18.12. Applying Network Filtering

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This section provides an introduction to libvirt's network filters, their goals, concepts and XML format.

18.12.1. Introduction

The goal of the network filtering, is to enable administrators of a virtualized system to configure and enforce network traffic filtering rules on virtual machines and manage the parameters of network traffic that virtual machines are allowed to send or receive. The network traffic filtering rules are applied on the host physical machine when a virtual machine is started. Since the filtering rules cannot be circumvented from within the virtual machine, it makes them mandatory from the point of view of a virtual machine user.
From the point of view of the guest virtual machine, the network filtering system allows each virtual machine's network traffic filtering rules to be configured individually on a per interface basis. These rules are applied on the host physical machine when the virtual machine is started and can be modified while the virtual machine is running. The latter can be achieved by modifying the XML description of a network filter.
Multiple virtual machines can make use of the same generic network filter. When such a filter is modified, the network traffic filtering rules of all running virtual machines that reference this filter are updated. The machines that are not running will update on start.
As previously mentioned, applying network traffic filtering rules can be done on individual network interfaces that are configured for certain types of network configurations. Supported network types include:
  • network
  • ethernet -- must be used in bridging mode
  • bridge

Example 18.1. An example of network filtering

The interface XML is used to reference a top-level filter. In the following example, the interface description references the filter clean-traffic.
    <interface type='bridge'>
      <mac address='00:16:3e:5d:c7:9e'/>
      <filterref filter='clean-traffic'/>
Network filters are written in XML and may either contain: references to other filters, rules for traffic filtering, or hold a combination of both. The above referenced filter clean-traffic is a filter that only contains references to other filters and no actual filtering rules. Since references to other filters can be used, a tree of filters can be built. The clean-traffic filter can be viewed using the command: # virsh nwfilter-dumpxml clean-traffic.
As previously mentioned, a single network filter can be referenced by multiple virtual machines. Since interfaces will typically have individual parameters associated with their respective traffic filtering rules, the rules described in a filter's XML can be generalized using variables. In this case, the variable name is used in the filter XML and the name and value are provided at the place where the filter is referenced.

Example 18.2. Description extended

In the following example, the interface description has been extended with the parameter IP and a dotted IP address as a value.
    <interface type='bridge'>
      <mac address='00:16:3e:5d:c7:9e'/>
      <filterref filter='clean-traffic'>
        <parameter name='IP' value=''/>
In this particular example, the clean-traffic network traffic filter will be represented with the IP address parameter and as per the rule dictates that all traffic from this interface will always be using as the source IP address, which is one of the purpose of this particular filter.
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