Recursion in BIND DNS

In the context of BIND (Berkeley Internet Name Domain) DNS, recursion refers to the process where a DNS server queries other DNS servers on behalf of a client to resolve a domain name. Here’s a breakdown of what this means:

What is Recursion in BIND DNS?

When a DNS server receives a query for a domain name that it doesn’t have information about, it can either respond with a referral to another DNS server or perform a recursive query to find the answer. In a recursive query, the DNS server takes on the responsibility of querying other DNS servers until it finds the authoritative answer and then returns that answer to the client.

How Recursion Works

1. Client Query: A client (such as a web browser) sends a DNS query to a recursive DNS server.
2. Recursive Query: If the recursive DNS server doesn’t have the answer in its cache, it queries other DNS servers starting from the root servers, then the top-level domain (TLD) servers, and finally the authoritative DNS servers for the domain.
3. Caching: Once the recursive DNS server obtains the answer, it caches the result for a specified period (determined by the TTL value) to speed up future queries for the same domain.
4. Response: The recursive DNS server returns the answer to the client.

Configuring Recursion in BIND

In BIND, recursion is enabled by default. You can control recursion behavior using the recursion option in the named.conf configuration file. Here’s how you can configure it:

1. Open the Configuration File:shCopysudo nano /etc/bind/named.conf.options
2. Enable or Disable Recursion:
   • To enable recursion:iniCopyoptions { recursion yes; allow-recursion { any; }; };
   • To disable recursion:iniCopyoptions { recursion no; };
3. Restrict Recursion to Specific Clients:
   • You can restrict recursion to specific IP addresses or networks:iniCopyoptions { recursion yes; allow-recursion { 192.0.2.0/24; 2001:db8::/32; }; };

Best Practices

• Do Not Operate an Open Resolver: Open resolvers can be exploited for DNS amplification attacks. Restrict recursion to known and authorized clients1.
• Separate Authoritative and Recursive Servers: It is recommended to run authoritative and recursive DNS services on separate servers to improve security and performance1.

By understanding and configuring recursion in BIND DNS, you can ensure efficient and secure resolution of domain names for your network