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What protocols or algorithms are Bitcoin clients using in order to find their peers? I learnt that BitTorrent clients may rely on central computers called trackers. Do Bitcoin clients need any kind of central servers or nodes in order to operate?
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Bitcoin clients use several methods to locate other clients. The primary method is a list of nodes from a previous connection to the network. The works very well for everything but your first connection or a connection after a very long period of disconnection.
For the case where you have no previous known IPs or they aren't usable, the primary fallback method is by DNS seeds. Several host names (such as dnsseed.bluematt.me) are maintained that resolve to a list of IP addresses known to be running nodes. Should that fail, the client has a list of IP addresses believed to point to stable nodes hard-coded into it.
One method that used to be used, but that has since been abandoned, was having the clients locate each other using an IRC server. That worked similar to a BitTorrent tracker.
Once you are connected to the network, you will begin to receive lists of IP addresses and ports of known clients.
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There's a couple of methods the client can use. However, unlike David's very good answer, I'll talk about them in chronological order:
when you guys mention IRC, what IRC network exactly? – knocte – 2013-04-14T12:52:40.613
1@knocte It used to use irc.lfnet.org, joining a random channel between
#bitcoin00and#bitcoin99. – David Schwartz – 2013-04-14T13:03:24.1001so if lfnet.org went down, that was a single point of failure? what now, who dictates what are the DNS names to lookup? – knocte – 2013-04-14T14:12:31.800
@knocte: It wouldn't have been a single point of failure because the client had several other bootstrapping methods including several hardcoded "well known" servers. Anyone who wants to can set their client to use whatever DNS names they want. The official client uses a list maintained by the developers. I believe the developers solicited volunteers to run DNS seeds. – David Schwartz – 2013-04-14T14:14:32.400
5When the Bitcoin.org client needs to discover peers, DNS seeds is the default for the client v0.6 and above. IRC was the default for earlier versions. – Stephen Gornick – 2012-05-01T15:10:44.040
funny how a ripple enthusiast came up with this question (me), googled it and found an answer by you :-).. off topic, but had to share! – Hedde van der Heide – 2017-08-25T13:04:59.977
@HeddevanderHeide I think most ripple enthusiasts are also bitcoin enthusiasts. – David Schwartz – 2017-08-25T17:53:11.680
Ok. So Bitcoins ARE vulnerable to China's DNS poisoning ? Good to know. – Taiko – 2017-10-19T14:23:06.260
@Taiko Not really, because DNS is only one method used. A bitcoin server needs only one connection to an honest node to be safe. – David Schwartz – 2017-10-19T20:07:31.423
Problem is. DNS poisoning will also prevent connection to hard coded IPs. So first time connections can't work of the GFW of China blocks it. @David – Taiko – 2017-10-23T16:22:39.027
3@Taiko DNS poisoning has no effect on connections to hard coded IPs. You are right that an entity that controls your Internet access can break your ability to use bitcoin, just as it can break your ability to use any other Internet service. (Bitcoin is especially resilient, however, because just one leak saves everyone.) – David Schwartz – 2017-10-24T18:37:54.670