Firewall basics (ufw, iptables) in Linux CLI - Step-by-Step Execution

Concept Flow - Firewall basics (ufw, iptables)

Start: No rules

↓

Add rule with ufw or iptables

↓

Rule stored in firewall config

↓

Firewall active: packets checked

↓

Packet matches rule?

No→Block or default action

Yes↓

Allow or reject packet

↓

End: Packet processed

The firewall starts with no rules, then rules are added using ufw or iptables. When active, each network packet is checked against these rules to decide if it is allowed or blocked.

Execution Sample

Linux CLI

sudo ufw allow 22
sudo ufw enable
sudo ufw status
sudo iptables -L

This sequence adds a rule to allow SSH (port 22), enables ufw, shows ufw status, then lists iptables rules.

Execution Table

Step	Command	Action	Result/Output
1	sudo ufw allow 22	Add rule to allow incoming TCP on port 22	Rule added: '22/tcp' ALLOW IN
2	sudo ufw enable sudo ufw status	Activate firewall and show current ufw rules	Status: active 22/tcp ALLOW IN Anywhere
3	sudo iptables -L	List iptables rules	Chain INPUT (policy DROP) ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
4	Packet arrives on port 22	Check rules	Packet allowed (matches rule)
5	Packet arrives on port 80	Check rules	Packet blocked (no matching allow rule)

💡 Firewall processes packets until all rules checked; unmatched packets follow default policy (block or allow).

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
ufw_rules	empty	allow 22/tcp added	show 22/tcp allow	unchanged	22/tcp allow
iptables_rules	default DROP	unchanged	INPUT chain has ACCEPT tcp dpt:ssh	unchanged	INPUT chain ACCEPT tcp dpt:ssh
packet_port_22	N/A	N/A	N/A	N/A	allowed
packet_port_80	N/A	N/A	N/A	N/A	blocked

Key Moments - 3 Insights

Why does 'sudo ufw allow 22' let SSH through but not HTTP on port 80?

Because the rule explicitly allows port 22 only. The execution_table row 5 shows packets on port 80 are blocked since no rule allows them.

What is the difference between ufw and iptables commands here?

ufw is a simpler interface to manage firewall rules, while iptables shows the detailed rules. Execution_table rows 2 and 3 show ufw status and iptables listing respectively.

What happens if a packet does not match any rule?

It follows the default policy, which is usually to block. Execution_table row 5 shows a packet on port 80 blocked due to no matching rule.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of 'sudo ufw status' at step 2?

A Status: active 22/tcp ALLOW IN Anywhere

B Status: inactive

C No rules configured

D Error: command not found

💡 Hint

Check execution_table row 2 under Result/Output.

At which step does the firewall allow packets on port 22?

A Step 1

B Step 4

C Step 5

D Step 3

💡 Hint

Look at execution_table row 4 where packet on port 22 is processed.

If you add a rule to allow port 80, how would the packet on port 80 be handled?

A Firewall would crash

B It would still be blocked

C It would be allowed

D Packet would be rejected with error

💡 Hint

Refer to variable_tracker row for packet_port_80 and how rules affect packet handling.

Concept Snapshot

Firewall basics:
- Use 'ufw allow <port>' to add simple rules
- Use 'ufw status' to see rules
- 'iptables -L' shows detailed rules
- Packets checked against rules in order
- Default policy blocks unmatched packets
- ufw is easier; iptables is detailed

Full Transcript

This visual execution shows how firewall rules are added and processed using ufw and iptables. First, a rule is added to allow SSH on port 22 using 'sudo ufw allow 22'. Then, 'sudo ufw status' confirms the rule is active. 'sudo iptables -L' lists the detailed rules, showing the ACCEPT rule for SSH. When a packet arrives on port 22, it matches the allow rule and is accepted. A packet on port 80 does not match any rule and is blocked by default. Variables track the rules and packet states step-by-step. Key moments clarify why only port 22 is allowed and the difference between ufw and iptables. The quiz tests understanding of rule effects and outputs. This helps beginners see how firewall rules control network traffic simply and clearly.