Title : Tor part 2: hidden service
Author: Solène
Date : 11 October 2018
Tags : openbsd68 openbsd unix tor security
In this second Tor article, I will present an interesting Tor feature
named **hidden service**. The principle of this hidden service is to
make available a network service from anywhere, with only
prerequisites that the computer must be powered on, tor not blocked
and it has network access.
This service will be available through an address not disclosing
anything about the server internet provider or its IP, instead, a
hostname ending by **.onion** will be provided by tor for
connecting. This hidden service will be only accessible through Tor.
There are a few advantages of using hidden services:
- privacy, hostname doesn't contain any hint
- security, secure access to a remote service not using SSL/TLS
- no need for running some kind of dynamic dns updater
The drawback is that it's quite slow and it only work for TCP
services.
From here, we assume that Tor is installed and working.
Running an hidden service require to modify the Tor daemon
configuration file, located in **/etc/tor/torrc** on OpenBSD.
Add the following lines in the configuration file to enable a hidden
service for SSH:
HiddenServiceDir /var/tor/ssh_service
HiddenServicePort 22 127.0.0.1:22
The directory **/var/tor/ssh_service** will be be created. The
directory **/var/tor** is owned by user **_tor** and not readable by
other users. The hidden service directory can be named as you want,
but it should be owned by user **_tor** with restricted
permissions. Tor daemon will take care at creating the directory with
correct permissions once you reload it.
Now you can reload the tor daemon to make the hidden service
available.
$ doas rcctl reload tor
In the **/var/tor/ssh_service** directory, two files are created. What
we want is the content of the file **hostname** which contains the
hostname to reach our hidden service.
$ doas cat /var/tor/ssh_service/hostname
piosdnzecmbijclc.onion
Now, we can use the following command to connect to the hidden service
from anywhere.
$ torsocks ssh piosdnzecmbijclc.onion
In Tor network, this feature doesn't use an exit node. Hidden services
can be used for various services like http, imap, ssh, gopher etc...
Using hidden service isn't illegal nor it makes the computer to relay
tor network, as previously, just check if you can use Tor on your
network.
Note: it is possible to have a version 3 .onion address which will
prevent hostname collapsing, but this produce very long
hostnames. This can be done like in the following example:
HiddenServiceDir /var/tor/ssh_service
HiddenServicePort 22 127.0.0.1:22
HiddenServiceVersion 3
This will produce a really long hostname like
tgoyfyp023zikceql5njds65ryzvwei5xvzyeubu2i6am5r5uzxfscad.onion
If you want to have the short and long hostnames, you need to specify
twice the hidden service, with differents folders.
Take care, if you run a ssh service on your website and using this
same ssh daemon on the hidden service, the host keys will be the same,
implying that someone could theoricaly associate both and know that
**this** public IP runs **this** hidden service, breaking anonymity.