refactor(transparent-proxy): simplify more tproxy configuration (#10206)

- Got rid of unnecessary internal `defaultConfig` function

  We had `defaultConfig` and `DefaultConfig`, where latter was just
  calling former function.

- Simplified configuration handling by removing `MergeConfigWithDefaults`

  This commit removed the unnecessary and hard-to-maintain
  `MergeConfigWithDefaults` function. Instead, we now use `DefaultConfig`
  directly and explicitly set modified values in commands like
  `kumactl install transparent-proxy` and CNI.

  I've also streamlined the `transparentProxyArgs` structure by removing
  parameters that can be directly configured via the `Config` structure.
  This paves the way for potentially removing this intermediate structure
  altogether in the future, although that would involve more complex
  changes in flag parsing.

- Added `StoreFirewalld` parameter to `Config` structure

  I think it's the logical place for this value as it's basically part of
  the configuration.

- Replaced writes to Writers with pure `fmt.Fprintln` to make code cleaner

  We don't have to cast string literals to `[]byte` now and don't have to
  explicitly ignore errors.

Signed-off-by: Bart Smykla <bartek@smykla.com>
Signed-off-by: Ilya Lobkov <ilya.lobkov@konghq.com>

app/cni/pkg/cni/injector_linux.go

@@ -69,6 +69,8 @@ func Inject(netns string, logger logr.Logger, intermediateConfig *IntermediateCo
 }
 
 func mapToConfig(intermediateConfig *IntermediateConfig, logWriter *bufio.Writer) (*config.Config, error) {
+	cfg := config.DefaultConfig()
+
 	port, err := convertToUint16("inbound port", intermediateConfig.targetPort)
 	if err != nil {
 		return nil, err
@@ -88,20 +90,13 @@ func mapToConfig(intermediateConfig *IntermediateConfig, logWriter *bufio.Writer
 		return nil, err
 	}
 
-	cfg := config.Config{
-		RuntimeStdout: logWriter,
-		Owner: config.Owner{
-			UID: intermediateConfig.noRedirectUID,
-		},
-		Redirect: config.Redirect{
-			Outbound: config.TrafficFlow{
-				Enabled:             true,
-				Port:                port,
-				ExcludePorts:        excludePorts,
-				ExcludePortsForUIDs: excludePortsForUIDsParsed,
-			},
-		},
-	}
+	cfg.Verbose = true
+	cfg.RuntimeStdout = logWriter
+	cfg.Owner.UID = intermediateConfig.noRedirectUID
+	cfg.Redirect.Outbound.Enabled = true
+	cfg.Redirect.Outbound.Port = port
+	cfg.Redirect.Outbound.ExcludePorts = excludePorts
+	cfg.Redirect.Outbound.ExcludePortsForUIDs = excludePortsForUIDsParsed
 
 	isGateway, err := GetEnabled(intermediateConfig.isGateway)
 	if err != nil {
@@ -133,12 +128,11 @@ func mapToConfig(intermediateConfig *IntermediateConfig, logWriter *bufio.Writer
 		if err != nil {
 			return nil, err
 		}
-		cfg.Redirect.Inbound = config.TrafficFlow{
-			Enabled:      true,
-			Port:         inboundPort,
-			PortIPv6:     inboundPortV6,
-			ExcludePorts: excludedPorts,
-		}
+
+		cfg.Redirect.Inbound.Enabled = true
+		cfg.Redirect.Inbound.Port = inboundPort
+		cfg.Redirect.Inbound.PortIPv6 = inboundPortV6
+		cfg.Redirect.Inbound.ExcludePorts = excludedPorts
 	}
 
 	useBuiltinDNS, err := GetEnabled(intermediateConfig.builtinDNS)
@@ -150,13 +144,13 @@ func mapToConfig(intermediateConfig *IntermediateConfig, logWriter *bufio.Writer
 		if err != nil {
 			return nil, err
 		}
-		cfg.Redirect.DNS = config.DNS{
-			Enabled:            true,
-			Port:               builtinDnsPort,
-			CaptureAll:         true,
-			ConntrackZoneSplit: true,
-		}
+
+		cfg.Redirect.DNS.Enabled = true
+		cfg.Redirect.DNS.Port = builtinDnsPort
+		cfg.Redirect.DNS.CaptureAll = true
+		cfg.Redirect.DNS.ConntrackZoneSplit = true
 	}
+
 	return &cfg, nil
 }