3.1.1. Lab 1 - TLS Version Control¶

3.1.1.1. Scenario¶

If you care about security at all, then you also need to care about SSL and TLS. These are the protocols that provide encryption to the Internet traffic. It’s the “S” in HTTPS, FTPS, and IMAPS. You get the picture!

Without encryption, all communication between a client and server is visible for any third party to see and this can have some pretty devastating effects. Encryption is a cornerstone of network security. If you’ve been paying attention to the news the last few years, you’re probably also aware that SSL and TLS are not without their own flaws. Exploits like BEAST, Heartbleed, and POODLE take advantage of specific holes in the SSL and TLS protocols. To stay ahead of these vulnerabilities, industry best practices suggest that we avoid some versions of these protocols specifically SSLv2, SSLv3, and now TLSv1. This also applies to certain ciphers such as MD5, RC4, and eventually the RSA key exchange.

The F5 BIG-IP platforms make it extremely easy to control and enforce these protocols and ciphers but at the same time, you may not simply want to “break” some users. If your income depends on Internet commerce, the last thing you may want to do is block customers from buying from you because they have a slightly out-of-date browser. Eventually, most of these older browsers will go away or at least enough of them that you can justify disabling lower encryption protocols and ciphers. But, how do you know?

F5 iRules have access to everything in the OSI layers from 3 to 7 that includes some pretty rich information about SSL and TLS. Let’s take a look at an iRule that will catalog the SSL/TLS versions that clients are using.

3.1.1.2. Requirements¶

• BIG-IP LTM, web server, client browser, and a SSL server certificate. If you don’t have certificates to test with, you can use the CA certificate, server certificate, and private key provided in the Client Certificate Inspection lab from Additional labs section.

3.1.1.3. The iRule¶

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when CLIENTSSL_HANDSHAKE {
    ISTATS::incr "ltm.virtual [virtual name] c [SSL::cipher version]" 1
}

3.1.1.4. Analysis¶

• iStats are user-created custom statistics, accessible from both the data plane (iRules) and the control plane (tmsh, on-box scripts, etc.). What we’re doing here is simply cataloging the SSL/TLS version from each client SSL handshake and storing these in an iStats table. That data can then be accessed from pretty much anywhere. For example, see David Holmes’ excellent guide on dumping this information to a Google pie chart:

  https://devcentral.f5.com/codeshare/categorize-ssl-traffic-by-version-display-as-graph/tag/istats

3.1.1.5. Testing¶

• Apply this iRule to an SSL VIP and test across several browsers and platforms. If at all possible, stage this iRule at someplace that it can be accessed by a larger audience.

• At any point you can access the data collected in the iStats by simply typing the following at the BIG-IP command line:

  istats dump

Here’s an example output:

[ ltm.virtual=/Common/stdsslvip ][TLSv1.1] = 54 (2015-09-01 13:52:20)
[ ltm.virtual=/Common/stdsslvip ][TLSv1.2] = 282 (2015-09-01 13:52:20)
[ ltm.virtual=/Common/stdsslvip ][TLSv1] = 32 (2015-09-01 13:52:20)

3.1.1.6. Bonus version¶

Now that you have a better idea of what protocols are being used on your site, you may now want to ease your customers into a better security posture than simply denying access with no warning. To do this, we’ll first test the SSL/TLS version. if it’s below our security threshold, we’ll redirect the user to another page or site to inform them that they need to upgrade their browser.

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when HTTP_REQUEST {
    if { (( [SSL::cipher version] equals "TLSv1" ) or ( [SSL::cipher version] equals "SSLv3" )) and not ( [HTTP::uri] equals "/insecure.html" ) } {
        set redirect "https://www.f5demolabs.com/insecure.html"
        HTTP::respond 302 Location "${redirect}"
   }
}

You’re still allowing SSLv3 and TLSv1 at this point, which is definitely bad, but you’re not allowing access to the application for anything less than TLSv1.1.