Firewall Issue

Problem

OpenVPN GUI v1.0.3 on a Windows laptop (client) talking to a OpenVPN (2.0.9-1.el4.rf) Linux server through two firewalls: an SMC2804WBRP-G wireless router (communicating over wireless), and a NetScreen 5GT. Large packets cause the connection to freeze.

Solution

Added to the client side:

# MTU issues
tun-mtu 1500
fragment 1400
mssfix

Added to the server side:

fragment 1400
mssfix

Note that it is not required for other clients to have the client changes in order to communicate properly with the server.

Commentary

The Netscreen 5GT is probably not at fault since I had many successful connections through other Netscreen 5GT and Netscreen-25 units without the change. However I have a smaller SMC router at home and didn't encounter this problem there, either, so honestly I don't know what is going on.