On a host running systemd-resolved, /etc/resolv.conf is a symbolic link into /run and contains the single entry `nameserver 127.0.0.53`. Your hand-edited nameserver lines vanish after every reboot. Where should the upstream DNS servers be set instead?
LPIC-1 Exam 102-500, objective 109. Networking fundamentals medium
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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 In the DNS= setting of /etc/systemd/resolved.conf (or a drop-in under /etc/systemd/resolved.conf.d/), then restart systemd-resolved.
Correct. resolved owns the generated file in /run; DNS= is the global upstream server list it reads at start-up, and a drop-in is the supported way to override it.
Not correct In /etc/hosts, one line per upstream server address.
Wrong. /etc/hosts maps names to addresses for the files NSS source. It cannot nominate a resolver.
Not correct By running `resolvectl dns eth0 192.0.2.1`, which writes the setting into resolved.conf so it survives a reboot.
Wrong. resolvectl dns does set the DNS servers for a link, but only in the running resolved: it writes nothing to disk, so the setting is lost when resolved restarts or the link is reconfigured.
Not correct On the hosts line of /etc/nsswitch.conf, appending the server addresses after `resolve`.
Wrong. nsswitch.conf names sources such as files, dns and resolve; its entries are module names, never server addresses.
Why
systemd-resolved runs a stub listener on 127.0.0.53 and publishes /run/systemd/resolve/stub-resolv.conf, to which /etc/resolv.conf is symlinked, so every application sends its queries to the local stub. The real upstream servers come from DNS= and FallbackDNS= in resolved.conf, plus whatever per-link servers arrive from DHCP through networkd or NetworkManager. Editing the generated file is pointless because resolved rewrites it; `resolvectl status` shows which servers are actually in force per link.
Where this comes from
- Cited
- manual page resolved.conf(5)
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
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- 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