I’m working on a project at the moment that requires me to observe traffic from an iOS/Android app to various external IPs.

The easiest way to do this is to setup a fake WiFi access point and use Wireshark to sniff the traffic. This is very easy in Kali Linux.

1. Connect the Kali box to the Internet

On my machine, this is as simple as connecting to my WiFi network “DoingAJob5G” using the built-in wireless card on my x220. I use the GUI provided with Kali.

Using ifconfig I can see that this adapter is called wlan0.

You could use wired Ethernet, then in all likelihood this will be eth0 instead.

2. Connect an external WiFi adapter that is supported by hostapd

I’m using a USB TP-LINK TL-WN722N which is using an Atheros AR9271 chipset. These are cheap (£8-£10), powerful and reliable.

I suspect many USB WiFi adapters are compatible with hostapd, unfortunately I can’t see a clear source documenting which ones.

Check it works by connecting to any network using Kali’s GUI. This will save you hassle later if there are any driver or hardware issues.

3. Bring up the new wireless interface.

Use ifconfig -a to see the new wireless interface name:

wlan3     Link encap:Ethernet  HWaddr c0:4a:00:1e:64:fd  
          BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

Bring this up as the gateway for your new wireless network. I am using 10.0.0.1/24 simply to avoid any chance of confusion with my internal NATed 192.168.0.1/24 network.

root@kali:~# ifconfig wlan3 10.0.0.1/24 up
root@kali:~# ifconfig wlan3
wlan3     Link encap:Ethernet  HWaddr c0:4a:00:1e:64:fd  
          inet addr:10.0.0.1  Bcast:10.0.0.255  Mask:255.255.255.0
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

4. Configure and run DHCP and DNS services

DHCP assigns IP addresses when clients connect, and DNS provides resolution of names to IPs.

Most wireless clients expect DHCP by default, so it is convenient to run a DHCP server. You can manually set IP addresses, but it’s really easier to do DHCP.

Running our own DNS server means that we can easily intercept and alter DNS queries, which can assist in setting up man-in-the-middle attacks.

A piece of software called dnsmasq does both DHCP and DNS and is very simple to setup.

First, install dnsmasq:

apt-get install dnsmasq

Next, create a config file dnsmasq.conf as follows:

interface=wlan3
dhcp-range=10.0.0.10,10.0.0.250,12h
dhcp-option=3,10.0.0.1
dhcp-option=6,10.0.0.1
server=8.8.8.8
log-queries
log-dhcp

This is about as simple as it gets. Only listen on wlan3, our additional wireless adapter. Hand out DHCP addresses from 10.0.0.10-10.0.0.250. DHCP option 3 is the gateway, DHCP option 6 is the DNS server – both of these should be set to our wlan3 IP of 10.0.0.1. server specifies upstream DNS servers that will handle most DNS queries – I have provided Google’s DNS server of 8.8.8.8. Finally, log DNS queries and DHCP requests – this just makes it easier to check everything is working.

We also want to create a file fakehosts.conf to allow us to spoof certain DNS requests:

10.0.0.9 neohub.co.uk

This will cause the dnsmasq DNS server to respond with 10.0.0.9 to any request for neohub.co.uk.

We then need to bring dnsmasq up. I want it to run with output to stderr, so this is done as follows:

dnsmasq -C dnsmasq.conf -H fakehosts.conf -d

5. Configure and run hostapd

Next, we need to get our wireless adapter to run as a access point.

hostapd allows us to do this.

Install hostapd:

apt-get install hostapd

Create a config file hostapd.conf:

interface=wlan3
driver=nl80211
ssid=Kali-MITM
channel=1

Again – really simple. Use our additional wireless adapter wlan3 with the nl80211 drivers (which seem to cover pretty much all modern adapters than can be APs), set the SSID to Kali-MITM and set the channel to 1. There is no encryption etc. but I really don’t need or want it for sniffing traffic.

Then start hostapd:

root@kali:~# hostapd ./hostapd.conf 
Configuration file: ./hostapd.conf
Failed to update rate sets in kernel module
Using interface wlan3 with hwaddr c0:4a:00:1e:64:fd and ssid 'Kali-MITM'

6. Setup routing for the access point

You want a very simple setup at the moment – act as a basic NAT gateway between wlan3 and wlan0.

Without going into any detail, the following commands will set this up:

sudo sysctl -w net.ipv4.ip_forward=1
sudo iptables -P FORWARD ACCEPT
sudo iptables --table nat -A POSTROUTING -o wlan0 -j MASQUERADE

At this stage, you should now be able to connect to Kali-MITM, get an IP address, and start using the Internet.