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Why a blanket STUN block can break VoIP or WebRTC
STUN helps an endpoint discover the public-facing address and port assigned by a NAT. ICE uses STUN for connectivity checks and can use TURN to relay traffic when a direct path is unavailable. As RFC 8489 puts it, “STUN is not a NAT traversal solution by itself. Rather, it is a tool to be used in the context of a NAT traversal solution.” RFC 8489
Blocking STUN can remove candidate information or connectivity checks that an ICE deployment relies on. It does not mean every call will fail: the effect depends on the application’s configuration, available host, server-reflexive, and relayed candidates, and how the network handles their paths. RFC 8445
Choose what “unneeded” means for your network
Decide which policy outcome you want before writing firewall rules. Restricting unknown public services, requiring mediated egress, and denying a particular application are different controls.
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| Policy goal | Practical approach | Main trade-off |
|---|---|---|
| Block unapproved public STUN | Allow known application or service endpoints and restrict other destinations. | Endpoint changes or undocumented services may interrupt connectivity. |
| Require organizationally controlled egress | Configure an approved proxy or enterprise TURN service, test it, then restrict direct paths as required. | The approved route must support the transports and traffic the applications need. |
| Deny a particular application | Use an application-aware policy where available rather than treating a port-only STUN rule as a complete application block. | Port rules may not identify all application configurations or traffic. |
RFC 8828 describes directing external WebRTC traffic through an organizational proxy or enterprise TURN server as a policy option; it does not establish one universal enterprise allowlist. RFC 8828
Which ports does STUN use?
RFC 8489 gives default ports of 3478 for STUN over UDP and TCP, and 5349 for STUN over TLS or DTLS. These are defaults, not a complete firewall allowlist: deployments can configure other ports, and TURN relay allocations and media paths have additional requirements. Confirm the actual service configuration rather than relying on port numbers alone. RFC 8489 also says server operators should publish the listening port in DNS service records. RFC 8489 RFC 8835
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Can you block UDP and still use WebRTC?
WebRTC implementations must support TURN over TCP and TURN over TLS-over-TCP for networks that block UDP, according to RFC 8835. That requirement does not create a working fallback by itself: the application must have a usable TURN configuration, and the firewall must permit the endpoint-to-relay route. TURN can carry relayed media, so permitting a STUN request alone does not ensure that calls or media will pass. RFC 8835 RFC 8656
Inventory services before changing the firewall
- List the VoIP and WebRTC applications in scope, along with their configured STUN and TURN server names, addresses, transports, and ports.
- Confirm those requirements with each application or service owner. Do not assume the RFC default ports describe the deployment’s full policy.
- Record whether the current design permits direct ICE connectivity or requires traffic to use an approved proxy or enterprise relay.
- For mediated egress, configure and verify the approved proxy or TURN service before removing direct paths.
Test the rule against real call paths
Validate the proposed policy on representative network conditions before broad enforcement. The relevant cases depend on your organization, but commonly include:
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- Two endpoints on the same LAN.
- Endpoints behind different NATs.
- A network that restricts or blocks UDP.
- Remote-access or split-tunnel paths used by staff.
Check candidate gathering and connectivity, then place calls and verify media in both directions. Signaling success or an ICE negotiation alone does not prove that the intended media path works. Roll the rule out to a small group, monitor call failures and quality, and keep a rollback path available.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Balance connectivity, privacy, and control
Compare rules by destination scope (unrestricted external services, approved endpoints, or an enterprise-controlled service), allowed transports, and whether direct paths are permitted or traffic must use a relay. Also consider whether endpoint address information or media should traverse third-party infrastructure. The standards describe protocol options, not vendor performance rankings or a single policy suitable for every organization. RFC 8489 RFC 8835 RFC 8828 RFC 8656
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For capacity planning, RFC 8445 gives an example of 1.7 bps per user, or 1.7 Mbps for one million users, under the assumptions in that 2018 example. This is not a general measured traffic rate or a current forecast; the RFC notes that TURN traffic is more substantial because it carries relayed data. RFC 8445
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