Technical data
4. Packages
Support informations: Shows all informations relevant when problems occur. You may
copy&paste these informations i.e. for a post on the newsgroups.
4.14.7. OpenVPN - Collaboration Of Different OpenVPN Versions
Please note that different versions of OpenVPN may use different default parameters for a
connection. In particular MTU fragment and MSSFIX settings may differ. If values „don’t
match“ connection establishment is not possible or no reliable connection can be made. Typical
error messages can be:
FRAG_IN error flags=0xfa2a187b: FRAG_TEST not implemented
FRAG_IN error flags=0xfa287f34: spurrious FRAG_WHOLE flags
Crucial parameters for a connection are:
OPENVPN_x_TUN_MTU MTU Values of the TUN device were set to 1300 for OpenVPN
1.x. As of OpenVPN 2.0 1500 is the default here.
OPENVPN_x_LINK_MTU Byte size of the connection in both OpenVPN daemons. The
default is depending on OpenVPN Version and operating system version.
OPENVPN_x_FRAGMENT Data packets (UDP or TCP) with a size bigger than the
fragment size will be fragmented to packets not bigger than byte size provided in
OPENVPN_x_FRAGMENT.
OPENVPN_x_MSSFIX To avoid fragmentation of data packets for TCP connections over
VPN a maximum size for TCp data packets can be set here. Up-to-date operating
systems will honorate this setting and make fragmentation unnecessary.
Different OpenVPN versions use the following settings as default values. Please obey these
values when connecting OpenVPN in varying versions. Default settings on fli4l routers are
shown in the second table.
OpenVPN Version/Option 1.xx 2.00
OPENVPN_x_TUN_MTU 1300 1500
OPENVPN_x_TUN_MTU_EXTRA unknown 32
OPENVPN_x_FRAGMENT unknown not configured
OPENVPN_x_MSSFIX not configured 1450
Table 4.10.: Different MTU parameters in different OpenVPN versions.
fli4l Version/Option up to 2.1.8 from 2.1.9 on
OPENVPN_x_TUN_MTU 1300 1500
OPENVPN_x_TUN_MTU_EXTRA 64 32
OPENVPN_x_FRAGMENT not configured 1300
OPENVPN_x_MSSFIX not configured 1300
Table 4.11.: Different MTU parameters in different fli4l router versions.
Based on this settings the defaults for your network should be determined and written to
config/openvpn.txt explicitely. These are the best values for your tests to start with:
180