[fpc-pascal] Buffer size for TCP DNS queries in netdb

[Michael Van Canneyt]

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.