Allowing Zone Drifting

A future release of firewalld will contain a behavioral change that may affect some users. Older version of firewalld had undocumented behavior that has come to be known as zone drifting.

The default value of AllowZoneDrifting is no. To regain the old, buggy behavior users can set this to yes.

What is zone drifting?

A user may configure a source-based zoneFoo - a zone to which you’ve added sources via --add-source. If zoneFoo uses a --set-target of default, then the packets will be allowed to ingress a second interface-based zone. This includes the catch-all default zone.

This is a result of how previous version of firewalld dispatched packets to zones.

Each dispatch type used to have it’s own dedicated chain.

chain filter_INPUT {
    jump filter_INPUT_ZONES_SOURCE  # The important
    jump filter_INPUT_ZONES         # parts are here
chain filter_INPUT_ZONES_SOURCE {
    ip6 saddr dead:beef::/54 goto filter_IN_internal
chain filter_INPUT_ZONES {
    iifname "dummy0" goto filter_IN_trusted
    goto filter_IN_public

They key bit, as noted by the comment, is that there are two different chains performing dispatch. One for source-based. One for interface-based. If the chain filter_IN_internal is non-terminal, as is the case if --set-target is default, then packet processing will return to the filter_INPUT chain and jump to the filter_INPUT_ZONES chain. This means a packet can ingress both the internal zone and the trusted/public zones.

Why disable it by default?

A tenant of zone based firewalls is that packets ingress one and only one zone. The above is a violation of that. It can cause traffic or services to be allowed unexpectedly.

This can be see by the various bug reports:

For these reasons AllowZoneDrifting defaults to no.

Slightly different than previous firewalld versions

Even with AllowZoneDrifting set to yes there is a slight behavioral difference between previous firewalld versions and current versions. This only applies to a subset of features; masquerade, forward-port, and helpers. For these features, zone drifting could occur for any value of --set-target including ACCEPT, DROP, REJECT even amongst interface-based zones.

This is the cause of the following issues:

It was deemed a security bug by firewalld developers and has no reasonable use case. As such, it was not carried forward when AllowZoneDrifting was featured.

How to re-enable zone drifting?

The firewalld developers strongly discourage enabling zone drifting. It is better to add the necessary services and configuration to the source-based zone.

That being said, if you would like to enable zone drifting it can easily be done by setting AllowZoneDrifting to yes in /etc/firewalld/firewalld.conf and restarting firewalld.

What versions are affected?

The following versions fixed the zone drifting bug and disallowed its occurrence:

  • 0.6.5
  • 0.7.0
  • 0.8.0

AllowZoneDrifting was introduced in the following versions allowing users to regain the old behavior.

  • 0.7.4
  • 0.8.2
  • 1.0.0