Traditional DNS Howto - Page 5

A Records

Up to now we have used the domain names centralsoft.org, server1.centralsoft.org, and ns0.centralsoft.org, but we did not specify anywhere to which IP addresses these domains should point to. That's where A records come into play. A records are the most important DNS records; with them you can create host addresses. Let's create our first A record:

centralsoft.org.   A 70.253.158.42

This means that centralsoft.org has the IP address 70.253.158.42. Remember my hint about the '.'? You can also write

                   A 70.253.158.42

which means exactly the same.

Now in a browser you are used to typing www.centralsoft.org instead of centralsoft.org, aren't you? www.centralsoft.org is technically totally different from centralsoft.org, but obviously you expect to see the same web site for both. Therefore we create this record:

www                A 70.253.158.42

which is the same as

www.centralsoft.org.              A 70.253.158.42

Finally we specify server1.centralsoft.org and ns0.centralsoft.org:

server1            A 70.253.158.42
ns0 A 70.253.158.45

ns0.centralsoft.org points to a different IP address which makes sense because it is our secondary nameserver which should be on a different system in case our primary nameserver goes down.

CNAME Records

CNAME is short for "canonical name", you can think of it as an alias to an A record. For example,

ftp                CNAME www

means, ftp.centralsoft.org is an alias for www.centralsoft.org, so ftp.centralsoft.org points to the same machine as www.centralsoft.org.

A CNAME must always point to an A record; a CNAME must not point to another CNAME! In addition to that, you must not use CNAME records for MX and SOA records. For example,

                   MX 10 ftp

is not allowed!

The usage of CNAMEs has its pros and cons. I can hear you say "If the usage of CNAME records has that many restrictions, why don't we always use A records?" That's true, but imagine you have hundreds of machine names as A records, pointing to the same IP address, and now you move the server to another data center where it is assigned a different IP address. You'd have to update every single A record. If you had just one A record and all other records were CNAMEs, you'd just have to update one A record...

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From: Anonymous at: 2006-03-12 15:55:03

IMO you should at least mention dynamic DNS updates, a method of editing your zones remotely from anywhere on the Internet:

http://linux.yyz.us/nsupdate/

http://linux.yyz.us/dns/ddns-server.html

From: Anonymous at: 2006-03-21 07:12:40

I don't think dynamic dns has much to do with traditional DNS configuration. It certainly deserves a Howto of its own, but I don't see it as applicable to this article.

From: Anonymous at: 2006-04-13 12:33:52

1) Restricting zonetransfers is useless. Anyone with secret data in public dns is doing something wrong anyway. Restringing zonetransfers just makes it harder to debug dns problems.

2) Make it clear that recursion should be disabled on nameservers. Never use the same dns as both nameserver and resolver. Public resolvers helps doing dns amplification DoS attacks. (http://www.us-cert.gov/reading_room/DNS-recursion121605.pdf)

From: Anonymous at: 2006-04-14 18:35:41

Though this work looks good, it is almost all written from a BIND perspective. Some focus on using tools to troubleshoot problems would be good.

Mention of alternate DNS packages would be good, especially for running the various DNS roles (caching, authorative). BIND is especially bad running as a cache and auth from the same daemon.

Finally, the expected plug: http://www.lifewithdjbdns.org/ .

From: Anonymous at: 2006-05-20 22:30:55

I have spent a lot of time trying to learn DNS and have read A LOT of "How-to's" on the subject. This how-to is hands-down the best I have seen. Well done!

From: Anonymous at: 2011-09-13 09:52:02

i am first timer but managed to configure to run! Well done, thanks a billions!

From: at: 2007-03-11 10:50:27

Thanks for this clarification on DNS.

Just want to add that reading the logs is basic for troubleshooting and if your sec. runs on another machine (shouldn't it be...) make sure that both machines (servers) are synchronized on time (helps if both have ntp) otherwise you'll wonder why zones don't transfer.

From: Makarand at: 2010-02-14 15:47:42

This tutorial is truly outstanding. I was able to follow it and get my DNS server up and running for my local LAN in less than an hour. Excellent work, guys. Deepest gratitude !

From: Anonymous at: 2009-12-14 22:05:03

This article is actually good, even if you only want to know about how DNS works.

Thank you.

From: Son Nguyen at: 2012-07-04 10:20:37

One billion thanks to author!
This is a very very very useful article.

From: Anonymous at: 2006-03-18 12:03:01

This is a superb howto. Also reminding us again the power of Linux that is we are free to configure it down to the configuration files. Many thanks :)

From: Anonymous at: 2006-04-11 04:20:25

the best tutorial for dns i've ever seen... woulda been nice to have this 2 weeks ago...

From: Anonymous at: 2006-06-14 05:48:33

wow that's great HOWTO ,job well done ,more power keep up the good work dudes!!!

From: Anonymous at: 2008-12-20 12:42:55

Excellent !

From: sujay at: 2012-11-27 22:55:07

It's really really awesome !!!!

From: Anonymous at: 2006-04-20 17:13:39

This is indeed the best DNS tutorial I have ever read.

I would definitely say that the author did a commendable job indeed.(SPF information was the crowning jewel

Well done,

Hope to see some more articles in the same tone.

From: Anonymous at: 2006-04-21 16:05:06

A DNS server on the Internet should normally only answer queries for the domains it is authoritative for. But unless you configure it otherwise bind will pass on requests for other domains up the hierarchy. This means it can potentially be used in a Denial of Service attack against other DNS servers. You can prevent this by restricting lookup via other DNS servers only to devices you trust. To do this add another line to the options section in the form...

options {

  ...

  allow-recursion { trusted.IP.subnet; };
It will still answer queries from anywhere for domains for which it is the authoritative server, but will now only do lookups via other DNS servers for requests from the trusted subnet.

From: Anonymous at: 2006-05-15 02:11:29

This HOWTO imho puts DNS in layman terms. I truly appreciate it.

Now to tackle my own little DNS venture :D

From: Anonymous at: 2006-06-14 05:52:31

wow that's great HOWTO,

job well done,keep the good work dudes!!! more power!!

From: RChan at: 2009-04-18 18:36:38

I've been a Unix SA for over 15 years now and I never took the time to really understand how to setup a DNS server.  At the companies I've worked for, they were usually setup before my time and I just had to maintain them and update A records or add CNAMEs.  This is by far the most informative writeup in very simple terms.  I would recommend this to any SA!

From: denu at: 2008-11-18 09:40:37

...very much for this great HowTo!

From: Anonymous at: 2009-03-18 18:35:32

Thank you very much for this HOWTO.  It is really really good for a SysAdmin.

From: Anonymous at: 2009-12-18 05:22:38

Very informative HOW-TO and very simple to follow.

From: Big Tone at: 2009-10-03 20:04:45

I thought I knew a little about DNS ... until I installed Bind(9) on FreeBSD for the first time. That's when I realized I knew what DNS did and that's about it!

Thanks for this very informative tutorial that answered a LOT of questions.

From: Richard B at: 2010-03-09 17:54:15

Fantastic job.  I have had problems due to reverse dns and other things that networksolutions doesn't support in it's hosted DNS... so I've had to setup my own DNS server.  I had already completed everything on my own before finding this, but I realize now that much of what I had setup was redundant and just wrong.  It still worked, but this has me down to just 4 files in my zone records and I have more stuff setup now for SPF and the tip mentioned here for "allow-recursion" within the options section of named.conf (though I had to do research to figure out how to list my ip block since I'm on what I now know to be a "moat" type setup and I only want the ip's on that network to contact me for dns lookup.

 Another tip I cam across... add the following to the options section in named.conf:

version "Nunyabeeswax";

Replace "Nunyabeeswax" with whatever you want.  This helps fight some hacking by hiding the version number of BIND... though it's probably mainly useful for those that refuse to upgrade old name servers.

 Again, excellent writeup.  I wanted to learn more and after digging through plenty of other articles, this was the most complete and understandable.

Other notes:

/var/named/ - default location in Slackware for Zone files

everything after ; in a zone file is a comment so be descriptive.

From: ed at: 2014-05-03 07:42:26

The settings on sec.centralsoft.org are  almost the same like in pri.centralsoft.org?