Networking / Stage 9 of 14

Why the address works and the name does not

Not running Starting usually ~20 s

Step 7 of 8 · lab · 15 min

Internal names do not resolve

The name app.internal does not resolve, although the DNS server runs on this very machine. Find out why the system does not ask it and fix that.

Situation

After the server moved, internal names stopped resolving: getent hosts app.internal prints nothing, and ping app.internal says Temporary failure in name resolution. The *.internal names on this network are served by a DNS server running on this very machine.

Task

The name app.internal resolves: getent hosts app.internal prints the address 10.0.0.10.
The address comes from the DNS server, not from a line in /etc/hosts.

There is no internet on the machine, and everything you need is installed. The app.internal service itself is not running in this lab — only the name is checked.

Lab · on your own

No commands are given in this lab: type them in the terminal yourself. Goals are checked on the machine after every command.

Goals

0 of 2

getent hosts app.internal prints 10.0.0.10 — not done yet
The address comes from the DNS server, not from /etc/hosts — not done yet

Ctrl+Enter

Starts the machine node1 · Ubuntu 24.04

The machine is starting

Hints

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Report a problem

Lab goals

0 of 2

getent hosts app.internal prints 10.0.0.10 — not done yet
The address comes from the DNS server, not from /etc/hosts — not done yet

node1 Ubuntu 24.04

Lab goals

0 of 2

getent hosts app.internal prints 10.0.0.10 — not done yet
The address comes from the DNS server, not from /etc/hosts — not done yet

Network · picked for this lab

DNS and name resolution

Cheat sheet

$_ — insert into the terminal

getent hosts example.com how the system actually resolves it (for apps)
resolvectl status DNS servers and search domains
dig +short example.com a quick answer (IP only)
dig example.com MX a specific record type
dig @8.8.8.8 example.com check via a public DNS server
dig +trace example.com where delegation breaks
resolvectl flush-caches clear the cache
local overrides go in /etc/hosts; permanent ones in netplan's nameservers

/etc/hosts — local overrides (usually take priority over DNS):

127.0.0.1     localhost
192.168.1.10  myserver myserver.local

bash

getent hosts myserver             # check the system sees the override
resolvectl flush-caches           # clear systemd-resolved's cache
resolvectl statistics             # cache hits/misses

dig / host / nslookup

bash

dig example.com                   # a detailed DNS query (package dnsutils/bind9-dnsutils)
dig +short example.com            # answer only (IP) — handy in scripts
dig +short example.com AAAA       # the IPv6 address
dig example.com MX                # a record type (A, AAAA, MX, TXT, NS, SOA, CNAME…)
dig @8.8.8.8 example.com          # ask a SPECIFIC server (bypassing the local one)
dig +trace example.com           # walk delegation from the root (where it breaks)
dig -x 1.2.3.4                    # a reverse lookup (PTR)
dig +noall +answer example.com   # only the answer section
host example.com                  # short and simple
host -t MX example.com           # a record type
nslookup example.com             # the classic (also interactive)

IPs ping, but names don't

DNS isn't working. Work down the ladder: 1. resolvectl status — does the interface have a DNS server (Current DNS Server)? 2. getent hosts example.com — does it resolve through the system at all? 3. dig @8.8.8.8 example.com — does a public DNS server answer (if yes, the problem is local/resolved)?

A name resolves to the wrong IP

An entry in /etc/hosts overrides DNS — check it (getent hosts name).
A stale cache: resolvectl flush-caches.
Multiple A records/round-robin — that's normal (load balancing).

resolv.conf gets overwritten after a reboot

Don't edit /etc/resolv.conf directly (it's a symlink to resolved). Configure DNS in netplan (nameservers:) or /etc/systemd/resolved.conf, then run sudo netplan apply / systemctl restart systemd-resolved.

Internal names don't resolve, external ones do

There's no search domain or the right DNS for the internal zone. Check resolvectl status (Search Domains) and add the domain/server in netplan.

`dig` works, but the application "can't find" the host

dig goes to DNS bypassing NSS. Check with getent hosts/resolvectl query instead — the cause is in /etc/hosts, nsswitch.conf, or resolved's cache.

Why the address works and the name does not

Situation

Task

Signing in is needed to start the machine

dig / host / nslookup

IPs ping, but names don't

A name resolves to the wrong IP

resolv.conf gets overwritten after a reboot

Internal names don't resolve, external ones do

dig works, but the application "can't find" the host

`dig` works, but the application "can't find" the host