Linux-Basics-Complete-Course-With-Notes-Slides

DNS

• The domain name system is a distributed way to share these name-to-IP associations instead of requiring each computer to synchronize a hosts file. A name server publishes the IP address for a domain and provides a single location to update when an IP changes.

#### Ping

• Ping Command is use to check the remote machine is reachable or not.

  [~]$ ping 192.168.1.11
  Reply from 192.168.1.11: bytes=32 time=4ms TTL=117
  Reply from 192.168.1.11: bytes=32 time=4ms TTL=117
  

• To Ping the remote host with a name instead of IP Address make an entry in /etc/hosts file

  [~]$ cat >> /etc/hosts
  192.168.1.11 db
  

  [~]$ ping db
  PING db (192.168.1.11) 56(84) bytes of data.
  64 bytes from db (192.168.1.11): icmp_seq=1 ttl=64 time=0.052 ms
  64 bytes from db (192.168.1.11): icmp_seq=2 ttl=64 time=0.079 ms
  

• You can configure as many hosts you want in the /etc/hosts file.

  [~]$ cat >> /etc/hosts
  192.168.1.10 web
  192.168.1.11 db
  192.168.1.12 nfs
  192.168.1.20 web
  192.168.1.21 db-1
  192.168.1.22 nfs-1
  192.168.1.30 web-1
  192.168.1.31 db-2
  192.168.1.32 nfs-2
  192.168.1.40 web-2
  192.168.1.41 sql
  192.168.1.42 web-5
  192.168.1.50 web-test
  192.168.1.61 db-prod
  192.168.1.52 nfs-4
  192.168.1.60 web-3
  192.168.1.61 db-test
  192.168.1.62 nfs-prod
  

• Every host has a DNS resolution file /etc/resolv.conf

  [~]$ cat /etc/resolv.conf
  nameserver 192.168.1.100    
  

• The /etc/nsswitch.conf file is used to configure which services are to be used to determine information such as hostnames, password files, and group files.There is a specific search order according to which it is performed. This order is set in this configuration file.

  [~]$ cat /etc/nsswitch.conf
  …
  hosts: files dns
  …
  

DOMAIN NAMES

• .com - Commerical or General Purpose.
• .net - Network or General Purpose.
• .edu - Education Purpose
• .org - Organizations for non profit organizations etc.

RECORD TYPES

• A - IP to host names.
• AAAA - Storing ipv6 to hostnames.

• CNAME - Mapping one name to another.

• To test the DNS resolution you can use nslookup command, this will query a hostname from a DNS Server.

  [~]$ nslookup www.google.com
  Server: 8.8.8.8
  Address: 8.8.8.8#53
  Non-authoritative answer:
  Name: www.google.com
  Address: 172.217.0.132

• Another useful tool to query a hostname from a DNS server is dig which return more detailed information as shown.

  [~]$ dig www.google.com
  ; <<>> DiG 9.10.3-P4-Ubuntu <<>> www.google.com
  ;; global options: +cmd
  ;; Got answer:
  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28065
  ;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 1
  ;; OPT PSEUDOSECTION:
  ; EDNS: version: 0, flags:; udp: 512
  ;; QUESTION SECTION:
  ;www.google.com. IN A
  ;; ANSWER SECTION:
  www.google.com. 245 IN A 64.233.177.103
  www.google.com. 245 IN A 64.233.177.105
  www.google.com. 245 IN A 64.233.177.147
  www.google.com. 245 IN A 64.233.177.106
  www.google.com. 245 IN A 64.233.177.104
  www.google.com. 245 IN A 64.233.177.99
  ;; Query time: 5 msec
  ;; SERVER: 8.8.8.8#53(8.8.8.8)
  ;; WHEN: Sun Mar 24 04:34:33 UTC 2019
  ;; MSG SIZE rcvd: 139