net: Dial always query DNS for both A and AAAA even if network is tcp4 or tcp6 when pure Go resolver is used

[zhangboyang]

When pure Go resolver is used, net.Dial always send two DNS query (one for A record and another for AAAA), even if the network parameter set to one of tcp4, tcp6, udp4, udp6. This will add extra latencies when making IPv4-only or IPv6-only connections. In the worst case, for example, a pure IPv4 host with firewall filtered out all AAAA queries, a dial with tcp4 will have to wait 10 second until AAAA query timed out.

The same problem was also found in cgo resolver (#25947), and it was fixed in c659be4.

My pull request will fix this.

[gopherbot]

Change https://golang.org/cl/301709 mentions this issue: net: make go resolver aware of network parameter

[zhangboyang]

Here is my test code: (build it with CGO_ENABLED=0 go build test46.go)

package main

import "net"

func main() {
	net.Dial("tcp4", "golang.org:80")
}

Dump from wireshark:

No.	Time	Source	Destination	Protocol	Length	Info
11	3.196111458	127.0.0.1	127.0.0.53	DNS	70	Standard query 0x668e AAAA golang.org
12	3.196261224	127.0.0.1	127.0.0.53	DNS	70	Standard query 0x5811 A golang.org

[cherrymui]

cc @bradfitz

[networkimprov]

cc @ekalinin