Lync (Skype for Business) Across Organisations

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Lync (Skype for Business) Across Organisations

L3 Networker

Hi guys,


Is anyone else having issues using Lync (Skype for Business) when messaging, or sending file transfers, to someone in a separate Skype Organisation in an external network?


Not sure if this is a bug or not, as the application is being seen as Incomplete, and looking at Packet Captures the SYN ACK isn't being received.



Kind regards



L4 Transporter



I'm using Skype for Business almost every day and I dont see any issues.

If You want please send me PM with Your skype login so I can send a message to You.




L2 Linker

Are you hosting your own SfB infrastructure, or cloud-based? Do you run decryption? Have some experience under these scenarios which doesn't work well through PAN, if this is applicable let me know and I will provide more detail.




Hi Shannon-Rowe,


It's on-premise so yes their own SFB infrastructure. Disabling decryption meant we could send files through 5 attempts but after that it failed. Looking into the PCAPs and flow basic shows that two internal IP addresses are trying to talk to one another. This is something to do with STUN right? Instead of STUN it's trying to use ports 50,000-59,000.



Hi Jack,


Yes that is just a behaviour of STUN, it is a bit confusing when you see that in the packet capture.




Hi Shannon,


Indeed, but it's not going through a port that STUN uses. STUN doesn't use any ports between 50,000-59,999 which is why it won't work I believe. Internal IPs can communicate over STUN for Lync file transfers but not anything else.



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