Impact and reason for enabling promiscuous mode


New Member
Jul 11, 2023

I have configured both SecurityOnion as well as PFSense in a VM on proxmox. Typical setup:
vmbr5 is the interface that is used as monitor interface in SecurityOnion. Since I want to capture all LAN pfSense traffic, I've configured mirroring from vmbr2 to vmbr5 using tc as explained here: .

However, a final step that was required was to enable promiscuous mode on vmbr2 (as also explained here: Skipping this step showed no traffic whatsoever on vmbr5.

My questions:
  • Why is enabling promiscuous mode on vmbr2 required? Using tc I understand all traffic is simply copied to vmbr5. As vmbr2 is the original recipient of that traffic, I do not see why I need to enable promiscuous mode.
  • What is the impact of enabling promiscuous mode on that interface? Does it mean it now captures and processes everything that is destined for my other interfaces (e.g. ELK pfSense, WAN pfsense, ...)?


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