How to make custom routes survive reboots?
See this script (don't forget to change ADDRESS/MASK GATEWAY with your values ): 

$ sudo mkdir /Library/StartupItems/AddRoute
# cd /Library/StartupItems/AddRoute
$ echo "{
        Description     = \"Add static routing tables\";
        Provides        = (\"AddRoutes\");
        Requires        = (\"Network\");
        OrderPreference = \"None\";
}" &gt; StartupParameters.plist
$ echo "#!/bin/sh

# Set up static routing tables 
. /etc/rc.common

StartService ()
{
        ConsoleMessage "Adding Static Routing Tables"
        sudo route add ADDRESS/MASK GATEWAY
}

StopService ()
{
        return 0
}

RestartService ()
{
        return 0
}

RunService "$1" &gt; AddRoutes
$ sudo chmod 755 AddRoutes