Linux Networking Important commands | Linux Network Configuration & Maintenance

Linux Networking Challenge - Config & Maintenance

📡 Linux Networking Challenge: Configuration & Maintenance

Test your knowledge of essential commands, configuration files, firewall management, and troubleshooting for Linux networking. **Pass with 80% to proceed to the next difficulty level.**

Tools & Interfaces: 0/10 (0%)

Routing & DNS: 0/10 (0%)

Firewall & Advanced: 0/10 (0%)

Level 1: Networking Tools & Interfaces

1.1 Which command is the modern, preferred utility for configuring IP addresses, routes, and interfaces on most current Linux distributions (replacing `ifconfig`)?

A. `netstat` B. `ip` C. `route`

**Correct Answer: B. `ip`**. The **`ip`** command (part of the iproute2 suite) is the standard modern tool, used for tasks like `ip addr show` and `ip route add`.

1.2 What does the command `ip a s eth0` display?

A. The current routing table for `eth0`. B. Active TCP/UDP sockets on `eth0`. C. IP address, broadcast address, and link status of the `eth0` interface.

**Correct Answer: C.** The `ip addr show` (or `ip a s`) command displays the configuration of network interfaces, including the **IP address** and **link status**.

1.3 To bring the `eth1` network interface up temporarily without making the change permanent, which command is most appropriate?

A. `ip set eth1 up` B. `ip link set eth1 up` C. `systemctl enable eth1`

**Correct Answer: B. `ip link set eth1 up`**. This command modifies the link state (Layer 2) of the interface, effectively turning it on.

1.4 Which file is typically used on Debian/Ubuntu systems to manage persistent network interface configurations?

A. `/etc/sysconfig/network-scripts/ifcfg-eth0` B. `/etc/network/interfaces` C. `/etc/netplan/*.yaml`

**Correct Answer: B. `/etc/network/interfaces`**. This is the traditional file used by the `ifupdown` package on Debian-based systems. (Note: Modern Ubuntu often uses Netplan, option C).

1.5 What is the purpose of the MAC address in networking?

A. To provide a logical address for routing across the internet. B. To manage multicast groups. C. To provide a unique hardware address for communication within a local network segment (Layer 2).

**Correct Answer: C.** The **MAC (Media Access Control) address** is a physical address used for local network communication (Layer 2), distinct from the IP address (Layer 3).

1.6 Which command would you use to view all active TCP and UDP connections and listening ports on your system?

A. `arp -a` B. `ss -tuln` (or `netstat -tuln`) C. `ip sockets`

**Correct Answer: B. `ss -tuln`**. The **`ss`** (Socket Statistics) command is the modern successor to `netstat` and is used to examine active connections (`t` for TCP, `u` for UDP, `l` for listening, `n` for numeric).

1.7 The `ping` command uses which Internet Protocol to test connectivity?

A. TCP B. ICMP (Internet Control Message Protocol) C. UDP

**Correct Answer: B. ICMP**. `ping` sends ICMP Echo Request packets and waits for ICMP Echo Reply packets to measure network reachability and latency.

1.8 Which file contains the hostname of the local Linux machine?

A. `/etc/resolv.conf` B. `/etc/hosts` C. `/etc/hostname`

**Correct Answer: C. `/etc/hostname`**. This file contains a single line specifying the machine's hostname. `/etc/hosts` maps hostnames to IP addresses.

1.9 You need to persistently assign a static IP address to an interface using NetworkManager. Which command would you use?

A. `ip addr add static ...` B. `nmcli con mod "connection_name" ipv4.address "ip/mask" ipv4.method manual` C. `netplan apply static`

**Correct Answer: B. `nmcli...`**. The **`nmcli`** (NetworkManager Command Line Interface) tool is used for persistent configuration management when NetworkManager is active.

1.10 What is the primary purpose of the ARP (Address Resolution Protocol) cache?

A. To map hostnames to IP addresses. B. To map **IP addresses** to **MAC addresses** for local network communication. C. To store the default gateway IP address.

**Correct Answer: B.** The ARP cache maintains a table of IP-to-MAC address mappings, which is necessary for data to be correctly encapsulated at the data-link layer (Layer 2).

**End of Tools & Interfaces. Click "Next Level" to continue.**

Level 2: Routing & DNS

2.1 How would you view the Linux kernel's IP routing table?

A. `ip routeshow` B. `netstat -r` C. `ip route show` (or `route -n`)

**Correct Answer: C. `ip route show`**. This command displays the current routing table entries, showing how the kernel will send traffic destined for different networks.

2.2 The default gateway is the entry in the routing table for which destination network?

A. The local loopback network (127.0.0.0/8) B. The `default` network (or `0.0.0.0/0`) C. The local subnet

**Correct Answer: B. The `default` network**. This route specifies the gateway for all traffic destined for networks not explicitly listed in the routing table (i.e., external traffic to the internet).

2.3 Which command would you use to trace the path a packet takes to reach the host `www.example.com`?

A. `pathfind www.example.com` B. `ip route trace www.example.com` C. `traceroute www.example.com` (or `tracepath`)

**Correct Answer: C. `traceroute`**. This tool identifies the path, measuring the time delay and displaying the IP addresses of the routers (hops) along the way.

2.4 Which file contains the list of DNS servers used by the local machine for name resolution?

A. `/etc/hosts` B. `/etc/resolv.conf` C. `/etc/dns.conf`

**Correct Answer: B. `/etc/resolv.conf`**. This file lists the IP addresses of the nameservers (DNS servers) that the system queries to resolve hostnames.

2.5 The command `dig @8.8.8.8 server.example.com` is used to:

A. Perform a Reverse DNS lookup. B. Check the status of the local DNS caching service. C. Query the IP address of `server.example.com` specifically using the Google DNS server (8.8.8.8).

**Correct Answer: C.** **`dig`** (Domain Information Groper) is a powerful tool. The `@8.8.8.8` syntax specifies that the query should be sent directly to that nameserver, bypassing the system's `/etc/resolv.conf` settings.

2.6 Which file can be edited to provide static, local hostname-to-IP mappings, overriding external DNS lookups?

A. `/etc/nsswitch.conf` B. `/etc/hosts` C. `/etc/hostname`

**Correct Answer: B. `/etc/hosts`**. Entries in this file are typically checked *before* external DNS is queried, allowing local overrides and definitions for hosts that are not in DNS.

2.7 The parameter metric in a routing table entry indicates:

A. The speed of the interface. B. The cost or preference of the route, where lower numbers are preferred. C. The destination network's netmask.

**Correct Answer: B.** When two or more routes exist for the same destination, the kernel uses the **lowest metric** to determine which route to use.

2.8 What is the command to view and clear the local DNS cache used by systems running `systemd-resolved`?

A. `dig -flush` B. `resolvectl statistics` and `resolvectl flush-caches` C. `systemctl restart dnsmasq`

**Correct Answer: B. `resolvectl`**. The **`resolvectl`** utility is the command-line client for the `systemd-resolved` DNS/name resolution service, which maintains a local cache.

2.9 When performing a host lookup, the `nsswitch.conf` file determines:

A. The default gateway IP address. B. The order in which name resolution sources (e.g., files, DNS) are consulted. C. Whether to use IPv4 or IPv6.

**Correct Answer: B. The order...**. The **`nsswitch.conf`** file configures the Name Service Switch, which controls how the system looks up hostnames, passwords, groups, and other information.

2.10 What command is used to add a static route for the network 192.168.50.0/24 via the gateway 192.168.1.1?

A. `route add net 192.168.50.0 gw 192.168.1.1` B. `ip route add 192.168.50.0/24 via 192.168.1.1 dev eth0` C. Both A and B are valid (though B is the modern, preferred method).

**Correct Answer: C.** Both **`route`** (older net-tools) and **`ip route`** (modern iproute2) can achieve this result, though `ip route` is recommended.

**End of Routing & DNS. Click "Next Level" to continue.**

Level 3: Firewall & Advanced Concepts

3.1 Which utility is used to manage the Linux kernel's firewall rules (Netfilter) on modern RHEL/CentOS systems?

A. `ufw` B. `firewall-cmd` (Firewalld) C. `iptables`

**Correct Answer: B. `firewall-cmd`**. **Firewalld** is the default firewall management service on Red Hat-based systems, using the `firewall-cmd` command-line tool.

3.2 In firewall configuration (Netfilter), which of the following is NOT one of the three primary built-in tables?

A. `filter` B. `route` C. `nat`

**Correct Answer: B. `route`**. The three main tables are **`filter`** (default for filtering), **`nat`** (Network Address Translation), and **`mangle`** (packet modification).

3.3 In `firewall-cmd`, what is a Zone primarily used for?

A. To define the geographic location of the server. B. To manage IPv6 traffic rules. C. To define a set of trust levels for different network connections (e.g., public, home, internal).

**Correct Answer: C.** Zones simplify firewall management by applying a predefined, sensible set of rules (like allowing SSH and DHCP) to an interface based on its location/trust level.

3.4 You want to forward incoming HTTP (port 80) traffic to a different internal port (8080) on the same server. This is an example of:

A. SNAT (Source NAT) B. Filtering C. DNAT (Destination NAT) / Port Forwarding

**Correct Answer: C. DNAT / Port Forwarding**. Since you are changing the destination port, this is a form of Destination Network Address Translation (DNAT), commonly called port forwarding.

3.5 What is the kernel mechanism that allows a Linux system to act as a router and forward packets between two different network interfaces?

A. Source Routing B. Bridging C. IP Forwarding (enabled via `net.ipv4.ip_forward=1`)

**Correct Answer: C.** IP forwarding must be explicitly enabled in the kernel settings (often via the `sysctl` command or `/etc/sysctl.conf`) for the Linux machine to behave as a router.

3.6 The utility `tcpdump` is primarily used for what purpose?

A. Restarting the TCP service. B. Intercepting and analyzing network traffic in real-time on a specific interface. C. Benchmarking TCP throughput.

**Correct Answer: B.** **`tcpdump`** is a packet analyzer. It captures packets matching given rules and displays their contents, essential for diagnosing complex network issues.

3.7 When setting up an Ethernet bridge (`br0`), which layer of the OSI model is being leveraged to group interfaces?

A. Layer 1 (Physical) B. Layer 3 (Network) C. Layer 2 (Data Link)

**Correct Answer: C.** Bridging operates at **Layer 2** (Data Link). It creates a virtual switch, allowing multiple physical or virtual interfaces to function as if they were on the same network segment.

3.8 Which `nmap` option is typically used for a stealthy, less noisy port scan?

A. `-sV` (Version detection) B. `-sS` (SYN scan or Stealth scan) C. `-p-` (All ports)

**Correct Answer: B. `-sS` (SYN scan)**. This scan sends only the SYN packet and watches for the reply, avoiding the full TCP handshake, making it less likely to be logged by the target system.

3.9 The MTU (Maximum Transmission Unit) setting on a network interface determines:

A. The maximum speed of the interface (e.g., 100 Mbps). B. The maximum number of simultaneous connections. C. The largest size of a frame (including data and header) that can be sent in one single packet without fragmentation.

**Correct Answer: C.** The **MTU** setting (usually 1500 bytes for Ethernet) is critical; mismatched MTUs between connected devices can cause packets to be dropped or fragmented, leading to performance issues.

3.10 When troubleshooting a service listening on a specific port, what is the best command to confirm the service is running and on which IP address it is listening?

A. `ps aux | grep service_name` B. `telnet server_ip port` C. `ss -tlnp` (or `netstat -tlnp`) | `grep port_number`

**Correct Answer: C. `ss -tlnp`**. This command explicitly shows the **listening sockets** (`-l`), their protocol (`-t` for TCP), the **process ID/name** (`-p`), and the local **IP/port** (essential for checking if it's bound to 0.0.0.0, 127.0.0.1, or a specific IP).

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

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Related Topics & Tags

Linux Networking IPRoute2 Firewalld TCPDump DNS Routing SysAdmin IP ICMP

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