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I2P support in Bitcoin ABC

It is possible to run Bitcoin ABC as an I2P (Invisible Internet Project) service and connect to such services.

This glossary may be useful to get started with I2P terminology.

Run Bitcoin ABC with an I2P router (proxy)

A running I2P router (proxy) with SAM enabled is required (there is an official one and a few alternatives). Notice the IP address and port the SAM proxy is listening to; usually, it is 127.0.0.1:7656. Once it is up and running with SAM enabled, use the following Bitcoin ABC options:

-i2psam=<ip:port>
     I2P SAM proxy to reach I2P peers and accept I2P connections (default:
     none)

-i2pacceptincoming
     If set and -i2psam is also set then incoming I2P connections are
     accepted via the SAM proxy. If this is not set but -i2psam is set
     then only outgoing connections will be made to the I2P network.
     Ignored if -i2psam is not set. Listening for incoming I2P
     connections is done through the SAM proxy, not by binding to a
     local address and port (default: 1)

In a typical situation, this suffices:

bitcoind -i2psam=127.0.0.1:7656

The first time Bitcoin ABC connects to the I2P router, its I2P address (and corresponding private key) will be automatically generated and saved in a file named i2p_private_key in the Bitcoin ABC data directory.

You may set the debug=i2p config logging option to have additional information in the debug log about your I2P configuration and connections. Run bitcoin-cli help logging for more information.

It is possible to restrict outgoing connections in the usual way with onlynet=i2p. I2P support was added to Bitcoin ABC in version 22.0 (mid 2021) and there may be fewer I2P peers than Tor or IP ones. Therefore, using onlynet=i2p alone (without other onlynet=) may make a node more susceptible to Sybil attacks. Use bitcoin-cli -addrinfo to see the number of I2P addresses known to your node.

There are several ways to see your I2P address in Bitcoin ABC:

To see which I2P peers your node is connected to, use bitcoin-cli -netinfo 4 or the getpeerinfo RPC (e.g. bitcoin-cli getpeerinfo).

To see which I2P addresses your node knows, use the getnodeaddresses 0 i2p RPC.

Compatibility

Bitcoin ABC uses the SAM v3.1 protocol to connect to the I2P network. Any I2P router that supports it can be used.

Ports in I2P and Bitcoin ABC

Bitcoin ABC uses the SAM v3.1 protocol. One particularity of SAM v3.1 is that it does not support ports, unlike newer versions of SAM (v3.2 and up) that do support them and default the port numbers to 0. From the point of view of peers that use newer versions of SAM or other protocols that support ports, a SAM v3.1 peer is connecting to them on port 0, from source port 0.

To allow future upgrades to newer versions of SAM, Bitcoin ABC sets its listening port to 0 when listening for incoming I2P connections and advertises its own I2P address with port 0. Furthermore, it will not attempt to connect to I2P addresses with a non-zero port number because with SAM v3.1 the destination port (TO_PORT) is always set to 0 and is not in the control of Bitcoin ABC.