FoxyProxy, Firefox 3.5, and DNS Leaking

[Update: Jan. 24, 2010] The DNS leaking problem described in this article applied to FoxyProxy v2.14. On Jan. 12, FoxyProxy v2.17 fixed the problem.

FoxyProxy is a popular Firefox extension that enables users to, setup, easily manage, and quickly switch between multiple proxy configurations. One of the most common uses of a proxy server is for security/privacy. By establishing an encrypted connection (usually via SSH) with a proxy server on a trusted network, you can have your web traffic go through an encrypted "pipe" to that server and have that server send and receive web requests on your behalf, relaying data to and from you only through that pipe. By doing this you eliminate the risk that someone on your current network could see your HTTP traffic. Maybe you don't trust other clients on the network, maybe you don't trust the gateway, it doesn't matter -- your HTTP(S) traffic is encrypted and shielded from prying eyes. (Readers unfamiliar with the concept of using HTTP proxies through SSH tunnels are encouraged to research the matter, there are many well-written tutorials available.)

There are many other popular uses of proxy servers, but the application of encrypted web traffic is of concern in this case for the following reason: A key problem that arises when using web proxy servers is the issue of handling DNS requests. DNS does not go through the HTTP protocol, so even if HTTP is being forwarded to a proxy the DNS requests may not be. If DNS requests are sent out over the network like normal then eavesdroppers can still read them. So although the actual traffic may be encrypted, the DNS queries behave normally and may cause the same problems that using an encrypted tunnel was designed to avoid in the first place. A situation in which HTTP traffic is encrypted but DNS is not is referred to as "DNS leaking". When using a proxy for the benefit of security or privacy, DNS leaking may be just as bad as non-encrypted traffic.

Solving the DNS leaking problem is simple. One type of proxy, SOCKS5, can forward DNS requests as well as HTTP(S) data. A user simply needs to tell their browser to use the SOCKS5 proxy for both HTTP(S) and DNS and then both will be routed through the encrypted stream, allowing the user to surf the web with greatly strengthened security and privacy.

However, Firefox users who use FoxyProxy at the moment will encounter a problem when using DNS forwarding to a SOCKS5 proxy. When using FoxyProxy, DNS leaking occurs even when it is configured not to, which has made many users very upset. Initially many people thought the problem was with Firefox 3.5, but others confirmed it was only present with FoxyProxy installed. Unfortunately, however, not everyone is convinced that this is FoxyProxy-related behavior and I have not found anyone who has presented a solution yet. I plan to do both.

This is the basic setup for my tests:

  • I set up an SSH server.
  • I established an SSH connection and used the built-in SOCKS5 functionality of the SSH server daemon:
    $ ssh username@myserver -D localhost:8080
    (For the non-SSH inclined: This command forwarded all traffic my client sends to itself on port 8080 through the SSH connection to the SSH server, which then acts as a SOCKS5 proxy and sends the data on to the destination.)
  • I used Wireshark to monitor all packets, specifically DNS requests, sent or received my network's interface. Note that DNS requests tunneled over the SSH connection to the SOCKS5 proxy are not visible to the packet sniffer.
  • I monitored my Firefox configuration in about:config. (All network proxy-related settings are under the filter network.proxy.)
  • I used Firefox v3.5.5 and FoxyProxy v2.14.

Using this I was able to monitor all DNS requests while I experimented with Firefox and FoxyProxy using a SOCKS5 proxy. I did a base test with no proxy configuration, a test using Firefox's included proxy management, and a test using Foxyproxy for proxy management.

Using no proxy

Starting with a default configuration (SSH SOCKS connection established but no proxy settings configured to use it) I visited several websites such as,, and This was the simple base test.

I checked to get my IP address.

The relevant about:config settings:

network.proxy.socks               ""
network.proxy.socks_port          0
network.proxy.socks_remote_dns    false
network.proxy.type                0

Via Wireshark I watched as the websites generated normal DNS requests over the standard network.

Using Firefox to configure proxy settings

I restarted Firefox to avoid any cached DNS entries. Then, without FoxyProxy installed, I setup my SOCKS5 proxy. (Note that FoxyProxy replaces the standard Firefox proxy editor, so it is impossible to not use FoxyProxy when it is installed.)

Under Firefox's Preferences/Tools (depending on your operating system) I went to the "Advanced" tab, "Network" sub-tab, and opened "Settings". I chose "Manual proxy configuration" and entered "localhost" for the SOCKS Host and "8080" for the port.

Unfortunately, Firefox v3.5 does not support a GUI method of enabling DNS SOCKS5 proxying, so I had to manually go to about:config and enable it by setting network.proxy.socks_remote_dns to "true".

I checked to ensure that my IP address displayed as coming from the server and not my client. It did show as coming from the server, so Firefox was using the proxy.

The final about:config settings were:

network.proxy.socks               localhost
network.proxy.socks_port          8080
network.proxy.socks_remote_dns    true
network.proxy.type                1

I visited the same websites. Via Wireshark, I did not see any DNS requests sent over the standard network. Firefox channeled both the HTTP and DNS data through the SSH tunnel perfectly.

Using FoxyProxy to configure proxy settings

I reset all the about:config settings back to their defaults. Then I installed FoxyProxy Standard v2.14. I went to FoxyProxy's options and, under the "Proxies" tab, created a new proxy entry whch I named "SSH SOCKS5". I set it to connect to "localhost" on port 8080. As well, I check-marked the "SOCKS proxy?" box and selected "SOCKS v5". I went to the "Global Options" tab and checked the box "Use SOCKS proxy for DNS lookups". To let this take effect, I had to restart Firefox.

When Firefox had restarted, I went to the Tools > FoxyProxy menu and selected to "Use proxy SSH SOCKS5 for all URLS". I checked to ensure that my IP address displayed as coming from the server and not my client. It did show as coming from the server, so Firefox was using the proxy.

I checked about:config:

network.proxy.socks               ""
network.proxy.socks_port          0
network.proxy.socks_remote_dns    false
network.proxy.type                0

The configuration was the same as the default, so apparently FoxyProxy does not adjust about:config to do its work.

Watching the DNS requests via Wireshark, I watched as all the website visits generated DNS requests over the normal network. Complete and thorough DNS leaking. And I would like to emphasize that I had selected "Use SOCKS proxy for DNS lookups", which is FoxyProxy's option to address the DNS leaking issue.

Fixing DNS leaking

There was no question about it, FoxyProxy caused the DNS leaking in my test. I wanted to solve the problem so I fiddled with about:config.

In about:config I manually set network.proxy.type to 1. I verified my IP address was from the server via

The new about:config:

network.proxy.socks_port          0
network.proxy.socks_remote_dns    false
network.proxy.type                1

I watched for DNS requests again via Wireshark. I saw none. It seemed that just manually setting network.proxy.type to 1 fixed the FoxyProxy DNS leaking problem.

I also tried other about:config settings, such as manually changing network.proxy.socks_remote_dns to "true", but that didn't work. The above was the only change in about:config that I found that fixed the problem.


I repeated the results above three times in different orders on different computers on both Linux and Windows to ensure I made no configuration mistakes and to verify that the behavior was consistent and cross-platform. All the tests yielded the same results. Here is the final summary:

  • Firefox v3.5 does not suffer from DNS leaking by itself.
  • DNS leaking occurs when FoxyProxy is managing the proxies.
  • FoxyProxy does not suffer from DNS leaking when network.proxy.type is manually set to 1.

It is obvious that FoxyProxy does not adjust about:config in order to configure proxy settings, but I do not know why. Many Firefox extensions adjust about:config in order to accomplish their goals and I know of no reason they should not. It's possible that FoxyProxy has not had a need to do so before, but in light of this serious problem that may need to change. The quickest/simplest solution for FoxyProxy may to set network.proxy.type to 1 if the currently enabled proxy is SOCKS5 and if the global options for FoxyProxy (or the about:config for Firefox) are set to enable DNS forwarding.

However, although this seems to indicate that FoxyProxy has made a mistake, I don't know that FoxyProxy is the party at fault. Clearly FoxyProxy does not have to alter about:config in order to change the other proxy settings, so why must network.proxy.type be set in about:config in order for DNS forwarding to work? Note that network.proxy.type isn't related to DNS forwarding, it just specifies which type of proxy is enabled. For all I know someone implemented a hack in Firefox that checks about:config when it shouldn't. Of course, I don't know that and I don't know if this is expected behavior from Firefox or not. It could be that FoxyProxy isn't setting whatever hidden configuration for DNS forwarding that exists on the same plane as the other invisible proxy settings it uses. Or maybe FoxyProxy is relying on an unreliable hack in order to avoid changing about:config. I don't know about any of that. What I do know is that Firefox by itself does not have this DNS leaking problem, FoxyProxy does, and a simple solution exists.

Again, I am certainly not the first person to note this problem, but a) I have seen many people blame Firefox for this bug, and b) I have not yet seen anyone else mention the solution that I noted above.

I leave it to someone with more time and knowledge about these software projects to determine which project should have which bug report filed. This needs to be fixed permanently.