What Is DNS Filtering?

DNS filtering involves the process of blocking or allowing access to specific websites or online content based on predefined criteria, typically managed through Domain Name System (DNS) servers.

DNS filters are often confused with URL filters. DNS filters work by offering DNS protection across an entire domain name work by redirecting DNS requests to filter out unwanted content, such as malicious websites or inappropriate content. These filters enhance network security and enforce acceptable use policies. URL filters instead offer a more granular level of control by filtering by the specific URL of a webpage.

DNS filtering works as a proactive measure to prevent users from accessing harmful or unauthorized content. By leveraging DNS filtering, organizations can enforce policies to block access to known unauthorized and malicious domains, phishing sites, or malicious content, reducing the risk of malware infections and data breaches.

The Components of DNS Filtering

Common components for DNS filtering include DNS servers, web filtering policies, and DNS filtering software.

These components help to effectively manage and control access to internet resources and provide additional security features for the organization’s network.

DNS Servers

DNS servers play a fundamental role in translating human-readable domain names into numerical IP addresses that computers use to communicate over the internet.

Acting as the internet’s address book, DNS servers facilitate the resolution of domain names to their corresponding IP addresses, which allows users to access websites and other online services seamlessly.

The Distribution of DNS Servers

DNS servers are distributed across the internet in a hierarchy, forming the backbone of the Domain Name System. At the top of the hierarchy are the root DNS servers, which maintain information about the top-level domains (TLDs) such as .com, .org, and .net.

These root servers direct queries to the appropriate TLD servers, which in turn delegate authority to domain name registrars and authoritative DNS servers responsible for individual domains. By efficiently resolving domain names to IP addresses and facilitating the flow of internet traffic, DNS servers enable users to navigate the internet and access online resources with ease and reliability.

Filtering Policies

Filtering policies are sets of rules or criteria used to manage and control access to internet resources within an organization’s network.

These policies dictate what content is allowed or restricted based on various factors, such as:

  • Website categories
  • User groups
  • Time of access
  • Security considerations

Organizations often implement filtering policies to enforce acceptable use guidelines, enhance network security layers, and optimize productivity by preventing access to inappropriate, harmful, or non-work-related content.

Filtering policies can be customized to align with the specific needs and objectives of an organization.

For instance, a company may choose to block access to categories such as gambling, adult content sites, or social media sites during work hours to maintain productivity and reduce distractions. Alternatively, filtering policies may focus on blocking known malicious websites or phishing domains to mitigate the risk of cyber threats and data breaches.

Filtering policies can be dynamically adjusted and updated to adapt to evolving internet usage patterns, emerging security threats, and regulatory requirements, ensuring that organizations maintain a secure and compliant online environment for their users.

DNS Filtering Software

DNS filtering software is a critical tool used by organizations to enforce filtering policies and control access to internet resources based on predefined criteria.

This software operates by intercepting DNS-level queries and applying filtering rules to determine whether to allow or block access to specific websites or content categories. By integrating with DNS servers or acting as standalone solutions, the filtering software provides granular control over internet access, allowing organizations to customize filtering policies to suit their unique requirements.

Real-Time Protection

One of the key advantages of DNS filtering software is its ability to provide real-time protection against various online threats, including malware, phishing attacks, and malicious websites. By leveraging threat intelligence feeds and continuously updating blacklists and whitelists, DNS filtering software can identify and block access to known malicious domains, helping to prevent infections and data breaches. DNS filtering software often offers reporting and analytics capabilities, allowing organizations to monitor internet usage patterns, track security incidents, and gain insights into potential risks.

This software serves as a proactive security measure that enhances network security, enforces acceptable use policies, and safeguards against a wide range of online threats.

How DNS Filtering Works

DNS filtering works by intercepting Domain Name System (DNS) queries made by devices on a network and applying predefined filtering policies to determine whether to allow or block access to specific websites or content categories.

By directing DNS requests through filtering servers equipped with blacklists, whitelists, and other filtering criteria, DNS filtering software controls access to internet resources, providing organizations with granular control over their network’s security features and content filtering.

DNS Resolution Process

The DNS resolution process is a fundamental mechanism that translates human-readable domain names into numerical IP addresses, enabling communication between devices over the internet.

When a user enters a domain name into their web browser, their device initiates a DNS resolution process to determine the corresponding IP address associated with that domain. Initially, the device checks its local DNS cache to see if it has recently resolved the domain name, saving time by avoiding unnecessary queries.

If the domain’s IP address is not found in the cache or has expired, the device sends a DNS query to a recursive DNS server, typically provided by the user’s internet service provider (ISP) or configured within the network settings.

The Recursive DNS Server

The recursive DNS server, acting on behalf of the client device, begins the resolution process by querying authoritative DNS servers starting from the root DNS servers, then moving to the appropriate top-level domain (TLD) servers, and finally reaching the authoritative name servers responsible for the specific domain.

Each step in this process involves iterative queries, where DNS servers provide referrals to other servers until the authoritative name server responsible for the queried domain is reached.

Once the authoritative name server is identified, it responds to the recursive DNS server with the corresponding IP address for the requested domain. The recursive DNS server then caches this information and returns the resolved IP address to the client device, allowing it to establish a connection with the desired web server and access the requested website or online service.

This hierarchical and distributed nature of the DNS resolution process ensures efficient and reliable translation of domain names into IP addresses, enabling seamless communication across the internet.

Filtering Mechanisms

Filtering mechanisms are pivotal tools in network security architectures, designed to regulate and manage the flow of data across networks based on specific criteria.

These mechanisms encompass various techniques and technologies aimed at controlling access to resources, preventing unauthorized access, and mitigating potential security risks.

Content Filtering

Content filtering, for instance, scrutinizes data packets for specific content types, enabling organizations to enforce policies regarding acceptable content and block access to malicious or inappropriate material.

URL Filtering

URL filtering focuses on managing access to websites by categorizing URLs and applying restrictions based on content categories or reputation, bolstering defenses against phishing attempts and malicious websites.

Application Filtering

Application filtering extends these capabilities to specific applications or protocols, enabling organizations to control access to critical applications and services, thereby reducing the attack surface and enhancing network security posture.

In essence, filtering mechanisms serve as vital components of cybersecurity strategies, empowering organizations to enforce access controls, safeguard sensitive data, and defend against emerging threats in today’s interconnected digital landscape.

Types of Filtering

DNS filtering utilizes types of filter mechanisms aimed at controlling access to internet resources based on predefined criteria.

Domain-Based filtering

One common type is domain-based filtering, which involves blocking or allowing access to specific domains or domain categories based on their reputation, content, or risk level.

Content-Based Filtering

Another type is content-based filtering, which inspects DNS requests and responses for specific content types, keywords, or patterns, allowing organizations to enforce policies regarding acceptable content and block access to malicious or inappropriate material.

Malware & Phishing Protection

DNS filtering can involve malware or phishing protection, where DNS queries are analyzed in real-time to identify and block requests to known malicious domains or URLs, thereby mitigating the risk of malware infections and phishing attacks.

These types of filtering mechanisms in DNS filtering enable organizations to enhance their network security posture, enforce acceptable use policies, and protect against various online threats.

Benefits of DNS Filtering

The benefits of DNS filtering include enhanced network security, improved regulatory compliance, and increased productivity by controlling access to inappropriate or harmful content.


DNS filtering helps improve security by blocking access to known harmful websites and domains, preventing users from inadvertently visiting harmful sites that could lead to malware infections or phishing email attacks sent by a bad actor.

By enforcing content-based filtering policies, DNS filtering also allows organizations to restrict access to risky or inappropriate content, reducing the likelihood of security breaches and enhancing overall network protection.

Additionally, DNS filtering can provide insights into network traffic patterns and detect suspicious activities, enabling organizations to respond proactively to potential security threats.

Content Control

DNS filters assist with content control by allowing organizations to enforce policies that dictate which websites and content categories users can access based on predefined criteria such as content type, category, or reputation.

This enables organizations to prevent access to inappropriate or non-work-related content, ensuring compliance with acceptable use policies and promoting a productive work environment.

DNS filtering provides flexibility to customize filtering policies according to organizational needs, allowing fine-grained control over the types of content that users can access, thus fostering a safe and secure browsing experience.

Malware Protection

DNS filtering services aid in malware protection by identifying and blocking access to known malicious sites and domains, preventing users from inadvertently downloading malware or falling victim to attacks from phishing websites.

By analyzing DNS queries in real-time, DNS filtering solutions can detect suspicious patterns and behaviors associated with malware activity, allowing organizations to proactively mitigate the risk of infections and protect their networks and users from various cyber threats.

DNS filtering can complement other cybersecurity measures by providing an additional layer of defense against malware propagation and enhancing overall security posture.

Challenges and Limitations

One possible challenge of DNS filtering is the potential for overblocking legitimate content or services due to overly strict filtering policies, which may inadvertently hinder user productivity or access to necessary resources.

False Positives

False positives in DNS filtering occur when legitimate websites or domains are incorrectly identified as malicious or inappropriate, leading to their unintentional blocking. These false positives can result from overly aggressive filtering policies or inaccurate categorization of websites, potentially disrupting user access to essential resources and impacting productivity.

Mitigating false positives requires careful tuning of filtering rules, regular updates to threat intelligence feeds, and continuous monitoring to ensure accurate identification of malicious content while minimizing the risk of blocking legitimate websites.

Evolving Threat Landscape

The threat landscape of DNS filtering is continuously evolving as cybercriminals devise new tactics to bypass traditional security measures and exploit vulnerabilities in DNS infrastructure. Sophisticated threats, such as DNS tunneling, DNS hijacking, and domain generation algorithms (DGA), pose significant challenges to DNS filtering solutions by disguising malicious activities within legitimate DNS traffic or creating dynamically changing domains to evade detection.

The increasing prevalence of encrypted DNS protocols complicates threat detection and mitigation efforts, such as:

  • DNS over HTTPS (DoH)
  • DNS over TLS (DoT)

…making it harder to enforce filtering policies and identify malicious behavior.

To address these evolving threats, organizations need to adopt advanced DNS filtering solutions capable of inspecting encrypted traffic, leveraging threat intelligence feeds, and employing machine learning techniques to adapt and respond effectively to emerging threats in real time.


The challenge of overblocking with DNS filtering arises when legitimate websites or services are incorrectly categorized or blocked due to overly aggressive filtering policies. This can hinder user access to essential resources, impede productivity, and potentially lead to frustration among users.

Overblocking may occur when filtering rules are not finely tuned or when there is insufficient granularity in categorizing websites, resulting in unintended consequences.

Addressing this challenge requires organizations to strike a balance between security and accessibility, implementing filtering policies that effectively mitigate risks while minimizing the impact on legitimate activities.

Regular review and adjustment of filtering rules, along with leveraging dynamic categorization techniques and user feedback, can help mitigate the risk of overblocking and ensure that DNS filtering solutions remain effective without unnecessarily restricting access to legitimate content.

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Implementing DNS Filtering

Once you’ve determined that a DNS filter fits the needs of your organization, you will need to make sure you take the proper steps to implement it properly.

Properly implementing a DNS filter can help protect from harmful domains, access to illegal content, and future potential loss prevention in case of any lapses in the initial setup process.

Deployment Options

Deployment options for DNS filtering include:

  • On-premises solutions, where filtering hardware or software is installed within the organization’s network infrastructure, providing direct control over filtering policies and data handling.
  • Cloud-based DNS filtering services, which offer scalable and easily accessible solutions that do not require on-site hardware deployment, enabling organizations to leverage the benefits of DNS filtering with reduced maintenance overhead and flexible deployment models.

Some organizations may opt for hybrid approaches, combining both on-premises and cloud-based solutions to achieve a balance between control, scalability, and ease of management.

Configuration Considerations

Configuration considerations for DNS filtering encompass defining filtering policies based on organizational needs and security requirements, including:

  • Selecting appropriate categories to block or allow
  • Setting up whitelists and blacklists
  • Configuring logging and reporting settings for monitoring and analysis

Organizations should consider integration with existing network infrastructure, ensuring compatibility with DNS servers, firewalls, and other security solutions, as well as evaluating scalability and performance requirements to accommodate growing network traffic and user demand.

Regular review and refinement of configuration settings are essential to maintaining the effectiveness of DNS filtering solutions in adapting to evolving threats and addressing changing organizational needs.

Best Practices

After implementing a DNS filter, best practices to make sure it’s offering the proper protection you need include:

  1. Conducting a thorough assessment of organizational requirements and security goals
  2. Selecting a DNS filtering solution that aligns with those needs
  3. Establishing clear policies and procedures for configuring
  4. Managing the solution effectively

You should prioritize user education and awareness to ensure understanding of DNS filtering policies and the importance of adhering to security protocols.

Regular monitoring and review of DNS traffic, along with collaboration with stakeholders and ongoing evaluation of the effectiveness of filtering policies, are crucial for maintaining a robust DNS filtering implementation.

Create a Bulletproof Security Strategy with Perimeter81

A key component of implementing a DNS filtering solution is finding the right partner. Perimeter81’s expertise can help your organization get started. The team at Perimeter81 has extensive knowledge of implementing DNS filtering solutions across a wide array of organizations at all sizes.

Contact us today to see how we can help protect your network as a partner for your DNS filtering solution!


What is a DNS filtering system?
DNS filtering, short for Domain Name System filtering, is the process of filtering malicious websites and other harmful or inappropriate content for a network.
What is DNS vs URL filtering?
DNS filtering works to block a website and all subsequent webpages, regardless of what the URL is. URL filtering is more granular and blocks only specific webpages from a website, instead of the entire website.
How do I set up DNS filters?
Working with an experienced team is key when setting up proper DNS filters for your organization. Perimeter81 is your partner to help ensure your network’s safety with robust DNS filtering options!
How to do DNS blocking?
Contact Perimeter81 today so we can help provide options that will help you properly implement DNS blocking and DNS filtering for your organization’s network.
Why use a DNS Filter?
DNS filters protect your network by preventing users from accessing unauthorized content. This can include malicious sites, social media sites, sites with adult content, or anything else that can harm your network or impact employee productivity.

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