Project 10: TCP Handshake with scapy (15 pts.)

What you need

Two Linux machines, real or virtual. I used two copies of Kali Linux.
You cannot use Windows for either role, because Microsoft has made the TCP handshake tineout too short and no longer allows the user to adjust it.

How to Make a Second Kali VM

You should have a Kali file with a file extension of .7z, which you downloaded in project 1. If you don't have it, download it here:

https://www.offensive-security.com/kali-linux-vmware-virtualbox-image-download/

Any version of Kali will do; in general, I recommend the 32-bit version.

Unzip the 7-zip file into a new folder. In S214, use the VMs drive, not drive C:.

Launch VMware Player and click "Open a Virtual Machine". Browse to the folder you unzipped the VM into, and launch the VM.

To run two VMs at the same time, launch two instances of VMware Player.

Starting a netcat Listener on the Linux Target Machine

On the Kali Linux target machine, in a Terminal window, execute this command (notice that the switch is a lowercase L):

nc -l -p 5555

Note: on some Linux versions you need to use this command instead: nc -l 5555

On the Linux target machine, open another Terminal window and execute this command:

watch "netstat -an | grep 5555"

This gives a realtime, constantly updated, report of connections to port 5555.

You should have two windows open now, one with netcat running, and the other running netstat. The netstat window shows a process in the LISTEN state on port 5555, as shown below on this page.

Finding the Target Linux Machine's IP Address

Open a third Terminal window on the Target machine.

Execute this command to find your IP address.

ifconfig

When I did it, my IP address was 172.16.1.175, as highlighted in the image below.

Changing Colors on the Linux Sender Machine

On the Linux Sender machine, open a Terminal window. From the menu bar, click Edit, "Profile Preferences", Colors, "Black on light yellow". Close the "Editing profile" windoow.

This will make it easier to see scapy's output.

Blocking RST Packets on the Linux Sender Machine

Scapy will be sending a "Raw" TCP SYN packet, but the Linux kernel will be offended by that, feeling like only kernel routines should be opening connections.

Then, like the Great Firewall of China, it will send a RST packet to cancel the improper connection.

To prevent that, we need to add an iptables firewall rule.

On the Linux Sender machine, open a Terminal window. From the menu bar, click Edit, "Profile Preferences", Colors. I used "Black on light yellow". execute these commands.

iptables -A OUTPUT -p tcp --tcp-flags RST RST -j DROP
iptables -L

You should see one rule in the OUTPUT section dropping RST packets, as shown below on this page.

Starting scapy on the Linux Sender Machine

On the Linux machine, use this command to start scapy:

scapy

If the colors are difficult to see, adjust them by clicking Edit, "Profile Preferences", Colors. I used "Black on light yellow".

Creating an IP Object on the Linux Sender Machine

In the Linux sender machine, in the Terminal window, at the >>> prompt, execute these commands, replacing the IP address in the second command with the IP address of your Linux target machine:

i = IP() i.dst = "172.16.1.175"
i.display()

scapy fills in both the source and destination addresses, as shown below on this page.

Creating a TCP Object on the Linux Sender Machine

Use these commands to create an object named t of type TCP with a destination port of 5555 and the flags set to S (for SYN):

t = TCP() t.dport = 5555 t.flags = "S"
t.display()

The properties of your TCP object should look like the example shown below on this page. Notice that the seq and ack numbers are both 0, as outlined in the image below.

Sending a SYN Packet from the Linux Sender Machine

Use this command to send the packet onto the network and listen to a single packet in response.

Note that the third character is the numeral 1, not a lowercase L:

sr1(i/t)

This command sends and receives one packet, of type IP at layer 3 and TCP at layer 4. As you can see in the image below, the response is shown, with flags=SA (a SYN/ACK reply). The ack number is 1, indicating that this is a reply to your SYN packet with seq = 0.

Find the seq number in the SYN/ACK reply. It's highlighted in the figure below. Later, you will add one to this number to find the ack value you must use in the ACK packet that completes the handshake.

Observing TCP Timeout

On the Target Linux machine, bring the window showing netstat to the front.

You should see a connection in the state SYN_RECV, as highlighted in the image below.

This means the Target has received a SYN, amd sent a SYN/ACK, and it is now waiting for the ACK to complete the handshake.

Wait for 60 seconds. You should see the connection time out--it will vanish from the netstat list.

This is the default setting in Kali Linux, to wait 60 seconds before giving up on the connection. You can complete the project this way if you work fast, but for most students 60 seconds isn't long enough to complete the handshake.

Lengthening the TCP Timeout on the Linux Target Machine

This is why hackers love Linux--you can adjust anything about it. There are several TCP timing settings, but the one important for this project is tcp_synack_retries --"The maximum number of times a SYN/ACK segment for a passive TCP connection will be retransmitted."

This setting defaults to 5, which means sessions time out after 60 seconds.

On the Linux target machine, in the third, unused Terminal window, excute this command to lengthen the timeout to 10 minutes:

echo 10 > /proc/sys/net/ipv4/tcp_synack_retries

Note: this setting is not wise for normal use, because I expect it to make your machine far more vulnerable to SYN floods. But it's not permanent--when the machine restarts it will reset to the default value of 5 (one minute).

Re-sending a SYN from the Linux Sender Machine

In scapy, execute this command again:

Note that the third character is the numeral 1, not a lowercase L:

sr1(i/t)

On the Sender Linux machine, you should see a SYN/ACK reply as shown below.

Look at your Target machine. It should now show a connection in the SYN-RECV state again:

Analyzing the SYN/ACK Packet

On your Linux Sender machine, examine the reply in Scapy, as shown below.

The important parts of the reply are highlighted in blue` above:

seq number. In my example, it is 2510685631. (The "L" indicates a long integer and is not part of the number.)
ack number (in my example, it is 1)
flags showing SA (SYN/ACK)

Find your ack number and make a note of it.

Find your seq number and add one to it. Ignore the "L" at the end.