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

What you need

Changing Colors

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.

Starting a netcat Listener

In the 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

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 listening, and the other running netstat. The netstat window shows a process in the LISTEN state on port 5555, as shown below.

Blocking RST Packets

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.

Open a new Terminal window and execute these commands. The first command flushes out the old rules, the second command addrs a rule to drop all RST packets sent out, and the third command lists the firewall rules.

iptables -F
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

Execute this command to start scapy:

Configuring Scapy for Local Traffic

In the Terminal window, at the >>> prompt, execute these commands, as shown below. They are required for traffic to the local host; I don't really know why.

Creating an IP Object

In the Terminal window, at the >>> prompt, execute these commands to create a default IP object, and to see its attributes.
i = IP()
Scapy fills in both the source and destination addresses with the loopback address, as shown below.

Creating a TCP Object

Execute these commands to create an object named t of type TCP with a destination port of 5555
t = TCP()
t.dport = 5555
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.

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:

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

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, open a new Terminal window and 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 use on a server, because it to makes 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

In scapy, execute this command again:

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

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

The window watching netstat now shows a connection in the SYN-RECV state, as shown below.

Analyzing the SYN/ACK Packet

Examine the reply in Scapy, as shown above. Note these items: 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.

Sending an ACK Packet from the Linux Sender Machine

On the Linux machine, at the >>> prompt, execute these commands.

Replace "1" with your ack number.

Replace "2510685632" with the value you calculated above; your seq value plus 1.

t.flags = "A"
t.seq = 1
t.ack = 2379023435

Observing the Session

On the Linux target machine, netstat should now show an ESTABLISHED connection, as shown below:

Saving the Screen Image

Make sure you can see the ESTABLISHED connection.

Save a screen image with the filename Proj 10 from Your Name.

Turning in Your Project

Email the image to with a Subject line of Proj 10 from Your Name.


Scapy fails when sending HTTP GET request to localhost

Last modified 7-5-17