Mahesh Puri

Day 08 — Networking Commands in Linux

4 min read

Day 08 — Networking Commands in Linux

🎯 Goal

Build a deep, practical understanding of how to debug networking issues in real systems using Linux tools.

In production, most failures are networking-related:

  • Service not reachable
  • DNS issues
  • Port not open
  • Latency / packet loss

This chapter gives you the tools to systematically diagnose them.

🧠 Mental Model

Networking issues can be broken into layers:

DNS → IP → Routing → Port → Application

Each command helps debug one layer of this chain.

1️⃣ Connectivity — ping

ping google.com
ping -c 4 google.com
ping -i 2 google.com

Explanation

ping sends ICMP echo requests to a remote host to check basic network connectivity.

It answers:

  • Is the host reachable?
  • How long does it take (latency)?

Use cases

  • Check if server is alive
  • Validate DNS resolution
  • Detect packet loss

Important notes

  • Some servers block ICMP → ping may fail even if server is up
  • Use as a first-level diagnostic, not final truth

2️⃣ IP & Interfaces — ip, ifconfig, hostname, iwconfig

ip (Modern tool)

ip addr
ip link
ip route
ip neigh

Explanation

ip is the modern unified networking tool.

  • ip addr → shows IP addresses
  • ip link → shows interfaces (up/down)
  • ip route → routing table
  • ip neigh → ARP table

Use cases

  • Check server IP
  • Debug routing issues
  • Verify interface status

ifconfig (Legacy)

ifconfig
ifconfig eth0

Explanation

Older tool for interface configuration.

⚠️ Not installed by default.

hostname

hostname
hostname -I

Explanation

  • Shows system hostname
  • -I → shows IP addresses

iwconfig

iwconfig

Explanation

Used for wireless (Wi-Fi) configuration.

3️⃣ DNS Debugging — nslookup, dig, whois

nslookup

nslookup google.com
nslookup google.com 8.8.8.8

Explanation

Queries DNS to resolve domain → IP.

dig (Preferred)

dig google.com
dig google.com +short
dig google.com @8.8.8.8

Explanation

Advanced DNS tool showing:

  • query time
  • DNS server used
  • record details

Use cases

  • Debug DNS issues
  • Verify DNS propagation

whois

whois google.com

Explanation

Shows domain ownership and registration info.

4️⃣ Path & Routing — traceroute, tracepath, mtr

traceroute

traceroute google.com
traceroute -n google.com

Explanation

Shows path packets take across networks (hops).

tracepath

tracepath google.com

Explanation

Simpler alternative to traceroute.

mtr (Best tool)

mtr google.com
mtr -c 10 google.com

Explanation

Combines ping + traceroute in real-time.

Use cases

  • Detect packet loss
  • Identify slow hops

5️⃣ Ports & Services — ss, netstat, nc, telnet

ss (Modern)

ss -tulnp
ss -tn
ss -l

Explanation

Shows:

  • listening ports
  • active connections

netstat (Legacy)

netstat -tulnp

Replaced by ss.

nc (Netcat)

nc -zv localhost 80
nc -zv google.com 443

Explanation

Tests if a port is open.

Use cases

  • Debug API availability
  • Check service ports

telnet

telnet google.com 80

Explanation

Manual connection tool (older).

6️⃣ ARP — arp

arp -a

Explanation

Maps IP → MAC addresses.

Used in local network debugging.

7️⃣ Routing Table — route, ip route

route -n
ip route

Explanation

Shows how packets are routed.

8️⃣ Monitoring & Utilities — watch, wget, curl

watch

watch -n 1 "ss -tulnp"

Explanation

Runs command repeatedly.

wget

wget http://example.com/file.zip
wget -c file.zip

Explanation

Downloads files.

curl

curl http://example.com
curl -I http://example.com
curl -X POST http://api

Explanation

Sends HTTP requests.

Use cases

  • API testing
  • debugging endpoints

curl vs wget

Featurecurlwget
API testing✅❌
Download files❌✅

JSON Processing — jq (Very Important)

Install (if not present)

sudo apt install jq

Basic Usage

curl https://api.github.com | jq

Explanation

jq formats and parses JSON data.

Without jq:

  • JSON is hard to read

With jq:

  • structured and readable

Extract fields

curl https://api.github.com | jq '.current_user_url'

Nested fields

curl https://api.github.com/repos/nodejs/node | jq '.owner.login'

Arrays

curl https://api.github.com/repos/nodejs/node/contributors | jq '.[0].login'

Pretty print JSON

cat file.json | jq

Why jq matters

Modern systems:

  • APIs return JSON
  • Logs often JSON
  • Kubernetes outputs JSON

👉 jq lets you filter, extract, transform data instantly

9️⃣ Network Scanning — nmap

nmap localhost
nmap -p 80,443 localhost

Explanation

Scans open ports and services.

🔟 Firewall — iptables

sudo iptables -L

Explanation

Controls inbound/outbound traffic.

🧪 Day-8 Lab

ping -c 4 google.com
 
ip addr
ip route
 
dig google.com +short
nslookup google.com
 
ss -tulnp
nc -zv localhost 80
 
traceroute google.com
mtr google.com
 
curl -I http://example.com
wget http://example.com/file.zip
 
nmap localhost
sudo iptables -L

🧠 Key Takeaways

  • Networking debugging is layered
  • ip, ss, dig are modern tools
  • nc replaces telnet
  • mtr is best for diagnosing network issues
  • curl is essential for APIs

✅ Outcome

You can now:

  • Debug connectivity issues
  • Analyze DNS problems
  • Inspect ports and services
  • Trace network paths
  • Test APIs and endpoints
  • Diagnose real production issues

Next → Pro CLI Tools (AWK, GREP, FIND, SED) 🚀

🔗 Links

← Back to Resume / Portfolio

Backlinks