What is a STUN server?

Modern internet con­nec­tions often run through routers that use Network Address Trans­la­tion (NAT). This helps reduce the number of IPv4 addresses needed but makes direct con­nec­tions between devices more difficult. A STUN server helps solve this problem, ensuring real-time com­mu­ni­ca­tion is still possible.

What is a STUN server?

A STUN server is a network service that helps resolve con­nec­tion issues caused by NAT. STUN stands for Session Traversal Utilities for NAT. As the name suggests, it allows ap­pli­ca­tions to work around network barriers created by NAT routers so two endpoints can connect.

STUN is defined in RFC 8489. Its main role is to tell a client how it appears on the internet. This includes the public IP address and port that other systems can use to reach it. This in­for­ma­tion is important because many devices are not directly connected to the internet. Instead, they sit behind routers that use Network Address Trans­la­tion, or NAT for short.

NAT allows multiple devices to share a single public IP address. While efficient, it hides internal IP addresses and ports from external networks. This makes direct con­nec­tions between devices harder to establish. A STUN server solves this by acting as a public reference point. It does not handle the con­nec­tion or transmit user data. Instead, it provides the in­for­ma­tion other tech­nolo­gies need to establish a con­nec­tion.

How does a STUN server work?

Think of a STUN server as a public in­for­ma­tion service on the internet. If a client isn’t sure how it can be reached from the internet, it sends a query—like someone checking how their phone number appears in the phone book. This analogy makes it easier to grasp how a STUN server helps devices figure out their public-facing address.

Step 1: STUN request

The process begins with the client sending a STUN request to the server. The request leaves the local network and passes through the router to reach the internet. If the client is behind a NAT router, the router replaces the private IP address with a public one and assigns an external port. Although the client doesn’t see this process, it de­ter­mines how the client appears on the internet.

Step 2: Server response

The STUN server receives the request and checks which IP address and port it came from. In other words, it de­ter­mines how the client appears on the internet. The STUN server then sends this in­for­ma­tion back to the client. The client now knows which address and port other systems can use to reach it.

Step 3: Es­tab­lish­ing the con­nec­tion

With this in­for­ma­tion, the client can now share its public address with another device. This allows both endpoints to establish a direct peer-to-peer con­nec­tion instead of routing all traffic through a central server. Whether this works depends on the type of NAT and how re­stric­tive it is. In simple network setups, direct con­nec­tions usually succeed. In more re­stric­tive en­vi­ron­ments, ad­di­tion­al methods may be required. The STUN server itself does not create a con­nec­tion. It simply provides the data needed to try to establish one.

What is a STUN server used for?

STUN servers are primarily used in real-time ap­pli­ca­tions where direct, low-latency com­mu­ni­ca­tion is critical. A typical use case is VoIP. In this case, STUN helps establish reliable voice con­nec­tions by providing the in­for­ma­tion needed for a direct peer-to-peer con­nec­tion, even when NAT routers are involved. Video calls and con­fer­ence tools also rely on STUN to transmit audio and video between par­tic­i­pants.

Online games use STUN for the same reason. It helps connect players, reducing latency since the data doesn’t have to be routed through central relay servers all the time. STUN also helps save bandwidth and lower server costs because it doesn’t handle media or ap­pli­ca­tion data. It’s just providing the in­for­ma­tion needed to set up the con­nec­tion.

Another key area is WebRTC. STUN is a core component in WebRTC ap­pli­ca­tions, allowing browsers and apps to establish real-time con­nec­tions. Without STUN servers, many of these con­nec­tions would have to go through in­ter­me­di­ate servers, which would slow things down and hurt both per­for­mance and scal­a­bil­i­ty.

What hardware do I need for a STUN server?

A STUN server has very low hardware re­quire­ments. It only handles small requests and responses, so hardly any CPU power or RAM is needed. A stable, reliable, and publicly ac­ces­si­ble network con­nec­tion is far more important than powerful hardware.

For this reason, VPS servers are a popular choice. They typically come with fixed public IP addresses and are always reachable from the internet. VPSs can also be scaled easily, so you can adjust resources as the number of users or requests grows over time. Costs usually remain man­age­able since there’s no need for any dedicated hardware. Thanks to vir­tu­al­iza­tion, VPSs are quick to set up, easy to manage and ideal for running a STUN sever.