Before switching a service to dual-stack, you want to see every address, IPv4 and IPv6, that the system's normal resolution path returns for `www.example.com`. Which command does that?
LPIC-1 Exam 102-500, objective 109. Networking fundamentals medium
Machine-checked — no person has signed for it. This question was read against the source cited below by an automated pass, which found no contradiction. That is a weaker claim than it sounds: the same kind of process wrote the question, so it can confirm its own mistake.
Treat it as a good draft rather than as settled fact, and read the source below before you rely on it. It is not used in mock exams here — only questions a person has signed for are.
How these questions are written — where each question comes from, what the verification ledger records, and what happens when one is found wrong.
The options
Correct getent ahosts www.example.com
Correct. The ahosts database calls getaddrinfo with the address family unspecified, so it lists both IPv6 and IPv4 results exactly as an application would receive them.
Not correct getent ahostsv4 www.example.com
Wrong. ahostsv4 pins the family to AF_INET, so any AAAA result is omitted.
Not correct getent networks www.example.com
Wrong. The networks database looks entries up in /etc/networks, which maps network names to network numbers, not hosts to addresses.
Not correct getent passwd www.example.com
Wrong. passwd is the account database; the key would be a user name or UID and this lookup simply fails.
Why
getent takes a database name and a key, and resolves the key through the sources configured for that database in /etc/nsswitch.conf. For hosts the family handling differs by database: hosts goes through gethostbyname2 and returns a single family, the IPv6 answer if there is one and otherwise the IPv4 one; ahostsv4 pins AF_INET and ahostsv6 AF_INET6; ahosts calls getaddrinfo with the family unspecified, so both are returned along with the socket type for each. Because all of them go through NSS, they also reflect /etc/hosts entries, which a pure DNS client would miss.
Where this comes from
- Cited
- manual page getent(1)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 109 and space the ones you get wrong.
More questions on this objective
- A subnet is defined as 192.168.10.0/26. How many addresses in it can actually be assigned to hosts? machine-checked
- A host is configured with the address 172.20.35.77/20. What is the network address of its subnet? machine-checked
- In IPv6, which address is the loopback address, equivalent in role to 127.0.0.1 in IPv4? machine-checked
- Which is the correct fully compressed form of the IPv6 address 2001:0db8:0000:0000:0000:ff00:0042:8329? machine-checked
- Which pairing of a service with its default port number is correct? machine-checked
- DNS normally uses UDP port 53. In which situation does a DNS client or server fall back to TCP on port 53? machine-checked