I think this issue might be related to the starting of the filebeat in the client, I have hanged its initialization time in rc.d with
sudo update-rc.d filebeat defaults 95 10
I read that in this post, it does work!
2016-10-20T15:59:51+02:00 DBG Flushing spooler because of timeout. Events flushed: 3
2016-10-20T15:59:51+02:00 DBG Publish: {
"@timestamp": "2016-10-20T13:59:44.233Z",
"beat": {
"hostname": "VPSA",
"name": "VPSA"
},
"count": 1,
"fields": null,
"input_type": "log",
},
"count": 1,
"fields": null,
"input_type": "log",
"message": "Oct 20 15:59:39 artguard GW[1401]: message repeated 2 times: [ #033[90m DEBUG#033[0m Handle PULL_DATA #033[90mGatewayID#033[0m=#033[90madapter#033[0m=gateway-semtech]",
"offset": 17654,
"source": "/var/log/syslog",
"type": "syslog"
}