anchor.diff:  
     register-hostname = {
       zoneId, zone, iamCredentialName,
  -    useLocalHostname, prefix ? if useLocalHostname then "local" else "public"
  +    useLocalHostname,
  +    query ? if useLocalHostname then "local-ipv4" else "public-hostname",
  +    recordType ? if useLocalHostname then "A" else "CNAME"
     }: pkgs.writeScript "ec2-register-hostname" ''
       date=$(${curl} -I https://route53.amazonaws.com/date | ${awk} '/^Date: / {sub("Date: ", "", $0); sub("\\r", "", $0); print $0}')
   
  
positive.code:              <Action>UPSERT</Action>
               <ResourceRecordSet>
                  <Name>$hostname</Name>
                  <Type>CNAME</Type>
                  <TTL>30</TTL>
                  <ResourceRecords>
                     <ResourceRecord><Value>$local_hostname</Value></ResourceRecord>
                  </ResourceRecords>
               </ResourceRecordSet>
            </Change>
  
negative.code:     signature=$(echo -n $date | ${openssl} dgst -binary -sha1 -hmac $1 | ${base64})
      auth_header="X-Amzn-Authorization: AWS3-HTTPS AWSAccessKeyId=$2,Algorithm=HmacSHA1,Signature=$signature"
      hostname=$(${hostname}).${zone}
      record_value=$(${wget} http://169.254.169.254/latest/meta-data/${query})
  
      ${curl} -d @/dev/stdin \
            -H "Content-Type: text/xml" \
  