Peer Discovery

The peer database is an in-memory store of peers with at least the following information about a peer, required for a response to the server.peers.subscribe() RPC call:

  • host name

  • IP address

  • TCP and SSL port numbers

  • protocol version

  • pruning limit, if any

Hard-coded Peers

A list of hard-coded, well-known peers seeds the peer discovery process. Ideally it should have at least 4 servers that have shown commitment to reliable service.

In Fulcrum the hard-coded peers come from here: servers.json.

server.peers.subscribe

server.peers.subscribe() is used by SPV clients to get a list of peer servers, in preference to a hard-coded list of peer servers in the client, which it will fall back to if necessary.

The server should craft its response in a way that reduces the effectiveness of server sybil attacks and peer spamming.

The response should only include peers it has successfully connected to recently. Only reporting recent good peers ensures that those that have gone offline will be forgotten quickly and not be passed around for long.

In Fulcrum, “recently” is taken to be servers it is actively able to maintain a connection to. All Fulcrum servers keep client connections to all of their peers at all times, in order to ensure that the returned list is as up-to-date as possible.

Maintaining the Peer Database

In order to keep its peer database up-to-date and fresh, Fulcrum maintains a constant connection to all of its peers, acting as an SPV client but not taking up much bandwidth. If connections are dropped or periodically fail, after some time has passed since the last successful connection to a peer, a Fulcrum server will make another attempt to connect, choosing either the TCP or SSL port.

On connecting it should issue server.peers.subscribe(), blockchain.headers.subscribe(), and server.features() RPC calls to collect information about the server and its peers. If the peer seems to not know of you, you can issue a server.add_peer() call to advertise yourself. Once this is done and replies received, terminate the connection.

The peer database should view information obtained from an outgoing connection as authoritative, and prefer it to information obtained from any other source.

On connecting, a server should confirm the peer is serving the same network, ideally via the genesis block hash of the server.features() RPC call below. Also the height reported by the peer should be within a small number of the expected value. If a peer is on the wrong network it should never be advertised to clients or other peers. Such invalid peers should perhaps be remembered for a short time to prevent redundant revalidation if other peers persist in advertising them, and later forgotten.

If a connection attempt fails, subsequent reconnection attempts should follow some kind of exponential backoff.

If a long period of time has elapsed since the last successful connection attempt, the peer entry should be removed from the database. This ensures that all peers that have gone offline will eventually be forgotten by the network entirely.

Fulcrum will connect to the SSL port if both ports are available. If that fails it will fall back to the TCP port. It tries to reconnect to a good peer at least once every hour. It forgets a peer entirely if 24 hours have passed since a successful connection. Fulcrum attempts to connect to onion peers through a Tor proxy that can be configured or that it will try to autodetect.

server.features

server.features() is a fairly new RPC call that a server can use to advertise what services and features it offers. It is intended for use by SPV clients as well as other peers. Peers will use it to gather peer information from the peer itself.

The call takes no arguments and returns a dictionary keyed by feature name whose value gives details about the feature where appropriate. If a key is missing the feature is presumed not to be offered.

server.add_peer

server.add_peer() is intended for a new server to get itself in the connected set.

A server receiving a server.add_peer() call should not replace existing information about the host(s) given, but instead schedule a separate connection to verify the information for itself.

To prevent abuse a server may do nothing with second and subsequent calls to this method from a single connection.

The result should be True if accepted and False otherwise.

Notes for Implementors

  • it is very important to only accept peers that appear to be on the same network. At a minimum the genesis hash should be compared (if the peer supports server.features()), and also that the peer’s reported height is within a few blocks of your own server’s height.

  • care should be taken with the server.add_peer() call. Consider only accepting it once per connection. Clearnet peer requests should check the peer resolves to the requesting IP address, to prevent attackers from being able to trigger arbitrary outgoing connections from your server. This doesn’t work for onion peers so they should be rate-limited.

  • it should be possible for a peer to change their port assignments - presumably connecting to the old ports to perform checks will not work.

  • peer host names should be checked for validity before accepting them; and localhost should probably be rejected. If it is an IP address it should be a normal public one (not private, multicast or unspecified).

  • you should limit the number of new peers accepted from any single source to at most a handful, to limit the effectiveness of malicious peers wanting to trigger arbitrary outgoing connections or fill your peer tables with junk data.

  • in the response to server.peers.subscribe() calls, consider limiting the number of peers on similar IP subnets to protect against sybil attacks, and in the case of onion servers the total returned.

  • you should not advertise a peer’s IP address if it also advertises a hostname (avoiding duplicates).