populate resolv.conf with dnsname responses when in usernamespace

when using usernamespace, dnsname respondes from cni were not making it into the containers /etc/resolv.conf because of a timing issue.  this corrects that behavior.

Fixes: containers#5256

Signed-off-by: Brent Baude <bbaude@redhat.com>

libpod/container_internal.go

@@ -914,6 +914,7 @@ func (c *Container) checkDependenciesRunning() ([]string, error) {
 }
 
 func (c *Container) completeNetworkSetup() error {
+	var outResolvConf []string
 	netDisabled, err := c.NetworkDisabled()
 	if err != nil {
 		return err
@@ -927,7 +928,37 @@ func (c *Container) completeNetworkSetup() error {
 	if c.config.NetMode == "slirp4netns" {
 		return c.runtime.setupRootlessNetNS(c)
 	}
-	return c.runtime.setupNetNS(c)
+	if err := c.runtime.setupNetNS(c); err != nil {
+		return err
+	}
+	state := c.state
+	// collect any dns servers that cni tells us to use (dnsname)
+	for _, cni := range state.NetworkStatus {
+		if cni.DNS.Nameservers != nil {
+			for _, server := range cni.DNS.Nameservers {
+				outResolvConf = append(outResolvConf, fmt.Sprintf("nameserver %s", server))
+			}
+		}
+	}
+	// check if we have a bindmount for resolv.conf
+	resolvBindMount := state.BindMounts["/etc/resolv.conf"]
+	if len(outResolvConf) < 1 || resolvBindMount == "" || len(c.config.NetNsCtr) > 0 {
+		return nil
+	}
+	// read the existing resolv.conf
+	b, err := ioutil.ReadFile(resolvBindMount)
+	if err != nil {
+		return err
+	}
+	for _, line := range strings.Split(string(b), "\n") {
+		// only keep things that dont start with nameserver from the old
+		// resolv.conf file
+		if !strings.HasPrefix(line, "nameserver") {
+			outResolvConf = append([]string{line}, outResolvConf...)
+		}
+	}
+	// write and return
+	return ioutil.WriteFile(resolvBindMount, []byte(strings.Join(outResolvConf, "\n")), 0644)
 }
 
 // Initialize a container, creating it in the runtime