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TCP for DNS security considerations

Discover the security vulnerabilities of using TCP for DNS queries and explore alternatives to mitigate risks. Learn about connection-flooding attacks, buffer issues, blind attacks, and more. Evaluate options like larger UDP responses, tuned TCP settings, or a DNS-specific transport protocol.

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TCP for DNS security considerations

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  1. TCP for DNS security considerations Fernando Gont project carried out on behalf of UK CPNI 76th IETF meeting, November 8-13, 2009 Hiroshima, Japan

  2. What are the issues? • TCP may be needed for DNS as a fall-back transport protocol when the answer to the query is large. • Relying on TCP for DNS is going from stateless to stateful • This opens the door to a number of Denial of Service vulnerabilities that are typical for a connection-oriented protocol: • Connection-flooding attacks: Naptha, FIN-WAIT-2 flooding, etc. • Send buffer issues: Netkill, closed windows, etc • Receive buffer issues: holes in the data stream • Blind attacks: that depend on predictable ISNs, ephemeral ports, etc. • Overhead issues arising from connection establishment/teardown

  3. Background needed to understand them • “Security Assessment of the Transmission Control Protocol (TCP)”, published by UK CPNI, available at: http://www.cpni.gov.uk/Docs/tn-03-09-security-assessment-TCP.pdf • An IETF I-D version of the document is currently a TCPM WG item: draft-ietf-tcpm-tcp-security (but you probably want to look at draft-gont-tcp-security instead)

  4. What alternatives exist? • Go with larger UDP responses – this might lead to fragmentation… and fragmentation brings a lot of other issues (see draft-ietf-opsec-ip-security) • Use some other connection-oriented protocol – probably with the same issues as TCP… (and additional issues for NAT traversal). • A DNS-specific transport protocol? (e.g., draft-barwood-dnsext-dns-transport) • Use a tuned/hardened TCP – i.e., configure TCP settings so that the aforementioned issues are less of an issue (better if much of the resiliency machinery is “built-in”).

  5. Choices that need to be made • If connection establishment/teardown is a concern, some might think (actually, *have* thought) about “persistent connections” (as in HTTP) • Do they really make sense?? • To minimize state at the DNS server, the client could be required to perform the active close. Otherwise (e.g., idle connection) the server resets (RST) it. • How “liberal” we are with respect to the client behavior? • Number of concurrent connections • Idle-connections • Connections in, e.g., FIN-WAIT-2 state, TIME-WAIT state, etc. • Timing issues: RTO, closed windows, etc.

  6. Possible way forward • TCP for DNS tuning document? (in DNSEXT or DNSOP?) • Update Section 4.2.2 (“TCP usage”) in RFC 1035 in a DNSEXT document? • draft-ietf-dnsext-dns-tcp-requirements currently has *some* text on the subject • Others?

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