{"id":1724,"date":"2011-01-30T13:13:43","date_gmt":"2011-01-30T12:13:43","guid":{"rendered":"http:\/\/mccltd.net\/blog\/?p=1724"},"modified":"2012-08-30T13:24:28","modified_gmt":"2012-08-30T12:24:28","slug":"dns-in-addr-arpa-reverse-mapping","status":"publish","type":"post","link":"http:\/\/darenmatthews.com\/blog\/?p=1724","title":{"rendered":"DNS – IN-ADDR.ARPA Reverse Mapping"},"content":{"rendered":"

IPv4 IN-ADDR.ARPA Reverse Mapping Domain<\/strong>
\nWe defined the normal domain name structure as a tree starting from the root. We write a normal domain name LEFT to RIGHT but the hierarchical structure is RIGHT to LEFT.<\/p>\n

domain name = www.example.com
\nhighest node in tree is = .com
\nnext (lower) = .example
\nnext (lower) = www<\/code><\/p>\n

An IPv4 address is written as:<\/p>\n

192.168.23.17<\/p>\n

This IPv4 address defines a host (17) which happens to be in a Class C address range (192.168.23.x). In this case the most important part (the highest node) is on the LEFT (192) not the RIGHT. This is a tad awkward and would make it impossible to construct a sensible tree structure that could be searched in a single lifetime.<\/p>\n

The solution is to reverse the order of the address and place the result under the special domain IN-ADDR.ARPA (you will see this also written as in-addr.arpa which is perfectly legitimate since domain names are case insensitive but the case should be preserved between query and response, so we will continue to use IN-ADDR.ARPA. You may elect to use whatever you wish including IN-addr.Arpa if that is your preference).<\/p>\n

Finally the last part of the IPv4 Address (17) is the host address and hosts, from our previous reading, are typically defined inside a zone file so we will ignore it and only use the Class C address base. The result of our manipulations are:<\/p>\n

IP address =192.168.23.17
\nClass C base = 192.168.23 ; omits the host address = 17
\nReversed Class C base = 23.168.192
\nAdded to IN-ADDR.ARPA domain = 23.168.192.IN-ADDR.ARPA<\/p>\n

This is show in the figure here:
\n\"IN-ADDR.ARPA<\/a>
\narpa organization<\/p>\n

IN-ADDR.ARPA Reverse Mapping<\/strong>
\nFinally we construct a zone file to describe all the hosts (nodes) in the Reverse Mapped zone using PTR Records ( see this post<\/a> for more about PTR records ). The resulting file will look something like this:<\/p>\n

$TTL 2d\u00a0 ; 172800 seconds
\n$ORIGIN 23.168.192.IN-ADDR.ARPA.
\n@\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 IN\u00a0\u00a0\u00a0\u00a0\u00a0 SOA\u00a0\u00a0 ns1.example.com. hostmaster.example.com. (
\n2003080800 ; serial number
\n3h\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 ; refresh
\n15m\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 ; update retry
\n3w\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 ; expiry
\n3h\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 ; nx = nxdomain ttl
\n)
\nIN\u00a0\u00a0\u00a0\u00a0\u00a0 NS\u00a0\u00a0\u00a0\u00a0\u00a0 ns1.example.com.
\nIN\u00a0\u00a0\u00a0\u00a0\u00a0 NS\u00a0\u00a0\u00a0\u00a0\u00a0 ns2.example.com.
\n1\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 IN\u00a0\u00a0\u00a0\u00a0\u00a0 PTR\u00a0\u00a0\u00a0\u00a0 www.example.com. ; qualified name
\n2\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 IN\u00a0\u00a0\u00a0\u00a0\u00a0 PTR\u00a0\u00a0\u00a0\u00a0 joe.example.com.
\n.....
\n17\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 IN\u00a0\u00a0\u00a0\u00a0\u00a0 PTR\u00a0\u00a0\u00a0\u00a0 bill.example.com.
\n.....
\n74\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 IN\u00a0\u00a0\u00a0\u00a0\u00a0 PTR\u00a0\u00a0\u00a0\u00a0 fred.example.com.
\n....
\n<\/code>
\nWe must use qualified names ending with a dot (in fact they are Fully Qualified Domain Names – FQDNs) in reverse mapped zone files because if we did not our $ORIGIN directive would lead to some strange results. For example, if we wrote an unqualified name such as:
\n
\n74\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 IN\u00a0\u00a0\u00a0\u00a0\u00a0 PTR fred<\/code><\/p>\n

Using the $ORIGIN substitution rule the above would expand to fred.23.168.192.IN-ADDR.ARPA. which is probably not what we intended.<\/p>\n

source of information<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

IPv4 IN-ADDR.ARPA Reverse Mapping Domain We defined the normal domain name structure as a tree starting from the root. We write a normal domain name LEFT to RIGHT but the hierarchical structure is RIGHT to LEFT. domain name = www.example.com highest node in tree is = .com next (lower) = .example next (lower) = www<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[87,84],"tags":[],"_links":{"self":[{"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1724"}],"collection":[{"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1724"}],"version-history":[{"count":3,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1724\/revisions"}],"predecessor-version":[{"id":1730,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1724\/revisions\/1730"}],"wp:attachment":[{"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1724"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1724"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/darenmatthews.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1724"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}