project

DNS Tunneler

This project implements a traffic tunneling method using DNS (Domain Name System) to transfer data over networks that block outgoing traffic. DNS traffic is typically allowed to pass through network restrictions, making it a suitable channel for data transmission. The tunneling technique involves encoding data into DNS packets to disguise it as regular traffic.

Implementation

The project is organized into the following directories:

• receiver: Contains the receiver source code.
• sender: Contains the sender source code.
• common: Includes common components such as the base64 implementation.

The source code is written in C11 and can be compiled using the provided Makefile. The communication protocol between the client and server is designed to be discreet and involves a handshake process to establish the connection.

Communication Protocol: Client-Server

The communication between the client (sender) and server (receiver) follows a three-step handshake process:

1. Hello UDP packet: The sender sends a valid DNS query packet using UDP, with the query type (QTYPE) set to A and the query name (QNAME) set to the BASE_HOST domain.
2. Hello UDP packet response: The receiver replies with a valid DNS response packet, setting the Truncated (TC) flag to 1, indicating that the client should switch to using TCP instead.
3. Opening TCP connection and filename packet: The client establishes a TCP connection and sends a DNS query packet with the filename encoded in the query name (QNAME) along with the BASE_HOST suffix.
4. Server checks the file: The server attempts to open the file and closes the connection if it fails.
5. Server is ready to receive file: If the file check is successful, the server is ready to receive data packets.

Data Encoding

Data is encoded using the base64 encoding algorithm. File chunks are encoded into alphanumeric characters (a-zA-Z0-9+-) with padding character =. The encoded data is then divided into 63-character-long labels with the BASE_HOST suffix, adhering to the specifications of RFC1035.

Extensions

The implementation supports both IPv4 and IPv6 addresses. Networking is designed to support dual-stack, where IPv4 addresses are mapped to IPv6 addresses. Additionally, the receiver acts as a DNS proxy for queries that do not use the BASE_HOST suffix, forwarding them to a configured DNS server.

Testing

The functionality of the receiver was tested using the dig command-line tool to perform DNS queries and the diff tool to compare the received file with the original file. Performance testing can be done by measuring the throughput of the tunnel using the provided macro in the sender code.

Functional Testing

The receiver has been tested using dig:

$ dig @127.0.0.1 vutbr.cz
; <<>> DiG 9.10.6 <<>> vutbr.cz
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10491
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 5

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;vutbr.cz.			IN	A

;; ANSWER SECTION:
vutbr.cz.		191	IN	A	147.229.2.90

;; AUTHORITY SECTION:
vutbr.cz.		1858	IN	NS	rhino.cis.vutbr.cz.
vutbr.cz.		1858	IN	NS	pipit.cis.vutbr.cz.

;; ADDITIONAL SECTION:
pipit.cis.vutbr.cz.	1	IN	A	77.93.219.110
pipit.cis.vutbr.cz.	1	IN	AAAA	2a01:430:120::4d5d:db6e
rhino.cis.vutbr.cz.	65	IN	A	147.229.3.10
rhino.cis.vutbr.cz.	65	IN	AAAA	2001:67c:1220:e000::93e5:30a

;; Query time: 54 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Nov 10 23:25:16 CET 2022
;; MSG SIZE  rcvd: 185

File transer

sender                                          receiver
...                                             $ ./dns_receiver dobron.sk tmp
$ ./dns_sender -u ::1 dobron.sk file bin        ...
...                                             ...
[CMPL] bin of 104254B                           [CMPL] tmp/file of 69240B^C
$                                               $ diff bin tmp/file
                                                $ echo $?
                                                0