How to discover your Transport Relay

[aswamy_martello]

Hello,

I was looking into how Teams works under the hood and I've viewed a few resources on how Media flow works and how Teams uses Transport Relays. I saw this one video (https://www.youtube.com/watch?v=p8ml3jYt9KI - note: this is not an official Microsoft resource) that talks about how you discover the first Transport Relay using an anycast IP. The video explicitly says you begin by hitting the endpoint 52.113.192.2. When I ran Wireshark on my system, followed by opening my Teams Client, I never saw this endpoint being hit.


There is an official Microsoft Resource that talks about ensuring reachability for certain Microsoft Endpoints (https://docs.microsoft.com/en-us/microsoft-365/enterprise/urls-and-ip-address-ranges?view=o365-world...). I am assuming those IP ranges are what all the Anycast IPs fall under. My question here is:

How does Teams know which Transport Relay to hit when it turns on?

Is there one specific IP? or is it a range of IPs? If its a range of IPs, how does it select one?

Thanks for any advice or links about this topic.

Some resources I looked at that talks about Transport Relays (but doesn't explicitly tell me which one(s) it hits):
https://docs.microsoft.com/en-us/microsoftteams/microsoft-teams-online-call-flows
https://docs.microsoft.com/en-us/microsoft-365/enterprise/urls-and-ip-address-ranges?view=o365-world...
https://tomtalks.blog/2019/06/where-in-the-world-will-my-microsoft-teams-meeting-by-hosted/
https://www.youtube.com/watch?v=1tmHMIlAQdo

[#2]

@aswamy_martello 

It could hit any of the ip addresses listed at https://docs.microsoft.com/en-us/microsoftteams/office-365-urls-ip-address-ranges all of these need to be allowed and bypassing proxies, inspection etc.

Controlling or filtering at different level is unsupported and likely to trip over issues.

[#3]

@Steven Collier Thanks for the help.

I see there are three IP ranges Teams uses (13.107.64.0/18, 52.112.0.0/14, 52.120.0.0/14). Just wondering how it picks an IP in that group.

Is there a suggestion from a Teams Server?

[#4]

Hello @ aswamy_martello,

Check out the following two links
https://www.youtube.com/watch?v=1tmHMIlAQdo

https://www.microsoft.com/en-us/download/details.aspx?id=103017
Tool here shows you the relay name your client is reaching to
you can use teams in browser and start capturing network within the browser using developer tools a.k.a HAR logs within that it will show which all endpoints teams is connecting to,
however finding TR = Transport Relay could be tricky in the logs but the tools shows all

BR,
/HS