LFCS Practice Questions 2 — 50 Questions

Question 1

A system administrator monitors a server that periodically becomes unresponsive for a few seconds. Investigation reveals that the kernel OOM killer is being invoked, but standard memory usage monitoring tools like 'free' and 'top' show less than 50% memory usage. The server runs a web server and a database. Which of the following is the most likely cause and diagnostic step?

Accepted Answer

The kernel is using a large amount of memory for slab caches. Use 'slabtop' to examine kernel memory allocations.. Option D is correct because the kernel OOM killer can be triggered by high slab cache usage, which is memory allocated for kernel data structures like inode and dentry caches. Standard tools like 'free' and 'top' may show low overall memory usage because they report only user-space memory, while slab caches are accounted separately. Using 'slabtop' allows the administrator to identify which kernel slab caches are consuming excessive memory, often due to a large number of small files or heavy filesystem metadata operations.

Answer

The server is experiencing a fork bomb. Use 'ps -ef | wc -l' to count processes.

Answer

The database is using swap space excessively. Use 'swapon -s' to check swap usage.

Answer

The web server is leaking file descriptors. Use 'lsof -n' to check open files.

Question 2

Which THREE of the following are valid Linux filesystem types that can be used for root partitions on a modern Linux system?

Accepted Answer

XFS. XFS is a high-performance 64-bit journaling filesystem created by Silicon Graphics, now widely supported in the Linux kernel. It is a valid choice for root partitions on modern Linux systems, especially for large file and high-concurrency workloads, and is the default filesystem in Red Hat Enterprise Linux 7 and later.

Answer

NTFS

Answer

FAT32

Question 3

A system administrator is configuring a custom systemd service that runs a Python script. The script logs output to stdout. The administrator wants to ensure that the service restarts automatically if it crashes, but only after a 10-second delay. Which directive should be added to the [Service] section of the unit file?

Accepted Answer

Restart=on-failure
RestartSec=10. Option B is correct because `Restart=on-failure` ensures the service restarts only when it exits with a non-zero exit code or is terminated by a signal (e.g., SIGKILL), which matches the 'crashes' scenario. Adding `RestartSec=10` introduces a 10-second delay before the restart attempt, as required. The other options either restart on success (irrelevant) or always restart (which would restart even on intentional stops), or omit the restart condition entirely.

Answer

Restart=on-success
RestartSec=10

Answer

Restart=always
RestartSec=10

Answer

RestartSec=10

Question 4

A system administrator notices that a new 500GB SSD (/dev/sdb) is not being recognized by the system after installation. The server uses UEFI and GPT partitioning. Which command should the administrator run first to verify that the disk is detected by the kernel?

Accepted Answer

lsblk. The `lsblk` command lists all block devices recognized by the kernel, including those without a filesystem or partition table. Since the disk is new and not yet partitioned, `lsblk` will show it if the kernel has detected it, making it the correct first diagnostic step.

Answer

fdisk -l /dev/sdb

Answer

cat /proc/cpuinfo

Answer

lsusb

Question 5

A Linux administrator needs to create a RAID 5 array using three disks: /dev/sdb, /dev/sdc, and /dev/sdd, each 2TB. The administrator wants to ensure the array can be reassembled automatically after a reboot. Which command should be used to create the array?

Accepted Answer

mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd --metadata=1.2. Option D is correct because it uses `--metadata=1.2`, which stores the superblock at a standard 4 KiB offset from the start of each device, enabling the kernel to automatically discover and assemble the RAID array on reboot without manual intervention. RAID 5 with three disks provides striping with distributed parity, offering a balance of performance and redundancy.

Answer

mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd --metadata=0.90

Answer

mdadm --create /dev/md0 --level=1 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd

Answer

mdadm --create /dev/md0 --level=0 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd

Question 6

A server runs a custom application that listens on TCP port 8080. The administrator wants to ensure the application starts automatically on boot and restarts if it crashes. Which systemd unit file directive should be used to achieve the restart behavior?

Accepted Answer

Restart=on-failure. The `Restart=on-failure` directive in a systemd unit file instructs systemd to automatically restart the service unit when it exits with a non-zero exit code, is terminated by a signal (including SIGKILL), or times out. This directly satisfies the requirement for the application to restart if it crashes, as a crash typically results in an unclean exit that triggers the restart condition.

Answer

RestartSec=5

Answer

Type=notify

Answer

RemainAfterExit=yes

Question 7

An administrator runs 'systemctl status sshd' and sees the output above. The administrator wants sshd to start automatically at boot. Which command should be used?

Accepted Answer

systemctl enable sshd. The `systemctl enable sshd` command creates the necessary symlinks in the systemd unit configuration directories (e.g., `/etc/systemd/system/multi-user.target.wants/`) so that the sshd service is started automatically at boot. This is the correct way to configure a service to start on boot in a systemd-based Linux distribution.

Answer

systemctl reenable sshd

Answer

systemctl mask sshd

Answer

systemctl start sshd

Question 8

A developer reports that a web application's logs are not being written to /var/log/myapp.log. The service runs as user 'myapp' and the log directory /var/log/myapp/ has permissions 755 owned by root. What is the most likely cause?

Accepted Answer

The service user 'myapp' does not have write permission to the log directory.. Option D is correct because the /var/log/myapp/ directory has permissions 755, which grants read and execute access to the 'others' category but not write. Since the service runs as user 'myapp', which is not the owner (root) and not in the root group, it falls under 'others' and thus lacks write permission. Without write permission on the directory, the service cannot create or write to /var/log/myapp.log, even if the file itself might have different permissions.

Answer

AppArmor is denying access.

Answer

SELinux is blocking the write.

Answer

The service is logging to systemd-journald instead of a file.

Question 9

An administrator wants to ensure that a custom service (myapp.service) starts only after the network is available and the PostgreSQL database service is running. Which systemd unit file directive should be used?

Accepted Answer

After=network.target postgresql.service. Option D is correct because the `After=` directive in a systemd unit file specifies ordering constraints, ensuring that `myapp.service` starts only after `network.target` and `postgresql.service` have reached the 'started' state. This does not create a dependency that forces those units to start; it only orders the startup sequence, which is exactly what the administrator needs to guarantee the service starts after the network and PostgreSQL are available.

Answer

Requires=network.target postgresql.service

Answer

Wants=network.target postgresql.service

Answer

BindsTo=network.target postgresql.service

Question 10

Which TWO statements are true about systemd service unit files? (Choose two.)

Accepted Answer

The [Service] section is mandatory for a service unit file.. Option C is correct because the [Service] section is mandatory in a systemd service unit file; without it, systemd cannot determine how to manage the service process. This section defines the service type, execution command, and other runtime parameters essential for the unit to function.

Answer

Environment variables can be loaded using EnvFile= directive.

Answer

The default service type is 'forking'.

Answer

The [Service] section must appear before the [Unit] section.

Question 11

A Linux administrator needs to temporarily stop a service named 'httpd' without disabling it from starting automatically on subsequent boots. Which command should be used?

Accepted Answer

systemctl stop httpd. The `systemctl stop httpd` command sends a SIGTERM signal to the main process of the httpd service, causing it to stop immediately. This action does not modify the service's enablement state, so the service will still start automatically on subsequent boots if it is enabled. This is the correct way to temporarily stop a service without altering its boot-time behavior.

Answer

systemctl mask httpd

Answer

systemctl disable httpd

Answer

systemctl kill httpd

Question 12

A system administrator receives an alert that disk /dev/sda is predicted to fail soon. The server uses LVM, and /dev/sda is part of a volume group named vg_system. Which of the following is the best course of action to replace the failing disk without downtime?

Accepted Answer

Use pvmove to move physical extents to another disk, then remove the old disk.. Option D is correct because pvmove relocates physical extents from /dev/sda to another physical volume in the same volume group while the filesystem remains online and accessible. This allows the failing disk to be removed from vg_system without any downtime, preserving LVM metadata and data integrity.

Answer

Use dd to clone /dev/sda to a new disk and then replace.

Answer

Use ddrescue to copy data, then replace the disk.

Answer

Remove /dev/sda from the volume group and add a new disk.

Question 13

The administrator wants to create a new logical volume of 5GB for /var/log. Which of the following is the most appropriate first step?

Accepted Answer

Add a new disk (e.g., /dev/sdc) to the system, create a PV, and extend vg_root.. Option B is correct because the volume group vg_root currently has no free space to create a new logical volume. Adding a new disk, creating a physical volume (PV) on it, and extending vg_root with vgextend provides the necessary free extents. Only then can lvcreate be used to create the 5GB lv_var_log logical volume for /var/log.

Answer

Use lvresize to shrink lv_root by 5G and then create lv_var_log

Answer

Mount a new filesystem directly on a new partition /dev/sdc1

Answer

Use lvcreate -L 5G vg_root -n lv_var_log

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

Which iptables command allows incoming SSH traffic only from the 10.0.0.0/8 network?

Answer

iptables -A INPUT -p tcp --dport 22 -s 10.0.0.0/8 -j ACCEPT

Answer

Edited: Option A: only ACCEPT, Option B: default DROP with ACCEPT (but missing state tracking for return traffic), Option C: ACCEPT from network then DROP others, Option D: completely wrong command.

Question 16

A system administrator needs to configure bonding in active-backup mode. Which line in /etc/sysconfig/network-scripts/ifcfg-bond0 defines the bonding mode and primary interface?

Accepted Answer

BONDING_OPTS="mode=1 primary=eth0 miimon=100". Option A is correct because it uses the BONDING_OPTS directive with mode=1 (which corresponds to active-backup mode in Linux bonding) and explicitly specifies the primary interface as eth0. The miimon=100 parameter is also included to enable link monitoring via MII (Media Independent Interface) polling every 100 milliseconds, which is essential for failover detection in active-backup mode.

Answer

BONDING_OPTS="miimon=100 mode=active-backup primary=eth0"

Answer

BONDING_OPTS="mode=0 primary=eth0"

Answer

BONDING_OPTS="mode=active-backup miimon=100"

Question 17

Refer to the exhibit. The /etc/sysconfig/network-scripts/ifcfg-eth0 file contains the above content. After reboot, eth0 still does not obtain an IP via DHCP. What is the most likely missing configuration?

Accepted Answer

TYPE=Ethernet is missing. Option C is correct because the `ifcfg-eth0` file is missing the `TYPE=Ethernet` directive. In RHEL/CentOS 7 and later, NetworkManager requires the `TYPE` parameter to correctly identify and manage the interface type. Without it, NetworkManager may ignore the file or fail to apply DHCP settings, even if `BOOTPROTO=dhcp` is present.

Answer

The MTU is not specified

Answer

DNS1 is not set

Answer

The gateway is not defined

Question 18

A server with two network interfaces (eth0 and eth1) needs to be configured as a bridge (br0) to allow KVM virtual machines to share the host's physical network. Which steps are correct?

Accepted Answer

Create bridge br0, add eth0 as port, assign IP to br0, remove IP from eth0.. Option A is correct because bridging requires the physical interface (eth0) to be added as a port to the bridge (br0) without an IP address, while the bridge itself gets the IP address to act as the network layer endpoint. This allows KVM virtual machines connected to br0 to communicate through eth0, which operates at layer 2. Removing the IP from eth0 prevents IP conflicts and ensures all traffic is handled by the bridge.

Answer

Create bridge br0, add eth0 as port, assign IP to eth0, do not assign IP to br0.

Answer

Create bond0 from eth0 and eth1, then add bond0 to bridge br0.

Answer

Create bridge br0, add eth0 and eth1 as bridge ports, assign IP to br0, remove IP from eth0 and eth1.

Question 19

An administrator is troubleshooting intermittent connectivity issues. Running 'ping -c 100 -i 0.2 10.0.0.1' shows about 5% packet loss. What is the primary purpose of the '-i 0.2' option?

Accepted Answer

It sets the interval between pings to 0.2 seconds. The '-i 0.2' option in the ping command sets the interval between sending ICMP Echo Request packets to 0.2 seconds. This allows the administrator to send pings more frequently than the default (typically 1 second), which helps in detecting intermittent connectivity issues over a shorter test duration. By sending 100 packets at a 0.2-second interval, the test completes in about 20 seconds, making it practical for troubleshooting transient packet loss.

Answer

It sets the TTL to 0.2

Answer

It sets the timeout to 0.2 seconds

Answer

It sets the packet size to 0.2 bytes

Question 20

A system administrator wants to allow incoming SSH connections from a specific IP range 192.168.10.0/24 using firewalld. Which command should be used?

Accepted Answer

firewall-cmd --zone=public --add-rich-rule='rule family="ipv4" source address="192.168.10.0/24" service name="ssh" accept' --permanent. Option D is correct because it explicitly targets the 'public' zone (the default zone for external-facing interfaces) and uses a rich rule to allow SSH traffic only from the 192.168.10.0/24 source IP range. The --permanent flag ensures the rule persists across reloads. This is the precise syntax required by firewalld for source-specific service access.

Answer

firewall-cmd --add-rich-rule='rule family="ipv4" source address="192.168.10.0/24" service name="ssh" accept' --permanent

Answer

firewall-cmd --add-service=ssh --add-source=192.168.10.0/24 --zone=internal --permanent

Answer

firewall-cmd --add-source=192.168.10.0/24 --add-service=ssh --permanent