STUN server: operation, uses and technique

Today, many Internet connections pass through routers using network address translation (NAT). While this saves IPv4 addresses, it does complicate the establishment of direct connections between terminals. It is precisely in this context that the STUN server intervenes, allowing real-time communication to function despite these constraints.

What is a STUN server?

A STUN server is a specialized service within the Internet infrastructure that helps to troubleshoot connection issues related to network configurations. STUN means Session Traversal Utilities for NAT. In French, it can be translated as “assistance tools for crossing the NAT during sessions”. This name describes its role: STUN helps applications overcome network obstacles created by NAT routers to allow connections to be established between two endpoints.

STUN is standardized in the RFC 8489. Its main purpose is to provide customers with information about their network visibility on the Internet. These include the public IP address as well as the port under which a device can be accessed from the outside. This information is essential because many endpoints are not directly connected to the Internet, but are behind routers using Network Address Translation, or NAT.

NAT is widely used because it allows multiple devices to share a single public IP address. However, it complicates direct communication between two endpoints, since internal addresses and ports are not visible from the outside. A STUN server addresses precisely this problem by serving as a public internet reference point. It does not itself establish connections or carry useful data: it only provides the necessary information on which other technologies and protocols can rely.

To better understand how a STUN server works, we can compare it to a public information terminal: a client who does not know exactly under what address it is accessible from the outside can query it directly, a bit like someone trying to find out the number under which it is listed in a directory. This analogy makes the technical process much more concrete.

Step 1: Send the STUN request

Concretely, the process begins when the customer sends a STUN request to the server. This request leaves the local network and is transmitted to the public network via the router. If the client is behind a NAT router, it modifies the connection by replacing the internal, private IP address with a public IP address, while assigning an external port. For the customer, this transformation generally remains invisible, but it determines how he appears on the Internet.

Step 2: receiving and processing the request

The STUN server receives the request over the public Internet and acts as a neutral observation point. It scans the IP address and port from which the request reaches it, that is to say the information under which the client is accessible from the outside, then sends them back to the customer. It then returns this data to the client in its response. The customer thus has reliable information on how he is perceived by other systems on the Internet.

Step 3: Initiating the connection

Using this information, the customer can then pass this data on to potential communication partners. The objective is toestablish a direct connection between two terminalswithout all traffic having to pass through a central server. The success of this approach, however, depends on the type of NAT used and its rules. In simple environments this method works reliably, while more restrictive NAT configurations may limit direct connection. A STUN server therefore does not impose any connection on its own: it only provides the information necessary to allow other components to establish communication.

What is a STUN server used for?

STUN servers are mainly used in real-time applications, where direct communication is essential. A typical example is Internet telephony via VoIP, where a STUN server facilitates the reliable establishment of voice communications despite NAT routers, providing clients with the necessary information for a direct Peer-to-Peer connection. Video chats and video conferencing also often rely on STUN to ensure smooth transmission of image and sound between participants.

Additionally, many online games use STUN to establish connections between players. This significantly reduces latency, since data traffic is not routinely routed through central relay servers. At the same time, STUN saves bandwidth and limits server costs, because it does not itself transmit media or application data, but only serves as a source of information for establishing the connection.

In the world of WebRTC applications in particular, STUN is a standard component. Without a STUN server, many connections would have to be handled entirely through intermediate servers, which would impact performance, scalability, and user experience.

What hardware is needed to install a STUN server?

A STUN server requires few material resources. As it is limited to receiving and processing small requests, it requires very little computing power. RAM requirements also remain minimal. On the other hand, a stable, reliable and publicly accessible network connection is essential.

For this reason, virtual private servers (VPS) constitute a popular choice. They have fixed public IP addresses and remain permanently accessible. Additionally, VPS can be easily scaled if the number of users or requests increases. Costs remain moderate, as no dedicated hardware is required. Thanks to virtualization, VPS are quickly deployed, simple to administer and suitable for operating a STUN server.