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In Transmission bittorrent client, there is an option to "enable blocklist", which has over 200,000 rules. I've tried searching but haven't found anything saying what the blocklist actually does.

Who is it blocking? And why might you need to block people? Will it affect my download speed?

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up vote 3 down vote accepted

Typically block lists in torrent clients are there to block any clients that show "abnormal" behaviour against your client - such as, too many connections per second, invalid connections amongst many others.

Although, I do not know about Transmission, some torrent clients have features of PeerGuardian build in which blocks known IP's that belong to law agencies and companies that are employed to attack, leech, infiltrate or sue users of P2P networks.

Can it slow down? Yes, probably, you may not be getting connections from clients capable of sending data - however, one of the items that it blocks is people sending repeatedly corrupt or invalid data, so it is not necessarily a bad thing.

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Blocklists generally block IP addresses belonging to companies that try to either track bittorrent users or send them bad data.

From what I've heard, they're useful as protection against tracking (and getting infringment letters), but the main reason to use a blocklist is that they generally speed up your downloads. The reason is that some companies run bittorrent clients with intentionally bad data and try to share it. Your client will automatically discard it, but it will slow you down. Your client will also eventually block them even without the blocklist, but it's faster to just block them from the start.

EDIT: So here's an article about how blocklists are helpful as protection against tracking (TorrentFreak suggests that if you're not using a blocklist, you're being tracked).

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