[Search] [txt|pdf|bibtex] [Tracker] [Email] [Nits]

Versions: 00 01                                                         
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,
              <https://www.rfc-editor.org/info/rfc1191>.

   [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>.





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, <https://ripe67.ripe.net/
              presentations/240-ipfragattack.pdf>.

   [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,
              <https://www.rfc-editor.org/info/rfc8078>.

Appendix A.  How to know path MTU value

   o  Linux: ip route get <IPv4/IPv6 address>

   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]