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 * [http://www.jacco2.dds.nl/networking/freeswan-l2tp.html Using a Linux L2TP/IPsec VPN server]
 * [http://dsas.blog.klab.org/archives/50133467.html DSAS開発者の部屋:Linux 2.4 と 2.6 で IPsec 接続 -- FreeS/WAN と racoon]