network: Setup DNS for sandbox

Sandbox has network but no dns. Set
dns, if received from runtime as part of
CreateSandboxRequest, in the shared kata
sandbox directory and bind mount it to
`/etc/resolv.conf` present in the VM
rootfs.

Fixes: #635

Signed-off-by: Nitesh Konkar <[email protected]>

network.go

@@ -13,6 +13,7 @@ import (
 	"reflect"
 	"strings"
 	"sync"
+	"syscall"
 
 	"golang.org/x/sys/unix"
 
@@ -25,11 +26,13 @@ import (
 )
 
 var (
-	errNoHandle = grpcStatus.Errorf(codes.InvalidArgument, "Need network handle")
-	errNoIF     = grpcStatus.Errorf(codes.InvalidArgument, "Need network interface")
-	errNoLink   = grpcStatus.Errorf(codes.InvalidArgument, "Need network link")
-	errNoMAC    = grpcStatus.Errorf(codes.InvalidArgument, "Need hardware address")
-	errNoRoutes = grpcStatus.Errorf(codes.InvalidArgument, "Need network routes")
+	errNoHandle             = grpcStatus.Errorf(codes.InvalidArgument, "Need network handle")
+	errNoIF                 = grpcStatus.Errorf(codes.InvalidArgument, "Need network interface")
+	errNoLink               = grpcStatus.Errorf(codes.InvalidArgument, "Need network link")
+	errNoMAC                = grpcStatus.Errorf(codes.InvalidArgument, "Need hardware address")
+	errNoRoutes             = grpcStatus.Errorf(codes.InvalidArgument, "Need network routes")
+	guestDNSFile            = "/etc/resolv.conf"
+	kataGuestSandboxDNSFile = "/run/kata-containers/sandbox/resolv.conf"
 )
 
 const (
@@ -609,8 +612,31 @@ func (s *sandbox) updateRoute(netHandle *netlink.Handle, route *types.Route, add
 // DNS //
 /////////
 
-func setupDNS(dns []string) error {
-	return nil
+func setupDNS(dns []string) (err error) {
+	var file *os.File
+
+	if len(dns) == 0 {
+		agentLog.Debug("Did not set sandbox DNS as DNS not received as part of grpc request.")
+		return nil
+	}
+	if file, err = os.Create(kataGuestSandboxDNSFile); err != nil {
+		return err
+	}
+	defer file.Close()
+
+	for i, line := range dns {
+		if i == (len(dns) - 1) {
+			_, err = file.WriteString(strings.TrimSpace(line))
+		} else {
+			_, err = file.WriteString(strings.TrimSpace(line) + "\n")
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+
+	return mount(kataGuestSandboxDNSFile, guestDNSFile, "bind", syscall.MS_BIND, "")
 }
 
 ////////////