Population of DNS records and DHCP

[James Howe]

I'm working to configure our DNS server for our small local network of
Windows XP/2k machines. Most of the machines use DHCP to get their
address and the DHCP server they connect to is on our firewall box. One
question I have about this is how does the DNS get updated if IP addresses
change for DHCP configured machines? Relatedly, I notice that when I look
at my forward zone records, I see a list of many of the computers on our
network, but not all of them. However, from any client machine, I'm able
to ping any of the machines on our network by name, regardless of whether
they show up in the list. On my machine, for example, I've configured DNS
to use the DNS running on our Win2k Server box. We have a machine called
'spam' on the network. I can ping spam, and things work fine. If I use
nslookup, however, the name isn't found. Who is resolving this name?
Also, how do names get into the forward zone list? Most of the names in
the list were not added by me, at least not through DNS. Were they added
through Active Directory?

Just trying to make sense of all this.

Thanks.

 

[barnski]

The DNS zone has an "allow dynamic updates" property -
basically, if this is set to "yes", then XP or "k clients
will update their own DNS records if their address
changes. If it's set to "Secure updates only" then only
2k and XP machines in the domain will update their DNS
records.

If you want NT or 9x machines to have records updated in
DNS, you must get the DHCP service to do it for them, as
they can't update the DNS server themselves. This is done
by checking "Automatically update DHCP client information
in DNS" under the DHCP scope properties. You may need to
select "Always update DNS" here as well - I don't
remember.

I suspect that you are seeing your XP and 2k machines
performing automatic updates to the DNS server, but not
your older NT or 9x machines.

If two machines are on the same subnet, they can resolve
each other by broadcast - this is how your machines
resolve hostnames that are not in DNS.

If your DHCP server specifies that the DHCP clients are h-
nodes (for example), then they will try to resolve a name
to an IP Address using the following methods, in the
following order:

local nbt cache
WINS
broadcast
lmhosts
hosts
dns

- a broadcast is a message sent to the entire local
subnet saying "is anyone called <hostname>? - if so,
reply to me with your IP address". The machine <hostname>
then replies.

Hope that helps,

Barnski.

 

[James Howe]

Thanks for the information, that fills in the gaps nicely. I'm still not
sure why some of our machines aren't registered in the DNS since all of
our machines are either Win2k or XP and the ones which weren't registered
had the 'Register this connections addresses in DNS' checked. I manually
did an ipconfig /registerdns and now the name is in the directory.

 

[Jonathan de Boyne Pollard]

JH> One question I have about this is how does the DNS get updated
JH> if IP addresses change for DHCP configured machines?

That is a leading question that takes a falsehood as its premise, and
so cannot be answered. Your DNS database is not necessarily updated.
It depends from whether your DHCP server supports such a thing and how
your DHCP clients are configured.

JH> I can ping spam, and things work fine. If I use
JH> nslookup, however, the name isn't found.

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