There are many things told about zone transfers and why it is important to restrict the use of zone transfers. The DNS zone can contain sensitive information like DKIM keys or information about the internal infrastructure. And because of this I actually thought it was not so common anymore.
A while ago I ran into a nameserver with an insecure zone transfer (AXFR) setting. Allowing zone transfers for the whole world will also allow the bad guys extract useful information from a zone that can be used to create a map of the network infrastructure. The bad guys can use this information to plan their attacks. Then I got curious about the number of nameservers that still offer the zone transfer option today.
My plan was to check all the nameservers that are used by the .NL gTLD. For this I started with a list of 3.3 million .NL domain names. I also wanted know the nameservers of domains that are related to the .NL domain names. You can have a domain like: yourcompany.nl which has is own set of nameservers, but you can use the mail servers of your ISP onder the ISP domain: yourisp.eu which has its own set of nameservers. So the domain yourisp.eu is related to yourcompany.nl for the email service that you user and that can also be interesting for possible attackers. I also did this for the domains of the nameserver hosts.
All together I found a total of 5,469,224 domain names. For each domain I extracted the nameservers from the DNSwhich I tested in combination with this domain.
The period I tested this was December 2017.
In total I found 72,656 nameservers. In these nameservers I found 10,524 nameservers on which I could perform a zone-transfer. So 12.65 % of all checked nameservers are leaking zone information of their domains.
When we take a look at the .NL domains. From the total of 3,271,088 domains that I tested it was possible to do an AXFR on 216,953 domains thru (one of) its nameservers. That is 6.6% of all .NL domains.
Later on, I did the same tests on 1,038,148 .BE domain names from Belgium and 2,212,192 .FR domain names for France. They had a better score. From the .BE domain names, 5.2% allowed an AXFR and from the .FR domain names was this 3.0% that allowed AXFR.
AXFR on .NL domains
AXFR on .BE domains
AXFR on .FR domains
Some notable situations
A big domain trading company had about 47,000 domain’s on 2 of their nameservers. I notified the company, and they secured the zone-transfers on the nameservers.
One of The Netherlands biggest ISP’s was leaking information of about 16,000 domains on one of their nameservers. They secured transfers the same day that I notified them.
There was also a well known MSP/Cloud provider leaking zones with very much interesting information of about 3700 domains on their nameservers. They noted my AXFR requests in their SOC and closed the zone transfer possibility almost the same time I notified them.
And last but not least, there is a large (and cheap) web hosting provider in The Netherlands. In their network I found 227 nameservers leaking information about 74,512 domains!!!
Distribution of domains on nameservers. Blue = hosting provider with 277 nameservers. Orange and Yellow = domain trading company. Green = large Dutch ISP. Red is “the rest”.
DNS enumeration is part of the reconnaissance phase (which is the first phase) of an attack. If you manage your own nameservers you can monitor them for AXFR requests from not trusted hosts. With this information you can be warned of a possible attack on your digital infrastructure.
If you want to check if one of your nameservers has zone transfers enabled, you can go this website and fill in your domain name: https://hackertarget.com/zone-transfer/.
How to secure zone transfers
Most nameservers have multiple options to secure zone transfers. The most common options are:
- Disable zone transfers completely (us an other method to synchronize)
- IP address filter / ACL
- TSIG (Transaction SIGnature) that works as a shared secret
- A combination of an IP access list and a TSIG
Choose the method that is safe and also works with your DNS infrastructure.
I will try to contact most of the owners of the nameservers and hope they will close down zone-transfers. Maybe I run this scan again in six months and compare the results.