DNS delegation

As with its predecessor, hostcount++ walks the DNS hierarchy - the domain name tree. This tree is divided into "zones of authority"; those parts managed by distinct people or organisations.  Nominet have authority over the country-code top-level domain (ccTLD) .uk zone, and also over the second-level domains (SLD) co.uk, org.uk, me.uk, net.uk, ltd.uk, plc.uk and sch.uk.  These are our zones of authority. Domain names are available for registration within these SLD zones. Once a domain name has been registered we delegate the DNS zone to the registrant so that they hold the authority for that zone. This allows them to advertise the contents of the zone through their authoritative DNS nameservers.

Reading a zone’s contents

Domain lookups are normally queries for single items of information: the address of a web server; or the destination for an email, etc. However, it is possible to read a whole zone with a single DNS command. The DNS command AXFR will transfer the contents of a whole zone from a nameserver to a client. This command is most often used to synchronize two nameservers. The hostcount and hostcount++ tools use AXFR to read whole zones within the DNS tree. The results are then anonymised, and analysed to determine the number of leaf nodes and the depth at which these nodes are found.

Third Parties Collect Local Data

Hostcount++ is more accurate and more data is collected when a 'local' machine collects the data. This usually means a machine that is geographically situated in the country that the TLD represents. The main reasons for this are:

• Connectivity external to the geographic region has no influence on the data collection
• The organisation collecting the data, specifically the IP address range of their network, is usually 'well-known' in the area and therefore more trusted. This means that zone transfers are refused less often, so that more data is found.

Because of the increased accuracy of third party data, RIPE NCC prefer that the data collection is done by a local site wherever possible, usually a Network Information Centre (NIC), University or similar institution. In the case of the .uk zone Nominet have been selected as most appropriate and trusted.

The Host tool

To collect the data, each local organisation runs the host tool on their TLD code. Host acts conceptually as follows:

1. Zone transfer the Top-Level Domain.
   • This means: ask one of the nameservers for the TLD to give out everything in its zone file for that TLD.
2. Remember any Nameserver records that are seen.
   • Namserver records indicate the existence of a sub-zone and tell us which nameservers should know something about the sub-zone.
3. Zone transfer all of the sub-zones.
4. Remember any Nameserver records that are seen.
5. Are there any more subzones ?
   • Yes - Go to step 3.
   • No - Stop.

Enabling hostcount++ in your zone

Most nameserver administrators should, quite correctly block AXFR from untrusted IP addresses. In order for this research to return reliable data, we need you to add our IP address to your trusted zone. An explanation of how to do this is given here.