Difference between revisions of "Tor"

Revision as of 15:50, 3 July 2016

Tor is, to put it simply, the world's largest anonymity service. Relied on by many, the Tor network is a group of volunteer-operated servers that allows people to improve their privacy and security while using the Internet.

Installation

Debian

To install Tor on Debian stable, Debian sid, or Debian testing, simply execute the following:

 
# apt-get install tor
 

Ubuntu

The Ubuntu repositories do not always have the most up-to-date versions of Tor. It is recommended that you use the official Tor Project repository or compile from source. To use the Tor Project's official repository, you need to add the following to /etc/apt/sources.list:

 
deb http://deb.torproject.org/torproject.org jessie main
deb-src http://deb.torproject.org/torproject.org jessie main
 

Next, you must add the Tor Project's GPG key used to sign the Tor packages:

 
gpg --keyserver keys.gnupg.net --recv 886DDD89
gpg --export A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89 | sudo apt-key add -
 

Now, you are able to install Tor from the official repository using:

 
# apt-get update
# apt-get install tor deb.torproject.org-keyring
 

Fedora/RHEL

This section is applicable to Fedora 22/23 and RHEL 6/7. The repositories in these distributions are frequently out-of-date. It is strongly recommended to use the Tor Project's official repository or compile from source.

To use the Tor Project's official repository, first, you must create /etc/yum.repos.d/tor.repo and insert the following:

 
[tor]
name=Tor repo
enabled=1
baseurl=https://deb.torproject.org/torproject.org/rpm/DISTRIBUTION/$basearch/     ##replace DISTRIBUTION with correct version (fc/22, fc/23, el/6, el/7)
gpgcheck=1
gpgkey=https://deb.torproject.org/torproject.org/rpm/RPM-GPG-KEY-torproject.org.asc
repo_gpgcheck=1
 
[tor-source]
name=Tor source repo
enabled=1
autorefresh=0
baseurl=https://deb.torproject.org/torproject.org/rpm/DISTRIBUTION/SRPMS     ##replace DISTRIBUTION with correct version (fc/22, fc/23, el/6, el/7)
gpgcheck=1
gpgkey=https://deb.torproject.org/torproject.org/rpm/RPM-GPG-KEY-torproject.org.asc
repo_gpgcheck=1
 

Next, you need to take care of a name clash in the repos to avoid the two packages from overwriting each other. To do this, add "Exclude=tor" to the relevant repo file.

For example, you may need to add "Exclude=tor" to the /etc/yum.repos.d/fedora.rep and /etc/yum.repos.d/fedora-updates.repo.

Next, install tor by executing:

 
# yum install tor
# service start tor
 

Gentoo

 
# emerge tor
 

Arch Linux

 
# pacman -S tor
 

Compile from Source

First, grab the Tor Source Code and verify you have the required dependencies including libevent, openssl, and the zlib packages.

After, extract and compile the source by executing:

 
# tar xzf tor-0.2.7.6.tar.gz; cd tor-0.2.7.6
# ./configure && make
# make install
 

How It Works

One takes a big chance using Tor. While privacy isn't guaranteed, anonymity can be if one changes their habits.

Tor originally stood for "The Onion Router". How Tor essentially works is, traffic gets wrapped in multiple layers of encryption, passes from the initial box to the first node in the chain where traffic gets decrypted once, and passed to the next node. It then gets decrypted again and passed to the exit node from where decryption occurs the last time, and routes traffic in the clear. Due to these multiple layers of encryption, each node only knows the last hop and the next hop in the chain.

Tor bridges are basically unofficial entry points into the tor network which are utilized by users in locations around the world, espcially in heavily oppressed and monitored countries (ie. China), in order to access Tor. This is because a vast majority of the official nodes are banned or traffic is heavily monitored.

Common Pitfalls

The onion structure undoubtedly has issues. Such problems can be read in a comical form here.

Without clicking links, exit node operators can sniff the traffic that passes through. Some operators choose to do so, and for this reason, it should be assumed that all Tor traffic is being monitored, and therefore, always use some form of end to end encryption such as sshing into a box over Tor.

Hidden services

When connecting to "normal" websites, the connection looks roughly like:

you -> tor node 1 -> tor node 2 -> exit node -> internet

Hidden services appear as:

you -> arbitrary # nodes -> rendezvous <- arbitrary # nodes <- hs box

While hidden service tends to be very slow, ssl is practically moot.

There was an article in 2600 a couple of years ago detailing use of the control port to change the length of tor circuits, and other uses.

One of the pitfalls with hidden services is a correlation attack. If someone controls enough nodes, they can send enough traffic to the hidden service to find its location. The best way to help prevent this is to make the hidden service a Tor node as well. At that point, it passes non-hs traffic and keeps anonymity static.

Hidden services use .onion as a pseudo-tld. An example being the hidden wiki at http://kpvz7ki2v5agwt35.onion/wiki/index.php/Main_Page .onion is a way of describing a hidden service without giving away its location. The string before the .onion is actually a key fingerprint, which ensures the service cannot be found, and it has the expected private key.

One of the more well known .onions is The Silk Road, a venue for buying and selling drugs.

Transparent Proxy

What is a transparent proxy? A transparent proxy forces all your outbound traffic through a proxy of your choosing, Tor is perfect for using this and we will cover setting one up in this section.

First we will need to add these four lines to the end of your torrc found at /etc/tor/torrc on most systems.

 
VirtualAddrNetworkIPv4 10.192.0.0/10
AutomapHostsOnResolve 1
TransPort 9040
DNSPort 5353
 

Now for our iptables rules to force all traffic through Tor:

 
#!/bin/bash
 
_non_tor="192.168.1.0/24 192.168.0.0/24"
 
_tor_uid="43"
 
_trans_port="9040"
 
iptables -F
iptables -t nat -F
 
iptables -t nat -A OUTPUT -m owner --uid-owner $_tor_uid -j RETURN
iptables -t nat -A OUTPUT -p udp --dport 53 -j REDIRECT --to-ports 5353
 
#allow clearnet access for hosts in $_non_tor
for _clearnet in $_non_tor 127.0.0.0/9 127.128.0.0/10; do
   iptables -t nat -A OUTPUT -d $_clearnet -j RETURN
done
 
iptables -t nat -A OUTPUT -p tcp --syn -j REDIRECT --to-ports $_trans_port
 
iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
 
for _clearnet in $_non_tor 127.0.0.0/8; do
   iptables -A OUTPUT -d $_clearnet -j ACCEPT
done
 
iptables -A OUTPUT -m owner --uid-owner $_tor_uid -j ACCEPT
iptables -A OUTPUT -j REJECT
 
iptables -I OUTPUT ! -o lo ! -d 127.0.0.1 ! -s 127.0.0.1 -p tcp -m tcp --tcp-flags ACK,FIN ACK,FIN -j DROP
iptables -I OUTPUT ! -o lo ! -d 127.0.0.1 ! -s 127.0.0.1 -p tcp -m tcp --tcp-flags ACK,RST ACK,RST -j DROP
 

The Tor UID varies from system to system and one of the easiest ways to find it is by running:

 
$ grep tor /etc/passwd
 

Be sure to save the config above to rules.sh in your home folder.

Now once you have done all that (saved Tor configuration, saved iptables rules to a file) you will need to run:

 
$ killall -HUP tor
 

Which restarts tor and:

 
$ chmod +x rules.sh
$ ./rules.sh
 

This sets the iptables rules and you should now be properly transparently proxying all your traffic through Tor. One thing you must think about is whether the router or network you are connecting to is IPv6 only, since Tor traffic only is IPv4, the iptables rules won't apply on IPv6 traffic, thus leaking this traffic you think is going through Tor to the clearnet. It's highley recommended to disable IPv6 at the kernel level by modifying config.x86_64 and commenting out all the IPv6 entries or by excluding it through "make menuconfig"

make menuconfig:

Networking Support ==> Networking Options ==> The IPv6 Protocol

Hit the "n" key to exclude it and continue building your kernel.

External Links

• The Tor Project's website
• EFF on Tor
• torservers.net