Tor is, to put it simply, the world's largest anonymity service. Relied on by many, the onion has developed into a staple of tech-security culture. With the immense popularity of Tor, several rumors have circulated regarding its security and effectiveness. This article serves to answer said speculations, and provide further insight into the workings of the Tor service.
# apt-get install tor
# emerge tor
- Arch Linux
# pacman -S tor
How It Works
One takes a big chance using tor. Privacy isn't assured in any way; anonymity however is, given that 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.
Tor bridges are basically unofficial entry points into the tor network which are utilized by users in locations such as China in order to access Tor because a vast majority of the official nodes are banned. Such was the case during the Middle East protests, in which online activists were creating exit bridges with haste.
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 Tor traffic is being monitored, and therefore, always use some form of end to end encryption such as sshing into a box over Tor.
If running a tor service (it's very simple, just requires one additional line to tor config), it should be run as a Tor node for security reasons. Services not on a node are vulnerable to certain attacks.
Getting Tor and Extra Uses
For more practical usage, the browser bundle can be downloaded and installed from the Tor Project's website. This being the easiest method.
A convenience for those who have Tor installed and/or running is to use it as a socks5 proxy. The proxy address should be set to 127.0.0.1, and the port to 9050.
For running command-line programs over tor, torsocks is recommended, which appears as:
$ torsocks ssh firstname.lastname@example.org
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.
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"
Networking Support ==> Networking Options ==> The IPv6 Protocol
Hit the "n" key to exclude it and continue building your kernel.