why ping registry-1.docker.io returns Request timeout for icmp_seq even when I can nslookup or dig?
The inability to ping the name does not in any way indicate a problem with your setup or anything else.
Using dig and nslookup allows you to look up the IP-addresses linked with the name registry-1.docker.io. This works fine for you and you get a list of IP-addresses.
Now when you ping the name - only one of the IP-addresses will be chosen to be pinged. If the system behind that IP-address has chosen not to respond to pings, you'll get the &quot;Request timeout&quot; message you're seeing.
It is perfectly valid for server operators to choose that they do not want to reply to pings. It does not indicate that the server is down, that the network is flawed, or anything of the kind.
The ImagePullBackOff message you're seeing can be caused by multiple things. I would advise using the kubectl describe command to get an idea of what happened.
It could be for example that you have specified an image that actually do not exist, or that you have specified a private image without giving the proper credentials.