Bangladesh Bank Assistant Maintenance Engineer Written Test: 2017প্রশ্ন ও সমাধান

• DHCP (Dynamic Host Configuration Protocol): an application-layer protocol that automatically assigns an IP address (and related configuration — subnet mask, default gateway, DNS server address) to each host joining an IP network, so devices don't need manual IP configuration. It operates via a four-step DORA process: Discover (client broadcasts for a DHCP server) leading to Offer (server proposes an IP) leading to Request (client requests that IP) leading to Acknowledge (server confirms the lease).

• SMTP (Simple Mail Transfer Protocol): an application-layer protocol used to send and relay outgoing email between mail servers (and from a mail client to its outgoing mail server). It runs over TCP, traditionally on port 25 (587 for authenticated submission). SMTP handles only outbound delivery — retrieving/reading received mail from a mailbox is done by a separate protocol, POP3 (downloads and typically deletes from server) or IMAP (keeps mail synced on the server, supports multiple devices).

graph LR
    C[New Client] -->|1. Discover| D[DHCP Server]
    D -->|2. Offer| C
    C -->|3. Request| D
    D -->|4. Acknowledge - lease IP| C

    MC[Mail Client] -->|SMTP: send mail| S1[Sending Mail Server]
    S1 -->|SMTP: relay| S2[Receiving Mail Server]
    S2 -->|POP3/IMAP: retrieve| MC2[Recipient Mail Client]

Exam takeaway: DHCP automatically leases IP configuration to hosts (via Discover-Offer-Request-Acknowledge); SMTP sends/relays outgoing mail between servers (port 25/587) — while POP3/IMAP handle the retrieval side.

Step	Action	Why
1	Use public-key (SSH key) authentication instead of passwords	Eliminates password brute-force risk on SSH
2	Enforce strong passwords everywhere else	Reduces credential-guessing attacks
3	Install/configure a firewall (e.g., CSF)	Blocks unauthorized ports/traffic
4	Install/configure Fail2Ban (or similar)	Automatically bans IPs after repeated failed login attempts
5	Install malware/rootkit scanning software	Detects compromise early
6	Keep OS and software fully updated/patched	Closes known vulnerabilities before they are exploited
7	Take regular, tested backups	Enables recovery from ransomware/compromise/failure
8	Monitor logs actively	Detects suspicious activity/intrusion attempts
9	Disable/remove unnecessary services	Shrinks the attack surface
10	Install a Web Application Firewall (e.g., ModSecurity)	Blocks common web attacks (SQLi, XSS) at the HTTP layer

graph TD
    A["Harden Access: SSH keys, strong passwords"] --> B["Harden Network: Firewall + Fail2Ban"]
    B --> C["Harden Software: Patch + disable unused services"]
    C --> D["Detect and Respond: Malware scan + log monitoring"]
    D --> E["Recover: Regular backups"]
    C --> F["Application layer: WAF/ModSecurity"]

Exam takeaway: web server security is layered — harden access (SSH keys, strong passwords), harden the network (firewall, intrusion prevention), keep software patched and minimal, monitor continuously, and always keep verified backups as the last line of defense.