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Pointer Record (PTR)

Pointer records are the opposite of A and AAAA RRs and are used in Reverse Map zone files to map an IP address (IPv4 or IPv6) to a host name.


name ttl  class   rr     name
15         IN     PTR

The number '15' (the base IP address) in the above example is actually a name and because there is no 'dot' BIND adds the $ORIGIN ( The example below which defines a reverse map zone file for the Class C address should make this clearer:

$TTL 2d ; 172800 secs
$ORIGIN 23.168.192.IN-ADDR.ARPA.
@             IN      SOA (
                              2003080800 ; serial number
                              12h         ; refresh
                              15m        ; update retry
                              3w         ; expiry
                              3h         ; minimum
              IN      NS
              IN      NS
; 2 below is actually an unqualified name and becomes
2             IN      PTR ; FDQN
15            IN      PTR
17            IN      PTR
74            IN      PTR


  1. Because the $ORIGIN reflects the reverse map domain, all right-hand names must use an FQDN format (they end with a dot). If the terminating dot on above were omitted in error it would become - not the desired result!.

  2. PTR RRs, like most other RRs, can have RRsets (RRs which have the same owner-name (left-hand-name) and RR type). If multiple names are assigned to a host using CNAME RRs, A RRs or AAAA RRs then they can all appear in the reverse map, for example:

    ; forward zone file for
    mail  IN  A
    www   IN  A
    ; OR
    mail  IN  A
    www   IN  CNAME
    # reverse map zone file for 0.168.192.IN-ADDR.ARPA
    7     IN  PTR
    7     IN  PTR

    is a prefectly legitimate zone file. However, in tests a number of mail systems, which invariably perform a reverse look up, did not handle RRsets and failed unless the mail server appeared first which is difficult to guarantee (see rrset-order). Thus, if a mail server ( and, say, a web server ( both have the same IP address (as illustrated in the example above) then since mail systems invariably use reverse lookups as a trivial security check it would be sensible to define the reverse map to only contain

  3. It is not essential, but considered good practise, to define all assigned IPs in the reverse map. Care should, however, be exercised to avoid huge PTR RRsets. For example, a web server may map thousands of virtual domains onto a single IP address (of the web server). If every one of these virtual domains appeared in the reverse map (a legitimate construct) then a reverse map query would lead to a huge RRset response and a DDoS attack would be a likely result.

  4. It is sensible to define the reverse map in order of IP addresses or some other fixed order to avoid subsequent errors or to simplify searching for a particular value.

  5. There are no A RRs for the defined NS names (respectively and since both are out-of-zone names. Any lookup is done via the forward zone file for in which suitable A RRs for these names must exist.

  6. It is sensible to define the reverse map in order of IP addresses or some other fixed order to avoid subsequent errors or to simplify searching for a particular value.

PTR Usage: While the overwhelming use of the PTR RR is in reverse mapping (for both IPv4 and IPv6) the PTR RR is not limited to this usage. Essentially, the PTR record may be thought of as a CNAME like RR. The significant difference is that a query which results in a CNAME RR will cause the DNS to automatically restart the query using the alias (or canonical) name if it resolves within the same domain. When a query results in a PTR RR(s) the response is immediately returned and no further DNS processing results. Other than reverse mapping, this author cannot immediately think of another use for such a capability. More imaginitive readers may find alternative applications.

PTR and IPv6

IPv6 and IPv4 addresses cannot be mixed in the same zone file as they can for forward-map zones. IPv6 addresses are reverse mapped under the domain IP6.ARPA whereas IPv4 addresses are mapped under the IN-ADDR.ARPA domain. IPv6 reverse-maps use a nibble domain name format defined in Chapter 3. The following fragment illustrates the use of the PTR RR to reverse-map the IPv6 addresses 2001:db8:0:1::1, 2001:db8:0:1::1, 2001:db8:0:2::1 and 2001:db8:0:1::1:

; reverse IPV6 zone file for
$TTL 2d    ; default TTL for zone 172800 secs
@         IN      SOA (
                        2003080800 ; sn = serial number
                        12h         ; refresh = refresh
                        15m        ; retry = update retry
                        3w         ; expiry = expiry
                        2h         ; min = minimum
; name servers Resource Recordsfor the domain
          IN      NS
; the second name servers is 
; external to this zone (domain).
          IN      NS
; PTR RR maps a IPv6 address to a host name
; hosts in subnet ID 1         IN      PTR         IN      PTR
; hosts in subnet ID 2         IN      PTR         IN      PTR

Notes: The IPv6 range 2001:db8:: is reserved for documentation purposes only by the great and mighty.

Problems, comments, suggestions, corrections (including broken links) or something to add? Please take the time from a busy life to 'mail us' (at top of screen), the webmaster (below) or info-support at zytrax. You will have a warm inner glow for the rest of the day.

Pro DNS and BIND by Ron Aitchison


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