Add nft_counters plugin

[penguinpee]

Plugin monitors nftables counters and can be configured to show
a subset of counters. Limiting output by type of counter (bytes
or packets) is also possible.

Plugin requires python3-nftables, which makes use of libnftables
and is part of Netfilter's nftables, but may need to be installed
separately. Plugin will bail out if the nftables module cannot be
loaded.

Plugin is built upon PyMunin, adapted for Python3. The version
listed in PIP is still using Python 2 syntax. Release of a Python 3
compatible version is pending.

I'm reaching out to the owner/maintainer of PyMunin for getting
the Python 3 changes implemented and having PyMunin updated in pip.
So far, there has been no response. If it remains quiet, I intent
to fork and publish the new version as PyMunin3. Advice on this
topic is welcome.

For now, to test the plugin, use the PR branch
in my fork of PyMunin. To install it to the user install directory
use pip install --user -e . from the repository root directory.
For system wide installation use sudo pip install --prefix /usr/local/ ..

[penguinpee]

There is now a Python 3 compatible test release of PyMunin on TestPyPI, if someone wants to give this plugin a testdrive. Install with:

pip install --user --index-url https://test.pypi.org/simple/ PyMunin

Or, if you want to make it available system wide:

pip install --prefix /usr/local --index-url https://test.pypi.org/simple/ PyMunin

I'm still hoping for an official release in the coming weeks.

[penguinpee]

Dependency is now available on PyPI. In order to use the plugin install the dependency, first, using:

pip install PyMunin3

To ensure there are some counters in nftables, store the following script in accounting.nft:

# Little trick to ensure we start with an empty table
table inet accounting
flush table inet accounting

table inet accounting {

    counter dns {
        comment "DNS traffic"
    }

    counter ntp {
        comment "NTP traffic"
    }

    chain input {
        type filter hook prerouting priority raw; policy accept;

        udp dport 53  counter name dns
        udp dport 123 counter name ntp
    }

    chain output {
        type filter hook postrouting priority raw; policy accept;

        udp dport 53  counter name dns
        udp dport 123 counter name ntp
    }

}

Then run it with sudo nft -f accounting.nft.

To reset the counters run sudo nft reset counters table inet accounting and to remove the accounting table all together run sudo nft delete table inet accounting.

[penguinpee]

With PyMunin3 >= 3.0.1 plugin supports dirtyconfig.

[penguinpee]

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