Edge Security Acceleration:DNS terms and concepts

Domain Name System (DNS) is a core Internet service that can translate domain names into IP addresses or translate IP addresses into domain names. Built on a distributed database, DNS makes it easier for people to access the Internet without the need to remember strings of machine-readable IP addresses.

To accommodate the increasing number of Internet users, a hierarchical tree structure is used for naming on the Internet. Any host or router connected to the Internet has a unique name in the DNS hierarchy. This name is a domain name. A domain is a manageable division in the DNS namespace. Grammatically, each domain name is a series of labels separated with dots (.). Domains can be divided into subdomains, and subdomains can be divided into lower-level subdomains. This creates top-level domains, primary domains, and subdomains. The following figure shows the domain hierarchy.

• .com is a top-level domain name.

• aliyun.com is a primary domain name.

• example.aliyun.com is a subdomain.

A site represents a specific domain name (such as example.com) and all its related DNS records (A records, CNAME records, MX records, etc.). A site is the smallest unit for domain name management. Site settings include DNS records, TTL settings, security settings, and traffic management rules of the domain name and its subdomains.

DNS records are used to map domain names to IP addresses or other associated resources. DNS record types such as A, AAAA, CNAME, TXT, MX, PTR, and SRV are supported.

Multiple servers are involved when you perform DNS resolution. In ESA, DNS server typically refers to the authoritative nameserver of ESA.

The time to live (TTL) specifies the longest period of time that a DNS record can be cached on a local DNS server. Once the TTL expires, the local DNS server deletes the record. If a user sends a request to the domain again afterward, the local DNS server makes a new recursive or iterative query.

CNAME flattening is a DNS optimization technique that allows you to add a CNAME record to a primary domain name (such as example.com). CNAME records are typically used to point one domain name to another domain name.

In traditional DNS systems, if a primary domain name uses a CNAME record, it typically cannot have other types of records, such as A records or AAAA records. The CNAME flattening feature provided by ESA solves this problem by allowing top-level domains to use CNAME records while maintaining the existence of other records. This way, after you initiate a DNS query, the system automatically returns the final IP address without performing CNAME resolution. This improves the performance of DNS resolution.