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If firewalls block all incoming connections by default how do p2p technologies work? Like torrents, how do you connect to everyone who shares a file and get the information from them? Does it go through a relay server or something?
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A lot of networks today are behind firewalls. In peer-to-peer (P2P) networking, firewall-protected peers may have to communicate with peers outside the firewall.
In peer-to-peer (P2P) networking, a group of computers are linked together with equal permissions and responsibilities for processing data. Unlike traditional client-server networking, no devices in a P2P network are designated solely to serve or to receive data.
In peer-to-peer systems like BitTorrent and Bitcoin, you start your local program on a networked computer, and you become connected to other instances of the same program running on lots of other computers. The local process starts listening on a local port, and finds IP:port pairs for those other processes.
If your router supports UPnP, it is possible that the p2p application that you are using is opening ports on the router without you needing to manually configuring open ports.
I'm not up on everything about BitTorrent, as I am about general P2P connectivity techniques. Typically clients in a P2P network rendezvous on a common signaling server (e.g. SIP, XMPP, tracking server, web site) to exchange IP addresses,other meta data, and messages to bootstrap direct connections. Then use any of the following techniques below to get a communications session going:
Both sides attempt to connect to each other simultaneously - in case one side can't accept incoming connections, but is allowed to make outbound connections. Such is the case for the firewall scenario.
Hole punching (used in conjunction with above). Relays are not required per se, but do help insure connectivity when both peers are behind network devices that are difficult to traverse. There's both UDP Hole Punching as well as TCP Hole Punching techniques. More info here.
Relays, including TURN servers, can be deployed into a P2P network when direct connectivity is not possible. All your favorite video call applications deploy relays for these scenarios, but do their best to get peers directly connected to avoid the cost of relaying.
Bing for the following topics: STUN, TURN, ICE (Interactive Connectivity Establishment), libjingle, pjnath, libnice.
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