Somehow this error is not explicit, I thought that sysctl was involved in this matter but it was clearly logstash's input's buffer.

input {
    udp {
        type => "netflow"
        port => 5555
        codec => netflow {
            versions => [5]
        }
        workers => 8
        receive_buffer_bytes => 2129920
        queue_size => 200000
    }
}

has fixed it

Apparemment ça fonctionne pas mal, a tester

'faudrait déjà que graylog soit un peu moins chiant à configurer/installer ;-)

je mets ça là.