fix: internal nsc should correctly work with dns (#443)

* external nsc should correctly work with dns

Signed-off-by: denis-tingaikin <denis.tingajkin@xored.com>

* apply review comments

Signed-off-by: denis-tingaikin <denis.tingajkin@xored.com>

internal/config/config.go

@@ -42,6 +42,8 @@ type Config struct {
 	LogLevel              string                  `default:"INFO" desc:"Log level" split_words:"true"`
 	OpenTelemetryEndpoint string                  `default:"otel-collector.observability.svc.cluster.local:4317" desc:"OpenTelemetry Collector Endpoint"`
 
+	CoreDNSConfigPath string `default:"/etc/coredns/Corefile" desc:"Default path to the Corefile source"`
+
 	LivenessCheckInterval time.Duration `default:"200ms" desc:"Dataplane liveness check interval"`
 	LivenessCheckTimeout  time.Duration `default:"1s" desc:"Dataplane liveness check timeout"`
 }

main.go

@@ -54,6 +54,7 @@ import (
 	"github.com/networkservicemesh/sdk/pkg/networkservice/common/mechanisms"
 	"github.com/networkservicemesh/sdk/pkg/networkservice/common/mechanisms/kernel"
 	"github.com/networkservicemesh/sdk/pkg/networkservice/common/mechanisms/sendfd"
+	"github.com/networkservicemesh/sdk/pkg/networkservice/common/null"
 	"github.com/networkservicemesh/sdk/pkg/networkservice/common/retry"
 	"github.com/networkservicemesh/sdk/pkg/networkservice/connectioncontext/dnscontext"
 	"github.com/networkservicemesh/sdk/pkg/networkservice/core/chain"
@@ -150,6 +151,12 @@ func main() {
 		),
 	)
 
+	var dnsClient = null.NewClient()
+
+	if _, err = os.Stat(c.CoreDNSConfigPath); err == nil {
+		dnsClient = dnscontext.NewClient(dnscontext.WithChainContext(ctx), dnscontext.WithCorefilePath(c.CoreDNSConfigPath))
+	}
+
 	nsmClient := client.NewClient(ctx,
 		client.WithClientURL(&c.ConnectTo),
 		client.WithName(c.Name),
@@ -168,7 +175,7 @@ func main() {
 				kernelmech.MECHANISM: chain.NewNetworkServiceClient(kernel.NewClient()),
 			}),
 			sendfd.NewClient(),
-			dnscontext.NewClient(dnscontext.WithChainContext(ctx)),
+			dnsClient,
 			excludedprefixes.NewClient(excludedprefixes.WithAwarenessGroups(c.AwarenessGroups)),
 		),
 		client.WithDialTimeout(c.DialTimeout),