Network Working Group K. Fujiwara
Internet-Draft JPRS
Intended status: Best Current Practice P. Vixie
Expires: October 15, 2020 Farsight
April 13, 2020
Fragmentation Avoidance in DNS
draft-fujiwara-dnsop-avoid-fragmentation-03
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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This Internet-Draft will expire on October 15, 2020.
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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 operator and DNS server operator . . . . . . 5
7. Considerations . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Protocol compliance . . . . . . . . . . . . . . . . . . . 6
7.2. DNS packet size . . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. How to retrieve path MTU value to a destination . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
DNS has EDNS0 [RFC6891] mechanism. It enables that DNS server can
send large size response using UDP. Now EDNS0 is 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 security implications of predictable
fragment identification values.
And more, Section 3.2 Message Side Guidelines of UDP Usage Guidelines
[RFC8085] specifies 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.
As a result, we 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. By
comparison, TCP is considered resistant against IP fragmentation
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attacks because TCP has a 32-bit sequence number and 32-bit
acknowledgement 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
target size. The 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 has no message 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 and for an IPv6 interface is 1280 octets (see
Section 5 of [RFC8200]). These are theoretic limits and no modern
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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 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 it been measurable.
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 actual or
estimated path MTU minus the estimated header space. When actual
path MTU information is not available, use the default maximum
DNS/UDP payload size described in following section.
o The maximum buffer size offered by an EDNS0 requestor SHOULD be no
larger than the estimated maximum DNS/UDP payload size. If the
response cannot be reasonably expected 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 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].
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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
answers received or transmitted by a full resolver, or else
fallback to TCP will occur too often. An operator of
authoritative servers would be also 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 faraway 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]
5. 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.
6. Request to zone operator and DNS server operator
Large DNS responses are the result of zone configuration. Zone
operators SHOULD seek configurations resulting in small responses.
For example,
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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 smaller signature / public key size algorithm for DNSSEC.
Notably, the signature size of ECDSA or EdDSA is smaller than RSA.
o Use 'minimal-responses' configuration: Some implementations have
'minimal responses' configuration that enables that DNS servers
make response packets smaller, mandatory and required data only.
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 noncompliant behaviour 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 noncompliant
servers.
7.2. DNS packet size
Many stub resolvers do not set the DNSSEC OK bit. In this case,
responses from full-service resolvers may be small.
With 'minimal-response' configuration, DNS servers can be forced to
emit small responses.
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Server |DNSSEC| Answer | Response data |response
type | OK | type | Answer/Authority/Add. | size
---------+------+----------+-----------------------+-----
Resolver | No | Exist | RRSet// |RRSet
Resolver | No | Not exist| /SOA/ |SOA
Resolver | Yes | Exist | RRSet+RRSIG// |RRSet+RRSIG
Resolver | Yes | Not exist| /SOA+NSEC+RRSIG/ |SOA+NSEC*2+RRSIG*3
Auth. | No | Referral | /NS/glue |NS+glue
Auth. | No | Exist | RRSet// |RRSet
Auth. | No | Not exist| /SOA/ |SOA
Auth. | Yes | Referral | /DS+RRSIG+NS/glue |NS+glue+DS+RRSIG
Auth. | Yes | Referral | /NSEC+RRSIG+NS/glue |NS+glue+NSEC+RRSIG
Auth. | Yes | Exist | RRSet+RRSIG// |RRSet+RRSIG
Auth. | Yes | Not exist| /SOA+NSEC*2+RRSIG/ |SOA+NSEC*2+RRSIG*3
Non-existent answers with DNSSEC are largest.
Without 'minimal responses' configuration, DNS servers may add
unnecessary NS RRset in authority section and nameservers' A/AAAA
RRSet in additional section.
However, with 'minimal-responses' configuration, zone operators can
control the authoritative server's response size (selection of DNSKEY
algorithm and size, and number of resource records).
8. IANA Considerations
This document has no IANA actions.
9. Security Considerations
10. References
10.1. 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.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
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[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>.
[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>.
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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.
[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
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))
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
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Paul Vixie
Farsight Security Inc
177 Bovet Road, Suite 180
San Mateo, CA 94402
Phone: +1 650 393 3994
Email: paul@redbarn.org
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