Add version of dns.lookup that returns all results

[jsha]

The dns module provides two sets of methods: the resolve family, which directly resolves using the DNS protocol, and the lookup function, which uses the system resolver (through getaddrinfo) and returns at most one address.

It's often preferable to use the system resolver, since it can use things like mDNS. However, using the dns.lookup API makes it impossible to receive multiple IP addresses in response to a DNS query (See #708 for motivating example).

Proposal: Add a new function dns.lookupAll that behaves like dns.lookup, but returns all addresses given by getaddrinfo.

[cjihrig]

This seems like a pretty simple addition. I'd be fine with working on it, if others find it useful. I think the window is currently open for a minor version bump too.

[bnoordhuis]

This seems like a perfectly reasonable addition to me. It doesn't even need a new function; dns.lookup() already looks up all the addresses but it's intentionally crippled to only report the first one. We can simply extend it like this:

dns.lookup('google.com', { all: true }, function(err, addrs) { /* ... */ });  // strawman

The callback gets the address family as its third argument now. For the { all: true } case, we probably want to drop that and report the family in the addrs array. It's a list of strings now, that would have to change to a list of objects.

[silverwind]

I'll start working on this according to @bnoordhuis's proposal, if that's ok with you, @cjihrig.

[cjihrig]

@silverwind sure. I was thinking you might just want to change this line to point to another function. And the callback would change from function(err, address, family) to function(err, addresses), where each element in addresses would be something like {ip: '127.0.0.1', family: 4}.

[silverwind]

My first approach would've been to modify onlookup with additional logic and add a req.all property. You think that a second function would be cleaner?

[cjihrig]

I figured it would be simpler, with less chance of breaking existing code, to just point to a new function. The two functions will have very little overlap except checking for an error.

[silverwind]

True, I'll add an second function then, definitely safer.

[silverwind]

Any opinions if the address field in the object should be called address or ip? I'm leaning more towards address for correctness, but @cjihrig suggested ip. Also, the docs usually call it address.

[cjihrig]

FWIW, I don't have a strong opinion. I just thought addresses.address seemed a little odd.

[bnoordhuis]

We call it address in most other places, e.g. socket.address().address.

[silverwind]

I'll put this up shortly for a PR, just have to test on OS X, as apparently my Windows machine isn't able to dns.lookup for an IPv6 (getting ENOENT), but dns.resolve6 strangely works. This is on v1.x too. Might be a separate issue.