Definitions limit on label length in UTF-8

Harald Alvestrand harald at
Mon Sep 14 12:24:02 CEST 2009

Martin J. Dürst wrote:
> Overall, we get a maximum label length in octets of 252 octets for 
> UTF-32 (with US-ASCII), and 224 octets in UTF-8 and UTF-16 (with Old 
> Italic and the like).
Documenting these 3 numbers as "an U-label can't get longer than that 
and fit into an A-label" seems sufficient to avoid the spectre of 
"unlimited length" to me. People with more expertise in actually writing 
the code to handle these names will have better info.

In the nsswitch case, we can't know the limitations of the other 
branches of the switch - some may allow longer labels, some may only 
allow a LOT shorter labels. Since we can't know, we should not make 
limits that we don't need for our primary mechanism (DNS with IDNA).

My thought.


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