The Tool Desk
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What STUN traffic can—and cannot—tell you
STUN (Session Traversal Utilities for NAT) helps other protocols work through network address translation. As the IETF puts it, “Session Traversal Utilities for NAT (STUN) is a tool for other protocols to deal with Network Address Translation (NAT).” It can help an application discover a NAT-mapped address and port, check connectivity, or maintain a NAT binding. See RFC 8489.
Legitimate applications may use STUN as part of ICE, SIP Outbound, or other usage contexts. STUN can run over UDP, TCP, TLS-over-TCP, and DTLS-over-UDP, so a search limited to one port or transport will miss possible activity. Encrypted transports may also limit which application-level fields are visible in a packet capture, depending on capture location and whether decryption is available.
Packet evidence can show decoded protocol details and flow behavior. It may not identify the process that opened the connection; that attribution usually requires endpoint telemetry. Treat unusual STUN as a lead to investigate, not a verdict.
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Capture traffic on the relevant Linux interface
With TShark installed and permission to capture, run this on the interface carrying the traffic:
sudo tshark -i eth0 -w stun-review.pcapngReplace eth0 with the interface under investigation. Stop the capture when you have a useful review window. Interface selection, capture placement, permissions, and packet loss affect what the capture contains; traffic not visible at that point cannot be assessed from the file.
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TShark supports both live capture and reading saved capture files. Its options and behavior are documented in the TShark manual.
Find packets decoded as STUN
Apply Wireshark’s STUN display filter to the saved capture:
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tshark -r stun-review.pcapng -Y stunThe -Y option applies a display filter, and stun selects packets TShark decodes as STUN. Do not rely on a port number alone: STUN may use different transports, and port-based identification does not establish that a packet is STUN.
Inspect the decoded packet details as well as the filtered list. Wireshark’s STUN display-filter reference includes fields such as stun.type, stun.type.class, and stun.type.method, along with attribute and malformed- or short-packet indicators. Available fields can vary by Wireshark/TShark version; check the installed version’s field support if a filter fails.
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Attribute each flow and compare it with expected use
For each candidate flow, record enough context to answer who initiated it, where it went, and whether the exchange behaved as expected. A packet capture alone may not reveal the originating process, so combine it with endpoint and network records where available.
- Host and direction: identify the local system and whether the traffic is outbound or inbound.
- Remote peer and transport: record the destination and whether the flow uses UDP, TCP, TLS-over-TCP, or DTLS-over-UDP.
- Timing and frequency: note timestamps, recurrence, and whether the activity falls outside that host’s normal pattern.
- Exchange behavior: review request and response patterns, transaction IDs, and whether repeated requests appear consistent with expected behavior.
- Application attribution: use host telemetry, process or socket information, and application logs to identify the software responsible.
- Business context: compare the application and destination with approved software, known network behavior, DNS and firewall records, and the organization’s inventory.
The STUN standard describes requests, responses, indications, and transaction IDs. It also permits multiple outstanding requests. These details help interpret a flow, but they do not independently determine whether the application is authorized.
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Decide which anomalies warrant follow-up
Investigate deviations from the host’s role and baseline, particularly when multiple independent observations point to the same unexplained activity. Useful leads include:
- A host with no expected real-time communications software contacting an unfamiliar destination.
- STUN activity at an unusual time, or at a rate or destination pattern that differs from that host’s baseline.
- Repeated requests without expected responses, especially when the application’s logs or network records do not explain the behavior.
- Malformed packets or short packets that merit validation against capture quality and the expected protocol usage.
- A flow whose responsible process, destination, or business purpose cannot be reconciled with approved software and normal operations.
Corroborate a packet-level lead with process information, application logs, DNS and network telemetry, firewall records, and the approved software inventory. The cited protocol and Wireshark documentation do not define universal alert thresholds or a standalone test for malicious STUN; establish thresholds against your own applications and network baseline.
Avoid false positives from normal STUN behavior
Retransmissions are not automatically suspicious
RFC 8489 describes retransmission behavior for UDP and DTLS-over-UDP. Its recommended initial retransmission timeout is at least 500 ms, with exceptions for some usages and environments. A repeated request—or a request without a visible response—needs to be interpreted in light of the transport, capture point, application behavior, and local baseline. A capture may not include both sides of an exchange.
A missing FINGERPRINT is not a universal warning
STUN’s optional FINGERPRINT mechanism can help distinguish STUN messages from other protocols multiplexed on the same transport address. Its use depends on the particular STUN usage. Do not treat the absence of a FINGERPRINT attribute by itself as evidence of malicious activity; see the mechanism described in RFC 8489.
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STUN supports TLS-over-TCP and DTLS-over-UDP as well as UDP and TCP. With secure transports, a capture may show flow metadata without exposing the same application attributes visible in an unencrypted exchange. Use endpoint logs or other authorized telemetry when packet decoding cannot answer which application is responsible or what it is doing.
Quick Recap
Choose the right evidence source for the question
| Question | Useful evidence | What it may not establish alone |
|---|---|---|
| Is STUN visible now? | Live TShark capture on an interface that sees the traffic. | Traffic on other interfaces or outside the capture window. |
| What STUN was present in an earlier event? | A saved capture read with tshark -r and filtered with -Y stun. |
Packets that were not captured or were lost. |
| Which protocol details are visible? | Wireshark/TShark decoded fields and packet details. | Encrypted application attributes when the capture cannot be decrypted. |
| Which process opened the flow? | Endpoint process/socket telemetry and application logs, correlated with the packet timestamps and endpoints. | Attribution from packet decoding alone. |
| Is the activity unauthorized or malicious? | Correlated application, endpoint, DNS, firewall, inventory, and baseline evidence. | A universal verdict based only on STUN, retransmissions, or a missing FINGERPRINT. |
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