How to make /etc/resolv.conf changes persistent in cloud environment

Environment

• Red Hat Enterprise Linux 8 (RHEL8)
• Cloud environment with cloud-init enabled

Issue

• DNS options in /etc/resolv.conf are lost during reboot
• ipv4.dns-options or ipv6.dns-options in NetworkManager connection settings lost during reboot
• cloud-init overwrites ifcfg-* files or keyfiles causing loss of customizations

Resolution

Disable cloud-init network configuration:

In `/etc/cloud/cloud.cfg` set:
    network:
       config: disabled

Configure the VM to pull the initial network configuration from the metadata server of the cloud (e.g. Azure):

# yum install NetworkManager-cloud-setup
# systemctl edit nm-cloud-setup.service
[Service]
Environment=NM_CLOUD_SETUP_AZURE=yes
################## end of nm-cloud-setup.service "snap-in" / override #########################
[Save the file and quit the editor.]

Environment is one of:

NM_CLOUD_SETUP_AZURE for Microsoft Azure
NM_CLOUD_SETUP_EC2 for Amazon EC2 (AWS)
NM_CLOUD_SETUP_GCP for Google Cloud Platform(GCP)
NM_CLOUD_SETUP_ALIYUN for Alibaba Cloud (Aliyun)

Reload the systemd configuration, enable and start the nm-cloud-setup service and turn on the nm-cloud-setup.timer:

# systemctl daemon-reload
# systemctl enable --now nm-cloud-setup.service
# systemctl enable --now nm-cloud-setup.timer

Ref: RHEL8 nm-cloud-setup and network management documentation

Now settings such as eth0 ipv4.dns-options will persist across reboots.

nmcli con mod "System eth0" +ipv4.dns-options "timeout:1, attempts:1 rotate"

Root Cause

cloud-init overwrote NM connection profiles and ifcfg-* files with default dhcp configurations, then NM overwrote /etc/resolv.conf without custom ipv4.dns-options.

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