home | O'Reilly's CD bookshelfs | FreeBSD | Linux | Cisco | Cisco Exam  

Book HomeDNS and BINDSearch this book
PreviousNext

9.5. Subdomains of in-addr.arpa Domains

Forward-mapping domains aren't the only domains that you can divide into subdomains and delegate. If your in-addr.arpa namespace is large enough, you may need to divide it, too. Typically, you divide the domain that corresponds to your network number into subdomains that correspond to your subnets. How that works depends on the type of network you have and on your network's subnet mask.

9.5.1. Subnetting on an Octet Boundary

Since Movie U. has just three /24 (class C-sized) networks, one per segment, there's no particular need to subnet those networks. However, our sister university, Altered State, has a class B-sized network, 172.20/16. Their network is subnetted between the third and fourth octet of the IP address; that is, their subnet mask is 255.255.255.0. They've already created a number of subdomains of their domain, altered.edu, including fx.altered.edu (okay, we copied them), makeup.altered.edu, and foley.altered.edu. Since each of these departments also runs its own subnet (their Special Effects department runs subnet 172.20.2/24, Makeup runs 172.20.15/24, and Foley runs 172.20.25/24), they'd like to divvy up their in-addr.arpa namespace appropriately, too.

Delegating in-addr.arpasubdomains is no different from delegating subdomains of forward-mapping domains. Within their db.172.20 zone data file, they need to add NS records like these:

2     86400    IN    NS    gump.fx.altered.edu.
  2     86400    IN    NS    toystory.fx.altered.edu.
  15    86400    IN    NS    prettywoman.makeup.altered.edu.
  15    86400    IN    NS    priscilla.makeup.altered.edu.
  25    86400    IN    NS    blowup.foley.altered.edu.
  25    86400    IN    NS    muppetmovie.foley.altered.edu.
delegating the subdomain that corresponds to each subnet to the correct name server in each subdomain.

A few important notes: the Altered States administrators could only use the third octet of the subnet in the owner name field because the default origin in this file is 20.172.in-addr.arpa. They needed to use the fully qualified domain names of the name servers in the right side of the NS records, though, to avoid having the origin appended. And they didn't need glue address records, since the names of the name servers they delegated the zone to didn't end in the domain name of the zone.

9.5.2. Subnetting on a Non-Octet Boundary

What do you do about networks that aren't subnetted neatly on octet boundaries, like subnetted /24 (class C-sized) networks? In these cases, you can't delegate along lines that match the subnets. This forces you into one of two situations: you have multiple subnets per in-addr.arpa zone, or you have multiple in-addr.arpa zones per subnet. Neither is particularly pleasing.

9.5.2.1. Class A and B networks

Let's take the case of the /8 (class A-sized) network 15/8, subnetted with the subnet mask 255.255.248.0 (a 13-bit subnet field and an 11-bit host field, or 8192 subnets of 2048 hosts). In this case, the subnet 15.1.200.0, for example, extends from 15.1.200.0 to 15.1.207.255. Therefore, the delegation for that single subdomain in db.15, the zone data file for 15.in-addr.arpa, might look like this:

200.1.15.in-addr.arpa.    86400    IN    NS    ns-1.cns.hp.com.
200.1.15.in-addr.arpa.    86400    IN    NS    ns-2.cns.hp.com.
201.1.15.in-addr.arpa.    86400    IN    NS    ns-1.cns.hp.com.
201.1.15.in-addr.arpa.    86400    IN    NS    ns-2.cns.hp.com.
202.1.15.in-addr.arpa.    86400    IN    NS    ns-1.cns.hp.com.
202.1.15.in-addr.arpa.    86400    IN    NS    ns-2.cns.hp.com.
203.1.15.in-addr.arpa.    86400    IN    NS    ns-1.cns.hp.com.
203.1.15.in-addr.arpa.    86400    IN    NS    ns-2.cns.hp.com.
204.1.15.in-addr.arpa.    86400    IN    NS    ns-1.cns.hp.com.
204.1.15.in-addr.arpa.    86400    IN    NS    ns-2.cns.hp.com.
205.1.15.in-addr.arpa.    86400    IN    NS    ns-1.cns.hp.com.
205.1.15.in-addr.arpa.    86400    IN    NS    ns-2.cns.hp.com.
206.1.15.in-addr.arpa.    86400    IN    NS    ns-1.cns.hp.com.
206.1.15.in-addr.arpa.    86400    IN    NS    ns-2.cns.hp.com.
207.1.15.in-addr.arpa.    86400    IN    NS    ns-1.cns.hp.com.
207.1.15.in-addr.arpa.    86400    IN    NS    ns-2.cns.hp.com.
That's a lot of delegation for one subnet!

Luckily, BIND 8.2 and later as well as BIND 9.1.0 and later name servers support a control statement called $GENERATE. $GENERATE lets you create a group of resource records that differ only by a numerical iterator. For example, you could create the 16 NS records just listed using these two $GENERATE control statements:

$GENERATE 200-207 $.1.15.in-addr.arpa.  86400  IN  NS  ns-1.cns.hp.com.
$GENERATE 200-207 $.1.15.in-addr.arpa.  86400  IN  NS  ns-1.cns.hp.com.
The syntax is fairly simple: when the name server reads the control statement, it iterates over the range specified as the first argument, replacing any dollar signs ($) in the template that follows the first argument with the current iterator.

9.5.2.2. Class C networks

Now let's look at the case of a subnetted /24 (class C-sized) network, say 192.253.254/24, subnetted with the mask 255.255.255.192. Here, you have a single in-addr.arpa zone, 254.253.192.in-addr.arpa, that corresponds to subnets 192.253.254.0/26, 192.253.254.64/26, 192.253.254.128/26, and 192.253.254.192/26. This can be a problem if you want to let different organizations manage the reverse-mapping information corresponding to each subnet. You can solve this in one of three ways, none of them pretty.

9.5.2.2.1. Solution 1

The first solution is to administer the 254.253.192.in-addr.arpa zone as a single entity and not even try to delegate. This requires either cooperation between the administrators of the four subnets involved or the use of a tool like Webmin (http://www.webmin.com/webmin) to allow each of the four administrators to take care of his or her own data.

9.5.2.2.2. Solution 2

The second is to delegate at the fourth octet. That's even nastier than the /8 delegation we just showed you. You'll need at least a couple of NS records per IP addressin the file db.192.253.254, like this:

1.254.253.192.in-addr.arpa.    86400    IN    NS    ns1.foo.com.
1.254.253.192.in-addr.arpa.    86400    IN    NS    ns2.foo.com.

2.254.253.192.in-addr.arpa.    86400    IN    NS    ns1.foo.com.
2.254.253.192.in-addr.arpa.    86400    IN    NS    ns2.foo.com.

...

65.254.253.192.in-addr.arpa.    86400    IN    NS    relay.bar.com.
65.254.253.192.in-addr.arpa.    86400    IN    NS    gw.bar.com.

66.254.253.192.in-addr.arpa.    86400    IN    NS    relay.bar.com.
66.254.253.192.in-addr.arpa.    86400    IN    NS    gw.bar.com.

...

129.254.253.192.in-addr.arpa.    86400    IN    NS    mail.baz.com.
129.254.253.192.in-addr.arpa.    86400    IN    NS    www.baz.com.

130.254.253.192.in-addr.arpa.    86400    IN    NS    mail.baz.com.
130.254.253.192.in-addr.arpa.    86400    IN    NS    www.baz.com.
and so on, all the way down to 254.254.253.192.in-addr.arpa.

You can pare that down substantially by using $GENERATE:

$GENERATE 0-63 $.254.253.192.in-addr.arpa  86400  IN  NS  ns1.foo.com.
$GENERATE 0-63 $.254.253.192.in-addr.arpa  86400  IN  NS  ns2.foo.com.

$GENERATE 64-127 $.254.253.192.in-addr.arpa.  86400  IN  NS  relay.bar.com.
$GENERATE 64-127 $.254.253.192.in-addr.arpa.  86400  IN  NS  gw.bar.com.

$GENERATE 128-191 $.254.253.192.in-addr.arpa.  86400  IN  NS  mail.baz.com.
$GENERATE 128-191 $.254.253.192.in-addr.arpa.  86400  IN  NS  www.baz.com.
Of course, in ns1.foo.com's named.conf, you'd also expect to see:

zone "1.254.253.192.in-addr.arpa" {
                type master;
                file "db.192.253.254.1";
};

zone "2.254.253.192.in-addr.arpa" {
                type master;
                file "db.192.253.254.2";
};
Or, if ns1.foo.com were running BIND 4, you'd expect to see these directives in named.boot:

primary    1.254.253.192.in-addr.arpa    db.192.253.254.1
primary    2.254.253.192.in-addr.arpa    db.192.253.254.2
and in db.192.253.254.1, just the one PTR record:

$TTL 1d
@    IN    SOA    ns1.foo.com.    root.ns1.foo.com.    (
                         1        ; Serial
                         3h       ; Refresh
                         1h       ; Retry
                         1w       ; Expire
                         1h       ; Negative caching TTL

    IN    NS    ns1.foo.com.
    IN    NS    ns2.foo.com.

    IN    PTR    thereitis.foo.com.
Notice that the PTR record is attached to the zone's domain name since the zone's domain name corresponds to just one IP address. Now, when a 254.253.192.in-addr.arpa name server receives a query for the PTR record for 1.254.253.192.in-addr.arpa, it refers the querier to ns1.foo.com and ns2.foo.com, which respond with the one PTR record in the zone.

9.5.2.2.3. Solution 3

Finally, there's a clever technique that obviates the need to maintain a separate zone data file for each IP address.[68] The organization responsible for the overall /24 network creates CNAME records for each of the domain names in the zone, pointing to domain names in new subdomains, which are then delegated to the proper servers. These new subdomains can be called just about anything, but names like 0-63, 64-127, 128-191, and 192-255 clearly indicate the range of addresses each subdomain will reverse map. Each subdomain then contains only the PTR records in the range the subdomain is named for.

[68]We first saw this explained by Glen Herrmansfeldt of CalTech in the newsgroup comp.protocols.tcp-ip.domains. It's now codified as RFC 2317.

Partial contents of file db.192.253.254:

1.254.253.192.in-addr.arpa.  IN  CNAME  1.0-63.254.253.192.in-addr.arpa.
2.254.253.192.in-addr.arpa.  IN  CNAME  2.0-63.254.253.192.in-addr.arpa.

...

0-63.254.253.192.in-addr.arpa.    86400    IN    NS    ns1.foo.com.
0-63.254.253.192.in-addr.arpa.    86400    IN    NS    ns2.foo.com.

65.254.253.192.in-addr.arpa. IN  CNAME 65.64-127.254.253.192.in-addr.arpa.
66.254.253.192.in-addr.arpa. IN  CNAME 66.64-127.254.253.192.in-addr.arpa.

...

64-127.254.253.192.in-addr.arpa.    86400    IN    NS    relay.bar.com.
64-127.254.253.192.in-addr.arpa.    86400    IN    NS    gw.bar.com.

129.254.253.192.in-addr.arpa.  IN  CNAME  129.128-191.254.253.192.in-addr. arpa.
130.254.253.192.in-addr.arpa.  IN  CNAME  130.128-191.254.253.192.in-addr. arpa.

...

128-191.254.253.192.in-addr.arpa.    86400    IN    NS    mail.baz.com.
128-191.254.253.192.in-addr.arpa.    86400    IN    NS    www.baz.com.
Again, you can abbreviate this with $GENERATE:

$GENERATE 1-63 $ IN CNAME $.0-63.254.253.192.in-addr.arpa.

0-63.254.253.192.in-addr.arpa.    86400    IN    NS    ns1.foo.com.
0-63.254.253.192.in-addr.arpa.    86400    IN    NS    ns2.foo.com.

$GENERATE 65-127 $ IN CNAME $.64-127.254.253.192.in-addr.arpa.

64-127.254.253.192.in-addr.arpa.    86400    IN    NS    relay.bar.com.
64-127.254.253.192.in-addr.arpa.    86400    IN    NS    gw.bar.com.
The zone data file for 0-63.254.253.192.in-addr.arpa, db.192.253.254.0-63, can contain just PTR records for IP addresses 192.253.254.1 through 192.253.254.63.

Partial contents of file db.192.253.254.0-63:

$TTL 1d
@    IN    SOA    ns1.foo.com.    root.ns1.foo.com.    (
                          1       ; Serial
                          3h      ; Refresh
                          1h      ; Retry
                          1w      ; Expire
                          1h )    ; Negative caching TTL

     IN    NS    ns1.foo.com.
     IN    NS    ns2.foo.com.

1    IN    PTR   thereitis.foo.com.
2    IN    PTR   setter.foo.com.
3    IN    PTR   mouse.foo.com.
...
The way this setup works is a little tricky, so let's go over it. A resolver requests the PTR record for 1.254.253.192.in-addr.arpa, causing its local name server to look that up. The local name server ends up asking a 254.253.192.in-addr.arpa name server, which responds with the CNAME record indicating that 1.254.253.192.in-addr.arpa is actually an alias for 1.0-63.254.253.192.in-addr.arpa and that the PTR record is attached to that name. The response will also include NS records telling the local name server that the authoritative name servers for 0-63.254.253.192.in-addr.arpa are ns1.foo.com and ns2.foo.com. The local name server then queries either ns1.foo.com or ns2.foo.com for the PTR record for 1.0-63.254.253.192.in-addr.arpa, and receives the PTR record.

PreviousHomeNext
9.4. How to Become a Parent: Creating SubdomainsBook Index9.6. Good Parenting
Library Navigation Links

Copyright © 2002 O'Reilly & Associates. All rights reserved.