問71
頻出度★★★
DNSサーバの機能に関する記述として, 適切なものはどれか。
平成26年度 春期 ITパスポート試験 問71
テクノロジ系ネットワーク › ネットワーク応用
This question's explanation and your answers so far.
解説
問 71
Incorrectア
Relaying packets destined for computers outside the same LAN is the function of a router (default gateway). A router looks at the destination IP address at the network layer, consults its routing table, and forwards packets between different networks. A DNS server does not relay packets itself; it only answers the correspondence between names and IP addresses.
Incorrectイ
Relaying Web access to external networks is the function of a proxy server. A proxy server accesses external Web servers on behalf of PCs inside the organization, and it may cache responses, perform access control, or keep logs. This is proxied access, not name resolution, so it is not a description of a DNS server.
Incorrectウ
Returning the MAC address corresponding to the queried IP address is the work of ARP (Address Resolution Protocol). ARP is a protocol for obtaining the MAC address, which is a physical address, from an IP address within the same LAN, and the decisive difference from DNS is that what it maps to is a MAC address. What DNS handles is the correspondence between host names and IP addresses.
- エ
正解 The correct answer is エ, “It returns the IP address corresponding to the queried host name.” Communication on the Internet is ultimately directed to IP addresses, but what people actually handle are host names (domain names) such as www.example.co.jp. Managing the correspondence between these two and returning the matching IP address in response to a query from a client is the role of a DNS (Domain Name System) server, and this process is called name resolution. For example, when a user types www.example.co.jp into a browser, the PC first asks the DNS server configured on it, “What is the IP address of this host name?”, and then connects to the Web server using the returned IP address as the destination. DNS servers manage the domains of the whole world in a hierarchical structure with divided responsibility, and for a name they do not know themselves they query a higher-level or another DNS server to obtain the answer. Incidentally, the reverse lookup that obtains a host name from an IP address is also a DNS function, but either way everything comes down to the single point of mapping between names and IP addresses.
ここがポイント
- A DNS server manages the correspondence between host names (domain names) and IP addresses, and returns an IP address in response to a query. This process is called name resolution.
- The key point is telling DNS apart from servers and protocols with similar roles: a router relays packets addressed to a different network, a proxy server acts on behalf of clients and relays Web access to the outside, and ARP is a mechanism for obtaining a MAC address from an IP address. All of them differ from DNS in what they map or in which layer they work.
用語