M18 — Networking basics for admins

Prerequisites#

• M13 comfort with processes helps when you ask “who owns this port?” later.
• Outbound HTTPS allowed for the curl lab (most home/cloud defaults allow it).

Learning objectives#

1. Print hostname and interface addresses with hostname and ip.
2. Read a simple default route.
3. Test connectivity with ping and/or curl.
4. List listening TCP ports with ss.
5. Resolve a name with getent hosts (and know that dig/host are optional extras).

Lab environment#

mkdir -p /tmp/linux-lab-pushpjeet/m18
cd /tmp/linux-lab-pushpjeet/m18

1. Hostname#

hostname
hostname -f 2>/dev/null || hostname

Lab run: grok-bot-vm-270875175 (your cloud or VM name will differ).
Change only when you know your org’s naming policy (hostnamectl set-hostname on systemd distros — optional reading).

2. Addresses with ip#

The older ifconfig still appears in blog posts; learn ip from the iproute2 suite.

ip -br addr          # brief overview
ip addr              # full detail
ip route             # routing table

Lab run (ip -br addr):

lo               UNKNOWN        127.0.0.1/8 ::1/128
enp0s3           UP             172.30.0.2/24 fe80::802b:38ff:fe3d:bcd0/64
docker0          DOWN           172.17.0.1/16

Lab default route:

default via 172.30.0.1 dev enp0s3

That via address is your gateway for off-subnet traffic.

Useful extras:

ip -br link          # layers 1–2 style summary
ip route get 1.1.1.1 # which path would a packet take?

3. Connectivity: ping and curl#

ping#

ping -c 3 127.0.0.1
ping -c 3 172.30.0.1          # your gateway from ip route
ping -c 3 example.com         # DNS + ICMP (may be blocked)

-c 3 sends three probes then stops (important — bare ping runs forever).

Note: Some containers and locked-down images omit ping (iputils-ping / inetutils-ping). On the drafting box it was not installed. Install via M16 if you want it, or use curl below.

curl / wget as reachability checks#

curl -sI --max-time 5 https://example.com | head -5

Lab run:

HTTP/2 200
date: Sat, 26 Sep 2026 05:35:37 GMT
content-type: text/html
server: cloudflare
...

HTTP/2 200 (or HTTP/1.1 200) means TCP+TLS+HTTP to that host worked — stronger proof of “internet useful” than ICMP alone when ping is blocked.

wget -qO- --timeout=5 https://example.com | head -c 100

4. Listening sockets with ss#

netstat is legacy; prefer ss.

ss -tln          # TCP listen, numeric ports
ss -uln          # UDP listen
ss -tlnp         # + process (may need sudo for all names)

Lab sample (ss -tln abridged):

State  Recv-Q Send-Q Local Address:Port  Peer Address:Port
LISTEN 0      128        127.0.0.1:8791       0.0.0.0:*
LISTEN 0      100          0.0.0.0:6080       0.0.0.0:*

When something cannot connect, ask: Is the process listening? On which address? Is a firewall in the way? (Firewalls are a follow-on topic; knowing ss comes first.)

5. DNS quick check#

getent hosts example.com

Lab run (IPv6 answers shown):

2606:4700:10::ac42:93f3 example.com
2606:4700:10::6814:179a example.com

Optional tools (install if needed): host example.com, dig example.com, nslookup example.com.

If curl works by IP but not by name, suspect DNS. If neither works, suspect route/firewall/connectivity.

6. Mini troubleshooting order#

1. ip -br addr — do I have an address?
2. ip route — is there a default gateway?
3. ping gateway or curl a known site.
4. getent hosts — do names resolve?
5. ss -tln — is my service listening where I expect?

Write that order on a sticky note; it saves hours over random guessing.

Common mistakes#

Hands-on lab (30–45 minutes)#

1. Record hostname and ip -br addr in your notes.
2. Identify default gateway from ip route.
3. curl -sI https://example.com and save the status line.
4. ss -tln | head and circle one loopback and one non-loopback listen (if present).
5. getent hosts example.com.
6. Stretch: Install iputils-ping (M16) and ping -c 3 your gateway.

Practice: check your understanding#

Multiple choice#

Q1. Which command shows a brief list of interfaces and IPs on modern Linux?

• A. chmod -br addr
• B. ip -br addr
• C. tar -br addr
• D. dnf -br addr

Answer: B.

Q2. A line default via 192.168.1.1 dev eth0 means:

• A. eth0 is down
• B. Packets to other networks go via gateway 192.168.1.1 on eth0
• C. DNS is disabled
• D. UID 192 is the network owner

Answer: B.

Q3. ss -tln is primarily for:

• A. Creating tar archives
• B. Listing listening TCP sockets
• C. Installing packages
• D. Editing sudoers

Answer: B.

Flashcards#

Match the columns#

Fill in the blank#

1. Brief addresses: ip ___ addr
   Answer: -br

2. Three pings: ping ___ 127.0.0.1
   Answer: -c 3

3. TCP listeners: ss ___
   Answer: -tln

4. Headers from example.com: curl ___ https://example.com
   Answer: -sI (or -I; -s silences progress)

Order the steps — “why can’t I reach the web?”#

1. Check ip -br addr for an address on the NIC
2. Check ip route for a default gateway
3. Probe gateway (ping) or try curl -sI https://example.com
4. If IP works but names fail, check getent hosts
5. If your server is unreachable to others, check ss -tln and firewall docs

Correct order: 1 → 2 → 3 → 4 → 5

Mini terminal challenges#

Challenge: Show IP and curl#

Goal: Document your address and prove outbound HTTPS.

Tasks:

1. Save hostname and ip -br addr to /tmp/linux-lab-pushpjeet/m18/net-notes.txt.
2. Append the first status line from curl -sI https://example.com.

Verify: File contains an interface line and HTTP/… 200 (or another informative status).

Stretch: Append ss -tln | wc -l (how many listen lines?).

Challenge: Gateway identity#

Goal: From ip route, write down the gateway IP and which device it uses.

Verify: ip route get 1.1.1.1 mentions that device (wording varies).

Module wrap#

You can report addresses and routes with ip, probe reachability with ping/curl, list listeners with ss, and resolve names with getent hosts. That completes the Linux from Scratch expansion track through M18.

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