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Name Server Reverse Lookup Configuration Method for IP Address Space
Smaller than /24
Japan Network Information Center
Superceded on: 22 October 1999
＊Outline of this document＊
This document is engineering reference material.
This document explains the name server reverse lookup configuration
method for IP address space smaller than /24.
＊Contents＊
1. Introduction
2. Name server configuration method
2.1 Outline
2.2 Configurations on the ISP side
2.3 Configurations on the end user side
2.4 Actual action
3. Notes
3.1 Effective timing for configuration
3.2 Problem of BIND-4.9.3-BETA17
3.3 Name servers for which reverse can not be set flexibly
4. Conclusion
1. Introduction
Current IP addresses are not assigned depending on the former IP
address class.
This section explains the classless name server reverse lookup
configuration method for cases where assignment is made in units
smaller than the 24-bit prefix (so-called "Class C" in the past) among
those assignments.
This configuration is handled only by the configuration method of the
name server, therefore it is no longer necessary to change the DNS
client.
2. Name server configuration method
2.1 Outline
The former method depended on an octet boundary used for reverse
configuration. However, if this is used as it is, it is not possible
to set for assignments smaller than the 24 bit prefix.
Therefore, a method utilizing CNAME is used for the name server
reverse lookup configuration part.
For example, if we assume an assignment like that shown below for
some-dom.co.jp, this means an assignment with a 27-bit prefix (1/8 of
Class C), however, the remaining part of 192.168.23.0/24 is used by
other organizations connected to the ISP to which some-dom.co.jp is
connected.
some-dom.co.jp 192.168.23.32/27
23.168.192.in-addr.arpa is controlled by the name server on the ISP
side, and 32/27.23.168.192.in-addr.arpa is controlled by the name
server on the user side.
2.2 configurations on the ISP side
For the configurations on the ISP side, named.boot is described as
shown below.
------------------------------------------------------------------------
primary 23.168.192.in-addr.arpa suba.rev
secondary 32/27.23.168.192.in-addr.arpa 192.168.23.34 bak/some-dom.rev
------------------------------------------------------------------------
suba.rev on the ISP side prepares the enumeration of CNAME as shown
below.
------------------------------------------------------------------------
$ORIGIN 23.168.192.in-addr.arpa
@ IN SOA ns.isp.ad.jp. hostmaster.......
32/27 IN NS gw.some-dom.co.jp.
33 IN CNAME 33.32/27.23.168.192.in-addr.arpa.
34 IN CNAME 34.32/27.23.168.192.in-addr.arpa.
35 IN CNAME 35.32/27.23.168.192.in-addr.arpa.
＜omission＞
61 IN CNAME 61.32/27.23.168.192.in-addr.arpa.
62 IN CNAME 62.32/27.23.168.192.in-addr.arpa.
------------------------------------------------------------------------
2.3 Configurations on the end user side
The following is set on the end user side (some-dom.co.jp).
If the NS of some-dom.co.jp is:
------------------------------------------------------------------------
gw.some-dom.co.jp 192.168.23.35
------------------------------------------------------------------------
according to this configuration, some-dom.co.jp where /27 is assigned
prepares the configuration shown below.
The following is written at named.boot:
------------------------------------------------------------------------
primary 32/27.23.168.192.in-addr.arpa some-dom.rev
------------------------------------------------------------------------
and, the following is written at some-dom.rev:
------------------------------------------------------------------------
$ORIGIN 32/27.23.168.192.IN-ADDR.ARPA.
@ IN SOA gw.some-dom.co.jp. hostmaster.some-dom.co.jp
IN NS gw.some-dom.co.jp.
33 IN PTR rt.some-dom.co.jp.
34 IN PTR www.some-dom.co.jp.
35 IN PTR gw.some-dom.co.jp.
------------------------------------------------------------------------
2.4 Actual action
According to the configuration shown above, when searching for a host
name from the IP address 192.168.23.34, for example:
34.23.168.192.in-addr.arpa.
->
It is
34.32/27.23.168.192.in-addr.arpa from CNAME of ISP server.
->
www.some-dom.co.jp from the server on the user side.
3. Notes
Note the following when using this method:
3.1 Effective timing for configuration
Under the normal configuration, it is possible to carry out reverse
lookup configuration without any problem if the configuration is
complete on the user side. However, when this method is used, reverse
lookup configuration can not be carried out if the configuration is
not completed on both the ISP and user sides.
3.2 Problem of BIND-4.9.3-BETA17
The resolver library of BIND-4.9.3 sends a warning message to syslog
if CNAME is found during the search of the PTR record. This problem is
solved in versions after BEAT18, so if the bind version is upgraded,
this problem can be solved.
However, as this is a problem on the resolver side, if the resolver
library is static linked, it is necessary to re-link these programs
using the resolver.
3.3 Name server for which reverse can not be set flexibly
As these items are set using a GUI, there is some dependence on the
octet boundary due to limitation of the GUI, so reverse configuration
can not be carried out using CNAME in some cases. The only option in
such cases is to abandon using the DNS.
4. Conclusion
This configuration is only an example; you can use other methods in
line with those used on the user and ISP sides. For example:
Apart from 32/27.23.168.192.in-addr.arpa,
The method using:
------------------------------------------------------------------------
A01.23.168.192.in-addr.arpa
32.23.168.192.in-addr.arpa
------------------------------------------------------------------------
is actually adopted.
JPNIC leaves the decision as to the method used, to the ISP and the
user.
End of document
