Explore the reasons why Docker does not update ARP

Docker technology has become one of the widely used container technologies in cloud computing since its release in 2013. It allows developers to easily deploy and run applications in different environments by packaging them into a portable container. However, in some scenarios, the network configuration of Docker containers can become a bit complicated. For example, when we need the container to communicate with the external network, we sometimes encounter situations where the container cannot automatically update the ARP table, causing network communication to be blocked. This article will explore why Docker doesn't update ARP and how to fix it.

ARP (Address Resolution Protocol) is a protocol used to resolve MAC addresses on the network. When a host needs to communicate with another host, it needs to first know the MAC address of the target host. In most cases, this is not a problem because when a host first communicates with another host, it sends an ARP request to the local network to obtain the target host's MAC address. However, in the case of Docker containers, this process can be problematic.

Docker is based on Linux kernel virtualization technology and uses Linux network namespace and network bridging technologies to provide network functions for containers. We can use the network namespace and network bridging functions to connect the virtual network card of the container to the physical network card of the host. However, when a container sends a packet out, Docker registers the container's IP address and MAC address into the Docker Host's ARP table when it creates the container. In this way, when other hosts need to communicate with the container, they can obtain the container's MAC address from the Docker Host's ARP table. However, when the IP address of the container changes, Docker does not automatically update the records in the ARP table, causing network communication to be blocked.

There are many ways to solve the problem of Docker not updating ARP. One method is to manually clear the Docker Host's ARP cache. We can achieve this by executing the following command:

sudo ip -s -s neigh flush all

This command will clear the ARP cache on the Docker Host, thus forcing Docker to update the records in the ARP table. In addition, we can also use the arping command to ping the container to force Docker to update the ARP table. For example, if we want to update the ARP table of the container with the IP address 172.17.0.3, we can execute the following command:

sudo arping -U 172.17.0.3

With this method, we can ensure that the Docker Host’s ARP table The records in are consistent with the actual IP address and MAC address of the container, allowing the container to communicate with the external network normally.

In addition, we can also use third-party network management tools in Docker containers, such as Open vSwitch and CNI (Container Network Interface) to manage the container's network connections. These tools can provide more flexible network control and management functions inside the container, and can automatically update records in the ARP table, thereby solving the problem of Docker not updating ARP.

In short, Docker has become an indispensable container technology in cloud computing. However, in some scenarios, the network configuration of Docker containers may become relatively complex, requiring us to spend some time troubleshooting and solving problems. This article explains why Docker does not update the ARP table and how to solve this problem by clearing the ARP cache, performing a Ping test, and using third-party network management tools. I hope these methods can help you with network problems you encounter when using Docker technology.

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