# I2P support in Bitcoin Core

It is possible to run Bitcoin Core as an
[I2P (Invisible Internet Project)](https://en.wikipedia.org/wiki/I2P)
service and connect to such services.

This [glossary](https://geti2p.net/en/about/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](https://geti2p.net/en/docs/api/samv3)
enabled is required (there is an [official one](https://geti2p.net) and
[a few alternatives](https://en.wikipedia.org/wiki/I2P#Routers)). 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 Core 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, its I2P address (and
corresponding private key) will be automatically generated and saved in a file
named `i2p_private_key` in the Bitcoin Core data directory.

## Additional configuration options related to I2P

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 Core 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](https://en.bitcoin.it/wiki/Weaknesses#Sybil_attack). Use
`bitcoin-cli -addrinfo` to see the number of I2P addresses known to your node.

## I2P related information in Bitcoin Core

There are several ways to see your I2P address in Bitcoin Core:
- in the debug log (grep for `AddLocal`, the I2P address ends in `.b32.i2p`)
- in the output of the `getnetworkinfo` RPC in the "localaddresses" section
- in the output of `bitcoin-cli -netinfo` peer connections dashboard

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](https://geti2p.net/en/docs/api/samv3) protocol
to connect to the I2P network. Any I2P router that supports it can be used.