fix DNS issues with unresolvable backends with ExternalName

[neerfri]

What this PR does / why we need it:

Currently there's a problem where if an Ingress is using a service with ExternalName but that external name cannot be resolved ingress-nginx is failing to sync the backend and will print a lot of log lines like:

2020/11/25 20:18:52 [error] 1707#1707: *51723 [lua] dns.lua:152: dns_lookup(): failed to query the DNS server for foo.unknown.com:
server returned error code: 3: name error
server returned error code: 3: name error, context: ngx.timer

This has been discussed in #6523 which was closed by the bot due to inactivity but is in fact still relevant.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

fixes #6523

How Has This Been Tested?

Not tested yet. I'm working to test this locally and will report here when I'm able to test it successfully

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

This issue is currently awaiting triage.

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[k8s-ci-robot]

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[k8s-ci-robot]

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[strongjz]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: neerfri
Once this PR has been reviewed and has the lgtm label, please assign gacko for approval. For more information see the Kubernetes Code Review Process.

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[neerfri]

Hi @strongjz
Thanks for the approval to test.
I've fixed the problems. looking forward for feedback on this

[neerfri]

@strongjz @cpanato please guide me on how to make progress here.
Thanks!