Utility script to help GCP Compute Engine VMs to set their ephemeral IP address to Cloud DNS zone on boot.

compute-engine-ddns.service

[Unit]
Description=Update current instance IP to Cloud DNS.
After=network-online.target auditd.service

[Service]
#ExecStart=/opt/compute-engine-ddns.sh <zone-name> <domain>
Type=oneshot

[Install]
WantedBy=multi-user.target

compute-engine-ddns.sh

#!/bin/bash

# Usage:
# compute-engine-ddns.sh <zone-name> <domain>
#
# Example:
# compute-engine-ddns.sh zone-1 instance-1.example.com

DNS_ZONE=$1
DOMAIN=$2

INSTANCE_NAME=$(curl -sS http://metadata.google.internal/computeMetadata/v1/instance/name -H Metadata-Flavor:Google)
INSTANCE_ZONE=$(curl -sS http://metadata.google.internal/computeMetadata/v1/instance/zone -H Metadata-Flavor:Google)
EXISTING=$(gcloud dns record-sets list --zone="$DNS_ZONE" --name="$DOMAIN." --format="value(rrdatas)")
NEW_IP=$(gcloud compute instances describe "$INSTANCE_NAME" --zone="$INSTANCE_ZONE" --format="value(networkInterfaces[0].accessConfigs.natIP)")

if [ -n "$EXISTING" ]; then
	gcloud dns record-sets update "$DOMAIN." --rrdatas="$NEW_IP" --type=A --zone="$DNS_ZONE"
else
	gcloud dns record-sets create "$DOMAIN." --rrdatas="$NEW_IP" --type=A --zone="$DNS_ZONE"
fi