Hello,
I found that we (probably) misunderstood each other. The sky-high level
overview of the proposal follow:
NO CHANGE:
1) LDAP stores all *unsigned* data.
2)
NO CHANGE:
a) bind-dyndb-ldap *on each server* fetches all unsigned data from LDAP and
store them in *in memory* database (we do it now)
THE DIFFERENCE:
b) All data will be stored in BIND's native RBT-database (RBTDB) instead of
our own in-memory database.
NEW PIECES:
3)
Mechanisms implemented in BIND's RBTDB will do DNSSEC signing etc. for us. The
BIND's feature is called 'in-line signing' and it can do all key/signature
maintenance for us, including periodical zone re-signing etc.
The whole point of this proposal is about code-reusage. I'm trying to avoid
re-inventing of the wheel.
Note that DNSSEC implementation in BIND has ~ 150 kiB of C code, stand-alone
signing utilities add another ~ 200 kiB of code (~ 7000 lines) . I really
don't want to re-write it again when it's not reasonable.
Further comments are in-line.
On 20.5.2013 14:07, Simo Sorce wrote:
On Wed, 2013-05-15 at 17:11 +0200, Petr Spacek wrote:
On 15.5.2013 10:29, Simo Sorce wrote:
I investigated various scenarios for DNSSEC integration and I would like to
hear your opinions about proposed approach and it's effects.
The most important finding is that bind-dyndb-ldap can't support DNSSEC
without rewrite of the 'in-memory database' component.
Can you elaborate why a rewrite would be needed ? What constraint we do not
meet ?
We have three main problems - partially with data structures and mostly with
the way how we work with the 'internal database':
1) DNSSEC requires strict record ordering, i.e. each record in database has to
have predecessor and successor (ordering by name and then by record data).
This can be done relatively simply, but it requires a full dump of the database.
2) On-line record signing requires a lot of data stored
per-record+per-signature. This would require bigger effort than point 1),
because many data structures and respective APIs and locking protocols have to
be re-designed.
3) Our current 'internal database' acts as a 'cache', i.e. records can appear
and disappear dynamically and the 'cache' is not considered as authoritative
source of data: LDAP search is conducted each time when some data are not
found etc. The result is that the same data can disappear and then appear
again in the cache etc.
Typical update scenario, with persistent search enabled:
a) DNS UPDATE from client is received by BIND
b) New data are written to LDAP
c) DN of modified object is received via persistent search
d) All RRs under the *updated name* are discarded from the cache
<-- now the cache is not consistent with data in LDAP
e) Object from LDAP is fetched by plugin
<-- a query for the updated name will enforce instant cache refresh, because
we know that the cache is not authoritative
f) All RRs in the object are updated (in cache)
The problem is that the cache in intermediate states (between <-- marks) can't
be used as authoritative source and will produce incorrect signatures. The
text below contains more details.
Database's in BIND has concept of 'versions' ('transactions') which our
internal cache do not implement ... It could be solved by proper locking, of
course, but it will not be a piece of cake. We need to take care of many
parallel updates, parallel queries and parallel re-signing at the same time.
I don't say that it is impossible to implement our own backend with same
properties as BIND's database, but I don't see the value (and I can see a lot
of bugs :-).
Well, we do not necessarily need all the same properties of bind's
database, only those that allow us to properly handle DNSSEC, so let's
try to uncover what those constrains are first, so I can understand why
you propose this solution as better than something else.
Fortunately, it seems
that we can drop our own implementation of the internal DNS database
(ldap_driver.c and cache.c) and re-use the database from BIND (so called
RBTDB).
I'm trying to reach Adam Tkac with the question "Why we decided to implement
it again rather than re-use BIND's code?".
Re-usage of BIND's implementation will have following properties:
== Advantages ==
- Big part of DNSSEC implementation from BIND9 can be reused.
- Overall plugin implementation will be simpler - we can drop many lines of
our code and bugs.
- Run-time performance could be much much better.
- We will get implementation for these tickets "for free":
-- #95 wildcard CNAME does NOT work
-- #64 IXFR support (IMHO this is important!)
-- #6 Cache non-existing records
And partially:
-- #7 Allow limiting of the cache
Sounds very interesting.
== Disadvantages ==
- Support for configurations without persistent search will complicate things
a lot.
-- Proposal => Make persistent search obligatory. OpenLDAP supports LDAP
SyncRepl, so it should be possible to make plugin compatible with 389 and
OpenLDAP at the same time. I would defer this to somebody from users/OpenLDAP
community.
Why the persistent search would be required ?
As I mentioned above - you need database dump, because DNSSEC requires strict
name and record ordering.
It is possible to do incremental changes when the 'starting snapshot' is
established, but it means that we need information about each particular
change => that is what persistent search provides.
Ok, so it is to have a complete view of the databse, I assume to reduce
the number of re-computations needed for DNSSEC.
I think it's inevitable, because you *need* the strict resource record
ordering (see point 1 above) for the NSEC/NSEC3 record generation [1] - i.e.
from the very beginning.
Partial re-signing is inevitable after each change, but you can't start the
signing process without the strict resource record ordering.
[1] http://tools.ietf.org/html/rfc4034#section-6
- Data from LDAP have to be dumped to memory (or to file) before the server
will start replying to queries.
=> This is not nice, but servers usually are not restarted often. IMHO it is
a
good compromise between complexity and performance.
I am not sure I understand what this means. Does it mean you cannot change
single
cache entries on the fly when a change happens in LDAP ? Or something else ?
Sorry, I didn't explained this part in it's full depth.
You can change everything run-time, but there are small details which
complicates loading of the zone and run-time changes:
1) A normal zones requires SOA + NS + A/AAAA records (for NSs) to load. It is
(hypothetically) possible to create empty zone, fill it with SOA, NS and A
records and then incrementally add rest of the records.
The problem is that you need to re-implement DNS resolution algorithm to find
which records you need at the beginning (SOA, NS, A/AAAA) and then load the
rest.
I would like to avoid this re-implementation. It is not possible to re-use
BIND's implementation because it is tied to the DB implementation ... but we
can't load the database because it is missing SOA, NS and A/AAAA records.
Chicken-egg problem.
To be honest I am not sure I understand what's your point here.
I'm not sure how to explain it without describing implementation in BIND. It
is implementation detail, we can discuss it when we agree on all other things.
2) The second reason why I want to make persistent search obligatory is that
each change in DNSSEC signed zone requires a lot of work, so it is not a good
idea to wait with the work to time when somebody asks for particular record.
How it works without persistent search (now):
1) Query from a client is received by BIND
2) Internal cache is consulted
3) Record is not found in the cache - LDAP search is done
4) Fetched records in saved to the cache
5) Reply to client is constructed
It is hard to work in the same way when DNSSEC is in place. Each change
implicates re-signing of the particular RRset and it's neighbours, i.e.:
1) Query from a client is received by BIND
2) Internal cache is consulted
3) Record is not found in the cache - LDAP search is done
4) Fetched records in saved to the cache
* 4b) New RRset is re-signed
* 4c) Records neighbouring with the new RR has to be updated and re-signed
5) Reply to client is constructed
Ok so the point here is that we want to do the signing at store time
rather than read time. That is understandable.
However we have 2 ways to look at it.
1. bind does the work
2. DS does the work
I haven't seen any reasoning from you why letting Bind do this work is
a better idea.
Simply said - because all the code is already in BIND (the feature is called
'in-line signing', as I mentioned above).
I actually see some security reasons why putting this into a DS plugin
can have quite some advantages instead. Have you considered doing this
It could improve the security a bit, I agree. But I don't think that it is so
big advantage. BIND already has all the facilities for key material handling,
so the only thing we have to solve is how to distribute keys from LDAP to
running BIND.
work in a DS plugin at all ? If you haven and have discarded the idea,
can you say why ?
1) It would require pulling ~ 200 kiB (~ 7000 lines) of DNSSEC signing code
into 389.
2) It would require pulling 'text->DNS wire format' parser into 389 (because
our LDAP stores plain text data but the signing process works with DNS wire
format).
3) It simplifies bind-dyndb-ldap, but we still need to re-implement DNS search
algorithm which takes DNSSEC oddities into account. (Note that the DNS search
algorithm is part of the database implementation. Bugs/limitations in our
implementation are the reason why wildard records are not supported...)
4) I'm not sure how it will work with replication. How to ensure that new
record will not appear in the zone until the associated RRset is (re)computed
by DS? (BIND has transaction mechanism built-in to the internal RBTDB.)
The point is that you *can* do changes run-time, but you need to know about
the changes as soon as possible because each change requires significant
amount of work (and magic/mana :-).
It opens a lot of opportunities for race condition problems.
Yes, I am really concerned about the race conditions of course, however
I really wonder whether doing signing in bind is really a good idea.
We need to synchronize these signatures to all masters right ?
No, because signatures are computed and stored only in memory - and forgotten
after BIND shutdown. Yes, it requires re-computing on each load, this is
definitely disadvantage.
Doesn't that mean we need to store this data back in LDAP ?
No, only 'normal' DNS updates containing unsigned data will be written back to
LDAP. RRSIG and NSEC records will never reach LDAP.
That means more round-trips before the data ends up being usable, and we
do not have transactions in LDAP, so I am worried that doing the signing
in Bind may not be the best way to go.
I'm proposing to re-use BIND's transaction mechanism built in internal
database implementation.
=> It should be possible to save old database to disk (during BIND shutdown
or
periodically) and re-use this old database during server startup. I.e. server
will start replying immediately from 'old' database and then the server will
switch to the new database when dump from LDAP is finished.
This look like an advantage ? Why is it a disadvantage ?
It was mentioned as 'proposed remedy' for the disadvantage above.
I think having dual authoritative data sources may not be a good thing.
Consistency is a reason why I want to make persistent search mandatory.
IMHO persistent storage could save the day if LDAP is down for some reason.
Old data in DNS are much better than no data in DNS.
=> As a side effect, BIND can start even if connection to LDAP server is down
- this can improve infrastructure resiliency a lot!
Same as above ?
The same here, it was mentioned as 'proposed remedy' for the disadvantage above.
When it comes to DNSSEC starting w/o LDAP may just mean that you have
different signatures for the same records on different masters. Is that
'legale' according to DNSSEC ?
1) You will have same signatures as long as records in LDAP and saved copy of
the database (on the disk) are equal.
2) I didn't find any new limitation imposed by DNSSEC. AFAIK some
inconsistency between servers is normal state in DNS, because zone transfers
take some time and the tree structure have many levels.
The problems arise when data *in single database* (i.e. on one server) are
inconsistent (e.g. signature != data in unsigned records). BIND solves this
with it's built-in transaction mechanisms.
== Uncertain effects ==
- Memory consumption will change, but I'm not sure in which direction.
- SOA serial number maintenance is a open question.
Why SOA serial is a problem ?
It simply needs more investigation. BIND's RBTDB maintains SOA serial
internally (it is intertwined to transactions in the DB), so the write-back to
LDAP could be very delicate operation.
It means all masters will often be out of sync, this is not very good.
I don't think so. BIND can use timestamp-based serials in exactly same way as
we do. The only problem is how to implement 'read from internal DB'->'write to
LDAP' operation. It still needs more investigation.
Decision if persistent search is a 'requirement' or not will have significant
impact on the design, so I will write the design document when this decision
is made.
I would like to know more details about the reasons before I can usefully
comment.
I forgot to one another 'Uncertain effect':
- Support for dynamically generated '_location' records will be a big
adventure. It probably means no change from the state without persistent
search :-) After basic exploration it seems doable, but still a bit uncertain.
I need more info here, does it mean you have to store _location records
when they are generated ?
I tend to do _location record generation during zone-load, so everything will
be prepared when the query comes in. As a benefit it will allow zone transfers
even for signed zones. This still needs more investigation.
> Maybe we can use the internal bind database
just for _location "zone" ?
I don't think that it is possible.
If _location.client-a and _location.client-b reside in the single database
then client-a and client-b have to reside in the same database. (The reason is
that _location.client-a and _location.client-b do not have immediate common
ancestor.)
My personal conclusion is that re-using of BIND's backend will save a huge
amount of work/code to maintain/bugs.
I can see that, unfortunately I fear it will make multi-master a lot
more difficult at the same time. And given we do want to have
multi-master properties we need to analyze that problem more carefully.
I agree. It is a delicate change and we should not hurry.
Also by welding ourselves to internal Bind infrastructure too much, it
will make it a lot more difficult for us to change the DNS
infrastructure. Bind10 will be completely different internally, and we
may simply decide to even not use bind10 at all and use a completely
different engine going forward. So I am quite wary of welding ourselves
even more to bind 9 internals.
Ehm ... how to say that ... 'to late'. I wasn't around when DNS design was
made, so I don't know all the reasons behind the decision, but IMHO we use
completely non-standard/obscure hacks all the time.
The proposal above doesn't extend our dependency on BIND, because we already
depend on BIND9 *completely*. It is about dropping our own internal database
implementation (buggy, incomplete, standard non-compliant) with the code from
the original BIND (which is at least standard compliant).
<sarcasm>
Do you want to go back to 'light side of the force'? So we should start with
designing some LDAP->nsupdate gateway and use that for zone maintenance. It
doesn't solve adding/reconfiguring of zones on run-time, but it could be
handled by some stand-alone daemon with an abstraction layer at proper place.
</sacrasm>
--
Petr^2 Spacek
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