For new kernels (3.18+), bridge module is split into two pieces: bridge
and br_netfilter. The latter provides firewall support for bridged
traffic, as well as the following sysctl knobs:
* net.bridge.bridge-nf-call-arptables
* net.bridge.bridge-nf-call-ip6tables
* net.bridge.bridge-nf-call-iptables
Before kernel 3.18, any brctl command was loading the 'bridge' module
with the knobs, so at the moment where we reached iptables setup, they
were always available.
With new 3.18+ kernels, brctl still loads 'bridge' module, but not
br_netfilter. So bridge existance no longer guarantees us knobs'
presence. If we reach _enable_netfilter_for_bridges before the new
module is loaded, then the code will fail, triggering agent resync. It
will also fail to enable bridge firewalling on systems where it's
disabled by default (examples of those systems are most if not all Red
Hat/Fedora based systems), making security groups completely
ineffective.
Systems that don't override default settings for those knobs would work
fine except for this exception in the log file and agent resync. This is
because the first attempt to add a iptables rule using 'physdev' module
(-m physdev) will trigger the kernel module loading. In theory, we could
silently swallow missing knobs, and still operate correctly. But on
second thought, it's quite fragile to rely on that implicit module
loading. In the case where we can't detect whether firewall is enabled,
it's better to fail than hope for the best.
An alternative to the proposed path could be trying
to fix broken deployment, meaning we would need to load the missing
kernel module on agent startup. It's not even clear whether we can
assume the operation would be available to us. Even with that, adding a
rootwrap filter to allow loading code in the kernel sounds quite scary.
If we would follow the path, we would also hit an issue of
distinguishing between cases of built-in kernel module vs. modular one.
A complexity that is probably beyond what Neutron should fix.
The patch introduces a sanity check that would fail on missing
configuration knobs.
DocImpact: document the new deployment requirement in operations guide
UpgradeImpact: deployers relying on agents fixing wrong sysctl defaults
will need to make sure bridge firewalling is enabled.
Also, the kernel module providing sysctl knobs must be
loaded before starting the agent, otherwise it will fail
to start.
Depends-On: Id6bfd9595f0772a63d1096ef83ebbb6cd630fafd
Change-Id: I9137ea017624ac92a05f73863b77f9ee4681bbe7
Related-Bug: #1622914
e83a44b96a