When one computer communicates with another over the internet, it first sends a signal to open a line of communication. This initial signal is a “SYN packet,” where SYN stands for synchronize. This packet is the first step in a process governed by the Transmission Control Protocol (TCP). Think of it as knocking on a door; the SYN packet announces the intent to connect.
The Function of SYN in the TCP Handshake
The process of establishing a reliable connection between two devices is called the “three-way handshake,” and the SYN packet initiates this sequence. First, the initiating computer (the client) sends a SYN packet to the destination computer (the server). This packet contains a randomly generated initial sequence number (ISN) to track data, essentially telling the server, “I want to start a conversation, and I’ll begin numbering my messages from this point.”
If the server receives the SYN packet and is available, it responds with a SYN-ACK packet, which stands for synchronize-acknowledge. The “ACK” part confirms receipt of the client’s initial SYN, and the “SYN” part is the server’s own synchronization signal with its own ISN. This packet communicates, “I hear you, I’m ready to talk, and here is my starting number.”
Upon receiving the SYN-ACK from the server, the client sends one final packet to complete the connection: an ACK (acknowledge) packet. This packet confirms that the client received the server’s response. The three-way handshake is now complete, and a stable, full-duplex connection is established, meaning data can flow in both directions. This process ensures both devices are ready before any data is exchanged.
Configuring Firewalls to Permit SYN Packets
For a computer to accept incoming connections, its firewall must be configured to allow SYN packets to reach their intended application. Firewalls act as security guards for a network, inspecting traffic and blocking anything that doesn’t meet predefined security rules. By default, many firewalls use a “deny all” state for unsolicited traffic, meaning they automatically drop unexpected SYN packets as a protective measure.
To run a service accessible from the internet, like a website or online game, an administrator must create a specific exception in the firewall’s ruleset. This involves creating an “allow rule” or “opening a port.” A port is a numerical identifier that directs traffic to a specific service on the server; for example, web traffic commonly uses port 443.
Creating this rule tells the firewall that incoming SYN packets for a particular port are legitimate and should be passed to the application. This action of “enabling” SYN packets is a highly specific instruction, not a general setting. The rule specifies which port to open and can also restrict access to certain source IP addresses for added security. Without this permission, connection requests would be ignored.
SYN Flood Attacks and Mitigation
The structured nature of the TCP handshake, while reliable, can be exploited. A common denial-of-service (DoS) attack known as a SYN flood takes advantage of this process. An attacker sends a massive volume of SYN packets to a target server, often from spoofed IP addresses. The server responds to each SYN request with a SYN-ACK packet and allocates a small amount of memory for each pending connection.
Because the SYN packets came from nonexistent IP addresses, the server never receives the final ACK packet to complete the handshake. These connections are left in a “half-open” state, consuming server resources. The attacker continues sending SYN requests, causing the server’s connection table to fill up until it can no longer accept connections from legitimate users.
To defend against these attacks, several mitigation techniques are used. One method is rate-limiting, where hardware restricts the number of SYN packets accepted from a single source in a given period. Another technique involves using SYN cookies. With this method, the server encodes connection information into a cryptographic “cookie” in its SYN-ACK response, avoiding memory allocation until the final ACK arrives. The server then validates the cookie to establish the connection, neutralizing the attack.