Wireshark Filtering S1AP to find Subscriber Signaling

The S1 interface can be pretty noisy, which makes it hard to find the info you’re looking for.

So how do we find all the packets relating to a single subscriber / IMSI amidst a sea of S1 packets?

The S1 interface only contains the IMSI in certain NAS messages, so the first step in tracing a subscriber is to find the initial attach request from that subscriber containing the IMSI.

Luckily we can filter in Wireshark to find the IMSI we’re after;

e212.imsi == "001010000000001"

The Wireshark e212 filter filters for ITU-T E.212 payloads (ITU-T E.212 is the spec for PLMN identifiers).

Quick note – Not all IntialUEMessages will contain the IMSI – If the subscriber has already established comms with the MME it’ll instead be using a temporary identifier – M-TMSI, unless you’ve got a way to see the M-TMSI -> IMSI mapping on the MME you’ll be out of luck.

Next up let’s take a look at the contents of one of these packets,

Inside the protocolIEs is the MME_UE_S1AP_ID – This unique identifier will identify all S1 signalling for a single user.

The MME_UE_S1AP_ID is a unique identifier, assigned by the MME to identify which signaling messages are for which subscriber.

(It’s worth noting the MME_UE_S1AP_ID is only unique to the MME – If you’ve got multiple MMEs the same MME_UE_S1AP_ID could be assigned by each).

So now we have the MME_UE_S1AP_ID, we can filter all S1 messaging containing that MME_UE_S1AP_ID, we’ll use this Wireshark filter to get it:

s1ap.MME_UE_S1AP_ID == 2

Boom, there’s a all the signalling for that subscriber.

Alternatively you can just right click on the value and apply it as a filter instead of typing everything in,

Hopefully that’ll help you filter to find what you’re looking for!

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