Network Working Group K. Fujiwara Internet-Draft JPRS Intended status: Informational February 15, 2019 Expires: August 19, 2019 Measures against cache poisoning attacks using IP fragmentation in DNS draft-fujiwara-dnsop-fragment-attack-00 Abstract Researchers proposed DNS cache poisoning attacks using IP fragmentation. This document shows feasible and adequate measures at full-service resolvers against these attacks. To protect resolvers from these attacks, avoid fragmentation (limit requestor's UDP payload size to 1220/1232), drop fragmented UDP DNS responses and use TCP at resolver side. 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 August 19, 2019. Copyright Notice Copyright (c) 2019 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 include Simplified BSD License text as described in Section 4.e of Fujiwara Expires August 19, 2019 [Page 1] Internet-Draft fragment-attack February 2019 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Current status . . . . . . . . . . . . . . . . . . . . . . . 3 3. Possible measures . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Use DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Limit requestor's UDP payload size to 1220/1232 on IPv6 . 4 3.3. Limit requestor's UDP payload size to 512 . . . . . . . . 5 3.4. Drop path MTU discovery or filter ICMP related to path MTU discovery . . . . . . . . . . . . . . . . . . . . . . 5 3.5. Drop all fragmented packets . . . . . . . . . . . . . . . 5 3.6. Drop fragmented UDP DNS responses at full-service resolvers . . . . . . . . . . . . . . . . . . . . . . . . 5 3.7. Use TCP only . . . . . . . . . . . . . . . . . . . . . . 6 4. Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Example firewall configuration . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7 9. Change History . . . . . . . . . . . . . . . . . . . . . . . 7 9.1. 00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 10.1. Normative References . . . . . . . . . . . . . . . . . . 7 10.2. Informative References . . . . . . . . . . . . . . . . . 8 Appendix A. How to know path MTU value . . . . . . . . . . . . . 8 Appendix B. How to generate crafted ICMP packets . . . . . . . . 8 B.1. Example of crafted ICMP Need Fragmentation and DF set packet . . . . . . . . . . . . . . . . . . . . . . . . . 8 B.2. Example of crafted ICMPv6 Packet Too Big . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction "Fragmentation Considered Poisonous" [Herzberg2013] proposed effective off-path DNS cache poisoning attacks using IP fragmentation. The attacks mainly depend on the use of UDP to retrieve long DNS responses, resulting in packet fragmentation. Recent full-service resolvers use good randomness for query source port numbers and ID field in DNS header to prevent cache poisoning attacks by off-path attackers. However, IP fragmentation is performed by OS kernel or routers that operators of DNS servers cannot control, and the query source number and ID field in DNS header exist only in first fragment. The attack depends on poor randomness of "Identification" field generated by IP fragmentation and some bugs in IP reassembly code. Attacker can know path MTU size Fujiwara Expires August 19, 2019 [Page 2] Internet-Draft fragment-attack February 2019 between authoritative servers and victim full-service resolvers, and responses from authoritative servers. If attackers know generation algorithm of "Identification" field, they can generate crafted second fragment packets that will be accepted by victim full-service resolvers. [Hlavacek2013] also discussed the attacks and pointed that attackers can control path MTU values between some authoritative servers and victim full-service resolvers by sending crafted ICMP packets (Fragmentation needed and DF set, or ICMPv6 Packet Too Big). The author proposed that the defense is DNSSEC and workarounds are ignoring ICMP type=3 code=4 fragmentation needed and DF set, limit response size and set EDNS0 buffer size fit to MTU value. And more, "Domain Validation++ For MitM-Resilient PKI" [Brandt2018] proved that off-path attackers can intervene in path MTU discovery [RFC1191] to perform intentionally fragment responses from authoritative servers. They also proved that they poisoned CAs' full-service resolvers and successfully issued some certificates. As a result, we cannot trust all fragmented UDP packets and path MTU discovery. By the way, TCP is considered strong against fragmentation attacks because TCP has sequence number and acknowledgement number in each sequence. This document describes resolver side countermeasure of cache poisoning attacks using IP fragmentation. 2. Current status DNS cache poisoning attacks using IP fragmentation are executed by combining the following two attacks. Path MTU attack targets are all authoritative DNS servers. Cache poisoning attack targets are full- service resolvers. [Brandt2018] showed that Linux version 3.13 and older versions are vulnerable to crafted ICMP fragmentation needed and DF set packet and off-path attackers can set some of authoritative servers' path MTU value to 296. The author tested Linux version 2.6.32, 4.18.20 and FreeBSD 12.0. Linux 2.6.32 accepts crafted "ICMP Need Fragmentation and DF set" packet and path MTU decreased to 552. Linux 2.6.32, Linux 4.18.20 and FreeBSD 12.0 accept crafted "ICMPv6 Packet Too Big" packet and path MTU decreased to 1280. Fujiwara Expires August 19, 2019 [Page 3] Internet-Draft fragment-attack February 2019 Linux version 4.18.20 may ignore crafted ICMP packet. FreeBSD and NetBSD accept "ICMP Need Fragmentation and DF set" packet related to established TCP and ignore "ICMP Need Fragmentation and DF set" packet related to UDP. Then, off-path attackers can decrease path MTU values from some IPv4 authoritative servers to 552 (or 296), and can decrease path MTU values from IPv6 authoritative servers to 1280 (minimal IPv6 MTU value). Furthermore, off-path attackers can know path MTU value related to authoritative servers and they can generate crafted fragmented DNS responses to victim full-service resolvers. Then, measures against these attacks at full-service resolvers is important. +---------------+---------+----------+---------+----------+ | OS / | crafted | minimal | crafted | minimal | | source | ICMPv4 | IPv4 MTU | ICMPv6 | IPv6 MTU | +---------------+---------+----------+---------+----------+ | [Brandt2018] | accept | 552/296 | unknown | unknown | +---------------+---------+----------+---------+----------+ | Linux 2.6.32 | accept | 552 | accept | 1280 | | Linux 4.18.20 | ignore? | | accept | 1280 | | FreeBSD 12 | ignore | | accept | 1280 | +---------------+---------+----------+---------+----------+ 3. Possible measures 3.1. Use DNSSEC DNSSEC is a measure against cache poisoning attacks. However, there are many unsigned zones and full-service resolver operator need to consider these zones. "Use DNSSEC" requires both authoritative side and resolver side support. 3.2. Limit requestor's UDP payload size to 1220/1232 on IPv6 Limiting EDNS0 requestor's UDP payload size [RFC6891] to 1220/1232 on IPv6 is countermeasure of path MTU attacks on IPv6 because minimal MTU value of IPv6 is 1280 and most of implementations ignore ICMPv6 packet too big packets whose MTU value is smaller than 1280. Fujiwara Expires August 19, 2019 [Page 4] Internet-Draft fragment-attack February 2019 3.3. Limit requestor's UDP payload size to 512 Limiting EDNS0 requestor's UDP payload size [RFC6891] to 512 may be countermeasure of path MTU attacks. However, since most of DNSSEC responses exceed 512 octets, limiting EDNS0 requestor's UDP payload size to 512 results truncated responses and resolvers need to retry queries by TCP. It always decreases name resolution performance. And more, [Brandt2018] showed that off-path attackers can set some of authoritative servers' path MTU cache to 296. This case, limiting EDNS0 payload size is not a countermeasure. Section 3 of [RFC4035] defines that A security-aware name server MUST support a message size of at least 1220 octets. 3.4. Drop path MTU discovery or filter ICMP related to path MTU discovery It is not a countermeasure of resolver side. All authoritative servers need to be changed. Changing all authoritative servers is impossible. TCP requires path MTU discovery. 3.5. Drop all fragmented packets To avoid fragmentation attacks, "drop all fragmented packets" is one idea. However, under path MTU discovery attacks, TCP packets may be fragmented and dropped. Then, "drop all fragmented UDP packets related to DNS" is the solution. 3.6. Drop fragmented UDP DNS responses at full-service resolvers Drop fragmented UDP DNS responses at full-service resolvers may be a countermeasure of cache poisoning attacks using IP fragmentation. To avoid fragmentation in normal condition, use EDNS0 requestor's UDP payload size as 1220 to avoid fragmentation. 1220 is the minimal value defined by [RFC4035]. Under path MTU discovery attacks and cache poisoning attacks using IP fragmentation, UDP DNS response packets are fragmented and dropped and name resolution fails. If resolver software retries by TCP, TCP is strong for fragmentation attacks and name resolution by TCP will success. Fujiwara Expires August 19, 2019 [Page 5] Internet-Draft fragment-attack February 2019 3.7. Use TCP only It is believed that TCP is not vulnerable to fragmentation attacks. Unbound has "tcp-upstream" option that changes the upstream queries use TCP only for transport. Some operators that support [RFC8078] said that they use TCP only for transport to avoid cache poisoning attacks. The full-service resolvers of multiple CAs issuing domain validation (DV) certificates are required to withstand cache poisoning attacks, it is better to implement their full-service resolvers use TCP upstream queries only. Section 11.2 "DNS security" of [I-D.ietf-acme-acme] recommends that servers SHOULD perform DNS queries over TCP, which provides better resistance to some forgery attacks than DNS over UDP. 4. Proposal To avoid cache poisoning attacks using IP fragmentation by full- service resolvers, o Full-service resolvers set EDNS0 requestor's UDP payload size to 1220. (minimal size defined by [RFC4035]) o Full-service resolvers drop fragmented UDP responses related to DNS. o Full-service resolvers may retry name resolution by TCP. 5. Example firewall configuration Linux iptables support dropping first fragment with UDP source port 53 by using m32 module. Other first fragments that is not UDP, not source port 53 are not dropped. Second and following fragments should not be dropped because they may relate to other protocols. Second fragments related to DNS will be dropped because their first fragments dropped. iptables -t raw -A PREROUTING -m u32 --u32 \\ "6&0xFFFF00FF=0x20000011&&18&0xffff=53" -j DROP or iptables -t raw -A PREROUTING -p udp -f -j DROP ip6tables -A INPUT -p udp -m frag --fragfirst -m udp --sport 53 -j DROP Fujiwara Expires August 19, 2019 [Page 6] Internet-Draft fragment-attack February 2019 Other OSs may not handle first fragments. Then, drop all fragmented UDP packets. On FreeBSD, 'ipfw' can drop all fragmented UDP packets (second fragments). ipfw deny log udp from any to me in frag 6. IANA Considerations This document has no IANA actions. 7. Security Considerations Under path MTU discovery and fragmentation attacks, most full-service resolver software do not retry name resolution by TCP, name resolution related to attacks fails. 8. Acknowledgments The author would like to specifically thank Mark Andrews. 9. Change History 9.1. 00 Initial version 10. References 10.1. Normative References [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990, . [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, . Fujiwara Expires August 19, 2019 [Page 7] Internet-Draft fragment-attack February 2019 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. [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, . [I-D.ietf-acme-acme] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", draft-ietf-acme-acme-18 (work in progress), December 2018. [RFC8078] Gudmundsson, O. and P. Wouters, "Managing DS Records from the Parent via CDS/CDNSKEY", RFC 8078, DOI 10.17487/RFC8078, March 2017, . Appendix A. How to know path MTU value o Linux: ip route get o FreeBSD: sysctl -o net.inet.tcp.hostcache.list Appendix B. How to generate crafted ICMP packets Let the crafted path MTU value be cMTU. B.1. Example of crafted ICMP Need Fragmentation and DF set packet Fujiwara Expires August 19, 2019 [Page 8] Internet-Draft fragment-attack February 2019 IP header: +-----------------------------------------------+ | V/HL 0x45 / TOS any / Total Length 20+8+20+8 | | Identification any / Flags/Offset 0 | | TTL any / Protocol 1 / Header checksum: calc | | Source Address: attack tool address or any | | Destination: target auth server address | +-----------------------------------------------+ ICMP header: +-----------------------------------------------+ | Type 3 / Code 4 / Checksum: calculate | | unused 0 / Next-Hop MTU: cMTU | +-----------------------------------------------+ Internet Header + 64 bits of Original Datagram: IP header: +-----------------------------------------------+ | V/HL 0x45 / TOS any / Total Length 1420 | | Identification any / Flags/Offset 0x4000(DF)| | TTL any / Protocol 17/ Header checksum: calc | | Source Address: target auth server address | | Destination: victim full-resolver address | +-----------------------------------------------+ UDP header: +-----------------------------------------------+ | Source Port 53 / Destination Port: any | | Length 1400 / Checksum: any | +-----------------------------------------------+ B.2. Example of crafted ICMPv6 Packet Too Big Fujiwara Expires August 19, 2019 [Page 9] Internet-Draft fragment-attack February 2019 IPv6 header: +----------------------------------------------------+ | Version/Traffic Class/Flow Label: 0x60000000 | |Payload Len: cMTU-40 / NextHeader 58 / HopLimit any | | Source Address: attack tool address or any | | Destination Address: target auth server address | +----------------------------------------------------+ ICMPv6 header: +----------------------------------------------------+ | Type 2 / Code 0 / Checksum: calculate | | MTU: (64bit) cMTU | +----------------------------------------------------+ Fake invoking packet IPv6 header: +----------------------------------------------------+ | Version/Traffic Class/Flow Label: 0x60000000 | |Payload Len: 1400 / NextHeader 17 / HopLimit any | | Source Address: target auth server address | | Destination Address: victim full-resolver address | +----------------------------------------------------+ UDP header: +----------------------------------------------------+ | Source Port 53 / Destination Port: any | | Length 1400 / Checksum: any | +----------------------------------------------------+ Rest: Fill zero to end of packet Author's Address 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 Expires August 19, 2019 [Page 10]