Test your DNS using Dig, Nmap, Tcpdump

For DNS resolution to succeed to 192.168.0.1, the DNS server at 192.168.0.1 will need to accept TCP and UDP traffic over port 53 from our server. A port scanner such as the nmap tool can be used to confirm if the DNS server is available on port 53 as shown below.

Note: To install nmap run ‘yum install nmap -y’.

[root@centos ~]# nmap -sU -p 53 192.168.0.1
Starting Nmap 6.40 ( http://nmap.org ) at 2015-08-26 15:22 AEST
Nmap scan report for 192.168.0.1
Host is up (0.00091s latency).
PORT STATE SERVICE
53/udp open|filtered domain
MAC Address: 02:00:79:55:00:0D (Unknown)
Nmap done: 1 IP address (1 host up) scanned in 0.29 seconds

[root@centos ~]# nmap -sT -p 53 192.168.0.1
Starting Nmap 6.40 ( http://nmap.org ) at 2015-08-26 15:22 AEST
Nmap scan report for 192.168.0.1
Host is up (0.00099s latency).
PORT STATE SERVICE
53/tcp open domain
MAC Address: 02:00:79:55:00:0D (Unknown)
Nmap done: 1 IP address (1 host up) scanned in 0.07 seconds

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NMAP Security Scanner on Linux

First of all, you should get the “nmap” package installed in your system.

On CentOS

yum install nmap

On Debian

apt-get install nmap

On Ubuntu

sudo apt-get install nmap

NMAP Commands:

sudo nmap -sS [IP address] or [website address]
sudo nmap -O --osscan-guess [IP address] or [website address]
sudo nmap -vv [IP address] or [website address]

sudo nmap -sV [IP address] or [website address]

nmap -sn 192.168.1.0/24 or nmap -sP 192.168.1.0/24

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