Network Working Group K. Fujiwara
Internet-Draft JPRS
Intended status: Best Current Practice P. Vixie
Expires: 25 December 2021 Farsight
23 June 2021
Fragmentation Avoidance in DNS
draft-ietf-dnsop-avoid-fragmentation-05
Abstract
EDNS0 enables a DNS server to send large responses using UDP and is
widely deployed. Path MTU discovery remains widely undeployed due to
security issues, and IP fragmentation has exposed weaknesses in
application protocols. Currently, DNS is known to be the largest
user of IP fragmentation. 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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This Internet-Draft will expire on 25 December 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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extracted from this document must include Simplified BSD License text
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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 . . . . . . . . . . . 4
3.2. Recommendations for UDP requestors . . . . . . . . . . . 4
3.3. Default Maximum DNS/UDP payload size . . . . . . . . . . 4
4. Incremental deployment . . . . . . . . . . . . . . . . . . . 6
5. Request to zone operators and DNS server operators . . . . . 6
6. Considerations . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Protocol compliance . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Weaknesses of IP fragmentation . . . . . . . . . . . 9
Appendix B. Details of maximum DNS/UDP payload size
discussions . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix C. How to retrieve path MTU value to a destination from
applications . . . . . . . . . . . . . . . . . . . . . . 11
Appendix D. How to retrieve minimal MTU value to a
destination . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix E. Minimal-responses . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
DNS has 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.
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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
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 IP_DONTFRAG / IPV6_DONTFRAG in DNS/UDP
messages in order to avoid IP fragmentation, and describes how to
avoid packet losses due to IP_DONTFRAG / IPV6_DONTFRAG.
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])
"Path MTU discovery" is defined by [RFC1191], [RFC8201] and
[RFC8899].
IP_DONTFRAG option is not defined by any RFCs. It is similar to
IPV6_DONTFRAG option defined in [RFC3542]. IP_DONTFRAG option is
used on BSD systems to set the Don't Fragment bit [RFC0791] when
sending IPv4 packets. On Linux systems this is done via
IP_MTU_DISCOVER and IP_PMTUDISC_DO.
Many of the specialized terms used in this document are defined in
DNS Terminology [RFC8499].
3. Proposal to avoid IP fragmentation in DNS
The methods to avoid IP fragmentation in DNS are described below:
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3.1. Recommendations for UDP responders
* UDP responders SHOULD send DNS responses with IP_DONTFRAG /
IPV6_DONTFRAG [RFC3542] options.
* If the UDP responder detects 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 MAY probe to discover the real MTU value per
destination.
* UDP responders SHOULD compose UDP responses that result in IP
packets that do not exceed the path MTU to the requestor. If the
path MTU discovery failed or is impossible, UDP responders SHOULD
compose UDP responses that result in IP packets that do not exceed
the default maximum DNS/UDP payload size described in Section 3.3.
The cause and effect of the TC bit is unchanged from EDNS0
[RFC6891].
3.2. Recommendations for UDP requestors
* UDP requestors SHOULD send DNS requests with IP_DONTFRAG /
IPV6_DONTFRAG [RFC3542] options.
* UDP requestors MAY probe to discover the real MTU value per
destination. Then, calculate their maximum DNS/UDP payload size
as the reported path MTU minus IPv4/IPv6 header size (20 or 40)
minus UDP header size (8). If the path MTU discovery failed or is
impossible, use the default maximum DNS/UDP payload size described
in Section 3.3.
* UDP requestors SHOULD use the requestor's payload size as the
calculated or the default maximum DNS/UDP payload size.
* 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,
UDP requestors may retry using TCP or UDP, per local policy.
3.3. Default Maximum DNS/UDP payload size
Fragmentation avoidance is achieved with the IP(V6)_DONTFRAG option.
The purpose of packet size limitation is to decrease packet loss due
to the effects of the IP(V6)_DONTFRAG option.
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Default maximum DNS/UDP payload size depends on the connectivity of
each node, it cannot be determined unconditionally. However, there
are good proposed values.
Operators MAY select a good number from Table 1. Details of proposed
values are described in Appendix B.
+========================+=============+===================+
| Source | IPv4 | IPv6 |
+========================+=============+===================+
| Minimal: RFC 4035 MUST | 1220 | 1220 |
+------------------------+-------------+-------------------+
| Software developers / | 1232 | 1232 (1280-40-8) |
| DNSFlagDay2020 propose | | |
+------------------------+-------------+-------------------+
| Authors' | 1400 | 1400 (1500 -40 -8 |
| recommendation | | - some headers) |
+------------------------+-------------+-------------------+
| Maximum: Ethernet MTU | 1472 | 1452 (1500-40-8) |
| 1500 [Huston2021] | (1500-20-8) | |
+------------------------+-------------+-------------------+
| Measured | MTU -20-8 | MTU -40-8 |
+------------------------+-------------+-------------------+
Table 1: Default maximum DNS/UDP payload size
However, operators of DNS servers SHOULD measure their path MTU to
the Internet at setting up DNS servers (and when network
configuration changes).
How to measure path MTU is described in Appendix D.
Operators of authoritative servers (that offer global DNS zones) and
full-service resolvers (that access authoritative servers of the
global DNS) SHOULD measure their path MTU to well-known locations on
the Internet, such as [a-m].root-servers.net or [a-m].gtld-
servers.net.
Operators of full-service resolvers would be well advised to measure
their path MTU to several authority name servers and to a random
sample of their expected stub resolver client networks, to find the
upper boundary on IP/UDP packet size in the average case. Or,
operators of ISPs know their customers' connectivity and customers'
MTU to ISPs' servers. This limit should not be exceeded by most
messages received or transmitted by a full resolver, or else fallback
to TCP will occur too often.
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DNS clients (stub resolvers) need to specify an appropriate
requestor's payload size when supporting EDNS0. In case of CPEs,
embedded devices, and user devices, network operators can not control
them, developers may choose small values such as 1220 and 1232.
Other DNS servers are out-of-scope of this document. (For example,
Forwarding only resolvers, or private DNS).
4. Incremental deployment
The proposed method supports incremental deployment.
When a full-service resolver implements the proposed method, its stub
resolvers (clients) and the authority server network will no longer
observe IP fragmentation or reassembly from that server, and will
fall back to TCP when necessary.
When an authoritative server implements the proposed method, its full
service resolvers (clients) will no longer observe IP fragmentation
or reassembly from that server, and will fall back to TCP when
necessary.
5. 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 smaller number of name servers (13 may be too large)
* Use smaller number of A/AAAA RRs for a domain name
* Use 'minimal-responses' configuration: Some implementations have
'minimal responses' configuration that causes DNS servers to make
response packets smaller, containing only mandatory and required
data (Appendix E).
* Use smaller signature / public key size algorithm for DNSSEC.
Notably, the signature size of ECDSA or EdDSA is smaller than RSA.
6. Considerations
6.1. Protocol compliance
In prior research ([Fujiwara2018] and dns-operations mailing list
discussions), there are some authoritative servers that ignore EDNS0
requestor's UDP payload size, and return large UDP responses.
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It is also well known that there are some authoritative servers that
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.
7. IANA Considerations
This document has no IANA actions.
8. Security Considerations
9. Acknowledgments
The author would like to specifically thank Paul Wouters, Mukund
Sivaraman, Tony Finch, Hugo Salgado, Peter van Dijk, Brian Dickson,
Puneet Sood and Jim Reid for extensive review and comments.
10. References
10.1. 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>.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, DOI 10.17487/RFC3542, May 2003,
<https://www.rfc-editor.org/info/rfc3542>.
[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>.
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[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>.
[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>.
[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>.
[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>.
10.2. 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.
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[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.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[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>.
[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>.
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.
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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 maximum DNS/UDP payload size discussions
There are many discussions for default path MTU size and maximum DNS/
UDP payload size.
* The minimum MTU for an IPv6 interface is 1280 octets (see
Section 5 of [RFC8200]). Then, we can use it as default path MTU
value for IPv6. The corresponding minimum MTU for an IPv4
interface is 68 (60 + 8) [RFC0791].
* 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, 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
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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], 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. How to retrieve path MTU value to a destination from
applications
Socket options: "IP_MTU (since Linux 2.2) Retrieve the current known
path MTU of the current socket. Valid only when the socket has been
connected. Returns an integer. Only valid as a getsockopt(2)."
(Quoted from Debian GNU Linux manual: ip(7))
"IPV6_MTU getsockopt(): Retrieve the current known path MTU of the
current socket. Only valid when the socket has been connected.
Returns an integer." (Quoted from Debian GNU Linux manual: ipv6(7))
Section 3.4 of [RFC1122] specifies FIND_MAXSIZES() as one of
"INTERNET/TRANSPORT LAYER INTERFACEs".
Appendix D. How to retrieve minimal MTU value to a destination
The Linux tool "tracepath" can be used to measure the path MTU to a
destination.
Or, "ping/ping6" command with "-D" Don't Fragment bit set / Disable
IPv6 fragmentation options.
Appendix E. Minimal-responses
Some implementations have '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
case of delegation, DNS servers compose response packets with
delegation NS RRSet in authority section and in-domain (in-zone and
below-zone) glue in the additional data section. In case of non-
existent domain name or non-existent type, the start of authority
(SOA RR) will be placed in the Authority Section.
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In addition, if the zone is DNSSEC signed and a query has the DNSSEC
OK bit, signatures are added in answer section, or the corresponding
DS RRSet and signatures are added in authority section. Details are
defined in [RFC4035] and [RFC5155].
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
Farsight Security Inc
177 Bovet Road, Suite 180
San Mateo, CA, 94402
United States of America
Phone: +1 650 393 3994
Email: vixie@fsi.io
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