Lab 20: Networking Basics (Capstone)

Objective

Understand Linux networking fundamentals: IP addresses, interfaces, routing, /etc/hosts, DNS, ping, curl, ss, and network troubleshooting. This capstone ties together everything from the Foundations level with a real-world network diagnostic workflow.

Time: 35 minutes | Level: Foundations | Docker: docker run -it --rm ubuntu:22.04 bash

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Step 1: Network Interfaces

ip addr show

📸 Verified Output:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 02:42:ac:11:00:06 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.6/16 brd 172.17.255.255 scope global eth0
       valid_lft forever preferred_lft forever

Key fields:

• lo: Loopback interface — 127.0.0.1, traffic stays on this machine

• eth0: Network interface — 172.17.0.6/16 is the container's IP

• /16 = subnet mask (covers 172.17.0.0 to 172.17.255.255)

💡 Every Linux server has at least lo (loopback). Real servers have eth0, ens3, enp0s3, etc. Cloud VMs may have ens3 or ens160. The name depends on the driver and BIOS.

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Step 2: Hostname and IP

hostname

📸 Verified Output:

eba630823a5f

hostname -i

📸 Verified Output:

172.17.0.6

# Container's DNS name and IPs
cat /etc/hosts

📸 Verified Output:

127.0.0.1	localhost
::1	localhost ip6-localhost ip6-loopback
fe00::	ip6-localnet
ff00::	ip6-mcastprefix
ff02::1	ip6-allnodes
ff02::2	ip6-allrouters
172.17.0.6	eba630823a5f

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Step 3: DNS Configuration

cat /etc/resolv.conf

📸 Verified Output:

# Generated by Docker Engine.
nameserver 8.8.8.8
search .

💡 nameserver 8.8.8.8 = Google's DNS. When you type curl google.com, the system asks 8.8.8.8 to resolve google.com to an IP address. Without a nameserver, hostnames won't resolve.

# Manual DNS lookup (using getent, available without extra packages)
getent hosts localhost

📸 Verified Output:

127.0.0.1       localhost

---

Step 4: Routing Table

ip route show

📸 Verified Output:

default via 172.17.0.1 dev eth0
172.17.0.0/16 dev eth0 proto kernel scope link src 172.17.0.6

• default via 172.17.0.1 — all traffic not matching other routes goes to this gateway

• 172.17.0.0/16 — this subnet is directly accessible via eth0

💡 The routing table is your network map. Traffic to 172.17.x.x goes directly. Everything else hits the default gateway. Misconfigured routing = no network access, even if the interface is up.

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Step 5: Checking Open Ports with ss

ss -tlnp

📸 Verified Output:

Netid  State   Recv-Q Send-Q Local Address:Port  Peer Address:Port

# All sockets (including UDP)
ss -alnp | head -10

📸 Verified Output:

Netid  State    Recv-Q Send-Q Local Address:Port   Peer Address:Port  Process
nl     UNCONN   0      0       0                    0
nl     UNCONN   0      0       0                    0

💡 ss replaced netstat on modern Linux. -t=TCP, -u=UDP, -l=listening, -n=numeric (no DNS lookups), -p=show process. On a real server: ss -tlnp shows you every open port and which process owns it — essential for security audits.

---

Step 6: curl — HTTP from the Command Line

apt-get install -y -qq curl 2>/dev/null
curl --version | head -1

📸 Verified Output:

curl 7.81.0 (x86_64-pc-linux-gnu) libcurl/7.81.0 OpenSSL/3.0.2 zlib/1.2.11 ...

# Fetch headers only (HEAD request)
curl -sI http://example.com | head -8

📸 Verified Output:

HTTP/1.1 200 OK
Content-Encoding: gzip
Accept-Ranges: bytes
Age: 604800
Cache-Control: max-age=604800
Content-Type: text/html; charset=UTF-8
Date: Thu, 05 Mar 2026 01:09:00 GMT
Etag: "3147526947"

# Download a file
curl -sL https://example.com -o /tmp/example.html
wc -l /tmp/example.html

📸 Verified Output:

46 /tmp/example.html

---

Step 7: ping — Connectivity Test

ping -c 3 127.0.0.1

📸 Verified Output:

PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.038 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.040 ms
64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.041 ms

--- 127.0.0.1 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2047ms
rtt min/avg/max/mdev = 0.038/0.039/0.041/0.001 ms

💡 ping sends ICMP echo requests. 0% packet loss = connectivity OK. High RTT = latency. Some hosts block ICMP (firewalls), so ping failure ≠ host is down — always verify with curl too.

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Step 8: Capstone — Network Diagnostic Script

cat > /tmp/netdiag.sh << 'SCRIPT'
#!/bin/bash
echo "================================================"
echo "  Network Diagnostic Report"
echo "  $(date)"
echo "================================================"
echo ""

echo "1. HOSTNAME & IP"
echo "   Hostname: $(hostname)"
echo "   IP:       $(hostname -i 2>/dev/null || ip addr show eth0 2>/dev/null | grep 'inet ' | awk '{print $2}')"
echo ""

echo "2. INTERFACES"
ip addr show | grep -E 'inet |^[0-9]' | while read line; do
    echo "   $line"
done
echo ""

echo "3. DEFAULT GATEWAY"
GW=$(ip route | grep default | awk '{print $3}')
echo "   Gateway: ${GW:-not found}"
echo ""

echo "4. DNS"
NS=$(grep nameserver /etc/resolv.conf | awk '{print $2}')
echo "   Nameserver: ${NS:-not configured}"
echo ""

echo "5. LOOPBACK TEST"
if ping -c 1 -W 1 127.0.0.1 > /dev/null 2>&1; then
    echo "   ✅ Loopback (127.0.0.1): OK"
else
    echo "   ❌ Loopback: FAILED"
fi
echo ""

echo "6. OPEN PORTS"
PORTS=$(ss -tlnp 2>/dev/null | grep -c LISTEN)
echo "   Listening TCP ports: $PORTS"
ss -tlnp 2>/dev/null | grep LISTEN | awk '{print "   " $4}' | head -5
echo ""

echo "7. INTERNET CONNECTIVITY"
if curl -sI --max-time 3 http://example.com > /dev/null 2>&1; then
    echo "   ✅ HTTP (example.com): reachable"
else
    echo "   ⚠️  HTTP (example.com): unreachable (may be expected in container)"
fi
echo ""

echo "================================================"
echo "  Diagnostic complete"
echo "================================================"
SCRIPT

chmod +x /tmp/netdiag.sh
/tmp/netdiag.sh

📸 Verified Output:

================================================
  Network Diagnostic Report
  Thu Mar  5 01:09:00 UTC 2026
================================================

1. HOSTNAME & IP
   Hostname: eba630823a5f
   IP:       172.17.0.6

2. INTERFACES
   1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 ...
   inet 127.0.0.1/8 scope host lo
   2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> ...
   inet 172.17.0.6/16 brd 172.17.255.255 ...

3. DEFAULT GATEWAY
   Gateway: 172.17.0.1

4. DNS
   Nameserver: 8.8.8.8

5. LOOPBACK TEST
   ✅ Loopback (127.0.0.1): OK

6. OPEN PORTS
   Listening TCP ports: 0

7. INTERNET CONNECTIVITY
   ✅ HTTP (example.com): reachable

================================================
  Diagnostic complete
================================================

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Summary

Command	Purpose
ip addr show	List network interfaces and IPs
ip route show	Routing table
hostname	Show hostname
hostname -i	Show primary IP
cat /etc/hosts	Local DNS overrides
cat /etc/resolv.conf	DNS server config
ss -tlnp	TCP listening ports
ping -c N host	ICMP connectivity test
curl -sI url	HTTP header check
curl -sL url -o file	Download file
getent hosts name	DNS lookup

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Foundations Level Complete! 🎉

You have completed all 20 Linux Foundations labs. You can now:

• Navigate the Linux filesystem and understand the FHS

• Create, copy, move, and delete files safely

• Read and modify file permissions and ownership

• Manage users, groups, and service accounts

• Use essential tools: grep, find, ps, df, du, curl

• Write shell scripts with variables, loops, and functions

• Understand I/O redirection and build command pipelines

• Diagnose network issues and understand Linux networking

Next Level: Linux Practitioner — system services, cron, SSH hardening, firewalls, and log management.