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DNS HOWTO : A real domain example Next Previous Contents

7. A real domain example

Where we list some real zone files

Users have suggested that I include a real example of a working domain as well as the tutorial example.

I use this example with permission from David Bullock of LAND-5. These files were current 24th of September 1996, and were then edited to fit BIND 8 restrictions and use extensions by me. So, what you see here differs a bit from what you find if you query LAND-5's name servers now.

7.1 /etc/named.conf (or /var/named/named.conf)

Here we find master zone sections for the two reverse zones needed: the 127.0.0 net, as well as LAND-5's 206.6.177 subnet, and a primary line for land-5's forward zone land-5.com. Also note that instead of stuffing the files in a directory called pz, as I do in this HOWTO, he puts them in a directory called zone.

// Boot file for LAND-5 name server

options {
        directory "/var/named";

controls {
        inet allow { localhost; } keys { rndc_key; };

key "rndc_key" {
        algorithm hmac-md5;
        secret "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K";

zone "." {
        type hint;
        file "root.hints";

zone "0.0.127.in-addr.arpa" {
        type master;
        file "zone/127.0.0";

zone "land-5.com" {
        type master;
        file "zone/land-5.com";

zone "177.6.206.in-addr.arpa" {
        type master;
        file "zone/206.6.177";

If you put this in your named.conf file to play with PLEASE put ``notify no;'' in the zone sections for the two land-5 zones so as to avoid accidents.

7.2 /var/named/root.hints

Keep in mind that this file is dynamic, and the one listed here is old. You're better off using a new one as explained earlier.

; <<>> DiG 8.1 <<>> @A.ROOT-SERVERS.NET. 
; (1 server found)
;; res options: init recurs defnam dnsrch
;; got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10
;; flags: qr aa rd; QUERY: 1, ANSWER: 13, AUTHORITY: 0, ADDITIONAL: 13
;;      ., type = NS, class = IN

.                       6D IN NS        G.ROOT-SERVERS.NET.
.                       6D IN NS        J.ROOT-SERVERS.NET.
.                       6D IN NS        K.ROOT-SERVERS.NET.
.                       6D IN NS        L.ROOT-SERVERS.NET.
.                       6D IN NS        M.ROOT-SERVERS.NET.
.                       6D IN NS        A.ROOT-SERVERS.NET.
.                       6D IN NS        H.ROOT-SERVERS.NET.
.                       6D IN NS        B.ROOT-SERVERS.NET.
.                       6D IN NS        C.ROOT-SERVERS.NET.
.                       6D IN NS        D.ROOT-SERVERS.NET.
.                       6D IN NS        E.ROOT-SERVERS.NET.
.                       6D IN NS        I.ROOT-SERVERS.NET.
.                       6D IN NS        F.ROOT-SERVERS.NET.

G.ROOT-SERVERS.NET.     5w6d16h IN A
J.ROOT-SERVERS.NET.     5w6d16h IN A
K.ROOT-SERVERS.NET.     5w6d16h IN A
L.ROOT-SERVERS.NET.     5w6d16h IN A
M.ROOT-SERVERS.NET.     5w6d16h IN A
A.ROOT-SERVERS.NET.     5w6d16h IN A
H.ROOT-SERVERS.NET.     5w6d16h IN A
B.ROOT-SERVERS.NET.     5w6d16h IN A
C.ROOT-SERVERS.NET.     5w6d16h IN A
D.ROOT-SERVERS.NET.     5w6d16h IN A
E.ROOT-SERVERS.NET.     5w6d16h IN A
I.ROOT-SERVERS.NET.     5w6d16h IN A
F.ROOT-SERVERS.NET.     5w6d16h IN A

;; Total query time: 215 msec
;; FROM: roke.uio.no to SERVER: A.ROOT-SERVERS.NET.
;; WHEN: Sun Feb 15 01:22:51 1998
;; MSG SIZE  sent: 17  rcvd: 436

7.3 /var/named/zone/127.0.0

Just the basics, the obligatory SOA record, and a record that maps to localhost. Both are required. No more should be in this file. It will probably never need to be updated, unless your nameserver or hostmaster address changes.

@               IN      SOA     land-5.com. root.land-5.com. (
                                199609203       ; Serial
                                28800   ; Refresh
                                7200    ; Retry
                                604800  ; Expire
                                86400)  ; Minimum TTL
                        NS      land-5.com.
1                       PTR     localhost.

If you look at a random BIND installation you will probably find that the $TTL line is missing as it is here. It was not used before, and only version 8.2 of BIND has started to warn about its absence. BIND 9 requires the $TTL.

7.4 /var/named/zone/land-5.com

Here we see the mandatory SOA record, the needed NS records. We can see that he has a secondary name server at ns2.psi.net. This is as it should be, always have a off site secondary server as backup. We can also see that he has a master host called land-5 which takes care of many of the different Internet services, and that he's done it with CNAMEs (a alternative is using A records).

As you see from the SOA record, the zone file originates at land-5.com, the contact person is root@land-5.com. hostmaster is another oft used address for the contact person. The serial number is in the customary yyyymmdd format with todays serial number appended; this is probably the sixth version of zone file on the 20th of September 1996. Remember that the serial number must increase monotonically, here there is only one digit for todays serial#, so after 9 edits he has to wait until tomorrow before he can edit the file again. Consider using two digits.

@       IN      SOA     land-5.com. root.land-5.com. (
                        199609206       ; serial, todays date + todays serial #
                        8H              ; refresh, seconds
                        2H              ; retry, seconds
                        4W              ; expire, seconds
                        1D )            ; minimum, seconds
                NS      land-5.com.
                NS      ns2.psi.net.
                MX      10 land-5.com.  ; Primary Mail Exchanger
                TXT     "LAND-5 Corporation"

localhost       A

router          A
land-5.com.     A
ns              A
www             A

ftp             CNAME   land-5.com.
mail            CNAME   land-5.com.
news            CNAME   land-5.com.

funn            A

;       Workstations
ws-177200       A
                MX      10 land-5.com.   ; Primary Mail Host
ws-177201       A
                MX      10 land-5.com.   ; Primary Mail Host
ws-177202       A
                MX      10 land-5.com.   ; Primary Mail Host
ws-177203       A
                MX      10 land-5.com.   ; Primary Mail Host
ws-177204       A
                MX      10 land-5.com.   ; Primary Mail Host
ws-177205       A
                MX      10 land-5.com.   ; Primary Mail Host
; {Many repetitive definitions deleted - SNIP}
ws-177250       A
                MX      10 land-5.com.   ; Primary Mail Host
ws-177251       A
                MX      10 land-5.com.   ; Primary Mail Host
ws-177252       A
                MX      10 land-5.com.   ; Primary Mail Host
ws-177253       A
                MX      10 land-5.com.   ; Primary Mail Host
ws-177254       A
                MX      10 land-5.com.   ; Primary Mail Host

If you examine land-5s nameserver you will find that the host names are of the form ws_number. As of late BIND 4 versions named started enforcing the restrictions on what characters may be used in host names. So that does not work with BIND 8 at all, and I substituted '-' (dash) for '_' (underline) for use in this HOWTO. But, as mentioned earlier, BIND 9 no longer enforces this restriction.

Another thing to note is that the workstations don't have individual names, but rather a prefix followed by the two last parts of the IP numbers. Using such a convention can simplify maintenance significantly, but can be a bit impersonal, and, in fact, be a source of irritation among your customers.

We also see that funn.land-5.com is an alias for land-5.com, but using an A record, not a CNAME record.

7.5 /var/named/zone/206.6.177

I'll comment on this file below

@               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 To find out the name of that machine we need to find 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 we have a ``PTR ws-177200.land-5.com'' record, meaning that the name that goes with 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 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 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.

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