On 12.6.2014 17:49, Petr Spacek wrote:
On 12.6.2014 17:19, Simo Sorce wrote:
On Thu, 2014-06-12 at 17:08 +0200, Petr Spacek wrote:
Hello list,
I have realized that we need to store certain DNSSEC metadata for every
(zone,key,replica) triplet. It is necessary to handle splits in replication
topology.
DNSSEC key can be in one of following states:
- key created
- published but not used for signing
- published and used for signing
- published and not used for signing but old signatures exist
- unpublished
Every state transition has to be postponed until relevant TTL expires, and of
course, we need to consider TTL on all replicas.
Example of a problem
====================
DNS TTL=10 units
Key life time=100 units
Replica=1 Key=1 Time=0 Published
Replica=2 Key=1 Time=0 Published
Replica=1 Key=1 Time=10 Published, signing
Replica=2 Key=1 Time=10 Published, signing
Replica=1 Key=2 Time=90 Generated, published, not signing yet
Replica=2 Key=2 Time=90 <replication is broken: key=2 is not on replica=2>
Replica=1 Key=1 Time=100
^^^ From time=100, all new signatures should be created with key=2 but that
can break DNSSEC validation because key=2 is not available on replica=2.
Can you explain how this break validation ?
Aren't signatures regenerated on each replica ?
They are.
And so isn't each replica self-consistent ?
Ah, sorry, I didn't mention one important detail. Keys published in the zone
'example.com.' have to match keys published in parent zone. There has to be a
mechanism for synchronizing this.
Validation will break if (keys published by parent) are not subset of (keys on
replicas).
Next logical step in the example above is to remove key1 from replica 1 so you
will end replica1 having key2 and replica2 having only key1.
How can we guarantee that synchronization mechanism will not wipe key1 from
parent? Or the other way around? How can we guarantee that key2 was uploaded?
Also, things will break is number of keys in parent exceeds reasonable number
(because DNS replies will be to big etc.).
Proposal 1
==========
- Store state and timestamps for (zone,key,replica) triplet
- Do state transition only if all triplets (zone,key,?) indicate that all
replicas reached desired state so the transition is safe.
- This implicitly means that no transition will happen if one or more replicas
is down. This is necessary otherwise DNSSEC validation can break mysteriously
when keys got out of sync.
dn: cn=<some-replica-id>,ipk11Label=zone1_keyid123_private, cn=keys, cn=sec,
cn=dns, dc=example
idnssecKeyCreated: <timestamp>
idnssecKeyPublished: <timestamp>
idnssecKeyActivated: <timestamp>
idnssecKeyInactivated: <timestamp>
idnssecKeyDeleted: <timestamp>
Why do you care for all 5 states ?
In short, to follow RFC 6781 and all it's requirements.
Simo and I have discussed this off-line. The final decision is to rely on
replication. The assumption is that if replication is broken, everything will
break soon anyway, so the original proposal is overkill.
We have to store one set of timestamps somewhere to be able to follow RFC
6781, so we decided to store it in the key-metadata object.
I added other attributes to object class definition so it contains all
necessary metadata. The new object class idnsSecKey is now complete.
Please note that DN in the previous example was incorrect. It is necessary to
store the metadata separately for pairs (zone, key) to cover the case where
key is shared between zones. This also nicely splits metadata from actual key
material.
All attributes are single-valued.
MUST attributes are:
idnsSecKeyRef
idnsSecKeyCreated
idnsSecAlgorithm
dn: cn=<z/ksk+keytag>, cn=keys, idnsname=example.com, cn=dns, dc=example
objectClass: idnsSecKey
idnsSecKeyRef: <DN of the PKCS#11 key object under cn=keys, cn=sec, dn=dns>
idnsSecKeyCreated: <timestamp>
idnsSecKeyPublish: <timestamp>
idnsSecKeyActivate: <timestamp>
idnsSecKeyInactive: <timestamp>
idnsSecKeyDelete: <timestamp>
idnsSecKeyZone: <boolean> equivalent to bit 7 (ZONE) in [1]
idnsSecKeyRevoke: <boolean> equivalent to bit 8 (REVOKE) in [1]
idnsSecKeySep: <boolean> equivalent to bit 15 (SEP) in [1]
idnsSecAlgorithm: <string> used as mnemonic in [2]
[1]
http://www.iana.org/assignments/dnskey-flags/dnskey-flags.xhtml#dnskey-flags-1
[2]
http://www.iana.org/assignments/dns-sec-alg-numbers/dns-sec-alg-numbers.xhtml#dns-sec-alg-numbers-1
It looks to me the only relevant states are Activated and (perhaps)
Deactivated.
But then again if replication is broken *many* other things are, so
should we *really* care to handle broken replication by adding all this
information ?
We need to keep track of timestamps anyway (to follow RFC 6781) so we need
some other way how to *make sure* that timestamps never go backwards, even if
replication was broken and later restarted.
What do you propose?
Effectively, state machine will be controlled by max(attribute) over all
replicas (for given key).
Replication traffic estimation
------------------------------
Number of writes to LDAP = (State transitions per key) * (Keys per zone) *
(Number of zones) * (Number of replicas)
The obvious problem is that amount of traffic grows linearly with all
variables.
State transitions per key: 5
Keys per zone: 10
Zones: 100
Replicas: 30
Key life time: 1 month
5*10*100*30 / 1 month
i.e.
150 000 writes / 1 month
i.e.
~ 1 write / 17 seconds
It seems like that this generates a lot of replication traffic. (Please note
that number of replicas/zones/keys per zone is also quite high but it will be
hard to improve scalability later if we decide to use LDAP in this way.)
Right, and for an edge case that is already a broken state. I would
rather spend time on a monitoring process that warns the amdin
replication is broken rather than adding all this unnecessary traffic.
And ... our favorite question :-)
What should I use for cn=<some-replica-id> ? I would propose use either FQDN
of replica or value returned by LDAP whoami.
Add to this dealing with dead/removed replicas ? I question the sanity
of this approach :)
Proposal 2
==========
Another possibility is to make timestamp attributes non-replicated and
(somehow) use DNS queries to determine if the desired key is available on all
other replicas before any state transition is allowed.
That would require:
- Full-mesh replica-to-replica connectivity
- Similar amount of DNS query/response round trips (multiply <small int>)
- Security is questionable: (Maybe, I'm not sure!) Attacker could spoof DNS
answers and break key rotation mechanism during bootstrap (when no keys are
available) and maybe even later.
It is easy to detect that key is:
- published
- unpublished
- used for signing
The problem is that there is no reliable way to detect that is a key was
created/is available on replica but is not published yet and similarly that
the key is published but not used for signing anymore (it would require to
check all names published in the zone).
I will think about it a bit more but I would like to know if full-mesh
replica-to-replica connectivity is acceptable requirement or not.
Not really, and again I question the need to do this.
We will do some limited set DNS queries to do sanity checking, mainly when new
KSK is to-be published. In that case we need to wait until parent zone picks
it up.
Petr^2 Spacek
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