[Dnsmasq-discuss] DNSMasq and DNS reflection attacks

Simon Kelley simon at thekelleys.org.uk
Fri Oct 25 11:08:50 BST 2013

On 24/10/13 23:03, /dev/rob0 wrote:
> On Thu, Oct 24, 2013 at 05:28:29PM +0100, Simon Kelley wrote:
>> On 24/10/13 17:03, Brian Rak wrote:
>>> We've recently undertaken a project to clean up our network, and
>>> lock down all the open DNS resolvers. As you may know, these are
>>> very frequently used for DDOS attacks:
>>> http://openresolverproject.org/ ,
>>> http://www.team-cymru.org/Services/Resolvers/ .
>>> I haven't been able to find any sort of configuration option that
>>> would prevent DNSMasq from being abused like this, and I've had to
>>> resort to iptables rules instead. Is there a configuration option
>>> that that would disable responding to DNS queries from certain
>>> interfaces? The other option that seems handy would be one to only
>>> reply to DNS queries from hosts that have a configured DHCP lease.
>>> Are there any features of DNSMasq that would prevent it from being
>>> abused to conduct attacks?
>> This is an important topic, and quite difficult to understand, so
>> I'm going to take this opportunity to try and put a definitive
>> statement on the record.
> Good stuff here, as usual, but questions still exist.
> Yes, I think of dnsmasq in its original mission mostly: as a DNS
> forwarder and provider of internal DNS for [DHCP usually] LAN hosts.
> That seems to be the gist of your response here: to keep dnsmasq safe
> from Internet attackers, don't expose dnsmasq to the Internet.
> However, in the coming age of IPv6 and [we hope] the decline of NAT,
> users will be more likely to want to expose dnsmasq to the Internet
> as an authoritative nameserver. I can see the potential need for
> serving both ip6.arpa zones and a zone such as "dh6.example.com" (the
> --domain in dnsmasq terms.)
> I have reviewed the "AUTHORITATIVE CONFIGURATION" section as well as
> all the --auth-* settings in the manual, and I still have two
> concerns:
> 1. Is there a way to designate interfaces which ONLY respond to
> queries for our authoritative names? (--auth-server looks like it
> might do this, but it does not quite say so.)

It does do exactly this, is you didn't understand that, it's a 
documentation bug.

> If I'm acting as NS
> host for dh6.example.com and<whatever>.ip6.arpa, I need to respond
> to those queries to anyone, but I don't want to let them look up
> google.com nor lists.thekelleys.org.uk. For that matter, they
> shouldn't even be able to get any names from /etc/hosts UNLESS those
> names are in my zone.
That's what happens. In addition, ANY queries are no responded to for 
auth zones.

> 2. Is there a way to limit the response rate on these queries on the
> external IP address[es]?

No. I'm watching and waiting on that, but I'm not sure it makes sense. 
The DNS amplification attacks work by sending a pre-crafted query with a 
forged source address which is the address of the victim. The query is 
designed to produce the biggest possible answer for the smallest 
possible query so that as much bandwidth lands on the victim as possible 
for the smallest bandwidth use by the attacker. This only really works 
for recursive nameservers In the case of an authoritative-only server, 
this won't work, since a query will not be in the zone, and therefore 
will get an empty answer. The only way to make it work is for the 
attacker to somehow determine which zone the server is authoritative 
for, and generate a suitable query on-the-fly.

> A third concern, which isn't quite relevant to this thread, but I
> might as well mention anyway:
> 3. what about DNSSEC signing, will dnsmasq ever be able to serve
> signed data?

The first priority for DNSSEC, indeed the first priority period, post 
2.67, is DNSSEC validation. Once that's done, I figure I'll have enough 
knowledge to approach the problem of signing.
> I'm a big fan of dnsmasq, BTW; it has made the jobs of SMB/SOHO
> network administrators much easier. The features I mention would
> improve safety while exposed to the Internet, but I am not sure
> they're worth the added size and code complexity.
> It could well be best that dnsmasq should stick to its original
> mission. Those who want advanced features could use it as a hidden
> master for BIND (or other nameservers.) That part can work safely,
> and the slave server could do inline signing.

An valid point. There's an internet draft around at the moment that I've 
had some input to which proposes more-or-less exactly that.




> PS to Simon, a note on your manual: you should stick to example names
> for zones, not "our.zone.com" or "secondary.myisp.com". Note that
> both zone.com and myisp.com exist (the former being owned by
> Microsoft.) I'd consider perhaps s/com/example/ for those. Reference:
> RFC 6761.
> Also in three places you make reference to "ipv6.arpa", which in
> other placess is correctly called "ip6.arpa".
> Another very minor nitpick: under --interface you mention "IP alias
> interfaces (eg "eth1:0")". Those aren't interfaces at all; only the
> brokenness of Linux ifconfig(8) displays them as such. Please
> consider changing "interfaces" there to "labels". Help stamp out
> ignorance ... and even Linux net-tools itself! :)
>> First the simple stuff.
>> Dnsmasq has --interface --except-interface and --listen-address
>> configuration options that disable response to DNS queries from
>> certain interfaces. The first thing that has to be done is to use
>> these. Mostly it's the only thing that needs to done.
>> Now, the complicated stuff.
>> Under certain circumstances, --interface=<interface>  degrades to
>> mean the same as --listen-address=<address on interface>. For
>> instance if eth0 has address and dnsmasq is configured
>> with --interface=eth0, then dnsmasq will reply to any query which
>> is sent to, no matter what interface it actually
>> arrives at. The circumstance under which happens is when the
>> --bind-interfaces flag is used.
>> Now, in the above example, this isn't a problem, since a botnet
>> can't direct traffic to an RFC-1918 address. If, on the other hand,
>> the address of an internal interface (ie one configured to accept
>> DNS queries) is globally routable, then queries which arrive via
>> another interface (ie one linked to the internet) with the
>> destination address of the internal interface _will_ be replied to,
>> and a DNS reflection attack is possible.
>> This has mainly been seen in libvirt and OpenStack installations
>> which use dnsmasq, since sometimes they are provisioned with "real"
>> addresses. I'd expect to see problems in the future with IPv6,
>> since far more people will be using globally routable addresses
>> with IPv6.
>> The reason that this happens is that --bind-interfaces uses the
>> bare-minimum BSD sockets API only. Detecting which interface a
>> packet arrived on, rather than the address to which it was sent,
>> needs non-portable API, and is impossible on some platforms
>> (openBSD, for instance) --bind-interfaces is a "works everywhere"
>> least common denominator. It's also useful when you're running
>> multiple instances of dnsmasq on one host, which is why most
>> people use it.
>> The fix is to use either the default listening mode, or if running
>> multiple instances, the new --bind-dynamic mode. --bind-dynamic is
>> only available on Linux, and --bind-interfaces is the only mode
>> available on openBSD, so BSD users have rather more problems here.
>> Summary. There's a problem is you want to accept queries in an
>> internal interface with a globally routable address and use
>> --bind-interfaces. The fix is to remove --bind-interfaces and, if
>> necessary, replace it with --bind-dynamic. This fix is not
>> applicable on all platforms,
>> The Real Soon Now 2.67 release logs a very prominent warning if the
>> dangerous combination is configured.

More information about the Dnsmasq-discuss mailing list