LPIC-1 Essential System Services and Networking Practice Test 3 — 15 Questions

Question 1

Refer to the exhibit. A system administrator notices that remote SSH connections are being blocked from all IP addresses except 10.0.0.5. Which configuration change would allow SSH from any IP?

Accepted Answer

Add a new rule to accept SSH from any source to the beginning of the chain.. The exhibit shows an iptables INPUT chain with a first rule that accepts SSH from 10.0.0.5 and a second rule that drops all other SSH traffic. Because iptables processes rules in order, the first rule accepts SSH from 10.0.0.5, but all other SSH packets are matched by the second rule and dropped. Adding a new rule to accept SSH from any source at the beginning of the chain ensures that SSH packets from any IP are accepted before reaching the drop rule, allowing connections from any IP.

Answer

Change the INPUT chain policy to ACCEPT.

Answer

Delete the first rule.

Answer

Move the second rule to be the first rule.

Question 2

A Debian server's /etc/crontab contains the line "30 2 * * 1-5 root /usr/local/bin/backup.sh". The administrator expects the script to run every weekday at 02:30, but it never executes, while other cron jobs on the same host run fine. The script is executable and produces no output. Which action will most likely resolve the problem?

Accepted Answer

Inspect /var/log/syslog or run journalctl -u cron to see whether cron logs an error for the job, and verify the file ends with a newline.. Because the script is executable and other jobs work, the issue is specific to this entry. A crontab whose last line lacks a terminating newline is silently discarded by cron, which is a classic cause of a single non-running job. Confirming via the cron log that the job never fires, then adding the missing newline, is the reliable fix.

Answer

Change the schedule to "30 2 * * 1,2,3,4,5" because ranges such as 1-5 are not supported in the fifth field.

Answer

Move the entry into /etc/cron.d/backup and change the user field to root, because /etc/crontab cannot contain per-user fields.

Answer

Add /usr/local/bin to the PATH variable at the top of /etc/crontab, since cron cannot execute scripts outside /bin or /usr/bin.

Question 3

A junior administrator needs to identify which processes are currently listening on TCP and UDP ports on a Linux server. She also wants to see the numeric port numbers and the associated PIDs without resolving service names or hostnames. Which TWO commands will provide this information? (Choose two.)

Accepted Answer

ss -tulpn. The commands ss and netstat with the -tulpn flags both list TCP and UDP listening sockets, show numeric ports, and include the owning process and PID. They are the standard tools for this exact task and remain valid even though ss is preferred on modern systems. The other commands either show different data or lack the necessary process and UDP details.

Answer

ip -s link show

Answer

nmap -sT -O localhost

Answer

lsof -i -P -n

Question 4

An administrator needs to inspect the journal for the sshd unit but only wants entries from the current boot and only messages at priority 'err' or higher. Which command accomplishes this?

Accepted Answer

journalctl -u sshd -b -p err. The journalctl utility uses -u to select a unit, -b for the current boot, and -p to filter by priority. Priority err corresponds to level 3, and specifying it includes err, crit, alert, and emerg messages. The other commands either use incorrect options, filter by time instead of boot, or misuse the -n switch, so they cannot deliver exactly current-boot err-or-worse sshd entries.

Answer

journalctl _SYSTEMD_UNIT=sshd.service -b --priority=3

Answer

journalctl -u sshd -b -n err

Answer

journalctl -u sshd --since today -p err

Question 5

A Debian 12 server uses systemd-resolved for DNS resolution. Users report that a recently added internal hostname resolves correctly when queried with dig against 127.0.0.53, but fails when applications call getaddrinfo(). An administrator must make the name resolvable by ordinary applications. (Choose two.)

Accepted Answer

Add the hostname and address to /etc/hosts so the local resolver database returns it before querying DNS.. Applications use getaddrinfo(), which consults the NSS sources configured in /etc/nsswitch.conf. A name that answers only when queried directly against the stub can still fail if no configured source actually serves it. Supplying the record through /etc/hosts, or pointing the correct per-link DNS server at the zone with resolvectl, gives the resolver a real authoritative answer that getaddrinfo() can return.

Answer

Run resolvectl flush-caches to clear stale negative entries for the hostname.

Answer

Add the mapping to the local stub resolver database using systemd-resolved's static host entry mechanism.

Answer

Point /etc/resolv.conf directly at the internal DNS server instead of the 127.0.0.53 stub listener.

Question 6

A Linux server has a GPT-partitioned disk with an existing XFS filesystem on /dev/sdb1. The administrator needs to add a new 200 GiB partition to the disk and have it mounted automatically at /data on every boot. Which sequence of commands will accomplish this?

Accepted Answer

parted /dev/sdb mkpart primary xfs 200GiB 400GiB ; mkfs.xfs /dev/sdb2 ; mount /dev/sdb2 /data ; add entry to /etc/fstab. Creating a new partition on a GPT disk requires a tool that supports GPT, such as parted or gdisk. The partition must be formatted with the desired filesystem (XFS in this case) and then mounted persistently via /etc/fstab. The correct sequence uses parted to create the partition, mkfs.xfs to format, mounts it, and adds an fstab entry. Other options either use the wrong filesystem, conflict with existing partitions, or use incompatible tools.

Answer

gdisk /dev/sdb ; n ; 2 ; [default] ; +200G ; w ; mkfs.ext4 /dev/sdb2 ; mount /dev/sdb2 /data ; echo '/dev/sdb2 /data ext4 defaults 0 0' >> /etc/fstab

Answer

fdisk /dev/sdb ; n ; p ; 1 ; [default] ; +200G ; w ; mkfs.xfs /dev/sdb2 ; mount /dev/sdb2 /data ; echo '/dev/sdb2 /data xfs defaults 0 0' >> /etc/fstab

Answer

parted /dev/sdb mkpart primary ext4 200GiB 400GiB ; mkfs.ext4 /dev/sdb2 ; mount /dev/sdb2 /data ; echo '/dev/sdb2 /data ext4 defaults 0 0' >> /etc/fstab

Question 7

A Linux server must run a script at 02:30 every day, but the administrator wants to avoid conflicts with other cron jobs and ensure that if the system is off at that time, the job runs after boot. Which cron syntax and directory should be used?

Accepted Answer

Create a systemd timer with OnCalendar=*-*-* 02:30:00 and Persistent=true, then enable it.. systemd timers with OnCalendar and Persistent=true satisfy both the exact time and the catch-up requirement. Cron entries in /etc/crontab do not run missed jobs, anacron cannot schedule a precise time, and placing scripts in /etc/cron.daily/ does not control the execution time. The systemd timer approach is the only one that guarantees the job runs at 02:30 daily and after boot if missed.

Answer

Add an entry to /etc/crontab with '30 2 * * * root /path/to/script' and rely on cron to run missed jobs.

Answer

Place a script in /etc/cron.daily/ with a 02:30 timestamp in the filename.

Answer

Use anacron with a period of 1 and delay of 0 in /etc/anacrontab, setting the start time to 02:30.

Question 8

A Linux administrator needs to allow incoming SSH connections on port 22 and block all other incoming traffic on a server using nftables. The current ruleset is empty. Which command sequence correctly creates a table, chain, and rules to achieve this?

Accepted Answer

nft add table inet filter; nft add chain inet filter input { type filter hook input priority 0 \; }; nft add rule inet filter input tcp dport 22 accept; nft add rule inet filter input drop. The correct sequence creates an inet table and input chain, accepts TCP port 22, and drops all other traffic. Using the inet family ensures both IPv4 and IPv6 are filtered, and the final drop rule enforces the default deny policy. This precisely matches the administrator's requirement to allow SSH and block everything else.

Answer

nft add table inet filter; nft add chain inet filter input { type filter hook input priority 0 \; }; nft add rule inet filter input tcp dport 22 drop; nft add rule inet filter input accept

Answer

nft add table inet filter; nft add chain inet filter input { type filter hook input priority 0 \; }; nft add rule inet filter input tcp dport 22 accept; nft add rule inet filter input counter drop

Answer

nft add table ip filter; nft add chain ip filter input { type filter hook input priority 0 \; }; nft add rule ip filter input tcp dport 22 accept; nft add rule ip filter input drop

Question 9

A Linux server has two network interfaces: eth0 (192.168.1.10/24) and eth1 (10.0.0.10/24). The default route is via 192.168.1.1. Users report that they cannot reach hosts on the 10.0.0.0/24 network from remote networks, but local communication on 10.0.0.0/24 works. Which command will allow the server to forward packets between the two interfaces?

Accepted Answer

sysctl -w net.ipv4.ip_forward=1. To allow a Linux server to forward packets between interfaces, IP forwarding must be enabled. The sysctl -w net.ipv4.ip_forward=1 command enables it at runtime. Firewall rules or routes alone do not enable forwarding. While writing to /proc/sys/net/ipv4/ip_forward also works, sysctl is the recommended method. Thus, enabling IP forwarding is the key step.

Answer

ip route add 10.0.0.0/24 dev eth1

Answer

iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT

Answer

echo 1 > /proc/sys/net/ipv4/ip_forward

Question 10

A technician must schedule a maintenance script to run once at 02:30 on the coming Sunday, and the job must survive a reboot if the machine is down at that moment. Which command should be used to create this job?

Accepted Answer

crontab -e with the line 30 2 * * 0 /usr/local/bin/maintenance.sh. A recurring weekly run at a precise clock time is expressed with cron's five fields, where the first is minute, the second hour, and the fifth day-of-week with 0 representing Sunday. Because crontab entries are persisted on disk and processed by the cron daemon, the schedule is re-established after a reboot without any additional action.

Answer

anacron with a weekly entry in /etc/anacrontab

Answer

systemd-run --on-calendar="Sun 02:30" /usr/local/bin/maintenance.sh

Answer

at 02:30 Sun

Question 11

An administrator is troubleshooting a server whose clock drifts. The output of timedatectl shows "System clock synchronized: no" and "NTP service: inactive". The administrator wants the host to synchronize its time automatically using chrony. Which command both enables and starts the chronyd service on this systemd host?

Accepted Answer

timedatectl set-ntp true. On a systemd host, timedatectl set-ntp true delegates to the installed NTP implementation, enabling and starting chronyd and marking the system as NTP-synchronized. This single command addresses both the enable and start requirements and is the distribution-agnostic way to activate time synchronization without manually manipulating the unit.

Answer

chronyc makestep

Answer

systemctl enable chronyd.service

Answer

hwclock --systohc

Question 12

A system administrator notices that the NTP service on a Linux server is not synchronizing time with external NTP servers. The administrator runs 'ntpq -p' and sees that all servers listed have a 'reach' value of 0. Which of the following is the most likely cause?

Accepted Answer

A firewall is blocking UDP port 123.. The `reach` value of 0 in `ntpq -p` output indicates that the NTP client has received no responses from any of the configured servers. Since NTP uses UDP port 123 for communication, a firewall blocking this port would prevent the client from sending or receiving NTP packets, resulting in zero reachability. This is the most common cause when all servers show a reach of 0.

Answer

The system timezone is incorrectly set.

Answer

The NTP service is configured to use the local clock.

Answer

The NTP server is using a different NTP version.

Question 13

Which of the following commands will display the default gateway of a Linux system?

Accepted Answer

ip route show. The `ip route show` command displays the kernel routing table, which includes the default gateway as a default route (typically `default via <gateway-IP>`). This is the standard modern tool for viewing routing information on Linux systems.

Answer

arp -a

Answer

netstat -i

Answer

ifconfig

Question 14

A database server on a Linux system is configured to listen on TCP port 3306. The administrator wants to restrict access to the database server to only the local network (192.168.1.0/24) using iptables. Which of the following iptables rules achieves this?

Accepted Answer

iptables -A INPUT -p tcp --dport 3306 -s 192.168.1.0/24 -j ACCEPT. It adds an INPUT chain rule that accepts TCP traffic destined for port 3306 only when the source address is within the 192.168.1.0/24 subnet. This effectively restricts incoming database connections to the local network, while all other sources are implicitly dropped by the default INPUT policy or subsequent rules.

Answer

iptables -A INPUT -p tcp --dport 3306 -d 192.168.1.0/24 -j DROP

Answer

iptables -A OUTPUT -p tcp --dport 3306 -d 192.168.1.0/24 -j ACCEPT

Answer

iptables -A OUTPUT -p tcp --sport 3306 -s 192.168.1.0/24 -j ACCEPT

Question 15

A user reports that they cannot connect to a remote server using SSH. The administrator checks the SSH server status and it is running. Which of the following is the most likely cause?

Accepted Answer

A firewall is blocking port 22.. SSH operates over TCP port 22 by default. If the SSH server is running but the client cannot connect, a firewall blocking port 22 is the most likely cause because it would prevent the TCP handshake from completing, even though the SSH daemon (sshd) is active and listening.

Answer

The client's subnet mask is incorrect.

Answer

The client cannot resolve the server's hostname.

Answer

The SSH server is using UDP instead of TCP.