This is an asy one. Let’s say, for instance, you’re living in Japan and connect to the internet via PPPoE. Your service provider informs you that you need to set your MTU to 1454, but when you connect you see it’s set to 1492. You can change this using ifconfig like so:

sudo ifconfig ppp0 mtu 1454

This obviously works for other interfaces and other MTU values too – just change ppp0 and 1454 respectively.