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package main
import (
"bufio"
"context"
"net"
"os"
"path/filepath"
"strings"
"time"
)
// configureResolver teaches Go's resolver where the nameservers are on Android.
//
// Android has no /etc/resolv.conf. Go's pure-Go resolver falls back to
// localhost, so every lookup fails with "dial tcp [::1]:53: connection
// refused". Termux does ship one, at $PREFIX/etc/resolv.conf, which Go never
// consults. Reading it here uses whatever nameservers are actually configured
// rather than hardcoding any.
//
// A no-op everywhere /etc/resolv.conf exists, which is every other platform we
// build for.
func configureResolver() {
if _, err := os.Stat("/etc/resolv.conf"); err == nil {
return
}
prefix := os.Getenv("PREFIX")
if prefix == "" {
return
}
servers := nameservers(filepath.Join(prefix, "etc", "resolv.conf"))
if len(servers) == 0 {
return
}
net.DefaultResolver = &net.Resolver{
PreferGo: true,
Dial: func(ctx context.Context, network, _ string) (net.Conn, error) {
d := net.Dialer{Timeout: 5 * time.Second}
var err error
for _, s := range servers {
var conn net.Conn
conn, err = d.DialContext(ctx, network, net.JoinHostPort(s, "53"))
if err == nil {
return conn, nil
}
}
return nil, err
},
}
}
func nameservers(path string) []string {
f, err := os.Open(path)
if err != nil {
return nil
}
defer f.Close()
var out []string
sc := bufio.NewScanner(f)
for sc.Scan() {
fields := strings.Fields(sc.Text())
if len(fields) >= 2 && fields[0] == "nameserver" {
out = append(out, fields[1])
}
}
return out
}
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