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dlogsdonmd's avatar
dlogsdonmd
Icon for Nimbostratus rankNimbostratus
Jun 14, 2017

Block Ciphers F5 LTM

Hello, I want to block specific ciphers on my LTM. We use a common SSL client profile for a good chunk of our sites/subdomains. Below are the two ciphers I want to block (SSL Labs reports them as weak). Below that is what we currently have on our SSL client profile.

We're running LTM 11.5.1 build 6.0 hotfix FH6

Ciphers need to block:

TLS_RSA_WITH_3DES_EDE_CBC_SHA (0xa)
TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA (0xc012)

Currently configured on LB:

DEFAULT:!SSLv3:!RC4

Should SLL profile be updated as below?

DEFAULT:!SSLv3:!RC4:!3DES

I don't want to guess so appreciate any assistance provided.

Thanks in advance.

Diane

application delivery
devops
LTM

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  • Leonardo_Souza's avatar
    Leonardo_Souza
    Icon for Cirrocumulus rankCirrocumulus
    Jun 14, 2017

    The first thing you need to understand is the fact that the "DEFAULT" changes between versions. If you update the F5 to 12.1.2, it will probably have a default that has removed the ciphers that are considered weak today.

    Secondly, no need to guess, as you can test the behaviour without applying the change. If you do this commands:

    tmm --clientciphers "DEFAULT:!SSLv3:!RC4"
tmm --clientciphers "DEFAULT:!SSLv3:!RC4:!3DES"

    And compare the output, you know which ciphers were removed.

    Lastly, some solutions to explain you a little bit more:

    https://support.f5.com/csp/article/K13156

    https://support.f5.com/csp/article/K13163

    Or the main solution for SSL profiles:

    https://support.f5.com/csp/article/K8802