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Forum Discussion

pstavr's avatar
pstavr
Icon for Cirrus rankCirrus
Jan 24, 2020
Solved

F5 Server SSL Profile using TLS 1.0 instead of TLS 1.2

Hi   I have an F5 virtual server that does SSL inspection so it has a client ssl profile and a server ssl profile. The backend server is running on a Windows Server 2019 / IIS and it only accepts...
application delivery
BIG-IP
Client Hello
LTM
RST ACK
server ssl profile
tls
  • pstavr's avatar
    pstavr
    Jan 31, 2020

    Hi all.

     

    I found the root cause. The problem was related to the .NET app using SNI. By default the F5 doesn't do that.

    https://devcentral.f5.com/s/articles/ssl-profiles-part-7-server-name-indication

     

    So basically I just followed the fix in the above article, I defined a server name and the backend service started sending Server Hello etc. Everything works fine now!

     

    Thank you all for your responses, as quite a few of them were helpful on identifying that the issue is with the app, and I could also spot a few things that were not proper on the negotiation part.

     

Rodrigo_Albuque's avatar
Rodrigo_Albuque
Icon for Cirrocumulus rankCirrocumulus
Jan 24, 2020

pstavr,

 

That is what I'm trying to explain.

 

The TLS1.0 Client Hello BIG-IP is sending is just a convention to signal which version it supports.

 

If your Windows client only sends TLS1.2/TLS1.2 it means it only supports TLS1.2 and nothing else.

 

Because BIG-IP supports TLS1.0, TLS1.1 and TLS1.2 it signals to server TLS1.0/TLS1.2.

 

This is expected behaviour.

 

And FYI, I took a capture on my Ubuntu box and this is my client hello:

Does it make sense?

Rodrigo_Albuque's avatar
Rodrigo_Albuque
Icon for Cirrocumulus rankCirrocumulus
Jan 31, 2020

If you're restricting BIG-IP to ONLY TLS1.2 then that might be a bug as BIG-IP is supposed to send TLS1.2/TLS1.2 in my understanding. However, the fact that BIG-IP signals IIS that it supports TLS1.0 to TLS1.2 is not supposed to trigger connection failure. The fact that BIG-IP also supports TLS1.0 and TLS1.1 is not supposed to break the connection. IIS is supposed to reply with TLS1.2 version and keep going. I'd say this is a broken behaviour from IIS and I believe the easiest thing to solve your issue is just to have a quick look at IIS log when the connection fails. Other than that, you can definitely open a low-priority (Sev4) Support ticket with F5 and ask them to file a low-priority bug for this behaviour.