cloudflare_domains.sh

#!/bin/bash
# cloudflare_domains.sh/pixnion/2017-02-25
# input domains to check on stdin or as cmdline arguments

die()
{
  printf '%s\n' "$1"
  exit 1
}

check()
{
  local r res
  res=()
  while read -r r; do
    if [[ $line =~ cloudflare ]]; then
      res+=("$r")
    elif printf '%s\n' "$r" | grepcidr -f <(printf '%s\n' "${n[@]}") >&-; then
      res+=("$r")
    fi
  done < <(dig "$1" ns +short; dig "$1" a +short; dig "www.$1" a +short)

  [ "${#res[@]}" == '0' ] &&  return 1
  printf '%s\n' "${res[@]}" | sort | uniq | xargs
}

for bin in curl grepcidr; do
  hash -r $bin &> /dev/null || \
    die "$bin is required."
done

n=($(curl -Ss 'https://www.cloudflare.com/ips-v4'))
[ "${#n[@]}" == '0' ] && \
  die 'Failed fetching cloudflare networks.'

[ "$#" -ge '1' ] && exec 0< <(printf '%s\n' "$@")

while read -r dom; do
  out="$(check "$dom")" || continue
  printf '%s: %s\n' "$dom" "$out"
done