Linux Networking from Scratch: TCP/IP, Sockets, Ports & DNS

Lab 08 of 20 · Networking & DNS · 5 hands-on activities · bilingual (English + සිංහල)

පිටුව 08: Linux Networking බිංදුවේ සිට: TCP/IP, Sockets, Ports සහ DNS Resolution

IP ලිපින, Ports සහ Sockets, TCP 3-Way Handshake (SYN, SYN-ACK, ACK), ss -tuln භාවිතය, සහ DNS වැඩ කරන හැටි.

Architectural Foundation & Systems Diagram

In distributed systems and microservices, applications do not run on isolated islands. They communicate across network sockets governed by the TCP/IP stack.

• 🚪 Ports & Sockets — An IP Address identifies a host machine. A Port (0-65535) identifies the specific application listening. A Socket is the binding of IP:Port (e.g. 127.0.0.1:8080).

• 🤝 TCP 3-Way Handshake — Before any HTTP data flows, a reliable bidirectional connection is negotiated:

1. SYN (Client $ ightarrow$ Server: "Can we talk?")
2. SYN-ACK (Server $ ightarrow$ Client: "Yes, I am ready!")
3. ACK (Client $ ightarrow$ Server: "Acknowledged! Sending data.")

• 🔍 DNS Resolution Workflow — DNS converts human names (google.com) to IP addresses (142.250.190.46). Linux checks /etc/hosts first, then queries nameservers defined in /etc/resolv.conf.

Step-by-step practical lab

Step 01: Inspect Available RAM & Swap Memory

සිංහලෙන් — RAM මතකය සහ Swap ඉඩ ප්‍රමාණය Megabytes වලින් මැනීම.

Display total, used, free, shared, buffer/cache, and available physical RAM and swap space in megabytes.

free -m

• -m : Megabytes (MiB) ඒකක වලින් මතකය ගණනය කරයි.

Expected output

               total        used        free      shared  buff/cache   available
Mem:            8192        3820        2480          48        1891        4120
Swap:           2048           0        2048

Step 02: Search Kernel Ring Buffer for Out-of-Memory (OOM) Kills

සිංහලෙන් — RAM ඉවර වී Kernel OOM Killer විසින් processes මරා ඇත්දැයි බැලීම.

Inspect kernel dmesg logs for invocations of the Out-of-Memory (OOM) killer terminating runaway processes.

dmesg | grep -i oom

• dmesg : Kernel ring buffer logs පෙන්වයි.
• grep -i oom : OOM killer ක්‍රියාත්මක වූ සිදුවීම් සොයයි.

Expected output

[ 4120.184] Out of memory: Kill process 18240 (python3) score 842 or sacrifice child
[ 4120.185] Killed process 18240 (python3) total-vm:4194304kB, anon-rss:3670016kB

Hands-on practice activities

Work through these yourself — tick them off as you go.

• Activity 36: Audit Active TCP/UDP Listening Ports (Drill) — Display all listening TCP and UDP sockets with port numbers and interface bindings.
• Activity 37: Audit Sockets via Legacy Netstat (Drill) — Query listening socket table via netstat to verify port 8080 and 80 bindings.
• Activity 38: Probe Web Server Response Headers via Curl (Drill) — Send an HTTP GET probe to confirm web listener is active and returning HTTP 200 OK.
• Activity 39: Inspect DNS Nameserver Configuration (Checkpoint) — Verify your system nameservers inside /etc/resolv.conf to troubleshoot DNS resolution timeouts.
• Activity 40: TCP 3-Way Handshake Order Quiz (Quiz) — What is the correct sequence of packets in the TCP 3-way handshake? Answer: SYN, SYN-ACK, ACK.

Keep going: ← Lab 07 · Lab 09 → · Track your progress in the Self Guide