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02-DNS.md

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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

DNS

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

Root

RECORD TYPES

Record

  • 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