Cambridge A-Level Computer Science (9618) · lesson 10 of 15 · 25 min
DNS in Depth
The DNS hierarchy has four levels of servers:
- Root servers: 13 logical root server clusters worldwide; know the addresses of all TLD servers.
- TLD servers: Handle top-level domains (.com, .uk, .org). Managed by registries.
- Authoritative servers: Hold the actual DNS records for specific domains.
- Recursive resolvers: Your ISP’s DNS server; queries the hierarchy on your behalf and caches results.
DNS record types: A (IPv4 address), AAAA (IPv6), MX (mail server), CNAME (alias), TXT (verification, SPF).
HTTP Methods and Status Codes
Cookies and Sessions
HTTP is stateless — each request is independent. Cookies are small text files stored by the browser that allow servers to maintain state across requests (login sessions, shopping carts, preferences).
Session cookies: Deleted when browser closes. Persistent cookies: Stored until expiry date. Secure cookies: Only sent over HTTPS. HttpOnly cookies: Not accessible to JavaScript (prevents XSS theft).
Which DNS record type maps a domain name to an IPv4 address?
- MX
- CNAME
- A
- TXT
Answer: A
An A record maps a hostname to an IPv4 address.
Which HTTP method is used to retrieve a resource without modifying it?
- POST
- PUT
- DELETE
- GET
Answer: GET
GET retrieves a resource; it should have no side effects.
An HttpOnly cookie attribute prevents:
- The cookie being sent over HTTP
- JavaScript from accessing the cookie
- The cookie from being stored
- The cookie from expiring
- Answer: 1
HttpOnly prevents JavaScript (document.cookie) from reading the cookie, protecting against XSS attacks.
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