OpenVPN fwmark RoutingSean Reifschneider, <jafo@tummy.com>Thursday November 27, 2003==========================These scripts can be used with OpenVPN up and down scripts to set uprouting on a Linux system such that the VPN traffic is sent via normalnetwork connectivity, but other traffic to that network runs over the VPN.The idea is to allow encryption of data to the network the remote host ison, without interfering with the VPN traffic.  You can't simply add a routeto the remote network, becaues that will cause the VPN traffic to also tryto run over the VPN, and breaks the VPN.These scripts use the Linux "fwmark" iptables rules to specify routingbased not only on IP address, but also by port and protocol.  This allowsyou to effectively say "if the packet is to this IP address on this portusing this protocol, then use the normal default gateway, otherwise use theVPN gateway.This is set up on the client VPN system, not the VPN server.  These scriptsalso set up all ICMP echo-responses to run across the VPN.  You cancomment the lines in the scripts to disable this, but I find this usefulat coffee shops which have networks that block ICMP.To configure this, you need to set up these scripts as your up and downscripts in the config file.  You will need to set these values in theconfig file:   up /etc/openvpn/fwmarkroute.up   down /etc/openvpn/fwmarkroute.down   up-restart   up-delay   setenv remote_netmask_bits 24Note: For this to work, you can't set the "user" or "group" config options,because then the scripts will not run as root.The last setting allows you to control the size of the network the remotesystem is on.  The remote end has to be set up to route, probably withmasquerading or NAT.  The network this netmask relates to is calculatedusing the value of "remote" in the conf file.Sean