BIND’s blackhole zone feature lets you deny DNS resolution for specific domains, effectively blocking access to malicious sites.
Let’s see it in action. Imagine you want to block malicious-site.com.
Here’s a snippet from a BIND zone file, db.blackhole:
$TTL 86400
@ IN SOA ns1.example.com. hostmaster.example.com. (
2023102701 ; Serial
3600 ; Refresh
1800 ; Retry
604800 ; Expire
86400 ) ; Minimum TTL
;
@ IN NS ns1.example.com.
ns1 IN A 192.168.1.10
malicious-site.com. IN A 0.0.0.0
malicious-site.com. IN AAAA ::
When a client requests malicious-site.com, BIND will respond with 0.0.0.0 for IPv4 and :: for IPv6. This is a "blackhole" address – it’s a valid IP address, but it doesn’t lead anywhere useful, effectively stopping the connection.
BIND’s blackhole functionality is powered by the zone directive in your BIND configuration (named.conf). You define a zone, point it to a specific zone file, and crucially, specify its type as master.
Here’s how you’d configure it in named.conf:
zone "malicious-site.com" {
type master;
file "/etc/bind/db.blackhole";
};
This tells BIND that malicious-site.com is a zone it’s authoritative for, and the records for it are located in /etc/bind/db.blackhole.
The real power comes when you aggregate these entries. Instead of creating a new zone for every single malicious domain (which would explode your named.conf), you can use a single "master" blackhole zone and include other zones into it.
Consider a scenario where you have multiple lists of malicious domains, perhaps from different threat intelligence feeds. You can manage these in separate files and then include them into your primary blackhole zone file.
Let’s say you have bad-ips.txt and phishing.txt.
db.blackhole would look like this:
$TTL 86400
@ IN SOA ns1.example.com. hostmaster.example.com. (
2023102701 ; Serial
3600 ; Refresh
1800 ; Retry
604800 ; Expire
86400 ) ; Minimum TTL
;
@ IN NS ns1.example.com.
ns1 IN A 192.168.1.10
$INCLUDE /etc/bind/bad-ips.txt
$INCLUDE /etc/bind/phishing.txt
And bad-ips.txt might contain:
bad-domain-1.com. IN A 0.0.0.0
another-bad.org. IN AAAA ::
And phishing.txt:
phishingsite.net. IN A 0.0.0.0
scam.biz. IN A 0.0.0.0
When BIND loads db.blackhole, it processes the $INCLUDE directives, effectively merging the contents of bad-ips.txt and phishing.txt into the malicious-site.com zone. This keeps your named.conf clean and allows for modular management of your blocklists.
The DNS protocol itself doesn’t have a concept of "blocking" a domain. What you’re doing here is making your BIND server authoritative for the malicious domain and providing a syntactically correct, but functionally useless, IP address. The client’s operating system or application then tries to connect to 0.0.0.0 or ::, which results in a connection failure, but importantly, the DNS lookup itself succeeds without error. This is often preferred over returning NXDOMAIN (non-existent domain), as NXDOMAIN responses can sometimes be cached aggressively by clients, making it harder to re-establish legitimate access if a domain is accidentally blackholed.
A common pitfall is forgetting to increment the serial number in the SOA record after making changes to your zone files. If the serial number doesn’t change, BIND won’t reload the zone, and your updates won’t take effect.
Another subtle point is the TTL (Time To Live) you set for your blackholed records. If you set a very high TTL (e.g., 86400 seconds, or 24 hours), clients will cache the 0.0.0.0 or :: response for that duration. While this reduces DNS query load, it means that if you need to unblock a domain quickly, clients might still be trying to reach the blackhole address for a long time. For dynamic blocklists where rapid updates are crucial, consider using lower TTLs, perhaps 60 or 300 seconds.
The next step is often managing these blocklists dynamically, perhaps by using tools that automatically fetch and update zone files, or by exploring BIND’s Response Policy Zones (RPZ) feature for more sophisticated control.