DNS Cache
DNS caching is a mechanism that improves the speed and efficiency of domain name resolution by storing previously retrieved DNS query results for a set period. When a DNS record is cached, future requests for the same domain can be resolved immediately without querying authoritative DNS servers, resulting in faster load times and reduced network traffic.
How it works
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 along with its Time to Live (TTL) value. If the same domain is requested again before the TTL expires, the resolver can provide the answer immediately from cache without querying other servers.
DNS caching occurs at multiple levels, forming a hierarchical structure that optimizes performance:
**Browser-level caching**: Modern web browsers maintain their own DNS cache, allowing them to quickly resolve domain names without needing to query the operating system or external DNS servers.
**Operating system (OS)-level caching**: The OS maintains a DNS cache that serves as the next line of resolution if the browser cache doesn't have the required DNS record.
**Router-level caching**: Network routers can cache DNS queries, allowing them to serve DNS queries locally for all connected devices on the network.
**DNS Resolver caching**: Recursive DNS resolvers (typically operated by ISPs or third-party providers like Google DNS or Cloudflare) maintain large caches that serve their entire user base, significantly reducing the need to query authoritative DNS servers repeatedly.
Each cached record includes a Time to Live (TTL) value that dictates how long the DNS record should be stored before it expires and must be refreshed. TTL is measured in seconds. 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.
Key Points
- DNS caching happens at multiple levels: browser, OS, router, and DNS resolver
- Each cached record has a TTL (Time to Live) value that determines how long it remains valid
- Short TTL values (30-300 seconds) allow for rapid changes but increase server load
- Long TTL values (hours or days) reduce server load and improve performance for stable domains
- Cached records are automatically discarded and refreshed when their TTL expires
Common Use Cases
- Performance Optimization: Caching dramatically reduces DNS lookup time for frequently visited websites, improving page load speeds from seconds to milliseconds
- Network Load Reduction: By serving queries from cache, DNS resolvers reduce the load on authoritative nameservers and decrease overall network traffic
- High-Traffic Websites: Popular websites with stable infrastructure benefit from high TTL values (24 hours or more), reducing billions of redundant DNS queries daily
- DNS Migration Management: Administrators lower TTL values before planned DNS changes (like server migrations) to ensure quick propagation of new records
code Example DNS Caching Lifecycle
| Type | Host / Name | Value / Points to | TTL |
|---|---|---|---|
| Query | Initial Request | example.com | — |
| Cache | Resolver Cache | 192.0.2.1 | 3600 |
| Query | Second Request (30 min later) | example.com | 1800 |
| Query | Third Request (65 min later) | example.com | — |
* This example shows how a DNS record with a 1-hour TTL is cached and refreshed across different levels.
Frequently Asked Questions
What is the difference between DNS cache and browser cache?expand_more
DNS cache specifically stores IP address mappings for domain names, while browser cache stores website content like images, CSS, and JavaScript files. DNS cache operates at the network level and is maintained by DNS resolvers and operating systems, whereas browser cache is application-level and managed by your web browser. Both improve performance, but they cache entirely different types of data.
How do I clear my DNS cache?expand_more
The method varies by operating system. On Windows, run
ipconfig /flushdns in Command Prompt. On macOS, use sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder in Terminal. On Linux, the command depends on your DNS service (e.g., sudo systemd-resolve --flush-caches). Browsers also maintain separate DNS caches that may require clearing browser data.What is an ideal TTL value for DNS records?expand_more
It depends on your use case. For stable production domains that rarely change, a TTL of 24-48 hours (86400-172800 seconds) is common and reduces server load. For domains undergoing frequent changes or migrations, use shorter TTLs of 5-30 minutes (300-1800 seconds). As a general rule, use the longest TTL that doesn't interfere with your operational needs. Before planned DNS changes, temporarily lower your TTL a day or two in advance.
Can DNS caching cause problems?expand_more
Yes, DNS caching can cause issues when DNS records change. If you update a DNS record but clients still have the old value cached, they'll continue using the outdated information until the TTL expires. This is why lowering TTL values before making changes is recommended. Additionally, some ISPs ignore TTL values and cache records longer than specified, which can delay propagation.
Does DNS cache improve security?expand_more
DNS caching itself is primarily a performance feature, not a security feature. However, it can indirectly improve security by reducing exposure to certain DNS-based attacks. When records are served from cache, there's less opportunity for DNS spoofing on the network path. That said, cached records can also propagate incorrect or malicious DNS data if the cache is poisoned. For security, use DNSSEC in conjunction with caching.