rbldnsd and DNSSEC compatibility issues - any suggestions?

Mark Andrews marka at isc.org
Mon Sep 14 00:12:43 UTC 2020

> On 13 Sep 2020, at 06:45, Fred Morris <m3047 at m3047.net> wrote:
> I'm a little confused by the "sky is falling" tone of all this. I'm
> pretty sure that the browser fetish for "search and URLs in the same
> box!" is pretty high up on the root server pain scale; on any given day
> .belkin/.cisco can be one of the top 10 NXDOMAIN TLDs (at least as of a
> couple of years ago); search lists are another source of noise (which
> doesn't always make it to the roots).
> It's not just quantity, there is also leakage occurring. My recent brain
> fart regarding how to turn off the real queries in BIND when something
> (typically generated because of a search list) is caught by an RPZ is a
> case in point. (This particular use case is to catch noise from stuff
> which could be generated from search lists; it's a good use case IMO for
> RPZ to generate NXDOMAIN responses locally.) Or, look in your passive
> DNS for
> I use DNS, the technology, as a distributed key/value store
> occasionally; there is no reason I should have to point it at a tree
> with ICANN's roots whatsoever, but for applications which need to be
> connected to ICANN's world that can be the past of least resistance.
> Seems to me that if some datacenter is doing beellions of DNS lookups to
> DNS as a K/V store some caching for hits as well as NX must be
> occurring, seems insane for that not to be the case. I'm trying to
> imagine scenarios where those beellions of queries would leak out to the
> root servers, or to any servers and none of them look sane to me
> (transport ain't free at scale). My biggest concern for the sane cases
> would be information leakage, such as the fingerprinting of machines via
> AV software doing hash lookups via The DNS; just an example.

Private names leak all the time to the DNS.  Take

zone “non-existant-tld” {
	type forward;
	forwarders { <address>; };

and have the server at <address> stop working or just be overloaded.  All
the queries to the recursive server with this configuration not answered by
the server will leak.  The configuration needs “forward only;” to be added
to prevent the leak.  We see this all the time.

zone “non-existant-tld” {
	type forward;
	forwarders { <address>; };
	forward only;

Remember forwarding started off as a performance measure.  Falling back to
talking to the root servers is desired in that scenario.

> There are too many borked configurations to enumerate them all. The most
> plausible architecture I come up with, perhaps unsurprisingly the one I
> would suggest in the absence of explicit mitigating factors, would be a
> caching resolver running on the same switch, if not the same machine, as
> the application generating the queries. I might secondary the zone
> there. Or I might just set it up as a static-stub or forward.
> What are the possible failure modes in this scenario? The application is
> configured to use the wrong DNS server: this is not a simple failure, it
> has to be configured to use a resolver or nothing resolves; if it's an
> authoritative then queries for what's not in bailiwick there are
> (hopefully) refused or referred back to the stub resolver (which won't
> follow them because it's RD not recursing itself), all of them, not just
> ones from the K/V store and it's going to look really really broken; if
> it's then surely gmrgle caches the NX response from the root for
> the bogus TLD; if it's the corporate caching resolver the same (plus I
> expect the phone to ring). The query generates a legitimate NX response
> from the zone: that should be the end of it (that's how DNSBLs work); if
> search list processing occurs, I'm wondering how one got configured but
> more than that how this solution made it into production (if I was
> writing the app, I'd probably eschew reliance on the operating system
> stub resolver for this particular use case entirely). The app is
> misconfigured not to use the correct domain (for the K/V domain): if
> it's the wrong domain (or none at all) it doesn't matter whether the
> "correct" domain is legitimate or locally concocted. Any others?
> I just don't see how beellions of queries are going to get directed at
> the roots or any auth server, regardless of what the TLD is, or if that
> occurs that the choice of TLD mitigates in any fashion whatsoever.
> There's always a way to make it happen, I just can't imagine it making
> it sanely into production even by accident. (This applies to DLV.ISC.ORG
> too, which returns an SOA, but they could make it NX if it suited their
> purposes.)

Actually we can’t remove the DLV.ISC.ORG zone as there is still traffic. That
traffic indicates that there are machines that have DLV.ISC.ORG configured
as a trust anchor which means we need to return a matching DNSKEY RRset or
cause all DNS resolution on those machines to fail.  Machines that are still
configured to use DLV talk to the servers about once a hour (negative and DNSKEY
response TTLs) and they talk to the root and ORG servers about once every 2 days
(referral TTL).  Machines using DLV are supposed to use aggressive negative
caching for the DLV lookups which significantly reduces the traffic to once a hour
compared with once a DNS resolution.

> Quizzically...
> --
> Fred Morris
> On 9/10/20 10:57 PM, Rob McEwen wrote:
>> Mark,
>> You gave me the "let them eat cake" answer I anticipated. Also, this
>> isn't fixing a problem that my services produce - it is preventing a
>> problem that a potential MISTAKE from a large customer would cause -
>> the type of mistake that is inevitable at some point, but likely
>> short-lived. That's on them, not me. But I can sleep well at night
>> knowing that such MISuse of my service isn't going to take out an
>> entire datacenter for hours (with MANY innocent bystanders taken out,
>> too!) with a DOS attack due to those queries NOT ending with a
>> valid/public domain name, thus making such an attack impossible.
>> (again, just referring to our very largest customers' DNSBL queries).
>> [...]
>> On 9/11/2020 1:32 AM, Mark Andrews wrote:
>>>> On 11 Sep 2020, at 15:04, Rob McEwen <rob at invaluement.com> wrote:
>>>> Mark,
>>>> The whole usage of DNS by the anti-spam industry in our DNSBLs - is
>>>> somewhat a hack on the DNS system from the start - I guess if you
>>>> think that is wrong, maybe you should take that up with Paul Vixie?
>>> And Paul will tell you to use a name you control.  We did that with
>>> DLV.ISC.ORG.  We are still absorbing that traffic despite there being
>>> no entries in the zone for several years now.  We knew we would have
>>> to do that going in.
>>>> And the whole purpose for MANY of us DNSBLs using ".local" in the
>>>> first place - was precisely to PREVENT the queries from possibly
>>>> leaking out of our largest customers LANs  - because in many cases,
>>>> that would an essential denial of service attack against us (and our
>>>> hosters, etc).
> [...]
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Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka at isc.org

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