allow/deny xfr requests by default?

[Spain, Dr. Jeffry A.]

> Do people have an opinion about whether BIND 10 should allow/deny AXFR/IXFR requests by default?  Currently b10-xfrout allows xfr requests by default just like BIND 9 does so.

> My general understanding is that this is a matter-of-opinion topic.

The National Institute of Standards and Technology touches on this issue in their April 2010 publication "Secure Domain Name System (DNS) Deployment Guide." See http://csrc.nist.gov/publications/nistpubs/800-81r1/sp-800-81r1.pdf. In section 8.1.2 "Restricting Zone Transfer Transaction Entities" on page 8-6:

"Authoritative name servers (especially primary name servers) should be configured with an allow-transfer access control substatement designating the list of hosts from which zone transfer requests can be accepted. These restrictions address the denial-of-service threat and potential exploits from unrestricted dissemination of information about internal resources. Based on the need to know, the only name servers that need to refresh their zone files periodically are the secondary name servers. Hence, zone transfer from primary name servers should be restricted to secondary name servers. The zone transfer should be completely disabled in the secondary name servers. The address match list argument for the allow-transfer substatement should consist of IP addresses of secondary name servers and stealth secondary name servers."

Based on this I would make an additional argument in favor of denying AXFR/IXFR requests by default. With denial by default, an administrator's failure to configure allowable zone transfers as recommended by NIST results in an obvious communications malfunction between master and slave name servers that the administrator would troubleshoot and fix. With AXFR/IXFR permitted to all hosts by default, this same failure to configure results in a set of master and slave name servers that is apparently communicating normally, but which is configured in a manner contrary to what NIST recommends.

Jeffry A. Spain
Network Administrator
Cincinnati Country Day School