DDNS system for Cloudflare: Update the DNS record if the webserver does not respond correctly

cloudflareddns.service

[Unit]
Description=Cloudflare DDNS
After=network.target

[Service]
#Environment=CLOUDFLARE_API_KEY=
Environment=ZONE_ID=1234
Environment=RECORD_ID=5678
Environment=RECORD_NAME=domain.example.com

ExecStart=/usr/local/bin/cloudflareddns /path/to/env.sh

WorkingDirectory=/tmp
Restart=on-failure
RestartSec=5s
SyslogIdentifier=ddns
Type=simple

[Install]
WantedBy=multi-user.target

cloudflareddns.sh

#!/bin/bash
PATH="/bin:/usr/bin";

[ -n "$1" ] && [ -e "$1" ] && source "$1";

check_environment() {
  [ -z "$CLOUDFLARE_API_KEY" ] && exec echo "CLOUDFLARE_API_KEY= must be set";
  [ -z "$RECORD_ID" ] && exec echo "RECORD_ID= must be set";
  [ -z "$RECORD_NAME" ] && exec echo "RECORD_NAME= must be set";
  [ -z "$ZONE_ID" ] && exec echo "ZONE_ID= must be set";
  return 0;
}

api() {
  echo "> curl $*" >&2;
  curl --no-progress-meter -H "Authorization: Bearer $CLOUDFLARE_API_KEY" -H "Content-Type: application/json" "$@";
}

curlx() {
  echo "> curl $*" >&2;
  curl --no-progress-meter "$@";
}

ensure_environment() {
  [ -z "$CHECK_INTERVAL" ] && CHECK_INTERVAL="10";
  [ -z "$API_BASE" ] && API_BASE="https://api.cloudflare.com/client/v4";
  [ -z "$WAN_IP_URL" ] && WAN_IP_URL="https://canhazip.com";
  return 0;
}

update_record() {
  local wan_ip; local data;
  wan_ip="$(curlx "$WAN_IP_URL")";
  [ -z "$wan_ip" ] && return 1;

  data="{'type':'A','name':'$RECORD_NAME','content':'$wan_ip'}";
  data="$(echo "$data" | sed 's/'\''/"/g')";
  api -X PUT "$API_BASE/zones/$ZONE_ID/dns_records/$RECORD_ID" --data "$data";
  echo;
}

main() {
  local record_check;
  while sleep "$CHECK_INTERVAL"; do
    record_check="$(curlx "https://$RECORD_NAME/$RECORD_NAME.txt")";
    [ "$record_check" = "$RECORD_ID" ] || update_record;
  done
}

check_environment && ensure_environment && main;