[fpc-pascal] netdb, DNS and TCP

[Michael Van Canneyt]

On Sun, 11 Oct 2020, Noel Duffy via fpc-pascal wrote:

> Hi all,
>
> To keep track of the work I've been doing on DNS in netdb, I created bug 
> #37906 in Mantis.
>
> https://bugs.freepascal.org/view.php?id=37906
>
> I've attached to a note on that bug report a proof-of-concept version of 
> netdb.pp and a small program that uses it to make text queries over TCP. This 
> version of netdb has different record types for TCP and UDP (technically, it 
> has three, one for UDP queries and replies, one for TCP queries, one for TCP 
> replies). The wire protocol for DNS over TCP has a mandatory two-octet length 
> field at the start for both queries and replies, and I didn't want to have to 
> use a full-length 64k buffer for both when a smaller 512 byte buffer 
> suffices.
>
> Overall I'm not really enthused with this implementation. Having separate 
> record types for the different payload types means having separate versions 
> of a number of helper functions as well. Functions like BuildPayload, NextRR, 
> and SkipAnsQueries.
>
> To avoid that duplication, the code would need to use a dynamic buffer and 
> fill it just before socket writing. The TCP variant would include its length 
> field in the buffer. Additionally, after reading the response from the 
> socket, the buffer contents would have to be copied into a record data 
> structure. It's a tradeoff between execution efficiency and simplicity.

It always is.

But I am inclined to think that DNS queries benefit more from execution
efficiency, hence the preference for avoiding the copying of buffers as much
as possible.

One could of course argue that with TCP, the speed suffers in each case.

If you have suggestions for a better implementation, I am all ears...

Michael.