Skip virtual interfaces when parsing route table

Since routing through virtual interfaces may trigger DNS loop in VPN
like observing in UnifiOS Site Magic VPN.

nameservers_linux.go

@@ -6,6 +6,7 @@ import (
 	"encoding/hex"
 	"net"
 	"os"
+	"strings"
 
 	"github.com/Control-D-Inc/ctrld/internal/dns/resolvconffile"
 )
@@ -28,6 +29,7 @@ func dns4() []string {
 
 	var dns []string
 	seen := make(map[string]bool)
+	vis := virtualInterfaces()
 	s := bufio.NewScanner(f)
 	first := true
 	for s.Scan() {
@@ -39,7 +41,10 @@ func dns4() []string {
 		if len(fields) < 2 {
 			continue
 		}
-
+		// Skip virtual interfaces.
+		if vis.contains(string(bytes.TrimSpace(fields[0]))) {
+			continue
+		}
 		gw := make([]byte, net.IPv4len)
 		// Third fields is gateway.
 		if _, err := hex.Decode(gw, fields[2]); err != nil {
@@ -63,12 +68,17 @@ func dns6() []string {
 	defer f.Close()
 
 	var dns []string
+	vis := virtualInterfaces()
 	s := bufio.NewScanner(f)
 	for s.Scan() {
 		fields := bytes.Fields(s.Bytes())
 		if len(fields) < 4 {
 			continue
 		}
+		// Skip virtual interfaces.
+		if vis.contains(string(bytes.TrimSpace(fields[len(fields)-1]))) {
+			continue
+		}
 
 		gw := make([]byte, net.IPv6len)
 		// Fifth fields is gateway.
@@ -95,3 +105,26 @@ func dnsFromSystemdResolver() []string {
 	}
 	return ns
 }
+
+type set map[string]struct{}
+
+func (s *set) add(e string) {
+	(*s)[e] = struct{}{}
+}
+
+func (s *set) contains(e string) bool {
+	_, ok := (*s)[e]
+	return ok
+}
+
+// virtualInterfaces returns a set of virtual interfaces on current machine.
+func virtualInterfaces() set {
+	s := make(set)
+	entries, _ := os.ReadDir("/sys/devices/virtual/net")
+	for _, entry := range entries {
+		if entry.IsDir() {
+			s.add(strings.TrimSpace(entry.Name()))
+		}
+	}
+	return s
+}

nameservers_linux_test.go

@@ -0,0 +1,10 @@
+package ctrld
+
+import (
+	"testing"
+)
+
+func Test_virtualInterfaces(t *testing.T) {
+	vis := virtualInterfaces()
+	t.Log(vis)
+}