Linux Iptables & firewalld interview question and answers | quiz test

Linux Firewall Challenge - iptables & firewalld

🔒 Linux Firewall Challenge: iptables & firewalld

Test your knowledge of the fundamental concepts, chains, actions, and management of the two major Linux firewall solutions. **Pass with 80% to proceed to the next difficulty level.**

iptables Basics: 0/10 (0%)

firewalld Basics: 0/10 (0%)

Advanced Concepts: 0/10 (0%)

Level 1: iptables Fundamentals

1.1 In iptables, what is the default action a packet takes if it does not match any rule in a chain?

A. It is always accepted (ACCEPT). B. It follows the chain's **Policy** (usually ACCEPT or DROP). C. It is sent to the next chain.

**Correct Answer: B.** The **Policy** is the ultimate fate of a packet that traverses the chain without matching a target.

1.2 Which of the following is NOT a standard built-in iptables chain?

A. `INPUT` C. `BRIDGE` B. `PREROUTING`

**Correct Answer: B. `BRIDGE`**. The standard chains are `PREROUTING`, `INPUT`, `FORWARD`, `OUTPUT`, and `POSTROUTING`.

1.3 Which iptables chain handles packets destined for the local system itself?

A. `OUTPUT` B. `INPUT` C. `FORWARD`

**Correct Answer: B. `INPUT`**. Packets entering the local system pass through the `INPUT` chain. `FORWARD` is for packets passing *through* the system.

1.4 What is the command-line flag used in iptables to append a new rule to the end of a specified chain?

A. `-I` (Insert) B. `-A` (Append) C. `-L` (List)

**Correct Answer: B. `-A` (Append)**. `-I` (Insert) places the rule at the beginning (or a specific line number).

1.5 Which action target simply drops the packet silently, without sending any response to the sender?

A. `REJECT` B. `ACCEPT` C. `DROP`

**Correct Answer: C. `DROP`**. `REJECT` sends an error message (like an ICMP Port Unreachable), which reveals the firewall's presence. `DROP` makes the packet vanish.

1.6 The iptables command `iptables -A INPUT -p tcp --dport 22 -j ACCEPT` opens which service?

A. HTTP (Port 80) B. DNS (Port 53) C. SSH (Port 22)

**Correct Answer: C. SSH (Port 22)**. `--dport 22` specifies the destination port 22, the standard port for Secure Shell (SSH).

1.7 What is the role of the `-m state --state RELATED,ESTABLISHED` rule, often used in the INPUT chain?

A. To allow connections only from the local network. B. To allow incoming traffic that is part of an already established or related connection (connection tracking). C. To limit the number of connections per minute from a single IP.

**Correct Answer: B.** This rule is critical for allowing replies to outbound requests (e.g., browsing a webpage) without having to explicitly open high-numbered reply ports.

1.8 Which command would you use to view the currently active iptables rules with numbering?

A. `iptables -check` B. `iptables -L -n -v --line-numbers` C. `iptables -L -show`

**Correct Answer: B.** `-L` (List), `-n` (numeric output), `-v` (verbose), and `--line-numbers` are standard flags for listing and managing rules by number.

1.9 Which iptables table is responsible for implementing Source Network Address Translation (SNAT) and Destination Network Address Translation (DNAT)?

A. `filter` table B. `nat` table C. `mangle` table

**Correct Answer: B. `nat` table**. The `nat` table handles network address translation, which modifies the source or destination IP/port of a packet.

1.10 In iptables, what happens to rules created in the running configuration if you do not save them?

A. They are automatically backed up to a default file. B. They are lost upon system reboot. C. They persist indefinitely until manually flushed.

**Correct Answer: B.** iptables rules are non-persistent by default, living only in kernel memory. Tools like `iptables-save` and `iptables-restore` are needed to manage persistence.

**End of iptables Basics. Click "Next Level" to continue.**

Level 2: firewalld Fundamentals

2.1 What is the primary conceptual difference between `iptables` and `firewalld`?

A. `iptables` uses service names; `firewalld` uses port numbers. B. `firewalld` is a dynamic daemon that manages rules using **zones**; `iptables` is a static tool that modifies kernel tables directly. C. `iptables` only supports IPv4; `firewalld` only supports IPv6.

**Correct Answer: B.** The dynamic nature and the concept of **zones** (trusted, home, public, etc.) are central to `firewalld`.

2.2 In `firewalld`, what concept is used to define the level of trust for a network interface or source IP address?

A. Chains C. Policies B. Zones

**Correct Answer: C. Zones**. Zones (`public`, `internal`, `trusted`) are used to group interfaces and determine which rules apply based on the security context.

2.3 To permanently allow incoming HTTP traffic (Port 80) on the `public` zone using `firewall-cmd`, what is the correct command?

A. `firewall-cmd --add-port=80/tcp --zone=public` B. `firewall-cmd --permanent --zone=public --add-service=http` C. `firewall-cmd --zone=public --set-rule=http`

**Correct Answer: B.** **`--permanent`** is essential for persistence across reboots. Using **`--add-service=http`** is cleaner than `--add-port=80/tcp`.

2.4 After adding a permanent rule using `firewall-cmd --permanent`, what final step is necessary for the change to become active immediately?

A. Reboot the system. B. Run `firewall-cmd --reload`. C. The change is effective immediately.

**Correct Answer: B.** Permanent changes are written to disk (`/etc/firewalld/`). **`--reload`** reads these changes into the running configuration without interrupting established connections.

2.5 Which command lists all the active zones and the network interfaces currently bound to them?

A. `firewall-cmd --list-interfaces` C. `firewall-cmd --get-active-zones` B. `firewall-cmd --list-all`

**Correct Answer: C. `firewall-cmd --get-active-zones`**. This is the quickest way to see which zones are currently being used and by which interfaces.

2.6 What is the default zone used for any new interface or connection that isn't explicitly assigned a zone?

A. `trusted` B. `public` C. `default`

**Correct Answer: B. `public`**. The **`public`** zone is often the default, as it represents external networks and has a high-security setting. Note: The actual default zone can be changed.

2.7 Which `firewall-cmd` command would you use to temporarily close a previously added port (e.g., port 8080/tcp) in the running configuration?

A. `firewall-cmd --purge-port=8080/tcp` C. `firewall-cmd --remove-port=8080/tcp` B. `firewall-cmd --delete-port=8080/tcp`

**Correct Answer: C. `firewall-cmd --remove-port=8080/tcp`**. The option is **`--remove-port`**. Since `--permanent` is not specified, the removal is only temporary (until reload or reboot).

2.8 What is a service in `firewalld` terms (e.g., `ssh`, `http`)?

A. An alias for an IP address. B. A predefined XML file that defines the ports and protocols required by a common application. C. A systemd unit file.

**Correct Answer: B.** Services simplify firewall configuration. Instead of opening TCP 22 and UDP 22 manually, you just add the `ssh` service.

2.9 The `firewalld` command `firewall-cmd --set-default-zone=home` changes which behavior?

A. It sets the zone for the currently active interface. C. It sets the zone that will be used by any network interface that has not been explicitly assigned a zone. B. It applies the 'home' zone's rules to all other zones.

**Correct Answer: C.** The default zone is the fallback zone for any connection that cannot be categorized, like an interface without a specified zone.

2.10 Which `firewalld` zone is typically used for a system where you trust all the computers on the network and want maximum openness?

A. `home` B. `internal` C. `trusted`

**Correct Answer: C. `trusted`**. The `trusted` zone is the most permissive, often allowing all incoming traffic. The `home` zone is slightly more restrictive.

**End of firewalld Basics. Click "Next Level" to continue.**

Level 3: Advanced Firewall Concepts

3.1 What is the primary purpose of the `FORWARD` chain in iptables?

A. To manage traffic originating from the firewall host. C. To manage traffic that passes *through* the firewall host, destined for another network (router/gateway function). B. To manage traffic destined for the firewall host itself.

**Correct Answer: C.** The **`FORWARD`** chain is only processed when the Linux box is acting as a router between two different networks.

3.2 Which iptables target is used for performing Destination Network Address Translation (DNAT) (Port Forwarding)?

A. `MASQUERADE` B. `DNAT` C. `SNAT`

**Correct Answer: B. `DNAT`**. DNAT is used in the `PREROUTING` chain of the `nat` table to change the destination IP/port of an incoming packet.

3.3 In `firewalld`, what is the term for a temporary block on an IP address that attempts to connect too often?

A. Permanent drop B. **Rich Rule** (with rate limiting) C. **Lockdown**

**Correct Answer: B.** A **Rich Rule** allows for advanced logic, including rate limiting connections per time period, which helps prevent brute-force attacks.

3.4 Which iptables command would flush (delete) all rules in all chains of the default `filter` table?

A. `iptables --reset` C. `iptables -F` B. `iptables -X`

**Correct Answer: C. `iptables -F`**. The `-F` (Flush) command deletes all rules in the specified (or default) chains. `-X` deletes *non-default* chains.

3.5 When using iptables for Source NAT (e.g., sharing internet connection from a local network), which specific target is often used instead of `SNAT` when the source IP address is dynamic?

A. `REDIRECT` B. `MASQUERADE` C. `DOCKER`

**Correct Answer: B. `MASQUERADE`**. `MASQUERADE` automatically uses the IP address of the outgoing interface, making it ideal for typical NAT scenarios where the public IP might change (like with DHCP).

3.6 What is the purpose of the `firewall-cmd --panic-on` command?

A. It starts the firewall daemon if it's currently stopped. B. It applies the least restrictive set of rules (e.g., `trusted` zone). C. It drops all incoming and outgoing packets immediately, effectively shutting down all network traffic except traffic to the firewall host.

**Correct Answer: C.** Panic Mode is an emergency lockdown, dropping all packets instantly. It can be disabled with `firewall-cmd --panic-off`.

3.7 In the iptables `filter` table, what is the default order of processing chains for an incoming packet destined for a local service?

A. `PREROUTING` -> `FORWARD` -> `OUTPUT` B. `PREROUTING` -> `INPUT` -> **(Local Application)** -> `OUTPUT` -> `POSTROUTING` C. `INPUT` -> `OUTPUT` -> `FORWARD`

**Correct Answer: B.** The correct path for locally destined traffic is: **`PREROUTING` (NAT before routing) -> Routing decision -> `INPUT` (filtering) -> Local Process -> `OUTPUT` (filtering) -> `POSTROUTING` (NAT after routing).**

3.8 What is the command used to install the persistence package necessary to save iptables rules permanently on a Debian/Ubuntu system?

A. `sudo apt install netfilter-save` B. `sudo apt install iptables-persistent` C. `sudo apt install firewall-saver`

**Correct Answer: B. `sudo apt install iptables-persistent`**. This package includes the necessary services and configuration files to save and restore rules automatically on boot/shutdown.

3.9 What is the purpose of the `conntrack` system often used with firewalls?

A. It tracks which network interfaces are active. C. It tracks the state of network connections, allowing firewalls to easily permit traffic that belongs to an existing session (like the `ESTABLISHED` state). B. It tracks the number of dropped packets.

**Correct Answer: C.** **Connection Tracking (`conntrack`)** is fundamental for **stateful firewalls**, enabling secure, efficient handling of two-way traffic.

3.10 In `firewalld`, where are custom service definition files typically stored?

A. `/usr/lib/firewalld/services/` B. `/var/firewalld/services/` C. `/etc/firewalld/services/`

**Correct Answer: C. `/etc/firewalld/services/`**. The `/etc/` directory is used for user-defined configuration files to override or supplement the system-provided files in `/usr/lib/`.

**Firewall Mastered! Click "Finish Quiz" to see your final summary.**

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Related Topics & Tags

Linux Security iptables firewalld NAT Networking Zones Chains SysAdmin Netfilter DNAT AccessControl

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