Custom LookupIPAddr function

[enchantner]

Continuing the work started at https://github.com/valyala/fasthttp/pull/689/files .

Turns out, specifying custom resolver doesn't allow to avoid resolving completely (as Dial() has to return a connection to a DNS server), so in this way it becomes possible to reimplement LookupIPAddr() function to specify resolved name manually or get it from some place other than actual DNS query.

[erikdubbelboer]

Instead of adding another field I would prefer to fix this differently. I think it's better if we change

type TCPDialer struct {
	// ...

	Resolver *net.Resolver
}

into

type Resolver interface {
	LookupAddr(context.Context, string) (names []string, err error)
}

type TCPDialer struct {
	// ...

	Resolver Resolver
}

That way net.Resolver implements this interface, but if you want to use a different LookupAddr function you can use your own struct that implements this interface with your own function.

[enchantner]

Wouldn't it better to use LookupIPAddr instead of LookupAddr? Afaik, LookupAddr only deals with IPv4 addresses, but with LookupIPAddr we're getting both. Anyway, here's a commit.

[erikdubbelboer]

I'm not sure I understand. LookupAddr does a reverse lookup from IP to name. That's not what we want?

One last change and then I'll merge it. Sorry for being pedantic.

[enchantner]

@erikdubbelboer LookupAddr spits out a list of strings, which are each an IPv4 address in "four numbers with dots" representation. That's totally okay, but IPAddr type is more common and compact, as you can get binary representations for both IPv4 and IPv6 from it, if present. It looks like fasthttp doesn't support IPv6 yet, but in this way we may be able to introduce it without changing this Resolver interface again.

Also, fixed your comment, thanks!

[erikdubbelboer]

LookupAddr goes from IP to domain. LookupIPAddr goes from domain to IP.

Where in fasthttp do you think we use LookupAddr?

fasthttp does support IPv6 just fine.

[enchantner]

@erikdubbelboer yeah, sorry, I just got everything mixed up looking at the example code you've provided above for the interface. Thanks for merging this!

[erikdubbelboer]

Ah sorry I didn't realize I put LookupAddr in my example interface. My bad!