Add the --default-targets flag

[bbl]

Description

This is a useful feature in environments with public global WAN addresses.
In our case we're using AKS which is Dnat'ed by Azure Firewall, hence ingress/services addresses are private. Only Azure Firewall public IP is exposed.

Fixes #1394
Fixes #1495

Basically, you can set the --default-targets flag which will override the source addresses, e.g.:

external-dns  --source=ingress  --provider azure-dns --default-targets 8.8.8.8

Checklist

• Unit tests updated
• End user documentation updated

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[bbl]

I signed it

[njuettner]

/lgtm

[njuettner]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bbl, njuettner

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[njuettner]

/lgtm

[njuettner]

Please don't forget to close the issues, thank you.

[njuettner]

Also in case documentation is needed please add this

[dmitriy-udoratin]

Is it possible to update this option so that hostnames can be used, not just IP addresses?

[illrill]

Thanks for this one @bbl! As @dmitriy-udoratin mentions, it would be awesome if the --default-targets flag accepted a hostname as well, and handled that scenario by creating a CNAME record - exactly like the Ingress annotation external-dns.alpha.kubernetes.io/target currently behaves.

[clayrisser]

@illrill I believe someone tried to work on this and submitted a pull request for it. I would be great if this feature was accepted.

#2621

#2621