[nsd-users] Unsecured zone transfers and open resolvers

[Olaf Kolkman]

On Jul 18, 2012, at 10:16 PM, Valentin Bud wrote:

> Hello,
> 
> My question is not related to NSD in particular, but I have seen here on the list a lot of people that work for TLDs and other Registrars and Registry operators I thought it would be a good place to ask this question. It is about DNS though, not completely off topic :).
> 
> I have encountered in my DNS studies a few name servers that let you transfer zones they are authoritative for. The name servers I am talking about are not under my control. I have noticed that in the majority of cases ns2.*, or whatever name the second NS has, lets you perform the zone transfer. This led me to the conclusion that the sys admins don't pay enough attention or don't really know or understand DNS technology. It is not my intention to offend any sys admin. I am just saying. Or maybe the people that set up those servers are not sys admins. Who knows.
> 
> Do you consider the above as being a security vulnerability?

There are different schools.

One school shares your thoughts:

> My thoughts on this. This isn’t necessarily bad if the only information provided is related to systems that are connected to the Internet and have valid hostnames, although it makes it that much easier for attackers to find potential targets. Almost all the time people use suggestive names like splunk, nagios, cpanel, switch-c2950, etc. That would give an attacker a good start. But on the other hand it can find those by himself by querying the name server for those names.
> 

The other schools says that information in the DNS is essentially public and while preventing *XFR will provide a bit of obscurity we all know that security through obscurity doesn't provide real security. (I am paraphrasing the school of thought).


> In some cases, as I have seen, there are entries that have private addresses. I consider this as being quite bad because it reveals the private address space of the company's/institution's IT infrastructure. 

The second school of thought would probably say that if you put the data in the DNS you do not mind disclosing the information. If you want this sort of information to be hidden you set up 'internal-only' infrastructure (with BIND you can use one server with two views, with other implementations you set up your nameserver infrastructure independently).

> 
> What about open resolvers? I am not talking here about OpenDNS or Google, who monitor their infrastructure and maybe they even rate limit the queries per source IP address if too many come from one particular source. I am talking about servers that are not being monitored. I say this because if you monitor your servers and if you understand the DNS technology you can see that someone has AXFR-ed your zone or queried whatever.domain.com recursively using your name server and put an end to it. 


In the context of this conversation I believe that if an open, and public facing, resolver has access to internal information (an internal view) such would probably be a mistake. But if the open resolver is configured to only see external facing data then I don't see any reason for the type of monitoring you describe above.

The type of monitoring that should always be taken place on open recursive nameservers is monitoring for being used as DOS amplification vector. 

--Olaf




NLnet
Labs
Olaf M. Kolkman

www.NLnetLabs.nl
olaf at NLnetLabs.nl

Science Park 400, 1098 XH Amsterdam, The Netherlands



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