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DNS Whisperer

DNS proxy for filtering out unwanted domains.

What it is

  • Run dnswhisperer on some box
  • Use this box as your DNS server
  • Add some domains that you don't like to dnswhisperer.txt
  • And voilà - all these domains will now shows up as "not found" on your machine

This is primarily meant for blocking online tracking and ad services. In particular, this works well for devices that can't run ad-blocking capable browsers themselves.

How it works

It listens for DNS requests on UDP 0.0.0.0:53, checks that they are queries and checks all names from the Question section against the blacklist. It remembers the result and forwards request to a real DNS server.

When it receives a response, if it was a query for a blacklisted domain it changes IP addresses in all A answer sections to 0.0.0.0 and forwards response to the client.

Note that earlier versions were changing the response code to NXDOMAIN ("not found") instead. This however stopped working with newer iOS versions as they appear to be falling back to hardcoded DNS servers in these cases, thus negating the blacklisting effort.

See code for details.

Example

Here's a single page load of https://reddit.com on iPad, each line is a DNS request:

        www.reddit.com
        www.redditstatic.com
        b.thumbs.redditmedia.com
        a.thumbs.redditmedia.com
        reddit.com
nope -- ssl-google-analytics.l.google.com
nope -- www-google-analytics.l.google.com
        www.redditmedia.com
nope -- events.redditmedia.com
        pixel.redditmedia.com
nope -- s.zkcdn.net
nope -- www-googletagmanager.l.google.com

Line marked with nope are the DNS requests that were blocked.

Caveats

This code is few hours worth of effort. It's stable and reasonably clean, but it could use lots of improvement.

  • Maxium number of in-flight (pending) requests is hardcoded to 256.
  • Real DNS address is hardcoded to 1.1.1.1, one of Cloudfare servers.
  • DNS server can be overriden with -s 1.2.3.4 (though it's IPv4 only for now).
  • Code uses single socket to talk to the real DNS server, so the absolute maximum of in-flight requests is 2^16, because the Request ID field in DNS packet is 16-bit wide. To lift this cap the code will need to maintain 2+ sockets and then track which request was forwarded to the server through which socket.
  • Blacklist matching is as dumb as it gets - a linear scan with no less linear substring search of each entry in the query name.
  • There's no support for proper clean up of timed out queries.
  • Can't be daemonized at the moment.
  • Logs to stdout.
  • Uses select() - mortifying, granted, but, hey, there are just two sockets!

Bulding

  1. Procure Linux
  2. Run make

Running

On console, logging to stdout:

sudo ./dnswhisperer

On console, logging to a file:

sudo ./dnswhisperer -l dnswhisperer.log

Daemonize, without a log:

sudo ./dnswhisperer -d

Daemonize, with a log:

sudo ./dnswhisperer -d -l /var/log/dnswhisperer.log

Sudo's needed because of listening on UDP/53, which is a privileged port.

Alternatively, use setuid or setcap (if on Linux), i.e. setcap cap_net_bind_service=+ep /path/to/dnswhisperer.