Application layer gateway facts for kids

An application layer gateway (often called an ALG) is a special kind of firewall. Think of a firewall as a security guard for a computer network. It decides what information can come in and go out. An ALG is unique because it can understand and help manage certain tricky ways computers talk to each other.

What is a Firewall?

A firewall is like a security guard at the entrance of a building. It checks everyone who tries to enter or leave. In the world of computers, a firewall checks all the data packets that try to go in or out of a network. It makes sure only safe and allowed information passes through. This helps protect computers from harmful programs or unwanted visitors.

How Firewalls Protect Your Network

Firewalls work by following a set of rules. These rules tell the firewall what kind of data is allowed and what should be blocked. For example, a firewall might block all incoming connections except for those from a trusted website.

• Packet Filtering: This is the simplest type. It looks at basic information like the source and destination of a data packet.
• Stateful Packet Inspection: This is smarter. It remembers past connections. If you ask for information from a website, the firewall remembers your request. It then allows the reply from that website to come back to you. It's like a bouncer at a club who remembers who went out for a moment and lets them back in without a new ticket.

Why Do We Need Application Layer Gateways?

Most computer programs use specific "doors" or ports to send and receive information. For example, web browsing usually uses port 80 or 443. Firewalls can easily manage these fixed ports.

However, some protocols (which are like languages computers use to talk) are more complex. A protocol called TFTP (Trivial File Transfer Protocol) is one example. When TFTP starts a conversation, it might pick a random port for the actual data transfer. This makes it hard for a regular firewall to know which ports to open.

How ALGs Help

This is where an Application Layer Gateway comes in handy. An ALG understands these complex protocols. When a program using TFTP wants to send a file, the ALG watches the conversation. It sees which random port TFTP chooses for the data. Then, the ALG tells the firewall to temporarily open that specific port.

• It acts like a translator and guide for the firewall.
• It ensures that only the necessary ports are opened, and only for the specific conversation.
• This makes the network more secure because random ports aren't left open for just anyone to use.

ALG vs. Proxy Firewall

It's important to know that an Application Layer Gateway is different from a proxy firewall.

• Application Layer Gateway (ALG): An ALG helps a regular firewall manage tricky protocols by understanding the communication and telling the firewall which ports to open. It doesn't usually look deep inside the actual data being sent.
• Proxy Firewall: A proxy firewall is like a middleman. All network traffic goes through it. It not only checks the ports but also looks closely at the actual content (the "payload data") of the information being sent. It can even stop certain types of content from passing through, like specific files or harmful code.

Both ALGs and proxy firewalls help make networks safer, but they do it in slightly different ways. ALGs are specialized helpers for firewalls dealing with dynamic port usage, while proxy firewalls are more like full-service security checkpoints.