Reasons and solutions for why Docker cannot ping the external network

When using Docker, we will encounter various problems, one of which is that ping from the external network cannot be reached from within Docker. This problem is very common, but it is also serious because it may prevent us from connecting to external resources.

There are many reasons why Docker cannot ping the external network. The following are some common solutions.

1. Check the network settings

First, we need to confirm whether the Docker network settings are correct. When running a Docker container, you often need to specify the network mode. If the network mode is not specified correctly, Docker may not be able to connect to the external network.

We can use the following command to view Docker’s network settings:

docker network inspect

With this command, we can view all Docker’s network settings. If the network settings are incorrect, we need to use the following command to reset the network:

docker network create \
    --driver bridge \
    --subnet 192.168.0.0/16 \
    mynetwork

The above command will create a Docker network named mynetwork and set the subnet to 192.168.0.0/16.

2. Check the firewall settings

The firewall settings may cause Docker to be unable to ping the external network. We need to check the firewall settings to make sure Docker access is allowed.

A commonly used tool for firewall settings is iptables. We can check the iptables settings using the following command:

sudo iptables -L

If there is a rule that blocks Docker’s network access, we need to add the following rule:

sudo iptables -A INPUT -p tcp --dport 2375 -j ACCEPT
sudo iptables -A OUTPUT -p tcp --dport 2375 -j ACCEPT

The above command will add a rule to allow Docker’s network access permissions.

3. Check DNS settings

DNS settings may prevent Docker from connecting to external networks. We need to confirm that Docker’s DNS settings are correct.

Use the following command to view Docker’s DNS settings:

docker exec -it container_id cat /etc/resolv.conf

Among them, container_id is the ID of the Docker container to be checked.

If the DNS settings are incorrect, we can add the correct DNS server address to Docker's /etc/docker/daemon.json file:

{
  "dns": ["1.1.1.1", "8.8.8.8"]
}

The above settings mean that 1.1.1.1 and 8.8.8.8 as the DNS server address.

4. Check the Docker Engine version

Finally, we need to confirm whether the Docker Engine version supports network connections.

Some older versions of Docker Engine may have network connection problems. To avoid this problem, we need to upgrade to the latest version of Docker Engine.

Check the version of Docker Engine using the following command:

docker version

If the version is not the latest, we need to upgrade the Docker Engine using the following command:

sudo apt-get update
sudo apt-get install docker-ce

The above command will update the Docker Engine to the latest version and resolve network connection issues.

To sum up, there may be many reasons why pinging from Docker cannot reach the external network. We need to troubleshoot one by one according to the above solutions. Only by confirming the root cause of the problem can you effectively solve network connection problems.

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