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. See Tor Project FAQ:TBBSocksPort for how to properly configure Tor.
There are several ways to see your local onion address in Bitcoin Core:
getnetworkinfo in the "localaddresses" section-netinfo peer connections dashboardYou may set the -debug=tor config logging option to have additional information in the debug log about your Tor configuration.
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. If SOCKS5 is selected (default), this proxy
server will be used to try to reach .onion addresses as well.
-onion=ip:port Set the proxy server to use for Tor onion services. You do not
need to set this if it's the same as -proxy. You can use -noonion
to explicitly disable access to onion services.
-listen When using -proxy, listening is disabled by default. If you want
to run an onion service (see next section), you'll need to enable
it explicitly.
-connect=X When behind a Tor proxy, you can specify .onion addresses instead
-addnode=X of IP addresses or hostnames in these parameters. It requires
-seednode=X SOCKS5. In Tor mode, such addresses can also be exchanged with
other P2P nodes.
-onlynet=onion Make outgoing connections only to .onion addresses. Incoming
connections are not affected by this option. This option can be
specified multiple times to allow multiple network types, e.g.
ipv4, ipv6, or onion.
In a typical situation, this suffices to run behind a Tor proxy:
./bitcoind -proxy=127.0.0.1:9050
If you configure your Tor system accordingly, it is possible to make your node also reachable from the Tor network. Add these lines to your /etc/tor/torrc (or equivalent config file): Needed for Tor version 0.2.7.0 and older versions of Tor only. For newer versions of Tor see Section 3.
HiddenServiceDir /var/lib/tor/bitcoin-service/
HiddenServicePort 8333 127.0.0.1:8334
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=7zvj7a2.htmlgkdbg4f2dryd5rgtrn7upivr5eeij4cicjh65pooxeshid.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
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 -upnp).
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=7zvj7a2.htmlgkdbg4f2dryd5rgtrn7upivr5eeij4cicjh65pooxeshid.onion -discover
Starting with Tor version 0.2.7.1 it is possible, through Tor's control socket API, to create and destroy 'ephemeral' onion services programmatically. Bitcoin Core has been updated to make use of this.
This means that if Tor is running (and proper authentication has been configured), Bitcoin Core automatically creates an onion service to listen on. This will positively affect the number of available .onion nodes.
This new feature is enabled by default if Bitcoin Core is listening (-listen), and requires a Tor connection to work. It can be explicitly disabled with -listenonion=0 and, if not disabled, configured using the -torcontrol and -torpassword settings. To show verbose debugging information, pass -debug=tor.
Connecting to Tor's control socket API requires one of two authentication methods to be configured. It also requires the control socket to be enabled, e.g. put ControlPort 9051 in torrc config file. For cookie authentication the user running bitcoind must have read access to the CookieAuthFile specified in Tor configuration. In some cases this is preconfigured and the creation of an onion service is automatic. If permission problems are seen with -debug=tor they can be resolved by adding both the user running Tor and the user running bitcoind to the same group and setting permissions appropriately. On Debian-based systems the user running bitcoind can be added to the debian-tor group, which has the appropriate permissions. Before starting bitcoind you will need to re-login to allow debian-tor group to be applied. Otherwise you will see the following notice: "tor: Authentication cookie /run/tor/control.authcookie could not be opened (check permissions)" on debug.log.
An alternative authentication method is the use of 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 (read the tor manual for more details).