DNS resolve workaround for android in pure go

setDefaultNS.go

package main

import (
	"fmt"

	"net"
	"sync"
	"time"
	_ "unsafe"
)

// DNS resolve workaround for android in pure go

//go:linkname defaultNS net.defaultNS
var defaultNS []string

// need to keep sync with go version
//go:linkname resolvConf net.resolvConf
var resolvConf resolverConfig

// copy from /src/net/dnsconfig_unix.go
type dnsConfig struct {
	servers    []string      // server addresses (in host:port form) to use
	search     []string      // rooted suffixes to append to local name
	ndots      int           // number of dots in name to trigger absolute lookup
	timeout    time.Duration // wait before giving up on a query, including retries
	attempts   int           // lost packets before giving up on server
	rotate     bool          // round robin among servers
	unknownOpt bool          // anything unknown was encountered
	lookup     []string      // OpenBSD top-level database "lookup" order
	err        error         // any error that occurs during open of resolv.conf
	mtime      time.Time     // time of resolv.conf modification
	soffset    uint32        // used by serverOffset
}

// copy from /src/net/dnsclient_unix.go
type resolverConfig struct {
	initOnce sync.Once // guards init of resolverConfig

	// ch is used as a semaphore that only allows one lookup at a
	// time to recheck resolv.conf.
	ch          chan struct{} // guards lastChecked and modTime
	lastChecked time.Time     // last time resolv.conf was checked

	mu        sync.RWMutex // protects dnsConfig
	dnsConfig *dnsConfig   // parsed resolv.conf structure used in lookups
}


//go:linkname (*resolverConfig).tryUpdate net.(*resolverConfig).tryUpdate
func (conf *resolverConfig) tryUpdate(name string)
// need to put a empty .s file


func setDefaultNS(addrs []string, loadFromSystem bool) {
	if resolvConf.dnsConfig == nil {
		resolvConf.tryUpdate("")
	}

	if loadFromSystem {
		now := time.Now()
		resolvConf.lastChecked = now.Add(-6 * time.Second)
		resolvConf.dnsConfig.mtime = now
	}

	resolvConf.dnsConfig.servers = addrs
	defaultNS = addrs
}

// --------------


func test() {
	ips, err := net.LookupIP("google.com")
	if err != nil {
		fmt.Printf("Could not get IPs: %v\n", err)
		return
	}
	fmt.Println("IPs:", ips)
}

func main() {
	// force use a non-exist DNS server
	setDefaultNS([]string{"127.0.0.1:5300"}, false)
	test() // error
	fmt.Println("------------------")


	// force use a list of DNS server
	setDefaultNS([]string{"8.8.8.8:53", "1.1.1.1:53"}, false)
	test() // ok
	fmt.Println("------------------")


	// set DNS & try reload from system config
	setDefaultNS([]string{"127.0.0.1:5300"}, true)

	// check & test net.defaultNS
	test() // ok
	fmt.Println("------------------")
}

setDefaultNS2.go

package main

import (
	"fmt"

	"net"
	_ "unsafe"
)

// DNS resolve workaround for android in pure go

// this only work before any Lookup call and net.dnsReadConfig() failed
//go:linkname defaultNS net.defaultNS
var defaultNS []string

func setDefaultNS2(addrs []string) {
	defaultNS = addrs
}

// --------------

func test() {
	ips, err := net.LookupIP("google.com")
	if err != nil {
		fmt.Printf("Could not get IPs: %v\n", err)
		return
	}
	fmt.Println("IPs:", ips)
}

func main() {
	// print net.defaultNS
	fmt.Println("use DNS", defaultNS)
	//test() // should setup before any call!!
	fmt.Println("------------------")

	// set net.defaultNS
	setDefaultNS2([]string{"8.8.8.8:53", "1.1.1.1:53"})

	// check & test net.defaultNS
	fmt.Println("use DNS", defaultNS)
	test()
	fmt.Println("------------------")
}

Thanks @cs8425, I've turned this into an importable library at https://github.com/mtibben/androiddnsfix