17. Why Learn All This Before Kali Linux?
17.2 From Networking to Nmap and Wireshark
Part 1 madhe shiklele ports, TCP/UDP, 3-way handshake aani OSI layers – he Nmap aani Wireshark che "bhasha" aahet.
| You learnt (Part 1) | You will use it for |
|---|---|
| Ports and services (22, 80, 443, 3306...) | Reading Nmap results: which service, which risk |
| TCP 3-way handshake (SYN, SYN-ACK, ACK) | Understanding SYN scan (-sS) vs connect scan (-sT), and why firewalls log half-open connections |
| TCP vs UDP | Why UDP scans (-sU) are slow and need different thinking |
| Private vs public IP, NAT, CIDR | Defining the scope of a test (192.168.56.0/24 lab only) |
| OSI layers | Placing attacks: ARP spoofing (L2), IP spoofing (L3), SYN flood (L4), SQLi/XSS (L7) |
| HTTP vs HTTPS, TLS | Why Wireshark can read HTTP passwords but not HTTPS content |
A small preview – run only against your own machine or your lab network:
nmap -sT -p 22,80,443,3306 127.0.0.1 # which of these ports are open on MY machine?
sudo tcpdump -i any -c 5 port 80 # watch 5 packets of your own HTTP traffic
Each line of the Nmap output (22/tcp open ssh) is something you already understand: a port, a protocol and a service you configured in Part 2 and Part 3.
Why this matters for security
Defenders use the same knowledge the other way: close every port that is not needed, restrict management ports (22, 3306) to specific sources in security groups, and watch for scanning patterns (many SYNs to many ports) in logs. Knowing the handshake is what lets you recognise a scan in Wireshark or a SIEM.
Ravindra Bagale's Tip
Many students don't understand the difference between filtered and closed in Nmap output. closed means the packet reached the server but nobody is listening there; filtered means a firewall/security group blocked it on the way. Remember the handshake from Part 1 – the answer is right there.
Ravindra Bagale's Tip – मराठी
Nmap output मध्ये filtered आणि closed मधला फरक बऱ्याच students ना कळत नाही. closed म्हणजे packet server पर्यंत पोहोचला पण तिथे कोणी ऐकत नाही; filtered म्हणजे मधेच firewall/security group ने अडवला. Part 1 चा handshake आठवा – उत्तर तिथेच आहे.
Ravindra Bagale's Tip – हिंदी
Nmap output में filtered और closed का फ़र्क बहुत से students को समझ नहीं आता. closed यानी packet server तक पहुँचा पर वहाँ कोई सुन नहीं रहा; filtered यानी बीच में firewall/security group ने रोक दिया. Part 1 का handshake याद करो – जवाब वहीं है.
Practice task
On your own EC2 instance, list listening ports with sudo ss -tulnp. Predict what Nmap would show from the internet given your security group, and write down which ports should be open and why.