Might be better to use the more generic 'net' package (instead of asaskevich/govalidator) to determine whether the address is IPv6, e.g.:

isIPv6 := net.ParseIP(addr.Address).To4() == nil
if addr.Type == v1.NodeInternalIP && isIPv6 == needIPv6 {
    return addr.Address, nil
}

hello @leblancd
I checked https://stackoverflow.com/questions/22751035/golang-distinguish-ipv4-ipv6

Please check in particular this answer: https://stackoverflow.com/a/48519490/1506396
It is the exaplanation why to use govalidator instead of the net package.

Hi @zioproto:
I would respectfully disagree with what the poster is claiming in this stackoverflow answer. Specifically, the list of addresses that are labeled "Valid IPv6 notations" should be labeled instead "IPv4-mapped IPv6 Notations" (RFC4291 and RFC4038). These addresses have an IPv6 format, but are intended to be internal representations of IPv4 addresses on the wire. They're essentially IPv4 addresses encapsulated in an IPv6 frame, and this is done so that dual-stack software can conveniently handle both IPv4 and IPv6 addresses as 16-byte entities. The net package's To4() takes care to look for this pattern in a 16-byte slice, and (correctly, I believe) creates an 4-byte (IPv4) slice from the lowest 4 bytes (since Golang can handle slices of arbitrary length, so there's no need to perpetuate the 16-byte internal representation of IPv4-mapped IPv6 addresses). In the examples given in the post, the only addresses that the net's To4() gets wrong, in my opinion, are the examples with :port appended, but To4() assumes that the caller has taken care of eliminating any :port (e.g. with splitHostPort()).

Same comment as before, can use 'net' package to test for IPv6, e.g.:

CIDRisV6 := IP.To4() == nil

With this last change, this function will only return InternalIPs. It should instead try ExternalIPs before giving up.

Aside: An alternate version might be to pass an address family down to getAddressesByName and thence nodeAddresses (with some value to indicate "all" families) and then do the filtering at the lowest level, when we know the address family from the openstack data structure. Happy to go with whatever version you think is best.

Both IP and CIDRisV6 should have (at least) a lowercased first letter, to conform with golang's public/private styleguide/rules.

Ditto lowercased first letter of local variable.