LFCS Networking Practice Test 3 — 15 Questions

Question 1

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 2

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.

Question 3

A company is designing a multi-homed server with two network interfaces: one for internal traffic (eth0) and one for external traffic (eth1). The server must prioritize traffic to the 10.1.0.0/16 network through eth0. What is the best practice to ensure traffic to 10.1.0.0/16 uses eth0?

Accepted Answer

Add a static route for 10.1.0.0/16 via the gateway on eth0.. Adding a static route for 10.1.0.0/16 via the gateway on eth0 directly tells the kernel to send traffic destined for that network through eth0. This is the simplest and most direct method, ensuring that the route is always preferred over the default route for that specific destination, without affecting other traffic.

Answer

Configure policy routing using ip rule and two routing tables.

Answer

Set a lower metric on the default route for eth0.

Answer

Use bonding to combine both interfaces.

Question 4

A user reports that they can access websites by IP address but not by domain name. Which command should the administrator use to diagnose the issue?

Accepted Answer

dig google.com. The user can access websites by IP address but not by domain name, indicating a DNS resolution failure. The `dig` command is the correct diagnostic tool because it directly queries DNS servers to test domain name resolution, bypassing the system's resolver cache and configuration. This allows the administrator to isolate whether the issue lies with DNS resolution or other network layers.

Answer

netstat -r

Answer

traceroute 8.8.8.8

Answer

ping google.com

Question 5

A financial firm requires all internal SSH connections to be encrypted with at least 256-bit ciphers. An administrator is configuring the SSH server. Which configuration line should be added to /etc/ssh/sshd_config?

Accepted Answer

Ciphers aes256-ctr. The Ciphers directive in sshd_config explicitly controls the symmetric encryption algorithms used to encrypt SSH session data. The cipher aes256-ctr provides 256-bit encryption, meeting the firm's requirement for at least 256-bit ciphers. Other directives like MACs, KexAlgorithms, or HostKeyAlgorithms do not directly set the encryption cipher strength.

Answer

MACs hmac-sha2-256

Answer

KexAlgorithms diffie-hellman-group-exchange-sha256

Answer

HostKeyAlgorithms ssh-rsa

Question 6

A server on a corporate network is intermittently losing connectivity. The administrator runs 'tcpdump -i eth0 icmp' and sees 'ICMP time exceeded in-transit' messages from a router. What is the most likely cause?

Accepted Answer

There is a routing loop or the TTL is too low.. The 'ICMP time exceeded in-transit' message indicates that a packet's TTL (Time to Live) has reached zero before reaching its destination. This is most commonly caused by a routing loop, where packets circulate endlessly between routers, or by an initial TTL value that is too low for the number of hops required. The router that decrements the TTL to zero sends this ICMP Type 11 Code 0 message back to the source, as defined in RFC 792.

Answer

There is a mismatch in MTU size.

Answer

The destination host is down.

Answer

A firewall is blocking the packets.

Question 7

An administrator wants to ensure that a service is listening on TCP port 8080 and accessible from remote hosts. Which command will confirm that the service is listening on the correct interface?

Accepted Answer

ss -tlnp. The `ss -tlnp` command displays listening (`-l`) TCP (`-t`) sockets with numeric addresses (`-n`) and the associated process (`-p`), which directly confirms that a service is bound to TCP port 8080 on a specific interface (e.g., 0.0.0.0:8080 or 192.168.1.10:8080). This ensures the service is listening on the correct interface and is reachable from remote hosts.

Answer

iptables -L

Answer

netstat -i

Answer

ip addr

Question 8

A company deploys a new web server with two network interfaces: one for public access (eth0) and one for database access (eth1). The database server is at 10.0.0.10. The web server's default gateway is via eth0, but traffic to 10.0.0.10 is being routed through eth0 instead of eth1. What is the simplest fix?

Accepted Answer

Add a static route for 10.0.0.10/32 via eth1's gateway.. Adding a static route for 10.0.0.10/32 via eth1's gateway forces traffic destined for the database server to use the eth1 interface, overriding the default route that sends all traffic through eth0. This is the simplest and most direct fix, as it does not alter the default gateway or require complex policy routing.

Answer

Remove the default gateway on eth0.

Answer

Configure policy routing to use both tables.

Answer

Change the default gateway metric to prefer eth1.

Question 9

A system administrator needs to securely transfer files between two Linux servers using port 22. The administrator uses the following command: 'scp file.txt user@remote:/tmp/'. The transfer fails with the error 'Permission denied (publickey)'. What is the most likely cause?

Accepted Answer

The client's public key is not in the remote user's authorized_keys file.. The error 'Permission denied (publickey)' indicates that the SSH key-based authentication failed. SCP uses SSH for transport, and by default, SSH on the remote server checks the client's public key against the remote user's ~/.ssh/authorized_keys file. If the client's public key is not listed there, the SSH server rejects the connection, causing the SCP transfer to fail.

Answer

The remote server does not have SSH installed.

Answer

The SSH service is not running on the remote server.

Answer

The remote server's firewall is blocking port 22.

Question 10

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

Accepted Answer

netstat -rn. Both `netstat -rn` and `route -n` display the kernel IP routing table. The `-r` flag in netstat shows the routing table, and `-n` disables DNS resolution, showing numeric addresses. The `route -n` command directly prints the routing table without resolving hostnames, making both commands suitable for viewing the current routing table.

Answer

ifconfig -a

Answer

ss -tuln

Answer

ip addr

Question 11

Which THREE conditions can cause an 'RTNETLINK answers: File exists' error when adding a static route?

Accepted Answer

The route exists with a different metric but same destination and netmask.. The Linux kernel's routing table uses the destination and netmask as a unique key for route entries. When adding a route with 'ip route add', if a route already exists with the same destination and netmask but a different metric, the kernel returns 'RTNETLINK answers: File exists' because the metric is not part of the uniqueness check — only the destination prefix and netmask are considered. The metric is a per-route property that does not disambiguate routes in the FIB (Forwarding Information Base).

Answer

The gateway is unreachable.

Answer

The interface is down.

Question 12

Which TWO configuration files are commonly used to set DNS resolver settings on a Linux system?

Accepted Answer

/etc/nsswitch.conf. /etc/nsswitch.conf controls the order of name resolution sources (e.g., files, dns, mdns) via the 'hosts' database line. Option E is correct because /etc/resolv.conf directly specifies the DNS resolver settings, including nameserver IP addresses, search domains, and options, as defined by RFC 1035.

Answer

/etc/hostname

Answer

/etc/sysconfig/network

Answer

/etc/hosts

Question 13

Given the routing table output in the exhibit, what will happen when the system tries to send a packet to 10.1.1.1?

Accepted Answer

The packet is sent to 192.168.1.1 via eth0.. The routing table shows a specific route for the 10.0.0.0/8 network via gateway 192.168.1.1 on eth0. Since 10.1.1.1 falls within this subnet, the packet is forwarded to 192.168.1.1 via eth0, not through the default route or any other interface.

Answer

The packet is sent via eth1.

Answer

The packet is sent via default route.

Answer

The packet is dropped because there is no subnet matching 10.1.1.1.

Question 14

Given the network interface configuration in the exhibit, which command should be run to apply the configuration without rebooting?

Accepted Answer

ifup eth0. The `ifup eth0` command reads the configuration file for the eth0 interface (typically `/etc/sysconfig/network-scripts/ifcfg-eth0` on RHEL/CentOS) and applies the settings (IP address, netmask, gateway, etc.) without requiring a system reboot. This is the standard way to activate a network interface with its configured parameters on Linux systems using the legacy network scripts.

Answer

systemctl restart network

Answer

ifconfig eth0 down && ifconfig eth0 up

Answer

ip link set eth0 up

Question 15

Based on the tcpdump output in the exhibit, what can be concluded about the TCP handshake?

Accepted Answer

The TCP three-way handshake completed successfully.. The TCP three-way handshake completes successfully when three packets are exchanged: SYN, SYN-ACK, and ACK. The tcpdump output shows exactly these three packets, confirming a successful handshake. The presence of the final ACK from the client to the server's SYN-ACK indicates that the connection is established.

Answer

The connection attempt failed because only three packets are shown.

Answer

The connection was reset by the remote host.

Answer

The handshake is incomplete because there is no ACK from the server.