SYN Flood

A SYN flood is a form of DoS attack in which an attacker sends a succession of SYN requests to a target's server in an attempt to consume enough server resources to make the system unresponsive to legitimate traffic^[1].

A SYN request and a SYN packet are the same things

How does a SYN flood attack work?

SYN flood attacks work by exploiting the handshake process of a TCP connection. Under normal conditions, TCP exhibits three distinct processes in order to make a connection (Figure 1a).

1. The client requests a connection by sending a SYN (synchronize) packet to the server.
2. The server then responds with a SYN-ACK packet, in order to ACK (acknowledge) the communication.
3. The client sends an ACK packet to acknowledge the receipt of the packet from the server and the connection is established.

After completing this sequence of packet sending and receiving, the TCP connection is open and able to send and receive data. This is called the TCP three-way handshake. This technique is the foundation for every connection established using TCP.

To create a DoS, an attacker exploits the fact that after an initial SYN packet has been received, the server will respond with one or more SYN-ACK packets and wait for the final step in the handshake. Cloudflare^[2] describes how it works (Figure 1b):

1. The attacker sends a high volume of SYN packets to the targeted server, often with spoofed IP addresses.
2. The server then responds to each one of the connection requests and leaves an open port ready to receive the response.
3. While the server waits for the final ACK packet, which never arrives, the attacker continues to send more SYN packets. The arrival of each new SYN packet causes the server to temporarily maintain a new open port connection for a certain length of time, and once all the available ports have been utilized the server is unable to function normally.

Network Setup

The attack in this repository was conducted in a VM. Three machines are needed to carry out the attack. One machine is used as the attacker, another machine is used as the victim (i.e. the server), and the third machine is used as an observer (i.e. the client).

To set up the attack, three VMs can be set up on the same host computer. Alternatively, two VMs can be set up on a host computer with the host machine itself acting as the third computer.

This attack was implemented under the assumption that the attackers are on the same physical network as the victims - thus simplifying the task of determining TCP sequence numbers and source port numbers. Sniffer tools can be used to collect the necessary information.

Attack

The attack in this repository was conducted using the netwox tool. Then, a sniffer tool such as Wireshark^[3] was used to capture the attacking packets. While the attack was ongoing, netstat -na was run on the victim machine to compare the result with that from before the attack.

# check the size of the queue for holding half-open connections
sudo sysctl -q net.ipv4.tcp_max_syn_backlog

# check the current usage of the queue;
# i.e., the number of half-open connections associated with some listening port
netstat -na

# one netwox tool that may be useful is tool number 76: synflood
netwox 76 --help

Countermeasures

SYN Cookie:

The attack in this repository was conducted by manipulating SYN cookies.

The use of SYN cookies allows a server to avoid dropping connections when the SYN queue fills up. Instead of storing additional connections, the SYN queue entry is encoded into the sequence number sent in the SYN-ACK response. If the server then receives a subsequent ACK response from the client with the incremented sequence number, the server is able to reconstruct the SYN queue entry using information encoded in the TCP sequence number and proceed as usual with the connection.

While this mitigation effort does lose some information about the TCP connection, it is better than allowing DoS to befall legitimate users as a result of an attack.

SYN cookies were created by D. J. Bernstein^[4] in direct response to SYN flooding.

Use the sysctl command to turn on/off the SYN cookie mechanism:

sudo sysctl -a | grep cookie                 # Display the SYN cookie flag
sudo sysctl -w net.ipv4.tcp_syncookies=0     # turn off SYN cookie
sudo sysctl -w net.ipv4.tcp_syncookies=1     # turn on SYN cookie

SYN cookies are one of many countermeasures that can be implemented to stop SYN flooding. Other mitigations include increasing the maximum size of the queue, reducing the SYN-RECEIVED timer, and recycling the oldest half-open TCP connection.

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References

1. ^ "CERT Advisory CA-1996-21 TCP SYN Flooding and IP Spoofing Attacks" Carnegie Mellon University

2. ^ "SYN Flood Attack" Cloudflare

3. ^ "About Wireshark" Wireshark Foundation

4. ^ "SYN cookies" D. J. Bernstein

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Thank you for your interest, this project was fun and insightful!