Concurrent use of the `portmap` and `firewall` plugins can result in
errors during iptables chain creation:
- The `portmap` plugin has a time-of-check-time-of-use race where it
checks for existence of the chain but the operation isn't atomic.
- The `firewall` plugin doesn't check for existing chains and just
returns an error.
This commit makes both operations idempotent by creating the chain and
then discarding the error if it's caused by the chain already
existing. It also factors the chain creation out into `pkg/utils` as a
site for future refactoring work.
Signed-off-by: Tim Gross <tim@0x74696d.com>
Distros often have additional rules in the their iptabvles 'filter' table
that do things like:
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
docker, for example, gets around this by adding explicit rules to the filter
table's FORWARD chain to allow traffic from the docker0 interface. Do that
for a given host interface too, as a chained plugin.
