DNS protocols were designed with the assumption that a certain
amount of trust could be presumed between the operators of primary
and secondary servers for a given zone. However, in current
practice some organizations have scenarios which require them to
accept zone data from sources that are not fully trusted (for
example: providers of secondary name service). A party who is
allowed to feed data into a zone (e.g. by AXFR, IXFR, or Dynamic DNS
updates) can overwhelm the server which is accepting data by
intentionally or accidentally exhausting that server's memory.