Google Public DNS Security Not Breaking New Ground, Some Say

By Brian Prince  |  Posted 2009-12-04 Print this article Print

Google has implemented security features in Google Public DNS to help prevent DNS cache poisoning and denial-of-service attacks. But while some applaud Google's approach, others--including OpenDNS--say Google is not breaking new ground.

Much has been made about how Google Public DNS will improve the speed of the Web. But what about security?

According to some, Google is on the right track-though others say the company is not exactly breaking new ground.

"DNS as a protocol is pretty terrible in terms of security, but from a cursory glance it appears that Google is doing all of the right things," said HD Moore, chief security officer at Rapid7. "Specifically, they are using strongly random transaction IDs and source ports for their outbound queries and they are not returning fraudulent responses, as many ISPs do."

According to Google, the company sought to take on DNS cache poisoning and denial-of-service attacks against DNS resolvers. To get this done, Google implemented and recommends a number of features to help mitigate these problems.

For starters, Google has implemented a few techniques for adding entropy to request messages, including randomizing source ports, the case of letters in domain name queries and the choice of nameservers. Google also appended nonce prefixes to name requests, which can help address the threat posed by exploits such as the one uncovered last year by researcher Dan Kaminsky.

"If a resolver cannot directly resolve a name from the cache, or cannot directly query an authoritative nameserver, then it must follow referrals from a root or TLD nameserver," Google stated on a Web page dedicated to Google DNS. "In most cases, requests to the root or TLD nameservers will result in a referral to another nameserver, rather than an attempt to resolve the name to an IP address. For such requests, it should therefore be safe to attach a random label to a query name to increase the entropy of the request, while not risking a failure to resolve a non-existent name."

"Although in practice such requests make up less than 3% of outgoing requests, assuming normal traffic (since most queries can be answered directly from the cache or by a single query), these are precisely the types of requests that an attacker tries to force a resolver to issue," Google continued. "Therefore, this technique can be very effective at preventing Kaminsky-style exploits."

Google has also taken steps to remove duplicate queries and rate-limiting requests to prevent DoS attacks.

But all this, said OpenDNS CTO David Ulevitch, is not exactly new.

"We were the only DNS company not vulnerable to the Kaminsky flaw," he told eWEEK. "We pioneered many of those techniques and have been doing things like source port randomization since our inception in 2005. The idea of adding entropy is not new, and again, is something we've been doing for a long, long time. We use embedded [extension mechanisms for DNS] options, which we think is better than their capitalization trick, though we've considered that one in the past."

Gartner analyst John Pescatore agreed that Google isn't offering anything here that isn't available elsewhere, and questioned Google's commitment to privacy.

"I read through Google's privacy statements around the DNS service, and they are saying they won't store any information long term, but it says nothing about if they will resell any of the trend information or sell advertising services based on the information they do see at every query," he said.

Google DNS stores two types of logs-permanent and temporary. The temporary logs store the full IP address of the machine being used, but are deleted within 24 to 48 hours. The permanent logs do not include personally identifiable information or IP information, though they do keep location information in order to perform debugging and analyze abuse to improve their prefetching feature.

"We don't correlate or combine your information from these logs with any other log data that Google might have about your use of other services, such as data from Web Search and data from advertising on the Google content network," according to Google. "After keeping this data for two weeks, we randomly sample a small subset for permanent storage."

"We built Google Public DNS to make the Web faster and to retain as little information about usage as we could, while still being able to detect and fix problems," the company added.


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