doc: document TLS_FORWARD issues with some public DNS providers

27 jobs for doc-google-public-dns in 7 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
canceled #282286
amd64 linux docker
archive

00:00:07

canceled #282287
amd64 linux docker
build

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canceled #282288
amd64 linux docker
build-asan

00:00:07

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docker
kres-gen

00:00:07

 
  Test
canceled #282290
docker delayed
build:darwin

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amd64 linux docker
deckard

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docker delayed
doc

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dind
docker

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docker delayed
lint:other

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amd64 linux docker delayed
lint:pedantic

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amd64 linux docker
lint:scan-build

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amd64 linux docker
lint:tidy

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privileged
pkg:epel-7

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privileged
pkg:fedora-29

canceled #282300
amd64 linux docker
respdiff:basic

canceled #282301
amd64 linux docker delayed
test:valgrind

 
  Extended
canceled #282303
respdiff
fwd-tls6-kresd.udp6

canceled #282304
respdiff
fwd-udp6-kresd.udp6

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respdiff
fwd-udp6-unbound.tcp6

canceled #282309
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fwd-udp6-unbound.tls6

canceled #282307
respdiff
fwd-udp6-unbound.udp6

canceled #282306
respdiff
iter.tls6

canceled #282305
respdiff
iter.udp6

canceled #282302
amd64 linux docker
pytests

canceled #282310
respdiff
resperf:fwd-tls6.udp-asan

canceled #282311
respdiff
resperf:fwd-udp6.udp-asan

canceled #282312
respdiff
resperf:iter.udp-asan