Network Working Group K. Fujiwara
Internet-Draft JPRS
Intended status: Best Current Practice P. Vixie
Expires: January 1, 2021 Farsight
June 30, 2020
Fragmentation Avoidance in DNS
draft-ietf-dnsop-avoid-fragmentation-00
Abstract
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proposal to avoid IP fragmentation in DNS . . . . . . . . . . 3
4. Maximum DNS/UDP payload size . . . . . . . . . . . . . . . . 5
5. Incremental deployment . . . . . . . . . . . . . . . . . . . 5
6. Request to zone operators and DNS server operators . . . . . 6
7. Considerations . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Protocol compliance . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. How to retrieve path MTU value to a destination from
applications . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Minimal-responses . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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.
However, "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.
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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 is considered resistant
against IP fragmentation attacks because TCP has a 32-bit sequence
number and 32-bit acknowledgment number in each segment. In TCP,
fragmentation should be avoided for performance reasons, whereas for
UDP, fragmentation should be avoided for resiliency and authenticity
reasons.
[I-D.ietf-intarea-frag-fragile] 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.
This document proposes to avoid IP fragmentation in DNS/UDP.
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])
Many of the specialized terms used in this document are defined in
DNS Terminology [RFC8499].
3. Proposal to avoid IP fragmentation in DNS
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
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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.
The minimum MTU for an IPv4 interface is 68 octets, and all receivers
must be able to receive and reassemble datagrams at least 576 octets
in size (see Section 2.1, NOTE 1 of [I-D.ietf-intarea-frag-fragile]).
The minimum MTU for an IPv6 interface is 1280 octets (see Section 5
of [RFC8200]). These are theoretic limits and no modern networks
implement them. In practice, the smallest MTU witnessed in the
operational DNS community is 1500 octets, the Ethernet maximum
payload size. While many non-Ethernet networks exist such as Packet
on SONET (PoS), Fiber Distributed Data Exchange (FDDI), and Ethernet
Jumbo Frame, there is currently no reliable way of discovering such
links in an IP transmission path. Absent some kind of path MTU
discovery result or a static configuration by the server or system
operator, a conservative estimate must be chosen, even if it is less
efficient than the path MTU would have been had that been
discoverable.
The methods to avoid IP fragmentation in DNS are described below:
o UDP requestors and responders SHOULD send DNS responses with
IP_DONTFRAG / IPV6_DONTFRAG [RFC3542] options, which will yield
either a silent timeout, or a network (ICMP) error, if the path
MTU is exceeded. Upon a timeout, UDP requestors may retry using
TCP or UDP, per local policy.
o The estimated maximum DNS/UDP payload size SHOULD be the
discovered or estimated path MTU minus the estimated header space.
Path MTU discovery [RFC1191], [RFC8201] and
[I-D.ietf-tsvwg-datagram-plpmtud] may discover real path MTU value
to destinations. One method to retrieve path MTU value is
described in Appendix A. When discovered path MTU information is
not available, a message sender SHOULD use the default maximum
DNS/UDP payload size described in following section.
o The maximum buffer size offered by an EDNS0 initiator SHOULD be no
larger than the estimated maximum DNS/UDP payload size. If the
desired response cannot be reasonably expected to fit into a
buffer of that size, the initiator should use TCP instead of UDP.
o Responders SHOULD compose UDP responses that result in IP packets
that do not exceed the path MTU to the requestor. Thus, if the
requestor offers a buffer size larger than responder's discovered
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or estimated maximum DNS/UDP payload size, then the responder will
behave as though the requestor had specified a buffer size equal
to the responder's estimated maximum DNS/UDP payload size.
o Fragmented DNS/UDP messages may be dropped without IP reassembly.
An ICMP error should be sent in this case, with rate limiting to
prevent this logic from becoming a DDoS amplification vector. If
rate limiting is not possible, then no ICMP error should be sent.
(This is a countermeasure against DNS spoofing attacks using IP
fragmentation.)
The cause and effect of the TC bit is unchanged from EDNS0 [RFC6891].
4. Maximum DNS/UDP payload size
o Most of the Internet and especially the inner core has an MTU of
at least 1500 octets. An operator of a full resolver 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. 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. An operator of
authoritative servers would also be well advised to measure their
path MTU to several full-service resolvers. The Linux tool
"tracepath" can be used to measure the path MTU to well known
authority name servers such as [a-m].root-servers.net or [a-
m].gtld-servers.net. If the reported path MTU is for example no
smaller than 1460, then the maximum DNS/UDP payload would be 1432
for IP4 (which is 1460 - IP4 header(20) - UDP header(8)) and 1412
for IP6 (which is 1460 - IP6 header(40) - UDP header(8)). To
allow for possible IP options and distant tunnel overhead, a
useful default for maximum DNS/UDP payload size would be 1400.
o [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.
o DNS flag day 2020 proposed 1232 as an EDNS buffer size.
[DNSFlagDay2020] By the above reasoning, this proposal is either
too small or too large.
5. Incremental deployment
The proposed method supports incremental deployment.
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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.
6. 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,
o Use smaller number of name servers (13 may be too large)
o Use smaller number of A/AAAA RRs for a domain name
o 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 B).
o Use smaller signature / public key size algorithm for DNSSEC.
Notably, the signature size of ECDSA or EdDSA is smaller than RSA.
7. Considerations
7.1. 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.
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.
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8. IANA Considerations
This document has no IANA actions.
9. Security Considerations
10. Acknowledgments
The author would like to specifically thank Paul Wouters, Mukund
Sivaraman for extensive review and comments.
11. References
11.1. Normative References
[I-D.ietf-intarea-frag-fragile]
Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile", draft-
ietf-intarea-frag-fragile-17 (work in progress), September
2019.
[I-D.ietf-tsvwg-datagram-plpmtud]
Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and
T. Voelker, "Packetization Layer Path MTU Discovery for
Datagram Transports", draft-ietf-tsvwg-datagram-plpmtud-22
(work in progress), June 2020.
[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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[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>.
[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>.
[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>.
[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>.
11.2. 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/>.
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[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>.
Appendix A. 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))
Appendix B. 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-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.
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].
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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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