# TOR SUPPORT IN BITCOIN

It is possible to run Bitcoin Core as a Tor onion service, and connect to such services.

The following directions assume you have a Tor proxy running on port 9050. Many distributions default to having a SOCKS proxy listening on port 9050, but others may not. In particular, the Tor Browser Bundle defaults to listening on port 9150.
## Compatibility

- Starting with version 22.0, Bitcoin Core only supports Tor version 3 hidden
  services (Tor v3). Tor v2 addresses are ignored by Bitcoin Core and neither
  relayed nor stored.

- Tor removed v2 support beginning with version 0.4.6.

## How to see information about your Tor configuration via Bitcoin Core

There are several ways to see your local onion address in Bitcoin Core:
- in the "Local addresses" output of CLI `-netinfo`
- in the "localaddresses" output of RPC `getnetworkinfo`
- in the debug log (grep for "AddLocal"; the Tor address ends in `.onion`)

You may set the `-debug=tor` config logging option to have additional
information in the debug log about your Tor configuration.

CLI `-addrinfo` returns the number of addresses known to your node per
network. This can be useful to see how many onion peers your node knows,
e.g. for `-onlynet=onion`.

You can use the `getnodeaddresses` RPC to fetch a number of onion peers known to your node; run `bitcoin-cli help getnodeaddresses` for details.

`bitcoin rpc` can also be substituted for `bitcoin-cli`.

## 1. Run Bitcoin Core behind a Tor proxy

The first step is running Bitcoin Core behind a Tor proxy. This will already anonymize all
outgoing connections, but more is possible.

    -proxy=ip[:port]
        Set the proxy server. It will be used to try to reach .onion addresses
        as well. You need to use -noonion or -onion=0 to explicitly disable
        outbound access to onion services.

    -proxy=ip[:port]=tor
    or
    -onion=ip[:port]
        Set the proxy server for reaching .onion addresses. You do not need to
        set this if it's the same as the generic -proxy. You can use -onion=0 to
        explicitly disable access to onion services.
        ------------------------------------------------------------------------
        Note: The proxy for DNS requests is taken from
        -proxy=addr:port or
        -proxy=addr:port=ipv4 or
        -proxy=addr:port=ipv6
        (last one if multiple options are given). It is not taken from
        -proxy=addr:port=tor or
        -onion=addr:port.
        If no proxy for DNS requests is configured, then they will be done using
        the functions provided by the operating system, most likely resulting in
        them being done over the clearnet to the DNS servers of the internet
        service provider.
        ------------------------------------------------------------------------

If -proxy or -onion is specified multiple times, later occurrences override
earlier ones and command line overrides the config file. UNIX domain sockets may
be used for proxy connections. Set `-onion` or `-proxy` to the local socket path
with the prefix `unix:` (e.g. `-onion=unix:/home/me/torsocket`).

    -listen
        When using -proxy, listening is disabled by default. If you want to
        manually configure an onion service (see section 3), you'll need to
        enable it explicitly.

    -connect=X
    -addnode=X
    -seednode=X
        When behind a Tor proxy, you can specify .onion addresses instead of IP
        addresses or hostnames in these parameters. Such addresses can also be
        exchanged with other P2P nodes.

    -onlynet=onion
        Make automatic outbound connections only to .onion 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, onlynet=cjdns.

In a typical situation, this suffices to run behind a Tor proxy:

    bitcoind -proxy=127.0.0.1:9050

`bitcoin node` or `bitcoin gui` can also be substituted for `bitcoind`.

## 2. Automatically create a Bitcoin Core onion service

Bitcoin Core makes use of Tor's control socket API to create and destroy
ephemeral onion services programmatically. This means that if Tor is running and
proper authentication has been configured, Bitcoin Core automatically creates an
onion service to listen on. The goal is to increase the number of available
onion nodes.

This feature is enabled by default if Bitcoin Core is listening (`-listen`) and
it requires a Tor connection to work. It can be explicitly disabled with
`-listenonion=0`. If it is not disabled, it can be configured using the
`-torcontrol` and `-torpassword` settings.

To see verbose Tor information in the bitcoind debug log, pass `-debug=tor`.

### Control Port

You may need to set up the Tor Control Port. On Linux distributions there may be
some or all of the following settings in `/etc/tor/torrc`, generally commented
out by default (if not, add them):

```
ControlPort 9051
CookieAuthentication 1
CookieAuthFileGroupReadable 1
DataDirectoryGroupReadable 1
```

Add or uncomment those, save, and restart Tor (usually `systemctl restart tor`
or `sudo systemctl restart tor` on most systemd-based systems, including recent
Debian and Ubuntu, or just restart the computer).

### Authentication

Connecting to Tor's control socket API requires one of two authentication
methods to be configured: cookie authentication or bitcoind's `-torpassword`
configuration option.

#### Cookie authentication

For cookie authentication, the user running bitcoind must have read access to
the `CookieAuthFile` specified in the Tor configuration. In some cases this is
preconfigured and the creation of an onion service is automatic. Don't forget to
use the `-debug=tor` bitcoind configuration option to enable Tor debug logging.

If a permissions problem is seen in the debug log, e.g. `tor: Authentication
cookie /run/tor/control.authcookie could not be opened (check permissions)`, it
can be resolved by adding both the user running Tor and the user running
bitcoind to the same Tor group and setting permissions appropriately.

On Debian-derived systems, the Tor group will likely be `debian-tor` and one way
to verify could be to list the groups and grep for a "tor" group name:

```
getent group | cut -d: -f1 | grep -i tor
```

You can also check the group of the cookie file. On most Linux systems, the Tor
auth cookie will usually be `/run/tor/control.authcookie`:

```
TORGROUP=$(stat -c '%G' /run/tor/control.authcookie)
```

Once you have determined the `${TORGROUP}` and selected the `${USER}` that will
run bitcoind, run this as root:

```
usermod -a -G ${TORGROUP} ${USER}
```

Then restart the computer (or log out) and log in as the `${USER}` that will run
bitcoind.

#### `torpassword` authentication

For the `-torpassword=password` option, the password is the clear text form that
was used when generating the hashed password for the `HashedControlPassword`
option in the Tor configuration file.

The hashed password can be obtained with the command `tor --hash-password
password` (refer to the [Tor Dev
Manual](https://2019.www.torproject.org/docs/tor-manual.html.en) for more
details).


## 3. Manually create a Bitcoin Core onion service

You can also manually configure your node to be reachable from the Tor network.
Add these lines to your `/etc/tor/torrc` (or equivalent config file):

    HiddenServiceDir /var/lib/tor/bitcoin-service/
    HiddenServicePort 8333 127.0.0.1:8334
    # If `tor --list-modules` shows "pow: yes", then enable PoW protection.
    # It is available in tor-0.4.8.1-alpha and newer when configured with
    # `./configure --enable-gpl`.
    HiddenServicePoWDefensesEnabled 1

The directory can be different of course, but virtual port numbers should be equal to
your bitcoind's P2P listen port (8333 by default), and target addresses and ports
should be equal to binding address and port for inbound Tor connections (127.0.0.1:8334 by default).

    -externalip=X   You can tell bitcoin about its publicly reachable addresses using
                    this option, and this can be an onion address. Given the above
                    configuration, you can find your onion address in
                    /var/lib/tor/bitcoin-service/hostname. For connections
                    coming from unroutable addresses (such as 127.0.0.1, where the
                    Tor proxy typically runs), onion addresses are given
                    preference for your node to advertise itself with.

                    You can set multiple local addresses with -externalip. The
                    one that will be rumoured to a particular peer is the most
                    compatible one and also using heuristics, e.g. the address
                    with the most incoming connections, etc.

    -listen         You'll need to enable listening for incoming connections, as this
                    is off by default behind a proxy.

    -discover       When -externalip is specified, no attempt is made to discover local
                    IPv4 or IPv6 addresses. If you want to run a dual stack, reachable
                    from both Tor and IPv4 (or IPv6), you'll need to either pass your
                    other addresses using -externalip, or explicitly enable -discover.
                    Note that both addresses of a dual-stack system may be easily
                    linkable using traffic analysis.

In a typical situation, where you're only reachable via Tor, this should suffice:

    bitcoind -proxy=127.0.0.1:9050 -externalip=7zvj7a2imdgkdbg4f2dryd5rgtrn7upivr5eeij4cicjh65pooxeshid.onion -listen

(obviously, replace the .onion address with your own). It should be noted that you still
listen on all devices and another node could establish a clearnet connection, when knowing
your address. To mitigate this, additionally bind the address of your Tor proxy:

    bitcoind ... -bind=127.0.0.1:8334=onion

If you don't care too much about hiding your node, and want to be reachable on IPv4
as well, use `discover` instead:

    bitcoind ... -discover

and open port 8333 on your firewall (or use port mapping, i.e., `-natpmp`).

If you only want to use Tor to reach .onion addresses, but not use it as a proxy
for normal IPv4/IPv6 communication, use:

    bitcoind -onion=127.0.0.1:9050 -externalip=7zvj7a2imdgkdbg4f2dryd5rgtrn7upivr5eeij4cicjh65pooxeshid.onion -discover

## 4. Privacy recommendations

- Do not add anything but Bitcoin Core ports to the onion service created in section 3.
  If you run a web service too, create a new onion service for that.
  Otherwise it is trivial to link them, which may reduce privacy. Onion
  services created automatically (as in section 2) always have only one port
  open.
- Operating a node that listens on multiple networks (e.g. IPv4 and Tor) can help
  strengthen the Bitcoin network, as nodes in this configuration (i.e. bridge nodes) increase
  the cost and complexity of launching eclipse and partition attacks. However, under certain
  conditions, an adversary that can connect to your node on multiple networks may be
  able to correlate those identities by observing shared runtime characteristics. It
  is not recommended to expose your node over multiple networks if you require
  unlinkability across those identities.
