Provide ability to set lo up without CNI

Signed-off-by: Michael Zappa <michael.zappa@gmail.com>

internal/cri/server/sandbox_run_linux.go

@@ -0,0 +1,37 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package server
+
+import (
+	"fmt"
+
+	"github.com/containernetworking/plugins/pkg/ns"
+	"github.com/vishvananda/netlink"
+)
+
+func (c *criService) bringUpLoopback(netns string) error {
+	if err := ns.WithNetNSPath(netns, func(_ ns.NetNS) error {
+		link, err := netlink.LinkByName("lo")
+		if err != nil {
+			return err
+		}
+		return netlink.LinkSetUp(link)
+	}); err != nil {
+		return fmt.Errorf("error setting loopback interface up: %w", err)
+	}
+	return nil
+}