DNS
Last updated
Last updated
A Domain Name System (DNS) translates a domain name such as www.example.com to an IP address.
DNS 就是一个翻译机，把网址转换成 IP 地址
DNS is hierarchical, with a few authoritative servers at the top level. Your router or ISP provides information about which DNS server(s) to contact when doing a lookup. Lower level DNS servers cache mappings, which could become stale due to DNS propagation delays. DNS results can also be cached by your browser or OS for a certain period of time, determined by the time to live (TTL).
NS record (name server) - Specifies the DNS servers for your domain/subdomain.
MX record (mail exchange) - Specifies the mail servers for accepting messages.
A record (address) - Points a name to an IP address.
CNAME (canonical) - Points a name to another name or CNAME (example.com to www.example.com) or to an A record.
Services such as CloudFlare and Route 53 provide managed DNS services. Some DNS services can route traffic through various methods:
Prevent traffic from going to servers under maintenance
Balance between varying cluster sizes
A/B testing
Latency-based
Geolocation-based
round robin 基本上就是 equally distribute request。所有在 pool 里的 host 挨个接收 request。而 weighted round robin 做了改进，比如有三个 hosts，他们的 weight 分别是 6:3:1。那么 每十个 requests，6 个 dispatch 到 host1，3 个 dispatch 到 host2，1 个 dispatch 到 host3。这在 hosts 他们的性能不一样的时候优化的效果明显。
Accessing a DNS server introduces a slight delay, although mitigated by caching described above.
DNS server management could be complex and is generally managed by governments, ISPs, and large companies.
DNS services have recently come under DDoS attack, preventing users from accessing websites such as Twitter without knowing Twitter's IP address(es).
