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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 15, 2020. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must Fujiwara & Vixie Expires October 15, 2020 [Page 1] Internet-Draft avoid-fragmentation April 2020 include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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 Fujiwara & Vixie Expires October 15, 2020 [Page 2] Internet-Draft avoid-fragmentation April 2020 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 Fujiwara & Vixie Expires October 15, 2020 [Page 3] Internet-Draft avoid-fragmentation April 2020 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]. Fujiwara & Vixie Expires October 15, 2020 [Page 4] Internet-Draft avoid-fragmentation April 2020 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, Fujiwara & Vixie Expires October 15, 2020 [Page 5] Internet-Draft avoid-fragmentation April 2020 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. Fujiwara & Vixie Expires October 15, 2020 [Page 6] Internet-Draft avoid-fragmentation April 2020 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, . Fujiwara & Vixie Expires October 15, 2020 [Page 7] Internet-Draft avoid-fragmentation April 2020 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . [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, . [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013, . [RFC7739] Gont, F., "Security Implications of Predictable Fragment Identification Values", RFC 7739, DOI 10.17487/RFC7739, February 2016, . [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, March 2017, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, . [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, . [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, January 2019, . Fujiwara & Vixie Expires October 15, 2020 [Page 8] Internet-Draft avoid-fragmentation April 2020 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., . [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, . 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 Fujiwara & Vixie Expires October 15, 2020 [Page 9] Internet-Draft avoid-fragmentation April 2020 Paul Vixie Farsight Security Inc 177 Bovet Road, Suite 180 San Mateo, CA 94402 Phone: +1 650 393 3994 Email: paul@redbarn.org Fujiwara & Vixie Expires October 15, 2020 [Page 10]