Day 3 - Primary Subject 3 Mock Exam: Weak Points in Using Networks and Services

📌 Key Points

• The last primary subject, "Using Networks and Services," mixes concepts + services + memorization, so it has the widest scope.
• The core strategy is to memorize the "service = daemon = port = configuration file" four-piece set together.
• The port table is the heart of this subject — SSH 22, SMTP 25, DNS 53, HTTP 80, HTTPS 443. FTP is control 21 / data 20.
• DNS records: A (name→IPv4), MX (mail), CNAME (alias), PTR (IP→name, reverse), SOA (zone authority).
• Mail is split into sending (SMTP) and receiving (POP3/IMAP). POP3 downloads and deletes; IMAP keeps mail on the server and synchronizes.
• The maximum first octet of Class A is 126, not 127 (127 is reserved for loopback). The private ranges are 10/8, 172.16/12, 192.168/16.

Weak Point 1: Networking Basics — OSI and TCP/IP

• The mapping between the 7 OSI layers and the 4 TCP/IP layers, and the devices and protocols of each layer, are asked.

• Let's also review subnet calculation. /24 = 255.255.255.0 = 256 addresses (254 usable).
• The prefix bit count is the network portion, and the rest is the host portion.

TCP vs UDP: TCP is connection-oriented and reliable (3-way handshake), while UDP is connectionless and fast (DNS lookups, streaming). The same service can use both (DNS uses 53/UDP and 53/TCP).

Weak Point 2: Port Number Roundup

• The port memorization table is the heart of this subject. Memorize it in full.

⚠️ FTP trap: Control is 21 and data is 20 (active mode). In passive mode, the data port changes dynamically. If the question just asks for the "FTP port," the answer is often 21.

Weak Point 3: DNS — BIND and Records

• The DNS server is BIND, and its configuration is split between /etc/named.conf and zone files.

• The 3 client configuration files: /etc/resolv.conf (specifies name servers), /etc/hosts (local static mappings), /etc/nsswitch.conf (lookup order).

Lookup tools: nslookup, dig (detailed), host (simple). Reverse lookup uses the PTR record, for IP→name conversion.

Weak Point 4: Web (Apache) and Mail

• Memorize the key directives of the Apache (httpd) configuration.

• Mail is split into sending (SMTP) and receiving (POP3/IMAP).

POP3 vs IMAP: POP3 downloads and deletes from the server (stored locally), while IMAP keeps mail on the server and synchronizes (multiple devices). IMAP is better for multiple devices.

Weak Point 5: FTP and SSH

• vsftpd is the representative FTP server, and telling the modes apart is the key.

• SSH is the standard for secure remote access.

⚠️ Security settings: In sshd_config, the key points are PermitRootLogin no (block direct root login), changing Port, and recommending key authentication. Telnet (23, plaintext) should be replaced with SSH.

Weak Point 6: Network Commands and Security

• Let's summarize the diagnostic and configuration commands.

Status check in one line: View open ports and LISTEN state with ss -tlnp (or netstat -tlnp). You can see at a glance which process is using which port.

Weak Point 7: DHCP and NTP

• Automatic IP assignment and time synchronization are also within the exam scope.

• The keywords are DHCP's IP lease concept and NTP's stratum structure.

The 4 DHCP steps (DORA): Discover → Offer → Request → Ack. The flow in which the client finds a server by broadcast and is leased an IP.

Weak Point 8: Network Configuration Files and Bonding

• Let's summarize the structure of interface configuration files on the RHEL family.

• Key items: BOOTPROTO (static/dhcp), IPADDR, NETMASK, GATEWAY, ONBOOT (activate at boot), DNS1.

ONBOOT trap: With ONBOOT=no, the interface does not come up at boot. Even if you set every IP value correctly, the network won't work if ONBOOT is no — that's the trap.

Weak Point 9: Security — Blocking Intrusions and Responding to Scans

• Network security concepts are also included in Subject 3.

Security in one line: Plaintext protocols (Telnet 23, FTP 21) send passwords as-is, so they are vulnerable to eavesdropping. The principle is to replace them with encrypted ones: SSH (22), SFTP, HTTPS (443).

Weak Point 10: The IP Addressing Scheme and Special Addresses

• IPv4 classes and private IP ranges are simple memorization but easy to confuse.

• Memorize the private IP ranges too — Class A 10.0.0.0/8, Class B 172.16.0.0/12, Class C 192.168.0.0/16.

127 vs 126: Class A is 1-126, and 127 is excluded because it is reserved for loopback. The answer to "What is the maximum first octet of Class A?" is 126, not 127.

Weak Point 11: IPv6 and Routing Basics

• IPv6 is 128 bits, written as 8 groups of hexadecimal (separated by colons).
• A run of consecutive zero groups can be abbreviated with :: once.

• Routing is split into static (route add/ip route add) and dynamic (RIP, OSPF, BGP).
• The default gateway is the exit to which packets are sent when the destination is not in the routing table.

The :: rule: In IPv6, :: (abbreviation of consecutive zeros) can be used only once in an address. Using it twice makes it ambiguous how many zeros each covers, so it is invalid.

Check yourself with the mock exam below.

📖 Glossary

• OSI 7 layers : A standard model that divides network communication by role. Routers operate at Layer 3 and switches at Layer 2.
• CIDR : A notation that expresses an address range by the number of bits in the network portion, like /24.
• TCP / UDP : A transport method that establishes a connection and guarantees ordering and retransmission / a transport method that sends quickly without a connection.
• Active / passive mode : In FTP, the method where the server opens the data connection to the client / the method where the client opens it to the server (firewall-friendly).
• MTA / MDA / MUA : The server that transfers mail / the program that delivers to the mailbox / the client the user reads with.
• POP3 / IMAP : A method that downloads mail and deletes it from the server / a method that keeps mail on the server and synchronizes with multiple devices.
• PTR record : A DNS record for reverse lookup that converts an IP to a name.
• DORA : The 4 steps by which DHCP leases an address — Discover, Offer, Request, Ack.
• stratum : A tier number in NTP indicating how many steps away you are from the reference clock.
• ONBOOT : A setting that decides whether the interface is brought up automatically at boot. If no, the network doesn't work even if the IP is set correctly.
• Private IP : Address ranges for internal use that don't go directly onto the Internet (10/8, 172.16/12, 192.168/16).
• Loopback (127.0.0.1 / ::1) : The address that points to itself. This is why 127 is excluded from the Class A range.
• :: abbreviation : A notation in IPv6 that omits a run of consecutive zero groups. It can be used only once in an address.
• Static routing / dynamic routing : The method where the administrator enters routes manually / the method of learning automatically via RIP, OSPF, and BGP.

📝 Practice Questions

Click a choice to reveal the answer and explanation.

Question 1

Among the 7 OSI layers, which layer is where routers operate and the IP and ICMP protocols belong?

• ALayer 2 (Data Link)
• BLayer 3 (Network)
• CLayer 4 (Transport)
• DLayer 7 (Application)

Question 2

Which service separates the control connection and the data connection and uses port 21 as the control port?

• ASSH
• BFTP
• CTelnet
• DSMTP

Question 3

Which DNS record is used for reverse lookup, converting an IP address to a hostname?

• AA
• BMX
• CPTR
• DCNAME

Question 4

Which protocol and port does a mail server's role of transferring mail to another server (MTA) use?

• APOP3 - 110
• BIMAP - 143
• CSMTP - 25
• DHTTP - 80

Question 5

Which receiving protocol is suited to synchronizing and using the same mailbox from multiple devices?

• APOP3
• BIMAP
• CSMTP
• DNNTP

Question 6

In a /24 subnet (255.255.255.0), how many IPs can actually be assigned to hosts?

• A256
• B254
• C128
• D62

Question 7

Which setting in /etc/ssh/sshd_config blocks direct root login for SSH server security?

• APermitRootLogin yes
• BPermitRootLogin no
• CAllowRoot off
• DRootLogin deny

Question 8

Which configuration file specifies the name servers a DNS client queries?

• A/etc/hosts
• B/etc/resolv.conf
• C/etc/named.conf
• D/etc/nsswitch.conf

Question 9

Which is an appropriate command to check the ports in the LISTEN state on the current system and their processes?

• Aping -c 4 localhost
• Bss -tlnp
• Ctraceroute 8.8.8.8
• Darp -a

Question 10

Which directive in the Apache (httpd) configuration specifies the top-level directory path of web documents?

• AServerName
• BListen
• CDocumentRoot
• DDirectoryIndex