Add minimal log support

[sbernard31]

This PR aims to implements #4.
This differs a lot from what it was specify initialy.
Finally there is 6 log levels :

CRITICAL => should never happened,
ERROR => unexpected state,
WARNING => unwanted traffic (invalid packet),
INFO => not supported traffic (fragmented ip, ip option used),
DEBUG => all about NAT,
TRACE => all output we can.

A new CLI option is added :

 -l {CRITICAL,ERROR,WARNING,INFO,DEBUG,TRACE}, --loglevel {CRITICAL,ERROR,WARNING,INFO,DEBUG,TRACE}
                        Use to set logging verbosity.

The output looks like this :

Loading real servers from ulb.ini file ...
...real servers loaded : 10.0.0.1, 10.0.0.2, 10.0.0.3.

Compiling & attaching bpf code ...
... compilation and attachement succeed.

Applying config to bpf ...
Add 10.0.0.1 at index 0
Add 10.0.0.2 at index 1
Add 10.0.0.3 at index 2
[1]=10.0.0.2
[2]=10.0.0.3
[0]=10.0.0.1
[10.0.0.2]=10.0.0.2
[10.0.0.1]=10.0.0.1
[10.0.0.3]=10.0.0.3
... config applied to bpf.

Load balancing UDP traffic over lo interface for port(s) [5683] from :
 Virtual Server           Real Server(s) 
      127.0.0.1 <───┬───> 10.0.0.1       
                    ├───> 10.0.0.2       
                    └───> 10.0.0.3 

                                    ┌ 00:00:00:00:00:00/10.0.0.1        Source NAT
    00:00:00:00:00:00/10.41.44.15 <─┘ 00:00:00:00:00:00/127.0.0.1       (NEW ASSOCIATION)
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/10.41.44.15     Unhandled traffic
    00:00:00:00:00:00/10.41.44.15 <─> 00:00:00:00:00:00/127.0.0.1       Not UDP packet
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/127.0.0.1       Not UDP packet
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/127.0.0.1       Not UDP packet
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/127.0.0.1       Not UDP packet
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/127.0.0.1       Not UDP packet
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/127.0.0.1       Not UDP packet
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/127.0.0.1       Not UDP packet
      00:00:00:00:00:00/127.0.0.1 ─┐  00:00:00:00:00:00/127.0.0.1       Destination NAT
                                   └> 00:00:00:00:00:00/10.0.0.2        (NEW ASSOCIATION)
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/10.0.0.2        Unhandled traffic
      00:00:00:00:00:00/127.0.0.1 ─┐  00:00:00:00:00:00/127.0.0.1       Destination NAT
                                   └> 00:00:00:00:00:00/10.0.0.2        (REUSED ASSOCIATION)
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/10.0.0.2        Unhandled traffic
                                    ┌ 00:00:00:00:00:00/10.0.0.1        Source NAT
    00:00:00:00:00:00/10.41.44.15 <─┘ 00:00:00:00:00:00/127.0.0.1       (REUSED ASSOCIATION)
      00:00:00:00:00:00/127.0.0.1 <─> 00:00:00:00:00:00/10.41.44.15     Unhandled traffic
    00:00:00:00:00:00/10.41.44.15 <─> 00:00:00:00:00:00/127.0.0.1       Not UDP packet
    00:00:00:00:00:00/10.41.44.15 <── 00:00:00:00:00:00/10.0.0.2        Not associated real server
    00:00:00:00:00:00/10.41.44.15 <── 00:00:00:00:00:00/10.0.0.2        Not associated real server

Stopping by signal SIGINT(2)...
Detaching bpf code ...
... code detached.
... sbulb stopped.

Of course more could/should? be done : #21

[oliwer]

Shouldn't we add the port number to logEvent?

[sbernard31]

As the idea is to have a minimal logging facilities, that's why I didn't add it for now.
But it is in the improvements list : #21 !

We could discuss about that to know if this should be in the v0.1.0.

[oliwer]

About the log levels, on AirVantage (and Loggly) we only use Error, Warning, Info and Debug. I would suggest doing the same. Debug and Trace are basically the same thing.
On MQTTFE, we used to differentiate Debug and Trace but we ended up merging them because it was too impractical.

[sbernard31]

DEBUG and TRACE is mainly for development time, not really for production.

I need 2 different log levels : one to track NAT, another for full tracing.

Here the explanation :
CRITICAL => should never happened,
ERROR => unexpected state,
WARNING => unwanted traffic (invalid packet),
INFO => not supported traffic (fragmented ip, ip option used),
DEBUG => all about NAT,
TRACE => all output we can.

[oliwer]

Please don't use the Critical level.

[sbernard31]

Any reason about that ?