Add Domain name system section

README.md

@@ -653,3 +653,41 @@ This topic is further discussed in the [Database](#database) section:
 
 * [Master-slave replication](#master-slave-replication)
 * [Master-master replication](#master-master-replication)
+
+## Domain name system
+
+<p align="center">
+  <img src="http://i.imgur.com/IOyLj4i.jpg">
+  <br/>
+  <i><a href=http://www.slideshare.net/srikrupa5/dns-security-presentation-issa>Source: DNS security presentation</a></i>
+</p>
+
+A Domain Name System (DNS) translates a domain name such as www.example.com to an IP address.
+
+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)](https://en.wikipedia.org/wiki/Time_to_live).
+
+* **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](https://www.cloudflare.com/dns/) and [Route 53](https://aws.amazon.com/route53/) provide managed DNS services.  Some DNS services can route traffic through various methods:
+
+* [Weighted round robin](http://g33kinfo.com/info/archives/2657)
+    * Prevent traffic from going to servers under maintenance
+    * Balance between varying cluster sizes
+    * A/B testing
+* Latency-based
+* Geolocation-based
+
+### Disadvantage(s): DNS
+
+* Accessing a DNS server introduces a slight delay, although mitigated by caching described above.
+* DNS server management could be complex, although they are generally managed by [governments, ISPs, and large companies](http://superuser.com/questions/472695/who-controls-the-dns-servers/472729).
+* DNS services have recently come under DDoS attack, preventing users from accessing websites such as Twitter without knowing Twitter's IP address(es).
+
+### Source(s) and further reading
+
+* [DNS architecture](https://technet.microsoft.com/en-us/library/dd197427(v=ws.10).aspx)
+* [Wikipedia](https://en.wikipedia.org/wiki/Domain_Name_System)
+* [DNS articles](https://support.dnsimple.com/categories/dns/)