DNS & DHCP: Services, Protocols & Real-World Systems

Core Concepts: DNS and DHCP Defined

DNS (Domain Name System)

DNS translates human-readable domain names (e.g., esewa.com.np) into machine-readable IP addresses (e.g., 103.12.123.45). Without DNS, users would memorize IPs like 172.217.160.46 for Google instead of typing google.com.

How DNS Works

DNS operates hierarchically, using a distributed database of name servers organized in zones. The process involves:

1. Recursive Resolution: Your device queries a local DNS resolver (e.g., ISP’s DNS server).
2. Iterative Queries: The resolver asks root servers → TLD (.com, .np) servers → authoritative servers for the domain.
3. Caching: Responses are cached to speed up future requests.

sequenceDiagram
    participant User
    participant LocalDNS
    participant RootServer
    participant TLDServer
    participant AuthoritativeServer

    User->>LocalDNS: Query for esewa.com.np
    LocalDNS->>RootServer: .np?
    RootServer->>LocalDNS: Refer to TLD server
    LocalDNS->>TLDServer: .com.np?
    TLDServer->>LocalDNS: Refer to authoritative server
    LocalDNS->>AuthoritativeServer: esewa.com.np?
    AuthoritativeServer-->>LocalDNS: 103.12.123.45
    LocalDNS-->>User: 103.12.123.45

DNS Record Types

DHCP (Dynamic Host Configuration Protocol)

DHCP automates IP address assignment, reducing manual configuration. It provides:

• IP address
• Subnet mask
• Default gateway
• DNS server addresses

DHCP Operation (4-Way Handshake)

sequenceDiagram
    participant Client
    participant DHCPServer

    Client->>DHCPServer: DHCPDISCOVER (Broadcast)
    DHCPServer-->>Client: DHCPOFFER (Unicast)
    Client->>DHCPServer: DHCPREQUEST (Accept offer)
    DHCPServer-->>Client: DHCPACK (Confirm lease)

DHCP Lease Process

1. Discovery: Client broadcasts DHCPDISCOVER to find servers.
2. Offer: Server responds with DHCPOFFER (proposed IP).
3. Request: Client accepts with DHCPREQUEST.
4. Acknowledgment: Server confirms with DHCPACK (or DHCPNAK if IP is invalid).

Screenshot of a DHCP scope setup in Windows Server or Linux (e.g., `isc-dhcp-server`), showing IP range, lease time, and exclusions. (Image: Patpat, CC BY-SA 3.0, via Wikimedia Commons)

In the Real World

1. eSewa (Nepal)

   • Uses DNS to resolve esewa.com.np to its web server IPs globally.
   • Relies on DHCP to assign dynamic IPs to its cloud servers (AWS/Azure) for scalability.

2. Ncell (Nepal)

   • DHCP assigns IPs to mobile users when they connect to 4G/5G networks, ensuring no IP conflicts in crowded areas like Thamel.

3. Daraz (Global)

   • DNS load balancing routes users to the nearest server (e.g., daraz.com.np → Singapore or Dubai) for faster delivery tracking.

Worked Example: NTC’s DNS for Traffic Management The Nepal Traffic Company (NTC) uses DNS to map ntc.gov.np to its servers hosting:

• Traffic camera feeds (A record → 192.0.2.100)
• Ticket booking system (CNAME → booking.ntc.gov.np)
• Email alerts (MX record → mail.ntc.gov.np)

If DNS fails, users can’t access real-time traffic updates or book tickets—causing delays like Kathmandu’s daily gridlock.

DNS vs. DHCP: Comparison

DNS and DHCP in Linux Administration

Configuring DNS on Linux

1. Edit /etc/resolv.conf to set DNS servers:

   nameserver 8.8.8.8   # Google DNS
   nameserver 1.1.1.1   # Cloudflare DNS
   

2. Install bind9 (BIND DNS server) for authoritative DNS:

   sudo apt install bind9
   

Configuring DHCP on Linux (ISC DHCP Server)

1. Install the server:

   sudo apt install isc-dhcp-server
   

2. Edit /etc/dhcp/dhcpd.conf:

   subnet 192.168.1.0 netmask 255.255.255.0 {
       range 192.168.1.100 192.168.1.200;
       option routers 192.168.1.1;
       option domain-name-servers 8.8.8.8, 1.1.1.1;
   }
   

3. Restart the service:

   sudo systemctl restart isc-dhcp-server
   

Security Considerations

DNS Attacks

1. DNS Spoofing (Cache Poisoning)

   • Attacker injects false IP records into DNS cache (e.g., redirecting khalti.com to a fake site).
   • Mitigation: Use DNSSEC (DNS Security Extensions).

2. DDoS on DNS

   • Overwhelming DNS servers (e.g., cloudflare.com DNS servers) to take down services.
   • Mitigation: Anycast routing (e.g., Cloudflare’s global DNS network).

DHCP Attacks

1. Rogue DHCP Server

   • Malicious server offers incorrect gateway/DNS, intercepting traffic.
   • Mitigation: Disable DHCP on unused interfaces, use static IPs for critical devices.

2. IP Exhaustion

   • DHCP server runs out of IPs, causing network outages.
   • Mitigation: Monitor lease times, expand IP pools.

Exam Tip

1. DNS Hierarchy: Always draw the root → TLD → authoritative flow in exams. Label each step (e.g., "Root server refers to .np TLD").
2. DHCP 4-Way Handshake: Memorize the DHCPDISCOVER → DHCPOFFER → DHCPREQUEST → DHCPACK sequence. Use it to explain how your laptop gets an IP at home.
3. Record Types: Know A, AAAA, MX, CNAME, NS and their real-world uses (e.g., MX for email routing in eSewa).
4. Linux Commands:
   • dig esewa.com.np (DNS lookup)
   • nslookup google.com (Alternative to dig)
   • sudo dhclient (Request DHCP lease manually)
5. Troubleshooting:
   • DNS fails? Check /etc/resolv.conf and test with ping 8.8.8.8 (if ping works but ping google.com fails, DNS is the issue).
   • DHCP fails? Verify dhcpd logs (/var/log/syslog) and firewall rules (port 67/68).

TAKEAWAYS:

• DNS translates names to IPs using a hierarchical, distributed system of root, TLD, and authoritative servers.
• DHCP automates IP assignment via a 4-way handshake, reducing manual configuration errors.
• DNSSEC and firewall rules are critical for securing DNS/DHCP against spoofing and exhaustion attacks.
• Linux tools like bind9, isc-dhcp-server, and dig are essential for administering these services in real-world networks (e.g., Ncell’s DHCP for mobile users).
• Real-world examples (eSewa’s DNS, NTC’s traffic systems) show how these protocols enable modern services—failures here cause outages like Kathmandu’s traffic jams.