DNS Tunneling Protection

Problem this snippet solves:

This snippet helps blocking DNS tunneling that may be observed in modern enterprise networks. There is a Data Group List that allows you to specify DNS domain names that are known to be used for DNS tunneling and you do not want to receive email alerts for them.

There is one known DNS domain used for tunneling - *.mac.sophosxl.net. We saw DNS tunnels from MacBooks with Sophos Antivirus installed.

How to use this snippet:

Just add this iRule to your Virtual Server (VS) that serves as DNS for your clients. This VS must have an DNS profile attached, so you need a BIG-IP DNS license.

Do not forget to add Data Group List, like this

ltm data-group internal DGL-DNS-TUNNEL {
    records {
        mac.sophosxl.net { }
    }
    type string
}

If you need to configure email alerts add this code to /config/user_alert.conf:

alert SECPOL_DNS_TUNNEL "Detected DNS tunneling for (.*)" {
snmptrap OID=".1.3.6.1.4.1.3375.2.4.0.302";
email toaddress="$ALERT_ADDRESS"
fromaddress="$ADC_ADDRESS"
body="Security Policy: Detected DNS tunneling"
}

Replace $ALERT_ADDRESS and $ADC_ADDRESS with appropriate email addresses that are valid in your environment

Code :

when DNS_REQUEST {
    if {[string length [DNS::question name]] > 231} {
        if {[class match [DNS::question name] ends_with DGL-DNS-TUNNEL]} {
            log local0. "Detected possible DNS tunneling for [IP::remote_addr]. Request [DNS::question name] dropped"
        } else {
            log local0. "Detected DNS tunneling for [IP::remote_addr]. Request [DNS::question name] dropped"
        }
        DNS::drop
    }
}

Tested this on version:

13.0