After setting up pptpd on a cheap VPS sat in New York to gain access to Netflix USA, I noticed some problems with several websites when connected to the VPN on Windows. Strangely these issues can not be replicated using a Mac.
After a little bit of playing around, I noticed an interesting difference between a Windows and Mac connection in the ifconfig output:
inet addr:10.6.0.1 P-t-P:10.6.0.101 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1396 Metric:1
RX packets:126 errors:0 dropped:0 overruns:0 frame:0
TX packets:71 errors:0 dropped:0 overruns:0 carrier:0
RX bytes:14384 (14.3 KB) TX bytes:6301 (6.3 KB)
ppp1 Link encap:Point-to-Point Protocol
inet addr:10.6.0.1 P-t-P:10.6.0.102 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1496 Metric:1
RX packets:2412 errors:0 dropped:0 overruns:0 frame:0
TX packets:1742 errors:0 dropped:0 overruns:0 carrier:0
RX bytes:1174836 (1.1 MB) TX bytes:644382 (644.3 KB)
The ppp0 connection is a Windows machine and ppp1 is a Mac. For some reason the Windows machine is using an MTU of 1396 while the Mac is using 1496. It seems to be a common problem that a lower than normal MTU causes some websites not to load (at least there are numerous similar or related articles on Google).
Fortunately there is a simple fix, I just added this line to the bottom of /etc/ppp/ip-up:
Future connections now all use an MTU of 1500 and the browsing issues are gone (I’m using Ubuntu 14.04 Server for the pptpd server).