How does the bitcoin client make the initial connection to the bitcoin network?

Question

Once the bitcoin client is connected to a machine (or multiple machines) on the bitcoin network, they can share information (e.g. the block-chain, IP addresses of other nodes, etc). I understand how this peer-to-peer architecture is robust to nodes joining/leaving/crashing and not relying on any central authority.

My question is: when I start my client, it has no information about the IP addresses of the other nodes, so how does it know who/where to connect to initially? How does one 'boot-strap' a peer-to-peer network?

Answer 1

The Bitcoin client has a number of sources that it uses to locate the network on initial startup. In order of importance:

1) The primary mechanism, if the client has ever run on this machine before and its database is intact, is to look at its database. It tracks every node it has seen on the network, how long ago it last saw it, and its IP address.

2) The client can use DNS to locate a list of nodes connected to the network. One such seed is bitseed.xf2.org. The client will resolve this and get a list of Bitcoin nodes.

3) The client has a list of semi-permanent nodes compiled into it.

4) The client can connect to a well-known IRC network, irc.lfnet.org, and find other nodes that way. (This method is now considered obsolete.)

Answer 2

There are several bootstrap methods. I know of two:

1. An specific IRC channel is joined by the client. In this channel, connected clients often broadcast their IP, to allow others to find them.
2. There is a list of hard-coded IPs in the client binary, which the client will connect to bootstrap its network.