DNS Config Under Linux

DNS usage on linux is done over a set of routines in the C library that provide access to the Internet Domain Name System (DNS). The resolver configuration file (resolv.conf) contains information that is read by the resolver routines the first time they are invoked by a process. In short each process requesting DNS will read /etc/resolv.conf over library. The NSS is layered on top of this, and is configured by /etc/nsswitch.conf.

Linux DNS config are located in the file /etc/resolv.conf BUT there are a number of programs/services that wants to automatically manage and handle the DNS configuration file at /etc/resolv.conf. In some situations you may want to manage this file yourself. Each program/service managing DNS have its own configuration files like /etc/dnsmasq.conf (for dnsmasq service) and append the DNS config at connection change and/or on other events... a quick solution is to lock the DNS config file with chattr +i /etc/resolv.conf but this is not recommended in certain case, a better solution is to setup correctly all the program/services using the DNS like (dnsmasq/network-manager/resolvconf/etc.)

Getting Back The Control Of DNS

Here is an exhaustive list of setups to get back the control of resolv.conf and avoid having it overwritten (how to disable/setup DNS from other location other than resolv.conf) note that resolvconf is an independent program from resolv.conf, also depending on your system/config you may not have one or many of the programs listed here.

1. Resolvconf:

Config files

cat /etc/resolvconf/resolv.conf.d/head
nameserver 8.8.4.4
cat /etc/resolvconf/resolv.conf.d/base
nameserver 8.8.4.4

Update the config

sudo resolvconf -u

Disable resolvconf

systemctl disable --now resolvconf.service 

2. Dnsmasq Service:

Config files

cat /etc/dnsmasq.conf
server=1.1.1.1
server=8.8.4.4

Update the config

sudo systemctl restart dnsmasq.service

3. Network Manager:

Config files

/etc/NetworkManager/*

Disable DNS

$ cat /etc/NetworkManager/conf.d/no-dns.conf
[main]
dns=none

Enable DNS

$ cat /etc/NetworkManager/conf.d/dns.conf
[main]
dns=default

[global-dns]
searches=example.com

[global-dns-domain-*]

Use resolved service

$ cat /usr/lib/NetworkManager/conf.d/resolved.conf 
[main]
dns=systemd-resolved

Use resolvconf

$ cat /usr/lib/NetworkManager/conf.d/resolvconf.conf 
[main]
rc-manager=resolvconf

Update the config

systemctl restart NetworkManager.service

4. Network Interfaces:

Config files

$ cat /etc/network/interfaces
#nameservers
# or dns-search like so
# dns-search x.y 
dns-nameservers 4.4.4.4 8.8.8.8

Update The Config

reboot

5. DHCP Client:

Config files

$ cat /etc/dhcp3/dhclient.conf
supersede domain-name-servers <dns_ip_address1>,<dns_ip_address2>;

Update The Config

reboot

6. Rdnssd Service:

Disable rdnssd

systemctl disable --now rdnssd.service

7. Resolved Service:

Disable resolved

systemctl disable --now systemd-resolved.service

8. Netconfig:

Config files

/etc/sysconfig/network/config

Disable netconfig

cat /etc/sysconfig/network/config
NETCONFIG_DNS_POLICY=""

Update The Config

reboot

Setting The DNS Server

Example of a /etc/resolv.conf configuration

#Cloudflare
nameserver 1.0.0.1

#Google
#nameserver 8.8.8.8
#nameserver 8.8.4.4

#Cloudflare 
#nameserver 1.1.1.1

#Classic Config
#nameserver 192.168.1.1
#search lan