Multiple PTRs for the same IP

This is an answer to the frequently asked question “Why should I avoid Multiple PTRs for the same IP?” by Mark Andrews of ISC.

They are not a good idea because they don’t scale. In the
past large web hosters attempted to put a PTR record for
every virtual site on their servers. This ends up exceeding
the limits of normal query resolution support. You get
truncated TCP responses. You have to resort to AXFR to
retrieve the PTR records.

The DNS does not impose a order on returned records. If you
have multiple PTR records and you are trying to do access
control by name the application has to either list all the
names (if it looks at h_name) or try all the aliases. Not
all lookup mechanism supply all the names which inturn
leads to maintenance issues on the access lists.

Think of PTR records in the reverse tree as returning the
canonical name of the machine. Usually this would be the
name the machine knows itself as (fully qualified). If you
do this you won’t break the API’s for returning the name of
the machine based on the address.