F5 sending packets to wrong destination?

Problem: You have a network with two upstream routers and an F5 LTM loadbalancer. Even though the default gateway points to router R1 the F5 LTM sends packets to the mac address of R2.

Discussion: “This is a feature not a bug”. This “Feature” is called “Auto Last Hop”. Which means the F5 answers packets allways to the mac address of the received packet. This may be usefull in some cases. But from the view of standards, compliance and security this behavior is a bug. In my case R2 sent some traffic to the F5 because of BGP multihoming, and received the answer allthough R1 should have received the traffic. Unfortunatly this setting is “Enabled” by default on F5.

If a hacker manages to inject  a request with a forged IP address, he will receive the answer even if the route to this IP points in a different direction.

Solution: This “Feature” can (and should) be disabled, if you don’t explicitly need it. It can be disabled globally, per VLAN, per Virtual Server or per SNAT policy.
You can find this setting globally in the web interface: System -> Configuration -> Local Traffic -> General
And for VLAN it can be disabled at:  Network -> VLANs -> Configuration [ Advanced]

Version: F5 LTM 11.5.1 8.0.175

Link: https://support.f5.com/kb/en-us/solutions/public/13000/800/sol13876.html