Adjusting Offsec’s VPN MTU’s values to solve network and reverse shells issues: A Practical Guide.

Are you an OSCP student not in the US? Perhaps you’re in Singapore or South Africa, struggling with VPN connectivity and unable to get a reverse shell back? Path MTU Discovery (PMTUD) is a technique used to determine the maximum transmission unit (MTU) size for a communication path between two devices. This guide, inspired by a real-world issue I encountered with the Offsec’s VPN, explores how to troubleshoot and resolve some network errors by adjusting MTU.

What is MTU and Why Does It Matter?

MTU refers to the largest size of a data packet that can be sent over a network without needing fragmentation. If a packet is too large and the DF (Don’t Fragment) flag is set, it cannot be transmitted, causing errors.

Path MTU Discovery identifies the largest packet size that can be sent end-to-end without fragmentation, ensuring efficient communication.

How to Test Path MTU Discovery on Linux

On Linux, the ping command supports options for setting the DF flag and…