The Internet, DNS and Internet security – A-level Computer Science exam questions with mark scheme

1Standard5 marks

A user types www.example.com into a web browser.

(a) Describe how the Domain Name System (DNS) is used so that the browser can load the page. [3]

(b) Explain why the DNS is used rather than requiring users to type IP addresses. [1]

(a) The domain name is sent (by the browser / computer) to a DNS server (allow: local cache checked first, then the DNS server queried if not found)(1)
(a) The DNS server looks up the matching IP address (or asks other DNS servers if it does not hold it) and returns the IP address(1)
(a) The browser uses the IP address to contact the web server and request the page(1)
(b) Domain names are easier for humans to remember than IP addresses (allow: the IP address of a server can change without users needing to know)(1)
(c) A port number reserved / assigned for a particular (standard) server-side application or protocol, so clients know which port to connect to (e.g. 80 for HTTP, 443 for HTTPS). Do not require the range 0–1023 or an example; reject 'a port that is commonly used' with no idea of it being reserved for a service(1)
Answer: (a) name sent to DNS server; IP address looked up and returned; browser uses IP address to request page from web server (b) names are easier to remember (c) a port number reserved/assigned to a standard server application protocol, e.g. 443 for HTTPS
Examiner tip: Describe the lookup as name to IP address; the DNS does not fetch the web page.

2Hard5 marks

A company uses a packet-filtering firewall and a proxy server.

(a) Describe how a packet-filtering firewall decides whether to allow a packet. [3]

(b) Explain two ways in which a proxy server can improve the security or performance of the network. [2]

(a) It examines header information of each packet, e.g. source / destination IP address, port number, protocol (at least one field needed)(1)
(a) It compares these with a set of rules (an access control list) set by the network administrator(1)
(a) The packet is allowed through if a rule permits it, otherwise it is dropped / rejected / blocked (ignore references to stateful inspection: not required and does not replace this point)(1)
(b) Any one way, with its benefit, from: hides internal IP addresses (requests appear to come from the proxy) so external attackers cannot directly target internal machines; filters / blocks access to unwanted or malicious websites or content; logs / monitors requests so misuse can be traced; caches frequently requested pages so they load faster and use less Internet bandwidth (allow any other valid way, e.g. all traffic must pass through one point that can be checked)(1)
(b) A second, different way, with its benefit, from the list above(1)
Answer: (a) header checked against rules, accept or drop (b) hides internal addresses/filters content; caching
Examiner tip: Say which header fields are checked; "checks for viruses" describes antivirus, not packet filtering.

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