LFCS Networking Practice Test 6 — 15 Questions

Question 1

A server needs to forward packets between two networks: 10.0.1.0/24 on eth0 and 10.0.2.0/24 on eth1. Which sysctl parameter must be enabled?

Accepted Answer

net.ipv4.ip_forward = 1. Enabling `net.ipv4.ip_forward = 1` allows the Linux kernel to forward IP packets between network interfaces, which is required for a server to route traffic between the 10.0.1.0/24 and 10.0.2.0/24 subnets. Without this parameter, the kernel drops any packet not destined for the local system, preventing inter-network communication.

Answer

net.ipv4.conf.all.rp_filter = 1

Answer

net.ipv4.conf.all.accept_source_route = 1

Answer

net.ipv4.conf.all.send_redirects = 0

Question 2

Which TWO commands can be used to display the routing table on a Linux system?

Accepted Answer

route -n. The `route -n` command displays the kernel IP routing table with numeric addresses, showing destination, gateway, netmask, and interface. The `ip route show` command is the modern equivalent from the iproute2 suite, which also displays the routing table with more detailed information. Both are standard tools for viewing routing decisions on a Linux system.

Answer

ip addr show

Answer

arp -a

Answer

ss -tln

Question 3

Which THREE conditions must be met for a Linux system to function as a router between two networks?

Accepted Answer

Each interface has an IP address in the respective subnet. Each interface must have an IP address in its respective subnet for the Linux system to receive packets from that network and forward them to the other. Without an IP address in the subnet, the interface cannot participate in ARP resolution or routing decisions for that network.

Answer

Both interfaces have the same MAC address

Answer

The system is configured as the default gateway for both networks

Question 4

Which TWO commands can be used to resolve a hostname to an IP address?

Accepted Answer

host. The `host` command is a simple DNS lookup utility that queries DNS servers to resolve a hostname to an IP address. It directly performs forward DNS resolution using the system's configured resolvers and returns the A or AAAA record for the given name.

Answer

traceroute

Answer

ping

Answer

ifconfig

Question 5

An administrator is unable to SSH into the server from a remote host at 192.168.1.100. Based on the exhibited iptables rules, what is the most likely reason?

Accepted Answer

The SSH rule only allows connections from 10.0.1.0/24, and 192.168.1.100 is not in that subnet. The exhibited iptables rules show an SSH rule that explicitly accepts incoming TCP traffic on port 22 only from the source subnet 10.0.1.0/24. The remote host at 192.168.1.100 is not within that subnet, so the SSH rule does not match, and the packet will fall through to the next rule or the default policy. Since no other rule permits SSH from 192.168.1.100, the connection is implicitly dropped or rejected, preventing SSH access.

Answer

SSH is not allowed from any source

Answer

The INPUT chain policy is ACCEPT, so SSH should be allowed

Answer

The DROP rule for SSH is not matching because of packet count zero

Question 6

You are a systems administrator for a company that runs a critical application on a Linux server with two network interfaces: eth0 (public IP 203.0.113.10/24, gateway 203.0.113.1) and eth1 (private IP 10.0.1.10/24, no gateway). The server must be accessible via SSH (port 22) from the internet, but only from a specific management subnet 198.51.100.0/24. Additionally, the server should be able to access the internet for package updates, but no other inbound traffic from the internet is allowed. The local firewall is iptables. After implementing rules, you find that the server cannot reach the internet (e.g., ping 8.8.8.8 fails), but SSH from the management subnet works. What is the most likely cause?

Accepted Answer

The iptables rules do not include a rule to allow established and related connections. Iptables is stateful: by default, the INPUT chain processes only the first packet of a connection. Without a rule allowing established and related connections (e.g., `-m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT`), return traffic from the server's outbound internet requests (e.g., ping to 8.8.8.8) is blocked by the INPUT chain, causing the failure. SSH from the management subnet works because the initial SYN packet is allowed, but the server's outbound traffic fails because the response packets are not recognized as part of an allowed flow.

Answer

The server's DNS resolver is not configured

Answer

The SSH rule is misconfigured and accidentally blocks all traffic

Answer

The default policy on the INPUT chain is DROP

Question 7

A company has a server with two network interfaces: eth0 (192.168.1.10/24, gateway 192.168.1.1) and eth1 (10.0.0.10/24, gateway 10.0.0.1). The server needs to reach a remote network 172.16.0.0/16 via a VPN tunnel that terminates at 10.0.0.5 on eth1. Which command should be used to add a route for this traffic?

Accepted Answer

ip route add 172.16.0.0/16 via 10.0.0.5 dev eth1. The VPN tunnel endpoint is at 10.0.0.5 on the eth1 network, so traffic to 172.16.0.0/16 must be forwarded via that next-hop IP address using the eth1 interface. The `ip route add` command with `via 10.0.0.5 dev eth1` explicitly sets the gateway and egress interface, ensuring packets are sent through the VPN tunnel.

Answer

ip route add 172.16.0.0/16 via 10.0.0.5 dev eth0

Answer

ip route add 172.16.0.0/16 via 192.168.1.1 dev eth0

Answer

ip route add 172.16.0.0/16 dev eth1

Question 8

A system administrator is troubleshooting network connectivity from a server that can reach internal resources but cannot access the internet. The server's /etc/sysconfig/network-scripts/ifcfg-eth0 file contains: BOOTPROTO=static, IPADDR=10.0.0.10, NETMASK=255.255.255.0, GATEWAY=10.0.0.1. The administrator runs 'ip route show' and sees: default via 10.0.0.1 dev eth0. However, 'ping 8.8.8.8' fails. Which is the most likely cause?

Accepted Answer

The gateway 10.0.0.1 is not configured to forward traffic to the internet (no NAT or upstream route).. The routing table shows a default gateway (10.0.0.1) is present, so the issue is not a missing route. Since the server can reach internal resources but not the internet, the most likely cause is that the gateway itself (10.0.0.1) is not configured to perform NAT or does not have an upstream route to forward traffic beyond the local subnet. Without this, packets destined for 8.8.8.8 are sent to the gateway but are then dropped because the gateway has no path to the internet.

Answer

The default gateway is missing from the routing table.

Answer

DNS resolution is not configured.

Answer

A firewall is blocking outbound ICMP traffic.

Question 9

A system administrator needs to permanently configure a network interface named ens33 with a static IPv4 address of 192.168.1.100/24 and a default gateway of 192.168.1.1 on a system using NetworkManager. Which command should the administrator use to achieve this?

Accepted Answer

nmcli connection modify 'ens33' ipv4.addresses 192.168.1.100/24 ipv4.gateway 192.168.1.1 ipv4.method manual. It uses the `nmcli` command to modify the NetworkManager connection profile for interface ens33, setting a static IPv4 address with CIDR notation and a default gateway, and explicitly setting the method to 'manual' to ensure the configuration persists across reboots. NetworkManager is the default network service on modern Linux distributions, and `nmcli` is the proper tool for permanent configuration changes.

Answer

ip addr add 192.168.1.100/24 dev ens33

Answer

ifconfig ens33 192.168.1.100 netmask 255.255.255.0 up

Answer

route add default gw 192.168.1.1 ens33

Question 10

Which TWO are valid methods to configure a network interface on a Linux system?

Accepted Answer

Editing /etc/network/interfaces. Option C is correct because /etc/network/interfaces is the standard configuration file used by the ifupdown toolset on Debian-based Linux distributions to define network interfaces, their addressing (static or DHCP), and related parameters. Option D is correct because nmcli connection add is the NetworkManager command-line method for creating a new connection profile that configures an interface, including its IP addressing, gateway, and DNS settings. Option A is not a valid interface configuration method because sysctl only tunes kernel runtime parameters such as net.ipv4.conf.eth0.forwarding (packet forwarding), not interface addressing or link settings. Option B is not valid because systemctl enable network.service merely sets the legacy network service to start at boot; it does not itself configure an interface. Option E is not valid because /etc/sysconfig/network is a Red Hat-style file that historically held global hostname and gateway settings, not per-interface configuration.

Answer

Using sysctl to set net.ipv4.conf.eth0.forwarding

Answer

Using systemctl enable network.service

Answer

Editing /etc/sysconfig/network

Question 11

Which THREE are common tools used for network troubleshooting on Linux?

Accepted Answer

traceroute. traceroute (A) is a standard Linux network diagnostic tool that maps the hop-by-hop path packets take to a destination by manipulating the IP TTL field and reading ICMP Time Exceeded replies, making it essential for locating routing problems. ping (C) is a core troubleshooting utility that sends ICMP Echo Request packets and measures Echo Reply responses to verify basic IP reachability and round-trip latency to a host. tcpdump (D) is a packet capture and analysis tool that uses libpcap to inspect live traffic at the frame/packet level, which is indispensable for diagnosing protocol-level and connectivity issues. The remaining options are not network troubleshooting tools: fdisk (B) is a disk partitioning utility for managing block devices, and useradd (E) is an account management command for creating local user accounts.

Answer

fdisk

Answer

useradd

Question 12

Order the steps to configure a static IP address on a CentOS/RHEL 7 system using ifcfg files.

Accepted Answer

Edit the ifcfg file, then restart the network service, then verify the configuration.. Static IP configuration requires editing the ifcfg file, restarting network, and verification.

Answer

Restart the network service, then edit the ifcfg file, then verify the configuration.

Answer

Edit the ifcfg file, then verify the configuration, then restart the network service.

Answer

Verify the configuration, then edit the ifcfg file, then restart the network service.

Question 13

Match each systemd unit type to its purpose.

Accepted Answer

service: Manages daemon processes that run in the background. Correct matches: service manages processes, socket handles socket activation, timer schedules events, target groups units. Common confusion: mixing socket and timer responsibilities is typical.

Answer

service: Manages IPC or network socket activation

Answer

socket: Manages time-based event scheduling

Question 14

A system administrator needs to configure a static IP address on a CentOS 7 server. Which file should be edited to set the IP address permanently?

Accepted Answer

/etc/sysconfig/network-scripts/ifcfg-eth0. On CentOS 7, network interfaces are configured via scripts located in /etc/sysconfig/network-scripts/, with each interface having a file named ifcfg-<interface>. The ifcfg-eth0 file stores static IP settings such as IPADDR, NETMASK, and GATEWAY, which are read by the network service (network.service) to apply persistent configuration.

Answer

/etc/netplan/01-netcfg.yaml

Answer

/etc/network/interfaces

Answer

/etc/hostname

Question 15

A Linux server cannot reach the internet, but internal LAN connectivity works. The output of 'ip route' shows a default gateway of 192.168.1.1, but pinging 8.8.8.8 fails. What is the most likely cause?

Accepted Answer

The default gateway is not reachable or has no internet connectivity.. The default gateway 192.168.1.1 is present in the routing table, but pinging 8.8.8.8 fails while internal LAN connectivity works. This indicates that the gateway itself either cannot be reached (e.g., due to a layer 2 issue or misconfiguration) or, more likely, it has no upstream internet connectivity. Since the default route is configured, the failure is not due to a missing gateway but rather the gateway's inability to forward traffic to external networks.

Answer

The ARP table is corrupted.

Answer

The default gateway is missing.

Answer

DNS resolution is failing.