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I2P support in Bitcoin Core

It is possible to run Bitcoin Core 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 Core with an I2P router (proxy)

A running I2P router (proxy) with SAM enabled is required. Options include:

i2prouter (I2P Router), the official implementation in Java
i2pd (I2P Daemon) (documentation), a lighter alternative in C++ (successfully tested with version 2.23 and up; version 2.36 or later recommended)
i2p-zero
other alternatives

Note the IP address and port the SAM proxy is listening to; usually, it is 127.0.0.1:7656.

Once an I2P router with SAM enabled is up and running, use the following Bitcoin Core configuration 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 Core connects to the I2P router, if -i2pacceptincoming=1, then it will automatically generate a persistent I2P address and its corresponding private key. The private key will be saved in a file named i2p_private_key in the Bitcoin Core data directory. The persistent I2P address is used for accepting incoming connections and for making outgoing connections if -i2pacceptincoming=1. If -i2pacceptincoming=0 then only outbound I2P connections are made and a different transient I2P address is used for each connection to improve privacy.

Persistent vs transient I2P addresses

In I2P connections, the connection receiver sees the I2P address of the connection initiator. This is unlike the Tor network where the recipient does not know who is connecting to them and can't tell if two connections are from the same peer or not.

If an I2P node is not accepting incoming connections, then Bitcoin Core uses random, one-time, transient I2P addresses for itself for outbound connections to make it harder to discriminate, fingerprint or analyze it based on its I2P address.

-debug=i2p

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

-onlynet=i2p

Make automatic outbound connections only to I2P addresses. Inbound and manual connections are not affected by this option. It can be specified multiple times to allow multiple networks, e.g. onlynet=onion, onlynet=i2p.

I2P support was added to Bitcoin Core in version 22.0 and there may be fewer I2P peers than Tor or IP ones. Therefore, using I2P alone without other networks may make a node more susceptible to Sybil attacks. You can use bitcoin-cli -addrinfo to see the number of I2P addresses known to your node.

Another consideration with onlynet=i2p is that the initial blocks download phase when syncing up a new node can be very slow. This phase can be sped up by using other networks, for instance onlynet=onion, at the same time.

In general, a node can be run with both onion and I2P hidden services (or any/all of IPv4/IPv6/onion/I2P/CJDNS), which can provide a potential fallback if one of the networks has issues.

There are several ways to see your I2P address in Bitcoin Core if accepting incoming I2P connections (-i2pacceptincoming):

in the "Local addresses" output of CLI -netinfo
in the "localaddresses" output of RPC getnetworkinfo
in the debug log (grep for AddLocal; the I2P address ends in .b32.i2p)

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 Core 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 Core

Bitcoin Core 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 Core 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 Core.

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