I'll comment on this file below
$TTL 3D @ IN SOA land-5.com. root.land-5.com. ( 199609206 ; Serial 28800 ; Refresh 7200 ; Retry 604800 ; Expire 86400) ; Minimum TTL NS land-5.com. NS ns2.psi.net. ; ; Servers ; 1 PTR router.land-5.com. 2 PTR land-5.com. 2 PTR funn.land-5.com. ; ; Workstations ; 200 PTR ws-177200.land-5.com. 201 PTR ws-177201.land-5.com. 202 PTR ws-177202.land-5.com. 203 PTR ws-177203.land-5.com. 204 PTR ws-177204.land-5.com. 205 PTR ws-177205.land-5.com. ; {Many repetitive definitions deleted - SNIP} 250 PTR ws-177250.land-5.com. 251 PTR ws-177251.land-5.com. 252 PTR ws-177252.land-5.com. 253 PTR ws-177253.land-5.com. 254 PTR ws-177254.land-5.com.
The reverse zone is the bit of the setup that seems to cause the
most grief. It is used to find the host name if you have the IP
number of a machine. Example: you are an FTP server and accept
connections from FTP clients. As you are a Norwegian FTP server you
want to accept more connections from clients in Norway and other
Scandinavian countries and less from the rest of the world. When you
get a connection from a client the C library is able to tell you the
IP number of the connecting machine because the IP number of the
client is contained in all the packets that are passed over the
network. Now you can call a function called gethostbyaddr that looks
up the name of a host given the IP number. Gethostbyaddr will ask a
DNS server, which will then traverse the DNS looking for the machine.
Supposing the client connection is from ws-177200.land-5.com. The IP
number the C library provides to the FTP server is 206.6.177.200. To
find out the name of that machine we need to find
200.177.6.206.in-addr.arpa
. The DNS server will first find the
arpa.
servers, then find in-addr.arpa.
servers, following
the reverse trail through 206, then 6 and at last finding the server
for the 177.6.206.in-addr.arpa
zone at LAND-5. From which it
will finally get the answer that for 200.177.6.206.in-addr.arpa
we have a ``PTR ws-177200.land-5.com
'' record, meaning that the
name that goes with 206.6.177.200
is ws-177200.land-5.com
.
The FTP server prioritizes connections from the Scandinavian
countries, i.e., *.no
, *.se
, *.dk
, the name
ws-177200.land-5.com
clearly does not match any of those, and the
server will put the connection in a connection class with less
bandwidth and fewer clients allowed. If there was no reverse
mapping of 206.2.177.200
through the in-addr.arpa
zone the
server would have been unable to find the name at all and would have
to settle to comparing 206.2.177.200
with *.no
, *.se
and *.dk
, none of which will match at all, it may even deny the
connection for lack of classification.
Some people will tell you that reverse lookup mappings are only important for servers, or not important at all. Not so: Many ftp, news, IRC and even some http (WWW) servers will not accept connections from machines of which they are not able to find the name. So reverse mappings for machines are in fact mandatory.