Preventing a DNS Leak and WebRTC Leak when using Tor in Linux

Background

I have added to my 2011 Tor post a note on how to avoid a DNS Leak and WebRTC Leak, but am repeating it here in a new post, along with a Bash script that can be used to toggle the relevant Firefox user preferences before and after using Firefox with Tor, which makes the process easier.

The original eleven steps I gave in my above-mentioned post will not prevent the so-called DNS Leak problem. If your Web browser is not configured correctly it will still use your ISP’s DNS servers instead of the DNS servers favoured by Tor, in which case your ISP will know which sites you are accessing. See What is a DNS leak? for details. Reference 1 at the end of this post is a link to an article about DNS leakage, and Reference 2 is a link to an article on the Tor Browser, a browser designed to help avoid DNS leakage.

Furthermore, now that WebRTC is incorporated in some browsers, a ‘WebRTC Leak‘ is also possible if you have not configured your browser correctly.

Using the Tor Browser

Instead of performing Steps 1 to 11 in my original Tor post, download the Tor Browser, unpack it (no installation is required) and use that browser. Reference 3 below is a link to the download page, and Reference 4 below is a link to the instructions on how to unpack the tarball and launch the browser.

If you want even more security, you could instead download the ISO for the Tails Linux distribution, burn a LiveDVD or LivePenDrive — see my post Help for Windows users: How to create a Linux LiveCD, LiveDVD or LivePenDrive from an ISO file if you don’t know how to do that — and launch the browser from a Live Environment.

Using Tor with Firefox

However, if you still want to use the method I gave in my original Tor post then you could try all the additional steps given below to stop DNS leakage and WebRTC leakage.

  1. Use the OpenDNS servers instead of your ISP’s DNS servers. That will not help, though, if your ISP is using a Transparent DNS Proxy.
  2. Make the following changes to the preferences in Firefox (enter about:config in the Firefox address bar):
    Preference Name                       Status   Type     Value
    network.dns.disableIPv6               default  boolean  false  Change to true
    network.dns.disablePrefetch           default  boolean  false  Change to true
    network.proxy.socks_remote_dns        default  boolean  false  Change to true
    browser.safebrowsing.enabled          default  boolean  true   Change to false
    browser.safebrowsing.malware.enabled  default  boolean  true   Change to false
    media.peerconnection.enabled          default  boolean  true   Change to false
    

    (When you have finished using Tor, set media.peerconnection.enabled back to true if you want to use WebRTC. If you also want Firefox to warn you of phishing Web sites and Web sites that download malware, also set browser.safebrowsing.enabled and browser.safebrowsing.enabled back to true after you have finished using Tor.)

    You may be wondering why I disable IPv6 DNS requests. It is because some IPv6-capable DNS servers may return an IPv4 address when an IPv6 address is requested. I disable the two ‘safe browsing’ preferences because, if enabled, they cause Firefox to compare visited URLs against a remotely-stored blacklist or submit URLs to a third party to determine whether a site is legitimate, and I don’t want the possibility of Firefox contacting other sites outside Tor or trying to find an IP address for a URL. The PeerConnection preference relates to WebRTC, and I disable that to stop Firefox contacting STUN servers (see Reference 5 below).

  3. Test if there is still leakage by visiting the DNS leak test Web site and clicking on the Standard test button, and visiting the IP/DNS Detect site.

Furthermore, do not forget to use a Private Browsing window in Firefox.

Automate the editing of Firefox user preferences

Using about:config to change the user preferences in Firefox is laborious, so I created a Bash script edit_firefox.sh to toggle the relevant user preferences:

#!/bin/bash
# Script to change Firefox user preferences rather than
# using about:config from within Firefox.
# Make sure you only run this script when Firefox is not running.
#
FILE="/home/fitzcarraldo/.mozilla/firefox/fm8q09x0.default/prefs.js"
#
#
STATE=$(grep media.peerconnection.enabled $FILE | cut -c 43- | cut -d')' -f1)
if ! grep -q media.peerconnection.enabled $FILE ; then
  echo 'user_pref("media.peerconnection.enabled", false);' >> $FILE
  echo 'Added media.peerconnection.enabled false (secure) to prefs.js'
elif [ $STATE = "true" ]; then
     sed -i s/^.*media.peerconnection.enabled.*$/'user_pref("media.peerconnection.enabled", false);'/ $FILE
     echo 'media.peerconnection.enabled changed to false (secure) in prefs.js'
  else
     sed -i s/^.*media.peerconnection.enabled.*$/'user_pref("media.peerconnection.enabled", true);'/ $FILE
     echo 'media.peerconnection.enabled changed to true (not secure) in prefs.js'
fi
#
STATE=$(grep browser.safebrowsing.malware.enabled $FILE | cut -c 51- | cut -d')' -f1)
if ! grep -q browser.safebrowsing.malware.enabled $FILE ; then
  echo 'user_pref("browser.safebrowsing.malware.enabled", false);' >> $FILE
  echo 'Added browser.safebrowsing.malware.enabled false (secure) to prefs.js'
elif [ $STATE = "true" ]; then
     sed -i s/^.*browser.safebrowsing.malware.enabled.*$/'user_pref("browser.safebrowsing.malware.enabled", false);'/ $FILE
     echo 'browser.safebrowsing.malware.enabled changed to false (secure) in prefs.js'
  else
     sed -i s/^.*browser.safebrowsing.malware.enabled.*$/'user_pref("browser.safebrowsing.malware.enabled", true);'/ $FILE
     echo 'browser.safebrowsing.malware.enabled changed to true (not secure) in prefs.js'
fi
#
STATE=$(grep browser.safebrowsing.enabled $FILE | cut -c 43- | cut -d')' -f1)
if ! grep -q browser.safebrowsing.enabled $FILE ; then
  echo 'user_pref("browser.safebrowsing.enabled", false);' >> $FILE
  echo 'Added browser.safebrowsing.enabled false (secure) to prefs.js'
elif [ $STATE = "true" ]; then
     sed -i s/^.*browser.safebrowsing.enabled.*$/'user_pref("browser.safebrowsing.enabled", false);'/ $FILE
     echo 'browser.safebrowsing.enabled changed to false (secure) in prefs.js'
  else
     sed -i s/^.*browser.safebrowsing.enabled.*$/'user_pref("browser.safebrowsing.enabled", true);'/ $FILE
     echo 'browser.safebrowsing.enabled changed to true (not secure) in prefs.js'
fi
#
STATE=$(grep network.proxy.socks_remote_dns $FILE | cut -c 45- | cut -d')' -f1)
if ! grep -q network.proxy.socks_remote_dns $FILE ; then
  echo 'user_pref("network.proxy.socks_remote_dns", true);' >> $FILE
  echo 'Added network.proxy.socks_remote_dns true (secure) to prefs.js'
elif [ $STATE = "true" ]; then
     sed -i s/^.*network.proxy.socks_remote_dns.*$/'user_pref("network.proxy.socks_remote_dns", false);'/ $FILE
     echo 'network.proxy.socks_remote_dns changed to false (not secure) in prefs.js'
  else
     sed -i s/^.*network.proxy.socks_remote_dns.*$/'user_pref("network.proxy.socks_remote_dns", true);'/ $FILE
     echo 'network.proxy.socks_remote_dns changed to true (secure) in prefs.js'
fi
#
STATE=$(grep network.dns.disablePrefetch $FILE | cut -c 42- | cut -d')' -f1)
if ! grep -q network.dns.disablePrefetch $FILE ; then
  echo 'user_pref("network.dns.disablePrefetch", true);' >> $FILE
  echo 'Added network.dns.disablePrefetch true (secure) to prefs.js'
elif [ $STATE = "true" ]; then
     sed -i s/^.*network.dns.disablePrefetch.*$/'user_pref("network.dns.disablePrefetch", false);'/ $FILE
     echo 'network.dns.disablePrefetch changed to false (not secure) in prefs.js'
  else
     sed -i s/^.*network.dns.disablePrefetch.*$/'user_pref("network.dns.disablePrefetch", true);'/ $FILE
     echo 'network.dns.disablePrefetch changed to true (secure) in prefs.js'
fi
#
STATE=$(grep network.dns.disableIPv6 $FILE | cut -c 38- | cut -d')' -f1)
if ! grep -q network.dns.disableIPv6 $FILE ; then
  echo 'user_pref("network.dns.disableIPv6", true);' >> $FILE
  echo 'Added network.dns.disableIPv6 true (secure) to prefs.js'
elif [ $STATE = "true" ]; then
     sed -i s/^.*network.dns.disableIPv6.*$/'user_pref("network.dns.disableIPv6", false);'/ $FILE
     echo 'network.dns.disableIPv6 changed to false (not secure) in prefs.js'
  else
     sed -i s/^.*network.dns.disableIPv6.*$/'user_pref("network.dns.disableIPv6", true);'/ $FILE
     echo 'network.dns.disableIPv6 changed to true (secure) in prefs.js'
fi

You will need to change the path to the Firefox prefs.js file in the sixth line of the script, to suit your installation. If you have the utility mlocate installed you can find the file easily by using the command:

$ locate prefs.js | grep firefox

You will also need to make the script executable:

$ chmod +x edit_firefox.sh

You can see below how the script works:

$ ./edit_firefox.sh
media.peerconnection.enabled changed to false (secure) in prefs.js
browser.safebrowsing.malware.enabled changed to false (secure) in prefs.js
browser.safebrowsing.enabled changed to false (secure) in prefs.js
network.proxy.socks_remote_dns changed to true (secure) in prefs.js
network.dns.disablePrefetch changed to true (secure) in prefs.js
network.dns.disableIPv6 changed to true (secure) in prefs.js
$ ./edit_firefox.sh
media.peerconnection.enabled changed to true (not secure) in prefs.js
browser.safebrowsing.malware.enabled changed to true (not secure) in prefs.js
browser.safebrowsing.enabled changed to true (not secure) in prefs.js
network.proxy.socks_remote_dns changed to false (not secure) in prefs.js
network.dns.disablePrefetch changed to false (not secure) in prefs.js
network.dns.disableIPv6 changed to false (not secure) in prefs.js
$

Procedure to use Tor

So, if I am not using the Tor Browser, in summary I do the following (refer to my 2011 Tor post for the details):

  1. Launch Polipo from a Konsole window.
  2. Launch Vidalia from a Konsole window.
  3. Launch edit_firefox.sh to make sure the relevant user preferences are set securely.
  4. Launch Firefox and change the network settings to enable use of Polipo and Vidalia.
  5. Launch a Firefox Private Browsing window and close the original window.
  6. Visit TorCheck at Xenobite.eu, What Is My IP Address?, DNS leak test and IP/DNS Detect to be sure I am using Tor and that there is no DNS leak or WebRTC leak.

The router provided by my ISP does not allow me to change its DNS server settings. Using the router’s Web browser interface I was able to view the IP addresses of the DNS servers the router uses (Whois Lookup is a good place to check to whom an IP address belongs), and they are indeed owned by the ISP. However, the leak test Web sites I mention above show me that there is no DNS leakage to the ISP’s DNS servers when I have performed all the steps above.

When I have finished using Tor, I do the following:

  1. Exit Firefox.
  2. Stop Tor from the Vidalia GUI, exit Vidalia and end the Konsole session.
  3. Stop Polipo and end the Konsole session.
  4. Launch edit_firefox.sh to set the relevant user preferences back to their original settings.
  5. Launch Firefox and change the network settings back to the original settings.

References

1. Preventing Tor DNS Leaks
2. Tor new advice (February 2014)
3. Download Tor Browser
4. Linux Instructions for Tor Browser
5. New Browser Based Flaw Leaks VPN Users’ IP Addresses

Secure deletion of a file via the KDE GUI

One of the various Linux tools for secure deletion of files is the srm command. In Gentoo Linux the srm command and other security-related commands are available by installing the package secure-delete. Now, srm is easy enough to use from the command line:

$ srm mysecrets.doc

You can do the same thing from the KDE GUI very easily by adding the srm command to the KDE Service Menu. To do this, first install KDE Service Menu Editor. Then launch it and do the following:

On the Actions tab:

1. Enter “Securely Delete” (without the quotes) in the Label box.

2. Enter srm -r %u in the Exec box. As you will see if you use the man srm command, the “-r” option means “recursive mode, deletes all subdirectories”. I’ve added this option so that I can securely delete directories and all their contents including sub-directories, as well as individual files.

3. Click on the blank Icon and choose a suitable icon. I selected System Icons > Actions, entered “delete” (without the quotes) in the Search box and selected the big red X (edit-delete) icon.

On the Conditions tab:

4. Against ‘Display the menu for:’, click on All Files.

5. Click OK then Close, and you’re done.

Now all you need to do in KDE is right-click on a file or directory and select Actions > Securely Delete.

The above configuration will only allow you to delete files and directories owned by your user name. However, if you also want to be able to delete files owned by other Linux accounts on your machine then you can modify the command in the KDE Service Menu to prompt you for the root user’s password. To do that, in Step 2 above you would use the following command instead:

kdesu srm -r %u

Now when you right-click on a file or directory, a window will pop up and prompt you to enter the root user’s password.

Of course, if you would prefer to use a different tool to securely delete a file or directory then you could substitute the srm command with e.g. wipe or bcwipe if you have those installed. Use man wipe and man bcwipe after installing them, to check which options you might want to use.

Note that some of your data might be in a temporary (deleted) copy of the file somewhere on the disk. If you installed the secure-delete package then you could use the sfill command to wipe free disk space on any mounted partition, but you would need to be root user to use it. Note also that, if a small file has already acquired a block with your data, no tool can remove that data. Anyway, if you also want to securely wipe any free disk space you can modify the aforementioned command in the KDE Service Menu even further:

kdesu srm -r %u && kdesu sfill directory/mountpoint

I have my /home directory on its own partition, so, in my case the KDE Service Menu entry would be:

kdesu srm -r %u && kdesu sfill /home

This would pop up a window twice prompting you to enter the root user’s password: once for the srm command, and once for the sfill command. Note that the sfill command can take a very, very long time to wipe the free space if it is large (I personally don’t bother using it, as I don’t want my hard disk thrashing for hours). The sfill command works by creating one big file to fill all the free space, then writes to that file using several steps to ensure all the previously-free areas of the disk have had their original contents erased. Once that is completed, the utility then removes the big file, releasing the free disk space.

If you think your swap space contains some of your data, you could use the sswap command — also available if you install the secure-delete package — for secure deletion of the swap space, but you would need to disable your swap space first. I have 4 GB of RAM and my swap partition is virtually never used, so I don’t bother putting my tinfoil hat on in the case of swap either.

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