Security/DNSSEC-TLS/Background
This page contains background information on TLS and DNSSEC. More comprehensive information can be found elsewhere.
Background
TLS sessions require a chain of trust to authenticate the server to the client. Currently this is achieved through a series of PKIX certificates that link the server's certificate to a trusted root certificate stored by the client. Trusted root certificates are issued by Certificate Authorities. Currently, no technical mechanism prevents any authority from signing a certificate for any domain name.
DNSSEC is a method of authenticating DNS records that also uses a chain of trust. In the case of DNSSEC, the trust structure is identical to the DNS hierarchy. For example, only the entity in charge of .com can sign the record that authenticates example.com, and only the (single) entity in charge of the (single) root can sign the record that authenticates .com.
To validate a DNS record, its RRSIG record and corresponding DNSKEY must be obtained. If the signature checks out (using the key), then the key record must be checked (recursively). This process eventually arrives at a key signing key for a given level in the DNS hierarchy. To go up the hierarchy, the DS record for the level must be authenticated by a DNSKEY in the next level up. If that succeeds, then the process continues (i.e. the DNSKEY in the next level up must be authenticated, and so forth). Eventually the root of trust is arrived at, whereupon the original DNS record has been validated.
Verifying a DNSSEC Chain
Verification of a DNSSEC chain can be started from the bottom up. For instance, to verify the A record for wiki.mozilla.org, the process can be started by requesting the A and RRSIG records for wiki.mozilla.org, as well as the DNSKEY records for mozilla.org (where mozilla.org is from whence we got the authoritative records for wiki.mozilla.org). One of the zone-signing DNSKEYs will verify the signature on the RRSIG (if all is well). The DNSKEY records then must be verified by the key-signing DNSKEY(s) for mozilla.org. At this point, to establish trust up to the root, a DS record from .org to mozilla.org must be verified. This requires the DNSKEYs from .org. Once the DS record checks out, the process recurses: .org's DNSKEYs are walked up to the DS record for .org, which leads to the root: .
At this point, since our root keys are the root of trust, if the .org DS record is verified, the entire chain has been verified. In the simplest case, this process requires the A and RRSIG records from wiki.mozilla.org, one zone-signing DNSKEY and corresponding RRSIG record for mozilla.org, one keys-igning DNSKEY and corresponding RRSIG record for mozilla.org, a DS record and corresponding RRSIG record for mozilla.org (from .org), a zone-signing DNSKEY and corresponding RRSIG record for .org, a key-signing DNSKEY and corresponding RRSIG record for .org, a DS record and corresponding RRSIG record for .org, and a copy of the root DNSKEYs. Thus 14 records would be required in a full DNSSEC chain.
In theory, it should not be necessary to have both a zone-signing key and a key-signing key for each level of the hierarchy. In practice, though, both are used (and sometimes multiples of each are used). In an example verification of wiki.mozilla.org, 22 records were sent for a total of 3832 bytes.
In theory, the DNSKEYs of TLDs could be cached by the client, meaning only everything up to the DS and corresponding RRSIG for mozilla.org need be sent. This could limit the number of records to 8 in normal operation (using ZSKs and KSKs). This could operate as follows:
- Verify wiki.mozilla.org A record using mozilla.org ZSK (requires the A and corresponding RRSIG records and the ZSK DNSKEY record)
- Verify ZSK from step 1 using KSK (requires in addition a DNSKEY record and the RRSIG record for the ZSK).
- Verify the KSK using mozilla.org's DS record (requires in addition the DS and corresponding RRSIG record, as well as the cached .org DNSKEY (and the RRSIG for the KSK, I think))
From there, we have already verified .org, so wiki.mozilla.org has been verified using a total of 8 transmitted records. From the trace, this reduces the data sent from 3832 to 1280 bytes. The amount of data required will vary by site (by domain name), but it will probably be in the range of 1K. Note: the TLS server hello from wiki.mozilla.org is 1063 bytes (sent in 1 packet). Adding the 1K of DNSSEC messages to this would fit in 1 more packet.
(Aside: CNAME records do not work well with this porposal. For instance, the A record for addons.mozilla.org is actually a CNAME for amo.glb.mozilla.net. The first problem is that amo.glb.mozilla.net has no RRSIG record, and thus cannot actually be verified through DNSSEC. The second problem is even if it did, we would have to send twice as much (more? can CNAMES point to CNAMES?) data.)
