dnsdist: Fix handling of proxy protocol payload outside of TLS for DoT

[rgacogne]

Short description

After reading the proxy protocol payload from the I/O buffer we were clearing the buffer but failed to properly reset the position, leading to an exception when trying to read the DNS payload after processing the TLS handshake:

Got an exception while handling (reading) TCP query from 127.0.0.1:59426: Calling tryRead() with a too small buffer (2) for a read of 18446744073709551566 bytes starting at 52

The huge value comes from the fact that the position (52 here) is larger than the size of the buffer (2 at this point to read the size of the incoming DNS payload), leading to an unsigned underflow. The code is properly detecting that the value makes no sense in this context, but the connection is then dropped because we cannot recover.

It turns out we had a end-to-end test for the "proxy protocol outside of TLS" case but only over incoming DoH, and the DoH case avoids this specific issue because the buffer is always properly resized, and the position updated. This PR adds a test for the DoT case as well.

Fixes #14631.

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)

[coveralls]

Pull Request Test Coverage Report for Build 10773196061

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 59 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.01%) to 64.646%

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Files with Coverage Reduction	New Missed Lines	%
pdns/dnsdistdist/dnsdist-tcp.cc	1	75.79%
pdns/pollmplexer.cc	1	83.66%
pdns/backends/gsql/gsqlbackend.hh	2	97.71%
pdns/iputils.cc	3	55.45%
pdns/fstrm_logger.cc	3	44.08%
pdns/recursordist/recpacketcache.hh	3	89.55%
pdns/iputils.hh	3	78.34%
pdns/shuffle.cc	4	39.71%
modules/godbcbackend/sodbc.cc	4	70.8%
pdns/recursordist/rec-tcpout.cc	6	50.79%

Totals
Change from base Build 10768211128:	-0.01%
Covered Lines:	124641
Relevant Lines:	162101

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💛 - Coveralls