Restricting zone transfers with IP addresses in BIND DNS Server

DNS server can be attacked using various techniques such as

[a] DNS spoofing

[b] Cache poisoning

[c] Registration hijacking

One of the simplest ways to defend is limit zone transfers between nameservers by defining ACL. I see many admin allows BIND to transfer zones in bulk outside their network or organization. There is no need to do this. Remember you don't have to make an attacker's life easier.

How to restrict zone trasfer with IP address?

You need to define ACL in /etc/named.conf file. Let us say IP 192.168.191.10 and 25.111.24.6 are allowed to transfer your zones.
# vi named.conf
Here is sample entery for domain nixcraft.com (ns1 configuration):

acl trusted-servers  {
        192.168.191.10;  //ns2
        25.111.24.6;   //ns3
};
zone nixcraft.com  {
        type master;
        file "zones/nixcraft.com";
        allow-transfer { trusted-servers; };
};

Next add zone nixcraft.com. Please note that you must use set of hosts later in each zone's configuration block i.e. put line allow-transfer { trusted-servers; }; for each zone / domain name. Restart named:
# /etc/init.d/named restart

How do I test zone transfers restrictions are working or not?

Use any UNIX dns tool command such as nslookup, host or dig. For example, following example uses host command to request zone transfer:
$ host -T axfr nixcraft.com
Output:

;; Connection to 74.86.49.133#53(74.86.49.133) for axfr failed: connection refused.

Transaction signatures (TSIG)

Another recommend option is to use transaction signatures (TSIG) to authorize zone transfers. This makes more difficult to spoof IP addresses.

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{ 3 comments… read them below or add one }

1 Ulrich Wisser October 16, 2007 at 11:36 am: Hi,
why would you like to restrict your zone transfer? You will allow any resolver to ask for the same data, but you won’t allow a transfer? I suggest you put only public data in your zone file and don’t care about the zone transfer. If you have to have private data in a zone file, set up an internal DNS master (or use split DNS) with a private zone file and restrict access for resolvers and zone transfer.
Ulrich
2 vivek October 16, 2007 at 12:36 pm: Yes this information is publicly available through BIND server, there is no reason to make an attacker’s life easier. There is no legitimate reason for anyone outside your organization to transfer your zones in bulk.
3 äijö December 28, 2008 at 6:26 pm: Ulrich: are you able to recognize authorative and resolve DNS server? You cannot run both on this same IP address, so if you need authorative server for your domains, you should restrict zone transfers only to slaves. If it’s necessary to having resolver in local network, run it on local address.