Google Public DNS plans to enable case randomization for cache poisoning protection

Tianhao Chi chitianhao at
Thu Aug 11 20:35:16 UTC 2022

Dear users and nameserver operators,

As part of our efforts to increase DNS cache poisoning protection for UDP
queries, we are planning to enable case randomization of DNS query names
sent to most authoritative nameservers (see our *security page description*
the feature and We
have been performing case randomization of query names since 2009 to a
small set of chosen nameservers. This set of servers handled a minority of
our query volume, so a year ago we started work on enabling case
randomization by default. As part of this, we’ve identified a small set of
nameservers (< 1000 distinct IPs) that do not handle case randomization
correctly and have exempted these from case randomization. We are confident
that case randomization will work without introducing significant increases
in DNS query volume or resolution failures.

The case-randomized query name in the request will be expected to exactly
match the name in the question section of the DNS server’s reply, including
the case of each ASCII letter (A–Z and a–z). For example, if “ExaMplE.CoM”
is the name sent in the request, the name in the question section of the
response must also be “ExaMplE.CoM” rather than, e.g., “”
Responses that fail to preserve the case of the query name may be dropped
as potential cache poisoning attacks. Thus, nameservers that fail to
preserve the query name in their response, or whose response to
case-randomized requests is an unexpected error (SERVFAIL, NOTIMP, FORMERR,
etc.) or a failure to respond, will negatively impact users' ability to
resolve names in the domains they serve.

Generally, when nameservers mishandle case-randomized queries, we recommend
asking the nameserver operator to correct their behavior. While our
exception list will work around the problem for now, it may not get
immediate updates for newly broken name servers.

We’ll have case randomization enabled in one or two regions starting on
August 29th and enabled globally by the end of October. Meanwhile, we’ve
already turned off case randomization to nameservers that we’ve identified
as not handling it correctly.

If you believe you have discovered name resolution failures with Google
Public DNS due to case randomization, please file a bug in our *issue
this announcement.
Let us know if there's any question via We've also posted
this in our discussion group:
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