dnsdist: Use 65535 instead of 255 to block all types via eBPF

[rgacogne]

Short description

Our eBPF filtering code used to treat the 255 (ANY) qtype as a special value meaning to block queries for all types. Unfortunately it prevented blocking queries for the ANY type only, which is a legitimate use-case.
After this commit 255 is no longer a special value, and 65535 (a reserved value) is used instead to represent all types.

This is a breaking change that should NOT be backported.

Fixes #13172

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[miodvallat]

I see no downside in this.

I wonder how easy it would be to accept -1 as a value - this way existing configurations could switch from 255 to -1 rather than 65535 and that will be future-proof...

[coveralls]

Pull Request Test Coverage Report for Build 13458110412

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 4 of 4 (100.0%) changed or added relevant lines in 3 files are covered.
• 43 unchanged lines in 12 files lost coverage.
• Overall coverage decreased (-0.001%) to 64.509%

---

Files with Coverage Reduction	New Missed Lines	%
modules/gpgsqlbackend/gpgsqlbackend.cc	1	88.62%
pdns/recursordist/recursor_cache.cc	1	84.29%
pdns/recursordist/syncres.cc	1	80.13%
pdns/opensslsigners.cc	2	61.07%
pdns/tcpiohandler.cc	2	68.18%
pdns/misc.hh	3	83.81%
pdns/recursordist/recpacketcache.hh	3	89.55%
pdns/shuffle.cc	4	53.93%
ext/json11/json11.cpp	5	62.72%
pdns/signingpipe.cc	5	84.99%

Totals
Change from base Build 13457319872:	-0.001%
Covered Lines:	127645
Relevant Lines:	166895

---

💛 - Coveralls

[rgacogne]

I wonder how easy it would be to accept -1 as a value - this way existing configurations could switch from 255 to -1 rather than 65535 and that will be future-proof...

I see the appeal in this but right now the type is carried around as a QType in several places, so an unsigned 16-bit, and I quite like it from a consistency point of view. I'm also not worried about 65535 being used for a different purpose one day (famous last words).