Troubleshooting Tip: Fix session drops in Azure FortiWeb VMs with MTU configuration

BenoitMTL · ‎12-21-2022

Description

This article explains recommended MTU settings for a FortiWeb VM in Azure to prevent session dropping.

Scope

FortiWeb VMs in Azure.

Solution

In some rare circumstances, sessions between a real server and a FortiWeb VM in Azure can be dropped.

Once the TCP connection is established between FortiWeb and the real server (such as SYN, SYN/ACK, ACK), a TCP reset is received from the server. The causes of this reset are unpredictable and some connections may continue to work correctly in spite of it.

According to Azure, it's recommended to set the MTU to 1,400. Run the following command in FortiWeb:

# config system interface

edit interface <index>

set mtu 1400

end

For more information about tuning virtual network performance in Azure, see https://learn.microsoft.com/en-us/azure/virtual-network/virtual-network-tcpip-performance-tuning.