DNS Extensions Working Group W. Wijngaards
Internet-Draft NLnet Labs
Intended status: Standards Track March 5, 2009
Expires: September 6, 2009
DNSSEC Trust Anchor History Service
draft-wijngaards-dnsext-trust-history-02
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Abstract
When DNS validators have trusted keys, but have been offline for a
longer period, key rollover will fail and they are stuck with stale
trust anchors. History service allows validators to query for older
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DNSKEY RRsets and pick up the rollover trail where they left off.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1. Introduction
DNSSEC [RFC4034] validators that have been offline of have missed an
(emergency) rollover can use trust history service to get back on
track. The trust history location is assumed available from the
validator configuration. The validator then fetches old DNSKEY
RRsets and checks they form a chain to the latest key.
Providers of trust history can fetch the DNSKEY data as published by
the zone they track, and copy-and-paste it. They need not sign nor
hold private keys safe. The algorithms for this are explained below.
2. Trust History Lookup
The algorithm is in steps.
Step 1. The validator performs a DNSKEY lookup to the target zone,
which looks like any other initial DNSKEY lookup for a trust anchor.
If the keyset verifies with the trust anchor currently held, the
keyset already works. Otherwise, store this result, the further
algorithm either ends with this result or fails.
Step 2. Fetch the trust history list end points. Query type PTR to
the location configured for trust history.
Step 3. Go backwards through the trust history list. Verify that
the keyset validly signs the next keyset. This is [RFC4034]
validation, but the RRSIG expiration date is ignored. Replace the
owner domain name of the DNSKEY with the target zone name for
verification. Query type PTR to get previous and next locations.
Step 4. When the trust anchor currently held by the validator
verifies the keyset, the algorithm is done. The initial DNSKEY from
Step 1 is the result. Use it as the new trust anchor (if using
[RFC5011], put it in state VALID). The validator SHOULD store the
new trust anchor on stable storage.
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3. Trust History Tracker
The tracker polls the target zone DNSKEY RRset from cron. Ignore
date changes in the RRSIG. If the tracker knows that the zone
performs responsible prepublish KSK rollovers [RFC4641] then also ZSK
changes can be ignored.
Copy the newly polled DNSKEY RRset and RRSIGs, change the owner name
to "h%d.example" or similar. With %d a new number, and .example the
history location. Publish the new RRset. Publish PTR records that
link list elements. Update PTR records for the list start and end.
The list is a double linked list, because this empowers low memory
hosts to perform consistency checks. Thus if there is x.example PTR
y.example then there MUST be y.example PTR x.example. Except at
start and end of list.
To find the start of the list, it MUST be a name before the end of
the list in canonical comparison ordering ([RFC4034]). With exactly
one list element, one PTR record points to both start and end.
4. Example
In this example tuhi.example.com provides trust history for
example.net. The DNSKEY rdata and RRSIG rdata is omitted for
brevity, it is a literal copy and paste of the data from example.net.
$ORIGIN tuhi.example.com.
@ PTR h0
@ PTR h2
h0 PTR h1
h0 DNSKEY ...
h0 RRSIG ...
h1 PTR h2
h1 PTR h0
h1 DNSKEY ...
h1 RRSIG ...
h2 DNSKEY ...
h2 RRSIG ...
h2 PTR h1
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5. Security Considerations
The trust history tracker only provides a cached copy of old data.
The history data can be altered or withheld; the lookup algorithm
then fails.
Ultimately the security depends on the key that the validator is
holding, and the keys in the chain up to the present. If the old key
held by the validator is too old, the validator MAY not accept this
risk, and then SHOULD perform out of band key priming.
The algorithm looks up the initial DNSKEY like other validators do,
and then walks the history in reverse. This avoids exposing the
validator on the network as a host with an older key and the key id.
Validators can (OPTIONAL) examine RRSIG dates. To account for clock
skew padding by the target zone, the middle of the inception and
expiration date SHOULD be used, if consistency checks are done.
The algorithm can be abused to provide a secure method to get (close
to) the current month or year. Validators without clock then know
the date that validates signatures currently in use by the zone.
6. IANA Considerations
-
7. References
7.1. Informative References
[RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
RFC 4641, September 2006.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", RFC 5011, September 2007.
7.2. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
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Author's Address
Wouter Wijngaards
NLnet Labs
Science Park 140
Amsterdam 1098 XG
The Netherlands
Phone: +31-20-888-4551
EMail: wouter@nlnetlabs.nl
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