LPIC-1 109.4 — Configure Client Side DNS

Exam weight: 2 — LPIC-1 v5, Exam 102

What You Need to Know

From the official LPIC-1 objectives:

Query remote DNS servers.
Configure local name resolution and use remote DNS servers.
Modify the order in which name resolution is done.
Debug errors related to name resolution.
Awareness of systemd-resolved.

Key files and commands: /etc/hosts, /etc/resolv.conf, /etc/nsswitch.conf, host, dig, getent.

Name Resolution Process

When a program resolves a hostname, it calls the GNU C library (glibc), which reads /etc/nsswitch.conf to determine where and in what order to look up the name.

DNS has three record classes:

IN — Internet (TCP/IP addresses; used in all standard lookups).
CH — ChaosNet (legacy; rarely encountered).
HS — Hesiod (used to store passwd/group data in DNS).

/etc/nsswitch.conf

Controls the sources and order used for each type of name database.

passwd:         compat
group:          compat
hosts:          dns [!UNAVAIL=return] files
networks:       nis [NOTFOUND=return] files
services:       nis [NOTFOUND=return] files

The hosts line determines how hostnames are resolved. Each column is a source. Columns in brackets provide conditional logic:

Notation	Meaning
`[!UNAVAIL=return]`	If the service IS available, stop here (even if not found).
`[NOTFOUND=return]`	If the lookup succeeded but entry was not found, stop.
`[SUCCESS=continue]`	If found, continue to the next source anyway.

Common hosts line examples:

hosts:    files dns          # check /etc/hosts first, then DNS
hosts:    dns files          # check DNS first, then /etc/hosts

/etc/resolv.conf

Configures DNS resolution. May be overwritten by NetworkManager or other tools (look for a # Generated by NetworkManager comment at the top).

search lpi.org
nameserver 10.0.0.53
nameserver fd00:ffff::2:53

Directive	Description
`nameserver IP`	DNS server to use (up to 3 entries)
`search DOMAIN`	Append domain for short name lookups (up to 6)
`domain DOMAIN`	Local domain name (mutually exclusive with `search`)
`options KEY:VALUE`	Resolver options, e.g., `options timeout:3`

search and domain are mutually exclusive — if both appear, the last one takes effect.

/etc/hosts

Maps IP addresses to hostnames locally. Checked before DNS when files appears first in /etc/nsswitch.conf.

127.0.0.1       localhost
::1             localhost ip6-localhost ip6-loopback
10.0.0.1        gateway.lpi.org gateway gw
10.0.1.53       dns1.lpi.org

Format: IP_ADDRESS primary-hostname [alias ...]

The IP address is always the first column.

systemd-resolved

systemd-resolved is a systemd service that provides DNS resolution on 127.0.0.53. It proxies queries to the servers configured in /etc/resolv.conf or /etc/systemd/resolv.conf.

To use it, set resolve as a source in /etc/nsswitch.conf:

hosts:    resolve [!UNAVAIL=return] files

Note: systemd-resolved does not replace a full DNS server; it only forwards requests.

getent

getent retrieves entries from NSS databases using the same resolver libraries that applications use, respecting /etc/nsswitch.conf.

getent hosts                  # list all entries from the hosts database
getent hosts dns1.lpi.org     # look up a specific hostname

getent -s files hosts learning.lpi.org   # force lookup from /etc/hosts
getent -s dns hosts learning.lpi.org     # force lookup via DNS

The -s option (glibc 2.2.5+) specifies which data source to use.

Key difference: getent uses the full NSS stack (respects nsswitch.conf); host and dig query DNS directly.

host

host is a simple DNS lookup tool. By default it returns A, AAAA, and MX records.

host wikipedia.org                    # A, AAAA, MX records
host 208.80.154.224                   # reverse lookup (PTR record)
host -t NS lpi.org                    # query specific record type
host -t MX lpi.org dns1.easydns.com  # use specific nameserver

Option	Description
`-t TYPE`	Record type: A, AAAA, MX, NS, SOA, PTR, etc.
Last argument	If it is an IP or hostname of a name server, use that server instead of `/etc/resolv.conf`

dig

dig is a verbose DNS diagnostic tool. By default it queries for A records.

dig learning.lpi.org              # A record lookup
dig -t SOA lpi.org                # query SOA record
dig +short lpi.org                # terse output (IP only)
dig +noall +answer +question lpi.org   # show only question and answer sections
dig +nocookie -t MX lpi.org       # disable EDNS cookie extension

dig Output Sections

Section	Contents
Header	Query metadata (opcode, status, flags, counts)
Question	The query that was sent
Answer	Resource records returned
Authority	NS records for the authoritative zone
Additional	A/AAAA records for servers in authority section

dig Options

Option	Description
`-t TYPE`	Record type to query
`+short`	Suppress all output except the result
`+noall`	Disable all output sections
`+answer`	Re-enable the answer section
`+question`	Re-enable the question section

~/.digrc

Default dig options can be set in ~/.digrc. For example, to always use +short:

+short

Quick Reference

/etc/nsswitch.conf:
  hosts: files dns        check /etc/hosts first, then DNS
  [!UNAVAIL=return]       stop if service is available (even if not found)
  [NOTFOUND=return]       stop if found but entry absent
  [SUCCESS=continue]      continue even if found

/etc/resolv.conf:
  nameserver IP           DNS server (up to 3)
  search DOMAIN           short-name search domain (up to 6)
  domain DOMAIN           local domain (mutex with search)
  options timeout:3

/etc/hosts:
  IP  hostname [alias]    left column is always IP

systemd-resolved:
  listens on 127.0.0.53; proxies to configured DNS
  use "resolve" in nsswitch.conf hosts line

getent hosts [NAME]          NSS lookup (respects nsswitch.conf)
getent -s files hosts NAME   force /etc/hosts
getent -s dns hosts NAME     force DNS

host NAME                    returns A, AAAA, MX
host -t TYPE NAME            specific record type
host NAME server             use specific nameserver

dig NAME                     verbose A lookup
dig -t TYPE NAME             specific record type
dig +short NAME              terse (IP only)
dig +noall +answer NAME      answer section only
~/.digrc                     default dig options

Exam Questions

What file controls the order in which name databases are consulted? → /etc/nsswitch.conf
What does [!UNAVAIL=return] mean in /etc/nsswitch.conf? → If the service is available (not unavailable), stop here and do not try the next source.
How many nameserver entries can appear in /etc/resolv.conf? → Up to 3.
How many search domains can be configured in /etc/resolv.conf? → Up to 6.
What happens if both search and domain appear in /etc/resolv.conf? → They are mutually exclusive; the last one in the file takes effect.
What is the correct format of an /etc/hosts entry? → IP address in the first column, followed by hostname and optional aliases: IP hostname [alias]
On what address does systemd-resolved listen? → 127.0.0.53
What nsswitch.conf source keyword enables systemd-resolved? → resolve
What is the key difference between getent and host/dig? → getent uses the full NSS resolver stack (respects nsswitch.conf); host and dig query DNS directly.
What getent option forces a lookup using /etc/hosts? → -s files
What does host wikipedia.org return by default? → A, AAAA, and MX records.
What host option specifies the DNS record type to query? → -t TYPE
How do you use host to query a specific name server? → Pass the server IP or hostname as the last argument: host NAME server.
What dig option shows only the IP address result? → +short
What file stores default dig options? → ~/.digrc
What dig combination shows only the question and answer sections? → dig +noall +answer +question NAME
What does dig -t MX lpi.org do? → Queries for the MX (Mail Exchanger) records for lpi.org.
Why might manual changes to /etc/resolv.conf not persist? → Tools like NetworkManager regenerate this file; changes are overwritten when the network is reconfigured.

Exercises

Exercise 1 — nsswitch.conf Order

Change the resolution order so that DNS is consulted before /etc/hosts for host lookups.

Answer

Edit /etc/nsswitch.conf and set:

hosts:    dns files

Exercise 2 — resolv.conf Configuration

Configure /etc/resolv.conf to use DNS servers 1.1.1.1 and 8.8.8.8, with search domain example.com.

Answer

nameserver 1.1.1.1
nameserver 8.8.8.8
search example.com

Exercise 3 — getent vs dig

Look up the address of myserver.local using the full NSS stack, then look it up using DNS only.

Answer

getent hosts myserver.local             # uses nsswitch.conf order
getent -s dns hosts myserver.local      # DNS only

Or using dig for the DNS-only lookup:

dig myserver.local

Exercise 4 — host Record Types

Use host to look up the NS records for lpi.org, then the SOA record.

Answer

host -t NS lpi.org
host -t SOA lpi.org

Exercise 5 — dig Terse Output

Use dig to display only the IP address of wikipedia.org.

Answer

dig +short wikipedia.org

LPIC-1 Study Notes | Topic 109: Networking Fundamentals