Enhance DNS failover reliability (#1637)

* Fix using wrong array index in log to avoid potential panic

* Increase gRPC connection timeout and add the timeout resolv.conf option

This makes sure the dns client is able to failover to a second
configured nameserver, if present. That is the case then when using the
dns `file` manager and a resolv.conf file generated for netbird.

* On file backup restore, remove the first NS if it's the netbird NS

* Bump dns mangager discovery message from debug to info to ease debugging

client/internal/dns/file_parser_linux_test.go

@@ -6,6 +6,8 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func Test_parseResolvConf(t *testing.T) {
@@ -172,3 +174,131 @@ nameserver 192.168.0.1
 		t.Errorf("unexpected resolv.conf content: %v", cfg)
 	}
 }
+
+func TestPrepareOptionsWithTimeout(t *testing.T) {
+	tests := []struct {
+		name     string
+		others   []string
+		timeout  int
+		attempts int
+		expected []string
+	}{
+		{
+			name:     "Append new options with timeout and attempts",
+			others:   []string{"some config"},
+			timeout:  2,
+			attempts: 2,
+			expected: []string{"some config", "options timeout:2 attempts:2"},
+		},
+		{
+			name:     "Modify existing options to exclude rotate and include timeout and attempts",
+			others:   []string{"some config", "options rotate someother"},
+			timeout:  3,
+			attempts: 2,
+			expected: []string{"some config", "options attempts:2 timeout:3 someother"},
+		},
+		{
+			name:     "Existing options with timeout and attempts are updated",
+			others:   []string{"some config", "options timeout:4 attempts:3"},
+			timeout:  5,
+			attempts: 4,
+			expected: []string{"some config", "options timeout:5 attempts:4"},
+		},
+		{
+			name:     "Modify existing options, add missing attempts before timeout",
+			others:   []string{"some config", "options timeout:4"},
+			timeout:  4,
+			attempts: 3,
+			expected: []string{"some config", "options attempts:3 timeout:4"},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			result := prepareOptionsWithTimeout(tc.others, tc.timeout, tc.attempts)
+			assert.Equal(t, tc.expected, result)
+		})
+	}
+}
+
+func TestRemoveFirstNbNameserver(t *testing.T) {
+	testCases := []struct {
+		name       string
+		content    string
+		ipToRemove string
+		expected   string
+	}{
+		{
+			name: "Unrelated nameservers with comments and options",
+			content: `# This is a comment
+options rotate
+nameserver 1.1.1.1
+# Another comment
+nameserver 8.8.4.4
+search example.com`,
+			ipToRemove: "9.9.9.9",
+			expected: `# This is a comment
+options rotate
+nameserver 1.1.1.1
+# Another comment
+nameserver 8.8.4.4
+search example.com`,
+		},
+		{
+			name: "First nameserver matches",
+			content: `search example.com
+nameserver 9.9.9.9
+# oof, a comment
+nameserver 8.8.4.4
+options attempts:5`,
+			ipToRemove: "9.9.9.9",
+			expected: `search example.com
+# oof, a comment
+nameserver 8.8.4.4
+options attempts:5`,
+		},
+		{
+			name: "Target IP not the first nameserver",
+			// nolint:dupword
+			content: `# Comment about the first nameserver
+nameserver 8.8.4.4
+# Comment before our target
+nameserver 9.9.9.9
+options timeout:2`,
+			ipToRemove: "9.9.9.9",
+			// nolint:dupword
+			expected: `# Comment about the first nameserver
+nameserver 8.8.4.4
+# Comment before our target
+nameserver 9.9.9.9
+options timeout:2`,
+		},
+		{
+			name: "Only nameserver matches",
+			content: `options debug
+nameserver 9.9.9.9
+search localdomain`,
+			ipToRemove: "9.9.9.9",
+			expected: `options debug
+nameserver 9.9.9.9
+search localdomain`,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			tempDir := t.TempDir()
+			tempFile := filepath.Join(tempDir, "resolv.conf")
+			err := os.WriteFile(tempFile, []byte(tc.content), 0644)
+			assert.NoError(t, err)
+
+			err = removeFirstNbNameserver(tempFile, tc.ipToRemove)
+			assert.NoError(t, err)
+
+			content, err := os.ReadFile(tempFile)
+			assert.NoError(t, err)
+
+			assert.Equal(t, tc.expected, string(content), "The resulting content should match the expected output.")
+		})
+	}
+}