Remove fqdn() function from libp2p-dns

[tomaka]

Fix #2022

See this comment

[mxinden]

Do I understand correctly that without fqdn(), i.e. without (falsely) making all domains fully-qualified domains, we do two queries for each domain, even though one might suffice for fully-qualified domains? Put differently, this change has a performance impact only, while restoring correctness across all cases?

[tomaka]

As far as I understand, yes

[mxinden]

@tomaka have you had the chance to test both the fully qualified and the not fully qualified case?

[tomaka]

I did not test this, as the fix seems obvious to me.
The trust-dns library expects a domain name. We do some pre-transformation on the domain name passed by the user. This PR removes this transformation so that trust-dns gets passed the exact value that the user requested.
If this introduces a bug, it can only either be in trust-dns or was already present before this PR.

[mxinden]

Just tested this with a non fully qualified domain locally with DnsConfig::system() which works as expected.

I can cut a new patch release of libp2p-dns. Is that good enough for you @montekki?

[montekki]

Yeah as soon as we can propagate it into the polkadot codebase

[mxinden]

libp2p-dns v0.28.1 is published and tagged. Thanks for the patch @tomaka.