Since my server is under a sustain DNS amplification attack (DDOS), I configured fail2ban and initially my outgoing traffic dropped markedly.

Anyway, after a few hours (mostly +10), fail2ban uses about 75% ram and seems to be crashed in some way, because the outgoing traffic raises imediatly after. When I searched the web for the memory problem, I found some people complaining about high fail2ban memory usages as well. But the recommended solution, to insert an ulimit command into a fail2ban config file, did not change that much for me.