Abbreviations (DNS and BIND, 4th Edition)

4.4. Abbreviations

At this point, we have created all the files necessary for a primary master name server. Let's go back and revisit the zone data files; there are shortcuts we didn't use. Unless you see and understand the long form first, though, the short form can look very cryptic. Now that you know the long form and have seen the BIND configuration file, we'll show you the shortcuts.

4.4.1. Appending Domain Names

The second field of a primary directive (BIND 4) or zone statement (BIND 8 and 9) specifies a domain name. This domain name is the key to the most useful shortcut. This domain name is the origin of all the data in the zone data file. The origin is appended to all names in the zone data file that don't end in a dot, and will be different for each zone data file since each file describes a different zone.

Since the origin is appended to names, instead of entering robocop.movie.edu's address in db.movie.edu like this:

robocop.movie.edu.    IN A     192.249.249.2

we could have entered it like this:

robocop    IN A     192.249.249.2

In the db.192.24.249 file we entered this:

2.249.249.192.in-addr.arpa.  IN PTR robocop.movie.edu.

Since 249.249.192.in-addr.arpa is the origin, we could have entered:

2  IN PTR robocop.movie.edu.

Remember our earlier warning not to omit the trailing dot when using the fully qualified domain names? Suppose you forget the trailing dot. An entry like:

robocop.movie.edu    IN A     192.249.249.2

turns into an entry for robocop.movie.edu.movie.edu, not what you intended at all.

4.4.2. The @ Notation

If a domain name is the same as the origin, the name can be specified as "@". This is most often seen in the SOA record in the zone data files. The SOA records could have been entered this way:

@ IN SOA terminator.movie.edu. al.robocop.movie.edu. (
                          1        ; Serial
                          3h       ; Refresh after 3 hours
                          1h       ; Retry after 1 hour
                          1w       ; Expire after 1 week
                          1h )     ; Negative caching TTL of 1 hour

4.4.3. Repeat Last Name

If a resource record name (that starts in the first column) is a space or tab, then the name from the last resource record is used. You use this if there are multiple resource records for a name. Here's an example in which there are two address records for one name:

wormhole   IN A     192.249.249.1
           IN A     192.253.253.1

In the second address record, the name wormhole is implied. You can use this shortcut even if the resource records are of different types.

4.4.4. The Shortened Zone Data Files

Now that we have shown you the abbreviations, we'll repeat the zone data files, making use of these shortcuts.

Here are the contents of the file db.movie.edu:

$TTL 3h
;
; Origin added to names not ending
; in a dot: movie.edu
;

@ IN SOA terminator.movie.edu. al.robocop.movie.edu. (
                          1        ; Serial
                          3h       ; Refresh after 3 hours
                          1h       ; Retry after 1 hour
                          1w       ; Expire after 1 week
                          1h )     ; Negative caching TTL of 1 hour

;
; Name servers (The name '@' is implied)
;
           IN NS  terminator.movie.edu.
           IN NS  wormhole.movie.edu.

;
; Addresses for the canonical names
;
localhost  IN A     127.0.0.1
robocop    IN A     192.249.249.2
terminator IN A     192.249.249.3
diehard    IN A     192.249.249.4
misery     IN A     192.253.253.2
shining    IN A     192.253.253.3
carrie     IN A     192.253.253.4

wormhole   IN A     192.249.249.1
           IN A     192.253.253.1

;
; Aliases
;
bigt       IN CNAME terminator
dh         IN CNAME diehard
wh         IN CNAME wormhole

;
; Interface specific names
;
wh249      IN A     192.249.249.1
wh253      IN A     192.253.253.1

Here are the contents of the file db.192.249.249 :

$TTL 3h
;
; Origin added to names not ending
; in a dot: 249.249.192.in-addr.arpa
;

@ IN SOA terminator.movie.edu. al.robocop.movie.edu. (
                          1        ; Serial
                          3h       ; Refresh after 3 hours
                          1h       ; Retry after 1 hour
                          1w       ; Expire after 1 week
                          1h )     ; Negative caching TTL of 1 hour

;
; Name servers (The name '@' is implied)
;
   IN NS  terminator.movie.edu.
   IN NS  wormhole.movie.edu.

;
; Addresses point to canonical names
;
1  IN PTR wormhole.movie.edu.
2  IN PTR robocop.movie.edu.
3  IN PTR terminator.movie.edu.
4  IN PTR diehard.movie.edu.

Here are the contents of the file db.192.253.253:

$TTL 3h
;
; Origin added to names not ending
; in a dot: 253.253.192.in-addr.arpa
;

@ IN SOA terminator.movie.edu. al.robocop.movie.edu. (
                          1        ; Serial
                          3h       ; Refresh after 3 hours
                          1h       ; Retry after 1 hour
                          1w       ; Expire after 1 week
                          1h )     ; Negative caching TTL of 1 hour

;
; Name servers (The name '@' is implied)
;
   IN NS  terminator.movie.edu.
   IN NS  wormhole.movie.edu.

;
; Addresses point to canonical names
;
1  IN PTR wormhole.movie.edu.
2  IN PTR misery.movie.edu.
3  IN PTR shining.movie.edu.
4  IN PTR carrie.movie.edu.

Here are the contents of the file db.127.0.0 :

$TTL 3h
@ IN SOA terminator.movie.edu. al.robocop.movie.edu. (
                          1        ; Serial
                          3h       ; Refresh after 3 hours
                          1h       ; Retry after 1 hour
                          1w       ; Expire after 1 week
                          1h )     ; Negative caching TTL of 1 hour

   IN NS  terminator.movie.edu.
   IN NS  wormhole.movie.edu.

1  IN PTR localhost.

While looking at the new db.movie.edu file, you may notice that we could have removed movie.edu from the host names of the SOA and NS records like this:

@ IN SOA terminator al.robocop (
                          1        ; Serial
                          3h       ; Refresh after 3 hours
                          1h       ; Retry after 1 hour
                          1w       ; Expire after 1 week
                          1h )     ; Negative caching TTL of 1 day

  IN NS  terminator
  IN NS  wormhole

You can't do this in the other zone data files because their origins are different. In db.movie.edu, we leave these names as fully qualified domain names so that the NS and SOA records are exactly the same for all the zone data files.