On Sat, 26 Sep 2020, Noel Duffy via fpc-pascal wrote:
Hi all,
I've spent some time this past couple of weeks reading through the DNS
resolver code in netdb and poring over RFCs that specify the protocol with a
view to adding support for additional resource record queries. Things like
TXT, SOA, and so forth. I'm using netdb from fpc 3.2.0 as the starting point.
That code only supports querying DNS over UDP. Because TXT records can be up
to 64k in size, they frequently get too big to fit inside a UDP packet. The
RFCs mandate that resolvers must fall back to TCP in that case. That's the
part I'm currently looking at.
The maximum size for a DNS response over TCP is 65,535 octets. The netdb code
defines a type TPayload that's a 512 octet buffer for receiving DNS
responses. That's the largest response possible over UDP. The simplest path
forward for me is to extend that buffer from 512 octets to 65,535. While that
increase in size would hardly register on most machines, I wonder whether it
might be too much on some of the more memory-restricted platforms that FPC
supports. DOS, for instance. I also see IFDEFs for Android in this code.
The alternative is to use different types for TCP queries, or to have the
buffer allocated dynamically.
To restate the question, is 64k too much overhead for DNS queries? That
overhead would be present on all queries, regardless of whether UDP or TCP
was used.
I would then prefer to allocate a dynamic buffer. A simple "Array of Byte" can
easily be managed with SetLength() and it is automatically disposed of when
not needed.
Michael.
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