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18.12.11.6. Sample custom filter

Although one of the rules in the above XML contains the IP address of the guest virtual machine as either a source or a destination address, the filtering of the traffic works correctly. The reason is that whereas the rule's evaluation occurs internally on a per-interface basis, the rules are additionally evaluated based on which (tap) interface has sent or will receive the packet, rather than what their source or destination IP address may be.

Example 18.12. Sample XML for network interface descriptions

An XML fragment for a possible network interface description inside the domain XML of the test guest virtual machine could then look like this:
   [...]
    <interface type='bridge'>
      <source bridge='mybridge'/>
      <filterref filter='test-eth0'/>
    </interface>
   [...]
To more strictly control the ICMP traffic and enforce that only ICMP echo requests can be sent from the guest virtual machine and only ICMP echo responses be received by the guest virtual machine, the above ICMP rule can be replaced with the following two rules:
  <!- - enable outgoing ICMP echo requests- ->
  <rule action='accept' direction='out'>
    <icmp type='8'/>
  </rule>
  <!- - enable incoming ICMP echo replies- ->
  <rule action='accept' direction='in'>
    <icmp type='0'/>
  </rule>

Example 18.13. Second example custom filter

This example demonstrates how to build a similar filter as in the example above, but extends the list of requirements with an ftp server located inside the guest virtual machine. The requirements for this filter are:
  • prevents a guest virtual machine's interface from MAC, IP, and ARP spoofing
  • opens only TCP ports 22 and 80 in a guest virtual machine's interface
  • allows the guest virtual machine to send ping traffic from an interface but does not allow the guest virtual machine to be pinged on the interface
  • allows the guest virtual machine to do DNS lookups (UDP towards port 53)
  • enables the ftp server (in active mode) so it can run inside the guest virtual machine
The additional requirement of allowing an FTP server to be run inside the guest virtual machine maps into the requirement of allowing port 21 to be reachable for FTP control traffic as well as enabling the guest virtual machine to establish an outgoing TCP connection originating from the guest virtual machine's TCP port 20 back to the FTP client (FTP active mode). There are several ways of how this filter can be written and two possible solutions are included in this example.
The first solution makes use of the state attribute of the TCP protocol that provides a hook into the connection tracking framework of the Linux host physical machine. For the guest virtual machine-initiated FTP data connection (FTP active mode) the RELATED state is used to enable detection that the guest virtual machine-initiated FTP data connection is a consequence of ( or 'has a relationship with' ) an existing FTP control connection, thereby allowing it to pass packets through the firewall. The RELATED state, however, is only valid for the very first packet of the outgoing TCP connection for the FTP data path. Afterwards, the state is ESTABLISHED, which then applies equally to the incoming and outgoing direction. All this is related to the FTP data traffic originating from TCP port 20 of the guest virtual machine. This then leads to the following solution:
<filter name='test-eth0'>
  <!- - This filter (eth0) references the clean traffic filter to prevent MAC, IP, and ARP spoofing. By not providing an IP address parameter, libvirt will detect the IP address the guest virtual machine is using. - ->
  <filterref filter='clean-traffic'/>

  <!- - This rule enables TCP port 21 (FTP-control) to be reachable - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='21'/>
  </rule>

  <!- - This rule enables TCP port 20 for guest virtual machine-initiated FTP data connection related to an existing FTP control connection - ->
  <rule action='accept' direction='out'>
    <tcp srcportstart='20' state='RELATED,ESTABLISHED'/>
  </rule>

  <!- - This rule accepts all packets from a client on the FTP data connection - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='20' state='ESTABLISHED'/>
  </rule>

  <!- - This rule enables TCP port 22 (SSH) to be reachable - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='22'/>
  </rule>

  <!- -This rule enables TCP port 80 (HTTP) to be reachable - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='80'/>
  </rule>

  <!- - This rule enables general ICMP traffic to be initiated by the guest virtual machine, including ping traffic - ->
  <rule action='accept' direction='out'>
    <icmp/>
  </rule>

  <!- - This rule enables outgoing DNS lookups using UDP - ->
  <rule action='accept' direction='out'>
    <udp dstportstart='53'/>
  </rule>

  <!- - This rule drops all other traffic - ->
  <rule action='drop' direction='inout'>
    <all/>
  </rule>

</filter>
Before trying out a filter using the RELATED state, you have to make sure that the appropriate connection tracking module has been loaded into the host physical machine's kernel. Depending on the version of the kernel, you must run either one of the following two commands before the FTP connection with the guest virtual machine is established:
  • #modprobe nf_conntrack_ftp - where available OR
  • #modprobe ip_conntrack_ftp if above is not available
If protocols other than FTP are used in conjunction with the RELATED state, their corresponding module must be loaded. Modules are available for the protocols: ftp, tftp, irc, sip, sctp, and amanda.
The second solution makes use of the state flags of connections more than the previous solution did. This solution takes advantage of the fact that the NEW state of a connection is valid when the very first packet of a traffic flow is detected. Subsequently, if the very first packet of a flow is accepted, the flow becomes a connection and thus enters into the ESTABLISHED state. Therefore a general rule can be written for allowing packets of ESTABLISHED connections to reach the guest virtual machine or be sent by the guest virtual machine. This is done writing specific rules for the very first packets identified by the NEW state and dictates the ports that the data is acceptable. All packets meant for ports that are not explicitly accepted are dropped, thus not reaching an ESTABLISHED state. Any subsequent packets sent from that port are dropped as well.
<filter name='test-eth0'>
  <!- - This filter references the clean traffic filter to prevent MAC, IP and ARP spoofing. By not providing and IP address parameter, libvirt will detect the IP address the VM is using. - ->
  <filterref filter='clean-traffic'/>

  <!- - This rule allows the packets of all previously accepted connections to reach the guest virtual machine - ->
  <rule action='accept' direction='in'>
    <all state='ESTABLISHED'/>
  </rule>

  <!- - This rule allows the packets of all previously accepted and related connections be sent from the guest virtual machine - ->
  <rule action='accept' direction='out'>
    <all state='ESTABLISHED,RELATED'/>
  </rule>

  <!- - This rule enables traffic towards port 21 (FTP) and port 22 (SSH)- ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='21' dstportend='22' state='NEW'/>
  </rule>

  <!- - This rule enables traffic towards port 80 (HTTP) - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='80' state='NEW'/>
  </rule>

  <!- - This rule enables general ICMP traffic to be initiated by the guest virtual machine, including ping traffic - ->
  <rule action='accept' direction='out'>
    <icmp state='NEW'/>
  </rule>

  <!- - This rule enables outgoing DNS lookups using UDP - ->
  <rule action='accept' direction='out'>
    <udp dstportstart='53' state='NEW'/>
  </rule>

  <!- - This rule drops all other traffic - ->
  <rule action='drop' direction='inout'>
    <all/>
  </rule>

</filter>