Network Working Group K. Fujiwara
Internet-Draft JPRS
Intended status: Best Current Practice P. Vixie
Expires: March 30, 2020 Farsight
September 27, 2019
Avoid IP fragmentation in DNS
draft-fujiwara-dnsop-avoid-fragmentation-01
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. Default maximum DNS/UDP payload size . . . . . . . . . . . . 4
5. Incremental deployment . . . . . . . . . . . . . . . . . . . 5
6. Request to zone operator . . . . . . . . . . . . . . . . . . 5
7. Considerations . . . . . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. Security Considerations . . . . . . . . . . . . . . . . . . . 5
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
10.1. Normative References . . . . . . . . . . . . . . . . . . 5
10.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. How to retrieve path MTU value to a destination . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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 packets, primarily due to
the low level of entropy provided by UDP port numbers and DNS message
identifiers, each of which being 16 bits in size. By comparison, TCP
is considered resistant against IP fragmentation attacks because TCP
has a 32-bit sequence number and 32-bit acknowledgement number in
each segment.
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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, in order to send a
smaller segment as necessary to keep the each IP datagram below a
target size. TCP's packetizing process is also elastic as to how
much queued data will fit into the next segment. 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 576 octets, and for an IPv6
interface, 1280 octets. 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 networks 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 less efficient.
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
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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, TCP should be used 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 estimated maximum DNS/
UDP payload size, then the responder will behave as though the
requestor had specified a buffer size equal to the estimated
maximum DNS/UDP payload size.
o Fragmented DNS/UDP messages may be dropped before IP assembly. An
ICMP error should 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. Default maximum DNS/UDP payload size
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 However, in practice, the smallest MTU witnessed in the
operational DNS community is 1500 octets. The estimated size of a
DNS message's UDP headers, IP headers, IP options, and one or more
set of tunnel, IP-in-IP, VLAN, and virtual circuit headers, SHOULD
be 100 octets. Then, the maximum DNS/UDP payload size may be
1400.
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5. Incremental deployment
The proposed method supports incremental deployment.
When a full-service resolver implements the proposed method, the
full-service resolver becomes to avoid IP fragmentation in DNS.
When an authoritative server implements the proposed method, the
authoritative server becomes to avoid IP fragmentation in DNS.
6. Request to zone operator
Fat DNS responses come from fat configurations of zones. Zone
operator SHOULD consider small response size configurations. 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 smaller signature / public key size algorithm for DNSSEC.
Signature size of ECDSA or EdDSA is smaller than RSA.
7. Considerations
In past researches ([Fujiwara2018] / dns-operations mailing list
discussions), there are some authoritative servers that ignore EDNS0
requestor's UDP payload size, and return large UDP responses.
And it is known that there are some authoritative servers that do not
support TCP transport.
8. IANA Considerations
This document has no IANA actions.
9. Security Considerations
10. References
10.1. Normative References
[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>.
[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>.
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.
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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
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: man 7 ip)
"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: man 7
ipv6)
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
Phone: +1 650 393 3994
Email: paul@redbarn.org
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