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

[Noel Duffy]

On 26/09/20 9:07 pm, Michael Van Canneyt via fpc-pascal wrote:
> 
> On Sat, 26 Sep 2020, Noel Duffy via fpc-pascal wrote:
>> 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.

An unintended consequence of attempting to replace the TPayLoad static array with a dynamic one is that the TQueryData record can no longer be passed to fpSendTo as a stream of octets by passing a pointer to it.

I.e,

   Function Query(Resolver : Integer; Var Qry,Ans : TQueryData; QryLen : Integer;
	Var AnsLen : Integer) : Boolean;
   begin
   [...]
     fpsendto(sock, at qry,qrylen+12,0, at SA,SizeOf(SA));

When TPayLoad is made dynamic, the final member of TQueryData changes from an array of octets to a pointer. Now passing a pointer to Qry results in the pointer to the dynamic array being serialized instead of the query payload. Naturally the DNS server will turn up its nose at that and return a sternly-worded error code in protest.

The payload data will have to be manually serialized to octets, I think. At least I don't know of any way to serialize a record that contains pointers using any feature of the language or api in the RTL. Doing manual serialization will also require more memory, of course. The quantities aren't huge, though, so I don't think it matters hugely. But more experienced FPC developers may be able to recommend a better way, of course!