August 13, 2014

MTU Size for PPTPD VPN Sessions

Users are getting a black screen when connecting to their Windows workstation using RDP through a PPTP VPN? After looking at all possible causes, I found out the issue had to do with the VPN PPP sessions’ MTU size. Here’s how to fix the MTU size of Ubuntu’s PPTPD sessions so that everyone can get their work done.

Edit /etc/ppp/ip-up and add the following line at the end:
/sbin/ifconfig $PPP_IFACE mtu 1500
You’ll see that new PPP connections now have a MTU of 1500 when using ifconfig.

It would appear that an MTU size of 1396 for PPP sessions will sometimes cause issues when using Windows 7’s RDP through the VPN, no matter what the display options are. Fixing the MTU to 1500 as explained above fixed the black screen glitch right away. The problem was on the server after all.