1 files changed, 67 insertions, 0 deletions
diff --git a/cmd/prognosis/resolver.go b/cmd/prognosis/resolver.go
new file mode 100644
index 0000000..095cc20
--- /dev/null
+++ b/cmd/prognosis/resolver.go
@@ -0,0 +1,67 @@
+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
+}