electrumx/docs/peer_discovery.rst

.. _Peer Discovery:

Peer Discovery
==============

This was imlpemented in ElectrumX as of version 0.11.0.  Support for
IRC peer discovery was removed in ElectrumX version 1.2.1.

The :dfn:`peer database` is an in-memory store of peers with at least
the following information about a peer, required for a response to the
:func:`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 ElectrumX this is a per-coin property in `lib/coins.py
<https://github.com/kyuupichan/electrumx/blob/master/lib/coins.py>`_.


server.peers.subscribe
----------------------

:func:`server.peers.subscribe` is used by Electrum 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 ElectrumX, "recently" is taken to be the last 24 hours.  Only one
peer from each IPv4/16 netmask is returned, and the number of onion
peers is limited.


Maintaining the Peer Database
-----------------------------

In order to keep its peer database up-to-date and fresh, after some
time has passed since the last successful connection to a peer, an
Electrum server should make another attempt to connect, choosing
either the TCP or SSL port.

On connecting it should issue :func:`server.peers.subscribe`,
:func:`blockchain.headers.subscribe`, and :func:`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 :func:`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
:func:`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.

ElectrumX 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 24 hours, and a failing
after 5 minutes but with exponential backoff.  It forgets a peer
entirely if a few days have passed since a successful connection.
ElectrumX attempts to connect to onion peers through a Tor proxy that
can be configured or that it will try to autodetect.


server.features
---------------

:func:`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 Electrum 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
---------------

:func:`server.add_peer` is intended for a new server to get itself in
the connected set.

A server receiving a :func:`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 :func:`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 :func:`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 :func:`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).