[dns-operations] TCP Re: FlagDay 2020 UDP Size (ofda.gov breakage)

Fred Morris m3047 at m3047.net
Sat Aug 8 05:07:04 UTC 2020

TLDR: Although DNS servers are supposed to support TCP, it is almost never 
utilized in practice unless a UDP response is first received with TC=1, 
and fragmentation exacerbates this.

I've discovered this myself during this recent period of intensive 
interaction with SOHO networks. Apparently it's always been like this. I 
was surprised to find it true, even reading venerable source code to 

On Sat, 8 Aug 2020, Viktor Dukhovni wrote:
> [...] So, while TCP actually works if used directly, there is
> no TCP fallback since no UDP packets are returned with TC=1. :-(

Basically the resolver protocol is unchanged since the 1980s. I think some 
further thinking should be done!


Fred Morris

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