The best method is via the response policy zone in Bind 9.8.1 or newer. It allows you to override single records in arbitrary zones (and there's no need to create a whole subdomain for that, only the single record you want to change), it allows you to override CNAMEs, etc. Other solutions such as Unbound cannot override CNAMEs.
https://www.redpill-linpro.com/sysadvent/2015/12/08/dns-rpz.html
EDIT: Let's do this properly then. I will document what I've done based on the tutorial linked above.
My OS is Raspbian 4.4 for Raspberry Pi, but the technique should work without any changes on Debian and Ubuntu, or with minimal changes on other platforms.
Go to where your Bind config files are kept on your system - here it's in /etc/bind
. Create in there a file called db.rpz
with the following contents:
$TTL 60
@ IN SOA localhost. root.localhost. (
2015112501 ; serial
1h ; refresh
30m ; retry
1w ; expiry
30m) ; minimum
IN NS localhost.
localhost A 127.0.0.1
www.some-website.com A 127.0.0.1
www.other-website.com CNAME fake-hostname.com.
What does it do?
- it overrides the IP address for
www.some-website.com
with the fake address 127.0.0.1
, effectively sending all traffic for that site to the loopback address
- it sends traffic for
www.other-website.com
to another site called fake-hostname.com
Anything that could go in a Bind zone file you can use here.
To activate these changes there are a few more steps:
Edit named.conf.local
and add this section:
zone "rpz" {
type master;
file "/etc/bind/db.rpz";
};
The tutorial linked above tells you to add more stuff to zone "rpz" { }
but that's not necessary in simple setups - what I've shown here is the minimum to make it work on your local resolver.
Edit named.conf.options
and somewhere in the options { }
section add the response-policy
option:
options {
// bunch
// of
// stuff
// please
// ignore
response-policy { zone "rpz"; };
}
Now restart Bind:
service bind9 restart
That's it. The nameserver should begin overriding those records now.
If you need to make changes, just edit db.rpz
, then restart Bind again.
Bonus: if you want to log DNS queries to syslog, so you can keep an eye on the proceedings, edit named.conf.local
and make sure there's a logging
section that includes these statements:
logging {
// stuff
// already
// there
channel my_syslog {
syslog daemon;
severity info;
};
category queries { my_syslog; };
};
Restart Bind again and that's it.
Test it on the machine running Bind:
dig @127.0.0.1 www.other-website.com. any
If you run dig on a different machine just use @the-ip-address-of-Bind-server instead of @127.0.0.1
I've used this technique with great success to override the CNAME for a website I was working on, sending it to a new AWS load balancer that I was just testing. A Raspberry Pi was used to run Bind, and the RPi was also configured to function as a WiFi router - so by connecting devices to the SSID running on the RPi I would get the DNS overrides I needed for testing.
With BIND, you need a fake root zone to do this. In named.conf
, put the following:
zone "." {
type master;
file "/etc/bind/db.fakeroot";
};
Then, in that db.fakeroot
file, you will need something like the following:
@ IN SOA ns.domain.com. hostmaster.domain.com. ( 1 3h 1h 1w 1d )
IN NS <ip>
* IN A <ip>
With that configuration, BIND will return the same IP address for all A
queries.
Best Answer
When overlapping zones are defined on an authoritative nameserver, the most specific zone is used to provide the answer.
example.com. IN A?
hits theexample.com
zone.foo.example.com. IN A?
hits thefoo.example.com
zone. Iffoo.example.com
is defined in theexample.com
zone, it will be ignored.sub.foo.example.com. IN A?
hits thefoo.example.com
zone, because it is more specific for the request thanexample.com
.In your specific example, the returned value for the query would be
NXDOMAIN
because thefoo.example.com
zone does not provide an A record definition for itself.To use DNS terminology, there is effectively a zone cut at the boundary between the parent zone and the more specific zone. RFC2181 ยง6.1 describes the proper behavior for your scenario:
As for which is a best practice, that's completely up to you. You can put all of your records in a single zone, or break out separate zone files for your subdomains if it would be more convenient. (say, due to them containing hundreds of records each) The one thing I would caution, which isn't very obvious, is that validation tools like
named-checkzone
typically have no awareness of subzones living on the same server and may generate incorrect warnings if the parent zone includes references to records inside the child zone. (say, CNAME aliases)