How DNS Caching Works ​
DNS caching is a mechanism that improves the speed and efficiency of domain name resolution by storing previously retrieved DNS query results. Without caching, every time a user requests a website, their computer would need to perform a full DNS lookup, which involves multiple queries to different servers. DNS caching reduces this need by keeping a temporary copy of DNS records.
What is DNS Caching? ​
DNS caching is the process of storing DNS query results for a set period, allowing future requests for the same domain to be resolved more quickly. This helps reduce response times and minimizes the load on upstream DNS servers.
When a user visits a website for the first time, their computer or DNS resolver must retrieve the IP address from authoritative DNS servers. Once resolved, the result is stored in cache, so if the same domain is requested again, the resolver can provide the answer immediately without querying other servers.
How Does DNS Caching Work? ​
When a DNS query is made, caching can occur at multiple levels:
- Browser Cache – Most modern web browsers store DNS lookups temporarily. If a user revisits a website, the browser can retrieve the IP address from its own cache, avoiding a new DNS request.
- Operating System Cache – The user’s device also maintains a local DNS cache. If the browser doesn’t have a cached entry, the operating system (e.g., Windows, macOS, Linux) will check its DNS cache before querying an external resolver.
- Recursive Resolver Cache – If the user’s device doesn’t have a cached response, it will send the query to a recursive DNS resolver (such as the UK DNS Privacy Project’s resolvers). These resolvers cache responses from authoritative servers, allowing them to respond to future queries more quickly.
- Authoritative Server Cache – Although authoritative DNS servers primarily provide official DNS records, some can implement caching to reduce the load from repeated queries.
Time-to-Live (TTL) and Cache Expiration ​
Each DNS record comes with a Time-to-Live (TTL) value, which determines how long it can be stored in cache. TTL is set by the authoritative DNS server and is typically measured in seconds. When the TTL expires, the cached record is discarded, and a fresh lookup is required.
For example, if a DNS record has a TTL of 3600 seconds (1 hour), any DNS resolver that caches the record will only store it for that duration. After an hour, a new lookup will be needed to ensure the data is still accurate.
Benefits of DNS Caching ​
DNS caching provides several advantages:
- Faster Browsing – Cached DNS records allow websites to load more quickly by reducing lookup times.
- Reduced DNS Traffic – Since queries are resolved from cache rather than sent to authoritative servers, caching reduces unnecessary network traffic.
- Improved Reliability – If an authoritative DNS server goes down, cached records allow users to continue accessing a site until the cache expires.
- Lower Latency – By serving responses locally, caching minimizes delays in name resolution.
Risks and Challenges of DNS Caching ​
While caching improves performance, it also introduces some challenges:
- Stale Records – If a domain’s IP address changes but cached records haven’t expired yet, users may be directed to an outdated or incorrect address.
- Cache Poisoning Attacks – Malicious actors can inject false DNS responses into a resolver’s cache, redirecting users to phishing sites or other harmful destinations.
- Delayed Updates – Websites that frequently change their DNS settings (e.g., content delivery networks) may require lower TTL values to ensure updates propagate quickly.
How the UK DNS Privacy Project Handles DNS Caching ​
The UK DNS Privacy Project ensures that its recursive DNS resolvers implement caching securely and efficiently. Our resolvers:
- Respect TTL values to maintain data accuracy.
- Validate DNSSEC to prevent cache poisoning attacks.
- Support encrypted DNS (DoH/DoT) to protect query privacy.
- Minimize unnecessary logging to enhance user security.