Out of the box, `curl` reports success (via exit code) even if the
server returns an error, e.g. _429 Too Many Requests_ which was
sometimes the case with `http://myip.dnsomatic.com`. So, errors went
undetected.
There is the `--fail` switch which enables us to use the exit code the
way we need:
> -f, --fail
> (HTTP) Fail silently (no output at all) on server
> errors. This is mostly done to better enable scripts etc
> to better deal with failed attempts. In normal cases
> when an HTTP server fails to deliver a document, it
> returns an HTML document stating so (which often also
> describes why and more). This flag will prevent curl
> from outputting that and return error 22.
Use it to make the `myip()` deliver results reliably.
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