VPN OpenSwan&Xl2tp problem with big udp packets

Hi all,

I'm having a problem with my vpn server using openswan et xl2tp on a
gentoo linux.

All the tcp traffic seems to work fine but my udp packets don't pass
correctly inside the vpn. The application tries to send packets of
1500bytes. The problem is the packets are cut but all the payload
doesn't pass to the ppp+ interface.

I saw it with iptraf and tcpdump tools.

I have set my LAN interface with a mtu of 1400 and the WAN interface
have a mtu of 1500 set by the dhcp of my ethernet modem.

In the options.l2tpd file, the mtu and the mru are set to 1400.

I don't have the possibility to change the protocole used by the
application because it's an electronic regulator and a non opensource
application.æ

The client of the server is a modem/router/vpn client. But the problem
is already in the server because I see the UDP packet truncated when
they come from my local network.

Are there options to force the VPN to rebuild the udp packet as they
arrived on the other side ?

Did I miss something in the configuration to force the application to
use smaller udp packets ?

Thanks for all the help or advice you can give me.

Pionnier