[Unbound-users] AAAA filter patch proposal

Stephane Lapie stephane.lapie at darkbsd.org
Thu Nov 13 03:08:10 UTC 2014


There was a post several days ago about AAAA filter, and questions about 
an implementation as a Python module by Christophe.


I work at the same company as him (a Japanese ISP, which are all 
subjected to NTT's flaky practices with use of IPv6), and have been 
working with him on this issue.

To sum up the scenario we are trying to fix :
- customers in Japan have a physical carrier (NTT in most cases) on top 
of which they get their own internet provider, to which they connect via 
PPPoE. In our case, we currently provide only IPv4 service at this point 
in time.
- some customers also get an on-demand video service via his carrier, 
NTT, which give them a default IPv6 route via IPoE, or they raise a 
second PPPoE session (NTT usage terms allow for up to 2 sessions for 
this scenario).
- the catch: said IPv6 default route does not go outside on the 
internet, and only enables access to NTT's closed network. Therefore, a 
customer in this configuration trying to access any IPv6 site is in for 
a world of pain as his browser times out and retries, hopefully 
fallbacking to IPv4. This prompted every Internet service provider in 
Japan to either provide native IPv6 or to filter AAAA records for 
"non-v6 only sites", which in this instance pretty much means everyone 
uses BIND.

To answer Bill Manning's earlier statement, "we can not change 
providers", first reason being because we ARE a provider, but also 
because even if we wanted to change carriers, everyone in Japan is 
entirely dependant on the majority physical carrier, i.e NTT. Since it 
happens at a lower layer than the ones we have control over (we work at 
PPPoE encapsulated level, they work at Ethernet level), we have no 
control whatsoever over said carrier-provided route, short of ourselves 
providing IPv6 service over or PPPoE and an overriding route, or via 
IPoE (and then tell NTT to buzz off and provide our route, if we had 
one). This is obviously scheduled as the proper solution, but requires 
overhauling all of the network infrastructure, which can not be done 

Also, thanks a lot to Daisuke Higashi for his statement, using 
"private-address/private-domain" is initially what we planned on doing, 
except this gets scary when we think about "what if NTT springs yet 
another domain on top of that, that we need to allow access to?" or 
"what if another customer tries to access yet another IPv6-only site in 
the future?", and the "whack-a-mole" administration nightmare it might 

In the meantime, we still need to get rid of BIND, which can't handle 
the resource exhaustion DDoS attacks 
(http://dnsamplificationattacks.blogspot.jp/2014/02/authoritative-name-server-attack.html) we are seeing since february of this year. This is where we wanted to go with unbound, except since it does not have AAAA-filter functionality, we could not use it in production for most of our customers.

This is where Christophe attempted to intercept queries with Python and 
we found out :
- Python API does not enable to spawn sub queries (for each AAAA query, 
the relevant A record has to exist in cache, for AAAA filter logic to 
- Python API does not enable to lookup RRset cache for a given record 
(in this case, for an A record matching the queried name)
- Python API does not allow for easy scrubbing of packets (it IS 
possible, but very painful)

I therefore came to the conclusion that the Python API was not 
appropriate to do these things, and that the most appropriate place to 
implement a filtering/scrubbing logic was the iterator module itself.

I coded the following patch (also attached) : 
It has been on tests and running in pre-production for roughly a month 
now (while undergoing some tuning as I got around to understanding how 
the state machine works).

The patch provides :
- a "aaaa-filter" config option which is off by default, so as to not be 
intrusive (I am fully aware this functionality is enough of an 
abomination as is). It can also be used in conjunction to 
private-address/private-domain without any issues.
- the relevant manual entry
- modifications to iterator/iter_scrub.c in scrub_sanitize() to remove 
AAAA records for queries that either are not AAAA type, OR that did 
return an A record, IF cfg->aaaa_filter is enabled.
- modifications to iterator/iter_utils.c to provide AAAA filter "on/off" 
info to the iterator environment.
- modifications to iterator/iterator.c :
-- a new ASN_FETCH_A_FOR_AAAA_STATE from which we branch into from 
QUERYTARGETS_STATE if this is a AAAA query (modifies iter_handle(), 
iter_state_to_string(), iter_state_is_responsestate(),
-- asn_processQueryAAAA() function that throws a subquery and flags the 
parent query as "already fetching an A subquery" so as to not loop
-- modifications to iter_inform_super() to handle the new state for AAAA 
parent queries having a A subquery.
-- asn_processAAAAResponse() function that basically takes after what 
error_supers() and processTargetResponse() do, except it does not alter 
target queries counters.
- modifications to iterator/iterator.h : declaration of new flags for 
iter_env (configuration option), iter_qstate (status flag), and the new 

At this point, the patch is pretty much stable and performing as 
expected, but I am looking for pointers as to stuff I could improve on 
that patch, especially style-wise, to ensure it is applicable as long as 
possible. In its current state, I can apply it up to 1.4.22.

I also know from previous postings that unbound development staff's 
opinion is that this functionality as a whole would harm IPv6 adoption, 
and therefore can probably not be officially endorsed, but I still 
intend to provide it freely (my company has given approval) to people 
suffering from the same scenario. (that is, mainly Japanese users at 
this point...)

Thanks for your time,
Stephane LAPIE, EPITA SRS, Promo 2005
"Even when they have digital readouts, I can't understand them."
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