What is a DNS Lookup?
When you type a URL into your browser, it initiates a series of steps to translate that domain name into an IP address. This process is a DNS lookup. The DNS lookup process involves several components, including your computer, the Recursive Resolver, the Root Nameserver, the Top-Level Domain (TLD) Nameserver, and the Authoritative Nameserver.
Let’s delve deeper into each stage to understand the intricacies involved.
Step 1: Your Computer’s Local Cache
The first place your computer looks when it needs to translate a domain name to an IP address is its local DNS cache. The local DNS cache stores recent DNS lookup information to speed up the process. If your computer finds the domain name in its cache, it can skip the rest of the DNS lookup process.
Step 2: Recursive Resolver
If the requested domain isn’t in your computer’s local cache, the search goes to the Recursive Resolver. The Recursive Resolver is a server designed to receive DNS queries and requests to translate a domain name into an IP address.
Internet Service Providers (ISPs) typically run Recursive Resolvers. If the Resolver also has the domain’s IP address cached from previous searches, it will return the IP address, concluding the DNS lookup process. However, it must make additional requests to find the IP address if it doesn’t.
Step 3: Root Nameservers
The Recursive Resolver requests the Root Nameserver if it doesn’t have the required domain’s IP address. The Root Nameserver doesn’t know the IP address but knows where to redirect the query. It guides the Resolver to the appropriate Top-Level Domain (TLD) Nameserver.
Step 4: TLD Nameservers
The TLD Nameservers store the information about the domain names that share the same extension. For instance, a TLD Nameserver might hold data for all ‘.com’ or ‘.org’ domains. The TLD Nameserver won’t have the IP address of the domain but can guide the Recursive Resolver to the server that does - the Authoritative Nameserver.
Step 5: Authoritative Nameservers
The Authoritative Nameserver is the final stage of the DNS lookup process. It holds the IP address for the specific domain name. The Recursive Resolver asks the Authoritative Nameserver for the domain’s IP, and once it receives this data, it stores it in its cache for future requests.
Then, it passes the IP address back to your computer. Your computer also stores this IP in its local DNS cache, loads it into your browser, and the website you requested appears.
DNS Record Types
While the most common use of DNS lookups is to translate domain names to IP addresses, the DNS system provides a wide range of record types that can provide different kinds of information. Some common types include:
- A Records: These are the most basic type of DNS records and are used to point a domain or subdomain to an IP address.
- CNAME Records: Canonical Name records are used to alias one name to another.
- MX Records: Mail Exchanger records designate the mail servers for a domain.
- NS Records: Name Server records delegate a subdomain to a set of name servers.
- TXT Records: These records can associate arbitrary and unformatted text with other names.
The DNS system was not originally designed with robust security mechanisms; several extensions have been added to address this. DNSSEC (DNS Security Extensions) provide data integrity and authentication to the DNS by attaching digital signatures to DNS data.
Despite these measures, DNS-based attacks, such as DNS spoofing or DNS cache poisoning, are still potential risks. DNS spoofing involves hackers presenting false DNS information to users, leading them to malicious sites. In a DNS cache poisoning attack, a hacker can inject false address records into a DNS resolver’s cache, causing the resolver to return an incorrect IP address and redirect traffic to the attacker’s computer.