Max TXT Size Entry in DNS

[Eric Wassenaar]

> Tristan> Is there a maximum size a TXT entry can be in DNS (using bind)

The amendment "using bind" is essential.

> The maxiumum length of a string in a  TXT record appears to be 255
> characters.

The rdata field of a TXT record may consist of multiple substrings,
each of which has a maximum of 255 characters.

As far as I'm aware, there is no RFC that restricts the size
of the rdata field of a resource record.

There may be practical limitations, however, such as the maximum size
of a single TCP packet.

> However there's an #ifdef in db_load.c (ALLOW_LONG_TXT_RDATA) which is
> switched on in 8.2.2P5. This allows for TXT records of up to MAXDATA -
> approx. 2Kbytes - to be loaded

The MAXDATA limit in BIND is rather arbitrary.
BIND restricts also the size of the answer buffer in TCP mode.
This may lead to truncated answers (which are not always handled
properly by applications).

There are alternative nameservers out there without such restrictions.
I have seen a TXT record of 17000 bytes :-)

-- Eric Wassenaar
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