The Domain Name System (DNS) is a elementary protocol used on the Internet to translate human-readable domains (e.g., www.instance.com) into numeric IP addresses (e.g., 192.0.2.1) in order that gadgets and servers can discover and talk with one another. When a person enters a site title of their browser, the DNS resolver (e.g. Google Public DNS) locates the authoritative DNS nameservers for the requested title, and queries a number of of them to acquire the IP handle(es) to return to the browser.
When DNS was launched within the early Nineteen Eighties as a trusted, content-neutral infrastructure, safety was not but a urgent concern, nevertheless, because the Internet grew DNS turned susceptible to varied assaults. In this put up, we’ll have a look at DNS cache poisoning assaults and the way Google Public DNS addresses the dangers related to them.
DNS lookups in most functions are forwarded to a caching resolver (which might be native or an open resolver like. Google Public DNS). The path from a consumer to the resolver is normally on an area community or might be protected utilizing encrypted transports like DoH, DoT. The resolver queries authoritative DNS servers to acquire solutions for person queries. This communication primarily happens over UDP, an insecure connectionless protocol, wherein messages might be simply spoofed together with the supply IP handle. The content material of DNS queries could also be sufficiently predictable that even an off-path attacker can, with sufficient effort, forge responses that seem like from the queried authoritative server. This response will likely be cached if it matches the mandatory fields and arrives earlier than the genuine response. This kind of assault is known as a cache poisoning assault, which might trigger nice hurt as soon as profitable. According to RFC 5452, the chance of success may be very excessive with out safety. Forged DNS responses can result in denial of service, or could even compromise software safety. For a wonderful introduction to cache poisoning assaults, please see “An Illustrated Guide to the Kaminsky DNS Vulnerability”.
Improving DNS safety has been a aim of Google Public DNS since our launch in 2009. We take a multi-pronged method to guard customers in opposition to DNS cache-poisoning assaults. There is not any silver bullet or countermeasure that fully solves the issue, however together they make profitable assaults considerably harder.
RFC 5452 And DNS Cookies
We have carried out the essential countermeasures outlined in RFC 5452 specifically randomizing question supply ports and question IDs. But these measures alone should not enough (see web page 8 of our OARC 38 presentation).
We have subsequently additionally carried out help for RFC 7873 (DNS Cookies) which might make spoofing impractical if it’s supported by the authoritative server. Measurements point out that the DNS Cookies don’t present enough protection, although round 40% of nameservers by IP help DNS Cookies, these account for lower than 10% of total question quantity. In addition, many non-compliant nameservers return incorrect or ambiguous responses for queries with DNS Cookies, which creates additional deployment obstacles. For now, we’ve enabled DNS Cookies via handbook configuration, primarily for chosen TLD zones.
Case Randomization (0x20)
The question title case randomization mechanism, initially proposed in a March 2008 draft “Use of Bit 0x20 in DNS Labels to Improve Transaction Identity”, nevertheless, is very efficient, as a result of all however a small minority of nameservers are suitable with question title case randomization. We have been performing case randomization of question names since 2009 to a small set of chosen nameservers that deal with solely a minority of our question quantity.
In 2022 we began work on enabling case randomization by default, which when used, the question title within the query part is randomized and the DNS server’s response is anticipated to match the case-randomized question title precisely within the request. For instance, if “ExaMplE.CoM” is the title despatched within the request, the title within the query part of the response should even be “ExaMplE.CoM” quite than, e.g., “example.com.” Responses that fail to protect the case of the question title could also be dropped as potential cache poisoning assaults (and retried over TCP).
We are joyful to announce that we’ve already enabled and deployed this characteristic globally by default. It covers over 90% of our UDP visitors to nameservers, considerably decreasing the danger of cache poisoning assaults.
Meanwhile, we preserve an exception listing and implement fallback mechanisms to forestall potential points with non-conformant nameservers. However we strongly suggest that nameserver implementations protect the question case within the response.
DNS-over-TLS
In addition to case randomization, we’ve deployed DNS-over-TLS to authoritative nameservers (ADoT), following procedures described in RFC 9539 (Unilateral Opportunistic Deployment of Encrypted Recursive-to-Authoritative DNS). Real world measurements present that ADoT has the next success price and comparable latency to UDP. And ADoT is in use for round 6% of egress visitors. At the price of some CPU and reminiscence, we get each safety and privateness for nameserver queries with out DNS compliance points.
Summary
Google Public DNS takes safety of our customers significantly. Through a number of countermeasures to cache poisoning assaults, we intention to offer a safer and dependable DNS decision service, enhancing the general Internet expertise for customers worldwide. With the measures described above we’re capable of present safety in opposition to passive assaults for over 90% of authoritative queries.
To improve DNS safety, we suggest that DNS server operators help a number of of the safety mechanisms described right here. We are additionally working with the DNS group to enhance DNS safety. Please see our displays at DNS-OARC 38 and 40 for extra technical particulars.