When you browse the internet at home or in the office, you may be used to the default network settings or deliberately change your DNS to a well-known public DNS, such as `8.8.8.8` or `1.1.1.1`, for faster speeds or more stable access. However, behind this seemingly ordinary "address lookup shortcut" lie some risks related to privacy, security, and even network stability. While public DNS services bring convenience, their centralized nature has made them a new focus for cyberattacks, privacy snooping, and network manipulation.

A responsible public DNS service provider will have a clear privacy policy, promising not to record or anonymize this data. However, the risk lies in the possibility of policy changes, misuse of data, or legal pressure to provide data. Worse still, if you encounter a malicious or unethical DNS service provider, this data could be used to build user profiles, targeted advertising, or even sold to third parties. You can use command-line tools like `dig` or `nslookup` to verify whether your DNS query actually went to the server you set, which at least helps you identify the source of the first hop's privacy risk.

dig +short www.example.com

If the returned result is clearly not the IP address you expected (for example, accessing a mainstream website but getting an unfamiliar address), this could be a warning sign.

In a man-in-the-middle attack, attackers use various methods to redirect you to an incorrect IP address. DNS hijacking typically occurs before your request reaches the public DNS server and can be caused by a compromised local router, malicious intervention by your network operator, or malware. For example, you might want to access online banking, but your DNS is tampered with and redirected to a phishing website that looks exactly the same, thus stealing your account password.

Another more covert and technically sophisticated method is DNS cache poisoning. Attackers poison the DNS server's cache by injecting forged responses. Once successful, all users querying the server for the poisoned domain will be redirected to the wrong address before the cache expires. Historically, there have been cache poisoning attacks targeting large public DNS servers with widespread impact. Preventing these attacks primarily relies on DNSSEC (Domain Name System Security Extensions). It uses digital signatures to verify the authenticity of DNS responses, ensuring that the "landmarks" you receive have not been tampered with. You can check if a domain has DNSSEC deployed using the following command:

dig +dnssec

Look for the `ad` (Authentic Data) flag in the returned results. If it's present, the response has been verified by DNSSEC.

Single point of failure and performance bottleneck: Relying on a few large public DNS servers is equivalent to centralizing the "address book" queries of a large number of users worldwide onto a few nodes. This introduces the risk of centralization. Once a major public DNS service goes down due to a large-scale DDoS attack, misconfiguration, or infrastructure failure, it will affect the internet experience of millions or even hundreds of millions of users worldwide, causing widespread "network outages" (websites become inaccessible). In comparison, using your local ISP's DNS, while perhaps slightly slower, usually limits the impact to a single region.

Performance isn't always optimal either. Public DNS providers use anycast technology to connect you to the geographically nearest data center, which is usually fast. However, for certain regions or localized services, the returned IP address may not be optimal and may even increase latency due to detours. A simple latency test can help you make a choice:

# In Windows PowerShell or Command Prompt

ping 8.8.8.8

ping 1.1.1.1

ping your local ISP's DNS address

Choose the one with the lowest average latency, which is usually most beneficial for ordinary users.

The DNS protocol was not designed with security in mind. Its queries and responses are mostly based on the connectionless UDP protocol and lack encryption, which makes it easy for eavesdropping and spoofing. In recent years, encrypted DNS protocols (such as DNS-over-HTTPS (DoH) and DNS-over-TLS (DoT)) designed to solve these problems have been rapidly gaining popularity. They wrap DNS queries in encrypted HTTPS or TLS tunnels, effectively preventing eavesdropping and tampering along the path. Major browsers and operating systems have begun to support them.

However, encrypted DNS has also raised new controversies. It makes it difficult for network administrators (such as companies and schools) to implement content filtering and network monitoring, and it also technically bypasses the DNS-level content control policies of some countries or regions, which may bring compliance risks. Simultaneously, it may concentrate more privacy data in the hands of a few technology companies providing encrypted DNS services.

Public DNS services do not operate in a vacuum. In some jurisdictions, governments may require public DNS service providers to block specific domains or resolve them to specific IPs (e.g., for traffic monitoring). Even if the service providers do not cooperate, governments can directly block access to these public DNS IP addresses at the national network border, forcing users to use controllable local DNS.

On the other hand, public DNS service providers themselves may also proactively block or redirect domains they deem malicious, fraudulent, or involved in infringing content based on their company policies. This partially shifts the power to filter network content from network operators to private technology companies, and whether their judgment standards are transparent and fair becomes a public policy issue.

Given these risks, completely rejecting public DNS is not a wise move, as they are often faster and cleaner than the DNS of some unscrupulous ISPs. The key is conscious selection and management.

1. Choose reputable service providers: Prioritize public DNS providers with clear and strict privacy commitments (such as a commitment not to log queries) and understand the legal environment of their headquarters' location.

2. Configure at the router level: Setting up DNS on your home or business router provides unified protection for all devices and facilitates easy replacement.

3. Consider using encrypted DNS: Enabling DoH or DoT on supported devices can significantly improve privacy and security during queries. For example, DoH can be enabled in the settings of the Firefox browser.

4. Keep local DNS as a backup: Configure multiple DNS servers in your device's network settings, prioritizing a reliable public DNS and using your local ISP's DNS as a backup to increase redundancy.

5. Keep software updated: Ensure your operating system, router, and security software are up-to-date to prevent malware from hijacking your DNS settings locally.

In conclusion, public DNS is a double-edged sword. It breaks the monopoly of local ISPs, bringing speed and freedom, but it also leads us to new central nodes, bringing new challenges in privacy, security, and controllability. As users, understanding these potential risks and making prudent choices based on your needs (whether you prioritize speed, privacy, or avoiding censorship) is a crucial step in protecting your digital rights in a complex network environment. While enjoying the convenience of technology, it is always beneficial to maintain a clear awareness of risks.