What is recursive DNS?
Recursive DNS is a process that finds the IP address connected to a domain name, so you can visit websites by entering their names (like nordvpn.com) instead of long strings of numbers (like 104.19.159.190).
When you type a web address into your browser, your device relies on the domain name system (DNS) to translate that name into an IP address that computers understand. DNS is like the internet’s address book and makes online navigation possible. A recursive DNS resolver acts as the middleman in this process and fetches the correct IP address so your browser can load the site.
Resolvers save IP addresses in the cache so they don’t have to look them up again every time you visit the same site. If the resolver has the answer stored, it returns the result instantly. If not, it sends a new DNS query to other DNS servers in a hierarchy, including root servers, TLD servers, and authoritative nameservers. The resolver queries them step by step until it finds the correct IP address. This lookup process happens in milliseconds, so websites load quickly.
How recursive DNS works
Recursive DNS works by retrieving the correct IP address for a website when you enter a domain name into your browser’s search bar. But how does it do that? Here’s the step-by-step process:
- 1.Initial query. Your device (the client) sends a DNS query to a recursive resolver, typically managed by your internet service provider (ISP) or a third-party DNS service.
- 2.Checking the cache. The recursive resolver first checks its cache for a stored IP address. If it finds the answer, it immediately returns it to the client. If not, it begins the lookup process.
- 3.Root server request. The resolver contacts a root nameserver, which helps determine which top-level domain (TLD) server (such as .com or .org) is responsible for the domain.
- 4.TLD server query. The root nameserver responds with the address of the appropriate TLD nameserver. The recursive resolver then queries this server to find out which second-level domain (SLD) nameserver holds the specific domain information.
- 5.SLD server query. The TLD nameserver provides the resolver with the SLD nameserver’s address. The resolver then contacts this server to obtain the correct IP address for the domain.
- 6.Retrieving the IP address. The SLD nameserver responds with the domain’s IP address. The recursive resolver stores this information in its cache for future lookups and sends it back to the client.
- 7.Website access. With the IP address in hand, the client connects to the website’s server, and the browser loads the page.
Recursive DNS resolvers not only speed up searches by caching results but also reduce the load on DNS servers, which helps the internet run smoothly.
Benefits of recursive DNS servers
Recursive DNS doesn’t just speed up DNS resolution and handle requests more efficiently — it offers even more benefits, including:
- Faster DNS resolution. Recursive DNS servers speed up domain lookups by caching previously resolved queries. If a requested domain is in the cache, the server provides the answer instantly, which results in a shorter response time without unnecessary lookups.
- Effective request distribution. Without caching, every DNS query would need to reach an authoritative nameserver, overloading the system. With thousands of recursive DNS servers caching data worldwide, DNS requests are distributed efficiently, keeping the internet fast and scalable.
- Lower network latency. Storing query results locally reduces the need for repeated communication with external DNS servers. This minimizes delays and makes web browsing smoother.
- Better security. Many recursive DNS servers offer malware blocking, phishing protection, and content filtering. By preventing access to malicious sites, they add an extra layer of defense against cyber threats.
To sum up, recursive servers speed up lookups by caching responses and handling the entire query process for users. Without these servers, every device would have to contact root and authoritative servers directly, making browsing unbearably slow. Still, like any other critical internet infrastructure, they face certain online threats.
What dangers recursive DNS servers face
Recursive DNS servers face various threats — from DDoS attacks to malware infections — that can disrupt service and compromise security. Awareness about these threats may help you get one step ahead of threat actors who perpetuate them.
Distributed denial-of-service (DDoS) attacks
DDoS attacks and DoS attacks overwhelm recursive DNS servers with a massive flood of traffic and DNS requests, which causes the servers to slow down or crash and makes them unable to provide services to legitimate users.
Denial-of-service (DoS) attacks
A DoS attack floods a recursive server with excessive traffic to slow it down or crash it. This prevents legitimate users from accessing websites or online services. Unlike a DDoS attack, which uses multiple sources for a more powerful impact, a DoS attack is easier to detect and block. For more on the differences, check out our blog post on DoS vs. DDoS.
Amplification attacks
An amplification attack is a type of DDoS attack that uses recursive DNS servers to overwhelm a target with excessive traffic. Attackers send small queries with a spoofed IP and trick the server into sending much larger data packets than the original query to the victim. This method increases attack impact, reduces the attacker’s bandwidth use, and is harder to block than traditional DDoS attacks.
Cache poisoning
Cache poisoning happens when an attacker inserts false DNS information into a recursive server’s cache. The server then unknowingly provides users with incorrect IP addresses of malicious websites.
Man-in-the-middle attacks
In a man-in-the-middle attack, a hacker places themselves between a user and a recursive DNS server to intercept the communication. They manipulate the DNS response to redirect the user to a fraudulent website or steal sensitive information.
Malware infections
Threat actors may infect a recursive DNS server with malware to redirect users to malicious sites. These sites then download harmful software onto the user’s device, which can result in data theft or system damage.
DNS spoofing
DNS spoofing threatens recursive DNS servers by injecting false DNS records into their cache, which causes them to return incorrect IP addresses. This attack allows threat actors to take control of user traffic and redirect it to malicious websites that steal data or distribute malware.
Recursive DNS vs. iterative DNS
Recursive and iterative DNS queries follow different processes to resolve domain names — recursive queries rely on a resolver to find the answer, while iterative queries require the client to contact multiple DNS servers itself.
In a recursive DNS query, the recursive resolver takes full responsibility for finding the IP address. It queries other DNS servers on behalf of the client and keeps searching until it gets a final answer, which it then returns to the client. The client simply waits for the result without needing to contact multiple servers on its own.
In an iterative DNS query, the client actively participates in the lookup process. Instead of tracking down the final answer, each DNS server responds with the address of the next server the client should ask. The client then continues querying until it finds the correct IP address.
Think of it this way: With recursive DNS, the client tells the resolver, “Find this IP for me, and don’t come back until you have it.” With iterative DNS, the client says, “Where should I look next?” and follows each step itself.
Recursive DNS vs. authoritative DNS
Recursive and authoritative DNS servers work together to keep the internet running smoothly. Instead of relying on a single central database, the Domain Name System distributes the workload across thousands of DNS servers worldwide. Authoritative servers store official domain records and provide the most up-to-date answers, but they can’t handle the overwhelming volume of DNS traffic alone.
Recursive DNS servers help by fielding initial requests and retrieving the correct IP addresses. They store previously retrieved data in cache to speed up responses, but when the information isn’t available, they query other DNS servers to find the answer. This setup keeps the internet fast and reliable by handling billions of DNS requests every day.
Best practices for securing recursive DNS servers
A poorly secured recursive DNS server can be an easy target for cyberattacks, but a few smart precautions can keep it safe and running smoothly. Let’s explore how you can strengthen DNS security to prevent threats like cache poisoning, DDoS attacks, and DNS leaks.
Implement DNSSEC
Domain name system security extensions (DNSSEC) are enhancements to the domain name system that protect recursive DNS servers from cache poisoning by verifying the authenticity of DNS responses. By implementing DNSSEC, you ensure that users receive accurate DNS records instead of manipulated or malicious ones.
Monitor traffic
Regularly analyzing DNS queries helps detect unusual patterns or signs of an attack. Suspicious spikes in traffic may indicate DDoS attempts, malware activity, or DNS tunneling.
Use rate limiting
Setting limits on the number of queries from a single source helps prevent recursive DNS server abuse. Rate limiting protects servers from being overwhelmed by excessive requests and reduces the risk of DDoS attacks.
Enable caching policies
Enabling caching policies helps secure recursive DNS servers by balancing performance and security. Shorter DNS time-to-live (DNS TTL) values reduce the risk of serving outdated or compromised records, while properly managing cache purging prevents attackers from poisoning stored data.
Check for DNS leaks
You should also regularly check for DNS leaks to make sure your queries go through the intended recursive DNS server and aren’t exposed to unauthorized servers. You can use online DNS leak test tools to see which servers handle your queries. If the results show unexpected servers, you may have a leak that needs fixing.
To stay in control of your network privacy, you should also know your DNS settings and verify that your device is using a secure and trusted server. For tips on the process, browse our blog post on how to know your DNS settings on Windows and Mac.
Restrict open resolvers
Limiting who can use the recursive server prevents cybercriminals from exploiting it for DNS amplification attacks. Simply put, if only trusted users can access the server, there’s less risk that someone will exploit it for malicious purposes.
Understand DNS vs. VPN for security
While a secure DNS setup protects against certain threats, it doesn’t encrypt internet traffic like a VPN. Using both together boosts privacy by preventing DNS-based attacks while also hiding browsing activity from prying eyes. For more on the topic, check out our article on DNS vs. VPN.
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