Security model: block DHT nodes

DHT node lookups can trigger P2P security warnings in firewalls. Since we block file sharing on the network, we will need to add TCP/UDP port 6881 to the security model.

We will look at blocking all announce ports in BEP 0005:

```
nodes = [["127.0.0.1", 6881], ["your.router.node", 4804], ["2001:db8:100:0:d5c8:db3f:995e:c0f7", 1941]]
```

[1] https://www.bittorrent.org/beps/bep_0005.html