update links, add dns

docs/source/index.rst

@@ -114,6 +114,7 @@ The organization of this site is as following:
    :name: sec-reference-network
 
    reference/network/network-devops-basic
+   reference/network/domain-name-and-dns
 
 
 .. toctree::

docs/source/links/cuddly_links_collection.md

@@ -764,6 +764,8 @@ Searching Engines in Japanese:
 
 * [Dig web interface](https://www.digwebinterface.com/)
 
+* [Domain Status](https://domain-status.com/)
+
 ### Miscellaneous Tools
 
 * [Archive](https://archive.org/web/)

docs/source/reference/network/domain-name-and-dns.md

@@ -0,0 +1,90 @@
+# Domain Name and DNS
+
+## Domain Name and Domain Name System
+
+Computers use IP addresses to communicate.
+But it is hard to memorize and manage those
+so humans use Domain Names.
+
+Domain Name System (DNS) translates the domain names to real addresses
+that the computer understands. One domain name can have different
+results returned based on different conditions for different queries
+(for example, open gmail.com is different from send email to @gmail.com).
+
+DNS is provided by DNS servers.
+
+One fully qualified domain name ([FQDN]) has 3 (usually) parts and is divided using period character, '.'.
+There are 4 period characters in total but the dot at the end is commonly omitted.
+
+[FQDN]:https://en.wikipedia.org/wiki/Fully_qualified_domain_name
+
+* Top-level domain
+  * .com or .org?
+* Second-level domain
+  * example.com
+  * something one needs to purchase from a company
+  * [Learn more from ICANN](https://www.icann.org/resources/pages/register-domain-name-2017-06-20-en)
+* host name
+  * controlled by the owner of the second-level domain
+  * like a specific server in a company
+
+Domains are case-insensitive.
+
+Somewhat duplicated with
+[another article](https://docs.snowme34.com/en/latest/reference/network/network-devops-basic.html) on this site.
+
+## DNS Clients
+
+Devices of end-users are clients, which do not store DNS
+information (but they might cache or override).
+
+Applications or services will use some low-level methods to
+resolve the domain names (by which it means a c program on Linux do
+not call `host` command directly).
+
+* [ngx_resolver.c](https://github.com/nginx/nginx/blob/master/src/core/ngx_resolver.c)
+* [resolver(5) - Linux man page](https://linux.die.net/man/5/resolver)
+* [getaddrinfo(3) - Linux man page](https://linux.die.net/man/3/getaddrinfo)
+* [res_query(3) - Linux man page](https://linux.die.net/man/3/res_query)
+
+DNS query of system:
+
+* Files
+  * `/etc/hosts`
+  * `/etc/networks`
+  * often taken advantage of by virus or Trojans
+  * usually have higher priority
+* DNS server
+  * `/etc/resolv.conf`
+  * see the config guide of your linux distribution
+  * [NetworkConfiguration - Debian Wiki](https://wiki.debian.org/NetworkConfiguration#Defining_the_.28DNS.29_Nameservers)
+* NIS(network information service)
+  * not popular
+
+The query order can be configured in `/etc/nsswitch.conf`
+
+```conf
+hosts: files dns
+```
+
+## DNS Query Commands
+
+Commands that directly send queries to DNS servers:
+
+* they skip `nsswitch.conf`
+
+```bash
+host
+dig # with more details
+```
+
+Others:
+
+* http://man7.org/linux/man-pages/man1/getent.1.html
+
+```bash
+getent hosts www.winaproduct.com
+getent ahosts google.com
+getent ahostsv4 www.google.com
+getent ahostsv6 www.google.com
+```