability that can yield failed name resolutions. If
cache poisoning attacks can be avoided, DNSSEC validation failures
will be avoided.
In Section 3.2 (Message Side Guidelines) of UDP Usage Guidelines
[RFC8085] we are told that an application SHOULD NOT send UDP
datagrams that result in IP packets that exceed the Maximum
Transmission Unit (MTU) along the path to the destination.
A DNS message receiver cannot trust fragmented UDP datagrams
primarily due to the small amount of entropy provided by UDP port
numbers and DNS message identifiers, each of which being only 16 bits
in size, and both likely being in the first fragment of a packet, if
fragmentation occurs. By comparison, TCP protocol stack controls
packet size and avoid IP fragmentation under ICMP NEEDFRAG attacks.
In TCP, fragmentation should be avoided for performance reasons,
whereas for UDP, fragmentation should be avoided for resiliency and
authenticity reasons.
Appendix B. Details of 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]). Then, 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].
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* 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 authors' recommendation of default
maximum DNS/UDP payload size is 1400.
* [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 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.
* [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 at 1,500 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 Buffer size to
IPv4 1472 octets and IPv6 1452 octets.
Appendix C. Minimal-responses
Some implementations have a 'minimal responses' configuration that
causes a DNS server to make response packets smaller, containing only
mandatory and required data.
Under the minimal-responses configuration, DNS servers compose
response messages using only RRSets corresponding to queries. In the
case of delegation, DNS servers compose response packets with
delegation NS RRSet in the authority section and in-domain (in-zone
and below-zone) glue in the additional data section. In case of a
non-existent domain name or non-existent type, the start of authority
(SOA RR) will be placed in the Authority Section.
In addition, if the zone is DNSSEC signed and a query has the DNSSEC
OK bit, signatures are added in the answer section, or the
corresponding DS RRSet and signatures are added in the authority
section. Details are defined in [RFC4035] and [RFC5155].
Authors' Addresses
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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@wide.ad.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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