Dnschef
DNSChef - DNS proxy for Penetration Testers and Malware Analysts
Install / Use
/learn @iphelix/DnschefREADME
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D O C U M E N T A T I O N
DNSChef is a highly configurable DNS proxy for Penetration Testers and Malware Analysts. A DNS proxy (aka "Fake DNS") is a tool used for application network traffic analysis among other uses. For example, a DNS proxy can be used to fake requests for "badguy.com" to point to a local machine for termination or interception instead of a real host somewhere on the Internet.
There are several DNS Proxies out there. Most will simply point all DNS queries a single IP address or implement only rudimentary filtering. DNSChef was developed as part of a penetration test where there was a need for a more configurable system. As a result, DNSChef is cross-platform application capable of forging responses based on inclusive and exclusive domain lists, supporting multiple DNS record types, matching domains with wildcards, proxying true responses for nonmatching domains, defining external configuration files, IPv6 and many other features. You can find detailed explanation of each of the features and suggested uses below.
The use of DNS Proxy is recommended in situations where it is not possible to force an application to use some other proxy server directly. For example, some mobile applications completely ignore OS HTTP Proxy settings. In these cases, the use of a DNS proxy server such as DNSChef will allow you to trick that application into forwarding connections to the desired destination.
Setting up a DNS Proxy
Before you can start using DNSChef, you must configure your machine to use a DNS nameserver with the tool running on it. You have several options based on the operating system you are going to use:
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Linux - Edit /etc/resolv.conf to include a line on the very top with your traffic analysis host (e.g add "nameserver 127.0.0.1" if you are running locally). Alternatively, you can add a DNS server address using tools such as Network Manager. Inside the Network Manager open IPv4 Settings, select Automatic (DHCP) addresses only or Manual from the Method drop down box and edit DNS Servers text box to include an IP address with DNSChef running.
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Windows - Select Network Connections from the Control Panel. Next select one of the connections (e.g. "Local Area Connection"), right-click on it and select properties. From within a newly appearing dialog box, select Internet Protocol (TCP/IP) and click on properties. At last select Use the following DNS server addresses radio button and enter the IP address with DNSChef running. For example, if running locally enter 127.0.0.1.
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OS X - Open System Preferences and click on the Network icon. Select the active interface and fill in the DNS Server field. If you are using Airport then you will have to click on Advanced... button and edit DNS servers from there. Alternatively, you can edit /etc/resolv.conf and add a fake nameserver to the very top there (e.g "nameserver 127.0.0.1").
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iOS - Open Settings and select General. Next select on Wi-Fi and click on a blue arrow to the right of an active Access Point from the list. Edit DNS entry to point to the host with DNSChef running. Make sure you have disabled Cellular interface (if available).
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Android - Open Settings and select Wireless and network. Click on Wi-Fi settings and select Advanced after pressing the Options button on the phone. Enable Use static IP checkbox and configure a custom DNS server.
If you do not have the ability to modify device's DNS settings manually, then you still have several options involving techniques such as ARP Spoofing, Rogue DHCP and other creative methods.
At last you need to configure a fake service where DNSChef will point all of the requests. For example, if you are trying to intercept web traffic, you must bring up either a separate web server running on port 80 or set up a web proxy (e.g. Burp) to intercept traffic. DNSChef will point queries to your proxy/server host with properly configured services.
Running DNSChef
DNSChef is a cross-platform application developed in Python which should run on most platforms which have a Python interpreter. You can use the supplied dnschef.exe executable to run it on Windows hosts without installing a Python interpreter. This guide will concentrate on Unix environments; however, all of the examples below were tested to work on Windows as well.
Let's get a taste of DNSChef with its most basic monitoring functionality. Execute the following command as root (required to start a server on port 53):
# ./dnschef.py
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iphelix@thesprawl.org
[*] DNSChef started on interface: 127.0.0.1
[*] Using the following nameservers: 8.8.8.8
[*] No parameters were specified. Running in full proxy mode
Without any parameters, DNSChef will run in full proxy mode. This means that all requests will simply be forwarded to an upstream DNS server (8.8.8.8 by default) and returned back to the quering host. For example, let's query an "A" record for a domain and observe results:
$ host -t A thesprawl.org
thesprawl.org has address 108.59.3.64
DNSChef will print the following log line showing time, source IP address, type of record requested and most importantly which name was queried:
[23:54:03] 127.0.0.1: proxying the response of type 'A' for thesprawl.org
This mode is useful for simple application monitoring where you need to figure out which domains it uses for its communications.
DNSChef has full support for IPv6 which can be activated using -6 or --ipv6* flags. It works exactly as IPv4 mode with the exception that default listening interface is switched to ::1 and default DNS server is switched to 2001:4860:4860::8888. Here is a sample output:
# ./dnschef.py -6
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iphelix@thesprawl.org
[*] Using IPv6 mode.
[*] DNSChef started on interface: ::1
[*] Using the following nameservers: 2001:4860:4860::8888
[*] No parameters were specified. Running in full proxy mode
[00:35:44] ::1: proxying the response of type 'A' for thesprawl.org
[00:35:44] ::1: proxying the response of type 'AAAA' for thesprawl.org
[00:35:44] ::1: proxying the response of type 'MX' for thesprawl.org
NOTE: By default, DNSChef creates a UDP listener. You can use TCP instead with the --tcp argument discussed later.
Intercept all responses
Now, that you know how to start DNSChef let's configure it to fake all replies to point to 127.0.0.1 using the --fakeip parameter:
# ./dnschef.py --fakeip 127.0.0.1 -q
[*] DNSChef started on interface: 127.0.0.1
[*] Using the following nameservers: 8.8.8.8
[*] Cooking all A replies to point to 127.0.0.1
[23:55:57] 127.0.0.1: cooking the response of type 'A' for google.com to 127.0.0.1
[23:55:57] 127.0.0.1: proxying the response of type 'AAAA' for google.com
[23:55:57] 127.0.0.1: proxying the response of type 'MX' for google.com
In the above output you an see that DNSChef was configured to proxy all requests to 127.0.0.1. The first line of log at 08:11:23 shows that we have "cooked" the "A" record response to point to 127.0.0.1. However, further requests for 'AAAA' and 'MX' records are simply proxied from a real DNS server. Let's see the output from requesting program:
$ host google.com localhost
google.com has address 127.0.0.1
google.com has IPv6 address 2001:4860:4001:803::1001
google.com mail is handled by 10 aspmx.l.google.com.
google.com mail is handled by 40 alt3.aspmx.l.google.com.
google.com mail is handled by 30 alt2.aspmx.l.google.com.
google.com mail is handled by 20 alt1.aspmx.l.google.com.
google.com mail is handled by 50 alt4.aspmx.l.google.com.
As you can see the program was tricked to use 127.0.0.1 for the IPv4 address. However, the information obtained from IPv6 (AAAA) and mail (MX) records appears completely legitimate. The goal of DNSChef is to have the least impact on the correct operation of the program, so if an application relies on a specific mailserver it will correctly obtain one through this proxied request.
Let's fake one more request to illustrate how to target multiple records at the same time:
# ./dnschef.py --fakeip 127.0.0.1 --fakeipv6 ::1 -q
[*] DNSChef started on interface: 127.0.0.1
[*] Using the following nameservers: 8.8.8.8
[*] Cooking all A replies to point to 127.0.0.1
[*] Cooking all AAAA replies to point to ::1
[00:02:14] 127.0.0.1: cooking the response of type 'A' for google.com to 127.0.0.1
[00:02:14] 127.0.0.1: cooking the response of type 'AAAA' for google.com to ::1
[00:02:14] 127.0.0.1: proxying the response of type 'MX' for google.com
In addition to the --fakeip flag, I have now specified --fakeipv6 designed to fake 'AAAA' record queries. Here is an updated program output:
$ host google.com localhost
google.com has address 127.0.0.1
google.
