DNS Tunneling POC

Introduction

This is a proof of concept for a DNS tunnelling client and server, for security education. The DNS server implemented here is woefully incomplete, as the point is not to resolve names, but to participate in file transfers. The server only handles A and AAAA queries with a single name, and responds to A queries with the same IP address (16.32.64.128), and to AAAA queries with a response indicating that an IPv6 address is not available, as per RFC 4074 section 3.

Theory of operation

The dns_send client can send a file or standard input to a remote name server controlled by an attacker. Each file transfer is identified by a uuid. Each payload of the data transfer consists of the uuid, a packet sequence number and 120 bytes of data. The payload is encoded in base 32, then included in a domain name. This is an example of such a name, which encodes a UUID, a sequence number and 120 bytes of data, and which obeys all of the rules for an Internet domain name:

QLFGR3VWU5D37KJMLXJOKUKDJERQAAAANNXHIZRIEJLXE33UMUQCKZBAMJ4X.IZLTEB2G6IBFOMQGC5BAN5TGM43FOQQCKZC4NYRCYIDQMF4WY33BMQWT43DF.NZTXI2BMEBTGS3DFNZQW2ZJMBIQCAIBAEAQCAIBAOBQXS3DPMFSC2PTTMVYX.KZLOMNSSAKRAGEZDAKJ3BJ6QU.badguy.io

In an attack scenario, an attacker would register the badguy.io domain (from the name above), point the DNS server for the domain to a server running the dns-server application in this project, so that it could receive and decode the specially crafted DNS queries. Since each name query is unique, there is no concern with caching.

Building

Run make. This will build dns-server and dns-send in the dist directory. See the Makefile for additional targets.

Running

Start the server by running dns-server. As the server binds to privileged port 53, it must be run as root (or sudo). The client usage is as follows:

dns_send [file [ip]]

Without any parameters, the program reads from stdin, and uses the default system resolver to eventually reach the attacker DNS server , provided the dns-send program is modified to resolve the correct domain (and not badguy.io).

A file name can be specified as the first argument, in which case the file will be transmitted through as many DNS requests as needed.

An IP address can be specified as the second parameter, which is useful in cases where you do not want to set up a domain and a server running dns-server. In that case, you can run dns-server on any machine, including the same machine on which dns-send is running, and point dns-send at the server. While this is not a realistic attack scenario in most cases, it will demonstrate transmitting large amounts of data over DNS.

Running on Linux

On systems using systemd, you will have to temporarily turn off the systemd resolver to free up port 53 (used by the POC server).

sudo systemctl stop systemd-resolved

Since the POC server really cannot resolve anything, you may want to edit /etc/resolv.conf to point to a real server, such as 8.8.8.8

Running on WSL

WSL requires Internet Connection Sharing which uses port 53. It is therefore not possible to run the server on WSL. The server can however run in a Windows command window.

Detecting DNS tunnelling

Due to the nature of DNS tunneling, there are several methods to detect it, and thereafter prevent it. See the Detecting DNS Tunelling paper from the SANS Institute for examples.

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