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Nov 30 23:42:52 Lisiano-Ubuntu kernel: [122378.362037] TCP: Possible SYN flooding on port 51300. Sending cookies.
So yes. Deluge is running on port 51300. I found that increasing net.ipv4.tcp_max_syn_backlog should help but what is still a sane number to assign to net.ipv4.tcp_max_syn_backlog? Currently it's on 204800, was 2048. Memory wise I have no problems. 8GB DDR3 and 6GB Swap, which is never touched unless I start using VMs.