The ABC's of DNS - Part 3

Continuing our glossary of DNS tips & tricks, we’re covering the letters G, H, and I this time.

Jul 11th, 2019

Continuing our glossary of DNS tips & tricks, we’re covering the letters G, H, and I this time.

G is for “glue records”

Glue records are DNS records (A records) created at the domain registrar, that returns references for the authoritative nameserver of the domain. They’re useful for those wishing to run their own authoritative DNS servers.

Normally, a DNS query

  • first goes to the root (which returns the top-level domain or TLD),
  • then to the TLD (which returns the authoritative nameserver),
  • and finally to the authoritative nameserver (that resolves the domain name).

Problem is, when the authoritative nameserver is part of the domain (like “” having nameservers such as “”) this creates a circular reference. Glue records allow for both resolving the domain name as well as listing the domain’s authoritative nameservers.

To check the validity of your glue records, you need to know the host and its assigned IP address, and use dig. The glue records will show up in the “additional section” part of dig’s output, listing the host names and their IPs.

Speaking of host names…

H is for “hosts”

The hosts file exists on every system that is connected (or capable to be connected) to a network. (On Linux and Mac it can be found in /etc; on Windows, it’s in %SystemRoot%System32driversetc.) It’s a plain text file whose only function is to provide local name resolution, mapping host names to IP addresses.

Usually, it’s managed automatically by the system, but it can be edited manually, bypassing the network’s own name resolution. For example, you can put127.0.0.1

into your hosts file. If you’re running a web server (like nginx or Apache) locally — and it’s configured to answer — you can display any content instead of the actual Google search page.

This can be useful when testing a website or web application, depending on using a specific domain name, locally. You can, for example, clone your website’s file structure to your local system, set up a web server, and test it without having to reconfigure the domain name.

FUN FACT: the hosts file comes from the ARPANET days when networks didn’t have standardized name resolution, and each connected system had its own hosts file. When DNS was developed, and the queries became increasingly complex (and thus carrying more data) one of the suggestions to solve the issue was to distribute hosts files on CDs.

Imagine that, and where the internet would be today if that had happened.( We talked about this (and many, many more things) with Geoff Huston from APNIC on our podcast.)

Speaking of organizations tasked with assigning Internet names and numbers ...

I is for “IANA”

IANA - Internet Assigned Numbers Authority - is responsible for global coordination of some of the key elements that keep the Internet running smoothly, specifically allocating and maintaining unique codes and numbering systems that are used in the technical standards (“protocols”) that drive the Internet.

It’s a  no-brainer: the Internet has become a mission-critical infrastructure for everything from business to banking to healthcare. Making sure it runs smoothly, and remains secure, apolitical, and free from centralized control, is essential. Yet there is a technical need for some key parts of the Internet to be globally coordinated. This is where organizations like IANA come into the picture.

One of the Internet’s oldest institutions (with functions dating back to the 1970s), IANA’s activities can be grouped broadly in three categories:

  • managing Domain Names, including management of the DNS Root,
  • coordinating global Number Resources, including providing IP and AS numbers to Regional Internet Registries (like APNIC and RIPE), and
  • certain Protocol Assignments, such as managing Internet protocols’ numbering systems in conjunction with standards bodies.

Since we’re talking about numbers, on the 14th of July of 2019 it will be exactly 20 years since IANA made this historical announcement regarding the delegation of IPv6 address to regional registries, which set in motion the beginning of the worldwide deployment of IPv6. (Perhaps - eventually - IPv6 will prove to be as revolutionary as the storming of the Bastille in France in 1789, also celebrated on the 14th of July? Only time will tell …)

Want to learn more?

This series is byte-sized (see what we did there?) — but a lot more can be said and done.

As mentioned, we recently talked with Geoff Huston from APNIC on our podcast (about, among many things, the hosts file) and we’ll continue bringing you DNS and networking-related content. Make sure you subscribe!

To learn more in-depth about DNS specifically, we offer a comprehensive DNS training program. You can enroll for different courses depending on your skill level:

  • If you’re new to DNS, we offer the DNS & BIND Fundamentals (DNSB-F) course. It’s part of the DNS & BIND Week (DNSB-W) and serves as a shorter introduction to the world of DNS and BIND.
  • If you’re already familiar with the basics, the full five-day DNS & BIND Week (DNSB-W) course takes you deeper into DNS, including a heavy emphasis on security, stopping just short of DNSSEC (for which we offer a separate course).
  • And if you're looking for even more, we offer the DNS & BIND Advanced (DNSB-A) program, getting into the deep end of things.

Check out our training calendar, and reach out to us with any questions.