I'm currently using bind and I have a unique scenario.
I have A records with multiple IPs (e.g xyz.mydomain.com has 200 different
IPs).
Recently some users encountered an issue while trying to resolve these
records, apparently due to the record size.
I was wondering if with Unbound I can control the response size and limit
it (so that for example when a user is resolving xyz.mydomain.com he will
only receive one or several records as per the round robin load balancing).
Looking forward for your insights.
It is not entirely clear from your question, so it may be that your question applies to NSD vs BIND rather than Unbound vs BIND. Note where BIND is a combined recursive server and authoritative server, Unbound is a standalone recursive server, and NSD is a standalone authoritative server.
If your hosting provider is handling the authoritative DNS, then you can use Unbound and python plug-ins to modify the records. You may find a reasonable starting point under “examples” here: http://unbound.net/documentation/pythonmod/index.html. Maybe do something similar to blacklist record manipulation and instead strip excess records.
I agree with Eric that your question might apply to the authoritative
name server rather than Unbound. Having said that, there are options to
reduce/minimize the authoritative and additional section in a DNS
response (minimal-responses option both in NSD and Unbound), but not for
the answer section.
For large answers though, TCP fail-over should work or an EDNS buffer
size of 4096 (if client supports EDNS).
An authoritative/recursive name server cannot cut an answer in pieces
and remove RRs. If your users still encounter this problem with a large
answer section, the suggestion of Eric would be your path forward and
you can develop a Python plugin to modify DNS replies such that if fits
with your requirements.