Fwd: Upcoming Adjustment to UDP Response Size for Google Public DNS Queries

[Ernesto Level]

Forwarding announcement on Google Public DNS queries for visibility:


---------- Forwarded message ---------
From: Ernesto Level <Unknown>
Date: Wednesday, March 19, 2025 at 6:56:02 AM UTC-4
Subject: Upcoming Adjustment to UDP Response Size for DNS Queries
To: public-dns-discuss <Unknown>


Dear Google Public DNS users,

If you send queries to our service over UDP please read since this change 
may affect you.

We would like to inform you about an upcoming change to our DNS 
configuration. This might impact you if you send UDP queries to Google 
Public DNS with an EDNS0 option setting a bufsize greater than 1400 bytes. 
The impact will be for responses larger than 1400 bytes.

In order to enhance reliability, we’ll reduce the maximum size of UDP 
responses to DNS queries from 4096 bytes to approximately 1400 bytes, which 
is the recommended maximum DNS/UDP payload size described in RFC 9715, 
section #3. Please note that the exact size might be subject to adjustment 
as we refine our approach.

When a UDP DNS response exceeds this limit, the truncated flag will be set 
(TC=1), which instructs DNS clients to switch to TCP in order to retrieve 
the complete response. Most DNS client software seamlessly handles this 
fallback process, so the majority of users should not notice any disruption.

This change will be rolled out gradually over the upcoming weeks to all 
public DNS queries. This change is expected to start on Monday, April 14, 
2025 and is expected to be live everywhere approximately by Friday, May 30, 
2025.

If you suspect this modification could affect your services or workflows, 
or if you encounter any issues, we encourage you to reach out for support 
by submitting a ticket through our issue tracker: 
https://developers.google.com/speed/public-dns/groups#issue_tracker. Our 
team will be glad to assist and work with you to address any concerns.

Ernesto Level,
on behalf of the Google Public DNS team.
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