Re: Assigning DNS based on DHCP

Subject: Re: Assigning DNS based on DHCP

From: Andre LaBranche <email@hidden>

Date: Thu, 7 Jun 2007 16:42:14 -0700

Delivered-to: email@hidden

On Jun 6, 2007, at 8:23 PM, Robert Everson wrote:

I tried searching the archives for this, because I'm sure it's been asked before, but I can't seem to find it.
How do I get my server to set domain names based on the DHCP address. In other words, if a computer connects to the network, I want them to get an internal DNS name, like 10-1-100-3.solon.k12.ia.us. Is this possible with OS X Server? Or do I just need to add each valid DHCP address into the DNS records manually?

Hello,

What you're looking for isn't really an OS X Server feature, it's a BIND feature. You want a "$GENERATE" statement in your zone file, something like:

$GENERATE 2-20 dhcp$ IN A 10.0.1.$

This would create address records named dhcpN, for IP addresses 10.0.1.N, where N is expanded to the range of numbers between 2 and 20 (inclusive).

$GENERATE also works within the reverse zone file for PTR records.

$GENERATE 2-20 $ IN PTR dhcp$.937.

(In this example, my tld is "937").

For more information on this directive, see:

http://www.bind9.net/manual/bind/9.3.2/Bv9ARM.ch06.html#id2566761

Unfortunately, use of the $GENERATE directive along with Server Admin's DNS module is uh... broken. Firstly, Server Admin won't be able to parse beyond the $GENERATE directive, so you may not see some existing and perfectly functional records through the GUI view. Worse, if you make a zone change via Server Admin, the zone file will be re-written without the $GENERATE directive. Be prepared to either not touch the zone file via Server Admin, or re-add the $GENERATE directive if you do. Finally, because the parsing is broken, and because SA seems to bail out as a result, you may find that your forward and reverse zone files become desynced.

If you want to retain the ability to use Server Admin to edit your DNS zones, I recommend instead that you use a small script to generate individual record statements which can then be pasted into your zone files. I've written a small sample, included below. Edit AddressPrefix, range, and tld as needed. If you want a different format than that produced by this script, edit inside the foreach loop.

----------script start----------

#!/usr/bin/perl -w

use strict;

# Place the first three octets here.

my $AddressPrefix = "10.0.1";

# Define the range of numbers, inclusive.

my $range = "2 .. 10";

# Your fqdn suffix here, e.g. "foo.com" or "k12.fl.us.gov"

my $tld = "937";

# Need a backwards representation for PTR records

my $octets = join(".", reverse(split(/\./, $AddressPrefix)));

# For our resulting address and ptr records

my @A, my @PTR = "";

foreach my $n ( eval $range ) {

push @A, "dhcp$n IN A $AddressPrefix.$n";

push @PTR, "$n.$octets.in-addr.arpa. IN PTR dhcp$n.$tld.";

}

print "Forward zone address records:\n";

print "$_\n" for @A;

print "\nReverse zone ptr records:";

print "$_\n" for @PTR;

----------script end----------

Example run:

{179} root@tiny [~] # ./gen.pl

Forward zone address records:

dhcp2 IN A 10.0.1.2

dhcp3 IN A 10.0.1.3

dhcp4 IN A 10.0.1.4

dhcp5 IN A 10.0.1.5

dhcp6 IN A 10.0.1.6

dhcp7 IN A 10.0.1.7

dhcp8 IN A 10.0.1.8

dhcp9 IN A 10.0.1.9

dhcp10 IN A 10.0.1.10

Reverse zone ptr records:

2.1.0.10.in-addr.arpa. IN PTR dhcp2.937.

3.1.0.10.in-addr.arpa. IN PTR dhcp3.937.

4.1.0.10.in-addr.arpa. IN PTR dhcp4.937.

5.1.0.10.in-addr.arpa. IN PTR dhcp5.937.

6.1.0.10.in-addr.arpa. IN PTR dhcp6.937.

7.1.0.10.in-addr.arpa. IN PTR dhcp7.937.

8.1.0.10.in-addr.arpa. IN PTR dhcp8.937.

9.1.0.10.in-addr.arpa. IN PTR dhcp9.937.

10.1.0.10.in-addr.arpa. IN PTR dhcp10.937.

See the following for a more complex example: http://www.perl.com/pub/a/2002/11/20/dns.html

After editing the zone file, restart named by sending it a SIGHUP, e.g. sudo killall -HUP named. Check system.log for errors.

HTH,

-Andre

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References:

>Assigning DNS based on DHCP (From: Robert Everson <email@hidden>)