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How Does DNSSEC Change DNS Lookup?

Traditional (insecure) DNS lookup is simple: a recursive name server receives a query from a client to lookup the name www.isc.org. The recursive name server tracks down the authoritative name server(s) responsible, sends the query to one of the authoritative name servers, and waits for the authoritative name server to respond with the answer.

With DNSSEC validation enabled, a validating recursive name server (a.k.a. a validating resolver) will ask for additional resource records in its query, hoping the remote authoritative name servers will respond with more than just the answer to the query, but some proof to go along with the answer as well. If DNSSEC responses are received, the validating resolver will perform cryptographic computation to verify the authenticity (origin of the data) and integrity (data was not altered during transit) of the answers, and even ask the parent zone as part of the verification. It will repeat this process of get-key, validate, ask-parent, parent, and its parent, and its parent, all the way until the validating resolver reaches a key that it trusts. In the ideal, fully deployed world of DNSSEC, all validating resolvers only need to trust one key: the root key.

The following example shows the DNSSEC validating process of looking up the name www.isc.org at a very high level:

Figure 1.1. DNSSEC Validation 12 Steps

DNSSEC Validation 12 Steps

  1. Upon receiving a DNS query from a client to resolve www.isc.org, the validating resolver follows standard DNS protocol to track down the name server for isc.org, sends it a DNS query to ask for the A record of www.isc.org. But since this is a DNSSEC-enabled resolver, the outgoing query has a bit set indicating it wants DNSSEC answers, hoping the name server who receives it speaks DNSSEC and can honor this secure request.
  2. isc.org name server is DNSSEC-enabled, and responds with the answer (in this case, an A record), and with a digital signature for verification purpose.
  3. The validating resolver needs to be able to verify the digital signature, and to do so it requires cryptographic keys. So it asks the isc.org name server for those keys.
  4. isc.org name server responds with the cryptographic keys (and digital signatures of the keys) used to generate the digital signature that was sent in #2. At this point, the validating resolver can use this information to verify the answers received in #2.

    Let's take a quick break here and look at what we've got so far... how could we trust this answer? If a clever attacker had taken over the isc.org name server(s), or course she would send matching keys and signatures. We need to ask someone else to have confidence that we are really talking to the real isc.org name server. This is a critical part of DNSSEC: at some point, the DNS administrators at isc.org had uploaded some cryptographic information to its parent, .org, maybe it was a secure web submit form, maybe it was through an email exchange, or perhaps it was done in person. No matter the case, at some point, some verifiable information about the child (isc.org) was sent to the parent (.org), for safekeeping.

  5. The validating resolver asks the parent (.org) for the verifiable information it keeps on its child, isc.org.
  6. The verifiable information is sent from the .org server. At this point, validating resolver compares this to the answer it received in #4, and the two of them should match, proving the authenticity of isc.org.

    Let's examine this process. You might be thinking to yourself, well, what if the clever attacker that took over isc.org also compromised the .org servers? Of course all this information would match! That's why we will turn our attention now to the .org servers, interrogate it for its cryptographic keys, and move on one level up to .org's parent, root.

  7. The validating resolver asks .org authoritative name servers for its cryptographic keys, for the purpose of verifying the answers received in #6.
  8. .org name server responds with the answer (in this case, keys and signatures). At this point, validating resolver can verify the answers received in #6.
  9. The validating resolver asks root (.org's parent) for verifiable information it keeps on its child, .org.
  10. Root name server sends back the verifiable information it keeps on .org. The validating resolver now takes this information and uses it to verify the answers received in #8.

    So up to this point, both isc.org and .org check out. But what about root? What if this attacker is really clever and somehow tricked us into thinking she's the root name server? Of course she would send us all matching information! So we repeat the interrogation process and ask for the keys from the root name server.

  11. Validating resolver asks root name server for its cryptographic keys in order to verify the answer(s) received in #10.
  12. Root name server sends keys, at this point, validating resolver can verify the answer(s) received in #10.

Chain of Trust

But what about the root server itself? Who do we go to verify root's keys? There's no parent zone for root. In security, you have to trust someone, and in the perfectly protected world of DNSSEC (we'll talk about the current imperfect state later and ways to work around it), each validating resolver would only have to trust one entity, that is the root name server. The validating resolver would already have the root key on file (and we'll talk about later how we got the root key file). So after answers in #12 are received, validating resolver takes the answer received and compare it to the key it already has on file, and these two should match. If they do, it means we can trust the answer from root, thus we can trust .org, and thus we can trust isc.org. This is known as "chain of trust" in DNSSEC.

We will revisit this 12-step process again later in the section called “How Does DNSSEC Change DNS Lookup (Revisited)?” with more technical details.

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