DNS - Caching

DNS, or Domain Name System is used to translate human-readable domain names (like "www.example.com") into numerical IPv4 or IPv6 addresses that computers use to communicate with each other. DNS caching is a critical aspect of this system.

  1. Purpose of DNS Caching: DNS caching is done to speed up the process of domain name resolution. Without caching, every time you requested a website, your computer would have to query a DNS server to find the corresponding IP address. Caching these records locally or on intermediate servers (like your ISP's DNS server) greatly reduces this overhead.

  2. Where Caching Occurs:

    • Browser Cache: Modern web browsers typically maintain a local DNS cache. This cache stores the results of recent DNS lookups to reduce the need for repeated queries for the same domain names.
    • Operating System Cache: The OS also maintains a DNS cache. For example, in Windows, you can view this cache using the command ipconfig /displaydns, and in Unix-like systems, the nscd service handles this.
    • Resolver Cache: When you make a DNS request, it typically goes to a recursive DNS server (like the one provided by your ISP). This server caches DNS records to improve efficiency for its users.
    • Authoritative DNS Servers: These servers can cache records from other authoritative servers, but this is less common.
  3. Time-to-Live (TTL): Each DNS record comes with a TTL value, which dictates how long a resolver is supposed to cache that record before discarding it. Once the TTL expires, the resolver must query the authoritative DNS server again to obtain a fresh record. TTL values are set by the administrators of the domain and can vary greatly depending on how often they expect the information to change.

  4. Types of Records Cached: Various types of DNS records can be cached, including A (Address) records, which map a domain name to an IPv4 address; AAAA records for IPv6 addresses; CNAME records for aliasing one domain name to another; MX records for email servers, and more.

  5. Benefits and Risks:

    • Benefits: Caching reduces latency in web browsing, decreases the load on authoritative DNS servers, and generally improves the efficiency of the DNS system as a whole.
    • Risks: Stale cache data can lead to issues. For example, if a site changes its IP address and a user's cache hasn't yet updated (because the TTL hasn't expired), the user might be directed to the old, incorrect address.
  6. Flushing DNS Cache: Sometimes, it's necessary to manually clear the DNS cache. This can be done on a browser level, operating system level, or on network devices. This action is often taken to resolve issues with stale or corrupt cache entries.

  7. Security Considerations: DNS caching is susceptible to certain types of attacks, such as cache poisoning, where an attacker inserts a false DNS entry into a cache. To mitigate this, DNSSEC (DNS Security Extensions) can be used to verify the authenticity of the returned DNS records.

A response to a DNS request that comes from a cached source is considered non-authoritative. Thus, such a response will always have its authoritative flag set to 0.

DNS caching is a key component in the efficiency and speed of the Internet, balancing the need for quick access with the dynamism of changing IP addresses and services.