Implementing a basic firewall in GoLang

A firewall is a crucial network security tool that helps protect computers and networks from malicious attacks. It acts as a barrier between a private internal network and the public internet, controlling incoming and outgoing traffic based on predetermined security rules.

This article will discuss how to implement a firewall using Go Lang.

Go is a modern programming language gaining popularity for network programming, including building firewalls.

Step 1: Define the firewall rules

The first step in building a firewall is to define the rules that will be used to filter network traffic. These rules should be based on the specific needs of your network. For example, block incoming traffic on specific ports, block traffic from specific IP addresses, or allow traffic only from specific IP addresses.

Step 2: Import necessary packages

In Go, several packages can be used to implement a firewall. The most common packages are net and os. The net package is used to handle network connections, and the os package is used to manage system-level functions.

import ( 
 "fmt" 
 "net" 
 "os" 
)

Step 3: Implement a function to filter traffic

The next step is to implement a function that will filter traffic based on the defined rules. This function will be called for each incoming network connection and will determine whether to allow or deny the relationship based on the rules.

func filterConnection(conn net.Conn, rules []string) { 
 remoteAddr := conn.RemoteAddr().String() 
 allowed := false 
 
 for _, rule := range rules { 
  if rule == remoteAddr { 
   allowed = true 
   break 
  } 
 } 
 
 if allowed { 
  fmt.Printf("Allowed connection from %s\n", remoteAddr) 
 } else { 
  fmt.Printf("Blocked connection from %s\n", remoteAddr) 
  conn.Close() 
 } 
}

This function takes two arguments: the incoming network connection (conn) and an array of rules (rules). It loops through each rule in the array and compares it to the remote address of the incoming connection. If there is a match, the connection is allowed. Otherwise, the connection is blocked, and the function closes the connection.

Step 4: Implement the main function

The final step is to implement the main function that will listen for incoming network connections and call the filterConnection function for each connection.

func main() { 
 port := "8080" 
 rules := []string{"192.168.0.1", "192.168.0.2"} 
 
 listener, err := net.Listen("tcp", ":"+port) 
 if err != nil { 
  fmt.Println("Error starting server:", err) 
  os.Exit(1) 
 } 
 fmt.Printf("Server listening on port %s\n", port) 
 
 for { 
  conn, err := listener.Accept() 
  if err != nil { 
   fmt.Println("Error accepting connection:", err) 
   continue 
  } 
  go filterConnection(conn, rules) 
 } 
}

In the main function, we first define the port that the firewall will listen on and the rules that will be used to filter traffic. We then create a network listener using the net.Listen function and start listening for incoming connections. We call each incoming connection the filterConnection function in a separate goroutine to handle the connection asynchronously.

Step 5: Test the firewall

To test the firewall, you can try connecting to the server from different IP addresses and verify that the connections are allowed or blocked based on the defined rules.

By following the steps outlined in this tutorial, you can build a basic firewall that can be customized to meet the specific needs of your network.

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Luis Soares

CTO | Head of Engineering | Fintech & Blockchain SME | Web3 | DeFi | Cyber Security

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