public class NameAuthorityPointerEncoder
extends Object
4. NAPTR RR Format
4.1 Packet Format
The packet format of the NAPTR RR is given below. The DNS type code
for NAPTR is 35.
The packet format for the NAPTR record is as follows
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ORDER |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PREFERENCE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ FLAGS /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ SERVICES /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ REGEXP /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ REPLACEMENT /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
<character-string> and <domain-name> as used here are defined in RFC
1035 [7].
ORDER
A 16-bit unsigned integer specifying the order in which the NAPTR
records MUST be processed in order to accurately represent the
ordered list of Rules. The ordering is from lowest to highest.
If two records have the same order value then they are considered
to be the same rule and should be selected based on the
combination of the Preference values and Services offered.
PREFERENCE
Although it is called "preference" in deference to DNS
terminology, this field is equivalent to the Priority value in the
DDDS Algorithm. It is a 16-bit unsigned integer that specifies
the order in which NAPTR records with equal Order values SHOULD be
processed, low numbers being processed before high numbers. This
is similar to the preference field in an MX record, and is used so
domain administrators can direct clients towards more capable
hosts or lighter weight protocols. A client MAY look at records
with higher preference values if it has a good reason to do so
such as not supporting some protocol or service very well.
The important difference between Order and Preference is that once
a match is found the client MUST NOT consider records with a
different Order but they MAY process records with the same Order
but different Preferences. The only exception to this is noted in
the second important Note in the DDDS algorithm specification
concerning allowing clients to use more complex Service
determination between steps 3 and 4 in the algorithm. Preference
is used to give communicate a higher quality of service to rules
that are considered the same from an authority standpoint but not
from a simple load balancing standpoint.
It is important to note that DNS contains several load balancing
mechanisms and if load balancing among otherwise equal services
should be needed then methods such as SRV records or multiple A
records should be utilized to accomplish load balancing.
FLAGS
A <character-string> containing flags to control aspects of the
rewriting and interpretation of the fields in the record. Flags
are single characters from the set A-Z and 0-9. The case of the
alphabetic characters is not significant. The field can be empty.
It is up to the Application specifying how it is using this
Database to define the Flags in this field. It must define which
ones are terminal and which ones are not.
SERVICES
A <character-string> that specifies the Service Parameters
applicable to this this delegation path. It is up to the
Application Specification to specify the values found in this
field.
REGEXP
A <character-string> containing a substitution expression that is
applied to the original string held by the client in order to
construct the next domain name to lookup. See the DDDS Algorithm
specification for the syntax of this field.
As stated in the DDDS algorithm, The regular expressions MUST NOT
be used in a cumulative fashion, that is, they should only be
applied to the original string held by the client, never to the
domain name produced by a previous NAPTR rewrite. The latter is
tempting in some applications but experience has shown such use to
be extremely fault sensitive, very error prone, and extremely
difficult to debug.
REPLACEMENT
A <domain-name> which is the next domain-name to query for
depending on the potential values found in the flags field. This
field is used when the regular expression is a simple replacement
operation. Any value in this field MUST be a fully qualified
domain-name. Name compression is not to be used for this field.
This field and the REGEXP field together make up the Substitution
Expression in the DDDS Algorithm. It is simply a historical
optimization specifically for DNS compression that this field
exists. The fields are also mutually exclusive. If a record is
returned that has values for both fields then it is considered to
be in error and SHOULD be either ignored or an error returned.
- Author:
- Apache Directory Project