Skip invalid DNS server #567

Job	Run time
build-lib-android	10m 30s
build-lib-apple	17m 13s
test (macos-latest)	4m 2s
test (ubuntu-latest)	4m 4s
build-cli-macos (x86_64-apple-darwin)	4m 4s
build-cli-cross (x86_64-unknown-linux-musl)	4m 33s
build-cli-cross (armv7-unknown-linux-musleabihf)	4m 37s
build-cli-cross (arm-unknown-linux-musleabi)	4m 57s
build-cli-cross (aarch64-unknown-linux-musl)	4m 41s
build-cli-cross (x86_64-pc-windows-gnu)	4m 59s
build-cli-cross (i686-unknown-linux-musl)	5m 9s
build-cli-macos (aarch64-apple-darwin)	3m 13s
	1h 12m 2s

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