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Four in five authoritative domain name system (DNS) servers across the world are vulnerable to types of hacking attacks that might be used by hackers to misdirect surfers to potentially fraudulent domains. A survey by net performance firm the Measurement Factory commissioned by net infrastructure outfit Infoblox of 1.3m internet name servers found that 84 per cent might be vulnerable to pharming attacks. Others exhibit separate security and deployment-related vulnerabilities.

Pharming attacks use DNS poisoning or domain hijacks to redirect users to dodgy urls. For example widespread attacks launched in April attempt to fool consumers into visiting potentially malicious web sites by changing the records used to convert domain names to IP addresses. These particular pharming attacks exploited name servers that allow recursive queries from any IP address. Recurssive queries are a form of name resolution that may require a name server to relay requests to other name servers.

Providing recursive queries to arbitrary IP addresses on the internet exposes a name server to both cache poisoning and denial of service attacks. Such requests should be restricted to trusted sources. But the study found that up to 84 per cent of the name servers investigated relayed requests from world + dog, violating best practices and opening the door to possible hacking attack.

The survey also revealed that more than 40 per cent of the name servers investigated provide zone transfers to arbitrary queries. Like recursive name services, zone transfers, which copy an entire segment of an organization's DNS data from one DNS server to another, should only be allowed for a designated list of trusted, authorised hosts. Network configuration errors in setting up redundant servers for extra availability were also uncovering during the study, which involved using a series of carefully designed queries in order to gauge the relative vulnerability of each name server to attacks or failures.

Cricket Liu, vice president of architecture at Infoblox and author of O'Reilly & Associates' DNS and BIND, DNS & BIND Cookbook, and DNS On Windows Server 2003, said "Given what enterprises are risking - the availability of all of their network services - these results are frightening, especially since there are easy ways to address these issues."

Infoblox has come up with a list of 'top tips' designed to help enterprises to guard against DNS vulnerabilities:

  1. If possible, split external name servers into authoritative name servers and forwarders.
  2. On external authoritative name servers, disable recursion. On forwarders, allow only queries from your internal address space.
  3. If you can't split your authoritative name servers and forwarders, restrict recursion as much as possible. Only allow recursive queries if they come from your internal address space.
  4. Use hardened, secure appliances instead of systems based on general-purpose servers and operating software applications (such as InfoBlox's appliance for DNS, we guess the firm is saying here, well it had to get a product pitch in there somewhere).
  5. Make sure you run the latest version of your domain name server software.
  6. Filter traffic to and from your external name servers. Using either firewall or router-based filters, ensure that only authorized traffic is allowed between your name servers and the Internet.

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