/cc @ianlancetaylor

I'm not sure what to do about this. The cgo code does a PTR lookup by calling the C function getnameinfo, which is the usual way to the name corresponding to an IP address. getnameinfo only returns a single answer. Is there a standard C interface that returns multiple PTR records?

Just to add some more thoughts here - as an end-user, the frustrating part for me wasn't really the ability to get multiple records. What frustrated me was the fact that the documentation states that I should receive multiple records for this call, and the default behavior will only return one.

LookupAddr performs a reverse lookup for the given address, returning a list of names mapping to that address.

The other really fun part of this is that (at least) BIND9 will return record results for a query in a random order. This means that if LookupAddr only returns one name, if you query the same IP repeatedly, if it has multiple PTRs, you'll get a different single result each time.

At minimum we could document the situation I suppose.

@bradfitz, it looks to me like we should document LookupAddr with something like

// LookupAddr performs a reverse lookup for the given address, returning a list
// of names mapping to that address.
//
// When using the host C library resolver, at most one result will be returned.
// To bypass the host resolver, use a custom Resolver. 
func LookupAddr(addr string) (names []string, err error) {

but that requires actually making (*Resolver).LookupAddr respect PreferGo, which it does not today. I filed #17532 for that.

CL https://golang.org/cl/31720 mentions this issue.