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Tag: advanced settings

Esxi Net.ReversePathFwdCheckPromisc Advanced setting

Esxi Net.ReversePathFwdCheckPromisc Advanced setting

During deployment of Cisco proxy appliance, we discovered a problem. According to cisco to resolve this problem qa“Net.ReversePathFwdCheckPromisc” should be set to “1” on ESX’s.

The question is – do you know any negative effects which such change could cause. We believe that there must be a reason why by default this option is set to 0 ? That’s why I decided to figure our what it is used for.

After some research I was able to find answer:

Setting – > Net.ReversePathFwdCheckPromisc = 1 — > this is when you are expecting the reverse filters to filter the mirrored packets, to prevent multicast packets getting duplicated.

Note: If the value of the Net.ReversePathFwdCheckPromisc configuration option is changed when the ESXi instance is running, you need to enable or re-enable the promiscuous mode for the change in the configuration to take effect.

The reason you would use promiscuous mode depends on the requirement and configuration. Please check the below KB Article:

  • This option is not enabled by default because we are not aware of the vSwitch configuration and can’t predict what it could be as it has configurable options.

VMware does not advise to enable this option if we do not have a use case scenario with teamed uplinks and have monitoring software running on the VMs ideally. As When promiscuous mode is enabled at the port group level, objects defined within that port group have the option of receiving all incoming traffic on the vSwitch. Interfaces and virtual machines within the port group will be able to see all traffic passing on the vSwitch causing VM performance impact.

Should the ESX server be rebooted for this change to take effect:  answer is – > Yes, and Yes you can enable this option with the VMs running on the existing portgroup.

Do you have any interesting virtualization related question?


SAP application on vSphere platform

SAP application on vSphere platform

This is a mini article to start our Q&A set, a set of not easy to find answer real life questions 😉 qa
Recently I received a question-related to advanced settings SAP app on vSphere platform:
“One of our customer ask us to set the following option to their virtual system: Misc.GuestLibAllowHostInfo This is according to SAP note: 1606643 where SAP requires reconfigure virtual system default configuration. I can’t find details information, which host data would be exposed to virtual system. Could you please point me to documentation or describe which information is being transferred from HOST to virtual systems?“

  • After some research I was able to find answer :

“Misc.GuestLibAllowHostInfo” and “tools.guestlib.enableHostInfo” these configurations if enabled allow the guest OS to access some of the ESXi host configurations, mainly performance metrics e.g. how many CPU cores the host has, their utilization and contention etc. There is no confidential information from other customers which would be visible, however, it may give the user of those SAP VMs access to performance/resource information which you may not want to share.

The following document outlines the effect of the changes as I have described above.

I believe the “might use the information to perform further attacks on the host” could only apply to other vulnerabilities which may exist for the particular hardware information that the guestOS can gather from the ESXi host.
Other than that I am not sure there is any other concern to worry about.

Do you have any interesting virtualization related question?