# F5 BIG-IP Application Level NTP Health Checks

When configuring a pool of NTP servers on a F5 BIG-IP load balancer you need to choose how to check if they are still up and running. There is no specific NTP monitor on a F5 BIG-IP that does an application layer health check (like there is for http or radius). The out-of-the-box options that can be used are only ICMP and UDP monitoring. Let’s first look at the pros and cons of using either (or both) of these monitors. Then let’s build a custom UDP monitor that does a better job at checking whether the NTP servers are still healthy.

# Load Balancing NTP via F5 BIG-IP LTM

As you hopefully already know, you should use at least three different NTP servers to get your time. However, there might be situations in which you can configure only one single NTP server, either via static IP addresses or via an FQDN. To overcome this single point of failure you can use an external load balancing server such as F5 LTM (in HA of course) to forward your NTP queries to one of many NTP servers. Here are some hints:

# F5 SSL Profile: “Single DH use” not working?

In the paper of the Logjam attack, a sentence about the F5 load balancers confused me a bit: “The F5 BIG-IP load balancers and hardware TLS frontends will reuse $g^{b}$ unless the “Single DH” option is checked.” This sounds like “it does NOT use a fresh/ephemeral diffie-hellman key for new connections”. I always believed, that when a cipher suite with EDH/DHE is chosen, the diffie-hellman key exchange always generates a new $b$ for computing $g^{b}$. Hm.

Therefore, I tested this “Single DH use” option on my lab F5 unit, in order to find out whether the same public key (as noted in Wireshark) is used for more than one session.