F5 Server SSL Profile using TLS 1.0 instead of TLS 1.2

Hi pstavr,

Is this issue resolved? i have the same problem with this.

What solution do you use to improve from the F5 side?

Thanks

Hi!

All tests failed today. But I have some extra clues.

1. The app running on the background is a .NET application
2. The registry tweaks for the .NET app basically failed. Same issue.
3. We also turned off the app its self, and tried to use IIS default pages, just to see if the communication between F5 and the backend server would be OK. And it is, it works perfectly fine. So its the ,NET app that breaks it.

The weird part is that without modifying anything on F5, it works with simple IIS on the backend, but it does not with the .NET app enabled. The only difference I can see in WireShark is that the successful Client Hello done from the F5 wowards the backend server, is done using TLS 1.2. Where one that fails uses TLSv1 instead for the Client Hello. Both mark 1.0 as the minimum and 1.2 as the maximum accewpted TLS though.

Have a look at the successful attempts against IIS, and compare them to the unsuccesful against the .NET app. Only difference is that the version used for the Client Hello part. Once again, thank you all for your ideas and feedback.

Well the pcap's if you've shared, i've seen this behavior too. Sometimes the IIS server schannel fails to reply back with the fatal alert message. It simply resets the the connection.

This puts hard to know what is causing the failure of SSL. There are so many SSL alert codes, which often gives us clue to where to look at for resolution, like protocol mismatch, cipher mismatch etc.

Regarding the version header protocols, Rodrigo has answered it clearly. As per the RFC, the lowest version has to be sent across in the record layer and the highest supported protocol in the handshake protocol.

Yes there have been cases where the record layers protocol version have caused issues too, but in those scenarios its the mistake of the server for being not compliant with the RFC standards.

As part of troubleshooting, can you please confirm if its a protocol issue or a cipher issue, sometime SHA1 could be blocked on the server side too.

Also you should think about the ssl session ID too, sometimes when the GTM/LTM sends health monitor probes, the session id gets stuck too.

To note, this SSL session ID gets cleared only when their is a proper reset packet coming back. But in schannel, the alert protocol never comes and this makes it hard for the LTM/GTM to understand clear it from its memory. But this is for health monitor. Lets shift gear to serverssl profile.

Long story short: I experienced this behavior too. I had to 1st check whether its a protocol problem or the SHA1 or some ciphers. Use openssl to confirm the same.

#To test tls1.0
openssl s_client -connect IP:PORT -tls1
 
#To test tls1.1
openssl s_client -connect IP:PORT -tls1_1
 
#To test tls1.2
openssl s_client -connect IP:PORT -tls1_2
 
#To test SHA1
openssl s_client -cipher 'SHA' -connect IP:PORT -tls1_2
 
#To test SHA2
openssl s_client -cipher 'SHA256' -connect IP:PORT -tls1_2

Once you have the above data, you can come to conclusion what could be the problem. If you suspect in the Client Hello, only a certain set of ciphers are being sent and your server is limited to a particular set of ciphers, you can check that too using ssldump.

Take a normal pcap and run ssldump to it.

Regarding the fix, once you find if its TLS 1.1 or SHA, try one after one and see the results,

#To block the weak ciphers
tmsh modify ltm profile client-ssl <client-ssl-profile> ciphers "!3DES:!DH:!ADH:!EDH:!MD5:!EXPORT:!DES:!RC4:!SSLv3:!TLSv1:!TLSv1_1:!SHA1"
 
#To block the old protocols
tmsh modify ltm profile client-ssl <client-ssl-profile> options { dont-insert-empty-fragments no-sslv2 no-sslv3 no-tlsv1 no-tlsv1.1 }

Keep us posted.

Is it a .NET application by any chance?

If you see the yellow-highlighted TLS version, that is F5 sending Client Hello. Its TLSv1. The exact same PCAP capture on Windows 10 clients accessing the Windows Server 2019 directly shows up TLSv1.2.

Hi Rodrigo

Thank you for your reply. We are already trying a few tweaks on the Windows Server 2019 / IIS environment for fixing this. I understand your comment and it makes sense. What I do not understand however is why would F5 use TLS 1.0 initially to contact the backend server during the Client Hello. I am using a server SSL profile which limits the ciphers to TLS 1.2 only. I should also mention that Windows 10 clients accessing the backend server directly work perfectly fine. While doing a Wireshark capture on the Windows Clients, the Client Hello is entirely on TLS 1.2 (both lowest and highest version). Hence my questioning on why F5 would send a TLS 1.0 Client Hello. The packet is marked as TLS1 on Wireshark.

Hi pstavr,

BIG-IP is NOT using TLS 1.0.

Although not specified in RFC, the de facto standard used by all major browsers and TLS stacks is to set the ClientHello as follows:

SSL Record:
  Content Type: Handshake (22)
  Version: $LOWEST_VERSION <----
  Handshake Record:
    Handshake Type: Client Hello (1)
    Version: $HIGHEST_VERSION <----

The BIG-IP system implementation tells the TLS peer that the system supports only TLS versions from the $LOWEST_VERSION through the $HIGHEST_VERSION.

Back end server is then supposed to reply with the selected version correctly and move on.

If your server is failing because of this, then there's an issue with your server but definitely NOT BIG-IP.

Please, Is there a Fatal Alert sent by the server with the error code? You should look into that.

Hi,

For the "Options" property. You can move "No TLSv1" from avaialbe list to enabled options. Then try again.

And here is the Cipher string I am using on the server ssl profile.

F5 is running on 14.1.2.1

Any help / ideas would be greatly appreciated. Thanks!