Virt-Manager Created QEMU VMs Break Host Internet via Routes VM

Introduction

In this article, we will discuss a common issue that can arise when creating virtual machines (VMs) using Virt-Manager and the QEMU backend with the "default" network interface set to NAT (Network Address Translation). Specifically, we will focus on the problem where the network connection on the host drops after starting a VM.

Understanding the Issue

When you create a VM using Virt-Manager and the QEMU backend with the "default" network interface set to NAT, a new virtual network interface is created for the VM. By default, Virt-Manager creates a route to direct traffic from the VM through this virtual network interface. However, this can sometimes cause issues with the host's network connection, especially if the host is using the same network interface for its own network traffic.

The NAT Networking Mode

NAT stands for Network Address Translation. It is a method of remapping one IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device. In the context of Virt-Manager and QEMU, NAT is used to allow the VM to access the host's network, but it can lead to conflicts with the host's own network traffic.

Virtual Network Interfaces

When you create a VM with Virt-Manager and QEMU, a new virtual network interface is created for the VM. This interface is separate from the host's physical network interface, but it can still cause conflicts if the host is using the same interface for its own network traffic.

The Problem: Network Connection Drops

After starting the VM, you may notice that the network connection on the host drops. This can be caused by the fact that the host's network traffic is being directed through the VM's virtual network interface. The host and the VM are now competing for access to the same physical network interface, leading to dropped connections.

Analyzing Network Traffic

To understand what's happening, you can use tools like tcpdump to analyze the network traffic. You will likely see that the host's network traffic is being directed through the VM's virtual network interface, which can lead to dropped connections.

Solving the Problem

One way to solve this problem is to use a different networking mode, such as the "bridge" mode, instead of NAT. This mode creates a bridge between the VM's virtual network interface and the host's physical network interface, which allows the VM and the host to share the same network interface without conflicts. However, this may not always be possible or desirable.

Modifying Routes

Another solution is to modify the routes on the host to prevent network traffic from being directed through the VM's virtual network interface. This can be done using the ip route command.

Step 1: Identify the Virtual Network Interface

The first step is to identify the virtual network interface that was created for the VM. You can do this by running the command:

Step 2: Find the Gateway Address

The next step is to find the gateway address for the host's network. You can do this by running the command:

Step 3: Add a New Route

Once you have the gateway address, you can add a new route that prevents network traffic from being directed through the VM's virtual network interface. You can do this by running the command: