How can I prevent NetworkManager from modifying the /etc/resolv.conf file?

Environment

• Red Hat Enterprise Linux 10
• Red Hat Enterprise Linux 9
• Red Hat Enterprise Linux 8
• Red Hat Enterprise Linux 7
• NetworkManager

Issue

• NetworkManager modifies the resolv.conf file and any manual changes there are lost
• How to prevent the /etc/resolv.conf file from being overwritten by NetworkManager?

Resolution

• In main section in /etc/NetworkManager/NetworkManager.conf file, define the dns parameter with a value of none:

  # vi /etc/NetworkManager/NetworkManager.conf 
  [main]
  dns=none
  

• Restart the NetworkManager service:

  # systemctl restart NetworkManager
  

• Or the following works for RHEL8 and RHEL9.

  # cat << 'EOF' > /etc/NetworkManager/conf.d/90-dns-none.conf
  [main]
  dns=none
  EOF
  # systemctl reload NetworkManager
  

Root Cause

• Per the NetworkManager.conf man page:

         dns
             Set the DNS (resolv.conf) processing mode. If the key is
             unspecified, default is used, unless /etc/resolv.conf is a symlink
             to /run/systemd/resolve/resolv.conf, /lib/systemd/resolv.conf or
             /usr/lib/systemd/resolv.conf. In that case, systemd-resolved is
             chosen automatically.
  
             default: NetworkManager will update /etc/resolv.conf to reflect
             the nameservers provided by currently active connections.
  
             dnsmasq: NetworkManager will run dnsmasq as a local caching
             nameserver, using a "split DNS" configuration if you are connected
             to a VPN, and then update resolv.conf to point to the local
             nameserver. It is possible to pass custom options to the dnsmasq
             instance by adding them to files in the
             "/etc/NetworkManager/dnsmasq.d/" directory. Note that when
             multiple upstream servers are available, dnsmasq will initially
             contact them in parallel and then use the fastest to respond,
             probing again other servers after some time. This behavior can be
             modified passing the 'all-servers' or 'strict-order' options to
             dnsmasq (see the manual page for more details).
  
             unbound: NetworkManager will talk to unbound and dnssec-triggerd,
             providing a "split DNS" configuration with DNSSEC support.
             /etc/resolv.conf will be managed by dnssec-trigger daemon.
  
             systemd-resolved: NetworkManager will push the DNS configuration
             to systemd-resolved
  
             none: NetworkManager will not modify resolv.conf. This implies
             rc-manager unmanaged
  

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