all: workaround problem with EdgeOS dnsmasq config

internal/router/dnsmasq/conf.go

@@ -0,0 +1,30 @@
+package dnsmasq
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"io"
+	"os"
+	"strings"
+)
+
+func InterfaceNameFromConfig(filename string) (string, error) {
+	buf, err := os.ReadFile(filename)
+	if err != nil {
+		return "", err
+	}
+	return interfaceNameFromReader(bytes.NewReader(buf))
+}
+
+func interfaceNameFromReader(r io.Reader) (string, error) {
+	scanner := bufio.NewScanner(r)
+	for scanner.Scan() {
+		line := scanner.Text()
+		after, found := strings.CutPrefix(line, "interface=")
+		if found {
+			return after, nil
+		}
+	}
+	return "", errors.New("not found")
+}

internal/router/dnsmasq/conf_test.go

@@ -0,0 +1,46 @@
+package dnsmasq
+
+import (
+	"strings"
+	"testing"
+)
+
+func Test_interfaceNameFromReader(t *testing.T) {
+	tests := []struct {
+		name      string
+		in        string
+		wantIface string
+	}{
+		{
+			"good",
+			`interface=lo`,
+			"lo",
+		},
+		{
+			"multiple",
+			`interface=lo
+interface=eth0
+`,
+			"lo",
+		},
+		{
+			"no iface",
+			`cache-size=100`,
+			"",
+		},
+	}
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			ifaceName, err := interfaceNameFromReader(strings.NewReader(tc.in))
+			if tc.wantIface != "" && err != nil {
+				t.Errorf("unexpected error: %v", err)
+				return
+			}
+			if tc.wantIface != ifaceName {
+				t.Errorf("mismatched, want: %q, got: %q", tc.wantIface, ifaceName)
+			}
+		})
+	}
+}