Introduction to DNS
DNS, or Domain Name System, is the foundational directory of the internet. It translates human-readable domain names like google.com into machine-readable IP addresses. For a deeper understanding of how this fits into the broader web experience, see The Hidden Magic Behind Accessing Your Favorite Websites.
Components of DNS
DNS comprises various server types in a hierarchical structure:
- DNS Resolver: The initial point a browser queries, often provided by an ISP or services like Cloudflare (1.1.1.1) and Google DNS (8.8.8.8). It checks cached entries before querying deeper layers.
- Root Name Servers: Thirteen logical servers holding IP addresses of all Top-Level Domain (TLD) servers, distributed globally with anycast routing for efficiency.
- TLD Name Servers: Manage IP addresses of authoritative nameservers for all domains within a TLD, e.g.,
.com,.org, country codes like.uk. - Authoritative Name Servers: Hold definitive DNS records for specific domains, updated through domain registrars or cloud providers like AWS. To understand how these servers fit into backend systems, see Understanding Backend Architecture: How Requests Travel and Why Backends Matter.
The DNS Query Process
- User enters a domain (e.g., google.com) into a browser.
- Browser checks its cache; if not found, queries the operating system cache.
- The OS requests the DNS resolver.
- Resolver checks its cache; if missing or expired, asks the root name server.
- Root server provides the IP addresses of the relevant TLD servers (e.g.,
.com). - Resolver queries the TLD name server, which returns the authoritative nameservers for the domain.
- Resolver contacts the authoritative nameserver to get the domain’s IP address.
- IP address is returned back to the browser for connection.
To see how your data travels through the internet during this process, visit The Hidden Magic Behind Browsing: How Your Data Travels the Internet.
Managing DNS Updates in Production
DNS changes require careful propagation management due to TTL (Time To Live) settings:
- Issue: Long TTLs cause delays in update propagation; some resolvers ignore TTLs.
- Best Practices:
- Lower TTL to about 60 seconds well before changes to speed up propagation.
- Maintain the old server alongside the new one post-update until traffic stabilizes.
Summary
DNS's hierarchical and decentralized design ensures robustness and scalability of the internet by efficiently resolving domain names to IP addresses. Understanding DNS operations and update management is essential for maintaining reliable web services.
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DNS, or Domain Name System, is the backbone of the internet, but few know exactly how it works. In this video, we will learn all about the system design of DNS. Let’s dive right in.
DNS is the internet’s directory. It translates human-readable domain names, such as google.com to machine-readable IP addresses. DNS is a little confusing because there are different types of DNS servers in the
DNS hierarchy, each serving a different purpose. When a browser makes a DNS query, it’s asking a DNS resolver. This DNS resolver could be from our ISP,
or from popular DNS providers like Cloudflare’s 1.1.1.1, or Google’s 8.8.8.8. If the DNS resolver does not have the answer in its cache, it finds the right authoritative nameserver and asks it.
The authoritative nameserver is the one that holds the answer. When we update a domain’s DNS records, we are updating its authoritative nameserver. How does the DNS resolver find the authoritative name server?
This is where the system of DNS gets interesting. There are three main levels of authoritative DNS servers. They are the root name servers, the top level domain (or TLD) name servers,
and the authoritative nameservers for the domains. The root name servers store the IP addresses of the TLD name servers. There are 13 logical root name servers.
Each root name server has a single IP address assigned to it. There are actually many physical servers behind each IP address. Through the magic of anycast, we get routed to the one closest to us.
The TLD name servers store the IP addresses of the authoritative name servers for all the domains under them. There are many types of TLD names.
We are all familiar with .com, .org and .edu. There are also country code TLDs like .de and .uk. There are many others.
The authoritative name servers for a domain provide, well, authoritative, answers to DNS queries. When we register a domain, the registrar runs the authoritative
nameservers by default, but we can change them to others. Cloud providers like AWS and Cloudflare run robust authoritative nameservers. This hierarchical design makes DNS highly decentralized and robust.
Let’s walk through the life of a typical DNS query. The user types google.com into the browser. The browser first checks its cache.
If it has no answer, it makes an operating system call to try to get the answer. The operating system call would most likely have its own cache. If the answer isn’t there, it reaches out to the DNS resolver.
The DNS resolver first checks its cache. If it’s not there or if the answer has expired, it asks the root name server. The root name server responds with the list of the .com TLD name servers.
Note that since .com is such a common TLD, the resolver most likely already caches the IP addresses for those .com TLD nameservers. The DNS resolver then reaches out to the .com TLD nameserver,
and the .com TLD nameserver returns the authoritative nameservers to google.com. And finally, the DNS resolver reaches out to google.com’s authoritative nameserver, and it returns the IP address of google.com
The DNS resolver then returns the IP address to the operating system, the operating system returns it to the browser. Finally, let’s go over some gotchas when updating DNS records for a live,
high-traffic production system. DNS propagation is slow because there is a TTL on each DNS record. And some of the default TTLs are pretty long. Also, not every DNS resolver is a good citizen.
There are some out there that don’t honor the TTL. To mitigate the risk, there are two practical steps to take. First, reduce the TTL for the record that we want to change to something very short,
say 60 seconds, well in advance before the update actually happens. This gives ample time for all the DNS servers to receive the shortened TTL which would allow the actual record update to take effect based on the new shortened TTL.
Second, leave the server running on the old IP address for a while. Only decommission the server when traffic dies down to an acceptable level. Because some DNS resolvers don’t honor the TTL, this could take a bit of time and patience.
This concludes our video on DNS. We hope you have a better understanding of how its hierarchical design makes it decentralized and robust.
Remember, DNS is what translates domain names to IP addresses, making it a critical component of the internet backbone. If you like our videos, you may like our system design newsletter as well.
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The DNS system consists of several key components: DNS Resolvers, which act as the first query point often provided by ISPs or services like Cloudflare and Google DNS; Root Name Servers, which provide IP addresses for Top-Level Domain (TLD) servers; TLD Name Servers that manage IP addresses of authoritative servers within each domain extension like .com or .org; and Authoritative Name Servers that hold the definitive DNS records for specific domains. Each component plays a critical role in efficiently translating domain names to IP addresses.
When you enter a domain name like google.com, your browser first checks its cache for the IP address. If not found, it queries the operating system, which then contacts the DNS resolver. The resolver checks its cache; if missing, it queries the root name server to get TLD server info, then the TLD server to obtain the authoritative nameservers, and finally the authoritative nameserver for the domain's IP address. This IP is returned to your browser to establish the connection.
DNS updates can propagate slowly due to high TTL (Time To Live) settings, causing cached information to remain outdated in resolvers. Some resolvers may even ignore TTL values. To manage this, it’s best to lower the TTL to around 60 seconds ahead of planned changes and maintain both old and new servers simultaneously until traffic stabilizes, ensuring seamless updates without service disruption.
Root Name Servers serve as the foundational layer in DNS, holding the IP addresses for all TLD servers like .com, .org, and country-specific domains. They help direct DNS resolvers to the correct TLD servers, enabling efficient domain resolution. There are thirteen logical root servers distributed globally using anycast routing to improve performance and reliability.
DNS resolvers improve lookup efficiency by caching DNS responses from previous queries, which reduces the need to repeatedly query higher DNS hierarchy levels. When a resolver receives a query, it first checks its cache for a valid IP address, quickly returning it if present, thereby speeding up the resolution process and reducing network traffic.
Web administrators should lower DNS TTL values to about 60 seconds well before making changes to ensure faster propagation. They should also keep the old server active alongside the new one after updates until traffic stabilizes. This approach minimizes downtime or service interruptions caused by DNS caching delays during transitions.
DNS’s hierarchical and decentralized design distributes the responsibility of resolving domain names across various servers globally, which ensures scalability to handle vast queries and robustness against failures. This structure prevents bottlenecks and single points of failure, maintaining reliable web service access worldwide even under heavy loads or isolated outages.
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